5 Preparation of the Catalysts
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solution in hydrothermal/solvothermal method, and accelerate the crystallization
rate, which is widely used to prepare nanostructured catalysts at present.
5.4 Post-Treatment and Activation of Catalysts
After decades of efforts, many catalysts with well-defined nanostructure and
enhanced performance have been synthesized by appropriate preparation techniques.
However, prepared by the most frequently used liquid-phase synthesis method, catalysts usually possess many surfactants coating on their surface, smooth surface structure and poor composition distribution. Proper post-treatments of the as-prepared
catalyst can optimize its structure, composition and surface, which will improve
their catalytic activity and structural stability, and that attract widespread attention
and application [1, 3, 74–77].
The commonly used post-treatments include cleaning, selective etching, surface
composition segregation, electrochemical activation and so on. As a notable example,
in 1924, American engineer Molly Raney treated a mixture of Ni and Si (Ni:Si =
1:1) with NaOH. As a result, Si reacted with NaOH, forming a porous catalyst,
whose catalytic activity for hydrogenation of cottonseed oil was five times higher
than that of Ni. Then, He treated Ni–Al alloy (Ni:Al = 1:1) with NaOH in high
concentration. In this process, Al reacted with NaOH and dissolved, leaving the
activated porous Raney Ni, which shows further enhanced catalytic activity. Due to
its strong adsorption of hydrogen, high catalytic activity and thermal stability, Raney
Ni is still widely used in many industrial processes and organic synthesis reactions
up to now.
5.4.1 Etching
The phenomenon that metals and products made of them are destroyed by chemical or
electrochemical action in natural environment or under working conditions is called
etching. Typically, etching can be divided into wet etching and dry etching, which
refer to the etching of metals in the presence of water or dry gases without liquid
water, respectively. Due to the general presence of water in the atmosphere and the
treatment of various aqueous solutions in chemical production, wet etching is most
commonly observed. However, the harm caused by dry etching in high temperature
operation can also not be ignored.
Etching of metals in aqueous solution is an electrochemical reaction, as an anode–
cathode isolated etching cell is formed on the surface of metal. The metal loses electrons in the solution and turns into positive metallic ions, and this process is an oxidation reaction, namely the anodic process. Meanwhile, the generated electrons could be
neutralized by some substance in the solution. The process of neutralizing electrons
199
solution in hydrothermal/solvothermal method, and accelerate the crystallization
rate, which is widely used to prepare nanostructured catalysts at present.
5.4 Post-Treatment and Activation of Catalysts
After decades of efforts, many catalysts with well-defined nanostructure and
enhanced performance have been synthesized by appropriate preparation techniques.
However, prepared by the most frequently used liquid-phase synthesis method, catalysts usually possess many surfactants coating on their surface, smooth surface structure and poor composition distribution. Proper post-treatments of the as-prepared
catalyst can optimize its structure, composition and surface, which will improve
their catalytic activity and structural stability, and that attract widespread attention
and application [1, 3, 74–77].
The commonly used post-treatments include cleaning, selective etching, surface
composition segregation, electrochemical activation and so on. As a notable example,
in 1924, American engineer Molly Raney treated a mixture of Ni and Si (Ni:Si =
1:1) with NaOH. As a result, Si reacted with NaOH, forming a porous catalyst,
whose catalytic activity for hydrogenation of cottonseed oil was five times higher
than that of Ni. Then, He treated Ni–Al alloy (Ni:Al = 1:1) with NaOH in high
concentration. In this process, Al reacted with NaOH and dissolved, leaving the
activated porous Raney Ni, which shows further enhanced catalytic activity. Due to
its strong adsorption of hydrogen, high catalytic activity and thermal stability, Raney
Ni is still widely used in many industrial processes and organic synthesis reactions
up to now.
5.4.1 Etching
The phenomenon that metals and products made of them are destroyed by chemical or
electrochemical action in natural environment or under working conditions is called
etching. Typically, etching can be divided into wet etching and dry etching, which
refer to the etching of metals in the presence of water or dry gases without liquid
water, respectively. Due to the general presence of water in the atmosphere and the
treatment of various aqueous solutions in chemical production, wet etching is most
commonly observed. However, the harm caused by dry etching in high temperature
operation can also not be ignored.
Etching of metals in aqueous solution is an electrochemical reaction, as an anode–
cathode isolated etching cell is formed on the surface of metal. The metal loses electrons in the solution and turns into positive metallic ions, and this process is an oxidation reaction, namely the anodic process. Meanwhile, the generated electrons could be
neutralized by some substance in the solution. The process of neutralizing electrons