Alumina-supported catalysts for propane oxidative dehydrogenation from mixed VXM(6−X)O19 (M=W, Mo) hexametalate precursors

γ-Al₂O₃ supported vanadium oxides were modified by tungsten and molybdenum oxides in order to improve dispersion and selectivity towards olefins in propane oxidative dehydrogenation (ODH). Both vanadium–tungsten and vanadium–molybdenum catalysts were obtained by adsorption of mixed isopolyanions (VW₅O₁₉⁵⁻, V₂W₄O₁₉⁴⁻, VMo₅O₁₉⁵⁻ and V₂Mo₄O₁₉⁴⁻) from aqueous solutions. The isopolyanion solutions were characterized by UV-Vis and ⁵¹V NMR spectroscopy. Vanadium, vanadium–tungsten and vanadium–molybdenum precursors and catalysts were also characterized by UV-Vis (diffuse reflectance) and solid state ⁵¹V NMR spectroscopy. An improved selectivity to propene in the presence of tungsten and molybdenum in VO_x/γ-Al₂O₃ was observed and attributed to dilution of vanadium by tungsten or molybdenum oxides on the γ-Al₂O₃ surface.     

Electrochemical characterization of ortho and meta-nitrotoluene derivatives in different electrolytic media. Free radical formation

This paper reports a comprehensive study by tast polarography, d.p.p., and cyclic voltammetry on the electrochemical reduction in different electrolytic media of ortho- and meta-nitrotoluene derivatives. Controlled potential electrolysis was used to generate the nitro radical anions and its detection by cyclic voltammetry and UV–Vis spectroscopy was performed. In protic media (30% ethanol/0.1 M Britton–Robinson buffer pH 2–12) both derivatives gave a sharp irreversible well-defined peak in all the pH range on Hg in a reaction involving four electrons to give the hydroxylamine derivative. In this medium meta-nitrotoluene is easier reduced in approximately 80 mV than that of the ortho-nitrotoluene. In mixed aqueous organic media (0.015 M aqueous citrate/DMF: 60:40, 0.3 M KCl and 0.1 M TBAI) at pH>8, the isolation and the electrochemical characterization of the one-electron reduction product, the nitro radical anion was achieved. At a 1 mM of nitrotoluene concentration, the average dismutation second-order rate constant values, k₂, were: 11,000±170 and 6900±72 M⁻¹ s⁻¹ for ortho-and meta-nitrotoluene, respectively. In aprotic media (0.1 M TBAI in DMF), the nitro radical anions were more stable than that of mixed media, with the following dismutation second-order rate constant values, k₂: 5800±35 and 4700±42 M⁻¹ s⁻¹ for ortho- and meta-nitrotoluene, respectively. Also, a comparison between nitrotoluene derivatives and some nitrocompounds of pharmacological relevance relating the effects of substituents on nitrobenzene and the electrolytic media composition on both the easiness of formation and stability of radicals is presented.     

On the formation of pyronitrate in molten salts

N₂O₅ reacts with O²⁻ ion in LiCl---KCl eutectic at 450° to give NO₃⁻. By analogy to the salts of the other oxides of Group V, NO₃⁻ can be considered as metanitrate and is expected to give—under appropriate conditions—the corresponding pyro-salt. Experiments are described in which the O²⁻ ion in LiCl---KCl melt is potentiometrically titrated with KNO₃. The titration curves show an inflexion at the composition corresponding to pyronitrate, N₂O₇⁴⁻.

The formation of pyronitrate in KNO₃ melts is also established. Strong oxide-ion donors, eg Na₂O₂ or NaOH, or electrolytically generated O²⁻ ion, react slowly with the melt to produce a compound of less basic character. The reaction is zero-order with respect to O²⁻ and has an activation energy of ca 6·17 Kcal/mole.

Pyronitrate in molten KNO₃ possesses a basicity comparable to that of the carbonate ion in the same melt. It readily lends its oxide ion to strong acids eg, Cr₂O₇²⁻ and PO₃⁻. X-ray diffraction patterns of NO₃⁻-N₂O₇⁴⁻ mixtures show peaks that can be correlated to the new anion.     

Spectroscopic study of vanadium(V) precipitation in the vanadium redox cell electrolyte

In this study, characterisation of vanadium electrolyte and vanadium(V) red precipitate formed in the positive half-cell electrolyte of a vanadium redox cell is present using different spectroscopic techniques. ⁵¹V solution NMR showed that the main peak at about −545 ppm in the spectra of the redox electrolyte could be attributed to the monomer species of VO₂⁺ ions and the intensity of the peak decreased with both temperature and aging time. These results confirmed the formation of a red precipitate in the redox electrolyte due to polymerisation of the monomer species of VO₂⁺ in the strong acidic media at elevated temperatures and with aging. Electron spin resonance (ESR) measurements showed that V(IV) ions were also present in the electrolyte. The presence of V(IV) ions may play an important role in the stability of the electrolyte. The static solid state ⁵¹V NMR spectrum of the thermal precipitate dried at room temperature showed a peak at −243 ppm which is characteristic of V(V) in distorted octahedral oxygen coordination similar to that formed for crystalline and gel forms of V₂O₅. Characteristics of V₂O₅ were also identified by X-ray diffraction (XRD) and had a fibrous morphology before heating. However, transition electron microscopy (TEM) showed that conversion of the fibrous morphology of V₂O₅ to the small crystalline morphology of V₂O₅ occurred after heating at 520°C.     

Passivity behavior of melt-spun Mg–Y Alloys

Several Mg–Y binary ribbons with Y content up to 17.9 at.% were fabricated by melt-spinning. X-ray diffraction (XRD) revealed that the phase structure changes with increasing Y content from extended solid solution to partially amorphous, and then fully intermetallic Mg₂₄Y₅. Anodic potentiodynamic polarization performed in 0.01 M NaCl electrolyte (pH=12) revealed improved anodic passivity behavior compared to pure Mg for all the Mg–Y alloys. X-ray photoelectron spectroscopy (XPS) revealed that the improved passivity of Mg–Y was more related to the elemental oxidation state rather than the concentration of the surface components. To study the effect of Cl⁻ ion on the passivity behavior, anodic potentiodynamic and potentiostatic polarization were performed on Mg–17.9 at.% Y in alkaline (pH=12) NaCl electrolytes containing Cl⁻ ion in the concentration range from 0.00 to 0.50 M. The passive films formed in 0.01 M NaCl electrolyte were similar to the native film, which were composed of MgO and Y₂O₃. No CO₃²⁻ and Cl⁻ ions were incorporated into the passive film. The passivity was significantly degraded in the electrolytes containing higher Cl⁻ concentration (0.1 and 0.5 M). Detailed XPS revealed that the surface films under these conditions were composed of much hydrated species Mg(OH)₂ and YOOH and/or Y(OH)₃ and CO₃²⁻ was incorporated into the surface film. The incorporation of Y₂O₃ in the passive film was given as the reason for the enhanced passivity properties of Mg–Y ribbons. The mechanism of Cl⁻ and CO₃²⁻ ions to the degradation of the passivity was discussed.     

Further studies on redoxokinetic titrations

The previously described “redoxokinetic effect” is used to indicate the end-points of titrations of Fe²⁺ with Cr₂O₇^2-, sulphuric acid with sodium hydroxide, AsO₂- with I₂ and Ag⁺ with Cl⁻. With the first three systems an accuracy of 0·1 per cent is possible. The method is not suited to the fourth system.

Abstract

Titrations of sulphuric acid vs. sodium hydroxide at 0·1 N concentration and ferrous ammonium sulphate vs. potassium dichromate at 0·05 N concentration can be carried out with an accuracy of ±0·1 per cent using the redoxokinetic technique. A very sharp end-point was obtained in the case of iodine vs. arsenite titration at 0·1 N concentration. Silver nitrate vs. chloride titrations cannot be carried out by the redoxokinetic technique.

Addition of MnSO₄ to the extent of 50 g/l. of the solution enhances the precision considerably in the titration of dilute solutions of ferrous ammonium sulphate with dichromate.     

A chronopotentiometric investigation of the dissociation of sulphate ions in molten equimolar NaCl-KCl at 750°C

The Lux—Flood acid—base equilibrium SO₃ + O²⁻ SO₄²⁻ in molten equimolar NaCl/KCl at 750°C has been investigated using conventional chronopotentiometry. The equilibrium constant for this reaction is shown to be very high (K > 10²). Thus the sulphate ion in solution in this melt does not decompose unless a very strong acid such as the metaphosphate ion is added to the melt. This removes oxide ions according to the reaction. 2PO₃⁻ + SO₄²⁻ → SO₃ + P₂O₇⁴⁻ The pyrophosphate anion is not a sufficiently strong acid to remove oxide from sulphate.     

Preparation of Au/TiO2 catalysts from Au(I)–thiosulfate complex and study of their photocatalytic activity for the degradation of methyl orange

Gold loaded on TiO₂ (Au/TiO₂) catalysts were prepared using Au(I)–thiosulfate complex (Au(S₂O₃)₂³⁻) as the gold precursor for the first time. The samples were characterized by UV–vis diffuse reflectance spectra, X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic absorption flame emission spectroscopy (AAS), and X-ray photoelectron spectroscopy (XPS) methods. Using Au(S₂O₃)₂³⁻ as gold precursor, ultra-fine gold nanoparticles with a highly disperse state can be successfully formed on the surface of TiO₂. The diameter of Au nanoparticles increases from 1.8 to 3.0 nm with increasing the nominal Au loading from 1% to 8%. The photocatalytic activity of Au/TiO₂ catalysts was evaluated from the analysis of the photodegradation of methyl orange (MO). With the similar Au loading, the catalysts prepared with Au(S₂O₃)₂³⁻ precursor exhibit higher photocatalytic activity for methyl orange degradation when compared with the Au/TiO₂ catalysts prepared with the methods of deposition–precipitation (DP) and impregnation (IMP). The preparation method has decisive influences on the morphology, size and number of Au nanoparticles loaded on the surface of TiO₂ and further affects the photocatalytic activity of the obtained catalysts.     

On the photooxidative behavior of TiO2 and PW12O40: OH radicals versus holes

Parallel experiments under similar conditions, using various substrates (atrazine, fenitrothion, 4-chlorophenol and 2,4-D) and OH radical scavengers (Br⁻, isopropyl alcohol, tertiary butyl alcohol and acetone), have shown that the photooxidizing mode of PW₁₂O₄₀³⁻ and TiO₂, i.e., OH radicals and/or holes (h⁺), depends on the nature of substrate and the mode of investigation. This provides an explanation for the controversial results reported in the literature. Atrazine shows that both PW₁₂O₄₀³⁻ and TiO₂ operate, mainly, via OH radicals and to a lesser extent with holes (h⁺), whereas, fenitrothion suggests that both systems operate almost exclusively, via OH radicals. Differences in the action of the catalysts are encountered in the photodegradation of 4-chlorophenol (4-ClPh) and 2,4-dichlorophenoxyacetic acid (2,4-D). PW₁₂O₄₀³⁻ appears to operate essentially via OH radicals, whereas, h⁺ appear to be the major oxidant with TiO₂. Overall, though, the action of OH radicals relative to h⁺ appears to be more pronounced with PW₁₂O₄₀³⁻ than TiO₂.     

Combined effect of H2O and SO2 on V2O5/AC catalysts for NO reduction with ammonia at lower temperatures

Combined effect of H₂O and SO₂ on V₂O₅/AC the activity of catalyst for selective catalytic reduction (SCR) of NO with NH₃ at lower temperatures was studied. In the absence of SO₂, H₂O inhibits the catalytic activity, which may be attributed to competitive adsorption of H₂O and reactants (NO and/or NH₃). Although SO₂ promotes the SCR activity of the V₂O₅/AC catalyst in the absence of H₂O, it speeds the deactivation of the catalyst in the presence of H₂O. The dual effect of SO₂ is attributed to the SO₄²⁻ formed on the catalyst surface, which stays as ammonium-sulfate salts on the catalyst surface. In the absence of H₂O, a small amount of ammonium-sulfate salts deposits on the surface of the catalyst, which promote the SCR activity; in the presence of H₂O, however, the deposition rate of ammonium-sulfate salts is much greater, which results in blocking of the catalyst pores and deactivates the catalyst. Decreasing V₂O₅ loading decreases the deactivation rate of the catalyst. The catalyst can be used stably at a space velocity of 9000 h⁻¹ and temperature of 250 °C.     

Catalytic activity and solid acidity of vanadium oxide thin layer loaded on TiO2, ZrO2, and SnO2

Vanadium oxide spread highly on TiO₂ (anatase, A) and SnO₂, and rather densely on TiO₂ (rutile, R) and ZrO₂ to make the monolayer in less than 4–5 V nm⁻². Profile of acid site of the monolayer was measured by temperature programmed desorption of ammonia, and its relation with the surface oxidation state was studied. The acid site density was high on the V₂O₅/TiO₂ (A) independent of the degree of oxidation. On the other hand, that of V₂O₅/TiO₂ (R) and V₂O₅/ZrO₂ depended on the oxidation state, and the high value of the concentration was observed on the oxidized one. The strength of acid site generated on the V₂O₅ monolayer on TiO₂ was as high as on the HZSM-5 zeolite. Turnover frequency (TOF) of propane conversion, and product selectivity were measured in propane oxidation. Among tested oxides, the V₂O₅/TiO₂ (A) showed the high TOF and selectivity to form propylene, while those loaded on TiO₂ (R) and ZrO₂ the small TOF and poor selectivity. Therefore, the reaction profile of activity and selectivity could be related with the extent of spreading and solid acidity. An idea of limit of the acid site density ca. 1.5 nm⁻² on the monolayer was elucidated.     

Thermoelectric characterization of NaxMx/2Ti1−x/2O2 (M=Co, Ni and Fe) polycrystalline materials

Layered -titanate materials, Na_xM_x/2Ti_1−x/2O₂ (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na_0.4Ni_0.2Ti_0.8O₂ were ca. 7×10³ S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm⁻¹ K⁻¹ at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na_0.4Ni_0.2Ti_0.8O₂ and Na_0.4Co_0.2Ti_0.8O₂ was about 1.9×10⁻⁴ and 1.2×10⁻⁴ K⁻¹ around 700 °C, respectively.     

Effect of pH, inorganic ions, organic matter and H2O2 on E. coli K12 photocatalytic inactivation by TiO2: Implications in solar water disinfection

The effect of different chemical parameters on photocatalytic inactivation of E. coli K12 is discussed. Illumination was produced by a solar lamp and suspended TiO₂ P-25 Degussa was used as catalyst. Modifications of initial pH between 4.0 and 9.0 do not affect the inactivation rate in the absence or presence of the catalyst. Addition of H₂O₂ affects positively the E. coli inactivation rate of both photolytic (only light) and photocatalytic (light plus TiO₂) disinfection processes. Addition of some inorganic ions (0.2 mmol/l) like HCO₃⁻, HPO₄²⁻, Cl⁻, NO₃⁻ and SO₄²⁻ to the suspension affects the sensitivity of bacteria to sunlight in the presence and in absence of TiO₂. Addition of HCO₃⁻ and HPO₄²⁻ resulted in a meaningful decrease in photocatalytic bactericidal effect while it was noted a weak influence of Cl⁻, SO₄²⁻ and NO₃⁻. The effect of counter ion (Na⁺ and K⁺) is not negligible and can modify the photocatalytic process as the anions. Bacteria inactivation was affected even at low concentrations (0.2 mmol/l) of SO₄²⁻ and HCO₃⁻, but the same concentration does not affect the resorcinol photodegradation, suggesting that disinfection is more sensitive to the presence of natural anions than photocatalytic degradation of organic compounds. The presence of organic substances naturally present in water like dihydroxybenzenes isomers shows a negative effect on photocatalytic disinfection. The effect of a mixture of chemical substances on photocatalytic disinfection was also studied by adding to the bacterial suspension nutrient broth, phosphate buffer and tap water.     

Solid acidity of metal oxide monolayer and its role in catalytic reactions

Such metal oxide as SO₄²⁻, MoO₃, WO₃, and V₂O₅ spread readily on supports like SnO₂, ZrO₂, and TiO₂ due to the different properties between acid and base oxides to generate the acid site on the monolayer. Number, strength, and structure of the acid site were characterized by temperature-programmed desorption (TPD) of ammonia principally, together with various physico-chemical techniques, and its role for catalytic reactions was studied. Approximately, one to two acid sites were stabilized on 1 nm² of the surface, which consisted of four to eight metal atoms. The limit in surface acid site density was estimated on the monolayer based on the concept of solid acidity on zeolites. Sequence of the metal oxide to show the strong acidity was, SO₄²⁻>WO₃>MoO₃>V₂O₅, and for the support oxide to accommodate the monolayer, SnO₂>ZrO₂>TiO₂>Al₂O₃. From these combinations, the metal oxide monolayer to show the adequate strength of acid site could be selected. Brønsted acidity was observed often, however, the Lewis acidity was prevailing on the reduced vanadium oxide. The structure of acid site, Brønsted or Lewis acid site, thus depended on the oxidation state. Relationship of the profile of solid acidity with various catalytic activities was explained. The solid acid site on the monolayer will possibly be applied to environment friendly technologies.     

Photodegradation catalyst screening by combinatorial methodology

In this work, a combinatorial methodology was developed for photodegradation catalyst screening. A fluorescence imaging detection system was designed for high throughput analysis, 1,6-hexamethylenediamine was used as the probe molecule for catalyst testing. The photodegradation activity of catalysts was evaluated by 1,6-hexamethylenediamine consumption during the photodegradation reaction. The methodology could provide reliable results. We found that pure TiO₂, ZrO₂, Nb₂O₅, MoO₃, and WO₃ did not show much activity for 1,6-hexamethylenediamine photodegradation under visible light. TiO₂ catalysts doped with different metal ions were tested. When TiO₂ was doped with Ta₂O₅, Nb₂O₅, V₂O₅, MoO₃, or WO₃, higher activity for photodegradation was observed. The doping of La³⁺, Ba²⁺, and Br⁻ to TiO₂ did not improve the catalytic activities. When doping TiO₂ with Mn²⁺, Cl⁻, Al³⁺, Cu²⁺, Fe³⁺, Na⁺, Mg²⁺, Li⁺, F⁻, Co²⁺, or K⁺, catalytic activity was lower than that of pure TiO₂. After elaborate catalysts screening, we discovered new catalysts, such as 50–70% TiO₂/0–20% WO₃/20–40% VO_2.5 and 20–30% TiO₂/30–50% MoO₃/40–60% VO_2.5 as well as 30% WO₃/20% ZrO₂/50% NbO_2.5 (synthesized from ZrCl₄, NbCl₅, and (NH₄)₅H₅[H₂(WO₄)₆]·H₂O in ethanol solution or suspension) and 60–70% WO₃/Nb₂O₅ (synthesized from WCl₆ and NbCl₅ in ethanol solution). We observed that the catalytic activity is sensitive to preparation methods and catalyst specific surface areas. When P123 (HO(CH₂CH₂O)₂₀(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H, designated EO₂₀PO₇₀EO₂₀) was used as template to synthesize mesoporous materials, the mesoporous catalysts showed higher activity than regular catalytic materials.     

Low temperature SCR of NO with NH3 over carbon nanotubes supported vanadium oxides

A novel multiwalled carbon nanotube (CNTs) supported vanadium catalyst was prepared. The structure of catalyst prepared was characterized by TEM, BET, FTIR, XRD and temperature-programmed desorption (TPD) methods. The results indicated that vanadium particles were highly dispersed on the wall of carbon nanotubes. The V₂O₅/CNT catalysts showed good activities in the SCR of NO with a temperature range of 373–523 K. The Lewis acid sites on the surface of V₂O₅/CNT are the active sites for the selective catalytic reduction (SCR) of NO with NH₃ at low temperatures. It was suggested that the reaction path might involve the adsorbed NH₃ species reacted with NO from gaseous phase and as well as the adsorbed NO₂ species. The diameter of CNTs showed positive effect on the activities of the catalysts. Under the reaction conditions of 463 K, 0.1 Mpa, NH₃/NO = 1, GHSV = 35,000 h⁻¹, and V₂O₅ loading of 2.35 wt%, the outer diameter of CNTs of 60–100 nm, the NO conversion was 92%.     

EPR study of electrochemical lithium intercalation in V2O5 cathodes

The evolution of the cathode material of Li/V₂O₅ cells upon lithium intercalation is studied by Electron Paramagnetic Resonance (EPR) spectroscopy. Below one lithium per V₂O₅(x < 1), where the electrochemical process is completely reversible, V^IV ions of the - and δ-Li_x V₂O₅ phases are detected by EPR. Within this intercalation range, the shape of the EPR signal is dominated by the strong exchange interactions between magnetically concentrated V^IV ions of the Li_x V₂O₅ phases. The intercalation range x > 1 is characterized by a new EPR signal attributed to magnetically diluted vanadyl ions VO²⁺. It is shown that these species do not belong to the Li_x V₂O₅ matrix, but most probably originate from the liberation of surface vanadyl ions during the irreversible transformation of δ-Li_x V₂O₅ into γ-Li_x V₂O₅. On the basis of these results, a mechanism to explain the partial irreversibility of the electrochemical processes is proposed. A third EPR signal is also observed for x 1. This signal is attributed to an electron-hole defect in Li_x V₂O₅, originating from a local charge compensation of the vanadyl vacancies.     

Direct evidence for purely silver ion conduction in CuI-doped silver oxysalt superionic systems: Combined electrolysis and EDS studies

Maiden attempt has been made for the direct estimation of the contributions of silver and copper ions to the ionic conductivity in superionic solids obtained in CuI-doped silver oxysalt systems. The application of the combined electrolysis and EDS techniques towards qualitative and quantitative analyses of the mobile ionic species in solid electrolyte systems having more than one possible mobile ion is reported. These studies confirmed that these electrolyte materials are purely Ag⁺ conducting up to 50 mol% CuI in xCuI–(100 − x)[2Ag₂O–0.7V₂O₅–0.3B₂O₃] and xCuI–(100 − x)[Ag₂O–0.7MoO₃–0.3WO₃] systems and small fraction of t_Cu+ exists above 60 mol% CuI. These solid electrolyte materials exhibited a high ionic transport numbers (t_i) of >0.985 and the t_i increases when two glass formers are used.     

Promotion of zirconia acidity by addition of sulfate ion

The promotion of zirconia by SO₄²⁻ is studied by percolating of zirconia with aqueous solutions of several sulfur compounds and several concentrations of H₂SO₄ as sources of sulfur. The presence of SO₄²⁻ is necessary to have catalytic activity to isomerize n-butane and produces a great increase in the stability of the physical texture to thermal treatments. The more convenient solution is 1N H₂SO₄·S0₄²⁻/ZrO₂ has the greatest catalytic activity after calcination at 893 K, where the tetragonal phase of ZrO₂ predominates. The catalytic activity was found proportional to the specific surface area and surface SO₄²⁻ concentration.     

Promoting effect of calcium on reduction of NO by C3H6 over V2O5/ZrO2 catalysts

The reduction of nitrogen monoxide by propene on V₂O₅/ZrO₂ doped with or without calcium has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Considerable promoting effect of calcium doping on the reduction of nitrogen monoxide by propene was observed on the V₂O₅/ZrO₂ catalysts. For the reaction of a mixture of NO+C₃H₆, carbonyl and carboxylate species were observed above 373 K, although nitrate species formed at room temperature on V₂O₅/ZrO₂ doped with calcium. No bands due to a compound including both carbon and nitrogen atoms were observed. Thus, the redox mechanism, i.e. propene reduces the catalyst and nitrogen monoxide oxidizes the catalyst, is confirmed on V₂O₅/ZrO₂ catalysts doped with or without calcium. The analysis of the V=O band in the region of 1100–900 cm⁻¹ indicates that this promotion is mainly due to new V=O species formed by the addition of calcium onto the catalyst. This species is easily reproduced in comparison with the other V=O species on the surface in the reoxidation process of the catalyst.