Narrow Results

In this study, we have calculated the basic nuclear properties such as binding energies, root mean square (rms) charge radii, and neutron and proton densities of the even–even natural ^92–100Mo isotopes. Investigations were performed using the Hartree–Fock–Bogoliubov (HFB) method with different Skyrme-like forces. Separation energies, which have an important role in nuclear structure, of neutron, proton, deuteron, triton, helium-3 and alpha were also investigated with TALYS 1.4 code. The calculated results were discussed and compared with experimental results.

The high-pressure strength of molybdenum (Mo) to 92 GPa has been studied by radial X-ray diffraction (RXRD) technique. The ratio of t/G is found to decrease above $\sim$24 GPa, showing the yield of Mo which is caused by plastic deformation at this pressure. Combined with high-pressure shear modulus, it was found that the differential stress corresponding to the yield of Mo at 24 GPa due to plastic deformation is 1.73 GPa. The second increase of t values occurs after $\sim$66 GPa, suggesting the strength of Mo with a differential stress of $\sim$1.93 GPa. In addition, the maximum difference stress of molybdenum at 87 GPa is 3.01 GPa.

Favored structures of SiO_n monolayers on Mo(112) surface have been studied using the total energy minimization technique based on DFT semirelativistic approach. In the [SiO₄] complexes, which form the c(2 × 2) silica structure on the Mo(112), the bonding of the Si atoms with the surface is accomplished through the oxygen atoms. The structure with a symmetric position of oxygen atoms has been found to be the most favorable. In this structure, two oxygen atoms occupy bridge-on-row sites on the Mo(112) surface, with Si atoms between them, while oxygen atoms in the troughs appear not in expected threefold sites, but adjust their positions along the middle lines of the troughs. Estimated main phonon frequency and density of states for the symmetric [SiO₄] structure agree well with experimental results.

Molybdenum thin films were sputter deposited under different conditions of DC power and chamber pressure. The structure and topography of the films were investigated using AFM, SEM and XRD techniques. Van der Pauw method and tape test were employed to determine electrical resistivity and interfacial strength to the substrate, respectively. All the films are of sub-micron thickness with maximum growth rate of 78 nm/min and crystallite size in the range of 4 to 21 nm. The films produced at high power and low pressure exhibit compressive residual strains, low electrical resistivity and poor adhesion to the glass substrate, whereas the converse is true for films produced at high pressure.

In current scenario, there is an ever increasing demand for alloys with increased wear resistance under severe working conditions. Ni–Cr–Mo alloy is suitable for surface modification and repairing applications. The Ni–Cr–Mo alloy clad region had cellular-dendrite with Laves phase and complex nitrides/carbides (MX) distributed in the interdendritic regions. Microindentation hardness variation was observed on the interdendritic regions as compared to that of dendritic regions. A new solidification path is confirmed through EDS analysis. Ni, Fe and Cr segregate to the dendritic area, whereas Nb and Mo segregate to the interdendritic region. Ni–Cr–Mo microstructure evolution was studied by optical microscope (OM), scanning electron microscope (SEM) and atomic force microscope (AFM). The Laves and G phases were identified in the Ni–Cr–Mo clad region, confirming superior resistance to corrosion. Pin-on-disc abrasion wear technique confirmed that cladding with low base metal dilution possesses better wear resistance. The mechanism of wear depends on load applied and sliding time. EBSD analysis revealed random texture and grains have same crystallographic orientation across the interface boundary line due to epitaxial growth.

Solution UV-visible electronic absorption and resonance Raman (RR) scattering spectra in 25 aprotic organic solvents have been measured and analyzed for a high-valent molybdenum(V) oxo complex of 5,10,15-triphenylcorrole, (Mo^VO)CorPh₃. Optical absorption data in the visible region show a significant positive solvatochromism upon increasing solvent polarizability (ca. 660 cm^-1 for the violet Soret band on going from acetonitrile to carbon disulfide), described by an excellent linear correlation between the bathochromic shifts of corrole π → π* electronic transitions and the solvent refractive index . Isotope labeling of the terminal oxo group with ¹⁸O revealed the oxo-Mo(V) stretch of (Mo^VO)CorPh₃ to vibrate at frequency ν(Mo^VO) ≈ 970 cm^-1 [Δ(¹⁶O-¹⁸O) = 46 cm^-1], consistent with a Mo^V≡O triple bond. This vibration is greatly enhanced in the RR spectra with excitations within the Soret absorption band and its frequency is sensitive to solute-solvent interactions that weaken the Mo^V≡O triple bond by inhibiting O^2- → Mo⁵⁺ electron donation. The ν(Mo^VO) frequency decreases in direct proportion to the Swain's solvent polarity parameter (A + B), from 969.4 cm^-1 in n-hexane solution down to 955.6 cm^-1 in dimethyl sulfoxide solution. However, only weak linear correlation was found when the ν(Mo^VO) frequencies were plotted as a function of the Gutmann's solvent acceptor numbers (ANs). A molecular association occurs between chloroform and (Mo^VO)CorPh₃ through Mo^V≡O⋯H-CCl₃ hydrogen-bonding interactions, manifested as an upshift of the ν(Mo^VO) RR band in deuteriochloroform solution. The ν(Mo^VO) stretching frequencies of (Mo^VO)CorPh₃ and its fluorinated derivative (Mo^VO)Cor(Ph(CF₃)₂)₃ {Cor(Ph(CF₃)₂)₃ = 5,10,15-tris-[3,5-bis(trifluoromethyl)phenyl]corrole} are compared, and the increased strength of the Mo^V≡O bond in the latter oxo-Mo(V) species is attributed to the cis effect of the electron-withdrawing -C₆H₃(CF₃)₂ groups, as judged by a 6 cm^-1 elevation of the ν(Mo^VO) frequency. Raman excitation profiles of ν(Mo^VO) and corrole skeletal vibrations show the presence of charge-transfer and π → π* electronic transitions within the Soret absorption band of oxomolybdenum(V) corroles.

The Soret maxima of a number of metallotriarylcorroles shift sensitively in response to varying substituents on the meso-aryl groups. The effect is most pronounced for copper corroles but is not seen for silver and gold corroles. In the copper case, the effect has been attributed to a small HOMO–LUMO gap. Chromium-oxo corroles share a small HOMO–LUMO gap with copper corroles, as well as a substantially metal-based LUMO, yet the Soret maxima of chromium-oxo triarylcorroles do not shift in response to substitution on the meso-aryl groups. Molybdenum-oxo corroles are substituent-insensitive in the same sense. TDDFT calculations, focusing on chromium-oxo and molybednum-oxo triarylcorroles, are reported here in an attempt to explain the divergent spectroscopic behavior of different metallocorrole families.

The solid state synthesis of molybdenum porphyrin using [Mo(NO)₂(py)₂(Cl)₂] as the metal source and H₂TPP (5,10,15,20-meso-tetraphenylporphyrin) in 3,5-dinitrobenzoic acid melt is described. Complex [Mo^VO(TPP)(3,5-DNB)] (1) (3,5-DNB = 3,5-dinitrobenzoate ion) is isolated in almost quantitative yield. 1 is characterized by single-crystal X-ray structure analysis, electron paramagnetic resonance, electronic, FT-IR spectroscopy, and by magnetic moment measurements. 1 in basic alumina column on elution with DCM/Ethyl acetate as solvent hydrolyzed to μ-oxo bridged dimeric molybdenum(V) complex [{Mo^V(O)(TPP)}₂O] (2). 2 shows a interesting triplet state ESR spectrum in the solid state with half field signal suggesting two independent Mo(V) centers in the complex.

Molybdenum has a relevant role in sustainability, owing to its being a relatively Earth-abundant metal and its presence in many biological systems. The broad range of easily accessible oxidation states confers on this metal, among others, a rich redox chemistry. In this review, it has been addressed in terms of the catalytic oxidation of alkenes and a reduction in the electrochemical reactivity of CO₂. The fact that Mo(II) complexes are the most important contributors to both approaches reveals their versatility. Catalytic steps are essential in the synthesis of chemical and pharmaceutical commodities for speeding up reactions. Homogeneous catalysts can be more selective, and their mechanistic issues are better understood, but are difficult to recycle, while heterogeneous catalysts are easier to separate from the products. Combining their advantages is the aim of much of the following work. Historically, alkene oxidation was catalyzed by Mo(VI) complexes or oxides in the presence of an oxygen donor, such as TBHP, but they were often difficult to synthesize and handle. It was found that Mo(II) complexes, easy to prepare and use, could behave as catalyst precursors, being oxidized in the first step to the Mo(VI) active species and continuing the reaction efficiently. The [CpMo^II(CO)₃X] family converted to [CpMo^VIO₂X] and [CpMo^VIO(O₂)X], both possible catalysts, as detected experimentally and confirmed by a mechanistic DFT study. This work addresses the two families of Mo(II) precursors, [Mo(η³-allyl)(LL)(CO)₂X] and [Mo(LL) (CO)₃X₂] (X = halide, LL = bidentate ligand), which are very easy to obtain and modify with suitable linkers to attach them to supports (silica-derived materials) and move from homogeneous to heterogeneous catalysis. Attempts to find out the nature of the active Mo(VI) catalyst have combined experimental and computational approaches and led to the formulation of a binuclear oxido-bridged Mo(VI) complex with terminal halides and the LL ligand, after the loss of the carbon ligands. The homogeneous catalysis studies involved the oxidation of simple substrates, such as cis-cyclooctene, styrene and 1-octene, to multifunctional ones, namely cis-hex-3-en-1-ol, trans-hex-2-en-1-ol, trans-hex-3-en-1-ol, geraniol, S-limonene and R-limonene, to analyze the conversion, selectivity and yield. Enantioselectivity was determined where possible. The same Mo(II) complexes could be immobilized in MCM-41, where they occupied the inner surface of the channels. This confinement, improved toward enantioselectivity by the synthesis of helical channels, led to better (or different) selectivity than using cheaper materials (SiO₂), where the catalyst binds on the outer surface. The heterogeneous catalyst proved to be resistant to leaching and recyclable, exhibiting, for many systems, a higher activity than the corresponding homogeneous catalyst.

The part on electrocatalytic reduction of CO₂ first introduces the intricate reduction path of complexes [Mo^II(η³-2-R-allyl)(x,x′-dmbpy) (CO)₂X] (x,x′ = 4–6; R = H, Me; X = pseudo-halide), which has only recently been resolved using a combination of cyclic voltammetry, IR/UV–Vis spectroelectrochemistry and supporting DFT calculations. The cathodic path bears striking similarity to the well-studied and highly promising family of catalysts based on [Mn(bpy)(CO)₃X]. Seemingly small, but systematic changes in the ligand sphere induce significant changes in the cathodic path, particularly in the reactivity of the primary 1e⁻-reduced state and the proclivity for the formation of the active catalyst, five-coordinate [Mo(η³-2-R-allyl)(x,x′-dmbpy)(CO)₂]⁻ versus deactivating dimerization. Under CO₂, catalytic conversion to CO and formate is observed. The excess of CO produced may convert the catalyst to Mo(0) tricarbonyl complexes lacking the π-bound 2-R-allyl ligand.

The last part of this chapter reviews in depth spectroelectrochemical studies of the [Mo⁰(x,x′-dmbpy)(CO)₄] (x,x′ = 4–6) series of complexes as catalyst precursors for the conversion of CO₂ to CO. A low-energy ECE pathway to the active catalyst was probed by systematic changes to the LL ligand, the donor solvent and the electrode. The best results were observed when a Au cathode was used in conjunction with N-methylpyrrolidone as the solvent. The data have revealed an important synergy between the solvent and the cathode for these catalysts, proving that, with careful, rational control, the Group-6 metals, and Mo in particular, are more promising than previously anticipated. Electrochemical and photochemical reduction of CO₂ are both well-established, independent catalytic routes toward producing value-added chemicals. The potential for any cross-reactivity has, however, scarcely been explored so far. Notably, photochemistry assists the cathodic activation of [Mo(6,6′-dmbpy)(CO)₄] to lower the catalytic overpotential needed to trigger the electrocatalytic reduction of CO2 to CO. Following the complete initial 1e^–-reduction of the parent complex, the key photochemical cleavage of a Mo−CO bond in [Mo(6,6′-dmbpy)(CO)₄]^•– generates the 2e^–-reduced, five-coordinate catalysts, [Mo(6,6′-dmbpy)(CO)₃]^2–, appreciably closer to the initial cathodic wave. Experiments under CO₂ have confirmed the activity of the electrocatalyst under photoirradiation with 365-nm light. This remarkable achievement corresponds to an approximately 500 mV positive shift in the catalytic onset compared to the exclusive standard electrocatalytic activation.

Thiometallic cluster continue to attract great interest not only because of their unusual catalytic activities in biological and industrial processes [1], but also their intriguing optical and electrical properties [2]. Among these physical properties, there is an interest in their optical limiting effect for potential application in protecting optical sensors from laser beams of high intensity. The design and synthesis of new materials with large optical limiting capability represents an active field in modern chemistry, physics and material science [3]. Thus, the synthetic and structural chemistry of heterometallic sulfide clusters containing d¹⁰ copper has been developed rapidly and some interesting Mo-Cu-S clusters with novel structural types have been synthesized [4]. Novel cluster compound MoS₄Cu₄(pz^Me2)₆Cl₂ was synthesized in acetone and its crystal structure has been characterized by X-ray diffraction [Fig.1], infrared, UV-Vis and ¹HNMR spectroscopy.

Mo/Cu functionally graded material (FGM) was fabricated by explosive powder consolidation technique utilizing under-water shock pressure, and microstructural aspects and mechanical properties of as-consolidated and sintered Mo/Cu FGMs were investigated. There were no cracks and no tears in the resultant compact in which continuously compositional change was achieved. The full densification and homogeneous structure were obtained by sintering at 1273 K for 14.4 ks. From the results of thermal shock and cycling test, it is concluded that the Mo/Cu FGM produced in the present study has superior thermal shock resistance and relaxation ability of thermal stress.

The promising role of structural ceramics in many useful devices and structures has in turn led to realization of the importance of developing ceramic joining techniques, which is perhaps one of the most important areas in the ceramic field for research and development. The increasing demand for joining ceramics arises from: a) fabrication of ceramic components of complicated shape or large size in one piece can be very expensive and in some cases impractical, and b) in many cases, because of the limitation of ceramics resulting from poor impact properties and lack of tensile ductility they have to be attached to metals, which must withstand stresses or temperature gradients too great for ceramics. This paper covers methods and problems of joining of ceramics with emphasis on brazing technique. This also includes examples of joining silicon nitride ceramic to itself and to molybdenum.

In comparison to the Cr-added hot-dip galvannealed DP steel with fully-recrystallized and equiaxed grains, the Mo-added hot-dip galvannealed DP steel shows small quasi-polygonal grains. The Mo-added hot-dip galvannealed DP steel shows better mechanical properties than Cr-added hot-dip galvannealed DP steel. The increased yield and tensile strengths in the Mo-added steel hot-dip galvannealed DP are mainly caused by grain refinement and martensite transformation. It is recognized that Cr and Mo suppress cementite transformation in the steels, but Cr is not as strong as that of Mo. Compared with Cr-steel, discontinuous yielding in the stress-strain curve did not appear in the Mo-steel because the martensite volume fraction was greater than the 5%.