Formation, Structure, and HDN Activity of Unsupported Molybdenum Phosphide

Influence of redox behavior of copper ions on dielectric and spectroscopic properties of Li2O-MoO3-B2O3 : CuO glass system

Characterization and Physical Properties of PbO–As2O3 Glasses Containing Molybdenum Ions

The relationship between the composition, structure and selected properties for four series of iron phosphate glasses containing MoO3 and SrO has been investigated. The three series of MoO3–Fe2O3–P2O5 glasses were prepared with increasing MoO3 content. In series A the molar Fe/P ratio was constant at 0.67, in series B the O/P ratio was 3.5, whereas, in series C the molar percentages of Fe2O3 was fixed at 40 mol%. In the fourth series, M, SrO was added to maintain the Fe/P ratio constant at 0.67. The structural changes in these glasses have been studied by Raman spectroscopy. The Raman spectra show that MoO3 is incorporated into the phosphate network. However, the structure/properties are strongly influenced by the overall O/P ratio. In glasses with O/P > 3.5 the addition of MoO3 depolymerized some of the bridging oxygens in pyrophosphate units to form isolated (PO4)3− orthophosphate groups. The decrease in glass temperature, Tg, and glass density, D, increase in thermal expansion coefficient, α, and glass density, D, for these glasses are attributed to an increase in the number of weaker P–O–Mo bonds connected to both orthophosphate and pyrophosphate units. The replacement of the stronger P–O–P bonds for the weaker Mo–O–P bonds imparts greater covalency to the network and contributes to the lower bond strength as the bond covalence decreases from P–O to Mo–O. The addition of up to 20 mol% SrO does not produce any changes in the Raman spectra and any further disruption of pyrophosphate chains.

Spectroscopic investigation of MoO3–Fe2O3–P2O5 and SrO–Fe2O3–P2O5 glasses. Part I

The coordination chemistry of molybdenum was investigated in nine series of synthetic silicate glasses, including sodium disilicate (NS2), sodium trisilicate (NS3), albite (Ab), anorthite (An), Ab50An50, Ab30An70, diopside (DI), rhyolite (RH), and basalt (BA), using electron paramagnetic resonance (EPR) and X-ray absorption fine structure (XAFS) spectroscopies. The Mo content of these glasses ranges from 300 ppm to 3 wt.%. On the basis of results derived from high-resolution X-ray absorption near-edge structure (XANES) spectroscopy, molybdenum is present primarily as molybdate moieties [Mo(VI)O42−] in most of the glass compositions prepared at f (O2) values ranging from 1 atm to 10−12 atm (temperatures ranging from 1100 to 1700°C, i.e., from air to IW+4). Analysis of extended XAFS (EXAFS) spectra of these glasses indicates an average Mo–O distance of ~1.76(1) A. No evidence for second-neighbor Si or Al around Mo was found in any of the glasses, confirming that molybdate moieties are not connected to the tetrahedral framework, in agreement with Pauling bond-valence predictions. The presence of molybdate moieties in regions of these glasses enriched in network modifiers helps explain why crystalline molybdates can nucleate easily in silicate glasses (and, by extension, in the corresponding melts). In the highly polymerized glass compositions (such as “Ab” or “RH”), Mo(VI)O66− moieties also exist, but at minor levels (<20% of the total Mo). In glasses prepared at low f (O2) (near IW), reduced species of Mo occur, such as molybdenyl [Mo(V) and Mo(IV)]. In glasses prepared at even lower f (O2) (near IW+4), Mo is present as a metallic precipitate. The prevalence of molybdate moieties in silicate glasses until relatively low oxygen fugacities (IW) are achieved appears to be at variance with the fact that molybdenite, Mo(IV)S2, is the dominant Mo-bearing mineral in the Earth’s crust. In a companion paper, we re-examine the speciation of molybdenum in more complex systems that are closer to geochemical reality, such as high-temperature melts, densified (high-pressure) glasses, and silicate glass compositions enriched in volatiles.

Structural environments around molybdenum in silicate glasses and melts. i. influence of composition and oxygen fugacity on the local structure of molybdenum

Glasses of P 2 O 5 -Li 2 MoO 4 -Li 2 O system with constant Li 2 MoO 4 molar contents (10%), containing different amounts of the alkaline oxide (Li 2 O) ranging from 0 to 30 mol%, have been studied by electron paramagnetic resonance (EPR) spectroscopy. It is found that less than 1% of molybdenum ions are reduced to Mo 5+ , this percentage decreasing with the Li 2 O content. The components of g - and A -tensors are determined by computer simulations and related to structural properties using molecular orbital method. The distorted octahedral environment of Mo 5+ sites is confirmed and minimal Mo-Mo distance is evaluated from the EPR linewidth as ≈0.38 nm. At Li 2 O contents ≥20 mol%, the coordination polyhedra of Mo 5+ become better ordered though more distorted. The Mo-O bond covalency increases with increasing Li 2 O content.

A. Nadiri

Papers

Mo5+ ions as EPR structural probes in molybdenum phosphate glasses

Structural Approach of Glasses Belonging to the Li2O-Li2WO4-P2O5 Ternary System