Sustainable photocatalytic cascaded reaction for H2 evolution over Co decorated ultrathin Cd0.5Zn0.5S nanosheets: Surface kinetics in aqueous NaOH and methanol

In photocatalysis, researchers mainly focus on the physical processes of light absorption and charge recombination, however, the surface chemical reaction process has been largely overlooked. Herein, a cascaded reaction strategy was employed to strengthen the surface reaction of water splitting over Co decorated ultrathin Cd0.5Zn0.5S nanosheets (Co/Cd0.5Zn0.5S), in which OH− from NaOH can be oxidized to OH by the holes of Cd0.5Zn0.5S, then it can be consumed by OH scavengers. Methanol (MeOH) was employed as a model scavenger to investigate surface redox kinetics of the system. The investigations show that OH− can reduce the apparent activation energy of MeOH photo-reforming by changing its oxidation pathway from a slow direct adsorption-oxidation path to a quick OH-mediated indirect path. The apparent MeOH decomposition kinetic via the direct path fits Langmuir-Hinshelwood model with a rate constant of 0.117 ± 0.007 mmol·h−1 at 30 °C. Whereas it is converted to zero-order reaction with rate constant of 0.734 ± 0.027 mmol·h−1 at 30 °C in the presence of OH−, where MeOH decomposition efficiency largely depends on the OH− adsorption and diffusion processes. The greatly enhanced surface charge extraction by OH− endows Co/Cd0.5Zn0.5S excellent anti-photocorrosion ability and highest activity of 178.4 ± 5.9 mmol‧g−1‧h−1, making a sustainable photocatalytic H2 evolution process. This work sheds light on rational design of reaction to achieve efficient photocatalytic H2 evolution.