Formation of bronzes during temperature-programmed reduction of MoO3 with hydrogen - an in situ XRD and XAFS study

The temperature-programmed reduction of molybdenum trioxide (MoO3) from 300 to 773 K in 50 vol% hydrogen in He (10(5) Pa) at different heating rates ( 0.1. 0.2, and 5 K/min) was investigated by in situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). At heating rates of similar to0 .1 and similar to0.2 K/min, the formation of the molybdenum bronze H0.34MoO 3 was observed by in situ XRD in the early stage of the reduction of MoO3. At a heating rate of 5 K/min, the formation of a more disordered bronze (Hx MoO3 with x similar to0.07) prior to the detection of the product phase MoO 2 was observed by in situ XAFS. In both studies, the consumption of the bro nze was found prior to the complete reduction of MoO3. A simplified mechani sm for the temperature-programmed reduction of MoO3 in hydrogen is proposed that includes: (i) incorporation of H-2 in the MoO3 bulk and formation of a more or less ordered bronze, (ii) consumption of the bronze and formation of nucleation site for MoO2. (iii) nucleation of MoO2 and nuclei growth. ( C) 2001 Elsevier Science B.V. All rights reserved.