Catalytic Wet Oxidation of H<Subscript>2</Subscript>S to Sulfur on V/MgO Catalyst

The V/MgO catalysts with different V₂O₅ loadings were prepared by impregnating MgO with aqueous vanadyl sulfate solution. All of the catalysts were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). It was observed that the H₂S removal capacity with respect to vanadia content increased up to 6 wt%, and then decreased with further increase in vanadia loading. The prepared catalysts had BET surface areas of 11.3 ~ 95.9 m²/g and surface coverages of V₂O₅ of 0.1 ~ 2.97. The surface coverage calculation of V₂O₅ suggested that a vanadia addition up to a monomolecular layer on MgO support increased the H₂S removal capacity of V/MgO, but the further increase of VO _x surface coverage rather decreased that. Raman spectroscopy showed that the small domains of Mg₃(VO₄)₂ could be present on V/MgO with less than 6 wt% vanadia loading. The crystallites of bulk Mg₃(VO₄)₂ and Mg₂(V₂O₇) became evident on V/MgO catalysts with vanadia loading above 15 wt%, which were confirmed by a XRD. The TPR experiments showed that V/MgO catalysts with the loading below 6 wt% V₂O₅ were more reducible than those above 15 wt% V₂O₅. It indicated that tetrahedrally coordinated V⁵⁺ in well-dispersed Mg₃(VO₄)₂ domains could be the active species in the H₂S wet oxidation. The XPS studies indicated that the H₂S oxidation with V/MgO could proceed from the redox mechanism (V⁵⁺ V⁴⁺) and that V³⁺ formation, deep reduction, was responsible for the deactivation of V/MgO.     

Effects of preparation methods for V<Subscript>2</Subscript>O<Subscript>5</Subscript>-TiO<Subscript>2</Subscript> aerogel catalysts on the selective catalytic reduction of NO with NH<Subscript>3</Subscript>

A series of V₂O₅-TiO₂ aerogel catalysts were prepared by the sol-gel method with subsequent supercritical drying with CO₂. The main variables in the sol-gel method were the amounts of V₂O₅ and when the vanadium precursor was introduced. V₂O₅-TiO₂ xerogel and V₂O₅/TiO₂ (P-25) were also prepared for comparison. The V₂O₅-TiO₂ aerogel catalysts showed much higher surface areas and total pore volumes than V₂O₅-TiO₂ xerogel and impregnated V₂O₅/TiO₂ (P-25) catalysts. The catalysts were characterized by N₂ physisorption, X-ray diffraction (XRD), FT-Raman spectroscopy, temperature-programmed reduction with H₂ (H₂-TPR), and temperature-programmed desorption of ammonia (NH₃-TPD). The selective catalytic reduction of NOx with ammonia in the presence of excess O₂ was studied over these catalysts. Among various V₂O₅-TiO₂ catalysts, V₂O₅ supported on aerogel TiO₂ showed a wide temperature window exhibiting high NOx conversions. This superior catalytic activity is closely related to the large amounts of strong acidic sites as well as the surface vanadium species with characteristics such as easy reducibility and monomeric and polymeric vanadia surface species. This work was presented at the 7 ^th Korea-China Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Phase cooperation of V<Subscript>2</Subscript>O<Subscript>5</Subscript> and Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> in the selective oxidation of H<Subscript>2</Subscript>S containing ammonia and water

The selective oxidation of hydrogen sulfide containing excess water and ammonia was studied over vanadium oxide-based catalysts. The investigation was focused on the role of V₂O₅, and phase cooperation between V₂O₅ and Bi₂O₃ in this reaction. The conversion of H₂S continued to decrease since V₂O₅ was gradually reduced by treatment with H₂S. The activity of V₂O₅ was recovered by contact with oxygen. A strong synergistic phenomenon in catalytic activity was observed for the mechanically mixed catalysts of V₂O₅ and Bi₂O₃. Temperature-programmed reduction (TPR) and oxidation (TPO) and two bed reaction tests were performed to explain this synergistic effect by the reoxidation ability of Bi₂O₃. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Vanadia supported on ceria: Characterization and activity in liquid-phase oxidation of ethylbenzene

Vanadia/ceria catalysts (2–10 wt% of V₂O₅) were prepared by wet impregnation of ammonium metavanadate in oxalic acid solution. Structural characterization was done with energy dispersive X-ray analysis (EDX), powder X-ray diffraction (XRD), BET surface area measurements, FT-IR spectroscopy and nuclear magnetic spectral analysis (⁵¹V MASNMR). XRD and ⁵¹V MASNMR results show highly dispersed vanadia species at lower loadings and the formation of CeVO₄ phase at higher V₂O₅ loading. The catalytic activity of catalysts was conducted in liquid phase oxidation of ethylbenzene with H₂O₂ as oxidant. The oxidation activity is increased with loading up to 8 wt% V₂O₅ and then decreased with further increase in V₂O₅ content to 10 wt%. Different vanadia species evidenced by various techniques were found to be selective towards ethylbenzene oxidation. The CeVO₄ formation associated with increased concentration of vanadia on ceria results the production of acetophenone along with 2-hydroxyacetophenone.     

Optimization of V2O5–MgO/TiO2 catalyst for the oxidative dehydrogenation of propane effect of magnesia loading and preparation procedure

The effect of magnesia loading and preparation procedure of vanadia on titania catalysts on the physicochemical characteristics and the performance in propane oxidative dehydrogenation were investigated. A series of magnesia promoted vanadia catalyst (5 wt% V₂O₅) with varying amounts of MgO (1.9--10 wt%) were synthesized by synchronous and sequential deposition on titania support. The catalysts were characterized using several techniques (BET, XRD, H₂-TPR and NH₃)-TPD). Both MgO loading and preparation procedure affect the catalyst surface properties and the behavior in the oxidative dehydrogenation of propane. Magnesia addition results in drastic increase in propene selectivity, while the effect on activity is negative. The activity is inversely related with the magnesia loading. Deposition of V₂O₅ on previously prepared MgO/TiO₂ presents a beneficial effect in the activity of the sample. The role of acidity and reducibility is explored. There is no correlation between reducibility and activity of the catalysts, whereas the acidity seems to influence the catalytic performance. Catalyst containing 5 wt% V₂O₅ and 1.9 wt% MgO prepared by sequential deposition of V₂O₅ on already doped with MgO titania exhibits the most interesting results.     

Catalytic performance of vanadia-doped titania-pillared clay for the selective catalytic oxidation of H2S

A series of vanadia-doped titania pillared clay (V/Ti-PILC) with various amounts of vanadia were prepared and their performance for the selective catalytic oxidation of H₂S was investigated in this study. V/Ti-PILCs were characterized using X-ray diffraction (XRD), BET apparatus, and X-ray photoelectron spectroscopy (XPS). V/Ti-PILCs showed better catalytic performance than as such Ti-PILC at temperatures ranging from 220 to 300 °C without any considerable SO₂ emission. The H₂S conversion over V/Ti-PILC increased with increasing vanadia content up to 5 wt.%. This superior catalytic performance might be related to the uniform dispersion of vanadia in the form of monomeric and polymeric species. However, it decreased at 10 wt.% vanadia loading due to the decrease of surface area and to the formation of crystalline V₂O₅ phase. The presence of water vapor in the reactant mixture resulted in the decrease of the H₂S conversion.     

Catalytic performance of vanadia-doped alumina-pillared clay for selective oxidation of H2S

The catalytic oxidation of hydrogen sulfide over V₂O₅ supported on Al-pillared clay (V/Al-PILCs) was studied. The synthesized catalysts were examined using a variety of characterization techniques such as XRD, BET, XPS, ⁵¹V NMR, H₂-TPR and NH₃-TPD. A catalytic activity study performed by using V/Al-PILC catalysts showed that H₂S was successfully converted to elemental sulfur without considerable emission of sulfur dioxide. The H₂S conversion over V/Al-PILCs increased with the vanadia content up to 6 wt.%. This superior catalytic performance might be related to the uniform dispersion of vanadia species on the Al-PILC support.     

Selective catalytic reduction of NO with ammonia over V2O5 doped TiO2 pillared clay catalysts

A series of vanadia doped TiO₂-pillared clay (TiO₂-PILC) catalysts with various amount of vanadia were studied for selective catalytic reduction (SCR) of NO by ammonia in the presence of excess oxygen. It was found that the V₂O₅/TiO₂-PILC catalysts were highly active for the SCR reaction. The catalysts showed a broad temperature window, and the maximum NO conversion was higher than that on V₂O₅/TiO₂ catalyst and was the same as the commercial V₂O₅ + WO₃/TiO₂ catalyst. The V₂O₅/TiO₂-PILC catalysts also had higher N₂/N₂O product selectivities as compared to V₂O₅ doped TiO₂ catalysts. In addition, H₂O + SO₂ slightly increased the activities at high temperatures (>350°C) for the V₂O₅/TiO₂-PILC catalysts. Addition of WO₃ to V₂O₅ further increased the activities of the PILC catalysts. These results indicate that TiO₂-PILC is a good support for vanadia catalysts for the SCR reaction. In situ FT–IR experiment indicated that both Brønsted acid sites and Lewis acid sites exist on the catalyst surface, but with a large proportion being Brønsted acid sites at low temperatures (e.g., 100°C). The reaction path for NO reduction by NH₃ on the V₂O₅/TiO₂-PILC is similar to that on V₂O₅/TiO₂ catalyst, i.e., N₂ originates from the reaction between gaseous NO and NH₃ adspecies.     

Effect of vanadia loading in propane oxidative dehydrogenation on V2O5/SiO2 catalysts

The influence of V₂O₅ loading on the catalytic behaviour of V₂O₅/SiO2₂ catalysts in the oxidative dehydrogenation of propane to propylene (POD) has been investigated. The different activity-selectivity pattern of low (5 wt%) and highly (>10 wt%) loaded V₂O₅/SiO₂ catalysts is explained in terms of different surface vanadia species.     

Phase evolution during mechanochemical coreduction of V<Subscript>2</Subscript>O<Subscript>5</Subscript> and TiO<Subscript>2</Subscript> with A

The direct preparation of V-Ti solid solution alloy by coreduction of V₂O₅ and TiO₂ with Al in an attritor mill was investigated. The reduction of V₂O₅ with Al is highly exothermic, whereas reduction of TiO₂ with Al is not sufficiently exothermic for a self-sustaining reaction. A range of compositions of a mixture of V₂O₅ and TiO₂ can be so chosen as to make the overall reduction of V₂O₅ and TiO₂ with Al sufficiently exothermic for a self-sustaining reaction. Initial studies were done to identify the reaction products obtained by reducing V₂O₅ with Al. The reaction yielded the intermetallic phase (Al₃V), V, and Al₂O₃. SEM images indicated melting and solidification of the phases, leading to agglomeration. Further experiments involved mixing appropriate amounts of TiO₂ with V₂O₅ and reducing the mixture with Al. XRD data for products showed the presence of V, V₅Al₈, and Al₂O₃. X-Ray Florescence (XRF) analysis and energy dispersive analyzer (EDAX) of SEM sample images indicated the formation of V-Ti solid solution. Microstructure of the milled charges taken out prior to reaction initiation indicated morphology change in Al powder and agglomeration/segregation of reactants. As a result, the reaction of V₂O₅ with the excess Al at certain regions also promoted the formation of vanadium aluminide.     

Infrared and raman characterization of V<Subscript>2</Subscript>O<Subscript>5</Subscript> on zirconia modified with WO<Subscript>3</Subscript> and activity for acid catalysis

Vanadium oxide supported on zirconia modified with WO₃ was prepared by adding Zr(OH)₄ powder into a mixed aqueous solution of ammonium metavanadate and ammonium metatungstate followed by drying and calcining at high temperatures. The characterization of prepared catalysts was performed by using FTIR, Raman, and XRD. In the case of calcination temperature at 773 K, for samples containing low loading V₂O₅ below 18 wt%, vanadium oxide was in a highly dispersed state, while for samples containing high loading V₂O₅ equal to or above 18 wt%, vanadium oxide was well crystallized due to the high V₂O₅ loading on the surface of ZrO₂. The ZrV₂O₇ compound was formed through the reaction of V₂O₅ and ZrO₂ at 873 K, and the compound decomposed into V₂O₅ and ZrO₂ at 1,073 K, these results were confirmed by FTIR and XRD. Catalytic tests for 2-propanol dehydration and cumene dealkylation have shown that the addition of WO₃ to V₂O₅/ZrO₂ enhanced both catalytic activity and acidity of V₂O₅-WO₃/ZrO₂ catalysts. The variations in catalytic activities for both reactions are roughly correlated with the changes of acidity.     

Selective oxidation of H<Subscript>2</Subscript>S to sulfur over CeO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> catalyst

The catalytic oxidation of hydrogen sulfide (H₂S) to elemental sulfur was studied over CeO₂-TiO₂ catalysts. The synthesized catalysts were characterized by various techniques such as X-ray diffraction, BET, X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of ammonia, and scanning electron microscopy (SEM). Catalytic performance studies of the CeO₂-TiO₂ catalysts showed that H₂S was successfully converted to elemental sulfur without considerable emission of sulfur dioxide. CeO₂-TiO₂ catalysts with Ce/Ti=1/5 and 1/3 exhibited the highest H₂S conversion, possibly due to the uniform dispersion of metal oxides, high surface area, and high amount of acid sites.     

Vanadium loaded carbon-based monoliths for the on-board No reduction: Influence of vanadia and tungsten loadings

Carbon-coated catalysts doped with tungsten and vanadia oxides with different V and W loadings have been prepared by the ionic exchange method and characterized. The surface, structure and composition have been investigated by XPS, Raman, N₂ sorption at 77 K, TPD-NH₃ and reactivity tests for the SCR of NO with NH₃ at low temperatures. Under reaction conditions, NO conversions were found to go through a maximum with vanadia surface coverage at approximately half a monolayer. The observed decrease in the SCR activity at higher vanadia loadings can be attributed to either a loss of dispersion or loss of textural properties. Maximum NO conversion is ascribed to the higher Brönsted proton acidity (V⁴⁺) of the centres that decreases with increasing vanadia loadings up to 3 wt% loading due to the increase of V⁴⁺/V⁵⁺ ratio.Large amounts of tungsten (5%, w/w) upon or before addition of vanadia do not provide an enhancement of activity. The results indicate that W addition increases surface acidity leading to stronger Brönsted or even Lewis acid centre creation.     

Characterization and activity of vanadia-promoted Pt/ZrO2 catalysts for the water–gas shift reaction

Pt/ZrO₂ catalysts for the water–gas shift (WGS) were promoted with various amounts of vanadia. Analyses by XRD, N₂ adsorption, Raman, and UV–vis DRS showed that vanadia is present below monolayer coverage as monovanadate and polyvanadate, with the former dominating at lower loadings, and that following monolayer formation, VO₅ species appear, with the eventual generation of V₂O₅ and ZrV₂O₇ for a vanadia weight loading of 13%. Though in all cases vanadia induced an enhancement in WGS activity, the best catalyst, that contained 3 wt.% of vanadia, gave a rate that was nearly double that of the unpromoted Pt/ZrO₂. That superior global activity probably results from the monovanadate that is the main species at low loadings. It is believed that monovanadate promotes the WGS by rendering the support's surface more oxidizing through its VOZr bonds.     

Catalytic Combustion of Benzene over Au Supported on Ceria and Vanadia Promoted Ceria

Complete oxidation of benzene over Au/CeO₂ and Au/V₂O₅/CeO₂ catalysts were studied. Gold was supported on CeO₂ from different sources by deposition precipitation method. The catalysts were characterized by XRD, BET, X-ray photoelectron spectroscopy, TEM, and H₂-TPR techniques. The catalytic activity toward the complete oxidation of benzene to CO₂ and water were strongly dependent on the kind of CeO₂ sources, loading amount of gold, modified amount of vanadia, and calcination temperature. High activities were obtained on 1% Au/CeO₂ and 2% vanadia modified Au/CeO₂ catalysts calcined at 300 °C. The nanometer size of Au particle and interaction between Au, V₂O₅, and CeO₂ play important roles in determining the activity of benzene complete oxidation.     

Effects of La<Subscript>2</Subscript>O<Subscript>3</Subscript> on ZrO<Subscript>2</Subscript> supported Ni catalysts for autothermal reforming of CH<Subscript>4</Subscript>

The effect of La₂O₃ content in Ni-La-Zr catalyst was investigated for the autothermal reforming (ATR) of CH₄. The catalysts were prepared by the coprecipitation method and had a mesoporous structure. Temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) indicated that a strong interaction developed between Ni species and the support with the addition of La₂O₃. Thermogravimetric analysis (TGA) and H₂-pulse chemisorption showed that the addition of La₂O₃ led to well dispersed NiO molecules on the support. Ni-La-Zr catalysts gave much higher CH₄ conversion than Ni-Zr catalyst. The Ni-La-Zr containing 3.2 wt% La₂O₃ showed the highest activity. The optimum conditions for maximal CH₄ conversion and H₂ yield were H₂O/CH₄=1.00, O₂/CH₄=0.75. Under these conditions, CH₄ conversion of 83% was achieved at 700 °C. In excess O₂ (O₂/CH₄>0.88), the catalytic activity was decreased due to sintering of the catalyst.     

Preparation of H<Subscript>5</Subscript>PMo<Subscript>10</Subscript>V<Subscript>2</Subscript>O<Subscript>40</Subscript> catalyst immobilized on spherical carbon and its application to the vapor-phase 2-propanol conversion reaction

Spherical carbon (SC) with a diameter of ca. 9 μm was synthesized by a hydrothermal method using sucrose as a carbon precursor. The spherical carbon was then modified to have a positive charge, and thus, to provide a site for the immobilization of H₅PMo₁₀V₂O₄₀ (PMo₁₀V₂) catalyst. The PMo₁₀V₂ catalyst was immobilized on the surface-modified spherical carbon by taking advantage of the overall negative charge of [PMo₁₀V₂O₄₀]⁵⁻. The PMo₁₀V₂ catalyst immobilized on the spherical carbon (PMo₁₀V₂/SC) was applied to the vapor-phase 2-propanol conversion reaction. In the catalytic reaction, the PMo₁₀V₂/SC catalyst showed a higher 2-propanol conversion than the unsupported PMo₁₀V₂ catalyst. Furthermore, the PMo₁₀V₂/SC catalyst showed enhanced oxidation catalytic activity (formation of acetone) and the suppressed acid catalytic activity (formation of propylene and isopropyl ether) compared to the unsupported PMo₁₀V₂ catalyst. The enhanced oxidation activity of PMo₁₀V₂/SC catalyst was due to the fine dispersion of [PMo₁₀V₂O₄₀]⁵⁻ on the spherical carbon formed via chemical immobilization.     

Studies on Preparation, Characterization and Ammoxidation Functionality of Zirconium Phosphate-Supported V2O5 Catalysts

Zirconium pyrophosphate (ZrP₂O₇) was synthesized from zirconyl chloride and phosphoric acid. A series of ZrP₂O₇-supported V₂O₅ catalysts, with the oxide loading ranging from 2 to 8 wt.%, was prepared by wet impregnation method. These catalysts were characterized by various techniques like X-ray diffraction, BET surface area, pore size distribution, FT-IR spectroscopy, acidity measurements and X-ray photoelectron spectroscopy. Their catalytic functionality was evaluated in the ammoxidation of 2-methyl pyrazine (MP) to 2-cyano pyrazine (CP). The V₂O₅/ZrP₂O₇ catalysts are found to be highly active and selective. FT-IR profiles of used catalysts indicate the interaction of ammonia with vanadia. The physico-chemical properties of the catalysts are correlated with their activity and nitrile selectivity. IICT Communication No. 050602.     

Influence of V2O5 content on ammoxidation of 3-picoline over V2O5/AlF3 catalysts

V₂O₅/AlF₃ catalysts with V₂O₅ loadings ranging from 2 to 15 wt% were prepared by the conventional wet impregnation method, using nonporous AlF₃·3H₂O sample as the support for impregnating NH₄VO₃. It was found that the catalysts evolve porous structures upon calcination at 723 K. The influence of V₂O₅ content was studied on ammoxidation of 3-picoline on the reduced catalysts. The catalyst with 15 wt% V₂O₅ exhibited the highest selective ammoxidation acitivity towards nicotinonitrile. The XRD and oxygen chemisorption studies revealed that vanadia is in a highly dispersed state in the catalysts.     

Influence of the V2O5 Loading on the Structure and Activity of V2O5/TiO2 SCR Catalysts for Vehicle Application

In this paper the effect of the vanadium oxide loading on the surface vanadia structure and the activity as well as selectivity in the catalytic reduction of NO with NH₃ was studied for a V₂O₅/TiO₂ model system. A series of TiO₂ (WO _x stabilized anatase) supported vanadia catalysts with varying loadings were characterized by laser Raman spectroscopy, ⁵¹V MAS-NMR, V K XANES. To determine the acidic properties, DRIFTS measurements were done with pyridine adsorbed on the samples. The measurements indicate that with increasing active phase loading square pyramidal coordinated surface vanadia species are replaced by an amorphous highly dispersed vanadium oxide phase with a coordination like V₂O₅. In addition, the ratio of Brønsted to Lewis acid sites is shifted from a comparatively low to an equal level at high loadings. This structural change is accompanied by a clearly improved catalytic activity and selectivity.