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

Abstract

The effect of the point of zero charge (PZC) of the support oxide (Al 2 O 3 , Nb 2 O 5 , SiO 2 and ZrO 2 ) on the molecular structure of hydrated vanadium oxide species has been investigated with EXAFS spectroscopy for low-loaded vanadium oxide catalysts. It was found that the degree of clustering (i.e., the V---V coordination number) and the V---V distance increase with decreasing PZC of the support oxide;i.e., Al 2 O 3 (8.7) < ZrO 2 (7) < Nb 2 O 5 (3.3) < SiO 2 (2). Upon hydration the silicasupported vanadium oxide exhibited a clear alteration in the position of the oxygen atoms surrounding the central vanadium atom and thenumber of oxygen atoms around vanadium increased to five. In contrast, only minor changes in the molecular structure were detected for the alumina-, niobia-and zirconia-supported vanadium oxide catalysts. Based on a detailed analysis of the EXAFS data a semi-quantitative distribution of vanadium oxide species present on the surface of the different support oxides can be obtained, which is in good agreement with earlier characterization studies making primarily use of Raman spectroscopy.

Structural characterization of vanadium oxide catalysts supported on nanostructured silica SBA-15 using X-ray absorption spectroscopy

“…The additional Si backscatterer resulted in a better agreement between theoretical and experimental FT(χ(k)*k 3 ) at distances of about 2.4 Å (not phase shift corrected). The resulting V-Si distance amounted to 2.54 Å, comparable to the distance obtained by Keller et al (2.61 Å [20]). However, it can be easily seen from Figure 5, bottom) that the amplitude in the FT(χ(k)*k 3 ) of dehydrated V x O y /SBA-15 between 2.4 and 4.0 Å is still not accounted for.…”

“…A seemingly expected contribution may arise from silicon backscatterers in the SiO 2 support at distances of less than 3.0 Å. This has been previously proposed by Keller et al [20]. Thus, in extension of the tetrahedron approach a silicon atom at a V-Si distance of 2.8 Å was included in the theoretical model.…”

Active sites of copper-complex catalytic materials for electrochemical carbon dioxide reduction

“…With S 0 2 known, the EXAFS data of the catalyst materials were fitted with such generated phase shifts and amplitudes. Accuracies of the obtained results presented here are as follows: Δ N (± 10%), Δ R (± 1%), Δ σ 2 (± 10%), and Δ E 0 (± 10%) 34 , 53 , 54 .…”

Mineralization of reactive Black 5 in aqueous solution by ozone/H2O2 in the presence of a magnetic catalyst

“…The transfer of RB5 and intermediates (formic acid, oxalic acid, and acetic acid) toward the SiO 2 /Fe 3 O 4 surface might be inhibited in alkaline solutions because RB5 and intermediates become negatively charged ions by losing hydrogen ions. The pH ZPC (pH of the zero point of charge) of SiO 2 was determined to be 2.0 [21]. The surface of SiO 2 /Fe 3 O 4 was also more negatively charged, because the solution pH was greater than the pH ZPC of SiO 2 /Fe 3 O 4 (Fe 3 O 4 (core) was covered by SiO 2 (shell)).…”