5 Preparation of the Catalysts
189
5.1.3 Sol–Gel Method
Sol–gel method has developed rapidly in the past 10 years, which has shown great
advantages and broad application prospects in the inorganic nonmetallic materials field [41, 42]. In the typical sol–gel synthesis, the first step is dispersing the
hydrolysable precursors, such as metal alkoxides and other components into solvents.
The active monomer is formed and turns into a stable transparent sol after hydrolysis
and condensation reaction. Then, a gel with a certain spatial network structure is
formed by aging. Finally, the desirable nanoparticles are prepared through drying
and heat treatment. Sol–gel method could fabricate various functional materials at
low temperature with uniform size, high purity and well-defined composition, and
it enables the design and engineering of nanomaterials at the molecular level. Benefiting from those advantages, sol–gel method has become an important way to prepare
self-supported nanocatalysts [43, 44].
In recent years, sol–gel method has developed as an effective and environmentally
friendly method for preparing noble metallic aerogel catalysts with high specific
surface area and large porosity [45–47]. As shown in Fig. 5.3, the preparation of
noble metallic aerogel catalysts is mainly achieved through two strategies. Strategy
I is that metal ions are reduced to form dispersed nanoparticles, which are gelled
after appropriate treatment to form hydrogels, and then porous aerogels with high
specific surface area are obtained through supercritical drying. In the second strategy
(Strategy II), the hydrogels are spontaneously formed by the in situ reduction of
noble metal precursors, and then porous aerogels are obtained through supercritical
drying. At present, a wide range of noble monometallic aerogel catalysts, including
Au, Ag, Pt and Pd, and multimetallic aerogel catalysts, including Au–Ag, Au–Pd,
Pt–Ag, Pd–Ag, Pt–Pd, Au–Ag–Pt, Au–Pt–Pd, Ag–Pt–Pd and Au–Ag–Pt–Pd have
been successfully prepared (Fig. 5.4) [48]. The drying process of hydrogels is very
important. If dried directly, the network structure of hydrogels will shrink or collapse.
As a widely used strategy in the preparation of noble metal aerogel catalysts by
sol–gel method, supercritical CO 2 drying method effectively ensures the preserved
internal structure of hydrogels during drying hydrogels into aerogels.
The main disadvantage of sol–gel method is that the process of sol preparation is
complex and the raw materials are expensive. Thus, it is only suitable for laboratory
research at present. Generally, the whole sol–gel process takes a long time, usually
several days or weeks. High contents of residual pores, residual hydroxyl and residual
carbon in the gel are retained. During drying process, gases and organics will escape,
lead to the shrink of hydrogels, and the organic solvents are harmful to human body.
Nevertheless, sol–gel method is still reliable and needs further development.
189
5.1.3 Sol–Gel Method
Sol–gel method has developed rapidly in the past 10 years, which has shown great
advantages and broad application prospects in the inorganic nonmetallic materials field [41, 42]. In the typical sol–gel synthesis, the first step is dispersing the
hydrolysable precursors, such as metal alkoxides and other components into solvents.
The active monomer is formed and turns into a stable transparent sol after hydrolysis
and condensation reaction. Then, a gel with a certain spatial network structure is
formed by aging. Finally, the desirable nanoparticles are prepared through drying
and heat treatment. Sol–gel method could fabricate various functional materials at
low temperature with uniform size, high purity and well-defined composition, and
it enables the design and engineering of nanomaterials at the molecular level. Benefiting from those advantages, sol–gel method has become an important way to prepare
self-supported nanocatalysts [43, 44].
In recent years, sol–gel method has developed as an effective and environmentally
friendly method for preparing noble metallic aerogel catalysts with high specific
surface area and large porosity [45–47]. As shown in Fig. 5.3, the preparation of
noble metallic aerogel catalysts is mainly achieved through two strategies. Strategy
I is that metal ions are reduced to form dispersed nanoparticles, which are gelled
after appropriate treatment to form hydrogels, and then porous aerogels with high
specific surface area are obtained through supercritical drying. In the second strategy
(Strategy II), the hydrogels are spontaneously formed by the in situ reduction of
noble metal precursors, and then porous aerogels are obtained through supercritical
drying. At present, a wide range of noble monometallic aerogel catalysts, including
Au, Ag, Pt and Pd, and multimetallic aerogel catalysts, including Au–Ag, Au–Pd,
Pt–Ag, Pd–Ag, Pt–Pd, Au–Ag–Pt, Au–Pt–Pd, Ag–Pt–Pd and Au–Ag–Pt–Pd have
been successfully prepared (Fig. 5.4) [48]. The drying process of hydrogels is very
important. If dried directly, the network structure of hydrogels will shrink or collapse.
As a widely used strategy in the preparation of noble metal aerogel catalysts by
sol–gel method, supercritical CO 2 drying method effectively ensures the preserved
internal structure of hydrogels during drying hydrogels into aerogels.
The main disadvantage of sol–gel method is that the process of sol preparation is
complex and the raw materials are expensive. Thus, it is only suitable for laboratory
research at present. Generally, the whole sol–gel process takes a long time, usually
several days or weeks. High contents of residual pores, residual hydroxyl and residual
carbon in the gel are retained. During drying process, gases and organics will escape,
lead to the shrink of hydrogels, and the organic solvents are harmful to human body.
Nevertheless, sol–gel method is still reliable and needs further development.