Iron oxide gel

As the gel is dehydrated the Weiss constant slowly rises. This must mean either that the chromium-chromium distance is decreasing slightly, or perhaps that the paramagnetic neighborhood is increasing. Some results on iron oxide gels to be described later indicate that the latter explanation may be more nearly correct. 

Nonferromagnetic samples of the iron oxide gel had a susceptibility of about 165 X 10 per gram of iron at 25°. (In the measurement of such large susceptibilities it is often convenient to dilute the sample with an inert diamagnetic substance such as alumina. In such cases the mixing must be thorough.) The magnetic moment of the iron in this substance was 4.6, and the Weiss constant was —20°. 

When the iron oxide gel was heated, water was, of course, lost, but the susceptibility per gram of iron remained constant until the glow temperature ( 270°) was reached, after which the susceptibility fell to about 32 X 10 per gram of iron at 25°, and began to vary directly, rather than inversely, with the absolute temperature. 

Wet preparation of red iron oxides can involve either a hydrothermal process (see Hydrothermal processing) or a direct precipitation and growth of iron oxide particles on specially prepared nucleating seeds of Fe202- In the hydrothermal process, iron(II) salt is chemically oxidized to iron(III) salt, which is further treated by alkahes to precipitate a hydrated iron(III) oxide gel. The gel can be dehydrated to anhydrous hematite under pressure at a temperature around 150°C. 

Other reactions taking place throughout the hardening period are substitution and addition reactions (29). Ferrite and sulfoferrite analogues of calcium monosulfoaluminate and ettringite form soHd solutions in which iron oxide substitutes continuously for the alumina. Reactions with the calcium sihcate hydrate result in the formation of additional substituted C—S—H gel at the expense of the crystalline aluminate, sulfate, and ferrite hydrate phases. 

The concentration of an oxidative gel breaker can be measured by colorimetric methods, by periodically or continuously sampling the gel [341]. The colorimetric reagent is sensitive to oxidizing agents. It contains iron ions and thiocyanate. Thus the quantity of breaker added to the fracturing fluid can be controlled. 

G. (1999) Structural chemistry of uranium associated with Si, Al, Fe gels in a granitic uranium mine. Chem. Geol. 158 81-103 Allen, G.C. Kirby, C. Sellers, R.M. (1988) The effect of the low-oxidation-state metal ion reagent tris-picolinatovanadium(II) formate on the surface morphology and composition of crystalline iron oxides. J. Chem. Soc. Faraday Trans. I. 84 355-364... 

P.M.A. Vandenberghe, R.E. (1994b) Synthesis and characterization of some iron oxides by sol-gel method. J. Solid State Chem. 113 405-412... 

Adsorption of alkaline earth, transition, and heavy metal cations by hydrous oxide gels of iron and aluminum. Soil Sci. Soc. Am. J. 40 796-799... 

Hida Y, Kozuka H (2005) Photo anodic properties of sol-gel-derived iron oxide thin films with embedded gold nanoparticles Effects of polyvinylpyrrolidone in coating solutions. Thin Solid Films 476 264-271... 

It has been reported that by heating in the presence of water vapor, silicon can be incorporated into the lattice of y-Fe203 (55,64). It is unlikely that this effect occurs under the butene oxidation conditions since a physical mixture of large size iron oxide and silica gel retains its selectivity for butadiene after a prolonged reaction time without any indication of changes in the catalyst. If silicon substitution takes place, the water from the oxidation reaction could catalyze the substitution. In fact, deliberate pretreatment of such a physical mixture with water vapor before reaction does not change the activity or selectivity (54). 

During the course of studying the effect of crystallite sizes, attempts were made to produce very small unsupported iron oxide powders by lowering the calcination temperature of the iron hydroxyl gel that was precipitated from iron nitrate with ammonium hydroxide. However, catalysts calcined below 300°C still contain hydroxide, and they show high selectivity in butadiene production. For this reason, two catalysts, calcined at 250°C and 300°C, respectively, were studied in more detail. 

Fig. 13. The activity in 1-butene oxidation, and the selectivity for butadiene of a 250°C calcined iron hydroxide gel (7).

Type IV isotherms (e.g. benzene on iron(III) oxide gel at 320 K) level off near the saturation vapour pressure and are considered to reflect capillary condensation in porous solids, the effective pore... 

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