Pt/TiH2 Catalyst for Ionic Hydrogenation via Stored Hydrides in the Presence of Gaseous H2

A Pt/TiH2 catalyst having hydrogen storage and release capability was investigated for selective hydrogenation of trans-cinnamaldehyde (CAL) to cinnamyl alcohol (COL) with gaseous dihydrogen. The catalytic behavior of this catalyst was significantly different from that of a reference Pt/TiO2 catalyst with respect to the product selectivity and the hydrogenation mechanism. The Pt/TiH2 catalyst showed a COL selectivity of 97% at a CAL conversion of 98%, which was ascribed to the function of a Pt crystallite–support boundary layer that caused the preferential adsorption of CAL with its carbonyl group. Furthermore, the carbonyl group was hydrogenated by hydride species (H+, H–) supplied from the support and the hydride species consumed were compensated from gaseous dihydrogen; hydrogen atoms were formed by ordinary homolytic cleavage on Pt and then these hydrogen atoms moved onto the surface of TiH2 and diffused into the bulk of the support, during which those simultaneously changed to hydride species (H+, H–) via electron transfer with titanium species and hydride vacancies therein. The surface and bulk diffusion of the hydrogen atoms from Pt to TiH2 support should be the dominant step rather than their addition to the carbonyl group of CAL (ordinary hydrogenation). That is, ionic hydrogenation occurs with Pt/TiH2 in the presence of gaseous dihydrogen.