pt/CeOx/ZrOx/γ-Al2O3 Ternary Mixed Oxide DeNOx Catalyst: Surface Chemistry and NOx Interactions

Surface chemistry and the nature of the adsorbed NOx species on a Pt/CeO2- ZrO2/Al2O3 catalyst were investigated by IR spectroscopy, X-ray diffraction, H-2-temperature programmed reduction, and NOx-temperature programmed desorption. Parallel studies were also carried out with benchmark samples such as CeO2/Al2O3, ZrO2/Al2O3, CeO2-ZrO2/Al2O3 and Pt-supported versions of these materials. All samples were studied in their reduced and nonreduced forms. The use of CO as a probe molecule revealed that during the synthesis of the mixed-metal oxide systems, deposited zirconia preferentially interacted with the alumina hydroxyls, while deposited ceria was preferentially located at the Lewis acid sites. Despite the limited extent of Zr4+ ions incorporated into the CeO2 lattice, the reduction of ceria was promoted and occurred at lower temperatures in the presence of zirconia. When deposited on ZrO2/Al2O3, platinum formed relatively big particles and existed in metallic state even in the nonreduced samples. The presence of ceria hindered platinum reduction during calcination and yielded a high platinum dispersion. Subsequent reduction with H-2 led to the production of metallic Pt particles. Consequently, NO adsorption on nonreduced Pt-containing materials was negligible but was enhanced on the reduced samples because of Pt-0-promoted NO disproportionation. The nature of the nitrogen-oxo species produced after NO and O-2 coadsorption on different samples was similar. Despite the high thermal stability of the NOx adsorbed species on the ceria and zirconia adsorption sites, the NO reduction in the presence of H-2 was much more facile over Pt/CeO2 ZrO2/Al2O3. Thus, the main differences in the NO reduction functionalities of the investigated materials could be related to the ability of the catalysts to activate hydrogen at relatively lower temperatures.