The Influence of Water on the Dispersion of Vanadia Supported on Silica SBA-15:  A Combined XPS and Raman Study

Abstract

The influence of water on the dispersion and structure of silica SBA-15-supported vanadia model catalysts has been studied using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, which were combined within one experimental setup, as well as UV-vis diffuse reflectance spectroscopy. By performing timedependent XPS experiments, the influence of UHV/X-ray radiation could be eliminated by extrapolation of the observed temporal changes to t ) 0. XPS characterization reveals that the V2p 3/2 emission consists of two contributions, which are assigned to vanadia with distinctly different cluster size. Dehydration by treatment in oxygen flow at elevated temperatures leads to a significant increase in total intensity and a substantial redistribution of spectral weight to higher binding energies as a result of an increase in the vanadia dispersion. The V/Si XPS intensity ratio of the dehydrated samples closely follows the corresponding bulk ratio over the whole range of vanadium loadings (0-22 wt % V) studied. This observation renders possible a correlation of XPS and Raman results allowing for quantification of Raman features such as the relative cross sections of the vanadyl surface species. It is shown that the observed changes in vanadia dispersion are directly associated with the changes in the molecular structure of the surface vanadia species.

Restructuring of silica supported vanadia during propane oxidative dehydrogenation studied by combined synchrotron radiation based in situ soft X-ray absorption and photoemission

“…The XPS peaks at 517.5 eV and 516.4 eV can readily be assigned to V 5+ and V 4+ species since the absolute BE, and in particular the BE difference of ΔE = 1.1 eV, agrees reasonably well with literature [27]. The peak centered at 518.7 eV is characteristic for highly dispersed V 5+ species as discussed in detail in Hess et al [19]. It can be concluded a loss of 3-dimensional character of the surface vanadium oxide species, i.e.…”

Potential of High-Frequency EPR for Investigation of Supported Vanadium Oxide Catalysts

“…It was shown in previous studies that in the case of V-SBA-15 no significant effects of vanadium loading on the structure of vanadium sites was observed for low vanadium loadings (<2.3 V nm -2 ). 24 In the case of the V-SBA-15 sample a higher reaction temperature was applied in order to reach comparable degrees of propane conversion at concurrent homogeneity of the catalyst part. The differences in the selectivity-conversion behavior at equal reaction conditions have been discussed in detail before.…”

“…24,38 However, recent time-dependent XPS experiments, which eliminate the influence of vacuum/X-ray radiation by extrapolating the temporal changes to t ) 0, indicate the absence (contribution ∼1%) of vanadium(III) in the case of dehydrated (hydrated) silica-supported vanadia samples. 24 …”

“…9,24 The grafting/ion-exchange procedure consists of (1) surface functionalization of SBA-15 using 3-aminopropyltrimethoxysilane (APTMS) and subsequent HCl treatment leading to formation of the corresponding ammonium salt (functionalized SBA-15), (2) ion exchange of decavanadate, and (3) a final calcination step at 550°C. Details of the synthesis procedure and its characterization can be found elsewhere.…”

The role of carbon species in heterogeneous catalytic processes: an in situ soft x-ray photoelectron spectroscopy study

“…multiple neighbouring reduced sites provide absorption sites for hydrocarbons, most likely unsaturated, causing coking and desorption hinderance for reaction intermediates [47]. Hess et al [58,59] observed an additional feature in their XPS of V x O y /SBA-15 samples, at even higher binding energy than expected for V…”