Effect of Ceria Surface Area, Reaction Temperature, and Oxyg

Oct 28, 2009 - Voice: 304-293-9337. Fax: 304-293-. 4139. ...... R. D.; Herman, I. P. Cerium Oxide Nanoparticles: Size-selective Formation and Structur...
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Ind. Eng. Chem. Res. 2009, 48, 10796–10802

Formation of Synthesis Gas from Propane Oxidation over Pt-on-Ceria: Effect of Ceria Surface Area, Reaction Temperature, and Oxygen/Fuel Ratio Tapan K. Das,† Edwin L. Kugler, and Dady B. Dadyburjor* Department of Chemical Engineering, West Virginia UniVersity, Morgantown, West Virginia 26505-6102

The formation of synthesis gas from propane oxidation has been investigated on 1.0% Pt/CeO2 catalysts in a fixed-bed flow reactor over a temperature range of 200-800 °C at atmospheric pressure using ∼90% inert. Three kinds of ceria, with different surface areas, have been used as the catalytic support for this reaction. The catalysts were characterized using X-ray diffraction (XRD), temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), H2 chemisorption, and BET surface area. For the catalysts with the highest surface area, a large decrease in BET surface area has been observed after calcinations at high temperature. The oxidation of propane occurs with two sets of products. At low temperatures (T e 500 °C), propane oxidizes exclusively into CO2 and H2O with little H2. Above 500 °C, O2 is completely consumed, and the selectivities of CO and H2 increase steadily with increase in temperature. In the region 500-700 °C, high-surface-area catalysts show higher activity and selectivity to H2 and CO than low-surface-area catalysts. Propane conversion at high temperatures (T g 750 °C) is similar for all the catalysts. The effect of O2/C3H8 has also been investigated. Propane conversion and carbon dioxide selectivity increase, and H2 and CO selectivities decrease, with increase in the O2/C3H8 ratio. 1. Introduction Fuel-cell technology is currently undergoing rapid development for both stationary and transportation applications. Protonexchanged membrane (PEM) fuel-cells operating with hydrogen are being increasingly accepted as an appropriate power source for future vehicles. When analyzing the commercial viability of the fuel-cell technology, the more-relevant issues are the process and feedstock for hydrogen to feed the fuel cell, rather than the fuel cell itself. Hydrogen can be produced from fossil fuels through a process of reforming using natural gas, naphtha, gasoline, or even heavier fuels like diesel, gasoil, etc.1 There are three major technologies used to produce hydrogen from hydrocarbon fuels. Using propane as an example, two of these routes are partial oxidation (PO) C3H8 + 1.5O2 f 3CO + 4H2,

∆H° ) -227 kJ/mol (1)

and steam reforming (SR) C3H8 + 3H2O f 3CO + 7H2O,

∆H° ) -497 kJ/mol (2)

Steam reforming of methane or natural gas is the conventional route for producing hydrogen on an industrial scale.2,3 The third route, autothermal reforming (ATR), is considered4-6 a combination of partial oxidation (eq 1) and total oxidation: C3H8 + 5O2 f 3CO2 + 4H2O,

∆H° ) -2043 kJ/mol (3)

Other reactions include: * To whom correspondence should be addressed. E-mail: [email protected]. Voice: 304-293-9337. Fax: 304-2934139. † Current address: Chevron Energy Technology Center, 100 Chevron Way, Richmond, CA 94801.

water-gas shift CO + H2O T H2 + CO2,

∆H° ) -41 kJ/mol (4)

methanation CO + 3H2 f CH4 + H2O,

∆H° ) -205 kJ/mol (5)

dry reforming C3H8 + 3CO2 f 6CO + 4H2,

∆H° ) 620 kJ/mol (6)

The relative importance of these reactions depends on the reactant composition, temperature, residence time, and the catalytic system involved. Additional side reactions include: water formation 2H2 + O2 f 2H2O, dehydrogenation C3H8 f C3H6 + H2, cracking C3H8 f C2H4 + CH4, carbon deposition 2CO f C(S) + CO2,

∆H° ) -483.6 kJ/mol

(7)

∆H° ) 124 kJ/mol

(8)

∆H° ) 89 kJ/mol

(9)

∆H° ) -172 kJ/mol

(10)

must also be considered. The carbon deposition reaction is a particularly unwanted reaction and generally occurs when the O2/C3H8 ratio in the reactant mixture becomes too low. The production of hydrogen by oxidation reactions is a rapid process and can be performed at short contact times. Methane oxidation has been studied extensively.7-12 However, the oxidation of propane to synthesis gas with steam and without steam has been studied by only a few research groups. Metal oxide,13 Pt/Pd/Al2O3,13-17 Ni/Al2O3,18 and Pt/Rh/CeO2/Al2O316,19 have been used as catalysts. The sequence of the activity is the following: Pt > Pd > Rh.14 The sequence of the activity of metals for steam reforming (corresponding to the reaction of water with

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unreacted propane after the oxygen is completely consumed) for propane is the following: Rh > Pd > Pt. Huff et al.15 observed that the reaction over Pd behaves similarly to that over Pt; however, the Pd catalyst is deactivated due to carbon deposition. Cerium oxide has been extensively used as a base component of automobile three-way catalysts to control pollutant emissions.20,21 Cerium oxide shows an easy transition between Ce4+ and Ce3+ and can store large amounts of O2 and promote the catalysis of oxidation reactions.22-26 Ceria promoted with noble metals also is very active for the water-gas shift reaction (eq 4).27 Ceria promotes stabilization of precious metals and prevents sintering of particles.28 This work reports on the oxidation of propane to synthesis gas over Pt supported on CeO2 catalysts with ∼90% dilution of feed gas. Three issues are investigated: (i) the effect of BET surface area of the catalyst; (ii) the effect of reaction temperature; and (iii) the effect of O2/C3H8 ratio on activity and selectivity. 2. Experimental Details 2.1. Catalyst Preparation. Three different methods were used to prepare the ceria support. Ceria obtained from Aldrich Chemical was designated as CeO2 (A). The ceria designated as CeO2 (B) was prepared by homogeneous precipitation from a cerium nitrate solution with aqueous ammonia.29 In this method, 30 g of Ce(NO3)3 · 6H2O (Alfa Aesar, 99.5%) was dissolved in 900 mL of deionized water, and NH4OH (Alfa Aesar, 28-30% NH3) was added dropwise (∼1 mL/min) to the solution. The precipitate was filtered, washed with 600 mL of deionized water, and dried in an oven at 100 °C overnight. The dried precipitate was crushed and calcined at 400 °C for 4 h. Finally, the ceria designated as CeO2 (C) was prepared by decomposition. In this method, Ce(NO3)3 · 6H2O was calcined at 500 °C in air for 4 h.30 All supports were loaded with platinum via incipient wetness impregnation. Appropriate amounts of Pt(NH3)4(NO3)2 (Aldrich) were dissolved in deionized water, and the solution was added dropwise to the dried support to obtain 1 wt % Pt. The catalysts were then dried at 100 °C for 4 h and then calcined at 400 °C for 4 h. 2.2. Temperature-Programmed Reduction. Temperatureprogrammed reduction (TPR) was carried out in a unit constructed in-house. A 100-mg catalyst sample was placed into a quartz tube (3 mm i.d.). The sample was preheated in situ with flowing argon at 400 °C for 2 h and then cooled to room temperature, still in flowing argon (30 cm3/min). A gas stream of 5 vol % H2 in argon at a total flow rate of 30 cm3/min was then passed through the catalyst. The temperature was increased at 10 °C/min from room temperature to 850 °C. The water produced by reduction was trapped in a dry-ice trap. The amounts of H2 in the inlet and outlet streams were detected by a thermal-conductivity detector (TCD) using 5 vol % H2 in argon as a reference. 2.3. Temperature-Programmed Desorption and Oxygen Titration. The apparatus for temperature-programmed desorption (TPD) was also constructed in-house. The TPD apparatus is similar to that used for TPR, except that now a six-way valve with a sample loop is placed before the sample tube, so that amounts of H2 and O2 can be measured by titration. Prior to the experiments, the sample loop size was measured with pulses of N2 in a helium flow and compared against a calibration line produced by using injections of N2 from a gastight syringe into a helium flow. For hydrogen chemisorption, about 0.1 g of material was activated for 2 h using pure hydrogen at 550 °C and then cooled

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under flowing pure hydrogen to 100 °C. The sample was held at 100 °C under flowing argon to remove physisorbed and weakly bound hydrogen species, i.e., until the TCD signal returned to the baseline. Then the temperature was increased to 550 °C at the rate of 20 °C/min in flowing argon and held for 20 min to desorb the remaining chemisorbed hydrogen. The number of moles of desorbed hydrogen was determined by comparing the area of the TPD spectrum to the areas of calibration pulses of hydrogen in argon. The TPD results yield a measure of chemisorbed H2 present in the catalyst material. The same unit was used for O2 pulse titration. A known amount of fresh sample was reduced under 5% H2/Ar at 500 °C for 2 h and then cooled to 300 °C under helium. It was assumed that some of the ceria is reduced to Ce3+. The sample was reoxidized at 300 °C by pulses of 5 vol % O2 in helium. When the entire O2 pulse was observed by the TCD, it was assumed that all of the Ce3+ was reoxidized to Ce4+. The number of moles of O2 consumed for this reoxidation was determined by comparing the total area of the pulses titrated to the areas of calibration pulses of O2. This then allowed the calculation of the percentage of ceria that was originally reduced to Ce3+ in H2 at 500 °C. 2.4. X-ray Diffraction. X-ray diffraction (XRD) measurements were conducted with a Philips X’pert instrument using nickel-filtered Cu KR radiation (λ ) 1.5406 Å). Calcined catalyst samples were used for the XRD measurements. The particle size of CeO2 was calculated from the line broadening of a CeO2 line (2θ ) 88.8°) using the Scherrer formula.31 2.5. Thermogravimetric Analysis. The amount of coke formed on the catalysts at different temperatures was examined by thermogravimetric analysis (TGA), using a TA Instruments thermoanalyzer (model Q500). The samples were pretreated at 150 °C under flowing nitrogen and heated at a rate of 20 °C/ min to 800 °C in a flow of air (40 cm3/min) and nitrogen (40 cm3/min). Then, the material was held at 800 °C for 20 min. The weight loss of the sample is assumed to be due to coke removed from the catalyst. 2.6. BET Surface Area. Nitrogen adsorption measurements were used to determine the surface area, using an Omnisorp 360 from Coulter Corporation. Approximately 300 mg of the calcined sample was degassed overnight at 300 °C at 10-5 torr prior to the measurements. The samples were then cooled to 77 K using liquid nitrogen, and the sorption of nitrogen was carried out. The results were fitted to the BET equation to obtain the surface areas. Anhydrous weights of the samples were used in the surface area calculations. 2.7. Thermal Stability. The thermal stability of all the ceria supports and catalysts was studied by heating the materials in air and measuring the BET surface area. Fresh support and catalyst materials were used for each temperature treatment. The temperature was increased at the rate of 10 °C/min from room temperature to the target temperature (400-900 °C), and then, the temperature was held for 3 h. 2.8. Catalytic Test. Reactions were carried out in a stainlesssteel fixed-bed reactor at atmospheric pressure. Approximately 100 mg of catalyst sample (20-30 mesh), diluted 5-fold with quartz of the same mesh size, was placed between two quartz wool plugs in the center of a stainless-steel tube of 10.9 mm internal diameter and 0.45 m length. Quartz chips were placed upstream and downstream of the dilute catalyst bed. The reaction temperature was measured by a chromel-alumel thermocouple centered within the catalyst bed. A propane-containing mixture (10% propane, 10% nitrogen, and the balance argon) and an oxygen-containing mixture (30% oxygen and balance argon)

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were used as reactants in the experiments. Pure argon gas (99.999%) was used to dilute the reactant gases further so that the final mixture in the reactor contained 90 vol % argon. The gas mixture was diluted in order to minimize axial temperature gradients in the catalyst bed. Three Brooks mass flow controllers were used to feed the reactant gases. The feed and products were analyzed online using a Varian 3600 gas chromatograph (GC) equipped with a thermalconductivity detector (TCD) and a flame-ionization detector (FID). A HaysepD packed column, 6 m × 3 mm, was used for the TCD, and an alumina capillary column, 30 m long, was used for the FID. Argon was used as the carrier gas; nitrogen in the propane feed was used as the internal standard for GC quantification. For each experiment, conversions (X) and selectivities (S) were calculated from the following equations: XC3H8 ) SH2 )

FC3H8,in - FC3H8,out , FC3H8,in

XO2 )

FO2,in - FO2,out FO2,in

FH2,out , 4[FC3H8,in - FC3H8,out] SCO )

SCO2 )

FCO,out 3[FC3H8,in - FC3H8,out]

FCO2,out , 3[FC3H8,in - FC3H8,out] SCH4

FCH4,out ) 3[FC3H8,in - FC3H8,out]

where Fi is the molar flow rate of species i. A few experiments were carried out in a reactor made of incoloy (an inert nickel-chromium alloy with titanium, copper, and molybdenum). The reactor is of the same dimensions as the stainless-steel tube reactor. 3. Results and Discussion 3.1. Characterization. XRD patterns of the three ceria samples (A, B, and C) with and without 1% Pt are shown in Figure 1. Only the fluoride oxide-type structure was identified. In order to assess the domain sizes of crystalline ceria, the broadening of the peak at 2θ ) 88.8° was analyzed.32 The results are provided in Table 1. The domain size for CeO2(A) is almost three times larger than those for CeO2(B) and CeO2(C). For all the catalysts, the domain size is not changed appreciably after impregnation of platinum to the ceria support. BET surface-area measurements are also summarized in Table 1. The BET surface area of fresh samples of CeO2(B) and CeO2(C) are 4-5 times larger than that of CeO2(A). Values of the BET surface area obtained after thermal treatments are presented in Figure 2. Surface areas of CeO2(B), CeO2(C), 1% Pt/CeO2(B), and 1% Pt/CeO2(C) drop drastically with increasing temperature. It is interesting to note that, although the BET surface area of fresh CeO2(C) is larger than that of CeO2(B), the latter has the larger BET surface area after heat treatment. The same qualitative results are obtained after deposition of Pt. The loss of BET surface areas can be attributed to loss of microporous surface followed by sintering of particles.33 TPR profiles of the differently prepared supports and platinum catalysts are reported in Figure 3. In the literature,34-37 the curves for CeO2 alone generally show two reduction peaks, at approximately 450 and 850 °C. Our values are higher by about 50 °C. The low-temperature peak corresponds to the reduction

Figure 1. X-ray diffraction pattern of CeO2 and 1.0% Pt/CeO2 materials calcined at 400 °C. Table 1. Physicochemical Characteristics of Support and Pt/CeO2 Material supports/catalysts CeO2(A) CeO2(B) CeO2(C) 1.0% Pt/CeO2(A) 1.0% Pt/CeO2(B) 1.0% Pt/CeO2(C)

domain size BET SA O2 consumption D, nma (m2/g) % dispersionb (µmol/g cat) 20.8 8.2 7.8 22.8 8.1 7.6

17.3 69.6 84.7 9.9 65.8 80.4

21.1 146 180 30.9 199 218

8.4 34 35

a From XRD line broadening at 2θ ) 88.8°. chemisorption, by difference.

b

From hydrogen

Figure 2. BET surface area of support and catalyst materials after calcinations at different temperature for 3 h.

of the most-easily reducible surface-capping oxygen anions (O2-) on the ceria surface while the high-temperature peak corresponds to the bulk reduction of the ceria lattice (Ce4+ f Ce3+).36 In Figure 3, CeO2(A) shows a very small lowtemperature peak, as the relative surface area is small. The corresponding peaks are more noticeable for the higher-surfacearea CeO2(B) and CeO2(C). When Pt is added, it catalyzes the

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Figure 3. TPR profile of supports and Pt/CeO2 catalysts.

reduction of the surface shell of ceria and, therefore, shifts the low-temperature peak to a lower temperature. However, Pt has little effect on the bulk, and so the high-temperature peak is not appreciably altered. In Figure 3, the shifted low-temperature peak corresponding to low-surface-area 1.0% Pt/CeO2(A) is barely discernible in the scale of the diagram, whereas the highsurface-area materials 1.0% Pt/CeO2(B) and 1.0% Pt/CeO2(C) show well-defined shifts in the positions of the low-temperature peaks and considerable reduction of surface ceria. The platinum dispersion cannot be directly estimated from H2 or CO chemisorption because both gases also adsorb on CeO2. To overcome this problem, chemisorbed H2 was measured separately for the support as well as for the Pt/CeO2 and the H2 chemisorbed on reduced Pt metal was calculated by difference. The values are presented in Table 1. The materials using CeO2(B) and CeO2(C) show similar values of Pt dispersion (35%), whereas the dispersion value is almost 4 times smaller in case of materials using CeO2(A). Finally, O2 consumption due to the reoxidation of surface Ce3+ (after reduction at 500 °C) back to Ce4+ is also presented in Table 1. After Pt loading, O2 consumption increases slightly for all Pt/CeO2 materials. This implies that Pt increases the ability of the ceria to be reduced in H2 at 500 °C. Further, the O2 consumption for CeO2(A) (with or without Pt) is very low, as compared to the corresponding values for B and C. This indicates that the percent reductions at 500 °C for CeO2(B) and CeO2(C) (with or without Pt) are much higher than that for the corresponding values of CeO2(A) material. 3.2. Catalytic Activity. 3.2.1. Blank Test. Prior to catalytic testing, blank reaction tests were performed, both with the stainless-steel reactor and with the reactor made from incoloy. In each case, the reactor was filled with quartz chips. Results for the blank runs (Figure 4) show that the light-off temperature (when the oxidation starts increasing dramatically) is around 600 °C. At 600 °C, the conversion of propane is 11% for the stainless-steel reactor and 10% for the incoloy reactor. This implies that the stainless-steel reactor has no catalytic effect. In both reactors, all the reacted propane is converted into CO2. At 750 °C, some CO and H2 are detected; the outlet volumetric flow rates of these species are 2-3 sccm. A small amount of O2 is observed in the outlet stream. 3.2.2. Equilibrium Calculations. The composition of reactants and products under thermodynamic equilibrium at reaction conditions are shown in Figure 5. The inlet conditions correspond to those in the blank test, Figure 4. The ChemCad option, “Gibbs free energy minimization reactor” (Chemstations

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Figure 4. Propane conversion (X, left-hand axis) and outlet component volumetric flow rates (right-hand axis) of the blank reactor as a function of temperature: total flow rate 303 sccm; feed composition (sccm) C3H8:O2: N2:Ar ) 10.8:19.2:10.8:262.9; S ) stainless-steel reactor; I ) incoloy reactor.

Figure 5. Thermodynamic equilibrium volumetric flow rate of reactants and products as a function of temperature for partial oxidation of propane: total flow rate 303 sccm; feed composition (sccm) C3H8:O2:N2:Ar ) 10.8: 19.2:10.8:262.9.

Inc., version 5.2.1) was used for the calculations. The equilibrium volumetric flow rates of H2 and CO increase with increase in temperature and level off at approximately 700 °C. However, the equilibrium values for CO2 and CH4 decrease with an increase in temperature and level off at the same temperature. Even at 200 °C, propane and O2 are completely consumed. Clearly, the blank reactors of Figure 4 are far removed from thermodynamic equilibrium. 3.2.3. Effect of Temperature. The outlet volumetric flow rates of reactants and products obtained during partial oxidation of propane over the three 1.0% Pt/CeO2 catalysts using the stainless-steel reactor are shown as a function of reaction temperature in Figure 6. Inlet conditions are as in Figures 4 and 5. As expected, the outlet volumetric flow rates lie between those of Figures 4 and 5. For all the Pt/CeO2 catalysts, the outlet volumetric flow rate of propane decreases with an increase in temperature, indicating an increase in conversion with temperature. At higher temperatures, Pt/CeO2(A) converts propane at least as well as do the other two catalysts. This is consistent with the changes in surface areas for the other two catalysts, as indicated in Figure 2. Unlike in Figure 5, oxygen is not completely consumed at all temperatures in Figure 6. Below 500 °C, some oxygen is observed, and propane oxidation produces only CO2 and H2O.

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Figure 7. Conversion and selectivity of the main products as a function O2/C3H8 for the oxidation of propane over 1% Pt/CeO2(C): reactor incoloy, total flow rate 303 sccm, reaction temperature 675 °C.

Figure 6. Volumetric outlet flow rate of reactants and products as a function of temperature for partial oxidation of propane: total flow rate 303 sccm; reactor stainless-steel; feed composition (sccm) C3H8:O2:N2:Ar ) 10.8:19.2: 10.8:262.9; (a) 1.0% Pt/CeO2(A), (b) 1.0% Pt/CeO2(B), (c) 1.0% Pt/ CeO2(C).

Below 500 °C, more oxygen and propane are observed over Pt/CeO2(C) than over Pt/CeO2 (B). This indicates that conversion of propane and oxygen is higher for Pt/CeO2(B) than Pt/CeO2(C) below 500 °C. At higher temperatures (T > 500 °C), O2 is completely consumed, and the volumetric flow rates of H2 and CO increase with increase in temperature. Maillet et al.14 and Barbier et al.19 observed a similar two-stage reaction process for the oxidation of propane under oxygen-deficient conditions over Pd/Al2O3 and Pt-Rh/CeO2/Al2O3 catalyst, respectively. For all the catalysts in Figure 6, production of hydrogen starts at 450 °C. This indicates that O2 and H2 could coexist over the catalyst during a small interval of temperature. The concentrations of hydrogen and carbon monoxide at least start leveling

off at 750 °C. The production of CO2 levels off at 550 °C and then decreases with temperature. These conversion and selectivity data are consistent with those in the literature. Liu et al.13 reported that NiO/γ-Al2O3 exhibits complete conversion of propane and oxygen at 873 K with a feed of O2/C3H8/Ar ) 1.65/1/5.3 and a space velocity of 57 000 h-1. Selectivities of H2, CO, CO2, and CH4 are 81%, 43%, 30%, and 27% respectively. When the support was changed to MgO or SiO2, activity and selectivity values were seen to be much smaller, and no CH4 was observed. Guimaraes et al.28 reported that a Pd/CeO2/Al2O3 catalyst is less active for propane oxidation at low temperature than is Pd/Al2O3, but suddenly becomes very active at higher temperatures, within about 150 K. 3.2.4. Effect of O2/C3H8 Ratio. The effects of the O2/C3H8 ratio on the performance of the 1.0% Pt/CeO2(C) in an incoloy reactor at 675 °C are shown in Figure 7. In these runs, the flow rate of propane is held constant (at 10.5 sccm), and the flow rates of O2 and Ar are changed such that the sum of these is held constant (at 292.5 sccm). Hence the total inlet flow rate is kept constant (at 303 sccm) and the space velocity is constant while the O2/C3H8 ratio is changed. The stoichiometric ratio for partial oxidation is 1.5 (eq 1) while the value equals 5.0 for total oxidation (eq 2). As O2/C3H8 increases from 0.5 to 5.0, the selectivities of H2, CO, and CH4 decrease while propane conversion, H2/CO ratio, and CO2 selectivity increase. The O2 conversion is 100% up to an O2/C3H8 ratio of 5.0 (stoichiometric for total oxidation); however, the conversion decreases at higher values of the ratio, since oxygen is now the excess reactant. The maximum amount of (H2 + CO) is generated at the O2/ C3H8 ratio of 1.8. 3.2.5. Effect of Time on Stream. Time-on-stream experiments were carried out using a value of the O2/C3H8 ratio equal to 1.78. Two temperatures, 700 and 800 °C, were used. Results over 1.0% Pt/CeO2(A) and 1.0% Pt/CeO2(C) are shown in Figure 8. At these high temperatures, Pt/CeO2(A) performs at least as well as Pt/CeO2(C). As before, this is consistent with the loss of surface area with temperature as shown in Figure 2. The oxygen conversion (not shown) reaches 100% for all runs under these conditions. In the case of the 1.0% Pt/CeO2(A) catalyst at 800 °C (Figure 8a), the conversion of propane is nearly constant over 15 h time on stream. A slight decrease in the selectivity to hydrogen and carbon monoxide, and a slight increase in selectivity to carbon dioxide, are observed at this temperature. However, these changes in product selectivity mainly occur during the first few hours, after which the propane activity remains stable.

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Figure 8. Conversion and selectivity to main products as functions of time on stream at various temperatures for partial oxidation of propane: total flow rate 303 sccm; feed composition C3H8:O2:N2:Ar ) 10.8:19.2:10.8:262.9; (a) 1.0% Pt/CeO2(A), T ) 800 °C, (b) 1.0% Pt/CeO2(C), T ) 800 °C, (c) 1.0% Pt/ CeO2(A), T ) 700 °C, (d) 1.0% Pt/CeO2(C), T ) 700 °C.

formation of coke during the reaction. This is consistent with the work of Schulze et al.,38 who observed more coke formation at 700 °C than at 800 °C in the partial oxidation of light paraffins over Ni-containing hydrotalcites. Figure 8 indicates that the Pt/ CeO2 catalysts are stable at 800 °C but show signs of deactivation at 700 °C. Hence, the tendency toward catalyst coking is linked to the deactivation of the catalyst. 4. Conclusions

Figure 9. Thermogravimetric analysis of spent catalysts after reaction at different temperatures.

For Pt/CeO2(C) at 800 °C (Figure 8b), the propane conversion and the selectivity to H2 and CO decrease with time on stream. The H2/CO ratio is 1.5, and it remains constant over 15 h time on stream for Pt/CeO2(C). This value is higher than the H2/CO ratio obtained over Pt/CeO2(A). At 700 °C for both the catalysts (Figure 8c and d), the deactivation rates seem to be slightly higher than those at 800 °C. The spent Pt/CeO2(C) catalysts, after about 16 h time on stream at various temperatures, were characterized by TGA in flowing air. The results are shown in Figure 9. The catalyst that was run at 700 °C shows a higher percent weight loss than the run at 800 °C, and the catalyst run at 600 °C has an even higher weight loss. The weight loss is probably due to the

Propane oxidation was investigated over different Pt/CeO2 catalysts. Pt catalyzes the reduction of the surface shell of the ceria support and, therefore, shifts the low-temperature reduction peak of the support to lower temperature. Pt has little effect on the bulk reduction of ceria, so the high-temperature reduction peak is unchanged. Low-surface-area ceria shows negligible surface reduction, whereas the high-surface-area materials show considerable reduction of surface ceria. For all three catalysts, the reactions occur with two sets of products. At reaction temperatures below 500 °C, propane oxidizes exclusively into CO2 and H2O. For reaction temperatures above 500 °C, O2 is completely consumed, and the volumetric flow rates of products H2 and CO increase with temperature. At temperatures between 500 and 700 °C, the higher-surface-area catalysts Pt/CeO2(B) and Pt/CeO2(C) exhibit slightly higher activity and higher selectivity to H2 and CO than does Pt/CeO2(A). A large decrease in BET surface area is observed in Pt/CeO2(B) and Pt/CeO2(C) after high-temperature treatment. This may be the reason for the decrease in activity and selectivity with temperature. The lower-surface-area Pt/CeO2(A) catalyst shows promising catalytic activity and selectivity during several hours of operation

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ReceiVed for reView March 17, 2009 ReVised manuscript receiVed October 1, 2009 Accepted October 13, 2009 IE900441D