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

Abstract

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 gRu–1, which is now close to that normalized by the weight of Pt in PtNPs (2.0 mol gPt–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.