X-RAY SPECTROSCOPY OF CHROMIUM FLUORIDATI ON CATALYSTS

CFC (chlorofluorocarbons) seem to be associated with the ozone depleti on. Attempts have been tackled to turn towards substitutes which do no longer contain chlorine atoms. The corresponding processes need perfo rming fluoration catalysts working in severe conditions. These catalys ts, based on trivalent chromium oxide and fluoride, are either bulky o r supported mainly on alumina or carbon. They are amorphous at the X-r ay diffraction scale from impregnation to activation. X-ray Absorption Spectroscopy (XAFS, X-ray Absorption Fine Structure), using synchrotr on radiation, gives information about short-range order and is thus we ll-adapted for non crystalline materials. We describe here the case of Cr(6% in weight)/C. After impregnation of an aqueous solution of CrO3 on carbon, and drying under N-2 the initial Cr(VI) phase is nearly to tally reduced into Cr(III). The activation process by HF occurs very s oon, the catalyst tends towards an amorphous CrF3, altough it still re mains oxygen. The nature of this oxygen will be discussed.