Understanding the role of Fe and Mo sites in a bio-inspired, bimetallic Mxene catalyst for the electrochemical nitrate reduction reaction

Daniel Abbott^a, Yuanzi Xu^a, Denis Kuznetsov^b, Priyank Kumar^c, Christoph Müller^b, Alexey Federov^b, Victor Mougel^a

^aDepartment of Chemistry and Applied Biosciences, ETH Zürich, Zürich, Switzerland

^bDepartment of Mechanical and Process Engineering, ETH Zürich, Zürich, Switzerland

^cDepartment of Mechanical and Process Engineering, School of Chemical Engineering, University of New South Wales Sydney, Sydney, Australia

Materials for Sustainable Development Conference (MATSUS)
Proceedings of MATSUS Fall 2023 Conference (MATSUSFall23)

#N2X - Recent advances on nitrogen activation and conversion

Torremolinos, Spain, 2023 October 16th - 20th

Organizers: Victor Mougel, Nella Vargas-Barbosa and Roland Marschall

Oral, Daniel Abbott, presentation 209
DOI: https://doi.org/10.29363/nanoge.matsus.2023.209
Publication date: 18th July 2023

Over the last decade there has been a growing interest in generating ammonia from nitrate with the aim of reducing eutrophication and acidication of water sources that has been exacerbated by anthropogenic activities.[1] In nature, the reduction of nitrate and nitrite ions is governed by Mo- and Fe-containing enzymes as a part of the nitrogen cycle. The key steps in the reduction of nitrate to nitrite are driven by nitrate reductase (Nars) enzymes involving the parent Mo(VI) oxo active site.[2] Subsequently, reduction of the nitrite to ammonium can be catalyzed by cytochrome c nitrite reductase (NrfA) enzymes, which features an Fe heme active site.[3] Inspired by this process, in the present work we introduce an Fe-substituted, two-dimensional molybdenum carbide of the MXene family (Mo₂CT_x:Fe, where T_x = O, OH, or F surface termination groups) for the electrochemical reduction of nitrate to ammonia.

Recent studies have demonstrated that MXenes efficiently enable various electrocatalytic reactions due to a high degree of control over the structure of their surface sites.[4] In this case, Mo₂CT_x:Fe contains isolated Fe sites in Mo positions of the host MXene (Mo₂CT_x), and as discussed above, the combination of these metals is known to facilitate the nitrate reduction reaction in naturally occurring reductase enzymes. Mo₂CT_x:Fe is shown here to outperform monometallic Mo₂CT_x for the electrochemical reduction of nitrate in both acidic and neutral electrolytes. Close examination of the electronic and local structure via operando X-ray absorption spectroscopy (Mo K-edge) reveals that substitution of Mo with Fe in the Mo₂CT_x lattice facilitates the defunctionalization of surface T_x groups upon reduction. This is shown to generate T_x vacancies, which then bind nitrate ions that subsequently fill the vacancies with O* via oxygen transfer. In accordance with experimental observations, density functional theory calculations provide further evidence that Fe sites promote the formation of T_x vacancies, which are thus identified as active sites and function in NO₃RR in a close analogy to the mechanism of the natural Mo-based nitrate reductase enzymes.

References:

[1] Abascal, E.; Gómez-Coma, L; Ortiz, I.; Ortiz, A. Global diagnosis of nitrate pollution in groundwater and review of removal technologies. Sci.Total Environ. 2022, 810, 152233

[2] Correia, C.; Besson, S.; Brondino, C.D.; González, P.J.; Fauque, G.; Lampreia, J.; Moura, I.; Moura, J.J.G. Biochemical and spectroscopic characterization of the membrane-bound nitrate reductase from Marinobacter hydrocarbonoclasticus 617. J. Biol. Inorg. Chem. 2008, 13, 1321-1333

[3] Einsle, O.; Messerschmidt, A.; Huber, R.; Kroneck, P.M.H.; Neese, F. Mechanism of the Six-Electron Reduction of Nitrite to Ammonia by Cytochrome c Nitrite Reductase. J. Am. Chem. Soc. 2002, 124, 11737-11745

[4] Tsounis, C.; Kumar, P.V.; Masood, H.; Kulkarni, R.P.; Gautam, G.S.; Müller, C.R.; Amal, R.; Kuznetsov, D.A. Advancing MXene Electrocatalysts for Energy Conversion Reactions: Surface, Stoichiometry, and Stability. Angew. Chem. Int. Ed. 2023. 62, 4, 02210828.

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