Oxygen-deficient sites

In general, it is accepted that recombination of electrons and holes, trapping of electrons by oxygen deficiency sites and a low mobility of the holes, cause a low conductivity and accordingly a low photoresponse for hematite. Electron mobility in the range 0.01 [60] to 0.1 cm2/V-s [17] has been reported. In the latter case, it was found that the electron mobility was independent of donor concentration. More recently, an electron mobility of about 0.1 cm2/V-s has been measured with doped single crystals and the mobility was also here independent of donor concentration [5]. A diffusion length of holes has been determined to be only of 2-4 nm [6], which is about 100 times lower than many other (III-V) oxides. 

Recent results showed that Au/Ce02 catalysts are subject to deactivation by poisoning or blockage of the active sites by either carbonates, formates or hydrocarbons [217,234,235]. These species appeared to be formed by CO and H2, and their formation was facilitated by oxygen-deficient sites on ceria [217]. However, regeneration of the WGS activity can be achieved by calcination 95% of the initial activity is recovered by heating in air at 673 K [217,234,235]. 

The next step is the insertion of a lattice oxygen into the allylic species. This creates oxide-deficient sites on the catalyst surface accompanied hy a reduction of the metal. The reduced catalyst is then reoxidized hy adsorbing molecular oxygen, which migrates to fill the oxide-deficient sites. Thus, the catalyst serves as a redox system. ... 

A mixture of LiOH and EMD is heated at 420 °C for 2-3 h in order to allow molten LiOH to penetrate into the pores of the EMD. The mixture is then heated from 650 to 800 °C to produce LiMn204. The amount of LiOH and EMD in the mixture must be stoichiometric (LiOH Mn02 = 1 2). The product, LiMn204, is usually tested by cyclic voltammetry (Fig. 22) a good LiMn204does not have peaks at a and b.(peak a (3.3 V) would be due to the oxygen deficiency and peak b (4.5 V) to replacement of the Li ion sites by Mn4+... 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

The most important of these are perovskite structure solids with a formula A2+b4+o3 that can be typified by BaCeC>3 and BaZrCV The way in which defects play a part in H+ conductivity can be illustrated by reference to BaCeCV BaCeC>3 is an insulating oxide when prepared in air. This is converted to an oxygen-deficient phase by doping the Ce4+ sites with trivalent M3+ ions (Sections 8.2 and 8.6). The addition of the lower valence ions is balanced by a population of vacancies. A simple substitution reaction might be formulated ... 

In the oxygen-deficient region, the predominant ionic defect is the oxygen vacancy, V". The charge neutrality in the solid is maintained by reduction of transition element in B-site to the lower valence state. This can be represented as [13] ... 

Environmental hazards are a function of the nature of the industrial site as well as a consequence of the work being performed there. They include (a) chemical exposure hazards, (b) fire and explosion hazards, (c) oxygen deficiency hazards, (d) ionizing radiation hazards, (e) biological... 

Another reaction of the flavylium cation has recently been demonstrated. " It involves concerted addition of compounds possessing a polarizable double bond on the electron-deficient site C-4 and the oxygen of the 5-hydroxyl group of the anthocyanin. The new pigments thus formed, showing a second pyran ring, have been referred to as vitisins, but the term pyranoanthocyanins proposed by Lu and Foo is preferred. 

When metal parts make contact, or when metals contact a nonmetallic part such as a gasket, the crevice formed at the point of contact can become a corrosion site. Corrosive attack at this site is usually more severe than the surrounding area due to factors such as oxygen deficiency, acidic changes, or inhibitor depletion. Using... 

Perovskites constitute an important class of inorganic solids and it would be instructive to survey the variety of defect structures exhibited by oxides of this family. Nonstoichiometry in perovskite oxides can arise from cation deficiency (in A or B site), oxygen deficiency or oxygen excess. Some intergrowth structures formed by oxides of perovskite and related structures were mentioned in the previous section and in this section we shall be mainly concerned with defect ordering and superstructures exhibited by these oxides. 

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