Promoted with Calcium, Strontium, or Barium

Catalysts Promoted with Calcium, Strontium, or Barium. The addition of these elements has been studied by two groups of workers. Spath and co-workers have added relatively large amounts of promoter, while Carberry and co-workers have studied catalysts containing trace amounts of calcium or strontium. When added as the peroxide, which is converted to the carbonate under reaction conditions, Spath et al. have shown that barium has two effects.Apart from stabilizing the particulate structure of the unsupported catalyst, a thin layer of barium carbonate is formed on the surface of the silver particles. A similar layer is also formed by the addition of calcium or strontium compounds which are convertible to carbonate. When the elements are added directly as the carbonate this layer is not formed. 

The layer is thought to be doped with interstitial silver atoms which act as electron donors and endow the layer with n-type semiconducting properties. The work function of such a catalyst falls progressively, up to 7 mol % addition of barium. The authors have argued that this effect will favour the adsorption of the more strongly polarized oxygen species, i.e. atomic rather than molecular oxygen, which results in the observed loss of selectivity and increase in activity. 

Carberry et al. have discovered that the presence of traces of surface calcium 

Spath and K. Torkar,/. Catalysis, 1972, 26, 163 H. T. Spath, Proceedings of the 5th International Congress on Catalysis , Miami Beach, 1972,North-Holland, Amsterdam, 1973, Vol. 2, p. 945. 

Figar and Haidinger have reported that the propene oxidation reaction may be modified by the use of hot electrons, whereby excited electrons are produced in a thin layer of gold. In the absence of hot electrons at 588 K, some acrolein was produced as well as traces of formic acid and propene oxide, but the major product was carbon dioxide. When hot electrons are introduced, propene oxide and formic acid become the major products, with a propene oxide selectivity of over 60%. A selectivity to propene oxide of up to 50%, at low conversions of 

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