(Invited) Electroreduction of Nitrogen to Ammonia at Iron Catalytic Sites Generated at Interfaces Utilizing Iron Phosphides and Heme-Type Complexes
Authors: Kulesza Pawel J. , Rytelewska Beata , Rutkowska Iwona A. , Sobkowicz Karolina , Chmielnicka Anna , Chouki Takwa , Emin Saim 
Published: November 2022

The feasibility of performing nitrogen electroreduction reaction, or nitrogen fixation, particularly in aqueous solutions, constitutes an attractive prospect to produce ammonia under ambient, or near ambient, conditions. Development of durable, specific and reasonably efficient low-cost catalysts remains a great challenge for electrochemical science and technology. Currently, most of electrochemical approaches to N2-fixation suffers from slow kinetics due to the difficulty of achieving the appropriate adsorption and activation of dinitrogen leading to cleavage of the strong, triple N≡N bond. This symposium will feature presentations on research dealing with the fundamental and applied aspects of nitrogen reduction reactions of relevance various aspects of science and existing technologies.

Transition metal phosphides are promising non-noble earth-abundant catalysts with broad electrocatalytic properties, in particular when it comes to electroreduction of such inert reactants as carbon dioxide. Herein, we report the successful electrocatalytic reduction of nitrogen (N2) using different phases of iron phosphide activated at 450 °C. For example, the FeP and Fe2P phases have been found to act as efficient catalysts for the formation of NH3 in alkaline and semineutral media. Detection of in-situ formed product has been achieved by probing the electrooxidation of NH3 to nitrogen (N2) using the additional working electrode modified with Pt nanoparticles. On mechanistic grounds, the iron (Fe0) sites seem to be electrocatalytic active during the reduction of nitrogen

The iron sites can also be generated within the porphyrin rings and related coordination structures. Their high electrocatalytic activity was historically demonstrated for the CO2-reduction. The results obtained here indicate the feasibility of horseradish peroxidase (HRP) to act as the biocatalyst, when deposited on the glassy carbon (an inert electrode substrate), capable of inducing electroduction of not only CO2 but also N2. 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 porphyrin units. On mechanistic grounds, the nitrogen molecules seems to be chemisorbed or attracted to the heme centers during the activation step. Indeed, only “adsorbates” of N2 are catalytically reduced. In other words, the HRP-surface-attached N2 molecules, rather than the bulk reactant, are reduced at the electrocatalytic interface. The reduction reaction of nitrogen is a multielectron and multiproton transfer process 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. Consequently, 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).