Promoters stability, effect

Besides the effect of the presence of alkali on CO adsorption, there is also a stabilizing effect of adsorbed CO on the adsorption state of alkali. Within the high alkali coverage range the number of CO molecules adsorbed on promoted surface sites becomes practically equal to the number of alkali metal species and their properties are not dependent on the CO coverage. In this region CO adsorption causes also stabilization of the adsorbed alkali, as indicated by the observed high temperature shift of the onset of alkali desorption. 

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

The EXAFS data recorded after exposure to air of the unsupported Co-Mo catalysts with different cobalt content allow one to examine the effect of cobalt. In spite of a great uncertainty in the coordination numbers, the promoted catalysts seem to have a somewhat smaller domain size than the unpromoted catalyst as indicated both by the smaller second shell coordination numbers and by the larger effect of air exposure (i.e., reduced sulfur coordination number in first shell). This influence of cobalt on the domain size may be related to the possibility that cobalt atoms located at edges of M0S2 stabilize the domains towards growth in the basal plane direction. Recent results on C0-M0/AI2O3 catalysts indicate that Co may also have a similar stabilizing effect in supported catalysts (36). 

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... 

Unpromoted ruthenium catalysts have been shown to become unstable toward metallization with increasing temperature and/or decreasing CO pressure. Ionic promoters, especially halides, are found to provide a large stabilizing effect toward precipitation of metal, and significantly higher... 

The air cell stabilizing effect of agglomerated fat globules, promoted by emulsifiers and the ice-crystal-growth-controlling effect of hydrocoiloid stabilize the foam structure of ice cream to a great extent. This is evident by melt down analysis (see section 5.2) of ice cream exposed to heat shock. 

This effect of polyhydroxyl compounds may not be quite as simple as it has been described, as the structure of the polyhydroxyl compound may play some part in effective stabilization of enzymes in wet systems. Thus Fujita et al, (20) reported that inositol was more effective than sorbitol in stabilizing lysozyme in aqueous solutions. Both compounds contain six hydroxyl groups, but inositol is cyclic in structure whereas sorbitol is linear, Fig 10. The interaction of polyhydroxyl compounds with water promotes a change in the molecular structure of water. Inositol was reported to have a larger structure-making effect than sorbitol, which accounted for the greater stabilization effect of this compound. 

Factor Effects Promotes coagulation Promotes stability... 

Perez Ramirez, J., Overeijnder, J., Kapteijn, F. and Moulijn, J. A. (1999). Structural promotion and stabilizing effect of Mg in the catalytic decomposition of nitrous oxide over calcined hydrotalcite-like compounds. 

The authors suggested that the effect of venlafaxine on cardiac conduction is mediated by its ability to block the fast inward sodium current in cardiac myocytes. This might promote membrane stabilizing effects in a similar way to tricyclic antidepressants. They recommended that the management of venlafaxine overdose should include cardiac monitoring. 

Metal ion template mediation in macrocyclic synthesis has been a part of the field since its inception, its importance having been realized early in the development of this area. Two specific roles for the metal ion in template reactions have been proposed. These are, in turn, kinetic and thermodynamic in origin." In the kinetic template effect, the arrangement of ligands already coordinated to the metal ion provides control in a subsequent condensation during which the macrocycle is formed. The thermodynamic effect serves to promote stabilization of a structure which would not be favored in the absence of a metal ion. Schiff base condensations tend to be dependent on this latter type of template effect. Some of the more routine and general synthetic procedures will be described here. A more in-depth treatment can be found in a review by Curtis, with particular emphasis on general methods as well as modifications of preformed macrocycles." ... 

Mechanism of the Promoter Effect. The action of the so-called structural promoters (stabilizers), such as A1203, is closely associated with their solubilities in the iron oxide matrix of the unreduced catalyst or with the capability of the regular crystallizing magnetite to form solid solutions with iron - aluminum spinels [33], [289]-[291]. The solid solutions of Fe304 and the spinel FeAl204 have a miscibility gap below 850 °C... 

A promoter is some third agent which when added, often in small amounts, results in desirable activity, selectivity or stability effects. It is perhaps in this regard that catalysis still deserves the term black art and the catalyst designer alchemist. Promoters are like spices in cooking, a pinch is added here and there because we know it makes the recipe taste better. Indeed, as in cooking, many promotional effects are discovered accidentally. We then glorify them with sciemific explanations. There is much work to be done in (his part of catalysis. 

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