(292c) Co-Oxidation of CO and Hydrocarbons on Pd/Ceria-Zirconia/Al2O3 Three-Way Catalysts

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/Al₂O₃was
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 [5].

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 [6].  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.

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