Synthesis of Ag nanoparticles/ordered mesoporous carbon as a highly efficient catalyst for the electroreduction of benzyl bromide

Article abstract of DOI:10.1039/c9ra08930f

To develop efficient catalysts for the electroreduction of organic halides, a facile one-pot synthesis of Ag nanoparticles/ordered mesoporous carbon electrode materials via the self-assembly of CH₃COOAg and resol in the presence of triblock copolymer is proposed. The resultant electrode materials possess uniform mesopore sizes (3.3 nm) and pore volumes (～0.28 cm³ g^-1), high specific surface areas (～500 m² g^-1), and uniformly dispersed Ag nanoparticles (12-36 nm) loaded within the carbon matrix. Cyclic voltammetry, measurements of electrochemically active surface area, and electrolysis experiments were conducted to understand the correlations between the catalytic ability and the structural and textural features of the catalysts. Excellent bibenzyl yield (98%) and remarkable reusability were obtained under mild conditions. The results confirm that the prepared nanocomposites show outstanding performance in the electroreduction degradation of PhCH₂Br to bibenzyl.

Full text of DOI:10.1039/c9ra08930f

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Synthesis of Ag nanoparticles/ordered mesoporous
carbon as a highly efficient catalyst for the
electroreduction of benzyl bromide†
Cite this: RSC Adv., 2020, 10, 756
*
*
Zhi-Xia Zhang, Shuo Wang, Shi-Ming Li, Si-Li Shan, Huan Wang and Jia-Xing Lu
To develop efficient catalysts for the electroreduction of organic halides, a facile one-pot synthesis of Ag
nanoparticles/ordered mesoporous carbon electrode materials via the self-assembly of CH3COOAg and
resol in the presence of triblock copolymer is proposed. The resultant electrode materials possess
uniform mesopore sizes (3.3 nm) and pore volumes (ꢀ0.28 cm3
g
ꢁ1), high specific surface areas (ꢀ500
m2 ꢁ1), and uniformly dispersed Ag nanoparticles (12–36 nm) loaded within the carbon matrix. Cyclic
g
voltammetry, measurements of electrochemically active surface area, and electrolysis experiments were
conducted to understand the correlations between the catalytic ability and the structural and textural
features of the catalysts. Excellent bibenzyl yield (98%) and remarkable reusability were obtained under
mild conditions. The results confirm that the prepared nanocomposites show outstanding performance
in the electroreduction degradation of PhCH2Br to bibenzyl.
Received 30th October 2019
Accepted 11th December 2019
DOI: 10.1039/c9ra08930f
rsc.li/rsc-advances
reduction of halogenated compounds tends to generate
multiple products and cannot achieve high transformation in
the theoretical electrolysis charge.13–15 Recently, our group ob-
1 Introduction
As important and multipurpose compounds in organic
synthesis, organic halides have been widely used as industrial
solvents, intermediates, and reagents. However, many organo-
halogen compounds are arbitrarily or inevitably emitted into
the environment and cannot be easily and rapidly degraded by
natural processes, causing serious damage to the ozone layer
and threatening ecological safety and human health, raising
concern about their use.1 In addition to biological2 and organic
chemistry methods,3 electrochemical approaches4–6 are valid
choices for the reductive dehalogenation of organic halides.
Electrosynthesis can successfully convert harmful halogenated
contaminants into environmentally friendly and potentially
more valuable compounds. This is particularly true in the case
of the formation of C–C bonds, one of the most challenging
aspects of electroorganic synthesis, which is employed in
coupling,7 electrocarboxylation,8 and electrocyclization.9 Over
the last decades, this process has boosted research on electrode
materials. For example, Ag and Ag nanoparticles are recognized
as powerful catalytic materials for the electroreductive dehalo-
genation of C–X bonds.10–12 However, the electrocatalytic
tained bibenzyl through the electrochemical reduction of
PhCH2Br on an Ag–zeolite Y electrode with much higher elec-
trocatalytic activity than obtained using an Ag electrode.
However, the yield was only 46%.16 De Souza et al.17–20 obtained
the best yield for bibenzyl production (89%) on Ag/graphite
powder macroelectrodes by examining the catalytic properties
of various powder macroelectrodes; however, the reaction
device was complicated. Thus, a new method for preparing
effective electrodes with high activity, selectivity, and stability
for the electrocatalytic reduction of C–X bonds is required.
The family of ordered mesoporous carbon (OMC) has great
importance because of its advantages over other materials,
including large specic surface area, uniform mesopore struc-
ture, good chemical stability, and high electrical conduc-
tivity.21–23 The crystallinity and conductivity of carbon materials
can be improved by heat treatment or by the use of graphiti-
zation catalysts.24,25 As a mesoporous material (diameter ¼ 2–50
nm), OMC is a good support or container for the deposition of
metal nanoparticles, resulting in outstanding mass transfer
performance of reactants and products.26,27 Therefore, OMC has
good application prospects as an electrode material in
electrosynthesis.
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, Shanghai
200062, P. R. China. E-mail: hwang@chem.ecnu.edu.cn; jxlu@chem.ecnu.edu.cn
† Electronic supplementary information (ESI) available: Chemicals and
instruments, electrochemical process, small-angle XRD pattern of the Ag-free
mesoporous carbon sample, FT-IR and TGA of Ag/OMC, XRD of
as-made-Ag/OMC, SEM of Ag/OMC and OMC, EDX-Mapping of Ag/OMC,
magnied TEM of Ag/OMC, cyclic voltammograms of OMC, Pb-UPD and
characterization of Ag/OMC-III aer using 8 times. See DOI: 10.1039/c9ra08930f
Recently, we found for the rst time that metal Ni nano-
particles supported by OMC showed excellent electrocatalytic
performance for the selective electroreduction of ketones into
alcohols.28 We previously demonstrated that the mesoporous
structure inuences the catalytic effect. However, the catalytic
effects of metal nanoparticles have not been discussed in detail.
756 | RSC Adv., 2020, 10, 756–762
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Additionally, metal Ag nanoparticles, a powerful economic for the synthesis of other Ag/OMC samples with the exception
catalytic material, grown in situ on OMC may be applied in the that the amount of CH3COOAg was 0.15, 0.2, or 0.25 g for Ag/
electroreduction of benzyl bromide. The synthesis and charac- OMC-II, Ag/OMC-III, or Ag/OMC-IV, respectively.
terization of the physicochemical properties of a series of Ag
nanoparticles/OMC (Ag/OMC) electrode materials with different
Ag contents but identical hole diameters and pore volumes
2.3 Preparation of Ag/OMC/GC modied electrodes
Before modication, a glassy carbon (GC) electrode (d ¼ 2 mm)
would be benecial for exploring the relationship between Ag
was polished with 0.5–0.7 mm alumina and then sonicated for
10 min in distilled water and then in acetone. Ag/OMC (0.2 mg)
was adhered to the electrode surface with 0.3 mL of 5% carboxyl
methyl cellulose (300–800 mPa s) aqueous solution as an
adhesive. The modied electrode, denoted Ag/OMC/GC, was
then air-dried.
nanoparticles and electrocatalytic reduction performance in
detail.
Herein, a series of Ag/OMC electrode materials with different
Ag contents but uniform mesopore sizes (3.3 nm) and pore
volumes (ꢀ0.28 cm3 gꢁ1) were synthesized, characterized, and
applied as highly efficient catalysts for the electroreduction of
organic bromides. These nanocomposites were prepared via the
one-pot, three-constituent self-assembly of commercially avail-
able F127 as the template, low-molecular-weight resol as the
carbon precursor, and CH3COOAg as the metal source. The
effects of the Ag content and nanoparticle size on the electro-
chemically active surface area (ECSA) were investigated in detail
by lead underpotential deposition (Pb-UPD) experiments. The
catalytic abilities of the Ag/OMC electrode materials were
characterized by cyclic voltammetry (CV) and bulk electrolysis.
The target product could be obtained in good to excellent yield
at room temperature on these new electrode materials.
3 Results and discussion
3.1 Structure and morphology
Ag/OMC nanocomposites can be obtained through the facile
one-pot self-assembly of low-molecular-weight resol, commer-
cially available F127, and CH3COOAg followed by thermopoly-
merization and carbonization. The typical small-angle X-ray
diffraction (XRD) patterns of the Ag/OMC nanocomposites with
different Ag contents show one distinct diffraction peak
(Fig. 1A), similar to the Ag-free OMC sample (Fig. S1†). This
peak is assigned to the (110) planes of the ordered body-
centered cubic structure in symmetry group Im3m. The well-
resolved XRD patterns indicate that the highly ordered meso-
structures of FDU-16 were well preserved. Upon increasing the
amount of CH3COOAg used to prepare Ag/OMC from 0.1 to
0.25 g, the diffraction peak shied distinctly toward higher 2q
values, and the unit cell parameter calculated from the (110)
planes decreased from 17.6 to 11.0 nm (Table 1), suggesting
a shrinkage of the unit cell. Fig. S2† shows the Fourier-
transform infrared spectrum of Ag/OMC. The bands at 1100
and 2800–3000 cmꢁ1 gradually disappeared as the Ag content
increased, further providing evidence for the shrinkage of the
framework. The wide-angle XRD patterns (Fig. 1B) show that the
Ag/OMC composites possessed well-crystallized Ag nano-
particles. The ve peaks at 38.2ꢂ, 44.3ꢂ, 64.5ꢂ, 77.4ꢂ, and 81.6ꢂ
were respectively ascribed to the diffraction reections from the
(111), (200), (220), (311), and (222) planes of the face-centered
cubic Ag structure (JCPDS card no. 04-0783). The Ag crystal
sizes calculated from the (111) plane were 10, 21, 27, and 34 nm
2 Experimental
2.1 Preparation of low-molecular-weight resol
Low-molecular-weight resol was prepared by a previously re-
ported method.29 Briey, 32.0 g (0.340 mol) of phenol was
melted at 45 ꢂC followed by the dropwise addition of 6.8 g (0.034
mol) of 20.0 wt% NaOH solution. The colorless and transparent
ꢂ
mixed solution was stirred at 50 C for 10 min followed by the
addition of 55.2 g (0.680 mol) of 37.0 wt% formaldehyde solu-
tion. The system was immediately warmed to 75 ꢂC and kept at
this temperature for 1 h. The product was cooled to room
temperature, adjusted to pH 6.0 with 2.0 M HNO3 solution,
dried by vacuum evaporation, and then centrifuged to remove
precipitates. The obtained resol was dissolved in 50.0 wt%
ethanol aqueous solution (20.0 wt%).
2.2 Preparation of monolithic Ag/OMC cathodes
The monolithic Ag/OMC cathodes were prepared by a facile one-
pot preparation method. First, 10.0 g of the 20.0 wt% solution of
resol was dissolved in a transparent solution of 1.0 g F127 and
10.0 g 50.0 wt% ethanol aqueous solution by stirring for 5 min.
Second, 0.1 g of CH3COOAg was added to the mixture, which
was then stirred for 2 h. Subsequently, the colorless, trans-
parent solution was transferred into glass panes to evaporate
the solvent for 12 h at 60 ꢂC followed by thermopolymerization
at 120 ꢂC in an oven for 36 h. Finally, the as-made Ag/OMC
samples were scrapped off, pressed into a coin (d ¼ 4 cm),
and carbonized under vacuum atꢁ9100 ꢂC for 3 h in the presence
of N2 at heating rates of 1 ꢂC min to 600 ꢂC and 5 ꢂC minꢁ1 to
900 ꢂC. The obtained monolithic Ag/OMC cathodes were
Fig. 1 (A) Small-angle and (B) wide-angle XRD patterns of the Ag
nanoparticle/ordered mesoporous carbon samples containing
denoted as Ag/OMC-I. The same procedure was implemented different Ag contents.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 756–762 | 757

View Article Online
RSC Advances
Paper
Table 1 Physical properties of the Ag nanoparticles/ordered mesoporous carbon samples
Metal amountd
(wt%)
Metal amounte
(wt%)
Ag-ECSAf
Ag-ECSA/AgNPg
(cm2 gꢁ1 AgNP)
b
b
c
Unit cell
SBET
Vp
Dp
Sample
parameter aa (nm)
(m2 gꢁ1
)
(cm3 gꢁ1
)
(nm)
(cm2 gꢁ1 Ag/OMC)
OMC
14.8
17.6
16.4
13.6
11.0
534
519
499
483
495
0.28
0.28
0.27
0.27
0.29
2.4
3.3
3.3
3.3
3.3
—
7.3
10.6
12.7
14.1
—
6.7
9.7
12.0
13.7
Ag/OMC-I
Ag/OMC-II
Ag/OMC-III
Ag/OMC-IV
2.1
15.5
30.0
15.1
31
159
250
110
a
c
Calculated from XRD results. b Calculated using the BJH model from sorption data in a P/P0 range from 0.04 to 0.2. Calculated using the BJH
model from the adsorption branches of the isotherms. d The Ag weight percentage obtained via TGA by combusting the carbon components. e The
Ag weight percentage obtained by ICP-AES. f Measured by lead underpotential deposition. Ag-ECSA divided by the Ag content of Ag/OMC.
g
for Ag/OMC-I, Ag/OMC-II, Ag/OMC-III, and Ag/OMC-IV, areas of 483–519 m2 gꢁ1 and high pore volumes of 0.27–0.29
respectively.
cm3 gꢁ1, similar to those of pure OMC (Table 1).
In other words, increasing the Ag content increased the sizes
of the Ag nanoparticles.
It is worth noting that the BET surface areas, pore sizes, and
pore volumes of the Ag/OMC composites with different Ag
To investigate the content of metallic Ag particles in the contents were nearly the same, which differs from the ndings
carbon framework, thermogravimetric analysis (TGA) was con- of previous studies.30–32 Traditionally, the surface areas, pore
ducted to record the weight losses of the mesoporous sizes, and pore volumes of most composites decrease with
ꢂ
ꢂ
composites from 30 C to 800 C in air atmosphere (Fig. S3†). increasing metal content, suggesting that the existence of
Ag/OMC showed excellent thermal stability in the range of 30 ꢂC hydrogen bonding involving the metal precursors affects the co-
ꢂ
to 340 C. The evident weight loss in the temperature range of assembly of resol (carbon precursor) and F127 (structure-
350 ꢂC to 500 ꢂC is attributed to the combustion of carbon. The directing agent). However, the wide-angle XRD pattern of the
ꢂ
weight-invariant phenomena above 550 C is attributed to the as-made (not calcined) Ag/OMC-III sample shows ve low-
residue of Ag; the Ag contents of Ag/OMC-I, Ag/OMC-II, Ag/ intensity diffraction peaks assigned to the (111), (200), (220),
OMC-III, and Ag/OMC-IV were determined to be 7.3, 10.6, (311), and (222) reections of cubic Ag (Fig. S4†). These results
12.7, and 14.1 wt%, respectively (Table 1). These contents agrees indicate that Ag nanoparticles were formed before calcination
well with those determined by inductively coupled plasma- rather than with the formation of the mesoporous structure.
atomic emission spectrometry (ICP-AES; Table 1).
Since there is no hydrogen bonding of the metal precursor when
Fig. 2(A) shows the N2 adsorption–desorption isotherms of the mesoporous structure is formed, the inuence of the Ag
the Ag/OMC materials. The resulting curves are all type-IV content on the mesoporous structure is small. The schematic of
isotherms with a well-dened capillary condensation step in the synthesis process is illustrated in Fig. 3. Ag/OMC can be
the relative pressure (P/P0) range from 0.3 to 0.6, characteristic synthesized through the one-pot, three-constituent self-
of high-quality mesoporous materials, and an H2 hysteresis assembly of commercially available F127, resol, and CH3-
loop, which can be attributed to the typical three-dimensional COOAg followed by thermopolymerization and decomposition
caged mesostructure with a regular pore size distribution. The of the Ag precursor, calcination to remove the templating agent,
pore size distribution curves (Fig. 2B) show that all Ag/OMC and carbonization. Ag/OMC retained a large surface area, which
samples had similar uniform pore distributions with average is a key factor in the dynamic mass transport performance of
pore sizes of 3.3 nm, larger than the average pore size in the Ag- the catalyst,33–35 indicating that Ag/OMC may have excellent
free OMC sample (Table 1). Aer introducing Ag, the compos- catalytic activity.
ites maintained large Brunauer–Emmett–Teller (BET) surface
Fig. 2 (A) N2 sorption/desorption isotherms and (B) the corresponding
pore size distribution curves of Ag/OMC samples with different Ag Fig.
3
Schematic illustration of the co-assembly route for the
contents.
synthesis of Ag/OMC.
758 | RSC Adv., 2020, 10, 756–762
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
The scanning electron microscopy (SEM) images (Fig. S5†) (Fig. S7†), a layered structure of graphite carbon is clearly
show that the Ag/OMC samples had similar micro- observed in the carbon matrix. This implies that the carbon
morphologies consisted of amorphous gray blocks, as for the walls are graphitized, providing the composites with excellent
OMC sample (FDU-16). The white balls/spots observed are Ag electrical conductivity.
nanoparticles, which were present as a large number of uniform
particles highly dispersed in the large-scale carbon matrix. No
signicant aggregation of Ag particles was observed, indicating
3.2 Electrochemical catalysis
that the Ag particles were well conned in the carbon matrix CV was conducted in MeCN-0.1 M TEABF4-5 mM PhCH2Br
without agglomeration. The nanoparticle density gradually solution under a nitrogen atmosphere to characterize the
increased with increasing Ag content. Energy-dispersive X-ray catalytic abilities of the Ag/OMC modied electrodes. As shown
spectroscopy (EDX) mapping provided evidence of a homoge- in Fig. 5, two successive one-electron reduction peaks corre-
neous dispersion of Ag (Fig. S6†). In The surface morphology of sponding to the two-electron transfer of C–X in PhCH2Br are
Ag/OMC was also examined using transmission electron observed on the Ag/OMC electrodes, similar to the curve of Ag.
microscopy (TEM). The TEM images of the Ag/OMC composites On the Ag electrode, two reduction peaks are detected at ꢁ0.68
(Fig. 4) all show the high-quality regular array of a mesoporous and ꢁ0.98 V. The rst peak corresponds to the reduction of
structure (Im3m) in large regions, in accordance with the small- PhCH2Br to PhCH2c and Brꢁ, while the second corresponds15 to
angle XRD and N2 physisorption results. Compared to pure the reduction of PhCH2c to PhCH2ꢁ. The PhCH2Br reduction
OMC, the Ag nanoparticles (indicated by the black spots in observed on a regular GC interface shows only one reduction
Fig. 4) were well dispersed inside the mesoporous carbon peak (Fig. 5). Since the reduction potential of benzyl bromide on
matrix, providing active sites for electroreduction, consistent GC is more negative (ꢁ1.4 V vs. Ag/AgI/Iꢁ) than the standard
with the SEM results. With increasing Ag content, the particle reduction potentiꢁal of PhCH2c,36 PhCH2c is immediately
density and size gradually increased. The particle size distri- reduced to PhCH2 as soon as it is formed; thus, the overall
bution histograms (insets in Fig. 4) indicate average particle process is a 2eꢁ reduction of PhCH2Br.37 Therefore, the peak
diameters of 12, 20, 30, and 36 nm for Ag/OMC-I, Ag/OMC-II, Ag/ current on GC is twice that of the rst reduction peak on Ag.
OMC-III, and Ag/OMC-IV, respectively, in good agreement with When the GC surface is modied with Ag/OMC, the two
the XRD analysis. The guest nanoparticles produced via the one- successive electron transfers to PhCH2Br indicate that the
pot synthetic method generally grew up in the mesoporous catalytic properties of Ag/OMC/GC are more similar to Ag than
framework with the pore wall thicknesses being smaller than to GC. Moreover, the onset potential of the rst reduction peak
the Ag nanoparticle size. This implies that the Ag nanoparticles gradually became more positive as the Ag loading increased.
rst grew in the mesoporous walls and then penetrated through This indicates that Ag/OMC has high catalytic activity toward
the walls into the mesochannels. In the magnied TEM image the electroreduction of organic halides. To eliminate the effect
of OMC itself, GC electrodes coated with only an OMC layer
(labeled OMC/GC, Fig. S8†) were used in electrochemical
investigations. One strong reduction peak was found at ꢁ1.33 V,
similar to the reduction peak on GC, indicating that OMC is
does not have catalytic ability for the electroreduction of benzyl
bromide. The peak current of OMC was signicantly larger than
Fig. 4 TEM images and grain diameter distributions (inset) of the Ag/ Fig. 5 Cyclic voltammograms recorded at a scan rate of 0.1 V sꢁ1
OMC samples containing different Ag contents: (a) Ag/OMC-I, (b) Ag/ using different working electrodes in MeCN-0.1 M TEABF4-5mM
OMC-II, (c) Ag/OMC-III, and (d) Ag/OMC-IV.
PhCH2Br.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 756–762 | 759

View Article Online
RSC Advances
Paper
that of GC, possibly because the larger specic surface area of which affected the active area of Ag and led to a decrease in
OMC provides more contact area with the substrate. The CV catalytic activity. Therefore, the appropriate size and content of
results show that Ag/OMC has better activity than the carbon metal nanoparticles are key to the electrocatalytic reduction of
matrix as an electrocatalyst for the reduction of benzyl bromide. PhCH2Br into biphenyl. The Ag/OMC-III sample had appro-
As shown in Fig. 5, the rst reduction peak potentials were priate Ag loading and nanoparticle size, resulting in optimal
found at ꢁ0.73, ꢁ0.69, ꢁ0.64, and ꢁ0.79 V for Ag/OMC-I/GC, catalytic performance.
Ag/OMC-II/GC, Ag/OMC-III/GC, and Ag/OMC-IV/GC, respec-
To test the electrocatalytic reduction ability of the Ag/OMC
tively. As the Ag loading increased, the reduction peak rst cathode, a series of potentiostatic electrolysis experiments
became more positive and then more negative. The most posi- were conducted in a mixture of PhCH2Br (0.05 M) substrate and
tive reduction peak of PhCH2Br was ꢁ0.64 V vs. Ag/AgI/Iꢁ on the supporting electrolyte TEABF4 (0.1 M) in 20 mL MeCN with an
Ag/OMC-III/GC electrode. In other words, the Ag/OMC-III Mg anode in the presence of N2 in an undivided cell.
modied electrode displayed the best electrocatalytic behavior
The electrolysis experiments were conducted at the rst
among the tested electrodes. In addition, the peak current of reduction peak potential in the presence of N2 to produce
Ag/OMC-III/GC was the largest among the samples, indicating bibenzyl (Table 2). The electrode material affected the electro-
that the sample resulted in the fastest reaction.
dimerization yield; 8% dimer was obtained on the Ag-free OMC
ECSA plays a decisive role in catalytic performance. Pb-UPD cathode (Table 2 entry 2), lower than the yield obtained on
experiments38–40 were used to measure the ECSAs of Ag (Ag- a bulk Ag electrode (24%; Table 2 entry 1), in accordance with
ECSA) in the Ag/OMC samples (Table 1, Fig. S9†). The curves the CV results. The yield results clearly show that the Ag/OMC
of all samples showed one characteristic peak attributing to Pb- cathodes resulted in large yields of bibenzyl, much higher
UPD on Ag. Ag/OMC-III had the largest Ag-ECSA (30.0 cm2 gꢁ1 than the yields obtained on the Ag electrode.
Ag/OMC), which was 14 times that of Ag/OMC-I (2.1 cm2 gꢁ1 Ag/
This shows that the catalytic properties of Ag were improved
OMC). The Ag-ECSAs of Ag/OMC-II and Ag/OMC-IV were nearly signicantly by loading the Ag nanoparticles into OMC. Ag/
the same (15.4 and 15.1 cm2 gꢁ1 Ag/OMC, respectively). Inter- OMC-III resulted in a 76% yield of bibenzyl as product (Table
estingly, as the loading of Ag increased, the electrocatalytic 2 entry 5), seven and three times higher than the yields obtained
ability rst increased and then decreased. In other words, on OMC and Ag electrodes under the same conditions, respec-
a higher Ag content in the Ag/OMC cathode did not strictly tively. The higher conversion and yield show that Ag/OMC-III is
correspond to better electrocatalytic ability. To eliminate the an extremely efficient electrocatalyst for reducing organic
inuence of Ag content, the effect of particle size on Ag-ECSA halides. The yields of bibenzyl were 32%, 45%, and 41% for Ag/
was examined. Dividing Ag-ECSA by the Ag content of Ag/ OMC-I, Ag/OMC-II, and Ag/OMC-IV, respectively, greater than
OMC gives the active area of each equivalent of Ag nano- that obtained on Ag electrode (Table 2 entries 3, 4, and 6). These
particles (Ag-ECSA/AgNP) (Table 1). The Ag-ECSA/AgNP value for ndings agree with the CV and Ag-ECSA results. The results
Ag/OMC-III (250 cm2 gꢁ1 AgNP) was eight times that of Ag/OMC- suggest that the potential, which represents the level of diffi-
I (31 cm2 gꢁ1 AgNP). Thus, to a certain extent, increasing the culty for the reaction to occur, and Ag-ECSA, which represents
nanoparticle size could dramatically increase the catalytic the number of electrochemically active sites, are the key factors
activity. However, when the nanoparticle size continued to determining the catalytic performance of the material.
increase, Ag-ECSA decreased. The Ag-ECSA/AgNP of Ag/OMC-IV
(110 cm2 gꢁ1 AgNP) was only half that of Ag/OMC-III.
The previous characterization of the composites demon-
Table 2 Effects of various synthetic factors on the yield of productsa
strated that the four Ag/OMC composites had nearly the same
pore sizes, pore volumes, and specic surface areas; the
composites differed in the size and content of the Ag nano- Entry Cathode
particles. Previous studies have found that the reaction is
Potential
(V)
Q
Conversionb
(%)
Yieldc
(%)
(F molꢁ1
)
1
Ag
ꢁ0.70
ꢁ1.30
ꢁ0.75
ꢁ0.70
ꢁ0.65
ꢁ0.80
ꢁ0.55
ꢁ0.60
ꢁ0.70
ꢁ0.65
ꢁ0.65
ꢁ0.65
ꢁ0.65
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.2
1.4
1.5
2.0
30
12
40
52
82
50
28
62
71
87
96
>99
>99
24
8
mainly concentrated in the pores;41 thus, the surface area of the
Ag nanoparticles exposed in holes can be in contact with the
substrate and may be the key factor in determining the catalytic
activity. As the nanoparticle size increased, the Ag particles rst
grew in the carbon mesoporous walls and then penetrated
through the walls into the channels. Tiny particles are easily
trapped in mesoporous walls; thus, small Ag nanoparticles have
low catalytic activity. The Ag/OMC-I sample had poor catalytic
performance due to its small nanoparticle size and content,
consistent with the TEM and Ag-ECSA results. As the Ag content
increased, the particle size also increased gradually. This
increase in particle size increased the surface area exposed to
the channel of nanoparticles, greatly improving the catalytic
activity. However, when the nanoparticles became too large, the
surface area of the Ag nanoparticles decreased drastically,
2
3
4
5
6
7
8
9
10
11
12
13
OMC
Ag/OMC-I
Ag/OMC-II
Ag/OMC-III
Ag/OMC-IV
Ag/OMC-III
Ag/OMC-III
Ag/OMC-III
Ag/OMC-III
Ag/OMC-III
Ag/OMC-III
Ag/OMC-III
32
45
76
41
18
55
65
80
91
98
96
a
Anode: Mg, solvent: MeCN (20 mL), supporting electrolyte: TEABF4
(0.1 M), substrate: PhCH2Br (0.05 M), T ¼ 298 ꢃ 2 K, PN ¼ 1 atm.
2
b
Some relatively large errors are due to the volatility of some of the
c
species (toluene and benzyl bromide). The yields were determined by
gas chromatography and based on the reactant.
760 | RSC Adv., 2020, 10, 756–762
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
of PhCH2Br were examined in detail while keeping other
conditions consistent. Among the Ag/OMC samples, Ag/OMC-III
possessed the optimal composition and maximum Ag-ECSA. Ag/
OMC-III displayed outstanding electrocatalytic performance
with a maximum bibenzyl yield of 98%. The results indicate that
functionalizing OMC by doping with metal nanoparticles
provides a promising opportunity for the electrocatalytic
reduction of organic halides.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Fig. 6 Reuse of the Ag/OMC-III cathode. Reaction conditions are
given in Table 2, entry 12.
Financial support from National Natural Science Foundation of
China (21773071 and 21673078) and the Fundamental Research
Funds for the Central Universities is gratefully acknowledged.
Under consistent conditions (except the electrolytic poten-
tial), the yield of benzyl bromide to bibenzyl was maximized
(76%) at ꢁ0.65 V on Ag/OMC-III (Table 2 entry 5). The yield was
lower at ꢁ0.55 V (Table 2 entry 7), likely because it is more
difficult reduce the substrate at this potential. As the potential
became more negative, the yield of the coupled product rst
increased and then decreased (Table 2 entries 7–9). The yield of
bibenzyl increased to 98% when the amount of charge passing
through the electrolysis cell (Q) was increased to 1.5 F molꢁ1
(Table 2 entry 12). The yield of bibenzyl increased with
increasing Q before 1.5 F molꢁ1 (Table 2 entries 10–12); aer
that point, the yield remained unchanged with increasing Q
(Table 2 entry 13). To test the catalytic ability upon recycling, the
Ag/OMC-III electrode was washed repeatedly with MeCN, dried
at 100 ꢂC, and reused under the same conditions. Fig. 6 shows
that the yield of bibenzyl remained around 94–98% without any
appreciable reduction in catalytic activity for eight reuses,
indicating that Ag/OMC possessed remarkable reusability. The
surface morphology of the Ag/OMC-III electrode was charac-
terized by TEM aer eight reuses (Fig. S10†). The highly ordered
mesostructure of the Ag/OMC-III composite was retained aer
repeated use, and the average nanoparticle diameter did not
change obviously, indicating that the carbon matrix immobi-
lized the Ag nanoparticles and limited their agglomeration
during the reaction.
Notes and references
1 H. J. Verhaar, C. J. Van Leeuwen and J. L. Hermens,
Chemosphere, 1992, 25, 471–491.
2 C. Holliger, C. Regeard and G. Diekert, in Dehalogenation,
Springer, 2004, pp. 115–157.
3 L. J. Matheson and P. G. Tratnyek, Environ. Sci. Technol.,
1994, 28, 2045–2053.
4 A. Wang, Y.-F. Huang, U. K. Sur, D.-Y. Wu, B. Ren,
S. Rondinini, C. Amatore and Z.-Q. Tian, J. Am. Chem. Soc.,
2010, 132, 9534–9536.
5 D. Niu, A. Zhang, T. Xue, J. Zhang, S. Zhao and J. Lu,
Electrochem. Commun., 2008, 10, 1498–1501.
6 V. Jouikov and J. Simonet, Electrochem. Commun., 2010, 12,
781–783.
7 T. Suga and Y. Ukaji, Org. Lett., 2018, 20, 7846–7850.
8 D.-T. Yang, M. Zhu, Z. J. Schiffer, K. Williams, X. Song, X. Liu
and K. Manthiram, ACS Catal., 2019, 9, 4699–4705.
9 F. Bellesia, A. J. Clark, F. Felluga, A. Gennaro, A. A. Isse,
F. Roncaglia and F. Ghel, Adv. Synth. Catal., 2013, 355,
1649–1660.
10 G. Fiori, S. Rondinini, G. Sello, A. Vertova, M. Cirja and
L. Conti, J. Appl. Electrochem., 2005, 35, 363–368.
ˇ
´
11 S. Rondinini, G. Aricci, Z. Krpetic, C. Locatelli, A. Minguzzi,
F. Porta and A. Vertova, Fuel Cells, 2009, 9, 253–263.
12 Y.-C. Lan, H. Wang, L.-X. Wu, S.-F. Zhao, Y.-Q. Gu and
J.-X. Lu, J. Electroanal. Chem., 2012, 664, 33–38.
4 Conclusions
In summary, Ag nanoparticles/OMC electrodes were synthe- 13 S. Rondinini, P. R. Mussini, P. Muttini and G. Sello,
sized via a facile one-pot solvent evaporation-induced self- Electrochim. Acta, 2001, 46, 3245–3258.
assembly method. A series of monolithic Ag/OMC electrode 14 P. Poizot and J. Simonet, Electrochim. Acta, 2010, 56, 15–36.
materials with different Ag contents were shown to have the 15 A. A. Isse, A. De Giusti and A. Gennaro, Tetrahedron Lett.,
same hole diameters and pore volumes. Both CV and electrol-
ysis experiments conrmed that the prepared nanocomposites 16 H. Wang, L. He, G.-J. Sui and J.-X. Lu, RSC Adv., 2015, 5,
show outstanding performance in the electrocatalytic reduction 42663–42665.
of PhCH2Br to bibenzyl. The effects of Ag nanoparticle loading 17 L. Santos de Oliveira, M. Navarro, R. Tavares Ribeiro and
2006, 47, 7735–7739.
˜
and particle size on the catalytic activity for the electroreduction
E. H. Lago Falcao, ChemistrySelect, 2017, 2, 7771–7775.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 756–762 | 761

View Article Online
RSC Advances
Paper
18 R. F. de Souza, M. Laurent, E. Leonel, C. Cachet-Vivier, 29 Y. Meng, D. Gu, F. Zhang, Y. Shi, L. Cheng, D. Feng, Z. Wu,
C. A. de Souza, M. C. Areias, L. W. Bieber and M. Navarro,
Z. Chen, Y. Wan and A. Stein, Chem. Mater., 2006, 18, 4447–
Electrochim. Acta, 2015, 167, 105–111.
4464.
19 R. F. De Souza, C. A. De Souza, M. C. Areias, C. Cachet-Vivier, 30 Y. Zhang, L.-G. Xing, X.-W. Chen and J.-H. Wang, ACS Appl.
´
M. Laurent, R. Barhdadi, E. Leonel, M. Navarro and
Mater. Interfaces, 2015, 7, 5116–5123.
L. W. Bieber, Electrochim. Acta, 2010, 56, 575–579.
20 C. A. de Souza, R. F. de Souza, M. Navarro, I. Malvestiti,
31 W. Wang, W. Jing, L. Sheng, D. Chai, Y. Kang and Z. Lei,
Appl. Catal., A, 2017, 538, 123–130.
A. P. da Silva, L. W. Bieber and M. C. Areias, Electrochim. 32 M. Guler, T. Dogu and D. Varisli, Appl. Catal., B, 2017, 219,
Acta, 2013, 89, 631–634. 173–182.
21 Z. Wang, Y. Zhu, W. Luo, Y. Ren, X. Cheng, P. Xu, X. Li, 33 J. Lee, J. Kim and T. Hyeon, Adv. Mater., 2006, 18, 2073–2094.
Y. Deng and D. Zhao, Chem. Mater., 2016, 28, 7773–7780.
34 H. Kim, A. W. Robertson, S. O. Kim, J. M. Kim and
22 S. W. Kang, K. Kim, D. H. Chun, J.-I. Yang, H.-T. Lee, H. Jung,
J. H. Warner, ACS Nano, 2015, 9, 5947–5957.
J. T. Lim, S. Jang, C. S. Kim and C.-W. Lee, J. Catal., 2017, 35 A. Stein, Z. Wang and M. A. Fierke, Adv. Mater., 2009, 21,
349, 66–74. 265–293.
23 P. Zhang, L. Wang, S. Yang, J. A. Schott, X. Liu, 36 Y. Meng, D. Gu, F. Zhang, Y. Shi, H. Yang, Z. Li, C. Yu, B. Tu
S. M. Mahurin, C. Huang, Y. Zhang, P. F. Fulvio and
M. F. Chisholm, Nat. Commun., 2017, 8, 15020–15029.
and D. Zhao, Angew. Chem., Int. Ed., 2005, 44, 7053–7059.
37 A. A. Isse, A. De Giusti, A. Gennaro, L. Falciola and
P. R. Mussini, Electrochim. Acta, 2006, 51, 4956–4964.
38 E. Kirowa-Eisner, D. Tzur and E. Gileadi, J. Electroanal.
Chem., 2008, 621, 146–158.
´
´
´
24 S. Perez-Rodrıguez, D. Torres and M. J. Lazaro, Powder
Technol., 2018, 340, 380–388.
25 K. P. Gierszal, M. Jaroniec, T. W. Kim, J. Kim and R. Ryoo,
New J. Chem., 2008, 32, 981–993.
39 X. Feng, K. Jiang, S. Fan and M. W. Kanan, J. Am. Chem. Soc.,
2015, 137, 4606–4609.
26 Z. Sun, B. Sun, M. Qiao, J. Wei, Q. Yue, C. Wang, Y. Deng,
S. Kaliaguine and D. Zhao, J. Am. Chem. Soc., 2012, 134, 40 C. Rogers, W. S. Perkins, G. Veber, T. E. Williams, R. R. Cloke
17653–17660.
and F. R. Fischer, J. Am. Chem. Soc., 2017, 139, 4052–4061.
41 W. Li, J. Liu and D. Zhao, Nat. Rev. Mater., 2016, 1, 16023–
16039.
27 A. Corma, Chem. Rev., 1997, 97, 2373–2420.
28 Z.-X. Zhang, Y. Liu, W.-J. Meng, J. Wang, W. Li, H. Wang,
D. Zhao and J.-X. Lu, Nanoscale, 2017, 9, 17807–17813.
762 | RSC Adv., 2020, 10, 756–762
This journal is © The Royal Society of Chemistry 2020