Dispersion and thermal stability of vanadium oxide catalysts supported on titania-alumina binary oxide (original) (raw)
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Thermochimica Acta, 2004
Titania supported vanadia catalysts (2.5, 5, and 11 wt.% V 2 O 5 ) were prepared by a wet impregnation technique and their thermal behavior, morphology as well as redox properties were examined by thermal analysis methods thermogravimetry (TGA), differential scanning calorimetry (DSC), temperature programmed-evolved gas analysis with mass spectroscopy, (EGA-MS), scanning electron microscopy (SEM), and temperature programmed reduction (TPR). The two Eurocat samples EL10V1 and EL10V8 containing 1 and 8 wt.% V 2 O 5 were also characterized using the same techniques. Thermal decomposition of vanadium oxide precursors (ammonium vanadyl oxalate) supported on TiO 2 as evidenced by thermal analysis, occurs in three successive steps, which are influenced by the surrounding atmosphere (oxidative, reductive, and inert). The presence of tower-like vanadia crystals in the sample with the highest vanadia loading (11 wt.% V 2 O 5 ) was identified by SEM. The H 2 -TPR experiments revealed that the reduction temperature is a factor of the vanadia loading and the type of support. Vanadia species supported on Norton titania are more reducible that those supported on Eurocat titania.
Applied Catalysis A: General, 1992
Different phases of titania were prepared and used to support ca. 1 wt.-% V,O,. The different tit&a phases prepared were: anatase (A22), rutile (F&28), brook& (BTllO) and B-phase (B18). Physical characterization of the various vanadia-tit&a catalysta was performed using X-ray photoelectron spectroscopy (XPS), in situ Raman and "'V solid stats nuclear magnetic resonance (NMR) spectroscopy. The XPS results reveal that the aiI the cataiysta contain various levele of impurities. In situ dehydration Raman shows, for all the samples, the stretching vibration of the terminai V= 0 bond at ca. 1030 cm-'. Solid state s'V NMR spectra of all the samples in the dehydrated state show basically the same powder pattern with a peak maximum around -660 to -670 ppm. The combined Raman and NMR results indicate that the same surface vanadium oxide species is present on aii the titania supports irrespective of the crystai structure of the bulk tit&a phase. Partial oxidation of methanol show similar activity and selectivity for the various vanadia-titania catalysts. The reaction selectivity was primarily to formaldehyde and methyl formate (9296% ) . The turnover number for methanol oxidation was essentially the same for aii the vanadia-titania cat&&s and ranged from 1.4 to 2.8 s-l. These results indicate that the type of titania phase used as the support is not criticaI for partial oxidation over vanadia-tit&a catalysts as long as other parameters (e.g. surface impurities ) are similar. Thus, the structure-reactivity Correspondence to: Prof. I.E. Wachs, ZettIemoyer Center for Surface Studies, Department of Chemical Engineering, 7 Asa Drive, Lehigh University, Bethlehem, PA 16015, USA. Tel. ( + l-215)7564274, fax. (+ l-215)7593079. 0926-3373/92/$05.00 0 1992 EIsevier Science Publishera B.V. Ail rights reserved.
Characterization and reactivity of vanadium oxide supported on TiO 2 -SiO 2 mixed oxide support
Molecular Catalysis, 2018
A TiO 2-SiO 2 support, 90Ti-Si, containing 90 wt.% TiO 2 was synthesized by the sol-gel method. Several supported vanadium oxide (vanadia) catalysts, xV90Ti-Si (x = 2-12.5 wt.%) were synthesized by incipient wetness impregnation method using this support. The catalysts and support were characterized and probed with the propane oxidative dehydrogenation (ODH) reaction. The support possesses no rutile phase and retains 30-40% of its initial surface area even after calcination at 1073 K. The surface area loss is accelerated in the presence of vanadia and the rutile phase appears below 1073 K. Surface vanadia species are present for the supported vanadia catalysts calcined at 673 K and the propane ODH activity and propene yield increases with vanadia loading. The ODH activity of the 2V90Ti-Si sample at 673 K is retained even after the catalyst is pre-calcined at 1023 K. Interestingly, the activity of the 4V90Ti-Si sample at 673 K reaches a maximum at an intermediate precalcination temperature. The higher vanadia loading samples deactivate with an increase in calcination temperature. The ODH reactivity reveals that the propene yield at iso-conversion increases with loading. Analysis of kinetic parameters also reveals that the rate constant ratio increases with vanadia loading.
2000
Ti-ZSM-5 is prepared via a chemical vapor deposition method by reacting HZSM-5 with TiCl 4 at temperatures of 200-400 • C. Ti-ZSM-5 is characterized by skeletal-and surface hydroxy-FT-IR, XPS, and XANES spectroscopy. It seems that Ti is incorporated in the zeolite surface with tetrahedral coordination. Contents of incorporated Ti atoms in Ti-ZSM-5 zeolite increase with increasing SiO 2 /Al 2 O 3 ratio and a CVD reaction temperature of 400 • C is optimal. Cyclohexanone ammoximation was used as test reaction and Ti-ZSM-5 catalysts show similar catalytic activities compared to TS-1.
Colloids and Surfaces, 1990
Solid state 51V wideline NMR studies show that under ambient conditions the vanadium (V) oxide surface phases on TiO,(anatase) and Ti02(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 Ti02 lattice structure (anatase versus rutile). Thus, the strong influence of surface impurities on the V205/Ti02 system is most likely responsible for the widely varying claims about differences in the catalytic properties of V,05/ Ti02 (anatase) versus V20S/Ti02 (rutile) samples. INTRODUCTION V,O, supported on TiOz is known to be an important oxidation catalyst [l-111, specifically for the partial oxidation of o-xylene to phthalic anhydride. Catalytic studies have suggested that V205/Ti02 (anatase) is a superior catalyst than V205/Ti02 (rutile) for this oxidation [ 121. Early studies attributed the higher activity of the V205/Ti02 (anatase) to the ease of oxygen evolution under inert environments [ 2,131. Vejux and Courtine [ 131 ascribe the higher activity of the Vz05/Ti02 (anatase) catalyst to the crystallographic fit between pure V205 (010 plane) and pure TiO, (anatase) (010 or 001 plane). Likewise, the lower activity of V205/Ti02 (rutile) was attributed to the misfit of the lattice parameters of the two corresponding bulk phases. Since then, these con
Journal of Solid State Chemistry, 1996
High surface area titania-supported materials prepared from V(IV) precursors and calcined at high temperatures have been characterized by Vis–UV diffuse reflectance, FT Raman, electron spin resonance, and X-ray photoelectron spectroscopies and tested in the partial oxidation of methane. Vanadium oxide loading and calcination temperature determine the structure of V2O5/TiO2materials. Below theoretical surface monolayer coverage, V(IV) species closely interacting with the support are observed. Vanadiam oxide species anchor by reaction with titanium oxide surface hydroxyl groups. The V(IV) species are stabilized by interaction with titania support and further stabilization occurs at high calcination temperatures by their location in titania (rutile) lattice. Larger loadings of vanadium decrease the temperatures required for conversion of titania (anatase) to titania (rutile). At higher vanadium loading segregation into bulk V2O5oxide takes place, thus decreasing interaction with titania support. This enables a larger population of V(V) species than samples with surface dispersed vanadium oxide species. Although partial oxidation of methane is nonselective on titania (anatase), partial oxidation products are observed on titania (rutile)-supported vanadium oxide catalysts. The higher selectivity to partial oxidation product formaldehyde appears to be related to the high stability of V(IV) cations located on rutile lattice and the absence of V(V) sites.
Surface reactivity and morphology of vanadia-titania catalysts
Surface Science Letters, 1991
Vanadia/ titania samples have been prepared from two different titania supports (P-25 from Degussa and anatase from Tioxide) by impregnation with aqueous solutions of NH,VO, and calcination at 723 K. A decrease in the specific surface area is observed on increasing the amount of vanadia; titania from Degussa being non-porous, only development of wide pores is observed upon supporting vanadia. However, while the unloaded Tioxide support shows pores with average diameters of 8 and 5 nm. these latter disappear upon incorporation of vanadia. The reactivity of the V,Os/TiO, sample prepared using the P-25 support in olefin oxidation has been studied by FTIR spectroscopy in the temperature range 150473 K. Ethylene and propene are adsorbed as such at low temperatures, become hydrated to alkoxy species at medium temperatures and later undergo oxidation to carbonyl compounds (acetaldehyde and acetone). Allylic oxidation of propene and oxidative breaking of the C=C bonds is also observed.