CO oxidation on Pd(100) at technologically relevant pressure conditions: First-principles kinetic Monte Carlo study

The possible significance of oxide formation for the catalytic activity of transition metals in heterogeneous oxidation catalysis has evoked a lively discussion over the recent years. On the more noble transition metals (such as Pd, Pt, or Ag), the low stability of the common bulk oxides primarily suggests subnanometer thin oxide films, so-called surface oxides, as potential candidates that may be stabilized under gas phase conditions representative of technological oxidation catalysis. We address this issue for the Pd(100) model catalyst surface with first-principles kinetic Monte Carlo simulations that assess the stability of the well-characterized (root 5 x root 5)R27 degrees surface oxide during steady-state CO oxidation. Our results show that at ambient pressure conditions, the surface oxide is stabilized at the surface up to CO:O-2 partial pressure ratios just around the catalytically most relevant stoichiometric feeds (p(CO):p(O2)=2:1). The precise value sensitively depends on temperature, so that both local pressure and temperature fluctuations may induce a continuous formation and decomposition of oxidic phases during steady-state operation under ambient stoichiometric conditions.