-Al

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Methane Reforming with Carbon Dioxide. The Behavior of Pd/r-Al2O3 and Pd-CeOx/r-Al2O3 Catalysts Pablo G. Schulz,† Marı´a G. Gonzalez,‡ Claudia E. Quincoces,‡ and Carlos E. Gigola*,† Planta Piloto de Ingenierı´a Quı´mica, Camino La Carrindanga Km 7, 8000 Bahı´a Blanca, Argentina, and Centro de Investigacio´ n y Desarrollo de Procesos Catalı´ticos, 47 No. 257, 1900 La Plata, Argentina

The catalytic reforming of methane with carbon dioxide on Pd (∼1%)/R-Al2O3 and Pd (∼1%)CeOx/R-Al2O3 catalysts was studied at 627 °C using a stoichiometric feed mixture. Characterization of fresh and used samples was carried out by TEM and TG analyses. Preliminary experiments in the 600-730 °C temperature range demonstrated that, on Pd/R-Al2O3, the conversion of CO2 was larger than that of CH4 because of the reverse water-gas shift (RWGS) reaction, although the CO/H2 ratio was close to 1. This behavior is attributed to the predominant CH4 decomposition reaction that leads to carbon deposition and catalyst deactivation. The decline in activity was also due to palladium sintering, a process favored by the reducing atmosphere generated by the presence of CO and H2. Kinetic information obtained in experiments carried out under differential conditions was used to evaluate the importance of mass- and heat-transport limitations. It is shown that intraparticle mass transfer and interparticle heat resistances can be controlled by using small catalyst particles and diluting the catalyst bed with an inert solid, respectively. On the other hand, gas-solid temperature gradients are expected if the reforming reaction is carried out at >600 °C at low flow rates. The catalytic activity, selectivity, and stability of Pd/R-Al2O3 and Pd-CeOx/R-Al2O3 were tested in 24-h runs. It was found that a Pd/R-Al2O3 sample with an average particle size of 5 nm exhibits an initial activity comparable to that of Pt/ZrO2 catalysts and a high and constant selectivity to H2 (CO/H2 ≈ 1). However, the catalytic activity decreased by 50% after a reaction time of 24 h due, in part, to metal sintering but mainly to carbon formation. A presintered Pd/R-Al2O3 sample with an average particle size of 19 nm deactivates more rapidly because of a rapid accumulation of carbon deposits. The process of carbon deposition can be suppressed to a very large extent by Ce addition, although the CO/H2 ratio becomes larger (CO/H2 ≈ 1.3). In addition, the presence of Ce reduces the sintering process. Introduction The reforming of CH4 with CO2 is a reaction that has attracted considerable and steady attention over the past 20 years, mainly because of the potential utilization of CH4/CO2 mixtures coming directly from natural gas wells to produce a balanced H2/CO ratio. A further advantage of this process is the possible location of synthesis gas plants in areas where water is not available in the quantity and quality required for conventional steam reforming. However, the commercial application of dry reforming has been limited by the strong endothermic character of the main reactions, which necessitates a high reaction temperature for a reasonable level of conversion and, consequently, gives rise to serious problems of catalyst deactivation due to carbon deposition and metal sintering. Several routes have been followed to overcome these problems. Supported noble metal catalysts have been prepared and tested at the laboratory scale with good results. The literature shows that Pt/ZrO21-3 and Rh supported on Al2O3 and MgO4,5 are highly active and stable catalysts for the title reaction, but the high cost of these metals limits their application. Group 8 metals on special supports were also found to be very active and selective. * To whom correspondence should be addressed. E-mail: [email protected]. † Planta Piloto de Ing. Quı´mica. ‡ Centro de Investigacio ´ n y Desarrollo de Procesos Catal´ıticos.

More than 10 years ago, Takayasu et al.6 reported a methane conversion level close to equilibrium and a CO/ H2 ratio of about 1 at 740 °C using Ni, Ru, Rh, Pt, and Pd supported on very fine MgO crystals. In line with this result, recent studies have clearly shown that the performance of the catalysts is strongly influenced by the method of preparation and/or the particle size of the support material. Very active and stable Ni catalysts supported on MgO nanocrystals,7,8 on Al2O3 aerogel,9 and on ZrO2 nanoparticles10 have been prepared. According to these studies, the particle size of the catalysts seems to play an important role in obtaining a high activity and stability for dry reforming. A clear advantage of using very small particles in laboratory studies is the negligible influence of mass- and heattransfer limitations, which are otherwise important under typical industrial conditions. Although the use of very small support particles obtained by special procedures appears as an appropriate route to obtain a high catalytic activity and stability in laboratory reactors, the cost of preparation might be too high for practical applications. Another promising approach is the modification of steam reforming catalysts by the addition of promoters to suppress the formation of carbon and/or to improve the catalytic activity. Nickel catalysts supported on γ-Al2O3 and R-Al2O3 have been found to exhibit better activity and sometimes better stability by the addition of MnO;11 CaO;12-15 and MgO, CeO2, and La2O3.12,16 The

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effect of CaO addition on Ni/Al2O3 catalysts has attracted considerable attention, but the results are not always consistent. In some studies,12,15 the amount of carbon formed was higher in the promoted catalysts. Regarding the effect of other promoters such as MgO, La2O3, and CeO2 on the catalytic behavior of Ni/γ-Al2O3, Wang and Lu12,16 found that MgO decreased the activity without improving the stability. On the other hand, the other promoters, La2O3 and CeO2, enhanced the stability, decreased the amount of carbon formed, and maintained the conversions of CH4 and CO2. These authors also found16 that, with 10% CeO2 on a Ni/γ-Al2O3 catalyst, the conversion of CH4 was increased, and the amount of carbon formed was below 5% in a 24-h run at 700 °C. The catalytic activity and selectivity of Pd catalysts for dry reforming has not been extensively studied despite the lower cost of Pd as compared with Pt or Rh. The catalytic activity of Pd supported on γ-Al2O3 and TiO2 for CH4 reforming with CO2 at 500 °C was first studied by Masai et al.17 They found that Pd (5%) showed an activity comparable to that of Rh/γ-Al2O3, but no information was given on the reaction selectivity. A patent on the use of Pd catalysts for CH4 reforming was issued in 1991.18 Solymosi et al.19 compared the specific activity of several alumina-supported Pt metals for CH4 reforming with CO2 at 550 °C and found that Ru and Pd exhibited the highest activities. In a subsequent paper, Solymosi et al.20 studied the interaction of CH4 with metals supported on SiO2. They observed that CH4 decomposition on Pd occurred at low temperature, 250 °C, leading to hydrogen production and the formation of carbon deposits. Erdo¨helyi et al.21 studied the kinetics of CH4 reforming with CO2 on several supported 1% Pd catalysts at 500 °C and found that the conversions of CH4 and CO2 was higher on Pd/TiO2, followed by Pd/γ-Al2O3, with CO/ H2 ratios of 1.68 and 1.47, respectively. Although these studies demonstrated that Pd/γ-Al2O3 catalysts were active and stable, the reaction temperature was low, and the reaction time was limited to 150 min. The actvity of Pd for the CH4 reforming reaction was also measured by Rostrup-Nielsen and Bak Hansen22 using an aluminastabilized magnesia support. A 1.2% Pd sample was tested in the 500-650 °C temperature range using a CO2/CH4 ratio of 4 in the presence of H2. At 550 °C, the turnover rate (TOR) for CH4 reforming was the lowest among other group VIII metals, and the TOR for the reverse water-gas shift (RWGS) reaction the highest. In addition, the work of Rostrup-Nielsen and Bak Hansen22 clearly showed that the activity of Pd for CH4 decomposition increases very rapidly with temperature and becomes comparable to those of Ni, Pt, and Rh above 600 °C. More recently, Nagaoka and Aika23 studied the effect of rare earth metal oxides on the catalytic activity and stability of a Pd (2%)/γ-Al2O3 catalyst at 750 °C for the reforming of methane with carbon dioxide. They found that the Pd/γ-Al2O3 catalyst was very active and selective, but carbon formation was the main cause of deactivation. On the other hand, they demonstrated that the addition of Ce or La suppressed the formation of coke. According to this review, the behavior of Pd/R-Al2O3 catalysts for the reforming of CH4 with CO2 at temperatures above 600 °C has not been investigated. R-Al2O3 is a very stable support widely used for steam reforming catalysts. A high reaction temperature is necessary to

obtain a level of CH4 conversion not limited by thermodynamic considerations. Taking into account these considerations, we undertook an investigation of the catalytic activity and stability of low-loaded Pd/R-Al2O3 and Pd-CeOx/R-Al2O3 catalysts. In this work, we have studied the dependence of the catalytic activity, the selectivity (CO/H2 ratio), the growth of the metal particles, and the formation of carbon on the reaction time for a Pd (∼1%)/R-Al2O3 catalyst. The effect of CeO2 addition on these parameters was also investigated. The characterization of fresh and used samples was mainly carried out by chemisorption, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). Special attention was paid to the presence of mass- and heat-transfer limitations on the catalytic activity and stability measurements. Experimental Section Preparation of Catalysts. A Pd catalyst with a metal content close to 1% was prepared by successive impregnation steps. The support material was a commercial R-Al2O3 support (Rhone Poulanc, Sg ) 10 m2/g, Vg ) 0.47 cm3/g), crushed and sieved to obtain particles with an average size of 0.36 mm corresponding to a 40-50 mesh fraction. Although R-Al2O3 is a stable support for hightemperature reactions, its low BET surface area precludes the use of a high metal loading if small metal particles are desired. To overcome this difficulty, an organometallic precursor, Pd(C5H7O2)2, and a recharging process were used to obtain a metal content of about 1%. By one-step impregnation using a heptane solution containing 3.0 × 10-3 g of Pd/mL, a sample with a metal loading of 0.24% was first obtained. The support was left in contact with the solution for 24 h at room temperature. The impregnated sample was then filtered, dried in N2 at 200 °C, calcined in air at 500 °C, and finally reduced in flowing H2 at 300 °C for 1 h. Two additional impregnation steps raised the metal content to 0.51 and finally to 0.95%. The metal loading was determined by atomic absorption spectroscopy. After each impregnation, the samples were submitted to the calcination and reduction treatments described above. A sample obtained by this procedure is denoted as Pd300. A portion of this catalyst was calcined in air at 736 °C for 1 h, followed by reduction in a H2 (5%)/Ar mixture at 736 °C for 1 h. In this way, sample Pd-736 was obtained. This high-temperature pretreatment in H2 was carried out in order to presinter the catalyst, limiting the growth of the palladium particles under reaction conditions. The monometallic palladium catalyst reduced at 300 °C, Pd-300, was used to prepare the Pd-CeOx/R-Al2O3 catalysts. An aqueous solution of Ce(NH4)2(NO3)6 containing the amount of Ce needed to obtain a Ce/Pd molar ratio of 2 was used to impregnate the palladium catalyst. The preparation was dried at 120 °C, calcined in air at 736 °C, and then reduced in H2 (5%)/Ar at the same temperature. Catalyst Characterization. Back sorption and desorption H2 isotherms were measured in a volumetric glass adsorption apparatus equipped with an MKS Baratron type PDR-A pressure gauge. A fresh catalyst sample (∼300 mg) was first reduced in flowing H2 at 300 °C for 1 h and evacuated overnight at the same temperature. After the sample had been cooled to 25 °C under vacuum, the first isotherm was measured

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Table 1. Main Properties of Pd/r-Al2O3 and Pd-CeOx/r-Al2O3 Catalysts catalyst

R-Al2O3 particle size (mm)

Pd-Ce content (wt %)

fraction of exposed Pd atoms (H/Pd)irr

Pd/R-Al2O3 (Pd-300)a Pd/R-Al2O3 (Pd-736)b Pd-CeOx/R-Al2O3b

0.36 0.36 0.36

0.95 0.95 0.95-2.5

0.23 0.06 0.075

a

Sample reduced in pure H2 at 300 °C;

b

Pd particle size (nm) (H/Pd)irr TEM 4.9 18.6 15

8 24 14

Samples reduced in H2 (5%) at 736 °C.

in the 10-100 Torr pressure range to determine the amount of adsorbed and absorbed H2. Subsequently, the sample was evacuated at room temperature for 30 min to remove the amount of weakly adsorbed H2 and that forming the R- or β- palladium hydride. These uptakes were then determined by measuring a second isotherm, and the amount of irreversibly adsorbed hydrogen (H/ Pd)irr was obtained by subtracting the second isotherm from the first. The fraction of exposed palladium atoms was calculated assuming that one hydrogen atom is adsorbed by a surface palladium atom. The crystallite size based on the chemisorption value was estimated from the equation d (nm) ) 112/(percentage of metal exposed). Fresh and used samples were also characterized by TEM in order to observe changes in the particle size distribution. Prereduced samples were ground and dispersed in distilled water. One drop of this suspension was then placed on holey carbon supported on a copper grid. Micrograph images of the samples were acquired with a JEOL 100 CX model microscope at 100 kV and a magnification of 100 000×. The average particle size was calculated as d ) ∑nidi3/∑nidi2. Because of the lack of contrast, it was not possible to distinguish between the cerium oxides and the alumina support. After reaction, the used catalysts were characterized by TGA in air flowing at 40 cm3/min by increasing the temperature linearly at 10 °C/min. In this way, the amount of carbon formed was determined. Activity and Selectivity Measurements. The activities and selectivities of the Pd/R-Al2O3 and Pd-CeOx/ R-Al2O3 samples were determined in a flow reactor operated at 890-1400 Torr in the 580-730 °C temperature range. A 1-m quartz tube (internal diameter ) 10 mm) with a central section that was 3.8 mm in diameter and 80 mm in length was placed in a LindbergBlue M electric furnace with three independent heating zones. Catalyst charges in the range of 80-300 mg were located in the center of the reactor between quartz wool plugs. The catalyst samples were diluted with equal amounts of pure R-Al2O3 ground to the same particle size. Some preliminary experiments were performed without dilution. The reaction temperature was monitored using two thermocouples in touch with the two ends of the catalyst bed. Prior to reaction, all samples were treated at room temperature with a H2 (5%)/Ar mixture and then heated to the desired temperature in pure Ar. Subsequently, a feed mixture consisting of CH4/ CO2/Ar ≈ 25/25/50 with a total flow rate in the range 100-600 cm3/min, depending on the purpose of the run and the amount of catalyst, was passed over the catalyst bed. Reactants and products were analyzed using two online gas chromatographs (GCs) equipped with thermal conductivity detector (TCD) cells. One GC used He as a carrier gas and a silica gel column (6 ft × 1/8 in.) held at 90 °C to separate CO2 from the other components. Another GC used Ar as a carrier gas and a Chromosorb 102 column (14 ft × 1/8 in.) at 35 °C to separate H2, CO,

and CH4. A silica gel bed was set after the reactor to remove the water produced by the RWGS reaction. The concentration of water in the product stream was calculated from the O2 material balance equations. Evaluation of internal mass-transfer limitations was carried out using the Weisz criterion24 based on the measured rates of reaction and the physical properties of the alumina particles. In addition, the importance of interphase and interparticle temperature gradients was estimated using the criterion developed by Mears.25 Results Table 1 summarizes the main properties of the Pd/ R-Al2O3 and Pd-CeOx/R-Al2O3 catalysts, in addition to the TEM and chemisorption results. The particle size distribution for fresh samples is presented in Figure 1; palladium particles of 6-10 nm predominate on the Pd300 sample, whereas on Pd-736, the particle size distribution is very broad, including a few particles below 10 nm and very large particles of 24-36 nm. Clearly, the palladium particles grew to a large extent because of the air and H2 treatment at 736 °C. Consequently, the fraction of exposed palladium atoms decreased from 0.23 to 0.06. For Pd-300, the average crystallite size determined from TEM (8 nm) was larger than that calculated from the chemisorption measurements (4.9 nm), because of our experimental limitations in observing particles below 1 nm by microscopy. In a previous study,26 a bimodal particle size was found on a Pd (0.2%)/R-Al2O3 catalyst, prepared by the procedure outlined above, but with the TEM analysis performed in a high-resolution instrument. On the other hand, for Pd-736, the average particle size determined by TEM (24 nm) was close to that calculated from the (H/Pd)irr value (19 nm). On the Pd-CeOx/R-Al2O3 catalyst treated at high temperature, the particle size distribution is also broad, as shown in Figure 1, but there are still many particles between 6 and 10 nm. The average particle size was 14 nm, and a few particles of about 24-26 nm were observed. This behavior indicates that the addition of Ce attenuates the sintering of the Pd particles relative to that of the Pd-300 sample subjected to the same pretreatment in air and hydrogen at 736 °C. The chemisorption measurements showed that the hightemperature pretreatment of Pd-CeOx/R-Al2O3 decreased the fraction of exposed Pd atoms to 7.5%. Using this value, an average Pd particle size of 15 nm was estimated, in very close agreement with the TEM value (14 nm). On the basis of this result, we conclude that the promoter does not cover the Pd active sites but is located on the support surface or on the metal-support interface. Consequently, the low H/Pd ratio on PdCeOx/R-Al2O3 is mainly due to the palladium sintering process. The fresh Pd-300 catalyst and the Pd-CeOx/R-Al2O3 catalyst (reduced at 736 °C) were subjected to a treatment in a CO (2%)/H2 (2%)/Ar mixture at 627 °C for 10

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Figure 1. Particle size distribution by TEM of fresh Pd/R-Al2O3 (Pd-300, Pd-736) and Pd-CeOx/R-Al2O3 catalysts.

Figure 2. Effect of CO + H2 at 627 °C on the particle size distribution by TEM of Pd/R-Al2O3 (Pd-300) and Pd-CeOx/R-Al2O3 catalysts.

h to observe the effect of the reaction products on the palladium particle size. The results presented in Figure 2 show that the particle size distributions for the two catalysts became similar, because of a marked sintering of the monometallic sample in the presence of a reducing gas mixture. On the other hand, the Pd-CeOx/R-Al2O3 catalyst exhibited a distribution comparable to that of the fresh sample shown in Figure 1. Preliminary activity and selectivity experiments were performed on Pd-300 at three different temperature levels with a gas hourly space velocity (GHSV) of 14 250 h-1 to observe the catalyst behavior. Figure 3 shows the CH4 and CO2 conversions versus time at 616, 671, and 730 °C. All conversion values are below those predicted by thermodynamics for the main reactions; CH4 reforming and the RWGS. The conversion level of CO2 was always larger than that of CH4, because the RWGS reaction and the deactivation process clearly increased with the reaction temperature. Figure 4 shows the dependence of product formation on temperature. The main products are CO and H2, with a CO/H2 ratio close to 1 but decreasing from 1.1 to 0.99 with increasing temperature while the amount of water decreased continuously, as expected from thermodynamic consid-

Figure 3. CH4 reforming with CO2 on Pd/R-Al2O3 (Pd-300). Effect of time on stream and temperature on CH4 (9) and CO2 (0) conversions. Reaction conditions: CH4/CO2/Ar ) 25/25/50, GHSV ) 14 250 h-1. Temperature ) (a) 616, (b) 671, (c) 730 °C.

erations. At 730 °C, the production of H2 slightly exceeds that of CO. However, because of the presence of the

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Figure 4. CH4 reforming with CO2 on Pd/R-Al2O3 (Pd-300). Effect of time on stream and temperature on CO ([), H2 (9), and H2O (b) formation. CH4/CO2/Ar ) 25/25/50, GHSV ) 14 250 h-1. Temperature ) (a) 616, (b) 671, (c) 730 °C.

RWGS reaction, the CO/H2 ratio should be >1. This apparent contradiction is due to the high activity of palladium for the CH4 decomposition reaction that generates H2 in addition to carbon deposits whose accumulation leads to catalyst deactivation. On the basis of these results, additional experiments were performed at 627 °C, to minimize the deactivation process. To determine whether the presence of concentration and temperature gradients influenced catalyst behavior, the Weisz24 and Mears25 criteria were used. Application of these criteria requires knowledge of the rate of reaction, which was measured by operating the reactor under differential conditions. Tests were carried out in the 579-632 °C temperature range with the mass of catalyst and the flow rate adjusted to keep the CH4 conversion below 15%. Catalyst samples were diluted with equal amounts of pure R-Al2O3. Initial rates of reaction were calculated by fitting the CH4 conversion versus time on stream data obtained in 4-5-h runs with a straight line. The calculated rates were used to construct the Arrhenius plot shown in Figure 5. The linearity of the plot seems to indicate the absence of diffusion limitations under the chosen experimental conditions. The apparent activation energy was found to be Eact ≈ 24 ( 2 kcal/mol for both Pd-300 and PdCeOx/R-Al2O3, but the high-temperature reduction performed on Pd-CeOx/R-Al2O3 led to a significant decrease in rate. Taking into account the initial metal dispersion, the TOR values for CH4 conversion at 627 °C for Pd300 and Pd-CeOx/R-Al2O3 were 2.5 and 3.6 s-1, respectively. These similar values correspond to samples with a large difference in palladium particle size, indicating that the rate of reaction is not structure sensitive. In other words, the rate of CH4 decomposition depends mainly on the fraction of exposed Pd atoms. To estimate the influence of intraparticle concentration gradients, the well-known Weisz criterion was applied

(rp)2rFc e1 CsDe

(1)

Figure 5. Arrhenius plot for CH4 conversion on Pd/R-Al2O3 (b, Pd-300) and Pd-CeOx/R-Al2O3 (O) catalysts. Reaction conditions: CH4/CO2/Ar ) 25/25/50, pressure ) 1000 Torr.

where rp is the particle radius, r is the rate of reaction per unit mass of catalyst, Fc is the catalyst density, Cs is the surface concentration of CH4 at reaction conditions, and De is the effective diffusivity of CH4. The catalyst’s density, Fc ) 1.4 g/cm3, was calculated from the pore volume (Vg ) 0.47 cm3/g) and the solid density (Fs ) 3.8 g/cm3). The effective diffusivity, estimated using the random-pore model,27 was found to be 0.10 cm2/s at 627 °C. It is shown in Table 2 that application of eq 1 to the measured rates of CH4 consumption confirms the absence of internal mass-transfer limitations. Interphase and interparticle heat-transfer limitations were also evaluated. The following equation must be satisfied in the absence of interphase temperature gradients

|∆H|rvrp 0.15RT < ) 0.011 hT Eact

(2)

where ∆H is the heat of reaction (49 328 cal/mol at 627 °C), rv ) rFc is the rate of reaction per unit volume of catalyst, and h is the gas-solid heat-transfer coefficient. This coefficient was estimated using the correlation of Cybulski et al.,28 which is recommended for low Reynolds numbers29

hdp ) 0.07Rep k

(3)

where k is the thermal conductivity of the gas mixture and Rep is the Reynolds number based on the particle diameter, dp. For the particle size and gas flow rate used in the kinetic measurements, Rep was found to be 4.2, leading to a heat-transfer coefficient of 0.0015 cal/(s °C cm2). Using this value, the criterion for the absence of interphase temperature gradients, at 627 °C, was not satisfied, as shown in the fifth column of Table 2. Consequently, the catalyst’s temperature was lower than that of the gas mixture. Finally, to determine whether interparticle temperature gradients are present, the following criterion

Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 9025 Table 2. Initial Rate of CH4 Conversion on Pd/r-Al2O3 and Pd-CeOx/r-Al2O3 Catalysts at 627 °C. Estimation of Mass- and Heat-Transport Limitations

a

catalyst

r × 103 a [gmol/(min gcat)]

TON (s-1)

intraparticle mass transport (eq 1)

interphase heat transport (eq 2)

interparticle heat transport (eq 4)

Pd-300 Pd-736 Pd-CeOx/R-Al2O3

3.1 1.2 1.4

2.5 3.6 3.5

0.28 < 1 0.11 < 1 0.16 < 1

0.095 > 0.011 0.037 > 0.011 0.043 > 0.011

0.026 > 0.015 0.010 < 0.015 0.012 < 0.015

Reaction conditions: CH4/CO2/Ar ) 25/25/50, pressure ) 1000 Torr.

Figure 6. CH4 reforming with CO2 on Pd/R-Al2O3 (s, Pd-300; - - -, Pd-736) catalysts. CH4 (9) and CO2 (0) conversions as a function of time at T ) 627 °C. Reaction conditions: CH4/CO2/Ar ) 25/25/50, pressure ) 920 Torr, GHSV ) 14 250 h-1.

introduced by Mears25 was used

|∆H|rbRo2 0.2RTw < ) 0.015 keTw Eact

(4)

where rb is the rate of reaction per unit bed volume, Ro is the reactor radius, ke is the effective conductivity of the bed, and Tw is the wall temperature. The effective thermal conductivity, ke ) 0.0015 cal/(s cm °C) at 627 °C, was estimated using the model derived by Yagi and Kunii30 and following the procedure recommended by Anderson and Pratt.29 Taking into account the fact that the catalyst’s charges were diluted with an equal amount of support material, the rate values given in Table 2 were estimated in terms of bed volume using the proper bed density. As shown in the last column of Table 2, eq 4 is not satisfied, but the values are quite similar, so we cannot provide a definitive answer about the influence of reactor temperature gradients. However, the previous analysis demonstrated that the main concern should be the resistance to the transport of heat from the gas phase to the catalyst surface. In other words, heat-transport limitations might influence the experiments carried out at 627 °C and higher temperatures, taking into account the low GHSV used in the high-conversion runs. The behaviors of the Pd-300 and Pd-736 catalysts in a 24-h run at 627 °C are shown in Figure 6. The GHSV was maintained at ∼15 000 h-1, and the catalyst’s particles were diluted with R-Al2O3. The main results

from these experiments are summarized in Table 3. For Pd-300, the initial CH4 conversion was about 44% and decreased first in a linear form and then more rapidly to reach a value of 18% in 24 h. The conversion of CO2 was higher, ∼53%, and followed the decrease of CH4 conversion closely. Both the CH4 and CO2 conversions were lower than those corresponding to thermodynamic equilibrium at 627 °C (49.6 and 71%, respectively). The CO/H2 ratio was about 1.1 and constant during the run. The evolution of the reaction products is observed in Figure 7. The production of CO was slightly higher than that of H2, and the CO/H2 ratio was nearly constant during the run. The production of water due to the RWGS reaction decreased slightly at the end of the run, indicating that it is not affected by the large decrease in activity for CH4 reforming. The behavior of the Pd736 sample, also shown in Figure 6, clearly demonstrates that the catalytic activity and stability are strongly affected by the increase in metal particle size. The initial conversions of CO2 and CH4 were lower than those for Pd-300, 42% and 36%, respectively, and these values decreased exponentially with the reaction time. The final CH4 conversion was about 8%, which indicates a much greater loss of activity (78%) than for sample Pd-300 (58%). The amount of carbon formed was also higher (see Table 3): 0.61 versus 0.48 g of C per gram of catalyst. The evolution of the reaction products is observed in Figure 7. In this case, the production of CO was slightly higher than that of H2, and the CO/H2 ratio was initially about 1.1 and increased to 1.5 during the run. On the other hand, the production of water due to the RWGS reaction was lower, showing the effect of the marked loss of metal surface area. The notable effect of Ce addition on the stability of the Pd/R-Al2O3 catalyst is clearly observed in Figure 8 and in Table 3, where the behaviors of the Pd-736 and Pd-CeOx/R-Al2O3 samples are compared. The initial conversions of CO2 and CH4 on Pd-CeOx/R-Al2O3 were similar to those on Pd-736, but the decay with time was much slower. For a period of 24 h, the decline in activity was reduced from 78% to about 21%, a significant improvement. On Pd-CeOx/R-Al2O3, both the conversion and the concentrations of products exhibited a slow trend toward constant values, as shown in Figures 8 and 9, respectively. On the other hand, the CO/H2 ratio was slightly higher than that on Pd-736; it increased from 1.2 to 1.3 during the run. As explained later, the rate of CO formation increased relative to that of H2 because of the reaction of C produced by methane decomposition with CeO2. Finally, the amount of water formed on the Pd-CeOx/R-Al2O3 catalyst was higher that that formed on P-736 and close to that formed on Pd-300. Examination of the spent catalysts by TEM revealed that extensive sintering under reaction conditions occurred on Pd-300 and to a lesser extent on Pd-736 and Pd-CeOx/R-Al2O3, as shown in Figure 10. For Pd-300, the distribution is asymmetric, and there is a significant

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Table 3. Reforming of CH4 with CO2 at 627 °C (GHSV ) 14250 h-1; CO2/CH4 ) 1). Effect of Time on Stream on the Activity, Selectivity, Metal Dispersion and Carbon Formation of Pd/r- Al2O3 and Pd-CeOx/r- Al2O3 Catalysts

a

catalyst

CH4 conv (%)

CO/H2

CO/H2O

(H/Pd)irr

carbon (g/gcat)

deacta (%)

Pd-300, t ) 0 h Pd-300, t ) 24 Pd-736, t ) 0 h Pd-736, t ) 24 h Pd-CeOx/R-Al2O3, t ) 0 h Pd-CeOx/R-Al2O3, t ) 24 h

44 18 36 8 33 26

1.1 1.1 1.1 1.5 1.2 1.3

10 7 15 6 7 6

0.23 0.06b 0.06 0.04b 0.075 0.07b

0.48 0.61