198
S. Luo et al.
catalyst prepared by this method shows better dispersion and higher electrocatalytic
activity than that prepared without ultrasound for methanol oxidation reaction [71].
5.3.3 Microwave Method
Microwave refers to the electromagnetic wave with a frequency of 300 MHz–
300 GHz, namely the electromagnetic wave with a wavelength ranging from 1
to 1 mm. With strong penetration ability, microwave does not need heat transfer
process and can realize internal and external heating simultaneously at the same
time. The principle of microwave heating is that the dielectric materials consist of
polar molecules and nonpolar molecules, and these polar molecules turn from random
distribution to polar alignment according to electric field under the action of electromagnetic field. Under the action of high-frequency electromagnetic fields, these
orientations change continuously according to the frequency of alternating electromagnetic fields, which causes the movement and friction of molecules and thus generates heat. Meanwhile, the field energy of the alternating electric field transformed
into the heat energy makes the temperature of the medium-rise continuously.
Microwave method has the advantages of simple operation, fast reaction speed,
mild reaction conditions, uniform heating, high reaction efficiency, and thus benefits
the production of nanocrystals in high purity and even distribution. Owing to the
significant advantages on the preparation of nanoparticles, microwave method has
been widely used in the synthesis of organic and inorganic nanomaterials. Yanhui
Yang et al. reported the preparation of PtM alloy catalysts (M = Fe, Co, Ni, Sn, Cu)
supported on multiwalled carbon nanotubes by microwave method. The microwave
method not only effectively reduces the reaction time but also strengthens the interaction between transition metals and the Pt active sites. The prepared alloy catalysts
show better selective hydrogenation performance than those prepared by traditional
impregnation method, among which the PtFe and PtCo alloy catalysts exhibit the
best performance [72].
The main problem of the traditional continuous microwave method is that it is difficult to control the temperature of the reaction system, which results in partial agglomeration of metal nanoparticles. Therefore, a new intermittent microwave heating
method has been developed. Peikang Shen et al. reported a highly dispersed Pt/C
catalyst prepared by an improved intermittent microwave heating method, and the
as-prepared Pt/C catalyst shows better catalytic performance than commercial Pt/C.
In addition, using an intermittent microwave heating procedure with 5 s-on and 5 soff and repeated that for six times, nanometer Pt–WC/C catalysts were prepared,
which exhibits enhanced catalytic performance than the relative Pt–WC/C catalysts
prepared by continuous microwave method for oxygen reduction reaction [73].
Another main disadvantage of microwave method is that it is tough to control
the crystal growth process and prepare catalysts with well-defined shapes and
structures. As an alternative strategy, the combination of microwave method and
hydrothermal/solvothermal method can solve the shortcoming of slow heating of
S. Luo et al.
catalyst prepared by this method shows better dispersion and higher electrocatalytic
activity than that prepared without ultrasound for methanol oxidation reaction [71].
5.3.3 Microwave Method
Microwave refers to the electromagnetic wave with a frequency of 300 MHz–
300 GHz, namely the electromagnetic wave with a wavelength ranging from 1
to 1 mm. With strong penetration ability, microwave does not need heat transfer
process and can realize internal and external heating simultaneously at the same
time. The principle of microwave heating is that the dielectric materials consist of
polar molecules and nonpolar molecules, and these polar molecules turn from random
distribution to polar alignment according to electric field under the action of electromagnetic field. Under the action of high-frequency electromagnetic fields, these
orientations change continuously according to the frequency of alternating electromagnetic fields, which causes the movement and friction of molecules and thus generates heat. Meanwhile, the field energy of the alternating electric field transformed
into the heat energy makes the temperature of the medium-rise continuously.
Microwave method has the advantages of simple operation, fast reaction speed,
mild reaction conditions, uniform heating, high reaction efficiency, and thus benefits
the production of nanocrystals in high purity and even distribution. Owing to the
significant advantages on the preparation of nanoparticles, microwave method has
been widely used in the synthesis of organic and inorganic nanomaterials. Yanhui
Yang et al. reported the preparation of PtM alloy catalysts (M = Fe, Co, Ni, Sn, Cu)
supported on multiwalled carbon nanotubes by microwave method. The microwave
method not only effectively reduces the reaction time but also strengthens the interaction between transition metals and the Pt active sites. The prepared alloy catalysts
show better selective hydrogenation performance than those prepared by traditional
impregnation method, among which the PtFe and PtCo alloy catalysts exhibit the
best performance [72].
The main problem of the traditional continuous microwave method is that it is difficult to control the temperature of the reaction system, which results in partial agglomeration of metal nanoparticles. Therefore, a new intermittent microwave heating
method has been developed. Peikang Shen et al. reported a highly dispersed Pt/C
catalyst prepared by an improved intermittent microwave heating method, and the
as-prepared Pt/C catalyst shows better catalytic performance than commercial Pt/C.
In addition, using an intermittent microwave heating procedure with 5 s-on and 5 soff and repeated that for six times, nanometer Pt–WC/C catalysts were prepared,
which exhibits enhanced catalytic performance than the relative Pt–WC/C catalysts
prepared by continuous microwave method for oxygen reduction reaction [73].
Another main disadvantage of microwave method is that it is tough to control
the crystal growth process and prepare catalysts with well-defined shapes and
structures. As an alternative strategy, the combination of microwave method and
hydrothermal/solvothermal method can solve the shortcoming of slow heating of