CO oxidation on Pd(100) vs. PdO(sqrt5xsqrt5)R27^o: First-Principles Kinetic Phase Diagrams and Bistability Conditions

TL;DR

This study uses first-principles kinetic Monte Carlo simulations to explore CO oxidation on Pd surfaces, revealing bistability between metallic and oxidized states under certain conditions, which explains experimental oscillations.

Contribution

It provides the first detailed kinetic phase diagrams showing bistability between Pd(100) and PdO surfaces during CO oxidation, challenging the notion of a single active phase.

Findings

Both Pd(100) and PdO surfaces can be metastable under certain conditions.

Bistability correlates with experimentally observed oscillations.

Surface reactivity is similar for both surface states.

Abstract

We present first-principles kinetic Monte Carlo (1p-kMC) simulations addressing the CO oxidation reaction at Pd(100) for gas-phase conditions ranging from ultra-high vacuum (UHV) to ambient pressures and elevated temperatures. For the latter technologically relevant regime there is a long-standing debate regarding the nature of the active surface. The pristine metallic surface, an ultra-thin (sqrt5xsqrt5)R27^o PdO(101) surface oxide, and thicker oxide layers have each been suggested as the active state. We investigate these hypotheses with 1p-kMC simulations focusing on either the Pd(100) surface or the PdO(101) surface oxide and intriguingly obtain a range of (T, p)-conditions where both terminations appear metastable. The predicted bistability regime nicely ties in with oscillatory behavior reported experimentally by Hendriksen and coworkers [Catal. Today 105, 234 (2005)]. Within this regime we find that both surface terminations exhibit very similar intrinsic reactivity, which puts doubts on attempts to assign the catalytic function to just one active state.