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 including ceria in PGM catalysts enhances the conversion of CO and HCs 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].
Pd/Al2O3 and Pd/CZO/Al2O3 monolith catalysts were synthesized and used in catalyst performance studies to elucidate the role of Pd and support (CZO, Al2O3) compositions and loadings on inhibition during CO and HC (propylene, toluene, acetylene) LO under near-stoichiometric conditions. Transient and steady-state bench scale reactor studies with simulated exhaust gas mixtures were conducted to extract oxidation kinetics and compare LO behavior using fresh and aged catalysts.
Comparing activity using fresh Pd/CZO/Al2O3 (support composition: 17 wt% CZO, balance Al2O3) and Pd/Al2O3 catalysts (1 wt% Pd, 3.0 g/in3 washcoat loading, turnover frequencies (TOFs) were calculated using isothermal, steady-state reaction data obtained at a feed concentration of 0.5% CO. Under the same set of operating conditions, the TOF using Pd/CZO/Al2O3 was higher than that using Pd/Al2O3, indicating higher activity using the CZO-containing catalyst. Table 1 lists the CO oxidation reaction rates and TOFs on the respective catalysts. The higher activity and lessened CO inhibition of ceria-containing catalysts is attributed to the contribution of oxygen supplied by ceria .
Catalyst activities were compared via transient temperature-programmed oxidation experiments using fresh and aged Pd/CZO/Al2O3 and fresh Pd/Al2O3 catalysts. 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 are higher using higher feed CO concentrations, showing that the reaction is negative order with respect to CO (i.e. self-inhibiting). LO temperatures are higher using the aged catalyst as sintering and loss of surface area are expected with aging . Comparing the use of fresh Pd/CZO/Al2O3 and Pd/Al2O3 catalysts with 1% CO in the feed, LO temperatures are lower using the ceria-containing catalyst.
Differential kinetics studies were conducted to quantify reaction orders and activation energies. As shown in Figure 2, the reaction order with respect to CO on the aged Pd/CZO/Al2O3 varied between -1.14 to -1.64 Reaction orders using fresh Pd/CZO/Al2O3 and fresh Pd/Al2O3 catalysts were found to be approximately -1, consistent with literature values using Pd and Pd/Al2O3 catalysts [7-9]. Figure 3 shows the results from the activation energy experiments while Table 2 lists slopes of linear fits and corresponding activation energies. The activation energy for CO oxidation on the aged catalyst ranged from 106 kJ/mol to 145 kJ/mol. Using fresh Pd/Al2O3, activation energies were found to range from 125 kJ/mol to 145 kJ/mol. Activation energies for CO oxidation on powder Pd/Al2O3 catalysts are reported in the literature to be approximately in the 100 kJ/mol to 126 kJ/mol range [7-9].
Steady-state and transient CO oxidation experiments using fresh Pd/Al2O3 and fresh and aged Pd/CZO/Al2O3 monolith catalysts were conducted to compare LO behavior and kinetic parameters (i.e. reaction orders and activation energies). Using aged Pd/CZO/Al2O3, CO LO temperatures were observed to be higher than those using fresh Pd/CZO/Al2O3 and Pd/Al2O3. Using a feed concentration of 1% CO, LO temperatures were lower using fresh Pd/CZO/Al2O3 than those using fresh Pd/Al2O3; higher activity was also observed in comparing TOFs using the two fresh catalysts.
Ongoing work involves the use of a mechanistic-based kinetic model incorporated into a low-dimensional monolith model. The kinetic model expands on established CO oxidation on Pd kinetics with steps involving ceria. The model helps to verify experimentally-observed oxidation behavior and predict catalyst performance. Finally, oxidation experiments involving mixtures of CO and HCs including propylene, acetylene, and toluene are being conducted to investigate mutual inhibition during LO, extract kinetic parameters for oxidation reactions, and provide a basis for validating modeling simulations.
Table 1. Fresh catalyst steady-state reaction rates and TOFs. SS170, 0.5% CO, 0.45% O2.
Figure 1: CO conversion (%) versus monolith temperature using fresh Pd/CZO/Al2O3 (solid lines), aged Pd/CZO/Al2O3 (long dashed lines), and fresh Pd/Al2O3 (short dashed lines) catalysts for feed concentrations of 0.5% CO (λ = 1.01) and 1% CO (λ = 1.01).
Figure 2: Log-log plot of steady-state reaction rate (mol/s·g) on aged Pd/CZO/Al2O3 vs. average CO concentration obtained at monolith temperatures of 198 °C (◊), 211 °C (▲), 222 °C (□).
Figure 3: Arrhenius plot of steady-state reaction rates (mol/s·g) on aged Pd/CZO/Al2O3 vs. inverse absolute temperatures (K) obtained at CO feed concentrations of 1000 ppm (diamonds), 2000 ppm (squares), 5000 ppm (triangles), and 8000 ppm (circles).
Table 2. Arrhenius plot slopes and corresponding activation energies for CO oxidation on aged Pd/CZO/Al2O3.
Activation Energy (kJ/mol)
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