Bimetallic Cu?Zn Co-Doped Porous N/C as Efficient Catalysts for Oxygen Reduction Reaction and Oxidation of 1,2-Propanediol

Article abstract of DOI:10.1002/cctc.201900975

The development of non-precious metal catalysts toward oxygen reduction reaction (ORR) is essential for widespread application of fuel cells. Herein, bimetallic Cu?Zn co-doped porous N/C catalysts (Cu?Zn?N/C) with high N content were synthesized via a simple metal-organic framework (MOF)-derived method. The as-prepared Cu?Zn?N/C catalyst presents superior catalytic activity toward ORR with an onset potential of 0.90 V, a half-wave potential of 0.79 V, and a diffusion limiting current density of 5.82 mA cm^?2. It is found that Cu exists in three forms: one exists in Cu particles, and the others exist in Cu single atoms (either in high valence or in zero valence) that embedded in the carbon substrate. The evaporation of Zn atoms during the pyrolysis induces the pore structure of the carbon matrix, making it possible to support for the zero valent Cu atoms. In addition, as an efficient Cu-based catalyst for many applications, we show that the as-prepared catalyst can selectively oxidize 1,2-propanediol to lactic acid with 75.7 % selectivity at 82.8 % conversion.

Full text of DOI:10.1002/cctc.201900975

Accepted Article
Title: Bimetallic Cu-Zn Co-doped Porous N/C as Efficient Catalysts for
Oxygen Reduction Reaction and Oxidation of 1,2-Propanediol
Authors: Li-Ping Yang, Jian-Li Mi, Hui-Jie Wang, Jia-Hao Liang, Xue-
Jing Yang, Yong-Hai Feng, Peng Zhang, and Lu Liu
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10.1002/cctc.201900975
ChemCatChem
Bimetallic Cu-Zn Co-doped Porous N/C as Efficient
Catalysts for Oxygen Reduction Reaction and Oxidation
of 1,2-Propanediol
Li-Ping Yang,[a] Jian-Li Mi,*[a] Hui-Jie Wang,[a] Jia-Hao Liang,[a] Xue-Jing Yang,[a] Yong-Hai Feng,[a]
Peng Zhang,*[a] and Lu Liu[b]
[a] Institute for Advanced Materials, School of Materials Science and Engineering, Jiangsu University,
Zhenjiang 212013, China. Email: jlmi@ujs.edu.cn (J.-L. Mi); zhangpjlxb@ujs.edu.cn (P. Zhang)
[b] School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China
Abstract: The development of non-precious metal catalysts toward oxygen reduction reaction (ORR) is
essential for widespread application of fuel cells. Herein, bimetallic Cu-Zn co-doped porous N/C
catalysts (Cu-Zn-N/C) with high N content were synthesized via a simple metal-organic framework
(MOF)-derived method. The as-prepared Cu-Zn-N/C catalyst presents superior catalytic activity toward
ORR with an onset potential of 0.90 V, a half-wave potential of 0.79 V, and a diffusion limiting current
density of 5.82 mA cm-2. It is found that Cu exists in three forms: one exists in Cu particles, and the
others exist in Cu single atoms (either in high valence or in zero valence) that embedded in the carbon
substrate. The evaporation of Zn atoms during the pyrolysis induces the pore structure of the carbon
matrix, making it possible to support for the zero valent Cu atoms. In addition, as an efficient Cu-based
catalyst for many applications, we show that the as-prepared catalyst can selectively oxidize
1,2-propanediol to lactic acid with 75.7% selectivity at 82.8% conversion.
Keywords: Cu-Zn-N/C; nonprecious metal catalyst; oxygen reduction reaction; selective oxidization of
1,2-propanediol
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Introduction
With the increasing demands for fuels and the increasing environmental problems caused by overuse and
exploitation of fossil fuels, many efforts have been devoted to developing sustainable energy conversion
devices. Among the most promising energy conversion technologies, fuel cells have been widely
investigated due to their zero-carbon emission and high fuel conversion efficiency.1-4 However, high
loading of Pt-based catalysts is required to accelerate the sluggish kinetics of the cathode oxygen
reduction reaction (ORR), which unfavorably hinders the large-scale commercialization of fuel cells.5
Besides, Pt-based catalysts always suffer from limited long-term stability, vulnerability to CO poison and
poor tolerance to methanol.6,7 Therefore, it has stimulated considerable enthusiasm for the development
of non-precious metal based catalysts with excellent activity and durability.8-12
Transition metals and nitrogen co-doped carbon materials (M-N/C) have received extensive attention
because of the low cost and promising activity toward the ORR.13-15 Tremendous progress has been
made in the catalyst synthesis methodologies such as metal-organic framework (MOF)-derived
method,16,17 and heat-treatment is normally identified as necessary for the activity and stability
improvement of M-N/C catalysts. The enhanced performance of the catalysts is dependent on the carbon
support, the source of metal and nitrogen, and the thermal treatment conditions.18 The transition metal
plays an important role in the improving the ORR activity of the N-doped carbon catalysts. It is known
that the ORR activity of M-N/C is very sensitive to the transition metals. It was reported that the ORR
activity of the M-N/C catalysts to follow the order of Fe > Co > Cu > Mn > Ni.19 Recently, Cu-N/C
catalysts were reported to be excellent electrocatalysts for ORR.20-23 For example, Cu-N/C catalyst with
abundant Cu single atom active sites was prepared and has shown superior electrocatalytic activity due
to the maximum Cu atom exposure of the catalyst.24
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In addition, Cu has many applications in chemical catalysts, not only due to its low price, but also due
to its ability of mediating a wide range of important oxidation reactions.25 Glycerol, a by-product from
bio-diesel manufacture, can be reduced to give 1,2-propanediol that can then be selectively oxidized to
lactic acid.26,27 The catalytic oxidation of 1,2-propanediol is generally carried under mild reaction
conditions using noble metals such as Pd and Pt modified by heavy metals such as Pb, Bi, Te, and
Sn.28,29 Nevertheless, development of catalysts with high efficiency and low cost is still highly desired.
In the present study, bimetallic Cu-Zn co-doped porous N/C catalysts with high N content were
synthesized via a simple pyrolysis method. The catalyst presents superior ORR catalytic activity, as well
as promising selective oxidation of 1,2-propanediol to lactic acid. The structure and composition of the
catalysts are carefully analyzed and the structure-activity relationship are discussed. The high activity of
the catalyst should be attributed to the high content of N species, the different Cu forms and the porous
structure resulting in abundant exposed active sites. The evaporation of Zn during the pyrolysis induces
the pore structure of the catalysts, facilitating the mass transfer during the catalytic process. The residual
Zn may further promote the activity of the catalysts.
Experimental Section
Chemicals
All of the chemicals used in the experiments were analytical grade. 2-methylimidazole (C4H6N2) was
obtained from Aladdin. Cu(NO3)2·3H2O, and Zn(CH3COO)2, 1,2-propanediol, n-butanol were purchased
from Sinopharm Chemical Reagent Co, LTD. Nafion (5 wt%) was from Dupont Corp (USA), and
commercial Pt/C (20 wt%) was from Johnson Matthey (UK).
Preparation of catalysts
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Cu-Zn-N/C catalysts were prepared by a metal-organic framework (MOF)-derived method. First, Cu
doped zeolite imidazole framework-8 (ZIF-8) was prepared by a NaOH-mediated method using
2-methylimidazole, NaOH, Cu(NO3)2·3H2O and Zn(CH3COO)2, similar to that described in our previous
study.30 In a typical synthesis, 12 mmol of 2-methylimidazole and a total amount of 6 mmol of
Cu(NO3)2·3H2O and Zn(CH3COO)2 were dissolved in 200 mL of deionized water. 12 mmol of NaOH
was then added to the above solution. After vigorously stirred for 1 h, the obtained precipitates were
centrifuged, washed by deionized water and ethanol, dried at 60 °C overnight, and Cu doped ZIF-8 was
obtained. Different molar ratios of Cu(NO3)2·3H2O and Zn(CH3COO)2 were performed in this study.
Cu-Zn-N/C catalysts were then obtained by carbonizing Cu doped ZIF-8 at 900 °C for 3 h under N2
atmosphere. The catalysts are nominated as Cu-Zn-N/C-x, where x denotes the molar ratio of
Cu2+/(Cu2++Zn2+) in the precursor.
Characterization
The morphology of the catalysts was characterized by a JSM-7800F scanning electron microscopy (SEM)
and a JEM-2100F transmission electron microscope (TEM). The elemental mapping was performed on
the energy-dispersive X-ray spectroscope attached to JEM-2100F. X-ray diffraction (XRD) was
performed on an X'pert PRO MPD X-ray diffractometer using Cu-Kα radiation (λ = 1.5406 Å). The
chemical states were characterized by X-ray photoelectron spectroscopy (XPS) on a Thermo Scientific
Escalab250Xi system.
Electrochemical measurements
The ORR activity was measured on an electrochemical workstation (CHI 760e, Chenhua Instrument Co.
Ltd, China) coupled with a rotating disk electrode (RDE) system (Pine Research Instrumentation, USA).
An Ag/AgCl electrode in saturated KCl solution was used as the reference electrode, and a graphite
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electrode was used as the counter electrode. All the measured potentials were converted to the reversible
hydrogen electrode (RHE). The catalyst loading on the glassy carbon working electrode was 0.255 mg
cm-2. All the electrochemical tests were performed in N2- or O2-saturated 0.1 M KOH solution at room
temperature. Cyclic voltammetry (CV) was recorded at a scan rate of 50 mV s-1 and linear sweep
voltammetry (LSV) was performed at 10 mV s-1. The electron transfer number (n) was analyzed using
Koutecky-Levich (K-L) equation:31
-1
j-1 = jL-1 + jK-1 = B-1-1/2 + jK
(1)
(2)
B = 0.2nFC0(D0)2/3v-1/6
where j, jL and jK are the measured current density, limiting current density and kinetic current density,
respectively,  is the electrode rotation speed (rpm), F is the Faraday constant (96485 C mol-1), C0 is the
bulk O2 concentration in 0.1 M KOH (1.2 × 10-6 mol cm-3), D0 is the O2 diffusion coefficient in 0.1 M
KOH (1.9 × 10-5 cm2 s-1), and v is the kinetic viscosity (0.01 cm2 s-1).
Catalytic oxidation of 1,2-propanediol
The catalytic oxidation of 1,2-propanediol was conducted in a 250 mL capacity stainless steel autoclave
equipped with a magnetically driven impeller, similar to our previous study.29 Typically, 0.06 g of
catalyst, 0.5 g of NaOH, 0.5 g of 1,2-propanediol and 40 mL of deionized water was added to the
autoclave. The autoclave was purged with O2 for 10 min and then pressurized to the desired pressure of 1
MPa. Afterwards, the autoclave was heated and maintained at 140 °C for 4 h accompanied with stirring
(600 rpm). After reaction, the autoclave was cooled to ambient temperature and depressurized. The
concentration of the remained 1,2-propanediol was analyzed on a gas phase chromatograph equipped
with a PEG-20 M packed capillary column (0.25 mm × 30 m) and flame ionization detector (FID) with
n-butanol as the internal standard. The selectivity of products was analyzed on an Agilent high
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performance liquid chromatography (HPLC) system equipped with a strong cation hydrogen exchange
column (carbonmix H-NP, 7.8 mm × 300 mm) and a UV detector at 35 °C. Before analysis, the pH of the
product solution was adjusted to 2-3 with hydrochloric acid. Sulfuric acid solution (5 mM) was used as a
mobile phase with a flow rate of 0.6 mL min-1. The concentrations of the products were analyzed by the
external standard method.
Results and Discussion
Structure and composition analysis
Bimetallic Cu-Zn co-doped porous N/C catalysts (Cu-Zn-N/C) were synthesized via a simple
metal-organic framework (MOF)-derived method by pyrolysis of Cu doped ZIF-8 at 900 °C under N2
flow. Figure 1a shows the XRD patterns of the as-prepared catalysts Cu-Zn-N/C-x (x = 0.025, 0.05, 0.1,
0.2, and 0.3). Only two broad diffraction peaks centered around 25° and 43° are detected for
Cu-Zn-N/C-0.025, which correspond to the (002) and (101) diffractions of carbon, respectively,
indicating the formation of amorphous carbon with partially graphitized. No Cu phase is detected
suggesting that the Cu may exist in single atoms that embedded in the carbon substrate in
Cu-Zn-N/C-0.025. Well crystallized Cu phase is observed for other samples. The relative intensity of the
Cu diffraction peaks increases from sample x = 0.05 to x = 0.3 due to the increase of Cu content in the
catalysts. No obvious Zn or its compounds are detected in the products, indicating that most of the Zn is
evaporated during the pyrolysis. The unit cell parameters of the Cu phase in different samples were
calculated from the XRD patterns (Figure 1b). It shows that the unit cell parameter (a) of Cu varies from
the catalysts, and a increases slightly with x in Cu-Zn-N/C-x (a = 3.601(1) Å for Cu-Zn-N/C-0.025, and
a = 3.614(1) Å for Cu-Zn-N/C-0.3). This could be attributed to the size effect on the surface stress,
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resulting a lattice contraction with reduced particle size. As can be seen from Figure 1b, the particle size
of Cu that calculated by the Scherrer equation basically increases with x from 30(3) nm for
Cu-Zn-N/C-0.05 to 42(1) nm for Cu-Zn-N/C-0.3.
Figure 1. (a) XRD patterns of the as-prepared catalysts Cu-Zn-N/C-x (x = 0.025, 0.05, 0.1, 0.2, and 0.3).
(b) Unit cell parameters a and the calculated particle sizes d of Cu in different catalysts.
The morphology of Cu-Zn-N/C-0.025 and Cu-Zn-N/C-0.1 was investigated using SEM and TEM.
Figure 2a and 2b show the SEM images of the Cu-Zn-N/C-0.025 and Cu-Zn-N/C-0.1, respectively.
Cu-Zn-N/C-0.025 is composed of uniform nanosized carbon particles, whereas large submicrosized
irregular carbon particles decorated with small metal nanoparticles are observed in Cu-Zn-N/C-0.1. Both
catalysts exhibit porous structures which enable more exposed active sites and fast mass transfer. As
further revealed by the TEM images, no Cu particles can be seen on the carbon substrate in
Cu-Zn-N/C-0.025 (Figure 2c and 2d), whereas Cu nanoparticles with a darker contrast compared with
the substrate are wrapped in the carbon substrate for Cu-Zn-N/C-0.1 (Figure 2e and 2f). The inset of
Figure 2e shows the corresponding size distribution of Cu particles and an average particle size of 35 nm
is obtained. The results are in good accordance with the XRD data.
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Figure 2. (a,b) SEM images of Cu-Zn-N/C-0.025 and Cu-Zn-N/C-0.1, respectively. (c,d) TEM images of
Cu-Zn-N/C-0.025. (e,f) TEM images of Cu-Zn-N/C-0.1. The inset of (e) is the corresponding size
distribution of Cu particles.
The composition and elemental distribution of Cu-Zn-N/C-0.025 and Cu-Zn-N/C-0.1 is further
analyzed by EDS elemental mapping (Figure 3). The results show that residual element Zn is present in
both Cu-Zn-N/C-0.025 and Cu-Zn-N/C-0.1. It confirms the uniform dispersions of C, N, O, Zn, and Cu
in the carbon substrate for Cu-Zn-N/C-0.025 (Figure 3a). Cu mainly forms particles in Cu-Zn-N/C-0.1,
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however, it is indicative that Cu is also distributed on the carbon substrate (Figure 3b).
Figure 3. EDS elemental mapping images of (a) Cu-Zn-N/C-0.025 and (b) Cu-Zn-N/C-0.1.
XPS was utilized to investigate the surface chemistry of Cu-Zn-N/C-0.025 and Cu-Zn-N/C-0.1. The
XPS survey spectra (Figure 4a) show the presence of C, N, O, Cu, and Zn in both catalysts. The high
resolution XPS spectra of C 1s (Figure 4b) can be deconvoluted into five peaks, corresponding to
various carbon moieties (C-C, C-N, C-O, C=O) and the π-π* interaction between the graphene
layers,32,33 indicating the existence of N and O heteroatoms in the carbon substrates for both
Cu-Zn-N/C-0.025 and Cu-Zn-N/C-0.1. The deconvolution of high resolution N 1s spectrum of
Cu-Zn-N/C-0.025 (Figure 4c) show four peaks at 398.49 eV, 399.24 eV, 400.75 eV, and 403.42 eV,
corresponding to pyridinic N (45.5%), pyrrolic N (16.2%), graphitic N (27.1%), and oxidized N (16.2%),
respectively.34 For Cu-Zn-N/C-0.1, the fractions are 36.6%, 21.2%, 33.9%, and 8.4% for pyridinic N at
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398.42 eV, pyrrolic N at 399.19 eV, graphitic N at 400.74 eV, and oxidized N at 403.70 eV, respectively.
It should be noted that the catalysts prepared in the present study have very high N content. The total N
contents are 16.7 at% and 15.3 at% for Cu-Zn-N/C-0.025 and Cu-Zn-N/C-0.1, respectively. In spite of
the large difference in the carbon morphology, the N content doesn’t vary much with Cu content in the
catalysts. This implies that the increase of Cu content from Cu-Zn-N/C-0.025 to Cu-Zn-N/C-0.1 doesn’t
change the content of embedded Cu atoms to form Cu-Nx species in the carbon substrate very much, but
mainly forms Cu particles. High resolution Cu 2p spectra (Figure 4d) show both zero valent Cu and high
valent Cu.35 The high valent Cu can be attributed to the presence of Cu-Nx and Cu-Ox species that
embedded in the carbon substrates. The zero valent Cu can be from the metallic Cu nanoparticles in
Cu-Zn-N/C-0.1, however, it is interesting to see that zero valent Cu is also detected in Cu-Zn-N/C-0.025,
indicating that the embedded Cu can either be N/O-bonded Cu-Nx/Cu-Ox species, or be present in zero
valence single atoms. Recently, it was reported the atomic catalysts comprising only isolated Fe/Ni zero
valence single atoms anchored on graphdiyne.36 Graphdiyne with natural uniform pores and triple bond
rich with strong reduction ability, is suggested to be an ideal support for single-atom catalysts. In
addition to sp2 hybridization, the sp hybridization of −C≡C− enables the arbitrary angle rotation of π/π*
to the axis, making it possible to point toward metal atoms and, thereby, chelate a single metal atom.
Herein, we find that Cu zero valence single atoms can also be formed on the carbon support by a
MOF-derived method. The evaporation of Zn during the pyrolysis may induce the similar pore structure
of the carbon matrix to graphdiyne, making it possible to support for the zero valent Cu atoms. The
valence bands of Cu-Zn-N/C-0.025 and Cu-Zn-N/C-0.1 were also measured. The matrix of the
Cu-Zn-N/C-x catalysts in the present study is N-doped amorphous carbon, which is very likely to be a
semiconductor.37,38 Due to the problem of obtaining the adequate reference level, the assignment of
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binding energies of the valence bands is difficult. Nevertheless, XPS valence band spectra allow the
comparison of the band electronic structure between different catalysts. As shown in Figure 4e, the top
of the valence band of Cu-Zn-N/C-0.1 is closer to the Fermi level compared with Cu-Zn-N/C-0.025. It
suggests that the Cu particles in the Cu-Zn-N/C-x catalysts can further adjust electronic structure and
improve the electrical conductivity, facilitating the electron transfer during the catalysis.
Figure 4. (a) XPS surveys, (b-d) high-resolution XPS spectra of C 1s, N1s and Cu 2p, and (e) valence
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band spectra of (a) Cu-Zn-N/C-0.025 and (b) Cu-Zn-N/C-0.1.
ORR catalytic activity
Figure 5a displays the CV curves of the catalysts in 0.1 M KOH. Obvious reduction peaks appear in
O2-saturated electrolyte, while featureless voltammetric currents are observed in N2-saturated electrolyte.
Figure 5b shows the LSV curves of Cu-Zn-N/C-x (x = 0.025, 0.05, 0.1, 0.2, and 0.3) and commercial
Pt/C in O2-saturated 0.1 M KOH electrolyte at 1600 rpm. To get the exact current response from ORR,
the currents were corrected by subtracting the corresponding currents in N2-saturated solution. Among
the bimetallic Cu-Zn co-doped porous N/C catalysts, Cu-Zn-N/C-0.1 shows the best ORR catalytic
activity with an onset potential (E0) of 0.90 V, a half-wave potential (E1/2) of 0.79 V, and the limiting
current density (jL) of 5.8 mA cm-2, comparable to that of commercial Pt/C with E0 = 0.96 V, E1/2 = 0.81
V, and jL = 5.7 mA cm-2. The ORR activity of Cu-Zn-N/C-0.1 is also comparable to the recent published
Cu-N/C catalysts.21,22,39,40 For example, Cu-N/C prepared by pyrolyzing Cu-doped
zeolitic-imidazolate-framework-8 was reported to have the ORR activity with E0 = 0.914 V, E1/2 = 0.813
V, and jL = 5.5 mA cm-2.39 And, a Cu-N/C catalyst with E0 = 0.883 V, E1/2 = 0.804 V, and jL = 5.06 mA
cm-2 was also reported.40 The half-wave potentials and the limiting current densities of different
Cu-Zn-N/C-x catalysts in the present study are summarized in Figure 5c. With the increase of Cu content
from x = 0.025 to x = 0.1, the half-wave potential for ORR only improves a little (from 0.76 V to 0.79 V),
while significant enhancement of limiting current density is observed (from 4.3 mA cm-2 to 5.8 mA cm-2).
It suggests that the inclusion of Cu nanoparticles contributes a lot to the enhancement of current density.
With the further increase Cu content from x = 0.1 to x = 0.3, both half-wave potential and current density
decrease which could due to the reduced surface area. As shown in Figure 5d, the good ORR activity of
Cu-Zn-N/C-0.1 is further evidenced by the small Tafel slope of 60.1 mV dec-1, which is much smaller
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than that of commercial Pt/C (82 mV dec-1). Figure 5e shows LSV curves of Cu-Zn-N/C-0.1 at various
rotating rates. The current density increases with the rotating rates, which is explained by the shortened
diffusion distance at high speed. The corresponding K-L plots at different potentials from 0.2 V to 0.5 V
are shown in Figure 5f. The K-L plots show good linearity, suggesting the first-order reaction kinetics of
Cu-Zn-N/C-0.1. The average electron transfer number (n) is calculated to be 3.98 for Cu-Zn-N/C-0.1,
indicating that the ORR proceeds with an efficient four-electron transfer pathway.
Figure 5. (a) CV curves of Cu-Zn-N/C-x (x = 0.025, 0.05, 0.1, 0.2, and 0.3) in O2-saturated (solid lines)
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and N2-saturated (dash lines) 0.1 M KOH solution. (b) LSV curves of Cu-Zn-N/C-x (x = 0.025, 0.05, 0.1,
0.2, and 0.3) and commercial Pt/C (20 wt%) measured by RDE at 1600 rpm. (c) Half-wave potentials
(E1/2) and the limiting current densities (jL) of different catalysts obtained from the LSV curves of 1600
rpm. (d) Tafel slopes of Cu-Zn-N/C-0.1 and Pt/C. (e) LSV curves of Cu-Zn-N/C-0.1 at various rotating
speeds, and (f) the corresponding K-L plots at varied potentials.
Overall, based on the above analysis, we suggest that the electrocatalytic performance of bimetallic
Cu-Zn co-doped porous N/C catalysts toward ORR is attributed to the synergistic effect of multiple
factors (Figure 6). First, the differernt N species (pyridinic N, graphitic N, and pyrrolic N), Cu-Nx and
Zn-Nx species are generally proposed to play essential roles for ORR of carbon-based catalysts. Second,
the Cu nanoparticles are believed to contribute a lot to the catalytic activity, especially to the high
limiting current density. Third, the impressive ORR catalytic performance should also be ascribed to the
active sites provided from the Cu zero valence single atoms. Finally, the pore structure and high surface
area created by the Zn evaporation during the pyrolysis not only provides abundant exposed active sites,
but also facilitates mass transfer during the catalytic process.41
Figure 6. Schematic illustration of the structure and ORR mechanism of bimetallic Cu-Zn co-doped
porous N/C catalysts.
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Selective oxidization of 1,2-propanediol
As Cu-Zn-N/C-x may be efficient catalysts for many applications, we studied the selective oxidization of
biomass derivative 1,2-propanediol to value-added lactic acid. Table 1 shows the activity of different
catalysts Cu-Zn-N/C-x (x = 0.025, 0.05, 0.1, 0.2, and 0.3) for the catalytic oxidation of 1,2-propanediol.
It is found that the conversion of 1,2-propanediol increases with Cu content from x = 0.025 to x = 0.05,
and then decreases with further increase of Cu content. The conversion rates of 82.8% and 67.4% are
achieved for Cu-Zn-N/C-0.05 and Cu-Zn-N/C-0.1, respectively. Formic acid, lactic acid and acetic acid
are the products of the catalytic oxidation of 1,2-propanediol. It seems that the presence of Cu
nanoparticles in the catalysts plays an important role in the selectivity of lactic acid. Cu-Zn-N/C-0.025
without Cu particles shows a lactic acid selectivity of 63.6%, whereas higher selectivity of above 75% is
achieved for other Cu-Zn-N/C-x catalysts with the presence of crystalline Cu nanoparticles. The results
show that as-prepared catalysts can selectively oxidize 1,2-propanediol to lactic acid efficiently.
Table 1 The catalytic activities of Cu-Zn-N/C-x (x = 0.025, 0.05, 0.1, 0.2, and 0.3) for the catalytic
oxidation of 1,2-propanediol.
Selectivity (%)
Catalysts
Conversions (%)
Formic acid
22.3
Lactic acid
63.6
Acetic acid
14.1
6.5
Cu-Zn-N/C-0.025
Cu-Zn-N/C-0.05
Cu-Zn-N/C-0.1
Cu-Zn-N/C-0.2
Cu-Zn-N/C-0.3
41.3
82.8
67.4
48.6
48.8
17.8
75.7
15.1
79.0
5.9
15.7
78.5
5.8
14.7
80.5
4.8
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Conclusions
In summary, bimetallic Cu-Zn co-doped porous N/C catalysts were prepared by a simple MOF-derived
method. Cu-Zn-N/C catalysts present superior catalytic activities toward ORR and selective oxidization
of 1,2-propanediol to lactic acid. Cu-Zn-N/C-0.1 exhibits the ORR activity with an onset potential of
0.90 V, a half-wave potential of 0.79 V, a diffusion limiting current density of 5.82 mA cm-2, and a Tafel
slope of 60.1 mV dec-1, which is comparable to that of commercial Pt/C (20 wt%). Cu-Zn-N/C-0.05 can
selectively oxidize 1,2-propanediol to lactic acid with 75.7% selectivity at 82.8% conversion. We
demonstrate that the catalytic activity is attributed to the synergistic effect of multiple factors, such as the
high N content, the different Cu forms (Cu particles, Cu-Nx species, and Cu zero valence single atoms)
in the catalysts, the pore structure and high surface area created by the Zn evaporation. Specially, the
presence of Cu particles can improve the electrical conductivity and enhance the catalytic activity.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (51401089), the Six
Talent Peaks Project of Jiangsu Province (2016-XCL-016), the Innovation and Creation Team Program
of Jiangsu Province (2016-32), and the Postdoctoral Research Funding Plan of Jiangsu Province
(1701029A).
Conflict of Interest
The authors declare no conflict of interest.
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Table of Contents
Bimetallic Cu-Zn co-doped porous N/C catalysts with the presence of different Cu forms show
promising activity toward oxygen reduction reaction and oxidation of 1,2-propanediol.
20
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