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CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream
Journal of Rare Earths xxx (2017) 1e6
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Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths
CeO2/CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream* Xiaolin Yan a, Aiai Zhang a, b, Meiyi Gao b, Shanghong Zeng b, * a b
College of Science, Inner Mongolia Agricultural University, Hohhot 010018, China School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2017 Received in revised form 11 May 2017 Accepted 15 May 2017 Available online xxx
The CeO2/CuO catalysts using different template agent (F68 L64, F127 and P123) were synthesized by the simple template and impregnation method. They were characterized by FESEM, XRD, N2 physisorption and H2-TPR techniques. It is found that the CeO2/CuO catalysts are double pore distribution, and CeO2 can enter into the gap of CuO supports and form the contact interface of copper and cerium. Among the asprepared catalysts, the CeO2/CuO-F127 catalyst displays better activity at lower temperature and the CeO2/CuO-P123 catalyst presents higher activity at higher temperature. The CeO2/CuO-P123 catalyst has the smallest crystallite sizes of CuO and CeO2 as well as the largest size of cubes, which may improve the interaction of copper and cerium and enhance the performance of CO oxidation. © 2017 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Keywords: Copperecerium oxides Template agent Interaction CO oxidation
1. Introduction Proton Exchange Membrane Fuel Cells (PEMFCs) have been regarded as one of the most attractive power sources in mobile applications because they are high efficient and environmentally friendly ways of energy generation.1e4 Preferential CO oxidation (CO-PROX) is a straightforward and effective method to realize acceptable CO concentrations and avoid poisoning Pt-based anodes which are employed on proton exchange membrane of fuel cells.5e9 CO-PROX reaction is a competitive process where CO and H2 compete molecular oxygen, and catalyst must be high selectivity for CO oxidation.8,10e12 CuOeCeO2 catalysts as alternative to noble metals appear high selectivity, thermally stable and low cost.8,13,14 Their good performance is mainly assigned to synergistic redox properties of coppereceria interfacial sites.8,10,15 The configurations of CuOeCeO2 catalysts can be classified into three kinds: CuO/CeO2, inverse CeO2/ CuO and supported CuOeCeO2.16,17 In 2010 Martinez-Arias et al. reported the inverse CeO2/CuO catalyst for preferential CO oxidation. It is found that the amount and properties of coppereceria
* Foundation item: Project supported by the National Natural Science Foundation of China (grant no. 21466024) and the Natural Science Foundation of Inner Mongolia (grant nos. 2014MS0217 and 2015MS0209). * Corresponding author. E-mail address: [email protected] (S. Zeng).
interfacial sites can be maintained in the inverse system to keep a high level of CO oxidation activity.18 For CuOeCeO2 catalysts, the template agent has a certain influence on structure and properties.19,20 Naskar et al. reported that optimum experimental conditions such as concentration of the surfactant, synthesis time and heating rate of calcination govern formation of multishell patterned structure of CuO particles.21 In this work, The CeO2/CuO catalysts using different template agent were synthesized by the simple template and impregnation method, and the multi-technique characterizations were employed to correlate template agent with catalytic performance for preferential oxidation of carbon monoxide. 2. Experimental 2.1. Catalyst preparation The CuO supports were prepared by a simple self-assembly template method.19 Four kinds of template agent were used in the process of preparation, respectively. They were F68 (SigmaeAldrich, Ma ¼ 8400), L64 (SigmaeAldrich, Ma ¼ 2900), F127 (SigmaeAldrich, Ma ¼ 12,600) and P123 (SigmaeAldrich, Ma ¼ 5800). The 4 mmol F68 (or L64, F127, P123) was dissolved in 70 mL deionized water under stirring at 80 C for 2 h, followed by the addition of 70 mL Cu(NO3)2 solution. The molar ratio of Cu2þ and F68 (or L64, F127, P123) was equal to 0.3. A 60 mL water solution of oxalic acid
http://dx.doi.org/10.1016/j.jre.2017.05.015 1002-0721/© 2017 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Please cite this article in press as: Yan X, et al., CeO2/CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream, Journal of Rare Earths (2017), http://dx.doi.org/10.1016/j.jre.2017.05.015
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X. Yan et al. / Journal of Rare Earths xxx (2017) 1e6
containing 2 mL of phosphoric acid was dropwise added into the above solution. Then, the flask was transferred to the ice-water bath immediately and maintained 3 h. After centrifuging and washing, the samples were dried at 80 C overnight and calcined at 400 C with a heating rate of 1 C min1 for 2 h. The catalysts using different template agent containing different amount hydrophilic group were prepared by an incipient wetness method. The CuO supports were impregnated in the Ce(NO3)3 solution for 24 h, and then dried overnight at 80 C. The samples were calcined at 400 C for 2 h in air to obtain the CeO2/CuO catalysts. The CeO2 content is 30 wt.%. The catalysts were named as CeO2/CuO-F68, CeO2/CuO-L64, CeO2/CuO-F127 and CeO2/CuO-P123, respectively. 2.2. Catalyst characterization Scanning electron microscopy analyses using secondary electrons to form the images were carried out on a Hitachi S-4800
scanning electron microscope equipped with energy-dispersive Xray (EDX) instrument. The samples were coated with a thin layer of Pt-Au before scanning. Powder X-ray diffraction patterns were recorded on a PANalytical X'pert PRO diffractometer with Cu Ka source (l ¼ 0.15406 nm) in the range of 2q between 5 C and 80 C. The average crystallite sizes were estimated from the line broadening using the Scherrer's equation. N2 adsorptionedesorption isotherms were obtained at liquid nitrogen temperature (196 C) using a Micrometrics ASAP2020 adsorption apparatus. The analysis procedure is fully automated and operated with the static volumetric technique. Prior to each measurement, the samples were outgassed at 200 C for 12 h. The surface area of the catalysts was determined by the BrunauereEmmetteTeller (BET) method. The pore size distribution was calculated from the desorption branch of the N2 isotherms using BJH algorithm.
Fig. 1. FESEM images of the CeO2/CuO catalysts: (A) CeO2/CuO-F68 (B) CeO2/CuO-L64 (C) CeO2/CuO-F127 (D) CeO2/CuO-P123. Bottom: EDS mapping of CeO2/CuO-F123 catalyst.
Please cite this article in press as: Yan X, et al., CeO2/CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream, Journal of Rare Earths (2017), http://dx.doi.org/10.1016/j.jre.2017.05.015
X. Yan et al. / Journal of Rare Earths xxx (2017) 1e6
CO2 ð%Þ ¼ ½O2 in ½O2 out
3
½O2 in 100
(2)
SCO2 ð%Þ ¼ CCO =lCO2 100
(3)
In all the tests, l ¼ 2 was used, because this value was previously found optimal for preferential oxidation of CO.26,27 3. Results and discussion 3.1. Scanning electron microscope
Fig. 2. XRD patterns of the CeO2/CuO catalysts: (A) CeO2/CuO-F68; (B) CeO2/CuO-L64; (C) CeO2/CuO-F127; (D) CeO2/CuO-P123.
Fig. 1 shows field emission scanning electron images of the CeO2/CuO catalysts using different template agent. It can be observed that the morphology of CeO2/CuO catalysts is cube shape with rough surface. EDX mapping indicates that the rough particles are CeO2 on the surface of the catalysts. For CeO2/CuO-F68, CeO2/ CuO-L64 and CeO2/CuO-F127, the size of cubes is 400e500 nm. And for CeO2/CuO-P123, the size of cubes is about 1 mm. It indicates that the different template agent had some influence on size and morphology of the CeO2/CuO catalysts. 3.2. X-ray powder diffraction
Temperature-programmed reduction experiments were carried out on a Micromeritics Apparatus (AutoChemⅡ2920). Hydrogen consumption was analyzed by a thermal conductivity detector. 30 mg sample was placed on top of some silica wool and pretreated at 200 C for 1 h in a N2 stream in order to remove the contaminants. After cooling to ambient temperature, the H2/Ar mixture was switched on and the sample was heated with a heating rate of 10 C min1. The reaction was performed from room temperature to 800 C. 2.3. Catalytic performance tests Performance tests were carried out in a fixed-bed reactor (i.d. ¼ 9 mm). The 100 mg catalyst diluted with quartz sands (particle size as same as catalyst) was loaded in quartz tubular reactor. The volume ratio was 1 for the catalyst and quartz sands. A K-type thermocouple was inserted into the catalyst bed to monitor the reaction temperature. The reaction gasses consisted of 1% O2, 1% CO, 50% H2 and N2 balance. The space velocity was 1 40,000 mL g1 cat h . The reaction was operated from 35 C to 215 C. A GC-2014C gas chromatograph equipped with a thermal conductivity detector was used to analyze the inlet and outlet composition. The 5A molecular sieve column was used to separate CO, O2 and N2, and CO2 was separated by the TDX-01 column. Water was trapped before the gasses entered the gas chromatograph. The conversion of CO (CCO), O2 conversion (CO2 ) and selectivity of CO2 (SCO2 ) were calculated according to the following equations (1)e(3), respectively.22e25
CCO ð%Þ ¼ ½COin ½COout
½COin 100
(1)
Fig. 2 shows XRD patterns of the CeO2/CuO catalysts using different template agent. The diffraction peaks at 35.5 , 38.7 and 48.8 were indexed to tenorite CuO. The diffraction peaks appeared at 2q of 28.4 , 32.7, 47.5 and 56.3 were assigned to the cubic fluorite CeO2. It can be observed that the intensity of CeO2 diffraction peaks for CeO2/CuO-P123 is weaker than those of the CeO2/CuO-F68, CeO2/CuO-L64 and CeO2/CuO-F127 catalysts, suggesting that CeO2 had a better dispersion over the CeO2/CuO-P123 catalyst. It was possibly related to the size of cubes, as shown in SEM measurements. Table 1 lists the average crystallite sizes of CeO2 and CuO, which were calculated from the line broadening of the most intense XRD reflection peaks according to Scherrer's equation. It can be seen that the average crystallite sizes of CeO2 are 5e7 nm, and the average crystallite sizes of CuO are 20e30 nm. It indicates that CeO2 clusters were dispersed on the surface of the CuO supports. It is consistent with the results of SEM. In addition, the CeO2 lattice parameters of the catalysts are smaller than 5.412 (pure CeO2), suggesting that Cu2þ entered into the crystal lattice of CeO2.22,23 3.3. N2 adsorptionedesorption analyses N2 adsorptionedesorption isotherms and pore-size distribution curves of the CuO supports are showed in Fig. 3. The isotherms are type-V isotherms according to the IUPAC classification, and the turning point does not appear within the scope of relatively low pressure. The pore structure is related to the shape of hysteresis loop on the adsorptionedesorption isotherm. The CuO-F68, CuOL64 and CuO-P123 supports present H3 hysteresis loops, corresponding to wedge-channel materials. The CuO-F127 support is H2 hysteresis loop with narrow mouth and wide body (often referred
Table 1 Structural properties of the CeO2/CuO catalysts. Sample
CeO2/CuO-F68 CeO2/CuO-L64 CeO2/CuO-F127 CeO2/CuO-P123
CeO2 Lattice parameter (Å)
CuO Lattice parameter (Å) a
b
c
4.684 4.671 4.690 4.690
3.426 3.428 3.424 3.428
5.145 5.139 5.130 5.130
5.410 5.409 5.405 5.406
Particle size (nm) d(CuO)a
d(CeO2)b
28.7 29.7 27.0 20.3
6.3 6.7 6.3 5.3
Please cite this article in press as: Yan X, et al., CeO2/CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream, Journal of Rare Earths (2017), http://dx.doi.org/10.1016/j.jre.2017.05.015
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Fig. 3. N2 adsorptionedesorption isotherms and pore size distribution curves of the CuO supports.
Fig. 4. N2 adsorptionedesorption isotherms and pore size distribution curves of the CeO2/CuO catalysts.
3.4. TPR measurements to as ink-bottle-like pores). As shown in Table 2, the CuO-F127 support has the largest pore volume. In addition, the pore-size distribution curves of the CuO supports are double pore distribution, including the smaller pores at 4 nm and larger pores at 20e40 nm. The larger pores of CuO-F127 support shift to the direction of smaller pores. Fig. 4 shows N2 adsorptionedesorption isotherms and pore-size distribution curves of the CeO2/CuO catalysts. The isotherms are type-IV isotherms with H3 hysteresis loops. Moreover, the poresize distribution curves of the CeO2/CuO catalysts also display double pore distribution. The maxima appear at 4 nm and 10 nm, suggesting that CeO2 entered into the gap of CuO supports and formed the contact interface of copper and cerium. BET surface area of the CeO2/CuO catalysts is improved after CeO2 impregnation, because CeO2 is much easier to obtain larger BET surface area than CuO.22
Fig. 5 shows H2-TPR profiles of the CeO2/CuO catalysts. For CeO2/ CuO-F68, CeO2/CuO-L64 and CeO2/CuO-F127 catalysts, there are two reduction peaks from 100 C to 350 C, corresponding to the reduction of CuO particles associated with CeO2 at about 200 C and bulk copper oxide species at about 280 C, respectively.28 However, there are three reduction peaks in H2-TPR profiles of the CeO2/CuOP123 catalysts, which shift to lower temperature compared with the other three catalysts. They were attributed to the reduction of highly dispersed CuO species, CuO particles interacted with CeO2 and bulk CuO from low to high temperature, respectively. In addition, the peaks at above 700 C were assigned to the reduction of bulk CeO2.9,16 It can be observed that bulk CeO2 over the CeO2/ CuO-P123 catalyst has lower reduction temperature in comparison with the other three catalysts, which is closely related to the crystallite sizes of CeO2, as shown in XRD measurements.
Table 2 Textural properties of the supports and catalysts. Supports
SBET (m2 g1)
Pore volume (cm3 g1)
Catalysts
SBET (m2 g1)
Pore volume (cm3 g1)
CuO-F68 CuO-L64 CuO-F127 CuO-P123
26.7 24.5 26.4 14.7
0.15 0.14 0.09 0.11
CeO2/CuO-F68 CeO2/CuO-L64 CeO2/CuO-F127 CeO2/CuO-P123
29.3 44.6 61.2 42.6
0.12 0.10 0.17 0.14
Please cite this article in press as: Yan X, et al., CeO2/CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream, Journal of Rare Earths (2017), http://dx.doi.org/10.1016/j.jre.2017.05.015
X. Yan et al. / Journal of Rare Earths xxx (2017) 1e6
5
Fig. 5. H2-TPR profiles of the CeO2/CuO catalysts.
other samples. It is possible that the CeO2/CuO-P123 catalyst has the larger size of cubes to improve the dispersion of CeO2, as seen in the results of SEM analysis. CO2 selectivity is directly related to competitive H2 oxidation over the catalysts.10,16 For CeO2/CuO-P123, it has the best selectivity for CO oxidation, which could be associated with its structure. As shown in Table 1 and SEM results, the CeO2/CuO-P123 catalyst has the smallest crystallite sizes of CuO and CeO2 as well as the largest size of cubes, which could improve the interaction of copper and cerium and enhance the activity of CO oxidation. 4. Conclusions
Fig. 6. CO conversion and CO2 selectivity over the catalysts ([CO]in ¼ 1%, [O2]in ¼ 1%, 1 [H2]in ¼ 50%, N2 balance; T ¼ 35e215 C, GHSV ¼ 40,000 mL g1 cat h ).
3.5. Catalytic performance Fig. 6 shows CO conversion and CO2 selectivity over the CeO2/ CuO catalysts using different template agent. It can be observed that CO conversion increases rapidly with the temperature until it reaches a maximum, and then CO conversion begins to decrease after 175 C due to competitive H2 oxidation. Among the as-prepared catalysts, the CeO2/CuO-F127 catalyst displays better activity at lower temperature and the CeO2/CuO-P123 catalyst presents higher activity at higher temperature. Moreover, the CeO2/CuO-P123 catalyst has the best selectivity for CO oxidation. Better activity at lower temperature over the CeO2/CuO-F127 catalyst could be from its textural properties, which are different than the other three samples, as shown in Fig. 4. It is favorable for the adsorption of reactive gasses inside the pores of catalyst. Higher activity at higher temperature over the CeO2/CuO-P123 catalyst was closely associated with the dispersion of CeO2 on the CuO support. XRD and TPR measurements indicate that CeO2 has a better dispersion on the CuO-P123 support compared with the
The CeO2/CuO catalysts using different template agent were synthesized by the simple template and impregnation method. SEM measurements show that the different template agent had some influence on size and morphology of the CeO2/CuO catalysts. XRD and TPR analyses indicate that CeO2 has a better dispersion on the CuO-P123 support compared with the other samples. It is possible that the CeO2/CuO-P123 catalyst has the larger size of cubes to improve the dispersion of CeO2. N2 adsorptionedesorption results display that the CeO2/CuO catalysts are double pore distribution, and CeO2 entered into the gap of CuO supports and formed the contact interface of copper and cerium. Among the as-prepared catalysts, the CeO2/CuO-P123 catalyst has the smallest crystallite sizes of CuO and CeO2 as well as the largest size of cubes, which could improve the interaction of copper and cerium and enhance the performance of CO oxidation. Acknowledgments The authors would like to acknowledge the National Natural Science Foundation of China (grant no. 21466024), the Natural Science Foundation of Inner Mongolia (grant nos. 2014MS0217 and 2015MS0209). References 1. Li J, Zhu PF, Zuo SF, Huang QQ, Zhou RX. Influence of Mn doping on the performance of CuO-CeO2 catalysts for selective oxidation of CO in hydrogen-rich streams. Appl Catal A Gen. 2010;381:261.
Please cite this article in press as: Yan X, et al., CeO2/CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream, Journal of Rare Earths (2017), http://dx.doi.org/10.1016/j.jre.2017.05.015
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ndez-García M, et al. Redox-catalytic cor2. Martínez-Arias A, Gamarra D, Ferna relations in oxidised copper-ceria CO-PROX catalysts. Catal Today. 2009;143: 211. 3. Gao MY, Jiang N, Zhao YH, Xu CJ, Su HQ, Zeng SH. Copper-cerium oxides supported on carbon nanomaterial for preferential oxidation of carbon monoxide. J Rare Earths. 2016;34:55. 4. Wu ZW, Zhu HQ, Qin ZF, Wang H, Huang LC, Wang JG. Preferential oxidation of CO in H2-rich stream over CuO/Ce1xTixO2 catalysts. Appl Catal B Environ. 2010;98:204. 5. Dufield CD, Chen R, Adcock PL. A carbon monoxide PROX reactor for PEM fuel cell automotive application. Int J Hydrogen Energy. 2001;26:763. 6. Han J, Kim HJ, Yoon S, Lee H. Shape effect of ceria in Cu/ceria catalysts for preferential CO oxidation. J Mol Catal A Chem. 2011;335:82. ndez WY, Arzamendi G, Gandía LM, Centeno MA, 7. Laguna OH, Herna Odriozola JA. Gold supported on CuOx/CeO2 catalyst for the purification of hydrogen by the CO preferential oxidation reaction (PROX). Fuel. 2014;118: 176. 8. Martínez-Arias A, Hungría AB, Fern andez-García M, Conesa JC, Munuera G. Preferential oxidation of CO ina H2-rich stream over CuO/CeO2 and CuO/(Ce,M) Ox (M ¼ Zr, Tb) catalysts. J Power Sources. 2005;151:32. 9. Moreno M, Baronetti GT, Laborde MA, Marino FJ. Kinetics of preferential CO oxidation in H2 excess (COPROX) over CuO/CeO2 catalysts. Int J Hydrogen Energy. 2008;33:3538. 10. Liu ZG, Zhou RX, Zheng XM. Preferential oxidation of CO in excess hydrogen over a nanostructured CuOeCeO2 catalyst with high surface areas. Catal Commun. 2008;9:2183. 11. Reyes-Carmona A, Arango-Díaz A, Moretti E, et al. CuO/CeO2 supported on Zr doped SBA-15 as catalysts for preferential CO oxidation (CO-PROX). J Power Sources. 2011;196:4382. 12. Zhang DS, Mai HL, Huang L, Shi LY. Pyridine-thermal synthesis and high catalytic activity of CeO2/CuO/CNT nanocomposites. Appl Surf Sci. 2010;256:6795. 13. Zeng SH, Wang Y, Liu KW, Liu FR, Su HQ. CeO2 nanoparticles supported on CuO with petal-like and sphere-flower morphologies for preferential CO oxidation. Int J Hydrogen Energy. 2012;37:11640. 14. Zhao FZ, Gong M, Zhang GY, Li JL. Effect of the loading content of CuO on the activity and structure of CuO/Ce-Mn-O catalysts for CO oxidation. J Rare Earths. 2015;33:604. 15. Laguna OH, Domínguez MI, Ora a S, et al. Influence of the O2/CO ratio and the presence of H2O and CO2 in the feed-stream during the preferential oxidation
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Please cite this article in press as: Yan X, et al., CeO2/CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream, Journal of Rare Earths (2017), http://dx.doi.org/10.1016/j.jre.2017.05.015
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Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths
CeO2/CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream* Xiaolin Yan a, Aiai Zhang a, b, Meiyi Gao b, Shanghong Zeng b, * a b
College of Science, Inner Mongolia Agricultural University, Hohhot 010018, China School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2017 Received in revised form 11 May 2017 Accepted 15 May 2017 Available online xxx
The CeO2/CuO catalysts using different template agent (F68 L64, F127 and P123) were synthesized by the simple template and impregnation method. They were characterized by FESEM, XRD, N2 physisorption and H2-TPR techniques. It is found that the CeO2/CuO catalysts are double pore distribution, and CeO2 can enter into the gap of CuO supports and form the contact interface of copper and cerium. Among the asprepared catalysts, the CeO2/CuO-F127 catalyst displays better activity at lower temperature and the CeO2/CuO-P123 catalyst presents higher activity at higher temperature. The CeO2/CuO-P123 catalyst has the smallest crystallite sizes of CuO and CeO2 as well as the largest size of cubes, which may improve the interaction of copper and cerium and enhance the performance of CO oxidation. © 2017 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Keywords: Copperecerium oxides Template agent Interaction CO oxidation
1. Introduction Proton Exchange Membrane Fuel Cells (PEMFCs) have been regarded as one of the most attractive power sources in mobile applications because they are high efficient and environmentally friendly ways of energy generation.1e4 Preferential CO oxidation (CO-PROX) is a straightforward and effective method to realize acceptable CO concentrations and avoid poisoning Pt-based anodes which are employed on proton exchange membrane of fuel cells.5e9 CO-PROX reaction is a competitive process where CO and H2 compete molecular oxygen, and catalyst must be high selectivity for CO oxidation.8,10e12 CuOeCeO2 catalysts as alternative to noble metals appear high selectivity, thermally stable and low cost.8,13,14 Their good performance is mainly assigned to synergistic redox properties of coppereceria interfacial sites.8,10,15 The configurations of CuOeCeO2 catalysts can be classified into three kinds: CuO/CeO2, inverse CeO2/ CuO and supported CuOeCeO2.16,17 In 2010 Martinez-Arias et al. reported the inverse CeO2/CuO catalyst for preferential CO oxidation. It is found that the amount and properties of coppereceria
* Foundation item: Project supported by the National Natural Science Foundation of China (grant no. 21466024) and the Natural Science Foundation of Inner Mongolia (grant nos. 2014MS0217 and 2015MS0209). * Corresponding author. E-mail address: [email protected] (S. Zeng).
interfacial sites can be maintained in the inverse system to keep a high level of CO oxidation activity.18 For CuOeCeO2 catalysts, the template agent has a certain influence on structure and properties.19,20 Naskar et al. reported that optimum experimental conditions such as concentration of the surfactant, synthesis time and heating rate of calcination govern formation of multishell patterned structure of CuO particles.21 In this work, The CeO2/CuO catalysts using different template agent were synthesized by the simple template and impregnation method, and the multi-technique characterizations were employed to correlate template agent with catalytic performance for preferential oxidation of carbon monoxide. 2. Experimental 2.1. Catalyst preparation The CuO supports were prepared by a simple self-assembly template method.19 Four kinds of template agent were used in the process of preparation, respectively. They were F68 (SigmaeAldrich, Ma ¼ 8400), L64 (SigmaeAldrich, Ma ¼ 2900), F127 (SigmaeAldrich, Ma ¼ 12,600) and P123 (SigmaeAldrich, Ma ¼ 5800). The 4 mmol F68 (or L64, F127, P123) was dissolved in 70 mL deionized water under stirring at 80 C for 2 h, followed by the addition of 70 mL Cu(NO3)2 solution. The molar ratio of Cu2þ and F68 (or L64, F127, P123) was equal to 0.3. A 60 mL water solution of oxalic acid
http://dx.doi.org/10.1016/j.jre.2017.05.015 1002-0721/© 2017 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Please cite this article in press as: Yan X, et al., CeO2/CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream, Journal of Rare Earths (2017), http://dx.doi.org/10.1016/j.jre.2017.05.015
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X. Yan et al. / Journal of Rare Earths xxx (2017) 1e6
containing 2 mL of phosphoric acid was dropwise added into the above solution. Then, the flask was transferred to the ice-water bath immediately and maintained 3 h. After centrifuging and washing, the samples were dried at 80 C overnight and calcined at 400 C with a heating rate of 1 C min1 for 2 h. The catalysts using different template agent containing different amount hydrophilic group were prepared by an incipient wetness method. The CuO supports were impregnated in the Ce(NO3)3 solution for 24 h, and then dried overnight at 80 C. The samples were calcined at 400 C for 2 h in air to obtain the CeO2/CuO catalysts. The CeO2 content is 30 wt.%. The catalysts were named as CeO2/CuO-F68, CeO2/CuO-L64, CeO2/CuO-F127 and CeO2/CuO-P123, respectively. 2.2. Catalyst characterization Scanning electron microscopy analyses using secondary electrons to form the images were carried out on a Hitachi S-4800
scanning electron microscope equipped with energy-dispersive Xray (EDX) instrument. The samples were coated with a thin layer of Pt-Au before scanning. Powder X-ray diffraction patterns were recorded on a PANalytical X'pert PRO diffractometer with Cu Ka source (l ¼ 0.15406 nm) in the range of 2q between 5 C and 80 C. The average crystallite sizes were estimated from the line broadening using the Scherrer's equation. N2 adsorptionedesorption isotherms were obtained at liquid nitrogen temperature (196 C) using a Micrometrics ASAP2020 adsorption apparatus. The analysis procedure is fully automated and operated with the static volumetric technique. Prior to each measurement, the samples were outgassed at 200 C for 12 h. The surface area of the catalysts was determined by the BrunauereEmmetteTeller (BET) method. The pore size distribution was calculated from the desorption branch of the N2 isotherms using BJH algorithm.
Fig. 1. FESEM images of the CeO2/CuO catalysts: (A) CeO2/CuO-F68 (B) CeO2/CuO-L64 (C) CeO2/CuO-F127 (D) CeO2/CuO-P123. Bottom: EDS mapping of CeO2/CuO-F123 catalyst.
Please cite this article in press as: Yan X, et al., CeO2/CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream, Journal of Rare Earths (2017), http://dx.doi.org/10.1016/j.jre.2017.05.015
X. Yan et al. / Journal of Rare Earths xxx (2017) 1e6
CO2 ð%Þ ¼ ½O2 in ½O2 out
3
½O2 in 100
(2)
SCO2 ð%Þ ¼ CCO =lCO2 100
(3)
In all the tests, l ¼ 2 was used, because this value was previously found optimal for preferential oxidation of CO.26,27 3. Results and discussion 3.1. Scanning electron microscope
Fig. 2. XRD patterns of the CeO2/CuO catalysts: (A) CeO2/CuO-F68; (B) CeO2/CuO-L64; (C) CeO2/CuO-F127; (D) CeO2/CuO-P123.
Fig. 1 shows field emission scanning electron images of the CeO2/CuO catalysts using different template agent. It can be observed that the morphology of CeO2/CuO catalysts is cube shape with rough surface. EDX mapping indicates that the rough particles are CeO2 on the surface of the catalysts. For CeO2/CuO-F68, CeO2/ CuO-L64 and CeO2/CuO-F127, the size of cubes is 400e500 nm. And for CeO2/CuO-P123, the size of cubes is about 1 mm. It indicates that the different template agent had some influence on size and morphology of the CeO2/CuO catalysts. 3.2. X-ray powder diffraction
Temperature-programmed reduction experiments were carried out on a Micromeritics Apparatus (AutoChemⅡ2920). Hydrogen consumption was analyzed by a thermal conductivity detector. 30 mg sample was placed on top of some silica wool and pretreated at 200 C for 1 h in a N2 stream in order to remove the contaminants. After cooling to ambient temperature, the H2/Ar mixture was switched on and the sample was heated with a heating rate of 10 C min1. The reaction was performed from room temperature to 800 C. 2.3. Catalytic performance tests Performance tests were carried out in a fixed-bed reactor (i.d. ¼ 9 mm). The 100 mg catalyst diluted with quartz sands (particle size as same as catalyst) was loaded in quartz tubular reactor. The volume ratio was 1 for the catalyst and quartz sands. A K-type thermocouple was inserted into the catalyst bed to monitor the reaction temperature. The reaction gasses consisted of 1% O2, 1% CO, 50% H2 and N2 balance. The space velocity was 1 40,000 mL g1 cat h . The reaction was operated from 35 C to 215 C. A GC-2014C gas chromatograph equipped with a thermal conductivity detector was used to analyze the inlet and outlet composition. The 5A molecular sieve column was used to separate CO, O2 and N2, and CO2 was separated by the TDX-01 column. Water was trapped before the gasses entered the gas chromatograph. The conversion of CO (CCO), O2 conversion (CO2 ) and selectivity of CO2 (SCO2 ) were calculated according to the following equations (1)e(3), respectively.22e25
CCO ð%Þ ¼ ½COin ½COout
½COin 100
(1)
Fig. 2 shows XRD patterns of the CeO2/CuO catalysts using different template agent. The diffraction peaks at 35.5 , 38.7 and 48.8 were indexed to tenorite CuO. The diffraction peaks appeared at 2q of 28.4 , 32.7, 47.5 and 56.3 were assigned to the cubic fluorite CeO2. It can be observed that the intensity of CeO2 diffraction peaks for CeO2/CuO-P123 is weaker than those of the CeO2/CuO-F68, CeO2/CuO-L64 and CeO2/CuO-F127 catalysts, suggesting that CeO2 had a better dispersion over the CeO2/CuO-P123 catalyst. It was possibly related to the size of cubes, as shown in SEM measurements. Table 1 lists the average crystallite sizes of CeO2 and CuO, which were calculated from the line broadening of the most intense XRD reflection peaks according to Scherrer's equation. It can be seen that the average crystallite sizes of CeO2 are 5e7 nm, and the average crystallite sizes of CuO are 20e30 nm. It indicates that CeO2 clusters were dispersed on the surface of the CuO supports. It is consistent with the results of SEM. In addition, the CeO2 lattice parameters of the catalysts are smaller than 5.412 (pure CeO2), suggesting that Cu2þ entered into the crystal lattice of CeO2.22,23 3.3. N2 adsorptionedesorption analyses N2 adsorptionedesorption isotherms and pore-size distribution curves of the CuO supports are showed in Fig. 3. The isotherms are type-V isotherms according to the IUPAC classification, and the turning point does not appear within the scope of relatively low pressure. The pore structure is related to the shape of hysteresis loop on the adsorptionedesorption isotherm. The CuO-F68, CuOL64 and CuO-P123 supports present H3 hysteresis loops, corresponding to wedge-channel materials. The CuO-F127 support is H2 hysteresis loop with narrow mouth and wide body (often referred
Table 1 Structural properties of the CeO2/CuO catalysts. Sample
CeO2/CuO-F68 CeO2/CuO-L64 CeO2/CuO-F127 CeO2/CuO-P123
CeO2 Lattice parameter (Å)
CuO Lattice parameter (Å) a
b
c
4.684 4.671 4.690 4.690
3.426 3.428 3.424 3.428
5.145 5.139 5.130 5.130
5.410 5.409 5.405 5.406
Particle size (nm) d(CuO)a
d(CeO2)b
28.7 29.7 27.0 20.3
6.3 6.7 6.3 5.3
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Fig. 3. N2 adsorptionedesorption isotherms and pore size distribution curves of the CuO supports.
Fig. 4. N2 adsorptionedesorption isotherms and pore size distribution curves of the CeO2/CuO catalysts.
3.4. TPR measurements to as ink-bottle-like pores). As shown in Table 2, the CuO-F127 support has the largest pore volume. In addition, the pore-size distribution curves of the CuO supports are double pore distribution, including the smaller pores at 4 nm and larger pores at 20e40 nm. The larger pores of CuO-F127 support shift to the direction of smaller pores. Fig. 4 shows N2 adsorptionedesorption isotherms and pore-size distribution curves of the CeO2/CuO catalysts. The isotherms are type-IV isotherms with H3 hysteresis loops. Moreover, the poresize distribution curves of the CeO2/CuO catalysts also display double pore distribution. The maxima appear at 4 nm and 10 nm, suggesting that CeO2 entered into the gap of CuO supports and formed the contact interface of copper and cerium. BET surface area of the CeO2/CuO catalysts is improved after CeO2 impregnation, because CeO2 is much easier to obtain larger BET surface area than CuO.22
Fig. 5 shows H2-TPR profiles of the CeO2/CuO catalysts. For CeO2/ CuO-F68, CeO2/CuO-L64 and CeO2/CuO-F127 catalysts, there are two reduction peaks from 100 C to 350 C, corresponding to the reduction of CuO particles associated with CeO2 at about 200 C and bulk copper oxide species at about 280 C, respectively.28 However, there are three reduction peaks in H2-TPR profiles of the CeO2/CuOP123 catalysts, which shift to lower temperature compared with the other three catalysts. They were attributed to the reduction of highly dispersed CuO species, CuO particles interacted with CeO2 and bulk CuO from low to high temperature, respectively. In addition, the peaks at above 700 C were assigned to the reduction of bulk CeO2.9,16 It can be observed that bulk CeO2 over the CeO2/ CuO-P123 catalyst has lower reduction temperature in comparison with the other three catalysts, which is closely related to the crystallite sizes of CeO2, as shown in XRD measurements.
Table 2 Textural properties of the supports and catalysts. Supports
SBET (m2 g1)
Pore volume (cm3 g1)
Catalysts
SBET (m2 g1)
Pore volume (cm3 g1)
CuO-F68 CuO-L64 CuO-F127 CuO-P123
26.7 24.5 26.4 14.7
0.15 0.14 0.09 0.11
CeO2/CuO-F68 CeO2/CuO-L64 CeO2/CuO-F127 CeO2/CuO-P123
29.3 44.6 61.2 42.6
0.12 0.10 0.17 0.14
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X. Yan et al. / Journal of Rare Earths xxx (2017) 1e6
5
Fig. 5. H2-TPR profiles of the CeO2/CuO catalysts.
other samples. It is possible that the CeO2/CuO-P123 catalyst has the larger size of cubes to improve the dispersion of CeO2, as seen in the results of SEM analysis. CO2 selectivity is directly related to competitive H2 oxidation over the catalysts.10,16 For CeO2/CuO-P123, it has the best selectivity for CO oxidation, which could be associated with its structure. As shown in Table 1 and SEM results, the CeO2/CuO-P123 catalyst has the smallest crystallite sizes of CuO and CeO2 as well as the largest size of cubes, which could improve the interaction of copper and cerium and enhance the activity of CO oxidation. 4. Conclusions
Fig. 6. CO conversion and CO2 selectivity over the catalysts ([CO]in ¼ 1%, [O2]in ¼ 1%, 1 [H2]in ¼ 50%, N2 balance; T ¼ 35e215 C, GHSV ¼ 40,000 mL g1 cat h ).
3.5. Catalytic performance Fig. 6 shows CO conversion and CO2 selectivity over the CeO2/ CuO catalysts using different template agent. It can be observed that CO conversion increases rapidly with the temperature until it reaches a maximum, and then CO conversion begins to decrease after 175 C due to competitive H2 oxidation. Among the as-prepared catalysts, the CeO2/CuO-F127 catalyst displays better activity at lower temperature and the CeO2/CuO-P123 catalyst presents higher activity at higher temperature. Moreover, the CeO2/CuO-P123 catalyst has the best selectivity for CO oxidation. Better activity at lower temperature over the CeO2/CuO-F127 catalyst could be from its textural properties, which are different than the other three samples, as shown in Fig. 4. It is favorable for the adsorption of reactive gasses inside the pores of catalyst. Higher activity at higher temperature over the CeO2/CuO-P123 catalyst was closely associated with the dispersion of CeO2 on the CuO support. XRD and TPR measurements indicate that CeO2 has a better dispersion on the CuO-P123 support compared with the
The CeO2/CuO catalysts using different template agent were synthesized by the simple template and impregnation method. SEM measurements show that the different template agent had some influence on size and morphology of the CeO2/CuO catalysts. XRD and TPR analyses indicate that CeO2 has a better dispersion on the CuO-P123 support compared with the other samples. It is possible that the CeO2/CuO-P123 catalyst has the larger size of cubes to improve the dispersion of CeO2. N2 adsorptionedesorption results display that the CeO2/CuO catalysts are double pore distribution, and CeO2 entered into the gap of CuO supports and formed the contact interface of copper and cerium. Among the as-prepared catalysts, the CeO2/CuO-P123 catalyst has the smallest crystallite sizes of CuO and CeO2 as well as the largest size of cubes, which could improve the interaction of copper and cerium and enhance the performance of CO oxidation. Acknowledgments The authors would like to acknowledge the National Natural Science Foundation of China (grant no. 21466024), the Natural Science Foundation of Inner Mongolia (grant nos. 2014MS0217 and 2015MS0209). References 1. Li J, Zhu PF, Zuo SF, Huang QQ, Zhou RX. Influence of Mn doping on the performance of CuO-CeO2 catalysts for selective oxidation of CO in hydrogen-rich streams. Appl Catal A Gen. 2010;381:261.
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Please cite this article in press as: Yan X, et al., CeO2/CuO catalysts using different template agent for preferential CO oxidation in H2-rich stream, Journal of Rare Earths (2017), http://dx.doi.org/10.1016/j.jre.2017.05.015