(582cj) The Effects of Aging on Steady State Ammonia Oxidation over Pt/Al2O3

Three-way catalyst (TWC) light-off (LO) performance is critical in cost-effectively meeting emission standards. Improvements in catalyst LO should include catalyst modifications that minimize inhibition effects of exhaust species such as CO and hydrocarbons (HCs), lower precious group metal (PGM) loading, or use a lower cost PGM such as Pd. Alumina (Al₂O₃) and zirconia (ZrO₂) supports provide high surface area and stability. Furthermore, ceria (CeO₂) provides 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. 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 and compared. Catalyst performance experiments were conducted to study the role of Pd and support compositions and loadings on inhibition during oxidation of CO, acetylene (C₂H₂), and propylene (C₃H₆) under near-stoichiometric conditions (λ = 1.01). 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. The Pd/CZO catalyst was tested when “fresh” (initial tests after synthesis) and 10+ months later (“deactivated”). 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 C₂H₂ LO (feed concentrations of 375 ppm C₂H₂ and 750 ppm C₂H₂) and C₃H₆ LO (feed concentrations of 250 ppm C₃H₆ and 500 ppm C₃H₆)using the deactivated Pd/CZO catalyst. Like CO, C₂H₂ and C₃H₆ are self-inhibiting. For the same concentrations of carbon in the feed mixture (750 ppm C₁or 1500 ppm C₁), C₂H₂ oxidation occurs at higher temperatures than C₃H₆ oxidation.

Exhaust hydrocarbons propylene and acetylene reveals some interesting differences during co-oxidation with CO. The individual LO features of 1% CO and 750 ppm C₂H₂ in Figures 1 and 2 are compared to their co-oxidation (1% CO + 750 ppm C₂H₂) in Figure 3. CO lights off at higher temperatures in the mixture, i.e. C₂H₂ inhibits CO oxidation. During co-oxidation, C₂H₂ oxidation is inhibited by the presence of CO below ~15% conversion, but is otherwise improved compared to oxidation of C₂H₂ alone. Whereas 1% CO lights off before 750 ppm of C₂H₂ during individual oxidation, the reverse is true during co-oxidation. This is in contrast to earlier findings of CO + C₃H₆ trends in which was found that CO inhibits C₃H₆; i.e. propylene conversion follows CO light off (see below).It is instructive to compare the behavior of the two hydrocarbons at the same C1 concentration. Though the same feed concentration of 1% CO + 1500 ppm C₁ (from HCs) is used, C₃H₆ and C₂H₂ co-oxidation with CO exhibits different trends in terms of LO temperatures and order of species oxidation. As shown in Figure 4, using the deactivated Pd/CZO catalyst, 1% CO lights off before 500 ppm C₃H₆ both during individual oxidation and mixture LO. During co-oxidation, 500 ppm C₃H₆ is 50% converted at ~ 246 °C, compared to ~ 266 °C for 750 ppm C₂H₂.

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 were found to be 134-213 kJ/mol using Pd/Al₂O₃ and Pd/CZO/Al₂O₃, and 25-70 kJ/mol using Pd/CZO. Using all three catalysts, the reaction order with respect to C₃H₆ was found to be approximately −0.89 to −0.75 and the activation energy in the range of 72-105 kJ/mol, approximately half of that observed for CO oxidation and similar to literature values [5]. Using the Pd/CZO catalyst, the reaction order with respect to C₂H₂ was found to be approximately −0.52 to −0.96 and the activation energy of C₂H₂ oxidation approximately 73 kJ/mol.

Steady-state and transient CO, C₂H₂, 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 LO temperatures decreased with increasing proportion of ceria used in the TWC, and C₂H₂ and C₃H₆ LO studies using Pd/CZO demonstrated self- and mutual-inhibition during species oxidation. Kinetics studies demonstrated further the self-inhibiting oxidation behavior.

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, C₂H₂, 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 to verify experimental LO behavior during individual and co-oxidation using catalysts and feed mixtures of different formulations. Finally, experiments involving C₂H₂ oxidation on Pd/Al₂O₃ and Pd/CZO/Al₂O₃ catalysts and other hydrocarbons including 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 Pd/Al₂O₃, Pd/CZO/Al₂O₃, and Pd/CZO catalysts for feed concentrations of 0.5% and 1% CO.

Figure 2: C₂H₂, C₃H₆ conversion (%) versus monolith temperature using deactivated Pd/CZO for feed concentrations of 375 ppm C₂H₂, 750 ppm C₂H₂, 250 ppm C₃H₆, and 500 ppm C₃H₆.

Figure 3: CO, C₂H₂ conversion (%) versus monolith temperature using deactivated Pd/CZO for feed concentrations of 750 ppm C₂H₂ and/or 1% CO.

Figure 4: CO, C₃H₆ conversion (%) versus monolith temperature using deactivated Pd/CZO for feed concentrations of 500 ppm C₂H₂ and/or 1% CO.

References

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