(661c) CO, C2H2, and C3H6 Oxidation on Pd/Ceria-Zirconia/Al2O3 Three-Way Catalysts

CO,
C2H2, and C3H6 oxidation on Pd/Ceria-Zirconia/Al2O3
Three-way Catalysts

Wendy
Lang¹, Michael P. Harold^1*, Yisun
Cheng², Carolyn Hubbard², and Paul Laing²

¹Department of Chemical and Biomolecular
Engineering, University of Houston, Houston, TX

²Research and Innovation Center, Ford Motor Company, Dearborn, MI

*mharold@uh.edu

Three-way catalyst (TWC) light-off (LO) performance is
critical in cost-effectively meeting emission standards. Improvements in
catalyst LO should include catalyst modifications that minimize inhibition
effects of exhaust species such as CO and hydrocarbons (HCs), lower precious
group metal (PGM) loading, or use a lower cost PGM such as Pd. Alumina (Al₂O₃)
and zirconia (ZrO₂) supports provide high
surface area and stability. Furthermore, ceria (CeO₂)   provides
the key benefit of oxygen storage, compensating for deviations from
stoichiometric TWC operating conditions by supplying oxygen for CO and HC
oxidation under rich transient conditions. Inclusion of ceria in PGM catalysts
enhances CO and HC conversion and minimizes self-inhibition; this improved
activity is mainly attributed to ceria containing more efficient oxygen
activation sites at the PGM-support interface relative to other support
materials such as Al₂O₃ [1-4].

Monolith catalysts having washcoats
with 1 wt% Pd, 3 g/in³ loading, 400 cpsi and
formulations Pd/Al₂O₃, Pd/CZO, and Pd/CZO/Al₂O₃
(17 wt% CZO, balance Al₂O₃) were synthesized
and compared. Catalyst performance experiments were conducted to study the role
of Pd and support compositions and loadings on inhibition during oxidation of CO,
acetylene (C₂H₂), and propylene (C₃H₆)
 under near-stoichiometric conditions (λ
= 1.01).
Transient and steady-state bench scale reactor studies with simulated exhaust
gas mixtures were conducted to extract oxidation kinetics, compare LO behavior,
and develop a predictive model for understanding and optimizing catalyst performance.

Catalyst activities were compared via temperature-programmed
oxidation experiments. Figure 1 compares CO LO curves using feed concentrations
of 0.5% CO and 1% CO. The Pd/CZO catalyst was tested
when “fresh” (initial tests after synthesis) and 10+ months later
(“deactivated”). Using all three catalysts, LO temperatures increase with feed
CO concentration, showing that CO oxidation is self-inhibiting. With increasing
CZO content, LO temperatures decrease, demonstrating
the promotional effects of using ceria.

Figure 2 shows C₂H₂
LO (feed concentrations of 375 ppm C₂H₂ and
750 ppm C₂H₂) and C₃H₆
LO (feed concentrations of 250 ppm C₃H₆ and 500 ppm C₃H₆)using the deactivated Pd/CZO catalyst. Like CO, C₂H₂
and C₃H₆ are self-inhibiting. For the same concentrations
of carbon in the feed mixture (750 ppm C₁or
1500 ppm C₁), C₂H₂ oxidation occurs
at higher temperatures than C₃H₆ oxidation.

Exhaust hydrocarbons
propylene and acetylene reveals some interesting differences during
co-oxidation with CO. The individual LO features of 1% CO and 750 ppm C₂H₂
in Figures 1 and 2   are compared to their co-oxidation (1% CO + 750
ppm C₂H₂)
in Figure 3. CO lights off at higher temperatures in the
mixture, i.e. C₂H₂ inhibits CO oxidation. During
co-oxidation, C₂H₂ oxidation is inhibited by the presence
of CO below ~15% conversion, but is otherwise improved compared to oxidation of
C₂H₂ alone. Whereas 1% CO lights off before 750 ppm
of C₂H₂ during individual oxidation, the reverse is true
during co-oxidation. This is in contrast to earlier findings of CO + C₃H₆
trends in which was found that CO inhibits C₃H₆;
i.e. propylene conversion follows CO light off (see below).It
is instructive to compare the behavior of the two hydrocarbons at the same C1
concentration. Though the same feed concentration of 1% CO +
1500 ppm C₁ (from HCs) is used, C₃H₆
and C₂H₂ co-oxidation with CO exhibits different trends
in terms of LO temperatures and order of species oxidation. As shown in Figure
4, using the deactivated Pd/CZO catalyst, 1% CO lights off before
500 ppm C₃H₆ both during individual oxidation and
mixture LO. During co-oxidation, 500 ppm C₃H₆ is 50%
converted at ~ 246 °C, compared to ~ 266 °C for 750 ppm C₂H₂.

Differential kinetics
studies were conducted to quantify reaction orders and activation energies. The
reaction order with respect to CO was found to be approximately −1
using Pd/Al₂O₃ and Pd/CZO/Al₂O₃ and
0 to slightly negative using Pd/CZO, consistent with literature values [5-7].
The activation energies for CO oxidation were found to be 134-213 kJ/mol using
Pd/Al₂O₃ and Pd/CZO/Al₂O₃, and 25-70
kJ/mol using Pd/CZO. Using all three catalysts, the reaction order with respect to C₃H₆
was found to be approximately −0.89 to
−0.75 and the activation energy in the range of 72-105 kJ/mol,
approximately half of that observed for CO oxidation and similar to literature
values [5]. Using the Pd/CZO catalyst, the
reaction order with respect to C₂H₂ was found to be
approximately −0.52 to −0.96 and the activation energy of C₂H₂
oxidation approximately 73 kJ/mol.

Steady-state and
transient CO, C₂H₂, and C₃H₆ oxidation
experiments using fresh Pd/Al₂O₃, Pd/CZO, andPd/CZO/Al₂O₃
monolith catalysts were conducted to compare LO behavior and kinetic
parameters (i.e. reaction orders and activation energies). CO LO temperatures
decreased with increasing proportion of ceria used in the TWC, and C₂H₂
and C₃H₆ LO studies using Pd/CZO demonstrated self- and mutual-inhibition
during species oxidation. Kinetics studies demonstrated further the
self-inhibiting oxidation behavior.

Ongoing work involves
the use of a mechanistic-based kinetic model incorporated into a
low-dimensional monolith model [8-9]. The global reactor
models are used with CO, C₂H₂, and C₃H₆
oxidation on Pd with steps involving ceria. The 1+1 dimensional model
formulation comprises coupled species balances with transverse-average
equations having external and internal mass transfer coefficients to verify
experimental LO behavior during individual and co-oxidation using catalysts and
feed mixtures of different formulations. Finally, experiments involving C₂H₂
oxidation on Pd/Al₂O₃ and Pd/CZO/Al₂O₃
catalysts and other hydrocarbons including toluene are being conducted to
investigate inhibition during LO, extract kinetic parameters for oxidation
reactions, and provide a basis for validating modeling simulations.

Figure 1: CO conversion (%) versus monolith temperature using Pd/Al₂O₃,
Pd/CZO/Al₂O₃, and Pd/CZO catalysts for feed
concentrations of 0.5% and 1% CO.

Figure 2: C₂H₂, C₃H₆
conversion (%) versus monolith temperature using deactivated Pd/CZO for feed
concentrations of 375 ppm C₂H₂,
750 ppm C₂H₂, 250 ppm C₃H₆,
and 500 ppm C₃H₆.

Figure 3: CO, C₂H₂ conversion (%) versus
monolith temperature using deactivated Pd/CZO for feed concentrations of 750
ppm C₂H₂ and/or 1% CO.

Figure 4: CO, C₃H₆ conversion (%) versus
monolith temperature using deactivated Pd/CZO for feed concentrations of 500
ppm C₂H₂ and/or 1% CO.

References

[1]
Fernandez-Garcia, M.; Martinez-Arias, A.; Iglesias-Juez,
A.; Hungria, A.B.; Anderson, J.A.; Conesa, J.C.; and Soria, J. App. Cat. B 2001, 31, 39-50.

[2]
Martinez-Arias, A., Fernandez-Garcia, M.; Iglesias-Juez,
A.; Hungria, A.B.; Anderson, J.A.; Conesa, J.C.; and Soria, J.. App. Cat. B
2001, 31, 51-60.

[3]
Sharma, S.; Hegde, M.S.; Das, R.N.; and Pandey, M. App. Cat.
A 2008, 337, 130-137.

[4] Harmsen, J.M.A.; Hoebink, H.B.J.;
and Shouten, J.C. Chem.
Eng. Sci. 2001, 56, 2019-2035.

[5]
Yao, Y-.F. Y. J. Catal.,
1984, 87, 152-162.

[6]
Cant, N.W.; Hicks, P.C.; and Lennon, B.S. J.
Catal. 1978, 54,
372-383.

[7]
Rainer, D.R.; Koranne, M.; Vesecky,
M.; and Goodman, D.W. J. Phys. Chem. B
1997, 101, 10769-10774.

[8]
Joshi, S.; Harold, M.P.; Balakotaiah, V. AIChE J., 2009, 55, 1771-1783.

[9]
Raj, R.; Harold, M.P.; Balakotaiah, V. Chem. Eng. J., 2015, 281, 322-333.