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 $(\sqrt{5}\times \sqrt{5})R27°$ 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 $\mathrm{C}\mathrm{O}:{\mathrm{O}}_2$ partial pressure ratios just around the catalytically most relevant stoichiometric feeds $(p_{\mathrm{C}\mathrm{O}}:p_{{\mathrm{O}}_2}=2:1)$. 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.

Figure 1

(Color online) In the upper panel, a schematic top view of the $\sqrt{5}$ surface oxide structure is shown. The large gray spheres represent Pd atoms, the small red (dark) ones O atoms. The Pd atoms of the Pd(100) substrate are darkened. The $\sqrt{5}$ surface unit cell as well as the $(1\times 1)$ surface unit cell of Pd(100) are indicated. As illustrated, the $\sqrt{5}$ surface unit cell consists of two halves that differ only with respect to the underlying Pd(100) substrate. We find the difference in binding energies at the equivalent adsorption sites in the two halves to be very small and, correspondingly, use the reduced unit cell in the kMC simulations. In the lower panel, these prominent adsorption sites and the final kMC lattice are illustrated. The three high-symmetry adsorption sites (bridge, top2f, and top4f) are indicated by the yellow (light) crosses. The hollow adsorption site of the upper O atoms [red (dark) cross] is likewise included as a site in the kMC lattice, whereas the reconstructed Pd layer as well as the lower O atoms are fixed. In the final kMC lattice, only the bridge and hollow sites are considered, as shown on the lower left (see text).

Figure 2

(Color online) Considered nearest neighbor pair interactions between adsorbates on the $\sqrt{5}$ surface oxide lattice. Large gray spheres represent Pd atoms. Small red spheres represent oxygen atoms included in the kMC lattice. Small yellow spheres represent adsorbed O and/or CO.

Figure 3

Schematic illustration of the diffusion barriers between energetically like and unlike sites.

Figure 4

(Color online) Thermodynamic surface ‘‘phase’’ diagram of the Pd(100) surface in a constrained equilibrium with ${\mathrm{O}}_2$ and CO gas phase as described in detail in Ref. 6. The differently shaded areas mark the stability regions of various surface structures for a given chemical potential of the ${\mathrm{O}}_2$ and the CO gas phase. Blue (solid) areas include $\mathrm{O}∕\mathrm{C}\mathrm{O}$ adsorption structures on Pd(100), red and white (hatched) areas comprise surface oxide structures, and the gray (crosshatched) area represents the stability range of bulk PdO. For convenience, the dependence of the chemical potential of the two gas phases is translated into pressure scales for $T=300\mathrm{K}$ and $T=600\mathrm{K}$ (upper $x$ axes and right $y$ axes). For the stability region of the surface oxide structures, additional schematic figures of the different surface configurations are shown. Large gray spheres represent Pd atoms, small red (dark) spheres represent oxygen atoms, and small yellow (light) spheres represent CO molecules. The white arrows indicate the pressure conditions of the kMC simulations for temperatures of $T=300\mathrm{K}$, $T=400\mathrm{K}$, and $T=600\mathrm{K}$. The oxygen pressure is always fixed at $p_{{\mathrm{O}}_2}=1\mathrm{atm}$, while the CO pressure is varied between ${10}^{-5}⩽p_{\mathrm{C}\mathrm{O}}⩽{10}^5\mathrm{atm}$.

Figure 5

(Color online) Average occupation of hollow sites by oxygen vs CO pressure for $T=300\mathrm{K}$, $T=400\mathrm{K}$, and $T=600\mathrm{K}$. The vertical black line marks the boundary between the surface oxide and a CO covered Pd(100) surface as determined within the constrained thermodynamics approach. The influence of the barrier for the ${\mathrm{O}}^{\mathrm{hol}}+\mathrm{C}{\mathrm{O}}^{\mathrm{br}}$ reaction is illustrated by showing the kMC simulation results for two barrier values of 0.9 and $0.8\mathrm{eV}$ (see text).