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Surface properties of palladium supported on cerium oxide and its catalytic activity for methanol decomposition
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
2315
Surface properties of palladium supported on cerium oxide and its catalytic activity for methanol decomposition Yasuyuki Matsumura,* Yuichi Ichihashi, Yasuo Morisawa, Mitsutaka Okumura and Masatake Haruta Osaka National Research Institute, AIST, Midorigaoka, Ikeda, Osaka 563-8577, Japan Cerium oxide can be partially reduced with hydrogen at 300~ in presence of palladium particles on the surface while palladium is rather oxidized in stead. The reduction may proceed in the following process; PdO + H 2 ~ Pd + H20 and 2 CeO 2 + Pd-- Ce203 + PdO. In the Pd/CeO 2 samples prepared by a deposition-precipitation method, the strong contact between palladium particles and the support can cause enhancement of carbon monoxide adsorption on cerium oxide at room temperature and higher catalytic activity for the methanol decomposition to carbon monoxide and hydrogen than that produced with the sample prepared by the conventional impregnation technique. 1. INTRODUCTION Palladium supported on cerium oxide is essentially used as catalysts for various reactions while the interaction between palladium and cerium oxide affects the catalytic activity remarkably [1-6]. The reduction temperature changes the catalytic properties to reactions such as hydrogenation of carbon dioxide [3], hydrogenation of benzene [4], and oxygen exchanging of carbon monoxide [5], suggesting that reduction state of the catalyst strongly affects the activity. In the present work, we will show that palladium particles and the surface of cerium oxide interact each other and reduction of palladium with hydrogen is suppressed in presence of chlorine impurity. Palladium in Pd/CeO 2 prepared by a deposition-precipitation technique can be reduced fairly well and it enhances adsorption of carbon monoxide on the support.
*Present address: ResearchInstitute of Innovative Technologyfor the Earth, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan.
2316 2.EXPERIMENTAL Three ceria-supported palladium samples were prepared by different methods. A sample (Pd-IMP) was prepared by the conventional impregnation technique from an aqueous solution of PdC12 (Kishida Chemicals, GR grade) in which CeO 2 (Daiichi Kigenso Kagaku Kogyo) was dispersed. The mixture was evaporated at around 80~ and the resulting wet solid was dried at 120~ for 5 h, then calcined at 450~ for 5 h in air. Two of them were prepared by a deposition-precipitation method from the same starting mixture as for Pd-IMP. Palladium hydroxide was exclusively deposited on the surface of cerium oxide by addition of 1 M NaOH solution to the PdC12 solution and the pH value of the solution was maintained at 10 for 1 h. One sample (Pd-UDP) was prepared with stirring by ultrasonic also with mechanical stirring, and another (Pd-DP) was only with a magnetic stirrer. The resulting solids were washed with distilled water, dried at 120~ for 5 h, and finally calcined at 450~ for 5 h in air. All the palladium contents of the three samples were 3 wt%. Spectra of XAFS (X-ray absorption fine structure) for the samples reduced with hydrogen at 300~ for 1 h were taken at room temperature in transmission mode for Pd K-edges at the beam-line BL01B1 of SPring-8 (Japan Synchrotron Radiation Research Institute, proposal No. 1998A0050-NX-NP). X-ray photoelectron spectra (XPS) were recorded at room temperature with a Shimadzu ESCA-KM. The samples were reduced with hydrogen at 300~ for I h in the spectrometer. Binding energies were determined by reference to the C ls binding energy of 284.6 eV. The surface molar ratio of Pd/Ce/O was calculated from the peak areas using the atomic sensitivity factors of Pd 3d (4.6), Ce 3d (10), and O ls (0.66) [6]. BET surface areas of the samples reduced with hydrogen at 300~ for I h were determined from the isotherms of nitrogen physisorption. Temperature-programmed desorption (TPD) of carbon monoxide from the palladium samples was carried out using a system equipped with TCD. A sample (0.20 g) was reduced with 5 vol% of hydrogen diluted with argon (3.6 dm 3 h 1) at 300~ for I h, then, it was kept under a stream of argon at 300 ~ for I h. After cooling the sample to room temperature, carbon monoxide was supplied for 0.3 h and excessive carbon monoxide on the sample was removed with a flow of argon for 0.5 h. In the measurement the sample was heated with a rate of600~ h 1 under an argon flow (3.6 dm 3 h-l). Catalytic tests were performed with a fixed bed continuous flow reactor. A catalyst (0.2 g) was sandwiched with quartz wool plugs in a quartz tube reactor of which i.d. was 6 mm. After the catalyst was reduced in a stream of 20 vol% hydrogen diluted with argon (1.8 dm 3 h ~) at 300~ for I h, 25 vol% of methanol diluted with argon was fed at 180~ (flow rate, 4.0 dm 3 hl). The outlet gas was analyzed with an on-line gas chromatograph whose column was Porapak T (4 m).
2317 3. RESULTS and DISCUSSION 3.1. Oxidation state of palladium
Profiles of XANES (X-ray absorption of near edge structure) for the Pd/CeO 2 samples reduced at 300~ are shown in Figure 1. The spectra for Pd-DP and Pd-UDP resembled to that of metallic palladium [7], but in the case of Pd-IMP, the peak at 24390 eV, which is not present in the XANES of PdO, was small, suggesting that palladium particles in Pd-IMP are partly oxidized. In the XPS of the samples reduced in situ, there were three Gaussian peaks of Pd 3d5~~ at 335.9-336.4 eV(Pd=), 337.9-338.5 eV(Pd~), and 341.2-342.0 eV(Pdv) (Figure 2 and Table 1). Other peaks deconvoluted are the corresponding lines of Pd 3d3~2 because the binding energies of these peaks were always higher than those of Pd 3d8/9 by 5.2-5.4 eV. Although the XANES of Pd-UDP suggests that metallic palladium is the major species, the value of Pd= (336.4 eV) was considerably higher than that of palladium metal (335.0 eV) [6]. Since nanometer-size palladium particles often results in the higher binding energy [7,8], it is supposed that palladium particles in Pd-UDP interact strongly with the surface of the support. The binding energies forPdp and Pdv are significantly higher than that of PdO (336.3 eV) [6], showing presence of palladium oxides interacting with the surface of cerium oxide. 3.2. Reducibility of cerium oxide In the XPS of Ce 3d region, peaks at 917-919 eV (u"') were recorded with the Pd/CeO 2 samples reduced at 300~ in situ (not shown). Since the peak is not present
PdB
( a ) P ~ Q r ,.a 0
[b)Pd-DP /
,.a
(c)Pd-UDP/'~------_.____J i
I
i
i
24200 24300 24400 24500 24600 Energy / eV Fig. 1. XANES of 3 wt% Pd/CeO~ reduced at 300 ~
355
i
i
i
350 345 340 335 Binding energy / eV
330
Fig. 2. XPS of Pd 3d region for 3 wt% Pd/CeO 2 reduced at 300~
2318 Table 1 S u m m a r y of XPS analyses for 3 wt% Pd/CeO 2 reduced at 300~ Sample Pd-IMP Pd-DP Pd-UDP
Binding energy / eV ~ Pd 3d5/2 O ls 335.9 (0.19) 336.0 (0.45) 336.4 (0.78)
338.5 (0.51) 337.9 (0.45) 338.0 (0.07)
341.2 b (0.31) 341.7 b (0.10) 342.0 b (0.15)
529.6 (0.25) 529.8 (0.34) 529.8 (0.51)
532.1 (0.49) 531.8 (0.50) 531.8 (0.42)
Surface molar ratio Pd/Ce O/Ce 534.7 (0.26) 534.3 (0.16) 534.0 (0.07)
0.06
2.0
0.09
2.5
0.07
2.9
In parentheses normalized peak intensities are given. b The energy is estimated from the peak position of Pd 3du2 assuming the separation of 5.3 eV. Table 2 Surface properties of 3 wt% Pd/CeO 2 reduced at 300~ Sample
%u'" /%
Ce 4§a /%
Pd-IMP Pd-DP Pd-UDP
6.9 7.5 10.7
50 60 73
and its catalytic activity
Surface CO desorbed area/m 2 g-1 /mmol g-1 70 92 102
0.2 1.1 0.8
Conversion of MeOH / % 4.2 8.5 9.9
aContent of Ce 4§ determined from the intensities of XPS peaks in O ls region. in the spectrum for Ce 3§ species, the percentage of the peak area for u'" in the whole Ce 3d region (%u'") is a parameter of the relative amount of Ce 4§ in the sample [9]. The values were considerably smaller than 13.7% which was reported for oxidized CeO 2 (Table 2) [9], showing the partial reduction of CeO 2 to Ce203. The spectra for the samples in O ls region were deconvoluted into three Gaussian peaks at 529.7 9 0.1 eV, 531.9 • 0.2 eV, and 534.3 ~: 0.4 eV as shown inFigure 3. Surprisingly the value of x 1/2 + y1~2was always 0.98-1.01, where x and y are the normalized intensities of the peaks at 530 and 534 eV, respectively (see Table 1). On the surface of cerium oxide, an oxygen atom connects mainly to two cerium ions. When a value, a, is the fraction of Ce 3§ to whole cerium atoms, the fractions of oxygen atoms connecting to two Ce a§ and to two Ce3+ will be (1- a)2 and a 2, respectively. If x and y are equal to (1- a) 2 and a 2, respectively, the value of x 1/2 + ym will be one. The surface fraction of Ce 4§ calculated corresponds well to the value of %u'" (see Table 2). Since palladium oxide can be reduced with hydrogen even at room temperature, presence of palladium oxide shows that reduction of palladium is suppressed by the
2319 presence of cerium oxide. Hence, it can be proposed that reduction of cerium oxide proceeds via reduction of palladium oxide, s PdO + H 2 - Pd + I-I~O and 2 CeO 2 + Pd -- Ce20 s + PdO. In the mechanism reduction of cerium oxide causes formation of palladium oxide. It is consistent with the dependence of the concentration of Ce4+ on the surface dencity of Pd~. The difference in the reducibility will be due to presence of chlorine in the samples, that is, the contents were 0.38, 0.033, and 0.020 wt% for Pd-IMP, PD-DP, and PD-UDP, respectively, because formation of CeOC1 can enhance reduction of cerium oxide [10]. Reduction of the support probably causes a decrease in the BET surface area (see Table 2). 3.3. Interaction between palladium and cerium oxide Temperature-programmed desorption (TPD) of carbon monoxide was carried out after reduction of the sample with hydrogen at 300~ followed by adsorption of carbon monoxide at room temperature. There was a desorption peak around 300~ in the profile of Pd-IMP (Figure 4). The amounts of carbon monoxide desorbed from Pd-DP and Pd-UDP were estimated as 1.1 mmol g-1 and 0.8 mmol gl, respectively, from the areas. Since these quantities are extremely larger than the content ofpaUadium (0.3 mmol gl), carbon monoxide is mainly adsorbed on the surface of the ceria support, t h a t is, spill-over of carbon monoxide takes place. It is known that irreversible adsorption of carbon monoxide takes place on unmodified ceria reduced at 400~ or above [11,12], but no significant adsorption occurs on ceria reduced at 300~ The adsorption of carbon monoxide on the surface of ceria without reduction is enhanced by elevation of adsorption temperature [13], suggesting that adsorption of carbon monoxide on cerium oxide is activated at room temperature on the samples modified
x s
.=_
f/#
Ir @
(~'~~
41~
r
c
m
( c ) P d - U ~ ~ ~ !
540
i
535 530 Binding energy / eV
i
5~
Fig. 3. XPS of O ls region for 3 wt% Pd/CeO 2 reduced at 300~
i
200 400 Temperature l "C
i
600
Fig. 4. TPD of CO adsorbed on 3 wt% Pd/CeO 2 reduced at 300~
2320 with palladium by deposition-precipitation. When zirconium oxide is employed as a support of palladium, Pd-O-Zr can be present in the sample prepared by the deposition-precipitation method [7]. This suggests that the preparation by deposition-precipitation often results in strong contact between palladium particles and the support compared with the impregnation method. The desorption peak of carbon monoxide for Pd-IMP was apparently small, implying a weaker interaction between palladium and the support than in Pd-DP and Pd-UDP. Methanol decomposition to carbon monoxide and hydrogen at 180~ was catalyzed over these samples pretreated with hydrogen at 300~ The activities of the samples prepared by deposition-precipitation were significantly higher than that produced with Pd-IMP (see Table 2). Since Pd, is considered to be the active species of the decomposition, the strong contact between palladium particles and cerium oxide will be an advantageous feature for the catalysis. REFERENCES 1. A. Trovarelli, Catal. Rev.- Sci. Eng., 38 (1996) 439. 2. S. Bernal, J.J. Calvino, M.A. Cauqui, J.M. Gatica, C. Larese, J.A. Pdrez Omil and J.M. Pintado, Catal. Today, 50 (1999) 175. 3. L. Fan and K. Fujimoto, J. Catal., 150 (1994) 217. 4. L. Kephiski, M. Wolcyrz and J. Okal, J. Chem. Soc. Faraday Trans., 91 (1995) 507. 5. S. Naito, S. Aida, T. Tsunematsu and T. Miyao, Chem. Lett., 1998, 941. 6. D. Briggs, M.P. Seah, Practical surface analysis (2nd edition) Vol. 1: Auger and X-ray photoelectron spectroscopy, John Willy & Sons, Inc., New York, 1990. 7. Y. Matsumura, M. Okumura, Y. Usami, K. Kagawa, H. Yamashita, M. Anpo and M. Haruta, Catal. Lett., 44, (1997) 189. 8. J. Goetz, M.A. Volpe, A.M. Sica, C.E. Gigola and R. Touroude, J. Catal., 153 (1995) 86. 9. J.Z. Shu, W.H. Weber and H.S. Gandhi, J. Phys. Chem., 92 (1988) 4964. 10. K. Kili, L. Hilaire and F. Le Normand, Phys. Chem. Chem. Phys., 1 (1999) 1623. 11. C. Li, Y. Sakata, T. Arai, K. Domen, K. Maruya and T. Onishi, J. Chem. Soc. Faraday Trans. 1, 85 (1989) 1451. 12. C. Binet, A. Badri, M. Boutonnet-Kizling and J.-C. Lavalley, J. Chem. Soc. Faraday Trans., 90 (1994) 1023. 13. C.Li, Y. Sakata, T. Arai, K. Domen, K. Maruya and T. Onishi, J. Chem. Soc. Faraday Trans. 1, 85 (1989) 929.
2315
Surface properties of palladium supported on cerium oxide and its catalytic activity for methanol decomposition Yasuyuki Matsumura,* Yuichi Ichihashi, Yasuo Morisawa, Mitsutaka Okumura and Masatake Haruta Osaka National Research Institute, AIST, Midorigaoka, Ikeda, Osaka 563-8577, Japan Cerium oxide can be partially reduced with hydrogen at 300~ in presence of palladium particles on the surface while palladium is rather oxidized in stead. The reduction may proceed in the following process; PdO + H 2 ~ Pd + H20 and 2 CeO 2 + Pd-- Ce203 + PdO. In the Pd/CeO 2 samples prepared by a deposition-precipitation method, the strong contact between palladium particles and the support can cause enhancement of carbon monoxide adsorption on cerium oxide at room temperature and higher catalytic activity for the methanol decomposition to carbon monoxide and hydrogen than that produced with the sample prepared by the conventional impregnation technique. 1. INTRODUCTION Palladium supported on cerium oxide is essentially used as catalysts for various reactions while the interaction between palladium and cerium oxide affects the catalytic activity remarkably [1-6]. The reduction temperature changes the catalytic properties to reactions such as hydrogenation of carbon dioxide [3], hydrogenation of benzene [4], and oxygen exchanging of carbon monoxide [5], suggesting that reduction state of the catalyst strongly affects the activity. In the present work, we will show that palladium particles and the surface of cerium oxide interact each other and reduction of palladium with hydrogen is suppressed in presence of chlorine impurity. Palladium in Pd/CeO 2 prepared by a deposition-precipitation technique can be reduced fairly well and it enhances adsorption of carbon monoxide on the support.
*Present address: ResearchInstitute of Innovative Technologyfor the Earth, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan.
2316 2.EXPERIMENTAL Three ceria-supported palladium samples were prepared by different methods. A sample (Pd-IMP) was prepared by the conventional impregnation technique from an aqueous solution of PdC12 (Kishida Chemicals, GR grade) in which CeO 2 (Daiichi Kigenso Kagaku Kogyo) was dispersed. The mixture was evaporated at around 80~ and the resulting wet solid was dried at 120~ for 5 h, then calcined at 450~ for 5 h in air. Two of them were prepared by a deposition-precipitation method from the same starting mixture as for Pd-IMP. Palladium hydroxide was exclusively deposited on the surface of cerium oxide by addition of 1 M NaOH solution to the PdC12 solution and the pH value of the solution was maintained at 10 for 1 h. One sample (Pd-UDP) was prepared with stirring by ultrasonic also with mechanical stirring, and another (Pd-DP) was only with a magnetic stirrer. The resulting solids were washed with distilled water, dried at 120~ for 5 h, and finally calcined at 450~ for 5 h in air. All the palladium contents of the three samples were 3 wt%. Spectra of XAFS (X-ray absorption fine structure) for the samples reduced with hydrogen at 300~ for 1 h were taken at room temperature in transmission mode for Pd K-edges at the beam-line BL01B1 of SPring-8 (Japan Synchrotron Radiation Research Institute, proposal No. 1998A0050-NX-NP). X-ray photoelectron spectra (XPS) were recorded at room temperature with a Shimadzu ESCA-KM. The samples were reduced with hydrogen at 300~ for I h in the spectrometer. Binding energies were determined by reference to the C ls binding energy of 284.6 eV. The surface molar ratio of Pd/Ce/O was calculated from the peak areas using the atomic sensitivity factors of Pd 3d (4.6), Ce 3d (10), and O ls (0.66) [6]. BET surface areas of the samples reduced with hydrogen at 300~ for I h were determined from the isotherms of nitrogen physisorption. Temperature-programmed desorption (TPD) of carbon monoxide from the palladium samples was carried out using a system equipped with TCD. A sample (0.20 g) was reduced with 5 vol% of hydrogen diluted with argon (3.6 dm 3 h 1) at 300~ for I h, then, it was kept under a stream of argon at 300 ~ for I h. After cooling the sample to room temperature, carbon monoxide was supplied for 0.3 h and excessive carbon monoxide on the sample was removed with a flow of argon for 0.5 h. In the measurement the sample was heated with a rate of600~ h 1 under an argon flow (3.6 dm 3 h-l). Catalytic tests were performed with a fixed bed continuous flow reactor. A catalyst (0.2 g) was sandwiched with quartz wool plugs in a quartz tube reactor of which i.d. was 6 mm. After the catalyst was reduced in a stream of 20 vol% hydrogen diluted with argon (1.8 dm 3 h ~) at 300~ for I h, 25 vol% of methanol diluted with argon was fed at 180~ (flow rate, 4.0 dm 3 hl). The outlet gas was analyzed with an on-line gas chromatograph whose column was Porapak T (4 m).
2317 3. RESULTS and DISCUSSION 3.1. Oxidation state of palladium
Profiles of XANES (X-ray absorption of near edge structure) for the Pd/CeO 2 samples reduced at 300~ are shown in Figure 1. The spectra for Pd-DP and Pd-UDP resembled to that of metallic palladium [7], but in the case of Pd-IMP, the peak at 24390 eV, which is not present in the XANES of PdO, was small, suggesting that palladium particles in Pd-IMP are partly oxidized. In the XPS of the samples reduced in situ, there were three Gaussian peaks of Pd 3d5~~ at 335.9-336.4 eV(Pd=), 337.9-338.5 eV(Pd~), and 341.2-342.0 eV(Pdv) (Figure 2 and Table 1). Other peaks deconvoluted are the corresponding lines of Pd 3d3~2 because the binding energies of these peaks were always higher than those of Pd 3d8/9 by 5.2-5.4 eV. Although the XANES of Pd-UDP suggests that metallic palladium is the major species, the value of Pd= (336.4 eV) was considerably higher than that of palladium metal (335.0 eV) [6]. Since nanometer-size palladium particles often results in the higher binding energy [7,8], it is supposed that palladium particles in Pd-UDP interact strongly with the surface of the support. The binding energies forPdp and Pdv are significantly higher than that of PdO (336.3 eV) [6], showing presence of palladium oxides interacting with the surface of cerium oxide. 3.2. Reducibility of cerium oxide In the XPS of Ce 3d region, peaks at 917-919 eV (u"') were recorded with the Pd/CeO 2 samples reduced at 300~ in situ (not shown). Since the peak is not present
PdB
( a ) P ~ Q r ,.a 0
[b)Pd-DP /
,.a
(c)Pd-UDP/'~------_.____J i
I
i
i
24200 24300 24400 24500 24600 Energy / eV Fig. 1. XANES of 3 wt% Pd/CeO~ reduced at 300 ~
355
i
i
i
350 345 340 335 Binding energy / eV
330
Fig. 2. XPS of Pd 3d region for 3 wt% Pd/CeO 2 reduced at 300~
2318 Table 1 S u m m a r y of XPS analyses for 3 wt% Pd/CeO 2 reduced at 300~ Sample Pd-IMP Pd-DP Pd-UDP
Binding energy / eV ~ Pd 3d5/2 O ls 335.9 (0.19) 336.0 (0.45) 336.4 (0.78)
338.5 (0.51) 337.9 (0.45) 338.0 (0.07)
341.2 b (0.31) 341.7 b (0.10) 342.0 b (0.15)
529.6 (0.25) 529.8 (0.34) 529.8 (0.51)
532.1 (0.49) 531.8 (0.50) 531.8 (0.42)
Surface molar ratio Pd/Ce O/Ce 534.7 (0.26) 534.3 (0.16) 534.0 (0.07)
0.06
2.0
0.09
2.5
0.07
2.9
In parentheses normalized peak intensities are given. b The energy is estimated from the peak position of Pd 3du2 assuming the separation of 5.3 eV. Table 2 Surface properties of 3 wt% Pd/CeO 2 reduced at 300~ Sample
%u'" /%
Ce 4§a /%
Pd-IMP Pd-DP Pd-UDP
6.9 7.5 10.7
50 60 73
and its catalytic activity
Surface CO desorbed area/m 2 g-1 /mmol g-1 70 92 102
0.2 1.1 0.8
Conversion of MeOH / % 4.2 8.5 9.9
aContent of Ce 4§ determined from the intensities of XPS peaks in O ls region. in the spectrum for Ce 3§ species, the percentage of the peak area for u'" in the whole Ce 3d region (%u'") is a parameter of the relative amount of Ce 4§ in the sample [9]. The values were considerably smaller than 13.7% which was reported for oxidized CeO 2 (Table 2) [9], showing the partial reduction of CeO 2 to Ce203. The spectra for the samples in O ls region were deconvoluted into three Gaussian peaks at 529.7 9 0.1 eV, 531.9 • 0.2 eV, and 534.3 ~: 0.4 eV as shown inFigure 3. Surprisingly the value of x 1/2 + y1~2was always 0.98-1.01, where x and y are the normalized intensities of the peaks at 530 and 534 eV, respectively (see Table 1). On the surface of cerium oxide, an oxygen atom connects mainly to two cerium ions. When a value, a, is the fraction of Ce 3§ to whole cerium atoms, the fractions of oxygen atoms connecting to two Ce a§ and to two Ce3+ will be (1- a)2 and a 2, respectively. If x and y are equal to (1- a) 2 and a 2, respectively, the value of x 1/2 + ym will be one. The surface fraction of Ce 4§ calculated corresponds well to the value of %u'" (see Table 2). Since palladium oxide can be reduced with hydrogen even at room temperature, presence of palladium oxide shows that reduction of palladium is suppressed by the
2319 presence of cerium oxide. Hence, it can be proposed that reduction of cerium oxide proceeds via reduction of palladium oxide, s PdO + H 2 - Pd + I-I~O and 2 CeO 2 + Pd -- Ce20 s + PdO. In the mechanism reduction of cerium oxide causes formation of palladium oxide. It is consistent with the dependence of the concentration of Ce4+ on the surface dencity of Pd~. The difference in the reducibility will be due to presence of chlorine in the samples, that is, the contents were 0.38, 0.033, and 0.020 wt% for Pd-IMP, PD-DP, and PD-UDP, respectively, because formation of CeOC1 can enhance reduction of cerium oxide [10]. Reduction of the support probably causes a decrease in the BET surface area (see Table 2). 3.3. Interaction between palladium and cerium oxide Temperature-programmed desorption (TPD) of carbon monoxide was carried out after reduction of the sample with hydrogen at 300~ followed by adsorption of carbon monoxide at room temperature. There was a desorption peak around 300~ in the profile of Pd-IMP (Figure 4). The amounts of carbon monoxide desorbed from Pd-DP and Pd-UDP were estimated as 1.1 mmol g-1 and 0.8 mmol gl, respectively, from the areas. Since these quantities are extremely larger than the content ofpaUadium (0.3 mmol gl), carbon monoxide is mainly adsorbed on the surface of the ceria support, t h a t is, spill-over of carbon monoxide takes place. It is known that irreversible adsorption of carbon monoxide takes place on unmodified ceria reduced at 400~ or above [11,12], but no significant adsorption occurs on ceria reduced at 300~ The adsorption of carbon monoxide on the surface of ceria without reduction is enhanced by elevation of adsorption temperature [13], suggesting that adsorption of carbon monoxide on cerium oxide is activated at room temperature on the samples modified
x s
.=_
f/#
Ir @
(~'~~
41~
r
c
m
( c ) P d - U ~ ~ ~ !
540
i
535 530 Binding energy / eV
i
5~
Fig. 3. XPS of O ls region for 3 wt% Pd/CeO 2 reduced at 300~
i
200 400 Temperature l "C
i
600
Fig. 4. TPD of CO adsorbed on 3 wt% Pd/CeO 2 reduced at 300~
2320 with palladium by deposition-precipitation. When zirconium oxide is employed as a support of palladium, Pd-O-Zr can be present in the sample prepared by the deposition-precipitation method [7]. This suggests that the preparation by deposition-precipitation often results in strong contact between palladium particles and the support compared with the impregnation method. The desorption peak of carbon monoxide for Pd-IMP was apparently small, implying a weaker interaction between palladium and the support than in Pd-DP and Pd-UDP. Methanol decomposition to carbon monoxide and hydrogen at 180~ was catalyzed over these samples pretreated with hydrogen at 300~ The activities of the samples prepared by deposition-precipitation were significantly higher than that produced with Pd-IMP (see Table 2). Since Pd, is considered to be the active species of the decomposition, the strong contact between palladium particles and cerium oxide will be an advantageous feature for the catalysis. REFERENCES 1. A. Trovarelli, Catal. Rev.- Sci. Eng., 38 (1996) 439. 2. S. Bernal, J.J. Calvino, M.A. Cauqui, J.M. Gatica, C. Larese, J.A. Pdrez Omil and J.M. Pintado, Catal. Today, 50 (1999) 175. 3. L. Fan and K. Fujimoto, J. Catal., 150 (1994) 217. 4. L. Kephiski, M. Wolcyrz and J. Okal, J. Chem. Soc. Faraday Trans., 91 (1995) 507. 5. S. Naito, S. Aida, T. Tsunematsu and T. Miyao, Chem. Lett., 1998, 941. 6. D. Briggs, M.P. Seah, Practical surface analysis (2nd edition) Vol. 1: Auger and X-ray photoelectron spectroscopy, John Willy & Sons, Inc., New York, 1990. 7. Y. Matsumura, M. Okumura, Y. Usami, K. Kagawa, H. Yamashita, M. Anpo and M. Haruta, Catal. Lett., 44, (1997) 189. 8. J. Goetz, M.A. Volpe, A.M. Sica, C.E. Gigola and R. Touroude, J. Catal., 153 (1995) 86. 9. J.Z. Shu, W.H. Weber and H.S. Gandhi, J. Phys. Chem., 92 (1988) 4964. 10. K. Kili, L. Hilaire and F. Le Normand, Phys. Chem. Chem. Phys., 1 (1999) 1623. 11. C. Li, Y. Sakata, T. Arai, K. Domen, K. Maruya and T. Onishi, J. Chem. Soc. Faraday Trans. 1, 85 (1989) 1451. 12. C. Binet, A. Badri, M. Boutonnet-Kizling and J.-C. Lavalley, J. Chem. Soc. Faraday Trans., 90 (1994) 1023. 13. C.Li, Y. Sakata, T. Arai, K. Domen, K. Maruya and T. Onishi, J. Chem. Soc. Faraday Trans. 1, 85 (1989) 929.