EFFECT OF LANTHANA AS A STABILIZING AGENT IN TITANIUM-DIOXIDE SUPPORT

The thermal stability of between 1 and 5 wt % lanthana-doped TiO2 was investigated. Three methods of aging the titania support were studied: stagnant air heating for 12 h at temperatures ranging from 450 to 900 degrees C, flowing humid 450 degrees C air for up to 25 days, and flo wing humid 650 degrees C air for up to 30 days. The thermal stability was assessed using BET surface area and XRD. Chlorodifluoromethane (HC FC22) oxidation was used as a probe reaction to determine the effects of thermal aging on the reactivity of the catalyst. The XRD results in dicate that the decrease in surface area corresponds to the phase tran sformation from anatase to rutile. In the fixed time results, all dopi ng levels achieved the same elevated stability, increasing the phase t ransition temperature from approximately 450 to about 650 degrees C. H owever, when the support was aged by exposure for an extended time, in creasing the lanthana loading increased the thermal stability. This su rface area thermal stability was shown to be related to the activity o f the catalyst in oxidizing HCFC22.