Frustrated Lewis pairs on metal-cation vacancy catalysts enhanced the electroreduction of NO to NH₃†

Honghong Yi^ab, Ruzhu Jia^a, Xiaolong Tang^ab, Dongjuan Kang^a, Qingjun Yu^ab, Fengyu Gao^ab, Shunzheng Zhao*^ab and Yunpeng Liu*^c
aSchool of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China. E-mail: zhaoshunzheng86@126.com; yhhtxl@163.com; Tel: +86-15810968502 Tel: +86-13911663278
^bBeijing Key Laboratory of Resource-oriented Treatment of Industrial Pollutants, Beijing 100083, China
^cBeijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China. E-mail: liuyunpeng@ihep.ac.cn; Tel: +86-10-88235982

First published on 5th August 2024

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1. Introduction

NO_x emissions resulting from fossil fuel combustion are a significant contributor to industrial emissions and remain a primary cause of environmental issues such as acid rain, photochemical smog, ozone depletion, global warming, and a serious threat to human health.^1–3 NO, which accounts for up to 95% of NO_x, is the dominant component of industrial flue gas.⁴ Converting NO from flue gas into harmless or valuable forms of N is crucial.^2,5 Selective catalytic reduction (SCR) is the most widely used and established technology worldwide for flue gas denitrification. However, the catalytic process occurs at high temperatures and employs precious NH₃ as a catalyst, which does not meet environmental friendliness requirements.^6,7 Consequently, developing alternative denitrification processes that are sustainable, energy-efficient, and environmentally friendly is crucial.

NH₃ is a zero-carbon fuel, a possible future hydrogen energy source, and a crucial chemical for manufacturing nitrogen fertilizers, paints, plastics, and more.^8,9 Industrial NH₃ is primarily produced by the highly energy-intensive Haber–Bosch process, which is responsible for approximately 1.5% of total global carbon dioxide (CO₂) emissions and consumes 2% of the worldwide annual energy supply.^10–12 The electrochemical nitrogen reduction reaction (NRR) is considered a promising alternative to the Haber–Bosch process, but its development has been impeded by the challenges associated with activating N₂ and its limited solubility in the electrolyte.^13,14 Nitrate possesses higher water solubility and lower NO bond dissociation energy, and electrocatalytic nitrate reduction to synthesize NH₃ for simultaneous nitrate wastewater treatment and NH₃ production has also been widely studied.^15–21 Some researchers have ingeniously employed the NO reduction reaction (NORR) for electrosynthesis of NH₃, which not only addresses the removal of NO but also generates NH₃ as a value-added chemical.^16,22 This highlights the significant potential of the NORR for practical applications.

Layered double hydroxides (LDHs) have garnered considerable attention as an essential catalyst in the areas of adsorption,²³ catalysis,²⁴ energy storage,²⁵ and electrochemistry²⁶ owing to their low raw material cost, versatility, customizable chemical composition, and unique layered structure. Nevertheless, the absence of accessible active sites and the low conductivity of the native LDH itself impede further enhancement of the catalytic performance. Fortunately, these deficiencies can be rectified through the precise adjustment of the electronic structure.²⁷ Generating cationic vacancies has recently been identified as an effective method for modulating the electronic configuration of the host material and the coordination environment of the metal sites, thereby tuning the electrocatalytic properties. These defects facilitate charge transfer and redox reaction kinetics and can also serve as additional host sites for the insertion of protons or alkali metal cations, thereby promoting ion diffusion during electrochemical cycling.^28,29 Cationic vacancies are regarded as a promising avenue for regulating catalytic selectivity.³⁰

In recent years, frustrated Lewis pairs (FLPs) constructed on catalyst surfaces have demonstrated significant potential for adsorption and activation of small molecules such as H₂,³¹ CO₂,^32–34 N₂,^35,36 and others. Lewis acids (LAs) with empty orbitals and Lewis bases (LBs) with lone-pair electrons in FLPs provide opportunities for dissociation and activation of gas molecules due to large steric hindrance formed by active regions instead of coordination bonds.³⁷ Research has demonstrated that FLPs may comprise oxygen vacancies (V_O) and neighboring hydroxyl (–OH) or lattice oxygen atoms, creating distinct reaction sites for catalytic activation.^38,39 Defect engineering represents a viable strategy in the design of FLPs for metal compounds that contain both a metal cation as the LA and an oxygen anion as the LB.^38,40

Inspired by this, metal cation vacancies were constructed on the catalyst surface by a simple alkali etching strategy to promote the generation of surface V_O and protons, and FLPs were successfully constructed in which the unsaturated Al sites on the surface of LDH and the neighboring –OH species acted as the LA and LB sites, respectively. The LA provided the empty orbitals and the LB provided the protons for the adsorption and activation of NO molecules, which enhanced the electrocatalytic NORR synthesis of NH₃ activity. As a result, the etched nanosheets exhibited better performance in NH₃ synthesis compared to pristine CoAl-LDH. Specifically, after alkali etching for 4 h, the NH₃ yield increased to 0.310 mg cm⁻² h⁻¹, which was 2.2 times higher than that of the pristine CoAl-LDH.

2. Experiments and calculations

2.1. Materials and chemicals

Cobaltous nitrate hexahydrate (Co(NO₃)₂·6H₂O), aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O) and sodium nitroferricyanide dihydrate (C₅FeN₆Na₂O·2H₂O) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Sodium hydroxide (NaOH) and urea (CH₄N₂O) were obtained from Sinopharm Chemical Reagent Co., Ltd (China). Salicylic acid (C₇H₆O₃), ammonium chloride (NH₄Cl), sodium citrate dihydrate (C₆H₅Na₃O₇·2H₂O), and sodium hypochlorite solution (NaClO) were purchased from Shanghai Macklin Biochemical Co., Ltd (China). N₂, Ar, and mixed gas of NO/Ar (1 vol% NO) were bought from Jing Hui Gas (China). Deionized water was used in all the experimental processes. All the chemicals were of analytical grade and used without further purification.

2.2. Catalyst preparation

2.3. Materials characterization

The crystalline phases were identified using a Rigaku Smartlab 3KW X-ray diffraction (XRD) instrument with Cu Kα radiation of 0.15418 nm wavelength (40 kV voltage, 100 mA current) at a scan rate of 10° min⁻¹ within the 2θ range of 10–80°. The morphology of the prepared samples was characterized using a Zeiss Gemini 300 scanning electron microscope (SEM). Fourier transform infrared spectroscopy (FTIR) was carried out on the iS10 FTIR spectrometer of Negoli, with a wave number range of 400–4000 cm⁻¹, a resolution of 4 cm⁻¹, and 32 scans. X-ray photoelectron spectroscopy (XPS) (Thermo Scientific K-Alpha) was employed for elemental mapping using a monochromatic Al Kα radiation source. Binding energy calibration was performed by referencing the C 1s main peak at 284.8 eV. The elemental ratio was investigated by inductively coupled plasma optical emission spectrometry (ICP-OES). N₂ adsorption–desorption isotherms were measured on an Autosorb iQ, Quantachrome Instruments, US analyzer. Pore volumes, sizes, and specific surface areas were determined by the Brunauer–Emmett–Teller (BET) method. Temperature-programmed desorption of NH₃/CO₂ (NH₃/CO₂-TPD) was performed using an Extrel MAX300 chemisorption analysis tester. K-edge X-ray absorption fine structure (XAFS) analysis was performed at the Beijing Synchrotron Radiation Facility 4B9A beamline.

2.4. Electrochemical measurements

Electrochemical measurements were performed at ambient temperature in an H-type cell with a three-electrode configuration separated by a Nafion 117 membrane (Electrochemical Workstation, CHI 660E, CH Instruments Inc., Shanghai). The electrolyte solution was 0.1 M K₂SO₄. The membrane was treated first in H₂O₂ (5 wt%) aqueous solution at 80 °C for 1 h. Then, it was treated in 0.5 M H₂SO₄ for 1 h at 80 °C and finally in water for 4 h. The carbon cloth utilized in this experiment was purchased from Canrd (WOS 1011 type). The prepared catalyst loaded on the carbon cloth (1 × 1 cm²) was used as the working electrode, and a Pt plate and Ag/AgCl (saturated KCl electrolyte) were used as the counter and reference electrodes, respectively. The potential without iR compensation was converted to the RHE scale using the following equation (E(V vs. RHE) = E(V vs. Ag/AgCl) + 0.21 + 0.0591 × pH). The electrochemical activity of catalysts was measured for the NORR. Generally, 20 mg of catalyst was ultrasonically dispersed in 750 μL of isopropanol, 100 μL of Nafion solution (5 wt%, Du Pont), and 2 mL of water to form a homogeneous ink. Then 1000 μL of the ink was dropped on the surface of carbon cloth (1 × 3 cm²) and dried at room temperature. To eliminate possible alkaline contaminants, the feed gas was passed through a saturator filled with a 0.05 M H₂SO₄ solution to remove impurities from the inlet gas. Before the electrochemical characterization, the electrolyte was purged with 1% NO/Ar at a rate of 30 sccm for 30 min and kept constant during electrochemical testing. The cyclic voltammetry (CV) test was conducted with a scan rate of 50 mV s⁻¹ ranging from 0 to −1.8 V vs. RHE. The linear sweep voltammetry (LSV) test was carried out with a scan rate of 5 mV s⁻¹. Subsequently, the chronoamperometric test was conducted at various potentials, and the stability experiment was executed at a potential of 1.0 V vs. RHE.

2.5. Determination of NH₃ concentration

The amount of NH₃ in the solution was determined by colorimetry using the indophenol blue method.⁴ A certain amount of electrolyte was taken out from the electrolytic cell and diluted to 2 mL to the detection range. Then, 2 mL of a 1 M NaOH solution that contained salicylic acid and sodium citrate was added. Then, 1 mL of 0.05 M NaClO containing 1 M NaOH and 0.2 mL of 1 wt% C₅FeN₆Na₂O·2H₂O were added to the above solution. After standing at room temperature for 2 h, the UV-Vis absorption spectrum was measured. The concentration of NH₃ was determined using the absorbance at a wavelength of 665 nm. The concentration–absorbance curve was calibrated using a series of standard NH₄Cl solutions. The fitting curve (y = 0.4453x − 0.0083, R² = 0.9998) showed a good linear relation of the absorbance value with NH₃ concentrations.

2.6. Calculations of NH₃ yield and Faraday efficiency

The average NH₃ yield (mg cm⁻² h⁻¹) was calculated using the following equation:⁴¹

NH3 yield = 10−3 × C × V/(S × t)

where C (mg L⁻¹) is the measured mass concentration of produced NH₃; V (100 mL) is the total volume of the cathodic electrolyte, S (1 cm²) is the geometric area of the working electrode, and t (h) is time for electrocatalysis.

The Faraday efficiency (FE) is the amount of charge used to produce NH₃ during electrolysis divided by the total amount of charge passing through the electrode. The FE was calculated according to the following equation:²

FE (%) = (10−6 × n × F × C × V)/(17 × Q) × 100

where n is the number of electrons required per mole of NH₃ produced; F (96485 C mol⁻¹) is the Faraday constant; C (mg L⁻¹) is the measured mass concentration of NH₃; V (100 mL) is the total volume of the cathodic electrolyte; Q is the total charge across the working electrode.

2.7. Computational details

The atomic models of the functionalized LDHs were designed assuming their complete surface termination by OH atoms. The periodic system had a vacuum thickness of 15 Å, which was used to eliminate spurious interactions between the adsorbate and the periodic image of the bottom layer of the surface. All the computations were performed with spin-polarized DFT implemented in the DMol³ code of the Materials Studio. A generalized gradient approximation (GGA) with the Perdew, Burke, and Ernzerhof (PBE) exchange–correlation functionals was employed. The double numerical plus polarization (DNP) was used as the basis set. Effective core potentials were adopted as the core treatment to conduct a metal relativistic effect. The geometry convergence tolerance for energy change was 2 × 10⁻⁵ Ha, the max force was 0.004 Ha Å⁻¹, and the max displacement was 0.005 Å. We chose 4.5 Å to be the real-space global orbital cutoff radius. Structural optimization was performed without any constraints. The Gibbs free energy change (ΔG) of each elemental step was calculated according to the CHE model, which used one-half of the chemical potential of hydrogen as the chemical potential of the proton–electron pairs. ΔG was calculated from the equation: ΔG = ΔE + ΔE_ZPE + ΔG_U − TΔS, where ΔE is the electronic energy difference directly obtained from DFT calculations, ΔE_ZPE is the zero-point energy, T is the temperature (T = 298.15 K), ΔS is the change in entropy, and ΔG_U represents the free energy contributions related to the applied electrode potential U.

3. Results and discussion

3.1. Effects of metal-cation vacancies on NO adsorption properties

Initially, DFT calculations were conducted to determine the activation of NO on the LDH surface and to investigate the crucial role of metal cation vacancies in facilitating NO adsorption. The active sites for NO adsorption on LDH are believed to be Al vacancies, and the adsorption energy of NO plays a vital role in the electrocatalytic synthesis of the NH₃ reaction.⁴² The calculated effect of LDH surface defects on NO adsorption performance is shown in Fig. 1. LDH containing V_O is more likely to attract NO molecules, providing active sites for molecular adsorption (Fig. 1a). The adsorption energy confirmed that LDH with V_O has more negative adsorption energy than that free of V_O, suggesting that NO adsorption is more favorable on LDH with V_O (Fig. 1b). This is consistent with literature studies reporting that unsaturated metal sites are the ones that are more likely to adsorb small molecules.^43–45 The higher adsorption energy could be attributed to the partial charge transfer of LDH and NO, thus weakening the N–O bond. Moreover, the effect of metal vacancies on the V_O formation energy was also calculated, and it was found that the formation energy of the V_O active site in LDH containing Al vacancies was lower (0.76 eV) than that of the defect-free LDH (1.59 eV) (Fig. 1c). Apparently, the Al vacancies in LDH are more favorable for the synthesis of V_O active sites, which promotes the adsorption and activation of NO, thereby improving the reaction kinetics of electrocatalytic NO reduction to NH₃.

3.2. Preparation and characterization of catalysts

DFT calculations reflected the advantage of Al vacancies in LDH electrocatalysts for NO adsorption reduction. The pristine CoAl-LDH sample was synthesized through a hydrothermal method. Subsequently, CoAl-LDH samples with Al vacancy defects were prepared by etching with NaOH aqueous solution for different durations using a facile alkali etching method (Fig. 2a). This paper presents the samples of pristine CoAl and etched CoAl (NaOH X h). The study investigated the changes in Co and Al atomic contents during the etching process using ICP-OES. The results showed that successful removal of partial Al sites and creation of metal cation defect sites were achieved after alkali etching of CoAl-LDH. This was evidenced by the significant decrease in Al content and increase in Co content compared to pristine CoAl (Fig. S1†).

The morphology of the CoAl-LDH samples was observed using SEM. The pristine CoAl exhibited a typical two-dimensional lamellar structure (Fig. 2b). It is noteworthy that the lamellar structure was maintained after 4 hours of alkali etching (CoAl (NaOH 4 h)) (Fig. S2a†). However, after 10 hours of alkali etching (CoAl (NaOH 10 h)), the catalyst structure was disrupted and etched into a flower-cluster-like shape (Fig. S2b†). The N₂ adsorption–desorption experiments indicate that all samples exhibit type IV adsorption isotherms according to the IUPAC classification, displaying H3 hysteresis loops, which correspond to mesoporous materials (Fig. S3a†).⁴⁶ The pore size distribution of most samples is around 3.82 nm (Fig. S3b†). The pore volume and surface area of the samples increased with the increase of alkali etching time (Table S1†). Will the high specific surface area of CoAl (NaOH 10 h) provide more active sites? This will be verified later in the electrochemical activity tests.

XRD was used to investigate the crystal structure of CoAl-LDH nanosheets (Fig. 2c). For pristine CoAl, typical diffraction peaks at 2θ = 11.6, 23.27, 34.42, 39.05, 46.48, 60.18, and 61.57 can be indexed to (003), (006), (012), (015), (018), (110), and (113), which correspond to the standard map JCPDS: 51-0045. These diffraction peaks and the absence of other peaks indicate the presence of a pure LDH phase.^47–49 The XRD pattern exhibits good crystallinity of the prepared samples with a low and stable baseline and high-intensity diffraction peaks. The distinct (110) and (113) peaks are evidence of well-dispersed Co²⁺ and Al³⁺ metal cations on the pristine CoAl layer.^49,50 Impressively, the (003) peaks weakened or even disappeared with increased etching time, indicating that the alkali etching treatment resulted in a smaller crystal size and a higher degree of disorder. The decrease in the intensity of the diffraction peaks of the etched CoAl-LDH samples is accompanied by the appearance of some stray peaks, which may be due to the diversification of some crystal surfaces by the lattice distortion caused by etching, resulting in the appearance of new diffraction peaks.

The functional groups present on the surface of CoAl-LDH nanosheets were identified using FTIR (Fig. 2d). The observed peaks at 3503 and 3382 cm⁻¹ indicate the stretching and deformation vibrations of O–H, respectively.⁵¹ Similarly, the absorption peak at 3628 cm⁻¹ is associated with the stretching vibration of H–O–H, and the peak at 2190 cm⁻¹ is the bending vibration of NHO⁻.⁵² The bands at 1358 cm⁻¹ in CoAl, CoAl (NaOH 4 h), and 1365 cm⁻¹ in CoAl (NaOH 10 h) are attributed to the CO antisymmetric stretching of CO₃²⁻.⁵³ The H₂O bending vibration is observed at 1545 cm⁻¹, while the remaining absorption bands below 800 cm⁻¹ can be attributed to metal–oxygen (M–O) and metal–hydroxyl (M–OH) vibrations in the LDH.⁵⁴ The peaks at 580 cm⁻¹ and 668 cm⁻¹ correspond to stretching vibrations of the Co–O and Al–O bonds respectively.⁵⁵ The M–O peaks in CoAl (NaOH 4 h) and CoAl (NaOH 10 h) are significantly enhanced, which may be due to the presence of more V_O resulting from cation vacancies after etching. This can be further confirmed by XPS.

The valence states of the elements in the CoAl-LDH nanosheets synthesized were investigated by using XPS. The corresponding results can be seen in Fig. 3. All elements have been normalized to carbon at 284.8 eV. Fig. S4† displays the composition of the CoAl-LDH nanosheets, mainly consisting of four elements, Co, Al, C, and O. Furthermore, Fig. 3 presents the fine XPS spectra of Co, O, and Al elements in the mentioned nanosheets. Fig. 3a illustrates that the Co 2p spectrum can be deconvoluted into two peaks at 780.78 eV and 797.24 eV, corresponding to Co 2p_3/2 and Co 2p_1/2, respectively. In addition, the presence of satellite peaks at 787.15 eV and 802.15 eV suggests the presence of a highly spinning divalent state of Co²⁺ in the sample.⁵⁶ The binding energies of CoAl (NaOH 4 h) and CoAl (NaOH 10 h) nanosheets are negatively shifted compared to those of pristine CoAl nanosheets, indicating an increase in the electron density caused by alkali etching. The Al 2p spectrum indicates the presence of Al³⁺ with a peak at 74.09 eV binding energy (Fig. 3b).⁵⁷ Moreover, the peak signals of CoAl (NaOH 4 h) samples display the highest binding energies, signifying the presence of numerous unsaturated Al sites generated as a result of alkali etching treatment compared to pristine CoAl.³⁶ The appearance of Co 2p and Al 2p peaks confirms the formation of CoAl-LDH. In CoAl (NaOH 4 h), the binding energy of Co 2p is negatively shifted while that of Al 2p is positively shifted in comparison to pristine CoAl. The electron density of Co 2p increases while that of Al 2p decreases, indicating a significant transfer of electrons from Co to the Al cation hole. Fig. 3c shows that the C–C peak is positioned at 284.8 eV based on the bonding between sp² hybridized carbon atoms, and using it as a reference peak, the peak with a bonding energy of 289.1 eV corresponds to the oxygen-containing functional group CO.⁴⁸ The V_O level can be determined from the O 1s nuclear energy spectrum obtained through XPS.⁵⁸ The O 1s spectrum can be deconvoluted into three peaks, where the peak at 530.12 eV is from oxygen atoms bound to the metal (M–OH), the peak at 531.36 eV is attributed to O atoms near the V_O, and the higher energy peak at 532.54 eV is chemisorbed or dissociated oxygen species (Fig. 3d).^59,60 Consistent with the FTIR results, the V_O peak of CoAl (NaOH 4 h) shifted to a higher binding energy compared to CoAl samples, which promoted the formation of new neighboring V_O with high electron attraction effects.⁶¹ Meanwhile, the M–OH peak of the CoAl (NaOH 4 h) sample shows the highest binding energy, indicating that the appropriate alkaline etching time is more favorable for the formation of –OH species. Based on the above results, it can be inferred that alkali etching creates metal cation defect sites by altering the Co and Al contents in CoAl-LDH nanosheets, which promotes the formation of nearby V_O.

It is interesting to investigate the alterations in the chemical state and coordination surrounding Co at an atomic level during alkali etching, utilizing X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). In the normalized XANES spectra, CoAl-LDH exhibits a characteristic Co K-edge spectrum with the pre-edge peak corresponding to the 1s → 3d transition and the white line peak resulting from the 1s → 4p transition (Fig. 4a).^62–64 The distinct absorption edge positions of the Co K-edge XANES spectra imply that the Co valence of CoAl-LDH nanosheets increases with the duration of alkali etching, and the oxidation state is between Co²⁺ and Co³⁺ (Fig. 4a and b, Table S2†). Alkali etching resulted in a significant change in the atomic valence state of Co in CoAl-LDH, consistent with the XPS results (Fig. 3a). The k³-weighted EXAFS spectra of the Fourier transform (FT) reveal notable dissimilarities in the local atomic structures around the Co atoms among the diverse specimens (Fig. 4c, Table S3†). Within the CoAl-LDH samples, the Co K-edge EXAFS spectra display two distinctive shells in the R-space spectrum, corresponding to the Co–O and Co–Co/Co–Al coordination shells. The Co–O peak, which underwent alkali etching treatment, exhibits a considerable reduction in intensity compared to the pristine CoAl nanosheet. The wavelet transform (WT) indicates significant WT peaks at about 6.0 and 10.0 Å associated with Co–O and Co–Co/Co–Al scattering, respectively (Fig. 4d). There is a continuous weakening of the Co–Al bond after alkali etching, which corresponds with the ICP-OES results (Fig. S1†). The first shell Co–O results in q-space were accurately fitted (Fig. 4e, Table S3†). The alkali etched CoAl (NaOH X h) exhibits a substantially lower Co–O coordination number N and bond length R than pristine CoAl, indicating the presence of hydroxyl vacancies (V_OH) in the layer after alkali etching (Fig. 4f, Table S3†).⁶⁵ The bond lengths of the Co–O in the samples showed no significant changes despite increasing the alkali etching time, while the coordination number experienced a slight decrease. The amount of disorder in the Debye–Waller factor σ² follows the order CoAl (NaOH 2 h) > CoAl (NaOH 6 h) > CoAl (NaOH 4 h) > CoAl > CoAl (NaOH 8 h) > CoAl (NaOH 10 h). The abundance of V_OH theoretically could supply further coordination of unsaturated sites for the NORR. Nevertheless, the low disorder amount of the Debye–Waller factor in CoAl (NaOH 10 h) may restrict its electrocatalytic activity. These XAFS data provide solid evidence for the existence of coordination unsaturation sites and significant structural distortions of etched CoAl-LDH.⁶⁶ Combined, these structural alterations would decrease the surface energy of LDH nanosheets and augment the abundance of NH₃-synthesizing active sites.

3.3. Performance of electrocatalytic NO reduction to NH₃

The electrocatalytic activity of CoAl-LDH nanosheets was investigated in an H-type electrolytic cell separated by a Nafion 117 membrane and coupled to a three-electrode configuration. Before the electrochemical experiments, the feed of 1% NO/Ar underwent strict experimental control to eliminate the interference of other impurity gases. The electrocatalytic performance of CoAl-LDH nanosheets was evaluated using linear scanning voltammetry (LSV). As shown in Fig. 5a and S5,† higher current densities were achieved with a NO-saturated electrolyte compared to an Ar-saturated solution. Additionally, Fig. 5b demonstrates that the reduction current densities of the etched CoAl-LDH were all higher than that of the pristine CoAl-LDH, especially for CoAl (NaOH 4 h), confirming the superior electrochemical performance of the etched CoAl-LDH for the NORR.⁶⁷ After 20 min of electrolysis, the cathodic electrolyte for the NORR was collected and stained with the indophenol blue indicator to quantify the amount of NH₃ produced (Fig. S6†). The results showed that CoAl (NaOH 4 h) had the highest NH₃ yield of 0.310 mg cm⁻² h⁻¹ at −1.1 V vs. RHE, with the corresponding highest FE (Fig. 5c and d). The NH₃ yield was 2.2 times increased compared to the pristine CoAl (0.097 mg cm⁻² h⁻¹). Meanwhile, Ar was passed to the cathode electrolyte for comparison purposes. It was observed that only a small amount of NH₃ was produced in the product, reaching 0.043 mg cm⁻² h⁻¹ at −1.1 V vs. RHE (Fig. 5d). This amount had a negligible effect on the experimental results. Additional constant potential electrolysis tests were performed at −1.0 V vs. RHE, and the obtained time-dependent current density curves indicated that the CoAl-LDH nanosheets exhibited excellent stability for 24 h (Fig. 5e). The results of the electrocatalytic activity of CoAl-LDH nanosheets provide compelling evidence that metal cation vacancies generated by alkali etching play a crucial role in enhancing the activity and selectivity of electrocatalytic NORR synthesis of NH₃.^68,69 It is proposed that the vacancies on the etched CoAl-LDH surface function as active sites, thereby facilitating the adsorption and activation of NO molecules. It is anticipated that this enhancement will elevate the current density, which will in turn result in an increased NH₃ yield. The observed increase in both current density and NH₃ yield in the etched CoAl-LDH is indicative of its enhanced electrochemical performance, thereby validating the hypothesis that these vacancies significantly contribute to the electrocatalytic activity.

To further evaluate the surface properties, and electrochemical kinetics and to determine the reason for the high NORR activity of CoAl (NaOH 4 h), electrochemical active surface area (ECSA) and Tafel tests were performed. ECSA was determined by calculating the double-layer capacitance (C_dl) using CV data obtained within the non-faradaic region. Fig. S7† displays the CV curves of CoAl-LDH nanosheets at different sweep speeds ranging from 20–120 mV s⁻¹ and the C_dl obtained from the fitting. The C_dl values for CoAl, CoAl (NaOH 4 h), and CoAl (NaOH 10 h) are 0.55, 0.41, and 0.51 mF cm⁻², respectively, with no significant difference. The Tafel test results indicate that CoAl (NaOH 4 h) exhibits a significantly lower Tafel slope (307.8 mV dec⁻¹) compared to CoAl (337.7 mV dec⁻¹) and CoAl (NaOH 10 h) (503.6 mV dec⁻¹) (Fig. 5f). This suggests that CoAl (NaOH 4 h) exhibits faster electrochemical reaction kinetics, resulting in a lower kinetic energy barrier to drive the NORR electrocatalytic reaction.⁷⁰

3.4. Exploration of the origin of the high-performance electrocatalyst

Metal cation vacancy defects were successfully created through the exploitation of the alkali solubility of Al, promoting the formation of V_O and enhancing the performance of the electrochemical NORR for NH₃ synthesis, as theoretically calculated. Both pristine CoAl and CoAl (NaOH 4 h) exhibit similar lamellar structures. In contrast, CoAl (NaOH 10 h) exhibits flower clusters and shows a greater surface area in N₂ adsorption–desorption measurements, which could potentially provide more active sites for reactants. However, it is noteworthy that the catalysts in the electrochemical performance tests exhibited similar ECSA. Nevertheless, CoAl (NaOH 4 h) was able to produce more NH₃, suggesting that the catalyst's performance enhancement is not solely due to morphology and surface area effects, but also due to its ability to activate the active sites of NO molecules massively. The lower energy barrier may be attributed to the fact that FLPs promote the adsorption and activation of NO molecules.³⁴ The structural characterization of the catalysts reveals unsaturated metal sites and neighboring –OH groups, which offer more opportunities for the formation of FLPs.

To further confirm the presence of acid–base sites of FLPs in CoAl-LDH nanosheets, NH₃/CO₂-TPD tests were conducted (Fig. S8†). The presence of two distinct peaks of NH₃ (around 320 °C and 419 °C) and CO₂ (around 324 °C and 432 °C) detachment indicate the occurrence of medium-strength and strong acidic and basic centers in the catalyst. The strong acidic and basic centers present in CoAl (NaOH 4 h) and CoAl (NaOH 10 h) disappeared following alkali etching. Moderately acidic and basic sites of medium-strength were detected in CoAl (NaOH 4 h) samples, and the optimal amount of LA and LB sites was more conducive to the generation of steric hindrance to promote the formation of FLPs.³⁶

DFT calculations further reveal the origin of the efficient NORR performance of the cation-defect-rich alkali-etched LDH nanosheet (D-LDH) electrocatalysts. Fig. 6a shows the Gibbs free energy distribution plots of NORR processes of LDH and D-LDH catalysts. The first protonation process of adsorbed NO molecules (*NO) is identified as the potential-determining step (PDS) of the NORR for both LDH and D-LDH catalysts (*NO + H⁺ + e⁻ → *NO). The corresponding Gibbs free energy change values of PDS are 0.71 eV (LDHs) and 0.32 eV (D-LDHs), as calculated. The previously mentioned DFT calculations indicate that D-LDH catalysts have lower PDS values. Better NORR electrocatalytic activity corresponds to lower PDS values, which is concordant with the results of catalyst characterization and electrochemical NORR performance tests. The LDH surface containing metal cation vacancies is more favorable for the formation of V_O, and the abundant LA (V_O) and LB (–OH) sites provide empty orbitals and protons respectively for NO molecules, lowering the reaction energy barriers and facilitating the synthesis of NH₃.⁷¹

Based on experimental results and theoretical calculations, FLPs can enhance the NH₃ synthesis performance of the electrochemical NORR in the following ways: alkali etching creates metal cation vacancies, promoting the formation of V_O on the surface. Metal cation vacancies (or V_O) and neighboring –OH form FLPs due to spatial site–barrier interaction. LA sites can be used as empty orbitals to adsorb activated NO molecules, and LB sites provide protons to generate the *NOH intermediates required for the reaction, facilitating the formation of NH₃. It is important to note that the FLP sites are regenerated when the product NH₃ is released after synthesis (Fig. 6b).

4. Conclusions

In summary, this work presents a novel strategy for targeted structural optimization of catalysts to boost the performance of the electrocatalytic NO synthesis of NH₃. CoAl-LDH nanosheets containing cationic vacancies were fabricated utilizing a straightforward alkali etching technique. After 4 h of alkali etching, the NH₃ yield of CoAl-LDH reached 0.310 mg cm⁻² h⁻¹ at −1.1 V vs. RHE, which is a 2.2-fold improvement compared with that of the pristine CoAl-LDH (0.097 mg cm⁻² h⁻¹). Experimental and theoretical results indicate that the V_O and neighboring –OH species in the artificial FLPs formed by LDH rich in cationic vacancies act synergistically to promote NO adsorption and activation, opening a new avenue for the use of LDH electrocatalysts to construct FLPs for electrocatalytic NORR synthesis of NH₃.

Data availability

Data will be made available on request.

Author contributions

Honghong Yi: conceptualization; formal analysis; funding acquisition; methodology; project administration; resources; writing – original draft. Ruzhu Jia: conceptualization; data curation; formal analysis; validation; writing – original draft. Xiaolong Tang: conceptualization; data curation; resources; supervision. Dongjuan Kang: methodology; resources; supervision. Qingjun Yu: investigation; resources; supervision. Fengyu Gao: conceptualization; methodology; resources; supervision. Shunzheng Zhao: conceptualization; formal analysis; funding acquisition; methodology; project administration; supervision; writing – review & editing. Yunpeng Liu: data curation; funding acquisition; project administration; supervision; visualization; writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (2021YFB3500702), National Natural Science Foundation of China (Project No. 12305372, 21677010, and 51808037), Fundamental Research Funds for the Central Universities (No. FRF-IDRY-20-018), and Special fund of Beijing Key Laboratory of Indoor Air Quality Evaluation and Control (No. BZ0344KF21-04). We also thank the 4B9A beamline (Beijing Synchrotron Radiation Facility, BSRF) for supporting the beam time of XAFS measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta04550e

This journal is © The Royal Society of Chemistry 2024