5 Preparation of the Catalysts
193
the metals used for deposition and the as-prepared seeds plays an important role in the
synthesis of multimetallic catalysts, as physical parameters such as lattice matching
degree, surface interaction, interface energy and electronegativity of the two metals
determine the nucleation and growth processes [64]. The deposition and growth of
metals are thermodynamically and dynamically controlled, and the precise control
of those processes could enable the preparation of as-designed catalyst. This can
be achieved by adjusting the reaction parameters, such as increasing temperature or
decreasing the speed of adding precursors for uniform deposition, adding appropriate
surfactants for the control of growth direction and speed, and using reductants with
different reducing abilities for the regulation of nucleation and growth rate [65].
Younan Xia et al. have performed a lot of work in preparing nanostructured catalysts by seed-mediated growth method. They prepared PtPd dendritic catalysts at
low temperature by using truncated octahedral Pd nanoparticles with a particle size
of 9 nm as seeds and ascorbic acid as reductant. By adopting higher temperature
and reducing the feed speed of Pt precursor, they avoided the formation of Pt island
structure and achieved the evenly deposition of Pt atoms on the surface of cubic Pd
nanoparticles to form PtPd core–shell structure (Fig. 5.5). By adjusting the feeding
amount of Pt precursor, the Pt shell was successfully controlled in 1–6 atomic layers.
Specially, the Pd@Pt 2–5 L catalyst exhibits the highest specific activity toward
oxygen reduction reaction, while Pd@Pt 1 L catalyst shows the highest mass activity,
which is three times higher than commercial Pt/C [63].
By using seed-mediated growth method, the deposition of alloy on seeds can also
be achieved, and catalysts with diverse structure and composition can be prepared.
For example, octahedral Pd@PtNi core–shell catalyst was formed by deposition of
PtNi alloy on octahedral Pd seeds (Fig. 5.6), and Au@PtCu core–shell catalyst was
formed by deposition of PtCu alloy on uniform Au nanoparticles [66].
The second metal can also diffuse into the lattice of seeds to form an alloy. Yadong
Li et al. reported the fabrication of uniform PtNi alloy by using multidendritic Pt
nanocrystals as seeds and the diffusion of Ni atoms into Pt seeds at high temperature
(Fig. 5.7). The as-obtained PtNi multidendritic nanostructures exhibited 3.6 times
higher catalytic activity than Pt nanoparticles for methanol oxidation reaction [67].
Seed-mediated growth method has great advantages in preparing complex structures and regulating the spatial distribution of components, thus attracting much
attention in the preparation of catalysts in the laboratory. However, the experimental
requirements for seed-mediated growth are very high, and the reaction conditions
and parameters require precise control, which is not easy to satisfy for industrial
large-scale synthesis at present.
5.3 Physical Techniques for Synthesis and Assistance
Compared with chemical synthesis like liquid-phase method, physical method could
synthesize pure phase and less defective catalysts, such as nanoparticles, films, wires
and other nanostructures in high vacuum environment, without chemical reactions
193
the metals used for deposition and the as-prepared seeds plays an important role in the
synthesis of multimetallic catalysts, as physical parameters such as lattice matching
degree, surface interaction, interface energy and electronegativity of the two metals
determine the nucleation and growth processes [64]. The deposition and growth of
metals are thermodynamically and dynamically controlled, and the precise control
of those processes could enable the preparation of as-designed catalyst. This can
be achieved by adjusting the reaction parameters, such as increasing temperature or
decreasing the speed of adding precursors for uniform deposition, adding appropriate
surfactants for the control of growth direction and speed, and using reductants with
different reducing abilities for the regulation of nucleation and growth rate [65].
Younan Xia et al. have performed a lot of work in preparing nanostructured catalysts by seed-mediated growth method. They prepared PtPd dendritic catalysts at
low temperature by using truncated octahedral Pd nanoparticles with a particle size
of 9 nm as seeds and ascorbic acid as reductant. By adopting higher temperature
and reducing the feed speed of Pt precursor, they avoided the formation of Pt island
structure and achieved the evenly deposition of Pt atoms on the surface of cubic Pd
nanoparticles to form PtPd core–shell structure (Fig. 5.5). By adjusting the feeding
amount of Pt precursor, the Pt shell was successfully controlled in 1–6 atomic layers.
Specially, the Pd@Pt 2–5 L catalyst exhibits the highest specific activity toward
oxygen reduction reaction, while Pd@Pt 1 L catalyst shows the highest mass activity,
which is three times higher than commercial Pt/C [63].
By using seed-mediated growth method, the deposition of alloy on seeds can also
be achieved, and catalysts with diverse structure and composition can be prepared.
For example, octahedral Pd@PtNi core–shell catalyst was formed by deposition of
PtNi alloy on octahedral Pd seeds (Fig. 5.6), and Au@PtCu core–shell catalyst was
formed by deposition of PtCu alloy on uniform Au nanoparticles [66].
The second metal can also diffuse into the lattice of seeds to form an alloy. Yadong
Li et al. reported the fabrication of uniform PtNi alloy by using multidendritic Pt
nanocrystals as seeds and the diffusion of Ni atoms into Pt seeds at high temperature
(Fig. 5.7). The as-obtained PtNi multidendritic nanostructures exhibited 3.6 times
higher catalytic activity than Pt nanoparticles for methanol oxidation reaction [67].
Seed-mediated growth method has great advantages in preparing complex structures and regulating the spatial distribution of components, thus attracting much
attention in the preparation of catalysts in the laboratory. However, the experimental
requirements for seed-mediated growth are very high, and the reaction conditions
and parameters require precise control, which is not easy to satisfy for industrial
large-scale synthesis at present.
5.3 Physical Techniques for Synthesis and Assistance
Compared with chemical synthesis like liquid-phase method, physical method could
synthesize pure phase and less defective catalysts, such as nanoparticles, films, wires
and other nanostructures in high vacuum environment, without chemical reactions