THE REACTION OF MOLECULAR-OXYGEN WITH SILVER AT TECHNICAL CATALYTIC CONDITIONS - BULK STRUCTURAL CONSEQUENCES OF A GAS-SOLID INTERFACE REACTION

Powder X-ray diffraction (XRD), microscopic examinations and temperatu re-programmed desorption (TDS) of molecular oxygen from polycrystallin e silver foils and granules were used to study its interaction with ox ygen in a pressure range between 0.01 mbar and 300 mbar and in a tempe rature interval between 525 K and 1000 K. Massive re-structuring in (1 10) oriented crystallites and the transformation of textured foils int o single-crystalline-like ordered materials were found in ambient air oxidation. High resolution XRD lineprofile analysis led to the detecti on 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 o xygen may diffuse into the bulk. At 300 mbar oxygen partial pressure t he re-structuring transforms into a preferential orientation in the (3 31) direction with an even better ordering of the crystallites than ob served in air oxidation. The TDS results confirmed the existence of th ree 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 th ree species were extracted. The role of the bulk-dissolved species in controlling the intermixing of the three species is discussed. The per formance of silver in the partial oxidation of methanol is directly re lated to the individual desorption features of the three oxygen specie s. 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 sur face atomic species also required in catalysis which would be fully de sorbed at reaction temperature, if only the gas-interface adsorption c hannel would exist for its population.