Isothermal Kinetics of Catalyzed Air Oxidation of Diesel Soot

To comply with the stringent emission regulations on soot, diesel vehicles manufacturers more and more commonly use diesel particulate filters (DPF). These systems need to be regenerated periodically by burning soot that has been accumulated during the loading of the DPF. Design of the DPF requires rate of soot oxidation. This paper describes the kinetics of catalytic oxidation of diesel soot with air under isothermal conditions. Kinetics data were collected in a specially designed mini-semi-batch reactor. Under the high air flow rate assuming pseudo first order reaction the activation energy of soot oxidation was found to be, Ea = 160 kJ/ mol. © 2010 BCREC UNDIP. All rights reserved. .


Introduction
The diesel engines find widespread applications as power source in both automotive and stationary applications.They are energy efficient, durable, driveable, etc. [1] but their emissions of particulate matter (PM) and NOx are responsible of severe environmental and health problems [2].Specifically, diesel PM has led the legislation to adopt stringent emission standards.Diesel particulate filters (DPF) are becoming wide spread as an effective measure to reduce PM (soot) emissions from diesel vehicles as they have filtration efficiencies of almost 100% [3].As the filters accumulate PM, it builds up backpressure that has many negative effects such as decreased fuel economy and possible engine and/or filter failure [4].To prevent these negative effects, the DPF have to be regenerated periodically, i.e. the combustion of the accumulated particulates.It is therefore, vital to understand the regeneration process in order to optimize the application and operation of DPF both for lifetime durability and fuel economy purposes.However, diesel soot elimination is known to be a hard task, since this material burns at roughly 550-600 0 C with air, while diesel exhaust gases temperature most of the time lies between 200 -450 0 C. Therefore, some artifice is needed to promote soot oxidation.Very often this is carried out by the use of an oxidation catalyst based DPF [5] in order to lower the required combustion temperatures.

Isothermal Kinetics of Catalyzed Air Oxidation of Diesel Soot
The catalysts are impregnated on the filter walls.The performance of catalytic traps is affected by the intrinsic catalytic activity and the soot-catalyst contact efficiency [6].The nature of the contact between soot and the catalyst depends on two important parameters: the relative concentration of the solids and the mixing method.For experimental studies the soot-catalyst mixture, in an appropriate ratio, are milled in an agate mortar for ''tight contact'' or mixed carefully with a spatula for ''loose contact'' [7].
In most of the studies reported in the literature, the catalyst performances are based on the light-off temperatures.The comparison of the catalyst performances may be consistent within one set of data, but the differences in the experimental conditions such as the heating rates and the gas flow rates may cause differences in the light-off temperatures.A better tool for catalyst performance comparison is to report the activation energy for the process [8].Furthermore, the activation energy data are needed for the modelling and design of the catalytic soot converters.Despite the significant progress in soot oxidation studies and the extended literature, there is still high uncertainty concerning the corresponding kinetic equations.Great scatter is observed regarding the reported activation energy (E), the order of reaction with respect to both the oxidants and the running soot mass [8][9][10] in the oxidation reaction.The reported activation energy (E) of the various soot-catalyst mixtures is given in Table 1.
The basic reasons, which lead to those discrepancies, are related with the sample and experimental setup characteristics.The commonly used synthetic soot samples are not necessarily equivalent to real diesel soot.Then again the quality of real soot is not constant and depends on engine and operational parameters.And finally, the experimental setup itself may impose uncertainties, such as rate controlling mass transfer limitations, etc.

Bulletin of Chemical Reaction Engineering & Catalysis, 5 (2), 2010, 96
Copyright © 2010, BCREC, ISSN 1978-2993 catalysts and in modelling of catalytic converters [10,21,24].A platinum group metals (PGM) coated DPF has been presented as an efficient after-treatment system [25,26].But in recent years, considering the high cost, limited availability of the noble metals and their instability with impurities, more attention has been focused on the base metal catalysts [27,28].In this context PGM-free, CuO-CeO2 based catalysts are expected to be a good alternative for diesel soot oxidation [29].
Thus, this study deals with the kinetics of catalyzed oxidation of diesel soot in a laboratory set-up, using a very sturdy and selective, Cu-Ag-Cr-K-Ce-Zr-Al catalyst [30] prepared by citric acid sol gel method.The soot oxidation was investigated under isothermal conditions at ambient pressure in a specially designed mini semi-batch reactor.

Catalyst preparation
Cu-Ag-Cr-K-Ce-Zr-Al mixed oxides, with Ce/Zr and Cu+Ag/(Cu+Cr+Ag+K+Ce+Zr+Al) molar ratio equal to 5/4 and 1/10 respectively were prepared by citric acid sol-gel methods.The nitrates Cu (NO3)2.3H2O,(NH4)2Cr2O7, AgNO3, KNO3, Ce(NO3)3.6H2O,ZrO(NO3)2.H2O, Al(NO3)3.9H2Owere mixed in deionised water according to the desired molar ratio.Citric acid was added as the complexing agent with a 1.3:1 ratio.Appropriate poly-glycol (poly ethylene glycol) is added to the weight of 10 % citric acid.The blended solution is sufficiently mixed in a magnetic stirrer and heated at 80 0 C till transparent gel was formed.The resulting gel was dried at 110 0 C in an oven overnight.The received powder was submitted to decompose at 300 0 C for 1 h and calcination at 500 0 C for 3 h under static air in a muffle furnace.The powder was cooled to room temperature in the furnace to obtain the so-called fresh samples.

Soot preparation
The soot was prepared by partial combustion of commercial diesel obtained from Hindustan petroleum (HP) in a lamp with a limited supply of air [31][32][33] as shown in Figure 1.The deposited soot on the inner walls of an inverted beaker was collected and then dried in an oven for overnight at 120 0 C.

Bench Scale Reactor
Experimental set-up for collecting kinetic data on isothermal air oxidation of soot is shown in Figure 2. It is consisted of a tubular furnace with a microprocessor based temperature controller, a quartz reactor and a condenser for condensing water vapour and cooling the product gas to room temperature.The reactor has a sample tube (as shown in Figure 2) attached to it by B-14 socket joint and a thermocouple well for measuring the temperature of the soot-catalyst bed.The bends of the sample tube and the tube connecting the sample tube to the reactor are such that when the sample tube is at position Y, the solid sample falls into the reactor.
The outlet of the reactor is connected to a cooler cum condenser as shown in the figure .The furnace temperature can be controlled to ±0.5 0 C and the temperature of the bed is measured by a separate temperature indicator (T).There is a hole in the lower part of the outer tube of the reactor, to take care of breakage due to the expansion or

Bulletin of Chemical Reaction Engineering & Catalysis, 5 (2), 2010, 97
Copyright © 2010, BCREC, ISSN 1978-2993 contraction of air in between co-axial tubes as the unit is subjected to the variation of temperature from ambient to the reaction temperature.The reactor was mounted vertically.

Experimental procedure
A known weight of the soot-catalyst mixture was taken in the sample tube and it was connected to the system (position X).Before the oxidation reaction, the soot-catalyst mixture, in a 1/10 weight ratio, were milled in an agate mortar for ''tight contact''.110 mg of this catalyst-soot mixture was placed in the sample tube.The heating of the furnace was started and the dry and CO2 free air was fed at flow rate of 150ml/ min.When the reactor attained the required temperature shown by the temperature indicator (T) the sample was dropped in the reactor by turning the sample tube by 180 0 (i.e.position Y) and the composition of the reactor outlet gases were analysed by a gas chromatograph as a function of time.Gaseous products were analysed for CO and CO2 with the help of an online gas chromatograph using Porapack-Q column, methaniser and FID detector.GC analysis of the product stream showed only peak for CO2, no peak was observed corresponding to CO retention time.This shows very high selectivity of the catalyst for CO2.The isothermal catalytic soot oxidation data were collected in the temperature range of 340-420 0 C.

Catalyzed soot oxidation
The kinetics of catalyzed air oxidation of diesel soot in laboratory conditions, using a very sturdy and selective, Cu-Ag-Cr-K-Ce-Zr-Al catalyst were studied under isothermal conditions in a semi batch reactor.The catalyst selectivity was 100% for CO2, as no CO was detected in the product stream.The experimental data, % CO2 Vs time plot at five different temperatures (340-420 0 C) are presented in Figure 3.The fraction conversion of soot, (α) is defined as: where, m and m0 are the running and the initial sample mass respectively.The value of α at various extent of reaction is calculated using the following formula: Where, k is the reaction rate constant and n is the order of reaction with respect to oxygen.In the present study flowing oxygen was in large excess, therefore the rate expression reduces to overall pseudo first order: The rate of oxidation reaction at any instance was obtained by taking slope of soot conversion (α) versus time plot (Figure 4).Linear plots passing through origin were obtained when rate of soot oxidation (dα/dt) as a function of (1-α) were plotted (Figure 5).Slopes of the lines gave reaction rate constant (k) at various temperatures.The values of reaction rate constants at various temperatures are reported in Table 2.The temperature dependence of the reaction rate is usually described by the Arrhenius equation: k = k 0 exp(-Ea/ R T ) ( 6 ) or ln(k) = ln(k0) -(Ea/RT) (7) where, T is the absolute temperature, k0 is the preexponential factor, R is the universal gas constant and Ea is the activation energy.Figure 6 shows a linear relationship between ln k and 1/T.Thus the reaction rate constants follow Arrhenius law.The activation energy (Ea) and frequency factor (k0) were calculated measuring the slope and intercept respectively of the plot ln (k) vs. 1/T (Figure 6).The values of Ea and k0 are found to be 160 kJ/mol and 3.52 x 10 12 sec -1 respectively under the conditions investigated in the present studies.On the basis of the experimental findings the rate of soot oxidation over Cu-Cr-Ag-K-Ce-Zr-Al catalyst in the temperature range of 613 -693 K can be expressed as follows: Rate = 3.52 x 10 12 exp(-160000/RT)(1-α) (8) This empirical rate expression can be adequately used in modeling of the diesel soot filter using the mentioned catalyst in the present studies.

Conclusion
Soot oxidation was studied in a specially designed mini semi-batch reactor.The study of soot oxidation kinetics in the mini-scale reactor revealed that it assures practically isothermal conditions in the sample (the difference between the inlet and outlet temperature was not observed).High flow rate of air indicate that there was no mass transfer limitations and that the chemical kinetics control the reaction under the investigated conditions.Intrinsic reaction rate of air oxidation of diesel soot over Cu-Cr-Ag-K-Ce-Zr-Al catalyst was determined as a function of temperature and fractional conversion.Under the high air flow rate assuming pseudo first order reaction the activation energy of soot oxidation was found to be, Ea = 160 kJ/ mol.

R
. Prasad * , and Venkateswara Rao Bella Department of Chemical Engineering & Technology, Institute of Technology Banaras Hindu University, Varanasi 221005, India

Table 1 .
The reported activation energy (E) of the various soot-catalyst mixtures

Table 2 .
Reaction rate constant as a function of temperature