Surface and oxidation

It is well known that the flotation of sulphides is an electrochemical process, and the adsorption of collectors on the surface of mineral results from the electrons transfer between the mineral surface and the oxidation-reduction composition in the pulp. According to the electrochemical principles and the semiconductor energy band theories, we know that this kind of electron transfer process is decided by electronic structure of the mineral surface and oxidation-reduction activity of the reagent. In this chapter, the flotation mechanism and electron transferring mechanism between a mineral and a reagent will be discussed in the light of the quantum chemistry calculation and the density fimction theory (DFT) as tools. 

Air oxidation layers on arsenopyrite are much thinner than layers produced from oxidizing mine waste-waters (Nesbitt and Muir, 1998, 141). The air-oxidized layers also contain Fe(III) hydroxides, Fe(III) arsenates, and Fe(III) arsenites, which are noticeably different than the Fe(III) oxyhydroxides that form in water-oxidized layers (Nesbitt and Muir, 1998, 141). An arsenopyrite sample from the Halen Mine of Wawa, Ontario, Canada, oxidized in air during 25 years of storage at the University of Western Ontario. The Fe As ratio of the arsenopyrite surfaces increased from 1 0.93 to 1 12.1 during the 25 years of air oxidation. Nesbitt and Muir (1998, 141-142) concluded that the increased ratio resulted from As(0) diffusing from the interior of the sample to its surfaces and oxidizing to As(III) and As(V). 

The oxide layer that forms on aluminum is more complex than with other metal substrates. Aluminum is a very reactive surface, and oxide forms almost instantaneously when a freshly machined aluminum surface is exposed to the atmosphere. Fortunately, the oxide is extremely stable, and it adheres to the base metal with strength higher than could be provided by most adhesives. The oxide is also cohesively strong and electrically nonconductive. These surface characteristics make aluminum a desirable metal for adhesive bonding, and they are the reasons why many adhesive comparisons and studies are done with aluminum substrates. 

In addition to the researches on oxygen adsorption at surfaces mentioned in the introduction, DIET has contributed significantly to the study of non-stoichio-metric and stoichiometric oxides surfaces, as well as the study of the reactivity of oxygen with oxidised metals and semiconductors. However, we shall restrict the scope of this review to oxidised surfaces and oxides. Henrich and Cox have reviewed the oxide surfaces up to 1993, which includes a chapter on ESD and PSD [108]. 

Studies of single-crystal surfaces under UHV conditions have allowed us to quantify fundamental interactions at surfaces, and the majority of surface-science studies have been conducted in this manner. Utilization of XPD and LEIS techniques require the studies to be conducted under high vacuum, and studies of clean surfaces or precisely controlled adsorbate layers require UHV conditions. Here we discuss a few examples of the use of these two techniques in studies of single-crystal surfaces, illustrating their power and limitations. The surfaces discussed are metal surfaces that contain controlled amounts of adsorbates, ultrathin metal films, two-component metal alloy surfaces, and oxide surfaces. 

Corrosion is fundamentally a chemical reaction between a metal and its environment. As such it is a heterogeneous reaction between a fluid and a solid. At higher temperatures (when the environment is a gas rather than a liquid), the reaction is typically a direct reaction between oxygen and the metal to form the metal oxide. The oxide will form as a solid on the metal surface," and oxidation will be controlled by the transport of oxygen and metal ions through the corrosion product. 

It appears that TiO2 particles in water are widely hydroxylated. Therefore holes escaping annihilation may migrate to the surface and oxidize adsorbed water molecules and hydroxyl ions. Oxygen adsorbed at the surface captures electrons preventing their recombination with h and therefore favouring the yield of OH radical (Fig. 5). 

Most PAHs in the atmosphere are associated with particulates (Baek et al. 1991). Vu-Duc and Huynh (1991) describe two types of chemical reactions that appear to be the predominant mode of transformation of these PAHs (1) reactions between PAHs adsorbed on the particle surfaces and oxidant gases like NO2, O, and SO3 that do not appear to be influenced by exposure to UV irradiation and (2) photooxidation of PAHs irradiated either under solar radiation or simulated sunlight which produces a variety of oxidized derivatives such as quinones, ketones, or acids. Kamens et al. (1990) estimate that, even in highly polluted air, photolysis is the most important factor in the decay of particle-sorbed PAHs in the atmosphere, followed by reaction with NO2, N2O5, and HNO3. 

In addition to the statistical nature of the interface instabilities active in diamond CVD, the orientation effect and anisotropic growth of crystals (i.e., evolutionary selection) play an important role in the observed instability phenomenon. Surface chemical reactions that occur preferentially between the growing diamond surface and oxidizing species in the combustion synthesis ambient also influence the development of the microstructure and morphology of crystals in diamond films. For example, in combustion CVD,... 

The development of such cracks is frequently accelerated by the environment to which the plastic is exposed. The initiating agent can be chemical exposure, heat exposure, surface-active agents, oils, greases, solvents (particularly those that cause softening or swelling of the plastic s surface), and oxidation. The stress crack can... 

The effect of surface orientation can be demonstrated very clearly when the aqueous phase contains accelerators of rancidity. Hughes and Rideal (1933) spread monolayers of oleic acid on the surface of aqueous solutions of potassium permanganate. When the double bond touched the water, oxidation proceeded very rapidly. In more compressed films, the reactive double bonds were removed from the water surface and oxidation was greatly retarded. Eleostearic acid, containing three conjugated double bonds, lay flat on the surface and oxidized very rapidly. Haurowitz and Schwerin (1941) called attention to the importance of interfacial orientation of linoleic acid at the oil water interface in the hemin-catalyzed oxidation of this fatty acid. 

Brundle, C.R., Chuang, T.J., Wandelt, K. Core and valence level photoemission studies of iron-oxide surfaces and oxidation of iron. Surf. Sci. 68, 459-468 (1977)... 

Metal cluster complexes were proposed to introduce mainly transition metals between the clay layers. Niobium, tantalum and molybdenum chlorides were exchanged on the clay surface and oxidized at 513 K under vacuum to transform... 

XPS has been applied to study the composition and formation of passivating layers on various pure metals and binary alloys. Usually, a multilayer structure is found with the lower valent species in the inner and the higher valent species in the outer part of the film. Generally, hydroxides are located at the surface and oxides at the inner part of these layers. The distribution and accumulation of cations within these films are characteristic for the alloy components and are closely related to their contribution to the improvement of passivity of these metals (Strehblow, 1997). XPS is a valuable tool to detect the role of alloy components and to understand their influence on the corrosion properties of metals. 

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