Local structure of stoichiometric and oxygen-deficient A_2Ti_6O_(13) (A = Li, Na, and K) studied by X-ray absorption spectroscopy and first-principles calculations

摘要

Oxygen vacancy defects (V-O) in Ti-based oxides play important roles in catalytic processes despite limited knowledge regarding their formation and characterization. Here, we demonstrate the use of X-ray absorption spectroscopy (XAS) measurements to compare the relative proportion of V-O defects in as-grown alkali hexatitanate A(2)Ti(6)O(13) (A = Li, Na, K). Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions were studied. The similarity of measured XANES spectra of Ti K-edge in all samples indicates the presence of (Ti4+) O-6 units in good agreement with reported X-ray diffraction results. The small influence of cations A at the tunnel was observed and can be well reproduced in the simulated spectra. In addition, we present a semi-quantitative approach to intuitively determine the content of V-O defects in oxygen-deficient K2Ti6O13-x by in situ time-resolved XAS measurements under reducing conditions (10%H-2/Ar, 50-650 degrees C). The in situ XANES measurements indicate that the oxidation state of bulk Ti remains the same as the as-grown sample, i.e., 4+, at elevated temperatures. By in situ EXAFS measurements, the relative number of V-O defects is highest at a reduction temperature of similar to 550 degrees C and slightly decreases after that. To confirm the formation of V-O defects, first-principles calculations were independently carried out using a 126-atom K2Ti6O13 supercell with V-O at various positions. Based on calculated EXAFS, the removal of the oxygen atom nearest to the tunnel, which is the lowest energy structure, provides a good match to the experimental spectra. Published by AIP Publishing.