459138 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. Harold1*, 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
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, and Pd/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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