Nanostructured Molybdenum Carbide: Sonochemical Synthesis and Catalytic Properties

The preparation of nanophase materials has been the focus of intense study in materials science.1,2 A variety of chemical and physical preparative methods have been applied to produce materials with nanometer structure, including metal evaporation,3 decomposition of organometallic compounds,4 and reduction of metal salts.5,6 Sonochemical decomposition of transition metal carbonyl compounds has also been proven to be a useful technique to generate nanophase transition metals.7,8 Recently, molybdenum and tungsten carbides have been examined as heterogeneous catalysts because their activity is often similar to that of the platinum group metals.9-11 For catalytic applications, high surface area materials are generally needed; the preparation of interstitial carbides of molybdenum and tungsten with high surface areas, however, is very difficult. We present here a simple sonochemical synthesis of nanostructured molybdenum carbide from the ultrasonic irradiation of molybdenum hexacarbonyl. In addition, we have examined the catalytic activity and selectivity of these materials for the dehydrogenation of alkanes. The chemical effects of ultrasound arise from acoustic cavitation: the formation, growth, and implosive collapse of bubbles in a liquid.12,13 The collapse of bubbles generates localized hot spots through adiabatic compression or shock wave formation within the gas of the collapsing bubble. This local heating produces a wide range of high-energy chemistry. The conditions formed in these hot spots have been experimentally determined, with transient temperatures of ∼5000 K, pressures of∼1800 atm, and cooling rates in excess of 1010 K/s.14,15 Using these extreme conditions, we have explored a variety of applications of ultrasound to materials chemistry.16 A slurry of molybdenum hexacarbonyl (1 g in 50 mL of hexadecane) was sonicated with a high-intensity ultrasonic horn (Sonic and Materials, model VC-600, 0.5 in Ti horn, 20 kHz, 100 W cm-2) at 90 °C for 3 h under argon to yield a black powder. The powder was filtered inside an inert atmosphere box (Vacuum Atmospheres, <1 ppm O2), washed several times with purified, degassed pentane, and heated at 100 °C under vacuum. X-ray powder diffraction17 (XRD) showed extremely broad peaks centered at a d spacing of 2.4, 1.5, and 1.3 A (Figure 1), which did not match body-centered cubic (bcc) lines of molybdenum metal. After the heat treatment at 450 °C under helium flow for 12 h, sharper peaks in the XRD were observed at d spacing values of 2.39, 1.49, and 1.27 A which accurately correspond to face-centered cubic (fcc) molybdenum carbide, Mo2C (Figure 1). The synthesis of Mo2C is particularly prone to substantial oxygen contamination.9 Even after heat treatment at 450 °C under helium, oxygen was still present at about 4 wt %. Since the presence of oxygen could poison the catalytic activity, it was removed prior to catalytic studies by heating in a flowing 1:1 CH4/H2 mixture at 300 °C for 1 h, then at 400 °C for 1 h, and finally at 500 °C for 48 h. The flow rate of the CH4/H2 mixture was 27.5 cm3 (STP)/min. After this carburization, excess carbon, hydrogen, and oxygen had been largely removed. The elemental analysis results showed the sample was very pure (theoretical for Mo2C, 94.11 % Mo, 5.89 % C; exptl., 93.86 % Mo, 5.68 % C, 0.08 % H, 0.06 % N), which corresponds to a stoichiometry of Mo2C0.97. The XRD was essentially unchanged by carburization.