Electrochemical ammonia production on molybdenum nitride nanoclusters

Electrochemical Nitrogen Reduction to Ammonia at Ambient Condition on the (111) Facets of Transition Metal Carbonitrides

We conducted Density Functional Theory calculations to investigate a class of materials with the goal of enabling nitrogen activation and electrochemical ammonia production under ambient conditions. The source of protons at the anode could originate from either water splitting or H2, but our specific focus was on the cathode reaction, where nitrogen is reduced into ammonia. We examined the conventional associative mechanism, dissociative mechanism, and Mars-van Krevelen mechanism on the (111) facets of the NaCl-type structure found in early transition metal carbonitrides, including Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, Sc, Y, and W. We explored the catalytic activity by calculating the free energy of all intermediates along the reaction pathway and constructing free energy diagrams to identify the steps that determine the reaction's feasibility. Additionally, we closely examined the potential for catalyst poisoning within the electrochemical environment, considering the bias required to drive the reaction. Furthermore, we assessed the likelihood of catalyst decomposition and the potential for catalyst regeneration among the most intriguing carbonitrides. Our findings revealed that the only carbonitride catalyst considered here exhibiting both activity and stability, capable of self-regeneration and nitrogen-to-ammonia activation, is NbCN with a low potential-determining step energy of 0.58 eV. This material can facilitate ammonia formation via a mixed associative-MvK mechanism. In contrast, other carbonitrides of this crystallographic orientation are likely to undergo decomposition, reverting to their parent metals under operational conditions.

Design Principle of Molybdenum-Based Metal Nitrides for Lattice Nitrogen-Mediated Ammonia Production

Chemical looping ammonia synthesis (CLAS) is a promising technology for reducing the high energy consumption of the conventional ammonia synthesis process. However, the comprehensive understanding of reaction mechanisms and rational design of novel nitrogen carriers has not been achieved due to the high complexity of catalyst structures and the unrevealed relationship between electronic structure and intrinsic activity. Herein, we propose a multistage strategy to establish the connection between catalyst intrinsic activity and microscopic electronic structure fingerprints using density functional theory computational energetics as bridges and apply it to the rational design of metal nitride catalysts for lattice nitrogen-mediated ammonia production. Molybdenum-based nitride catalysts with well-defined structures are employed as prototypes to elucidate the decoupled effects of electronic and geometrical features. The electron-transfer and spin polarization characteristics of the magnetic metals are constructed as descriptors to disclose the atomic-scale causes of intrinsic activity. Based on this design strategy, it is demonstrated that Ni3Mo3N catalysts possess the highest lattice nitrogen-mediated ammonia synthesis activity. This work reveals the structure-activity relationship of metal nitrides for CLAS and provides a multistage perspective on catalyst rational design.

Impact of Vacancy Defects on Electrochemical Nitrogen Reduction Reaction Performance of MXenes

We investigated electrochemical nitrogen reduction reaction (eNRR) on MXenes consisting of the vacancy defects in the functional layer using density functional theory calculations. We considered Mo2C, W2C, Mo2N, and W2N MXenes with F, N, and O functionalization and investigated distal and alternative associative pathways. We analyzed these MXenes for eNRR based on N2 adsorption energy, NH3 desorption energy, NRR selectivity, and electrochemical limiting potential. While we find that most of the considered MXenes surfaces are more favorable for eNRR compared to hydrogen evolution, these surfaces also have strong NH3 binding (>-1.0 eV) and thus will be covered with NH3 during operating conditions. Amongst all considered MXenes, only W2NF2 is found to have a low NH3 desorption energy along with low eNRR overpotential and selectivity towards eNRR. The obtained eNRR overpotential and NH3 desorption energy on W2NF2 are superior to those reported for pristine W2N3 as well as functionalized MXenes.

Powerful Orbital Hybridization of Copper-Silver Bimetallic Nanosheets for Electrocatalytic Nitrogen Reduction to Ammonia

Electrochemical nitrogen reduction (eNRR) is a promising strategy to replace the energy- and capital-intensive Haber-Bosch process. Unfortunately, the low selectivity of the eNRR process impedes the industrial application of this approach. In this work, a highly efficient and stable NRR electrocatalyst is obtained via coreduction of Cu and Ag precursors using the holly leaves as reducing agents. The as-obtained Cu₃Ag bimetallic nanosheets exhibit excellent NRR performance with an NH₃ production rate of 31.3 μg h^-1 mg^-1_cat. and a Faradaic efficiency of 31.3% at -0.2 V vs RHE. According to density functional theory (DFT) calculation, the outstanding performance of Cu₃Ag bimetallic nanosheets could be caused by the fact that Ag optimizes the 3d orbital occupation of Cu and synergistically enhances the charge transfer during the NRR process, resulting in a suitable adsorption strength of the intermediates.

Recent Progress in Electrochemical Nitrogen Reduction on Transition Metal Nitrides

Distributed electrochemical nitrogen reduction reaction (ENRR) powered by renewable energy for the on-site production of ammonia is an attractive alternative to the industrial Haber-Bosch process, which is responsible for roughly 2 % of global energy consumption. In this Review, we summarize recent progress in the ENRR catalyzed by transition metal nitrides (TMNs). The unique electronic structures of TMNs make them promising ENRR catalysts for active and selective ammonia production, which have been predicted theoretically and demonstrated experimentally. Reaction pathways and deactivation mechanisms of the ENRR on different TMNs are surveyed, and current understanding of structure-activity relations is discussed. To develop highly active, selective, and stable TMN catalysts for industrial-scale ENRR, membrane electrode assembly configuration is recommended in catalyst evaluation. Furthermore, we highlight the importance of developing mechanistic understanding on ENRR with different operando spectroscopic techniques.

Understanding potential-dependent competition between electrocatalytic dinitrogen and proton reduction reactions

A key challenge to realizing practical electrochemical N2 reduction reaction (NRR) is the decrease in the NRR activity before reaching the mass-transfer limit as overpotential increases. While the hydrogen evolution reaction (HER) has been suggested to be responsible for this phenomenon, the mechanistic origin has not been clearly explained. Herein, we investigate the potential-dependent competition between NRR and HER using the constant electrode potential model and microkinetic modeling. We find that the H coverage and N2 coverage crossover leads to the premature decrease of NRR activity. The coverage crossover originates from the larger charge transfer in H+ adsorption than N2 adsorption. The larger charge transfer in H+ adsorption, which potentially leads to the coverage crossover, is a general phenomenon seen in various heterogeneous catalysts, posing a fundamental challenge to realize practical electrochemical NRR. We suggest several strategies to overcome the challenge based on the present understandings.

Nanoporous Nickel-Molybdenum Oxide with an Oxygen Vacancy for Electrocatalytic Nitrogen Fixation under Ambient Conditions

The electrochemical nitrogen reduction reaction (NRR) is regarded as a sustainable method for N₂ fixation. N₂ adsorption and N≡N cleavage are the main challenges for the NRR. Herein, we propose a potential approach to enhance N₂ activation via introducing oxygen vacancies (OVs) into nanoporous NiO/MoO₃. Nanoporous NiO/MoO₃ with OVs (np-OVs-NiO/MoO₃) is prepared by a two-step process of dealloying and solid-state reaction. np-OVs-NiO/MoO₃ exhibits a high NH₃ yield of 35.4 μg h^-1 mg_cat^-1 and a Faradaic efficiency (FE) of 10.3% in 0.1 M PBS solution. The introduction of OVs enhances the conductivity, N₂ adsorption, and catalytic performance of np-NiO/MoO₃. The dual-metal sites with OVs have a unique electronic structure in favor of the "π back-donation" behavior, which decreases the energy barrier of protonation steps and improves the whole NRR process. This approach provides new insight into the design of composite transition metal oxides with OVs for the NRR catalyst under ambient conditions.

Identification and elimination of false positives in electrochemical nitrogen reduction studies

Ammonia is of emerging interest as a liquefied, renewable-energy-sourced energy carrier for global use in the future. Electrochemical reduction of N2 (NRR) is widely recognised as an alternative to the traditional Haber-Bosch production process for ammonia. However, though the challenges of NRR experiments have become better understood, the reported rates are often too low to be convincing that reduction of the highly unreactive N2 molecule has actually been achieved. This perspective critically reassesses a wide range of the NRR reports, describes experimental case studies of potential origins of false-positives, and presents an updated, simplified experimental protocol dealing with the recently emerging issues.

Biomimetic Nitrogen Fixation Catalyzed by Transition Metal Sulfide Surfaces in an Electrolytic Cell

The nitrogen reduction reaction was investigated on the surfaces of 18 different stable transition metal sulfides using density functional theory calculations. YS, ScS, and ZrS were modeled in the rocksalt structure with the (1 0 0) facet; TiS, VS, CrS, NbS, NiS, and FeS in NiAs-type structure with the (1 1 1) facet; and MnS₂ , CoS₂ , IrS₂ , CuS₂ , OsS₂ , FeS₂ , RuS₂ , RhS₂ , and NiS₂ in pyrite structure for both the (1 0 0) and (1 1 1) orientations. As the first step towards determination of sulfides that are less prone to hydrogen evolution, the competition between adsorption of NNH and H (for the associative mechanism), and between adsorption of N and H (for the dissociative mechanism) on these surfaces was considered. The catalytic activity through both the associative and dissociative mechanisms was explored and the overpotential required for electrochemical ammonia formation is reported. The scaling relations and volcano plots were constructed with free energy of adsorption of NNH or N on the surface as the descriptor. RuS₂ was observed as the most active sulfide that could catalyze nitrogen reduction to ammonia at potentials around -0.3 V through the associative mechanism. NbS, CrS, TiS, and VS are also promising candidates for both the associative and dissociative mechanisms with overpotentials for nitrogen reduction around 0.7-1.1 V.

Systematic exploration of N, C configurational effects on the ORR performance of Fe–N doped graphene catalysts based on DFT calculations

Metal single-atom catalysts (MSATs), such as Fe–N coordination doped sp²-carbon matrices, have emerged as a promising oxygen reduction reaction (ORR) catalyst to replace their costly platinum (Pt) based counterparts in fuel cells. In this work, we employ density functional theory (DFT) to systematically discuss the electronic-structure and surface-stress effects of N, C configurations on Fe–N doped graphene in single and double vacancy. The formation energy (E_f) of Fe–N-gra dropped off with the increase of N atoms incorporated for both single and double vacancy groups. The theoretical overpotentials on Fe–N–C sites were calculated and revealed that moderate N-doping levels and doping configuration could enhance the ORR activity of Fe–N coordination structures in the double vacancy and that doping N atoms is not effective for ORR activity in single vacancy. By exploring the d-band centers, we found that ligand effects and surface tension effects contribute to the modification of the d-band centers of metal Fe atoms. An optimum Fe–N–C ORR catalyst should exhibit moderate surface stress properties and an ideal N, C ligand configuration. This study provides new insight into the effects of N atom doping in Fe–N-gra catalysts and could help guide the rational design of high-performance carbon-based ORR electrocatalysts.

An optimum Fe–N–C ORR catalyst should exhibit a moderate surface stress property and an ideal N, C ligand configurations that results in a moderate interaction between the ORR intermediates and its surface sites.

Single transition metal atom embedded into a MoS2 nanosheet as a promising catalyst for electrochemical ammonia synthesis

The electrochemical reduction of N2 to NH3 (NRR) under ambient conditions is significant for sustainable agriculture. Here, by means of density functional theory (DFT) computations, the potential of a series of single transition metal (TM) atoms embedded into a MoS2 monolayer with an S-vacancy (TM/MoS2) as electrocatalysts for NRR was systematically investigated. Our DFT results revealed that among all these considered candidate catalysts, the single Mo atom embedded into the MoS2 nanosheet was found to be the most active catalyst for NRR with an onset potential of -0.53 V, in which the hydrogenation of the adsorbed N2* to N2H* is the potential-determining step. The high stabilization of the N2H* species is responsible for the superior performance of the embedded Mo atom for the NRR, which is well consistent with its d-band center. Our findings may facilitate the further design of single-atom electrocatalysts with high efficiency for NH3 synthesis at room temperature.

A theoretical study of the effect of a non-aqueous proton donor on electrochemical ammonia synthesis

Ammonia synthesis is one of the most studied reactions in heterogeneous catalysis. To date, however, electrochemical N₂ reduction in aqueous systems has proven to be extremely difficult, mainly due to the competing hydrogen evolution reaction (HER). Recently, it has been shown that transition metal complexes based on molybdenum can reduce N₂ to ammonia at room temperature and ambient pressure in a non-aqueous system, with a relatively small amount of hydrogen output. We demonstrate that the non-aqueous proton donor they have chosen, 2,6-lutidinium (LutH⁺), is a viable substitute for hydronium in the electrochemical process at a solid surface, since this donor can suppress the HER rate. We also show that the presence of LutH⁺ can selectively stabilize the *NNH intermediate relative to *NH or *NH₂via the formation of hydrogen bonds, indicating that the use of non-aqueous solvents can break the scaling relationship between limiting potential and binding energies.

Nitrogen electroreduction and hydrogen evolution on cubic molybdenum carbide: a density functional study

The (111) surface of cubic MoC was found to be active for nitrogen electroreduction to ammonia via an associative Heyrovsky path.

Impact of H-termination on the nitrogen reduction reaction of molybdenum carbide as an electrochemical catalyst

H-Terminals can remarkably affect the performance of catalysts in nitrogen reduction.

CO2electroreduction performance of a single transition metal atom supported on porphyrin-like graphene: a computational study

Single transition metal atoms supported by porpyrin-like graphene exhibit high catalytic activity for the electroreduction of CO₂.

Can boron antisites of BNNTs be an efficient metal-free catalyst for nitrogen fixation? – A DFT investigation

Nitrogen fixation is a challenging reaction under ambient conditions.

On the mechanism of electrochemical ammonia synthesis on the Ru catalyst

We theoretically investigate the electrochemical N2 reduction reaction (NRR) mechanism to produce NH3 on the Ru catalyst. All possible N-N dissociation steps during the reduction processes were evaluated along with the conventional associative and dissociative pathways. Based on the calculated free energy diagrams, it is revealed that the kinetically facile intermediate dissociative pathways during the NRR require a thermodynamic limiting potential (-0.71 V) similar to the associative pathway (-0.68 V), although the initial dissociative pathway as in the Haber-Bosch process has a substantial kinetic barrier for the N-N bond dissociation. The competitive hydrogen evolution is found to be a major hurdle for achieving a high efficiency for the electrochemical nitrogen reduction. In the low overpotential region, the hydrogen adsorption is thermodynamically more favorable than the protonation of N2, thereby reducing the number of active sites for the N2 activation. A comparison of free energies in the presence of different H-coverages on the Ru further demonstrates that the H-coverage can significantly increase the energy barrier for the first protonation of N2, resulting in a change of the potential determining step and an increase in the overpotentials.

Chemical looping of metal nitride catalysts: low-pressure ammonia synthesis for energy storage

The activity of many heterogeneous catalysts is limited by strong correlations between activation energies and adsorption energies of reaction intermediates. Although the reaction is thermodynamically favourable at ambient temperature and pressure, the catalytic synthesis of ammonia (NH3), a fertilizer and chemical fuel, from N2 and H2 requires some of the most extreme conditions of the chemical industry. We demonstrate how ammonia can be produced at ambient pressure from air, water, and concentrated sunlight as renewable source of process heat via nitrogen reduction with a looped metal nitride, followed by separate hydrogenation of the lattice nitrogen into ammonia. Separating ammonia synthesis into two reaction steps introduces an additional degree of freedom when designing catalysts with desirable activation and adsorption energies. We discuss the hydrogenation of alkali and alkaline earth metal nitrides and the reduction of transition metal nitrides to outline a promoting role of lattice hydrogen in ammonia evolution. This is rationalized via electronic structure calculations with the activity of nitrogen vacancies controlling the redox-intercalation of hydrogen and the formation and hydrogenation of adsorbed nitrogen species. The predicted trends are confirmed experimentally with evolution of 56.3, 80.7, and 128 μmol NH3 per mol metal per min at 1 bar and above 550 °C via reduction of Mn6N2.58 to Mn4N and hydrogenation of Ca3N2 and Sr2N to Ca2NH and SrH2, respectively.

Enabling electrochemical reduction of nitrogen to ammonia at ambient conditions through rational catalyst design

Investigation of transition metal nitrides reveals extremely promising electrocatalysts for high-yield ammonia production in aqueous electrolytes under ambient conditions.

The role of oxygen and water on molybdenum nanoclusters for electro catalytic ammonia production

The presence of water often gives rise to oxygen adsorption on catalyst surfaces through decomposition of water and the adsorbed oxygen or hydroxide species often occupy important surfaces sites, resulting in a decrease or a total hindrance of other chemical reactions taking place at that site. In this study, we present theoretical investigations of the influence of oxygen adsorption and reduction on pure and nitrogen covered molybdenum nanocluster electro catalysts for electrochemical reduction of N2 to NH3 with the purpose of understanding oxygen and water poisoning of the catalyst. Density functional theory calculations are used in combination with the computational hydrogen electrode approach to calculate the free energy profile for electrochemical protonation of O and N2 species on cuboctahedral Mo13 nanoclusters. The calculations show that the molybdenum nanocluster will preferentially bind oxygen over nitrogen and hydrogen at neutral bias, but under electrochemical reaction conditions needed for nitrogen reduction, oxygen adsorption is severely weakened and the adsorption energy is comparable to hydrogen and nitrogen adsorption. The potentials required to reduce oxygen off the surface are -0.72 V or lower for all oxygen coverages studied, and it is thus possible to (re)activate (partially) oxidized nanoclusters for electrochemical ammonia production, e.g., using a dry proton conductor or an aqueous electrolyte. At lower oxygen coverages, nitrogen molecules can adsorb to the surface and electrochemical ammonia production via the associative mechanism is possible at potentials as low as -0.45 V to -0.7 V.