Hydrothermal synthesis of potassium molybdenum oxide bronzes: structure-inheriting solid-state route to blue bronze and dissolution/deposition route to red bronze

The hydrothermal syntheses of the alkali metal molybdenum bronzes from starting solids (H_xMoO₃) with structural affinities to the desired products were investigated. Single-phase potassium blue and red bronzes were prepared by the hydrothermal treatments at around 430 K, and characterized by powder X-ray diffraction, IR spectroscopy, and SEM. The formation processes of these two bronzes during the hydrothermal treatments were found to differ. The blue bronze was formed by a structure-inheriting solid-state route from H_xMoO₃ with x<0.3, whereas the red bronze was formed for x>0.3 through a solution dissolution/deposition route via the formation of MoO₃+MoO₂.     

Dispersed platinum supported by hydrogen molybdenum bronze-modified carbon as electrocatalyst for methanol oxidation

A new electrocatalyst, Pt/H_xMoO₃-C, for methanol oxidation, was prepared by dispersing platinum nano-particles on Vulcan XC-72 modified by hydrogen molybdenum bronze (H_xMoO₃, 0 ≤ x ≤ 2). The modification of Vulcan XC-72 with H_xMoO₃ on was accomplished by reducing the adsorbed molybdic acid and the platinum nano-particles were dispersed on the modified carbon by reducing chloroplatinic acid, with formaldehyde as the reductant. The prepared Pt/H_xMoO₃-C was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersion spectrometer, cyclic voltammetry (CV), chronoamperometry (CA), and single-cell test, with a comparison of the electrocatalyst, carbon-supported platinum (Pt/C) prepared under the same condition but without the modification. The results obtained from XRD and SEM showed that the modification of Vulcan XC-72 with H_xMoO₃ reduced the platinum particle size and improved distribution uniformity of platinum on carbon. The results, obtained from CV, CA, and the single-cell test, showed that Pt/H_xMoO₃-C exhibited better electrocatalytic activity toward methanol oxidation than Pt/C.     

Structural and magnetic study of the cation-ordered perovskites Ba2−xSrxErMoO6

A series of perovskite phases have been prepared from the appropriate carbonates and oxides by heating under reducing conditions at temperatures up to 1300 °C. Complete ordering between ErO₆ and MoO₆ octahedra and a disordered distribution of Sr²⁺ and Ba²⁺ occur in all compounds. Neutron powder diffraction experiments show that the substitution of Sr²⁺ into Ba₂ErMoO₆ introduces a progressive reduction in symmetry from Fm3¯m (x=0) to I4/m (x=0.5, 0.8) to P2₁/n (x=1.25, 1.75, 2.0). Magnetic susceptibility measurements indicate that all of these compounds show Curie-Weiss paramagnetism and that for x<1.25 this behaviour persists down to 2 K. The monoclinically distorted compounds show magnetic transitions at low temperature and neutron diffraction has confirmed the presence of long-range antiferromagnetic order below 2.5 and 4 K in Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆, respectively. Ba_0.75Sr_1.25ErMoO₆, Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆ do not undergo structural distortion on cooling from room temperature.     

Selective skeletal isomerization of alkanes over partially reduced MoO<Subscript>3</Subscript>

The surface area and the pentane isomerization activity of Pt/MoO₃ were enlarged by H₂ reduction. The enlargements was observed only when the reduction proceeded through the formation of hydrogen molybdenum bronze, H_xMoO₃. The catalytic activities of H₂-reduced MoO₃ with different noble metals for pentane isomerization and 2-propanol dehydration depended on the ability of noble metal to produce the H_xMoO₃ phases. H₂-reduced Pt/MoO₃ was more active for pentane isomerization than Pt/H, and its activity was comparable to that of Pt/HZSM-5. In heptane isomerization, H₂-reduced Pt/MoO₃ exhibited a lower activity than Pt/H, although heptane was isomerized very selectively. Strong adsorption of heptane onto H₂-reduced Pt/MoO₃ is likely to be a reason for its lower heptane isomerization activity.     

Structures,thermal expansion properties and phase transitions of ErxFe2−x(MoO4)3 (0.0 ≤ x ≤ 2.0)

Structures, thermal expansion properties and phase transitions of Er_xFe_2−x(MoO₄)₃ (0.0 ≤ x ≤ 2.0) have been investigated by X-ray diffraction and differential thermal analysis. The partial substitution of Er³⁺ for Fe³⁺ induces pronounced decreases in the phase transition temperature from monoclinic to orthorhombic structure. Rietveld analysis of the XRD data shows that both the monoclinic and orthorhombic Fe₂(MoO₄)₃, as well as the orthorhombic Er_xFe_2−x(MoO₄)₃ (x ≤ 0.8) have positive thermal expansion coefficients. However, the linear thermal expansion coefficients of Er_xFe_2−x(MoO₄)₃ (x = 0.6–2.0) decrease with increasing content of Er³⁺ and for x ≥ 1.0, compounds Er_xFe_2−x(MoO₄)₃ show negative thermal expansion properties. Attempts for making zero thermal expansion coefficient materials result in that very low negative thermal expansion coefficient of −0.60 × 10⁻⁶/°C in Er_1.0Fe_1.0(MoO₄)₃ is observed in the temperature range of 180–400 °C, and zero thermal expansion is observed in Er_0.8Fe_1.2(MoO₄)₃ in the temperature range of 350–450 °C. In addition, anisotropic thermal expansions are found for all the orthorhombic Er_xFe_2−x(MoO₄)₃ compounds, with negative thermal expansion coefficients along the a axes.     

The influence of hydrogen-containing molybdenum and tungsten bronzes on the catalytic activity of palladium composite catalysts for the oxidation of H<Subscript>2</Subscript>, CO,and CH<Subscript>4</Subscript>

The influence of hydrogen-containing molybdenum and tungsten bronzes on the catalytic activity of palladium composite catalysts for the oxidation of H₂, CO, and CH₄ was studied. It was found that the composite catalysts containing H _x MO₃ phases (M = W or Mo), which were formed by the reduction of MoO₃ and WO₃ oxides with hydrogen in the presence of deposited Pd, showed higher catalytic activity in the oxidation of small molecules (H₂, CO, and CH₄) with excess oxygen than the traditional Pd/Al₂O₃ deposited catalyst with the same content of the deposited metal. It was shown that the thermal stability of the H _x MO₃ phases was the limiting factor influencing the activity of these composite catalysts.     

Preparation and structural study from neutron diffraction data of R2MoO6 (R=Dy, Ho, Er, Tm, Yb, Y)

The title compounds have been prepared as polycrystalline powders by thermal treatments of stoichiometric mixtures of R₂O₃ and MoO₃ in air. The room-temperature crystal structure for all the series has been refined from high-resolution neutron powder diffraction data. All the phases are isostructural (space group C2/c, Z=8) with the polymorph α-R₂MoO₆, typified by Sm₂MoO₆. The structure contains four zigzag, one-dimensional MoO₅ polyhedral rows per unit cell, running through the RO₈ polyhedral framework along the [001] direction. MoO₅ form discrete units (i.e. do not share common oxygen), with Mo-O distances ranging from 1.77 to 2.24 Å, although the oxygen coordination can be extended to distances of about 3.1 Å, giving rise to strongly distorted MoO₈ scalenohedra. Thus, MoO₈ and RO₈ polyhedra are fully ordered in R₂MoO₆ compounds, which in fact can be considered as superstructures of fluorite (M₃O₆), containing 24 MO₂ fluorite units per unit cell, with unit-cell parameters related to that of cubic fluorite ( Å). A bond valence study demonstrates that the present crystal structure is especially stable for small rare-earth cations, and becomes more unstable when the R³⁺ size increases, thus explaining the observed preference of the large rare-earth molybdates for polymorphs β and γ with the same stoichiometry.     

Ethene Protonation Over Silica-Grafted Metal (Cr,Mo, and W) Oxide Catalysts: A Comparative Nanocluster Modeling Study

Fundamental insights into the ethene protonation reaction was obtained over different cluster models of acidic CrO_x/SiO₂, MoO_x/SiO₂, and WO_x/SiO₂ catalysts at the M06/Def2-TZVP level of theory. The clusters varied from MSiO₄H₃ structures (all-fixed, H-optimized, and all-relaxed) to MSi₄O₄H₉ (saturated with H atoms) and further to MSi₄O₁₃H₉ (saturated with OH atoms). The formation of the ethene protonation adducts followed the order of WO_x/SiO₂ < MoO_x/SiO₂ < CrO_x/SiO₂ in terms of the thermodynamic favorability which agreed well with the partial charges and the global softness data. The natural bond orbital analysis revealed a partial flow of electrons from the bridging O atom to the hydrocarbon fragment than to the metal during the initiation. Although the interbond angles were comparably different in the largest cluster, the bond lengths and orbital energy levels did not change significantly from a cluster to another. Concerning the thermochemical properties, any of the cluster models would be utilized within a 2 kcal/mol confidence limit.     

X-ray and neutron powder diffraction studies of Ba(NdxY2−x)CuO5

Ba(R,R′)₂CuO₅ (R,R′=lanthanides and Y) plays an important role as a flux-pinning agent in enhancing the superconducting properties of the Ba₂(R,R′)Cu₃O_6+x (R,R′=lanthanides and Y) coated conductors. Using X-ray diffraction and neutron diffraction, we found that the Ba(Nd_xY_2−x)CuO₅ solid solution adopts two structure types. In the Nd-rich region (1.8?x?2.0), the materials are of brown color (commonly referred to as the ‘brown phase’), and the structure is tetragonal with space group I4/mbm (no. 127). In the Y-rich region (0.0?x?1.4), the materials are green (commonly referred to as the ‘green phase’) and the structure is orthorhombic with space group Pnma (no. 62). A two-phase region (1.4<x<1.8) exists between the orthorhombic and tetragonal phases. The crystal chemistry and crystallography of the orthorhombic ‘green phase’ series, Ba(Nd_xY_2−x)CuO₅ (isostructural to BaY₂CuO₅), are discussed in this paper.     

First electrochemical growth of Tb16O30 single crystal

Single crystals of Tb₁₆O₃₀ (TbO_1.875) were successfully grown for the first time by DC electrolysis of the Tb³⁺ ion conducting Tb₂(MoO₄)₃ solid electrolyte at 11 V, 900 °C. The Tb₁₆O₃₀ phase is the intermediate phase of fluorite-related rare earth oxides and it is extremely difficult to grow in a single crystal form, because this intermediate phase is usually obtained as one of the mixture of the fluorite related TbO_x phases. Because there are many non-stoichiometric phases in the terbium oxide system, it is impossible to grow a specific intermediate phase in a single crystal form by the conventional methods via melt. Although single crystals of TbO_x have been recently obtained by anodic electrocrystallization from alkaline hydroxide melts containing TbCl₃, the composition has been confirmed to be TbO_x with 1.75<x<1.82. On the contrary, the presently developed DC electrolysis method can be simply applicable at moderate temperatures around 900 °C to artificially grow an intermediate phase of Tb₁₆O₃₀ (x=1.875) in a single crystal form, which was evidenced by the electron diffraction patterns for each particle.     

X-ray and neutron diffraction study of intermediate phases in nonstoichiometric cerium dioxide

Powder samples of reduced ceria, CeO_2?x, of known compositions in the range 0 < x < 0.3 have been examined by X-ray and neutron diffraction techniques in order to determine which intermediate phases belonging to the homologous series Ce_nO_2n?2 (with n = integer) truly exist. Through the appearance of superlattice lines in the neutron diffraction patterns, the existence of four distinct phases, corresponding to n = 7, 9, 10 and 11 was established. Aside from the phase Ce₇O₁₂, the structures of these phases cannot be accounted for with rhombohedral cells based on 〈111〉 vacancy strings, but indicate lower (monoclinic or triclinic) symmetry. The structure of Ce₉O₁₆ and Ce₁₀O₁₈ do not agree with structures proposed for the analogous Pr_nO_2n?2 compounds.     

Structural phase transition in mixed crystals WxMO1−xO3

Crystals of W_xMo_1?xO₃ have been grown. The phase diagram shows 12 different phases. Their structures are closely related to the corresponding WO₃ phases except for x < 0.05% where an MoO₃-like structure was found. The structural parameters are given and discussed on the basis of their group theoretical sequences. Raman and infrared spectra revealed a strong dependence of the lowest energetic phonon branch (50 cm^?1) and the 650 cm^?1 mode on chemical composition. Thermal soft modes were found for W_0.78MO_0.22O₃.     

Hydrogen in polar intermetallics: Syntheses and structures of the ternary Ca5Bi3D0.93, Yb5Bi3Hx, and Sm5Bi3H∼1 by powder neutron or single crystal X-ray diffraction

The syntheses of the title compounds are described in detail. Structural characterizations from refinements of single crystal X-ray diffraction data for Yb₅Bi₃H_x and Sm₅Bi₃H_∼1 and of powder neutron diffraction data for Ca₅Bi₃D_0.93(3) are reported. These confirm that all three crystallize with the heavy atom structure type of β-Yb₅Sb₃, and the third gives the first proof that the deuterium lies in the center of nominal calcium tetrahedra, isostructural with the Ca₅Sb₃F-type structure. These Ca and Yb phases are particularly stable with respect to dissociation to Mn₅Si₃-type product plus H₂. Some contradictions in the literature regarding Yb₅Sb₃ and Yb₅Sb₃H_x phases are considered in terms of adventitious hydrogen impurities that are generated during reactions in fused silica containers at elevated temperatures.     

Low-temperature synthetic route based on the amorphous nature of giant species for preparation of lower valence oxides

We examined low-temperature synthetic route based on the amorphous nature of giant species to succeed to prepare Cs blue bronze (Cs_0.3MoO₃), which has never obtained by usual high-temperature methods, at ca. 680 K. Solid solutions (K_1−xRb_x)_0.28MoO₃ and (Li_1−xNa_x)_0.9Mo₆O₁₇ were also obtained at lower temperatures (ca. 670 K). For the latter system consisting of non-isostructural end members, Li_0.9Mo₆O₁₇-structure type solid solution was formed even when 0.25<x<0.70, unlike the case by the usual high-temperature methods. Metastable mixed oxides Ln₂Mo₃O₉ (Ln=La, Gd) were obtained, but not as single phases.     

Platinum and molybdenum oxide deposited carbon electrocatalyst for oxidation of hydrogen containing carbon monoxide

Carbon-supported Pt/MoO_x catalysts for use in PEFC anodes were prepared and their catalytic activity for the oxidation of CO-contaminated H₂ was examined based on the fuel cell performance in PEFC single cell arrangements. Based on the XRD pattern and XPS measurements of the prepared Pt/MoO_x/C catalysts, it was found that the deposited MoO_x exists as an amorphous oxide phase. The MoO_x phase shows a redox peak at around 0.45 V, which was revealed by the cyclic voltammogram of the Pt/MoO_x/C in sulfuric acid solution. The PEFC performance of the cell with Pt/MoO_x/C was improved under 100 ppm CO-contaminated H₂ conditions compared to the Pt/C catalyst, and was almost comparable to the PtRu(1:1)/C catalyst.     

Changes in the phase composition of a multicomponent Co-Mo-Bi-Fe-Sb-K-O catalyst under conditions of the partial oxidation of isobutylene to methacrolein: A comparison with the oxidation of propylene to acrolein

The partial oxidation of isobutylene to methacrolein on a multicomponent multiphase Co-Mo-Bi-Fe-Sb-K-O catalyst and on catalysts from which some components were absent was studied. Activity and selectivity changes in the case of isobutylene oxidation were the same as in the oxidation of propylene; however, the rate of propylene oxidation was higher than that of isobutylene oxidation. The X-ray diffraction analysis of the catalysts before and after the reaction indicated the occurrence of a number of phases in the samples: α-CoMoO₄, β-CoMoO₄, Fe₂(MoO₄)₃, reduced bismuth molybdate species, MoO₃, and reduced MoO _x species. Under catalytic reaction conditions, redox phase transformations occurred. Iron molybdate and molybdenum oxide phases underwent the largest transformations. Of two molybdenum oxide phases, a β-Mo₄O₁₁ phase with a structure of a crystallographic shift is formed in the course of catalysis, whereas the second phase (MoO₃) almost does not participate in catalysis and occurs in an excess.     

Synthesis and electrical properties of variable-composition phases Rb1 − x A1 − x R1 + x (MoO4)3 (0 ≤ x ≤ 0.2; A = Mg, Mn, Co, Ni; R = Sc, Yb, Lu)

Subsolidus region of the ternary systems Rb₂MoO₄-AMoO₄-R₂(MoO₄)₃, in which variable-composition phases Rb_{1 ? x} A_{1 ? x} R_{1 + x} (MoO₄)₃ crystallizing in the monoclinic system (space group C2) are formed, was studied. Their crystallographic parameters were calculated; temperature dependences of the electrical conductivity, dielectric constant, and dielectric loss tangent were analized.     

Structural variations and cation distributions in Mn3−xCoxO4 (0 ≤ x ≤ 3) dense ceramics using neutron diffraction data

Single phase ceramics of cobalt manganese oxide spinels Mn_3?xCo_xO₄ were structurally characterized by neutron powder diffraction over the whole solid solution range. For x < 1.75, ceramics obtained at room temperature by conventional sintering techniques are tetragonal, while for x ≥ 1.75 ceramics sintered by Spark Plasma Sintering are of cubic symmetry. The unit cells, metal–metal and metal–oxygen average bonds decrease regularly with increasing cobalt content. Rietveld refinements using neutron data show that cobalt is first preferentially substituted on the tetrahedral site for x < 1, then on the octahedral site for increasing x values. Structural methods (bond valence sum computations and calculations based on Poix's work in oxide spinels) applied to our ceramics using element repartitions and [M–O] distances determined after neutron data refinements allowed us to specify the cation distributions in all phases. Mn²⁺ and/or Co²⁺ occupy the tetrahedral site while Mn³⁺, Co²⁺, Co^III (cobalt in low-spin state) and Mn⁴⁺ occupy the octahedral site. The electronic conduction mechanisms in our highly densified ceramics of pure cobalt and manganese oxide spinels are explained by the hopping of polarons between adjacent Mn³⁺/Mn⁴⁺ and Co²⁺/Co^III on the octahedral sites.     

Synthesis and characterization of molybdenum carbides using propane as carbon source

Bulk face-centered-cubic (fcc)-based η-MoC_1−x and hexagonal-close-packed (hcp)-based β-Mo₂C have been prepared using C₃H₈/H₂ by temperature-programmed reaction method and a rapid heating method. In this work, direct carburization of MoO₃ produces η-MoC_1−x or MoO_xC_y with excess carbon, different from that with CH₄/H₂ or C₂H₆/H₂ as carburization reagent. A successive post-treatment by hydrogen causes the phase transformation from fcc-based η-MoC_1−x or MoO_xC_y to hcp-based β-Mo₂C. Amorphous SiO₂-supported β-Mo₂C is also successfully prepared by the two methods and passes through the same route as the bulk one. HRTEM, BET surface area measurements and thiophene hydrodesulfurization reaction are conducted for the comparison of the two methods. The results indicate that different ramping rates bring slight difference in specific surface area and initial catalytic activity but obvious difference in particle size to the final product supported β-Mo₂C.     

Oxidation process of MoOxCy to MoO3: kinetics and mechanism

A non-isothermal kinetic study of the oxidation of “carbon-modified MoO₃” in the temperature range of 150-550°C by simultaneous TGA-DTA was investigated. During the oxidation process, two thermal events were detected, which are associated with the oxidation of carbon in MoO_xC_y and MoO₂ to MoO₃. The model-free and model-fitting kinetic approaches have been applied to TGA experimental data. The solid state-kinetics of the oxidation of MoO_xC_y to MoO₃ is governed by F1 (unimolecular decay), which suggests that the reaction is of the first order with respect to oxygen concentration. The constant (Ea)_α value (about 115±5 kJ/mol) for this first stage can be related to the nature of the reaction site in the MoO₃ matrix. This indicates that oxidation occurs in well-defined lattice position sites (energetically equivalent). On the other hand, for the second stage of oxidation, MoO₂ to MoO₃, the isoconversional analysis shows a complex (Ea)_α dependence on (α) and reveals a typical behavior for competitive reaction. A D2 (two-dimensional diffusion) mechanism with a variable activation energy value in the range 110-200 kJ/mol was obtained. This can be interpreted as an inter-layer oxygen diffusion in the solid bulk, which does not exclude other simultaneous mechanism reactions.