Nitrogen Reduction Reaction to Ammonia on Transition Metal Carbide Catalysts

Abstract

The development of a low‐cost, energy‐efficient, and environmentally friendly alternative to the currently utilized Haber‐Bosch process to produce ammonia is of great importance. Ammonia is an essential chemical used in fertilizers and a promising high‐density fuel source. The nitrogen reduction reaction (NRR) has been explored intensively as a potential avenue for ammonia production using water as proton source, but to this day a catalyst capable of producing this chemical at high Faradaic efficiency (FE) and commercial yield and rates has not been reported. Here, we investigate the activity of transition metal carbide (TMC) surfaces in the (100) facets of the rocksalt (RS) structure as potential catalysts for the NRR. In this study, we use density functional theory (DFT) to model reaction pathways, estimate stability, assess kinetic barriers, and compare adsorbate energies to determine the overall performance of each TMC surface. For pristine TMC surfaces (with no defects) we find that none of the studied TMCs possess both exergonic adsorption of nitrogen and the capability to selectively protonate nitrogen to form ammonia in the desired aqueous solution. ZrC, however, is shown to be a potential catalyst if used in a non‐aqueous electrolyte. To circumvent the endergonic adsorption of nitrogen onto the surface, a carbon vacancy was introduced. This provides a well‐defined high coordination active site on the surface. In the presence of a vacancy VC, NbC, and WC showed efficient nitrogen adsorption, selectivity towards ammonia, and a low overpotential (OP). NbC did, however, display an unfeasible kinetic barrier to nitrogen dissociation for ambient‐condition purposes, and thus it is suggested for high tempearture/pressure ammonia synthesis. Both WC and VC in their RS (100) structure are promising materials for experimental investigations in aqueous electrolytes, and ZrC could potentially be interesting for non‐aqueous electrolytic systems.

Beyond the thermodynamic volcano picture in the nitrogen reduction reaction over transition-metal oxides: Implications for materials screening

“…The reason for the choice of organic solvents refers to the higher solubility of dinitrogen therein compared to water, which may result in higher ammonia selectivity. Despite this, aqueous electrolytes are most promising for future scale up due to their simplicity, non-toxicity, and low cost, which is a significant advantage over organic solvents [14], and this justifies the search of efficient electrocatalyst for selective formation of ammonia in aqueous media.…”

Theoretical exploration of the nitrogen fixation mechanism of two-dimensional dual-metal FeTM@GY (TM = Fe, Mo, Co, and V) electrocatalysts

“…8 Additionally, because of a high overpotential as well as the competing hydrogen evolution reaction (HER), most reported catalysts for NRR exhibit poor activity and selectivity. 9 To date, a number of NRR electrocatalysts are based on transition metals (TMs), 10–12 such as oxides, 13,14 carbides, 15,16 nitrides 17,18 and sulfides, 19 as well as metal-free catalysts 20,21 have been developed and widely noted. Wang et al found that Fe 3 O 4 @rGO hybrids synthesized by in situ redox hydrothermal method have excellent activity, selectivity and stability as NRR catalysts.…”

Catalytic Nitrogen Reduction on the Transition Metal Carbonitride (110) Facet: DFT Predictions and Mechanistic Insights

“…23,24 Consequently, the majority of NRR endeavors report a limited ammonia yield and faradaic efficiency. So far, a variety of electrocatalysts for the NRR have been developed, including noble metals, 25,26 transition metal sulfides, 27 transition metal nitrides (TMN), 28−31 transition metal oxides (TMO), 32,33 transition metal oxynitrides (TMON), 34,35 transition metal carbides (TMC), 36 transition metal carbonitrides (TMCN), 37,38 and metal-free materials. 39−44 Enhancing NRR performance requires comprehensive optimization of the entire electrochemical system, encompassing the cell configuration, electrolyte, and electrocatalysts.…”

“…So far, a variety of electrocatalysts for the NRR have been developed, including noble metals, , transition metal sulfides, transition metal nitrides (TMN), − transition metal oxides (TMO), , transition metal oxynitrides (TMON), , transition metal carbides (TMC), transition metal carbonitrides (TMCN), , and metal-free materials. − Enhancing NRR performance requires comprehensive optimization of the entire electrochemical system, encompassing the cell configuration, electrolyte, and electrocatalysts . Notably, electrocatalysts play a crucial role in accelerating reaction rates and improving the selectivity and efficiency in electrochemical NRR systems.…”