Poisoning of catalytic sites

Preferential Poisoning of Catalytic Sites. In order to show that yields and Cs-lBO F octanenumbers... 

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. 

Poisoning of catalytic reaction sites for the water gas shift reaction, and... 

Several examples showing the effects of adatoms on activity and selectivity of a given catalytic reaction were observed. In most cases, this effect can be rationalized as a selective poisoning of undesirable sites. Usually, the presence of adatoms leads to a simultaneous decrease of the global activity and to a significant increase of selectivities in favor of the desired products. We describe here two examples. 

Poisoning experiments. Poisoning of acidic sites by adsorption of bases is another technique that has been applied to identify the centers of catalytic activity for various reactions. The results of many of the earlier studies were interpreted in terms of interaction of the adsorbed bases with... 

Methanol oxidation is a self-poisoning process, in which the intermediate adsorbate CO, formed from the dissociation of methanol, poisons the catalytic sites of Pt. The mechanism of methanol oxidation is as follows ... 

In Older to explain the diverse deactivation behavior of the catalyst in processing these three types of feedstocks, we formulated a simplified dewaxing model which assumes that the dewaxing reaction can be described as an irreversible reaction (ix., cracking of waxy paraffinic molecules) coupled with a order catalyst deactivation reaction. The deactivation reaction is assumed to be conceniratioti independent while the fractional catalytic activity at any time Is a function of a number of variables including number of catalytic sites and concentration of poisons in the feedstock. 

Uniform Distribution of Poison Suppose the rate of the adsorption (or reaction) process which poisons the catalytic site is slow with respect to intrapellet diffusion. Then the surface will be deactivated uniformly through the pellet. If a is the fraction of the surface so poisoned, the rate constant will become A i(l — a). The rate per pellet, according to Eq. (11-44), is... 

Hydroxyethyl)-pyridine was dehydrated to 2-vinyl-pyridine in liquid phase over solid acid catalysts, with very high selectivity and fairly good reaction rate at relatively low reaction temperature (160°C). The catalytic activity is well correlated with the presence on the catalyst surface of medium to weak Bronsted acid sites. The analysis of coke left behind onto the catalyst and the effect of partial poisoning of catalytic activity by CO2 indicate that the reaction takes place through two mechanisms, involving either a Bronsted acid site or a couple of acid-base sites. 

A series of catalytic tests was performed in the presence of benzoic acid (poison of basic sites) and tripropylamine (poison of acidic sites). The poison weight is equal to 15 % of the catalyst weight. Those poisons were chosen for practical reasons GC retention time, boiling temperature and chemical inertness of the poisons. 

Several adatoms effects were observed, with various reactions. This effect can usually be depicted as poisoning of undesirable sites, resulting simultaneously in a decrease of the global catalytic activity and in a significant increase of the selectivities for the desired products. We describe here three examples, the hydrogenation of a, -unsaturated aldehydes (the same reaction as above but now in the presence of a very small amount of tin), the isomerization of 3-carene into 2-carene, and the dehydrogenation of butan-2-ol into methyl ethyl ketone. 

Poisoning of catalytic reaction sites for the water gas shift reaction, and Oxidation to SO2 in a combustion reaction, and subsequent reaction with carbonate ions in the electrolyte. 

The ionic liquid may interact with the solid catalytic sites and thereby modify its adsorption and reaction properties [90, 91] this includes the possibility that the ionic liquid may poison specific catalytic sites of the catalytic material, thus suppressing unwanted side reactions [92]. 

Poisoning is a deactivation pathway in which at least one component of the reaction mixture adsorbs in a very strong - often irreversible - manner to the catalytic active center (Figure 2.3.6a). Kinetically speaking, the number and concentration of catalytic sites for this process reduces over time. In cases in which the catalytic material is characterized by different catalytic centers of different reactivity the poisoning process can be selective for one sort of center. By selective poisoning... 

Another aspect of surface area effects deals with the poisoning of catalytically active sites by competing adsorbates. The species most pertinent to... 

There are several assumptions involved in this The reaction rate expression probably contains a nominator For the isomerization case it is justified to take the nominator in the front of the matrix and include in it terms related to catalyst activity However, this is less clear for the disproportionation Doing so is equivalent to assuming that all reactions occur at the same type of catalytic site This was true for their case, but is an undesirable assumption in general In fact, we hope it is not true. If it is correct, the main parameter effecting selectivity is temperature On the other hand, should the disproportionation reaction be favored by a different type of site, we could hope to improve selection by changing reaction concentration or finding a selective poison for this site. 

A selective poison is one that binds to the catalyst surface in such a way that it blocks the catalytic sites for one kind of reaction but not those for another. Selective poisons are used to control the selectivity of a catalyst. For example, nickel catalysts supported on alumina are used for selective removal of acetjiene impurities in olefin streams (58). The catalyst is treated with a continuous feed stream containing sulfur to poison it to an exacdy controlled degree that does not affect the activity for conversion of acetylene to ethylene but does poison the activity for ethylene hydrogenation to ethane. Thus the acetylene is removed and the valuable olefin is not converted. 

The mechanism of poisoning automobile exhaust catalysts has been identified (71). Upon combustion in the cylinder tetraethyllead (TEL) produces lead oxide which would accumulate in the combustion chamber except that ethylene dibromide [106-93-4] or other similar haUde compounds were added to the gasoline along with TEL to form volatile lead haUde compounds. Thus lead deposits in the cylinder and on the spark plugs are minimized. Volatile lead hahdes (bromides or chlorides) would then exit the combustion chamber, and such volatile compounds would diffuse to catalyst surfaces by the same mechanisms as do carbon monoxide compounds. When adsorbed on the precious metal catalyst site, lead haUde renders the catalytic site inactive. 

Fig. 6. Catalyst inhibition mechanisms where ( ) are active catalyst sites the catalyst carrier and the catalytic support (a) masking of catalyst (b) poisoning of catalyst (c) thermal aging of catalyst and (d) attrition of ceramic oxide metal substrate monolith system, which causes the loss of active catalytic material resulting in less catalyst in the reactor unit and eventual loss in performance.

Poisoning is operationally defined. Often catalysts beheved to be permanently poisoned can be regenerated (5) (see Catalysts, regeneration). A species may be a poison ia some reactions, but not ia others, depending on its adsorption strength relative to that of other species competing for catalytic sites (24), and the temperature of the system. Catalysis poisons have been classified according to chemical species, types of reactions poisoned, and selectivity for active catalyst sites (24). 

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