Fluorinated chromia properties

Fundamental Properties of Fluorinated Chromia 198 Attempts to Describe Active Sites on Fluorinated Chromia 199 Synthesis of 1,1,1,2-Tetrafluoroethane Over Fluorinated Chromia 200 Oxidation States of Chromium 200 Doped Chromia Catalysts 201... 

It will be apparent from the topics described in the sections above that fluorinated chromia has subtleties that are not completely understood and in some catalytic systems have so far been described incompletely. Some factors are clear however. The surface properties depend crucially on the extent of surface prefluorination. Although many laboratory studies have employed CFC or better HFC reagents, such surfaces are normally lightly fluorinated only, and in the case of CFCs will inevitably contain surface chloride species that complicate interpretation of the catalysis. For large-scale applications, anhydrous HF is the prefluorination (and fluorination) reagent of choice. The process is slow and, even at the surface level, is probably never complete. This is beneficial, since complete conversion to a chromium(in) fluoride layer, it is generally agreed, would result in a catalyst whose activity was very low or which was inactive. 

On the basis of temperature programmed reduction and oxidation (TPR-TPO) measurements, it was proposed that the calcination of chromia used as a catalyst for the selective fluorination of CF3CH2C1 into CF3CH2F depends strongly on the gas used to calcine the precursor [50], Furthermore, the fluorination of CF3CH2C1 depends directly on the number of reversibly oxidizible chromium atoms in the catalysts. The oxidation/reduction properties are related closely to the atmosphere of pre-treatment and a linear relationship between the catalytic activity and the hydrogen uptake during the second reduction step has been found [51],... 

In order to mimic the properties of a partially fluorinated working chromia catalyst, several model-compound-based studies using an instrumental surface analysis approach have been reported. Laboratory-prepared CrjOj is treated under carefully chosen conditions with a reagent that leads to incorporation of fluorine and the resulting material analyzed, for example, by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES). Although of academic interest, such studies relate to models rather than to real catalysts and are probably some distance from describing the real situation. The approach is illustrated with two examples. 

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