Platinum catalysts sulfur poisoning

These chemisorbed sulfur species on platinum surfaces are very stable and so are not easily detached. As a result, the presence of SO2 causes permanent damage to the catalyst [46-48]. The introduction of sulfur-containing species in a continuous manner and/or at high concentration was shown to irreversibly damage the catalysts [49,50]. The rate at which a platinum catalyst is poisoned depends strongly on the SO2 concentration in the bulk gas... 

R SiH and CH2= CHR interact with both PtL and PtL 1. Complexing or chelating ligands such as phosphines and sulfur complexes are exceUent inhibitors, but often form such stable complexes that they act as poisons and prevent cute even at elevated temperatures. Unsaturated organic compounds are preferred, such as acetylenic alcohols, acetylene dicarboxylates, maleates, fumarates, eneynes, and azo compounds (178—189). An alternative concept has been the encapsulation of the platinum catalysts with either cyclodextrin or in thermoplastics or siUcones (190—192). 

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

The precious-metal platinum catalysts were primarily developed in the 1960s for operation at temperatures between about 200 and 300°C (1,38,44). However, because of sensitivity to poisons, these catalysts are unsuitable for many combustion apphcations. Variations in sulfur levels of as Httle as 0.4 ppm can shift the catalyst required temperature window completely out of a system s operating temperature range (44). Additionally, operation withHquid fuels is further compHcated by the potential for deposition of ammonium sulfate salts within the pores of the catalyst (44). These low temperature catalysts exhibit NO conversion that rises with increasing temperature, then rapidly drops off, as oxidation of ammonia to nitrogen oxides begins to dominate the reaction (see Fig. 7). 

Once the reaction conditions have been decided upon, the feed preparation steps and product purification steps must be determined. The designer must decide how much of which compounds must be removed from the feed and product streams. The latter has already been set by the product composition specified in the scope. The former is often determined by how the impurities affect the reaction. For instance, when platinum catalysts are used all sulfur and heavy metals must be removed or this very expensive catalyst will be poisoned. 

Catalyst poisoning. The more or less permanent deactivation of a catalyst by chemical reaction with a contaminant. Sulfur will poison platinum catalysts vanadium will poison zeolyte catalysts. 

Figure 12. Transient HCN yield over platinum catalysts. A and B represent one experiment (Exp. 1) in which a sulfur-poisoned catalyst was regenerated on admission of 1% O2 into the inlet gas mixture between times t, and C shows the resistance of the catalyst to poisoning by SO when oxygen is simultaneously present in the inlet gas mixture. At t only oxygen is removed from the inlet gas mixture. (See Ref. 16 for details.)...

Platinum was found to be the most efficient hydrogenating component for the isomerization catalyst from the standpoint of amount required and resistance to sulfur poisoning. 

In the catalytic reforming of naphthas there are a number of nonhydrocarbon materials which play an important part in the performance of the catalyst. Sulfur is a poison for the reforming catalyst. There appears to be evidence developing that the platinum-rhenium catalysts may be more sensitive to sulfur than the conventional catalysts. Effective pretreatment of the feed stock to maintain sulfur at low levels is desirable. 

The chief advantages of the contact process are the high purity of the product and the fact that the product is a concentrated acid. Disadvantages are the high cost of the catalysts and the fact that if sulfides are used as raw materials, costly purification of the sulfur dioxide is necessary, because impurities such as arsenic trioxide and selenium dioxide poison the catalyst (i.e., render the catalyst inactive). Platinum catalysts are particularly sensitive to these impurities, while vanadium catalysts are claimed to be free from this disadvantage. 

Such electronic transfer induced by sulfur adsorption was also pointed out by using cinnamic acid as a probe molecule (48). The UV-visible reflexion spectra of adsorbed cinnamic acid on nonpoisoned and partly poisoned platinum catalysts shows that adsorption on pure platinum induces a shift of the peaks toward the higher wavelengths and an appearance of fine structure. Sulfurization of platinum induces a further enhancement of higher wavelength peaks. Binding energy of cinnamic acid is thus increased by sulfur adsorption on Pt catalysts. 

A desulfurization step is necessary in the reforming of natural gas because natural gas is odorized with a sulfur-containing substance - usually methyl mercaptan. This step will prevent damage to (i.e., deactivation of) the nickel or platinum catalysts in downstream processes. Some of these feed streams may also contain chlorides that are not only poisons to the downstream catalysts, but they may also contribute to stress corrosion in the reactors or piping. 

Zeolite-based hydrogenation catalysts containing platinum and palladium have increased resistance toward sulfur poisoning (101-104), and a higher activity (95, 105) than many other supports. In recent years there has been some effort devoted to attempt to explain this phenomenon. Although there is general agreement that the catalytic surface of the zeolites most probably... 

The composition of a reforming catalyst is dictated by the composition of the feedstock and the desired reformate. The catalysts used are principally molybdena-alumina, chromia-alumina, or platinum on a silica-alumina or alumina base. The nonplatinum catalysts are widely used in regenerative process for feeds containing, for example, sulfur, which poisons platinum catalysts, although pretreatment processes (e.g., hydrodesulfurization) may permit platinum catalysts to be employed. 

Catalytic reduction in the presence of platinum or palladium is a very mild method for splitting trityl ethers. The products are an alcohol and tritane. The latter may be separated from the alcohol by taking advantage of its solubility in petroleum ether. If the trityl ether contains sulfur, poisoning of the platinum or palladium catalyst may occur. ... 

The high theoretical efficiency of a fuel cell is substantially reduced by the finite rate of dynamic processes at various locations in the cell. Substantial efficiency losses at typical operating temperatures occur already in the anodic and cathodic catalyst layers due to the low intrinsic reaction rates of the oxygen reduction and, in the case of the DMFC, of the methanol oxidation reaction. (The catalytic oxidation of hydrogen with platinum catalysts is very fast and thus does not limit PEFC performance.) In addition, at low temperatures, turnover may be limited by noble metal catalyst poisoning due to sulfur... 

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