Surface F centres

Size-selected palladium atoms were deposited on an in. sv /M-prepared MgO(lOO) thin film at 90 K the palladium surface concentration was about 1% of a monolayer. Comparison of ab initio calculations and FTIR studies of CO adsorption provided evidence for single Pd atoms bond to F centres of the MgO support with two CO molecules attached to each palladium atom.24... 

The kinetics of these processes in the time range 102-106s is well described by equations of the type illustrated by eqn. (30) of Chap. 4 [69, 70]. With a decrease in temperature from 77 to 4.2 K the rate of recombination of electron and hole centres decreases by less than a factor of 100, which corresponds to a formal activation energy of less than 40 cal mol i.e. the reaction proceeds practically without activation. In the absence of contact between the F+-centres stabilized in the volume and the Yr-centres stabilized on the surface these data point to the tunneling mechanism of recom-... 

The authors of ref. 72 have detected and studied the decay of F +-centres located in the bulk of MgO by reaction at 4.2 and 77 K with particles of the electron acceptor N20 adsorbed on the surface of MgO... 

With increasing extent of coverage of the surface with N20 molecules, the rate of decay of F+ -centres at 77 K has been found to rise. Since reaction (20) proceeds even at T = 4.2 K and no direct contact of the F+-centre, located in the bulk of MgO, with the adsorbed N20 seems to be possible, the most probable mechanism of the reaction in question is electron tunneling from the F -centres in the bulk of the MgO to the adsorbed molecules of nitrous oxide. The distance of the transfer at 77 K within the time of 105s has been estimated to exceed 15 A. 

Specifically adsorbed ions are those which are attached (albeit temporarily) to the surface by electrostatic and/or van der Waals forces strongly enough to overcome thermal agitation. They may be dehydrated, at least in the direction of the surface. The centres of any specifically adsorbed ions are located in the Stern layer - i.e. between the surface and the Stern plane. Ions with centres located beyond the Stern plane form the diffuse part of the double layer, for which the Gouy-Chapman treatment outlined in the previous section, with 0o replaced by (f/d, is considered to be applicable. 

Fig. 2.13. Comparison of shrinkage parameter, (Vo— V)/Vo> as a function of time at the surface and centre of pellets of different sizes for two different heat transfer conditions [53]. (a) U = 70 Btu/(h ft2 °F) (b) = 15 Btu/(h ft2 °F). (Copyright American Society for Metals and The Metallurgical Society of AIME, 1974.)...

Fig. 14 Band structure of a fully oxygen defective (1 x 1) MgO(lOO) surface along the three symmetry lines J-F-M of the 2D Brillouin Zone, as obtained through the FP-LMTO calculation (Full Potential- Linear MufiSn-Tin Orbital method). The dashed horizontal line represents the Fermi level, black dots (st indicate the energy positions of the filled (empty) Bloch states at F calculated in a (2v x 2- /2) supercell. The dashed line in the gap of the projected bulk bandstructure gives the dispersion of the F, centre band. The dashed-dotted line is used for the surface conduction band of lowest energy (from Ref. 69).

Combined quantum mechanical/molecular mechanics calculations have become popular over the last decade or so in solid state modelling. Typically a molecule attaching to a surface or the side of a cage structure or an isolated defect is treated quantum mechanically as a cluster embedded in a bulk solid described by an interatomic potential method. One of the earliest examples of this approach was Vail s work on F-centres which began in 1983" " and which treated a cluster around the F-centre quantum-mechanically and the rest of the solid via an interatomic potential method. 

The defect aggregates themselves, depending on the nature of the defect, can acquire a net charge on the surface by trapping either electrons or positive holes. This allows them to trap further defects to increase the aggregate size. The step in this reaction may be sym-bohzed as follows, using the simple case of aggregation of F-centre defects to produce metal M from an ionic solid MX ... 

The nuclei form both at the surface and in the interior of the cry tal, but the theory developed by Mott (49) assumes that the defects are interstitial Ba - ions and that the nuclei grow at the surface of the grains only. As in the formation of the latent image in sensitized Ag halide grains, the assumption of pure Frenkel defects could only lead to the formulation of surface nucleation, but recent developments have led Mitchell to consider the role of Schottky defects (F-centres), which may forn internal aggregates ultimately bi caking away from the parent lattice as minute nuclei of the new metal phase. It is possible that a similar state of affairs exists in azides (36) unfortunately, nothing is known of the nature of... 

No physico-chemical methods exist that allow differences in one-electron donor strength for F centres, formed by alkali metal evaporation on the surfaces of various oxides, to be measured. However, there must be differences in one-electron donor ability because of their different catalytic activities. 

The latter implies that under the above conditions the qnantnm yield depends on k, the concentration of surface-active centres [S], and the snrface concentration of charge carriers, ns (A is the fraction of light absorbed, i.e. absorptance, and p is the photon flow). It is therefore possible to query those factors that govern the snrface concentration of charge carriers, and the activity of the photocatalyst through the quantum yield (f>. For this, the functionality of the surface charge-carrier concentration and the parameters that affect it need to be examined. 

In the third approach, the total number of surface-active centres was obtained by extrapolation of the kinetic curve that represents the accumulation of photoadsorbed oxygen species obtained from the TPD spectra (see Fig. 5.56). This gave 8.8 x 10 such centres at the limit of f °o, corresponding to the maximal number of surface-active centres, also in fair accord with the above estimates. 

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