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

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Reduction of MoO 3 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 MoO 3 to MoO 2 is a one-step process without the formation of crystalline intermediates. At reduction temperatures above 723 K and H 2 concentrations higher than 10 vol %, Mo metal is the final product of the reduction of MoO 3 . In addition, at temperatures higher than 698 K, the formation of Mo 4 O 11 was observed. However, Mo 4 O 11 is not an intermediate in the reduction of MoO 3 but is being formed in a parallel reaction from MoO 3 and MoO 2 at temperatures above 698 K. It is shown that Mo 4 O 11 can be obtained from a reaction of MoO 3 and MoO 2 at temperatures above 773 K, affording the same phase ratio of monoclinic and orthorhombic Mo 4 O 11 as the reduction of MoO 3 with hydrogen. Quantitative XRD analysis reveals a sigmoidal shape of the evolution of the MoO 3 and MoO 2 phases during reduction of MoO 3 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 MoO 3 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.