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

The silver-catalyzed, oxidative coupling of methane to C2 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 Oγ 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 C2 hydrocarbons suggests that the reaction is limited by the formation of Oγ via surface segregation of bulk dissolved oxygen. Catalysis over fresh silver catalysts indicates an initially preferential oxidation of CH4 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 COx 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.