ABSTRACT
The precipitation/co-precipitation route usually affords products with pure phase, and the experimental procedures are relatively simple. The metal ions are first precipitated from solutions as hydroxides, carbonates, and oxalates, which undergo the subsequent calcination treatment to form the products. Binary compounds, such as CeO2 and REF3, can be readily obtained by this method; yet for those complex systems (doped systems and ternary systems, for example), the precipitation procedure requires particular care because the precipitation rates can vary for different metal ions. In such cases, certain coordination reagents may be necessary to adjust the synchronicity in the subtle co-precipitation procedure, so as to obtain homogeneous products with predesigned compositions. Due to the relatively simple operations involved in this route, mass production is easily achieved. The rare earth compound ceria (CeO2) is currently under most extensive and intensive investigation. It adopts a cubic fluorite phase in a wide temperature range (from ambient temperature up to its melting point), and the fluorite structure can be preserved to a considerable extent under reductive atmospheres. The reduction from Ce4+ ions to Ce3+ ions can generate oxygen vacancies, which act as highly reactive sites for plenty of catalysis redox reactions. When the size of ceria is reduced down to the nanometer dimensions, the catalytic activity is much elevated due to the enlarged surface area, enhanced oxygen storage capacity, which caters to the demands of three-way catalysts, fuel cells, and so on. Due to the cubic phase, nanoceria tends to expose low-index crystal surfaces, i.e., {100}, {110}, and {111}, and usually takes the shape of nanocubes, nanooctahedra, nanowires, and nanotubes.
