Phase relations in the system V/Nb/O. V. Investigation of mixed crystals V1-xNbxO2

Mixed crystals V_1-xNb_xO₂ exist over the whole area of the quasibinary line VO₂-NbO₂. The existence of Nb_{5+ beside V3+ and V4+ on the V-rich side and V3+ beside Nb5+ and Nb4+ on the Nb-rich side of the mixed crystals is demonstrated by XANES-measurements. The compound VNbO}4_(V0.5_Nb0.5_O2_{) is described as a double oxide with vanadium only as V3+ and niobium only as Nb5+. At this point the electric resistivity of the solid solution shows a maximum.}     

Phase relations in the system V/Nb/O. V. Investigation of mixed crystals V1-xNbxO2

Mixed crystals V_1-xNb_xO₂ exist over the whole area of the quasibinary line VO₂-NbO₂. The existence of Nb_{5+ beside V3+ and V4+ on the V-rich side and V3+ beside Nb5+ and Nb4+ on the Nb-rich side of the mixed crystals is demonstrated by XANES-measurements. The compound VNbO}4_(V0.5_Nb0.5_O2_{) is described as a double oxide with vanadium only as V3+ and niobium only as Nb5+. At this point the electric resistivity of the solid solution shows a maximum.} Received: 11 May 1998 / Revised: 4 August 1998 / Accepted: 10 August 1998     

Zu den Phasenverhältnissen im System V/Nb/O. I. Koexistenzbeziehungen im Bereich V2O5/Nb2O5/VO2/NbO2

The Phase Relations in the System V/Nb/O. I. Coexistence Relations in the Section V₂O₅/Nb₂O₅/VO₂/NbO₂ . Phase relations in the section Nb₂O₅/V₂O₅/VO₂/NbO₂ of the ternary system V/Nb/O have been studied by X-ray diffraction. The investigated samples were prepared by high temperature synthesis at 900°C–1000°C. The section Nb₂O₅/V₂O₅/VO₂/NbO₂ is charakterized by fife three phase regions: The limits of solubility of the pseudobinary system were ascertained by determination of lattic parameters of powder samples:      

Thermochemische Untersuchungen im System V/Nb/O. II Zum chemischen Transportverhalten im Bereich V2O5/Nb2O5/VO2/NbO2

Thermochemical Investigations in the System V/Nb/O. II. Chemical Transport in the Region V₂O₅/Nb₂O₅/VO₂/NbO₂ Transport experiments were used to support the phase relationships of the V₂O₅/Nb₂O₅/VO₂/NbO₂ system, which were established by annealing experiments of powder mixtures. The phase relations were studied in the NbO₂-rich region of the system by means of X-ray and ESMA methods. The NbO₂-rich section is characterized by the following two phase and three phase regions: Two phase region: V₃Nb₉O₂₉/rutile mixed crystal V_1?xNb_xO₂ Two phase region: BI-mixed crystal/V_xNb_1?xO₂ Three phase region: V₃Nb₉O₂₉/solubility limit LG1 (V_1?xNb_xO₂)/BI-mixed crystal Three phase region: solubility limit LG1 (V_1?xNb_xO₂)/BI-mixed crystal/solubility limit LG2 (V_xNb_1?xO₂). The composition of the solubility limits LG1 and LG2 was ascertained by means of ESMA-investigation: LG1: 57.5 ± 5 mol% NbO₂/43.5 ± 5 mol% VO₂ LG2: 22.5 ± 5 mol% NbO₂/78.5 ± 5 mol% VO_2?     

TPD study of NH3 adsorption/desorption on the surface of V/Ti, V/Al based catalysts for selective catalytic reduction of Nox by ammonia 2. TPD test of commercial V/W/Ti, V/Al, Pd/V/Al catalysts

The effect of temperature on the adsorption/desorption of ammonia from the air mixture on the surface of commercial binary V/Al and ternary Pd/V/Al, V(0.65 wt.%) /W(6.73 wt.%) /Ti and V(1.8 wt.%) /W(7.3 wt.%) /Ti de-NO_x catalysts has been investigated by temperature-programmed desorption (TPD) method. The ability of the commercial catalysts to adsorb ammonia in the most stable surface species was shown to correlate well with their suppression of the NH₃ oxidation.     

Heterocyclic tautomerism. V. 2-Guanidinobenzimidazole

A single-crystal X-ray structure analysis of 2-guanidinobenzimidazole shows the molecule exists in the solid state as tautomer 1 , with an intramolecular hydrogen bond between the benzimidazole-N3 and a guanidino-NH₂. The guanidino group is inclined at an angle of 13.8° to the benzimidazole plane. Other nitrogen atoms and their attached hydrogens are involved in additional intermolecular hydrogen bonds that connect the molecules in a complex three-dimensional network.     

Zur Kenntnis von Ba(MgZn)V2O8, BaMn2V2O8 und Ba1/2Sr1/2Ni2V2O8

On Ba(MgZn)V₂O₈, BaMn₂V₂O₈, and Ba_1/2Sr_1/2Ni₂V₂O₈ Ba(Mg, Zn)V₂O₈ (A), BaMn₂V₂O₈ (B) and Ba_1/2Sr_1/2Ni₂V₂O₈ (C) were prepared by solid state reactions (A and B) and crystallization from a melt (C) respectively. (A? C) crystallize in the space group: D-I4₁/acd, Nr. 142. [Lattice constants (A): a = 12.4524(57) Å, c = 8.4408(36) Å; (B): a = 12.5563(14) Å, c = 8.5942(9) Å; (C): a = 12.2248(20) Å, c = 8.3245(15) Å]. (A), (B) and (C) are isotypic to SrNi₂V₂O₈ but showing higher symmetry.     

同时脱除烟气中硫和硝的V2O5/AC催化剂研究

通过在固定床微反应器上对同时脱硫脱硝催化剂的研究,发现将V2O5担载在活性焦(AC)上制得的V2O5/AC催化剂可在200 ℃同时脱除烟气中的SO2和NO,其活性明显高于纯AC。V2O5/AC催化剂的脱硫脱硝活性与催化剂中钒的质量分数有关,随着V2O5质量分数从0.5%增至9%,硫容从3%增加到12%,脱硝率在V2O5质量分数为0.5%到3%时脱硝率稳定在60%左右,继续增加V2O5质量分数,脱硝率降低。程序升温脱附 (TPD) 和红外光谱 (FTIR) 表征结果显示在脱硫脱硝过程中,催化剂表面有H2SO4, 铵盐和VOSO4生成, VOSO4的质量分数随催化剂中V2O5质量分数的提高而升高。使用后的催化剂可通过5%NH3在300 ℃再生,再生后催化剂的脱硝活性明显增加,NO转化率从新鲜样的67%提高到接近100%,对SO2的吸附也比新鲜样有所增加。     

Alkali-resistant performance of V2O5-MoO3/TiO2 catalyst modified by SiO2

以陶瓷为骨架,用溶胶凝胶法和浸渍法制备了V2O5-MoO3/TiO2催化剂,并用SiO2进行改性.采用浸渍法模拟碱K中毒,研究了SiO2改性前后催化剂中毒脱硝效率的变化,并通过BET、H2-TPR、NH3-TPD等技术手段对催化剂进行了表征.结果表明,加入SiO2后,催化剂的比表面积明显提高.SiO2改性后新鲜催化剂还原温度向低温方向迁移10℃左右,氧化还原能力得到了提高,其表面酸强度和酸量也得到较大的提高.反应评价结果表明,SiO2改性可以提高催化剂抗碱中毒的件能.     

Oxidative dehydrogenation of propane over V2O5/MoO3/Al2O3 and V2O5/Cr2O3/Al2O3: structural characterization and catalytic function

The structure and catalytic properties of binary dispersed oxide structures prepared by sequential deposition of VO(x) and MoO(x) or VO(x) and CrO(x) on Al(2)O(3) were examined using Raman and UV-visible spectroscopies, the dynamics of stoichiometric reduction in H(2), and the oxidative dehydrogenation of propane. VO(x) domains on Al(2)O(3) modified by an equivalent MoO(x) monolayer led to dispersed binary structures at all surface densities. MoO(x) layers led to higher reactivity for VO(x) domains present at low VO(x) surface densities by replacing V-O-Al structures with more reactive V-O-Mo species. At higher surface densities, V-O-V structures in prevalent polyvanadates were replaced with less reactive V-O-Mo, leading to lower reducibility and oxidative dehydrogenation rates. Raman, reduction, and UV-visible data indicate that polyvanadates predominant on Al(2)O(3) convert to dispersed binary oxide structures when MoO(x) is deposited before or after VO(x) deposition; these structures are less reducible and show higher UV-visible absorption energies than polyvanadate structures on Al(2)O(3). The deposition sequence in binary Mo-V catalysts did not lead to significant differences in structure or catalytic rates, suggesting that the two active oxide components become intimately mixed. The deposition of CrO(x) on Al(2)O(3) led to more reactive VO(x) domains than those deposited on pure Al(2)O(3) at similar VO(x) surface densities. At all surface densities, the replacement of V-O-Al or V-O-V structures with V-O-Cr increased the reducibility and catalytic reactivity of VO(x) domains; it also led to higher propene selectivities via the selective inhibition of secondary C(3)H(6) combustion pathways, prevalent in VO(x)-Al(2)O(3), and of C(3)H(8) combustion routes that lead to low alkene selectivities on CrO(x)-Al(2)O(3). VO(x) and CrO(x) mix significantly during synthesis or thermal treatment to form CrVO(4) domains. The deposition sequence, however, influences catalytic selectivities and reduction rates, suggesting the retention of some of the component deposited last as unmixed domains exposed at catalyst surfaces. These findings suggest that the reduction and catalytic properties of active VO(x) domains can be modified significantly by the formation of binary dispersed structures. VO(x)-CrO(x) structures, in particular, lead to higher oxidative dehydrogenation rates and selectivities than do VO(x) domains present at similar surface densities on pure Al(2)O(3) supports.     

Mechanism by which Tungsten Oxide Promotes the Activity of Supported V2O5/TiO2 Catalysts for NOX Abatement: Structural Effects Revealed by 51V MAS NMR Spectroscopy

The selective catalytic reduction (SCR) of NO_x with NH₃ to N₂ with supported V₂O₅(‐WO₃)/TiO₂ catalysts is an industrial technology used to mitigate toxic emissions. Long‐standing uncertainties in the molecular structures of surface vanadia are clarified, whereby progressive addition of vanadia to TiO₂ forms oligomeric vanadia structures and reveals a proportional relationship of SCR reaction rate to [surface VO_x concentration]², implying a 2‐site mechanism. Unreactive surface tungsta (WO₃) also promote the formation of oligomeric vanadia (V₂O₅) sites, showing that promoter incorporation enhances the SCR reaction by a structural effect generating adjacent surface sites and not from electronic effects as previously proposed. The findings outline a method to assess structural effects of promoter incorporation on catalysts and reveal both the dual‐site requirement for the SCR reaction and the important structural promotional effect that tungsten oxide offers for the SCR reaction by V₂O₅/TiO₂ catalysts.     

NO-NH3-O2 reaction catalyzed by V2O5/AC at low temperatures

The effects of SO₂, V₂O₅ loading and reaction temperature on the activity of activated carbon supported vanadium oxide catalyst have been studied for the reduction of NO with NH₃ at low temperatures (150—250°C). It is found that SO₂ significantly promotes the catalyst activity. Both V₂O₅ loading and reaction temperature are vital to the promoting effect of SO₂. The catalysts with V₂O₅ loadings of 1—5 weight percent have a positive effect on the promotion of SO₂, while the catalysts with V₂O₅ loadings of above 7 weight percent have not such an effect or show a negative effect. At lower temperatures (<180°C) SO₂ poisons the catalyst but at higher temperatures promotes it. The reason of the SO₂ promotion was also discussed; it may results from the formation of SO₄ ^2? on the catalyst surface, which increases the surface acidity and hence the catalytic activity.     

Neue Verbindungen mit SrNi2V2O8-Struktur: BaCo2V2O8 und BaMg2V2O8

New Compounds of the SrNi₂V₂O₈-Type: BaCo₂V₂O₈ and BaMg₂V₂O₈ For the first time BaCo₂V₂O₈ (A) and BaMg₂V₂O₈ (B) were prepared and investigated by X-ray methods. Space group: D–I4₁/acd, Z = 8. ((A): a = 12.4441, c = 8.4153 Å; (B): a = 12.4189, c = 8.4657 Å). A and B crystallize with higher symmetry, but they are isotypic with SrNi₂V₂O₈. The differences in crystal chemistry in respect to the BaNi₂V₂O₈-type are discussed.     

V2O5/TiO2 Catalyst Xerogels: Method of Preparation and Characterization

This work describes a modified sol-gel method for the preparation of V₂O₅/TiO₂ catalysts. The samples have been characterized by N₂ adsorption at 77 K, X-ray Diffractometry (XRD), Scanning Electronic Microscopy (SEM/EDX) and Fourier Transform Infrared Spectroscopy (FT-IR). The surface area increases with the vanadia loading from 24 m² g^–1 for pure TiO₂ to 87 m² g^–1 for 9 wt% of V₂O₅. The rutile form is predominant for pure TiO₂ but becomes enriched with anatase phase when vanadia loading is increased. No crystalline V₂O₅ phase was observed in the diffractograms of the catalysts. Analysis by SEM showed heterogeneous granulation of particles with high vanadium dispersion. Two species of surface vanadium were observed by FT-IR spectroscopy: a monomeric vanadyl and polymeric vanadates. The vanadyl/vanadate ratio remains practically constant. Ethanol oxidation was used as a catalytic test in a temperature range from 350 to 560 K. The catalytic activity starts around 380 K. For the sample with 9 wt% of vanadia, the conversion of ethanol into acetaldehyde as the main product was approximately 90% at 473 K.     

NO-NH_3-O_2 reaction catalyzed by V_2O_5/AC at low temperatures——Effects of SO_2, V_2O_5 loading and reaction temperature

The effects of SO2, V2O5 loading and reaction temperature on the activity of activated carbon supported vanadium oxide catalyst have been studied for the reduction of NO with NH3 at low temperatures (150-250℃). It is found that SO2 significantly promotes the catalyst activity. Both V2O5 loading and reaction temperature are vital to the promoting effect of SO2. The catalysts with V2O5 loadings of 1 -5 weight percent have a positive effect on the promotion of SO2, while the catalysts with V2O5 loadings of above 7 weight percent have not such an effect or show a negative effect. At lower temperatures (<180℃) SO2 poisons the catalyst but at higher temperatures promotes it. The reason of the SO2 promotion was also discussed; it may results from the formation of SO42- on the catalyst surface, which increases the surface acidity and hence the catalytic activity.