The mechanism of the action of alkaline earth compounds as additives to silver catalysts in the direct oxidation of ethylene

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The mechanism of the action of SrO 2, CaO 2, SrCO 3, barium formate and barium oxalate used as additives to silver catalysts (2–20 mole %) in the oxidation of ethylene to ethylene oxide and CO 2 + H 2O has been studied. Two methods of preparation were employed, i.e., mixing of the components (AgO + additive) in alcohol (type I) and acetone (type II), respectively, and the characteristic differences are discussed. In a stream of ethylene/air the additives in presence of silver transform into their respective carbonates of increased surface area, the actual catalyst matrix consisting of the carbonates highly doped with Ag atoms and covering with a thin layer the network structure of metallic silver. The processes occurring during the conditioning period are favored if the components are vibrated in alcohol (type I) instead of acetone (type II). The resulting catalysts have largely increased activity but lower selectivity than have properly defined standard pure silver catalysts. The two principal mechanisms, i.e., a primary effect with additives stable to decomposition (SrCO 3), and a secondary effect with additives that transform into their carbonates in a stream of ethylene/air at 300 °C have been well established. The data of different catalyst series have been correlated by a normalization method discussed in the Appendix.