202
S. Luo et al.
the edges and corners exhibit superior catalytic activity and stability. Many methods
have been developed to prepare alloy nanocatalysts, which usually possess uniformly
distributed components [25, 92–98]. To further improve their activity and stability,
appropriate post-treatment is needed to ensure composition segregation, forming a
nanostructure with optimized composition distribution. It is well known that, surface
composition in the alloy is different from the corresponding bulk material, and the
surface composition depends on the separation energy and surface mixing energy.
In addition, the difference in the adsorption energy of gas on two metals can induce
the separation of components. Héctor D. Abruña et al. described a new class of Pt–
Co nanocatalysts composed of ordered Pt 3 Co intermetallic cores with a two to three
atomic-layer-thick platinum shell that segregated in hydrogen. This ordered catalysts
prepared at 700 °C are more active and durable than the disordered Pt 3 Co/C alloy
prepared at 400 °C [24]. Zhichuan J. Xu et al. reviewed the surface segregation of
bimetallic catalysts, discussed the causes of surface segregation and introduced some
characterization methods. The rational control and application of surface segregation
benefit the development of efficient multimetallic catalysts [99, 100].
5.4.3 Surface Cleaning
In the typical synthetic process, organic compounds or polymers are often used
as surfactants, ligands, capping agents or organic solvents in the controllable
synthesis of catalysts with well-defined structure and uniform dispersion (Fig. 5.11).
Specially, surfactants can selectively adsorb on specific crystal surfaces, promoting
the anisotropic growth of nanoparticles and forming unique structures. Ligands can
coordinate with precursors and affect the reaction kinetics, thus determining the
formation of nanostructures. Capping agents adsorb on the surface of nanoparticles,
forming steric hindrance to prevent the agglomeration of nanoparticles, thus forming
highly dispersed or monodisperse colloidal catalysts. On the other hand, oleylamine,
dimethylformamide and ethylene glycol are widely used as effective multifunctional
solvents to prepare a series of efficient catalysts [17, 18, 39].
Generally, washing the as-obtained nanocrystals by solvents (e.g., anhydrous
ethanol for oleylamine/oleic acid, acetone for PVP) and high-speed centrifugation
is used for the surface cleaning. Repeated washing by centrifugation can effectively
remove excess capping agents on the surface of nanoparticles, but there will still be a
certain amount of residual [2]. Annealing can effectively remove organic compounds
from the surface, but high temperature could cause agglomeration and transforming
of nanoparticles.
S. Luo et al.
the edges and corners exhibit superior catalytic activity and stability. Many methods
have been developed to prepare alloy nanocatalysts, which usually possess uniformly
distributed components [25, 92–98]. To further improve their activity and stability,
appropriate post-treatment is needed to ensure composition segregation, forming a
nanostructure with optimized composition distribution. It is well known that, surface
composition in the alloy is different from the corresponding bulk material, and the
surface composition depends on the separation energy and surface mixing energy.
In addition, the difference in the adsorption energy of gas on two metals can induce
the separation of components. Héctor D. Abruña et al. described a new class of Pt–
Co nanocatalysts composed of ordered Pt 3 Co intermetallic cores with a two to three
atomic-layer-thick platinum shell that segregated in hydrogen. This ordered catalysts
prepared at 700 °C are more active and durable than the disordered Pt 3 Co/C alloy
prepared at 400 °C [24]. Zhichuan J. Xu et al. reviewed the surface segregation of
bimetallic catalysts, discussed the causes of surface segregation and introduced some
characterization methods. The rational control and application of surface segregation
benefit the development of efficient multimetallic catalysts [99, 100].
5.4.3 Surface Cleaning
In the typical synthetic process, organic compounds or polymers are often used
as surfactants, ligands, capping agents or organic solvents in the controllable
synthesis of catalysts with well-defined structure and uniform dispersion (Fig. 5.11).
Specially, surfactants can selectively adsorb on specific crystal surfaces, promoting
the anisotropic growth of nanoparticles and forming unique structures. Ligands can
coordinate with precursors and affect the reaction kinetics, thus determining the
formation of nanostructures. Capping agents adsorb on the surface of nanoparticles,
forming steric hindrance to prevent the agglomeration of nanoparticles, thus forming
highly dispersed or monodisperse colloidal catalysts. On the other hand, oleylamine,
dimethylformamide and ethylene glycol are widely used as effective multifunctional
solvents to prepare a series of efficient catalysts [17, 18, 39].
Generally, washing the as-obtained nanocrystals by solvents (e.g., anhydrous
ethanol for oleylamine/oleic acid, acetone for PVP) and high-speed centrifugation
is used for the surface cleaning. Repeated washing by centrifugation can effectively
remove excess capping agents on the surface of nanoparticles, but there will still be a
certain amount of residual [2]. Annealing can effectively remove organic compounds
from the surface, but high temperature could cause agglomeration and transforming
of nanoparticles.