Combustion synthesis of CuO-ZnO-ZrO2 catalysts for the hydrogenation of carbon dioxide to methanol

Combustion synthesis of CuO–ZnO–ZrO 2catalysts for the hydrogenation of carbon dioxide to methanol

Xiaoming Guo a,b ,Dongsen Mao a,*,Song Wang a,b ,Guisheng Wu a ,Guanzhong Lu a,b,*

a Research Institute of Applied Catalysis,Department of Chemical Engineering,Shanghai Institute of Technology,Shanghai 200235,China b

Research Institute of Industrial Catalysis,East China University of Science and Technology,Shanghai 200237,China

a r t i c l e i n f o Article history:

Received 10February 2009

Received in revised form 4May 2009Accepted 6May 2009

Available online 12May 2009Keywords:

CuO–ZnO–ZrO 2catalyst Combustion synthesis Urea/nitrate ratio CO 2hydrogenation Methanol

a b s t r a c t

Various CuO–ZnO–ZrO 2(CZZ)catalysts have been synthesized by urea–nitrate combustion method and their performances for methanol synthesis from CO 2hydrogenation have been investigated.The effects of urea/nitrate ratio on physicochemical and catalytic properties were studied by XRD,BET,H 2-TPR and reactive N 2O adsorption techniques.The results show that when 50%of stoichiometric amount of urea was used,the CZZ catalyst exhibited an optimum catalytic behavior.The combustion method was found to be a simple,fast and valuable route for the preparation of CZZ catalyst for methanol synthesis from CO 2hydrogenation.

ó2009Elsevier B.V.All rights reserved.

1.Introduction

It is well known that development of technologies for the ?xa-tion and disposal of CO 2,one of the main greenhouse gases,is of great signi?cance and urgency at present.Recently,synthesis of valuable chemicals by CO 2hydrogenation has received much attention as a technology to solve the CO 2problem.Among them,methanol synthesis holds a central position since methanol can be used as a starting feedstock for the chemical industries and as an alternative to fossil fuels [1].

Although Cu/ZnO/Al 2O 3catalysts have a considerably high activity for methanol synthesis from syngas (CO +H 2),they exhibit a poor activity for the hydrogenation of CO 2[2,3].Therefore,great efforts have been made to ?nd an ideal catalyst for CO 2hydrogena-tion,in which CuO–ZnO–ZrO 2(CZZ)catalysts were reported to have higher activity [1,3–5].In order to further improve the perfor-mance of CZZ catalysts,promoters such as Mn [1,6],Mg [6]and Ga [7]have been reported recently.Besides catalyst compositions,preparation methods have also a considerable in?uence on the cat-alytic performance [4,7].Conventional methods such as co-precip-itation,impregnation and decomposition of citrate complexes were employed to prepare the CZZ catalysts [1–7].Very recently,combustion synthesis,based on the principles of the propellant chemistry,has been used to prepare metal oxide powders exten-sively for its advantages of applying inexpensive raw materials,

maintaining a relatively simple and fast preparation process,and achieving ?ne powders with high homogeneity [8,9].In the syn-thesis process,a thermally induced redox reaction takes place be-tween an oxidant and a fuel.Many types of combustion synthesis exist which differ mainly in the physical state of the reactants or in the combustion modality [10].Urea–nitrate combustion synthesis,in which the fuel is urea and metal nitrates are oxidant,is a very popular method.Despite the existence of some reports concerning catalysts produced with urea–nitrate combustion synthesis [11,12],no articles regarding the synthesis of CZZ catalysts using this method for the synthesis of methanol from CO 2hydrogenation were found in the literature,to our knowledge.

The aim of this paper is to present a urea–nitrate combustion method for the preparation of CZZ catalysts for methanol synthesis from CO 2hydrogenation.Since fuel/oxidant ratio is an important parameter in combustion synthesis,this study emphasizes the ef-fects of urea/nitrate ratios on the properties of the derived CZZ cat-alysts.The prepared CZZ catalysts were characterized by BET,XRD,H 2-TPR and reactive N 2O adsorption techniques and their catalytic properties were examined.Furthermore,the catalytic perfor-mances of the CZZ catalysts were discussed in relation to the re-sults of physicochemical characterization.2.Experimental 2.1.Catalyst preparation

Metal nitrates were dissolved in deionized water to form a solu-tion in which Cu 2+,Zn 2+and Zr 4+concentrations meet the formula

1566-7367/$-see front matter ó2009Elsevier B.V.All rights reserved.doi:10.1016/j.catcom.2009.05.004

*Corresponding authors.Tel./fax:+862164941386(D.Mao).

E-mail addresses:dsmao1106@https://www.360docs.net/doc/5b3754717.html, (D.Mao),gzhlu@https://www.360docs.net/doc/5b3754717.html, (G.Lu).

Catalysis Communications 10(2009)

1661–1664

Contents lists available at ScienceDirect

Catalysis Communications

j ou r na l h om e pa ge :w w w.e lse v ie r.c om /lo c at e /c at c o

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of(CuO)0.5(ZnO)0.2(ZrO2)0.3.The urea solution was slowly added to the metal nitrate solution under constant stirring.The resulting mixture was kept in an ultrasound bath operating at47kHz with a power of30W for0.5h and white slurry was obtained.Then the basin was transferred to an open muf?e furnace preheated at 673K.The slurry started boiling with frothing and foaming,and in a couple of minutes ignited spontaneously with rapid evolution of a large quantity of gases,yielding a foamy,voluminous powder. Because the time for the autoignition is rather short,to remove traces of undecomposed urea,nitrates,and their decomposition products,the powder was further heated at673K for3h.Urea addition was set from150%to40%of the stoichiometric amount which is2.17moles for1mol CZZ catalyst according to propellant chemistry[13].The resulted CZZ powders were termed as150-CZZ, 125-CZZ,100-CZZ,75-CZZ,50-CZZ and40-CZZ,respectively.In addition,in order to make a comparison,two CZZ catalysts of the same composition were prepared by the conventional oxalate and carbonate co-precipitation methods described previously[4,7].

2.2.Catalyst characterization

XRD patterns were recorded on a Bruker D8Focus instrument

by using Ni?ltered Cu K a radiation.The crystallite size was calcu-lated from the XRD spectra by using the Scherrer equation.Full nitrogen adsorption/desorption isotherms at77K were obtained after outgassing of the sample under vacuum at473K for3h,using a Micromeritics ASAP2020M+C adsorption apparatus.The active surface area of Cu(S Cu)was determined by the technique of N2O reactive frontal chromatography at333K assuming a Cu:N2O=2 titration stoichiometry and a surface atomic density of 1.46?1019copper atoms per m2,respectively[5].H2-TPR mea-surement was carried out by using10%H2/N2as a reducing gas in a quartz microreactor with30mg catalyst.The?ow rate of the reducing gas was kept at30ml/min and the temperature was raised from373to673K at a rate of5K/min.The amount of con-sumed H2was measured by a TCD.

2.3.Catalytic activity test

Activity measurements were carried out in a continuous-?ow,?xed-bed reactor.A0.5g catalyst diluted with0.5g quartz sand (both in20–40mesh)was packed into the stainless steel tubular reactor(5mm i.d.).Prior to the catalytic measurements,the cata-lyst was reduced in a stream of10%H2/N2(30ml/min)at573K for3h under atmospheric pressure.Then the reactor was cooled to433K and the reactant gas(CO2:H2=1:3,molar ratio)?ow of 15ml/min was introduced,raising the pressure to3.0MPa and the temperature to a given temperature.Ef?uent products were analyzed on-line with a gas chromatograph(Agilent6820).Con-version and selectivity values were calculated by mass-balance methods and the steady-state values are quoted as the average of four different analyses taken after4h on stream operation.

3.Results and discussion

3.1.Characterization of catalysts

The XRD patterns of the different CZZ catalysts are presented in Fig.1.The peaks at2h of35.6°,38.8°and48.9°were attributed to CuO phase(JCPDS80-1268),while those at2h of34.5°and36.3°could be assigned to ZnO phase(JCPDS36-1451).As shown in Fig.1,with the decrease of urea amount from150%to50%of stoi-chiometry,the intensity of the diffraction peaks of CuO and ZnO weakened gradually,while the line width broadened slightly. However,the comparison of XRD peaks between40-CZZ and50-CZZ samples reveals that further decrease in urea amount resulted in the reappearance of stronger and sharper peaks.These results illustrate that the crystallization degree of the CZZ samples reached a minimum when the amount of urea was50%of stoichi-ometry.The changes of diffraction peaks could be explained in terms of reaction temperatures during the combustion process. As the amount of urea decreases,combustion reaction enthalpy de-creases.Small reaction enthalpy means low reaction temperature which inclines to form ill-de?ned crystalline phases with weak and broad XRD peaks.On the other hand,the gases evolved during the combustion process may dissipate some of heat and lower the ?ame temperature.The amount of gases decreases as the urea amount decreases and results in a higher reaction temperature. There could be competition between the effect of the reaction en-thalpy and that of the amount of the evolved gases[10].The results indicate that the former plays a more dominant role as the amount of urea changes from150%to50%of stoichiometry and the later becomes more important as the urea amount is less than50%of stoichiometry.

It is interesting that a signi?cantly different variation behavior of the structural evolution as a function of the amount of urea has been observed in the zirconia phase.For40-CZZ and50-CZZ, a weak and broad XRD peak corresponding to tetragonal zirconia (t-ZrO2,2h=30.3°,JCPDS88-1007)appeared and no peaks of monoclinic ZrO2(m-ZrO2,2h=24.1°,28.2°,31.5°,JCPDS83-0940) can be discerned.This was attributed to low urea content in the combustion reaction resulting in low reaction temperature.Under such a low reaction temperature,ZrO2existed in an amorphous or semicrystalline state[1,14].As the urea amount reached75%of stoichiometry the intensity of t-ZrO2peak increased drastically. When the amount of urea increased further,the intensity of t-ZrO2peak decreased whereas the intensity of m-ZrO2peak in-creased.The phenomena demonstrate that partial transformation from tetragonal to monoclinic phase occurred with the rise of reac-tion temperature[15].

The mean crystallite sizes of CuO are listed in Table1.With a decrease in urea content,the crystallite size decreased from 35.9nm of150-CZZ to14.7nm of50-CZZ.But for40-CZZ,the crys-tallite size is larger than that of50-CZZ because of the higher reac-tion temperature during combustion,which is helpful for the crystal growth.

The BET surface areas are listed in Table1as well.A slight in-crease of the BET surface area can be observed from150-CZZ to 75-CZZ.Then,a marked increase appears and a maximum surface

1662X.Guo et al./Catalysis Communications10(2009)1661–1664

area of54.8m2/g is obtained for50-CZZ.The change trends of sur-face area are consistent with the results of XRD measurements. Moreover,as shown in Table1,the active surface area of Cu in-creases with the decrease of urea amount and reaches a maximum for50-CZZ.

As shown in Fig.2,there are two reduction peaks in the TPR pro-?les of the catalysts.One occurs at about459K and the other ap-pears in the range of485–502K.They are ascribed to the reduction of the highly dispersed CuO species and the bulk-like CuO phases[16,17],which are denoted as a and b peak,respec-tively.These reduction peaks change with the variation of urea amount.For150-CZZ and125-CZZ samples,a peak is too weak to be discerned.A remarkable increase of the intensity of a peak can be observed for100-CZZ and75-CZZ at the expense of the b peak.Only a peak occurred in50-CZZ and40-CZZ samples.This suggests the fractions of the highly dispersed CuO species,which are considered to contribute mainly to catalytic activity[16],in-crease as the decrease of urea amount used in the preparation pro-cess.Meanwhile,it can be seen that the b peak shifted from502K of sample150-CZZ to485K of sample75-CZZ.The shift of b peak indicated that the particle size became smaller from150-CZZ to 75-CZZ because the easier to be reduced for CuO particles,the smaller they are[17].The inference is in good agreement with the results of XRD measurements.

3.2.Catalytic activity and selectivity

The performance of the resulting catalysts for methanol synthe-sis from CO2hydrogenation is shown in Table1.CO and methanol are the only carbon-containing products under the reaction condi-tions and traces of methane can be detected when the temperature is higher than513K.It can be seen that the CO2conversion in-creased with the decrease of urea amount and reached a maximum for sample50-CZZ.The trend is similar to that of crystallite size of CuO species and active surface area of Cu of the catalysts.This is because smaller crystallite size of CuO species and higher active surface area of Cu lead to more active catalysts[16,18].At the same time,the reduction temperatures of CuO showed in the TPR pro-?les provide a strong evidence for the sequence of activity.On the other hand,it is noteworthy that the CH3OH selectivity in-creased monotonically with the increase of urea amount although the enhancement was not signi?cant.It can be ascribed to partial transformation from t-ZrO2to m-ZrO2occurred as the urea amount increased during catalyst preparation which demonstrated by the XRD spectra.The methanol selectivity over m-ZrO2-supported cat-alysts is higher than that over t-ZrO2-supported catalysts.Similar results for CO hydrogenation had been reported by Rhodes and Bell [19].As seen in Table1,50-CZZ catalyst exhibits the highest yield of methanol among all the catalysts investigated here.

The catalytic performances of CZZ catalysts with the same com-position prepared by carbonate and oxalate co-precipitation meth-ods as well as combustion method are shown in Fig.3,for the purpose of comparison.It is obvious that the catalysts prepared by combustion method are more active for methanol synthesis from CO2hydrogenation.On the other hand,the temperature at which the maximum methanol yield obtained over the catalyst prepared by combustion method was evidently lower than those over the catalysts prepared by carbonate and oxalate co-precipita-tion methods,indicating that the former has better low tempera-ture activity.The high activity described above of the CZZ catalyst prepared by combustion method may be due to the high temperature treatment during combustion synthesis process, which persists for a few minutes and renders rapid quenching ef-fect[20]resulting in a more favorable interaction between copper species and ZnO,ZrO2.

Table1

Physicochemical and catalytic properties of the CuO–ZnO–ZrO2catalysts.

Catalysts Moles of urea/1mol CZZ S BET(m2/g)S Cu(m2/g)CuO crystallite size(nm)CO2conversion(%)CH3OH selectivity(%)CH3OH yield(%)

40-CZZ0.8747.79.016.815.855.48.8

50-CZZ 1.0854.810.614.717.056.29.6

75-CZZ 1.6217.2 4.320.614.956.88.5

100-CZZ 2.1710.5 2.725.012.357.17.0

125-CZZ 2.71 5.4 1.926.69.259.1 5.4

150-CZZ 3.26 4.6 1.635.97.859.7 4.7

Reaction conditions:T=513K,CO2:H2=1:3,P=3.0MPa,GHSV=3600hà1.

X.Guo et al./Catalysis Communications10(2009)1661–16641663

4.Conclusions

A series of CZZ catalysts were prepared via the urea–nitrate combustion method with urea ranging from150%to40%of stoichi-ometric amount and tested for the hydrogenation of CO2to meth-anol.The fuel-de?cient samples were found to have favorable characteristics such as small grain size,large surface area and low reduction temperature.The optimal CZZ catalyst(50-CZZ) exhibits excellent activity and remarkable selectivity for the syn-thesis of methanol from CO2hydrogenation.The urea–nitrate com-bustion method,therefore,appears to be a simple,fast and effective method for the preparation of CZZ catalysts. Acknowledgement

The authors thank Shanghai Educational Development Founda-tion(06SG35)for?nancial support.

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