Characterization by X-ray absorption spectroscopy of bimetallic Re–Pd/TiO2 catalysts efficient for selective aqueous-phase hydrogenation of succinic acid to 1,4-butanediol

Re–Pd catalysts supported on TiO2 (P25 or DT51) prepared by successive impregnation (SI) or by Re deposition on Pd/TiO2 by catalytic reduction (CR) were characterized by XANES and EXAFS at the Pd and Re edges. These samples were shown to be efficient for the selective hydrogenation in aqueous solution of succinic acid (SUC) to 1,4-butanediol (BDO) (T = 160 °C, P(H2) = 150 bar). This study clearly highlights the need for in situ re-activation of Re-based systems before their characterization or use, since Re species are fully re-oxidized (Re7+) by contact with air. The XANES quantitative analysis at the Re LIII-edge reveals that after in situ reduction, the Re average reduction state is in most cases inferior or equal to +3, indicating the presence of a mixture of Re3+ and Re0 species in all reduced catalysts. EXAFS study indicates that the Re species are mainly located at the Pd-support interface for the CR sample, whereas a part of Re entities is also deposited as isolated forms on the support in the case of the SI catalyst. The Re–Pd/DT51-SI system leading to the best BDO yield displays a specific arrangement of the Pd and Re species (Re deposit on Pd sites of high coordination). In this sample, the Re0 species, present in none negligible proportion, would contribute similarly as Pd metal to the efficient transformation of SUC towards GBL, which is further converted to BDO on sites involving Pd0 and oxidized Re3+ species nearby.