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

We used temperature programmed desorption (TPD) to investigate the influence of molecular O-2 and H2O on the adsorption of CH4 on the PdO(101) surface. We find that a strongly-bound state of molecularly adsorbed O-2 suppresses methane adsorption on PdO(101), whereas methane readily displaces a more weakly-bound form of molecular O-2 during adsorption on PdO(101). Pre-adsorbing O-2 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 O-2 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 O-2 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 similar to 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 sigma-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 O-2 is likely to inhibit methane activation on PdO(101) under conditions relevant to commercial applications of Pd-catalyzed combustion of natural gas. (C) 2013 Elsevier B.V. All rights reserved.