Transition Bands as a Source of Structural Insight

When oxide catalysts contain TMl in d d d configurahon, additional detailed information on the structure of metal ion sites can be provided by the analysis of the features of their d-d bands. As indicated in Sechon 2.1, for such an analysis a huge background of literature is available. Selected examples of the different spectral features that can be considered for the derivation of structural and chemical insight for TMl sites in catalysts, or on the evoluhon of the TMl sites during catalyst preparahon, will be presented. 

In the case of semiconductor systems initially containing TMI in a d state and subsequently reduced, the presence of band-like absorptions assignable to d-d transitions, instead of edge-bke features, arises from the formation of localized reduced centers and not the injection of electrons into the conduction band of the support This has been discussed in the case of the V205/W03/Ti02 EUROCAT SCR catalyst [72]. 

Besides asymmetry, band width may be a source of information also. The width of the d-d bands of TMI complexes adsorbed on an oxidic support can be comparable to or even narrower than those observed for the aqueous precursor complexes. This can be related to a high homogeneity of the molecular environment of TMI on the surface, suggesting a possible molecular recognition character of the adsorption process [73]. 

Furthermore, the comparison of the values of the Racah parameter, B, obtained from the d-d spectra of a TMI dispersed on a series of supports can give information on the difference in the covalence level of the TMI-support interaction among 

Reprinted from ref [74] with permission from Elsevier. 

---