Catalysts platinum metals

Allyl Complexes. Allyl complexes of thorium have been known since the 1960s and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a thorium haUde and an aHyl grignard. This synthetic method was utilized to obtain a rare example of a naked aHyl complex, Th(Tj -C2H )4 [144564-74-9] which decomposes at 0°C. This complex, when supported on dehydroxylated y-alumina, is an outstanding heterogeneous catalyst for arene hydrogenation and rivals the most active platinum metal catalysts in activity (17,18). 

The mechanism of this strongly inhibited reaction has not yet been explained in detail, even with platinum, the most intensively studied of all catalysts 26>. The results obtained to date show that the course of the reaction is not the same on all catalysts and that other factors, such as oxygen absorption on platinum metal catalysts, play an important part. In most of the reaction mechanisms hitherto formulated and discussed H2O2 occurs as an intermediate stage 26> ... 

Organothorium complexes such as [Th(r 3-allyI )4] supported on dehydroxylated y-alumina have been shown to exhibit activities rivaling those of the most active platinum metal catalysts.123 Thorium maintains its original +4 oxidation states at all times that is, the mechanism does not follow the usual oxidative addition-reductive elimination pathway. Partially hydrogenated products cannot be detected... 

As it is clear that the higher the exchange current density, the lower the penetration depth and catalyst utilization, it is evident that for the more active platinum metal catalysts, an improvement of catalyst performance by using highly porous coatings is neither expected nor observed, as the normal coating roughness of 1 /im already corresponds to the penetration depth in a nanopore (82). 

Cadmium chromium(III) selenide can be prepared by the direct combination of the elements or by the reaction of cadmium selenide with chromium(III) selenide. Crystals of cadmium chro-mium(III) selenide have been prepared by flux growth1 in cadmium chloride, by vapor transport,2 and by a liquid-transport method3 with a platinum metal catalyst. The synthesis given... 

The hydrogenation of a,/ -unsaturatcd aldehydes to saturated aldehydes is readily achieved over most platinum metal catalysts under mild conditions. The selective synthesis of unsaturated alcohols is, however, much more difficult to perform and several attempts have been made to develop a suitable catalytic system. 

Particularly, the formation of C02 by the oxidation of CO over platinum metal catalysts is the reaction that has been studied most extensively in the past years and whose elementary steps appear to be best understood. This reaction is an important process in automobile catalytic converters [92] and its mechanism proceeds as follows [129] ... 

Olefins may undergo a facile double bond migration in the presence of hydrogen and a platinum metal catalyst. A relative order (palladium ruthenium > rhodium > platinum >> iridium) established (2) for... 

Uchida, H., S. Katoh and M. Watanabe (1998). Photocatalytic degradation of trichlorobenzene using immobilized Ti02 films containing poly(tetrafluoroethylene) and platinum metal catalyst. Electrochimica Acta, 43(14-15), 2111-2116. 

Platinum metal catalysts have been employed either in the form of unsupported fine particles of metal, usually referred to as blacks, or in the state supported on an inert... 

Unhindered simple olefins are usually rapidly hydrogenated under very mild conditions over platinum metal catalysts such as platinum, palladium, and rhodium as well as over active nickel catalysts such as Raney Ni, nickel boride, and Urushibara Ni. For example, 0.1 mol of cyclohexene is hydrogenated in 7 min over 0.05 g of Adams platinum oxide in ethanol at 25°C and 0.2-0.3 MPa H2 (eq. 3.1).5 1-Octene and cyclopentene (eq. 3.2) are hydrogenated in rates of 11.5 and 8.6 mmol (258 and 193 ml H2 at STP) g Ni 1-min 1, respectively, over P-1 Ni in ethanol at 25°C and 1 atm H2.18 Hydrogenation of cyclohexene over active Raney Ni proceeds at rates of 96-100 ml H2 at STP (4.3-4.5 mmol) g Ni min-1 in methanol at 25°C and 1 atm H2 49,50 and can be completed within a short time, although usually larger catalyst substrate ratios than required for platinum catalyzed hydrogenations are employed (eq. 3.3).50... 

Riesz and Weber compared the selectivities of commercial platinum, palladium, rhodium, and nickel catalysts for hydrogenation of linolenic components in soybean oil.110 Representative results are summarized in Table 3.8. Certain platinum metal catalysts showed higher selectivities than nickel catalysts, as indicated by the values of SL (A in Scheme 3.13) = 2.4-2.7. Generally, nickel catalysts showed selectivities... 

Over platinum metal catalysts, however, the hydrogenation of benzonitrile in the presence of butylamine forms benzylbutylamine, the amount of which was greatly dependent on the catalyst and solvent used, although the hydrogenation in the presence of diethylamine afforded no benzyldiethylamine.17... 

Hydrogenation of aliphatic nitriles over platinum metal catalysts often results in ex-tensive formation of secondary and tertiary amines. For example, hydrogenation of butyronitrile over Rh-C in water at 100-125°C and 2.76-4.14 MPa H2 gave a mixture of 51.5% of butylamine and 48.5% of dibutylamine, while in hydrogenation with addition of ammonia (167 mol% based on butyronitrile) dibutylamine was formed quantitatively. Under similar conditions the hydrogenations with Pd-C and Pt-C gave tributylamine in yields exceeding 95%.7... 

Hydrogenations in methanol tend to be poisoned by ammonia. It is noteworthy that, in contrast to the hydrogenation with nickel and cobalt catalysts, the yields of secondary and tertiary amines even increased in the presence of ammonia over these platinum metal catalysts. 

TABLE 9.4 Hydrogenation of Nitrobenzene over Platinum Metal Catalysts Produced In Situ by Borohydride Reduction 2 ... 

The benzoic acid prepared by an air oxidation of toluene contains small amounts of various compounds that decrease the catalytic activity of platinum metal catalysts. The benzoic acid purified by sublimation is hydrogenated much more rapidly than an unpurified one. On a large scale, however, treatment of commercial benzoic acid with Pd-C at 100-200°C under high hydrogen pressure in a solvent for hydrogenation or, better, treatment with 0.2-10% (for benzoic acid) of concentrated sulfuric acid at... 

Mochida et al. studied the hydrogenation of fluoranthene (39) over carbon-sup-ported platinum metal catalysts in decalin at elevated temperatures and pressures.267 Fluoranthene was hydrogenated much more readily than pyrene, and was highly selective for the formation of tetrahydrofluoranthene 40 over all the catalysts investi-... 

Scheme 13.20 Hydrogenolysis of phenylcyclopropane over platinum metal catalysts.

Degussa Also called BMA. The process by which this large German company is best known is its version of the Andrussov process for making hydrogen cyanide. Methane and ammonia are reacted in the absence of air, at approximately 1,400°C, over a platinum metal catalyst ... 

---