Kinetic Study of Vapor-Phase Preparation of Orthorhombic Molybdenum Trioxide

Abstract

A new kinetic approach to study the vapor-phase preparation of orthorhombic molybdenum trioxide (MoO3) has been established in this work. Using a simple experimental setup, the vaporization and condensation of MoO3 can be investigated with sufficient oxygen background to ensure the stoichiometry of the compound. The empirical condensation rate of the MoO3 obtained in the temperature range 642−660 °C is RE = 1.20 × 10-3ΔP 2.07, where ΔP is the partial pressure difference of MoO3 between the source and condensed crystals. On the basis of the observed crystal morphology and XRD results, a more precise MoO3 condensation kinetic model has been developed to explain the layer-by-layer growth of orthorhombic structure. The staged growth phenomena can be interpreted by this mechanistic model, which correlates the condensed MoO3 weight data to the experimental setup. The driving force of the condensation growth is investigated with respect to the current experimental conditions. For the MoO3 partial pressure ratios of 3.86−2.97 between the source and condensed MoO3, free energy changes determined for the growth are in the range −10.7 to −8.8 kJ/mol.