Ce-doped MoS_2−x nanoflower arrays for electrocatalytic nitrate reduction to ammonia

Abstract

Electrocatalytic reduction of nitrate to ammonia (NO₃RR) represents a highly appealing route to achieve both value-added NH₃ synthesis and facile removal of waste nitrate. In this study, Ce-doped and S-vacancy-enriched MoS_2−x nanoflower arrays are reported as a highly efficient NO₃RR catalyst, exhibiting an NH₃-faradaic efficiency of 96.6% with a corresponding NH₃ yield rate of 7.3 mg h⁻¹ cm⁻². Theoretical calculations highlight the synergy of the S-vacancy and Ce-dopant to enhance the NO₃RR activity of Ce–MoS_2−x, in which the S-vacancy-induced unsaturated Mo serves as an active site to activate NO₃⁻, while the Ce-dopant jointly enhances NO₃⁻ activation and reduces the thermodynamic energy barrier for the rate-determining step of *NO → *NOH.

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1. Introduction

Ambient electrocatalytic N₂ fixation emerges as a promising method for sustainable NH₃ synthesis.^1–5 However, the NRR efficiency is greatly hindered by the severely competitive hydrogen evolution reaction (HER), very low N₂ solubility and strong N≡N bond (945 kJ mol⁻¹), causing a dramatically low NH₃ yield and NH₃-faradaic efficiency (FE_NH3).^6–13 Compared with inert N₂, NO₃⁻ possesses a low N=O cleavage energy (204 kJ mol⁻¹) and a high aqueous solubility, and thus electrocatalytic nitrate-to-ammonia conversion (NO₃RR) is thermodynamically more effective than the NRR for NH₃ electrosynthesis.¹⁴ Besides, NO₃⁻ is a common environmental pollutant that has a large abundance in industrial and sanitary wastewater, and thus converting waste NO₃⁻ into value-added ammonia is clearly a “turn waste into treasure” strategy.¹⁵ Meanwhile, it has been demonstrated that NH₃ capture is more simple and easy than NO₃⁻ removal.¹⁶ Therefore, the NO₃RR represents a highly appealing route to achieve both value-added NH₃ synthesis and facile removal of waste NO₃⁻, leading to a win–win scenario. Nevertheless, NO₃RR activity is restricted by its complex eight-electron process and some side reactions including the HER, and thus efficient NO₃RR catalysts are required to accelerate the NO₃RR reaction kinetics while suppressing the side reactions.^17–30

MoS₂ has been extensively confirmed to be able to effectively electroreduce nitrogenous molecules into diverse value-added chemicals owing to the nitrogenase-analogous characteristic of Mo active sites,^31–35 as well as its other merits of a layered structure, natural abundance, ease of synthesis and high electrochemical stability.³⁶ Nevertheless, the investigations of MoS₂-based electrocatalysts are quite limited for the NO₃RR due possibly to their low electrical conductivity and poor intrinsic NO₃RR activity.^31,37 Metal doping is considered a facile but powerful strategy to enhance electrocatalytic performance,³ while lanthanide rare-earth metals have attracted considerable interest as fascinating metal dopants due to their unique 4f structure, rich redox capability and high chemical activity and stability.³⁸ Previous investigations have demonstrated that rare-earth metal doping can significantly improve the catalyst activity for various electrocatalytic reactions.^39,40 Nonetheless, rare-earth metals serving as active dopants for the NO₃RR have not been reported so far.

Herein, we regulate the electronic structure of MoS₂ by Ce-doping, which spontaneously generates enriched S-vacancies (V_S). The designed Ce-doped and V_S-enriched MoS_2−x nanoflower arrays grown on carbon cloth (Ce–MoS_2−x/CC) can be a highly efficient NO₃RR catalyst. Theoretical calculations using a combination of density functional theory (DFT) and molecular dynamics (MD) simulations unveil that V_S and Ce-dopant synergistically activate NO₃⁻ and reduce the thermodynamic energy barrier, resulting in the greatly enhanced NO₃RR activity of Ce–MoS_2−x.

2. Results and discussion

Ce–MoS_2−x/CC was prepared through a facile hydrothermal approach. X-ray diffraction (XRD, Fig. 1a) patterns show that the diffraction peaks of both MoS₂/CC and Ce–MoS_2−x/CC are well matched to the 2H-MoS₂ phase (JCPDS no. 37-1492).^41,42 A scanning electron microscopy (SEM, Fig. 1b) image of Ce–MoS_2−x/CC shows abundant nanoflower arrays grown on CC. The magnified image (Fig. 1c) reveals that the nanoflowers consist of a large number of vertically aligned nanosheets. Transmission electron microscopy (TEM, Fig. 1d and e) images further reveal the nanoflower morphology and layered structure of Ce–MoS_2−x. The selected area electron diffraction (SAED, Fig. 1f) pattern of Ce–MoS_2−x presents distinct diffraction rings assigned to (110), (002), and (100) facets of 2H-MoS₂.⁴³ The high-resolution TEM (HRTEM, Fig. 1g) image of Ce–MoS_2−x shows an interplanar spacing of 0.65 nm, which is associated with the (002) plane. Given that the actual layer spacing of MoS₂ is 0.62 nm (Fig. S1†), the introduction of Ce-dopants can apparently enlarge the interplanar spacing due to the larger atomic radius of Ce (0.18 nm) than that of Mo (0.14 nm). The corresponding fast Fourier inverse transform image (IFFT, Fig. 1h, yellow dotted circles) and 3D surface plot (Fig. 1i, red dotted circles) show the existence of plentiful surface defects,^44–46i.e., V_S. Fig. 1j displays the elemental mapping images of Ce–MoS_2−x/CC (Fig. S2†), signifying the even distribution of Ce, Mo and S elements throughout Ce–MoS_2−x/CC.

Fig. 1 (a) XRD patterns of MoS₂/CC and Ce–MoS_2−x/CC. (b)–(j) Characterization of Ce–MoS_2−x: (b) and (c) SEM images, (d) and I TEM images, (f) SAED pattern, (g) HRTEM images of Ce–MoS_2−x basal planes and corresponding (h) IFFT image and (i) surface plot, and (j) element mapping images.

X-ray photoelectron spectroscopy (XPS) was used to investigate the surface chemical states of MoS₂ and Ce–MoS_2−x scraped from the CC. The Ce 3d spectra of Ce–MoS_2−x (Fig. 2a) can be split into four characteristic peaks, in which the two dominant brown peaks belong to Ce³⁺ 3d_5/2, while the other two green peaks are ascribed to Ce⁴⁺ 3d_3/2, indicating the coexistence of Ce³⁺ and Ce⁴⁺ in Ce–MoS_2−x. The presence of Ce⁴⁺ is attributed to the oxidation of surface Ce³⁺ to Ce⁴⁺ as a result of the inevitable surface contact with air and moisture during the XPS measurements. For pristine MoS₂, doublet Mo 3d peaks (Fig. 2b) are located at 229.4 eV and 232.4 eV, corresponding to Mo 3d_5/2 and Mo 3d_3/2, respectively. The peaks at 162.3 eV and 163.5 eV in the S 2p spectra (Fig. 2c) are assigned to S 2p_3/2 and S 2p_1/2 orbits of S²⁻, respectively.⁴⁷ With the introduction of the Ce³⁺-dopant, it can be seen that the Mo 3d_5/2 and S 2p_3/2 spectra of Ce–MoS_2−x are positively shifted compared to those of MoS₂, which is attributed to the electron transfer from the Mo/S atoms to neighboring Ce-dopant-induced V_S, resulting in the electron-deficient nature of the MoS₂ component in Ce–MoS_2−x. Moreover, Ce–MoS_2−x, in exhibiting a much stronger signal (g = 2.003) than MoS₂, can be observed in the electron paramagnetic resonance (EPR, Fig. 2d) spectra, further affirming the existence of rich V_S in Ce–MoS_2−x.

Fig. 2 (a)–(c) XPS spectra of (a) Ce 3d, (b) Mo 3d and (c) S 2p for MoS₂ and Ce–MoS_2−x. (d) EPR spectra of MoS₂ and Ce–MoS_2−x. (e) and (f) Electron contour maps of MoS₂ and Ce–MoS_2−x, blue: depletion, red: accumulation. (f) PDOS profile of MoS₂ and Ce–MoS_2−x. (g) Calculated work functions of MoS₂ and Ce–MoS_2−x.

The electronic structure of Ce–MoS_2−x was further studied by the DFT calculations. Electron contour maps display the electron accumulation around both V_S and the Ce-dopant in Ce–MoS_2−x (Fig. 2e and f), in line with the differential charge-density plots (Fig. S3†). The projected density of states (PDOS, Fig. 2g) profile shows that MoS₂ possesses an obvious band gap at the conduction band minimum of the Fermi level, suggesting its semiconductor nature, while the states of Ce–MoS_2−x at the Fermi level consist of hybridized S 3p, Mo 4d and Ce 5d orbitals, implying its metallic character has enhanced electronic conductivity.^48–51 Meanwhile, Ce–MoS_2−x (4.943 eV) possesses a lower work function than MoS₂ (5.111 eV), further confirming the improved conductivity of Ce–MoS_2−x (Fig. 2h and Fig. S4†).^52–54 Consequently, the synergy of the Ce-dopant and V_S renders the greatly improved conductivity of Ce–MoS_2−x, which is favorable for boosted NO₃RR catalytic kinetics.

The electrocatalytic NO₃RR activity of Ce–MoS_2−x/CC was evaluated using an H-type cell in a 0.5 M Na₂SO₄ + 0.1 M NaNO₃ electrolyte. Colorimetric methods were applied to detect all possible reductive products (Fig. S5–S7†). The linear sweep voltammetry (LSV, Fig. 3a) curves show that Ce–MoS_2−x/CC exhibits a higher current density with NO₃⁻ than it does without NO₃⁻, implying the outstanding catalytic activity of Ce–MoS_2−x/CC toward the NO₃RR. We then combined the chronoamperometric and colorimetric methods for quantitatively determining the NH₃ yields and FE_NH3 of Ce–MoS_2−x/CC. Fig. 3b shows that the FE_NH3 is continuously increased by increasing the potentials, and reached a maximum value of 96.6% at −0.7 V, with the corresponding NH₃ yield rate achieved being 7.3 mg h⁻¹ cm⁻², outperforming many other reported NO₃RR catalysts (Table S1†). The decreased FE_NH3 beyond −0.7 V is due to the boosted HER. As a comparison, we also assessed the NO₃RR performance of pristine MoS₂/CC (Fig. 3c), which is substantially inferior to that of Ce–MoS_2−x/CC. Fig. S8† shows that Ce–MoS_2−x/CC possesses a higher electrochemical active surface area (95.9 cm²) than MoS₂/CC (63.6 cm²), which is conducive to exposing more active sites. Besides, Fig. S9† shows that compared to MoS₂/CC, Ce–MoS_2−x/CC delivers a lower charge-transfer resistance, implying the faster charge-transfer efficiency and boosted NO₃RR kinetics of Ce–MoS_2−x/CC,^55–57 in line with the theoretical calculations (Fig. 2g and h). These results demonstrate that Ce-doping can simultaneously increase the number of active sites and the charge-transfer efficiency, which are favorable for the expedited NO₃RR activity of Ce–MoS_2−x/CC.

Fig. 3 (a) LSV curves of Ce–MoS_2−x/CC in 0.5 M Na₂SO₄ with and without 0.1 M NaNO₃. (b) NH₃ yields and FE_NH3 of Ce–MoS_2−x/CC toward the NO₃RR at different potentials. (c) Comparison of the maximum NH₃ yields and highest FE_NH3 between MoS₂/CC (3.2 mg h⁻¹ cm⁻², 65.8%) and Ce–MoS_2−x/CC (7.3 mg h⁻¹ cm⁻², 96.6%). (d) ¹H NMR spectra of the electrolyte after the NO₃RR adopting ¹⁵NO₃⁻ and ¹⁴NO₃⁻ as nitrogen sources. (e) FEs of NO₂⁻, H₂ and NH₃ for Ce–MoS_2−x/CC toward the NO₃RR. (f) Cycling tests of Ce–MoS_2−x/CC for NO₃RR at −0.7 V. (g). Long-term durability test (15 h electrolysis) on Ce–MoS_2−x/CC at −0.7 V.

Some experiments were performed to confirm the source of generated NH₃ during the NO₃RR. The ¹H nuclear magnetic resonance (¹H NMR) spectra (Fig. 3d) present a triplet coupling for ¹⁴NH₄⁺ and a doublet coupling for ¹⁵NH₄⁺ when employing ¹⁴NO₃⁻ and ¹⁵NO₃⁻, respectively, indicating that the generated NH₃ comes from the Ce–MoS_2−x/CC-catalyzed NO₃RR process. Besides, as shown in Fig. S10,† negligible NH₃ was detected in the electrolyte without NO₃⁻ and at an open-circuit potential (OCP) with 0.1 M NO₃⁻. These results affirm that the detected NH₃ entirely arises from NO₃⁻ electroreduction.

We then examined the NO₃RR selectivity of Ce–MoS_2−x/CC. Fig. 3e shows that Ce–MoS_2−x/CC exhibits much higher FE_NH3 than and FE_H2 at all the considered potentials, while N₂H₄ was undetectable throughout the experiments (Fig. S11†). These findings indicate that Ce–MoS_2−x/CC possesses a high NO₃RR selectivity for the electroreduction of NO₃⁻ to NH₃. Moreover, to evaluate the stability of Ce–MoS_2−x/CC for practical applications, we performed the cycling electrolysis test and the long-time chronoamperometry test. After repeating the NO₃RR electrolysis seven times, Fig. 3f shows no remarkable fluctuations in FE_NH3 and NH₃ yield rates, suggesting the favorable cycling stability of Ce–MoS_2−x/CC. Ce–MoS_2−x/CC also demonstrates good long-term durability (Fig. 3g), as is evident from no obvious decrease in current density and stable FE_NH3 and NH₃ yield rates during 15 h of continuous electrolysis.

Theoretical computations were performed to reveal the NO₃RR mechanism of Ce–MoS_2−x. MD simulations were first employed to investigate the dynamic NO₃⁻ adsorption on MoS₂ and Ce–MoS_2−x.^58–62 After simulations, in comparison with MoS₂ (Fig. 4a), more NO₃⁻ coverage on Ce–MoS_2−x can be observed in Fig. 4b. The calculated radial distribution function (RDF) curves show an obviously increased g(r) value related to the NO₃⁻/Ce–MoS_2−x interaction compared to the NO₃⁻/MoS₂ interaction (Fig. 4c), and a similar trend can also be found in their corresponding integrated RDF plots (Fig. 4d).⁶³ These findings suggest that the co-presence of Ce-dopants and V_S effectively enhance the interaction of NO₃⁻ with the Ce–MoS_2−x catalyst, which is kinetically beneficial for the NO₃RR process.

Fig. 4 (a) and (b) MD snapshots of absorbed NO₃⁻ on (a) MoS₂ and (b) Ce–MoS_2−x at the initial and final states. (c) Corresponding RDF curves and (d) integrated RDF curves.

DFT calculations were carried out to obtain atomic-level understandings of the NO₃RR mechanism on Ce–MoS_2−x. The (002) plane was selected as the facet for both MoS₂ and Ce–MoS_2−x based on the HRTEM observation (Fig. 1g). Fig. S12† shows the optimized structures of adsorbed NO₃⁻ (*NO₃) on all the considered catalysts. It was found that V_S-induced unsaturated Mo (V_S–Mo) sites on MoS_2−x and Ce–MoS_2−x can directly absorb NO₃⁻, resulting in the distinct N=O bond elongation of 0.10 Å on MoS_2−x and 0.11 Å on Ce–MoS_2−x, whereas weak NO₃⁻ adsorption can be observed on pristine MoS₂. The charge-density difference maps of *NO₃ on MoS_2−x (Fig. S13b†) and Ce–MoS_2−x (Fig. 5a) show −0.53 |e| and −0.58 |e| charge transfer from V_S to *NO₃, respectively, while the electron transfer (−0.17 |e|, Fig. S13a†) is significantly reduced from pristine MoS₂ to *NO₃. In addition, the differently oriented electron counter maps of Ce–MoS_2−x (Fig. 5b) reveal a significant electron donation to *NO₃ from both V_S–Mo and Ce-dopant around V_S, which can also be confirmed by the detailed charge analysis (Fig. S14†). Thus, although the Ce-dopant of Ce–MoS_2−x does not directly absorb NO₃⁻, it joins with V_S–Mo to donate more electrons to *NO₃ compared to Ce-dopant-free MoS_2−x (Fig. S13b†), leading to the expedited NO₃⁻ activation and dissociation.⁶⁴ This result can be further verified by the PDOS (Fig. 5c) analysis, showing a largely enhanced orbital hybridization between Ce–MoS_2−x and *NO₃ at the Fermi level compared to the pristine MoS₂ and MoS_2−x counterparts, implying a maximum NO₃⁻ activation being enabled by Ce–MoS_2−x.⁶⁵ Therefore, the Ce-dopant and V_S on Ce–MoS_2−x can synergistically promote NO₃⁻ absorption and activation, which is greatly beneficial for the subsequent NO₃RR process.

Fig. 5 (a) Charge density difference map of *NO₃ on Ce–MoS_2−x. (b) Differently oriented electron counter maps of *NO₃ on Ce–MoS_2−x. (c) PDOS of *NO₃ on MoS₂, MoS_2−x and Ce–MoS_2−x. (d) Free energy diagrams of NO₃RR on Ce–MoS_2−x.

The NO₃RR energetic processes of MoS_2−x and Ce–MoS_2−x were then investigated from a thermodynamics perspective through two typical NOH and NHO pathways (Fig. S16 and S17†).^37,66,67 For both MoS_2−x (Fig. S15†) and Ce–MoS_2−x (Fig. 5d), the NOH pathway is thermodynamically preferred due to its lower rate-determining step (RDS) energy barrier relative to the NHO pathway. Meanwhile, the RDS energy barrier of Ce–MoS_2−x was reduced by 0.07 eV compared to MoS_2−x, demonstrating that the introduced Ce-dopant on Ce–MoS_2−x is able to further decrease the RDS energy barrier of the NO₃RR process. Therefore, the above DFT and MD results demonstrate that the Ce-dopant and V_S synergistically enhance the NO₃RR activity of Ce–MoS_2−x, in which V_S-induced unsaturated Mo acts as the active site to absorb and activate NO₃, while the Ce-dopant can jointly enhance NO₃⁻ activation and reduce the RDS energy barrier.

3. Conclusion

In conclusion, Ce–MoS_2−x/CC has been confirmed as an efficient NO₃RR catalyst. Theoretical calculations demonstrate the synergy of V_S and a Ce-dopant to enhance the NO₃RR activity of Ce–MoS_2−x, where V_S-induced unsaturated Mo mainly serves as an active site to activate NO₃⁻, while the Ce-dopant jointly enhances NO₃⁻ activation and leads to a reduced RDS energy barrier. The present work may open a new avenue to explore efficient MoS₂-based NO₃RR catalysts from the perspective of vacancy/doping engineering.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work is supported by the Fundamental Research Top Talent Program of Lanzhou Jiaotong University (2022JC03).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qi01798a

‡ These authors contributed equally to this work.

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