In Situ XAS and XRD Studies on the Formation of Mo Suboxides during Reduction of MoO3

Abstract

Reduction of MoO3 with hydrogen (5−100 vol %) in the temperature range from 573 to 823 K was studied by in situ X-ray diffraction and X-ray absorption spectroscopy. The experiments performed focused on elucidating phase composition and evolution with time under isothermal as well as temperature-programmed reduction conditions. At reaction temperatures below 698 K, the reduction of MoO3 to MoO2 is a one-step process without the formation of crystalline intermediates. At reduction temperatures above 723 K and H2 concentrations higher than 10 vol %, Mo metal is the final product of the reduction of MoO3. In addition, at temperatures higher than 698 K, the formation of Mo4O11 was observed. However, Mo4O11 is not an intermediate in the reduction of MoO3 but is being formed in a parallel reaction from MoO3 and MoO2 at temperatures above 698 K. It is shown that Mo4O11 can be obtained from a reaction of MoO3 and MoO2 at temperatures above 773 K, affording the same phase ratio of monoclinic and orthorhombic Mo4O11 as the reduction of MoO3 with hydrogen. Quantitative XRD analysis reveals a sigmoidal shape of the evolution of the MoO3 and MoO2 phases during reduction of MoO3 and an increase in the crystallite size of the phases present. This Oswald ripening indicates that a nucleation−growth kinetic mechanism governs the reduction of MoO3 under the conditions studied. The results presented in this work clearly demonstrate the potential of a combined application of in situ XRD and XAFS to reveal phase composition and kinetics of solid-state reactions.