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Electrochemical Dinitrogen Reduction to Ammonia by Mo2N: Catalysis or Decomposition? Bo Hu, Maowei Hu, Lance C. Seefeldt, and T. Leo Liu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00648 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019
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ACS Energy Letters
Electrochemical Dinitrogen Reduction to Ammonia by Mo2N: Catalysis or Decomposition? Bo Hu, Maowei Hu, Lance Seefeldt, T. Leo Liu* Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84321, United States *Corresponding author:
[email protected] ABSTRACT Instead of catalyzing electrochemical reduction of N2, we found that tetragonal Mo2N undergoes decomposition and results in the generation of ammonia. The present results call urgent attention to the need to carefully evaluate the catalytic nature of dinitrogen reduction reaction (NRR) by nitrogen containing materials. TOC
The century-old Haber-Bosch process remains to be the only industrial process to achieve large scale N2 reduction to ammonia production more than 150 million tons per year.1-3 Despite its wide application, the Haber-Bosch process is massively energy consuming for the production of H2 feedstock (ca. 2% of the world’s energy supply), is heavily dependent on fossil fuel (3% - 5% of the world’s natural gas production), and is a significant greenhouse gas emitter.1-3 Therefore, milder, more energy efficient, and CO2-neutral alternative methods are of great interest for both scientific research and industrial applications.1-3 Electrochemical synthesis of ammonia from dinitrogen (N2) in the presence of catalysts represents a highly attractive approach.1-3 Electrochemical devices for the dinitrogen reduction reaction (NRR) could be easily integrated into renewable energy systems, thus eliminating the utilization of fossil fuel and the emission of CO2. Due to these potential benefits, electrocatalytic N2 fixation has been under intensive exploration over the last few years and has become a hot topic in catalysis. A number of
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heterogeneous electrocatalysts have been reported with various claimed performance for NRR, including noble metals,4-5 metal oxides,6 metal sulfides,7 metal nitrides,8-9 and metal-free catalysts.10 Homogeneous electrocatalytic NNR was also recently demonstrated.11 Metal nitrides have been studied both theoretically and experimentally indicating their potential capability of electrocatalytic N2 reduction.8-9, 12 However, presence of nitrogen in the catalyst and the chemical stability problem of the nitride materials could result in ambiguous results.13 Herein, we carefully examined the catalytic ability of tetragonal Mo2N for NRR. It is revealed that Mo2N undergoes fast chemical decomposition in aqueous electrolytes and showed no catalytic activity for NRR. Tetragonal Mo2N was synthesized via the well-known “urea glass” route according to the literature.14 In Figure S1, X-ray diffraction (XRD) analysis revealed a pure tetragonal phase Mo2N with a good crystallinity that is in line with the standard PDF card. The morphology of the material was studied by scanning electron microscope (SEM) displaying partially aggregated nanoparticles (30-50 nm) with a homogeneous distribution of Mo and N elements (Figure S2).
Figure 1. (A) LSV curves for Mo2N@Ti electrode. Scan rate: 50 mV s-1. (B) Chronoamperometric curves for the electrolysis at various controlled potential with N2 as the feeding gas. (C) Indophenol-blue assay for ammonium after electrolysis at various controlled potential. (D) Faradaic efficiency and reaction rate at different potentials.
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ACS Energy Letters
The synthesized Mo2N was evaluated for NRR at ambient conditions. Although carbon paper is the most common substrate for catalysts loading NRR studies, titanium was selected as the substrate in this study due to its inertness against hydrogen evolution reaction (HER). To be in accordance with other reported metal nitride catalyzed NRR experiments, our electrochemical studies were conducted in 0.1 M HCl solution (pH = 1). Linear sweeping voltammograms (LSV) were collected on a Mo2N@Ti electrode in both argon and dinitrogen saturated electrolytes. As shown in Figure 1A, a slight current increase in the dinitrogen saturated electrolyte indicated the possibility of catalytic N2 reduction taking place at more negative potential less than - 0.22 V. When the catalyst was subjected to controlled potential electrolysis, the current density experienced dramatic change from less than 10 µA cm-2 at 0.05 V to 3.3 mA cm-2 at -0.25 V vs RHE, which was attributed to the more favorable HER. After bulk electrolysis for 1 hour at potential from -0.25 V to 0.05 V vs RHE (Figure 1B), the amount of ammonia was detected by UV-vis spectroscopy using the Indophenol-blue assay (Figure 1C, S4). Assuming N2 was eletrocatalytically reduced to ammonia, the highest faradaic efficiency (FE) was obtained at - 0.05 V vs RHE up to 42.3% with a yield of 1.99 µg mg-1 h-1 (Figure 1D). At 0.05 V, the reaction showed the highest yield of 2.73 µg mg-1 h-1 with a good FE of 28.4 %. When the potential was applied to -0.15 V, faradaic efficiency dramatically dropped to 1.93 %. In all the experiments, no hydrazine product was detected. The obtained results which appeared very exciting prompted us to conduct
15N
2
controlled
experiments to confirm the catalytic nature of NRR. It is worth noting that non-negligible amount of 15NH4+, 15NO2-, and other impurities exist in the commercialized 15N2 gas.15 It is also noted that the reported 15N2 controlled experiments seldom conducted pretreatment, bringing in ambiguous results. Thus, before the test, the 15N2 gas was treated by 50 mM sulfuric acid overnight to remove any NH3 impurity by formation of NH4+. After bulk electrolysis under the purified 15N2 gas for 1 hour, 1H-NMR experiments were carried out to quantify the formation of 15NH4+. To our surprise, no 15NH4+ was detected (no doublet peaks), while a significant amount of 14NH4+ was observed (Figure 2A). The finding suggested the ammonia formation likely originate from a chemical decomposition process of Mo2N rather than a catalytic process. In order to further elucidate the chemical decomposition of Mo2N, the Mo2N@Ti electrodes were treated in 0.1 M HCl, 0.1 M NaOH, and 0.5 M NaCl solutions, respectively. After the electrodes were incubated for 1 hour, the detected ammonia for all conditions was just slightly less than the electrolysis at 0.05 V vs RHE 3
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(see Figure 2B). We assume that the decomposition of Mo2N could be accelerated under the electrochemical reduction condition. Moreover, it is found that Mo2N displayed much poorer stability in alkaline condition than in acidic and neutral conditions. As shown in Figure S3, after 96 hours’ treatment, much more ammonia was detected in the NaOH solution than in the HCl and NaCl solutions. Mo2N was even totally dissolved in NaOH solution after 1 week.
Figure 2. (A) 1H NMR spectra of ammonium in the sampled electrolytes after one hour’s electrolysis in the presence of saturated 15N2 and 14N2, respectively. (B) Indophenol-blue assay for ammonium after the electrodes were immersed in the different supporting electrolytes (labeled) for 1 hour.
The failure of the 15N2 isotope labeling experiment and the chemical decomposition lead us to draw the conclusion that Mo2N has no catalytic activity for electrochemical N2 reduction. The present results raise an urgent alert to the application of other metal nitrides and even nitrogen contained materials for NRR. In addition to the decomposition of potential catalytic materials, we would like to highlight several other apparent pitfalls to avoid in claiming catalytic NRR. First, nitrogen contaminants in dinitrogen gas, solvents, and other reactants need pretreatment to mitigate background ammonia.15 Second,
15N
labeling experiments must be conducted to confirm the
formation of ammonium from dinitrogen. Third, turnover number (> 1) for a catalytic process needs to be demonstrated, which requires carefully evaluation of the loading of a catalyst and the yield of ammonium. ASSOCIATED CONTENT Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXX. Experimental details and additional data AUTHOR INFORMATION Corresponding Author Email:
[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank Utah State University for providing faculty startup funds to the PI (T. Leo Liu) for supporting this work. Bo Hu and Maowei Hu are grateful to the China CSC Study Abroad program for supporting their graduate program. LCS is funded by a grant from the Department of Energy, Office of Science, Basic Energy Sciences (DESC0010687). REFERENCES 1. Chen, J. G.; Crooks, R. M.; Seefeldt, L. C.; Bren, K. L.; Bullock, R. M.; Darensbourg, M. Y.; Holland, P. L.; Hoffman, B.; Janik, M. J.; Jones, A. K.; Kanatzidis, M. G.; King, P.; Lancaster, K. M.; Lymar, S. V.; Pfromm, P.; Schneider, W. F.; Schrock, R. R., Beyond fossil fuel–driven nitrogen transformations. Science 2018, 360 (6391), eaar6611. 2. Suryanto, B. H. R.; Du, H.-L.; Wang, D.; Chen, J.; Simonov, A. N.; MacFarlane, D. R., Challenges and prospects in the catalysis of electroreduction of nitrogen to ammonia. Nat. Catal. 2019, 2, 290-296. 3. Minteer, S. D.; Christopher, P.; Linic, S., Recent Developments in Nitrogen Reduction Catalysts: A Virtual Issue. Acs Energy Lett 2019, 4 (1), 163-166. 4. Wang, J.; Yu, L.; Hu, L.; Chen, G.; Xin, H.; Feng, X., Ambient ammonia synthesis via palladium-catalyzed electrohydrogenation of dinitrogen at low overpotential. Nat. Commun. 2018, 9 (1), 1795. 5. Bao, D.; Zhang, Q.; Meng, F.-L.; Zhong, H.-X.; Shi, M.-M.; Zhang, Y.; Yan, J.-M.; Jiang, Q.; Zhang, X.-B., Electrochemical Reduction of N2 under Ambient Conditions for Artificial N2 Fixation and Renewable Energy Storage Using N2/NH3 Cycle. Adv. Mater. 2017, 29 (3), 1604799. 6. Zhang, G.; Ji, Q.; Zhang, K.; Chen, Y.; Li, Z.; Liu, H.; Li, J.; Qu, J., Triggering surface oxygen vacancies on atomic layered molybdenum dioxide for a low energy consumption path toward nitrogen fixation. Nano Energy 2019, 59, 10-16. 7. Zhang, L.; Ji, X.; Ren, X.; Ma, Y.; Shi, X.; Tian, Z.; Asiri, A. M.; Chen, L.; Tang, B.; Sun, X., Electrochemical Ammonia Synthesis via Nitrogen Reduction Reaction on a MoS2 Catalyst: Theoretical and Experimental Studies. Adv. Mater. 2018, 30 (28), 1800191.
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