Effect of TiO2 Crystal Phase on the Catalytic Transfer Hydrogenation Using Formic Acid over the Pd/TiO2 Catalyst

The influence of crystal phase of TiO2 on the catalytic performance of transfer hydrogenation using formic acid (HCOOH) over Pd/TiO2 catalyst has not been clarified before. In this study, Pd/TiO2 catalysts with four typical TiO2 crystal phases, that is, rutile, anatase, brookite, and TiO2(B), were synthesized and analyzed. The catalytic results showed that the TiO2 crystal phase could vary the transfer hydrogenation performance of the Pd/TiO2 catalysts by several to tens of times, which was in the following tendency: Pd/rutile > Pd/anatase > Pd/brookite > Pd/TiO2(B). Detailed comparison manifests the Pd dispersion state is obviously different on various TiO2 surfaces, which the relatively small particle size and more metallic are observed on Pd/rutile and Pd/anatase catalysts, contributing to the high catalytic performance. The reaction mechanism study further indicates that HCOOH on Pd/rutile is more liable to be dehydrogenated than that on other Pd/TiO2 catalysts, especially for the transformation of bidentate to monodentate formate. Pd/brookite shows the relatively highest selectivity of HCOOH dehydration. However, it is difficult for the HCOOH dehydrogenation to occur on Pd/TiO2(B). This work elucidates the roles of TiO2 crystal phase in Pd/TiO2 catalysts for the transfer hydrogenation reaction, which is expected to facilitate the development of efficient catalysts.