Promoter effect oxygenation catalysts

It should be mentioned here that Sn sites are not considered to be the solitary source for OHad, which could be adsorbed on Pt sites owing to the influence of adjunct Sn atoms [Stamenkovic et al., 2005], The promotional effect of Sn was later confirmed on a PtSn/C nanocatalyst [Arenz et al., 2005], which exhibits similar behavior that was assigned primarily to the formation of reactive OH species at much lower potential than on pure Pt catalysts. Based on these findings, the bifunctional effect was unambiguously confirmed for Pt-Sn surfaces, where Sn sites serve as a source of oxygenated species that boost CO oxidation at low potentials and allow these surfaces to be employed as CO-tolerant catalysts. 

Unlike boron fluoride, titanium tetrachloride does not catalyze the liquid phase polymerization of isobutylene under anhydrous conditions (Plesch et al., 83). The addition of titanium tetrachloride to a solution of the olefin in hexane at —80° failed to cause any reaction. Instantaneous polymerization occurred when moist air was added. Oxygen, nitrogen, carbon dioxide, and hydrogen chloride had no promoting effect. Ammonia and sulfur dioxide combined with the catalyst if these were added in small quantity only, subsequent addition of moist air permitted the polymerization to occur. Ethyl alcohol and ethyl ether, on the other hand, prevented the polymerization even on subsequent addition of moist air. They may be regarded as true poisons. 

It is of importance to draw a distinction between additions which are sensitive toward oxygen and those which are not sensitive toward oxygen. For instance, metallic potassium, under the influence of oxygen, forms potassium oxide, and the promoting effect of the latter differs from that of the alkali-metal. The same element can act on the catalyst differently, depending on whether it is present as the oxide, chloride, or other compound. ... 

The studies performed over promoted manganese molybdate catalysts have shown significant changes in catalytic behavior due to presence of the promoter. The preliminary results suggest that the pronounced differences observed in selectivity and activity may be related to the effect of the promoter cations on the reactivity of the lattice oxygen and the availability of adsorbed oxygen. 

The Bi203—Sn02 combination was studied by Solymosi and Bozso [299] and by Seiyama et al. [284,285]. The former carried out pulse experiments in the absence of oxygen and report that even small amounts of Sn02 added to Bi203 have a promoting effect and shift the product spectrum from hexadiene to benzene. The best combination is a mechanical mixture of the two oxides in a 1/1 ratio. With this catalyst, a selectivity of 80% (benzene) is reached at a 40% conversion level (at 500° C),... 

On the basis of these observations, the coke formed in MTO and DTO is classified into two categories unreactive coke, formed from adsorbed olefins having a deactivating effect on DTO and MTO and reactive coke, formed from oxygenates, having a promoting effect on DTO and MTO. The activities of the catalyst for the MTO and DTO reactions at various coke contents depend on the nature of the coke, in particular on the ratio of the reactive to the unreactive coke (7). 

More recently, bis-silylation of various ,p-unsaturated ketones with Cl2PhSiSiMe3 was reported to be promoted effectively by the phosphine-palladium catalyst in a similar manner (Eq.34) [63]. In products 68, the geometry of the double bonds and the regioselectivity with the TMS group attached to the oxygen atom are nearly completely controlled in the reaction. Use of BINAP lig-... 

The Cl promoter lowers the Ag-O bond strength to its lowest measured value of 105 kJ/mol. The corollary of this is that the heat of adsorption of ethylene on to the Cl-promoted, oxidised Ag/a-A Os catalyst is also at its lowest value so that the coverage of Cl-promoted the oxidised catalyst by ethylene at operating temperatures will be reduced, thus lowering the activity of the catalyst. The effect of Cl is kinetic and thermod)mamic on the chemisorption of ethylene and not site-blocking in oxygen chemisorption. 

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