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

Abstract The influence of crystal phase of TiO 2 on the catalytic performance of transfer hydrogenation using formic acid (HCOOH) over Pd/TiO 2 catalyst has not been clarified before. In this study, Pd/TiO 2 catalysts with four typical TiO 2 crystal phases, that is, rutile, anatase, brookite, and TiO 2 (B), were synthesized and analyzed. The catalytic results showed that the TiO 2 crystal phase could vary the transfer hydrogenation performance of the Pd/TiO 2 catalysts by several to tens of times, which was in the following tendency: Pd/rutile > Pd/anatase > Pd/brookite > Pd/TiO 2 (B). Detailed comparison manifests the Pd dispersion state is obviously different on various TiO 2 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/TiO 2 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/TiO 2 (B). This work elucidates the roles of TiO 2 crystal phase in Pd/TiO 2 catalysts for the transfer hydrogenation reaction, which is expected to facilitate the development of efficient catalysts.