Characterization of the synthesis and reactivity behavior of nanostructured vanadia model catalysts using XPS and vibrational spectroscopy

Abstract

Nanostructured vanadia model catalysts, i.e. highly dispersed vanadium oxide supported on mesoporous silica SBA-15 (VO x /SBA-15), were prepared. The mechanism for the synthesis of VO x /SBA-15 was elucidated by detailed characterization of the individual synthesis steps using XPS and vibrational spectrocopy. The resulting surface vanadium oxide species (0 -2.3 V/nm 2 ), grafted on the inner pores of the SBA-15 silica matrix, consists of tetrahedrally coordinated vanadia as inferred from UV-VIS-and Raman spectroscopy. The prepared vanadia model catalysts were tested in the partial oxidation of methanol to formaldehyde yielding high formaldehyde selectivities of 94% at 350ºC. XPS and Raman analysis of the catalyst after reaction reveal the presence of methoxy as well as a significant amount of carbonaceous species on the surface. Our results demonstrate that a detailed understanding of partial oxidation reactions requires the combination of complementary spectroscopic techniques ultimately within one experimental set-up.

Redox Properties of Vanadium Ions in SBA-15-Supported Vanadium Oxide: An FTIR Spectroscopic Study

“…Importantly, the formation of bridged nitrates points to the presence of associated vanadium surface species (dimmers or polymers) on 2.7 wt % V/SBA-15, i.e., well below the V loading at which V 2 O 5 crystal formation has been observed by Raman spectroscopy (≈9wt%V). 33 These findings are supported by the results of the CO adsorption experiments, as discussed above. …”

“…35 Details of the catalyst preparation are described elsewhere. 33 Functionalization of SBA-15 was achieved by stirring 2.5 g of SBA-15 in 100 mL of toluene at 338 K. 42 To this suspension was added 6.5 g of 3-aminopropyltrimethoxysilane (APTMS) with stirring. After stirring for 12 h, the contents were filtered, washed with toluene, and stirred in 150 mL of 0.3 M HCl for 12 h. The contents were filtered again, washed with water, and dried in air overnight.…”

“…33,34 SBA-15 is a promising new support material as (i) it possesses a well-defined structure based on the presence of uniform hexagonally arrayed channels and a narrow pore size distribution, 35 (ii) its large internal surface area (>800 m 2 /g) allows for the dispersion of a high number of catalytically active sites, 33 and (iii) its thick framework walls (3.1-6.4 nm) provide considerable hydrothermal stability, which exceeds those of the thinner-walled MCM-41 materials. Only recently, we have demonstrated the excellent performance of this catalyst system in methanol oxidation to formaldehyde and in propane oxidation to acrylic acid.…”

Oxidative dehydrogenation of propane over low-loaded vanadia catalysts: Impact of the support material on kinetics and selectivity

“…The impact of mass-and heat transfer effects on the ODP even over a low (< 2 V nm -²) loaded alumina supported catalyst has recently been reported [23]. The focus of this study are highly dispersed supported vanadia catalysts with the objective of a comparison of the gathered kinetic data with that originating from quantum chemical calculations [26] and other well characterized catalyst systems [28]. The disadvantage of catalysts providing a low surface density of vanadium is the possibility of propene adsorption on acidic sites of the bare support material, its consecutive decomposition and deep oxidation.…”

The influence of water on the dispersion of vanadia supported on silica SBA-15

“…Therefore, we have prepared vanadium oxide model catalysts via controlled grafting/anion exchange, which consist of highly dispersed vanadium oxide supported on the inner pores of the well-ordered mesoporous matrix of silica SBA-15 [18][19][20]. Only recently, we have demonstrated the excellent catalytic properties of this catalyst system in the propane partial oxidation to acrylic acid as well as methanol partial oxidation to formaldehyde [21,22].…”