Vanadium pentoxide dispersion

The preparation of lyophilic sols is easy and most of the time a mixture of the dispersion medium and the substance to be dispersed need only be stirred. Gelatine, for example, disperses almost spontaneously in water. The hydroxides of iron, aluminium, chromium and zirconium as well as vanadium pentoxide and silicic acids all belong to the group of hydrophilic colloids. 

Several catalysts on the market today contain special vanadium traps or vanadium scavengers in order to protect the active ingredients against poisoning and/or destruction by Vanadium. These "Metal Traps" limit the mobility of the vanadium pentoxide compounds under FCC conditions (2, 10). The nickel problem needs to be approached differently and more recently, progress has been made towards reducing the dehydrogenation activity of nickel dispersed on FCC catalysts (11). 

Although the forward reaction is favored by increase in pressure, this is not employed in practice since 97 to 99% conversion of sulfur dioxide to sulfur trioxide can be accomplished at the temperature specified here, provided suitable catalysts are used. The first catalyst used for this reaction consisted of finely divided platinum dispersed in asbestos, anhydrous magnesium sulfate, or silica gel. Other catalysts were later discovered. Mixtures of ferric and cupric oxides are useful, but these are less efficient than platinum. Certain mixtures containing vanadium pentoxide (V205) and other compounds of vanadium appear to be as good as or better than platinum. There has been much controversy over the relative merits of platinum and vanadium catalysts, and only time will provide the answer as to which is best. 

In some cases a catalyst consists of minute particles of an active material dispersed over a less active substance called a support. The active material is frequently a pure metal or metal alloy. Such catalysts are called supported catalysts, as distinguished from unsupported catalysts, whose active ingredients are major amounts of other substances called promoters, which increase the activity. Examples of supported catalysts are the automobile-muffler catalysts mentioned above, the platinum-on-alumina catalyst used in petroleum reforming, and the vanadium pentoxide on silica used to oxidize sulfur dioxide in manufacturing sulfuric acid. On the other hand, the platinum gauze for ammonia oxidation, the promoted iron for ammonia synthesis, and the silica-alumina dehydrogenation catalyst used in butadiene manufacture typify unsupported catalysts. 

Electrolytic oxidation of anthracene in 20 per cent sulfuric acid solution with 1 per cent of vanadium pentoxide present is carried out at 80° C. with lead electrodes and a current density of 300 amperes per square meter at 1.6 volts. Good yields have been claimed 10 for this process. Air under pressure has been used for the oxidation of anthracene in the form of dispersions in aqueous ferric sulfate solutions,20 or as a solution iu pyridine or dispersion in aqueous alkaline solutions preferably in the presence of catalysts 21 of copper, cobalt, nickel or lead compounds. Vanadium compounds have been found more active than chromium compounds for use as oxidation catalysts in the form of suspensions in the liquid phase, as in the preparation of aniline black.22 Anthracene suspended in water or dilute sulfuric arid or dissolved in a solvent as acetone is oxidized with ozone, or ozonized oxygen at ordinary temperatures.28... 

When minute particles of an active material are dispersed on a less active substance to produce a catalytic effect, such catalysts are called supported catalysts. The active material is usually a pure metal or a metal alloy. Examples of supported catalysts are the platinum on-alumina catalyst used in petroleum reforming and vanadium pentoxide on silica catalyst used in oxidation of sulphur dioxide. 

Flow birefringence can be understood properly with the help of a commonplace example. A curious thing happens when an aged colloidal dispersion of vanadium pentoxide is stirred slowly the path of the stirnng rod lights up since the colloidal particles orient themselves along the stream lines. This effect is observed because of the difference in the amount of light reflected by the symmetrically oriented particles as compared to the randomly oriented ones. In a similar... 

Went, G., Leu, L. and Bell, A. (1992). Quantitative Structural Analysis of Dispersed Vanadia Species in Ti02(anatase)-supported Vanadium Pentoxide, J. Catal, 134, pp. 479-491. 

In most cases hydrophobic sols are NEWTONian liquids with a viscosity only slightly deviating from that of the dispersion medium. In the flocculated state more or less clear indications of non-NEWTONian behaviour, especially the presence of a yield value, may be detected. This is most evident for sols containing blade-shaped or rodshaped particles like iron oxide or vanadium pentoxide. When these sols are not too dilute and electrolyte is added to them in quantities insufficient to obtain flocculationp gel formation is often observed. The who.le mass becomes solid, probably becauste by partial flocculation the particles of the sol form a loose network, in the meshes of which the intermiccllar liquid is retained. 

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