159 Studies in Surface Science and Catalysis, volume 159 Hyun-Ku Rhee, In-Sik Nam and Jong Moon Park (Editors) (Editors) © 2006 Elsevier B.V. All rights reserved
Catalytic removal of diesel soot participates over LaMnO 3 perovskite-type oxides Seong-Soo Hong*and Gun-Dae Lee Division of Applied Chemical Engineering, Pukyong National University, 100 Yongdang-dong, Nam-ku, Pusan, 608-739, Korea
Catalytic combustion of diesel soot particulates over LaMnO3 perovskite-type oxides prepared by malic acid method has been studied. In the LaMnO3 catalyst, the partial substitution of alkali metal ions into A site enhanced the catalytic activity in the combustion of diesel soot particulates and the activity was shown in following order;Cs>K>Na. In the Lai.xCsxMnOj catalyst, the catalytic activity increased with an increase of x value and showed constant activity at the substitution of x>0.3
1. INTRODUCTION One of principal problems in larger urban centers is the presence of particulate material in the atmosphere due to the emission of diesel engine[l]. One of the most dangerous components of diesel exhaust is particulate, which consists of agglomerates of small carbon particles with a number of different hydrocarbons and sulfates adsorbed on their surface. A potential way to face the related environmental problem is that of filtering the particulate and burning it out in catalyzed traps before any emission of diesel exhausts in the environment. The combustion temperature of soot particulates can be lowered by the addition of an oxidation catalyst in the form of fuel additives, by spraying metal salt solution on an accumulated soot or by the impregnation of filter walls with an oxidation catalyst. For the last option, oxides of supported metals are considered to be the most promising candidates. Several researchers have focused their attention on the application of oxide materials to lower the oxidation temperature of soot particulates. It was reported that active soot oxidation catalysts are PbO, Co3O4, V2O5, MoO 3 , CuO, and perovskite type oxides. In this paper, we prepared LaMnO3 perovskite-type oxides using the malic acid method and investigated their physical properties. It has been also investigated the effect of partial substitution of metal ions into La and Mn sites and the reaction conditions on the activity for the combustion of soot particulates.
2. EXPERIMENTAL The preparation method of perovskite-type oxides was taken from the previous paper. Malic acid was added into mixed aqueous solution of metal nitrates in a desired proportion so as for the molar ratio of malic *To whom correspondence should be addressed E-mail: sshongj2ipknu.ac.kr
262 acid to the total metal cations to be unity. The solution was then evaporated to dryness with stirring, and further dried at 1501. The precursor was ground and then calcined in air at 200 °C for 30 min, 350 °C for 30 min and 600 °C for 12h. Experiments were performed with a model carbon(Printex-U) which was obtained from Degussa AG. The properties(primary particle size, BET surface area, oxidation rate) of this model soot were similar to those of a real diesel soot paniculate. The catalyst and the soot(5wt.%) were well mixed in an agate mortar with a pestle for more than 20 min. Although the soot/catalyst contact was known to affect significantly experimental results, this mixing procedure gave reproducible results under present experimental conditions. The catalyst/soot mixture(0.2g) was placed in a quartz-tube reactor, preheat-treated at 300 °C for 2 h, and then cooled down to 200 "C in a He stream. After that, the temperature programmed reaction(TPR) was started with the linear heating rate of l°C/min in a gaseous mixture of C>2(4%) and He(balance)(fiow rate; 100cm3/ min). The outlet gases were analyzed by gas chromatograph(HP 5890) at intervals of about 20min. The soot was almost oxidized into CO2. Very small amount of CO was detected but it was neglected. In addition, the carbon mass balances were generally better than 97%. 3. RESULTS AND DISCUSSION The carbon removal reaction supposedly takes place at two-phase boundary of a solid catalyst, a solid reactant(carbon particulate) and gaseous reactants(C>2, NO). Because of the experimental difficulty to supply a solid carbon continuously to reaction system, the reaction have been exclusively investigated by the temperature programmed reaction(TPR) technique in which the mixture of a catalyst and a soot is heated in gaseous reactants. The thermogravimetric analysis of catalyst/carbon mixtures was carried out to elucidate the combustion properties of carbon particulates with a catalyst and the result is shown in Fig. 1. While noncatalytic oxidation of carbon particulates occurs above 500°C, ignition temperature remarkably decreases in the presence of a catalyst. The combustion is initiated at 520°C in the absence of a catalyst but the ignition temperature decreases to 330°C in the presence of Lao.8Cso.2Mn03 catalyst. With increasing temperature, the charged carbon particulate is progressively consumed and finally exhausted. The effect of substitution of metal ion into A and B sites of LaMnO3 on the oxidation of soot particulate was examined and the result is shown in Table 1. The substitution of alkali metal ions decreases the ignition temperature of soot particulates and the catalytic activity is shown in the order of Cs>K>Na. From simple geometric considerations, unit cells may expand upon the substitution of larger alkali metal ions for smaller magnesium ions. Therefore, the substitution of larger alkali metal ions brings about the formation of a large number of ion vacancies in their lattice. It suggests that the partial substitution of Cs gives rise to easy reduction of oxides and forms oxide ion vacancies on the surface, and then increases the adsorption rate of active oxygen on the catalyst surface. This result can be verified by the TPRCtemperature programmed reduction) result(Fig. 2), As shown in Fig. 2, they show one reduction peak and the reduction peak appears at lowest temperature in the LaojCso.sMnOs catalyst. This result suggests that the substitution of Cs into A site gives rise to easy reduction of oxides and forms oxide ion vacancies on the surface and then increases the adsorption rate of active oxygen on the catalyst surface.
263 Table 1. Perovskite-type oxides prepared by malic acid method and their catalytic performances Catalyst
" Ignition temperature estimated by extrapolating the steeply ascending portion of the COj formation curve to zero CO2 concentration Temperature which shows the maximum CO2 concentration In the Lai.,CsxMnO3 catalyst, the Tm decreases with an increase of x value and shows an almost constant value upon substitution of x>0.3. It is thought that the oxygen vacancy sites of perovskite oxide increase with an increase of amount of Cs and the oxidation activity also increases. This result is also verified by the TPR result of these catalysts(Fig. 3). As shown in Fig. 3, the reduction peak appears at low temperature with an increase of x value and no change is shown at more than x=0.3. It can thus be concluded that the catalytic performance of these oxides increases as the amount of Cs in the crystal lattice increases. However, the substitution of Cs to more than x=0.3 leads to excess Cs, which is present on the surface of mixed oxides might have no effect on the catalytic activity
Carbon only 60 La0.7K0.3MnO3
Fig. 1. TG spectra of carbon particulates with catalyst; heating rate=l K/min.
o Temperature ((°C) C)
Fig. 2. TPR profiles measured for various perovskite type oxides; heating rate=10 K/min, gas mixture= 5% Ha/He.
Outlet CO2 Concentration(mole)
La^Cs^MnO, La 0.6Cs0.4MnO3 La^Cs^MnO, La 0.7Cs0.3MnO3 La^Cs^MnO, La Cs0.2MnO3 0.8 La^Cs^MnO, La Cs0.1MnO3 0.9
Fig. 3. TPR profiles measured for various perovskite type oxides; heating rate=10 K/min, gas mixture= 5% H2/He.
o Temperature ((°C) C)
Temperature ((°C) C)
Fig. 4. Temperature programmed reaction on combustion of carbon particulates over Lao^Cso^MnOs catalyst: heating rate= 1 K/min, NO= 500 ppm, O2=4%, a)carbon, b)carbon+oxygen, c)carbon+catalyst, d)carbon+NO+oxygen+catalyst, e)carbon+oxygen+catalyst.
Fig. 4 shows outlet CO 2 concentration over LaojCso.jsMnOs catalyst at the various reactant compositions. In the presence of carbon without the catalyst and oxygen(Fig. 4(a)), carbon dioxide cannot be produced at even high temperature, hi the absence of oxygen(Fig. 4(c)), carbon dioxide can be produced relatively high temperature, which is thought to be due to the carbon particulate oxidation by lattice oxygen. The similar tendency was shown in the previous report with the presence of perovskite-type catalyst and carbon particulate. In the absence of the catalyst(Fig. 4(b)), a small amount of carbon dioxide can be produced at very high temperature. This result indicates that the catalyst play an important role on the combustion of carbon particulates. In the presence of NO and oxygen(Fig. 4(d)), outlet CO2 concentration goes through a maximum at about 350 °C and similar result is obtained in the absence of NO(Fig. 4(e)). It is demonstrated that NO has little effect on the catalytic oxidation of carbon particulate. In this study, catalytic combustion of diesel soot particulates over LaMnOj perovskite-type oxides prepared by malic acid method has been carried out. In the LaMnO3 catalyst, the partial substitution of alkali metal ions into A site enhanced the catalytic activity in the combustion of diesel soot particulates and the activity was shown in following order;Cs>K>Na.
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