High-Performance Electrohydrogenation of N2 to NH3 Catalyzed by

Aug 14, 2018 - High-Performance Electrohydrogenation of N2 to NH3 Catalyzed by Multishelled Hollow Cr2O3 Microspheres under Ambient Conditions...
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High-Performance Electrohydrogenation of N2 to NH3 Catalyzed by Multishelled Hollow Cr2O3 Microspheres at Ambient Conditions Ya Zhang, Wei-Bin Qiu, Yongjun Ma, Yonglan Luo, Ziqi Tian, Guanwei Cui, Fengyu Xie, Liang Chen, Tingshuai Li, and Xuping Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02311 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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High-Performance Electrohydrogenation of N2 to NH3 Catalyzed by Multishelled Hollow Cr2O3 Microspheres at Ambient Conditions Ya Zhang,†,‡ Weibin Qiu,† Yongjun Ma,§ Yonglan Luo,† Ziqi Tian,₤ Guanwei Cui,∫ Fengyu Xie,⊥ Liang Chen,*,₤ Tingshuai Li*,║ and Xuping Sun,*,† †

Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China, ‡College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China, §Analytical and Test Center, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China, ₤Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, Zhejiang, China, ∫College of Chemistry, ⊥ Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, Shandong, China, College of ║ Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, Sichuan, China, and School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, Sichuan, China ABSTRACT: Electrohydrogenation of N2 to NH3 is emerging as an environmentally-benign strategy to tackle the issues associated with the energy-intensive, CO2-emitting Haber–Bosch process. However, the method is severely challenged by N2 activation and needs efficient N2 reduction reaction (NRR) catalysts. Here, we report that multishelled hollow Cr2O3 microspheres (MHCMs), which are synthesized by a facile synthetic route, can serve as efficient and selective non-noble metal electrocatalysts for NRR. In 0.1 M Na2SO4 solution, the MHCMs achieve a high Faradaic efficiency (6.78%) and a large NH3 yield (25.3 µg h−1 mg−1cat.) at –0.9 V vs. reversible hydrogen electrode. The MHCMs also exhibit high stability during the reaction. Density functional theory calculations suggest that NRR over MHCMs takes place via both distal associative and partially alternative routes.

KEYWORDS: multishelled hollow Cr2O3 microspheres, N2 reduction reaction, NH3 electrosynthesis, ambient conditions, density functional theory calculations

INTRODUCTION NH3 plays a key role in the Earth’s ecosystem and is widely used as an activated N2 building block to manufacture fertilizers and other products.1−3 NH3 is also regarded as an important energy storage medium and a carbon-free energy carrier.4−7 However, making NH3 from N2 is difficult because N2 is quite unreactive due to its strong N≡N bond (with bond energy of 941 kJ mol−1), low polarizability, and lack of dipole moment.8 Hence, NH3 is industrially synthesized at high temperature and pressure, usually using the Haber–Bosch process.9−11 This process not only consumes a large amount of energy, but also indirectly causes a significant emission of the greenhouse gas CO2. Thus, finding environmentally-benign and sustainable alternative synthetic methods that can produce NH3 is currently of paramount importance. So far, biological,12,13 chemical,14,15 photocatalytic,16,17 photoelectrochemical,18,19 and electrochemical20−24 approaches have been considered for NH3 synthesis. Among these methods, electrochemical route holds great promise for artificial NH3 synthesis, particularly because the reaction can be driven by electrical energy derived from sustainable energy sources such as solar or wind energy.25 This and the challenges associated with N2 activation underscore the major need of efficient electrocatalysts for N2 reduction reaction (NRR).25−28 Although noble metal catalysts offer favorable activity for the NRR,29−32 they suffer from scarcity and high price. Meanwhile, transition metal oxides (TMOs) are earth-

abundant and can be easily obtained on a large scale;33 thus, they can be more attractive as NRR catalysts if they can catalyze the reaction effectively. However, so far, only a few TMOs have been investigated for electrocatalytic NRR, including γ-Fe2O3,34 Fe2O3-CNT,35 Bi4V2O11/CeO236 and MoO3.37 Besides, TMO materials that can effectively catalyze NRR in neutral media are needed in order to overcome the corrosion issues that are common in acidic and alkaline electrolytes. Although Fe2O3-CNT can electrocatalyze NRR in KHCO3 solution, it does so only with a very low Faradaic efficiency (FE) of 0.15%.35 Cr2O3 has recently attracted much attention due to its unique properties of thermodynamic stability, high hardness and great resistance against chemical attack.38 Besides, these as well as its versatile properties have allowed Cr2O3 to find potential applications in gas sensors, catalysis and lithium ion batteries.38−40 To the best of our knowledge, the ability of Cr2O3 to electrochemically catalyze N2 fixation to NH3 has not been explored. In this Letter, we report the first such study for Cr2O3 and we show that multishelled hollow Cr2O3 microspheres (MHCMs) serve as a robust catalyst for ambient electrohydrogenation of N2 with excellent selectivity to NH3 product. In 0.1 M Na2SO4, the MHCMs enable electrocatalytic NRR with a high FE of 6.78% and a large NH3 yield (VNH3) of 25.3 µg h−1 mg−1cat. at −0.9 V vs. reversible hydrogen electrode (RHE). Density functional theory (DFT) calculations reveal that both distal associative and partially alternative routes are

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involved in the reaction pathways during NRR over the MHCMs.

Cr2O3.42 All the above observations strongly support the synthesis of MHCMs.

RESULTS AND DISCUSSION

Figure 2. XPS spectra of MHCMs showing peaks in the (a) Cr 2p and (b) O 1s regions.

Figure 1. (a) XRD pattern of MHCMs. SEM images of (b) the precursor of MHCMs and (c) MHCMs. (d, e) TEM images in different magnifications of a single MHCM. (f) HRTEM image of MHCM. (g) STEM image of MHCM and EDX elemental mapping of (h) Cr and (i) O.

MHCMs were synthesized by calcination in air of the corresponding hydrothermally prepared precursor (see ESI for details). The X-ray diffraction (XRD) pattern of MHCMs (Figure 1a) shows peaks that are characteristic of Cr2O3 phase (JCPDS No. 38-1479). Scanning electron microscopy (SEM) analysis depicts that the precursor consists of microspheres with smooth surface, as displayed in Figure 1b. Subsequent calcination of the precursor in air leads to broken spheres with smaller size (Figure 1c). Moreover, the shell of MHCMs displays some cockles, which are most likely formed due to the asymmetric constriction of the precursor during the calcination process. The transmission electron microscopy (TEM) image of MHCM (Figure 1d) shows a contrast between darker periphery and lighter regions, further suggesting the formation of multishelled hollow Cr2O3 microsphere. Detailed structural analysis of the outer shell of a hollow microsphere (Figure 1e) reveals that the shell is porous and composed of nanocrystals. High-resolution TEM (HRTEM) image (Figure 1f) shows lattice fringes with interplane spacing of 0.248 nm, corresponding to the (110) plane of Cr2O3 phase. Figure 1g shows the scanning TEM (STEM) image of MHCM. Energydispersive X-ray (EDX) analysis shown in Figure 1h,i reveal that Cr and O elements are uniformly distributed on the MHCM. X-ray photoelectron spectra (XPS) of MHCMs show peaks in the Cr 2p and O 1s regions (Figure 2a,b) confirming presence of Cr and O elements in the microspheres. For the Cr 2p region (Figure 2a), the peaks fitted at 576.8 and 586.4 eV correspond to Cr 2p3/2 and Cr 2p1/2, respectively, indicating the presence of Cr3+ associated with Cr2O3.41 The peak at 530.2 eV in the O 1s region (Figure 2b) is attributed to surface lattice oxygen (O2−) combined with Cr3+.41 And the peak at 531.9 eV originates from the absorbed H2O or O2 on the surface of

Figure 3. (a) Chronoamperometry curves of NRR over MHCMs/CP in 0.1 M Na2SO4 solution at different potentials. (b) UV-Vis absorption spectra of the electrolytes stained with indophenol indicator after NRR electrolysis at different potentials for 2 h. (c) VNH3 and (d) FEs of NRR over MHCMs/CP at different potentials. (e) VNH3 and FEs over different electrodes at a potential of −0.9 V after electrolysis for 2 h. (f) Cycling test of MHCMs/CP during NRR at −0.9 V. The working electrode used for NRR over MHCMs was prepared by depositing MHCMs on a carbon paper (MHCMs/CP, loading: 0.12 mg cm−2). All the potentials were corrected using the automatic IR compensation function on the potentiostat. The potentials for NRR were all reported based on a RHE scale, and the performance of MHCMs/CP for NRR was evaluated with chronoamperometric tests in N2-saturated electrolyte for 2 h. The chronoamperometry curves of MHCMs/CP at different potentials suggest that the current densities remain almost constant for 2 h (Figure 3a). The desired product NH3 and the possible by-product hydrazine (N2H4)43,44 were spectrophotometrically identified by the indophenol blue method35 (Figure S1) and the Watt and Chrisp method45 (Figure S2), respectively. N2H4 is not detected after

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NRR over MHCMs/CP at all potentials (Figure S3), suggesting the excellent selectivity of MHCMs/CP for NH3 product. Figure 3b shows the UV-Vis absorption spectra at different potentials. The result clearly shows that electrocatalytic NRR takes place in a potential range from −0.7 to −1.1 V. The VNH3 and FEs at various potentials are calculated and shown in Figure 3c and d, respectively. The highest VNH3 (25.3 µg h−1 mg−1cat.) and FE (6.78%) are obtained at −0.9 V. It is worth noting here that these values are higher than the corresponding values reported for other electrocatalysts (see Table S1). When the potential is below –0.9 V, the competing H2 evolution reaction reduces the FEs of NRR;18 thus, this potential was chosen as the optimum potential to carry out the reaction or the studies further. To verify NH3 is generated by electrocatalytic NRR over MHCMs/CP, control experiments were performed including N2-saturated solution at open circuit potential and Ar-saturated solution at −0.9 V (Figure S4). The results show that no NH3 is produced in either case, indirectly indicating that detected NH3 stems only from electrochemical NRR catalyzed by MHCMs/CP. It is worth adding that the bare CP has almost no catalytic activity to NRR (Figure S5). The hollow structure of the MHCMs are expected to facilitate the diffusion of N2 and the reaction intermediates and products during NRR.32,46,47 Such structures may also allow for the exposure of more active sites in MHCMs to effectively contact the surface of the carbon paper and form a large density of three-phase contact points between N2, electrolyte and catalyst48 for more efficient NRR. In order to assess this possible scenario, we synthesized solid Cr2O3 microspheres (SCMs) (Figure S6) and Cr2O3 nanoparticles (CNPs) (Figure S7) for comparison. Figure 3e shows that the VNH3 and FE of MHCMs/CP at −0.9 V is higer than those of SCMs/CP (11.4 µg h−1 mg−1cat., 2.94%) and CNPs/CP (13.8 µg h−1 mg−1cat., 4.73%). As shown in Figure S8, the double-layer capacitance (Cdl) of MHCMs/CP (2.22 mF cm−2) is larger than that of CNPs/CP (1.95 mF cm−2) and SCMs/CP (1.95 mF cm−2), suggesting that MHCMs/CP have more exposed electrocatalytically active sites for NRR.49 Of note, MHCMs/CP show enhanced electrocatalytic activity for NRR, which may be affected by several factors. First, in some situations, the internal surface may not be as well capped as the external surface, and thus be more catalytically active.46,47,50 Second, the confinement effect of the cage can increase the steady-state concentration of the species in the rate-determining step of the reaction.32,47 Besides, the N2 trapped in the cavity of MHCMs is more likely to experience high frequency collisions with hollow inner surface of the microspheres and greater residence time in there, which is beneficial to electrohydrogenation of N2 to NH3.32 As stability is a critical parameter that needs to be evaluated for catalysts, the stability of MHCMs/CP was also investigated. Figure 3f shows that the VNH3 and FEs barely change during five consecutive NRR tests (UV-Vis absorption spectra and current densities are shown in Figure S9). The 24h long-term test of MHCMs/CP shows no obvious change in current density (Figure S10). We further tested the NRR performance of the MHCMs/CP after long-term test. The UV-Vis absorption spectrum of the electrolyte (Figure S11) exhibits approximately the same intensity as that shown in Figure 3b,

suggesting MHCMs/CP is still highly active for the NRR. SEM image (Figure S12) and TEM image (Figure S13) of MHCM show that it still retains its multishelled hollow microsphere morphology after stability test. XRD pattern (Figure S14) and XPS spectra (Figure S15) of MHCMs show barely any change after the stability test, indincating that they are still Cr2O3 in nature. All these results indicate that MHCMs have great electrochemical and structural stability during NRR.

Figure 4. (a) Free energy profile of N2 electroreduction on Cr2O3 (110) surface. An asterisk (*) denotes as the adsorption site. (b) The structure of *NNH. Color code: white, H; blue, N; gray, Cr; red, O.

We performed DFT calculations to gain deep insights into the mechanism by which NRR takes place on the particles. The surface energies of a series of facets were examined, as listed in Table S2, indicating that (001) and (110) facets are much more stable than any other surfaces. Moreover, in the experimental HRTEM image, the (110) surface was observed while there is no (001) surface. The reaction path on (001) surface (Figure S16) shows that the energy barrier of PSD is 1.55 eV, and much higher than that on (110) surface. Thus, we used the (110) surface to build a slab model to investigate the reaction pathways (Figure S17) and to study the free energy profile of NRR process on Cr2O3 (110) surface based on computational hydrogen electrode (CHE) model,51 as depicted in Figure 4a. Due to the strong triple bond, the direct dissociation of N2 molecule should hardly take place on the oxide surface. We tried numerous geometries to locate the most possible structures on the reaction path. The most stable *NNH intermediate is shown in Figure 4b, where NN bond length and two N-Cr bond distances are 1.227, 1.998, and 2.000 Å, respectively. This geometry suggested that two neighboring exposed Cr sites attract and activate the inert N2 molecule, resulting in an associative route. For the following steps, we considered both distal and alternative routes. All the steps leading to *NH are exothermic and yield the first NH3 molecule; so, both paths are viable. Based on the calculations and results, the following possible reaction mechanism is proposed: first, N2 is adsorbed on Cr3+, and then the hydrogenation process proceeds via addition of (H + e–) pair. Thus, it is believed that NRR takes place over the MHCMs/CP via both distal associative and partially alternative routes. CONCLUSION In summary, Cr2O3-based nanomaterials, dubbed MHCMs, have been synthesized and experimentally proven to be superior non-noble metal electrocatalysts for artificial N2 fixation to NH3 at ambient condition. The catalyst attains a

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high FE (6.78%) and a large VNH3 (25.3 µg h−1 mg−1cat.) at −0.9 V, with remarkably high stability. DFT calculations reveal that both the distal associative and partially alternative route are operating during NRR. This study not only offers an efficient catalyst for electrohydrogenation of N2 to NH3 at mild pH, but also opens up a new way to rational design of sustainable materials as advanced catalysts for N2 fixation. Our next research will focus on the structure optimization of the Cr2O3 catalyst by constructing Cr2O3/carbon nanohybrids with enhanced conductivity and engineered surface oxygen vacancy for more efficient molecular N2 adsorption and activation and ultimately for most efficient NRR.17,19,36

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Supporting Information Experimental details; UV-Vis absorption spectra; calibration and chronoamperometry curves; cyclic voltammograms; XRD pattern; SEM and TEM images; slab model; Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

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Corresponding Author *E-mail: [email protected] (X.S.); [email protected] (L.C.); [email protected] (T.L.)

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Notes The authors declare no competing financial interest.

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

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Multishelled hollow Cr2O3 microspheres (MHCMs) can serve as an efficient and selective electrocatalysts for N2 reduction reaction (NRR). In 0.1 M Na2SO4 solution, the MHCMs achieve a high Faradaic efficiency (6.78%) and a large NH3 yield (25.3 µg h−1 mg−1cat.) at –0.9 V vs. reversible hydrogen electrode, with high stability. Density functional theory calculations suggest that the NRR over the MHCMs takes place via both distal associative and partially alternative routes.

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