Deactivation poisoning mode

An example of reversible deactivation is presented in Figure 7, as a variation of the poisoning mode of deactivation shown in Figure 4. The catalyst promoting the reaction A->B involves dynamic adsorption of Component A, wherein molecules of A continually adsorb on and desorb from the surface. While adsorbed, some of the molecules are converted to Component B. The rate of reaction is proportional to the surface coverage of Component A. 

This heterogeneous process may constitute an interesting alternative to the classical synthetic route. Catalyst deactivation can be slowed down by deliberately poisoning the acidic surface site with added pyridine in the reactant feed. A feasible operation mode for a continuous heterogeneous process consists of reaction and subsequent reoxidation cycles of the catalyst. 

Reforming catalysts may suffer deactivation either in reversible or irreversible mode. Deactivation is irreversible when catalyst is exposed to permanent poisons or very high temperature during regeneration. Directional concentration of specific permanent poisons in... 

As mentioned above, an area in which the concepts and techniques of statistical physics of disordered media have found useful application is the phenomenon of catalyst deactivation. Deactivation is typically caused by a chemical species, which adsorbs on and poisons the catalyst s surface and frequently blocks its porous structure. One finds that often reactants, products and reaction intermediates, as well as various reactant stream impurities, also serve as poisons and/or poison precursors. In addition to the above mode of deactivation, usually called chemical deactivation (2 3.), catalyst particles also deactivate due to thermal and mechanical causes. Thermal deactivation (sintering), in particular, and particle attrition and break-up due to thermal and mechanical causes, are amenable to modeling using the concepts of statistical physics of disordered media, but as already mentioned above the subject will not be dealt with in this paper. 

In Modes 1 and 2 the limitation or the blockage is due to chemical reasons i.e., the coke molecules are (1) reversibly or (2) quasi-irreversibly adsorbed on the acid sites (site poisoning or site coverage) and/or to steric reasons the diffusion of reactant molecules through the cavity or the channel intersection is (1) limited or (2) blocked. With these modes the toxicity of the coke molecules is low as only the sites located in the cavity or at the chaimel intersection, often only one site, are partially (Mode 1) or totally (Mode 2) deactivated. 

Chemical deactivation of oxygen storage materials occurs primarily by two modes sulfur poisoning and poisoning from oil additives. Each of these is discussed in turn below. 

In the final deactivation mode reported by the authors, the active acidic sites of the catalyst are poisoned (7 = 145°C, P = 50 bar) by continuous addition of a very dilute solution of pyridine to the reacting mixture over a period of 12 h (see figure 11.10). The catalyst can be reactivated by heating and compressing the reaction mixture to conditions well within the mixture critical region (7 = 250°C, P = 500 bar). Tiltscher and coworkers report that the catalyst poison is precipitated from the product solution as pyridinium chloride. Presumably only a very small amount of pyridinium chloride is needed to deactivate the catalyst since supercritical hexene probably would not be able to solubilize much of this salt. It is surprising, however, that supercritical hexene can overcome the acid-base interactions that are occurring on the catalyst surface and, hence, remove the pyridinium chloride. 

Another mode of deactivation is termed poisoning (see Figure 4), which results from a chemical reaction of a gaseous species with the active metal (or, alternatively, a strong chemisoiption bond). For example, under certain conditions sulfur can fonn a compound with some metals. Phosphorous, halogens, and lead are common poisons for noble metal catalysts. 

Cinchonidine, being a bulky molecule, reduces the accessible active platinum surface as it adsorbs and should causes some deactivation with respect to racemic hydrogenation. The decrease in formation rate of the main product after the maximum can be a result of poisoning by adsorbed spectator species, which inhibit enantiodifferentiating substrate-modifier interaction. Adsorbed cinchonidine in parallel mode (active form) provides an enantioselective site (Figure 7.8) and when the reactant is adsorbed in the vicinity, interaction between reactant and modifier leads to such orientation that hydrogenation towards the main product (e.g. B or 1-R enantiomer) is preferred. However, when the tilted form (Figure 7.8) of... 

Use of simulated rigs enabled study of the mode of catalyst poisoning since evaluation of poisoning deactivation curves provides insight into the mechanism by which noble metal oxidation catalysts become deactivated by the acquisition of poisons from the gas phase. 

Sulfur is a well-known poison for most catalysts [12]. The deactivation of metal oxide based catalysts is usually attributed to the formation of extremely stable sulfate species at the surface of the catalyst. This mode of deactivation has been reported for catalysts in car exhaust converters [13]. However, sulfate species on noble metals are less stable than those formed on transition metals and usually this deactivation is reversible [14]. 

Ej-Jennane et al. [156] compared H-ZSM-5, H-MOR, and H-Y zeolites in the disproportionation of toluene at long TOS. They found that the catalytic activity was related to the munber of acid sites for the H-ZSM-5 structure which did not show much deactivation, that there was a strong diffusional limitation with H-MOR due to pore blocking, and that the reactions over H-Y were limited by site poisoning. These results show that measurements after a long stabilization period are very hard to compare to acidity values which are measured on the fresh catalysts. If no initial activity is taken for comparison, different modes of deactivation over various catalysts will make it nearly impossible to evaluate the acidity of zeoHtes with different structures by means of this test reaction. 

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