Catalytic Ammonia Synthesis Research Articles

Higher Activity of Ni/γ-Al2O3 over Fe/γ-Al2O3 and Ru/γ-Al2O3 for Catalytic Ammonia Synthesis in Nonthermal Atmospheric-Pressure Plasma of N2 and H2

Developing a novel ammonia synthesis process from N2 and H2 is of interest to the catalysis and hydrogen research communities. γ-Alumina-supported nickel was determined capable of serving as an efficient catalyst for ammonia synthesis using nonthermal plasma under atmospheric pressure without heating. The catalytic activity was almost unrelated to the crystal structure and the surface area of the alumina carrier. The activity of Ni/Al2O3 was quantitatively compared with that of Fe/Al2O3 and Ru/Al2O3, which contained active metals for the conventional Haber–Bosch process. The activity sequence was Ni/Al2O3 > Al2O3 > Fe/Al2O3 > no additive > Ru/Al2O3, surprisingly indicating that the loading of Fe and Ru decreased the activity of Al2O3. The catalytic activity of Ni/Al2O3 was dependent on the amount of loaded Ni, the calcination temperature, and the reaction time. XRD, visual, and XPS observations of the catalysts before the plasma reaction indicated the generation of NiO and NiAl2O4 on Al2O3, the latter of which was generated upon high-temperature calcination. The NiO species was readily reduced to Ni metal in the plasma reaction, whereas the NiAl2O4 species was difficult to reduce. The catalytic behavior could be attributed to the production of fine Ni metal particles that served as active sites. The PN2/PH2 ratio dependence and rate constants of formation and decomposition of ammonia were finally determined for 5.0 wt% Ni/Al2O3 calcined at 773 K. The ammonia yield was 6.3% at an applied voltage of 6.0 kV, a residence time of reactant gases of 0.12 min, and PH2/PN2 = 1.

Plasma-Catalytic Ammonia Synthesis beyond the Equilibrium Limit

We explore the consequences of non-thermal plasma activation on product yields in catalytic ammonia synthesis, a reaction that is equilibrium-limited at elevated temperatures. We employ a minimal m...

(Invited) Electrochemistry Meets Heterogeneous Catalysis for the Conversion of CO2 and N2 to Fuels and Chemicals

Electrochemical reactors operating at intermediate temperatures (200 – 400 °C) can potentially have several advantages over their high temperature (600- 1000 °C) and low temperature (< 100 °C) counterparts. For example their potential of an increased catalytic activity enabling a decrease in material cost compared to low temperature cells, and can be thermally integrated into other chemical processes like synthesis of synthetic fuels and chemicals, such as methanol and ammonia.Electrochemical reduction of CO2 to chemical building blocks such as CO, CH3OH, and CH4 is a dream for electrochemists, and high faradaic efficiencies have been reported for liquid electrochemical cells operated at ambient temperature, using e.g. Cu electrodes [1]. However, selectivity and electrochemical activity are far from being technically relevant, so that heterogeneous catalysis processes still are the matter of choice. Even more difficult is the electrochemical reduction of nitrogen f.i. to ammonia.In this contribution, activities at DTU Energy are summarized using solid state electrochemical cells under operating conditions close to the well-known catalytic synthesis of methanol and ammonia. For CO2 reduction, cells based on CsH2PO4 as electrolyte and Cu based cathodes have been investigated towards their electrochemical activity in both H2/H2O and H2/H2O/CO2 containing atmospheres at elevated temperatures of 240 °C. Proton conducting ceramics cells based on barium cerate with Mo and Fe electrodes have been characterized in H2/N2 atmospheres between 300 and 500 °C. Electrochemical impedance analysis point towards a predominant hydrogen evolution and the role of adsorption and desorption processes as well as overpotential will be discussed.[1] Y. Hori, Modern Aspects of Electrochemistry: Electrochemical CO2 Reduction on Metal Electrodes, Springer New York, USA 42 (2008), 89-189.

Catalytic Hydrogenation of a Manganese(V) Nitride to Ammonia.

The catalytic hydrogenation of a metal nitride to produce free ammonia using a rhodium hydride catalyst that promotes H2 activation and hydrogen-atom transfer is described. The phenylimine-substituted rhodium complex (η5-C5Me5)Rh(MePhI)H (MePhI = N-methyl-1-phenylethan-1-imine) exhibited higher thermal stability compared to the previously reported (η5-C5Me5)Rh(ppy)H (ppy = 2-phenylpyridine). DFT calculations established that the two rhodium complexes have comparable Rh-H bond dissociation free energies of 51.8 kcal mol-1 for (η5-C5Me5)Rh(MePhI)H and 51.1 kcal mol-1 for (η5-C5Me5)Rh(ppy)H. In the presence of 10 mol% of the phenylimine rhodium precatalyst and 4 atm of H2 in THF, the manganese nitride (tBuSalen)Mn≡N underwent hydrogenation to liberate free ammonia with up to 6 total turnovers of NH3 or 18 turnovers of H• transfer. The phenylpyridine analogue proved inactive for ammonia synthesis under identical conditions owing to competing deleterious hydride transfer chemistry. Subsequent studies showed that the use of a non-polar solvent such as benzene suppressed formation of the cationic rhodium product resulting from the hydride transfer and enabled catalytic ammonia synthesis by proton-coupled electron transfer.

Solid solution for catalytic ammonia synthesis from nitrogen and hydrogen gases at 50\u2009\xb0C

The lack of efficient catalysts for ammonia synthesis from N2 and H2 gases at the lower temperature of ca. 50 °C has been a problem not only for the Haber–Bosch process, but also for ammonia production toward zero CO2 emissions. Here, we report a new approach for low temperature ammonia synthesis that uses a stable electron-donating heterogeneous catalyst, cubic CaFH, a solid solution of CaF2 and CaH2 formed at low temperatures. The catalyst produced ammonia from N2 and H2 gases at 50 °C with an extremely small activation energy of 20 kJ mol−1, which is less than half that for conventional catalysts reported. The catalytic performance can be attributed to the weak ionic bonds between Ca2+ and H− ions in the solid solution and the facile release of hydrogen atoms from H− sites.

The Power of Hydrides

Qianru Wang is a graduate student pursuing her Ph.D. in physical chemistry at Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Her research focuses on hydride materials for catalytic ammonia synthesis. Jianping Guo is a professor at Dalian Institute of Chemical Physics, Chinese Academy of Sciences. His research focuses on exploring metal hydrides, imides, and amides in the activation and transformation of small molecules including N2, H2, and NH3. Ping Chen is a professor and division head of Hydrogen Energy and Advanced Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. The primary research interests of her group include materials development for hydrogen storage and catalysis.

ДОСЛІДЖЕННЯ КІНЕТИКИ ПРОЦЕСУ ОКИСНЮВАЛЬНОГО АМОНОЛІЗУ МЕТАНУ

The main results of research of the kinetics of the process of methane oxidative ammonolysis are presented. Industrial production of hydrogen cyanide is the basis for the production of one of the important components for gold production – sodium cyanide. Now, the main method of production of sodium cyanide is based on the neutralization of hydrocyanic acid, obtained by catalytic synthesis of methane, ammonia and oxygen in a platinum catalyst, by alkali solution. The purpose of the investigation is to research kinetic parameters of the process of methane oxidative ammonolysis, namely to determine the dependence of contact time and reaction rate on technological parameters of the process. It is known that the content of platinized metals in the catalyst, as well as the number of nets in the package and its surface development, have the greatest influence on the process of methane oxidative ammonolysis. It is established that with the reduced content of the one of reagents in the initial gas mixture, namely NH3, after cyanide hydrogen synthesis reactor, a considerable amount of unreacted components of the mixture is present in contact gas. The time of the contact for platinum nets for different developed surfaces at different pressures and constant temperature of 1193 K is calculated. The number of nets in the platinum catalyst package have the greatest influence on contact time. It is proved that with increasing a linear velocity of the gas stream, the cyanide hydrogen output is increased. Based on the experimental data, the rate of the reaction of HCN formation and the flow of the diffusion process in the first order are determined. At a volumetric ammonia concentration 10,75 % vol., the rate of the reaction of cyanide hydrogen formation is 0,75 s.

Deciphering the synergy between plasma and catalyst support for ammonia synthesis in a packed dielectric barrier discharge reactor

Plasma-assisted ammonia synthesis in a packed dielectric barrier discharge (DBD) reactor at atmospheric pressure is presented in this work. A broad range of materials (commonly used as catalyst supports) with various chemical properties (acidic α-Al2O3, anatase TiO2 and basic MgO, CaO), surface area and porosity (α-Al2O3 and γ-Al2O3), dielectric properties (quartz wool, TiO2, and BaTiO3), have been investigated for synergetic effects by packing them in the discharge zone of the DBD reactor. All the materials showed a substantial effect on ammonia production, which can be explained solely as a result of the effect of packing on plasma formation and not by a synergy between plasma and surface catalysis. Size and shape of packing material are found to be the key parameters in enhancing the performance. Quartz wool, closely followed by γ-Al2O3, produces the highest concentration of ammonia at 2900 and 2700 ppm, respectively, due to their ability to generate dense filamentary microdischarges. Particles with a diameter of 200 µm yielded a 64% higher concentration of NH3 than 1300 µm particles—because of amplified electric field strength from increased particle-particle contact points. The specific energy input per unit volume also displayed a significant impact on ammonia production. The process parameters such as N2/H2 feed flow ratio, total flow rate and argon dilution were also investigated. In contradiction to catalytic ammonia synthesis, plasma-assisted synthesis favors a N2/H2 feed ratio ⩾2 instead of the stoichiometric feed ratio of 0.33. At 0.4 l min−1, 3500 ppm of ammonia was produced with an energy efficiency of 1.23 g NH3 kWh−1. Dilution with 2–5 vol% of argon yielded a 2% improvement in the concentration and energy efficiency, which seems insignificant considering the added practical challenges posed by gas separation. To achieve even higher ammonia concentration and energy efficiencies, it is recommended to support transition metal on γ-Al2O3.

Pseudo catalytic ammonia synthesis by lithium–tin alloy

In this work, nitrogenation, ammonia generation, regeneration reactions of lithium-tin alloy is investigated as pseudo catalytic process of ammonia synthesis. Li17Sn4 synthesized by thermochemical method at 500 °C can react with 0.1 MPa of N2 below 400 °C. Nano or amorphous lithium nitride would be formed by the nitrogenation. By reaction of the nitrogenated samples and H2, ammonia is generated at 300 °C under 0.1 MPa. The initial alloy phase Li17Sn4 is regenerated below 350 °C from the products after the ammonia generation process. Based on the above three step process, ammonia can be pseudo-catalytically synthesized from N2 and H2 below 400 °C under ambient pressure. Furthermore, the reactivity for the ammonia synthesis using Li–Sn alloy is preserved during the NH3 synthesis cycles due to the characteristic reaction process based on the Li extraction and insertion.

Plasma-driven catalysis: green ammonia synthesis with intermittent electricity

Plasma-driven catalytic ammonia synthesis from renewable electricity has recently gained traction as an alternative to the Haber–Bosch process for decentralized applications. We summarize the state-of-the-art in literature and provide avenues for improvement.

Pure Siliceous Zeolite-Supported Ru Single-Atom Active Sites for Ammonia Synthesis

As the most active new frontier, a single-atom catalyst (SAC) combining the merits of heterogeneous and homogeneous catalysts would have a significant effect on the 100 year history of ammonia synthesis research. Here, the Ru SAC has been demonstrated to be active and efficient for ammonia synthesis. To this end, an ideal model catalyst, pure siliceous zeolite-supported Ru SAC (Ru SAs/S-1), which shows surprising catalytic ammonia synthesis activity compared to that of a conventional Ru catalyst, was designed. Both atomic-resolution scanning transmission electron microscopy, X-ray absorption spectrometric analysis, and in situ diffuse reflectance infrared Fourier transform spectroscopy identify the single-Ru-atom nature of Ru SAs/S-1 before and after the reaction. Further DFT calculations reveal that the reaction mechanism is different from traditional mechanisms. Therefore, this paper provides an alternative SAC strategy to design high-performance Ru catalysts for ammonia synthesis.

Plasma-Enhanced Catalytic Synthesis of Ammonia over a Ni/Al2O3 Catalyst at Near-Room Temperature: Insights into the Importance of the Catalyst Surface on the Reaction Mechanism.

A better fundamental understanding of the plasma-catalyst interaction and the reaction mechanism is vital for optimizing the design of catalysts for ammonia synthesis by plasma-catalysis. In this work, we report on a hybrid plasma-enhanced catalytic process for the synthesis of ammonia directly from N2 and H2 over transition metal catalysts (M/Al2O3, M = Fe, Ni, Cu) at near room temperature (∼35 °C) and atmospheric pressure. Reactions were conducted in a specially designed coaxial dielectric barrier discharge (DBD) plasma reactor using water as a ground electrode, which could cool and maintain the reaction at near-room temperature. The transparency of the water electrode enabled operando optical diagnostics (intensified charge-coupled device (ICCD) imaging and optical emission spectroscopy) of the full plasma discharge area to be conducted without altering the operation of the reactor, as is often needed when using coaxial reactors with opaque ground electrodes. Compared to plasma synthesis of NH3 without a catalyst, plasma-catalysis significantly enhanced the NH3 synthesis rate and energy efficiency. The effect of different transition metal catalysts on the physical properties of the discharge is negligible, which suggests that the catalytic effects provided by the chemistry of the catalyst surface are dominant over the physical effects of the catalysts in the plasma-catalytic synthesis of ammonia. The highest NH3 synthesis rate of 471 μmol g–1 h–1 was achieved using Ni/Al2O3 as a catalyst with plasma, which is 100% higher than that obtained using plasma only. The presence of a transition metal (e.g., Ni) on the surface of Al2O3 provided a more uniform plasma discharge than Al2O3 or plasma only, and enhanced the mean electron energy. The mechanism of plasma-catalytic ammonia synthesis has been investigated through operando plasma diagnostics combined with comprehensive characterization of the catalysts using N2 physisorption measurements, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), NH3-temperature-programmed desorption (TPD), and N2-TPD. Four forms of adsorbed NHx (x = 0, 1, 2, and 3) species were detected on the surfaces of the spent catalysts using XPS. It was found that metal sites and weak acid sites could enhance the production of NH3 via formation of NH2 intermediates on the surface.

Vibrationally Excited Activation of N2 in Plasma-Enhanced Catalytic Ammonia Synthesis: A Kinetic Analysis

Plasma-enhanced catalytic ammonia synthesis has been proposed as an alternative pathway for green nitrogen fixation in the case of medium- and small-scale operation. Recently, Mehta et al. [ Nat. C...

Governing factors of supports of ammonia synthesis in an electric field found using density functional theory

Efficient ammonia synthesis at low temperatures is anticipated for establishing a hydrogen carrier system. We reported earlier that application of an electric field on the Cs/Ru/SrZrO3 catalyst enhanced catalytic ammonia synthesis activity. It is now clear that N2 dissociation is activated by hopping protons in the electric field. Efficient ammonia synthesis proceeds by an “associative mechanism” in which N2 dissociates via an N2H intermediate, even at low temperatures. The governing factor of ammonia synthesis activity in an electric field for active metals differed from that in the conventional mechanism. Also, N2H formation energy played an important role. The effects of dopants (Al, Y, Ba, and Ca) on this mechanism were investigated using activity tests and density functional theory calculations to gain insights into the support role in the electric field. Ba and Ca addition showed positive effects on N2H formation energy, leading to high ammonia synthesis activity. The coexistence of proton-donating and electron-donating abilities is necessary for efficient N2H formation at the Ru–support interface.

Surface enrichment phenomenon in the Ba-doped cobalt catalyst for ammonia synthesis

Barium promoted cobalt catalysts (Co/Ba) for NH3 synthesis were synthesized by co-precipitation of a mixture of cobalt and barium carbonates. Various pretreatment method of the obtained precursors were applied: subsequent calcination and in situ reduction or in situ reduction only (without calcination). The catalytic materials were subjected to detailed characterization studies by nitrogen physisorption, X-ray powder diffraction (XRPD), temperature-programmed techniques (TPR-MS, TPD-H2, TPD-N2, TPSR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM-EDX). The Co/Ba catalysts were tested in catalytic ammonia synthesis reaction under conditions close to the industrial ones (400 °C, 6.3 MPa, H2/N2 = 3). Studies revealed that the method of catalysts precursors pretreatment has a significant influence on the properties of the final Co/Ba systems. The more effective method is the one without subsequent calcination – this material exhibited the most favorable catalytic properties. Detailed surface studies showed that calcination process of the co-precipitated carbonates mixture adversely affects the final properties of the catalyst, because it favors a migration of the barium promoter to the catalyst's surface during its reduction (surface enrichment phenomenon) limiting the amount of active phase (metallic cobalt) available to reactants during the catalytic reaction and thus substantially decreasing the catalyst's activity.

Recent progress in heterogeneous ammonia synthesis

Ammonia is feeding nearly half the world population and also holds the promise as a carbon-free energy carrier to store and transport renewable energy. However, the catalytic synthesis of ammonia from N2 and H2 is currently realized industrially under harsh conditions (10−25 MPa, 350−550°C), consuming 1%−2% of the power produced globally. This fossil fuel-based process also results in the emissions of 670 million tonnes of CO2. The development of more sustainable and environmentally benign processes is highly desirable. To this end, heterogeneous catalysts that allow ammonia synthesis under lower temperature and pressure, solid materials for chemical looping ammonia synthesis, as well as electron-, photon- and plasma-driven routes have been actively explored in recent years. In this review article, some recent progress on these topics are summarized and discussed. Major effort has been devoted to searching for more active heterogeneous catalysts under milder conditions. The tailoring of particle size and shape of transition metals as well as the control of metal-support and -promoter interactions are key factors for activity enhancement. Surface science and theoretical results reveal that the scaling relations among the adsorption energies of reacting species place limitations on the activity enhancement of a catalytic process. To develop more active catalysts, the scaling relations should be circumvented or broken. Some strategies including the introduction of a non-transition metal site, or construction of surface cluster site have been shown to be effective approaches. Chemical-looping ammonia synthesis may provide an alternative approach, which enables ammonia production under ambient pressure by decoupling a catalytic process into several separated reactions. This process circumvents the scaling relations and the competitive adsorption of N2 and H2 or H2O on metal catalysts. Challenges arise from tuning the thermodynamic and kinetic properties of all of the separated steps by selecting materials containing H, N, O, etc. The electron-, photon- and plasma-driven routes for ammonia synthesis have been received increasing attention recently. The poor selectivity and pretty low ammonia production rates are the two key challenges in electro- or photon-chemical processes. The ammonia synthesis rates should be carefully measured and quantified by combining multiple methods. The strategy for designing catalysts with high efficiency may also be focused on the modification of catalyst surface structural and electronic properties to circumvent the scaling relations. Through these efforts, processes that are relied on renewable energy, which are quite different from the conventional Haber-Bosch process, may come to the fore in the future.

N-H Bond Formation in a Manganese(V) Nitride Yields Ammonia by Light-Driven Proton-Coupled Electron Transfer.

A method for the reduction of a manganese nitride to ammonia is reported, where light-driven proton-coupled electron transfer enables the formation of weak N-H bonds. Photoreduction of (saltBu)MnVN to ammonia and a Mn(II) complex has been accomplished using 9,10-dihydroacridine and a combination of an appropriately matched photoredox catalyst and weak Brønsted acid. Acid-reductant pairs with effective bond dissociation free energies between 35 and 46 kcal/mol exhibited high efficiencies. This light-driven method may provide a blueprint for new approaches to catalytic homogeneous ammonia synthesis under ambient conditions.

Thermodynamic Properties of Ammonia Production from Hydrogenation of Alkali and Alkaline Earth Metal Amides.

Thermodynamic properties of alkali and alkaline earth metal amides are critical for their performance in hydrogen storage as well as catalytic ammonia synthesis. In this work, the ammonia equilibrium concentrations of LiNH2 , KNH2 and Ba(NH2 )2 at ca.10 bar of hydrogen pressure and different temperatures were measured by using a high-pressure gas-solid reaction system equipped with a conductivity meter. Hydrogenation of KNH2 gives the highest ammonia equilibrium concentration, followed by Ba(NH2 )2 and LiNH2 . Based on these data, the entropy and enthalpy changes of the reaction of ANH2 +H2 →AH+NH3 (A=Li, K, and Ba) were obtained from the van't Hoff equation. These thermodynamic parameters provide important information on the understanding of metal amides in catalytic ammonia synthesis reaction.

RESEARCH OF THE INFLUENCE OF THE COMPOSITION OF THE INITIAL GAS MIXTURE ON THE FORMATION OF HYDROGEN CYANIDE

The main results of research on the influence of the composition of the initial gas mixture on the formation of hydrogen cyanide by the oxidative ammonolysis of methane is considered. Industrial production of hydrogen cyanide is the basis for the production of one of the important components in gold mining, sodium cyanide. Today, the main method of production of sodium cyanide is based on the neutralization of hydrocyanic acid obtained by the catalytic synthesis of methane, ammonia and oxygen of the air on a platinum catalyst, with an alkali solution. This method has been little studied and has great prospects for its further improvement. The purpose of the study was to establish the optimal ratio of the components of the initial gas mixture and their influence on the output of hydrogen cyanide. In oxidative ammonolysis of methane, taking into account the uneven diffusion of gases and lower cost of methane, compared with ammonia, the process of producing hydrogen cyanide is carried out with a small excess of methane relative to ammonia. It has been established that with a reduced content of one of the reactants in the initial gas mixture after the reactor of hydrogen cyanide synthesis, a significant amount of unreacted components of the mixture is present in the contact gas. It was revealed that the degree of conversion of reagents into hydrogen cyanide depends not only on the initial concentration, but also on their ratio in the initial mixture. It is proved that the maximum degree of conversion of ammonia and methane is achieved when the ratio of ammonia / methane in the reaction mixture is 0.9-0.95. The maximum yield of hydrogen cyanide by oxidative ammonolysis of methane is 62–72 % and is achieved when the ratio of components in the initial reaction mixture when the ratio of ammonia / methane is 0.8–0.9. The effect of temperature on the formation of hydrogen cyanide by the Andrussov method has been investigated. It has been established that increasing the temperature of the process for producing hydrogen cyanide by the oxidative ammonolysis of methane has a positive effect on the yield of the target product. The results obtained can be used in modern nitrogen-fertilizer plants to optimize the process of synthesis of hydrocyanic acid.

Enhanced Catalytic Ammonia Synthesis with Transformed BaO

We report that a simple mixture of BaO and CaH2 powders with ruthenium nanoparticles acts as an effective heterogeneous catalyst for low-temperature ammonia synthesis with a very small activation energy (41 kJ mol–1). The high catalytic performance of this material is not due to either the original CaH2 or BaO particles but to BaO transformed by reaction with CaH2. The transformed BaO contains BaH2, a stable and strong electron-donating material that facilitates a reversible hydrogen storage-release reaction and significantly enhances electron donation to Ru, which results in high catalytic performance.