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Quantum Mechanical Screening of Single-Atom Bimetallic Alloys for the Selective Reduction of CO2 to C1 Hydrocarbons

† The Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
‡ Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, United States
§ Department of Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, California 94720, United States
ACS Catal., 2016, 6 (11), pp 7769–7777
DOI: 10.1021/acscatal.6b01393
Publication Date (Web): September 23, 2016
Copyright © 2016 American Chemical Society
OpenURL UNIV OF NORTH TEXAS
*E-mail: alexbell@berkeley.edu (A. T. Bell)., *E-mail: mhg@cchem.berkeley.edu (M. Head-Gordon).

Abstract

Abstract Image

Electrocatalytic reduction of CO2 to energy-rich hydrocarbons such as alkanes, alkenes, and alcohols is a very challenging task. So far, only copper has proven to be capable of such a conversion. We report density functional theory (DFT) calculations combined with the Poisson–Boltzmann implicit solvation model to show that single-atom alloys (SAAs) are promising electrocatalysts for CO2 reduction to C1 hydrocarbons in aqueous solution. The majority component of the SAAs studied is either gold or silver, in combination with isolated single atoms, M (M = Cu, Ni, Pd, Pt, Co, Rh, and Ir), replacing surface atoms. We envision that the SAA behaves as a one-pot tandem catalyst: first gold (or silver) reduces CO2 to CO, and the newly formed CO is then captured by M and is further reduced to C1 hydrocarbons such as methane or methanol. We studied 28 SAAs, and found about half of them selectively favor the CO2 reduction reaction over the competing hydrogen evolution reaction. Most of those promising SAAs contain M = Co, Rh, or Ir. The reaction mechanism of two SAAs, Rh@Au(100) and Rh@Ag(100), is explored in detail. Both of them reduce CO2 to methane but via different pathways. For Rh@Au(100), reduction occurs through the pathway *CO → *CHO → *CHOH → *CH + H2O(l) → *CH2 + H2O(l) → *CH3 + H2O(l) → * + H2O(l) + CH4(g); whereas, for Rh@Ag(100), the pathway is *CO → *CHO → *CH2O→ *OCH3 → *O + CH4(g) → *OH + CH4(g) → * + H2O(l) + CH4(g). The minimum applied voltages to drive the two electrocatalytic systems are −1.01 and −1.12 VRHE for Rh@Au(100) and Rh@Ag(100), respectively, at which the Faradaic efficiencies for CO2 reduction to CO are 60% for gold and 90% for silver. This suggests that SAA can efficiently reduce CO2 to methane with as small as 40% loss to the hydrogen evolution reaction for Rh@Au(100) and as small as 10% for Rh@Ag(100). We hope these computational results can stimulate experimental efforts to explore the use of SAA to catalyze CO2 electrochemical reduction to hydrocarbons.

Keywords:

CO2 reduction; density functional theory; electrocatalysis; one-pot tandem catalyst; single-atom alloys;

1 Introduction

Global energy consumption is projected to triple by the end of the 21st century, relative to the present rate, because of population and economic growth.(1, 2) While this increased energy demand can be met through the use of fossil fuels (petroleum and coal), doing so is expected to triple the level of atmospheric CO2 by the end of the century.(1) Such a dramatic increase in atmospheric CO2 concentration in the atmosphere is likely to perturb global ecosystems on an unprecedented scale.(3) A potential means for reducing CO2 concentration and achieving carbon neutrality is the electrochemical reduction of CO2 to fuel using electricity produced from wind, solar radiation, and geothermal heat. To achieve this goal, it is necessary to discover electrocatalysts for CO2 reduction that are inexpensive (ideally composed of Earth-abundant materials), efficient (having low overpotentials), selective, and stable.(4-13) With regard to selectivity, the objective is to obtain high Faradaic efficiencies to the products of CO2 reduction while reducing the Faradaic efficiency for the hydrogen evolution reaction (HER), which can dominate, particularly at less negative applied voltages.(14-18)
In thermodynamic terms, the outcome of the electrocatalysis is determined by the stability of the first hydrogenation intermediates (*COOH or *OCHO for CO2RR, and *H for HER; see Scheme 1).(19-21) If *COOH or *OCHO is more stable than *H, it will occupy the active sites, and then the CO2RR is the primary pathway; on the other hand, if *H is more stable, then the HER will be the dominant pathway. Since the formation of these intermediates all involve the transfer of one (H+ + e–) pair, variations in the applied voltage (assuming the interaction between the dipole of the adsorbate and the space-charge field is small and can be ignored)(22) or pH will not affect the relative stability of the *COOH, *OCHO, or *H intermediates. As a consequence, the key to enhancing the CO2RR and to suppressing the HER is discovery of electrocatalysts that bind *COOH or *OCHO more strongly than *H (ΔG*COOH or ΔG*OCHO < ΔG*H, where ΔG*COOH, ΔG*OCHO, and ΔG*H are the Gibbs free energies of formation for surface-bonded COOH, OCHO, and H species, respectively, referenced to a clean surface, H2(g), and CO2(g)).
figure
Scheme 1. Schematic Description of the First Two Hydrogenation Steps for the Electrochemical CO2 Reduction Reaction and Hydrogen Evolution Reactiona

aThe asterisk symbol (*) represents the surface, and “*A” represents A adsorbed on the surface.

Meeting this criterion has proven to be a very challenging task. Karamad et al. have used density functional theory (DFT) to predict the performance of 34 intermetallic alloys for CO2RR, relative to that for HER.(20) They found that ΔG*COOH (or ΔG*OCHO) > ΔG*H for all 34 materials, meaning that HER would be the dominant reaction pathway in all cases. Changing the electrocatalysis to consider the competition between the CO reduction reaction (CORR) versus HER on the same 34 catalysts showed that four of them can perform CORR preferentially to HER. In other words, four cases satisfy ΔG*CHO (or ΔG*COH) < ΔG*H, where *CHO and *COH are the first hydrogenated intermediates of CORR (see Scheme 2), and ΔG*CHO, ΔG*COH, and ΔG*H are their Gibbs free energies of formation, relative to a clean surface, and H2(g), and CO(g).(20) Based on those discoveries, the authors proposed a two-pot tandem reaction for CO2RR, in which CO2 is first reduced to CO by well-defined existing protocols such as the reverse water–gas shift reaction,(23) and then CO is further reduced to hydrocarbons by those alloys. However, the capital cost for the proposed two-pot process is expected to be higher than that for an integrated system. Therefore, there is an opportunity to explore new types of electrochemical catalysts that can first convert CO2 to CO, and then to hydrocarbons, while suppressing the HER.
figure
Scheme 2. Schematic Description of the First Hydrogenation Step for the Electrochemical CO Reduction Reaction and Hydrogen Evolution Reaction
In this study, quantum mechanical DFT calculations are used to screen 28 single-atom alloys (SAAs) as novel electrochemical catalysts for CO2RR. Those catalysts are composed of isolated single atoms, M (M = Cu, Ni, Pd, Pt, Co, Rh, and Ir), embedded into the (111) and (100) surfaces of Au or Ag (Figure 1). The total number of SAAs is 28 (7 × 2 × 2 = 28), and we will denote these catalysts as M@Au(111), M@Au(100), M@Ag(111), or M@Ag(100).
figure

Figure 1. Surface models used to simulate the single-atom alloys: (a) M@Au(111), M@Ag(111), and (b) M@Au(100), M@Ag(100). Each model is composed of a three-layer slab, using a 3 × 3 periodic cell, where one Au or Ag atom on the topmost layer is replaced by M (M = Cu, Ni, Pd, Pt, Co, Rh, and Ir). This means that the doping concentration in the top layer is 11.1% (1/9). The periodic boundary is represented by the black dotted line.

Au and Ag are known for their ability to reduce CO2 to CO with high Faradaic efficiencies,(17, 18, 24-28) in preference to the HER, and are the major component of our proposed catalysts. Therefore, we expect that CO2 will first be selectively reduced to CO by the surrounding Au or Ag (Scheme 3). While Ag and Au cannot further reduce CO, because of its much stronger binding interaction with M than with Au or Ag,(29) the newly formed CO will be preferentially bound to the single atom alloy site, M. There CO may be further reduced to hydrocarbons, if the CORR can be favored versus HER.
figure
Scheme 3. Schematic Description of Our Proposed One-Pot Tandem Catalytic Reactiona

aThe proposed electrocatalyst is composed primarily of gold or silver (colored gold), alloyed with small amounts of M (M = Cu, Ni, Pd, Pt, Co, Rh, and Ir). The feedstock CO2 is first reduced to CO by gold or silver, and is subsequently captured and further reduced to C1 products by M.

There are at least two advantages for this type of catalyst. First, since CO2 reduction to CO, and the further reduction of CO to hydrocarbons are catalyzed by the same catalyst (the one-pot tandem catalyst), these SAAs are intrinsically simpler than a two-pot tandem approach, such as the one proposed by Karamad et al.(20) Second, because individual M atoms can accommodate only a single CO, and are isolated from each other by gold or silver, C–C coupling is unlikely. Therefore, the catalyst is expected to be selective for C1 hydrocarbons (such as CH4 and CH3OH), and may even be selective for a single C1 product, simplifying the challenge of product separation.
While our work is entirely computational, there are encouraging experimental precedents for the use of SAAs to catalyze a variety of chemical reactions, such as the electrochemical reduction of O2 to H2O2, the selective hydrogenation of organic compounds, and the Ullmann reaction of aryl halides.(30-39) For example, Jirkovsky et al. have synthesized Pd@Au SAA and used it for selective reduction of O2 to H2O2 with ∼95% selectivity.(38) Sykes and co-workers have synthesized Pd@Cu for the selective hydrogenation of styrene and acetylene(36) and Pt@Cu for 1,3-butadiene hydrogenation to butenes.(34) Also, Blaylock et al. have found Pd@Ag to be an efficient catalyst for the selective hydrogenation of acetylene and acrolein.(39)
The remainder of this paper is organized as follows. In Section 2, the computational approach that we employ is described. Our results are presented in Section 3, beginning with computational screening of 14 Au-based SAAs and 14 Ag-based SAAs, searching for alloys that exhibit high selectivity for CORR versus HER, as well as a relatively low applied bias to enable the endergonic CORR to proceed past the critical first reduction step. We then turn to a detailed mechanistic examination of two of the most promising alloys, showing that both of them appear to be selective for methane formation (over both methanol and hydrogen), as well as a discussion of some of the considerations that are relevant to possible future experimental preparation and study of these alloys.

2 Computational Details

The revised PBE functional (RPBE)(40) and the projector augmented wave pseudo-potentials,(41, 42) as implemented in the Vienna ab initio Simulation Package (VASP),(43-46) were employed for all slab and molecule calculations. The Gaussian-smearing technique (smearing temperatures kBT = 0.1 eV for slabs and 0.01 eV for molecules) was used to accelerate SCF convergence, after which all calculated values of energy were extrapolated to kBT = 0. A Monkhorst–Pack k-point net of 3 × 3 × 1 was chosen to sample the reciprocal space for the slab calculations, whereas only the gamma point was sampled for the molecule calculations. Test calculations (see Computational Details in the Supporting Information) indicated that a 2 × 2 × 1 Monkhorst–Pack k-point net yields chemically significant changes in binding energy (as large as 0.22 eV for ΔG*COH), while the changes upon extension to 5 × 5 × 1 are at least two times smaller. Extending the slab from three layers to four layers also yields similar energy changes of no more than ∼0.1 eV (see the Supporting Information), which are likely to be similar to inherent errors in the density functional itself.
To prevent interactions between the periodic replicas along the z-direction, a vacuum separation of at least 15 Å between adjacent images was used for the slab calculations, and a 20 Å × 20 Å × 20 Å box was used for molecular calculations. Spin-polarized wave functions were used for all surface calculations, while spin-restricted wave functions were used for molecular calculations. A dipole correction was not employed in this study, since test calculations showed that it has an insignificant effect on the binding energies of ΔG*H, ΔG*CHO, and ΔG*COH.
As already shown in Figure 1, the slab models for all SAAs were composed of three metal layers, where each layer contains a 3 × 3 periodic cell. One Au or Ag atom on the topmost layer is substituted by M, leading to a surface substituent concentration of 1/9 (or 11.1%). During the geometry optimization, the atoms on the bottom layer were fixed in their bulk positions (a separation of 4.073 Å for Au, and 4.086 Å for Ag), whereas the atoms on the two top layers and all adsorbate atoms were allowed to relax. Geometries were optimized in water, simulated using the Poisson–Boltzmann implicit solvation model with a dielectric constant of ε = 80.(47)
All energies discussed in this work are free energies. For slabs, they were calculated asThe vibrational frequencies were evaluated for only surface adsorbates and were calculated by evaluating the partial Hessian matrix by finite differences. Based on the calculated vibrational frequencies, the zero-point vibrational energy (ZPVE), and vibrational contributions to the internal energy and entropy at 25 °C were calculated. The free energy of molecules was evaluated aswhere n = 8 for nonlinear molecules and n = 7 for linear molecules. Eelecsolv was obtained using the same approach as that for the slab systems. However, the ZPVE, the vibrational component of internal energy, and Svib, Srot, and Strans were calculated by the Q-Chem package, using the same functional with the 6-31G* Gaussian-type basis set. Finally, it is well-known that gas-phase thermochemical reaction energies computed with the RPBE functional exhibit systematic deviations with experimental data.(48) Therefore, following previous theoretical work, a sensitivity analysis was performed for 21 chemical reactions,(49) and we found that a +0.38 eV correction for the electronic energy for CO2 is needed.
For the investigation of reaction mechanisms, we assumed that proton and electron transfer are coupled. We also assumed that the barriers for hydrogenation through electrochemical means are small and easily surmountable at room temperature; therefore, we only considered thermodynamics and ignored kinetics. This assumption is supported by recent theoretical work showing the kinetic barriers are small, particularly under the influence of the applied voltages.(50) To relate our calculated free-energy diagrams at U = 0.0 VRHE to different applied voltages, the computational hydrogen electrode (CHE) model proposed by Nørskov and co-workers was used.(51) In the CHE, the zero of voltage is defined to correspond to a proton and an electron in equilibrium with H2(g) at a pressure of 101 325 Pa. Using the standard relation between applied bias and free energy, the chemical potential of the proton and the electron at an applied bias (U) may then be expressed as μ(H+) + μ(e–) = 1/2μ(H2(g)) – eU, where e is the electron charge.
To evaluate the performance of the DFT methods, we first calculated the equilibrium potentials for CO2 reduction to formic acid, CO, formaldehyde, methanol, and methane (Table 1), all of which are common intermediates during the reduction. As shown in Table 1, the equilibrium potentials were calculated to be −0.22, −0.11, −0.09, 0.03, and 0.17 VRHE for the reduction to formic acid, CO, formaldehyde, methanol, and methane, respectively. [Note: VRHE is the unit of applied voltage (vs RHE).] Those numbers are in excellent agreement with the experimental values of −0.20, −0.12, −0.07, 0.03, and 0.17 VRHE, respectively.(52)
Table 1. Experimental(52) and Calculated Equilibrium Potential for Some Two-, Four-, Six-, and Eight-Electron Reductions
  Equilibrium Potential, U (eV, vs RHE)
 reactionexperimenttheory
1CO2(g) + 2(H+ + e–) → HCOOH(aq)–0.20–0.22
2CO2(g) + 2(H+ + e–) → CO(g) + H2O(l)–0.12–0.11
3CO2(g) + 4(H+ + e–) → CH2O(aq) + H2O(l)–0.07–0.09
4CO2(g) + 6(H+ + e–) → CH3OH(aq) + H2O(l)0.030.03
5CO2(g) + 8(H+ + e–) → CH4(G) + 2H2O(l)0.170.17

3 Results and Discussion

3.1Identification of Catalysts with a High Selectivity to CORR
As stated in the Introduction, the requirement for a catalyst to be selective for CORR over HER is that either ΔG*CHO < ΔG*H or ΔG*COH < ΔG*H, meaning that the catalyst binds with *CHO or *COH more tightly than with *H. Therefore, as the first step in our investigation, ΔG*CHO, ΔG*COH, and ΔG*H were calculated for all 28 SAAs and the corresponding applied voltages needed to hydrogenate *CO to *CHO and *COH were evaluated. We considered all possible adsorption sites; however, the values of ΔG*CHO, ΔG*COH, and ΔG*H reported in Table 2 are based on the most stable binding site (see Tables S1 and S2 in the Supporting Information).
Table 2. Calculated Binding Free Energy of *CHO (ΔG*CHO), *COH (ΔG*COH), and *H (ΔG*H), and the Applied Bias Necessary for the Reduction of *CO to *CHO (U*CHO) and *COH (U*COH)
catalystΔG*H (eV)ΔG*CHO (eV)ΔG*COH (eV)U*CHO (VRHE)U*COH (VRHE)
Cu@Au(111)0.370.731.22–0.61–1.10
Ni@Au(111)0.290.460.70–1.10–1.33
Pd@Au(111)0.380.521.06–0.69–1.23
Pt@Au(111)0.000.040.70–0.70–1.35
Co@Au(111)0.14–0.040.02–1.21–1.27
Rh@Au(111)–0.10–0.320.22–0.96–1.50
Ir@Au(111)–0.50–0.56–0.37–1.19–1.39
Cu@Au(100)0.230.641.34–0.75–1.44
Ni@Au(100)0.010.200.36–1.11–1.27
Pd@Au(100)0.090.240.89–0.69–1.34
Pt@Au(100)–0.14–0.210.20–0.75–1.16
Co@Au(100)–0.12–0.20–0.36–1.23–1.08
Rh@Au(100)–0.22–0.46–0.34–1.01–1.13
Ir@Au(100)–0.57–0.73–0.73–1.22–1.22
Cu@Ag(111)0.300.841.44–0.81–1.42
Ni@Ag(111)0.020.180.74–1.18–1.74
Pd@Ag(111)0.240.501.44–0.80–1.74
Pt@Ag(111)–0.14–0.060.93–0.71–1.70
Co@Ag(111)–0.10a–0.23a–1.50
Rh@Ag(111)–0.12–0.360.23–1.14–1.73
Ir@Ag(111)–0.56–0.71–0.36–1.28–1.63
Cu@Ag(100)0.310.771.69–0.88–1.80
Ni@Ag(100)–0.08–0.030.54–1.26–1.82
Pd@Ag(100)0.100.251.22–0.87–1.84
Pt@Ag(100)–0.15–0.240.78–0.83–1.84
Co@Ag(100)–0.26a–0.41a–1.44
Rh@Ag(100)–0.27–0.59–0.11–1.12–1.60
Ir@Ag(100)–0.63–0.89–0.63–1.26–1.52
a

Geometry optimization of *CHO leads to the formation of surface-bound H and CO species on Co.

Analyzing the results, we found that there are 10 alloys that favor CORR via the *CHO intermediate (i.e., ΔG*CHO < ΔG*H): Co@Au(111), Rh@Au(111), Ir@Au(111), Pt@Au(100), Rh@Au(100), Rh@Ag(111), Ir@Ag(111), Pt@Ag(100), Rh@Ag(100), and Ir@Ag(100). Three alloys favor CORR via the *COH intermediate (i.e., ΔG*COH < ΔG*H): Co@Au(100), Co@Ag(111), and Co@Ag(100). For Ir@Au(100), ΔG*CHO and ΔG*COH are the same, and both of them are smaller than ΔG*H. Two interesting trends are observed. First, most of the alloys (except Pt@Au(100) and Pt@Ag(100)) that favor CORR have M = Co, Rh, and Ir. This means that choosing group 9 transition metals as M effectively enhances CO reduction and suppresses HER. Second, for most of the 28 SAAs (except Co@Au(100), Co@Ag(111), and Co@Ag(100)), *CHO is more stable than *COH. This means that *CO → *CHO is the most energetically favorable hydrogenation pathway for *CO, similar to what was observed in the Cu(211) system.(49)
Also, it should be mentioned that very recent theoretical studies using the surface charging technique to simulate the applied voltage show that the binding energy of *OCHO is voltage-dependent, although the variation is small.(53, 54) It is possible that the binding energies of *H, *CHO, and *COH may also be slightly potential-dependent. This effect may affect our results for Ir@Au(111), Pt@Au(100), and Pt@Ag(100), since the values of ΔG*CHO for these catalysts are only slightly smaller than that of ΔG*H (by 0.06, 0.07, and 0.09 eV, respectively), and the stability of *CHO and *H could consequently be reversed for very negative applied voltages (e.g., −1.0 VSHE).
To understand the reason why alloys with M = Co, Rh, and Ir favor CORR over the HER, we plotted the scaling relations between ΔG*H and ΔG*CHO (Figure 2), and ΔG*H and ΔG*COH (Figure 3). Figures 2 and 3 show that the slopes for ΔG*CHO vs ΔG*H and ΔG*COH vs ΔG*H are 1.61 and 2.10, respectively. This indicates that when alloys bind strongly with H, they also bind with CHO and COH even more strongly, making CORR the major pathway. The larger slope for ΔG*COH vs ΔG*H (2.10), compared to that for ΔG*CHO vs ΔG*H (1.61), can be rationalized by the number of bonds formed between the COH and CHO and the surface: COH can form three bonds to the surface, while CHO can only form one.(55)
figure

Figure 2. Gibbs free binding energy of *CHO (ΔG*CHO) against that of *H (ΔG*H). The equation for the linear fitting line is ΔG*CHO = 1.61 × ΔG*H + 0.07, and the correlation coefficient (R2) is 0.90.

figure

Figure 3. Gibbs free binding energy of *COH (ΔG*CHO) against that of *H (ΔG*H). The equation for the linear fitting line is ΔG*COH = 2.10 × ΔG*H + 0.54, and R2 = 0.71.

3.2Mechanisms of CO Reduction to Methane Catalyzed by Rh@Au(100) and Rh@Ag(100)
Having identified several promising SAAs for CORR, we then selected two of them to study the reaction mechanism for further reduction of *CHO or *COH to C1 products. Rh@Au(100) was chosen from the Au-based alloys for two reasons. First, Rh@Au(100) requires a smaller applied voltage for the reaction *CO → *CHO (−1.01 V vs RHE), which suggests that its energy consumption is less than the others. Second, *CHO on the surface of this catalyst is 0.24 eV more stable than *H, which guarantees that most of the electrical energy is directed to CORR, in preference to HER. From among the Ag-based SAAs, we chose Rh@Ag(100) for detailed mechanistic study, because *CHO for Rh@Ag(100) is 0.32 eV more stable than *H, guaranteeing selectivity for CORR over HER. In addition, although the required applied voltage for *CO → *CHO (−1.12 VRHE) for Rh@Ag(100) is higher than that for Pt@Ag(100) (−0.83 VRHE), the method to synthesize Rh@Ag is known.(56)
There are numerous possible CORR routes leading to the formation of either the 4 × (H+ + e–) reduction product, CH3OH(aq), or the 6 × (H+ + e–) reduction product, CH4(g), from *CO, as depicted in Scheme 4. The first hydrogenation products are *CHO and *COH, while the second hydrogenation products are *CH2O, *CHOH, and *C + H2O(l), and the third hydrogenation products include *OCH3, *CH2OH, and *CH + H2O(l). The fourth hydrogenation products are *O + CH4(g), *CH3OH, and *CH2 + H2O(l), the fifth hydrogenation products are *OH + CH4(g), and *CH3 + H2O(l), and the sixth hydrogenation product is CH4(g) + H2O(l). Desorption of surface-bound CH3OH (*CH3OH) results in the formation of CH3OH(aq). In the absence of experimental information, we considered the thermodynamics for all possible pathways for Rh@Au(100) and Rh@Ag(100). However, the activation barrier for each elementary step was not determined.
figure
Scheme 4. Schematic Description of Possible Reduction Pathways for the Surface-Bound CO (*CO)a

aFour-electron reduction of *CO leads to the formation of methanol, whereas six-electron reduction results in the formation of methane.

Numerous DFT-based studies have been devoted to the investigation of the mechanisms of CO2RR catalyzed by heterogeneous catalysts.(20, 21, 29, 49, 50, 57-64) Since copper is the only material that can reduce CO2RR to hydrocarbons with significant conversion,(26, 65) most of the previous theretical studies have focused on the mechanism of CORR on Cu.(49, 50, 57) Two main pathways have emerged from these studies, differing primarily in the reaction intermediate formed by the reduction of *CO. Based on their investigation of CO2RR catalyzed by the Cu(211) surface, Nørskov and co-workers proposed a reaction mechanism involving the following transformations:(colored blue in Scheme 4).(49) By contrast, based on their analysis of CORR on Cu(111), Janik and co-workers proposed that CH4 involves the following transformations:(colored red in Scheme 4).(57) Recently, Goddard and co-workers have used implicit solvation plus applied electric voltage to revisit CO2RR catalyzed by the Cu(111) surface.(50) They propose that the hydrogenation of *CO leads to the formation of *COH, in agreement with Janik;(57) however, the subsequent reduction of *COH leads to the formation of *CHOH, which is different from the pathways identified by Nørskov(49) and Janik.(57)
Our calculations show that along the most favorable pathway (Figure 4) for CORR catalyzed by Rh@Au(100), the first (H+ + e–) pair is added to the carbon of *CO to form *CHO, the second one is added to the oxygen to form *CHOH, and the third one is added to the oxygen to liberate H2O(l), leading to the formation of *CH. (The free-energy surface for all possible reduction pathways can be found in Figure S1 in the Supporting Information). These steps are followed by further hydrogenation of *CH to *CH2, *CH3, and eventually to the formation of gaseous CH4(g) and the regeneration of the surface alloy site (*). For the *CO, *CHO, *CHOH, and *CH3 intermediates, the adsorbates prefer to bind on the top site of Rh. However, *CHOH prefers to bind in the 4-fold hollow site formed by three Au and one Rh, and *CH2 prefers to bind on the bridged site formed by Au and Rh. Analyzing the potential energy surface, we found that the most uphill step (the potential-determining step) is the initial reduction: *CO → *CHO with ΔG = 1.01 eV. Since each step involves only a single electron transfer, with an applied bias of −1.01 VRHE, all the steps become downhill or thermoneutral.
figure

Figure 4. Gibbs free-energy diagram for *CO electrochemical reduction to methane catalyzed by Rh@Au(100) at 0.00 VRHE (black) and −1.01 VRHE (red). At −1.01 VRHE, the free-energy change for each step is either zero or downhill.

In contrast, CORR catalyzed by Rh@Ag(100) takes a very different pathway, one that is similar to that proposed by Nørskov and co-workers (Figure 5).(49) The free-energy surface for all possible reduction pathways is detailed in Figure S2 in the Supporting Information. In the beginning, four (H+ + e–) pairs are successively added to the carbon of *CO to reduce it to *CHO, *CH2O, *OCH3, and then to * + free CH4(g). After those hydrogenation steps, a surface-bound oxygen (*O) is left behind, which is further reduced by two other (H+ + e–) pairs, first to form *OH, and then to liberate H2O(l) and regenerate *. All of the adsorbates prefer to bind on the top site of Rh. From the calculated potential energy surface (PES), we found that the potential-determining step is *CO → *CHO with ΔG = 1.12 eV, and therefore, all the steps along the pathway become downhill or thermal neutral once a voltage of −1.12 VRHE is applied.
figure

Figure 5. Gibbs free-energy diagram for *CO electrochemical reduction to methane catalyzed by Rh@Ag(100) at 0.00 VRHE (black) and −1.12 VRHE (red). At an applied bias of −1.12 VRHE, the free-energy change for each step is either zero or downhill.

3.3Experimental Viability and Discussion
For the proposed M@Au (or Ag) SAAs, the single active site M is only involved in converting CO to C1 products, while the surrounding Au or Ag reduces CO2 and supplies CO for the M site. Therefore, another critical requirement for success of this type of one-pot tandem catalyst is whether the minimal electric voltage for CO reduction to CH4(g) or CH3OH(aq) by M matches the voltage for efficient CO2-to-CO reduction catalyzed by Au or Ag. If the minimum applied voltage required for the reduction CO to C1 products is too high or too low, compared to the voltage for the maximum Faradaic efficiency for CO2 reduction to CO, the SAAs will not be efficient and may not be selective for the CORR vs the HER.
For one of our proposed catalysts, Rh@Au(100), the minimum voltage needed to drive the CORR is −1.01 VRHE. Experimental measurements show that, at this voltage, the Faradaic efficiency for CO2 reduction to CO over Au is ∼60%.(25) Assuming all CO produced by surrounding Au is reduced by M, then this means that only ∼40% of the Faradaic efficiency is diverted to the HER. For Rh@Ag(100), the minimum bias needed to drive the CORR is −1.12 VRHE, for which the Faradaic efficiency for CO2 reduction to CO over Ag is as high as ∼90%.(17) This suggests that the HER channel may consume as little as 10% of the Faradaic efficiency (assuming that the reduction of CO on M is 100% efficient). These results suggest that our proposed one-pot tandem SAA catalysts are potentially efficient for CO2RR to CH4. A critical issue remaining unanswered by the present work is whether or not the rate of CO conversion to CH4 on the Rh sites embedded in the surface of Au or Ag is sufficiently active and sufficiently numerous to consume all, or a large fraction, of the CO generated on the host metal. To address this point, it will be necessary to determine the turnover frequencies (TOFs) for CO2 reduction to CO on Au and Ag, as well as the TOF for CO conversion to CH4. If the TOF for the latter reaction is sufficiently high, then it would be possible to achieve high overall Faradaic efficiencies for the reduction of CO2 to CH4 using Rh@Au or Rh@Ag catalysts.
Another important question about the viability of our proposed Rh@Au and Rh@Ag SAA catalysts is whether or not they can be synthesized and whether they would be stable. Rh and Au, and Rh and Ag are immiscible: while they can be mixed at high temperatures, phase separation occurs upon cooling.(66, 67) However, recent advances in synthetic techniques have overcome this difficulty.(56, 68, 69) In particular, Humphrey and co-workers have discovered a microwave-assisted method that enables the synthesis of stable, homogeneous Rh@Au and Rh@Ag nanoparticles with broadly tunable Rh:Au and Rh:Ag ratios.(56)
Subsurface segregation of M is another problem, which may deactivate the proposed SAA catalysts. Co (2.522 J/m2), Rh (2.659 J/m2), and Ir (3.048 J/m2) have larger surface free energies than Au (1.506 J/m2) and Ag (1.246 J/m2); therefore, they are likely to segregate into the subsurface under vacuum,(70, 71) leading to an inactive catalyst. However, since it is well-known that CO binds with Co, Rh, and Ir more strongly than Au and Ag do (indeed, that is the basis of the usefulness of the SAAs for CORR), those Group 9 metals may prefer to be present on the surface under CO-rich conditions.(29) Indeed, Sansa et al. have used DFT to study CO-induced segregation in gold-based alloys, and they have predicted that, under CO environments, Co, Rh, and Ir will remain on the surface of Au(111) and Au(100).(72)
The aggregation of active sites into islands is another potential problem for the stability of SAAs. To test whether or not this is the case, additional calculations were performed for Rh@Au(100), one of the two most promising SAAs for CO2RR. Rh@Au(100) was modeled by a three-layer 5 × 5 surface and the reciprocal space was sampled by a 2 × 2 × 1 Monkhorst-pack k-point net. During the geometry optimizations, the atoms in the bottom layer were fixed at their bulk positions, whereas the rest of atoms were allowed to relax. Two Au atoms on the top layer were replaced by Rh atoms, corresponding to an Rh surface concentration of 8%. Three surface configurations were considered: (a) two Rh atoms adjacent to each other along the y-axis, (b) two Rh atoms adjacent to each other along the xy-direction, and (c) two Rh atoms separated by Au atoms (see Figure 6). Our calculated results show that the three configurations are almost degenerate energetically, with configuration (c) being slightly more stable than configurations (a) and (b), by 0.01 and 0.02 eV, respectively. However, configuration (c) is expected to be more stable when entropy is taken into account. These results suggest that the proposed SAAs are stable to island formation.
figure

Figure 6. Configurations considered in evaluating the tendency of active sites toward island formation.

Finally, it is appropriate to revisit some of the uncertainties in the computational protocol itself. For example, as discussed in the Computational Details of the Supporting Information, we observe an ∼0.1 eV change in the difference in binding energy between H and CHO upon changing from a three-layer slab to a four-layer slab, as well as a similar shift upon increasing the slab thickness from three layers to four layers. While such shifts are not insignificant, they do not affect our main conclusions. The key point is that we find a free energy binding difference of nearly 0.4 eV in favor of CO2RR over HER for the best single-atom alloys (e.g., Rh@Au(100)). This gap is large enough that potential errors on the order of 0.1 eV will not qualitatively change it, even if they lead directly to stabilization of the HER pathway (which, of course, is not clear). The same argument suggests that our main conclusions are likely to be robust to other possible errors imposed by computational limitations, such as deficiencies of the RPBE density functional itself.

4 Conclusions

Twenty-eight (28) single-atom alloys (SAAs) were studied and evaluated as catalysts for the CO2 electrochemical reduction reaction to energy-rich hydrocarbons such as alkanes and alcohols in aqueous phase, using density functional theory (DFT) calculations together, with the Poisson–Boltzmann implicit solvation model. To be useful for the CO2 reduction reaction (CO2RR), the SAAs should contain isolated single-atom surface sites of M (M = Cu, Ni, Pd, Pt, Co, Rh, and Ir), as surface substitutions in Au or Ag. Initial reduction of CO2 to CO occurs on the Au or Ag host, and CO then binds preferentially to M. We found that 14 SAAs favor the formation of *CHO or *COH over *H, which, for those systems, means that CO reduction is preferred over proton reduction. This suggests that, for these catalysts, CO2 reduction is the major pathway, and the competitive evolution of hydrogen is suppressed. Most of the 14 promising SAAs contain M = Co, Rh, or Ir. This indicates that it is the identity of M rather than the surrounding metal that determines the performance of the SAA catalysts. The fact that M can bind only a single CO molecule also means that the SAAs will be selective for C1 products, relative to larger hydrocarbons.
We also performed a detailed thermodynamic study of the reaction mechanism for two of the most promising SAAs: Rh@Au(100) and Rh@Ag(100). We found that both of these SAAs reduce CO2 to methane, although the reduction occurs via different pathways. For Rh@Au(100), the reaction proceeds through the pathwaywhereas, for Rh@Ag(100), it isThe minimum applied voltages needed to drive the two electrocatalytic systems are −1.01 and −1.12 VRHE for Rh@Au(100) and Rh@Ag(100), respectively, for which the Faradaic efficiencies for CO2 reduction to CO are 60% for gold and 90% for silver. This suggests that SAA can efficiently reduce CO2 to methane with as little as 40% Faradaic efficiency loss in the hydrogen evolution reaction for Rh@Au(100) and as little as 10% for Rh@Ag(100). We believe that this computational work provides encouraging motivation for experimental efforts to prepare and characterize SAAs for the electrochemical reduction of CO2.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01393.

  • Additional tables and details regarding computational methods (PDF)

The authors declare no competing financial interest.

Acknowledgment

This material is based on work performed in the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy, under Award No. DE-SC0004993.

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    Schouten, K. J. P.; Kwon, Y.; van der Ham, C. J. M.; Qin, Z.; Koper, M. T. M.
    Chemical Science (2011), 2 (10), 1902-1909 CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The authors have studied the reaction mechanism of the electrochem. redn. of CO2 to hydrocarbons on Cu electrodes. This reaction occurs via two pathways: a C1 pathway leading to methane, and a C2 pathway leading to ethylene. To identify possible intermediates in the redn. of CO2 the authors have studied the redn. of small C1 and C2 org. mols. contg. O. The authors followed the formation and consumption of intermediates during the reaction as a function of potential, using online mass spectrometry. For the C1 pathway probably CHOads is the key intermediate towards the breaking of the C-O bond and, therefore, the formation of methane. For the C2 pathway probably the 1st step is the formation of a CO dimer, followed by the formation of a surface-bonded enediol or enediolate, or the formation of an oxametallacycle. Both the enediol(ate) and the oxametallacycle would explain the selectivity of the C2 pathway towards ethylene. This new mechanism is significantly different from existing mechanisms but it is the most consistent with the available exptl. data.
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    OpenURL UNIV OF NORTH TEXAS
  12. 12.
    Jedidi, A.; Rasul, S.; Masih, D.; Cavallo, L.; Takanabe, K. J. Mater. Chem. A 2015, 3, 19085– 19092, DOI: 10.1039/C5TA05669A
    [CrossRef], [CAS]
    12.
    Generation of Cu-In alloy surfaces from CuInO2 as selective catalytic sites for CO2 electroreduction
    Jedidi, Abdesslem; Rasul, Shahid; Masih, Dilshad; Cavallo, Luigi; Takanabe, Kazuhiro
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (37), 19085-19092 CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    The lack of availability of efficient, selective and stable electrocatalysts is a major hindrance for scalable CO2 redn. processes. Herein, the authors report the generation of Cu-In alloy surfaces for electrochem. redn. of CO2 from mixed metal oxides of CuInO2 as the starting material. The material successfully generates selective active sites to form CO from CO2 electroredn. at mild overpotentials. D. functional theory (DFT) indicates that the site occupation of the inert In occurs more on the specific sites of Cu. While In atoms do not preferentially adsorb H or CO, Cu atoms, which neighbor the In atoms, alters the preference of their adsorption. This preference for site occupation and altered adsorption may account for the improved selectivity over that obsd. for Cu metal. This study demonstrates an example of a scalable synthesis method of bimetallic surfaces used with the mixed oxide precursor having the diversity of metal choice, which may drastically alter the electrocatalytic performance, as presented herein.
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  13. 13.
    Rasul, S.; Anjum, D. H.; Jedidi, A.; Minenkov, Y.; Cavallo, L.; Takanabe, K. Angew. Chem., Int. Ed. 2015, 54, 2146– 2150, DOI: 10.1002/anie.201410233
    [CrossRef], [CAS]
    13.
    A Highly Selective Copper-Indium Bimetallic Electrocatalyst for the Electrochemical Reduction of Aqueous CO2 to CO
    Rasul, Shahid; Anjum, Dalaver H.; Jedidi, Abdesslem; Minenkov, Yury; Cavallo, Luigi; Takanabe, Kazuhiro
    Angewandte Chemie, International Edition (2015), 54 (7), 2146-2150 CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The challenge in the electrochem. redn. of aq. CO2 is in designing a highly selective, energy-efficient, and nonprecious-metal electrocatalyst that minimizes the competitive redn. of proton to form H during aq. CO2 conversion. A nonnoble metal electrocatalyst based on a Cu-In (Cu-In) alloy that selectively converts CO2 to CO with a low overpotential is reported. The electrochem. deposition of In on rough Cu surfaces led to Cu-In alloy surfaces. DFT calcns. showed that the In preferentially located on the edge sites rather than on the corner or flat sites and that the d-electron nature of Cu remained almost intact, but adsorption properties of neighboring Cu was perturbed by the presence of In. This prepn. of nonnoble metal alloy electrodes for the redn. of CO2 provides guidelines for further improving electrocatalysis.
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    OpenURL UNIV OF NORTH TEXAS
  14. 14.
    Elgrishi, N.; Chambers, M. B.; Fontecave, M. Chem. Sci. 2015, 6, 2522– 2531, DOI: 10.1039/C4SC03766A
    [CrossRef], [CAS]
    14.
    Turning it off! Disfavouring hydrogen evolution to enhance selectivity for CO production during homogeneous CO2 reduction by cobalt-terpyridine complexes
    Elgrishi, Noemie; Chambers, Matthew B.; Fontecave, Marc
    Chemical Science (2015), 6 (4), 2522-2531 CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Understanding the activity and selectivity of mol. catalysts for CO2 redn. to fuels is an important scientific endeavour in addressing the growing global energy demand. Cobalt-terpyridine compds. have been shown to be catalysts for CO2 redn. to CO while simultaneously producing H2 from the requisite proton source. To investigate the parameters governing the competition for H+ redn. vs. CO2 redn., the cobalt bisterpyridine class of compds. is first evaluated as H+ redn. catalysts. We report that electronic tuning of the ancillary ligand sphere can result in a wide range of second-order rate consts. for H+ redn. When this class of compds. is next submitted to CO2 redn. conditions, a trend is found in which the less active catalysts for H+ redn. are the more selective towards CO2 redn. to CO. This represents the first report of the selectivity of a mol. system for CO2 redn. being controlled through turning off one of the competing reactions. The activities of the series of catalysts are evaluated through foot-of-the-wave anal. and a catalytic Tafel plot is provided.
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    OpenURL UNIV OF NORTH TEXAS
  15. 15.
    Zhang, Y. J.; Sethuraman, V.; Michalsky, R.; Peterson, A. A. ACS Catal. 2014, 4, 3742– 3748, DOI: 10.1021/cs5012298
    [ACS Full Text ACS Full Text], [CAS]
    15.
    Competition between CO2 Reduction and H2 Evolution on Transition-Metal Electrocatalysts
    Zhang, Yin-Jia; Sethuraman, Vijay; Michalsky, Ronald; Peterson, Andrew A.
    ACS Catalysis (2014), 4 (10), 3742-3748 CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The known H evolution reaction (HER) volcano plot describes the relation between H binding energy and the corresponding H evolution catalytic activity, which depends on the species of metal. Under CO2/CO redn. conditions or in cases where CO impurities enter electrodes, the catalyst may exist under a high coverage of coadsorbed CO. The authors present DFT calcns. that suggest that coadsorbed CO during H evolution will weaken the binding strength between H and the catalyst surface. For metals on the right-hand side (too weak of H binding) this should lead to a suppression of the HER, as is reported for metals such as Cu and Pt. However, for metals on the left-hand side of the volcano (too strong of H binding), this may actually enhance the kinetics of the H evolution reaction, although this effect will be countered by a decreased availability of sites for HER, which are blocked by CO. The authors performed expts. in Ar and CO2 environments of two representative metals that bind CO on the far right- and left-hand side of the volcano, namely, Cu and Mo (resp.). On Cu, the CO2 environment suppresses HER, which is consistent with previous findings. However, on Mo the CO2 environment enhances HER in the kinetically active region. This helps to explain the outstanding performance of Cu in CO2 redn. and suggests that searches for high-selectivity CO2/CO redn. catalysts may benefit from focusing on the right-hand side of the HER volcano. This also suggests principles for assessing the activity of catalysts for fuel cell and electrolysis reactions in which impurities such as CO may be present.
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    OpenURL UNIV OF NORTH TEXAS
  16. 16.
    Li, C. W.; Kanan, M. W. J. Am. Chem. Soc. 2012, 134, 7231– 7234, DOI: 10.1021/ja3010978
    [ACS Full Text ACS Full Text], [CAS]
    16.
    CO2 Reduction at Low Overpotential on Cu Electrodes Resulting from the Reduction of Thick Cu2O Films
    Li, Christina W.; Kanan, Matthew W.
    Journal of the American Chemical Society (2012), 134 (17), 7231-7234 CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Modified Cu electrodes were prepd. by annealing Cu foil in air and electrochem. reducing the resulting Cu2O layers. The CO2 redn. activities of these electrodes exhibited a strong dependence on the initial thickness of the Cu2O layer. Thin Cu2O layers formed by annealing at 130° resulted in electrodes whose activities were indistinguishable from those of polycryst. Cu. In contrast, Cu2O layers formed at 500° that were ≥ ∼3 μm thick resulted in electrodes that exhibited large roughness factors and required 0.5 V less overpotential than polycryst. Cu to reduce CO2 at a higher rate than H2O. The combination of these features resulted in CO2 redn. geometric current densities >1 mA/cm2 at overpotentials <0.4 V, a higher level of activity than all previously reported metal electrodes evaluated under comparable conditions. Also, the activity of the modified electrodes was stable over several hours, whereas a polycryst. Cu electrode exhibited deactivation within 1 h under identical conditions. The electrodes described here may be particularly useful for elucidating the structural properties of Cu that det. the distribution between CO2 and H2O redn. and provide a promising lead for the development of practical catalysts for electrolytic fuel synthesis.
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    OpenURL UNIV OF NORTH TEXAS
  17. 17.
    Hatsukade, T.; Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. Phys. Chem. Chem. Phys. 2014, 16, 13814– 13819, DOI: 10.1039/C4CP00692E
    [CrossRef], [PubMed], [CAS]
    17.
    Insights into the electrocatalytic reduction of CO2 on metallic silver surfaces
    Hatsukade, Toru; Kuhl, Kendra P.; Cave, Etosha R.; Abram, David N.; Jaramillo, Thomas F.
    Physical Chemistry Chemical Physics (2014), 16 (27), 13814-13819 CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The electrochem. redn. of CO2 could allow for a sustainable process by which renewable energy from wind and solar are used directly in the prodn. of fuels and chems. In this work we investigated the potential dependent activity and selectivity of the electrochem. redn. of CO2 on metallic silver surfaces under ambient conditions. Our results deepen our understanding of the surface chem. and provide insight into the factors important to designing better catalysts for the reaction. The high sensitivity of our exptl. methods for identifying and quantifying products of reaction allowed for the observation of six redn. products including CO and hydrogen as major products and formate, methane, methanol, and ethanol as minor products. By quantifying the potential-dependent behavior of all products, we provide insights into kinetics and mechanisms at play, in particular involving the prodn. of hydrocarbons and alcs. on catalysts with weak CO binding energy as well as the formation of a C-C bond required to produce ethanol.
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    OpenURL UNIV OF NORTH TEXAS
  18. 18.
    Kim, D.; Resasco, J.; Yu, Y.; Asiri, A. M.; Yang, P. D. Nat. Commun. 2014, 5, 4948, DOI: 10.1038/ncomms5948
    [CrossRef], [PubMed], [CAS]
    18.
    Synergistic geometric and electronic effects for electrochemical reduction of carbon dioxide using gold-copper bimetallic nanoparticles
    Kim Dohyung; Resasco Joaquin; Yu Yi; Asiri Abdullah Mohamed; Yang Peidong
    Nature communications (2014), 5 (), 4948 ISSN:.
    Highly efficient and selective electrochemical reduction of carbon dioxide represents one of the biggest scientific challenges in artificial photosynthesis, where carbon dioxide and water are converted into chemical fuels from solar energy. However, our fundamental understanding of the reaction is still limited and we do not have the capability to design an outstanding catalyst with great activity and selectivity a priori. Here we assemble uniform gold-copper bimetallic nanoparticles with different compositions into ordered monolayers, which serve as a well-defined platform to understand their fundamental catalytic activity in carbon dioxide reduction. We find that two important factors related to intermediate binding, the electronic effect and the geometric effect, dictate the activity of gold-copper bimetallic nanoparticles. These nanoparticle monolayers also show great mass activities, outperforming conventional carbon dioxide reduction catalysts. The insights gained through this study may serve as a foundation for designing better carbon dioxide electrochemical reduction catalysts.
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    OpenURL UNIV OF NORTH TEXAS
  19. 19.
    Cheng, M. J.; Kwon, Y.; Head-Gordon, M.; Bell, A. T. J. Phys. Chem. C 2015, 119, 21345– 21352, DOI: 10.1021/acs.jpcc.5b05518
    [ACS Full Text ACS Full Text], [CAS]
    19.
    Tailoring Metal-Porphyrin-Like Active Sites on Graphene to Improve the Efficiency and Selectivity of Electrochemical CO2 Reduction
    Cheng, Mu-Jeng; Kwon, Youngkook; Head-Gordon, Martin; Bell, Alexis T.
    Journal of Physical Chemistry C (2015), 119 (37), 21345-21352 CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    D. functional theory (DFT) calcns. were performed to study the energetics of the CO2 electrochem. redn. on metal (M) porphyrin-like motifs incorporated into graphene layers. The objective is to develop strategies that enhance CO2 redn. while suppressing the competitive H evolution reaction (HER). There exists a scaling relation between the binding energy of the catalyst to H and to COOH, a key intermediate in the redn. of CO2 to CO; however, the M-H bond is stronger than the M-COOH bond, driving the reaction toward the HER rather than the redn. of CO2 to CO. This scaling relation holds even with axial ligation to the metal cation coordinated to the porphyrin ring. When 4f lanthanide or 5f actinide elements were used as the reactive center, the scaling relation still holds but the M-COOH bond is stronger than the M-H bond, and the reaction favors the redn. of CO2 to CO. By contrast, there is no scaling relation between the binding energy of the catalyst to H and that to OCHO, the key intermediate in CO2 redn. to formic acid. Coordination of a ligand to an unoccupied axial site can make the M-OCHO bond stronger than the M-H bond, resulting in preferential formic acid formation. This means that the axial ligand effectively enhances CO2 redn. to formic acid and suppresses the HER. The authors' DFT calcns. have also identified several promising electrocatalysts for CO2 redn. to HCOOH with almost zero overpotentials.
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    OpenURL UNIV OF NORTH TEXAS
  20. 20.
    Karamad, M.; Tripkovic, V.; Rossmeisl, J. ACS Catal. 2014, 4, 2268– 2273, DOI: 10.1021/cs500328c
    [ACS Full Text ACS Full Text], [CAS]
    20.
    Intermetallic Alloys as CO Electroreduction Catalysts-Role of Isolated Active Sites
    Karamad, Mohammadreza; Tripkovic, Vladimir; Rossmeisl, Jan
    ACS Catalysis (2014), 4 (7), 2268-2273 CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    One of the main challenges assocd. with the electrochem. CO or CO2 redn. is poor selectivity toward energetically rich products. In order to promote selectivity toward hydrocarbons and alcs., most notably, the hydrogen evolution reaction (HER) should be suppressed. To achieve this goal, we studied intermetallic compds. consisting of transition metal (TM) elements that can reduce CO (Ru, Co, Rh, Ir, Ni, Pd, Pt, and Cu) sepd. by TM and post transition metal elements (Ag, Au, Cd, Zn, Hg, In, Sn, Pb, Sb, and Bi) that are poor HER catalysts. In total, 34 different stable binary bulk alloys forming from these elements were investigated using d. functional theory calcns. The electronic and geometric properties of the catalyst surface can be tuned by varying the size of the active centers and the elements forming them. We have identified six different potentially selective intermetallic surfaces on which CO can be reduced to methanol at potentials comparable to or even slightly pos. than those for CO/CO2 redn. to methane on Cu. Common features shared by most of the selective alloys are single TM sites. The role of single sites is to block parasitic HER and thereby promote CO redn.
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  21. 21.
    Tripkovic, V.; Vanin, M.; Karamad, M.; Björketun, M. E.; Jacobsen, K. W.; Thygesen, K. S.; Rossmeisl, J. J. Phys. Chem. C 2013, 117, 9187– 9195, DOI: 10.1021/jp306172k
    [ACS Full Text ACS Full Text], [CAS]
    21.
    Electrochemical CO2 and CO Reduction on Metal-Functionalized Porphyrin-like Graphene
    Tripkovic, Vladimir; Vanin, Marco; Karamad, Mohammedreza; Bjorketun, Maarten E.; Jacobsen, Karsten W.; Thygesen, Kristian S.; Rossmeisl, Jan
    Journal of Physical Chemistry C (2013), 117 (18), 9187-9195 CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Porphyrin-like metal-functionalized graphene structures were studied as possible catalysts for CO2 and CO redn. to methane or MeOH. The late transition metals (Cu, Ag, Au, Ni, Pd, Pt, Co, Rh, Ir, Fe, Ru, Os) and some p (B, Al, Ga) and s (Mg) metals comprised the center of the porphyrin ring. A clear difference in catalytic properties compared to extended metal surfaces was obsd. owing to a different electronic nature of the active site. The preference to bind H, however, becomes a major obstacle in the reaction path. A possible soln. to this problem is to reduce CO instead of CO2. Volcano plots were constructed from scaling relations of reaction intermediates, and from these plots the reaction steps with the highest overpotentials were deduced. The Rh-porphyrin-like functionalized graphene was identified as the most active catalyst for producing MeOH from CO, featuring an overpotential of 0.22 V. Addnl., the authors have also examd. the H evolution and oxidn. reaction, and in their case, too, Rh-porphyrin turned out to be the best catalyst with an overpotential of 0.15 V.
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    OpenURL UNIV OF NORTH TEXAS
  22. 22.
    Rossmeisl, J.; Logadottir, A.; Norskov, J. K. Chem. Phys. 2005, 319, 178– 184, DOI: 10.1016/j.chemphys.2005.05.038
    [CrossRef], [CAS]
    22.
    Electrolysis of water on (oxidized) metal surfaces
    Rossmeisl, J.; Logadottir, A.; Norskov, J. K.
    Chemical Physics (2005), 319 (1-3), 178-184 CODEN: CMPHC2; ISSN:0301-0104. (Elsevier B.V.)
    D. functional theory calcns. are used as the basis for an anal. of the electrochem. process, where by H2O is split to form mol. hydrogen and hydrogen. The authors develop a method for obtaining the thermochem. of the electrochem. H2O splitting process as a function of the bias directly from the electronic structure calcns. The authors consider electrodes of Pt(111) and Au(111) in detail and then discuss trends for different metals. The difficult step in the H2O splitting process is the formation of superoxy-type (OOH) species on the surface by the splitting of a H2O mol. on top an adsorbed O atom. One conclusion is that this is only possible on metal surfaces that are (partly) oxidized. The binding energies of the different intermediates are linearly correlated for a no. of metals. In a simple anal., where the linear relations are assumed to be obeyed exactly, this leads to a universal relation between the catalytic rate and the oxygen binding energy. Finally, for systems obeying these relations, there is a limit to how good a H2O splitting catalyst an oxidized metal surface can become.
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    OpenURL UNIV OF NORTH TEXAS
  23. 23.
    Carrasquillo-Flores, R.; Ro, I.; Kumbhalkar, M. D.; Burt, S.; Carrero, C. A.; Alba-Rubio, A. C.; Miller, J. T.; Hermans, I.; Huber, G. W.; Dumesic, J. A. J. Am. Chem. Soc. 2015, 137, 10317– 10325, DOI: 10.1021/jacs.5b05945
    [ACS Full Text ACS Full Text], [CAS]
    23.
    Reverse Water-Gas Shift on Interfacial Sites Formed by Deposition of Oxidized Molybdenum Moieties onto Gold Nanoparticles
    Carrasquillo-Flores, Ronald; Ro, Insoo; Kumbhalkar, Mrunmayi D.; Burt, Samuel; Carrero, Carlos A.; Alba-Rubio, Ana C.; Miller, Jeffrey T.; Hermans, Ive; Huber, George W.; Dumesic, James A.
    Journal of the American Chemical Society (2015), 137 (32), 10317-10325 CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We show that MoOx-promoted Au/SiO2 catalysts are active for reverse water-gas shift (RWGS) at 573 K. Results from reactivity measurements, CO FTIR studies, Raman spectroscopy, and X-ray absorption spectroscopy (XAS) indicate that the deposition of Mo onto Au nanoparticles occurs preferentially on under-coordinated Au sites, forming Au/MoOx interfacial sites active for reverse water-gas shift (RWGS). Au and AuMo sites are quantified from FTIR spectra of adsorbed CO collected at subambient temps. (e.g., 150-270 K). Bands at 2111 and 2122 cm-1 are attributed to CO adsorbed on under-coordinated Au0 and Auδ+ species, resp. Clausius-Clapeyron anal. of FTIR data yields a heat of CO adsorption (ΔHads) of -31 kJ mol-1 for Au0 and -64 kJ mol-1 for Auδ+ at 33% surface coverage. Correlations of RWGS reactivity with changes in FTIR spectra for samples contg. different amts. of Mo indicate that interfacial sites are an order of magnitude more active than Au sites for RWGS. Raman spectra of Mo/SiO2 show a feature at 975 cm-1, attributed to a dioxo (O=)2Mo(-O-Si)2 species not obsd. in spectra of AuMo/SiO2 catalysts, indicating preferential deposition of Mo on Au. XAS results indicate that Mo is in a +6 oxidn. state, and therefore Au and Mo exist as a metal-metal oxide combination. Catalyst calcination increases the quantity of under-coordinated Au sites, increasing RWGS activity. This strategy for catalyst synthesis and characterization enables quantification of Au active sites and interfacial sites, and this approach may be extended to describe reactivity changes obsd. in other reactions on supported gold catalysts.
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  24. 24.
    Lu, Q.; Rosen, J.; Zhou, Y.; Hutchings, G. S.; Kimmel, Y. C.; Chen, J. G. G.; Jiao, F. Nat. Commun. 2014, 5, 3242, DOI: 10.1038/ncomms4242
    [CrossRef], [PubMed], [CAS]
    24.
    A selective and efficient electrocatalyst for carbon dioxide reduction
    Lu Qi; Rosen Jonathan; Zhou Yang; Hutchings Gregory S; Kimmel Yannick C; Jiao Feng; Chen Jingguang G
    Nature communications (2014), 5 (), 3242 ISSN:.
    Converting carbon dioxide to useful chemicals in a selective and efficient manner remains a major challenge in renewable and sustainable energy research. Silver is an interesting electrocatalyst owing to its capability of converting carbon dioxide to carbon monoxide selectively at room temperature; however, the traditional polycrystalline silver electrocatalyst requires a large overpotential. Here we report a nanoporous silver electrocatalyst that is able to electrochemically reduce carbon dioxide to carbon monoxide with approximately 92% selectivity at a rate (that is, current) over 3,000 times higher than its polycrystalline counterpart under moderate overpotentials of <0.50 V. The high activity is a result of a large electrochemical surface area (approximately 150 times larger) and intrinsically high activity (approximately 20 times higher) compared with polycrystalline silver. The intrinsically higher activity may be due to the greater stabilization of CO2 (-) intermediates on the highly curved surface, resulting in smaller overpotentials needed to overcome the thermodynamic barrier.
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    OpenURL UNIV OF NORTH TEXAS
  25. 25.
    Hahn, C.; Abram, D. N.; Hansen, H. A.; Hatsukade, T.; Jackson, A.; Johnson, N. C.; Hellstern, T. R.; Kuhl, K. P.; Cave, E. R.; Feaster, J. T.; Jaramillo, T. F. J. Mater. Chem. A 2015, 3, 20185– 20194, DOI: 10.1039/C5TA04863J
    [CrossRef], [CAS]
    25.
    Synthesis of thin film AuPd alloys and their investigation for electrocatalytic CO2 reduction
    Hahn, Christopher; Abram, David N.; Hansen, Heine A.; Hatsukade, Toru; Jackson, Ariel; Johnson, Natalie C.; Hellstern, Thomas R.; Kuhl, Kendra P.; Cave, Etosha R.; Feaster, Jeremy T.; Jaramillo, Thomas F.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (40), 20185-20194 CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    The authors synthesize and study AuPd alloys for the electrocatalytic redn. of CO2. Thin films of AuPd were synthesized using an electron-beam co-deposition method, which yields uniform, phase-pure metal alloys with compn. control. Scanning electron microscope images show that the thin films are relatively uniform and flat in morphol. X-ray diffraction showed alloying and phase homogeneity within the AuPd thin films. Elemental mapping of Au and Pd with scanning TEM shows that AuPd thin films are uniform in compn. on the nanometer scale. XPS characterization indicates that AuPd alloys are slightly Au-rich on the surface and follow a similar trend to the bulk compn. as detd. by Vegard's Law. CO2 redn. activity and selectivity were studied across the AuPd system. All AuPd alloys are more active and selective for formate prodn. than either of the pure metals, indicating that Au and Pd can act synergistically to yield new electrocatalytic properties.
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    OpenURL UNIV OF NORTH TEXAS
  26. 26.
    Hori, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O. Electrochim. Acta 1994, 39, 1833– 1839, DOI: 10.1016/0013-4686(94)85172-7
    [CrossRef], [CAS]
    26.
    Electrocatalytic process of CO selectivity in electrochemical reduction of CO2 at metal electrodes in aqueous media
    Hori, Yoshi; Wakebe, Hidetoshi; Tsukamoto, Toshio; Koga, Osamu
    Electrochimica Acta (1994), 39 (11-12), 1833-9 CODEN: ELCAAV; ISSN:0013-4686.
    Electrochem. redn. of CO2 at metal electrodes yields CO, HCOO-, CH4, C2H4, and alcs. in aq. media. Metal electrodes are roughly divided into two groups, CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and HCOO- ones (Pb, Hg, In, Sn, Tl). Foreign atom modified electrode (the coverage is virtually unity) showed variable product selectivity between CO and HCOO-, which depends upon the combination of modifier atom and substrate electrode. Crit. investigation of foreign atom modified electrodes derived a series of CO selectively, as Au > Ag > Cu > Zn » Cd > Sn > In > Pb > Tl > Hg. The electrode potentials of CO2 redn. are well correlated with the heat of fusion of metals and the potential of H2 evolution. The order of CO selectivity agrees roughly with that of the electrode potential of CO2 redn., and is rationalized in terms of stabilization of intermediate species CO2·- at the electrode surface. CO is produced from stably adsorbed CO2·-, and HCOO- is formed from free or weakly adsorbed CO2·-.
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    OpenURL UNIV OF NORTH TEXAS
  27. 27.
    Chen, Y. H.; Li, C. W.; Kanan, M. W. J. Am. Chem. Soc. 2012, 134, 19969– 19972, DOI: 10.1021/ja309317u
    [ACS Full Text ACS Full Text], [CAS]
    27.
    Aqueous CO2 Reduction at Very Low Overpotential on Oxide-Derived Au Nanoparticles
    Chen, Yihong; Li, Christina W.; Kanan, Matthew W.
    Journal of the American Chemical Society (2012), 134 (49), 19969-19972 CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Carbon dioxide redn. is an essential component of many prospective technologies for the renewable synthesis of carbon-contg. fuels. Known catalysts for this reaction generally suffer from low energetic efficiency, poor product selectivity, and rapid deactivation. It is shown that the redn. of thick Au oxide films results in the formation of Au nanoparticles (oxide-derived Au) that exhibit highly selective CO2 redn. to CO in water at overpotentials as low as 140 mV and retain their activity for at least 8 h. Under identical conditions, polycryst. Au electrodes and several other nanostructured Au electrodes prepd. via alternative methods require at least 200 mV of addnl. overpotential to attain comparable CO2 redn. activity and rapidly lose their activity. Electrokinetic studies indicate that the improved catalysis is linked to dramatically increased stabilization of the CO2•- intermediate on the surfaces of the oxide-derived Au electrodes.
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    OpenURL UNIV OF NORTH TEXAS
  28. 28.
    Zhu, W. L.; Michalsky, R.; Metin, O.; Lv, H. F.; Guo, S. J.; Wright, C. J.; Sun, X. L.; Peterson, A. A.; Sun, S. H. J. Am. Chem. Soc. 2013, 135, 16833– 16836, DOI: 10.1021/ja409445p
    [ACS Full Text ACS Full Text], [CAS]
    28.
    Monodisperse Au nanoparticles for selective electrocatalytic reduction of CO2 to CO
    Zhu, Wenlei; Michalsky, Ronald; Metin, Onder; Lv, Haifeng; Guo, Shaojun; Wright, Christopher J.; Sun, Xiaolian; Peterson, Andrew A.; Sun, Shouheng
    Journal of the American Chemical Society (2013), 135 (45), 16833-16836 CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We report selective electrocatalytic redn. of carbon dioxide to carbon monoxide on gold nanoparticles (NPs) in 0.5 M KHCO3 at 25 °C. Among monodisperse 4, 6, 8, and 10 nm NPs tested, the 8 nm Au NPs show the max. Faradaic efficiency (FE) (up to 90% at -0.67 V vs reversible hydrogen electrode, RHE). D. functional theory calcns. suggest that more edge sites (active for CO evolution) than corner sites (active for the competitive H2 evolution reaction) on the Au NP surface facilitates the stabilization of the redn. intermediates, such as COOH*, and the formation of CO. This mechanism is further supported by the fact that Au NPs embedded in a matrix of butyl-3-methyl-imid-azolium hexafluorophosphate for more efficient COOH* stabilization exhibit even higher reaction activity (3 A/g mass activity) and selectivity (97% FE) at -0.52 V (vs RHE). The work demonstrates the great potentials of using monodisperse Au NPs to optimize the available reaction intermediate binding sites for efficient and selective electrocatalytic redn. of CO2 to CO.
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    OpenURL UNIV OF NORTH TEXAS
  29. 29.
    Peterson, A. A.; Norskov, J. K. J. Phys. Chem. Lett. 2012, 3, 251– 258, DOI: 10.1021/jz201461p
    [ACS Full Text ACS Full Text], [CAS]
    29.
    Activity Descriptors for CO2 Electroreduction to Methane on Transition-Metal Catalysts
    Peterson, Andrew A.; Noerskov, Jens K.
    Journal of Physical Chemistry Letters (2012), 3 (2), 251-258 CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The electrochem. redn. of CO2 into hydrocarbons and alcs. would allow renewable energy sources to be converted into fuels and chems. However, no electrode catalysts were developed that can perform this transformation with a low overpotential at reasonable current densities. The authors compare trends in binding energies for the intermediates in CO2 electrochem. redn. and present an activity volcano based on this anal. This anal. describes the exptl. obsd. variations in transition-metal catalysts, including why Cu is the best-known metal electrocatalyst. The protonation of adsorbed CO is singled out as the most important step dictating the overpotential. New strategies are presented for the discovery of catalysts that can operate with a reduced overpotential.
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    OpenURL UNIV OF NORTH TEXAS
  30. 30.
    Lucci, F. R.; Lawton, T. J.; Pronschinske, A.; Sykes, E. C. H. J. Phys. Chem. C 2014, 118, 3015– 3022, DOI: 10.1021/jp405254z
    [ACS Full Text ACS Full Text], [CAS]
    30.
    Atomic Scale Surface Structure of Pt/Cu(111) Surface Alloys
    Lucci, Felicia R.; Lawton, Timothy J.; Pronschinske, Alex; Sykes, E. Charles H.
    Journal of Physical Chemistry C (2014), 118 (6), 3015-3022 CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Pt-Cu bimetallic alloys are a key component in many heterogeneous catalysts that have the potential to be used in a range of industrially important reactions. Given the catalytic differences between Pt and Cu, the surface compn. and geometry of Pt-Cu alloys can have a large influence on their chem. Extensive characterization of bulk Pt-Cu alloys has been performed; however, only a few studies have addressed surface and subsurface alloying of Pt with Cu, and none have examd. the at. scale surface structure of Pt-Cu. In this study, scanning tunneling microscopy was used to det. the local structure of surface alloys formed by phys. vapor deposition of Pt onto Cu(111) over a range of alloying temps. (315-550 K). Our results indicated that Pt and Cu were capable of intermixing at 315 K and forming multiple metastable states. Increasing the temp. of the Cu surface during the deposition of Pt altered the surface geometry and further enhanced the dispersion of Pt. The results are compared to the well-characterized Pd/Cu(111) surface alloy. A distinguishing feature of the Pt/Cu(111) surface alloy is the ability of Pt atoms to alloy directly into the Cu surface. Pt alloys as individual isolated atoms, well sepd. from each other, rather than more localized in regions at step edges, as is the case with Pd. This work indicates that the highly dispersed nature of Pt-Cu surface alloys should render them useful for understanding the surface chem. of Pt at the single atom level.
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    OpenURL UNIV OF NORTH TEXAS
  31. 31.
    Lucci, F. R.; Marcinkowski, M. D.; Lawton, T. J.; Sykes, E. C. H. J. Phys. Chem. C 2015, 119, 24351– 24357, DOI: 10.1021/acs.jpcc.5b05562
    [ACS Full Text ACS Full Text], [CAS]
    31.
    H2 Activation and Spillover on Catalytically Relevant Pt-Cu Single Atom Alloys
    Lucci, Felicia R.; Marcinkowski, Matthew D.; Lawton, Timothy J.; Sykes, E. Charles H.
    Journal of Physical Chemistry C (2015), 119 (43), 24351-24357 CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Platinum is a key component in many heterogeneous hydrogenation catalysts. Because of its high price, fairly strong interaction with intermediates, and susceptibility to CO poisoning, it is often mixed with other elements. These bimetallic alloys have complex surface structures, and the at. structure of their active sites is not well understood. In this study, we examine the effect of the geometric arrangement of dil. Pt-Cu alloys on H2 activation, spillover, and release. Using scanning tunneling microscopy, we directly visualize the at. arrangement of Pt-Cu alloys and show that small amts. of Pt (∼1%) exists as isolated atoms in the Cu surface. These Pt monomers are capable of facile H2 dissocn. and spillover to Cu at temps. as low as 85 K. Addnl., the low-temp. desorption of H2 (230 K) suggests a reduced desorption barrier compared to monometallic Pt or Cu. We find these single atom alloy surfaces are robust to multiple adsorption/desorption and heating cycles to 450 K. Larger Pt ensembles in Cu exhibit higher temp. desorption profiles due to the stronger binding of H to extended Pt ensembles, demonstrating how the geometric arrangement of Pt atoms in Cu impacts the binding of H to catalytic surface sites. Overall, dil. Pt-Cu alloys contg. only isolated Pt atoms are most favorable for H2 activation, spillover, and release and hence should be capable of catalyzing hydrogenation reactions with a greatly reduced concn. of the precious metal.
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  32. 32.
    Pei, G. X.; Liu, X. Y.; Wang, A. Q.; Li, L.; Huang, Y. Q.; Zhang, T.; Lee, J. W.; Jang, B. W. L.; Mou, C. Y. New J. Chem. 2014, 38, 2043– 2051, DOI: 10.1039/c3nj01136d
    [CrossRef], [CAS]
    32.
    Promotional effect of Pd single atoms on Au nanoparticles supported on silica for the selective hydrogenation of acetylene in excess ethylene
    Pei, Guang Xian; Liu, Xiao Yan; Wang, Aiqin; Li, Lin; Huang, Yanqiang; Zhang, Tao; Lee, Jonathan W.; Jang, Ben W. L.; Mou, Chung-Yuan
    New Journal of Chemistry (2014), 38 (5), 2043-2051 CODEN: NJCHE5; ISSN:1144-0546. (Royal Society of Chemistry)
    A Pd single-atom alloy (SAA) structure was constructed by alloying Pd with Au supported on silica. The XRD and HRTEM results demonstrated that the addn. of a small amt. of Pd efficiently prevented the sintering of Au nanoparticles. The DRIFTS and EXAFS results confirmed that the Pd SAA structure was formed when the at. ratios of Pd/Au were lower than 0.025. The Pd SAA structure exhibits a much better catalytic performance for the selective hydrogenation of acetylene in excess ethylene than the corresponding monometallic Au or Pd systems.
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    OpenURL UNIV OF NORTH TEXAS
  33. 33.
    Pei, G. X.; Liu, X. Y.; Wang, A. Q.; Lee, A. F.; Isaacs, M. A.; Li, L.; Pan, X. L.; Yang, X. F.; Wang, X. D.; Tai, Z. J.; Wilson, K.; Zhang, T. ACS Catal. 2015, 5, 3717– 3725, DOI: 10.1021/acscatal.5b00700
    [ACS Full Text ACS Full Text], [CAS]
    33.
    Ag Alloyed Pd Single-Atom Catalysts for Efficient Selective Hydrogenation of Acetylene to Ethylene in Excess Ethylene
    Pei, Guang Xian; Liu, Xiao Yan; Wang, Aiqin; Lee, Adam F.; Isaacs, Mark A.; Li, Lin; Pan, Xiaoli; Yang, Xiaofeng; Wang, Xiaodong; Tai, Zhijun; Wilson, Karen; Zhang, Tao
    ACS Catalysis (2015), 5 (6), 3717-3725 CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Semihydrogenation of acetylene in an ethylene-rich stream is an industrially important process. Conventional supported monometallic Pd catalysts offer high acetylene conversion, but they suffer from very low selectivity to ethylene due to overhydrogenation and the formation of carbonaceous deposits. Herein, a series of Ag alloyed Pd single-atom catalysts, possessing only ppm levels of Pd, supported on silica gel were prepd. by a simple incipient wetness compregnation method and applied to the selective hydrogenation of acetylene in an ethylene-rich stream under conditions close to the front-end employed by industry. High acetylene conversion and simultaneous selectivity to ethylene was attained over a wide temp. window, surpassing an analogous Au alloyed Pd single-atom system we previously reported. Restructuring of AgPd nanoparticles and electron transfer from Ag to Pd were evidenced by in situ FTIR and in situ XPS as a function of increasing redn. temp. Microcalorimetry and XANES measurements support both geometric and electronic synergetic effects between the alloyed Pd and Ag. Kinetic studies provide valuable insight into the nature of the active sites within these AgPd/SiO2 catalysts, and hence, they provide evidence for the key factors underpinning the excellent performance of these bimetallic catalysts toward the selective hydrogenation of acetylene under ethylene-rich conditions while minimizing precious metal usage.
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    OpenURL UNIV OF NORTH TEXAS
  34. 34.
    Lucci, F. R.; Liu, J. L.; Marcinkowski, M. D.; Yang, M.; Allard, L. F.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H. Nat. Commun. 2015, 6, 8550, DOI: 10.1038/ncomms9550
    [CrossRef], [PubMed], [CAS]
    34.
    Selective hydrogenation of 1,3-butadiene on platinum-copper alloys at the single-atom limit
    Lucci, Felicia R.; Liu, Jilei; Marcinkowski, Matthew D.; Yang, Ming; Allard, Lawrence F.; Flytzani-Stephanopoulos, Maria; Sykes, E. Charles H.
    Nature Communications (2015), 6 (), 8550pp. CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Platinum is ubiquitous in the prodn. sectors of chems. and fuels; however, its scarcity in nature and high price will limit future proliferation of platinum-catalyzed reactions. One promising approach to conserve platinum involves understanding the smallest no. of platinum atoms needed to catalyze a reaction, then designing catalysts with the minimal platinum ensembles. Here, we design and test a new generation of platinum-copper nanoparticle catalysts for the selective hydrogenation of 1,3-butadiene,, an industrially important reaction. Isolated platinum atom geometries enable hydrogen activation and spillover but are incapable of C-C bond scission that leads to loss of selectivity and catalyst deactivation. γ-Alumina-supported single-atom alloy nanoparticle catalysts with <1 platinum atom per 100 copper atoms are found to exhibit high activity and selectivity for butadiene hydrogenation to butenes under mild conditions, demonstrating transferability from the model study to the catalytic reaction under practical conditions.
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    OpenURL UNIV OF NORTH TEXAS
  35. 35.
    Boucher, M. B.; Zugic, B.; Cladaras, G.; Kammert, J.; Marcinkowski, M. D.; Lawton, T. J.; Sykes, E. C. H.; Flytzani-Stephanopoulos, M. Phys. Chem. Chem. Phys. 2013, 15, 12187– 12196, DOI: 10.1039/c3cp51538a
    [CrossRef], [PubMed], [CAS]
    35.
    Single atom alloy surface analogs in Pd0.18Cu15 nanoparticles for selective hydrogenation reactions
    Boucher, Matthew B.; Zugic, Branko; Cladaras, George; Kammert, James; Marcinkowski, Matthew D.; Lawton, Timothy J.; Sykes, E. Charles H.; Flytzani-Stephanopoulos, Maria
    Physical Chemistry Chemical Physics (2013), 15 (29), 12187-12196 CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    We report a novel synthesis of nanoparticle Pd-Cu catalysts, contg. only trace amts. of Pd, for selective hydrogenation reactions. Pd-Cu nanoparticles were designed based on model single atom alloy (SAA) surfaces, in which individual, isolated Pd atoms act as sites for hydrogen uptake, dissocn., and spillover onto the surrounding Cu surface. Pd-Cu nanoparticles were prepd. by addn. of trace amts. of Pd (0.18 at. (at)%) to Cu nanoparticles supported on Al2O3 by galvanic replacement (GR). The catalytic performance of the resulting materials for the partial hydrogenation of phenylacetylene was investigated at ambient temp. in a batch reactor under a head pressure of hydrogen (6.9 bar). The bimetallic Pd-Cu nanoparticles have over an order of magnitude higher activity for phenylacetylene hydrogenation when compared to their monometallic Cu counterpart, while maintaining a high selectivity to styrene over many hours at high conversion. Greater than 94% selectivity to styrene is obsd. at all times, which is a marked improvement when compared to monometallic Pd catalysts with the same Pd loading, at the same total conversion. XPS and UV-visible spectroscopy measurements confirm the complete uptake and alloying of Pd with Cu by GR. Scanning tunneling microscopy and thermal desorption spectroscopy of model SAA surfaces confirmed the feasibility of hydrogen spillover onto an otherwise inert Cu surface. These model studies addressed a wide range of Pd concns. related to the bimetallic nanoparticles.
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    OpenURL UNIV OF NORTH TEXAS
  36. 36.
    Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.; Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H. Science 2012, 335, 1209– 1212, DOI: 10.1126/science.1215864
    [CrossRef], [PubMed], [CAS]
    36.
    Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations
    Kyriakou, Georgios; Boucher, Matthew B.; Jewell, April D.; Lewis, Emily A.; Lawton, Timothy J.; Baber, Ashleigh E.; Tierney, Heather L.; Flytzani-Stephanopoulos, Maria; Sykes, E. Charles H.
    Science (Washington, DC, United States) (2012), 335 (6073), 1209-1212 CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Facile dissocn. of reactants and weak binding of intermediates are key requirements for efficient and selective catalysis. However, these two variables are intimately linked in a way that does not generally allow the optimization of both properties simultaneously. By using desorption measurements in combination with high-resoln. scanning tunneling microscopy, the authors show that individual, isolated Pd atoms in a Cu surface substantially lower the energy barrier to both hydrogen uptake on and subsequent desorption from the Cu metal surface. This facile hydrogen dissocn. at Pd atom sites and weak binding to Cu allow for very selective hydrogenation of styrene and acetylene as compared with pure Cu or Pd metal alone.
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    OpenURL UNIV OF NORTH TEXAS
  37. 37.
    Zhang, L.; Wang, A.; Miller, J. T.; Liu, X.; Yang, X.; Wang, W.; Li, L.; Huang, Y.; Mou, C. Y.; Zhang, T. ACS Catal. 2014, 4, 1546– 1553, DOI: 10.1021/cs500071c
    [ACS Full Text ACS Full Text], [CAS]
    37.
    Efficient and Durable Au Alloyed Pd Single-Atom Catalyst for the Ullmann Reaction of Aryl Chlorides in Water
    Zhang, Leilei; Wang, Aiqin; Miller, Jeffrey T.; Liu, Xiaoyan; Yang, Xiaofeng; Wang, Wentao; Li, Lin; Huang, Yanqiang; Mou, Chung-Yuan; Zhang, Tao
    ACS Catalysis (2014), 4 (5), 1546-1553 CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Ion exchange resin-supported Au alloyed Pd single atoms have been explored to serve as an effective and robust catalyst for the Ullmann reaction of aryl halides under mild conditions in aq. media, in particular for the activation of less reactive aryl chlorides. The catalysts were prepd. with an ion exchange-NaBH4 redn. method and submitted to extensive characterizations by HRTEM, XRD, EXAFS, and DRIFTS techniques. XRD patterns demonstrated the formation of Au-Pd alloys. EXAFS and DRIFTS characterization results showed that with an increase of Au/Pd molar ratios, the continuous Pd ensembles on the surface were gradually sepd. and eventually isolated by Au atoms, confirming that the Au alloyed Pd single-atom catalyst was formed. The catalysts exhibited excellent performance for the Ullmann reaction of aryl chlorides, and the turnover no. (TON) increased exponentially with a decrease of the amt. of Pd in the catalysts. On the basis of these characterization and catalytic results, the Au alloyed Pd single-atom was proposed as the active site for the reaction. The catalyst exhibited excellent catalytic performance for a broad scope of substrates and could be reused at least 8 times with no change in yield. This Au alloyed Pd single-atom catalyst bridges the gap between homogeneous and heterogeneous catalysis in org. transformations and may open a new vision to develop other efficient single-atom catalysts for green synthesis of fine chems.
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    OpenURL UNIV OF NORTH TEXAS
  38. 38.
    Jirkovsky, J. S.; Panas, I.; Ahlberg, E.; Halasa, M.; Romani, S.; Schiffrin, D. J. J. Am. Chem. Soc. 2011, 133, 19432– 19441, DOI: 10.1021/ja206477z
    [ACS Full Text ACS Full Text], [CAS]
    38.
    Single Atom Hot-Spots at Au-Pd Nanoalloys for Electrocatalytic H2O2 Production
    Jirkovsky, Jakub S.; Panas, Itai; Ahlberg, Elisabet; Halasa, Matej; Romani, Simon; Schiffrin, David J.
    Journal of the American Chemical Society (2011), 133 (48), 19432-19441 CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A novel strategy to direct the oxygen redn. reaction to preferentially produce H2O2 is formulated and evaluated. The approach combines the inertness of Au nanoparticles toward oxidn., with the improved O2 sticking probability of isolated transition metal "guest" atoms embedded in the Au "host". DFT modeling was employed to screen for the best alloy candidates. Modeling indicates that isolated alloying atoms of Pd, Pt, or Rh placed within the Au surface should enhance the H2O2 prodn. relative to pure Au. Consequently, Au1-xPdx nanoalloys with variable Pd content supported on Vulcan XC-72 were prepd. to investigate the predicted selectivity toward H2O2 prodn. for Au alloyed with Pd. Increasing the Pd concn. to 8% leads to an increase of the electrocatalytic H2O2 prodn. selectivity up to nearly 95%, when the nanoparticles are placed in an environment compatible with that of a proton exchange membrane. Further increase of Pd content leads to a drop in H2O2 selectivity, to below 10% for x = 0.5. The enhancement in H2O2 selectivity is caused by the presence of individual surface Pd atoms surrounded by gold, whereas surface ensembles of contiguous Pd atoms support H2O formation. The results are discussed in the context of exergonic electrocatalytic H2O2 synthesis in Polymer Electrolyte Fuel Cells for the simultaneous cogeneration of chems. and electricity, the latter a credit to prodn. costs.
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    OpenURL UNIV OF NORTH TEXAS
  39. 39.
    Aich, P.; Wei, H. J.; Basan, B.; Kropf, A. J.; Schweitzer, N. M.; Marshall, C. L.; Miller, J. T.; Meyer, R. J. Phys. Chem. C 2015, 119, 18140– 18148, DOI: 10.1021/acs.jpcc.5b01357
    [ACS Full Text ACS Full Text], [CAS]
    39.
    Single-Atom Alloy Pd-Ag Catalyst for Selective Hydrogenation of Acrolein
    Aich, Payoli; Wei, Haojuan; Basan, Bridget; Kropf, A. Jeremy; Schweitzer, Neil M.; Marshall, Christopher L.; Miller, Jeffrey T.; Meyer, Randall
    Journal of Physical Chemistry C (2015), 119 (32), 18140-18148 CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Pd-Ag alloy catalysts with very dil. amts. of Pd were synthesized. EXAFS results demonstrated that when the concn. of Pd was as low as 0.01 wt %, Pd was completely dispersed as isolated single atoms in Ag nanoparticles. The activity for the hydrogenation of acrolein was improved by the presence of these isolated Pd atoms due to the creation of sites with lower activation energy for H2 dissocn. In addn., for the same particle size, the 0.01% Pd/8% Ag alloy nanoparticles exhibited higher selectivity than their monometallic counterparts, suggesting that the Pd atom may act as a site for the favorable bonding of the acrolein mol. for facile hydrogenation of the aldehyde functionality.
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    OpenURL UNIV OF NORTH TEXAS
  40. 40.
    Hammer, B.; Hansen, L. B.; Norskov, J. K. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 7413– 7421, DOI: 10.1103/PhysRevB.59.7413
    [CrossRef], [CAS]
    40.
    Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals
    Hammer, B.; Hansen, L. B.; Norskov, J. K.
    Physical Review B: Condensed Matter and Materials Physics (1999), 59 (11), 7413-7421 CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    A simple formulation of a generalized gradient approxn. for the exchange and correlation energy of electrons has been proposed by J. Perdew et al. (1996). Subsequently, Y. Zhang and W. Wang (1998) have shown that a slight revision of the Perdew-Burke-Ernzerhof (PBE) functional systematically improves the atomization energies for a large database of small mols. In the present work, we show that the Zhang and Yang functional (revPBE) also improves the chemisorption energetics of atoms and mols. on transition-metal surfaces. Our test systems comprise at. and mol. adsorption of oxygen, CO, and NO on Ni(100), Ni(111), Rh(100), Pd(100), and Pd(111) surfaces. As the revPBE functional may locally violate the Lieb-Oxford criterion, we further develop an alternative revision of the PBE functional, RPBE, which gives the same improvement of the chemisorption energies as the revPBE functional at the same time as it fulfills the Lieb-Oxford criterion locally.
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    OpenURL UNIV OF NORTH TEXAS
  41. 41.
    Blochl, P. E. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953– 17979, DOI: 10.1103/PhysRevB.50.17953
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    41.
    Projector augmented-wave method
    Blochl
    Physical review. B, Condensed matter (1994), 50 (24), 17953-17979 ISSN:0163-1829.
    There is no expanded citation for this reference.
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    OpenURL UNIV OF NORTH TEXAS
  42. 42.
    Kresse, G.; Joubert, D. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758– 1775, DOI: 10.1103/PhysRevB.59.1758
    [CrossRef], [CAS]
    42.
    From ultrasoft pseudopotentials to the projector augmented-wave method
    Kresse, G.; Joubert, D.
    Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775 CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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    OpenURL UNIV OF NORTH TEXAS
  43. 43.
    Kresse, G.; Furthmuller, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169– 11186, DOI: 10.1103/PhysRevB.54.11169
    [CrossRef], [PubMed], [CAS]
    43.
    Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
    Kresse, G.; Furthmueller, J.
    Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186 CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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    OpenURL UNIV OF NORTH TEXAS
  44. 44.
    Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15– 50, DOI: 10.1016/0927-0256(96)00008-0
    [CrossRef], [CAS]
    44.
    Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set
    Kresse, G.; Furthmuller, J.
    Computational Materials Science (1996), 6 (1), 15-50 CODEN: CMMSEM; ISSN:0927-0256. (Elsevier)
    The authors present a detailed description and comparison of algorithms for performing ab-initio quantum-mech. calcns. using pseudopotentials and a plane-wave basis set. The authors will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temp. d.-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N2atoms scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge d. including a new special preconditioning optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. The authors have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio mol.-dynamics package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
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    OpenURL UNIV OF NORTH TEXAS
  45. 45.
    Kresse, G.; Hafner, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558– 561, DOI: 10.1103/PhysRevB.47.558
    [CrossRef], [PubMed], [CAS]
    45.
    Ab initio molecular dynamics of liquid metals
    Kresse, G.; Hafner, J.
    Physical Review B: Condensed Matter and Materials Physics (1993), 47 (1), 558-61 CODEN: PRBMDO; ISSN:0163-1829.
    The authors present ab initio quantum-mech. mol.-dynamics calcns. based on the calcn. of the electronic ground state and of the Hellmann-Feynman forces in the local-d. approxn. at each mol.-dynamics step. This is possible using conjugate-gradient techniques for energy minimization, and predicting the wave functions for new ionic positions using sub-space alignment. This approach avoids the instabilities inherent in quantum-mech. mol.-dynamics calcns. for metals based on the use of a factitious Newtonian dynamics for the electronic degrees of freedom. This method gives perfect control of the adiabaticity and allows one to perform simulations over several picoseconds.
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    OpenURL UNIV OF NORTH TEXAS
  46. 46.
    Kresse, G.; Hafner, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 14251– 14269, DOI: 10.1103/PhysRevB.49.14251
    [CrossRef], [PubMed], [CAS]
    46.
    Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium
    Kresse, G.; Hafner, J.
    Physical Review B: Condensed Matter and Materials Physics (1994), 49 (20), 14251-69 CODEN: PRBMDO; ISSN:0163-1829.
    The authors present ab initio quantum-mech. mol.-dynamics simulations of the liq.-metal-amorphous-semiconductor transition in Ge. The simulations are based on (a) finite-temp. d.-functional theory of the 1-electron states, (b) exact energy minimization and hence calcn. of the exact Hellmann-Feynman forces after each mol.-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nose' dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows the authors to perform simulations over >30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liq. and amorphous Ge in very good agreement with expt.. The simulation allows the authors to study in detail the changes in the structure-property relation through the metal-semiconductor transition. The authors report a detailed anal. of the local structural properties and their changes induced by an annealing process. The geometrical, bounding, and spectral properties of defects in the disordered tetrahedral network are studied and compared with expt.
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    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2cXkvFKrtL4%253D&md5=c5dddfd01394e53720fb4c3a3ccfd6c0
    OpenURL UNIV OF NORTH TEXAS
  47. 47.
    Mathew, K.; Hennig, R. G. ArXiv 2016, 1601.03346
    There is no corresponding record for this reference.
    OpenURL UNIV OF NORTH TEXAS
  48. 48.
    Blaylock, D. W.; Ogura, T.; Green, W. H.; Beran, G. J. O. J. Phys. Chem. C 2009, 113, 4898– 4908, DOI: 10.1021/jp806527q
    [ACS Full Text ACS Full Text], [CAS]
    48.
    Computational Investigation of Thermochemistry and Kinetics of Steam Methane Reforming on Ni(111) under Realistic Conditions
    Blaylock, D. Wayne; Ogura, Teppei; Green, William H.; Beran, Gregory J. O.
    Journal of Physical Chemistry C (2009), 113 (12), 4898-4908 CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The reaction pathways and kinetics of steam methane reforming (SMR) over Ni(111) are investigated using plane wave d. functional theory. The thermochem. data are used to develop a microkinetic model of SMR that allows for the investigation of reforming pathways and the most abundant reaction intermediates on the catalyst surface at industrially relevant temps. and pressures. Pairing the kinetic model with a statistical thermodn. treatment, surface behavior under a wide range of temps., pressures, and initial concns. can be examd. We present our results at T = 800° and P = 10 bar with an initial H2O/CH4 ratio of 2.5:1. Sensitivity anal. is used to provide information about rate-limiting steps in the reaction network. The reaction intermediate CH* is the most important carbon-contg. intermediate. CH4(g) adsorption as well as the reactions CH* + O* → CHO* and CH* + OH* → CHOH* are the most sensitive reactions in the mechanism. Consistent accounting for entropic effects was important in obtaining reasonable surface coverages of reaction intermediates, which can influence the detn. of active reforming pathways on the catalyst surface.
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    OpenURL UNIV OF NORTH TEXAS
  49. 49.
    Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Norskov, J. K. Energy Environ. Sci. 2010, 3, 1311– 1315, DOI: 10.1039/c0ee00071j
    [CrossRef], [CAS]
    49.
    How copper catalyzes the electroreduction of carbon dioxide into hydrocarbon fuels
    Peterson, Andrew A.; Abild-Pedersen, Frank; Studt, Felix; Rossmeisl, Jan; Norskov, Jens K.
    Energy & Environmental Science (2010), 3 (9), 1311-1315 CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    D. functional theory calcns. explain copper's unique ability to convert CO2 into hydrocarbons, which may open up (photo-)electrochem. routes to fuels.
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    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXht1yrsbnP&md5=313d823d1b2899ca40d1312681f1ad15
    OpenURL UNIV OF NORTH TEXAS
  50. 50.
    Xiao, H.; Cheng, T.; Goddard, W. A.; Sundararaman, R. J. Am. Chem. Soc. 2016, 138, 483– 486, DOI: 10.1021/jacs.5b11390
    [ACS Full Text ACS Full Text], [CAS]
    50.
    Mechanistic Explanation of the pH Dependence and Onset Potentials for Hydrocarbon Products from Electrochemical Reduction of CO on Cu (111)
    Xiao, Hai; Cheng, Tao; Goddard, William A.; Sundararaman, Ravishankar
    Journal of the American Chemical Society (2016), 138 (2), 483-486 CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Energy and environmental concerns demand development of more efficient and selective electrodes for electrochem. redn. of CO2 to form fuels and chems. Since Cu is the only pure metal exhibiting redn. to form hydrocarbon chems., the authors focus here on the Cu (111) electrode. The authors present a methodol. for d. functional theory calcns. to obtain accurate onset electrochem. potentials with explicit const. electrochem. potential and pH effects using implicit solvation. The authors predict the atomistic mechanisms underlying electrochem. redn. of CO, finding that (1) at acidic pH, the C1 pathway proceeds through COH to CHOH to form CH4 while C2 (C3) pathways are kinetically blocked; (2) at neutral pH, the C1 and C2 (C3) pathways share the COH common intermediate, where the branch to C-C coupling is realized by a novel CO-COH pathway; and (3) at high pH, early C-C coupling through adsorbed CO dimerization dominates, suppressing the C1 pathways by kinetics, thereby boosting selectivity for multi-C products.
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    OpenURL UNIV OF NORTH TEXAS
  51. 51.
    Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. J. Phys. Chem. B 2004, 108, 17886– 17892, DOI: 10.1021/jp047349j
    [ACS Full Text ACS Full Text], [CAS]
    51.
    Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode
    Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H.
    Journal of Physical Chemistry B (2004), 108 (46), 17886-17892 CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    A method to obtain the stability of reaction intermediates of electrochem. processes using electronic structure calcns. is presented. This method is used in combination with detailed d. functional calcns. to develop a detailed description of the free-energy landscape of the electrochem. O redn. reaction over Pt(111), as a function of applied bias. This allowed identification of the origin of the overpotential of this reaction. Adsorbed O and hydroxyl are stable intermediates at potentials close to equil., and the calcd. rate const. for the activated p/electron transfer to adsorbed O or hydroxyl can account quant. for the obsd. kinetics. From a database of calcd. O and hydroxyl adsorption energies, the trends in the O redn. rate for a large no. of different transition and noble metals can be explained. Alternative reaction mechanisms involving p/electron transfer to adsorbed mol. O were also considered, and this peroxide mechanism dominates for most noble metals. The model suggests ways to improve the electrocatalytic properties of fuel cell cathodes.
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    OpenURL UNIV OF NORTH TEXAS
  52. 52.
    Oloman, C.; Li, H. ChemSusChem 2008, 1, 385– 391, DOI: 10.1002/cssc.200800015
    [CrossRef], [PubMed], [CAS]
    52.
    Electrochemical processing of carbon dioxide
    Oloman, Colin; Li, Hui
    ChemSusChem (2008), 1 (5), 385-391 CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    With respect to the neg. role of CO2 on climate, it is clear the time is ripe for developing processes which convert CO2 into useful products. Electro-redn. of CO2 is a prime candidate here, since the reaction at near-ambient conditions yields org. compds., e.g., formic acid, methanol, and CH4. Recent lab. work on the 100-A scale showed CO2 redn. to formate (HCO2-) may be conducted in a trickle-bed continuous electro-chem. reactor under industrially viable conditions. Presuming problems of cathode stability and formate crossover can be overcome, this type of reactor is proposed as the basis for a com. operation. The viability of corresponding processes for electro-synthesis of formate salts and/or formic acid from CO2 was examd. through conceptual flow sheets for 2 process options, each converting CO2 at a rate of 100 tonnes/day.
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    OpenURL UNIV OF NORTH TEXAS
  53. 53.
    Steinmann, S. N.; Michel, C.; Schwiedernoch, R.; Sautet, P. Phys. Chem. Chem. Phys. 2015, 17, 13949– 13963, DOI: 10.1039/C5CP00946D
    [CrossRef], [PubMed], [CAS]
    53.
    Impacts of electrode potentials and solvents on the electroreduction of CO2: a comparison of theoretical approaches
    Steinmann, Stephan N.; Michel, Carine; Schwiedernoch, Renate; Sautet, Philippe
    Physical Chemistry Chemical Physics (2015), 17 (21), 13949-13963 CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Since CO2 is a readily available feedstock throughout the world, the utilization of CO2 as a C1 building block for the synthesis of valuable chems. is a highly attractive concept. However, due to its very nature of energy depleted "carbon sink", CO2 has a very low reactivity. Electrocatalysis offers the most attractive means to activate CO2 through redn.: the electron is the "cleanest" reducing agent whose energy can be tuned to the thermodn. optimum. Under protic conditions, the redn. of CO2 over many metal electrodes results in formic acid. Thus, to open the road to its utilization as a C1 building block, the presence of water should be avoided to allow a more diverse chem., in particular for C-C bond formation with alkenes. Under those conditions, the intrinsic reactivity of CO2 can generate carbonates and oxalates by C-O and C-C bond formation, resp. On Ni(111), almost exclusively carbonates and carbon monoxide are evidenced exptl. Despite recent progress in modeling electrocatalytic reactions, detg. the actual mechanism and selectivities between competing reaction pathways is still not straight forward. As a simple but important example of the intrinsic reactivity of CO2 under aprotic conditions, we highlight the shortcomings of the popular linear free energy relationship for electrode potentials (LFER-EP). Going beyond this zeroth order approxn. by charging the surface and thus explicitly including the electrochem. potential into the electronic structure computations allows us to access more detailed insights, shedding light on coverage effects and on the influence of counterions.
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    OpenURL UNIV OF NORTH TEXAS
  54. 54.
    Steinmann, S. N.; Michel, C.; Schwiedernoch, R.; Filhol, J. S.; Sautet, P. ChemPhysChem 2015, 16, 2307– 2311, DOI: 10.1002/cphc.201500187
    [CrossRef], [PubMed], [CAS]
    54.
    Modeling the HCOOH/CO2 Electrocatalytic Reaction: When Details Are Key
    Steinmann, Stephan N.; Michel, Carine; Schwiedernoch, Renate; Filhol, Jean-Sebastien; Sautet, Philippe
    ChemPhysChem (2015), 16 (11), 2307-2311 CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The authors' 1st principles simulations of the electrooxidn. of formic acid over Ni identify the reorientation of the formate intermediate and the desorption of CO2 as the rate-limiting steps. Although they are not assocd. with an electron transfer, these barriers are strongly modified when the electrochem. potential is explicitly accounted for and when modeling the influence of the solvent. Hence, such a level of modeling is key to understand the kinetic limitations that penalize the reaction.
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    OpenURL UNIV OF NORTH TEXAS
  55. 55.
    Abild-Pedersen, F.; Greeley, J.; Studt, F.; Rossmeisl, J.; Munter, T. R.; Moses, P. G.; Skulason, E.; Bligaard, T.; Norskov, J. K. Phys. Rev. Lett. 2007, 99, 016105, DOI: 10.1103/PhysRevLett.99.016105
    [CrossRef], [CAS]
    55.
    Scaling Properties of Adsorption Energies for Hydrogen-Containing Molecules on Transition-Metal Surfaces
    Abild-Pedersen, F.; Greeley, J.; Studt, F.; Rossmeisl, J.; Munter, T. R.; Moses, P. G.; Skulason, E.; Bligaard, T.; Noerskov, J. K.
    Physical Review Letters (2007), 99 (1), 016105/1-016105/4 CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    D. functional theory calcns. are presented for CHx, x = 0,1,2,3, NHx, x = 0,1,2, OHx, x = 0,1, and SHx, x = 0,1 adsorption on a range of close-packed and stepped transition-metal surfaces. We find that the adsorption energy of any of the mols. considered scales approx. with the adsorption energy of the central, C, N, O, or S atom, the scaling const. depending only on x. A model is proposed to understand this behavior. The scaling model is developed into a general framework for estg. the reaction energies for hydrogenation and dehydrogenation reactions.
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    OpenURL UNIV OF NORTH TEXAS
  56. 56.
    Garcia, S.; Zhang, L.; Piburn, G. W.; Henkelman, G.; Humphrey, S. M. ACS Nano 2014, 8, 11512– 11521, DOI: 10.1021/nn504746u
    [ACS Full Text ACS Full Text], [CAS]
    56.
    Microwave Synthesis of Classically Immiscible Rhodium-Silver and Rhodium-Gold Alloy Nanoparticles: Highly Active Hydrogenation Catalysts
    Garcia, Stephany; Zhang, Liang; Piburn, Graham W.; Henkelman, Graeme; Humphrey, Simon M.
    ACS Nano (2014), 8 (11), 11512-11521 CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Noble metal alloys are important in large-scale catalytic processes. Alloying facilitates fine-tuning of catalytic properties via synergistic interactions between metals. It also allows for diln. of scarce and expensive metals using comparatively earth-abundant metals. RhAg and RhAu are classically considered to be immiscible metals. We show here that stable RhM (M = Ag, Au) nanoparticles with randomly alloyed structures and broadly tunable Rh:M ratios can be prepd. using a microwave-assisted method. The alloyed nanostructures with optimized Rh:M compns. are significantly more active as hydrogenation catalysts than Rh itself: Rh is more dil. and more reactive when alloyed with Ag or Au, even though the latter are both catalytically inactive for hydrogenation. Theor. modeling predicts that the obsd. catalytic enhancement is due to few-atom surface ensemble effects in which the overall reaction energy profile for alkene hydrogenation is optimized due to Rh-M d-band intermixing.
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    OpenURL UNIV OF NORTH TEXAS
  57. 57.
    Nie, X. W.; Esopi, M. R.; Janik, M. J.; Asthagiri, A. Angew. Chem., Int. Ed. 2013, 52, 2459– 2462, DOI: 10.1002/anie.201208320
    [CrossRef], [CAS]
    57.
    Selectivity of CO2 Reduction on Copper Electrodes: The Role of the Kinetics of Elementary Steps
    Nie, Xiaowa; Esopi, Monica R.; Janik, Michael J.; Asthagiri, Aravind
    Angewandte Chemie, International Edition (2013), 52 (9), 2459-2462 CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    DFT calcns. of the activation barriers of elementary steps have resolved that the redn. of CO is the key selectivity-detg. step for CO2 electroredn. on Cu (111). The dominant path proceeds through redn. of CO to COH, which eventually leads to CHi species, and can produce both methane and ethylene. This path does not occur during the nonelectrochem. methanol synthesis from syngas owing in part to the lack of an aq. environment to promote CO-H bond formation, but also because the applied potential is needed to drive the forward reaction through COH to methane/ethylene. The authors proposed path is able to reconcile the exptl. data on CO2, CO, and CH2O redn. on Cu electrodes. While other side reactions may still play a role, and coverage effects, step edges, Cu surface facet, and the role of addnl. solvation need to be explored in the future, this contribution makes a crit. step forward in our fundamental understanding of CO2 electroredn. on Cu surfaces. This anal. suggests that future design of heterogeneous catalysts for CO2 (photo)electroredn. should focus on the relative energetics of COH vs. CHO formation to tailor the selectivity of the desired products towards liq. fuels such as methanol.
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    OpenURL UNIV OF NORTH TEXAS
  58. 58.
    Back, S.; Kim, H.; Jung, Y. ACS Catal. 2015, 5, 965– 971, DOI: 10.1021/cs501600x
    [ACS Full Text ACS Full Text], [CAS]
    58.
    Selective Heterogeneous CO2 Electroreduction to Methanol
    Back, Seoin; Kim, Heejin; Jung, Yousung
    ACS Catalysis (2015), 5 (2), 965-971 CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Catalytic electroredn. of CO2 to useful chem. feedstocks is an environmentally and technol. important process, yet the low energy efficiency and difficulty in controlling product selectivity are great challenges. The reason for part of the latter is that there are presently no catalyst design principles to selectively control CO2 electroredn. toward a desired product. As a 1st attempt, the authors suggest combining a few criteria (CO binding energy, OH binding energy, and H binding energy) that can be collectively used as activity- and selectivity-detg. descriptors to preferentially produce MeOH over methane from CO2 electroredn. The authors then apply these concepts to near-surface alloys (NSAs) to propose efficient and selective CO2 electrochem. redn. catalysts to produce MeOH. The W/Au alloy is identified as a promising candidate to have increased catalyst efficiency (decreased CO2 redn. overpotential and increased overpotential for unwanted H evolution) as well as improved product selectivity toward MeOH, in comparison to conventional Cu catalyst.
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    OpenURL UNIV OF NORTH TEXAS
  59. 59.
    Shi, C.; O’Grady, C. P.; Peterson, A. A.; Hansen, H. A.; Norskov, J. K. Phys. Chem. Chem. Phys. 2013, 15, 7114– 7122, DOI: 10.1039/c3cp50645b
    [CrossRef], [PubMed], [CAS]
    59.
    Modeling CO2 reduction on Pt(111)
    Shi, Chuan; O'Grady, Christopher P.; Peterson, Andrew A.; Hansen, Heine A.; Norskov, Jens K.
    Physical Chemistry Chemical Physics (2013), 15 (19), 7114-7122 CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    D. functional theory was used to model the electrochem. redn. of CO2 on Pt(111) with an explicit solvation layer and the presence of extra hydrogen atoms to represent a neg. charged electrode. We focused on the electronic energy barriers for the first four lowest energy proton-electron transfer steps for reducing CO2 on Pt(111) beginning with adsorbed *CO2 and continuing with *COOH, *CO + H2O, *COH, and ending with *C + H2O. We find that simple elementary steps in which a proton is transferred to an adsorbate (such as the protonation of *CO to *COH) have small barriers on the order of 0.1 eV. Elementary steps in which a proton is transferred and a C-O bond is simultaneously cleaved show barriers on the order of 0.5 eV. All barriers calcd. for these steps show no sign of being insurmountable at room temp. To explain why these barriers are so small, we analyze the charge d. and the d. of states plots to see that first, the electron transfer is decoupled from the proton transfer so that in the initial state, the surface and adsorbate are already charged up and can easily accept the proton from soln. Also, we see that in the cases where barriers are on the order of 0.1 eV, electron d. in the initial state localizes on the oxygen end of the adsorbate, while electron d. is more spread out on the surface for initial states of the C-O bond cleaving elementary steps.
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    OpenURL UNIV OF NORTH TEXAS
  60. 60.
    Luo, W. J.; Nie, X. W.; Janik, M. J.; Asthagiri, A. ACS Catal. 2016, 6, 219– 229, DOI: 10.1021/acscatal.5b01967
    [ACS Full Text ACS Full Text], [CAS]
    60.
    Facet Dependence of CO2 Reduction Paths on Cu Electrodes
    Luo, Wenjia; Nie, Xiaowa; Janik, Michael J.; Asthagiri, Aravind
    ACS Catalysis (2016), 6 (1), 219-229 CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Exptl. results have shown that CO2 electroredn. is sensitive to the surface morphol. of Cu electrodes. We used d. functional theory (DFT) to evaluate the thermodn. and kinetics of CO2 redn. pathways on Cu(100) and Cu(111) with the aim of understanding the exptl. reported differences in CO2 redn. products. Results suggest that the hydrogenation of CO* to hydroxymethylidyne (COH*) or formyl (CHO*) is a key selective step. Cu(111) favors COH* formation, through which methane and ethylene are produced via a common CH2 species under high overpotential (<-0.8 V vs RHE). On Cu(100), formation of CHO* is preferred and ethylene formation goes through C-C coupling of two CHO* species followed by a series of redn. steps of the C2 intermediates, under relatively lower overpotential (-0.4 to -0.6 V vs RHE). Further redn. of these C2 intermediates, however, require larger potentials (∼-1.0 V vs RHE) and conflicts with the exptl. obsd. low potential pathway to C2 products on Cu(100). Calcns. show that the presence of (111) step sites on the flat (100) terrace can reduce the overpotential for C2 prodn. on the Cu electrode, which may be present on Cu(100) due to reconstruction. On Cu(100), a change in CO* coverage from low to high with increasing neg. applied potential can trigger a switch from ethylene/ethanol to methane/ethylene as the redn. products by affecting the relative stability of CHO* and COH*.
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    OpenURL UNIV OF NORTH TEXAS
  61. 61.
    Karamad, M.; Hansen, H. A.; Rossmeisl, J.; Norskov, J. K. ACS Catal. 2015, 5, 4075– 4081, DOI: 10.1021/cs501542n
    [ACS Full Text ACS Full Text], [CAS]
    61.
    Mechanistic Pathway in the Electrochemical Reduction of CO2 on RuO2
    Karamad, Mohammadreza; Hansen, Heine A.; Rossmeisl, Jan; Noerskov, Jens K.
    ACS Catalysis (2015), 5 (7), 4075-4081 CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    RuO2 is reported to reduce CO2 electrochem. to MeOH at low overpotential. Herein, the authors used d. functional theory (DFT) to gain insight into the mechanism for CO2 redn. on RuO2(110). The authors have studied the thermodn. stability of various surface terminations in the electrochem. environment and found CO covered surfaces to be particularly stable, although their formation might be kinetically limited under mildly reducing conditions. The authors have identified the lowest free energy pathways for CO2 redn. to formic acid (HCOOH), MeOH, and methane (CH4) on partially reduced RuO2(110) covered with 0.25 and 0.5 ML of CO*. CO2 is reduced to formic acid, which is further reduced to MeOH and methane. At 0.25 ML of CO*, the redn. of formate (OCHO*) to formic acid is the thermodynamically most difficult step and becomes exergonic at potentials <-0.43 V vs. the reversible H electrode (RHE). However, at 0.5 ML of CO*, the redn. of formic acid to H2COOH* is the thermodynamically most difficult step and becomes exergonic at potentials <-0.25 V vs. RHE. CO2 redn. activity on RuO2 changes with CO coverage, which suggests that CO coverage can be used as a tool to tune the CO2 redn. activity. The authors showed the mechanism for CO2 redn. on RuO2 to be different from that on Cu. On Cu, hydrocarbons are formed at high faradaic efficiency through redn. of CO* at ∼1 V overpotential, while on RuO2, MeOH and formate are formed through redn. of formic acid at lower overpotentials. Using the authors' understanding of the CO2 redn. mechanism on RuO2, the authors suggest redn. of formic acid on RuO2, which should lead to MeOH and methane prodn. at relatively low overpotentials.
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    OpenURL UNIV OF NORTH TEXAS
  62. 62.
    Hirunsit, P.; Soodsawang, W.; Limtrakul, J. J. Phys. Chem. C 2015, 119, 8238– 8249, DOI: 10.1021/acs.jpcc.5b01574
    [ACS Full Text ACS Full Text], [CAS]
    62.
    CO2 Electrochemical Reduction to Methane and Methanol on Copper-Based Alloys: Theoretical Insight
    Hirunsit, Pussana; Soodsawang, Wiwaporn; Limtrakul, Jumras
    Journal of Physical Chemistry C (2015), 119 (15), 8238-8249 CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The authors performed a systematic study of CO2 electroredn. to CH4 and MeOH on Cu-based alloys stepped surfaces using d. functional theory calcns. assocd. with the std. H electrode model. The authors detd. the correlations between CO adsorption energy and the other key CxHyOz intermediates adsorption energy, the overpotential, the limiting-potential elementary step, and selectivity to CH4, MeOH, HCOOH, and H2. The electrode efficiency decrease by OH* poisoning and the H2 evolution is also studied. The CO* protonation is the limiting-potential step on most surfaces, with the exception on Cu3Au and Cu3Co surfaces. In spite of the excessive strong CO* interaction on some surfaces, the overpotentials reduce when the degree of CO* adsorption energy and HCO*/COH* adsorption energy decoupling increases. The CO* adsorption energy is a good descriptor for linear scaling correlations with the other CxHyOz intermediates due to the similar charge transfer characteristics of the C-O bond in CO* and those intermediates. The formic acid prodn. can be efficiently catalyzed on Cu3Pt, Cu3Ni, Cu3Co, and Cu3Rh surfaces. MeOH prodn. is favorable on Cu3Pd and Cu3Pt surfaces, yet they show high overpotential (∼0.7 V). The key of MeOH selectivity is CH2OH* intermediate formation favorability assocd. with the preference of CH2OH* protonation at the C atom over the O atom. The calcns. reveal that the electroredn. activity on Cu-based alloys catalysts do not show a volcano-type relation as was previously found on pure metal catalysts.
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    OpenURL UNIV OF NORTH TEXAS
  63. 63.
    Lim, H. K.; Shin, H.; Goddard, W. A.; Hwang, Y. J.; Min, B. K.; Kim, H. J. Am. Chem. Soc. 2014, 136, 11355– 11361, DOI: 10.1021/ja503782w
    [ACS Full Text ACS Full Text], [CAS]
    63.
    Embedding Covalency into Metal Catalysts for Efficient Electrochemical Conversion of CO2
    Lim, Hyung-Kyu; Shin, Hyeyoung; Goddard, William A.; Hwang, Yun Jeong; Min, Byoung Koun; Kim, Hyungjun
    Journal of the American Chemical Society (2014), 136 (32), 11355-11361 CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    CO2 conversion is an essential technol. to develop a sustainable carbon economy for the present and the future. Many studies have focused extensively on the electrochem. conversion of CO2 into various useful chems. However, there is not yet a soln. of sufficiently high enough efficiency and stability to demonstrate practical applicability. In this work, we use first-principles-based high-throughput screening to propose silver-based catalysts for efficient electrochem. redn. of CO2 to CO, while decreasing the overpotential by 0.4-0.5 V. We discovered the covalency-aided electrochem. reaction (CAER) mechanism in which p-block dopants have a major effect on the modulating reaction energetics by imposing partial covalency into the metal catalysts, thereby enhancing their catalytic activity well beyond modulations arising from d-block dopants. In particular, sulfur or arsenic doping can effectively minimize the overpotential with good structural and electrochem. stability. We expect this work to provide useful insights to guide the development of a feasible strategy to overcome the limitations of current technol. for electrochem. CO2 conversion.
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    OpenURL UNIV OF NORTH TEXAS
  64. 64.
    Bernstein, N. J.; Akhade, S. A.; Janik, M. J. Phys. Chem. Chem. Phys. 2014, 16, 13708– 13717, DOI: 10.1039/C4CP00266K
    [CrossRef], [PubMed], [CAS]
    64.
    Density functional theory study of carbon dioxide electrochemical reduction on the Fe(100) surface
    Bernstein, Nicole J.; Akhade, Sneha A.; Janik, Michael J.
    Physical Chemistry Chemical Physics (2014), 16 (27), 13708-13717 CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    CO2 electroredn. offers the possibility of producing hydrocarbon fuels using energy from renewable sources. Herein, the authors use d. functional theory to analyze the feasibility of CO2 electroredn. on a Fe(100) surface. Exptl., Fe is nonselective for hydrocarbon formation. A simplistic anal. of low-coverage reaction intermediate energies for the paths to produce CH4 and MeOH from CO2 suggests Fe(100) could be more active than Cu(111), currently the only metallic catalyst to show selectivity towards hydrocarbon formation. Impediments to CO2 electroredn. on Fe(100) including O*/OH* (* denotes surface bound species) blockage of active surface sites; competitive adsorption effects of H*, CO* and C*; and Fe carbide formation are considered. The authors' results indicate that under CO2 electroredn. conditions, Fe(100) is predicted to be covered in C* or CO* species, blocking any C-H bond formation. Further, bulk Fe is predicted to be unstable relative to FeCx formation at potentials relevant to CO2 electroredn. conditions.
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    OpenURL UNIV OF NORTH TEXAS
  65. 65.
    Kuhl, K. P.; Hatsukade, T.; Cave, E. R.; Abram, D. N.; Kibsgaard, J.; Jaramillo, T. F. J. Am. Chem. Soc. 2014, 136, 14107– 14113, DOI: 10.1021/ja505791r
    [ACS Full Text ACS Full Text], [CAS]
    65.
    Electrocatalytic Conversion of Carbon Dioxide to Methane and Methanol on Transition Metal Surfaces
    Kuhl, Kendra P.; Hatsukade, Toru; Cave, Etosha R.; Abram, David N.; Kibsgaard, Jakob; Jaramillo, Thomas F.
    Journal of the American Chemical Society (2014), 136 (40), 14107-14113 CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Fuels and industrial chems. that are conventionally derived from fossil resources could potentially be produced in a renewable, sustainable manner by an electrochem. process that operates at room temp. and atm. pressure, using only water, CO2, and electricity as inputs. To enable this technol., improved catalysts must be developed. Herein, trends are reported in the electrocatalytic conversion of CO2 on a broad group of seven transition metal surfaces: Au, Ag, Zn, Cu, Ni, Pt, and Fe. Contrary to conventional knowledge in the field, all metals studied are capable of producing methane or methanol. Reaction rates are quantified for these two products and catalyst activity and selectivity are described in the framework of CO binding energies for the different metals. While selectivity toward methane or methanol is low for most of these metals, the fact that they are all capable of producing these products, even at a low rate, is important new knowledge. This study reveals a richer surface chem. for transition metals than previously known and provides new insights to guide the development of improved CO2 conversion catalysts.
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    OpenURL UNIV OF NORTH TEXAS
  66. 66.
    Zarkevich, N. A.; Tan, T. L.; Johnson, D. D. Phys. Rev. B 2007, 75, 104203, DOI: 10.1103/PhysRevB.75.104203
    [CrossRef], [CAS]
    66.
    First-principles prediction of phase-segregating alloy phase diagrams and a rapid design estimate of their transition temperatures
    Zarkevich, N. A.; Tan, Teck L.; Johnson, D. D.
    Physical Review B: Condensed Matter and Materials Physics (2007), 75 (10), 104203/1-104203/12 CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The transition temp. (Tc) vs. concn. binary phase diagrams are calcd. for several phase-segregating alloys (fcc. Ca-Sr, Au-Pt, and Rh with Pd, Cu, Ag or Au) by using a multi-scale method combining first-principles calcns. and Monte Carlo simulation via the cluster expansion. The Pd-Rh phase diagram, with its well-known high-temp. miscibility gap, was studied to verify the method's reliability. Ca-Sr is predicted to segregate at low temps. A rapid est. of Tc is obtained from enthalpy anal. derived from a cluster expansion, and with thermodn. integration under what circumstances this mean-field est. is accurate compared to Monte Carlo results. Also, how an electronegativity difference of the alloying elements is rapidly assessed when vibrational entropy effects should be included in the est. of Tc is discussed.
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    OpenURL UNIV OF NORTH TEXAS
  67. 67.
    Davies, R. H.; Dinsdale, A. T.; Gisby, J. A.; Robinson, J. A. J.; Martin, S. M. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2002, 26, 229– 271, DOI: 10.1016/S0364-5916(02)00036-6
    [CrossRef], [CAS]
    67.
    MTDATA - thermodynamic and phase equilibrium software from the national physical laboratory
    Davies, R. H.; Dinsdale, A. T.; Gisby, J. A.; Robinson, J. A. J.; Martin, S. M.
    CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry (2002), 26 (2), 229-271 CODEN: CCCTD6; ISSN:0364-5916. (Elsevier Science Ltd.)
    In the past, the complexity of the chem. and phase equil. established during many industrial processes has prevented the kind of in-depth understanding of their thermodn. necessary for successful and efficient process control. Predictive thermochem., as embodied within MTDATA, makes such an understanding possible. MTDATA allows equil. to be calcd. for multicomponent systems of practical interest, contg. many different types of phase, from critically assessed data for their component binary and ternary subsystems. Very complicated calcns. can be undertaken with as many as thirty different components and 500 phases being considered simultaneously. A no. of modules are incorporated for critically assessing, manipulating and retrieving the data, making various types of calcn. and plotting binary, ternary, multicomponent, and predominance area diagrams. Facilities are also available for users to link the complexities of phase equil. calcns. within MTDATA to their own software or to third party com. software packages enabling users to simulate unit operations within an industrial plant or to integrate kinetic effects. Special derivs. of MTDATA are also now being developed to provide the sophisticated capabilities within MTDATA to non-expert users for special applications using carefully designed graphical user interfaces, which are both easy to use, and which provide output specifically designed for the particular industrial need.
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    OpenURL UNIV OF NORTH TEXAS
  68. 68.
    Essinger-Hileman, E. R.; DeCicco, D.; Bondi, J. F.; Schaak, R. E. J. Mater. Chem. 2011, 21, 11599– 11604, DOI: 10.1039/c0jm03913f
    [CrossRef], [CAS]
    68.
    Aqueous room-temperature synthesis of Au-Rh, Au-Pt, Pt-Rh, and Pd-Rh alloy nanoparticles: fully tunable compositions within the miscibility gaps
    Essinger-Hileman, Elizabeth R.; DeCicco, Danielle; Bondi, James F.; Schaak, Raymond E.
    Journal of Materials Chemistry (2011), 21 (31), 11599-11604 CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    Many binary late transition metal systems have large bulk miscibility gaps, and a variety of synthetic strategies were developed to generate these nonequil. alloys as nanoparticles. While many of these methods strive to co-nucleate both elements by exploiting fast redn. kinetics or co-sequestration within a confined space, the authors show here that simple room-temp. borohydride co-redn. of appropriate aq. metal salt solns. yields alloy nanoparticles in the bulk-immiscible Au-Rh, Au-Pt, Pt-Rh, and Pd-Rh systems. The compns. can be tuned across the entire Au1-xRhx, Au1-xPtx, Pt1-xRhx, and Pd1-xRhx solid solns. by varying the ratio of metal salt reagents, and they form in the presence of a variety of mol. and polymeric surface stabilizers. Reaction pathway studies on the model Au-Rh system suggest that the alloy nanoparticles form via a conversion chem. mechanism: Au nanoparticle templates nucleate 1st, followed by diffusion of Rh to form homogeneous Au-Rh alloy nanoparticles. The alloy nanoparticles tend to be agglomerated, but this can be minimized by forming the nanoparticles directly on catalytically relevant high surface area C and biol. supports, e.g. Vulcan C and wild-type M13 bacteriophage.
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    OpenURL UNIV OF NORTH TEXAS
  69. 69.
    Kusada, K.; Yamauchi, M.; Kobayashi, H.; Kitagawa, H.; Kubota, Y. J. Am. Chem. Soc. 2010, 132, 15896– 15898, DOI: 10.1021/ja107362z
    [ACS Full Text ACS Full Text], [CAS]
    69.
    Hydrogen-storage properties of solid-solution alloys of immiscible neighboring elements with Pd
    Kusada, Kohei; Yamauchi, Miho; Kobayashi, Hirokazu; Kitagawa, Hiroshi; Kubota, Yoshiki
    Journal of the American Chemical Society (2010), 132 (45), 15896-15898 CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Rh and Ag are the elements neighboring Pd, which is well known as a hydrogen-storage metal. Although Rh and Ag do not possess hydrogen-storage properties, can Ag-Rh alloys actually store hydrogen. A chem. redn. method was used to obtain Ag-Rh alloys, and XRD and STEM-EDX gave clear evidence that the alloys mixed at the at. level. From the measurements of hydrogen pressure-compn. isotherms and solid-state 2H NMR, the Ag-Rh solid-soln. alloys absorb hydrogen. The total amt. of hydrogen absorbed reached a max. at the Ag-50 at.% Rh alloy whose electronic structure is expected to be similar to that of Pd.
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    OpenURL UNIV OF NORTH TEXAS
  70. 70.
    Tyson, W. R.; Miller, W. A. Surf. Sci. 1977, 62, 267– 276, DOI: 10.1016/0039-6028(77)90442-3
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    70.
    Surface free energies of solid metals: estimation from liquid surface tension measurements
    Tyson, W. R.; Miller, W. A.
    Surface Science (1977), 62 (1), 267-76 CODEN: SUSCAS; ISSN:0039-6028.
    An equation is derived on semitheor. grounds which expresses the solid-vapor surface free energy as a function of the liq. surface tension and the solid-liq. interfacial free energy. A d. method of calcg. the solid-liq. energy is presented, which then allows an accurate est. of solid surface energy at the melting temp., Tm, to be made for the large no. of elements for which dependable liq. surface tension data exist. A method of estg. surface entropy is presented, and was used to calc. the energies typical of av., high-index surfaces at temps. from 0 K to Tm. This is the most accurate method presently available for the calcn. of the surface energy of solids in the absence of direct exptl. measurement.
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    OpenURL UNIV OF NORTH TEXAS
  71. 71.
    Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollar, J. Surf. Sci. 1998, 411, 186– 202, DOI: 10.1016/S0039-6028(98)00363-X
    [CrossRef], [CAS]
    71.
    The surface energy of metals
    Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollar, J.
    Surface Science (1998), 411 (1/2), 186-202 CODEN: SUSCAS; ISSN:0039-6028. (Elsevier Science B.V.)
    The authors used d. functional theory to establish a database of surface energies for low index surfaces of 60 metals in the periodic table. The data may be used as a consistent starting point for models of surface science phenomena. The accuracy of the database is established in a comparison with other d. functional theory results and the calcd. surface energy anisotropies are applied in a detn. of the equil. shape of nano-crystals of Fe, Cu, Mo, Ta, Pt and Pb.
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    OpenURL UNIV OF NORTH TEXAS
  72. 72.
    Sansa, M.; Dhouib, A.; Guesmi, H. J. Chem. Phys. 2014, 141, 064709, DOI: 10.1063/1.4891869
    [CrossRef], [CAS]
    72.
    Density functional theory study of CO-induced segregation in gold-based alloys
    Sansa, Myriam; Dhouib, Adnene; Guesmi, Hazar
    Journal of Chemical Physics (2014), 141 (6), 064709/1-064709/8 CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    This paper reports a systematic study of the effect of CO gas on the chem. compn. at the surface of gold-based alloys. Using DFT periodic calcns. in presence of adsorbed CO the segregation behavior of group 9-10-11 transition metals (Ag, Cu, Pt, Pd, Ni, Ir, Rh, Co) substituted in semi-infinite gold surfaces is investigated. Although, CO is found to be more strongly adsorbed on (100) than on the (111) surface, the segregation of M impurities is found to be more pronounced on the (111) surface. The results reveal two competitive effects: the effect of M on CO and the effect of CO on M. Thus, on one hand, if M exists on the (100) gold facet, CO would be strongly adsorbed on it. But if M is initially located in the bulk, it would segregate to the (111) facet instead of the (100) in order to bind to CO. (c) 2014 American Institute of Physics.
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    OpenURL UNIV OF NORTH TEXAS
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