(Invited) Reduction of Carbon Dioxide and Activation of Nitrogen at Heme Type Porphyrin-Complexes of Iron Existing in Enzymes

Abstract

A choice of the carbon dioxide reduction reaction has been dictated by the growing problem of continuously rising levels of this greenhouse gas released through burning of fossil fuels, thus becoming a critical environmental issue and concern over climate change. The results obtained indicate the feasibility of HRP to act as the biocatalyst, when deposited on the glassy carbon (an inert electrode substrate), capable of inducing electroduction of CO2. HRP is a metalloenzyme, in which a large alpha-helical protein which binds heme as a redox cofactor. The actual electrocatalytic properties shall be attributed to the existence of heme groups, i.e. complexes of iron ions coordinated to porhyrin units. On mechanistic grounds, the carbon dioxide molecules seems to be chemisorbed or attracted to the heme centers during the activation step. Indeed, only “adsorbates” of CO2 are catalytically reduced. In other words, the HRP-surface-attached CO2 molecules, rather than the bulk reactant, are reduced at the electrocatalytic interface. For comparison, the activity of certain metal-oxide-based systems (Cu/Cu2O, WO3, Fe2O3, and Nb2O5) has also been studied during the CO2-reduction. All of them exhibit electrocatalytic activity but at different potentials. Among them, niobium(V) oxide acts as an active support for HRP, and its use permits good surface distribution of the biocatalyst and utilization of heme catalytic sites. Finally, in addition to the expected activity toward oxygen reduction, the HRP-based system seems to exhibit electrocatalytic activity toward highly inert nitrogen (N2) presumably to its strong interactions with the heme catalytic centers. The reduction reaction of nitrogen is a multielectron and multiproton transfer proces suffering from high over-potentials and limited selectivity The N2 molecule with triple bond requires special means of activation through strong adsorption at the electrocatalytic interface. Obviously most of electrocatalysts will be more active toward hydrogen evolution than ammonia production. Despite some reports available, this area of research is largely unexplored: typically ammonia is produced at trace levels, and the reaction efficiency is very low. On the whole, the presence of the α-helix HRP secondary structure (composed of backbone N−H groups that hydrogen-bonded to the backbone C=O groups of the amino acid network) is likely to contribute to the system’s good stability and selectivity (e.g., with respect to the competing hydrogen evolution reaction).