Rhodium-oxide species formed on progressive oxidation of rhodium clusters dispersed on alumina

Samples of rhodium crystallites finely dispersed on alumina were prepared by impregnating an alumina support with different amounts of rhodium chloride. Reduced samples were oxidized at different temperatures (T-o) in air and the oxidized rhodium crystallites were characterized by the temperature-programmed reduction technique (TPR). According to the reduction peaks found by TPR, five kinds of oxidation products, i.e., oxygen-chemisorbed rhodium (RhOc), surface rhodium-oxide (RhsO), bulk rhodium-oxides (RhOx), rhodium-oxides interacting with support (RhiOx) and rhodium aluminate [Rh(AlO2)(y)], have been progressively formed on increasing T-o temperature. The abundance of these products on oxidized samples varied with T-o temperature and the dispersion of supported rhodium crystallites. Rhodium atoms in the Rh(AlO2)(y) structure, formed on severe oxidation conditions, would segregate from the bulk to the surface of alumina upon reductions at temperature above 750 degrees C. (C) 1999 Elsevier Science B.V. All rights reserved.