In situ-Raman studies on thermally induced structural changes of porous MoO3 prepared in vapor phase under He and H2

Abstract The structural transformations induced by heating on MoO 3 mesoporous samples grown in vapor-phase under helium and hydrogen were studied by in situ Raman spectroscopy. The samples were continuously irradiated by a He–Ne laser of 5.5 mW for 105 min and Raman spectra were dynamically acquired every 5 min, in order to evaluate the laser effects. The He-grown sample did not undergo structural transformations due to the laser irradiation while the H 2 -grown sample underwent changes after just 10 min of irradiation. On the other hand, each type of sample was heated in air from room temperature up to 450 °C using a heating rate of 5 °C min −1 and Raman spectra were recorded each 25 °C. The He-grown sample remained structural unchanged up to 250 °C revealing a high temperature state of its amorphous matrix whereas the H 2 -grown sample exhibited changes around 70 °C. These changes were attributed to the crystallization onset of its amorphous matrix to β-MoO 3 . However, this same transition was detected between 250 °C and 275 °C for the He-grown sample. A second transformation was detected in both samples in a temperature range more similar associated to the transition of the formed β-MoO 3 phase to α-MoO 3 . After the heat treatment, the oxide of both samples was slightly reduced, as revealed by XPS analysis. The metastability at low temperatures of the amorphous phase of the H 2 -grown sample could be associated to release of hydrogen trapped inside the compound and to the break of weak bonds between Mo and OH groups. At high temperatures, above 225 °C, the rapid conversion to β-MoO 3 of the amorphous matrix of the H 2 -grown sample occurred due to the dehydration of the sample.