Yukun Bai,Yuqi Wang*,Weijian Yuan,Wen Sun,Guoxia Zhang,Lan Zheng,Xiaolong Han,Lifa Zhou

School of Chemical Engineering,Northwest University,Xi'an 710069,China

Shaanxi Provincial Institute of Energy Resources&Chemical Engineering,Xi'an 710069,China

Keywords:Perovskite-like La2?xCexCoO4±y Methane dry reforming Cerium Syngas

A B S T R A C T A series of oxides(La2?xCexCoO4±y)with perovskite-like structure were prepared by the Pechini sol-gel method for dry reforming of methane reaction(DRM).The prepared catalysts were characterized by BET,XRD,TGA,H2-TPR and SEM.Experimental results indicate that the addition of Ce can impact both sample morphology and catalytic performance significantly compared with La2CoO4catalyst,and LaCeCoO4presented the highest catalytic ability among all the samples.The Ce addition tends to increase the specific surface area of La2?xCexCoO4± yfrom 0.2 to 8.5 m2·g?1,suggesting that LaCeCoO4catalyst contained more well-dispersed active sites and more space to reaction.Moreover,the catalytic performance and anti-coking ability were substantiallyimprovedafterCeadditionduringDRM,whichmaybeattributedtothedecreaseofLaCoO3particlesizeand growth of oxygen storage capacity,respectively.

1.Introduction

Dry reforming of methane(DRM)is one of the most favorable methodsforgreenhousegasutilization.DRMreactioncanprovideaneffective way to transferthe CO2and CH4to syngas(H2and CO)(Eq.(1)),which may be used as feedstock in chemical processes such as Fischer-Tropsch synthesis[1,2].Although many scholars have intensively studied DRM for many years,the catalysts still suffer from poor thermal stability,coke deposition,and agglomeration of active metals at high temperature,as shown in Eqs.(2)-(4)[3,4].

The perovskite-like structures(A2BO4oxides of K2NiF4)are studied and show their high catalytic activity,which presents their low cost and high thermal stability as new materials for catalytic reaction[5].The perovskiteoxidesformaA2BO4structurecomprisingsupportandactive components,which can enhance the interaction between supports and active metals,and thus can be used in catalytic reaction process[6-9].Unfortunately,only a few reports on catalytic reaction were found about catalytic properties of LaSrCoO4in the oxidation of CO and C3H8,which have been researched and exhibit high performance[10].SmCoO3perovskite was synthesized as catalyst for methane dry reforming and good results have been achieved[11].

In our former study[12,13],rare earth elements can significantly improve the structure and the dispersion of catalysts for DRM.In this work,perovskite-like oxide(La2CoO4)and cerium doped were tried as catalysts for DRM,and therefore a new perovskite-like oxide(La2?xCexCoO4±y)was introduced and experimentally studied,which was found to have more activity and potential for the DRM reaction.Physical and chemical properties of the catalysts were characterized by XRD,BET,SEM,H2-TPR and TGA.The activity and stability of catalysts were evaluated by thefixed-bed differential reactor activity evaluation device.

2.Experimental

2.1.Catalysts preparation

La(2?x)CexCoO4±ycatalysts(x=0,0.10.2,0.4,0.6,0.8,1)wereprepared by using the Pechini sol-gel method.First,La(NO3)3,Ce(NO3)3,and Co(NO3)2(analytical grade,Sinopharm Chemical Reagent Co.,Ltd)were dissolved in 400 ml of distilled water by continuous stirring.After 1 h of stirring,CA:citric acid and EG:ethylene glycol(analytical grade,Sinopharm Chemical Reagent Co.,Ltd)were added to the mixed solution of metal nitrates with a molar ratio of CA and EG to the total metal cations of 2:2:1.CTAB:cetyl trimethyl ammonium bromide was added to the aqueous solutions to disperse all of the cations.Then,the mixed solutions were heated and stirred in a water bath at 80 °C for 8 h.These gels were then dried at 110 °C for 12 h and calcined under air flow at 800°C for 4 h.All the catalyst samples were crushed and sieved to particle sizes ranging from 0.45 mm to 0.90 mm.

2.2.Evaluation and calculation of the DRM reaction

The catalytic performances towards CH4&CO2reforming were experimentally investigated in a quartz tube fixed bed reactor(?10 mm)supplied with CH4/CO2/N2=3:3:2(total flow rate 80 ml·min?1)at WHSV=16000 ml·h?1·(g cat)?1and the reactive temperatures ranged from 400-800°C.Before the reaction,300 mg of catalyst particles was pre-reduced at 700°C for 1 h in the stream of H2:N2=2:1(total flow rate 30 ml·min?1),and then the catalyst bed was heated to the reaction temperature(800 °C,heating rate 10 °C·min?1).

Theeffluentgas was analyzedbygas chromatography(GC2060system)equipped with a thermal conductivity detector(TCD)and hydrogen flame detector(FID).The produced gases were separated by a TDX column(CO2,CH4)and 5A molecular sieves(H2,N2,CO),and all gases could be detected by a thermal conductivity detector(TCD)and hydrogen flame ionization detector(FID).CH4and CO2conversion and the molar ratio of H2/CO can be calculated as follows:

2.3.Characterization methods

The crystalline phases of the catalysts were identified by X-ray diffraction(XRD),using a D/MAX-3C X-ray powder diffractometer with Cu Kαradiation(λ =0.154056 nm)at a scanning rate of 8(°)·min?1from 10°to 80°under atmospheric pressure.The specific surface areas and the pore size distributions of the samples were calculated by Brunauer-Emmett-Teller(BET)method in a Micrometrics ASAP2020 instrument.Scanning electron microscopy(SEM)(ZEISS EVO MA10)analysis was used to survey the surface morphology of fresh and spent catalysts for ammonia synthesis.Thereducibility of catalysts was investigated by H2temperature programmed reduction(H2-TPR).The amount of carbon deposition on each spent catalyst was measured by TGA(NETZSCHSTA 449 F3).

3.Results and Discussion

3.1.Catalytic activity of La2?xCexCoO4±yin DRM

From Fig.1 we can see that catalytic activity of LaCeCoO4was the highest during 12 h reaction time and that La1.8Ce0.2CoO4maintained a high activity and stability during DRM reaction.

Fig.1.Catalytic performance of La2?xCexCoO4±ycatalysts with a 12 h reaction time on the stream.Reaction conditions:0.3 g catalyst,1073 K,P=0.1 MPa,WHSV=16000 ml·h?1·(g cat)?1,CH4:CO2=1:1.

AsshowninTable1,comparedwiththeinitialandfinalCH4andCO2conversion,a loss of 34.7%,22.2%,10.3%and 14.8%of activity for CH4conversion of La2CoO4,La1.8Ce0.2CoO4,La1.4Ce0.6CoO4and LaCeCoO4was observed,respectively.The catalytic activity of other differentcatalysts,such as La1.6Ce0.4CoO4and La1.2Ce0.8CoO4,was closed with La1.4Ce0.6CoO4.Owing to the Ce addition,the catalytic activity of samples for CH4and CO2conversions improved largely compared to La2CoO4.This was because CeO2had a strong capacity to store and transportoxygensothatthecokecouldbereducedbylatticeoxygenaccording to the mechanism[13-15]:

Table 1 Conversion comparison of catalysts inDRM and crystalline size,as well as mass loss registered in the TGA

The oxygen vacancies produced by CeO2?npromoted the adsorption of CO2and facilitated its reaction with CHx(x=1,2,3,4),and these vacancies were replenished by the dissociation of CO2:

The catalytic performance of LaCeCoO4was the highest among the catalysts,given that the LaCoO3particle size decreased(proved by Table 1 LaCoO3crystalsizes)and the specific surface areawasincreased through doping Ce into the voids of La and CoO,which was in good agreement with BET data(Table 2).The catalysts was not stable during the12 hDRM reactionandtheconversiondecreasedtoa certainextent,which might be because the catalyst did not construct a stable skeleton structure due to doping of Ce[16].

Table 2 The textural parameters of fresh catalysts

3.2.Characterization of catalysts

Fig.2 shows the XRD patterns of La2?xCexCoO4±y(x=0,0.2,0.4,0.6,0.8,1)catalystsaftercalcinationwiththeDRMreaction.Thecharacteristic peaks of the perovskite oxide LaCoO3were observed for all catalysts in the XRD image,and the characteristic peaks were 2θ=32.9°,47.5°,and 59.2°PDF480123(111),respectively.The formation of perovskite oxide(LaCoO3)demonstrates that the Pechini method successfully combined the two different metals into LaCoO3,resulting in La in sufficient contact with Co.We can found that characteristic peaks of La2CoO4was weaker with the Ce increasing at 2θ=30.2°,indicating that Ce addition weaken crystal structure of La2CoO4and improved the interaction between La and Co with Ce doped.The characteristic peaks of CeO2could be seen in the XRD of catalyst La2?xCexCoO4±y(x=0,0.2,0.4,0.6 0.8,1)at 2θ =28.6°,47.5°,and 56.3°PDF340394(111 cubic phase),which was strongly enhanced with Ce addition.Moreover,characteristic peaks of LaCoO3in LaCeCoO4and La1.8Ce0.2CoO4were weaker thanthoseinLa2CoO4,indicatingthatCeadditionimprovesthedispersion of LaCoO3.The characteristic peaks of CoO in La1.8Ce0.2CoO4with Ce addition were sharpened and enhanced and the particle size also increased,indicating that CoO was rallied and sintered to some extent during the synthesis and calcination process.

Both fresh and spent catalysts of La2?xCexCoO4±y,were detected by XRD,andtheresultswereshowninFig.3.Comparedwithfreshandspent La2CoO4catalysts,wecanwitnessthatcharacteristicpeaksofCoappeared at 2θ=44.7°,47.6°,75.9°,and these peaks were enhanced remarkably after a 12-h reaction.This was probably because when LaCoO3was reduced to Co,it tended to sinter and agglomerate at high temperature,resultinginthepoordispersionofCo.Moreover,noobviouscharacteristic peak of Co was observed in spent La1.8Ce0.2CoO4and LaCeCoO4,which was due to the fact that the doped-metal such as Ce has evident antisintering ability[12,14,15],leading to the well-dispersion of active metals and increase of active sites.

According to the data in Table 2,the specific surface areas of La1.8Ce0.2CoO4,La1.4Ce0.6CoO4,LaCeCoO4,and La2CoO4were 8.55,8.34,8.21 and 0.25 m2·g?1,respectively.The average pore size follows the order:La2CoO4＞LaCeCoO4=La1.8Ce0.2CoO4=La1.8Ce0.2CoO4.It is generally accepted that the surface structure of the catalysts has been greatly changed with Ce addition and adding a small amount of Ce to La2CoO4caused the surface area to improve from 0.25 m2·g?1to 8.55 m2·g?1,while the average pore size decreased due to Ce doping into the space of La3+and CoO,indicating that much more mesoporous regionsexistedinthecatalystswithCeaddition.Highersurfaceareaand lower pore diameter caused active sites to be easily exposed to the outside and crystal particle sizetobe reduced(LaCoO3particle sizedata recorded in Table 1)[17].Furthermore,the smaller LaCoO3particle size gave rise to well-dispersed catalysts with increasing surface area.By comparison with the IUPAC desorption curve in Fig.4,N2physical adsorption isotherms of La2CoO4,La1.8Ce0.2CoO4,La1.4Ce0.6CoO4,and LaCeCoO4belong to II-type(i.e.,weak interaction),which indicated the existence of a considerable number of mesoporous regions in the catalysts[18].With increased Ce addition,the specific surface area did change significantly,indicating that a small amount of Ce can greatly improve the catalyst's surface.

To obtain information of the behavior of La2?xCexCoO4±yunder a reductive atmosphere,TPR studies were presented in Fig.5.The different hydrogen consumption peaks can be observed in Fig.5,indicating the presence of Co species with different particle sizes and the formation of oxygen intermediates,which was in agreement with XRD data.TheoverallreductionfromCo3+toCo0representedbypeaks1and2occurred in three steps in equation[19]:

Peak 1 perovskite reduction

Fig.3.XRD patterns of La2?xCexCoO4±y(1)fresh and spent La2CoO4,(2)fresh and spent La1.8Co0.2O4,(3)fresh and spent LaCeCoO4.

Fig.4.N2adsorption-desorption isotherms of fresh catalysts.

Fig.5.Temperature-programmed reduction(TPR)profiles of La2?xCexCoO4±ycatalysts.

It was observed that the similarity of the reduction profiles suggests that Ce did not have a strong effect on the process since the reduction depends mainly on the reduction of Co.However,reduction peak 1 of LaCoO3was postponed to some extent and divided into two reduction peaks with increasing Ce addition since greater amounts of La3+in LaCoO3were partially replaced by Ce.

As shown in Table 1 the crystal sizes for the LaCoO3phase were calculated based on the Scherrer equation and followed the order:LaCeCoO4＜La1.4Ce0.6CoO4＜La1.8Ce0.2CoO4＜La2CoO4,indicating that nano-sized particles of LaCoO3were more highly dispersed with increasing Ce addition and thus improved catalytic performance.The amount of carbon deposition on spent catalysts was detected and listed in Table 1.La1.8Ce0.2CoO4exhibited the lowestamount of carbon formation,which was largely due to its stability towards the DRM reaction,which was consistent with initial and final conversion data.It was evident that the carbon deposition on La1.8Ce0.2CoO4was less than 6.3%,while La2CoO4and LaCeCoO4formed higher carbon depositions of 16.5%and 11.9%,respectively,indicating that a small Ce addition to La2CoO4could resist carbon formation compared with other catalysts.In Table 1,all the catalyst samples with different Ce addition showed less carbon deposition amount and more favorable anti-coking ability than La2CoO4(without Ce addition).Since the coke on the catalyst surface cannot be removed in time(Eq.(9))at high CH4conversions,the overall carbon deposition tended to ascend with increasing Ce amount accordingly.

From Fig.6 we could see that surface morphology of perovskite-like oxide La2?xCexCoO4±yhas changed to some extent due to doping Ce after calcination.Compared with La2CoO4(a),the particle size of La1.8Ce0.2CoO4(b)and LaCeCoO4(c)became more uniform and the sheet structure of perovskite-like oxide(A2BO4)decreased relative to thepartialreplacementofLabyCe,resultinginanirregularmesoporous structure and high catalytic activity.After Ce addition,many irregular pore structures of LaCeCoO4(Fig.6(c))can be clearly seen compared with La2CoO4(Fig.6(a)).From SEM images of Fig.6(e),(f),La1.8Ce0.2CoO4(e)andLaCeCoO4(f)stillretainedacertainporestructure,which shows that the catalysts'structures were not destroyed during the 12 h reaction.Moreover,coke deposition was not observed distinctly in the SEM(d,e,f)of the series of spent catalysts,indicating that the carbon deposited may have reacted with lattice oxygen provided by CeO2,CeO2→CeO2?n+(n/2)O2,nC+(n/2)O2→nCO[14,15].Furthermore,Ce doped into La2CoO4could improve the dispersion of active the component on the catalysts and enhance anti-sintering and anti-coking disposition,resulting in good catalytic performance during the reaction process[20,21].

3.3.Effect of reaction temperature on catalytic performance

Fig.7 shows the change of CH4&CO2conversion rates as reaction temperature extends.Meanwhile,both CH4and CO2conversion rates ofLa2?xCexCoO4±yalso obeyed the similarorder(La2CoO4＜La1.8Ce0.2CoO4＜LaCeCoO4)with increasing of Ce content.

Based on the thermodynamic data,DRM was a reversible and endothermic reaction.In Fig.7,both conversions witnessed a sharp increase from 873 K,approaching 100%at 1173 K.Nevertheless,even at 973 K the catalytic activity of La2CoO4,La1.8Ce0.2CoO4and LaCeCoO4still presented large difference of 3.3%,39.3%and 50.1%,respectively,indicating that Ce addition may decrease reaction temperature and improve the catalytic performance significantly.This was largely because the rare earth metals could absorb heat and stimulate themselves to donate electrons for active metals under high temperature.Moreover,it can be inferred that Ce addition could effectively reduce the activation energy,whichwasinconsistentwithexperimentalresultsthattheactivity of catalysts with Ce addition(La1.8Ce0.2CoO4,LaCeCoO4)exceeded those without Ce addition(La2CoO4)apparently.Furthermore,the CH4conversion rate was still higher than that of CO2at various temperatures,suggesting that the La2?xCexCoO4±yseries of catalysts have stronger CH4decomposition ability and weaker CO2adsorption ability[22].

Fig.6.SEM images of fresh catalysts(La2CoO4(a),La1.8Ce0.2CoO4(b),LaCeCoO4(c))and spent catalysts after 12 h reaction at 800°C(d,e,f),scanning voltage 10 kV.

Fig.7.The relationship between temperature and conversion after 12 h.Reaction conditions:0.3 g catalyst,1073 K,0.1 MPa,16000 ml·h?1·(g cat)?1,CH4:CO2=1:1.

Moreover,asshowninFig.8,bothH2andCOyieldsclimbedgradually with increasing temperature.It can be found from the figure that the yields of H2and CO increased apparently before 800°C,and both yields tended to be stable afterwards.This could be attributed to the fact that DRM is endothermic reaction which thermodynamically favor at high temperature.Meanwhile,the H2yield was lower than that of CO all the time,and H2/CO ratio inclined to reach the value of 1:1 after 800°C,which is favorable to alcohol production by carbonyl synthesis[23].

3.4.Effect of WSHVs on catalytic performance

Fig.8.Effect of temperature on the yield of H2and CO.Reaction condition:0.3 g catalyst,1073 K,0.1 MPa,16000 ml·h?1·(g cat)?1,CH4:CO2=1:1.

Fig.9.Effects of CH4&CO2conversion on WHSV(CO2:CH4:N2=1:1:0),0.3 g catalyst,1073 K,0.1 MPa.

From the Fig.9 we can see that the CH4&CO2conversions increased a little with increasing of WHSVs between 6000 and 8000 ml·h?1·(g cat)?1,which wasdue to the enhanced temperature distribution of catalyst bed as WHSVs extended within 8000 ml·h?1·(g cat)?1.Nevertheless,a sharp decrease of CH4&CO2conversions was observed when WHSVs exceeded 10000 ml·h?1·(g cat)?1,which was mainly due to the short residence time and insuffi cientcontactofrawgases.BothcatalyticperformancesofLa1.8Ce0.2CoO4and LaCeCoO4overwhelmed La2CoO4at all WHSVs,and CH4&CO2conversions remained above 80%,compared with less than 60%conversion of La2CoO4within WHSVs 10000 ml·h?1·(g cat)?1.This may be explained as La1.8Ce0.2CoO4and LaCeCoO4have more active sites and well-dispersed active metals(proved by XRD,Fig.2),resulting in the increaseofcontactsitesandreactionactivityduringDRM.Duetothelackof enough active sites and contact time inside the reaction bed,La2CoO4exhibited the poorest catalytic activity at high WHSV(about 40%).The experimental results of LaCeCoO4at different WHSVs further proved its dominant catalytic performance.As we know,the carbon deposition is easy to occur at high conversion rates,which is largely because the deepcrackingofmethaneappearedatlowWHSV(highconversion),producing a large amount of carbon deposition and is not beneficial to the longtime catalytic reaction.Hence the optimal WHSV for LaCeCoO4would be determined based on the actual requirements.

4.Conclusions

The perovskite-like oxide(La2?xCexCoO4±y)series of catalysts were prepared by Pechini sol-gel method,and the catalytic performance and stability of catalysts with Ce addition for the DRM reaction were investigated.LaCeCoO4showed the top reaction performance among all the catalysts,which was due to the descent of LaCoO3particles and ascent of oxygen storage capacity.Furthermore,catalyst specific surface area increased greatly from 0.2 m2·g?1to 8.5 m2·g?1due to doping with Ce,resulting in a well-dispersed active component.As a result of doped CeO2into the void between La2O3and CoO,the LaCoO3crystal particle size diminished and active sites increased in order to improve catalytic performance.