Boron-Dependency of Molybdenum Boride Electrocatalysts for the Hydrogen Evolution Reaction

Molybdenum-based materials have been considered as alternative catalysts to noble metals, such as platinum, for the hydrogen evolution reaction (HER). We have synthesized four binary bulk molybdenum borides Mo₂B, α-MoB, β-MoB, and MoB₂ by arc-melting. All four phases were tested for their electrocatalytic activity (linear sweep voltammetry) and stability (cyclic voltammetry) with respect to the HER in acidic conditions. Three of these phases were studied for their HER activity and by X-ray photoelectron spectroscopy (XPS) for the first time; MoB₂ and β-MoB show excellent activity in the same range as the recently reported α-MoB and β-Mo₂C phases, while the molybdenum richest phase Mo₂B show significantly lower HER activity, indicating a strong boron-dependency of these borides for the HER. In addition, MoB₂ and β-MoB show long-term cycle stability in acidic solution.     

The molybdenum-molybdenum triple bond—XVII. Syntheses,spectroscopic and structural characterizations of Mo4F4(O—t-Bu)8 and Mo2F2(O—t-Bu)4(PMe3)2 (MoMo)

Hydrocarbon solutions of Mo₂(O—t-Bu)₆ and PF₃ (2 equiv) yield Mo₄F₄(O—t-Bu)₈, I, and PF₂(O—t-Bu). Compound I contains a bisphenoid of molybdenum atoms with two short MoMo distances, 2.26 Å, and four long MoMo distances, 3.75 Å, corresponding to localized MoMo triple bonding and non-bonding distances, respectively. The tetranuclear compound may be viewed as a dimer, [Mo₂(μ₂-F)₂(O-t-Bu)₄]₂, and addition of PMe₃ to hydrocarbon solutions of I yields Mo₂F₂(O—t-Bu)₄(PMe₃)₂, II, which contains an unbridged MoMo triple bond of distance 2.27 Å. Each molybdenum atom is coordinated to two oxygen atoms, one fluorine atom and the phosphorus atom of the PMe₃ ligand in a roughly square planar manner. The overall central Mo₂O₄F₂P₂ skeleton has C₂ symmetry and NMR studies (¹H, ¹⁹F and ³¹P) are consistent with the maintenance of this type of structure in solution. Infrared and electronic absorption spectral data are reported. These are the first compounds containing fluorine ligands attached to the (MoMo)⁶⁺ unit.     

Composition space analysis of templated molybdates

A systematic investigation of the factors governing reaction product composition and three-dimensional structure was conducted in the MoO₃/H₂N(CH₂)_nNH₂/H₂O (n = 3–7) systems. Composition space analysis was performed through approximately 30 reactions using each amine under mild hydrothermal conditions. Ten new compounds were synthesized, of which single crystals of nine were grown. Five different molybdate structures were observed in these ten compounds, including β-[Mo₈O₂₆]⁴⁻ molecular anions, two distinct [Mo₃O₁₀]_n²ⁿ⁻ chain polymorphs, [Mo₈O₂₆]_n⁴ⁿ⁻ chains and [Mo₅O₁₆]_n²ⁿ⁻ layers. The relative phase stabilities of the reaction products and associated molybdate architectures are dependent upon the concentrations of each reactant in solution.     

Structural and spectroscopic characterization of Mo1−xWxO3−δ mixed oxides

Mo_1−xW_xO₃ oxides with different cationic fraction (x=0.2, 0.5 and 0.8) and, for comparison purposes, pure MoO₃ and WO₃ were prepared. Along with textural and structural characterizations, absorbance FT-IR, diffuse reflectance UV-vis-NIR and EPR spectroscopies were employed to study the changes in the electronic properties of these materials passing from Mo_1−xW_xO₃ in oxidizing atmosphere to Mo_1−xW_xO_3−δ in reducing conditions. XRD analysis showed that the Mo-W mixed oxides are constituted by two or three crystalline phases, whose abundance and composition are well characterized by structural refinement with the Rietveld method. Only the sample with the highest Mo content (x=0.2) shows a predominant mixed phase and also a superior ability to lose oxygen with respect to the other mixed oxides. The oxygen loss in the reduced oxides induces the formation of defects with electronic levels in the band gap of the material, in particular, electrons trapped in oxygen vacancies and/or at cationic sites (polarons). While the nature of defect sites induced in the mixed and in the pure oxides is similar, the photo-ionization energies, the ratio between surface and bulk defects and the stability of the defects in oxygen at increasing temperature are peculiar of each mixed oxide.     

Function of the hydrogen bond in the conversion of α- to β-Mo8O264− ions: Formation and structure of tetra(tributylhydrogenammonium) β-octamolybdate

Complex β-(Bu₃NH)₄Mo₈O₂₆ has been transformed from α-(Bu₄N)₄Mo₈O₂₆ in the presence of o-mercaptophenol (H₂mp) and CuCl₂ in MeOH, and structurally characterized. It can be considered as a dimer [(Bu₃NH)₂Mo₄O₁₃]₂ with a center of symmetry. Each molybdenum atom is in a severely distorted octahedral environment. Two types of H bond between the cations and the anions have been found in the complex: N–H⋯O and C–H⋯O, which may play important roles in the α→β transformation.     

Subsolidus phase diagram of Cu2OCuOMoO3 system

Five chemical compounds, CuMoO₄, Cu₃Mo₂O₉, Cu₂Mo₃O₁₀, Cu₆Mo₄O₁₅, and Cu_4?x Mo₃O₁₂ (0.10 ? x ? 0.40), were identified in the system Cu₂OCuOMoO₃ and characterized by DTA, X-ray powder patterns, ir spectra, and magnetic properties. Cupric molybdates CuMoO₄ and Cu₃Mo₂O₉ are stable in air up to 820 and 855°C, respectively, melting at these temperatures with simultaneous decomposition (oxygen loss). Congruent mp of cuprous molybdates Cu₂Mo₃O₁₀ and Cu₆Mo₄O₁₅, in argon, are 532 and 466°C, respectively. Nonstoichiometric phase Cu_4?x Mo₃O₁₂ = Cu²⁺₃Cu⁰_1?xMo⁶⁺₃O₁₂, melts in argon between 630 and 650°C depending on the value of x and at 525–530°C undergoes polymorphic transformation. Areas of coexistence of the above-mentioned phases are determined. The μ_eff of Cu²⁺ ions and θ values are: 1.80 B.M. and 28°K for CuMoO₄, 1.71 B.M. and ? 12°K for Cu₃Mo₂O₉, and 1.74 B.M. and ? 93°K for Cu_4?xMo₃O₁₂. Below 200°K CuMoO₄ becomes antiferromagnetic. Cu₂Mo₃O₁₀ and Cu₆Mo₄O₁₅ show weak temperature-independent paramagnetism.     

Komplexe des zweiwertigen Molybdäns,Derivate des Oktahalodimolybdat(II)-Ions, 2. Mitt.

Cs₃Mo₂Br₇·2H₂O reacts with pyridine at room temperature giving Mo₂Br₄(C₅H₅N)₄. Dry ammonia substitutes pyridine from Mo₂ X ₄(C₅H₅N)₄ (X=Cl, Br) at 100°C. The resulting products are Mo₂ X ₄(NH₃)₄. No reaction takes place under the same experimental conditions with 2.2′-bipyridine complexes Mo₂ X ₄(C₁₀N₂H₈)₂. Mo₂Br₄(C₅H₅N)₄ can also be obtained reacting Mo₂Cl₄(C₅H₅N)₄ with LiBr. New compounds were identified on the basis of IR spectra, powder patterns, magnetic measurements and chemical evidence.     

Synthesis, crystal structure, and physical properties of the monoclinic form of the R4Mo4O11 compounds (R=Yb and Lu) containing infinite chains of trans-edge-shared octahedral clusters

Polycrystalline samples and single crystals of the R₄Mo₄O₁₁ compounds (R=Yb and Lu) were synthesized by solid-state reactions at high temperature in sealed Mo crucibles. The structure of Lu₄Mo₄O₁₁ (a=10.5611(1), b=5.61930(5), c=15.6877 (2), β=99.5131(4) and Z=4) was determined by single crystal X-ray diffraction and refined by least squares on F² converging to R₁=0.0425, wR₂=0.0980 for 3508 intensities. Contrary to the R₄Mo₄O₁₁ compounds with lighter rare earths, which crystallize in the orthorhombic space group Pbam, the Yb and Lu compounds crystallize in the monoclinic space group P2/m. The R₄Mo₄O₁₁ compounds contain distorted infinite oxide-molybdenum chains of trans-edge-sharing Mo₆ octahedra diluted with the rare earths. Magnetic susceptibility measurements indicate that the oxidation state of the Yb atoms is +3, affording 14 metallic valence electrons per Mo₄ fragment and, the absence of localized moments on the Mo network. Resistivity measurements on single crystals show that the Yb₄Mo₄O₁₁ and Lu₄Mo₄O₁₁ compounds are small band-gap semi-conductors.     

Molybdenum(V) dimeric complexes with dialkyl carbamates

Syntheses are reported for a number of novel oxo and sulphido bridged molybdenum(V) complexes with N-methyl-N-cyclohexyl carbamate and N,N-dicyclohexyl carbamate as ligands, and we have compared these complexes with the molybdenum(V) complexes with dialkyldithiocarbamates as ligands. These complexes were identified by IR and electronic spectra, magnetic susceptibility and analytical data, and were assigned the formulae [Mo₂O₃(LL)₄], [Mo₂O₄(LL)₂], [Mo₂O₂S₂(LL)₂] and [Mo₂O₃S(LL)₂]. IR and electronic spectra of these compounds are sensitive to substitution of sulphur atoms into the bridge system. It is suggested that the low magnetic moments observed are due, at least in part, to intramolecular metal-metal interactions.     

Expanding the Hierarchy of Metal-Oxide Building Blocks from Fragments via Clusters to Networks: A ∞2 [({Mo17(NO)2}3{MoV2}3{FeIII}6)(FeII1.5)]-Type Layer Compound

A planar network consisting of {Mo₁₇(NO)₂}₃{Mo^V ₂}₃{Fe₆^III} cluster entities that are interlinked to layers via {Fe^II(H₂O)₄}²⁺ groups is formed stepwise from building units. The corresponding mixed-valence compound exhibits a variety of different formal oxidation states: {MoNO}³⁺, Mo^V, Mo^VI, Fe^II, and Fe^III. This compound also represents an extension of building-block hierarchy from the molecular level to extended networks.     

Planar defects in β-alumina

Sodium β-alumina crystals were elaborated by melting of a mixture of Na₂CO₃ and Al₂O₃ or by PbO flux evaporation and were studied by transmission electron microscopy. They exhibit regular planar defects lying in the {11.0} prismatic planes. These defects are described as antiphase boundaries for the cationic sublattice with fault vectors $\text{1}\text{2}\text{〈10.0〉}$ (such faults do not affect the anionic sublattice). As a consequence it would be interesting to study precisely the structure of the sodium β cationic lattice in the vicinity of the melting point.     

Structure and properties of La2Mo2O7: A quasi-two-dimensional metallic oxide with strong MoMo bonds

Single crystals of La₂Mo₂O₇, prepared by fused salt electrolysis, were used for structural and electronic characterization. La₂Mo₂O₇ is orthorhombic with a = 6.034Å, b = 12.236 Å, and c = 3.888 Å. The dominant feature of the structure, which was refined in space group Pnnm, is Mo₂O₁₀ units formed by edge-sharing MoO₆ octahedra which contain MoMo distances of only 2.478 Å. These groups then share corners in two dimensions to give rise to MoO layers which are held together by the lanthanum ions. The relationship of the La₂Mo₂O₇ structure to those of other reduced oxides is discussed. La₂Mo₂O₇ is a metallic conductor down to 125 K where a phase transition takes place. A similar transition is seen in the magnetic susceptibility. The anomalous electric and magnetic behavior of this compound may be associated with a charge density wave instability of the type often found in quasi-two-dimensional materials.     

Preparation, crystal structure, and magnetic studies of Na3Fe2Mo5O16, a new oxide containing Mo3O13 clusters

The complex oxide Na₃Fe₂Mo₅O₁₆ has been synthesized, and its crystal structure was determined by single-crystal X-ray diffraction (space group (SG) P-3m1; a=5.7366(6) Å, c=22.038(3) Å; Z=2). The compound can be considered as a new structure type containing Mo₃O₁₃ cluster units, which can be derived from the Na₂In₂Mo₅O₁₆ structure model by doubling of the cell along the c-axis. Na₃Fe₂Mo₅O₁₆ crystallizes in centrosymmetric SG (P-3m1) and the positions of the sodium atoms are fully occupied in contrast to the proposed Na₂In₂Mo₅O₁₆ model SG (P3m1). Magnetic properties of Na₃Fe₂Mo₅O₁₆ were studied by superconducting quantum interference device measurements, revealing antiferromagnetic ordering below Tχ_max=10(1) K. Thermal stability in air was investigated by in situ high-temperature X-ray powder diffraction. Structural relationships to Na₂In₂Mo₅O₁₆ and NaFe(MoO₄)₂ are discussed.     

Molybdenum cluster chalcogenides Mo6X8: Electrochemical intercalation of closed shell ions Zn2+, Cd2+, and Na

The intercalation ofd¹⁰ ions Zn²⁺ and Cd²⁺ by electron/ion transfer reactions into the Chevrel-type molybdenum cluster chalcogenidesMo₆X₈ (X =S, Se) demonstrates the competitive influence of electronic and steric factors upon these processes. The following rhombohedral phases have been identified: Zn₁Mo₆S₈, Zn₂Mo₆S₈, Zn₁Mo₆Se₈, Zn₂Mo₆Se₈, Cd₁Mo₆S₈, Cd₁Mo₆Se₈, and Cd₂Mo₆Se₈. Thermodynamic data and chemical diffusion coefficients are given. The intercalation of Na⁺, which has an ionic radius close to that of Cd²⁺, exhibits a strong influence of kinetics leading to the partial irreversibility of the reaction and the formation Na₁Mo₆S₈ and Na₁Mo₆Se₈, the first cubic phases among the molybdenum cluster chalcogenidesA_xMo₆X₈.     

Bi2n+4MonO6(n+1) with n=3, 4, 5, 6: A new series of low-temperature stable phases in the mBi2O3 - MoO3 system (1.0<m<1.7): Structural relationships and conductor properties

Four low-temperature phases with compositions Bi₁₀Mo₃O₂₄, Bi₆Mo₂O₁₅, Bi₁₄Mo₅O₃₆ and Bi₈Mo₃O₂₁ have been prepared by the n-butylamine wet synthesis method. They have been characterized by powder X-ray diffraction and transmission electron microscopy, mainly by selected area electron diffraction. The four phases present a close structural relationship and a common basic fluorite-type structure and are members of a homologous series of phases with general formula Bi_2n+4Mo_nO_6(n+1), being n=3, 4, 5 and 6, respectively. The matrices relating their superstructures and the basic fluorite type unit cell are given, as well as a general one for the whole series. The conductor behavior of these phases is characterized by impedance spectroscopy being all these materials very good ionic conductors.     

Tetrathiafulvalene-hydroxyamides and -oxazolines: hydrogen bonding, chirality, and a radical cation salt

Racemic and enantiopure ethylenedithio-tetrathiafulvalene (EDT-TTF) derivatives featuring β-hydroxyamide or oxazoline (OX) groups bearing methyl or isopropyl substituents have been synthesized starting from the corresponding amino alcohols. Crystal structure analysis shows in the case of the racemic methyl-β-hydroxyamide donor the development of a unique hydrogen bond network, characterized by short CO?H-O and N-H?O-H intermolecular distances. The enantiopure (S)-EDT-TTF-methyl-OX crystallizes in the monoclinic non-centrosymmetric space group P2₁, whereas the isopropyl counterparts, (R)-and (S)-EDT-TTF-isopropyl-OX, crystallize in the orthorhombic non-centrosymmetric space group P2₁2₁2₁. All of them adopt a s-trans conformation in which TTF and oxazoline units are coplanar. Electrocrystallization experiments with the racemic EDT-TTF-methyl-OX, in the presence of (nBu₄)₂Mo₆Cl₁₄ as supporting electrolyte, afford a radical cation salt, formulated as [(±)-EDT-TTF-methyl-OX]₂Mo₆Cl₁₄, in which the donors associate in strong dimers, which further stack along the b direction to form quasi-homochiral helix-like ribbons.     

Structural refinement of T2Mo3O8 (T=Mg, Co, Zn and Mn) and anomalous valence of trinuclear molybdenum clusters in Mn2Mo3O8

Crystal structure of a series of mixed-metal oxides, T₂Mo₃O₈ (T=Mg, Co, Zn and Mn; P6₃mc; a=5.7628(1) Å, c=9.8770(3) Å for Mg₂Mo₃O₈; a=5.7693(3) Å, c=9.9070(7) Å for Co₂Mo₃O₈; a=5.7835(2) Å, c=9.8996(5) Å for Zn₂Mo₃O₈; a=5.8003(2) Å, c=10.2425(5) Å for Mn₂Mo₃O₈) was investigated by X-ray diffraction on single crystals. Structural analysis, magnetization measurements, X-ray photoemission spectroscopy and cyclic voltammetry showed that the Mn ions at the tetrahedral and octahedral sites in Mn₂Mo₃O₈ adopt different valences of +2 and 2+δ (δ>0), respectively. The formal valence of the Mo₃ in Mn₂Mo₃O₈ is 12−δ to retain electric neutrality of the compound. In contrast, the T ions and Mo₃ in T₂Mo₃O₈ (T=Mg, Co and Zn) adopt the valences of +2 and +12, respectively.     

Application of Solid-State Molybdenum Sulfide Clusters with an Octahedral Metal Framework to Catalysis: Ring-Opening of Tetrahydrofuran to Butyraldehyde

Solid-state molybdenum sulfide clusters with an octahedral metal framework, the superconducting Chevrel phases, are applied to catalysis. The cluster of copper salt, Cu _x Mo₆S₈ (x = 2.94), stored in air is treated in a hydrogen stream above 300 °C. The activated cluster exhibits catalytic activity for the ring-opening of tetrahydrofuran, yielding butyraldehyde. Cyclic ethers such as trimethylene oxide and tetrahydropyran are also converted to the corresponding aldehydes. The cluster contains nonstoichiometric defects of sulfur atoms. Oxygen atoms are incorporated at the sulfur-deficient sites upon storage in air, but they are removed from the sites by the activation in a hydrogen stream. The resulting coordinatively unsaturated molybdenum atoms are catalytically active for the ring-opening reaction. The molybdenum atom in an intermediate oxidation state around 2+ is moderately coordinated by the oxygen of tetrahydrofuran and favorably releases the produced aldehyde. The neutral cluster Mo₆S₈, which has such sulfur-deficient sites, also catalyzes the reaction.     

Some reduced ternary and quaternary oxides of molybdenum. A family of compounds with strong metal-metal bonds

Several new, reduced ternary and quaternary oxides of molybdenum are reported, each containing molybdenum in an average oxidation state <4.0. All are prepared by reactions between a molybdate salt; metal oxide, if needed; and MoO₂ sealed in Mo tubes held at 1100°C for ca. 7 days. Refinement of the substructure of the new compound Ba_0.62Mo₄O₆ was based on an orthorhombic cell, witha = 9.509(2), b = 9.825(2), c = 2.853(1)Å, Z = 2 in space groupPbam; weak supercell reflections indicate the true structure hasc = 8(2.853) Å. The chief structural feature is closely related to that of NaMo₄O₆ (C. C. Torardi, R. E. McCarley,J. Amer. Chem. Soc.101, 3963 (1979)), which consists of infinite chains of Mo₆ octahedral clusters fused on opposite edges, bridged on the outer edges by O atoms and crosslinked by MoOMo bonding to create four-sided tunnels in which the Ba²⁺ ions are located. The structure of Ba_1.13Mo₈O₁₆ is triclinic,a = 7.311(1), b = 7.453(1), c = 5.726(1)Å, α = 101.49(2), β = 99.60(2), γ = 89.31(2)°,Z = 1, space groupP1¯. It is a low-symmetry, metal-metal bonded variant of the hollandite structure, in which two different infinite chains, built up from Mo₄O^2?₈ and Mo₄O^0.26?₈ cluster units, respectively, are interlinked via MoOMo bridge bonding to create again four-sided tunnels in which the Ba²⁺ ions reside. Other compounds prepared and characterized by analyses and X-ray powder diffraction data arePb_xMo₄O₆(x ～ 0.6), LiZn₂Mo₃O₈, CaMo₅O₈, K₂Mo₁₂O₁₉, and Na₂Mo₁₂O₁₉.     

Luminescence and dielectric properties of c-axis oriented (Bi1.90Eu0.10)(V1−zMoz)O5.5 ferroelectric thin films

(Bi_1.90Eu_0.10)(V_1−zMo_z)O_5.5 (z = 0, 0.05, 0.10, 0.15 and 0.20) thin films with c-axis oriented were prepared on Pt(111)/Ti/SiO₂/Si substrates by using chemical solution deposition method. The effect of Mo⁶⁺ concentration on the structure, luminescence properties and dielectric properties of the thin films were characterized systematically. X-ray diffraction data indicates that the thin films with low Mo⁶⁺-doping content can remain Bi₂VO_5.5 structure. When the Mo⁶⁺-doping content z reaches to 0.15, the thin films are a mixture of diphase with the main phase Bi₂VO_5.5 and secondary phase Bi₂MoO₆. Under UV irradiation, all the thin films emit a bright red or orange emission which origin from Eu³⁺. With increasing Mo⁶⁺-doping content z, the relative intensity of the Red and Orange emissions show obviously change. The value of Red/Orange ratio first decrease, and it reached minimum when z is 0.15, then it recover to the initial value. The variation trend of the Red/Orange ratio reflects the change of the lattice symmetry. Dielectric constant of the thin films increased with the increasing of the Mo⁶⁺ concentration while dielectric loss decreased. The decrease of the quantities of oxygen vacancies and the generation of Bi₂MoO₆ phase are responsible for the improvement of electric properties. These results explain that Eu³⁺ion can be used as an effective luminescent probe in (Bi_1.90Eu_0.10)(V_1−zMo_z)O_5.5 (z = 0, 0.05, 0.10, 0.15 and 0.20) thin films, and the electric properties of the thin films can be improved by Mo⁶⁺ doping.