V2O5 System Research Articles

Phase composition of a selective V2O5−Fe2O3 catalyst of methanol oxidation to formaldehyde

The formation of the binary Fe2O3·V2O5 catalyst for selective methanol oxidation was investigated by IR and X-ray measurements on samples obtained at different calcination temperatures. It was found that the most selective catalyst of the binary Fe2O3−V2O5 system consists of a single phase of the composition Fe2O3·V2O5 which is formed at 600°C. The vanadium-oxygen bonds of this phase are not of the orthovanadate type.

Phase Equilibria in the Er2O3–V2O3–V2O5 System at 1200 °C

Abstract The phase equilibria in the Er2O3–V2O3–V2O5 system have been established at 1200 °C. In this system, Er2O3, Er8V2O17(4Er2O3·V2O5), ErVO3, ErVO4, VnO2n−1(n: 2 to 7), and VO2 found to be stable, Er8V2O17, ErVO4, V2O3, and VO2 of which had non-stoichiometric compositions. On the basis of the phase equilibria, the standard Gibbs energies for the reactions, ErVO3+1⁄2O2=ErVO4, (1) 3Er2O3+2ErVO3+O2=Er8V2O17, (2) have been determined to be −121±1 and −256±1 kJ, respectively. It has been shown that the standard Gibbs energy for Sm, Er, and Lu in Eq. 1 decreases linearly with increasing ionic radius of lanthanoid.

Struktur von Gläsern des Systems TeO2−VO2

Glasses from the TeO2−VO2 system and α- and β-crystalline modifications of TeO2·VO2 (TeVO4) are studied by IR-spectroscopy in the 1400–400 cm−1 range. Similarity is established in the area near 970 cm−1 of the spectrum between TeVO4 glass and the β-form. The continuous shift of the band from 970 cm−1 to 950 cm−1 with increasing TeO2 content in the glasses is connected with the indirect influence on the nonbridging V−O bond. It is shown that the local environment of the V in some glasses from TeO2−VO2 and TeO2−V2O5 system is similar: VO5 polyhedra.

Phase Equilibria in Sm2O3–V2O3–V2O5 System at 1200 °C

Abstract The phase equilibria in the Sm2O3–V2O3–V2O5 system was established at 1200 °C. In this system, Sm2O3, Sm10V2O20, SmVO3, SmVO4, V2O3, V3O5, V4O7, V5O9, V6O11 and VO2 were stable and Sm10V2O20, SmVO4, V2O3, and VO2 had the non-stoichiometry. On the basis of the phase equilibria, the Gibbs free energy for the reactions, SmVO3+1⁄2O2=SmVO4 and 4 Sm2O3+2 SmVO3+O2=Sm10V2O20, were determined to be −30.4±0.1 and −76.8±0.1 kcal, respectively. Also, standard Gibbs free energies of oxidation of various vanadium oxides were calculated.

The Oxidation Activity and Acid-base Properties of Mixed Oxide Catalysts Containing Titania. I. The TiO2–MoO3 and TiO2–V2O5 Systems

Abstract The amounts of both the acidic and basic sites of two series of catalysts, TiO2–MoO3 and TiO2–V2O5, with different compositions were measured by studying the adsorption of the basic and acidic molecules in the gas phase, using both the static and pulse methods. The acidity of TiO2–MoO3 catalysts is very high at Mo=40–60 atom%, though those of the TiO2-rich (Mo<30 atom%) and MoO3-rich (Mo>80 atom%) catalysts are fairly low. The acidity of the TiO2–V2O5 increases steadily with the V2O5 content. The basicity of the catalysts is remarkably enhanced by the introduction of a small amount of MoO3 or V2O5. It can be said that the catalysts are basic at Mo<20 atom% or V<10 atom%, while the MoO3-rich or V2O5-rich catalysts are acidic. The vapor-phase oxidation of 1-butene, butadiene,and acetic acid, the dehydration and dehydrogenation of isopropyl alcohol (IRA), and the isomerization of 1-butene were carried out in the presence of an excess of air, and the relationship between the catalytic behavior and the acid-base properties was investigated. It was concluded that the activities and selectivities can be relatively well explained by the acid-base properties between the catalyst and the reactant.

GROWTH OF TIN (IV) OXIDE CRYSTALS FROM FLUX OF B2O3–V2O5 SYSTEM

Abstract Needle crystals of SnO2 were grown from the melt of B2O3–V2O5 system containing Zn2SnO4. Increasing the soaking temperature and the V2O5 content of B2O3–V2O5 system tended to increase the size of crystals. The growth direction of the crystals was along the c-axis of SnO2.

Influence of alkaline promoters on the selectivity of vanadium catalysts in the oxidation of o-xylene to phthalic anhydride

The influence of the alkali metals Na, K, Rb, and Cs on the catalytic properties of the system V2O5−TiO2 has been studied in the oxidation of o-xylene. When introducing the promoter into the catalyst, and as the alkaline nature of the metals increases, the initial selectivity to partial oxidation products increases. A correlation is observed between the selectivity and the acidity of the surface, which was determined in a model raction of cumene cracking. The most acidic sites of the surface are assumed to be responsible for the destructive oxidation of o-xylene. The rate of oxidation of phthalic anhydride remaines constant when the nature of the additive varied.

Le Systeme Tl2OV2O5

AbstractThe System Tl2OV2O5The Tl2OV2O5 system has been studied by thermal analysis and x‐ray analysis. The entire phase diagram is reported. The symmetry and cell dimensions of most of the phases are derived from single crystal photographs. The compounds rich in vanadium are isomorphous with potassium analogues. Such an isomorphism does not exist for compounds like Tl4V2O7 and Tl3VO4; in these cases, the lone pair of thallium(I), doubtless, is stereochemically active.

Viscosities of Melts of V2O5 and V2O5-Beryl(Cr3+) System

The viscosities of melts of V205 and V20s-beryl(Cr3+)system were measured by a Broo: kfield type visc meter in the temperature range fr m 700 t 1070 The results with in a maximum error of 9% were oftainedThe flows o th melts were found to be Newtonian above ca 880 C and non-Newtonian be1 w 870 q which vaXied.300 t 36 centip isesr The l garithms f vlscosity f both rnelts are plotted agdinst the reciprocal of absolute temperature in Fig 3a d 4 Two linear relationsof lo9 na 1/T were fo ti nd below ca, 870 c and above ca, 880, From the slopes of these linear relations, the energies of activation for viscousbowsof melts of V205 and V20 bery1(Cr3) system were 23.8 24.2kcal/m 1 be1 w-ca 870. C and 2, 0, 3, 4kca1/m l above ca.880C respectively.The molar volume as a flow unit in V2O5 melt was calculated from Eyring's transition state theory, using the estimated entr py f activati n(dSdi several ca1/deg m 1), and assumed t be about 5 10 times la ger than a tolar volume of a unit cell of V20 below ca 870 C and ca.1 3 times abovea.880 C.

Reactions of potassium monoxide with the oxides of vanadium

The alkali-rich portion of the K2O–V2O5 system has been investigated by the reaction of various potassium oxo-compounds with vanadium pentoxide. The compounds KVO3, K32V18O61, K4V2O7, and K3VO4 lose oxygen and three forms of the compound K3VO4 were established. Both oxidations and disproportionations were observed for the K2O–VO2 system, and a new compound KVO2 was found in the reaction of equimolar mixtures of the monoxide K2O and vanadium dioxide. The lower oxides of vanadium and vanadium metal were oxidised by potassium monoxide to the vanadate K3VO4. The reactions of potassium monoxide are compared with those of sodium oxide for the same stoicheiometry.

The Phase Equilibria in the FeO-Fe2O3-V2O3 System at 1500°K

Abstract The phase equilibria in the FeO–Fe2O3–V2O3 system have been determined at 1500°K by varying the oxygen partial pressure. The following three oxide phases have been stable in equilibrium: a sesquioxide solid solution with a corundum-type structure (approximate composition Fe2O3–V2O3), a ternary solid solution with a spinel type structure (approximate composition FeO·Fe2O3–FeO·V2O3), and a ternary wüstite solid solution with a periclase-type structure which dissolves vanadium ions up to about 10 mol per cent. The extent of the solid solution areas and the location of the oxygen isobars have been determined. The spinel phase coexisted partly with metallic iron and partly with the wüstite solid solution, corresponding to lower and higher iron contents respectively. The standard free energies of the Fe+V2O3+\frac12O2=FeV2O4 and 0.05 Fe+V2O3+Fe0.95O=FeV2O4 reactions to produce FeV2O4 have been calculated to be −45500±200 cal and −5800±400 cal respectively at 1500°K on the basis of the equilibrium oxygen partial pressure.

Optical Absorption Properties of Vanadate Glasses

The optical absorption of vanadate glasses based on the system V2O5–P2O5 was measured in the range 20 cm−1 to 25 000 cm−1 at room and liquid-nitrogen temperatures. The samples were blown films of thickness about 1–2 μ and of composition 70.0, 80.0, and 87.5 mole % V2O5. Vibrational absorption peaks were observed at about 360, 420, 680, 1010, and 1100 cm−1 with additional structure likely at about 900 cm−1 in the 70.0 mole% V2O5 films. Peaks were observed at about 330, 435, 635, 810, 1007, and 1085 cm−1 in the 87.5 mole % films. Absorption tails were observed extending from the lowest-energy peaks to 20 and 33 cm−1 in the 70.0 and 87.5 mole % V2O5 samples, respectively. No noticeable temperature effects on spectra shape and peak positions were observed. Absorption peaks were also observed at 1038 and 1277 cm−1 in crystalline V2O5 at room temperature and at 915, 1040, and 1274 cm−1 and possibly at 1256 cm−1 at liquid-nitrogen temperature. The peaks at about 1010 cm−1 in the glasses are thought to be the V–O stretching vibrations, and the peaks at about 1090 cm−1 are assigned to a phosphorous–oxygen vibration. Other peaks are unassigned. A broad absorption tail which is responsible for the dark black color of bulk samples was observed between the apparent fundamental absorption edge of the glasses in the short-wavelength region of the visible and about 4000 cm−1. The cause of this absorption is not definitely ascertained, though V4+ ions may contribute. The absorption edge of both the 70.0 and 87.5 mole % V2O5 glasses fits the condition for direct forbidden transitions as does the edge of crystalline V2O5. Eg values were determined to be 2.38 and 2.41 eV for 87.5 mole % V2O5 films at room and liquid-nitrogen temperatures, respectively, and 2.47 and 2.51 eV for the 70.0 mole % V2O5 films at room and liquid-nitrogen temperatures, respectively.

Metal-insulator Transition in Transition Metal Oxides

The metal-insulator transition in the V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> system is discussed. A recent series of experiments on V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> and (V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1−x</inf> Cr <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> ) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> is reviewed. The phase diagram for the system is described. The Cr-doped mixed oxides are insulating at room temperature for x ≥ 0.009 and transform to a metal with the application of pressure. This phase transition is identified as a Mott transition. A comparison is made between the experimental results and theoretical predictions of the Mott transition.

The Phase Diagram of the V2O3--V2O5 System and its Magnetism

The Phase Diagram of the V2O3--V2O5 System and its Magnetism