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

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


Dhillon, P. S. - Presenter, University of Houston
Harold, M., University of Houston
Wang, D., University of Houston
Kumar, A., Cummins Inc.

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.


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