CO2 Activation on Lindqvist-type Polyoxotantalate: Structural Analysis by  In Situ HERFD-XANES

T. Matsuyama, S. Kikkawa, N. Kawamura, K. Higashi, N. Nakatani, K. Kato, S. Yamazoe

J. Phys. Chem. C, accepted.

Polyoxotantalates, clusters of tantalum (Ta) oxide, are promising catalysts for CO2 fixation reactions because of their exceptional Lewis basicity facilitating CO2 activation. Here, we unveiled the CO2 activation on [H2Ta6O19]6– (Ta6) by high-energy-resolution fluorescence detection (HERFD) X-ray absorption near-edge structure (XANES). The white line of Ta L3-edge HERFD-XANES identified three peaks not discernible with conventional XANES. In situ HERFD-XANES study of CO2 adsorption on Ta6 showed the disappearance of the high-energy shoulder peak and the emergence of a new lower-energy peak. Theoretical calculations showed that the three peaks before CO2 adsorption correspond to transitions to molecular orbitals with high contributions from Ta 5d orbitals split by the ligand field of distorted Oh symmetrical {TaO6} units. The observed peak replacement reflects both destabilization of the dx2–y2 orbitals and stabilization of the dz2 orbital induced by centering from the off-center arrangement of {TaO6} units by CO2 attachment on terminal oxygen.