Two-dimensional Vanadia Research Articles

Topology of silica supported vanadium–titanium oxide catalysts for oxidative dehydrogenation of propane

Two-dimensional vanadia and titania surface clusters were hosted on the walls of the hierarchical pore system of mesoporous silica SBA-15. The topology of the catalyst surface was varied by sequential grafting of vanadium and titanium alkoxides generating an extended library of mixed (VOx)n–(TiOx)n/SBA-15 catalysts. The surface of the catalysts was analyzed by FTIR, UV-vis, Raman, and NEXAFS spectroscopy. Electron microscopy, X-ray fluorescence, X-ray diffraction, and nitrogen adsorption have been applied to characterize chemical composition, micro- and meso-structure of the materials. Segregation of nano-crystalline vanadia and titania particles was excluded by UV-vis, Raman and NEXAFS spectroscopy. Monolayer coverage of titanium oxide surface species has been achieved in the range between 17 and 19 wt% Ti loading corresponding to 6–8 Ti atoms per nmcat2 and Si/Ti ratios between 3.3 and 2.8. Up to a critical total metal loading, vanadia is grafted on both the silica surface and surface titania species yielding tetrahedrally coordinated vanadium oxo-species characterized by low nuclearity and moderate catalytic activity. A volcano-type dependency with respect to the propylene space-time yield has been observed in the oxidative dehydrogenation of propane. The maximum in productivity of propylene is attributed to a particular surface topology that is characterized by (VOx)n islands embedded in a matrix of dispersed titania species forming an almost complete combined vanadia–titania monolayer on the silica surface.

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

Vanadia “monolayer”-type catalysts supported on reducible oxides such as ceria previously have shown high activity for the selective oxidation of alcohols. Here, a model system consisting of vanadia particles deposited on well-ordered CeO 2(1 1 1) thin films has been employed. Scanning tunneling microscopy (STM), photoelectron spectroscopy (PES), and infrared reflection absorption spectroscopy (IRAS) were used to characterize the VO x /CeO 2 surface as a function of vanadia loading. The formation of isolated monomeric species as well as two-dimensional vanadia islands that wet the ceria support was directly observed by STM. The vanadia species exhibit V in a +5 oxidation state and expose vanadyl (V O) groups with stretching vibrations that blue-shift from ∼1005 cm −1 to ∼1040 cm −1 with increasing coverage. Temperature programmed desorption (TPD) of methanol revealed three peaks for formaldehyde production. One is correlated with reactivity on the ceria support (565–590 K). Another is correlated with reactivity on large vanadia particles (475–505 K) similar to that previously observed on vanadia/silica and vanadia/alumina model systems. A low temperature reaction pathway (∼370 K) is observed at low coverage, which is assigned to the reactivity of isolated vanadia species surrounded by a reduced ceria surface. It is concluded that strong support effects reported in the literature for the real catalysts are likely related to the stabilization of small vanadia clusters by reducible oxide supports.

Surface acidity of supported vanadia catalysts

The surface acidity of SiO2, γ-Al2O3 and TiO2 supported vanadia catalysts has been studied by the microcalorimetry and infrared spectroscopy using ammonia as the probe molecule. The acidity in terms of nature, number and strength was correlated with surface structures of vanadia species in the catalysts, characterized by X-ray diffraction and UV-Vis spectroscopy. It was found that the dispersion and surface structure of vanadia species depend on the nature of supports and loading and affect strongly the surface acidity. On SiO2, vanadium species is usually in the form of polycrystalline V2O5 even for the catalyst with low loading (3%) and these V2O5 crystallites exhibit similar amount of Bronsted and Lewis acid sites. The 25%V2O5/SiO2 catalyst possesses substantial amount of V2O5 crystallites on the surface with the initial heat of 105 kJ mol-1 and coverage of about 600 mmol g-1 for ammonia adsorption. Vanadia can be well dispersed on g-Al2O3and TiO2 to form isolated tetrahedral species and polymeric two-dimensional network. Addition of vanadia on γ-Al2O3 results in the change of acidity from that associated with g-Al2O3 (mainly Lewis sites) to that associated with vanadia (mainly Bronsted sites) and leads to the decreased acid strength. The 3%V2O5/TiO2 catalyst may have the vanadia structure of incomplete polymeric two-dimensional network that possesses the Ti-O-V-OH groups at edges showing strong Bronsted acidity with the initial heat of about 140 kJ mol-1 for ammonia adsorption. On the other hand, the 10%V2O5/TiO2 catalyst may have well defined polymeric two-dimensional vanadia network, possessing V-O-V-OH groups that exhibit rather weak Bronsted acidity with the heat of 90 kJ mol-1 for NH3 adsorption. V2O5 crystallites are formed on the 25%V2O5/TiO2 catalyst, which exhibit the acid properties similar to those for 25%V2O5 on SiO2 and γ-Al2O3.

In Situ UV–vis–NIR Diffuse Reflectance and Raman Spectroscopic Studies of Propane Oxidation over ZrO2-Supported Vanadium Oxide Catalysts

The molecular structures and oxidation states of supported 1–5% V2O5/ZrO2 catalysts during propane oxidative dehydrogenation (ODH), with varying propane/O2 ratios, were examined by in situ UV–vis–NIR diffuse reflectance and in situ Raman spectroscopic studies. The results indicate that the reduction extent of surface V5+ cations to V3+/V4+ cations under steady-state reaction conditions increases with the propane/O2 ratio. At the same propane/O2 ratio, the relative extent of reduction of the supported V2O5/ZrO2 catalysts generally increases with the surface vanadia loading, and the polymerized surface VO4 species are more extensively reduced than the isolated surface VO4 species during steady-state propane oxidation. The reactivity studies reveal that at the same reaction conditions, both polymerized and isolated surface V cations are active sites for propane oxidation and that the specific catalytic reactivity (as measured by turnover frequency; TOF) is independent of the surface density of the two-dimensional vanadia overlayer on the ZrO2 support. Furthermore, the relatively constant TOF with surface vanadia coverage demonstrates that propane ODH to propylene requires only one surface VO4 site. However, the propylene selectivity increases with increasing surface vanadia loading due to the removal of nonselective surface sites, possibly terminal Zr–OH groups, on the ZrO2 surface by the deposition of surface vanadia species. The propane/O2 ratio greatly affects the selectivity of these catalysts. Highly oxygen-rich environments (e.g., propane/O2 ratio = 1/10) give rise to the highest propylene selectivity, revealing that propylene production is favored on highly oxidized surface vanadia (+5) sites. Small V2O5 crystallites above monolayer surface vanadia coverage do not contribute to propane ODH because of their low dispersion and low number of active surface sites (spectator vanadia species).

A Study of the Structure of Vanadium Oxide Dispersed on Zirconia

Raman and UV−vis diffuse reflectance spectroscopy were used to characterize the structure of vanadia dispersed on high surface area zirconium oxide. Two-dimensional vanadia species with tetrahedral coordination appear on the surface of the ZrO2 and expand in size with increasing V loading. Crystalline V2O5 appears when the vanadia loading exceeds an apparent surface density of 7.0 V atoms/nm2, and ZrV2O7 is formed as a consequence of zirconia migration into the V2O5 crystallites. A model for the structure of two-dimensional vanadia overlayer is proposed based on the experimental data and information taken from the literature. Vanadia is found to absorb the light scattered by the support, and this gives rise to a reduction in the intensity of the Raman bands for zirconia as the surface loading of vanadia increases. Partial reduction of the dispersed vanadia increases the absorbance of the vanadia and alters the profile of absorbance versus frequency in such a manner as to increase the intensity of the Raman bands for zirconia relative to those for vanadia. Examination of Raman spectra taken after repeated reduction−oxidation cycles suggests that reduction occurs via removal of oxygen from the vanadia monolayer without agglomeration or reorganization of the surface species.

Vanadia on titania prepared by vapour deposition of vanadyl alkoxide: Influence of preparation variables on structure and activity for the selective ca

Vanadyl-triisopropoxide (VOTIP) has been used for vapour deposition out of a carrier gas stream onto titania to prepare titania supported vanadia. The catalysts were characterized by means of temperature-programmed reduction (TPR), Raman- and 51V nuclear magnetic resonance (NMR) spectroscopy and tested for the selective catalytic reduction of nitric oxide by ammonia. The influence of different preparation parameters on the structural and catalytic properties has been studied: (i) degree of dehydration (dehydroxylation) of the support; (ii) exposure to the VOTIP vapour phase and effect of successive depositions; and, (iii) calcination. The degree of dehydration of the support, which can be controlled by pretreatments, influences significantly the amount of deposited alkoxide. The upper limit of vanadia deposition that can be achieved by the used vapour deposition conditions is markedly lower than the one that can be reached by immersion of the carrier into liquid VOTIP. All preparation steps, including carrier pretreatment, deposition and calcination influence the structural and catalytic properties of the final catalysts. Highest intrinsic catalytic activity of the deposited vanadia has been observed above a threshold coverage, at which the formation of extended two-dimensional vanadia surface layers was revealed by Raman spectroscopy. Temperature-programmed desorption of ammonia showed a significant dependence of the profiles on the vanadia loading. On the pure titania support two distinct desorption maxima occurring at 405 K and at 520 K (broad) are observed. With increasing vanadia loading the broad maximum decreases, and simultaneously new sites are formed which correspond to ammonia weakly bound to Brønsted sites desorbing at 350 K. The latter sites are considered to be most active for selective catalytic reduction of nitric oxide.

Selective catalytic reduction of nitric oxide over vanadia grafted on titania. Influence of vanadia loading on structural and catalytic properties of catalysts

The selective catalytic reduction (SCR) of NO by NH3 has been studied over vanadia/ titania catalysts prepared by selective immobilization of vanadyl alkoxide species on two structurally different titania supports. The loading of vanadia was varied from 1.8 to 7.5 μ,mol V5+ per m2 surface area. Comparative kinetic measurements at 150 °C show that the NO turnover frequencies increase by more than an order of magnitude when the vanadia loading is increased from 1.8 to ≈ 3 μmol V5+/m2. In the region of lower SCR activity, i.e. at lower coverages (≈ 2 μmol V5+/m2), small clusters and ribbons of vanadia are detected in the Raman spectra, whereas at loadings where maximum NO turnovers are achieved (≈ 3 μmol V5+/m2) the prevalent vanadia species are well-developed two-dimensional vanadia layers bound to titania.