Abstract
While being highly active for the CO oxidation reaction already at low temperatures, Au/CeO2 catalysts suffer from continuous deactivation with time on stream, with the activity and deactivation depending on the initial catalyst activation procedure. In previous X-ray absorption measurements at the Au LIII edge, which focused on changes in the electronic and geometric changes of Au, we found a modest increase of the Au particle size during reaction, with the Au nanoparticles (NPs) present in a dominantly metallic state during reaction, regardless of the pretreatment. Here we aim at expanding on these insights by examining the changes in electronic and chemical composition of the CeO2 support induced by different pretreatment procedures and during subsequent CO oxidation at 80 °C, by following changes at the Ce LIII near edge region in time-resolved operando X-ray absorption measurements. The results indicate a strong dependence of the initial concentration of Ce3+ ions on the pretreatment, while during subsequent reaction this rapidly approaches a steady-state value which depends on the oxidative/reductive character of the reaction gas mixture, but is largely independent of the pretreatment. These results are discussed and related to earlier finding on the electronic properties of Au nanoparticles under identical reaction conditions.
Highlights
IntroductionFor over 3 decades, the intriguing catalytic properties of supported gold catalysts have inspired a large number of scientists, both from a fundamental and application-oriented points-of-view [1,2,3]
In the inset we show XANES date collected at the Au LIII edge at (11919 eV) after oxidative (O400—blue spectrum) and reductive (H400—red spectrum) pretreatments, which were taken from [30,35], respectively, together with spectra from Au2O3 and a Au foil used as reference materials
The chemical composition and the catalytic activity of the differently pretreated Au/CeO2 catalysts during the CO oxidation reaction were characterized as a function of reaction time by recording the CO2 formation during the X-ray absorption spectroscopy (XAS) experiments
Summary
For over 3 decades, the intriguing catalytic properties of supported gold catalysts have inspired a large number of scientists, both from a fundamental and application-oriented points-of-view [1,2,3]. While there is ample evidence that each of these effects somehow affects the reaction, direct correlations between the temporal development of these features and of the activity of Au/CeO2 catalysts in the CO oxidation reaction, under working conditions, have rarely been established [33,39] This is the topic of the present contribution, where we report results of a time-resolved operando XAS study at the Ce LIII near edge region on the oxidation state of the ceria support during the CO oxidation reaction in different gas mixtures. This study follows a recent report of similar XAS measurements at the Au LIII edge, and additional in situ FTIR measurements, focusing on the electronic, chemical, and structural properties of the Au nanoparticles on an Au/CeO2 catalyst Those measurements indicated that under present reaction conditions the deactivation of the Au/CeO2 catalysts originates from a modest growth of. Set-up and procedures is given in Section 3, which is followed by the conclusions (Section 4)
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