Abstract
CO oxidation on O-precovered Pd(111) surfaces exhibits remarkably different reactivities at different temperatures, which correlate with structural changes in the atomic O overlayer. Stoichiometric titration experiments by Nakai et al. (J. Chem. Phys. 2006, 124, 224712) show that although the p(2 × 2) ordered phase is inert, the (√3 × √3) and p(2 × 1) phases that form at 320 and 190 K, respectively, have different apparent activation energies and reaction orders with respect to O coverage. In this work, we perform first-principles-based kinetic Monte Carlo (kMC) simulations to understand the behavior of this catalytic system and shed light on the origin of the changes in reactivity. Accounting explicitly for lateral interactions among adsorbates and for their impact on the activation energies of the elementary processes, our simulations reproduce quantitatively the main features of the experimental measurements, and we show that the relative rates of CO adsorption and surface reaction are different as the temperature changes. We find that ordering of the adsorbate layer strongly depends on the strength of the lateral interactions but does not have a significant role on the catalytic properties of the system.
Highlights
Lateral interactions among adsorbates on metal surfaces are known to play an important role in heterogeneous catalysis.[1]
The accuracy of our approach is such that our simulations reproduce quantitatively the mean features of earlier experiments performed on the same system
We were able to rationalize the change in reaction order with respect to O coverage and the change in apparent activation energy seen in experiments as the temperature is lowered from 320 to 190 K
Summary
Lateral interactions among adsorbates on metal surfaces are known to play an important role in heterogeneous catalysis.[1]. Typical examples include the formation of oxygen p(2 × 2) domains on closed pack surfaces of metals like Ag, Ru, Ni, Pt, and Pd. Lateral interactions influence the adsorption energy of atoms and molecules, usually leading to a decrease of the adsorption energy at increasing coverages,[2] and have a large role in determining the phase diagram of the adsorbate overlayer on a catalytic surface.[3] Affecting the adsorbates’ adsorption energy, these interactions influence the structural properties of an overlayer and modify the rates of elementary reactions, because activation energies correlate with adsorption energies through the Brønsted−Evans−Polanyi (BEP) relationship.[4,5]. At around 320 K, the appearance of the (√3 × √3) phase correlates with an extremely low apparent activation energy of 0.04 ± 0.02 eV The reaction, in this regime, is found to have approximately order 1/2 with respect to oxygen coverage, albeit at coverages below 0.05 ML. Around 190 K, the oxygen domains give rise to ordered p(2 × 1) structures, and the reaction proceeds with an apparent activation energy of 0.29 ± 0.03 eV and exhibits approximately order 1 with respect to oxygen coverage
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