- Email: [email protected]
Performance of Si-modified Pd catalyst in acetylene hydrogenation: catalyst deactivation behavior
Applied Catalysis A: General 251 (2003) 305–313
Performance of Si-modified Pd catalyst in acetylene hydrogenation: catalyst deactivation behavior Woo Jae Kim, Eun Woo Shin, Jung Hwa Kang, Sang Heup Moon∗ School of Chemical Engineering, Institute of Chemical Processes, Seoul National University, San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744, South Korea Received 25 December 2002; received in revised form 23 April 2003; accepted 23 April 2003
Abstract The deactivation behavior of Si-modified Pd catalysts in acetylene hydrogenation was studied using thermogravimetric analysis (TGA), infrared (IR) spectroscopy, and reaction tests. TGA and IR analyses of green oil produced on the catalyst indicate that it is produced in smaller amounts and its average chain length is shorter on a Si-modified catalyst than on an unmodified one. The above findings are due to deposition of Si species on the Pd surface; such deposits effectively block multiply-coordinated adsorption sites on the catalyst and suppress the formation of green oil on the catalyst surface, specifically on or in the vicinity of Pd. The Si species also retard the sintering of Pd crystallites during the regeneration step and allow for the slow deactivation of the catalyst during acetylene hydrogenation, after regeneration. The improvement in the deactivation behavior of the Si-modified catalyst is believed to arise from the geometric modification of the Pd surface with small clusters of the Si species. © 2003 Elsevier B.V. All rights reserved. Keywords: Acetylene hydrogenation; Pd catalyst; Si modification; Deactivation; Green oil; Sintering
1. Introduction Supported Pd catalysts that are used for the selective hydrogenation of acetylene are deactivated relatively easily due to the accumulation of green oil on the catalyst surface. Industrial processes are usually operated in a narrow range of reaction conditions to maintain high ethylene selectivity and to minimize the amount of green oil that accumulates on the catalyst surface [1]. Carbon monoxide is sometimes added to the reactant stream to improve ethylene selectivity, but the presence of carbon monoxide on the catalyst usually accelerates green ∗ Corresponding author. E-mail address: [email protected] (S.H. Moon).
oil formation [2], thus shortening the lifetime of the catalyst. The green oil is not produced when only acetylene is adsorbed on the surface, but is produced when acetylene is co-adsorbed with hydrogen [3,4]. The amount of green oil increases with the H2 /C2 H2 ratio when the hydrogen pressure is very low [5] but eventually decreases with the H2 /C2 H2 ratio at high hydrogen pressures, as is the case in industrial processes [6,7]. Green oil formation is enhanced when many multiply-coordinated sites are present, since these favor the adsorption of acetylene as neighbors [8] and consequently the polymerization of C2 species on the catalytic surface [9]. Green oil consists of paraffin and olefinic hydrocarbons in the C8 –C24 range and has a H/C ratio of about 1.9 [5,10]. It has been reported
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00367-3
306
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
that the volatile fraction of green oil, on average, is composed of molecules containing 8–10 carbon atoms [11]. Used catalysts are regenerated by burning off the green oil in air; after a series of such cycles, the catalysts become inactive due to the sintering of Pd crystallites. The time between regeneration cycles is typically less than a year, sometimes several months. Various promoters have been examined to improve catalyst lifetime as well as ethylene selectivity [8,12–16], but improvements are still needed to achieve a longer lifetime. We previously reported [17–19] that the ethylene selectivity of a Pd catalyst is improved when the catalyst surface is modified with Si species deposited by the decomposition of silane and followed by oxidation in air. We attributed the selectivity improvement to the geometric modification of the Pd surface with the Si species, which changed the relative rates of reaction paths in acetylene hydrogenation. The adsorption and desorption behavior of ethylene, acetylene, and hydrogen on the Si-modified catalyst suggests that ethylene desorption is facilitated, the amount of chemisorbed hydrogen is reduced, and C2 polymerization is suppressed. All the above trends can be regarded as contributing to the improved selectively. This paper presents the results of our continued work on the performance of the Si-modified catalyst in acetylene hydrogenation [19], particularly on the catalyst deactivation behavior. The deactivation behavior of the Si-modified and unmodified catalysts before and after catalyst regeneration were compared, and the green oil produced on the catalysts was analyzed by thermogravimetric analysis (TGA), infrared (IR) spectroscopy. The results can be explained based on the role of the Si species in the catalyst deactivation process.
Si-modified catalyst, designated as Pd-Si/SiO2 , was prepared from Pd/SiO2 by depositing Si on the catalyst and subsequently oxidizing the Si species in air. The Si/Pd atomic ratio in the catalyst was 0.83, as estimated from the amounts of silane added to the reactor for decomposition. The catalysts were tested for acetylene hydrogenation at atmospheric conditions in a quartz reactor. The reaction conditions were more severe than those used in our previous study [19], which was focused on the catalytic selectivity of ethylene production, and as a result the catalysts were deactivated to greater extents. The reaction temperature was 343 K instead of 313 K, the reactant stream contained 4.1% acetylene in ethylene instead of 1.01%, the H2 /C2 H2 ratio in the reaction stream was 1.0 instead of 2.0, and the space velocity was 800 cm3 /(min g). The parameters for the gas chromatography (GC) in the product analysis were the same as those in the previous study [19].
2. Experimental
3. Results
2.1. Catalyst preparation and reaction tests
3.1. Deactivation rates
Two sample catalysts, Si-modified and unmodified Pd/SiO2 , were prepared and their deactivation characteristics in acetylene hydrogenation were compared. Unmodified Pd/SiO2 , containing 1.0 wt.% Pd, is identical to that prepared in a previous study [19]. The
As shown in Table 1, the activity of fresh Pd/SiO2 for acetylene hydrogenation is slightly lowered when the catalyst is modified with the Si species because the latter, which are inactive for the reaction, partially cover the Pd surface. We reported in our previous study
2.2. TGA, DTGA, and IR The catalysts used for the same accumulated conversion of acetylene were analyzed thermogravimetrically (TA Instruments, TGA2050) in air flowing at 40 cm3 /min. The temperature was raised from 303 to 1123 K at a rate of 10 K/min. DTGA results were obtained from the TGA curves by differentiating the latter with respect to temperature. The infrared spectra of either CO adsorbed to or green oil deposited on the catalyst surface after the reaction were recorded on a Midac 2100 spectrometer in an IR cell with CaF2 windows attached at both ends. The spectrum of the green oil was obtained from the difference in the spectra of used and fresh catalysts. Details of the IR apparatus and the measurement procedures have been described previously [20].
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313 Table 1 Reaction rates and CO-IR results obtained with fresh and regenerated catalysts Catalyst
Pd/SiO2 Pd-Si/SiO2
Reaction ratea
Acb /Al + ib b
Fresh
After regeneration
Fresh
After regeneration
0.91 0.85
0.84 0.84
0.23 0.21
0.83 0.62
a Reaction conditions: temperature = 343 K; H /C H ratio in 2 2 2 the reaction stream = 1.0; space velocity = 2200 min−1 . b Area of compressed-bridged band (A )/(area of linear band cb (A1 ) + area of isolated-bridged band (Aib )).
[17] that Si species deposited on the Pd surface consisted of Si and SiO2 , the latter being the major component because the catalyst had been exposed to air after the Si deposition. Catalysts are deactivated during their use in acetylene hydrogenation. Fig. 1 shows the reaction rates normalized to the initial values. The rates were plotted against the accumulated amounts of acetylene con-
307
verted in the reaction instead of the reaction time, so that they can be compared based on the same load in the reaction. It is apparent in Fig. 1 that catalyst deactivation is retarded in the case where the catalysts are modified with the Si species. Pd catalysts used in acetylene hydrogenation are largely deactivated as a result of the accumulation of green oil on the catalyst surface; such oil is produced by the polymerization of the C2 species [9]. Consequently, the above results suggest that green oil is produced in smaller amounts on the Si-modified catalyst than on the Pd-only catalyst. 3.2. TGA and DTGA To analyze the amounts of green oil deposited on the catalyst during the reaction more accurately, we performed the TGA of Pd/SiO2 and Pd-Si/SiO2 after they were used for the same accumulated acetylene conversion. Fig. 2 shows that Pd/SiO2 used under the conditions specified in Table 1 loses 66.3% of its initial
Fig. 1. Deactivation of catalysts with accumulated amounts of converted acetylene: catalyst weight = 0.05 g, H2 /C2 H2 = 1, reaction temperature = 70 ◦ C. (a) Pd/SiO2 , (b) Pd-Si/SiO2 .
308
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
Fig. 2. Thermogravimetric analysis of used catalysts in air: air flow rate = 40 ml/min, heating rate = 10 ◦ C/min. (a) Pd/SiO2 , (b) Pd-Si/SiO2 .
Fig. 3. Differential thermogravimetric analysis of used catalysts in air: (a) Pd/SiO2 , (b) Pd-Si/SiO2 .
weight at 850 ◦ C, while Pd-Si/SiO2 loses 61.5% of its weight. The difference in weight loss between the two catalysts is not due to the removal of volatile hydrocarbons or water at low temperatures, because the TGA curves are almost identical in this region. A reasonable explanation is that the difference occurs mostly due to the burn-off of green oil deposited on the catalysts, as indicated by the TGA curves, which deviate from each other at temperatures above 250 ◦ C. To study further the properties of the green oil removed in the oxidation process, we differentiated the TGA curves to obtain the DTGA results given in Fig. 3. Three major peaks are observed in different temperature regions: below 300, 300–500 ◦ C, and above 500 ◦ C. According to Larsson et al. [5], who obtained similar results in the analysis of coke deposited on Pd catalysts used for acetylene hydrogenation by temperature-programmed oxidation (TPO), the peak below 300 ◦ C, designated as Peak I in this study, originates from heavy hydrocarbons that are adsorbed on the catalyst surface or absorbed in the catalyst pores. Peak II, observed between 300 and 500 ◦ C, represents coke on or in the vicinity of Pd. Peak III, above 500 ◦ C, corresponds to coke produced on the support without the influence of Pd.
Two changes are of interest in Fig. 3 as a result of the Si modification: peak shift to lower temperatures, which is particularly large for Peak II, and characteristic changes in the intensity of the three major peaks. The intensity and the position of Peak I seem to change slightly with Si modification, but a definite conclusion is not available because the peak appears as a shoulder to the more intense Peak II. The peaks may be deconvoluted as was done by Larsson et al. [5], but the data herein are not sufficiently accurate for a quantitative analysis. Peak II consists of two minor peaks that are split by about 70 ◦ C on Pd/SiO2 , but are merged into a single intense peak on Pd-Si/SiO2 . The average position of Peak II is significantly shifted to lower temperatures, by 67 ◦ C, due to the Si modification. The peak shift may occur for two reasons. One is because coke, or green oil, produced on the catalyst becomes more volatile with lower molecular weight, and the other is because the coke particles are present in smaller sizes, as suggested by Querini and Fung [21], on the Si-modified catalyst. This issue will be discussed further in Section 3.3, based on the IR analysis of the surface carbonaceous species. Peak III is only slightly shifted to lower temperatures and its intensity is reduced by about 20% after catalyst modification.
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
309
Fig. 4. Infrared spectra of green oil deposited on used catalysts: catalysts were used for the conversion of 0.022 mol of acetylene/0.05 g of catalyst. (a) Pd/SiO2 , (b) Pd-Si/SiO2 .
The effect of the Si modification can be summarized as follows, based on the above DTGA results and peak assignments proposed by Larsson et al. [5]. Coke that is deposited on or in the vicinity of Pd is removed at significantly lower temperatures, that deposited on SiO2 is removed at slightly lower temperatures, and the amounts of coke on the SiO2 are reduced by about 20% as a result of the catalyst modification. Changes in the amounts of heavy hydrocarbons produced on the catalyst and the coke associated with Pd due to the Si modification are not clear, because Peaks I and II overlap with each other. 3.3. IR of green oil The infrared spectrum of the carbonaceous species, green oil, that is accumulated on the catalyst surface during the reaction shows many peaks due to the C–H and C–C bonds included in the species. Fig. 4 compares the spectra of green oil obtained from two used catalysts in the region between 2800 and 3050 cm−1 . The peak at 2930 cm−1 is due to the C–H stretching of the CH2 group and the peak at 2960 cm−1 is due to the C–H stretching of the CH3 group [11]. Follow-
ing the analysis by Sheppard and Ward [22], one may estimate the average chain length of the hydrocarbon species in green oil from the intensity ratio of two C–H peaks, CH2 /CH3 . Table 2 shows that the intensity ratio is smaller, 2.9, for Pd-Si/SiO2 than for Pd/SiO2 , 4.0, and therefore green oil accumulated on the former catalyst has a shorter chain length than that on the latter catalyst. The average carbon number of green oil molecules should be estimated from three times the intensity ratio, 3CH2 /CH3 , because the extinction coefficient of the IR peak due to the CH3 group is about triple that of the CH2 group [23]. The estimated carbon numbers are 19.4 for Pd-Si/SiO2 and 26 for Pd/SiO2 ; such values fall in the range known for common green oil [10]. The above carbon numbers, 19.4–26, are larger Table 2 Intensity ratios of ν(CH2 ) to ν(CH3 ) for green oil produced on different catalysts Catalyst
Peak area ratio (CH2 /CH3 )
Pd/SiO2 Pd-Si/SiO2
4.0 2.9
310
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
than those, 4.46–4.86, obtained with a catalyst subjected to acetylene TPD in our previous study [19]. The latter simply represents dimers of C2 species. The IR results indicate that the shift in the DTGA peaks to lower temperatures, as observed in Fig. 3, originates from changes in the average chain length of green oil molecules.
worthy that Pd/SiO2 is deactivated at higher rates after catalyst regeneration, while Pd-Si/SiO2 retains almost the same deactivation rate irrespective of the regeneration. Accordingly, Si modification retards catalyst deactivation even after catalyst regeneration.
3.4. Reaction rates after regeneration
Fig. 6 shows infrared spectra of CO adsorbed on the catalysts before and after regeneration. The spectra show four bands between 1800 and 2200 cm−1 , corresponding to CO adsorbed in different modes, as shown in Table 3 [24]: linear (2100–2050 cm−1 ), compressed-bridged (1995–1975 cm−1 ), isolatedbridged (1960–1925 cm−1 ), and tri-coordinated (1890–1870 cm−1 ) modes. A strong CO band at 2094 cm−1 is observed on fresh Pd/SiO2 because Pd is dispersed in small crystallites on the catalyst. On Pd-Si/SiO2 , the overall spectral intensity is reduced and the band at 1995 cm−1 is significantly suppressed. This suggests that the Si species deposited on the Pd surface preferentially block multiply-coordinated Pd
We regenerated used catalysts by burning the green oil in oxygen at 600 ◦ C for 2 h, after which the catalysts were tested again for acetylene hydrogenation. The reaction rates obtained with fresh and regenerated catalysts are compared in Table 1. The initial activity of a fresh catalyst is reduced after the regeneration, but the extent of the activity decrease is smaller with Pd-Si/SiO2 than with Pd/SiO2 . Thus, the catalyst deactivation caused by the regeneration treatment is retarded by the Si modification. The deactivation rates of the catalysts before and after regeneration are also compared in Fig. 5. It is note-
3.5. CO-IR of regenerated catalysts
Fig. 5. Deactivation of catalyst, before and after regeneration, with accumulated amounts of converted acetylene: regeneration condition: O2 = 20 ml/min, temperature = 600 ◦ C for 2 h. (a)Pd/SiO2 , (b) Pd-Si/SiO2 ; (1) fresh, (2) after regeneration.
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
311
Fig. 6. Infrared spectra of CO adsorbed on the catalysts before and after the catalyst regeneration: (a) Pd/SiO2 , (b) Pd-Si/SiO2 ; (1) fresh, (2) after regeneration.
sites [17], which are responsible for CO adsorption in the compressed-bridged mode. Multiply-coordinated sites represent large Pd ensembles that are populated on the surface of large Pd crystallites.
Table 3 Assignments of IR bands of CO adsorbed on Pd [24] Band
Wavenumber (cm−1 )
Linear
2100–2050
Compressed-bridged
1995–1975
Species
The spectral changes shown in Fig. 6 indicate the sintering of Pd crystallites after the regeneration treatment. The ratio of the intensity of the CO band at 1995 cm−1 , corresponding to the compressed-bridged mode, to that of bands at 2094 cm−1 , corresponding to the linear modes, and at 1925 cm−1 , isolated-bridged mode, is an index for estimating the relative fraction of multiply-coordinated sites on the Pd surface. That is, the ratio increases when the size of Pd crystallites grows. The ratio obtained from Fig. 6 increases from 0.23 to 0.83, a factor of 3.61, on Pd/SiO2 and from 0.21 to 0.62, a factor of 2.95, on Pd-Si/SiO2 , as summarized in Table 1. The results indicate that Pd crystallites are sintered to a lesser extent when the catalyst surface is modified with the Si species. 4. Discussion
Isolated-bridged
Tri-coordinated
1960–1925
4.1. Green oil accumulation
1890–1870
The rate of deactivation of the Pd catalyst during acetylene hydrogenation is reduced when its surface is modified with the Si species. The reason for this is because the carbonaceous species, or green
312
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
oil, is produced in smaller amounts and becomes more volatile on the Si-modified catalyst than on the Pd-only catalyst. Si modification allows for the green oil produced on or in the vicinity of Pd to be removed at relatively low temperatures, as can be observed in the DTGA results. The IR analysis of green oil indicates that its average chain length is shorter on the Si-modified surface than on the unmodified one. Previous studies [25,26] have indicated that green oil formation is enhanced when Pd is dispersed in large crystallites on the support, so that the Pd surface contains many multiply-coordinated adsorption sites. The latter sites allow for the polymerization of C2 hydrocarbons, which are adsorbed on the Pd surface as neighbors, eventually leading to the formation of green oil. Accordingly, most commercial Pd catalysts used for acetylene hydrogenation are prepared at high metal dispersions [26] so that the Pd is dispersed in small crystallites containing relatively few multiply-coordinated sites. It has been previously reported [17,19,27] that Si species, when added to catalysts by silane decomposition, are deposited selectively on the Pd surface, thus blocking the surface from the adsorption of gases in specific modes. An example is the suppression of CO adsorption in the compressed-bridged mode, as shown in previous studies [17,19] and in Fig. 6 of this study. The suppression occurs because the Si species are dispersed over the multiply-coordinated Pd sites that are responsible for CO adsorption in the compressed-bridged mode. Accordingly, the Si deposition reduces the amount of green oil produced on the catalyst. One of the reasons why the Si species are distributed effectively on the Pd surface may be the formation of a Pd–Si alloy during the deposition process, as proposed by Smith et al. [27]. Alloy formation, which is thermodynamically feasible under the conditions of this study [28], has been predicted in many studies of Pd–Si systems [29]. XPS analysis of the Si-modified catalyst in our previous study [17] indicated that metallic Si is present in the catalyst even after its oxidation. The reason for this is because Si, deposited on the Pd surface, is not completely oxidized and a fraction remains in the form of a Pd–Si alloy at the interface between the Pd and SiO2 . Such alloy formation should contribute to the distribution of the Si species as small clusters on the Pd surface.
We attempted to estimate the size of the SiO2 clusters by the XRD analysis of Pd-Si catalysts supported on Al2 O3 but the signals were too low to permit size estimation, which indirectly indicates that the clusters are very small. In summary, the effective blocking of the multiplycoordinated adsorption sites of Pd by the Si species suppresses the formation of green oil on the surface and consequently reduces catalyst deactivation rates during acetylene hydrogenation. 4.2. Sintering of Pd crystallites The activity of a Pd catalyst is reduced after regeneration, but the extent of reduction in activity is smaller for the Si-modified catalyst than for the unmodified one because the sintering of Pd crystallites is retarded by the Si species, as indicated by the IR results of this study. The retardation of metal sintering in supported catalysts by the addition of a promoter is a well-known technique with many commercial applications [30]. The Si species modifying the Pd surface in this study would be expected to reduce the mobility of Pd crystallites as well as their vapor pressure at high temperatures so as to retard metal sintering. The effect should be large particularly when the Si species are uniformly distributed on the Pd surface in the form of small clusters, as proposed above. The Si species retard not only the sintering of Pd crystallites during catalyst regeneration but the deactivation rate of the catalyst while it is used in acetylene hydrogenation after the regeneration as well. Fig. 5 shows that the deactivation of the Si-modified catalyst, which proceeds more slowly than in the case of the Pd-only catalyst, is nearly unaffected by the regeneration, while the deactivation of the Pd-only catalyst is accelerated after the regeneration. Accordingly, the suppression of green oil formation on the Si-modified catalyst is preserved even after catalyst regeneration, whereas the Pd-only catalyst is deactivated at higher rates after regeneration. The reason for green oil suppression due to the Si species has been discussed in Section 4.1. 5. Conclusions The deactivation behavior of Pd catalyst in acetylene hydrogenation is improved when the catalyst
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
surface is modified with Si species, deposited by silane decomposition, followed by oxidation in air. We compared the deactivation rates of the Si-modified and the unmodified catalysts before and after catalyst regeneration, analyzed the green oil produced on the catalysts, and reached the following conclusions. 1. Si modification reduces the amount of green oil produced on the catalyst and makes a portion of the green oil, specifically that produced on or in the vicinity of Pd, more volatile such that it can be removed at relatively low temperatures. The average chain length of green oil molecules, estimated from IR observations, is shorter on the Si-modified surface than on the unmodified one. 2. Si species retard not only the sintering of Pd crystallites during catalyst regeneration but also the deactivation rates of the catalyst during the hydrogenation process after the regeneration. The above promotional effect by the Si modification occurs because the Si species cover the Pd surface in the form of small clusters, possibly due to Pd–Si alloy formation, in the Si deposition process. The effective blocking of the multiply-coordinated adsorption sites of Pd by the Si species suppresses the formation of green oil on the surface and consequently reduces the catalyst deactivation rate during acetylene hydrogenation. The Si species on the Pd surface would also be expected to reduce the mobility and the vapor pressure of Pd crystallites at high temperatures such that the crystallites are sintered to smaller extents in the regeneration step.
Acknowledgements This work was supported by Daelim Industrial Co. Ltd., the Brain Korea 21 project, and also by the National Research Laboratory program. References [1] W.J. Kim, C.H. Choi, S.H. Moon, Korean J. Chem. Eng. 19 (2002) 617.
313
[2] D.L. Trimm, Design of Industrial Catalysts, Elsevier, Amsterdam, 1980, p. 229. [3] A.S. Al-Ammer, G. Webb, J. Chem. Soc. Faraday 74 (1978) 195. [4] A.S. Al-Ammer, G. Webb, J. Chem. Soc. Faraday 74 (1978) 657. [5] M. Larsson, J. Jansson, S. Asplund, J. Catal. 178 (1998) 49. [6] G.C. Battiston, L. Dalloro, G.R. Tauszik, Appl. Catal. 2 (1982) 2. [7] Z.E. Gandman, M.E. Aerov, V.A. Men’shchikov, V.S. Getmantsev, Int. Chem. Eng. 15 (1975) 183. [8] S. Leviness, V. Nair, A.H. Weiss, Z. Schay, L. Guczi, J. Mol. Catal. 25 (1984) 131. [9] A. Sarkany, A.H. Weiss, L. Guczi, Appl. Catal. 10 (1984) 369. [10] J.P. Boitiaux, J. Cosyns, M. Derrien, G. Leger, Hydrocarbon Process 64 (1985) 51. [11] A. Sarkany, A.H. Weiss, T. Szilagyi, P. Sandor, L. Guczi, Appl. Catal. 12 (1984) 373. [12] A. Sarkany, Z. Zsoldos, G. Stefler, J.W. Hightower, L. Guczi, J. Catal. 157 (1995) 179. [13] P. Miegge, J.L. Rousset, B. Tardy, J. Massardier, J.C. Bertolini, J. Catal. 149 (1994) 404. [14] Y.H. Park, G.L. Price, Ind. Eng. Chem. Res. 31 (1992) 469. [15] J.H. Kang, E.W. Shin, W.J. Kim, J.D. Park, S.H. Moon, Catal. Today 63 (2000) 183. [16] Q. Zhang, J. Li, X. Liu, Q. Zhu, Appl. Catal. A 197 (2000) 221. [17] E.W. Shin, C.H. Choi, K.S. Chang, Y.H. Na, S.H. Moon, Catal. Today 44 (1998) 137. [18] E.W. Shin, S.-I. Cho, J.H. Kang, W.J. Kim, J.D. Park, S.H. Moon, Korean J. Chem. Eng. 17 (2000) 468. [19] E.W. Shin, J.H. Kang, W.J. Kim, J.D. Park, S.H. Moon, Appl. Catal. A 223 (2002) 161. [20] S.H. Moon, H. Windawi, J.R. Katzer, Ind. Eng. Chem. Fundam. 20 (1981) 396. [21] C.A. Querini, S.C. Fung, Catal. Today 37 (1997) 277. [22] N. Sheppard, J.W. Ward, J. Catal. 15 (1969) 50. [23] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1975, p. 223. [24] D. Tessier, A. Rakai, B. Verduraz, Chem. Soc. Faraday Trans. 88 (1992) 741. [25] G.A. Somorjai, D.W. Blakelv, Nature 258 (1975) 580. [26] A.J. Den Hartog, M. Deng, F. Jongerius, V. Ponec, J. Mol. Catal. 60 (1990) 99. [27] G.V. Smith, S. Tjardra, M. Mosoin, T. Woltowski, F. Notheisz, M. Bartok, I. Hannus, J. Catal. 161 (1996) 441. [28] G. Ottaviani, J. Vac. Sci. Technol. 16 (5) (1979) 1112. [29] W. Juszczyk, Z. Karpinski, J. Catal. 117 (1989) 519. [30] R.A. van Santen, Surf. Sci. 251 (1991) 6.
Performance of Si-modified Pd catalyst in acetylene hydrogenation: catalyst deactivation behavior Woo Jae Kim, Eun Woo Shin, Jung Hwa Kang, Sang Heup Moon∗ School of Chemical Engineering, Institute of Chemical Processes, Seoul National University, San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744, South Korea Received 25 December 2002; received in revised form 23 April 2003; accepted 23 April 2003
Abstract The deactivation behavior of Si-modified Pd catalysts in acetylene hydrogenation was studied using thermogravimetric analysis (TGA), infrared (IR) spectroscopy, and reaction tests. TGA and IR analyses of green oil produced on the catalyst indicate that it is produced in smaller amounts and its average chain length is shorter on a Si-modified catalyst than on an unmodified one. The above findings are due to deposition of Si species on the Pd surface; such deposits effectively block multiply-coordinated adsorption sites on the catalyst and suppress the formation of green oil on the catalyst surface, specifically on or in the vicinity of Pd. The Si species also retard the sintering of Pd crystallites during the regeneration step and allow for the slow deactivation of the catalyst during acetylene hydrogenation, after regeneration. The improvement in the deactivation behavior of the Si-modified catalyst is believed to arise from the geometric modification of the Pd surface with small clusters of the Si species. © 2003 Elsevier B.V. All rights reserved. Keywords: Acetylene hydrogenation; Pd catalyst; Si modification; Deactivation; Green oil; Sintering
1. Introduction Supported Pd catalysts that are used for the selective hydrogenation of acetylene are deactivated relatively easily due to the accumulation of green oil on the catalyst surface. Industrial processes are usually operated in a narrow range of reaction conditions to maintain high ethylene selectivity and to minimize the amount of green oil that accumulates on the catalyst surface [1]. Carbon monoxide is sometimes added to the reactant stream to improve ethylene selectivity, but the presence of carbon monoxide on the catalyst usually accelerates green ∗ Corresponding author. E-mail address: [email protected] (S.H. Moon).
oil formation [2], thus shortening the lifetime of the catalyst. The green oil is not produced when only acetylene is adsorbed on the surface, but is produced when acetylene is co-adsorbed with hydrogen [3,4]. The amount of green oil increases with the H2 /C2 H2 ratio when the hydrogen pressure is very low [5] but eventually decreases with the H2 /C2 H2 ratio at high hydrogen pressures, as is the case in industrial processes [6,7]. Green oil formation is enhanced when many multiply-coordinated sites are present, since these favor the adsorption of acetylene as neighbors [8] and consequently the polymerization of C2 species on the catalytic surface [9]. Green oil consists of paraffin and olefinic hydrocarbons in the C8 –C24 range and has a H/C ratio of about 1.9 [5,10]. It has been reported
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00367-3
306
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
that the volatile fraction of green oil, on average, is composed of molecules containing 8–10 carbon atoms [11]. Used catalysts are regenerated by burning off the green oil in air; after a series of such cycles, the catalysts become inactive due to the sintering of Pd crystallites. The time between regeneration cycles is typically less than a year, sometimes several months. Various promoters have been examined to improve catalyst lifetime as well as ethylene selectivity [8,12–16], but improvements are still needed to achieve a longer lifetime. We previously reported [17–19] that the ethylene selectivity of a Pd catalyst is improved when the catalyst surface is modified with Si species deposited by the decomposition of silane and followed by oxidation in air. We attributed the selectivity improvement to the geometric modification of the Pd surface with the Si species, which changed the relative rates of reaction paths in acetylene hydrogenation. The adsorption and desorption behavior of ethylene, acetylene, and hydrogen on the Si-modified catalyst suggests that ethylene desorption is facilitated, the amount of chemisorbed hydrogen is reduced, and C2 polymerization is suppressed. All the above trends can be regarded as contributing to the improved selectively. This paper presents the results of our continued work on the performance of the Si-modified catalyst in acetylene hydrogenation [19], particularly on the catalyst deactivation behavior. The deactivation behavior of the Si-modified and unmodified catalysts before and after catalyst regeneration were compared, and the green oil produced on the catalysts was analyzed by thermogravimetric analysis (TGA), infrared (IR) spectroscopy. The results can be explained based on the role of the Si species in the catalyst deactivation process.
Si-modified catalyst, designated as Pd-Si/SiO2 , was prepared from Pd/SiO2 by depositing Si on the catalyst and subsequently oxidizing the Si species in air. The Si/Pd atomic ratio in the catalyst was 0.83, as estimated from the amounts of silane added to the reactor for decomposition. The catalysts were tested for acetylene hydrogenation at atmospheric conditions in a quartz reactor. The reaction conditions were more severe than those used in our previous study [19], which was focused on the catalytic selectivity of ethylene production, and as a result the catalysts were deactivated to greater extents. The reaction temperature was 343 K instead of 313 K, the reactant stream contained 4.1% acetylene in ethylene instead of 1.01%, the H2 /C2 H2 ratio in the reaction stream was 1.0 instead of 2.0, and the space velocity was 800 cm3 /(min g). The parameters for the gas chromatography (GC) in the product analysis were the same as those in the previous study [19].
2. Experimental
3. Results
2.1. Catalyst preparation and reaction tests
3.1. Deactivation rates
Two sample catalysts, Si-modified and unmodified Pd/SiO2 , were prepared and their deactivation characteristics in acetylene hydrogenation were compared. Unmodified Pd/SiO2 , containing 1.0 wt.% Pd, is identical to that prepared in a previous study [19]. The
As shown in Table 1, the activity of fresh Pd/SiO2 for acetylene hydrogenation is slightly lowered when the catalyst is modified with the Si species because the latter, which are inactive for the reaction, partially cover the Pd surface. We reported in our previous study
2.2. TGA, DTGA, and IR The catalysts used for the same accumulated conversion of acetylene were analyzed thermogravimetrically (TA Instruments, TGA2050) in air flowing at 40 cm3 /min. The temperature was raised from 303 to 1123 K at a rate of 10 K/min. DTGA results were obtained from the TGA curves by differentiating the latter with respect to temperature. The infrared spectra of either CO adsorbed to or green oil deposited on the catalyst surface after the reaction were recorded on a Midac 2100 spectrometer in an IR cell with CaF2 windows attached at both ends. The spectrum of the green oil was obtained from the difference in the spectra of used and fresh catalysts. Details of the IR apparatus and the measurement procedures have been described previously [20].
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313 Table 1 Reaction rates and CO-IR results obtained with fresh and regenerated catalysts Catalyst
Pd/SiO2 Pd-Si/SiO2
Reaction ratea
Acb /Al + ib b
Fresh
After regeneration
Fresh
After regeneration
0.91 0.85
0.84 0.84
0.23 0.21
0.83 0.62
a Reaction conditions: temperature = 343 K; H /C H ratio in 2 2 2 the reaction stream = 1.0; space velocity = 2200 min−1 . b Area of compressed-bridged band (A )/(area of linear band cb (A1 ) + area of isolated-bridged band (Aib )).
[17] that Si species deposited on the Pd surface consisted of Si and SiO2 , the latter being the major component because the catalyst had been exposed to air after the Si deposition. Catalysts are deactivated during their use in acetylene hydrogenation. Fig. 1 shows the reaction rates normalized to the initial values. The rates were plotted against the accumulated amounts of acetylene con-
307
verted in the reaction instead of the reaction time, so that they can be compared based on the same load in the reaction. It is apparent in Fig. 1 that catalyst deactivation is retarded in the case where the catalysts are modified with the Si species. Pd catalysts used in acetylene hydrogenation are largely deactivated as a result of the accumulation of green oil on the catalyst surface; such oil is produced by the polymerization of the C2 species [9]. Consequently, the above results suggest that green oil is produced in smaller amounts on the Si-modified catalyst than on the Pd-only catalyst. 3.2. TGA and DTGA To analyze the amounts of green oil deposited on the catalyst during the reaction more accurately, we performed the TGA of Pd/SiO2 and Pd-Si/SiO2 after they were used for the same accumulated acetylene conversion. Fig. 2 shows that Pd/SiO2 used under the conditions specified in Table 1 loses 66.3% of its initial
Fig. 1. Deactivation of catalysts with accumulated amounts of converted acetylene: catalyst weight = 0.05 g, H2 /C2 H2 = 1, reaction temperature = 70 ◦ C. (a) Pd/SiO2 , (b) Pd-Si/SiO2 .
308
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
Fig. 2. Thermogravimetric analysis of used catalysts in air: air flow rate = 40 ml/min, heating rate = 10 ◦ C/min. (a) Pd/SiO2 , (b) Pd-Si/SiO2 .
Fig. 3. Differential thermogravimetric analysis of used catalysts in air: (a) Pd/SiO2 , (b) Pd-Si/SiO2 .
weight at 850 ◦ C, while Pd-Si/SiO2 loses 61.5% of its weight. The difference in weight loss between the two catalysts is not due to the removal of volatile hydrocarbons or water at low temperatures, because the TGA curves are almost identical in this region. A reasonable explanation is that the difference occurs mostly due to the burn-off of green oil deposited on the catalysts, as indicated by the TGA curves, which deviate from each other at temperatures above 250 ◦ C. To study further the properties of the green oil removed in the oxidation process, we differentiated the TGA curves to obtain the DTGA results given in Fig. 3. Three major peaks are observed in different temperature regions: below 300, 300–500 ◦ C, and above 500 ◦ C. According to Larsson et al. [5], who obtained similar results in the analysis of coke deposited on Pd catalysts used for acetylene hydrogenation by temperature-programmed oxidation (TPO), the peak below 300 ◦ C, designated as Peak I in this study, originates from heavy hydrocarbons that are adsorbed on the catalyst surface or absorbed in the catalyst pores. Peak II, observed between 300 and 500 ◦ C, represents coke on or in the vicinity of Pd. Peak III, above 500 ◦ C, corresponds to coke produced on the support without the influence of Pd.
Two changes are of interest in Fig. 3 as a result of the Si modification: peak shift to lower temperatures, which is particularly large for Peak II, and characteristic changes in the intensity of the three major peaks. The intensity and the position of Peak I seem to change slightly with Si modification, but a definite conclusion is not available because the peak appears as a shoulder to the more intense Peak II. The peaks may be deconvoluted as was done by Larsson et al. [5], but the data herein are not sufficiently accurate for a quantitative analysis. Peak II consists of two minor peaks that are split by about 70 ◦ C on Pd/SiO2 , but are merged into a single intense peak on Pd-Si/SiO2 . The average position of Peak II is significantly shifted to lower temperatures, by 67 ◦ C, due to the Si modification. The peak shift may occur for two reasons. One is because coke, or green oil, produced on the catalyst becomes more volatile with lower molecular weight, and the other is because the coke particles are present in smaller sizes, as suggested by Querini and Fung [21], on the Si-modified catalyst. This issue will be discussed further in Section 3.3, based on the IR analysis of the surface carbonaceous species. Peak III is only slightly shifted to lower temperatures and its intensity is reduced by about 20% after catalyst modification.
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
309
Fig. 4. Infrared spectra of green oil deposited on used catalysts: catalysts were used for the conversion of 0.022 mol of acetylene/0.05 g of catalyst. (a) Pd/SiO2 , (b) Pd-Si/SiO2 .
The effect of the Si modification can be summarized as follows, based on the above DTGA results and peak assignments proposed by Larsson et al. [5]. Coke that is deposited on or in the vicinity of Pd is removed at significantly lower temperatures, that deposited on SiO2 is removed at slightly lower temperatures, and the amounts of coke on the SiO2 are reduced by about 20% as a result of the catalyst modification. Changes in the amounts of heavy hydrocarbons produced on the catalyst and the coke associated with Pd due to the Si modification are not clear, because Peaks I and II overlap with each other. 3.3. IR of green oil The infrared spectrum of the carbonaceous species, green oil, that is accumulated on the catalyst surface during the reaction shows many peaks due to the C–H and C–C bonds included in the species. Fig. 4 compares the spectra of green oil obtained from two used catalysts in the region between 2800 and 3050 cm−1 . The peak at 2930 cm−1 is due to the C–H stretching of the CH2 group and the peak at 2960 cm−1 is due to the C–H stretching of the CH3 group [11]. Follow-
ing the analysis by Sheppard and Ward [22], one may estimate the average chain length of the hydrocarbon species in green oil from the intensity ratio of two C–H peaks, CH2 /CH3 . Table 2 shows that the intensity ratio is smaller, 2.9, for Pd-Si/SiO2 than for Pd/SiO2 , 4.0, and therefore green oil accumulated on the former catalyst has a shorter chain length than that on the latter catalyst. The average carbon number of green oil molecules should be estimated from three times the intensity ratio, 3CH2 /CH3 , because the extinction coefficient of the IR peak due to the CH3 group is about triple that of the CH2 group [23]. The estimated carbon numbers are 19.4 for Pd-Si/SiO2 and 26 for Pd/SiO2 ; such values fall in the range known for common green oil [10]. The above carbon numbers, 19.4–26, are larger Table 2 Intensity ratios of ν(CH2 ) to ν(CH3 ) for green oil produced on different catalysts Catalyst
Peak area ratio (CH2 /CH3 )
Pd/SiO2 Pd-Si/SiO2
4.0 2.9
310
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
than those, 4.46–4.86, obtained with a catalyst subjected to acetylene TPD in our previous study [19]. The latter simply represents dimers of C2 species. The IR results indicate that the shift in the DTGA peaks to lower temperatures, as observed in Fig. 3, originates from changes in the average chain length of green oil molecules.
worthy that Pd/SiO2 is deactivated at higher rates after catalyst regeneration, while Pd-Si/SiO2 retains almost the same deactivation rate irrespective of the regeneration. Accordingly, Si modification retards catalyst deactivation even after catalyst regeneration.
3.4. Reaction rates after regeneration
Fig. 6 shows infrared spectra of CO adsorbed on the catalysts before and after regeneration. The spectra show four bands between 1800 and 2200 cm−1 , corresponding to CO adsorbed in different modes, as shown in Table 3 [24]: linear (2100–2050 cm−1 ), compressed-bridged (1995–1975 cm−1 ), isolatedbridged (1960–1925 cm−1 ), and tri-coordinated (1890–1870 cm−1 ) modes. A strong CO band at 2094 cm−1 is observed on fresh Pd/SiO2 because Pd is dispersed in small crystallites on the catalyst. On Pd-Si/SiO2 , the overall spectral intensity is reduced and the band at 1995 cm−1 is significantly suppressed. This suggests that the Si species deposited on the Pd surface preferentially block multiply-coordinated Pd
We regenerated used catalysts by burning the green oil in oxygen at 600 ◦ C for 2 h, after which the catalysts were tested again for acetylene hydrogenation. The reaction rates obtained with fresh and regenerated catalysts are compared in Table 1. The initial activity of a fresh catalyst is reduced after the regeneration, but the extent of the activity decrease is smaller with Pd-Si/SiO2 than with Pd/SiO2 . Thus, the catalyst deactivation caused by the regeneration treatment is retarded by the Si modification. The deactivation rates of the catalysts before and after regeneration are also compared in Fig. 5. It is note-
3.5. CO-IR of regenerated catalysts
Fig. 5. Deactivation of catalyst, before and after regeneration, with accumulated amounts of converted acetylene: regeneration condition: O2 = 20 ml/min, temperature = 600 ◦ C for 2 h. (a)Pd/SiO2 , (b) Pd-Si/SiO2 ; (1) fresh, (2) after regeneration.
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
311
Fig. 6. Infrared spectra of CO adsorbed on the catalysts before and after the catalyst regeneration: (a) Pd/SiO2 , (b) Pd-Si/SiO2 ; (1) fresh, (2) after regeneration.
sites [17], which are responsible for CO adsorption in the compressed-bridged mode. Multiply-coordinated sites represent large Pd ensembles that are populated on the surface of large Pd crystallites.
Table 3 Assignments of IR bands of CO adsorbed on Pd [24] Band
Wavenumber (cm−1 )
Linear
2100–2050
Compressed-bridged
1995–1975
Species
The spectral changes shown in Fig. 6 indicate the sintering of Pd crystallites after the regeneration treatment. The ratio of the intensity of the CO band at 1995 cm−1 , corresponding to the compressed-bridged mode, to that of bands at 2094 cm−1 , corresponding to the linear modes, and at 1925 cm−1 , isolated-bridged mode, is an index for estimating the relative fraction of multiply-coordinated sites on the Pd surface. That is, the ratio increases when the size of Pd crystallites grows. The ratio obtained from Fig. 6 increases from 0.23 to 0.83, a factor of 3.61, on Pd/SiO2 and from 0.21 to 0.62, a factor of 2.95, on Pd-Si/SiO2 , as summarized in Table 1. The results indicate that Pd crystallites are sintered to a lesser extent when the catalyst surface is modified with the Si species. 4. Discussion
Isolated-bridged
Tri-coordinated
1960–1925
4.1. Green oil accumulation
1890–1870
The rate of deactivation of the Pd catalyst during acetylene hydrogenation is reduced when its surface is modified with the Si species. The reason for this is because the carbonaceous species, or green
312
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
oil, is produced in smaller amounts and becomes more volatile on the Si-modified catalyst than on the Pd-only catalyst. Si modification allows for the green oil produced on or in the vicinity of Pd to be removed at relatively low temperatures, as can be observed in the DTGA results. The IR analysis of green oil indicates that its average chain length is shorter on the Si-modified surface than on the unmodified one. Previous studies [25,26] have indicated that green oil formation is enhanced when Pd is dispersed in large crystallites on the support, so that the Pd surface contains many multiply-coordinated adsorption sites. The latter sites allow for the polymerization of C2 hydrocarbons, which are adsorbed on the Pd surface as neighbors, eventually leading to the formation of green oil. Accordingly, most commercial Pd catalysts used for acetylene hydrogenation are prepared at high metal dispersions [26] so that the Pd is dispersed in small crystallites containing relatively few multiply-coordinated sites. It has been previously reported [17,19,27] that Si species, when added to catalysts by silane decomposition, are deposited selectively on the Pd surface, thus blocking the surface from the adsorption of gases in specific modes. An example is the suppression of CO adsorption in the compressed-bridged mode, as shown in previous studies [17,19] and in Fig. 6 of this study. The suppression occurs because the Si species are dispersed over the multiply-coordinated Pd sites that are responsible for CO adsorption in the compressed-bridged mode. Accordingly, the Si deposition reduces the amount of green oil produced on the catalyst. One of the reasons why the Si species are distributed effectively on the Pd surface may be the formation of a Pd–Si alloy during the deposition process, as proposed by Smith et al. [27]. Alloy formation, which is thermodynamically feasible under the conditions of this study [28], has been predicted in many studies of Pd–Si systems [29]. XPS analysis of the Si-modified catalyst in our previous study [17] indicated that metallic Si is present in the catalyst even after its oxidation. The reason for this is because Si, deposited on the Pd surface, is not completely oxidized and a fraction remains in the form of a Pd–Si alloy at the interface between the Pd and SiO2 . Such alloy formation should contribute to the distribution of the Si species as small clusters on the Pd surface.
We attempted to estimate the size of the SiO2 clusters by the XRD analysis of Pd-Si catalysts supported on Al2 O3 but the signals were too low to permit size estimation, which indirectly indicates that the clusters are very small. In summary, the effective blocking of the multiplycoordinated adsorption sites of Pd by the Si species suppresses the formation of green oil on the surface and consequently reduces catalyst deactivation rates during acetylene hydrogenation. 4.2. Sintering of Pd crystallites The activity of a Pd catalyst is reduced after regeneration, but the extent of reduction in activity is smaller for the Si-modified catalyst than for the unmodified one because the sintering of Pd crystallites is retarded by the Si species, as indicated by the IR results of this study. The retardation of metal sintering in supported catalysts by the addition of a promoter is a well-known technique with many commercial applications [30]. The Si species modifying the Pd surface in this study would be expected to reduce the mobility of Pd crystallites as well as their vapor pressure at high temperatures so as to retard metal sintering. The effect should be large particularly when the Si species are uniformly distributed on the Pd surface in the form of small clusters, as proposed above. The Si species retard not only the sintering of Pd crystallites during catalyst regeneration but the deactivation rate of the catalyst while it is used in acetylene hydrogenation after the regeneration as well. Fig. 5 shows that the deactivation of the Si-modified catalyst, which proceeds more slowly than in the case of the Pd-only catalyst, is nearly unaffected by the regeneration, while the deactivation of the Pd-only catalyst is accelerated after the regeneration. Accordingly, the suppression of green oil formation on the Si-modified catalyst is preserved even after catalyst regeneration, whereas the Pd-only catalyst is deactivated at higher rates after regeneration. The reason for green oil suppression due to the Si species has been discussed in Section 4.1. 5. Conclusions The deactivation behavior of Pd catalyst in acetylene hydrogenation is improved when the catalyst
W.J. Kim et al. / Applied Catalysis A: General 251 (2003) 305–313
surface is modified with Si species, deposited by silane decomposition, followed by oxidation in air. We compared the deactivation rates of the Si-modified and the unmodified catalysts before and after catalyst regeneration, analyzed the green oil produced on the catalysts, and reached the following conclusions. 1. Si modification reduces the amount of green oil produced on the catalyst and makes a portion of the green oil, specifically that produced on or in the vicinity of Pd, more volatile such that it can be removed at relatively low temperatures. The average chain length of green oil molecules, estimated from IR observations, is shorter on the Si-modified surface than on the unmodified one. 2. Si species retard not only the sintering of Pd crystallites during catalyst regeneration but also the deactivation rates of the catalyst during the hydrogenation process after the regeneration. The above promotional effect by the Si modification occurs because the Si species cover the Pd surface in the form of small clusters, possibly due to Pd–Si alloy formation, in the Si deposition process. The effective blocking of the multiply-coordinated adsorption sites of Pd by the Si species suppresses the formation of green oil on the surface and consequently reduces the catalyst deactivation rate during acetylene hydrogenation. The Si species on the Pd surface would also be expected to reduce the mobility and the vapor pressure of Pd crystallites at high temperatures such that the crystallites are sintered to smaller extents in the regeneration step.
Acknowledgements This work was supported by Daelim Industrial Co. Ltd., the Brain Korea 21 project, and also by the National Research Laboratory program. References [1] W.J. Kim, C.H. Choi, S.H. Moon, Korean J. Chem. Eng. 19 (2002) 617.
313
[2] D.L. Trimm, Design of Industrial Catalysts, Elsevier, Amsterdam, 1980, p. 229. [3] A.S. Al-Ammer, G. Webb, J. Chem. Soc. Faraday 74 (1978) 195. [4] A.S. Al-Ammer, G. Webb, J. Chem. Soc. Faraday 74 (1978) 657. [5] M. Larsson, J. Jansson, S. Asplund, J. Catal. 178 (1998) 49. [6] G.C. Battiston, L. Dalloro, G.R. Tauszik, Appl. Catal. 2 (1982) 2. [7] Z.E. Gandman, M.E. Aerov, V.A. Men’shchikov, V.S. Getmantsev, Int. Chem. Eng. 15 (1975) 183. [8] S. Leviness, V. Nair, A.H. Weiss, Z. Schay, L. Guczi, J. Mol. Catal. 25 (1984) 131. [9] A. Sarkany, A.H. Weiss, L. Guczi, Appl. Catal. 10 (1984) 369. [10] J.P. Boitiaux, J. Cosyns, M. Derrien, G. Leger, Hydrocarbon Process 64 (1985) 51. [11] A. Sarkany, A.H. Weiss, T. Szilagyi, P. Sandor, L. Guczi, Appl. Catal. 12 (1984) 373. [12] A. Sarkany, Z. Zsoldos, G. Stefler, J.W. Hightower, L. Guczi, J. Catal. 157 (1995) 179. [13] P. Miegge, J.L. Rousset, B. Tardy, J. Massardier, J.C. Bertolini, J. Catal. 149 (1994) 404. [14] Y.H. Park, G.L. Price, Ind. Eng. Chem. Res. 31 (1992) 469. [15] J.H. Kang, E.W. Shin, W.J. Kim, J.D. Park, S.H. Moon, Catal. Today 63 (2000) 183. [16] Q. Zhang, J. Li, X. Liu, Q. Zhu, Appl. Catal. A 197 (2000) 221. [17] E.W. Shin, C.H. Choi, K.S. Chang, Y.H. Na, S.H. Moon, Catal. Today 44 (1998) 137. [18] E.W. Shin, S.-I. Cho, J.H. Kang, W.J. Kim, J.D. Park, S.H. Moon, Korean J. Chem. Eng. 17 (2000) 468. [19] E.W. Shin, J.H. Kang, W.J. Kim, J.D. Park, S.H. Moon, Appl. Catal. A 223 (2002) 161. [20] S.H. Moon, H. Windawi, J.R. Katzer, Ind. Eng. Chem. Fundam. 20 (1981) 396. [21] C.A. Querini, S.C. Fung, Catal. Today 37 (1997) 277. [22] N. Sheppard, J.W. Ward, J. Catal. 15 (1969) 50. [23] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1975, p. 223. [24] D. Tessier, A. Rakai, B. Verduraz, Chem. Soc. Faraday Trans. 88 (1992) 741. [25] G.A. Somorjai, D.W. Blakelv, Nature 258 (1975) 580. [26] A.J. Den Hartog, M. Deng, F. Jongerius, V. Ponec, J. Mol. Catal. 60 (1990) 99. [27] G.V. Smith, S. Tjardra, M. Mosoin, T. Woltowski, F. Notheisz, M. Bartok, I. Hannus, J. Catal. 161 (1996) 441. [28] G. Ottaviani, J. Vac. Sci. Technol. 16 (5) (1979) 1112. [29] W. Juszczyk, Z. Karpinski, J. Catal. 117 (1989) 519. [30] R.A. van Santen, Surf. Sci. 251 (1991) 6.