An XPS study of microporous and mesoporous titanosilicates

In this contribution we report on an XPS study of microporous and mesoporous titanosilicates, in particular microporous titanium silicalite TS-1, ordered mesoporous Ti-MCM-41 and [Ti]-MCM-41 and amorphous mesoporous silica-titania (MST) catalysts. Our aim was to obtain both photoemission and x-ray-excited Auger data for Ti species on these catalysts and use them in a Ti Wagner plot to rationalize the dependence of the local electronic structure on the atomic environment. Isolated Ti(IV) species coordinated to four and six oxygen anions and segregated TiO2 clusters were detected on all catalysts by a curve-fitting procedure of Ti 2p, O 1s and related peaks. The presence of the Si 2p peak excited by an O Ka ghost makes the detection of Ti LMM Auger transitions in mesoporous samples impossible due to the low Ti loadings and its homogeneous distribution in the silica matrix. Small TiO2 clusters are eventually segregated within the mesopores of the catalysts and not at their external surface. On TS-1 microporous catalysts with similar Ti loadings to the mesoporous catalysts we were able to detect Ti LMM Auger transitions, and by the Ti Wagner plot we clearly identify the presence of octahedrally coordinated Ti(IV) species. Thus, it is suggested that on TS-1 the in-framework (-O)4Ti species are easily changed to (-O)4(H2O)2Ti species by insertion of water molecules from the atmosphere. Small TiO2clusters (diameter <5 nm), eventually present on samples with Ti loading >2 wt.%, are segregated at their external surface and present spectroscopic features similar to (-O)4(H2O)2Ti species.