First In Situ Raman Study of Vanadium Oxide Based SO2 Oxidation Supported Molten Salt Catalysts

In situ Raman spectroscopy at temperatures up to 500°C is used for the first time to identify vanadium species on the surface of a vanadium oxide based supported molten salt catalyst during SO₂ oxidation. Vanadia/silica catalysts impregnated with Cs₂SO₄ were exposed to various SO₂/O₂/SO₃ atmospheres and in situ Raman spectra were obtained and compared to Raman spectra of unsupported model V₂O₅–Cs₂SO₄ and V₂O₅–Cs₂S₂O₇ molten salts. The data indicate that (1) the V^V complex V^VO₂(SO₄)₂ ^3– (with characteristic bands at 1034 cm^–1 due to (V=O) and 940 cm^–1 due to sulfate) and Cs₂SO₄ dominate the catalyst surface after calcination; (2) upon admission of SO₃/O₂ the excess sulfate is converted to pyrosulfate and the V^V dimer (V^VO)₂O(SO₄)₄ ^4– (with characteristic bands at 1046 cm^–1 due to (V=O), 830 cm^–1 due to bridging S–O along S–O–V and 770 cm^–1 due to V–O–V) is formed and (3) admission of SO₂ causes reduction of V^V to V^IV (with the (V=O) shifting to 1024 cm^–1) and to V^IV precipitation below 420°C.     

Selective oxidation of methanol to dimethoxymethane over acid-modified V2O5/TiO2 catalysts

Selective oxidation of methanol to dimethoxymethane (DMM) was conducted in a fixed-bed reactor over an acid-modified V₂O₅/TiO₂ catalyst. The influence of the acid modification on its structure, redox and acidic properties, and catalytic performance for methanol oxidation were investigated. The results indicated that the content of vanadia in the catalyst exhibits a vital influence on the dispersion of vanadium species, while the acid modification can enhance its surface acidity. Proper amounts of the acid (W() = 15%) and V₂O₅ (W(V₂O₅) = 15%) components loaded in the acid-modified V₂O₅/TiO₂ catalyst are able to build a bi-functional circumstance that is favorable for the formation of DMM with high activity and selectivity. As a result, for the selective oxidation of methanol, the H₂SO₄-modified V₂O₅/TiO₂ catalyst gives a much higher DMM yield at 150 °C than the unmodified one.     

Molten V2O5/Cs0.9K0.9Na0.2S2O7 and V2O5/K2S2O7 catalysts as electrolytesin an electrocatalytic membrane separation device for SO2 removal

Bench scale fuel cell tests have been carried out on the SO₂ oxidation catalyst systems V₂O₅/M₂S₂O₇ (M = alkali) used as electrolytes in a standard molten carbonate fuel cell (MCFC) fuel cell setup for removal of SO₂ from power plant flue gases. Porous Li _x Ni_(1–x)O electrodes were used both as anode and cathode. The cleaning cell removes SO₂ when a potential is applied across the membrane, potentially providing cheap and ecological viable means for regeneration of SO₂ from off-gases into high quality H₂SO₄. Results show that successful removal of up to 80% SO₂ at 450 °C can be achieved at approximately 5 mAcm^–2. However, the data obtained during the experiments explain the current limitations of the process, especially in terms of electrolyte wetting capability and acid/base chemistry of the electrolyte.     

A VOx/meso-TiO2 catalyst for methanol oxidation to dimethoxymethane

Mesoporous TiO₂ was prepared by simply controlling the hydrolysis of Ti(OBu)₄ with the help of acetic acid. The mesoporous TiO₂ had a well-crystallized anatase phase and a high surface area of 290 m² g⁻¹ with a pore size of about 4 nm. The anatase phase and the mesoporous structure were maintained in the VO_x/TiO₂ catalyst with a monolayer dispersion of V₂O₅, however, the surface area decreased to 126 m² g⁻¹. The catalyst was highly active and selective for methanol oxidation, giving about 55% conversion of methanol and 85% selectivity to dimethoxymethane at 423 K.     

Low temperature selective catalytic reduction of NO by NH3 over V2O5 supported on TiO2–SiO2–MoO3

The V₂O₅ catalysts supported on TiO₂–SiO₂–MoO₃ (TSM) prepared by the coprecipitation method were investigated for the selective catalytic reduction (SCR) of NO by NH₃ at low temperatures. The V₂O₅/TSM catalyst with 7–13 wt% SiO₂ was found to exhibit a superior SCR activity and a good sulfur tolerance at low temperatures (<250 °C). The presence of highly active polymeric vanadates formed by the incorporation of MoO₃ to TiO₂–SiO₂ and superior redox properties seems to enhance SCR activity, and furthermore the very lower SO₂ oxidation activity due to the higher acidity leads to the remarkable improvement of sulfur tolerance.     

Ionic Liquid Promoted Efficient and Rapid One-pot Synthesis of Pyran Annulated Heterocyclic Systems

Aluminovanadate oxide, “V–Al–O”, has been studied by X-ray photoelectron spectroscopy (XPS) with the emphasis to reveal chemical modifications as a function of the X-irradiation time. Considerable damage was found for V–Al–O and less so for vanadium pentoxide, V₂O₅, and sapphire, α-Al₂O₃, both serving as reference samples. Modifications in V–Al–O were seen even at low radiation doses. Absolute and relative shifts in binding energies along with changes of peak intensities and widths demonstrate that an appreciable amount of V⁵⁺ is reduced to lower oxidation states. X-ray induced chemical modifications extend at least to the depth sampled by the V3p electrons. It is suggested that the damage is caused by electron-hole pair generation and Auger decay. Al–O–H in V–Al–O is also affected by X-rays. This causes O₂ and water desorption as followed by mass spectrometry of the residual gas.     

Gas-phase elemental mercury capture by a V2O5/AC catalyst

Gas-phase elemental mercury (Hg⁰) capture by an activated coke (AC) supported V₂O₅ (V₂O₅/AC) catalyst was studied in simulated flue gas and compared with that by the AC. The study on the influences of V₂O₅ loading, temperature, capture time and flue gas components (O₂, SO₂, H₂O and N₂) shows that the Hg⁰ capture capability of V₂O₅/AC is much higher than that of AC. It increases with an increase in V₂O₅ loading and is promoted by O₂, which indicates the important role of V₂O₅ in Hg⁰ oxidation and capture; it is promoted slightly by SO₂ but inhibited by H₂O; it increases with an increase in temperature up to 150 °C when Hg desorption starts. X-ray photoelectron spectroscopy analysis and sequential chemical extraction experiments indicate that the main states of Hg captured on V₂O₅/AC are HgO and HgSO₄. Temperature programmed desorption experiments were also made to understand the stability of the Hg captured.     

Oxidation of toluene to benzoic acid over modified V2O5-TiO2 catalyst with SeO2, TeO2 and Sb2O3

It was found that the addition of SeO₂, TeO₂ or Sb₂O₃ to a V₂O₅/TiO₂ catalyst greatly improved the catalytic activity in the vapor phase oxidation of toluene to selectively form benzoic acid.     

Oxidation of CH4 over Pd supported on TiO2-doped SiO2: Effect of Ti(IV) loading and influence of SO2

Titania-modified silicas with different weight% of TiO₂ were prepared by sol–gel method and used as supports for Pd (1 wt%) catalysts. The obtained materials were tested in the oxidation of methane under lean conditions in absence and in presence of SO₂. Test reactions were consecutively performed in order to evaluate the thermal stability and poisoning reversibility. Increasing amounts of TiO₂ improved the catalytic activity, with an optimum of the performance for 10 wt% TiO₂ loading. Moreover, the titania-containing catalysts exhibited a superior tolerance towards SO₂ by either adding it to the reactants or feeding it as a pure pretreatment atmosphere at 350 °C. Catalysts were characterized by XPS, XRD, FT-IR and BET measurements. According to the structural and surface analyses, the mixed oxides contained Si–O–Ti linkages which were interpreted as being responsible for the enhanced intrinsic activity of supported PdO with respect to PdO on either pure SiO₂ or pure TiO₂. Moreover, the preferential interaction of the sulfur molecule with TiO₂ and the easy SO_x desorption from high surface area silica were the determining factors for the superior SO₂ tolerance of the TiO₂-doped catalysts.     

Investigation of the sulfidation of Mo/TiO2-Al2O3 catalysts by TPS and LRS

A series of Mo/Al₂O₃ and Mo/TiO₂-Al₂O₃ catalysts were investigated by temperature programmed sulfiding (TPS) and laser Raman spectroscopy (LRS). The effect of TiO₂ on the sulfidability of molybdena was studied in detail. It is found that Mo/Al₂O₃ catalysts can be partially sulfided by O-S exchange at low temperature, forming molybdenum oxysulfide. The Mo-S bond subsequently ruptures in the presence of H₂ to produce H₂S. At 530–550 K deep sulfiding of molybdenum oxysulfide occurs forming crystalline MoS₂. When the surface of Al₂O₃ was covered by a monolayer of TiO₂, the sulfiding rate of molybdena at low temperature was not only greatly increased, but H₂S produced in the reduction of Mo-S species caused deep sulfiding of the catalyst which resulted in a decrease of the TPS peak temperature by 80–100 K. The results indicate that this promotion of the sulfiding of molybdena is enhanced with TiO₂ loading. The function of TiO₂ is explained by the weakened interaction between MoO₃ and Al₂O₃ due to the coverage of the Al₂O₃ surface by TiO₂.     

SO2 and NO removal from flue gas over V2O5/AC at lower temperatures — role of V2O5 on SO2 removal

Supporting V₂O₅ onto an activated coke (AC) has been reported to significantly increase the AC's activity in simultaneous SO₂ and NO removal from flue gas. To understand the role of V₂O₅ on SO₂ removal, V₂O₅/AC is studied through SO₂ removal reaction, surface analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) techniques. It is found that the main role of V₂O₅ in SO₂ removal over V₂O₅/AC is to catalyze SO₂ oxidation through a VOSO₄-like intermediate species, which reacts with O₂ to form SO₃ and V₂O₅. The SO₃ formed transfers from the V sites to AC sites and then reacts with H₂O to form H₂SO₄. At low V₂O₅ loadings, a V atom is able to catalyze as many as 8 SO₂ molecules to SO₃. At high V₂O₅ loadings, however, the number of SO₂ molecules catalyzed by a V atom is much less, due possibly to excessive amounts of V₂O₅ sites in comparison to the pores available for SO₃ and H₂SO₄ storage.     

Selective oxidation of H2S to sulfur over vanadia supported on mesoporous zirconium phosphate heterostructure

Vanadium oxide supported on mesoporous zirconium phosphate catalysts has been synthesized, characterized and tested in the selective oxidation of H₂S to sulfur. The nature of the vanadium species depends on the V-loading of catalyst. Catalysts with a V-content lower than 4wt% present both isolated vanadium species and V₂O₅ crystallites. However, V₂O₅ crystallites have been mainly observed in catalysts with higher V-content, although the presence of isolated V-species on the surface of the metal oxide support cannot be completely ruled out. The catalytic behaviour also depends on V-loading of catalysts. Thus, while the catalytic activity of catalysts can be related to the number of V-sites, the catalyst decay is clearly observed in samples with low V-loading. The characterization of catalysts after the catalytic tests indicates the presence of sulfur on the catalyst, which is favoured on catalysts with low V-loading. However, a clear transformation of V₂O₅ to V₄O₉ can be proposed according to XRD and Raman results of used catalysts with high V-loading. The importance of V⁵⁺–O–V⁴⁺ pairs in activity and selectivity is also discussed.     

Effect of ceria on the structure and catalytic activity of V2O5/TiO2–ZrO2 for oxidehydrogenation of ethylbenzene to styrene utilizing CO2 as soft oxidant

The present study was undertaken to investigate the influence of ceria on the physicochemical and catalytic properties of V₂O₅/TiO₂–ZrO₂ for oxidative dehydrogenation of ethylbenzene to styrene utilizing CO₂ as a soft oxidant. Monolayer equivalents of ceria, vanadia and ceria–vanadia combination over TiO₂–ZrO₂ (TZ) support were impregnated by a coprecipitation and wet impregnation methods. Synthesized catalysts were characterized by using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, temperature programmed reduction, transmission electron microscopy and BET surface area methods. The XRD profiles of 550 °C calcined samples revealed amorphous nature of the materials. Upon increasing calcination temperature to 750 °C, in addition to ZrTiO₄ peaks, few other lines due to ZrV₂O₇ and CeVO₄ were observed. The XPS V 2p results revealed the existence of V⁴⁺ and V⁵⁺ species at 550 and 750 °C calcinations temperatures, respectively. TEM analysis suggested the presence of nanosized (<7 nm) particles with narrow range distribution. Raman measurements confirmed the formation ZrTiO₄ under high temperature treatments. TPR measurements suggested a facile reduction of CeO₂–V₂O₅/TZ sample. Among various samples evaluated, the CeO₂–V₂O₅/TZ sample exhibited highest conversion and nearly 100% product selectivity. In particular, the addition of ceria to V₂O₅/TZ suppressed the coke deposition and allowed a stable and high catalytic activity.     

Enhancement of the visible light photocatalytic performance of C-doped TiO2 by loading with V2O5

V₂O₅ was loaded on the surface of C-doped TiO₂ (C-TiO₂) by incipient wetness impregnation in order to enhance the visible light photocatalytic performance. The physicochemical properties of the C-TiO₂/V₂O₅ composite were characterized by XRD, Raman, TEM, XPS, UV–vis diffuse reflectance spectra, and PL in detail. The result indicated that a heterojunction between C-TiO₂ and V₂O₅ was formed and the separation of excited electron–hole pairs on C-TiO₂/V₂O₅ is greatly promoted. Thus, this composite photocatalyst exhibited enhanced visible light photocatalytic activity in degradation of gas-phase toluene compared with the pristine C-TiO₂.     

Surface alteration of (VO)2P2O7 by α-Sb2O4 as a route to control the n-butane selective oxidation

The catalytic properties of (VO)₂P₂O₇/α-Sb₂O₄ mixed oxides system for n-butane mild oxidation have been investigated on two mechanical mixtures (M1 and M2) of the same well crystallized (VO)₂P₂O₇ (reference vanadyl pyrophosphate) with two different morphologies of α-Sb₂O₄.The M1 mixture of (VO)₂P₂O₇ with α-Sb₂O₄ (1), prepared by oxidation of Sb₂O₃, leads to the oxidative dehydrogenation (ODH) of n-butane, whereas the M2 mixture of (VO)₂P₂O₇ with a commercial α-Sb₂O₄ (2) (Aldrich) with a different morphology improves the maleic anhydride selectivity as compared to the reference (VO)₂P₂O₇ catalyst (synergetic effect). After reaction, no ternary VPSbO phase is detected by XRD and DTA and it was controlled that the two α-Sb₂O₄ oxides are catalytically inactive.The (VO)₂P₂O₇ reference catalyst which produced only maleic anhydride as mild oxidation product shows by XPS a slightly oxidized surface (14% V⁵⁺–86% V⁴⁺).Contamination of the (VO)₂P₂O₇ phase by migration of Sb species occurs after catalytic reaction in the case of the M1 mixture as shown by XPS, LEIS and TEM–EDX analysis. XPS showed that (VO)₂P₂O₇ is partially superficially reduced (86% V⁴⁺–14% V³⁺). This feature is consistent with the decrease of acidity as observed by pyridine adsorption–desorption.In opposition with the M1 mixture, no contamination of the (VO)₂P₂O₇ phase is observed after catalytic reaction in the case of the M2 mixture. The XPS study shows, in this case, that (VO)₂P₂O₇ is partially oxidized (30% V⁵⁺–70% V⁴⁺) at a higher level than for the reference (VO)₂P₂O₇ catalyst. This situation is associated with the increase of selectivity observed for maleic anhydride (synergetic effect).The difference in the catalytic results for the two M1 and M2 mixtures, as compared to the (VO)₂P₂O₇ reference catalyst, can be explained by the alteration of the surface composition of (VO)₂P₂O₇ and the distribution of vanadium oxidation state due to different interaction between Sb₂O₄and (VO)₂P₂O₇, depending on the orientation of the α-Sb₂O₄ crystals.     

Promoting Effect of Pt Addition to V2O5/ZrO2 Catalyst on Reduction of NO by C3H6

The effect of Pt addition to a V₂O₅/ZrO₂ catalyst on the reduction of NO by C₃H₆ has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Pt loading promoted the catalytic activity remarkably. FTIR spectra of NO adsorbed on the catalysts doped with Pt show the presence of two different types of Pt sites, Pt oxide and Pt cluster, on the surface. The amount of these sites depends on Pt contents and the catalyst state. Pt atoms highly disperse on the surface as Pt oxide at low Pt content, being aggregated into Pt metal clusters by increasing Pt amount or reducing the catalysts. The spectral behavior of V=O bands on the surface also supports the formation of Pt clusters. It is concluded that Pt promotes the NO–C₃H₆ reaction through a reduction–oxidation cycle between its oxide and cluster form.     

Sorption of ethene and ethane on the V2O5(001)/TiO2(001) anatase interface

Simulation techniques have been employed to investigate the differences in the low energy adsorption configurations of ethene and ethane on the TiO₂ supported and unsupported V₂O₅(001) surface. We find that the ethene molecule approaches much closer to thesupported V₂O₅(001) surface which is reflected in the 40 kj mol^–1 higher adsorption energy. The low energy adsorption configuration located for ethane on the supported V₂O₅ shows that the molecule does not approach as close to the supported V₂O₅ surface as does ethene, resulting in the adsorption energy of ethane being 52 kJ mol^–1 lower than that of ethene on the supported V₂O₅ surface.     

Effect of the metal oxide loading on the activity of silica supported MoO3 and V2O5 catalysts in the selective partial oxidation of methane

Precipitated silica catalysts loaded with either MoO₃ (0.2–4.0 wt%) or V₂O₅ (0.2–5.3 wt%) have been studied in the selective partial oxidation of methane to formaldehyde with molecular oxygen at 520 °C. The functionality of the SiO₂ surface towards the formation of HCHO is significantly promoted by V₂O₅, while it is depressed by the MoO₃.     

The Effect of a Changing Lean Gas Composition on the Ability of NO2 Formation and NOx Reduction over Supported Pt Catalysts

Three model catalysts (Pt/Al₂O₃, Pt/TiO₂, Pt/V₂O₅/TiO₂) were examined in regard to their NO₂ formation ability under a changing lean gas composition. The results show that the NO to NO₂ oxidation function as well as the NO _x reduction under lean gas conditions is affected by a change in the lean gas atmosphere. The NO oxidation activity also decreased with time, for Pt/Al₂O₃ and Pt/TiO₂, and a possible explanation may be platinum oxide formation. This deactivation was not observed for Pt/V₂O₅/TiO₂.     

An exploratory study of diesel soot oxidation with NO2 and O2 on supported metal oxide catalysts

A number of supported metal oxide catalysts were screened for their catalytic performance for the oxidation of carbon black (CB; a model diesel soot) using NO₂ as the main oxidant. It was found that contact between the carbon and catalyst was a key factor in determining the rate of oxidation by NO₂. Oxides with low melting points, such as Re₂O₇, MoO₃ and V₂O₅ showed higher activities than did Fe₃O₄ and Co₃O₄. The activities of MoO₃ and V₂O₅ on various supporting materials were also examined. MoO₃/SiO₂ was the most active catalyst among the supported MoO₃ examined, whereas, V₂O₅/MCM-41 showed the highest activity among the supported V₂O₅. Different performances of the supported MoO₃ catalysts were explained by the interaction of MoO₃ with the supports: a strong MoO₃/support interaction may result in a poor mobility of MoO₃ and a poor activity for oxidation of carbon by NO₂. The high activity of V₂O₅/MCM-41 was associated with its catalysis of the oxidation of SO₂ by NO₂ to form SO₃, which substantially promotes oxidation of carbon by NO₂. Addition of transition metal oxides or sulfates to supported MoO₃ and V₂O₅ was also investigated. Combining MoO₃ or V₂O₅ with CuO on SiO₂, adding VOSO₄ to MoO₃/SiO₂ or MoO₃/Al₂O₃ and adding TiOSO₄ or CuSO₄ to V₂O₅/Al₂O₃ improved the catalytic performance.