Architecture of the Ti(IV) Sites in TiAlPO-5 Determined Using Ti K-Edge X-ray Absorption and X-ray Emission Spectroscopies

Abstract

X-ray absorption and emission spectroscopies, supported by quantum mechanical calculations, were used to determine the architecture of Ti(IV) sites in Ti aluminophosphate AlPO 4 -5. The chemical sensitivity of the Kβ 2,5 X-ray emission lines of Ti was exploited to monitor its local environment, proving that it mainly substitutes P in the framework, with a not negligible fraction substituting Al. Local structure of the substituted sites was found to be considerably deformed with respect to the AlPO 4 -5 original framework.

Atomic Observations of Microporous Materials Highly Unstable under the Electron Beam: The Cases of Ti‐Doped AlPO4‐5 and Zn–MOF‐74

“…According to the latest results achieved by X‐ray absorption and emission spectroscopies, if Ti atoms would preferentially occupy the P positions it would result in a large framework deformation as a result of their difference in ionic radii, as Ti−O bonds would increase 0.33 Å. The images provided here do not reveal any kind of structural deformation; however, the distortion proposed by Gallo et al . would fall within the experimental error of the electron microscope.…”

“…The original structure of AlPO 4 ‐5 alternates Al and P units as shown in Figure a in which the Al atoms appears in gray, P in blue, and O in red. It has been reported the isomorphic substitution of P by Ti; P and Al by 2 Ti, and Al by Ti . According to the latest results achieved by X‐ray absorption and emission spectroscopies, if Ti atoms would preferentially occupy the P positions it would result in a large framework deformation as a result of their difference in ionic radii, as Ti−O bonds would increase 0.33 Å.…”

“…Interestingly,t he Si atoms in zeolites can be substituted by Po nes creatingafamily of microporous materials known as AlPOs. [7] AlPO 4 -5 has an AFI (aluminophosphate-five, IZA code [8] )f ramework type, in which the "T atoms" alternate Al and P, which can be doped with otherh eteroatoms; [9] in the case of Ti,i tc an replace either the Al or P. [10] However,t he family of AlPO 4 materials has not been deeply analyzed as ac onsequence of their higher electron beam sensitivity compared with zeolites. Therefore, the number of papers dealing with the analysis of AlPO 4 -type structures is scarce because of their high complexity.…”

Ti‐Based Catalysts and Photocatalysts: Characterization and Modeling

“…This peculiarity represents an important advantage of XES spectroscopy with respect to the other X‐ray based techniques such as XRD, EXAFS and XANES. Gallo et al . used valence to core XES spectroscopy, supported by DFT calculations, to prove that then insertion of Ti atoms in the AlPO‐5 framework mainly occurs at the P position with a smaller fraction substituting Al (10 %), see Figure .…”

“…c) Representation of the calculated most important orbitals of cluster with Ti in P position, responsible for the valence to core ‐XES signal using the B3LYP functional. Reproduced with permission from Ref., copyright ACS 2014.…”