Integrated selective nitrite reduction to ammonia with tetrahydroisoquinoline semi-dehydrogenation over a vacancy-rich Ni bifunctional electrode

Abstract

The development of efficient electrocatalysts for nitrite reduction to ammonia, especially integrated with a value-added anodic reaction, is important. Herein, Ni nanosheet arrays with Ni vacancies (Ni-NSA-V_Ni) were demonstrated to exhibit outstanding electrocatalytic performances toward selective nitrite reduction to ammonia (faradaic efficiency: 88.9%; selectivity: 77.2%) and semi-dehydrogenation of tetrahydroisoquinolines (faradaic efficiency: 95.5%; selectivity: 98.0%). The origin and quantitative analyses of ammonia were performed by ¹⁵N isotope labeling and ¹H NMR experiments. The decrease in electronic cloud density induced by the Ni vacancies was found to improve the NO₂⁻ adsorption and NH₃ desorption, leading to high nitrite-to-ammonia performance. In situ Raman results revealed the formation of Ni^II/Ni^III active species for anodic semi-dehydrogenation of tetrahydroisoquinolines on Ni-NSA-V_Ni. Importantly, a Ni-NSA-V_Ni‖Ni-NSA-V_Ni bifunctional two-electrode electrolyzer was constructed to simultaneously produce ammonia and dihydroisoquinoline with robust stability and high selectivity.

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Nitrite (NO₂⁻), as one of the most common contaminants in groundwater, can cause serious health problems.^1–4 Electro-catalytic reduction has emerged as a promising strategy to remove nitrite.^5–8 Simultaneously, ammonia/ammonium has been widely used in agriculture and industry.^9–11 Interestingly, nitrite is difficult to be recycled, while ammonia can be easily reclaimed from its aqueous solution.^12,13 Thus, adopting nitrite as the nitrogen source and water as the hydrogen source to produce ammonia is promising for the artificial nitrogen cycle. Recently, a series of metal complexes as homogeneous electro-catalysts exhibited high activity for selective nitrite reduction to ammonia.^14–16 Considering the separation difficulties of homogeneous electrocatalysts, a heterogeneous electrocatalyst for selective nitrite reduction to ammonia is urgently needed.

The counter reaction to nitrite reduction is the kinetically sluggish oxygen evolution reaction (OER), which requires large overpotential, and its product O₂ does not bear a high price tag.¹⁷ An alternative strategy is to develop a thermodynamically favorable anodic reaction to produce value-added fine chemicals.¹⁸ Dihydroisoquinolines (DHIQs), as an important intermediate in the pharmaceutical industry, display a wide range of bioactivities in anti-tumor, anti-fungal, vasodilation and nonoamine oxidase inhibition.¹⁹ DHIQs can be prepared through the dehydrogenation of tetrahydroisoquinolines (THIQs). The challenge of this reaction lies in the controlled semi-dehydrogenation of THIQs to avoid undesirable over-dehydrogenation.^20–22 Recently, our group reported that Ni₂P could electrocatalyze semi-dehydrogenation of THIQs into DHIQs and the in situ formed Ni^II/Ni^III redox served as the active species.²³ But, expensive phosphorus sources or/and dangerous gaseous PH₃ byproducts are big concerns for the synthesis of Ni₂P nanostructures.^23–25 Pure Ni is one of the cheap Ni-based materials, but its electrocatalytic performance is low. Interestingly, introducing anion or cation vacancies into materials could regulate their electronic structures to improve electrocatalytic performances.^26–33 However, Ni nanostructures with Ni vacancies are rarely touched due to the wet-chemical synthetic difficulty. Therefore, the development of a facile method to synthesize nickel with rich Ni vacancies as a bifunctional electrocatalyst for integrating semi-dehydrogenation of THIQs with selective nitrite electroreduction to ammonia is significant but still remains a great challenge.

Here, Ni nanosheet arrays with Ni vacancies, denoted as Ni-NSA-V_Ni, were prepared through plasma-assisted synthesis and exhibited high performance for selective nitrite electroreduction and semi-dehydrogenation of THIQs. The experimental and density functional theory (DFT) calculation results revealed that Ni vacancies promoted the conversion of nitrite to ammonia. In situ Raman spectra confirmed the formation of Ni^II/Ni^III active species for semi-dehydrogenation of THIQs. Ni-NSA-V_Ni could act as efficient bifunctional electrodes for simultaneously producing ammonia and DHIQs with high stability and selectivity in a Ni-NSA-V_Ni‖Ni-NSA-V_Ni two-electrode electrolyzer.

Ni-NSA-V_Ni were synthesized through a facile H₂-plasma chemical conversion (see the ESI† for details). First, Ni(OH)₂ nanosheet arrays supported on Ni foam were prepared by a typical solution-chemical method (Fig. S1†) and then reduced to Ni nanosheet arrays (Ni-NSA) using ethylene glycol.³⁴ Finally, Ni-NSA-V_Ni could be obtained through the H₂-plasma treatment of Ni-NSA because of the etching role of plasma. A series of advanced techniques were adopted to characterize Ni-NSA-V_Ni. Scanning electron micrograph (SEM) images (Fig. S2a and b†) show the nanosheet array morphology of Ni-NSA and Ni-NSA-V_Ni, suggesting the morphology retention during the plasma-assisted conversion synthesis. The transmission electron micrograph (TEM) image of Ni-NSA-V_Ni (inset in Fig. 1a) displayed a transparent structure, indicating ultrathin thickness. The typical high-resolution TEM (HRTEM) image of Ni-NSA-V_Ni demonstrated a crystalline structure with a clear lattice spacing of 0.21 nm belonging to the (111) facet of Ni (Fig. 1a). Some amorphous regions within the yellow circles could be clearly observed, indicating the presence of vacancies.³⁵ To exclude the influence of the Ni substrate on X-ray diffraction (XRD), Ni-NSA-V_Ni was synthesized on Cu foam. The XRD pattern of Ni-NSA-V_Ni/Cu (Fig. 1b) exhibited exclusive signals of pure Ni (JCPDS no. 04-0850), which were the same as that of the Ni foam (Fig. S3b†).³⁴ The atomic force microscopy (AFM) image and the height configurations (Fig. S4†) revealed the thickness of Ni-NSA-V_Ni to be 1.8 nm, confirming their ultrathin thickness. X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) were performed to analyze Ni-NSA-V_Ni. In Ni 2p XPS spectra (Fig. 1c), the peaks at 852.9 and 870.3 eV could be assigned to Ni⁰, while the two peaks of 855.6 and 873.5 eV arising from Ni²⁺ could be ascribed to the unavoidable oxidation during the XPS test.³⁶ The similarity of the Ni K-edge radial structure functions to those of the Ni foam verified that Ni-NSA-V_Ni was metallic nickel (Fig. 1d). The weaker peak intensity of the Ni–Ni shell (2.4 Å) in Ni-NSA-V_Ni compared to Ni foam arose from its decreased coordination number as a result of the nickel vacancies (Table S1†).³⁵ The aforementioned results demonstrated that the H₂-plasma treatment of Ni-NSA could produce abundant nickel vacancies with the maintenance of the morphology and structure.

Fig. 1 (a) HRTEM image of Ni-NSA-V_Ni. Inset is the corresponding TEM image. (b) XRD pattern of Ni-NSA-V_Ni/Cu foam. (c) XPS spectra of Ni-NSA-V_Ni. (d) Ni K-edge radial structure functions of Ni foam and Ni-NSA-V_Ni.

The performance of nitrite electroreduction was recorded versus the saturated calomel electrode (SCE) with 200 ppm nitrite-N (Fig. 2). All electrochemical tests were performed in a standard three-electrode electrochemical cell by using 0.2 M Na₂SO₄ solution as the electrolyte. A typical H-type electrolytic cell was used. The as-prepared samples on the Ni foam, Pt plate and SCE were used as the working electrode, the counter electrode and the reference electrode, respectively. The reaction for calculation of conversion and faradaic efficiency was carried out for 2 hours. Colorimetric methods were adopted to determine the concentration of nitrite and ammonia in pre- and post-test electrolytes (Fig. S5†).^{9–11,37–39} The linear sweep voltammetry (LSV) curves of Ni-NSA-V_Ni showed obvious enhancement in the current density in the presence of NaNO₂, and the current density of Ni-NSA-V_Ni was much higher than that of Ni foam (Fig. S6† and 2a).

Fig. 2 (a) LSV curves of Ni Foam and Ni-NSA-V_Ni in 0.2 M Na₂SO₄ with 200 ppm nitrite. (b) Time-dependent concentration of nitrite and ammonia over Ni-NSA-V_Ni at −1.2 V vs. SCE. (c) Conversion yield of nitrite and faradaic efficiency of ammonia over Ni-NSA-V_Ni at different potentials. (d) Faradaic efficiency and selectivity of ammonia, and conversion rate of nitrite at −1.2 V vs. SCE. (e) Selectivity and faradaic efficiency of ammonia during consecutive recycling tests for Ni-NSA-V_Ni at −1.2 V vs. SCE. (f) ¹H NMR spectra of standard samples (¹⁵NH₄)₂SO₄ and (¹⁴NH₄)₂SO₄ and electrolyte after the NO₂⁻ reduction using ¹⁴NO₂⁻ and ¹⁵NO₂⁻ as the N-source.

With prolonging the reaction time, the concentration of nitrite decreased, while the concentration of ammonia increased, suggesting nitrite reduction to ammonia (Fig. 2b). The selectivity of ammonia over Ni-NSA-V_Ni kept increasing from −1.0 to −1.4 V, while the faradaic efficiency displayed a volcanic shape curve with a maximum at −1.2 V (Fig. 2c). Thus, we chose −1.2 V as the operation voltage for investigation of nitrite electroreduction. The electrocatalytic performance of Ni-NSA-V_Ni (conversion rate: 73.4%, faradaic efficiency: 88.9%, selectivity: 77.2%) was remarkably higher than those of Ni foam (conversion rate: 10.8%, faradaic efficiency: 3.9%, and selectivity: 23.9%) at −1.2 V vs. SCE (Fig. 2d). Notably, the faradaic efficiency and yield rate of ammonia were much higher than all the reported values of nitrogen electroreduction and among the best for electroreduction of nitrite to ammonia (Table S2†). The electroreduction of 200 ppm nitrite in 2 h was regarded as one cycle. The selectivity and faradaic efficiency of ammonia exhibited no decay during five cycling tests (Fig. 2e). Moreover, the morphology and structure of Ni-NSA-V_Ni were maintained well after the durability test, suggesting the high stability (Fig. S7†). Negligible ammonia was detected without the addition of nitrite (Fig. S8†), indicating that the produced ammonia originated from nitrite reduction. Meanwhile, ¹⁵N isotope labelling experiments were conducted and the yield of ammonium was quantified using ¹H nuclear magnetic resonance (NMR) spectra.⁴⁰ The ¹H NMR spectra of electrolyte adopting ¹⁵NO₂⁻ as the reactant showed typical double peaks of ¹⁵NH₄⁺, while the ¹H NMR spectra of electrolyte adopting ¹⁴NO₂⁻ as the reactant displayed typical triple peaks of ¹⁴NH₄⁺ (Fig. 2f).^41,42 The ¹H NMR results further confirmed that the produced ammonia was derived from nitrite. For quantification, maleic acid (C₄H₄O₄) was chosen as the ¹H NMR external standard. Based on the standard curve of the integral area (NH₄⁺–N/C₄H₄O₄) against the NH₄⁺–N concentration, the ammonia was quantified (Fig. S9 and S10†). The generated ¹⁵NH₄⁺–¹⁵N and ¹⁴NH₄⁺–¹⁴N quantified by ¹H NMR were very close to the results quantified by colorimetric methods (Table S3†), suggesting the quantitative accuracy.

To further probe the origin of high performance of Ni-NSA-V_Ni for selective nitrite reduction to ammonia, a series of in situ and ex situ experiments and theoretical calculations were performed. First, the ammonia yield rate of Ni-NSA-V_Ni was much higher than that of Ni foam (Fig. 3a). Furthermore, the electrochemical impedance spectroscopy (EIS) results illustrated that vacancy-rich Ni-NSA-V_Ni possessed smaller interfacial charge-transfer resistance than Ni foam (Fig. S11†). Online differential electrochemical mass spectrometry (DEMS) measurements were performed for the detection of the molecular intermediate and product over Ni-NSA-V_Ni (Fig. 3b).^43–45 The m/z signals of NO (30) and NH₃ (17) combined with the large desorption free energies (ΔG_des) of NO on the surfaces of Ni foam (ΔG_des = 2.53 eV) and Ni-NSA-V_Ni (ΔG_des = 2.61 eV) obtained by DFT calculations indicated NO as one of the intermediates. To gain insights into the nitrite electroreduction process, the surface electron cloud density and reaction Gibbs free energies (ΔG) were calculated based on DFT. Compared to Ni foam, Ni-NSA-V_Ni possessed lower surface electron cloud density (Fig. 3c and d). As displayed in Fig. 3e, the adsorption energy of NO₂⁻ on Ni-NSA-V_Ni (−0.44 eV) was much lower than that of Ni foam (0.12 eV). For further hydrogenation processes, the reaction Gibbs free energy of each step was downhill or uphill with a small degree (ΔG_max = 0.25 and 0.16 eV over Ni foam and Ni-NSA-V_Ni, respectively), guaranteeing the conversion from to . The last desorption of NH₃ from Ni-NSA-V_Ni (ΔG = 1.12 eV) was also easier than that on Ni foam (ΔG = 1.34 eV). Thus, the decreased surface electron cloud density in Ni-NSA-V_Ni improved the adsorption and activation of NO₂⁻ and the desorption of NH₃, leading to high nitrite-to-ammonia catalytic performance over the Ni-NSA-V_Ni cathode.

Fig. 3 (a) The yield rate of ammonia over Ni foam and Ni-NSA-V_Ni. (b) DEMS of Ni-NSA-V_Ni for nitrite electroreduction. Calculated surface electron density distribution of (c) Ni foam and (d) Ni-NSA-V_Ni. (e) Free energy diagram of the nitrite electroreduction process over Ni foam and Ni-NSA-V_Ni at 0 V vs. RHE.

Next, the electrocatalytic semi-dehydrogenation of THIQs over the Ni-NSA-V_Ni anode was carried out in 1.0 M KOH aqueous solution. The 1,2,3,4-tetrahydroisoquinoline (1a) was selected as a model substrate. Without 1a, the OER proceeded on Ni-NSA-V_Ni (Fig. 4a). To achieve the current density of 50 mA cm⁻² for the OER, Ni-NSA-V_Ni required 1.61 V vs. the reversible hydrogen electrode (RHE). Notably, the oxidation peak before 1.45 V arose from the Ni^II/Ni^III redox, which was demonstrated as active species for the electrocatalytic semi-dehydrogenation conversion of THIQs.²³ After adding 1a, Ni-NSA-V_Ni, exhibited increased current density, indicating the oxidation of THIQs (Fig. 4a). Compared with Ni foam (Fig. S12†), Ni-NSA-V_Ni exhibited much higher current density, indicating the superior oxidation ability of THIQs. Time-dependent Raman spectra initially showed two peaks of NiOOH at 472 and 553 cm⁻¹ for Ni-NSA-V_Ni (Fig. 4b).^23,46 The two peaks of NiOOH in Ni-NSA-V_Ni were maintained well after the addition of 1a. This proved the superior stability of Ni^II/Ni^III species on Ni-NSA-V_Ni during the electrooxidation process of 1a. Ni-NSA-V_Ni could efficiently drive a semi-dehydrogenation conversion of 1a into 2a with 98.0% selectivity and 95.5% faradaic efficiency. 6-Methoxy tetrahydroisoquinoline (1b) and hexachlorotetrahydroisoquinoline (1c) were efficiently converted to the corresponding dihydroisoquinolines with 96–99% selectivity, suggesting the good group tolerance of the tetrahydroisoquinoline semi-dehydrogenation over the Ni-NSA-V_Ni anode (Fig. 4c and S13†). Our results demonstrated that Ni-NSA-V_Ni could act as an efficient bifunctional electrode material for both cathodic nitrite-to-ammonia reduction and anodic tetrahydroisoquinoline semi-dehydrogenation. Thus, a Ni-NSA-V_Ni‖Ni-NSA-V_Ni two-electrode electrolyzer was constructed to simultaneously produce ammonia at the cathode and DHIQs at the anode (Fig. 4d). After adding 1a, the current density increased for Ni-NSA-V_Ni, indicating that the overall potential can be decreased by adopting oxidation of THIQs to replace the OER (Fig. S14†). The selectivity of 2a and ammonia over the Ni-NSA-V_Ni bifunctional electrodes achieved 91.5% and 82.7% at the voltage of 1.5 V vs. counter electrode (CE). During three continuous cycles, no decrease in their selectivities was observed (Fig. 4e), reflecting the good durability of the bifunctional Ni-NSA-V_Ni electrodes. Despite a great advance in using value-added reactions to replace the OER in hydrogen production,^18,47–50 this work is the first report on the development of high-value organic oxidation as an alternative anodic reaction at a low voltage for ammonia synthesis.

Fig. 4 (a) LSV curves of the Ni-NSA-V_Ni anode at a scan rate of 5 mV s⁻¹ in 1.0 M KOH electrolyte with and without 0.5 mmol 1a. (b) Time-dependent in situ Raman spectra of Ni-NSA-V_Ni recorded at 1.4 V versus RHE in 0.1 M KOH solution without 1a (initial) and after the addition of 1a. (c) The substrate scope of the selective semi-dehydrogenation of THIQs over a Ni-NSA-V_Ni anode. (d) Schematic illustration for coupling electrocatalytic semi-dehydrogenation of THIQs with nitrite electroreduction to ammonia. (e) Cycle-dependent selectivities of 2a and ammonia over the Ni-NSA-V_Ni bifunctional electrodes.

Conclusions

In conclusion, we successfully introduced Ni vacancies into Ni nanosheets through the H₂-plasma treatment. Ni vacancies could not only promote the cathodic selective nitrite electroreduction to ammonia (faradaic efficiency: 88.9% and selectivity: 77.2%), but also improve the anodic semi-dehydrogenation transformation of THIQs into DHIQs (selectivity: 98.0%; faradaic efficiency: 95.5%) over the Ni-NSA-V_Ni electrodes. The combined results of in situ and ex situ experiments and theoretical calculations revealed the enhanced mechanism of vacancies. The vacancy-induced decrease in the electronic cloud density of Ni improved the adsorption of NO₂⁻ and the desorption of NH₃, leading to high catalytic performance for nitrite reduction. Moreover, the Ni^II/Ni^III active species for the semi-dehydrogenation of THIQs were maintained well during the electrooxidation process. Notably, the bifunctional Ni-NSA-V_Ni electrodes can be assembled into a two-electrode electrolyzer for simultaneously producing ammonia and DHIQs with robust stability and high selectivity. This work opens a facile electrocatalytic strategy to integrate nitrite-to-ammonia reduction with the oxidative transformation of organic compounds with high efficiency.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 22071173 and 21871206).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details, supplementary characterization and electrochemical measurements. See DOI: 10.1039/d0ta09590g

‡ These authors contributed equally to this work.

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