Size- and Shape-Dependent Activity of Metal Nanoparticles as Hydrogen-Evolution Catalysts: Mechanistic Insights into Photocatalytic Hydrogen Evolution

The catalytic activity of Pt nanoparticles (PtNPs) with different sizes and shapes was investigated in a photocatalytic hydrogen‐evolution system composed of the 9‐mesityl‐10‐methylacridinium ion (Acr+–Mes: photocatalyst) and dihydronicotinamide adenine dinucleotide (NADH: electron donor), based on rates of hydrogen evolution and electron transfer from one‐electron‐reduced species of Acr+–Mes (Acr.–Mes) to PtNPs. Cubic PtNPs with a diameter of (6.3±0.6) nm exhibited the maximum catalytic activity. The observed hydrogen‐evolution rate was virtually the same as the rate of electron transfer from Acr.–Mes to PtNPs. The rate constant of electron transfer (ket) increased linearly with increasing proton concentration. When H+ was replaced by D+, the inverse kinetic isotope effect was observed for the electron‐transfer rate constant (ket(H)/ket(D)=0.47). The linear dependence of ket on proton concentration together with the observed inverse kinetic isotope effect suggests that proton‐coupled electron transfer from Acr.–Mes to PtNPs to form the PtH bond is the rate‐determining step for catalytic hydrogen evolution. When FeNPs were used instead of PtNPs, hydrogen evolution was also observed, although the hydrogen‐evolution efficiency was significantly lower than that of PtNPs because of the much slower electron transfer from Acr.–Mes to FeNPs. Rate of exchange: The catalytic activity of metal nanoparticles (MNPs) of different size and shape is studied based on the rate constant of electron transfer (ket) from one‐electron‐reduced species of the 9‐mesityl‐10‐methylacridinium ion (Acr+–Mes) to MNPs. The linear dependence of ket on proton concentration and the inverse kinetic isotope effect (KIE) suggest that proton‐coupled electron transfer (PCET) from Acr.–Mes to PtNPs is the rate‐determining step for catalytic hydrogen evolution (see picture).