Enhanced redox stability of tittanium doped molybdenum dioxide

Qian He

Molybdenum Dioxide (MoO2) is a transition metal oxide which can be used not only in heterogeneous catalysis, but also as an anode material in lithium-ion batteries and internal-reforming solid oxide fuel cells. MoO2 has shown high catalytic activity, excellent coking resistance and sulfur tolerance in the partial oxidation of various liquid hydrocarbons. Moreover, MoO2 also has shown metallic-like electrical conductivity and good ionic conductivity. However, the drawback of this material, and others that can exist in multiple valence states, is its limited phase stability range. The risk of being transformed to volatile MoO3 or Mo2C is a challenge in the use of MoO2. Therefore, the starting point of this work was to improve the redox stability of MoO2 by doping with redox stable TiO2. Solution-based method combined with hydrothermal synthesis was initially used to develop Ti4+ doped MoO2 (<100 nm). Scale-up direct hydrothermal synthesis was developed to prepare undoped MoO2 (<10 nm) and Ti4+ doped MoO2 (10~20 nm). 3 at.% Ti doping has reached solid solubility limit of MoO2. Lattice parameters of MoO2 slightly changed upon doping. Annealed (up to 1000 ºC) undoped and Ti4+ doped MoO2 still remain monoclinic structure. It was confirmed that doping with Ti4+ can kinetically enhance the redox stability of MoO2 in H2 and O2 rich environment respectively. Furthermore, it was found that addition of Ti4+ in MoO2 can enhance CH4 activation which could be related to the presence of higher surface density of strong Lewis acid sites and stronger Mo-O bonds compared to that of the undoped MoO2. The undoped MoO2 was carburized to thermodynamically stable beta-Mo2C, whereas Ti4+ doped MoO2 transformed to alfa-MoC0.54. The carburization of Ti4+ doped MoO2 showed lower amount of CO production and higher amount of oxygen inclusion in the final carbide compared to undoped MoO2, indicating Ti4+ doping could decrease lattice oxygen mobility of MoO2. Therefore, doping with Ti4+ could kinetically stabilize lattice oxygen of MoO2 in H2, O2, and C-rich environment.

Enhanced redox stability of tittanium doped molybdenum dioxide

Files and links (1)

Abstract

Metrics

Details