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

Abstract

Silica-supported vanadium oxide (V2O5/SiO2) and alkali-ion (Na+, K+, or Rb+)-modified V2O5/SiO2 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 V2O5/SiO2 has been confirmed to be of a (VO)O3 tetrahedron. The addition of alkali ions to V2O5/SiO2 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 V2O5/SiO2 samples, the structure of vanadium species has also been a VO4 tetrahedron, and the bond lengths of V−O have been longer than those of V2O5/SiO2. 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 V2O5/SiO2 and alkali-ion-modified V2O5/SiO2 samples are reasonable. In addition, it has been clarified that by adsorption of water molecules, the structure of vanadium species in alkali-ion-modified V2O5/SiO2 samples is kept tetrahedral, while that in V2O5/SiO2 changes to a distorted octahedral one.

Cyclohexene photo-oxidation over vanadia catalyst analyzed by time resolved ATR-FT-IR spectroscopy

“…Evidence is based on the observation of transient loss of V-O-H absorption upon photoreaction and the fact that hydroxylated vanadyl centers absorb in the blue spectral region while isolated vanadyl groups in a completely dehydrated state do not. On the other hand, it has been reported that vanadia centers in the hydrated state are not active in photo-oxidation reactions [3,24]. This is confirmed in the present study, since accumulation of water on the catalyst leads to a significantly lower activity in the photo-oxidation of cyclohexene.…”

Molecular Structure of a Supported VO4 Cluster and Its Interfacial Geometry as a Function of the SiO2, Nb2O5, and ZrO2 Support

“…[11][12][13][14][15] This classical model is characterized by a vibration at ∼1020-1040 cm -1 , as measured with Raman spectroscopy. 16 The exact frequency depends on the support material.…”

Atomic XAFS as a Tool To Probe the Reactivity of Metal Oxide Catalysts: Quantifying Metal Oxide Support Effects

“…In general, the VO 4 monomeric species in dehydrated supported vanadium oxide catalysts has been envisaged as a distorted tetrahedral structure with one Vd O bond and three VsO b sM support bonds. [17][18][19][20][21] Although this classical VO 4 model is most widely accepted in literature, recent studies demonstrated, with results obtained from EXAFS at 77 K in combination with structural models, that for supported vanadium oxide species a single VsO b sM support bond can be formed on alumina, silica, niobia, and zirconia supports. [22][23][24] The structure of the interface between the support oxide material and the VO 4 cluster has also been recently determined.…”