Rationalizing the Relation between Adlayer Structure and Observed Kinetics in Catalysis

Relating the kinetic behavior of catalytic reactions with adsorbate overlayer structure is a long-standing challenge in catalysis. Even for simple systems such as CO oxidation on Pd(111), recent studies have observed rich behavior. In particular, titration experiments by Kondoh and co-workers on this system (J. Chem. Phys. 2006, 124, 224712), demonstrated first order reaction kinetics with respect to oxygen coverage at 190 K. Yet, the observed kinetics appeared as half-order at 320 K, a phenomenon that was attributed to island formation. We apply a theoretical approach to rationalize these observations, using first-principles kinetic Monte Carlo simulations. We employ an analysis that decomposes the overall order into two contributions: one pertaining to the adlayer structure and one quantifying coverage effects on the elementary events' rates. It is shown that at both temperatures, the layer appears as almost well-mixed, whereas coverage effects due to adsorbate lateral interactions result in the distinct reaction orders.