Platinum catalysis shift

Although the mechanism of the platinum catalysis is by no means completely understood, chemists do know a lot about how it works. It is an example of a dual catalyst platinum metal on an alumina support. Platinum, a transition metal, is one of many metals known for its hydrogenation and dehydrogenation catalytic effects. Recently bimetallic platinum/rhenium catalysts are now the industry standard because they are more stable and have higher activity than platinum alone. Alumina is a good Lewis acid and as such easily isomerizes one carbocation to another through methyl shifts. 

The impurities usually found in raw hydrogen are CO2, CO, N2, H2O, CH, and higher hydrocarbons. Removal of these impurities by shift catalysis, H2S and CO2 removal, and the pressure-swing adsorption (PSA) process have been described (vide supra). Traces of oxygen in electrolytic hydrogen are usually removed on a palladium or platinum catalyst at room temperature. 

Fiirstner and coworkers developed a new Pt- and Au-catalyzed cycloisomerization of hydroxylated enynes 6/4-141 to give the bicylo[3.1.0]hexanone skeleton 6/4-143, which is found in a large number of terpenes [317]. It can be assumed that, in the case of the Pt-catalysis, a platinum carbene 6/4-142 is formed, which triggers an irreversible 1,2-hydrogen shift. The complexity of the product/substrate relationship can be increased by using a mixture of an alkynal and an allyl silane in the presence of PtCl2 to give 6/4-143 directly, in 55 % yield (Scheme 6/4.36). 

Metal chemical shifts have not found extensive use in relation to structural problems in catalysis. This is partially due to the relatively poor sensitivity of many (but not all) spin 1=1/2 metals. The most interesting exception concerns Pt, which is 33.7% abundant and possesses a relatively large magnetic moment. Platinum chemistry often serves as a model for the catalytically more useful palladium. Additionally, Pt NMR, has been used in connection with the hydrosilyla-tion and hydroformylation reactions. In the former area, Roy and Taylor [82] have prepared the catalysts Pt(SiCl2Me)2(l,5-COD) and [Pt()i-Cl)(SiCl2Me)(q -l,5-COD)]2 and used Pt methods (plus Si and NMR) to characterize these and related compounds. These represent the first stable alkene platinum silyl complexes and their reactions are thought to support the often-cited Chalk-Harrod hydrosilylation mechanism. 

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

There have been several recent studies of homogeneous catalysis of the water gas shift reaction (equation 7) by mononuclear and cluster catalysts, including mononuclear platinum complexes (15). 

There have been fewer reports on the particle size dependence of catalysis by platinum-catalyzed redox reactions. A report by Sharma et al. [21] showed that platinum colloidal nanoparticles do not demonstrate the same dependence on particle size as gold nanoparticles do for the reduction of hexacyanoferrate (III) by thiosulfate [19]. Platinum nanoparticles protected by sodium di(2-ethylhexyl) sulfosuccinate (synthesized by a reverse micelle technique) exhibit an optimum size ( 38 nm) for the reduction of ferricyanide by thiosulfate (Fig. 18.2). The reason for an optimum particle size is not fully understood however, they proposed the following explanation a shift in the Fermi level occurs as the diameter is increased. 

Metal NMR is interesting in catalysis because of the relation of the (spin) Knight shift with the Pauli susceptibility, which in turn is related to the local density of states at the Fermi energy at the site of the nucleus. In practice, a detailed analysis of spin-shift-related effects requires that the orbital shift be relatively weak. This is not the general case for transition metals, but fortunately it applies to the catalytically important metal platinum. This is the subject of the next section. 

C. H. Cheng, R. Eisenberg, Homogeneous catalysis of the water gas shift reaction using a platinum chloride-tin chloride system, J. Am. Chem. Soc. 100 (1978) 5968-5970. 

The strong interactions with metal ions extend to the use of metal-modified electrodes in electrocatalysis. Catalysis has been demonstrated with four systems. Chromium treatment results in as much as a 200 mV positive shift in the reduction peak for lO in acetate buffer This has been compared to the necessity for prior oxidation of the platinum electrode surface Ruthenium pretreatment of (SN), electrodes results in a catalytic current for the I / couple in phosphate buffer, pH 7.6. These electrodes also photoelectrochemically reduce protons to hydrogen at —0.05 V versus SCE in dilute sulfuric acid solution Molybdate treated electrodes have been used to electrochemically reduce acetylene at potentials of 1.5 V versus SCE in borate and hydroxide solutions. Iron treated electrodes show some ability to facilitate this reaction, but the rate is slower than with the molybdate treated electrodes... 

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