ΛO₄ Upside Down:  A New Molecular Structure for Supported VO₄ Catalysts

Abstract

Vanadium oxide (1 wt %) supported on γ-Al 2 O 3 was used to investigate the interface between the catalytically active species and the support oxide. Raman, UV-vis-NIR DRS, ESR, XANES, and EXAFS were used to characterize the sample in great detail. All techniques showed that an isolated VO 4 species was present at the catalyst surface, which implies that no V-O-V moiety is present. Surprisingly, a Raman band was present at 900 cm -1 , which is commonly assigned to a V-O-V vibration. This observation contradicts the current literature assignment. To further elucidate on potential other Raman assignments, the exact molecular structure of the VO 4 entity (1 VdO bond of 1.58 Å and 3 V-O bonds of 1.72 Å) together with its position relative to the support O anions and Al cation of the Al 2 O 3 support has been investigated with EXAFS. In combination with a structural model of the alumina surface, the arrangement of the support atoms in the proximity of the VO 4 entity could be clarified, leading to a new molecular structure of the interface between VO 4 and Al 2 O 3 . It was found that VO 4 is anchored to the support oxide surface, with only one V-O support bond instead of three, which is commonly accepted in the literature. The structural model suggested in this paper leaves three possible assignments for the 900 cm -1 band: a V-O-Al vibration, a V-O-H vibration, and a V-(O-O) vibration. The pros and cons of these different options will be discussed.

Elucidation of the molecular structure of hydrated vanadium oxide species by X-ray absorption spectroscopy: correlation between the V⋯V coordination number and distance and the point of zero charge of the support oxide

“…The total R-space fits are displayed The EXAFS data-analysis and the vanadium oxide cluster geometry for the dehydrated samples have been given and discussed extensively in previous studies. [28,29,38] The coordination parameters for the dehydrated samples are given in Tables 2-5 for 1V-Si, 1V-Nb, 1V-Zr and 1V-Al, respectively. After dehydration no V---V interactions were observed for the low loaded (1 wt%) samples, implying the presence of single isolated VO 4 clusters.…”

Determining the Structure of Silica-Supported Monomeric Vanadium Oxide Catalysts Based on Synthesis Method and Spectral Data from Theoretical Calculations

“…6,[22][23][24] There is, however, no definite model that can unambiguously explain all the experimental data collected over the years, although a predominant tendency for the pyramid model [25][26][27][28][29][30][31] exists ( Figure 1A). Magg et all.…”

“…However, we think that the explanation of the spectra should be sought in a different model or perhaps in the presence of multiple molecular structures as discussed in earlier papers. 23,33,34 There are a number of routes to synthesize the vanadium oxide catalyst, each leading to (perhaps not surprisingly) a different structure of the active species.…”

“…There is a consensus that the monomeric structure present at low loadings is a distorted tetrahedral around the central vanadium atom and that there are one or more VdO bonds present. 29,[44][45][46][47] For the incipient wetness technique, it has been concluded from EXAFS experiments 23,48 that the V-atom is surrounded by three O-atoms at 1.78 Å and one O-atom at 1.58 Å. This is as far as the consensus goes.…”

“…[25][26][27][28][29][30][31] Another structure, which has two Si-O s -V bonds and two VdO bonds, has been presented by Kozlowski et al 45 This structure has already been discarded as a candidate structure by Bond et al 49 and later by EXAFS experiments. 23,48 The hydrogenated variant of this structure ( Figure 1B) in which one of the VdO bonds is replaced by a V-OH has also been presented in literature. [49][50][51][52][53] In 2004, we introduced the umbrella model 23,33,34 (Figure 1C) in which the vanadium atom is linked to the surface via only one Si-O s -V bond and further consists of a VdO and a perturbed O 2 molecule linked to the central vanadium atom.…”