Spectroscopic Identification of Adsorbed Intermediates Derived from the CO+H2O Reaction on Zeolite-Encapsulated Gold Catalysts

Abstract

Identification of reaction intermediates in the water–gas shift reaction (WGSR: H2O+CO→H2+CO2) on Aun+ (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 Aun+/zeolites to a gas mixture consisting of CO+H2O at 323 K produced IR carbonyl spectra characterized by Au+–CO at 2192 cm−1 and Au0–CO at 2128 cm−1. On Aun+/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 H2 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 CO2 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 CO2 on Au+/NaY compared to those on Au+/Na–mordenite and Au+/Na–ZSM-5 catalysts.