Ammonia Synthesis Activity Research Articles

Catalytic Ammonia Synthesis by Supported Molybdenum Nitride: Insight into the Support Effect

AbstractThe influence of the support on the performance of Mo nitrides has been investigated in ammonia synthesis and decomposition. A series of Mo–N catalysts supported on different materials, namely SiO2 (commercial, SBA‐15), Al2O3, and CeO2, were prepared. The results indicated that, despite the high dispersion of Mo species in all catalysts, large disparities in the activity for ammonia synthesis exist. Initial rates of ∼1208, ∼481, and ∼372 µmol gactive phase−1 h−1 are obtained over 10‐Mo–N/SBA‐15, 10‐Mo–N/SiO2, and 10‐Mo–N/Al2O3 respectively. However, no catalytic activity was registered when Mo species were supported on CeO2. Furthermore, 10‐Mo–N/Al2O3 deactivated after few hours of reaction. The surface composition was studied by means of XPS to probe the origin of the catalytic activity differences, and the results indicated that a range of various oxidation states of Mo was detected depending on the support. The difference in catalytic behavior could not be solely explained by the differences in Mo–N species concentrations. In situ EPR analysis exhibited that the mechanism of MoO3 nitridation could differ depending on the support, leading to the formation of different Mo–N species. The effect of support was, however, not as severe in ammonia decomposition as it was the case of ammonia synthesis.

Structures and ammonia synthesis activity of hexagonal ruthenium iron nitride phases

A series of ruthenium iron nitride phases with Ru:Fe ratios of ca. 1:3 were synthesized by ammonolysis. When the ammonolysis temperature was above 500°C, the obtained RuxFe3Ny materials had a ε-Fe3N (P6322) structure, while two similar phases were present when the ammonolysis was lower than 500°C. Powder neutron diffraction identified one phase as relating to the ε-Fe3N structure, while the other had a disordered NiAs-type (P63/mmc) structure. These ternary metal nitrides show ammonia synthesis activity at low temperature (200°C-300°C) and ambient pressure, which can be related to the loss of lattice nitrogen. Steady state catalytic performance at 400°C is associated with ruthenium-iron alloy. Additionally, density functional theory calculations were performed using an approximate model for the disordered hexagonal phase, revealing that this phase is more stable than a cubic anti-perovskite phase which has been previously investigated computationally, and corroborating the experimental findings of the present work.

Unlocking the Potential of MXene in Catalysis: Decorated Mo2CTx Catalyst for Ammonia Synthesis under Mild Conditions.

Ammonia, which is one of the most important chemicals for the synthesis of dyes, pharmaceuticals, and fertilizers, is produced by the reaction of molecular hydrogen with nitrogen, over an iron-based catalyst at 400-500 °C under pressure of over 100 bar. Decreasing the operating temperature and pressure of this highly energy-intensive process, developed by Haber and Bosch over 100 years ago, would decrease energy consumption in the world. In this work, we used two-dimensional Mo2CTx MXene as a support for a cobalt-based catalyst. The MXene functionalized by Co showed catalytic activity for ammonia synthesis from H2 and N2 at temperatures as low as 250 °C, without any pretreatment. The developed catalyst was highly active for ammonia synthesis, demonstrating a high rate of up to 9500 μmol g-1activephase h-1 at 400 °C under ambient pressure in steady-state conditions, and did not suffer from any deactivation after 15 days of reaction. The apparent activation energy (Ea) was found to be in the range of 68-74 kJ mol-1, which is in line with values reported for highly active catalysts. This improved catalyst may decrease the energy consumption in the synthesis of ammonia and its derivatives, as well as facilitate the use of ammonia as a hydrogen carrier for renewable energy storage.

Modulating the Electrolyte Microenvironment in Electrical Double Layer for Boosting Electrocatalytic Nitrate Reduction to Ammonia.

Electrochemical nitrate reduction reaction (NO3RR) is a promising approach to achieve remediation of nitrate-polluted wastewater and sustainable production of ammonia. However, it is still restricted by the low activity, selectivity and Faraday efficiency for ammonia synthesis. Herein, we propose an effective strategy to modulate the electrolyte microenvironment in electrical double layer (EDL) by mediating alkali metal cations in the electrolyte to enhance the NO3RR performance. Taking bulk Cu as a model catalyst, the experimental study reveals that the NO3 --to-NH3 performance in different electrolytes follows the trend Li+<Cs+<Na+<K+. Theoretical studies illustrate that the proton transport rate in NO3RR and the activity of the rate-determining step (NO3 - to NO2 -) increase in the order Li+<Cs+<Na+<K+. The cation effects are also general for two typical nanostructured catalysts including copper/cuprous oxide and nickel phosphides, achieving near-100 % Faradaic efficiency and over 99 % conversion of NO3 - to NH3. Furthermore, we demonstrate that NO3 - can be converted to high-purity NH4Cl by copper/cuprous oxide catalyst in K+-containing electrolyte.

Modulating the Electrolyte Microenvironment in Electrical Double Layer for Boosting Electrocatalytic Nitrate Reduction to Ammonia

AbstractElectrochemical nitrate reduction reaction (NO3RR) is a promising approach to achieve remediation of nitrate‐polluted wastewater and sustainable production of ammonia. However, it is still restricted by the low activity, selectivity and Faraday efficiency for ammonia synthesis. Herein, we propose an effective strategy to modulate the electrolyte microenvironment in electrical double layer (EDL) by mediating alkali metal cations in the electrolyte to enhance the NO3RR performance. Taking bulk Cu as a model catalyst, the experimental study reveals that the NO3−‐to‐NH3 performance in different electrolytes follows the trend Li+&lt;Cs+&lt;Na+&lt;K+. Theoretical studies illustrate that the proton transport rate in NO3RR and the activity of the rate‐determining step (NO3− to NO2−) increase in the order Li+&lt;Cs+&lt;Na+&lt;K+. The cation effects are also general for two typical nanostructured catalysts including copper/cuprous oxide and nickel phosphides, achieving near‐100 % Faradaic efficiency and over 99 % conversion of NO3− to NH3. Furthermore, we demonstrate that NO3− can be converted to high‐purity NH4Cl by copper/cuprous oxide catalyst in K+‐containing electrolyte.

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.

Synthesis of Cobalt Molybdenum Nitrides From Precursors Precipitated at Different pH

AbstractCobalt molybdenum nitrides can be obtained from the oxidic precursor by reaction with gaseous ammonia. Despite the large number of works regarding the catalytical properties of these compounds, there are relatively few reports about the initial transformation from oxide to nitride. In this work, the influence of the precursor structure on the active phase formation and catalytic properties of cobalt molybdenum nitrides is examined. Oxidic precursors were obtained by precipitation at different pH values controlled by the addition of aqueous ammonia. The parameters of the precipitation process affected the composition and properties of the materials. The ammonolysis of NH4Co2OH(MoO4)2 ⋅ H2O was investigated in situ with the use of X‐ray powder diffraction and thermogravimetry. The ammonia synthesis activity of the obtained catalysts was compared to the reference iron catalyst. The formation of the Co3Mo intermetallic phase was suggested as the factor governing high activity.

Engineering the Metal‐Support Interaction and Oxygen Vacancies on Ru@P‐Fe/Fe3O4 Nanorods for Synergetic Enhanced Electrocatalytic Nitrate‐to‐Ammonia Conversion

AbstractRuthenium (Ru) loaded catalysts show high activity and selectivity for ammonia (NH3) synthesis via electrochemical reduction of nitrate (NO3−), but their practical application is still restricted by their high cost and insufficient stability. Herein, a multi‐component electrocatalyst of Ru nanoclusters loaded on phosphorus‐doped/phosphate‐modified and oxygen vacancy (OV)‐rich Fe/Fe3O4 composite nanorods (Ru@P‐Fe/Fe3O4) to synergistically promote electrocatalytic NO3− reduction reaction (NO3−RR)‐to‐NH3 performance via strong metal‐support interaction (SMSI) is reported. Impressively, the best Ru@P‐Fe/Fe3O4 catalyst exhibits outstanding NO3−RR activity, selectivity, and durability in 0.1 M KNO3 + 0.5 M KOH solution, with an NH3 yield rate of 14.37 ± 0.21 mgNH3 h−1 cm−2 (1710.71 ± 25 mgNH3 h−1 mgRu−1) at −0.75 V versus reversible hydrogen electrode (vs. RHE), an NH3 Faradaic efficiency (FE) of 97.2% at −0.55 V vs. RHE, and a superior stability over 50 h, suppressing most of reported Fe‐based and Ru‐based electrocatalysts. The characterizations and theoretical calculations unveil that the SMSI between Ru nanoclusters and P‐Fe/Fe3O4 composite nanorods can promote the generation of OV, tune the electronic structure of Ru species, and stabilize Ru nanoclusters, thereby reducing the reaction energy barrier of NO3−RR‐to‐NH3, inhibiting the competitive hydrogen evolution reaction, and boosting the NH3 yield rate NH3 FE, and stability.

Effects of low crystallinity cerium oxide on ammonia synthesis activity for cerium oxide supported ruthenium catalyst

AbstractRuthenium (Ru) catalysts supported on cerium oxide (CeO2), which was composed of high crystallinity (HC) CeO2 covered by low crystallinity (LC) CeO2 were investigated. The ammonia synthesis activities of a series of Ru/CeO2(LC)/CeO2(HC), having different LC/HC ratios were evaluated, and various characterizations, such as N2 adsorption, X‐ray diffraction, hydrogen temperature‐programmed reduction (H2‐TPR), and hydrogen temperature‐programmed desorption (H2‐TPD), were performed. Ru/CeO2(LC)/CeO2(HC) (0.1) demonstrated higher activity. H2‐TPR revealed that H2 consumption increased as the LC/HC ratio increased. This indicates that the larger the amount of low crystallinity CeO2, the higher the reducibility. However, the results of H2‐TPD imply that the larger the amount of low crystallinity CeO2, the higher the interaction between adsorbed H2 and the catalyst surface. These results indicate that the balance of reducibility and hydrogen adsorption strength plays an important role in increasing the ammonia synthesis activity.

Transient heating synthesis of a highly ordered Ga–Cu intermetallic antiperovskite for efficient ammonia electrosynthesis and ultrastable zinc–nitrate fuel cells

Strong p–d intermetallic hybridization was performed to construct an ordered antiperovskite via transient heating strategy, exhibiting high and stable electrochemical activity in both ammonia synthesis and zinc–nitrate battery.

Regulating oxygen vacancies by Zn atom doping to anchor and disperse promoter Ba on MgO support to improve Ru-based catalysts activity for ammonia synthesis.

In heterogeneous catalysis, surface defects are widely regarded as an effective means to enhance the catalytic performance of catalysts. In this study, the oxygen vacancy-rich Mg(1-X)ZnXO solid solution support was successfully prepared by doping a small amount of Zn into MgO nanocrystals. Based on this support, Ru/Ba-Mg(1-X)ZnXO catalyst for ammonia synthesis was prepared. Characterization using TEM, EPR, XPS, and DFT calculations confirmed the successful substitution of Zn atoms for Mg atoms leading to the formation of more oxygen vacancies (OVs). N2-TPD, SEM and TEM analyses revealed that a small amount of Zn had minimal influence on the surface morphology and the size of Ru nanoparticles. The abundance of OVs in the support was identified as the primary factor enhancing the catalytic activity. XPS, H2-TPD and kinetics experiment studies further elucidated the mechanism by which OVs promote the reaction, with OVs serving as an anchor point for the promoter Ba on the MgO support and promoted the dispersion of Ba. This anchoring effect not only enhanced the electron density on Ru, favoring the dissociation of the N[triple bond, length as m-dash]N bond, but also mitigated hydrogen poisoning. As a result,the ammonia synthesis rate reached 1.73 mmol g-1 h-1. Furthermore, the CO2-TPD and H2-TPR analyses indicated that Zn doping effectively promotes the metal-support interaction (MSI) and surface alkalinity. The findings of this study offers valuable insights for the design of defective modified catalyst supports.

Barium-doped iron nanoparticles supported on MgO as an efficient catalyst for ammonia synthesis under mild reaction conditions

A Ba-doped Fe/MgO catalyst pre-reduced at 700 °C, much higher than that of conventional Fe catalysts, forms an Fe0-core and BaO-shell structure. This enhances ammonia synthesis activity due to electron donation from BaO to N2via Fe0.

Effect of CeO2 support structure on the catalytic performance of ammonia synthesis in an electric field at low temperatures.

Ammonia is an extremely important storage and transport medium for renewable energy, and technology is expected to produce it on demand and onsite using renewable energy. Applying a DC (direct current) to a solid catalyst layer with semiconducting properties makes ammonia synthesis highly efficient, even at low temperatures (approximately 400 K). In this process, oxide supports with semiconducting properties play important roles as metal supports and conduction fields for electrons and protons. The influence of the degree of particle aggregation on the support properties and ammonia synthesis using an electric field was evaluated for CeO2, which is the best material for this purpose because of its semiconducting properties. The results showed that controlling the aggregation structure of the crystalline particles could significantly influence the surface conductivity of protons and electrons; thus, the activity could be largely controlled. The Ru-CeO2 interaction could also be controlled by changing the crystallinity, which suppressed the aggregation of the supported Ru and significantly improved the ammonia synthesis activity using an electric field at low temperatures.

Origin of electrocatalytic nitrogen reduction activity over transition metal disulfides: critical role of in situ generation of S vacancy

We identified an interesting electrochemistry-driven “in situ” generation of S-vacancies of transition metal disulfides, which serves as an origin of promising electrochemical ammonia synthesis activity.

Au Nanoparticle-Loaded UiO-66 Metal–Organic Framework for Efficient Photocatalytic N2 Fixation

In order to achieve efficient photocatalytic N2 reduction activity for ammonia synthesis, a photochemical strategy was used in this work. UiO-66 was prepared through the solvothermal method and further loaded with Au nanoparticles (Au NPs) onto the UiO-66 (Zr) framework. The experimental results verified that there were metal–support interactions between Au NPs and UiO-66; this could facilitate charge transfer among Au NPs and UiO-66, which was beneficial to enhance the photocatalytic activity. The best N2 fixation effect of Au/UiO-66 with a loading of 1.5 wt% was tested, with a photocatalytic yield of ammonia of 66.28 μmol g−1 h−1 while maintaining good stability. The present work provides a novel approach to enhancing photocatalytic N2 fixation activity by loading NPs onto UiO-66.

Theoretical Insight into the Special Synergy of Bimetallic Site in Co/MoC Catalyst to Promote N2 -to-NH3 Conversion.

The catalytic mechanisms of nitrogen reduction reaction (NRR) on the pristine and Co/α-MoC(001) surfaces were explored by density functional theory calculations. The results show that the preferred pathway is that a direct N≡N cleavage occurs first, followed by continuous hydrogenations. The production of second NH3 molecule is identified as the rate-limiting step on both systems with kinetic barriers of 1.5 and 2.0 eV, respectively, indicating that N2 -to-NH3 transformation on bimetallic surface is more likely to occur. The two components of the bimetallic center play different roles during NRR process, in which Co atom does not directly participate in the binding of intermediates, but primarily serves as a reservoir of H atoms. This special synergy makes Co/α-MoC(001) have superior activity for ammonia synthesis. The introduction of Co not only facilitates N2 dissociation, but also accelerates the migration of H atom due to the antibonding characteristic of Co-H bond. This study offers a facile strategy for the rational design and development of efficient catalysts for ammonia synthesis and other reactions involving the hydrogenation processes.

Catalytic Behavior of K-doped Fe/MgO Catalysts for Ammonia Synthesis Under Mild Reaction Conditions.

An important part of realizing a carbon-neutral society using ammonia will be the development of an inexpensive yet efficient catalyst for ammonia synthesis under mild reaction conditions (<400 °C, <10 MPa). Here, we report Fe/K(3)/MgO, fabricated via an impregnation method, as a highly active catalyst for ammonia synthesis under mild reaction conditions (350 °C, 1.0 MPa). At the mentioned conditions, the activity of Fe/K(3)/MgO (17.5 mmol h-1 gcat -1 ) was greater than that of a commercial fused iron catalyst (8.6 mmol h-1 gcat -1 ) currently used in the Haber-Bosch process. K doping was found to increase the ratio of Fe0 on the surface and turnover frequency of Fe in our Fe/K(3)/MgO catalyst. In addition, increasing the pressure to 3.0 MPa at the same temperature led to a significant improvement of the ammonia synthesis rate to 29.6 mmol h-1 gcat -1 , which was higher than that of two more expensive, benchmark Ru-based catalysts, which are also potential alternative catalysts. A kinetics analysis revealed that the addition of K enhanced the ammonia synthesis activity at ≥300 °C by changing the main adsorbed species from NH to N which can accelerate dissociative adsorption of nitrogen as the rate limiting step in ammonia synthesis.

Barium Oxynitride Electride as Highly Enhanced Promotor for Ruthenium Catalyst in Ammonia Synthesis: Comparative Study with Barium Oxide

AbstractBarium oxide is a conventional promoter for various transition‐metal (TM) catalysts in ammonia synthesis. Ba‐promoted Ru catalysts are widely used for ammonia synthesis under mild reaction conditions. However, the promotion mechanism, including the active factor, remains unclear. In this study, the excellent promotion effect of barium oxynitride is reported, which creates anionic electrons in the lattice, forming an electride (BaOxNy:e−z) with the same rocksalt structure as BaO. The BaOxNy:e−z enhances the ammonia synthesis activity of the supported Ru catalyst by 40–100 fold compared with the BaO promotor. This study reveals that a low‐work‐function electron formed in the BaO lattice plays a pivotal role in the electronic promotion effect in catalytic ammonia synthesis, suggesting that BaOxNy:e−z formed near the surface during the ammonia synthesis reaction is an active species for the promotion effect of conventional BaO.

A Comparison of the Reactivity of the Lattice Nitrogen in Tungsten Substituted Co3 Mo3 N and Ni2 Mo3 N.

The effect of the partial substitution of Mo with W in Co3 Mo3 N and Ni2 Mo3 N on ammonia synthesis activity and lattice nitrogen reactivity has been investigated. This is of interest as the coordination environment of lattice N is changed by this process. When tungsten was introduced into the metal nitrides by substitution of Mo atoms, the catalytic performance was observed to have decreased. As expected, Co3 Mo3 N was reduced to Co6 Mo6 N under a 3 : 1 ratio of H2 /Ar. Co3 Mo2.6 W0.4 N was also shown to lose a large percentage of lattice nitrogen under these conditions. The bulk lattice nitrogen in Ni2 Mo3 N and Ni2 Mo2.8 W0.2 N was unreactive, demonstrating that substitution with tungsten does not have a significant effect on lattice N reactivity. Computational calculations reveal that the vacancy formation energy for Ni2 Mo3 N is more endothermic than Co3 Mo3 N. Furthermore, calculations suggest that the inclusion of W does not have a substantial impact on the surface N vacancy formation energy or the N2 adsorption and activation at the vacancy site.

Rapid Joule-Heating fabrication of oxygen vacancies and anchor of Ru clusters onto BiVO4 for greatly enhanced photocatalytic N2 fixation

Herein, we applied a rapid Joule heating method to form oxygen vacancies and further induced the deposition of Ru clusters onto BiVO4. The rapid Joule heating method fabricated oxygen vacancy on BiVO4, resulting in the formation of electron-rich active Ru sites and unsaturated V sites. The pristine BiVO4 was almost no activity in photocatalytic ammonia synthesis. The photocatalytic production rate of ammonia was increased to 3.88 μmol g-1h-1 after the formation of oxygen vacancies, and further increased to 16.57 μmol g-1h-1 with Ru clusters loaded on BiVO4. It was indicated that unsaturated V sites near the oxygen vacancy and Ru clusters might act as the reactive sites for N2 adsorption and activation. And the strong metal-support interactions between Ru and BiVO4 resulted in charge redistribution between Ru clusters and BiVO4 supports, thus reducing energy barrier of the rate-determining step for the N2 hydrogenation. Furthermore, DFT confirmed that associative alternating pathway was suitable for this process.