Photocatalytic hydrogen evolution with Ni nanoparticles by using 2-phenyl-4-(1-naphthyl)quinolinium ion as a photocatalyst

Photocatalytic hydrogen evolution with 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh(+)-NA) as a photocatalyst and dihydronicotinamide adenine dinucleotide (NADH) as a sacrificial electron donor has been made possible for the first time by using nickel nanoparticles (NiNPs) as a non-precious metal catalyst. The hydrogen evolution rate with the most active Ni nanoparticles (hexagonal close-packed (hcp) structure, 6.6 nm) examined here was 40% of that with commercially available Pt nanoparticles (2 nm) using the same catalyst weight. The catalytic activity of NiNPs depends not only on their sizes but also on their crystal phases. The hydrogen-evolution rate normalized by the catalyst weight increased as the size of NiNPs becomes smaller, with regard to the crystal phase, the hydrogen-evolution rate of the NiNPs with hcp structure is more than 4 times higher than the rate of the NiNPs with face-centred cubic (fcc) structure of similar size. NiNPs act as the hydrogen-evolution catalyst under the pH conditions between 4.5 and 8.0, although the hydrogen-evolution rate at pH > 7.0 was much lower as compared with the hydrogen-evolution rate at pH 4.5. A kinetic study revealed that the rate of electron transfer from photogenerated QuPh(center dot)-NA to NiNPs was much higher than the rate of hydrogen evolution, indicating that the rate-determining step may be proton reduction or desorption of hydrogen.