Ni-Cu alloy nanoparticles loaded on various metal oxides acting as efficient catalysts for photocatalytic H₂ evolution

Catalysis of Al₂O₃-SiO₂, TiO₂, SiO₂ and CeO₂ (MOx) impregnated with pre-formed Ni-Cu alloy nanoparticles (Ni-CuNPs/MOx) for photocatalytic hydrogen (H₂) evolution was compared with that of MOx impregnated with Ni²⁺ and Cu²⁺ ions followed by calcination and reduction (Ni-Cu/MOx). The photocatalytic H₂ evolution was conducted by photoirradiation (λ > 340 nm) of a deaerated mixed solution of a phthalate buffer (pH 4.5) and acetonitrile [1:1 (v/v)] containing β-dihydronicotinamide adenine dinucleotide (NADH), 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh⁺-NA), and Ni-CuNPs/MOx or Ni-Cu/MOx as an electron donor, a photosensitiser and an H₂-evolution catalyst, respectively. Ni-CuNPs/Al₂O₃-SiO₂ exhibited activity for the photocatalytic H₂ evolution, whereas Ni-Cu/Al₂O₃-SiO₂ showed no activity. Such precursor dependent catalysis can be elucidated by the ion-exchangeable nature and high surface area of Al₂O₃-SiO₂, on which Ni-Cu alloy particles hardly form from metal salts. On the other hand, Ni-Cu/TiO₂ and Ni-Cu/SiO₂ exhibited higher activity than Ni-CuNPs/TiO₂ and Ni-CuNPs/SiO₂, respectively, resulting from formation of smaller Ni-Cu alloy nanoparticles on TiO₂ and SiO₂ by reducing Ni²⁺ and Cu²⁺ on the surfaces. When CeO₂ was used as the support, no catalytic activity was observed for either Ni-CuNPs/CeO₂ or Ni-Cu/CeO₂. Kinetic study for thermal H₂ evolution suggested that Ni-CuNPs were severely deactivated for H₂ evolution by being loaded on CeO₂.