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CeO2 system
Materials Science and Engineering C 25 (2005) 516 – 520 www.elsevier.com/locate/msec
Preparation, characterization and catalytic properties of CuO/CeO2 system Xiu-Cheng Zheng, Shu-Ping Wang, Shu-Rong Wang, Shou-Min Zhang, Wei-Ping Huang, Shi-Hua Wu* Department of Chemistry, Nankai University, Tianjin 300071, China Received 15 June 2004; received in revised form 19 October 2004; accepted 25 March 2005 Available online 1 June 2005
Abstract CeO2 nano-particles and CuO/CeO2 system were prepared by sol-gel and impregnation methods and characterized using combined spectroscopic techniques of XRD, XPS, TPR, FT – Raman, BET and HRTEM. It was found the CeO2 was cubic phase with fluorite structure and CuO was highly dispersed on the CeO2 particles. Temperature-programmed reduction (TPR) showed a two-step reduction for CuO/CeO2 catalysts. XPS analysis indicated the presence of redox couple Ce4+/Ce3+ and reduced copper species in the CuO/CeO2 catalysts. The factors, such as calcination temperature, calcination time and CuO loading, influenced on the catalytic properties of CuO/CeO2 catalysts. D 2005 Elsevier B.V. All rights reserved. Keywords: CeO2; CuO/CeO2; CO oxidation
1. Introduction Today, the protection of the environment is receiving much more attention than ever before. One of the major efforts of environmental cleanup is focused on controlling the emission of toxic pollutant produced during the combustion of fuels in automotive engines. The so-called ‘‘three-way catalysts’’ (TWC), in which CeO2 is a crucial component, are commonly used to reduce the emissions of CO, NOx , and hydrocarbons from automobile exhaust. The promoting of ceria was originally believed to be included both structural and chemical aspects, like the enhancement of the metal dispersion or its participation in the water – gas shift reaction or the decomposition of nitrogen oxides, and processes involving the oxygen storage capacity provided by the redox couple Ce4+/Ce3+, making more oxygen available for the oxidation processes [1]. On the other hand, precious metals are well-known oxidation catalysts with high activity and stability, and are widely used for CO oxidation reaction. However, due to the high cost of precious metals and their sensitivity to sulfur * Corresponding author. Tel.: +86 22 23505896; fax: +86 22 23502458. E-mail address: [email protected] (S.-H. Wu). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.03.005
poisoning, attention has been given to base metal catalysts, such as copper oxide [2 – 8]. Several methods for the preparation of CuO/CeO2 catalysts, such as thermal decomposition, co-precipitation and sol-gel starting from CeCl3 I 7H2O, have been reported [2,7 –9]. The present paper describes the preparation of CeO2 nano-particles via a sol-gel method starting from Ce(NO3)3 I 6H2O and CuO/CeO2 system via impregnation the obtained CeO2 powders with Cu(NO3)2 solution. Furthermore, the catalytic performance of the prepared CuO/CeO2 system was described.
2. Experimental CeO2 was prepared by sol-gel method [10]. At room temperature, 10.19 g Ce(NO3)3 I 6H2O was dissolved in 25 mL H2O in a flask, then 1 mL HNO3 was added to the solution. After the solution was heated to 65 -C in a water bath, 105 g citric acid was slowly added into it with vigorous stirring within 1 h. The reaction was run for 24 h until the light yellow wet gel was obtained. The gel was dried at 65 -C to remove the solvents and then calcined at 320 -C for 2.5 h to give the yellow-white CeO2 nano-particles.
3. Results and discussion
(400)
60
70
(331) (420)
(222)
(311)
(220) (200) CuO/CeO2 8 h
CuO/CeO2 5 h
CuO/CeO2 2 h CeO2
10
20
30
40
50
80
2 Theta (degree) Fig. 2. XRD patterns of CeO2 and CuO/CeO2 catalysts calcined at 700 -C.
1(a), the adjacent CeO2 nanocrystals intruded on each other and the existence of the grain boundary indicated that it was crystallized. Furthermore, The prepared CeO2 was almost regular spherical morphology particles with similar size. Fig. 2 shows the XRD patterns of 1.26 wt.% CuO/CeO2 catalysts calcined at 700 -C in air for 2, 5, and 8 h. The pattern of CeO2 was also exhibited for comparison purpose. It can be seen that only CeO2 characteristic peaks appeared and no copper oxide reflections were found by XRD for all the present samples in Fig. 2. This indicates that copper oxide exhibits in a highly dispersed form [11]. Fig. 3 shows the XRD patterns of CuO/CeO2 catalysts with various CuO loadings calcined at 500 -C for 3.5 h. Only CeO2 XRD peaks were observed when CuO loading was lower than 6.30 wt.%. The weak peaks attributed to CuO crystal phase can be observed on 10.08 wt.% CuO/ CeO2 catalyst, indicating that bulk CuO had already formed on the 10.08 wt.% CuO/CeO2 catalyst in addition to the dispersed clusters of copper oxide [11]. As expected, the
60
70
(331) (420)
(311)
(400)
CuO
15.12 wt.% CuO/CeO2
(222)
(200)
Intensity (cps)
(220)
(111)
Fig. 1 shows the typical HRTEM micrograph of the obtained CeO2 nano-particles. As can be seen from Fig.
Intensity (cps)
The CuO/CeO2 system was prepared by the conventional wet impregnation method using an aqueous solution of Cu(NO3)2. The prepared samples were dried at 80 -C in an oven and then heated at 500 and 700 -C in air for 2, 3.5, 5, and 8 h. The CuO loading was 1.26– 15.12 wt.%. The surface area of the samples was calculated by BET method from N2 adsorption isotherms recorded at 196 -C using a ChemBET-3000 physical Autosorbmeter. The X-ray powder diffraction (XRD) analysis was performed using a Dmax-2500 X-ray diffractometer with CuKa radiation. The electron microscopy study was taken using Philips T20ST high-resolution transmission electron microscope. Fourier transform (FT) Raman spectra were recorded with a BRUKER RTS 100/S FT –RAMAN spectrometer, using the 1064-nm excitation line of Nd – YAG laser. Temperature-programmed reduction (TPR) experiments were performed on the Micromeritics temperature-programmed chemisorption instrument under the mixture of 5% H2 in N2 flowing (30 mL min 1) at a heating rate of 10 -C/min. X-ray photoelectron spectra (XPS) were recorded on a PHI1600 spectrometer equipped with a MgKa radiation for exciting photoelectrons. The catalytic activities of the catalysts for CO oxidation were measured in a fixed bed flow microreactor (7 mm I.D.) under atmospheric pressure using 50 mg of catalyst powders doping with 2 g fine quartz sand in order to avoid temperature differences and bed channeling. The total gas flow rate was 33.6 mL/min, contained 1.0% CO balanced with air. The reactant and product composition were analyzed on-line by a GC-508A gas chromatograph equipped with a thermal conductivity detector (TCD).
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12.60 wt.% CuO/CeO2 10.08 wt.% CuO/CeO2
6.30 wt.% CuO/CeO2
10
20
30
40
50
80
2 Theta (degree) Fig. 1. HRTEM micrograph of the prepared CeO2 nano-particles.
Fig. 3. XRD patterns of CuO/CeO2 catalysts with various CuO loadings.
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Table 1 Crystallite sizes of CeO2 and CuO in CuO/CeO2 catalysts Sample
CeO2 crystallite size (nm)
CuO crystalline size (nm)
CeO2 1.26 wt.% CuO/CeO2 (700 -C 2 h) 1.26 wt.% CuO/CeO2 (700 -C 5 h) 1.26 wt.% CuO/CeO2 (700 -C 8 h) 6.30 wt.% CuO/CeO2 (500 -C 3.5 h) 10.08 wt.% CuO/CeO2 (500 -C 3.5 h) 12.60 wt.% CuO/CeO2 (500 -C 3.5 h) 15.12 wt.% CuO/CeO2 (500 -C 3.5 h)
9.8 27.4 44.5 61.6 12.6 13.4 13.9 13.4
– – – – – 46.5 46.5 46.5
IV
S BET (m2 g
500 500 500 700 700 700
2 5 8 2 5 8
51.0 62.2 41.2 14.2 12.1 10.4
100
200
300
400
500
600
700
800
900
1000
Temperature (ºC) Fig. 4. TPR pattern of 6.30 wt.% CuO/CeO2 catalyst.
H2-TPR of pure CeO2 showed a low temperature reduction with peak maximum around 500 -C and a high temperature reduction with peak maximum around 800 -C. The low temperature peak around 500 -C was attributed to the removal of surface capping oxygen ions and the high temperature peak around 800 -C was attributed to bulk oxygen ions during the reduction reaction with H2. Our results proved the above viewpoint. In other words, we think that I peak can be attributed to the reduction of dispersed copper oxide (non-crystalline forms), II peak attributed to the reduction of bulk copper oxide (crystalline forms). III and IV peaks can be attributed to the low and high temperature reduction of freely CeO2 powders. In recent years, FT – Raman spectroscopy has become an increasingly valuable tool for the investigation of the supported catalysts [13]. The FT – Raman spectra of CeO2, CuO (thermal decomposition of Cu(NO3)2 at 500 -C for 3.5 h) and 6.30 wt.% CuO/CeO2 catalyst are comparatively shown in Fig. 5. CeO2 was characterized by the strong 460
Raman intensity (a.u.)
Table 2 Surface area of 1.26 wt.% CuO/CeO2 catalysts Calcination time (h)
I
III
intensity of CuO crystal phase peaks increased with the increase of CuO loading. The average particle size of CeO2 and CuO on CuO/CeO2 catalysts (see Table 1) was 12.6 – 13.9 and 46.5 nm, respectively. the particle size did not change obviously with increasing CuO loading. The measured data of the BET surface areas of 1.26 wt.% CuO/CeO2 catalysts are listed in Table 2. The BET surface area of the catalysts calcined at 500 -C for 2, 5, and 8 h were 51.0, 62.2, and 41.2 m2 g 1. While the BET surface area of the catalysts calcined at 700 -C for 2, 5, and 8 h were 14.2, 12.1, and 10.4 m2 g 1. It was noted that the BET surface area decreased with the increase of calcination time except the catalyst calcined at 500 -C for 2 h, and the BET surface areas of the catalysts calcined at 500 -C were obviously larger than that of those calcined at 700 -C. This was in accordance to the results determined from XRD (shown in Table 1), which showed that the crystallite size of CeO2 on CuO/CeO2 catalysts calcined at 700 -C increased with calcination time. In order to determine the existing states of CuO and CeO2, we have employed H2-TPR characterization. Fig. 4 shows the typical TPR profile of 6.30 wt.% CuO/CeO2 catalyst calcined at 500 -C for 3.5 h. Two overlapping reduction peaks with higher intensity at about 160 and 210 -C (called I and II, respectively) were presented. Two broad reduction peaks with lower intensity at 400 – 500 -C and 700 – 900 -C (called III and IV, respectively) were presented, too. Rao et al. [12] reported that pure CuO showed a single reduction at 380– 392 -C while CuO supported on CeO2 showed a two-step reduction at much lower temperatures due to finely dispersed CuO strongly interacting with CeO2 (130 –175 -C) and bulk CuO particles (180 – 200 -C). The
Calcination temperature (-C)
TCD signal (a.u.)
II
6.30 wt.% CuO/CeO2
298
CuO
1
)
CeO2
800
700
600
500
400
300
200
Wavenumber (cm-1) Fig. 5. FT – Raman patterns of CeO2, CuO and 6.30 wt.% CuO/CeO2 catalysts.
X.-C. Zheng et al. / Materials Science and Engineering C 25 (2005) 516 – 520
(a)
519
100
O1s
CO conversion (%)
529.3
C (S)
4
531.4
80 1.26 wt.% CuO/CeO2 3.78 wt.% CuO/CeO2 6.30 wt.% CuO/CeO2 10.08 wt.%CuO/CeO2 12.60 wt.%CuO/CeO2 15.12 wt.%CuO/CeO2
60 40 20 0 80
0
535
530
525
100
120
520
140
160
180
200
220
Temperature (ºC)
Binding Energy (eV) Fig. 7. Catalytic activities of CuO/CeO2 catalysts with various CuO loadings.
(b) Ce3d
882.5
C (S)
05
0
910
905
900
895
890
885
880
875
Binding Energy (eV)
(c) 932.5
Cu2P
C (S)
5
in air for 3.5 h, are shown in Fig. 6. It can be observed from Fig. 6(a) that O 1s showed in two main peaks at about 531.6 eV contributed to the absorbed oxygen and 529.5 eV contributed to lattice oxygen. The Ce 3d showed in five peaks at about 882, 888, 898, 900, and 907 eV in the present Fig. 6(b), which were consistent with the previous report of Ce4+ [14], indicating the main valence of CeO2 in the sample was + 4. Furthermore, there were two weak characteristic peaks of Ce2O3 at about 883 and 903 eV. This proved that existence of the redox couple Ce+4/Ce+3 in the CuO/CeO2 catalysts. On the other hand, the weak peaks in XPS spectra and no Ce2O3 crystallite peaks appearing on XRD pattern in Figs. 2 and 3 indicated that Ce2O3 content was very low. The peaks of Cu 2p3/2 and Cu 2p1/2 were centered at 932.5 and 952.5 eV, respectively. The Cu 2p3/2 value was lower than that of CuO, i.e. 933.6 eV [7]. This suggests the presence of reduced copper species in the CuO/ CeO2 catalysts. The surface composition, represented as the Cu / (Cu + Ce) atomic ratio, estimated by XPS was 0.35, while the nominal composition value of the 6.30 wt.% CuO/CeO2 catalyst was 0.13. The former was nearly three times higher
960
950
940
930
920
Binding Energy (eV) Fig. 6. XPS spectra of 6.30 wt.% CuO/CeO2 catalyst. (a) O 1s, (b) Ce 3d, (c) Cu 2p.
1
feature at ¨ 460 cm due to Raman active F2g mode of CeO2, the typical band of a fluorite structural material. CuO was characterized by the band at ¨ 298 cm 1. For the 6.30 wt.% CuO/CeO2 catalyst, no new feature appeared and this was consistent with the XRD analytic results (Fig. 3). XPS spectra of the O 1s, Cu 2p and Ce 3d binding energies of 6.30 wt.% CuO/CeO2 catalyst calcined at 500 -C
CO conversion (%)
100 80 500 ºC 2 h 500 ºC 5 h 500 ºC 8 h 700 ºC 2 h 700 ºC 5 h 700 ºC 8 h
60 40 20 0 80
100
120
140
160
180
200
220
240
260
Temperature (ºC) Fig. 8. Catalytic activities of 1.26 wt.% CuO/CeO2 catalysts.
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Table 3 Comparison of CO oxidation activities of CuO/CeO2 catalysts CeO2 preparation method CuO/CeO2 preparation method
T 50% (-C)
References
Thermal decomposition Precipitation Insert gas condensation Urea – nitrate combustion Sol-gel method Sol-gel method Sol-gel method
120 80 255 107 120 90 105
[2] [3] [6] [7] [8] [9] Present work
Impregnation Ultrasonication Insert gas condensation Urea – nitrate combustion Sol-gel method Sol-gel method Impregnation
T 50% is the light-off temperature for 50% CO conversion.
than that of the latter, indicating copper oxide species were enriched on the surface. Fig. 7 presents the catalytic activities of CuO/CeO2 catalysts calcined at 500 -C in air for 3.5 h with various CuO loadings. The activity of 1.26 wt.% CuO/CeO2 was the lowest whose full conversion temperature was 230 -C. The activities of CuO/CeO2 catalysts with 3.78 –15.12 wt.% CuO loading were obviously higher than that of 1.26 wt.% CuO/CeO2 catalyst. They reached full conversion at 130– 140 -C. On the basis of the present activity results, it was clear that increasing the CuO loading can promote the catalytic activity to a certain extent but it was not always so. This due to that the catalyst activity derives primarily from the combination of finely dispersed copper – cerium oxide system, while the bulk CuO particles have negligible contribution. The explanation was also confirmed in the previous work [2,3]. Fig. 8 shows the catalytic activities of 1.26 wt.% CuO/ CeO2 catalysts calcined at 500 and 700 -C in air for 2, 5, and 8 h. Under the experimental conditions described in the present paper, CO conversion increased with increasing temperature on the present catalysts. However, the CO conversion data were different. The CuO/CeO2 catalysts calcined at 500 -C were more active than those calcined at 700 -C. Furthermore, the CuO/CeO2 catalyst calcined at 500 -C for 5 h exhibited the highest activity. The activity of CuO/CeO2 catalysts calcined at 700 -C decreased with calcination time. This probably due to the crystallite size increasing of CeO2 with calcination time and the active CuO species sinter. In addition, correlating with the results in Table 2, it seemed that the catalytic activity of the CuO/ CeO2 catalysts was closely related to their surface area. At least once during each burning cycle, the temperature within the stove should be raised high enough to cause the catalyst to become active. This is the definition of the catalyst light-off temperature. Table 3 shows the comparison of CO oxidation activities, referring the light-off tempera-
ture for 50% CO conversion, on some CuO/CeO2 catalysts prepared by various methods. Compared to CuO/CeO2 catalysts prepared with other techniques the catalysts prepared via sol-gel combined with impregnation method exhibited similar or even higher catalytic performance.
4. Conclusions CeO2 nano-particles and CuO/CeO2 system were successfully prepared by sol-gel combined with impregnation method. The obtained spherical CeO2 was cubic phase and CuO was highly dispersed on it. TPR showed a two-step reduction for CuO/CeO2 catalysts. XPS analysis indicated the presence of redox couple Ce4+/Ce3+ and reduced copper species in the CuO/CeO2 catalysts. The prepared CuO/CeO2 system exhibits highly catalytic activity and the CuO loading, calcination temperature and calcination time affect the activity.
Acknowledgements This work has been supported by the National Natural Science Foundation of China (No. 20271028) and Tianjin Natural Science Foundation (No. 033602511). References [1] A. Martinez-Arias, R. Cataluna, J.C. Conesa, J. Soria, J. Phys. Chem., B 102 (1998) 809. [2] M.-F. Luo, Y.-J. Zhong, X.-X. Yuan, X.-M. Zheng, Appl. Catal., A Gen. 162 (1997) 121. [3] W. Liu, M. Flytzani-stephanopoulos, J. Catal. 153 (1995) 304. [4] S.-M. Zhang, W.-P. Huang, X.-H. Qiu, B.-Q. Li, X.-C. Zheng, S.-H. Wu, Catal. Lett. 80 (2002) 41. [5] P.G. Harrison, I.K. Ball, W. Azelee, W. Daniell, D. Goldfarb, Chem. Mater. 12 (2000) 3715. [6] S. Bjo¨rn, G. Didier, E.B. Robert, H. Andreas, A. Arne, L.R. Wallenberg, J. Catal. 211 (2002) 119. [7] G. Avgouropoulos, T. Ioannides, Appl. Catal., A Gen. 24 (2003) 155. [8] G. Avgouropoulos, T. Ioannides, C. Papadopoulou, J. Batista, S. Hocevar, H.K. Matralis, Catal. Today 75 (2002) 157. [9] G. Sedmak, S. Ho*evar, J. Levec, J. Catal. 213 (2003) 135. [10] X.-C. Zheng, W.-P. Huang, S.-M. Zhang, S.-R. Wang, S.-H. Wu, Chin. J. Catal. 24 (2003) 887. [11] M. Kobayashi, M. Flytzani-stephanopoulos, Ind. Eng. Chem. Res. 41 (2002) 3115. [12] G.R. Rao, H.R. Sanjan, B.G. Mishra, Colloids Surf., A Physicochem. Eng. Asp. 220 (2003) 261. [13] Y. Brik, M. Kacimi, B.V. Francois, M. Ziyad, J. Catal. 211 (2002) 470. [14] Y.-W. Zhang, R. Si, C.-S. Liao, C.-H. Yan, J. Phys. Chem., B 107 (2003) 10159.
Preparation, characterization and catalytic properties of CuO/CeO2 system Xiu-Cheng Zheng, Shu-Ping Wang, Shu-Rong Wang, Shou-Min Zhang, Wei-Ping Huang, Shi-Hua Wu* Department of Chemistry, Nankai University, Tianjin 300071, China Received 15 June 2004; received in revised form 19 October 2004; accepted 25 March 2005 Available online 1 June 2005
Abstract CeO2 nano-particles and CuO/CeO2 system were prepared by sol-gel and impregnation methods and characterized using combined spectroscopic techniques of XRD, XPS, TPR, FT – Raman, BET and HRTEM. It was found the CeO2 was cubic phase with fluorite structure and CuO was highly dispersed on the CeO2 particles. Temperature-programmed reduction (TPR) showed a two-step reduction for CuO/CeO2 catalysts. XPS analysis indicated the presence of redox couple Ce4+/Ce3+ and reduced copper species in the CuO/CeO2 catalysts. The factors, such as calcination temperature, calcination time and CuO loading, influenced on the catalytic properties of CuO/CeO2 catalysts. D 2005 Elsevier B.V. All rights reserved. Keywords: CeO2; CuO/CeO2; CO oxidation
1. Introduction Today, the protection of the environment is receiving much more attention than ever before. One of the major efforts of environmental cleanup is focused on controlling the emission of toxic pollutant produced during the combustion of fuels in automotive engines. The so-called ‘‘three-way catalysts’’ (TWC), in which CeO2 is a crucial component, are commonly used to reduce the emissions of CO, NOx , and hydrocarbons from automobile exhaust. The promoting of ceria was originally believed to be included both structural and chemical aspects, like the enhancement of the metal dispersion or its participation in the water – gas shift reaction or the decomposition of nitrogen oxides, and processes involving the oxygen storage capacity provided by the redox couple Ce4+/Ce3+, making more oxygen available for the oxidation processes [1]. On the other hand, precious metals are well-known oxidation catalysts with high activity and stability, and are widely used for CO oxidation reaction. However, due to the high cost of precious metals and their sensitivity to sulfur * Corresponding author. Tel.: +86 22 23505896; fax: +86 22 23502458. E-mail address: [email protected] (S.-H. Wu). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.03.005
poisoning, attention has been given to base metal catalysts, such as copper oxide [2 – 8]. Several methods for the preparation of CuO/CeO2 catalysts, such as thermal decomposition, co-precipitation and sol-gel starting from CeCl3 I 7H2O, have been reported [2,7 –9]. The present paper describes the preparation of CeO2 nano-particles via a sol-gel method starting from Ce(NO3)3 I 6H2O and CuO/CeO2 system via impregnation the obtained CeO2 powders with Cu(NO3)2 solution. Furthermore, the catalytic performance of the prepared CuO/CeO2 system was described.
2. Experimental CeO2 was prepared by sol-gel method [10]. At room temperature, 10.19 g Ce(NO3)3 I 6H2O was dissolved in 25 mL H2O in a flask, then 1 mL HNO3 was added to the solution. After the solution was heated to 65 -C in a water bath, 105 g citric acid was slowly added into it with vigorous stirring within 1 h. The reaction was run for 24 h until the light yellow wet gel was obtained. The gel was dried at 65 -C to remove the solvents and then calcined at 320 -C for 2.5 h to give the yellow-white CeO2 nano-particles.
3. Results and discussion
(400)
60
70
(331) (420)
(222)
(311)
(220) (200) CuO/CeO2 8 h
CuO/CeO2 5 h
CuO/CeO2 2 h CeO2
10
20
30
40
50
80
2 Theta (degree) Fig. 2. XRD patterns of CeO2 and CuO/CeO2 catalysts calcined at 700 -C.
1(a), the adjacent CeO2 nanocrystals intruded on each other and the existence of the grain boundary indicated that it was crystallized. Furthermore, The prepared CeO2 was almost regular spherical morphology particles with similar size. Fig. 2 shows the XRD patterns of 1.26 wt.% CuO/CeO2 catalysts calcined at 700 -C in air for 2, 5, and 8 h. The pattern of CeO2 was also exhibited for comparison purpose. It can be seen that only CeO2 characteristic peaks appeared and no copper oxide reflections were found by XRD for all the present samples in Fig. 2. This indicates that copper oxide exhibits in a highly dispersed form [11]. Fig. 3 shows the XRD patterns of CuO/CeO2 catalysts with various CuO loadings calcined at 500 -C for 3.5 h. Only CeO2 XRD peaks were observed when CuO loading was lower than 6.30 wt.%. The weak peaks attributed to CuO crystal phase can be observed on 10.08 wt.% CuO/ CeO2 catalyst, indicating that bulk CuO had already formed on the 10.08 wt.% CuO/CeO2 catalyst in addition to the dispersed clusters of copper oxide [11]. As expected, the
60
70
(331) (420)
(311)
(400)
CuO
15.12 wt.% CuO/CeO2
(222)
(200)
Intensity (cps)
(220)
(111)
Fig. 1 shows the typical HRTEM micrograph of the obtained CeO2 nano-particles. As can be seen from Fig.
Intensity (cps)
The CuO/CeO2 system was prepared by the conventional wet impregnation method using an aqueous solution of Cu(NO3)2. The prepared samples were dried at 80 -C in an oven and then heated at 500 and 700 -C in air for 2, 3.5, 5, and 8 h. The CuO loading was 1.26– 15.12 wt.%. The surface area of the samples was calculated by BET method from N2 adsorption isotherms recorded at 196 -C using a ChemBET-3000 physical Autosorbmeter. The X-ray powder diffraction (XRD) analysis was performed using a Dmax-2500 X-ray diffractometer with CuKa radiation. The electron microscopy study was taken using Philips T20ST high-resolution transmission electron microscope. Fourier transform (FT) Raman spectra were recorded with a BRUKER RTS 100/S FT –RAMAN spectrometer, using the 1064-nm excitation line of Nd – YAG laser. Temperature-programmed reduction (TPR) experiments were performed on the Micromeritics temperature-programmed chemisorption instrument under the mixture of 5% H2 in N2 flowing (30 mL min 1) at a heating rate of 10 -C/min. X-ray photoelectron spectra (XPS) were recorded on a PHI1600 spectrometer equipped with a MgKa radiation for exciting photoelectrons. The catalytic activities of the catalysts for CO oxidation were measured in a fixed bed flow microreactor (7 mm I.D.) under atmospheric pressure using 50 mg of catalyst powders doping with 2 g fine quartz sand in order to avoid temperature differences and bed channeling. The total gas flow rate was 33.6 mL/min, contained 1.0% CO balanced with air. The reactant and product composition were analyzed on-line by a GC-508A gas chromatograph equipped with a thermal conductivity detector (TCD).
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12.60 wt.% CuO/CeO2 10.08 wt.% CuO/CeO2
6.30 wt.% CuO/CeO2
10
20
30
40
50
80
2 Theta (degree) Fig. 1. HRTEM micrograph of the prepared CeO2 nano-particles.
Fig. 3. XRD patterns of CuO/CeO2 catalysts with various CuO loadings.
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Table 1 Crystallite sizes of CeO2 and CuO in CuO/CeO2 catalysts Sample
CeO2 crystallite size (nm)
CuO crystalline size (nm)
CeO2 1.26 wt.% CuO/CeO2 (700 -C 2 h) 1.26 wt.% CuO/CeO2 (700 -C 5 h) 1.26 wt.% CuO/CeO2 (700 -C 8 h) 6.30 wt.% CuO/CeO2 (500 -C 3.5 h) 10.08 wt.% CuO/CeO2 (500 -C 3.5 h) 12.60 wt.% CuO/CeO2 (500 -C 3.5 h) 15.12 wt.% CuO/CeO2 (500 -C 3.5 h)
9.8 27.4 44.5 61.6 12.6 13.4 13.9 13.4
– – – – – 46.5 46.5 46.5
IV
S BET (m2 g
500 500 500 700 700 700
2 5 8 2 5 8
51.0 62.2 41.2 14.2 12.1 10.4
100
200
300
400
500
600
700
800
900
1000
Temperature (ºC) Fig. 4. TPR pattern of 6.30 wt.% CuO/CeO2 catalyst.
H2-TPR of pure CeO2 showed a low temperature reduction with peak maximum around 500 -C and a high temperature reduction with peak maximum around 800 -C. The low temperature peak around 500 -C was attributed to the removal of surface capping oxygen ions and the high temperature peak around 800 -C was attributed to bulk oxygen ions during the reduction reaction with H2. Our results proved the above viewpoint. In other words, we think that I peak can be attributed to the reduction of dispersed copper oxide (non-crystalline forms), II peak attributed to the reduction of bulk copper oxide (crystalline forms). III and IV peaks can be attributed to the low and high temperature reduction of freely CeO2 powders. In recent years, FT – Raman spectroscopy has become an increasingly valuable tool for the investigation of the supported catalysts [13]. The FT – Raman spectra of CeO2, CuO (thermal decomposition of Cu(NO3)2 at 500 -C for 3.5 h) and 6.30 wt.% CuO/CeO2 catalyst are comparatively shown in Fig. 5. CeO2 was characterized by the strong 460
Raman intensity (a.u.)
Table 2 Surface area of 1.26 wt.% CuO/CeO2 catalysts Calcination time (h)
I
III
intensity of CuO crystal phase peaks increased with the increase of CuO loading. The average particle size of CeO2 and CuO on CuO/CeO2 catalysts (see Table 1) was 12.6 – 13.9 and 46.5 nm, respectively. the particle size did not change obviously with increasing CuO loading. The measured data of the BET surface areas of 1.26 wt.% CuO/CeO2 catalysts are listed in Table 2. The BET surface area of the catalysts calcined at 500 -C for 2, 5, and 8 h were 51.0, 62.2, and 41.2 m2 g 1. While the BET surface area of the catalysts calcined at 700 -C for 2, 5, and 8 h were 14.2, 12.1, and 10.4 m2 g 1. It was noted that the BET surface area decreased with the increase of calcination time except the catalyst calcined at 500 -C for 2 h, and the BET surface areas of the catalysts calcined at 500 -C were obviously larger than that of those calcined at 700 -C. This was in accordance to the results determined from XRD (shown in Table 1), which showed that the crystallite size of CeO2 on CuO/CeO2 catalysts calcined at 700 -C increased with calcination time. In order to determine the existing states of CuO and CeO2, we have employed H2-TPR characterization. Fig. 4 shows the typical TPR profile of 6.30 wt.% CuO/CeO2 catalyst calcined at 500 -C for 3.5 h. Two overlapping reduction peaks with higher intensity at about 160 and 210 -C (called I and II, respectively) were presented. Two broad reduction peaks with lower intensity at 400 – 500 -C and 700 – 900 -C (called III and IV, respectively) were presented, too. Rao et al. [12] reported that pure CuO showed a single reduction at 380– 392 -C while CuO supported on CeO2 showed a two-step reduction at much lower temperatures due to finely dispersed CuO strongly interacting with CeO2 (130 –175 -C) and bulk CuO particles (180 – 200 -C). The
Calcination temperature (-C)
TCD signal (a.u.)
II
6.30 wt.% CuO/CeO2
298
CuO
1
)
CeO2
800
700
600
500
400
300
200
Wavenumber (cm-1) Fig. 5. FT – Raman patterns of CeO2, CuO and 6.30 wt.% CuO/CeO2 catalysts.
X.-C. Zheng et al. / Materials Science and Engineering C 25 (2005) 516 – 520
(a)
519
100
O1s
CO conversion (%)
529.3
C (S)
4
531.4
80 1.26 wt.% CuO/CeO2 3.78 wt.% CuO/CeO2 6.30 wt.% CuO/CeO2 10.08 wt.%CuO/CeO2 12.60 wt.%CuO/CeO2 15.12 wt.%CuO/CeO2
60 40 20 0 80
0
535
530
525
100
120
520
140
160
180
200
220
Temperature (ºC)
Binding Energy (eV) Fig. 7. Catalytic activities of CuO/CeO2 catalysts with various CuO loadings.
(b) Ce3d
882.5
C (S)
05
0
910
905
900
895
890
885
880
875
Binding Energy (eV)
(c) 932.5
Cu2P
C (S)
5
in air for 3.5 h, are shown in Fig. 6. It can be observed from Fig. 6(a) that O 1s showed in two main peaks at about 531.6 eV contributed to the absorbed oxygen and 529.5 eV contributed to lattice oxygen. The Ce 3d showed in five peaks at about 882, 888, 898, 900, and 907 eV in the present Fig. 6(b), which were consistent with the previous report of Ce4+ [14], indicating the main valence of CeO2 in the sample was + 4. Furthermore, there were two weak characteristic peaks of Ce2O3 at about 883 and 903 eV. This proved that existence of the redox couple Ce+4/Ce+3 in the CuO/CeO2 catalysts. On the other hand, the weak peaks in XPS spectra and no Ce2O3 crystallite peaks appearing on XRD pattern in Figs. 2 and 3 indicated that Ce2O3 content was very low. The peaks of Cu 2p3/2 and Cu 2p1/2 were centered at 932.5 and 952.5 eV, respectively. The Cu 2p3/2 value was lower than that of CuO, i.e. 933.6 eV [7]. This suggests the presence of reduced copper species in the CuO/ CeO2 catalysts. The surface composition, represented as the Cu / (Cu + Ce) atomic ratio, estimated by XPS was 0.35, while the nominal composition value of the 6.30 wt.% CuO/CeO2 catalyst was 0.13. The former was nearly three times higher
960
950
940
930
920
Binding Energy (eV) Fig. 6. XPS spectra of 6.30 wt.% CuO/CeO2 catalyst. (a) O 1s, (b) Ce 3d, (c) Cu 2p.
1
feature at ¨ 460 cm due to Raman active F2g mode of CeO2, the typical band of a fluorite structural material. CuO was characterized by the band at ¨ 298 cm 1. For the 6.30 wt.% CuO/CeO2 catalyst, no new feature appeared and this was consistent with the XRD analytic results (Fig. 3). XPS spectra of the O 1s, Cu 2p and Ce 3d binding energies of 6.30 wt.% CuO/CeO2 catalyst calcined at 500 -C
CO conversion (%)
100 80 500 ºC 2 h 500 ºC 5 h 500 ºC 8 h 700 ºC 2 h 700 ºC 5 h 700 ºC 8 h
60 40 20 0 80
100
120
140
160
180
200
220
240
260
Temperature (ºC) Fig. 8. Catalytic activities of 1.26 wt.% CuO/CeO2 catalysts.
520
X.-C. Zheng et al. / Materials Science and Engineering C 25 (2005) 516 – 520
Table 3 Comparison of CO oxidation activities of CuO/CeO2 catalysts CeO2 preparation method CuO/CeO2 preparation method
T 50% (-C)
References
Thermal decomposition Precipitation Insert gas condensation Urea – nitrate combustion Sol-gel method Sol-gel method Sol-gel method
120 80 255 107 120 90 105
[2] [3] [6] [7] [8] [9] Present work
Impregnation Ultrasonication Insert gas condensation Urea – nitrate combustion Sol-gel method Sol-gel method Impregnation
T 50% is the light-off temperature for 50% CO conversion.
than that of the latter, indicating copper oxide species were enriched on the surface. Fig. 7 presents the catalytic activities of CuO/CeO2 catalysts calcined at 500 -C in air for 3.5 h with various CuO loadings. The activity of 1.26 wt.% CuO/CeO2 was the lowest whose full conversion temperature was 230 -C. The activities of CuO/CeO2 catalysts with 3.78 –15.12 wt.% CuO loading were obviously higher than that of 1.26 wt.% CuO/CeO2 catalyst. They reached full conversion at 130– 140 -C. On the basis of the present activity results, it was clear that increasing the CuO loading can promote the catalytic activity to a certain extent but it was not always so. This due to that the catalyst activity derives primarily from the combination of finely dispersed copper – cerium oxide system, while the bulk CuO particles have negligible contribution. The explanation was also confirmed in the previous work [2,3]. Fig. 8 shows the catalytic activities of 1.26 wt.% CuO/ CeO2 catalysts calcined at 500 and 700 -C in air for 2, 5, and 8 h. Under the experimental conditions described in the present paper, CO conversion increased with increasing temperature on the present catalysts. However, the CO conversion data were different. The CuO/CeO2 catalysts calcined at 500 -C were more active than those calcined at 700 -C. Furthermore, the CuO/CeO2 catalyst calcined at 500 -C for 5 h exhibited the highest activity. The activity of CuO/CeO2 catalysts calcined at 700 -C decreased with calcination time. This probably due to the crystallite size increasing of CeO2 with calcination time and the active CuO species sinter. In addition, correlating with the results in Table 2, it seemed that the catalytic activity of the CuO/ CeO2 catalysts was closely related to their surface area. At least once during each burning cycle, the temperature within the stove should be raised high enough to cause the catalyst to become active. This is the definition of the catalyst light-off temperature. Table 3 shows the comparison of CO oxidation activities, referring the light-off tempera-
ture for 50% CO conversion, on some CuO/CeO2 catalysts prepared by various methods. Compared to CuO/CeO2 catalysts prepared with other techniques the catalysts prepared via sol-gel combined with impregnation method exhibited similar or even higher catalytic performance.
4. Conclusions CeO2 nano-particles and CuO/CeO2 system were successfully prepared by sol-gel combined with impregnation method. The obtained spherical CeO2 was cubic phase and CuO was highly dispersed on it. TPR showed a two-step reduction for CuO/CeO2 catalysts. XPS analysis indicated the presence of redox couple Ce4+/Ce3+ and reduced copper species in the CuO/CeO2 catalysts. The prepared CuO/CeO2 system exhibits highly catalytic activity and the CuO loading, calcination temperature and calcination time affect the activity.
Acknowledgements This work has been supported by the National Natural Science Foundation of China (No. 20271028) and Tianjin Natural Science Foundation (No. 033602511). References [1] A. Martinez-Arias, R. Cataluna, J.C. Conesa, J. Soria, J. Phys. Chem., B 102 (1998) 809. [2] M.-F. Luo, Y.-J. Zhong, X.-X. Yuan, X.-M. Zheng, Appl. Catal., A Gen. 162 (1997) 121. [3] W. Liu, M. Flytzani-stephanopoulos, J. Catal. 153 (1995) 304. [4] S.-M. Zhang, W.-P. Huang, X.-H. Qiu, B.-Q. Li, X.-C. Zheng, S.-H. Wu, Catal. Lett. 80 (2002) 41. [5] P.G. Harrison, I.K. Ball, W. Azelee, W. Daniell, D. Goldfarb, Chem. Mater. 12 (2000) 3715. [6] S. Bjo¨rn, G. Didier, E.B. Robert, H. Andreas, A. Arne, L.R. Wallenberg, J. Catal. 211 (2002) 119. [7] G. Avgouropoulos, T. Ioannides, Appl. Catal., A Gen. 24 (2003) 155. [8] G. Avgouropoulos, T. Ioannides, C. Papadopoulou, J. Batista, S. Hocevar, H.K. Matralis, Catal. Today 75 (2002) 157. [9] G. Sedmak, S. Ho*evar, J. Levec, J. Catal. 213 (2003) 135. [10] X.-C. Zheng, W.-P. Huang, S.-M. Zhang, S.-R. Wang, S.-H. Wu, Chin. J. Catal. 24 (2003) 887. [11] M. Kobayashi, M. Flytzani-stephanopoulos, Ind. Eng. Chem. Res. 41 (2002) 3115. [12] G.R. Rao, H.R. Sanjan, B.G. Mishra, Colloids Surf., A Physicochem. Eng. Asp. 220 (2003) 261. [13] Y. Brik, M. Kacimi, B.V. Francois, M. Ziyad, J. Catal. 211 (2002) 470. [14] Y.-W. Zhang, R. Si, C.-S. Liao, C.-H. Yan, J. Phys. Chem., B 107 (2003) 10159.