An assessment of Pt and Pd model catalysts for low temperature NOx adsorption

• NOx storage on Pd/CZO, Pd/Al2O3, Pt/CZO, and Pt/Al2O3 low temperature NOx adsorbers.
• Measured NOx storage on transient temperature test with and without H2O and C2H4.
• Pt/Al2O3 produced high levels of NO2 and N2O.
• Pd/CZO provided good NO storage with C2H4 and H2O above 100 °C, low NO2 and N2O.

Low temperature NOx adsorbers (LTNA) are being developed to adsorb the cold start NOx emissions from diesel engines and release the NOx when the urea/SCR system is operational. To assess the contributions of the precious metal and washcoat, model catalysts with 0.43 g/L (26 gpcf) of platinum (Pt) or palladium (Pd) on washcoats of alumina (Al2O3) or ceria/zirconia (CZO) were evaluated for lean NO storage and release with and without C2H4 and H2O. The degreened samples were evaluated on a reactor-based transient test that partially simulated the exhaust temperatures during the first two phases of the light-duty Federal Test Procedure (FTP) followed by a Supplemental FTP (US06) test. After oxidation, the Pd/CZO catalyst stored NO at the lowest temperatures of the test (∼90 °C) without C2H4 and H2O, and the NO storage was enhanced by the presence of C2H4 but inhibited by H2O at bed temperature under 100 °C. Most of the NOx emissions during the FTP simulation were in the form of NO, and the catalyst produced low N2O levels on tests with C2H4. The oxidized Pd/Al2O3 sample exhibited poor NO storage with or without C2H4 and H2O, suggesting that cerium is important for NOx storage when the catalyst is oxidized. Both Pd technologies stored more NO when the samples were reduced, indicating that metallic Pd is effective for storing NO. The Pt/Al2O3 sample exhibited high NO storage efficiency between 90 & 120 °C without H2O & C2H4 but lower efficiency with these species. Above 120 °C, the catalyst produced high NO2 levels and high N2O levels on the tests with C2H4. The Pt/CZO sample performed similarly to the Pd/CZO sample except for higher NO2 and N2O levels. The Pd/CZO catalyst provided the best combination of NO storage with C2H4 and H2O above 100 °C, essentially complete NOx release at the maximum bed temperature of the US06 simulation (400 °C), low NO2 and N2O formation, and cost effectiveness.

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