Influence of Ceria on the Dispersion and Reduction/Oxidation Behaviour of Alumina-Supported Copper Catalysts

Abstract

A study using TEM combined with electron diffraction (ED), several spectroscopies (IR, ESR, XPS), and mass spectrometry-temperature programmed surface reaction of adsorbed NO is carried out on a Cu/CeOx/Al2O3catalyst and on similar specimens without Ce or Cu. TEM shows a preferential nucleation of the oxidized Cu phase on the ceria-rich regions, leading to a particle size in the Cu-containing phase smaller than that found in the Al2O3-supported system and decreasing the formation of the CuAl2O4spinel. The Cu–Ce interaction developed in the calcination treatment also gives significant stability against sintering of the metallic copper phase formed during H2reduction. After reduction, the Ce-containing sample shows higher resistance to reoxidation of the surface 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 partial coverage of the metallic Cu by reduced ceria in a way similar to that currently accepted for the SMSI effect; a capability of the Ce-containing 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 identified as a (Ce,Cu)–Al perovskite. The copper-cerium interaction affects also the reactivity of the catalyst towards NO, increasing (partly through a dissociative mechanism) the amount of adsorbed species which leads to low temperature NO and N2/N2O desorption, as well as shifting the decomposition temperatures of surface species containing N–N bonds (N2/N2O) formed upon NO adsorption. It is proposed that the addition of ceria may help to enhance the reductive elimination of NO by copper in automobile exhaust gases.