Activity Function for Describing Alteration of Three-Way Catalyst

ARTICLE pubs.acs.org/IECR

Activity Function for Describing Alteration of Three-Way Catalyst Performance over Palladium-Only Three-Way Catalysts by Catalyst Mileage Sung Bong Kang, Hyuk Jae Kwon,† and In-Sik Nam* Department of Chemical Engineering/School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyoja-dong, Pohang 790-784, Korea

Young Il Song Exhaust Emission Engineering Team, Power Train Research and Development Center, Hyundai-Kia Motors Company, Jangduk-dong, Hwaseong 445-706, Korea

Se H. Oh General Motors Research and Development Planning Center, Warren, Michigan 48090-9055, United States ABSTRACT: A simple three-way catalyst (TWC) activity function based upon the alteration of the Pd metallic surface area (MSA) of Pd-only commercial monolith TWCs has been developed to describe the change of the catalytic performance with respect to the Pd metal loading varying from 5 to 10 g/L and the field-aged catalyst mileage from 4K (stabilized) to 98K miles. Particularly, the change of the Pd MSA of TWC with respect to the catalyst mileage has been well-correlated by the second-order sintering kinetics, regardless of the Pd content of the TWC. The TWC activity function was incorporated into the primary reaction kinetics developed for the stabilized 4K Pd-only TWCs to predict the deactivation of the Pd-only TWC performance with respect to the field-aged mileage. The overall reaction kinetic model with the TWC activity function developed in the present study is capable of describing the alteration of TWC performance with respect to the catalyst mileage, regardless of the catalyst metal content.

’ INTRODUCTION The development of the catalyst activity function is useful for predicting the life of the catalyst for designing a commercial catalytic reactor, since more than 90% of the heterogeneous catalytic process experiences a decay of the reaction rate with respect to the reactor on-stream time.1 The activity function can be obtained by correlating the rates at any time to time zero, although it requires exhaustive time and effort including the collection and test of the deactivated catalyst samples under practical operating conditions with respect to the duration of their uses. Another approach may be the independent development of an activity function based upon the alteration of the physicochemical properties of the catalyst including the content of the deactivation precursors, such as coke, sulfur, and contaminants, the metal dispersion, and the catalyst surface area.24 The deactivation of the activity and durability of three-way catalyst (TWC) installed into gasoline driven cars is apparent as the catalyst mileage increases.5 Furthermore, the technical and market demands for developing the deactivation model to predict the life of TWC becomes inevitable for the design of the TWC catalytic converter to meet the ever-tightening emission regulations throughout the world and the mileage guarantee by automakers.6 However, a TWC activity function directly describing the alteration of TWC performance has been rarely reported, mainly due to the difficulty in collecting of the catalyst samples with respect to the catalyst mileage and the complexity r 2011 American Chemical Society

of the deactivation of commercial TWC under the real driving condition. Ekstr€om et al. predicted the decline of the catalytic activity of TWC aged under accelerated engine bench condition by the simple adjustment of the frequency factor of the rate constant included in the primary kinetics.7 Baba et al. developed a TWC deactivation kinetics based upon the change of the frequency factor attributed to both sintering of Pt metal and phosphorus poisoning under accelerated engine bench condition.8 They reported that the sintering of noble metal is the primary cause for the deactivation of TWC due to the decrease of noble metal surface area (MSA). Recently, the decrease of the catalytic activity over the Pt-based diesel oxidation catalyst (DOC) has been examined by the incorporation of the normalized activity based upon the catalyst active MSA with respect to the Pt loading and aging temperatures.9,10 The change of the Pt MSA determined by CO chemisorption is capable of predicting the alteration of the catalytic activity of the Pt-based DOC aged under the laboratory aging program including 10% H2O and air at 850 and 950 °C for 16 h. However, it has long been desired to directly develop the TWC activity function, a, with respect to the catalyst Received: January 14, 2011 Accepted: March 16, 2011 Revised: March 9, 2011 Published: March 25, 2011 5499

dx.doi.org/10.1021/ie200083f | Ind. Eng. Chem. Res. 2011, 50, 5499–5509

Industrial & Engineering Chemistry Research

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practical usefulness of the activity function developed in this work.

Table 1. Commercial Pd-Only TWCs metal content of catalyst mileage

4K TWCs (wt %)

Pd metal content (g of Pd loaded in 1 L of monolith)

a

a

41K

b

98K

Pd

Ce

Zr

b

Pd 5

4K

0.7

5.1

5.3

Pd 7

4K a

21K b

55K b

1.0

5.0

5.2

Pd 10

4K a

41K b

98K b

1.4

4.9

5.3

4K: laboratory-aged. b 21K98K: field-aged.

mileage and noble metal loading prepared under real driving condition, not by the simple tuning of the frequency factors of the reaction rate constants included in the primary kinetics. The decline of TWC activity is extremely complex under commercial operating conditions, since the deactivation can arise, mainly from the poisoning by the contaminants included in fuel and engine lubricant and the sintering of noble metal by the high operating temperature of gasoline engine.11 Contaminants such as phosphorus (P), lead (Pb), zinc (Zn), and sulfur (S) can deactivate the TWC performance due to the strong deposition onto the active reaction sites of TWC. P and S can form deactivating agents including AlPO4, CePO4, and Ce2(SO4)3 on the catalyst surface.12,13 However, the automotive catalytic converter including commercial TWC is typically operated in a wide temperature range from 150 to 1000 °C. The higher the exhaust gas temperature from the engine, the sintering of the noble metals loaded on TWC becomes more significant, particularly in the temperature region above 650 °C.14 Since the loss of the active reaction sites through the growth of the metal particle size is directly related to the decrease of the active metal surface area, the sintering of the noble metal included in TWC has been widely regarded as the most detrimental factor for the deactivation of TWC.11,15 Indeed, the atomic and crystallite migrations of noble metal including Pd, Pt, and Rh onto its support by the increase of mobility have been proposed as the primary mechanism for sintering of a TWC.16 In addition, the stability of the metal crystallite was seriously affected by a variety of the engine operating conditions including fuel and lubricant compositions, the driving region, and the driver’s driving habit. It has been further complicated by the recent variation trend of the TWC formulation including Pd, Pt, and Rh.5,17 Shinjoh et al. investigated the effect of aging atmosphere on the sintering behavior for automotive catalysts.18 They reported that the thermal stability of Pd is superior to that of Pt and Rh under the automotive exhaust condition at 1100 °C attributed to relatively lower vapor pressure of palladium oxide than rhodium and platinum oxides. In the present study, an activity function for commercial Pdonly TWCs has been independently developed to predict the degradation of TWC activity upon aging, by correlating the Pd MSA with the catalytic activity for a range of the Pd loading and the field-aged mileage of the catalysts. The change of TWC performance upon aging has been numerically simulated by incorporating the activity function into the primary kinetics for the 4K Pd-only TWCs in the kinetic model. Results of the kinetic model simulation have shown a good agreement with the experimental data, further validating the concept and the

’ EXPERIMENTAL SECTION Catalyst Preparation and Characterization. The nine commercial Pd-only TWC monoliths whose Pd loadings and catalyst mileages vary from 5 to 10 g/L and from 4000 to 98 000 miles, respectively, were supplied by Hyundai-Kia Motors Co. The samples were stabilized (4K miles equivalent) by the laboratory aging program developed at GM Research and Development and the field-aged samples from 21K to 98K miles were collected from the customers. Listed in Table 1 are the metal contents and accumulated mileages of three Pd-only TWCs examined in this study. The catalyst mileages for Pd 5 and Pd 10 catalysts are 4000 (4K stabilized), 41 000 (41K), and 98 000 (98K) miles, while those for Pd 7 catalysts are 4000 (4K), 21 000 (21K), and 55 000 (55K) miles. Note that the Pd 5, 7, and 10 TWCs were installed into an identical vehicle model and contained no other noble metals (Pt or Rh) besides Pd. The TWCs contained similar amounts of oxygen storage components (OSC) such as Ce and Zr, regardless of the Pd metal loading. The powder samples of the TWCs employed in the present study for their activity test were obtained by uniformly grinding the monolith samples. The contents of Pd and OSC (Ce and Zr) were analyzed by an inductively coupled plasma-optical emission spectrometer (ICPFlame-EOP, SPECTRO Co.). The MSA, dispersion, and particle size of Pd on the catalyst surface were measured by pulse CO chemisorption (Autochem II 2920, Micromeritics Co.). About 0.2 g of the catalyst sample charged into a quartz U tube was employed for the measurements with the following catalyst pretreatment procedure: reducing by H2 (5%)/Ar at 350 °C for 2 h f purging with He at 400 °C for 1 h f cooling to 35 °C f pulsing with 0.5 cm3 CO (10%)/He every 3 min. The dispersion of Pd was calculated, assuming 1 molecule of CO adsorbed per surface Pd atom.19 Reaction System and Experimental Procedure. The alteration of TWC activity has been examined with respect to both the Pd metal loading and catalyst mileage in a packed-bed U-tube type flow reactor system charged with 1 g of 20/30 mesh size of catalyst. Details of the reactor system are described elsewhere.13 The TWC activity was examined over a wide range of reaction temperature (150450 °C) and reactor space velocity (50 000 170 000 h1) under steady-state condition. The feed gas mixture employed in this study to simulate the exhaust gas composition is comprised of 1% CO, 500 ppm C3H6, 0.3% H2, 1% O2, 500 ppm NO, 10% CO2, 10% H2O, and Ar balance (λ = 1.009).20 Concentrations of NO, N2O, and NH3 were determined by FTIR analyzer with a 2 m gas cell and DTGS/KBr detector (Nicolet 6700, Thermo Electrons Co.), and those of CO, H2, C3H6, CO2, and O2 were analyzed by gas chromatography (GC) equipped with TCD and FID (6890N, Agilent Co.).

’ RESULTS TWC Performance with Respect to the Catalyst Mileage and Pd Loading. Figure 1 shows the effect of the catalyst mileage

on the TWC activities over the Pd 5 catalyst. The TWC activities apparently decrease as the catalyst mileage increases. The major deactivation occurs in low catalyst mileage from 4K to 41K and gradual deactivation from 41K to 98K has been observed as also examined in Table 2. The increase of the light-off temperatures 5500

dx.doi.org/10.1021/ie200083f |Ind. Eng. Chem. Res. 2011, 50, 5499–5509


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Figure 1. TWC performance of Pd 5 catalyst with respect to the catalyst mileage. Feed gas composition: 1% CO, 500 ppm C3H6, 0.3% H2, 1% O2, 500 ppm NO, 10% CO2, 10% H2O, and Ar balance (λ = 1.009). Reactor SV: 100 000 h1.

(LOTs) based upon T50 for the TWC reactions within the low mileage from 4K up to 41K miles is more significant than that within the high mileage from 41K to 98K miles. The deactivation trend of the CO oxidation reaction with respect to the catalyst mileage is similar to that of the C3H6 oxidation reaction within the entire range of the reaction

temperatures covered in the present study. Moreover, the initial deactivation of the TWC reactions, particularly the oxidation of H2 and the reduction of NO in the low catalyst mileage from 4K to 41K, is apparent. For example, the increase of LOT of the CO oxidation reaction over the Pd 5 catalyst in the mileage range from 4K to 41K is 15 °C, whereas that of the NO reduction 5501



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reaction is 35 °C from 173 to 208 °C. The deactivation of the CO and C3H6 oxidation reactions seems to be milder than that of NO reduction and H2 oxidation reactions as the catalyst mileage increases. The TWC activities over the Pd 7 and Pd 10 were also examined with respect to the catalyst mileage. The trend of declining TWC activity over the Pd 10 catalyst is similar to that of the Pd 5, probably due to their identical catalyst mileage (4K, 41K, and 98K) of both Pd 5 and 10 catalysts employed. Particularly, the catalyst deactivation of the Pd 7 catalysts within Table 2. Alteration of LOTs (Light-Off Temperatures), T50, of the TWC Reaction with Respect to the Catalyst Mileage over Pd 5, 7, and 10 Catalysts T50 (°C) sample Pd 5

Pd 7

Pd 10

catalyst mileage (miles)

CO

C3H6

H2

NO

4K

205

211

179

173

41K

220

220

193

208

98K

225

224

202

210

4K

201

205

171

162

21K

208

210

176

171

55K

214

216

190

200

4K 41K

200 210

203 211

162 175

156 188

98K

215

213

192

198

the medium catalyst mileage up to 55K miles was examined. Again, a similar deactivation trend was observed within the medium catalyst mileage. The increase of the LOT is apparent with respect to the catalyst mileage, regardless of the Pd content as listed in Table 2. Moreover, for Pd-only TWCs employed in the present study, the apparent deactivation within the low catalyst mileage that ranged from 4K to 41K can be clearly observed in Figure 2. For the Pd 10 catalyst, the conversion of CO decreased from 90 to 63% and from 63 to 52% at 215 °C as the catalyst mileage increased from 4K to 41K and from 41K to 98K, respectively. Similarly, both Pd 5 and 7 catalysts were also severely deactivated within the low catalyst mileage, regardless of the reactions, oxidations of CO, C3H6, and H2 and reduction of NO. The TWC activity over the 4K Pd 5 catalyst including the lowest catalyst mileage and Pd content (0.7 wt %) is similar to those over both 4K Pd 7 (1.0%) and 10 (1.4%) catalysts as depicted in Figure 2ad. About 10% of the difference in conversions between 4K Pd 5, 7, and 10 catalysts can be observed, regardless of the gas components including CO, C3H6, H2, and NO. It may be probably due to the relatively high Pd content of the catalyst employed in the present study.21 However, the TWC activity over the Pd 5 catalyst is lower than that over both Pd 7 and Pd 10 catalysts upon aging, particularly within their medium and high catalyst mileage region. For example, at the medium catalyst mileage (41K miles) the conversions of CO, C3H6, H2 and NO increase from 44, 11, 54, and 39% to 63, 50, 74, and 54, respectively, as the Pd metal

Figure 2. Performance alteration of Pd-only TWC with respect to the catalyst mileage and Pd metal content. 5502



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Table 3. Alteration of MSA with Respect to the Pd Loading and the Catalyst Mileage catalyst sample Pd 5

Pd 7

Pd 10

mileage

MSA of Pd

(miles)

(m2/g)

particle dispersion (%)

size (nm)

a ≈ S/S0

4K

0.243

7.9

14.2

1

41K

0.076

2.5

45.4

0.31

98K

0.037

1.2

93.1

0.15

4K

0.261

6.0

18.8

1

21K

0.118

2.7

41.6

0.45

55K

0.078

1.8

62.7

0.30

4K

0.274

4.4

25.3

1

41K 98K

0.104 0.038

1.7 0.6

66.3 183.7

0.38 0.14

content increases from Pd 5 to Pd 10. The beneficial effect of the Pd loading on the TWC activity within the medium and high catalyst mileage may be primarily attributed to the number of the active metal sites included in the Pd catalysts with respect to the Pd loading.8,13 Alteration of Pd Metallic Surface Area. To understand the main cause for the deactivation of the Pd-only TWCs used under field-aging condition, Heo et al. extensively examined the deactivation mechanism in view of the change of the physicochemical properties of the identical TWCs including the Pd dispersion and the level of contaminants.13 Sintering of Pd was the dominant cause for the deactivation of the Pd-only TWC, while no systematic dependence of the content of the contaminants including sulfur (S) and lead (Pb) on the catalyst deactivation with respect to the catalyst mileage was observed.11,15 CO chemisorption was then conducted to observe the trend of the alteration of the Pd MSA and particle size upon field-aging with respect to the catalyst mileage and the metal content for elucidating the change of the TWC activity of the Pd-only catalysts as a function of the Pd loading and catalyst mileage. The MSA, dispersion, and particle size of Pd can be determined by22,23 Vs f 2 MSA=ðm =gÞ ¼ nSA ð1Þ 22414Ws

Vs S dispersion=ð%Þ ¼ 100 m 22414Ws F particle size=ðnmÞ ¼

6 109 Ca m FDn

ð2Þ

ð3Þ

where Vs is the CO volume adsorbed (mL at STP), f is the stoichiometric factor (=1), Ws is the sample weight (g), n is Avogadro’s number (6.02 1023), and SA is the specific surface area of Pd atom (0.0787 nm2).23 Ca is the concentration of the surface metal atoms (1.27 1019 atoms/m2), m is the Pd atomic mass (106.42 g/mol), F is the metal density (Pd = 12.02 106 g/m3), and D is the Pd metal dispersion.22 The MSA, dispersion, and particle size of the Pd metal included in Pd 5, 7, and 10 TWCs with respect to the Pd loading and the catalyst mileage determined by CO chemisorption are listed in Table 3. The MSA of the 4K Pd-only catalyst increased slightly over 10%, from 0.243 to 0.274 m2/g, when the Pd metal

loading of the 4K Pd-only catalyst increased 2 times from Pd 5 to Pd 10. The nonlinear increase of the MSA for the 4K Pd-only TWCs simply indicates that there is an optimum MSA over the present catalytic system. Also, this trend may describe the TWC performance hardly altered over the 4K Pd-only TWCs with respect to the Pd metal loading. As the catalyst mileage increases, the MSA of Pd of the Pd-only TWCs decreases and the particle size of Pd increases. In addition, the alteration of the MSA and particle size of Pd mainly occurs during the initial period of the catalyst mileage from 4K to 41K miles, whereas it gradually changes up to 98K miles, the highest catalyst mileage employed in the present study. Since the MSA of Pd has apparently decreased with the increasing catalyst mileage, regardless of the Pd content, the catalyst deactivation trend of Pd-only TWCs may be wellcorrelated with the alteration of the MSA of Pd with respect to the catalyst mileage. Indeed, the decrease of the MSA of TWC is a critical parameter for directly describing the loss of the active metal surface included in TWC upon aging.810 The Pd metal dispersion of TWC levels off at nearly 1%, particularly for the catalyst with the highest mileage, 98K miles employed in the present study. This asymptotic value of the Pd dispersion may be attributed to the lower diffusion coefficient of the larger metal particle formed by the particle migration mechanism.24 It may then be anticipated that the catalytic performance also levels off to a certain degree of the TWC activity. Primary and Deactivation Kinetics. Primary Kinetics over 4K Pd-Only TWCs. On the basis of the detailed reaction kinetics recently developed for a commercial Pd-only TWC in terms of microkinetics,25 the primary kinetics was derived for the 4K Pdonly TWCs employed in the present study. Their fresh activities were predicted with the reliable kinetic parameters determined from the experimental data shown in Figure 1 and Table 2. Note that stabilized TWCs (4K miles equivalent) have been used for determining the fresh catalytic activity of the TWCs with respect to the catalyst Pd content.2528 Figure 3 shows the predictions of the TWC performance by the primary kinetic model developed for the 4K Pd 5 catalyst at the reactor space velocities of 50 000, 100 000, and 170 000 h1. The detailed kinetic model well-predicts the measured experimental data on the conversions of CO, C3H6, H2, and NO as well as the formation of NH3 and N2O at the reactor space velocities varied from 50 000 to 170 000 h1. The primary kinetic model developed is capable of predicting the general trend of the TWC performance under full feed stream within the wide range of the reactor space velocity and reaction temperature as shown in Figure 3. The kinetic parameters for the 4K Pd 7 and 4K Pd 10 catalysts have been also estimated to predict their fresh TWC activities. Their primary reaction kinetic models again welldescribe the general trend of the TWC performances of the Pd-only catalyst with respect to the Pd metal content. The kinetic parameters listed in Table 4 were estimated by using nonlinear regression to minimize the sum of squares of the difference between the experimental and calculated data. A regression routine for the minimization employs the Marquardt algorithm.29 The MATLAB (version 6.1, the MathWorks, Inc.) was used as the numerical subroutine program for solving the nonlinear ODE. The variation of the kinetic parameters may not be apparent with respect to the catalyst Pd content as listed in Table 4. The activation energies and heat of adsorptions are hardly varied, while the frequency factors of the rate constants regarded as the 5503



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Figure 3. Comparison of predicted and measured conversions over 4K Pd 5 catalyst with respect to the reactor space velocities. Feed gas composition: 1% CO, 500 ppm C3H6, 0.3% H2, 1% O2, 500 ppm NO, 10% CO2, 10% H2O, and Ar balance (λ = 1.009).

key reaction steps for the rate equations derived are altered with respect to the amount of Pd loading onto the catalyst. In addition, the activation energies of the CO, C3H6, and H2 oxidations are 18.8, 21.1, and 11.6 kcal/mol, respectively, as also reported in the previous studies.25,27 It is quite consistent with the TWC oxidation activity ranking observed in the present study and widely accepted in the following order: H2 > CO > C3H6.26 The

activation energy of the NO dissociation (Ea,16) is 25.4 kcal/mol, which is also within the similar range of the activation energy reported in the literature.25 Development of TWC Activity Function. The sintering model has been widely employed to appropriately describe the decrease of the TWC activity with respect to the reactor on-stream time and temperature. Wanke and Flynn developed a power-law 5504



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Table 4. Kinetic Parameters Estimated from the Experimental Data over 4K Pd 5, 7, and 10 TWCs kinetic parametersa 4K Pd 5

4K Pd 10

4K Pd 5, 7 and 10

CO

K0,1

2.3 10

ΔH1

11.2

C3H6

K0,2

1.5 101

ΔH2

7.8

H2

K0,3

1.1 103

ΔH3

17.7

O2

K0,4

2.0 101

ΔH4

24.5

NO

K0,5

4.5 10°

ΔH5

20.1

H2O

K0 0,6

1.0 107

ΔH6

22.3

NH3

K0,7

3.0 102

ΔH7

8.1

N2O H2O 3 S þ S f OH 3 S þ H 3 S

K0,8 k0,9

1.1 101 3.9 103

ΔH8 Ea,9

12.2 34.8

Ea,10

33.6

Ea,11

32.0

Ea,12

18.8

H2O 3 S þ O 3 S f 2OH 3 S

1

2.1 104

k0,10 2.0 10

5.0 1010

7.0 1010

H 3 S þ O 3 S f OH 3 S þ S

k0,11

CO oxidation

k0,12

C3H6 oxidation

k0,13

1.6 1010

2.1 1010

2.3 1010

Ea,13

21.1

H2 oxidation

k0,14

5.6 109

6.2 109

7.3 109

Ea,14

11.6

CO 3 S þ 2OH 3 S f CO2 þ H2O þ 2S

k0,15

2.0 103

Ea,15

10.3

k0,16 k0,17

6.7 104 1.1 107

Ea,16 Ea,17

25.4 14.0

Ea,18

19.8

Ea,19

19.8

NO 3 S þ S f N 3 S þ O 3 S NO 3 S þ N 3 S f N2O 3 S þ S

NO 3 S þ N 3 S f N2 þ O 3 S þ S NO 3 S þ N 3 S f N 3 S þ OH 3 S

a

4K Pd 7

10

5.3 107

8.0 109

k0,18 k0,19

6.0 10

1.3 105

5

1.9 105

N 3 S þ 3H 3 S f NH3 3 S þ 3S

k0,21

1.6 104

Ea,21

12.2

NH3 oxidation (N2)

k0,22

1.1 107

Ea,22

15.2

NH3 oxidation (NO)

k0,23

4.0 109

Ea,23

15.2

NH3 oxidation (N2O)

k0,24

2.5 104

Ea,24

19.3

NH3 SCR N2O 3 S f N2 þ O 3 S

k0,25 k0,26

2.5 104 2.0 104

Ea,25 Ea,26

27.8 28.0

K0,i (cm3/mol), ΔHi (kcal/mol), k0,i [cm3/(mol 3 s)], and Ea,i (kcal/mol).

model to predict the changing trend of the metal dispersion of supported Pt catalysts upon sintering with respect to the catalyst aging duration and temperature.4,30 Hughes et al. conducted CO chemisorption to examine the alteration of the Pt dispersion over a 0.4% Pt/Al2O3 catalyst under H2 atmosphere at 900 and 1000 °F. They reported that the alteration of the amount of CO uptake onto the catalyst with respect to the sintering time could be well-correlated with a simple power-law model.31 Another way to develop the TWC activity function may be the use of the catalyst mileage as an independent variable for developing the sintering kinetics, instead of the reaction time and temperature which are readily varied by the catalyst operating condition employed.32 However, no activity function has been reported so far that can describe the sintering of TWC as a function of the catalyst mileage. Indeed, the mileage is the primary parameter for predicting automobile’s life in the automotive industry. For describing the catalyst activity as a function of the catalyst mileage, an activity function for TWC, a, as a normalized activity is defined as33 a ¼ r=r0

Figure 4. Trend of TWC activity function with respect to the catalyst mileage.

ð4Þ

where r is the reaction rate at any mileage, M, and r0 is the reaction rate at the initial mileage, particularly 4K stabilized miles of TWC. Indeed, the 4K stabilized TWC has been commonly employed as a fresh TWC.26,27

The activity function of TWC may be independently developed by correlating the change of the MSA of Pd by sintering, the representative cause for the deactivation of TWC as the catalyst mileage increases.8,13,34 The normalized MSA of Pd, S/S0 listed in Table 3 has been directly utilized for describing the alteration 5505



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Figure 5. Comparison of predicted and measured conversions over Pd 5 catalyst with respect to the catalyst mileage. Feed gas composition: 1% CO, 500 ppm C3H6, 0.3% H2, 1% O2, 500 ppm NO, 10% CO2, 10% H2O, and Ar balance (λ = 1.009). Reactor SV: 100 000 h1.

of TWC activity with respect to the catalyst mileage.30 a ¼ r=r0 S=S0

ð5Þ

The changing rate of the normalized MSA of Pd with respect to the catalyst mileage, M, can be described by

dðS=S0 Þ ¼ kd ðS=S0 Þn dM

ð6Þ

where S is the MSA of Pd at any M and S0 is the MSA of Pd at the initial mileage, particularly 4K miles. By substitution of eq 5 into 5506



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Figure 6. Comparison of predicted and measured conversions over Pd 7 and Pd 10 catalysts with respect to the catalyst mileage. Feed gas composition: 1% CO, 500 ppm C3H6, 0.3% H2, 1% O2, 500 ppm NO, 10% CO2, 10% H2O, and Ar balance (λ = 1.009). Reactor SV: 100 000 h1.

eq 6, a becomes da ¼ kd an ð7Þ dM where kd is the sintering rate constant and n, an exponent, is the sintering rate order. As shown in Figure 4, the second-order

sintering kinetics well-describes the decrease of the catalyst activity based upon the alteration of the Pd MSA with respect to the catalyst mileage, regardless of the Pd content. Indeed, Xu et al. observed no Pd loading effect on the sintering rate of Pd/θ-Al2O3 containing 1.17.0 wt.% of Pd.35 In addition, the second-order sintering kinetics had been reported previously for 5507



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representatively describing the deactivation of TWC.34 The TWC activity function with n = 2 then becomes a ¼ 1=ð1 þ kd MÞ

ð8Þ

With the MSA data listed in Table 3, the sintering rate constant, kd = 0.063 miles1 has been estimated from the slope of the linear plot for the second-order sintering kinetics. Particularly, the TWC activity function developed in the present study has been capable of capturing the decrease of MSA with the increasing catalyst mileage over the low mileage region as well as the activity function over the high-mileage region, even 98K miles. This result simply indicates that the alteration of the MSA of the Pd with respect to the catalyst mileage can be well-described by the second-order sintering kinetics, regardless of the Pd metal content.

’ DISCUSSION The activity function derived has been incorporated into the detailed reaction kinetics developed for the fresh catalyst, 4K stabilized Pd-only TWCs in order to predict the alteration of the catalytic activity over the Pd-only commercial TWC with respect to the catalyst mileage. The activity function, eq 8, has been simultaneously employed for predicting the rates of the disappearances of CO, C3H6, O2, H2, and NO and of the formations of NH3 and N2O. The overall reaction kinetics can be expressed in terms of the TWC activity function by the following equation: ri, M ¼ a 3 ri, M0

’ CONCLUSIONS A simple empirical activity function for the commercial Pdonly TWC has been independently derived from the change of the Pd MSA of TWC upon aging with respect to the content of Pd and the catalyst mileage. Indeed, the MSA determined by CO chemisorption can reflect that there is no major difference in the fresh activity over the Pd-only TWC as the Pd metal loading increases from Pd 5 to Pd 10. The second-order sintering kinetics developed well-expresses the TWC activity function, a, with respect to the catalyst mileage, regardless of the Pd loading. To describe the alteration of the TWC performance under realistic feed-stream conditions with respect to the catalyst mileage, an overall kinetic model has been developed by incorporating the TWC activity function developed into the primary steady-state reaction rates based on the detailed reaction kinetics. Consequently, the activity function for the commercial Pd-only TWCs developed in the present study should serve as a useful tool for predicting the long-term performance of TWC without expensive and time-consuming experimental efforts. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: 82-54-279-2264. Fax: 82-54-279-8299. E-mail: isnam@ postech.ac.kr. Present Addresses †

Emerging Technology Research Center/Environment group, Samsung Advanced Institute of Technology, San 14-1, Nongseo-dong, Giheung-gu, Yongin, Gyeonggi-do 446-712, Korea.

ð9Þ

where ri,M is the rate of the reaction for i at any catalyst mileage, M, ri,M0 is the primary rate of the reaction for i over the stabilized 4K TWC, and i refers to the gas composition involved in TWC reaction including CO, C3H6, H2, O2, NO, NH3, and N2O.25 Figures 5 and 6 show the model prediction of the TWC activity over the Pd-only TWCs (5, 7, and 10) with respect to the catalyst mileage. The overall kinetic model developed on the basis of the primary and sintering kinetics independently derived in the present study well-predicts the experimental data and describes the trend of the catalyst deactivation with respect to the catalyst mileage. Particularly, the model prediction seems to be quite appropriate for describing the fast deactivation rate during the initial period of the catalyst mileage and the subsequent gradual decrease of the TWC activity in the high mileage, regardless of the catalyst Pd content. Note that no further adjustment of the kinetic parameters has been made for predicting the decrease of the TWC activity with respect to the catalyst field mileage. It reflects that the normalized MSA utilized in the present study for developing the activity function is a primary deactivation variable to directly relate the decrease of the reaction rates for each reactant and product species during the course of the deactivation process under the field-aging conditions. It is remarkable that the long-term catalytic activity of Pd-only TWCs may now be directly predicted by the use of the activity function, which has been derived in this study from the alteration of the metallic surface area upon aging as a representative of the available number of active reaction sites with respect to the catalyst mileage.

’ ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST; Grant No. 20100028712). ’ NOMENCLATURE a =TWC activity function Ca =concentration of surface metal atoms, atoms/m2 D =Pd metal dispersion, % Ea =activation energy, kJ/mol f =stoichiometry factor F =fraction of sample weight ΔHi =heat of adsorption of i species, kJ/mol k0,i =frequency factor of rate constant for a reaction i, cm3/ (mol 3 s) K0,i =preexponential factor of adsorption equilibrium constant, cm3/mol m =Pd atomic mass, g/mol M =catalyst mileage ri =reaction rate for i species based on catalytic volume, mol/ (cm3 3 s) SA =specific surface area of Pd atom (0.0787 nm2) T50 =light-off temperature (the temperature at which 50% conversion of reactant is achieved), °C Vs =CO volume adsorbed, mL Ws =sample weight, g Greek Letters

G =metal density, g/m3 5508



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Activity Function for Describing Alteration of Three-Way Catalyst

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