Catalyst migration

In every gas/solid catalytic cycle, at least one of the reactants must at some point be adsorbed on the catalyst surface. Let us consider the reaction A + B —> C. There are two options (Figure 4.2) In the first, both reactants A and B are first adsorbed on the catalyst, migrate to each other, and react on the surface, giving the product C, which is desorbed into the gas phase. This pathway, which we have already met in Chapter 2, is the Langmuir-Hinshelwood mechanism. The other option is that A is adsorbed on the catalyst surface, and B subsequently reacts with it from the gas phase to give C (the so-called Eley-Rideal mechanism [22]). The Langmuir-Hinshelwood mechanism is much more common, partly because many reactants are activated by the adsorption on the catalyst surface. 

When dispersion systems are used some residual dispersants can remain in the plastic and then migrate out. In the case of homogeneous catalysts migration can also occur. In particular this applies to the residual organic parts of the organo-metal compounds which remain as breakdown products in the plastic after completion of polymer synthesis and destruction of the catalyst. This also applies to residual solvents that are not fully evaporated. 

Cesium fluoride as fluoride ion catalyst Migration of fluorine Perfluoro compounds... 

Once the low surface area monolith body has been produced, a catalyst carrier needs to be deposited onto the monolith, which may be achieved in most instances by washcoating (see Section 4.1.3). The materials of ceramic monoliths are very compatible with catalyst coatings. They do not migrate into the catalyst coatings and neither do the active species of the precious metal catalysts migrate into the monolith body [57]. 

The compounds used are semirigid and, in some cases, ABS blends. In the case of a straight PVC compound, 30-40 phr of a low-volatility phthalate or trimellitate plasticizer is used. From 15 to 20 phr calcium carbonate is supplemented with 1 phr titanium dioxide, 1-2 phr antimony trioxide for improved flame resistance, 2-3 phr ESO, colorants as required, and about 0.1 phr (or less) stearic acid. The stabilizers used are barium/zinc or calcium/zinc powders supplemented with mineral acid absorbers and also a source of perchlorate anion. The latter is active in scavenging amine catalysts migrating fiom the PUR layer, which rapidly discolor PVC. If need be, minor amounts of dispersion resin or of acryhc processing aid can be added to compounds such as the above to improve flowability. Although principally used in the United States for instrument panels, European and Japanese usage in arm and head rests has also been reported. 

With electrons flowing from ethylene to zirconium the Zr—CH3 bond weakens the carbons of ethylene become positively polarized and the methyl group migrates from zirconium to one of the carbons of ethylene Cleavage of the Zr—CH3 bond is accom panied by formation of a ct bond between zirconium and one of the carbons of ethylene m Step 3 The product of this step is a chain extended form of the active catalyst ready to accept another ethylene ligand and repeat the chain extending steps... 

There aie a number of major indusuial problems in the operation of the steam reforming of metlrane. These include the formation of carbon on the surface of the catalyst, the sulphidation of the catalyst by the H2S impurity in commercial natural gas, and die decline of catalytic activity due to Ostwald ripening of the supported catalyst particles by migration of catalyst atoms from the smaller to tire larger particles, as the temperamre is increased. A consideration of tire thermodynamics of the principal reaction alone would suggest that the reaction shifts more favourably to the completion of the reaction as the temperature is increased. 

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