Rare Metals2016年第10期

Morphology and activity relationships of macroporous CuO-ZnO-ZrO₂ catalysts for methanol synthesis from CO₂ hydrogenation

Yu-Hao Wang Wen-Gui Gao Hua Wang Yan-E Zheng Kong-Zhai Li Ru-Gui Ma

State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization Engineering,Faculty of Metallurgy and Energy Engineering,Kunming University of Science and Technology

收稿日期：4 March 2014

基金：financially supported by the National Key Technologies Research & Development Program of China(No.2011BAC01B03);the National Natural Science Foundation of China(No.51304099);the Applied Basic Research Program of Yunnan Province(No.2013FZ035);the Testing and Analyzing Foundation of Kunming University of Science and Technology(No. 2010213);

Morphology and activity relationships of macroporous CuO-ZnO-ZrO₂ catalysts for methanol synthesis from CO₂ hydrogenation

Yu-Hao Wang Wen-Gui Gao Hua Wang Yan-E Zheng Kong-Zhai Li Ru-Gui Ma

State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization Engineering,Faculty of Metallurgy and Energy Engineering,Kunming University of Science and Technology

Abstract：

A series of macroporous CuO-ZnO-Zr0₂(CZZ) catalysts with different Zn/Zr ratios were successfully prepared by template method and characterized by X-ray diffraction(XRD),N₂ adsorption,reactive N₂O adsorption,scanning electron microscopy(SEM),H₂ temperature-programmed reduction(H₂-TPR),and transmission electron microscopy(TEM).The activity of the catalysts was tested for methanol synthesis from CO₂ hydrogenation.It is found that the increase in the Zn/Zr ratio could lead to the sintering of the catalysts,destroying the macroporous structure integrity.The macroporous CZZ catalysts own lower Zn/Zr ratio,exhibiting a better morphology and activity.For comparison,the conventional nonporous CZZ catalysts were also investigated.The results show that the CZZ catalysts with macroporous structure own smaller particles,higher CO₂ conversion,and CH₃OH yield.It reveals that the macroporous structure could inhibit the growth of the particle size,and the special porous structure is favorable for diffusion and penetration of CO₂,which could improve the catalytic activities.

Keyword：

Macroporous structure; CuO-ZnO-ZrO2 catalysts; CO2 hydrogenation; Methanol; Activity;

Author： Wen-Gui Gao,e-mail:gao_wengui@126.com;

Received： 4 March 2014

1 Introduction

It is well known that methanol synthesis from CO₂ hydrogenation attracts much attention as a promising method to mitigate the global warming effect [ 1] .Catalysts based on Cu,especially Cu-ZnO-ZrO₂ (CZZ),are widely used for the synthesis of methanol from CO₂ hydrogenation and investigated extensively [ 2, 3, 4, 5, 6] .It is reported that the CZZ catalysts exhibit high activity for CO₂ hydrogenation.A series of methods such as co-precipitation [ 4, 7, 8, 9] ,impregnation [ 10] ,and sol-gel [ 11, 12] were investigated extensively.However,the catalysts are easy to sinter prepared by this method,and the Cu particles grow rapidly,which will reduce the catalytic activity.

Apart from catalyst compositions and prepared methods,the morphology of the catalysts also has a considerable influence on the catalytic activity.The special structure of the catalysts including nanotubes [ 13] ,nanorod [ 14] ,mesoporous [ 15] ,and macroporous structures [ 16] was developed.All the above catalysts own high specific surface area,which may improve their c atalytic performances.In the last dec ade,much attention has been attracted to macroporous materials with uniform pore size and well-defined periodic structure in catalysis,because the big pore size (>50 nm) can permit reactant to enter the inner pores of materials and to easily transport and diffuse [ 17] .However,macroporous Cu-ZnO-ZrO₂ catalysts were rarely reported.To better understand the relationships of morphology and activity of CuO-ZnO-ZrO₂catalysts for methanol synthesis from CO₂ hydrogenation,it is necessary to investigate the macroporous Cu-ZnO-ZrO₂catalysts.

In this paper,a series of macroporous Cu-ZnO-ZrO₂catalysts with different Zn/Zr ratios were prepared via a template method and were used as catalysts in the methanol synthesis from CO₂ hydrogenation.The influences of the Zn/Zr ratio and the macroporous structure on the catalytic activity were both investigated.Much attention was devoted to discussing the influence of macroporous structure,and thus the nonporous catalyst with the same Zn/Zr ratio was also prepared by traditional co-precipitation method.

2 Experimental

2.1 Catalysts preparation

The macroporous Cu-ZnO-ZrO₂ catalysts were prepared by a template method.Firstly,polymethyl methacrylate (PMMA)spheres used as the template were synthesized by emulsifierfree emulsion polymerization [ 18] .After that,analyticalgrade Cu(NO₃)₂.3H₂O,Zn(NO₃)₂·6H₂O and Zr(NO₃)₄·5H₂O were dissolved in deionized water to form a transparent solution.And then,citric acid was added to the precursor and was dissolved at 60℃for 1 h under stirring.Secondly,the dried PMMA templates were soaked in the precursor solutions for some hours.The precursors were dried at 60℃for 24 h.Finally,the precursors was calcined at 350℃for 2 h with a ramp rate of 2℃·min^-1 and then heated at 2℃.min up to450℃with a dwell time of 2 h.The obtained samples prepared by template method were (CuO)_0.5-(ZnO)_0.1-(ZrO₂)_0.4,(CuO)_0.5-(ZnO)_0.2(ZrO₂)_0.3,(CuO)_0.5-(ZnO)_0.3-(ZrO₂)_0.2,and (CuO)_0.5-(ZnO)_0.4-(ZrO₂)_0.1;the content of Cu was constant,but the relative amounts of ZrO₂ were 80 mol%,60 mol%,40 mol%,and 20 mol%,respectively,labeled as CZZ-80-M,CZZ-60-M,CZZ-40-M,and CZZ-20-M.

The nonporous catalyst was synthesized by co-precipitation method [ 19] .The precursor solutions produced during the process of preparation of macroporous catalysts were heated in a muffle furnace under the same calcination conditions as the synthesis of macroporous samples.The obtained samples (CuO)0.5(ZnO)0.1(ZrO₂)0.4 were labeled as CZZ-80-N.

2.2 Catalysts characterization

The morphology of the catalyst was observed by scanning electron microscopy (SEM) on a Hitachi S4800 instruments using accelerating voltages of 3 kV.The powders X-ray diffraction (XRD) was performed on a Japan Science D/maxR diffractometer using Cu Kαradiation (λ=0.15406 nm).The X-ray tube was operated at 40 kV and 45 mA.The XRD patterns were recorded with 2θrange of 10°-90°at a scanning rate of 10 (°)·min^-1.Cu contents of the calcined catalysts were determined by atomic absorption spectroscopy(AAS) on acid-digested samples,using a SpectrAA-220FS/Z atomic absorption spectrometer (VARIAN).

Specific surface area of oxygen carrier was determined by N₂ adsorption isotherm at-196℃calculated according to the Brunauer-Emmett-Teller (BET) method,using a Quantachrome ASIQ-C instrument.The metallic copper surface area (S_Cu) was measured by a nitrous oxide decomposition method followed by H₂ temperature-programmed reduction (H₂-TPR) [ 20] .The catalysts (10 mg)were first reduced with 10 vol%H₂+90 vol%Ar at280℃for 1 h followed by purging with He for 30 min and cooling to 60℃.After that,a flow of 10 vol%N₂O of(N₂O+He) gas mixture was fed into the reactor for 1 h.Then,temperature-programmed reduction (TPR) was performed under a 10 vol%H₂+90 vol%Ar flow to 300℃with a ramp rate of 10℃min^-1.A surface copper density of 1.47×10¹⁹ atoms·m^-2 and the adsorption stoichiometry mole ratio of Cu:N₂O=2:1 were taken into consideration for the copper surface area calculations.TPR was performed on a ChemBET Pulsar&TPR/TPD apparatus (Quantachrome Instruments) with a thermal conductivity detector (TCD).The amount of catalyst used was50 mg at a heating rate 10℃·min^-1 in all cases.After a standard cleaning pretreatment,TPR was carried out in a flow of H₂/Ar (20 ml·min^-1) up to 550℃.

The microstructure was carried on a JEOL JEM-2100(UHR) transmission electron microscopy (TEM) at200 keV.The specimens were crushed into powder and immersed in a small volume of ethanol.After sonicating the mixture for 10 min,a droplet of the suspension was allowed to dry on a holey carbon-/Formvar-coated copper TEM grid.

2.3 Activity test

The activity and selectivity measurements for CO₂ hydrogenation to methanol were tested in a high-pressure fixed bed flow stainless steel reactor.1 g catalyst diluted with quartz sand (both in 420-841μm) was packed into the stainless steel tubular reactor.Prior to the catalytic measurements,the catalyst was reduced in a stream of 10 vol%H₂+90 vol%N₂ at 280℃for 3 h under atmospheric pressure.Subsequently,the temperature in the catalyst bed was cooled down to 50℃under the flow of diluted H₂,and then the reductive gas was replaced by the reaction gas consisting of 24.4 vol%CO₂ and 75.6 vol%H₂,the pressure was increased to 3.0 MPa,gas hourly space velocity(GHSV) was set to 3600 h^-1,and the temperature was increased up to the reaction temperature of 250℃.The reaction was conducted at the above conditions for about8 h.The reactants and products flowing out of the reactors were passed through the gas/liquid separator connected to a heat exchanger (0℃) and then were weighed and analyzed by gas chromatography (GC,Agilent Technologies 6890A)equipped with flame ionization detector (FID,HP-PLOT/Q capillary column).Uncondensed gases and light hydrocarbons (CO₂,CO,DME (dimethyl ether) and C1-C6(alkane which contains one to six carbon atoms)) were analyzed online at different time intervals in a GC equipped with a TCD for quantification of CO,CO₂ and N₂ and a FID for the analysis of hydrocarbons,using HP-PLOT/Q capillary column and HP-MOLESIEVE capillary column in series for separation.The CO₂ conversion and CH₃OH selectivity were obtained from the GC data,in which N₂was the internal standard gas.

3 Results and discussion

3.1 Structure properties of catalysts

Figure 1 shows the SEM images of CZZ catalysts with different Zn/Zr ratios.It can be seen that the CZZ-80-M owns open,periodic,and interconnected three-dimensionally macroporous frameworks.The macropores,existing everywhere,are regular with uniform average diameters and wall thicknesses,and these macropores are in a highly periodic array and are interconnected through small windows.With the increase in the Zn/Zr ratio,the macroporous structure collapses.Although the CZZ-20-M represents obvious sintering,its particle size is much larger than those of other samples,and it still exists some porous structure.It can be seen that lower Zn/Zr ratio is beneficial to the formation of the macroporous sample,and thus CZZ-80-N sample with the same Zn/Zr ratio as CZZ-80-M was prepared.It can be seen that the CZZ-80-N does not show any porous structure,and it exhibits obvious agglomeration and sintering,leading to the growth of the particle size.Figure 1f shows the pore size distribution of CZZ-80-M catalyst.It can be seen that the macropores with size in the range of 150-220 nm in this sample are very regular.The average pore diameter (d_ave) of this material is about200 nm,indicating that the reactants can easily diffuse in the materials.

Figure 2 shows the XRD patterns of CZZ catalysts with different Zn/Zr ratios and Table 1 presents the physicochemical and catalytic properties of the CZZ catalysts with different Zn/Zr ratios.For the CZZ-80 sample,it presents some weak and broad diffraction peaks of CuO phase at35.4°,38.8°,and 48.9°.While a much broader and weaker diffraction peak presented at 30.2°can be attributed to the tetragonal zirconia (t-ZrO₂),and no diffraction peak of ZnO phase can be detected.These results indicate that CZZ-80 sample owns a low crystallization degree.With the increase in the Zn/Zr ratio,the diffraction peaks of CuO become stronger and sharper,and the crystallite size of CuO (D_Cu) increases from 11.25 nm of CZZ-80 to22.36 nm of CZZ-20 (Table 1).This phenomenon indicates a continuous increase in the crystallization degree of CuO,which implies that the decrease in Zn/Zr ratio could enhance the dispersion of Cu [ 21] .For the phase of metallic oxide,the diffraction peaks of CZZ-80-N are much sharper and narrower.It can be seen from Table 1that the CuO crystallite size of CZZ-80-N is much larger than that of CZZ-80-M,concluding that the macroporous structure inhibits the growth of the grain size and avoids the sintering,which agree well with the findings of SEM.Besides,the formation of the macroporous structure is beneficial to the dissipation of the reaction heat,which could also effectively suppress the sintering.

Fig.1 SEM images of CZZ catalysts with different Zn/Zr ratios:a CZZ-80-M,b CZZ-60-M,c CZZ-40-M,d CZZ-20-M,and e CZZ-80-N;f pore size distribution of CZZ-80-M

Fig.2 XRD patterns of CZZ catalysts with different Zn/Zr ratios

The Cu contents in the catalysts,as determined by AAS,are given in Table 1.The values of Cu contents for all the samples are very close to the theoretical value (CZZ-80-M:32.8,CZZ-60-M:34.3,CZZ-40-M:35.9,CZZ-20-M:37.7,CZZ-80-N:32.8;wt%),indicating that the composition of CZZ catalysts can be precisely controlled.

It shows that in Table 1,for the macroporous samples,the BET surface area (S_BET) decreases from 60.32 to36.21 m²·g^-1 with the increase in Zn/Zr ratios.This variation trend is similar with the change of D_Cu.The metallic copper surface areas (S_Cu) were measured by a nitrous oxide decomposition method followed by H_2-TPR.The increase in the Zn/Zr ratio reduces S_Cu,and this trend is similar with the change of S_BET.The maximum S_Cu of11.18m²·g^-1is found in the CZZ-80-M catalyst,and the minimum S_Cu of 4.13 m²·g^-1 is observed over the CZZ-20-M sample,evidencing that lower Zn/Zr ratio could improve the dispersion of Cu on the surface of catalysts.The value of metallic copper surface area is a mirror of the dispersion of CuO [ 22] .The CZZ-80-M obtains significantly higher SBET and S_Cu,indicating good dispersion.Although with the same component,there still exists an obvious difference between CZZ-80-M and CZZ-80-N for S_BET and S_Cu.For CZZ-80-N,the values of S_BET and S_Cu are 20.89 and2.73 m²·g^-1,respectively,which are much lower than those of CZZ-80-M.Although the decrease in Zn/Zr ratios could enhance S_BET and S_Cu,the S_BET and S_Cu of the nonporous CZZ-80-N are even lower than those of the macroporous sample CZZ-20-M.These phenomena indicate that both the Zn/Zr ratio and the porous structure have influences on S_BET and S_Cu,while the porous structure plays a much more important role.

Fig.3 H_2-TPR profiles of CZZ catalysts with different Zn/Zr ratios

The H_2-TPR profiles of the studied catalysts prepared by a template method are shown in Fig.3,and Table 2 shows the TPR-derived hydrogen consumptions and temperatures for CZZ catalysts.All the samples with different Zn/Zr ratios exhibit a main reduction peak in the narrow range of287-320℃.Since ZnO and ZrO₂ are not reduced within the experimental region [ 23, 24, 25] ,these peaks should be attributed to the reduction of CuO phase.The reduction peak of the CZZ-80-M sample is more symmetrical than those of other samples,implying that CuO crystallites of the CZZ-80-M are more homogeneously distributed [ 20] .The area of the lower temperature region is obviously bigger than those of other samples,which can be attributed to the reduction of amorphous and well-dispersed surface CuO crystallites [ 22, 26, 27] .

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Table 1 Physicochemical and catalytic properties of the CZZ catalysts with different Zn/Zr ratios

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Table 2 TPR-derived hydrogen consumptions and temperatures for CZZ catalysts

As shown in Table 2,with the increase in Zn/Zr ratios,the peak temperature gradually shifts toward higher temperature,and simultaneously the H₂ consumption reduces.It is clear that,CZZ-80-N shifts to higher temperature obviously compared with all the macroporous samples,indicating a lower reducibility.In addition,as to the reduction temperature,many researchers believed that the smaller the CuO particle was,the lower the reduction temperature was [ 24, 28, 29] .The CZZ-80-M presents higher reducibility,indicating the well-dispersed surface CuO species on it [ 22, 26, 27] .

The mole ratio of H₂/Cu decreases with ZrO₂ content decreasing from 80 mol%to 20 mol%(CZZ-80-M to CZZ-20-M).In CZZ-80-M and CZZ-60-M samples,the H₂/Cu ratios are higher than 1,evidencing the incipient reduction of ZnO and/or ZnO-ZrO₂ carriers in CZZ-80-M and CZZ-60-M [ 2, 30, 31] .By comparing the H₂/Cu ratio of CZZ-80-M with that of CZZ-80-N,CZZ-80-M owns better reduction behavior.

3.2 Catalytic performance

The catalytic activity for CO₂ hydrogenation to methanol over various catalysts is presented in Fig.4.It can be seen that,the increase in Zn/Zr ratios reduces the CO₂ conversion.The maximum CO₂ conversion of 23.63%is found for the CZZ-80-M catalyst,and the minimum of 15.58%is observed over the CZZ-20-M catalyst.This variation trend can be explained in terms of the dispersion of Cu in CZZ catalysts,because many researchers believed that a higher dispersion of Cu usually resulted in a higher catalytic activity for copper-based catalysts [ 29, 32] .The CZZ-80-M sample presents the highest dispersion of Cu,leading to the highest catalytic activity.However,it is found that the variations of CH₃OH selectivity take an in verse-volcano variation with the increase in Zn/Zr ratios.The CZZ-20-M exhibits the maximum CH₃OH selectivity of 48.23%,and the CZZ-40-M exhibits the minimum of 37.51%.In the case of CH₃OH yield,it has the similar trend with the CO₂ conversion,and the CH₃OH yield of the CZZ-80-M sample could reach11.29%,which is much higher than those of other samples.

Fig.4 Catalytic activities for CO₂ hydrogenation to methanol over CZZ catalysts

It is found that the CZZ-80-M sample with lower Zn/Zr ratios owns prominent catalytic activity.For comparison,the catalytic performance of CZZ-80-N was also investigated.It is clear that the CO₂ conversion,CH₃OH selectivity and yield of CZZ-80-N are all lower than those of CZZ-80-M,and even lower than those of CZZ-20-M,which owns some irregular porous structure.This phenomenon demonstrates that the macroporous structure plays a much more important role than the Zn/Zr ratios in catalytic performance.

3.3 Influence of macroporous structure on activity

It is well known that,there are many factors influencing the activity,such as the macroporous structure,Zn/Zr ratios,D_Cu,S_BET,S_Cu.At the same time,these factors are also interacted with each other.The macroporous structure cannot influence the activity directly,but it can affect the activity by changing other factors.The relationships among these factors were investigated as following.

The TEM images of CZZ-80-M sample are shown in Fig.5.CZZ-80-M presents a well-ordered structure,attributed to the lower Zn/Zr ratios,which favors to the formation of macroporous structure.The holes in the sample form by the combustion and decomposition of the PMMA template in the calcining process.Figure 5b presents the wall of the macroporous structure clearly.Cu,Zn,and Zr elements are all detected simultaneously in Fig.5b by the TEM—energy-dispersive spectrometer (EDS),evidencing the homogeneous agglomerations of the particles.It can be seen that,some grain particles grow beyond the wall boundary,but not too much.This phenomenon can be explained that the particles grow rapidly under calcinations,but for the macroporous samples,the slow decomposition of PMMA template restrains the growth of the particles randomly in space,and then an ordered macroporous structure forms,which maintains well dispersion and avoids agglomeration.Therefore,the macroporous structure could inhibit the growth of D_Cu,and improve,S_Cu and S_BET.Besides,the special porous could accelerate the diffusion of the CO₂,which is favorable for the catalytic reaction.

Fig.5 TEM image of CZZ-80-M catalyst a and corresponding magnified image b

Fig.6 Effect of S_BET a and S_Cu b on CH₃OH yield of CZZ catalyst

Figure 6 shows the effect of S_BET and S_Cu,on the CH₃OH yield of the CZZ catalyst.It can be seen that although the CH₃OH yield is not in a linear relationship with S_BET S_Cu,it increases with the increase in the two factors which are influenced by the Zn/Zr ratios and the structure of the catalyst.

It can be concluded that lower Zn/Zr ratios could enhance S_BET and S_Cu,and more importantly,it is beneficial to the formation of the macroporous structure.The special structure not only inhibits the growth of the crystalline grain,but also enhances S_BET and S_Cu to a great degree,thus leading to a higher catalytic activity.

4 Conclusion

Macroporous CZZ catalysts with different Zn/Zr ratios were prepared by the template method,which were used asmethanol synthesis catalysts from CO2 hydrogenation.The macroporous CZZ catalysts with lower Zn/Zr ratios exhibit a better structural morphology.The ordered macroporous structure is beneficial to improving the dispersion state of the catalyst,leading to a higher catalytic activity.However,with the increase in Zn/Zr ratios,the macroporous structure collapses and the catalysts exhibit obvious sintering,resulting in the decrease in the activity.Both the macroporous structure and the Zn/Zr ratios have an influence on the catalytic activity,but the macroporous structure plays a much more important role than the Zn/Zr ratios.With the same composite,the CZZ-80-M presents more prominent catalytic performance than the nonporous sample CZZ-80-N.To further explore the influence of macroporous structure on the activity,the synthesis mechanism of the macroporous structure was investigated.The formation process of the porous structure inhibits the growth of the particle size and enhances S_BET and S_Cu,leading to a higher catalytic activity.

Acknowledgments This study was financially supported by the National Key Technologies Research&Development Program of China (No.201 1BAC01B03),the National Natural Science Foundation of China (No.51304099),the Applied Basic Research Program of Yunnan Province (No.2013FZ035),and the Testing and Analyzing Foundation of Kunming University of Science and Technology (No.2010213).

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