Accelerating N₂H_4(ads) formation by frustrated Lewis pairs in an oxyhydroxide for electrocatalytic ammonia oxidation into N₂

Abstract

Conventional catalysts based on the individual Oswin and Salomon (O–S) or Gerischer and Mauerer (G–M) mechanism cannot achieve direct electrocatalytic ammonia (NH₃) oxidation into nitrogen (N₂) with high activity and selectivity. Herein, a bimetallic nickel–cobalt oxyhydroxide (Ni_0.5–Co_0.5-OOH) with frustrated Lewis pairs was developed through an elaborate analysis of the binding types of NH₃ with the metal-oxide anode, efficiently integrating O–S and G–M mechanisms for converting NH₃ into N₂ with high activity (94%) and selectivity (63%), which is much superior to the anodes in the previous reports. The evidence of batch experiments, in situ characterization, and theoretical calculations confirms that two NH₃ molecules bind to Co³⁺ sites (Lewis acid) in CoOOH and hydroxy sites (Lewis base) in NiOOH, respectively. Then, the NH_2(ads) generated on the Lewis acid sites can quickly recombine with the NH_2(ads) desorbed from the Lewis base sites, accelerating the formation of N₂H_4(ads) and preventing the peroxidation of NH₃. The electrocatalytic system assembled with the Ni_0.5–Co_0.5-OOH anode shows excellent performance for NH₃ elimination in the secondary aerobic process effluent. Our work provides precious guidance for the design of novel anodes and sheds light on further promoting the performance of ammonia conversion.

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Introduction

The electrocatalytic ammonia oxidation reaction (EAOR) is viewed as a promising conversion measure due to its ability to address the growing problem of ammonia-containing wastewater pollution.^1–4 Nitrate and nitrite are the common products during the EAOR process, which could exacerbate secondary pollution.^5–7 As a comparison, nitrogen (N₂) is the most desirable product of the EAOR.^8–11 Therefore, it is urgent to exploit effective strategies to efficiently convert ammonia into N₂.

Direct electrocatalytic oxidation, based on anodic electron transfer, has been widely applied to ammonia pollution purification.^12–14 It is more environmentally friendly than indirect oxidation, which is related to the electrochlorination process because there are no toxic secondary products (chlorine gas and chlorine-containing intermediates) produced.^15–17 Until now, two possible conversion paths of the EAOR by direct electrocatalytic oxidation have been proposed. One of them is the Oswin and Salomon (O–S) mechanism,¹⁸ in which the adsorbed NH₃ dehydrogenates to N_(ads) step by step and then converts into N₂ after the recombination of N_(ads) (eqn (1)–(5)). During this process, the active sites in the anode have high Lewis acidity and mainly bind to the N atom in NH₃, ensuring strong binding energy for subsequent dehydrogenation. Unfortunately, excessive binding prevents the generated N_(ads) from recombining, making it highly susceptible to peroxidation to generate NO₂⁻ and NO₃⁻.^19–21 The O–S mechanism demonstrates strong activity for the EAOR but extremely low selectivity for N₂ generation. The other reaction path for the EAOR is the Gerischer and Mauerer (G–M) mechanism,^22,23 in which the formation of N₂ is derived from the recombination of N_xH_y(ads) intermediates (eqn (1)–(3) and (6)). The G–M mechanism shows a higher selectivity for N₂ formation than the O–S mechanism because the anodic active sites have high Lewis basicity and mainly bind to the H atom in NH₃, which is easily desorbed from the anodic surface after dehydrogenation, further efficiently converting into N₂. However, the binding energy is relatively weak when the active site binds to the H atom in NH₃, resulting in low efficiency of NH₃ dehydrogenation to NH_2(ads) and sluggish EAOR kinetics.^24,25 The anode materials reported so far do not exhibit simultaneously high selectivity and high activity of the EAOR. Therefore, it is necessary to develop novel anode materials to realize the highly efficient and selective conversion of NH₃ into N₂ through direct oxidation.

NH_3(aq) → NH_3(ads) (1)

NH_3(ads) → NH_2(ads) + H⁺ + e⁻ (2)

NH_2(ads) → NH_(ads) + H⁺ + e⁻ (3)

NH_(ads) → N_(ads) + H⁺ + e⁻ (4)

N_(ads) + N_(ads) → N₂ (5)

NH_x(ads) + NH_y(ads) → N₂ + (x + y)H⁺ + (x + y)e⁻, x, y = 1 or 2 (6)

Inspired by the above mechanisms, it is found that the dehydrogenation of NH₃ into NH_2(ads) is the rate-limiting step of the EAOR, while the recombination of NH_x(ads) and NH_y(ads) is the determining step for the selectivity of N₂ generation.²⁶ Theoretically, if the dehydrogenation in the O–S mechanism and the recombination of NH_x(ads) and NH_y(ads) in the G–M mechanism can be integrated together, the oxidation of NH₃ into N₂ can be effectively achieved with high activity and selectivity. Among the numerous anode materials,^27–30 metal oxyhydroxides (MOOH) are the desired anode materials for the efficient and selective conversion of NH₃ into N₂ due to the presence of frustrated Lewis pairs.^31,32 In detail, the metal atoms as Lewis acid sites can preferentially bind to the N atom due to the strong interaction between empty d-orbitals and lone pair electrons in the N atom, while the oxygen-containing functional groups as Lewis base sites can bind to the H atom (O–H–N) through hydrogen bonding. NH_x(ads) generated on the Lewis acid sites will quickly recombine with NH_y(ads) desorbed from the Lewis base sites, which can prevent the peroxidation of NH₃, thereby promoting EAOR efficiency and N₂ generation selectivity. More importantly, the presence of surface hydroxy groups enables oxyhydroxides to effectively resist the erosion of Cl⁻, thereby demonstrating high reliability in actual wastewater treatment. Although a small number of hydroxyl oxide electrodes (NiOOH or FeOOH) have been reported for use in the EAOR process, the activity of the EAOR and the selectivity for N₂ generation are poor (below 30%).^33,34 Therefore, it is urgent to clarify the conversion law of NH₃ on different oxyhydroxides and develop highly active and selective oxyhydroxide anodes to achieve the EAOR.

Herein, a series of mono-metallic oxyhydroxide electrodes (MOOH, M: Mn, Co, Fe, Cu, Ni, and Zn) were synthesized, and their activity and selectivity of N₂ formation in the EAOR were investigated. Combined with the electronic structure of different transition metal ions, the transformation law between NH₃ and the active sites with different Lewis acidity/basicity was summarized. Based on this, a bimetallic oxyhydroxide was designed and synthesized, which can bind to the N and H atoms in NH₃, respectively, integrating the O–S and the G–M mechanisms together, further enhancing the EAOR efficiency and the selectivity of N₂ formation. The synergistic effect of the bimetallic oxyhydroxide was explored by batch performance evaluation and structural characterization. Moreover, through XPS, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and theoretical calculations, the conversion mechanism of NH₃ mediated by the bimetallic oxyhydroxide was further demonstrated. This work provides a new avenue for the design of anodic materials for the EAOR, which is in favor of the technological development of ammonia pollution purification.

Materials and methods

Materials and characterization

All chemicals were commercially obtained and used without further purification and the detailed information of materials is shown in Text S1 in the SI. The results of electrochemical analysis and characterization for catalyst structure and morphology are displayed in Text S2 in the SI. The evolution of conversion performance is displayed in Text S3 in the SI. The analytical methods for electrochemical characterization are displayed in Text S4 in the SI.

Preparation of the anodes

Nickel foam (NF) was first cleaned through sonicating consecutively in acetone, 3 M HCl, ethanol, and deionized water for 15 min to remove the oxide layer and degrease the surface of the NF. Then, the treated NF was directly used as the substrate to deposit Ni–Co precursors. In a typical procedure of Ni–Co precursor fabrication, 2.0 mmol (582.06 mg) of Co(NO₃)₂·6H₂O, 2.0 mmol (497.68 mg) of Ni(CH₃COO)₂·4H₂O, 3.75 mmol (138.8 mg) of NH₄F and 15 mmol (900.7 mg) of CO(NH₂)₂ were well dissolved in 50 mL of deionized water to form a mixture solution. The obtained solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave. After immersing the washed NF (1.5 × 4 cm²) substrate in the homogeneous solution, the autoclave was sealed and heated at 90 °C for 7 h. After the autoclave cooled down to room temperature, the Ni–Co precursor loaded NF was taken out from the autoclave and rinsed with ultrapure water several times. The black electrode of Ni–Co oxyhydroxide was obtained by immersing the precursors in 5% NaClO solution as an oxidant for 30 min. The chemical oxidation of the precursors was carried out at pH 3–4 adjusted with H₂SO₄ or NaOH.

Catalytic experiments

The initial concentration of ammonia was 20 mg N L⁻¹ and 50 mM Na₂SO₄ electrolyte was added into a 100 mL undivided cylindrical glass electrolytic cell. The different anodes and the Pt cathode were fixed at a gap of 1 cm in the reactor and connected to a potentiostat (CHI 660E electrochemical workstation (Shanghai, Chenhua, China)). Ag/AgCl was used as the reference electrode. The immersed area of the electrode was 4.5 cm². During the reaction process, the samples were taken with syringes at designated time intervals and immediately filtered through 0.45 μm polytetrafluoroethylene (PTFE) membranes. All experiments were conducted three times.

Analysis methods

The concentration of ammonia was measured by Nessler's reagent colorimetric method. The samples were distilled first and the absorbance was obtained from a UV-vis spectrophotometer (UV-9000S, Metash Instruments Ltd, China) at the wavelength of 420 nm.³⁵ The concentrations of nitrate (NO₃⁻) and nitrite (NO₂⁻) were determined by the ultraviolet spectrophotometry and N-(1-naphthyl)-ethylenediamine dihydrochloride spectrophotometric method, respectively.³⁶ The concentration of total nitrogen (TN) was measured by the traditional alkaline persulfate oxidation method.³⁷ The pH value was measured with a pH meter (Mettler Toledo-FE28, Swit). The ammonia removal (R) and selectivity of nitrogen conversion (S_N) were calculated using equations below:where [N] denotes the concentration of nitrogen products, including [NO₂⁻–N], [NO₃⁻–N] and [N₂–N].

Current efficiency is expressed as eqn (9):

Here, C₀ is the initial concentration of NH₃–N, C_t is the concentration of NH₃–N at degradation time t (s), V is the volume of the electrolyte (0.1 L), M is the molar mass of NH₃–N (14 g mol⁻¹), I is the current density (A m⁻²), A is the effective area of the electrodes (4.5 m⁻²), F is the Faraday constant (96 485.3 C mol⁻¹) and n is the number of electrons needed for the oxidation of one mole of ammonia (n = 3).

The energy consumption (EC, kWh m³) was calculated through eqn (10).

where U is the applied voltage (V), I is the average current (A), t is the time (h), V is the solution volume (L), and log(C₀/C_t) is the logarithm of the initial and instant concentrations of the ammonia.

Theoretical calculation

The present first principle DFT calculations are performed with the Vienna Ab initio Simulation Package (VASP) with the projector augmented wave (PAW) method. Detailed information about the calculation procedure is provided in Text S5 in the SI.

Results and discussion

Lewis acidity/basicity on NH₃ conversion

The structure characterization studies of different MOOH are shown in Fig. S1 and S2. The prototypical MOOH oxyhydroxide has a typical cubic crystal structure. Metal atoms are located at the centre of MO₆ octahedra and the layers are linked by hydrogen bonds (Fig. 1a). When there are many unoccupied d orbitals, metal atoms in MOOH can bind with N atoms in NH₃ during the adsorption process.³⁸ In contrast, the O atoms in MOOH may bind with H in NH₃ through a hydrogen bond (Fig. 1b).³⁹ With the outer 3d orbitals being gradually filled by electrons, the Lewis acidity of metal sites decreases and the Lewis basicity of hydroxy sites increases (Fig. 1c, e and S3). The EAOR performance of MOOH (from Mn³⁺ to Zn³⁺) increases first and then decreases (CoOOH > MnOOH > NiOOH > FeOOH > ZnOOH > CuOOH). When the d orbitals of the metal atom in MOOH are before half-filled, the adsorption energies of NH₃ at the metal sites are higher than those at the hydroxy sites (Fig. 1d), which can strongly bind with the N atom in NH₃, thus giving it strong EAOR performance. However, the overly strong force leads to the peroxidation of NH_x(ads) intermediates into NO₂⁻/NO₃⁻, which follows the typical O–S mechanism and exhibits low N₂ formation selectivity. As a comparison, after the d orbital is half-filled, NH₃ adsorbs more strongly at the hydroxy sites than at the metal sites. Though the EAOR performance of MOOH is relatively sluggish, the N₂ formation selectivity is dramatically increased, which can be attributed to the change of the catalytic mechanism from O–S to G–M. In the two stages before and after a half-filled electronic orbital, CoOOH and NiOOH exhibited optimal Lewis acidity/basicity and superior activity and selectivity for the EAOR compared to other MOOH or bimetallic systems (Fig. S4), respectively. Hence, Co and Ni elements were used to assemble the subsequent bimetallic oxyhydroxide catalysts to integrate the O–S and G–M mechanisms on a catalyst.

Fig. 1 (a) Crystal structure of MOOH; (b) different adsorption types with NH₃; (c) EAOR activity and selectivity of MOOH with different transition metal ions (E = 1.8 V, pH = 11, 0.05 M Na₂SO₄); (d) calculated adsorption energies of ammonia molecules at metal and hydroxyl sites in mono-metallic oxyhydroxides; (e) diagram of d orbital electron configuration of different transition metal ions.

Synthesis and characterization of Ni–Co-OOH

The Ni–Co oxyhydroxide (Ni–Co-OOH) was synthesized following the schematic diagram in Fig. 2a. The XRD pattern of Ni–Co-OOH shows the presence of characteristic peaks attributed to NiOOH and CoOOH, respectively (Fig. S5). As shown in Fig. 2b, Raman spectra further demonstrate that the peaks located at 474 and 557 cm⁻¹ are attributed to the E_g bending and A_1g stretching vibrations of Ni–O of NiOOH, respectively, whereas the peaks observed at 506 and 680 cm⁻¹ are assigned to the CoOOH in Ni–Co-OOH.^40,41 The Raman spectra show a slight shift in the vibrational frequencies of Ni–O and Co–O bonds compared to NiOOH and CoOOH, indicating there is electronic interaction between NiOOH and CoOOH. Besides, X-ray photoelectron spectroscopy (XPS) shows that two peaks in Co 2p_3/2 at 780.8 and 781.9 eV are assigned to Co³⁺ and Co²⁺,^42–44 respectively. For Ni 2p_3/2 spectra, the peaks at 855.36 and 856.66 eV are attributable to the Ni²⁺ and Ni³⁺,^45–47 respectively (Fig. 2c and d). Ni/Co–O bonds are also identified by the characteristic peak at 529.7 eV from O 1s spectra (Fig. S6). The peaks of Ni³⁺ and Co³⁺ are observed to shift by 0.3 eV and 0.5 eV, respectively, towards a lower binding energy compared to their mono-metallic oxyhydroxide, proving the electron transfer from Co³⁺ to Ni³⁺. The coupling of Ni³⁺ and Co³⁺ via the bridging O²⁻ facilitates π-donation from O²⁻ to Ni³⁺, resulting in the overall electron transfer from Co³⁺ to Ni³⁺, which is in agreement with the Raman spectra results. In addition, scanning electron microscopy (SEM) images show that the morphology of Ni–Co-OOH is a nanoplate structure covered with nano-flower spheres, where NiOOH and CoOOH are nanoplates and nano-flower spheres, respectively, confirming that Ni–Co-OOH is a composite and CoOOH is loaded on the surface of NiOOH (Fig. 2e, S7 and S8). A further inspection of these nanosheet structures with high-resolution TEM and spherical aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was conducted. As shown in Fig. 2f and g, the images demonstrate the heterojunction region of Ni–Co-OOH, indicating that there are two different phases on both sides of the interface. The lattice distances are 1.6, 2.1 and 2.3 Å, which are consistent with the (2 2 0) facet of NiOOH, (0 0 6) facet of CoOOH and (1 0 2) facet of NiOOH, respectively. Therefore, the aforementioned characterization evidently confirms the successful preparation of the Ni–Co bimetallic oxyhydroxide catalyst by a two-stage method of hydrothermal and chemical oxidation and it is a biphasic composite with a heterojunction structure.

Fig. 2 (a) Synthesis diagram of Ni–Co-OOH; (b) Raman spectra of Ni–Co-OOH, CoOOH and NiOOH and nickel foam (NF); (c) Ni 2p XPS of Ni–Co-OOH and NiOOH; (d) Co 2p XPS of Ni–Co-OOH and CoOOH; (e) SEM images of Ni–Co-OOH; (f) TEM images of Ni–CoOOH; (g) HAADF-STEM image of Ni–Co-OOH.

EAOR activity, N₂ selectivity and practical application

The EAOR activity of CoOOH and NiOOH is 93.8% and 24.1%, whereas the selectivity of N₂ is 1.3% and 35.7% and the selectivity of NO₃⁻ is 74% and 24%, respectively, which typically follows the O–S and G–M mechanisms, respectively (Fig. 3a). Ni_x–Co_1−x-OOH catalysts with different Ni/Co ratios were obtained by varying the precursor dosages based on the similar solubility products of Ni(OH)₂ and Co(OH)₂ (k_sp(Ni(OH)₂) = 2 × 10⁻¹⁵, k_sp(Co(OH)₂) = 1.6 × 10⁻¹⁵) (Table S1). Ni_0.5–Co_0.5-OOH (Ni : Co = 0.5 : 0.5) exhibits higher EAOR activity (94.0%) and N₂ formation selectivity (63.0%) than Ni_0.75–Co_0.25-OOH, Ni_0.25–Co_0.75-OOH and the physically mixed CoOOH/NiOOH, which is superior to the electrocatalysts reported in the previous literature (Fig. S9–S11 and Table S2).^48–54 Since the strong interaction between the Co-based Lewis acid sites and the N atom can cause difficulty in the desorption of N-containing intermediates from the anodic surface, the introduction of excessive CoOOH into NiOOH inhibits the formation of N₂, while generating more NO₃⁻ during the EAOR process. In addition, as shown in Fig. 3b, the NiOOH anode shows low faradaic efficiency (7.5%) and high energy consumption (150 kWh kg N⁻¹) for NH₃ conversion. Conversely, the current efficiency of Co-based oxyhydroxides significantly improves to 20–40%, while the low N₂ selectivity makes the NO₂⁻ and NO₃⁻ predominate the energy consumption. The Ni_0.5–Co_0.5-OOH can reach the highest faradaic efficiency (42%) and the energy consumption reaches a minimum value of 26.3 kWh kg N⁻¹, which further verifies the superior activity of Ni_0.5–Co_0.5-OOH for the EAOR.

Fig. 3 (a) EAOR activity and product selectivity with different anodes (E = 1.8 V, pH = 12, 0.05 M Na₂SO₄); (b) Faradaic efficiency and energy consumption of the EAOR with different anodes (E = 1.8 V, pH = 12, 0.05 M Na₂SO₄); (c) EAOR activity and product selectivity of Ni_0.5–Co_0.5-OOH under different conditions; (d) ten cycles of Ni_0.5–Co_0.5-OOH (E = 1.8 V, pH = 12, 0.05 M Na₂SO₄); (e) EAOR activity of Ni_0.5–Co_0.5-OOH for practical wastewater (E = 2.0 V, 0.05 M Na₂SO₄).

With the increase of applied voltage from 1.6 V to 2.1 V vs. RHE, the EAOR activity of Ni_0.5–Co_0.5-OOH increases, while the N₂ formation selectivity is optimal (63.0%) at 1.8 V (Fig. 3c and S12). When excessive voltage is applied, the anodic oxygen evolution reaction (OER) may be severe, and NH₃ can be easily peroxided into NO₃⁻ and NO₂⁻, resulting in a low N₂ formation selectivity. Besides, the removal of NH₃ and the formation selectivity of N₂ is significantly inhibited with increasing NH₃ concentrations (Fig. 3c and S13), which may be due to the saturation of the active sites on the electrode surface. In addition, the oxidation efficiency of NH₃ increases from 3.9% to 99.3%, whereas the N₂ formation selectivity declines from 100% to 38.4% with the increase in pH from 10 to 13 (Fig. 3c and S14). At a relatively low pH, NH₃ can be protonated into NH₄⁺ (pK_a(NH₄⁺/NH₃) = 9.25), leading to the sluggish anodic oxidation of NH₄⁺ caused by electrostatic repulsion. In contrast, at a high pH, N₂ production selectivity is significantly inhibited due to the serious OER and NH₃ peroxidation (eqn (S4)–(S10)) at the anode, leading to the generation of NO₂⁻ and NO₃⁻. These results indicate that the best performance of the Ni_0.5–Co_0.5-OOH anode can be obtained when the voltage is 1.8 V, the NH₃ concentration is 20 mg L⁻¹ and the pH is 12.

To evaluate the practical application of the Ni_0.5–Co_0.5-OOH anode, ten cycles of experiments were carried out. As shown in Fig. 3d, the NH₃ conversion is around 90%, while the N₂ formation selectivity is still over 60% within 10 cycles, indicating that Ni_0.5–Co_0.5-OOH shows remarkable stability. Besides, there are no leaching concentrations of cobalt/nickel ions detected even in chloride-containing solutions and no change is observed in the metal value state and structure during ten cycles (Fig. S15–S18), which further confirms the stability of Ni_0.5–Co_0.5-OOH. Unlike the lattice oxygen mechanism involved in the OER processes (Fig. S19), surface reconstruction does not occur during the EAOR process. In addition, the electrocatalytic system assembled with the Ni_0.5–Co_0.5-OOH anode was applied to treat the practical ammonia-containing wastewater, which was the secondary aerobic process effluent. As shown in Fig. 3e and S20, the NH₃ conversion is around 99%, while the N₂ formation selectivity (55%) is much greater than that of nitrate (31%) and nitrite (11%). In conclusion, the Ni_0.5–Co_0.5-OOH electrode has excellent stability and shows good application potential in the treatment of practical ammonia-containing wastewater.

Electrochemical characterization

Diverse electrochemical characterization studies were performed to explore the EAOR activity of Ni_0.5–Co_0.5-OOH. As shown in the linear sweep cyclic voltammetry (LSV) and Tafel curves (Fig. 4a and S21–S23), a lower onset potential (1.28 V) and a lower Tafel slope (128 mV dec⁻¹) are observed in the presence of NH₃, demonstrating that Ni_0.5–Co_0.5-OOH is more susceptible to the EAOR than the OER (1.45 V, 244 mV dec⁻¹). Besides, the polarization current of Ni_0.5–Co_0.5-OOH in the presence of NH₃ is beyond that of CoOOH and the corresponding Tafel slope is the lowest derived from these catalysts, which further indicates exceptional activity of Ni_0.5–Co_0.5-OOH for the EAOR (Fig. 4b, S24 and S25). Moreover, the electrochemical surface area is calculated from the values of double layer capacitance (C_dl) to be 35.5, 31.75, 50.25, 38 and 69.5 cm² for NiOOH, CoOOH, Ni_0.75–Co_0.25-OOH, Ni_0.25–Co_0.75-OOH and Ni_0.5–Co_0.5-OOH, respectively (Fig. 4c and S26). This result confirms that there are more catalytically active sites on the Ni_0.5–Co_0.5-OOH than other oxyhydroxides because of the synergistic effect of CoOOH and NiOOH. Furthermore, Ni_0.5–Co_0.5-OOH displays the smallest Nyquist semicircle diameter, suggesting the lowest charge transfer resistances (1.2 Ω) during the reaction process (Fig. 4d). Meanwhile, the in situ electrochemical impedance spectroscopy (EIS) tests were conducted to uncover the dynamic interfacial changes of the catalyst (Fig. 4e and f). It is observed that the phase angle of the HF region in the NH₃ solution sharply decreased from 1.26 V, indicating the fast charge-transfer kinetics of the EAOR. Thus, the Ni_0.5–Co_0.5-OOH/NF electrocatalyst exhibits extraordinary activity for the EAOR.

Fig. 4 (a) LSV curves of Ni_0.5–Co_0.5-OOH with or without NH₃; (b) LSV curves of different oxyhydroxides in the presence of NH₃; (c) electrochemical double-layer capacitance (C_dl) of different oxyhydroxides; (d) electrochemical impedance spectroscopy of different oxyhydroxides, the inset is the equivalent circuit for EIS fitting; (e and f) Bode phase plots of Ni_0.5–Co_0.5-OOH for the EAOR and OER.

Direct NH₃ oxidation mechanism

XPS and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were used to detect the variation of Ni_0.5–Co_0.5-OOH/NF and NH₃ molecules during the EAOR process. As shown in Fig. 5a and b, the proportions of Ni³⁺/Ni²⁺ and Co³⁺/Co²⁺ first decrease and then increase close to the initial value during the EAOR process (Tables S3 and S4). Regardless of O–S and G–M mechanisms, trivalent metal ions with high oxidative potential in oxyhydroxides act as the electron acceptors to induce the dehydrogenation of NH_3(ads) into NH_2(ads). As a result, the decrease of Ni³⁺ and Co³⁺ in Ni_0.5–Co_0.5-OOH occurs at the initial stage of the EAOR. Subsequently, the formed Ni²⁺ and Co²⁺ in Ni_0.5–Co_0.5-OOH can be further oxidized into Ni³⁺ and Co³⁺ through anodic electron transfer. Besides, it can be seen from the potential-dependent in situ DRIFTS that the peak around 1102 cm⁻¹ is ascribable to the symmetric N–H bending vibration, which stems from the NH_3(ads) or NH_x(ads) intermediates (Fig. 5c and S27). Notably, there is a new peak at 1244 cm⁻¹ with increasing potential, which is assigned to the stretching of H₂N–NH_2(ads).^55,56 This phenomenon manifests that the formation of N₂ during the EAOR process is derived from the further conversion of N₂H_4(ads). A negligible peak around 1605 cm⁻¹ in DRIFTS, corresponding to the N–O vibration, is also observed, resulting from a few NO₂⁻ or NO₃⁻ generated from the peroxidation of NH₃. In addition, as shown in Fig. 5d, in situ Raman spectra of CoOOH and NiOOH show an increasing intensity of Ni–OH and Co–N peaks, respectively, with the potential increasing from 1.2 V to 1.9 V vs. RHE. It is noted that the stretching vibrations of Co–N and Ni–OH bonds are observed in Ni–Co-OOH throughout the reaction, which serves to further verify the synergistic effect of the frustrated Lewis pairs. The intensity of the Ni–OH peak, which arises from strong hydrogen bonding between NiOOH and NH₃,^57–59 initially increases and then decreases as the potential is raised from 1.2 V to 1.9 V. This trend suggests that NiOOH undergoes reversible transformation to Ni(OH)₂ and eventually returns to its initial state, accompanied by the dehydrogenation of NH₃, formation of N₂H_4(ads), and generation of N₂ on frustrated Lewis pairs. A similar reversible behavior is observed for CoOOH in the Ni–Co-OOH system. The change in Co–N strength also confirms that Co³⁺ is the site for NH₃ oxidation. The frustrated Lewis acid–base pair composed of Co³⁺ and hydroxy facilitates N₂H_4(ads) formation, thus promoting the selective ammonia oxidation into N₂.

Fig. 5 (a) Ni 2p XPS of the Ni_0.5–Co_0.5-OOH system within 4 h; (b) Co 2p XPS of the Ni_0.5–Co_0.5-OOH system; (c) in situ DRIFTS spectra of the Ni_0.5–Co_0.5-OOH system with applied potential positive scanning from 0.91 to 2.0 V vs. RHE; (d) in situ Raman test of CoOOH (left), NiOOH (middle) and Ni_0.5–Co_0.5-OOH (right) systems; (e) the modeled N₂H_4(ads) formation pathway of the Ni_0.5–Co_0.5-OOH and NiOOH system, respectively; (f) the pivotal potential gaps of RDS and the formation step of N₂H_4(ads) in Ni–Co-OOH, CoOOH and NiOOH systems; (g) Gibbs free energy diagrams of the EAOR by G–M and O–S mechanisms of the Ni_0.5–Co_0.5-OOH and CoOOH system, respectively.

DFT calculations were performed to explore the conversion path of NH₃ mediated by Ni_0.5–Co_0.5-OOH. According to the structural characterization, the Ni_0.5–Co_0.5-OOH model with the heterojunction is simulated (Fig. S28). It is known that NiOOH following the G–M mechanism exhibits high selectivity for N₂ generation, which is primarily derived from the conversion of N₂H_4(ads) intermediates. The N₂H_4(ads) generation energy barrier is compared to determine the N₂ generation selectivity of Ni–Co-OOH. As shown in Fig. 5e and f, for Ni_0.5–Co_0.5-OOH, two NH₃ molecules bind to Co³⁺ sites and hydroxy sites (the adsorption model as displayed in Fig. S29), which is the integration of O–S and G–M mechanisms. Then, the NH_2(ads) generated on the Co³⁺ sites can quickly recombine with the NH_2(ads) desorbed from the hydroxy sites into N₂H_4(ads) (eqn (11)–(16)). As a comparison, for NiOOH, the formation of N₂H_4(ads) is derived from the recombination of two NH_2(ads) at hydroxy sites. The lower free energy of the N₂H_4(ads) formation confirms that the Ni_0.5–Co_0.5-OOH anode exhibits a higher N₂ selectivity for the EAOR than NiOOH. In addition, the NH₃ conversion path is further analysed to explore the EAOR activity of the Ni_0.5–Co_0.5-OOH anode. As shown in Fig. 5f and g, after the formation of the N₂H_4(ads) intermediate, its continuous dehydrogenation into N₂ occurs at Co³⁺ sites. The rate-limiting step of the EAOR with the Ni_0.5–Co_0.5-OOH anode is the formation of N₂H_2(ads) (1.56 eV), which is the lowest in comparison with the O–S mechanism of CoOOH (2.02 eV) and G–M mechanism of NiOOH (3.25 eV). This result further verifies that the EAOR activity of the Ni_0.5–Co_0.5-OOH anode is superior to that of CoOOH. In summary, due to the synergetic effect of the frustrated Lewis pairs in Ni_0.5–Co_0.5-OOH, the O–S and G–M mechanisms are integrated, and the recombination of NH_2(ads) intermediates into N₂H_4(ads) intermediates is facilitated, thereby immensely promoting the EAOR activity and N₂ formation selectivity.

Co^IIIOOH + NH₃ → Co^IIIOOH–NH_3(ads) (11)

Co^IIIOOH–NH_3(ads) → Co^II(OH)₂–NH_2(ads) (12)

Ni^IIIOOH + NH₃ → Ni^IIIOOH–NH_3(ads) (13)

Ni^IIIOOH–NH_3(ads) → Ni^II(OH)₂–NH_2(ads) (14)

Co^II(OH)₂–NH_2(ads) + Ni^II(OH)₂–NH_2(ads) → Co^II(OH)₂–N₂H_4(ads) + Ni^II(OH)₂ (15)

Co^II(OH)₂–N₂H_4(ads) + 5OH⁻ → Co^IIIOOH + N₂ + 4H₂O + 5e⁻ (16)

Conclusions

In conclusion, this study developed a bimetallic nickel–cobalt oxyhydroxide (Ni–Co-OOH) to integrate O–S and G–M mechanisms together. The synergy of the frustrated Lewis acid–base pairs is utilized for the selective conversion of NH₃ into N₂. Noteworthily, two NH₃ molecules bind to Co³⁺ sites (Lewis acid) in CoOOH and hydroxy sites (Lewis base) in NiOOH. Then, the NH_2(ads) desorbed from the hydroxy sites can rapidly recombine with the NH_2(ads) generated in Co³⁺ into N₂H_4(ads), leading to the formation of N₂. The Ni–Co-OOH anode exhibits excellent activity (94%) and selectivity (63%) for the EAOR beyond other catalysts. This work provides a new idea for designing and preparing novel catalysts to improve the performance of ammonia conversion technology.

Author contributions

Meng-Ying Yin: writing – original draft, formal analysis, data curation, conceptualization. Xing-Yuan Xia: formal analysis, data curation. Ting Dai: formal analysis, data curation, software. Xia Chen: formal analysis, conceptualization, software. Qiu-Ju Xing: resources, supervision, visualization. Lei Tian: project administration, writing – review & editing, methodology. Jian-Ping Zou: writing – review & editing, methodology, resources, project administration, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be made available on request.

The general information, methods and supplementary data associated with this article are provided in the supplementary information (SI). See DOI: https://doi.org/10.1039/d5sc06524k.

Acknowledgements

We gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 52470079, 52170082, 52300081, 51938007, and 52100186), Natural Science Foundation of Jiangxi Province (Grant No. 20212ACB203008), Science and Technology Department Project of Jiangxi Province (No. 20223AEI91001), and Key Laboratory of Jiangxi Province for Persistent Pollutants Prevention Control and Resource Reuse (No. 2023SSY02061).

Notes and references

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