STAGES IN THE MODIFICATION OF A SILVER SURFACE FOR CATALYSIS OF THE PARTIAL OXIDATION OF ETHYLENE

The interaction of silver foil with oxygen over a wide range of temper atures and O2 pressures has been studied by XPS, TPD, and TPR. It has been shown that depending on the treatment conditions two adsorbed oxy gen states with a different ionicity of the Ag-O bond can be formed, a s well as oxygen dissolved in the silver bulk. ''Ionic'' oxygen [E(b)( O 1s) = 528.4 eV] is formed in the initial step of O2 adsorption, its incorporation into uppermost silver layer proceeding at T > 420 K. Thi s state is responsible for ethylene adsorption, followed by its destru ction and complete oxidation to CO2 and H2O. Dissolution of oxygen ato ms in the silver bulk starts at T > 470 K. No effect of the dissolved oxygen on the electronic and catalytic properties of the ''ionic'' oxy gen has been revealed. Accumulation of ''covalent'' oxygen [E(b) (O 1s = 530.5 eV] occurs at higher temperature and pressure. A possible mec hanism for its formation which includes the creation of specific defec tive sites on silver surface is discussed. Despite a large variation o f pressures and temperatures used for the modification of clean silver foil by O2, all attempts to produce a surface active in ethylene epox idation have failed. (C) 1994 Academic Press, Inc.