Group VIII metals, oxidation

We consider next perhaps the bet understood catalyzed reaction the oxidation of CO over group VIII metal catalysts. The reaction is an important environmental one since it involves the conversion of CO to CO2 in automobile catalytic converters. The mechanism is straightforward ... 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

G.L. Haller, and D.E. Resasco, Metal-Support Interaction Group VIII Metals and Reducible Oxides, Advances in Catalysis 36, 173-235 (1989). 

Bayer and Schretzmann 25) came to the conclusion that reversible oxygenation is a characteristic property of group VIII metals. However, work has shown that the cadmium complex CdEt2 can take up dioxygen reversibly in the ratio 1 2 (Cd O2). But it was found that the oxygenated complex (II(P) or 11(G) orientation) can undergo spontaneous catalytic oxidation to form bis(ethylperoxy) cadmium ... 

The selectivity of RNH2 on M/A1203 and Raney catalysts decreased in the order Co Ni Ru>Rh>Pd>Pt. This order corresponds to the opposite sequence of reducibility of metal-oxides [8] and standard reduction potentials of metalions [9], The difference between Group VIII metals in selectivity to amines can probably been explained by the difference in the electronic properties of d-bands of metals [3], It is interacting to note that the formation of secondary amine, i.e. the nucleophilic addition of primary amine on the intermediate imine can also take place on the Group VIII metal itself. Therefore, the properties of the metal d-band could affect the reactivity of the imine and its interaction with the amine. One could expect that an electron enrichment of the metal d-band will decrease the electron donation from the unsaturated -C=NH system, and the nucleophilic attack at the C atom by the amine [3], Correlation between selectivity of metals in nitrile hydrogenation and their electronic properties will be published elsewhere. 

It is a matter of speculation as to whether or not the activity would pass through a significant maximum at a surface composition between 0 and 30% Rh. It is interesting to note in this connection that the magnetic susceptibility (156, 157) and the electronic specific heat coefficient (156) increase from low values at 60% Ag-Pd through pure palladium and reach a maximum at - 5% Rh-Pd, thereafter decreasing smoothly to pure rhodium. Activity maxima have also been reported for reduced mixed oxides and supported alloys of group VIII metal pairs. For example, in the... 

The oxidation of CO by Oj over group VIII metal catalysts has been the subject of a large body of ultrahigh vacuum surface science and high pressure catalysis work due to its importance in pollution control. Currently, the removal of CO as CO2 from automobile exhaust is accomplished by catalytic converters which employ a supported Pt, Pd, and Rh catalyst. The importance of CO oxidation has led to numerous recent studies of the kinetics of this reaction on supported metal catalysts and transient kinetic studies on polycrystalline foils , which have sought to identify and quantify the parameters of the elementary mechanistic steps in CO oxidation. 

Catalysts. - Group VIII metals, conventional base metal catalysts (Ni, Co, and Fe) as well as noble metal catalysts (Pt, Ru, Rh, Pd) are active for the SR reaction. These are usually dispersed on various oxide supports. y-Alumina is widely used but a-alumina, magnesium aluminate, calcium aluminate, ceria, magnesia, pervoskites, and zirconia are also used as support materials. The following sections discuss the base metal and noble metal catalysts in detail, focusing on liquid hydrocarbon SR for fuel cell applications. 

Transition metals are used as catalysts for a variety of reactions hydrogenation, hydrogenolysis and isomerization of hydrocarbons (group VIII metals), oxidation of... 

Examples for electrochromic behavior upon electrochemical oxidation can be found among group VIII metal oxides. Thin films of transparent hydrated iridium oxide turn blue-black, whereas nickel oxide switches from pale green to brown-black, possibly due to the absorbance of Ni3+ centers [26]. The systems are much less thoroughly investigated and a detailed mechanistic explanation is not known. However, proton extraction and anion insertion have been suggested. 

Oxidation of Tetramethylethylene. Tetramethylethylene, TME, was an excellent model olefin since it was rapidly and selectively oxidized in the presence of many transition metal complexes (12). Oxidation of TME in the presence of the group VIII metal complexes [MCI(CO)-(Ph3P)2] (M = Rh, Ir) at 50°C gave two major products 2,3-dimethyl-2,3-epoxybutane, I, and 2,3-dimethyl-3-hydroxy-l-butene, II (Reaction 5). Reaction mixtures were homogeneous with no observable deposits of insoluble materials. Little oxidation occurred under these conditions in the absence of the metal complexes, but low yields of I and II were obtained in the presence of a radical initiator (Table I). Reactions were severely inhibited by hydroquinone. The ruthenium (II) complex, [RuCl2(Ph3P)3]2, also promoted efficient oxidation of TME yielding I... 

In work reported elsewhere (31) we have shown that the oxidation of styrene under mild conditions is promoted by many group VIII metal complexes. The product profile depends on the nature of the metal center and often differs from that observed when radical initiators are used (Table IX). Substantial quantities of styrene oxide are found in some cases but not in others (31). The epoxide which is formed, however, seems to arise via the co-oxidation of styrene and formaldehyde which is formed by oxidative cleavage of the double bond. Formaldehyde may be oxidized to performic acid or formylperoxy radicals which are efficient epoxidizing agents. Reactions of styrene with oxygen in the presence of group VIII complexes exhibit induction periods and are severely retarded by radical inhibitors (31). Thus, the initial step in the oxidation of styrene in the presence of the Ir(I),Rh(I), Ru(II), and Os(II) com-... 

The author thanks John O. Turner, who collaborated with him in early studies of the group VIII metal catalyzed oxidation of TME and styrene, Caroline Link for technical assistance, and Arthur Brown for iodometric titrations and experimental assistance. 

Hydroperoxo species intervene as reactive species in the Group VIII metal-catalyzed oxidation of alkenes by 02 or H202. 

As shown by Table 3, most of the Group VIII metal-peroxo complexes are obtained from the direct interaction of dioxygen with the corresponding reduced forms. A considerable effort has been devoted to this subject in the last decade with the hope that selective oxidations of hydrocarbons could be achieved by the activation of molecular oxygen under mild conditions12,56 133,184 and several such examples have actually been shown to occur. 

A large number of Group VIII metal-dioxygen complexes catalyze the oxidation of phosphine to phosphine oxide or isocyanides to isocyanates by molecular oxygen.6,12,56,140,141 146,184 However, their use as catalysts for the oxidation of alkenes generally leads to the same products as those obtained from free radical chain autoxidations.184,196-198... 

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