Simulations of the reaction kinetics on nanometer supported catalyst particles

Real catalysts frequently consist of nm-sized (1–100 nm) metal particles deposited on the internal surface of a more or less inactive porous support. The complexity of such systems in combination with relatively high (atmospheric) reactant pressures, typical for practical catalysis, hinders the application of surface science methods to their full potential and makes it difficult to interpret the measured reaction kinetics on the basis of kinetic data for single-crystal samples. To bridge these pressure and structure gaps and to form a conceptual basis for the understanding of reactions occurring on supported catalyst, we have performed simulations scrutinizing qualitatively new effects in the reaction kinetics on the nm scale. The attention is focused on the particle size range, where the catalyst particles have reached sufficient size to essentially have the electronic properties of bulk crystals, but where they are still small enough that the kinetic effects connected with the particle size are important. We treat in detail such factors inherent for nm chemistry as reactant supply via the support, interplay of the reaction kinetics on different facets of supported particles due to interfacet diffusion, adsorbate-induced reshaping of catalyst particles, and oscillation and chaos on the nm scale. All these factors are demonstrated to be especially important in the case of rapid catalytic reactions occurring far from the adsorption–desorption equilibrium. The kinetics of chemically induced Ostwald ripening of nm catalyst particles and also the gas-phase diffusion limitations in reactions on model supported nm catalysts, prepared by employing electron beam lithography, are discussed as well. As a general conclusion, we find that kinetic effects alone (i.e., no special kinetic effects or “active sites”) can cause large differences in reaction kinetics on nm particles and single crystals. Practical consequences and experimental opportunities to measure these effects are briefly discussed.