Catalysts For N2 Reduction Reaction Research Articles

High spin polarization ultrafine Rh nanoparticles on CNT for efficient electrochemical N2 fixation to ammonia

The electroreduction of nitrogen (N2) to ammonia (NH3) is gravely restricted by its inherent kinetic complexity and energy-intensive multi-electron steps. Most literature has reported that the first step of N2 adsorption and activation is the bottleneck of electrochemistry N2 reduction reaction (NRR). However, it is a considerable challenge to understand the design rule of catalysts with perfect performances for NRR. Here, as a proof-of-concept experiment, we apply the theoretical calculations with the experimental studies to reveal the influence of the high spin related to the size effect and the charge density related to the substrate effect on the catalytic activity of NRR catalysts. The desired ultrafine Rh nanoparticles anchored on CNT exhibit excellent NRR performances, especially high NH3 yield (26.91 μg h−1 mgcat.−1), Faradaic efficiency (23.48 %) and energy efficiency (20.50 %), outperforming most reported NRR electrocatalysts under ambient conditions, which can be attributed to excellent synergism of the high-spin polarization of Rh NPs and the charge exchange between Rh NPs and the substrate that can promote N2 adsorption and activate its intrinsic NN triple bond.

Electrochemical N2 reduction at ambient condition – Overcoming the selectivity issue via control of reactants’ availabilities

Ammonia production via the electrochemical N2 reduction reaction (NRR) at ambient conditions is highly desired as an alternative to the Haber-Bosch process, but remains a great challenge due to the low efficiency and selectivity caused by the competing hydrogen evolution reaction (HER). Herein we investigate the effect of availabilities of reactants (protons, electrons and N2) on NRR using a FeOx-coated carbon fiber paper cathode in various electrochemical configurations. NRR is found viable only under the conditions of low proton- and high N2 availabilities, which are achieved using 0.12 vol% water in LiClO4-ethyl acetate electrolyte and gaseous N2 supplied to the membrane-electrode assembly cathode. This results in an NRR rate of 29 ± 19 pmolNH3 s−1 cm−2 at a Faradaic efficiency of 70 ± 24% at the applied potential of −0.1 V vs. NHE. Other conditions (high proton-, or low N2-availability, or both) yield a lower or negligible amount of ammonia due to the competing HER. Our work shows that promoting NRR by suppressing the HER requires optimization of the operational variables, which serves as a complementary strategy to the development of NRR catalysts.

Electrocatalytic Mechanism of N2 Reduction Reaction by Single-Atom Catalyst Rectangular TM-TCNQ Monolayers.

Herein, the catalytic properties and reaction mechanisms of the 3d, 4d, and 5d transition metals embedded in 2D rectangular tetracyanoquinodimethane (TM-rTCNQ) monolayers as single-atom catalysts (SACs) for the electrocatalytic N2 reduction reaction (NRR) were systematically investigated, using first-principles calculations. A series of high-throughput screenings were carried out on 30 TM-rTCNQ monolayers, and all possible NRR pathways were explored. Three TM-rTCNQ (TM = Mo, Tc, and W) SACs were selected as promising new NRR catalyst candidates because of their high structural stability and good catalytic performance (low onset potential and high selectivity). Our results show that the Mo-rTCNQ monolayer can catalyze NRR through a distal mechanism with an onset potential of -0.48 V. Surprisingly, the NH3 desorption energy on the Mo-rTCNQ monolayer is only 0.29 eV, the lowest one reported in the literature so far, which makes the Mo-rTCNQ monolayer a good NRR catalyst candidate. In-depth research studies on the structures of N2-TM-rTCNQ (TM = Mo, Tc, and W) found that strong adsorption and activation performance of TM-rTCNQ for N2 may be due to the strong charge transfer and orbital hybridization between the TM-rTCNQ catalyst and the N2 molecules. Our work provides new ideas for achieving N2 fixation under environmental conditions.

(Invited) Critical Effect of Particle Size in Ru-Catalyzed Electrosynthesis of Ammonia

Electrochemical reduction of N2 to NH3 has recently received considerable attention, because it may enable sustainable, distributed production of NH3 when powered by solar- or wind-generated electricity. However, typical catalysts show a low activity and selectivity for N2 reduction reaction (NRR) due to the barrier for N2 activation and the competing hydrogen evolution reaction (HER). A rational design of NRR catalysts relies on our understandings of structure-activity relationships and active sites for the NRR, and such study requires model catalysts with well-defined structures. Here we present a study of size-dependent activity for the NRR on Ru nanoparticle catalysts. We first tried colloidal synthesis method with polyvinylpyrrolidine (PVP) as a surfactant to control the size of Ru nanoparticles, while the derived Ru catalysts showed negligible activity for NRR, which was attributed to residual surfactant molecules that blocked catalyst surfaces. Therefore, we used atomic layer deposition (ALD) method to prepare Ru nanoparticles with controlled sizes and clean surfaces. We also quantified the electrochemical active surface areas of Ru samples and measured surface-area-normalized activity for the NRR. Consequently, the effect of Ru nanoparticle size on the NRR activity and Faradaic efficiency was revealed, which can provide insights into the active sites for the NRR and guidance on the design of NRR catalysts via surface site engineering.

Atomically Dispersed Iron Metal Site in a Porphyrin-Based Metal-Organic Framework for Photocatalytic Nitrogen Fixation.

The rational design of photocatalysts for efficient nitrogen (N2) fixation at ambient conditions is important for revolutionizing ammonia production and quite challenging because the great difficulty lies in the adsorption and activation of the inert N2. Inspired by a biological molecule, chlorophyll, featuring a porphyrin structure as the photosensitizer and enzyme nitrogenase featuring an iron (Fe) atom as a favorable binding site for N2via π-backbonding, here we developed a porphyrin-based metal-organic framework (PMOF) with Fe as the active center as an artificial photocatalyst for N2 reduction reaction (NRR) under ambient conditions. The PMOF features aluminum (Al) as metal node imparting high stability and Fe incorporated and atomically dispersed by residing at each porphyrin ring promoting the adsorption and the activation of N2, termed Al-PMOF(Fe). Compared with the pristine Al-PMOF, Al-PMOF(Fe) exhibits a substantial enhancement in NH3 yield (635 μg g-1cat.) and production rate (127 μg h-1 g-1cat.) of 82% and 50%, respectively, on par with the best-performing MOF-based NRR catalysts. Three cycles of photocatalytic NRR experimental results corroborate a stable photocatalytic activity of Al-PMOF(Fe). The combined experimental and theoretical results reveal that the Fe-N site in Al-PMOF(Fe) is the active photocatalytic center that can mitigate the difficulty of the rate-determining step in photocatalytic NRR. The possible reaction pathways of NRR on Al-PMOF(Fe) were established. Our study of porphyrin-based MOF for the photocatalytic NRR will provide insight into the rational design of catalysts for artificial photosynthesis.

Recent advances in TiO2‐based catalysts for N2 reduction reaction

AbstractNitrogen (N2) fixation under mild conditions is a promising approach for green production of ammonia (NH3). In the past decades, various advanced catalysts have been fabricated to achieve this goal through electrocatalytic and photocatalytic processes. Among them, the TiO2‐based catalysts have been recognized as promising candidates due to their high activity, low cost, chemical stability, and nontoxicity. In this review, recent advances in the fabrication of high‐performance TiO2‐based materials for N2 reduction reaction (NRR) under mild conditions are summarized, including electrocatalytic and photocatalytic NRR. The design principles, synthetic strategies, and corresponding chemical/physical properties of TiO2‐based NRR catalysts are described in detail. Moreover, the key challenges and potential opportunities in this field are presented and discussed.

Theoretical Study on P-coordinated Metal Atoms Embedded in Arsenene for the Conversion of Nitrogen to Ammonia.

The conversion of gaseous N2 to ammonia under mild conditions by artificial methods has become one of the hot topics and challenges in the field of energy research today. Accordingly, based on density function theory calculations, we comprehensively explored the d-block of metal atoms (Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo, Ru, Rh, W, and Pt) embedded in arsenene (Ars) for different transition systems of phosphorus (P) coordination as potential electrocatalysts for N2 reduction reaction (NRR). By adopting a “two-step” strategy with stringent NRR catalyst screening criteria, we eventually selected Nb@P3-Ars as a research object for a further in-depth NRR mechanism study. Our results show that Nb@P3-Ars not only maintains the thermodynamic stability at mild temperatures but also dominates the competition with the hydrogen evolution reaction when used as the electrochemical NRR (e-NRR) catalyst. In particular, while the NRR process occurs by the distal mechanism, Nb@P3-Ars has a low overpotential (0.36 V), which facilitates the efficient reduction of N2. Therefore, this work predicts the possibility of Nb@P3-Ars as an e-NRR catalyst for reducing N2 from a theoretical perspective and provides significant insights and theoretical guidance for future experimental research.

Defected MoS2: An efficient electrochemical nitrogen reduction catalyst under mild conditions

Although the electrochemical N2 reduction reaction (NRR) to NH3 provides a promising avenue for artificial N2 fixation, building of efficient NRR electrocatalysts with high activity and stability remains a tremendous challenge. The combined experimental results and theoretical calculation reveal that MoS2 with sulfur-vacancy defect (VS-MoS2) is an admirable NRR catalyst. Electrochemical results demonstrate that VS-MoS2 (5:2) possesses outstanding NRR performances, including a superior NH3 average yield rate (46.1 × 10−11 mol s−1 cm−2 at -0.5 V vs reversible hydrogen electrode (RHE)), an excellent faraday efficiency (FE: 4.58% at -0.4 V vs RHE), good electrochemical stability (at least 24 h), and high selectivity (no by-product of N2H4). Density functional theory (DFT) calculations demonstrate that the rate determining steps (RDSs) of all VS-MoS2 (010)-2, VS-MoS2 (010)-1, and MoS2 (010) are the first hydrogenation step to form *NNH intermediate, as reflected by the energy barriers of 0.06 eV, 0.20 eV, and 0.93 eV, the significantly charge transfer (1.02e, 0.45e vs. 0.41e) from two Mo atoms to N2, as well as the stretching of NN bond (1.200 Å, 1.140 Å vs. 0.970 Å), respectively. These results reveal that VS-MoS2 (010)-2 can effectively activate inert N2 molecule and have the potential application in nitrogen fixation under mild conditions.

A systematic computational investigation of the water splitting and N2 reduction reaction performances of monolayer MBenes.

Recently, transition metal borides (MBenes, analogous to MXenes) have attracted interest due to their potential applications in energy conversion and storage. In this work, we performed density functional theory calculations to systematically explore the exfoliation properties of 14 MAlB phases and their water splitting and N2 reduction reaction (NRR) performances. Results showed a linear relationship between the binding energy and exfoliation energy with the coefficient (R2) of 0.95, indicating that the lower the binding energy of element Al in MAlB (M2AlB2), the higher the exfoliation energy required to synthesize monolayer MB from MAlB (M2AlB2). NiB (B site) was predicted to possess the best electrocatalytic activity for water splitting, hydrogen evolution reaction (HER), and oxygen evolution reaction (OER) among the studied MBenes, and overpotentials on the NiB surface were calculated to be 0.08 V (for HER) and 0.37 V (for OER), respectively. The electronic properties and dynamic simulations indicated that NiB is the best candidate catalyst for water splitting. Conversely, the Fe site on FeB (FeB-Fe) was predicated to have the highest nitrogen reduction reaction (NRR) activity among the studied MBenes, with the overpotential ηNRR of 0.11 V. Furthermore, the B site of TaB (TaB-B) was identified as the best NRR catalyst against HER among the studied MBenes considering the HER side reaction.

Catalytic Performance of Two-Dimensional Bismuth Tuned by Defect Engineering for Nitrogen Reduction Reaction

An ideal electro-catalyst is required to possess high activity and high selectivity for N2 reduction reaction (NRR). Bismuth (Bi), a hydrogen-insensitive metal crystal, has recently received attention as a potential catalyst for NRR. We, here, evaluated NRR performance of Bi-based catalysts by performing density functional theory (DFT) calculations. Our results indicate that the NRR activity of pristine Bi(012) surface is slightly higher than that of the (001) surface. Bi catalysts achieve enhanced catalytic performance with defect engineering strategies, such as the introduction of vacancies and heteroatoms into the lattice. Remarkably, single boron atom doping on Bi surface could give rise to impressive catalytic performance, which can potentially act as a promising NRR catalyst. Our work deepens the understanding of catalytic behavior of Bi-based catalysts for NRR and provides a novel way to design efficient NRR catalysts.

Electrocatalytic N2 reduction to NH3 with high Faradaic efficiency enabled by vanadium phosphide nanoparticle on V foil

To develop highly efficient electrochemical catalysts for N2 fixation is important to sustainable ambient NH3 production through the N2 reduction reaction (NRR). Herein, we demonstrate the development of vanadium phosphide nanoparticle on V foil as a high-efficiency and stable catalyst for ambient NH3 production with excellent selectivity. The high Faradaic efficiency of 22% with a large NH3 yield of 8.35 × 10−11 mol·s−1·cm−2 was obtained at 0 V vs. the reversible hydrogen electrode in acid solution, superior to all previously studied V-based NRR catalysts. Density functional theory calculations are also utilized to have an insight into the catalytic mechanism.

Nano‐Ferric Oxide Embedded in Graphene Oxide: High‐performance Electrocatalyst for Nitrogen Reduction at Ambient Condition

Nitrogen (N2) fixation at ambient condition by electrochemical N2 reduction reaction (NRR) is energy‐efficient and eco‐friendly as compared to the traditional Harber–Bosch process, but it is extremely challenging. Development and design of high‐performance NRR electrocatalysts are indispensable to achieve the goal. In this work, a strongly coupled hybrid of nano‐Fe3O4 with reduced graphene oxide (rGO) is synthesized via an in situ redox hydrothermal approach, and the synthesized Fe3O4@rGO hybrid has excellent activity, selectivity, and stability as an NRR catalyst. The NH3 yield rate of 28.01 μg h‐1 mg‐1 at −0.3 V and the Faradaic efficiency (FE) of 19.12% at −0.1 V are obtained in 0.1 M Na2SO4 solutions at ambient conditions. The superior NRR performance is attributed to the chemical coupling effect between rGO and nano‐Fe3O4 particles, which leads to the enhancement of the binding affinity to N2 molecules, improvement of the conductivity, and lowering the free energy of reaction for the limiting reaction step. This work provides a facile route in fabricating hybrid NRR catalysts with superior performance and shed lights on the reaction mechanism with theoretical mechanistic calculations.

FeMo sub-nanoclusters/single atoms for neutral ammonia electrosynthesis

Electrochemical N 2 reduction reaction (NRR) has long been regarded as a promising process to generate NH 3 under ambient conditions. Therefore, developing cost-effective and high-performance non-noble-metal catalysts for NRR is highly desirable. Inspired by the biological nitrogenase structure, we here designed and synthesized a catalyst with iron-molybdenum sub-nanoclusters and single atoms on porous nitrogen-doped carbon (FeMo/NC). The catalyst features porous structure beneficial to active site exposure and accessibility to electrolyte as well as FeMo sub-nanoclusters and single atoms enabling to activate N 2 molecular. In situ near-ambient pressure X-ray photoelectron spectroscopy tests reveal that during the process from vacuum to nitrogen saturation, N 2 was close to, adsorbed on and interacted with Fe and Mo in FeMo/NC. The Fe and Mo through electron transfer play a key role in activating the N 2 molecules. Therefore, when tested for NRR, FeMo/NC achieves the maximum Faradaic efficiency (FE) of 11.8 ± 0.8% at −0.25 V and NH 3 yield rate of 26.5 ± 0.8 μg h −1 mg cat. −1 at −0.3 V in neutral electrolyte. Moreover, the catalyst exhibits ignorable variations in the FE and a slight decrease in current density for 100,000 s. This work develops a non-precious bimetallic electrocatalyst with synergetic effect capability for efficient NH 3 production and provides a guideline for the design of efficient and robust catalysts with coexistence of sub-nanoclusters and single atoms. Inspired by the biological nitrogenase, we have designed a novel catalyst composed of FeMo sub-nanoclusters/single atoms on N-doped carbon that can effectively catalyse electroreduction of N 2 to generate NH 3 in neutral media. • FeMo sub-nanoclusters/single atoms are developed from the inspiration of biological nitrogenase. • The catalyst shows excellent activity toward neutral NH 3 electrosynthesis. • The catalyst also displays an outstanding stability during the electrolysis. • Near-ambient pressure X-ray photoelectron spectroscopy confirms that the coexistence of Fe and Mo activates the N 2 molecules.

Regulating the coordination environment through doping N atoms for single-atom Mn electrocatalyst of N2 reduction with high catalytic activity and selectivity: A theoretical study

Electrochemical N2 reduction reaction (NRR) is a promising way for ammonia synthesis, and developing novel electrocatalyst with high catalytic activity and selectivity is an urgent matter. Herein, the catalytic performances of single-atom Mn catalysts supported with nitrogen-doped graphene substrates (MnSA@Vx-Ny, x = s, d. y = 0, 1, 2, 3, 4.) for NRR have systematically investigated through density functional theory (DFT) calculations. The catalytic activity and selectivity of MnSA@Vx-Ny were discussed through reaction path, Gibbs free energy variation, hydrogen evolution reaction (HER), electron transfer and projected density of states. The result reveals the adsorption energy of N2 and catalytic activity of MnSA@Vx-Ny were effectively regulated through modulating the coordination environment of Mn atom. MnSA@Vs-N1 has the highest catalytic activity and selectivity among all studied catalysts for NRR at room temperature. MnSA@Vs-N1 can facilitate NRR by the distal mechanism, and the potential determining step is *N2 + H++ e− → *NNH with a free energy variation of 0.77 eV. Simultaneously, HER is suppressed by the high selectivity of N2 adsorption on MnSA@Vs-N1. This adjusting method can provide a guideline for developing robust electrocatalyst for NRR.

NH2–MIL-88B–Fe for electrocatalytic N2 fixation to NH3 with high Faradaic efficiency under ambient conditions in neutral electrolyte

The global NH3 production is dominated by Haber–Bosch process, requiring high temperature and pressure. Electrochemical N2 reduction reaction (NRR) under ambient conditions is a greener path for artificial N2 fixation to NH3 but calling for efficient catalyst to increase activity and selectivity. Herein, we report the iron-based metal–organic frameworks (MOFs), i.e., MIL-88B–Fe and amine-functionalized MIL-88B–Fe (NH2–MIL-88B–Fe) as efficient catalysts for electrochemical NRR under ambient temperature and pressure in neutral electrolyte. NH2–MIL-88B–Fe shows higher NH3 yield rate of 1.205 × 10–10 mol s−1 cm−2 than MIL-88B–Fe (3.575 × 10–11 mol s−1 cm−2). Furthermore, NH2–MIL-88B–Fe exhibits the highest Faradaic efficiency of 12.45% at 0.05 V versus RHE. The control experiments prove that NH3 is produced through electrocatalytic NRR. This work may trigger the interest of using MOFs as highly efficient catalysts for electrochemical NH3 production.

Self-supporting CoMoC nanoporous catalysts for N2 reduction reaction under ambient conditions

Electrochemical nitrogen reduction is expected to replace the industrial Haber-Bosch process due to its environment-friendly and sustainable development route, while the main challenge is to develop active and selective N2 reduction reaction (NRR) electrocatalysts. Here, we prepare the self-supporting CoMoC nanoporous catalyst by electrochemical dealloying method. Due to the nanoporous structure and MoC compositions, the CoMoC catalyst exhibits high catalytic activity and outstanding selectivity to NRR. The intimate between Co and MoC favors in electron transport, resulting in promoting the activity of the whole nitrogen reduction reaction process. In 0.1 M PBS (PH = 7), the NH3 yield rate and faradaic efficiency (FE) at 0.1 V (vs. RHE) are 18.9 ug cm−2 h−1 and 8.9%, respectively. This work opens up new opportunities to develop self-supporting CoMoC nanoporous catalysts with good conductivity and activity for NRR.

Rational Design of Hydroxyl‐Rich Ti3C2Tx MXene Quantum Dots for High‐Performance Electrochemical N2 Reduction

AbstractTo enable an efficient and cost‐effective electrocatalytic N2 reduction reaction (NRR) the development of an electrocatalyst with a high NH3 yield and good selectivity is required. In this work, Ti3C2Tx MXene‐derived quantum dots (Ti3C2Tx QDs) with abundant active sites enable the development of efficient NRR electrocatalysts. Given surface functional groups play a key role on the electrocatalytic performance, density functional theory calculations are first conducted, clarifying that hydroxyl groups on Ti3C2Tx offer excellent NRR activity. Accordingly, hydroxyl‐rich Ti3C2Tx QDs (Ti3C2OH QDs) are synthesized as NRR catalysts by alkalization and intercalation. This material offers an NH3 yield and Faradaic efficiency of 62.94 µg h−1 mg−1cat. and 13.30% at −0.50 V, respectively, remarkably higher than reported MXene catalysts. This work demonstrates that MXene catalysts can be mediated through the optimization of both QDs sizes and functional groups for efficient ammonia production at room temperature.

Boosting electrosynthesis of ammonia on surface-engineered MXene Ti3C2

Abstract Seeking a breakthrough in the development of efficient nitrogen fixation catalysts has become the frontier of energy and chemical conversion schemes. Here, we report that the MXene Ti3C2 can serve as a promising catalyst for the electrochemical N2 reduction reaction (NRR) under ambient conditions. The electrocatalytic performance of Ti3C2 can be further optimized through surface engineering. Specifically, Ti3C2 with the increased surface hydroxyl moieties demonstrates enhanced production of NH3 with a yield rate of 1.71 μg h−1 cm−2, a Faradaic efficiency of 7.01% at −0.2 V vs. RHE at 20 °C and an even higher yield rate of 12.46 μg h−1 cm−2 together with a Faradaic efficiency of 9.03% at −0.2 V vs. RHE at 60 °C. The detailed electrochemical analysis suggests that the surface hydroxyl modification can effectively facilitate the electron transfer, surface adsorption and activation of dinitrogen. Our work sheds light on the development of efficient NRR catalysts based on earth-abundant elements.

The Crucial Role of Charge Accumulation and Spin Polarization in Activating Carbon‐Based Catalysts for Electrocatalytic Nitrogen Reduction

AbstractCost‐effective carbon‐based catalysts are promising for catalyzing the electrochemical N2 reduction reaction (NRR). However, the activity origin of carbon‐based catalysts towards NRR remains unclear, and regularities and rules for the rational design of carbon‐based NRR electrocatalysts are still lacking. Based on a combination of theoretical calculations and experimental observations, chalcogen/oxygen group element (O, S, Se, Te) doped carbon materials were systematically evaluated as potential NRR catalysts. Heteroatom‐doping‐induced charge accumulation facilitates N2 adsorption on carbon atoms and spin polarization boosts the potential‐determining step of the first protonation to form *NNH. Te‐doped and Se‐doped C catalysts exhibited high intrinsic NRR activity that is superior to most metal‐based catalysts. Establishing the correlation between the electronic structure and NRR performance for carbon‐based materials paves the pathway for their NRR application.

The Crucial Role of Charge Accumulation and Spin Polarization in Activating Carbon-Based Catalysts for Electrocatalytic Nitrogen Reduction.

Cost-effective carbon-based catalysts are promising for catalyzing the electrochemical N2 reduction reaction (NRR). However, the activity origin of carbon-based catalysts towards NRR remains unclear, and regularities and rules for the rational design of carbon-based NRR electrocatalysts are still lacking. Based on a combination of theoretical calculations and experimental observations, chalcogen/oxygen group element (O, S, Se, Te) doped carbon materials were systematically evaluated as potential NRR catalysts. Heteroatom-doping-induced charge accumulation facilitates N2 adsorption on carbon atoms and spin polarization boosts the potential-determining step of the first protonation to form *NNH. Te-doped and Se-doped C catalysts exhibited high intrinsic NRR activity that is superior to most metal-based catalysts. Establishing the correlation between the electronic structure and NRR performance for carbon-based materials paves the pathway for their NRR application.