http://pubs.acs.org/journal/aelccp
Recent Developments in Nitrogen Reduction Catalysts: A Virtual Issue the solution. Spectroscopic and fluorimetric assays are most commonly used, but they require secondary verification using N15-labeled nitrogen gas, because none of the spectroscopic and fluorimetric assays are completely selective and they are impacted by changes in the solution conditions (pH, ionic strength, chemical additives, etc.).2 Using N15-labeled nitrogen gas as a feedstock followed by NMR analysis is a good qualitative secondary verification that ammonia is being generated from N2 and not from other nitrogen-based compounds in solution. However, it is important to note that N15-labeled nitrogen gas is usually produced from N15labeled ammonia; therefore, there is often a small contaminant of N15-labeled ammonia in the N15-labeled nitrogen gas from the manufacturer, and therefore, ammonia scrubbing of N15labeled nitrogen gas is needed before use. Other contamination of N15-labeled ammonia is also possible and should be considered. Other control experiments that should be included in any analysis of nitrogen reduction to ammonia include studying the process without the catalyst and just the background binder, catalyst support, and electrode/current collector. One direction that the field of nonconventional ammonia synthesis has been embracing recently has been to utilize biological redox catalysts and molecular electrocatalysts as model systems to understand the mechanisms of nitrogen reduction to ammonia, with the overall goal to translate these systems to heterogeneous electrocatalysis at nanostructured metal-based catalytic materials. We note that some microorganisms contain an enzyme nitrogenase that can reduce nitrogen to ammonia at ambient pressures and temperatures.3 Molecular electrocatalysts are easy to tailor and therefore help researchers understand the challenges of nitrogen binding. Peters and co-workers have recently utilized an iron-based molecular catalyst to experimentally verify the terminal iron nitride intermediate that has been proposed in the Chatt-type biocatalytic mechanisms as well as in some molecular catalytic mechanisms.4 These measurements suggested that there are common mechanistic features between these two classes of catalysts. Because the most common nitrogenase enzyme capable of nitrogen reduction to ammonia contains an FeMo cofactor, there is a wealth of interest in using the cofactor as biological inspiration. This effort has resulted in a number of iron and molybdenum molecular electrocatalysts over the last several decades, and there are several reviews covering the past work.5−7 Recently, Schrock and co-workers have been studying conformationally rigid pyridine-based diamido ligands for producing molybdenum molecular catalysts for nitrogen
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ver 100 years ago, Haber and Bosch invented and scaled up the process for the formation of ammonia from nitrogen and hydrogen.1 This revolutionary technology was necessary to produce the required quantities of ammonia, which is critical for the production of fertilizers. Unfortunately, the Haber−Bosch process is not particularly efficient, consuming 1−3% of the world’s energy produced annually. While many reports consider the difficult activation of N2 as the primary source for the energy inefficiencies and CO2 emission in ammonia synthesis, it is important to realize that estimates by the International Energy Agency show that over ∼70% of CO2 emissions and energy use are derived from the use of steam methane reforming to produce H2, which is reacted with N2. The difficulty in activating the N−N bond requires the process to be operated at high temperature and pressure to achieve desired catalytic rates. The required hightemperature and -pressure operation decreases equilibrium conversion and increases the capital costs. The negative impact of the required temperature and pressure has motivated researchers to consider alternative approaches for N2 activation and fixation, through biological and nonthermal catalytic processes. In this Virtual Issue, we highlight some of the work published in ACS Energy Letters, ACS Catalysis, and the Journal of the American Chemical Society in 2017 and 2018 that focuses on improving the efficiency, performance, and mechanistic understanding of nonconventional catalysts and catalytic processes for nitrogen reduction to ammonia. This is an expanding field in terms of photocatalysis and electrocatalysis, as shown by Figure 1. It is important to start by noting that there are some controversies in the nonthermal catalytic nitrogen reduction community, mainly related to the difficulties associated with accurately measuring ammonia produced through nonthermal routes. These difficulties stem from relatively low rates of ammonia production in lab-scale measurements. Ammonia is present in the background of many environments and can be electrochemically or photochemically produced from nitrogenbased contaminants in many solutions (i.e., not from the N2 reactant). Also, many of the methods for detecting ammonia have large interferences from other nitrogen-containing compounds. Therefore, appropriate blanks and controls are critically important to standardize the data reporting in this field. Renner, Greenlee, and co-workers recently explained the need for doing experiments with an argon blank (no added nitrogen gas) to minimize the effect of environmental ammonia that might be in the solution or adsorbed to the laboratory materials when analyzing electrocatalytic nitrogen reduction (these issues also apply to photochemical systems described below).2 This issue is clearly illustrated by the example in Figure 2. It is also important to utilize an assay that has high sensitivity under relevant experimental conditions and minimal interferences from other molecules that could be in © XXXX American Chemical Society
Received: November 13, 2018 Accepted: November 26, 2018
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DOI: 10.1021/acsenergylett.8b02197 ACS Energy Lett. 2019, 4, 163−166
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Cite This: ACS Energy Lett. 2019, 4, 163−166
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ACS Energy Letters
Figure 1. (Left) Histogram of the number of publications per year on nitrogen photofixation classified by the type of catalyst. Reproduced from ref 17. (Right) Publications that fall under the term “electrochemical+ammonia+synthesis” in a Web of Science search and the total number of citations for all of the articles in the search per year for the past 10 years. Reproduced from ref 2.
Figure 2. Example data sets used to demonstrate calculation of Faradaic efficiency with and without background Ar measurements. (a,b) Two different bimetallic compositions of an iron−nickel nanoparticle catalyst. Reproduced from ref 2.
reduction to ammonia.8 These molecular catalyst systems are known for their very high electron efficiency (43%). Note that electron efficiency is not the same as Faradaic efficiency, which would likely be significantly lower. The electron transfer in these systems was mediated by cobaltocene rather than directly communicating with an electrode. Peters and co-workers have also been using cobaltocene for mediating iron-based molecular catalyst systems and have shown from their density functional theory calculations that cobaltocene can promote ammonia formation.9 This shows that cobaltocene may have a role as not just a mediator but a cocatalyst. Peters and coworkers have also used this strategy when developing highly active osmium complexes that have shown that 120 equiv of ammonia is produced per Os complex in a small-batch reactor.10 In the molecular catalysts’ area, pincer-type ligands have become extremely popular for designing nitrogen reduction catalysts, as shown by the recent work by Miller, Siewert, Schneider, and co-workers studying the mechanism of rhenium pincer complexes for nitrogen reduction.11 It is important to note that molecular catalysts are typically studied in solution rather than immobilized. Future application would require their immobilization onto electrode surfaces, as well as a focus on improving their long-term stability, as the total turnover is typically low. In electrocatalysis, selectivity is critically important. Nitrogen reduction is typically plagued with low Faradaic efficiencies due to the large amounts of hydrogen evolution/proton reduction catalyzed at the same potential. Miller and coworkers have shown that ammonia is barely thermodynamically favorable (63 mV) over hydrogen in acetonitrile;12
therefore, catalyst and interface design will be very important for improving the performance of nitrogen reduction electrocatalysts. Jonas Peters and co-workers have shown that the ligand structure is critically important in the selectivity between hydrogen evolution and nitrogen reduction in molecular catalyst systems.13 One of the most recent translations between molecular electrocatalysts and heterogeneous electrocatalysts has been the emerging field of single-atom heterogeneous catalysts. Jung and co-workers have improved the selectivity of nitrogen reduction versus proton reduction with heterogeneous single-atom titanium and vanadium catalysts.14 It has also been observed recently that catalyst design is not the only way to alter selectivity. MacFarlane and co-workers have shown that the electrolyte can alter selectivity.15 They report improved ammonia selectivity in an aprotic fluorinated solvent−ionic liquid mixture at iron nanorod electrodes. Shao and co-workers have been studying this selectivity at heterogeneous electrocatalysts (i.e., gold and platinum). They have developed an in situ infrared spectroelectrochemistry system for studying the reaction mechanism.16 It was suggested that this technique will improve the ability to study structure/ activity relationships in the design of heterogeneous electrocatalysts. In the photocatalysis area, nitrogen reduction to ammonia is also a challenge for a variety of reasons. As pointed out by Medford and Hatzell, the choice of photocatalyst should consider the thermodynamics. Specifically, the band gap of the semiconductor must be larger than the voltage of the overall redox reaction, and the band edges should be aligned with the half-reaction potentials.17 Medford and Hatzell plot this data 164
DOI: 10.1021/acsenergylett.8b02197 ACS Energy Lett. 2019, 4, 163−166
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ACS Energy Letters Notes
for a wide variety of semiconductors, which shows why TiO2 is popular but that other semiconductors can be considered and that band gap engineering could provide better semiconductor photocatalysts.17 In the photocatalysis area, benchmarking and controls are just as important as those in the electrocatalysis area. Medford and Hatzell specifically point out the need for testing all photocatalysts under the same conditions. They propose measuring ammonia production at neutral pH, ambient temperature, atmospheric pressure, with no scavengers, and with an AM 1.5 solar simulator.17 It should also be noted that recent work by Medford and Hatzell has suggested that adventitious carbon can serve as an active participant in nitrogen reduction, which could not occur on the clean catalyst surface.18 This further highlights the importance of cleanliness and careful analysis in catalyst evaluation. Plasma synthesis of ammonia has also been an interesting new development in recent years. Iwamoto and co-workers at Chuo University have used transition metal wools to produce ammonia from nitrogen at atmospheric pressures.19 This area will likely see an expansion in the coming years as researchers learn more about mechanisms and strategies for controlling selectivity of products in plasma-mediated processes. There has also been recent research focused on improving on the fused iron and supported Ru catalysts that are traditionally used in thermocatalytic Haber−Bosch plants. For instance, Chen and co-workers developed BaH2−Co/ carbon nanotube (CNT) catalysts that operate at significantly lower temperatures (i.e., 150C), which could dramatically improve the energy efficiency of the industrial process.20 Norskov and co-workers have made recent progress in the computational design of better catalysts for nitrogen reduction and have shown that active site engineering is needed to force dissociated adsorption of nitrogen onto two isolated reactive metal atoms in a nonreactive framework.21 This work is guiding experimentalists into understanding what active site structures are necessary for more efficient nitrogen reduction catalysis. This Virtual Issue showcases the recent work published in ACS Energy Letters, ACS Catalysis, and the Journal of the American Chemical Society in 2017 and 2018 that focuses on improving the efficiency and performance of catalysts for nitrogen reduction to ammonia.
Views expressed in this Energy Focus are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest.
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(1) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. How a Century of Ammonia Synthesis Changed the World. Nat. Geosci. 2008, 1, 636. (2) Greenlee, L. F.; Renner, J. N.; Foster, S. L. The Use of Controls for Consistent and Accurate Measurements of Electrocatalytic Ammonia Synthesis from Dinitrogen. ACS Catal. 2018, 8 (9), 7820−7827. (3) Cai, R.; Minteer, S. D. Nitrogenase Bioelectrocatalysis: From Understanding Electron-Transfer Mechanisms to Energy Applications. ACS Energy Lett. 2018, 3, 2736−2742. (4) Thompson, N. B.; Green, M. T.; Peters, J. C. Nitrogen Fixation via a Terminal Fe(IV) Nitride. J. Am. Chem. Soc. 2017, 139 (43), 15312−15315. (5) Hazari, N. Homogeneous Iron Complexes for the Conversion of Dinitrogen into Ammonia and Hydrazine. Chem. Soc. Rev. 2010, 39 (11), 4044−4056. (6) Č orić, I.; Holland, P. L. Insight into the Iron−Molybdenum Cofactor of Nitrogenase from Synthetic Iron Complexes with Sulfur, Carbon, and Hydride Ligands. J. Am. Chem. Soc. 2016, 138 (23), 7200−7211. (7) Schrock, R. R. Catalytic Reduction of Dinitrogen to Ammonia by Molybdenum: Theory versus Experiment. Angew. Chem., Int. Ed. 2008, 47 (30), 5512−5522. (8) Wickramasinghe, L. A.; Ogawa, T.; Schrock, R. R.; Müller, P. Reduction of Dinitrogen to Ammonia Catalyzed by Molybdenum Diamido Complexes. J. Am. Chem. Soc. 2017, 139 (27), 9132−9135. (9) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Peters, J. C. Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pKa Effects and Demonstrating Electrocatalysis. J. Am. Chem. Soc. 2018, 140 (19), 6122−6129. (10) Fajardo, J.; Peters, J. C. Catalytic Nitrogen-to-Ammonia Conversion by Osmium and Ruthenium Complexes. J. Am. Chem. Soc. 2017, 139 (45), 16105−16108. (11) Lindley, B. M.; van Alten, R. S.; Finger, M.; Schendzielorz, F.; Würtele, C.; Miller, A. J.M.; Siewert, I.; Schneider, S. Mechanism of Chemical and Electrochemical N2 Splitting by a Rhenium Pincer Complex. J. Am. Chem. Soc. 2018, 140 (25), 7922−7935. (12) Lindley, B. M.; Appel, A. M.; Krogh-Jespersen, K.; Mayer, J. M.; Miller, A. J. M. Evaluating the Thermodynamics of Electrocatalytic N2 Reduction in Acetonitrile. ACS Energy Lett. 2016, 1 (4), 698−704. (13) Matson, B. D.; Peters, J. C. Fe-Mediated HER vs N2RR: Exploring Factors That Contribute to Selectivity in P3EFe(N2) (E = B, Si, C) Catalyst Model Systems. ACS Catal. 2018, 8 (2), 1448− 1455. (14) Choi, C.; Back, S.; Kim, N.-Y.; Lim, J.; Kim, Y.-H.; Jung, Y. Suppression of Hydrogen Evolution Reaction in Electrochemical N2 Reduction Using Single-Atom Catalysts: A Computational Guideline. ACS Catal. 2018, 8 (8), 7517−7525. (15) Suryanto, B. H.R.; Kang, C. S.M.; Wang, D.; Xiao, C.; Zhou, F.; Azofra, L. M.; Cavallo, L.; Zhang, X.; MacFarlane, D. R. Rational Electrode−Electrolyte Design for Efficient Ammonia Electrosynthesis under Ambient Conditions. ACS Energy Lett. 2018, 3 (6), 1219− 1224. (16) Yao, Y.; Zhu, S.; Wang, H.; Li, H.; Shao, M. A Spectroscopic Study on the Nitrogen Electrochemical Reduction Reaction on Gold and Platinum Surfaces. J. Am. Chem. Soc. 2018, 140 (4), 1496−1501. (17) Medford, A. J.; Hatzell, M. C. Photon-Driven Nitrogen Fixation: Current Progress, Thermodynamic Considerations, and Future Outlook. ACS Catal. 2017, 7 (4), 2624−2643. (18) Comer, B. M.; Liu, Y.-H.; Dixit, M. B.; Hatzell, K. B.; Ye, Y.; Crumlin, E. J.; Hatzell, M. C.; Medford, A. J. The Role of
Shelley D. Minteer*,† Phillip Christopher‡ Suljo Linic§ †
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REFERENCES
Department of Chemistry, and Department of Materials Science & Engineering, University of Utah, Salt Lake City, Utah 84112, United States ‡ Department of Chemical Engineering, University of CaliforniaSanta Barbara, Santa Barbara, California 93106, United States § Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States
AUTHOR INFORMATION
ORCID
Shelley D. Minteer: 0000-0002-5788-2249 Phillip Christopher: 0000-0002-4898-5510 Suljo Linic: 0000-0003-2153-6755 165
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ACS Energy Letters Adventitious Carbon in Photo-catalytic Nitrogen Fixation by Titania. J. Am. Chem. Soc. 2018, 140, 15157. (19) Iwamoto, M.; Akiyama, M.; Aihara, K.; Deguchi, T. Ammonia Synthesis on Wool-Like Au, Pt, Pd, Ag, or Cu Electrode Catalysts in Nonthermal Atmospheric-Pressure Plasma of N2 and H2. ACS Catal. 2017, 7 (10), 6924−6929. (20) Gao, W.; Wang, P.; Guo, J.; Chang, F.; He, T.; Wang, Q.; Wu, G.; Chen, P. Barium Hydride-Mediated Nitrogen Transfer and Hydrogenation for Ammonia Synthesis: A Case Study of Cobalt. ACS Catal. 2017, 7 (5), 3654−3661. (21) Singh, A. R.; Montoya, J. H.; Rohr, B. A.; Tsai, C.; Vojvodic, A.; Nørskov, J. K. Computational Design of Active Site Structures with Improved Transition-State Scaling for Ammonia Synthesis. ACS Catal. 2018, 8 (5), 4017−4024.
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