Photocatalytic hydrogen evolution using 9-phenyl-10-methyl-acridinium ion derivatives as efficient electron mediators and Ru-based catalysts

Photocatalytic hydrogen evolution has been performed by photoirradiation (λ>420nm) of a mixed solution of a phthalate buffer and acetonitrile (MeCN) (1:1 (v/v)) containing EDTA disodium salt (EDTA), [Ru^II(bpy) ₃]²⁺ (bpy=2,2′-bipyiridine), 9-phenyl-10- methylacridinium ion (Ph-Acr⁺-Me), and Pt nanoparticles (PtNPs) as a sacrificial electron donor, a photosensitiser, an electron mediator, and a hydrogen-evolution catalyst, respectively. The hydrogen-evolution rate of the reaction system employing Ph-Acr⁺-Me as an electron mediator was more than 10 times higher than that employing a conventional electron mediator of methyl viologen. In this reaction system, ruthenium nanoparticles (RuNPs) also act as a hydrogen-evolution catalyst as well as the PtNPs. The immobilization of the efficient electron mediator on the surface of a hydrogen-evolution catalyst is expected to enhance the hydrogen-evolution rate. The methyl group of Ph-Acr⁺-Me was chemically modified with a carboxy group (Ph-Acr ⁺-CH2COOH) to interact with metal oxide surfaces. In the photocatalytic hydrogen-evolution system using Ph-Acr⁺-CH2COOH and Pt-loaded ruthenium oxide nanoparticles (Pt/RuO₂NPs) as electron donor and hydrogen-evolution catalyst, respectively, the hydrogen-evolution rate was 1.5-2 times faster than the reaction system using Ph-Acr⁺-Me as an electron mediator. On the other hand, no enhancement in the hydrogen-evolution rate was observed in the reaction system using Ph-Acr ⁺-CH2COOH with PtNPs. Thus, the enhancement of hydrogen-evolution rate originated from the favourable interaction between Ph-Acr ⁺-CH2COOH and RuO₂NPs. These results suggest that the use of Ph-Acr⁺-Me as an electron mediator enables the photocatalytic hydrogen evolution using PtNPs and RuNPs as hydrogen-evolution catalysts, and the chemical modification of Ph-Acr⁺-Me with a carboxy group paves the way to utilise a supporting catalyst, Pt loaded on a metal oxide, as a hydrogen-evolution catalyst.