Comparative study of the bond energy of oxygen at the surface of supported silver catalysts and of the activity of these catalysis for ethylene epoxidation

A heat-flow microcalorimeter equipped with a pulsed flow reactor has been used to study the reaction of oxygen, at 473 K, with a series of silica-supported silver catalysts. Combustion of impurities with the production of carbon dioxide and water was detected and allowed to correct the calorimetric data. At 473 K, adsorption of oxygen at the surface of silver is a fast process; incorporation of oxygen into deeper metal layers, though present, is a slow process. The differential heats of interaction decrease with increasing amounts of consumed oxygen but always exceed the heat of formation of bulk silver oxides. The average heat of formation of an oxygen monolayer varies from sample to sample and is correlated with the intrinsic activity of the catalysts for ethylene oxidation: surface oxygen species are apparently more catalytically active when they are less energetically bonded at the silver surface.