Heterogeneous deactivation

Trimm, D.L. The regeneration or disposal of deactivated heterogeneous catalyst... 

Surface active substances deactivate heterogeneous particles and thus increase the width of the metastable region (165,183]. The extent of this action Is given by the amount and catalytic activity of foreign partKdes. An opposite Influence [203.204] can be explained by the fact that surface active substances decrease the surface energy so that the nucleation rate can Increase. The shape of the curve of nucleation rate vs. the admixture concentration resembles the adsorption Isotherms of surface active substances on solid surfaces so that there may be expected a direct link of the nucleation rate rise with the adsorption of the admixture on the surface. 

Supported aqueous phase (SAP) catalysts (16) employ an aqueous film of TPPTS or similar ligand, deposited on a soHd support, eg, controlled pore glass. Whereas these supported catalysts overcome some of the principal limitations experienced using heterogeneous catalysts, including rhodium leaching and rapid catalyst deactivation, SAP catalysts have not found commercial appHcation as of this writing. 

Very few examples have been described for the non-covalent immobilization of chiral porphyrin complexes (Fig. 26). In the first case, the porphyrin-dichlororutheninm complex was encapsulated in silica, which was prepared around the complex by a sol-gel method [78], in an attempt to prevent deactivation observed in solution in the epoxidation of different alkenes with 2,6-dichloropyridine N-oxide. In fact, the heterogeneous catalyst is much more active, with TON up to 10 800 in the case of styrene compared to a maximum of 2190 in solution. Enantioselectivities were about the same imder both sets of conditions, with values aroimd 70% ee. 

The catalyst deactivates, but after four runs the conversion is still significantly higher (> 99% after 2 h) as compared with the uncatalyzed reaction. Moreover, the Z-selectivity in all four runs is higher than 80%, whereas in the uncatalyzed reaction, it is typically only 30% (Z). The fact that the solid powder can be used several times furthermore supports the fact that the reaction mechanism is heterogeneous. The reason for the deactivation is unknown. A disadvantage of the nanoparticles is the difficulty of separation. Thus, in some cases the particles form col-... 

Zeolites have led to a new phenomenon in heterogeneous catalysis, shape selectivity. It has two aspects (a) formation of an otherwise possible product is blocked because it cannot fit into the pores, and (b) formation of the product is blocked not by (a) but because the transition state in the bimolecular process leading to it cannot fit into the pores. For example, (a) is involved in zeolite catalyzed reactions which favor a para-disubstituted benzene over the ortho and meso. The low rate of deactivation observed in some reactions of hydrocarbons on some zeoUtes has been ascribed to (b) inhibition of bimolecular steps forming coke. 

The analysis carried out allows us to state the general conditions needed to properly determine the coefficient of heterogeneous deactivation by the diffusion method ... 

Being an integral magnitude, the heterogeneous deactivation coefficient does not allow us to judge of the particular mechanisms of EEP... 

Using the experimentally obtained coefficient of heterogeneous deactivation of 02( Aj) on glass (lO" ) and the adsorption heat... 

The first step of surveying was represented by studying the nature of heterogeneous deactivation of singlet oxygen on a surface of oxidized and partially reduced zinc oxide [102] with the aid of semiconductor sensors. At room temperature and the pressure of O2 10 Torr, the value of y for de-exciting 02( A ) in ZnO is equal or approximately... 

Fig. 5.12. Coefficient of heterogeneous deactivation of singlet oxygen versus the temperature of zinc oxide surface at Pqj = 0.1 Torr [102]...

Because of these factors, the fundamental experimental information about the interaction of metastable atoms with semiconductors and dielectrics is meant for the reflection coefficients that are determined with the aid of beam methods and for the coefficients of heterogeneous deactivation which are evaluated under diffusion conditions. However, the data in this event are fairly scarce and conflicting. The results obtained by the methods of electronic beams do not agree with diffusion experiments. Thus, Allison et al [ 137] report that the coefficients dealing with... 

Note that a similar situation arises in the study of heterogeneous deactivation of electron-excited molecules of N2. Thus, an opinion expressed by Clark et al. [152] states that the coefficients of heterogeneous deactivation of N2(A S, v = 0.1) for all surfaces are close to unity. On the other hand, Vidaud with his coworkers [59, 153] have obtained 3 10 2 and (1.8 + 1.2) 10 values for these coefficients shown by platinum and Pyrex, respectively. Tabachnik and Shub [154] investigated heterogeneous decay of NaC A SJJ ) molecules on a quartz surface by the method of bulk-luminescence spectroscopy. The authors carried out a series of experiments within a broad (about four orders of magnitude) range of active particle concentrations and arrived at a conclusion that at a concentration of N2( A 2 ) in excess of 10 mole/cm , the... 

An important role in the mechanism is plaid by the applied particles of metal. One can conclude so from the experiments in studying heterogeneous deactivation of RGMAs on a pure and Au microcrystal-activated surfaces of glass and zinc oxide [162], The experiments were conducted by the Smith method, Au/ZnO sensors being used as RGMA detectors. The results of these investigations are tabulated in Tables 5.3. 

Surface Weight thickness of Au film, Nm Average size of particle in film, Nm Metal-coated fraction of surface Coefficient of heterogeneous deactivation, r He Kr Xe ... 

---