Spectroscopic Identification of Adsorbed Intermediates Derived from the CO+H(2)O Reaction on Zeolite-Encapsulated Gold Catalysts.
Identification of reaction intermediates in the water-gas shift reaction (WGSR: H(2)O+CO-->H(2)+CO(2)) on Au(n+) (1</=n<3) incorporated into NaY, Na-mordenite, and Na-ZSM-5 zeolites has been studied by means of in situ FT-IR spectroscopy. Exposure of Au(n+)/zeolites to a gas mixture consisting of CO+H(2)O at 323 K produced IR carbonyl spectra characterized by Au(+)-CO at 2192 cm(-1) and Au(0)-CO at 2128 cm(-1). On Au(n+)/NaY, a unidentate formate species (1710, 1620, and 1340 cm(-1)) is produced as a surface intermediate, which was very readily removed upon evacuation at 323 K. The reduction of the catalyst by H(2) at 423 K prior to the admission of the reacting gas mixture caused inhibition of formate species formation. This result suggests that Au(+) is the dominant species on which the reaction took place. Au(+)/Na-mordenite displayed unidentate formate and organic carbonate species (1936 and 1850 cm(-1)). The latter bands showed a significant increase in intensity with time at the expense of the adsorbed CO(2) bands at 2384 and 2338 cm(-1). On the other hand, Au(+)/Na-ZSM-5 resulted in the formation of different carbonate-like species (1936, 1850, 1730, and 1400 cm(-1)) as well as the rapid appearance of deformation vibrations of adsorbed water molecules at 1630 cm(-1) that built up very quickly compared to that in the Au(+)/Na-mordenite sample. These results were consistent with the catalytic activity data that showed the highest formation rates of CO(2) on Au(+)/NaY compared to those on Au(+)/Na-mordenite and Au(+)/Na-ZSM-5 catalysts. Copyright 2000 Academic Press.
Mohamed
MM
Faculty of Science, Chemistry Department, Benha University, Benha, Egypt
Salama
TM
Ichikawa
M