An effective preparation method of composite photocatalysts for hydrogen evolution using an organic photosensitizer and metal particles assembled on alumina-silica

Composite catalysts for photocatalytic hydrogen (H₂) evolution were prepared by loading an organic electron donor–acceptor linked dyad [2-phenyl-4-(1-naphthyl)quinolinium ion, QuPh⁺–NA] as an organic photosensitizer and Pt or Cu particles as H₂-evolution catalysts on alumina-silica. The composite catalysts loading Pt particles were prepared by two different methods; first, Pt particles were deposited by reduction of PtCl₆ ²⁻ owing to photocatalysis of QuPh⁺–NA supported on alumina-silica (PD method), and second, alumina-silica was impregnated with the PtCl₆ ²⁻ and calcined, and then QuPh⁺–NA was loaded on the Pt/alumina-silica by a cation exchange method (IMP method). When a composite catalyst was prepared by the IMP method, a high Pt-loading amount of 4.2 wt% was necessary to achieve the highest H₂-evolution rate of 0.27 μmol h⁻¹. On the other hand, a composite catalyst prepared by the PD method exhibited three times faster H₂ evolution (0.83 μmol h⁻¹) even though the loading amount of Pt was as low as 0.4 wt%. The activity of composite catalysts prepared by the PD method highly depends on the electric charges of precursors for Pt particles. A composite catalyst prepared with positively charged Pt(NH₃)₄ ²⁺ as a precursor of Pt particles exhibited low catalytic activity with the H₂-evolution rate of 0.10 μmol h⁻¹, which is significantly lower than the rate (0.27 μmol h⁻¹) for the composite catalyst prepared with PtCl₆ ²⁻. However, such precursor-dependence was not observed for composite catalysts employing Cu particles as an H₂-evolution catalyst, because the Cu precursors are more labile than the Pt precursors in a reaction solution. The electrostatic interaction between the precursors of metal particles and negatively charged surfaces of alumina-silica should be taken into account to construct efficient H₂-evolution catalysts.