Platinum deactivation

High toxicities were obtained with very low lead coverages (0pb < 0.05), equal to about 20-50 atoms of platinum deactivated by one lead adatom. Such high toxicities cannot be explained neither by ensemble effects nor by ligand effects. A fast diffusion of lead adatoms on the platinum surface could account for this result. A plateau in activity is found, for medium lead coverages (0.05 < 0pb < 0.30-0.50) which could be ascribed to the formation oflead islands on the platinum surface. 

For the intimate (5m particle) composite, such control of the X-function results in the results shown in Fig. 16. The fresh platinum, under these conditions leads to complete hydrogenolysis of methylcyclopentane. Platinum deactivation controls the reaction, optimizing the isomerization products (benzene -p cyclohexane), until the generating step itself becomes insufficient. 

Jambunathan, K., Hillier, A.C. Scanning electrochemical microscopy of hydrogen electro-oxidation. Part II. Coverage and potential dependence of platinum deactivation by carbon monoxide. J. Electroanal. Chem. 2002, 524-525, 144-156. 

Hydrosdylation can also be initiated by a free-radical mechanism (227—229). A photochemical route uses photosensitizers such as peresters to generate radicals in the system. Unfortunately, the reaction is quite sluggish. In several apphcations, radiation is used in combination with platinum and an inhibitor to cure via hydro sdylation (230—232). The inhibitor is either destroyed or deactivated by uv radiation. 

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. 

Low pressure operation became routine with the appHcation of new catalysts that are resistant to deactivation and withstand the low pressures. The catalysts are bimetallic most incorporate rhenium as well as platinum (95). The stmctures of these catalysts are stiU not well understood, but under some conditions the two metals form small alloylike stmctures, which resist deactivation better than the monometallic catalyst. 

Catalyst Deactivation. Catalyst deactivation (45) by halogen degradation is a very difficult problem particularly for platinum (PGM) catalysts, which make up about 75% of the catalysts used for VOC destmction (10). The problem may weU He with the catalyst carrier or washcoat. Alumina, for example, a common washcoat, can react with a chlorinated hydrocarbon in a gas stream to form aluminum chloride which can then interact with the metal. Fluid-bed reactors have been used to offset catalyst deactivation but these are large and cosdy (45). 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

Hydrofining is applied to virgin naphthas mainly in the form of a pretreatment step for the feed to catalytic reformers (Powerforming). Sulfur levels of 5 parts per million (ppm) or less are required to avoid deactivation of the platinum reforming catalyst. 

Platinum, especially platinum oxide, has been used by many investigators (5), Platinum oxide, when used with aldehydes is apt to be deactivated before reduction is completed. Deactivation is inhibited by small amounts of ferrous or stannous chlorides (59,82). This type of promoter can also sharply curtail hydrogenolysis if it is a troublesome reaction (Rylander and Starrick, 1966). Deactivated systems can often be regenerated by shaking the reaction mixture with air (2,8,21 J3,96). The usefulness of this regenerative technique transcends aldehyde reductions it frequently is worth resorting to. 

Note that a similar situation arises in the study of heterogeneous deactivation of electron-excited molecules of N2. Thus, an opinion expressed by Clark et al. [152] states that the coefficients of heterogeneous deactivation of N2(A S, v = 0.1) for all surfaces are close to unity. On the other hand, Vidaud with his coworkers [59, 153] have obtained 3 10 2 and (1.8 + 1.2) 10 values for these coefficients shown by platinum and Pyrex, respectively. Tabachnik and Shub [154] investigated heterogeneous decay of NaC A SJJ ) molecules on a quartz surface by the method of bulk-luminescence spectroscopy. The authors carried out a series of experiments within a broad (about four orders of magnitude) range of active particle concentrations and arrived at a conclusion that at a concentration of N2( A 2 ) in excess of 10 mole/cm , the... 

Hydrogenation of 3,4-hexanedione was used to compare the behaviour of different supported platinum catalysts. The highest rate has been obtained over Pt/MCM-41 catalyst. It was the only catalysts, where the rate constant k2 exceeded ki, i.e., there was no catalyst deactivation during the catalytic run. It is... 

During hydrogenation of aldehydes, especially over platinum oxide, catalyst deactivation occurs. The reasons for this deactivation are not well understood and several theories exist.6... 

IRRAS spectrum collected at 0.4 V. In this way the build-up of C=Onds with adsorption time was monitored, as shown in Figure 3.36(b). From Figures 3.36(a) and (b) it is clear that the deactivation of the platinum electrode is closely related to the increase in C=Oads. 

By comparing the initial activities of I and E samples and the TEM results, it can be concluded that the Pt amount on the outer surface of E sample is at least 10 times lower than that on I sample. With Pt/Al203 alone or mixed with HMCM-22 zeolite, a very fast initial deactivation can be observed, followed by a plateau in activity. This behaviour suggests a very fast initial formation of carbonaceous compounds, most likely with the co-participation of adjacent platinum and acid sites. 

The plateau in activity (samples E and I) or the slow decrease (sample M) could be related to n-hexane transformation on the cups located on the outer surface of the crystals. A better balance in I sample between the active (hence located on the outer surface) platinum and acid sites explains its higher activity ( 1.5 times than that of the E sample) at the plateau. Furthermore, the longer distance between acid and metallic sites on sample M can explain both its lower activity and its faster deactivation. 

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