Photocatalytic hydrogen production using Me/Cd_0.3Zn_0.7S (Me = Au, Pt, Pd) catalysts: Transformation of the metallic catalyst under the action of the reaction medium

The activity and stability of Me/Cd_0.3Zn_0.7S (Me = Au, Pt, Pd) photocatalysts in the course of hydrogen production from water under the action of visible radiation have been investigated. The mechanism of activation and deactivation of the catalysts have been elucidated for the first time using X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. An increase in the hydrogen evolution rate is observed for all of the catalysts at the early stages of testing. The highest hydrogen evolution rate, 5.4 μmol/min, is afforded by the 1%Pt/Cd_0.3Zn_0.7S catalyst. The activity of the Au/Cd_0.3Zn_0.7S and Pt/Cd_0.3Zn_0.7S catalysts becomes constant 7.5–9 h after the beginning of the photocatalytic test, while in the case of Pd/Cd_0.3Zn_0.7S the hydrogen evolution rate increases over the initial 6 h and then decreases. These specific features of the catalysts likely correlate with the initial state of the metals on the support surface. In particular, supported palladium is in the form of PdO, while gold and platinum are in the metallic state. The Au/Cd_0.3Zn_0.7S and Pt/Cd_0.3Zn_0.7S photocatalysts are activated due to metal encapsulation; the 1%Pd/Cd_0.3Zn_0.7S catalyst, due to the partial reduction of PdO to PdO_x. The 1%Pd/Cd_0.3Zn_0.7S catalyst is deactivated because of the aggregation of nanoparticles of the cadmium sulfide–zinc sulfide solid solution.