SiO2 catalysts

Applied Catalysis A: General 244 (2003) 83–91

Selective hydrogenation of 1,3-butadiene on TiO2 -modified Pd/SiO2 catalysts Doh Chang Lee, Jae Hyung Kim, Woo Jae Kim, Jung Hwa Kang, Sang Heup Moon∗ Surface Engineering Laboratory, School of Chemical Engineering and Institute of Chemical Processes, Seoul National University, San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744, South Korea Received 16 July 2002; received in revised form 26 October 2002; accepted 26 October 2002

Abstract The properties of Pd/SiO2 catalysts modified with titanium oxide were examined by determining their activity with respect to the partial hydrogenation of 1,3-butadiene included in an excess amount of butenes and by characterizing their surfaces using infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), H2 chemisorption, and temperature-programmed desorption (TPD). The TiO2 -modified catalysts had an improved selectivity for the conversion of 1,3-butadiene to 1-butene, particularly when the catalysts were reduced at high temperatures, e.g., 500 ◦ C. IR and chemisorption results suggest that, when the catalyst is reduced at 500 ◦ C, the Pd surface is decorated with partially-reduced TiOx similar to the case of TiO2 -supported catalysts, which show strong metal-support interaction (SMSI). XPS and TPD results indicate that the Pd surface is also modified electronically, in that the charge is transferred from the Ti species to Pd and the adsorption of 1-butene to the Pd surface becomes weaker. It can be concluded that the strong interaction between the Pd surface and partially reduced TiO2 is responsible for the improved selectivity of the catalyst for the conversion of 1,3-butadiene to 1-butene. © 2002 Elsevier Science B.V. All rights reserved. Keywords: 1,3-Butadiene; Palladium; Selective hydrogenation; Strong metal-support interaction (SMSI); Titanium oxide

1. Introduction The selective hydrogenation of 1,3-butadiene in a 1-butene rich stream from a steam cracker is an important industrial process in the purification of 1-butene [1,2], which is used as a feed for polymerization processes. Because even a trace amount of 1,3-butadiene in the feed functions as a poison and consequently reduces the quality of the polymers produced, the complete elimination of 1,3-butadiene is necessary for the ∗ Corresponding author. Tel.: +82-2-880-7409; fax: +82-2-875-6697. E-mail address: [email protected] (S.H. Moon).

production of polymer-grade 1-butene. The improvement of 1-butene selectivity is a key issue in this process because even a small increase in the selectivity of a catalyst can lead to considerable savings [3]. Pd catalysts have good activity and stability in 1,3-butadiene hydrogenation but its selectivity for 1-butene needs to be further improved. For this, various metals or metal oxides have been suggested as promoters of the Pd catalysts [3–7]. In this study, we report on the addition of TiO2 to Pd/SiO2 catalysts as a potential promoter for improving 1-butene selectivity during the hydrogenation of 1,3-butadiene. Tauster et al. [8], and Tauster and Fung [9] reported that TiO2 , when used as a support,

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interacts with the dispersed metal particles after the catalyst is reduced at high temperatures. This is referred to as a strong metal-support interaction (SMSI). Most previous studies of SMSI phenomenon involved the use of catalysts prepared using transition metal oxides as supports [10–18]. However, TiO2 has not generally been considered to be a good support material because it is not as stable as other conventional support materials, e.g., SiO2 , both in thermal and physical properties [19]. In this study, we modified Pd/SiO2 with TiO2 , by adding the latter to the catalyst instead of using it as a support, and examined the performance of the TiO2 added catalysts in the hydrogenation of 1,3-butadiene. Data on the reaction were correlated with the properties of the catalyst surface, as evidenced by H2 chemisorption, Fourier-transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD) measurements.

1,3-butadiene, 2.25% 1-butene with the balance of N2 . We varied the flow rate of the reactant mixture in the range between 60 and 200 ml/min to control 1,3-butadiene conversion. Reaction products were analyzed by a gas chromatograph (HP Model 5890 with FID) using a capillary column.

2. Experimental

2.3. XPS and TPD

2.1. Catalyst preparation and 1,3-butadiene hydrogenation

XPS was measured with VG ESCA LAB-5 equipped with an Al K␣ (1486.6 eV) anode. After reduction either at 300 or 500 ◦ C, the catalysts were protected from air oxidation by wetting with isooctane, and then placed in a UHV chamber for analysis. All data were corrected using the C 1s peak at 284.6 eV as an internal standard. TPD of 1-butene from the sample catalyst was carried out after exposure of the reduced catalyst to a mixture of 1-butene and helium at room temperature. The sample was heated at a rate of 10 ◦ C/min in a 20 ml/min flow of helium, and the effluent gas was monitored by means of a mass spectrometer (VG Sensorlab 200).

One weight percent Pd/SiO2 was prepared by an ion-exchange method using Pd(NH3 )4 (OH)2 as a Pd precursor. SiO2 was obtained from the Catalysis Society of Japan (JRC-SIO-6, surface area = 109 m2 /g), and was added to an aqueous solution of Pd(NH3 )4 (OH)2 for Pd-ion exchange. The resulting catalyst was dried overnight in an oven at 110 ◦ C, and then calcined in air at 300 ◦ C for 2 h. Titanium oxide was added to Pd/SiO2 by an excess wetness impregnation method. That is, a hexane solution of the titanium precursor, Ti(O-iC3 H7 )2 (dipivaloylmethanate)2 , was mixed with Pd/SiO2 catalyst and agitated for 3 h. The TiO2 -modified catalyst was then separated by filtration, calcined in air at 300 ◦ C for 2 h, and finally reduced at different temperatures for 1 h prior to its use in 1,3-butadiene hydrogenation. 1,3-Butadiene hydrogenation was performed in a pyrex microreactor containing 8 mg of the catalyst mixed with SiO2 (JRC-SIO-6), as a diluent (catalyst/SiO2 weight ratio = 1/4). The composition of the reactor-inlet stream was 1.9% H2 , 0.25%

2.2. H2 chemisorption and FT-IR Chemisorption was carried out in a glass vacuum system using H2 as a probing gas. The measurement was made at room temperature, following the double isotherm method proposed by Benson and Boudart [20]. IR spectra of the CO adsorbed on the catalysts were obtained using an IR cell with CaF2 windows [21] and by means of an IR spectrometer (Midac, model 2000). The sample catalyst was pressed into a self-supporting disc, placed in the cell, reduced at either 300 or 500 ◦ C, and then exposed to 10 Torr of CO. IR spectra of the adsorbed CO were recorded after removing gaseous CO from the cell by evacuation.

3. Results and discussion 3.1. 1,3-Butadiene hydrogenation Table 1 lists typical conversions of 1,3-butadiene to 1-butene under identical reaction conditions on Pd-only and TiO2 -added Pd catalysts. The TiO2 -added catalysts are designated as “Pd-xTi/T”, with x denoting

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Table 1 1,3-Butadiene conversions on various catalysts obtained under identical reaction conditionsa Catalyst

Conversion

Pd/300 Pd/500 Pd-1Ti/300 Pd-1Ti/500

0.907 0.848 0.890 0.829

a Reaction temperature = 50 ◦ C, weight of the catalyst = 8 mg, total flow rate = 60 ml/min.

the atomic ratio of Ti to Pd and T denoting the reduction temperature. For example, Pd-1Ti/500 contains TiO2 with a Ti/Pd atomic ratio of 1 and was reduced at 500 ◦ C for 1 h. Pd/500 shows a lower conversion than Pd/300, which is obviously due to the sintering of Pd crystallites after the reduction of the catalyst at 500 ◦ C. Pd-1Ti/300 shows almost the same, or slightly lower, conversion as Pd/300, indicating that the Pd surface is not significantly modified by the added TiO2 . The conversion on Pd-1Ti/SiO2 is lowered after the catalyst is reduced at 500 ◦ C, but the extent of lowering is nearly the same as that of the Pd-only catalyst. As a whole, the above reaction results show that the activity of the TiO2 -added catalysts is maintained at a nearly comparable level to that of the Pd-only catalysts, which is not usually obtained with TiO2 -supported catalysts because Pd is not well dispersed on TiO2 [22]. Fig. 1 shows the relationships between 1-butene selectivity and the conversion of 1,3-butadiene for Pd/SiO2 and Pd-1Ti/SiO2 catalysts reduced at either 300 or 500 ◦ C. The selectivity of 1-butene is defined as moles of 1-butene produced per mole of 1,3-butadiene converted. Negative values of the selectivity are obtained when the net amount of 1-butene, which is present in excess in the feed, is lost in the reaction stream as a result of hydrogenation. Selectivity data are plotted versus the conversion because the reaction proceeds by a typical series-type scheme and consequently the selectivity is strongly dependent on the conversion. Fig. 1 shows that the 1-butene selectivity of the Pd/SiO2 catalyst is significantly decreased when the reduction temperature is raised from 300 to 500 ◦ C. 1-Butene selectivity is decreased because Pd particles are sintered during the high temperature reduction, thus generating many low-index surfaces [23].

Fig. 1. Variation in 1-butene selectivity with the conversion in 1,3-butadiene hydrogenation on Pd/SiO2 and Pd-1Ti/SiO2 reduced at either 300 or 500 ◦ C (H2 /1,3-butadiene = 2, reaction temperature = 50 ◦ C).

Pd-1Ti/300 exhibits almost the same selectivity as that of Pd/300, but Pd-1Ti/500 shows a higher selectivity over a wide range of conversions. To further study the improvement in the selectivity of the TiO2 -added catalysts, we observed the 1-butene selectivity for catalysts containing different amounts of TiO2 and reduced at 500 ◦ C. In Fig. 2, the selectivity increases up to a Ti/Pd atomic ratio of 2 but is eventually lowered when the Ti/Pd ratio reaches 2.5 or higher. In the case of Pd-4Ti/500, both selectivity and conversion are lower than those of Pd-2Ti/500, suggesting that the Pd surface is covered with an excess of Ti species. We also examined the effect of catalyst reduction temperature on the performance of Pd-2Ti/SiO2 , which shows a maximum selectivity among catalysts containing different amounts of TiO2 . In Fig. 3, the 1-butene selectivity of Pd-2Ti/300 or Pd-2Ti/400 is almost the same as or slightly higher than that on Pd/300, but the selectivity is significantly enhanced when the TiO2 -added catalyst is reduced at temperatures higher than 450 ◦ C. The selectivity for the production of either 2-butene or n-butane, given in Figs. 4 and 5, also shows a similar dependence on the reduction temperature, although, in these cases, the selectivity is

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Fig. 2. Variation in 1-butene selectivity with the conversion in 1,3-butadiene hydrogenation on various catalysts containing different amounts of TiO2 (H2 /1,3-butadiene = 2, reaction temperature = 50 ◦ C).

Fig. 4. Variation in 2-butene selectivity with the conversion in 1,3-butadiene hydrogenation on Pd/SiO2 and Pd-2Ti/SiO2 reduced at different temperatures (H2 /1,3-butadiene = 2, reaction temperature = 50 ◦ C).

lowered when the catalyst is reduced at high temperatures. Accordingly, the results suggest that a threshold temperature exists between 400 and 450 ◦ C, beyond which the catalyst surface is significantly modified

by added TiO2 , such that 1-butene selectivity is improved. In the following section, we discuss the nature of Pd surface modification by the Ti species based on the results of catalyst surface characterization.

Fig. 3. Variation in 1-butene selectivity with the conversion in 1,3-butadiene hydrogenation on Pd/SiO2 and Pd-2Ti/SiO2 reduced at different temperatures (H2 /1,3-butadiene = 2, reaction temperature = 50 ◦ C).

Fig. 5. Variation in butane selectivity with the conversion in 1,3-butadiene hydrogenation on Pd/SiO2 and Pd-2Ti/SiO2 reduced at different temperatures (H2 /1,3-butadiene = 2, reaction temperature = 50 ◦ C).

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Table 2 The amounts of H2 chemisorption on various catalysts Catalyst

H/Pd

Pd/300 Pd/500 Pd-2Ti/300 Pd-2Ti/400 Pd-2Ti/450 Pd-2Ti/500 Pd-1Ti/500 Pd-2.5Ti/500 Pd-4Ti/500

0.65 0.48 0.42 0.32 0.10 0.07 0.09 0.06 0.04

3.2. H2 chemisorption and IR spectra of adsorbed CO Table 2 lists the amounts of H2 irreversibly adsorbed on the catalysts after reduction at different temperatures. On Pd/SiO2 , the amount of H2 uptake decreases due to the sintering of Pd particles when the reduction temperature is raised from 300 to 500 ◦ C. H2 uptake is also lowered when the TiO2 -added catalyst is reduced at high temperatures, but in this case the extent of decrease is much larger than that observed for Pd/SiO2 . For example, the H/Pd ratio is as low as 0.07 for Pd-2Ti/500, while it is 0.48 for Pd/500. The remarkable decrease in H2 uptake after the catalyst reduction at 500 ◦ C has been reported as a characteristic pattern of behavior of TiO2 -supported catalysts that show an SMSI phenomenon [8,9]. Fig. 6 shows IR spectra of CO adsorbed on Pd/SiO2 and Pd-2Ti/SiO2 after reduction at 300 or 500 ◦ C. The IR bands corresponding to CO adsorbed on Pd can be divided into four modes: linear, compressed-bridged, isolated-bridged and tri-coordinated modes, which are observed at 2100–2050, 1995–1975, 1960–1925, and 1890–1870 cm−1 , respectively [24]. The latter three modes constitute the multiply-bound adsorption of CO on Pd. The overall spectral intensity decreases as the reduction temperature is raised from 300 to 500 ◦ C. In the case of Pd-2Ti/500, the intensity of bands at positions below 2000 cm−1 , assigned to the multiply-bound CO species, decreases rapidly, and consequently the ratio of the intensity of the multiplyto linearly-bound CO peaks becomes smaller than the ratio obtained with Pd/300, as listed in Table 3. The above results suggest that the Ti species are distributed on the Pd surface and accordingly dilute the

Fig. 6. Infrared spectra of CO adsorbed on various catalysts.

multiply-coordinated Pd sites after the catalyst reduction at 500 ◦ C, which is another characteristic of the SMSI effect, as reported previously [25]. In summary, both the H2 chemisorption and IR results support the hypothesis that TiO2 added to Pd/ SiO2 strongly interacts with Pd after the reduction of the catalyst at high temperatures, showing the characteristic SMSI behavior observed with TiO2 -supported catalysts. The interaction becomes significant beyond the threshold temperature between 400 and 450 ◦ C and is observed over a wide range of Ti/Pd ratios. 3.3. XPS and 1-butene TPD In Fig. 7, XPS peaks representing the binding energy of Pd 3d are observed at almost the same locations Table 3 The ratios of multiply- to linearly-bound CO peaks on various catalysts Catalyst

Am /Al a

Pd/300 Pd/500 Pd-2Ti/300 Pd-2Ti/500

3.23 3.45 1.92 0.69

a

Intensity of multiply-bound peak (Am )/intensity of linearlybound peak (Al ).

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Fig. 7. X-ray photoelectron spectra of Pd/SiO2 and Pd-xTi/SiO2 .

for Pd/SiO2 and Pd-1Ti/SiO2 when the catalysts are reduced at 300 ◦ C, but the peaks shift to lower energies, when Pd-1Ti/SiO2 is reduced at 500 ◦ C. The peak shift can be induced by changes in the Pd particle size after catalyst reduction at 500 ◦ C, as suggested by Takasu et al. [26]. However, we believe that the shift is largely due to charge transfer from the partiallyreduced TiOx to Pd because the shift is larger for Pd-10Ti/500 than for Pd-1Ti/500, although the two catalysts have the same Pd particle size distribution because they were prepared from an identical Pd/SiO2 catalyst and are different only in their Ti content. Similar results consistent with a charge transfer between the Ti species and metal were obtained for catalysts showing the SMSI phenomenon, e.g., Pt/TiO2 and Rh/TiO2 [27,28]. In Fig. 8, 1-butene is desorbed from Pd/300, showing a major peak at about 180 ◦ C. On Pd-2Ti/500, however, the peak significantly loses its intensity and is observed at lower temperatures, indicating that reduced amounts of 1-butene are adsorbed on the Pd surface and are easily desorbed from the Pd. This result can be explained as follows. The partial distribution of Ti species on the Pd surface, as evidenced by the CO-IR results, decreases the amounts of adsorbed species on the Pd. Additionally, the charge

Fig. 8. Temperature programmed desorption of 1-butene from Pd/SiO2 and Pd-2Ti/SiO2 reduced at different temperatures.

transfer from the Ti species to Pd, as evidenced by XPS observations, reduces the adsorption strength of hydrocarbons on the Pd surface and, particularly, the adsorption of 1-butene is affected to a larger extent than that of 1,3-butadiene because the adsorption of the former is weaker than that of the latter [29]. 3.4. The role of Ti oxide To understand the reason for the improvement in selectivity of the TiO2 -modified catalysts, it is necessary to consider the individual reaction steps involved in the 1,3-butadiene hydrogenation process, a simplified scheme of which is presented in Fig. 9 [30]. Gaseous 1,3-butadiene is adsorbed to the catalyst surface as a mono-␲-bonded species (I) or a di-␲-bonded species (II), which are converted to half-hydrogenated species (III) by the addition of hydrogen [31,32]. The half-hydrogenated species (III) gives 1-butene after the addition of another hydrogen atom or is converted to a ␲-allylic species (V), which eventually produces chemisorbed 2-butene (VI) after a double-bond shift. This double-bond shift is known to occur on the Pd(1 1 1) surface [23]. Chemisorbed 2-butene (VI) is desorbed as gaseous 2-butene or isomerized to a carbene species (VII), which is further hydrogenated to

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Fig. 9. Reaction paths involved in 1,3-butadiene hydrogenation.

produce n-butane. Accordingly, the above steps constitute a typical series-type reaction scheme and, in order to achieve higher 1-butene selectivity, it is necessary to promote the desorption of 1-butene as well as to suppress the formation of the ␲-allylic species (V), which triggers the double-bond shift. The results of this study consistently show that the 1-butene selectivity increases, which is accompanied by a decrease in selectivity for 2-butene and n-butane, on Pd catalysts containing TiO2 and reduced at temperatures higher than 450 ◦ C. The H2 chemisorption data indicate that H2 uptake is significantly suppressed in the case of TiO2 added catalysts reduced at high temperatures. Such a decrease is similar to the results obtained with TiO2 supported catalysts showing the SMSI phenomenon [33]. Accordingly, we may expect that the Ti species

modifies the Pd surface in the same manner as in the case of the TiO2 -supported catalysts. That is, the partially reduced TiOx species migrates onto and decorates the Pd surface, and simultaneously affects the electronic state of Pd. The geometric blocking of the large Pd ensembles by the Ti species is confirmed by the IR observation of adsorbed CO, and the electronic modification of the Pd surface is indicated both by the XPS result, suggesting a charge transfer from the Ti species to Pd, and by the TPD of 1-butene, indicating the weakening of 1-butene adsorption on Pd. The above surface properties of TiO2 -modified catalysts can be correlated with the reaction results obtained in this study as follows. The decoration of the Pd surface with the Ti species would suppress the formation of ␲-allylic species (V) on the surface because it has been reported that the allylic species

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are formed preferentially on the Pd surface of large ensembles [23]. The facilitated desorption of 1-butene from the Pd surface would be expected to promote the production of gaseous 1-butene. The reduced levels of H2 chemisorption on the TiO2 -modified catalysts would retard the rates of hydrogenation of both the half-hydrogenated species (III) and the ␲-allylic species (V), eventually leading to the production of 1-butene and 2-butene/n-butane, respectively. It would be expected that the hydrogenation rate of the ␲-allylic species (V) would be retarded to a larger extent than that of the half-hydrogenated species (III) on TiO2 -modified catalysts because the former species is singly bonded and therefore more weakly adsorbed to the catalyst surface than the doubly-bonded latter species. All of the above changes in the Pd surface properties, caused by the Ti modification, contribute to the promotion of the 1-butene selectivity, as observed in this study.

4. Conclusion TiO2 -added Pd/SiO2 catalysts were tested for the selective hydrogenation of 1,3-butadiene in a 1-butene-rich stream, and show an improved 1-butene selectivity when they are reduced at high temperatures, e.g., 500 ◦ C. The added Ti species interacts strongly with the Pd surface after the reduction of the catalyst at high temperatures, similar to the case of TiO2 -supported catalysts showing the SMSI phenomenon. The Ti species are incorporated onto the Pd surface such that multiply-coordinated sites of Pd are partially blocked and, as a result, the isomerization of 1-butene on the catalyst is retarded. The Ti species also electronically modifies the Pd surface, as evidenced by XPS analysis, which expedites the desorption of 1-butene from Pd. H2 chemisorption is significantly suppressed on the TiO2 -added catalysts, which would be expected to retard the hydrogenation of the ␲-allylic species, leading to the production of either 2-butene or n-butene, to a larger extent than that of the half-hydrogenated species, leading to the production of 1-butene. All the above properties of the TiO2 -added catalysts contribute to the improvement of 1-butene selectivity, as observed in the experimental tests. The TiO2 added catalyst has advantages over the TiO2 -supported

catalyst because the former maintains its catalytic activity at a level nearly comparable to that of Pd/SiO2 , unlike the case of the latter, which shows a relatively poor metal dispersion.

Acknowledgements This work was supported by LG Chem. Co. Ltd., Brain Korea 21 project, and National Research Laboratory program.

References [1] H. Arnold, F. Doeber, J. Gaube, in: G. Ertl, H. Knoezinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, 1997, vol. 5, VCH, Weinheim, p. 2165. [2] M.L. Derrien, Stud. Surf. Sci. Catal. 27 (1986) 613. [3] B. Furlong, J.W. Hightower, T.Y.-L. Chan, A. Sarkany, L. Guczi, Appl. Catal. 117 (1994) 41. [4] A. Borgna, B. Moraweck, J. Massadier, A.J. Renouprez, J. Catal. 128 (1991) 99. [5] M.M. Pereira, F.B. Noronha, M. Schmal, Catal. Today 16 (1993) 407. [6] A. Sarkany, Z. Zsoldos, G. Stefler, J.W. Hightower, L. Guczi, J. Catal. 157 (1995) 179. [7] M. Schmal, D.A.G. Aranda, R.R. Soares, F.B. Noronha, A. Frydman, Catal. Today 57 (2000) 169. [8] S.J. Tauster, S.C. Fung, R.L. Garten, J. Am. Chem. Soc. 100 (1) (1978) 170. [9] S.J. Tauster, S.C. Fung, J. Catal. 55 (1978) 131. [10] J.A. Horsley, J. Am. Chem. Soc. 101 (11) (1979) 2870. [11] P. Meriaudeau, O.H. Ellestad, M. Dufaux, C. Naccache, J. Catal. 75 (1982) 243. [12] B. Chen, J.M. White, J. Phys. Chem. 86 (1982) 3534. [13] J.-G. Choi, H.-K. Rhee, S.H. Moon, Korean J. Chem. Eng. 1 (2) (1984) 159. [14] T.H. Fleisch, R.F. Hicks, A.T. Bell, J. Catal. 87 (1984) 398. [15] T.-C. Chang, J.-J. Chen, C.-T. Yeh, J. Catal. 96 (1985) 51. [16] T.J. Lee, Y.G. Kim, Korean J. Chem. Eng. 2 (2) (1985) 119. [17] T. Sheng, X. Guoxing, W. Hongli, J. Catal. 111 (1988) 136. [18] G.L. Haller, D.E. Resasco, Adv. Catal. 36 (1989) 173. [19] E.K. Poels, R. Koolstra, J.M. Gens, V. Ponec, in: B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin, J.C. Vedrine (Eds.), Metal-Support and Metal-Additive Effects in Catalysis, Elesevier, Amsterdam, 1982, p. 232. [20] J.E. Benson, M. Boudart, J. Catal. 4 (1965) 704. [21] S.H. Moon, H. Windawi, J.R. Katzer, Ind. Eng. Chem. Fundam. 20 (1981) 396. [22] J.H. Kang, E.W. Shin, W.J. Kim, J.D. Park, S.H. Moon, J. Catal. 208 (2002) 310. [23] J. Goetz, M.A. Volpe, R. Touroude, J. Catal. 164 (1996) 369.

D.C. Lee et al. / Applied Catalysis A: General 244 (2003) 83–91 [24] D. Tessier, A. Rakai, B. Verduraz, J. Chem. Soc. Faraday Trans. 88 (1992) 741. [25] A.J. Simoens, R.T.K. Baker, D.J. Dwyer, C.R.F. Lund, R.J. Madon, J. Catal. 86 (1984) 359. [26] Y. Takasu, R. Unwin, B. Tesche, A.M. Bradshaw, Surf. Sci. 77 (1978) 219. [27] S.C. Fung, J. Catal. 76 (1982) 225. [28] S.-H. Chien, B.N. Shelimov, D.E. Resasco, E.H. Lee, G.L. Haller, J. Catal. 77 (1982) 301.

91

[29] T. Ouchaib, J. Massardier, A.J. Renouprez, J. Catal. 119 (1989) 517. [30] J.P. Boitiaux, J. Cosyns, E. Robert, Appl. Catal. 49 (1989) 235. [31] J.J. Philipson, P.B. Wells, G.R. Wilson, J. Chem. Soc. (A) (1969) 1351. [32] Y. Soma, Bull. Chem. Soc. Jpn. 50 (8) (1977) 2119. [33] S.J. Tauster, S.C. Fung, R.T.K. Baker, J.A. Horsley, Science 211 (1981) 1121.