The topotactic mechanism for the reduction of MoO3 to MoO2 has been well-su
pported by experiments, though the detection of Mo4O11 during the process h
as been a challenge to the notion of direct conversion. In this study, high
quality in-situ Neutron Powder Diffraction (NPD) has been used to monitor
phase composition during the reduction Of MoO3 in a flow of hydrogen at 550
degreesC. Three phases have been detected: MoO3, MoO2, and Mo4O11. Multiph
ase Rietveld refinement has been used to obtain concentration profiles for
all three oxides throughout the reduction process. Examination of these pro
files reveals the following: (1) a higher content, initially, of MoO2 than
Mo4O11; (2) a period of constant MoO2 content; (3) an induction period of M
o4O11 formation; and (4) a resumption in growth of MoO2 but with different
unit cell dimensions.