5 Preparation of the Catalysts
185
Qin Xin et al. used XC-72R carbon powder as carrier, H 2 PtCl 6 ·6H 2 O as precursor,
and formaldehyde as reducing agent, respectively. In the typical synthesis, pH value of
the mixed solution is adjusted to 14 by adding NaOH, and the solution is impregnated
for 15 min and heated to 90 °C. After reaction for 3 h, sedimentation promoters are
added, and the solid 40 wt% Pt/C catalyst is fabricated by washing and drying. The
as-obtained Pt nanoparticles are uniformly dispersed with an average particle size of
2.6 nm (Fig. 5.1). However, if the impregnation time is changed to 36 h, the catalyst
will be agglomerated with uneven particle sizes (1.5–400 nm), indicating the great
influence of impregnation time on the dispersion and size of Pt particles [26].
The impregnation method can also be used to prepare supported bimetallic catalysts [28]. Deli Wang et al. prepared a highly active Pt–Co/C catalyst with structurally
ordered nanostructure by ultrasonic impregnating H 2 PtCl 6 ·6H 2 O, CoCl 2 ·6H 2 O with
XC-72 carbon support in aqueous solution, following evaporating/drying/grinding
steps, and reducing at high temperature in H 2 . Using the impregnation method with
H 2 reduction, other homogeneous bimetallic catalysts such as Pt–Cu and Pt–Ru
are synthesized as well. In the preparation of PtRu/C by impregnation method,
it is revealed that the alloying degree of Pt–Ru is lowering with the increase of
drying temperature, which is probably owing to the partial conversion of RuCl 3 into
unreducible RuO x under high drying temperature [27].
More importantly, the impregnation process can be further simplified. Xiaoqing
Huang et al. used a facile one-pot method to directly prepare octahedral PtNi/C and
PtNiCo/C catalysts, which eliminated the evaporation, drying, calcination and other
steps. This one-pot method could reduce metal ions to metal atoms by using dimethyl
formamide (DMF) as both solvent and reducing agent, which in-situ depositing and
growing into octahedral alloy on various carbon supports (carbon black, carbon
nanotubes, reduced graphene oxide, etc.). The octahedral PtNi and PtNiCo alloy are
uniformly distributed and exhibit enhanced activity than commercial Pt/C catalyst
(Fig. 5.2) [29].
5.1.2 Hydrothermal/Solvothermal Method
Hydrothermal/solvothermal method refers to the chemical synthesis of materials in
aqueous or nonaqueous under high temperature and pressure. The reaction usually
takes place in sealed pressure vessels, such as polytetrafluoroethylene (PTFE),
contains sealed with stainless steel. For security purposes, the volume of contain is
typically less than 1000 mL, the reaction temperature is less than 220 °C, the pressure
is less than 10 MPa, and the feeding coefficient should be less than 0.8. Under high
temperature and pressure, insoluble reagents in atmospheric conditions can be well
dissolved or reacted. Originating from the temperature difference of reaction solution
in the autoclave, supersaturated state of metal ions would be reached by convection,
forming nucleates and then growing into various nanostructures in the presence of
surfactant. This method is very simple to handle, which can effectively prevent the
volatilization of toxic substances and prepare air-sensitive precursors in a closed
185
Qin Xin et al. used XC-72R carbon powder as carrier, H 2 PtCl 6 ·6H 2 O as precursor,
and formaldehyde as reducing agent, respectively. In the typical synthesis, pH value of
the mixed solution is adjusted to 14 by adding NaOH, and the solution is impregnated
for 15 min and heated to 90 °C. After reaction for 3 h, sedimentation promoters are
added, and the solid 40 wt% Pt/C catalyst is fabricated by washing and drying. The
as-obtained Pt nanoparticles are uniformly dispersed with an average particle size of
2.6 nm (Fig. 5.1). However, if the impregnation time is changed to 36 h, the catalyst
will be agglomerated with uneven particle sizes (1.5–400 nm), indicating the great
influence of impregnation time on the dispersion and size of Pt particles [26].
The impregnation method can also be used to prepare supported bimetallic catalysts [28]. Deli Wang et al. prepared a highly active Pt–Co/C catalyst with structurally
ordered nanostructure by ultrasonic impregnating H 2 PtCl 6 ·6H 2 O, CoCl 2 ·6H 2 O with
XC-72 carbon support in aqueous solution, following evaporating/drying/grinding
steps, and reducing at high temperature in H 2 . Using the impregnation method with
H 2 reduction, other homogeneous bimetallic catalysts such as Pt–Cu and Pt–Ru
are synthesized as well. In the preparation of PtRu/C by impregnation method,
it is revealed that the alloying degree of Pt–Ru is lowering with the increase of
drying temperature, which is probably owing to the partial conversion of RuCl 3 into
unreducible RuO x under high drying temperature [27].
More importantly, the impregnation process can be further simplified. Xiaoqing
Huang et al. used a facile one-pot method to directly prepare octahedral PtNi/C and
PtNiCo/C catalysts, which eliminated the evaporation, drying, calcination and other
steps. This one-pot method could reduce metal ions to metal atoms by using dimethyl
formamide (DMF) as both solvent and reducing agent, which in-situ depositing and
growing into octahedral alloy on various carbon supports (carbon black, carbon
nanotubes, reduced graphene oxide, etc.). The octahedral PtNi and PtNiCo alloy are
uniformly distributed and exhibit enhanced activity than commercial Pt/C catalyst
(Fig. 5.2) [29].
5.1.2 Hydrothermal/Solvothermal Method
Hydrothermal/solvothermal method refers to the chemical synthesis of materials in
aqueous or nonaqueous under high temperature and pressure. The reaction usually
takes place in sealed pressure vessels, such as polytetrafluoroethylene (PTFE),
contains sealed with stainless steel. For security purposes, the volume of contain is
typically less than 1000 mL, the reaction temperature is less than 220 °C, the pressure
is less than 10 MPa, and the feeding coefficient should be less than 0.8. Under high
temperature and pressure, insoluble reagents in atmospheric conditions can be well
dissolved or reacted. Originating from the temperature difference of reaction solution
in the autoclave, supersaturated state of metal ions would be reached by convection,
forming nucleates and then growing into various nanostructures in the presence of
surfactant. This method is very simple to handle, which can effectively prevent the
volatilization of toxic substances and prepare air-sensitive precursors in a closed