Vanadia and Water Coadsorption on Tetragonal Zirconia Surfaces

Abstract

The surface structure of vanadia supported on zirconia is examined by density functional theory with periodic boundary conditions. Adsorption of V 2 O 5 and H 2 O on the (101) and (001) surfaces of tetragonal zirconia is studied for different loadings up to (V 2 O 5 ) 4 or V 2 O 5 • 6H 2 O per eight Zr surface sites (monolayer coverage). The considered surface species include vanadia monomers, dimers, and polymers on water-free surfaces as well as different combinations of coadsorbed vanadia, hydroxyls, and water. Calculated vibrational spectra do not show unique features that could be used for identifying certain surface species. Statistical thermodynamics is used to evaluate the relative stability of different surface structures as function of vanadia loading, water partial pressure, and temperature (surface phase diagrams). The presence of water has a strong influence on the surface species. Under water free conditions and for Θ(V 2 O 5 ) ) 0.25 vanadia is present as a monomeric species on the (001) surface and as dimeric species on the (101) surface. For water partial pressures typical of oxidative dehydrogenation of propane (10 -2 bar, 775 K), hydrolysis of the dimeric species on the (101) surface is observed. The presence of water also affects the reactivity in partial oxidations as characterized by oxygen defect formation or hydrogen attachment energies. At Θ(V 2 O 5 ) ) 0.25 the presence of water stabilizes less reactive species, whereas at high vanadia coverage more reactive species are present.

Partial oxidation of ethanol on vanadia catalysts on supporting oxides with different redox properties compared to propane

“…Whereas for isolated dimeric vanadia species, the calculated values for zirconia (389 kJ/mol) are hardly different from the ones for alumina (386-401 kJ/mol, Figs. 6 and 7), significantly lower values (150-190 kJ/mol) have been obtained by DFT only for polymeric vanadia species on zirconia supports [75].…”

“…DFT calculations have also been performed for vanadia supported on zirconia [75]. Whereas for isolated dimeric vanadia species, the calculated values for zirconia (389 kJ/mol) are hardly different from the ones for alumina (386-401 kJ/mol, Figs.…”

“…As mentioned previously, several authors tried, more or less successfully, to correlate the ODH rate with the reducibility of the surface vanadyl species [6,79]. Oxygen defect formation also leads to a (partially) reduced catalyst, and hence, the oxygen defect formation energy has been also suggested as reactivity descriptor [13,15,75,80].…”

Optical switching of radical pair conformation enhances magnetic sensitivity

“…Azobenzene is an organic molecule composed of two phenyl rings linked by a N=N double bond, and occurs in two isomeric forms, trans and cis azobenzene, having a stretched, respectively contracted, structure. The synthesis of azobenzene derivatives having different chemical functional groups extending from the phenyl rings is routinely performed [18,19]. Preliminary checks have to verify that the chemistry of the radical pair is not drastically altered by the presence of the azobenzene and that the constraints on the RP distance are satisfied.…”

Stability of V2O5 Supported on Titania in the Presence of Water, Bulk Oxygen Vacancies, and Adsorbed Oxygen Atoms

“…Ti-O-V bridges; the hydrogen atom is bound to one of the yellow oxygens at the basis of the pyramid (Figure 2b). A study by Hofmann et al 40 that act as Lewis bases. [15][16][44][45] Here we examine the effect of an O vacancy present in the third layer of the TiO2 slab.…”