Particle size effects EXAFS analysis

Once that the enhancement of the anisotropy due to the nanometric dimensions of the particle has been proven in our samples beyond doubt, the pertinent question is whether we can find the physical origin of this enhancement. A number of possible mechanisms can give rise to this strong variation in anisotropy size effects due to the increasing ratio of surface to bulk atoms, electronic confinement within the cluster leading to 3d band narrowing, surface oxidation, stress induced anisotropy, or even shape anisotropy due to a departure of sphericity. The two latter ones can be readily disregarded. The interatomic Co distance is estimated from the analysis of EXAFS spectra of these clusters. They are shorter than the bulk fee Co and comparable to that found for free Co clusters [11,12], Therefore... 

Figure 10 The effect of particle size on the change in the normalized Pt J-band vacancies d-band vacancies/% surface atoms) (...) in going from 0.54 to 0.0 V. The change in the Pt-Pt coordination number as determined from the Pt L3 edge EXAFS analysis is also shown (—). The ordinate axis refers to the change in the J-band vacancy/atom determined from XANES spectra normalized with respect to total number of surface atoms present (based on cluster calculations on a cubo-octahedron model). 

EXAFS is also well suited for the study of finely divided metal (or metal oxide or metal sulfide) clusters supported within the pore structure (see Chapter 6). These particles are readily observed by X-ray spectroscopy, even if they are disordered throughout the solid. Analysis can even determine the average particle size of such clusters, which is of vital importance in catalytic preparation. Typically, for example, platinum supported on zeolites (and other solid acids) is a highly effective catalyst in the reforming of hydrocarbons. 

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