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Oxygen Vacancies in Ta2O5 Nanorods for High-Efficient Electrocatalytic N2 Reduction to NH3 under Ambient Conditions Wenzhi Fu, Peiyuan Zhuang, Mason OliverLam Chee, Pei Dong, Mingxin Ye, and Jianfeng Shen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01178 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 29, 2019

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ACS Sustainable Chemistry & Engineering

Oxygen Vacancies in Ta2O5 Nanorods for High-Efficient Electrocatalytic N2 Reduction to NH3 under Ambient Conditions Wenzhi Fua, Peiyuan Zhuanga, Mason OliverLam Cheeb, Pei Dongb, Mingxin Ye*a, Jianfeng Shen*a

aInstitute

of Special Materials and Technology, Fudan University, Shanghai, 200433,

P. R. China bDepartment

of Mechanical Engineering, George Mason University, VA 22030, USA

Corresponding Author: *[email protected], [email protected].

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Abstract Electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions is emerging as a potential alternative to Haber-Bosch process when considering cost and environmental protection. Furthermore, the grand challenge on N2 activation encourages development of efficient NRR catalysts. Herein, we report the positive effect of oxygen vacancies in Ta2O5 nanorods for effective N2 reduction. Density functional theory calculations reveal that N2 can be efficiently activated at the O-vacancy site via coordination with two Ta atoms adjacent to the O-vacancy. Electrocatalytic NRR experiments verify the superior catalytic performance of O-vacancy engineered Ta2O5 nanorods (NH3 yield: 15.9 μg h-1 mg-1cat., Faradaic efficiency: 8.9%). This work clearly demonstrates the significance of defect engineering on NRR field.

Key words: N2 reduction reaction, oxygen vacancies, Ta2O5 nanorods, N2 activation, density functional theory calculations


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Introduction Ammonia (NH3), one of the most highly produced chemicals, is widely used as an essential building block to manufacture inorganic fertilizers, plastics and other products.1-3 NH3 is also considered an important carbon-free energy carrier due to its 17.6% gravimetric hydrogen density.4,5 The conversion of N2 to NH3 is a promising and significant way to alleviate the energy crisis.6 Although the availability of N2 from the atmosphere is unlimited, N2 fixation is still a great challenge because of the high bond energy (941 kJ mol−1) of inert N2.7,8 In industry, NH3 is made though the Haber-Bosch process by using a Fe-based catalyst, but needs harsh reaction conditions, accompanied by large consumption of fossil fuels and heavy CO2 emissions.9,10 Thus, the development of other environmentally benign ways for N2 fixation is in urgent need. Electrocatalytic N2 reduction reaction (NRR) under ambient conditions has recently emerged as a potential alternative strategy for NH3 synthesis, because it can reduce the energy consumption and avoid large carbon emission.11-18 Moreover, the electrical energy used to drive the N2 fixation reaction can be derived from renewable sources such as wind and solar energy.19 Noble-metal catalysts (Au20-22, Pd23,24 and Ru25) demonstrate favorable catalytic activities on NRR, but limited resources and high cost hinder large-scale use. Nowadays, metal oxides are receiving increasing attention as promising alternative NRR catalysts due to their availability and respectable catalytic activities.26-31 However, only a few metal oxides were reported for NRR, such as MoO3,32 Fe3O4,33 Nb2O534 and Cr2O3.35 Although they presented efficient catalytic activities on NRR, the Faradaic efficiency (FE) needs to be improved. Furthermore, the grand challenge on the step of activating inert N≡N triple bond appeals to develop more high-efficient NRR catalysts. To realize the efficient activation of inert N2 molecule, introducing oxygen vacancies in oxide semiconductors is proposed. Firstly, O-vacancies are considered to be effective structures for enhanced adsorption and activation of inert gas molecules due to the abundant localized electrons.36-39 For example, the strategies of direct activation of O2 and CO2 molecules at O-vacancies were employed for efficient



oxygen reduction40-42 and CO2 reduction reactions.43,44 Secondly, the in-situ activation of adsorbates on O-vacancies can lower the energy barrier for faster electron transfer, which may lead to a superior electrocatalytic performance.45 In this work, we prepared Ta2O5 nanorods with rich O-vacancies to investigate the electrocatalytic performance on NRR. In 0.1 M HCl electrolyte, Ta2O5 nanorods could efficiently catalyze NRR with a high NH3 yield of 15.9 μg h-1 mg-1cat. and an excellent FE of 8.9%. Furthermore, this catalysis also exhibited good stability.

Experimental Section Materials. Commercial tantalum oxide (Ta2O5), Tantalum ethoxide (Ta(C2H5O)5), Salicylic acid (C7H6O3), p-dimethylaminobenzaldehyde (p-C9H11NO), and Sodium nitroferricyanide (Na2[Fe(CN)5NO]·2H2O) were purchased from Macklin (China). Sodium citrate (C6H5Na3O7·2H2O), Sodium hypochlorite (NaClO), Ethylene glycol, NH4F, NH4Cl, KOH, HCl and Na2SO4 were obtained from Sinopharm Chemical Reagent Co., Ltd. Nafion membrane 211 was purchased from DuPont. Preparation of Ta2O5 nanorods. Briefly, 22 mg NH4F was dissolved to 6 mL ethylene glycol in an inner container. Then, 300 mg Ta(C2H5O)5 was added to the solution and stirred for about 1 minute. The small inner container was put into a 100 mL stainless-steel autoclave containing 10 mL distilled water. After sealing the autoclave, it was heated at 220 oC for 24 h in an oven. After cooling down to room temperature, the yellow residue was collected by centrifugation and thoroughly washed with ethanol, 0.1 M HCl and water for three times each. Then, the O-vacancies engineered Ta2O5 nanorods were obtained after drying at 60 oC in vacuum for 12 h. For comparison, annealed Ta2O5 was obtained by heating O-vacancies engineered Ta2O5 nanorods at 450 °C in air atmosphere for 3 h. Electrocatalytic N2 reduction measurements. Electrocatalytic NRR tests were conducted in 0.1 M HCl solution in a two-compartment cell separated by Nafion membrane 211 with a standard three-electrode system (Pt foil as the counter electrode, catalyst working electrode and Ag/AgCl/saturated KCl as the reference electrode). All NRR tests were performed at room temperature (25 oC). Before the electrocatalytic


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process, 40 mL HCl electrolyte was purged with N2 for 30 min. Then, N2 was continuously bubbled into the cathodic compartment during the electrocatalytic process. After the electrocatalytic experiment, the indophenol blue method was employed to determine the NH3 concentration under an UV-vis spectrophotometer. Preparation of working electrode. Typically, 5 mg catalyst and 30 μL Nafion solution (5 wt%) were added into 970 μL H2O/C2H5OH (1:1) solvent followed by sonication for 30 min to form a uniform solution. Then, 200 μL as-obtained catalyst solution was loaded onto a carbon paper electrode with an area of 1x1 cm2 and dried in vacuum at room temperature. Characterization. The crystal structures of the as-prepared samples were determined by X-ray diffraction (XRD, D/max-γB diffractometer with Cu Kα radiation handled at 40 kV and 40 mA). The surface morphologies of the as-prepared samples were imaged by field emission scanning electron microscopy (SEM, Philips XL30FEG handled at 15 kV) and transmission electron microscopy (TEM, JEM-2100F handled at 200 kV). The binding energies of elements were measured by X-ray photoelectron spectroscopy (XPS, XR 5 VG using a monochromatic Mg X-ray source). The electron paramagnetic resonance spectra were conducted on an EPR Spectrometer (Bruker EMX-10/12). N2 temperature programmed desorption experiments (N2-TPD) were carried out on DAS7000 instrument. H2 quantifications were calculated by a gas chromatograph (Agilent 7890B). 1H NMR spectra were obtained via a 500 MHz NMR spectrometer (Bruker Advance III HD). The electrochemical impedance spectroscopy (EIS) and NRR experiments were performed using an electrochemical workstation (CHI 660E) equipped with a standard three-electrode system. The UV-vis absorption spectra were acquired from an UV-vis spectrophotometer (Shimadzu, UV-3600).

Results and Discussion To confirm the possible N2 activation on O-vacancies of Ta2O5 nanorods, density functional theory (DFT) calculations were performed on (001) facets of Ta2O5. In Figure 1a, after introducing an O-vacancy site, the Bader charge of these two



connected Ta atoms increased from 2.45 e to 2.92 and 3.00 e due to the reduction of Ta ions by localized electrons. As expected, N2 can be adsorbed at the O-vacancy site via coordination with two Ta atoms adjacent to the O-vacancy (Figure 1b). The charge density difference demonstrates that electrons can be exchanged and transferred between two partially reduced Ta atoms adjacent to the O-vacancy site and adsorbed N2 molecule (Figure 1c, d). The interesting electron back-donation process generally takes place in some transition metal-N2 complexes.46 The available d-orbital electrons of these partially reduced Ta atoms are donated into π N-N antibonding, which can lead to the activation of an inert N≡N triple bond. Due to the electrons transferring between Ta atoms and adsorbed N2, the efficient-activated N2 molecule can be reflected by an increased bond length of 1.381 Å, which contrasts sharply with 1.098 Å in free N2 and lies between N-N bond length in diazene (1.201 Å) and hydrazine (1.47 Å). As activation of the inert N≡N triple bond plays a key role in the electrocatalysis of N2 to NH3, the strategy of engineering O-vacancies would be effective for NRR. To further identify the intrinsic N2 molecules active sites, DFT calculations of N2 hydrogenation in the whole NRR process were performed on pristine (001) surface of defect Ta2O5 and defect-free Ta2O5 (Figure S1). The results show that the step of N2 adsorption on the surface of defect Ta2O5 is easier than defect-free Ta2O5. Moreover, the subsequent step of N2 hydrogenation on defect-free Ta2O5 possesses very high energy barrier, indicating the inferior NRR activity on defect-free Ta2O5. As a comparison, the step of N2 hydrogenation can be performed smoothly on the surface of defect Ta2O5. Therefore, the structure of oxygen vacancies possesses positive effect for improving NRR catalytic activity.


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Figure 1. (a) Model of (001) surface of Ta2O5 with an O-vacancy site. (b) The adsorption geometry of N2 on O-vacancy site of Ta2O5 (001) surface. (c) Side and (d) top view of charge density difference of the N2 adsorbed (001) surface.

Ta2O5 nanorods were synthesized by a facile vapor hydrolysis method. The oxygen vacancies were directly produced during the hydrolysis process due to the reduction by ethylene glycol at high temperature. The crystal structure of the defect Ta2O5 nanorods (Figure 2a) was determined by X-ray diffraction (XRD). The XRD pattern of defect Ta2O5 nanorods well agrees with the characteristic peaks of the Ta2O5 phase (JCPDS No.25-0922). Scanning electronmicroscopy (SEM) was used to reveal the surface morphologies of as-prepared defect Ta2O5 nanorods. In Figure 2b and c, the Ta2O5 catalyst shows the typical rod-like structure with a uniform size. After annealing at 450 °C in air atmosphere, annealed Ta2O5 nanorods also retain the main morphologies (Figure S2). Energy-dispersive X-ray (EDX) elemental mapping images (Figure 2d) illustrate the uniform distribution of Ta and O elements in defect Ta2O5 nanorods. EDX elements analysis (Figure 2e) demonstrates the elements ratio of defect Ta2O5 nanorods. The transmission electron microscopy (TEM) image further confirms the structure of defect Ta2O5 nanorods with a diameter of ~40 nm (Figure 2f). The lattice fringe spacing of 0.39 nm is consistent to the (001) plane of Ta2O5 (Figure 2g). Moreover, some defect structures can be observed, which are ascribed to



the escape of oxygen element from the lattice.47 Figure 2h shows the selected area electron diffraction (SAED) pattern of defect Ta2O5 nanorods, and the result illustrates the polycrystalline structure.

Figure 2. (a) XRD, (b, c) SEM images, (d) EDX-mapping, (e) EDX element analysis, (f) TEM image, (g) HRTEM image and (h) SAED pattern of defect Ta2O5 nanorods.

X-ray photoelectron spectroscopy (XPS) was employed to investigate the valence and composition about Ta2O5 samples. For defect Ta2O5 nanorods, the Ta 4f high-resolution XPS spectrum (Figure 3a) shows two peaks at 25.8 and 27.8 eV, belonging to Ta 4f7/2 and Ta 4f5/2 respectively. Importantly, the Ta 4f peaks of defect Ta2O5 nanorods shift to lower binding energies compared to the Ta 4f peaks of annealed Ta2O5 (ca. 0.3 eV), resulting from the low valence state Ta species accompanied by oxygen vacancies, which can further confirm the reduction of Ta ions. In Figure 3b, the XPS spectrum of O 1s shows three peaks at 530.0, 531.3 and 533.0 eV, arising from lattice oxygen, oxygen atoms in the vicinity of oxygen vacancies and adsorbed water.48,49 However, after annealing process, the O 1s peak of oxygen atoms close to oxygen vacancies dramatically decreases, confirming the rich O-vacancies in Ta2O5 nanorods and poor O-vacancies in annealed Ta2O5. The


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presence of O-vacancies was further verified by low-temperature electron paramagnetic resonance (EPR) spectra (Figure 3c). Compared to annealed Ta2O5 and commercial Ta2O5, the as-prepared defect Ta2O5 nanorods show a strong EPR signal at g=2.004, demonstrating the rich O-vacancies in Ta2O5 nanorods.48,50 However, the weak EPR signal of annealed Ta2O5 illustrates the reduced O-vacancy sites. Furthermore, electrochemical impedance spectroscopy (EIS) was employed to investigate the resistance for electron transfer of catalysts. In Figure 3d, a smaller arc radius for defect Ta2O5 nanorods illustrates faster electron transfer which results from rich O-vacancies. Thus, the faster electron transfer to adsorbed N2 may accelerate the NRR process. N2 temperature programmed desorption (TPD) experiment (Figure S3) was used to assess N2 activation on the surface of catalysts. For defect Ta2O5 nanorods, the weak peak at 100 oC and strong adsorption peak at 360 oC are ascribed to the N2 physisorption and chemisorption, respectively. As a comparison, the N2 adsorption ability of commercial Ta2O5 is almost negligible. Therefore, the strong N2 chemisorption demonstrates the efficient activation of N2 on the surface of defect Ta2O5 nanorods.

Figure 3. XPS spectra of (a) Ta 4f and (b) O 1s for Ta2O5 samples. (c) low-temperature EPR spectra and (d) EIS spectra of Ta2O5 samples.



Electrocatalytic NRR tests were performed in 0.1 M HCl electrolyte to investigate the catalytic activities of O-vacancy engineered Ta2O5 nanorods. The NH3 concentrations were calculated via the calibration curve as shown in Figure S4. Linear-sweep voltammetric (LSV) curves were first measured in N2 and Ar-saturated electrolytes. The higher current density in N2 atmosphere indicates the reduction of N2 to NH3 (Figure S5).34,51 The NRR activities of defect Ta2O5 nanorods were evaluated at different potentials against a reversible hydrogen electrode (RHE). In Figure 4a, the highest NH3 yield (15.9 μg h-1 mg-1cat.) was obtained when the potential was at -0.7 V, with a FE of 8.9%. The corresponding UV-vis absorption curves of the electrolytes stained with indophenol52 are presented in Figure 4b. It is worth noting that the defect Ta2O5 nanorods exhibit competitive catalytic performance compared to other previously reported catalysts (Table S1). It is known that hydrogen evolution reaction (HER) is major competition side reaction for NRR process. The amount of produced H2 and corresponding FEs at different potentials were determined by a gas chromatography (Figure S6). The results demonstrate that the higher potentials were beneficial to the competitive HER. Although HER counts against NRR process, the abundant O-vacancy sites can greatly boost the catalytic activity of NRR under proper potential, and increase NRR selectivity. Moreover, Figure 4c exhibits the good catalytic current stability of defect Ta2O5 nanorods at different potentials under N2 atmosphere. To verify the generated NH3 was obtained from the electrocatalytic N2 reduction process, the UV-vis absorption curves of electrolytes were measured after NRR tests conducted at an open-circuit and Ar-saturated atmosphere (Figure S7). As a result, almost no NH3 was produced in open-circuit, and only a little NH3 yield was acquired in an Ar atmosphere. Furthermore,

15N

isotope labeling experiment was

carried out by using 15N2 enriched gas as the feeding gas (Figure S8). After comparing with a standard 15NH4+ sample, 1HNMR spectra reveal that 15NH4+ could be detected when 15N2 was fed, confirming that the produced NH3 derived from the supplied N2 in NRR tests. The catalytic activities of defect Ta2O5 nanorods were also investigated in different electrolytes (Figure S9). The results demonstrate that the NH3 yields and corresponding FEs were lower in KOH and Na2SO4 electrolytes, when compared to


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HCl electrolyte.

Figure 4. (a) The yields of NH3 and Faradaic efficiencies of defect Ta2O5 nanorods at different potentials vs. RHE. (b) UV-vis absorption curves of the electrolytes stained with indophenol indicator using defect Ta2O5 nanorods at different potentials. (c) Time-dependent current density curves of defect Ta2O5 nanorods at different potentials. (d) Comparison of NH3 yields from different samples at -0.7 V. (e) UV-vis absorption curves of the electrolytes estimated by the method of Watt-Chrisp before and after 2 h electrolysis in N2 atmosphere at -0.7 V. (f) Consecutive recycling tests of defect Ta2O5 nanorods at -0.7 V.

To further highlight the significance of O-vacancy structure on Ta2O5 nanorods, we compared the catalytic activities with the annealed Ta2O5 and commercial Ta2O5 (Figure 4d). As expected, the commercial Ta2O5 only showed a low NH3 yield of 1.2 μg h-1 mg-1cat.. In contrast, the defect Ta2O5 nanorods achieved 13 times of commercial Ta2O5 in NH3 yield. Moreover, the NH3 yield of annealed Ta2O5 sharply decreased due to the loss of O-vacancy sites. The UV-vis absorption curves of post-tested electrolyte from the method of Watt-Chrisp53 suggest no N2H4 was generated under a potential of -0.7 V, which indicates the excellent catalytic selectivity of defect Ta2O5 nanorods (Figure 4e). The consecutive recycling tests were employed to evaluate the catalyst stability. In Figure 4f, the NH3 yield rate and corresponding FEs of defect Ta2O5 nanorods show a downward trend, but still retains 86.7% of the original level for NH3 yield after five repeated tests, illustrating the good catalyst stability. To further confirm the stability of the catalyst, we also conducted a long-term test. The result shows that the current density of defect Ta2O5 nanorods occurs only a slight



decrease after 24-hour electrolysis (Figure S10). XPS analysis (Figure S11) and XRD pattern (Figure S12a) of reused catalyst demonstrate that it maintains the original Ta2O5 structure. TEM imaging (Figure S12b) also shows that the defect Ta2O5 catalyst retains its rod-like morphology. Thus, the above results indicate the superior NRR activities and stabilities of O-vacancy engineered Ta2O5 nanorods.

Conclusions In summary, Ta2O5 nanorods with rich O-vacancies were successfully synthesized and proved to be an efficient electrocatalyst on NRR based on density functional theory (DFT) calculations and experimental verification. DFT calculations revealed that the O-vacancy site could enable a highly activated N2, reflected by an extended N-N bond length. NRR tests demonstrated the extraordinary catalytic activities of O-vacancy engineered Ta2O5 nanorods, with a NH3 yield of 15.9 μg h-1 mg-1cat. and an excellent FE of 8.9%. The defect Ta2O5 nanorods also showed high NH3 selectivity and good stability. This work could promote the artificial fixation of atmospheric N2 to NH3 and provide guidance for more rationally designed catalysts on NRR with defect engineering.

Supporting information NRR reaction pathways, SEM images, H2 quantifications, N2-TPD of Ta2O5 samples, NMR data, UV-vis absorption, long time-dependent current density curve, calibration curves and linear sweep voltammetric curve, XRD pattern, XPS analysis and TEM image of reused catalyst, Tables S1.

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Graphical Abstract:

Synopsis The positive effect of oxygen vacancies in Ta2O5 nanorods greatly promotes electrocatalytic N2 conversion to NH3 with a high Faradaic efficiency.


Oxygen Vacancies in Ta2O5 Nanorods for Highly ... - ACS Publications

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