Chromia surface

FIG. 2. (a) Possible model for a completely dehydroxylated (001) face of a-chromia. O, O2 ions of the underlying layer , Cr3+ ions , O2" ions of the upper layer, (b) Possible model of a partially hydrated (001) face of a-chromia. , surface hydroxyl. [Reproduced with permission from Zecchina et al. (145).]... 

Very complex spectra are obtained on C02 adsorption by partially dehydroxylated a-chromia surfaces (272). Seven surface species have been identified by... 

Reaction of a CHCIFt molecule with hydroxyl groups on a chromia surface... 

Fig. 13. Intermediates proposed for mono- and di-exchange of Cl by F in CC14 at a fluorinated chromia surface (reproduced with permission from J. Catal. 174 (1998) 219 [104]).

Carbondioxide adsorption is another example of molecular adsorption which has been studied quite extensively in the past. While for some time is was generally accepted that CO2 forms carbonates with chromia surfaces very readily upon exposure, it was recently observed that carboxylate species may be formed. TDS spectra indicate [111, 112] that there are more weakly and less weakly bound CO2 species on the surfaces. We have studied the nature of those species by various techniques including infrared spectroscopy. Fig. 23 shows several sets of IR spectra. The pair of sharp bands around 2300 cm can easily be assigned to the more weakly bound CO2 with only slightly distorted structure... 

Magnetic susceptibility studies of chromia-alumina catalysts described in Section V,B indicate that the chromia is present, at least partially, as small crystallites on the surface of the alumina. The question naturally arises, therefore, as to how much chromia surface... 

It is known that chromia chemisorbs a number of gases such as hydrogen, oxygen, and carbon monoxide in substantial quantities, while alumina does not (33). It is reasonable to suppose, therefore, that the extent of chemisorption on a chromia-alumina catalyst would be indicative of the fraction of chromia surface that is present. This approach has been used to analyze the surfaces of a number of multi-component catalysts, and has proved to be a useful technique in catalysis research. The present section will be concerned with its application to the chromia-alumina system. 

A rather different approach has been used by the present authors. Rather than rely simply on analyses of product distributions, a radiotracer approach using [ F] (ti/2 = 110 min emitter) and [ Cl] (ti/2 = 3 X 10 year P emitter) has been employed to determine the quantities and labilities of halogen deposited on a prefluorinated chromia surface by direct radiochemical counting of the catalyst. By using radio gas chromatography, the distribution of a [ F] or [ C] label in the product mixtures has been obtained. 

Figure 3.4 Modified chromium stability diagram at given temperamre and chromium activity [10]. In atmospheres located in Zone I neither oxide nor carbide is stable thus decarburization occurs and in Zone II the chromia scale cannot prevent decarbutization. Zone III is carburizing but the formation of a stable chromia surface layer protects against significant carbon uptake (most satisfactory atmosphere). The surface scales in Zone IV and even more Zone V offer no significant protection against carburization.

Figure 7.3 Two Possible Pathways for Catalytic Reactions at a Fluorinated Chromia Surface Reversible Halogen Exchange, that is, F-for-CI and CI-for-F Oehydrochlorination Followed by...

Treatment of chromia under flow conditions and at moderate temperatures with a variety of C, CFCs or hydrochlorofluorocarbons (HCFCs) or Cj HFCs is an alternative way of producing a fluorinated chromia surface that is catalytically active [24]. It has the advantage that corrosive reagents such as anhydrous HE are avoided. XPS and X-ray excited Auger electron spectroscopy (XAES) demonstrate clearly the points in the processes where activation occurs formation of amorphous highly fluorinated regions at the surface is indicated. Description of local structure is less easily made, although a... 

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