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 MoO3 mesoporous samples grown in vapor-phase under helium and hydrogen were studied by in situ Raman spectroscopy. The samples were continuously irradiated by a HeeNe 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 H2-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 H2-grown sample exhibited changes around 70 C. These changes were attributed to the crystallization onset of its amorphous matrix to b-MoO3. 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 b-MoO3 phase to a-MoO3. 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 H2-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 b-MoO3 of the amorphous matrix of the H2-grown sample occurred due to the dehydration of the sample.