Dynamic Phenomena during Reduction of α-NiMoO4 in Different Atmospheres:  In-Situ Thermo-Raman Spectroscopy Study

A study by in-situ thermo-Raman spectroscopy was carried out to understand the structural changes occurring during the reduction of α-NiMoO4 in different atmospheres (viz., H2, CO, and C3H6) at different temperatures and time. In addition, the dynamics of α-NiMoO4 during reduction/reoxidation were also investigated. The products from reoxidation of reduced α-NiMoO4 in different atmospheres were analyzed by XRD. During the reaction of H2 with α-NiMoO4 at 400 °C a new band was detected at 455 cm-1, which is characteristic of reduced surface molybdenum oxide species. Thus, the formation NiMoO3 and/or Ni2Mo3O8 suboxide intermediates was suggested. On the other hand, reversible H2 reduction/O2 reoxidation dynamics was observed, where the main Raman features of H2-reduced α-NiMoO4 were restored completely after reoxidation by oxygen. On the contrary, during the reaction of C3H6 with α-NiMoO4 the formation of a stable suboxide was difficult to detect, and irreversible C3H6 reduction/O2 reoxidation dynamics was suggested as the extensive heating of the C3H6-reduced sample in 10% O2/He for 2 h at 400 °C did not restore the Raman features of the α-NiMoO4 observed prior to reduction. No significant change was observed in the Raman spectra of α-NiMoO4 during heating in a flow of 10% CO/He, except for the steady decrease in the intensities of all bands with increasing temperature. This may be due to the fact that at high temperatures CO decomposed and the produced carbon deposited on the sample surface.