Controlling the performance of a silver co-catalyst by a palladium core in TiO2-photocatalyzed alkyne semihydrogenation and H2 production

Abstract

Titanium (IV) oxide (TiO2) having palladium (Pd) core-silver (Ag) shell nanoparticles (Pd@Ag/TiO2) was prepared by using a two-step (Pd first and then Ag) photodeposition method. The core-shell structure of the nanoparticles having various Ag contents (shell thicknesses) and the electron states of Pd and Ag were investigated by transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. The effect of the Pd core and the Ag shell was evaluated by hydrogenation of 4-octyne in alcohol suspensions of a photocatalyst under argon and light irradiation. 4-Octyne was fully hydrogenated to 4-octane over Pd/TiO2, whereas 4-octyne was selectively hydrogenated to cis-4-octene over Pd(0.2)@Ag(0.5)/TiO2. Further increase in the Ag content resulted in a decrease in the conversion of 4-octyne. Pd-free Ag/TiO2 was inactive for hydrogenation of alkyne and induced coupling of active hydrogen species (H2 production). Photocatalytic reactions at various temperatures revealed that the change in selectivity (semihydrogenation or H2 production) can be explained by the difference in values of activation energy of the two reactions. An applicability test showed that the Pd@Ag/TiO2 photocatalyst can be used for hydrogenation of various alkynes to alkenes.