Oxidative CO2 Reforming of Methane on Alumina-Supported Co−Ni

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Oxidative CO2 Reforming of Methane on Alumina-Supported Co-Ni Catalyst Say Yei Foo, Chin Kui Cheng, Tuan-Huy Nguyen, and Adesoji A. Adesina* Reactor Engineering and Technology Group, School of Chemical Engineering, The UniVersity of New South Wales, Sydney, New South Wales 2052, Australia

CO2 reforming of CH4 in the presence of O2 in the feed has been investigated in a fixed bed reactor containing a Co-Ni catalyst. The reforming rate increased with O2 partial pressure before leveling out at O2:CH4 ) 1 at temperature greater than 823 K. Although CO production decreased with O2 addition, H2 formation initially rose to a maximum before a slow decline. An expression relating the optimum O2 partial pressure for H2 production as a function of temperature was obtained as, PO2,max ) 1.008 × 10-3e8420/T. The H2:CO product ratio increased from 0.9 for pure CO2 reforming and peaked at 1.73 as O2 partial pressure increased to an equimolar level in the feed. The increased reaction temperature resulted in lower H2:CO due to increased CO2 reforming kinetics. The complete consumption of O2 in the final product stream means that the oxidative CO2 reforming of CH4 may be used to generate nearly ideal syngas composition for Fischer-Tropsch synthesis if feed CO2:CH4 is unity. In particular, the overall heat demand for the reforming reaction could be reduced (or become exothermic) for a judicious combination of CO2/CH4/O2. The postreaction analysis revealed that even mild O2 dosing leads to negligible carbon deposition. Thus, this form of reactor operation is energetically attractive and provided efficient carbon utilization. 1. Introduction Synthesis gas (H2/CO) is primarily produced from catalytic steam reforming of natural gas, a highly energy and capital intensive operation. Furthermore, the product ratio of the steam reforming process (H2:CO > 3) is higher than that required for downstream methanol and hydrocarbon conversion processes.1 With increasing concern on the rise of anthropogenic greenhouse gas emissions, there has been renewed interest in the replacement of steam as a reactant with carbon dioxide. Implementation of CO2 reforming rather than conventional steam reforming is also attractive in areas where water is not readily available. However, similar to steam reforming, the dry reforming route is an endothermic reaction and suffers from carbon-induced catalyst deactivation on conventional Ni-based catalysts. Noble metals such as Ru, Rh, Pt, Pd, and Ir have been found to be highly active with low carbon formation;2-4 however, due to the high cost and limited availability of these metals, it is more practical to develop Ni-based catalysts. CO2 reforming also yields product H2:CO ratios lower than 1, which is below the ideal for a downstream Fischer-Tropsch synthesis plant. In order to reduce energy consumption for synthesis gas production, the catalytic partial oxidation process has attracted significant interest. Since the oxidation of hydrocarbons to synthesis gas mixtures is exothermic, this process is deemed more energy efficient than both the steam and dry reforming options. However, the oxidation reaction involves cofeeding hydrocarbon-oxygen mixtures under flammable or explosive conditions.5,6 Flames in the reaction zone may also lead to local hot spots with possible catalyst sintering. To overcome these problems, the combined reaction of CO2 reforming and partial oxidation may be utilized. Ashcroft et al. have shown that exothermic partial oxidation may be combined with endothermic CO2 reforming to achieve a thermally neutral reaction, with high synthesis gas yields.2 The other advantages of the oxidative CO2 reforming reaction are increased CH4 conversion,7 high yields at low temperatures,8,9 the ability to control the thermal behavior * To whom correspondence should be addressed. Tel.: +61 2 9385 5268. Fax: +61 2 9385 5966. E-mail: [email protected].

of the system,10 and improved stability and resistance to deactivation.11 Downstream Fischer-Tropsch hydrocarbon selectivity may also be manipulated by controlling the product H2:CO ratio via oxygen addition.12,13 The uniqueness of the present investigation lies in three distinct operational attributes: (1) the ability to predict optimum O2 partial pressures from the kinetics of the participating reactions; (2) tunability of the H2:CO ratio to meet varying downstream ends; (3) carbon deposition control without shutdown or any other adverse operational effects without losing gaseous products selectivity. Bimetallic Co-Ni catalysts have been proven to offer superior performance for CH4 dry reforming in terms of activity and stability compared to monometallic and other Ni-based bimetallic combinations.14 Previous studies in our laboratory have also shown that alumina-supported bimetallic Co-Ni catalysts exhibit synergistic effects during hydrocarbon reforming and superior coking resistance compared to monometallic Ni/Al2O315,16 and have therefore been employed in the current investigation. Although the net reaction for oxidative dry reforming is 2CH4 + CO2 + 0.5O2 f 4H2 + 3CO

(1)

the complex reaction network includes CO2 reforming of CH4 CH4 + CO2 f 2H2 + 2CO

(2)

Partial oxidation of CH4 CH4 + 0.5O2 f 2H2 + CO

(3)

Reverse water-gas shift reaction CO2 + H2 f H2O + CO

(4)

CH4 combustion reactions CH4 + 2O2 f 2H2O + CO2

(5)

H2 oxidation CO oxidation

H2 + 0.5O2 f H2O

(6)

CO + 0.5O2 f CO2

(7)

10.1021/ie100460g  2010 American Chemical Society Published on Web 06/03/2010

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Table 1. Constants to Calculate ∆H(T) from Equations 8a and 8b reaction

R0 × 10-3 (J mol-1)

R1 (J mol-1 K-1)

R2 × 103 (J mol-1 K-2)

R3 × 106 (J mol-1 K-3)

R4 × 10-5 (J K mol-1)

1 2 3 4 5 6 7

189.98 238.26 -48.28 48.82 -416.76 -237.72 -286.54

103.5 50.64 52.82 -15.464 28.41 -13.29 2.174

-66.93 -33.95 -32.98 2.25 -25.56 3.222 0.9769

11.99 5.997 5.997 0 5.997 0 0

12.55 10.48 2.066 9.677 -3.833 1.260 -8.418

The heat of reaction for reactions 1-7 at temperature, T, may be obtained from eqs 8a and 8b: ∆H(T) ) ∆H298 +

∫

T

298

∆Cp dT

(8a)

which may be expressed as: ∆H(T) ) R0 + R1T + R2T2 + R3T3 - R4T-1

(8b)

The constants in eqs 8a and 8b for reactions 1-7 are shown in Table 1 and were derived from standard thermodynamic tables.17 2. Experimental Section The catalyst support, γ-alumina (Saint-Gobain Norpro, USA), was first crushed and sieved to 140-425 µm before pretreatment at 1073 K for 6 h. 5Co-15Ni/80Al2O3 was then prepared via sequential impregnation, first with aqueous Co(NO3)2 and then with Ni(NO3)2 (Sigma Aldrich, Australia) on the thermally treated alumina under a constant pH of 2 in a Mettler Toledo T90 Titrator. Each impregnation step was followed by 3 h of stirring at ambient conditions, with subsequent drying for 24 h in an oven at 393 K. The resulting dried catalysts were calcined at 1073 K in air for 5 h, at a heating rate of 5 K min-1. The calcined solid was then crushed and sieved to 140-250 µm, before being activated in situ in the reactor. Multipoint BET surface area and pore volume measurements were obtained from N2 adsorption at 77 K using a Quantachrome Autosorb-1 unit. A Micromeritics Autochem 2910 was used to perform pulse H2-chemisorption at 383 K, as well as to determine basic and acidic character of the catalyst via CO2 and NH3 temperature-programmed desorption (TPD), respectively. CO2 and NH3 were adsorbed at 323 and 423 K, respectively, before TPD experiments at four heating rates of 10, 15, 20, and 30 K min-1. Also, 10% CO2/He and 10% NH3/N2 were used as probe gases for the TPD measurements. Powder X-ray Diffraction (XRD) analysis was conducted on a Philips X’Pert system using a Nifiltered Cu KR radiation (λ ) 1.542 Å) at 40 kV and 40 mA. The X-ray diffractograms were analyzed using an X’Pert ScorePlus software. Temperature-programmed experiments (calcination and reduction) were performed in a ThermoCahn TherMax 200 unit

to study the weight change profiles. Temperature-programmed calcination was carried out in 55 mL min-1 of air and ramped to 973 at 5 K min-1. Temperature-programmed reduction was conducted in 55 mL min-1 of 50% H2/Ar mixture using the same temperature-program scheme. The total carbon content of spent catalysts was determined using a Shimadzu TOC Analyzer 5000A coupled to a solid sample module SSM-5000A. Reaction runs were conducted on a computer-controlled experimental rig consisting of a gas manifold station, a stainless steel fixed-bed reactor (o.d. ) 6.25 mm, i.d. ) 4.57 mm, packed with 0.1 g of catalyst) and a TCD-equipped Shimadzu GC-17A gas chromatograph fitted with an Alltech CTR-1 column. Gas flow rates were regulated by Brooks Smart mass flow controllers. Prior to each run, the calcined catalyst was reduced in situ in 50 mL min-1 of 50% H2/N2 mixture at a temperature ramp of 5 K min-1. The catalyst was held for 2 h at 1063 K which is higher than that during reaction in order to avoid any temperature-induced phase transformations during the actual reaction. Following catalyst activation, the reactor was cooled under a blanket of N2 to the reaction temperature. In order to minimize transportdisguised kinetics, the reactor was operated using a gas-hourly space velocity of 20 000 h-1 over the bed of catalyst particles in the size range 140-250 µm. The reaction was conducted at temperatures between 823 and 973 K and a constant total pressure of 110 kPa with CO2 and CH4 partial pressures, PCO2 ) PCH4 ) 20 kPa, while the O2 partial pressure varied between 0 and 20 kPa. N2 was employed as a diluent gas. 3. Results and Discussion 3.1. Catalyst Characterization. Figure 1a shows the derivative weight profile during temperature-programmed calcination. The main peak at about 473 K corresponds to the decomposition of the metal nitrates to their respective oxides:

Figure 1. Derivative weight profiles of the catalyst during (a) calcination and (b) reduction.

Ni(NO3)2 f NiO + N2O5

(9)

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Co(NO3)2 f CoO + N2O5

(10)

while the shoulder at 500 K represents the oxidation of NiO and CoO to Co3O4 and NiCo2O4: 3CoO + 0.5 O2 f Co3O4

(11)

NiO + 2CoO + 0.5O2 f NiCo2O4

(12)

The broad shoulder at 560 K indicates the formation of the metal aluminate phase (NiO/CoO + Al2O3 f NiAl2O4/ CoAl2O4). The H2-TPR profile (cf. Figure 1b) implicates the reduction of Co3O4 and NiO to CoO and Ni (at 435 K) and NiCo2O4 to Ni and CoO, at 600 K. The peaks at 740 and 973 K represent the reduction of CoO to Co and the reduction of the metal aluminates phase, respectively. XRD pattern of the unreduced catalyst shown in Figure 2 reveals the existence of multiple oxide phases such as spineltype NiCo2O4 (2θ ) 31.2°), CoAl2O4 (2θ ) 36.8° and 59.1°), NiAl2O4 (2θ ) 36.8°, 44.8°, 59.1°, and 65.7°). In comparison, the peak intensities for Co3O4 (2θ ) 31.2° and 55.3°) and NiO (2θ ) 43.2°) phases were significantly smaller, indicating strong metal-support interactions to form aluminates. Table 2 summarizes data obtained from N2 physisorption and H2 chemisorption. BET surface area and pore volume obtained were similar to those previously prepared earlier in our laboratory,15,16,18 suggesting the conversion of γ-alumina to δ-alumina during pretreatment at 1073 K. H2-chemisorption results (metal dispersion, metal surface area, and active particle size) were consistent with the high metal loading (20 wt %) used. Table 3 displays the acidic and basic

Figure 3. NH3-TPD profile for Co-Ni catalyst.

Figure 4. CO2-TPD profile for Co-Ni catalyst.

Figure 2. X-ray diffractogram of calcined 5Co-15Ni/Al2O3. Table 2. N2-Physisorption and H2-Chemisorption Results BET area (m2 g-1) pore volume (cm3 g-1) average pore size (nm) metal dispersion (%) metal surface area (m2 g-1) active particle size (nm)

110.8 0.4962 17.91 0.5802 0.7758 173.8

Table 3. Acidic and Basic Properties of the Catalyst

NH3 heat of desorption, ∆Hd (kJ mol-1) acid site concentration (µmol m-2) CO2 heat of desorption, ∆Hd (kJ mol-1) basic site concentration (µmol m-2)

peak I

peak II

43.3

71.0

1.079 51.3 0.202

peak III

2.893 68.0 0.209

total

3.972 73.4 1.032

1.443

properties of the catalyst, with Figures 3 and 4 showing the corresponding NH3-TPD and CO2-TPD profiles for the Co-Ni catalyst. NH3-TPD revealed the two distinct peaks with heats of desorption, ∆Hd, of 43.3 and 71.0 kJ mol-1, respectively, suggesting the presence of multiple acid sites on the catalyst. The low temperature peak could be attributed to a weak Lewis acid site, while the high temperature peak may arise from the presence of strong Lewis and Brønsted acid sites.19,20 However, in this catalyst, it is more likely that the high temperature desorption of ammonia represents a Lewis site, since Brønsted acid sites typically exhibit ∆Hd greater than 125 kJ mol-1.21 Similarly, the CO2-TPD profile (cf. Figure 4) showed that CO2 desorption is characterized by multiple peaks, indicating the presence of weak, intermediate, and strong basic sites.22,23 Compared to the acid sites, the basic sites are stronger in strength (∆Hd ) 51.3, 68.0, and 73.4 kJ mol-1); however, the total basic site concentration (1.443 µmol m-2) is lower than the total acid site concentration (3.972 µmol m-2), indicating a net acidic catalyst. 3.2. Reaction Runs. Reactor operating conditions chosen ensured that the rate data collected were free from transport intrusions as confirmed by the satisfaction of the diagnostic criteria provided below in Table 4. Preliminary runs using an empty reactor also showed no detectable reaction products at temperatures up to 1023 K in the presence of CO2 and CH4. 3.2.1. Dry Reforming. In the first set of runs, rate data from CH4 dry reforming at different CO2 inlet partial pressures were obtained as shown in Figures 5-7. The CH4 consumption rate increased almost linearly with CO2 partial pressure while the

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Table 4. Criteria for the Absence of Transport-Limiting Resistances in Laboratory Reactors mass transfer criteria

heat transfer value

Mears (external) (-rexp)Fbdpn 8 kPa (depending on temperature), i.e. when CO2 consumption is zero. Significantly, CO2 became a reaction product beyond this critical point as seen from the negative values in Figure 8b. The H2 production rate appeared to go through a maximum depending on temperature (cf. Figure 9a). The initial increase in H2 formation above that due to pure dry reforming was probably due to additional oxidation of the accompanying carbon residue, CxH1-x (formed from dry reforming), namely;

(16)

and predicts the asymptotic values of SH2:CO ) ∞ for PCO2 ) 0 (conditions for direct CH4 dehydrogenation) and SH2:CO ) 0.5 for high PCO2 (maximum of 90 kPa with PCH4 ) 20 kPa when there is no N2 diluent). 3.2.2. Effect of O2 Addition during Dry Reforming. Figure 8a and b show the CH4 and CO2 rate profiles for oxidative dry reforming for different inlet O2 partial pressures using PCH4 ) PCO2 ) 20 kPa. O2 was fully consumed under all the reaction conditions studied and its presence increased the CH4 consumption rate significantly. However, apparent CO2 consumption decreased such that a net production of CO2 was registered at higher O2 partial pressures. In particular, the CH4 reaction rate seemed to have a first-order dependence on PO2 at low temperature but changed to a nonlinear functionality beyond 873 K. At T > 873 K, an increase in CH4 consumption rate started to level off at PO2 greater than

1-x x CxH1-x + O2 f xCO + H2 2 2

(

)

(17)

However, as PO2 increased, H2 oxidation (cf. eq 6) would become dominant leading to a decrease in overall H2 production and, hence, the appearance of a maximum in H2 rate versus PO2 profile. Interestingly, the experimental PO2 corresponding to the maximum H2 rate, PO2,max shifted to lower values with increasing temperature following an Arrhenius-type dependency as shown in Figure 9b, thus PO2,max ) kmaxe-γ/T

(18)

or ln(PO2,max) ) ln kmax - γ

( T1 )

(19)

where the slope and intercept of the plot are characteristic parameters of the reaction network. Figure 9c shows that the CO formation rate is at its height during dry reforming but decreased with increasing PO2 during oxidative reforming, indicating that any CO produced from dry reforming and eq 17 would not survive in an O2 environment. As a result of these observations, a general empirical relation that can capture the essential features for the rate of each

Figure 8. (a) CH4 and (b) CO2 consumption rates during oxidative dry reforming.

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Figure 9. (a) H2 production rates, (b) relationship between ln PO2,max and 1/T, and (c) CO production rates during oxidative dry reforming.

participating species during oxidative dry reforming may be written:

Table 6. Parameter Estimates for the Empirical Model species parameter

ri ) a0 + a1PO2 - a2PO2b

(20)

where a0 represents the dry reforming contribution, a1 is the pseudo-rate constant due to O2 enhancementsthe removal of surface carbon residue accompanying dry reforming (initial linear increase, via eq 17), while a2 stands for the apparent rate constant for the direct total oxidation of the species (detrimental role of O2) with bth-order kinetics. Given the temperature-dependency of these parameters, eq 20 is rewriten as ri ) k0e-E0/RT + k1e-E1/RTPO2 - k2e-E2/RTPO2b

(21)

Parameter estimates using the data at all four temperatures and the six different PO2 values were obtained via nonlinear regression (Polymath 6.0) as summarized in Table 6. We note that the activation energy, E0, for H2, CO, and CH4 are nearly the same as that obtained for the pure dry reforming runs (EDR). The similarity in E0 values for all species suggests is symptomatic of similar rate-determining steps in formation of both CO and H2 during pure dry reforming. However, during oxidative dry reforming, the oxidation of CxH1-x species appear to be relatively facile since E1 values are low compared to E2 estimates.

-1

-1

ko × 10 (mol gcat s ) E0 (kJ mol-1) k1 × 105 (mol gcat-1 s-1) E1 (kJ mol-1) k2 × 103 (mol gcat-1 s-1) E2 (kJ mol-1) b EDR (kJ mol-1) 3

H2

CO

CH4

17.4 36.4 33.4 27.7 4.52 57.8 1.43 38.4

13.9 33.8

4.67 30.2 0.923 7.39 1475 131 2 30.9

2.29 64.3 1.57 36.3

CO2 2.78 27.1 0.0123 6.41 1.05 34.0

Although H2 production declined at high O2 partial pressure, Figure 10 shows that the H2:CO ratio continued to increase, suggesting that the rate of CO oxidation is more rapid than that of H2. At lower temperatures, catalytic CH4 combustion appeared to be the dominant reaction, producing CO2 and H2O, while at higher temperatures, the endothermic CH4 dry reforming reaction became increasingly favored.24 The decrease in H2:CO ratio with increased temperature at a fixed PO2 arose from increased CO production as CH4 dry reforming became important. Stoichiometric CH4 dry reforming produces H2:CO ratios somewhat lower than unity, due to the reverse water-gas shift reaction (eq 4). Thus, the formation of total oxidation products (H2O from H2 and CO2 from CO/CH4) may be manipulated by temperature and indeed PO2 in view of the difference in total oxidation kinetics for each species (cf. b-values in Table 6). This is a significant tool for H2:CO ratio control if the product

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Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 Table 7. Computed Heat of Reaction for CH4 Dry Reforming and Partial Oxidation ∆Hr (kJ mol-1)

Figure 10. Influence of O2 partial pressure on H2:CO ratio.

Figure 11. Residual plot for H2 production rate during oxidative dry reforming.

gas were to be used for downstream gas-to-liquid (GTL) conversion and in fact attractive, since O2 was completely consumed under all conditions and would not need to be removed from the syngas mixture (O2 is an inhibitor for Fischer-Tropsch reaction). The adequacy of the model given by eq 21 is confirmed by the relatively higher R2-values obtained (R2 > 0.95) and the pattern of the residual plot as demonstrated in Figure 11. In particular, this model predicts the temperature dependency of PO2,max. At the maximum rate, eq 21 requires drH2

) 0 ) a1 - a2bPO2b-1

dPO2

temperature (K)

CH4 + CO2 f 2H2 + 2CO

CH4 + 0.5O2 f 2H2 + CO

823 873 923 973

259.4 259.8 260.0 260.1

-24.0 -23.5 -23.1 -22.8

where the intercept (-6.9) in Figure 9b is (1/(b - 1)) ln(k1/ (k2b)) and slope (8420) is (E1 - E2)/((1 - b)R). The agreement between the parameter estimates in Figure 9b and data in Table 6 also lends credence to the reliability of eq 21. Table 7 shows the heat of reaction (∆Hr) for the stoichiometric CO2 reforming and partial oxidation (as calculated from standard thermochemical tabless17cf. eq 8b), and Figure 12 plots the experimental ∆Hr values (based on energy balance with the actual feed and product compositions). It is evident that the calculated ∆Hr for CH4 dry reforming, which varies between 259.4 and 260.1 kJ mol-1, is in agreement with the experimental counterpart (221.7-240.1 kJ mol-1). With the addition of O2, heat requirement for the reaction decreased such that the reaction eventually became exothermic at PO2 > 8 kPa as may be seen from Figure 12. At PO2 ) 20 kPa, the reaction was strongly exothermic, with -∆Hr greater than 135 kJ mol-1 and significantly larger than the theoretical value for partial oxidation of methane (which is between 22.8 and 24 kJ mol-1). This suggests that besides methane partial oxidation, other oxidation reactions such as H2 and CO oxidation were also taking place concurrently. Figure 12 also shows that that the difference in ∆Hr was small for runs carried out between 873 and 973 K with runs performed at 823 K being substantially more exothermic. This is because dry reforming is thermodynamically unfavorable at low temperature (∆G823K ) 26 kJ mol-1) while CH4 combustion seemed to dominate, resulting in a large difference in ∆Hr for T ) 823 K. Importantly, the thermal behavior of the reactor system may be controlled by varying the amount of O2 cofed to the reactor. 3.2.3. Mechanistic Inferences. Spectroscopic and pulse reaction techniques have shown that reactants involved in oxidative dry reforming may be adsorbed on transition metals.25-29 Osaki and co-workers,25,26 using pulse surface reaction analysis of CH4 dry reforming on various supported Ni catalysts, found that adsorbed CHy is produced via sequential

(22)

from whence PO2,max )

( ) a1 a2b

1/(b-1)

)

( ) k1 k2b

1/(b-1)

e-(E1-E2)/(b-1)RT

(23)

and consequently, ln(PO2,max) )

( )

(E1 - E2) 1 k1 1 ln + b-1 k2b (1 - b)R T

(24)

Figure 12. Experimental heat of reaction as a function of PO2 during oxidative dry reforming of CH4.

Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 Table 8. TOC (%) of Spent CO2 Reforming Catalysts at Various Temperatures, without O2 in Feed (PCH4 ) PCO2 ) 20 kPa) temperature (K)

TOC (%)

823 873 923 973

58.6 57.5 40.6 17.8

elimination of hydrogen atoms (where y varied between 1 and 2.7 in their study), eventually leading to surface carbon, suggesting that hydrocarbon chemisorption is dehydrogenative in nature. Stevens and Chuang,27 via combined in situ infrared (IR) spectroscopic and mass spectrometric study, proposed that the first step of CH4 decomposition on Rh/Al2O3 is decomposition of CH4 into CHy and H2. Erdo¨helyi et al.28,29 studied CH4 and CO2 activation over supported Pd and Rh using IR methods and found that CO2 adsorption was dissociative in nature as confirmed by the presence of adsorbed CO-band signals (wavenumber 1750-2080 cm-1). We therefore propose that the sequence of elementary steps describing oxidative CH4-CO2 reforming proceeds as follows:

typical of carbon formation from hydrocarbons at the reaction temperatures studied, in which negative activation energy is expected at temperatures between 823 and 923 K where carbon deposition decreased with increasing temperature, while, out of this range, a positive activation energy is obtained.30 Previous work on propane steam reforming in our group have also reported similar observations in the deactivation rate coefficients.18 Nakano et al.31 have also reported negative activation energy during CO disproportionation and attributed it to the greater adsorption energy of CO than the dissociation barrier. Combining this analysis with the eq 15, leads to rcarbondeposition ) 5.265 × 105e6127.4/TPCO2-7.36

(27)

as the kinetic expression for carbon deposition for CH4 dry reforming between 823 to 973 K. At all the temperatures studied, addition of even the lowest amount of O2 (5 kPa) gave a used catalyst with no detectable carbon, showing that O2 cofeeding during dry reforming may indeed suppress carbon deposition. 4. Conclusions

CH4 + X T CH3-X + H-X

(25a)

CH3-X + X f CH2-X + H-X l CH-X + X f C-X + H-X

(25b)

CO2 + 2X T CO-X + O-X

(25c)

O2 + 2X T 2O-X

(25d)

C-X + O-X f CO-X + X

(25f)

This study has demonstrated the beneficial effects of cofeeding small amounts of O2 during CO2 reforming of CH4. O2 addition resulted in improved CH4 reaction rate with attendant negligible coke deposition. With O2 addition, H2:CO ratio increased from less than 1 for stoichiometric CO2 reforming to between 1 and 2, depending on reaction conditions, showing that product syngas composition may be manipulated to one more suited to downstream processes such as the Fischer-Tropsch synthesis. The increase in overall reaction exothermicity shows that by managing the amount of O2 added, the thermal behavior of the reactor may be controlled. Moreover, an empirical relation describing the oxygen partial pressure for optimum H2 production as a function of temperature was validated by our data.

2H-X T H2 + 2X

(25g)

Acknowledgment

CO-X T CO + X

(25h)

H-X + O-X f OH-X

(25i)

H-X + OH-X T H2O + 2X

(25j)

The authors are grateful to the Australian Research Council for financial support. SYF and CKC are recipients of the Australian Postgraduate Award and University International Postgraduate Award scholarships respectively.

CHy-X + O-X f CHy-1O-X + H-X

1eye3 (25e)

where X denotes an active site. On the basis of this mechanism, the Langmuir-Hinshelwood kinetic model (for constant PCH4 and PCO2) for oxidative CH4 dry reforming was derived as rCH4 )

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krxnPO21/2 (1 + KO21/2PO21/2)2 (with eq 25e as the rate-determining step)

(26)

in which Ea,rxn ) 49.7 kJ mol-1 and -∆Hads,O2 ) 112 kJ mol-1 are associated with krxn (kinetic constant) and KO2 (equilibrium adsorption constant for O2), respectively. 3.2.4. Carbon Content Analysis. Table 8 displays the total carbon content of the spent catalysts following 4 h of CH4 dry reforming, in the absence of O2 in the feed. It seems that carbon lay-down from CH4 dehydrogenation was removed via CO2 gasification (cf. eq 14) and appeared to be more facile at high temperatures. The decrease in total carbon content with temperature suggests that carbon gasification rate was probably higher than CH4 dehydrogenation as temperature increased. Arrhenius treatment of carbon deposition gave an apparent activation energy of -50.9 kJ mol-1. This behavior is indeed

Nomenclature b ) inert volume fraction of bed Biw ) wall Biot number CAs ) surface concentration of reactant (mol m-3) dp ) catalyst particle diameter (m) dr ) reactor tube diameter (m) Deff ) effective diffusivity (m2 s-1) Ea ) activation energy (kJ mol-1) EDR ) activation energy for pure dry reforming (kJ mol-1) ∆Hd ) heat of desorption (kJ mol-1) ∆Hr ) heat of reaction (kJ mol-1) kc ) mass transfer coefficient (m s-1) R ) ideal gas constant (J mol-1 K-1) (-rexp) ) reaction rate (mol s-1 gcat-1) Tb ) bulk gas-phase temperature (K) Ts ) particle temperature (K) Tw ) wall temperature (K) Greek Letters Fb ) catalyst particle bulk density (kg m-3) λeff ) effective bed thermal conductivity (W m-1 K-1) εbed ) bed voidage

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ReceiVed for reView March 1, 2010 ReVised manuscript receiVed May 17, 2010 Accepted May 19, 2010 IE100460G