Deactivation and Coke Formation on Nickel−Tungsten Supported on

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Deactivation and Coke Formation on Nickel-Tungsten Supported on Silica-Alumina Catalysts Yacine Rezgui* and Miloud Guemini Laboratoire de Recherche de Chimie Applique´e et Technologie des Mate´riaux, UniVersite´ d’Oum El Bouaghi, B.P. 358, Route de Constantine Oum El Bouaghi 04000, Algeria

Nickel-tungsten oxides supported on silica-alumina catalysts were submitted to the coking reaction with n-octane at 150, 250, and 300 °C. The characteristics of deposited coke with different times on stream were studied using the temperature-programmed oxidation (TPO) technique. The results revealed that the strength and density of acid sites as well as the reaction temperature affect the nature of the coke formed: at 300 °C, it was observed that for the WT7 catalyst, the most acidic solid, there were two peaks corresponding to two kinds of coke recorded in TPO profiles, while at 150 °C only one peak was observed. On the other hand, for the WT4 catalyst, the less acidic sample, only the soft coke was observed. In addition, the oxidation temperature of coke shifted to higher values with high medium acid sites concentration and high temperature values with the increase of coke deposits. Besides, coke molecules interacted preferentially with the most acidic sites, which was confirmed by the strong deactivation in the first reaction stages, especially for the WT7 catalyst. For the regeneration tests, the collected data suggested that the coked WT4 catalyst showed a good combustion behavior; it was regenerated at 500 °C for 1 h, whereas the coked WT7 could not be regenerated at this temperature. 1. Introduction One of the most serious problems in the practice of industrial catalytic processes is presented by the loss of catalytic activity during reaction within a porous catalyst medium;1 this catalytic decay is a result of a number of unwanted chemical and physical changes and may be induced by several causes such as the sintering of a porous carrier,2,3 pore-plugging by poisons in the feed or by poisonous byproducts,4 and the deposition of reaction residues on active sites (coke formation).5–10 Costs to industry for catalyst replacement and process shutdown total billions of dollars per year; thus, there is a considerable motivation to understand and treat catalyst decay.1 In many industrial processes, the activity decay is caused by the formation of carbonaceous deposits (coke) on the catalyst surface; thus, the development of catalysts that are less prone to deactivation requires understanding the mechanisms by which such deposits are formed and what structures they exhibit.1 Coke has several complex structures by nature, and its forms may vary from high molecular weight hydrocarbons to primarily carbons such as graphite, depending upon the conditions under which the coke was formed and aged.1 However, it was mentioned that coke which burnt at about the same temperature is suggested to have a similar structure and that the more graphitic the structure, the higher the combustion temperature.11 In the past few years, enormous efforts have been devoted to study coke deposits (location and total amount) and to obtain information regarding deactivation mechanism and regeneration conditions; to this purpose, several characterization techniques have been used. One of the most widely used techniques is temperature-programmed oxidation (TPO).12–21 In a previous work, we have demonstrated that Ni-WOx/ SiO2-Al2O3 catalysts can be excellent candidates for catalytic hydroisomerization (up to 79.2 and 70% selectivity to isomers in the case of n-hexane and n-heptane, respectively)22,23 and dewaxing (the selectivity to isomers, in the case of n-decane, * Corresponding author. E-mail: [email protected].

amounts to 55% at a conversion of 42.3%);24,25 however, they exhibit a decay in activity due to carbonaceous deposits. Thus, in this contribution, we report a study of the nickel-tungsten supported on amorphous silica-alumina catalysts deactivation. For this pupose, we try to identify relationships between the amount of coke deposits and catalyst composition by investigating the factors affecting TPO technique. 2. Experimental Section 2.1. Catalyst Preparation. The catalysts used throughout the experiments consisted of nickel-tungsten oxide supported on silica-alumina, hereafter abbreviated WTx (WT1, WT2, . . . WT7) with ponderal compositions listed in Table 1. They were prepared by a sol-gel method as described in refs 24 and 25, and a series of 7 catalysts containing 12, 15, and 17% of nickel and 8-10% of tungsten were prepared as follows: To an aqueous solution of nickel nitrate (Ni(NO3)2 · 6H2O), preliminarily acidified by nitric acid, aqueous solutions of aluminum sulfate and sodium tungstanate were added. Afterward, an aqueous solution of sodium silicate was added to the obtained sol, under vigorous stirring. To exchange undesirable ions, such as Na+, the prepared gel was activated under reflux conditions in a thermostat with ammonium sulfate (liquid to solid ratio of 30) at 60 °C over a period of 48 h (this unit operation was repeated several times), washed with hot water (60 °C), dried at 120 °C for 4 h, and finally calcined at 500 °C for 5 h. A heating rate of 10 °C/min was used. Table 1. Acidity and Chemical Composition of the WTx Catalysts25 composition catalyst

Ni

W

WT1 WT2 WT3 WT4 WT6 WT7

12 12 15 15 17 17

08 10 08 10 08 10

total acidity medium acidity Lewis acidity (mmol NH3/g) (mmol NH3/g) (µmol Pyr/g) 0.29 0.25 0.32 0.28 0.34 0.38

10.1021/ie071407g CCC: $40.75  2008 American Chemical Society Published on Web 05/20/2008

0.15 0.11 0.17 0.11 0.24 0.26

140 121 148 100 154 89

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2.2. Catalyst Characterization. The prepared materials were characterized using different techniques. The most common characterization protocols have already been mentioned in refs 24 and 25. 2.2.1. Ammonia Temperature Programmed Desorption (NH3-TPD). The acidic properties of the samples were measured by using the temperature-programmed desorption of ammonia (TPD of NH3). Prior to TPD experiments, the sample was pretreated, for 3 h at 500 °C, with an oxygen flow, then purged by flowing helium at 300 °C, and then reduced, at 430 °C for 2 h, in a hydrogen flow. After reduction, the sample was cooled to room temperature. When the system became steady, ammonia was adsorbed at 100 °C for 30 min (using a 10% NH3/He carrier gas) and then the sample was subsequently purged, at the same temperature, by flowing He (100 mL/min) for 1 h to remove the excess and physically adsorbed NH3. The TPD spectrum was obtained by heating the sample from 100 to 700 °C at a heating rate of 10 °C/min under a He flow. Evolved NH3 was monitored with a TCD detector. 2.2.2. FT-IR Spectroscopy. FT-IR spectra of adsorbed pyridine were recorded on a 170-SX Nicolet FT-IR spectrometer. The sample was finely grounded and pressed into a selfsupporting wafer (14 mg/cm2) and then placed in a heatable glass cell equipped with KBr windows, which permitted us to follow the changes of the spectrum with thermal treatments. Prior to IR measurements, the sample was treated in situ (in the cell) with a hydrogen flow at 430 °C for 2 h, evacuated to 0.1 Pa at the same temperature for 1 h, and then exposed to pyridine (1 Torr) at 423 K for 20 min, outgassed 1 h at room temperature (pressure 0.1 Pa), and then heated to the desired temperature using a linear program. 2.2.3. Temperature Programmed Desorption (TPO). As mentioned by Herrera et al.26 and other authors,27–29 TPO can be used to probe carbonaceous deposits on catalyst surfaces. Analysis of the evolved CO2/CO allows both qualitative and quantitative assesments of the type and reactivity of these deposits.30,31 With the use of the protocol mentioned in ref 26, TPO measurements were carried out by passing a continuous flow of 5% O2/He over catalysts as the temperature was increased linearly at 12 °C/min to 750 °C. The CO2 and CO thus formed were quantitatively converted to methane in a methanator by mixing the effluent with 0.83 cm3 s-1 H2 and passing over a 15% Ni/Al2O3 catalyst held at 673 K. CH4 formation rates were measured by a flame ionization detector, calibrated with 100 µl pulses of CO2 and by the combustion of known amounts of graphite. 2.3. Catalytic Test. The n-octane isomerization was performed at atmospheric pressure using a continuous flow fixedbed quartz reactor, with an inner diameter of 10 mm, operated under isothermal conditions, and heated by a controlled temperature electrical oven. The reactor outlet passed through a sampling valve connected to a gas chromatograph. The operating conditions were as follows: • Mass of the catalyst, 1 g. • Reaction temperature ranging from 150 to 300 °C with a step of 50 °C. • Weight hourly space velocity, 4 h-1. • Hydrogen/hydrocarbon (H2/n-octane) molar ratio equal to 5. 2.4. Regeneration Procedure. After reaction, the catalyst was regenerated by heating it in oxygen from the reaction temperature to the desired regeneration temperature (in our case 350, 400, 450, 500, or 550 °C) in 1 h and maintaining this

temperature for 1 additional hour. Afterward, the catalyst was cooled back to room temperature and reduced according to the procedure mentioned in ref 23. Finally, the reaction procedure was repeated. 3. Results and Discussion 3.1. Coke Characterization. In previous works,22–25 we found that Ni-WOx/SiO2-Al2O3 catalysts are deactivated during straight chain paraffin’s hydroisomerization because of the formation of carbon deposits that block pores and surface active sites. To quantify the amount of coke deposited during the reaction, after reaching the steady state (120 min) and to obtain information about how coke distributes over the catalyst, TPO analyses of the carbonaceous deposits were conducted on all the spent catalysts. On the basis of the TPO spectra (Figure 1 parts a and b), we can speculate that for samples with 17% of nickel (WT7 and WT6, see Table 1) at 300 °C, we can categorize coke into two groups. The first group is the one which appeared at temperatures of 418 and 420 °C, for WT6 and WT7, respectively, while the second one is for the peak which appeared around 550 and 570 °C, for WT6 and WT7, respectively. As mentioned in several papers dealing with the nature and structure of coke deposited on catalysts, in the TPO profiles, peaks which appeared in the temperature range of 350-450 °C may be identified as the combustion of coke on metallic site,32,33 while peaks which appeared around 450-600 °C may ascribed to coke deposited on the support. Thus in the case of WT6 and WT7, the coke is deposited on both metallic site and support. In contrast, at 150 °C the two above-mentioned solids exhibited only one peak at 494 and 504 °C, for WT6 and WT7, respectively, which were assigned to support coking. On the other hand, despite the difference in reaction temperature (150 or 300 °C), in the TPO spectra of the samples with 15 and 12% of nickel, one major CO2 evolution peak was observed in the temperature range of 460-504 °C suggesting that metallic sites were not coked. In addition, from TPO profiles, it is obvious that, whatever the catalyst, increasing reaction temperature induced a rise in the intensity of both low and high temperature peaks and a shift in their positions toward high temperature values; the phenomenon was more pronounced for the coke, which burns at high temperatures. These effects suggest that at high reaction temperatures greater amounts of coke and more graphitic structures were obtained. In our previous works, we have reported that Ni-WOx/ SiO2-Al2O3 catalysts were deactivated and that solids with 17% Ni (WT6 and WT7) exhibited the higher deactivation rates while materials with 15% Ni exhibited the lowest deactivation rates, which is consistent with TPO data where coke amounts were the highest on the first solids and the lowest on the latter ones. On the other hand, we have mentioned that in the case of Ni-WOx/SiO2-Al2O3 materials, when nickel exceeded a certain threshold (in our case, 15% Ni), the optimal balance of metallic site-acidic site was altered and consequently the intermediate species residence time increased and the possibility that these species could be cracked was greater. Hence, the polymerization reactions (coke formation) would be more favored in such conditions, and thus, in the case of WT6 and WT7 catalysts, the metallic sites were coked. In addition, increasing temperature increased conversion, hydrogenolysis, and cracking reactions and consequently increased the coke formation rate. For evidencing the fact that WT6 and WT7 metallic sites were coked at 300 °C, while the other samples metallic sites were not affected, isomerization, hydrogenolysis, and cracking reactions were thoroughly studied over the prepared catalysts

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Figure 1. (a) TPO profiles of coke deposits over Ni-WOx/SiO2-Al2O3 catalysts. Reaction conditions: reaction temperature ) 150 °C; time on stream (TOS) )120 min. (b) TPO profiles of coke deposits over Ni-WOx/ SiO2-Al2O3 catalysts. Reaction conditions: reaction temperature ) 300 °C; TOS )120 min. (c) Effect of TOS on Ni-WOx/SiO2-Al2O3 catalysts performance. Reaction temperature ) 300 °C.

at 300 °C and at different times on stream (TOS). The reaction pathways with respect to these reactions are depicted in Figure 1.c. The collected data show clearly that during the run, the cracking reactions decreased with TOS, the effect being more drastic during the first few minutes particularly when the number of medium acid sites present on the sample increased (the WT (17,y) catalysts with the highest density of medium acid 0.24

and 0.26 mmol NH3/g showed the highest decay in the rate of cracking reactions with TOS). Thus, it seems to be a correlation between the density of moderate acid sites and the cracking. On the other hand, the results evidence that with increasing TOS, isomerization reactions increased with a significant rise during the first few minutes; this effect was more pronounced in the case of WT(17,10) due to its higher deactivation, which decreased cracking and allowed more isomerization of the adsorbed species. Concerning hydrogenolysis reactions, catalyst deactivation was observed only on WT6 and WT7 samples, which suggests that the metallic function of these solids was poisoned. 3.2. Effect of TOS. The effect of TOS was studied at a reaction temperature of 250 °C. As shown in Figure 2, all catalysts exhibited identical behaviors in that the amount of coke deposited, reported as the surface carbon concentration, increased with increasing TOS, the rise being more rapid during the first few minutes (20 min). Besides, the deposited coke amount was the highest for WT7 and the lowest for WT4. Furthermore, the slopes of the curves became smaller at the high values of TOS and remained nearly steady after 100 min; this feature suggests that the rate of coke deposition decreased with TOS, the phenomenon being more rapid during the first few minutes (20 min), and stationary amounts were obtained after 100 min. These phenomena may be explained by the fact that during the initial reaction stages, the number of medium acid sites present on the sample was high; these acids sites are likely to deactivate more rapidly as compared with weak acid sites, which infers that, at initial reaction stages, the deactivation rate and consequently the amount of deposited coke were high. With increasing TOS, the number of medium acid sites decreased and thus the deactivation rate and consequently the amount of deposited coke decreased with TOS. Similar results were reported by Rossi et al.33 during their study of the n-butane isomerization over WOx/ZrO2 catalysts. 3.3. Effect of Acidity. From the open literature, it is wellknown that the number and density of strong and medium acid sites is crucial for the rate of coke formation and the amount of coke deposited on the catalyst.34–36 But also, the nature and location of the coke already deposited is of influence. Thus, in this section we describe the study of the effect of acidity on the coke formation. The study was performed at a reaction temperature of 250 °C and at different times on stream. As shown in Figure 3, whatever the TOS, the amount of deposited coke increased with increasing the catalyst medium acidity, which rules out the idea that the high acidity of the catalysts catalyzes the production of carbonaceous deposits. The same results were reported by Yori et al.37 and Grau et al.38–41 during their studies on straight chain paraffins hydroisomerization and by Kim et al.42 and Kang and Inui43 during their studies on the effects of the decrease in number of acid sites located on the external surface of Ni-SAPO-34 crystalline catalyst, where it was mentioned that the amount of deposited coke was closely related to the acid site concentration. These data may be explained by the fact that, in the presence of site heterogenity, the sites with higher site activity tend to deactivate at a faster rate. On the other hand, for the same medium acidity concentrations (see Table 1), the coke formation was promoted by an increase in Lewis acid sites density. This result is in accordance with those reported by Arenamnart and Wimonrat44 and by Chang et al.,45 who mentioned that the reduction of the concentration of Lewis acid centers leads to lower deactivation by coke, since coke formation seems to be faster on this type of site. In addition, it can be clearly seen from the TPO spectra,

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Figure 2. Effect of TOS on the amount and the rate of deposition of deposited coke.

Figure 3. Evolution of deposited coke amount with the catalysts medium acidity at different times on stream.

that with increasing the density of medium acid sites, the peak positions shift toward high temperature values; the phenomenon was more noticeable for the coke on the support (second peak). This observation implies that more graphitic coke was obtained on more acidic catalysts. 3.4. Regeneration of Spent Catalysts. As shown in Figure 4a-c, for WT1, WT2, WT3, and WT4 catalysts, at regeneration temperatures of 350 and 400 °C, the catalytic conversion level does not present any observable trend to change as compared to spent catalysts (same behavior was observed for WT6 and WT7 samples for a regeneration temperature of 300 °C), while in the regeneration temperature range of 400-550 °C, increasing the regeneration temperature induced an increase in the conversion. On the other hand, from Figure 4a, solids with 17% of nickel (WT7 and WT6) exhibited the same trends, as hydro-

genolysis reactions increased with regeneration temperature to reach a maximum at 450 °C and then diminished, while isomerization reactions presented a contrary pathway, as they decreased with regeneration temperature to reach a minimum at 450 °C, then increased until 500 °C was reached; a further increase in regeneration temperature (550 °C) induced a decrease in these reactions. In addition, the cracking reactions were lowered with a rise in the regeneration temperature until 450 °C was reached, while the opposite effect was observed by a further increase in regeneration temperature from 500 to 550 °C. The phenomena in the case of hydrogenolysis and cracking reactions were more pronounced for WT6. At their turn, catalysts WT3 and WT1 behaved similarly (Figure. 4b), as hydrogenolysis reactions were constant in the regeneration temperature range of 350-400 °C, and then they diminished

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Figure 4. (a) Dependence of WT7 and WT6 catalysts conversion, hydyrogenolysis, isomerization, and cracking reactions on regeneration temperature. Reaction conditions: exposure time ) 2 h, TOS ) 20 min, and reaction temperature ) 300 °C. (b) Dependence of WT3 and WT1 catalysts conversion, hydrogenolysis, isomerization, and cracking reactions on regeneration temperature. Reaction conditions: exposure time ) 2 h, TOS ) 20 min, and reaction temperature ) 300 °C. (c) Dependence of WT2 and WT4 catalysts conversion, hydrogenolysis, isomerization, and cracking reactions on regeneration temperature. Reaction conditions: exposure time ) 2 h, TOS ) 20 min, and reaction temperature )300 °C. (d) Dependence of WTx catalysts Brønsted/Lewis ratio on regeneration temperature. Reaction conditions: exposure time ) 2 h, TOS ) 20 min, and reaction temperature )300 °C.

in the regeneration temperature range of 400-500 °C to reach nearly stable values in the range of 500-550 °C. For isomerization reactions, an increase in the regeneration temperature from 350 to 450 °C led to a rise in these reaction rates, while a further increment induced a decrease in isomerization selectivity. Concerning cracking reactions, they were nearly stable at 350-450 °C and then were boosted with an increment in the regeneration temperature in the range of 450-500 °C to reach constant values in the range of 500-550 °C. On the other hand, the two materials of WT2 and WT4 exhibited the same behavior in the case of hydrogenolysis reactions, as these latter had stable values from 350 to 400 °C, and they diminished from 400 to 450 °C to regain stable values from 450 to 550 °C. On the other hand, isomerization reactions were nearly constant in the regeneration temperature range of 350-450 °C, as they decreased continuously with increasing regeneration temperature in the case of WT2, while they decreased to reach a plateau for WT4. Besides, the cracking reaction exhibited an opposite trend to the one observed for the isomerization reactions for the two solids. In order to obtain more information on the correlation acidityregenerated catalysts performance, the Brønsted to Lewis (Br/ Le) ratio was measured after each regeneration test and the results are depicted in Figure 4d. The collected data evidenced that in the case of WT6 and WT7 solids, regeneration temperatures in the range of 350-450 °C do not practically affect the Br/Le ratio; thus, the increase in the WT6 and WT7 conversions may be attributed to the regeneration of metallic sites, and this fact is proved by the rise in hydrogenolysis reactions (Figure

4a). With increasing regeneration temperature (500 °C), the Br/ Le ratio increased and consequently the isomerization reactions were boosted whereas the cracking ones decreased; a further increase in regeneration temperature (550 °C) caused the opposite effect. Besides, all the other prepared samples behaved similarly, in that their Br/Le ratios were practically constant for regeneration temperatures of 350 and 400 °C, they increased at 450 °C which caused the enhancement of isomerization reactions and the lowering of cracking, and finally they (Br/Le ratios) decreased with increasing regeneration temperature (500 and 550 °C), which favored the cracking reactions. The obtained correlations are in agreement with the open literature where it was mentioned that isomerization and cracking reactions are correlated to the Brønsted to Lewis (Br/Le) ratio. The higher this ratio, the higher the isomerization reaction rates and the lower the cracking reaction rates (Lewis acidity is more favorable for cracking). Figure 5 presents the effect of exposure time on conversion and on percentage recovery in initial conversion for samples having the highest medium acid sites density (WT7) and for those having the lowest medium acid sites density (WT2 and WT4). This effect was studied at a TOS of 20 min, a regeneration temperature of 500 °C, and a reaction temperature of 300 °C. The data presented in this figure infer that for WT7 catalyst, increasing the exposure time from 2 to 6 h induced an increase in conversion and consequently in the percentage of recovery; however, this percentage can not exceed 64%. For WT2 catalyst, a rise in the exposure time from 2 to 5 h boosted the conversion, this phenomenon was more noticeable from 2

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Figure 5. Effect of exposure time on conversion and on percentage recovery in initial conversion for WT7, WT2, and WT4 catalysts. Reaction conditions: TOS ) 20 min, regeneration temperature ) 500 °C, and reaction temperature ) 300 °C.

to 4 h, and then the conversion reach a stable value from 4 to 6 h. The maximum recovery in this case was obtained at 99.5%. In the case of WT4 catalyst, the catalytic activity increased with the exposure time from 2 to 3 h and then stationary conversions were obtained without any further observable trend to change. The maximum recovery obtained for WT4 was 99.6%. According to these results, an oxidation treatment at 500 °C for 2 and 3 h is necessary to eliminate practically all the coke deposited on WT4 and WT2, respectively, while the same treatment can not eliminate completely the coke formed on WT7, which is more graphitic and consequently needs a higher temperature to be eliminated. 4. Conclusion This study provides supporting evidence of the following: • At 300 °C, samples with 17% of nickel (WT7 and WT6), exhibited two CO2 evolution peaks, and the lower temperature peak was identified as the combustion of coke on the metallic site and the higher one was ascribed to coke deposited on the support. In contrast, at 150 °C, the two above-mentioned solids exhibited only one peak assigned to support coking. In contrast, whatever the reaction temperature, the catalysts with 12 and 15% nickel do not exhibit the coking of the metallic sites. • Whatever the catalyst, increasing reaction temperature induced a rise in the intensity of both low and high temperature peaks and a shift in their positions toward high temperature values, and the phenomenon was more pronounced for the coke which burns at high temperatures. These effects suggest that at high reaction temperatures greater coke amounts and more graphitic structures were obtained. • All catalysts exhibited identical behaviors in that the amount of coke deposited increased with increasing TOS, the rise being more rapid during the first few minutes. This feature suggests that the rate of coke deposition decreased with TOS, the phenomenon being more rapid during the first few minutes (20 min), and stationary amounts were obtained after 100 min. • The amount of deposited coke was strongly correlated with the density and the nature of acid sites. It increased with

increasing the catalyst medium acidity, and for the same medium acidity concentrations, the coke formation was promoted by an increase in Lewis acid sites density. In addition, with increasing the density of medium acid sites, the peak positions shift toward high temperature values, and the phenomenon was more noticeable for the coke on the support, which suggested that the higher the medium acidity, the higher the amount of coke deposited and the more graphitic its structure. • Whatever the catalyst, at a regeneration temperature of 350 °C,, the catalytic conversion level does not present any observable trend to change as compared to that of spent catalysts, whereas it was improved by a rise in the regeneration temperature range of 400-550 °C. • The conversion and its recovery are found to be roughly a function of time exposure and of the catalysts acidity (strength and nature), and an oxidation treatment at 500 °C for 2 and 3 h is necessary to practically eliminate all the coke deposited on WT4 and WT2, respectively, while the same treatment cannot completely eliminate the coke formed on WT7, which is more graphitic and consequently needs a higher temperature to be eliminated. Literature Cited (1) Bartholomew, C. H. Mechanisms of catalysts deactivation. Appl. Catal. 2001, 212, 17. (2) Prasher, B. D.; Gabriel, G. A.; Ma, Y. H. Catalyst Deactivation by Pore Structure Changes.The Effect of Coke and Metal Depositions on Diffusion Parameters. Ind. Eng. Chem. Process Des. DeV. 1978, 17, 266. (3) Jens, R.-N. 40 years in catalysis. Catal. Today 2006, 111, 4. (4) Oudar, J.; Wise, H. DeactiVation and poisoning of catalysts; Marcel Dekker: New York,1985. (5) Souza, M. V. M; Aranda, D. A. G; Schmal, M. Coke Formation on Pt/ZrO2/Al2O3Catalysts during CH4 Reforming with CO2. Ind. Eng. Chem. Res. 2002, 41, 4681. (6) Bitter, J. H.; Hally, W.; Seshan, K.; Van Ommen, J. G.; Lercher, J. A. The role of the oxidic support on the deactivation of Pt catalysts during the CO2 reforming of methane. Catal. Today 1996, 29, 349. (7) Stagg, S. M.; Romeo, E.; Padro, C.; Resasco, D. E. Effect of Promotion with Sn on Supported Pt catalysts for CO2Reforming of CH4. J. Catal. 1998, 178, 137. (8) Bitter, J. H.; Seshan, K.; Lercher, J. A. Deactivation and Coke Accumulation during CO2/CH4Reforming over Pt Catalysts. J. Catal. 1999, 183, 336. (9) Nagaoka, K.; Seshan, K.; Aika, K.; Lercher, J. A. Carbon Deposition during Carbon Dioxide Reforming of Methane-Comparison between Pt/ Al2O3 and Pt/ZrO2. J. Catal. 2001, 197, 34. (10) Butt, J. B.; Peterson, E. E. , ActiVation, DeactiVation, and Poisoning of Catalysts; Academic Press: San Diego, 1988. (11) Mongkhonsi, T.; Prasertdham, P.; Saebgpoo, A.; Pinitniyom, N.; Jaikaew, B.; Korean, J. Chem. Eng. 1998, 15 (5), 486. (12) Bayraktar, O.; Kugler, E. L. Characterisation of coke on equilibrium fluid catalytic cracking catalysts by temperature-programmed oxidation. Appl. Catal. 2002, 233, 197. (13) Choudhary, V. R.; Devadas, P.; Sansare, S. D.; Guisnet, M. Temperature Programmed Oxidation of Coked H-Gallosilicate (MFI) Propane Aromatization Catalyst: Influence of Catalyst Composition and Pretreatment Parameters. J. Catal. 1997, 166, 236. (14) Brasco, G. N.; Comelli, R. A. Deactivation of ferrierite during the skeletal isomerization of linear butenes. Catal. Lett. 2001, 71, 111. (15) Fung, S. C.; Querini, C. A. A Highly sensitive detection method for temperature programmed oxidation of coke deposits: methanation of CO2 in the presence of O2. J. Catal. 1992, 138, 240. (16) Querini, C. A.; Fung, S. C. Coke characterisation by temperatureprogrammed techniques. Catal. Today 1997, 37, 277. (17) Querini, C. A.; Fung, S. C. Temperature-programmed oxidation technique: kinetics of coke-O2 reaction on supported metal catalysts. Appl. Catal. 1994, 117, 53. (18) Takanabea, K.; Nagaokab, K.; Aikaa, K. Improved resistance against coke deposition of titania supported cobalt and nickel bimetallic catalysts for carbon dioxide reforming of methane. Catal. Lett. 2005, 102, 153.

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ReceiVed for reView October 17, 2007 ReVised manuscript receiVed March 16, 2008 Accepted March 20, 2008 IE071407G