Platinum-silica catalysts catalytic activity

More than three decades ago, skeletal rearrangement processes using alkane or cycloalkane reactants were observed on platinum/charcoal catalysts (105) inasmuch as the charcoal support is inert, this can be taken as probably the first demonstration of the activity of metallic platinum as a catalyst for this type of reaction. At about the same time, similar types of catalytic conversions over chromium oxide catalysts were discovered (106, 107). Distinct from these reactions was the use of various types of acidic catalysts (including the well-known silica-alumina) for effecting skeletal reactions via carbonium ion mechanisms, and these led... 

Specifically, catalysts are typically in the form of a ceramic support carrying small amounts of metals such as chromium, nickel, or platinum. Alumina and silica are commonly used in the construction of the ceramic support. Die catalysts lose their activity progressively via various deactivation mechanisms (Pavel and Elvin, 1994). Tliermal regeneration is often employed for regaining catalytic activity, if applicable, but some of the particles break during this process. Once the catalyst particles become too small to be useful, they constitute a waste disposal problem, since catalysts may contain heavy metals that are considered hazardous, or other harmful components. 

Support. In multiphase catalysts, the active catalytic material is often present as the minor component dispersed upon a support sometimes called a carrier. The support may be catalyticaliy inert but it may contribute to the overall catalytic activity. Certain bifunctional catalysts ( 1.2.8) constitute an extreme example of this. In naming such a catalyst, the active component should be listed first, the support second and the two words or phrases should be separated by a solidus, for example, platinum/silica or platinum/silica-alumina. The solidus is sometimes replaced by the word on, for example, platinum on alumina. 

In addition to palladium, the catalysts used commercially always contain alkali salts, preferably potassium acetate. Additional activators include gold, cadmium, platinum, rhodium, barium, while supports such as silica, alumina, aluminosilicates or carbon are used. The catalysts remain in operation for several years but undergo deactivation. The drop in activity is due to a gradual sintering of the palladium particles which causes the catalytically active area to decrease progressively. Under reaction conditions potassium acetate is slowly lost from the catalyst and must continuously be replaced. 

Shown in Table 9.7 are some examples of incorporating catalysts into porous ceramic membranes. Both metal and oxide catalysts have been introduced to a variety of ceramic membranes (e.g., alumina, silica, Vycor glass and titania) to make them catalytically active. The impregnation/heat U eatment procedures do not appear to show a consistent cause-and-effeci relationship with the resulting membrane permeability. For example, no noticeable change is observed when platinum is impregnated into porous Vycor glass... 

The catalytic activation of CO2 and its reaction with C2H4 and H2O was studied over several silica-supported platinum-tin catalysts under different reaction conditions. The lactic acid production is related to the content of the PtSn alloy in the catalyst. 

Platinum-iron on alumina catalysts were characterized by Mbssbauer spectroscopy (Section 4) and their activity tested. Iron in clusters with high Pt Fe ratios, about 5, and fully combined with platinum, was catalytically inert for the CO-H2 synthesis reaction, attributed to a decrease in the electron density of the iron as indicated by the Mbssbauer isomer shift. The direction of electron transfer was opposite to that proposed for alkali-metal promoted iron catalysts. At low Pt Fe ratio, 0.1, ferromagnetic iron as well as Fe " ions and PtFe clusters were produced and dominated the activity/selectivity pattern. Rhodium on silica catalysts produced C2-compounds containing oxygen, specifically acetic acid, acetaldehyde and ethanol, with methane as the other major product. The addition of iron moved the C2-product formation sharply in favour of ethanol and now methanol was also formed. ... 

An aminated silica (13) was treated wiA HjPtCls and RhCl(PPh3)3 to give the aminated silica-supported platinum (13-Pt) and rhodium (13-Rh) catalysts, respectively. The metal content in the immobilized catalysts was 0.05 wt% for Pt and 0.35-0.49 wt% for Rh depending on the structure of amine moiety. The hydrosilyiation of 1-alkenes, allyl chloride, and allyl chloroacetate by HSiCl3 and HSifOEtj) was studied 13-Rh (NR2 = morpholino) retains its high catalytic activity even after nine times of use. ... 

Platinum catalysts have been the subject of several studies with that perspective. Using Pt on various soUds (silicas, alumina, with various porosities etc.), Praliaud et al. [15] found a very clear correlation between dispersion and catalytic activity, whatever the support and its morphology. The lower the dispersion, and thus the bigger the particle size (in the range 1-20 nm), the more active the catalyst The selectivity (i.e. the reaction pathway itself) was not affected by particle size. It is noteworthy that the excellent correlation with metal dispersion on the solid was not observed for particle size, determined by TEM. This is probably related to the low accuracy of the measurement of size by TEM, which is a local method, whereas dispersion measurement is done by global analysis, thus providing a perfect statistical average. 

High reaction temperatures in catalytic processes can lead to loss of active components by evaporation. This does not only occur with compounds that are known to be volatile (e. g., P2O5 in H3PO4, silica gel, HgCl2/activated carbon), but also by reaction of metals to give volatile oxides, chlorides, or carbonyls. In the oxidation of ammonia on Pt/Rh net catalysts (Ostwald nitric acid process), the catalyst reacts with the gas phase to form volatile Pt02- Furthermore, porous platinum growths are observed on the surface. This can be prevented by addition of rare earth oxides. 

The solid catalysts are usually of none noble metal catalysts and noble types. None-noble metal catalysts include base metals and zeolites. Nobel metal catalyst includes a variety of precious metals from the platinum group. In many cases, catalytically active metals are combined with a solid support such as alumina, silica-alumina, zeolites, carbon, etc. 

Studies on the use of complexes immobilized on quinoline-carboimine-functionalized FSM-16 mesoporous silica for Mizoroki-Heck-type reactions between methyl acrylate (1) and various aryl iodides highlighted the superior catalytic activity of a mthenium(III) catalyst compared with the corresponding platinum(IV) complex [44]. 

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