Insights into the Thermo-Photo Catalytic Production of Hydrogen from Water on a Low-Cost NiOx-Loaded TiO2 Catalyst

Thermo-photo catalytic water splitting, where the introduction of thermal energy increases the oxidation driving force for narrow-band-gap photocatalysts (with a low valence band potential), exhibited significantly advanced performance for hydrogen production as compared to general water splitting at room temperature. Herein, a low-cost NiOx-loaded TiO2 catalyst was reported for thermo-photo catalytic water splitting with methanol as the sacrificial agent. The catalyst with an optimal Ni ratio of 5 wt % achieved a hydrogen evolution rate of 53.7 mmol/h/g under simulated AM 1.5G sunlight at 260 °C, which was 2.5 times more than that without illumination, with apparent quantum efficiencies of 66.24%, 33.55%, 32.52%, and 15.35% at 380, 420, 450, and 500 nm, respectively. More impressively, under the irradiation of visible light (λ > 420 nm) at this temperature, the photohydrogen yield could still reach 26.9 mmol/h/g, which was 5 orders of magnitude greater than that (0.0011 mmol/h/g) conducted at room temperature. Isotope tracer experiments demonstrated that the introduction of photoenergy promoted the hydrogen production mainly by enhancing hydrogen evolution from water splitting rather than methanol decomposition or reformation. Furthermore, the stepwise reaction mechanism was revealed with insights into the synergistic roles of thermo-energy and photoenergy for production of hydrogen from water. Those findings highlight the great promise of thermo-photo catalysis and should inspire more efforts for water splitting.