(326k) Electrokinetics and Operational Stability of the Homogeneous Molecular Catalyst Co(DIM) for Electrochemical Nitrate Reduction

Haber-Bosch ammonia (NH₃) production enables large-scale food production, but demands high energy inputs, yields direct atmospheric carbon emissions, and outpaces nitrogen removal from wastewaters, which threatens global water quality.^1–3 The electrochemical nitrate reduction reaction (NO₃RR) can couple water treatment with ammonia production from wastewaters, helping close the loop on nitrogen commodity production. A central challenge to NO₃RR is selective conversion to ammonia, which requires eight electron transfers and nine proton transfers. Homogeneous molecular catalysts are uniquely posed to promote NO₃RR over the competing hydrogen evolution reaction and NH₃ formation over nitrogenous byproducts (e.g., NO₂^-, N₂) because of the atomically precise reaction environment created by catalyst active sites. Homogeneous molecular catalysts have been historically understudied in the context of wastewater treatment due to the need of separating catalyst, product, and treated wastewater. Recent developments in electrochemical reactive separations – the co-location of selective reactions with selective separations – can help facilitate the integration of homogeneous molecular catalysts into water treatment processes. Framed in this use-informed context, in this talk we focus on fundamental investigations of the cobalt–centered macrocycle [Co(DIM)Br₂]⁺ (DIM = 2,3-dimethyl-1,4,8,11-tetraazacyclotetradeca-1,3-diene: abbreviated as Co(DIM)) for its aqueous solubility and ability to yield ammonia from NO₃RR with near perfect selectivity (> 95%).^4–6

A dearth of electroanalytical investigations has prevented the rational design and implementation of NO₃RR molecular platforms such as Co(DIM). At the micron scale, Co(DIM) molecules catalyze NO₃RR adjacent to the cathode surface within a reaction-diffusion layer (RDL, Figure 1). We electrochemically control the RDL through cyclic voltammetry (CV) to achieve a steady state between diffusive transport of Co(DIM) and catalytic reaction. We fit data obtained under this regime to a reaction-diffusion model derived from first principles to rigorously benchmark the kinetics of Co(DIM)-mediated NO₃RR. Two key findings emerge from the kinetic analysis. First, we find that Co(DIM)-mediated NO₃RR is pH-independent between pH values of ~ 3.7-11.0, implicating the rate-limiting step to ammonia conversion does not involve proton transfer. The pH-independent kinetics subvert typical trends observed on heterogeneous NO₃RR catalysts, in which reaction activity changes substantially with pH.^7–10 Rather, we identify metal to substrate charge transfer (MSCT) as the rate-limiting step to ammonia conversion and provide guidance on rational improvements that can be made to enhance the kinetics of MSCT. Second, we solve for the Tafel-like turnover frequency (TOF) vs. overpotential (η) relationship for Co(DIM)-mediated NO₃RR to obtain provide approximations for TOF₀, a measure of intrinsic kinetics, and TOF_max, the TOF achievable by Co(DIM) in the absence of interfacial nitrate depletion and catalyst degradation.¹¹ These findings contextualize the performance of Co(DIM) relative to state-of-the-art molecular electrocatalysts and facilitate optimization goals for next-generation NO₃RR molecular catalysts.

Salient to any homogeneous molecular catalyst, we also study the operational stability of Co(DIM) under various reaction durations and electrolyte pH values. Catalyst degradation is observed in alkaline environments, as demonstrated via characterization of the electrode (XPS, SEM-EDX, and XRD) and electrolyte (UV-VIS, IC). Through in situ CVs and ex situ rinse tests (CPE involving electrodes with surface immobilized degradation products and no Co(DIM) in solution), we demonstrate that the degradation products likely play a small catalytic role compared to Co(DIM), highlighting the importance of maintaining the structural integrity of Co(DIM) during CPE operation. In total, this use-informed work takes an innovative approach to water treatment via fundamental electrochemical investigations of the homogeneous molecular catalyst Co(DIM). We address several critical knowledge gaps surrounding Co(DIM)-mediated NO₃RR that will guide its use in reactive separation processes to treat wastewaters, helping to meet the scale and urgency of electrochemically remediating nitrogen emissions and producing ammonia.

References

(1) Smith, C.; Hill, A. K.; Torrente-Murciano, L. Current and Future Role of Haber–Bosch Ammonia in a Carbon-Free Energy Landscape. Energy Environ. Sci. 2020, 13 (2), 331–344. https://doi.org/10.1039/C9EE02873K.

(2) MacFarlane, D. R.; Cherepanov, P. V.; Choi, J.; Suryanto, B. H. R.; Hodgetts, R. Y.; Bakker, J. M.; Ferrero Vallana, F. M.; Simonov, A. N. A Roadmap to the Ammonia Economy. Joule 2020, 4 (6), 1186–1205. https://doi.org/10.1016/j.joule.2020.04.004.

(3) Kyriakou, V.; Garagounis, I.; Vourros, A.; Vasileiou, E.; Stoukides, M. An Electrochemical Haber-Bosch Process. Joule 2020, 4 (1), 142–158. https://doi.org/10.1016/j.joule.2019.10.006.

(4) Jackels, S. C.; Farmery, Keith.; Barefield, E. Kent.; Rose, N. J.; Busch, D. H. Tetragonal Cobalt(III) Complexes Containing Tetradentate Macrocyclic Amine Ligands with Different Degrees of Unsaturation. Inorg. Chem. 1972, 11 (12), 2893–2901. https://doi.org/10.1021/ic50118a008.

(5) Xiang, Y.; Zhou, D.-L.; Rusling, J. F. Electrochemical Conversion of Nitrate to Ammonia in Water Using Cobalt-DIM as Catalyst. Journal of Electroanalytical Chemistry 1997, 424 (1), 1–3.

(6) Xu, S.; Ashley, D. C.; Kwon, H.-Y.; Ware, G. R.; Chen, C.-H.; Losovyj, Y.; Gao, X.; Jakubikova, E.; Smith, J. M. A Flexible, Redox-Active Macrocycle Enables the Electrocatalytic Reduction of Nitrate to Ammonia by a Cobalt Complex. Chem. Sci. 2018, 9 (22), 4950–4958. https://doi.org/10.1039/C8SC00721G.

(7) McEnaney, J. M.; Blair, S. J.; Nielander, A. C.; Schwalbe, J. A.; Koshy, D. M.; Cargnello, M.; Jaramillo, T. F. Electrolyte Engineering for Efficient Electrochemical Nitrate Reduction to Ammonia on a Titanium Electrode. ACS Sustainable Chem. Eng. 2020, 8 (7), 2672–2681. https://doi.org/10.1021/acssuschemeng.9b05983.

(8) Yang, J.; Sebastian, P.; Duca, M.; Hoogenboom, T.; Koper, M. T. M. PH Dependence of the Electroreduction of Nitrate on Rh and Pt Polycrystalline Electrodes. Chem. Commun. 2014, 50 (17), 2148–2151. https://doi.org/10.1039/C3CC49224A.

(9) Wu, Z.-Y.; Karamad, M.; Yong, X.; Huang, Q.; Cullen, D. A.; Zhu, P.; Xia, C.; Xiao, Q.; Shakouri, M.; Chen, F.-Y.; Kim, J. Y. (Timothy); Xia, Y.; Heck, K.; Hu, Y.; Wong, M. S.; Li, Q.; Gates, I.; Siahrostami, S.; Wang, H. Electrochemical Ammonia Synthesis via Nitrate Reduction on Fe Single Atom Catalyst. Nature Communications 2021, 12 (1), 2870. https://doi.org/10.1038/s41467-021-23115-x.

(10) Katsounaros, I. On the Assessment of Electrocatalysts for Nitrate Reduction. Current Opinion in Electrochemistry 2021, 28, 100721. https://doi.org/10.1016/j.coelec.2021.100721.

(11) Costentin, C.; Savéant, J.-M. Multielectron, Multistep Molecular Catalysis of Electrochemical Reactions: Benchmarking of Homogeneous Catalysts. ChemElectroChem 2014, 1 (7), 1226–1236. https://doi.org/10.1002/celc.201300263.

Figure 1 Caption. Nitrate conversion to ammonia in the reaction-diffusion layer and chemical structures of Co(DIM) in precatalytic form (P) and active catalytic form (Q). In situ generation of the catalytically active, low-valence Co species Q occurs via electron transfer from the glassy carbon cathode to species P. The active catalyst Q binds nitrate and facilitates conversion to nitrite. A subsequent cascade of analogous catalytic turnovers converts nitrite to nitroxyl, nitroxyl to hydroxylamine, and hydroxylamine to ammonia.