Oxidants, formation

Alonso C, Salvarezza R C, Vara J M and Arvia A J 1990 The meohanism of silver (I) oxide formation on polyorystalline silver in alkaline solution. Determination of nuoleation and growth rates Electrochim. Acta 35 489-96... 

Only thallium of the Group III elements is affected by air at room temperature and thalliumflll) oxide is slowly formed. All the elements, however, burn in air when strongly heated and, with the exception of gallium, form the oxide M2O3 gallium forms a mixed oxide of composition GaO. In addition to oxide formation, boron and aluminium react at high temperature with the nitrogen in the air to form nitrides (BN and AIN). 

Alternative approaches to nitric oxide formation include irradiation of air in a nuclear reactor (72) and the oxidation of ammonia to nitric oxide in a fuel cell generating energy (73). Both methods indicate some potential for commercial appHcation but require further study and development. 

Naphthalene (qv) from coal tar continued to be the feedstock of choice ia both the United States and Germany until the late 1950s, when a shortage of naphthalene coupled with the availabihty of xylenes from a burgeoning petrochemical industry forced many companies to use o-xylene [95-47-6] (8). Air oxidation of 90% pure o-xylene to phthaUc anhydride was commercialized ia 1946 (9,10). An advantage of o-xylene is the theoretical yield to phthaUc anhydride of 1.395 kg/kg. With naphthalene, two of the ten carbon atoms are lost to carbon oxide formation and at most a 1.157-kg/kg yield is possible. Although both are suitable feedstocks, o-xylene is overwhelmingly favored. Coal-tar naphthalene is used ia some cases, eg, where it is readily available from coke operations ia steel mills (see Steel). Naphthalene can be produced by hydrodealkylation of substituted naphthalenes from refinery operations (8), but no refinery-produced napthalene is used as feedstock. Alkyl naphthalenes can be converted directiy to phthaUc anhydride, but at low yields (11,12). 

Scrap that is unsuitable for recycling into products by the primary aluminum producers is used in the secondary aluminum industry for castings that have modest property requirements. Oxide formation and dross buildup are encountered in the secondary aluminum industry, and fluxes are employed to assist in the collection of dross and removal of inclusions and gas. Such fluxes are usually mixtures of sodium and potassium chlorides. Fumes and residues from these fluxes and treatment of dross are problems of environmental and economic importance, and efforts are made to reclaim both flux and metal values in the dross. 

Direct Oxidation of Propylene to Propylene Oxide. Comparison of ethylene (qv) and propylene gas-phase oxidation on supported silver and silver—gold catalysts shows propylene oxide formation to be 17 times slower than ethylene oxide (qv) formation and the CO2 formation in the propylene system to be six times faster, accounting for the lower selectivity to propylene oxide than for ethylene oxide. Increasing gold content in the catalyst results in increasing acrolein selectivity (198). In propylene oxidation a polymer forms on the catalyst surface that is oxidized to CO2 (199—201). Studies of propylene oxide oxidation to CO2 on a silver catalyst showed a rate oscillation, presumably owing to polymerization on the catalyst surface upon subsequent oxidation (202). 

The hterature suggests that more than one mechanism may be operative for a given antiozonant, and that different mechanisms may be appHcable to different types of antiozonants. All of the evidence, however, indicates that the scavenger mechanism is the most important. All antiozonants react with ozone at a much higher rate than does the mbber which they protect. The extremely high reactivity with ozone of/)-phenylenediamines, compared to other amines, is best explained by their unique abiUty to react ftee-tadicaHy. The chemistry of ozone—/)-PDA reactions is known in some detail (30,31). The first step is beheved to be the formation of an ozone—/)-PDA adduct (32), or in some cases a radical ion. Pour competing fates for dissociation of the initial adduct have been described amine oxide formation, side-chain oxidation, nitroxide radical formation, and amino radical formation. 

Electrochemical Process. Several patents claim that ethylene oxide is produced ia good yields ia addition to faradic quantities of substantially pure hydrogen when water and ethylene react ia an electrochemical cell to form ethylene oxide and hydrogen (206—208). The only raw materials that are utilized ia the ethylene oxide formation are ethylene, water, and electrical energy. The electrolyte is regenerated in situ ie, within the electrolytic cell. The addition of oxygen to the ethylene is activated by a catalyst such as elemental silver or its compounds at the anode or its vicinity (206). The common electrolytes used are water-soluble alkah metal phosphates, borates, sulfates, or chromates at ca 22—25°C (207). The process can be either batch or continuous (see Electrochemicalprocessing). 

The Schiff base intermediate (57) permits the oxidative formation of an imino intermediate which can then be converted to the 6a-methoxy derivative (Scheme 45) (76MI51100). 

If this energy is positive, the material is stable if negative, it will oxidise. The bar-chart of Fig. 21.1 shows the energies of oxide formation for our four categories of materials numerical values are given in Table 21.1. 

Describe the role of hydrocarbons in photochemical oxidant formation. 

Modulation Spectroscopy can be very usefiil in evaluating strains induced by growth (lattice-mismatched systems) or processing procedures, such as reactive-ion etching or oxide formation. The size and magnitude of the strain can be evaluated from the shifrs and splitdngs of various spectral lines, such as. ) or... 

The preparation of e/n-difluoro compounds by the oxidative fluorodesul-furization ot 1,3-dithiolanes readily proceeds by treatment with a pyridinium polyhydrogen fluoride-Af-halo compound reagent the latter serves as a bromonium ion source [2], l,3-Dibromo-5,5-dimethylhydantoin is the most effective of several At-halo oxidants. It is believed that /V-halo compounds combine with hydrogen fluoride to generate in situ halogen fluorides, the oxidants. Formation of gem-difluorides from dithiolanes derived from ketones is efficient and rapid, even at -78 °C, whereas the reaction of dithiolanes derived from aldehydes requires higher temperature (0 °C) (equation 4). 

For all three halates (in the absence of disproportionation) the preferred mode of decomposition depends, again, on both thermodynamic and kinetic considerations. Oxide formation tends to be favoured by the presence of a strongly polarizing cation (e.g. magnesium, transition-metal and lanthanide halates), whereas halide formation is observed for alkali-metal, alkaline- earth and silver halates. 

---