Inhibition of methane adsorption on PdO(101) by water and molecular oxygen

We used temperature programmed desorption (TPD) to investigate the influence of molecular O2 and H2O on the adsorption of CH4 on the PdO(101) surface. We find that a strongly-bound state of molecularly adsorbed O2 suppresses methane adsorption on PdO(101), whereas methane readily displaces a more weakly-bound form of molecular O2 during adsorption on PdO(101). Pre-adsorbing O2 in the strongly-bound state limits the subsequent adsorption of methane to only 10% of the saturation coverage in a pure CH4 layer on PdO(101) obtained at 85 K. In contrast, adsorbing O2 onto a CH4-saturated layer lowers the methane coverage to 49% of its saturation value, demonstrating that oxygen-induced displacement of methane from PdO(101) is only moderately effective at 85 K. Water adsorbed on the coordinatively unsaturated (cus) Pd sites is more effective than O2 at both hindering methane adsorption and displacing adsorbed methane from PdO(101). We find that chemisorbed water limits the adsorption of methane on PdO(101) to a coverage as low as ~ 3% of the saturation coverage of a pure CH4 layer on the PdO(101) surface. We also show that pre-adsorbed water on the cus-Pd sites completely suppresses the formation and dissociation of n-butane σ-complexes on PdO(101). These results demonstrate that methane preferentially binds on the cus-Pd sites of PdO(101) at 85 K, and suggest that occupation of the cus-Pd sites by H2O or O2 is likely to inhibit methane activation on PdO(101) under conditions relevant to commercial applications of Pd-catalyzed combustion of natural gas.