Catalysts For Ammonia Synthesis Research Articles

Enhanced Activity and Selectivity for Nitrogen Reduction Reaction in Electrode-based Heterostructures: A DFT Computational Study.

Developing sustainable and efficient catalysts for ammonia synthesis from atmospheric molecular N2 under ambient conditions presents a significant 21st-century challenge. Two-dimensional heterostructures, particularly single-atom catalysts (SACs) supported on two-dimensional materials, have emerged as a promising avenue due to their remarkable catalytic activity and selectivity. Electrides, characterized by an abundance of free electrons and high surface activity, have attracted substantial attention in this context. Through density functional theory (DFT) calculations, this study proposes electride-graphene heterostructures (EGHS) as catalysts to effectively regulate charge distribution at the catalytic center, facilitating the optimization of catalytic performance. The EGHS model addresses challenges related to excessive adsorbate binding, mitigating electron transfer compared to electride monolayer adsorption. This novel approach utilizes heterogeneous heterostructures to finely tune the catalytic site, optimizing electron input for enhanced catalysis.Based on the optimized charge transfer for N2 activation, the Cr-doped EGHS (Cr@EGHS) exhibits a promising performance in the nitrogen reduction reaction, leading to, a relatively low limiting potential of -0.85 V and high selectivity. The hypothesis charge transfer depend on N2 activation is further supported by modulating the distance between component layers of heterostructure. These findings contribute to design principles for 2D heterostructure catalysts and offer a reference for experimental synthesis.

NbC Nanoparticles Decorated Carbon Nanofibers as Highly Active and Robust Heterostructural Electrocatalysts for Ammonia Synthesis.

Transition-metal carbides with metallic properties have been extensively used as electrocatalysts due to their excellent conductivity and unique electronic structures. Herein, NbC nanoparticles decorated carbon nanofibers (NbC@CNFs) are proposed as an efficient and robust catalyst for electrochemical synthesis of ammonia from nitrate/nitrite reduction, which achieves a high Faradaic efficiency (FE) of 94.4% and a large ammonia yield of 30.9 mg h-1 mg-1cat.. In situ electrochemical tests reveal the nitrite reduction at the catalyst surface follows the *NO pathway and theoretical calculations reveal the formation of NbC@CNFs heterostructure significantly broadens density of states nearby the Fermi energy. Finite element simulations unveil that the current and electric field converge on the NbC nanoparticles along the fiber, suggesting the dispersed carbides are highly active for nitrite reduction.

Tuning clusters-metal oxide promoters electronic interaction of Ru-based catalyst for ammonia synthesis under mild conditions

Ammonia (NH3) is an excellent candidate for hydrogen storage and transport. However, producing NH3 under mild conditions is a long-term, arduous task. Atomic cluster catalysts (ACCs) have been shown to be effective for catalytic N2-to-NH3 conversion, opening the door to the development of efficient catalysts under mild conditions. Still, ACC formation with thermally stable catalytic sites remains a challenge because of their high surface free energy. Herein, we report anchoring Ba and/or Ce onto Ru ACCs (2 wt% Ru atomic clusters supported on N-doped carbon) to form so-called clusters–metal oxide promoters electronic interaction (CMEI) to stabilize the Ru atomic clusters. The resulting Ba/Ce/Ru ACCs significantly boost the NH3 synthesis rate to 56.2 mmolNH3 gcat−1 h−1 at 400 °C and 1 MPa, which is 7.5-fold higher than that of Ru ACC. The strengthened CMEI between the Ba/Ce and Ru atomic clusters across the Ba/Ce/Ru ACC enables electron transfer from Ba and/or Ce to Ru atomic clusters. As such, the electron-enriched Ru atom could facilitate electron transfer to N≡N bond π* orbitals, which would weaken the N≡N bond and drive the eventual conversion of N2 to NH3. This study offers insight into the role of CMEI in Ru ACCs and provides an effective approach for designing stable atomic cluster catalysts for NH3 synthesis.

Boosting electrocatalytic nitrogen fixation on Ni-P codoped MoS2 nanosheets

Electrocatalytic nitrogen fixation in an aqueous solution is considered a promising approach to ammonia synthesis. Still, this process has low Faraday efficiency and ammonia yield, so it is necessary to develop an efficient, clean, and safe catalyst to promote nitrogen reduction process. Doping can be easily carried out during the crystal growth owing to the weak interaction between the sandwiched layers (due to van der Waals forces) of MoS2. Here, a Ni-P codoped MoS2 nanosheets is designed as a highly efficient catalyst for electrochemical nitrogen fixation with superior selectivity. Especially, the doping of Ni achieved the transformation of MoS2 from 2 H phase to 1 T phase, improving the conductivity of the material. In addition, the doping of P creates more sulfur vacancies in the catalyst, thereby producing more H*, which promotes the adsorption and activation of nitrogen. The as-synthesized Ni-P codoped MoS2 nanosheets show excellent catalytic performance with a high yield of electrocatalytic ammonia synthesis (84.29 μgNH3 h−1 mgcat−1) and Faraday efficiency (2.39 %) at −0.5 V versus reversible hydrogen electrode in acidic electrolytes (0.005 M H2SO4), which outperformed most electrocatalytic ammonia synthesis catalysts. Furthermore, the Ni-P codoped MoS2 catalyst also shows outstanding electrochemical stability and durability. This work may offer a hopeful lead for designing effective non-noble-metal NRR electrocatalysts.

Progress in Green Ammonia Synthesis Technology: Catalytic Behavior of Ammonia Synthesis Catalysts

AbstractAmmonia as a green energy source has attracted a lot of attention in recent years. Despite its industrial intensity, the Haber‐Bosch process remains a primary ammonia source, emitting significant CO2 (≈2.9 tons per ton of ammonia). Future ammonia synthesis methods aim to surpass the Haber‐Bosch process by operating under milder conditions. These methods encompass chemical looping, thermal catalysis, electrochemical catalysis, photocatalysis, and plasma catalysis, albeit with inherent limitations. Although thermal catalysis has reduced conditions to ≈5 MPa, innovative catalysts are still scarce. Electrochemical catalysis produces hydrogen via water electrolysis but encounters challenges in Faraday efficiency and ammonia yield. Photocatalytic synthesis, while energy‐efficient, suffers from sluggish reaction rates. Plasma synthesis, while achieving low temperatures and pressures, faces difficulties in ammonia yield amidst competitive reactions. Chemical looping synthesis, enabling independent nitrogen fixation and hydrogenation, lacks efficient nitrogen transport catalysts. Effective catalysts are a common requirement across these methods. This review explores recent advancements, elucidating reaction mechanisms, nitrogen activation, and catalyst performance, while discussing the strengths, weaknesses, and future prospects of ammonia synthesis technologies to foster further innovation in the field.

Electron transfer from yttrium hydride to Mo-carbonitride boosts low-temperature ammonia synthesis

Ammonia is pivotal to various chemical industries, which is produced using the Haber-Bosch process. This process requires harsh conditions (e.g., 20.0 MPa and (500–600)°C) to generate ammonia and thus, is an energy intensive process. The growing consensus is to mild the conditions for ammonia generation. Herein, we demonstrate the use of hydride to increase the activity of molybdenum carbonitride (MCN). YH@MCN (95:5, w/w) generates ammonia at a rate of 448μmolg−1h−1 at 255 °C and 0.1 MPa. To address the stability issue of hydride, we incorporate iron. The experiment demonstrates two-fold rates for such catalyst, comparing MCN alone. The activation energy (19.73kJmol-1) for YFeH@MCN is considerably low, owing to the electron transfer from YH to MCN that facilitates nitrogen dissociation. Hence, the use of distinct sites for easy dissociation of nitrogen molecules, and the synergy between them, results in an activity that exceeds of conventional molybdenum and iron-based catalysts, and is comparable to ruthenium-based catalysts. Our results illustrate the potential of using synergistic multiple active sites in catalysts, and introduce a design concept for ammonia synthesis catalysts, using readily available elements.

Spin-mediated promotion of Co catalysts for ammonia synthesis.

Over the past two decades, there has been growing interest in developing catalysts to enable Haber-Bosch ammonia synthesis under milder conditions than currently pertain. Rational catalyst design requires theoretical guidance and clear mechanistic understanding. Recently, a spin-mediated promotion mechanism was proposed to activate traditionally unreactive magnetic materials such as cobalt (Co) for ammonia synthesis by introducing hetero metal atoms bound to the active site of the catalyst surface. We combined theory and experiment to validate this promotion mechanism on a lanthanum (La)/Co system. By conducting model catalyst studies on Co single crystals and mass-selected Co nanoparticles at ambient pressure, we identified the active site for ammonia synthesis as the B5 site of Co steps with La adsorption. The turnover frequency of 0.47 ± 0.03 per second achieved on the La/Co system at 350°C and 1 bar surpasses those of other model catalysts tested under identical conditions.

Advancements in Electrocatalytic Nitrogen Reduction: A Comprehensive Review of Single-Atom Catalysts for Sustainable Ammonia Synthesis.

Electrocatalytic nitrogen reduction technology seamlessly aligns with the principles of environmentally friendly chemical production. In this paper, a comprehensive review of recent advancements in electrocatalytic NH3 synthesis utilizing single-atom catalysts (SACs) is offered. Into the research and applications of three categories of SACs: noble metals (Ru, Au, Rh, Ag), transition metals (Fe, Mo, Cr, Co, Sn, Y, Nb), and nonmetallic catalysts (B) in the context of electrocatalytic ammonia synthesis is delved. In-depth insights into the material preparation methods, single-atom coordination patterns, and the characteristics of the nitrogen reduction reaction (NRR) are provided. The systematic comparison of the nitrogen reduction capabilities of various SAC types offers a comprehensive research framework for their integration into electrocatalytic NRR. Additionally, the challenges, potential solutions, and future prospects of incorporating SACs into electrocatalytic nitrogen reduction endeavors are discussed.

Vacancy enhanced proton preference in bimetallic phosphide catalysts for electrochemical ammonia synthesis and energy supply in Zn-NO3− battery

Electrochemical ammonia synthesis via nitrate reduction reaction (NO3RR) under ambient condition is increasingly recognized as a sustainable alternative to the conventional Haber-Bosch process. Transition metal phosphides have attracted extensive attention as promising electrocatalysts owing to their unique proton provision. However, a challenge persists as the protons tend to dimerize to form H2 through the hydrogen evolution reaction (HER). In this work, we manipulate the proton preference in a Ni-Co bimetallic phosphide through vacancy engineering, leading to a notable enhancement in NH3 production. The developed NiCoP-VP with electron-rich surface demonstrates high selectivity, faradaic efficiency, and yield rate for ammonia, achieving a maximum yield rate of 16.63 mgNH3 h−1 cm−2 (ca. 83.17 mgNH3 h−1 mgcat.−1) at − 0.7 V vs. RHE. Density functional theory calculation reveal an increased energy barrier for proton dimerization and a more favorable rate-determining protonation step of *NO in NO3RR. Furthermore, a Zn-NO3− battery system was constructed based on NiCoP-VP, which showed dual functionality of generating electricity and producing NH3. With an open circuit voltage of 1.39 V and a peak-power density of 1.14 mW cm−2, the battery system presents a promising application for sustainable energy supply and green NH3 synthesis.

Potassium promoter regulates electronic structure and hydrogen spillover of ultrasmall Ru nanoclusters catalyst for ammonia synthesis

The effects of promoters on Ru nanoparticles (≥2 nm) catalysts for NH3 synthesis have been extensively elaborated, but their roles on ultrasmall Ru nanoclusters (NCs, 1–2 nm) remain largely unknown and need to be further uncovered. Herein, a series of K-promoted MgO supported Ru NCs were synthesized and investigated for NH3 synthesis. The addition of 5wt.%K onto Ru/MgO NCs leads to a significantly high NH3 synthesis rate of 21.7 mmolNH3 gcat−1h−1 at 400 °C and 0.2 MPa, close to the thermodynamic equilibrium. Out studies reveal that anchoring K onto Ru NCs can increase the electron density and cause an upshift of the d-band center of Ru entities. Moreover, the addition of K regulates the hydrogen affinity and accelerates the migration of hydrogen from the Ru NCs surface to MgO support, which is crucial in avoiding the hydrogen poisoning effect on Ru NCs. With the synergistic effect of the Ru NCs sites bridged by H-spillover, makes the K-mediated Ru/MgO NCs catalysts efficient for NH3 synthesis at mild conditions.

Unraveling the deactivation mechanism of Co-LiH composite catalyst for ammonia synthesis at milder conditions

An efficient ammonia synthesis catalyst that operates under mild conditions has been a long-cherished goal in heterogeneous catalysis. The challenges associated with low-temperature ammonia synthesis arise from both the activation of inert N2 molecules and the presence of scaling relations between reactant dissociation and intermediate adsorption energies. Transition metals-metal hydride (TM-MH) composite catalysts have shown promise to overcome these challenges as both TM and MH actively participate in the reaction, enabling improved ammonia synthesis rates at lower temperatures (150 ºC). Considering this prospect, two types of Co-LiH catalyst were synthesized by ball-milling of precursors (CoCl2 and LiH- denoted as Co-LiH(rxn)) and pure components (Co and LiH- denoted as Co-LiH(pur)), and their catalytic activity was evaluated at several reaction conditions to investigate the stability for long-term operation. Co-LiH(rxn) was found to deactivate in less than 150 h whereas Co-LiH(pur) displayed a continuous production although the synthesis rate was 3 times lower than the former. Structural characterization techniques, such as XRD, SEM, and TEM were used, and it was found that Co metal in the FCC phase was primarily sintered due to its small particle size in the fresh catalyst. Temperature-programmed desorption (TPD) test on the spent Co-LiH(rxn) catalyst also displayed an ammonia release profile similar to that of LiNH2 decomposition, which confirms the presence of ammonia in the spent catalyst structure. Hence, another major finding is speculated that irreversible ammonia absorption by LiH at the target ammonia reaction conditions contributes to the deactivation of the catalyst and the formation of intermediate compounds that are required for continuous ammonia generation in this composite catalyst.

Platinum group nanoparticles doped BCN matrix: Efficient catalysts for the electrocatalytic reduction of nitrate to ammonia

The effective treatment of nitrate (NO3−) in water as a nitrogen source and electrocatalytic NO3− reduction to ammonia (NH3) (NRA) have become preferred methods for NO3−-to-NH3 conversion. Achieving efficient NO3−-to-NH3 conversion requires the design and development of electrode materials with high activity and efficiency for the electrocatalytic NRA reaction. Herein, based on the special properties of dodecahydro-closo-dodecaborate anions, a BCN matrix, loaded with platinum-group nanoparticles (namely, Pd/BCN, Pt/BCN, and Ru/BCN), was prepared using a simple method for the electrocatalytic NRA reaction. Results showed that Pd/BCN exerts the best catalytic effect on the NRA reaction. The NH3 production rate reached 12.71 mg h−1 mgcat.−1 at −1.0 V vs. RHE. Faraday efficiency reached 91.79 %, which can be attributed to the more uniform distribution of the nanoparticles. Furthermore, Pd/BCN exhibited high cycling stability and resistance to ionic interference. Moreover, the density functional theory calculations indicated that small and well-distributed Pd nanoclusters in the BCN matrix have a large active surface area and promote the catalytic process. This study provides a new strategy to design catalysts for green ammonia synthesis.

Mechanocatalytic Synthesis of Ammonia: State of the Catalyst During Reaction and Deactivation Pathway.

Ammonia synthesis holds significant importance for both agricultural fertilizer production and emerging green energy applications. Here, we present a comprehensive characterization of a catalyst for mechanochemical ammonia synthesis, based on Cs-promoted Fe. The study sheds light on the catalyst's dynamic evolution under reaction conditions and the origin of deactivation. Initially, elemental Cs converts to CsH, followed by partial CsOH formation due to trace oxygen impurities on the surface of the Fe metal and the equipment. Concurrently, the mechanical milling process comminutes Fe, exposing fresh metallic Fe surfaces. This comminution correlates with an induction period observed during ammonia formation. Critical to the study, degradation of active Cs promoter species (CsH and CsNH2) into inactive CsOH emerged as the primary deactivation mechanism. By increasing the Cs content from 2.2 mol % to 4.2 mol %, we achieved stable, continuous ammonia synthesis for nearly 90 hours, showcasing one of the longest-running mechanocatalytic gas phase reactions. Studies of the temperature dependence of the reaction revealed negligible bulk temperature influence in the range of -10 °C to 100 °C, highlighting the dominance of mechanical action over bulk thermal effects. This study offers insights into the complex interplay between mechanical processing, reactive species, and deactivation mechanisms in mechanocatalytic ammonia synthesis.

Mechanocatalytic Synthesis of Ammonia: State of the Catalyst During Reaction and Deactivation Pathway

AbstractAmmonia synthesis holds significant importance for both agricultural fertilizer production and emerging green energy applications. Here, we present a comprehensive characterization of a catalyst for mechanochemical ammonia synthesis, based on Cs‐promoted Fe. The study sheds light on the catalyst‘s dynamic evolution under reaction conditions and the origin of deactivation. Initially, elemental Cs converts to CsH, followed by partial CsOH formation due to trace oxygen impurities on the surface of the Fe metal and the equipment. Concurrently, the mechanical milling process comminutes Fe, exposing fresh metallic Fe surfaces. This comminution correlates with an induction period observed during ammonia formation. Critical to the study, degradation of active Cs promoter species (CsH and CsNH2) into inactive CsOH emerged as the primary deactivation mechanism. By increasing the Cs content from 2.2 mol % to 4.2 mol %, we achieved stable, continuous ammonia synthesis for nearly 90 hours, showcasing one of the longest‐running mechanocatalytic gas phase reactions. Studies of the temperature dependence of the reaction revealed negligible bulk temperature influence in the range of −10 °C to 100 °C, highlighting the dominance of mechanical action over bulk thermal effects. This study offers insights into the complex interplay between mechanical processing, reactive species, and deactivation mechanisms in mechanocatalytic ammonia synthesis.

Single-atom Ti doping on S-vacancy two-dimensional CrS2 as a catalyst for ammonia synthesis: A DFT study

Single-atom Ti doping on S-vacancy two-dimensional CrS2 as a catalyst for ammonia synthesis: A DFT study

Thermodynamics of Iron Ammonia Synthesis Catalyst Sintering

The sintering of iron ammonia synthesis catalysts (nanocrystalline iron promoted with: Al2O3, CaO and K2O) was studied under a hydrogen atmosphere, in a temperature range of 773 to 973 K to obtain stationary states. The catalysts were characterized by measuring the nitriding reaction rate under an ammonia atmosphere at 748 K to obtain steady states and the measurement of specific surface area. Chemical processes were conducted in a tubular differential reactor enabling thermogravimetric measurements and the chemical composition analysis of a gas phase under conditions allowing experiments to be carried out in the kinetic region of chemical reactions. An extended model of the active surface of the iron ammonia synthesis catalyst was presented, taking into account the influence of the gas phase composition and process temperature. The surface of iron nanocrystallites was wetted using promoters in an exothermic process associated with the formation of the surface Fes-O- bond and the change in the surface energy of iron nanocrystallites. Promoters formed on the surface of iron nanocrystallites with different structures of chemisorbed dipoles, depending on the composition of the gas phase. The occupied sites stabilized the structure, and the free sites were active sites in the process of adsorption of chemical reagents and in sintering. Based on the bonding energy of the promoter oxides and the difference in surface energy between the covered and uncovered surfaces, the wetting abilities of promoters, which can be arranged according to the order K2O > Fe3O4 > Al2O3 > CaO, were estimated. By increasing the temperature in the endothermic sintering process, the degree of surface coverage with dipoles of promoters decreased, and thus the catalyst underwent sintering. The size distribution of nanocrystallites did not change with decreasing temperature. Only the equilibrium between the glass phase and the surface of iron nanocrystallites was then established.

Experimental discovery of novel ammonia synthesis catalysts via active learning

Active learning based on literature and experimental data enabled the discovery of highly active novel catalysts for ammonia synthesis. Pathway analysis implies that these activities have been achieved by both structural and electronic promotion.

In situ exsolved CoFe alloys over perovskite toward enhanced ammonia synthesis

This study via the synthesis of in situ exsolved CoFe alloy NPs for use as NRR catalysts for electrochemical synthesis of ammonia. Good results have been obtained.

Towards green and efficient chemical looping ammonia synthesis: design principles and advanced redox catalysts

This review provides insights into the design and application of redox catalysts in chemical looping ammonia synthesis.

Single-atom and cluster catalysts for thermocatalytic ammonia synthesis at mild conditions.

Ammonia (NH3) is closely related to the fields of food and energy that humans depend on. The exploitation of advanced catalysts for NH3 synthesis has been a research hotspot for more than one hundred years. Previous studies have shown that the Ru B5 sites (step sites on the Ru (0001) surface uniquely arranged with five Ru atoms) and Fe C7 sites (iron atoms with seven nearest neighbors) over nanoparticle catalysts are highly reactive for N2-to-NH3 conversion. In recent years, single-atom and cluster catalysts, where the B5 sites and C7 sites are absent, have emerged as promising catalysts for efficient NH3 synthesis. In this review, we focus on the recent advances in single-atom and cluster catalysts, including single-atom catalysts (SACs), single-cluster catalysts (SCCs), and bimetallic-cluster catalysts (BCCs), for thermocatalytic NH3 synthesis at mild conditions. In addition, we discussed and summarized the unique structural properties and reaction performance as well as reaction mechanisms over single-atom and cluster catalysts in comparison with traditional nanoparticle catalysts. Finally, the challenges and prospects in the rational design of efficient single-atom and cluster catalysts for NH3 synthesis were provided.