The Reaction of Molecular Oxygen with Silver at Technical Catalytic Conditions: Bulk Structural Consequences of a Gas-Solid Interface Reaction

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Abstract
Powder X-ray diffraction (XRD), microscopic examinations and temperature-programmed desorption (TDS) of molecular oxygen from polycrystalline silver foils and granules were used to study its interaction with oxygen in a pressure range between 0.01 mbar and 300 mbar and in a temperature interval between 525 K and 1000 K. Massive re-structuring in (110) oriented crystallites and the transformation of textured foils into single-crystalline-like ordered materials were found in ambient air oxidation. High resolution XRD lineprofile analysis led to the detection of a symmetry lowering of the initially fee cubic silver lattice to an orthorhombic distorted form (a = 409.2 pm, b = 289.6 pm, c = 288.6 pm) with a slight deviation of the atomic co-ordinates of the silver creating channels perpendicular to the (110) direction through which oxygen may diffuse into the bulk. At 300 mbar oxygen partial pressure the re-structuring transforms into a preferential orientation in the (331) direction with an even better ordering of the crystallites than observed in air oxidation. The TDS results confirmed the existence of three atomic oxygen species located at the surface, (T-des 575 K) in the interface (T-des 900 K) and as bulk-dissolved species (T-des 700 K). Pre-saturation of the samples allowed to obtain consistent isothermal TDS data sets from which the unperturbed desorption features of all three species were extracted. The role of the bulk-dissolved species in controlling the intermixing of the three species is discussed. The performance of silver in the partial oxidation of methanol is directly related to the individual desorption features of the three oxygen species. It was confirmed that the strongly held interface species plays an important role in the overall conversion performance of the catalyst. The bulk-dissolved species provides a significant abundance of the surface atomic species also required in catalysis which would be fully desorbed at reaction temperature, if only the gas-interface adsorption channel would exist for its population.