Synergistic effects of Ni and Cu supported on TiO₂ and SiO₂ on photocatalytic H₂ evolution with an electron donor-acceptor linked molecule

Synergistic effects of Ni and Cu supported on metal oxides on their catalytic activity for hydrogen evolution were observed in photocatalytic hydrogen evolution with 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh⁺-NA) and β-dihydronicotinamide adenine dinucleotide (NADH) as a photocatalyst and an electron donor, respectively. Among the catalysts of Ni and Cu supported on TiO₂, SiO₂, SiO₂-Al₂O₃ and CeO₂, Ni and Cu supported on TiO₂ and SiO₂ exhibited high catalytic activity at a wide range of Ni contents [Ni/(Ni + Cu)] from 30% to 70%, while Ni or Cu solely supported on TiO₂ and SiO₂ showed insignificant catalytic activity. The catalytic activity of Ni and Cu supported on TiO₂ and SiO₂ depends on the preparation methods of the catalysts. The catalysts prepared by a co-impregnation method, in which a precursor solution containing both Ni(NO₃)₂ and Cu(NO₃)₂ was used for the impregnation, showed high catalytic activity, whilst catalysts prepared by a sequential impregnation method, in which Ni(NO₃)₂ and Cu(NO₃)₂ were loaded and calcined successively, exhibited low catalytic activity. TEM observations with energy-dispersive X-ray spectroscopy (EDS) elemental mapping of these catalysts revealed that Ni and Cu were closely located on the support surfaces of a catalyst prepared by the co-impregnation method, whereas Ni and Cu were separated in the catalyst prepared by the sequential impregnation method. These results suggest that the close location of Ni and Cu is necessary to exhibit the high catalytic activity. Such a synergistic effect among base metals and metal oxide supports would be a key to develop active catalysts for hydrogen evolution without using platinum group metals.