Active Au Species During the Low-Temperature Water Gas Shift Reaction on Au/CeO₂: A Time-Resolved Operando XAS and DRIFTS Study

Abstract

Operando XAS measurements in the near (XANES) and the extended (EXAFS) Au LIII edge as well as in situ diffuse reflectance FTIR (DRIFTS) spectroscopy were employed in combination with kinetic measurements in a further attempt to identify the nature of the active Au species responsible for the high activity of Au/CeO2 catalyst in the low-temperature water gas shift (LT-WGS) reaction. The changes in the reaction behavior during the LT-WGS were followed at 180 °C for different initial states of the catalyst, prepared by either reducing or oxidizing pretreatments at different temperatures. Findings from kinetic and deactivation measurements were correlated with experimental data on the Au particle size, the Au oxidation state, and the CO-Au adsorption properties directly after different pretreatments and during the subsequent LT-WGS reaction obtained by operando/in situ spectroscopy measurements. The combined experimental results show that the use of different pretreatments can significantly influence the electronic state of the Au species (Auδ-, Au0, Auδ+). Exposure to the reaction atmosphere under the present reaction conditions, however, results in the rapid formation of extremely small, (sub)nanometer-sized Au0 nanoparticles, which are the dominant Au species and responsible for the high WGS activity. Small amounts of oxidic gold species (Au3+) persisting during reaction after the strongly oxidative O400 pretreatment, in the few percent range, are too little to be responsible for the catalytic activity of that catalyst and changes therein with time on stream.