(120g) CO Oxidation on O-Rich Pd(111): How Does Adlayer Structure Affect Reactivity?

The CO oxidation reaction is crucial in the function of catalytic converters found in personal vehicles. Despite its simplicity, several aspects of this reaction are still under investigation. Thus, a recent experimental study by Nakai et al. [J. Chem. Phys. 124, 224712 (2006)] proposed that CO oxidation on O-precovered Pd(111) exhibits markedly different rates at different temperatures as a result of different phases in the O overlayer exhibiting disparate reactivities. Thus, the p(2x2) phase appears as inert, whereas the (√3x√3)R30° and p(2x1) are reactive and exhibit distinct apparent activation energies and reaction orders with respect to O coverage. It still unclear however, whether there is a causal relation between the changes in the overlayer structure and the different reactivity, and if this is the case what is the mechanism giving rise to this effect.

To shed light into this question we perform first principles-based kinetic Monte Carlo (KMC) simulations using Zacros, our software package implementing the Graph-Theoretical KMC approach. These simulations incorporate a detailed cluster expansion Hamiltonian and Brønsted-Evans-Polanyi relations, in order to explicitly account for lateral interactions and their impact on the activation energies of elementary processes. Our simulations reproduce quantitatively the main features of the experimental measurements, and in particular the temperature dependence of CO adsorption and surface reaction rates. The ordering of the adsorbate layer is found to strongly depend on the strength of the lateral interactions but does not seem to have a significant role on the catalytic properties of the system. By analysing the appearance and realisation statistics of the elementary CO oxidation processes, we show that only a small subpopulation of events with rather low activation energies contribute to the observed rate, in agreement with previous observations on a different system, namely the NO oxidation reaction on Pt(111) [Wu et al. J. Catal. 286, 88 (2012)].