Active phase effects

A3.8.2 THE ACTIVATION FREE ENERGY AND CONDENSED PHASE EFFECTS... 

An additional effect of the use of an organic medium in the catalyst preparation is creation of mote defects in the crystalline lattice when compared to a catalyst made by the aqueous route (123). These defects persist in the active phase and are thought to result in creation of strong Lewis acid sites on the surface of the catalysts (123,127). These sites ate viewed as being responsible for the activation of butane on the catalyst surface by means of abstraction of a hydrogen atom. 

For comparison purposes activation energies for reaction 12, in both gas and liquid phase, are collected in Table 5. The value for the a-toluenesulfonyl radical is in line with the activation energies of the other RS02 desulfonylations, provided allowance is made for differences in the stabilization energies of the organic radicals that are produced. However, electrostatic repulsions within the radical molecule and phase effect may considerably influence activation energies. 

In all these cases the support has a dramatic effect on the activity and selectivity of the active phase. In classical terminology all these are Schwab effects of the second kind where an oxide affects the properties of a metal. Schwab effects of the first kind , where a metal affects the catalytic properties of a catalytic oxide, are less common although in the case of the Au/Sn02 oxidation catalysts9,10 it appears that most of the catalytic action takes place at the metal-oxide-gas three phase boundaries. 

Density functional theory study of aqueous-phase rate acceleration and endo/exo selectivity of the butadiene and acrolein Diels-Alder reaction72 shows that approximately 50% of the rate acceleration and endo/exo selectivity is attributed to hydrogen bonding and the remainder to bulk-phase effects, including enforced hydrophobic interactions and cosolvent effects. This appears to be supported by the experimental results of Engberts where a pseudothermodynamic analysis of the rate acceleration in water relative to 1-propanol and 1-propanol-water mixtures indicates that hydrogen-bond stabilization of the polarized activated complex and the decrease of the hydrophobic surface area of the reactants during the activation process are the two main causes of the rate enhancement in water.13... 

Nowadays, most of the car manufacturers are interested to replace noble metals by cheaper active phases or at least to significantly lower their content without altering their tolerance to poisoning effects. Recent developments using gold and silver as active components specifically for low temperature applications could be promising, but their sulphur tolerance is questionable in those temperature conditions [12-14], Apparently, such an aspect may exclude an extensive development of those silver-based catalysts for NO abatement processes for stationary sources. 

Presently the catalytic selective NOx reduction by ammonia is efficient and widespread through the world for stationary sources. The remarkable beneficial effect of 02 for the complete reduction of NO into nitrogen is usually observed between 200 and 400°C. However, such a technology is not applicable for mobile sources due to the toxicity of ammonia and vanadium, which composes the active phase in vanadia-titania-based catalysts. Main drawbacks related to storing and handling of ammonia as well as changes in the load composition with subsequent ammonia slip considerably affect the reliability of such a process. On the other hand, the use of urea for heavy-duty vehicles is of interest with the in situ formation of ammonia. 

The results presented in this paper therefore show that V and Mo species supported on alumina can give rise to a catalyst which has a high selectivity for the oxidation of propane to propene and a reasonable selectivity to acrolein and that both species are essential to give the optimal behaviour. Contrary to our previous observations and what observed for bulk catalysts [5], the presence of Nb and W seem to have little effect, perhaps because the methods used here restrict the active phase to a monolayer whereas previously prepared materials may have contained multilayer oxidic species. 

Many drugs, toxins, and diseases exert their clinical effects by altering some phase of synaptic activity. These effects may occur by means of ... 

Our data can be used to estimate the effective temperatures reached in each site through comparative rate thermometry, a technique developed for similar use in shock tube chemistry (32). Using the sonochemical kinetic data in combination with the activation parameters recently determined by high temperature gas phase laser pyrolysis (33), the effective temperature of each site can then be calculated (8),(34) the gas phase reaction zone effective temperature is 5200 650°K, and the liquid phase effective temperature is 1900°K. Using a simple thermal conduction model, the liquid reaction zone is estimated to be 200 nm thick and to have a lifetime of less than 2 usee, as shown in Figure 3. 

In addition to metals, other effects and poisons causes deactivation of the HDT catalysts. We already mentioned the danger of over reduction of the active phase during... 

Pore size optimization is one area where developmental efforts have been focused. Unimodal pore (NiMo) catalysts were found highly active for asphaltene conversion from resids but a large formation of coke-like sediments. Meanwhile, a macroporous catalyst showed lower activity but almost no sediments. The decrease of pore size increases the molecular weight of the asphaltenes in the hydrocracked product. An effective catalyst for VR is that for which average pores size and pore size distribution, and active phase distribution have been optimized. Therefore, the pore size distribution must be wide and contain predominantly meso-pores, but along with some micro- and macro-pores. However, the asphaltene conversion phase has to be localized in the larger pores to avoid sediment formation [134],... 

The choice of the catalyst is an important factor in PTC. Very hydrophilic onium salts such as tetramethylammonium chloride are not particularly active phase transfer agents for nonpolar solvents, as they do not effectively partition themselves into the organic phase. Table 5.2 shows relative reaction rates for anion displacement reactions for a number of common phase transfer agents. From the table it is clear that the activities of phase transfer catalysts are reaction dependent. It is important to pick the best catalyst for the job in hand. The use of onium salts containing both long and very short alkyl chains, such as hexade-cyltrimethylammonium bromide, will promote stable emulsions in some reaction systems, and thus these are poor catalysts. 

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