Kinetic consideration of non-catalytic and catalytic oxidation of soot
Advancing the chemical engineering fundamentals
Catalysis (T2-13P)
Keywords: soot oxidation, kinetics, thermo-oxidative degradation
Diesel engines are considered as the main source of emission of the particulate matter (soot particles) originating from the uncomplete fuel combustion (Neeft et al., 1996; Stanmore et al., 2001). These particles are very harmful to human health, due to their carcinogenic effect. One of the technologies available to control diesel particulate emissions employs the filters which capture soot particles from the exhaust stream. However, the filters must be regenerated periodically or continuously by combustion/oxidation of the trapped soot to prevent the back-pressure build-up. Soot oxidation takes place at high temperatures (> 600˚C) while the temperature of diesel exhaust gases for small engines may be as low as 200˚C. Therefore, the catalyst is used to prevent accumulation of soot on the monolithic filter. The catalyst is introduced by addition of the oxidation catalyst precursors as fuel additives or by impregnation of the filter walls with the oxidation catalyst.
The aim of this work was to investigate the influence of the applied catalysts (Mn- and Pt-based catalyst) on the soot oxidation. The manganese-based catalyst was prepared by wet impregnation of Mn(SO4)·4 H2O solution on the alumina support. The Pt-based catalyst was supplied by Degussa-Huls AG. The prepared catalysts were characterized by several techniques, such as XRD, SEM, surface measurements and mercury porosimetry. The commercial active carbon («Kemika», Zagreb) was the model soot. The soot sample was mixed with powder catalyst in the ratio of 1:1. The thermo-oxidative degradation and catalytic oxidation of the soot were investigated by means of dynamic thermogravimetric method of analysis (TGA), using Perkin-Elmer TGS-2 Thermobalance. The mass loss and the sample temperature were continuously recorded by the computerised data acquisition system. An appropriate amount of the active carbon (AC, 7-10 mg) or AC+ catalyst mixture was loaded in the aluminium crucible and heated from 40˚C to 1000˚C. The experiments were carried out at constant heating rates (5, 10, 15, 20 and 25 ˚C min-1). The oxidant stream was dry air (30-150 cm3 min-1), flowing downwards onto the cylindrical sample holder.
In this study special emphasis was put on the kinetics of both non-catalyzed and catalyzed reaction. After comparison of the results obtained during non-catalytic and catalytic oxidation of the soot at constant heating rate, it was found that the characteristic temperature of the soot oxidation was shifted to the low temperature region in the presence of the Pt-based catalyst. Maximum temperature for catalytic oxidation of the AC-Pt catalyst mixture was by approximately 180-200˚C lower than for thermo-oxidative degradation in the absence of the catalyst. Conversely, the Mn-based catalyst had no influence on combustion rate of the active carbon sample. The values of kinetic parameters, such as activation energy (Ea) and Arrhenius pre-exponential factor (A) were calculated using Kissinger-Akahira-Sunose (KAS) isoconversional method. The obtained values of the activation energy were in good agreement with the literature data (López-Fonseca et al., 2005). At 50 % conversion the activation energies were 88.39 kJ mol-1 and 173 kJ mol-1 for the catalytic and non-catalytic soot oxidation respectively. Linear relationship between activation energy and pre-exponential factor was observed and defined by the compensation effect.
References
Kissinger, H.E. (1957). Reaction kinetics in differential thermal analysis. Anal. Chem. 29, 1702.
López-Fonseca, R., Elizundia, U., Landa, I., Gutiérrez-Ortiz, M.A., González-Velasco, J.R. (2005). Kinetic analysis of non-catalytic and Mn-catalysed combustion of diesel soot surrogates. Appl. Catal. B: Environ. 61, 150.
Neeft, J.P.A., Makkee, M., Moulijn, J.A. (1996). Diesel Particulate Emission Control. Fuel Process. Technol. 47 (1) 1.
Stanmore, B.R., Brilhac, J.F., Gilot, P. (2001). The Oxidation of Soot: A review of Experiments, Mechanisms and Models. Carbon 39, 2247.
Presented Wednesday 19, 13:30 to 15:00, in session Catalysis (T2-13P).
