Photocatalytic hydrogen production on nickel-loaded LaxNa1−xTaO3 prepared by hydrogen peroxide-water based process

Abstract

A green production process for producing nickel-loaded La_xNa_1−xTaO₃, using a hydrogen peroxide-water based solvent system (HW-derived), is reported. The H₂ evolution of the HW-derived sample is about 1.8 times higher than samples made using ethanol as a solvent. The activity of the sample can be further increased 9.3 times by depositing nanosized nickel as a co-catalyst on the surface of the La_0.02Na_0.98TaO₃. Possible mechanisms of H₂ evolution from pure water and from aqueous methanol solutions using nickel in three states (i.e.Ni metal, NiO oxide, and Ni/NiO core/shell)-La_0.02Na0_0.98TaO₃, are discussed systematically for the first time. It is clearly shown that the activity of hydrogen generation from pure water is in sequence: Ni/NiO > NiO > Ni, whereas the activity sequence with respect to aqueous methanol is: Ni > Ni/NiO > NiO. Metallic Ni presents the most active sites and favors the formation of hydrogen from aqueous methanol. The Ni in Ni/NiO core/shell induce the migration of photogenerated electrons from the bulk to catalyst surface, while NiO act as H₂ evolution site and prevent water formation from H₂ and O₂. The recombination is interrupted by the effective capture of the holes by methanol acting as a sacrificial reagent, thereby leading to higher hydrogen evolution. In contrast, the competition between the recombination and the charge-transfer reaction occurs in pure water leading to a possible back reaction between H₂ and O₂ on the photocatalyst's surface. The photocatalyst synthesis avoids the use of organic solvents and thereby contributes to the environmentally friendly production of hydrogen.