An XPS study of microporous and mesoporous titanosilicates

Abstract

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 Kalpha 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)(4)Ti species are easily changed to (-O)(4)(H2O)(2)Ti species by insertion of water molecules from the atmosphere. Small TiO2 clusters (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)(2)Ti species. Copyright (C) 2004 John Wiley Sons, Ltd

XPS characterization of a synthetic Ti‐containing MFI zeolite framework: the titanosilicalites, TS‐1

“…The samples synthesized at the 'Sapienza' University of Rome [4,6] were the reference silicalite S-1 and two titanosilicalites, TS-1 with different Ti loading, Ti 1.94 and Ti 2.61 wt%. The 'as received' 'samples were stored in a container with a constant relative humidity (79% from saturated NH 4 Cl aqueous solution) and provided with the specifications reported in Table 1, from bulk analyses.…”

“…The TS-1 and S-1 samples were thermally treated in an oven at 550 • C for 5 h, thus repeating the calcination step reported in the synthesis procedure to remove the organic template, TPA, [4,6] and immediately transferred for XPS analysis to avoid any possible rehydration. This treatment was aimed to verify the environmental influence on the surface state of TS-1 and S-1 samples during storage.…”

“…[6] An overall uncertainty of ±0.2 eV is estimated for the peak positions derived from curve-fitting and, similarly, for the evaluation of Auger parameters, due to the propagation of the error associated with determination of peak maximum (±0.1 eV, i.e. equal to the energy step used for spectra acquisition).…”

“…[5] The scientific and technological importance of TS-1 have already been discussed in our previous publication related to microand mesoporous titanosilicates where a number of physical analyses devoted to the bulk characterization of these materials have been compared to their surface analyses using XPS/X-rayexcited Auger electron spectroscopy (XAES). [6] The simultaneous existence of different Ti (IV) centers was there recognized with relative intensities dependent on sample structure and synthesis procedures.…”

“…The aim was that of implementing the results previously obtained [6] and providing a more accurate qualitative and quantitative analysis of the XPS/XAES data using a well-established curve fitting procedure. [7] The results are compared with those obtained by X-ray diffraction (XRD) on the same samples [6] and a representation of the possible Ti-site distribution in TS-1 is proposed.…”