In situ X-ray analysis of MoO3 reduction

The reduction of MoO3 to MoO2 under hydrogen/argon atmosphere (5 vol. % H-2/95 vol. % Ar) in the temperature range 323 K...623 K was studied in situ by means of wide-angle X-ray scattering. It has been found that the starting material, MoO3, consists of two different orthorhombic MoO3 phases A and B with nearly the same structure parameters. The phase A (fraction of 37.1 wt%) describes the larger crystallites whereas the phase B (fraction of 62.9 wt.%) describes the smaller crystallites. Under the reduction to monoclinic MoO2 phase during the heating, the thermal evolution of the phase fractions is different. A conclusion is drawn that MoO2 is formed preferably in big crystallites. About 10 Wt % of MoO2 has been found to form at 623 K resulting in about 69 wt. % after cooling to room temperature followed by holding in Ar/H-2 atmosphere about 24 h. Additionally, about 4.4 wt. % of the Mo4O11 oxide probably formed in large crystallites was detected in the reduced powder after the cooling.