Highly Dispersed Supported Oxo-Species and TMI

1 LMCT Transition Bands as Source of Structural Insight 

Highly dispersed surface species, with the limiting form of single-site active centers, play a primary role in a number of catalytic materials because of their peculiar features in terms of activity and selectivity. Both oxo-species and transition metal ions supported on oxides (or in zeotype materials) belonging to these types 

For each type of coordination number, the position of LMCT bands are sensitive to the presence of M —O—M linkages. This is found, for example, for species ranging from supported chromates to dichromates and polychromates [66, 67], and for isolated or polymeric VO4 tetrahedra present in Na3V04 and NH4VO3, respectively, which are considered as reference compounds for the interpretation of the spectra of supported species [68]. 

As a further aspect, it must be considered that in the context of a type of structure, differences in shape and positions of LMCT bands can monitor the occurrence of peculiar local geometries or distortions. Such spectral features can usually be analyzed in more detail, with a consequent higher information outpuL in the case of catalysts with active centers that are quite homogeneous in structure and with simpler spectra, as more commonly occurs in the case of highly isolated species. These are the conditions for the observation of spectral behavior that can be rationalized in terms of differences in the bond angles connecting the metal 

Figu re 2.11 Evolution of DR UV-Vis spectra as a function of hydration time (under ambient conditions) for a VO,/MCM-48 catalyst (V/Si = 0.05) (a) time 0, (b) lOmin, (c) 30min, 

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