Photocatalytic hydrogen evolution under highly basic conditions by using Ru nanoparticles and 2-phenyl-4-(1-naphthyl)quinolinium ion

Photocatalytic hydrogen evolution with a ruthenium metal catalyst under basic conditions (pH 10) has been made possible for the first time by using 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh ⁺-NA), dihydronicotinamide adenine dinucleotide (NADH), and Ru nanoparticles (RuNPs) as the photocatalyst, electron donor, and hydrogen-evolution catalyst, respectively. The catalytic reactivity of RuNPs was virtually the same as that of commercially available PtNPs. Nanosecond laser flash photolysis measurements were performed to examine the photodynamics of QuPh ⁺-NA in the presence of NADH. Upon photoexcitation of QuPh ⁺-NA, the electron-transfer state of QuPh ⁺-NA (QuPh ^•-NA ^•+) is produced, followed by formation of the π-dimer radical cation with QuPh ⁺-NA, [(QuPh ^•-NA ^•+) (QuPh ⁺-NA)]. Electron transfer from NADH to the π-dimer radical cation leads to the production of 2 equiv of QuPh ^•-NA via deprotonation of NADH ^•+ and subsequent electron transfer from NAD ^• to QuPh ⁺-NA. Electron transfer from the photogenerated QuPh ^•-NA to RuNPs results in hydrogen evolution even under basic conditions. The rate of electron transfer from QuPh ^•-NA to RuNPs is much higher than the rate of hydrogen evolution. The effect of the size of the RuNPs on the catalytic reactivity for hydrogen evolution was also examined by using size-controlled RuNPs. RuNPs with a size of 4.1 nm exhibited the highest hydrogen-evolution rate normalized by the weight of RuNPs.