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Hydrogen Production from Ammonia Using Sodium Amide

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Abstract

This paper presents a new type of process for the cracking of ammonia (NH3) that is an alternative to the use of rare or transition metal catalysts. Effecting the decomposition of NH3 using the concurrent stoichiometric decomposition and regeneration of sodium amide (NaNH2) via sodium metal (Na), this represents a significant departure in reaction mechanism compared with traditional surface catalysts. In variable-temperature NH3 decomposition experiments, using a simple flow reactor, the Na/NaNH2 system shows superior performance to supported nickel and ruthenium catalysts, reaching 99.2% decomposition efficiency with 0.5 g of NaNH2 in a 60 sccm NH3 flow at 530 °C. As an abundant and inexpensive material, the development of NaNH2-based NH3 cracking systems may promote the utilization of NH3 for sustainable energy storage purposes.

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... Cyclical dissociation and recombination of ammonia to its constituent elements using endothermic and exothermic reactions, respectively [136e138]. Using ammonia as hydrogen (fuel) carrier [137,139]. ...
... Additionally, the cost of operation and performance of hydrogen-based energy technologies are now becoming comparable to those of batteries and fossil fuels [162]. Yet, the lack of cost-effective modes of storage and transportation has hindered its commercialization [139]. Storing hydrogen in gaseous form is inefficient due to its low volumetric energy density. ...
... However, ruthenium is a precious metal, the catalyst is costly, and therefore other low-cost catalysts can be explored such as nickel-based catalysts [172]. Catalysts of the formula LiNH x and NaNH x , in particular, NaNH 2 [139], have exhibited strong affinity for ammonia decomposition and the same is true for bimetallic catalysts. For example, NiePt and NieMo catalysts, as well as, MoN x / a-Al 2 O 3 and NiMoN x / a-Al 2 O 3 have been suggested as potential catalyst materials. ...
Article
Ammonia as an energy storage medium is a promising set of technologies for peak shaving due to its carbon-free nature and mature mass production and distribution technologies. In this paper, ammonia energy storage (AES) systems are reviewed and compared with several other energy storage techniques. It is shown that once optimized for commercial use, AES systems have the potential for cost-effectiveness and efficiency. Its independence of topographic or climatic resource availability makes it an ideal option for many locales that has no provisions for pumped-hydro storage or with too harsh a climate for electrochemical batteries to survive. The range of applications for AES systems covers common utility-scale storage and includes electric vehicles applications. In this review, the viability of ammonia as a hydrogen carrier is discussed in detail, especially as a thermochemical energy storage media, and as a fuel for fuel cells and internal combustion engines. The health and safety impacts of ammonia are also highlighted and discussed.
... 26 David et al. found that NaNH 2 was active for NH 3 decomposition when tested in a stainless steel reactor. 27 A more recent development is the use of composites of lithium imide and transition metals (nitrides). Guo et al. reported that Li 2 NH combined with 3d transition metals or their nitrides leads to high catalytic activities for NH 3 decomposition, with the MnN−Li 2 NH composite having an activity similar to that of 5 wt % Ru/CNTs. ...
... Similar activity losses were observed in previously reported The Journal of Physical Chemistry C Article amide-based catalysts and this was attributed to the loss of NaNH 2 and KNH 2 species due to their volatility at high temperatures. 27,29,30 We therefore investigated the thermal stability of our catalysts by performing TGA on the Ni− NaNH 2 /GNP and Ni−KNH 2 /GNP in an argon flow ( Figure S10). Both samples started to lose weight upon heating and (8−20%) in the catalysts was lost upon heating, thus showing improved thermal stability. ...
Article
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The development of efficient catalysts for hydrogen generation via ammonia decomposition is crucial for the use of ammonia as an energy carrier. Here we report the effect of pore confinement of NaNH2 and KNH2 on ammonia decomposition catalysis. For the first time, Ni or Ru doped NaNH2 and KNH2 were confined in carbon nanopores using a combination method of solution impregnation and melt infiltration. Structure characterizations indicate the nanoscale intimacy between transition metals and alkali metal amides inside the pores of carbon support. As a result, 8 wt% Ni doped NaNH2 and KNH2 nanocomposites give NH3 conversions of 79% and 60% respectively at 425 °C, close to the performance of a 5 wt% Ru/C reference catalyst. 0.8 wt% Ru doped nanocomposites exhibit even better catalytic performance, with about 95 % NH3 conversion at a moderate temperature of 375 °C. The hydrogen production rates of these Ni and Ru doped nanocomposites in a pure NH3 flow are about 3-4 times higher than for recently reported novel catalysts such as Ni-Li2NH and Ru-Li2NH/MgO. Interestingly, the apparent activation energies of the Ru or Ni based catalysts decrease 20-30 kJ mol-1 by co-confinement with alkali metal amides. The strategy of nanoconfinement of alkali metal amides in porous hosts may open a new avenue for effectively generating H2 from NH3 at low temperatures.
... For example, a nanoconfined NaAlH 4 was claimed to enable the catalytic hydrogenation of a range of alkynes and alkenes at 150 °C and under 10 MPa hydrogen pressure [329]. NaNH 2 and LiNH 2 have also been reported to enable the catalytic decomposition of NH 3 at 530 °C [330]. Interestingly, improved catalytic activity for the decomposition of NH 3 was achieved at a relatively lower temperature of 350 °C after doping LiNH 2 with Mn [330]. ...
... NaNH 2 and LiNH 2 have also been reported to enable the catalytic decomposition of NH 3 at 530 °C [330]. Interestingly, improved catalytic activity for the decomposition of NH 3 was achieved at a relatively lower temperature of 350 °C after doping LiNH 2 with Mn [330]. In another report, using a combination of transition metals such as Cr, Mn, Fe or Co with LiH resulted in the synthesis of NH 3 under milder conditions (300 °C) [331]. ...
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The metal-hydrogen interaction and its equilibrium conditions allow for distinct properties in metal hydrides (MHs). Based on these properties, MHs have been found to enable a range of novel technologies from thermal compression to sensors, and catalysis. Ni-MH batteries are currently the main application of hydrides in the market. However, new battery concepts such as Li-MgH2, Mn-MH, and hydride-based solid electrolytes have emerged. Fuel cells based on hydrides have also been proposed to convert the chemical “hydride energy” into electrical energy. Based on their unique thermodynamic properties, MHs are also the basis of new concepts in hydrogen compression, heat pumps, cooling systems, and thermal energy storage. Other important applications, including catalysis and chemical speciation have also been considered owing the chemical properties of hydrides. Sensors and smart mirrors based on the dynamic optical, structural, and electrical properties of MHs have been developed. This review summarizes current state-of-the-art along the multiple applications of MHs and provides recommendations on the future progress required to enable a more widespread adoption of MHs beyond their use as hydrogen storage materials.
... These high temperatures are not only detrimental to operating costs, more expensive reactor materials must be used as well [136]. Recently, new catalysts based on light metal amides or imides have been suggested, which, while less costly than catalysts based on transition metals, still require high temperatures (>500 C) to reach close to full conversion of ammonia [137]. Although early results using these amide or imide catalysts are encouraging, there are still technical problems to be solved regarding their industrial application [138]. ...
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The large-scale storage of hydrogen plays a fundamental role in a potential future hydrogen economy. Although the storage of gaseous hydrogen in salt caverns already is used on a full industrial scale, the approach is not applicable in all regions due to varying geological conditions. Therefore, other storage methods are necessary. In this article, options for the large-scale storage of hydrogen are reviewed and compared based on fundamental thermodynamic and engineering aspects. The application of certain storage technologies, such as liquid hydrogen, methanol, ammonia, and dibenzyltoluene, is found to be advantageous in terms of storage density, cost of storage, and safety. The variable costs for these high-density storage technologies are largely associated with a high electricity demand for the storage process or with a high heat demand for the hydrogen release process. If hydrogen is produced via electrolysis and stored during times of low electricity prices in an industrial setting, these variable costs may be tolerable.
... 32 Using the amide NaNH 2 , a conversion equivalent to that of Ru/Al 2 O 3 was obtained. 373 Moreover, NaNH 2 showed higher conversion than LiNH 2 at temperatures below ca. 420°C, while at higher temperature the two amides presented a similar activity. ...
... For these new catalyst systems, two parallel paths of investigation have been explored: metal amide/imide as individual catalysts [108,110,111] and in composites with transition metals and their nitrides [109,112e116]. The catalytic activity has been rationalized by the cyclic formation and decomposition of ternary lithium nitrides (e.g. ...
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Globally, the accelerating use of renewable energy sources, enabled by increased efficiencies and reduced costs, and driven by the need to mitigate the effects of climate change, has significantly increased research in the areas of renewable energy production, storage, distribution and end-use. Central to this discussion is the use of hydrogen, as a clean, efficient energy vector for energy storage. This review, by experts of Task 32, “Hydrogen-based Energy Storage” of the International Energy Agency, Hydrogen TCP, reports on the development over the last 6 years of hydrogen storage materials, methods and techniques, including electrochemical and thermal storage systems. An overview is given on the background to the various methods, the current state of development and the future prospects. The following areas are covered, porous materials, liquid hydrogen carriers, complex hydrides, intermetallic hydrides, electrochemical storage of energy, thermal energy storage, hydrogen energy systems and an outlook is presented for future prospects and research on hydrogen-based energy storage.
... For these new catalyst systems, two parallel paths of investigation have been explored: metal amide/imide as individual catalysts [108,110,111] and in composites with transition metals and their nitrides [109,112e116]. The catalytic activity has been rationalized by the cyclic formation and decomposition of ternary lithium nitrides (e.g. ...
Preprint
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Magnesium hydride owns the largest share of publications on solid materials for hydrogen storage. The Magnesium group of international experts contributing to IEA Task 32 Hydrogen Based Energy Storage recently published two review papers presenting the activities of the group focused on magnesium hydride based materials and on Mg based compounds for hydrogen and energy storage. This review article not only overviews the latest activities on both fundamental aspects of Mg-based hydrides and their applications, but also presents a historic overview on the topic and outlines projected future developments. Particular attention is paid to the theoretical and experimental studies of Mg-H system at extreme pressures, kinetics and thermodynamics of the systems based on MgH2,nanostructuring, new Mg-based compounds and novel composites, and catalysis in the Mg based H storage systems. Finally, thermal energy storage and upscaled H storage systems accommodating MgH2 are presented.
... In comparison with all carbonaceous compounds, ammonia has the highest gravimetric H 2 density, of 17.75wt% (Podila et al. 2016), and energy density of 3000Wh/kg which is greater than those of methanol and other carbon-based fuels (Bell and Torrente-Murciano 2016). Also, it has a very high volumetric H 2 density of 121kgH 2 /m 3 in the liquid form (David et al. 2014) and successfully meets the DOE criteria as a hydrogen storage material. Ammonia is safe (narrow combustion air), easy to store and transport, and has variety of uses (fertilizer, cleaning agent) (Wojcik et al. 2003;Zamfirescu and Dincer 2009). ...
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The need for COx-free H2 in proton-exchange membrane fuel cells (PEMFC) has driven ammonia (NH3) decomposition to the forefront of H2 production technologies, taking NH3 as a potential and viable hydrogen storage material. Herein, a detailed derivation of thermodynamics governing equations has been applied to analyze the thermodynamics of ammonia decomposition reaction. The study utilizes MATLAB optimization tool ‘fmincon’ to solve the objective function, in a bid to find Gibbs free energy minima. The present study supports that if NH3 decomposition proceeds without molecular hindrance, almost 100% ammonia conversion, with close to 99.85% H2 yield, is achievable at 1 bar pressure and ≥ 700 K (427 ℃) temperature but also noticeable that 98% NH3 conversion is achievable at 600 K (327 ℃). The total free energy of ammonia decomposition system becomes more negative with increasing extent of reaction until equilibrium is reached. As the reaction temperature increases at a pressure of 1 bar, the extent of ammonia decomposition reaction also increases, reaching 0.61, 0.84, 0.91, 0.97 and 0.99 mol at 450, 500, 600, 700, and 773 K, respectively. The conversion of ammonia increases with increasing temperature and a negative effect of pressure was observed as per Le-Chatelier’s principle.Graphical abstract
... The system has been applied to the real-time measurement of ammonia storage in halide salts [71] and in metal-organic frameworks (MOFs) [72]. Ammonia is well known as a potential hydrogen storage material owing to is very high gravimetric (17.8 wt%) and volumetric (121 kg/m 3 ) H 2 density [73]. MOFs are multi-dimensional network structures comprising metal ions or clusters connected by organic links. ...
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The unique properties of microwaves make them useful in many diverse applications across a wide range of fields, spanning much of engineering and science. In the chemical sciences, microwaves provide a toolkit of electric and magnetic effects with which, at high power, unconventional heating modes can be used to produce new materials not obtainable by conventional heating. At low power, unique microwave properties can be used to make revealing spectroscopic measurements. In this review, we consider the current outlook for microwaves in chemistry beginning with the theoretical framework for our understanding of microwaves interactions and the causes of results observed. We then survey major application areas including in synthesis and emerging areas in catalysis, energy, and environmental applications. Finally, we review new concepts in dielectric and magnetic spectroscopy at microwave frequencies with a focus upon dielectric property measurement and electron paramagnetic resonance. This nonexhaustive review seeks to highlight important and emerging areas in the chemical sciences and put into context recent developments and advances in our understanding of microwave applications in this diverse area of science and engineering.
... Ammonia decomposition is an endothermic reaction that yields high conversions at high temperature and low-pressure conditions in the presence of heterogeneous catalysts. Ru-based catalyst series were reported to show the highest activities for the ammonia cracking reaction [7]; however, numerous studies have focused on enhancing the reaction conversions at low temperature using novel nonprecious metal catalysts [6,[8][9][10]. Hence, the overall operation performance can be improved, decreasing at the same time the process temperature. ...
Article
A flat-plate microreactor was designed, examined and modelled for the on-board hydrogen generation via the ammonia decomposition reaction over CoCeAlO mixed oxide catalyst. The tested catalyst was synthesised and immobilised using an inkjet printing-assisted self-assembly method. Inkjet printing technology is proved to be an efficient way for the direct synthesis, deposition and rapid screening of heterogeneous catalysts. A series of kinetic experiments were performed to analyse the influence of reaction temperature and NH3 flow rate on the microreactor performance. Two kinetic rate expressions were developed based on the conversion data and implemented in the CFD model. The simulation results showed good accuracy with respect to the experimental data, and the kinetic models could effectively predict the reacting flow properties along the microchannel. The microreactor is especially designed to operate under the kinetic–control conditions with negligible mass transfer resistance. Furthermore, the microreactor configuration eases the reassembly of the design parts and probing the fresh catalysts for the kinetic studies.
... This observation quantitatively explains why so many material-energy nexus studies have been launched to address problems at these two hot-spots. Previous studies have been focused on the supply-demand of rare materials and metals needed for wind turbine manufacture, fuel cells, and EV batteries [62,63], the catalysts for both green ammonia synthesis [64] and cracking [65], and ESI, emergy sustainability index; EWR, emergy waste ratio; BIR, biotic impact ratio. 5. ...
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Ammonia is a renewable energy medium appropriate for distant trading; therefore, many countries and companies have formulated ambitious strategies to develop energy transitions to use green ammonia for transportation systems. However, the associated social, economic and environmental impacts, and the overall viability and sustainability of these transitions are still a mystery, because of the lack of sufficiently complicated evaluations. To fill this gap, an integrated life cycle assessment and emergy evaluation (LCA-EME) method was developed and applied to synthesize, compare and recognize the hotspot nodes of resource depletion, emissions and impacts, and to quantify the exploitation and utilization efficiencies, environmental loadings and sustainability of the Australia-Japan telecoupling of wind power-based ammonia for electric vehicles (EV) and hydrogen fuel cell vehicles (FCV) used for passenger transportation, compared with two fossil fuel-based EV transportation systems. The results revealed that the transition to ammonia-based fuels can reduce nonrenewable energy consumption by >29.64% and Greenhouse Gas (GHG) emissions by >10.00%; however, the demand for emergy resources >2.03 times and biotic endpoint impacts >1.56 times, both of which mainly occurred in the sending subsystem of the telecoupling interaction. The results highlighted the necessity of internalizing the ‘external’ resource stress and its biotic impacts, increasing the utilization efficiency and the recycling rate of minerals and fresh water, and decreasing the endpoint impacts to guarantee the sustainability of the telecoupled energy transitions. Integrated LCA-EME was confirmed as a valuable tool for handling complex, multi-nodal nexus problems of telecoupling, which is widely needed for energy transition strategy making.
... As a result, the decomposition efficiency of 90% was obtained at 500 C using 0.5 g NaNH 2 in a 60 sccm NH 3 flow, which is higher than that of the Ru (82%) and Ni (58%) catalysts. 106 Table 3 summarizes the ADR catalysts performance mentioned above. ...
Article
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Nitrogen, one of most important elements on the Earth, plays an essential role in shaping the modern society. The natural nitrogen looping, however, is insufficient to satisfy the high demand of the large‐scale human activities. To achieve a more sustainable and efficient utilization of nitrogen, artificial nitrogen looping by photo(electro)catalytic processes has been considered as a feasible strategy. In this context, the rational design on the high‐performance catalysts for nitrogen looping becomes increasingly important and urgent. On this basis, herein, we provide a timely review on the recent progress, achievements, and essential challenges for the artificial nitrogen looping process, mainly including photo(electro)catalytic transformations among dinitrogen, ammonia, gaseous nitrogen oxides, nitrate, and so on. Especially, the photo(electro)catalysts used in various reactions involved in nitrogen looping, including nitrogen reduction reaction, nitrogen oxidation reaction, ammonia oxidation reaction, ammonia decomposition reaction, etc., are systematically introduced. Finally, we hope that this review will help us deepen the understanding of nitrogen looping‐related photo(electro)catalysts, and further pave a way toward the sustainable development on energy and environment. Artificial nitrogen looping process, mainly including photo(electro)catalytic transformations among dinitrogen, ammonia, gaseous nitrogen oxides, and nitrate, is very essential to the sustainable developments of energy and environment. Especially, high‐performance photo(electro)catalysts used in various reactions involved in nitrogen looping, including nitrogen reduction reaction, nitrogen oxidation reaction, ammonia oxidation reaction, ammonia decomposition reaction, etc., are greatly significant to realize above‐mentioned artificial nitrogen looping process.
... NaNH 2 was then found to be decomposed directly into Na and NH 3 with a small amount of N 2 and H 2 as shown in Figure 24. 278,323 Additionally, the decomposition product Na was proven to be able to react with NH 3 to generate NaNH 2 and H 2 , 324 indicating the decomposition of NaNH 2 is reversible. 5.3.2. ...
Article
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... Its use as Responsible Editor: Santiago V. Luis a carrier for hydrogen storage and transportation needs to address the problem of high energy consumption because its synthesis and liquefaction processes require considerable energy. Spontaneous decomposition of ammonia requires the provision of high temperature heating, generally above 650°C for complete conversion, and even in the presence of a catalyst, it still requires no less than 500°C to achieve complete conversion (David et al. 2014). By comparison, LOHC has the advantages of chemical stability, highly pure hydrogen production, and mild reaction conditions for hydrogen storage and release. ...
Article
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Dehydrogenation reactions are critical in hydrogen storage based on a liquid organic hydrogen carrier (LOHC) system. Speeding up the dehydrogenation rate and lowering the reaction temperature are the main focuses of LOHC dehydrogenation catalysts. In this paper, Pd/SBA-15 catalysts (Pd-IP/S15) were prepared by NaOH treatment of surface hydroxyl groups on SBA-15, the ion exchange of Na⁺ with Pd(NH3)4²⁺, and then reduction of Pd ions via glow discharge plasma. The dehydrogenation performance of dodecahydro-N-ethylcarbazole on the prepared catalysts is studied. The turnover frequency of Pd-IP/S15 is 13.94 min⁻¹ at 170°C, which is 10.25 times that of commercial Pd/C. It is ensured via the ion exchange method that Pd(NH3)4²⁺ could be precisely targeted at the Si-OH of SBA-15 to form Si-O-Pd(NH3)4²⁺, which effectively prevents the aggregation and uncontrollable growth of Pd nanoparticles (NPs) during the in situ reduction by plasma. Pd NPs with high dispersion are obtained on SBA-15, which enhances the catalytic activity of Pd-IP/S15. The coordination of Pd NPs with O of Si-OH on SBA-15 enabled Pd-IP/S15 to exhibit excellent catalytic stability. After 7 dehydrogenation cycles at 180°C, the dehydrogenation efficiency remained above 97%.
... The chemical decomposition of ammonia starts to cycle between sodium amide and sodium metal; however, further details were not discussed. 45 In an analogous manner, Li 2 NH can be used to produce carbon-free hydrogen. Even though an imide is used, it first reacts with ammonia to form lithium amine: ...
Article
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Ammonia can be applied as a hydrogen carrier and used directly as a hydrogen production supply. In this paper, a technique for mass-producing hydrogen from ammonia water is proposed by applying a liquid phase plasma (LPP) discharge technique and a photocatalyst. In this reaction, N- and Fe ion codoped TiO2 (N/Fe/TiO2) photocatalysts were prepared and applied as a visible light-sensitive photocatalyst. N/Fe/TiO2 (NFT) had a similar crystal shape and size to anatase TiO2, but the surface was doped with metal ions. The bandgap of the NFT photocatalyst obtained from the spectrum measured by photoluminescence spectroscopy was approximately 2.4 eV. Nitrogen and Fe ions played a role in narrowing the gap between the conduction band (CB) and valence band (VB) of TiO2, effectively reducing the bandgap. In the decomposition reaction of ammonia water by LPP irradiation, the NFT photocatalyst showed the highest hydrogen evolution rate. The amount of hydrogen produced from ammonia water by LPP irradiation on the NFT photocatalyst was approximately 133 L/h. The hydrogen production rate obtained from ammonia water by the photocatalyst and LPP irradiation was significantly higher than that obtained by the ammonia electrolysis process.
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This review article deals with the challenge to identify catalyst materials from literature studies for the ammonia decomposition reaction with potential for application in large‐scale industrial processes. On the one hand, the requirements on the catalyst are quite demanding. Of central importance are the conditions for the primary reaction that have to be met by the catalyst. Likewise, the catalytic performance, i.e., an ideally quantitative conversion, and a high lifetime are critical as well as the consideration of requirements on the product properties in terms of pressure or by‐products for potential follow‐up processes, in this case synthesis gas applications. On the other hand, the evaluation of the multitude of literature studies poses difficulties due to significant varieties in catalytic testing protocols. Ammonia decomposition over heterogeneous catalysts is one of the central pillars of the strategy to utilize ammonia as hydrogen vector in a greener energy economy and chemical industry. A suitable catalyst material is still to be identified, but with the recent surge in research on this topic, this challenge is likely to be tackled in the near future.
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High surface area tungsten nitride catalysts synthesized from ammonium meta-tungstate and employed as catalysts for ecofriendly H2 production from NH3. A series of tungsten nitride catalysts synthesized by using CiA (citric acid) as chelating agent with different molar ratio of W and CiA. The synthesized materials characterized using BET-surface area, X-ray diffraction, X-ray photoelectron spectroscopy and SEM techniques. The BET value of as-synthesized tungsten nitride was raised from 25 to 80 m² g⁻¹. The influence of amount of CiA in preparation on the catalyst's surface area was investigated. The catalyst performance measured within the desired range of temperature 300–600 °C. A pure phase of tungsten nitride was formed by this preparation method. The catalyst with the ratio of CiA/W = 3 exhibited the best catalytic performance. The increased activity of WN-31 catalyst was mainly due to increased surface area, decreased particle size and high surface concentration. The WN-31 catalyst showed stable performance during time on study for 25 h. These bulk tungsten-based materials are easy to synthesize and highly stable material in the reaction atmosphere.
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Hydrogen energy is a highly efficient and renewable energy carrier. The rapid and sophisticated development of nanotechnologies has promoted the transition of hydrogen storage systems from gaseous/liquid to solid-state. In order to clarify the intrinsic relationship between structure and performance, and to understand the hydrogen absorption and desorption mechanism of materials, electron microscopy (EM) can effectively help us obtain a series of information such as particle size, phase and composition determination, morphology and structure of the materials at nanoscale. The most recent progress of advanced EM techniques applied in solid-state hydrogen storage materials are summarized, which should also inspire future research on energy storage related materials.
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We report that (TMP)Ru(NH3)2 (TMP = tetramesit-ylporphryin) is a molecular catalyst for oxidation of ammonia to dinitrogen. An aryloxy radical, tri-tert-butylphenoxyl (ArO•), abstracts H atoms from a bound ammonia ligand of (TMP)Ru(NH3)2, lead-ing to the discovery of a new catalytic C-N cou-pling to the para position of ArO•, forming 4-amino-2,4,6-tri-tert-butylcyclohexa-2,5-dien-1-one. Modification of the aryloxy radical to contain a trityl group at the para position, 2,6-di-tert-butyl-4-tritylphenoxyl radical, prevents C-N coupling and diverts the reaction to catalytic oxidation of NH3 to give N2. We achieve 125(±5) turnovers at 22 °C for oxidation of NH3, the highest reported to date for a molecular catalyst.
Chapter
Ammonia is one of the best potential hydrogen storage materials, having a high volumetric (121 kg H2/m³) and gravimetric (17.75 wt%) hydrogen capacity. Its properties fully correspond to the DOE’s (Department of Environment, USA) hydrogen storage requirements as a commercial hydrogen storage material. Ammonia can be used for onboard clean (COX free) hydrogen generation (2NH3 ⇔ N2 + 3H2) for fuel cell-driven vehicles. The main challenge of using ammonia to produce clean hydrogen via an onboard catalytic decomposition process necessitates a catalyst able to decompose 100% ammonia at a low temperature (≥400 °C) and supply pure hydrogen to the fuel cell. Currently, only ruthenium-based catalysts showed activity to complete decomposition of ammonia at 400 °C and above but the scarcity of precious ruthenium put an economic constraint in the wide application of ruthenium-based catalysts and drive researchers to look for alternative (non-precious) catalytic materials for this reaction. This chapter describes briefly about hydrogen and current hydrogen production and storage technologies, the cost of hydrogen production from different processes, ammonia and current status of ammonia production followed by a detailed discussion of different ammonia decomposition catalysts.
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A comprehensive DFT study of the electrocatalytic oxidation of ammonia to dinitrogen by a ruthenium polypyridyl complex, [(tpy)(bpy)RuII(NH3)]2+ (complex a), and its NMe2-substituted derivative (b), is presented. The thermodynamics and kinetics of electron (ET) and proton transfer (PT) steps and transition states are calculated. NMe2 substitution on bpy reduces the ET steps on average 8 kcal/mol for complex b as compared to a. The calculations indicate that N–N formation occurs by ammonia nucleophilic attack/H-transfer via a nitrene intermediate, rather than a nitride intermediate. Comparison of the free energy profiles of Ru-b with its first-row Fe congener reveals that the thermodynamics are less favorable for the Fe-b model, especially for ET steps. The N-H bond dissociation free energies (BDFEs) for NH3 to form N2 show the following trend: Ru-b <Ru-a <Fe-b, indicating the lowest and most favorable BDFEs for Ru-b complex.
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Functional materials are the key enabling factor in the development of clean energy technologies. Materials of particular interest, which are reviewed herein, are a class of hydrogenous compound having the general formula of M(XHn)m, where M is usually a metal cation and X can be Al, B, C, N, O, transition metal (TM), or a mixture of them, which sets up an iono‐covalent or covalent bonding with H. M(XHn)m is generally termed as a complex hydride by the hydrogen storage community. The rich chemistry between H and B/C/N/O/Al/TM allows complex hydrides of diverse composition and electronic configuration, and thus tunable physical and chemical properties, for applications in hydrogen storage, thermal energy storage, ion conduction in electrochemical devices, and catalysis in fuel processing. The recent progress is reviewed here and strategic approaches for the design and optimization of complex hydrides for the abovementioned applications are highlighted. The rich chemistry between H and B/C/N/O/Al/transition metal (TM) allows complex hydrides of diverse composition and electronic configuration, and thus tunable physical and chemical properties, with applications in hydrogen storage, thermal energy storage, ion conduction, and catalysis. The recent progress is reviewed and the strategic approaches for the design and optimization of complex hydrides for the abovementioned applications are highlighted.
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Ammine metal borohydrides show potential for solid-state hydrogen storage and can be tailored toward hydrogen release at low temperatures. Here, we report the synthesis and structural characterization of seven new ammine metal borohydrides, M(BH4)3·nNH3, M = La (n = 6, 4, or 3) or Ce (n = 6, 5, 4, or 3). The two compounds with n = 6 are isostructural and have new orthorhombic structure types (space group P21212) built from cationic complexes, [M(NH3)6(BH4)2]+, and are charge balanced by BH4-. The structure of Ce(BH4)3·5NH3 is orthorhombic (space group C2221) and is built from cationic complexes, [Ce(NH3)5(BH4)2]+, and charge balanced by BH4-. These are rare examples of borohydride complexes acting both as a ligand and as a counterion in the same compound. The structures of M(BH4)3·4NH3 are monoclinic (space group C2), built from neutral molecular complexes of [M(NH3)4(BH4)3]. The new compositions, M(BH4)3·3NH3 (M = La, Ce), among ammine metal borohydrides, are orthorhombic (space group Pna21), containing molecular complexes of [M(NH3)3(BH4)3]. A revised structural model for A(BH4)3·5NH3 (A = Y, Gd, Dy) is presented, and the previously reported composition A(BH4)3·4NH3 (A = Y, La, Gd, Dy) is proposed in fact to be M(BH4)3·3NH3 along with a new structural model. The temperature-dependent structural properties and decomposition are investigated by in situ synchrotron radiation powder X-ray diffraction in vacuum and argon atmosphere and by thermal analysis combined with mass spectrometry. The compounds with n = 6, 5, and 4 mainly release ammonia at low temperatures, while hydrogen evolution occurs for M(BH4)3·3NH3 (M = La, Ce). Gas-release temperatures and gas composition from these compounds depend on the physical conditions and on the relative stability of M(BH4)3·nNH3 and M(BH4)3.
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The effect of nitrogen content within the hydrogen fuel supplied to a polymer electrolyte fuel cell (PEFC) operating in dead-ended anode mode is examined, with a view to using an ammonia decomposition product gas mix (containing 75H 2 :25N 2) as the hydrogen-containing fuel. The impact of this impure hydrogen stream, supplied to the anode, was evaluated in terms of mean cell voltage and in relation to actual operating conditions (purge interval, dead-ended interval and fuel cell load). Design of Experiments (DoE) methodology, using multi-linear models, assessed hydrogen utilisation in terms of stack efficiency and identified an effective and viable dead-ended anode purge strategy for this nitrogen-containing hydrogen fuel.
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In the past decade, producing chemicals from renewable energy for use as fuel has gained considerable interest. Renewable hydrogen production (PtH) is the backbone of this power-to-x concept, while further conversion to methanol (PtM) or ammonia (PtA) serves to increase energy density. In this article, we review production and utilization technologies for PtH, PtM, and PtA in the context of the energy and transportation sectors. Specifically, each technology’s basic operating principals, state of development, energy efficiency, dynamic flexibility, and deployment outlook is discussed. We also review recent process systems engineering research of PtH, PtM, and PtA. At the process level, this research largely aims to improve economics through optimal synthesis and design of novel processes as well as coupled real-time operation and control for dynamic operation. At the facility or supply chain level, combined capacity planning and scheduling to optimally use intermittent renewable resources is the major focus.
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Metal amides are promising candidates for hydrogen storage, hydrogen production, NH3 synthesis and cracking etc. However, the decomposition behaviors and mechanisms of metal amides remain unclear. In this study, the decomposition properties of three metal amides, including LiNH2, Mg(NH2)2 and NaNH2, are studied by thermogravimetry, mass spectroscope and in-situ X-ray diffraction techniques combining with density functional theory calculations. It is found that Mg(NH2)2, LiNH2 and NaNH2 exhibit very different metal-N and N-H bond strengths, which precipitates various formations energies of different kinds of vacancies. As a result, LiNH2 releases a major amount of NH3, with a small amount of N2 at a temperature as high as 350 oC. Mg(NH2)2 releases NH3 and N2 synchronously at a temperature range of 300-400 oC without emission of H2. NaNH2 synchronously releases H2, NH3, and a small amount of N2, at a narrow temperature range of 275-290 oC. Using density functional theory calculations, the decomposition behaviors and the corresponding decomposition mechanisms for LiNH2, Mg(NH2)2 and NaNH2 have been well understood.
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Hydrogen storage technology is essentially necessary to promote renewable energy. Many kinds of hydrogen storage materials, which are hydrogen storage alloys, inorganic chemical hydrides, carbon materials and liquid hydrides have been studied. In those materials, ammonia (NH3) is easily liquefied by compression at 1 MPa and 298 K, and has a highest volumetric hydrogen density of 10.7 kg H2/100 L. It also has a high gravimetric hydrogen density of 17.8 wt%. The theoretical hydrogen conversion efficiency is about 90%. NH3 is burnable without emission of CO2 and has advantages as hydrogen and energy carriers.
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Ein molekularer Ru‐Komplex oxidiert NH3 katalytisch zu Distickstoff unter Umgebungsbedingungen. Die Spaltung von sechs N‐H‐Bindungen und die Bildung einer N≡N‐Bindung wurde durch die Kopplung von Protonen‐ und Elektronentransfers unter Verwendung des 2,4,6‐Tri‐tert‐butylphenoxyl‐Radikals (tBu3ArO.) als H‐Atomakzeptor erreicht, resultierend in bis zu 10 katalytischen Umläufen. Abstract Catalysts for the oxidation of NH3 are critical for the utilization of NH3 as a large‐scale energy carrier. Molecular catalysts capable of oxidizing NH3 to N2 are rare. This report describes the use of [Cp*Ru(PtBu2NPh2)(¹⁵NH3)][BArF4], (PtBu2NPh2=1,5‐di(phenylaza)‐3,7‐di(tert‐butylphospha)cyclooctane; ArF=3,5‐(CF3)2C6H3), to catalytically oxidize NH3 to dinitrogen under ambient conditions. The cleavage of six N−H bonds and the formation of an N≡N bond was achieved by coupling H⁺ and e⁻ transfers as net hydrogen atom abstraction (HAA) steps using the 2,4,6‐tri‐tert‐butylphenoxyl radical (tBu3ArO.) as the H atom acceptor. Employing an excess of tBu3ArO. under 1 atm of NH3 gas at 23 °C resulted in up to ten turnovers. Nitrogen isotopic (¹⁵N) labeling studies provide initial mechanistic information suggesting a monometallic pathway during the N⋅⋅⋅N bond‐forming step in the catalytic cycle.
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Catalysts for the oxidation of NH3 are critical for the utilization of NH3 as a large‐scale energy carrier. Molecular catalysts capable of oxidizing NH3 to N2 are rare. This report describes the use of [Cp*Ru(PtBu2NPh2)(15NH3)][BArF4], (PtBu2NPh2 = 1,5‐di(phenylaza)‐3,7‐di(tert‐butylphospha)cyclooctane; ArF = 3,5‐(CF3)2C6H3), to catalytically oxidize NH3 to dinitrogen under ambient conditions. The cleavage of six N‐H bonds and the formation of an N•••N bond was achieved by coupling H+ and e‐ transfers as net hydrogen atom abstraction (HAA) steps using the 2,4,6‐tri‐tert‐butylphenoxyl radical (tBu3ArO•) as the H atom acceptor. Employing an excess of tBu3ArO• under 1 atm of NH3 gas at 23 °C resulted in up to ten turnovers. Nitrogen isotopic (15N) labeling studies provide initial mechanistic information suggesting a monometallic pathway during the N•••N bond forming step in the catalytic cycle.
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A study was conducted on the decomposition reaction of liquid hydrocarbons induced by liquid plasma and photocatalyst. This study is to obtain useful resources by recycling waste liquid-hydrocarbons. Titanium dioxide and TiO2-doped with a metal (and nitrogen atoms) were used as photocatalysts individually in the study. First, H2 and carbon were simultaneously produced in the decomposition reaction of liquid hexane and benzene induced by liquid plasma. Notably, only H2 was obtained as a gaseous product, and no carbon dioxide was produced in this reaction. However, when photocatalyst was injected into this decomposition, the corresponding reactivity improved. In particular, the amount of H2 and carbon produced increased in the order of: the amount of H2 and carbon produced with N/Ni/TiO2 > Ni/TiO2 > TiO2 photocatalyst, respectively, when the photocatalyst was added in an equal amount to the reactant of the decomposition. The highest carbon yield and hydrogen evolution under these experimental conditions were about 1 %/(g∙h) and about 180 L/(g∙h), respectively. This order is because the strong visible light generated by liquid plasma discharge improved the photoreactivity of the N/Ni/TiO2 photocatalyst that has a high sensitivity to visible light and narrow band gap. Carbon particles produced from the decomposition reaction of liquid hexane and benzene, respectively, induced by liquid plasma and photocatalyst were small and uniform, with a size of 10 nm or less. In addition, the BET surface area of the carbon produced in this reaction was greater than 500 m²/g, and the properties of the carbon particles were almost the same regardless of the lapse of reaction time. Hence, this decomposition reaction of liquid hexane and benzene, respectively, induced by liquid plasma and photocatalyst was judged to be forming an efficient technology that can simultaneously obtain high-purity hydrogen energy and useful basic materials from liquid hydrocarbons.
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Ammonia is a promising material as a direct source of green hydrogen production. This paper reports a method for mass production of hydrogen from liquid NH3(NH4OH) through a photocatalytic decomposition reaction using liquid plasma. In this reaction, the highest hydrogen production rate was observed in the TiO2 photocatalyst doped with N and metal ions as a photocatalyst sensitive to visible light with a low bandgap. At this time, the hydrogen production rate was obtained as about 142 L/g∙h. This is due to the high photoactivity of the visible light-sensitive photocatalyst in liquid plasma emitting strong visible light and ultraviolet light. The H2 production rate obtained from the decomposition of liquid NH3 by plasma discharge to the catalyst was higher than the H2 production rate obtained from the NH3 electrolysis process.
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Ammonia is an important chemical for human beings for the synthesis of chemical fertilizers and products, while it is also a hazardous compound, which causes undesirable odors, several diseases and environmental problems. So there is an urgent need to control and remove ammonia pollutant from water, air and soil. Hence, the clean process using photocatalysis to convert ammonia into H2 and N2 have been an important research topic in recent years. Up to now, only some metal-loaded common photocatalysts, such as TiO2, ZnO, C3N4, graphene and other carbon-based materials, together with their hybrid materials have been reported as active photocatalysts for the decomposition of an aqueous ammonia solution. In this review, we summarize these recent advantages of heterogeneous nanostructures for photocatalytic ammonia decomposition. Particular emphasis also will be given to the metal-loading along with their based heterojunction. Furthermore, the recent efforts toward the development of building heterogeneous nanostructures for photocatalytic ammonia decomposition in this direction have been discussed and appraised. At last, the perspective and future opportunities on the challenges and future directions in the area of heterogeneous photocatalysts for ammonia decomposition are also provided.
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Chemical looping combustion (CLC) between Fe2O3 and NH3 can lead to the generation of H2O, H2, NO, and N2 as the main product under different reaction stages, reasonable control of which can realize effective conversion and utilization of NH3. To initiate the CLC reaction, NH3 is chemically adsorbed on the perfect Fe2O3 surface with a hybrid between N and Fe, leading to the dehydrogenation of NH3 into *NH2 as the first reaction step. Then the second dehydrogenation step (*NH2→*NH) acts as the speed‐control step for the oxidation of NH3 into H2O and N2, leading to the reduction of Fe2O3. The reduction of Fe2O3 promotes the further adsorption of NH3, especially the intermediate species *NH2, *NH, and *N, which favors the generation of H2O and N2. Further reduction of Fe2O3 into the oxidation state lower than ~Fe2O2.25 shows lower surface oxygen potential, which is beneficial to the formation of H2 and N2. Results suggest that reasonable control of the oxidation state of iron oxide can optimize the NH3 CLC process for H2 production.
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The amides Na 3 RE (NH 2 ) 6 have been obtained from the metals in supercritical ammonia under ammonobasic conditions at 573 K and 70 MPa for RE = La–Nd, and at 473 K and 40 MPa for RE = Er, Yb. All compounds are formed in the hot zone within a temperature gradient, indicating a retrograde solubility under the applied process conditions. These amides represent soluble intermediates in ammonothermal binary rare earth metal nitride synthesis. All compounds were obtained as microcrystalline powders, while single crystals of those amides containing the heavier rare earth metals could be isolated. The crystal structures were solved and refined from single-crystal and powder X-ray diffraction intensity data. The results of vibrational spectroscopy are reported. Thermal analysis measurements under inert gas atmosphere demonstrated a decomposition to the respective black binary rare earth metal nitrides RE N 1− δ .
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Earth‐abundant metal‐catalyzed oxidative conversion of ammonia into dinitrogen is a promising process to utilize ammonia as a transportation fuel. Herein, we report the manganese‐catalyzed ammonia oxidation under chemical and electrochemical conditions using manganese complex bearing (1S,2S)‐N,N'‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediamine. Under chemical conditions using oxidant, up to 17.1 equivalents of N2 per catalyst are generated. In addition, the catalytic reaction proceeds with 96% faradaic efficiency (FE) in bulk electrolysis. Also, mechanistic studies including density functional theory (DFT) calculations reveal that a nucleophilic attack of ammonia on manganese imide complexes occurs to form a nitrogen–nitrogen bond leading to dinitrogen.
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On-site produced hydrogen from ammonia decomposition can directly fuel solid oxide fuel cells (SOFCs) for power generation. The key issue in ammonia decomposition is to improve the activity and stability of the reaction at low temperatures. In this study, proton-conducting oxides, Ba(Zr,Y) O3-δ (BZY), were investigated as potential support materials to load Ni metal by a one-step impregnation method. The influence of Ni loading, Ba loading, and synthesis temperature, of Ni/BZY catalysts on the catalytic activity for ammonia decomposition were investigated. The Ni/BZY catalyst with Ba loading of 20 wt%, Ni loading of 30 wt%, and synthesized at 900 °C attained the highest ammonia conversion of 100% at 600 °C. The kinetics analysis revealed that for Ni/BZY catalyst, the hydrogen poisoning effect for ammonia decomposition was significantly suppressed. The reaction order of hydrogen for the optimized Ni/BZY catalyst was estimated as low as −0.07, which is the lowest to the best of our knowledge, resulting in the improvement in the activity. H2 temperature programmed reduction and desorption analysis results suggested that a strong interaction between Ni and BZY support as well as the hydrogen storage capability of the proton-conducting support might be responsible for the promotion of ammonia decomposition on Ni/BZY. Based on the experimental data, a mechanism of hydrogen spillover from Ni to BZY support is proposed.
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This paper focuses on the challenges, opportunities and future potentials with ammonia as a carbon-free fuel, and covers recent technological solutions to overcome the barriers with the production, storage and usage of green ammonia. One way to decarbonize the energy industry is by converting electrical energy into chemical energy via water electrolysis to produce hydrogen. Hydrogen can then be stored, used in a fuel cell to generate electricity, or burnt cleanly with air to generate heat, steam, producing only water as a by-product. However, hydrogen has an extremely low density, even when compressed, which means that its energy density on a volumetric basis remains distinctly substandard to most liquid fuels, hydrogen also has a much wider range of concentrations over which it remains potentially explosive. Ammonia alternatively is ~ 18% hydrogen by weight, which means that in terms of hydrogen density, it is ~ 50% higher than compressed or liquefied hydrogen. One major advantage is that there is an existing infrastructure for the production, transport and distribution of ammonia worldwide. Although ammonia in theory can be combusted to produce only nitrogen and water as emissions, in practice, several challenges arise, nitrous oxides (NOx) are often generated, especially if the combustion happens at higher temperatures and/or under pressure, in vehicle engines, gas turbines and as rocket fuel. To overcome such challenges, further research into ammonia combustion phenomena is required. This review sheds light on recent technological advancements with ammonia from the production point to the utilization end point. Moreover, the study concludes with a techno-economic evaluation and global market trends of ammonia in the COVID-19 crises.
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ConspectusThe fixation of dinitrogen to ammonia is critically important for the biogeochemical cycle on earth. Ammonia also holds promise as a sustainable energy carrier. Tremendous effort has been devoted to the development of green processes and advanced materials for ammonia synthesis and decomposition under milder conditions, and encouraging progress has been made.The reduction of dinitrogen to ammonia needs electrons and protons, which hydridic hydrogen H- could supply. Polarized, electron-rich N x H y intermediates, on the other hand, can be stabilized by alkali or alkaline earth metal cations to lower kinetic barriers in the transformation. The inherent properties of alkali/alkaline earth metal hydrides (denoted as AH) endow them with a unique function in ammonia synthesis.In this Account, recent efforts in the exploration of alkali or alkaline earth metal hydrides (denoted as AH), amides, and imides (denoted as ANH hereafter) for ammonia synthesis and decomposition reactions will be summarized and discussed. We begin with an introduction to the chemistry of A with N2, NH3, and H2, highlighting the interconversion between AH and ANH that has profound implications on the formation and decomposition of NH3. We then present our finding on the strong synergistic effect between ANH and transition metals (TM) in ammonia decomposition catalysis, which stimulated our subsequent research on AH for ammonia synthesis. We discuss the effect and function mechanism of AH in the thermocatalytic and chemical looping ammonia synthesis processes. In the thermocatalytic process, AH cooperates with both early and late TM forming either composite catalysts with two active centers or complex metal hydride catalysts with electron- and hydrogen-rich ionic centers facilitating ammonia synthesis with high activities at lower temperatures. Very interestingly, AH levels the catalytic performances of TMs and intervenes in the energy-scaling relations of TM-only catalysts. Moreover, ANH serves as a new type nitrogen carrier effectively mediating ammonia synthesis via a low-temperature chemical looping process, in which N2 is fixed by AH forming ANH. Subsequently, ANH is hydrogenated to ammonia and AH. Late TMs have a strong catalytic effect on the chemical looping process. The unique interplay of A, N, TM, and H- offers plenty of opportunities for achieving dinitrogen conversion under mild conditions, while further efforts are needed to address the challenges in the fundamental understanding and practical application.
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Efficient storage and conversion of renewable energies is of critical importance to the sustainable growth of human society. With its distinguishing features of high hydrogen content, high energy density, facile storage/transportation, and zero‐carbon emission, ammonia has been recently considered as a promising energy carrier for long‐term and large‐scale energy storage. Under this scenario, the synthesis, storage, and utilization of ammonia are key components for the implementation of ammonia‐mediated energy system. Being different from fossil fuels, renewable energies normally have intermittent and variable nature, and thus pose demands on the improvement of existing technologies and simultaneously the development of alternative methods and materials for ammonia synthesis and storage. The energy release from ammonia in an efficient manner, on the other hand, is vital to achieve a sustainable energy supply and complete the nitrogen circle. Herein, recent advances in the thermal‐, electro‐, plasma‐, and photocatalytic ammonia synthesis, ammonia storage or separation, ammonia thermal/electrochemical decomposition and conversion are summarized with the emphasis on the latest developments of new methods and materials (catalysts, electrodes, and sorbents) for these processes. The challenges and potential solutions are discussed.
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The spectroscopic properties and anharmonic force field of NaNH2 are studied in present work by DFT (B3P86 and B3PW91) and MP2 methods in combination with 6-311++G(2d, 2p) and 6-311++G(3df, 2pd) basis sets. The calculated equilibrium geometry, ground state rotational constants and centrifugal distortion constants of NaNH2 at B3P86/6-311++G(3df, 2pd) theoretical level agree very well with the corresponding experimental values. Noteworthy, some spectroscopic constants and anharmonic force fields of NaNH2, which have not been experimentally measured, are firstly predicted. In addition, the spectroscopic properties of KNH2 are also predicted at the B3P86/6-311++G(3df, 2pd) level of theory. The influences of metal atoms on the equilibrium geometry, anharmonic constants, rotational constants, centrifugal distortion constants of MNH2 (M=Li, Na, K) are analyzed intuitively. One can find that the metal atoms affect the rotational constants, part of centrifugal distortion constants (DK, DJK, HK, and HKJ), M-N bond length and some anharmonic constants of MNH2.
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The La based perovskite type LaMO3 (M=Ni, Co) oxides were prepared by combustion synthesis method using citric acid as organic fuel. These catalyst precursors tested for ammonia decomposition. The LaNiO3 and LaCoO3 catalysts showed good activity for NH3 decomposition. The LaNiO3 catalyst displayed greater activity than LaCoO3. This due to high surface area and easily reducibility of Ni species. A 50% of La was substituted by Ce in both LaNiO3 and LaCoO3 catalysts. A remarkable effect on catalytic performance was observed with the partial substitution of La by Ce in perovskite catalyst especially at lower temperatures. The La0.5Ce0.5NiO3 catalyst exhibited highest activity among all prepared samples. The achieved superior activity is due to boost in surface area, reducibility and suitable basicity. The SEM elemental mapping of La0.5Ce0.5NiO3 catalyst concluded that metal oxide constituents dispersed homogeneously. The La0.5Ce0.5NiO3 catalyst showed excellent stable catalytic performance during 50h time on study at 550°C.
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Electrides are materials in which electrons serve as anions. Here, the concept of inorganic electrides is extended in several respects: from ionic crystals to intermetallic compounds in host materials, from crystalline to amorphous solids, and from 0-dimensional to 1- and 2-dimensional materials in electron-confined spaces. In particular, 2D electrides, in which anionic electrons are sandwiched by cationic slabs, can form a bulk crystal of a 2-dimensional electron gas, thus exhibiting a large electron mobility and providing a platform for topological materials. Exploration of new electrides by computation and high pressure has advanced, revealing that an electride is a stable equilibrium phase of many elements and compounds under high pressure. This review describes the history and current status of electride research and next summarizes the chemical application of electrides and relevant materials. An emphasis is placed on catalysts for ammonia synthesis from N2 and H2 at mild conditions. This subject is accelerated by a demand for on-site ammonia synthesis using hydrogen produced by renewable energy sources. A wide applicability of electride for chemical reactions such selective hydrogenation and carbon-carbon coupling is shown by extending the concept of electrides. Finally, a view for the relationship between electrides and crystallographic voids and current issues are described.
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Hydrogen is ideal for producing carbon-free and clean-green energy with which to save the world from climate change. Proton exchange membrane fuel cells use to hydrogen to produce 100% clean energy, with water the only by-product. Apart from generating electricity, hydrogen plays a crucial role in hydrogen-powered vehicles. Unfortunately, the practical uses of hydrogen energy face many technical and safety barriers. Research into hydrogen generation and storage and reversibility transportation are still in its very early stages. Ammonia (NH3) has several attractive attributes, with a high gravimetric hydrogen density of 17.8 wt.% and theoretical hydrogen conversion efficiency of 89.3%. Ammonia storage and transport are well-established technologies, making the decomposition of ammonia to hydrogen the safest and most carbon-free option for using hydrogen in various real-time applications. However, several key challenges must be addressed to ensure its feasibility. Current ammonia decomposition technologies require high temperatures, pressures and non-recyclable catalysts, and a sustainable decomposition mechanism is urgently needed. This review article comprehensively summarises current knowledge about and challenges facing solid-state storage of ammonia and decomposition. It provides potential strategic solutions for developing a scalable process with which to produce clean hydrogen by eliminating possible economic and technical barriers.
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A series of Co–Ce–Al–O mesoporous catalysts synthesised by self-assembly is shown to be efficient for the hydrogen production via ammonia decomposition reaction. The evaporation-induced self-assembly method is utilised to effectively synthesise mesoporous multicomponent catalysts with tuned composition and improved performance. The prepared catalyst materials were characterised by several techniques including X-ray diffraction, scanning electron microscope, transmission electron microscope and N2 physisorption analysis. The kinetic performance of the solid catalysts was evaluated at different temperatures and flow rates under atmospheric pressure. The optimised Co0.5Ce0.1Al0.4O(sa) catalyst remained highly active below 550 °C which represents a competitive performance among state-of-the-art catalysts. The promising performance of the catalyst and the use of nonprecious metals in the structure represent a feasible approach towards the application of ammonia as a hydrogen carrier for on-board hydrogen production.
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Shipping, which accounts for 2.6% of global CO2 emissions, is urged to find clean energy solutions to decarbonize the industry and achieve IMO’s GHG emission targets by 2050. It is generally believed that hydrogen will play a vital role in enabling the use of renewable energy sources. However, issues related with hydrogen storage and distribution currently obstruct its implementation. Alternatively, an energy-carrier such as ammonia with its carbon neutral chemical formula, high energy density and established production, transportation and storage infrastructure could provide a practical short-term next generation power solution for maritime industry. This paper presents an overview of the state-of-the-art and emerging technologies for decarbonising shipping using ammonia as a fuel. The review covers general properties of ammonia, the current production technologies with an emphasis on green synthesis methods, safety and environmental aspects, onboard storage, and ways to generate power from it. The challenges for the adaptation of technology to maritime structure as well as an insight for the level of costs during fuel switching are also discussed to provide perspectives and a roadmap for future development of the technology.
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Over the past years, hydrogen has been identified as the most promising carrier of clean energy. In a world that aims to replace fossil fuels to mitigate greenhouse emissions and address other environmental concerns, hydrogen generation technologies have become a main player in the energy mix. Since hydrogen is the main working medium in fuel cells and hydrogen-based energy storage systems, integrating these systems with other renewable energy systems is becoming very feasible. For example, the coupling of wind or solar systems hydrogen fuel cells as secondary energy sources is proven to enhance grid stability and secure the reliable energy supply for all times. The current demand for clean energy is unprecedented, and it seems that hydrogen can meet such demand only when produced and stored in large quantities. This paper presents an overview of the main hydrogen production and storage technologies, along with their challenges. They are presented to help identify technologies that have sufficient potential for large-scale energy applications that rely on hydrogen. Producing hydrogen from water and fossil fuels and storing it in underground formations are the best large-scale production and storage technologies. However, the local conditions of a specific region play a key role in determining the most suited production and storage methods, and there might be a need to combine multiple strategies together to allow a significant large-scale production and storage of hydrogen.
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Oxidation of ammonia by molecular complexes is a burgeon-ing area of research, with critical scientific challenges that must be addressed. A fundamental understanding of individu-al reactions steps is needed, particularly for cleavage of N-H bonds and formation of N-N bonds. This perspective evaluates the challenges of designing molecular catalysts for oxidation of ammonia, and highlights recent key contributions to realiz-ing the goals of viable energy storage and retrieval based on the N-H bonds of ammonia in a carbon-free energy cycle.
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Double hydrogen atom abstraction from (TMP)OsII(NH3)2 (TMP = tetramesitylporphyrin) with phenoxyl or nitroxyl radicals leads to (TMP)OsIV(NH2)2. This unusual bis(amide) complex is diamagnetic and displays an N-H resonance at 12.0 ppm in its 1H NMR spectrum. 1H-15N correlation experiments identified a 15N NMR spectroscopic resonance signal at -267 ppm. Experimental reactivity studies and density functional theory calculations support relatively weak N-H bonds of 73.3 kcal/mol for (TMP)OsII(NH3)2 and 74.2 kcal/mol for (TMP)OsIII(NH3)(NH2). Cyclic voltammetry experiments provide an estimate of the pKa of [(TMP)OsIII(NH3)2]+. In the presence of Barton's base, a current enhancement is observed at the Os(III/II) couple, consistent with an ECE event. Spectroscopic experiments confirmed (TMP)OsIV(NH2)2 as the product of bulk electrolysis. Double hydrogen atom abstraction is influenced by π donation from the amides of (TMP)OsIV(NH2)2 into the d orbitals of the Os center, favoring the formation of (TMP)OsIV(NH2)2 over N-N coupling. This π donation leads to a Jahn-Teller distortion that splits the energy levels of the dxz and dyz orbitals of Os, results in a low-spin electron configuration, and leads to minimal aminyl character on the N atoms, rendering (TMP)OsIV(NH2)2 unreactive toward amide-amide coupling.
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DOE Position Paper on the potential role of ammonia in a hydrogen economy. Focused on vehicular applications
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The possibility of using ammonia as a hydrogencarrier is discussed. Compared to other hydrogenstorage materials, ammonia has the advantages of a high hydrogen density, a well-developed technology for synthesis and distribution, and easy catalyticdecomposition. Compared to hydrocarbons and alcohols, it has the advantage that there is no CO2 emission at the end user. The drawbacks are mainly the toxicity of liquid ammonia and the problems related to trace amounts of ammonia in the hydrogen after decomposition. Storage of ammonia in metal ammine salts is discussed, and it is shown that this maintains the high volumetric hydrogen density while alleviating the problems of handling the ammonia. Some of the remaining challenges for research in ammonia as a hydrogencarrier are outlined.
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On 13 October 1908, Fritz Haber filed his patent on the ``synthesis of ammonia from its elements'' for which he was later awarded the 1918 Nobel Prize in Chemistry. A hundred years on we live in a world transformed by and highly dependent upon Haber-Bosch nitrogen.
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The possible role of ammonia in a future energy infrastructure is discussed. The review is focused on the catalytic decomposition of ammonia as a key step. Other aspects, such as the catalytic removal of ammonia from gasification product gas or direct ammonia fuel cells, are highlighted as well. The more general question of the integration of ammonia in an infrastructure is also covered.
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Nanosized Na2Ti3O7, K2Ti6O13 and Cs2Ti6O13 materials were prepared and used as supports of ruthenium nanoparticles for catalytic ammonia decomposition. It is shown that these catalysts exhibit higher catalytic activity than ruthenium supported on TiO2 nanoparticles promoted with cesium. The difference is attributed to the use of nanostructured materials with incorporated alkali metals in the crystal lattice, which apparently gives a higher effect of the promoter. All samples were characterized by X-ray powder diffraction, transmission electron microscopy and N2 physisorption measurements. Furthermore, the effect of ruthenium loading on the catalytic decomposition of ammonia was investigated.
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Here we report the first in situ high-pressure study of sodium amide (NaNH(2)) as an agent in potential hydrogen storage applications by using combined Raman and infrared (IR) spectroscopies at room temperature and pressures up to ~16 GPa. Starting with an orthorhombic crystal structure at ambient pressure, sodium amide was found to transform to two new phases upon compression as evidenced by the changes in the characteristic Raman and IR modes as well as by examining the pressure dependences of these modes. Raman and IR measurements on NaNH(2) collectively provided consistent information about the structural evolutions of NaNH(2) under compression. Upon decompression, all Raman and IR modes were completely recovered, indicating the reversibility of the pressure-induced transformations in the entire pressure region. The combined Raman and IR spectroscopic data together allowed for the analysis of possible structures of the new high-pressure phases of NaNH(2).
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The prerequisite for widespread use of hydrogen as an energy carrier is the development of new materials that can safely store it at high gravimetric and volumetric densities. Metal borohydrides M(BH4)n (n is the valence of metal M), in particular, have high hydrogen density, and are therefore regarded as one such potential hydrogen storage material. For fuel cell vehicles, the goal for on-board storage systems is to achieve reversible store at high density but moderate temperature and hydrogen pressure. To this end, a large amount of effort has been devoted to improvements in their thermodynamic and kinetic aspects. This review provides an overview of recent research activity on various M(BH4)n, with a focus on the fundamental dehydrogenation and rehydrogenation properties and on providing guidance for material design in terms of tailoring thermodynamics and promoting kinetics for hydrogen storage.
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Mobility--the transport of people and goods - is a socioeconomic reality that will surely increase in the coming years. It should be safe, economic and reasonably clean. Little energy needs to be expended to overcome potential energy changes, but a great deal is lost through friction (for cars about 10 kWh per 100 km) and low-efficiency energy conversion. Vehicles can be run either by connecting them to a continuous supply of energy or by storing energy on board. Hydrogen would be ideal as a synthetic fuel because it is lightweight, highly abundant and its oxidation product (water) is environmentally benign, but storage remains a problem. Here we present recent developments in the search for innovative materials with high hydrogen-storage capacity.
Article
We report the experimental investigation of hydrogen storage and release in the lithium amide-lithium hydride composite (Li-N-H) system. Investigation of hydrogenation and dehydrogenation reactions of the system through in situ synchrotron X-ray powder diffraction experiments allowed for the observation of the formation and evolution of non-stoichiometric intermediate species of the form Li1+xNH2-x. This result is consistent with the proposed Frenkel-defect mechanism for these reactions. We observed capacity loss with decreasing temperature through decreased levels of lithium-rich (0.7 ≤ x ≤ 1.0) non-stoichiometric phases in the dehydrogenated material, but only minor changes due to multiple cycles at the same temperature. Annealing of dehydrogenated samples reveals the reduced stability of intermediate stoichiometry values (0.4 ≤ x ≤ 0.7) compared with the end member species: lithium amide (LiNH2) and lithium imide (Li2NH). Our results highlight the central role of ionic mobility in understanding temperature limitations, capacity loss and facile reversibility of the Li-N-H system.
Article
Materialization of a hydrogen economy could provide a solution to significant global challenges. In particular, the possibility of improving the efficiency and simultaneously minimizing the environmental impact of energy conversion processes, together with the opportunity to reduce the dependency of fossil fuels, are main drivers for the currently increasing research and development efforts. However, significant technological breakthroughs are necessary for making a hydrogen economy feasible. In particular, it is necessary to develop appropriate hydrogen storage and transportation technologies. Recently, metal ammine salts were proposed as safe, reversible, high-density and low-cost hydrogen carriers. Here, we discuss how this development could provide a platform for using ammonia as a fuel for the hydrogen economy. We do that by comparing various possible hydrogen carriers with respect to energy and cost efficiency, infrastructure requirements, safety concerns and also environmental impact. Based on this, it appears that in several scenarios, the use of metal ammines offers significant new opportunities.
Article
The strong efforts devoted to the exploration of BNH compounds for hydrogen storage have led to impressive advances in the field of boron chemistry. This review summarizes progress in this field from three aspects. It starts with the most recent developments in using BNH compounds for hydrogen storage, covering NH3BH3, B3H8− containing compounds, and CBN compounds. The following section then highlights interesting applications of BNH compounds in hydrogenation and catalysis. The last part is focused on breakthroughs in the syntheses and discovery of new BNH organic analogues. The role of N–Hδ+Hδ−–B dihydrogen interactions in molecule packing, thermal hydrogen evolution, and syntheses is also discussed within the review.
Article
Energy production and combating climate change are among some of the most significant challenges we are facing today. Whilst the introduction of a hydrogen economy has its merits, the associated problems with on-board hydrogen storage are still a barrier to implementation. Ammonia and related chemicals may provide an alternative energy vector. Besides ammonia and metal amine salts, some other ammonia related materials such as hydrazine, ammonia borane, ammonia carbonate and urea also have the potential for use as alternative fuels. These materials conform to many of the US DOE targets for hydrogen storage materials.Similar to hydrogen, ammonia itself is carbon-free at the end users, although CO2 emission on production of ammonia is dependent on the source of energy. Both hydrogen and ammonia utilised similar energy sources for production: fossil fuels, biomass, renewable electricity, nuclear and solar energy.While a number of papers have been published on the catalytic decomposition of ammonia or related chemicals to produce hydrogen, the use of fuel cells directly fed by ammonia and related chemicals would have a higher efficiency. In recent years significant progress has been made on direct ammonia, hydrazine and urea fuel cells to generate electricity from these materials for transport applications. With the development and application in these technologies, reduction of CO2 emissions in transportation would be possible.In this review, we propose the use of ammonia and related chemicals as potential indirect hydrogen storage materials. The progress on fuel cells using these fuels is also briefly reviewed.
Article
Carbon-supported ruthenium catalysts promoted with Ba or Cs were studied in ammonia decomposition. Under the experimental conditions (p=1 bar, H2:N2=3:1, 5–50% NH3, 370–400°C), the reaction rates over Cs–Ru/carbon were found to be higher than those over Ba–Ru/carbon, the difference being larger for the high dispersion samples. The effect of the ruthenium precursor (carbonyl, chloride) proved to be unessential for the activity. At 20% NH3 (400°C), TOF of NH3 decomposition over Cs–Ru/carbon was about 3×102 times higher than for K–Fe/carbon, both based on H2 chemisorption. The apparent activation energies of 134 and 158 kJ/mol were determined for Cs–Ru/carbon and Ba–Ru/carbon, respectively. The temperature-programmed desorption studies revealed that the amount of nitrogen desorbed from Ba–Ru/carbon was much smaller and the peak position was shifted to higher temperatures when compared to Cs–Ru/carbon. The promoting mechanism of both Ba and Cs is discussed.
Article
Following preparative work, the behavior of amides towards excess ammonia (ammoniate formation) and on removal of ammonia (degradation to imides)was studied particularly with regard to the solid state (crystal structure) and the transition to the liquid phase (melting point). Energy relationships are discussed on the basis of enthalpies of formation. Studies on ternary amides and imides are still in the initial stage. – Emphasis is laid here on the establishment of parallels between amides and hydroxides as well as between the aquo and ammono systems, and also on the differences between the amides with their dipolar NH2- anion, and the halides. The results, although still incomplete, permit a survey to be given of experimental data with the aid of information obtained from the pertinent literature.
Article
Hydrogen, free from trace compounds that poison PEM fuel cell electrodes, is produced by catalytic decomposition of ammonia at low temperature in micro-fabricated reactors; this constitutes a possible option for easy and compact supply of fuel to miniaturized devices. A versatile method for depositing support materials in micro-fabricated reactors that also allows convenient introduction of active components by conventional impregnation methods is reported. This allows for easy screening of various metal catalyst since conventional catalyst preparation techniques can be applied in miniaturized form. For the catalytic decomposition of ammonia with transition metal catalysts, a volcano-type relation is found to exist analogously to that for the synthesis reaction but apparently shifted towards metals with lower metal–nitrogen bond energies. By use of supported ruthenium catalysts promoted with barium, highly efficient hydrogen production for miniaturized, and possibly mobile, units is demonstrated. If appropriate sequestration strategies are implemented at the central ammonia production facilities this provides a CO2-free energy currency.
Article
This paper proposes the use of ammonia as a multipurpose energy vector. Synthesized from hydrogen produced in a large, centralized facility using nuclear process heat from a high-temperature, gas-cooled reactor (HTGR), the ammonia serves as a low-cost vehicle for energy storage and transmission, via pipeline, to remote demand centers where some of it serves as a clean-burning fuel for local cogeneration and process heat applications, and some of it is used for direct agricultural application or as feedstock for production of nitrogen-based fertilizers or other chemical processes.
Article
The interaction between Mg(NH2)2 and NaH enable Mg–Na–N–H potential candidate for hydrogen storage. Upon heating to 120 °C the mixture of Mg(NH2)2 and NaH gives out hydrogen and converts to a new Mg–Na–N–H structure, which can re-absorb hydrogen at temperature as low as 60 °C. Increasing NaH content enhances the chemical stability of the Mg–Na–N–H system; however, it also leads to the high-temperature shift of hydrogen absorption. The desorption plateau is relatively flat, however, pressures are generally low at temperature below 200 °C.
Article
The reactions xLiNH2 + (1 − x)LiBH4 and xNaNH2 + (1 − x)NaBH4 have been investigated and new phases identified. The lithium amide–borohydride system is dominated by a body centred cubic compound of formula Li4BH4(NH2)3. In the sodium system, a new hydride of approximate composition Na2BH4NH2 has been identified with a primitive cubic structure and lattice parameter a ≈ 4.7 Å. The desorption of gases from the two amide–borohydrides on heating followed a similar pattern with the relative proportions of H2 and NH3 released depending critically on the experimental set-up: in the IGA, ammonia release occurred in two steps – beginning at 60 and 260 °C for Li4BH4(NH2)3 – the second of which was accompanied by hydrogen release; in the TPD system the main desorption product was hydrogen—again at 260 °C for Li4BH4(NH2)3 accompanied by around 5% ammonia. We hypothesize that the BH4− anion can play a similar role to LiH in the LiNH2 + LiH system, where ammonia release is suppressed in favour of hydrogen. The reaction xLiNH2 + (1 − x)LiAlH4 did not result in the production of any new phases but TPD experiments show that hydrogen is released from the mixture 2LiNH2 + LiAlH4, over a wide temperature range. We conclude that mixed complex hydrides may provide a means of tuning the dehydrogenation and rehydrogenation reactions to make viable storage systems.
Chapter
Anhydrous ammonia is discussed as an easy to handle and commercially widely available energy carrier. Facilities for storage and transport by sea and land from ammonia producers to major customers are available throughout the world. Pure liquid ammonia, stored in low pressure vessels, is comparable in energy density as well as in price with methanol. Ammonia is cracked on-site or on-board into hydrogen and nitrogen. Traces of residual ammonia in the feed stream have to be removed when an acidic electrolyte fuel cell is used. For alkaline fuel cells no gas purification is necessary.
Article
Magnesia–carbon nanotubes (abbreviated as MgO–CNTs) nanocomposites were prepared by impregnation of CNTs with Mg(NO3)26H2O in ethanol solution, followed by drying at 353 K and calcination at 873 K, respectively. The nanocomposites are thermally more stable than CNTs in a H2 flow. The use of the nanocomposites as support yielded more efficient Ru catalysts for the generation of CO x -free hydrogen from NH3 decomposition.
Article
In recent years there has been increasing focus on using metal ammine complexes for ammonia storage. In this paper a fuel system for ammonia fuelled internal combustion engines using metal ammine complexes as ammonia storage is analyzed. The use of ammonia/hydrogen mixtures as an SI-engine fuel is investigated in the same context. Ammonia and hydrogen were introduced into the intake manifold of a CFR-engine. Series of experiments with varying excess air ratio and different ammonia to hydrogen ratios was conducted. This showed that a fuel mixture with 10 vol.% hydrogen performs best with respect to efficiency and power. A comparison with gasoline was made, which showed efficiencies and power increased due to the possibility of a higher compression ratio. The system analysis showed that it is possible to cover a major part of the necessary heat using the exhaust heat. It is proposed to reduce the high NOx emissions using SCR as exhaust after treatment.
Article
Ammonia (NH3) is a non-polluting fuel which produces only water and nitrogen as products of combustion. Therefore, it could be an alternative to hydrogen for vehicle motive power in the hydrogen economy. For this role ‘electrolytic ammonia’ would be prepared by catalytic combination of electrolytic hydrogen and atmospheric nitrogen. The background and developmental status of hydrogen and ammonia as motor-vehicle fuels are reviewed. Engine tests have demonstrated that ammonia can replace gasoline or diesel fuel for motor vehicles, giving near-theoretical values of engine power and efficiency. Ammonia is superior to hydrogen as a vehicle fuel for several reasons: it can be stored and transported as a liquid at ambient temperatures in low-pressure containers; per unit volume ammonia has 1.3 times the heating value of liquid hydrogen; ammonia is distributed internationally in quantities of over 100 million tons per year, and procedures and facilities are established world-wide for its safe handling and distribution. These factors would greatly facilitate the commercial adoption of ammonia as a practical replacement for carbonaceous fuels. The projected cost of supplying ‘electrolytic ammonia’ to motor vehicle filling stations is estimated to be roughly half the cost of supplying electrolytic liquid hydrogen for the same purpose, i.e. $10.5–12.5 GJ−1 for ammonia vs $25–30 GJ−1 for LH2 (1988$). A summary is presented of the physical and thermo-chemical characteristics and estimated costs of ammonia in comparison with hydrogen, as liquid, compressed gas or stored as metal hydride. Properties of gasoline, methanol, ethanol and liquified methane are also listed.
Article
This paper gives an overview of the current status and future potential of hydrogen storage from a chemistry perspective and is based on the concluding presentation of the Faraday Discussion 151--Hydrogen Storage Materials. The safe, effective and economical storage of hydrogen is one of the main scientific and technological challenges in the move towards a low-carbon economy. One key sector is transportation where future vehicles will most likely be developed around a balance of battery-electric and hydrogen fuel-cell electric technologies. Although there has been a very significant research effort in solid-state hydrogen storage, high-pressure gas storage combined with conventional metal hydrides is still seen as the current intermediate-term candidate for car manufacturers. Significant issues have arisen in the search for improved solid-state hydrogen storage materials; for example, facile reversibility has been a major challenge for many recently studied complex hydrides while physisorption in porous structures is still restricted to cryogenic temperatures. However, many systems fulfil the majority of necessary criteria for improved hydrogen storage--indeed, the discovery of reversibility in multicomponent hydride systems along with recent chemistry breakthroughs in off-board and solvent-assisted regeneration suggest that the goal of both improved on-board reversible and off-board regenerated hydrogen storage systems can be achieved.
Article
The Polaris instrument at the ISIS spallation neutron source operates as a medium resolution powder diffractometer. The high incident neutron flux enables datasets to be collected with comparatively short counting times or from extremely small sample volumes. Examples of recent experiments performed on Polaris, which exploit the high count rate and the particular advantages offered by fixed geometry diffraction measurements performed on a pulsed neutron source, are presented.
Article
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