Amorphous Vanadia Research Articles

High performance vanadia–anatase nanoparticle catalysts for the Selective Catalytic Reduction of NO by ammonia

Highly active nanoparticle SCR deNO X catalysts composed of amorphous vanadia on crystalline anatase have been prepared by a sol–gel, co-precipitation method using decomposable crystallization seeds. The catalysts were characterized by means of XRPD, TEM/SEM, FT-IR, nitrogen physisorption and NH 3-TPD. Due to the high-surface area anatase particles, loading of 20 wt% vanadia could be obtained without exceeding monolayer coverage of V 2O 5. This resulted in unprecedented high deNO X SCR activity corresponding to a factor of two compared to an industrial reference and to other V 2O 5/TiO 2 catalysts reported in the literature in the examined temperature range of 200–400 °C. The catalysts showed very high resistivity towards potassium poisoning maintaining a 15–30 times higher activity than the equally poisoned industrial reference catalyst, upon impregnation by 280 μmole potassium/g of catalyst.

Vanadia–titania thin films for photocatalytic degradation of formaldehyde in sunlight

Thin films of vanadia–titania with good adhesion to the substrates have been deposited on various substrates such as glass slides, glass helix and silica raschig rings by simple sol–gel dip coating process using vanadium and titanium peroxide gel. The optimum concentration of vanadia in titania for obtaining good uniform viscous gel was found to be 0.5–4 wt% beyond which the vanadia particles disturb the gel network, resulting in the formation of a gelatinous precipitate. The films of vanadia–titania as well as the dried powder of the bulk gel were characterized by different characterization techniques. Optical characterization by UV–vis spectrophotometer showed a shift in optical absorption wavelength to the visible region that may be due to the incorporation of vanadia into titania structure. The XRD revealed the formation of anatase phase in pure titania as well as titania with up to 2% vanadia loading, whereas formation of rutile as minor phase along with anatase as major phase was observed at higher vanadia loading. The XRD did not show any peaks of vanadia phase up to 5% vanadia loading indicating either incorporation of vanadia into titania structure or high dispersion of amorphous vanadia on titania support. The pure and vanadia doped TiO 2 thin films were evaluated for their photocatalytic activity for degradation of methylene blue as a model pollutant under sunlight. Doping of V 2O 5 in TiO 2 showed an increase in the photo-degradation rate of methylene blue by a factor of 3–6.6 times compared to pure TiO 2. The highest rate has been obtained for 4% V 2O 5-doped TiO 2 films. Vanadia doped TiO 2 thin films were also found to be very active for photocatalytic degradation of formaldehyde from aqueous solution in sunlight.

ALD of Vanadium Oxide

Experiments on the atomic layer deposition (ALD) of vanadium oxide from vanadyl triisopropoxide are presented. Water and oxygen plasma based processes are compared to the thermal process which uses water as a reagent. The plasma enhanced processes enable a fast ALD of films in a wide temperature range (50-200{degree sign}C). Pure, crystalline V2O5 can be grown with oxygen plasma ALD at 150{degree sign}C. Water based ALD resulted in carbon-contaminated, amorphous vanadia. The plasma steps were monitored with optical emission spectroscopy.

Nanoparticles of vanadia–zirconia catalysts synthesized by polyol-mediated route: Enhanced selectivity for the oxidative dehydrogenation of propane to propene

A polyol-mediated route was employed to obtain nanoparticles of vanadia–zirconia (∼10 nm) and Ti4+-modified zirconia catalyst for the selective oxidative dehydrogenation reaction of propane to propene. The catalytic activity and selectivity of samples thus prepared were compared with the values for the sample synthesized by the conventional impregnation method. More dispersed amorphous vanadia species on zirconia support could be obtained by polyol method compared to those obtained by conventional impregnation route, as discerned from transmission electron microscopy (TEM) and Raman studies. Raman spectra of samples prepared by polyol method indicated the presence of monovanadate and polyvanadate species on the zirconia support surface, whereas the impregnated sample showed the existence of aggregated vanadia besides mono and polyvanadate species though the vanadia loading was the same on all samples. XPS studies revealed that vanadia existed as both V5+ and V4+ in the samples prepared by the polyol method, whereas only V5+ state was seen in the impregnated sample. The catalysts prepared by polyol method exhibited enhanced selectivity for propene formation compared to the sample prepared by impregnation method. The enhanced selectivity is attributed to the presence of dispersed vanadia species with lower valence state of vanadium. The present results demonstrate that the polyol-mediated synthesis is an efficient method for the preparation of supported vanadia catalysts containing such active species.

Molecular Dynamics Simulations of Lithium Diffusion in Silica-Doped Nanocrystalline V2O5

Molecular dynamics computer simulations are used to study lithium-ion diffusion in doped amorphous vanadia intergranular films (IGFs) in nanocrystalline cathodes. Previous simulations showed rapid transport paths for Li intercalation from a solid electrolyte into vanadia cathodes via amorphous IGFs separating the vanadia crystals as a function of IGF thickness. However, ordering at the IGF/crystalline vanadia interface affected Li intercalation and required a minimum IGF thickness for rapid transport. The current simulations evaluate the role of Si (as ) as an impurity in the IGF on interface ordering and Li transport. The simulations are done on the three different concentrations of Si ions in the IGF: 5, 10 and 15% in molar percentage. Results show that the presence of Si in the IGF retards the lithium-ion diffusion in IGF and hence, intercalation.

Reducibility, heats of re-oxidation, and structure of vanadia supported on TiO 2 and TiO 2–Al 2O 3 supports used as vanadium traps in FCC

V/TiO 2 and V/TiO 2–Al 2O 3 (1:1 w/w basis) supports were characterized by TPR, Raman spectroscopy, and heats of re-oxidation of samples pre-reduced in CO at 770 K with a heat-flow calorimeter. Supports were pure anatase or rutile dispersed with hydrated aluminas (boehmite, gibbsite, bayerite) subsequently calcined at 870 K. Raman spectroscopy of fully oxidized, air-exposed samples show the presence of polymeric polyvanadate species, but not of isolated monomeric species. Sample loadings were 4 wt.% and show different reduction and structural features. During TPR, vanadia reduced to V(III) and V(IV) in V/rutile and V/anatase, respectively, and multiple reduction peaks were observed due to crystalline V 2O 5 and amorphous vanadia. In V/TiO 2–Al 2O 3 samples, vanadium coverages were 6–8 μmol V m −2 yielding well-dispersed, amorphous vanadia. Trends observed during TPR were: addition of bayerite phase to anatase or rutile increases H 2 consumption by 100%, implying formation of V(III) and V(II), respectively. However, with addition of boehmite or gibbsite to either titania phase, vanadia reduces only to V(IV). Oxygen doses at 473 K of pre-reduced samples titrated about one-third of total vanadia content. Re-oxidation heat values range from 400 to 500 kJ mol −1 O 2 and represent oxygen–vanadium ion bond strengths within the dispersed vanadia. The heat values are higher than expected for re-oxidation of a bulk phase, and are indicative of the degree of stabilization provided by the support.

Molecular Dynamics Simulations of Li Insertion in a Nanocrystalline V 2 O 5 Thin Film Cathode

The behavior of lithium ion diffusion from an electrolyte into a polycrystalline layered cathode has been studied using molecular dynamics computer simulations. Lithium silicate glass was the model solid electrolyte while the cathode was a nanocrystalline vanadia with amorphous intergranular films (IGF) between the crystals. Nanosized crystals were aligned with their (001) planes parallel to electrolyte/cathode interface, rotated 90° from each other around this interface’s normal in order to present two different orientations between the crystal planes for lithium intercalation via the amorphous vanadia IGF. A series of nanocrystalline vanadia cathodes with different IGF thicknesses was simulated to examine the effects of the IGF thickness on lithium transport into the cathodes. Results showed preferential diffusion of Li from the electrolyte into the amorphous vanadia IGF, with some of those Li diffusing into the crystalline from the IGF. Results also showed easier lithium diffusion from the IGF into the crystal along the 〈010〉 direction than along the 〈100〉 direction. Additionally, an optimum IGF thickness of 2.5-3.0 nm is suggested as being neither too thick to decrease the capacity of the cathode nor too thin to impede the transport of lithium from glassy electrolyte into the cathode. © 2005 The Electrochemical Society. All rights reserved.

Molecular dynamics simulations of Li transport between cathode crystals

The molecular dynamics (MD) computer simulation technique has been used to study the effect of an amorphous intergranular film (IGF) present in a polycrystalline cathode on Li transport. The solid electrolyte is a model lithium silicate glass while the cathode is a nanocrystalline vanadia with an amorphous V 2O 5 IGF separating the crystals. Thin (∼1 to a few nanometer thick) IGFs are known to be present in most polycrystalline oxide materials. However, the role of such a film on Li transport in oxide cathodes has not been addressed. Current scanning probe microscopy (SPM) studies have shown that the orientation of the layered nanocrystalline vanadia crystals near the cathode/solid electrolyte interface is not optimized for Li ion transport. While the precise structure of the material between the crystals has not been identified, initially it can be initially considered as likely to be a thin non-crystalline (amorphous) film. This is based on the ubiquitous presence of such a structure in other polycrystalline oxides. Also, and with more relevance to the materials used in thin film batteries, an amorphous film can be expected to form between nanocrystals that crystallized from an amorphous matrix, as would be the case in a deposited thin film cathode. Consistent with simulations of Li transport in amorphous vanadia, the current simulations show that Li ions diffuse more rapidly into the amorphous intergranular thin film than into the layered vanadia with the (0 0 1) planes parallel to the cathode/electrolyte interface.

Oxidative Dehydrogenation of Ethylbenzene over Sm2O3–V2O5 System

Abstract Sm2O3/V2O5 catalysts were synthesized by wet impregnation of samaria with an aqueous solution of ammonium trioxovanadate(V). The characterization of the prepared samples was carried out using EDX, XRD, FTIR and TG-DTA as well as a measurement of the surface area and pore volume. The catalysts were found to be stable up to 800 °C. Both amorphous and crystalline vanadate species were detected in the supported system. Vanadia species became anchored to the surface through an interaction with basic hydroxy groups and Lewis acid sites of the surface. Lewis acid and base sites decreased upon vanadia addition along with a concomitant enhancement of Brønsted acid sites. The oxidative dehydrogenation of ethylbenzene was carried out over Sm/V catalysts, which exhibited a styrene selectivity > 80%. The active species present in the supported system was concluded to be amorphous vanadia in tetrahedral coordination. The higher vanadia-loaded systems exhibited constant styrene selectivity, where crystalline tetraoxovanadates were predominantly formed.

Mechanism of surface spreading in vanadia-titania system

Spreading of vanadia over TiO2 (anatase) supports of different specific surfaces has been investigated by XRD, XPS and ESR. It has been found that vanadia migration is favoured over well-developed anatase crystal planes. At 723 K spontaneous spreading takes place, controlled by diffusion of vanadium species through the vanadia monolayer. In reducing atmosphere an induction period is observed, possibly initiated by formation of a structurally modified titania surface layer. Amorphous vanadia is postulated as a transient form between crystalline V2O3 and the vanadia monolayer.

Ammoxidation of Propane over Antimony Vanadium-Oxide Catalysts

Catalysts belonging to the Sb-V-O system were prepared with various Sb/V ratios and were used for propane ammoxidation to acrylonitrile. XRD patterns of freshly prepared samples show those with excess vanadia to consist of V2O5 and SbVO4, while SbVO4 and α-Sb2O4 are constituents in the samples with a Sb/V ratio above unity. High rate and selectivity for propylene formation at low conversion are characteristic for samples with excess vanadia and considering XRD, Raman, infrared, and XPS results, this is explained by formation of amorphous vanadia spread over the surface of SbVO4. Catalysts with both α-Sb2O4 and SbVO4 phases are the most selective for acrylonitrile formation, a function that is linked to their ability to selectively transform intermediate propylene. XPS data suggest this function to be associated with the formation of suprasurface antimony sites on SbVO4 as a result of migration of antimony from α-Sb2O4 during the catalytic process. Raman and infrared spectral features revealed that compared with SbVO4, the samples with both α-Sb2O4 and SbVO4 are more efficiently reoxidised during propane ammoxidation. Rate dependences on the partial pressures of reactants over a sample with excess α-Sb2O4 show that the adsorption of propane is the rate limiting step for propylene formation, and that acrylonitrile and carbon oxides are predominantly formed from the intermediate propylene in routes comprising nonequilibrated steps. Addition of water vapour results in an increase of rate and selectivity for acrylonitrile formation. The kinetic dependences indicate that for acrylonitrile formation it is advantageous to have a feed rich in propane and to use recirculation for obtaining high productivity.

Zirconia-supported vanadium oxide catalysts for ammoxidation and oxidation of toluene: A characterization and activity study

A series of samples of vanadia supported on monoclinic zirconia were prepared with nominal loadings from a half up to sixteen theoretical vanadia layers. The samples were characterized with X-ray diffraction, scanning electron microscopy combined with energy dispersive X-ray analysis, high-resolution electron microscopy, Raman and diffuse reflectance infrared spectroscopy, and were used in the oxidation and the ammoxidation of toluene. At loadings in the monolayer range, Raman and infrared bands from decavanadate-like and dehydrated tetrahedral vanadia species were at ca. 990 and ca. 1025 cm−1, respectively. Raman bands at 821 and 880 cm−1 were present only at the lowest loading and are characteristic of orthovanadate and pyrovanadate species, respectively. X-ray diffraction, Raman and infrared spectroscopic results revealed formation of some crystalline V2O5 and ZrV2O7 at loadings exceeding a theoretical monolayer. In this case, consideration of Raman intensity variations allowed the conclusion that additional non-crystalline vanadia must be present. According to high-resolution electron micrographs, this vanadia consists of an amorphous overlayer, 4–8 atomic layers thick. In toluene oxidation zirconia-supported vanadia compared with crystalline V2O5 was found less selective for benzaldehyde formation. In toluene ammoxidation, on the other hand, vanadia on zirconia was found to possess good activity and selectivity for benzonitrile formation. Amorphous vanadia was the most active structure on zirconia, while the selectivities for nitrile and aldehyde formations were almost independent of the loading for one theoretical layer and above.

Vanadia, vanadia-titania, and vanadia-titania-silica gels: Structural genesis and catalytic behavior in the reduction of nitric oxide with ammonia

Metal oxide xerogels of pure vanadia, vanadia-titania, and vanadia-titania-silica were prepared by the hydrolysis of vanadium, titanium, and silicon alkoxides. The structural and chemical transformations of the gels occurring during thermal treatment under oxidizing and inert gas atmosphere were studied using thermoanalytical methods (TG, DTA) combined with mass spectroscopy, X-ray diffraction, and electron microscopy. Crystallization of the amorphous vanadia was found to be considerably retarded in the mixed gels compared to the pure vanadia gel. The mixed gels remain amorphous at 520 K, whereas crystallization already occurs in the pure vanadia gel at this temperature. All mixed gel phases separate after stabilization at 870 K, with the formation of V 2O 5 platelets and globular TiO 2 (anatase) particles. No other phases were detected by X-ray diffraction at this temperature. Addition of silica sol increases the overall surface area of the gel catalysts. This, however, does not increase the dispersion of vanadia in the amorphous matrix or stabilize it sufficiently to prevent crystallization under oxidizing conditions. Heat treatment of the gels at temperatures close to the melting point of vanadia results in rapid transformation of anatase to rutile. The stability and catalytic activity of the gels for the selective catalytic reduction of NO with NH 3 (SCR) were studied following thermal treatments under in situ and oxidizing (7% O 2) atmosphere. Low temperature SCR testing ( T ⩽ 520 K) showed that the same kinetic behavior typical of titania-supported vanadia exists in all gels, with activity behavior being a function of gel composition and dispersion.

Vanadia catalysts on anatase, rutile, and TiO 2(B) for the ammoxidation of toluene: An ESR and high-resolution electron microscopy characterization

Three titania polymorphs, anatase, rutile, and TiO 2(B), were used as supports for vanadia. Catalysts with nominal loadings corresponding to 1.5 and 5 theoretical layers were prepared. True monolayer samples were obtained by NH 3(aq)-treatment of the samples with 5 nominal vanadia layers. Prepared catalysts were characterized by chemical analysis, ESR, and high-resolution electron microscopy. It was observed that an almost complete monolayer of vanadia can be formed on anatase and TiO 2(B). The monolayer on TiO 2(B) consists of V 4+ species. A magnetic species attributable to tetrahedral states of vanadium was found to dominate the ESR resonance. Chemical analysis of the monolayer on anatase showed the presence of both V 4+ and V 4+ species. The ESR spectrum indicated a high degree of V 4+-V 4+ interaction. From the characterizations of rutile-supported samples it seems that the support interface is a solid solution of V 4+. On all supports, vanadium on top of the monolayer is present as V 5+. High-resolution micrographs of the samples with the highest loading revealed the formation of amorphous vanadia on both anatase and rutile. On TiO 2(B), vanadia was found to grow coherently at the titania interface. The catalysts were used in the oxidation and ammoxidation of toluene to benzaldehyde and benzonitrile, respectively. Considering both activity and selectivity, multilayers of vanadia supported on TiO 2(B) show a good performance for both reactions.

Methanol oxidation on vanadium oxide catalyst prepared by in situ activation of amorphous vanadium pentoxide precursor

Amorphous vanadium pentoxide was prepared using the sol-gel process and vanadyl triisobutoxide as precursor. The catalytic properties of this material for methanol oxidation to formaldehyde were investigated in the temperature range 300 to 520 K. At 520 K the initial activity of the amorphous V 2O 5 was very low. However, during in situ activation at this temperature the activity increased steadily and reached a steady-state value which was about 30 times higher than the initial activity of the amorphous V 2O 5 and also considerably higher than the activity of crystalline V 2O 5. X-ray diffraction indicated that upon exposure to methanol oxidation conditions the amorphous vanadia crystallized slowly and was partly reduced. These changes were accompanied by drastic alterations of the bulk and surface structure of the amorphous precursor as evidenced by electron microscopy. The final catalyst exhibiting steady-state activity consisted of crystalline V 2O 5 and V 3O 7. Partial reduction of the bulk was not observed with crystalline vanadia catalysts and appears to be characteristic for the amorphous vanadia. The results indicate that amorphous vanadia undergoes structural changes more easily than its crystalline counterpart and may thus be an interesting catalyst precursor.

Selective catalytic reduction of nitric oxide with ammonia: I. Monolayer and Multilayers of Vanadia Supported on Titania

Titania supported vanadium oxide layers have been prepared and tested as catalysts for the selective catalytic reduction (SCR) of nitric oxide with ammonia. SCR was performed in a continuous tubular fixed-bed microreactor in the temperature range 400–600 K and at atmospheric pressure. Highly active and selective catalysts for SCR were obtained by successive impregnations using the specific reaction of vanadyl triisobutoxide with surface hydroxyl groups followed by subsequent calcination. One such impregnation resulted in a catalyst covered with an incomplete monolayer of strongly disordered vanadium oxide species. Upon successive impregnations the vanadium oxide content increased, and after four to five impregnations, an additional vanadium oxide phase was formed, which exhibited a significantly different reduction behavior, as was indicated by the appearance of a second reduction peak at higher temperature in the temperature programmed reduction profile. High-resolution electron microscopy showed that the vanadia was preferentially deposited at the edges of the titania and that even after several successive impregnations the amorphous vanadia does not form a compact layer. The activity expressed as moles nitric oxide converted to nitrogen per moles vanadium (V) species and per time increased markedly upon a second impregnation and was highest for catalysts covered with three and four layers of vanadium oxide.