Catalyst: NH3 as an Energy Carrier

Jianping Guo is an associate professor at the Dalian Institute of Chemical Physics. He received his PhD from the University of Chinese Academy of Sciences in 2015. His research focuses on exploring alkali and alkaline earth-metal hydrides and imides in catalytic ammonia decomposition and synthesis.Ping Chen is a professor and division head of Hydrogen Energy and Advanced Materials at the Dalian Institute of Chemical Physics. She received her BS, MS, and PhD degrees in chemistry in 1991, 1994, and 1997, respectively, from Xiamen University. Her primary research interests are material design and development for hydrogen storage and heterogeneous catalysis. Jianping Guo is an associate professor at the Dalian Institute of Chemical Physics. He received his PhD from the University of Chinese Academy of Sciences in 2015. His research focuses on exploring alkali and alkaline earth-metal hydrides and imides in catalytic ammonia decomposition and synthesis. Ping Chen is a professor and division head of Hydrogen Energy and Advanced Materials at the Dalian Institute of Chemical Physics. She received her BS, MS, and PhD degrees in chemistry in 1991, 1994, and 1997, respectively, from Xiamen University. Her primary research interests are material design and development for hydrogen storage and heterogeneous catalysis. There are strong pushes toward a more efficient, renewable, and cleaner energy system for the sustainable growth of human society. This calls for the identification and development of carriers that can harvest, store, and transport those primary and secondary energy resources while releasing the energy on the site of demand without greenhouse gas emission. Hydrogen is undoubtedly an ideal candidate because of its abundance, ubiquity, zero emission, and high energy density, etc. It can be centrally produced in large quantities from fossil fuels via the steam reforming of natural gas and coal gasification (referred to as fossil H2), where byproduct CO2 can be collected, converted, and/or sequestrated; it can also be produced via water electrolysis and photolysis to enable storage of the energies from nuclear power and renewable sources (referred to as nuclear and renewable H2, respectively) and to provide solutions for the electricity intermittence and surplus problems. The energy release from the reaction of hydrogen and oxygen via combustion or fuel cells holds the merits of high efficiency and fewer environmental consequences. Such a hydrogen energy system shows great promise, but technical bottlenecks, particularly methods of storing and transporting hydrogen effectively, hinder the realization of its full potential. Hydrogen has a relatively large compressibility factor and low boiling point, and thus the compression and liquefaction of hydrogen consume energy. The past two decades have witnessed strong efforts in material development for onboard hydrogen storage with a focus on hydrides of lightweight elements and high-surface-area physisorbents. For the large-scale storage, long-distance transportation, and on-site production of hydrogen, carriers of high hydrogen and energy densities and economic and technical feasibility are of equivalent importance.1He T. Pachfule P. Wu H. Xu Q. Chen P. Nat. Rev. Mater. 2016; 1: 16059Crossref Scopus (401) Google Scholar Ammonia (ΔfHo298 K = −46 kJ⋅mol−1) is neither as stable as H2O (ΔfHo298 K = −286 kJ⋅mol−1) nor as unstable as H2. It has a high hydrogen content (17.7 wt %) and gravimetric energy density (3 kWh⋅kg−1). It can be produced heterocatalytically from N2 and fossil, nuclear, or renewable H2 or directly from N2 and H2O via electrochemical, photochemical, or chemical looping processes; energy is thus stored simultaneously. The release of energy from ammonia does not accompany COx emission. Ammonia has many other merits as an energy and hydrogen carrier. For instance, ammonia is one of the most highly produced chemicals—it had an annual production of ca. 180 million tons in 2015 and has a scalable production capacity. The liquefaction, storage, and transport of ammonia are technically ready. Moreover, NH3 has a relatively narrow explosion range and low odor threshold, which could help with safety control. The ammonia cycle, comprising its production, storage, transportation, and conversion (Figure 1) could supplement the hydrogen energy regime well.2International Energy Agency. (2017). Energy Technology Perspectives 2017. http://www.iea.org/etp2017/summary/.Google Scholar Ammonia formation from N2 and H2 is a mild exothermic reaction (ΔH = −46 kJ⋅mol−1 NH3) that is thermodynamically favored at lower temperatures, i.e., it has nearly 100% equilibrium conversion at 300 K and 1.0 bar. The well-established Haber-Bosch process, however, normally occurs under harsh conditions (673–773 K and 150–250 bar) and consumes more than 1% of the energy produced globally and releases more than 400 Mt of CO2 annually. The mild-condition ammonia synthesis from N2 and H2 is a long-sought-after scientific task and a practical need, especially for distributed ammonia production using renewable or nuclear H2. On the other hand, ammonia formation from N2 and H2O or H2 via electrochemical, photochemical, and chemical looping processes is more sustainable but associates with additional scientific challenges that have yet to be overcome.3US Department of Energy Office of Science. (2016). Sustainable Ammonia Synthesis. https://science.energy.gov/∼/media/bes/pdf/reports/2016/SustainableAmmoniaReport.pdf.Google Scholar The success of the ammonia cycle will also depend on the effective release of energy from ammonia. This will require technical viability to ensure the decomposition of ammonia to H2 and/or the conversion of NH3 directly to electricity or power. Herein, we discuss the relevant scientific challenges and opportunities. Fossil, nuclear, or renewable H2 can be converted to ammonia at the site of its production on either a large or small scale that would ideally operate under ambient temperature and pressure. This would require a catalyst to have strong activation (small Eact) to N2 but relatively weak binding (small adsorption energy, ΔE) to −NHx (x = 0–3) adspecies. Unfortunately, such a scenario can hardly be attained in processes catalyzed by transition metals (TMs). The existence of linear correlations between Eact and ΔE of reacting species (kinetic scaling relations) has been disclosed, which places restrictions preventing the optimization of Eact and ΔE independently and thus results in a volcano-type plot of catalytic activity versus ΔE of key adspecies over TMs.4Nørskov J.K. Bligaard T. Rossmeisl J. Christensen C.H. Nat. Chem. 2009; 1: 37-46Crossref PubMed Scopus (2719) Google Scholar Comparatively, TMs with moderate activation to N2 and binding to −NHx perform well, which nicely explains why, after a century of investigation, only promoted Fe and supported Ru catalysts are employed in industry. The manipulation of the electronic properties of TM catalysts via the formation of alloy (Co-Mo-N, for instance), the use of functional supports,5Kitano M. Inoue Y. Yamazaki Y. Hayashi F. Kanbara S. Matsuishi S. Yokoyama T. Kim S.W. Hara M. Hosono H. Nat. Chem. 2012; 4: 934-940Crossref PubMed Scopus (864) Google Scholar and the exposure of specific facets, etc., would benefit the optimization or identification of catalysts with activity positioned on the top of the volcano plot. Pending issues such as the function of electronic promoters and the nature of the active site could be elucidated with the help of recently developed in situ and operando techniques. However, strategies that could circumvent the scaling relations would hold great promise for addressing the long-sought-after mild-condition ammonia synthesis. Hereinafter, we discuss a few considerations for modulating the catalysis of ammonia formation. The formation of NH3 from N2 and H2 catalyzed by Fe or Ru is composed of six elementary reactions that can be grouped into activation, hydrogenation, and desorption steps. Intuitively, conducting the activation and hydrogenation steps on different catalytic centers would change the solo-TM-mediated catalysis. That would require one center (referred to as center A) to have the functionality of activating N2 and the other center (center B) to be able to draw the activated N species away from center A and conduct hydrogenation to NH3. The energetics of the hydrogenation steps are thus less dependent on the electronic nature of center A. Metals or compounds that can interact with N2 can be the candidates of center A, whereas alkali or alkaline earth hydrides that can reduce TM nitrides to form amides or imides that can undergo self-hydrogenation to NH3 are potential candidates of center B. The composite catalysts composing LiH and 3d TMs (V to Ni) show universal activities in ammonia synthesis.6Wang P. Chang F. Gao W. Guo J. Wu G. He T. Chen P. Nat. Chem. 2017; 9: 64-70PubMed Google Scholar The dissociations of N2 and H2 on a clean Fe or Ru surface for the formations of N and H adspecies release large amounts of energy, some of which unfortunately disperses into the environment instead of “powering” the follow-up reaction. We would also like to discuss another way to approach the mild-condition synthesis via the so-called hydrogen-associative mechanism, where the setup of the N–H bond takes place synchronically with the weakening of the N≡N bond to favor the conservation of internal energy. Such catalysis is similar to biological and homogeneous processes except that it uses H2 as a hydrogen source. The H-associative N2 activation on a TM surface has been discussed before but was later found to be energetically unfavorable. To make this H-associative activation feasible, the catalytic center again might need to be multi-functional to be able to supply activated H species to the chemisorbed N2 and to mediate the electron, configuration, and energy for the setup of the N–H bond to form −N2Hy (y = 0–4). It should be noted that both biological and homogeneous NH3 formation consume energy. It is very intriguing to formulate an energy-effective hydrogen-associative N2 activation in the heterogeneous catalysis regime where dihydrogen is applied. The theoretical prediction of potential catalysts would be highly directive. Increasing attention has been given to electrochemical and photochemical ammonia syntheses from N2 and H2O (dihydrogen can also be used as hydrogen source) with the use of renewable- or nuclear-derived electricity or energy.7Kyriakou V. Garagounis I. Vasileiou E. Vourros A. Stoukides M. Catal. Today. 2017; 286: 2-13Crossref Scopus (425) Google Scholar, 8Medford A.J. Hatzell M.C. ACS Catal. 2017; 7: 2624-2643Crossref Scopus (355) Google Scholar These ammonia are essentially carbon free and/or sustainable. At present, these processes have a low NH3 synthesis rate and poor faradic efficiency, i.e., the reported NH3 synthesis rates are usually in the order of 10−9 mol⋅s−1⋅cm−2, which is far below the suggested industrial level (10−7 mol⋅s−1⋅cm−2). Highly active, selective, and stable photo- or electrocatalysts and suitable electrolytes are being actively pursued. Nowadays, investigated electrocatalysts, i.e., Pd, Pt, Fe, Ru, or conductive oxides, usually have high overpotentials. Recent density functional theory calculations have suggested that early TM nitrides could be promising candidates.9Abghoui Y. Garden A.L. Howat J.G. Vegge T. Skulason E. ACS Catal. 2016; 6: 635-646Crossref Scopus (263) Google Scholar Similarly, the scaling relations are also dominant in electrocatalysis, which calls for the design and development of a novel electrocatalyst to intervene in the solo-TM-mediated process. The low faradic efficiency also comes from the H2 evolution reaction. This requires the electrocatalyst to adjust its binding preference to N rather than H. In the meantime, the electrolyte should be designed to have good proton and/or oxygen-ion conductivity and suitable hydrogen evolution potential to favor ammonia formation. The recent advancements in electrolytes for batteries could have certain implications for the selection of electrolyte for electrochemical ammonia synthesis. Different from the aforementioned catalytic routes, NH3 formation from N2 and H2O via the metal-, nitride-, or oxide-mediated chemical loop offers an option that could address some of the problems associated with the catalytic processes (e.g., competitive activation and adsorption of N2 and H2). It comprises three sequential steps separating the N2 reduction and the hydrogenation reactions, i.e., (1) nitridation of metal to nitride, (2) hydrolysis of nitride to NH3 and oxide, and (3) reduction of oxide to metal.10Michalsky R. Pfromm P.H. Steinfeld A. Interface Focus. 2015; 5: 20140084Crossref PubMed Scopus (49) Google Scholar The identification of materials with fast rates of nitridation and hydrolysis and with thermodynamic properties favoring energy efficiency of the whole loop are important. In most cases, the third step requires temperatures up to 1,800 K and a reducing reagent. Al, AlN, and Al2O3 and Mn, MnN, and MnO2 have been investigated. Compositional and morphological optimizations of the known materials are certainly worthy of exploration. In addition, other H-, N- and O-containing materials, such as hydroxides, oxynitrides, amides, imides, and hydrides, etc., could be potential candidates for this process. N2 fixation has been a focal research topic for a century, whereas in the NH3-mediated energy cycle, the energy release from NH3 through a thermal, electrochemical, or photochemical route is of equivalent importance. Ammonia has been proposed as an alternative fuel to petroleum and diesel for internal combustion engines. The burning products are mainly water and nitrogen. Ammonia can also be used directly as a fuel in solid oxide fuel cells and alkaline membrane fuel cells for the generation of electricity. More attention should be given to these topics to enhance the energy efficiency and technical viability. The decomposition of ammonia produces COx-free H2, a potential method of supplying H2 on the site of demand. Such a process is mildly endothermic, but obtaining an appropriate H2 production rate requires that temperatures higher than 773 K be applied with the most active Ru-based catalysts. Although ammonia decomposition is the reverse reaction of its synthesis, catalyst developed for the latter might not always perform well for the former because of the change in reaction condition. Electrochemical and photochemical NH3 decomposition are also possible routes for hydrogen production, and more attention should be given to these topics. Ammonia-derived hydrogen usually contains certain amounts of ammonia, a contaminant that poisons the membrane of proton-exchange membrane fuel cells. The separation and purification techniques that can rigorously reduce NH3 concentration below 1 ppm in the hydrogen stream are critically important for NH3 as a hydrogen carrier. To this end, membrane separation and effective sorbents will find their uses. The growing attention toward using ammonia as an energy carrier renews the research interests and provides plenty of opportunities for the advancement of related sciences and technologies. Will the “ammonia-mediated” energy system be a solution for a sustainable future? Ecological consequences apart from scientific and technological challenges should be seriously evaluated. We hope this short piece will create more discussion on this exciting topic and call for cooperation of different disciplines to address the challenging issues so as to realize the full potential of ammonia. Reaction: “Green” Ammonia ProductionYe et al.ChemNovember 09, 2017In BriefDr. Lin Ye is a postdoctoral researcher at the Wolfson Catalysis Centre (WCC) under Prof. Edman Tsang and has expertise in the development of new catalysts.Richard Nayak-Luke gained his MEng from the University of Oxford in 2013. After working at Roland Berger Strategy Consultants, he is now pursuing a DPhil degree on green ammonia synthesis and quantification of energy storage requirements in islanded and semi-islanded networks under Prof. Banares-Alcantara.Prof. Rene Banares-Alcantara is a reader in engineering science at the University of Oxford. Full-Text PDF Open Archive