Molybdenum surfaces, chemically modified

Much of surface science research to date has focussed on the physical and chemical properties of clean metal surfaces, a state of matter that is only obtainable under ultrahigh vacuum. However, under practical, real world conditions most metals are covered by an oxide layer or take the form of various compounds, eg. sulfide, carbide, etc. For the last several years my research group has investigated the properties of "chemically modified" molybdenum surfaces which serve as models for the surface of molybdenum compounds. Surfaces that are models for the oxides, carbides, sulfides, and borides of molybdenum are fabricated by the reaction,... 

The present paper reviews the physical and chemical evidence for the above rules obtained over the last several years from ultrahigh vacuum surface science studies of molybdenum single crystals chemically modified by 0, C, S, and B. Additionally, the results of recent studies of methylcyclopropane hydrogenolysis will be presented which illustrate the influence of surface acid/base sites on catalytic hydrocarbon conversions. The surface coverage of each modifier was determined by quantitative Auger electron spectroscopy or x-ray photoelectron spectroscopy (XPS). The atomic structure of oxygen, carbon, and sulfur adlayers below one monolayer (ML)... 

XPS measurements of chemically modified metal surfaces contain contributions from both surface and bulk metal atoms. The component due to surface molybdenum atoms was determined using a procedure established by Citrin et. al. (10) which is based on the changing contribution of surface and bulk components to the measured signal as a function of photoemission takeoff angle. The surface... 

The physical and chemical properties of the oxygen modified molybdenum surfaces described aboye indicate the formation of acidic sites with yariable strength and hard/soft character as a function of oxygen coyerage. The hydrogenolysis of methylcyclopropane (MCP) was inyestigated to probe the catalytic properties of these surfaces. A full account of this study will appear elsewhere (Touyell, M. S. Stair, P. C. J. Catal. submitted). 

The flotation process is applied on a large scale in the concentration of a wide variety of the ores of copper, lead, zinc, cobalt, nickel, tin, molybdenum, antimony, etc., which can be in the form of oxides, silicates, sulfides, or carbonates. It is also used to concentrate the so-called non-metallic minerals that are required in the chemical industry, such as CaF2, BaS04, sulfur, Ca3(P03)2, coal, etc. Flotation relies upon the selective conversion of water-wetted (hydrophilic) solids to non-wetted (hydrophobic) ones. This enables the latter to be separated if they are allowed to contact air bubbles in a flotation froth. If the surface of the solids to be floated does not possess the requisite hydrophobic characteristic, it must be made to acquire the required hydrophobicity by the interaction with, and adsorption of, specific chemical compounds known as collectors. In separations from complex mineral mixtures, additions of various modifying agents may be required, such as depressants, which help to keep selected minerals hydrophilic, or activators, which are used to reinforce the action of the collector. Each of these functions will be discussed in relation to the coordination chemistry involved in the interactions between the mineral surface and the chemical compound. 

The materials currently used in the production of medical devices include stainless steels, cobalt-base alloys, titanium-base alloys, platinum-base alloys, and nickel-titanium alloys. Steels were the first modern metallic alloys to be used in orthopedics and initial problems with corrosion were overcome by modifying the composition of the steel with the addition of carbon, chromium, and molybdenum. Carbon was added at low concentrations (ca. 0.03-0.08%) to initiate carbide formation, while the addition of chromium (17-19%) facilitated the formation of a stable surface oxide layer and the presence of molybdenum (2.0-3.0%) was found to control corrosion. The compositions of stainless steels used can vary widely. Table V shows the limits for the chemical compositions of three different alloys containing eleven different elements together with the mechanical properties for the samples after annealing and cold working. 

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