INFLUENCE OF CERIA ON THE DISPERSION AND REDUCTION OXIDATION BEHAVIOROF ALUMINA-SUPPORTED COPPER-CATALYSTS/

A study using TEM combined with electron diffraction (ED), several spe ctroscopies (IR, ESR, XPS), and mass spectrometry-temperature programm ed surface reaction of adsorbed NO is carried out on a Cu/CeOx/Al2O3 c atalyst and on similar specimens without Ce or Cu. TEM shows a prefere ntial nucleation of the oxidized Cu phase on the ceria-rich regions, l eading to a particle size in the Cu-containing phase smaller than that found in the Al2O3-supported system and decreasing the formation of t he CuAl2O4 spinel. The Cu-Ce interaction developed in the calcination treatment also gives significant stability against sintering of the me tallic copper phase formed during H-2 reduction. After reduction, the Ce-containing sample shows higher resistance to reoxidation of the sur face copper phase by NO to the Cu2+ State (in comparison to Cu/Al2O3), On the basis of XPS-Ar+ sputtering data, this may be related to parti al coverage of the metallic Cu by reduced ceria in a way similar to th at currently accepted for the SMSI effect; a capability of the Ce-cont aining support to stabilize the Cu+ state may also contribute to this behaviour. This stable copper state appears in subsurface regions and is likely related to a new phase detected by TEM-ED and tentatively id entified as a (Ce,Cu)-Al perovskite, The copper-cerium interaction aff ects also the reactivity of the catalyst towards NO, increasing (partl y through a dissociative mechanism) the amount of adsorbed species whi ch leads to low temperature NO and N-2/N2O desorption, as well as shif ting the decomposition temperatures of surface species containing N-N bonds (N-2/N2O) formed upon NO adsorption. It is proposed that the add ition of ceria may help to enhance the reductive elimination of NO by copper in automobile exhaust gases. (C) 1997 Academic Press.