Robustness of Ru/SiO₂ as a Hydrogen-Evolution Catalyst in a Photocatalytic System Using an Organic Photocatalyst

Effects of various metal oxide supports (SiO₂, SiO₂–Al₂O₃, TiO₂, CeO₂, and MgO) on the catalytic reactivity of ruthenium nanoparticles (RuNPs) used as a hydrogen-evolution catalyst have been evaluated in photocatalytic hydrogen evolution using 2-phenyl-4-(1-naphthyl)­quinolinium ion (QuPh⁺–NA) and dihydronicotinamide adenine dinucleotide (NADH) as a photocatalyst and an electron donor, respectively. The 3 wt % Ru/SiO₂ catalyst freshly prepared by an impregnation method exhibited the highest catalytic reactivity among RuNPs supported on various metal oxides, which was nearly the same as that of commercially available Pt nanoparticles (PtNPs) with the same metal weight. However, the initial catalytic reactivity of 3 wt % Ru/SiO₂ was lost after repetitive use, whereas the catalytic reactivity of PtNPs was maintained under the same experimental conditions. The recyclability of the 3 wt % Ru/SiO₂ was significantly improved by employing the CVD method for preparation. The initial catalytic reactivity of 0.97 wt % Ru/SiO₂ prepared by the CVD method was higher than that of 2 wt % Ru/SiO₂ prepared by the impregnation method despite the smaller Ru content. The total amount of evolved hydrogen normalized by the weight of Ru in 0.97 wt % Ru/SiO₂ was 1.7 mol g_Ru^–1, which is now close to that normalized by the weight of Pt in PtNPs (2.0 mol g_Pt^–1). Not only the preparation method but also the morphology of SiO₂ supports affected significantly the catalytic activity of Ru/SiO₂. The Ru/SiO₂ catalyst using nanosized SiO₂ with undefined shape exhibited higher catalytic activity than Ru/SiO₂ catalysts using mesoporous SiO₂ or spherical SiO₂. The kinetic study and TEM observation of the Ru/SiO₂ catalysts suggest that the microenvironment of RuNPs on SiO₂ surfaces plays an important role to exhibit the high catalytic performance in the photocatalytic hydrogen production