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The photoelectric work function Ψ of silver catalysts modified by addition of 2–20 mole% of Ba and Sr peroxide, formate and carbonate is markedly reduced, the steepest decrease in Ψ being at small additive concentration. At 6–8 mole% addition Ψ approaches a stationary value. Changes in Ψ are more pronounced with peroxides and type I (alcohol) preparation (ΔΨ ∼- −0.6 eV) as compared to addition of the inert carbonate (ΔΨ ∼- −0.3 eV). An intermediate situation is met with formates and type II (acetone) preparation. On conditioning the catalyst components in a stream of ethylene/air at 300 °C, the peroxides and formates transform into their respective carbonate. The carbonate covers the underlying silver with a thin film which, by incorporation of silver atoms acting as donors, is endowed with semiconducting properties. The interpretation of the work function vs mole percent additive characteristics is based upon the model of a metal/semiconductor film contact. Calculations of the electrostatic potential across the film and at its catalytically active outer surface (in the absence or presence of an acceptor gas) satisfactorily reflect the experimental findings.

The mechanism of the action of BaO2 and BaCO3 used as additives to silver catalysts (2–36 wt %) in the oxidation of ethylene to ethylene oxide and CO2 has been studied. As starting material for the silver component AgO was used. Two different effects exerted by these additives have been distinguished. On the one hand, an effect termed the “primary effect”, as exerted by the almost inert BaCO3 additive, consists in stabilizing the catalyst matrix, which is made up of a loosely bound network of interlaced silver rods and lumps. The resulting catalysts had a somewhat increased activity and good selectivity. On the other hand, BaO2, being not stable under reaction conditions, completely transforms into BaCO3 of highly increased surface area. The carbonate is covering the silver matrix with a semiconducting film containing Ag atoms as donors and is considered to be the actual catalyst. This effect was termed the “secondary effect”. The resulting catalysts have largely increased activity with a maximum around 8 wt % BaO2, but a lower selectivity compared to a proper standard silver catalyst. Besides catalytic studies carried out in a reactor where both thermal effects and longitudinal diffusion were absent, the catalysts have been investigated by X-ray analysis, thermogravimetry, BET studies, and scanning electron microscopy. The results, as obtained in this study, can be generalized, to some extent, to additives other than BaCO3 and BaO2.

The mechanism of the action of SrO2, CaO2, SrCO3, barium formate and barium oxalate used as additives to silver catalysts (2–20 mole %) in the oxidation of ethylene to ethylene oxide and CO2 + H2O 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 (SrCO3), 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.