Molybdenum trioxide
Molybdenum trioxide (MoO₃) is the most stable and common oxide of the transition metal molybdenum, existing as an odorless solid that ranges in color from white to pale yellow.[1] It features a layered crystal structure composed of distorted MoO₆ octahedra sharing edges and vertices, forming double layers with van der Waals gaps between them, primarily in its stable orthorhombic α-phase, though a metastable monoclinic β-phase also exists.[2] With a molecular weight of 143.95 g/mol, a density of 4.69 g/cm³, a melting point of 795 °C, and a sublimation point around 1155 °C, it exhibits slight solubility in water (approximately 1.07 g/L at 18 °C) and behaves as an n-type semiconductor with a band gap of about 3 eV.[1][2] Commercially, molybdenum trioxide is produced on a large scale by the roasting of molybdenite (MoS₂) ore in air, which oxidizes the sulfide to the trioxide, followed by purification through sublimation or other methods such as ignition of molybdenum metal, sulfides, or lower oxides.[1] This compound serves as a key intermediate in the production of pure molybdenum metal via hydrogen reduction and is essential in various industrial sectors, including as an alloying agent in stainless and high-strength steels to enhance corrosion resistance and strength, and in superalloys for aerospace applications.[1][2] In catalysis, molybdenum trioxide is a vital component in processes like petroleum hydrodesulfurization, methanol oxidation to formaldehyde, and epoxidation reactions, owing to its ability to undergo topotactic redox chemistry and intercalate guest species.[2] It also finds use in ceramics for pigments and enamels, as a smoke suppressant in polyvinyl chloride (PVC), and in emerging applications such as electrochromic devices and lithium-ion battery electrodes due to its electrochemical properties.[1][2] Safety considerations include its classification as a possible carcinogen (IARC Group 2B) and its irritant effects on the eyes and respiratory tract, with occupational exposure limits set at 5 mg/m³ (OSHA PEL) and 0.5 mg/m³ (ACGIH TLV).[1]Properties
Physical properties
Molybdenum trioxide (MoO₃) is typically observed as a white to light yellow crystalline powder or as needle-like crystals, with the color shifting to yellow at elevated temperatures.[1] This appearance corresponds to its orthorhombic crystal structure, which consists of layered sheets.[1] The compound exhibits a density of 4.69 g/cm³ for its orthorhombic form at 25°C.[1] It has a melting point of 795°C, above which it begins to decompose rather than fully liquefy.[1] The boiling point is approximately 1155°C, though MoO₃ primarily sublimes under standard conditions rather than boiling.[1] MoO₃ is slightly soluble in water (solubility ≈ 1 g/L at 20 °C) and most organic solvents, but it shows slight solubility in alkaline solutions such as alkali hydroxides.[1] The material is hygroscopic, readily absorbing atmospheric moisture to form the dihydrate MoO₃·2H₂O, which is bright yellow and loses water upon heating.[3] The specific heat capacity of solid MoO₃ is described by the Shomate equation for temperatures from 298 K to 1700 K, yielding values around 0.51 J/g·K at 298 K.[4] Thermal conductivity for bulk orthorhombic MoO₃ is approximately 5.3 W/m·K at room temperature, though it is anisotropic due to the layered structure.[5] Regarding vapor behavior, MoO₃ has low vapor pressure under ambient conditions but evaporates effectively under vacuum at around 400–500°C, with a vapor pressure reaching 10⁻⁴ Torr near 900°C.[6]| Property | Value | Conditions |
|---|---|---|
| Density | 4.69 g/cm³ | Orthorhombic, 25°C |
| Melting point | 795°C (decomposes >800°C) | - |
| Sublimation point | ~1155°C | 760 mmHg |
| Water solubility | ≈ 1 g/L | 20°C |
| Specific heat capacity | ~0.51 J/g·K | 298 K |
| Thermal conductivity | ~5.3 W/m·K | Bulk, room temperature |