Loading CuO on the surface of MgO with low-coordination basic O2- sites for effective enhanced CO2 capture and photothermal synergistic catalytic reduction of CO2 to ethanol

Ting Li,Hongxia Guo,Xiao Wang,Huan Wang,Li Liu,Wenquan Cui,Xiaoran Sun,Yinghua Liang

College of Chemical Engineering, Hebei Key Laboratory for Environment Photocatalytic and Electrocatalytic Materials, North China University of Science and Technology,Tangshan 063210, China

Keywords:CO2 reduction Photothermal catalysis Ethanol MgO CuO

ABSTRACT The higher capacity of CO2 adsorption on the surface of magnesium oxide (MgO) with low-coordination O2-sites would effectively enhance the catalytic reduction of CO2.Herein,a series of copper oxide(CuO)and MgO composites with different mass ratios have been prepared by hydrothermal method and used for photothermal synergistic catalytic reduction of CO2 to ethanol.The catalyst with CuO mass ratio of 1.6% shows the best yield (15.17 μmol·g-1·h-1) under 3 h Xenon lamp illumination.The improved performance is attributable to the loose nano-sheet structure,uniform dispersion of active sites,the increased specific surface area,medium-strength basicity,the high separation efficiency of electrons and holes,and the formation of Mg-O-Cu species.The synthesized CuO and MgO composites with loose nano-sheet structure facilitate the diffusion of reactants CO2,so an excellent CO2 adsorption performance can be obtained.Meanwhile,the introduction of CuO in the form of bivalence provides higher specific surface area and porosity,thus obtaining more active sites.More importantly,the Mg-O-Cu species make the donation of electrons from MgO to CO2 easier,resulting in the breaking of the old Mg-O bond and the formation of C—O bond,thus promoting the adsorption and conversion of CO2 to ethanol.

1.Introduction

Modern society is highly dependent on fossil fuel for power generation and transportation,the combustion of which will release CO2into the atmosphere,resulting in global warming and a large number of related environmental problems [1-10].Therefore,the excessive emission of CO2has been paid more and more attention globally,but still remains a long-term task.With the potential benefits of simultaneously alleviating the environmental deterioration and the energy consumption,catalytic conversion of CO2into various chemicals has been identified as the most promising strategies to solve the issue,especially the longer chain hydrocarbons (C2+) with higher industrial value.

Solar energy is considered to be one of the cleanest energy sources to directly convert CO2into liquid fuels and valuable commodity chemicals,thus in line with the concept of environmental protection [11-16].The photothermal reduction can significantly promote the conversion of CO2,with better catalytic performance than single photocatalysis and thermal catalysis [17].Unfortunately,the photocatalysts developed for CO2reduction suffer from high cost as a result of the use of low-abundance elements despite intensive efforts nowadays.Meanwhile,photocatalysts require high affinity,high thermal stability and high specific surface area of active sites for the conversion of CO2into valuable products[4,18].Cu compounds(i.e.,Cu2O,CuO and Cu)are the most widely reported CO2photocatalytic raw materials with favorable multielectron transfer properties because of their loosely bonded d electrons,which can accelerate C—C coupling to generate the longer chain hydrocarbons (C2+) [19,20].At the same time,they are inexpensive materials based on relatively abundant elements,and the narrow band gaps enable the absorption of visible light that constitutes most of the solar spectrum.Therefore,CuO shows great potential for facilitating CO2activation and conversion to ethanol or other liquid fuels.MgO is an abundantly available alkaline metal oxide and an inert component with a wide band gap [21].It has the characters of low cost,high CO2adsorption capacity,low toxicity,good renewable effect,and wide application,such as photocatalytic hydrogen production and photocatalytic degradation of pollutants [22,23].Daubet al.[24] found a lowdensity adsorbed monolayer in which each CO2molecule bounds to two Mg2+ions on the MgO substrate.Most important of all,the acidic CO2reacts with basic O2-sites depending on their coordination during CO2adsorption,and a higher capacity can be obtained on the surface of MgO with the higher density of lowcoordination oxygen(edge and corners)[25].The MgO can not only enhance the performance of CO2capture,but also act as an excellent substrate for supporting metal catalyst.Loderet al.[26] prepared a bifunctional Ni/MgO catalyst for CO2methanation,and the effect of Ni loading mass on reaction rate was tested in the temperature range of 533-648 K.MgO activated CO2through chemical adsorption,and the increase of Ni content in the catalyst improved the CO2conversion.When the CO2conversion was 87%,the selectivity of methane reached 99%.

What is more,it is well-known that integrated carbon capture and conversion rely heavily on the information about the CO2adsorption occurring at the interface.Therefore,it is anticipated that the sites for CO2adsorption associate with the lowcoordinated Mg2+-O2-on the surface of MgO,which can effectively improve the subsequent photothermal synergistic catalytic reduction of CO2.However,so far to our best knowledge,there is few relevant research works designing photothermal catalyst for the reduction of CO2to produce hydrocarbon fuel from the perspective of low-coordinated O2--Mg2+sites on the surface of MgO.

In this work,to validate the exciting predictions of photothermal synergistic catalytic reduction of CO2with the lowcoordinated Mg2+-O2-sites on the surface of MgO,a series of CuO and MgO composites with different mass ratios have been prepared by hydrothermal method and used for photothermal synergistic catalytic reduction of CO2to ethanol under visible light irradiation.The structures of the catalysts were well characterized to reveal the contribution of MgO and CuO to the catalytic performance of reduction of CO2to hydrocarbon fuel.Furthermore,the mechanism of photothermal catalytic reduction of CO2was proposed.Our results can provide a unique way to design the photothermal catalyst for the reduction of CO2,focusing on how to effectively improve the catalytic performance by the lowcoordinated Mg2+-O2-sites on the surface of MgO.

2.Experimental

2.1.Synthesis of MgO and xCuO/MgO

The copper acetate (Cu(CH3COO)2·H2O,Shanghai McLean Biochemical Technology Co.,Ltd.) and MgO (Tianjin Bailuns Biotechnology Co.,Ltd.) were dissolved in 150 ml ethanol (Tianjin Yongda Chemical Reagent Co.,Ltd.),then stirred in a 50 °C water bath for 30 min.Subsequently,the mixture was put into a teflon reactor and reacted at 150 °C for 20 h.Finally,it was washed with deionized water and dried at 80°C for 24 h.The obtained samples were denoted asxCuO/MgO(xis the mass percentage of CuO in the catalyst).

2.2.Characterization

The crystal structure was identified by X-ray diffraction (XRD,D8 ADVANCE) using a Cu Kα as the radiation,and the scan rate and step size were 10 (°)·min-1and 0.2°,respectively.The chemical bond was determined by Fourier Transform Infrared (FT-IR,VERTEX70).The morphology was characterized by a fieldemission scanning electron microscope(SEM,Hitachi S-4800)with the accelerating voltage of 15.0 kV.The microstructure was revealed by a transmission electron microscope (TEM,JEM-2010)with the operating voltage of 200 kV.The porosity properties were obtained by N2adsorption-desorption isotherms on Micrometics Auto-sorb-iQA3200-4,in which the specific surface area was calculated by Brunauer-Emmett-Teller (BET) model.The CO2adsorption-desorption performance was determined by temperature programmed desorption (CO2-TPD,ChemBET Pulsar TPR/TPD).The sample was pretreated at 400 °C for 1 h under a He flow(100 ml·min-1),then it was cooled to 50 °C and exposed to CO2(100 ml·min-1) for 2 h,followed by He purging for 30 min to remove the physically adsorbed CO2.Subsequently,the CO2-TPD was performed with a heating rate of 10 °C·min-1in He atmosphere,and the desorbed CO2was detected by a mass spectrometer.

Ultraviolet visible diffuse reflectance spectroscopy(UV-vis,TU-1901) was recorded by a UV-vis spectrophotometer using barium sulfate(BaSO4)as the background.The photoluminescence spectra were obtained by a fluorescence spectrometer (PL,F-7000).The photocurrent test was carried out on the electrochemical workstation(CHI660E).A platinum wire and a saturated calomel electrode were used as the counter electrode and the reference electrode,respectively.The sample coated with sand chip on indium-tin oxide (ITO) glass electrode was used as the working electrode,and the electrolyte was Na2SO4(0.5 mol·L-1) aqueous solution.The elemental states were analyzed by X-ray photoelectron spectroscopy (XPS,Thermo Fisher ESCALAB 250Xi).The carbonaceous C 1s line (284.8 eV) was used as a reference for the calibration of binding energy values.

2.3.Evaluation of photothermal catalytic activity

The photothermal catalytic CO2reduction conversion performances were performed by a 250 ml high temperature and high pressure CEL-HPR reactor (Beijing Zhongjiao Jinyuan Technology Co.,Ltd.).30 mg catalyst was dispersed into 100 ml ultrapure water,and then poured into quartz reactor before sealing.Highpurity CO2(99.999%) was introduced into the suspension for 30 min to ensure the removal of air in the reactor and make CO2saturate in the solution.After that,the reactor was filled with high-purity CO2to a pressure of 0.8 MPa and the adsorption of CO2was performed for 60 min until an equilibrium between adsorption and desorption obtained.Finally,the suspension was mixed continuously with a magnetic stirrer and the reaction lasted for 3 h with a 300 W Xenon lamp(15 A,300 mW·cm-2)as an external light source,which was applied through a quartz window at the top of the reactor.The light source was 11 cm away from the quartz window,and the light intensity was measured by CELNP2000 strong power meter(Beijing Zhongjiao Jinyuan Technology Co.,Ltd.).The photocatalytic and thermal catalytic CO2reduction reactions were carried out under the same condition.

The generated methanol and ethanol were detected by a GC7920 gas chromatograph system (Beijing Zhongjiao Jinyuan Technology Co.,Ltd.) equipped with a capillary column and flame ionization detector (FID),and the yields of methanol and ethanol were obtained by Eq.(1).

whereR(CH3OH,CH3CH2OH) is the reaction rate within a certain time,nis the molar content of methanol or ethanol,andmis the mass of catalyst per reaction.The selectivity of methanoland ethanolcan be calculated by Eqs.(2) and (3).

3.Results and Discussion

3.1.The textural properties of the samples

To investigate the crystal structure of the catalysts,the XRD was performed (Fig.1),and the diffraction peaks at 2θ of 36.9°,42.8°,62.3°,74.6° and 78.6° corresponded to (1 1 1),(2 0 0),(2 2 0),(3 1 1) and (2 2 2) crystal plane of MgO [27].No diffraction peak of CuO was found in the samples with small mass ratio of CuO,suggesting the well-dispersion of CuO on the surface of the catalysts.The characteristic peak of CuO appeared in 30% CuO/MgO,but no change of diffraction peak of MgO,indicating the maintenance of the crystal structure of MgO with the addition of CuO.It is worth mentioning that there were diffraction peaks of Mg(OH)2,which was ascribed to the reaction of MgO powder with moisture in the air.

Fig.1.The XRD patterns of the samples.

To explore the type of chemical bond in the catalyst,the infrared spectra were investigated(Fig.S1 in Supplementary Material).The strong characteristic vibration peak at 1438 cm belonged to monodentate carbonate on the defect [28].At the same time,consistent with the XRD results,there was an obvious peak at 3697 cm-1ascribed to —OH bond in the CuO-incorporated MgO,which further demonstrated the production of Mg(OH)2[29].However,the peak at 3697 cm-1was not observed in MgO and CuO.In addition,the characteristic vibration peak at～860 cm-1indicates the formation of Mg2+-O2-active species according to the literature[30].

From the SEM images of the samples,the MgO exhibited an obvious accumulated nanoparticles morphology with irregular shape (Fig.2(a)),whereas block structure was observed in CuO(Fig.2(g)).At the same time,the synthesized CuO/MgO samples showed an irregular loose nano-sheet structure,facilitating the diffusion and migration of CO2,so an excellent CO2adsorption performance can be obtained (Fig.2(b)-(f)).However,when the mass percentage of CuO reached 30%,most of the particles were irregular aggregations with no obvious flake structure and pores.

Fig.2.SEM images of the samples: (a) MgO,(b) 1.4%CuO/MgO,(c) 1.6%CuO/MgO,(d) 1.8%CuO/MgO,(e) 2.0%CuO/MgO,(f) 30%CuO/MgO,(g) CuO.

To study the structural feature in detail,TEM and the energy disperse spectroscopy (EDS) mapping analyses were performed(Fig.3).On the one hand,the MgO presented irregular shape(Fig.3(a)),and serious agglomeration occurred in the sample of CuO(Fig.3(h)).On the other hand,it can be observed that the synthesized CuO/MgO samples showed obvious nano-flake structure,CuO intimately contacting with layered MgO (Fig.3(b)-(e).However,when the mass percentage of CuO reached 30%,the nano-flakes were not obvious,with the agglomeration of nanoparticles in the sample of 30%CuO/MgO(Fig.3(f)).As verified by highresolution TEM (HRTEM) image and fast Fourier transform (FFT)pattern,obvious lattice fringes with the lattice spacing d of 0.25 nm and diffractions were observed,confirming the distribution of CuO within the MgO particle matrix,which was consistent with the XRD results (Fig.3(h)).In addition,the element mapping(attached to HRTEM) of 1.6%CuO/MgO is presented in Fig.4,indicating the uniform distribution of Mg,O,and Cu components.The homogeneity effectively ensures the interaction of each element in the catalyst,thus promoting the photothermal catalytic reduction of CO2.

Fig.3.TEM images of the samples:(a)MgO,(b)1.4%CuO/MgO,(c,h)1.6%CuO/MgO,(d)1.8%CuO/MgO,(e)2.0%CuO/MgO,(f)30%CuO/MgO,(g)CuO,(h)atomic lattice images and Fourier transforms for 1.6%CuO/MgO.

Fig.4.The energy disperse spectroscopy (EDS) mapping (attached to HRTEM) of the sample (1.6%CuO/MgO).orange spots,magnesium;blue spots,oxygen;yellow spots,copper.

Typical BET isotherms and corresponding pore size distribution curves for various samples are shown in Fig.S2.Evidently,all samples exhibited characteristics of types II(IUPAC classification),suggesting the existence of mesoporous structure in the accumulation of the flake-like particles.The BET specific surface area,pore volume,and pore diameter values are shown in Table 1.It is apparent that the introduction of CuO makes the increment of the specific surface area and pore volume as the decrease of pore diameter,improving the the adsorption of CO2molecules.However,the 1.6%CuO/MgO with the best activity of photothermal catalytic reduction does not have the highest specific surface area and pore volume,thus being not the decisive factor for the reduction conversion of CO2.

Table 1 The specific surface area,pore volume and pore size of the samples

To explore the effect of CO2adsorption on photocatalytic activity,the CO2-TPD test was carried out(Fig.S3).The MgO signal peak was located at the position of medium strong base,including Mg2+-O2-ion pair with medium strong base and —OH with weak base[31].The CuO-incorporated MgO showed two CO2desorption peaks,of which the first one at～300°C was ascribed to the chemical adsorption of CO2,whereas the second one with low intensity at～680 °C,attributed to the two basic sites of MgO.Meanwhile,compared with MgO,the adsorption performance of CO2for the 1.6%CuO/MgO was effectively improved by incorporation of CuO into MgO,therefore the favorable alkalinity made it the most suitable for CO2conversion.

3.2.The absorbance of the samples

Evidenced by UV-Vis diffuse reflectance spectroscopy,MgO almost had no light absorption in the wavelength range of 200-900 nm,whereas that of CuO exhibited the strongest light absorption (Fig.S4(a)).Meanwhile,it can be found that the light absorption gradually shifted to the visible wavelength range with the increase of CuO mass ratio,thus improving the utilization of visible light.Moreover,the band gap width of the samples became narrower with the addition of CuO,thus facilitating the separation of electrons and holes (the band gap width of MgO is 5.6 eV)(Fig.S4(b)-(c)).

To validate the separation efficiency of charge carriers,the fluorescence spectra of the samples were tested under the condition of excitation wavelength of 470 nm,with the appearance of the characteristic emission peak at～708 nm (Fig.5).The fluorescence peak of the sample significantly decreased with the introduction of CuO,thus inhibiting the recombination of carriers and effectively improving the charge separation efficiency.The higher the fluorescence intensity,the lower the separation efficiency of photogenerated carriers,therefore,a high separation efficiency of electrons and holes in the sample of 1.6%CuO/MgO with low fluorescence intensity can be obtained.

Fig.5.The fluorescence spectra of the samples.

To confirm the separation efficiency of photogenerated charges in the catalysts,the responses of transient photocurrent were investigated through five cycles of intermittent visible light irradiation (Fig.6(a)).Obviously,the photocurrent density of the samples gradually increased with the incorporation of CuO,and the excellent photocurrent response showed that the introduction of CuO would be of great benefit to the separation of photogenerated charges.More significantly,the photocurrents generated by the 1.6%CuO/MgO working electrode remained relatively greater level,which was twice as much as that of MgO,indicating its superior charge separation efficiency.The charge transfer resistance were measured by means of electrochemical impedance,which increased with increasing arc radius of a Nyquist curves obtained by electrochemical impedance spectra(EIS,Fig.6(b)).It is apparent that the arc radii of the nanocomposites are smaller than that of MgO,and that of 1.6%CuO/MgO is the smallest among all the samples,suggesting the rapid transfer of photogenerated charge and the highest concentration of photoexcited charges,thus promoting the photocatalytic reduction of CO2.

Fig.6.The photocurrent measurement of the samples: (a) transient photocurrent and (b) electrochemical impedance spectra curve of CuO/MgO composites with different mass ratios.

3.3.The electronic structure of the samples

To investigate the surface composition and chemical state of the catalysts,the XPS analysis was adopted,and the spectrum of the elemental survey scan displayed typical peaks of Cu,Mg and O respectively (Fig.7(a)).The binding energies of Cu 2p1/2(953.3 eV) and Cu 2p3/2(933.8 eV) characteristic peaks are representative of Cu2+as the predominant valence of copper species in the sample of 1.4%CuO/MgO (Fig.7(b)) [32-34].To incorporate MgO with CuO,the binding energy of Cu2+showed a positive shift,which may be caused by the high dispersion of CuO or the strong interaction between CuO and MgO (Fig.7(c)).The characteristic peaks of Mg2+with binding energy of 1307.2 eV and 1303.4 eV in Mg 1s spectra are clearly observed in the samples.Moreover,with the introduction of CuO into the MgO,the density of electron cloud of Mg atom increased,so the Mg 1s spectra shifted a little to the lower binding energy,suggesting the formation of Mg-O-Cu species through the electron-transfer from Cu to Mg (Fig.7(d)).The Mg-O-Cu species made the donation of electrons from MgO to CO2easier,resulting in the breaking of the old Mg-O bond and the formation of C—O bond,thus promoting the adsorption and conversion of CO2.In addition,determined from the O 1s spectrum,O existed in the form of O2-ion(Fig.7(e)).

Fig.7.XPS spectra of the samples: (a) full spectrum,(b) Cu 2p,(c) Cu LM2,(d) Mg 1s and (e) O 1s.

3.4.The performance of photocatalytic reduction of CO2

To study the effect of introduction of CuO into MgO with the low-coordinated Mg2+-O2-on photothermal catalytic reduction of CO2,the yield of the samples with various CuO mass ratios was tested.It is evident that pure MgO has no catalytic activity,and CuO has very low yields of methanol (2.21 μmol·g-1·h-1)and ethanol (4.37 μmol·g-1·h-1) (Fig.8(a)).Meanwhile,the CO2reduction activity of the CuO/MgO composites increased first and then reduced with the introduction of CuO,in which the activity of 1.6%CuO/MgO was the best,with the yield of methanol and ethanol being 2.38 μmol·g-1·h-1and 15.17 μmol·g-1·h-1,respectively.When the mass ratio of CuO was 30%,the yields of methanol and ethanol were less than that of 1.6%CuO/MgO.Combined with the results of TEM and SEM,the accumulation and agglomeration were serious with the increase of the amount of CuO,thus hindering the adsorption of CO2and reducing the conversion of CO2.Therefore,it is clearly reflected that appropriate amount of CuO loading on MgO (1.6%CuO/MgO) can effectively improve the yield of ethanol in the photothermal catalytic reduction of CO2,being 3.47 times that of CuO.

Fig.8.The yields of CO2 photocatalytic reduction over the CuO/MgO composites: (a) with different mass ratios,(b) at different temperatures (1.6%CuO/MgO),(c) with or without light (1.6%CuO/MgO).

To investigate the effect of temperature on the photothermal catalytic reduction of CO2,the photothermal catalytic activities of 1.6%CuO/MgO at different temperatures were tested (Fig.8(b)).Obviously,with the increase of reaction temperature,the catalytic activity increased first and then reduced.The activity of 1.6%CuO/MgO was the best at 120°C,with the yields of methanol and ethanol being 2.38 μmol·g-1·h-1and 15.17 μmol·g-1·h-1,while that at the temperature of 30 °C were only 0.56 μmol·g-1·h-1and 5.23 μmol·g-1·h-1,respectively.It can be proven that temperature plays an important role in the catalytic reduction of CO2.On the one hand,the lower reaction temperature is not enough to promote the activation of CO2.On the other hand,the higher temperature can provide more energy for the improvement of photogenerated electrons and desorption of the product,but hinder the photothermal effect of carrier separation.To prove the catalytic effect of light on the reduction of CO2in photothermal catalysis,the yield of 1.6%CuO/MgO with no light was also tested,with the yields of methanol and ethanol being 0.399 μmol·g-1·h-1and 6.43 μmol·g-1·h-1,respectively (Fig.8(c)).It is thus clear that the yield of ethanol with light just mentioned above is 2.36 times that with no light,indicating the significant role of photocatalysis in the catalytic reduction of CO2.

In order to verify whether the product is affected by the solvent used in the stage of catalyst preparation,the activity of pure MgO treated by hydrothermal process was also investigated,with no methanol and ethanol detected,exhibiting no influence of the solvent on the product of photocatalysis reduction.Meanwhile,the ethanol selectivity over the 1.6%CuO/MgO photocatalyst was calculated (～92.7%),without CO and CH4detected in the gas after reaction.

Various MgO-based catalysts in the literature for the CO2photocatalytic reduction were summarized,as shown in Table 2,indicating that MgO and the semiconductor played the role of promoting CO2adsorption and electron transfer (or light utilization),respectively.However,relatively few literature of the longer chain hydrocarbons (C2+) with higher industrial value as the product is reported.The magnesium-based catalyst used for photothermal synergistic catalytic reduction of CO2to ethanol without organic solvents and sacrificial agents exhibits a promising prospect of practicability,with economic and environmental benefits in the long run.

Table 2 MgO-based composites studied for CO2 photocatalytic reduction

3.5.Reaction mechanism for effective enhanced CO2capture and reduction to ethanol

The contribution of the CuO/MgO catalyst was further illustrated to understand the mechanism for its improved performance of photothermal catalytic CO2reduction (Fig.9).According to the experimental results mentioned above,the methanol and ethanol are not detected in the product of CO2reduction over pure MgO,indicating that MgO only plays the role of CO2adsorption and hindering the recombination of photogenerated carriers.The adsorption of CO2occurs on the surface of MgO as the form of magnesium carbonate species,which would effectively enhance the following catalytic reduction of CO2,because CO2as a linear molecule is more difficult to activate than carbonate.In addition,the CuO as the active site for the conversion of CO2to ethanol in the catalyst enhances the light absorption and increases the specific surface area,the conduction band electrons of which is favorable for photocatalytic reduction reaction.Meanwhile,combined with the results of XPS,the Mg-O-Cu species make the donation of electrons from MgO to CO2easier,resulting in the breaking of the old Mg-O bond and the formation of C—O bond,thus also promoting the adsorption of CO2.

Fig.9.The schematic illustration of photothermal catalytic CO2 reduction to ethanol over the CuO/MgO catalyst [7,42,43].

As shown in Fig.9,the corresponding reaction path of CO2reduction to ethanol was also proposed according to the literature[7,42,43].The formation of a CO2monolayer on the surface of MgO has been revealed,and the little change in the geometry or the electronic configuration of CO2at the regular surface sites consists with physical adsorption,whereas the CO2at low-coordinated sites of MgO tends to chemical adsorption forming surface carbonates[24].A higher capacity can be obtained on the surface of MgO with the higher density of low-coordination oxygen(edge and corners),which effectively improve the following CO2conversion by the low-coordinated Mg2+-O2-sites on the surface of MgO.In the whole reaction process,MgO is favorable for adsorption,and photoexcited electrons in CuO trigger CO2activation.

4.Conclusions

In summary,a series of CuO/MgO catalysts with lowcoordination O2-sites were synthesized for photothermal catalytic reduction of CO2to ethanol under visible light irradiation,indicating that the favorable capacity of CO2adsorption on the surface of MgO effectively enhanced the following catalytic reduction of CO2.Among them,1.6%CuO/MgO photocatalyst exhibits the best photocatalytic reduction activity with the ethanol yield of 15.6 μmol·g-1·h-1.The synthesized CuO/MgO samples with an irregular loose nano-sheet structure facilitate the diffusion and migration of CO2,so an excellent CO2adsorption performance can be obtained.Meanwhile,the CuO with a bivalent state as the active site for the conversion of CO2to ethanol enhances the light absorption and increased the specific surface area,the conduction band electrons of which is favorable for photocatalytic reduction reaction.Moreover,the Mg-O-Cu species can not only provide more electrons for the photocatalytic reduction of CO2,but also make the donation of electrons from MgO to CO2easier,resulting in the breaking of the old Mg-O bond and the formation of C—O bond,thus promoting the adsorption of CO2and conversion to ethanol.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial supports by National Natural Science Foundation of China (21908052),the Key Program of Natural Science Foundation of Hebei Province (B2020209017),the Project of Science and Technology Innovation Team,Tang shan(20130203D) and Youth Program of Natural Science of Hebei Province (B2020209065) are gratefully acknowledged.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.03.008.