Enhanced oxygen storage capacity of Ce_0.65Hf_0.25M_0.1O_2-δ (M=rare earth elements): Applications to methane steam reforming with high coking resistance

To develop efficient materials possessing high oxygen storage capacity (OSC) for a number of catalytic applications, ceria-hafnia based metal oxides, Ce_0.65Hf_0.25M_0.1O_2-δ, (CH-M, M=Tb, Sm, Nd, Pr, and La), were prepared by the EDTA-citrate method. The structural and textural properties of the as-synthesized materials were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analyses, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and high resolution transmission electron microscopy (HRTEM). These analyses indicated that the doping of the rare earth elements could potentially induce enhanced OSC compared to the undoped CeO₂-HfO₂ solid solution, and particularly, the CH-Pr, CH-La, and CH-Tb materials were suggested to possess greater OSC than other CeO₂-HfO₂ based solid solutions. The redoxability and size of the dopants were proposed to play a pivotal role in enhancing the OSC of the as-prepared materials. Soot oxidation experiments were further performed to examine the relative OSC of the ceria-hafnia based solid solutions and revealed that soot oxidation activities increased in the order of CH-Pr>CH-La>CH-Tb>CH-Nd>CH-Sm>CH (undoped material). To elucidate the effect of these materials on suppressing carbon coking, Ni/CH and Ni/CH-Pr catalysts were synthesized by a solvothermal method and their catalytic activities towards steam reforming of methane with steam to carbon ratios of 1-2 at 700°C were carried out. The Ni/CH-Pr catalyst exhibited superior activity and stability over Ni/CH for the steam reforming of methane, which likely resulted from the enhanced oxygen mobility of the CH-Pr support.