Supported CuO plus Ag/partially stabilized zirconia catalysts for the selective catalytic reduction of NOx under lean burn conditions - 1. Bulk and surface properties of the catalysts

摘要

Thermally stable cubic mesoporous zirconia samples stabilized by the alkaline-earth cations (Ca, Sr, Pa) were synthesized via the coprecipitation route followed by refluxing in the presence of surfactants. These systems were used as supports for copper cations and then modified by the addition of silver nanoparticles using impregnation or photoassisted deposition techniques. The structural, textural, and surface features of these nanosystems were studied by using TEM, X-ray diffraction, EXAFS, nitrogen adsorption isotherms, SAXS, FTIRS of adsorbed CO, and TPD of adsorbed NOx species. Partially stabilized zirconia samples were found to possess a disordered cubic structure. A higher tendency of bulky Pa cation to segregate in the surface layer is reflected in a higher degree of surface disordering, higher concentration of hydroxyls, and greater coordination unsaturation of isolated copper cations. In contrast to such traditional supports as gamma -alumina, stabilized zirconia supports appear to favor formation of small reactive (probably, three-dimensional) clusters of copper cations possessing an increased reactivity and decreased strength of oxygen bonding with these cations. It is reflected in decreased thermal stability of surface nitrite and nitrate species located at these centers as compared with such species on the surface of CuO/alumina catalysts. This feature seems to be primarily determined by the specificity of the surface structure of fluorite-like supports (ceria, zirconia). Silver incorporation into copper oxidic clusters decreases the strength of copper-oxygen bonds as well as the thermal stability of adsorbed nitrite-nitrate species. For samples prepared via the photodeposition route, the clustering degree of copper cations is usually lower than in the case of samples obtained by traditional impregnation procedure. (C) 2001 Academic Press. [References: 24]