Monomeric Vanadium Oxide on a θ-Al₂O₃Support: A Combined Experimental/Theoretical Study

Abstract

A combined experimental and theoretical study of vanadium oxide monomers on a θ-alumina surface under different environments has identified four different structures. Deep UV Raman results suggest that vanadia is attached predominantly to an aluminum site that was an isolated terminal Al−OH group on the θ-alumina surface. The preresonance Raman spectra for vanadium oxide supported on θ-alumina with a very low VO x surface density show three distinct VO bands under dehydrated conditions. The observed frequencies match well with the calculated stretching frequencies from B3LYP density functional theory for tridendate, bidendate, and molecular structures of vanadium oxide monomers on a dehydrated surface. The free energies calculated for these three structures from density functional theory as a function of temperature suggest that all three could exist on the surface with the tridentate structure being the most stable of the three on the dehydrated surface. Different structures and different degrees of vibrational coupling of V−O to V=O modes may cause the appearance of three VO bands in the preresonance Raman spectra. On the hydrated surface, the Raman spectra show a V−O band, in agreement with the calculated frequency for a monodentate structure on this surface. Finally, the calculated free energies of hydrated and dehydrated surfaces indicate a transition from a hydrated to a dehydrated θ-alumina surface occurs at around 600 K at 10−6 atm pressure of H2O.

Phthalocyanine- and Calixarene-Templating Effect on the Catalytic Performance of Solid Supported Vanadates

“…Such behavior corresponds to low values of apparent activation energy (*14 kJ/mol) and low pre-exponential coefficient (*1.5) characteristic for the presence of highly active metal sites with low accessibility. Presence of different types of Al-OH binding sites on the surface of h-Al 2 O 3 is consistent with a reported theoretical and experimental study [35], which suggested that at the surface density of 0.16 V/nm 2 , vanadium tends to bind with the terminal Al-OH configuration, which is the most basic among all possible Al m OH configurations (m = 1, 2, 3). The possibility of different modes of vanadate binding to c-Al 2 O 3 at higher surface densities (4 V/nm 2 ) was also proved by the 51 V-NMR [14].…”

Role of dispersion of vanadia on SBA-15 in the oxidative dehydrogenation of propane

“…[30,31] Recent resonant Raman spectroscopy studies with different excitation-laser wavelengths even point to the presence of three different monomeric VO 4 species on alumina at a loading as low as 0.16 V/nm², although only a single band is resolved in the UV/Vis spectrum. [32,33] The presence of a mixture of different species, including non-monomeric vanadia, has been transmitted also to vanadia on SBA-15, based on near-edge X-ray absorption fine structure (NEXAFS) measurements. [34,35] The shift in the edge energy and adsorption maxima of the dehydrated V/SBA-15 samples in Fig.…”

“…[21] The band at 925 cm -1 is thus assigned to an out-of-phase vanadium-silicon support vibration, in agreement with recent literature. [30][31][32][33]39] The presence of three-dimensional (bulklike) vanadia can be sensitively monitored using Raman spectroscopy, mainly by vibrational resonances at 144, 284, 303, 404, 482, 525, 702 and 994 cm -1 . [19,22] All these peaks are visible in the spectrum of 14V/SBA-15 in Fig.…”

Relating methanol oxidation to the structure of ceria-supported vanadia monolayer catalysts

“…Evidence for the presence of monomeric VO 4 species on alumina has been found using x -ray absorption spectroscopy [12] , although Raman, infrared, and ultravioletvisible spectroscopies are the most frequently used techniques for characterization of vanadia surface species (e.g., see [9,[13][14][15][16][17][18] and references therein). The nature of many absorption bands in the spectra is generally understood (e.g., vanadyl or V=O), but the precise determination of the fraction of isolated versus polymeric vanadate species is not trivial.…”