Study on the structure and formation mechanism of molybdenum carbides

The synthesis of high-surface-area molybdenum carbides has been studied by the temperature-programmed carburization of molybdenum trioxide MoO3. The feedstocks used were mixtures of methane and ethane with hydrogen. The solid reaction products were characterized at selected intervals using thermogravimetric analysis differential scanning calorimetry (TGA-DSC), surface area measurement (BET), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The gaseous products of the carburization process were monitored using a gas chromatograph equipped with a mass spectrometer (GC-MS). The structural properties of the product carbides are shown to depend on the conditions of synthesis. The C2H6/H-2 feedstock gave the highest-surface-area material. The presence of H-2 in the feed mixture reduced the amount of amorphous carbon deposited an the molybdenum carbide material. The surface area was found to increase most rapidly during the initial H-2-reduction stage. Initially, the MoO3 is reduced to form MoO3-x. This material has structural defects, which can account for a decrease in the average particle size and an increased porosity, resulting in an increased surface area. During the carburization process, three phase transitions are observed. At higher temperatures, the rate of deposition of graphitic and amorphous carbons derived from CH4 or CO is much greater than the rate of hydrogenation of the deposited carbon, resulting in the formation of surface graphitic carbon.