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

Abstract

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 (VO) by changing the doping content. The presence of VO 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 (SmxCe1–xO2−δ, x = 0–1) are investigated to explore the doping effect of Sm3+ on the ceria-based nanoctrystals. The SmxCe1–xO2−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 VO concentration and catalytic property of SmxCe1–xO2−δ are unsynchronized. The lattice structures of SmxCe1–xO2−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 VO 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 VO.