The Development of a Model Capable of Predicting Diesel Lean NOx Catalyst Performance Under Transient Conditions

Reaction Kinetics and the Development of Catalytic Processes G.F. Froment and K.C. Waugh (Editors) 91999 Elsevier Science B.V. All rights reserved.

157

The Development of a Model Capable of Predicting Diesel Lean N O x Catalyst Performance Under Transient Conditions GP Anseii, PS Bennett, JP Cox, JM Evans*, JC Frost, PG Gray, A-M Jones, M Litorell, RR Rajaram, G Smedler and AP Walker Johnson Matthey Technology Centre, Blount's Court, Sonning Common, Reading, RG4 9NI~ UK Abstract Steady state kinetics data from a commercial Pt-based lean NOx catalyst have been used to formulate a kinetic model to describe the performance of the catalyst. It is clear from this analysis that steady state kinetics m isolation are not sufficient to provide a full picture of the operational performance of such a catalyst. However, when this kinetic analysis is combined with mechanistic information obtained over the catalyst, the resulting model is extremely powerful. Within this paper, the development of the kinetic model is described, and the requirement for both accurate mechanistic information and detailed kinetic measurements is clearly demonstrated. The use of the model to predict the performance of a light-duty diesel vehicle under light-off conditions is described, and the power and flexibility of the model within the lean NOx area are emphasised.

Introduction Minimising the emissions of hydrocarbons (HC's), CO and NOx from motor vehicles is a major goal in today's increasingly environmentally conscious society. Today, all new gasoline-powered vehicles sold in the USA and the EC are fitted with so-called three-way catalysts which provide very high conversions of HC, CO and NOx, and lead to significant improvements in air quality. The diesel engine is an inherently low emission technology which, until recently, has not required secondary emission control such as catalytic converters. However, the impending introduction of more stringent legislation in the EC and USA will require the implementation of such systems. The diesel vehicle operates under strongly oxidising conditions, which means that the oxidative catalytic removal of CO and HC's is relatively straightforward. Unfortunately, the reduction of NOx under oxidising conditions is difficult, but the legislation demands that significant NOx conversion be achieved. In recent years Pt-based catalysts capable of performing "lean NOx" reduction within the temperature range of relevance to the light-duty diesel vehicle (150-300~ have been developed (eg [ 1]). While the performance of these systems is encouraging, the peak NOx conversion obtained over such a catalyst is around 40-50%, and this conversion efficiency can only be sustained over a relatively narrow temperature window (Figure 1). In the presence of such intrinsic limitations on the performance of the catalyst, it is clearly desirable to optimise 100 not only the catalyst, but also the rest of the emission control system, of which the NOx / . . . . . 80 catalyst forms a part. Such optimisation NO is only possible if a comprehensive .~_ 6 0 understanding of the characteristics of the catalyst under operating conditions can be g 40 developed. This paper describes the 0 construction of such an understanding, 20 and shows that the combination of accurate mechanistic information and _ _ 0 -: = detailed kinetics is essential if one is to 450 200 250 300 350 400 150 construct a model capable of predicting Inlet Temperature [C] the kinetic response of a lean NOx Fig. 1. The NO, NOx, and propene conversionprofiles obtained catalyst under actual operating over a 1% Pt/AI203 catalyst during temperature programmed conditions. reaction under a full gas mix. N.B. Above270 ~ NO -+ NO2

158

Experimental The steady state kinetics measurements were made using a spinning basket reactor which behaves as a continuous stirred tank reactor (CSTR) [2]. The commercial, Pt-based catalyst in monolith form was loaded into the reactor (4 blocks with dimensions of 40xl 0x5 mm at 400cpsi). The sample was exposed to a simplified gas feed (C3H6, NO, 02, CO and N2) at a gas hourly space velocity (GHSV) of 22,000hr-~. The speed of rotation (2500 rpm) was selected after ensuring that any further increases did not alter the measured steady state rate of reaction i.e. no mass transfer limitation. The conversion of propene was measured using gas chromatography and the CO and NO levels were monitored using a non-dispersive IR system and a chemiluminescent analyser respectively. The steady state experiments took the form of a stepped temperature ramp where the reactor was heated by an external furnace. The system was allowed to reach equilibrium at each temperature (all other inlet conditions remain constant). At an inlet temperature producing a fixed reactant conversion (typically 10%) a concentration span was carried out running from low to high reactant concentration, ie C3H6:300-1000 ppm, NO: 100-600 ppm. Temperature programmed reaction experiments were also performed using a monolith sample. The dimensions of this monolith core were 25.4 mm diameter by 38.1 mm long. Within these experiments, the catalyst was heated at a rate of 5~ from 120~ to 400~ under a simulated leanbum exhaust gas, at a GHSV of 30,000 hr1. This mixture comprised 200ppm NO, 200ppm CO, 400ppm C3H6, 12% O2, 4.5% H20 in N2. The gas phase concentrations were varied from this base case as follows: C3H6 200-800ppm; NO 100-400ppm; CO 200-5000ppm; 02 1-12%. In addition, the space velocity was varied between 15,000 and 60,000 hr-1 and the heating rate between 2 and 10~ -1. The engine emission data were obtained using a diesel 2.5L TDI engine run on a test bench (500ppm fuel S). The engine was set up without Exhaust Gas Recirculation (EGR) but with additional fuel injection into the exhaust. It is now well accepted that the HC levels emitted from diesel engines are not high enough to effect substantial lean NOx conversion. Therefore, additional fuel needs to be presented to the catalyst to boost the HC/NOx ratio. This was done by directly injecting fuel into the exhaust pipe, upstream of the catalyst. The HC:NOx ratio was kept constant within each test and was controlled to a set point by a fuel injection system The test protocol comprised a series of stepped lightoff tests; the temperature was increased by approximately 15~ at a time by increasing the engine load and the system allowed to reach steady state at each temperature. Exhaust gas was sampled for analysis using the same method employed during FTP/ECE vehicle te~ting i.e. constant volume sampling, where the exhaust gas sample is diluted with air to a constant volume to prevent water condensation in the pipework since exhaust gas contains a lot of water (typically 6%).

Modelling To enable comparison of the predictions of the propene and NOx conversions with the experimental data, a 1-dimensional dynamic mathematical model was written to describe the catalyst monolith sample used in the temperature programmed reaction experiments. We are using the model to extract kinetic data and Heck et al [3] have verified that a 1-dimensional model is adequate for this purpose. In addition, a fast turn around of simulations can be achieved using this approach, since 1dimensional models run relatively rapidly. The following assumptions have been made in the model: (a) Uniform flow distribution at entrance to monolith. (b) Negligible radial temperature and concentration profiles. (c) Adiabatic operation. (d) Transport of mass and energy in the gas is by convection. (e) Transport of energy in the solid is by conduction. (f) Mass and energy transfer between phases is accounted for using expressions by Ullah et al [4]. (g) No radiative transport of energy to or from the inlet/outlet monolith faces occurs. (h) No diffusion resistance is present in the washcoat. The following expressions are the one-dimensional equations describing a monolith reactor:

159 Gas mass balance"

OvCg, c3z

Gas energy balance"

+ k=Sv(Cg, C~j) = 0 (1)

OTg pg CpgV-~z + h Sv (Tg TA = 0 OT,

Solid energy balance" He) Ps Cp, Ot Solid mass balance

(le)

0 Csi

Ot

=

s (~Z2

(2)

+ ZHi RATE, Sa + hSv (TgTQ

(3)

= kmS,, (Cg, C~ORATE, Sa (4)

Where Ratei can be substituted by one of the kinetic rate expressions mentioned in the following sections of this paper. The partial differential equations above were discretized in the spatial domain using a finite difference technique to convert them into ordinary differential equations. These were then solved using a standard numerical Ordinary Differential Equation (ODE) solver. Results and Discussion

a)

"=

Steady State Kinetic Model

NO EXP

T .

Power law rate expressions have been used extensively in catalytic converter flow modelling studies. Figure 2 shows Arrhenius plots obtained from spinning basket reactor experiments over the commercial, Pt-based catalyst. The concentration span data were used to quantify the dependence of the reaction rates on the gas phase concentrations of the different reactants by linear regression analysis in power law form. These were then incorporated into the Arrhenius model to redetermine the activation energies and preexponential factors. The power law expressions for C3H6 and NO were found to be of the following form:

.

C3H6 EXP .

.

.

(Linear Fit) I

.

NO Rate IZ ..J -12

HC Rate

-14

-16 0.0021

~

I 0.00215

~.

~

J

I

0.0022 0.00225 1/Temperature [1/K]

~

! 0.0023

t

" 0.00235

Fig. 2. Arrhenius plots for C3H6 and NO rates. Temperature dependence of the lean NOx reaction rates from the spinning

basket reactor data showingthe linear regression analysisfor power laws.

The orders of reaction, a, b, c, and d are positive numbers between 0 and 1. The ~v,jC~v, C-NbO (5) power law equations show that propene RATEc3n, = Ac~m ex ~ Eac Rg Ts concentration has a positive effect on its own conversion but NO concentration has a negative effect on propene conversion over ( EaNo1 (6) R A T E No = ANO exp R gTs) Cc3m C a the conditions of the experiment. Propene also shows a promoting effect on NO conversion as shown in equation (6). Initially the power law equations (5) and (6) were inserted into equations (3) and (4) of the monolith model and the predicted reactant conversions were compared with the experimental data from the temperature programmed reaction experiments (Figure 3). Although the power law kinetics are capable of qualitatively predicting the observed experimental trends at low temperatures, it is clear that the derived expressions do not satisfactorily fit the data. This is particularly true for the NOx conversion profile which decreases with increasing temperature far more slowly in the model than it does

1

160 experimentally. It is obvious from this comparison that a more rigorous description of the kinetics is required. This has been achieved by obtaining a greater understanding of the reaction mechanism.

lOO _

80

m

_ m

m m

mllm__

r........

c 60 .o

"/ EXP NO b) Mechanistic Studies of the Lean NOx > 9e ,-- 40 Reaction o The mechanistic studies were o carried out using a Temporal Analysis of 20 Products (TAP) reactor [5]. Experiments designed to generate a full understanding 0 ~ mmm of the mechanism of the lean NOx 1O0 150 200 250 300 350 400 reaction over Pt-based catalysts were Inlet T~.mn~.mttJre f('.l carried out; these experiments are Fig. 3. Power law kineticfit to leanNOx data. Comparisonof the described in detail in reference [6]. experimentally observed HC and NOx conversions with those Figure 4 outlines the mechanistic steps predicted in a 1-dimensionalmonolithmodel. derived from this mechanistic investigation. The hydrocarbon species reduces a patch of adjacent Pt atoms from Pt-O to metallic Pt. NO adsorption and subsequent dissociation then occurs on the reduced Pt sites. Once the NO dissociation has begun, there are two ways in which the adsorbed N adatom species can be removed from the catalyst. At low temperatures, where the rate of NO dissociation is low, most of the NO species associated with the surface will be present in the molecular form (rather than as dissociated N and O adatoms). Therefore, under these conditions, the most likely means of removal of the N adatoms is via reaction with molecular NO to yield NzO. As the temperature increases, the rate of NO dissociation increases and at higher temperatures, most of the NO-derived species associated with the catalyst surface will be in the form ofN adatoms. Under such conditions, the major route ofN adatom removal is by reaction with another N adatom to form Nz.

The key feature of the mechanism in the context of kinetic modelling concerns the temperature dependence of the NOx conversion. When the HC removes oxygen atoms to generate a reduced patch

161 of Pt, there is an immediate competition between NO and 0 2 to re-oxidise the Pt sites. At low temperatures, NO tends to win this competition, and significant adsorption of NO molecules occurs leading to NOx conversion. However, at high temperatures the oxygen wins the competition, partly because the dissociative adsorption of oxygen becomes increasingly rapid as the temperature is increased, and partly because the rate at which molecular NO desorbs from the catalyst increases rapidly as the temperature rises. As a result of this competition, the surface of the Pt particles is predominantly oxidised at the higher temperature and any NO conversion is oxidation to NO2 rather than reduction (to N2 and N20); the overall NOx conversion is then diminished (see Figure 1). This temperature dependence of the NO/O2 competitive adsorption explains the profile of the lean NOx conversion traces obtained over Pt-based catalysts. Our initial kinetic model based on power law expressions provides an inadequate description of the catalyst performance at high temperatures because this temperature dependent NO/O2 competition was not explicitly incorporated into the model.

C) Mechanistically based model Historically, mechanistically based modelling of catalytic reactions has involved testing estimated mechanisms against experimental data. Voltz et al [7] pioneered this approach by deriving kinetic rate equations for various possible mechanisms of heterogeneous catalysis and then using the equations which best fit the experimental data in a catalytic converter model. However, in this case we already have a mechanism for the lean NOx reactions and so can go straight to a specific formalism. The mechanistic studies reveal that the rate of NOx reaction depends upon the rate of reduction of the platinum surface i.e. the rate of NOx reaction is strongly dependent on the rate of the propene oxidation. Therefore the propene and NOx reaction rate equations were coupled together. Additionally, an improved kinetic description can be obtained by describing more fully the self and cross-inhibition effects that arise as the surface concentrations of reactants and products change. This is done by fitting the data from the reactor to equations in a Langrnuir-Hinshelwood formalism where self- and crossinhibition effects are incorporated as site adsorption terms in the kinetic expressions. Combining these two improvements gives : When these expressions for Ratei were used within the ~Eac~H,] Cc3m 1 (7) one-dimensional model, the RATEc~, = Ac,.m ext R, T] (1 + X Cc~,) (1 + Y CNO) agreement between model and experiment data shown KNO CNO (8) RA TE No = RA TE c~m in figure 5 was obtained. (1 + Z CNo) 100

The Langrnuir-Hinshelwood expressions allow a much better agreement with the experimental data than did the power law equations. During the light-off phase, the equations model the data very closely indicating that the kinetic parameters (i.e. activation energies and pre-exponential factors) are more accurate than previously. However, there is still a major discrepancy in the NOx rate equations since the model curve does not taper down as steeply as the experimental data points, highlighting the fact that there is still a major process being omitted from the kinetic equations.

80

P HC EXP NOx

oe- 60

L-H fit

| =~ 40 0o

~176176176176 o

o

20 0 100

J" 150

%

I

200

:

1

250

',

I ;=1;

300

=I

=-----I

350

400

Inlet Temperature [C]

Fig. 5. Comparison of experimentally observed HC and NOx conversions with those predicted using a Langmuir-Hinshelwood kinetic formalism

162 The mechanism suggests that this process is the high temperature competition between the NO and 02 for the reduced platinum sites. The NO/O2 competition for surface sites at higher temperatures causes a decline in the NOx conversion, so it should be accounted for as an inhibitory term within the kinetic formalism. The Langmuir-Hinshelwood type equations described earlier, which fitted the data satisfactorily at low temperatures and conversions, were modified to account for this process. Using the one-dimensional model and the experimental data an integral type analysis was performed to find values for the parameters shown in equations (9) and (10).

C ) RATEc~m = Ac~m exP Eac~.m X' Cc,mCo~ " Y' ~, RgTs (1+ Cc~vr)(l+ CNo) RATENo = RATEc~n,

(

(9)

(EaN~176I(I+Z'CNo)

Kxo

CNO

(10)

1 + Kxoaa,e x p l ~ l C o ~ '

LR

Equation (9) shows that over the ranges of oxygen concentrations tested there is a positive oxygen effect on the hydrocarbon oxidation rate even at very high levels of oxygen. Apart from this, the propene reaction rate equation has not changed significantly from equation (7). However, an exponential, temperature-dependent NO adsorption term has been added into the denominator of the NOx equation (10). This term acts to decrease the predicted NOx conversion sharply at higher temperatures, which is consistent with the experimental observations. Figure 6 shows a comparison of the predicted and observed propene and NOx conversion profiles using these improved kinetic expressions. The match between the model prediction and the experimental data is now very good. In addition, the model is capable of accurately predicting the effect of variations in reactant concentration, space velocity and heating rate over the whole range

T,)

100 _

_..,

mmm

m ~ -mmm -

_

80 EXP HC

,- 60 .9

l-

~ 40 o

"

EXP NOx L-H-M fit

20 0 100

150 9

200 250 300 350 Inlet Temperature [C]

400

Fig. 6. Comparisonof experimentally observed HC and NOx conversions with those predicted in a 1-dimensionalmonolith model using the Langmuir-Hinshelwoodformalism based on meehani.~tic information

of experimental conditions evaluated.

d) Application of the Model to Vehicle Tests Ageing and evaluating catalysts using engines and vehicles is costly and time-consuming. Often different permutations of exhaust system design and refinement leading to potential improvements in catalyst performance are overlooked because of the expense involved in considering them experimentally. This study has been aimed at obtaining kinetic expressions for different catalysts in the laboratory and employing them within an exhaust system engineering model. The model uses engine outlet data as input and predicts catalyst performance and outlet emissions levels. This capability would greatly compliment and serve to reduce the amount of engine and vehicle testing currently being carried out to evaluate catalysts, and can therefore lead to substantial reductions in costs. A one-dimensional systems model was developed containing (optionally) one or two catalytic monoliths and a length of exhaust pipe. The equations describing the heat and mass balance in a monolith are described earlier by equations (1)-(4). The assumptions and heat and mass transfer parameters used are the same as for the laboratory monolith model described above. The onedimensional equations for the pipe are as follows:

163 c3T. P. Cv~ Ot

=

02T~ Ri hi 2~ o,Z2. + R~ AR (Tg T.)

OTg 2hi PgCwv Oz + R, (Tg T~) = 0

Ro ho R. AR (T.

T~..)

(11)

(12)

The kinetic expressions for the hydrocarbon reaction rate were obtained using propene as a representative hydrocarbon. The hydrocarbon species emitted from the engine do not behave in exactly the same way as propene, so the kinetic 100 i I'1' I"' I' 'il I 9 I parameters in equations (9) and (10) needed to be refined to account for the different reactivity of the engine-out HC species. The form of the 80 equations was kept exactly the same but the = EXP E 'P HC value of the HC pre-exponential factor was o~ 60 altered by fitting to data collected from a stepped light-off experiment performed using the diesel 2.5 L TDI engine. The stepped light40 j EXP NOx off was achieved by increasing the engine load L) in stages and holding it at each point until a steady state was obtained. This was 20 approximated to a very slow temperature ramp in the model. A 0.144m x 0.152m monolith 0 was used with a space velocity of 60,000/hr. It 200 250 300 350 400 450 150 must be stressed that all of the other kinetic Temperature [C] terms used in the model were those obtained during the microreactor tests. Figure 7 shows Fig. 7. Comparison of HC and NOx conversions observed during the fit of the simulated light-off to the engine bench tests for the 2.5L TDI diesel engine with those experimental data. predicted by the model following adjustment of the HC preexponential factor to take account of the different reactivities of The systems model was then used to the engine-out HC species and propene. predict the performance of the arrangement shown in Figure 8. The system comprises two catalyst bricks separated by 0.4m of exhaust HC HC [. . . . [ ~J I [LJ' pipe. HC was injected before each catalyst to 1%~'~/~0] CatalYst i Catalyst ! 1 give a constant HC/NOx of 2:1 and the G.H.S.V. was 30,000/hr over the whole system. Figure 9 shows good agreement between predicted and experimentally Fig. 8. Dual brick exhaust configuration with intermediate pipe observed HC emissions during the vehicle used for simulations. test. Figure 10 shows the predicted NOx emissions over the vehicle test compared with the experimental values. Again the excellent agreement between predicted and the experimental data demonstrates the level of accuracy to which the mechanistically based lean NOx kinetic expressions have been determined.

i • 1

t"

"

Conclusions

Steady state kinetic data have been used to construct a model to predict the lean NOx performance of catalysts under dynamic conditions. A simple power law model fitted to the steady state kinetic data from a gradientless reactor, was not capable of predicting the NOx conversion profile, suggesting that critical features were missing from the model.

164 1000

600 .

' ~'

.

.

.

.

.

.

.

.

.

~ 0 400

600

I!;,

.00 :, r

.

ooo.

3O0

~ 200 Z

200

T 0

1001

0

2

4 6 Time/lO00

8 [s]

10

12

Fig. 9. Inlet, experimental and predicted catalyst outlet hydrocarbon concentrations over vehicle test

0

,

f'

2

,

I '",

t"

~

I'

4 6 8 Time/lO00 [S]

I

i

10

,I

12

Fig. 10. Inlet, experimental and predicted catalyst outlet NOx concentrations over vehicle test

Mechanistic studies revealed that the missing features related to the competition between adsorbed reactants and decomposition products, the most important feature being the strong competition between NO and 02 for reduced Pt sites at high temperatures. Once the mechanistic features were incorporated into the model, excellent agreement was obtained between the model predictions and experimental data obtained using a laboratory microreactor. Following the success of the modelling approach under laboratory conditions, the model was extended to describe the light-off performance on a diesel 2.5L TDI engine. Once again there was excellent agreement between the model prediction and the experimental data, reinforcing our confidence in the power of this modelling approach. It is clear that such a model can be used to optimise the design of lean NOx catalyst systems, to investigate HC injection strategies which maximise NOx reduction whilst minimising any fuel economy penalties. References

[1] [2] [3] [4] [5] [6] [7]

G. Smedler, S. Fredholm, J.C. Frost, P. Loof, P. Marsh, A.P. Walker, D. Winterbom SAE 950750 C.N. Satterfield, "Heterogeneous Catalysis in Practice", McGraw-Hill, New York, 1980,p.359. R.H. Heck, J. Wei and J.R. Katzer, A.I.Ch.E. Journal 2._22(1976) 477. U. Ullah, S.P Waldram, C.J. Bennett and T.J. Truex, Chem. Eng. Sci. 47 (1992) 2413. J.T. Gleaves, J.R. Ebner and T.C. Keuchler, Catal. Rev.-Sci. Eng., 30 (1988) 49. R. Burch, P.J. Millington and A.P. Walker, Appl. Catal. B: Env., 4 (1994) 65. S.E. Voltz, C.R. Morgan, D. Leiderman and S.M. Jacob. Ind. Eng. Chem. Prod. Dev. 12(1973)

Nomenclature

Ai, Ai', Ai" Pre-exponential factors Cgi, Csi Gas & Solid species concentration Cp~ Gas specific heat capacity Ea~, Ea~', Ea~" Activation Energies Heat of reaction for species i h Heat transfer coefficient Mass transfer coefficient kmi Pipe inner and outer radii R~,Ro R~ Logarithmic mean pipe radius AR Pipe wall thickness

Rg Universal gas constant Sv Specific surface area Sa Specific surface area of catalyst Tg, Ts Gas and Solid temperatures X, Y, Z, X', Y', Z' Adsorption parameters z Spatial axial co-ordinate 3' Porosity pgi, Psi Gas & Solid densities ~Ls,Lw Solid & Wall thermal conductivities v Velocity