Nature of dispersed vanadium oxide: influence of the silica support structure and synthesis methods

Abstract

Dispersed vanadium oxide samples were prepared on the basis of two differently structured high surface area silica materials (Aerosil 300, SBA-15). For each support material incipient wetness impregnation and a grafting/ion exchange procedure were applied to prepare catalyst samples with comparable vanadium density. The influence of the silica support material and preparation method on the vanadium oxide structure and dispersion has been studied using diffuse reflectance UV-Vis spectroscopy, visible Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). By independent spectroscopic characterization a fully consistent picture regarding the relation of the structure (UV-Vis, Raman) and dispersion (XPS) is developed. Based on the present structural data and recent findings the dispersed vanadium oxide is proposed to consist of monomers and oligomers with a distorted tetrahedral (VO 4 ) structure containing one short VQO bond. A different degree of hydroxylation of vanadium gives rise to two VQO stretching bands at 1027 and 1040 cm À1. The structure and dispersion of vanadium oxide are more strongly influenced by the support material than by the synthesis method. In this regard the Aerosil 300 samples show a higher degree of oligomerization, i.e. less dispersion of the surface vanadium oxide species, than the SBA-15 samples. Likewise incipient wetness impregnation leads to more oligomerized species than grafting/ion-exchange.

Properties of BioRoot RCS, a tricalcium silicate endodontic sealer modified with povidone and polycarboxylate

“…) and 509 cm −1 (Chlosta et al . ), respectively; the former is also attributable to zirconia and bisphenol A epoxide resin, the latter to zirconia.…”

Insights into Redox Dynamics of Vanadium Species Impregnated in Layered Siliceous Zeolitic Structures during Methanol Oxidation Reactions

“…It can be assumed that when a strong interaction between vanadium oxide and the support occurs (e. g. TiO 2 ), a monolayer of surface species can be reached without the formation of nanoparticles . However, when the reactivity of the support surface is low (e. g. SiO 2 ), nanoparticles are also present below monolayer coverage . Some strategies to improve dispersion over silica‐based supports involve the preparation of highly ordered mesoporous silicas, like MCM‐41, or thermally treated commercial silicas, or by generation of surface reactive isolated silanol groups.…”

“…[19] However, when the reactivity of the support surface is low (e. g. SiO 2 ), nanoparticles are also present below monolayer coverage. [20] Some strategies to improve dispersion over silica-based supports involve the preparation of highly ordered mesoporous silicas, like MCM-41, [21] or thermally treated commercial silicas, [16] or by generation of surface reactive isolated silanol groups. Increasing the surface area and surface hydroxyl density can directly affect electrostatic interactions between the precursor and support during impregnation, and the stability of sites during thermal post-treatments like drying and calcination is also important.…”

Selective Oxidation of Methanol to Dimethoxymethane at Low Temperatures through Size‐controlled VTiOx Nanoparticles

“…[10] The bands at ñ = 285, 531, 705, and 1005 cm À1 assigned to crystalline V 2 O 5 are detected for VTiS-CTAB-0, but these bands gradually weaken in intensity upon the addition of CTAB. For VTiS-CTAB-0.8/1.0/1.3, none of the signals for V 2 O 5 crystal can be observed, except for one shoulder at ñ = 531 cm À1 .A tthes ame time, the band at ñ = 1034 cm À1 ,a ssigned to the V = O( vanadyl) stretching vibration of tetrahedrally coordinated vanadium in O = V(-O) 3 , [19] and the band at ñ = 947 cm À1 ,a ttributable to the VÀOÀVl inkage for polymericv anadate species, [20] appear for VTiS-CTAB-0.6 and are further intensified for VTiS-CTAB-1.0. It is reasonable to deduce that vanadium species exist as crystalline V 2 O 5 for VTiS-CTAB-0 and are then transformed into polymeric vanadate speciesf or VTiS-CTAB-0.6/1.0 and finally form monomeric species for VTiS-CTAB-1.3.…”