Construction of hollow carbon polyhedron supported Pt catalyst for methanol electrocatalytic oxidation

Article abstract of DOI:10.1016/j.electacta.2021.138790

In this work, the hollow Pt/CP (carbon polyhedron) catalyst derived from ZIF-8 was obtained via a simple hydrothermal method for electrocatalytic methanol oxidation reaction (MOR). Compared with Pt/CB (carbon black) catalyst, the hollow Pt/CP had the higher ESCA, which was 60 m² g^?1, while the Pt/CB was only 41 m² g^?1. Similarly, the I_f/I_b of the hollow Pt/CP was 7.5, which was also obviously about 3 times higher than the Pt/CB (2.5). All the results verified that the electrocatalytic oxidation activity of hollow Pt/CP was greatly better than Pt/CB. According to a series of characterizations, it can be discovered that the hollow Pt/CP catalyst derived from ZIF-8 had more rich oxygen-containing functional groups, smaller Pt nanoparticles size, more Pt content, larger specific surface area, and so on, which were good for the improvement of MOR activity.

Full text of DOI:10.1016/j.electacta.2021.138790

Electrochimica Acta 390 (2021) 138790
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Construction of hollow carbon polyhedron supported Pt catalyst for
methanol electrocatalytic oxidation
Chengxiu Dinga,b, Fang Donga, Zhicheng Tang a,∗
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, National Engineering Research Center for Fine Petrochemical Intermediates, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China
b University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, the hollow Pt/CP (carbon polyhedron) catalyst derived from ZIF-8 was obtained via a simple
hydrothermal method for electrocatalytic methanol oxidation reaction (MOR). Compared with Pt/CB (car-
Received 16 October 2020
Revised 7 June 2021
bon black) catalyst, the hollow Pt/CP had the higher ESCA, which was 60 m2
g
−1, while the Pt/CB was
Accepted 12 June 2021
Available online 16 June 2021
only 41 m2 −1. Similarly, the If/Ib of the hollow Pt/CP was 7.5, which was also obviously about 3 times
g
higher than the Pt/CB (2.5). All the results verified that the electrocatalytic oxidation activity of hollow
Pt/CP was greatly better than Pt/CB. According to a series of characterizations, it can be discovered that
the hollow Pt/CP catalyst derived from ZIF-8 had more rich oxygen-containing functional groups, smaller
Pt nanoparticles size, more Pt content, larger specific surface area, and so on, which were good for the
improvement of MOR activity.
Keywords:
MOR
ZIF-8
Hollow
Pt nanoparticles
© 2021 Elsevier Ltd. All rights reserved.
Oxygen-containing functional groups
1. Introduction
the lowest cost. The anode catalytic material of DMFC is a key fac-
tor that determines its performance and commercial application.
The pressure on the environment was mounting with the
world’s population being bigger. Besides, the pressure on energy
was also very serious [1]. Therefore, the requirements of sustain-
able development were also very important and necessary. So, the
current exploration and research on new energy were very urgent.
Nowadays, the application of nuclear energy, hydrogen energy, so-
lar energy, and other new energy source had achieved initial re-
sults. Also, due to the need for sustainable energy development
and energy demand, fuel cells were one of the research hotspots.
Fuel cells can directly transform the energy in chemical fuels to
electricity, and this reaction had high efficiency and was friendly
to the environment. Generally, hydrogen, as well as liquids such as
alcohols, aldehydes, and so on can be used as fuel. Among them,
methanol has become the most promising fuel due to its simple
molecular structure, low cost, and low CO2 emissions. The direct
methanol fuel cell (DMFC) was very promising research which had
some advantages such as high energy density, low operating tem-
perature, convenient fuel storage, low pollutant emission, low cost,
and so on [2,3]. Compared with traditional battery devices, DMFC
had higher power density with long service life and rechargeable
which was almost moment. Also, methanol fuel cell vehicles had
To maximize energy efficiency, researchers had conducted exten-
sive exploration of catalysts. And the transition metals’ application
was more common at present. Among them, Pt was the most ef-
fective active ingredient. However, the existing Pt-based catalysts
have the following problems: (1) Pt is easily adsorbed by reaction
intermediate products (such as CO), resulting in catalyst poisoning;
(2) the cost of Pt-based catalysts is too high. So, it was very nec-
essary to find effective and low-cost materials that can reduce the
Pt’s content.
Based on the shortcomings of Pt-based catalysts, to maximize
the active advantage of a small amount of Pt, the carrier’s charac-
teristic structure was used to achieve the requirement which had
low content and high activity, such as carbon paper, graphene ox-
ide, multi-walled carbon nanotubes, carbon nanofiber and so on
[4–8]. At the same time, MOFs have the characteristics of high
porosity, rich pore structure, high specific surface area, pores that
can be easily controlled, various skeleton components and struc-
ture changes, easy to realize functionalization, and so on, whose
application was very pervasive [9]. Currently, Ni-MOF, Co-MOF,
HKUST, and other MOFs were widely used [10–13]. Some MOFs
structures with special morphology, such as 3D flower-like [14],
polyhedron [15], and so on, also have good application potential.
Based on the above advantages, the use of carbon materi-
als derived from MOFs as carriers to prepare electro-catalysis for
methanol oxidation reaction (MOR) has a promising application
∗
Corresponding author.
E-mail address: tangzhicheng@licp.cas.cn (Z. Tang).
https://doi.org/10.1016/j.electacta.2021.138790
0013-4686/© 2021 Elsevier Ltd. All rights reserved.

C. Ding, F. Dong and Z. Tang
Electrochimica Acta 390 (2021) 138790
prospect. The most important carbonization method was to use
high-temperature calcination to obtain a carbon material with a
MOF’s morphology. The materials with MOFs’ unique morphology
which can provide a special reaction channel were very advan-
tageous for molecular’s mass transfer, adsorption, desorption, and
such advantages were particularly useful for MOR. Because of the
special morphology and active components’ dispersion displayed
activity tends to be better.
used to find the elements’ interaction. SEM (Scanning electron mi-
croscopy) image was obtained on SU-8020 to observe the morphol-
ogy of Co-MOFs.
Electrochemical experiments were done at room temperature
on a CHI660c electrochemical workstation (Chenhua, Shanghai),
and counter electrode was a platinum wire and the reference elec-
trode was Ag/AgCl. The preparation of working electrode: 5.0 mg
catalyst and 25 mL of 0.05% Nafion solution was dispersed in 1 mL
ethanol through ultrasound for 30 min. Then dropping ink 25 μL
of the above on the GCE surface, and then let it dry naturally,
which was used as the working electrode.
In this paper, we prepared a hollow CP dodecahedron structure
derived from ZIF-8, and test the electrocatalytic activity of the cat-
alyst for MOR in alkaline medium. Compared with carbon black,
it was discovered that hollow CP support had obvious structural
advantages, which were conducive to the subsequent loading and
dispersion of Pt, and the electron transport in the reaction. When
the hollow Pt/CP was used in MOR, the catalytic performance was
better than Pt/CB.
3. Result and discussion
3.1. Electrochemical performance
3.1.1. CV curve
All the tests were done under an N2-saturated condition at
room temperature. In this system, the counter electrode was plat-
inum wire and the Ag/AgCl electrode was used as a reference elec-
trode, respectively. Glassy carbon electrode (GCE, 3.0 mm diame-
ter) was buffed sequentially with 0.3 and 0.05 mm alumina ox-
ide (Al2O3) powder 20 min, severally. Then it was ultrasonic for
2 min to wash (first use deionized water, then used deionized wa-
ter and ethanol mixed solution, last use deionized water), respec-
tively, then catalysts dropped to this electrode, which was treated
as the working electrode. The working electrode was prepared by
dropping 25 μL of the catalyst ink on the GCE surface, and then
evaporated the ethanol. The as-prepared working electrode was ac-
tivated by CV (cyclic voltammetry) in 1 M NaOH and the potential
range was from −1 V to 0.6 V (vs. Ag/AgCl) with a scan rate 50 mV
2. Experimental
2.1. Synthesis of catalysts
2.1.1. Synthesis of ZIF-8
Refer to the previous literature [15] and make some adjust-
ments to get ZIF-8. In a typical synthesis, 1.116 g Zn(NO3)2•6H2O
was dissolved in methanol (15 mL) to form uniform solution, 2-
Methylimidazole (1.232 g) was dissolved with stirring in methanol
(30 mL) to get other clear solution. Then the two solutions with
methanol were put together under stirring for 5 min. The mixture
was kept reacting at hydrothermal reactor for 4 h. The powders
were collected by centrifugation, washed with methanol for sev-
eral times, and dried overnight at 110 °C.
s
−1. As the catalysts were used in the MOR, the electro-catalytic
activities of catalysts were got by CV in a condation which contain-
ing 1 M NaOH and 1 M methanol from −1 to 0.4 V with a scan rate
50 mV s−1 (vs. Ag/AgCl), respectively. The CA (Chronoamperome-
try) for the MOR was obtained at a potential of −0.2 V for 3600 s.
In a frequency scope from 100 kHz to 0.1 Hz with stationary poten-
tial mode amplitude of 5 mV, the EIS (electrochemical impedance
spectroscopy) data were tested in Nyquist diagrams form.
2.1.2. Synthesis of hollow CP
The above ZIF-8 was put in the tube furnace, carbonized at
900°C under argon atmosphere. The heating rate was 1 °C min−1
and heating time was 2 h. The purpose of this calcination condi-
tion was to completely remove Zn.
,
2.1.3. Synthesis of hollow dodecahedron Pt/CP
In the following experiments, to further get the electrochemical
behaviors of hollow Pt/CP and Pt/CB. Fig. 1A depicted the CV curves
of the two catalysts in N2-saturated with 1 M NaOH. The sweep
potential was −1–0.6 V, scan rate was 50 mV s−1. According to lit-
eratures [16–18], the current peak between −1 V-0.6 V is caused
by the typical hydrogen adsorption and desorption on the surface
of the Pt catalyst (vs Ag/AgCl), The oxidation peak between −1–
0.6 V was caused by the absorption and desorption of hydrogen
on the Pt surface, it can be found that hollow Pt/CP had a larger
electric double layer than Pt/CB, indicating that hollow Pt/CP had
a larger electrochemically active surface area (ESCA). Also, this can
Subsequently, 100 mg of hollow CP was dissolved in 100 mL
ethylene glycol solution under ultrasonic conditions. After, added
3.3 mL of H2PtCl6•6H2O (38.6 mM L−1) and added dropwise 1 M
NaOH to adjust the pH of the mixed solution to 8, the above so-
lution was heated to 130 °C and reacted for 3 h. Then, using cen-
trifugation (10,000 rpm min−1, 3 min), washed with distilled water
and ethanol for three times, severally, and dried overnight at 60°C.
The above experimental process was shown in Scheme 1.
2.2. Apparatus and measurements
be used to get the ESCA of hollow Pt/CP and Pt/CB. We used the
-2
The XRD (X-ray diffraction) analysis was characterized by a
Rigaku D/MAX-RB diffractometer to analyze the catalyst structure
with Cu Kα (λ=0.154 nm; 60 kV and 55 mA) diffraction source. To
observe the catalysts’ morphology and Pt nano-particles (NPs)’ dis-
tribution size we used the TEM (transmission electron microscope,
JEOL JEM-2010 transmission electron microscope at 200 kV). Ra-
man spectrometer with 532 nm laser was employed to obtain Ra-
man spectra (A RM 2000 microscope confocal). In order to know
the composition on the samples’ surface, the XPS (X-ray photo-
electron spectroscopy) was performed on a VG ESCALAB 210 em-
ploying Mg Kα as X-ray. Also, to detect the information of surface
area and pore size distribution, N2 adsorption-desorption isotherm
was inspected by adsorbing N2 at 76.2 K on a Micromeritics ASAP
2020 determinator for viewing the pore structure and specific sur-
face area. FTIR (Fourier transform infrared spectroscopy) was also
following equation to obtain the ESCA. where QH (in mC cm
)
was the charge for hydrogen desorption, 0.21 (in mC cm-2) repre-
sent the charge required to oxidize a monolayer of H2 on bright Pt
and [Pt] was the loading of Pt on the electrode (in mg cm−2)] [19],
and the ꢀQH.was the average value of QH.
ꢀQH
ESCA =
0.21∗ Pt
[
]
The calculated data are shown in the Table 1. And the Accord-
ing to the above formula, we can get the ESCA of hollow Pt/CP
was 60 m2
g g
−1, and the Pt/CB was 41 m2 −1. From this re-
sult, we can know the hollow Pt/CP had better ESCA and a larger
electric double layer, which was helpful for the methanol oxida-
tion. The bigger ESCA might be due to the hollow CP structure
had a large specific surface area and excellent pore size channels
2

C. Ding, F. Dong and Z. Tang
Electrochimica Acta 390 (2021) 138790
Scheme 1. The catalyst preparation process and simple reaction process.
Table 1
Hydrogen adsorption and desorption charges and mean values.
catalyst
Pt (mg cm−2
)
Q’ (mC cm−2
)
Q’’ (mC cm−2
)
QH (mC cm−2
)
QH/[Pt] (mC mg−1
)
Pt/CP
Pt/CB
0.24
0.24
45
26
15
16
30
21
125
87
Table 2
Kinetic parameters for the MOR at selected potentials and electrode active surface area
(ECSA) and If/Ib of the electrocatalysts.
Sample
ESCAH m2g−1
Methanol oxidation current density mAcm−2
If/Ib
Pt/CB
41
60
0.30
0.25
7.5
2.5
Hollow Pt/CP
and had smaller Pt particles with good dispersion. These structural
advantages might be beneficial to the molecules’ adsorption and
activation.
Fig. 1C showed the CV curves in 1 M NaOH with 1 M methanol
at different scan rate (10mV s-1, 20 mV s-1, 50 mV s-1, 80 mV
s-1, 100 mV s−1). The first half of the CV was selected to ana-
lyze changes in current density. It can be discovered that as the
sweep rate increased, the methanol oxidation peak gradually in-
creased, which meant the peak current also increased. The square
root of sweep speed had a linear relationship with the current
density, which meant the diffusion was the controlling step in the
methanol oxidation process [23]. As Fig. 1D shown, there was a
linear relationship between Ep (peak potential) and ln(ʋ) at differ-
ent scanning speeds, which meant this methanol oxidation reac-
tion was not reversible [24]. And in Fig. 1E had shown the spe-
cific activity and mass activity of the two simples, as the results
shown, the hollow Pt/CP had higher specific activity, which was
about 1.5 times higher than Pt/CB, and the mass activity of hol-
low Pt/CP was also better than Pt/CB. This result may indicate that
under the same load conditions, the Pt on hollow CP was more
uniformly dispersed so that having a smaller Pt nanoparticle size,
and more active sites were exposed.
Fig. 1B showed the CV curves in the 1 M NaOH and 1 M
methanol, the oxidation peak at −0.4–0.2 V was attributed to
the methanol oxidation on the catalyst, the other peak at −0.2–
0.6 V belonged to the oxidation of intermediate species such as
CO [20]. The first oxidation peak of hollow Pt/CP was 0.3 mA
cm−2, which was about 1.5 times higher than the Pt/CB (0.25 mA
cm−2). Hollow Pt/CP had good catalytic activity, which might be
due to its large specific surface area and smaller Pt, which was
conducive to the reaction of methanol on the surface. Besides,
the Pt0 was also a favorable factor that was good for improving
activity and was conducive to better contact between methanol
and active sites. And the hollow Pt/CP has more Pt. To obtain
the CO tolerance of the catalyst, it was expressed by the ra-
tio of If (forward peak current density) and Ib (backward peak
current density) [21]. The larger of the If/Ib ratio, the stronger
the catalyst’s resistance to the CO-like substances produced dur-
ing the CH3OH oxidation process. The If/Ib of the hollow Pt/CP
was 7.5, which was higher than the Pt/CB (2.5) about 3 times,
higher If/Ib meant there was more CH3OH was oxidized to CO2,
more intermediates of carbonaceous were effectually removed
[22]. And Table 2 summarized and compared the ESCA data of the
two catalysts, and the hollow Pt/CP had better ESCA and higher
If/Ib.
3.1.2. Chronoamperometric curve and electro-chemical impedance
tests
The chronoamperometry can test the long-term stability of hol-
low Pt/CP and Pt/CB in methanol oxidation at −0.2 V, which was
in 1.0 M NaOH containing 1.0 M methanol for 3600 s. As Fig. 2A
3

C. Ding, F. Dong and Z. Tang
Electrochimica Acta 390 (2021) 138790
Fig. 1. The CV curves of hollow Pt/CP and Pt/CB in 1 M NaOH without methanol (A) the insert was Pt/CB CV curves with methanol (B) at 50 mv/s, and at different scan
rate for the forward scan of hollow Pt/CP and Pt/CB (C). Insert: plot of peak current density vs. square root of scan rate and the plot of Ep vs. ln(ʋ) (D) of hollow Pt/CP. (E)
Corresponding activity of hollow Pt/CP and Pt/CB.
shown, the hollow Pt/CP had higher current density at the first,
but decreased after 200 s. The reason may be that the intermedi-
ate products produced during the reaction poisoned the catalyst,
so its activity decreased with time. In order to compare the con-
ductivity of the two catalysts, the EIS impedance spectrogram was
used. As Fig. 2B shown, the hollow Pt/CP had a smaller radius.
The diameter of the semicircle can be used to explain the con-
ductivity of catalyst, and to illustrate the charge transferability in
the methanol oxidation reaction [25]. From the experimental data,
got equivalent circuit (inserted in Fig. 2B), where the RS represents
the solution resistance, the capacitance of electrode double layer
used Cdl to present, and RCT meant the charge-transfer’s resistance
[26]. The Rs of hollow Pt/CP was 11 ꢁ, and the Pt/CB was 9 ꢁ,
and RCT was 166.6 ꢁ, 202.9 ꢁ, respectively. And it can be found
that when compared with Rct, the Rs can be ignored, which meant
that the resistance of solution can be ignored. From the results,
the smaller radius of hollow Pt/CP meant its high conductivity and
strong charge transferability. And from the above results, we can
know the activity of hollow Pt/CP was better than the Pt/CB. In
order to further explain the physical structure of catalysts, the dis-
tribution of elements and the carbon materials’ composition, and
other structural features were as follows.
4

C. Ding, F. Dong and Z. Tang
Electrochimica Acta 390 (2021) 138790
Fig. 2. Chronoamperometric curves (A) and EIS (B): insert: the equivalent circuit diagram of hollow Pt/CP and Pt/CB.
Fig. 3. (A) Nitrogen adsorption-desorption isotherms and (B) Pore-size distribution of hollow Pt/CP and Pt/CB.
Table 3
Textural properties of the catalysts.
a
a
b
d
Sample
SBET (m2g−1
)
Pore Volume (cm3g−1
)
Average pore diameter (nm)a
Pt nanoparticle size (nm)
Pt crystallite size (nm)c
ID/IG
Pt/CB
156.72
365.73
0.385
0.496
18.58
34.51
3.27
2.62
2.8
1.17
1.03
Hollow Pt/CP
1.63
a
From the BET.
b
from TEM.
c
from XRD pattern.
d
from Raman spectrum.
3.2. Characterization of the catalysts
3.2.1. BET analysis
which was higher than the Pt/CB. Table 3 summarized the two
samples’ structural properties. Due to the hollow structure of CP,
its specific surface area is much larger than CB. It was precise
because of the large specific surface area and large pore diam-
eter of CP that it was conducive to the loading and dispersion
of Pt. Also, the big surface area was good for the methanol’s ad-
sorption and its activation. These results indicate that the hol-
low CP obtained by carbonization well maintains the character-
istics of ZIF-8, that is, it has a large specific surface area and a
good pore size. Moreover, the hollow Pt/CP had better pore vol-
ume and bigger pore diameter, which may be also useful for trans-
mitting electronics and transferring. Generally, it was considered
that the large surface area may be good for the adsorption and
activation of the reaction molecule. So, the hollow Pt/CP may be
more beneficial for improving the activity of MOR under the same
content.
To investigate the specific surface area and pore size of the two
catalysts, the nitrogen isotherms of adsorption-desorption were ob-
tained and the results were shown in Fig. 3A. The isotherms of N2
adsorption-desorption were carried out, and the results exhibited
a type IV hysteresis, with a typical H1 hysteretic loop, which in-
dicated the samples having mesoporous structure also including
capillary condensation [27]. Hollow Pt/CP’s isotherm was typical
for mesoporous materials, but included step at low relative pres-
sures, reflecting the framework’s dynamic character [28,29]. Also,
the BET surface area of hollow Pt/CP (365.7 m2
g
−1) was big-
ger than the Pt/CB (156.7 m2 −1). As the data made clear that
g
the specific surface area of hollow Pt/CP was nearly 2.3 times
that of Pt/CB. The pore size distribution was got by BJH which
was shown in Fig. 3B. The hollow Pt/CP was at about 10–60 nm,
5

C. Ding, F. Dong and Z. Tang
Electrochimica Acta 390 (2021) 138790
Fig. 4. The SEM (a), TEM (b) picture of ZIF-8 and TEM figures of hollow Pt/CP (c), (insert picture was the HRTEM of hollow Pt/CP), Pt/CB (d). Particle size analysis of Hollow
Pt/CP (e), Pt/CB (f). The (g–l) EDX elemental mapping of hollow Pt/CP.
3.2.2. Topography characterization
cubic (fcc) Pt [31]. Besides, we selected one of the dodecahedron
structures to display the EDX elemental mapping of the hollow
Pt/CP (Fig. 4g–l). It can be discovered that the C spread all over
the area, the distribution of O and N was similar, and the Pt was
evenly distributed throughout the frame. The resulting carbon ma-
terial (hollow CP), holding the advantages of the ZIF-8 and N, Pt
active species was highly dispersed and good conductivity, which
was highly desired for the MOR [32]. From the distribution map
of Pt elements, it can be found that the distribution of Pt at the
edge of the dodecahedron is more than that in the interior. At the
same time, we think that the distribution range of Pt is based on
the structure of carbon in CP, which indicates that the CP has a
hollow structure. And from the TEM results, we can know that the
C material was amorphous carbon, which had a big surface area.
This result was consistent with the BET results.
To get the two catalysts’ morphology information and analyze
the Pt’s particle size in the catalysts, SEM and TEM were con-
ducted. The results were shown in Fig. 4. From the SEM and TEM
(Fig. 4a and Fig. 4 b), a dodecahedron ZIF-8 was formed. After be-
ing carbonized under an argon atmosphere, the precursor dodec-
ahedron’s morphology was still maintained, and then the Pt was
evenly loading onto the surface. To compare the active component
Pt’s particle size distribution on the two supports, statistics were
made on the Pt in the two catalysts [Hollow Pt/CP (Fig. 4e), Pt/CB
(Fig. 4f)]. As the results have shown, the hollow Pt/CP had smaller
Pt nanoparticles than Pt/CB and the mean size which was 2.62 nm.
And the smaller Pt was good for the methanol oxidation [30]. And
the HRTEM (High-resolution TEM) of hollow Pt/CP was showed in
Fig. 4c Fig. 4 c insert, and the lattice spacing was 0.23 nm which
was fit into the Pt (111), and the lattice plane was face-centered
6

C. Ding, F. Dong and Z. Tang
Electrochimica Acta 390 (2021) 138790
Fig. 5. The XRD patterns of hollow Pt/CP and Pt/CB and the XRD pattern belongs to the 13–42°data of hollow Pt/CP (B).
Fig. 6. FTIR spectrums of Hollow Pt/CP and Pt/CB.
3.2.3. XRD analysis
XRD pattern, the Pt/CB was 2.8 nm, the hollow Pt/CP was 1.63 nm.
This result showed that the Pt’s dispersion in hollow Pt/CP was
very good. This result may due to the big surface area and appro-
priate pore size which was in favor of the Pt’s fine dispersion, and
Pt was evenly distributed on the carbon, which was agreed with
TEM results. And the XRD pattern of the hollow Pt/CP also illus-
trated this result again. The diffraction peak at 24.6° was ascribed
to the C (002), which was attributed to the amorphous carbon. To
better understand the various functional groups on the carbon sur-
face, we used FTIR to make a more detailed analysis.
The XRD was used to get the catalysts’ phase structure. And the
XRD patterns of the two catalysts were showed in Fig. 5. The re-
sults had shown that there were different crystal face peaks of C
and Pt. Also, the XRD exhibited the characteristic face-centered cu-
bic structure of Pt (JCPDS 87–0647) [33], which was agreed with
the HRTEM result. There were three peaks of Pt in the spectrum.
The diffraction peaks were at 39.6 ° for Pt (111), 46.2 ° for Pt (200),
67.2 ° for Pt (220) [34,35]. The Pt (111) can be seen. As for the hol-
low Pt/CP, insert the corresponding pattern into Fig. 5. It can be
found from Fig. 5 that there are two peaks in the range of 13°−42°,
one of which belongs to C (002) and the other belongs to Pt (111).
The different crystal planes’ of corresponding Pt peaks were not
very obvious. In the hollow Pt/CP, Pt (111) crystal plane mainly ex-
isted. But the crystal faces of Pt’s characteristics (200), (220), (311)
were almost inconspicuous. To explain the dispersion degree of Pt
in the catalyst and get the fine crystallite size of Pt (111). From the
3.2.4. FTIR analysis
We used the FTIR since FTIR was highly sensitive to the cat-
alyst’s surface chemistry. As Fig. 6 shown, there were mainly
four distinct characteristic peaks. The broad peak presence of -
OH (ʋO–H at about 3400 cm−1), C = O can be discovered (ʋC=O
7

C. Ding, F. Dong and Z. Tang
Electrochimica Acta 390 (2021) 138790
Fig. 7. O1s (A), Pt4f (B) XPS, and C1s (C) XPS spectra of Hollow Pt/CP and Pt/CB.
at 1627 cm−1) [36]. As the study had showed, they were C H of
Table 4
–
Relative content of species in O1s peaks, O/C atomic ratio and Pt0/(Pt0+Pt2+) in
–
olefin, C = O, -OH, and C H of saturated alkanes. And the peak
Pt4f determined by XPS spectra of samples.
at around 1300 cm−1 belonged to the C N stretch [37]. As the
–
Relative O1s peak area (%)
–
Pt0/(Pt0+Pt2+)(%)
results showed, oxygen-containing functional groups’ intensity in
hollow Pt/CP was higher than the Pt/CB. In particular, as for the
hollow Pt/CP, the -OH was higher than the Pt/CB, which meant
the hollow Pt/CP had better hydrophilicity. Also, the -OH was good
for adhering more Pt4+ in H2PtCl6•6H2O to the carbon material
derived from ZIF-8, thereby the hollow Pt/CP obtaining more Pt,
which was the main reason for its higher activity than Pt/CB. And
the C = O band intensity of the hollow Pt/CP was also higher than
Pt/CB, which made the catalyst favor the oxidation of the one-
carbon compound. And the detailed analysis of the catalyst surface
elements will be showed below.
Samples
O/C
C-OH
C
O
C = O
-OH
Pt/CB
0.48
0.63
36
41
15
17
38
27
11
15
32
48
Hollow Pt/CP
tional groups such as hydroxyl had a certain increase in the cat-
alyst’s hydrophilicity. And these groups’ intensity in the hollow
Pt/CP was higher than that of Pt/CB. As for the C1s spectrum,
the peak at 285.08 ev was the carbon (sp3 C C), and the peak at
–
–
284.18 ev was ascribed to the C N [40,41]. To have a visualized
3.2.5. XPS technique
analysis, we listed the mass ratio of O/C and the oxygen relative
percentage in the four different functional groups in Table 4. For
the hollow Pt/CP, the content of the oxygen on the surface was
higher than that of Pt/CB. The most important was that the car-
boxyl group was the key addition of functional groups in oxygen
was as the data showed. The results indicated that the -COOH will
legitimately fix the Pt ions precursor via electrostatic or coordina-
tion action, thereby more Pt nanoparticles were dispersed on the
The XPS was normally used to obtain the surface composition
of catalysts and their electronic structure. Fig. 7 had shown the
O1s (A) Pt4f (B), and C1s (C) spectrum. Form the O1s spectrum,
there were four peaks at 532.08ev, 532.88ev, 533.5ev, 534.38 ev.
–
The 532.08 ev was attributed to C O, and 533.58 ev belonged to
C = O, which was higher than the Pt/CB. The peak at 532.88 ev
–
was C OH. The 534.58 ev was carboxyl’s O atom [38,39]. Func-
8

C. Ding, F. Dong and Z. Tang
Electrochimica Acta 390 (2021) 138790
Fig. 8. Raman spectrums of Hollow Pt/CP and Pt/CB, insert was the Raman spectrums from 1300 to 1700 cm−1 of Pt/CB.
hollow CP surface after the reduction which was the ethylene gly-
col method. Also, this result was agreed with former research [42].
As for the Pt4f spectrum, the Pt4f7/2 and Pt4f5/2 existed. The more
intense doublet (71.0 eV and 74.5 eV) was deemed to Pt(0) species
[43] which was in favor of the catalytic activity. The other two
peaks belonged to Pt (Ⅱ), which may be the PtO or Pt(OH)2 [44].
As the results had shown, the intensity of Pt(0) in the hollow Pt/CP
was higher than the Pt/CB. We calculated the proportion of Pt(0)
in the entire spectrum and found the proportion of Pt(0) in hollow
Pt/CP was 48%, which was higher than that of Pt/CB (32%). This
meant that the hollow CP was more conducive to the formation of
Pt(0) which was good for increasing catalytic activity. This was due
to the structural advantage of hollow CP over carbon black, which
had more carboxyl groups made it easier for the precursor of Pt to
be fixed to the CP and be reduced to Pt. Such consequences also
displayed that the oxidation activity of hollow Pt/CP was higher
than that of Pt/CB in MOR. The carbonized carbon could supply
more electronic traps which made the e− can easily transfer to the
surface of porous carbon [45], which was good for the MOR.
onally bonded, and the carbon frames were distorted on the defect
sites in the graphitic networks, respectively [51]. In order to know
the graphitization degree, using the D-band to G-band intensity ra-
tio, and the higher intensity of ratio meant the sample with higher
defects or disorder [52–54], and the disorder of the carbon struc-
ture may come from carbon’s other electronic structure, such as
sp2, sp3 on carbon chain or carbon ring in the hollow CP, which
was good for the later reaction. The results had shown that hol-
low Pt/CP had higher ID/IG, and the carbonation at a high temper-
ature can lead to more defects or disorder. More structural defects
made the obtained hollow CP material more conducive to the dis-
persion of precious metal-Pt and were useful for catalytic reactions
to provide more active sites and facilitated better contact between
methanol and catalyst. The above characterization results indicate
that the activity can be influenced by the material’s surface area,
surface properties, the active components’ particle size, and so on
[55].
3.3. Catalytic mechanism
3.2.6. Raman spectroscopy
As the above results had shown, the hollow Pt/CP catalyst was
the best in all two samples. From the characterization results, the
hollow CP had a big surface area and favorable pore size which
was good for dispersing metal-Pt. And the hollow CP also con-
tained more hydroxyl groups, which connected more Pt precursors
to the carrier, resulting in more elemental Pt. Besides, the hollow
Pt/CP had more Pt0 which was the most important active ingredi-
ent in MOR. All of these advantages made the hollow Pt/CP catalyst
showed better methanol oxidation activity than Pt/CB. And the ox-
idation of methanol on the catalyst was mainly through methanol
dehydrogenation into intermediates, dehydrogenation, and oxida-
tion of these intermediates ultimately into CO2. Adsorption and de-
hydrogenation mainly occur at Pt sites. To better illustrate the per-
formance of the hollow Pt/CP, the oxidation process of methanol on
the catalysts was put down. The process of adsorption occurred in
To explore the structural defects difference of the hollow Pt/CP
and Pt/CB, the Raman was conducted. The results were showed in
Fig. 8. The two strong peaks were at 1369 cm−1 and 1584 cm−1
,
which were respectively called D (Disordered carbon) and
G
(Graphtic carbon) peak. The G peak belonged to the bond’s stretch-
ing for all chain structure or ring structure’s sp2 atom pairs. The D
peak was attributed to disordered carbon atom structure [46,47].
Graphene-like carbon has the G and D peaks, which lie at~1560
and 1360 cm−1, respectively [48]. The G peak position shows dis-
persion (excitation wavelength dependence). The D peak position
means disordered sp3 carbon, which gives a broad feature in 1100
cm−1–1350 cm−1, the shape of which corresponds to the density of
states [49]. Also, it redounded to the amorphous or disordered car-
bon [50]. The D and G meant the carbon atoms which were hexag-
9

C. Ding, F. Dong and Z. Tang
Electrochimica Acta 390 (2021) 138790
Fig. 9. The mechanism diagram of hollow Pt/CP.
the voids of CP, which was further enhanced by the presence of Pt.
Moreover, the pores of hollow Pt/CP also adsorbed water molecules
which facilitated the oxidation of intermediates to CO2. The big
surface area and porosity of the hollow Pt/CP played an important
role in accelerating the methanol adsorption and subsequent oxi-
dation by providing an extended active surface area [56–58]. The
processes can be predicted as the following proposed mechanism,
and the corresponding mechanism diagram was shown in Fig. 9.
4. Conclusions
In this work, the hollow CP support was successfully derived
from ZIF-8, and it was discovered that the ZIF-derived hollow CP
support had many structural advantages, such as a large specific
surface area, good pore size, abundant oxygen-containing func-
tional groups, like carboxyl, which were necessary for the subse-
quent loading and dispersion of Pt nanoparticles, and maybe ben-
efit for the transport of electron. Based on the results of charac-
terizations, we discovered that the hollow Pt/CP due to the special
morphology had the bigger specific surface area, which was 365.7
CH3OH + Pt → Pt − CH3OHad
Pt − CH3OHad → Pt − COad+H+
Pt + OH− → Pt − OHad+e−
m2 g−1 and the Pt/CB was 156.7 m2 −1, which was The high sur-
g
face area was good for the Pt dispersion, the adsorption and acti-
vation of reactants. More importantly, the hollow Pt/CP had richer
oxygen-containing functional groups, higher Pt0/(Pt0+Pt2+), which
were beneficial to improve catalyst activity. In addition, the hollow
Pt/CP had more structural defects, which was in favor of the ac-
Pt − OHad+Pt − COad → 2Pt + CO2+H2O
10

C. Ding, F. Dong and Z. Tang
Electrochimica Acta 390 (2021) 138790
tivity. Therefore, the hollow Pt/CP showed better activity than the
Pt/CB, and a better ability to against CO-like substances toxic. This
also shown that the ZIF-derived carbon material was very mean-
ingful material as a substrate, which was useful for improving the
activity and stability of the catalyst.
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Declaration of Competing Interest
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The authors declare that they have no known competing finan-
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Chengxiu Ding: Writing – original draft. Fang Dong: Conceptu-
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Acknowledgments
This authors wish to express their sincere appreciation for
the National Natural Science Foundation of China (51908535,
52070182), Province Natural Science Foundation of GanSu
(20JR10RA053, 20JR10RA046), the Major Project of Inner Mongolia
Science and Technology (2019ZD018), and the K.C. Wong Education
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12