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In honor of Prof. Dr. Wolfgang Sachtler´s 75 th birthday. AbstractThe exceptional catalytic activity of silver for a number of partial oxidation reactions has been known for nearly a century. Despite the wid espread 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 economi c motivation for understanding these reactions and for unraveling the secret to the exceptional catalytic a ctivity of silver. This contribution in honor of Prof. W. Sachtler´s 75 th 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 apparant 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 (111) surfaces at which oxygen rapidly reacts with methane.

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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 O γ 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 CH 4 to complete oxidation products. This is a result of the reaction of methane with surface bound atomic oxygen whic h 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 co mplete 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.

of the catalyst. The bulk-dissolved species provides a significant abundance of the surface atomic species also required in catalysis which would be fully desorbed at reaction temperature, if only the gas-interface adsorption channel would exist for its population.Die Reaktion von polykristallinem Silbermetall mit molekularem Sauerstoff wurde im Druckbereich 0.01 mbar bis 300 mbar und bei Temperaturen von 525 bis 1000 untersucht. Dazu wurden Röntgenbeugung an Pulvern (XRD), thermische Desorption (TDS) und mikroskopische Methoden eingesetzt. Proben waren Bieche und elektrolytisch hergestelltes Silbergranulat.Bei Reaktionen in synthetischer Luft wurden erhebliche strukturelle Modifikationen festgestellt, die zum einen eine Texturierung der Proben bis zur Qualitat von Einkristallen in die (110) Richtung umfaBte. Genaue Strukturbestimmungen fiihren zur Detektion einer Symmetrieerniedrigung der Atomanordnung von kubisch nach orthorhombisch (a = 409.2 pm, b = 289.6 pm, c = 288.6 pm), die in (110) Richtung eine kanalartige Verzerrung schafft, welche die Volumendiffusion von Sauerstoff erleichtert. Bei Erhöhung des Partialdruckes wandelt sich die Texturierung in die (311) Richtung um. Die TDS Resultate bestátigen literaturbekannte Daten und weisen drei, in ihrer Konzentration voneinander abhangige Formen von atomarem Sauerstoff in Silber mit Desorptionstemperaturen von 575 , 700 und 900 nach. Durch Vorsàttigung des Volumens mit Sauerstoff wurde die Auflösung der TDS-Daten soweit verbessert, dafi die drei Desorptionprozesse einzeln isoliert werden konnten. Die Analyse dieser Daten zeigte, daB alle drei Formen benötigt werden, um die Wirkung von Silber als Selektivkatalysator in der Partialoxidation von Methanol zu Formaldehyd zu erklàren. Die Bedeutung der drei Formen von Sauerstoff für die Epoxidation von Ethylen wird diskutiert.

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