Chemical Engineering Journal xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Activity function describing the effect of Pd loading on the catalytic performance
of modern commercial TWC
Sung Bong Kang a, Seok Jun Han a, Sung Bang Nam a, In-Sik Nam a,⇑, Byong K. Cho a, Chang Hwan Kim b,
Se H. Oh b
a
Department of Chemical Engineering/School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyoja-dong,
Pohang 790-784, Republic of Korea
General Motors Research and Development Planning Center, Warren, MI 48090-9055, United States
b
h i g h l i g h t s
" The activity function has been developed on the basis of the alteration of the Pd MSA.
" The catalytic activity is well described by an empirical exponential function of Pd loading.
" The overall kinetics combined with the activity function well predicts the TWC performance.
a r t i c l e
i n f o
Article history:
Available online xxxx
Keywords:
Pd-based TWC
Pd loading
Activity function
TWC reaction kinetics
a b s t r a c t
A catalyst activity function for the commercial Pd-based three-way catalyst (TWC) has been developed to
describe the effect of Pd loading ranging from 20 to 240 g/ft.3 on its catalytic performance. Derived from
the alteration of the Pd metallic surface area (MSA) due to the variation of the Pd loading, the activity
function reasonably well captures the nonlinear dependence of catalytic activity on the Pd loading of
the Pd-based TWCs. The overall reaction kinetics for the commercial TWC obtained from the combination
of the catalyst activity function developed and the primary reaction kinetics over the 4k Pd240 reference
catalyst has proven to be capable of well predicting the effect of Pd loading on the catalytic performance
of the Pd-based TWCs. The accuracy of model predictions has been excellent for the oxidation of CO, C3H6
and H2, while leaving some room for improvement in the model accuracy for the reduction of NO and
related byproducts.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
The chemical formulation of commercial three-way catalysts
(TWCs) has been shifting in recent years from the traditional Ptbased to a Pd-based system due to the higher cost of platinum (Pt)
than that of palladium (Pd), which has prompted heightened interest in the kinetic behavior and the optimum design of the Pd-based
TWCs [1]. In fact, the optimum design of the Pd-based TWC is closely
related to its kinetic behavior, and an effective way for the optimum
design of a commercial TWC converter is to develop an accurate and
reliable reaction kinetics model that can describe the TWC performance in terms of the catalyst mileage (i.e., the time-on-stream)
and the noble metal loading of the catalyst [2].
Baba et al. developed the deactivation kinetics for a Pt-based
TWC with an adjustable pre-exponential factor included in the
reaction rate constants [3]. They reported that the sintering of no⇑ Corresponding author. Tel.: +82 54 279 2264; fax: +82 54 279 8299.
E-mail address: [email protected] (I.-S. Nam).
ble metals was the primary cause of the TWC deactivation involving the decrease of the active metal surface area (MSA). Recently, a
simple TWC deactivation function has been developed to describe
the change of the catalytic performance as a function of the fieldaged catalyst mileage from 4k (stabilized) to 98k miles, based on
the alteration of the Pd metallic surface area (MSA) of Pd-based
commercial TWCs [4]. However, to the best of our knowledge,
the TWC activity function correlating the conversion performance
of commercial Pd-based TWC catalysts with the noble metal loading has not been reported in the literature, probably due to the lack
of available catalyst samples covering a wide range of noble metal
loadings, although the amount of noble metal loaded in a commercial TWC widely varies depending upon the size, model and performance of the engine to be installed [5].
In the present study, the effect of noble metal loading on the Pdbased TWC performance was investigated by using temperature
run-up tests under full-feed conditions simulating the real exhaust
gas composition of the gasoline engine. Based upon the alteration of
the active metallic surface area (MSA) of the Pd-based catalysts with
1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cej.2012.06.003
Please cite this article in press as: S.B. Kang et al., Activity function describing the effect of Pd loading on the catalytic performance of modern commercial
TWC, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.003
2
S.B. Kang et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
various Pd loadings, a simple activity function has been developed to
describe the variation of the TWC activity of the 4k stabilized Pd (20,
50, 80, 140, 160, 200 and 240 g/ft.3) based catalysts. In a series of
numerical simulations of the overall reaction kinetics, the alteration
of the TWC performance with respect to the Pd loading was predicted reasonably well by the activity function incorporated into
the primary kinetic model developed for the 4k Pd240 TWC.
2. Experimental
2.1. Catalyst preparation and characterization
S¼
Six samples of commercial Pd-based TWCs were employed in
this study, each of them containing 20, 80, 140, 160, 200 and
240 g/ft.3 of Pd loadings, respectively. In addition, a Pd/c-Al2O3 catalyst containing 50 g/ft.3 of Pd (Pd50) was also prepared by the
incipient wetness method with an aqueous solution of Pd(NO3)2
Ò
(ALDRICH) for impregnating Pd onto c-Al2O3 support (CATALOX
SBA-200, Sasol) as a probing model catalyst to further examine systematically the alteration of the catalyst TWC performance as a
function of the Pd loading. The total seven Pd-based TWCs with
the Pd loading ranging from 20 to 240 g/ft.3 were stabilized with
4k miles equivalent aging by using the laboratory aging program
developed by GM R&D. To determine the Pd content of the catalysts, an inductively coupled plasma-optical emission spectrometer (ICP-Flame-EOP, SPECTRO Co.) was employed. The catalyst
samples were dissolved in a mixture of acids containing HCl and
HNO3 (3:1) at 60 °C for 6 h prior to the ICP analysis. Listed in Table 1
is the Pd content in the catalysts examined in the present study.
The active metallic surface area (MSA) and the dispersion of Pd
were measured by a pulse CO chemisorption apparatus (Autochem
II 2920, Micromeritics Co.). Details of the procedure for CO chemisorption are described elsewhere [4]. The dispersion and active
metal surface area (MSA) and of Pd were calculated as follows [6]:
Dispersion ð%Þ ¼ 100 ½ðVs f Þ=ðCs Ws 22; 414Þ m
ð1Þ
MSA ðm2 =gÞ ¼ D Cs ðn=mÞ SA
ð2Þ
where Vs is the CO volume adsorbed (mL at STP), f is the stoichiometric factor (=1), Cs is Pd metal content (wt.%), Ws is the sample
weight (g), D is Pd metal dispersion (%), n is Avogadro’s number
(6.02 1023), SA is the specific surface area of a Pd atom
(0.0787 nm2) and m is the Pd atomic mass (106.42 g/mol).
2.2. Catalyst activity test
The steady-state TWC activity was examined by increasing the
reaction temperature stepwise from 150 to 450 °C over a packedbed U-tube type flow reactor immersed in a molten salt bath to
maintain an isothermal reactor condition [7]. Then 0.5 g of powder
catalyst, including cordierite in the 20/30 mesh size, diluted with
inert glass beads (0.710 mm) was charged into a 3/8 in. o.d. stainTable 1
Physicochemical properties of 4k Pd-based TWCs employed.
a
b
4k Pd-based TWCs
(g of Pd loaded on 1 ft3 of monolith)
Pd metal
contentsa (wt.%)
Pd20 (g/ft.3)
Pd50b (g/ft.3)
Pd80 (g/ft.3)
Pd140 (g/ft.3)
Pd160 (g/ft.3)
Pd200 (g/ft.3)
Pd240 (g/ft.3)
0.10
0.26
0.42
0.73
0.86
1.02
1.24
Determined by ICP.
Model catalyst.
less steel tube reactor to minimize the internal diffusion resistance
while maintaining the isothermal condition of the reactor. The
overall reactor space velocity was maintained typically at
100,000 h1, depending on the total bulk volume of the powder
catalysts (0.55 cc) employed with the total volumetric flow rate
of 917 cc/min. The feed gas stream for the ‘‘full-feed’’ operating
condition contained 1% CO, 500 ppm C3H6, 0.3% H2, 500 ppm NO,
1% O2, 10% CO2 and 10% H2O in Ar balance. The stoichiometric factor for the ‘‘full-feed’’ operating condition (S = 1.17) has been calculated as follows [8]:
2½O2 þ ½NO
½CO þ ½H2 þ 9½C3 H6 ð3Þ
The concentrations of CO, H2, C3H6, CO2 and O2 were analyzed by
gas chromatography (GC) equipped with a TCD and an FID (Agilent,
model 6890N). The TCD was employed for the quantification of H2
and O2, while the FID was used to measure the concentrations of
CO, CO2 and C3H6 at the inlet and outlet of the reactor. The concentrations of NO and the products including N2O and NH3 were determined by an on-line FTIR analyzer with a 2 meter gas cell and a
DTGS/KBr detector (Nicolet 6700, Thermo Electrons Co.) in the
range of 500–4000 cm1. The catalysts examined were always
pre-calcined in a stoichiometric gas mixture containing 0.9% CO,
0.6% O2, 0.3% H2, 10% CO2, 10% H2O and Ar balance (S = 1) at
450 °C for 2 h and then cooled in Ar atmosphere to the reaction
temperature prior to the activity measurement.
3. Results and discussion
3.1. Alteration of TWC performance as a function of Pd loading
The alteration of TWC activity was examined over 4k Pd-based
catalysts in a wide range of the Pd loading from 20 to 240 g/ft.3 as
depicted in Fig. 1. As expected, with higher Pd loading for a commercial TWC system, higher TWC performance has been observed
for simultaneously removing CO, C3H6, H2 and NO. Interestingly,
however, the enhancement of TWC activity is not linearly proportional to the Pd loading, particularly over the catalyst containing a
relatively high amount of Pd above 80 g/ft.3.
As listed in Table 2, the light-off temperatures (LOTs, T50) of CO,
C3H6, and H2 and the temperature for the maximum NO conversion
(Tmax.NO) shift to a lower temperature region from 234, 243, 195
and 219 °C to 202, 211, 153 and 186 °C, respectively, with the
increasing Pd content from 20 to 80 g/ft.3. On the other hand, the
differences in T50.CO, C3H6, H2 and Tmax.NO between the 4k Pd80 and
4k Pd240 catalyst (3-fold increase in Pd loading) are only about
10 °C. In a similar observation, Dong et al. reported that the CO oxidation activity of Pd/CeO2–TiO2 catalysts leveled off in the range of
Pd loading higher than 0.5 wt.% [9].
For the 4k Pd50 catalyst (Pd/c-Al2O3) prepared in the present
study as a probing model catalyst, the general trends for the oxidation of CO, C3H6 and H2 are well correlated with the variation of Pd
loading from 20 to 80 g/ft.3, where the NO reduction activity is relatively lower than that over the 4k Pd20 catalyst, the lowest Pdcontaining catalyst employed in this study. It is most likely due
to the absence of catalyst promoters such as Ce, Ba and La in this
model catalyst. Note that all commercial Pd-based TWCs employed
in the present study typically contain 3 wt.% Ce, 1 wt.% Ba and
1 wt.% La, as they are commonly used as catalyst promoters to enhance the NO reduction activity of commercial TWCs [10,11].
3.2. Development of catalyst activity function
It has been widely reported that the active metal surface area
(MSA) representing the number of active sites formed on the
Please cite this article in press as: S.B. Kang et al., Activity function describing the effect of Pd loading on the catalytic performance of modern commercial
TWC, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.003
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S.B. Kang et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Fig. 1. Effect of the Pd loading on the TWC performance over 4k Pd20, 50, 80, 160 and 240 (g/ft.3) catalysts: Feed gas composition: 1% CO, 500 ppm C3H6, 0.3% H2, 500 ppm NO,
1% O2, 10% CO2, 10% H2O and balance Ar (S = 1.17): Reactor SV: 100,000 h1.
Table 2
The light-off temperatures (LOTs) and the temperatures for the maximum NO
conversion.
4k Pd-based TWCs
Pd20
Pd50
Pd80
Pd160
Pd240
T50 (°C)
Tmax. (°C) (Conversion%)
CO
C3H6
H2
NO
234
222
202
199
195
243
228
211
207
199
195
177
153
n.d.
n.d.
219
238
186
175
173
(40)
(37)
(55)
(66)
(70)
The Pd MSA of the 4k Pd-based TWCs was normalized with respect to that over the 4k Pd240, so that the catalyst containing the
highest MSA (0.57 m2/g) became the reference catalyst in the
development of the catalyst activity function as listed in Table 3.
As the Pd metal loading increased fourfold from Pd20 to Pd80,
the normalized MSA increased proportionally to the Pd metal
content. However, the increasing trend of the normalized MSA
Table 3
MSA and dispersion of Pd over Pd-based TWCs as a function of the Pd loading.
n.d.: Not determined (the relevant conversions are higher than 50%).
catalyst surface plays a critical role in determining its catalytic performance [4,12]. Accordingly, the alteration of Pd MSA determined
by CO chemisorption has been systematically examined to develop
an activity function applicable to commercial Pd-based TWCs in
describing their relevant catalytic activity as a function of the Pd
loading.
Sample
(4k Pd-based TWCs)
Pd MSA
(SL, m2/g)
Dispersion
(%)
Normalized Pd MSA
a = S4k Pd L/S4k Pd240
Pd20
Pd50
Pd80
Pd140
Pd160
Pd200
Pd240
0.10
0.24
0.41
0.49
0.56
0.55
0.57
22.22
21.33
22.78
15.56
15.10
12.28
10.56
0.18
0.42
0.72
0.86
0.98
0.96
1.00
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S.B. Kang et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
with respect to the Pd metal loading revealed a nonlinear increase
(only about 1.4-fold increase) as the Pd metal loading increased
further from Pd80 to Pd240. There was no appreciable difference
in the Pd MSA between Pd160 and Pd240, mainly due to the
decreased Pd dispersion with the increasing Pd loading on the
catalyst surface as clearly shown in Table 3.
Based upon these observations along with an assumption that
the rate of TWC reactions is proportional to the metal surface area
at a given Pd loading, a TWC activity function has been developed
to describe the alteration of TWC activity of the 4k Pd-based catalysts as a function of either the Pd metal loading or the metal surface area in such a way as [13]:
a¼
Fig. 2. Model prediction of catalytic activity by using activity function.
r 4k PdL
S4k PdL
¼
r4k Pd240 S4k Pd240
ð4Þ
Note that the catalyst activity function (a) has been automatically
normalized with its value ranging from zero to one by choosing
the reaction rate over the Pd240 catalyst as the primary rate. Since
the normalized Pd MSA is a function of the Pd loading (L), a corre-
Fig. 3. Comparison of predicted and measured data over the 4k Pd-based TWCs as a function of the Pd loading: Feed gas composition: 1% CO, 500 ppm C3H6, 0.3% H2, 500 ppm
NO, 1% O2, 10% CO2, 10% H2O and balance Ar (S = 1.17): Reactor SV: 100,000 h1.
Please cite this article in press as: S.B. Kang et al., Activity function describing the effect of Pd loading on the catalytic performance of modern commercial
TWC, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.003
S.B. Kang et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Table 4
Kinetic parameters estimated from the experimental data over the 4k Pd240 catalyst
[4,14].
Adsorption and desorption of reactants
CO + S M COS
C3H6 + S M C3H6S
K1
H2 + 2S M 2HS
O2 + 2S M 2OS
NO + S M NOS
H2O + S M H2OS
NH3 + S M NH3S
N2O + S M N2OS
K2
K3
K4
K5
K6
K7
K8
Surface reactions
H2OS + S ? OHS + HS
H2OS + OS ? 2OHS
HS + OS ? OHS + S
COS + OS ? CO2 + 2S
C3H6S + 9OS ? 3CO2 + 3H2OS + 7S
HS + OHS ? H2OS + S
COS + 2OHS ? CO2 + H2OS + 2S
NOS + S ? NS + OS
NOS + NS ? N2OS + S
NOS + NS ? N2 + OS + S
NOS + HS ? NS + OHS
2NOS + HS ? N2OS + OHS
NS + 3HS ? NH3S + 3S
2NH3S + 3OS ? 2N2 + 3H2OS + 2S
2NH3S + 5OS ? 2NOS + 3H2OS + 2S
2NH3S + 4OS ? N2OS + 3H2OS + 2S
2NH3S + 2NOS + OS ? 2N2 + 3H2OS + 2S
N2OS ? N2 + OS
k9
k10
k11
k12
k13
k14
k15
k16
k17
k18
k19
k20
k21
k22
k23
k24
k25
k26
2.3 101
1.5 101
D H1
D H2
11.2
7.8
5.1 103
2.0 101
2.0 100
1.0 107
3.0 103
2.0 102
D H3
D H4
D H5
D H6
D H7
D H8
16.3
24.6
20.0
23.9
3.5
5.2
3.9 103
2.1 104
3.2 1010
5.0 104
5.4 106
9.6 109
2.0 103
1.7 103
1.5 106
2.5 1012
1.9 108
2.5 102
4.5 1015
1.1 107
1.0 106
1.0 1011
2.5 104
8.0 104
Ea,9
Ea,10
Ea,11
Ea,12
Ea,13
Ea,14
Ea,15
Ea,16
Ea,17
Ea,18
Ea,19
Ea,20
Ea,21
Ea,22
Ea,23
Ea,24
Ea,25
Ea,26
34.8
33.5
30.2
11.9
12.9
8.0
10.3
19.4
20.5
17.9
22.9
12.0
29.5
12.9
14.0
11.9
27.8
8.3
K0,i (cm3/mol), k0,i (cm3/mols), DHi (kcal/mol) and Ea,i (kcal/mol).
lation between the catalyst activity function (a) and the Pd loading
(L) can be obtained from Eq. (4), leading to an empirical correlation
as follows:
a ¼ 1:05 1:08 exp
2:0 L
70:0
ð5Þ
where L is the Pd metal loading on the TWC. As shown in Fig. 2, the
TWC activity function, Eq. (5) developed in the present study well
describes the nonlinear increase of the normalized Pd MSA of 4k
Pd-based TWCs over a wide range of the Pd metal loading from
20 to 240 g/ft.3. Boll et al. also examined the change of MSA for a
Pt-based diesel oxidation catalyst with an increasing Pt loading
from 20 to 120 g/ft.3, which yielded a linear relationship between
the Pt loading and the corresponding MSA, probably attributable
to the narrow range of the Pt loading on their catalysts [12].
3.3. Prediction of TWC performance
To predict the alteration of the TWC performance over the Pdbased commercial TWCs with the increasing Pd metal loading from
20 to 240 g/ft.3, the TWC activity function, Eq. (5) has been incorporated into the detailed reaction kinetics previously developed
for the Pd-based commercial catalyst [14]. The overall reaction
kinetics over a TWC with a specific Pd loading can be expressed
by a combination of the catalyst activity function (a) and the primary reaction kinetics over the 4k Pd240:
r i;4k
Pd L
¼ a r i;4k
Pd240
ð6Þ
where ri,4k Pd L is the rate of the reaction for species i at a specific Pd
metal loading, L, ri,4k Pd240 is the primary rate of the reaction for species j over the 4k Pd240 catalyst employed as the reference catalyst
in this study and species i refers to the gas species involved in the
TWC reaction including CO, C3H6, H2, O2, NO, NH3 and N2O.
Fig. 3 shows the model prediction of the TWC activity over the
4k Pd-based catalysts as a function of the Pd metal loading. Note
5
that the primary kinetic parameters estimated over the 4k Pd240
catalyst listed in Table 4 have been directly utilized for predicting
the alteration of the TWC performance with respect to the catalyst
Pd loading without any further modification.
The oxidation activities of CO, C3H6 and H2 are well captured by
the overall kinetic model obtained from the combination of the primary kinetics for the 4k Pd240 catalyst and the catalyst activity
function, whereas the predictions of NO reduction activity and
the formations of NH3 and N2O are slightly overestimated at the
reaction temperature above 250 °C as shown in Fig. 3d–f. Specifically, it is remarkable that both the gradual increase of the TWC
performance with the increasing Pd loadings from 80 to 240 g/
ft.3 for the high-loading catalysts and its rapid increase therewith
from 20 to 80 g/ft.3 for the low-loading catalysts are appropriately
captured by the use of the rather simple activity function developed in this work. Further investigation will focus on improving
the accuracy of model predictions for NO reduction and related
byproducts through a better understanding of the NO reduction
kinetics over the Pd-based TWCs.
4. Conclusion
A TWC activity function based upon the alteration of the Pd
MSA has been developed and used in predicting the alteration
of the TWC performance as a function of the Pd metal loading.
The nonlinear correlation between the catalytic activity and the
Pd loading is reasonably well described by an empirical exponential function over a wide range of Pd loading from 20 to 240 g/ft.3.
Consequently, the overall reaction kinetics resulting from the
combination of the activity function and the primary reaction
kinetics is capable of predicting the alteration of TWC performance with respect to the Pd metal loading. The TWC activity
function developed for the commercial Pd-based TWCs in the
present study should provide a useful tool for the optimal design
of a modern catalytic converter, compared to the currently common practice of simple adjustments in the reaction rate constants
to describe the alteration of the TWC performance with respect to
the Pd loading.
Acknowledgments
This work was supported by General Motors Corporation (Project 4.0007049.01: Effect of Sintering and Hydrocarbon Oxidation
Performance of Next-Generation Three-Way Catalysts) and National Research Foundation (No. 2011-0029806: Emission Control
Catalytic System for Next Generation Energy-Efficient Vehicle) of
Korea (NRF) grant funded by the Korean government (MEST).
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TWC, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.003