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

Authors: 
Lang, W., University of Houston
Harold, M., University of Houston
Cheng, Y., Ford Motor Company
Hubbard, C., Ford Motor Company
Laing, P., Ford Motor Company

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

Wendy
Lang1, Michael P. Harold1*, 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.

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