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The structure of bimetallic catalysts based on silver and indium has been studied during the hydrogenation of acrolein by in situ X-ray absorption spectroscopy. A novel in situ cell, capable of performing X-ray absorption experiments in transmission geometry on catalyst powders under reaction conditions (i.e., temperatures up to 500 °C and pressures up to 20 bar, corrosive atmosphere), was developed for this purpose. Ag K- and In K-spectra were recorded during the acrolein hydrogenation. Changes in the indium valence were observed during the reaction by XANES analysis, whereas silver remained redox inert. The evaluation of the EXAFS spectra exhibited a characteristic peak in r-space at ca. 3 A during acrolein hydrogenation, which can be understood by a structure model consisting of AgIn clusters partly covered with adsorbed acrolein on the catalyst surface. It could be shown that the adsorbate-metal-distance is different for Ag/SiO2 and AgIn/SiO2 catalysts, which is most likely due to different adsorption geometries. Structural parameters of the catalyst under reaction conditions were determined by a quantitative EXAFS analysis. |
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