Supports modifier effect

Small metal particles are frequently expected (however, the evidence is sometimes questionable) to experience an electron transfer with the carrier, which modifies the adsorption and catalytic properties of the metal particles [sometimes called the Schwab effect (108-116)]. In other cases, by special conditions under preparations of the catalysts, a so-called strong metal support interaction effect (SMSI) (117-121) was evoked. In particular, with zeolites as carriers, there are pieces of experimental evidence reported (115, 116) in support of the existence of such transfer (for remarks on those conclusions, see 122, 123). 

Early studies of carbon monoxide chemisorbed on copper-nickel alloys were complicated by the failure to realize the importance of dissolved hydrogen when the experiments were conducted (10). However recent infrared studies have shown that addition of 1 to 2% copper to nickel causes the band to shift to lower frequencies due to linear chemisorbed carbon monoxide (11). This shift supports the idea that copper and dissolved hydrogen have similar modifying effects on the electronic properties of the nickel. An argument developed below, based on the spectral changes, shows that these modifying effects are consistent with the view that electrons are transferred to the nickel. 

The term monofunctional does not exclude the possibility that the intrinsic activity of a catalytic material may be influenced by chemical contact with a second material. Thus the intrinsic hydrogenation activity of a metal may differ depending on the nature of the support. Such effects may be due to varying degrees of metal dispersion, or due to more profound effects of electronic interaction which modify the electronic properties of the metal. In such cases, although the activity of the metal may depend on the nature of the support, the locus of activity is still at the metal constituent, and we still have a case of monofunctionality. 

Because of the intricate network of reactions in consideration, the global effect is very difficult to interpret without a detailed investigation on each step of the reaction. Kinetics of each reaction step should be studied with the standard catalyst, the mechanical mixture of catalyst and supplementary support (modified or not) and with the modified catalyst. 

The catalytic transfer reductions of ethyl methyl ketone, isopropyl methyl ketone, and 4-methylacetophenone were studied over the wide series of basic, acidic and semiconducting oxides supported on Si02. Most of them exhibited remarkable activity in the studied transformations. The existence of the strong oxide oxide interaction between deposited phases and Si02 was noted. The nature of catalytic active sites was identified using catalytic titration with Hammett indicators and tetracyanoethylene. An unforeseen modifying effect of n-propylamine, o-nitroaniline, and TCNE onto AI2O3 was observed which led to the enhancement of catalyst activity. 

Alumina supported on silica undergoes unforeseen modifying effect during catalytic titration with organic acid, base and electron acceptor. The observed phenomenon seems to be interesting from the practical point of view, however it would be an essential limitation for the catalytic titration method using the mentioned poisons. 

As expected, the primary amines on the unmodified APS and the APS-phenylalanine were more sensitive to pH change than tertiary amine modified APS-dimethylaniline and APS-pyridine phases. When pH was increased on the primary amine supports, ionic effects were reduced while hydrophobic effects were increased. In comparing the APS and APS-... 

The property of these additives of forming a coating on the particle surface which has the effect of masking the morphology further supports the effect. This means that deviations in the surface structure of the particle from that of an ideal crystal, such as cavities or projections for example cannot bring about interlocking or mechanical bonding between particles. In this manner the surface-modified particles can move past each other without interference when an external force is applied. 

Platinum containing cataljd ic membrane was prepared in the following way aqueous ammonia was added to a 0.01 M hexacUoroplatinic add solution until the pH of the solution reaches a definite value. The dosed vessel was kept in a cool place for one day until the complex ion of platinum and ammonia was formed. The membrane tube was dipped into this solution for at least 0.5 h, taken out and the surface was carefully washed with deionized water. It was dried at room temperature for one night and air caldned at 723 K In order to study the effects of pH and dipping time on the microstructure and activity of the membranes, platinum containing membranes modified at different conditions were also prepared. Non-supported modified membranes were prepared as well. 

Polystyrene supports modified with the similar triazine-based dendrons were used by Marsh et al. as insoluble scavengers for removal of excess nucleophilic or electrophilic reagents from the reaction mixtures. Dendrons, decorated with tertiary amines, effectively removed electrophiles (protons), whereas the similar resin with chlorotriazine terminal groups sequestered the excess nucleophiles (amines). 

SbVOx based catalysts are well known to be effective in propane ammoxidation [1-3] and have recently been appUed to the new reaction of glycerol transformation to acrylonitrile [4], It has been previously reported that the chemical and physical properties of the support modify the structure of SbV04 and its catalytic behaviour [5-7], Therefore, the use of proper support and formation of a mixed oxide phase desired is a crucial point in preparation of effective catalyst. 

The deposition of small amounts of Sb ions on the surface of the supports modifies considerably their catalytic properties. When compared with pure supports, the yield and the selectivity in methacrolein are significantly increased. In the case of Sn02, this effect is dramatic. It suffices that an amount of Sb ions equivalent to that necessary to form one monolayer be present for observing an increase of 700% in the yield. The selectivity is increased from 3 to 25%. (The two effects together bring about a diminution of the conversion to half the value observed for pure SnC>2). 

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