Catalysis at the Nanoscale

Scientists are learning how to make effective catalysts from nanoparticles.
 

Germanium beads on a zinc oxide nanowire. Image from Zhengwei Pan

In 1897 French chemist Paul Sabatier discovered that passing a mixture of ethylene and hydrogen over a column of heated nickel transformed the ethylene into ethane. Sabatier, who won the Nobel Prize for chemistry in 1912, later used "finely divided" nickel to catalyze other hydrogenations, the process employed today on oils for food production. Some suggest that Sabatier's heterogeneous catalysis was among the earliest predecessors of today's nanotechnology.

In heterogeneous catalysis, chemical reactions in gases or liquids are accelerated by introducing a solid phase that ideally contains large enough amounts of the right kind of site for chemical reactants to adsorb, react, and desorb. Because optimization of the catalyst requires increasing the numbers of sites to expand surface area, the catalytic particle size must be decreased. In contemporary laboratories, active catalysts tend to consist of carefully prepared nanometer-sized particles on supports with nanometer-sized pores or structural features. Modern catalysts typically consist of multiple-component active phases that may include a support tailored to disperse, isolate, or otherwise enhance the structure or properties of individual catalytic particles.

One goal of catalysis research is to understand how decreasing the size of catalytic particles alters the intrinsic catalytic performance beyond simply expanding surface area. A corollary goal is learning how to design and prepare catalysts with the most effective size and structure.

Gallium-catalyzed silicon dioxide nanowire assembly. Image from Zhengwei Pan

Achieving these goals is the aim of research conducted by the Heterogeneous Catalysis and Surface Chemistry Group in ORNL's Chemical Sciences Division and at the Center for Nanophase Materials Sciences.

One surprising catalytic material studied at ORNL is gold. Because gold is chemically inert, the metal does not react with oxygen (i.e., rust) in air. This property makes gold valuable as jewelry. Intriguingly, gold, once thought to have no catalytic activity because of its inertness, is now known to be extremely active in catalyzing certain oxidation reactions, but only if the gold is in the form of particles 2 to 5 nanometers in diameter. Researchers are seeking to determine the reasons for this size constraint on the catalytic activity of gold. Because gold particles tend to grow readily by migrating and merging under high-temperature conditions, a major goal of research is to stabilize the sizes of gold particles by, for example, trapping them in nanosized pores or tailoring the surface properties of the material supporting the gold particles.

Sheng Dai and Wenfu Yan have found ways to adjust surface properties of oxides by introducing single-layer coatings of various compositions, thereby tailoring the interaction between gold particles and the oxide support. Gold particles supported in this way can be highly stable and can exhibit high catalytic activity for oxidation of carbon monoxide, an automotive exhaust pollutant. Properties of the gold particles have been determined through the analysis of X-ray absorption by David Mullins and Viviane Schwartz, who can directly assess both the gold oxidation state and the average gold particle sizes. From analysis by high-resolution Z-contrast electron microscopy, Steve Pennycook and Andy Lupini, both of ORNL's Condensed Matter Sciences Division, have demonstrated that gold particles apparently must not only be very small but should also have a raft-like shape, to exhibit optimum catalytic activity. Computational approaches by Sergey Rashkeev are now revealing how the stability and adsorption energetics of gold particles are related to their morphologies.

At ORNL's nanocenter, four laboratories will be used to study the relationship between nanoscale structure and catalysis. Researchers in two of the labs will use state-of-the-art methods to synthesize candidate catalytic materials, allowing users to try their ideas for preparing and modifying support materials. Multiple techniques will be used to analyze the composition, structure, and particle and pore sizes of nano-structured catalysts. Several catalytic reactors will be available for testing the catalytic performance of candidate catalysts for various chemical reactions.

One can imagine that Sabatier would have felt right at home in Oak Ridge.—Steve Overbury, Chemical Sciences Division

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