Chlorine promoters in selective ethylene epoxidation over Ag(111): A comparison with Ag(110)

Abstract

Chlorine promoters are added in the silver-catalyzed oxidation of ethylene (C 2H 4 + 1 2 O 2 → C 2H 4O) in order to improve the selectivity for ethylene epoxide production. We have recently (Campbell, C. T., and Paffett, M. T., Appl. Surf. Sci. 19, 28, 1984; Campbell, C. T., and Koel, B. E., J. Catal. 92, 272, 1985) modeled the role of chlorine promoters in this reaction by depositing chlorine adatoms onto the Ag(110) surface in ultrahigh vacuum (UHV) and monitoring the resultant effects on the medium-pressure (~1 atm) kinetics of the epoxidation reaction. This was accomplished with an apparatus that allowed rapid (<20 s) transfer of the model catalyst between the UHV chamber for surface analysis (XPS, AES, LEED, AES) and a high-pressure microreactor for kinetics. That work led to a detailed picture of the role of chlorine promoters in this reaction. This present study is an extension of that work to include the Ag(111) surface, which behaves much differently than Ag(110) in dissociative oxygen adsorption [(a) Campbell, C. T., Surf. Sci. 157, 43, 1985; (b) Campbell, C. T., submitted for publication but nevertheless displays very similar catalytic behavior for this reaction (Campbell, C. T., J. Catal. 94, 436, 1985). The results indicate that the effects of chlorine addition are very similar on Ag(111) and (110). They can be explained with the same reaction mechanism involving adsorbed oxygen and ethylene. The steady-state atomic oxygen coverage (under catalytic reaction conditions) goes through a large maximum with chlorine coverage on Ag(111), not seen with Ag(110). This is due to the very low reactivity of atomic oxygen on Ag(111) in the presence of chlorine. Evidence is presented that suggests that the standard method of measuring the Ag surface area of supported catalysts via oxygen chemisorption is very unreliable.