Chemical and spectroscopic study of the nature of a vanadium oxide monolayer supported on a high-surface-area TiO₂ anatase

Abstract

Vanadium titanium oxides with a vanadium coverage up to 1.6 of the theoretical monolayer (bidimensional sheet of vanadium oxide on the Ti02 surface), prepared by wet impregnation, grafting, and exchange techniques, are studied by X-ray diffraction, electron spin resonance, infrared, UV-vis-near-IR diffuse reflectance, and chemical analysis of the valence state and of the total amount of the vanadium present, before and after washing with a basic solution. Results indicate that three vanadium oxide surface structures can be formed. At the lower coverages, vanadium reacts with surface OH groups of Ti02, forming strongly interacting and stabilized Vlv ions, part of which are coordinatively unsaturated. This species is present also at higher coverages together with a second vanadium oxide species, interpreted as a bidimensional cluster of Vv ions weakly interacting with the Ti02 surface. The vanadium oxide in excess of a monolayer is present as multilayer crystalline V205. The nature of the vanadium oxide monolayer is discussed in relation to the presence of these species and to IR analysis of the first overtone of i/(V=0), which suggests the presence of V ions with (i) a single oxo group and (ii) a higher frequency of y(V=0) as compared to the bulk V208. The shift to lower frequencies found by other authors is probably related to the presence of adsorbed water.

Chemical and structural characterization of V2O5/TiO2 catalysts

“…The band at 1030 cm Ϫ1 is attributed to monomeric vanadyls species bound directly to the TiO 2 support. [30][31][32][33][34] The number of bonds anchoring the vanadyl group to the support cannot be determined from the Raman spectra. This assignment is based on the similarity in the position of these bands and those for terminal VvO bonds of polyvanadate anions in solution.…”

Study of the V2O5-WO3/TiO2 Catalyst Synthesized from Waste Catalyst on Selective Catalytic Reduction of NOx by NH3

“…In addition, the weak band at 1063 cm −1 , which showed increased intensity with increasing V2O5 content, was associated with the (S-O) stretching of the SO4 2− species interacting with the TiO2 surface [34]. The broad bands at 988-993 cm −1 were associated with W=O and V=O stretching modes of wolframyl and vanadyl species that are superimposed on each other, and the bands increased slightly in intensity with increasing V2O5 content for the resynthesized catalysts [37][38][39]. The bands in the range of 873-895 cm −1 and the weak broad bands at 942 cm −1 were assigned to the V-O stretching mode of the metavanadate species [37,38] and the intensity of both bands decreased slightly with increasing V2O5 content for the resynthesized catalysts.…”

“…The bands in the range of 873-895 cm −1 and the weak broad bands at 942 cm −1 were assigned to the V-O stretching mode of the metavanadate species [37,38] and the intensity of both bands decreased slightly with increasing V2O5 content for the resynthesized catalysts. This was because with increasing V2O5 content, the strongly interacting and stabilized isolated vanadium ions produced by the reaction with surface OH groups of TiO2, was gradually transformed into vanadium oxide clusters, more weakly interacting with the TiO2 surface [37]. …”

“…The broad bands at 988-993 cm −1 were associated with W=O and V=O stretching modes of wolframyl and vanadyl species that are superimposed on each other, and the bands increased slightly in intensity with increasing V2O5 content for the resynthesized catalysts [37][38][39]. The bands in the range of 873-895 cm −1 and the weak broad bands at 942 cm −1 were assigned to the V-O stretching mode of the metavanadate species [37,38] and the intensity of both bands decreased slightly with increasing V2O5 content for the resynthesized catalysts. This was because with increasing V2O5 content, the strongly interacting and stabilized isolated vanadium ions produced by the reaction with surface OH groups of TiO2, was gradually transformed into vanadium oxide clusters, more weakly interacting with the TiO2 surface [37].…”

Hydration effects on the molecular structure of silica-supported vanadium oxide catalysts: A combined IR, Raman, UV–vis and EXAFS study

“…Furthermore, other species (d) and (e) are proposed to be formed under various measurement conditions. The di-oxo structure (a) is commonly rejected based on the results of EXAFS analysis [30][31][32][33] and the absence of a symmetric and anti-symmetric V O stretching band in the vibrational spectra [59][60][61]. The single V O nature of the VO 4 unit has been confirmed with 18 O labeling experiments in conjunction with Raman spectroscopic analysis [60,61].…”