Acetate induced enhancement of photocatalytic hydrogen peroxide production from oxalic acid and dioxygen.

The addition of acetate ion to an O2-saturated mixed solution of acetonitrile and water containing oxalic acid as a reductant and 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh(+)-NA) as a photocatalyst dramatically enhanced the turnover number of hydrogen peroxide (H2O2) production. In this photocatalytic H2O2 production, a base is required to facilitate deprotonation of oxalic acid forming oxalate dianion, which acts as an actual electron donor, whereas a Brønsted acid is also necessary to protonate O2(•-) for production of H2O2 by disproportionation. The addition of acetate ion to a reaction solution facilitates both the deprotonation of oxalic acid and the protonation of O2(•-) owing to a pH buffer effect. The quantum yield of the photocatalytic H2O2 production under photoirradiation (λ = 334 nm) of an O2-saturated acetonitrile-water mixed solution containing acetate ion, oxalic acid and QuPh(+)-NA was determined to be as high as 0.34, which is more than double the quantum yield obtained by using oxalate salt as an electron donor without acetate ion (0.14). In addition, the turnover number of QuPh(+)-NA reached more than 340. The reaction mechanism and the effect of solvent composition on the photocatalytic H2O2 production were scrutinized by using nanosecond laser flash photolysis.

Photocatalytic production of hydrogen peroxide by two-electron reduction of dioxygen with carbon-neutral oxalate using a 2-phenyl-4-(1-naphthyl)quinolinium ion as a robust photocatalyst.

Efficient photocatalytic production of hydrogen peroxide (H(2)O(2)) from O(2) and oxalate has been made possible by using a 2-phenyl-4-(1-naphthyl)quinolinium ion as a robust photocatalyst in an oxygen-saturated mixed solution of a buffer and acetonitrile with a high quantum yield of 14% (maximum 50% for the two-electron process) at λ = 334 nm and a high H(2)O(2) yield of 93% at λ > 340 nm.

Photocatalytic hydrogen evolution from carbon-neutral oxalate with 2-phenyl-4-(1-naphthyl)quinolinium ion and metal nanoparticles.

Photocatalytic hydrogen evolution has been made possible by using oxalate as a carbon-neutral electron source, metal nanoparticles as hydrogen-evolution catalysts and the 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh(+)-NA), which forms the long-lived electron-transfer state upon photoexcitation, as a photocatalyst. The hydrogen evolution was conducted in a deaerated mixed solution of an aqueous buffer and acetonitrile (MeCN) [1:1 (v/v)] by photoirradiation (λ > 340 nm). The gas evolved during the photocatalytic reaction contained H(2) and CO(2) in a molar ratio of 1:2, indicating that oxalate acts as a two-electron donor. The hydrogen yield based on the amount of oxalate reached more than 80% under pH conditions higher than 6. Ni and Ru nanoparticles as well as Pt nanoparticles act as efficient hydrogen-evolution catalysts in the photocatalytic hydrogen evolution. The photocatalyst for hydrogen evolution can be used several times without significant deactivation of the catalytic activity. Nanosecond laser flash photolysis measurements have revealed that electron transfer from oxalate to the photogenerated QuPh˙-NA˙(+), which forms a π-dimer radical cation with QuPh(+)-NA [(QuPh˙-NA˙(+))(QuPh(+)-NA)], occurs followed by subsequent electron transfer from QuPh˙-NA to the hydrogen-evolution catalyst in the photocatalytic hydrogen evolution. Oxalate acts as an efficient electron source under a wide range of reaction conditions.

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.