EXAFS surface sites

CP/MAS NMR study of CH4 activation on [(=SiO)2Ta(H)J shows the formation even at 150 °C of methyUdene and methyUdyne species by an a-H elimination process on several sites that should correspond to the tris-hydride on other sites a methyl group is transferred to the surface, leading to the formation of (=Si-Me) and of [(=SiO)3Ta]. Correlation with EXAFS suggests that the tris-hydride should exist on surface sites (=20%) quite distant from siloxy bridges whereas methyl transfer to the surface should happen on the specific sites (=80%) close to the siloxy bridges. The latter, which are formally 10 electrons species, exhibit a moderate to weak activity in methane C-H activation. To the best of our knowledge, this is the first observation of methyl group transfer on a surface (Scheme 2.18). 

EXAFS data showed that cations and oxyanions (e.g. selenite and arsenite) can form two kinds of bidentate, inner sphere complexes on iron oxides depending upon the surface site at which the adsorbate adsorbs (Manceau, 1995 Randall et al.. 

The third model is a shift to higher frequencies and concomitant broadening of the resonance peak, due to increased 6s electron density in the eluster. Here use was made of the expected increase of 6s electron density due to the decrease in cluster volume, as obtained from EXAFS [39,40,41]. But as we have pointed out above, despite the decrease in cluster volume, the I.S. of the core sites indicates a decrease of 6s electron density. The excellent prediction of the I.S. values of the surface sites, given above, utilizing a sizeable increase in d-character of the electrons associated with (in the vicinity of) the surface sites, means that the postulated blue shift and flattening should really be a red shift and concomitant sharpening of the resonance, which should make the resonance more visible, if present. The MES I.S. results thus refute this as a possible explanation. 

The adsorption site, i.e. the chemisorption position of the adatoms on (within, below) the substrate surface, thanks to the polarisation dependence of SEXAFS. Often a unique assignment can be derived from the analysis of both polarisation dependent bond lengths and relative coordination numbers. The relative, polarisation dependent, amplitudes of the EXAFS oscillations indicate without ambiguity the chemisorption position if such position is the same for all adsorbed atoms. More than one chemisorption site could be present at a time (surface defect sites or just several of the ideal surface sites). If the relative population of the chemisorption sites is of the same order of magnitude, then the analysis of the data becomes difficult, or just impossible. 

Spadini, L. et al., Hydrous ferric oxide Evaluation of Cd-HFO surface complexation models combining Cd EXAFS data, potentiometric titration results and surface site structures identified from mineralogical knowledge, J. Colloid Interf. Sci., 266, 1, 2003. 

In the case of metallic adsorbates (metal deposits, underpotentially deposited upd-layers, catalytically active metal deposits), the type of coordination to surface sites (one-, two- or three-fold) and the distance to these sites may be of interest. Vice versa the same type of data may be of importance in the case of adsorbed ions on metal electrodes or about the atomic environment of a given atom/ion in an interphase. Analysis of the fine structure of X-ray absorption (EXAFS, XANES) close to the X-ray absorption edge of the species (atom) of interest will yield this data provided the sample can be prepared in a very thin layer in order to exclude unwanted bulk interference. Otherwise the experiment can be done in reflection (SEXAFS). Information about the distance between the atom of interest and its first and sometimes even second shell of surrounding species can be derived from the spectra [95]. Availability of a suitable light source, generally a synchrotron (for details see p. 15), is an experimental prerequisite. The method has been applied in studies of passive and corrosion layers on various metals [96-102] and of molecular and ionic adsorbates on single crystal surfaces [103]. 

Co(II) sorption complexes on rutile surfaces appear to form extensions of the bulk structure, with EXAFS-derived Co-Ti distances very similar to Ti-Ti distances in rutile. The small number of Ti second neighbors detected by EXAFS spectroscopy indicates that Co occurs dominantly in Ti-equivalent sites at the rutile surface (21). A bond-valence analysis of Co(II) sorption on rutile also indicates that Co(II) can bond stably to a variety of surface oxygen sites (26). When the sorption density of Co is high on rutile, O Day et al (21) found Co-Ti distances characteristic of anatase. The observed differences in the style of Co sorption on quartz, rutile, and alumina under similar conditions and sorption densities are consistent with structural differences among the three sorbents that result in different reactivities of available surface sites. 

Similarly detailed information on the chemistry of atoms at. surface sites and on the relationships of such sites to surface reactivity may be obtained for oxide, silicate, aluminosilicate and other minerals using XPS in a.s.sociation with EXAFS and adsorption studies. The reviews by Bancroft and Hyland [481. and Brown et al. [16], provide many examples of the types of information obtainable for these surfaces. 

Fig. 4 Osmium clusters supported on MgO(OOl) a OssC/MgisOs and b OS5C at a surface point Vs defect site [33] these were represented by density functional theory, and the samples were characterized by EXAFS spectroscopy, transmission electron microscopy, and other techniques [15]...

---