Ceria-samarium binary metal oxides: A comparative approach towards structural properties and soot oxidation activity

Influence of Surface Properties of Nanostructured Ceria-Based Catalysts on Their Stability Performance

As the poor cycling stability of CeO₂ catalysts has become the major obstacle for applications of diesel particulate filters (DPF), it is necessary to investigate how to reduce their structural and compositional changes during soot oxidation. In this study, different ratios of Samarium (Sm) were doped into the lattice of CeO₂ nanoparticles to improve the catalytic performance as well as surface properties. The stability was investigated by recycling the catalyst, mixing it with soot again, and repeating the thermogravimetric analysis (TGA) tests seven times. Consistent observations were expected for more cycles. It was found that doping 5%, 10%, and 20% samarium into the CeO₂ lattice can improve the catalyst stability but at the cost of losing some activity. While the catalyst became more stable with the increasing Sm doping, the 10% Sm-doped catalyst showed the best compromise between stability and activity. Ce³⁺ and O_α were found to play important roles in controlling catalytic soot oxidation activity. These two species were directly related to oxygen vacancies and oxygen storage capacity of the catalyst. Sm-doped catalysts showed a minimized decrease in the Ce³⁺ and O_α content when the fresh and spent catalysts were compared.

Finely Tuned Structure and Catalytic Performance of Cerium Oxides by a Continuous Samarium Doping from 0 to 100

Cerium oxides are prevalent catalytic materials, and the lanthanide-doped ceria have attracted special interest since it is easy to tune the concentration of oxygen vacancies (V_O) by changing the doping content. The presence of V_O is generally believed to favor a catalytic reaction, but the formation of dopant-vacancy associations at a high doping concentration might produce an adverse effect. Herein, evolutions of the structural properties and catalytic performances in Sm-doped ceria (Sm_xCe_1-xO_2-δ, x = 0-1) are investigated to explore the doping effect of Sm³⁺ on the ceria-based nanoctrystals. The Sm_xCe_1-xO_2-δ films composed of nanoctrystals are elaborately prepared via electrodeposition under mild conditions to prevent phase separation. A combination of studies, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman, photoluminescence (PL), and methanol electro-oxidation (MEO) reaction, reveals that variation trends for the V_O concentration and catalytic property of Sm_xCe_1-xO_2-δ are unsynchronized. The lattice structures of Sm_xCe_1-xO_2-δ nanoctrystals undergo a smooth and steady transition from F-type (fluorite CeO₂) to C-type (cubic Sm₂O₃) with the increase of Sm³⁺ contents. The structural transition occurs in the Sm³⁺ concentration range of 64-84%, within which the V_O concentration reaches the maximum as well. However, the optimal MEO performance is obtained at a relatively lower doping concentration of 24%. Above this concentration, significant dopant-vacancy associates are observed by XRD, Raman, and PL characterizations. It is inferred that, for these ceria-based nanocrystals, the dopant-vacancy association induced by high doping would impede the growth of catalytic performance despite all the benefits of V_O.