188
S. Luo et al.
Hydrothermal/solvothermal method possesses the advantages of simple operation, reproducible, low cost in production, wide range of raw materials, low reaction
temperature, one-step in synthesis and multiple simultaneous reactions, etc. It is
effective in preparing multimetallic catalysts with high crystalline, high purity, good
dispersion and diverse morphology. In addition, the size, morphology, and composition of products can be well regulated by choosing different surfactants or capping
agents. Surfactant and capping agent are usually adsorbed on the specific surface of
reduced metal atoms, which can effectively prevent the agglomeration of nanoparticles and ensure the anisotropic growth to form nanostructured catalysts. Surfactant
and capping agent can also coordinate with metal ions and manipulate the reduction rate to control the size, morphology, composition and surface of nanocrystals
[39, 40].
Nanfeng Zheng’s group has developed the small adsorbate-assisted shape control
of Pt and Pd-based catalysts with intriguing morphology and catalytic performance. For example, carbon monoxide, halogen anion, formaldehyde and other
small molecules are added in the reaction system to prepare catalysts with various
morphologies such as sheets, rods, cubes and tetrahedrons. Among them, carbon
monoxide can be specifically adsorbed on the (111) surface of Pd and the (100)
surface of Pt. Halogen anions can be adsorbed on the (100) surface of Pd and can also
coordinate with noble metal ions, forming PdBr 4
2− and PdI 4
2− , and thus affecting
the morphology and surface structure of catalysts [31]. At present, it is still difficult to understand the adsorption behavior and mechanism of small molecules on
specific crystal surfaces of metals at molecular scale. In the process of synthesis,
small molecules cannot prevent nanoparticles from agglomeration, so polymer and
surfactant need to be added together. Therefore, before testing the catalytic performance, the absorbents on the surface of nanoparticles should be removed as much
as possible.
In recent years, some new technologies, such as microwave, have been introduced into hydrothermal/solvothermal method to prepare a series of unique catalyst
nanomaterials, making it widely used and one of the most important preparation
methods. However, there are some main problems existing, such as: difficult in theoretical simulation, in-situ observation, and mechanism analyses; high equipment
requirements (resistance to acid, alkali, high temperature and high pressure); sensitive to reactant concentration, temperature, reaction time, selected reagents and other
factors; difficult to adjust parameters and conditions; difficult to produce catalysts
in large scale. Besides, the introduction of some organic reagents such asoleylamine
and polyvinyl pyrrolidone also makes it hard to clean the as-obtained nanoparticles
and limits their catalytic applications [2].
S. Luo et al.
Hydrothermal/solvothermal method possesses the advantages of simple operation, reproducible, low cost in production, wide range of raw materials, low reaction
temperature, one-step in synthesis and multiple simultaneous reactions, etc. It is
effective in preparing multimetallic catalysts with high crystalline, high purity, good
dispersion and diverse morphology. In addition, the size, morphology, and composition of products can be well regulated by choosing different surfactants or capping
agents. Surfactant and capping agent are usually adsorbed on the specific surface of
reduced metal atoms, which can effectively prevent the agglomeration of nanoparticles and ensure the anisotropic growth to form nanostructured catalysts. Surfactant
and capping agent can also coordinate with metal ions and manipulate the reduction rate to control the size, morphology, composition and surface of nanocrystals
[39, 40].
Nanfeng Zheng’s group has developed the small adsorbate-assisted shape control
of Pt and Pd-based catalysts with intriguing morphology and catalytic performance. For example, carbon monoxide, halogen anion, formaldehyde and other
small molecules are added in the reaction system to prepare catalysts with various
morphologies such as sheets, rods, cubes and tetrahedrons. Among them, carbon
monoxide can be specifically adsorbed on the (111) surface of Pd and the (100)
surface of Pt. Halogen anions can be adsorbed on the (100) surface of Pd and can also
coordinate with noble metal ions, forming PdBr 4
2− and PdI 4
2− , and thus affecting
the morphology and surface structure of catalysts [31]. At present, it is still difficult to understand the adsorption behavior and mechanism of small molecules on
specific crystal surfaces of metals at molecular scale. In the process of synthesis,
small molecules cannot prevent nanoparticles from agglomeration, so polymer and
surfactant need to be added together. Therefore, before testing the catalytic performance, the absorbents on the surface of nanoparticles should be removed as much
as possible.
In recent years, some new technologies, such as microwave, have been introduced into hydrothermal/solvothermal method to prepare a series of unique catalyst
nanomaterials, making it widely used and one of the most important preparation
methods. However, there are some main problems existing, such as: difficult in theoretical simulation, in-situ observation, and mechanism analyses; high equipment
requirements (resistance to acid, alkali, high temperature and high pressure); sensitive to reactant concentration, temperature, reaction time, selected reagents and other
factors; difficult to adjust parameters and conditions; difficult to produce catalysts
in large scale. Besides, the introduction of some organic reagents such asoleylamine
and polyvinyl pyrrolidone also makes it hard to clean the as-obtained nanoparticles
and limits their catalytic applications [2].