P-051

Beata Rytelewska

b.rytelewska@uw.edu.pl

Anna Chmielnicka, Saim Emin, Dominik Eder, Shaghayegh Naghdi, Pawel J. Kulesza, Iwona A. Rutkowska

Faculty of Chemistry, University of Warsaw, Poland

Structure and activity of iron-phosphide-based electrocatalytic systems for reduction of nitrogen to ammonia

The feasibility of pursuing  nitrogen electroreduction reaction, or nitrogen fixation, particularly in aqueous solutions, constitutes an attractive prospect to produce ammonia under ambient conditions. 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. 

Electrocatalytic systems based on iron phosphides (Fe2P, Fe3P) have exhibited electrocatalytic activities toward N2-reduction reaction in alkaline (NaOH) and semi-neutral (phosphate buffers) media. The results are consistent with the view that, in alkaline medium, the interfacial reduced-iron (Fe0) centers within Fe2P, while existing within the network of P sites, induce activation and reduction of nitrogen, parallel to the water splitting (reduction) to hydrogen. The catalytic system exhibits certain tolerance with respect to the competitive hydrogen evolution; namely,  during electrolysis at -0.4 V vs. RHE, the molar efficiency toward production of NH3 is on the level of 60%. Under such conditions, the NH3-yield rate has been found to be equal to 7.5 µmol cm-2 h-1. By referring to classic concepts of electrochemical kinetic analysis, the rate constant in heterogeneous units has been found to be on the moderate level of 1-2*10-4 cm s-1 (at -0.4 V). Furthermore, by supporting iron phosphides (e.g. Fe3P) onto hexagonal tungsten nanowires (capable of controlling hydrogen evolution), electroreduction of nitrogen has been feasible in the semi-acid medium (pH=4.6). Finally special attention has been paid to the determination and verification of the identity of the nitrogen-electroreduction products. In this respect, in addition to spectrophotometric and NMR methods, electroanalytical approaches have been explored. A series of diagnostic electrocatalytic experiments have been proposed and performed to control purity of nitrogen gas and to probe presence of potential contaminants such as ammonia, nitrogen oxo-species and oxygen.  

Acknowledgements

This work was financially supported by the National Science Center (NCN, Poland) under Opus Lap Project 2020/39/I/ST5/03385.