Modeling periodic NOx storage–reduction in the presence of CO2

Chemical Engineering Science 62 (2007) 5167 – 5175 www.elsevier.com/locate/ces

Modeling periodic NOx storage–reduction in the presence of CO2 V.R. Gangwal, C.M.L. Scholz, M.H.J.M. de Croon, J.C. Schouten ∗ Laboratory of Chemical Reactor Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Received 20 June 2006; received in revised form 14 December 2006; accepted 26 December 2006 Available online 17 January 2007

Abstract The influence of temperature on periodic NOx storage–reduction in the presence of CO2 has been investigated. A dynamic plug flow model and experiments performed in a packed bed reactor with a Pt–Ba/-Al2 O3 powder catalyst (1 wt% Pt and 30 wt% Ba) with different lean/rich cycle timings at different temperatures are compared. The model is based on a multiple storage site mechanism. The rate of NOx storage increases with temperature. NOx storage below 300 ◦ C mainly occurs through NO adsorption as nitrites. The oxidation of nitrites to nitrates by NO2 , with release of NO, is the predominant storage step, and its rate increases with temperature. The number of sites taking part in NOx storage is independent of temperature and cycle time. The reduction of stored NOx with H2 is efficient. However, with an increase in temperature, thermal decomposition of nitrites/nitrates results in a decrease of NOx reduction. 䉷 2007 Elsevier Ltd. All rights reserved. Keywords: NOx storage; NOx reduction; Kinetic model

1. Introduction Diesel and lean burn engines are fuel efficient and have lower emissions of green-house gases compared to stoichiometric gasoline engines. However, due to use of high air-to-fuel ratios, the control of NOx (NO and NO2 ) emissions remains a challenge. Presently the most promising solution for passenger cars is the use of NOx storage–reduction (NSR) catalysts along with mixed lean operation, where the air-to-fuel ratio is altered between lean (oxygen excess) and rich (fuel excess) conditions (Bogner et al., 1995; Matsumoto, 2000). During the relatively long lean periods, NOx is stored in the form of nitrates/nitrites on a storage component, e.g. barium. During short rich periods, the storage material is regenerated and the released NOx reacts with hydrocarbons, CO, and H2 to produce CO2 , H2 O, and N2 over the noble metal sites. The lean and rich phases are alternated to achieve the highest NOx storage/reduction efficiencies (Li et al., 2001). To optimize the overall NOx reduction efficiency, a good understanding of the mechanism of storage is necessary (Fridell et al., 1999; ∗ Corresponding author. Tel.: +31 40 247 2850; fax: +31 40 244 6653.

E-mail address: [email protected] (J.C. Schouten) URL: http://www.chem.tue.nl/scr 0009-2509/$ - see front matter 䉷 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2006.12.078

Lietti et al., 2001). This mechanism can be implemented in a quantitative model of the NOx storage process to predict the extent of storage and the NO and NO2 emissions as a function of time and varying operating conditions. The oxidation of NO to NO2 is an important part of the overall process. It is believed that the rate of NO2 adsorption on the trapping material of the catalyst is higher than that of NO and the final state of the stored NOx is in the form of barium nitrate. The kinetic limitations at low temperatures and the thermodynamic equilibrium of the oxidation at high temperatures prevent high access to NO2 (Daw et al., 2003). In addition the temperature affects the storage of NOx as the thermodynamic stability of nitrites and nitrates decreases at elevated temperatures, while the adsorption is simultaneously increased. These phenomena result in an optimum temperature for the NSR catalyst. Extensive research has been done in the past decade to understand and describe the NOx storage–reduction mechanism. The review paper of Epling et al. (2004) offers a useful survey of the subject. Detailed kinetic models (Olsson et al., 2001; Laurent et al., 2003) and global kinetics with diffusion models (Tuttlies et al., 2004; Olsson et al., 2005) have been used to describe the process. However, these models are based on a single storage site mechanism, while there is growing evidence that NOx storage occurs through multiple storage

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(240, 300, and 370 ◦ C) with a Pt–Ba/-Al2 O3 powder catalyst (1 wt% Pt and 30 wt% Ba) have been used. 2. Modeling

Fig. 1. Pictorial representation of surface, semi-bulk, and bulk barium sites.

sites (Epling et al., 2004; Scholz et al., 2007a). Moreover, most of these studies do not consider the influence of CO2 and/or water, which are known to have a detrimental effect on the NOx storage–reduction process (Balcon et al., 1999; Rodrigues et al., 2001; Epling et al., 2003). A systematic study by Epling et al. (2003) with varying temperature in-between 175 and 420 ◦ C suggests that CO2 strongly affects both the storage and reduction abilities of NSR catalysts at all temperatures, whereas the water effect is limited to the storage process mainly at low temperatures (< 300 ◦ C). Balcon et al. (1999) pointed out that CO2 inhibits NO2 storage and promotes release of stored NOx . They interpreted this in terms of the existence of the equilibrium CO2(g) + Ba nitrates ↔ NO2(g) + Ba carbonates. Sedlmair et al. (2003) also found that NO2 shows inactivity for carbonate replacement. Interestingly, NO was able to replace carbonate by nitrites with the release of CO2 . Further they proposed that oxidation of the surface nitrites into surface nitrates by NO2 is the predominant step during the storage of NOx on the catalyst. Recently, we have also found that in the presence of CO2 , NOx storage on the surface Ba-sites mainly occurs through NO adsorption as nitrites and the role of NO2 is to oxidize nitrites to nitrates (Scholz et al., 2007a). The purpose of this work is to explain the influence of temperature on the periodic NOx storage–reduction process in the presence of CO2 . For this purpose a recently developed model (Scholz et al., 2007a) and lean–rich experimental data obtained in a packed bed reactor at three different temperatures

The plug flow reactor model uses a global reaction kinetic model, which is described in detail elsewhere (Scholz et al., 2007a). The kinetic model considers that NOx storage occurs on three types of barium sites, viz. surface, semi-bulk, and bulk barium sites. The surface, semi-bulk, and bulk sites not only differ in physical appearance (as shown in Fig. 1), but also in chemical reactivity; surface sites are being the most reactive, whereas bulk sites are the least reactive. Fast NOx storage occurs at the surface sites, which is determined by the kinetics, while slow NOx storage occurs at the semi-bulk and bulk sites, where NOx diffusion inside barium cluster plays a major role. It is considered that in the presence of CO2 , barium is present in the carbonate form. The global reaction steps with the rate equations are presented in Table 1. 2.1. Reactor model The packed bed reactor is regarded as an ideal plug flow reactor, under isothermal conditions, as experimental data show a maximum increase in temperature of 5 K. The pressure is taken uniform and equal to ambient throughout the packed bed. The gas bulk in the packed bed reactor is discretized in the axial direction z and the gas in the spherical barium clusters with the partial co-ordinate . Both parts are connected by diffusion from the bulk gas to the inner side of the barium clusters. The change in the concentration of the different components as a function of time at each axial position can be described as follows:  Fv jCi jCi jCi  b =− − Deff ABa jt Ar jz j =R  + LPt i,j RPt,j j

+ LBa,surf



i,k RBa_surf,k ,

(1)

k

Table 1 The global reaction steps and rate equations used for modeling NOx storage and reduction Reactions

Rate equation

Lean phase NO + 1/2 O2 ⇔ NO2 BaCO3 + 2 NO + 0.5 O2 → Ba(NO2 )2 + CO2 Ba(NO2 )2 + 2 NO2 → Ba(NO3 )2 + 2 NO BaCO3 + 3NO2 → Ba(NO3 )2 + NO + CO2

Rox = kox_f CNO CO0.52 − kox_b CNO2 2 C 0.5 Rst_NO = ks _NO,i BaCO3,i CNO O2 2 Rst_oxi = ks _dis,i Ba(NO2)2,i CNO2 Rst_NO2 = ks _NO2,i BaCO3,i CNO2

(I) (II) (III) (IV)

Rich phase Ba(NO2 )2 + H2 + CO2 → BaCO3 + 2NO + H2 O Ba(NO3 )2 + 3H2 + CO2 → BaCO3 + 2NO + 3H2 O Ba(NO3 )2 + CO2 → BaCO3 + 2NO2 + 0.5 O2 NO + H2 → 1/2 N2 + H2 O

Rreg,nitrite = kreg,nitrite,i Ba(NO2)2,i CH2 Rreg,nitrate = kreg,nitrate,i Ba(NO3)2,i CH2 Rreg,nitrate = kreg,nitrate,thermal Ba(NO3)2,i Rred = kred CNO CH2

(V) (VI) (VIa) (VII)

The reaction rate parameters ks _NO,i , ks _dis,i , ks _NO2 ,i , kreg,nitrite,i , and kreg,nitrate,i have different units and values for the surface, semi-bulk, and bulk barium sites with i = surface, semi-bulk or bulk. See also Table 2.

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Table 2 Model parameters and constants Catalyst parameters ( Scholz et al., 2007a) LPt LBa,surface LBa,semi-bulk LBa,bulk ABa dBa

Description 9.67 1.34 × 102 4.01 × 102 1.53 × 103 2.20 × 107 10 × 10−9

−3 Moles of active Pt surface sites per m3 reactor (mol mreactor ) −3 3 Moles of active Ba surface sites per m reactor (mol mreactor ) −3 Moles of active Ba semi-bulk sites per m3 reactor (mol mreactor ) −3 Moles of active Ba bulk sites per m3 reactor (mol mreactor ) 2 m −3 Specific barium surface (mBa reactor ) Diameter of semi-bulk barium cluster (mBa )

0.35 1.40 × 10−4 3.54 × 10−5 1.50 × 10−2 3.64 × 10−15 0.5 0.055

3 m −3 Bed porosity (mgas reactor ) 2 Surface area of the reactor (mreactor ) 3 −1 Volumetric flow rate (mgas s ) Reactor length (mreactor ) 3 m −1 s−1 ) Diffusion coefficient (mgas Ba −3 3 Cluster porosity (mgas mBa ) 3 m −3 Volume fraction (mBa reactor )

Reactor parameters (Scholz et al., 2007a)

b Ar Fv Lreactor Deff

cluster f

NO oxidation parameters (Olsson et al., 2005) exp(−(−58000 + 76T)/(RT)) Koxi_equilibrium kox_f 2.72 × 103 exp(−39200/RT) kox_f /Koxi_equilibrium kox_b Storage parameters Surface barium sitesa ks _NO2 ks _NO ks _dis Semi-bulk barium sitesa ks _NO2 ks _NO ks _dis Bulk barium sites (Scholz et al., 2007a) ks _NO2

NO oxidation equilibrium 4.5 mol−0.5 mol−1 s−1 ) Forward NO oxidation (mgas Pt i 3 mol−1 s−1 ) Backward NO oxidation (mgas Pt

1.60 × 104 2.54 × 1010 exp(−78016/RT) 3.6 × 103 exp(−48670/RT)

3 mol−1 s−1 ) NO2 storage (mgas i 7.5 NO storage (mgas mol−2.5 s−1 ) i 6 −1 Nitrite oxidation (mgas mol−2 i s )

3.91 × 104 1.95 × 103 9.10 × 102

3 3 mol−1 s−1 mol NO2 storage (mgas Ba mBa ) i −2.5 −1 3 ) 7.5 NO storage (mgas moli s molBa mBa 3 6 mol−2 s−1 mol Nitrite oxidation (mgas Ba mBa ) i

2.10 × 10−3

3 3 mol−1 s−1 mol NO2 storage (mgas Ba mBa ) i

Regeneration parameters Surface barium sitesa kreg,nitrite kreg,nitrate kreg,nitrate,thermal (370 ◦ C)

1.9 × 102 exp(−41096/RT) 1.2 × 103 exp(−37680/RT) 1.0 × 10−2

3 mol−1 s−1 ) Ba(NO2 )2 regeneration (mgas i 3 −1 Ba(NO3 )2 regeneration (mgas mol−1 i s ) Ba(NO3 )2 thermal decomposition (s−1 )

Semi-bulk barium sitesa kreg,nitrite kreg,nitrate kreg,nitrate,thermal (370 ◦ C)

1.45 × 102 2.7 × 106 exp(−33994/RT) 2.0

3 3 mol−1 s−1 mol Ba(NO2 )2 regeneration (mgas Ba mBa ) i −1 −1 3 3 Ba(NO3 )2 regeneration (mgas moli s molBa mBa ) Ba(NO3 )2 thermal decomposition (s−1 )

Bulk barium sites (Scholz et al., 2007a) kreg,nitrate

1.96 × 10−1

3 3 mol−1 s−1 mol Ba(NO3 )2 regeneration (mgas Ba mBa ) i

Reduction parameters (Scholz et al., 2007a) 2.9 × 103 kred a Obtained

6 mol−2 s−1 ) NOx reduction (mgas i

by fitting the reactor model to the experimental data obtained at 240, 300, and 370 ◦ C.

where i = NO, NO2 , O2 , H2 , and N2 , RPt,j is the NO oxidation (Rox ) and NO reduction rate (Rred ) as given in Eqs. (I) and (VII) in Table 1, and RBa_surf,k is the NOx storage (Rst_NO , Rst_NO2 ) and NOx decomposition (Rreg,nitrite , Rreg,nitrate ) rate at barium surface sites determined from the reaction Eqs. (II)–(VI). The rate of change of the nitrite/nitrate species concentration on the surface barium sites is given as jm,surf  = i,k RBa_surf,k , jt k

(2)

where m denotes either Ba(NO2 )2 , Ba(NO3 )2 , or BaCO3 . The site balance for the surface barium sites is BaCO3 ,surface = 1 − Ba(NO2 )2 ,surface − Ba(NO3 )2 ,surface .

(3)

The NOx storage–reduction at the semi-bulk and bulk barium sites is limited by the diffusion of gaseous species inside the barium cluster, the respective balance equations are as follows:    jCi jCi 1 j cluster i,k RBa_x,k , (4) = 2 Deff 2 + jt j  j k

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Fig. 2. Lean phase NO and NO2 reactor outlet concentrations at different temperatures. (a) 240 ◦ C, (b) enlarged view of (a), (c) 300 ◦ C, (d) enlarged view of (c), (e) 370 ◦ C, and (f) enlarged view of (e). Continuous lines show experimental data and dashed-dotted lines (NO) along with dashed lines (NO2 ) show model predictions.

where x means semi-bulk or bulk barium sites, and RBa_x,k is the NOx storage and NOx decomposition rate for the semi-bulk and bulk sites, which can be determined from the rate equations (II)–(VI), mentioned in Table 1. The time-dependent changes of the different species on the semi-bulk and bulk barium sites are given as, respectively: LBa,semi-bulk jm,semi-bulk  = i,k RBa_semi-bulk,k , f jt

(5)

LBa,bulk jm,bulk  = i,k RBa_bulk,k , f jt

(6)

k

k

with the site balances for the semi-bulk and bulk barium sites are shown in Eqs. (7) and (8): BaCO3 ,semi-bulk = 1 − Ba(NO2 )2 ,semi-bulk − Ba(NO3 )2 ,semi-bulk ,

(7)

BaCO3 ,bulk = 1 − Ba(NO3 )2 ,bulk .

(8)

The initial and boundary conditions are: Ci = 0, Ba(NO3 )2 ,y = 0, and Ba(NO2 )2 ,y = 0 at t = 0, where y is surface, semi-bulk, or bulk barium sites; Ci = Cin at t > 0 and z = 0; jCi,z, /j = 0 at  = 0 at any t.

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Fig. 3. Lean phase simulated Ba-site coverages at the reactor outlet at 240 ◦ C. (a) Surface Ba-site coverages, (b) semi-bulk Ba(NO2 )2 coverages, and (c) semi-bulk Ba(NO3 )2 coverages.

The above system of equations is solved using gPROMS (Process Systems Enterprise) software. Table 2 gives the kinetic parameters and constants of the model. The activation energy for NO oxidation (reaction (I)) is taken from Olsson et al. (2005). The pre-exponential factor for the NO oxidation and the storage and regeneration kinetic parameters are obtained by manually fitting the results of the model to the results of the 9 h/15 h lean/rich cycling experiments at different temperatures. The distribution of surface, semi-bulk, and bulk sites is taken from Scholz et al. (2007a): 7% of the total barium sites correspond with surface barium sites, 20% with semi-bulk barium sites, and 73% with bulk barium sites.

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Fig. 4. Rich phase NO, NO2 , N2 , and H2 reactor outlet concentrations at different temperatures. (a) 240 ◦ C, (b) 300 ◦ C, and (c) 370 ◦ C. Continuous lines show experimental data and dashed-dotted lines (N2 ), dashed lines (NO), dotted lines (NO2 ) along with H2 show model predictions.

3. Experimental Lean/rich cycling experiments have been performed in a packed bed reactor, which is described in detail elsewhere (Scholz et al., 2007b). A NOx storage catalyst, Pt–Ba/Al2 O3 (1/30/100, w/w/w), is used in powder form as provided by Engelhard. The total gas flow during the experiments is kept constant at 0.743 mmol/s, resulting in a gas space velocity (GHSV) of 29, 000 h−1 (standard conditions 298 K, 1 bar). Measurements were done at temperatures of 240, 300, and 370 ◦ C with a lean mixture of 0.2 vol% NO, 4 vol% O2 , 10 vol% CO2 , and with a rich mixture of 0.8 vol% H2 , 10 vol% CO2 .

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4. Results and discussion 4.1. Long-time NOx storage Figs. 2a, c, and e show the lean phase experimental NO and NO2 breakthrough profiles as a function of time, at temperatures of 240, 300, and 370 ◦ C, respectively. Three different time periods can be distinguished. The first period shows complete NOx storage on the catalyst. This period increases from 300 to 385 s with an increase in temperature from 240 to 370 ◦ C. The second period shows NOx breakthrough with still a considerable NOx storage. During this period the NO concentration passes through a maximum, which becomes more pronounced with increasing temperature. The final period shows a slow but still measurable NOx storage. The extent of NO oxidation to NO2 increases with an increase in temperature. Interestingly, the temperature seems to have a negligible effect on the total amount of Ba utilized during 9 h of NOx storage. A maximum amount of 30% Ba is utilized. In order to understand and explain the above results, the model is fitted to the experimental data. It appeared that NO2 storage could be best described through the disproportionation mechanism. The temperature dependent kinetic parameters are presented in Table 2. Fig. 2 also shows the model outcomes in case of the NO and NO2 breakthrough profiles at different temperatures. From Figs. 2b, d, and f it can be seen that the model is able to describe the NO storage, NO oxidation to NO2 ,

and NO2 storage. It elucidates that the initial rapid complete NOx uptake occurs on the surface Ba-sites, in which NO and NO2 are stored as nitrite and nitrate, respectively. Fig. 3a shows the simulated surface Ba-site coverage as a function of time at 240 ◦ C. The BaCO3 sites are rapidly converted to mainly Ba(NO2 )2 with a minor amount of Ba(NO3 )2 . This conversion progresses from reactor inlet to the outlet and as soon as BaCO3 at the outlet starts converting, NO breakthrough can be seen in Fig. 2b. The time required to convert outlet BaCO3 (Fig. 3a) corresponds to the NO dead time of approximately 300 s (Fig. 2b), which increases to 385 s (Fig. 2f) with an increase in temperature to 370 ◦ C. The NO storage as nitrites continues with the involvement of semi-bulk Ba-sites but at a lower rate due to diffusion limitation, as is presented in Fig. 3b. Meanwhile NO2 is consumed in oxidizing surface nitrites to nitrates (Fig. 3a) and by getting stored on semi-bulk Ba-sites as nitrates (Fig. 3c) through the disproportionation mechanism. As a result, a delay in NO2 breakthrough can be seen in Fig. 2b. Both steps result in NO release, which increases with an increase in temperature. Consequently a maximum in the NO concentration is observed (Figs. 2a, c, and e), which corresponds to Ba(NO2 )2 ,surface = Ba(NO3 )2 ,surface = 0.5. Fig. 3a shows that at 240 ◦ C, the surface nitrite to nitrate transformation is complete after 300 min, whereas the semi-bulk sites transformation takes more than 540 min (Fig. 3c). At 300 ◦ C, the same transformation is complete in 80 min for the surface sites and 150 min for the semi-bulk sites, whereas at 370 ◦ C, the transformation is complete in 50 min for the surface sites and 90 min for the semi-bulk sites. This indicates that the catalyst is saturated with NOx and further storage is not possible. The bulk Ba-sites (not shown) show negligible storage activity at all temperatures. 4.2. Long-time stored–NOx reduction

Fig. 5. Rich phase simulated surface Ba-site coverages at the reactor outlet at 240 ◦ C. (a) Surface Ba-site coverages and (b) semi-bulk BaCO3 coverages.

From the previous section it is clear that at the end of the 9 h lean phase, the stored NOx is essentially present as nitrates on the catalyst. Figs. 4a–c show the rich phase experimental and model N2 , NO, NO2 , and H2 outlet concentrations as a function of time, at temperatures of 240, 300, and 370 ◦ C, respectively. The catalyst regeneration with NOx release and subsequent reduction is rapid, as can be seen from the instantaneous NO and N2 formation. NH3 formation is seen at the end of the N2 formation experimentally, but is not included in the further analysis (Scholz et al., 2007a). Fig. 4a shows that at 240 ◦ C, the N2 formation starts with 0.16 vol% and stabilizes at 0.18 vol%. The observed lower initial N2 concentration might be due to the fact that some amount of H2 is consumed in reducing the oxidized Pt sites and less H2 is available for nitrate decomposition and NOx reduction (Lietti et al., 2001). However, at 300 ◦ C (Fig. 4b) a higher NO release with relatively lower N2 formation, starting with 0.13 vol% and stabilizing at 0.16 vol%, is seen. With an increase of the temperature to 370 ◦ C (Fig. 4c), besides an increased NO and less N2 formation (0.12 vol%), a considerable amount of NO2 is also released. The observed decrease in the reduction efficiency might be due to increased catalyst regeneration through a thermal decomposition of nitrates (James et al., 2003). Figs. 4a and b also show that the model adequately

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Fig. 6. Periodic lean (240 s)/rich (60 s) reactor operation at 240 ◦ C. Continuous lines show experimental data and discontinuous lines show model predictions. (a) NO and NO2 outlet concentrations, (b) enlarged view of (a) showing first six storage–reduction cycles, (c) N2 outlet concentrations, (d) simulated surface Ba-site coverages, and (e) simulated semi-bulk Ba-site coverages.

describes (H2 assisted) NO release and its subsequent reduction to N2 . At 370 ◦ C (Fig. 4c) the model slightly over predicts N2 formation, while it slightly under predicts NO release. However, the model adequately describes the observed NO2 release through the thermal decomposition, as given in step (VIa) in Table 1. Figs. 5a and b show the simulated surface Ba-sites and the semi-bulk site coverage’s at 240 ◦ C. Stored nitrates are rapidly converted to carbonates and within 15 min all surface sites are regenerated, while regeneration of the semi-bulk sites takes nearly 20 min. The regeneration rate increases with temperature. At 370 ◦ C all sites are regenerated within 10 min. 4.3. Periodic NOx storage–reduction Figs. 6–8 show the model prediction and experimental results of periodic reactor operation, for 20 cycles with 240 s lean

and 60 s rich phase, at 240, 300, and 370 ◦ C, respectively. As mentioned in the previous section, during the first few minutes of rich exposure, part of the H2 is probably consumed reducing Pt surface. This leads to less available H2 for NOx reduction. Therefore, the simulation results show initially a higher N2 concentration as experimentally observed. For this reason, the inlet H2 concentration used in the modeling of the transient experiments was set to 0.6 vol%. Fig. 6a shows the NO and NO2 reactor outlet concentrations at 240 ◦ C. The first two storage/reduction cycles show complete NOx uptake (Fig. 6b). During the rich phase, N2 formation and NO desorption are observed. No NH3 formation is seen. Fig. 6c shows that the model adequately predicts observed N2 formation. NOx stored during lean exposure is not completely reduced during the subsequent rich period and NO breakthrough is observed (Figs. 6a and b). This can be seen

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Fig. 7. Periodic lean (240 s)/rich (60 s) reactor operation at 300 ◦ C. Continuous lines show experimental data and discontinuous lines show model predictions. (a) NO and NO2 outlet concentrations, (b) enlarged view of (a) showing first six storage–reduction cycles, and (c) N2 outlet concentrations.

Fig. 8. Periodic lean (240 s)/rich (60 s) reactor operation at 370 ◦ C. Continuous lines show experimental data and discontinuous lines show model predictions. (a) NO and NO2 outlet concentrations, (b) enlarged view of (a) showing first six storage–reduction cycles, and (c) N2 outlet concentrations.

from the accumulation of mainly nitrites and negligible nitrates on the surface barium sites (Fig. 6d). Part of the barium will be inactive for further NOx storage. The NO storage contin-

ues with the involvement of semi-bulk barium sites (Fig. 6e). Meanwhile NO2 is consumed in oxidizing surface nitrites into nitrates and by getting stored on semi-bulk barium sites. After

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a number of lean/rich cycles, NO2 breakthrough can be seen. The experimental results show that even after 20 cycles, the steady state is not reached yet. Model simulations predict that the cyclic steady state is reached only after 80 cycles, when the semi-bulk sites are saturated with NOx . At the cyclic steady state, only fraction of the surface barium sites participates in the NOx storage/reduction process. With an increase in the temperature to 300 and 370 ◦ C , the NOx storage rate increases, due to increased NO2 formation (Figs. 7a and 8a). However, N2 formation remains the same (Figs. 7c and 8c). The model adequately predicts NO, NO2 , and N2 reactor outlet concentrations at both temperatures. 5. Conclusions The influence of the temperature on periodic NOx storage–reduction in the presence of CO2 has been investigated both experimentally and by modeling. A recently developed dynamic plug flow model and experiments performed in a packed bed reactor with a Pt–Ba/-Al2 O3 powder catalyst (1 wt% Pt and 30 wt% Ba) with different lean/rich cycle timings at different temperatures (240, 300, and 370 ◦ C) have been compared. The model uses a global reaction kinetic model based on a multiple NOx storage–reduction sites mechanism. Long cycling, 9 h lean and 15 h rich, experimental data have been used for elucidating the NOx storage–reduction chemistry and for determining the kinetic parameters. The model is further used to predict the influence of the temperature on periodic lean (240 s)–rich (60 s) reactor operation. The model adequately describes not only NO and NO2 breakthrough profiles observed during the lean phase but also NOx reduction data during rich phase operation. The NOx storage at temperatures below 300 ◦ C mainly occurs through NO adsorption as nitrites. The oxidation of nitrites to nitrates by NO2 , with release of NO, is the predominant storage step. The number of sites taking part in NOx storage is independent of the temperature and the cycle time. The reduction of stored NOx with H2 is efficient. However, at relatively higher temperatures, the thermal decomposition of nitrates results in a decreased NOx reduction. Acknowledgments This project is financed by the Dutch Technology Foundation (STW). Support of PSA, Ford, Engelhard, PD&E Automotive solutions, Shell, IPCOS, E.P. Controls and Toyota is gratefully acknowledged.

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