Chemical and structural characterization of V2O5/TiO2 catalysts

Abstract

A series of V 2 O 5 /TiO 2 samples was synthesized by sol-gel and impregnation methods with different contents of vanadia. These samples were characterized by x-ray diffraction ͑XRD͒, Raman spectroscopy, x-ray photoelectron spectroscopy ͑XPS͒, and electronic paramagnetic resonance ͑EPR͒. XRD detected rutile as the predominant phase for pure TiO 2 prepared by the sol-gel method. The structure changed to anatase when the vanadia loading was increased. Also, anatase was the predominant phase for samples obtained by the impregnation method. Raman measurements identified two species of surface vanadium: monomeric vanadyl (V 4ϩ ) and polymeric vanadates (V 5ϩ ). XPS results indicated that Ti ions were in octahedral position surrounded by oxygen ions. The V/Ti atomic ratios showed that V ions were highly dispersed on the vanadia/titania surface obtained by the sol-gel method. EPR analysis detected three V 4ϩ ion types: two of them were located in axially symmetric sites substituting for Ti 4ϩ ions in the rutile structure, and the third one was characterized by magnetically interacting V 4ϩ ions in the form of pairs or clusters. A partial oxidation of V 4ϩ to V 5ϩ was evident from EPR analysis for materials with higher concentrations of vanadium.

Tungsten-Vanadium mixed oxides for the oxidehydration of glycerol into acrylic acid

“…This may be related to the higher presence of W O bonds generated by a structure defect due to vanadium incorporation 32, 35 and/or V O bonds associated to polymeric V-O-W chains. 36 In addition, a similar band is reported to correspond to the mode W-O, which is also a bond of tungsten oxide hydrates (WO 3 •nH 2 O), when a water molecule occupies one axial position, 32,37 but our Raman spectra are obtained in dehydration conditions. On the other hand, no changes in the Raman spectra were observed in these catalysts after catalytic tests (Fig.…”

High yield lactic acid selective oxidation into acetic acid over a Mo-V-Nb mixed oxide catalyst

“…The pattern for the Raman spectra is shown in Figure 4, The phases observed by XRD were also confirmed by Raman spectroscopy. In fact, the catalyst shows characteristic peaks at 139 and 282 cm −1 for the orthorhombic vanadium oxide (V 2 O 5 ) [42,43]. For the orthorhombic MoO 3, more intense characteristic peaks are observed at 114, 129, 281, 665, 819 and 997 cm −1 [44,45].…”

Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis

“…Although Eberhardt et al [97] observed only the presence of V 5+ and V 3+ on the surface after reduction with H 2 or CO, it has often been concluded that the alumina support also stabilizes reduced V 4+ species. The presence of V 4+ species have indeed been measured by many authors for supported vanadium oxides on alumina by Table 6 Results of peak deconvolution of the vanadium photoelectron signal after calcination of a 5 wt.% V/Al 2 O 3 catalyst in air and after reduction with H 2 , CO and CH 4 at 580 • C for 30 min, together with average oxidation states calculated from these fractions [193,196] ESR [87][88][89][90][91][92][93][94][95][96]. As an example, the ESR spectra of the same 5 wt.% V/Al 2 O 3 catalyst after calcination and reduction with CO and H 2 is shown in Fig.…”

“…Electron spin resonance (ESR) is a powerful and quantitative technique to study especially the presence and coordination geometry of the paramagnetic V 4+ (d 1 ) species even under in situ conditions [87][88][89][90][91][92][93][94][95][96]. Vanadium has a nuclear spin of 7 2 , resulting in a complex spectrum with a high number of hyperfine lines.…”