Carbon dioxide hydrogenation to methanol over the pre-reduced LaCr0.5Cu0.5O3 catalyst

Carbon dioxide hydrogenation to methanol over the pre-reduced LaCr 0.5Cu 0.5O 3catalystLishan Jia a,c,*,Jing Gao a,c ,Weiping Fang b ,Qingbiao Li a,caDepartment of Chemical Engineering and Biochemical Engineering,College of Chemistry and Chemical Engineering,Xiamen University,Xiamen 361005,Fujian,China bDepartment of Chemistry,College of Chemistry and Chemical Engineering,Xiamen University,Xiamen 361005,Fujian,China cNational Engineering Laboratory for Green Chemical Productions of Alcohols,Ethers and Esters,Xiamen 361005,Fujian,Chinaa r t i c l e i n f o Article history:Received 26May 2009Received in revised form 17July 2009Accepted 20July 2009Available online 23July 2009Keywords:CO 2hydrogenation Methanol synthesisLaCr 0.5Cu 0.5O 3perovskitea b s t r a c tThe synthesis of methanol from CO 2hydrogenation was carried out over the pre-reduced Cu-based LaCr 0.5Cu 0.5O 3catalyst.It showed a much higher catalytic performance (X CO 2=10.4%and S MeOH =90.8%)at 250°C than over 13%Cu/LaCrO 3prepared by wet-impregnation method (X CO 2=4.8%and S MeOH =46.6%).XRD,H 2/CO 2-TPD and XPS measurements illustrated that hydrogen was adsorbed on the Cu a +sites and that CO 2was activated on the medium basic sites for the reduced LaCr 0.5Cu 0.5O 3.This phenomenon was responsible for its catalytic activity.Ó2009Elsevier B.V.All rights reserved.1.IntroductionCarbon dioxide emissions which induce greenhouse effect in-crease with the development of economic activity.Thus,exploita-tion of CO 2as a major source of carbon is of importance [1,2].One of the promising methods is catalytic hydrogenation of carbon dioxide to methanol.Methanol is a starting material for several important chemicals such as acetic acid and formaldehyde.As an excellent liquid fuel,methanol could also provide convenient stor-age of energy in the fields of transportation and fuel cell applica-tion [3,4].Among the various suitable catalytic systems,Cu-based cata-lysts prepared by a conventional coprecipitation method have been widely investigated for methanol synthesis from carbon dioxide and hydrogenation [5–9].Recently,much attention has been re-ceived in obtaining fine dispersion of ‘‘metal-on-oxide”catalyst by reduction of AB 1Àx B x 0O 3perovskite.This catalyst often contains mixed valence ions and catalytic active sites,responsible for its high activity for several reactions.For example,Tien-Thao and co-workers [10–12]were engaged in studying a series of LaCo 1Àx Cu x O 3Àd perovskites for CO hydrogenation to higher-alco-hol synthesis.They confirmed that the cobalt–copper,bimetallic catalyst,significantly improved the selectivity and productivity ofhigher-alcohols.Bourzutschky [13]reported the catalytic perfor-mance of LaMn 1Àx Cu x O 3+k which displayed more than 90%selectiv-ity toward alcohols for CO hydrogenation.Bedel [14,15]achieved more active catalytic system for Fischer–Tropsch synthesis,using LaCo x Fe 1Àx O 3perovskites prepared by a sol–gel method.Araujo [16]pointed out that LaNi 1Àx Ru x O 3was active in the methane reforming with CO 2and was effective at resisting against coke deposition.The aim of the present work was to investigate the develop-ment of lanthanum chromite perovskite doped with 50%copper which showed a high catalytic behavior for CO 2hydrogenation to methanol.The catalytic surface sites and reaction paths were also studied.2.Experimental 2.1.Catalyst preparationLaCr 1Àx Cu x O 3oxides with x =0and 0.5,were prepared by a sol–gel method using citric acid as a complexing agent.The amounts of precursor salts (La(NO 3)3Án H 2O (La 2O 3P 44%),Cr(NO 3)3Á9H 2O,Cu(NO 3)2Á3H 2O)along with citric acid were dissolved in water at a molar ratio of 2:1(metal cations:citric acid).The total metal cat-ionic concentration was fixed to be 0.2mol/L.The solution was heated to 150°C until it ignited.The resulting powder was finally calcined under air at 400°C for 2h and then at 800°C for 4h.The catalyst 13wt%Cu/LaCrO 3was prepared by the wet-impregnation method.The copper nitrate was dissolved in distilled water and was stirred.And the appropriated weight of LaCrO 3was1566-7367/$-see front matter Ó2009Elsevier B.V.All rights reserved.doi:10.1016/j.catcom.2009.07.017*Corresponding author.Address:Department of Chemical Engineering and Biochemical Engineering,College of Chemistry and Chemical Engineering,Xiamen University,Xiamen 361005,Fujian,China.Tel.:+865922188283;fax:+865922184822.E-mail address:Jials@ (L.Jia).Catalysis Communications 10(2009)2000–2003Contents lists available at ScienceDirectCatalysis Communicationsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /c at c omadded.The slurry was heated to90°C until the water was evapo-rated.The residue was dried at120°C for24h and was subse-quently calcined at350°C for3h.2.2.Catalyst characterizationPowder X-ray diffraction(XRD)patterns were recorded on a Panalytical X’Pert Pro X-ray diffractometer with Cu K a radiation, in the step mode(0.0167°,15s)in the range20°<2h<80°.The voltage and current were40kV and30mA,respectively.X-ray photoelectron spectroscopy(XPS)measurements were done on a Quantum2000Scanning ESCA Microprobe instrument with Al K a radiation(15kV,150W,h m=1486.6eV)under ultra-high vacuum(10À7Pa),calibrated internally by the carbon deposit C(1s)(E b=284.6eV).H2-temperature-programmed desorption(H2-TPD)was con-ducted on Micromeritics AutoChem2920II instrument connected to a ThermoStar GSD301T2mass spectrometer.About100mg of catalyst sample was used for each test.The precursor of catalyst wasfirstly pre-reduced in situ in the TPD equipment by pure H2 at350°C for2h,followed by switching to a H2(99.999%,purity) stream for adsorption at45°C for1h.The H2desorption was then performed from100°C to700°C at a rate of10°C/min.CO2-TPD was carried out under the same condition as H2-TPD,conducting from50°C to400°C.2.3.Catalyst evaluationThe methanol synthesis reaction was carried out in afixed-bed flow reactor.Prior to the reaction,the precursor of catalyst(0.5g) was pre-reduced with pure H2at350°C for2h under atmospheric pressure.Then the reactor was cooled to200°C and the reaction mixture was admitted,raising the pressure to2Mpa.The reaction of CO2hydrogenation was tested over a temperature range of250–300°C.A CO2–H2reaction mixture in the molar ratio equal to1:3 was fed at a space velocity(GHSV)of9000ml/h g.The products were analyzed by an on-line gas chromatograph(GC-950)using TDX-01column and GDX-401column,connected to TCD and FID, respectively.The former column was used for the separation of N2,CO and CO2,while the latter was used for the separation of hydrocarbons and alcohols.Conversion and selectivity values were calculated by the internal standard and mass-balance methods. 3.Results and discussion3.1.Catalytic activityMethanol synthesis from CO2hydrogenation was tested over the reduced LaCr0.5Cu0.5O3catalyst.For comparison,the activity of13%Cu/LaCrO3was investigated.The copper amount(13wt%) was chosen to be equal to the amount of Cu weight in the LaCr0.5Cu0.5O3.The major product was methanol,and the by-products were CO and methane.The amount of unidentified high-boiling products did not exceed1%and they were neglected. Table1presents the results of catalytic performance.It was found that reaction temperature had a little influence on the selectivity of CH4which was in the range of2–5%over LaCr0.5Cu0.5O3.And the selectivity of CH4was up to30%over13%Cu/LaCrO3at250°C, decreasing significantly with the increasing amount of CO.The selectivity of CO showed an up trend with the increasing temper-ature due to‘‘reverse-water–gas-shift(RWGS)reaction”over both LaCr0.5Cu0.5O3and13%Cu/LaCrO3.The selectivity of methanol and the conversion of carbon dioxide over LaCr0.5Cu0.5O3were much higher than those over13%Cu/LaCrO3.The catalytic behavior ofLaCr0.5Cu0.5O3with10.4%of(X CO2and90.8%of S MeOH was twiceas many as that of13%Cu/LaCrO3with4.8%of(X CO2and46.6%ofS MeOH at250°C.3.2.XRD analysesThe main characteristics of catalysts before and after reductiontreatment are reported in Fig.1A and B,respectively.According tothefile of the Joint Committee on Powder Diffraction System(JCPDS),LaCr0.5Cu0.5O3had mixed phases,indexed to CuO(JCPDS00-005-0661),La2CrO6(JCPDS00-026-0817)and LaCrO3perov-skite structure(JCPDS01-074-1961).On the one hand,thisphenomenon implied that a noticeable amount of copper ionsTable1Summary of the results from activity tests.Catalysts Reactiontemperature(°C)Conversion of CO2(%)Selectivity of product(%)CH3OH CO CH4LaCr0.5Cu0.5O325010.490.8 5.5 3.427012.175.619.6 4.430016.149.347.7 2.513%Cu/LaCrO3250 4.846.622.830.12707.056.417.725.83008.216.180.1 3.5Reaction conditions:P=2MPa,GHSV=9000ml/h g,CO2:H2=1:3(molarratio).Fig.1.XRD patterns of LaCr0.5Cu0.5O3and13%Cu/LaCrO3before(A)and after(B)being reduced.(j)LaCrO3;(N)La2CrO6;(d)CuO;and(s)Cu.L.Jia et al./Catalysis Communications10(2009)2000–20032001was still located out of the perovskite lattice.On the other hand,it indicated that oxygen vacancies for x =0.5were so high that CuO appeared,with a remaining fraction of Cr–Cu perovskite [17].For 13%Cu/LaCrO 3,the diffractograms displayed a single LaCrO 3phase and crystalline phase of CuO.Moreover,a small shift of diffraction lines to lower 2h values was observed for LaCr 0.5Cu 0.5O 3.This resultcould be attributed to the difference of ionic radii (r 3þCu =0.615Åand r 2þCu =0.73Å),leading to a small change in unit cell of perov-skite lattice [12].The main diffraction peak at 2h =32.68°for LaC-rO 3shifted to the lower diffraction angle 2h =32.58°for LaCr 0.5Cu 0.5O 3,while it did not change for 13%Cu/LaCrO 3.It can be seen from Fig.1B that LaCrO 3perovskite structure was preserved for LaCr 0.5Cu 0.5O 3after being reduced.The peaks attrib-uted to metallic copper (2h =43.45°and 50.57°,JCPDS 00-003-1005)were observed.Meanwhile,La 2CrO 6phases disappeared,decomposing to LaCrO 3at 350°C under the H 2atmosphere [18].3.3.Temperature-programmed desorption (H 2-TPD)experiments Fig.2shows the TPD profiles of hydrogen adsorbed on the pre-reduced CrO 3displayed no desorption peak in the whole temperature range and so it did not adsorb H 2.However,there was no H 2desorption peak for 13%Cu/LaCrO 3.According to the literature data,the lower peak around room temperature re-sulted from the desorption of weakly molecularly adsorbed hydro-gen on the Cu sites,while the higher peak in the range 150–500°C was attributed to the desorption of strongly adsorbed species such as dissociatively adsorbed hydrogen [19].Thus,there might be H 2adsorption on the Cu sites at room temperature and did not be de-tected due to our spanning from 100°C to 700°C.A much broader signal around 400°C was monitored over LaCr 0.5Cu 0.5O 3.3.4.CO 2-TPDFig.3depicts the concentration of surface basic sites and basic strength on CO 2-TPD spectra.One desorption peak (I)was observed around 100°C for all the samples.This peak originated from the weak CO 2desorption.The TPD profiles of LaCr 0.5Cu 0.5O 3perovsk-ites showed a desorption peak (II)around 250°C corresponding to the medium basic sites,while the CO 2-TPD patterns of LaCrO 3and 13%Cu/LaCrO 3showed no moderate-strength peak [20].This result disclosed a contribution of medium basic sites associated with the Cu—O j—Cr species which could be able to adsorb CO 2for the reduced LaCr 0.5Cu 0.5O 3.The catalytic performance demon-strated the crucial role of medium basic site.3.5.XPS investigationsXPS analyses of the reduced catalysts revealed the differences between LaCr 0.5Cu 0.5O 3and 13%Cu/LaCrO 3catalysts.The fitted XPS spectra of Cr 2p 3/2levels are shown in Fig.4A.In the 13%Cu/LaCrO 3catalyst,the peak at 575.3eV in Cr 2p 3/2spectrum was a single peak,which was assigned to Cr 3+.The Cr 2p 3/2spectra of LaCr 0.5Cu 0.5O 3displayed peaks characteristic of the presence of two distinct valence states of chromium.The peak at 575.5eV was ascribed to Cr 3+.The other peak at 576.6eV was likely attrib-uted to a higher oxidation state Cr species,lower than that of Cr 6+[21–23].According to the literature,for LaCr 1Àx M x O 3upon B-site doping of perovskites with divalent cations,charge neutrality tended to be maintained owing to an increase in Cr 4+content [24–27].Fig.4B illustrates a typical XPS spectrum of Cu 2p 3/2core level.The profile of 13%Cu/LaCrO 3and LaCr 0.5Cu 0.5O 3was similar in the shape of peak.It consisted of a principal peak at 931.7eV which was the characteristic of reduced Cu +/Cu 0species,and the shoulder peak centered at 934.5eV which was assigned to Cu 2+,indicating that some copper was still oxidised [28].Additional evidence for the presence of Cu 2+ions was provided by the satellite structure caused by electron shake-up processes.In combination with the CO 2-TPD studies,there might be Cu 2þ—O j—Cr 4þspecies after LaCr 0.5Cu 0.5O 3being reduced.Differentiation between reduced Cu +and Cu 0species at 931.7eV was impossible on the basis of binding energy analysis alone.The distinction between these spe-cies was only feasible through the examination of the modified Au-ger parameter.The value of 918.7eV for the species with a binding energy of 931.7eV value matched with KE of Cu 0,while the value of Cu LMM for LaCr 0.5Cu 0.5O 3shifted to a higher kinetic energy of 919.7eV,close to value of Cu a +.3.6.Suggested reaction mechanismWhen the reduced LaCr 0.5Cu 0.5O 3perovskite was used as a start-ing material,H 2was adsorbed on Cu a +sites resulting from the reduction of extra-perovskite framework copper species and part of intra-perovskite species at the reaction conditions.Meanwhile,CO 2was activated on Cu 2þ—O j—Cr 4þspecies in the lattice.ThenFig.2.TPD profiles of H 2on thecatalysts.Fig.3.TPD profiles of CO 2on the catalysts.2002L.Jia et al./Catalysis Communications 10(2009)2000–2003activated hydrogen shifted from Cu a +to the surface of perovskite.The reactive intermediate such as methoxyl group was formed [29].A simplified sketch of the catalysts surface and reaction paths is illustrated in Fig.5.4.ConclusionsThe reduced Cu-based LaCr 0.5Cu 0.5O 3catalyst showed much higher catalytic performance in comparison with 13%Cu/LaCrO 3for methanol synthesis from CO 2hydrogenation.The Cu a +sites and medium basic sites Cu 2þ—O j—Cr 4þwhich adsorbed/activated H 2and CO 2played a crucial role on the selectivity of methanol.AcknowledgementsThis work was supported by the National Engineering Labora-tory for Green Chemical Productions of Alcohols,Ethers and Esters.The authors were also grateful for the State Key Laboratory for Physical Chemistry of Solid Surface of Xiamen University.References[1]Xu J.A.Moulijn,Energy Fuels 10(1996)305.[2]R.Raudaskoski,E.Turpeinen,R.Lenkkeri,E.Pongrácz,R.L.Keiski,Catal.Today144(2009)318.[3]G.A.Olah,Catal.Lett.93(2004)1.[4]J.P.Breen,J.R.H.Ross,Catal.Today 51(1999)521.[5]J.Toyir,P.R.Piscina,J.L.G.Fierro,N.Homs,Appl.Catal.B 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