Pt/CeOx/ZrOx/γ-Al2O3 ternary mixed oxide DeNOx catalyst: surface chemistry and NOx interactions

Abstract

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, H2-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 H2 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 Pt0-promoted NO disproportionation. The nature of the nitrogen-oxo species produced after NO and O2 coadsorption on different samples was similar. Despite the high thermal stability of the NOx adsorbed species on the ceria and zirconia adsorption sites, the NOx reduction in the presence of H2 was much more facile over Pt/CeO2-ZrO2/Al2O3. Thus, the main differences in the NOx reduction functionalities of the investigated materials could be related to the ability of the catalysts to activate hydrogen at relatively lower temperatures.