CALORIMETRIC STUDY ON OXIDATION OF ALUMINA-SUPPORTED RHODIUM BY DIOXYGEN

Rhodium particles in nanometer size were prepared by impregnating alum ina powders with aqueous solutions containing rhodium salts. The dispe rsion (D) of rhodium crystallites on the prepared samples was estimate d by dioxygen adsorption measured at 300 K. Phenomena of oxidizing the supported crystallites with 2.5 X 10(4) Pa O-2 in a temperature range between 280 and 870 K were calorimetrically studied. Extent of oxidat ion may be distinguished into three stages, i.e., adsorption on surfac e (T< 300 K), progressive penetration into bulk, and formation of a st able bulk oxide (T> 700 K), on raising the oxidation temperature. Heat of dioxygen adsorption varies only slightly with the dispersion (D) o f rhodium and has a value of 294 +/- 6 kJ (mol O-2)(-1). Chemical stoi chiometry of the bulk oxide formed, however, varies with the dispersio n of rhodium crystallites. A dioxide (RhO2) (Delta(f)H = 225 +/- 3 kJ (mol O-2)(-1)) and a sesquioxide (Rh2O3) (Delta(f)H = 273 +/- 3 kJ(mel O-2)(-1)) was formed at D < 60% and D > 80%, respectively.