RETRACTED ARTICLE: Preparation of hollow Au_x-Cu₂O nanospheres by galvanic replacement to enhance the selective electrocatalytic CO₂ reduction to ethanol

Abstract

Electrocatalytic CO₂ reduction to fuel is one of the important ways to solve energy and environmental problems. In this work, the preparation of hollow Au_x-Cu₂O electrocatalyst and the performance of electrocatalytic CO₂ reduction to ethanol were studied. Hollow Cu₂O nanospheres were prepared by a soft template method, and Au_x-Cu₂O composites were prepared by galvanic replacement. The characterization results of XRD and XPS reveal that Cu⁺ is the main chemical state of Cu in the catalysts. The results of electroactive surface area demonstrate that the electroactive surface area of Au_0.51-Cu₂O is the largest. The performance evaluation of electrocatalytic CO₂ reduction shows that the Faraday efficiency of H₂ on Au_0.51-Cu₂O is the lowest (∼19.5%) and the Faraday efficiency of ethanol can reach ∼18.8% at −1.2 V vs. RHE. Compared with hollow Cu₂O nanospheres, Au_x-Cu₂O catalysts have an earlier onset for ethanol production and promote the CO₂ reduction to ethanol with high efficiency, while the hydrogen evolution reaction is significantly inhibited. Our study demonstrates an effective approach to develop Cu-based electrocatalysts favourable toward ethanol in electrocatalytic CO₂ reduction.

Keywords:

• Electrocatalytic CO₂ reduction
• hollow Cu₂O nanospheres
• Au_x-Cu₂O composites
• ethanol

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Statement of Retraction

1. Introduction

Electrocatalytic CO₂ reduction (ECR) to C₂₊ products with high energy density is a current research hotspot. At present, the main challenge is that it is difficult to effectively realize C–C bond coupling reaction [1]. The results show that copper-based materials are the only catalysts that can reduce CO₂ to hydrocarbons and oxygen-containing compounds, but the catalytic activity and selectivity need to be improved to meet the requirements of industrial application [2–7].

Many studies have shown that the copper-based catalysts designed based on the double active sites or tandem catalysis mechanism can significantly improve the selectivity of C₂₊ products [8–10]. For example, Ag, Au and Zn all have high activity and selectivity for electrocatalytic CO₂ reduction to CO. Cu-M and other copper-based catalysts exhibits the performance of promoting the formation of C–C bond coupling reaction to C₂₊ products [11–19]. Therefore, the introduction of active sites to promote the formation of CO on Cu-based catalysts can effectively improve the CO₂ reduction to C₂₊ products.

Recent studies have shown that Cu(I) species on copper-based catalysts can remain relatively stable in the process of electrocatalytic CO₂ reduction and play an important role in promoting CO₂ activation and *CO intermediate dimerization or protonation, and then the production of C₂ products increases [20–26]. In addition, it has also been found that the catalyst with a hollow structure has the advantages of large specific surface area and low density compared with single-component solid nanoparticles [27,28]. Especially as an electrocatalyst the hollow structure provides a larger electrolyte-electrode contact area and abundant electrochemical active sites for rapid diffusion and reaction [29–31].

In our study, hollow Au_x-Cu₂O electrocatalysts were designed based on the tandem catalysis mechanism. The role of Au is to promote the conversion of CO₂ to CO, which is beneficial to increase the local concentration of the key intermediate *CO in order to promote the C–C bond coupling reaction, and then enhance the formation of multi-carbon products. Firstly, hollow Cu₂O nanospheres were prepared with CTAB (Hexadecyl trimethyl ammonium Bromide) as a soft template, and hollow Au_x-Cu₂O electrocatalysts were prepared by galvanic replacement. The physical and chemical properties and electrochemical properties of the prepared electrocatalysts were characterized, and the performance of CO₂ reduction to ethanol was evaluated.

2. Experiment

2.1. Materials and method

Copper(II) sulphate pentahydrate (CuSO₄·5H₂O, AR, J&K Scientific LTD), Sodium citrate (Na₃C₆H₅O₇·2H₂O, AR, J&K Scientific LTD), Sodium borohydride (NaBH₄, 98%, J&K Scientific LTD), Ascorbic acid (AA, 98%, HEOWNS), Hexadecyl trimethyl ammonium Bromide (CTAB, 97.0%, HEOWNS), isopropanol ((CH₃)₂CHOH, AR, HEOWNS), Chloroauric Acid (HAuCl₄, 99.0%, HEOWNS). Deuterium Oxide (D₂O, 99.79%, HEOWNS), Potassium bicarbonate (KHCO₃, 99%, HEOWNS), Sodium hydroxide (NaOH, AR, Aladdin), Ethanol absolute (C₂H₅OH, AR, Tianjin Yuanli Chemical Co., Ltd.), Deionized water. All reagents were used directly without further purification and deionized water was used in our experiments.

2.2. Synthesis of hollow Cu₂O

Hollow Cu₂O nanospheres were prepared by a soft template method using Hexadecyl trimethyl ammonium Bromide (CTAB) as soft template [32]. In a typical synthesis procedure, 50 mg CuSO₄ ·5H₂O was added to the 0.13 M CTAB aqueous solution (100 mL), then 0.18 g ascorbic acid (AA) was add the above solution and heated to 60 °C. After stirring for 20 min at 60 °C, 0.2 M NaOH (10 mL) aqueous solution was added to the above solution. After 10 min, the mixture was cooled to room temperature, and the yellow Cu₂O precipitate was separated and washed sequentially using deionized water and ethanol five times by centrifuging at 9000 rpm. The obtained Cu₂O was dried at 60 °C for 12 h under vacuum.

2.3. Synthesis of hollow Au_x-Cu₂O

Hollow Cu₂O nanospheres were prepared following the procedure in the 2.2. section. The schematic diagram of the formation process of Au_x-Cu₂O composites is illustrated in Figure 1. 10 mg Cu₂O nanospheres was dispersed into the mixed solution of 20 mL water and ethanol (volume ratio 1:1). After ultrasonic for 10 min, 0.05 M sodium citrate aqueous solution (1.5 mL) was added to the above solution drop by drop. After ultrasonic for 10 min, 0.25 mL HAuCl₄ (20 mg•mL⁻¹) aqueous solution was added to the mixed solution drop by drop. After 20 min, the precipitate was separated and washed sequentially using deionized water and ethanol five times by centrifuging at 9000 rpm. The obtained Au-Cu₂O was dried at 60 °C for 12 h under vacuum. The sample was named Au_0.17-Cu₂O. By changing the addition amount of HAuCl₄ (0.50 mL, 0.75 mL), the samples were named Au_0.34-Cu₂O and Au_0.51-Cu₂O, respectively. 0.17, 0.34 and 0.51 represents the molar ratio of Au to Cu₂O, respectively.

Figure 1. Schematic diagram of the formation process of Au_x-Cu₂O composites. (The main galvanic replacement reaction is 3Cu⁺ + AuCl₄⁻ → 3Cu²⁺ + Au + 4Cl⁻. The whole reaction is carried out under normal temperature and pressure).

2.4. Characterization

X-ray diffraction (XRD, Rigaku D/MAX-2500 diffractometer, Japan, 2θ = 20°∼85°, scanning rate 8°/min) with Cu Kɑ radiation was recorded to determine the phase composition of the tested sample. The microscopic morphologies of the prepared catalysts were observed by scanning electron microscope (SEM, Hitachi, S-4800, Japan) and Transmission electron microscopy (TEM, Japan, JEOLJ, EM-2100F). Analysis of element composition and chemical environment of sample surface by X-ray photoelectron spectroscopy (XPS, Britain, Thermo Fisher Scientific, K-Alpha⁺) and all the XPS spectra were calibrated by the binding energy with contaminated carbon C1s (284.8 eV as the correction value).

2.5. Electrochemical measurements

All electrochemical measurements were controlled on an electrochemical station (Switzerland, Metrohm AutoLab 302 N) in an H-type two-chamber electrochemical cell separated by proton exchange Nafion membrane (Tianjin Incole Union Technology Co., Ltd.). The catalyst Ink was prepared by ultrasonic dispersion of 5 mg catalyst and 50 μL Nafion in isopropanol solution. 105 μL Ink was dripped on the glassy carbon electrode (1.0 cm × 1.0 cm), which was as the working electrode. The saturated calomel electrode (SCE) was used as the reference electrode, and the platinum plate (1.0 cm × 2.0 cm) was used as the counter electrode. Electrochemical tests were carried out in 0.1 M KHCO₃ solution. The applied potential is converted to reversible hydrogen electrode potential (RHE) by using the formula: E_(RHE) = E_(SCE) + 0.0591 × pH + 0.24. Before electrocatalytic CO₂ reduction, N₂ and CO₂ were purged into the electrolyte for 50 min. The working electrode was scanned by LSV at N₂ atmosphere for 2 h to achieve the purpose of pre-reduction and surface activation, and then the CO₂ reduction performance of each electrode was tested at five potentials (−0.9 V, −1.1 V, −1.2 V, −1.3 V, −1.5 V vs. RHE). CO₂ was continuously purged into the cathode chamber at a flow rate of 20 mL/min. The gas phase products were detected using the gas chromatography (GC, Agilent 7890B) with a TCD and the liquid phase products were detected by 400 MHz 1H 1 D liquid NMR spectrometer (Bruker, AVANCE III). The Faraday efficiency of each product is calculated according to the formula (1)–(3). (1) $$j_{H_2\text{or}CO}=V_{H_2\text{or}CO}\times q\times \frac{zF{P}_0}{RT}$$(1) (2) $$F{E}_{H_2\text{or}CO}=\frac{{\mathrm{j}}_{H_2\text{or}CO}}{{\mathrm{j}}_{\text{total}}}\times 100\%$$(2) (3) $$F{H}_{\mathit{HCOOH}\text{or}{\mathrm{C}}_2{H}_{5}OH}=\frac{\mathit{znF}}{Q}\times 100\%$$(3) where q is the flow rate of CO₂ during ECR, F is the Faraday constant (96,485.3 C/mol), P₀ is pressure, T is room temperature, R is the ideal gas constant (8.314 J mol⁻¹ K⁻¹), z is the number of electrons transferred during ECR, n is the molar mass of the products, and Q is the total electric quantity during ECR.

3. Results and discussion

3.1. Structural characterisation

The X-ray diffraction (XRD) patterns of Cu₂O and Au_x-Cu₂O composites are shown in Figure 2. For hollow Cu₂O catalyst, the diffraction peaks at 29.55°, 36.41°, 42.30°, 61.34°and 73.53° correspond to the (110), (111), (200), (220) and (311) of Cu₂O, respectively. For Au_x-Cu₂O composites, except for the typical peaks of Cu₂O, the diffraction peaks observed at 38.18°, 44.39°, 64.58°, 77.55°and 81.72° correspond to the face-centred cubic crystal Au (PDF#04-0784), indicating that Au nanoparticles have been successfully supported on the hollow Cu₂O nanospheres by galvanic replacement. In addition, with the increase of the amount of HAuCl₄, the area of the diffraction peak of Au increases and the area of Cu₂O diffraction peak decreases gradually, which is due to the increase of the number of Au nanoparticles formed by Cu₂O replacement and the increase of Cu₂O consumption as a sacrificial template in the galvanic replacement reaction [33]. No impurity peak was observed in the Cu₂O and Au_x-Cu₂O composites, indicating that the purity of the prepared catalysts was high.

Figure 2. XRD patterns of Cu₂O and Au_x-Cu₂O.

Figure 3 shows the SEM and TEM images of hollow Cu₂O nanospheres. The size of hollow Cu₂O nanospheres is about 150–180 nm, and the surface is uneven. Because the surface of the shell is composed of many Cu₂O particles attached along the CTAB vesicles [34]. From the TEM Figure 3c, the single-shell and double-shell structure of nanospheres were observed clearly, the latter of which was dominated. From the HRTEM Figure 3d, the clear crystal plane spacing of 0.241 nm is consistent with the lattice spacing of Cu₂O (111) plane.

Figure 3. (a) and (b) SEM images of hollow Cu₂O nanospheres; (c) and (d) HRTEM images of hollow Cu₂O nanospheres.

Figure 4 shows the SEM and TEM images of Au_0.17-Cu₂O composite. The sample basically maintains the hollow shell structure of Cu₂O. The surface of the sample with an average particle size of about 150–200 nm, and small particles can be observed on the surface. The relatively brighter particles in Figure 4c are Au nanoparticles produced by galvanic replacement reaction, which are dispersed on the surface of Cu₂O. From the HRTEM Figure 4e, it can be seen that the particle size of Au nanoparticles is about 5–7 nm. In addition, 0.241 nm crystal plane spacing corresponding to Cu₂O planes and 0.235 nm crystal plane spacing corresponding to Au planes can be observed. From the TEM-mapping, it can see that Au nanoparticles are almost uniformly dispersed on the surface of Cu₂O. Figure S1 shows the SEM and TEM images of Au_0.34-Cu₂O composite. The sample basically maintains the hollow shell structure of Cu₂O, and the average particle size is about 140–180 nm. In addition, small particles and more uniform dispersion can be observed on the sample surface. Although the variable controlled in our study is the amount of HAuCl₄, the pH of the whole system is changed by adding more HAuCl₄. It is speculated that the environment of the system may be more suitable for galvanic replacement reaction at this time [35]. It can be also seen from the TEM-mapping that Au nanoparticles are basically uniformly dispersed on the surface of Cu₂O. Figure S2 shows the SEM and TEM images of Au_0.51-Cu₂O composite. The average particle size is about 180–200 nm and the surface of the sample has some pores and more small particles, indicating that under the condition of a certain amount of carriers, the increase of HAuCl₄ addition leads to the overreaction of the shell and the exposure of pores. On the one hand, the increase of carrier consumption is due to the fact that the increase of AuCl₄⁻ aggravates the galvanic replacement reaction, and the other is that the increase of H⁺ accelerates the consumption of Cu₂O. From the HRTEM Figures S2c and 2d, it can be seen that the number of Au nanoparticles loaded on Cu₂O surface increases and the particle size of Au nanoparticles is larger (about 10–20 nm), which may be due to the increase of Au nanoparticles generated by galvanic replacement reaction with the increase of HAuCl₄ addition and partial aggregation on the surface of Cu₂O.

Figure 4. (a) and (b) SEM images of Au_0.17-Cu₂O composite; (c)–(e) HRTEM images of Au_0.17-Cu₂O composite; (f)–(h) Element mapping of Cu, Au and O of Au_0.17-Cu₂O composite.

Furthermore, the surface compositions and chemical states of the catalysts were determined by XPS characterization. The XPS characterization results of Au_x-Cu₂O and Cu₂O catalysts are shown in Figures 5 and S3–S5. Take Au_0.17-Cu₂O as an example, from the spectrum of Figure 5a, there are three elements Cu, Au and O, indicating that Au_0.17-Cu₂O electrocatalyst was successfully prepared by galvanic replacement reaction. From Figure 5c, there are four typical peaks of Cu in the Cu 2p spectrum, indicating that there is a mixed oxidation state of copper [36,37]. The peaks at the binding energies of 952.3 eV and 932.5 eV correspond to Cu 2p_3/2 and Cu 2p_1/2, respectively [38]. Combined with the Auger spectrum of Cu in Figure 5b, it indicates that Cu⁺ is the main chemical state of Au_0.17-Cu₂O. In addition, the peak at 933.8 eV belongs to CuO [39], which is due to the galvanic replacement reaction of AuCl₄⁻ with Cu⁺ and the formation of trace CuO on the surface. The Au 4f spectrum of Au_0.17-Cu₂O is shown in Figure 5d. There are two peaks at Au 4f_7/2 and Au 4f_5/2, which are located at 84.1 eV and 87.7 eV, respectively, indicating the existence of Au⁰ [40,41]. According to the XPS results, the atomic composition of Cu₂O and Au_x-Cu₂O catalysts is summarized in Table S1. The change trend of the surface atomic composition of the catalysts is consistent with that of the theoretical Au/Cu atomic ratio.

Figure 5. Au_0.17-Cu₂O (a) Survey spectra, (b) Cu LMM, (c) Cu 2p, (d) Au 4f.

3.2. Electrochemical measurements

After the galvanic replacement of hollow Cu₂O nanospheres with AuCl₄⁻, part of the surface of Cu₂O will be covered by Au, which leads to the change of electrochemical reaction characteristics of the electrode surface. Therefore, the redox behaviour of the Au_x-Cu₂O nanospheres was investigated by cyclic voltammetry (CV). The prepared Cu₂O and Au_x-Cu₂O electrodes were scanned by CVs in the potential range of −0.75 to + 1.25 V vs. RHE, respectively. Then, the same CV tests were carried out on the electrode after surface activation treatment. By comparing the CVs obtained twice, the effect of different loading amount of Au nanoparticles on the redox properties of copper on the surface of Au_x-Cu₂O can be investigated (there is no redox reaction in the test potential range of Au nanoparticles).

As shown in Figure 6, there are two oxidation peaks (A1 and A2) and three reduction peaks (C1, C2 and C3) before and after activation of Cu₂O and Au_x-Cu₂O electrodes. The two anodic peaks at 0.45 V (A1) and 0.69 V (A2) correspond to the oxidation of Cu⁰ to Cu⁺ and Cu⁺ to Cu²⁺, respectively. The three reduction peaks at 0.22 V, −0.19 V and −0.41 V are attributed to the reduction of CuO to Cu₂O (C1) and further to Cu (C2) and HCuO₂⁻ (C3) [42,43]. Among them, the C3 peaks of Au_0.34-Cu₂O and Au_0.51-Cu₂O electrodes are weak and not prominent. As shown in Figure 6a, the Cu₂O electrode has two typical reduction peaks at 0.22 V and −0.19 V. Comparing the CVs before and after activation, the current densities of both oxidation and reduction reactions on the activated electrode increase, indicating that clear Cu₂O reduction occurs on the surface of the electrode during activation, resulting in a significant change in the redox properties of the surface. In contrast, as shown in Figure 6b–d, the corresponding reduction peak changes little in the CV curve measured by Au_x-Cu₂O electrode before and after activation, which may be due to the fact that after the galvanic replacement of Cu₂O and AuCl₄⁻, part of the Cu on the surface of the sample is covered, the Cu₂O reduction reaction is weakened to a certain extent in the process of electrode activation, and the redox reaction on the surface is relatively stable. Because of the high loading of Au, the CV curve of Au_0.51-Cu₂O electrode is close to coincidence before and after activation. The results show that a small amount of Cu⁰ can be formed after electrode activation, but most of the Cu₂O is still remained. In addition, with the increase of the amount of HAuCl₄, the exposed copper on the surface of the sample decreased, and the loading of Au nanoparticles improved the stability of the electrode to some extent.

Figure 6. (a) Cu₂O, (b) Au_0.17-Cu₂O, (c) Au_0.34-Cu₂O, (d) Au_0.51-Cu₂O in N₂-saturated 0.1 M KHCO₃ solution, scan rate = 10 mV/s.

The electrochemical active surface area (EASA) of Cu₂O and Au_x-Cu₂O composites was measured by double layer capacitance method. The CV test results of different scanning speeds are shown in Figure 7, and a straight line is obtained by analysing and fitting the current density and scanning rate of the midpoint of the curve (−0.33 V). The slope of the fitting line in Figure 8 is the double layer capacitance of each electrode. Among these catalysts, Au_0.51-Cu₂O has the largest double-layer capacitance of 0.66 mF. Because the double-layer capacitance is proportional to the electroactive surface area, Au_0.51-Cu₂O has the largest electroactive surface area. It may be due to the severe galvanic replacement reaction on the surface of Au_0.51-Cu₂O sample, which provides more active sites for electrocatalytic CO₂ reduction, therefore the Au_0.51-Cu₂O catalyst has better electrocatalytic performance.

Figure 7. Cyclic voltammograms at various scan rates for (a) Cu₂O, (b) Au_0.17-Cu₂O, (c) Au_0.34-Cu₂O, (d) Au_0.51-Cu₂O.

Figure 8. The linear relationship between current density and scan rate.

3.3. Electrocatalytic CO₂ reduction

After Cu₂O and Au_x-Cu₂O electrodes were activated, electrocatalytic CO₂ reduction was conducted at five potentials (−0.9 V, −1.1 V, −1.2 V, −1.3 V, −1.5 V vs. RHE, respectively). The electrolysis time was 4 h. The ECR catalytic performance of each catalyst was investigated by H₂, CO, HCOOH and C₂H₅OH products, as shown in Figure 9. Figure 9a shows the distribution of ECR products of Cu₂O electrode. The hydrogen evolution reaction of Cu₂O electrode is serious (∼40%). At the potential of −0.9 V, CO was detected but no liquid product was detected in the ECR product of the Cu₂O electrode. At the relatively negative reduction potential of −1.1 V and above, the Faraday efficiency of HCOOH and C₂H₅OH increased to about 15% and 6%, respectively. At the same time, as the Faraday efficiency of CO decreased from about 41.5% to 28.9%, the production of HCOOH and C₂H₅OH increased. These results show that the prepared hollow Cu₂O nanospheres have active sites for further reduction of CO molecules to ethanol, but the selectivity is low and requires a higher reduction potential [44]. The distribution of ECR products of Au_x-Cu₂O electrodes is shown in Figure 9b–d. Compared with Cu₂O, the hydrogen evolution reaction of the three Au_x-Cu₂O electrodes are weakened, indicating that the existence of Au helps to reduce the coverage of *H [45], so that intermediates (such as *CO, *CHO or *CH₂O) have more adsorption sites on the surface of the catalyst, thus promoting the formation of C₂ products. In addition, the three Au_x-Cu₂O catalysts all have an earlier onset for ethanol production than Cu₂O. Among them, the Faraday efficiency of H₂ on Au_0.34-Cu₂O electrode (Figure 9c) is the lowest (∼17.1%) at −1.1 V, and the Faraday efficiency of ethanol is ∼16.5%. The Faraday efficiency of H₂ on Au_0.51-Cu₂O electrode (Figure 9d) is the lowest (∼19.5%) at −1.2 V, and the Faraday efficiency of ethanol is ∼18.8%. It is speculated that while the loading of Au nanoparticles increases, the Au_0.51-Cu₂O catalyst may also have some pores on the surface due to the over-intense galvanic replacement reaction, which is beneficial to the electrocatalytic CO₂ reduction. All the Au_x-Cu₂O catalysts can promote the CO₂ reduction to ethanol with high efficiency.

Figure 9. The FEs of H₂, CO, HCOOH and C₂H₅OH at different potentials on (a) Cu₂O, (b) Au_0.17-Cu₂O, (c) Au_0.34-Cu₂O, (d) Au_0.51-Cu₂O.

In addition, the Faraday efficiency and current density of CO₂ reduction products of Cu₂O and Au_x-Cu₂O catalysts at different potentials (−0.9 V, −1.1 V, −1.2 V, −1.3 V, −1.5 V vs. RHE) were compared and analysed. As shown in Figures S6–S9, it can be seen that for each electrode, the Faraday efficiency of each product was kept at a stable level during the whole electrolysis process and the current density was stable within 4 h, indicating that the prepared catalysts had a good electrocatalytic stability.

4. Conclusions

In summary, hollow Cu₂O nanospheres with particle size of about 150–180 nm were synthesized by a soft template method, and hollow Au_x-Cu₂O electrocatalysts were successfully prepared by galvanic replacement reaction. The characterization results of SEM and TEM show that Au_x-Cu₂O basically maintained the hollow shell structure of Cu₂O, and Au nanoparticles were uniformly dispersed on the surface of Cu₂O nanospheres. Besides, the density of Au nanoparticles on the Cu₂O surface increases clearly with the increase of HAuCl₄ addition, and some pores appear on the surface of Au_0.51-Cu₂O samples due to the intense galvanic replacement reaction. The characterization results of XRD and XPS reveal that Cu⁺ is the main chemical state of Cu of the prepared catalysts. The effect of Au nanoparticles loading on surface Cu₂O was investigated by cyclic voltammetry. The results show that with the increase of HAuCl₄ addition, the coverage of Au nanoparticles on Cu₂O surface is higher, the reduction degree of Cu₂O was weaker and the stability is better. The results of electroactive surface area demonstrate that the electroactive surface area of Au_0.51-Cu₂O was the largest. The performance evaluation of ECR shows that Au_x-Cu₂O catalysts promote the high efficiency of CO₂ reduction to ethanol, while the hydrogen evolution reaction is significantly inhibited. Among them, the Faraday efficiency of ethanol on Au_0.51-Cu₂O can reach ∼18.8% at −1.2 V vs. RHE. Our study shows that Au_x-Cu₂O composites prepared by galvanic replacement reaction is beneficial to the electrocatalytic CO₂ reduction to ethanol.

Supplementary materials

The following are available online at www.mdpi.com/xxx/s1, Figure S1: SEM and TEM images of Au_0.34-Cu₂O composite, Figure S2: SEM and TEM images of Au_0.51-Cu₂O composite, Figure S3: XPS results of Au_0.34-Cu₂O, Figure S4: XPS results of Au_0.51-Cu₂O, Figure S5: XPS results of Cu₂O, Figure S6: Current density and Faraday efficiency of each product of Cu₂O, Figure S7: Current density and Faraday efficiency of each product of Au_0.17-Cu₂O, Figure S8: Current density and Faraday efficiency of each product of Au_0.34-Cu₂O, Figure S9: Current density and Faraday efficiency of each product of Au_0.51-Cu₂O, Table S1: The XPS semiquantitative analysis of Cu₂O and Au_x-Cu₂O.

Acknowledgments

I am very grateful to the School of Chemical Engineering and Technology of Tianjin University and my postgraduate research group for providing a good research platform. The experimental part of this article was completed with the support of School of Chemical Engineering and Technology and my research group of Tianjin University. I am very grateful to the teachers and classmates of my graduate research group. I am grateful to the analysis and test center of Tianjin University for providing XRD, SEM, XPS characterizations. Writing, revising, and responding to reviewers' comments of this article was done with the participation of my colleagues and me who are working in SINOPEC Dalian Research Institute of Petroleum and Petrochemicals now. Therefore, the research results of this dissertation also belong to School of Chemical Engineering and Technology of Tianjin University and my postgraduate research group.

Disclosure statement

No potential conflict of interest was reported by the authors.

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work is financially supported by SINOPEC (No.112006).

Notes on contributors

Lijie Zhang

Lijie Zhang graduated from Tianjin University. She is working in Dalian Petrochemical Research Institute as an assistant engineer now.

Ying Zhang

Ying Zhang graduated from Tianjin University. She is working in Dalian Petrochemical Research Institute as a professor level senior engineer now.

Hongtao Wang

Hongtao Wang and Jianbing Chen are working in Dalian Petrochemical Research Institute as senior engineers now.

Jianbing Chen

Hongtao Wang and Jianbing Chen are working in Dalian Petrochemical Research Institute as senior engineers now.

Zhongqi Cao

Zhongqi Cao graduated from Beijing University of Chemical Technology. He is woring in Dalian Petrochemical Research Institute as an engineer now.