Structure and Reactivity of Gas‐Phase Molybdenum Oxide Cluster Ions

Combining the ion cyclotron resonance method and a Knudsen effusion source, we obtained a series of Mo_xO_y ⁺ (x = 1 – 5, y = 1 – 15) molybdenum oxide cluster ions. We studied the dependence of the concentrations of these ions on the trapping time and their reactions with carbon monoxide. It is shown that Mo_xO_y ⁺ ions with x > 3 contain a cyclic Mo₃O₉ fragment in their structure. The oxygen bond energies in Mo_xO_y ⁺ ionic clusters are estimated.     

Mechanism of the Formation of Molybdenum Metallocarbene Complexes in the Gas Phase

The reactions of Mo⁺ ions and Mo _x O _y ⁺ oxygen-containing molybdenum cluster ions (x = 1-3; y = 1-9) with methane, ethylene oxide, and cyclopropane were studied using ion cyclotron resonance. The formation of a number of organometallic ions, including the metallocarbene MoCH₂ ⁺ , as well as molybdenum oxometallocarbenes Mo _x O _y CH₂ ⁺ (x = 1-3; y = 2, 4, 5, or 8) and Mo _x O _y (CH₄)⁺ ions (x = 1-3; y = 2, 5, or 8), was detected. The upper and lower limits of bond energies in oxometallocarbene complexes were evaluated: 111 > D ⁰ (Mo _x O _y ⁺-CH₂) > 82 kcal/mol (x = 1-3; y = 2, 5, 8).     

Formation of mixed Fe-Mo oxo clusters in the gas phase

Reactions of oxygen-containing molybdenum clusters Mo_xO_y (x = 1–3, y = 1–9) with iron carbonyl ions Fe(CO) _n ⁺ (n = 1–3) were studied by the ion cyclotron resonance technique. The reactions were found to yield mixed Fe-Mo oxo clusters Mo_xO_yFe⁺ (x = 2, 3; y = 5, 6, 8, 9).     

Possibility of Mo<Subscript>2</Subscript>O<Subscript>6</Subscript> formation at high temperatures in the range of pressures from 1 to 1 × 10<Superscript>−5</Superscript> bar

Gibbs free energy minimization was used to consider the formation of complex molybdenum oxide (Mo₂O₆) at 2400 K in the range of pressures from 1 to 1 to 1 × 10⁻⁵ bar for the basic component ratio Mo: O₂ = 1: 1. Several ways are shown to lead to Mo₂O₆ formation: when P = 1 bar, a synthesis reaction involving simple molybdenum oxides (MoO, MoO₂, MoO₃) is the main way; when P = 1 × 10⁻³ bar or lower, reactions of (MoO₃) _n (n = 3−5) complex oxides with metallic molybdenum and molybdenum monoxide (MoO) are.     

Influence of Adding Molybdenum on Structure and Performance of FexOy/SBA-15 Catalysts in Selective Oxidation of Propene

Mixed iron and molybdenum oxide catalysts supported on nanostructured silica, SBA-15, were synthesized with various Mo/Fe atomic ratios ranging from 0.07/1.0 to 0.57/1.0. Structural characterization of as-prepared Mo_xO_y_Fe_xO_y/SBA-15 samples was performed by nitrogen physisorption, X-ray diffraction, and DR-UV-Vis spectroscopy. Adding molybdenum resulted in a pronounced dispersion effect on supported iron oxidic species. Increasing atomic ratio up to 0.21Mo/1.0Fe was accompanied by decreasing species sizes. Strong interactions between iron and molybdenum during the synthesis resulted in the formation of Fe−O−Mo structure units, possibly Fe₂(MoO₄)₃-like species. Reducibility of Mo_xO_y_Fe_xO_y/SBA-15 catalysts was investigated by temperature-programmed reduction experiments with hydrogen as reducing agent. The lower reducibility obtained when adding molybdenum was ascribed to both dispersion and electronic effect of molybdenum. Catalytic performance of Mo_xO_y_Fe_xO_y/SBA-15 samples was studied in selective gas-phase oxidation of propene with O₂ as oxidant. Adding molybdenum resulted in an increased acrolein selectivity and a decreased selectivity towards total oxidation products.     

Zur Existenz des Seandiummonoxides

Zusammenfassung Durch Reduktion von Sc₂O₃ mit Silicium oder Magnesium bildet sich ein Produkt mit B1-Typ und einer Gitterkonstantea=4,45 Å. Die früher bestimmten Gitterparameter im Bereich 4,48–4,54 Å gelten für Produkte, die etwas Kohlenstoff enthalten, d. h. für SxO_xC_y mitx=0,95–1,03 undy=0,03–0,05.

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Existence of scandium oxide

The reduction of Sc₂O₃ with silicon or magnesium yields a product of B1-type, lattice-constanta=4,45 Å. Lattice parameters, obtained before, at 4,48–4,54 Å are typical for substances containing some carbon, i.e. for ScO_xC_y, withx=0,95–1,03 andy=0,03–0,05.

     

Structure and properties of Y5Mo2O12 and Gd5Mo2O12: Mixed valence oxides with structurally equivalent molybdenum atoms

Crystals of Ln₅Mo₂O₁₂ (Ln = Y, Gd) were grown by electrochemical reduction of alkali-molybdate/rare-earth oxide melts at 1075–1100°C. A single crystal of Y₅Mo₂O₁₂, used for structure determination, was found to be monoclinic with a = 12.2376(7) Å, b = 5.7177(8) Å, c = 7.4835(5) Å, β = 108.034(5)°, and Z = 2. Although the structure was refined in space group C2/m, the true space group appears to be P2₁/m. In Y₅Mo₂O₁₂, rutile-like sheets of edge-shared MoO₆ chains linked by YO₆ octahedra are interconnected with YO₇ monocapped trigonal prisms. The Mo atoms in the chains have alternating distances of 2.496 and 3.221 Å and in that respect are similar to MoO₂. However, in contrast to metallic MoO₂ both the Y and Gd compounds are n-type semiconductors with room temperature resistivities of the order of 10³ ohm-cm. Magnetic susceptibility measurements confirm the presence of one unpaired electron per Mo₂ unit. The semiconducting behavior can be explained in terms of an unfavorable bridging oxygen coordination which prevents electron delocalization through metal-oxygen pi bonding as in MoO₂.     

Relative stability of isomers of polynuclear complexes of molybdenum oxides

Molybdenum(VI) oxide MoO₃ has been studied and the composition of polynuclear molybdenum oxides in the gas phase Mo _x O_{3x ? y} (x = 1–6, y = 0–2) has been determined by laser desorption/ionization time-of-flight mass spectrometry. Quantum-chemical calculations of bond energies, interatomic distances, charge distributions, and molybdenum-molybdenum bond orders for the isomers of neutral polynuclear molecular compounds Mo _x O3 _{x ? y} have been performed with the use of the PBE functional with a relativistically corrected potential implemented as the PRIRODA program package. On the basis of the bond energies, the relative stability of the isomers has been estimated. For the Mo _x O _y isomers (x ≥ 3), cyclic structures have been predicted to be more favorable. For the predicted most stable isomers of each Mo _x O _y composition, the bond energies of their positive and negative ions have been calculated. The positive ionization of Mo _x O _y leads to a considerable decrease in the bond energy of the isomer and the negative ionization, to its increase by about 0.1 au.     

Control over the self-assembly of polymers and layered molybdenum oxide: from the micrometer scale to the molecular scale

We report adjustment on the self-assembly between polymer of polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA) and inorganic molybdenum oxide layers from the micrometer scale to the nanometer scale. Our method is to break the strong interactions between the organic polymers by introducing suitable bridging agents and adjust the reaction speeds of the two competitive reactions in the reaction system. We use I₂ to complex with PVA and break the strong hydrogen interactions between the PVA chains, resulting in a PVA-I₂/(Mo_xO_y)_∞ⁿ⁻ complex, in which the organic and inorganic species self-assemble homogenously on the molecular scale. We also adjust the thickness of the inorganic (Mo_xO_y)_∞ⁿ⁻ layers in the hybrid of PVP/(Mo_xO_y)_∞ⁿ⁻ by controlling the reaction speeds of the two competitive reactions: hydrolysis of Mo₇O₂₄ ⁶⁻ into (Mo_xO_y)_∞ⁿ⁻ and packing into thick inorganic layers on the one hand, and hybridization of (Mo_xO_y)_∞ⁿ⁻ and PVP into layered hybrid on the other hand. Experimental results proved that when the hydrolysis is overwhelming, the inorganic molybdenum oxide chains pack into heavy layers and self-assemble with PVP polymers on the micrometer scale, and when the hybrid reaction dominates, the organic polymer and molybdenum oxide hybridize on the molecular scale. These findings open new routes to disperse organic polymer and inorganic species homogenously and fabricate novel organic/inorganic hybrid nanomaterials in situ.     

C–C Bond Rupture in the Oxidation of Lower (C2–C6) Alcohols with Hydrogen Peroxide Mediated by Iron-Containing Compounds

C–C bond rupture upon the oxidation of alcohols in the Fe(ClO₄)₃+ H₂O₂system in aqueous acetonitrile at room temperature is found. The relative yield of the products of C–C bond rupture is 20–30% under standard conditions for C₂and C₃alcohols and decreases in the series C₂> C₃> C₄> C₆. The alkyl radical and carboxylic acid are the products of C–C bond rupture in alcohol oxidation. Cyclohexane is a competitive inhibiting agent for C–H bond oxidation in 1-propanol, and it does not affect the yield of the products of C–C bond rupture. When H₂O₂is replaced by tert-BuOOH, the fraction of the products of C–C bond rupture decreases by an order of magnitude. Our data suggest that a non-radical intermediate, likely Fe(III) hydroperoxo complex, is responsible for C–C bond rupture in alcohol under the reaction conditions.     

Enhanced lithium electrochromism of atmospheric pressure plasma jet-synthesized tungsten/molybdenum oxide films for flexible electrochromic devices

Enhanced lithium electrochromic performances of mixed organo-tungsten oxide (W _x O _y C _z )/organo-molybdenum oxide (Mo _x O _y C _z ) films by a rapid codeposition onto 40 Ω/□ flexible polyethylene terephthalate/indium tin oxide substrates at a short exposed duration of 23 s using an atmospheric pressure plasma jet (APPJ) at various mixed concentrations of hexacarbonyl precursors [W(CO)₆ and Mo(CO)₆] are investigated. The flexible organo-tungsten–molybdenum oxide (WMo _x O _y C _z ) films demonstrated noteworthy electrochromic performance for 200 cycles of reversible Li⁺ ion intercalation and deintercalation in a 1 M LiClO₄–propylene carbonate electrolyte by the switching measurements of potential sweep from ?1 to 1 V at a scan rate of 50 mV/s and the potential step at ?1 and 1 V, even after being bent 360^o around a 2.5-cm diameter rod for 1,000 cycles. The optical modulation (ΔT) of 61.3 % for MoO _y C _z films at a wavelength of 795.6 nm was significantly improved up to 72.5 % for WMo _x O _y C _z films cosynthesized with an APPJ.     

The Mechanism of Action of a Multicomponent Co-Mo-Bi-Fe-Sb-K-O Catalyst for the Partial Oxidation of Propylene to Acrolein: II. Changes in the Phase Composition of the Catalyst under Reaction Conditions

The structures of a complex multicomponent Co-Mo-Fe-Bi-K-Sb-O catalyst for the oxidation of propylene to acrolein and simpler catalysts from which some catalyst components were absent were studied by X-ray diffraction analysis. The phases of α-CoMoO₄, β-CoMoO₄, Fe₂(MoO₄)₃, Bi₂O₃ ⋅ MoO₃, Bi₂O₃ ⋅ 2MoO₃, Bi₂O₃ ⋅ 3MoO₃, oxidized molybdenum oxide, and reduced molybdenum oxide are the main components of the catalyst. Ternary compounds were not detected. Under catalytic reaction conditions, the relative amounts of the phases changed; this change suggests the occurrence of redox transformations with the participation of these phases, probably, at the interface.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 580–584.Original Russian Text Copyright © 2005 by Shashkin, Udalova, Shibanova, Krylov.     

Heterometallic Alkoxide Complexes of Variable Composition—A New Way to Ultrafine Powders of Metal Alloys

The anodic oxidation of Re metal in MeOH (Me = CH₃) provides a mixture of Re₂O₃(OMe)₆ and Re(V) oxoalkoxides that on storage or on heating give insoluble and air stable Re₄O_6–y (OMe)_12+y (I). I can be also obtained by reaction of Re₂O₇ with MeOH. In the presence of MoO(OMe)₄, a heterometallic complex ReMoO₂(OMe)₇(II) is formed as intermediate, the final product being Re_4–x Mo _x O_6–y (OMe)_12+y (III). The electrosynthesis in the presence of WO(OMe)₄ gives Re_4–x W _x O_6–y (OMe)_12+y (IV) only at very high Re : W ratios in solutions and the W content varies in one and the same sample. The dissolution of Re₂O₇ in the solutions of MO(OMe)₄, M = Mo,W in toluen on reflux yields Re_4–x M _x O_6–y (OMe)_12+y with uniform Re : M distribution. The cocrystallization of MoO(OMe)₄ and WO(OMe)₄ yields (Mo,W)O(OMe)₄ (V) with almost uniform Mo : W distribution. The thermal decomposition of II and III in inert atmosphere gives fine powder of the (Re,Mo)O₂ phase. The reduction with hydrogen gas converts II and III into an ultrafine powder of Re–Mo alloy at temperatures below 400°C. The latter can be sintered into compact metal at 800–900°C.     

Thermal analysis of transition metal and rare earth oxide system-gas interactions by a solid electrolyte-based coulometric technique

The oxidation-reduction behaviour of transition metal and rare earth oxide systems in oxygen potential controlled atmospheres was investigated by means of a solid electrolyte-based coulometric technique (SEC) in carrier gas mode to obtain information concerning the extent of oxygen stoichiometry, thep-T-x diagram of any mixed oxide phase, the kinetics of the oxygen exchange and the phase transitions.The direct coupling of SEC and electrical conductivity measurements provides further information about the relationship between oxygen deficiency and conductivity, especially as concerns the oxygen mobility and the transition from ionic to mixed ionic/electronic conductivity in any system.In the fluorite-type phases PrO_2–x, Ce_0.8Pr_0.2O_y–x and Ce_0.8Sr_0.08Pr_0.12O_y–x, the higher oxidation state of Pr is stabilized and the electrical conductivity increases in this sequence. The perovskite-type phase Sr_1–yCe_yFeO_3–x, shows transitions and a second phase reflected in the temperature-programmed spectrum of this substance. The electrical conductivity of Sr_0.9Ce_0.1FeO_3–x changes fromn-type top-type with increasing oxygen pressure.     

Heterometallic Alkoxides of Molybdenum and Heavy Transition Metals—the Synthesis and Prospects of Application

The alkoxides of molybdenum and other heavy transition elements such as Ta or Nb were found to be unreactive towards each other. The bimetallic derivatives could be obtained either via partial hydrolysis that gave Mo₂Ta₄O₈(OMe)₁₆ (I) or via partial thermolysis that provided access to Mo₄Ta₂O₈(OⁱPr)₁₄ (II), Mo₃Ta₂O₈(OⁱPr)₁₀ (III), Mo₄Ta₄O₁₆(OⁱPr)₁₂ (IV), Mo₄Nb₂O₈(OⁱPr)₁₄ (V) and Mo₄W_2–x Mo _x O₁₀(OⁱPr)₁₂ (VI). I–VI can be isolated only from hydrocarbon media as the presence of alcohols leads to precipitation of insoluble homometallic derivatives of molybdenum. The cathodic reduction of MoO(OR)₄ (R = Me, Et) in the presence of LiCl and M(OR)₅ (M = Nb, Ta) leads only to formation of LiMo₂O₂(OMe)₇(MeOH) (VII) or LiMo₂O₂(OEt)₇ (VIII) respectively.     

Synthesis and Structure of the Supramolecular Adduct Formed by the [Mo3S4(H2O)7Cl22+ Cluster Complex with Cucurbituril: {[Mo3S4(H2O)7Cl2](C36H36N24O12)} Cl2·10H2O

Slow crystallization of an HCl solution containing cucurbituril (C₃₆H₃₆N₂₄O₁₂) and a triangular molybdenum cluster aqua complex [Mo₃S₄(aq)]⁴⁺ yielded a supramolecular adduct of { [Mo₃S₄(H₂O)₇Cl₂]×(C₃₆H₃₆N₂₄O₁₂₎Cl₂·10H₂O composition. The molecular and crystal structure of the adduct were established by single crystal X-ray diffraction. Monoclinic crystal system, space group P2₁/c, a = 21.4762(2) Å, b = 14.6853(1) Å, c = 24.6480(3) Å; β = 112.8366(5)°, V _cell = 7164.26(12) ų, Z = 4, ρ_calc = 1.725 g/cm³.Original Russian Text Copyright © 2004 by E. V. Chubarova, D. G. Samsonenko, J. H. Platas, M. N. Sokolov, and V. P. Fedin__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 950–954, September–October, 2004.     

Farbe und Konstitution bei anorganischen Feststoffen, 14. Mitt.: Die Lichtabsorption des zweiwertigen Kupfers in oxidischen Wirtsgittern

Zusammenfassung Um die Beziehungen zwischen der Lichtabsorption des zweiwertigen Kupfers nach isomorphem Einbau in ein oxidisches Wirtsgitter und dessen Konstitution aufzufinden, wurde Cu²⁺ in oktaedrischer (Cu _x Mg _1–x TiO₃, Cu _x Cd _1–x TiO₃, Cu _x Mg _1–x CaSiO₄, Cu _x Mg _1–x CaGeO₄, Cu _x Mg _2–x SiO₄, Cu _x Mg _2–x GeO₄) und tetraedrischer Koordination (Cu _x Zn_2–x SiO₄, Cu _x Mg _1–x Cr₂O₄) spektralphotometrisch untersucht. Die Farbkurven besitzen mindestens 2 Absorptionsbanden (Kristallfeldbanden) im längerwelligen und eine oft gut ausgeprägte Elektronenübergangsbande (charge transfer) im kürzerwelligen Spektralbereich. In einigen Fällen ist noch eine zweite Elektronenübergangsbande als Schulter zu erkennen. Es wurden auch Cu-haltige 2,3- und 2,4-Spinelle spektralphotometrisch untersucht (Cu _x Mg _1–x Al₂O₄, Cu _x Mg _1–x Ga₂O₄, Cu _x Cd _y Zn _1–x–y Al₂O₄, Cu _x Mg _2–x SnO₄, Cu _x Mg _2–x TiO₄, Cu _x Zn _1–x MgTiO₄, Cu _x Mg _1–x Cd _y TiO₄). Es zeigte sich, daß Cu²⁺ immer auf Tetraeder- und Oktaederlücken verteilt ist. Eine Aufweitung des Wirtsgitters durch isomorphen Einbau größerer Kationen bewirkt nicht immer eine IR-Verschiebung der Banden, sondern in einigen Fällen (Spinellphasen) auch eine UV-Verschiebung. Eine Sonderstellung nimmt das ägyptisch-Blau CuCaSi₄O₁₀ ein, da hier das Cu²⁺ von 4 O_2– in planarer Anordnung umgeben ist. Die Farbkurve weist 3 Maxima auf im Einklang mit der Kristallfeldtheorie.

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In order to find out relations between the lightabsorption of bivalent copper isomorphously incorporated into an oxidic host lattice and the constitution of this lattice, the spectrum of Cu²⁺ has been investigated in octahedral (Cu _x Mg_1–x TiO₃, Cu _x Cd _1–x TiO₃, Cu _x Mg _1–x CaSiO₄, Cu _x Mg _1–x CaGeO₄, Cu _x Mg _2–x SiO₄, Cu _x Mg _2–x GeO₄) and tetrahedral coordination (Cu _x Zn _2–x SiO₄, Cu _x Mg _1–x Cr₂O₄). The colour curves show at least 2 absorption bands within the region of longer wave length and a charge transfer band often well developed in the range of shorter wavelength. In some cases also a second charge transfer band becomes conspicuous as a shoulder. Copper containing 2,3- and 2,4-spinels have been also investigated (Cu _x Mg _1–x Al₂O₄, Cu _x Mg _1–x Ga₂O₄, Cu _x Cd _y Zn _1–x–y Al₂O₄, Cu _x Mg _2–x SnO₄, Cu _x Mg _2–x TiO₄, Cu _x Zn _1–x MgTiO₄, Cu _x Mg _1–x Cd _y Zn _1–y TiO₄). From the colour curve one can infer that Cu²⁺ occupies in the spinels always tetrahedral as well as octahedral interstices. A widening of the lattice does not effect always a shifting of the absorption bands towards IR but in some cases (spinel phases) also the inverse shifting will occur. An exceptional case represents the egyptian blue CuCaSi₄O₁₀ since in this lattice the Cu₂₊ are surrounded by four O^2– in a coplanar arrangement. The colour curve shows three absorption bands in agreement with the crystal field theory.

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Mit 20 Abbildungen     

FT-IR and FT-FIR studies of vanadium,molybdenum and tungsten oxides supported on different carriers

In the far IR region at low molybdenum loadings, Mo-SiO₂ catalysts present a pseudomolybdate or a polymolybdate species, while bulk-like MoO₃ appears at loadings close to the geometrical monolayer coverage. W-SiO₂ and V-SiO₂ spectra show bands close to those observed on the corresponding bulk oxides.In the case of TiO₂, Al₂O₃, ZrO₂ supported catalysts, a band is observed near 1000 cm^–1 which is assigned to the Mo=O stretching vibration of coordinatively unsaturated Mo ⁿ⁺ ions showing a stronger interaction with the support than one observed on silica.     

MALDI-TOF Mass Spectrometry of Nanosized MoO<Subscript>2</Subscript>. Structure and Relative Stability of Isomers of Lower Molybdenum Oxide Cations

MALDI-TOF was used to study molybdenum dioxide (MoO₂) containing a nanosized fraction. The composition of cationic clusters of nonstoichiometric lower molybdenum oxides in the gas phase was determined, and the thermodynamic stabilities and configurations of isomers were calculated for selected symmetric molecular structures and for cations MoSO ₈ ⁺ and Mo₅O ₉ ⁺ . Molecular orbital analysis was performed for two trigonal-bipyramidal clusters Mo₅O₈ and Mo₅O₉. Changes in molybdenum–molybdenum interatomic distances in going from MoO ₈ ⁺ and Mo₅O ₉ ⁺ cations to neutral clusters are discussed.     

Epoxidation of cycloolefins with hydrogen peroxide in the presence of heteropoly acids combined with phase transfer catalyst

Oxidation of cycloolefins (cyclohexene, cyclooctene, and cyclododecene) with a 30% solution of hydrogen peroxide at 65 °C in the presence of heteropoly acids (HPA) H₃PW_12–x Mo _x O₄₀ (x = 0—12), which are precursors of active peroxo complexes, and phase transfer catalysts Q⁺Cl^–, where Q⁺ is the quaternary ammonium cation containing C₄—C₁₈ alkyl groups or [C₅H₅NC₁₆H₃₃]⁺, was studied. The catalytic activity decreases in the HPA series: H₃PW₁₂O₄₀ > H₃PW₉Mo₃O₄₀ > H₃PW₆Mo₆O₄₀ > H₃PW₃Mo₉O₄₀ > H₃PMo₁₂O₄₀. The state of the H₃PW₁₂O₄₀—I₂I₂ system was studied using UV, IR, and ³¹P NMR spectroscopies with variation of the [H₂O₂] : [HPA] ratio from 2 to 200 during cyclohexene epoxidation. Despite different catalytic precursors, the reaction proceeds through the same peroxo complex.