Titanium defective sites in TS-1: structural insights by combining spectroscopy and simulation
Titanium Silicalite-1 (TS-1) is a titanium zeolite, whose peculiarity is the presence of Ti atoms isomorphously substituting the Si ones at tetrahedral framework positions. However, real TS-1 samples are characterized by the co-presence of other Ti sites, ranging from extended TiO2phases down to defective Ti sites. The “defective Ti” label covers a broad range of possible Ti moieties, whose structural description is in most of the cases barely qualitative in the literature. In this work, we combined experimental and theoretical approaches, aiming to unravel the exact structure of defective Ti sites. In detail, the outcomes from electronic spectroscopies: i) UV-vis, X-ray absorption techniques measured in Elettra synchrotron at ii) Ti K-edge XAS at XAFS beamline and at iii) L2,3-edges NEXAFS at APE-HE beamline are compared to simulation results, allowing a comprehensive electronic description of potential defective Ti sites to be correlated to their structure. The proposed structural models, as showed in Figure 1, were derived from the periodic structure of a perfect tetrahedral Ti site, sitting at the T10 crystallographic position of the TS-1 framework. This site was altered in order to obtain partially hydrolyzed, higher (penta- or hexa-) coordinated or tetrahedral vicinal sites. UV-vis, Ti K-edge XAS and L2,3-edges NEXAFS data were collected on two representative samples (a perfect TS-1, named Perf, and a defective one, Def) and compared to the spectra simulated on the aforementioned structural models (Figure 2).
Figure 1. Structural models for: A, perfect tetracoordinated Ti; B, bipodal tetracoordinated Ti; C, H-terminated pentacoordinated Ti; D, Na-terminated pentacoordinated Ti; E, H-terminated hexacoordinated Ti; F, Na-terminated hexacoordinated Ti; and G, dimeric tetracoordinated Ti. Atoms color code: H grey, O red, Na blue, Si yellow and Ti cyan.
On the experimental side, all the techniques highlight the specific features of defective Ti sites, which can be easily visualized in the difference spectra D. In order to infer the structure of these defective sites, a comparison with simulated data is necessary. None of the techniques, if singularly taken, is able to univocally pinpoint a precise defective structure. However, by simultaneously considering all the outcomes, the structure F (Na-terminated hexacoordinated Ti) seemed to provide the better description of the defective site on the basis of the experimental-theoretical combined approach. Our assignment was finally crosschecked via first-shell biphasic EXAFS fitting of the Ti K-edge XAS data for the Def, that provided an optimal agreement in terms of structural parameters with the F model, as well as in compositional terms.
Figure 2. Experimental (Exp) and simulated (Sim) UV-vis, Ti K-edge XAS and Ti L2,3-edges NEXAFS, Experimental spectra are collected for a perfect (Perf) and a defective (Def) TS-1 sample. A difference spectrum Dis calculated subtracting the contribution of perfect sites from Perf from the spectrum of Def, weighted on the perfect Ti concentration (i.e. Δ= Def–0.89×Perf, eventually multiplied by a factor for the sake of visualization). The simulated spectra are computed for the seven models reported in Figure 1: A, perfect tetracoordinated Ti; B, bipodal tetracoordinated Ti; C, H-terminated pentacoordinated Ti; D, Na-terminated pentacoordinated Ti; E, H-terminated hexacoordinated Ti; F, Na-terminated hexacoordinated Ti; and G, dimeric tetracoordinated Ti.
This research was conducted by the following research team:
Matteo Signorile1, Luca Braglia2, Valentina Crocellà1, Piero Torelli2, Elena Groppo1, Gabriele Ricchiardi1, Silvia Bordiga1, Francesca Bonino1
2 CNR-IOM, Trieste, Italy
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