Catalytic mechanisms of hydrogen evolution with homogeneous and heterogeneous catalysts

This perspective focuses on reaction mechanisms of hydrogen (H-2) evolution with homogeneous and heterogeneous catalysts. First, photocatalytic H-2 evolution systems with homogeneous catalysts are discussed from the viewpoint of how to increase the efficiency of the two-electron process for the H-2 evolution via photoinduced electron-transfer reactions of metal complexes. Two molecules of the one-electron reduced species of [Rh-III(Cp*)(bpy)(H2O)](SO4) (bpy 2,2'-bipyridine) and [Ir-III(Cp*)(H2O) (bpm) Ru-II(bpy)(2)](SO4)(2) (bpm 2,2'-bipyrimidine) produced by photoinduced electron-transfer reactions are converted to the two-electron reduced complexes suitable for H-2 generation by disproportionation. The photocatalytic mechanism of H-2 evolution using Pt nanoparticles as a catalyst is also discussed based on the kinetic analysis of the electron-transfer rates from a photogenerated electron donor to Pt nanoparticles, which are comparable to the overall H-2 evolution rates. The electron-transfer rates become faster with increasing proton concentrations with an inverse kinetic isotope effect, when H+ is replaced by D+. The size and shape effects of Pt nanoparticles on the rates of hydrogen evolution and the electron-transfer reaction are examined to optimize the catalytic efficiency. Finally, catalytic H-2 evolution systems from H-2 storage molecules are described including shape dependent catalytic activity of Co3O4 particles for ammonia borane hydrolysis and a large tunneling effect observed in decomposition of formic acid with [(IrCp)-Cp-III(*)(H2O)(bpm)Ru-II(bpy)(2)](SO4)(2).