Powder directed metal oxidation

Experiments with different ionol concentrations in oxidized T-6 with the copper powder showed another important peculiarity of the metal surface. The reaction of direct ionol oxidation by hydroperoxide on the catalyst surface was found to occur with the rate... 

This transformation is carried out by intimately mixing metal oxide powders with carbon, again as with the pure metals, at temperatures between 1500-2300 K, with or without the presence of a hydrocarbon gas. The reactions of oxides with carbon are thermodynamically favored, but high temperatures are again needed because the transformations are limited by diffusion. The direct transformation of oxides to carbides is economically advantageous over the use of metals since the need to separately reduce the oxide phases is avoided. Wide application is found for the commercial production of carbides of molybdenum, tungsten, and tantalum. 

The reduction of phosphates by carbon is a classical method, but the purity of phosphides obtained this way can be suspect. This also holds for replacement reactions, in which a redox process between a transition metal and a metal phosphide is used for the preparation of phosphides with high thermal stabihty. Binary main-group element and transition metal phosphides hke AIP, CrP, NbP, MoP, or WP can be prepared by the reaction of the powdered metals with a melt of hthium metaphosphate LiP03. Bulk samples of transition metal phosphides hke C02P or NiMoP can be obtained via reduction of metal oxide/phosphate mixtures in a mixture of 5% H2 in N2. MoP, WP, Fc2P, M2P, FeP, and RuP can be synthesized by direct reduction of the transition metal phosphates in hydrogen atmosphere between 670 and 1320 K. ... 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 jim in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fme oxide particles can be prepared from metals, alloys, and intermciallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-mcthoxycthanol at 473-523 K yields ultrafine ceria particles (ca 2 nm). 

This contribution focuses primarily on those metal oxides that are viewed as immediately relevant to electrocatalysis, but for additional information on the subject the reader is directed to a recent review article [2j]. It has been shown that after exposure to air, nanosized (e.g., 3-5 nm) colloidal Fe Co and Nl particles cannot be redispersed because they are immediately oxidized both in solution and in powder form. When the addition of oxygen is precisely controlled, e.g., by using stoichiometric amounts of O2 diluted in large excess of argon, an... 

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