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CeO2 water–gas shift reaction catalyst by generating mobile electronic carriers
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 8 4 7 e1 1 8 5 2
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Promotion effect of Nb5D for Cu/CeO2 wateregas shift reaction catalyst by generating mobile electronic carriers Xingyi Lin*, Chongqi Chen, Juntao Ma, Xing Fang, Yingying Zhan, Qi Zheng National Engineering Research Center of Chemical Fertilizer Catalysts, Fuzhou University, Gongye Road 523, Fuzhou, Fujian 350002, PR China
article info
abstract
Article history:
A series of CuO/CeO2 catalysts doping with Nb2O5 were fabricated by co-precipitation
Received 18 March 2013
method. It is found that the introduction of Nb5þ will result in the substitution of Ce4þ
Received in revised form
with Nb5þ, thus creating mobile electronic carriers in the as-prepared catalysts. The
23 June 2013
characterization results correlating with the catalytic activity evaluation disclose that the
Accepted 1 July 2013
catalyst added with 1 wt. % Nb2O5 shows the most mobile electronic carries, certain
Available online 1 August 2013
amount of weak, medium basic sites and enhanced reducibility and chemical adsorption of
Keywords:
addition prevents the incorporation of Cu2þ into CeO2 lattice and partially covers the
CO, thus the best catalytic activity for wateregas shift reaction. However, excessive Nb2O5 CuO/CeO2 catalyst
surface of CuO and CeO2, resulting in weaken their reducibility and interaction between
Nb2O5
them, thus leading to inferior catalytic performance.
Wateregas shift
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Mobile electronic carriers
reserved.
Surface basicity
1.
Introduction
Hydrogen as a fuel source of energy, which makes only water without greenhouse gas emissions and other environmental pollutants, is a promising alternative to fossil fuels [1,2]. The proton exchange membrane fuel cells (PEMFCs) is supposed to be an efficient way of converting hydrogen energy to electricity; whereas, hydrogen energy from reforming of hydrocarbon often contains some degree of CO that will poison the Pt-based electrode employing in the PEMFCs system [3,4]. The wateregas shift (CO þ H2O / CO2 þ H2, WGS) reaction, which plays a role in eliminating CO and producing H2, has been receiving extensive research attentions for the fuel cell processing.
It was reported that the Cu-based catalysts exhibited outstanding catalytic performances for the WGS reaction [3e5] and undisputed with lower price comparing with the precious metal (e.g. Au, Pt) based catalysts. As one of the most important rare earth oxides, CeO2 has been extensively applied in catalysis, electrochemistry, and optics, due to its unique physical and chemical properties. Recently, CueCeO2 catalyst has also been widely studied because of their high oxygen storage capacities (OSC), strong metal-support interactions (SMSI) and rich oxygen vacancies [5]. Meanwhile, many efforts have been carried out focusing on CueCeO2 catalyst for the WGS reaction, such as investigations on the surface sensitivity of CueCeO2 [6], fabricating CueCeO2
* Corresponding author. Tel.: þ86 591 8373 1234x8112; fax: þ86 591 8373 8808. E-mail address: [email protected] (X. Lin). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.07.001
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catalyst with template method [5], and adding of promoter, like La2O3 [7] ZrO2 [8] and tungsten [9], aiming to improve the catalytic performances of CeO2-supported Cu catalysts for the WGS reaction. Niobium compounds, which are of great interests in enhancing the catalytic performance of heterogeneous catalysts, are commonly used as promoters [10e13]. The Pd/Al2O3 catalysts modified with niobium oxide showed good catalytic performances for the methane oxidation [10] and hydrodechlorination of dichloro- difluoromethane [11]. The Nb2O5WOx synthesized by Okumura et al. [12] utilizing a hydrothermal method revealing a long nano-crystalline structure, and showing high catalytic activities in the FriedeleCrafts alkylations and acylations. It was also reported that the methane conversion could be greatly enhanced by the addition of small amounts of niobium oxides to ceria [13,14]. However, to the best of our knowledge, till now, no work has been focused on investigating the niobium oxide modified CuO/CeO2 catalysts for the WGS reaction. In present study, a series of CuO/CeO2 catalysts modified with different amount of Nb2O5 were fabricated by a parallel co-precipitation method. The catalytic activity test indicates that Nb2O5 has positive effects on the performance of CuO/ CeO2 catalyst for the WGS reaction. Furthermore, the influences of Nb2O5 on the structures and catalytic performance of CuO/CeO2 catalyst are illustrated by virtue of XRD, Cyclic Voltammetry (CV), CO- and CO2-TPD techniques. It is found that the microstructures, reducibilities and surface properties of the as-synthesized catalysts are varied by the addition of Nb2O5, thus affecting their catalytic performance.
2.
Experimental
2.1.
Catalyst preparation
The niobium oxide modified CuO/CeO2 catalysts with fixed CuO content (25 wt.%) were prepared by parallel coprecipitation method. Nitrate salts of copper and cerium were dissolved in deionized water firstly; subsequently, the aqueous solution was co-precipitated with KOH aqueous solution under vigorous stirring; meanwhile, a certain amount (0, 1, 5, 10 wt.% Nb2O5 in the final samples) of ethanolic solutions of niobium pentachloride was added dropwise to the mixture. The temperature was held at 80 C, pH ¼ 10, and aged with continuous stirring for 6 h. The resulting precipitate was centrifuged and washed by deionized water for several times, dried at 110 C for 12 h and finally calcined at 650 C for 4 h (heating rate is 5 C/min) in air. The X wt.% Nb2O5 modified catalysts were denoted as Cu/CN-X (X ¼ 0, 1, 5, 10).
2.2.
Characterizations
Powder XRD patterns of the as-synthesized samples were recorded by a PANalytical X’pert Pro diffractometer equipped with CoeKa (l ¼ 0.1789 nm) radiation operating at 40 kV and 40 mA for 2q angles ranging from 25 to 75 . The cyclic voltammetric (CV) experiments were performed at ambient temperature using a CHI 660B electrochemical workstation (CH Instrument Company, China) with three-electrode cell
configuration. About 10 mg of catalysts were suspended in 2 mL of ethanol and 30 mL of Nafion solution to prepare catalyst ink. Then 25 mL of ink was transferred with an injector to glassy carbon disk electrode. The counter and reference electrodes are Pt foil and Hg/Hg2SO4/H2SO4 (0.5 M), respectively. The electrodes were cycled from þ0.8 to 0.6 V at a scan rate of 100 mV/S. TPD experiments were performed with AutoChem 2910 instrument, 50 mg of spent catalysts (after the catalytic activity test) for CO-TPD or fresh catalysts for CO2TPD were firstly pretreated with He (30 mL/min) at 300 C for 1 h and then adsorption with CO (CO-TPD) or CO2 (CO2-TPD) at Room-temperature for 30 min. After that, it was heated to desire temprature with a heating rate of 10 C/min in He (30 mL/min). A mass spectrometer (Omnistar/Pfeiffer Vacuum) was employed to monitor the effluents of CO-TPD, while the CO2-TPD desorption was monitored using a thermal conductivity detector (TCD). The catalytic activities of the catalysts for WGS reaction were tested in a fixed bed reactor at atmospheric pressure. The catalysts (20e40 mesh) was placed between two layers of quartz granules inside a stainless steel tube (i.d. ¼ 12 mm). The experiment was directly performed under a feed gas (10% CO, 60% H2, 8% CO2 and balance N2) without pre-reduction in the temperature range 200e400 C at an interval of 50 C. The ratio of vapor to feed gas was maintained at 1:1; and space velocity was kept for 4500 h1. The residual water of the outlet was removed by a condenser before entering a gas chromatograph equipped with a thermal conductivity detector (TCD). The activity was expressed by the conversion of CO, defined as: XCO (%) ¼ (1 VʹCO/VCO) 100%/(1 þ VʹCO), where VCO and VʹCO are the inlet and outlet content of CO, respectively.
3.
Results and discussion
3.1.
Crystal structure of the CuO/CeO2eNb2O5 catalysts
The XRD patterns of the series CuO/CeO2eNb2O5 catalysts are presented in Fig. 1. All the diffraction peaks are indexed to fluorite-type oxide of CeO2 (labeled with “*”, JCPDS file no. 340394) and monoclinic CuO (labeled with “#”, JCPDS file no. 050661), no diffraction peaks of niobium and/or niobia can be detected, even when the addition of Nb2O5 is up to 10 wt.%. In order to further study their micro-structure, cell parameter and crystal size are calculated, as are shown in Table 1. As clearly seen in Table 1, after the doping of niobium, the crystal structures of the series CuO/CeO2 catalysts are varied apparently. When small amount of Nb2O5 was added, for the Cu/CN-1, the crystallite sizes of CuO and CeO2 are dramatically decreased; however, when more Nb2O5 are introduced, as for the Cu/CN-5, it gives negative effects on restraining the crystallite size of CuO and CeO2, as may be ascribed to the interaction between CuO and CeO2 is disturbed and aggregation occurs; but for Cu/CN-10, the crystallite sizes of CuO and CeO2 are slightly reduced compared with the Cu/CN-5. When carefully examine the cell parameters of CeO2, it can be found that the cell parameters of Cu/CN-0 and Cu/CN-1 are almost the same, e.g. smaller cell parameters are observed for those catalysts added with more Nb2O5. The shrinkage of the cell
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 8 4 7 e1 1 8 5 2
Fig. 1 e X-ray diffraction patterns of CuO/CeO2eNb2O5 catalysts: (a) Cu/CN-0, (b) Cu/CN-1, (c) Cu/CN-5 and (d) CuCN-10.
parameters must be ascribed to the incorporation of Cu2þ to CeO2 lattice and the substitution of Nb5þ with Ce4þ. The incorporation of Cu2þ may result in the expansion of lattice parameter of CeO2 [15], while the substitution of Nb5þ with 5þ Ce4þ (r4þ Ce ¼ 0:092 nm, rNb ¼ 0:070 nm) has an opposite effect. For the Cu/CN-1 sample, the synergy effect of incorporation and substitution, resulting in the similar cell parameter of CeO2 in Cu/CN-1 to the Cu/CN-0. Whereas, cell parameter of CeO2 kept at a constant of 0.5408 in Cu/CN-5 and Cu/CN-10 samples are ascribed to: (i) more Ce4þ is substituted by Nb5þ; (ii) the incorporation of Cu2þ ions is confined by the surplus Nb2O5. Therefore, it can be concluded that introduction of appropriated amount of Nb5þ will effectively suppress the crystallite growth of CuO and CeO2, but the surplus Nb2O5 additives may restrain the incorporation of Cu2þ and its dispersion on the surface of CeO2, thus weakening the interaction between CuO and CeO2 and showing larger crystal size.
3.2. Reduction properties of the CuO/CeO2eNb2O5 catalysts Cyclic voltammetry (CV) is a dynamic electrochemical method for probing the redox property of various materials, particularly for those heterogeneous catalysts for the
Table 1 e Microstructure results of the as-synthesized catalysts. Sample
Cu/CN-0 Cu/CN-1 Cu/CN-5 Cu/CN-10
Crystal size (nm)
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oxidationereduction reactions [16e18]. Our previous work has exactly assigned the intensive anodic and cathodic peaks in cyclic voltammograms of CuO/CeO2 catalyst to the oxidation of Cu / Cu2þ and reduction of Cu2þ / Cu, respectively; moreover, the findings of CV test are well consistent with the results of H2-TPR, indicating that CV technique can be served as a useful tool in characterization of the reduction property of CuO/CeO2 catalyst [18]. In the present work, the cyclic voltammetry characterization has also been carried out, and the results are shown in Fig. 2A. It is evidence that anodic and cathodic peaks of CuO/ CeO2eNb2O5 catalysts are dramatically changed with the variation of doping content of Nb2O5. For the Cu/CN-1 catalyst, it shows the largest redox peaks in the series of catalysts, and the peak areas of anodic and cathodic peaks are almost symmetrical, suggesting that the reduction and oxidation of Cu spices in Cu/CN-1 is reversible, as may result in high catalytic stability for the as-obtained catalyst. However, the redox peaks become smaller when more Nb2O5 was added, ranking as: CuO/CN-1 > CuO/CN-5 > CuO/CN-10 > Cu/CN-0, as suggests that the amounts of Cu2þ that can be easily reduced in the as-prepared catalysts also follow the sequence. When coming to Fig. 2B, the peak area and location of peak b are obviously distinguish from each other. For the undoped one, the peak b is ascribed to the reduction of moderate copper oxide (crystalline) interacted with ceria via surface oxygen vacancies, and it is thought to be the most effective CuO for the WGS reaction [15]. As for those doping with Nb5þ ions, which is a kind of cations with high chemical valence, it can increase the concentration of mobile electronic carriers and fill the oxygen vacancies compares to undoped ceria [12]:
Nb2 O5 þ VO ¼ 2NbCe þ 5OxO
(1)
1 Nb2 O5 ¼ 2NbCe þ 4OxO þ O2 þ 2e 2
(2)
Then, a question comes about, among all the three samples, why the Cu/CN-1 possesses the largest peak area of peak b? Based on the above XRD analysis, when the addition of Nb2O5 is more than 1 wt. %, the incorporation of Cu2þ ions is confined, thus resulting in the decrease of mobile electronic carriers to attain neutral chemical valence. Therefore, in this case, the peak b should be assigned to the reduction of moderate copper oxide (crystalline) interacted with ceria via mobile electronic carriers rather than with the surface oxygen vacancies. Moreover, it can also seen in Fig. 2B, for the peak g of Cu/CN-5 shifts to high reduction temperature, as is due to the surplus Nb2O5 partially covers on the surface of copper and/or cerium crystallites; for Cu/CN-10, the reduction peak g is almost symmetrical, indicating that there is almost no mobile electronic carriers in Cu/ CN-10, and the higher reduction temperature is ascribed to the aggregated CuO and CeO2 [18,19].
Cell parameter
CeO2
CuO
aCeO2 (nm)
13.2 8.9 9.3 8.2
32.7 17.2 19.6 18.1
0.5419 0.5419 0.5408 0.5408
3.3. CO-adsorption properties of the CuO/CeO2eNb2O5 catalysts For the WGS reaction, CO adsorption is a crucial step during the reaction processes, so CO-TPD technique was employed. The TPD profiles of CO and CO2 after CO adsorption over the
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Fig. 2 e Cyclic voltammograms (A) and TPR profiles (B) of the CuO/CeO2eNb2O5 catalysts: (a) Cu/CN-0, (b) Cu/CN-1, (c) Cu/CN5, (d) Cu/CN-10.
spent catalysts are presented in Fig. 3A and B. Except for CO and CO2, no other gas was detected in the effluent during the CO-TPD characterization. CO physically adsorbed on catalyst will desorb as molecular CO [20,21], as is shown in Fig. 3A. The desorption temperatures are 54 C, 60 C, 61 C and 63 C for CCN-0, CCN-1, CCN-5 and CCN-10 catalysts, respectively. And the areas of CO-desorption peaks become smaller when increasing the Nb-doping amount. The rest of adsorbed CO would interact with surface hydroxyl groups and/or oxygen species (including surface oxygen, lattice oxygen, interfacial oxygen, etc.) of the as-prepared catalysts, and desorbed as CO2 during the heating process [20e23], as is considered as chemical bonded CO, shown in Fig. 3B. It is interesting to find that unlike the CO-desorption curves, the peak areas in CO2-desorption profiles are extremely distinguish from each other. With a tail extended to 250 C, the only desorption peak (located at 127 C) of CO2 for Cu/CN-0 is slightly asymmetric; the CO2 desorption profile of Cu/CN-1 consists of three peaks at about 90 C, 158 C and 231 C; the CO2 desorption peak areas decrease with the further addition of Nb2O5. Although the additive of Nb2O5 has side effect on the CO desorption ability, the CO2 desorption is promoted by adding of Nb2O5. As seen in Fig. 3, the Cu/CN-1 catalyst has the best adsorption capability of CO, i.e. the largest peak areas of CO-
plus CO2-desportion, in the series of catalysts, while the CO adsorption capabilities of those as-obtained catalysts decrease with a further addition of Nb2O5. There is a close relation between reducibility and CO chemical adsorption of the Cu-based catalysts. The CO adsorption site of CuO/CeO2 catalyst is found to be the finely dispersed Cu-species [24]. The adsorbed CO may interact with Cu-ions and form instable Cu(þx)-carbonyl species (Cu(þx)eC]O), which are easily decomposed to generate Cuþ(x1) (e.g. Cuþ1 and Cu0) and CO2 with elevating temperatures [22,23]. The CuO/CeO2 catalyst with stronger reducibility also will lead to CO2 desorption at lower temperature [20e24]. Cu/CN-1 presents the larger desorption peak area of CO2 and lower desorption temperature indicate that it has the greatest reducibility in the series of CuO/CeO2 catalyst, as is correlated well with the above CV and TPR analysis.
3.4. Surface basic properties of the CuO/CeO2eNb2O5 catalysts CO2 is extensively used as probe molecule for the investigation of the basic property of catalyst [21,25e27]. The adsorbed CO2 would desorb at different temperatures during the TPD process based on the interaction intensity between CO2 and different types of basic sites of the catalyst. The temperature
Fig. 3 e CO-TPD profiles of CuO/CeO2eNb2O5 catalysts: (A) desorption profiles of CO after CO adsorption; (B) desorption profiles of CO2 after CO adsorption. (a) Cu/CN-0, (b) Cu/CN-1, (c) Cu/CN-5 and (d) CuCN-10.
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ranges of 100e250, 250e400, and 400e650 C in CO2-TPD curves are responsible for the weak, medium and strong basic sites for the binary and ternary CeO2-based catalysts [25]. Fig. 4 presents the CO2-TPD analysis results of the CuO/ CeO2eNb2O5 catalysts. It is evident from Fig. 4 that the basic properties of CuO/ CeO2 catalysts are influenced by the introduction of Nb2O5. It is well known that Nb2O5 is a kind of acidic oxide, by adding of Nb2O5, the peak areas of CO2-TPD profiles become smaller. The one added with 1 wt. % Nb2O5 (Cu/CN-1) show large peak area in the temperature range of 100e350 C, which is assigned to the weak, medium basic sites. For the strong basic sites, which located at 400e650 C, the adsorbed CO2 would form unidentate carbonates with low-coordination O2 ions [26]. The unidentate carbonates are uneasily degraded at temperatures below 400 C, thus the catalytic active sites of WGS catalyst would be hindered when the concentration of such basic sites are relatively high [21,26,27].
3.5. Catalytic performance of the CuO/CeO2eNb2O5 catalysts over WGS reaction The catalytic performances of the series CuO/CeO2 catalysts modified with different content of Nb2O5 v.s. temperature are presented in Fig. 5. The catalytic activity of Cu/CN-0.5 is also shown for comparison. From Fig. 5, it can be easily found that the performance of CuO/CeO2 catalyst was improved by adding Nb2O5. Cu/CN-1 catalyst demonstrates the greatest improvements in the activities during the temperature range, but the improvements decrease with the further increase of niobium content. The CO conversions of Cu/CN-0.5, 1, 5 and 10 catalysts reach the maximum value (89.8%, 95.3%, 93.8% and 90.7%, respectively) at 250 C. However, the CO conversion of the unmodified CuO/CeO2 catalyst is only 83.5% at 250 C and reaches its maximum 88.1% at 350 C. The results indicate that the niobium doping not only enhances
Fig. 5 e WGS activity of CuO/CeO2eNb2O5 catalysts (-) CCN-0, (>) CCN-0.5, (:) CCN-1, (C) CCN-5, (;) CCN-10.
the CO conversion but also shifts the optimal reaction temperature to lower range. Base on the above characterization results, the improvements in catalytic performance of Cu/CN catalyst are ascribed to the substitution of Nb5þ with Ce4þ, thus resulting in the variation of crystal structure, reduction property and surface properties of the CuO/CeO2eNb2O5 catalysts. For the Cu/CN-1 catalyst, mobile electronic carries are induced by the incorporation of Cu2þ and substitution of Nb5þ with Ce4þ, favoring to obtain CuO and CeO2 with small crystal size and large lattice strain; at the same time, the reducibility and chemical adsorption CO are also enhanced; moreover, the catalyst possesses certain amount of weak, medium basic sites, as is figured out elsewhere, the temperate basicity of catalyst benefits the catalyst applied for WGS reaction. Therefore, the Cu/CN-1 catalyst has the best catalytic performance with respect to the WGS reaction. For the Cu/CN-5 and Cu/CN-10 catalysts, the excessive Nb2O5 covers the surface of CuO and CeO2, resulting in lesser amount of Cu2þ incorporated in the CeO2 lattice and weaken the reducibility and interaction between them, thus leading to inferior catalytic performance. More details about the doping effect of Nb5þ in the CuO/CeO2 catalyst are being carried out.
4.
Fig. 4 e CO2-TPD profiles of CuO/CeO2eNb2O5 catalysts: (a) Cu/CN-0, (b) Cu/CN-1, (c) Cu/CN-5 and (d) CuCN-10.
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Conclusions
A series of CuO/CeO2 catalysts doped with Nb2O5 were successfully fabricated by co-precipitaiton method. The crystal structure, reduction property and surface property of the asprepared can be modulated by the introduction of niobium. The Cu/CN-1 catalyst exhibits the best catalytic activity due to its largest amount of mobile electronic carriers and COadsorption capacity, as well as the smallest CuO and CeO2 particles, greatest reducibility and appropriate amount of weak and medium basic sites. The surplus Nb-additives in Cu/ CN-5 and Cu/CN-10 catalyst disturbs the interactions between CuO and CeO2 and leads to lower catalytic activity.
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Acknowledgements The authors acknowledge the financial support from the Department of Science of the People’s Republic of China (20771025) and the Technology Development Program of Fuzhou University (2011-XY-5).
references
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[12] Okumura K, Tomiyama T, Shirakawa S, Ishida S, Sanada T, Arao M, et-al. J Mater Chem 2011;21:229e35. [13] Ram0 ırez-Cabrera E, Laosiripojana N, Atkinson A, Chadwick D. Catal Today 2003;78:433e8. [14] Ram0 ırez-Cabrera E, Atkinson A, Chadwick D. Appl Catal B Environ 2002;36:193e206. [15] Li L, Zhan Y, Zheng Q, Zheng Y, Chen C, She Y, et-al. Catal Lett 2009;130:532. [16] Rusling JF, Suib SL. Adv Mater 1994;6:922e30. [17] Zhu J, Zhao Z, Xiao D, Li J, Yang X, Wu Y. Electrochem Commun 2005;7:58e61. [18] Li L, Zhan Y, Zheng Q, Zheng Y, Lin X, Li D, et-al. Catal Lett 2007;118:91e7. [19] Papavasiliou J, Avgouropoulos G, Ioannides T. Appl Catal B: Environ 2007;69:226e34. [20] Nielson FPR, Mariana MVMS, Martin S. J Power Sources 2008;179:329e34. [21] Avgouropoulos G, Ioannides T. Catal Lett 2007;116:15e22. [22] Martinez-Arias A, Ferna´ndez-Garcia M, Galvez O, Coronado JM, Anderson JA, Conesa JC, et-al. J Catal 2000;195:207e16. [23] Avgouropoulos G, Ioannides T. J Mol Catal A: Chem 2008;296:47e53. [24] Luo MF, Zhong YJ, Yuan XX, Zheng XM. Appl Catal A: Gen 1997;162:121e7. [25] Istadi Amin NAS. J Mol Catal A: Chem 2006;259:61e6. [26] Zhu XL, Shen M, Lobban LL, Mallinson RG. J Catal 2011;278:123e32. [27] Guo Y, Azmat MU, Liu XH, Wang YQ, Lu GZ. Appl Energy 2012;92:218e23.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Promotion effect of Nb5D for Cu/CeO2 wateregas shift reaction catalyst by generating mobile electronic carriers Xingyi Lin*, Chongqi Chen, Juntao Ma, Xing Fang, Yingying Zhan, Qi Zheng National Engineering Research Center of Chemical Fertilizer Catalysts, Fuzhou University, Gongye Road 523, Fuzhou, Fujian 350002, PR China
article info
abstract
Article history:
A series of CuO/CeO2 catalysts doping with Nb2O5 were fabricated by co-precipitation
Received 18 March 2013
method. It is found that the introduction of Nb5þ will result in the substitution of Ce4þ
Received in revised form
with Nb5þ, thus creating mobile electronic carriers in the as-prepared catalysts. The
23 June 2013
characterization results correlating with the catalytic activity evaluation disclose that the
Accepted 1 July 2013
catalyst added with 1 wt. % Nb2O5 shows the most mobile electronic carries, certain
Available online 1 August 2013
amount of weak, medium basic sites and enhanced reducibility and chemical adsorption of
Keywords:
addition prevents the incorporation of Cu2þ into CeO2 lattice and partially covers the
CO, thus the best catalytic activity for wateregas shift reaction. However, excessive Nb2O5 CuO/CeO2 catalyst
surface of CuO and CeO2, resulting in weaken their reducibility and interaction between
Nb2O5
them, thus leading to inferior catalytic performance.
Wateregas shift
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Mobile electronic carriers
reserved.
Surface basicity
1.
Introduction
Hydrogen as a fuel source of energy, which makes only water without greenhouse gas emissions and other environmental pollutants, is a promising alternative to fossil fuels [1,2]. The proton exchange membrane fuel cells (PEMFCs) is supposed to be an efficient way of converting hydrogen energy to electricity; whereas, hydrogen energy from reforming of hydrocarbon often contains some degree of CO that will poison the Pt-based electrode employing in the PEMFCs system [3,4]. The wateregas shift (CO þ H2O / CO2 þ H2, WGS) reaction, which plays a role in eliminating CO and producing H2, has been receiving extensive research attentions for the fuel cell processing.
It was reported that the Cu-based catalysts exhibited outstanding catalytic performances for the WGS reaction [3e5] and undisputed with lower price comparing with the precious metal (e.g. Au, Pt) based catalysts. As one of the most important rare earth oxides, CeO2 has been extensively applied in catalysis, electrochemistry, and optics, due to its unique physical and chemical properties. Recently, CueCeO2 catalyst has also been widely studied because of their high oxygen storage capacities (OSC), strong metal-support interactions (SMSI) and rich oxygen vacancies [5]. Meanwhile, many efforts have been carried out focusing on CueCeO2 catalyst for the WGS reaction, such as investigations on the surface sensitivity of CueCeO2 [6], fabricating CueCeO2
* Corresponding author. Tel.: þ86 591 8373 1234x8112; fax: þ86 591 8373 8808. E-mail address: [email protected] (X. Lin). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.07.001
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catalyst with template method [5], and adding of promoter, like La2O3 [7] ZrO2 [8] and tungsten [9], aiming to improve the catalytic performances of CeO2-supported Cu catalysts for the WGS reaction. Niobium compounds, which are of great interests in enhancing the catalytic performance of heterogeneous catalysts, are commonly used as promoters [10e13]. The Pd/Al2O3 catalysts modified with niobium oxide showed good catalytic performances for the methane oxidation [10] and hydrodechlorination of dichloro- difluoromethane [11]. The Nb2O5WOx synthesized by Okumura et al. [12] utilizing a hydrothermal method revealing a long nano-crystalline structure, and showing high catalytic activities in the FriedeleCrafts alkylations and acylations. It was also reported that the methane conversion could be greatly enhanced by the addition of small amounts of niobium oxides to ceria [13,14]. However, to the best of our knowledge, till now, no work has been focused on investigating the niobium oxide modified CuO/CeO2 catalysts for the WGS reaction. In present study, a series of CuO/CeO2 catalysts modified with different amount of Nb2O5 were fabricated by a parallel co-precipitation method. The catalytic activity test indicates that Nb2O5 has positive effects on the performance of CuO/ CeO2 catalyst for the WGS reaction. Furthermore, the influences of Nb2O5 on the structures and catalytic performance of CuO/CeO2 catalyst are illustrated by virtue of XRD, Cyclic Voltammetry (CV), CO- and CO2-TPD techniques. It is found that the microstructures, reducibilities and surface properties of the as-synthesized catalysts are varied by the addition of Nb2O5, thus affecting their catalytic performance.
2.
Experimental
2.1.
Catalyst preparation
The niobium oxide modified CuO/CeO2 catalysts with fixed CuO content (25 wt.%) were prepared by parallel coprecipitation method. Nitrate salts of copper and cerium were dissolved in deionized water firstly; subsequently, the aqueous solution was co-precipitated with KOH aqueous solution under vigorous stirring; meanwhile, a certain amount (0, 1, 5, 10 wt.% Nb2O5 in the final samples) of ethanolic solutions of niobium pentachloride was added dropwise to the mixture. The temperature was held at 80 C, pH ¼ 10, and aged with continuous stirring for 6 h. The resulting precipitate was centrifuged and washed by deionized water for several times, dried at 110 C for 12 h and finally calcined at 650 C for 4 h (heating rate is 5 C/min) in air. The X wt.% Nb2O5 modified catalysts were denoted as Cu/CN-X (X ¼ 0, 1, 5, 10).
2.2.
Characterizations
Powder XRD patterns of the as-synthesized samples were recorded by a PANalytical X’pert Pro diffractometer equipped with CoeKa (l ¼ 0.1789 nm) radiation operating at 40 kV and 40 mA for 2q angles ranging from 25 to 75 . The cyclic voltammetric (CV) experiments were performed at ambient temperature using a CHI 660B electrochemical workstation (CH Instrument Company, China) with three-electrode cell
configuration. About 10 mg of catalysts were suspended in 2 mL of ethanol and 30 mL of Nafion solution to prepare catalyst ink. Then 25 mL of ink was transferred with an injector to glassy carbon disk electrode. The counter and reference electrodes are Pt foil and Hg/Hg2SO4/H2SO4 (0.5 M), respectively. The electrodes were cycled from þ0.8 to 0.6 V at a scan rate of 100 mV/S. TPD experiments were performed with AutoChem 2910 instrument, 50 mg of spent catalysts (after the catalytic activity test) for CO-TPD or fresh catalysts for CO2TPD were firstly pretreated with He (30 mL/min) at 300 C for 1 h and then adsorption with CO (CO-TPD) or CO2 (CO2-TPD) at Room-temperature for 30 min. After that, it was heated to desire temprature with a heating rate of 10 C/min in He (30 mL/min). A mass spectrometer (Omnistar/Pfeiffer Vacuum) was employed to monitor the effluents of CO-TPD, while the CO2-TPD desorption was monitored using a thermal conductivity detector (TCD). The catalytic activities of the catalysts for WGS reaction were tested in a fixed bed reactor at atmospheric pressure. The catalysts (20e40 mesh) was placed between two layers of quartz granules inside a stainless steel tube (i.d. ¼ 12 mm). The experiment was directly performed under a feed gas (10% CO, 60% H2, 8% CO2 and balance N2) without pre-reduction in the temperature range 200e400 C at an interval of 50 C. The ratio of vapor to feed gas was maintained at 1:1; and space velocity was kept for 4500 h1. The residual water of the outlet was removed by a condenser before entering a gas chromatograph equipped with a thermal conductivity detector (TCD). The activity was expressed by the conversion of CO, defined as: XCO (%) ¼ (1 VʹCO/VCO) 100%/(1 þ VʹCO), where VCO and VʹCO are the inlet and outlet content of CO, respectively.
3.
Results and discussion
3.1.
Crystal structure of the CuO/CeO2eNb2O5 catalysts
The XRD patterns of the series CuO/CeO2eNb2O5 catalysts are presented in Fig. 1. All the diffraction peaks are indexed to fluorite-type oxide of CeO2 (labeled with “*”, JCPDS file no. 340394) and monoclinic CuO (labeled with “#”, JCPDS file no. 050661), no diffraction peaks of niobium and/or niobia can be detected, even when the addition of Nb2O5 is up to 10 wt.%. In order to further study their micro-structure, cell parameter and crystal size are calculated, as are shown in Table 1. As clearly seen in Table 1, after the doping of niobium, the crystal structures of the series CuO/CeO2 catalysts are varied apparently. When small amount of Nb2O5 was added, for the Cu/CN-1, the crystallite sizes of CuO and CeO2 are dramatically decreased; however, when more Nb2O5 are introduced, as for the Cu/CN-5, it gives negative effects on restraining the crystallite size of CuO and CeO2, as may be ascribed to the interaction between CuO and CeO2 is disturbed and aggregation occurs; but for Cu/CN-10, the crystallite sizes of CuO and CeO2 are slightly reduced compared with the Cu/CN-5. When carefully examine the cell parameters of CeO2, it can be found that the cell parameters of Cu/CN-0 and Cu/CN-1 are almost the same, e.g. smaller cell parameters are observed for those catalysts added with more Nb2O5. The shrinkage of the cell
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 8 4 7 e1 1 8 5 2
Fig. 1 e X-ray diffraction patterns of CuO/CeO2eNb2O5 catalysts: (a) Cu/CN-0, (b) Cu/CN-1, (c) Cu/CN-5 and (d) CuCN-10.
parameters must be ascribed to the incorporation of Cu2þ to CeO2 lattice and the substitution of Nb5þ with Ce4þ. The incorporation of Cu2þ may result in the expansion of lattice parameter of CeO2 [15], while the substitution of Nb5þ with 5þ Ce4þ (r4þ Ce ¼ 0:092 nm, rNb ¼ 0:070 nm) has an opposite effect. For the Cu/CN-1 sample, the synergy effect of incorporation and substitution, resulting in the similar cell parameter of CeO2 in Cu/CN-1 to the Cu/CN-0. Whereas, cell parameter of CeO2 kept at a constant of 0.5408 in Cu/CN-5 and Cu/CN-10 samples are ascribed to: (i) more Ce4þ is substituted by Nb5þ; (ii) the incorporation of Cu2þ ions is confined by the surplus Nb2O5. Therefore, it can be concluded that introduction of appropriated amount of Nb5þ will effectively suppress the crystallite growth of CuO and CeO2, but the surplus Nb2O5 additives may restrain the incorporation of Cu2þ and its dispersion on the surface of CeO2, thus weakening the interaction between CuO and CeO2 and showing larger crystal size.
3.2. Reduction properties of the CuO/CeO2eNb2O5 catalysts Cyclic voltammetry (CV) is a dynamic electrochemical method for probing the redox property of various materials, particularly for those heterogeneous catalysts for the
Table 1 e Microstructure results of the as-synthesized catalysts. Sample
Cu/CN-0 Cu/CN-1 Cu/CN-5 Cu/CN-10
Crystal size (nm)
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oxidationereduction reactions [16e18]. Our previous work has exactly assigned the intensive anodic and cathodic peaks in cyclic voltammograms of CuO/CeO2 catalyst to the oxidation of Cu / Cu2þ and reduction of Cu2þ / Cu, respectively; moreover, the findings of CV test are well consistent with the results of H2-TPR, indicating that CV technique can be served as a useful tool in characterization of the reduction property of CuO/CeO2 catalyst [18]. In the present work, the cyclic voltammetry characterization has also been carried out, and the results are shown in Fig. 2A. It is evidence that anodic and cathodic peaks of CuO/ CeO2eNb2O5 catalysts are dramatically changed with the variation of doping content of Nb2O5. For the Cu/CN-1 catalyst, it shows the largest redox peaks in the series of catalysts, and the peak areas of anodic and cathodic peaks are almost symmetrical, suggesting that the reduction and oxidation of Cu spices in Cu/CN-1 is reversible, as may result in high catalytic stability for the as-obtained catalyst. However, the redox peaks become smaller when more Nb2O5 was added, ranking as: CuO/CN-1 > CuO/CN-5 > CuO/CN-10 > Cu/CN-0, as suggests that the amounts of Cu2þ that can be easily reduced in the as-prepared catalysts also follow the sequence. When coming to Fig. 2B, the peak area and location of peak b are obviously distinguish from each other. For the undoped one, the peak b is ascribed to the reduction of moderate copper oxide (crystalline) interacted with ceria via surface oxygen vacancies, and it is thought to be the most effective CuO for the WGS reaction [15]. As for those doping with Nb5þ ions, which is a kind of cations with high chemical valence, it can increase the concentration of mobile electronic carriers and fill the oxygen vacancies compares to undoped ceria [12]:
Nb2 O5 þ VO ¼ 2NbCe þ 5OxO
(1)
1 Nb2 O5 ¼ 2NbCe þ 4OxO þ O2 þ 2e 2
(2)
Then, a question comes about, among all the three samples, why the Cu/CN-1 possesses the largest peak area of peak b? Based on the above XRD analysis, when the addition of Nb2O5 is more than 1 wt. %, the incorporation of Cu2þ ions is confined, thus resulting in the decrease of mobile electronic carriers to attain neutral chemical valence. Therefore, in this case, the peak b should be assigned to the reduction of moderate copper oxide (crystalline) interacted with ceria via mobile electronic carriers rather than with the surface oxygen vacancies. Moreover, it can also seen in Fig. 2B, for the peak g of Cu/CN-5 shifts to high reduction temperature, as is due to the surplus Nb2O5 partially covers on the surface of copper and/or cerium crystallites; for Cu/CN-10, the reduction peak g is almost symmetrical, indicating that there is almost no mobile electronic carriers in Cu/ CN-10, and the higher reduction temperature is ascribed to the aggregated CuO and CeO2 [18,19].
Cell parameter
CeO2
CuO
aCeO2 (nm)
13.2 8.9 9.3 8.2
32.7 17.2 19.6 18.1
0.5419 0.5419 0.5408 0.5408
3.3. CO-adsorption properties of the CuO/CeO2eNb2O5 catalysts For the WGS reaction, CO adsorption is a crucial step during the reaction processes, so CO-TPD technique was employed. The TPD profiles of CO and CO2 after CO adsorption over the
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Fig. 2 e Cyclic voltammograms (A) and TPR profiles (B) of the CuO/CeO2eNb2O5 catalysts: (a) Cu/CN-0, (b) Cu/CN-1, (c) Cu/CN5, (d) Cu/CN-10.
spent catalysts are presented in Fig. 3A and B. Except for CO and CO2, no other gas was detected in the effluent during the CO-TPD characterization. CO physically adsorbed on catalyst will desorb as molecular CO [20,21], as is shown in Fig. 3A. The desorption temperatures are 54 C, 60 C, 61 C and 63 C for CCN-0, CCN-1, CCN-5 and CCN-10 catalysts, respectively. And the areas of CO-desorption peaks become smaller when increasing the Nb-doping amount. The rest of adsorbed CO would interact with surface hydroxyl groups and/or oxygen species (including surface oxygen, lattice oxygen, interfacial oxygen, etc.) of the as-prepared catalysts, and desorbed as CO2 during the heating process [20e23], as is considered as chemical bonded CO, shown in Fig. 3B. It is interesting to find that unlike the CO-desorption curves, the peak areas in CO2-desorption profiles are extremely distinguish from each other. With a tail extended to 250 C, the only desorption peak (located at 127 C) of CO2 for Cu/CN-0 is slightly asymmetric; the CO2 desorption profile of Cu/CN-1 consists of three peaks at about 90 C, 158 C and 231 C; the CO2 desorption peak areas decrease with the further addition of Nb2O5. Although the additive of Nb2O5 has side effect on the CO desorption ability, the CO2 desorption is promoted by adding of Nb2O5. As seen in Fig. 3, the Cu/CN-1 catalyst has the best adsorption capability of CO, i.e. the largest peak areas of CO-
plus CO2-desportion, in the series of catalysts, while the CO adsorption capabilities of those as-obtained catalysts decrease with a further addition of Nb2O5. There is a close relation between reducibility and CO chemical adsorption of the Cu-based catalysts. The CO adsorption site of CuO/CeO2 catalyst is found to be the finely dispersed Cu-species [24]. The adsorbed CO may interact with Cu-ions and form instable Cu(þx)-carbonyl species (Cu(þx)eC]O), which are easily decomposed to generate Cuþ(x1) (e.g. Cuþ1 and Cu0) and CO2 with elevating temperatures [22,23]. The CuO/CeO2 catalyst with stronger reducibility also will lead to CO2 desorption at lower temperature [20e24]. Cu/CN-1 presents the larger desorption peak area of CO2 and lower desorption temperature indicate that it has the greatest reducibility in the series of CuO/CeO2 catalyst, as is correlated well with the above CV and TPR analysis.
3.4. Surface basic properties of the CuO/CeO2eNb2O5 catalysts CO2 is extensively used as probe molecule for the investigation of the basic property of catalyst [21,25e27]. The adsorbed CO2 would desorb at different temperatures during the TPD process based on the interaction intensity between CO2 and different types of basic sites of the catalyst. The temperature
Fig. 3 e CO-TPD profiles of CuO/CeO2eNb2O5 catalysts: (A) desorption profiles of CO after CO adsorption; (B) desorption profiles of CO2 after CO adsorption. (a) Cu/CN-0, (b) Cu/CN-1, (c) Cu/CN-5 and (d) CuCN-10.
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ranges of 100e250, 250e400, and 400e650 C in CO2-TPD curves are responsible for the weak, medium and strong basic sites for the binary and ternary CeO2-based catalysts [25]. Fig. 4 presents the CO2-TPD analysis results of the CuO/ CeO2eNb2O5 catalysts. It is evident from Fig. 4 that the basic properties of CuO/ CeO2 catalysts are influenced by the introduction of Nb2O5. It is well known that Nb2O5 is a kind of acidic oxide, by adding of Nb2O5, the peak areas of CO2-TPD profiles become smaller. The one added with 1 wt. % Nb2O5 (Cu/CN-1) show large peak area in the temperature range of 100e350 C, which is assigned to the weak, medium basic sites. For the strong basic sites, which located at 400e650 C, the adsorbed CO2 would form unidentate carbonates with low-coordination O2 ions [26]. The unidentate carbonates are uneasily degraded at temperatures below 400 C, thus the catalytic active sites of WGS catalyst would be hindered when the concentration of such basic sites are relatively high [21,26,27].
3.5. Catalytic performance of the CuO/CeO2eNb2O5 catalysts over WGS reaction The catalytic performances of the series CuO/CeO2 catalysts modified with different content of Nb2O5 v.s. temperature are presented in Fig. 5. The catalytic activity of Cu/CN-0.5 is also shown for comparison. From Fig. 5, it can be easily found that the performance of CuO/CeO2 catalyst was improved by adding Nb2O5. Cu/CN-1 catalyst demonstrates the greatest improvements in the activities during the temperature range, but the improvements decrease with the further increase of niobium content. The CO conversions of Cu/CN-0.5, 1, 5 and 10 catalysts reach the maximum value (89.8%, 95.3%, 93.8% and 90.7%, respectively) at 250 C. However, the CO conversion of the unmodified CuO/CeO2 catalyst is only 83.5% at 250 C and reaches its maximum 88.1% at 350 C. The results indicate that the niobium doping not only enhances
Fig. 5 e WGS activity of CuO/CeO2eNb2O5 catalysts (-) CCN-0, (>) CCN-0.5, (:) CCN-1, (C) CCN-5, (;) CCN-10.
the CO conversion but also shifts the optimal reaction temperature to lower range. Base on the above characterization results, the improvements in catalytic performance of Cu/CN catalyst are ascribed to the substitution of Nb5þ with Ce4þ, thus resulting in the variation of crystal structure, reduction property and surface properties of the CuO/CeO2eNb2O5 catalysts. For the Cu/CN-1 catalyst, mobile electronic carries are induced by the incorporation of Cu2þ and substitution of Nb5þ with Ce4þ, favoring to obtain CuO and CeO2 with small crystal size and large lattice strain; at the same time, the reducibility and chemical adsorption CO are also enhanced; moreover, the catalyst possesses certain amount of weak, medium basic sites, as is figured out elsewhere, the temperate basicity of catalyst benefits the catalyst applied for WGS reaction. Therefore, the Cu/CN-1 catalyst has the best catalytic performance with respect to the WGS reaction. For the Cu/CN-5 and Cu/CN-10 catalysts, the excessive Nb2O5 covers the surface of CuO and CeO2, resulting in lesser amount of Cu2þ incorporated in the CeO2 lattice and weaken the reducibility and interaction between them, thus leading to inferior catalytic performance. More details about the doping effect of Nb5þ in the CuO/CeO2 catalyst are being carried out.
4.
Fig. 4 e CO2-TPD profiles of CuO/CeO2eNb2O5 catalysts: (a) Cu/CN-0, (b) Cu/CN-1, (c) Cu/CN-5 and (d) CuCN-10.
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Conclusions
A series of CuO/CeO2 catalysts doped with Nb2O5 were successfully fabricated by co-precipitaiton method. The crystal structure, reduction property and surface property of the asprepared can be modulated by the introduction of niobium. The Cu/CN-1 catalyst exhibits the best catalytic activity due to its largest amount of mobile electronic carriers and COadsorption capacity, as well as the smallest CuO and CeO2 particles, greatest reducibility and appropriate amount of weak and medium basic sites. The surplus Nb-additives in Cu/ CN-5 and Cu/CN-10 catalyst disturbs the interactions between CuO and CeO2 and leads to lower catalytic activity.
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Acknowledgements The authors acknowledge the financial support from the Department of Science of the People’s Republic of China (20771025) and the Technology Development Program of Fuzhou University (2011-XY-5).
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