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Energy & Fuels 1998, 12, 870-874
Catalytic Activities of K2CO3 Supported on Several Oxides for Carbon Combustion Tatsuro Miyazaki,*,† Nobuyuki Tokubuchi,† Masahiro Inoue,† Masaaki Arita,† and I. Mochida‡ Device Research Laboratory, Kyushu Matushita Electric Co., Ltd., Minosima, Hakata-ku, Fukuoka 812, Japan, and Institute of Advanced Material Study, Kyushu University, Kasugakoen, Kasuga-shi, Fukuoka 816, Japan Received November 20, 1997
Catalytic activities of K2CO3 supported on several oxides and carbon itself were studied for the combustion of carbon at 673-773 K. K2CO3 supported on a perovskite oxide, La0.8Sr0.2Cr0.5Mn0.45Pt0.05O3 (LSCMP), exhibited higher activity than that supported on the other supports including carbon itself used in this study, the carbon conversion achieving nearly 100% above 723 K. K2CO3 supported on another perovskite oxide, LaMn0.8Cu0.2O3 (LMC), exhibited a high activity comparable to that of LSCMP, although LSCMP and LMC alone showed very limited activity. The mixture of LSCMP with K2CO3/carbon increased the activity and achieved the conversion of 100%. Catalytic active species were found to hardly flow out from the reaction zone when LSCMP was present in the carbon layer. In contrast, they easily flew out downstream as proved by the catalytic combustion of carbon bed placed at downstream when LSCMP was absent in the upstream carbon bed. Roles of LSCMP in the catalytic combustion may be to accelerate reductive activation of alkali metal salts to the active species, to allow sublimation them to the carbon, and to hold the active species in the reaction zone under the catalytic combustion conditions.
1. Introduction The thermal efficiency of diesel engines is better than that of spark ignition engines. Because of this, the number of diesel-powered vehicles is expected to increase in the future. However the black smoke containing carbon particulates (referred to as carbon) found in the exhaust gases of diesel engines has been recognized as a pollutant that should be eliminated.1 Its elimination can be accomplished by complete removal of polycondensed aromatic hydrocarbons from the diesel fuel, by improving the engine, trap, and combustion in the exhaust. There are many reports concerning the trap and combustion of the exhaust.2 However, combustion requires rather high temperatures which restrict the materials available for the trap and filter in long periods of service. The combustion catalyst is expected which can burn off the black smoke, or carbon particulates, at low temperature. Alkali metal salts have been reported as the best catalyst for the combustion and gasification of carbon through their activation into corresponding metals or metal oxides.3-8 However, their active species have †
Kyushu Matushita Electric Co., Ltd. Kyushu University. (1) Walsh, M.P. SAE paper 1989, No. 890168. (2) Saito, K.; Ichihara, S. Shokubai 1989, 31, 52 and many cited review papers. (3) Mckee, D. W. Chemistry and Physics of Carbon; Marcel Dekker: New York, 1981; Vol. 16, pp 1-118. (4) Lang, R. J. Fuel 1986, 15, 1324-1329. (5) Johnson, J. L. Fuel 1986, 15, 1324-1329. (6) Veraa, M. J.; Bell, A. T. Fuel 1978, 57, 194-200. (7) Sato, H.; Akamatsu, H. Fuel 1954, 33, 195-202. ‡
been recognized to flow out from the combustion zone, resulting in the rapid loss of the activity.9,10 It is dispensable for the practical catalyst not only to exhibit a high activity at the initial reaction stage but also to keep the activity high in a long term. It is important to obtain the catalyst which contacts repeatedly with the substrate carbon for its complete and continuous combustion. Highly dispersing active species on the carbon surface, selecting the composition of alkali salts, preparation of the catalysts, and their impregnation onto carbon, have been studied.11,12 In most cases, active species directly supported on the carbon substrate suffer problems from their disappearance from the reaction zone, their reactivity with minerals, and the difficulty in their recovery and regeneration. In the present study, the catalytic activity of K2CO3 supported on several oxides or an active carbon itself for its combustion was studied to discover a way to realize the repeated contact of active species with carbon through their sublimation and returning precipitation over the surface of support oxides. Such a catalytic scheme is applicable to the catalytic gasification of coal in the fluidized bed as well as the catalytic combustion of carbon particulates in the fixed bed. Some perovskite type oxides have been studied for combustion, and the (8) Hasimoto, K.; Miura, K.; Xu, J. J.; Watanabe, A.; Masukai, H. Fuel 1986, 65, 489-493. (9) Marsh, H.; Mochida, I. Fuel 1981, 60, 231-239. (10) Huhn, F.; Klein, J.; Juntgen, H. Fuel 1983, 62, 196-199. (11) Haga, T.; Nogi, K.; Amaya, T.; Nishiyama, Y. Appl. Catal. 1991, 67, 189-202. (12) Mchida, I.; Gao, Y.-Z.; Fujitsu, H. Sekiyu Gakkaishi 1991, 34, 178-181.
S0887-0624(97)00213-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/25/1998
Catalytic Activities of K2CO3
solid oxide fuel cell was studied for the redox cycles of component transition metal ions.13,14 Their redox activity was expected to activate alkali metal salts in contact with carbon on their surface. Their acidity can provide sites to capture the active species for their stay in the carbon bed. Thus, influences of supports, including perovskite type oxides and carbon itself, mixing ratio with the oxide, as well as the reaction temperature were examined. The roles of oxide supports could be concluded by comparing the activity of K2CO3 on the oxides to that on the carbon itself. The latter data have been extensively accumulated in the literature.3-10 It was also attempted to measure the range where the catalysts could migrate into the carbon plate. No elution of active species was also confirmed by observing the catalytic combustion of the carbon bed placed downstream of the catalyst-containing carbon bed. 2. Experimental Section Supported catalysts were prepared by the impregnation. A prescribed amount of K2CO3 was dissolved in ethanol/H2O. The several oxides or carbon used as the carrier material were dispersed in the solution and heated to dryness. Perovskite type oxides were prepared from mixtures of component metal nitrates or acetates, freezed-dried, and calcined at 1123 K for 5 h. X-ray diffraction analysis showed perovskite type structure. Compositions of these oxides were calculated from the amounts of starting materials. Activated carbon (Nakalai tesque inc; surface area 370 m2/ g) was used for combustion. The catalyst loading was adjusted typically to 0.65 mmol of potassium per gram of support to compare the effects of supports. A continuous flow reactor apparatus of quartz tube (12 mm diameter) with a fixed carbon bed (20 mm long) was operated at 673-773 K under atmospheric pressure. Supported K2CO3 was physically mixed with carbon. The weight ratio of catalyst to carbon was kept at 1.0. To prevent the local accumulation of combustion heat, the carbon, K2CO3 on the supports, and γ-Al2O3 particles (the weight ratio of the carbon to γ-Al2O3 was 1.0) were dispersed within a ceramic foam. The pressure drop through the catalyst bed was negligible. Nitrogen was flowed during heating to the reaction temperature, and the oxidant gas (1.0-4.0 vol % O2 in N2) was introduced for the carbon combustion. The concentration of oxygen was adjusted according to the activity of the catalyst. High activity of potassium salt was measured at 1 vol % O2, while low activity of the support alone was measured at 4 vol % O2. The gaseous products (CO and CO2) were analyzed by an IR spectrometer at the reactor outlet. The amount of carbon combusted was calculated from the gaseous product.
3. Results 3.1. Influences of Supports on the Catalytic Activity for Carbon Combustion. Figure 1 shows the combustion of carbon catalyzed by potassium carbonate supported on the various supports as function of time at 723 K by 1% O2 in N2. Though the combustion rates were different with the different catalysts, the carbon conversion always increased as the reaction time increased to a certain saturation level after sufficient reaction time. (13) Tascon, J. M. D.; Tejuca, L. G. React. Kinet. Catal. Lett. 1980, 15, 185-191. (14) Vasanthacharya, N. Y.; Sigh, K. K.; Ganguly, P. Rev. Chim. Minerale 1981, 18, 333-343.
Energy & Fuels, Vol. 12, No. 5, 1998 871
Figure 1. Combustion profiles of carbon catalyzed by K2CO3 supported on several supports: reaction temperature, 723 K; oxygen concentration, 1.3 vol %; loading amount of potassium: 0.65 mmol/g-support.
Table 1 summarized the saturated carbon conversion and the product gas ratio of CO to CO2 by K2CO3 supported on various supports at 673-773 K by 1% O2 in N2. The activity of supported K2CO3 catalyst was significantly influenced by the supports; particularly, K2CO3 supported on La0.8Sr0.2Cr0.5Mn0.45Pt0.05O3 (LSCMP) and on LaMn0.8Cu0.2O3 (LMC) showed the largest conversion among the supports tested. The oxidation of the carbon certainly took place on LSCMP and LMC alone, although their activities were not so high under the present conditions. When the ratio of the carbon to the catalyst at their packing volume was as high as 10 or more, K2CO3 supported on LSCMP or LMC achieved the conversion up to c.a. 100% within 3 h. Even at 673 K, the catalysts on the supports achieved 55% and 48% conversion, respectively, within 6 h. In contrast, the saturated conversions by K2CO3 supported on other oxides and on carbon were 50% or less at 723 K. The product ratio of CO to CO2 by K2CO3 supported on LSCMP or LMC was as low as 0.01-0.1 at 723 K to compare with those over the same catalyst supported on the other oxides and on carbon (0.2-0.3). The ratio of K2CO3/LMC was 0.01, indicating the complete oxidation to CO2 for this particular combination of the catalyst and support. Figure 2 shows the reaction rate of the carbon combustion by these catalysts as function of the carbon conversion. The rates of K2CO3 supported on LSCMP and LMC were much higher than those of K2CO3 supported on other oxides and remained constant in the first half of the conversion and then decreased according to the conversion. In contrast, the rates of K2CO3 supported on the other oxides and on the carbon were much lower and decreased monotonically with the increase of the carbon conversion. The activity of supports alone at 723 K was also included in Table 1, where the oxygen concentration was 4% because of low activity. The carbon alone without the catalyst exhibited a conversion as low as 2% within 25 000 s. Except for LMC and LSCMP, the conversion was below 10% for the oxide supports examined in the present study within 25 000 s. The conversions by LMC and LSCMP were extraordinarily high among the
872 Energy & Fuels, Vol. 12, No. 5, 1998
Miyazaki et al.
Table 1. Saturated Conversion of Carbon Catalyed by K2CO3 Supported on Oxide Supportsa support
particle size (µm)
Al2O3 TiO2 SiO2-Al2O3 LMF LMC LSCMP carbon
8 2 5 7 4 5 13
conversion (%)
CO/CO2 ratio
Vc/Vcat
673 K
723 K
773 K
673 K
723 K
773 K
8.3 10.2 8.1 13.1 12.3 12.5
1.3 1.1 2.7 0.9 70.0 89.4 19.5
25.9 (2.3) 10.9 (3.8) 4.0 (3.1) 34.9 (8.9) 96.7 (56.9) 97.8 (40.0) 40.3 (1.9)
43.7 39.2 4.7 50.4 100 100 65.9
0.28 0.22 0.20 0.40 0.14 0.24 0.21
0.28 (0.34) 0.21 (0.30) 0.24 (0.39) 0.22 (0.16) 0.01 (0.05) 0.10 (0.08) 0.2 (0.40)
0.27 0.12 0.06 0.27 0.01 0.05 0.21
a Potassium loading amount: 0.65 mmol/g-support. PO : 1.3 vol %. V : packing volume of carbon. V : packing volume of catalyst. 2 c cat Parentheses: support alone at 4 vol % PO2. LMF: LaMn0.8Fe0.2O3. LMC: LaMn0.6Cu0.4O3. LSCMP: La0.8Sr0.2Cr0.5Mn0.45Pt0.05O3.
Figure 3. Carbon conversion catalyzed by K2CO3 in the presence of LSCMP of various amounts: reaction temperature, 723 K; oxygen concentration, 1.0 vol %; loading amount of potassium, 1.30 mmol/g-support; LSCMP, La0.8Sr0.2Cr0.5Mn0.45Pt0.05O3. Table 2. Combustion of Carbon in Two Bedsa catalyst in the upstream bed
presence of downstream carbon bed (mg)
absence of downstream carbon bed (mg)
K2CO3/LSCMP K2CO3/carbon
10.1 3.4
9.7 1.6
a The carbon amount in the bed: 10 mg. The carbon amount in the downstream bed: 10 mg. Reaction temperature: 773 K. Oxygen concentration: 1 %. Reaction time: 10 000 s. LSCMP: La0.8Sr0.2Cr0.5Mn0.45Pt0.05O3.
Figure 2. Reaction rates for carbon combustion by K2CO3 supported on oxides depending on degree of carbon conversion: reaction temperature, 723 K; oxygen concentration, 1.3 vol %; loading amount of potassium, 0.65 mmol/g-support; LSCMP, La0.8Sr0.2Cr0.5Mn0.45Pt0.05O3.
supports, reaching 57% and 40%, respectively, although such supports showed the conversion of 17% and 14%, respectively, within 60 000 s in 1% O2. CO/CO2 ratios for the oxide supports alone were 0.30.4, while the perovskite type oxides exhibited 0.050.16, indicating complete combustion on the latter oxides. 3.2. Repeated Combustion over K2CO3/LSCMP. The combustion of carbon mixed with K2CO3/LSCMP was measured repeatedly by mixing fresh carbon three times. The combustion was performed by heating from room temperature to 823 K in 1% O2 at a rate of 5 K/min to avoid the rapid liberation of heat. Each run provided the same profiles of carbon combustion. The K2CO3/LSCMP left after the third run of the above combustion was mixed with carbon and alumina and charged to the ceramic foam to examine the activity in the fourth run under the same conditions as those of Figure 1. The same profile of the combustion was reproduced. These two sets of experiments confirmed the activity of K2CO3/LSCMP in the repeated runs.
3.3. Effects of LSCMP Presence in the Reaction Zone. Figure 3 shows the saturated conversion of K2CO3/carbon blended with LSCMP at 723 K by 1% O2 in N2 as a function of the blending ratio of LSCMP to K2CO3/carbon. K2CO3/carbon was physically mixed with LSCMP, and then the mixture was diluted with more carbon without the catalyst to fix the total carbon amount (i.e., carbon support plus carbon reactant). The conversions by K2CO3/carbon and LSCMP alone were as low as 40% and 27%, respectively. The blend of K2CO3/carbon with LSCMP exhibited the strong synergistic effects to increase markedly the carbon conversion. When LSCMP was blended in 25-65% to K2CO3/ carbon, the conversion achieved 100%, while more blending of 75% reduced the conversion to 90%. It is definite that LSCMP’s role is to keep the active species in the reaction zone to increase the turnover. 3.4. Migration of Active Species in the Carbon at the Combustion. The conversion of two separate carbon beds, one containing the catalyst being placed at upstream and another bed without catalyst at downstream, was examined in order to estimate the migration of the active species from the upstream to the downstream bed. The catalysts examined were K2CO3/ LSCMP and K2CO3/carbon. Table 2 summarizes the total carbon combustion amount in the two beds at
Catalytic Activities of K2CO3
Energy & Fuels, Vol. 12, No. 5, 1998 873
Table 3. Reduction of Potassium after the Carbon Combustiona
a
catalyst
percentage
K2CO3/carbon K2CO3/La0.8Sr0.2Cr0.5Mn0.45Pt0.05O3
17 4
Reaction temp: 773 K; PO2 ) 1%.
Figure 4. Combustion profile of graphite plate catalyzed by K2CO3 placed on the surface.
Figure 5. Migration depth of potassium into graphite plate: reaction temperature, 823 K; oxygen concentration, 4.0 vol %; loading amount of potassium, 1.30 mmol/g-support; LSCMP, La0.8Sr0.2Cr0.5Mn0.45Pt0.05O3.
10 000 s after the reaction started. The combusted amounts of K2CO3/LSCMP were similar whether the downstream carbon bed was placed or not, indicating that the combustion basically took place only in the upstream bed. In contrast, K2CO3/carbon increased the conversion of carbon when the catalyst-free bed was placed downstream. Some active species are suggested to be transferred from the upstream bed to the downstream bed during the reaction. The potassium remaining in the beds carrying K2CO3/ carbon and K2CO3/LSCMP was extracted with H2O and analyzed by capillary zone electrophoresis after the carbon combustion at 773 K. Table 3 showed the reduction of the potassium amount after the reaction. The bed carrying K2CO3/carbon lost its 17% of potassium after the reaction, while the bed carrying K2CO3/ LSCMP lost only 4%. The combustion of graphite plate was studied by placing a K2CO3/LSCMP disk on the surface at 823 K by 4% O2 in N2. After the reaction, the graphite surface was eroded by catalytic combustion as illustrated in Figures 4 and 5 which show the depth of the erosion from the surface as a function of reaction time. The K2CO3/LSCMP disk increased the depth of erosion with the reaction time to 50 µm by 2 h, 175 µm by 12 h, and 190 µm by 16 h, respectively. The last depth may indicate the range where the active species can migrate under the influence of LSCMP support. No erosion of the graphite plate was observed even at the early stages of reaction when K2CO3 or LSCMP alone was placed on the plate.
4. Discussion K2CO3 has been reported to catalyze the combustion of carbon through its reduction into metal.11,12 The alkali metal can sublime to be mobile over the carbon substrates, maintaining the intimate contact for the catalytic activity.15,16 On the other hand, sublimed metal may flow out from the reaction zone as observed in previous9 and present work, no catalytic activity being maintained. Thus, the reduction of alkali carbonate to the metal is primarily essential for its high catalytic activity. The sublimation and precipitation of the active species should be balanced in the carbon bed for the high catalytic turnover. If the conservation of high catalytic activity was ensured by supporting on the metal oxides instead of supporting on the carbon directly, the recovery and regeneration of the catalyst becomes unnecessary, solving serious problems in the gasification of coal as well as the purification of the exhaust gas from diesel engine. In this study, K2CO3 supported on LSCMP or LMC was proved to show higher activity compared with those of K2CO3 supported on other oxides or on even carbon directly. K2CO3 supported on LSCMP or LMC achieved 100% conversion of carbon at 723 K or higher when the volume ratio of the carbon to catalyst is c.a. 12.5. Although the activities of LSCMP and LMC alone were higher compared with those of other oxides when 4% O2 in N2 was flowed, their contribution under 1% O2 was limited. It should be noted that the conversion by K2CO3/carbon stayed low under present conditions. Very interestingly, the blending of LSCMP into the carbon bed containing K2CO3/carbon increased the conversion to 100%. All these results suggest that LSCMP and LMC activate K2CO3 into the active species effectively, allowing their contact with carbon and holding them in the reaction zone for high catalytic turnover. The present catalytic combustion is of value for preliminary kinetic analysis. The rate is believed to be dependent on the carbon concentration, temperature, partial pressures of oxidant and products, as well as the amount of the catalysts. In the present fixed bed flow reactor of low oxygen content, the partial pressures of oxygen and products are assumed constant and negligible. Thus the combustion rate is described in a simple form as a function of reactant carbon and temperature, assuming a constant amount of the catalyst.
r ) dc/dt ) kcn ) k(1 - X)n
(1)
In eq 1, r ) rate, k ) rate constant, c ) carbon amount, n ) reaction order in carbon, and X ) conversion of carbon. Such kinetics have been applied in the coal gasification catalyzed by alkaline compounds, and several values of n have been reported.10,16,17 The amount of (15) Shadaman, F.; Sama, D. A.; Punjak, W. A. Fuel 1987, 66, 16581663. (16) Matsukata, M.; Fujikawa, T.; Kikuchi, E.; Morita, Y. Energy Fuels 1988, 2, 750-756. (17) Morita, Y. Reports of special project research on energy Japan 1984; pp 69-78. (18) Takarada, T.; Ogiwara, M.; Kato, K. J. Chem. Eng. Jpn. 1992, 25, 45-49. (19) Juntgen, H.; van Heek, K. H.; Erdol; Kohle 1985, 38, 22-26.
874 Energy & Fuels, Vol. 12, No. 5, 1998
the catalyst active and present in the reaction bed should be noted different from its changed one and variable according to the reaction conditions. When all carbon atoms in the substrates are equally reactive, accessing the catalyst and oxidant, a first-order kinetic can be applicable (n ) 1). No loss and high mobility of the catalyst should be satisfied. When carbon in the neighborhood of the catalytically active species is only combusted, the first-order kinetics is applicable within a limited conversion. When the activation or mobility of the catalyst defines the combustion, zero order in carbon can be assumed. The catalyst may be lost during the combustion according to the level of conversion because the conversion is related to reduction of both alkaline compounds to volatility, and to the number of trapping carbon. Such a loss of catalyst is reflected in the higher order of kinetics in carbon, no constant rate constant being obtained. The loss of the catalyst is essentially ruled out in the case of K2CO3/LSCMP. Figure 2 suggests the constant rate up to 50-60% conversion and monotonic reduction by high conversion in the cases of K2CO3/LSCMP and K2CO3/LMC. Thus the zero-order kinetics are applicable up to 60% for the combustion by these catalysts. The rate at high conversions can be described by first order. The reduced carbon amount may lead to the sufficient active species available to the all remaining carbon particles. When K2CO3 and LSCMP were put alone with the surface of the graphite plate, they did not erode. In contrast, when K2CO3/LSCMP was placed, it eroded the surface of the graphite plate to a certain depth. The activation of the salt and the effective range to hold the (20) Kasaoka, S.; Sakata, Y.; Tong, G. Int. Chem. Eng. 1985, 25, 160-175.
Miyazaki et al.
active species by LSCMP are suggested by the depth of the erosion. The catalytic activity is influenced by the following factors in the fixed bed: (1) the number of active species and their retention in the carbon bed and (2) the contact of carbon with the active species. The particle sizes of TiO2, Al2O3, SiO2-Al2O3, and LSCMP were much the same. Vc/Vcat was smaller with the former three oxides than with LSCMP as described in Table 1. Nevertheless K2CO3/LSCMP exhibited much higher activity. K2CO3 supported directly on carbon should have the best contact with carbon, whereas the achieved conversion was low compared with that achieved with K2CO3/ LSCMP. Hence physical contact of the catalyst and solid carbon is not the major factor in the present case of catalytic gasification. Thus LSCMP and LMC are concluded to accelerate the reduction of K2CO3 to active species to sublime them to the carbon surface and to hold them in the reaction zone, resulting in repetition of its effective interaction with carbon for the complete conversion of the carbon present in the reaction bed. So far, reasons why these perovskite type oxides can perform such supporting roles are not fully answered in the present study. Their redox activity may oxidize carbon on the surface to reduce the potassium salt in the neighbor. Their appropriate acidity may play some roles to maintain the active species within the bed. Metal ions substituting La and Mn in the perovskite appear very influential on the catalytic activity. The combination of redox activity and acidity in the other supports may be valuable to be further examined in the future work. EF970213I
