Boron-Doped TiO2 for Efficient Electrocatalytic N2 Fixation to NH3 at

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Boron-Doped TiO2 for Efficient Electrocatalytic N2 Fixation to NH3 at Ambient Conditions Yuan Wang, Kun Jia, Qi Pan, Yadi Xu, Qian Liu, Guanwei Cui, Xiao-Dong Guo, and Xuping Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05332 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 20, 2018

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ACS Sustainable Chemistry & Engineering

Boron-Doped TiO2 for Efficient Electrocatalytic N2 Fixation to NH3 at Ambient Conditions Yuan Wang,†,‡ Kun Jia,†,‡ Qi Pan,†,‡ Yadi Xu,† Qian Liu,§ Guanwei Cui,║ Xiaodong Guo,*,† Xuping Sun*,‡ †Chemical

Engineering Institute, Sichuan University, Chengdu 610065, 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, Sichuan, China, ║College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, Shandong, China ABSTRACT: NH3 synthesis almost relies on energy-intensive Haber-Bosch process with abandoned CO2 emission. Electrochemical N2 reduction offers an environmentally friendly process for sustainable artificial N2 fixation at ambient conditions, but stable and efficient catalysts are demanded for the N2 reduction reaction (NRR). In this Letter, we report that boron-doped TiO2 acts as an efficient non-noble-metal NRR catalyst for electrochemical N2 reduction to NH3 at ambient conditions. This catalyst delivers remarkable NRR performance in 0.1 M Na2SO4 with a NH3 yield of 14.4 μg h-1 mg-1cat. and a Faradic efficiency of 3.4% at -0.8 V vs. reversible hydrogen electrode (RHE). Notably, it is also excellent in electrochemical durability and selectivity for NH3 formation. Keywords: TiO2, B doping, N2 reduction reaction, NH3 synthesis, ambient conditions NH3, as a nitrogen-compound precursor and hydrogen storage production, has played an important role in agriculture, pharmacy and textile industries.1-5 Reduction of atmospheric N2 to NH3 is one of the essential processes for earth’s ecosystem.6 Although N2 is greatly abundant in atmosphere (the percentage is 78%), it is extremely difficult to cleave the strong N≡N triple bond (bond energy: 940.95 kJ mol-1) for fixation of N2 to NH3.7-9 In industry, NH3 synthesis almost relies on the long-standing Haber-Bosch process which is energy-intensive and requires high temperatures (300-400 ℃) and pressures (150-250 atm). Furthermore, carbon emission (1.87 tons per ton of NH3 produced) is substantial and cannot be ignored.10,11 Recently, electrochemical N2 reduction reaction (NRR) has been paid considerable attention owing to the utilization of renewable energy resources from solar energy, wind wave power, etc.12-14 Compared with traditional Haber-Bosch process, electrochemical NRR possesses following advantages: ambient operation conditions, earthabundant raw materials namely N2 and water, simplified equipment and extremely little carbon emission,15-17 but it requires efficient electrocatalysts for NRR. Noble metals such as Au,18,19 Ru,20 Ag,21 and Rh22 perform efficiently for electrolytic N2 reduction, whereas the expensiveness of these catalysts limit their wide uses. Hence, much attention has focused on designing and researching nonnoble-metal catalyst, especially transition metal oxides,23-31 which are low cost and easily prepared. As an abundant and non-toxic substance, TiO2 has become one of the hot spots in photochemistry.32,33 Our recent work indicates that TiO2 nanosheets array is also active for NRR with a NH3 yield of 9.16 × 10-11 mol·s-1·cm-2 and a Faradaic efficiency (FE) of 2.5%.34 Zheng and co-workers report that boron doping can enhance the NRR performances of graphene and such doped catalyst

attains a high FE of 10.8% and a NH3 yield of 9.8 μg h-1 mg1 35 It suggests that boron can break the intrinsic equilibrium cat. of graphene molecular orbitals and the positively charged boron is conducive to the adsorbtion of N2 to imporve the NRR activity. Additionally, boron doping is also capable of promoting the electrochemical performance of TiO2 due to the formation of oxygen vacancy which increases the conductivity of TiO2.36,37 We thus anticipate enhanced NRR performances for TiO2 after boron doping, which, however has not been explored before. In this work, boron-doped TiO2 micro-particles (B-TiO2) (see SI for preparative detail) are reported as an efficient nonnoble-metal catalyst for electrochemical N2 fixation to NH3 at ambient conditions. In 0.1 M Na2SO4, this catalyst attains a NH3 yield of 14.4 μg h-1 mg-1cat. and a FE of 3.4% at -0.8 V vs. reversible hydrogen electrode (RHE), superior to its pristine TiO2 (NH3 yield: 5.4 μg h-1 mg-1cat.; FE: 2.2 %). It is also excellent in electrochemical durability and selectivity for NH3 formation. The X-ray diffraction (XRD) patterns of pristine TiO2 and B-TiO2 are displayed in Figure 1a. Clearly, all peaks exhibit the characteristic of anatase TiO2 (JCPDS No. 21-1272), but the peak of B2O3 is not detected in B-TiO2 simple. The peaks of B-TiO2 are larger and shaper than that of pure TiO2 revealing the increase of TiO2 crystallinity, which results from the reduction of grain boundaries and amorphous regions serving as charge-carrier recombination centers.38,39 Powder XRD patterns as well as Rietveld refinements for TiO2 and BTiO2 are shown Figure S1. Table S1 displays the detailed data for two samples, indicating the larger unit cell of B-TiO2 (136.29 Å3) than that of pure TiO2 (136.14 Å3). The expanding unit cell could be caused by interstitial B atoms.40,41 The scanning electron microscopy (SEM) images of B-TiO2 are shown in Figure 1b, which indicates that their morphology

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is sphere with strong agglomeration. Figure S2 illustrates the SEM of pure TiO2, indicating almost no morphology difference in two samples. The EDX elemental mapping images for B-TiO2 (Figure 1c) reveal that Ti and O elements were uniformly distributed and low content B were detected in the surface of particles. Figure 1d shows the transmission electron microscopy (TEM) image indicating the sphere nature of B-TiO2 (Figure 1d). And the high-resolution TEM (HRTEM) image of B-TiO2 exhibits a lattice fringe with a dspacing of 0.35 nm corresponding to the (101) plane of TiO2 (Figure 1e). Furthermore, the selected area electron diffraction (SAED) pattern of B-TiO2 (Figure 1f) exhibits four diffraction rings indexed to the (101), (103), (200) and (211) planes of TiO2 phase.

by the existence of substitutional site B-Ti-O, in which the negativity of B (2.04) is greater than that of Ti (1.54) leading to electron transfer from Ti to B.44,45 For O 1s XPS spectra (Figure 2c), a single peak appears at 529.92 eV for B-TiO2, which can be deconvoluted into three peaks for Ti-O (529.95 eV), hydrogen group H-O (531.56 eV) and B-O (533.00 eV) bond.42 Compared with B-TiO2, only two O 1s XPS peaks corresponding to Ti-O and H-O are observed for TiO2 sample, implying the existence of B-O in B-TiO2. For B 1s XPS spectra (Figure 2d), the peak of B-TiO2 at around 188-196 eV was deconvoluted into two peaks with BE of 191.2 and 192.2 eV corresponding to B-O-Ti and B-O-B, respectively.41

Figure 2. (a) XPS spectrum of B-TiO2. XPS spectra of B-TiO2 and TiO2 in the (b) Ti 2p and (c) O 1s and (d) B 1s regions.

Figure 1. (a) XRD patterns for B-TiO2 and TiO2. (b) SEM images of B-TiO2. (c) SEM and EDX elemental mapping images of Ti, O and B for B-TiO2. (d) TEM image for one single B-TiO2 particle. (e) HRTEM image and (f) SAED pattern taken from B-TiO2.

The surface chemical composition of B-TiO2 and TiO2 are explored by X-ray photoelectron spectroscopy (XPS). The wide XPS spectrum of B-TiO2 (Figure 2a) shows that the binding energies (BEs) of Ti 2p3/2, O 1s, B 1s and C 1s are 458.72, 529.92, 191.52 and 284.82 eV, respectively, and C signal could arise from the residual carbon from alcohol and extraneous hydrocarbon under XPS measurement.42 Figure 2b displays the Ti 2p peaks for both B-TiO2 and TiO2 samples. Two peaks are at 458.28 and 463.98 eV corresponding to Ti 2p3/2 and Ti 2p1/2, respectively. Compared with TiO2 sample, Ti 2p peaks show positive shift of 0.4 eV for B-TiO2 sample. This indicates that B in the TiO2 lattice influences the local chemical states of Ti4+.43,44 According to previous report,43,44 one B atom in TiO2 lattice reduces three Ti4+ into Ti3+ with the simultaneous formation of B3+. While there is no Ti3+ detected in B-TiO2 in our present study, possibly due to the low amount of Ti3+. The positive shift of BE for Ti 2p could be explained

To evaluate the NRR performance, B-TiO2 and TiO2 were deposited on carbon paper electrode (B-TiO2/CPE and TiO2/CPE with loading of 0.1 mg) as working electrode. All potentials were reported on a RHE scale. Both product NH3 and possible by-product hydrazhine (N2H4) were spectrophotometrically evaluated by the indophenol blue method46 and the Watt and Chrisp method,47 respectively. Figure S3 and S4 show the corresponding calibration curves. Linear sweep voltammograms (LSVs) were firstly collected in both N2 and Ar saturated electrolytes to determine the occurrence of NRR on B-TiO2. As shown in Figure S5, when the potential is more negative than -0.5 V vs. RHE, the LSV in N2 saturated electrolyte delivers lowered onset of reductive currents and higher current density, indicative of a N2 reduction event. The chrono-amperometry curves of BTiO2/CEP at various potentials are displayed in Figure 3a, revealing the stabilization of current densities for 2 h electrolysis under N2 bubbling. Their electrolytes stained with indophenol indicator were measured by ultraviolet spectrophotometer and the corresponding spectra are shown in Figure 3b. Clearly, the electrocatalytic N2 reduction can be achieved ranging from -0.75 to -0.95 V. Figure 3c displays its NH3 yields and FEs under different potentials. As observed, both NH3 yield and FEs increase with the potentials raise until


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-0.8 V, where the maximum NH3 yield of 14.4μg h-1 mg-1cat. with the highest FE of 3.4% are achieved. Below the potential of -0.8 V, the NH3 yield decreases significantly with a slight decrease of FE which is mainly caused by the competition of the hydrogen evolution reaction.23 For comparison, NRR measurement of TiO2/CPE (Figure S6) was carried out at -0.8 V, revealing only NH3 yield of 5.4 μg h-1 mg-1cat. and FE of 2.2%. More detailed comparison is listed at Table S2. Figure 3d indicates the amount of NH3 generated with different electrodes at -0.8 V after 2 h electrolysis. Bare CPE only delivers quit a low catalytic activity for NRR and TiO2/CPE shows a poor eletrocatalytic NRR activity while an outstanding performance is obtained when B-TiO2/CPE was applied. This indicates that B doping is significant to TiO2 for NRR.

Figure 3. (a) Chrono-amperometry curves of B-TiO2/CPE at various potentials for 2 h in N2-saturated 0.1 M Na2SO4. (b) UVV is absorption spectra of the electrolytes stained with indophenol indicator after NRR electrolysis at a series of potentials for 2h. (c) NH3 yields and FEs of B-TiO2 for NRR at a series of potentials. (d) Amount of NH3 generated with different electrodes at -0.8 V after 2 h electrolysis under ambient conditions.

To confirm that NH3 is generated from NRR over B-TiO2 catalyst, control experiments were measured at -0.8 V in Arsaturated solution and open circuit potential in N2-saturated solutions (Figure S7). Clearly, almost no NH3 is detected in either case. We further performed electrolysis at -0.8 V with alternating 2 h per cycles between Ar-saturated and N2saturated solution for 14 h (Figure S8). In addition, the nuclear magnetic resonance (NMR) spectrum shows that 15NH3 was detected after electrolysis when using 15N2 as feeding gas (Figure S9). All results strongly support that NRR only occurs in N2-saturated solution. Moreover, no by-product N2H4 is detected in final electrolytes at all potentials (Figure S10), indicating B-TiO2 has good selectivity for NH3 formation. A stable performance is critical to the catalyst for NRR. As shown in Figure 4a, consecutive recycling tests at -0.80 V have almost no influence on NH3 yields and FEs which indicates the prominent stability of B-TiO2/CPE for NRR. Furthermore, almost no fluctuation of current density occurs at -0.8 V for 24 h electrolysis (Figure 4b), further indicating the excellent stability of B-TiO2/CPE. In addition, the SEM and HRTEM images of B-TiO2 after 24-h test suggest the

preservation of the initial morphology of B-TiO2 with a lattice fringe of 0.35 nm (Figure S11).

Figure 4. (a) Recycling test at potential of -0.80 V for B-TiO2. (b)

Chrono-amperometry curve for B-TiO2 catalyst at the potential of –0.8 V for 24 h.

The superior NRR activity of B-TiO2 could be attributed to following three reasions. Firstly, interstitial B in TiO2 serves as a three-electron donor and the resulting positively charged B favours N2 adsorption for more effective NRR.35 Secondly, theoretical calcultions suggest that with boron doping, peaks may apear around the Fermi level in the density of states (DOS) of B-TiO2, indicating the aggregation of electrons around the Fermi level.48,49 Appropriate B doping in TiO2 may change the semi-conductor property of intrinsic TiO2 to semi-metal property. The significantly altered electronic structure of semi-metal B-TiO2 benifits the electron transfer from BTiO2 to N2. Indeed, electrochemical impedance spectroscopy (EIS) analysis (Figure S12) shows that BTiO2 has much lower impedance and thus faster NRR kinetics than pure TiO2. Finally, the double layer capacitances at the solid/liquid interface of B-TiO2 and TiO2 were measured to be 0.96 and 0.06 mF cm-2, respectively (Figure S13), revealing much higher surface area with more exposed active sites for B-TiO2. In summary, B-TiO2 has been demonstrated as a stable electrocatalyst for NRR. Compared to TiO2 without B doping, such catalyst delivers a higher NH3 yield of 14.4 μg h-1 mg-1cat. and a Faradic efficiency of 3.4% at -0.8 V vs. RHE. This work not only provides us a low-cost NRR catalyst for electrochemial NH3 production, but would open up exciting new avenues to ration design of dopedTiO2 nanocatalysts and their graphene-based nanohybrids50 for artifical N2-fixing applications at ambient conditions.

ASSOCIATED CONTENT Supporting Information Experimental Section; SEM and HRTEM images; UV-Vis absorption spectra; calibration, LSV, and CV curves; Nyquist plots. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected] (X.G.); [email protected]

(X.S.)



Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 21506133, 21878195, and 21575137) and the Youth Foundation of Sichuan University (No. 2017SCU04a08). We also appreciate Hui Wang from the Analytical & Testing Centre of Sichuan University for her helping with SEM characterization.

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B-TiO2 shows high performance for N2 reduction reaction under ambient conditions. In 0.1 M Na2SO4, this catalyst delivers a NH3 yield of 14.4 μg h-1 mg-1cat. and a Faradaic efficiency of 3.4% at -0.8 V. It is also excellent in electrochemical durability and selectivity for NH3 formation.


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Boron-Doped TiO2 for Efficient Electrocatalytic N2 Fixation to NH3 at

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