Chlorocarbon surfaces

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

Much effort has gone into development of catalysts for photochemical reactions, initially with the objective of converting solar energy into storable fuels (typically H2 from the photolysis of water) but, more recently, mainly for the destruction of noxious pollutants such as chlorocarbons. There are two ways in which a catalyst may be involved in a photochemical reaction it may simply provide a surface on which the reactants can be adsorbed, so that, when a molecule of one reactant is activated by absorption of light, a molecule of the other is held in close proximity to facilitate reaction (a catalyzed photoreaction)-, or it may itself be excited by the absorption of light and then activate the adsorbed molecules (a sensitized photoreaction). The latter mode is the more relevant to the theme of this chapter, and is exemplified by the photocatalytic properties of titanium dioxide, Ti02-14 15... 

Coupons of Type 304 stainless steel were prepared by mechanical abrasion and rinsed with methanol. Each sample was analyzed by XPS prior to treatment to ensure that no detectable casually-introduced chlorine was present. Two separate series of laboratory experiments were done one series (a) followed the effects of short-term contact between chlorocarbon and the alloy surface, a second series (b) investigated the effects of prolonged vapor and liquid contact with the alloy in a glass refluxer. In series (a) the clean alloy surface was swabbed using trichloroethane-soaked tissue and immediately inserted into the vacuum chamber of an XPS spectrometer for analysis. After analysis, the same coupon was exposed to the atmosphere for periods of 72 and 336 hours... 

For computation of ASA the computer program SURFAC was used. It requires geometry of a molecule and van der Waals radii of every atom and the solvent moleculeas the main input. Standard tabulated values (Weast 1974) of van der Waals radii of atoms were used. For water, the radius used was 1.5A (Pearlman 1981). In order to calculate SA, the solvent radius is assumed to be zero (Bultsma 1980). For chlorophenols, the geometry, optimized by MNDO MO calculations (Gombar Richards 1985), was input to SURFAC whereas for chlorobenzenes and acyclic chlorocarbons, planar and fully staggered conformations, respectively, were assumed and standard angles and distances used to generate the cartesian co-ordinates. 

TABLE 4 Correlations between LC50 for guppy and grouped accessible surface area (ASA) contributions for chlorophenols, chlorobenzenes and acyclic chlorocarbons. 

Mechanisms Controlling Chlorocarbon Reduction at Iron Surfaces... 

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