Biochemical and artificial pathways for the reduction of carbon dioxide, nitrite and the competing proton reduction: effect of 2nd sphere interactions in catalysis

Reduction of oxides and oxoanions of carbon and nitrogen are of great contemporary importance as they are crucial for a sustainable environment. Substantial research has been dedicated to these areas in the last few decades. These reductions require both electrons and protons and their thermodynamic potentials often make them compete with hydrogen evolution reaction i.e., the reaction of protons and electrons to generate H2. These reactions are abundant in the environment in microorganisms and are facilitated by naturally occurring enzymes. This review brings together the state-of-the-art knowledge in the area of enzymatic reduction of CO2, NO2− and H+ with those of artificial molecular electrocatalysis. A simple ligand field theory-based design principle for electrocatalysts is first described. The electronic structure considerations developed automatically yield the basic geometry required and the 2nd sphere interactions which can potentially aid the activation and the further reduction of these small molecules. A systematic review of the enzymatic reaction followed by those reported in artificial molecular electrocatalysts is presented for the reduction of CO2, NO2− and H+. The review is focused on mechanism of action of these metalloenzymes and artificial electrocatalysts and discusses general principles that guide the rates and product selectivity of these reactions. The importance of the 2nd sphere interactions in both enzymatic and artificial molecular catalysis is discussed in detail.