Au nanoparticles with oxygen vacancies on BiVO₄ for electrocatalytic nitrate reduction to ammonia

Abstract

Ammonia (NH₃) is an important energy carrier and agricultural fertilizer. Development of electrocatalysts for efficient NH₃ electrosynthesis via the nitrate reduction reaction (NitRR) is highly desirable but remains a key challenge. In this work, we successfully loaded Au nanoparticles on BiVO₄ by a one-step hydrothermal method. It is demonstrated that by using Au nanoparticles (10–15 nm) embedded on BiVO₄ (Au/BiVO₄) with oxygen vacancies (Au loading is 1.3 wt%), the electrocatalytic NitRR is indeed possible under ambient conditions. Unexpectedly, at −1.35 V (vs. RHE), the yield rate for NH₃ of Au/BiVO₄ reached 3320.9 ± 89.9 μg h⁻¹ cm⁻², which was far superior to (11.3 μg h⁻¹ cm⁻²) pristine BiVO₄. The ¹⁵N isotope labeling experiments confirmed that the produced NH₃ indeed originated from the nitrate reduction reaction catalyzed by Au/BiVO₄. The comprehensive analysis further confirms that the oxygen vacancies in Au/BiVO₄ can effectively weaken the N–O bonding and restrain the formation of by-products, resulting in high faradaic efficiency and NH₃ selectivity. Furthermore, in situ differential electrochemical mass spectrometry (DEMS) was adopted to monitor the electrochemical separation of the NitRR products on the surface of Au/BiVO₄.

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Ammonia (NH₃) has attracted considerable interest as a sustainable nitrogen fertilizer in its anhydrous and salt forms, as well as a viable alternative fuel for transportation and an efficient hydrogen energy carrier.^1–3 Currently, ammonia synthesis remains predominantly dependent on the energy- and capital-intensive Haber-Bosch process (150–350 atm, 350–550 °C), in which hydrogen and energy inputs are primarily sourced from fossil fuels, leading to substantial CO₂ emissions.^4–7 With the depletion of fossil fuel reserves and escalating concerns over greenhouse gas emissions, the development of more sustainable and cost-effective ammonia production technologies is imperative to meet the increasing global demand. In recent years, electrocatalytic and photo(electro)catalytic conversion of nitrogen gas and water into ammonia under ambient conditions has garnered significant attention.^8–11 However, the faradaic efficiency (FE) remains severely constrained by the high dissociation energy of the N≡N bond (941 kJ mol⁻¹), the limited solubility of nitrogen gas in electrolytes, and the competing hydrogen evolution reaction (HER).^7,12,13 Thus, developing a new route for ammonia synthesis under benign conditions is urgently desired. Given that the N–O bond dissociation energy in nitrate is relatively low (204 kJ mol⁻¹) and ammonia can be readily recovered from aqueous solutions, utilizing nitrate contaminants as a nitrogen source and water as a hydrogen source for the electrochemical synthesis of value-added NH₃ has attracted considerable interest.^14–16 However, the competing hydrogen evolution reaction (HER) and the inherently complex eight-electron transfer process significantly limit the faradaic efficiency (FE) and selectivity of NH₃ in electrocatalytic nitrate reduction reactions (NitRR).^17–19 Therefore, the rational design and precise engineering of high-performance electrocatalysts are essential to overcoming these challenges.

Theoretical studies have demonstrated that oxygen vacancies (OVs) exhibit unique electronic modulation functions in catalytic reactions. By optimizing the adsorption energy of reaction intermediates and lowering the activation barrier, thereby significantly promoting reaction pathways toward lower energy consumption.²⁰ Surface characterization and theoretical calculations have confirmed that NO_x molecules can be effectively activated into highly reactive intermediate states under different coverage conditions and adsorption configurations.²¹ By precisely constructing electron-donating semiconductor catalytic systems enriched with oxygen vacancies, it is possible not only to overcome the kinetic limitations of conventional nitrogen oxide reduction reactions but also to enhance surface adsorption strength and electron transfer efficiency, thereby significantly improving the catalytic activity for nitrogen oxide reduction. Recently, oxygen vacancies (OVs) have been widely used to improve the performance of electrocatalysts. For example, Zeng and co-workers demonstrated that LaCoO₃ with OVs is proposed for efficient electrocatalytic NRR.²² Density functional theory calculations revealed that enhanced activation of N₂ over V_o-LaCoO₃ originated from the increased charge density around the valence band edge via the introduction of OVs.²² Moreover, Zhang et al. reported the TiO₂ nanotubes with rich oxygen vacancies that exhibited enhanced high FE (85.0%) and selectivity (87.1%) toward the NH₃ synthesis from nitrate electroreduction.²³ Theoretical calculations combined with in situ measurements reveal that the oxygen atoms in nitrate can occupy the oxygen vacancies in TiO_2-x, thereby weakening the N–O bond. Furthermore, a higher concentration of oxygen vacancies enhances NH₃ selectivity by modulating the interaction between intermediates and the catalyst.²³

Therefore, combined with the theoretical calculations with experimental observations, we have designed and constructed the Au nanoparticles embedded on BiVO₄ (Au/BiVO₄) with oxygen vacancies through a facile and controllable strategy. And the as-prepared Au/BiVO₄ exhibits brilliant NitRR performances, including high NH₃ yield rate of 3320.9 ± 89.9 μg h⁻¹ cm⁻², high FE (up to 59.6 ± 2.4%) at −1.35 V (vs. RHE), and good stability (up to 10 h). The enhanced NitRR electrocatalytic activities mainly originate from the enhanced nitrate adsorption energy and a more favorable active reaction site due to the introduction of OVs in Au/BiVO₄, which can effectively weaken the N–O bonding and restrain the formation of by-products, resulting in high faradaic efficiency and NH₃ selectivity.

The Au nanoparticles (NPs) prepared by reducing HAuCl₄·4H₂O with NaBH₄ were loaded into BiVO₄ to form Au/BiVO₄ as shown in Fig. 1a. The transmission electron microscopy (TEM) image of Au/BiVO₄ shows that BiVO₄ has a fusi form structure, Au NPs are uniformly distributed on the surface of the BiVO₄ support (Fig. 1b). Furthermore, the well-resolved lattice fringes of 0.246 nm in High-resolution transmission electron microscopy (HR-TEM) image of Au/BiVO₄ corresponded to the (111) plane of cubic Au phase,^24,25 demonstrated the successful deposition of Au NPs on BiVO₄ surface (Fig. 1c). X-ray diffraction (XRD) was used to analyse the crystal structure of the as-prepared Au/BiVO₄ sample. As shown in Fig. 1d, the present XRD pattern of Au/BiVO₄ has three strong diffraction peaks at 30.7°, 32.9°, and 48.6°, corresponding to (211), (112), and (312) crystal planes of BiVO₄ (PDF No. 14-0688).²⁶ The diffraction peaks located at 38.2° belong to the (111) crystal planes of Au (PDF No. 04-0784),²⁷ which is in good agreement with the HR-TEM results. However, due to the low gold content, the intensity of the diffraction peaks is not strong. Moreover, the corresponding element mapping images displayed that Au NPs was uniformly distributed on BiVO₄ (Fig. 1e). The loading content of Au NPs in Au/BiVO₄ is 1.3 wt% from Fig. S1 (ESI†), corresponding to the weak diffraction peak in XRD pattern. The pore size distribution of Au/BiVO₄ demonstrates its micro- and mesoporous structure (Fig. 1f), which allows for the exposure of more active sites are beneficial for the transport of electrons and the mass transport of electrolytes during electrolysis.²⁸

Fig. 1 (a) Schematic for diagram illustrating synthetic procedure of Au/BiVO₄. (b) TEM and (c) HR-TEM images of Au/BiVO₄. (d) XRD pattern of Au/BiVO₄. (e) HAADF-STEM and corresponding elemental mapping images of Au/BiVO₄. (f) Pore size distribution curve of Au/BiVO₄.

X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical composition and valence state variations of the catalyst, providing further insights into the electronic interactions between Au NPs and BiVO₄. The survey spectra in Fig. S2 (ESI†) exhibit distinct peaks corresponding to Bi, Au, V, and O, indicating that these elements are the primary constituents of the catalyst, which is consistent with the element mapping characterization. The high-resolution Au 4f spectrum (Fig. 2a) contains two predominant peaks at 83.9 eV and 87.5 eV indicate the successful formation of Au.²⁹ In the XPS spectra of Bi 4f spectra (Fig. 2b), two distinctive peaks corresponded to Bi 4f_7/2 in pristine BiVO₄ are observed at 158.7 eV and 160.0 eV, respectively, while the Bi 4f_7/2 in Au/BiVO₄ has only one characteristic peak at 159.1 eV.³⁰ For the O 1s XPS spectra in Fig. 2c, where two peaks at 529.7 eV and 531.3 eV correspond to lattice oxygen and oxygen vacancy in pristine BiVO₄, respectively. Compared to pristine BiVO₄, the O 1s binding energies of the lattice oxygen and oxygen vacancy peaks of the Au/BiVO₄ were reduced by 0.1 eV. Studies have demonstrated that under high-temperature and reducing conditions, lattice oxygen atoms are prone to detachment, leading to the formation of oxygen vacancies. Moreover, the strong reducing agent NaBH₄ can further induce the removal of oxygen atoms from the BiVO₄ crystal lattice, contributing to the generation of additional oxygen vacancies.^31,32 In the V⁵⁺ 2p spectra (Fig. 2d), the peaks of V⁵⁺ in Au/BiVO₄ located at 516.6 eV and 524.3 eV show a slight low-energy shift in contrast to pristine BiVO₄, indicates that V species in BiVO₄ are stable after loading Au. More importantly, the related changes in the valence state further demonstrate that the charge transferred from BiVO₄ to Au NPs. The induced local charge redistribution contributes to the targeted adsorption of reactant species and thus enhances the electrocatalytic NitRR to NH₃ production ability.³³

Fig. 2 High-resolution XPS spectra of (a) Au 4f, (b) Bi 4f, (c) O 1s, (d) V 2p and of pristine BiVO₄ and Au/BiVO₄.

The NitRR performance of the catalyst was evaluated using a three-electrode H-type electrolytic cell. Initial voltammetric verification of NitRR on Au/BiVO₄ was carried out by linear sweep voltammetry (LSV). Fig. 3a shows the LSV curves of the catalyst in electrolytes of 0.1 M K₂SO₄ and 0.1 M K₂SO₄ + 0.1 M KNO₃. Compared with the LSV curve for 0.1 M K₂SO₄, the current density was markedly enhanced following the introduction of KNO₃, indicating a strong reduction reaction at the cathode after the addition of nitrate. By analyzing the two LSV curves, we chose to perform the NitRR at 0.05 V intervals within the applied potential range of −1.1 V to−1.35 V (vs. RHE). The obtained current density curves and corresponding UV-vis absorption spectra are shown in Fig. S3 (ESI†) and the produced NH₃ was determined by the indophenol blue method (Fig. S4, ESI†). Fig. 3b shows the R_NH3 and FE at different applied potentials. It can be observed that as the negative potential increases, the R_NH3 increases from 1385.5 ± 158.0 μg h⁻¹ cm⁻² at −1.1 V (vs. RHE) to a maximum of 3320.9 ± 89.9 μg h⁻¹ cm⁻² at −1.35 V (vs. RHE) with an FE of 59.6 ± 2.4%. The results demonstrate that the Au/BiVO₄ catalyst has a good performance towards the NitRR to NH₃. Except for the NH₃ main product, the by-product NO₂⁻ was also detected in the electrolyte (Fig. S5, ESI†).

Fig. 3 (a) LSV curves of Au/BiVO₄ catalyst in 0.1 M K₂SO₄ and 0.1 M K₂SO₄ + 0.1 M KNO₃ solution. (b) R_NH3 and FE of Au/BiVO₄ catalyst obtained at different potentials for 2 h NitRR measurement. (c) Comparison of R_NH3 and FE under different conditions. (d) ¹H NMR spectra of Au/BiVO₄ catalyst using ¹⁵NO₃⁻ as nitrogen source for NitRR and standards (¹⁵NH₄)₂SO₄. (f) Recycling tests for Au/BiVO₄ catalyst during NitRR at −1.35 V (vs. RHE). (e) The FE and NH₃ yield were monitored over a 10 h NitRR conducted at −1.35 V (vs. RHE).

A series of controlled experiments were conducted to eliminate the influence of other factors on NH₃ production (Fig. 3c, ESI†). Firstly, the NitRR was carried out using Au/BiVO₄ catalyst in electrolyte of 0.1 M K₂SO₄ at −1.35 V (vs. RHE). The results demonstrate that there is a negligible quantity of ammonia produced. Furthermore, the presence of ammonia was not discernible when the electrolyte was 0.1 M K₂SO₄ + 0.1 M KNO₃ but without applied potential (OCP). The above experiment eliminated the effect of environmental factors on ammonia production. In order to eliminate any potential interference from carbon paper, pure carbon paper was employed in the NitRR process. The lower R_NH3 and FE can infer that the excellent ammonia production activity is mainly attributed to the prepared Au/BiVO₄ catalyst. Moreover, to investigate the effect of the interaction between Au and BiVO₄ on the catalysis of NitRR to ammonia, BiVO₄ was used as the working electrode under 0.1 M K₂SO₄ and 0.1 M KNO₃ for 2 h at −1.35 V (vs. RHE). Only negligible NH₃ concentration was detected. This demonstrates that addition of Au nanoparticles can facilitate the generation and stabilization of oxygen vacancies, while these vacancies, in turn, anchor the Au nanoparticles and optimize their electronic structure, significantly enhancing electron transfer efficiency. This synergistic interaction effectively boosts the catalytic activity and selectivity for NO₃⁻ reduction, thereby enabling high-efficiency and stable catalytic performance.^34,35 To investigate the synergistic effect of gold loading on oxygen vacancy concentration and the electrochemical probe reaction performance of the composites, we optimized the Au-to-oxygen vacancy ratio by adjusting the Au loading. Fig. S6† is the R_NH3 and FE of catalyst at different Au loading during a 2 h NitRR measurement. It can be observed that Au/BiVO₄ exhibits superior performance, suggesting that, at this specific loading, the oxygen vacancy concentration on the catalyst surface achieves an optimal balance with the distribution of active sites. To further verify the source of NH₃, K¹⁵NO₃ was used instead of K¹⁴NO₃ for the NitRR, and the resulting products were qualitatively analyzed for NH₃ yield using ¹H nuclear magnetic resonance (NMR) spectra. The results indicate that the ¹H NMR spectrum of the electrolyte containing ¹⁵NO₃⁻ as the reactant exhibits the characteristic doublet of ¹⁵NH₄⁺(Fig. 3d). This result reveals that the produced NH₃ is completely derived from nitrate. Besides superior activity, the stability of the catalyst is another crucial parameter for practical applications. Cyclic stability of the electrocatalyst is an important factor for practical application. For the evaluation of the cyclic stability of Au/BiVO₄, the consecutive NitRR test was performed for 10 times at −1.35 V (vs. RHE). The R_NH3 and FE of the Au/BiVO₄ have no obvious change after 10 consecutive electrolysis cycles (Fig. 3f). The sustainability of the catalyst was further tested by reacting at −1.35 V (vs. RHE) for 10 h (Fig. S7, ESI†). It can be seen that throughout the experiment, the NH₃ yielded increases linearly with time and the FE decreases uniformly (Fig. 3e). This phenomenon can be attributed to the accumulation of reaction by-products on the electrode surface, which impedes the accessibility of reactants to active sites, consequently resulting in a reduction in the FE of the reaction. This indicates that the Au/BiVO₄ catalyst has good stability. To determine the structural stability, careful examination of XPS characterization on Au/BiVO₄ after NitRR test (Fig. S8, ESI†) confirms the high structural stability of Au/BiVO₄. To evaluate the scalability of the catalyst for potential large-scale applications, we systematically assessed the performance of Au/BiVO₄ by constructing an expanded reaction system (50 mL–100 mL) and gradually increasing the nitrate concentration (0.1 M–0.2 M). Fig. S9† presents the R_NH3 and FE of Au/BiVO₄ at different scales. In the 100 mL electrolysis system, the catalyst retained its catalytic activity comparable to that in the smaller-scale system, demonstrating excellent scalability. When the nitrate concentration was increased to 0.2 M, the R_NH3 was enhanced compared to that at 0.1 M, revealing a significant mass transfer enhancement effect. These findings strongly validate the feasibility of this catalyst for large-scale applications.

Furthermore, the in situ differential electrochemical mass spectrometry (DEMS) is employed for in situ detection of molecular intermediate and product over Au/BiVO₄.³⁶ The gaseous molecules can be identified based on m/z values, and the relative quantities of intermediates and products can be estimated in real-time based on signal intensity.³⁷ In order to investigate the electrocatalytic NitRR pathway throughout the system, in situ electrochemical differential mass spectrometer (DEMS) was employed for the detection of intermediates and reduction products. During the electrocatalytic experiments, reaction products and intermediates were transported to the online mass spectrometry analysis system via a vacuum pump for real-time monitoring and analysis based on mass-to-charge ratio (m/z). The signal intensity enables analysis of the real-time concentration of intermediates and products. As shown in Fig. 4, the signal peaks at m/z 17, 30, 31, 32, 33 and 46 clearly indicate the presence of *NH₃, *NO, *HNO, *NH₂O, *NH₂OH and *NO₂ intermediates during the reaction. Based on these results, the electrocatalytic NitRR pathway on Au/BiVO₄ catalyst is proposed to be the following: NO₃⁻ → *NO₃ → *NO₂ → *NO → *NHO → *NH₂O → *NH₂OH → *NH₃ → NH₃ (g).

Fig. 4 In situ DEMS measurements of NitRR over Au/BiVO₄.

In summary, Au nanoparticles embedded on BiVO₄ (Au/BiVO₄) with oxygen vacancies (OVs) have been successfully synthesized through a facile and controllable strategy. Owing to the enhanced NO₃⁻ adsorption energy and a more favourable active reaction site at the OVs due to the introduction of Au NPs to BiVO₄. The as-synthesized Au/BiVO₄ catalyst exhibits superior NitRR performances, including high NH₃ yield rate of 3320.9 ± 89.9 μg h⁻¹ cm⁻² and high FE of 59.6 ± 2.4% at −1.35 V (vs. RHE), and good stability (up to 10 h) at room temperature and atmospheric pressure. This work is not only the excellent report on noble metal NitRR electrocatalyst achieving both high NH₃ yield rate and high FE imultaneously, but also opens up a new avenue for exploring the practical applications of the transition metal vanadium oxides and vanadates family as very attractive low-cost NitRR electrocatalysts.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Kui Hu: conceptualization, investigation, visualization, writing – original draft. Shengbo Zhang: data curation, resources, supervision, writing – review & editing. Zhixian Mao: investigation, resources, formal analysis. Dongnan Zhao: investigation, resources, formal analysis. Daopeng Li: investigation, resources, formal analysis. Zhongjun Li: software. Qiang Li: software. Qiong Tang: data curation, resources, supervision. Tongfei Shi: funding acquisition, supervision, resources, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Anhui Provincial Natural Science Foundation (Grant No. 2408085MB021).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ra00886g

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