Figure 1

Surface phase diagram for the Pd(100) surface in “constrained thermodynamic equilibrium” with an environment consisting of ${\mathrm{O}}_2$ and CO. The atomic structures underlying the various stable (co-)adsorption phases on Pd(100) and the surface oxide, as well as a thick bulklike oxide film (indicated by the bulk unit-cell), are also shown ($\mathrm{Pd}=\mathrm{\text{large}}$ spheres, $\mathrm{O}=\mathrm{\text{small}}$ spheres, $\mathrm{C}=\mathrm{\text{white}}$ spheres). Phases involving the bulk oxide are crosshatched, phases involving the surface oxide are hatched. The dependence on the chemical potentials of ${\mathrm{O}}_2$ and CO in the gas phase is translated into pressure scales at 300 and 600 K. The thick black line marks gas phase conditions representative of technological CO oxidation catalysis, i.e., partial pressures of 1 atm and temperatures between 300–600 K. The three vertical gray (green) lines correspond to the gas phase conditions employed in the kinetic Monte Carlo simulations shown in Fig. 2 (see text).Reuse & Permissions

Figure 2

Kinetic Monte Carlo (kMC) results for the stability of the $\sqrt{5}$ surface oxide. Shown is the average occupation of the hollow sites with oxygen in the $\sqrt{5}$ lattice structure, ${\overline{\Theta }}_{{\mathrm{O}}^{\mathrm{hol}}}$. The chosen gas phase conditions ($T=300$, 400, 600 K, $p_{{\mathrm{O}}_2}=1\mathrm{\text{atm}}$, variable CO pressure) correspond to the three vertical lines drawn in Fig. 1. In the intact $\sqrt{5}$ surface oxide structure ${\overline{\Theta }}_{{\mathrm{O}}^{\mathrm{hol}}}=100\%$, and the reduction obtained in the kMC simulations occurs at CO partial pressures quite close to those corresponding to the stability boundary as obtained in the constrained thermodynamic approach (transition from the hatched to the plain areas in Fig. 1), marked here by the corresponding vertical lines. Using the onset of reduction, we thus arrive at critical pressures for the structural decomposition of approximately $5\times {10}^{-2}$, ${10}^{-1}$, and 10 atm at 300, 400, and 600 K, respectively.Reuse & Permissions