Mo K-edge X-ray Absorption Spectroscopy Research Articles

Structural Distortion in the Wadsley-Roth Niobium Molybdenum Oxide Phase Triggering Extraordinarily Stable Battery Performance.

Wadsley-Roth niobium oxide phases have attracted extensive research interest recently as promising battery anodes. We have synthesized the niobium-molybdenum oxide shear phase (Nb, Mo)13 O33 with superior electrochemical Li-ion storage performance, including an ultralong cycling lifespan of at least 15000 cycles. During electrochemical cycling, a reversible single-phase solid-solution reaction with lithiated intermediate solid solutions is demonstrated using in situ X-ray diffraction, with the valence and short-range structural changes of the electrode probed by in situ Nb and Mo K-edge X-ray absorption spectroscopy. This work reveals that the superior stability of niobium molybdenum oxides is underpinned by changes in octahedral distortion during electrochemical reactions, and we report an in-depth understanding of how this stabilizes the oxide structure during cycling with implications for future long-life battery material design.

The role of transformation in the risks of chemically exfoliated molybdenum disulfide nanosheets to the aquatic environment

While the effects of environmental factors (e.g., coexisting organic macromolecules and solar irradiation) on the phase transformation and oxidative dissolution of chemically exfoliated molybdenum nanosheets (ceMoS2) have been recognized, the effects of environmental processes on the subsequent biological impacts of ceMoS2 are still poorly understood. In this study, the bioavailability and transitions in chemical speciation occurring during the aging process are demonstrated to be key factors causing ceMoS2 to affect aquatic organisms. The lower survival rate of embryonic zebrafish with aged (i.e., sunlight-irradiated and dark-ambient-aged) ceMoS2, compared to that with freshly prepared ceMoS2, was due to the release of ionic aging products (mainly acidic Mo species) throughout the oxidative dissolution of ceMoS2. The released soluble molybdenum interacted with natural organic matter (NOM) depending on their functionality, and this attenuated the toxicity caused by ceMoS2 to different degrees. Toxicity triggered by aged ceMoS2 under both dark and irradiated conditions was significantly reduced by Suwannee River NOM due to the formation of complexes with ionic Mo species, which was established by Mo K-edge X-ray absorption spectroscopy. The findings provide useful insights for comprehending the impacts of ceMoS2 on aquatic organisms and guidance for the prevention measures necessary in the applications of MoS2 nanosheets.

The structural evolution of Mo2C and Mo2C/SiO2 under dry reforming of methane conditions: morphology and support effects.

The thermal carburization of MoO3 nanobelts (nb) and SiO2-supported MoO3 nanosheets under a 1 : 4 mixture of CH4 : H2 yields Mo2C-nb and Mo2C/SiO2. Following this process by in situ Mo K-edge X-ray absorption spectroscopy (XAS) reveals different carburization pathways for unsupported and supported MoO3. In particular, the carburization of α-MoO3-nb proceeds via MoO2, and that of MoO3/SiO2via the formation of highly dispersed MoOx species. Both Mo2C-nb and Mo2C/SiO2 catalyze the dry reforming of methane (DRM, 800 °C, 8 bar) but their catalytic stability differs. Mo2C-nb shows a stable performance when using a CH4-rich feed (CH4 : CO2 = 4 : 2), however deactivation due to the formation of MoO2 occurs for higher CO2 concentrations (CH4 : CO2 = 4 : 3). In contrast, Mo2C/SiO2 is notably more stable than Mo2C-nb under the CH4 : CO2 = 4 : 3 feed. The influence of the morphology of Mo2C and its dispersion on silica on the structural evolution of the catalysts under DRM is further studied by in situ Mo K-edge XAS. It is found that Mo2C/SiO2 features a higher resistance to oxidation under DRM than the highly crystalline unsupported Mo2C-nb and this correlates with an improved catalytic stability. Lastly, the oxidation of Mo in both Mo2C-nb and Mo2C/SiO2 under DRM conditions in the in situ XAS experiments leads to an increased activity of the competing reverse water gas shift reaction.

Complementary combinative strategy of defect engineering and graphene coupling for efficient energy-functional materials.

The synergetic combination of defect engineering and graphene coupling enables to develop an effective way of exploring efficient bifunctional electrocatalyst/electrode materials. Defect-engineered amorphous MoO2 -reduced graphene oxide (rGO) nanohybrid was synthesized by soft-chemical reduction of K2 MoO4 in graphene oxide colloids. Mo K-edge X-ray absorption spectroscopy clearly demonstrates the rutile-type local atomic structure of amorphous MoO2 with significant oxygen vacancies and intimate electronic coupling with rGO. The defect-introduced MoO2 -rGO nanohybrid shows excellent bifunctionality as electrocatalyst for hydrogen evolution reaction and electrode for sodium-ion batteries, which are superior to those of crystalline MoO2 -rGO homologue. The beneficial effect of simultaneous defect control and rGO coupling can be ascribed to the provision of oxygen vacancies acting as active sites, the increase of electrical conductivity, and the improvement of reaction kinetics.

Controlling MoO2 and MoO3 phases in MoOx/CNTs nanocomposites and their application to anode materials for lithium-ion batteries and capacitors

Molybdenum oxides (MoO2 and MoO3) are attractive anode materials for Li- and Na- ion batteries. Although there have been extensive studies on them individually, systematic and comparative studies are still lacking. In this work, we demonstrate a facile and straightforward synthesis method to control the phase and oxidation state in the MoOx/CNTs nanocomposites via hydrothermal reaction followed by heat-treatment. By changing the gas atmosphere during the annealing process, well-dispersed MoO2/CNTs and MoO3/CNTs nanocomposites are formed without altering their overall morphology. This strategy enables us to investigate the true structure-property correlation of MoOx/CNTs nanocomposites by comparing the structure and electrochemical properties of MoO2/CNTs and MoO3/CNTs. When tested as anode materials for lithium-ion batteries, both HT-MoO2&3/CNTs electrodes show much-improved cycling stability and rate performance compared to the rod-shaped bulk MoO3 electrode. In situ Mo K-edge x-ray absorption spectroscopy (XAS) has been further employed to compare and elucidate Li+ storage mechanisms of both electrodes. When employed to the negative electrode of a high-power lithium-ion capacitor (LIC), the LIC full-cell composed of HT-MoO3/CNTs negative and activated carbon positive electrodes demonstrates impressive energy and power densities (~ 90 Wh kg−1 with 2000 W kg−1) and excellent cycling stability (96.8 % capacity retention after 300 cycles), revealing the versatility of the MoOx/CNTs electrodes in energy applications.

X-ray absorption near edge spectroscopy of thermochromic phase transition in CuMoO4

Thermochromic phase transition was studied in CuMoO4 using the Cu and Mo K-edge x-ray absorption spectroscopy in the temperature range of 10–300 K. The hysteretic behavior has been evidenced from the temperature dependence of the pre-edge shoulder intensity at the Mo K-edge, indicating that the transition from brownish-red γ-CuMoO4 to green α-CuMoO4 occurs in the temperature range of 230–280 K upon heating, whereas the α-to-γ transition occurs between 200 and 120 K upon cooling. Such behavior of the pre-edge shoulder at the Mo K-edge correlates with the change of molybdenum coordination between distorted tetrahedral in α-CuMoO4 and distorted octahedral in γ-CuMoO4. This result has been supported by ab initio full-multiple-scattering x-ray absorption near edge structure (XANES) calculations.

Sequestration of As and Mo in uranium mill precipitates (pH 1.5–9.2): An XAS study

As- and Mo- bearing secondary mineral phases formed during the neutralization of uranium mill wastes require characterization. Previous studies indicate that arsenate and molybdate adsorbed to ferrihydrite are the dominant controls in the tailings materials. A lab-scale plant was employed to characterize secondary precipitates from a variety of ore blends. Through total elemental analysis of precipitates and As and Mo K-edge X-ray absorption spectroscopy, different ratios of contributing phases were determined for each pH stage (4.2, 6.5, and 9.2) of the neutralization process. Overall, arsenate adsorbed to ferrihydrite was the dominant As mineral phase regardless of pH or sample blend (53–77%), with fractional contribution from ferric arsenates, and adsorption to aluminum phases. Molybdate adsorbed to ferrihydrite was the dominant Mo mineral phase, with fractional contribution decreasing with increasing pH (100–69%). The characterization of these phases in the secondary precipitates provides further understanding of the contributing mineral species in tailing facilities.

Molybdenum L-Edge XAS Spectra of MoFe Nitrogenase.

A molybdenum L-edge X-ray absorption spectroscopy (XAS) study is presented for native and oxidized MoFe protein of nitrogenase as well as Mo-Fe model compounds. Recently collected data on MoFe protein (in oxidized and reduced forms) is compared to previously published Mo XAS data on the isolated FeMo cofactor in NMF solution and put in context of the recent Mo K-edge XAS study, which showed a MoIII assignment for the molybdenum atom in FeMoco. The L3-edge data are interpreted within a simple ligand-field model, from which a time-dependent density functional theory (TDDFT) approach is proposed as a way to provide further insights into the analysis of the molybdenum L3-edges. The calculated results reproduce well the relative spectral trends that are observed experimentally. Ultimately, these results give further support for the MoIII assignment in protein-bound FeMoco, as well as isolated FeMoco.

Molybdenum site structure of MOSC family proteins.

Mo K-edge X-ray absorption spectroscopy has been used to probe as-isolated structures of the MOSC family proteins pmARC-1 and HMCS-CT. The Mo K-edge near-edge spectrum of HMCS-CT is shifted ∼2.5 eV to lower energy compared to the pmARC-1 spectrum, which indicates that as-isolated HMCS-CT is in a more reduced state than pmARC-1. Extended X-ray absorption fine structure analysis indicates significant structural differences between pmARC-1 and HMCS-CT, with the former being a dioxo site and the latter possessing only a single terminal oxo ligand. The number of terminal oxo donors is consistent with pmARC-1 being in the MoVI oxidation state and HMCS-CT in the MoIV state. These structures are in accord with oxygen-atom-transfer reactivity for pmARC-1 and persulfide bond cleavage chemistry for HMCS-CT.

Synthesis and Electrochemical Reactivity of Molybdenum Dicarbonyl Supported by a Redox-Active α-Diimine Ligand

Low-valent molybdenum dicarbonyl complexes with a diazabutadiene [(mes)DAB(R); [ArN═C(R)C(R)═NAr]; Ar = 2,4,6-trimethylphenyl (mes), R = H or CH3] ligand have been synthesized and fully characterized. The title complexes exhibit elongated DAB C-N and shortened C-C bond lengths over the free ligand and other zerovalent molybdenum complexes of DAB. Compared to known examples theoretically described as iminato π-radicals (L(•-)), the oxidation state assignment fits a molybdenum(II) description. However, Mo K-edge X-ray absorption spectroscopy indicates that the complexes are best described as molybdenum(0). This example demonstrates that caution should be exercised in assigning the oxidation state based on structural parameters alone. Cyclic voltammetry studies reveal an electrochemical-chemical process that has been identified by in situ Fourier transform infrared spectroelectrochemistry as cis-to-trans isomerization.

X-ray Absorption Spectroscopic Investigation of Molybdenum Multinuclear Sorption Mechanism at the Goethite−Water Interface

Understanding in situ metalloid surface speciation on mineral surfaces is critical to predicting the bioavailability in surface and subsurface environments. In this study, Mo K-edge X-ray absorption spectroscopy (XAS) was used to elucidate Mo(VI) surface speciation at the goethite-water interface. Effects of pH and loading levels were investigated. X-ray absorption near edge structure (XANES) analysis indicated that the Mo(VI) coordination environment changes from tetrahedral to octahedral with decreasing pH. At near neutral pH, Mo(VI) predominantly remains as tetrahedral molecules, forming inner-sphere surface species via corner- and edge-sharing attachment with iron octahedral structures (interatomic distance of Mo-Fe (R(Mo-Fe)) at ∼2.8 and ∼3.4 Å, respectively). In contrast, a mixture of surface species comprising tetrahedrally and octahedrally coordinated Mo(VI) exists at pH ∼3-4. While the same Mo(VI) tetrahedral surface species are present at acidic pH, there was an additional MoO(6) polymer attachment on iron octahedral structures, resulting in a R(Mo-Fe) at 3.53 Å. The coordination number (CN) of a Mo-Mo backscatterer gradually increased with increasing loading level, suggesting the formation of surface polymerization. Overall, there seems to be a transition from Mo(VI) tetrahedral to octahedral coordination environment with decreasing pH. The XAS findings further support a Mo(VI) inner-sphere adsorption mechanism that was previously suggested in the pressure-jump relaxation study by Zhang and Sparks (Soil Sci. Soc. Am. J. 1989, 53 (4), 1028-1034). pH-Dependent multinuclear Mo(VI) surface speciation may be important in predicting Mo(VI) transport process in the soil-water environment.

Framework fluxionality of organometallic oxides: synthesis, crystal structure, EXAFS, and DFT studies on [[Ru(eta6-arene)]4Mo4O16] complexes.

Reactions of the molybdates Na(2)MoO4.2 H2O and (nBu(4)N)2[Mo2O7] with [[Ru(arene)Cl(2)](2)] (arene=C(6)H5CH3, 1,3,5-C6H3(CH3)(3), 1,2,4,5-C6H2(CH3)4) in water or organic solvents led to formation of the triple-cubane organometallic oxides [[Ru(eta(6)-arene)](4)Mo4O16], whose crystal and molecular structures were determined. Refluxing triple cubane [[Ru(eta(6)-C6H5CH3)](4)Mo4O16] in methanol caused partial isomerization to the windmill form. The two isomers of [[Ru(eta(6)-C6H5CH3)](4)Mo4O16] were characterized by Raman and Mo K-edge X-ray absorption spectroscopy (XAS), both in the solid-state and in solution. This triple-cubane isomer was also used as a spectroscopic model to account for isomerization of the p-cymene windmill [[Ru(eta(6)-1,4-CH3C6H4CH(CH3)2)](4)Mo4O16] in solution. Using both Raman and XAS techniques, we were then able to determine the ratio between the windmill and triple-cubane isomers in dichloromethane and in chloroform. Density functional calculations on [[Ru(eta(6)-arene)](4)Mo4O16] (arene=C6H6, C6H5CH3, 1,3,5-C6H3(CH3)3, 1,4-CH3C6H4CH(CH3)2, C6(CH3)6) suggest that the windmill form is intrinsically more stable, provided the complexes are assumed to be isolated. Intramolecular electrostatic interactions and steric bulk induced by substituted arenes were found to modulate but not to reverse the energy difference between the isomers. The stability of the triple-cubane isomers should therefore be accounted for by effects of the surroundings that induce a shift in the energy balance between both forms.

Tetrathiomolybdate causes formation of hepatic copper-molybdenum clusters in an animal model of Wilson's disease.

Wilson's disease is an autosomal recessive human illness in which large quantities of copper accumulate in various organs, including the brain and the liver. If left untreated, it results in hepatitis, neurological complications, and death. Long-Evans Cinnamon (LEC) rats have a homologous mutation to Wilson's disease and thus provide an animal model. Liver lysosomes from tetrathiomolybdate-treated LEC rats were isolated and analyzed by Cu and Mo K-edge X-ray absorption spectroscopy. The lysosomes contained a Cu-Mo-S cluster in which the Mo is coordinated by four sulfurs at 2.24 A with approximately three copper neighbors at 2.70 A. Each Cu is coordinated to 3-4 sulfurs at 2.28 A with approximately one Mo neighbor at 2.70 A. These results indicate the formation of a biologically novel molybdenum-copper-sulfur cluster.

Dehydration of the molybdenum trioxide hydrates MoO3·nH2O: in situ x-ray absorption spectroscopy study at the Mo K edge

The process of dehydration for the molybdenum trioxide hydrates, MoO32H2O 2-MoO3H2O -MoO3, was studied for the first time by in situ Mo K-edge x-ray absorption spectroscopy. A regularization procedure was applied to reconstruct the radial distribution functions around the molybdenum at all stages of the process. Strong variations in the local environment of the molybdenum were found at both phase transitions and were attributed to the changes in direction of the off-centre Mo displacement and in the degree of symmetrization of the Mo-O bonds. Also, an increase of the Mo-O-Mo angle, leading to the appearance of strong multiple-scattering effects in the x-ray absorption fine structure, was observed at the second phase transition -MoO3H2O-MoO3. The final -MoO3 product had nanocrystalline structure with an average grain size of about 15 Å.

Metallocene-derived, isolated MoVI active centres on mesoporous silica for the catalytic dehydrogenation of methanol

Isolated MoVI active sites have been grafted onto the inner surfaces of MCM-41 mesoporous silica via a molybdocene dichloride precursor, to generate a catalyst which has been tested for the oxidative dehydrogenation of methanol. Mo K-edge X-ray absorption spectroscopy shows that, after calcination, at low Mo loadings (ca. 1 mol% with respect to SiO2), isolated MoO4 species are generated on the surface, whereas at higher loadings (ca. 4 mol%) there is some evidence for the formation of polymeric oxo-molybdenum species. Catalysis tests show that lower Mo loadings on the MCM-41 silica lead to higher selectivity for the production of formaldehyde, in contrast to the results of previous studies of isolated Mo species on pure silica, which report that methyl formate is the major product.

X-ray absorption spectroscopy of dimethylsulfoxide reductase from Rhodobacter capsulatus

Mo K-edge X-ray absorption spectroscopy (XAS) has been used to probe the environment of Mo in dimethylsulfoxide (DMSO) reductase from Rhodobacter capsulatus in concert with protein crystallographic studies. The oxidised (MoVI) protein has been investigated in solution at 77 K; the Mo K-edge position (20006.4 eV) is consistent with the presence of MoVI and, in agreement with the protein crystallographic results, the extended X-ray absorption fine structure (EXAFS) is also consistent with a seven-coordinate site. The site is composed of one oxo-group (Mo=O 1.71 A), four S atoms (considered to arise from the dithiolene groups of the two molybdopterins, two at 2.32 A and two at 2.47 A, and two O atoms, one at 1.92 A (considered to be H-bonded to Trp 116) and one at 2.27 A (considered to arise from Ser 147). The Mo K-edge XAS recorded for single crystals of oxidised (MoVI) DMSO reductase at 77 K showed a close correspondence to the data for the frozen solution but had an inferior signal:noise ratio. The dithionite-reduced form of the enzyme and a unique form of the enzyme produced by the addition of dimethylsulfide (DMS) to the oxidised (MoVI) enzyme have essentially identical energies for the Mo K-edge, at 20004.4 eV and 20004.5 eV, respectively; these values, together with the lack of a significant presence of MoV in the samples as monitored by EPR spectroscopy, are taken to indicate the presence of MoIV. For the dithionite-reduced sample, the Mo K-edge EXAFS indicates a coordination environment for Mo of two O atoms, one at 2.05 A and one at 2.51 A, and four S atoms at 2.36 A. The coordination environment of the Mo in the DMS-reduced form of the enzyme involves three O atoms, one at 1.69 A, one at 1.91 A and one at 2.11 A, plus four S atoms, two at 2.28 A and two at 2.37 A. The EXAFS and the protein crystallographic results for the DMS-reduced form of the enzyme are consistent with the formation of the substrate, DMSO, bound to MoIV with an Mo-O bond of length 1.92 A.