Structure of Active Species in Alkali-Ion-Modified Silica-Supported Vanadium Oxide

Abstract

Silica-supported vanadium oxide (V 2 O 5 /SiO 2 ) and alkali-ion (Na +, K +, or Rb + )-modified V 2 O 5 /SiO 2 samples have been characterized by diffuse reflectance UV−vis spectroscopy, Raman spectroscopy, X-ray absorption spectroscopy (XANES and EXAFS), and XPS. The structure of vanadium species in V 2 O 5 /SiO 2 has been confirmed to be of a (V O)O 3 tetrahedron. The addition of alkali ions to V 2 O 5 /SiO 2 has caused neither the change of the coordination number of vanadium ions nor the formation of alkali vanadates on the silica support. In alkali-ion-modified V 2 O 5 /SiO 2 samples, the structure of vanadium species has also been a VO 4 tetrahedron, and the bond lengths of V−O have been longer than those of V 2 O 5 /SiO 2 . A rubidium ion has located at ca. 3.3 Å from a vanadium atom and interacted with two V−O bonds directly. Ab initio molecular orbital calculations have indicated that the proposed structures of vanadium species in V 2 O 5 /SiO 2 and alkali-ion-modified V 2 O 5 /SiO 2 samples are reasonable. In addition, it has been clarified that by adsorption of water molecules, the structure of vanadium species in alkali-ion-modified V 2 O 5 /SiO 2 samples is kept tetrahedral, while that in V 2 O 5 /SiO 2 changes to a distorted octahedral one.