  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.
