In-situ X-ray absorption spectroscopy of supported Ru catalysts in the aqueous phase

The size of supported Ru metal particles on various supports was monitored by in situ X-ray absorption spectroscopy under aqueous phase conditions typical of biomass conversion reactions. In particular, Ru/gamma-Al2O3, Ru/C, Ru/TiO2, and Ru/SiO2 were evaluated at the Ru K edge at 473 K in neutral water and 0.4 M NaOH. The as-prepared samples exposed to air contained oxidized Ru that was subsequently reduced by H-2-saturated water solution. Significant growth of the metal particles was observed on the Ru/gamma-Al2O3 and Ru/SiO2 samples during the aqueous treatments, whereas the Ru/TiO2 and Ru/C samples were quite stable under the conditions used here. Results from post-treatment X-ray diffraction and surface area analysis revealed major structural changes of the alumina and silica supports. The structural stability of the carbon and titania account for the lack of metal particle growth on those supports. Because a reference Pt/C catalyst revealed metal particle growth under the same conditions, the results for the Ru/C cannot be generalized to other carbon-supported catalysts and underscores the importance of in situ characterization for heterogeneous catalysts in the aqueous phase.