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 [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 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 [Ir^III(Cp*)(H₂O)(bpm)Ru^II(bpy)₂](SO₄)₂.