Enhanced CO2 electroreduction to ethylene via strong metal-support interaction

Mengen Chu,Chunjun Chen,Yhui Wu,Xupeng Yn,Shuiqing Ji,Ruting Feng,Hihong Wu,,Mingyun He,,Buxing Hn,,

a Shanghai Key Laboratory of Green Chemistry and Chemical Processes,School of Chemistry and Molecular Engineering,East China Normal University,Shanghai,200062,China

b Beijing National Laboratory for Molecular Sciences,CAS Key Laboratory of Colloid and Interface and Thermodynamics,CAS Research/Education Center for Excellence in Molecular Sciences,Institute of Chemistry,Chinese Academy of Sciences,Beijing,100190,China

Abstract The CuO/CeO2 composites with strong metal-support interaction were synthesised,which can efficiently electroreduct CO2 to C2H4.The Faradaic efficiency(FE)of C2H4 could reach 50.5%with a current density of 18 mA cm-2.The strong metal-support interaction could not only enhance the adsorption and activation of CO2,but also can stablize the CuO.

Keywords: Carbon dioxide reduction;Ethyene;Electrocatalyst;Copper-ceria interaction

1.Introduction

Electroreduction of carbon dioxide (CO2RR) to high-value chemicals and fuels is a promising technology which can not only reduce the accumulation of CO2to some extent but also can store intermittent renewable energy [1-5].However,the conversion efficiency and product selectivity need to be improved,especially for the production of multi-carbon(C2+)product such as ethylene,ethanol,and n-propanol [6].The formate,methane,and byproduct H2were usually generated in the process of CO2RR [7-9].Therefore,design of effective catalysts to produce C2+products are desired.

Many studies showed that copper and copper-based catalysts were very promising for producing C2+products [10-13].Recently,the copper oxide catalysts have attracted much attention because they can decrease the overpotential and increase the selectivity towards C2+products[14,15].According to previous studies[16],the abundant high valence state of Cu can enhance the selectivity of C2+products.Various strategies have been carried out to stablize the high valence state of copper,including modifying morphology [17],alloying [18],and doping [19].However,the Cu oxide species were usually not stable at negative potentials during CO2RR.

The supported catalysts with strong metal-support interaction (SMSI) have shown some obvious advantages,such as improving CO oxidation[20],CO/CO2hydrogenation[21,22],and reducing temperature for water-gas shift reaction (WGS)[23].Recently,SMSI has been carried out to enhance the activity of CO2RR [24-27].However,the product is mainly limited for C1chemicals(e.g.,CO and CH4).According to the previous report [22],the electronic structure of metal can be significantly influenced by the support through the SMSI,thus we can assume that the high valence state of Cu can be stabilized via the SMSI between the Cu oxides and supports.The supported catalysts have distinct inherit characteristics,such as little metal catalysts content,high utilization rate of metal catalysts,high stability and environmental benign [28].Thus,it is very desirable to design and study the supported copperbased catalysts with SMSI for CO2RR to produce C2+products.

As a common support,the CeO2have distinct inherit characteristics,such as the non-toxic,abundant storage and rich oxygen vacancy to stablize the metal catalyst [29].Especially,the dispersion and chemical state of the copper can be tuned via the strong interaction between the Cu and CeO2[30,31].Thus,the Cu oxides supported on CeO2are suitable candidate to study the effect of the interface interaction on producing C2+products in CO2RR.

In this study,the CuO/CeO2catalysts with strong metalsupport interaction (CuO/CeO2-SMSI) and weak metal-support interaction (CuO/CeO2-WMSI) were prepared.It was shown that metal-support interaction affected the catalytic performance considerably.The CuO/CeO2-SMSI could efficiently electroreduction of CO2to C2H4.The faradaic efficiency(FE)of C2H4could reach 50.5%with a current density of 18 mA cm-2.

2.Experimental

2.1.Materials and reagents

Cu(NO3)2·3H2O(98.0-102.0%)was purchased from Arcos Organics.Ce(NO3)3·6H2O (99.5%) was obtained from Innocom.NaOH(96%),Na2CO3,CsI were provided by Sinopharm Chemical Reagent Company(Shanghai,China).Toray carbon paper (CP,TGP-H-60,19 cm × 19 cm),Nafion D-521 dispersion(5%w/w in water and 1-propanol,≥0.92 meg g-1exchange capacity) and Nafion N-117 membrane (0.180 mm thick,≥0.90 meg g-1exchange capacity) were purchased from Alfa Aesar China Co.,Ltd.(USA).Carbon dioxide(CO2,99.999%) and nitrogen (N2,99.999%) were supplied by Shanghai Pujiang Gases Company.Deionized water was used in the experiments.All reagents were employed without further treatment.

2.2.Catalyst preparation

Synthesis of CeO2rod:The procedures were similar to that reported previously [32,33].In a typical experiment,0.88 g Ce(NO3)3·6H2O was dissolved in 20 mL distilled water,and the solution was stirred for 5 min.At the same time,8.44 g NaOH was dissolved in 15 mL distilled water.After that,the solution of NaOH was added dropped into the solution of Ce(NO3)3·6H2O.The mixed solution was stirred for another 30 min and then transfered into a 100 mL Teflon autoclave.The autoclave was hydrothermally heated in oven at 100°C for 24 h.After cooling,the products were collected by centrifugation,washed with deionized water until the pH of supernatant was less than 7,and the product was washed using ethanol for several times,and then dried at 60°C overnight.The obtained yellow powder was calcined in a tube furnace under H2/Ar atmosphere at 400°C for 4 h to get CeO2.

Synthesis of Cu/CeO2: Above mentioned CeO2was used to prepare CuO/CeO2catalyst by deposition-precipitation (DP)method.In a typical,0.2 g CeO2was dispersed in 10 mL of deionized water with ultrasonication for 30 min,and then 63.6 mg Cu(NO3)2·3H2O was added into the CeO2/H2O suspension under stirring.After stirring for 10 min,10 mL Na2CO3aqueous solution (0.5 mol L-1) was added dropwise.The obtained green suspension was continuous stirring for 9 h at room temperature before centrifugation and washed by deioned water.The product was dried in oven at 60°C overnight and then calcined in different conditions.In this work,the CuO/CeO2-SMSI samples was obtained under air at 400°C for 4 h at a heating rate of 2°C min-1.The CuO/CeO2-WMSI samples was obtained under 5%H2/Ar mixture atmosphere at 300°C for 3 h(heating rate: 2°C min-1) in a tube furnace.

2.3.Catalyst characterizations

Power X-ray diffraction(XRD)pattern was conducted on a Rigaku Ultima VI X-ray diffractometer with Cu-Kα radiation at a scan speed of 10°min-1between 20°and 80°.The morphologies of CuO/CeO2catalysts were characterized by scanning electron microscopy(SEM)S-4800 and transmission electron microscopy TEM (JEOL-2100F).X -ray photoelectron spectroscopy (XPS) analysis was implemented using the AXIS Supra surface analysis instrument with an X -ray monochromatic source (combined Al/Ag anode,energy 1486.6/2984.2 eV) 200 W monochromated Al Kα radiation.Typically,the hydrocarbon C1s line at 284.8 eV.The adsorption isotherms of CO2for CuO/CeO2catalysts were obtained at 298 K within the pressure range of 0-1 atm on a BELSORPmax II device.The Nitrogen adsorption/desorption isotherms were obtained using a Quadrasorb SI-MP system.The content of metal in the catalysts was measured by inductively coupled plasma optical emission spectroscopy (Thermo IRIS Intrepid II).X-ray adsorption spectroscopy(XAS) measurements were performed at the 1W1B,1W2B beamline at Beijing Synchrotron Radiation Facility (BSRF).

2.4.Electrode preparation

To prepare the electrode,5 mg catalyst was dispersed in 0.5 mL ethanol and 30 uL 5% Nafion D-521 to form homogeneous ink with the help of ultrasound.The catalyst ink was spread onto carbon paper (1 cm) to investigate the electrochemical performance.The amount of CuO/CeO2catalyst on the electrode was 4 mg.

2.5.Linear sweep voltammetry (LSV) and electrolysis experiments

All the electrochemical experiments were performed on the electrochemical workstation (CHI 660E,Shanghai CH Instruments Co.,China).The controlled potential electrolysis of CO2experiments and linear sweep voltammetry (LSV) measurements was conducted in a H-shaped cell with a threeelectrode system,which consisted of working electrode,a platinum gauze as counter electrode,and Ag/AgCl (3 mol L-1KCl) as reference electrode.Nafion-117 proton exchange membrane was used to separate the cathode and anode compartments and simultaneously permit H+to transferred from anode compartment to cathode compartment.KHCO3aqueous solution (0.1 mol L-1) and CsI solution (0.1 mol L-1) were used as anodic and cathodic electrolytes,respectively.Prior to experiments,the 0.1 mol L-1CsI (pH=4.04) electrolyte was bubbled with N2or CO2at least 30 min to form N2or CO2saturated solution.During constant potential CO2electrolysis,CO2was continously bubbled through the electrolyte at a flow rate 10 sccm.The flow rate of CO2was controlled with a mass flow controller (SevenStar D07-19B).CV measurement was carried out within gas-saturated electrolytes in the potential range of 0.44 V to-1.8 V(vs.Ag/AgCl)at appropriate sweep rate of 50 mV s-1sweep rate.In this work,all potentials were converted to RHE by considering the pH of solution by the following equation:

The pH values of CO2saturated 0.1 mol L-1CsI electrolyte at ambient temperature was determined by an electronic pH meter (INESA PHS-3C).Prior to measurment,CO2was bubbled into solution under stirring at least 30 min.

2.6.Product analysis

Gas chromatography (GC,agilent 8890D) equipped with FID and TCD detectors was used to analyze the gaseous product.The liquid product was detected by1H NMR(Bruker Avance III 400 HD spectrometer) with TMS as an internal standard.

Calculations of Faradaic efficiencies of gasous and liquid products:

Gasous products：

(v:flow rate of CO2;A: mol fraction of product;I: total current;F: Faraday's constant;n: transfer electron number).

Theamounts ofgaseous productswerecalculatedfromthe GC peak areas.The peak areas were definited by standard calibration gas with known concentration of different component.The theoretical moles were calculated accoring to the current density and the amount of transfered electron in the process.

Liquid products:

After elecrolysis,liquid product in the electrolyte was quantified by NMR spectra.Concretely,1 mL cathode electrolyte was mixed with 100 μL of phenol(200 mmol L-1),and then took out 300 μL sample from the electrolyte and mixed with and 250 μL D2O.The quantification was based on the peak area against the internal standard.The Faradaic Efficiency of liquid products can be calculated using the following equation:

(A: peak area of internal standard in NMR;V: the volume of catholyte;F: Faraday's constant;n: transfer electron number;k:coefficient of peak area/molar concentration;Q:quantity of electric charge.)

2.7.Electrochemical active surface area (ECSA)measurement

According to the literature,the electrochemical active surface area is proportional to Cdlvalue.Cdlcan be determined in H-type electrolysis cell by measuring the capacitive current associated with double-layer charging from the scan-rate dependence of cyclic voltammetric (CV).For this,the potential window of cyclic voltammetric stripping was ranged from-0.35 V to -0.45 V (vs.RHE).The Cdlwas estimated by plotting the Δj=(ja-jc)at-0.40 V(vs.RHE)against the scan rates,in which the jaand jcwere the anodic and cathodic current densities,respectively.The scan rates were 20,30,50,80,and 100 mV s-1.

2.8.Electrochemical impedance spectroscopy (EIS)

The electrochemical impedance spectroscopic studies were conducted in CO2saturated 0.1 mol L-1KHCO3aqueous solution at an open circuit potential (OCP) within the frequency extent of 0.1-100,000 Hz and at a amplitude of 5 mV.

3.Results and discussion

3.1.Catalyst characterization

CuO/CeO2catalyst was first prepared by deposition of copper oxide on the CeO2(Fig.S1) [32].The two kinds of CuO/CeO2catalysts with different degrees of metal-support interaction were prepared via different post-processing methods [31].The CuO/CeO2-SMSI was prepared by calcination under air.In contrast,the CuO/CeO2-WMSI was prepared by calcination under 5% H2of Ar.The Cu contents in CuO/CeO2catalysts were determined by ICP-AES,and the Cu loading contents in CuO/CeO2-SMSI and CuO/CeO2-WMSI were similar,which were about 11 wt% (Table S1).

Fig.1.Morphology and structure characterization: (a)TEM image of CuO/CeO2-SMSI;(b)HR-TEM image of CuO/CeO2-SMSI;(c)TEM image and elemental mappings of CuO/CeO2-SMSI;(d) XRD patterns of CuO/CeO2 catalysts.

The obtained CeO2nanorods had a length of 50-100 nm and diameter of about 10 nm (Fig.S2).The transmission electron microscopy (TEM) revealed that the CuO/CeO2-SMSI also exhibited the nanorod morphology,which was similar to the original CeO2(Fig.1(a)).No obvious CuO clusters could be observed on the surface of CeO2in CuO/CeO2-SMSI (Fig.1(b)),due to the size of clusters was too small or the substrate of Cu and Ce is similar [32,33].The lattice spacing of 0.31 nm belongs to (111) facets of CeO2.The CuO/CeO2-WMSI also exhibited the nanorod morphology,and CuO clusters could not be detected neither(Fig.S3).The energy dispersive X-ray spectroscopy maps(EDS)show that the elements of Cu,O and Ce were uniformly dispersed over the catalyst (Fig.1(c) and Fig.S4),which confirm that the Cu oxides were successfully dispersed on the CeO2.The X-ray diffraction(XRD)pattern of the as-prepared samples are shown in Fig.1(d).It shows that CeO2exhibited fcc Fluorite CeO2phase and peaks affiliated to CuO could not be observed,this may result from small size of the CuO clusters [31],which is consistent with the result of HR-TEM.The BET specific surface areas of different CuO/CeO2catalysts are shown in Fig.S5,indicating that they possessed similar surface areas [34].

X-ray photoelectron spectroscopy (XPS) was used to identify the chemical states of Cu and Ce on CuO/CeO2.For the CuO/CeO2-SMSI,we can observe that valence state of Cu was Cu2+(Fig.2(a)).The valence state of Cu in CuO/CeO2-WMSI was also Cu2+,although it was obtained under 5% H2of Ar (Fig.S6).This is due to the size of Cu species is too small,which can be rapidly oxidized by air at room temperature.The XPS of Ce 3d mainly consist of Ce 3d3/2and Ce 3d5/2[30,31].As shown in Fig.2(b),the Ce of CuO/CeO2-SMSI can be divided to Ce3+and Ce4+.The content of Ce3+in CuO/CeO2-SMSI was higher than that in CuO/CeO2-WMSI(Fig.S7 and Table S2).Due to the obtained CeO2nanorod was first treated by H2atmosphere at 400°C before loading of CuO,the amount of Ce3+will not increase by the further H2treatment after loading of CuO.In contrast,calcination in air results in strong interaction between CuO and CeO2.Thus,we can assume that the increase of Ce3+in CuO/CeO2-SMSI is attribute to the strong interfacial interaction between CuO and CeO2[32].In addition,the electronic structure of the Cu on catalysts were characterized by X-ray absorption spectroscopy(XAS) [35,36].It can be known from X-ray absorption near edge structure(XANES)spectroscopy(Fig.2(c))that the preedge peak of CuO/CeO2-SMSI and CuO/CeO2-WMSI were close to CuO,indicating the valence state of Cu was Cu2+[37].This was consistent with the results of XPS.From extended X-ray absorption fine structure (EXAFS) spectroscopy (Fig.2(d)),Cu-O coordination peak was observed in CuO/CeO2-SMSI and CuO/CeO2-WMSI,which were similar to CuO [34].

3.2.CO2RR performance of CuO/CeO2 catalysts

The CO2electroreduction over CuO/CeO2catalysts were carried out in an H-cell.The linear sweep voltammetry(LSV)curves were recorded in N2or CO2saturated electrolyte at a scan rate of 50 mV s-1in a potential window between 0 and-1.5 V vs.reversible hydrogen electrode (RHE).The current density over CuO/CeO2catalysts under CO2atmosphere were significantly higher than that in N2saturated electrolyte(Fig.3(a) and Fig.S8),demonstrating the electroreduction of CO2over the CuO/CeO2catalysts [38].

Fig.2.X-ray photoelectron spectroscopy(XPS)and X-ray adsorption fine characterizations for CuO/CeO2 catalysts:(a)XPS spectra of Cu 2p of CuO/CeO2-SMSI catalyst;(b) XPS spectra of Ce 3d of CuO/CeO2-SMSI catalyst;(c) Normalized Cu K-edge XANES spectra of CuO/CeO2 catalyst,CuO and Cu foil;(d) The cor111responding Fourier transforms FT (k3w(k)) for CuO/CeO2 catalysts,CuO and Cu foil.

Fig.3.CO2RR performance of CuO/CeO2 catalysts.(a)LSV curves in CO2 and N2-saturated 0.1 mol L-1 CsI electrolyte over CuO/CeO2-SMSI electrode;(b)FE of C2H4 in the potential range from-0.96 to-1.46 V(vs.RHE)over CuO/CeO2-SMSI electrode;(c)FE of C2H4 in the potential range from-0.96 to-1.46 V(vs.RHE) at CuO/CeO2-WMSI electrode;(d) The current density and FE of C2H4 over CuO/CeO2 electrode with 5 h electrolysis at -1.16 V (vs.RHE).

To further analyze the products of CO2electroreduction over the CuO/CeO2catalysts,controlled potential electrolysis of CO2at different potentials was conducted.The gas products were analyzed by gas chromatography (GC) and the liquid product by nuclear magnetic resonance (NMR) spectroscopy.As shown in Fig.3(b) and 3(c) and Figs.S9 and S10,C2H4,CO and H2are the mainly products,and the total FE were around 100% for all the studied catalysts.CuO/CeO2-SMSI displayed high selectivity for C2H4,and the maximum FE of C2H4could reach 50.5% at -1.16 V (vs.RHE),which was higher than that of CuO/CeO2-WMSI at the same potential.Meanwhile,the partial current density for C2H4over CuO/CeO2-SMSI was also higher than that of CuO/CeO2-WMSI(Fig.S11).

Fig.4.Characterizations of electrochemical performance and electronic structure over CuO/CeO2 catalysts.(a)CO2 adsorption isotherms for CuO/CeO2 catalyst at 25°C;(b) Charging current density differences plotted against the square of scan rates;(c) Normalized Cu K-edge XANES spectra of CuO/CeO2 electrode measured at -1.2 V (vs.RHE);(d) The Fourier transforms FT (k3w(k)) for CuO/CeO2 electrodes at -1.2 V (vs.RHE) during CO2RR.

The effect of content of CuO in the CuO/CeO2-SMSI on the reaction was also investigated (Fig.S12).The FE of C2H4gradually increased with the loading of CuO on the CeO2and reached the maximum when the content was 11 wt%.A further increase of CuO content led to decrease of the selectivity of C2H4.

Furthermore,the stability of catalyst was evaluated at a potential of -1.16 V (vs.RHE).The CuO/CeO2-SMSI electrode was stable in 5 h electrolysis (Fig.3(d)),there was no obvious decrease in both current density and FE of C2H4.However,current density and FE (C2H4) over CuO/CeO2-WMSI decreased after electrolysis 2 h.These results indicate that the CuO/CeO2-SMSI not only exibited higher selectivity and activity for CO2reduction to C2H4,but also had excellent stability.

The intrinsic reason that CuO/CeO2-SMSI exhibited high selectivity and activity for C2H4was further investigated.Firstly,the CO2adsorption was characterized over CuO/CeO2catalysts(Fig.4(a)).It can be seen that CuO/CeO2-SMSI had a CO2adsorption capacity of 10 cm3g-1at 1 atm,which was roughly 1.3 times higher than that of CuO/CeO2-WMSI.This indicates that more CO2could be adsorbed on CuO/CeO2-SMSI[39,40].The electrochemical surface area (ECSA) of the catalysts was measured on the basis of cyclic voltammograms(CV),which is a crucial factor in affecting the catalytic performance[41].As shown in Fig.4(b),the CuO/CeO2-SMSI had larger ECSA than that of CuO/CeO2-WMSI,indicating that CuO/CeO2-SMSI could provide more catalytic active sites [42].Combining with the result of BET (Table S1),we can assume that the higher ECSA and CO2adsorption for CuO/CeO2-SMSI is attribute to the strong interaction between CuO and CeO2,rather than the negligible change of BET surface area.Electrochemical impedance spectroscopy(EIS)was used to measure the charge transfer resistance (Rct) for the catalysts at an open circuit potential.As shown in the Nyquist plots in Fig.S13,CuO/CeO2-SMSI showed lower interfacial Rct,indicating that electron transfer was more facile on CuO/CeO2-SMSI than that of CuO/CeO2-WMSI during CO2RR [43].

Due to the electronic structure of Cu can influence the CO2reduction activity,we carried out in situ X-ray adsorption spectroscopy (XAS) to monitor the local structure of Cu during CO2RR [44].The operando-XAFS measurements were conducted during the CO2RR at-1.2 V(vs.RHE).As shown by the normalized Cu K-edge XANES(Fig.4(c)),the valence state of Cu in CuO/CeO2-SMSI during the CO2RR was Cu2+.In contrast,for the CuO/CeO2-WMSI,the valence state of Cu was lower than that of CuO/CeO2-SMSI,indicating that the CuO could be partially reduced during CO2RR.As showed in the Fourier-transform(FT)of EXAFS(Fig.4(d)),intensity of Cu-O in CuO/CeO2-WMSI was lower than that of CuO/CeO2-SMSI,indicating that some Cu-O was reduced during CO2RR [45].According to previous report [32],the strong interaction over interface between oxides and supports can stabilize the oxides.Thus,we can assume that the high stability of CuO in CuO/CeO2-SMSI was attributed to the strong interaction between CuO and CeO2.

4.Conclusions

In summary,CuO/CeO2-SMSI was prepared,which could efficiently convert CO2to C2H4by electroreduction.The FE of C2H4could reach 50.5%with a current density of 18 mA cm-2.The detailed study indicates that the strong metal-support interaction could provide large number of active sites,strong CO2adsorption and low interfacial charge transfer resistance.In addition,the CuO could be stabilized by the strong metal-support interaction,leading to better stability of the catalyst.We believe that design of supported catalysts with strong metal-support interaction is promising strategy for enhancing CO2RR.

Conflict of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the National Key Research and Development Program of China(2017YFA0403102),National Natural Science Foundation of China (21573073,21733011),Beijing Municipal Science ＆Technology Commission(Z191100007219009),the Chinese Academy of Sciences(QYZDY-SSW-SLH013).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2020.12.001.