Catalyst site activation/deactivation

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

An unstabilized high surface area alumina siaters severely upon exposure to temperatures over 900°C. Sintering is a process by which the small internal pores ia the particles coalesce and lose large fractions of the total surface area. This process is to be avoided because it occludes some of the precious metal catalyst sites. The network of small pores and passages for gas transfer collapses and restricts free gas exchange iato and out of the activated catalyst layer resulting ia thermal deactivation of the catalyst. 

Many chemical elements exhibit catalytic activity (5) which, within limits, is inversely related to the strength of chemisorption of the VOCs and oxygen, provided that adsorption is sufficiently strong to achieve a high surface coverage (17). If the chemisorption is too strong, the catalyst is quickly deactivated as the active sites become irreversibly covered. If the chemisorption is too weak, only a small fraction of the surface is covered and the activity is very low (17) (Fig. 2). 

Catalyst deactivation may affect the performance of a reactor in several ways. A reduction in the number of catalyst sites can reduce catalytic activity and decrease fractional... 

E6 Deactivation activation energy, s, Total catalyst sites... 

Since the decline of catalytic activity is owing to the decrease in the number of active sites, a should be correlated to the fraction N/N0, where Nt is the number of active sites after deactivating time t and N0 is the number of active sites of the fresh catalyst. This correlation is derived from experimental observations, or an empirical form is used (Forzatti and Lietti, 1999). 

This review will only focus on the modeling efforts in pore diffusion and reaction in single-catalyst pellets which have incorporated pore plugging as a deactivation mechanism. A broad literature exists on the deactivation of catalysts by active site poisoning, and it has been reviewed by Froment and Bischoff (1979). The behavior of catalytic beds undergoing deactivation is... 

We can also distinguish a few lines of approach regarding catalyst poisons which deactivate the catalyst by coke formation, that is by blocking of pores and the catalyst active sites,... 

Sulfur compounds adsorb onto surface-active metal (or metal oxide) sites, causing deactivation in a large number of petroleum, petrochemical, and chemical catalytic applications. Acidic catalysts such as zeolites and promoted aluminas are poisoned by nitrogen compounds by chemisorption onto active sites also located on the surface. 

The oxidation of feed C required a higher initial temperature (385 vs 360 °C), and the catalyst deactivated steadily, requiring a temperature increase from 385 to 418 C over 201 days-on-stream. This history is shown in Figure 5. The catalyst lost activity primarily for the oxidation of benzene and trichloroethylene (Figure 6), while conversion of other components remained > 99%. The proportion of CO was 11-13% of total CO, and remained constant with time, as for feed B. Although it is apparent that the number of active sites is decreasing here, as shown by the deactivation,... 

Deactivation of zeolites, like that of the other porous catalysts occurs in two ways, the first one in which at the maximum one active site per coke molecule is deactivated, the second in which several active sites are deactivated. The two modes of zeolite deactivation are shown in Figure 7. The effect of coke on the activity and on the pore volume accessible to the reactant is also indicated. 

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