Busha Assaba Fayisa,Yushan Xi,Youwei Yang,Yueqi Gao,Antai Li,Mei-Yan Wang,2,3,Jing Lv,2,,Shouying Huang,2,3,Yue Wang,2,3,,Xinbin Ma,2,3

1 Key Laboratory for Green Chemical Technology of Ministry of Education,Collaborative Innovation Center of Chemical Science and Engineering,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China

2 Zhejiang Institute of Tianjin University,Ningbo 315201,China

3 Joint School of National University of Singapore and Tianjin University,International Campus of Tianjin University,Fuzhou 350207,China

Keywords:CO2 Ethylene carbonate Hydrogenation Pt-Cu/SiO2 catalyst Ethylene glycol Methanol

ABSTRACT Copper-based catalysts were widely used in the heterogeneous selective hydrogenation of ethylene carbonate(EC),a key step in the indirect conversion of CO2 to methanol.However,a high H2/EC molar ratio in feed is required to achieve favorable activity and the methanol selectivity still needs to be improved.Herein,we fabricated a series of Pt-modulated Cu/SiO2 catalysts and investigated their catalytic performance for hydrogenation of EC in a fixed bed reactor.By modulating the Pt amount,the optimal 0.2Pt-Cu/SiO2 catalyst exhibited the highest catalytic performance with～99%EC conversion,over 98%selectivity to ethylene glycol and 95.8%selectivity to methanol at the H2/EC ratio as low as 60 in feed.In addition,0.2Pt-Cu/SiO2 catalyst showed excellent stability for 150 h on stream over different H2/EC ratios of 180-40.It is demonstrated a proper amount of Pt could significantly lower the H2/EC molar ratio,promote the reducibility and dispersion of copper,and also enhance surface density of Cu+ species.This could be due to the strong interaction of Cu and Pt induced by formation of alloyed Pt single atoms on the Cu lattice.Meanwhile,a relatively higher amount of Pt would deteriorate the catalytic activity,which could be due to the surface coverage and aggregation of active species.These findings may enlighten some fundamental insights for further design of Cu-based catalysts for the hydrogenation of carbon–oxygen bonds.

1.Introduction

CO2is one of the major contributors of global warming and climate change[1–3].Its utilization as a primary carbon resource has attracted great attention in recent years,especially on converting CO2to methanol [4].Building up a ‘methanol economy’ becomes so fascinating concept as methanol is a superb hydrogen carrier,an alternative liquid fuel and additive for fuel cells,a useful feedstock for producing wide-range of valuable chemicals including olefins and aromatics [5–9].Substantial efforts have been made in the direct hydrogenation of CO2to methanol.However,the high CO2activation energy required,low equilibrium conversion rate of CO2,and low selectivity of methanol made the direct CO2hydrogenation challenging for large-scale industrial applications [10–12].

An alternative approach for CO2conversion,which has recently got a keen interest due to the high CO2conversion and high methanol selectivity,is an indirect hydrogenation of CO2to methanol and diol via ethylene carbonates (EC) intermediate [13–15].EC is industrially produced by cycloaddition of CO2with ethylene oxide(EO),which is an efficient and environmentally benign process(Eq.(1))[14–17].Subsequently,catalytic hydrogenation of EC produces methanol and ethylene glycol(EG)as the major products(Eq.(2)).The development of suitable heterogeneous catalyst for efficient hydrogenation of EC has recently received extensive attention.

Copper-based catalysts are the suitable and widely used catalysts in selective hydrogenation of C＝O and C-O bonds without excessive side reactions.In case of EC hydrogenation reaction,ethanol is produced as a bi-product from further hydrogenation of EG at higher reaction temperature.Besides,CO2and CO could also be produced during the hydrogenation reaction [8–11].Liu and coworkers [18] examined the effects of different oxide supports on the catalytic hydrogenation of EC.SiO2is suggested as a suitable support for Cu-based catalysts due to its weak aciditybasicity level and suitable interaction with the active metal.Kim et al.[19] also studied the promotion effect of Lewis oxide centers on the Cu-based catalysts for the hydrogenation of propylene carbonate to methanol.It is observed that methanol selectivity showed a volcano-trend as a function of Lewis acidity,in which the maximum selectivity (＞60%) was observed for Cu/ZrOx@Al2O3with intermediate acidity.This could be because oxides with low Lewis acidity promote unselective decarbonylation of carbonate reactant,while those with strong Lewis acidity sites favor a decarboxylation pathway [19].

In the copper catalyzed hydrogenation of carbon–oxygen bonds,Cu+and Cu0are considered as active species where Cu0sites could activate hydrogen,whereas the Cu+sites may act as electrophilic or Lewis acid sites to polarize the C＝O/C-O bonds [18,20–23].Cubased catalysts for EC hydrogenation have been modified by adding different polyhydroxy compounds and metal promoters.A suitable amount of glucose or β-cyclodextrin could assist to enhance copper dispersion and alleviate catalyst deactivation [8,24].Additionally,Cu/SiO2catalyst with a suitable amount of molybdenum dopant could greatly increase the EC conversion as well as products selectivities [25].However,Cu-catalyzed EC hydrogenation reaction is still performed under high H2/EC molar ratio of typically 200,which is much higher than the stoichiometric ratio of 3[16,26].This would result in massive circulation of hydrogen in the system,extensive energy consumption and also increase the costs of devices such as compressors for large-scale industrial application.

Herein,we fabricated a series of Pt-modulated Cu/SiO2catalysts and investigated their behaviors in EC hydrogenation to produce methanol and EG.By modulating Pt-dopant on Cu/SiO2catalyst,we found that H2adsorption and dissociation was significantly promoted and thus exhibited excellent catalytic activity in the hydrogenation of EC at significantly lowered H2/EC molar ratio.The optimal 0.2Pt-Cu/SiO2catalyst furnished almost full conversion of EC,98%EG and 95.8%MeOH selectivities at low H2/EC ratio of 60.The addition of optimal Pt promoter also enhanced the dispersion of copper and increased the amount of surface Cu+species.This effect could arise from the strong Pt-Cu interaction induced by the formation of Pt-Cu single atom alloy (SAA) structure.This insight to structure–activity correlation may help as a guidance for further design of Cu-based catalysts for C-O/C＝O bonds hydrogenation.

2.Experimental

2.1.Catalyst preparation

2.1.1.Materials

All chemicals and reagents were analytical grade and were used without any purification.Ethylene carbonate (＞99%) was purchased from Aladdin Industrial Co.Cu(NO3)2·3H2O(＞99%)was purchased from Sinopharm Chemical Reagent Co.Aqueous ammonia(25%,mass)was purchased from Tianjin Kemiou Chemical Reagent Co and silica sol(30%,mass)was from Qingdao Grand Chemical Co.

2.1.2.Preparation of Pt-Cu/SiO2catalysts

The Cu/SiO2catalyst with 40% (mass) nominal copper loading was synthesized via the typical ammonia evaporation(AE)method[27].Briefly,a desired amount of Cu(NO3)2.3H2O was dissolved in 100 ml deionized water and aqueous ammonia solution (35%,mass) was added and stirred for 10 min to form a copperammonia complex solution.Thereafter,the required amount of 30% (mass) silica sol was added dropwise to the copper–ammonia complex solution and stirred for 4 h.The above procedure was performed at room temperature,then the temperature was raised to 80°C after 4 h stirring to evaporate ammonia.The evaporation process was terminated after the pH value of the suspension decreased to 6–7.The precipitates as obtained were filtered,washed with deionized water and dried at 110 °C overnight.

The as-synthesized Cu/SiO2catalysts were modulated with Pt via incipient wetness impregnation (IWI) method by using Pt(NH3)4Cl2as a precursor.Finally,the catalysts were dried at 110 °C overnight and calcined in air at 350 °C for 3 h.The Ptmodulated catalysts were denoted as xPt-Cu/SiO2,where x refers to the weight percentage of Pt (0.1,0.2 and 0.5% (mass) respectively).For comparison,0.1Pt/SiO2catalyst was also synthesized by the aforementioned method without copper.

2.2.Catalyst characterization

The Cu loading of all the calcined catalysts was analyzed by inductively coupled plasma optical emission spectrometry (ICPOES) on a Varian Vista-MPX.The N2adsorption–desorption isotherms were performed by using a physisorption analyzer Micromeritics Tristar 3000 apparatus at -196 °C.Before the measurement,the samples were degassed at 300 °C for 4 h to remove any possible impurities.The specific surface area was calculated by the Brunauer-Emmett-Teller (BET)method and pore-size distribution was obtained from the adsorption branch of the isotherm using the BJH method.

The X-ray diffraction(XRD)patterns were obtained on a Rigaku C/max-2500 diffractometer using Cu Kα radiation(λ=0.15418 nm)with a scanning angle(2θ)ranging from 10°to 90°.Before the test,all the calcined samples were reduced at 350 °C in the flowing H2for 4 h and protected by N2from air oxidation.The particle sizes of catalysts were calculated based on Scherrer equation.Transmission electron microscopy (TEM) images were analyzed by using a JEM-2100F microscope.The X-ray photoelectron spectra (XPS)and Auger electron spectra (AES) tests were performed by using PHI 1600 ESCA instrument equipped with an AlKα X-ray radiation source (1486.6 eV).Before the measurement,the catalyst was reduced in H2flow at 350°C for 4 h,and after cooling to room temperature,the sample was transferred to the analysis chamber.Binding energy was calibrated by using C1s peak at 284.6 eV as the reference.The experimental error was within ±0.2 eV.

H2-TPR analysis was conducted on Autochem II 2920 Micromeritics.The calcined sample (50 mg) was placed in the Ushaped quartz reactor and firstly heated at 200 °C in Ar for 1 h.After cooling to 50 °C,the gas was switched to 10% H2/Ar,and the sample was heated to 800 °C at a ramp rate of 10 °C·min-1.The temperature-programmed desorption of hydrogen (H2-TPD)was performed on the same device with about 100 mg of a sample.The samples were in situ reduced at 350 °C in 10% H2-Ar flow for 1.5 h followed by cooling to 50°C under Ar flow and subsequently isothermal adsorption of H2was performed for 1 h.After purged with Ar flow,H2-TPD was measured from 50 °C to 800 °C at a ramping rate of 10 °C·min-1using Ar flow and recorded by TCD detector.

The N2O titration using Micromeritics Autochem II 2920 instrument based on the stoichiometry of[2Cu(s)+N2O →Cu2O(s)+N2,with copper surface area of 1.47 × 1019Cu atoms·m-2],where Cu(s) refers to the surface copper atom [27].For each measurement,50 mg of the calcined sample was loaded into the reactor and reduced in 10% H2-Ar flow for 1.5 h at 350 °C.Then,the reduced sample was purged with Ar during cooling.After cooling to 90°C,10%N2O-Ar was introduced for 1 h to allow the entire oxidation of surface metallic copper.After purged with Ar flow,the sample was again reduced with 10% H2/Ar at 350 °C and the corresponding TCD signal was recorded.

In situ FTIR of CO adsorption experiment was conducted on the Nicolet 6700 spectrometer.Briefly,about 20 mg catalyst was pressed into self-supporting disks and reduced under hydrogen flow at 350 °C for 1.5 h.Then the background spectrum was obtained under He flow after cooling to 30 °C and the sample was exposed to CO flow for 30 min.After that,the sample was purged by He while collecting the spectra until the last several peaks appeared unchanged.

2.3.Catalytic activity

The gas-phase hydrogenation reaction of EC was performed in a stainless-steel fixed-bed reactor,in which a thermocouple is inserted in catalyst bed to monitor reaction temperature.Typically,about 0.5 g catalyst pellets (300-450 μm) were placed in the middle section of the reactor.Both sides of the catalyst bed were packed with silica sand and separated from the catalyst by a thin layer of silica wool.Before the reaction,the catalyst was reduced in situ with H2at 350°C for 4 h.After cooling to the reaction temperature of 180 °C,then EC solution (10% EC dissolved in 1,4-dioxane solvent) was injected into the reactor at certain weight hourly space velocity (WHSV).The steady and condensed liquid products were collected and analyzed on Shimadzu GC 2010 pro instrument equipped with a flame ionization detector (FID).To ensure repeatability of the samples,every single experimental figure was averaged from 2 to 3 separate samples for each reaction condition.The gas-phase samples were analyzed by the Shimadzu GC 2014 C instrument with both TCD and FID detectors.For the catalyst stability test,the reaction was performed continuously over different H2/EC ratio of 180–40 and WHSV of 1.0 h-1,and the sample was collected every 2 or 3 h.

3.Results and Discussion

3.1.Morphology and textural properties of the catalysts

The metal contents of calcined samples determined by ICP-OES are listed in Table 1.The copper loadings of all the catalysts are about 40% (mass),and the platinum contents vary in the range of0.1%–0.5% (mass).The N2adsorption and desorption isotherm and pore size distribution curves of Cu/SiO2and xPt-Cu/SiO2catalysts are displayed in Fig.S1(see Supplementary Material).All the samples showed Langmuir type IV isotherms with H1-type hysteresis loop showing the existence of a mesoporous structure,which is also attributed to a cylindrical nanotube material[28,29].As shown in Table S1,all the catalysts have similar specific surface area,pore volume and pore size,indicating the modification with platinum species has no obvious effect on the porosity of the catalysts[28].The TEM images of the calcined samples were obtained to illustrate the morphology of as-synthesized catalysts.As shown in Fig.S2,both the samples before and after modification by platinum exhibited the cylindrical nanotube structure,consistent with the porosity characterization results.

Table 1 Physicochemical properties of Cu/SiO2 and xPt-Cu/SiO2 catalysts

3.2.Chemical states and structure of surface active species

XRD patterns of reduced catalysts are displayed in Fig.1.The broad peak that appeared at 2θ of～22° belongs to amorphous silica.The characteristic diffraction peak at 36.4°,61.4°are attributed to Cu2O phase,while the peaks centered at 2θ of 43.3°,50.4°,and 74.1° are ascribed to the (111),(200) and (220) planes of metallic Cu,respectively [23].Moreover,no diffraction peaks of Pt species are observed,due to the low amount of Pt or high dispersion of Pt species.It was observed that diffraction peaks of Cu2O and metallic copper became broader and weaker when the amount of Pt increased to 0.1% and 0.2% (mass),and slightly sharper for 0.5Pt-Cu/SiO2catalyst,suggesting the addition of a small amount of Pt promoted the dispersion of copper species [30].According to the Scherrer equation,the average crystallite sizes of metallic copper were calculated,and it followed the order 0.2Pt-Cu/SiO2(3.0 nm) ＜0.1Pt-Cu/SiO2(3.1 nm) ＜Cu/SiO2(3.4 nm) ＜0.5Pt-Cu/SiO2(3.8 nm).This result illustrates that modulating proper amount of Pt could reduce the particle size and promote the dispersion of Cu particles,which may result from the strong interaction of Cu and Pt bimetals [30,31].Accordingly,the highest dispersion of copper species is achieved on 0.2Pt-Cu/SiO2catalyst.

TEM images of the reduced Cu/SiO2and xPt-Cu/SiO2catalysts are shown in Fig.2.It is noted that the nanotube structure of unmodified Cu/SiO2is destroyed,while the structure of xPt-Cu/SiO2catalysts are maintained after reduction.The dark dots indicate that Cu particles are uniformly distributed over the cylindrical silica nanotube.The particle size distributions (inset) of the reduced catalysts are also measured from TEM images.The average particle diameter of catalysts first slightly decreased with the increase of Pt amount and raised on further increase of Pt content,similar with the XRD result.This result verifies that a small amount of Pt dopant could reduce the Cu particle size and thus promote dispersion of Cu particles.

Fig.1.XRD patterns of the reduced CuSiO2 and xPt-Cu/SiO2 catalysts.

H2-TPR of as-prepared samples are shown in Fig.S3.The reduction peak of Cu/SiO2is concentrated at 209 °C,which is attributed to the overlapped reduction peak of well-dispersed CuO to Cu0and copper phyllosilicate to Cu2O [28,32].With the increase of Pt amount,the reduction temperature shifted to significantly lower temperature.For instance,the reduction temperature for 0.5Pt-Cu/SiO2catalyst is peaked at 178°C,which is 31°C lower than that of Cu/SiO2.It is indicated that a very low amount of Pt dopant could markedly promote the reduction of copper oxides [30].This effect of lowering the reduction temperature could be due to the interaction occurred between Cu and Pt species or enhanced capability for H2activation by Pt dopant.

To further investigate the chemical states and active copper species distribution,N2O titration,XPS and AES were measured.As shown in Table 1,the surface area of Cu0(SCu(0)) determined from N2O titration first decreased and slightly increased with increasing Pt content,in contrary with aforementioned XRD and TEM results.This inconsistency could be induced by the introduction of Pt,which may inhibit the oxidation of Cu by N2O as some surface copper particles could be covered by Pt species [31,33].Thus,here we used the size of Cu nanoparticles to calculate the metallic copper surface area,which first increased and then decreased with increasing Pt loading and reached the largest amount of 44.6 m2·(g cat)-1for 0.2Pt-Cu/SiO2.

The surface chemical states of copper species of the reduced catalysts were examined by XPS.As shown in Fig.3(a),the two peaks centered at 932.5 eV and 952.4 eV are typically assigned to the binding energies of Cu 2p3/2and Cu 2p1/2,respectively [34].The absence of the Cu 2p satellite peak at 942–944 eV indicates that Cu2+is completely reduced to the lower valances [23].Moreover,it was noted that with the increase of Pt loading,there is a slight shift of Cu 2p3/2peak to the higher binding energy,suggesting a strong interaction and charge transfer between the Cu and Pt species,as shown by H2-TPR [28].

Fig.2.TEM images of reduced Cu/SiO2 and xPt-Cu/SiO2 catalysts.

Fig.3.Cu 2p XPS spectra (a) and Cu LMM AES (b) spectra of the reduced Cu/SiO2 and xPt-Cu/SiO2 catalysts.

Cu LMM AES was performed to distinguish the different copper species[22].As shown in Fig.3(b),from the deconvolution of broad peak,there are two overlapping peaks at the binding energies of 570.3 eV and 573.9 eV,which are attributed to Cu0and Cu+species,respectively.The measurement of the areas below each peak showed that surface distribution of Cu+and Cu0species first increased and slightly decreased with Pt content (Table 1).The Pt-free Cu/SiO2catalyst contained relative low content of Cu+(37.2%) and upon increasing Pt loading,the Cu+content reached as high as 44.9% on 0.2Pt-Cu/SiO2catalyst.It is reported that doping of Pt species could increase the surface density of Cu+sites[30].The reason for the Cu 2p3/2peak shift and difference in the Cu species valence distribution may be ascribed to the electron exchange and formation of Pt-Cu alloy,in which Pt single atoms are dispersed on the copper particles surface [30,35].

In situ FTIR of CO adsorption was used to explore the surface structure of Pt-Cu catalysts and the spectra were shown in Fig.4.The adsorption of CO on Cu2+and Cu0species is considered weak and here,Cu2+sites couldn’t exist after reduction,and CO adsorbed on Cu0sites is considered to be purged out by He gas at room temperature [36].The adsorption band in the range of 2100–2200 cm-1could be ascribed to the Cu+–CO and Pt-CO adsorption domains [22,33].For comparison,Pt/SiO2catalyst was prepared in similar method to evaluate the CO adsorption on Pt/SiO2without copper.For Pt/SiO2catalyst,a typical linear Pt–CO adsorption vibration is observed at about 2116 cm-1[36–38].As Pt dopant gets diluted with Cu (Pt:Cu ratio decreased),the band of CO linearly adsorbed on Pt (Pt–CO) showed a redshift from 2116 cm-1(for pristine Pt/SiO2)to 2121 cm-1(for 0.1Pt-Cu/SiO2)(Fig.4a).This band is attributed to CO linearly adsorbed on single atoms of Pt in the metallic state in the surface of the Cu host [39–41].As illustrated in Fig.4a,the weak band of bridge-adsorbed CO on two adjacent Pt atoms is clearly observed around 1855 cm-1on Pt/SiO2and absent on the Pt-Cu bimetallic catalysts [38,42].This could be caused by the considerable loss of adjacent Pt atoms,forming Pt-Cu single atom alloy (SAA) species [30,43].In the three Pt-Cu bimetallic catalysts,it is showed shoulder peaks at 2130 cm-1and 2111 cm-1that could be ascribed to CO-Cu+and CO-Pt in the Pt-Cu SAA catalysts,respectively [30,43].Compared to the reported Cu/SiO2catalysts,the CO-Cu+vibration frequency of bimetallic catalysts had a blueshift.The blueshift effect could be due to the electronic and geometric effects on Cu species induced by Pt modulation [35].The electronic effect could be induced by the transfer of electrons from Cu to Pt,consistent with the XPS results,that could produce a blue shift in the C-O stretching frequency.In another way,the doping of Pt significantly promotes the dispersion of Cu,as evidenced by both XRD and TEM,and thus made the Cu species more uncoordinated,leading to the geometric effect that results in a blueshift [30,44].

In addition,for bimetallic catalysts a broadened band is observed at 2021,2030 and 2046 cm-1for 0.1Pt-Cu/SiO2,0.2Pt-Cu/SiO2,and 0.5Pt-Cu/SiO2catalysts,respectively.These peaks could be attributed to CO adsorbed on Pt0nanoparticles with different sizes,which showed higher blueshift and peak intensity as the Pt amount increased.And it is obviously that this peak becomes much more intensive when the Pt addition is increased to 0.5% (mass),indicating a portion of Pt formed nanoparticles rather than highly dispersed atomic Pt species appeared in the 0.5Pt-Cu/SiO2sample.It also could reduce the interaction between Cu and Pt and lead to the vibration frequency of CO-Cu+in 0.5Pt-Cu/SiO2being lower than that in the 0.2Pt-Cu/SiO2sample.

3.3.Catalytic performance

The catalytic performances of Cu/SiO2and xPt-Cu/SiO2catalysts were tested to investigate the effect of Pt on vapor-phase EC hydrogenation to MeOH and EG in a fixed-bed reactor under different reaction conditions.The reaction is performed at temperature of 180 °C,pressure of 3 MPa and different H2/EC molar ratios of 180–40.As shown in Fig.5,for H2/EC of 180–60,almost full conversion of EC and～98% selectivity of EG were achieved over the Cu/SiO2,0.1Pt-Cu/SiO2and 0.2Pt-Cu/SiO2catalysts,while the selectivity of MeOH significantly varied with changing H2/EC ratio and Pt content.Upon further decrease of the H2/EC ratio to 40,the conversion of EC rapidly decreased(Fig.5(a)).Whereas,the selectivity of MeOH increased with decreasing the H2/EC molar ratio and decreased with further decrease of the H2/EC ratio to 40,and the highest methanol selectivity is achieved at H2/EC ratio of 60 over all catalysts (Fig.5(b)).

The products selectivity significantly enhanced with proper amount of Pt promoter.However,with further increase of Pt loading,the conversion of EC gradually declined.It is noted that the selectivity of MeOH is remarkably enhanced with increasing Pt content and declined at 0.5%(mass)Pt content,while the selectivity of EG is maintained at about 98%for all the catalysts(Fig.S4a).Notably,0.2Pt-Cu/SiO2catalyst exhibited a superior catalytic activity furnishing 98.7% EC conversion,98% EG and most importantly 95.8% MeOH selectivity at the H2/EC ratio as low as 60.As shown in Table S2,this value of methanol selectivity is the highest value reported for EC hydrogenation at a significantly lowered H2/EC ratio of 60.We also provided time on stream data of the bestperforming 0.2Pt-Cu/SiO2catalyst in the hydrogenation of EC in Fig.5(d).It is showed that the catalyst exhibited excellent stability over different H2/EC molar ratios of 180-40 during 150 h time on stream.The EC conversion maintained ＞92% even for the harsh condition of H2/EC of 40 and gradually increased back to complete conversion on increasing H2/EC ratio to 180.The selectivity of MeOH reached about 95% for the suitable H2/EC ratio of 60,whereas,the EG selectivity maintained at about 98%.The conversion of EC and selectivity of products showed similarity for the same reaction conditions repeated at different time on stream.Moreover,the test of gas-phase product confirmed the presence of some CO and CO2bi-products,which could be generated either from hydrolysis of EC or methanol decomposition via the decarbonylation and decarboxylation reactions [25,33].

Fig.4.In situ FTIR spectra of CO adsorbed on Pt/SiO2 and xPt-Cu/SiO2 catalysts.

Fig.5.The catalytic performance of Cu/SiO2 and xPt-Cu/SiO2 catalysts:Correlation of EC conversion(a),methanol selectivity(b)and EG selectivity(c)versus H2/EC molar ratio and (d) time on stream catalytic performance of the 0.2Pt-Cu/SiO2 catalyst (Reaction conditions:180 °C,3 MPa,WHSVEC=1.0 h-1).

Fig.6.H2-TPD profiles of Cu/SiO2 and xPt-Cu/SiO2 samples.

3.4.Effect of Pt dopant on Cu-based catalysts and the catalytic performance for EC hydrogenation

H2adsorption and activation properties on the Cu/SiO2and xPt-Cu/SiO2catalysts were examined using the H2-TPD.As shown in Fig.6,the H2-TPD profiles of Cu/SiO2and xPt-Cu/SiO2catalysts showed two desorption peaks positioned within 50–120 °C and 350–650 °C temperature ranges.The narrow symmetrical desorption peak at lower temperature is attributed to the chemisorbed H2on the surface of metal and the peak intensity gradually increased with Pt content over Pt-Cu catalysts[44].The desorption peaks at higher temperature corresponds to the strong adsorption,activation and desorption of H2species[32,44,45].The higher temperature H2desorption peak for Cu/SiO2is very low compared to the Pt-Cu SAA catalysts.As the amount of Pt increases,the intensity of the H2desorption peak increases and the peak broadens with the peak center shifted to higher temperature,indicating the increase in binding strength of H to Pt-Cu SAA sites and larger Pt clusters [44].This indicates the modulation of Pt significantly improves H2activation and surface concentration of H species.Moreover,0.2Pt-Cu/SiO2catalyst exhibited the strongest peak intensity with the highest chemisorbed hydrogen (334.7 μmol·(g cat)-1as shown in Table 1,which could result from both the Pt-Cu SAA sites [46–48] and the enhanced copper dispersion and the metallic copper surface area [20,28].

Moreover,the very low amount of Pt addition to form a single atom alloy Pt-Cu catalyst showed an effect on chemical state and structure of Cu species.The formation of single atom alloy is reported to maximize the efficiency of noble metals in heterogeneous catalysts [49,50].The introduction of low amount of Pt significantly promoted the reducibility of Cu as determined by H2-TPR.Modulating a suitable amount of Pt reduced the Cu particle size and thus enhanced metallic Cu surface area and dispersion.It is worth noting that the modulation of Pt also benefited to alter the surface chemical states of the copper species,clearly evidenced by the slight peak shift of Cu 2p3/2to the higher binding energy and the enhanced surface Cu+over a suitable Pt loading as determined from XPS and Cu LMM.Remarkably,with the increase of Cu+surface area,methanol selectivity first increased and dropped when Cu+surface content decreased.Accordingly,the highest methanol selectivity is attained over 0.2Pt-Cu/SiO2catalyst with the highest Cu+surface area of 36.3 m2·(g cat)-1and highest Cu+/(Cu0+Cu+)of 44.9% (Fig.S4(b)),consistent with published literature [23,33].It has been commonly accepted that the strong synergetic effect of Cu0and Cu+promotes the catalytic activity,in which Cu+site polarize the C＝O/C-O bonds and Cu0sites could activate and cleave hydrogen[19,22].Pt atom could also facilitate the activation and adsorption of H2and spillover to Cu0[48,51,52].Thus,when the surface amount of Pt and Cu0species are enough to adsorb and activate H2,the presence of more Cu+species could greatly enhance methanol selectivity.

4.Conclusions

In this work,we prepared a series of Pt-modulated Cu/SiO2catalysts by ammonia evaporation method followed by incipient wetness impregnation method and found that Pt could be an effective promoter to enhance the catalytic performance of Cu/SiO2for the hydrogenation of CO2-derived EC to produce MeOH and EG.The optimal modulated 0.2Pt-Cu/SiO2catalyst exhibited the highest catalytic performance with～99%EC conversion,over 98%EG selectivity and up to 95.8%selectivity to MeOH and presented excellent stability for 150 h long-term reaction at a various H2/EC of 180-40.Notably,by adding such a low amount of Pt,there are Pt-Cu SAA species formed.The strong interaction between Pt and Cu in Pt-Cu SAA significantly altered the surface chemical states and structure of Cu species.Consequently,modulation of Pt highly promoted the reducibility and dispersion of copper,and also increased the amount of surface Cu+sites.Moreover,a low amount of Pt modulation remarkably enhanced the H2adsorption and activation,thus significantly lowered the H2/EC molar ratio.Therefore,when the surface amount of Pt and Cu0species are sufficient to activate H2,the presence of more Cu+species could greatly enhance methanol selectivity.It is also noticed that the introduction of excess amount of Pt slightly deteriorated the catalytic performance,which could be resulted from the surface coverage and aggregation of Cu active sites by excess Pt species.

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

This work was supported by the National Natural Science Foundation of China (22022811,U21B2096 and 21938008),the National Key Research &Development Program of China(2018YFB0605803).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.10.024.