SiO₂ assisted Cu⁰–Cu⁺–NH₂ composite interfaces for efficient CO₂ electroreduction to C₂₊ products

Abstract

The electrochemical CO₂ reduction reaction (CO₂RR) for high value-added multi-carbon product (C₂₊) production over copper oxide-based catalysts is an important way to realize the carbon cycle. However, developing effective reaction interfaces and microenvironments to improve the Faraday efficiency (FE) and current density of C₂₊ remains a major challenge. Herein, we construct Cu⁰–Cu⁺–NH₂ composite interfaces with the assistance of SiO₂. Using Cu₂O nanoparticles as a model catalyst, a layer of porous SiO₂ is first coated on the surface of the particles, and then, a silane coupling agent containing –NH₂ is bonded on the surface of SiO₂. The strong interaction between SiO₂ and Cu₂O at the interface induces the oxidation effect of low valent Cu, and even under the CO₂RR, part of Cu⁺ is reduced to Cu⁰ and part of Cu⁺ still maintains positive valence, forming the interface of Cu⁰–Cu⁺. SiO₂ also acts as a bridge between copper species and –NH₂ to create a Cu catalyst–NH₂ group interface. With the help of the synergistic effect of the composite interfaces, the optimized Cu₂O@SiO₂–NH₂ catalyst achieves a FE of 81.2% for C₂₊ products with a current density of 292 mA cm⁻² at −1.7 V versus a reversible hydrogen electrode. In situ Raman and attenuate total reflectance-infrared absorption spectroscopy spectra show that the interaction between surface –NH₂ and CO₂ molecules enhances the adsorption and activation process of CO₂ and promotes the formation of CO intermediates (*CO). On the Cu⁰–Cu⁺ interface, the C–C coupling process between *CO is accelerated, and the two interfaces synergistically promote the generation of C₂₊ products. This work provides a new strategy for constructing composite interfaces to improve the CO₂RR to C₂₊ products.

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1. Introduction

The excessive emission of CO₂ breaks the balance of the natural carbon cycle;¹ therefore, it is urgent to develop new technologies for CO₂ recycling and utilization to realize the sustainable development of resources and energy, and mitigate global warming. The electrochemical CO₂ reduction reaction (CO₂RR) coupled with renewable electric energy can not only synthesize high-value-added products from CO₂ but also realize energy storage at the same time, which is a promising way of CO₂ utilization.^2,3 Up to now, it has been reported that at least 16 distinct products are formed from the CO₂RR, such as CO, CH₄, HCOOH, C₂H₄, C₂H₅OH, C₃H₇OH, and so on.^4,5 Among the CO₂RR products, C₂₊ can be used as a chemical raw material and fuel, leading to wide application.^1,6 Consequently, researchers have made significant endeavors in the synthesis of C₂₊ through the CO₂RR. However, CO₂ is a linear and inert molecule with a C=O bond energy of up to 750 kJ mol⁻¹.⁷ Thus, achieving C=O activation under milder conditions requires the use of appropriate catalysts. Currently reported catalysts for CO₂RR synthesis of C₂₊ products are mainly Cu-based catalysts, because they have a moderate adsorption strength for C₁ intermediates in the CO₂RR process, which is neither too strong for desorption nor too weak for further adsorption activation for subsequent reactions, to promote the dimerization between C₁ intermediates and generate C₂₊ products.^1,8

Among Cu-based catalysts, the oxides of Cu and their derivatives are an important type of catalyst with high reactivity.^9–11 During the reduction and reconstruction process, abundant metal-oxide (Cu⁰–Cu^δ+) interfaces are generated for Cu oxides, which significantly enhance the activity of the catalysts and improve the rate of C–C coupling.¹² Therefore, their electronic structure¹³ and morphology^14,15 have been widely studied to improve the activity of the CO₂RR to C₂₊ products. Especially, recent studies have found that the adsorption strength of the CO intermediate (*CO) on Cu^δ+ species (0 < δ < 2) is stronger than that on Cu⁰ species at the metal-oxide interface,^16–18 which is beneficial to increase the concentration of *CO on the catalyst surface and promote the *CO dimerization step. Therefore, it is important to improve the stability of Cu^δ+ species under reduction conditions to produce C₂₊ products. For example, Zhou et al.¹⁶ reported a B-doped Cu catalyst to regulate the local electronic structure of Cu and improve the stability of positive valence Cu. Consequently, the adsorption and dimerization of *CO can be controlled by adjusting the average oxidation valence of Cu, which makes the Cu(B) catalyst achieve 79% faradaic efficiency (FE) of C₂ in the CO₂RR process. In addition, Yan et al.¹⁹ reported a hexagonal boron nitride (h-BN) modified Cu₂O catalyst, where the strong electron interaction between the two components of Cu₂O and BN makes the electrons on Cu₂O transfer to BN to strengthen the Cu–O bond, thus stabilizing the Cu⁺ species during the CO₂RR. Upon applying the Cu₂O-BN catalyst for the CO₂RR process, the ratio of C₂H₄/CO increased by 1.62 times compared with that of the Cu₂O catalyst. Similarly, Zang et al.²⁰ designed a carbon-coated CuO_x (CuO_x@C) catalyst and the carbon layer on the catalyst surface effectively stabilized Cu⁺ species, thereby facilitating the C–C coupling process. In the CO₂RR process, the FE of ethanol reached 46%, and the partial current density reached 166 mA cm⁻². A series of studies have proven that the interfaces between Cu⁺ and Cu⁰ regions could promote C₂₊ production. Additionally, organic molecules containing amino groups (–NH₂) are often used as surface modifiers to regulate the surface properties of catalysts, thereby altering the reaction microenvironment during the CO₂RR. Li et al.²¹ constructed a molecular–catalyst interface by modifying a layer of an N-aryl-dihydropyridine-based oligomer on the surface of the Cu catalyst, which made the catalyst exhibit excellent ethylene selectivity with an FE of 72% at −0.83 V versus the reversible hydrogen electrode (vs. RHE; the same potential scale is used in the following discussion unless otherwise specified) during the CO₂RR. Similarly, Chen et al.²² incorporated polyamine on Cu electrodes. Due to the strong binding force between the CO₂/CO molecule and –NH₂, the polyamine–Cu interface formed a microenvironment with locally high concentrations of CO₂ and CO, which accelerates the formation of ethylene and the FE reached 72% at −0.97 V. According to the above discussion, the Cu⁰–Cu⁺ reaction interface and –NH₂ induced molecular–catalyst interface both can promote the formation of C₂₊ products in the CO₂RR.

The widely suggested CO₂-to-C₂₊ conversion mechanisms show that the promoted CO₂ activation, the faster formation and adsorption of the C₁ intermediate, and the accelerated C–C coupling process are essential to increase the selectivity of C₂₊ products. However, developing a reaction interface that simultaneously meets the requirements of three aspects still faces great challenges. In this work, we construct a Cu⁰–Cu⁺–NH₂ composite reaction interface with the assistance of SiO₂, which includes both the Cu-based Cu⁰–Cu⁺ interface and the Cu catalyst–NH₂ group interface. We have fully utilized two characteristics of amorphous SiO₂: firstly, it can uniformly coat inorganic nanoparticles, and secondly, it can bond with silane coupling agents to introduce organic functional groups. Using Cu₂O nanoparticles as a model catalyst, a layer of porous SiO₂ is first coated on the surface of the particles (named Cu₂O@SiO₂). Under the conditions of electrochemical reduction, part of Cu⁺ is reduced to Cu⁰, and part of Cu⁺ maintains positive valence under the strong interaction of SiO₂, forming the interface of Cu⁰–Cu⁺. Then, a silane coupling agent containing –NH₂ is bonded on the surface of SiO₂ (named Cu₂O@SiO₂–NH₂), and the SiO₂ coating acts as a bridge between copper species and –NH₂ to form a Cu catalyst–NH₂ group interface. With the help of the synergistic effect of the Cu⁰–Cu⁺–NH₂ composite interfaces, the optimized Cu₂O@SiO₂–NH₂ catalyst achieves a selectivity of 81.2% for C₂₊ products at a current density of 292 mA cm⁻² at −1.7 V (without iR compensation). In situ Raman and attenuate total reflectance-infrared absorption spectroscopy (ATR-IRAS) spectra show that the interaction between surface –NH₂ and CO₂ molecules enhances the adsorption and activation process of CO₂ and promotes the formation of *CO, which increases the local concentration of surface *CO. On the Cu⁰–Cu⁺ interface, the C–C coupling process between *CO is accelerated, and the two interfaces synergistically promote the generation of C₂₊ products. This work provides a new strategy for constructing composite interfaces to promote the CO₂RR to C₂₊ product conversion under high current density.

2. Experimental section

2.1 Materials

All reagents involved in this study were commercially available and used without further purification: CuCl₂·2H₂O (AR, JiangTian), NaOH (AR, MACKLIN), Na₂CO₃ (AR, JiangTian), KCl (AR, JiangTian), ethyl orthosilicate (GC, Aladdin), aminopropyl triethoxysilane (AR, HEOWNS), ascorbic acid (AR, JiangTian), n-octane (AR, MERYER), L-arginine (AR, JiangTian), polyvinylpyrrolidone (MW = 58 000, MACKLIN). All aqueous solutions were prepared with deionized water. A proton exchange membrane (Nafion 211, DuPont), Nafion (5 wt%, Sigma-Aldrich), isopropanol (>99%, Aladdin), carbon NPs (50 nm, Sigma-Aldrich), and graphite (Aladdin) were obtained.

2.2 Catalyst preparation

Cu₂O dodecahedron nanoparticles were synthesized by a method reported in the literature.²³ 1.11 g PVP was dissolved into deionized water, and then 10 mL of 0.1 mol L⁻¹ CuCl₂·2H₂O was also added into the above solution dropwise, under stirring and heating to 55 °C. Then 10 mL of 2 mol L⁻¹ NaOH solution was added. After stirring at 55 °C for 30 min, 10 mL of 1 mol L⁻¹ ascorbic acid solution was added to the mixture and continued to stir at 55 °C for 3 h. Then, the precipitation was centrifuged and washed in deionized water and anhydrous ethanol 3 times to obtain Cu₂O dodecahedron nanoparticles.

Then the Cu₂O nanoparticles were modified with SiO₂ and –NH₂.²⁴ 0.73 g n-octane and 0.014 g L-arginine were added into 15 mL deionized water and stirred vigorously at 60 °C. n-Octane was used as the solvent to prevent excessive polymerization of SiO₂, and to control the formation process of SiO₂ on the surface of Cu₂O. After that, 25 μL ethyl orthosilicate (TEOS) was added to the mixture and stirred at 60 °C for 4 h. Then 0.2 g Cu₂O dodecahedron nanoparticles were added into the mixture and stirred at room temperature for 20 h. The precipitation was centrifuged and washed three times with deionized water and anhydrous ethanol to obtain Cu₂O@SiO₂ nanoparticles. The obtained Cu₂O@SiO₂ nanoparticles were evenly dispersed into 25 mL anhydrous ethanol, and then 25 μL aminopropyl triethoxysilane (APTES) was added, and stirred at room temperature for 24 h. The precipitation was centrifuged, washed three times with anhydrous ethanol and deionized water, and dried by vacuum at room temperature for 12 h. Cu₂O@SiO₂–NH₂ nanoparticles were obtained. For the control sample of Cu₂O@SiO₂, the amount of TEOS was increased to 50 μL during the SiO₂ coating process to maintain a coating thickness similar to that of the Cu₂O@SiO₂–NH₂ sample.

2.3 Catalyst characterization

Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4800 field emission scanning electron microscopy. Transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were conducted on a JEOL F200 transmission electron microscope with an acceleration voltage of 200 kV. The samples were dispersed in ethanol and ultrasonically treated for 20 min, and then the samples were added dropwise onto a Mo grid ultrathin carbon film. X-ray diffraction was conducted on a Bruker D8 Focus X-ray diffractometer using Cu Kα radiation (λ = 0.15418 nm). The tube voltage and current were 40 kV and 40 mA, respectively. The diffraction angle of samples was recorded from 20° to 80° (2θ) with a scan rate of 5° min⁻¹. N₂ physisorption analysis was conducted at −196 °C using a Tristar 3000 Micromeritics instrument. XPS was conducted on a Thermo-Fisher Scientistic K-Alpha+ instrument. The X-ray radiation source was Al Kα (hν = 1486.6 eV) with an X-ray power of 150 W and the spot size was 400 μm. The pass energy was 50 eV. The XPS was calibrated with a C 1s binding energy of 284.8 eV. X-ray absorption fine structure (XAFS) measurements were performed to probe the valence state and the bonding information of Cu species on a TableXAFS-500 X-ray absorption fine structure spectrometer. The CO₂ adsorption experiment was carried out on a BEL SORP-max at 298 K.

2.4 Electrode preparation

We first prepared a conductive gas-diffusion layer by sputtering the Cu layer on a PTFE film, copper target (99.999%). To prepare the catalyst ink, 10 mg of the pre-catalyst and 2 mg of carbon were dispersed in a mixture of 1 mL propanol and 30 μL of 5 wt% Nafion solution (Sigma-Aldrich); 10 mg carbon was dispersed in a mixture of 1 mL of propanol and 50 μL of 5 wt% Nafion solution; 10 mg graphite was dispersed in a mixture of 1 mL of propanol and 70 μL of 5 wt% Nafion solution, and then they were sonicated for at least 1 hour. The ink was airbrushed onto a 2 × 3 cm² Cu/PTFE substrate with a loading of ∼1 mg cm⁻²; the carbon–graphite mixture was sprayed on the catalyst surface in turn. The PTFE-based gas diffusion electrode could enhance CO₂ gas mass transfer through hydrophobic PTFE channels and the carbon powder and graphite powder with certain hydrophobicity on the surface of the catalyst could tailor the wettability of the electrolyte and prevent the catalyst from being flooded by aqueous electrolyte. A stainless-steel mesh was used as the anode. Before the reaction, the steel (1.5 × 2 cm²) was sonicated in acetone and deionized water for 30 min, respectively, and then dried by nitrogen purging for further use.

2.5 Electrochemical CO₂ reduction measurement

The CO₂RR activity of the catalyst was investigated by performing electrolysis in a flow-cell configuration using 1 M KCl as the cathodic and 2 M KOH as the anodic electrolyte. Cl⁻ anions can specifically adsorb on the inner Helmholtz plane (IHP) of the catalyst surface, which not only confines CO₂ and facilitates electron transfer from the electrode to CO₂via the X_ad–C bond but also improves the *CO adsorption for favorable C–C coupling.²⁵ Moreover, the formation of OH⁻ during the CO₂RR in the KCl electrolyte leads to a local alkaline environment, and the use of high pH can greatly accelerate the production rates for C₂₊ products.²⁶ The flow cell consists of a gas diffusion layer as the working electrode (0.5 cm²), a proton exchange membrane (Nafion 211), a stainless-steel mesh (1500 mesh) as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode. An electrochemical workstation (CHI660, Chenhua, Shanghai) was used to perform the CO₂RR test. CO₂ was supplied into gas chambers with a constant rate of 10 mL min⁻¹ by using a mass-flow controller, and the outlet gas flow rate was also recorded by the flow controller. The cathodic electrolyte (30 mL) was circulated through the electrolyte chambers under a constant flow (5 mL min⁻¹) via a peristaltic pump. The anodic electrolyte was circulated through the anodic chamber by a gas–liquid mixed flow pump. Reactions were tested via chronoamperometry for 30 min at different applied potentials from −1.1 V to −1.9 V. Gas products were analyzed via online GC (Shimadzu 2010) with a Thermal conductivity detector (TCD) (column: Agilent Carbon Plot (30 m × 0.32 mm × 3 μm)). The FE of gas products was calculated based on the following:
where C_product is the concentration of the gas-phase products (mol L⁻¹), ν_CO2 is the flow rate of CO₂, t is the reaction time, e is the number of transferred electrons for the product, F the Faraday constant 96 485 C mol⁻¹, V_m is the gas mole volume, and Q is the total quantity of electric charge.

The liquid products were determined by H NMR (JEOL JNM ECZ600R 600 MHz), in which 300 μL electrolyte was mixed with 300 μL D₂O and 10 μL diluent dimethyl sulfoxide (DMSO), wherein DMSO served as an internal standard. The concentration of liquid products was calculated based on the following:

where V is the volume of DMSO; ρ_DMSO is the density of DMSO (1.1 g cm⁻³ at 20 °C); M is the molar mass; c_X is the concentration of X product; n_X(H) is the number of hydrogen atoms in the product X in the nuclear magnetic spectrum used to calculate the concentration of X; is the ratio of the product peak area to DMSO area in the nuclear magnetic spectrum.

The FE of liquid products was computed from:

FE = e × F × n/Q

where n is the total product (in moles).

Potentials were referenced to the RHE based on the following:

E_RHE = E (vs. Ag/AgCl) + 0.197 V + 0.059 × pH

2.6 In situ Raman test

In situ Raman experiments were conducted by Confocal Raman Microscopy (Horiba) with a 785 nm near-infrared laser in a homemade electrolyzer cell. A platinum electrode and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The catalyst for the cathode was sprayed on carbon paper (0.5 × 1 cm²) using ionomer solution as a binder. The electrolyte (1 M KCl) was saturated with CO₂ solution and CO₂ was continued to inject at a flow rate of 5 mL min⁻¹. A long focal length lens (Leica, 50×) was used for focusing and collecting the incident and scattered laser light. Electrolysis at different cathodic potentials was performed for 10 min before signal collection.

2.7 In situ ATR-IRAS measurement

In situ ATR-IRAS experiments were performed on a Nicolet iS20 spectrometer equipped with a HgCdTe (MCT/A) detector and a VeeMax III (PIKE Technologies) accessory in a homemade single-cell electrolyzer. A platinum electrode and Ag/AgCl electrode were used as the counter and reference electrodes. A fixed-angle Si prism (60°) coated with a catalyst embedded into the bottom of the cell served as the working electrode. Before testing, the detector was cooled with liquid nitrogen for at least 30 min to maintain a stable signal. Electrolysis at different cathodic potentials was carried out for 3 min with chronoamperometry by spectrum collection (32 scans, 4 cm⁻¹ resolution). All spectra were subtracted from the background.

3. Results and discussion

3.1 Synthesis of catalysts and characterization analysis

As shown in Fig. 1, the synthesis of Cu₂O@SiO₂–NH₂ can be divided into three steps. Firstly, Cu₂O dodecahedron nanoparticles were synthesized by a precipitation method. Second, the Cu₂O NPs were mixed with TEOS uniformly, and the SiO₂ formed during the hydrolysis of TEOS is uniformly covered on the surface of Cu₂O NPs to form Cu₂O@SiO₂ samples. Finally, the Cu₂O@SiO₂ nanoparticles and ATPES were evenly dispersed with anhydrous ethanol to obtain Cu₂O@SiO₂–NH₂.²⁴ The detailed procedure is discussed in the Experimental section.

Fig. 1 The schematic illustration of the Cu₂O@SiO₂–NH₂ synthesis process.

The morphology of the catalyst was characterized by SEM. As shown in Fig. 2a–c and S1,† the Cu₂O nanoparticles exhibit a regular dodecahedral shape. After SiO₂ coating, the surface of the nanoparticles becomes rough, indicating the successful coating of SiO₂, and there may be a porous structure (Fig. 2b and S1b†). In the subsequent modification process of –NH₂, the whole particle still maintained a complete dodecahedral structure. The results show that the SiO₂ coating and –NH₂ modification have no significant effect on the overall structure of Cu₂O nanoparticles (Fig. 2a–c and S1†). The structure of Cu₂O@SiO₂–NH₂ and control samples were characterized by TEM. As shown in Fig. S2,† the Cu₂O nanoparticle exhibits a smooth surface in the TEM image, and the high-resolution TEM (HR-TEM) image shows that there is a lattice fringe d = 0.213 nm on the catalyst surface, which is assigned to the Cu₂O (200) surface (Fig. S2b†). The Cu and O elements overlap with the STEM image in EDX element mapping (Fig. S2c†). In the TEM images of the Cu₂O@SiO₂ sample (Fig. S3a–c†), a lattice fringe of d = 0.246 nm can be seen in the inner layer, which belongs to the Cu₂O (111) crystal plane, and an amorphous SiO₂ coating with a thickness of about 10 nm located at the outer layer can be seen, which construct the obvious Cu₂O–SiO₂ interface. At the same time, the STEM and EDX element distribution maps further confirm the formation of the coating structure (Fig. S3d and e†). For the Cu₂O@SiO₂–NH₂ sample, a uniform coating layer with a thickness of about 13 nm can be seen on the surface of Cu₂O nanoparticles (Fig. 2a–f). By comparison with Cu₂O@SiO₂, it can be preliminarily confirmed that the Cu₂O@SiO₂ sample surface has been successfully modified with –NH₂. The lattice fringe of d = 0.302 nm of the Cu₂O (110) facet in close contact with the SiO₂–NH₂ coating layer in HR-TEM images (Fig. 2g) can confirm the formation of a Cu₂O–SiO₂–NH₂ interface. Furthermore, the EDX elemental maps show that the Si and N elements are mainly distributed on the surface of Cu₂O nanoparticles, and the overlay image of element distribution further proves the formation of Cu₂O@SiO₂–NH₂. The surface pore distribution on the surface of Cu₂O and Cu₂O@SiO₂–NH₂ catalysts was characterized by using N₂ isothermal adsorption–desorption curves. As illustrated in Fig. S4a,† the surface of Cu₂O exhibits almost no pore structure. In contrast, the N₂ isothermal adsorption–desorption curve of the Cu₂O@SiO₂–NH₂ catalyst reveals a gradual uptake of nitrogen gas with a hysteresis loop, indicating the presence of irregular pores on its surface. At the same time, combined with the pore size distribution diagram in Fig. S4b,† it shows that there are mesoporous pores with an average pore size of 3.32 nm distributed on the Cu₂O@SiO₂–NH₂ catalyst surface. And its specific surface area increases from 6 m² g⁻¹ of Cu₂O to 40 m² g⁻¹ of Cu₂O@SiO₂–NH₂.

Fig. 2 The SEM images of (a) Cu₂O, (b) Cu₂O@SiO₂, and (c) Cu₂O@SiO₂–NH₂; and the structural characterization of Cu₂O@SiO₂–NH₂ (d and e) TEM images, (f) HR-TEM image and (g) EDX mapping.

X-ray diffraction (XRD) was conducted to analyze the chemical compositions of Cu₂O@SiO₂–NH₂ and the control samples. As shown in Fig. 3a and S5,† five obvious diffraction peaks in the XRD pattern of Cu₂O are located at 2θ = 29°, 36°, 42.5°, 62° and 74°, which belong to the (110), (111), (200), (220) and (311) crystal planes of Cu₂O, respectively. However, the position of the diffraction peaks of Cu₂O dodecahedron nanoparticles have no obvious change after SiO₂ coating and –NH₂ modification, indicating that the process of SiO₂ coating and –NH₂ modification have no significant effect on the crystal phase structure of Cu₂O nanoparticles. The surface chemical state of the samples was characterized by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 3b, for the Cu 2p XPS spectrum, there are only two peaks at the binding energies of 932.3 and 952.0 eV for the Cu₂O catalyst, which are ascribed to Cu⁺ 2p_3/2 and 2p_1/2, respectively.²⁷ Meanwhile for Cu₂O@SiO₂ and Cu₂O–SiO₂–NH₂ catalysts, a satellite peak at 943 eV appears in the Cu 2p XPS spectra, indicating the presence of Cu²⁺ species.²⁸ Meanwhile, the asymmetric peaks of Cu 2p can be deconvoluted into two sets of peaks. The first group of peaks with binding energies of 932.3 and 952.0 eV are attributed to 2p_3/2 and 2p_1/2 of Cu⁺ species. The second set of peaks at 934.6 and 954.3 eV belong to 2p_3/2 and 2p_1/2 of Cu²⁺ species.²⁹ Compared with Cu₂O, both Cu²⁺ and Cu⁺ species exist on the surface of Cu₂O@SiO₂ and Cu₂O@SiO₂–NH₂, indicating strong interaction between SiO₂ and Cu species. This result is further confirmed by the Cu LMM X-ray induced Auger transition spectra (XAES) (Fig. 3c), where the Cu LMM XAES of Cu₂O sample shows a symmetrical peak at 916.8 eV, which belongs to Cu⁺ species, while the Cu LMM XAES of Cu₂O@SiO₂ and Cu₂O@SiO₂–NH₂ show an asymmetric peak with a wider full width at half maximum (FWHM), indicating the coexistence of Cu⁺ and Cu²⁺ species at 916.8 and 917.9 eV.³⁰ As is well known, XRD is the analysis of the entire bulk phase, while XPS is surface analysis. No CuO phase is observed in the XRD pattern (Fig. 3a), indicating a low content of Cu²⁺. XPS spectra (Fig. 3b) show a clear Cu²⁺ peak, indicating that Cu²⁺ is mainly present on the surface, while for Cu₂O@SiO₂–NH₂ and Cu₂O@SiO₂ catalysts, it is at the interface between Cu₂O and SiO₂. The production of Cu²⁺ indicates that the strong interaction between SiO₂ and Cu₂O at the interface induces the oxidation effect of low valent Cu, which will affect the reduction of Cu₂O in the CO₂RR.³¹ Moreover, XPS analysis was also performed on the Cu₂O@ sample that was treated under the same conditions without TEOS and APTES, and there is no significant difference in Cu 2p XPS between the Cu₂O@ and Cu₂O samples, which indicates that the treatment process did not oxidize Cu⁺ to Cu²⁺. Therefore, it can be confirmed that the strong interaction between SiO₂ and Cu species at the Cu₂O–SiO₂ interface promotes the formation of Cu²⁺ species. In addition, the chemical state of the –NH₂ group on the surface was characterized by N 1s XPS. As shown in Fig. 3d, the spectrum of N 1s XPS can be divided into two peaks located at the binding energies of 400 and 403.2 eV, respectively. According to a previous report, N 1s XPS at low binding energy belongs to the –NH₂ group, while the peak at high binding energy belongs to –NH₃⁺.³² This means that part of the –NH₂ group on Cu₂O@SiO₂–NH₂ has been protonated, which may increase the local pH on the catalyst surface.²²

Fig. 3 Chemical composition characterization of Cu₂O, Cu₂O@SiO₂ and Cu₂O@SiO₂–NH₂. (a) XRD patterns, (b) Cu 2p XPS spectra, (c) Cu LMM XAES spectra, (d) N 1s XPS spectrum, (e) XANES spectra and (f) Fourier transformed EXAFS spectra of the Cu K-edge.

To explore more detailed structural information, the samples are further investigated by X-ray absorption fine structure spectroscopy (XAFS). Fig. 3e shows the X-ray absorption near-edge structure (XANES) spectra of Cu. In the Cu K-edge spectra, the pre-edge peak at 8987 eV is attributed to the dipole-forbidden 1s to 3d electron transition, which represents the fingerprint of Cu²⁺. Moreover, the absorption edge of the curve located between those for Cu₂O and CuO shows the averaged valence state of Cu species in Cu₂O@SiO₂–NH₂ between +1 and +2, which is consistent with XPS data. Moreover, the extended X-ray absorption fine structure (EXAFS) spectra of the Cu K-edge show that compared to pure Cu₂O, the Cu₂O@SiO₂–NH₂ catalyst exhibits a shorter Cu–O bond length in the first shell, indicating that SiO₂ enhances the bonding between Cu²⁺/Cu⁺ and O (Fig. 3f). To demonstrate the interaction between –NH₂ groups and CO₂, the CO₂ adsorption capacity of the catalyst was tested. As shown in Fig. S6,† compared to Cu₂O@SiO₂, Cu₂O@SiO₂–NH₂ exhibits stronger CO₂ adsorption, indicating that the modification of –NH₂ significantly enhances the CO₂ adsorption capacity, which will play an important role in promoting the activity of the CO₂RR.

3.2 Electrochemical CO₂ reduction performance

The catalytic performance of Cu₂O@SiO₂–NH₂ for the CO₂RR was evaluated in a flow-cell with 1 M KCl cathode electrolyte (Fig. S7†). The gas and liquid phase products were detected by online gas chromatography (GC) and ¹H nuclear magnetic resonance (¹H NMR), respectively, (Fig. S8 and S9†). Fig. 4a shows the linear sweep voltammetry (LSV) curves of Cu₂O@SiO₂–NH₂ and the control samples. The cathode current density increases sharply after CO₂ gas is introduced, corresponding to the catalytic CO₂RR. When CO₂ is replaced by N₂, the cathodic current of Cu₂O@SiO₂–NH₂ decreases significantly, which indirectly indicates the low HER activity of Cu₂O@SiO₂–NH₂. Under a N₂ atmosphere, the HER current density of the Cu₂O@SiO₂–NH₂ catalyst is relatively low compared to Cu₂O. Another possible reason is that the modification of SiO₂ and –NH₂ groups hinders the diffusion of H₂O molecules to the catalyst surface, while the CO₂ atmosphere eliminates this obstacle and accelerates the adsorption and diffusion of H₂O and CO₂. The current density of the CO₂RR on Cu₂O@SiO₂–NH₂ is significantly higher than that on Cu₂O and Cu₂O@SiO₂, suggesting that Cu₂O@SiO₂–NH₂ has a strong CO₂RR activity. Due to the exposure of the Cu₂O catalyst to the electrolyte solution, its HER current density under the N₂ atmosphere is slightly higher than that of Cu₂O@SiO₂ and Cu₂O@SiO₂–NH₂, indicating that the HER is more likely to occur in the Cu₂O catalyst.

Fig. 4 CO₂RR performance on Cu₂O@SiO₂–NH₂, Cu₂O@SiO₂ and Cu₂O catalysts. (a) LSV curves toward the CO₂RR; product distributions and corresponding faradaic efficiencies produced by Cu₂O@SiO₂–NH₂ (b), Cu₂O@SiO₂ (c), and Cu₂O (d), (e) the partial current density of C₂₊ products, and (f) stability test of Cu₂O@SiO₂–NH₂ at 280 mA cm⁻² in 1 M KCl electrolyte.

The product distribution of the CO₂RR over Cu₂O@SiO₂–NH₂ and the control samples in the cathode potential range of −1.1 to −1.9 V as shown in Fig. 4b–d. The total FE of C₂₊ products exhibits a volcanic trend relative to the cathodic potential change over Cu₂O@SiO₂–NH₂ (Fig. 4b), and the total current density increases from 80 to 375 mA cm⁻² (Fig. S10†). Under a current density of 292 mA cm⁻² at −1.7 V, the FE of C₂₊ products reaches the highest value of 81.2% (including C₂H₄: 40.2%, C₂H₅OH: 29%, CH₃COOH: 9%, and C₃H₇OH: 3%), which has a higher activity than most reported catalysts (Table S1†). The FE of H₂ and C₁ is only 12% and 6%, respectively. In comparison, the FE of C₂₊ is 68.3% (C₂H₄: 35%, C₂H₅OH: 24.3%, CH₃COOH: 6.5%, and C₃H₇OH: 2.5%) over Cu₂O@SiO₂ at −1.7 V (Fig. 4c). Meanwhile for Cu₂O dodecahedron nanoparticles, the C₂₊ FE of Cu₂O is only 61.7% (C₂H₄: 31%, C₂H₅OH: 21%, CH₃COOH: 7%, and C₃H₇OH: 2.7%) at the optimum potential of −1.6 V (Fig. 4d). As a result, the ratio of C₂₊/C₁ increases significantly from 4 in Cu₂O to 13.5 in Cu₂O@SiO₂–NH₂ (Fig. S11a†). In addition, the partial current densities of C₂₊ products of different samples increase from 147 mA cm⁻² of Cu₂O to 168 mA cm⁻² of Cu₂O@SiO₂ during the CO₂RR process, and then it further increases to 237 mA cm⁻² over Cu₂O@SiO₂–NH₂ (Fig. 4e and S11b–d†). These results indicate that the Cu₂O@SiO₂–NH₂ catalyst has a high selectivity of C₂₊ products in the process of the CO₂RR.

By comparison, it can be found that when Cu₂O nanoparticles are coated with SiO₂, the HER is inhibited and the selectivity of C₂₊ products increases in the full cathodic potential range. This indicates that the SiO₂ coating layer helps to promote the C–C coupling process. According to the Cu 2p XPS results, the Cu species on the surface will maintain a high valence state after coating with SiO₂, thus improving the adsorption strength of *CO on Cu species and increasing the surface coverage of *CO,³³ followed by enhanced C–C coupling. After modifying the surface of the Cu₂O@SiO₂ sample with the –NH₂ group, the CO selectivity increases at low cathodic potential, indicating that the CO₂RR process is accelerated. According to previous reports, the interaction between nucleophilic N in –NH₂ and electrophilic C in CO₂ molecules can enrich CO₂, thus increasing the local CO₂ concentration on the catalyst surface,^22,34,35 and increasing the conversion rate of CO₂ to CO. Therefore, the reaction environment with a high local concentration of *CO can be provided at higher cathodic potential. Meanwhile, according to the N 1s XPS results (Fig. 3d), part of the –NH₂ group on the catalyst surface interacts with H₂O to form protonated –NH₃⁺ and release OH⁻ at the same time, leading to a higher local pH value of the reaction micro-environment,²² which is beneficial to promote the dimerization process of the C₁ intermediate.³⁶ Therefore, the positively charged Cu species coupled with the reaction microenvironment with sufficient CO supply and a high local pH value created by the surface –NH₂ group significantly enhance the formation of C₂₊ products from the CO₂RR and increase the FE of C₂₊ to 81.2% (Fig. 4b). However, in the Cu₂O catalyst without SiO₂ coating, the FE of CH₄ increases at high potential (Fig. 4d). According to a previous report, the CO₂RR is conducive to the formation of the C₁ product over the bulk Cu catalyst in neutral electrolyte.^37,38 Therefore, it can be inferred that Cu₂O is rapidly reduced to form Cu in the reaction process, which leads to the formation of CH₄ and promotes the HER process at the same time, while the SiO₂ coating promotes the formation of C₂₊ by stabilizing the positively charged Cu species in the catalyst.³¹ To eliminate the influence of the Cu/PTFE substrate on the analysis results, the CO₂RR performance of the Cu/PTFE substrate was tested under the same conditions. It is found that the CO₂RR to C₂₊ product conversion over Cu/PTFE shows a maximum FE of 26.2% at −1.5 V, and the FE of H₂ reaches 58% with a total current density of only 67 mA cm⁻² (Fig. S12†). The partial current density of C₂₊ products over Cu/PTFE is only 17 mA cm⁻², compared to 237 mA cm⁻² over Cu₂O@SiO₂–NH₂, which shows that the main activity source of the CO₂RR to C₂₊ products is originated from the target catalyst, and the Cu/PTFE substrate has little effect on the activity of the catalysts. The pH value of the electrolyte was also tested during the CO₂RR. Due to the generation of OH⁻ at the cathode and the reaction of some OH⁻ with CO₂ to generate CO₃²⁻, the pH increased from 6.82 at 0 V to 10.11 at −1.9 V (Fig. S13†), and the local alkaline environment may facilitate C–C coupling.² In addition to the excellent FE of C₂₊, Cu₂O@SiO₂–NH₂ also exhibits high stability at high current densities. The FE of C₂₊ from the CO₂RR over Cu₂O@SiO₂–NH₂ does not decrease significantly for 15 h with 280 mA cm⁻² current density in 1 M KCl electrolyte and remains above 75% (Fig. 4f).

3.3 Characterization of the samples after the activity test

Considering the reconstruction phenomenon of the oxidation state Cu-based catalyst during the CO₂RR, it is necessary to further explore the activity source of Cu₂O@SiO₂–NH₂ and the control samples, so the structure and composition of the samples after the activity test were characterized (the samples after the test were stored in a vacuum). As shown in TEM images of Fig. S14–S16,† the morphologies of Cu₂O@SiO₂–NH₂ and Cu₂O@SiO₂ catalysts have no obvious change after the CO₂RR, except that the thickness of the SiO₂–NH₂ layer slightly decreases, indicating that the coating layer is partially dissolved during the reaction. It can be explained by the fact that the formation of OH⁻ during the CO₂RR increases the pH of the solution, and SiO₂ reacts with OH⁻ leading to a decrease in the coating layer thickness. Meanwhile, the HR-TEM images of Cu₂O@SiO₂–NH₂ and Cu₂O@SiO₂ samples (Fig. S14c, d and S15c, d†) show lattice fringes of d = 0.302, 0.213 and 0.209 nm, which belong to the (110) and (200) of Cu₂O and Cu (111) crystal facets, respectively. This indicates that the oxidation state Cu on the catalyst surface is partially reduced, forming the Cu–Cu₂O interface; therefore, the metal-oxide interface effect can effectively improve the formation rate of C₂₊ from the CO₂RR.²⁰ At the same time, the elements overlap each other in the EDX mapping of Cu₂O@SiO₂–NH₂ and Cu₂O@SiO₂ samples, which proves that the Cu₂O@SiO₂–NH₂ catalyst still maintains a complete coating structure after the CO₂RR (Fig. S14e, f and S15e†). In contrast, the morphology of the Cu₂O catalyst after the CO₂RR is significantly changed (Fig. S16a and b†), and only the lattice fringe of the Cu (111) facet (d = 0.2086 nm) can be observed in the HR-TEM image (Fig. S16c†). This indicates that Cu₂O nanoparticles are completely reduced to Cu during the CO₂RR. Notably, irregular holes can be observed on the surface of Cu₂O nanoparticles (Fig. S16d†), which further indicates that Cu₂O nanoparticles are reduced, leading to a change in the morphology of nanoparticles. The EDX mapping also confirmed the morphology change (Fig. S16e†), where the Cu element is not evenly distributed. By comparing the TEM images of Cu₂O@SiO₂–NH₂, Cu₂O@SiO₂ and Cu₂O catalysts, the results indicate that SiO₂ coating can stabilize the morphology of Cu₂O nanoparticles, inhibit the reduction of oxidized Cu in the catalyst, and form a stable metal-oxide interface in the reduction process, which accelerates the C–C coupling step during the CO₂RR.

XRD characterization was carried out to analyze the chemical composition of the catalyst after the CO₂RR. As shown in Fig. 5a, there is only one diffraction peak at 2θ = 43.2° in the XRD pattern of the Cu₂O catalyst, which is attributed to metal Cu (PDF# 97-004-3493), indicating that Cu₂O is completely reduced during the CO₂RR. There are two sets of diffraction peaks in the XRD patterns of Cu₂O@SiO₂–NH₂ and Cu₂O–SiO₂ catalysts, which are located at 2θ = 36.4°, 42.5° and 2θ = 43.2°. They belong to the Cu₂O (PDF# 97-018-0846) and Cu phases, respectively. The XRD results show that Cu₂O nanoparticles coated with SiO₂ inhibit the reduction of Cu⁺ species and form a metal-oxide interface in the CO₂RR process. Combined with TEM characterization results, it is further demonstrated that SiO₂ coating is beneficial to stabilize the oxidized copper in Cu₂O nanoparticles during the CO₂RR, due to the strong interaction between SiO₂ and Cu₂O species. At the same time, the Cu 2p XPS spectrum of Cu₂O after the CO₂RR (Fig. 5b) shows only two peaks located at 932.0 and 951.8 eV, belonging to Cu⁰ species.³⁹ Meanwhile for Cu 2p XPS of Cu₂O@SiO₂ and Cu₂O@SiO₂–NH₂, there is weak satellite peaks at 942–944 eV and acromial peaks at 934.7 eV, indicating the presence of a small amount of Cu²⁺ on the surface.⁴⁰ We further analyzed the Cu LMM XAES spectra of Cu₂O@SiO₂–NH₂ and control samples after the CO₂RR. As shown in Fig. 5c, there is only one peak located at 918.1 eV in the Cu LMM XAES of the Cu₂O catalyst, which belongs to Cu⁰ species, suggesting that all Cu⁺ species are reduced to Cu⁰ species. In contrast, the Cu LMM XAES of Cu₂O@SiO₂ and Cu₂O@SiO₂–NH₂ catalysts can be fitted into two peaks at 916.2 and 918.1 eV, belonging to the Cu⁺ and Cu⁰ species, respectively.^19,41 Compared with the Cu 2p XPS before the CO₂RR (Fig. 3b), the surface of the Cu₂O@SiO₂–NH₂ catalyst is mainly Cu²⁺, and after the CO₂RR, it is mainly mixed Cu⁺ and Cu⁰, indicating that SiO₂ can stabilize part of Cu⁺ and form Cu⁺–Cu⁰ interfaces. The results are consistent with TEM and XRD images. Meanwhile, the Si 2p XPS also can be detected in Cu₂O@SiO₂–NH₂ and Cu₂O@SiO₂ catalysts, which confirms the presence of SiO₂ on the catalyst surface after the CO₂RR (Fig. S17†). Moreover, the N 1s XPS spectrum shows the retention of –NH₂ groups on the surface and the presence of groups in two states of –NH₂ and –NH₃⁺, indicating that the surface –NH₂ continued to play a role in the enrichment of CO₂ during the reaction process and accelerated the conversion rate of CO₂ (Fig. 5d).

Fig. 5 Chemical composition characterizations of Cu₂O, Cu₂O@SiO₂ and Cu₂O@SiO₂–NH₂ after the CO₂RR. (a) XRD patterns, (b) Cu 2p XPS spectra, (c) Cu LMM XAES spectra and (d) N 1s XPS spectrum.

3.4 Reaction mechanism study

In situ Raman experiments were performed to study the valence changes of Cu species of Cu₂O@SiO₂–NH₂ and control samples, and to detect the key intermediates in the CO₂RR process. As shown in Fig. 6, under open-circuit potential conditions, the two Raman peaks at 145 and 213 cm⁻¹ belong to the Cu₂O phase.¹³ When the cathodic potential is applied, the peaks of the Cu₂O phase still exist in the Raman spectrum of Cu₂O@SiO₂–NH₂, indicating that the Cu^δ+ species can be well preserved in the CO₂RR process. Similarly, this phenomenon has also been observed on the Cu₂O@SiO₂ catalyst, where Cu₂O species can exist stably with the increase of cathodic potential. In contrast, for the Cu₂O catalyst, the two Raman peaks at 145 and 213 cm⁻¹ only can be observed under the open-circuit potential conditions. However, when a cathodic potential of −1.1 V is applied, the two peaks disappear, suggesting that the Cu₂O on the catalyst surface is completely reduced to metal Cu. The above analysis results imply that the strong interaction between SiO₂ and Cu₂O can improve the stability of Cu^δ+ species on the catalyst surface. This is consistent with the results of TEM and XRD of samples after the CO₂RR. More importantly, in the Raman spectrum of Cu₂O@SiO₂–NH₂, an obvious peak at 529 cm⁻¹ is observed, which is related to the chemisorption of CO₂ on the copper surface (*CO_2ad),^42,43 and it further enhances with the increase of cathodic potential applied. This shows that the catalyst has strong CO₂ adsorption and activation ability (Fig. 6a). In contrast, the peak intensity of *CO_2ad at 529 cm⁻¹ on Cu₂O@SiO₂ and Cu₂O catalysts decreases sharply (Fig. 6b and c), indicating that *CO_2ad is less adsorbed on the surface. Therefore, this result shows that –NH₂ enhances the ability of the catalyst to adsorb and activate CO₂. As mentioned above, the –NH₂ group can enrich CO₂ molecules, thus improving the CO₂ coverage on the catalyst surface.²²

Fig. 6 In situ Raman spectra of the catalysts under CO₂RR conditions. (a and d) Cu₂O@SiO₂–NH₂, (b and e) Cu₂O@SiO₂, and (c and f) Cu₂O.

Moreover, the peaks at 297 and 373 cm⁻¹ are attributed to the rotation and stretching vibrations of *CO on Cu (Cu–CO) in the Raman spectra,^12,44 which indicate the formation of CO and the adsorption of *CO on the catalyst surface (Fig. 6a–c). At the same time, the most obvious peak at 2000–2080 cm⁻¹ is associated with *CO_a (Fig. 6d).²⁷ It is worth noting that the adsorption peak of *CO first increases and then decreases with the increase of cathodic potential, indicating that the coverage of *CO on the catalyst surface increases first with the cathodic potential, which promotes the C–C coupling step. However, after the C–C coupling reaction, the *CO adsorbed on the surface will be consumed, thus weakening the adsorption peak of *CO.⁴⁴ In contrast, the Cu–CO Raman signal on Cu₂O@SiO₂ is weaker (Fig. 6b and e), indicating the low surface coverage of *CO. Moreover, the Cu–CO Raman signal is the weakest on the Cu₂O catalyst (Fig. 6c and f). The results of in situ Raman show that the interaction between –NH₂ and CO₂ creates a high local concentration of the CO₂ microenvironment at the Cu⁰–Cu⁺–NH₂ composite interfaces, which accelerates the activation of CO₂ and generation of CO, and enhances the adsorption of *CO.⁴⁵ At the same time, the Cu⁰/Cu⁺ synergistic effect at the interface may promote the C–C coupling step. However, the Cu⁰–Cu⁺ interface without –NH₂ on the Cu₂O@SiO₂ surface leads to a low local concentration of the CO₂ microenvironment, which suppresses the CO generation rate, followed by a slow C–C coupling step. The lack of a SiO₂ coating layer on the Cu₂O catalyst leads to the quick reduction of Cu₂O to metal Cu, thus reducing the adsorption strength of CO.

In situ attenuated total reflectance-infrared absorption spectroscopy (ATR-IRAS) was carried out under CO₂RR conditions to monitor the adsorption intermediates and get insight into the CO₂RR mechanism more precisely. As shown in Fig. 7, when the applied cathodic potential is higher than −1.1 V, new infrared (IR) absorption peaks begin to appear in the ATR-IRAS of all catalysts. The most obvious peak appears at 1650 cm⁻¹ in the ATR-IRAS of Cu₂O@SiO₂–NH₂ and Cu₂O@SiO₂ (Fig. 7a, b), which is related to the IR absorption peak of H₂O.⁴⁶ The intensity of the IR absorption peak increases with the increase of cathodic potential, indicating that the process of H₂O adsorption and activation is accelerated at high cathodic potential. In contrast, on the surface of the Cu₂O catalyst, the intensity of this absorption peak decreases sharply. This suggests that the surface of Cu₂O@SiO₂ can promote the adsorption and activation of H₂O. This can be explained by the fact that the metal-oxide interface (Cu⁰–Cu⁺) can regulate the dissociation process of H₂O, resulting in high coverage of *H species on the catalyst surface.^47,48 The presence of a metal-oxide interface is also confirmed by the XRD and TEM results. From the CO₂RR equations in Table S2,† it can be seen that the CO₂RR must involve the participation of active *H, such as the transfer of 12 protons and 12 electrons to generate ethylene and ethanol, where the protons obtain electrons and become active *H. Therefore, the peak intensity of the H₂O peak represents the ability to activate H₂O to produce active *H, which is partially involved in the CO₂RR and partially in the HER. For catalysts with high CO₂RR activity, most of the active *H is involved in the CO₂RR. In contrast, for catalysts with poor CO₂RR activity, most of the active *H is involved in the HER. From Table S2,† it can also be seen that the number of electrons transferred is directly proportional to the number of protons consumed, which means that the number of active *H is directly proportional to the current density. In Fig. S11,† the current density of C₂₊ in the CO₂RR process of the Cu₂O@SiO₂–NH₂ catalyst is 237 mA cm⁻², and the current density of H₂ is only 35.32 mA cm⁻², while for the Cu₂O catalyst, the current density of C₂₊ is only 147 mA cm⁻², but the current density of H₂ is 76.55 mA cm⁻². From this result, it can be seen that more active *H is generated in the Cu₂O@SiO₂–NH₂ catalyst, and most of it is involved in the CO₂RR. Although the HER current density of Cu₂O is high, the total current density is low, resulting in less active *H, which is consistent with the H₂O peak intensity in the ATR-IRAS spectra.

Fig. 7 In situ ATR-IRAS spectra of catalysts under CO₂RR conditions and the reaction mechanism. (a) Cu₂O@SiO₂–NH₂, (b) Cu₂O@SiO₂, and (c) Cu₂O and (d) reaction mechanism of the CO₂RR to C₂₊ product formation over the Cu₂O@SiO₂–NH₂ catalyst.

In addition to the H₂O absorption peak, the other strong peak located at 1390 cm⁻¹ belongs to the *COOH species,⁴⁹ implying that the Cu₂O@SiO₂–NH₂ catalyst has strong adsorption and activation ability for CO₂ molecules. The simultaneous adsorption and activation of H₂O and CO₂ molecules indicate a good coupling between H₂O dissociation and CO₂ reduction. The activated *CO₂ species and surface *H_ad species promote the formation of *CO species, which can be confirmed by the *CO absorption peak appearing around 2070 cm⁻¹ in the ATR-IRAS spectrum.⁴⁶ There is an obvious *CO IR absorption peak on the Cu₂O@SiO₂–NH₂ surface, indicating the high coverage of *CO on the Cu₂O@SiO₂–NH₂ surface, which promotes the formation of C₂₊ products (Fig. 7a). In contrast, the ATR-IRAS of Cu₂O@SiO₂ shows a weak *CO IR absorption peak, which corresponds to a low CO coverage (Fig. 7b). However, no obvious IR absorption peak of *CO was found on the surface of Cu₂O, indicating weak *CO adsorption capacity on the surface, because Cu₂O was reduced to Cu⁰, followed by low *CO coverage (Fig. 7c). This finding proves that the presence of –NH₂ can improve the adsorption and activation of CO₂ on the catalyst surface, and the Cu⁺ species can promote the adsorption of CO, thus improving the surface coverage of *CO.

In addition, the peaks of ATR-IRAS at 1260 cm⁻¹, 1205 cm⁻¹, and 1530 cm⁻¹ are related to the intermediate species of *CHO and *OCCOH on the catalyst, respectively (Fig. 7).⁴⁹ The absorption peak intensity of *OCCOH increases with the cathodic potential, which is consistent with the enhancement trend of the C₂₊ product formation rate with the change of the cathodic potential. Compared with Cu₂O@SiO₂ and Cu₂O catalysts, the IR absorption peak of the C₂ intermediate on Cu₂O@SiO₂–NH₂ is more intense, indicating that the C₂ intermediate has a stable adsorption structure on the catalyst surface, which is conducive to promoting C–C coupling, followed by higher C₂₊ product selectivity. Therefore, on the surface of Cu₂O@SiO₂–NH₂, the enhancement of the CO₂ adsorption activation process improves the surface coverage of *CO, thus accelerating the C–C coupling process. The high coverage of *H_ad species can enhance the protonation process of C₂₊ intermediates and desorption of products, thus promoting the efficient formation of C₂₊ products from the CO₂RR process.^50,51

According to the above analysis results, the CO₂RR mechanism on the surface of Cu₂O@SiO₂–NH₂ can be reasonably proposed (Fig. 7d). Firstly, CO₂ reaches the three-phase reaction interface through the gas diffusion layer, and then is enriched by –NH₂ at the Cu⁰–Cu⁺–NH₂ composite interfaces, forming a local microenvironment with a high concentration of CO₂.³⁴ CO₂ molecules diffused through the porous SiO₂ coating layer to reach the Cu⁰–Cu⁺ interface, and adsorbed and activated at the active site to form *CO_2ad. At the same time, the H₂O molecules are activated by the Cu⁰–Cu⁺ interface to increase the concentration of *H, which couples with CO₂ activation to form *COOH, and promotes the formation of *CO. Then the Cu⁺ enhances the adsorption of *CO, which facilitate the C–C coupling process at the Cu^δ+/Cu⁰ interface. The C₂ intermediate is protonated by *H on the catalyst surface, which promotes product desorption and thus accelerates the formation of C₂₊ products. This mechanism is consistent with the theoretical calculation results reported in the literature. For example, the electron density around the C atoms in *CO on the Cu₂O–Cu interface is higher than that on Cu₂O, which reduces the reaction energy barrier of C–C coupling to form *OCCO. Meanwhile, the energy barrier of H₂O dissociation on Cu, Cu₂O, and Cu₂O–Cu is 2.33, 2.15, and 1.64 eV, respectively; therefore, the faster H₂O dissociation and promoted C–C coupling contribute to accelerated C₂₊ product formation from the CO₂RR on the Cu₂O–Cu interface.⁴⁸

4. Conclusion

In this work, dodecahedral Cu₂O@SiO₂–NH₂ nanoparticles have been synthesized by hydrolysis of TEOS and APTES on the surface of Cu₂O particles for the CO₂RR to C₂₊ products. A series of characterization results show electronic interaction between Cu₂O and SiO₂–NH₂, and the Cu⁰–Cu⁺–NH₂ composite interface is formed in the CO₂RR process. With the help of the interface effect, the FE of C₂₊ products reaches 81.3% with a current density of 290 mA cm⁻², and is stable for 15 h without significant degradation of activity. Combined with XPS and in situ Raman spectroscopy, the high activity and stability originate from the interaction between SiO₂ and Cu₂O, which stabilizes the positively charged Cu species and creates a stable Cu⁰–Cu⁺ interface under CO₂RR conditions. Meanwhile, the surface –NH₂ can enrich CO₂ and promote the adsorption and activation of CO₂ on the catalyst surface, which accelerates the formation of *CO; the –NH₂ protonation process increases the local pH, which creates a reaction microenvironment that facilitates C–C coupling at the Cu⁰–Cu⁺–NH₂ composite interface, and improves the efficiency of the CO₂RR to C₂₊. This research provides a new idea for the surface modification of Cu-based catalysts and the design of an efficient Cu⁰–Cu⁺–NH₂ composite interface.

Author contributions

Conceptualization: Z.-L. W.; methodology: Z.-Y. Z. and H. T.; software: H. T.; validation: Z.-Y. Z. and H. J.; formal analysis: X. W., L. B., and Y. L.; investigation: Y. L. and N. K.; resources: Z.-L. W.; data curation: all authors; writing – original draft: all authors; writing – review & editing: all authors; visualization: Z.-Y. Z. and Z.-L. W.; supervision: Y. Y. and Z.-L. W.; project administration: Y. Y. and Z.-L. W.; funding acquisition: N. K., Y. Y., and Z.-L. W.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (NSFC) [grant numbers 22075201 and 21962014] and the National Key Research and Development Program of China [2022YFB4101800]. N. K. acknowledges the Office of the Ministry of Higher Education, Science, Research and Innovation (MHESRI) under the Reinventing University System/Visiting Professor Program 2023 for partial support. This work used the Queensland node of the NCRIS-enabled Australian National Fabrication Facility (ANFF).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ta05652j

‡ The authors contributed equally to this work.

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