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)(H ₂O)](SO ₄) (bpy = 2,2′-bipyridine) and [Ir ^III(Cp*)(H ₂O)(bpm) Ru ^II(bpy) ₂](SO ₄) 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 Co ₃O ₄ particles for ammonia borane hydrolysis and a large tunneling effect observed in decomposition of formic acid with [Ir ^III(Cp*)(H ₂O)(bpm)Ru ^II(bpy) ₂](SO ₄) ₂.