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TixSn1−xO2 catalysts for low-temperature carbon monoxide oxidation
Catalysis Communications 9 (2008) 2131–2135
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The preparation and catalytic behavior of CuO/TixSn1 xO2 catalysts for low-temperature carbon monoxide oxidation Jiang Huang, Shurong Wang, Xianzhi Guo, Da Wang, Baolin Zhu, Shihua Wu * Department of Chemistry, NanKai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 29 February 2008 Received in revised form 17 April 2008 Accepted 19 April 2008 Available online 25 April 2008 Keywords: TixSn1 xO2 CuO/TixSn1 xO2 catalysts Low-temperature CO oxidation
a b s t r a c t A series of TixSn1 xO2 (x = 1, 0.7, 0.6, 0.5, 0.3, 0) powders were synthesized by co-precipitation method and the 8 wt.% CuO/TixSn1 xO2 catalysts were prepared via deposition–precipitation method. The samples were characterized by TG-DTA, XRD, XPS and TPR techniques. The influences of the proportion of TiO2 and SnO2 and calcination temperature on the catalytic activities of the catalysts were investigated. The CuO/Ti0.6Sn0.4O2 catalyst calcined at 300 °C for 3 h showed the best catalytic performance. TPR analysis indicated that the addition of SnO2 could improve the reducibility of the catalysts. The XPS measurement showed that the copper species were mainly dispersed as Cu2+ ions. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The serious environmental implication of CO emission from automotive sources has stimulated a huge amount of pure and applied researches directed at catalytic abatement of such emission. TiO2 used as a catalyst support has been received great attention because the catalyst supported on TiO2 presents a relatively high catalytic activity, and TiO2 is stable, inexpensive and innocuous [1–5]. Nevertheless, the increment of the catalytic efficiency of catalysts remains as an important goal. In order to further improve the performance of TiO2 as a catalyst support, several structural and chemical modifications of this material have been assayed. In this way, significant increments of CO oxidation rate have been attained using either porous TiO2 materials, or by the incorporation of noble metals into the TiO2 lattice [6–9]. On the other hand, mixed oxides had received great attention in recent years. It has been realized that such systems may benefit from the combination of the best sensing properties of their pure components. The growing number of papers [10–15] reporting on the successful application of these oxides in practical works shows the important role they play in various research fields. Among them, TixSn1 xO2 system has recently emerged as attractive materials for gas sensor and low-voltage varistor application [16–20]. Zakrzewska [16] reported that the operating temperature for hydrogen sensor was lowered to about 500 °C by addition of small amount of SnO2 to TiO2 lattice. Radecka’s work [17,18] had showed that the response of SnO2 sensors containing small additions of Ti was comparable with that of undoped SnO2, however the long* Corresponding author. Tel.: +86 22 23505896; fax: +86 22 23502458. E-mail address: [email protected] (S. Wu). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.04.011
term stability was improved markedly. In addition, the use of TixSn1 xO2 system in environmental catalysis has also been examined because of it is high quantum yield and good photocatalytic activity. Lin [21] found that small Sn substitution for Ti in rutile TiO2 increased the photoactivity of TiO2 by 15 times for the oxidation of acetone. Compared to the experimental investigations, few systemic works have been reported on the studies of TixSn1 xO2 system used as a catalyst support for CO oxidation. Sermon and Walton have reported that Pt/TiO2–SnO2 (at higher TiO2 contents) appeared to be a good catalyst for CO oxidation [22]. However, the optimum TiO2:SnO2 balance for the ideal CO sensors was not assessed. Our previous work [26] has shown that the CuO/TiO2 catalyst with 8 wt.% CuO loading exhibited higher catalytic activity than that of the catalysts with other CuO loadings. In this paper, a series of TixSn1 xO2 with different Ti/Sn ratio were prepared by co-precipitation method. The 8 wt.% CuO/TixSn1 xO2 catalysts were prepared by deposition–precipitation (DP) method. The samples have been characterized by TG-DTA, XRD, XPS and TPR. The catalytic activity in induction of CO oxidation was determined by a microreactorGC system. For comparison, pure TiO2 and SnO2 were also prepared by the same method, and the catalytic performance of CuO/TiO2 and CuO/SnO2 was also studied.
2. Experimental 2.1. TixSn1 xO2 preparation TixSn1 xO2 (x = 0, 0.2, 0.3, 0.4, 0.5, 0.7, 1) powders were prepared by the co-precipitation method. The mixed solution of Ti(SO4)2 and
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SnCl4 5H2O together with the solution of ammonia were simultaneously dropped into a buffer solution (pH 8) under vigorous agitation, keeping the pH value of the solution at 8–9 during the
precipitation process. After aging for 1 h, the resultant precipitate was filtered, washed with deionize water and then dried at 100 °C overnight. The obtained sample was calcined at 500 °C for 3 h in air.
a DTA
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Fig. 1. TG-DTA curves: (a) TiO2; (b) Ti0.7Sn0.3O2; (c) Ti0.6Sn0.4O2; (d) Ti0.5Sn0.5O2; (e) Ti0.3Sn0.7O2.
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2.2. Catalyst preparation The 8 wt.% CuO/TixSn1 xO2 catalysts were prepared by deposition–precipitation (DP) method. At room temperature, 0.5 g TixSn1 xO2 powders were suspended in the aqueous solution of Cu(NO3)2 3H2O (0.1316 g), then, 0.25 mol/L Na2CO3 aqueous solution was gradually added to the above suspended solution until the pH of the mixed solution reached 9.0. The mixture was stirred for another 1 h, then filtered and washed with distilled water. The resulting solid was dried at 80 °C for 4 h and subsequently calcined at 300 °C for 3 h.
lation process of surface-attached H2O and OH groups. The peaks deferred and became more and more broad and weak as the proportion of Sn increased.
Anatase Rutile
TiO2(sol-gel) TiO2 Ti0.7Sn0.3O2
2.3. Catalyst characterization
Ti0.6Sn0.4O2
X-ray powder diffraction (XRD) analyses were obtained from a D/MAX-2500 diffractometer using CuKa radiation (40 kV, 100 mA) and scanning in the region 2h = 3–80°. The average crystallite sizes were calculated from the peak width using Scherrer’s equation. Thermogravimetry and differential thermal analyses (TG-DTA) were performed on a ZRY-2P thermal analyser. The thermograms were recorded from ambient temperature to 1173 K at a linear heating rate of 10 K/min under nitrogen atmosphere flowing at a rate of 40 cm3/min. a-Al2O3 was used as reference. X-ray photoelectron spectra (XPS) were recorded on a PHI-1600 spectrometer (USA) equipped with a MgKa radiation for exciting photoelectrons. X-ray source was operated at an accelerating voltage of 15 kV and 250 W. The pressure in the ion-pumped analysis chamber was maintained at 8 10 10 Torr during data acquisition. All binding energies (BE) were referenced to the adventitious C1s line at 284.6 eV (1 eV = 1.602 10 19 J). Temperature-programmed reduction (TPR) experiments were performed under the mixture of 5% H2 in N2 flowing (30 cm3/ min) over 0.1 g catalyst at a heating rate of 10 °C/min. Prior to TPR, the samples were treated in air at 200 °C for 30 min. The H2 uptake amount during the reduction was measured by using a thermal conductivity detector (TCD).
Ti0.5Sn0.5O2 Ti0.3Sn0.7O2
SnO2 10
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2θ (deg) Fig. 2. X-ray diffraction profile of TixSn1 xO2.
Cu2p3/2 Cu2p
933.6eV
Cu2p1/2
934.3eV
953.7eV 954.3eV
sat
sat c
b a
2.4. Measurement of catalytic activity The catalytic activity measurements of the catalysts were carried out in a fixed bed flow microreactor (7 mm i.d.) under atmospheric pressure using 0.1 g catalyst powder. The airflow rate was 33.3 cm3/min, and the CO gas flow rate was 0.5 cm3 min. The reactant and product composition were analysed on-line by a GC-508A gas chromatograph equipped with a thermal conductivity detector (TCD).
970
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Binding Energy (eV) Fig. 3. Cu2p XP spectra of (a) CuO/TiO2; (b) CuO/Ti0.6Sn0.4O2; (c) CuO/SnO2.
o
ß (206 C) 3 o
(191 C) a
3. Results and discussion 3.1. TG-DTA analysis of TixSn1 xO2
3
a ß (179oC) 2
o
The thermal behavior of TixSn1 xO2(x = 1, 0.7, 0.6, 0.5, 0.3) was investigated by TG-DTA, and the results were shown in Fig. 1. The DTA curve of TiO2 (x = 1) (Fig. 1a) exhibits a endothermic event at about 100 °C, where a weight-loss occurred in its TG curve. This peak could be due to the elimination of adsorbed water. The broad and weak endothermic feature centered at 190 °C is attributed to the dehydroxylation process of surface-attached H2O and OH groups [23,24]. At the temperature higher than 500 °C, no significant weight loss was observed in the TG curve. The TG-DTA profile of the TixSn1 xO2 (x = 0.7, 0.6, 0.5, 0.3) Fig. 1b–e is essentially similar to that observed for the TiO2 (x = 1), except the position and intensity of the endothermic peaks associated with the dehydroxy-
(164 C) a 2
b
o
(196 C) δ
o
(150 C) a
1
o
1 ß (216 C) 1
c 100
200
300
400
o
Temperature/ C Fig. 4. TPR pattern of (a) CuO/TiO2; (b) CuO/Ti0.6Sn0.4O2; (c) CuO/SnO2.
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3.2. XRD
3.4. TPR
The XRD patterns of TixSn1 xO2 (x = 1, 0.7, 0.6, 0.5, 0.3, 0), along with TiO2 prepared by sol–gel method (discussed before [26]) used as reference, were displayed in Fig. 2 (all of the samples calcined at 500 °C for 3 h). Both TiO2 and Ti0.7Sn0.3O2 had mostly amorphous character. The incomplete crystallization of these two samples might have been caused by inadequate time needed for better crystallization [27]. The XRD pattern of Ti0.6Sn0.4O2 corresponded to anatase structure (JCPDS PDF [28] card 21-1272), with a dominant 011 diffraction line at 2h = 25.01°. In contrast, Ti0.5Sn0.5O2 and Ti0.3Sn0.7O2 showed only reflections corresponding to rutile phase. No evidence of the presence of segregated SnO2 was found in any of these samples. The results indicated that the incorporation of Sn facilitates the crystallization transformation as previously reported [20,29,30]. It was observed that the crystallite size calculated by Scherrers’s equation increased with the increase of Sn content. The broad peaks on the XRD pattern of Ti0.6Sn0.4O2 indicated the crystallite diameter of the sample was very small (1.8 nm), whereas the crystallite diameter of Ti0.5Sn0.5O2 was 2.2 nm, and Ti0.3Sn0.7O2 was 3.2 nm. Oliveira [24] has reported the similar results which can signify that the presence of tin might be used to control the particle size of the oxides.
In order to investigate the catalyst structures further, temperature-programmed reduction studies were performed. The typical TPR profiles of 8 wt.% CuO/TiO2, 8 wt.% CuO/Ti0.6Sn0.4O2 and 8 wt.% CuO/SnO2 catalysts were shown in Fig. 4. When temperature was below 400 °C, the TPR profiles of 8 wt.% CuO/TiO2 showed mainly two reduction peaks at about 191 and 206 °C (marked a3 and b3, respectively). Similarly, the TPR profiles of 8 wt.% CuO/ Ti0.6Sn0.4O2 showed two overlapping peaks at about 164 and 179 °C (marked a2 and b2, respectively). Compared with the two peaks in above two samples, there were three peaks at 150,196 and 216 °C (marked a1, d1 and b1, respectively) in 8 wt.% CuO/ SnO2 catalyst. According to general interpretation [33,34], the peaks a1, a2 and a3 could be attributed to the reduction of highly dispersed CuO (amorphous form) interacting with support strongly. This phase is highly dispersed and, by reduction, can generate isolated copper atoms, keeping a partial positive charge and regarded as the active sites for CO oxidation. The peaks b1, b2 and b3 should correspond to the reduction of larger CuO particles associated with the support. The peak d3 in the 8 wt.% CuO/SnO2 was attributed to the reduction of the highly dispersed SnO2. On the other hand, the TPR analyses showed that the reduction temperature of the CuO supported on Ti0.6Sn0.4O2 and SnO2 was lower than that on TiO2, and the intensity of a2 was much higher than that of a1 and a3. This might be due to the synergistic effect between CuO and the support. The results were also conceivable that the added SnO have the synergistic ability in TiO2 support and is promoting the reduction of CuO [25]. This may make the CuO/ Ti0.6Sn0.4O2 catalysts have higher catalytic activity than CuO/TiO2 and CuO/SnO2 catalysts under the same reaction condition.
3.3. XPS The XP spectra of Cu2p binding energies for 8 wt.% CuO/TiO2, 8 wt.% CuO/Ti0.6Sn0.4O2 and 8 wt.% CuO/SnO2 were shown in Fig. 3. Obvious differences between the two catalysts can be seen from this figure. The binding energies of Cu 2p3/2 and Cu 2p1/2 in 8 wt.% CuO/SnO2 were 933.6 eV and 953.7 eV, respectively. This means that the copper species in 8 wt.% CuO/SnO2 were mainly dispersed as Cu2+ ions [31]. However, the Cu 2p3/2 and Cu 2p1/2 binding energies of 8 wt.% CuO/TiO2 and 8wt.% CuO/Ti0.6Sn0.4O2 were 934.3 and 954.3 eV, respectively. They were higher than that of 8 wt.% CuO/SnO2 and shift at higher binding energy. This indicated that there was a charge transfer from the metal ion to the support oxide [32]. Córdoba et al. [33] observed an analogous shift in the XP spectra of CuO/TiO2 samples in which dispersed CuO species (933 eV) and those of -Cu–O–Ti–O- (935 eV) have been recognized. The presence of Cu 2p3/2 and Cu 2p1/2 peaks along with their shakeup satellites confirmed further that copper exists as Cu2+ ions [31,32].
a
3.5. Catalytic activity for CO oxidation The catalytic activities of CuO/TixSn1 xO2 (x = 1, 0.7, 0.6, 0.5, 0.3, 0) catalysts with different x value calcined at 300 °C for 3 h were shown in Fig. 5a. It displayed that all the catalysts supported on the binary composition had better catalytic performance than those supported on the pure composition TiO2 and SnO2 while the CuO/TiO2 catalyst cannot reach complete conversion under the same experimental condition. The results were consistent with the TPR analysis which indicated that the addition of Sn improved the reducibility of the catalysts. Comparing with the performance
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CO Conversion(%)
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Fig. 5. Catalytic activities of (a) 8 wt.% CuO/TixSn1 xO2 catalysts; (b) 8 wt.% CuO/Ti0.6Sn0.4O2 catalysts calcined at different temperature for 3 h.
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J. Huang et al. / Catalysis Communications 9 (2008) 2131–2135
of the catalysts prepared by Dong et al. [25], our catalysts displayed higher catalysis activities for CO oxidation. On the other hand, it could be seen from Fig. 5a that the temperature for 100% CO conversion (T100%) of the samples with x = 0.7, 0.6, 0.5, and 0.3 were 170, 110, 130 and 160 °C, respectively. It is noted that the T100% decreased with the decrease of the x value from 1 to 0.6, however, it increased when the x value decreased from 0.6 to 0. This indicated that the anatase crystal phase and smaller particle size of support were responsible for the superior catalytic property in the present study, which was in good agreement with the result of XRD analyse. For the purpose of comparison, the 8 wt.% CuO/Ti0.6Sn0.4O2 catalyst was selected to continue the following study. The influence of calcination temperature on the catalytic activity of the 8 wt.% CuO/Ti0.6Sn0.4O2 catalysts for low-temperature CO oxidation was investigated and the results were shown in Fig. 5b. It can be seen that the T100% of the sample calcined at 400 °C for 3 h was 180 °C and when the calcination temperature was 200 °C, the T100% was 130 °C. Both of them were higher than the T100% of the sample calcined at 300 °C (T100% = 110 °C). The results could be due to the fact that higher temperature resulted in sintering of the components, and lower temperature was unfavorable to the highly disperse of ion copper oxide and the interaction between copper oxide and the support [26].
4. Conclusions TixSn1 xO2 (x = 1, 0.7, 0.6, 0.5, 0.3, 0) were prepared by the coprecipitation method. The 8 wt.% CuO/TixSn1 xO2 catalysts were prepared by deposition–precipitation (DP) method. The XRD analysis indicated that the addition of SnO2 could accelerate the anatase-rutile transformation of the crystal phase markedly, and the TixSn1 xO2 was formed with lower crystallite size than the pure SnO2. The lower crystallite size of doped particles was attributed to CO oxidation. Among the prepared samples, Ti0.6Sn0.4O2 was in anatase structure and had the smallest particle size. The XPS measurement showed that the copper species were mainly dispersed as Cu2+ ions on the support. The study of the catalytic activity for CO oxidation indicated that the catalytic activity of the CuO catalysts supported on the binary composition (TixSn1 xO2) was higher
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than that on the pure composition (TiO2 and SnO2), which might be due to the fact that the addition of SnO2 lowered the reduction temperature of the CuO as revealed by TPR analyse. From the results of the catalytic activity measurements, we also found that the suitable proportion of TiO2 and SnO2 and calcination temperature of the catalysts were responsible for better catalytic activity. In our present work, the CuO/Ti0.6Sn0.4O2 catalyst calcined at 300 °C for 3 h represented the best catalytic activity. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
M. Valden, S. Paka, X. Lai, et al., Catal. Lett. 56 (1998) 7. M.V. Tsodikov, Y.A. Trusova, Y.V. Slivinski,Stud. Surf. Sci. Catal. 118 (1998) 679. B. Schumacher, V. Plzak, M. Kinne, et al., Catal. Lett. 89 (2003) 109. F. Boccuzzi, A. Chiorino, M. Manzoli, Mater. Sci. Eng. C – Biomimetic Supramol. Syst. 15 (2001) 215. S. Carrettin, Y. Hao, V. Aguilar-Guerrero, et al., Chem. Eur. J. 13 (2007) 7771. V. Idakiev, T. Tabakova, Z.Y. Yuan, et al., Appl. Catal. A: Gen. 270 (2004) 135. H.Q. Zhu, Z.F. Qin, W.J. Shan, et al., Catal. Today 126 (2007) 382. J. Raskó, J. Kiss, R. Kinet, Catal. Lett. 90 (2007) 389. S. Funk, U. Burghaus, Catal. Lett. 118 (2007) 118. Z. Ma, S. Brown, S.H. Overbury, et al., Appl. Catal. A – Gen. 327 (2007) 226. G. Dagan, S. Sampath, O. Lev, Chem. Mater. 7 (1995) 446. J. Purans, A. Azens, C.G. Granqvist, Electrochim. Acta. 46 (2001) 2055. S. Komornicki, M. Radecka, M. Rekas, J. Mater. Sci. Mater. Electron. 12 (2001) 11. V. Dusastre, D.E. Williams, J. Mater. Chem. 9 (1999) 445. C.M. Carney, S. Yoo, S.A. Akbar, Sens. Actuator, B – Chem. 108 (2005) 29. K. Zakrewska, Thin Solid Films 391 (2001) 229. M. Radecka, K. Zakrzewska, M. Rekas, Sens. Actuator, B – Chem. 47 (1998) 194. M. Radecka, K. Zakrzewska, M. Rekas, Solid State Ionics 119 (1999) 43. P.R. Bueno, M.R. Cassia-Santos, L.G.P. Simões, et al., J. Am. Ceram. Soc. 85 (2002) 282. F. Fresno, J.M. Coronado, D. Tudela, et al., Appl. Catal. B – Environ. 55 (2005) 159. J. Lin, J.C. Yu, D. Lo, et al., J. Catal. 183 (1999) 368. P.A. Sermon, T.J. Walton, Solid State Ionics 101–103 (1997) 673. L.J. Alemany, M.A. Bañares, E. Pardo, et al., Mater. Charact. 44 (2000) 271. M.M. Oliveira, D.C. Schnitzler, A.J.G. Zarbin, Chem. Mater. 15 (2003) 1903. G.L. Dong, C.L. Yang, Y.B. Gao, et al., Huanjing Kexue 4 (1999) 71. J. Huang, S.R. Wang, Y.Q. Zhao, et al., Catal. Commun. 7 (2006) 1029. J. Lukácˇ, M. Klementová, P. Bezdicˇka, et al., J. Mater. Sci. 42 (2007) 9421. JCPDS PDF-2 release 2001, ICDD Newtown Square, PA, USA. K.N.P. Kumar, K. Keizer, A.J. Burgraaf, et al., J. Mater. Chem. 3 (1993) 923. S.K. Kulshreshtha, R. Sasikala, V. Sudarsan, J. Mater. Chem. 11 (2001) 930. A. Gervasini, M. Manzoli, G. Martra, et al., J. Phys. Chem. B 110 (2006) 7851. J.P. Espinós, J. Morales, A. Barranco, et al., J. Phys. Chem. B 106 (2002) 6921. G. Córdoba, M. Viniegra, J.L.G. Fierro, et al., J. Solid State Chem. 138 (1998) 1. B.L. Zhu, X.X. Zhang, S.R. Wang, Micropor. Mesopor. Mater. 102 (2007) 333.
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
The preparation and catalytic behavior of CuO/TixSn1 xO2 catalysts for low-temperature carbon monoxide oxidation Jiang Huang, Shurong Wang, Xianzhi Guo, Da Wang, Baolin Zhu, Shihua Wu * Department of Chemistry, NanKai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 29 February 2008 Received in revised form 17 April 2008 Accepted 19 April 2008 Available online 25 April 2008 Keywords: TixSn1 xO2 CuO/TixSn1 xO2 catalysts Low-temperature CO oxidation
a b s t r a c t A series of TixSn1 xO2 (x = 1, 0.7, 0.6, 0.5, 0.3, 0) powders were synthesized by co-precipitation method and the 8 wt.% CuO/TixSn1 xO2 catalysts were prepared via deposition–precipitation method. The samples were characterized by TG-DTA, XRD, XPS and TPR techniques. The influences of the proportion of TiO2 and SnO2 and calcination temperature on the catalytic activities of the catalysts were investigated. The CuO/Ti0.6Sn0.4O2 catalyst calcined at 300 °C for 3 h showed the best catalytic performance. TPR analysis indicated that the addition of SnO2 could improve the reducibility of the catalysts. The XPS measurement showed that the copper species were mainly dispersed as Cu2+ ions. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The serious environmental implication of CO emission from automotive sources has stimulated a huge amount of pure and applied researches directed at catalytic abatement of such emission. TiO2 used as a catalyst support has been received great attention because the catalyst supported on TiO2 presents a relatively high catalytic activity, and TiO2 is stable, inexpensive and innocuous [1–5]. Nevertheless, the increment of the catalytic efficiency of catalysts remains as an important goal. In order to further improve the performance of TiO2 as a catalyst support, several structural and chemical modifications of this material have been assayed. In this way, significant increments of CO oxidation rate have been attained using either porous TiO2 materials, or by the incorporation of noble metals into the TiO2 lattice [6–9]. On the other hand, mixed oxides had received great attention in recent years. It has been realized that such systems may benefit from the combination of the best sensing properties of their pure components. The growing number of papers [10–15] reporting on the successful application of these oxides in practical works shows the important role they play in various research fields. Among them, TixSn1 xO2 system has recently emerged as attractive materials for gas sensor and low-voltage varistor application [16–20]. Zakrzewska [16] reported that the operating temperature for hydrogen sensor was lowered to about 500 °C by addition of small amount of SnO2 to TiO2 lattice. Radecka’s work [17,18] had showed that the response of SnO2 sensors containing small additions of Ti was comparable with that of undoped SnO2, however the long* Corresponding author. Tel.: +86 22 23505896; fax: +86 22 23502458. E-mail address: [email protected] (S. Wu). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.04.011
term stability was improved markedly. In addition, the use of TixSn1 xO2 system in environmental catalysis has also been examined because of it is high quantum yield and good photocatalytic activity. Lin [21] found that small Sn substitution for Ti in rutile TiO2 increased the photoactivity of TiO2 by 15 times for the oxidation of acetone. Compared to the experimental investigations, few systemic works have been reported on the studies of TixSn1 xO2 system used as a catalyst support for CO oxidation. Sermon and Walton have reported that Pt/TiO2–SnO2 (at higher TiO2 contents) appeared to be a good catalyst for CO oxidation [22]. However, the optimum TiO2:SnO2 balance for the ideal CO sensors was not assessed. Our previous work [26] has shown that the CuO/TiO2 catalyst with 8 wt.% CuO loading exhibited higher catalytic activity than that of the catalysts with other CuO loadings. In this paper, a series of TixSn1 xO2 with different Ti/Sn ratio were prepared by co-precipitation method. The 8 wt.% CuO/TixSn1 xO2 catalysts were prepared by deposition–precipitation (DP) method. The samples have been characterized by TG-DTA, XRD, XPS and TPR. The catalytic activity in induction of CO oxidation was determined by a microreactorGC system. For comparison, pure TiO2 and SnO2 were also prepared by the same method, and the catalytic performance of CuO/TiO2 and CuO/SnO2 was also studied.
2. Experimental 2.1. TixSn1 xO2 preparation TixSn1 xO2 (x = 0, 0.2, 0.3, 0.4, 0.5, 0.7, 1) powders were prepared by the co-precipitation method. The mixed solution of Ti(SO4)2 and
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SnCl4 5H2O together with the solution of ammonia were simultaneously dropped into a buffer solution (pH 8) under vigorous agitation, keeping the pH value of the solution at 8–9 during the
precipitation process. After aging for 1 h, the resultant precipitate was filtered, washed with deionize water and then dried at 100 °C overnight. The obtained sample was calcined at 500 °C for 3 h in air.
a DTA
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800
0
200
400
600 o
Temperature/ C
Fig. 1. TG-DTA curves: (a) TiO2; (b) Ti0.7Sn0.3O2; (c) Ti0.6Sn0.4O2; (d) Ti0.5Sn0.5O2; (e) Ti0.3Sn0.7O2.
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2.2. Catalyst preparation The 8 wt.% CuO/TixSn1 xO2 catalysts were prepared by deposition–precipitation (DP) method. At room temperature, 0.5 g TixSn1 xO2 powders were suspended in the aqueous solution of Cu(NO3)2 3H2O (0.1316 g), then, 0.25 mol/L Na2CO3 aqueous solution was gradually added to the above suspended solution until the pH of the mixed solution reached 9.0. The mixture was stirred for another 1 h, then filtered and washed with distilled water. The resulting solid was dried at 80 °C for 4 h and subsequently calcined at 300 °C for 3 h.
lation process of surface-attached H2O and OH groups. The peaks deferred and became more and more broad and weak as the proportion of Sn increased.
Anatase Rutile
TiO2(sol-gel) TiO2 Ti0.7Sn0.3O2
2.3. Catalyst characterization
Ti0.6Sn0.4O2
X-ray powder diffraction (XRD) analyses were obtained from a D/MAX-2500 diffractometer using CuKa radiation (40 kV, 100 mA) and scanning in the region 2h = 3–80°. The average crystallite sizes were calculated from the peak width using Scherrer’s equation. Thermogravimetry and differential thermal analyses (TG-DTA) were performed on a ZRY-2P thermal analyser. The thermograms were recorded from ambient temperature to 1173 K at a linear heating rate of 10 K/min under nitrogen atmosphere flowing at a rate of 40 cm3/min. a-Al2O3 was used as reference. X-ray photoelectron spectra (XPS) were recorded on a PHI-1600 spectrometer (USA) equipped with a MgKa radiation for exciting photoelectrons. X-ray source was operated at an accelerating voltage of 15 kV and 250 W. The pressure in the ion-pumped analysis chamber was maintained at 8 10 10 Torr during data acquisition. All binding energies (BE) were referenced to the adventitious C1s line at 284.6 eV (1 eV = 1.602 10 19 J). Temperature-programmed reduction (TPR) experiments were performed under the mixture of 5% H2 in N2 flowing (30 cm3/ min) over 0.1 g catalyst at a heating rate of 10 °C/min. Prior to TPR, the samples were treated in air at 200 °C for 30 min. The H2 uptake amount during the reduction was measured by using a thermal conductivity detector (TCD).
Ti0.5Sn0.5O2 Ti0.3Sn0.7O2
SnO2 10
20
30
40
50
60
70
80
2θ (deg) Fig. 2. X-ray diffraction profile of TixSn1 xO2.
Cu2p3/2 Cu2p
933.6eV
Cu2p1/2
934.3eV
953.7eV 954.3eV
sat
sat c
b a
2.4. Measurement of catalytic activity The catalytic activity measurements of the catalysts were carried out in a fixed bed flow microreactor (7 mm i.d.) under atmospheric pressure using 0.1 g catalyst powder. The airflow rate was 33.3 cm3/min, and the CO gas flow rate was 0.5 cm3 min. The reactant and product composition were analysed on-line by a GC-508A gas chromatograph equipped with a thermal conductivity detector (TCD).
970
965
960
955
950
945
940
935
930
Binding Energy (eV) Fig. 3. Cu2p XP spectra of (a) CuO/TiO2; (b) CuO/Ti0.6Sn0.4O2; (c) CuO/SnO2.
o
ß (206 C) 3 o
(191 C) a
3. Results and discussion 3.1. TG-DTA analysis of TixSn1 xO2
3
a ß (179oC) 2
o
The thermal behavior of TixSn1 xO2(x = 1, 0.7, 0.6, 0.5, 0.3) was investigated by TG-DTA, and the results were shown in Fig. 1. The DTA curve of TiO2 (x = 1) (Fig. 1a) exhibits a endothermic event at about 100 °C, where a weight-loss occurred in its TG curve. This peak could be due to the elimination of adsorbed water. The broad and weak endothermic feature centered at 190 °C is attributed to the dehydroxylation process of surface-attached H2O and OH groups [23,24]. At the temperature higher than 500 °C, no significant weight loss was observed in the TG curve. The TG-DTA profile of the TixSn1 xO2 (x = 0.7, 0.6, 0.5, 0.3) Fig. 1b–e is essentially similar to that observed for the TiO2 (x = 1), except the position and intensity of the endothermic peaks associated with the dehydroxy-
(164 C) a 2
b
o
(196 C) δ
o
(150 C) a
1
o
1 ß (216 C) 1
c 100
200
300
400
o
Temperature/ C Fig. 4. TPR pattern of (a) CuO/TiO2; (b) CuO/Ti0.6Sn0.4O2; (c) CuO/SnO2.
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3.2. XRD
3.4. TPR
The XRD patterns of TixSn1 xO2 (x = 1, 0.7, 0.6, 0.5, 0.3, 0), along with TiO2 prepared by sol–gel method (discussed before [26]) used as reference, were displayed in Fig. 2 (all of the samples calcined at 500 °C for 3 h). Both TiO2 and Ti0.7Sn0.3O2 had mostly amorphous character. The incomplete crystallization of these two samples might have been caused by inadequate time needed for better crystallization [27]. The XRD pattern of Ti0.6Sn0.4O2 corresponded to anatase structure (JCPDS PDF [28] card 21-1272), with a dominant 011 diffraction line at 2h = 25.01°. In contrast, Ti0.5Sn0.5O2 and Ti0.3Sn0.7O2 showed only reflections corresponding to rutile phase. No evidence of the presence of segregated SnO2 was found in any of these samples. The results indicated that the incorporation of Sn facilitates the crystallization transformation as previously reported [20,29,30]. It was observed that the crystallite size calculated by Scherrers’s equation increased with the increase of Sn content. The broad peaks on the XRD pattern of Ti0.6Sn0.4O2 indicated the crystallite diameter of the sample was very small (1.8 nm), whereas the crystallite diameter of Ti0.5Sn0.5O2 was 2.2 nm, and Ti0.3Sn0.7O2 was 3.2 nm. Oliveira [24] has reported the similar results which can signify that the presence of tin might be used to control the particle size of the oxides.
In order to investigate the catalyst structures further, temperature-programmed reduction studies were performed. The typical TPR profiles of 8 wt.% CuO/TiO2, 8 wt.% CuO/Ti0.6Sn0.4O2 and 8 wt.% CuO/SnO2 catalysts were shown in Fig. 4. When temperature was below 400 °C, the TPR profiles of 8 wt.% CuO/TiO2 showed mainly two reduction peaks at about 191 and 206 °C (marked a3 and b3, respectively). Similarly, the TPR profiles of 8 wt.% CuO/ Ti0.6Sn0.4O2 showed two overlapping peaks at about 164 and 179 °C (marked a2 and b2, respectively). Compared with the two peaks in above two samples, there were three peaks at 150,196 and 216 °C (marked a1, d1 and b1, respectively) in 8 wt.% CuO/ SnO2 catalyst. According to general interpretation [33,34], the peaks a1, a2 and a3 could be attributed to the reduction of highly dispersed CuO (amorphous form) interacting with support strongly. This phase is highly dispersed and, by reduction, can generate isolated copper atoms, keeping a partial positive charge and regarded as the active sites for CO oxidation. The peaks b1, b2 and b3 should correspond to the reduction of larger CuO particles associated with the support. The peak d3 in the 8 wt.% CuO/SnO2 was attributed to the reduction of the highly dispersed SnO2. On the other hand, the TPR analyses showed that the reduction temperature of the CuO supported on Ti0.6Sn0.4O2 and SnO2 was lower than that on TiO2, and the intensity of a2 was much higher than that of a1 and a3. This might be due to the synergistic effect between CuO and the support. The results were also conceivable that the added SnO have the synergistic ability in TiO2 support and is promoting the reduction of CuO [25]. This may make the CuO/ Ti0.6Sn0.4O2 catalysts have higher catalytic activity than CuO/TiO2 and CuO/SnO2 catalysts under the same reaction condition.
3.3. XPS The XP spectra of Cu2p binding energies for 8 wt.% CuO/TiO2, 8 wt.% CuO/Ti0.6Sn0.4O2 and 8 wt.% CuO/SnO2 were shown in Fig. 3. Obvious differences between the two catalysts can be seen from this figure. The binding energies of Cu 2p3/2 and Cu 2p1/2 in 8 wt.% CuO/SnO2 were 933.6 eV and 953.7 eV, respectively. This means that the copper species in 8 wt.% CuO/SnO2 were mainly dispersed as Cu2+ ions [31]. However, the Cu 2p3/2 and Cu 2p1/2 binding energies of 8 wt.% CuO/TiO2 and 8wt.% CuO/Ti0.6Sn0.4O2 were 934.3 and 954.3 eV, respectively. They were higher than that of 8 wt.% CuO/SnO2 and shift at higher binding energy. This indicated that there was a charge transfer from the metal ion to the support oxide [32]. Córdoba et al. [33] observed an analogous shift in the XP spectra of CuO/TiO2 samples in which dispersed CuO species (933 eV) and those of -Cu–O–Ti–O- (935 eV) have been recognized. The presence of Cu 2p3/2 and Cu 2p1/2 peaks along with their shakeup satellites confirmed further that copper exists as Cu2+ ions [31,32].
a
3.5. Catalytic activity for CO oxidation The catalytic activities of CuO/TixSn1 xO2 (x = 1, 0.7, 0.6, 0.5, 0.3, 0) catalysts with different x value calcined at 300 °C for 3 h were shown in Fig. 5a. It displayed that all the catalysts supported on the binary composition had better catalytic performance than those supported on the pure composition TiO2 and SnO2 while the CuO/TiO2 catalyst cannot reach complete conversion under the same experimental condition. The results were consistent with the TPR analysis which indicated that the addition of Sn improved the reducibility of the catalysts. Comparing with the performance
b 100
100
80
60
TiO2 Ti0.7Sn0.3O2
40
Ti0.6Sn0.4O2 Ti0.5Sn0.5O2
60
200 300 400
40
20
Ti0.3Sn0.7O2
20
SnO2 0
CO Conversion(%)
CO Conversion(%)
80
100
120
140
160 o
Temperature/ C
180
0
200
100
120
140
160
180
Temperature/ oC
Fig. 5. Catalytic activities of (a) 8 wt.% CuO/TixSn1 xO2 catalysts; (b) 8 wt.% CuO/Ti0.6Sn0.4O2 catalysts calcined at different temperature for 3 h.
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of the catalysts prepared by Dong et al. [25], our catalysts displayed higher catalysis activities for CO oxidation. On the other hand, it could be seen from Fig. 5a that the temperature for 100% CO conversion (T100%) of the samples with x = 0.7, 0.6, 0.5, and 0.3 were 170, 110, 130 and 160 °C, respectively. It is noted that the T100% decreased with the decrease of the x value from 1 to 0.6, however, it increased when the x value decreased from 0.6 to 0. This indicated that the anatase crystal phase and smaller particle size of support were responsible for the superior catalytic property in the present study, which was in good agreement with the result of XRD analyse. For the purpose of comparison, the 8 wt.% CuO/Ti0.6Sn0.4O2 catalyst was selected to continue the following study. The influence of calcination temperature on the catalytic activity of the 8 wt.% CuO/Ti0.6Sn0.4O2 catalysts for low-temperature CO oxidation was investigated and the results were shown in Fig. 5b. It can be seen that the T100% of the sample calcined at 400 °C for 3 h was 180 °C and when the calcination temperature was 200 °C, the T100% was 130 °C. Both of them were higher than the T100% of the sample calcined at 300 °C (T100% = 110 °C). The results could be due to the fact that higher temperature resulted in sintering of the components, and lower temperature was unfavorable to the highly disperse of ion copper oxide and the interaction between copper oxide and the support [26].
4. Conclusions TixSn1 xO2 (x = 1, 0.7, 0.6, 0.5, 0.3, 0) were prepared by the coprecipitation method. The 8 wt.% CuO/TixSn1 xO2 catalysts were prepared by deposition–precipitation (DP) method. The XRD analysis indicated that the addition of SnO2 could accelerate the anatase-rutile transformation of the crystal phase markedly, and the TixSn1 xO2 was formed with lower crystallite size than the pure SnO2. The lower crystallite size of doped particles was attributed to CO oxidation. Among the prepared samples, Ti0.6Sn0.4O2 was in anatase structure and had the smallest particle size. The XPS measurement showed that the copper species were mainly dispersed as Cu2+ ions on the support. The study of the catalytic activity for CO oxidation indicated that the catalytic activity of the CuO catalysts supported on the binary composition (TixSn1 xO2) was higher
2135
than that on the pure composition (TiO2 and SnO2), which might be due to the fact that the addition of SnO2 lowered the reduction temperature of the CuO as revealed by TPR analyse. From the results of the catalytic activity measurements, we also found that the suitable proportion of TiO2 and SnO2 and calcination temperature of the catalysts were responsible for better catalytic activity. In our present work, the CuO/Ti0.6Sn0.4O2 catalyst calcined at 300 °C for 3 h represented the best catalytic activity. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
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