Platinum-catalyzed oxidations mechanism

Mhadeshwar AB, Aghalayam P, Papavassihou V, Vlachos DG Surface reaction mechanism development for platinum-catalyzed oxidation of methane, Proc Combust Inst 29 997-1004, 2002. 

The platinum-catalyzed oxidation of methanol apparently proceeds through formation of formaldehyde and then carbon monoxide. The following mechanism is proposed for the oxidation of the carbon monoxide ... 

A mixture of Pt(ll) and metallic Pt in an aqueous medium was shown to oxidize ethane to yield acetic and glycolic acids. A series of deuterium-exchange processes enabled a complex mechanism to be elucidated metallic platinum catalyzes the oxidation of intermediate alcohols to acid products, whereas the Pt(ll) salt activates the initial alkene (Scheme 7X29... 

Experimental and theoretical studies on gold- and platinum-catalyzed reactions " explained the mechanism of this phenol synthesis. Oxepines and arene oxides have been observed in the reaction catalyzed by Au(III). ... 

The mechanism of platinum-catalyzed hydrosilylation was studied in detail by Chalk and Harrod (Scheme 18.10). The catalyticaUy active species [PtCl4] is obtained by a reduction of [PtClg] with isopropanol. One of the Cl ligands is then replaced with the olefin (A), whereby a 16 VE (VE = valence electron) r -olefin complex forms. Subsequent oxidative addition of external... 

Mechanism of Homogeneously Catalyzed Hydrosilylation. A mechanism analogous to the homogeneous hydrogenation reaction has been proposed by Chalk and Harrod (200) (Scheme 47) for the platinum catalyzed hydrosilylation. In the first step, oxidative addition of the silane occurs and leads to an hexa-coordinate intermediate complex 126, in which both silane and olefin are cis oriented. Hydrogen migration produces a o-alkyl complex 127 from which the hydrosilylation products 128 are obtained by reductive elimination. 

The Pt-catalyzed oxidation of NO to NOa is a well-known reaction [10], FTIR-studies have shown that with NO surface nitrate species can be formed on alumina supports. The presence of platinum promoted the surface nitrate formation [11]. Furthermore, Burch and Watling concluded from steady-state kinetic studies that Pt-NOa species take part in the mechanism for the selective reduction of NO with propane, being a highly abundant species on the platinum surface [6]. Thus we ascribe the gap in the i N-balance to the formation of Pt-i NOa species (9) and to an eventual spill-over of i NOa to the alumina surface. 

The individual steps in the mechanism proposed for the ruthenium catalyzed oxidation of triphenylphosphine are similar to those of the platinum catalyzed reaction. However, the stoichiometry of one step and the relative rates of several other steps appear to be somewhat different. The observed rate laws are quite different from those of the platimun complex. The rate equations for the proposed mechanism are shovm in equation (71) for the dioxygen complex and equation (72) for the carbonyl complex (RuOj = [Ru(NCSXN0XPPh3)j(02)] RuCO = [Ru(NCSXCOXNOXPPh3)2]). 

Several mechanisms have been proposed for the platinum-catalyzed homogeneous hydrosilylation reaction. The most commonly invoked mechanism, proposed by Chalk and Harrod in 1965, consists of elementary steps similar to homogeneous hydrogenation, oxidative addition, migratory insertion, and reductive elimination (226). However, this mechanism fails to describe the indnction period or the presence of colloidal species at the end of the reaction. Lewis proposed an alternative mechanism based on the intermediacy of colloids that were detected by transmission electron microscopy after evaporation of catalsdically active solutions (227,228). 

Hydrosilylation reactions are also catalyzed by some complexes where the metal is not platinum. A general mechanism for the hydrosilylation reaction is shown in Figure 5.8. It involves the oxidative addition of HSiRj to a metal center M, followed by insertion of the alkene into the M-H bond. 

CO oxidation, an important step in automotive exhaust catalysis, is relatively simple and has been the subject of numerous fundamental studies. The reaction is catalyzed by noble metals such as platinum, palladium, rhodium, iridium, and even by gold, provided the gold particles are very small. We will assume that the oxidation on such catalysts proceeds through a mechanism in which adsorbed CO, O and CO2 are equilibrated with the gas phase, i.e. that we can use the quasi-equilibrium approximation. 

Cg Dehydrocyclization. Arguments have been put forward that primary ring closure produces six-membered rings over three important catalyst types oxides, supported platinum, and bimetallic catalysts (107). The postulation of metal catalyzed Cg ring closure does not involve any definite suggestion whether its mechanism is direct or stepwise. ... 

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