Efficient electrochemical coupling of nitrate and biomass-derived acetone to acetoxime at a high current density over a Zn/Cu hexagonal nanosheet catalyst

Abstract

Acetoxime, as an important type of organic compound containing a C=N bond, is commonly employed as a boiler chemical deoxygenation agent and boiler acid pickling passivator due to its low toxicity and high reducibility characteristics. The conventional synthesis of acetoxime involves the initial catalysis of NO_x and H₂ under harsh conditions by noble metal catalysts to generate unstable NH₂OH, which further reacts with acetone to form acetoxime. Herein, we report a Zn/Cu/CF hexagonal ultrathin nanosheet catalyst capable of efficiently catalyzing the coupling reaction between NH₂OH produced in situ from electrocatalytic NO₃⁻ reduction and CH₃COCH₃ to generate an oxime under ambient conditions. The presence of Cu facilitates the conversion of NO₃⁻, while the introduction of Zn is favourable to accelerate the electron transfer rate, enhance the number of active sites, and suppress the reduction hydrogenation of *CH₃COCH₃, thereby significantly improving the activity of oxime. Through catalyst screening and the optimization of electrolysis conditions, a faradaic efficiency of 44.5% with a yield rate of 415.5 μmol cm⁻² h⁻¹ is achieved at an industrially relevant current density of −150 mA cm⁻² in the electrolyte containing 0.2 M NO₃⁻ + 0.2 M CH₃COCH₃ over the Zn/Cu-CF-6 electrocatalyst, corresponding to a C selectivity approaching 100%. A series of control experiments and in situ characterization studies indicate that *NH₂OH and *CH₃COCH₃ are crucial intermediates for the synthesis of acetoxime, proposing a plausible reaction pathway. This work presents a facile synthesis method for Zn/Cu/CF hexagonal nanosheet catalysts and proposes a green strategy for the one-pot efficient electrochemical synthesis of acetoxime from pollutants NO₃⁻ and biomass-derived acetone, offering a green pathway for high-value-added C=N product synthesis.

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Introduction

Acetoxime, a significant chemical raw material, is employed in a multitude of industrial applications.^1–3 (1) Due to its low toxicity and high reducibility, it is commonly employed as a boiler chemical deoxygenation agent, replacing the more toxic hydrazine;^4,5 (2) as a passivator after boiler acid pickling, it effectively prevents secondary corrosion of metals, and avoids the use of toxic passivators such as ammonia and sodium nitrite;⁶ (3) as an organic synthesis intermediate, it belongs to a crucial class of organonitrogen compounds used in pharmaceuticals, pesticides, fuels, and organosilane coupling agent synthesis;^7,8 (4) it is also used as an analytical reagent for measuring Ni and Co metal elements.^9,10 Currently, the traditional synthesis of acetoxime mainly involves two steps. The initial step is to utilize a reducing agent such as H₂ under the catalysis of the noble metal catalyst Pd to reduce nitrogen oxides (NO_x) to NH₂OH (route 1 in Fig. 1a).^11,12 Due to its easy decomposition and explosive properties,¹³ hydroxylamine requires to be further neutralized with H₂SO₄ or HCl to obtain hydroxylamine hydrochloride or hydroxylamine sulfate for easy storage and transportation, alternatively, the Raschig process can be employed to reduce bisulfite and nitrite to hydroxylamine sulfate. The above strategies lead to low yield, high energy consumption and more waste emission.^14–16 The second step involves the reaction of acetone with hydroxylamine hydrochloride or hydroxylamine sulfate under alkaline conditions to produce acetoxime.^17,18 Apparently, these methods involve complex procedures, the use of toxic reducing agents or noble metal catalysts, and the generation of large amounts of chloride or sulfate by-products.¹⁹ Recently, researchers have developed alternative strategies, and acetone ammoxidation is a commonly used method in the industry,²⁰ where H₂ and O₂ are first synthesized into oxidant H₂O₂ using a Pd catalyst under high temperature and pressure, and then, NH₃ is oxidized by H₂O₂ to generate NH₂OH in situ over a titanium silicalite molecular sieve (TS-1) catalyst, which further nucleophilically attacks acetone to produce acetoxime^21–23 (route 2 in Fig. 1a). However, this strategy requires excess NH₃ and H₂O₂ and suffers from slow kinetics which needs to be compensated by raising the temperature and pressure, leading to more energy consumption.²⁴ Moreover, NH₃ is usually produced by energy-intensive industries,^25–27 making the strategy of synthesizing acetoxime from NH₃ and acetone less environmentally friendly. Therefore, there is an urgent need to establish a green synthesis pathway for acetoxime and develop low-cost, high-performance electrocatalysts to achieve energy-saving processes for one-pot synthesis of acetoxime from environmental waste NO_x and biomass-derived acetone.

Fig. 1 (a) Schematic diagram of two traditional routes for the synthesis of acetoxime. (b) Schematic diagram of direct electrosynthesis of acetoxime from waste nitrate and biomass-derived acetone driven by renewable energy sources.

Global warming and energy crises, exacerbated by fossil fuel consumption, as well as environmental pollution, necessitate a transition in modern industry from fossil fuel dependency to using renewable energy sources.^28,29 Electrocatalytic technology, considered as an important means to alleviate energy and environmental issues, driving green innovations in the synthesis of high-value compounds,^30,31 has been extensively researched in recent years.^32,33 Among the compounds, organonitrogen compounds driven by renewable energy have attracted widespread attention due to their significant academic research and economic application value.^34–42 Currently, the top 200 pharmaceuticals and the top ten agricultural chemicals globally are organonitrogen compounds.⁴³ Since Jiao successfully constructed acetamide from CO and NH₃ using an electrocatalytic method over Cu-based catalysts,⁴⁴ the green synthesis of organonitrogen compounds using readily available carbon/nitrogen-containing small molecules under mild conditions has become an emerging field.^8,45–48 In the field of electrosynthesis of oximes, researchers have successfully synthesized cyclohexanone oxime,^49,50 benzaldoxime,⁵¹ formaldoxime,⁵² cyclopentanoxime,⁵³ and pralidoxime⁴² from C-containing (cyclohexanone, benzaldehyde, formaldehyde, acetone, pyridine aldehyde) and N-containing (NO₃⁻, NO₂⁻, NO) small molecules as reactants. Although there has been some progress in the electrocatalytic C–N/C=N field, this reaction typically entails multi-step elementary reactions, rendering it a complex catalytic system. For instance, challenges such as preventing the individual reduction of the N or C source to produce separate N/C-containing products, regulating the formation of high-value C=N bond oximes from unstable key intermediates, and inhibiting the further reduction of oximes to secondary amines under a high current density all need to be addressed.^38,54 Hence, establishing a suitable electrocatalytic system and comprehending the mechanism steps involved in the co-catalysis of NO₃⁻ and CH₃COCH₃ through screening and designing efficient electrocatalysts are crucial for enhancing the electrochemical activity of the desired product.

In this study, we report an efficient electrochemical approach for the one-pot synthesis of acetoxime from nitrate and acetone by employing foam copper-based hexagonal Zn/Cu nanosheets (Zn/Cu/CF) as the working electrode (Fig. 1b). This process utilizes water as the proton source and achieves the green electrosynthesis of acetoxime under mild conditions. Electrochemical characterization studies indicate that Zn/Cu/CF nanosheets possess a larger electrochemically active surface area and a faster electron transfer rate, enabling the efficient electrochemical synthesis of acetoxime by capturing *NH₂OH generated in situ from NO₃⁻ electroreduction by *CH₃COCH₃. Through a series of condition optimizations, a faradaic efficiency of 44.5% and a yield rate of 415.5 μmol cm⁻² h⁻¹ are achieved at a current density of −150 mA cm⁻². Additionally, under such an industrial-relevant current density (>100 mA cm⁻²), oximes are not further reduced to secondary amines, and the C selectivity of acetoxime in the carbon-containing liquid products approaches 100% (with minimal isopropanol production, FE < 1%). A continuous 10-cyclic test and catalyst characterization results after the reaction demonstrate high catalyst durability. Results of a series of control experiments, in situ electrochemical attenuated total reflectance-infrared absorption spectroscopy (ATR-IRAS), and online differential electrochemical mass spectrometry (DEMS) indicate that *NH₂OH generated in situ from NO₃⁻ nucleophilically attacks *CH₃COCH₃ to produce the target product, oxime. The introduction of Zn not only transforms the morphology of Cu nanowires into Zn/Cu hexagonal nanosheets, but also boosts the electrochemically active surface area, accelerates electron transfer, enhances *NH₂OH generation, diminishes *CH₃COCH₃ hydrogenation, and thus improves the electrosynthesis performance of acetoxime.

Results and discussion

Materials synthesis and characterization

Electrocatalytic co-reduction of acetone and NO₃⁻ for C=N coupling selective formation of oximes (e-NARR) was tested in an H-type cell containing 30 mL of 0.5 M KOH electrolyte, where both acetone and NO₃⁻ were added at a concentration of 0.2 M. At the initial stage of our investigation, we discovered that acetoxime possessed two chemical environments for its methyl groups under strong alkaline conditions, corresponding to the appearance of double peaks (∼1.72, 1.67 ppm) of the ¹H nuclear magnetic resonance (¹H NMR) spectrum. However, as the pH of the electrolyte gradually decreased, the proton promoted the cis–trans isomerization of the C=N double bond and the chemical environment of the two methyl groups of acetoxime tended to be the same, and thus, only one peak (∼1.78 ppm) could be observed from the ¹H NMR spectra (Fig. S1†). In view of this, to standardize the quantitative results, the electrolyte was bubbled with CO₂ to a pH of approximately 8 at the end of each test. At a constant current density of −100 mA cm⁻², Co, Fe, Ag, Zn, Pb, Sn, Pt, Ni, and Cu catalysts were screened via a 2 h electrochemical performance test. Acetoxime and isopropanol were the only two categories of liquid C-containing products identified by ¹H NMR. Therefore, standard curves of isopropanol and acetoxime at different concentrations were obtained by 400M ¹H NMR for the qualitative analysis of carbon-containing liquid products (Fig. S2†). Among the screened metal catalysts, Zn (FE = 19.2%, yield rate = 119.5 μmol cm⁻² h⁻¹) and Cu (FE = 13.9%, yield rate = 86.6 μmol cm⁻² h⁻¹) exhibited the most favourable performance, with a C selectivity approaching 100% (Fig. S3 and 4†). Although the Zn catalyst exhibits optimal performance in catalyzing the synthesis of acetoxime from acetone and NO₃⁻ undergoing C=N coupling, Zn is an amphoteric metal which reacts with OH⁻ in alkaline systems to produce metazincate (ZnO₂²⁻) dissolved in the electrolyte, diminishing the durability of the catalyst (<4 h at −100 mA cm⁻²) (Fig. S5†).

Accordingly, this work envisages well-designed Zn/Cu-based nanocatalysts that are expected to accelerate the electron transfer rate, enhance the reaction kinetics, increase the electrochemically active sites of the catalyst to improve the performance of acetoxime electrosynthesis and prolong the lifetime of the catalyst. Consequently, a combination procedure of alkaline etching, wet impregnation, calcination, and in situ electrochemical conversion was employed to prepare Zn/Cu hexagonal nanosheet electrocatalysts (Fig. 2a), which was achieved by first utilizing porous copper foam (CF) with a larger specific surface area as the substrate, followed by treatment of the CF with an alkaline oxidative etching solution (AOES) until a light blue color appeared on the surface⁵⁵ (Fig. S6–8†). X-ray diffraction (XRD) and scanning electron microscopy (SEM) results demonstrated that Cu(OH)₂ nanowires were obtained after etching (Fig. S9 and S10†). Cu(OH)₂ nanowires were then immersed in a Zn²⁺ solution for different periods (3, 6, 12 h) to facilitate the attachment and deposition of Zn ions. The results of SEM images revealed that a transformation was observed in the structural arrangement of Cu(OH)₂/CF nanowires, undergoing a transition to that of Zn²⁺/Cu(OH)₂/CF hexagonal ultrathin nanosheets (Fig. S11†), which may be more favourable for the adsorption of reactants and electron transfer, and the XRD results indicated the presence of Zn(OH)₂/Cu(OH)₂ over the catalyst surface (Fig. S12†), possibly attributed to a partial hydrolysis reaction of Zn²⁺ during the process. The samples were then named according to the different impregnation times, respectively, Zn²⁺/Cu(OH)₂/CF-3, Zn²⁺/Cu(OH)₂/CF-6, and Zn²⁺/Cu(OH)₂/CF-12. In order to stabilize Zn²⁺ on the surface of Zn²⁺/Cu(OH)₂/CF, calcination was carried out at 220 °C under an air atmosphere for a period of two hours. The XRD results demonstrated that the phases on the surface of the catalysts had undergone a transformation into ZnO and CuO (Fig. S13†), and the SEM images showed that the catalysts retained their hexagonal nanosheet morphology and the stacked agglomerations of multiple nanosheets transformed into nanoflowers (Fig. 2b and S14†). The materials obtained at this juncture were named ZnO/CuO/CF-3, ZnO/CuO/CF-6, and ZnO/CuO/CF-12, respectively. Subsequently, the obtained materials were in situ reduced with 0.25 M Na₂SO₄ as the electrolyte at a current density of −36 mA cm⁻² for 45 minutes. The SEM results demonstrated that their morphology retained the hexagonal nanosheet structure (Fig. 2c and S15†), with the nanoflower structure of the clusters unfolding, possibly exposing an increased number of electrochemically active sites. XRD analysis revealed that the phases of catalysts had transformed into Zn and Cu metallic states at this stage (Fig. 2d). The Zn/Cu/CF catalysts were finally obtained, which were designated as Zn/Cu/CF-3, Zn/Cu/CF-6, and Zn/Cu/CF-12.

Fig. 2 (a) Schematic diagram of the synthesis of CuNWs/CF and Zn/Cu/CF catalysts. SEM images of (b) ZnO/CuO/CF-6 and (c) Zn/Cu/CF-6. (d) XRD patterns of different Zn/Cu/CF electrocatalysts. (e) Performance of acetoxime electrosynthesis using different Zn/Cu/CF catalysts. (f) SEM and the corresponding EDS images of Zn/Cu/CF-6. (g) TEM and (h) HRTEM images of Zn/Cu/CF-6. (i) Cu 2p and (j) Zn2p XPS results of ZnO/CuO/CF-6 and Zn/Cu/CF-6.

Then, the electrochemical performance of Zn/Cu/CF catalysts impregnated for different times was investigated under a constant current of −120 mA cm⁻². Zn/Cu/CF-6 exhibited the optimal acetoxime electrosynthesis performance, with a faradaic efficiency of 42.69% (Fig. 2e). Furthermore, the results of performance characterization studies at different current densities, linear scanning voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) all indicated that Zn/Cu/CF-6 exhibited significant electrochemical advantages, which will be discussed subsequently in this article. Consequently, more infinite characterization studies of Zn/Cu/CF-6 were conducted. SEM and the corresponding energy dispersive X-ray spectroscopy (EDS) revealed an ultrathin nanosheet morphology of the catalyst, with Zn and Cu elements distributed uniformly on the surface (Fig. 2f g and S16†). The TEM images also demonstrated the nanosheet structure of the catalyst. The corresponding EDS showed that the Zn content accounted for 19.5% (Fig. S17†), which was considerably higher than the inductively coupled plasma (ICP) result of 2.7% (Table S1†), further showing Zn was primarily located on the catalyst surface and uniformly distributed with Cu to form nanosheets. High-resolution transmission electron microscopy (HRTEM) revealed the presence of Cu (111) and Zn (100) lattice planes, confirming that Zn and Cu were distributed uniformly in their metallic state (Fig. 2h and S18†). Furthermore, X-ray photoelectron spectroscopy (XPS) was employed to analyze ZnO/CuO/CF-6 and Zn/Cu/CF-6, and compared with ZnO/CuO/CF-6, the binding energy of Cu2p shifted negatively by 1.2 eV and the characteristic satellite peak of Cu²⁺ disappeared (Fig. 2i), while a 0.6 eV positive shift appeared for the Zn2p binding energy (Fig. 2j) which indicated that Zn and Cu mainly existed in the metallic state. However, a small number of high-valence peaks of Cu2p still appeared after electroreduction, possibly due to the limited oxidation of Zn/Cu/CF-6 exposed to air. Additionally, the Cu LMM spectra showed that the kinetic energy was found to have a positive shift of 0.5 eV after electroreduction, which also demonstrated that Cu predominantly existed as the metal phase (Fig. S19†). In addition, control samples were prepared under identical synthesis conditions for the calcination of Cu(OH)₂NWs/CF to acquire CuO NWs/CF (Fig. S20 and S21†) and in situ electrochemical treatment to obtain CuNWs/CF (Fig. S22–24†).

Target product identification

Subsequently, Zn/Cu/CF-6, Hg/HgO, and Pt mesh were employed as working, reference and counter electrodes, respectively. The electrolyte environment consisted of a solution of 0.5 M potassium hydroxide, 0.2 M acetone, and 0.2 M nitrate ions. Electrolysis was performed for two hours at a current density of −150 mA cm⁻² and the resulting solution was analyzed via¹H NMR (Fig. 3a), gas chromatography (GC) (Fig. 3b), and ¹³C nuclear magnetic resonance (¹³C NMR) (Fig. 3c) to ascertain the successful synthesis of acetoxime. Furthermore, when ¹⁵N-labeled NO₃⁻ was used as the nitrogen source, gas chromatography–mass spectrometry (GC–MS) results revealed the presence of ¹⁵N-labelled acetoxime (m/z = 74.05), thereby indicating that the nitrogen in acetoxime originated from NO₃⁻ (Fig. S25†).

Fig. 3 (a) ¹H NMR results before and after electrolysis of acetone and NO₃⁻. (b) GC diagram of the electrolyte after electrolysis and the standard acetoxime sample. (c) ¹³C NMR spectra before and after electrolysis and the standard sample.

Performance optimization and electrochemical characterization

To optimize the performance of acetoxime from acetone and nitrate, the electrolyte pH and the concentration of NO₃⁻ were investigated. The electrolyte was initially switched from an alkaline system (0.5 M KOH) to a neutral (0.25 M K₂SO4) or acidic system (0.1 M H₂SO₄), and it was observed that the alkaline system was the most conducive to the formation of acetoxime with an FE_(acetoxime) of 19.7%, followed by the neutral system of 11.0%. However, only isopropanol was detected as the liquid carbon-containing product under acidic conditions which may be attributed to the protonation of the key intermediate NH₂OH to NH₃OH⁺, thereby diminishing its nucleophilic attack capability.⁵⁶ Moreover, the oxime cannot remain stable under acidic conditions and will decompose into ketone and NH₂OH spontaneously.⁵¹ Additionally, a higher adsorption of protons *H on the catalyst surface under acidic conditions facilitates the hydrogenation reduction of *CH₃COCH₃, resulting in the detection of only isopropanol (Fig. 4a). Subsequently, with the acetone concentration held constant at 0.2 M, the NO₃⁻ concentration was gradually increased from 0 to 500 mM. It was found that the FE of acetoxime increased as the NO₃⁻ concentration increased, while the performance of isopropanol was significantly inhibited. However, until reaching a NO₃⁻ concentration of 0.2 M, further increments showed minimal change (from FE_acetoxime = 19.7% at 0.2 M to FE_acetoxime = 21.2% at 0.5 M) (Fig. 4b). Thus, a NO₃⁻ concentration of 0.2 M was adopted for subsequent tests.

Fig. 4 (a) Performance of carbon-containing liquids under different pH electrolyte conditions. (b) Performance of carbon-containing liquids with different NO₃⁻ concentrations. (c) The current density-dependent faradaic efficiency and the yield rate of products under the optimized conditions (0.5 M KOH + 0.2 M CH₃COCH₃ + 0.2 M NO₃⁻) over the Zn/Cu/CF-6 catalyst. (d) The partial current density of acetoxime (j_(acetoxime)) under different applied current densities. (e) LSV curves and the corresponding (f) Tafel slopes for different catalysts. (g) LSV and the corresponding (h) Tafel slopes for different electrolytes using Zn/Cu/CF-6. (i) Nyquist plots of different catalysts.

Performance tests were conducted over different catalysts: CuNWs/CF, Zn/Cu/CF-3, Zn/Cu/CF-6, Zn/Cu/CF-12, and Zn, under different current densities (−20, −50, −80, −120, −150, −200 mA cm⁻²). Zn/Cu/CF-6 exhibited the optimal performance, achieving a faradaic efficiency of 44.5% with a productivity of 415.5 μmol cm⁻² h⁻¹ at −150 mA cm⁻², corresponding to a partial current density of −66.8 mA cm⁻² for acetoxime (Fig. 4c and d), outperforming the results of most of the reported articles in aspects of FE and current density (Table S2†), while CuNWs/CF exhibited the least favourable performance, with an optimal FE of 24.8% and a corresponding yield rate of 123.4 μmol cm⁻² h⁻¹ at an applied current density of −80 mA cm⁻², and the incorporation of Zn resulted in a notable enhancement in the electrosynthesis performance of acetoxime (Fig. S26†).

As shown in Fig. 4e, LSV tests were conducted under optimal conditions (0.5 M KOH + 0.2 M CH₃COCH₃ + 0.2 M NO₃⁻) over different catalysts. Compared to CuNWs/CF and Zn, Zn/Cu nanosheets significantly improved the current density. At −100 mA cm⁻², the corresponding potentials were −0.52 V vs. RHE (Zn), −0.11 V vs. RHE (CuNWs/CF), −0.10 V vs. RHE (Zn/Cu/CF-3), 0.04 V vs. RHE (Zn/Cu/CF-6), and −0.01 V vs. RHE (Zn/Cu/CF-12), respectively, with Tafel slopes of 277.88 mV dec⁻¹ (Zn), 737.16 mV dec⁻¹ (CuNWs/CF), 218.05 mV dec⁻¹ (Zn/Cu/CF-3), 146.28 mV dec⁻¹ (Zn/Cu/CF-6), and 150.59 mV dec⁻¹ (Zn/Cu/CF-12) (Fig. 4f). Zn/Cu/CF-6 exhibited the highest current density and the lowest Tafel slope, implying the fastest electron transfer rate, which is likely more favorable for promoting the formation of acetoxime, consistent with the performance test results mentioned above. Similarly, LSV tests were conducted using Zn/Cu/CF-6 as the working electrode under different electrolyte conditions (Fig. 4g). In the absence of acetone and NO₃⁻ in the electrolyte, the HER occurred with a current density of −100 mA cm⁻² and a corresponding potential of −0.46 V vs. RHE. The introduction of nitrate resulted in a decrease in the potential by 580 mV, indicating that Zn/Cu/CF-6 is more conducive to the electrochemical nitrate reduction reaction (e-NO₃RR) kinetically. However, when sole acetone was added, the potential shifted negatively by 40 mV compared to the HER at −100 mA cm⁻², showing that the absorbed *CH₃COCH₃ was less susceptible to individual acetone electroreduction (e-ARR) to isopropanol, which facilitated subsequent C=N coupling reactions. The addition of both nitrate and acetone into the reaction system resulted in a decrease in current density compared to e-NO₃RR alone which demonstrated that acetone adsorbed on the catalyst surface may have occupied some of the active sites for e-NO₃RR, thereby reducing its reaction rate. Despite a decrease in current density after acetone addition, it remained significantly higher than individual ARR and HER, suggesting that coupling reactions between acetone and nitrate still occurred. The Tafel slope under different electrolyte conditions indicates the kinetic order to be e-NO₃RR > HER > e-ARR (Fig. 4h), further confirming that NO₃⁻ is more easily electro-reduced to the key nitrogen-containing intermediate, while acetone adsorbed on the catalyst surface hinders further hydrogenation to the by-product isopropanol, thus improving the electrosynthesis performance of acetoxime.

In addition, electrochemical impedance spectroscopy (EIS) of different catalysts was carried out to analyze the kinetics during the co-reduction process (Fig. 4i). The results showed that Zn/Cu/CF exhibited a smaller semicircular radius compared to CuNWs/CF and Zn electrodes, also indicating that the introduction of Zn significantly reduced the electron transfer impedance and increased the electron transfer rate. Furthermore, cyclic voltammetry (CV) tests were carried out in the non-faradaic region at different scan rates using different catalysts. The results showed that Zn/Cu nanosheets significantly increased the double layer capacitance (C_dl) and the electrochemically active surface area compared to CuNWs/CF and Zn. Specifically, Zn/Cu/CF-6 exhibited the highest electrochemically active surface area, approximately 9.5 times higher than CuNWs/CF and 4.8 times higher than Zn, signifying that more active sites existed on the catalyst surface, thereby increasing the probability of C=N coupling to form acetoxime (Fig. S27 and Table S3†).

Nitrogen/carbon-containing liquid by-products analysis

Understanding the by-products of the C=N coupling reaction is beneficial for assuming a plausible reaction mechanism and guiding the subsequent catalyst design. Therefore, we first analyzed the nitrogen-containing liquid by-products by carrying out UV-vis absorption spectroscopy colorimetric tests for different concentrations of NH₄⁺, NO₂⁻, and NH₂OH using sodium salicylate, p-amino benzenesulfonamide, and 8-quinolinol method,⁵⁷ respectively (Fig. 5a–c). Then the absorbance values at specific wavelengths (NH₄⁺∼650 nm, NO₂⁻∼540 nm, and NH₂OH∼705 nm) and the concentrations were linearly fitted for qualification (Fig. S28†). Results showed that NH₄⁺ was the main by-product at different current densities over various catalysts, and the faradaic efficiency of NO₂⁻ by-products was about 10% (Fig. 5d and e and S29†). Notably, the presence of NH₂OH was barely discernible after the reaction (Fig. S30†), which may be attributed to two factors, one is that NH₂OH is unstable in strongly alkaline systems, undergoing spontaneous decomposition into N₂ and NH₃,⁵⁸ and the other is that NH₂OH may be predominantly present on the catalyst surface in an adsorbed state, a hypothesis that will be discussed subsequently. In addition, the FE of isopropanol was less than 1% in the C-containing liquid by-products and the presence of isopropylamine was not detected in all the tests, indicating that acetoxime would hardly undergo further hydrogenation reduction. The ratios for the performance of acetone oxime as well as the main nitrogenous by-products NH₃ and NO₂⁻ were then compared between Zn/Cu/CF-6 and CuNWs (Fig. 5f). It was found that the introduction of Zn with a hexagonal nanosheet structure was able to achieve acetoxime electrosynthesis with remarkable selectivity and inhibit the generation of NH₃ from the e-NO₃RR, which was conducive to the reduction of subsequent purification and isolation costs.

Fig. 5 UV-vis absorption spectroscopy of different concentrations of (a) NH₃, (b) NO₂⁻, and (c) NH₂OH. The current density-dependent faradaic efficiency and the yield rate of carbon/nitrogen-containing liquid products using (d) CuNWs/CF and (e) Zn/Cu/CF-6. (f) Performance ratios of acetoxime, NH₃, and NO₂⁻ of Zn/Cu/CF-6 relative to CuNWs/CF at a current density of −150 mA cm⁻².

Stability test

To assess the stability of the electrocatalyst, Zn/Cu/CF-6 was employed as the working electrode to conduct 10 continuous cyclic stability tests under the conditions of 0.2 M CH₃COCH₃ + 0.2 M KNO₃, where each cyclic test lasted for 2 hours. As shown in Fig. 6a, the potential–time curve did not exhibit significant changes after 10 cycles. The ¹H NMR spectra of the liquid-phase carbon-containing products after each reaction cycle are presented in Fig. 6b. The faradaic efficiency and yield rate of acetoxime showed no noticeable variation (Fig. 6c). Subsequent to stability tests, a series of characterization studies were performed. ICP results indicated that the Zn content was 2.4 wt% (Fig. 6d), which was consistent with the pre-reaction content, suggesting that Zn remained stable after 10 cyclic tests. SEM, TEM, HRTEM, and EDS images of the catalyst showed no obvious changes in the morphology and both Zn and Cu were uniformly distributed on the catalyst surface (Fig. 6e and S31, 32†). The XRD results demonstrated that the catalyst structure remained unchanged (Fig. 6f), and post-reaction XPS analysis showed that Zn and Cu also persisted mainly in the metallic form on the catalyst surface (Fig. 6g and h). All the aforementioned results indicated that the structure and morphology of the catalyst remained almost unchanged before and after the stability tests (Fig. S33†), demonstrating excellent durability of Zn/Cu/CF-6 for the electrosynthesis of acetoxime from acetone and nitrate.

Fig. 6 (a) The potential–time curves of Zn/Cu/CF-6 from 1^st to 10^th cyclic stability test. (b) ¹H NMR spectra of ten cyclic tests. (c) The faradaic efficiency and the yield rate of acetoxime. (d) ICP results of Zn content of Zn/Cu/CF-6 before and after the stability tests. (e) SEM and the corresponding EDS results of Zn/Cu/CF-6 after the stability test. (f) XRD pattern of Zn/Cu/CF-6 after the stability test. Cu2p (g) and Zn2p (h) XPS results of Zn/Cu/CF-6 after the stability test.

Mechanistic analysis

To elucidate the mechanistic pathways involved in the electrosynthesis of acetoxime from acetone and nitrate, a series of control experiments were conducted to identify the key intermediates of both the carbon and nitrogen feedstocks. As shown in Table S4 and Fig. S34,† experiments were performed using 0.5M KOH as the electrolyte at −150 mA cm⁻². Initially, with acetone as the C source and NO₃⁻ as the N source, subsequent replacement of NO₃⁻ with NO₂⁻, N₂, NH₂OH, and NH₃ (entries 1–5 in Table S4†) revealed that no acetoxime was produced when NH₃ was used as the N source, suggesting NH₂OH was the crucial nitrogen-containing intermediate. However, acetoxime was undetectable when N₂ was employed as the nitrogen reactant, likely due to its low solubility in the electrolyte and the high dissociation energy of the N≡N bond (941 kJ mol⁻¹), making its activation challenging. Upon the introduction of hydroxylamine (NH₂OH) as the N source, a substantial amount of acetoxime was generated. However, the nucleophilic addition–elimination reaction between NH₂OH and acetone does not involve electron transfer, and thus, the yield rate was utilized to represent the performance of acetoxime (entry 4 in Table S4†). Further experiments with NO₃⁻ as the N source and isopropanol as the C source showed no acetoxime production, suggesting that *CH₃COCH₃ could be a key carbon intermediate (entry 6 in Table S4†). The cessation of the current application also resulted in no acetoxime production, showing acetoxime was formed driven by electricity (entry 7 in Table S4†). In order to ascertain whether C=N coupling occurs at the electrode surface or in the electrolyte due to the desorption of key intermediates, we conducted an examination to evaluate the presence of NH₂OH. It is worth noting that NH₂OH was almost undetectable in the post-reaction solution (Fig. S35†) and NH₂OH was unstable under alkaline conditions which decomposed into N₂ and NH₃ spontaneously,⁵¹ suggesting that C=N coupling occurred between the adsorbed *NH₂OH and *CH₃COCH₃ on the electrode surface, rather than *NH₂OH desorbing into the solution to couple with acetone. To elucidate the cumulative effect of Zn introduction on *CH₃COCH₃, electrochemical studies were conducted using Zn/Cu/CF-6, CuNWs/CF, and Zn as working electrodes to evaluate the individual activity of e-ARR (Fig. S36†). The results demonstrated that the incorporation of Zn inhibited the formation of isopropanol. Concurrently, considering the results from Fig. 4c and Fig. S26a,† it can be inferred that, compared to CuNWs/CF, the introduction of Zn was detrimental to the hydrogenation of *CH₃COCH₃, and thus, it is more prone to undergo a coupling reaction with surface-generated *NH₂OH, thereby promoting the formation of acetoxime.

Moreover, the Zn/Cu/CF-6 catalyst was employed as the working electrode for in situ ATR-IRAS and DEMS investigations to monitor the intermediates of e-NO₃RR and e-NARR. When only e-NO₃RR occurred, the peaks of *NH₂OH, *NO₂⁻, and *NH₄⁺ were detected at 1112 cm⁻¹,^59,60 1229 cm⁻¹,⁶¹ and 1458 cm⁻¹,^25,62,63 respectively (Fig. 7a), with 1880 cm⁻¹ and 2670 cm⁻¹ corresponding to the CaF₂ substrate.⁶⁴ Upon the introduction of CH₃COCH₃ into the system, the *NH₂OH signal significantly weakened (Fig. S37 and S38†), accompanied by the appearance of signals located at 1683 cm⁻¹ of the C=N bond,^40,53,65 1027 cm⁻¹ for the N–O stretching of C=N–OH,⁴¹ and 1227 cm⁻¹ attributed to the C–C–C asymmetric stretch peak of adsorbed *CH₃COCH₃^66,67 (Fig. 7b). To further verify the production of *NH₂OH with *CH₃COCH₃, quasi in situ electron paramagnetic resonance (EPR) trapping experiments of hydrogen and carbon radicals were conducted over Zn/Cu/CF-6 (Fig. S39†). During the e-NO₃RR test, only H radicals were detected, which were attributed to the protonation of NO₃⁻. With the addition of acetone, significant C radicals were observed, confirming the adsorption and activation of *CH₃COCH₃ on the electrode surface, which provides additional evidence for the production of *NH₂OH with *CH₃COCH₃. The results of the DEMS data indicated that when only NO₃⁻ was present in the solution, a significant signal rise/fall of m/z = 17 and m/z = 33 (corresponding to NH₃ and NH₂OH, respectively) was observed with the application/withdrawal of the current (Fig. 7c). Upon the introduction of acetone to the reaction system, the signals of m/z = 33 and m/z = 17 were significantly weakened (Fig. 7d), suggesting that acetone was able to capture the in situ formed *NH₂OH, which generated the target product oxime and inhibited the formation of the by-product NH₄⁺. Furthermore, CuNWs/CF was also tested for e-NO₃RR and e-NARR DEMS. The NH₂OH intensity was found to be weaker than that of Zn/Cu/CF-6 when only e-NO₃RR occurred (Fig. S40a†). This indicated that the introduction of Zn was conducive to the generation of NH₂OH. As for the introduction of acetone, the intensity of m/z = 33 also exhibited a significant attenuation (Fig. S40b†), meaning further reduction of *NH₂OH was diminished. The in situ ATR-IRAS and DEMS findings collectively suggested that Zn/Cu/CF-6 facilitated *NH₂OH generation and *CH₃COCH₃ accumulation. It can be concluded that the hydrogenation of *NH₂OH and *CH₃COCH₃ is less facile than the spontaneous C=N coupling reaction, enabling acetoxime electrosynthesis with high faradaic efficiency at large current densities. Based on the aforementioned conclusions and existing literature reports, we propose a plausible reaction pathway for the synthesis of acetoxime, as illustrated in Fig. 7e: NO₃⁻ → *NO₂ → *NO → *HNO → *HNHO → *NH₂OH, followed by C=N coupling between *NH₂OH and *CH₃COCH₃ to yield acetoxime.

Fig. 7 In situ ATR-IRAS spectra of Zn/Cu/CF-6 for (a) e-NO₃RR and (b) e-NARR. DEMS results of Zn/Cu/CF-6 for (c) e-NO₃RR and (d) e-NARR. (e) Proposed plausible reaction pathways of acetoxime from nitrate and acetone.

Conclusions

In summary, this work reports a method for the synthesis of hexagonal Zn/Cu/CF nanosheets through wet-impregnation, calcination, and in situ electroreduction. Through optimization of the electrolyte pH, precursor concentration, and current densities, a faradaic efficiency of 44.5% for acetoxime production was achieved at a near-industrial current density of −150 mA cm⁻², corresponding to a yield rate of 415.5 μmol cm⁻² h⁻¹ with a C selectivity approaching 100%. A series of electrochemical characterization studies revealed that the introduction of Zn leads to a faster electron transfer and a larger ECSA with more active sites and promotes the generation of *NH₂OH while inhibiting further hydrogenation of *CH₃COCH₃, thereby improving the performance of acetoxime electrosynthesis. Results of a series of control experiments, ATR-IRAS and DEMS indicate that the C=N bond arises from the coupling of key intermediates *NH₂OH and *CH₃COCH₃, and a reasonable pathway, NO₃⁻ → *NO₂ → *NO → *HNO → *HNHO → *NH₂OH → CH₃C=NOHCH₃, was proposed. This work not only provides a facile method for the synthesis of hexagonal Zn/Cu ultrathin nanosheets, but also elucidates the mechanism by which Zn/Cu synergistically promotes the electrosynthesis of acetoxime. Furthermore, a green transformation of the inorganic pollutant NO₃⁻ and biomass-derived acetone into a high-value organonitrogen chemical is achieved, which expands the research scope of electrocatalytic C–N coupling reactions and offers a new pathway for the one-step green synthesis of acetoxime from inorganic nitrogen sources.

Data availability

The datasets supporting this article have been uploaded as part of the ESI† (QI-RES-05-2024-001234).

Author contributions

Ya-Wen Zhang conceived the idea and directed the project. Jiang Shao designed the experiments and carried out the materials synthesis and characterization. Zi-Yuan Li and Xin-Li helped in the TEM data collection, Bin Liang contributed to the SEM image capturing, and Sheng-Zhi Xue contributed to the drawing of the schematic diagram and result discussion. Ting-Zhou Li assisted in the XPS test. Yi-Fei Zhang helped in the electrochemical tests and mechanism analysis. Jiang Shao wrote the paper. Ya-Wen Zhang and Hao Dong revised the manuscript. All authors discussed the results and commented on the manuscript.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was supported by the National Key R&D Program of China (No. 2023YFA1506800 and 2021YFA1501100), the National Natural Science Foundation of China (No. 22293042), and the Beijing National Laboratory for Molecular Sciences (BNLMSCXXM-202104). We greatly acknowledge Prof. Jie Zheng from Peking University for the help on the in situ ATR-IRAS and DEMS test and sincerely thank Dr Shuai Ma for his assistance with GC-MS.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01234h

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