- Email: [email protected]
Kinetics of propane combustion over La0.66Sr0.34Ni0.3Co0.7O3 perovskite
Applied Catalysis A: General 213 (2001) 113–121
Kinetics of propane combustion over La0.66Sr0.34 Ni0.3Co0.7O3 perovskite Kwang Sup Song1 , Danilo Klvana∗ , Jitka Kirchnerova Department of Chemical Engineering, Ecole Polytechnique, P.O. Box 6079, Station Center-ville, Montreal, Canada QC H3C 3A7 Received 18 August 2000; received in revised form 17 November 2000; accepted 26 November 2000
Abstract La0.66 Sr0.34 Ni0.3 Co0.7 O3 (LSNC) perovskite is an excellent catalyst in several oxygen involving reactions, including methane combustion. Catalytic combustion of propane based on efficient technologies using low cost catalysts could serve as a non-polluting source of energy in remote regions. To optimize reactors for catalytic combustion (combusters), reliable kinetic data are needed. In this paper are presented kinetics of a complete propane oxidation over LSNC studied at a steady state in a plug-flow reactor. The experimental data were obtained for 0.5 g catalyst at temperatures between 473 and 613 K, with propane concentration varied from 0.58 to 4.76 vol.% oxygen between 7.5 and 98 vol.%, and flowrate between 100 and 400 ml/min. The LSNC catalyst, prepared with high specific surface area of 15 m2 /g, shows again an excellent stable activity which competes favorably with that of noble metals. Several kinetic models have been tested. Best fit was obtained with the Mars–van Krevelen kinetic model. Nevertheless, the complete set of obtained data can also be fitted adequately by a simple power law model with 0.5 order in propane and 0.3 order in oxygen and apparent activation energy of 71 kJ/mol. Both water and carbon dioxide added to the feed have slight inhibiting effect, which was taken into account in a final extended Mars–van Krevelen kinetic model. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Propane catalytic combustion; Kinetics of propane catalytic combustion; Perovskite catalyst; La0.66 Sr0.34 Ni0.3 Co0.7 O3 perovskite; Combustion catalysts
1. Introduction Catalytic combustion of fossil fuels, in particular of light hydrocarbons, represents one of the least polluting and economical means of heat and energy generation. For remote areas where natural gas is not readily available, propane is an attractive alternative.
∗ Corresponding author. Tel.: +514-340-4711/4927; fax: +514-340-4159. E-mail address: [email protected] (D. Klvana). 1 On leave from Korea Institute of Energy Research, P.O. Box 5, Taedok Science Town, Taejon 305-343, Korea.
However, development of efficient technologies using low cost catalysts is needed. Catalytic combustion (total oxidation) of light hydrocarbons, specifically of propane, has been investigated for decades both as part of fundamental and development studies [1–14]. These studies have covered a wide range of materials including noble metals such as platinum and palladium [5–7,10,11], different transition metal oxides, transition metal containing spinels and perovskites [1–4,8]. Among these catalysts, platinum is known as the most active [1,5], its activity exceeding that of palladium. This contrasts with the catalytic combustion of methane which is much less reactive than other light hydrocarbons and
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 8 8 4 - X
114
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
for which the most active catalyst is palladium in the form of an oxide. Because of their easier activation, light hydrocarbons can be readily oxidized over a much wider range of transition metal based oxides. However, only a few of the single metal oxides exhibit very high activity and selectivity to the complete oxidation. In comparison with simple transition metal oxides and even with spinels, perovskites offer the advantage of higher thermal, chemical and structural stability and in many cases improved catalytic activity [15]. Furthermore, perovskites also exhibit exceptionally high selectivity to complete oxidation [16]. Although a number of transition metal based perovskites are potentially suitable for propane oxidation, most of the work on perovskites in catalytic combustion concerns mainly methane [15]. Similarly, kinetic studies of catalytic combustion have over the last decades been focused on methane combustion. With the exception of a recent work concerning kinetics of ethane and propane combustion over supported palladium catalyst [9], reliable data suitable for reactor design are rarely available in literature. Our previous work has shown that La0.66 Sr0.34 Ni0.3 Co0.7 O3 (LSNC) perovskite is an excellent catalyst not only for oxygen reduction and evolution in alkaline media [17], but also in catalytic combustion of methane [18,19]. The exceptionally high catalytic activity in oxygen involving reactions of this perovskite composition is most likely related to the high electronic and ionic conductivity [20] and facile sorption–desorption of oxygen [21]. This catalyst also exhibits a reasonable resistance to poisoning by sulfur oxides [22], and can be employed in a variety of forms [23,24], although, its use is limited to lower temperatures, i.e. to rather lean fuel/air mixtures. In this paper, results of a complete kinetic study of propane combustion over LSNC are presented. At low temperatures, the kinetics is best described by a Mars–van Krevelen type model, extended to include inhibition by both products.
2. Experimental 2.1. Catalyst preparation and characteristics The catalyst used in this study was prepared by a method based on an aqueous slurry of lanthanum
hydroxide in a solution of metal (strontium, nickel and cobalt) nitrates which was processed by freeze-drying. To form the perovskite phase, the highly homogeneous freeze-dried stoichiometric precursor mixture was calcined 12 h at 863 K and 5 h at 923 K. The formation of perovskite phase was confirmed by powder X-ray spectroscopy. Details of the preparation are described elsewhere [25]. Before use, the slightly agglomerated powder was dry-milled in a polyethylene bottle using zirconia cylinders to a particle size of less than 5 m. Specific surface area (SSA) of the powder was 15 m2 /g. It was determined by a single point BET method with 30% nitrogen in helium as an adsorbate, on a Micrometritics FlowSorb 2300 apparatus.
2.2. Catalytic performance Catalytic activity was studied at ambient pressure in a steady state using a U-shape plug-flow type reactor consisting of a 30 cm long stainless steel tube having 0.7 cm inner diameter. The volume of the reaction zone (catalytic bed) was 7 ml [18,19]. The reactor was heated in a cylindrical electrical furnace with forced air circulation to assure a uniform temperature, in intervals of 25◦ C. The temperature of the catalytic bed was monitored by two thermocouples inserted in the reactor and touching the front and end of the catalytic bed. To keep the catalytic bed quasi isothermal, the catalyst (between 0.25 and 1 g) was homogeneously mixed with 7 ml of inert pumice particles (300–500 m) having SSA less than 1 m2 /g after calcination 10 h at 1000 K. The required propane concentrations between 0.58 and 4.76% were obtained by mixing pure propane with air or with nitrogen and oxygen at corresponding flowrates controlled by mass-flowmeters. Total flowrates were varied between 100 and 400 ml/min. When needed for determination of its inhibition effect, water concentration in the feed was fixed by passing the air through a saturator kept at a required temperature. The catalyst activity was monitored as propane conversion to carbon dioxide at several steady temperatures between 473 and 613 K. Analysis of the reaction mixture, striped of water by passing it over a desiccant, was made by gas chromatography.
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
3. Results and discussion
115
To confirm the inert character of pumice, initial tests were run with 2% propane in air flowing over 7 ml of pumice at 200 ml/min. No measurable conversion (to carbon dioxide) was observed up to 570 K. At 633 K, the conversion was less than 2%, reaching 3% at 673 K. When 0.25 g of LSNC catalyst was admixed with 7 ml pumice, propane oxidation was detected starting at 475 K, the conversion reaching about 10% at 540 K. Thus, the very weak catalytic effect of pumice can be neglected, similarly as was found in the case of methane [19]. Independent calculations carried out before experiments [26] have indicated that the reaction was free of external and internal heat and mass transport limitations. Lack of external mass transfer limitation was confirmed experimentally by using different amount of catalyst (0.25, 0.5 and 1 g), but keeping the contact time constant by selecting appropriate flowrate of 2% propane in air. Basically the same results were obtained for the three experiments as shown in Fig. 1, representing propane conversions as a function of temperature. In view of the small size of catalyst particles (<5 m) internal mass transfer limitation would not be anticipated. Fig. 1 is also a good example of the catalytic activity of LSNC perovskite. As expected, in comparison
with methane, this conversion — temperature curve is shifted, for the comparable reaction conditions, by about 200◦ C to lower temperatures illustrating much easier oxidation of propane. Furthermore, similarly to the case of methane, no carbon containing product of oxidation other than carbon dioxide has been detected even when higher propane concentrations were used. The presence of only one product, namely carbon dioxide, was further confirmed by carbon balance on propane and carbon dioxide which was better than 100 ± 1% for the lower propane concentrations and within 100 ± 3% for the high propane concentrations. This very important characteristic which has practical implications for potential use of LSNC perovskite contrasts with Mn3 O4 , over which, in spite of its high SSA = 24 m2 /g, significant quantities of partial propane oxidation by-products such as propene and traces of propanol were reported [12]. While the relatively easy activation of propane may, in the initial step, be assumed to proceed through the interaction of the weaker secondary C–H bond with the surface oxygen [12], subsequent oxidation steps are apparently faster on LSNC and do not involve the desorption of intermediates. This possibly reflects higher availability of active oxygen species on perovskites as compared to Mn3 O4 . Although, no direct quantitative comparison of the activity of LSNC with that of other materials cited in literature can be made, the conversion data in Fig. 1
Fig. 1. Conversion of propane in air as a function of temperature at different experimental conditions.
Fig. 2. Conversion of propane in air as a function of time, 0.5 g catalyst, 1% propane in air, 100 ml/min, 653 K.
3.1. Activity of LSNC perovskite
116
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
suggest an excellent activity, approaching, within an order of magnitude, that of the best available materials such as high SSA Co3 O4 , MgCr2 O4 or platinum. For example, in a mixture of 2% propane and 10% oxygen in helium passing at 100 ml/min over 0.1 g MgCr2 O4 (SSA = 53 m2 /g) full conversion of propane was observed only at 720 K [9]. Over 0.05 g 1% Pt/Al2 O3, in a gas mixture of 0.4% propane and 4% oxygen in helium, flowing at 180 ml/min, Burch et al. observed a complete propane oxidation at 723 K [7]. The excellent activity of LSNC will become more
apparent from the rates (rate constants) of combustion discussed below. Before starting the kinetic study, stability of the catalyst activity (0.5 g) was evaluated at high conversion of 1% propane (100 ml/min, 653 K) by a continuous operation of the reactor for about four days, while periodically monitoring the conversion. Although decrease in conversion was observed over the initial 10–15 h, the activity had stabilized within about 24 h and no further change was observed over 60 additional hours as shown in Fig. 2. Such
Table 1 Kinetic models tested in fitting the propane combustion Model
Reaction rate equation
Regression for: xcal = a·xexp b
Rate parametersa
1
r = k 1 PP
2
r = k2 PP0.5 PO0.3
3
r=
r=
4
5
r=
6
r=
k3 KO3 PP PO 1 + KO3 PO
k4 KO4 PP PO0.5 1 + KO4 PO 0.5
k 5 K P 5 K O 5 PP PO 1 + KP5 PP + KO5 PO k6 KP6 KO6 PP PO0.5 1 + KP6 PP + KO6 PO0.5
−8710 T −8388 k2 = 11.3 exp T −9195 k3 = 699 exp T 1677 KO3 = 0.287 exp T −10215 k4 = 74.4 exp T 5820 KO4 = 3.15 × 10−5 exp T k1 = 118 exp
Negative parameter k6 = 67.8 exp
−7335 T
KP6 = 14.2 exp r=
k7P k7O PP PO 5k7P PP + k7O PO
8 r= a b
k8P k8O PP PO0.5 5k8P PP + k8O PO0.5
1225 T
2246 T
0.903
0.833
1.009
0.989
–
–
1.164
0.915
–
–
1.070
0.920
–
–
–
–
1.022
0.988
–
–
–
0.967
1.092
–
1.001
–
–
0.988
−8763 T −8078 k7O = 27.3 exp T −8768 k8P = 352 exp T −7934 k8O = 6.82 exp T k7P = 284 exp
R2
KO6 = 2.67 × 10−2 exp
7
ab
k1 , k3 , k4 , k6 , k7P , k7O , k8P : mol/(g s bar); k2 : mol/(g s bar0.8 ); k8O : mol/(g s bar0.5 ); KO3 , KP6 : bar−1 ; KO4 , KO6 : bar−0.5 . Calculated (xcal ) versus experimental (xexp ) conversion; a: slope of the regression line; R2 : sum of the squares.
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
117
decrease in activity is commonly observed with many laboratory and commercial catalysts. Data for kinetic analysis were collected after this period. The activity remained stable over the entire study lasting several weeks with intermittent periods without operation. 3.2. Kinetic study, search for the best kinetic model To obtain a reliable kinetic equation representing a wide range of conditions, conversions were determined at five different temperatures for a wide range of conditions mentioned before. At each temperature 21 different data points were obtained. To find the most suitable rate model, all the collected data were used. Several kinetic models listed in Table 1 were tested. Integrated forms of individual models were used in the fitting procedure. Note that in Table 1, we express the kinetic constants as mol/(g s bar) (or mol/(g s bar0.5 )), whereas elsewhere they are given as mol/(g s bar) (or mol/(g s bar0.5 )). We prefer to use specific rate constants (per gram catalyst), rather than areal rate constants (per m2 ). Although specific surface area plays an important role in the activity, in the case of oxides, the real value of areal rates is questionable. Apparent activity is directly proportional to SSA only up to about 10 m2 /g [18,25,27]. Knowing the SSA (15 m2 /g), specific rate constants can be easily converted to those based on SSA. As a first step, the data were analyzed by using a simple first-order model. Integrated first-order kinetic parameters are plotted in Fig. 3. Although for each set of individual experimental conditions an excellent fit was obtained, with apparent activation energy, Eapp, of 71 ± 1 kJ/mol, it is clear that this model is not sufficiently representative. Nevertheless, both the values of k1 and of the Eapp agree very well with those determined previously under single experimental condition for an iron doped (2 at.%) LSNC [25]. From the dependence of calculated (integrated) values of k1 on propane and oxygen concentration, the apparent reaction order of both reactants was found fractional, specifically that of propane being 0.5, whereas that of oxygen being around 0.25. Correspondingly, two power law rate models r = k2 PPm POn with m = 0.5 and n = 0.25 and 0.3 were tested. Both provided adequate results, but the latter (n = 0.3) resulted in a slightly better fit. The Arrhenius plot for data of this model, for which the integrated values of k2 were
Fig. 3. Arrhenius plot of integrated first-order kinetic parameters k1 for several experimental conditions.
obtained numerically, is presented in Fig. 4. The resulting apparent activation energy was the same as that for the pseudo-first-order model, i.e. 71 kJ/mol. This value is rather low, but falls in the range of usual activation energies reported for complete hydrocarbon combustion. It agrees with the one reported for propane combustion over platinum [1,7], but it is at least 30 kJ/mol lower than the value reported for
Fig. 4. Arrhenius plot of the integrated values of kinetic parameters k2 (model 2 in Table 1) of the power law model for several experimental conditions, m = 0.5, n = 0.3.
118
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
Co3 O4 or NiO or for other transition metal oxides [1]. While the deduced reaction order in oxygen agrees with that obtained by Moro-oka et al. [1] for Co3 O4 and for other transition metal oxides for which values between 0.16 and 0.4 were reported, the order in propane is rather low in comparison with that reported for Co3 O4 (0.94) or NiO (0.89). Although, the power law model is only mathematical and provides no direct insight into the mechanism of the reaction, because of its simplicity it is convenient and for engineering purposes often yields useful results. While real surfaces, especially those of metal oxides, invariably depart from the ideal Langmuir adsorption model, classical Langmuir–Hinshelwood kinetics is widely used to describe the rates of catalytic reactions, including those of oxidation and has been successfully used, in the form of the Eley–Rideal model, for catalytic combustion of methane over perovskites [28,29]. The two Eley–Rideal type models tested in this work (models 3 and 4) provided a reasonable, but far from satisfactory fit. Of the two Langmuir–Hinshelwood type models tested (models 5 and 6), only the one which assumes a dissociative adsorption of oxygen yielded positive parameters and it provided a very good fit. However, although, this model yielded a reasonable value for the heat of propane adsorption, 10.1 kJ/mol, the deduced heat of oxygen adsorption, 18.7 kJ/mol, seems in view of expected oxygen chemisorption rather low, in spite of being in agreement with the recently published value of oxygen adsorption on LaMn0.8 Mg0.2 O3 [29]. We have, therefore, examined other reaction models. In the area of oxidation reactions, one of the highly successful and often preferred rate model is the so-called redox model proposed by Mars and van Krevelen [30]. This model is based on the assumption that the catalyst surface is successively reduced by the hydrocarbon and oxidized by oxygen at comparative rates. In other words, a given hydrocarbon reacts with a surface bound oxygen species at a given rate rh , which is subsequently replenished by surface oxidation, i.e. chemisorption of oxygen at a rate rO : r=
kh Ph kO POn (νkh Ph + kO POn )
where ν is a stoichiometry factor. When νkh P h k O POn , r becomes equal to kh Ph .
Fig. 5. Calculated versus experimental conversions x using the Mars–van Krevelen model (model 8 in Table 1).
Although the original model allows different orders in oxygen, we have considered the two that are mechanistically best suited: 0.5 and 1. Of the two Mars–van Krevelen models tested in fitting the data (models 7 and 8), the one with the 0.5 reaction order in oxygen gave a better fit. In fact, model 8 provided the best fit among all tested models and yielded reasonable values for the two involved kinetic parameters (Table 1). The apparent activation energies of the two kinetic constants kP (k8P ) and kO (k8O ) are 73 and 66 kJ/mol, respectively. The goodness of fit of this model is evident from Fig. 5, where conversions calculated using the derived parameters are plotted versus those determined experimentally. The fact that the model with half order in oxygen provided the best fit permits to assume that it is the atomic oxygen species which takes part in the reaction, possibly serving as an active center for propane oxidation. In addition, the dissociative surface adsorption of oxygen to form atomic species is slow comparatively to propane oxidation. Indeed, we have shown previously that from LSNC, at temperatures between 300 and 950 K at least two energetically different forms of oxygen desorb [21]. This is known for several other perovskites. One form, generally considered as an α-oxygen, desorbs from LSNC with the maximum at about 580 K, the other form at about 900 K [21]. In the case of methane combustion, which is considerably slower than that of propane, and typically
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
119
proceeds at measurable rates only above 600 K, i.e. kCH4 PCH4 kP PP , the rate of surface reoxidation to form active oxygen centers, as well as oxygen ion mobility is comparatively much faster, i.e. kCH4 PCH4 kO POn . Consequently, the rates of methane combustion over this catalyst could have been fitted satisfactorily by a simple pseudo-first-order model [19]. 3.3. Effect of combustion products In most of the literature on propane catalytic combustion, inhibition by the combustion products has been generally neglected. However, in a recent study of the propane combustion over alumina supported palladium, the authors found (experimentally) that added carbon dioxide had no effect, whereas added water significantly inhibited the reaction [10]. No prior information was available for transition metal based catalysts, although, inhibition of methane combustion by carbon dioxide has been known [31]. In our previous work on methane combustion over LSNC catalyst, which takes place at temperatures between 600 and 950 K, we have also found that carbon dioxide inhibits the reaction [23]. Nevertheless, in the case of toluene combustion over iron doped LSNC perovskite catalyst proceeding at temperatures between 523 and 660 K, the effect of carbon dioxide was negligible [32]. In view of these apparently conflicting observations, and in spite of an excellent fit obtained by kinetic model neglecting the inhibition of products, it was of interest to determine their effect experimentally. This was done by adding them separately or together to the feed containing different oxygen concentrations. The partial pressures of water varied between 1.23 and 3.36 vol.%, while CO2 was varied between 0.53 and 6.4 vol.%. An example of the effect of individual products and when combined is shown in Fig. 6. As can be seen, both products have an inhibiting effect on the activity. This inhibition, when expressed as a proportional loss of activity, i.e.X(CO2 ,or H2 O) /X(original) decreases in the studied range with temperature. Upon removing carbon dioxide or water from the feed, the original activity was always restored within a few minutes. It can also be noted that this inhibition seems to be a function of oxygen concentration, suggesting that the adsorption of either product takes place via some adsorbed oxygen species. Thus, to take the observed inhibition into account, an extended
Fig. 6. Effect of water and of carbon dioxide added to the feed on propane conversions. 0.5 g LSNC, 2 vol.% propane in air; with 6.5 vol.% CO2 and/or 3.36 vol.% H2 O, 100 ml/min.
Mars–van Krevelen model such as suggested by Golodets [31] has been used to fit the data and to derive the corresponding parameters. Arrhenius plot of these parameters is shown in Fig. 7 and the derived kinetic parameters are listed in Table 2. Alternative rate expressions did not produce satisfactory fit. As can be seen in Fig. 7, the effect of water inhibition is slightly stronger. It is also apparent that the logarithm of the
Fig. 7. Arrhenius plot of kinetic parameters of the extended Mars–van Krevelen model.
120
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
Table 2 Parameters for the final kinetic modela Parameter
Eapp (kJ/mol)
ln A
−H (kJ/mol)
S (J/mol K)
Kp kO KH 2 O
77 66 –
20.88 mol/(g s bar) 15.68 mol/(g s bar0.5 ) –
KCO2
–
–
– – 52 44b 39 31b
– – 56 40b 40 24b
a b
r=
kP kO PP PO0.5 5kP PP +kO PO0.5 (1+KH2 O PH2 O +KCO2 PCO2 )
.
Values based on the regression of data between 473 and 573 K (0.00211 and 0.00175 K−1 ).
two derived adsorption constants does not follow a strictly linear dependence on 1/T over the whole temperature range. Above 573 K, both KH2 O and KCO2 seem to decrease considerably faster with temperature. This could possibly be explained by a changing mechanism in the adsorption. In fact, at higher temperatures, the inhibition by water is expected to become negligible. By restricting the regression to a lower temperature region (473–573 K), the deduced values of apparent heats of adsorption and of corresponding entropies are lower (Table 2). In the case of methane combustion, effect of water was not evaluated directly, but water was found to slightly diminish the poisoning effect by mercaptan [22]. The inhibition of carbon dioxide observed in this work contrasts the inhibition by the activated adsorption occurring at higher temperature [22,24]. In view of the various modes of carbon dioxide, such a behavior it is not surprising [33].
4. Conclusions La0.66 Sr0.34 Ni0.3 Co0.7 O3 perovskite is an excellent catalyst for propane catalytic combustion. Based on the analysis of complete kinetic data collected at low temperatures, the reaction rate, which is slightly inhibited by relatively strong adsorption of both products, water and carbon dioxide, is best described by an extended Mars–van Krevelen model: r=
kP PP kO PO0.5 (5kP PP + kO PO0.5 (1 + KCO2 PCO2 + KH2 O PH2 O ))
This suggests that dissociatively adsorbed oxygen takes part in the reaction and that at the lower tem-
peratures, surface re-oxidation, or regeneration of the active oxygen sites is slow in comparison with the reaction of propane. In addition, the water and carbon dioxide are adsorbed via an atomic oxygen species.
Acknowledgements This work has been made possible by research grant from Natural Sciences and Engineering Research Council of Canada and by a postdoctoral fellowship from Korea Science and Engineering Foundation to one of the authors, Kwang Sup Song. References [1] Y. Moro-oka, Y. Morikawa, A. Ozaki, J. Catal. 7 (1967) 23. [2] R. Prasad, L.A. Kennedy, E. Ruckenstein, Combust. Sci. Technol. 22 (1980) 271. [3] T. Nitadori, M. Misono, J. Catal. 93 (1985) 459. [4] T. Nitadori, S. Kurihara, M. Misono, J. Catal. 98 (1986) 221. [5] K. Otto, J.M. Andino, C.L. Parks, J. Catal. 131 (1991) 243. [6] C.P. Hubbard, K. Otto, H.S. Ganghi, K.Y.S. Ng, J. Catal. 139 (1993) 268. [7] R. Burch, E. Halpin, M. Hayes, K. Ruth, J.A. Sullivan, Appl. Catal. B: Environ. 19 (1998) 199. [8] E. Finocchio, G. Busca, V. Lorenzelli, R.J. Wiley, J. Chem. Soc., Faraday Trans. 90 (21) (1994) 3347. [9] E. Finocchio, G. Busca, V. Lorenzelli, R.J. Wiley, J. Catal. 151 (1995) 204. [10] L. van de Beld, M.C. van der Ven, K.R. Westerterp, Chem. Eng. Processing 34 (1995) 469. [11] Y. Yazawa, H. Yoshida, N. Takagi, S.-I. Komai, A. Satsuma, T. Hattoir, Appl. Catal. B: Environ. 19 (1998) 261. [12] M. Baldi, E. Finocchio, F. Milella, G. Baldi, Appl. Catal. B: Environ. 16 (1998) 43. [13] G. Busca, E. Finocchio, V. Lorenzelli, G. Ramis, M. Baldi, Catal. Today 49 (1999) 453.
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121 [14] M. Baldi, V.S. Esribano, J.M.G. Amores, F. Milella, G. Busca, Appl. Catal. B: Environ. 17 (1998) L175. [15] T. Seiyama, in: R.J. Tejuca, J. L.G. Fierro (Eds.), Properties and Applications of Perovskite-Type Oxides, Chem. Ind. 50 (1993). [16] V.R. Choudhary, V.H. Rane, J. Catal. 130 (1991) 411. [17] J. Kirchnerova, D. Klvana, Int. J. Hydrogen Energy 19 (1994) 501. [18] J. Kirchnerova, J. Vaillancourt, D. Klvana, J. Chaouki, Catal. Lett. 21 (1993) 77. [19] D. Klvana, J. Vaillancourt, J. Kirchnerova, J. Chaouki, Appl. Catal. A: Gen. 109 (1994) 181. [20] K. Huang, H.Y. Lee, J.B. Goodenough, J. Electrochem. Soc. 145 (1998) 3220. [21] J.-P. Joly, D. Klvana, J. Kirchnerova, React. Kinet. Catal. Lett. 68 (2) (1999) 249. [22] D. Klvana, J. Delval, J. Kirchnerova, J. Chaouki, Appl. Catal. A: Gen. 165 (1997) 171. [23] D. Klvana, J. Kirchnerova, P. Gauthier, J. Delval, J. Chaouki, Can. J. Chem. Eng. 75 (1997) 509.
121
[24] D. Klvana, J. Kirchnerova, J. Chaouki, J. Delval, W. Yaici, Catal. Today 47 (1999) 115. [25] J. Kirchnerova, D. Klvana, Solid State Ionics 123 (1999) 307. [26] H.S. Fogler, Elements of Chemical Reaction Engineering, Prentice-Hall, Englewood Cliffs, NJ, 1991. [27] J. Kirchnerova, D. Klvana, Catal. Lett. 67 (2000) 175. [28] H. Arai, T. Yamada, K. Eguchi, T. Seiyama, Appl. Catal. 26 (1986) 265. [29] G. Saracco, F. Geobaldo, G. Baldi, Appl. Catal. B: Environ. 20 (1999) 277. [30] P. Mars, D.W. van Krevelen, Chem. Eng. Sci. 3 (Special Suppl.) (1954) 41. [31] G.I. Golodets, Heterogeneous Catalytic Reactions Involving Molecular Oxygen, Stud. Surf. Sci. Catal., Vol. 15, Elsevier, Amsterdam, 1983, p. 437. [32] D. Klvana, S. Vantomme, J. Kirchnerova, J. Chaouki, S. Vigneron, Odours VOC’s J. (1997) 510. [33] J.M.D. Tascón, L.G. Tejuca, J. Chem. Soc., Faraday Trans. 77 (1981) 591.
Kinetics of propane combustion over La0.66Sr0.34 Ni0.3Co0.7O3 perovskite Kwang Sup Song1 , Danilo Klvana∗ , Jitka Kirchnerova Department of Chemical Engineering, Ecole Polytechnique, P.O. Box 6079, Station Center-ville, Montreal, Canada QC H3C 3A7 Received 18 August 2000; received in revised form 17 November 2000; accepted 26 November 2000
Abstract La0.66 Sr0.34 Ni0.3 Co0.7 O3 (LSNC) perovskite is an excellent catalyst in several oxygen involving reactions, including methane combustion. Catalytic combustion of propane based on efficient technologies using low cost catalysts could serve as a non-polluting source of energy in remote regions. To optimize reactors for catalytic combustion (combusters), reliable kinetic data are needed. In this paper are presented kinetics of a complete propane oxidation over LSNC studied at a steady state in a plug-flow reactor. The experimental data were obtained for 0.5 g catalyst at temperatures between 473 and 613 K, with propane concentration varied from 0.58 to 4.76 vol.% oxygen between 7.5 and 98 vol.%, and flowrate between 100 and 400 ml/min. The LSNC catalyst, prepared with high specific surface area of 15 m2 /g, shows again an excellent stable activity which competes favorably with that of noble metals. Several kinetic models have been tested. Best fit was obtained with the Mars–van Krevelen kinetic model. Nevertheless, the complete set of obtained data can also be fitted adequately by a simple power law model with 0.5 order in propane and 0.3 order in oxygen and apparent activation energy of 71 kJ/mol. Both water and carbon dioxide added to the feed have slight inhibiting effect, which was taken into account in a final extended Mars–van Krevelen kinetic model. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Propane catalytic combustion; Kinetics of propane catalytic combustion; Perovskite catalyst; La0.66 Sr0.34 Ni0.3 Co0.7 O3 perovskite; Combustion catalysts
1. Introduction Catalytic combustion of fossil fuels, in particular of light hydrocarbons, represents one of the least polluting and economical means of heat and energy generation. For remote areas where natural gas is not readily available, propane is an attractive alternative.
∗ Corresponding author. Tel.: +514-340-4711/4927; fax: +514-340-4159. E-mail address: [email protected] (D. Klvana). 1 On leave from Korea Institute of Energy Research, P.O. Box 5, Taedok Science Town, Taejon 305-343, Korea.
However, development of efficient technologies using low cost catalysts is needed. Catalytic combustion (total oxidation) of light hydrocarbons, specifically of propane, has been investigated for decades both as part of fundamental and development studies [1–14]. These studies have covered a wide range of materials including noble metals such as platinum and palladium [5–7,10,11], different transition metal oxides, transition metal containing spinels and perovskites [1–4,8]. Among these catalysts, platinum is known as the most active [1,5], its activity exceeding that of palladium. This contrasts with the catalytic combustion of methane which is much less reactive than other light hydrocarbons and
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 8 8 4 - X
114
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
for which the most active catalyst is palladium in the form of an oxide. Because of their easier activation, light hydrocarbons can be readily oxidized over a much wider range of transition metal based oxides. However, only a few of the single metal oxides exhibit very high activity and selectivity to the complete oxidation. In comparison with simple transition metal oxides and even with spinels, perovskites offer the advantage of higher thermal, chemical and structural stability and in many cases improved catalytic activity [15]. Furthermore, perovskites also exhibit exceptionally high selectivity to complete oxidation [16]. Although a number of transition metal based perovskites are potentially suitable for propane oxidation, most of the work on perovskites in catalytic combustion concerns mainly methane [15]. Similarly, kinetic studies of catalytic combustion have over the last decades been focused on methane combustion. With the exception of a recent work concerning kinetics of ethane and propane combustion over supported palladium catalyst [9], reliable data suitable for reactor design are rarely available in literature. Our previous work has shown that La0.66 Sr0.34 Ni0.3 Co0.7 O3 (LSNC) perovskite is an excellent catalyst not only for oxygen reduction and evolution in alkaline media [17], but also in catalytic combustion of methane [18,19]. The exceptionally high catalytic activity in oxygen involving reactions of this perovskite composition is most likely related to the high electronic and ionic conductivity [20] and facile sorption–desorption of oxygen [21]. This catalyst also exhibits a reasonable resistance to poisoning by sulfur oxides [22], and can be employed in a variety of forms [23,24], although, its use is limited to lower temperatures, i.e. to rather lean fuel/air mixtures. In this paper, results of a complete kinetic study of propane combustion over LSNC are presented. At low temperatures, the kinetics is best described by a Mars–van Krevelen type model, extended to include inhibition by both products.
2. Experimental 2.1. Catalyst preparation and characteristics The catalyst used in this study was prepared by a method based on an aqueous slurry of lanthanum
hydroxide in a solution of metal (strontium, nickel and cobalt) nitrates which was processed by freeze-drying. To form the perovskite phase, the highly homogeneous freeze-dried stoichiometric precursor mixture was calcined 12 h at 863 K and 5 h at 923 K. The formation of perovskite phase was confirmed by powder X-ray spectroscopy. Details of the preparation are described elsewhere [25]. Before use, the slightly agglomerated powder was dry-milled in a polyethylene bottle using zirconia cylinders to a particle size of less than 5 m. Specific surface area (SSA) of the powder was 15 m2 /g. It was determined by a single point BET method with 30% nitrogen in helium as an adsorbate, on a Micrometritics FlowSorb 2300 apparatus.
2.2. Catalytic performance Catalytic activity was studied at ambient pressure in a steady state using a U-shape plug-flow type reactor consisting of a 30 cm long stainless steel tube having 0.7 cm inner diameter. The volume of the reaction zone (catalytic bed) was 7 ml [18,19]. The reactor was heated in a cylindrical electrical furnace with forced air circulation to assure a uniform temperature, in intervals of 25◦ C. The temperature of the catalytic bed was monitored by two thermocouples inserted in the reactor and touching the front and end of the catalytic bed. To keep the catalytic bed quasi isothermal, the catalyst (between 0.25 and 1 g) was homogeneously mixed with 7 ml of inert pumice particles (300–500 m) having SSA less than 1 m2 /g after calcination 10 h at 1000 K. The required propane concentrations between 0.58 and 4.76% were obtained by mixing pure propane with air or with nitrogen and oxygen at corresponding flowrates controlled by mass-flowmeters. Total flowrates were varied between 100 and 400 ml/min. When needed for determination of its inhibition effect, water concentration in the feed was fixed by passing the air through a saturator kept at a required temperature. The catalyst activity was monitored as propane conversion to carbon dioxide at several steady temperatures between 473 and 613 K. Analysis of the reaction mixture, striped of water by passing it over a desiccant, was made by gas chromatography.
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
3. Results and discussion
115
To confirm the inert character of pumice, initial tests were run with 2% propane in air flowing over 7 ml of pumice at 200 ml/min. No measurable conversion (to carbon dioxide) was observed up to 570 K. At 633 K, the conversion was less than 2%, reaching 3% at 673 K. When 0.25 g of LSNC catalyst was admixed with 7 ml pumice, propane oxidation was detected starting at 475 K, the conversion reaching about 10% at 540 K. Thus, the very weak catalytic effect of pumice can be neglected, similarly as was found in the case of methane [19]. Independent calculations carried out before experiments [26] have indicated that the reaction was free of external and internal heat and mass transport limitations. Lack of external mass transfer limitation was confirmed experimentally by using different amount of catalyst (0.25, 0.5 and 1 g), but keeping the contact time constant by selecting appropriate flowrate of 2% propane in air. Basically the same results were obtained for the three experiments as shown in Fig. 1, representing propane conversions as a function of temperature. In view of the small size of catalyst particles (<5 m) internal mass transfer limitation would not be anticipated. Fig. 1 is also a good example of the catalytic activity of LSNC perovskite. As expected, in comparison
with methane, this conversion — temperature curve is shifted, for the comparable reaction conditions, by about 200◦ C to lower temperatures illustrating much easier oxidation of propane. Furthermore, similarly to the case of methane, no carbon containing product of oxidation other than carbon dioxide has been detected even when higher propane concentrations were used. The presence of only one product, namely carbon dioxide, was further confirmed by carbon balance on propane and carbon dioxide which was better than 100 ± 1% for the lower propane concentrations and within 100 ± 3% for the high propane concentrations. This very important characteristic which has practical implications for potential use of LSNC perovskite contrasts with Mn3 O4 , over which, in spite of its high SSA = 24 m2 /g, significant quantities of partial propane oxidation by-products such as propene and traces of propanol were reported [12]. While the relatively easy activation of propane may, in the initial step, be assumed to proceed through the interaction of the weaker secondary C–H bond with the surface oxygen [12], subsequent oxidation steps are apparently faster on LSNC and do not involve the desorption of intermediates. This possibly reflects higher availability of active oxygen species on perovskites as compared to Mn3 O4 . Although, no direct quantitative comparison of the activity of LSNC with that of other materials cited in literature can be made, the conversion data in Fig. 1
Fig. 1. Conversion of propane in air as a function of temperature at different experimental conditions.
Fig. 2. Conversion of propane in air as a function of time, 0.5 g catalyst, 1% propane in air, 100 ml/min, 653 K.
3.1. Activity of LSNC perovskite
116
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
suggest an excellent activity, approaching, within an order of magnitude, that of the best available materials such as high SSA Co3 O4 , MgCr2 O4 or platinum. For example, in a mixture of 2% propane and 10% oxygen in helium passing at 100 ml/min over 0.1 g MgCr2 O4 (SSA = 53 m2 /g) full conversion of propane was observed only at 720 K [9]. Over 0.05 g 1% Pt/Al2 O3, in a gas mixture of 0.4% propane and 4% oxygen in helium, flowing at 180 ml/min, Burch et al. observed a complete propane oxidation at 723 K [7]. The excellent activity of LSNC will become more
apparent from the rates (rate constants) of combustion discussed below. Before starting the kinetic study, stability of the catalyst activity (0.5 g) was evaluated at high conversion of 1% propane (100 ml/min, 653 K) by a continuous operation of the reactor for about four days, while periodically monitoring the conversion. Although decrease in conversion was observed over the initial 10–15 h, the activity had stabilized within about 24 h and no further change was observed over 60 additional hours as shown in Fig. 2. Such
Table 1 Kinetic models tested in fitting the propane combustion Model
Reaction rate equation
Regression for: xcal = a·xexp b
Rate parametersa
1
r = k 1 PP
2
r = k2 PP0.5 PO0.3
3
r=
r=
4
5
r=
6
r=
k3 KO3 PP PO 1 + KO3 PO
k4 KO4 PP PO0.5 1 + KO4 PO 0.5
k 5 K P 5 K O 5 PP PO 1 + KP5 PP + KO5 PO k6 KP6 KO6 PP PO0.5 1 + KP6 PP + KO6 PO0.5
−8710 T −8388 k2 = 11.3 exp T −9195 k3 = 699 exp T 1677 KO3 = 0.287 exp T −10215 k4 = 74.4 exp T 5820 KO4 = 3.15 × 10−5 exp T k1 = 118 exp
Negative parameter k6 = 67.8 exp
−7335 T
KP6 = 14.2 exp r=
k7P k7O PP PO 5k7P PP + k7O PO
8 r= a b
k8P k8O PP PO0.5 5k8P PP + k8O PO0.5
1225 T
2246 T
0.903
0.833
1.009
0.989
–
–
1.164
0.915
–
–
1.070
0.920
–
–
–
–
1.022
0.988
–
–
–
0.967
1.092
–
1.001
–
–
0.988
−8763 T −8078 k7O = 27.3 exp T −8768 k8P = 352 exp T −7934 k8O = 6.82 exp T k7P = 284 exp
R2
KO6 = 2.67 × 10−2 exp
7
ab
k1 , k3 , k4 , k6 , k7P , k7O , k8P : mol/(g s bar); k2 : mol/(g s bar0.8 ); k8O : mol/(g s bar0.5 ); KO3 , KP6 : bar−1 ; KO4 , KO6 : bar−0.5 . Calculated (xcal ) versus experimental (xexp ) conversion; a: slope of the regression line; R2 : sum of the squares.
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
117
decrease in activity is commonly observed with many laboratory and commercial catalysts. Data for kinetic analysis were collected after this period. The activity remained stable over the entire study lasting several weeks with intermittent periods without operation. 3.2. Kinetic study, search for the best kinetic model To obtain a reliable kinetic equation representing a wide range of conditions, conversions were determined at five different temperatures for a wide range of conditions mentioned before. At each temperature 21 different data points were obtained. To find the most suitable rate model, all the collected data were used. Several kinetic models listed in Table 1 were tested. Integrated forms of individual models were used in the fitting procedure. Note that in Table 1, we express the kinetic constants as mol/(g s bar) (or mol/(g s bar0.5 )), whereas elsewhere they are given as mol/(g s bar) (or mol/(g s bar0.5 )). We prefer to use specific rate constants (per gram catalyst), rather than areal rate constants (per m2 ). Although specific surface area plays an important role in the activity, in the case of oxides, the real value of areal rates is questionable. Apparent activity is directly proportional to SSA only up to about 10 m2 /g [18,25,27]. Knowing the SSA (15 m2 /g), specific rate constants can be easily converted to those based on SSA. As a first step, the data were analyzed by using a simple first-order model. Integrated first-order kinetic parameters are plotted in Fig. 3. Although for each set of individual experimental conditions an excellent fit was obtained, with apparent activation energy, Eapp, of 71 ± 1 kJ/mol, it is clear that this model is not sufficiently representative. Nevertheless, both the values of k1 and of the Eapp agree very well with those determined previously under single experimental condition for an iron doped (2 at.%) LSNC [25]. From the dependence of calculated (integrated) values of k1 on propane and oxygen concentration, the apparent reaction order of both reactants was found fractional, specifically that of propane being 0.5, whereas that of oxygen being around 0.25. Correspondingly, two power law rate models r = k2 PPm POn with m = 0.5 and n = 0.25 and 0.3 were tested. Both provided adequate results, but the latter (n = 0.3) resulted in a slightly better fit. The Arrhenius plot for data of this model, for which the integrated values of k2 were
Fig. 3. Arrhenius plot of integrated first-order kinetic parameters k1 for several experimental conditions.
obtained numerically, is presented in Fig. 4. The resulting apparent activation energy was the same as that for the pseudo-first-order model, i.e. 71 kJ/mol. This value is rather low, but falls in the range of usual activation energies reported for complete hydrocarbon combustion. It agrees with the one reported for propane combustion over platinum [1,7], but it is at least 30 kJ/mol lower than the value reported for
Fig. 4. Arrhenius plot of the integrated values of kinetic parameters k2 (model 2 in Table 1) of the power law model for several experimental conditions, m = 0.5, n = 0.3.
118
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
Co3 O4 or NiO or for other transition metal oxides [1]. While the deduced reaction order in oxygen agrees with that obtained by Moro-oka et al. [1] for Co3 O4 and for other transition metal oxides for which values between 0.16 and 0.4 were reported, the order in propane is rather low in comparison with that reported for Co3 O4 (0.94) or NiO (0.89). Although, the power law model is only mathematical and provides no direct insight into the mechanism of the reaction, because of its simplicity it is convenient and for engineering purposes often yields useful results. While real surfaces, especially those of metal oxides, invariably depart from the ideal Langmuir adsorption model, classical Langmuir–Hinshelwood kinetics is widely used to describe the rates of catalytic reactions, including those of oxidation and has been successfully used, in the form of the Eley–Rideal model, for catalytic combustion of methane over perovskites [28,29]. The two Eley–Rideal type models tested in this work (models 3 and 4) provided a reasonable, but far from satisfactory fit. Of the two Langmuir–Hinshelwood type models tested (models 5 and 6), only the one which assumes a dissociative adsorption of oxygen yielded positive parameters and it provided a very good fit. However, although, this model yielded a reasonable value for the heat of propane adsorption, 10.1 kJ/mol, the deduced heat of oxygen adsorption, 18.7 kJ/mol, seems in view of expected oxygen chemisorption rather low, in spite of being in agreement with the recently published value of oxygen adsorption on LaMn0.8 Mg0.2 O3 [29]. We have, therefore, examined other reaction models. In the area of oxidation reactions, one of the highly successful and often preferred rate model is the so-called redox model proposed by Mars and van Krevelen [30]. This model is based on the assumption that the catalyst surface is successively reduced by the hydrocarbon and oxidized by oxygen at comparative rates. In other words, a given hydrocarbon reacts with a surface bound oxygen species at a given rate rh , which is subsequently replenished by surface oxidation, i.e. chemisorption of oxygen at a rate rO : r=
kh Ph kO POn (νkh Ph + kO POn )
where ν is a stoichiometry factor. When νkh P h k O POn , r becomes equal to kh Ph .
Fig. 5. Calculated versus experimental conversions x using the Mars–van Krevelen model (model 8 in Table 1).
Although the original model allows different orders in oxygen, we have considered the two that are mechanistically best suited: 0.5 and 1. Of the two Mars–van Krevelen models tested in fitting the data (models 7 and 8), the one with the 0.5 reaction order in oxygen gave a better fit. In fact, model 8 provided the best fit among all tested models and yielded reasonable values for the two involved kinetic parameters (Table 1). The apparent activation energies of the two kinetic constants kP (k8P ) and kO (k8O ) are 73 and 66 kJ/mol, respectively. The goodness of fit of this model is evident from Fig. 5, where conversions calculated using the derived parameters are plotted versus those determined experimentally. The fact that the model with half order in oxygen provided the best fit permits to assume that it is the atomic oxygen species which takes part in the reaction, possibly serving as an active center for propane oxidation. In addition, the dissociative surface adsorption of oxygen to form atomic species is slow comparatively to propane oxidation. Indeed, we have shown previously that from LSNC, at temperatures between 300 and 950 K at least two energetically different forms of oxygen desorb [21]. This is known for several other perovskites. One form, generally considered as an α-oxygen, desorbs from LSNC with the maximum at about 580 K, the other form at about 900 K [21]. In the case of methane combustion, which is considerably slower than that of propane, and typically
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
119
proceeds at measurable rates only above 600 K, i.e. kCH4 PCH4 kP PP , the rate of surface reoxidation to form active oxygen centers, as well as oxygen ion mobility is comparatively much faster, i.e. kCH4 PCH4 kO POn . Consequently, the rates of methane combustion over this catalyst could have been fitted satisfactorily by a simple pseudo-first-order model [19]. 3.3. Effect of combustion products In most of the literature on propane catalytic combustion, inhibition by the combustion products has been generally neglected. However, in a recent study of the propane combustion over alumina supported palladium, the authors found (experimentally) that added carbon dioxide had no effect, whereas added water significantly inhibited the reaction [10]. No prior information was available for transition metal based catalysts, although, inhibition of methane combustion by carbon dioxide has been known [31]. In our previous work on methane combustion over LSNC catalyst, which takes place at temperatures between 600 and 950 K, we have also found that carbon dioxide inhibits the reaction [23]. Nevertheless, in the case of toluene combustion over iron doped LSNC perovskite catalyst proceeding at temperatures between 523 and 660 K, the effect of carbon dioxide was negligible [32]. In view of these apparently conflicting observations, and in spite of an excellent fit obtained by kinetic model neglecting the inhibition of products, it was of interest to determine their effect experimentally. This was done by adding them separately or together to the feed containing different oxygen concentrations. The partial pressures of water varied between 1.23 and 3.36 vol.%, while CO2 was varied between 0.53 and 6.4 vol.%. An example of the effect of individual products and when combined is shown in Fig. 6. As can be seen, both products have an inhibiting effect on the activity. This inhibition, when expressed as a proportional loss of activity, i.e.X(CO2 ,or H2 O) /X(original) decreases in the studied range with temperature. Upon removing carbon dioxide or water from the feed, the original activity was always restored within a few minutes. It can also be noted that this inhibition seems to be a function of oxygen concentration, suggesting that the adsorption of either product takes place via some adsorbed oxygen species. Thus, to take the observed inhibition into account, an extended
Fig. 6. Effect of water and of carbon dioxide added to the feed on propane conversions. 0.5 g LSNC, 2 vol.% propane in air; with 6.5 vol.% CO2 and/or 3.36 vol.% H2 O, 100 ml/min.
Mars–van Krevelen model such as suggested by Golodets [31] has been used to fit the data and to derive the corresponding parameters. Arrhenius plot of these parameters is shown in Fig. 7 and the derived kinetic parameters are listed in Table 2. Alternative rate expressions did not produce satisfactory fit. As can be seen in Fig. 7, the effect of water inhibition is slightly stronger. It is also apparent that the logarithm of the
Fig. 7. Arrhenius plot of kinetic parameters of the extended Mars–van Krevelen model.
120
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121
Table 2 Parameters for the final kinetic modela Parameter
Eapp (kJ/mol)
ln A
−H (kJ/mol)
S (J/mol K)
Kp kO KH 2 O
77 66 –
20.88 mol/(g s bar) 15.68 mol/(g s bar0.5 ) –
KCO2
–
–
– – 52 44b 39 31b
– – 56 40b 40 24b
a b
r=
kP kO PP PO0.5 5kP PP +kO PO0.5 (1+KH2 O PH2 O +KCO2 PCO2 )
.
Values based on the regression of data between 473 and 573 K (0.00211 and 0.00175 K−1 ).
two derived adsorption constants does not follow a strictly linear dependence on 1/T over the whole temperature range. Above 573 K, both KH2 O and KCO2 seem to decrease considerably faster with temperature. This could possibly be explained by a changing mechanism in the adsorption. In fact, at higher temperatures, the inhibition by water is expected to become negligible. By restricting the regression to a lower temperature region (473–573 K), the deduced values of apparent heats of adsorption and of corresponding entropies are lower (Table 2). In the case of methane combustion, effect of water was not evaluated directly, but water was found to slightly diminish the poisoning effect by mercaptan [22]. The inhibition of carbon dioxide observed in this work contrasts the inhibition by the activated adsorption occurring at higher temperature [22,24]. In view of the various modes of carbon dioxide, such a behavior it is not surprising [33].
4. Conclusions La0.66 Sr0.34 Ni0.3 Co0.7 O3 perovskite is an excellent catalyst for propane catalytic combustion. Based on the analysis of complete kinetic data collected at low temperatures, the reaction rate, which is slightly inhibited by relatively strong adsorption of both products, water and carbon dioxide, is best described by an extended Mars–van Krevelen model: r=
kP PP kO PO0.5 (5kP PP + kO PO0.5 (1 + KCO2 PCO2 + KH2 O PH2 O ))
This suggests that dissociatively adsorbed oxygen takes part in the reaction and that at the lower tem-
peratures, surface re-oxidation, or regeneration of the active oxygen sites is slow in comparison with the reaction of propane. In addition, the water and carbon dioxide are adsorbed via an atomic oxygen species.
Acknowledgements This work has been made possible by research grant from Natural Sciences and Engineering Research Council of Canada and by a postdoctoral fellowship from Korea Science and Engineering Foundation to one of the authors, Kwang Sup Song. References [1] Y. Moro-oka, Y. Morikawa, A. Ozaki, J. Catal. 7 (1967) 23. [2] R. Prasad, L.A. Kennedy, E. Ruckenstein, Combust. Sci. Technol. 22 (1980) 271. [3] T. Nitadori, M. Misono, J. Catal. 93 (1985) 459. [4] T. Nitadori, S. Kurihara, M. Misono, J. Catal. 98 (1986) 221. [5] K. Otto, J.M. Andino, C.L. Parks, J. Catal. 131 (1991) 243. [6] C.P. Hubbard, K. Otto, H.S. Ganghi, K.Y.S. Ng, J. Catal. 139 (1993) 268. [7] R. Burch, E. Halpin, M. Hayes, K. Ruth, J.A. Sullivan, Appl. Catal. B: Environ. 19 (1998) 199. [8] E. Finocchio, G. Busca, V. Lorenzelli, R.J. Wiley, J. Chem. Soc., Faraday Trans. 90 (21) (1994) 3347. [9] E. Finocchio, G. Busca, V. Lorenzelli, R.J. Wiley, J. Catal. 151 (1995) 204. [10] L. van de Beld, M.C. van der Ven, K.R. Westerterp, Chem. Eng. Processing 34 (1995) 469. [11] Y. Yazawa, H. Yoshida, N. Takagi, S.-I. Komai, A. Satsuma, T. Hattoir, Appl. Catal. B: Environ. 19 (1998) 261. [12] M. Baldi, E. Finocchio, F. Milella, G. Baldi, Appl. Catal. B: Environ. 16 (1998) 43. [13] G. Busca, E. Finocchio, V. Lorenzelli, G. Ramis, M. Baldi, Catal. Today 49 (1999) 453.
K.S. Song et al. / Applied Catalysis A: General 213 (2001) 113–121 [14] M. Baldi, V.S. Esribano, J.M.G. Amores, F. Milella, G. Busca, Appl. Catal. B: Environ. 17 (1998) L175. [15] T. Seiyama, in: R.J. Tejuca, J. L.G. Fierro (Eds.), Properties and Applications of Perovskite-Type Oxides, Chem. Ind. 50 (1993). [16] V.R. Choudhary, V.H. Rane, J. Catal. 130 (1991) 411. [17] J. Kirchnerova, D. Klvana, Int. J. Hydrogen Energy 19 (1994) 501. [18] J. Kirchnerova, J. Vaillancourt, D. Klvana, J. Chaouki, Catal. Lett. 21 (1993) 77. [19] D. Klvana, J. Vaillancourt, J. Kirchnerova, J. Chaouki, Appl. Catal. A: Gen. 109 (1994) 181. [20] K. Huang, H.Y. Lee, J.B. Goodenough, J. Electrochem. Soc. 145 (1998) 3220. [21] J.-P. Joly, D. Klvana, J. Kirchnerova, React. Kinet. Catal. Lett. 68 (2) (1999) 249. [22] D. Klvana, J. Delval, J. Kirchnerova, J. Chaouki, Appl. Catal. A: Gen. 165 (1997) 171. [23] D. Klvana, J. Kirchnerova, P. Gauthier, J. Delval, J. Chaouki, Can. J. Chem. Eng. 75 (1997) 509.
121
[24] D. Klvana, J. Kirchnerova, J. Chaouki, J. Delval, W. Yaici, Catal. Today 47 (1999) 115. [25] J. Kirchnerova, D. Klvana, Solid State Ionics 123 (1999) 307. [26] H.S. Fogler, Elements of Chemical Reaction Engineering, Prentice-Hall, Englewood Cliffs, NJ, 1991. [27] J. Kirchnerova, D. Klvana, Catal. Lett. 67 (2000) 175. [28] H. Arai, T. Yamada, K. Eguchi, T. Seiyama, Appl. Catal. 26 (1986) 265. [29] G. Saracco, F. Geobaldo, G. Baldi, Appl. Catal. B: Environ. 20 (1999) 277. [30] P. Mars, D.W. van Krevelen, Chem. Eng. Sci. 3 (Special Suppl.) (1954) 41. [31] G.I. Golodets, Heterogeneous Catalytic Reactions Involving Molecular Oxygen, Stud. Surf. Sci. Catal., Vol. 15, Elsevier, Amsterdam, 1983, p. 437. [32] D. Klvana, S. Vantomme, J. Kirchnerova, J. Chaouki, S. Vigneron, Odours VOC’s J. (1997) 510. [33] J.M.D. Tascón, L.G. Tejuca, J. Chem. Soc., Faraday Trans. 77 (1981) 591.