Effects of rhodium and platinum on the reactivity of lanthanum phases

Rhodium and platinum catalysts were prepared by wet impregnation of Anedra lanthanum oxide. The polymorph forms of La2O2CO3 have also been prepared by an appropriate treatment of lanthanum compounds in carbon dioxide. The characterisation of the phases present in these solids was performed by XRD, FT-IR, LRS, TPD and TGA. One of the untreated supports (Aldrich) was La(OH)3, and the other (Anedra) consisted mainly of II-La2O2CO3 with traces of type la oxycarbonate and lanthanum hydroxide. Catalysts with a low rhodium loading showed an increase of Ia-La2O2CO3 already present on the support, whereas those with a low platinum loading would not modify the phases of the lanthanum solid. A different behaviour was observed in high metal loading catalysts. In this case, metals would catalyse the decomposition of type Ia polymorph and also prevent the hydration of the support. The treatment with CO2 provides additional information about the effect of the gas upon the metal containing solids. Pt would favour the carbon dioxide interaction with the support to form monoclinic polymorph (type Ia), while rhodium would prevent the further formation of the oxycarbonate phase. (C) 2004 Elsevier B.V. All rights reserved.