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.     

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.     

Characterization of tungsten oxide supported on TiO2 and activity for acid catalysis

Tungsten oxide-titania catalysts were prepared by drying powdered Ti(OH)4 with ammonium metatungstate aqueous solution, followed by calcining in air at high temperature. Characterization of prepared catalysts was performed by using FTIR, Raman, XPS, XRD, and DSC and by measuring surface area. Upon the addition of tungsten oxide to titania up to 20 wt%, the specific surface area and acidity of catalysts increased in proportion to the tungsten oxide content due to the interaction between tungsten oxide and titania. Since the TiO₂ stabilizes the tungsten oxide species, for the samples equal to or less than 20 wt%, tungsten oxide was well dispersed on the surface of titania, but for the samples containing 25 wt% or above 25 wt%, the triclinic phase of WO₃ was observed at calcination temperature above 400 °C. The catalytic activities for 2-propanol dehydration and cumene dealkylation were correlated with the acidity of catalysts measured by ammonia chemisorption method. This paper was presented at the 8th APCChE (Asia Pacific Confederation of Chemical Engineering) Congress held at Seoul between August 16 and 19, 1999.     

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.     

Performance of Ni catalyst supported on La-hexaaluminate in CO<Subscript>2</Subscript> reforming of CH<Subscript>4</Subscript>

CO₂ reforming of CH₄ was performed using Ni catalyst supported on La-hexaaluminate which has been an well-known material for high-temperature combustion. La-hexaaluminate was synthesized by sol-gel method at various conditions where different amount of Ni (5–20 wt%) was loaded. Ni/La-hexaaluminate experienced 72 h reaction and its catalytic activity was compared with that of Ni/Al₂O₃, Ni/La-hexaaluminate shows higher reforming activity and resistance to coke deposition compared to the Ni/Al₂O₃ model catalyst. Coke deposition increases proportionally to Ni content. Consequently, Ni(5)/La-hexaaluminate(700) is the most efficient catalyst among various Ni/La-hexaaluminate catalysts regarding the cost of Ni in Ni(X)/La-hexaaluminate catalysts. BET surface area, XRD, EA, TGA and TPO were performed for surface characterization. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

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.     

CO<Subscript>2</Subscript>/O<Subscript>2</Subscript>-oxidative dehydrogenation of ethane to ethylene over highly dispersed vanadium oxide on MgO-promoted sulfated-zirconia nanocatalyst: Effect of sulfation on catalytic properties and performance

The ZrO₂ was treated by various molarities of H₂SO₄ solution (0, 0.5, 1 and 2) then mixed by MgO and impregnated with 5 wt% of V₂O₅. The synthesized catalysts were characterized by XRD, FESEM, PSD, EDX, BET and FTIR techniques. According to the results obtained by characterization studies, the modification of MgO-ZrO₂ support by various molarities of H₂SO₄ solution had a great impact on the crystallinity, morphology and functional groups of prepared nanocatalysts. On the other hand, the catalytic activity of synthesized nanocatalysts in the oxidative dehydrogenation of ethane to ethylene is affected by the sulfur content on the support. The crystalline structures of MgO and ZrO₂ were confirmed by XRD analysis. The crystallinity of tetragonal ZrO₂ was decreased by increasing H₂SO₄ molarity used in ZrO₂ (Sx) synthesising. The highest catalytic performance and ethylene productivity (C₂H₄ yield of 48% and ethane conversion of 79% at 700 °C) were obtained on the V₂O₅/MgO-ZrO₂ (S1) nanocatalyst. This could be related to the superior acid-base property, smaller particles, better dispersion of active phase and uniform morphology of V₂O₅/MgO-ZrO₂ (S1).     

Catalytic cracking of isobutane over HZSM-5, FeHZSM-5 and CrHZSM-5 catalysts with different SiO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ratios

Several systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with different ratios of SiO₂/Al₂O₃ (25,38,50,80, and 150) were prepared and they were characterized by means of X-ray diffraction (XRD), UV–Vis, NH₃-TPD and BET techniques. The results indicated that, compared with uncalcined HZSM-5 zeolites, the total acid amounts, acidic site density and acidic strength of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts obviously decreased, while those of weak acid amounts obviously enhanced with the decrease of SiO₂/Al₂O₃ molar ratio. When the ratio of SiO₂/Al₂O₃ is less than 50, the three systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with same ratio of SiO₂/Al₂O₃ gave similar and high isobutane conversions. However, when the ratio of SiO₂/Al₂O₃ was equal to or greater than 80, these three systems of catalysts possessed different altering tendencies of isobutane conversions, thus their isobutene conversions were different. High yields of light olefins were obtained over the FeHZSM-5 and CrHZSM-5 zeolite catalysts with high ratio of SiO₂/Al₂O₃ (≥80). The ratio of SiO₂/Al₂O₃ has large effects on the surface area, and acidic characteristics of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolites catalysts, and thus further affect their catalytic performances for isobutane cracking. That is the nature of SiO₂/Al₂O₃ ratio effect on the catalytic performances.     

Structure and electrochemical characterization of LiNi<Subscript>0.3</Subscript>Co<Subscript>0.3</Subscript>Mn<Subscript>0.3</Subscript>Fe<Subscript>0.1</Subscript>O<Subscript>2</Subscript> cathode for lithium secondary battery

A lithium insertion material having the composition LiNi_0.3Co_0.3Mn_0.3Fe_0.1O₂ was synthesized by simple sol-gel method. The structural and electrochemical properties of the sample were investigated using X-ray diffraction spectroscopy (XRD) and the galvanostatic charge-discharge method. Rietvelt analysis of the XRD patterns shows that this compound can be classified as α-NaFeO₂ structure type (R3m; a=2.8689(5) Å and 14.296(5) Å in hexagonal setting). Rietvelt fitting shows that a relatively large amount of Fe and Ni ion occupy the Li layer (3a site) and a relatively large amount of Li occupies the transition metal layer (3b site). LiNi_0.3Co_0.3Mn_0.3Fe_0.1O₂ when cycled in the voltage range 4.3–2.8 V gives an initial discharge capacity of 120 mAh/g, and stable cycling performance. LiNi_0.3Co_0.3Mn_0.3Fe_0.1O₂ in the voltage range 2.8–4.5 V has a discharge capacity of 140 mAh/g, and exhibits a significant loss in capacity during cycling. Ex-situ XRD measurements were performed to study the structure changes of the samples after cycling between 2.8–4.3 V and 2.8–4.5 V for 20 cycles. The XRD and electrochemical results suggested that cation mixing in this layered structure oxide could be causing degradation of the cell capacity.     

Effect of vanadium surface density and structure in VOx/TiO<Subscript>2</Subscript> on selective catalytic reduction by NH<Subscript>3</Subscript>

We investigated the correlation between vanadium surface density and VOx structure species in the selective catalytic reduction of NOx by NH₃. The properties of the VOx/TiO₂ catalysts were investigated using physicochemical measurements, including BET, XRD, Raman spectroscopy, FE-TEM, UV-visible DRS, NH₃-TPD, H₂-TPR, O₂-On/Off. Catalysts were prepared using the wet impregnation method by supporting 1.0-3.0 wt% vanadium on TiO₂ thermally treated at various calcination temperatures. Through the above analysis, we found that VOx surface density was 3.4 VOx/nm², and the optimal V loading amounts were 2.0-2.5 wt% and the specific surface area was 65-80m²/g. In addition, it was confirmed that the optimal VOx surface density and formation of vanadium structure species correlated with the reaction activity depending on the V loading amounts and the specific surface area size.     

Catalytic hydrolysis of chlorofluorocarbon (CFC-12) over WO3/ZrO2

The catalytic decomposition of CFC-12 (CCl₂F₂) in the presence of water vapor was investigated over a series of solid acids WO₃/ZrO₂. Compared with tungstic acid, ammonium metatungstate is a better source of tungsten oxide for the preparation of WO₃/ZrO₂ catalysts. CFC-12 decomposition activities of WO₃/ZrO₂ catalysts are in good agreement with their acidities. Enhancing the acidities of catalysts is favorable to increase their CFC-12 decomposition activities. WO₃/ZrO₂ catalysts calcined at higher temperature exhibit good catalytic activity and stability for the hydrolysis of CFC-12, and show better structural stability during the reaction.     

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.     

Reduction of NO by CO on Cu/ZrO2/Al2O3 catalysts: Characterization and catalytic activities

ZrO₂, γ-Al₂O₃ and ZrO₂/γ-Al₂O₃-supported copper catalysts have been prepared, each with three different copper loads (1, 2 and 5 wt%), by the impregnation method. The catalysts were characterized by nitrogen adsorption (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR) with H₂, Raman spectroscopy and electronic paramagnetic resonance (EPR). The reduction of NO by CO was studied in a fixed-bed reactor packed with these catalysts and fed with a mixture of 1% CO and 1% NO in helium. The catalyst with 5 wt% copper supported on the ZrO₂/γ-Al₂O₃ matrix achieved 80% reduction of NO. Approximately the same rate of conversion was obtained on the catalyst with 2 wt% copper on ZrO₂. Characterization of these catalysts indicated that the active copper species for the reduction of NO are those in direct contact with the oxygen vacancies found in ZrO₂.     

Synthesis,structural, and electrochemical performance of V<Subscript>2</Subscript>O<Subscript>5</Subscript> nanotubes as cathode material for lithium battery

Various vanadium oxide nanostructures are currently drawn interest for the potential applications of Li batteries, super capacitors, and electrochromic display devices. In this article, the synthesis of V₂O₅ nanotubes by hydrothermal method using 1-hexadecylamine (HDA) and PEO as a template and surface reactant were reported, respectively. The structural properties and electrochemical performances of these nanostructures were investigated for the application of Li batteries. Structure and morphology of the samples were investigated by XRD, FTIR, SEM, and TEM analysis. The battery with V₂O₅ nanotubes electrode showed initial specific capacity of 185 mAhg⁻¹, whereas the PEO surfactant V₂O₅ nanotubes exhibited 142 mAhg⁻¹. It was found that PEO surfactant V₂O₅ nanotubes material showed less specific capacity at initial stages but better stability was exhibited at higher cycle numbers when compared to that of V₂O₅ nanotubes. The cyclic performance of the PEO surfactant material seems to be improved with the role of polymeric component due to its surface reaction with V₂O₅ nanotubes during the hydrothermal process.     

Selective oxidation of H<Subscript>2</Subscript>S to ammonium thiosulfate and elemental sulfur using mixtures of V-Bi-O and Sb<Subscript>2</Subscript>O<Subscript>4</Subscript>

The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was investigated by using vanadium-bismuth based mixed oxide catalysts. Synergistic effect on catalytic activity was observed for the mechanical mixtures of V-Bi-O and Sb₂O₄. Temperature programmed oxidation (TPO), X-ray photoelectron spectroscopy (XPS), and two separated bed reactivity test results supported the role of Sb₂O₄ for reoxidizing the reduced V-Bi-O during the reaction. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Simultaneous dehydrogenation and isomerization of n-butane to isobutene over Cr/WO3–ZrO2 catalysts

A series of WO₃-promoted Cr₂O₃-based catalysts were prepared and tested for the simultaneous dehydrogenation and isomerization of n-butane to isobutene. It is found that a Cr₂O₃/WO₃–ZrO₂ system is an effective catalyst for this reaction; however, the catalytic behavior is dependent on Cr₂O₃ and WO₃ contents, space velocity and temperature. 10 wt% Cr₂O₃/20 wt% WO₃–ZrO₂ can give high initial conversion and isobutene selectivity, but it deactivates rapidly due to the variation of surface properties and pore structure caused by carbon deposition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective catalytic oxidation of H<Subscript>2</Subscript>S using nonhydrolytic vanadia-titania xerogels

A series of vanadia-titania (V-Ti) xerogel catalysts were prepared by nonhydrolytic sol-gel method. These catalysts showed much higher surface area and total pore volumes than the conventional V₂O₅-TiO₂ xerogel. Two species of surface vanadium in the xerogel catalysts were identified by Raman measurements: monomeric vanadyl and polymeric vanadates. The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over these catalysts. Xerogel catalysts from the nonhydrolytic method showed very high conversion of H₂S without harmful emission of SO₂. The conversion of H₂S increased with increasing vanadia loading up to 10V-Ti; however, it decreased at higher vanadia loading (12V-Ti and 18V-Ti) probably due to the formation of crystalline V₂O₅.     

How oxide carriers affect the reactivity of V2O5 catalysts in the oxidative dehydrogenation of propane

The catalytic pattern of several oxide carriers (MgO, Al₂O₃, ZrO₂, TiO₂, SiO₂, HY zeolite) and supported V₂O₅ (4.7–5.3 wt%) catalysts in the oxidative dehydrogenation of propane to propylene (PODH) has been comparatively investigated. The fundamental role of the oxide support on both reducibility and reactivity of vanadia catalysts has been assessed. A direct relationship between the specific surface activity of oxide carriers and that of vanadia catalysts is discussed. The inverse relationship between the specific activity and the onset temperature of reduction marks the prevailing redox behaviour of V₂O₅ catalysts in the PODH reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization and catalytic properties of MoO3–V2O5/Nb2O5 catalysts

The influence of molybdenum oxide on the dispersion of vanadium oxide supported on niobia was investigated. A series of MoO₃–V₂O₅/Nb₂O₅ catalysts with varying MoO₃ content ranging from 1% to 5% (w/w) with fixed V₂O₅ content were prepared by impregnation of previously prepared 5 wt% V₂O₅/Nb₂O₅ with requisite amounts of ammonium molybdate solution. X-ray diffraction patterns indicate the presence of β-(Nb,V)₂O₅ phase with the addition of MoO₃ up to a loading of 3 wt%. Temperature-programmed reduction (TPR) results suggest that the reducibility is decreasing with MoO₃ loading. The results of temperature programmed desorption (TPD) of ammonia suggest that the acidity of the catalysts increased with the addition of MoO₃. The catalytic properties of the catalysts in the ammoxidation of 3-picoline were correlated with the characterization data.     

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.