Rationalising the Relation Between Adlayer Structure and Observed Kinetics in CO Oxidation on Pd(111)

Relating the kinetic behaviour 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 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 experimental observations. We thus perform first-principles kinetic Monte Carlo simulations of the titration experiment, and employ an analysis scheme that decomposes the overall/apparent order into two contributions: one pertaining to the adlayer structure and another one quantifying coverage effects on the elementary events’ rates. It is shown that at both temperatures the layer appears as almost well-mixed contrary to the island formation hypothesis, whereas coverage effects due to adsorbate lateral interactions result in the distinct reaction orders. We further calculate spatial correlation functions that provide insight into the average environment around the reactive CO-O pairs, as well as the enviroments that induce the highest contributions to the overall reaction rate.