Pt- and Pd-Promoted CeO2-ZrO2 for Passive NOx Adsorber Applications.

Pt- and Pd-promoted CexZr1-xO2 mixed oxides were characterized and investigated for passive NOx adsorber applications. X-ray diffraction analysis revealed a phase transition from tetragonal to cubic with increasing cerium content in CexZr1-xO2, while H2 and CO chemisorption data in all cases indicated average Pt and Pd particle sizes of close to 2 nm. H2-temperature programmed reduction (TPD) measurements revealed a shift of the Pt reduction peak to higher temperature with increasing Ce content, consistent with a corresponding increase in the degree of Pt oxidation. According to microreactor data, doping Ce into the ZrO2 lattice resulted in a significant improvement in low temperature (80-160 °C) NOx storage efficiency. Diffuse reflectance infrared Fourier transform spectroscopy measurements on Pt/CexZr1-xO2 showed that as Ce content increased, relatively more nitrite species were generated during NOx storage. However, oxidation of nitrite to nitrate during subsequent NOx-TPD increasing the concentration of more thermally stable nitrate also correlated with increased Ce content. The use of Pd as a promoter resulted in decreased NOx storage efficiency compared to Pt, although low-temperature NOx desorption behavior was improved. This is attributed to decreased formation of nitrate during NOx storage compared to that of Pt, as well as the lower activity of Pd for oxidation of nitrite to nitrate during subsequent NOx-TPD. To achieve more balanced NOx storage and desorption behavior, Ce0.2Zr0.8O2 was promoted with both Pt and Pd, resulting in superior overall NOx performance relative to its Pt and Pd analogues. After hydrothermal aging at 750 °C for 16 h, the copromoted sample still maintained excellent NOx adsorption-desorption performance. [ABSTRACT FROM AUTHOR]