Physico-chemicals and catalytic properties of manganese-promoted vanadium phosphate (VPO) catalyst

The addition of 1% Mn promoter to vanadium phosphate catalyst led to doubling of the specific surface area from 20.3 (unpromoted) to 39.4 m² g⁻¹. The XRD pattern of the Mn-promoted catalyst gave only the characteristics of the (VO)₂P₂O₇ phase, indicating that the Mn was incorporated into the crystal lattice of the catalyst. The Mn-promoted catalyst was also twice as active in removing the total amount of oxygen. However, since the only oxygen species related to V⁴⁺ being removed and no oxygen species associated with V⁵⁺ was observed, the n-butane conversion was not much improved as compared to the unpromoted counterpart. A necessary amount and distribution of the V⁵⁺ phase in a well crystalline V⁴⁺ phase is essential in order to enhance the catalytic performance in the mild oxidation of n-butane to maleic anhydride.     

Synthesis, V K β5β″ spectra, and magnetic properties of M4 ± x V6O16 ± x · n H2O (M = K, Rb, Cs) polyvanadates

Tetragonal polyvanadates M_{4 ± x} V₆O_{16 ± x} · nH₂O (M = K, Rb, Cs) have been synthesized under hydrothermal conditions. According to the VKβ₅β″ spectra of the hydrates, vanadium atoms have an average valence state (the V⁵⁺ ⇄ V⁴⁺ charge-fluctuation frequency is higher than 10⁻¹⁵ s). After dehydration, the phases do not change their crystal system. The thermal properties of the compounds have been studied in air and under an inert atmosphere. K_4.2V₆O_16.2 and Rb_4,1V₆O_16.1 melt congruently at 720 and 690°C, respectively. Cs_3.8V₆O_15.8 melts incongruently at 675°C. The magnetic susceptibility of all phases obeys the Curie-Weiss law in the range from 77 to 673 K. Original Russian Text ? V.L. Volkov, N.V. Podval’naya, V.M. Cherkashenko, S.N. Nemnonov, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 8, pp. 1272–1276.     

Ultra‐deep desulfurization of a model fuel using novel VOHPO4 0.5H2O/boehmite catalysts

A high‐surface‐area boehmite was used as the support for a series of vanadium phosphate catalysts. The catalysts were prepared by heating of V₂O₅ in an isobutyl alcohol and benzyl alcohol mixture at 140°C for 5 h to reduce V⁵⁺ to more active V⁴⁺ in the presence of phosphoric acid. Then a series of catalysts with various VPO loadings on boehmite were synthesized. The catalysts were characterized using various techniques. The catalysts were utilized for extraction combined with catalytic oxidation of dibenzothiophene. The important factors influencing the desulfurization process, including reaction time, temperature, H₂O₂, catalyst loading, catalyst amount and solvents, were systematically investigated. Under the optimized reaction conditions, i.e. 30 mg of catalyst loading at 50°C and in 60 min, sulfur removal reached 94%. The catalyst was recycled and reused five times.     

Synthesis, Crystal Structure and Physical Properties of LixMg0.857−xCu2.143O3−y

Solid solutions of Li-doped Mg_0.857Cu_2.143O₃ (Li_xMg_0.857−xCu_2.143O_3−y) were prepared at 950°C for 12 h in air by the solid-state method using Li₂CO₃, MgO and CuO powders. The solid solutions were obtained as the single α phase with the güggenite structure in 0≦x≦0.06 region. With the increasing of the Li content x, the lattice parameters a, b and unit cell volume V decreased, while c increased. On the basis of the charge neutrality, hole carrier estimated by the oxygen content increased with the Li substitution. The Seebeck coefficient at room temperature of x = 0.03 sample was +400 μV/K. The electrical resistivity ρ at room temperature drastically decreased with the increasing x. Temperature dependences of ρ for x = 0.01, 0.03 and 0.06 samples were semi-conductive behavior from room temperature to about 12 K. Interaction between Cu²⁺ and Cu²⁺ through O²⁻ seems to be somewhat large antiferromagnetic one. Sperconducting transition was not detected in the temperature range.     

Study of Ce0.7Sn0.3O2 supported PdO catalysts for CO oxidation

Palladium supported by Ce_0.7Sn_0.3O₂ has been prepared by an impregnation method, and used for low temperature carbon monoxide oxidation. They were characterized by means of XRD and H₂-TPR techniques. For PdO/Ce_0.7Sn_0.3O₂ catalyst, three reduction peaks (α, β and γ) are observed. The β peak contributes to the reduction of PdO species and Sn⁴⁺ species on the surface of Ce_0.7Sn_0.3O₂; β peak to the reduction of bulk SnO₂ and surface Ce⁴⁺and the γ peak to the reduction of bulk CeO₂. The increase of Pd loading from 0 to 0.75% enhances oxidation of CO, further increase of the Pd content affects the catalytic activity but slightly. XRD and TPR results show that highly dispersed Pd on the surface of the support is the active species for CO oxidation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Speciation of the Vanadium(III) Complexes with 1,10-Phenanthroline, 2,2′-Bipyridine,and 8-Hydroxyquinoline

The complex species formed between vanadium(III) and 1,10-phenanthroline (phen), 2,2′-bipyridine (bipy), and 8-hydroxyquinoline (8hq) were studied in aqueous solution by means of electromotive forces measurements, emf(H), at 25 °C with 3.0 mol⋅dm⁻³ KCl as the ionic medium. The potentiometric data were analyzed using the least-squares computational program LETAGROP, taking into account the hydrolytic vanadium(III) species formed in solution. Analysis of the vanadium(III)–phen system data shows the formation of [VHL]⁴⁺, [V(OH)L]²⁺, [V₂OL₂]⁴⁺ and [V₂OL₄]⁴⁺ complexes. In the vanadium(III)–bipy system the [VHL]⁴⁺, [V(OH)L]²⁺, [V₂OL₂]⁴⁺ and [V₂OL₄]⁴⁺ complexes were observed, and in the vanadium(III)–8hq system the complexes [V(OH)L]⁺, [V(OH)₂L], [VL₂]⁺ and [VL₃] were detected.     

FT-IR study of the nature and stability of NOx surface species on ZrO2, VOx/ZrO2 and MoOx/ZrO2 catalysts

The adsorption of NO, NO/O₂ mixtures and NO₂ on pure ZrO₂ and on two series of catalysts supported on ZrO₂, one containing vanadia and the other molybdena (ZV and ZMo, respectively), has been investigated. The V and Mo surface contents of the latter were ≤3 atoms nm⁻² and ≤5 atoms nm⁻², respectively. All samples had been previously submitted to a standard oxidation treatment. On all samples, only extremely minor amounts of NO_x surface species are formed by NO interaction at room temperature (RT). NO_x surface species are formed in greater amounts on pure ZrO₂ when NO and O₂ are coadsorbed at RT; they are mainly nitrites, small amounts of nitrates, and small amounts of (O₂NO−H)^δ− species; when ZrO₂ is warmed to 623 K in the NO/O₂ mixture, nitrites decrease, nitrates and (O₂NO−H)^δ− species increase. The same NO_x species as on the ZrO₂ surface free from V (or Mo) are formed on ZV (or ZMo) samples with surface V (or Mo) density <1.5 atoms nm⁻²; however, they occur in decreased amount with increasing V (or Mo) coverage. On ZV samples with a surface V density of 1.5–3 atoms nm⁻² (or ZMo samples with a surface Mo density of 1.5–5 atoms nm⁻²) when NO and O₂ are coadsorbed at RT, there is formation of small amounts of nitrites, nitrates (both on ZrO₂ surface free from V (or Mo) and at the edges of V- or Mo-polyoxoanions) and NO₂ ^δ+ species, associated with V⁵⁺ (or Mo⁶⁺) of very strong Lewis acidity; when samples are warmed up 623 K in the NO/O₂ mixture, nitrites disappear, nitrates increase, NO₂ ^δ+ species remain constant or slightly decrease. When NO₂ is allowed into contact at RT with oxidized samples, surface situations almost identical to those obtained for each sample warmed to 623 K in NO/O₂ mixture is reached. The NO_x surface species stable at 623 K, the temperature at which catalysts show the best performance in the selective catalytic reduction (SCR) of NO by NH₃, are nitrates, both on ZrO₂ and on polyvanadates or polymolybdates at high nuclearity. On the contrary, nitrites and NO₂ ^δ+ species are unstable at 623 K.     

Introduction of vanadium species in <Emphasis Type="Italic">β</Emphasis> zeolite by solid-state reaction: spectroscopic study of V speciation and molecular mechanism

V-containing β zeolites were prepared by solid-state reaction between V₂O₅ and β zeolite. The zeolite structure was analysed by XRD and N₂ physisorption. The V speciation was studied by chemical analysis and different spectroscopies (FT-IR, ²⁷Al-NMR, UV-Vis, EPR, photoluminescence). After calcination of V₂O₅-β zeolite mechanical mixtures at 500°C, three kinds of V species were identified: (i) oligomeric vanadates with octahedral V⁵⁺ easily removed by treatment with NH₄OAc, (ii) isolated vanadyl (V=O)²⁺ ions in axially distorted octahedral or square pyramidal environment, interacting with framework and/or extraframework Al nuclei and (iii) isolated V⁵⁺ in tetrahedral and octahedral environments, localized in framework defect sites. The amount of the latter species is higher when water vapor is present during calcination and when parent β zeolite contains a high concentration of defect sites generated by a strong acid pretreatment. Isolated V⁵⁺ are easily reduced to tetrahedral V⁴⁺ or to square pyramidal (V=O)²⁺. Possible models of the mechanism of formation of V species by solid-state reaction and further reduction are proposed.     

Hierarchical Ni−Al hydrotalcite supported Pt catalyst for efficient catalytic oxidation of formaldehyde at room temperature

Catalytic oxidation at room temperature is recognized as the most promising method for formaldehyde (HCHO) removal. Pt-based catalysts are the optimal catalyst for HCHO decomposition at room temperature. Herein, flower-like hierarchical Pt/NiAl-LDHs catalysts with different [Ni²⁺]/[Al³⁺] molar ratios were synthesized via hydrothermal method followed by NaBH₄ reduction of Pt precursor at room temperature. The flower-like hierarchical Pt/NiAl-LDHs were composed of interlaced nanoplates and metallic Pt nanoparticles (NPs) approximately 3–4 nm in diameter were loaded on the surface of the Pt/NiAl-LDHs with high dispersion. The as-prepared Pt/NiAl21 nanocomposite was highly efficient in catalyzing oxidation of HCHO into CO₂ at room temperature. The high activity of the hierarchical Pt/NiAl21 nanocomposite was maintained after seven recycle tests, suggesting the high stability of the catalyst. Based on in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies, a reaction mechanism was put forward about HCHO decomposition at room temperature. This work provides new insights into designing and fabricating high-performance catalysts for efficient indoor air purification.     

A Novel Catalytic Procedure for the Determination of Ultratrace Zirconium

 A novel catalytic procedure for zirconium was proposed based on Zr(IV) catalyzed oxidation of gallocyanine by hydrogen peroxide in hexamethylene tetramine-hydrochloric acid buffer medium. The calibration graph is linear for 0–110 ngċml⁻¹, and the detection limit is 0.4 ngċml⁻¹ Zr(IV). Most foreign ions do not interfere with the determination, except for Cu²⁺, Fe³⁺ and Cr(VI). The interferences of Cu²⁺ and Fe³⁺ could be eliminated by masking with EDTA and mannitol, and that of Cr(VI) by reducing to Cr(III) with ascorbic acid. The typical features of this procedure are that it is sensitive for zirconium, and the determination could be carried out at room temperature. It had been used to the determination of zirconium in zirconium bronze, simulated samples and a certified reference material. The recoveries were 98.6 ∼ 102%, and relative standard deviations (R.S.D.) were 0.9 ∼ 1.5%, respectively. Received September 12, 1999. Revision April 10, 2000.     

Highly Efficient Mesoporous V<Subscript>2</Subscript>O<Subscript>5</Subscript>/WO<Subscript>3</Subscript>–TiO<Subscript>2</Subscript> Catalyst for Selective Catalytic Reduction of NO<Subscript>x</Subscript>: Effect of the Valence of V on the Catalytic Performance

Mesoporous WO₃–TiO₂ support was synthesized by hydrothermal method, mesoporous V₂O₅/WO₃–TiO₂ catalyst was synthesized by impregnation method and used for selective catalytic reduction (SCR) of NO_x with a excellent NO_x conversion at a wider operating temperature ranging from 200 to 460?°C. In the range of 260–440?°C, NO_x conversion reached to 98.6%, and nearly a complete conversion. Even with the existence of 300 ppm SO₂, NO_x conversion was only a little decline. The catalyst was characterized by a series of techniques, such as XRD, BET, XPS, TEM, Raman and H₂-TPR. It was concluded that V₂O₅/WO₃–TiO₂ catalyst was ascribe to antase TiO₂, and also the high crystallinity of anatase TiO₂ could improve the SCR performance. More interested, V₂O₅/WO₃–TiO₂ catalyst exhibited the typical mesoporous structure according to the BET results. In addition, the TEM results indicated that the active components of V and W were well-dispersed on the surface of TiO₂, while the enhancement of dispersion could improve the activity of catalysts. More importantly, the concentration ratio of V⁴⁺/(V⁵⁺?+?V⁴⁺?+?V³⁺) performed the key role in improving the activity of V₂O₅/WO₃–TiO₂ catalyst.     

V?Mo?Ox/SiO2 catalysts for acrolein oxidation I. The nature of active sites

The influence of the annealing temperature and the activation of V–Mo–O_x/SiO₂ catalysts for acrolein oxidation has been studied. The catalyst activity decreases with increasing temperature of annealing due to lowering of the number of active sites of V⁴⁺ and the content of active component VMo₃O_11+x.     

Synthesis and characterization of a reduced heteropoly-tungstovanadate: (NH4)7[VvO4W 10IV V 2IV O36].ca. 22H2O

The compound (NH₄)₇[ V^vO₄W ₁₀ ^VI V ₂ ^VI O₃₆]·ca.22H₂O (1) has been synthesized from an aqueous ammonium acetate buffer (pH 4) containing sodium vanadate, sodium rungstate_and sodium dithionite. Compound (1) crystallizes in a cubic space groupFm — 3, witha = 22.2001(6) ? and Z = 8. The anion [V^vO₄W ₁₀ ^VI V ₂ ^IV O₃₆]^7- is a typical Keggin type structure with V^VO₄ as the central tetrahedron. (1) has further been characterized by elemental analyses, redox titration, IR, EPR, and electronic spectroscopy and room temperature magnetic moment measurement.     

Kinetic Peculiarities in the Low-Temperature Oxidation of H2S on Vanadium Catalysts

H₂S oxidation by oxygen on catalysts V₂O₅/Al₂O₃, V₂O₅/TiO₂, V₂O₅/Al₂O₃/TiO₂ was studied at temperatures below the sulfur dew point. High activity and the oscillation character of the oxidation were demonstrated by catalysts with low contents of V₂O₅ (3–5 wt.%). The increase in the V₂O₅ concentration to 10–20 wt.% results in the reduction of the catalytic activity and oscillation ability. On a pure V₂O₅ catalyst, the oscillations were not detected. The difference between the catalysts with the high and low concentrations of V₂O₅ is explained in terms of the structures of the V⁵⁺ species formed in the catalysts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Hydrolytic precipitation of magnesium polyvanadates in vanadium (IV, V) solutions

The phase and chemical composition of precipitates formed in Mg(VO₃)₂-VOSO4-H₂O system at initial pH from 1 to 7 and temperature from 80 to 90°C was studied. Polyvanadates of variable composition Mg _x V _y ⁴⁺V_12-y ⁵⁺¹O_31–δ · nH₂O (0.7 ≤ x ≤ 1.3, 1.2 ≤ y ≤ 2.4, 0.7 ≤ δ = 1.4) were formed at pH from 1 to 4 and V⁴⁺/V⁵⁺ ratio from 0.43 to 9. Compounds with the general formula Mg _x V _y ⁴⁺V_6-y ⁵⁺O_16-δ · nH₂O (0.7 ≤ x ≤ 0.65, y = 1.0, 0.8 ≤ δ ≤ 0.85) were formed at pH from 6.0 to 7.0 and V⁴⁺/V⁵⁺ ratios from 0.11 to 0.25. The maximum V⁴⁺ concentration (y = 2.4) in the precipitates was achieved at the VV⁴⁺/V⁵⁺ solution ratio of 1.0 and pH = 3. The precipitates in solutions with pH 3 were formed only upon addition of VO²⁺ ions with the maximum rate at a V⁴⁺/V⁵⁺ ratio of 0.33. These processes were limited by second-order reactions on the surface of polyvanadates.     

Partial charge ordering in the mixed-valent compound

Crystals of the mixed-valent compound were grown from a flux. The room temperature conductivity of a crystal was 3 S/cm but decreased smoothly with decreasing temperature to 10⁻⁵ S/cm at 25 K. Magnetic susceptibility data indicate a localized moment for Rh⁴⁺. A Seebeck coefficient at 200 K of +280 μV/K further confirms that this compound is a semiconductor rather than a metal with a partially filled 4d t_2g band. A structure refinement based on single crystal X-ray diffraction data obtained at 173 and 296 K provided Rh–O distances sufficiently accurate to indicate the nature of the charge ordering between Rh³⁺ and Rh⁴⁺. The large Seebeck coefficient coupled with the high electrical conductivity indicates that this may be a promising low-temperature thermoelectric material.     

Complete oxidation of ethanol over vanadium based catalysts

V2O5/γ-Al2O3-TiO2 catalysts were prepared by the mixing sol-gel and co-impregnation method. The performance of the catalysts for complete oxidation of ethanol was performed in a conventional fixed-bed quartz reactor. And the effects of support, preparation methods and vanadium content have been investigated. The results showed that 5% V2O5 catalyst supported on γ-Al2O3-TiO2 possessed the best ethanol conversion under the considered temperature. This may be ascribed to the highly dispersible active component, mutual function between the active component and the carriers. The nature of the best performance for 5%V/γ-Al2O3-TiO2 catalyst may be related to the high V4+ amounts on the surface. And the surface V4+ species may play an important role in the formation of active site for the total ethanol oxidation.     

Cu、N共掺杂TiO2纳米管光催化重整甘油制备合成气

通过碱性水热-离子交换法制备了Cu、N共掺杂TiO₂纳米管(Cu/N-TNT),对其光催化重整甘油制备合成气性能进行了研究。结果表明,Cu/N-TNT具有富含氧空位(O_V)的管状结构,N以Ti-N形式取代部分O形成杂质能级,Cu以Cu²⁺形式掺杂在催化剂晶格间隙和表面,Cu、N共掺杂促进TiO₂表面电荷有效分离,有利于其光催化重整甘油制备合成气活性和选择性的提高。紫外光照射8 h时,掺Cu量为0.15%的Cu/N-TNT催化剂上CO和H₂产量分别为7.3和8.5 mmol·g^-1,是原始TiO₂的9.1和70.8倍,n_H2/n_CO从0.52提高为1.18,n_CO/n_CO2从0.21提高至0.42。Cu/N-TNT表面N和O_V为醛类脱羰和甲酸脱水生成CO提供反应活性位点,Cu作为浅势阱提高光生电子-空穴分离效率。光生空穴(h⁺)是光催化重整甘油制备合成气过程中的主要活性物种,大量羟基自由基(·OH)和超氧自由基(·O₂^-)会导致甘油过度氧化,使CO选择性降低。     

Effect of direct ultrasound synthesis via a sesquihydrate route on bismuth-promoted vanadyl pyrophosphate catalysts

A series of 1, 3, and 5% Bi-doped vanadium phosphate catalyst catalysts were prepared via sesquihydrate route using direct ultrasound method and were denoted as VPSB1, VPSB3, and VPSB5, respectively. These catalysts were synthesized solely using a direct ultrasound technique and calcined in a n-butane/air mixture. This study showed that catalyst synthesis time can be drastically reduced to only 2 hr compared to conventional 32–48 hr. All Bi-doped catalysts exhibited a well-crystallized (VO)₂P₂O₇ phase. In addition, two V⁵⁺ phases, that is, β-VOPO₄ and α_II-VOPO₄, were observed leading to an increase in the average oxidation state of vanadium. All catalysts showed V2p_3/2 at approx. 517 eV, giving the vanadium oxidation state at approx. 4.3–4.6. Field-emission scanning electron microscopy micrographs showed the secondary structure consisting of thin and small plate-like crystal clusters due to the cavitation effect of ultrasound waves. VPSB5 showed the highest amount of oxygen species removed associated with the V⁵⁺ and V⁴⁺ species in temperature-programmed reduction in H₂ analyses. TheX-ray absorption near edge structure (XANES) measurement showed the occurrence of vanadium oxide reductions in hydrogen gas flow, indicating the presence of V⁴⁺ and V⁵⁺ species. Higher average valence states of V⁵⁺, indicating more V⁵⁺ phases, were present. The addition of bismuth has increased the activity and selectivity to maleic anhydride.     

Free radical/singlet dioxygen system under the conditions of catalytic hydrogen peroxide decomposition

Hydrogen peroxide decomposition and the oxidation of unsaturated compounds (anthracenes, alkenes, etc.) in the H₂O₂/V^(V)/AcOH system occur via a molecular mechanism. H₂O₂ decomposes to yield singlet dioxygen¹O₂(¹Δ_g). Among V^V peroxo complexes of different compositions, coordinated Superoxide radical anions V^(V)(O2/∸) are found in a steady-state concentration in the system under investigation. Styrene oxidation in the H₂O₂V^(V)/AcOH system unusually accelerates in the presence of 2,6-di-tert-butyl-4-methylphenol (BHT), which is an inhibitor of radical chain reactions. This is explained by a decrease in the V^(V)(O ₂ ^∸ ) concentration and an increase in the concentration of dissolved¹O₂ in the presence of ionol. A new phenomenon in the chemistry of singlet dioxygen is found: the ESR signal from the paramagnetic system upon its interaction with¹O₂ broadens in an unusually drastic manner (up to 10 G). This broadening is virtually independent of the nature of the radicals, the acidity of the medium, and the nature of the metal catalysts used for the generation of¹O₂(¹Δ_g).