Titanium silicalite-1 (TS-1) has been shown to be a heterogeneous catalyst with remarkable efficiency and selectivity; however, the nature of the active Ti site in the MFI framework remains elusive. Here we report combined experimental and theoretical research on Ti distribution in the 12 crystallographically distinct T sites of the MFI framework in high-Ti-loaded TS-1 (2.7 wt % in TiO2). Using a multishell fit to extended X-ray absorption fine structure, we show that T4 is the most populated site, in marked contrast to the preferential substitution sites and the definitely excluded sites assumed hitherto by diffraction studies. The identification is supported by a good agreement between calculated and experimental X-ray absorption near-edge structure studies and by full periodic density functional theory (DFT) computation. In spite of having the identical most favored site, the preference order for the remaining sites predicted by DFT does not fully match the experimental results. This suggests that Ti distribution in the resulting TS-1 framework is positively correlated with the thermodynamic stability of pure material but can be affected by other factors such as interdefects. These new insights may facilitate the bottom-up design of new zeolites with tailored catalytic performance and studies on mechanisms of various oxidation reactions.

In combination with a single-crystal diamond anvil cell (DAC), a polycapillary half-lens (PHL) re-focusing optics has been used to perform high-pressure extended X-ray absorption fine-structure measurements. It is found that a large divergent X-ray beam induced by the PHL leads the Bragg glitches from single-crystal diamond to be broadened significantly and the intensity of the glitches to be reduced strongly so that most of the DAC glitches are efficiently suppressed. The remaining glitches can be easily removed by rotating the DAC by a few degrees with respect to the X-ray beam. Accurate X-ray absorption fine-structure (XAFS) spectra of polycrystalline Ge powder with a glitch-free energy range from -200 to 800 eV relative to the Ge absorption edge are obtained using this method at high pressures up to 23.7 GPa, demonstrating the capability of PHL optics in eliminating the DAC glitches for high-pressure XAFS experiments. This approach brings new possibilities to perform XAFS measurements using a DAC up to ultrahigh pressures.

Despite the great importance in fundamental and industrial fields, understanding structural changes for pressure-induced polyamorphism in network-forming glasses remains a formidable challenge. Here, we revisited the local structural transformations in GeO2 glass up to 54 GPa using x-ray absorption fine structure (XAFS) spectroscopy via a combination diamond anvil cell and polycapillary half-lens. Three polyamorphic transitions can be clearly identified by XAFS structure refinement. First, a progressive increase of the nearest Ge-O distance and bond disorder to a maximum at ~5-16 GPa, in the same pressure region of previously observed tetrahedral-octahedral transformation. Second, a markedly decrease of the nearest Ge-O distance at ~16-22.6 GPa but a slight increase at ~22.6-32.7 GPa, with a concomitant decrease of bond disorder. This stage can be related to a second-order-like transition from less dense to dense octahedral glass. Third, another decrease in the nearest Ge-O distance at ~32.7-41.4 GPa but a slight increase up to 54 GPa, synchronized with a gradual increase of bond disorder. This stage provides strong evidence for ultrahigh-pressure polyamorphism with coordination number >6. Furthermore, cooperative modification is observed in more distant shells. Those results provide a unified local structural picture for elucidating the polyamorphic transitions and densification process in GeO2 glass.