Surface Vanadia Species Research Articles

Characterization of surface species of supported V 2O 5Al 2O 3 catalysts by 51V NMR

51VNMR spectra were used for the study of the interaction between vanadium oxocomplexes in solution and the Al 2O 3 surface, and for the identification of surface vanadium species formed over Al 2O 3 surface for different preparation conditions (pH, concentration of supported V, treatment temperature). At low vanadium concentration, mainly isolated tetrahedral surface complexes are formed over the Al 2O 3 surface, bound with one or two bonds. The increase in vanadium concentration results in the formation of surface clusters with a part of the V in a distorted octahedral environment. After treatment at 500 °C and at a V concentration of ∼ 10 wt.%, the formation of AlVO 4 and V 2O 5 crystals over the Al 2O 3 surface takes place.

Spectroscopic investigation of the structure of silica-supported vanadium oxide catalysts at submonolayer coverages

Surface vanadia species on a high-surface-area silica support have been investigated by UV/visible diffuse reflectance, infrared and laser Raman spectroscopies. The catalysts were prepared and characterized under water-free conditions, as the hydration state of the surface strongly influences the structure of the dispersed vanadia. Two types of surface species have been identified. Monomeric and oligomeric vanadia species, with a tetrahedral coordination geometry of oxygen ligands around the central vanadium ion, give rise to a narrow VO stretching absorption at 1040 cm–1. The second species is characterized by a Raman band at 920 cm–1 and a charge-transfer absorption at 28 000 cm–1, and corresponds to vanadia ribbons of limited lateral extent, with the vanadium ion situated in the centre of a square pyramid. Vibrational data from aqueous vanadate ions are inadequate as a reference for describing the vanadia species on a dehydrated surface.

Structure and acidity of vanadium oxide layered on titania (anatase and rutile)

The adsorption of pyridine on, and the structure of, two series of V2O5/TiO2(anatase and rutile) samples, prepared by the gas-phase method (1.2–3.4 wt.%), have been studied. V–Ti oxides show both Brønsted- and Lewis-acidic sites. The Brønsted sites increases with increasing vanadium loadings. The number of Lewis sites on the anatase samples is greater than on the rutile form. From Raman and in situ FTIR studies the vanadium oxides on both series are composed mainly of surface vanadia species. These are stable during heat treatment at 673 K. The VO character of a monolayer rutile sample is different from that of a monolayer anatase sample, and resembles that of multilayer samples.

Comparison of grafted vanadia species on ZrO2, TiO2, SiO2 and TiO2/SiO2 mixed oxides

Vanadia layers have been deposited onto several zirconia, titania, silica and titania—silica mixed-oxide supports, using the selective reaction of vanadyl triisopropoxide with surface hydroxy groups of the carrier material. The influence of the support, its crystallinity and its crystallographic modification, on the structure of the immobilized vanadia layers were investigated. Surface vanadia species formed upon single and multiple graftings were characterized by Raman, diffuse reflectance FTIR (DRIFT) and UV–VIS diffuse reflectance spectroscopy. On ZrO2, the rapid formation of multilayer structures was observed, as a consequence of the weak interaction between this support and the dispersed vanadia. Comparing the vanadia grafted on low-surface area crystalline ZrO2 with the one grafted on high-surface-area amorphous ZrO2, we noted that the crystallinity and surface area, which are of lesser importance at low coverage, exert an increasing influence on the vanadia structure at higher loadings. On TiO2, small clusters, two-dimensional layer structures and multilayers are identified. The high-surface-area silica support favours a high dispersion of vanadia in the form of clusters and ribbons of limited lateral extent; the influence of the surface hydration state and of thermally induced structural changes of the vanadia species are investigated. On the TiO2/SiO2 mixed-oxide gel, vanadia species are anchored on TiO2 at low loadings and spread out over the SiO2 fractions of the surface at higher coverages.

Solid state 51V NMR structural studies of vanadium(V) oxide catalysts supported on TiO 2(anatase) and TiO 2(rutile). The influence of surface impurities on the vanadium(V) coordination

Solid state 51V wideline NMR studies show that under ambient conditions the vanadium (V) oxide surface phases on TiO 2(anatase) and TiO 2(rutile) supports predominantly possess distorted-octahedral coordination. However, the coordination environment of vanadia is markedly influenced by the presence of impurities in the support materials. Surface contaminants promote the formation of tetrahedral surface vanadia species, which preferentially form at low surface coverages. The presence of these surface impurities depends on the titania preparation method and overshadows the influence, if any, of the bulk TiO 2 lattice structure (anatase versus rutile). Thus, the strong influence of surface impurities on the V 2O 5/TiO 2 system is most likely responsible for the widely varying claims about differences in the catalytic properties of V 2O 5/TiO 2(anatase) versus V 2O 5/ TiO 2 (rutile) samples.

Metal contaminant effects on the properties of a silica-rich fluid cracking catalyst

A fluid cracking catalyst (FCC) was prepared by spray drying a solid slurry obtained by first titrating diluted sodium silicate with an acid alum solution to pH = 3.0 and then by adding to the hydrogel a partially dealuminated kaolin mineral, together with calcined rare earth exchanged zeolite Y (CREY). Catalyst contamination by surface vanadium and nickel species is correlated with observed luminescence quenching of the zeolite. Steaming causes Ni migration from luminescence centers and restoration of band intensity. In contrast, vanadium irreversibly quenches REY emission bands. XPS data indicate that steaming causes nickel (and vanadium) migration to the cracking catalyst surface where nickel sinters and nickel crystallites 50 to 70 μm in size are observed by TEM. The SiO 2-rich matrix prevents good Ni distribution on the surface; large Ni crystallites and inert NiSiO 3 formation are believed responsible for the catalyst's low carbon and hydrogen generation during gas oil cracking.

The interaction of V 2O 5 with Ti0 2(anatase): Catalyst evolution with calcination temperature and O-xylene oxidation

The interaction of V 2O 5 with the surface of TiO 2(anatase) was studied over the temperature range 110–750 °C. The V 2O 5 TiO 2 (anatase) system was characterized with laser Raman spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, X-ray diffraction, thermal gravimetric analysis, BET, and catalytic performance for o-xylene oxidation to phthalic anhydride. The state of V 2O 5 TiO 2 (anatase) possessing high loadings of vanadia is strongly dependent on calcination temperature. In the presence of vanadia the TiO 2(anatase) support exhibits a simultaneous loss in surface area and structural transformation to rutile at elevated calcination temperatures. The morphology of the supported vanadia phase also depends on calcination temperature. At low calcination temperatures, 110–200 °C, the vanadia exists as vanadyl oxalate, the starting vanadia salt. At intermediate calcination temperatures, 350–575 °C, vanadia is present as a complete monolayer of surface vanadia species coordinated to the titania support and V 2O 5 crystallites. At calcination temperature of 575 °C and above, the supported vanadia phase reacts with the TiO 2(anatase) support to yield V x Ti 1 − x O 2(rutile). These structural changes have a pronounced effect on the catalytic performance of V 2O 5 TiO 2 (anatase) catalysts for the oxidation of o-xylene. The optimum catalytic performance is observed for prolonged calcination at intermediate temperatures, 350–575 °C, where a complete monolayer of surface vanadia exists on the TiO 2(anatase) support. The complete monolayer of surface vanadia and crystalline vanadia phases remain intact during the o-xylene oxidation reaction and become partially reduced by the reaction environment.

The interaction of vanadium pentoxide with titania (anatase): Part I. Effect on o-xylene oxidation to phthalic anhydride

Two types of vanadia are present in active V 20 5/TiO 2(anatase) catalysts:a surface vanadia species coordinated to the TiO 2 support and crystallites of V 20 5. The surface vanadia is the active site in V 20/TiO 2(anatase)catalysts for the oxidation of o-xylene to phthalic anhydride. The TiO 2(anatase)support must be covered by a complete monolayer of the surface vanadia species since exposed titania sites lead to complete combustion of the partial oxidation products. The surface vanadia species possess a higher activity and selectivity than crystalline V 20 5 for this oxidation reaction. Moderate amounts of crystalline V 90 5 do not significantly affect the catalytic performance of V 20 5/TiO 2(anatase) because of the low effective surface area and poor catalytic activity of this phase. The unique properties of the surface vanadia species are related to the vanadiatitania interaction.