Subsolidus phase equilibria in the V9Mo6O40-Cr2(MoO4)3 system

The V₉Mo₆O₄₀?Cr₂(MoO₄)₃ system has been investigated using the differential thermal analysis (DTA) and X-ray phase diffraction methods. The system has been found not to be a real two-component system over the whole component system.     

Thin films of V2O5/MoO3 and their applications in electrochromism

Journal of Solid State Electrochemistry - Pure and doped vanadium pentoxide (V2O5, V2O5/MoO3) thin films were prepared by sol-gel method and dip coating technique. Furthermore, they were...     

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.     

Gibbs energies of formation of MoO2 and Mo4O11

The Gibbs energies of formation of MoO₂ and Mo₄O₁₁ have been determined by means of e.m.f. measurements using the cells Mo, MoO₂/ZrO₂· CaO or ThO₂· Y₂O₃/Fe_0.95O, Fe and Fe, Fe_0.95O/ZrO₂· CaO/Mo₄O₁₁, MoO₂. The data may be represented by the equations ^f ΔG⁰ 〈MoO₂〉 = −571.8 + 0.1662T ± 1.1 (kJ mol⁻¹) between 1070 and 1320 K and ^fΔG^o〈Mo₄O₁₁〉 = −2743 + 0.853T ± 5 (kJ mol⁻¹) between 840 and 1090 K. The enthalpies of formation and standard entropies of MoO₂ and Mo₄O₁₁ at 298 K are calculated by second law evaluations. ^fΔH⁰₂₉₈ 〈MoO₂〉 = −(586 ± 3) kJmol⁻¹S⁰₂₉₈ 〈MoO₂〉 = (49 ± 2) JK⁻¹mol^−1fΔH⁰₂₉₈〈Mo₄O₁₁〉 = −(2807 ± 12) kJmol⁻¹S⁰₂₉₈〈Mo₄O₁₁〉 = (290 ± 15) JK⁻¹mol⁻¹ The Gibbs energies of formation of MoO₂ and Mo₄O₁₁ are given by ^fΔG⁰〈MoO₂〉 = −592 − 0.0387TlogT + 0.303T (kJmol⁻¹) between 298 and 2000 K and ^fΔG⁰〈Mo₄O₁₁〉 = −2776 + 0.886T (kJmol⁻¹) between 298 and 1091 K.     

Importance of the O-M-O bridges (M = V5+, Mo6+) for the spin-exchange interactions in the magnetic oxides of Cu2+ ions bridged by MO4 tetrahedra: spin-lattice models of Rb2Cu2(MoO4)3, BaCu2V2O8, and KBa3Ca4Cu3V7O28

The spin-lattice models relevant for the magnetic oxides Rb2Cu2(MoO4)3, BaCu2V2O8, and KBa3Ca4Cu3V7O28 were determined by evaluating the relative strengths of the spin-exchange interactions between their Cu2+ ions on the basis of spin dimer analysis. Our study shows that the O-M-O bridges (M = V5+, Mo6+) between the magnetic ions Cu2+, provided by the MO4 tetrahedra, are crucial for the spin-exchange interactions and hence for deducing the spin-lattice models needed to interpret the magnetic properties of these oxides. The spin-lattice model of Rb2Cu2(MoO4)3 is not a uniform chain but two interpenetrating spin ladders that interact weakly with geometric spin frustration. The spin-lattice model of BaCu2V2O8 is an alternating chain as expected, but the spin-exchange paths responsible for it differ from those expected. With respect to the strongest spin exchange of BaCu2V2O8, the spin exchange of KBa3Ca4Cu3V7O28 is only slightly weaker, but the strongest spin exchange of Rb2Cu2(MoO4)3 is much weaker. This difference in the spin-exchange strengths is caused by the difference in the bridging modes of the MO4 tetrahedra leading to these spin-exchange interactions.     

Solid state interaction between V2O5 and MoO3: specific features related to minor vanadia transfer

Vanadia transport, which is a minor reaction flux in the solid state reaction between V₂O₅ and MoO₃, was studied using chemical and neutron activation analyses and electron spectroscopy for chemical analysis. It was found that negligible quantities of vanadia were transferred in a molybdena briquette during the reaction. Vanadia was presumably localized in thin external layers of molybdena grains. The reaction potential difference U _r across a Pt|MoO₃|V₂O₅|Pt cell was studied. It was shown that in this cell U _r was produced at the molybdena briquette and was due to vanadia transport. The U _r value changed with time in two stages. The reaction potential difference U _r was constant (or diminished slightly) at the first stage and dropped abruptly at the second stage. The duration of the first stage depended on the initial thickness of the MoO₃ briquette: the thicker the briquette, the longer the U _r value was nearly constant. Causes and probable mechanisms of U _r generation are discussed in different terms: chemical reaction, variation of a _{O 2} at the boundary between the reaction product and initialoxides, or surface spreading of the minor (V₂O₅ or V₉Mo₆O₄₀) diffusant. The last mechanism, which received the least study in the general case, was shown to be the most probable one for the reaction at hand. Electronic Publication     

Synergy effects in mixed Bi2O3, MoO3 and V2O5 catalysts for selective oxidation of propylene

In this work, the possible synergy effects between Bi₂O₃, MoO₃ and V₂O₅, and between Bi₂Mo₃O₁₂ and BiVO₄, were investigated. The catalytic activity of the ??mechanical mixture?? of these compounds was measured. The mixture containing 36.96?mol% Bi₂O₃, 39.13?mol% MoO₃ and 23.91?mol% V₂O₅ (21.43?mol% Bi₂Mo₃O₁₂ and 78.57?mol% BiVO₄), corresponding to the compound Bi_1?x/3V_1?x Mo _x O₄ with x?=?0.45 (Bi_0.85V_0.55Mo_0.45O₄), exhibited the highest activity for the selective oxidation of propylene to acrolein. The mixed sample prepared chemically by a sol?Cgel method possessed higher activity than that of mechanical mixtures.     

Study of the solid solution of MoO3 in Fe2V4O13

Differential thermal and phase X-ray analyses have shown that MoO₃ and Fe₂V₄O₁₃ form a solid substitution solution, in which Mo⁶⁺ ions are incorporate into the crystal lattice of Fe₂V₄O₁₃ in place of V⁵⁺ ions. The solubility limit of MoO₃ in Fe₂V₄O₁₃ at ambient temperature is 18 mole % of MoO₃. The phase equilibria in the system Fe₂V₄O₁₃-FeVMoO₇, were also studied. Results are presented in the form of a phase diagram.

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Zusammenfassung Durch DTA und Röntgenphasenanalyse wurde gezeigt, daß MoO₃ und Fe₂V₄O₁₃ Substitutionsmischkristalle bilden, in denen Mo⁶⁺-Ionen anstelle von V⁵⁺-Ionen in das Kristallgitter von Fe₂V₄O₁₃ eingebaut sind. Die Löslichkeitsgrenze von MoO₃ in Fe₂V₄O₁₃ beträgt bei Umgebungstemperatur 18 Mol-% MoO₃. Ebenfalls wurden die Phasengleichgewichte im System Fe₂V₄O₁₃-FeVMoO₇ untersucht. Die Ergebnisse sind in Form eines Phasendiagramms dargestellt.

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- , ₃ Fe₂V₄O₁₃ , o⁶⁺ V^5– Fe₂V₄O₁₃. ₃ Fe₂V₄O₁₃ 18 %. Fe₂V₄O₁₃-FeVMoO₇ .

     

ChemInform Abstract: From 1∞ [(UO2)2O(MoO4)4]6‐ to 1∞ [(UO2)2(MoO4)3(MoO5)] 6‐ Infinite Chains in A6U2Mo4O21 (A: Na,K, Rb,Cs) Compounds: Synthesis and Crystal Structure of Cs6 [(UO2)2(MoO4)3(MoO5)] .

Single crystals of the title compound are obtained from a melt of U₃O₈, MoO₃, and excess Cs₂CO₃ (Pt crucible, 950 °C, 12 h, cooling rate 5 °C/h).     

Phase equilibria up to the solidus line in the V2O5−Cr2(MoO4)3 system

Phase equilibria being established in the subsolidus area of the V₂O₅?Cr₂(MoO₄)₃ system at the whole component concentration range have been studied basing on DTA and X-ray phase powder diffraction. It has been established that the system is not a real two-component system in the subsolidus area. The fact has been proved by the presence of fields in that area, where three solid phases remain in mutual equilibrium.     

铬和钼掺杂对HCa2Nb3O10/Pt光催化性能的影响

通过高温固相反应合成了铌酸盐KCa2Nb3O10及Cr3+和Mo6+掺杂(摩尔分数5%)的KCa2Nb3O10,并通过离子交换反应制备出HCa2Nb3O10及Cr3+和Mo6+掺杂的HCa2Nb3O10,采用X射线衍射、原子吸收光谱、扫描电镜等对所制得的样品进行了表征.在甲醇为电子给体、Pt为助催化剂的情况下,研究了催化剂HCa2Nb3O10及Cr3+和Mo6+掺杂的HCa2Nb3O10在紫外光辐射下分解水产氢的光催化活性,并讨论了引起催化剂活性差异的原因.     

Studies on the Fe2(MoO4)3-V2O5 system

The phase equilibria established in solid state in the whole range of component concentrations in the Fe₂(MoO₄)₃-V₂O₅ system were studied by DTA and X-ray powder diffraction. This system is not a real two-component system.

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Zusammenfassung Im gesamten Konzentrationsbereich der Komponenten des Systems Fe₂(MoO₄)₃-V₂O₅ wurde das im festen Aggregatzustand festgestellte Phasengleichgewicht mittels DTA und Pulverröntgendiffraktionsverfahren untersucht. Dieses System ist kein wirkliches Zweikomponentensystem.

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F₂(₄)₃-V₂O₅ . , .

     

Pressure-induced first-order phase transition in (NH4)2V3O8 fresnoite: a double coordination change for V4+ and V5+

The high-pressure behaviour of (NH(4))(2)V(3)O(8) with the fresnoite structure (P4bm, Z = 2) has been studied at room temperature with single-crystal X-ray diffraction in diamond anvil cells using laboratory and synchrotron facilities. At ambient conditions, the crystal structure is composed of layers of corner-sharing V(5+)O(4) tetrahedra and V(4+)O(5) square pyramids separated by layers of the NH(4)(+) cations. At about 3 GPa, there occurs a reversible first-order phase transition to a three-dimensional structure (P4/mbm, Z = 2) built of corner-sharing V(5+)O(5) trigonal bipyramids and V(4+)O(6) octahedra. The NH(4)(+) cations fill up the interstitial sites in the tunnels formed by the vanadate framework. Up to the phase transition, the a lattice parameter of the low-pressure polymorph does not change while the contraction perpendicular to the stacking of the V(3)O(8) slabs accounts entirely for the bulk compressibility. Above the phase transition, the a lattice parameter slightly expands. The structural features of the high-pressure phase of (NH(4))(2)V(3)O(8) are compared to those of other vanadium oxides.     

具有四帽假Keggin结构"簇阴离子对"的多金属氧簇[Co(2,2-bipy)3]4[MoⅤ5MoⅥ5VⅣ6O40(PO4)]2·4H2O的水热合成与晶体结构

金属氧簇合物在催化、医药和材料等方面的应用越来越成为无机化学研究的热点[1～4]. 在众多的金属氧簇合物中, 只有几种双帽及四帽Keggin结构被合成出来[5 ～12], 而含有四帽假Keggin结构的钼钒氧簇合物尚未见报道.