This review focuses on the mechanisms of CO2 and CO methanation, and catalysts' deactivation.

The effects of preparation methods, calcination times, and La doping concentrations on the crystallinity, visible light absorption, and photocatalytic water splitting performance of Rh- and La-codoped SrTiO3 (SrTiO3:La/Rh) were investigated. Applying a two-step solid state reaction in which SrTiO3 acted as a perovskite-type host produced core/shell structured SrTiO3:La/Rh, the surface of which was enriched with the dopants. La doping suppressed the formation of oxygen vacancies and inactive Rh4+ species. Under visible light irradiation (λ > 420 nm), SrTiO3:La/Rh exhibited 3.5 and 3.8 times higher rates of H2 evolution in an aqueous methanol solution and during redox-free Z-scheme overall water splitting in combination with Ir/CoO x /Ta3N5, respectively, compared to SrTiO3:Rh. The solar-to-hydrogen efficiency of the Z-scheme system as measured under illumination with simulated sunlight (AM1.5G) was found to have improved by a factor of 3.

Tantalum nitride (Ta(3)N(5)) is a promising nitride semiconductor photocatalyst for solar water splitting because it has band edge potentials capable of producing hydrogen and oxygen from water under visible light (λ < 590 nm). However, the photocatalytic performance of Ta(3)N(5) has been far below expectations because insufficient crystallization upon thermal nitridation of the oxide precursors enhances undesirable charge recombination limiting the quantum efficiency of the photocatalytic reaction. This problem was successfully rectified in this study by modifying the surface of the starting Ta(2)O(5) with a small amount of alkaline metal (AM) salts. Compared with conventional Ta(3)N(5), Ta(3)N(5) nitrided from AM salt-modified Ta(2)O(5) had better crystallinity and smaller particles with smoother surfaces and, most importantly, demonstrated a 6-fold improvement in photocatalytic activity for O(2) evolution under visible light. AM salt modification was compatible with the loading of an O(2) evolution cocatalyst, such as CoO(x), yielding an apparent quantum efficiency of 5.2% at 500-600 nm. This indicates that the effects of AM modification were attributable to the changes in the crystallinity and the morphology of Ta(3)N(5) rather than to catalytic effects. Detailed characterization of the Na(2)CO(3)-modified Ta(3)N(5) suggested partial dissolution of Ta(2)O(5) and nucleation of NaTaO(3) in the early stages of nitridation, which gave rise to the characteristic particle morphologies and improved the crystallinity of the nitridation products. This study demonstrates that a facile pretreatment of a starting material can improve the physical and photocatalytic properties of photocatalysts drastically, enabling the development of advanced photocatalysts for solar water splitting.

An (oxy)nitride-based heterostructure for powdered Z-scheme overall water splitting is presented. Compared with the single MgTa2O(6-x)N(y) or TaON photocatalyst, a MgTa2O(6-x)N(y)/TaON heterostructure fabricated by a simple one-pot nitridation route was demonstrated to effectively suppress the recombination of carriers by efficient spatial charge separation and decreased defect density. By employing Pt-loaded MgTa2O(6-x)N(y)/TaON as a H2-evolving photocatalyst, a Z-scheme overall water splitting system with an apparent quantum efficiency (AQE) of 6.8% at 420 nm was constructed (PtO(x)-WO3 and IO3(-)/I(-) pairs were used as an O2-evolving photocatalyst and a redox mediator, respectively), the activity of which is circa 7 or 360 times of that using Pt-TaON or Pt-MgTa2O(6-x)N)y) as a H2-evolving photocatalyst, respectively. To the best of our knowledge, this is the highest AQE among the powdered Z-scheme overall water splitting systems ever reported.

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