Chemistry Reference
In-Depth Information
compared to “dry” methods, which commonly resort to rigorous
conditions such as high temperature, high pressure, or high vacuum,
the “wet” syntheses are usually carried out under relatively mild
conditions, which also help to lower energy consumption and total
cost. Therefore, the discussion in the following sections will be
mainly focused on wet methods, including precipitation, sol-gel,
hydrothermal/solvothermal, thermal decomposition, microwave,
microemulsion routes, and so on.
1.2
Precipitation/Co-Precipitation Route
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 CeO
, 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 (CeO
and REF
2
3
) 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 Ce
2
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.
4+
ions to Ce
3+
