Ammonia Synthesis Activity Research Articles

Synergistic effect of coordinating interface and promoter for enhancing ammonia synthesis activity of Ru@N-C catalyst.

Triruthenium dodecacarbonyl (Ru3(CO)12) was applied to prepare the Ru-based ammonia synthesis catalysts. The catalyst obtained from this precursor exhibited higher activity than the other Ru salts owing to its unique atomic reorganization under mild temperatures. Herein, Ru3(CO)12 as a guest metal source incorporated into the pore of ZIF-8 formed the Ru@N-C catalysts. The results indicated that the Ru nanoparticle (1.7 nm) was dispersed in the confined N coordination environment, which can increase the electron density of the Ru nanoparticles to promote N[triple bond, length as m-dash]N bond cleavage. The promoters donate the basic sites for transferring the electrons to Ru nanoparticles, further enhancing ammonia synthesis activity. Ammonia synthesis investigations revealed that the obtained Ru@N-C catalysts exhibited obvious catalytic activity compared with the Ru/AC catalyst. After introducing the Ba promoter, the 2Ba-Ru@N-C(450) catalyst exhibited the highest ammonia synthesis activity among the catalysts. At 360 °C and 1 MPa, the activity of the 2Ba-Ru@N-C(450) is 16 817.3 µmol h-1 gRu-1, which is 1.1, 1.6, and 2 times higher than those of 2Cs-Ru@N-C(450) (14 925.4 µmol h-1 gRu-1), 2K-Ru@N-C(450) (10 736.7 µmol h-1 gRu-1), and Ru@N-C(450) (8604.2 µmol h-1 gRu-1), respectively. A series of characterizations were carried out to explore the 2Ba-Ru@N-C(450) catalysts, such as H2-TPR, XPS, and NH3-TPD. These results suggest that the Ba promoter played the role of an electronic and structural promoter; moreover, it can promote the NH3 desorption from the Ru nanoparticles.

Ammonia Synthesis on Ternary LaSi-based Electrides: Tuning the Catalytic Mechanism by the Third Metal.

Intermetallic electrides have recently drawn considerable attention due to their unique electronic structure and high catalytic performance for the activation of inert chemical bonds under mild conditions. However, the relationship between electride (anionic) electron abundance and catalytic performance is undefined; the key deciding factor for the performance of intermetallic electride catalysts remains to be addressed. Here, the secret behind electride catalysts La-TM-Si (TM=Co, Fe and Mn) with the same crystal structure but different anionic electrons was studied. Unexpectedly, LaCoSi with the least anionic electrons showed the best catalytic activity. The experiments and first-principles calculations showed that the electride anions promote the N2 dissociation which alters the rate-determining step (RDS) for ammonia synthesis on the studied electrides. Different reaction mechanisms were found for La-TM-Si (TM=Fe, Co) and LaMnSi. A dual-site module was revealed for LaCoSi and LaFeSi, in which transition metals were available for the N2 dissociation and La accelerates the NHx formation, respectively, breaking the Sabatier scaling relation. For LaMnSi, which is the most efficient for the N2 activation, the activity for ammonia synthesis is limited and confined by the scaling relations. The findings provide new insight into the working mechanism of intermetallic electrides.

Electrocatalytic Urea Synthesis with 63.5 % Faradaic Efficiency and 100 % N-Selectivity via One-step C-N coupling.

Electrocatalytic urea synthesis via coupling N2 and CO2 provides an effective route to mitigate energy crisis and close carbon footprint. However, the difficulty on breaking N≡N is the main reason that caused low efficiencies for both electrocatalytic NH3 and urea synthesis, which is the bottleneck restricting their industrial applications. Herein, a new mechanism to overcome the inert of the nitrogen molecule was proposed by elongating N≡N instead of breaking N≡N to realize one-step C-N coupling in the process for urea production. We constructed a Zn-Mn diatomic catalyst with axial chloride coordination, Zn-Mn sites display high tolerance to CO poisoning and the Faradaic efficiency can even be increased to 63.5 %, which is the highest value that has ever been reported. More importantly, negligible N≡N bond breakage effectively avoids the generation of ammonia as intermediates, therefore, the N-selectivity in the co-electrocatalytic system reaches100 % for urea synthesis. The previous cognition that electrocatalysts for urea synthesis must possess ammonia synthesis activity has been broken. Isotope-labelled measurements and Operando synchrotron-radiation Fourier transform infrared spectroscopy validate that activation of N-N triple bond and nitrogen fixation activity arise from the one-step C-N coupling process of CO species with adsorbed N2 molecules.

Electrocatalytic Urea Synthesis with 63.5 % Faradaic Efficiency and 100 % N‐Selectivity via One‐step C−N coupling

AbstractElectrocatalytic urea synthesis via coupling N2 and CO2 provides an effective route to mitigate energy crisis and close carbon footprint. However, the difficulty on breaking N≡N is the main reason that caused low efficiencies for both electrocatalytic NH3 and urea synthesis, which is the bottleneck restricting their industrial applications. Herein, a new mechanism to overcome the inert of the nitrogen molecule was proposed by elongating N≡N instead of breaking N≡N to realize one‐step C−N coupling in the process for urea production. We constructed a Zn−Mn diatomic catalyst with axial chloride coordination, Zn−Mn sites display high tolerance to CO poisoning and the Faradaic efficiency can even be increased to 63.5 %, which is the highest value that has ever been reported. More importantly, negligible N≡N bond breakage effectively avoids the generation of ammonia as intermediates, therefore, the N‐selectivity in the co‐electrocatalytic system reaches100 % for urea synthesis. The previous cognition that electrocatalysts for urea synthesis must possess ammonia synthesis activity has been broken. Isotope‐labelled measurements and Operando synchrotron‐radiation Fourier transform infrared spectroscopy validate that activation of N−N triple bond and nitrogen fixation activity arise from the one‐step C−N coupling process of CO species with adsorbed N2 molecules.

Introducing trace Co to molybdenum carbide catalyst for efficient ammonia synthesis

AbstractDevelopment of the cost‐effective catalysts with excellent catalytic performance is highly demanded for ammonia synthesis, and the strong adsorption of ammonia greatly hinders the design of cost‐effective and high‐performance ammonia synthesis catalysts. Herein, we report that the addition of a small amount of Co species (0.1 wt%) into Mo2C catalyst, which can provide electrons to Mo2C, not only leads to improvement of the adsorption and migration of hydrogen, but also facilitates the adsorption and desorption of ammonia. Consequently, Mo2C catalyst with 0.1 wt%Co offers a 40% higher ammonia synthesis activity and a lower negative reaction order with respect to NH3 in comparison to Mo2C. This work stresses the importance of the minor components in improving the ammonia synthesis activity by accelerating the migration of reactants over the catalyst surface and the escape of products from the catalyst.

Ammonia Synthesis via an Associative Mechanism on Alkaline Earth Metal Sites of Ca3 CrN3 H.

Typically, transition metals are considered as the centers for the activation of dinitrogen. Here we demonstrate that the nitride hydride compound Ca3 CrN3 H, with robust ammonia synthesis activity, can activate dinitrogen through active sites where calcium provides the primary coordination environment. DFT calculations also reveal that an associative mechanism is favorable, distinct from the dissociative mechanism found in traditional Ru or Fe catalysts. This work shows the potential of alkaline earth metal hydride catalysts and other related 1 D hydride/electrides for ammonia synthesis.

Splitting of Hydrogen Atoms into Proton-Electron Pairs at BaO-Ru Interfaces for Promoting Ammonia Synthesis under Mild Conditions.

Ru catalysts promoted with alkali and alkaline earth have shown superior ammonia (NH3) synthesis activities under mild conditions. Although these promoters play a vital role in enhancing catalytic activity, their function has not been clearly understood. Here, we synthesize a series of Ba-Ru/MgO catalysts with an optimal Ru particle size (∼2.3 nm) and tailored BaO-Ru interfacial structures. We discover that the promoting effect is created through the separate storage of H+/e- pairs at the BaO-Ru interface. Chemisorbed H atoms on Ru dissociate into H+/e- pairs at the BaO-Ru interface, where strongly basic, nonreducible BaO selectively captures H+ while leaving e- on Ru. The resulting electron accumulation in Ru facilitates N2 activation via enhanced π-backdonation and inhibits hydrogen poisoning during NH3 synthesis. Consequently, the formation of intimate BaO-Ru interface without an excessive loss of accessible Ru sites enables the synthesis of highly active catalysts for NH3 synthesis.

Oxygen-Induced Activation of a Ceria-Supported Ru Catalyst for Enhancing Ammonia Synthesis Activity

Oxygen-Induced Activation of a Ceria-Supported Ru Catalyst for Enhancing Ammonia Synthesis Activity

Efficient Ru/MgO–CeO2 catalyst for ammonia synthesis as a hydrogen and energy carrier

CeO 2 -supported Ru (Ru/CeO 2 ) is considered an efficient catalyst for ammonia synthesis but the use of CeO 2 (expensive rare earth oxide) increases the cost of the catalyst limiting its application at large/plant scale. The objective of this study was to develop a relatively low-cost efficient catalyst for plant scale ammonia synthesis. For this purpose, in this study, a mixed oxide of MgO–CeO 2 (Mg/Ce molar ratio = 1/1) was prepared and used as a support for the Ru catalyst. The prepared 3 wt-% Ru/MgO–CeO 2 catalyst was evaluated for ammonia synthesis efficiency and was also compared with the Ru/CeO 2 . Though the Ru/MgO–CeO 2 had a lower CeO 2 concentration than Ru/CeO 2 , it showed equivalent ammonia synthesis activity. The activation energies, and H 2 , N 2 , and NH 3 orders for Ru/MgO–CeO 2 and Ru/CeO 2 were very close to each other which confirmed the equivalent efficiency of both catalysts for ammonia synthesis. Moreover, H 2 -TPR results showed very close temperature ranges for the appearance of reduction peaks for both catalysts. Containing a lower amount of CeO 2 , Ru/MgO–CeO 2 was evaluated as an efficient and cost-effective catalyst as compared to Ru/CeO 2 . The optimization study shows that H 2 /N 2 ratio, temperature, and pressure conditions were strongly dependent on each other affecting the ammonia synthesis efficiency. The reaction conditions were optimized to 375 °C, 2.5 MPa gauge pressure using a reactant feed with H 2 /N 2 ratio 1 for Ru/MgO–CeO 2 catalyst. • Ru/MgO–CeO 2 with Mg/Ce molar ratio 1/1 is an efficient catalyst for ammonia synthesis. • Ru/MgO–CeO 2 showed equivalent efficiency to that obtained with pure CeO 2 supported Ru. • Ea and H 2 , N 2, and NH 3 orders of both Ru/MgO–CeO 2 and Ru/CeO 2 were very close. • Optimization of reaction temperature, pressure, and H 2 /N 2 ratio was conducted. • H 2 /N 2 molar ratio strongly influenced the ammonia synthesis activity.

Switching on/off molybdenum nitride catalytic activity in ammonia synthesis through modulating metal-support interaction.

Modulating the interaction between Mo nanoparticles and their support is an elegant approach to finely tune the structural, physico-chemical, redox and electronic properties of the active site. In this work, a series of molybdenum nitride catalysts supported on TiO2, and SBA-15 has been prepared and fully characterized. The results of characterization confirmed the high dispersion of Mo and the formation of small molybdenum nanoparticles in both the 10-Mo-N/SBA-15 and 10-Mo-N/TiO2 catalysts. In this context, we have shown that the catalytic activity of Mo species was strongly impacted by the nature of the catalytic support. Amongst the studied supports, SBA-15 was found to be the most appropriate for Mo dispersion. In comparison, when supported on a reducible oxide (TiO2), Mo species showed poor catalytic activity in both ammonia synthesis and decomposition and were prone to quick deactivation in the ammonia synthesis reaction. Evidence of charge transfer from the reducible support to the active phase, indicative of possible SMSI behaviour, has been observed by XPS and EPR. Differences in the oxidation states, redox behaviours, and electronic properties have been further studied by means of EPR, H2-TPR and H2-TPD.

Experimental and theoretical investigations on the anti-perovskite nitrides Co3CuN, Ni3CuN and Co3MoN for ammonia synthesis.

The ammonia synthesis activities of the anti-perovskite nitrides Co3CuN and Ni3CuN have been compared to investigate the possible metal composition-activity relationship. Post-reaction elemental analysis showed that the activity for both nitrides was due to loss of lattice nitrogen rather than a catalytic process. Co3CuN was observed to convert a higher percentage of lattice nitrogen to ammonia than Ni3CuN and was active at a lower temperature. The loss of lattice nitrogen was revealed to be topotactic and Co3Cu and Ni3Cu were formed during the reaction. Therefore, the anti-perovskite nitrides may be of interest as reagents for the formation of ammonia through chemical looping. The regeneration of the nitrides was achieved by ammonolysis of the corresponding metal alloys. However, regeneration using N2 was shown to be challenging. In order to understand the difference in reactivity between the two nitrides, DFT techniques were applied to investigate the thermodynamics of the processes involved in the evolution of lattice nitrogen to the gas phase via conversion to N2 or NH3, revealing key differences in the energetics of bulk conversion of the anti-perovskite to the alloy phase, and in loss of surface N from the stable low-index N-terminated (111) and (100) facets. Computational modelling of the density of states (DOS) at the Fermi level was performed. It was shown that the Ni and Co d states contributed to the density of states and that the Cu d states only contributed to the DOS for Co3CuN. The anti-perovskite Co3MoN has been investigated as comparisons with Co3Mo3N may give an insight into the role structure type plays in the ammonia synthesis activity. The XRD pattern and elemental analysis for the synthesised material revealed that an amorphous phase was present that contained nitrogen. In contrast to Co3CuN and Ni3CuN, the material was shown to have steady state activity at 400 °C with a rate of 92 ± 15 μmol h-1 g-1. Therefore, it appears that metal composition has an influence on the stability and activity of the anti-perovskite nitrides.

Amorphous nickel–iron hydroxide nanosheets for effective electroreduction of nitrate to ammonia

Amorphous NiFe nanosheets synthesized through a simple one-step electrodeposition approach showed excellent activity and stability for selective ammonia synthesis by nitrate electroreduction.

Low temperature ammonia synthesis by surface protonics over metal supported catalysts.

Low-temperature ammonia synthesis by applying an electric field to a solid heterogeneous catalyst was investigated to realize an on-demand, on-site catalytic process for converting distributed renewable energy into ammonia. By applying an electric field to the catalyst, even at low temperatures, the reaction proceeds efficiently by an "associative mechanism" in which proton-conducting species on the support surface promote the formation of N2Had intermediates through surface protonics. Kinetics, isotope exchange, infrared spectroscopy, X-ray spectroscopy, and AC impedance analysis were performed to clarify the effect of metal and catalyst support structure on the reaction, and an evaluation method for the surface protonics of the support was established to analyze the reaction mechanism, and further analysis using computational chemistry was also conducted. The elementary step determining catalytic activity changed from N2 dissociation to N2H formation, and this difference resulted in high activity for ammonia synthesis at low temperatures even when using base metal catalysts such as Fe and Ni.

Li-intercalated CeO2 as an ideal substrate for boosting ammonia synthesis.

We present a Li-intercalated-CeO2 catalyst that exhibits outstanding activity for ammonia synthesis. The incorporation of Li significantly reduces the activation energy and suppresses hydrogen poisoning of the Ru co-catalysts. As a result, the lithium intercalation enables the catalyst to achieve ammonia production from N2 and H2 at substantially lower operating temperatures.

Co nanoparticles supported on mixed magnesium-lanthanum oxides: effect of calcium and barium addition on ammonia synthesis catalyst performance.

The synthesis of ammonia in the Haber-Bosch process produces millions of tons of ammonia annually needed for producing fertilisers required to feed the growing population. Although this process has been optimised extensively, it still accounts for about 2% of global energy consumption. It is, therefore, desirable to develop an efficient ammonia synthesis catalyst. Over the last decades, many attempts have been made to improve the ammonia synthesis catalyst efficiency under mild conditions. Here, we studied the effect of adding Ca and Ba to the cobalt ammonia synthesis catalyst. The combination of the different experimental results allows concluding that Ca served as an inactive additive, whereas Ba served as an electronic promoter. The Ca addition did not change the textural, structural, and chemisorptive properties of the Ca-doped Co catalyst. On the other hand, the Ba addition had a major effect on the nature of active Co sites. It contributed to the formation of new active sites for hydrogen and nitrogen adsorption and dissociation. Barium addition also contributed to the generation of new basic sites, particularly the strong ones. These unique characteristics were ascribed to the formation of Co(core)-BaO(shell) structures. It is likely that the donation of electrons from BaO to N2 via Co markedly promoted ammonia synthesis. This catalyst exhibited ammonia synthesis activity 4 times higher than that of the undoped Co catalyst and 2 times higher than that of the industrial Fe catalysts under identical conditions.

Boosting the ammonia synthesis activity of ceria-supported Ru catalysts achieved through trace Pr addition.

The amount of dopant used in conventional cases for improving catalytic performance is higher than 5%. In this work, a strategy to enhance the ammonia synthesis performance of a Ru/CeO2 catalyst by using trace Pr (0.1 mol%) is reported. Owing to the improvement of oxygen defects, Ce3+ concentration and interfaced Ru species, the hydrogen adsorption was enhanced, and the desorption of hydrogen species would be promoted. As a result, Ru/CeO2 with 0.1 mol% Pr shows 1.4 times higher ammonia synthesis rate and excellent stability compared to Ru/CeO2 or the sample with high Pr loading (50 mol% Pr). This study provides a new idea for the design of high-efficiency ammonia synthesis catalysts.

Enhancing ammonia catalytic production over spatially confined cobalt molybdenum nitride nanoparticles in SBA-15

Ternary Co3Mo3N nitrides are reported to exhibit high catalytic activity in ammonia synthesis. However, synthesis of ternary nitrides requires thermal treatments at elevated temperatures and reactive atmospheres that lead to unavoidable surface reduction (∼ 10 m2 g-1). In this work, we have developed a novel approach to improve the catalytic activity of Co3Mo3N through its dispersion into a high surface area silica-based support (SBA-15). During ammonolysis and ammonia synthesis conditions reaction, SBA-15 demonstrated good thermal and chemical stability maintaining an ordered porous structure and high surface area (> 500 m2 g-1). For application in ammonia synthesis, SBA-15 supported cobalt molybdenum catalysts with different metal loading (10, 20 and 30 wt%) were prepared by a modified impregnation-infiltration protocol and their catalytic activity studied. The dispersion of CoMo nitride nanoparticles into SBA-15 structures resulted in the improvement of their structural and textural properties of nitrides as evidenced by XRD analysis, STEM-EDS, and N2- physisorption (e.g. 10-CoMo-N/SBA-15: 348 m2 g-1). Nevertheless, the surface composition of CoMo-N/SBA-15 catalysts was found to be similar to the non-supported Co3Mo3N. Furthermore, supported CoMo-N/SBA-15 displayed enhanced catalytic activity in ammonia synthesis (1714, 1429 and 810 µmol gactivephase-1 h-1 corresponding to the CoMo oxide loadings of 10, 20, 30 wt% respectively) that outperform the classical Co3Mo3N catalyst (259 µmol gcatalyst-1 h-1). The results reported in this work highlights a novel approach for the design of nitride-based catalysts with superior catalytic properties in ammonia synthesis.

Toward Sabatier Optimal for Ammonia Synthesis with Paramagnetic Phase of Ferromagnetic Transition Metal Catalysts.

The Sabatier principle defines the essential criteria for being an ideal catalyst in heterogeneous catalysis, while approaching the Sabatier optimal is a major pursuit in catalyst design. The Haber-Bosch (H-B) process, converting nitrogen (N2) and hydrogen (H2) to ammonia (NH3), is a holy grail reaction for humans and also a great model reaction for fundamental research, where the established volcano plot between ammonia synthesis activity and nitrogen binding energy among metals has successfully guided new catalyst design. However, reaching the top of the activity volcano is still very challenging. Herein, we identify an elegant strategy to promote the ferromagnetic (FM) catalysts to be the Sabatier optimal of ammonia synthesis via a second-order ferromagnetic-paramagnetic phase transition, which represents an ideal and novel interdisciplinary of the aforementioned century-old classic principle, reaction, and theory in chemistry, physics, and material science. The paramagnetic (PM) Co and Ni metals could have 2-4 orders of magnitude higher ammonia synthesis activity than their ferromagnetic counterparts, holding the potential to achieve a near-ambient H-B process. We believe that our discovery will open a novel avenue for revisiting the catalytic performances of paramagnetic phases of ferromagnetic materials in heterogeneous catalysis.

Ammonia synthesis using Co catalysts supported on MgO–Nd2O3 mixed oxide systems: Effect of support composition

This study presents the use of emerging support materials based on mixed magnesium–neodymium oxides for the dispersion of the Co metal for ammonia synthesis. A series of cobalt catalysts using mixed MgO–Nd2O3 oxide supports with different Mg/Nd molar ratios (2, 4, 7, 10, and 12) was successfully prepared. The N2 physisorption, XRD, HR-TEM, STEM-EDX, TPR/TG-MS, H2-TPD, and CO2-TPD studies of these systems revealed a strong correlation between the composition of the support and the overall catalyst performance. It was found that varying the Mg/Nd molar ratio caused a gradual change in the textural, structural and adsorption properties. The difference in the activity of the Co/MgO–Nd2O3 catalysts towards ammonia synthesis was mainly attributed to the different surface adsorption properties. The highest number of basic sites on the catalyst surface was noted for the support with the Mg/Nd molar ratio of 10. Also, the highest Co dispersion was obtained on this support, leading to the best distribution of Co particles with the smallest Co particle size. As a result, the Co/10MgO–Nd2O3 catalyst demonstrated superior activity for ammonia synthesis compared to other catalysts. The use of a binary composite support consisting of MgO and Nd2O3 with the properly selected ratio of each component is beneficial in terms of catalyst activity, stability and cost compared to single-oxide supports.

Improved Electrocatalytic Selectivity and Activity for Ammonia Synthesis on Diporphyrin Catalysts

The electrocatalytic nitrogen reduction reaction (NRR) under mild conditions is one of the most essential challenges in chemistry. Catalysts for electrochemical NRR play a crucial role in realizing this NH3 synthesis. In this work, we use density functional theory simulations to investigate the electrocatalytic NRR selectivity and activity on dual-atom catalysts, especially diporphyrins. We classify catalysts on the basis of adsorption of *N2 versus *H. Our results demonstrate the possibility of diporphyrins to bind and reduce N2 without producing H2 under ambient conditions, promoting high selectivity toward NH3 formation. This is due to chelating adsorption of N2, where N2 sits between two metal atoms, enhancing the binding of *N2. Additionally, the chelating adsorption of N2 activates N–N bond breaking and provides more favorable scaling relations on the adsorption energies of key intermediates, leading to enhanced NRR activity.