Chapter 3 - The Oxide Chemistry of Molybdenum

The growing energy demands of daily life necessitate innovative solutions. Green approaches, which are universally accepted, advocate the use of renewable energy sources in conversion and storage systems. Research in nanomaterials has continually advanced, with the structures and shapes of these materials diversifying for a broad spectrum of applications. Supercapacitor-based systems are emerging as leading alternatives in contemporary trends among the various energy storage devices. In particular, transition metal dichalcogenides (TMDCs) are surfacing as novel supercapacitive electrode materials, thanks to ongoing research efforts. This review focuses on the technological development, characteristics, and properties of various transition metal chalcogenides used for supercapacitors. Mainly, molybdenum chalcogenides have been observed to be an emerging member of the TMDCs family that recently gained considerable attention for supercapacitor applications. This study briefly describes different synthesis methods and their parameters for molybdenum chalcogenide nanomaterials. The electrochemical properties like electrochemical impedance spectroscopy (EIS), specific capacitance (SC), cycling stability, energy density, and the power density of molybdenum chalcogenide-based supercapacitors have been discussed thoroughly with appropriate literature support.

Proton conductors Mo-Eu-Zr mixed oxide systems were synthesized and further mixed with a conventional Pt/CeO₂/Al₂O₃ catalyst to develop a highly efficient water-gas-shift (WGS) catalyst. The designed catalyst, once structured, allows reach the equilibrium conversion at medium temperatures (∼350 °C) at 80 L·g⁻¹ h⁻¹ space velocity. The ability of the proton conductor to maintain an elevated water concentration at the metal-support interface by Grotthuss’ mechanism boosts the catalytic activity in WGS reaction.

The Mo-containing proton conductor is extensively characterized allowing to establish the formation of molybdenum oxide phases nucleating on top of the Eu sites in Eu-Zr oxide solid solution. [MoO₄]²⁻ to [Mo₇O₂₄]⁶⁻ clusters nucleates at low Mo contents resulting in a α-MoO₃ layer on increasing its content. In presence of H₂, Mo-bronzes are formed from ∼200 °C enhancing water concentration at the surfaces and boosting the catalytic activity in the WGS reaction. These results pave the way for developing lower volume WGS reactors.

A green and environmentally friendly route uses air to fabricate the hollow structured nanocomposites, which successfully self-assembled the uniform MoO₃ nanodots (denote as MoO₃ NDs, about 1.8 nm) into the mesoporous SiO₂ hollow nanosphere (denote as MoO₃/SiO₂ HN) in a homogeneous solution. The new type of NDs-supported nanocomposite is fully characterized with different techniques (XPS, NH₃-TPD, EPR, EIS and UV–vis). The result is that the small size of NDs enhances the number of active sites exposed on the surface of the carrier or mesoporous channels, and the novel catalyst of MoO₃/SiO₂ HN exhibits rapid electron transfer capabilities and abundant O-vacancies. Besides, the possible mechanism of desulfurization reaction has also been studied in detail. It is found that the O-vacancy of the catalyst facilitates the adsorption of H₂O₂ and the rapid electron transfer of NDs promotes decomposition of H₂O₂ to produce more active oxygen (O^-), thereby improving the catalytic oxidation desulfurization performance.

The oxidation mechanism of a Mo–Si–B alloy in two different oxygen partial pressure ranges was investigated between 820 and 1200 °C. Oxygen partial pressures between 10⁻¹⁹ and 10⁻¹² bar were applied in order to suppress Mo oxide formation. Weight gain kinetics were determined resulting from simultaneous external and internal oxidation. Silica scale formation was found to lead to a droplet shape because of the high evaporation rates of B₂O₃ and limited wetting of the silica. In the oxygen partial pressure range 10⁻⁶–10⁻⁴ bar Mo–Si–B alloys suffer from severe degradation due to continuous formation of volatile MoO₃. Catastrophic oxidation was observed as a consequence of the formation of a highly porous and non-protective silica scale.

Transparent and colourless α-MoO₃ crystal platelets have been prepared in vapour phase from various alkali-metal-containing molybdenum sources. With XRD technique, it is evident that the α-MoO₃ is the only crystalline phase in the as-grown crystals. Furthermore, FTIR and elemental analysis results reveal that alkali metals are not intercalated in the layered crystal lattice of α-MoO₃. Factors that lead to the success of this purification growth are identified. In view of its simplicity, this method can be efficiently used for laboratory preparation of high quality α-MoO₃ crystals from low-grade molybdenum sources.

Platelet crystals of molybdenum trioxide have been grown by a new simple vapour phase method using a purified air at normal atmospheric pressure. The grown crystals are transparent and colourless. Using the XRD and FTIR methods, it is confirmed that platelet crystals have layered structure of α-MoO₃ (orthorhombic). The nucleation-growth of crystal can be described by a two-dimensional layer-by-layer mechanism, in view of presences of ledges, kinks, terraces, and islands on {0 1 0} surfaces. Morphological study indicates that rates of planar growth along axes of the crystal follows the hierarchy {0 0 1}>{1 0 0}>{0 1 0}. Four types of boundary planes are found along 〈0 0 1〉 directions: {0 0 1}, {1 0 1}, {1 0 2}, and {2 0 1}. Defect distribution has been investigated with 0.1 M NaOH. Crystallographic symmetry and layered structure of α-MoO₃ have been verified by the pyramid-like etch pits. The etching rate on {1 0 0} planes is slightly faster than that on the {0 0 1}. Technical features of the current growth setting are also addressed.