Transient Kinetic Analysis (TKA)
Quick Specs
- Fast gas switching and data acquisition rate
- Broad temperature range of 200 K to 770 K
- Contains entire catalyst sample in the common reactor volume
EMSL recently constructed a transient kinetic analysis (TKA) apparatus (in collaboration with the University of Washington) to elucidate reaction mechanisms and identify the rate-determining steps in heterogeneous catalysis for processes related to reactions of the hydrogen economy. Systems of interest include, but are not limited to, reforming of small oxygenates, PROX, and water-gas shift chemistries. These investigations are made possible using an apparatus that can perform a transient interrogation of catalyst-adsorbed species by rapid-scan transmission Fourier-transform ion resonance (FTIR) in conjunction with a mass spectrometric reactor effluent analysis. This is accomplished by pressing a small portion of the supported catalyst into an optically thin (0.1mm path length) tungsten grid, thus having it be largely infrared transparent, with additional catalyst co-located in the reactor present as millimeter-sized beads. Our reactor system is unique because of its fast gas switching and data acquisition rate (~3 Hz), broad temperature range (200-770 K), and that it contains the entire catalyst in the common reactor volume seen by the FTIR. The infrared diagnostics are performed using a Bruker IFS/66S instrument equipped with rapid-scanning capabilities. The mass spectral analyses are accomplished using an Extrel Labgas instrument with a capillary feed input. This enables a more powerful version of the well-known steady-state isotopic kinetic analysis (SSITKA) technique in which the vibrational spectra of the adsorbed species and gas phase are also probed: transmission FTIR-MS-TKA. An array of eight mass flow controllers (MFC's) are used to direct gases to switching valves and the reactor.
Figure 1 shows a photo of the layout of the apparatus. The FTIR is on the far right and the reactor, wrapped with fiberglass insulation, is shown as the white instrument just to the left of the FTIR. Transfer optics direct collimated IR light to the reactor (lower right in photo) and the IR light exiting the reactor is focused onto an MCT detector (behind the reactor). The MFC's are in the back of the photo and the 1/16" tubing employed is used to direct gases to the switching valves, two before reactor and one after the reactor. The mass spectrometer has subsequently been installed after this photo was taken and is located on the left side by the data acquisition computer.
The gas handling equipment is shown Figure 2 below, and comprises valves, gas filters, MFC's, 4-port 2-position VICI Metronics switching valves, pressure gauges, and back pressure regulators (BPR's). The BPR's ensure a constant line pressure while gases flow though the system (1.2<P<2.7 atm). The third switching valve (SV3) after the reactor enables the mass spectral quantitation of "sticky" gases to be obtained, such as water. This is accomplished by pulsing SV3 from signal to an inert gas, such as He, to obtain an effective zero signal. The signal difference has been shown to be proportional to the sticky gas concentration [1].
Reactor design was critical to the successful operation of the apparatus. A key design feature is to minimum reactor volume while maximizing cross-sectional area for transmission IR analyses. The minimum reactor volume required arises as it is known the theoretical effective reactor response time is equal to reactor volume divided by gas volumetric flow rate; for the 0.5 cm3 reactor volume and 40 sccm flow, the theoretical response time is ~0.6 second. The reactor volume is cylindrically shaped (on end relative to optics table). In addition, it is important to minimize the IR optical pathlength (4.0 mm chosen) so that gas phase IR absorption does not dominate compared to catalyst-adsorbed species. The reactor volume diameter is 1.2 cm. As mentioned briefly above, the reactor volume was constructed out of block tungsten and via cartridge heating enabled heating to ~770 K. The overall tungsten block size is ~5" x 3.5" x 3.0" and the IR optics are sealed with readily demountable/resealable GTY Grafoil gaskets (EGC Inc.). The tungsten grid with pressed catalyst is located in the center of the reactor and held in place by a wave-washer/flat-washer combination. The Grafoil gaskets make a very good seal for positive pressure experiments so that little if any ambient atmosphere contamination is observed. Multiple 1mm catalyst beads can be placed in the reactor volume upstream of the grid. This enables a larger conversion to product to occur making mass spectral analysis, and the study of re-uptake phenomena, to be made. The IR optics used are typically calcium fluoride, however we have also employed CVD diamond that offers the advantage of enhanced transmission at <1000 cm-1.
A single PC is used to control the acquisition of FTIR, mass spectrometer, and thermocouple data. In addition, there is a computer interface to control the gas switching valves, although manual operation of these is often used instead. The heating of the reactor and input lines is performed using custom Watlow PID-current regulator units fed to cartridge or heating tapes, respectively.
[1] C.O. Bennett, Catalysis Under Transient Conditions, Division of Coll. & Surf. Chem., 2nd Chemical Congress, American Chem. Soc. Publ., A.T. Bell, L.L. Hegedus (eds.), v.178, p.1-32, 1982.
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