Showing papers on "Chemisorption published in 1970"

Low-temperature oxidation

Abstract: Low-temperature oxidation is a reaction, occurring at or below room temperature, between a solid and a gas. It usually involves the combination of oxygen with metals, and it has the greatest commercial impact in the presence of moisture, as in corrosion. Cabrera and Mott put forward a theory of low-temperature oxidation, based on the assumption that cation migration occurs under the influence of a potential built up across the growing oxide film. Recent experimental results require that this theory be expanded to explain recent observations such as anion migration during oxide growth and the transition from the initial chemisorbed monolayer to a bulk, threedimensional oxide. The additional ideas put forward in the present paper may be summarized as follows. Low-temperature oxidation is controlled by the nature of the oxide; whether it is a network former or a modifier. A period of fast, linear oxidation is followed by a slow logarithmic reaction whose rate, in turn, can increase if the oxide film crystallizes to form grain boundaries. The initial fast oxidation is a continuation of the chemisorption process. Place exchange (anions and cations interchanging positions) occurs when the energy due to the image force of an oxygen ion is greater than the bond energy holding the ion in place. A stable film forms when this bond energy is greater than the image force energy. The oxygen ions formed on the oxide surface then set up a potential across the film. This potential provides the driving force for continued reaction. Oxide growth during this later stage is a slow, logarithmic process. A barrier to ion transport exists at the gas-oxide interface in the case of anion migration and at the metal-oxide interface in the case of cation migration. In both cases, the field built up across the oxide lowers the barrier sufficiently so that ion migration can occur. Network modifiers allow cation migration. The reaction rate is sensitive to crystallographic orientation of the metal, but not to oxygen pressure. A constant voltage is maintained across the film, so that the Cabrera-Mott theory explains the logarithmic kinetics. Network-forming oxides allow onion migration. The number of anions, and hence, the rate of reaction, is sensitive to oxygen pressure, but not crystallographic orientation of the metal substrate. Since the potential is a result of the mobile anions, the film tends to grow under constant field. The logarithmic kinetics then must be explained by an increasing activation energy for ion transport, as proposed by Eley and Wilkinson. The logarithmic growth rate can be increased by the presence of water vapor if the water introduces “dangling” bonds into an oxide network structure. Crystallization of the oxide film also increases its rate of growth and results in the formation of oxide islands.

Adsorption of inert gases on tungsten: measurements on single crystal planes.

Abstract: The adsorption of Ar, Kr, and Xe on the (110), (120), (100), (211), and (111) planes of a tungsten field emitter was studied under conditions of immobile adsorption and also under conditions of surface equilibrium. The relative dipole moments and heats of binding could be determined, and it was found that both these quantities were largest on the (110) plane. A discussion in terms of charge transfer bonding is presented to rationalize these results. It was found that chemisorption of oxygen prevented the relatively strong first layer adsorption observed on the clean surface, and also the dipole moment associated with it.

Chemisorption of Carbon Monoxide on (110) and (100) Nickel Crystal Faces

Abstract: Adsorption equilibria, adsorption rates, and exchange rates of carbon monoxide have been investigated on nickel single‐crystal faces of the (110) and (100) orientation using radiotracer techniques. On the clean annealed surfaces, carbon monoxide is uniformly bound, is mobile, and occupies an area of 9 A2 in the saturated layer. The surface equilibria and the kinetic phenomena are both quantitatively accounted for by a theory utilizing the Stockmayer potential and a cell approximation for describing the lateral interaction of the adsorbed molecules. The ion‐bombarded nonannealed surfaces are unstable and heterogeneous. The contaminated surfaces adsorb only minute amounts of carbon monoxide. The chemisorption of CO thus provides a useful criterion for the degree of cleanliness of nickel surfaces.

Adsorption of carbon dioxide by magnesia studied by use of infrared and isotherm measurements

Abstract: The adsorption of carbon dioxide on magnesia (prepared by hydrolysis of magnesium methoxide) has been studied by measurements of the i.r. spectra of carbon dioxide which had been adsorbed at various temperatures, supplemented by gravimetric measurements of adsorption isotherms at temperatures ranging from –97·9 to 500 °C. Three types of adsorption were encountered: (I) a rapid physical adsorption; (II) a slow chemisorption involving the formation of a bidentate carbonate ion; and (III) a still slower chemisorption which resulted in the formation of a carbonate ion similar to that present in bulk magnesium carbonate. (I) and (II) occur at temperatures up to and including room temperature; and (II) and (III) at temperatures from that of the room upwards.

Surface chemistry of sulphides. Part 1.—Infra-red study of molybdenum and germanium sulphides and of their reaction with H2, H2O, thiophene and ethanethiol

Abstract: SH stretching vibrations on the surfaces of reduced GeSx(x≃ 1.4) and MoS2 have been observed in the infra-red domain of 2500–2700 cm–1. In GeSx, a single broad band at 2575 cm–1 shifts towards higher frequencies on heating to 200°C. In MoS2 two bands are observed at 2640 and 2500 cm–1; they decrease in intensity at increasing temperature. Chemisorption of an electron donor, such as H2O, on reduced GeSx produces a doubling of the SH band, with a high-frequency component above 2700 cm–1 and a low-frequency component around 2500 cm–1. The same observation has been made by adsorbing thiophene below 200°C. The study of the CH stretching region permits the assignment of the infra-red features in this temperature range to 2-point (or 4-point) attachment of thiophene, and eventually, to butene attached by the double bond. Above 200°C, in GeSx and from 20–400°C on MoS2, the infra-red spectra show the transformation of chemisorbed thiophene into 1- or 2-butene and into acetylenic and vinyledenic radicals. The same radicals are observed on adsorbing ethanethiol on GeSx. At low temperature, however, ethyl surface groups are probably more abundant. Conductivity measurements performed during the chemisorption of thiophene on GeSx support the conclusion that the adsorbate transfers electrons to the solid at low temperature.

Immobilization of Elastomers at the Carbon Black Particle Surface

Abstract: Determinations have been made of the amount of elastomer unextractable from unvulcanized masticated mixes with carbon black by a given solvent at boil, expressed as immobilized elastomer. Saturated and unsaturated elastomers varying in molecular weights from 2000 to 325,000, were used, while solvents of greatly differing boiling points and solvent power were employed. It could be shown that the bonding between elastomers and carbon black is not a simple adsorption, but involves a higher energy interaction, defined as chemisorption. At successively higher temperatures elastomer is increasingly removed from the carbon black surface. The temperature Tm, obtained by extrapolation of the linear relationship between amounts immobilized and temperature of extraction, represents the temperature theoretically required to eliminate all bonds between carbon black and elastomer and is therefore indicative of the bond strength. Data suggest the existence of a bonding energy spectrum. Upon graphitization, blac...

Chemisorption on Single Crystal Molybdenum (112) Surfaces

Abstract: Chemisorption trends established using LEED techniques have shown that, at least for tungsten and melybdenum, the most densely packed planes are not the most reactive. The body-centered cubic (110), (100), and (112) planes are the most densely packed in this system and they contain surface atoms in density ratios of 1.4:1.0:0.8. Adsorption studies on the molybdenum (112) plane were concerned mainly with O2, N2, H2, and CO. Oxygen was the most reactive gas on this particular surface and formed (2×1), (2×1)+(1×2), (1×2), (1×3), and C(4×2) patterns. The (2×1), (2×1)+(1×2), and (1×2) coverages were obtained upon exposing the sample at room temperature to oxygen while the latter two patterns were obtained by heating the sample after very long oxygen exposures. No adsorption was found upon room temperature exposure to nitrogen while room temperature hydrogen exposure gave rise to a (1×2) LEED pattern. Carbon monoxide adsorbed only in an amorphous state at room temperature. Slight heating caused ordering of this...