Homogeneously Dispersed Ceria Nanocatalyst Stabilized with Ordered Mesoporous Alumina

As one of the most important functional rare earth oxides, ceria (CeO2) has been widely applied in catalysis, fuel cells, optical materials, gas sensors, and so forth. In particular, nanostructured ceria plays an active role in catalysis applications because of its reduced dimensions, increased relative surface area, highly active facets, large number of active sites, and changeable valence state. In the past decade, the controlled synthesis of ceria nanocrystals has become one of the essential topics in rare earth materials science, since high selectivity and activity can be achieved by size and morphology design. One of the well-known cases is the role of CeO2 in CO oxidation. In this case, traditional bulk ceria materials had been reported to be inadequate for CO oxidation as a catalyst support. However, as shown by Corma et al. the activity for CO oxidation increased by two orders of magnitude when the particle size of ceria decreased to the nanosize region. Hydrothermal, solvothermal, and thermolysis approaches, all of which are based on solution-phase methods, are widely utilized for nanostructured ceria synthesis. Recent examples showed that sub-10-nm ceria can be synthesized using capping agents. Using oleic acid as the stabilizing agent, Gao and coworkers obtained monodisperse ceria nanocubes with an average size of approximatley 4 nm. However, an obstruction for further application lies in that CeO2 nanoparticles with a size smaller than 5nm tend to aggregate during thermal treatments, forming secondary large particles, and thus, the active sites decrease rapidly owing to reduced surfaces. Therefore, up to now, the synthesis of thermally stable ceria nanoparticles with a uniform small size still remains as a challenge. In the case of catalysis reaction, catalyst deactivations caused by sintering of catalysts at high temperature are very common, which may hinder their further industrial applications. Confinement effect can be a solution to address this tough problem. Materials with different nanostructures, especially those that have pores or hollows, are ideal candidates to provide confined microenvironments. With ordered channels of 2–50nm, mesoporous structured materials are very suitable for this purpose. Current methodologies for the assembly of metal or metal oxide nanoparticles in mesoporous materials include, for example, conventional incipient wetness impregnation, post-grafting, and metal–organic chemical vapor deposition. Somorjai et al. incorporated Pt nanocrystals into SBA-15 silica during hydrothermal synthesis. The Pt particles were observed to be located within surfactant micelles during silica formation, which led to their dispersion throughout the silica structure. Bao and co-workers reported an in situ autoreduction route for the fabrication of monodisperse silver nanoparticles on silica-based materials. Features of the narrow channels in hexagonal mesostructures (for example MCM-41, SBA-15) being utilized, one-dimensional nanomaterials such as nanorods or nanowires can be anticipated. Gold nanowires have been reported in the channels of mesoporous SBA-15 by hydrogen flow reduction, electroless reduction, and seed-mediated growth processes. CeO2 nanoparticles were also embedded into mesoporous SiO2 during the mesostructure formation and then stabilized by this ordered mesostructure. Although many efforts have been devoted to explore the confinement effect of mesoporous materials, almost all of these works are focused on silica-based mesoporous materials except for a few works on carbon-based cases. Despite its obvious virtues, the lack of acid/base sites as well as the chemical inactive inertness of the silica surface limits its use for the purpose of catalysis application. As a promising candidate for a three-way catalyst, CeO2–Al2O3 composites have attracted world-wide attention. The sol–gel process was employed to synthesize the ceria-doped alumina materials reported by Viveros et al. In our previous study, the ordered mesoporous aluminas have proved to possess wide applications in catalysis with the benefits of their large surface areas, high thermal stability, large surface Lewis acid sites, and tunable pore size. They are proposed to be good candidates for the fabrication and stabilization of nanometer-scaled ceria, since the channels can prevent the possible growth and reuniting of nanocrystals. Based on all the advantages mentioned above, herein, using the sol–gel method combined with an evaporation-induced self-assembly process, we explore a new strategy to synthesize uniform ceria nanocatalysts stabilized by ordered mesoporous alumina. Using the sample with 8 mol% Ce (denoted as meso-8CeAl) as an example, evidence for the formation of mesostructures is provided by small-angle X-ray diffraction (XRD) patterns shown in Figure 1a. The sample calcined at 400 8C shows a very strong diffraction peak around 1.08 and one weak peak around 1.78, which, associated with transmission electron microscopy (TEM) observation, can be attributed to p6mm hexagonal symmetry. The mesoscopic ordering is sustained well even after calcination at