The Role of Subsurface Oxygen in the Silver-Catalyzed, Oxidative Coupling of Methane

摘要

The silver-catalyzed, oxidative coupling of methane to C_2 hydrocarbons (OCM) is shown to be an extremely structure-sensitive reaction. Reaction-induced changes in the silver morphology lead to changes in the nature and extent of formation of various bulk and surface-terminating crystal structures. This, in turn, impacts the adsorption properties and diffusivity of oxygen in silver which is necessary to the formation of subsurface oxygen. A strongly bound, Lewis-basic, oxygen species which is intercalated in the silver crystal structure is formed as a result of these diffusion processes. This species is referred to as O_#gamma# and acts as a catalytically active site for the direct dehydrogenation of a variety of organic reactants. It is found that the activation energy for methane coupling over silver of 138 kJ/mol is nearly identical to the value of 140 kJ/mol for oxygen diffusion in silver measured under similar conditions. This correlation between the diffusion kinetics of bulk-dissolved oxygen and the reaction kinetics of the oxidative coupling of methane to C_2 hydrocarbons suggests that the reaction is limited by the formation of O_#gamma# via surface segregation of bulk dissolved oxygen. Catalysis over fresh silver catalysts indicates an initially preferential oxidation of CH_4 to complete oxidation products. This is a result of the reaction of methane with surface bound atomic oxygen which forms preferentially on high-index terminating crystalline planes. Reaction-induced facetting of the silver results in a restructuring of the catalyst from one which initially catalyzes the complete oxidation of methane to CO_x and water to a catalyst which preferentially catalyzes the formation of coupling products. This represents an extremely dynamic situation in which a solid-state restructuring of the catalyst results in the formation of a Lewis-basic, silver-oxygen species which preferentially catalyzes the dehydrogenation of organic molecules.