Heat treatment of molybdenum under vacuum conditions

Oxide coverage of molybdenum plays an important role in several applications, for example in lighting industry. Surface conditioning procedures were simulated in an XPS instrument by in situ heat treatments while monitoring the surface composition and changes in the chemical states of molybdenum. Heat treatments have been made at different temperatures between 435 and 690 °C under vacuum conditions. It has been observed that during heating the molybdenum test samples the native MoO3 layer on the surface dissociates, and a layer of suboxides forms on the surface. This layer hinders the further reduction of the surface, thus reaction speed decreases after the initial phase. It has been established that in the second phase of the heat treatment the activation energy of the process is 1.1 ± 0.2 eV. Reduction of MoO3 to elemental molybdenum runs through two intermediate states: Mo6+ → Mo5+ → Mo4+ → Mo0.