Chromia, deposition

The selectivity of catalysts in COj is always lower than 3 % in all the temperature range investigated (410 - 490°C). An excess of chromia deposited on the support reduces the activity of the catalysts which become more oxidant and less selective in nitriles. Sample containing 50 % of chromium tends to behave as pure Cr O which favours deep oxidation and yields 71.5 % of CO2 at 440°C (see Table 2). 

The X-Ray Diffraction results shows the presence of only AI2O3, Zr02 and Th02 and in none of the cases Cr203 is observed for the chromia loaded sample. This probably indicates that the chromia deposited is well dispersed and probably in an amorphous form. 

L. Ma, A.J. Smith, T. Tran and M.S. Wainwright, Development of skeletal copper chromia catalysts II. Kinetics of leaching of aluminum and chromia deposition. Chemical Engineering and Processing, 40 59-69, 2001. 

Promoter deposition through different mechanisms can account for different catalyst properties. In particular, chromate depositing as chromia does not easily redissolve but, zinc oxide does redissolve once the leach front passes and the pH returns to the bulk level of the lixiviant. Therefore, chromate can provide a more stable catalyst structure against aging, as observed in the skeletal copper system. Of course, promoter involvement in catalyst activity as well as structural promotion must be considered in the selection of promoters. This complexity once again highlights the dependence of the catalytic activity of these materials on the preparation conditions. 

Initially tests were conducted in glass equipment at atmospheric pressure. It was discovered that a more durable catalyst could be made if the Group VI metal oxide were deposited on an alumina support. The best support found for this reaction was alumina, and the first commercial catalyst was made by impregnating a material very similar to activated alumina 1 with a molybdenum salt solution, followed by drying and calcining at a temperature above 1000° F. Interestingly enough, the supported chromia catalyst which showed a marked superiority over the supported molybdena catalyst at atmospheric... 

The above authors have established that the procedure with the polymeric membrane is not satisfactory. At high current densities, metallic nickel nucleates on the polymeric membrane. Although a reasonable loading of a suspended silica carrier could be achieved, a considerable fraction of the nickel was deposited on the polymeric membrane. The chromia layer method was observed to yield much better results. 

In the discussion of the subject Balandin mentions (15) that Fischer previously postulated that methylene radicals may be produced as an intermediate in the formation of hydrocarbons by his method (116). This mechanism of carbon deposition on platinum supported on oxides of nickel and chromium (oxidized nichrome) through the intermediate formation of methylenes was thought by Balandin to be similar to the mechanism of dehydrogenation over this type of catalyst in that both occur on the boundaries of platinum-nickel and of platinum-chromia and were brought in agreement by him with his multiplet theory (26). 

A comparison of the UV Raman spectrum measured for coke deposited during the MTH reaction with that deposited during butane dehydrogenation catalyzed by chromia on alumina (66) shows clear differences in the spectral intensity distribution (Fig. 11). In particular, the intensity of the features in the regions 1340-1440cm and 1560 1630 cm are nearly equal for the MTH reaction. 

A series of papers was published by Vansant and coworkers dealing with the gas-phase deposition and thermal transformation of Cr(acac)3 to chromia on the surface of alumina and silica supports. Cr(acac)3 binds to the hydroxyl-terminated alumina surface by hydrogen bonding and/or a donor-acceptor interaction with coordinatively unsaturated Al + ions as outlined in Figure 23 . ... 

DEACTIVATION MECHANISMS OF A CHROMIA-ALUMINA CATALYST BY COKE DEPOSITION... 

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