(617gi) Co-Oxidation of CO and Hydrocarbons on Pd/Ceria-Zirconia/Al2O3 Three-Way Catalysts

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

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
C₃H₆ adsorption 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
inhibiting 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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