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RESEARCH NOTES Coke Formation on Pt/ZrO2/Al2O3 Catalysts during CH4 Reforming with CO2 Mariana M. V. M. Souza,† Donato A. G. Aranda,†,‡ and Martin Schmal*,†,‡ NUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502, 21945-970 Rio de Janeiro, Brazil, and Escola de Quimica, Universidade Federal do Rio de Janeiro, C.P. 68542, 21940-900 Rio de Janeiro, Brazil
The CO2 reforming of methane was studied over Pt supported on Al2O3, ZrO2, and x % ZrO2/ Al2O3 (1 e x e 20 wt %). The Pt/Al2O3 deactivated very quickly during 20 h onstream at 1073 K and a CH4/CO2 ratio of 1:1, while the catalysts with a ZrO2 content above 5 wt % presented improved stability during 60 h. Temperature-programmed oxidation studies showed that the amount of carbon on Pt/Al2O3 is much larger than that on zirconia-containing catalysts. Deactivation is attributed to carbon formation surrounding the metal-support perimeter. The high stability of zirconia-based catalysts is probably due to strong Pt-Zrn+ interactions, which reduce carbon formation during the reaction by promoting CO2 dissociation. High ratios of CH4/ CO2 were also used in an attempt to accelerate deactivation, but even under these severe deactivation conditions, the catalyst with 10 wt % ZrO2 exhibited excellent stability. 1. Introduction The catalytic reforming of CH4 with CO2, rather than H2O, for the production of synthesis gas, i.e., a mixture of CO and H2, has attracted substantial interest.1-4 The reaction is well suited to a lower H2/CO product ratio, which is preferable as feed for Fischer-Tropsch plants5 and for the synthesis of acetic acid, dimethyl ether, and oxoalcohols.6 Moreover, dry reforming is of interest for environmental reasons because it reduces both CO2 and CH4 emissions, which are so-called greenhouse gases.7 The major obstacle for larger diffusion of this process in industry is the high thermodynamic potential to form coke under elevated temperatures.5 Temperatures around 1073 K are required to reach high conversions because of the high endothermic nature of the process. Thus, the catalyst deactivation is a serious challenge and must be overcome by effective catalysts.
CH4 + CO2 T 2CO + 2H2
∆H298K ) 247.3 kJ/mol (I)
Although the most commonly used support for CO2/ CH4 reforming is alumina,2,8,9 it has been found that Pt reaches much higher conversions and better stability when supported on ZrO2.10-12 Van Keulen et al.13 showed that Pt/ZrO2 is very stable for a period of over 1000 h, at 923-973 K and a feed ratio of CO2/CH4 ) 2. The coking resistivity of Pt/ZrO2 is associated with strong Pt-Zrn+ interactions, which result in the formation of ZrOx species in close contact with the Pt * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (5521) 2562-8300. † NUCAT/PEQ/COPPE. ‡ Escola de Quimica.
surface.11 Therefore, the reforming reaction proceeds basically on the metal-support perimeter.14 When zirconia is dispersed on alumina, it provides a better dispersion for Pt particles because these systems combine the unique chemical properties of ZrO2 with the high surface area and mechanical stability of Al2O3. We showed earlier that Pt/ZrO2/Al2O3 catalysts exhibit high activity and suffer less deactivation than Pt/Al2O3 or even Pt/ZrO2.15 In the present work we tried to develop a better understanding of the nature of coking in CO2 reforming of CH4, by doing temperatureprogrammed oxidation (TPO) analysis of carbon deposits and varying the CH4/CO2 ratio in the feed. We investigated the relation between coking and catalyst performance at a temperature typically used for commercial operations (1073 K). 2. Experimental Section 2.1. Catalyst Preparation and Characterization. Al2O3 (Harshaw) and ZrO2 were used as supports. Al2O3 was calcined in air at 823 K for 16 h (BET area ) 200 m2/g), and ZrO2 was prepared by calcination of zirconium hydroxide (MEL Products) in air at 823 K for 2 h (BET area ) 62 m2/g). ZrO2/Al2O3 samples were prepared by impregnation over an alumina powder with a nitric acid solution (50%) of zirconium hydroxide, as described elsewhere.15 Zirconia loading was varied between 1 and 20 wt %. The catalysts were prepared by an incipient wetness technique, using an aqueous solution of H2PtCl6‚6H2O (Aldrich), followed by drying at 393 K for 16 h and calcination in air at 823 K for 2 h. For all catalysts, the platinum content was around 1 wt %. The prepared catalysts will be referred to as PtAl for Pt/Al2O3, PtZr for Pt/ZrO2, and PtxZr for Pt/x % ZrO2/Al2O3.
10.1021/ie010970a CCC: $22.00 © 2002 American Chemical Society Published on Web 08/06/2002
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H2 and CO chemisorptions were measured on all catalysts at room temperature after reduction by 10% H2/Ar at 773 K using an ASAP 2000 apparatus (Micromeritics). Because of the metal-support interaction,14 chemisorption on the PtZr catalyst was also carried out after reduction at 573 K to better estimate the Pt dispersions.15 2.2. Catalytic Test. The reaction was carried out in a fixed-bed flow-type quartz reactor, loaded with 20 mg of catalyst. A thermocouple was placed on top of the catalyst bed to measure the catalyst temperature. The catalysts were dried in situ with flowing nitrogen at 423 K, before reduction with 10% H2/N2 for 1 h at 773 K. After reduction, the sample was purged with nitrogen for 30 min at the same temperature. All catalytic tests were performed under atmospheric pressure, and the total feed flow rate was 200 cm3/min (WHSV ) 160 h-1), over the temperature range 723-1173 K. Stability tests were carried out at 1073 K with stoichiometric conditions (CH4:CO2 ) 1:1) as well as with an excess of CO2 for PtAl and an excess of CH4 for the Pt10Zr catalyst, maintaining the total feed flow rate of 200 cm3/min with helium. The reaction products were analyzed by an online gas chromatograph (CHROMPACK CP9001), equipped with a Hayesep D column and a thermal conductivity detector. Temperature-programmed surface reaction (TPSR) was also performed to investigate CO2 reforming of CH4 over platinum catalysts, using a dynamic mode apparatus. After reduction at 773 K, the catalyst was purged with He at this same temperature during 1 h and cooled to room temperature. The amount of catalyst and total feed flow rate were the same as those used in stability tests, with a CH4/CO2/He ratio of 1:1:18. TPSR was performed by heating the catalyst at 10 K/min up to 823 K, maintained for 30 min, and subsequently ramped to 973 K, remaining at this temperature for 1 h. The effluent gas composition was monitored online by a quadrupole mass spectrometer (Dycor MA100M Ametek). TPO of carbonaceous deposits was carried out in the same dynamic mode apparatus as that used for TPSR. After reaction, the samples were cooled to room temperature under a helium flow and then heated to 1073 K at a rate of 10 K/min in a 5% O2/He mixture (30 cm3/ min). Integration of the CO2 evolution spectra allowed the quantification of carbon deposition. 3. Results and Discussion The amounts of irreversibly adsorbed H2 and CO, at room temperature, were reported in a previous paper.15 Alumina-supported catalysts with zirconia loading up to 10% presented high H/Pt values (around 0.85). The lower dispersion of the Pt20Zr catalyst (H/Pt ) 0.60) can be attributed to the presence of large ZrO2 crystallites on the support. After high-temperature reduction, the H2 chemisorption on PtZr was markedly decreased compared to that on PtZr reduced at 573 K (H/Pt varied from 0.34 to 0.57). This type of behavior suggests the migration of partially reduced zirconia onto the platinum surface (a SMSI type state). However, the presence of ZrOx moieties did not cause any decrease in CO chemisorption on the PtZr catalyst. The high values of the CO/H2 ratio on zirconia-containing catalysts and in particular on the PtZr catalyst reduced at 773 K (CO/ H2 ) 6.9) predict an interaction of CO with the Pt-ZrOx interface, shown by IR of CO adsorbed on PtZr systems, as reported in ref 15.
Figure 1. CO formation during TPSR over Pt catalysts. Reaction conditions: CH4:CO2:He ) 1:1:18, total feed flow rate ) 200 cm3/ min.
The activity of the catalysts was evaluated under reforming conditions, with a CH4/CO2 ratio of 1:1, over a temperature range of 723-1173 K. The activity is influenced by the nature of the support.15 At higher temperatures, PtAl and Pt1Zr catalysts are less active than Pt10Zr, the most active catalyst over the whole temperature range investigated: the CH4 conversion ranged from 5.5% at 723 K to 93.5% at 1173 K. TPSR measurements were carried out under similar conditions. Figure 1 presents the TPSR profiles of CO production on PtAl, PtZr, and Pt10Zr catalysts. As shown, the PtZr catalyst exhibits higher initial activity, mainly at lower temperatures, but deactivates very fast, while PtAl and Pt10Zr catalysts present good stability at 823 K during the first 30 min onstream. On the other hand, the Pt10Zr catalyst showed the best performance at 973 K. The support influences strongly the stability of the catalysts, as reported previously.15 The order of activity maintenance at 1073 K was Pt10Zr > Pt5Zr > Pt20Zr and PtZr . PtlZr > PtAl. As reported, PtAl and Pt1Zr catalysts exhibited high linear deactivation rates of 4.0 + 0.5%/h and 3.3 + 0.4%/h, respectively, during the first 20 h onstream at 1073 K because of the rapid deposition of inactive carbon, which will be discussed later. The Pt10Zr catalyst deactivated only at a rate of 0.1%/h during 60 h onstream at this temperature.15 There are two potential causes of deactivation: coke deposition and sintering of metal particles. Most authors agree that the coke formation is the main source of deactivation.10,16-18 Bitter et al.10,17 showed that sintering of Pt particles during reforming conditions can be excluded based on EXAFS results of fresh and used Pt/ Al2O3 and Pt/ZrO2. Thus, the fast deactivation of PtAl should be associated with carbon deposition. Figure 2 shows the oxidation profiles of carbon deposited on the PtAl, PtZr, and Pt10Zr catalysts after 21 h of reaction at 1073 K and a CH4/CO2 ratio of 1:1. For the PtAl catalyst, the oxidation started at 520 K, exhibiting a major peak around 700 K and a smaller one at 830 K. For zirconia-containing catalysts, the onset of the oxidation was at lower temperature, around 373 K. The amount of carbon deposited at 1073 K, as quantified by integration of the CO2 formation during TPO runs, was normalized to surface Pt, and the results are displayed in Table 1. It shows that after 21 h onstream the Pt surface is not entirely covered by carbon; thus, the high values of C/Pt, mainly for the PtAl catalyst, must be due to accumulation of carbon on the support.
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Figure 2. TPO profiles for PtAl, PtZr, and Pt10Zr catalysts, after exposure to a CH4:CO2:He ) 1:1:18 mixture at 1073 K for 21 h. Table 1. Amount of Coke As Quantified by TPO mg of coke/ mg of coke/ catalyst (g of catalyst)‚h C/Pt catalyst (g of catalyst)‚h C/Pt PtAl PtZr
6.3 0.6
10.2 0.9
Pt10Zr
0.3
0.5
Noronha et al.19 have suggested that the various TPO peaks are not due to different forms of carbon but rather to different locations on the catalyst surface, for Pt/Al2O3 and Pt/ZrO2. The low-temperature peaks observed in TPO of coked Pt/Al2O3 have been typically ascribed to carbon surrounding the metal particles, while those at high temperatures are ascribed to the carbon deposition over the support.20 Deactivation is attributed to carbon formation surrounding the metal-support perimeter. On zirconia-containing catalysts, coke has a higher reactivity and does not cause any blockage of Pt-ZrOx interfacial sites. The higher stability and coking resistivity of Pt/ZrO2 have been related to strong Pt-Zrn+ interactions, which result in the formation of ZrOx species on the Pt surface.11 Indeed, our TPR results15 indicated that zirconia can be reduced at lower temperatures than 500 K, resulting in ZrOx species that may decorate the Pt surface, diminishing the hydrogen chemisorption capacity. The interfacial sites on Pt-ZrOx are active for CO adsorption and CO2 dissociation, providing active species of oxygen that may react with carbon formed by CH4 decomposition on the metal particle, suppressing carbon accumulation.11,15,16 Moreover, zirconia is a wellknown oxygen supplier, and its oxygen mobility is about 3 times higher than that of alumina,21 which helps to keep the metal surface free of carbon. When zirconia is dispersed over alumina, the Pt surface is not extensively recovered by ZrOx, as shown by chemisorption measurements.15 Thus, the Pt-ZrOx interface in ZrO2/Al2O3 systems appears to be more active and stable for CO2 reforming of methane. Carbon deposition during methane reforming can be originated from either methane decomposition (reaction II) or CO disproportionation (Boudouard reaction III), which are thermodynamically favorable below 1173 K.5,22
CH4 T C + 2H2
∆H298K ) 75 kJ/mol
2CO T C + CO2
∆H298K ) -172 kJ/mol (III)
(II)
Figure 3. Effect of the ratio CO2/CH4 on the stability of PtAl at 1073 K. Reaction conditions: flow rates of CO2 ) 10 cm3/min and CH4 + He ) 190 cm3/min.
Kinetically, both the methane decomposition reaction and the Boudouard reaction, which give undesirable carbon, are known to be exceptionally slow in the absence of a catalyst, but both can be readily catalyzed by many transition metals.23 Thermodynamic calculations5 showed that the extent of carbon deposition during reforming decreases at higher reaction temperatures, in agreement with several experimental observations.2,13,24 These results suggest that CO disproportionation is the main contributor to carbon deposition because it is exothermic and the equilibrium constant decreases with increasing temperature. Despite these evidences, there are disagreements concerning the source of carbon deposition. Swaan et al.25 and Efstathiou et al.26 showed by TPO with isotopic mixtures that most of the carbon accumulated during reforming reaction over Ni/SiO2 and Rh/Al2O3, respectively, is derived from a CO2 molecule. Other authors claimed that carbon is formed from methane, over a large variety of catalysts, including Pt/Al2O3 and Pt/ZrO2.17,18,27,28 So, there is not a consensus in the literature about the origin of the coke formation during reforming conditions. We tried to elucidate this question by investigating the PtAl catalyst, which exhibited the fastest deactivation under stoichiometric conditions, in a more detailed way. Stability tests were performed with an excess of CO2 in the feed, as shown in Figure 3. It shows that a surplus of CO2 improves the performance of the catalyst and an excess of 50% is enough to reach good stability. On the other hand, when methane flowed alone over the catalyst at 1073 K, the maximum conversion was only 0.8%, which suggests that PtAl is not active for CH4 decomposition. Therefore, it is possible to ascribe the coke deposition to CO disproportionation because an excess of CO2 favors the reverse reaction, which minimizes carbon formation. We also examined the behavior of Pt10Zr, which showed the best stability under stoichiometric conditions,15 with higher ratios of CH4/CO2, to accelerate the catalyst deactivation. Stability tests under severe conditions were carried out with CH4/CO2 ratios of 2:1 and 3:1, and the results are shown in Figures 4 and 5, respectively. The Pt10Zr catalyst exhibited excellent stability even under severe deactivation conditions. With the CH4/CO2 ratio of 2:1, the rate of deactivation was 0.15%/h, and with the ratio of 3:l, this rate increased to 0.4%/h, based on CO2 consumption. These deactivation rates are much lower than those reported by Stagg-Williams et al.29 for Pt/ZrO2. In this case the
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Desenvolvimento Cientifico, Brazil) for financial support during this work. Literature Cited
Figure 4. Stability test with Pt10Zr at 1073 K and a feed ratio of CH4:CO2 ) 2:1. Reaction conditions: flow rates of CH4 ) 10 cm3/min, CO2 ) 5 cm3/min, and He ) 185 cm3/min.
Figure 5. Stability test with Pt10Zr at 1073 K and a feed ratio of CH4:CO2 ) 3:1. Reaction conditions: flow rates of CH4 ) 10 cm3/min, CO2 ) 3.3 cm3/min, and He ) 187 cm3/min.
rate of deactivation was 1.5%/h during 23 h of reforming reaction at 1073 K and the CH4/CO2 ratio of 2:1, when the X(CH4)/X(CO2) conversion ratio was greater than 0.5. On the basis of the conversion rate, Stagg et al.16 proposed that the CH4 decomposition rate is initially greater than the CO2 dissociation rate and that the CH4 decomposition is responsible for the deactivation of the catalyst. In the present study, the X(CH4)/X(CO2) conversion ratio was 0.47, only slightly lower than 0.5 due to the reverse water gas shift reaction, whose influence is small at high temperatures. It reinforces our previous statement that CO disproportionation is determinant for coke deposition because the presence of CO2 below the stoichiometric value favors this reaction (2CO T C + CO2). 4. Conclusions CO2 reforming of methane is strongly affected by the nature of support oxides. The Pt/ZrO2/Al2O3 and Pt/ZrO2 catalysts presented very high stability compared to the Pt/Al2O3 at 1073 K. The deactivation is primarily caused by coke deposition surrounding the metal-support perimeter, although carbon deposits are located basically on the support. The principal route of carbon deposition is the CO disproportionation. The catalyst with 10 wt % ZrO2 exhibited excellent stability even under severe deactivation conditions. Acknowledgment M.M.V.M.S. and D.A.G.A. are grateful to FAPERJ (Fundac¸ a˜o de Amparo a` Pesquisa do Estado do Rio de Janeiro) and CNPq (Conselho Nacional de Pesquisa e
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Received for review November 30, 2001 Revised manuscript received May 29, 2002 Accepted June 13, 2002 IE010970A
