Activity enhancement, deactivation

As discussed below in the section on Cu catalysts, it has been noted that Cu/ZnO catalysts have low activities and deactivate quickly in the decomposition reaction. However, the inclusion of nickel, introduced in co-precipitation preparation, has been reported to enhance initial activity and retard deactivation.29 The role of nickel has been ascribed to an increase in dispersion of the active Cu species and also the prevention of deactivation via the formation of Cu-Zn alloys. 

The enhanced control can be ascribed to the dynamic stabilization of growing cations. The stabilization can be understood as an extension of the lifetime of the growing species which only for a small fraction of time are in the carbocationic form and during the vast majority of time they are in the dormant form. The nature and concentrations of various activators and deactivators define the equilibrium position and dynamics of the exchange processes. There are three general approaches to controlled systems (Section IV.B) ... 

Catalyst performance may be influenced by oxidative pretreatment of carbon supports. The AC samples shown in Table 12 were used for the preparation of the Mo/AC and NiMo/AC catalysts. The activity of the catalysts was tested in the flow reactor at 3 MPa and 623 K using the 7% solution of pyridine in cyclohexane. Figure 39 shows that for the catalyst consisting of the pretreated supports, the overall conversion (to C5 hydrocarbons and piperidine) decreased, indicating an enhanced deactivation of catalyst. However, in the presence of H2S, the activity difference was much less evident. For pretreated... 

In a more recent study, Charleux et studied the theoretical features of the activation-deactivation equilibrium in nitroxide-mediated copolymerization and applied it to the SGl-mediated copolymerization of methyl methacrylate with a low percentage of styrene (typically in the 4-9 mol.% range). They actually demonstrated that the system exhibited all the characteristics of a living/controlled polymerization, which was explained by the following features (1) the overall concentration of propagating radicals was strongly reduced by the copolymerization effect and the irreversible termination reactions undergone by the MMA/SGf system were hence slowed down (2) isolated styrene subunits were incorporated into the chains and the terminal one promoted the reversible deactivation by the SGI nitroxide and (3) the MMA penultimate unit effect enhanced deactivation of the so-formed styryl-SGf... 

Carbon monoxide has been found to poison cobalt molybdate catalysts. It causes not only instantaneous deactivation but a cumulative deactivation as well. It should be removed from treat gas entirely or at least reduced to a very low value. Carbon dioxide also must be removed since it is converted to CO in the reducing atmosphere employed in Hydrofining. Liquid water can damage the structural integrity of the catalyst. Water, in the form of steam does not necessarily hurt the catalyst. In fact 30 psig steam/air mixtures are used to regenerate the catalyst. Also, steam appears to enhance the catalyst activity in... 

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