Iron-alumina catalysts wetting

A large number of phenomena such as sintering, wetting, shape changes, splitting, etc., occur during heating of iron-on-alumina catalysts in H2 or 02 environments. Some of these phenomena are discussed below ... 

A study of the product selectivites of variously supported Co catalysts (kieselguhr, silica, alumina, bentonite, Y-zeolite, mordenite, and ZSM-5) was carried out by Bessel (37). AAdiereas the lower acidity supports such as silica and alumina produced mainly linear hydrocarbons, the acidic supports produced more branched products. At higher temperatures, the latter produced aromatics as well. The isomerization and aromatization are secondary, acid-promoted reactions of the FT olefins. This is then equivalent to a combination of the FT and the Mobil olefins to gasoline process. (With iron-based catalysts, this approach is unlikely to be successful because alkali promotion is essential and the alkali would neutralize the required acid sites on the zeolite support.) Calleja and coworkers (38) studied the FT performance of Co/HZSM-5 prepared by incipient wetness impregnation. Promotion with thorium, being basic, decreased the acidity of the zeolite and so less aromatics were formed and consequently more of the heavier hydrocarbons emerged from the reactor because of the depressed level of secondary reactions. 

In a much earlier patent, the removal of organics from exhaust gases by oxidation over a supported uranium oxide catalyst was reported by Hofer and Anderson [39]. The catalyst was 4% U3O8 supported on alumina spheres. The authors used the incipient wetness technique to impregnate alumina with uranyl nitrate solution. In this case the catalyst precursors were calcined at 700°C for 3 h to decompose the uranium salt. The use of other uranium compounds as starting materials was mentioned and these included uranyl acetate, uranium ammonium carbonate and uranyl chloride. The alumina-supported catalyst had a surface area of ca 400m g and further added components, such as copper, chromium and iron, were highlighted as efficient additives to increase activity. 

Relevant to this issue is dehydrogenation of ethylbenzene for the manufacture of styrene which uses alumina supported iron oxide as the preferred catalyst in most cases. Therefore, when an alumina membrane is used in conjunction with stainless steel piping or vessels as the membrane reactor, caution should be exercised. An estimate of the effects of their exposure to the reaction mixuire at the application temperature of 600 to 640 C is desirable. Wu et al. [1990b] estimated that the alumina membrane contributes to less than 5% conversion of ethylbenzene and the stainless steel tubing or piping could account for as much as 20% conversion. The high activity of the stainless steel is attributed to iron and chromium oxide layers that may form on the wetted surface. 

The scope of the present paper is to emphasize the role of wetting and spreading in the aging by sintering, and in the redispersion of supported metal catalysts. In the next section, some experimental results regarding the behavior of iron supported on alumina are presented to demonstrate that surface phenomena do play a major role. This is followed by stability considerations which are employed to explain the coexistence of multilayer surface films with crystallites in an oxygen atmosphere and the rupture of thin films into crystallites in a hydrogen atmosphere. 

Carbon fibrils can be produced rather easily, e.g., by exposing supported, finely dispersed iron or nickel particles to reducing carbon containing gas flows. To this end, one has to produce first finely dispersed iron or nickel particles on a support material, such as alumina or silica. The desired catalyst can be prepared, e.g., by incipient wetness impregnation of the support material with a suitable metal salt solution or by means of homogeneous deposition-precipitation of the metal ions onto the carrier. 

The broadening of the spectrum upon treatment at 790 K is similar to observations made when calcium carbonate single crystals are subject to argon ion bombardment. The defects induced in the calcium oxide component of the catalyst cannot be caused by an initial reduction of the oxide, because calcium oxide is stable to hydrogen at 790 K. The defects may be indicative of the formation of a ternary calcium iron oxide since there is no observed increase in the dispersion of calcium, which would be associated with a disintegration of the CaO crystals. It should be pointed out that the other structural promoter element, aluminum, exhibits the same spectral changes in its A12 emission. The dispersion of the alumina increases, however, with reduction of the catalyst, particularly when the wet reduction method is applied (see Table 2.1). 

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