Showing papers on "Ammonia published in 2019"

A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements

Abstract: The electrochemical synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative1–4 to the energy-intensive Haber–Bosch process, which dominates industrial ammonia production. However, there are considerable scientific and technical challenges5,6 facing the electrochemical alternative, and most experimental studies reported so far have achieved only low selectivities and conversions. The amount of ammonia produced is usually so small that it cannot be firmly attributed to electrochemical nitrogen fixation7–9 rather than contamination from ammonia that is either present in air, human breath or ion-conducting membranes9, or generated from labile nitrogen-containing compounds (for example, nitrates, amines, nitrites and nitrogen oxides) that are typically present in the nitrogen gas stream10, in the atmosphere or even in the catalyst itself. Although these sources of experimental artefacts are beginning to be recognized and managed11,12, concerted efforts to develop effective electrochemical nitrogen reduction processes would benefit from benchmarking protocols for the reaction and from a standardized set of control experiments designed to identify and then eliminate or quantify the sources of contamination. Here we propose a rigorous procedure using 15N2 that enables us to reliably detect and quantify the electrochemical reduction of nitrogen to ammonia. We demonstrate experimentally the importance of various sources of contamination, and show how to remove labile nitrogen-containing compounds from the nitrogen gas as well as how to perform quantitative isotope measurements with cycling of 15N2 gas to reduce both contamination and the cost of isotope measurements. Following this protocol, we find that no ammonia is produced when using the most promising pure-metal catalysts for this reaction in aqueous media, and we successfully confirm and quantify ammonia synthesis using lithium electrodeposition in tetrahydrofuran13. The use of this rigorous protocol should help to prevent false positives from appearing in the literature, thus enabling the field to focus on viable pathways towards the practical electrochemical reduction of nitrogen to ammonia. A protocol for the electrochemical reduction of nitrogen to ammonia enables isotope-sensitive quantification of the ammonia produced and the identification and removal of contaminants.

Promoting nitrogen electroreduction to ammonia with bismuth nanocrystals and potassium cations in water

Abstract: The electrochemical nitrogen reduction reaction (ENRR) can allow the production of ammonia from nitrogen and water under ambient conditions and is regarded as a sustainable alternative to the industrial Haber–Bosch process. However, electrocatalytic systems that selectively and efficiently catalyse nitrogen reduction remain elusive due to the strong competition with the hydrogen evolution reaction. Here, we report a strategy to simultaneously promote ENRR selectivity and activity using bismuth nanocrystals and potassium cations. Bismuth exhibits higher intrinsic ENRR activity than transition metals due to the strong interaction between the Bi 6p band and the N 2p orbitals. Potassium cations stabilize key nitrogen-reduction intermediates and regulate proton transfer to increase the selectivity. A high Faradaic efficiency of 66% and ammonia yield of 200 mmol g–1 h–1 (0.052 mmol cm–2 h–1) are obtained in aqueous electrolyte under ambient conditions. This strategy represents a general method to expand the library of catalysts and promoters for the selective electrochemical reduction of stable molecules. The electrochemical reduction of nitrogen to ammonia represents a challenge of major interest that would substantially decrease the burden of the energy-consuming Haber–Bosch process. Now, Yin, Yan, Zhang, Si and colleagues achieve high ammonia yield and Faradaic efficiency over 66% using bismuth nanocatalysts promoted by alkali cations.

Molybdenum-catalysed ammonia production with samarium diiodide and alcohols or water

Abstract: The production of ammonia from nitrogen gas is one of the most important industrial processes, owing to the use of ammonia as a raw material for nitrogen fertilizers. Currently, the main method of ammonia production is the Haber–Bosch process, which operates under very high temperatures and pressures and is therefore very energy-intensive1. The transition-metal-catalysed reduction of nitrogen gas2–6 is an alternative method for the formation of ammonia. In these reaction systems, metallocenes or potassium graphite are typically used as the reducing reagent, and conjugate acids of pyridines or related compounds are used as a proton source. To develop a next-generation nitrogen-fixation system, these reagents should be low cost, readily available and environmentally friendly. Here we show that the combination of samarium(ii) diiodide (SmI2) with alcohols or water enables the fixation of nitrogen to be catalysed by molybdenum complexes under ambient conditions. Up to 4,350 equivalents of ammonia can be produced (based on the molybdenum catalyst), with a turnover frequency of around 117 per minute. The amount of ammonia produced and its rate of formation are one and two orders of magnitude larger, respectively, than those achieved in artificial reaction systems reported so far, and the formation rate approaches that observed with nitrogenase enzymes. The high reactivity is achieved by a proton-coupled electron-transfer process that is enabled by weakening of the O–H bonds of alcohols and water coordinated to SmI2. Although the current reaction is not suitable for use on an industrial scale, this work demonstrates an opportunity for further research into catalytic nitrogen fixation. The molybdenum-catalysed reduction of nitrogen to ammonia is achieved under ambient conditions, using samarium(ii) diiodide in combination with either simple alcohols or water as the proton source.

Defect Engineering in Photocatalytic Nitrogen Fixation

Abstract: Approximately 2% of the energy consumed by humans each year is used to make nitrogen-based fertilizers, with ammonia (NH3) production being the most significant contributor to this energy demand. C...

Catalyst-free, highly selective synthesis of ammonia from nitrogen and water by a plasma electrolytic system

Abstract: There is a growing need for scalable ammonia synthesis at ambient conditions that relies on renewable sources of energy and feedstocks to replace the Haber-Bosch process. Electrically driven approaches are an ideal strategy for the reduction of nitrogen to ammonia but, to date, have suffered from low selectivity associated with the catalyst. Here, we present a hybrid electrolytic system characterized by a gaseous plasma electrode that facilitates the study of ammonia formation in the absence of any material surface. We find record-high faradaic efficiency (up to 100%) for ammonia from nitrogen and water at atmospheric pressure and temperature with this system. Ammonia measurements under varying reaction conditions in combination with scavengers reveal that the unprecedented selectivity is achieved by solvated electrons produced at the plasma-water interface, which react favorably with protons to produce the key hydrogen radical intermediate. Our results demonstrate that limitations in selectivity can be circumvented by using catalyst-free solvated electron chemistry. In the absence of adsorption steps, the importance of controlling proton concentration and transport is also revealed.

Strategies toward Selective Electrochemical Ammonia Synthesis

Abstract: The active and selective electroreduction of atmospheric nitrogen (N2) to ammonia (NH3) using energy from solar or wind sources at the point of use would enable a sustainable alternative to the Hab...

Metal-free electrocatalyst for reducing nitrogen to ammonia using a Lewis acid pair

Abstract: Electrocatalytic nitrogen reduction reaction (NRR) is a promising way for the sustainable production of ammonia. However, it suffers from slow kinetics due to the difficulty of NN bond activation and the side hydrogen evolution reaction. Herein, on the basis of the concept of “Lewis acid pair”, we propose a metal-free electrocatalyst based on boron-decorated black phosphorus by using extensive first-principles calculations. In the integrated structure of the catalyst, the doped B atoms serve as Lewis acid pairs and catalytic centers, while the channels of BP provide structural advantages for hosting the pair and activating the NN bond. A new strategy of nitrogen activation based on the pull–pull effect is thus developed. Performance evaluations show that this metal-free catalyst is highly efficient for electrocatalytic nitrogen reduction with an ultra-low onset-potential of 0.19 V. This work opens a new possible avenue for nitrogen activation and can be generally applied to other two-dimensional or bulk materials.

Ammonium nitrate particles formed in upper troposphere from ground ammonia sources during Asian monsoons

Abstract: The rise of ammonia emissions in Asia is predicted to increase radiative cooling and air pollution by forming ammonium nitrate particles in the lower troposphere. There is, however, a severe lack of knowledge about ammonia and ammoniated aerosol particles in the upper troposphere and their possible effects on the formation of clouds. Here we employ satellite observations and high-altitude aircraft measurements, combined with atmospheric trajectory simulations and cloud-chamber experiments, to demonstrate the presence of ammonium nitrate particles and also track the source of the ammonia that forms into the particles. We found that during the Asian monsoon period, solid ammonium nitrate particles are surprisingly ubiquitous in the upper troposphere from the Eastern Mediterranean to the Western Pacific—even as early as in 1997. We show that this ammonium nitrate aerosol layer is fed by convection that transports large amounts of ammonia from surface sources into the upper troposphere. Impurities of ammonium sulfate allow the crystallization of ammonium nitrate even in the conditions, such as a high relative humidity, that prevail in the upper troposphere. Solid ammonium nitrate particles in the upper troposphere play a hitherto neglected role in ice cloud formation and aerosol indirect radiative forcing. Solid ammonium nitrate particles are formed in the upper troposphere during the Asian monsoons, which bring large amounts of ground ammonia to this altitude, according to integrated analyses of measurements on ammoniated aerosol, together with model simulations.

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

Abstract: 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...

Efficient Ammonia Decomposition in a Catalytic Membrane Reactor To Enable Hydrogen Storage and Utilization

Abstract: Liquid ammonia is a high-density (17.7 wt %) hydrogen carrier with a well-established production and distribution infrastructure. Efficient decomposition and purification are essential for its use ...

Electrocatalytic Nitrogen Reduction to Ammonia by Fe2O3 Nanorod Array on Carbon Cloth

Abstract: Electrocatalytic nitrogen fixation provides a green and facile strategy for efficient synthesis of ammonia under ambient conditions, but it lacks efficient and inexpensive electrocatalysts for the ...

Electrochemical Hydrogenation with Gaseous Ammonia.

Abstract: As a carbon-free and sustainable fuel, ammonia serves as high-energy-density hydrogen-storage material. It is important to develop new reactions able to utilize ammonia as a hydrogen source directly. Herein, we report an electrochemical hydrogenation of alkenes, alkynes, and ketones using ammonia as the hydrogen source and carbon electrodes. A variety of heterocycles and functional groups, including for example sulfide, benzyl, benzyl carbamate, and allyl carbamate were well tolerated. Fast stepwise electron transfer and proton transfer processes were proposed to account for the transformation.

Homogeneous electrocatalytic oxidation of ammonia to N2 under mild conditions

Abstract: We report that ruthenium polypyridyl complexes can catalyze ammonia oxidation to dinitrogen at room temperature and ambient pressure. During bulk electrolysis experiments, gas chromatography and mass spectrometry analysis of the headspace in the electrochemical cell showed that dinitrogen and dihydrogen are generated from ammonia with high faradaic efficiencies. A proposed mechanism where a hydrazine complex is the initial N–N bonded intermediate is supported by chemical and electrochemical experiments. This is a well-defined system for homogeneous electrocatalytic ammonia oxidation. It establishes a platform for answering mechanistic questions relevant to using ammonia to store and distribute renewable energy.

Direct Activation of Cobalt Catalyst by 12CaO·7Al2O3 Electride for Ammonia Synthesis

Abstract: Cobalt is well-known as an active component of heterogeneous solid catalysts or metal-complexes for the reduction of dinitrogen into ammonia. However, the activity of bare Co metal itself is not hi...

Nonthermal Plasma Synthesis of Ammonia over Ni-MOF-74

Abstract: Herein, we demonstrate a synergistic approach with radiofrequency plasma to synthesize ammonia in the presence of Ni-MOF-74 as catalyst. The Ni-MOF displayed higher ammonia yields as compared to the pure Ni metal. Specifically, ammonia yields as high as 0.23 g-NH3 (g-catalyst·kWh)−1 and energy cost of 265 MJ mol–1 over Ni-MOF were observed. The enhanced catalytic activity of the Ni-MOF in the presence of plasma was attributed to the presence of pores, which improved mass transfer of guest and product molecules during reaction, the presence of open Ni metal sites, and lower surface hydrogen recombination. Furthermore, the ammonia energy yield of our plasma-Ni MOF catalyst is superior to those of the state-of-the-art RF plasma catalytic systems.