Accelerated Semihydrogenation of Alkynes over a Copper/Palladium/Titanium (IV) Oxide Photocatalyst Free from Poison and H2 Gas

Abstract Selective hydrogenation of alkynes to alkenes (semihydrogenation) without the use of a poison and H 2 is challenging because alkenes are easily hydrogenated to alkanes. In this study, a titanium (IV) oxide photocatalyst having Pd core‐Cu shell nanoparticles (Pd@Cu/TiO 2 ) was prepared by using the two‐step photodeposition method, and Pd@Cu/TiO 2 samples having various Cu contents were characterized by electron transmission microscopy, X‐ray photoelectron spectroscopy and UV‐vis spectroscopy. Thus‐prepared Pd@Cu/TiO 2 samples were used for photocatalytic hydrogenation of 4‐octyne in alcohol and the catalytic properties were compared with those of Pd/TiO 2 and Cu/TiO 2 . 4‐Octyne was fully hydrogenated to octane over Pd/TiO 2 at a high rate and 4‐octyne was semihydrogenated to cis ‐4‐octene over Cu/TiO 2 at a low rate. Rapid semihydrogenation of 4‐octyne was achieved over Pd(0.2 mol%)@Cu(1.0 mol%)/TiO 2 , indicating that the Pd core greatly activated the Cu shell that acted as reaction sites. A slight increase in the reaction temperature greatly increased the rate with a suppressed rate of H 2 evolution as the side reaction. Changes in the reaction rates of the main and side reactions are discussed on the basis of results of kinetic studies. Reusability and expandability of Pd@Cu/TiO 2 in semihydrogenation are also discussed.