Metals and Metal Oxides

From a practical standpoint, metal-metal friction is probably the most intensely studied system. In fact, the study of metal friction is in reality the study of two topics—metal friction and metal oxide friction—because except in very special (and seldom very practical) systems, a truly clean metal-metal contact is never encountered. In the best of situations, a normal metal surface will be covered with at least a monolayer of adsorbed gas molecules or other contaminants. As a result, frictional forces will be less than would be expected in their absence. 

For truly clean metal surfaces that have, for example, been treated by electron bombardment at elevated temperatures and ultrahigh vacuum, coefficients of friction in the range of 3-6 have been recorded. Sometimes, when good contact is achieved between clean surfaces of the same metal, the two surfaces will in fact weld or seize, to the point that the union is as strong as the bulk metal. For dissimilar metals, a similar result may be encountered if there exists a degree of mutual miscibility. If the two metals are mutually immiscible, there may or may not be such seizure. 

The behavior of metal surfaces in the presence of air will be quite different from the clean surfaces. When oxygen is present, all but the most noble of the metals will rapidly develop a layer of metal oxide on the surface. The oxide, then, will have frictional properties significantly different from those of the metal. It is found, for example, that a clean copper surface has a coefficient of friction of about 6.8. After exposure to air, the coefficient falls to the range of 0.8. 

In practice, then, the friction between unlubricated metal surfaces should probably be considered that between oxide layer or composite surfaces in which there are oxide-oxide, oxide-metal, and metal-metal components. If one considers only two of the three—metal-metal and oxide-oxide friction— one may estimate surface composition from the frictional force using the relationship 

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Chemisoq)tion bonding to metal and metal oxide surfaces has been treated extensively by quantum-mechanical methods. Somoijai and Bent [153] give a general discussion of the surface chemical bond, and some specific theoretical treatments are found in Refs. 154-157 see also a review by Hoffman [158]. One approach uses the variation method (see physical chemistry textbooks) ... 

Formaldehyde is readily reduced to methanol by hydrogen over many metal and metal oxide catalysts. It is oxidized to formic acid or carbon dioxide and water. The Cannizzaro reaction gives formic acid and methanol. Similarly, a vapor-phase Tischenko reaction is catalyzed by copper (34) and boric acid (38) to produce methyl formate ... 

Inorganic membranes (29,36) are generaUy more stable than their polymeric counterparts. Mechanical property data have not been definitive for good comparisons. IndustriaUy, tube bundle and honeycomb constmctions predominate with surface areas 20 to 200 m. Cross-flow is generaUy the preferred mode of operation. Packing densities are greater than 1000 /m. Porous ceramics, sintered metal, and metal oxides on porous carbon support... 

The radicals are then involved in oxidations such as formation of ketones (qv) from alcohols. Similar reactions are finding value in treatment of waste streams to reduce total oxidizable carbon and thus its chemical oxygen demand. These reactions normally are conducted in aqueous acid medium at pH 1—4 to minimize the catalytic decomposition of the hydrogen peroxide. More information on metal and metal oxide-catalyzed oxidation reactions (Milas oxidations) is available (4-7) (see also Photochemical technology, photocatalysis). 

A number of basic materials such as hydroxides, hydrides and amides of alkaline and alkaline earth metals and metal oxides such as zinc oxide and antimony oxide are useful catalysts for the reaction. Acid ester-exchange catalysts such as boric acid, p-toluene sulphonic acid and zinc chloride are less... 

This chapter presents detailed and thorough studies of chemical synthesis in three quite different chemical systems zinc ferrite, intermetallic, and metal oxide. In addition to different reaction types (oxide-oxide, metal-metal, and metal oxide), the systems have quite different heats of reaction. The oxide-oxide system has no heat of reaction, while the intermetallic has a significant, but modest, heat of reaction. The metal oxide system has a very large heat of reaction. The various observations appear to be consistent with the proposed conceptual models involving configuration, activation, mixing, and heating required to describe the mechanisms of shock-induced solid state chemistry. 

Leclercq, M., The Electrochemical Determination of the Proportions of Metal and Metal Oxide in Zinc-base Coatings Obtained by Hot Spraying , Anii -Corr. Methods Mater., 25 No. 9, 5 (1978)... 

Where minerals, metals, and metal oxides are picked up, they are, for the most part, undissolved and exist in colloidal or particulate form. When this matter is transported through the steam-condensate system, the result may be any of a range of pre-boiler section problems such as ... 

Surface adsorption reaction, which occurs on the surface of the metal and metal oxides ... 

Alkalis are the most important electropositive promoters of metal and metal oxide catalysts. They are used in many important industrial catalysts but are also quite suitable for fundamental studies since they can be easily introduced under vacuum conditions on well-characterized model metal surfaces. 

Many important industrial reactions rely on heterogeneous catalysts, yet there are inherent drawbacks to heterogeneous reactions. Such reactions occur only when the reactants contact the solid surface of the catalyst. Catalysis can be much more efficient when the catalyst is dissolved in the solvent where the reaction occurs. Unfortunately, the main catalysts used by industry, metals and metal oxides, are not soluble in traditional solvents. 

SPECTROSCOPY OF THE POTENTIAL ENERGY SURFACES FOR C-H AND C-O BOND ACTIVATION BY TRANSITION METAL AND METAL OXIDE CATIONS... 

Spectroscopy of the Potential Energy Surfaces for C-H AND C-O Bond Activation by Transition Metal and Metal Oxide Cations 331 By R. B. Metz... 

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

Surfactants and Colloids in Supercritical Fluids Because very few nonvolatile molecules are soluble in CO2, many types of hydrophilic or lipophilic species may be dispersed in the form of polymer latexes (e.g., polystyrene), microemulsions, macroemulsions, and inorganic suspensions of metals and metal oxides (Shah et al., op. cit.). The environmentally benign, nontoxic, and nonflammable fluids water and CO2 are the two most abundant and inexpensive solvents on earth. Fluorocarbon and hydrocarbon-based surfactants have been used to form reverse micelles, water-in-C02... 

Gong X-Q, Liu Z-P, Raval R, Hu P. 2003. A systematic study of CO oxidation on metals and metal oxides Density Functional Theory calculations. J Am Chem Soc 126 8-9. 

First there are the physical chemists, chemical engineers, and surface scientists, who study mainly nonpolar hydrocarbon reactions on clean and relatively clean metals and metal oxides. These have been the traditional studies formerly driven by the petroleum industry and now driven by environmental concerns. These workers typically treat the surface as a real entity composed of active sites (usually not identified, but believed in). These investigators typically, although not always, interpret mechanisms in terms of radical reactions on metals and in terms of acid-base reactions on metal oxides. 

An intimately powdered mixture, usually ignited by magnesium ribbon as a high-temperature fuse, reacts with an intense exotherm to produce molten iron and was used formerly (before the advent of gas or arc welding) in the commercial thermite welding process. Incendive particles have been produced by this reaction on impact between aluminium and rusty iron. (The term thermite reaction has now been extended to include many combinations of reducing metals and metal oxides) [1], Some accidents in demonstrating the thermite reaction are described [2],... 

The complexity and inhomogenicity of catalytic sites of metals and metal oxides make it difficult to interpret the mechanism of catalytic reactions on solid surfaces. Investigations that may lead to a better characterization of adsorbed species on catalytic sites could add much to our understanding of heterogeneous catalysis. 

Bio-Nanohybrids Based on Metal and Metal Oxide Nanoparticles... 

Mark A. Barteau is Robert L. Pigford Professor and Chair of the Department of Chemical Engineering at the University of Delaware. He received his B.S. degree from Washington University in 1976 and his M.S. (1977) and Ph.D. (1981) from Stanford University. His research area is chemical engineering with specialized interests in application of surface techniques to reactions on nonmetals, hydrocarbon and oxygenate chemistry on metals and metal oxides, scanning probe microscopies, and catalysis by metal oxides. 

Metals and metal oxides, as a rule, accelerate the liquid-phase oxidation of hydrocarbons. This acceleration is produced by the initiation of free radicals via catalytic decomposition of hydroperoxides or catalysis of the reaction of RH with dioxygen (see Chapter 10). In addition to the catalytic action, a solid powder of different compounds gives evidence of the inhibiting action [1-3]. Here are a few examples. The following metals in the form of a powder retard the autoxidation of a hydrocarbon mixture (fuel T-6, at T= 398 K) Mg, Mo, Ni, Nb V, W, and Zn [4,5]. The retarding action of the following compounds was described in the literature. 

Airborne particulate matter may be associated with many carcinogenic and other toxic agents. Hazardous materials include coal dust, fly-ash from power stations, metals and metal oxides from mining, extraction and refining and materials used as catalysts in industrial processes, as well as particulate matter from, for example, diesel exhausts. 

Tetenyi, P. Comparison of metals and metal-oxides from the viewpoint of their effect in effect in some reactions of hydrocarbons. Catal. Today 1993,17, 439 147. 

In the specific case of biomass gasification, several alkaline salts and heavy metals and metal oxides particles may act as additional poisons by enhancing the sintering of the Ni crystallites or by being adsorbed on the Ni sites [44]. While acid supports such as alumina react with alkali to form crystalline phases, basic supports (like MgO) do not react directly with them however, alkali causes coverage of the surface and plugging of the pores. 

Preparation. Industrially, cobalt is normally produced as a by-product from the production of copper, nickel and lead. The ore is roasted to form a mixture of metals and metal oxides. Treatment with sulphuric acid leaves metallic copper as a residue and dissolves out iron, cobalt and nickel as the sulphates. Iron is separated by precipitation with lime (CaO) while cobalt is produced as the hydroxide by precipitation with sodium hypochlorite. The trihydroxide Co(OH)3 is heated to form the oxide and then reduced with carbon (as charcoal) to form cobalt metal. 

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