Molybdenum Bronze Research Articles

Low and High-Temperature In Situ X-Ray Absorption Study of the Local Order in Orthorhombic α-MoO3Upon Hydrogen Reduction

In-situ x-ray absorption spectroscopy has been applied to the study of the modifications of the short range order in orthorhombic α-MoO 3 under low and high-temperature hydrogen reduction. It was observed that at low-temperature (T 110°C) reduction, a formation of molybdenum bronzes H x MoO 3 (x ≤ 2) occurs while at high-temperature (T ≥ 400°C) reduction, α-MoO 3 transforms into MoO 2 . Both processes have great applied importance since they are related to electrochromic and catalytic properties of α-MoO 3 . The information on the local atomic and electronic structure, obtained from the Mo K-edge XAFS and XANES analyses, allowed us to follow all steps of the reduction process and to formulate a mechanism of hydrogen interaction with the layered-type structure of α-MoO 3 .

Structural Analysis of Molybdenum Bronzes

Hydrogen molybdenum bronzes, H 0.21 MoO 3 (type I) and H0 .91 MoO 3 (type II) were characterized by Mo K-edge XAFS spectroscopy. XANES spectra showed that type I has an axially symmetric MoO 6 unit which is mainly connected to each other with vertices sharing and a MoO 6 unit in type II are similar to that in MoO 2 . EXAFS spectra suggested that the MoOx sheet in type I is similar to MoO 3 while in type II, a MoOx sheet is similar to that in rutile MoO 2 in which MoO 5 is connected to form a chain by sharing the edges of opposite sides. The curve fitting analysis of Mo-O bonds for type I supported the earlier results obtained from diffraction and IR methods; a MoO 6 unit has C 4v symmetry. In case of type II, D 4h structure is possible and hydrogen is captured by Mo=O bonds resulting in the formation of Mo-OH bonds.

Soft Chemical Modification of Blue Potassium Molybdenum Bronze

Hydrogen insertion compounds of general formula HxK0.28MoO3·zH2O have been obtained from blue potassium molybdenum bronze (K0.28MoO3) by the spillover technique using hydrogen gas in the presence of a Pt catalyst. The hydrogen insertion compounds are formed topotactically and the original blue bronze is regenerated by standing in air for at least >2 weeks. Although the values ofxexceeding approximately 0.42 could not be determined by redox titration, the value ofxwas estimated to be 1.3 immediately after the hydrogen insertion for about 20 h. The hydrated hydrogen insertion compound undergoes intercalation reactions withn-alkylamines. The basal spacing of the intercalation compounds increases linearly with the number of carbon atoms. The electric resistivity of the hydrogen insertion compounds was increased by the order of 104as a result of amine intercalation.

Preparation of hydrated rubidium molybdenum bronze from hydrated sodium molybdenum bronze by ion exchange, and its characterization and thermal decomposition

A new type of hydrated rubidium molybdenum bronze has been synthesized, which was very unstable. It gave a orthorhombic structure with lattice constantsa=3.74(1), b=22.82(7), and c=3.85(1) A. Exchange of Na+ ion with Rb+ ion took place within 30 s. The exchange reaction, [Na+(H2O)n]x[MoO3x–]+Rb+ ⇆[Rb+(H2O)n]x[MoO3x–]+Na+ , took place in both directions by changing the Na+/Rb+ ratio in the mixed solution. A concentration of about 0.05 mol Na in the sample was the minimum value necessary to retain the structure of hydrated sodium molybdenum bronze. We obtained the blue rubidium bronze, Rb0.30MoO3 , by heating hydrated rubidium bronze in nitrogen. This is a new route to producing the metal bronze at low temperature. We also obtained rubidium decamolybdate by heating it in air. This is also a new method for producing rubidium decamolybdate by solid-state reaction.

THERMOELECTRIC POWER PROPERTIES OF QUASI TWO DIMENSIONAL POTASSIUM/SODIUM PURPLE BRONZE- CRYSTALS A0.9MoO17(A=K,Na)

Temperature dependence of thermoelectric power S in quasi-two dimensional potassium/sodium purple bronzes has been studied in the temperature range 80—280K.It′s found that:(1)in potassium purple bronze,S has a sign change at 105K,above which S is the small negative value and can be described by the form of S=AT+B,the dominant carriers are free electrons,in the CDW state below 105K,S is positive,and can be described by S=A′T+B′/T,the phonon-drag of the carriers is dominant; (2)in sodium purple bronze,in the studied temperature range above 80K,S has a good linear relation with temperature,no phase transition was observed,electrons may be the dominant carriers.In addition,the origins of the difference of the properties for both these isostructural crystals are also discussed briefly.

Solid-state chemistry of early transition-metal oxides containing d0 and d1 cations

This paper presents a brief survey of the structures and properties of early transition-metal oxides containing d0 and d1–d0 electronic configurations. The metal–oxygen (MO6) octahedron, which is the essential structure building unit of these materials, exhibits a characteristic out-of-centre distortion for the d0 configuration in many instances, the degree of distortion increasing with decreasing HOMO–LUMO gap. Several characteristic properties of d0 oxides, which include low-dimensional structures (that give rise to intercalation, ion-exchange and acidity), ferroelectricity and non-linear optical response, arise from the out-of-centre distortion of d0 MO6 octahedra. Oxides containing d1–d0 electronic configurations exhibit an equally impressive array of electronic properties that owe their origin to the nature of d states near the Fermi level. While three-dimensional (3-D) oxides containing 5d and 4d electrons exhibit itinerant electron properties, 3d1 oxides, especially with low-dimensional (low-D) structures, display localized electron magnetism and semiconduction. Low-dimensional oxides containing 4d electrons, typified by molybdenum bronzes and Mo4O11 , exhibit charge-density-wave (CDW)-driven electronic instabilities arising from electron–phonon interactions.

Polymer-Oxide Anode Materials

ABSTRACTThe adaptable layer structure of molybdenum trioxide was exploited to insert the amino derivative form of the conductive polymer poly(para-phenylene) (PPPNH2) within the van der Waals gap. Two polymer insertion routes were designed that yield novel PPPNH2-MoO3 materials of different composition. Characterization of these materials using powder XRD, thermal analysis, and FTIR spectroscopy shows insertion of the polymer has occurred. The properties of the nanocomposites for low potential electrochemical lithium insertion were compared to those of the sodium molybdenum bronze using the materials as cathodes in conventional lithium cells. Initial results indicate the specific charge capacity and irreversibility during the first charge are effected by polymer content whereas polarization is not.

Lamellar polymer–LixMoO3 nanocomposites via encapsulative precipitation

With Li x MoO 3 (x=0.31–0.40) as a host material, a new family of polymer–molybdenum bronze nanocomposites has been synthesized using an exfoliation/encapsulation methodology. Nanocomposites with poly(ethylene oxide), poly(ethylene glycol), poly(propylene glycol), poly(vinylpyrrolidinone), methyl cellulose, polyacrylamide, and nylon-6 were prepared and characterized by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), powder X-ray diffraction, FTIR spectroscopy, UV–VIS spectroscopy, variable-temperature 7 Li and 13 C solid-state NMR spectroscopy and magnetic susceptibility measurements. The electrical conductivity of these materials lies in the range from 10 -2 to 10 -7 S cm -1 , and decreases as the interlayer separation increases. The intercalated polymer imparts both mechanical strength and ease of processing to these materials. The water-soluble polymer–Li x MoO 3 nanocomposites can be cast into films and other shapes, which may provide opportunities for applications. Factors affecting the intercalation reaction and the structure of nanocomposites, such as variations in the preparation procedure, the polymer molecular mass and the annealing behaviour of the products are discussed.

Formation of secondary crystallites during electrochemical cycling of hydrogen molybdenum bronze

Powder samples of hydrogen molybdenum bronzes (H xMoO 3) were effectively stabilized by the addition of 30 mass% polyethylene (PE) powder, pressing and a following thermal treatment. The composite material does not degrade in 0.05 mol 1 −1 H 2SO 4 but shows 1–3 μm wide cracks in the surface and the formation and growth of small hydrated bronze crystals (1–10 μm) during electrochemical cycling. An electrochemical dissolution/precipitation mechanism is presented and discussed.

Charge Density Wave Properties of Quasi Low-Dimensional Transition Metal Oxide Bronzes

Several families of transition metal oxide bronzes including the quasi one-dimensional molybdenum blue bronzes A 0.3 MoO 3 , the quasi two-dimensional (2D) Mo purple bronzes AMo 6 O 17 and the quasi 2D monophosphate tungsten bronzes (PO 2 ) 4 (WO 3 ) 2m show Peierls transitions leading to commensurate or incommensurate charge density wave (CDW) states. The properties of the monophosphate tungsten bronzes (PO 2 ) 4 (WO 3 ) 2m are reviewed. These series of compounds provide a model system where both the low-dimensional character and the average electron concentration are function of the m parameter. The low m compounds (m = 4, 6) show conventional CDW instabilities as well as giant positive magnetotransport and quantum oscillations at low temperature. Detailed information on the Fermi surface in the CDW state is obtained from transport data, the size of the pockets left by the CDW gap openings being found of the order of 1% of the high temperature two-dimensional Brillouin zone. The m = 5 compound, which may be considered as an intergrowth of m = 4 and 6, shows also instabilities with slightly different properties. The absence of CDW long range order affects the properties of the m = 8 member which shows low temperature properties possibly due to weak localisation effects. Finally, studies on the 4 ≤ m ≤ 14 compounds show that the Peierls temperatures are increasing with m, reaching high values, up to 550 K, for m = 13. This is attributed to a 2D character increasing with m. The increase of the room temperature resistivity with m is discussed in terms of enhanced electron-phonon or electron-electron interactions.

Fermi surfaces and single-particle spectral functions of low-dimensional inorganic non-cuprate compounds: the molybdenum bronzes

The Fermi surfaces and single-particle spectral functions of several low-dimensional materials have been measured as part of an effort to assess the occurrence of non-Fermi-liquid behaviour in non-cuprate materials.

Cyclic voltammetry of polyethylene stabilized hydrogen molybdenum bronzes

Powder samples of hydrogen molybdenum bronzes (H x MoO 3) were effectively stabilized by the addition of 30 mass% polyethylene powder, pressing and a following thermal treatment. The composite material does not degrade in 0.05 mol l 1 H 2SO 4, so that cyclic voltammetry could be carried out in 0.05 moll 1 H 2SO 4 to give reproducible waves and peak potentials. Extrapolation to the scan rate ν = 0 yields the following redox potentials (all versus the SHE): + 277 mV for phase II in its maximum reduction degree ( x max), + 67 mV for phase III ( x min), -91 0mV for phase III ( xx), -204mV for phase IV and -358 0mV for hydrogen at phase IV. In 0.5 mol l 1 H 2SO 4 the bronze is partially soluble, which leads to a strong decrease of the electrochemical activity.

Sodium ion sensitive electrode based on a molybdenum oxide bronze

New sodium ion sensitive metal—oxide-type sensors have been developed and tested with a direct solid state contact method. Performance was demonstrated at ambient temperature with single crystals of Na0.9Mo6O17 and Li0.33MoO3. The sodium molybdenum bronze sensor responded reproducibly and fast to changes of Na+ concentration in the range of 1–10−4 mol dm−3. Cross sensitivity tests to other ions such as H+ or K+ have shown that the new sodium ion sensor may be used when the concentration of other ions is an order of magnitude smaller than the Na+ concentration.

New metal-oxide-type pH sensors

Two new types of metal-oxide pH sensors with a direct solid-state contact have been developed and tested. Their performance was demonstrated near ambient temperatures with a samarium substituted ceria ceramic membrane and single crystals of several molybdenum oxide bronzes. The pH sensors with Na-molybdenum-oxide bronzes show near-ideal Nernstian behaviour in the pH range 3–9. The response was not affected by the direction of the pH change. In contrast, the pH sensor with the Na-tungsten bronze sensitive electrode (SE) shows a deviation from linearity, as well as non-Nernstian behaviour with a slope − 76.3 mV/pH at 25 °C. The response time of most molybdenum bronze pH sensors is less than 5 s for 90% response. The pH sensors with stabilized ceria membrane show close to ambient temperature non-Nernstian behaviour in the pH range 3–10 with a slope − 23.8 mV/pH at 25 °C. With increasing temperatures the behaviour of ceria membrane pH sensors approaches more and more a Nernstian response with slope − 39.9 mV/pH at 60 °C. Both pH sensors also can be used to follow acid-base titrations.

ELECTRONIC STRUCTURES OF PURPLE BRONZE KMo6O17 STUDIED BY X-RAY PHOTOEMISSION SPECTRA

X-ray photoemission spectroscopy study has been performed for the purple bronze KMo 6 O 17. The structures of conduction band and valence band are analogous to the results of ultraviolet photoemission spectra and are also consistent with the model of Travaglini et al., but the gap between conduction and valence band is insignificant. The shape of asymmetric and broadening line of O-1s is due to unresolved contributions from the many inequivalent oxygen sites in this crystal structure. Mo 3d core-level spectrum reveals that there are two kinds of valence states of Molybdenum (Mo+5 and Mo +6). The calculated average valence state is about +5.6, which is consistent with the expectation value from the composition of this material. The tail of Mo-3d spectrum toward higher binding energy is the consequence of the excitation of electron-hole pairs with singularity index of 0.21.

Shubnikov-de Haas effect and magnetic breakdown in the low-temperature charge-density-wave state for the thallium purple bronze TlMo6O17.

Magnetoresistance has been studied for quasi-two-dimensional thallium purple bronze T1Mo6017 at a temperature of 4.2 K in fields up to 12 T. Shubnikov-de Haas oscillations are observed. The calculated Fermi surface size is about 5x10(-4) in the high-temperature Brillouin zone and consistent with hidden Fermi surface nesting theory. The field dependence of magnetoresistance is studied in the vicinity of magnetic breakdown. The characteristic field of breakdown is around 0.08 T corresponding to an energy gap of about 2 meV. The effective mobility and cyclotron mass are about 0.025 m(2) V-1 s(-1) and 0.3m(e), respectively.

Structure and electrical resistivity of the new yellow oxygenated molybdenum bronze GdMo 16O 44

The crystalline structure and electrical resistivity of the new yellow molybdenum bronze GdMo 16O 44 are presented. Single crystal X-ray diffraction analysis indicate a rhombohedral space group R 3 with lattice parameters a = 10.780(2) A ̊ and c = 27.493(4) A ̊ . The outstanding features are; i) all polyhedra share corners with their neighbours; ii) the structure is built up from clusters of eight MoO 6 octahedra separated from other clusters and from GdO 8 cubes by MoO 4 tetrahedra; iii) six-fold and four-fold coordinated molybdenum atoms have a valence state close to +5 and +6 respectively. As expected by the structural results the compound is found semiconducting.

Observation of double Pake doublet in a sodium insertion compound of hydrogen molybdenum bronze

It was revealed by computer-simulation that a rigid lattice proton NMR spectrum in a new sodium insertion compound of hydrogen molybdenum bronze, [Na(H 2O) y ] 0.25H 0.21MoO 3, consists of two Pake doublets and a Gaussian. One of the Pake doublets originates in pairs of intralayer protons and the other in a proton in H 2O.

Electrocatalytic properties of mixed-valence molybdenum oxide thin film modified microelectrodes

A compact blue conducting mixed-valence Mo (VI,V) oxide film was grown on the surface of a carbon fibre (CF) microelectrode by cycling the potential between +0.20 and ∼ 0.70 V SCE in freshly prepared Na 2 MoO 4 solution in H 2 SO 4 (pH2). The thickness of the oxide film was controlled by the charge passed. X-ray photoelectron spectroscopy (XPS) examination confirmed that the components of the molybdenum oxide film on the CF electrode surface are mainly a mixture of Mo 6+ and Mo 5+ complex, together with a small amount of Mo 4+; no Mo 3+ peak was observed. Well-defined redox transitions have been attributed to the reductive formation of hydrogen molybdenum bronzes [MoO 3−x (OH) x] and re-oxidation in acidic media. The coating is easily permeable to ions. The molybdenum oxide film catalyses the electroreduction of chlorate to chloride and bromate to bromide in H 2SO 4. At a bare CF microelectrode, chlorate and bromate are not reduced prior to the onset of hydrogen evolution at about −1.0 V. At the modified surface, a chlorate or bromate reduction peak appears at about −0.4 V SCE. The peak current is linear with concentration from 10 −5 to 10 −3 M chlorate or from 10 −6 to 10 −4 M bromate, and is nearly diffusion controlled. The electrocatalytic activity of H 0.34MoO 3 towards chlorate or bromate ions is even stronger than that of H 0.93MoO 3. The reactivity sequence determined by comparing current magnitudes on the molybdenum oxide film modified CF microelectrode is BrO 3 −> ClO 3 −.

Electrochemical and Chemical Insertion/Extraction of Lithium into/from Potassium Molybdenum Bronze

Electrochemical and Chemical Insertion/Extraction of Lithium into/from Potassium Molybdenum Bronze