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S. Luo et al.
highly controllable and reliable protocols are required to avoid great waste in work
force and material resources. More importantly, environment, energy and economic
benefits should be seriously considered during the design and synthesis of catalysts
in large-scale and green synthesis methods should be developed in priority [6, 7].
5.1 Preparation Methods in Laboratories
To prepare catalysts, solid-phase method, gas-phase method and liquid-phase method
are mainly adopted in the laboratory [8–15]. Solid-phase method produces materials
by solid-phase reaction, which is simple to produce catalysts in large quantities.
However, this method possesses disadvantages of high energy consumption (high
temperature is required for solid phase reaction), low purity of products, and it cannot
achieve the precise control over the structure of product. The gas-phase method can
fabricate nanoparticles with high purity and controllability by gaseous chemical
reaction. However, it is limited by the use of complex equipment and low yield of
product [11, 16–18]. On the other hand, liquid-phase method is the ideal way to
synthesize nanocrystals by chemical reaction in mixed solution, which is not only
convenient in operation but also flexible in controlling size, morphology and composition [19–23]. The liquid-phase method typically includes impregnation method,
hydrothermal/solvothermal method and sol–gel method. Specially, in the preparation of multimetallic catalysts, coreduction method, seed-mediated growth method
and replacement method are most used. In addition, physical techniques such as
sputtering, ultrasonic, microwave and illumination contribute to the preparation of
catalysts. Examples are given as below.
5.1.1 Impregnation Method
Impregnation method is the main way to prepare supported metal catalysts by means
of that the active components are impregnated and then loaded onto the support materials. Usually, the support material is first impregnated with metal salts in aqueous
solution, then the metal salts permeate into the carrier through capillary pressure, and
finally the active components could gradually diffuse into the holes and adsorb on the
surface of carrier. When the impregnation reaches balance, the solution is removed
and the solid product is obtained by drying, grinding, calcining and activating (reduction), forming supported catalysts [24–27]. The impregnation method is economical
and easy in optimizing the shape, surface area and porosity of catalysts. To prepare
uniformly supported catalysts, the reaction parameters such as impregnation time,
temperature, pH value and reactant concentration should be finely controlled. Occasionally, competitive adsorbents are introduced to ensure evenly adsorbed active
components over the whole carrier. In addition, selecting the reasonable solvent and
reducing agent is also very important [26].
S. Luo et al.
highly controllable and reliable protocols are required to avoid great waste in work
force and material resources. More importantly, environment, energy and economic
benefits should be seriously considered during the design and synthesis of catalysts
in large-scale and green synthesis methods should be developed in priority [6, 7].
5.1 Preparation Methods in Laboratories
To prepare catalysts, solid-phase method, gas-phase method and liquid-phase method
are mainly adopted in the laboratory [8–15]. Solid-phase method produces materials
by solid-phase reaction, which is simple to produce catalysts in large quantities.
However, this method possesses disadvantages of high energy consumption (high
temperature is required for solid phase reaction), low purity of products, and it cannot
achieve the precise control over the structure of product. The gas-phase method can
fabricate nanoparticles with high purity and controllability by gaseous chemical
reaction. However, it is limited by the use of complex equipment and low yield of
product [11, 16–18]. On the other hand, liquid-phase method is the ideal way to
synthesize nanocrystals by chemical reaction in mixed solution, which is not only
convenient in operation but also flexible in controlling size, morphology and composition [19–23]. The liquid-phase method typically includes impregnation method,
hydrothermal/solvothermal method and sol–gel method. Specially, in the preparation of multimetallic catalysts, coreduction method, seed-mediated growth method
and replacement method are most used. In addition, physical techniques such as
sputtering, ultrasonic, microwave and illumination contribute to the preparation of
catalysts. Examples are given as below.
5.1.1 Impregnation Method
Impregnation method is the main way to prepare supported metal catalysts by means
of that the active components are impregnated and then loaded onto the support materials. Usually, the support material is first impregnated with metal salts in aqueous
solution, then the metal salts permeate into the carrier through capillary pressure, and
finally the active components could gradually diffuse into the holes and adsorb on the
surface of carrier. When the impregnation reaches balance, the solution is removed
and the solid product is obtained by drying, grinding, calcining and activating (reduction), forming supported catalysts [24–27]. The impregnation method is economical
and easy in optimizing the shape, surface area and porosity of catalysts. To prepare
uniformly supported catalysts, the reaction parameters such as impregnation time,
temperature, pH value and reactant concentration should be finely controlled. Occasionally, competitive adsorbents are introduced to ensure evenly adsorbed active
components over the whole carrier. In addition, selecting the reasonable solvent and
reducing agent is also very important [26].