Physical deactivation

Figure 3.1 Physical deactivation of excited states of organic molecules...

An electronically excited atom must lose its energy either by emission of radiation or by collisional deactivation chemical decomposition is not possible, and radiationless degradation (involving an increase in translational energy) is extremely improbable. At low enough pressures, therefore, fluorescent emission is expected from all atoms. Many molecular species, however, either do not exhibit fluorescence or fluoresce weakly even when bimolecu-lar reaction or physical deactivation does not occur. Some general principles... 

When spirooxazines were present in toluene (or acetonitrile) solutions of the endoperoxide, after periods of time longer than ca. 20 half-lives, no appreciable degradation of spirooxazines was observed. This would seem to indicate that even when 02 is independently generated, SOs quench it through a mosdy physical deactivation process, and the chemical reaction component is small or exceedingly small. 

The excited chromophore may undergo physical deactivation, which lowers the quantum yield of the reaction. 

Cerium-zirconium mixed metal oxides are used in conjunction with platinum group metals to reduce and eliminate pollutants in automotive emissions control catalyst systems. The ceria-zirconia promoter materials regulate the partial pressure of oxygen near the catalyst surface, thereby facilitating catalytic oxidation and reduction of gas phase pollutants. However, ceria-zirconia is particularly susceptible to chemical and physical deactivation through sulfur dioxide adsorption. The interaction of sulfur dioxide with ceria-zirconia model catalysts has been studied with Auger spectroscopy to develop fundamental information regarding the sulfur dioxide deactivation mechanism. 

For reaction (5.18) it has been shown from a crossed molecular beam experiment that the main process is CH2OH + H, and the process of CH3O + H is not important (Lin et al. 1998). Also for the physical deactivation pathway (5.20), Wine and Ravishankara (1982) and Takahashi et al. (1996) reported that the ratio is less than a few%, while the recent high precision experiment of Vranckx et al. (2008b) by using the chemiluminescence method showed that it is 0.2 0.3 % and that it is negligible as an atmospheric reaction. 

The pathways of the reactions of 0( D) and CFCs can be conceived as CIO radical formation and physical deactivation. For example, for CCI2F2 the following reaction pathways are known to be. 

While the understanding of the other modes of physical deactivation is quite limited, certain useful conclusions can be made on the effect of physical deactivation on the catalyst activity as related to reactor design, provided that the physical deactivation takes place uniformly throughout the pellet. Therefore, this general case will be treated first before proceeding to sintering. 

It has been pointed out that the exposed surface area is reduced in a shell-progressive manner when the deposition of particles causes the physical deactivation. On the... 

Physical deactivation Demanding reaction, 20 Design of reactor. See Fixed-bed Fluidized-bed design Multiphase reactors Differential reactor, 72 Diffusion... 

The physical deactivation processes can be reversed to a certain extent, but only after the damage has been done. Cyclic treatment of crystallites in an oxidizing atmosphere and then a reducing atmosphere has been found to redisperse large crystallites. Dissolved metal atoms can be made to migrate back to the support surface by proper heat treatment. The deposited particulates can be removed either by combustion or by dissolving them with a suitable solvent. 

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