(148a) Co-Oxidation of CO and Hydrocarbons on Pd/Ceria-Zirconia/Al2O3 Three-Way Catalysts: Experiments and Modeling

Co-oxidation
of CO and Hydrocarbons on Pd/Ceria-Zirconia/Al2O3

Three-way
Catalysts: Experiments and Modeling

Wendy Lang¹,
Michael P. Harold^1*, Yisun Cheng²,
Carolyn Hubbard², Manish Sharma², 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 performance is critical in cost effectively meeting
future emission standards. Improvements in catalyst light-off (LO) should
include catalyst modifications that minimize inhibition effects of exhaust
species such as CO and hydrocarbons (HCs), lowering precious group metal (PGM)
loading, or using a lower cost PGM such as Pd. Alumina (Al₂O₃)
and zirconia (ZrO₂) supports provide high surface area and
stability. Furthermore, ceria (CeO₂) supports for PGMs provide 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. Studies have demonstrated that 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. Catalyst performance studies were conducted to elucidate the
role of Pd and support (CZO, Al₂O₃) compositions and
loadings on inhibition during CO and C₃H₆ LO under
near-stoichiometric conditions. 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
(λ = 1.01 in both cases). 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 C₃H₆ LO using feed concentrations of 250 ppm C₃H₆
and 500 ppm C₃H₆ (λ = 1.01). Like CO, C₃H₆
is self-inhibiting. The shoulder feature in the LO curves using Pd/Al₂O₃
and Pd/CZO/Al₂O₃ is speculated to be due to a change in
the oxidation state of Pd and/or the accumulation of inhibiting carbonaceous
surface species. LO begins at lower temperatures and the shoulder becomes less
prominent with increasing CZO content and is absent with Pd/CZO.

Comparing
the individual oxidations of 1% CO and 500 ppm C₃H₆ in
Figures 1 and 2 to LO behavior during their co-oxidation (1% CO + 500 ppm C₃H₆)
in Figure 3, LO temperatures for both species are higher in the mixture, i.e.
CO and C₃H₆ are mutually-inhibiting. During co-oxidation,
CO lights off before C₃H₆ and the C₃H₆
shoulder does not appear, suggesting that CO inhibits C₃H₆adsorption and the accumulation of carbonaceous
species on the catalyst. Once CO lights off and temperatures increase, C₃H₆
oxidation proceeds. Significantly lower LO temperatures are achieved using
Pd/CZO, and higher conversions of CO and C₃H₆ are reached
at lower temperatures using catalysts containing CZO.

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 was found to be 125-149 kJ/mol using Pd/Al₂O₃ and Pd/CZO/Al₂O₃,
and 29-44 kJ/mol using Pd/CZO. Using the Pd/Al₂O₃
catalyst, the reaction order with respect to C₃H₆ was
found to be approximately −1 and activation energy 77 kJ/mol.

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 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. As shown in Figure 4, the model verifies the experimental LO of
CO on Pd/Al₂O₃. For example, it validates additional
experimentally-observed oxidation behavior such as the switch in order of
ignition between individual and mixture oxidations on Pd/Al₂O₃
and Pd/CZO/Al₂O₃ as well as the promotional effect of
ceria.

Steady-state
and transient CO 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 and C₃H₆
individual and mixture LO behavior was improved with increasing catalyst ceria
content. A reactor model confirms experimentally-observed oxidation behavior
and predicts catalyst performance. Finally, experiments involving other hydrocarbons
including acetylene and 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 fresh Pd/CZO, Pd/Al₂O₃, and Pd/CZO/Al₂O₃
catalysts for feed concentrations of 0.5% CO (λ = 1.01) and
1% CO (λ = 1.01).

Figure
2: C₃H₆
conversion (%) versus monolith temperature using fresh Pd/CZO, Pd/Al₂O₃,
and Pd/CZO/Al₂O₃ catalysts for feed concentrations of 250
ppm C₃H₆ (λ = 1.01) and 500
ppm C₃H₆ (λ = 1.01).

Figure
3: CO, C₃H₆
conversion (%) versus monolith temperature using fresh Pd/CZO, Pd/Al₂O₃,
and Pd/CZO/Al₂O₃ catalysts for feed concentration of 500
ppm C₃H₆, 1% CO (λ = 1.01).

Figure
4: CO conversion (%) versus
temperature using fresh Pd/Al₂O₃ catalysts for
experimental and simulated feed concentration of 1% CO (λ = 1.01).

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