Enhancing Electrocatalytic N2 Reduction to NH3 ... - ACS Publications

Letter pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Enhancing Electrocatalytic N2 Reduction to NH3 by CeO2 Nanorod with Oxygen Vacancies Bo Xu,† Li Xia,‡ Fuling Zhou,† Runbo Zhao,‡ Hongyu Chen,‡ Ting Wang,‡ Qiang Zhou,∥ Qian Liu,∥ Guanwei Cui,§ Xiaoli Xiong,*,† Feng Gong,*,∥ and Xuping Sun*,‡ †

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, Sichuan, China Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China ∥ School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, China § College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, Shandong, China

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‡

S Supporting Information *

ABSTRACT: As a sustainable and environmentally friendly route, electrochemical N2 reduction has been expected to replace the traditional N2 fixation process of Haber−Bosch, which is energy-consuming and capital-intensive. In this work, we demonstrate that CeO2 nanorod with oxygen vacancies achieves a great enhancement for NRR with excellent selectivity and stability. Such electrocatalyst delivers NH3 yield of 16.4 μg h−1 mg−1cat. at −0.5 V vs reversible hydrogen electrode (RHE) and Faradaic efficiency of 3.7% at −0.4 V vs RHE using aqueous 0.1 M Na2SO4 electrolyte, higher than that of pristine CeO2 counterpart (5.4 μg h−1 mg−1cat.; 2.1%).

KEYWORDS: CeO2 Nanorod, Oxygen Vacancies, N2 reduction reaction, Electrocatalysis, Ambient conditions

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remarkable NRR activity.17−21 However, their extensive use is limited due to the scarcity and high cost of noble metal. Nonnoble-metal catalysts are promissing candidates with good activity such as Mo nanofilm,22 VN,23 MoN,24 defect-rich MoS2,25 Ti3C2Tx.26 Metal oxides are low cost and easily prepared on a large scale, and great recent attention has paid on developing such catatalys.27−34 Cerium(IV) oxide (CeO2) has good electronic/ionic conductivity because of the flexible transition between the Ce3+ and Ce4+ oxidation states.35,36 The groups of exposed Ce3+ act as surface catalysis sites for adsorbed gases or catalytic reaction intermediates.36 Defect engineering is efficient to manipulate the electronic structures of metal oxides and regulate the catalytic activity in solar-driven nitrogen fixation which could offer the coordinatively unsaturated sites (CUS) for the delivery of electrons to the adsorbed N2 molecules as well as weaken the NN bond.37−39 Moreover, oxygen vacancy defects can be formed and eliminated rapidly on surface.36 CeO2 is thus an ideal material for developing defectrich catalyst.36 To our knowledge, however, few catalyst with oxygen vacancies has been reported for electrochemical NRR.

INTRODUCTION As the second largest chemical product in the world, NH3 is indispensable in agriculture and some sectors of industry including fertilizer, medicament, dye, explosive, and resin.1−5 Besides, NH3 has a promising prospect as a potential superior hydrogen carrier and combustion fuel.6,7 The ever-increasing NH3 demands have stimulated intensive interest in N2-to-NH3 conversion.8 Although the atmosphere consists largely of N2, it remains a chanllenge to convert N2 to NH3 due to the high thermodynamic stability of triple bond in N2 molecule.3,9 In industrial synthesis of NH3, the Haber−Bosch process is operated under elevated temperature (300−500 °C) and high pressure (200−300 atm) with iron- or ruthenium-based catalysts.10−12 Meanwhile, the acquisition of the necessary H2 is largely derived from fossil fuels, which also leads to substantial carbon emission.13 Therefore, it is necessary to grope for the environmental protection technology of N2 fixation with low energy consumption under environmental conditions. The electrochemical N2 reduction reaction (NRR), using water as the hydrogen source under ambient conditions, has emerged as an alternative approach toward artificial N2 fixation.14−16 But this process requires effective electrocatalysts to break strong nonpolar NN covalent triple bond.10 Many studies have shown that noble-metal-based catalysts have © XXXX American Chemical Society

Received: September 29, 2018 Revised: January 8, 2019

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DOI: 10.1021/acssuschemeng.8b05007 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

plane of CeO2. As shown in Figure 1e, the HRTEM of r-CeO2 nanorod shows a blurry lattice spacing of 0.313 nm, corresponding to the (111) plane of CeO2.41 Obviously, after the treatment of hydrogen-reduction, the oxygen element escapes from the lattice which leads to the formation of oxygen vacancies, indicating that the point defect causes nearby lattice distortion.41,42 To acquire the valence states of elements in the samples, we carried out X-ray photoelectron spectroscopy (XPS) characterizations for pristine CeO2 and r-CeO2. The wide XPS spectra of r-CeO2 shown in Figure 2a, the binding energies (BEs) of C

In this Letter, we utilized hydrogen reduced CeO2 nanorods (r-CeO 2 nanorods) as catalyst for electrochemical N 2 reduction. When electrolyzed in 0.1 M Na2SO4, this catalyst achieves NH3 yield of 16.4 μg h−1 mg−1cat. at −0.5 V vs reversible hydrogen electrode (RHE) and Faradaic efficiency (FE) of 3.7% at −0.4 V vs RHE. It is worth noting that the rCeO2 nanorods also show good durability and excellent selectivity for electrochemical NRR.

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RESULTS AND DISCUSSION Pristine CeO2 nanorods were treated using H2 in tube furnace for 6 h to synthesize CeO2 nanorods with oxygen vacancies (preparation details demonstrated in Supporting Information). Figure 1a shows the X-ray diffraction (XRD) patterns of CeO2

Figure 2. (a) XPS survey spectrum of r-CeO2. XPS spectra of CeO2 and r-CeO2 in the (b) Ce 3d and (c) O 1s. (d) room-temperature PL spectra.

1s, O 1s, and Ce 3d are located at 284.8, 529.1, and 897.71 eV, respectively. Among them, C could be attributed to the residue from ethanol under XPS measurement. Figure 2b displays the XPS spectra of Ce 3d. The peaks of Ce4+ 3d5/2 locates two peaks with the BEs of 881.9, 888.2 eV, while the Ce4+ 3d3/2 region locates two peaks at 883.2, 902.0 eV. The two peaks at BEs of 897.7 and 900.3 eV are assigned to Ce3+ 3d5/2 and Ce3+ 3d3/2.43,44 According to density functional theory (DFT) calculations (Figure S4a), Ce3+ possesses higher NRR activity for NRR than that of Ce4+. Combing the calculation of characteristic peak area at Ce3+ 3d5/2 and Ce3+ 3d3/2, we found that r-CeO2 possesses a higher concentration of Ce3+, which results from the treatment of hydrogen reduction.42,44 Further calculations show that pontential determining step of the NRR reactions on r-CeO2 is *NHNH2-*NH2NH2 with an energy barrier of ∼0.77 eV, consistent well with the experimental results. When applying a potential of −0.77 eV, all the NRR reaction steps are downhill (Figure S4b). For O 1s XPS spectra (Figure 2c), two different bands at 529.0 and 531 eV are observed, the peak of 531.2 eV is commonly associated with oxygen vacancies.44,45 Of note that their peak area of 531.2 eV has a great difference, which indicates that the r-CeO2 nanorod obtained much more oxygen vacancies in hydrogen-reduction possess compared with the pristine CeO2 nanorod.44,46 In addition, the photoluminescence (PL) spectra were also applied. The peak at 420 nm corresponds to the recombination of two-electron-trapped oxygen vacancies with photogenerated holes.47,48 The corresponding emission peak intensity elevated further confirm the formation of oxygen vacancies (Figure 2d).

Figure 1. (a) XRD patterns for r-CeO2 nanorods and CeO2 nanorods. (b) SEM image for r-CeO2 nanorods. (c) SEM image and EDX elemental mapping images of Ce and O elements of r-CeO2 nanorods. (d) TEM image of r-CeO2 nanorods (e) HRTEM image taking from single r-CeO2 nanorod.

and r-CeO2. As observed, both samples show the same diffraction peaks at 28.5°, 33.0°, 47.4°, and 56.3° corresponding to the (111), (200), (331), and (222) planes of CeO2 (JCPDS No. 81-0792),40 respectively. Scanning electron microscopy (SEM) reveals that r-CeO2 has regular rod-like morphology with a diameter of about 100−200 nm and a side length of 5−10 μm, as shown in Figure 1b. The r-CeO2 nanorods still maintain the nanorod feature (Figure S1) after heat-treatment. Figure S2 displays the energy-dispersive X-ray (EDX) spectrum indicating the existence of Ce and O elements. Figure 1c (EDX elemental mapping) reveals that the distribution of Ce and O was homogeneous. Figure 1d and S3a display the transmission electron microscopy (TEM) images of r-CeO2 and CeO2, respectively, further confirming their nanorod nature. Figure S3b exhibits the high-resolution TEM (HRTEM) image of pristine CeO2 suggesting the pristine CeO2 nanorod has excellent crystallinity. The interplanar distance is 0.313 nm, corresponding to the (111) B


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our present work, revealing excellent selectivity for N2 conversion to NH3. The NH3 yields and corresponding FEs of r-CeO2/CP at potentials between −0.3 and −0.7 V are plotted in Figures 3d, respectively. As observed, the NH3 yield and corresponding FE increase with the decrease of negative potential. FE for NH3 production earlier reaches the highest value of 3.74% at −0.4 V, then the maximum NH3 yield of 16.4 μg h−1 mg−1cat. was achieved at −0.5 V, which attains a higher performance comparing with those reported electrocatalysts, including Au nanorods (1.648 μg h−1 mg−1cat.),51 α-Au/CeOx-RGO (8.31 μg h−1 mg−1cat.)18 and γ-Fe2O3 (0.212 μg h−1 mg−1cat.).52 A detained comparison is summarized in Table S1. As two competitive processes, hydrogen evolution reaction (HER) and NRR are take place simultaneously on the catalyst surface.53,54 When the potential moves below −0.5 V, the HER becomes the primary process in this system, leading to the decline of the yield rate and FE. We tested the performance of different kinds of catalysts after 2 h electrolysis at −0.5 V (Figure 3e). Clearly, bare CP shows a poor electrocatalytic NRR activity. Of note, CeO2/CP is efficient to electrochemically catalyze the N2 reduction with a moderate NH3 formation rate of 5.7 μg h−1 mg−1cat. In sharp contrast, r-CeO2/CP shows greatly enhanced NRR activity with a NH3 formation yield over 2.8 times of CeO2/CP. The enhanced NRR activity of rCeO2/CP may be attributed to the exposed Ce3+ ions and Ce4+ ions in vacancy clusters, which could act as CUS for facilitating N2 activation through delivering the electrons to the adsorbed N2 molecules as well as weakening the NN bond.37,39 Combining the electrochemical impedance spectroscopy (EIS) data (Figure 3f), a smaller semicircle radius of r-CeO2 suggests that r-CeO2 possesses a lower charge transfer resistance55 and thus faster NRR kinetics. These results demonstrate that oxygen vacancies can remarkably enhance the NRR activity of CeO2 and accomplish a higher NH3 yield and FE at a suitable potential. To avoid the deviation caused by the NH3 in the atmosphere and supplying gas, we continually supply high-purified gas (Ar or N2, purify: 99.999%) to the sealed electrolytic cell without applied voltage. The results proved that no NH3 is formed in either case (Figure S9). To confirm that detected NH3 originates from the NRR over r-CeO2/CP, we performed alternating 2 h cycles between N2- and Ar-saturated electrolytes for a total of 12 h at −0.5 V. As shown in Figure S10, NH3 is detected in the duration of N2-saturated electrolytes, while NH3 is not produced in Ar-saturated electrolytes. As shown in Figure S11, the mass of NH3 increases with electrolysis time. All above results testify that NH3 detected only stems from electrochemical conversion of N2 over r-CeO2/CP. As another parameter for evaluating the performance of catalyst, stability and durability are also critical. During the consecutive recycling tests, NH3 yield and FE only show ignorable variation (Figure 4a). In Figure 4b, the timedependent current density curve of r-CeO2 nanorod only occurs a slight decrease after 24 h of electrolysis. As shown in Figure S12, the XRD pattern confirms that such catalyst maintains CeO2 in nature after reaction. SEM image (Figure S13) shows that r-CeO2 nanorods still maintains its morphology of nanorod after stability test. All these results show that r-CeO2 nanorod possess high mechanical strength and chemical stability.

In evidence, r-CeO2 nanorod possesses the high concentration of oxygen vacancies. To evaluate electroreduction of N2 on r-CeO2 nanorods, a rCeO2/Carbon paper electrode (r-CeO2/CP; r-CeO2 loading: 0.1 mg cm−2) was tested in a N2-saturated 0.1 M Na2SO4 solution using a Nafion membrane separated from H-shaped cells. N2 inflow is conveyed to the cathode, where N2 and proton (H+) combine with electrons to produce NH3. Same operation were conducted on pristine CeO2. All potentials were converted to RHE scales. In Figure 3a, the linear sweep voltammetric (LSV) curves of r-CeO2/CP in Ar-saturated (black curve) and N2-saturated

Figure 3. (a) LSV curve of r-CeO2/CP in Ar-saturated and N2saturated 0.1 M Na2SO4. (b) Time-dependent current density curves for r-CeO2/CP at different potentials in 0.1 M Na2SO4. (c) UV−vis absorption spectra of the electrolytes stained with indophenol indicator after electrolysis at a series of potentials for 2 h. (d) NH3 yields and FEs for r-CeO2/CP at each given potential. (e) Amount of NH3 generated NH3 with different electrodes at potential of −0.5 V after 2 h electrolysis under ambient conditions. (f) Nyquist plots of CeO2/CP and r-CeO2 /CP.

(red curve) electrolytes were conducted. Obviously, the current density of r-CeO2/CP is higher in N2-saturated solution, manifesting the electrocatalytic N2 reduction process is started-up.48,49 A similar tendency for pristine CeO2 (Figure S5) is also observed. Figure 3b provides the current density curves of r-CeO2/CP obtained in a range potentials from −0.3 to −0.7 V, demonstrating that the current densities are stable for 2 h electrolysis. After electrolysis, the indophenol blue method18 and the Watt and Chrisp method were applied to determined and quantified the produced NH3 and possible byproduct (N2H4),50 respectively. The corresponding calibration curves are shown in Figures S6 and S7. Figure 3c demonstrates the ultraviolet−visible (UV−vis) absorption spectra of the product solutions colored with indophenol indicator. Of note, Figure S8 shows that no N2H4 is detected in C


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(4) Smil, V. Detonator of the Population Explosion. Nature 1999, 400 (6743), 415. (5) Schlögl, R. Catalytic Synthesis of Ammonia−A ‘Never-Ending Story’? Angew. Chem., Int. Ed. 2003, 42 (18), 2004−2008. (6) Pickett, C. J.; Talarmin, J. Electrosynthesis of Ammonia. Nature 1985, 317 (6038), 652. (7) Christensen, C. H.; Johannessen, T.; Sørensen, R. Z.; Nørskov, J. K. Towards an Ammonia-mediated Hydrogen Economy? Catal. Today 2006, 111 (1−2), 140−144. (8) Brown, K. A.; Harris, D. F.; Wilker, M. B.; Rasmussen, A.; Khadka, N.; Hamby, H.; Keable, S.; Dukovic, G.; Peters, J. W.; Seefeldt, L. C.; King, P. W. Light-driven Dinitrogen Reduction Catalyzed by a CdS: Nitrogenase MoFe Protein Biohybrid. Science 2016, 352 (6284), 448−450. (9) Smil, V. Global Population and the Nitrogen Cycle. Sci. Am. 1997, 277 (1), 76−81. (10) Liu, Y.; Su, Y.; Quan, X.; Fan, X.; Chen, S.; Yu, H.; Zhao, H.; Zhang, Y.; Zhao, J. Facile Ammonia Synthesis from Electrocatalytic N2 Reduction under Ambient Conditions on N-doped Porous Carbon. ACS Catal. 2018, 8 (2), 1186−1191. (11) Cheng, H.; Ding, L.; Chen, G.; Zhang, L.; Xue, J.; Wang, H. Molybdenum Carbide Nanodots Enable Efficient Electrocatalytic Nitrogen Fixation under Ambient Conditions. Adv. Mater. 2018, 30 (43), 1803694. (12) MacLeod, K. C.; Holland, P. L. Recent Developments in the Homogeneous Reduction of Dinitrogen by Molybdenum and Iron. Nat. Chem. 2013, 5 (7), 559. (13) Bao, D.; Zhang, Q.; Meng, F.; Zhong, H.; Shi, M.; Zhang, Y.; Yan, J.; Jiang, Q.; Zhang, X. 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. (14) Chen, G.; Cao, X.; Wu, S.; Zeng, X.; Ding, L.; Zhu, M.; Wang, H. Ammonia Electrosynthesis with High Selectivity under Ambient Conditions via a Li+ incorporation Strategy. J. Am. Chem. Soc. 2017, 139 (29), 9771−9774. (15) Luo, Y.; Chen, G.; Ding, L.; Chen, X.; Ding, L.; Wang, H. Efficient Electrocatalytic N2 Fixation with MXene under Ambient Conditions. Joule 2018, 3, 1−11. (16) Guo, C.; Ran, J.; Vasileff, A.; Qiao, S. Rational Design of Electrocatalysts and Photo (electro) Catalysts for Nitrogen Reduction to Ammonia (NH3) under Ambient Conditions. Energy Environ. Sci. 2018, 11 (1), 45−56. (17) Song, Y.; Johnson, D.; Peng, R.; Hensley, D. K.; Bonnesen, P. V.; Liang, L.; Huang, J.; Yang, F.; Zhang, F.; Qiao, R.; Baddorf, A. P.; Tschaplinski, T. J.; Engle, N. L.; Hatzell, M. C.; Wu, Z.; Cullen, D. A.; Meyer, H. M., III; SUmpter, B. G.; Rondinone, A. J. A Physical Catalyst for the Electrolysis of Nitrogen to Ammonia. Sci. Adv. 2018, 4 (4), No. e1700336. (18) Li, S.; Bao, D.; Shi, M.; Wulan, B.; Yan, J.; Jiang, Q. Amorphizing of Au Nanoparticles by CeOx−RGO Hybrid Support towards Highly Efficient Electrocatalyst for N2 Reduction under Ambient Conditions. Adv. Mater. 2017, 29 (33), 1700001. (19) Huang, H.; Xia, L.; Shi, X.; Asiri, A. M.; Sun, X. Ag Nanosheet for Efficient Electrocatalytic N2 Fixation to NH3 at Ambient Conditions. Chem. Commun. 2018, 54, 11427−11430. (20) 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. (21) Lee, H.; Koh, C. S.; Lee, Y.; Liu, C.; Phang, I.; Han, X.; Tsung, C.; Ling, X. Favoring the Unfavored: Selective Electrochemical Nitrogen Fixation Using a Reticular Chemistry Approach. Sci. adv. 2018, 4 (3), No. eaar3208. (22) Yang, D.; Chen, T.; Wang, Z. Electrochemical Reduction of Aqueous Nitrogen (N2) at a Low Overpotential on (110)-oriented Mo Nanofilm. J. Mater. Chem. A 2017, 5 (36), 18967−18971. (23) Zhang, R.; Zhang, Y.; Ren, X.; Cui, G.; Asiri, A. M.; Zheng, B.; Sun, X. High-efficiency Electrosynthesis of Ammonia with High Selectivity under Ambient Conditions Enabled by VN Nanosheet Array. ACS Sustainable Chem. Eng. 2018, 6, 9545−9549.

Figure 4. (a) NH3 yields and FEs at potential of −0.5 V vs RHE during recycling tests for 5 times. (b) Time-dependent current density curve for r-CeO2/CP at the potential of −0.5 V.

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CONCLUSION In summary, we report that CeO2 nanorod with oxygen vacancies achieves a greatly enhanced NRR activity with a NH3 yield over 2.8 times that of pristine CeO2 nanorod under ambient conditions. In 0.1 M Na2SO4, the r-CeO2 nanorod attains a NH3 yield of 16.4 μg h−1 mg−1cat. at −0.5 V and a FE of 3.7% at −0.4 V. The result described here not only presents an efficient catalyst for NRR but also opens up attracting avenues to the rational design and development of metal-oxide catalysts for artificial N2 fixation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b05007. Experimental section; SEM and TEM images; XRD pattern; UV−vis absorption, EDX, and XPS spectra; calibration curves; Tables S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.X.). *E-mail: [email protected] (F.G.). *E-mail: [email protected] (X.S.). ORCID

Qian Liu: 0000-0002-7217-5083 Xiaoli Xiong: 0000-0002-5407-4350 Xuping Sun: 0000-0002-5326-3838 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21575137). We also appreciate Hui Wang from the Analytical & Testing Center of Sichuan University for her help with SEM characterization.

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