High temperature partial oxidation reactions over silver catalysts Original Research Article

The exceptional catalytic activity of silver for a number of partial oxidation reactions has been known for nearly a century. Despite the widespread use of silver in heterogeneous catalysis, there still remain unresolved questions about the mechanistic details of reaction. The ethylene epoxidation and formaldehyde synthesis reactions are the two industrially-relevant reactions which have received, by far, the most attention. The importance of these reactions cannot be underestimated. Both ethylene epoxide and formaldehyde serve as primary chemicals for a wide variety of materials which find use in an enormous number of products. There is, therefore, a great scientific and economic motivation for understanding these reactions and for unraveling the secret to the exceptional catalytic activity of silver. This contribution in honor of Prof. W. Sachtler’s 75th birthday summarizes recent results obtained in our laboratory on the formaldehyde synthesis reaction and the oxidative coupling of methane as a laboratory test reaction. The article shows that the formation of bulk-dissolved oxygen species is critical to activation of the silver for reaction. Both the chemical composition and the bulk morphology of the catalyst are strong functions of the gas-phase composition and reaction temperature. Similar apparent activation energies determined for oxygen diffusion and oxidative coupling of methane suggest that the rate-limiting step of the oxidative coupling of methane over silver is the diffusion of bulk-dissolved oxygen to silver (1 1 1) surfaces at which oxygen rapidly reacts with methane.