Two-Component Catalysts for Low-Temperature CO Oxidation: A Monte Carlo Study

The reaction of CO and O 2 at low temperature over composite, noble-metal/reducible-oxide catalysts is simulated using Monte Carlo techniques. High activity for CO oxidation can be obtained aver a composite material composed of a highly interspersed mixture of one type of site that adsorbs CO and O 2 and another type of site that adsorbs O 2 without significant CO inhibition. For example, the rate over Pd under 1% of an atmosphere of CO at room temperature is predicted to increase 10 orders-of-magnitude with addition of 1% of surface sites which adsorb O 2 but not CO. For most reaction rules and parameter values, a roughly 50-50 mixture of the two types of sites gives the greatest activity per unit total surface area. This result is determined by the reaction stoichiometry and the fact that the two reactants primarily adsorb separately on the two different types of sites. In a randomly distributed mixture, the two types of sites have widely differing activities which depend on the local site configurations. The local site configurations of the most active sites in a random surface are similar to site configurations found in a search for optimal configurations. The site configurations found in the search for optimal configurations were about 20% more active than the random surfaces of the same overall composition. This relatively small increase may be due to the simple steric requirements of CO and O 2 adsorption. We expect that similar searches for optimal site configurations will be more fruitful for reactions with more complex steric requirements.