Morphology and activity relationships of macroporous CuO-ZnO-ZrO2 catalysts for methanol synthesis from CO2 hydrogenation
来源期刊:Rare Metals2016年第10期
论文作者:Yu-Hao Wang Wen-Gui Gao Hua Wang Yan-E Zheng Kong-Zhai Li Ru-Gui Ma
文章页码:790 - 796
摘 要: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.
稀有金属(英文版) 2016,35(10),790-796
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);
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
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
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
The nonporous catalyst was synthesized by co-precipitation method
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 (SCu) was measured by a nitrous oxide decomposition method followed by H2 temperature-programmed reduction (H2-TPR)
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 (dave) 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 (DCu) 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
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 (SBET) 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 DCu.The metallic copper surface areas (SCu) were measured by a nitrous oxide decomposition method followed by H2-TPR.The increase in the Zn/Zr ratio reduces SCu,and this trend is similar with the change of SBET.The maximum SCu of11.18m2·g-1is found in the CZZ-80-M catalyst,and the minimum SCu 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
Fig.3 H2-TPR profiles of CZZ catalysts with different Zn/Zr ratios
The H2-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
Table 1 Physicochemical and catalytic properties of the CZZ catalysts with different Zn/Zr ratios
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
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
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
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,DCu,SBET,SCu.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 DCu,and improve,SCu and SBET.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 SBET a and SCu b on CH3OH yield of CZZ catalyst
Figure 6 shows the effect of SBET and SCu,on the CH3OH yield of the CZZ catalyst.It can be seen that although the CH3OH yield is not in a linear relationship with SBET SCu,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 SBET and SCu,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 SBET and SCu 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 SBET and SCu,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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