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

Abstract

The local structure of vanadium oxide supported on nanostructured SiO2 (VxOy/SBA-15) was investigated by in situ X-ray absorption spectroscopy (XAS). Because the number of potential parameters in XAS data analysis often exceeds the number of "independent" parameters, evaluating the reliability and significance of a particular fitting procedure is mandatory. The number of independent parameters (Nyquist) may not be sufficient. Hence, in addition to the number of independent parameters, a novel approach to evaluate the significance of structural fitting parameters in XAS data analysis is introduced. Three samples with different V loadings (i.e. 2.7 wt %, 5.4 wt %, and 10.8 wt %) were employed. Thermal treatment in air at 623 K resulted in characteristic structural changes of the V oxide species. Independent of the V loading, the local structure around V centers in dehydrated VxOy/SBA-15 corresponded to an ordered arrangement of adjacent V2O7 units. Moreover, the V2O7 units were found to persist under selective oxidation reaction conditions.

Active Sites for Propane Oxidation: Some Generic Considerations

“…A careful EXAFS analysis [122] of grafted monolayer systems revealed that the expected dimers are clearly compatible with the observations. In addition, the dimers seem to occur in an ordered form with close proximity as to form dimers of dimers.…”

“…Larger oligomers [122], mimicking the active site in M1 seem not to be frequently present on these systems. This is also true for the titaniavanadia mixed system where EPR clearly showed [116] redox exchange between vanadium and titanium sites.…”

Structure of molybdenum oxide supported on silica SBA-15 studied by Raman, UV–Vis and X-ray absorption spectroscopy

“…Single scattering and multiple scattering paths in the hexagonal MoO 3 model structure were calculated up to 6.0 Å with a lower limit of 4.0% in amplitude with respect to the strongest backscattering path. EXAFS refinements were performed in R space simultaneously to magnitude and imaginary part of a Fourier transformed k 3 -weighted and k 1 -weighted experimental (k) using the standard EXAFS formula [33]. This procedure strongly reduces the correlation between the various XAFS fitting parameters.…”

“…As the molybdenum oxide precursor is held electrostatically within the channels of the mesoporous matrix in a prearranged geometry, this method allows one to precisely control the amount and dispersion of the metal oxide introduced into SBA-15 within a broad range of densities. Recently, by such an approach new insight into the structure of silica SBA-15 supported vanadium oxide catalysts has been obtained [32][33][34][35][36].…”

Macromodel of interfacial transition layer in ceramic matrix composites

“…Specialists emphasize the necessity to develop and use new research methods, along with standard ones, for studying ceramics made from technogenic raw materials with new structural and mechanical characteristics [17][18][19][20][21]. The novelty of this research compared to other studies is selectivity while studying interphase boundaries in ceramic matrix composites using a transition layer macromodel.…”