Promoters high dispersion

Agricultural Products. Pesticides are frequendy appHed as emulsiftable concentrates. The active insecticide or herbicide is dissolved in a hydrocarbon solvent which also contains an emulsifier. Hydrocarbon solvent selection is critical for this appHcation. It can seriously impact the efficacy of the formulation. The solvent should have adequate solvency for the pesticide, promote good dispersion when diluted with water, and have a dash point high enough to minimise dammabiUty ha2ards. When used in herbicide formulas, low solvent phytotoxicity is important to avoid crop damage. Hydrocarbon solvents used in post-harvest appHcation require special testing to ensure that polycycHc aromatics are absent. 

M0S2 is one of the most active hydroprocessing catalysts, but it is expensive, and the economical way to apply it is as highly dispersed material on a support, y-Al202. The activity of the supported catalyst is increased by the presence of promoter ions, Co " or Ni ". The stmctures of the catalysts are fairly well understood the M0S2 is present in layers only a few atoms thick on the support surface, and the promoter ions are present at the edges of the M0S2 layers, where the catalytic sites are located (100,101). 

Electrochemical Promotion with Highly Dispersed Catalysts... 

Further, a stabilization of the total surface of the main catalyst by added substances may explain some promoter effects, but this explanation holds only for a few multicomponent catalysts. For the iron-alumina catalyst, a beneficial stabilizing effect of the promoter alumina on the fine structure of the iron has to be accepted as a partial explanation. The fact that highly dispersed pure iron sinters at temperatures above 300°C. to a considerable extent, and that sintering practically does not occur with iron of the same high dispersion which contains 1 to 2% of alumina, is a strong qualitative support for this concept. In a quantitative way, the work of P. H. Emmett (47) and his associates has proved this point beyond any doubt it gives similarly valuable... 

The finding of the low-temperature activation of propane on zeolite Ga/HZSM-5 indicates a bifunctional reaction mechanism 179,181. The highly dispersed gallium oxide species in close vicinity to the Bronsted acid sites promote the initial activation of propane. Derouane et al. 179,181 further showed that hydrogen inhibits the activation of propane. This effect was explained by a competitive adsorption of hydrogen on the gallium species or even by a reduction of Ga to Ga species. 

Structural promotion A highly dispersed support can provide and (or) stabilize a high surface area of the catalyst supported by it. A typical example is ammonia synthesis where the thermal sintering of the iron catalyst is inhibited by alumina (although the phase configuration is different). 

The length of the induction period is affected by all the same factors which affect the coking rate of nickel catalysts in the steam reforming of hydrocarbons. A high dispersion of nickel, an increase in MgO content in the support or a small additive of a promoter all cause a prolongation of the induction time. 

It is the reprecipitation of Zn(OH)2 in the porous skeletal copper which provides promotion in methanol synthesis, water gas shift, and other reactions. The highly dispersed reprecipitated Zn(OH)2 decomposes at around 400 K to form ZnO which is an active promoter of copper catalysts. 

It is clear that the encapsulated Pt particles exhibit higher reactivity than the clean Pt particles. The difference must be even higher if one normalizes the reactivity to the particles surface area, which obviously decreases at 850 K due to particle sintering, albeit not measured in those experiments. The same effect was observed also at the stoichiometric ratio (40 mbar CO + 20 mbar O2) [83], Such promotional effect of encapsulation seems counterintuitive since the FeO film covering Pt particles and exposing a close-packed O layer must be essentially inert. In order to rationalize these findings on highly dispersed systems, we have to address the structure-reactivity relationships observed for extended, well-ordered FeO(lll) films on Pt(lll). 

Alumina promoted FCC catalysts are commercially viable if, and only if, the alumina component produces the desired properties without detrimentally affecting the attrition resistance and the cracking activity of the finished catalyst particle. Previous work (8.9.11) indicated that attrition resistant catalysts containing alumina could be formed only if a highly dispersed, pseudoboehmitic alumina was used. Other studies have demonstrated catalytic performance improvement without determining the attrition character of the catalyst (1-7). 

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