Global Kinetic Modelling of a Supplier Barium- and Potassium

Ind. Eng. Chem. Res. 2006, 45, 8883-8890

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Global Kinetic Modelling of a Supplier Barium- and Potassium-Containing Lean NOx Trap Louise Olsson* Competence Centre for Catalysis, Chalmers UniVersity of Technology, 412 96 Go¨teborg, Sweden

David Monroe and Richard J. Blint Chemical and EnVironmental Sciences Laboratory, General Motors Research and DeVelopment Center, 30500 Mound Road, Warren, Michigan 48090-9055

Kinetic modeling, in combination with flow reactor experiments, was used in this study to simulate a supplier lean NOx trap (LNT). The LNT catalyst used is a commercial catalyst that contains barium and potassium as storage components. The results presented in this paper are a continuation of a previous study, where a global kinetic model for NOx storage was developed for a model Pt/Rh/BaO/Al2O3 catalyst. In this work, a simplified model is used, where NO oxidation, nitrite, and nitrate formation are lumped together into one reaction, and the model predicts the total NOx. In this model, only one reaction step must be tuned for each storage component: that is, in our case, one reaction for the formation of barium nitrate and one reaction for potassium nitrate. A broad range of experimental conditions was used when developing this model; five temperatures (200, 300, 400, 500, and 600 °C) and three different inlet NO concentrations (100, 200, and 300 ppm) were used. Water and CO2 were present in all experiments, because these can affect the storage behavior. The reductant used in the regeneration period was CO. Long lean and rich cycles were used to capture the kinetics of the reactions accurately. The model was able to describe all 15 experiments well and could adequately capture the amount of stored NOx during the lean period and the NOx conversion during the rich period, including the NOx breakthrough peak that occurred at the beginning of the rich period. The model was validated with short lean-rich cycling experiments, where the lean period was 30 s and the rich period was 2 s. The model could predict the outlet NOx concentration well, and the error for the average conversion was only 1%-2% in the validation simulations. 1. Introduction Diesel engines and lean-burn gasoline engines have better fuel economy, compared to stoichiometric gasoline engines. However, the conventional three-way catalyst used with today’s stoichiometric gasoline engines cannot reduce the nitrogen oxides (NOx) when the exhaust is lean, because the reductants in the exhaust (CO, hydrocarbons (HCs), H2) have a tendency to react preferentially with oxygen, rather than with NO. One possible solution to this problem may be the use of lean NOx traps (LNTs) ,1-11 where the catalyst contains both a NOx adsorbing material (e.g., barium oxide) and noble metals (platinum and rhodium). Mixed lean operation is needed to use the LNT concept. During the lean periods, NOx is adsorbed on the storage component, forming nitrates; during short rich pulses, the nitrates are decomposed and the NOx is reduced on the noble-metal sites to nitrogen. Several studies have been presented in the literature where NOx adsorbers have been simulated/modeled with different techniques.1,5-17 Olsson et al.5,6 presented a detailed kinetic model for NO oxidation and NOx storage over a Pt/BaO/ Al2O3 model catalyst. Schneider7 performed density functional theory (DFT) calculations and determined that the adsorption energies from first-principle calculations were very similar to those derived by Olsson et al.5 using detailed kinetic simulations. There are also several studies available in the literature that involve global mechanisms and different control-oriented models.1,9-17 Hepburn and co-workers9,10 developed a kinetic * To whom correspondence should be addressed. Fax: +46-317723035. E-mail address: [email protected].

model for NOx storage on a barium-containing catalyst, and they used a shrinking-core model to describe mass transport in the barium particles. Tuttlies et al.11 also included mass-transfer limitations in the barium particles in their model. In an earlier publication,1 we presented a global kinetic model for NOx storage on a model Pt/Rh/BaO/Al2O3 catalyst, using a shrinking-core model to describe the mass-transfer resistance. The model could adequately describe the experimental data, where temperature, as well as NO and oxygen concentrations, were varied. The objective of the present study is to simulate NOx storage and regeneration for a barium- and potassiumcontaining supplier LNT catalyst, using a more-complex inlet gas with both CO2 and H2O present. 2. Experimental Procedure A supplier NOx storage catalyst that contained barium, potassium, and platinum was used in this study. The sample was a cored-out monolith 1 in. (25.4 mm) long and ∼0.75 in. (19.05 mm) in diameter. The sample was placed in a reactor, and the gas mixture was produced using mass-flow controllers. The construction of the setup made it possible to produce very rapid switching between lean and rich environments. The NOx was analyzed with a chemiluminescense detector (Horiba model CLA 220). Prior to NOx storage measurements (as in Figure 1), the catalyst was aged in air with 10% water and 10% CO2 at 700 °C for 16 h. Then, prior to each cycle, the sample was pretreated under rich conditions for 240 s to remove NOx from the surface. Repeated cycles after these pretreatments resulted in similar NOx-out profiles, which indicated that no residual NOx was

10.1021/ie0608105 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/22/2006

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Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006

Table 1. Reaction and Rate Expression for CO Oxidationa reaction

reaction rate

r1

r1 ) k1(CCOCO2/G1), where G1 ) T(1 + K1CCO + K2CC3H6)2(1 + K3C2CO CC3H6)(1 + K4C 0.7 NO)

CO + 0.5O2 98 CO2 a

Data taken from ref 18.

present. The NOx storage/regeneration was investigated using experiments, where the feed gas was cycled between lean (660 s) and rich periods (240 s), using three different NO concentrations: 100, 200, and 300 ppm. The concentrations in the respective periods were as follows: Lean period: 10% O2, 5% CO2, 5% H2O, and 100 (or 200 or 300) ppm NO Rich period: 3% CO, 5% CO2, 5% H2O, and 100 (or 200 or 300) ppm NO The experiments were conducted at temperatures of 200, 300, 400, 500, and 600 °C. The total feed flow rate was 6 L/min, yielding a space velocity of 50 000 h-1. Two additional experiments were performed at 350 and 500 °C with short lean and rich cycles on a catalyst aged by reaction conditions up to 600 °C. The experimental procedure was first subjected to pre-conditioning in a rich feed gas for 60 s, followed by nine cycles with 30 s under lean conditions and 2 s under rich conditions. The concentrations in the respective periods were as follows: Lean period: 10% O2, 250 ppm NO, 50 ppm HC (2:1 C3H6:C3H8), 5% CO2, and 5% H2O Rich period: 100 ppm NO, 20 ppm HC (2:1 C3H6:C3H8), 3.6% CO, 1.2% H2, 5% CO2, and 5% H2O The total feed flow rate was 6 L/min, yielding a space velocity of 50 000 h-1. 3. The Model This work is a continuation of an earlier publication,1 where NOx storage on a model Pt/BaO/Al2O3 catalyst was simulated using a global LNT mechanism. A tanks-in-series model, with 15 tanks, was used to describe the reactions over the LNT. The differential equations for the material balances and heat balance were solved numerically using Matlab and Simulink software programs. The Arrhenius equation is used to describe the rate constant, k:

( )

EA k ) A exp RT

(1)

where A is the pre-exponential factor, EA the activation energy (expressed in units of J/mol), R the gas constant (8.314 J/(mol K)), and T the temperature (expressed in Kelvin). The response time characteristics of the reactor system were investigated by making step changes in gas composition using an empty reactor (no catalyst). The system response was simulated using a first-order filter in the Simulink program. All simulated NOx concentrations in this work are compensated for the system response. 3.1. Kinetics for Oxidation Reaction. We have used the rate expression that was derived by Volz et al.18 for CO oxidation. This has been used in several studies19-22 and is given in Table 1. In the table, Ci is the concentration of species i (expressed as a molar fraction) and G1 is a factor that considers the inhibiting effect of CO, C3H6, and NO on CO oxidation. 3.2. Kinetics for NOx Storage. 3.2.1. Previous LNT Model for NO and NO2. In a previous publication,1 three reaction steps were used to describe the NOx storage: NO oxidation to NO2,

formation of barium nitrate from NO2, and formation of barium nitrite from NO:

NO + 0.5O2 T NO2

(2)

0.5BaCO3 + NO2 + 0.25O2 T 0.5Ba(NO3)2 + 0.5CO2 (3) 0.5BaCO3 + NO + 0.25O2 T 0.5Ba(NO2)2 + 0.5CO2 (4) Many studies2,23,24 have shown that NO oxidation is important for the storage of NOx; therefore, this step was included in the model. Barium nitrates have been observed in several Fourier transform infrared (FTIR) studies25,26 when barium-containing catalysts have been exposed to NO2. In NO + O2 environment, surface nitrites are initially observed on platinum- and bariumcontaining catalysts;27 however, for prolonged exposure, nitrates dominate the surface. In the initial simulations, only NO oxidation and the formation of barium nitrate was used (eqs 2 and 3); however, with this model, it was not possible to describe the rapid initial storage at the same time as the low NO oxidation activity observed experimentally. When one reversible reaction step is included for the formation of barium nitrite, the model was able to describe the experiments adequately. The barium nitrites can probably be oxidized to barium nitrates; however, this reaction was not needed to describe the experimental features observed, and, therefore, this reaction was not added to the model. Mass transfer in the wash coat12 or in the barium particles1,9-11 have been used in some NOx storage simulations. Hepburn and co-workers9,10 used a shrinking-core model to consider the mass transport in the barium particles. Tuttlies et al.11 also included mass transfer in the particles in their model. Theis et al.28 also explained their experimental results with a mechanism that was based on a shrinking-core model. Kojima et al.12 assumed masstransfer limitations in the wash coat instead, but used a mathematical expression that was similar to that used by Hepburn and co-workers9,10 for mass-transport limitations in the particles. We have chosen to implement a shrinking-core model, for mass-transport in the barium particles, and this model was used to simulate NOx storage on a model Pt/BaO/Al2O3 catalyst.1 The model was able to describe experimental results obtained at different temperatures, as well as different NO and oxygen concentrations. The basic concept of the shrinking-core model is that nitrates are first formed on the surface and then penetrate further and further into the barium carbonate particle. This is described numerically by a decreasing rate constant as the nitrates increase, because the NO and NO2 have a longer diffusion distance (see Olsson et al.1 for more details and derivation of the equations). 3.2.2. Modified LNT Model for Total NOx. In this work, a simplified model was investigated with only one reaction step, where NO oxidation, and formation of barium nitrites and barium nitrates, were lumped together into one reaction. This model considers the total NOx coming into and out of the system and does not distinguish the NO and NO2 concentrations. Consequently, the exponent of CO2 was selected to be 1/4, for consistency with the more-explicit nitrite and nitrate formation steps. The choice of this exponent is somewhat arbitrary but should be quite low, because the storage amount is only weakly

Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006 8885 Table 2. Reaction and Rate Expressions for the NOx Storage Reactions reaction

reaction rate

r2

r2 )

0.5BaCO3 + NO + 0.75O2 798 0.5Ba(NO3)2 + 0.5CO2

kbulk,/ 2,f

kbulk,/ ) 2,f kbulk,/ ) 2,b r3

kbulk,/ ) 3,f kbulk,/ ) 3,b

dependent on the oxygen concentration. The same form as that given in the previous model was used for the reaction rates in this case.1 The advantage of this model is that only one reaction step must be tuned,which makes it more convenient when using several storage components in the catalyst. In this work, a supplier catalyst is used that contains both barium and potassium. In regard to the reaction rate for NOx storage on potassium, the same form as that for the storage on barium has been applied. The reactions and rates for storage on the barium and potassium are given in Table 2. In this table, Ci is the molar fraction of species i (which is dimensionless), l is the thickness of the nitrate layer, Rtot is the total radius of the barium particle, and r1 is the radius of the barium carbonate (r1 ) Rtot - l). D is a constant (expressed in terms of kmol/(s m2)) in which the diffusivity (given in units of m2/s), the total concentration (P/RT, given in terms of kmol/(m3 s)), and an area per volume (expressed in units of m2/m3) are lumped together. The mean coverage of barium nitrate and barium carbonate in the entire particle are described using the parameters θBa(NO3)2,m and θBaCO3,m, respectively. This observation means that

θBa(NO3)2,m ) number of moles of Ba(NO3)2 number of moles Ba(NO3)2 + number of moles BaCO3 The same nomenclature was applied for potassium. 3.3. Kinetic Model for NOx Regeneration. A kinetic model for three-way catalysts was developed by Montreuil et al.,29 based on experiments where the proportion of actual to stoichiometric air/fuel ratios (λ ) (air/fuel)actual/(air/fuel)stoich) varies between λ ) 0.5 and λ ) 4.5. They found it necessary to introduce different kinetics for NO reduction under rich and lean conditions; therefore, they used a blending function β1 to switch between the reaction rates (see eqs 5-8).

R1 ) CCO + 9CC3H6 + 10CC3H8 + CH2 - CO2 - CNO

1/2 - kbulk,/ CCO θBa(NO3)2,m, where 2,b 2

k2,f 1 + k2,f(l/D)(r1/Rtot) k2,b 1 + k2,f(l/D)(r1/Rtot)

1/2 r3 ) kbulk,/ CNOCO1/42 θK2CO3,m - kbulk,/ CCO θKNO3,m, where 3,f 3,b 2

0.5K2CO3 + NO + 0.75O2 798 KNO3 + 0.5CO2

β1 ) 2500R1 + 0.5

CNOCO1/42 θBaCO3,m

(5) (6)

if β1 < 0, set β1 ) 0

(7)

if β1 > 1, set β1 ) 1

(8)

where Ci is the molar fraction of species i. Shamim et al.20 used this kinetic model, together with oxygen storage reactions and simulated test cycles, according to the U.S. Federal Test Procedure (FTP). The air-fuel ratio was also considered by Kim et al.13 in their LNT simulations. They used a release rate of

k3,f 1 + k3,f(l/D)(r1/Rtot) k3,b 1 + k3,f(l/D)(r1/Rtot)

Table 3. Reaction and Rate Expressions for NO Reduction and Nitrate Regeneration with CO reaction

reaction rate r4

0.5Ba(NO3)2 + 1.5CO 98 0.5BaCO3 + NO + CO2 r5

KNO3 + 1.5CO 98 0.5K2CO3 + NO + CO2 r6

CO + NO 98 CO2 + 0.5N2

r4 ) β1k4CCOθBa(NO3)2,m0.3 r5 ) β1k5CCOθKNO3,m0.3 r6 ) β1k6(CCO1.9CNO0.13/G2), where G2 ) T-0.17(T + K5CCO)2

NOx from the adsorber, which was a function of the λ value. Kojima et al.12 simulated NOx storage and regeneration, and they turned off the regeneration reactions during lean operation. We have chosen, as in our earlier study,1 to use β129 for the NOx reduction reactions, including the LNT regeneration. The function R1, which is defined in eq 6, is modified for the gases that are present in our experiments. The reaction steps for the reduction of NO with CO and for the regeneration of barium nitrate and potassium nitrate with CO are summarized in Table 3. These are global reactions where the barium nitrate (or potassium nitrate) first reacts with CO, which results in the formation of barium carbonate (or potassium carbonate) and NO. The dependence of these rates on coverages (θ) was adjusted to fit the data within the simulations. The same form of the reaction rate has been applied for the regeneration of barium nitrate and potassium nitrate. The NO formed reacts with CO to produce N2, and, for this step, the kinetics that have been derived by Subramanian and Varma30 are used. 4. Results and Discussion The experiments in this study are performed with a barium-, potassium-, and noble metal-containing supplier catalyst. Figure 1a shows the measured NOx concentration coming from the catalyst from one lean cycle (10% O2, 5% CO2, 5% H2O, and 200 ppm NO) and one rich cycle (3% CO, 5% CO2, 5% H2O, and 200 ppm NO). The gray field in this figure marks the amount of stored NOx during the lean period. The integrated amount of stored NOx during the lean period of 660 s for the five temperatures and different NO levels are given in Figure 1b. The maximum storage occurs at 400 °C; at lower temperatures, the storage is lower, because of the kinetics of the reactions. At high temperatures, the nitrates begin to be unstable and carbonate formation is favored. However, at 500 °C, the storage is still very high on this catalyst. This is due to the addition of potassium, which forms more high-temperature stable nitrates,31 compared to barium, which is not stable at 500 °C.4,31 Figure 1b also shows a comparison between the stored

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Figure 1. (a) NOx concentrations from a NOx storage experiment at 400 °C; the gray area indicates the amount of stored NOx during lean period. (b) Amount of stored NOx during the lean period versus temperature for three different NO inlet concentrations.

Figure 2. Catalyst-out NOx concentrations for experiments (solid lines) and simulations (dashed lines) for NOx storage cycles at 200 °C using a Ba-K supplier catalyst. Three inlet NO concentrations (dashed-dotted lines) have been used: (a) 100 ppm, (b) 200 ppm, and (c) 300 ppm. Region legend: 0-8 s, rich conditions; 8-670 s, lean conditions; and 670-900 s, rich conditions.


amounts of NOx using three different levels of NO in the feed gas. The storage is increased when the NO concentration is increased. Because the catalyst, in most cases, is far from saturation during the lean cycle of 660 s, it is natural that a larger storage exists using a higher NO concentration. However, at 200 and 600 °C, the NOx that is coming from the catalyst reaches the inlet concentration; thus, the catalyst is saturated for these two cases. At these temperatures, it is also observed that the storage is increased as the NO concentration is increased. Thus, there is a real effect of the NO concentration on the maximum storage capacity. Figure 2 shows the measured (solid lines), simulated (dashed lines), and inlet (dashed-dotted lines) NOx concentrations for

three NOx storage experiments at 200 °C, where the NO concentration in the feed gas was varied (100, 200, and 300 ppm). The corresponding results for 300, 400, 500, and 600 °C are shown in Figures 3-6, respectively. The model is able to describe the 15 experiments adequately. At 200 °C, the storage is low and the conversion of NOx during the regeneration period is not complete. When the rich period is started, the NOx is released very rapidly from the storage component and not all of it is reduced on the noble-metal sites, resulting in a NOx breakthrough peak. The model can predict this at temperatures in the range of 300-600 °C. However, the small peaks observed at 200 °C are not reproduced by the model. When the temperature is increased, both the storage and NOx

Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006 8887




reduction in the rich period increases and the storage reaches a maximum at 400 °C. The storage is still high at 500 °C and, in the model, this is mainly due to potassium nitrates. This is in accordance with previous experiments in the literature,31 which showed that potassium forms more high-temperature stable nitrates than barium. Furthermore, at 500 °C, the NOx breakthrough peaks that occur when switching to the rich period are the largest. The reason for this can be that, at temperatures this high, the decomposition of the nitrates is very rapid, in combination with the fact that the storage is very large. When the temperatures are increased further, the storage decreases significantly, resulting in a reduction of the NOx breakthrough peaks when switching to rich conditions. In the model, no barium nitrates are formed at 600 °C, because they are unstable at this high temperature and the small amount of NOx that is stored occurs on the potassium sites.

Figure 7 shows the amount of stored NOx during the lean cycle (660 s) for both the experiments (solid lines) and the simulations (dashed lines). The integrated total amount of NOx breakthrough, as a function of temperature during both the lean period (660 s) and the rich period (240 s), are shown in Figure 8, where the solid lines represent the experimental results and the dashed lines represent the simulations for the three different NO feed gas concentrations. The model predicts the conversion of NOx during both the lean and rich period very well over the broad range of temperatures and feed NO concentrations. The parameters for the model are given in Tables A1 and A2 of the Appendix. Eleven parameters were fitted using the least-squares method, and the 95% confidence intervals for the fitted parameters are given in Tables A1 and A2. Five experiments were used in the fitting procedure. The inlet gas contained 10% O2, 5% CO2,

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Figure 7. NOx stored during the lean period using three different NO inlet concentrations: 100, 200, and 300 ppm NO. The experimental results are shown by the solid lines, and the simulations are given by the dashed lines.

Figure 8. Integrated amount of NOx breakthrough during lean period (660 s) and rich period (240 s) using three different NO inlet concentrations: 100, 200, and 300 ppm NO. The experimental results are shown by the solid lines, and the simulations are given by the dashed lines.

5% H2O, and 200 ppm NO during the lean period and 3% CO, 5% CO2, 5% H2O, and 200 ppm NO during the rich period; the temperatures for the five experiments were 200, 300, 400, 500, and 600 °C, respectively. The model then was validated with the other 10 experiments.

The enthalpy for the formation of barium nitrate from NO and barium carbonate was calculated to be 172.2 kJ/mol, using the thermodynamic values for bulk species.32 The surface species on the catalyst can have different energies, compared to the bulk values; however, because there are no reported thermodynamic values for these species, the bulk values were used as an approximation. The same approximation was used for the formation of potassium nitrates, resulting in an enthalpy of 195.0 kJ/mol.32 The barium and potassium particles in this catalyst are likely to be quite small, because it is a supplier catalyst with high storage capacity. In addition, the sample has been pretreated under quite mild conditions (700 °C), which will not cause the particles to sinter. Asanuma et al.33 measured sulfate particles from an aged lean NOx trap, using X-ray diffraction (XRD), and concluded that, when using a maximum of 30 ppm sulfur, the sulfate particle size was

Global Kinetic Modelling of a Supplier Barium- and Potassium

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