Molybdenum dioxide (MoO2) is a mixed conductivity oxide that has been found to exhibit high activity as well as coking resistance and sulfur tolerance for the partial oxidation of liquid fuels such as jet fuel and gasoline. However, stability issues prevent the use of MoO2 for long periods of time. This problem seems to be related to the phase transition resulting from the interaction between the oxide and the carbon-containing fuel, which results in the formation of a carbide phase. Different approaches are discussed in this work to increase the stability of the oxide phase such as doping and the use of supporting materials. Thus, titanium doping showed to be able to broaden the oxygen-to-carbon ratio operating range of MoO2 for the partial oxidation of long-chain hydrocarbons by increasing the redox stability. The modification in the structure causes the hydrogen reduction mechanism to change from a three-dimensional nuclei growth to a three-dimensional hydrogen diffusion limited model.
Unsupported MoO2 nanoparticles synthesized at 180°C using a hydrothermal approach exhibit loss of surface area when exposed for long periods of time to typical reforming temperatures (~850C). However, the particle size of MoO2 nanoparticles was found to become stable when supported on silica through a synthesis process involving a sol gel approach. Compared to large particle size MoO2, the selectivity to syngas of silica-supported MoO2 seems to be significantly enhanced, as deduced from the results obtained using dodecane as probe molecule. Long-term activity tests using methyl oleate as biodiesel surrogate showed a stable performance for at least 200 h, using an O2/C=0.75, producing a H2 yield of 80% and 100% conversion.
Mixed conductivity and high reforming activity, as well as significant coking resistance and sulfur tolerance displayed by this metal oxide, suggest MoO2 as a potential candidate as anode material for solid oxide fuel cells. Conventional materials such as nickel–yttrium-stabilized zirconia (Ni–YSZ) cermet anodes operating with complex liquid hydrocarbon fuels display severe carbon formation on the Ni surface and sulfur poisoning. Molybdenum dioxide (MoO2)-based anodes with an optimum microporous structure were able to produce a maximum power density over 3 W cm−2 at 750 °C under the direct feeding of n-dodecane fuel mixture. The cell resisted the carbon (coke) formation for over 24 h of continuous cell testing and was also resistant to deactivation with 500 ppm of sulfur in the fuel stream. Using this anode, a fuel cell running with commercial gasoline demonstrated a power density >3.0 W cm−2 at 0.6 V. Over a 24 h period of operation, the open cell voltage remained stable at ∼0.9 V. At the cell voltage of 0.6 V, its current density dropped over the first 7 h to a value of ∼3.0 A cm−2, where it stayed for the remaining 17 h of the test with a minor fluctuation. Power density of ∼2.0 W cm−2 at 0.6 V was still measured after 24 h on stream with a continuous feed of gasoline. Scanning electron microscopy (SEM) examination of the anode surface pre- and post-testing showed no evidence of coking, which hints at the reason for the observed stability under such harsh cell operating conditions. Thus, a SOFC using a MoO2-based anode has potential for generating electrical power from liquid fuels for future hybrid electric vehicles.
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