Modulating oxygen vacancies of CeO₂ nanospheres by Zn-doping: an efficient electrocatalyst for N₂ reduction under ambient conditions

Abstract

At present, most industrial NH₃ production comes from the Haber–Bosch process, which causes a series of serious environmental pollution problems. Electrochemical N₂ reduction is regarded as a green pathway to deal with this problem. Recently, CeO₂ has attracted much attention due to its high thermal stability. Metal doping with smaller ion radius is an effective strategy to regulate oxygen vacancies, increase the defect concentration and enhance the catalyst activity. Herein, we developed Zn-doped CeO₂ nanospheres (Zn–CeO₂) for the electrochemical NRR. In 0.1 M Na₂SO₄, Zn–CeO₂ achieves a large NH₃ yield of 29.01 μg h⁻¹ mg_cat.⁻¹ and a high faradaic efficiency of 10.3% at −0.20 V vs. reversible hydrogen electrode, and it also shows good structure stability. The density functional theory (DFT) calculations revealed the reaction mechanism of NRR activity regulated by the doping metal.

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As an important cornerstone of world energy, ammonia gas has been widely used in many fields, such as fertilizers, medicaments, dyes, etc.¹ NH₃ is also gradually being recognized as a potential alternative to fossil fuels as a transport fuel characterized by high energy density and no CO₂ emissions.² Although N₂ has a very high content in the atmosphere, it is difficult to convert it into NH₃ at room temperature and pressure due to its high nitrogen triple bond energy and strong dipole moment.^3–5 For nearly a century, most industrial NH₃ production has come from the Haber–Bosch process, which uses Fe or Ru as a catalyst and nitrogen and hydrogen as raw materials.^6–9 However, the temperature and pressure required for this process are very high, and large amounts of carbon dioxide are emitted, leading to a series of environmental problems. Therefore, it is very necessary to explore a low energy consumption and environmentally friendly NH₃ synthesis method.

As an important oxide catalyst, cerium oxide material has been a research hotspot in the field of ternary catalysts, environmental catalysis and catalyst supports for decades.^10,11 However, the reported NRR activity of CeO₂ related catalysts is still not high, which is mainly caused by their low electrical conductivity and poor N₂ adsorption and activation.⁹ Recent reports have highlighted that oxygen vacancies (O_V) can receive foreign electrons to enhance N₂ adsorption and activation, thereby improving the NRR performance of the catalysts.^12–15 Moreover, a large number of applications of CeO₂ in the field of electrocatalysis mainly benefit from the existence of O_V.¹⁶ Furthermore, a recent report by Liu et al.¹⁷ showed that O_Vs of CeO₂ can be significantly enhanced via doping with transition elements; meanwhile, Liu et al.¹⁸ also proved that ZnO is an efficient NRR electrocatalyst. The above studies further motivated us to use elements with smaller ionic radii as dopants to improve the NRR performance of pure CeO₂.

In this work, we report Zn-doped CeO₂ nanospheres to modulate oxygen vacancies for promoting the NRR performance. As observed, in 0.1 M Na₂SO₄, the NH₃ yield rate of Zn–CeO₂ (29.01 μg h⁻¹ mg cat.⁻¹) is much better than that of undoped CeO₂, with a high faradaic efficiency (FE) of 10.3%. It also shows good stability. Density functional theory (DFT) calculations reveal the mechanism that doped pairs are the active sites.

The XRD patterns of CeO₂ and Zn–CeO₂ nanospheres are shown in Fig. 1a, and all the XRD peaks positioned in each sample correspond to (111), (200), (220), (311), (222), (400), (331), and (420) planes. It is worth noting that no other secondary or impurity peaks are observed in Fig. 1a, indicating that Zn has been successfully doped with CeO₂. Scanning electron microscopy (SEM) reveals the CeO₂ nanospheres before and after Zn doping, as shown in Fig. 1b and c. The high-resolution TEM (HRTEM) image reveals the crystallographic fringes of 0.308 nm and can be well assigned to the (111) lattice plane of CeO₂ (Fig. 1e). Furthermore, the corresponding elemental mapping images (Fig. 1f and g) of Zn–CeO₂ also demonstrate that Zn is successfully doped in CeO₂. From the above characterization results, it can be seen that we successfully synthesized a Zn–CeO₂ nanosphere catalyst.

Fig. 1 (a) XRD patterns of pristine CeO₂ and Zn–CeO₂. SEM images of (b) pristine CeO₂ and (c) Zn–CeO₂. TEM (d) and HRTEM (e) images of Zn–CeO₂. (e) HRTEM image for Zn–CeO₂. Corresponding EDX elemental mapping images of (f) Zn and (g) Ce.

In order to further determine the element valence in the composite material, the XPS spectrum is necessary, and the results show the presence of Ce, O, and Zn elements in the Zn–CeO₂ materials. Fig. 2a shows the survey scan of Ce 3d, Zn 2p and O 1s. The XPS spectra of Zn 2p_3/2 and Zn 2p_1/2 correspond to the binding energies of 1021.6 eV and 1043.8 eV (Fig. 2b). The spin-orbital splitting of 23.1 eV, between the peaks, confirmed that Zn exists as pure metal on the CeO₂ matrix rather than its oxide form.¹⁹Fig. 2c shows the Ce 3d spectrum, and the peaks located in the range of 881–902 eV correspond to Ce 3d_5/2; meanwhile, the peaks in the range of 901–921 eV correspond to Ce 3d_3/2.^20–22 The above results can be attributed Ce³⁺ and Ce⁴⁺.²³ And they are match well with the previous reports.^24,25Fig. 2d shows the XPS spectrum of O 1s. The peak seen at low binding energy (529.9 eV) corresponds to the oxygen atom in the CeO₂ lattice,²⁶ and the other peak at high binding energy (532.4 eV) corresponds to chemisorbed oxygen of the surface hydroxyl group.²⁷ From the above results, it can be concluded that Ce element exists in the +3 or +4 oxidation state in the composite. For example, in the original cerium oxide, Zn exists in the 0 oxidation state, and oxygen exists in the −2 oxidation state and in the 0 oxidation state. Fig. S1 (ESI†) shows the Raman spectra of Zn–CeO₂ and pure CeO₂. Notably, the intensity in Zn–CeO₂ is less than pure CeO₂, implying that more oxygen vacancies are present in Zn–CeO₂. Meanwhile, Fig. S2 (ESI†) shows the room temperature electron spin resonance (ESR) spectra of pure CeO₂ and Zn–CeO₂. The latter shows a definite oxygen vacancy signal at g = 2.018, indicating the formation of a large number of oxygen vacancies after the Ce³⁺ center.

Fig. 2 XPS spectra of Zn–CeO₂ in the (a) survey scan, (b) Zn 2p, (c) Ce 3d, and (d) O 1s regions.

Our electrochemical tests are carried out in U-shaped electrolyzers separated by membranes. The optimum catalyst load is 0.1 mg cm⁻². For more accurate determination of ammonia concentration, as shown in Fig. S3a (ESI†), the time-dependent current density curves of Zn–CeO₂ were obtained. Fig. S3b (ESI†) presents the UV-Vis absorption. After electrolysis, the obtained NH₃ in the solution phase was spectrophotometrically determined by the indophenol blue method,²⁸ and another possible by-product (N₂H₄) was detected by the method of Watt and Chrisp.²⁹ The corresponding calibration curves are shown in Fig. S4 (ESI†), respectively. It is worth noting that we did not detect the byproduct hydrazine in the reaction solution (Fig. S5, ESI†). The ammonia production rate and Faraday efficiency of the catalyst at different voltages are shown in Fig. 3a, and the ammonia production rate (29.01 μg h⁻¹ mg_cat.⁻¹) and Faraday efficiency (10.3%) of the catalyst were the highest at −0.2 V, outperforming most reported NRR electrocatalysts listed in Table S1 (ESI†). To better demonstrate the NRR performance of the Zn–CeO₂, we compare the electrochemical activity between the material and the precursor by the amount of ammonia produced (Fig. 3b). The results showed that the composites Zn–CeO₂/CP showed the best NRR activity, and its NRR performance is much higher than that of CeO₂/CP and bare CP. Stability is another indicator of electrochemical performance; the ammonia production rate and Faraday efficiency were tested after six cycles at −0.6 V (Fig. 3c). And UV-vis absorption spectra (Fig. S6, ESI†) confirm the high stability of Zn–CeO₂/CP. After the cycling test, we found that the ammonia production rate and Faraday efficiency of the catalyst did not change basically. In addition, the current density of the catalyst did not fluctuate after 24 hours of electrolysis (Fig. 3d). This further shows that the catalyst has good electrochemical stability. The amount of NH₃ measured in the Ar-saturated electrolyte at each potential was very small and insignificant compared to the N₂-saturated electrolyte (Fig. S7, ESI†), indicating that the NH₃ product is mainly generated by the supply of N₂ electrocatalyzed by Zn–CeO₂.

Fig. 3 (a) NH₃ yields and FEs at each given potential. (b) NH₃ yields at −0.20 V. (c) Stability test of Zn–CeO₂ for 6 cycles at −0.20 V. (d) The curve for Zn–CeO₂ at −0.20 V.

In order to explore the mechanism by which Zn, as a dopant, regulates defect concentration to enhance NRR activity from a microscopic perspective, the reaction mechanism of the NRR on the CeO₂(111) surface was observed by DFT calculation. There are several well-established reaction pathways for the NRR depending on the specific adsorption modes of N₂ molecules.^30,31 To evaluate the potential of Zn–CeO₂ as an electrocatalyst for nitrogen reduction, typical reaction paths through the distal mechanism were optimized and the corresponding free energy profiles/structures are summarized in Fig. 4a. Importantly, the N₂ adsorption energy on the Zn–CeO₂(111) surface with V_O is higher than that on pure MnO₂ (−0.24 vs. −0.58 eV). These results are attributed to the stronger electronic interaction between V_O, N₂ and Ce atoms. However, the barrier of NNH* protonation is greatly reduced after Zn doping. The second NH₃* formation is the Zn–CeO₂ limiting step with a critical energy barrier of 0.24 eV (NH₂* + H⁺ + e⁻ → NH₃*). As shown in Fig. S8 (ESI†), the results show that the Zn doped surface has more vacant orbitals, which can enhance the adsorption energy of N₂. The density of states (DOS) indicates that two orbitals hybridize after N₂ adsorption by adding H atoms to the adsorbent, and N₂ hydrogenation is performed by adding H atoms to the adsorbent based on a distal or alternate mechanism (Fig. S9, ESI†). The top panel of Fig. S10 (ESI†) shows the end-to-end adsorption configuration (a) with V_O and (b) without V_O for the N₂ molecule on the CeO₂(111) surface. For the former, the N₂ molecule occupies V_O, and one of its terminal N atoms interact directly with V_O. In general, our theoretical calculations point out that Zn atoms can significantly improve the NRR performance, which is in good agreement with the experimental electrochemical results.

Fig. 4 Free energy diagram and the optimized structures for the NRR at zero and applied potential (limiting potential) through distal mechanisms on the (111) before doping (a) and after doping (b) surface.

In conclusion, Zn has been shown to be an effective dopant to regulate the CeO₂ defect concentration to enhance the NRR performance. In 0.1 M Na₂SO₄, Zn–CeO₂ attains the largest NH₃ yield of 29.01 μg h⁻¹ mg_cat.⁻¹ and highest FE of 10.3% at −0.20 V. Moreover, it has good electrochemical stability, and its catalytic activity is basically unchanged after 24 hours of electrolysis. DFT calculation shows that doping Zn element with small ion radius in CeO₂(111) can regulate and increase the concentration of oxygen vacancies, thus promoting the adsorption and activation of N₂. Moreover, Ce³⁺ formed by oxygen vacancy defects is more likely to capture electrons, thus improving the NRR activity of the catalyst. This work not only provides an attractive scheme for the construction of the defect concentration on the catalyst surface but also opens new opportunities to explore cerium-based catalysts for N₂ fixation applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21575137).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental section and supplementary figures. See DOI: https://doi.org/10.1039/d2ya00241h

‡ Yuyao Ji and Mingyu Yang contribute equally to this manuscript.

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