Catalytic mechanisms of hydrogen evolution with homogeneous and heterogeneous catalysts

Abstract

This perspective focuses on reaction mechanisms of hydrogen (H2) evolution with homogeneous and heterogeneous catalysts. First, photocatalytic H2 evolution systems with homogeneous catalysts are discussed from the viewpoint of how to increase the efficiency of the two-electron process for the H2 evolution via photoinduced electron-transfer reactions of metal complexes. Two molecules of the one-electron reduced species of [RhIII(Cp*)(bpy)(H2O)](SO4) (bpy = 2,2′-bipyridine) and [IrIII(Cp*)(H2O)(bpm)RuII(bpy)2](SO4)2 (bpm = 2,2′-bipyrimidine) produced by photoinduced electron-transfer reactions are converted to the two-electron reduced complexes suitable for H2 generation by disproportionation. The photocatalytic mechanism of H2 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 H2 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 H2 evolution systems from H2storage 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 [IrIII(Cp*)(H2O)(bpm)RuII(bpy)2](SO4)2.