Basic Reaction Model of Automobile Exhaust Gas Treatment over Pt

Sep 2, 2010 - (Pattaya, Thailand, November 21-24, 2007). 2007; CO-01, pp 114-119. (9) Aghalayam, P.; Park, Y. K.; Fernandes, N.; Papavassiliou, V.;...
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Ind. Eng. Chem. Res. 2010, 49, 10341–10347

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Basic Reaction Model of Automobile Exhaust Gas Treatment over Pt-Rh Catalyst Motoaki Kawase,*,† Hiroyasu Fujitsuka,† Hitoshi Nakanishi,† Tatsuya Yoshikawa,‡ and Kouichi Miura† Department of Chemical Engineering, Kyoto UniVersity, Katsura, Kyoto 615-8510, Japan, and ICT Co., Ltd., 992-1 Nishioki, Okihama, Aboshi-Ku, Himeji 671-1292 Japan

A reaction model, which is comprehensive and simple enough to be a base of the dynamic model, of the after-treatment of automobile exhaust gas was proposed. The studied catalyst was Pt- and Rh-supported alumina washcoated over a honeycomb. For attaining a short residence time, a small reactor, 2-73 mm long, was employed. To formulate the reaction rate, the subsets of reactions, CO oxidation, C3H6 oxidation, and N2O and NO reduction by CO or C3H6, were carried out separately over a wide range of gas composition at 140-340 °C. An overall model was built by combining the subset models. The results calculated with the proposed model well represented the experimental results. 1. Introduction The automobile exhaust emission regulations are getting more and more severe every year. To meet the stringent future standards, further improvement of the catalytic converter is needed. Since the exhaust gas contains a lot of components, the after-treatment of automobile exhaust gas is a complex reaction system. Moreover, the reaction conditions including the inlet gas composition are changed over time greatly and rapidly according to a driver’s operation. To estimate the behavior of the catalytic converter, a dynamic model is desired.1,2 The dynamic simulation would be of help to enable more efficient development of the catalytic converter. In this study, a steady-state reaction model, which is comprehensive and simple enough to be a base of the dynamic model, was developed on the basis of a wide range of experimental results. Of the reactions of concern, CO oxidation has been studied most intensively.3–8 Although several reaction models have been proposed,3–6 it is still unclear which model is the best. One reason is the investigated partial pressure range was too narrow. The overall rate equations derived with some models give the reaction rates close to each other in the narrow reactant pressure range. Oxidation of some light hydrocarbons including methane,9 ethylene,10 acetylene,11 and propylene8 has been reported. The hydrocarbon reactions have not been clearly revealed. They consist of so many elementary reactions that even overall rate equations could be a reasonable solution in modeling the reaction. NO reduction in the automotive exhaust gas is more complex. Burch and co-workers12 reported that the NO reduction was too complex to understand the mechanism of N2 and N2O formation. Hoebink and co-workers13 carried out numerical simulation with an assumed elementary reaction model although the overall reaction rate equations were not presented. Granger and co-workers14–16 proposed a detailed reaction model for the exhaust gas composition near the stoichiometric condition. The objective of this study is to formulate the overall rates of reactions between CO, O2, C3H6, and NO to build a basic reaction model of the after-treatment of the automobile exhaust * To whom corresponding should be addressed. Tel: +81-75-3832683. Fax: +81-75-383-2653. E-mail: [email protected]. † Kyoto University. ‡ ICT Co., Ltd.

gas. The reaction rates of CO oxidation, propylene oxidation, and NO reduction by C3H6 or CO over Pt, Rh/Al2O3 in a wide range of gas composition were measured and analyzed for developing the reaction model. Although reactions including hydrogen and steam are also important, they are not investigated yet in this study. 2. Experimental Section A schematic of the reactor is shown in Figure 1. To attain the uniform temperature distribution and to lower the reactant conversion for the analysis purpose, a small reactor was employed in this study. The catalyst used in experiments was cut out of a commercial 1 mm pitch square-channel honeycomb catalyst. The length and diameter were 2-73 and 10 mm, respectively. Pt and Rh supported Al2O3 particles are washcoated on the inner wall of the cordierite honeycomb. An 89.9 kg/m3 precious-metal-supported alumina layer is coated; the amount of Pt is 0.7 kg/m3 and Pt/Rh ) 3:1. Although 30 kg/m3 of Ce0.2Zr0.8O2 is also coated for oxygen storage, the experiments were carried out at steady state and it did not affect the measured reaction rate. The reactor is heated by a three-section electric furnace. The flow rate of each feed gas component is regulated with a thermal

Figure 1. Schematic of a thin catalytic converter.

10.1021/ie1005564  2010 American Chemical Society Published on Web 09/02/2010

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by strong CO adsorption. By investigating the wide partial pressure range, in this study, we observed the maximum CO oxidation rate around the CO partial pressure of 30 Pa. Several reaction mechanisms of CO oxidation have been proposed, four of which were examined. Model M1 is the dissociative adsorption model,3 which is often employed,20–23 described as follows: CO + σ a COσ

(1)

O2 + 2σ a 2Oσ

(2)

COσ + Oσ f CO2 + 2σ

(3)

where σ represents the surface site. Model M2 is the slow molecular adsorption model,4 which is described as follows: Figure 2. Comparison of CO oxidation models at 150 °C (SV ) 2.0 × 104 h-1): (a) CO partial pressure dependency (pO2 ) 0.50 kPa); (b) O2 partial pressure dependency (pCO ) 0.30 kPa).

mass flow controller. N2 is used as carrier gas in all the experiments. Total gas flow rate is ranged so that the space velocity (SV) is 104 to 7 × 105 h-1. The typical flow rate is 2 dm3/min (SV ) 2.1 × 105 h-1). For estimating the kinetic parameters, the subsets of the aftertreatment reactions are carried out separately. For instance, only CO and O2 are supplied with N2 for examining the kinetics of CO and O2. As the light-off easily takes place in the oxidation reactions and the uniform temperature is not attained, only the reduction experiments are carried out above 190 °C. Oxidation of CO and C3H6 by oxygen is carried out at 140-215 °C. In CO oxidation experiments, the CO fraction is 0.0003-0.6% and the O2 fraction is 0.2-2.0%. C3H6 as representative hydrocarbon species is oxidized with the C3H6 fraction 0.1-1.2% and the O2 fraction 0.2-2.0%. NO reduction by C3H6 and CO is carried out at 275-340 °C with the NO fraction 0.05-0.28%, the C3H6 fraction 0.15-1.15%, and the CO fraction 0.03-0.3%. N2O reduction by CO is carried out at 215-340 °C with the N2O fraction 0.005-0.03% and a CO fraction of 0.01%. Gas analysis is carried out with a micro gas chromatograph (Varian micro GC CP4900) and a quadrupole mass spectrometer (ANELVA M-100GC-DM). The micro GC is equipped with a Parapak Q column (10 m) and a Molsieve 5A column (10 m). The column temperature is 80 °C. Carrier gas for GC is helium. The mole fractions of components are monitored at the inlet and outlet of the reactor. After reaching the steady state, the mole fractions are measured and used for kinetic analysis. The reaction rate is calculated as a rate of the increase in the molar flow rate of a component per unit mass of the washcoated layer of the catalyst. 3. Results and Discussion 3.1. CO Oxidation. CO oxidation was carried out with CO, O2, and N2 supplied below the light-off temperature. The temperature in the reactor was uniform, and the CO conversion was no higher than 16%. The measured CO oxidation rate at a fixed O2 partial pressure is plotted against the CO partial pressure in Figure 2. The CO partial pressure ranged over 4 orders of magnitude. It is usually observed that the CO oxidation rate has a negative reaction order with respect to the CO partial pressure in the higher partial pressure region due to the inhibition

CO + σ a COσ

(4)

O2 + σ f O2σ

(5)

O2σ + σ f 2Oσ

(6)

COσ + Oσ f CO2 + 2σ

(7)

Model M3 is the adsorbed molecular oxygen reaction model6 described as follows: CO + σ a COσ

(8)

O2 + σ a O2σ

(9)

COσ + O2σ f CO2 + Oσ + σ

(10)

COσ + Oσ f CO2 + 2σ

(11)

Model M4 is the adsorbed atomic oxygen reaction model5 described as follows: CO + σ a COσ O2 + σ a O 2 σ

θCO ) KCOpCOθv

(12)

θO2 ) KO2pO2θv

(13)

O2σ + σ f 2Oσ COσ + Oσ f CO2 + 2σ

rm1 ) km1θO2θv

(14)

rm2 ) km2θCOθO

(15)

The overall CO oxidation rate expressions are respectively as follows:

(M1)

-rm,CO )

(

2km1pO2 1 + KCOpCO +

2km1pO2 km2KCOpCO

)

2

(16)

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Figure 3. Light-off in CO oxidation (pCO ) 0.30 kPa, pO2 ) 0.50 kPa, SV ) 2.0 × 104 to 7.4× 105 h-1).

(M2) -rm,CO ) 2km0pO2(km1km2KCOpCO - km0km2KCOpCOpO2 - 2km0km1pO2) km1km2KCOpCO(1 + KCOpCO)

(17) (M3)

-rm,CO )

(

2km1KCOpCOKO2pO2

1 + KCOpCO + KO2pO2 +

(M4)

-rm,CO )

(

)

km1 K p km2 O2 O2

2

(18)

2km1KO2pO2

1 + KCOpCO + KO2pO2 +

2km1KO2pO2

)

2

km2KCOpCO (19)

where the partial equilibrium for adsorption/desorption of CO and O2 (except for M2) as well as the steady state for the oxygen adatom, Oσ, is assumed. The best fit results with the four rate equations are also shown in Figure 2. The O2 dissociative adsorption model M1 cannot represent the CO oxidation rate dependency on the CO partial pressure as shown in Figure 2a. The slow molecular adsorption model M2 cannot account for the rate dependency on the O2 pressure.7 The model M3, in which the adsorbed oxygen molecule reacts with adsorbed CO, was proposed for the high ratio of the CO partial pressure to the O2 partial pressure. The model M3 did not represent the reaction rate at low CO partial pressure, although the dependency was explained qualitatively. The model M4, in which rapid molecular adsorption of O2, ratecontrolling O2σ dissociation, and the reaction of adsorbed CO and O adatom are assumed, represented the experimental data excellently over the full CO pressure range measured, as shown in Figure 2a,b. The previously reported data were measured at the CO fraction above 0.22%,5 0.55%,24 0.1%,25 or 0.5%.26 For the high CO pressure, eqs 16 and 19 yield an identical approximate equation, -rm,CO ) krpO2/(KCOpCO)2. By carrying out the experiments in a wide range of the CO partial pressure over 4 orders of magnitude including very low CO pressure, we successfully determined the most reasonable model. The CO oxidation rate is plotted against the inlet temperature in Figure 3. At a certain inlet temperature, the reaction rate increased remarkably. Granger and co-workers calculated the conversion by numerical simulation under isothermal conditions at various temperature levels and found the light-off behavior occurs at a certain temperature even without the exothermic effect.16 The steep increase in the reaction rate in Figure 3 was observed at a temperature lower than the light-off temperature estimated by Granger et al. The observed rise in the reaction rate resulted from the too fast exothermic reaction, which raised

Figure 4. CO oxidation rates: (a) CO partial pressure dependency (pO2 ) 0.50 kPa); (b) O2 partial pressure dependency (pCO ) 0.30 kPa).

the catalyst temperature above the controlled temperature. Under such conditions, accurate kinetic analysis is impossible. Therefore, the measurement was carried out below this critical temperature. To expand the temperature range, a high space velocity of 7.4 × 105 h-1 was attained by shortening the catalyst honeycomb to 2 mm, which enabled the kinetic measurement up to 200 °C, as shown in Figure 3. The experimental results of the temperature dependency of CO oxidation are shown in Figure 4a,b. The rate constants in the model were determined at each temperature first, and the frequency factor and the activation energy were determined by an Arrhenius plot, which were taken as initial values in nonlinear-least-squares optimization of the parameters using all the experimental data. The reaction rates recalculated with the parameters were also shown in Figures 4a,b. Dependency of the CO oxidation rate on the CO partial pressure as well as the O2 pressure is in good agreement with the experimental results. 3.2. C3H6 Oxidation. Parts a and b of Figure 5 show the C3H6 oxidation rate measured in C3H6-O2 reaction experiments. Inhibition by C3H6 adsorption was observed. Although the detailed analysis of the C3H6 oxidation over the three-way catalyst was reported recently,27,28 it is too complicated for the overall rate expression to be derived; it was approximated with the assumption that the first dissociation step of adsorbed propylene is rate-controlling and leads rapidly to CO2 and H2O. That is, the following equations are assumed in addition to eqs 2 and 3: C3H6 + σ a C3H6σ

θC3H6 ) KC3H6pC3H6θv

(20)

C3H6σ + Oσ f ... + 8 Oσ f 3CO2 + 3H2O + 10σ rm7 ) km7θC3H6θO

(21)

The overall reaction rate is expressed as follows:

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rm,N2O ) km5KNO2pNO2θv2

(28)

where

[

θv ) 1 +

Figure 5. C3H6 oxidation rate: (a) C3H6 partial pressure dependency (pO2 ) 0.50 kPa, SV ) (1.1-4.1) × 105 h-1); (b) O2 partial pressure dependency (pC3H6 ) 0.20 kPa, SV ) 1.1 × 105 h-1).

-rm, C3H6 )

(2/9)km1KO2pO2

(

1 + KC3H6pC3H6 + KO2pO2 +

2km1KO2pO2 9km7KC3H6pC3H6

)

2

(22)

The slow O2 molecular adsorption kinetics reproduced the experimental results as well as the CO oxidation case. The results recalculated with the kinetic parameters determined from the measured data are shown in Figure 5a,b. The measured and calculated rates are in fair agreement. 3.3. NO Reduction by C3H6. The NO reduction by C3H6 experiments was carried out at 275-340 °C. In addition to N2 formation, the generation of N2O was observed. Some reaction mechanisms of NO reduction by hydrocarbons have been proposed or investigated.11–19 The surface species, Nσ and NOσ, might react to form N2 and N2O. If N2 and N2O are generated in different paths as follows: NO + σ a NOσ

θNO ) KNOpNOθv

NOσ + σ f Nσ + Oσ NOσ + Nσ f N2 + Oσ + σ

rm3 ) km3θNOθv

(23) (24)

rm4 ) km4θNOθN

(25) 2NOσ f N2O + Oσ + σ

rm5 ) km5θNO2

(26)

the N2 and N2O formation rates are formulated, with the partial equilibrium approximation for adsorption/desorption and the steady-state approximation for Nσ and Oσ, as follows: rm,N2 ) km4KNOpNOθv2

(27)

km3 + KNOpNO + KC3H6pC3H6 + km4 (2km3 + km5KNOpNO)KNOpNO 9km7KC3H6pC3H6

]

-1

(29)

In this case, the ratio of the N2O formation rate, rm,N2O, to the N2 formation rate, rm,N2, should be proportional to the NO pressure as far as the N2O decomposition leading to N2 formation can be ignored. If N2 and N2O are both formed from NOσ and Nσ, no pNO dependency should be observed. As shown by good linearity in Figure 6, the former one is supported by the experimental results. Another possible mechanism is that N2 is formed from 2Nσ and N2O is formed from 2NOσ. Although the kinetic parameters were also determined for this mechanism, the former one yielded better agreement with the experimental results. With KC3H6 already determined from the C3H6 oxidation results, the other kinetic parameters were determined. The measured and calculated N2 formation rate and N2O formation rate are plotted against the NO partial pressure in Figure 7a,b. The results calculated with the determined kinetic parameters are in good agreement with the experimental results. 3.4. NO Reduction by CO. The NO reduction by CO was carried out at 275-340 °C.8 Combining the reaction model of the CO oxidation and NO reduction by C3H6, i.e., eqs 12, 15, and 23-26, the experimental results of NO reduction by CO were well represented. Dependency of the selectivity on the NO partial pressure, as shown in Figure 6, is similar to that in the case of NO reduction by C3H6. When the NO partial pressure is higher than ca. 0.2 kPa, the observed rm,N2O/rm,N2 became lower than the linear trend due to a decrease in the N2O formation rate. Since the reaction of CO with Oσ is faster than that of C3H6 with Oσ, the consumption of N2O is more remarkable in the presence of CO. The weak dependency of the N2O selectivity on the NO partial pressure was also observed in the high NO pressure range by Granger et al.15 The N2O partial pressure is so high that N2O is consumed in reaction with C3H6, although it is neglected in the analysis of this study and in the literature.15 The overall reaction rates of N2 and N2O are expressed by eqs 27 and 28 whereas the vacant site fraction θv is as follows:

[

θv ) 1 +

km3 + KNOpNO + KCOpCO + km4 (2km3 + km5KNOpNO)KNOpNO km2KCOpCO + 9km7KC3H6pC3H6

]

-1

(30)

With KCO and km2 determined from the CO oxidation experiments, the other kinetic parameters were determined by fitting the calculated rate with the measured rate. It was found that km3/km4 in the denominator in eq 30 was negligible compared with 1. The calculated and measured results are in good agreement as shown in Figure 8a,b. 3.5. N2O Reduction by CO. Although N2O formation was observed in the experiments of NO-C3H6 reaction and NO-CO reaction, the experiments were carried out at low conversion and, therefore, the N2O decomposition was not accurately observed in those experiments. To determine the kinetic parameter of the N2O decomposition, N2O reduction by CO was

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Figure 6. NO pressure dependency of ratio of N2O formation to N2 formation rates in NO-C3H6 and NO-CO experiments (SV ) 2.1 × 105 h-1).

Figure 8. NO partial pressure dependency of rates in reaction of NO and CO (pCO ) 0.15 kPa, SV ) 2.1 × 105 h-1): (a) N2 formation rate; (b) N2O formation rate.

Figure 7. NO partial pressure dependency of rates in reaction of NO and C3H6 (pC3H6 ) 0.15 kPa, SV ) 2.1 × 105 h-1): (a) N2 formation rate; (b) N2O formation rate.

carried out at 215 and 230 °C. The measured N2O consumption rate, which was equal to the N2 formation rate, is plotted in Figure 9. By assuming eqs 12 and 15 in addition to the following elementary reactions: N2O + σ a N2Oσ N2Oσ f N2 + Oσ

θN2O ) KN2OpN2Oθv

(31)

rm6 ) km6θN2O

(32)

we derived the overall rate equation as follows: -rm,N2O ) rm,N2 ) km6θN2O )

(

km6KN2OpN2O 1 1 + KCOpCO

km6KN2OpN2O

km2KCOpCO + KN2OpN2O

)

(33)

Figure 9. N2O reduction rate in N2O-CO reaction (pCO ) 10 Pa, SV ) 2.1 × 105 h-1).

The second term in parentheses in eq 33 represents the coverage of the oxygen adatom, which can be approximated to zero. Good linearity was observed in the Langmuir plot, pN2O/(-rm,N2O) vs pN2O, of these experimental data (the plot not shown). This indicates that N2 is formed from a single adsorbed species, supporting the assumed mechanism. 3.6. Reaction Model. On the basis of the experimental and analysis results described above, the basic reaction model illustrated in Figure 10 is proposed. The model is composed of seven reactions: (1) dissociation of adsorbed O2, (2) oxidation of adsorbed CO, (3) dissociation of adsorbed NO, (4) formation of N2 from adsorbed NO and N, (5) formation of N2O from adsorbed NO, (6) decomposition of adsorbed N2O, and (7) oxidation of adsorbed hydrocarbons. The numbers in circles in Figure 10 represent the index of the reactions. All the reactions are connected by an oxygen adatom. With the steady-state approximation for Oσ and Nσ, the following overall rate equations of respective components are derived:

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Figure 10. Proposed reaction model.

CO consumption:

-rm,CO ) km2KCOpCOθOθv

(34)

C3H6 ) km7KC3H6pC3H6θOθv

(35)

CO consumption: -rm,C3H6 O formation: N2 rm,N2O ) km5KNO pNO2θv2 - km6KN2OpN2Oθv 2

(36)

formation: N2 rm,N2 ) km3KNOpNOθ2v + km6KN2OpN2Oθv

(37)

where θO )

2km1KO2pO2 + 2km3KNOpNO + km5KNO2pNO2 km2KCOpCO + 9km7KC3H6pC3H6 km6KN2OpN2O

θv +

km2KCOpCO + 9km7KC3H6pC3H6

(38)

θv ) 11+

∑ j

Kjpj +

Figure 11. Determined kinetic parameters.

km6KN2OpN2O km2KCOpCO + 9km7KC3H6pC3H6

2km1KO2pO2 + 2km3KNOpNO + km5KNO2pNO2 km2KCOpCO + 9km7KC3H6pC3H6

(39) The determined parameters are shown in the Arrhenius plot in Figure 11 with symbols different for the reactions used for analysis. The values below and above 190 °C were determined respectively from the oxidation experiments and the reduction experiments. Figure 12 shows the comparison between the measured and calculated reaction rates of all 663 runs. Even though the parameters were determined from different reactions, the recalculated values are in fairly good agreement with the experimental results. A total of 331 data are within (50% error. The values determined from C3H6 reactions are not so accurate compared with the other values. Excluding C3H6 reactions, 227 of 306 data are in the (50% error regime. The average error in the CO oxidation rate is 29%. In this analysis, an identical rate constant km1 of O2σ decomposition was determined both from CO oxidation and from C3H6 oxidation results. Agreement of km1 determined from CO oxidation and C3H6 oxidation is not good, as shown in Figure 11. This is the most significant reason for the errors in the

Figure 12. Comparison between the measured and calculated reactions rates of all 663 runs.

calculated reaction rates. For further refinement, detailed analysis of hydrocarbon reactions is necessary. 4. Conclusions A basic reaction model of the after-treatment of the automobile exhaust gas was proposed on the basis of the experimental

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results. A total of 663 runs of experiments were carried out for the subsets of after-treatment reaction, i.e., CO oxidation, C3H6 oxidation, NO reduction by CO or C3H6, and N2O reduction by CO. By employing the small reactor, we suppressed the lightoff in the CO oxidation experiments, which enabled the kinetic analysis up to 200 °C. By carrying out the CO oxidation reaction with a wide range over 4 orders of magnitude of the CO partial pressure, we determined successfully the most plausible reaction mechanism by the macroscopic rate analysis. The determined mechanism is that O2 is adsorbed as molecular oxygen, its decomposition is the rate-controlling step, and the O adatom is reacted with adsorbed CO, yielding CO2. In the experiments of NO reduction by C3H6 and CO, it was demonstrated that N2 and N2O are generated in different reactions, i.e., Nσ + NOσ f N2 + Oσ + σ and 2NOσ f N2Oσ + Oσ. Even without a N2 formation reaction, 2Nσ f N2 + 2σ, the proposed model reproduced the experimental results well. The overall rate equations were derived by assuming a single sort of active site, partial equilibrium approximation for adsorption/desorption, and steady-state approximation for the oxygen adatom and nitrogen adatom. A single set of kinetic parameters of the proposed model were estimated from the experiments. The numerical simulations using the determined model well represented the experimental results. The proposed model consists of only seven reactions. It is simple enough to be a base of dynamic model in which oxygen storage capacity is taken into account as well as to be used for optimization of the catalytic converter. For further improvement, refinement of the C3H6 oxidation model and investigation of the hydrogen and steam reaction models are necessary and being in progress. Nomenclature Variables km ) catalytic reaction rate constant (mol kg-1 s-1) K ) adsorption equilibrium constant (Pa-1) p ) partial pressure (Pa) rm ) catalytic reaction rate (mol kg-1 s-1) θ ) surface coverage Subscripts j ) component index v ) vacant active site

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ReceiVed for reView March 9, 2010 ReVised manuscript receiVed July 29, 2010 Accepted July 30, 2010 IE1005564