Types of catalyst poisoning

The phenomenon of catalyst poisoning may be divided into three types according to the different effect reversible poisoning, irreversible poisoning and selective poisoning. 

Because poisoning is due to the interaction between a poison and the active component in a catalyst, this interaction may be strong or weak. According to the strength or weakness of this interaction, poisoning may be divided into two kinds. One is that the interaction is so weak that the poison on the active site can be 

For example, oxygen and water vapor are poisons to ammonia s3mthesis catalysts. Because these poisons can be removed by reduction by H2 or by treatment using a fresh synthesis gas without poisons, this is reversible poisoning. However, poisoning caused by sulfide, chlorine, phosphorus and heavy metal is very difficult to be removed so the poisoning is irreversible. 

Although a poisoned catalyst becomes inactive to some particular reaction, it can be active for other reactions. This is known as the selective poisoning. In a series-reaction, if the poisoned catalyst is only deactivating to the last reaction in the series, then the process can stay in the intermediate stage, the 3deld of the intermediate product could be expected to be high. For certain catalysts, the introduction of a small amount of poison can therefore selectively enhance the catalytic activity and 

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Poisons ndInhibitors. Catalyst poisons and inhibitors can come from the fuel, the lube oil, from engine wear and corrosion products, and from air ingestion. There are two types of catalyst poisons one poisons active sites, the other is a masking agent. 

It should be mentioned that most authorities (17) consider the solid solubility of silicon in nickel to be several per cent in the temperature region of this study. The present sample contained only 0.3 per cent Si. This would indicate that a temperature-dependent fraction of the total finds it more economical, from the free energy standpoint, to occur as a surface phase. It may be that certain types of catalyst poisoning consist of the formation of surface phases of this kind on normally active regions of the catalyst. 

The main strategies for addressing these types of catalyst poisons are summarized in Table XI. 

The hydrogenation of 2-ethyl-5,6,7,8-tetrahydroanthraqumone (THEAQ) at the oxygen in the presence of a palladium supported catalyst is a key step in the industrial production of hydrogen peroxide. In industrial plants, the performance of the catalyst slowly decreases because of deactivation. Two types of catalyst poisoning are operative, a reversible one, related to the presence of water, and a permanent one, probably due to the condensation of two or more anthraquinone molecules on the palladium surface. The kinetic data obtained from laboratory runs are used to simulate the performance in industrial plants. 

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). 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

Sulfur is a potential problem even at low levels for synthesis gas systems using certain types of catalysts. The production of methanol from synthesis gas, for example, uses catalysts that are poisoned by sulfur. Some tar cracking catalysts are also sulfur sensitive. In those systems, thorough removal of sulfur will be required. Fuel cell systems are also sulfur sensitive. 

We will use the term deactivation for all types of catalyst decay, both fast and slow and we will call any material which deposits on the surface to lower its activity a poison. 

This type of N-doped soot catalyst is of particular interest for the development of advanced fuel cells. As this type of catalyst is not poisoned by carbon monoxide, it is a promising candidate for O2 cathodes in methanolconsuming fuel cells (132). In methanol-combusting cells, diffusive transport of methanol from the anode to the cathode cannot be avoided, with the consequence that the activity of Pt-activated cathodes becomes severely impaired by CO poisoning of the Pt catalyst therefore, a CO-insensitive cathodic electrocatalyst seems to be indispensible. Yet the longevity of this type of catalyst is still in dispute (133). 

Special care has to be taken, however, that the quinoline titer truly represents the minimum amount of catalyst poison. In most cases this type of base is adsorbed by inactive as well as active sites. Demonstration of indiscriminate adsorption is furnished by the titration results of Roman-ovskii et al. (52). These authors (Fig. 13) showed that introduction of a given dose of quinoline at 430°C in a stream of carrier gas caused the activity of Y-zeolite catalyst (as measured by cumene conversion) to drop with time, reach a minimum value, then slowly rise as quinoline was desorbed. The decrease in catalytic activity with time is direct evidence for the redistribution of initially adsorbed quinoline from inactive to active centers. We have observed similar behavior in carrying out catalytic titrations of amorphous and crystalline aluminosilicates with pyridine, quinoline, and lutidine isomers. In most cases, we found that the poisoning effectiveness of a given amine can be increased either by lengthening the time interval between pulse additions or by raising the sample temperature for a few minutes after each pulse addition. 

Two kinds of poison distributions must be distinguished. One distribution is that along the catalyst bed, the other one is within the porous system of the catalyst. It may be reasonably anticipated that under most conditions there will be a gradient of contaminant concentration which decreases in the direction from inlet to outlet also that there will be a decreasing concentration of contaminants from the outer confines of each separate catalyst body inwards into the pore system. The contaminant distribution will, however, differ for different types of catalysts and contaminants. 

Certain types of catalyst uranium oxide and chromium oxide may be used as a promoter. This is reported to give a higher resistance to catalyst poisoning by sulfur components and a lower tendency to form carbon deposits. 

There are a number of basic factors to be considered in polymerization in addition to process variables, types of catalyst, and catalyst poisons. Possibly the most important is that essentially all polymerization... 

This type of unit requires very efficient sulfur removal from the feed, as sulfur acts as a catalyst poison. Furthermore this catalyst is poisoned by the same materials which poison the Solid Phosphoric Acid type of catalyst, which contains no copper. 

Two reactions for which specific poisoning experiments have contributed to the elucidation of the reaction mechanisms and permit evaluation of the possibilities and pitfalls of the technique are discussed as examples in this section. The first example is the dehydration of alcohols on alumina catalysts, and the second, the isomerization of olefins on the same type of catalyst. 

Some catalysts suffer a different type of alkyne poisoning. Chlorotris(triphenylphosphine)rhodium(I) is an effective terminal alkyne polymerization catalyst. When this complex is used in the reduction of these alkynes, it gradually loses its activity because of the competing polymerization reaction. Even initially the rate of alkyne hydrogenation is much slower than that of the corresponding alkene because of the greater binding constant of the former substrate. 

The high-cost of materials and efficiency limitations that chemical fuel cells currently have is a topic of primaiy concern. For a fuel cell to be effective, strong acidic or alkaline solutions, high temperatures and pressures are needed. Most fuel cells use platinum as catalyst, which is expensive, limited in availability, and easily poisoned by carbon monoxide (CO), a by-product of many hydrogen production reactions in the fuel cell anode chamber. In proton exchange membrane (PEM) fuel cells, the type of fuel used dictates the appropriate type of catalyst needed. Within this context, tolerance to CO is an important issue. It has been shown that the PEM fuel cell performance drops significantly with a CO con-... 

While it is possible to treat an atmospheric residue from a Libyan crude direct, trace contaminants, particularly sodium, tend to poison the catalyst. However, this atmospheric residue can be vacuum-distilled to give a ca. 50 wt % yield of a 350°-550°C distillate containing 34 wt % wax, 72% of which is normal-paraffinic, and which is essentially free of catalyst poisons. This material can be catalytically dewaxed with Pt-H-mordenite in a ca. 70 wt % yield, resulting in a reduction of its pour-point from 115°F to values in the range 30° to 60 °F. This low-pour-point material can be blended back with the vacuum residue, which contains the catalyst poisons and only ca. 2 wt % n-parafEnic wax, to form a low-sulfur, low-pour-point fuel oil blending component (6). With this type of operation, catalyst activity can be maintained for long periods. 

The understanding of the interaction of S with bimetallic surfaces is a critical issue in two important areas of heterogeneous catalysis. On one hand, hydrocarbon reforming catalysts that combine noble and late-transition metals are very sensitive to sulphur poisoning [6,7]. For commercial reasons, there is a clear need to increase the lifetime of this type of catalysts. On the other hand. Mo- and W-based bimetallic catalysts are frequently used for hydrodesulphurization (HDS) processes in oil refineries [4,5,7,8]. In order to improve the quality of fuels and oil-derived feedstocks there is a general desire to enhance the activity of HDS catalysts. These facts have motivated many studies investigating the adsorption of S on well-defined bimetallic surfaces prepared by the deposition of a metal (Co, Ni, Cu, Ag, Au, Zn, A1 or Sn) onto a single-crystal face of anodier metal (Mo, Ru, Pt, W or Re) [9-29]. 

Venezia and co-workers also used bimetallic Pd-Au catalysts supported on ASA for the hydrogenation of aromatic compounds [208]. The simultaneous hydrogenation of toluene and naphthalene in the presence of dibenzothiophene was studied. This type of catalyst was shown to be resistant to sulphur poisoning (at 523 K, until 113 ppm S in the form of dibenzothiophene). 

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