3.+Deactivation+Methods

__Methods of Deactivation __
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The following is about different deactivation processes.

=Deactivation =

Catalyst deactivation is a complex phenomenon, but can be defined as the loss of catalytic activity and/or selectivity over time. It is a problem of great and continuing concern in the practice of industrial catalytic processes. Costs to industry for catalyst replacement and process shutdown total billions of dollars per year. Time scales for catalyst deactivation vary considerably; for example, in the case of cracking catalysts, catalyst mortality may be on the order of seconds, while in ammonia synthesis the iron catalyst may last for 5–10 years. The three most common causes of catalyst deactivation are fouling, poisoning, thermal degradation and sintering. This site will mainly focus on deactivation by fouling because of its prevalence in numerous catalytic processes. But poisoning and thermal degradation are often caused by fouling and subsequent removal of fouling, therefore they are summarily explained (Bartholomew. C.H. 2001;Ertl G. and others.1997).

=Deactivation by Fouling =

Generally fouling covers all phenomena where the surface is covered with a deposit. The most widely known form of fouling of catalysts is coke formation. There are many reactions and mechanisms of coke formation depending on the nature of the catalyst therefore it is not clearly defined. But one can say that for the most part coke formation arises as a result of carbonaceous residues covering the active sites of a heterogeneous catalyst surface, subsequently decreasing the active surface area of the catalyst. In addition the deposition of rust and scale from elsewhere in the catalytic system is not uncommon. Coke-forming processes also involve chemisorption  of different kinds of carbons or condensed hydrocarbons that may act as catalyst poisons causing the chemical deactivation of the catalyst. The fouling of zeolite catalysts occurs in the form of coke molecules l imiting the access of the reactant to the active sites of a cavity or a pore intersection of the zeolite. (  Trimm D. 2001 ;  Gulsnet M., Magnoux P.1997;  Ertl G. and others.1997) . The following figure exemplifies the desired function of the <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 10pt;">MCM-68 zeolite <span style="font-family: 'Palatino Linotype',serif; font-size: 12pt;"> ( <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 13px;">Satoshi Inagaki, and others. 2010 <span style="font-family: 'Palatino Linotype',serif; font-size: 16px;">)  <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; line-height: normal;">. The pores in which the hexane molecules are rearranged to form the desired products end up being filled with the coke molecules resulting in the zeolite’s deactivation.

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=<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 80%;">Chemical Deactivation - Poisoning =

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">Poisoning occurs when there is a strong chemical interaction of reactants, products, or impurities with active sites on the catalyst surface. Sulfur Poisoning being the most widely cited example. Thus, poisoning has operational meaning; that is, whether a species acts as a poison depends upon its adsorption strength relative to the other species competing for catalytic sites. For example, oxygen can be a reactant in partial oxidation of ethylene to ethylene oxide on a silver catalyst and a poison in hydrogenation of ethylene on nickel. In addition to physically blocking adsorption sites,adsorbed poisons may induce changes in the electronic or geometric structure of the surface.( Trimm D. 2001 )

=<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 80%;">Deactivation by Thermal Degradation and Sintering =

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">For a catalyst to be effective it must have an effective interface with the reactants, thus heterogeneous catalysts are prepared with high surface areas. Large surface areas are thermodynamically unstable, thus given suitable conditions such as high temperatures the catalysts will rearrange to form the most favorable lower surface area agglomerates. These rearrangements are often accelerated by particular chemical environments. For example, moist atmospheres accelerate structural changes in oxide catalyst supports. This agglomeration of catalysts is known as sintering, which causes a decrease in the availability of active sites on a catalyst.( Trimm D. 2001 )

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