Comer defects

It was found that nanosize Ti02 particles experience an adjustment in the coordination geometry of the Ti atoms near the particle surface from octahedral to square-pyramidal in order to accommodate the large surface curvature [57]. X-ray absorption near edge structure reveals that surface modification with enediol ligands (ascorbate, ortho-hydroxy cyclobutene dione, catechol, etc.) restores the pre-edge features of octahedrally coordinated Ti in the anatase crystal environment. Specific binding of the enediol modifiers to surface comer defects ... 

As the calculated adsorption energy of acetylene on regular 0 sites of (001) terraces is notably lower than that on edge and comer defects, adsorbates at terraces should desorb already at lower temperature. Therefore, IR bands related to C2H2 at edge and comer sites can be observed if the concentration of the latter sites in an MgO sample is sufficiently high. 

Figure 13. Oxygen Is photoemission spectra (left panel) and oxygen 2s plus valence band photoemission spectra (right panel) of MgO(lOO) after sequential interactions with water vapor (3 min exposure) at different p(H20) (reported in torr to left of each spectrum) showing growth of low kinetic energy feature at 81 eV in the Is spectra and at 47 and 60 eV in the 2s + VB spectra. Also shown are Is and 02s + VB spectra of MgO(lOO) after immersion in liquid water. The 2s + VB spectrum of brucite is also shown. A schematic view of the MgO (100) surface is shown on the left before and after reaction with water vapor. Edge and comer defects are shown together with a vacancy, which are the most reactive sites on the MgO (100) surface. The structure of bmcite is shown at the upper left. These data were taken on SSRL beam line 10-1. (after Liu et al. 1998a)...

Investigating the role of the surface structures (hydroxyl groups, Lewis acid sites, basic sites, and combinations thereof) and their location (faces, edges, comers, defects) in determining the dispersion and the structure of the supported species in alumina-supported catalysts. Also the nature of the anchoring bonds between surface species (e.g., metal particles) and the support should be clarified. 

For example, let us consider the case of a planar piece containing a planar defect located near to the surface opposite to the probe (back of the specimen). In such a case, the defect may produce two diffracted echoes arising from the defect tips and supplementary echoes, the so-called comer echoes, that involve two successive reflections one on the... 

Schematic representation of defect clusters in Fei- jO. The normal NaCl-type structure (a) has Fe (small open circles) and O (large dark circles) at alternate comers of the cube. In the 4 1 cluster (h), four octahedral Fe" sites are left vacant and an Fe" ion (grey) occupies the cube centre, thus being tetrahedrally coordinated by the 40. In (c) a more extended 13 4 cluster is shown in which, again, all anion sites are occupied but the 13 octahedral Fe sites are vacant and four Fe occupy a tetrahedral array of cube centres.

Beta radiation Electron emission from unstable nuclei, 26,30,528 Binary molecular compound, 41-42,190 Binding energy Energy equivalent of the mass defect measure of nuclear stability, 522,523 Bismuth (m) sulfide, 540 Blassie, Michael, 629 Blind staggers, 574 Blister copper, 539 Blood alcohol concentrations, 43t Body-centered cubic cell (BCC) A cubic unit cell with an atom at each comer and one at the center, 246 Bohrmodd Model of the hydrogen atom... 

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. 

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