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 coworkers on this system (J. Chem. Phys. 2006, 124, 224712), demonstrated firstorder 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.