Oxides defect sites

Wang L S, Nicholas J B, Dupuis M, Wu FI and Colson S D 1997 SijO (x = 1-6) models for oxidation of silicon surfaces and defect sites in bulk oxide materials Phys. Rev. Lett. 78 4450... 

Catalytic oxidations on the surface of oxidic materials usually proceed according to the Mars-Van Krevelen mechanism [P. Mars and D.W. van Krevelen, Chem. Eng. Sci. 3 (1954) 41], as illustrated in Fig. 9.17 for the case of CO oxidation. Instead of a surface reaction between CO and an adsorbed O atom, CO2 is formed by reaction between adsorbed CO and an O atom from the metal oxide lattice. The vacancy formed is filled in a separate reaction step, involving O2 activation, often on defect sites. 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

These results seem likely to be general—metal clusters on metal oxides are expected to be present predominantly at defect sites [33]. 

CO oxidation is a highly structure-sensitive reaction that needs steps and defect sites. MobUity of CO on the electrode surface does not seem to play a role on... 

An important route to solubilization of carbon nanotubes is to functionalize their surface to form groups that are more soluble in the desired solvent environment. It has been shown that acid treatment of nanotube bundles, particularly with HC1 or HNO3 at elevated temperatures, opens up the aggregate structure, reduces nanotube length, and facilitates dispersion (An et al., 2004 Kordas et al., 2006). Nitric acid treatment oxidizes the nanotubes at the defect sites of the outer graphene sheet, especially at the open ends (Hirsch, 2002 Alvaro et al., 2004), and creates carbonyl, carboxyl, and hydroxyl groups, which aid in their solubility in polar solvents. 

BET results are reported in Table 19.1. A decrease in the BET surface areas was observed with addition of Pt and Na to zirconia however, the surface areas remained well above 100 m2/g. The activation of reduced defect sites over zirconia has been suggested49 to occur via either (1) the formation of an oxygen vacancy defect or (2) the formation of a type II bridging OH group. The latter is interpreted to be the result of the dissociative adsorption of H20 at the oxygen vacancy sites, or from the dissociation and spillover of H2 from the metal to the oxide surface. TPR experiments49 demonstrated that 2% Pt addition shifted reduction peaks for the zirconia surface to <200°C. 

Adsorption of water is thought to occur mainly at steps and defects and is very common on polycrystalline surfaces, and hence the metal oxides are frequently covered with hydroxyl groups. On prolonged exposure, hydroxide formation may proceed into the bulk of the solid in certain cases as with very basic oxides such as BaO. The adsorption of water may either be a dissociative or nondissociative process and has been investigated on surfaces such as MgO, CaO, TiOz, and SrTi03.16 These studies illustrate the fact that water molecules react dissociatively with defect sites at very low water-vapor pressures (< 10 9 torr) and then with terrace sites at water-vapor pressures that exceed a threshold pressure. Hydroxyl groups will be further discussed in the context of Bronsted acids and Lewis bases. 

The ESR signal due to 02 demonstrates that the heterolytic activation of R-H has occurred. An interesting feature of the H-D exchange reactions over the MgO surface is the low activation energy, i.e., Ea 2 kcal/mol. This is lower than the gas phase for the reaction H + D2 —> 2 D + HD. The high activity of the ionic oxides has been attributed to the presence of basic sites and in particular to defect sites that are formed during the oxide preparation that persist even at elevated temperatures.41 42... 

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