Halogens, surface chemistry

Aluminas are extensively used as catalysis and supports for catalytic materials. It is well known that the surface chemistry of adatoms may be influenced profoundly by addition on the alumina of minute amounts of electropositive or electronegative ions like alkalies or halogens (ref 1), Asa result the catalytic action of the active surface phase appears modified (refs 2-5), Such modiftcation may be due to (1) modified symmetry of the active sites, (2) different degree of dispersion of sites, or (3) altered surface coverage (refs. 6-9). Additives absorbed on the active sites of alumina may act as poisons. Such a poisoning action cun be expressed in formal terms, as a function of the concentration of added modifier. 

During this study, we have found that laser intensity is one of the important factors that control laser surface chemistry. At a small laser intensity, molecules adsorbed on solid surfaces dissociate into atoms and radicals. Some of these atoms or radicals react with atoms of the solid substrates. At a large laser intensity, atoms are photoablated from the solid surfaces to react with the molecules adsorbed or in the gas phase. Hence, we describe in this paragraph a) the dynamical study of UV laser photodissociation of halogen or metal-containing molecules on solid surfaces, b) reactions of atoms generated in the photodissociation of an adsorbate with solid surfaces, and c) reactions of molecules in the gas phase with the photoelectrons or metal atoms generated on intense laser irradiation of solid surfaces. 

Interfaces are tailored with molecules. Heterogeneous catalysts can be made from molecules that are known to be homogeneous catalysts by immobilizing them on the surfaces of solids. Surfaces can be made hydrophobic with halogenized silane derivatives. Molecules can act as antenna dyes in novel types of solar cells or in nanoelectronic materials for optical devices. Surface chemistry is molecular chemistry. 

Two main topics are selected (1) study of the surface chemistry of raw carbonaceous materials, their behavior during thermal treatment (pyrolysis) and oxidation (2) study of the modifications of surface chemistry after chemical treatments, such as nitrogenation or halogenation. 

The presence of ionizing radiation in the upper regions of the earth s atmosphere and the realization that atmospheric chemistry can occur on the surface of ice and dust particles have lead many authors to study on the interaction of LEE with molecular solids of ozone [203], HCl [236], and halogen-containing organic compounds [176,177,195-197,199-202,205,214,217,224-234] in an effort to shed new light on the problem of ozone depletion. In a recent series of experiments, Lu and Madey [297,298] found that the and CG yields... 

In short, the overall features of the chemistry involved with the massive destruction of ozone and formation of the ozone hole are now reasonably well understood and include as a key component heterogeneous reactions on the surfaces of polar stratospheric clouds and aerosols. However, there remain a number of questions relating to the details of the chemistry, including the microphysics of dehydration and denitrification, the kinetics and photochemistry of some of the C10x and BrOx species, and the nature of PSCs under various conditions. PSCs and aerosols, and their role in halogen and NOx chemistry, are discussed in more detail in the following section. 

Figure 12.43 summarizes the model-predicted relative importance of these cycles for conditions at 20-km altitude and 43.5°N, assuming total chlorine and bromine levels found in 1990 (Solomon et al., 1996). The importance of the HOx cycles, removal of NOx by the hydrolysis of N205, and the increased importance of halogen chemistry as the particle surface area available for heterogeneous reactions increase, are all illustrated. 

Hanson, D.R., and Ravishankara, A.R. (1993) Reactions of halogen species on ice surfaces, in The Tropospheric Chemistry erf Ozone in the Polar Regions, Ed. H. Niki and K.-H. Becker, NATO ASI Series Vol. 1-17, Springer-Verlag Berlin, Heidelberg, 281-290. 

Addition chemistry has developed into a promising tool for the modification and derivatization of the surface of nanotubes [24, 26], However, it is difficult to achieve chemoselectivity and regioselectivity control of addition reactions, requiring hot addends such as arynes, carbenes, radicals, nitrenes or halogens under drastic reaction conditions. 

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