Surface oxygen vacancies

Equipped with surface models, one can determine those surfaces that are catalytically most active in that they proffer labile oxygen species. For the atomistic simulator this translates into the question which surface has the lowest oxygen vacancy formation energy Such direct interpretation is a view not necessarily held by the experimentalist as the simulator views the problem with respect to an atomistically perfect, pristine material. 

Calculations using such pristine models have been performed and revealed that the (110) and (310) surfaces are more reactive than (111). The result is perhaps intuitive because the coordination number of ions at the surface of (110) and (310) is lower than (111). Thus, simulation was able to offer an important prediction selectively expose the (110) or (310) surfaces to make a more reactive catalyst . However, one is left with a conundrum because the 

It is perhaps gratifying to note that the perceived simplicity of ceria surfaces, surface steps and surface vacancies, based upon inspection of the atomistic models, for example Fig. 5.2, are not that dissimilar to experimental images measured more recently. Fig. 5.3, using AFM.  

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The possible active species are OH radicals, the photo-produced holes (h+) as suggested by Draper and Fox [9], the surface oxygen vacancies or anions (02 ) suggested by Lu et al. [4], and chlorine radicals (Cl ) when chloroolefins (e. g. TCE) are present [1-3, 5, 6]. We may anticipate several possible behaviors for plots of photocatal5dic rate vs. kinetic variable ... 

M. Gillet, C. Lemire, E. Gillet and K. Aguir, The role of surface oxygen vacancies upon WO3 conductivity. Surf. Sci. 532-535, 519-525 (2003). 

Of great interest is the question of the role of trigonal aluminum, which is usually assumed to act as a LAS. Such a center should be quite typical of A1203, where it may appear as a result of surface oxygen vacancy formation. These vacancies may either develop due to dehydroxylation or be of a biographical nature. A similar situation may take place in the case of such mixed oxides as amorphous aluminosilicates. Uytterhoeven, Cristner, and Hall 123) have concluded that trigonal aluminum could also appear as a LAS upon dehydroxylation of H forms of zeolites. Their scheme was criticized, however, by Kiihl 124), who has undertaken X-ray fluorescence studies of the dehydroxylated forms of faujasites and found that the dehydroxylation was accompanied by dealumination of a zeolite framework with formation of extralattice aluminum which could also exhibit the Lewis acidity. 

In the first step the hydrocarbon is oxidized by lattice oxygen from the oxide-containing catalyst to create surface oxygen vacancies in the oxide. The second step involves filling the vacancies by oxidization of the reduced oxide surface with... 

The RAIRS study of this system [87] has probed the orientation of formate on Ti02(l 10)1x1, the substrate being prepared via sputter/UHV anneal cycles with a final anneal in 1x10" mbar of oxygen to minimise surface oxygen vacancies, p-polarised RAIRS measurements were recorded as a function of azimuthal incidence angle. Two bands were observed, one absorption the second transmission, which were assigned to the antisymmetric and symmetric vibrational modes of formate, respectively. These data evidence the presence of not one but two surface formate species (A and B). A, which is the majority species, is oriented such that its molecular plane is in the [001] azimuth, which... 

The extent of lattice oxygen incorporation into the decomposition products is linked to the surface redox properties of the oxide. Recent SSIMS and FT-RAIRS studies have provided initial evidence for the transient formation of oxygen vacancies on the surface of TiO2(110). Henderson has proposed that surface oxygen vacancies may explain the formation of trace amounts of formaldehyde from formic acid on Ti02(l 10) (figure 3) [41]. Both reactions (6) and (7) are proposed to occur below 500 K the water produced from formic acid exposure desorbed at 475 K, before the onset of formate decomposition to form CO. Formaldehyde and CO2 were produced in minor quantities relative to the production of CO [41]. Formaldehyde was formed from formic acid on reduced... 

The dark features ( B in Fig. 5 are less common ( 1 - 2% per surface unit cell) than the oxygen vacancies. They can extend over several unit cells. Upon adsorption of oxygen, the number of the dark spots stays constant within the statistical error. They were tentatively assigned as sub-surface oxygen vacancies [46], however, first-principles total-energy calculations show that such a configuration is highly unlikely [67]. Their nature is unclear at this point. 

Representing the surface perturbations arising from electron-electron interactions, surface oxygen vacancy sites, and changes in the interlayer spacing... 

Examination of Eqs. (35) to (41) does not reveal any parameter that is obviously dependent on flow rate, provided that the applied voltage is maintained constant and vacancy condensation dominates the induction time. Thus, neither the thermodynamics of absorption of X into a surface oxygen vacancy nor the ejection of a cation from the film is expected to depend on flow velocity, nor are the events (e.g., vacancy condensation) that occur at the metal/film interface expected to be sensitive to fluid motion. Thus, the PDM predicts that the breakdown ( pitting ) potential for passive alloys that are of interest to the thermal power industry should not be sensitive to flow rate. The PDM also predicts that the induction time should be insensitive to fluid flow velocity, provided that the induction period is dominated by vacancy condensation at the metal/film interface. 

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