Salt template-assisted: In situ construction of Ru nanoclusters and porous carbon: Excellent catalysts toward hydrogen evolution, ammonia-borane hydrolysis, and 4-nitrophenol reduction

Article abstract of DOI:10.1039/c9gc03986d

Sustainable yet effective manufacture of supported metal nanoclusters (NCs) with high performance for catalytic applications is worthwhile but remains a big challenge. Here, we report a salt template-assisted approach through direct calcination of a mixture of ruthenium oleate precursor and sodium sulfate for one-step synthesis of Ru NCs solidly loaded on the simultaneously formed porous carbon (Ru/PC). In this case, both the salt template of sodium sulfate and one-step approach play key roles in forming surface-clean Ru NCs with good dispersion to supply abundant accessible active sites and constructing a three-dimensional architecture to boost the mass and/or electron transport during applications. Taking advantage of these features, Ru/PC as a catalyst shows excellent activities for electrocatalytic hydrogen evolution with an overpotential of 21 mV at 10 mA cm^-2 in 1.0 M KOH and ammonia-borane hydrolysis with a high turnover frequency of 744 min^-1 and a high activity for 4-nitrophenol reduction with a reaction rate of 1.29 min^-1 at room temperature. The catalyst also shows high stability and reusability for the investigated reactions. This study presents a green yet sustainable method to produce supported metal NCs for versatile catalytic applications.

Full text of DOI:10.1039/c9gc03986d

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DOI: 10.1039/C9GC03986D
ARTICLE
Salt template-assisted in situ construction of Ru nanoclusters and
porous carbon: Excellent catalysts toward hydrogen evolution,
ammonia-borane hydrolysis, and 4-nitrophenol reduction
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Rong Ding a, Qian Chen a, Qian Luo a, Lingxi Zhou a,Yi Wang a, Yun Zhang *a, and Guangyin Fan*a
Sustainable yet effective manufacture of supported metal nanoclusters (NCs) with high performance for catalytic
applications is worthwhile but remains a big challenge. Here, we report a salt template-assisted approach through direct
calcination of the mixture of ruthenium oleate precursor and sodium sulfate for one-step synthesis of Ru NCs solidly loaded
on the simultaneously formed porous carbon (Ru/PC). In this case, both the salt template of sodium sulfate and one-step
approach play key roles in forming surface-clean Ru NCs with a good dispersion to supply abundant accessible active sites,
and constructing three-dimensional architecture to boost the mass and/or electron transport during the applications. Taking
advantages of these features, Ru/PC as the catalyst shows excellent activities for electrocatalytic hydrogen evolution with
an overpotential of 21 mV at 10 mA cm-2 in 1.0 M KOH, ammonia-borane hydrolysis with a high turnover frequency of 744
min-1, and a high activity for 4-nitrophenol reduction with a reaction rate of 1.29 min-1 at room temperature. The catalyst
also shows high stability and reusability for the investigated reactions. This study presents a green yet sustainable method
to produce supported NCs for versatile catalytic applications.
9
problems.8,
Furthermore, most of previous developed
Introduction
synthetic methods for SMNCs generally need large amount of
solvents and aggressive reduction atmosphere (H2) or chemical
agents (e.g. NaBH4, hydrazine hydrate, alcohols), both of which
are cost-ineffectiveness and environmental-friendlessness.10
Therefore, developing green and sustainable strategy to
produce SMNCs in a good distribution is of considerable
importance.
Supported metal nanoclusters (SMNCs) with metal size less
than 2 nm have received continuous interest for catalytic
applications because of their abundant surface active sites
originated from the increased atom utilization compared with
their counterpart of supported metal nanoparticles (NPs).1-3
Owing to the high surface energy, however, the aggregation and
heterogeneous distribution of metal nanoclusters (NCs) are
generally unavoidable during synthetic procedure, which
considerably prohibit their practical applications. Thus,
stabilizing molecules and/or surface capping agents are
employed for prohibiting the aggregation and controlling the
size of metal NCs. However, these used agents also cover the
active sites, resulting in the decline of catalytic activity.
Moreover, the extra operation for removal of the organic
species unavoidably causes the sintering of metal NCs, leading
to the loss of active surface areas.4 In contrast, the introduction
of heteroatoms on solid substrates has been demonstrated as
an efficient strategy for synthesis of metal NCs due to the
supplement of an appropriate coordination environment from
the doped-heteroatoms.5-7 However, the generation of
hazardous gases attributing to the application of heteroatom
containing functional agents arouses seriously environmental
Notably, carbon materials with the features of large specific
surface areas (SSAs), tunable physicochemical properties and
cost-effective scaled-up synthesis as the catalyst supports to
prepare SMNCs have sparked a surge of interest.11,
increase the SSAs for a good load of metal NCs in a fine
size/distribution and construct three-dimensional (3D)
12
To
a
architecture for an efficient mass and/or electron transport
during the applications, hard templates, such as porous silica,
layered metal oxide, etc., have been widely introduced in the
fabrication procedure of nanostructured carbon materials.13
Though many types of porous carbon materials have been
obtained successfully through such route, however, the
operation procedures are relatively complex and time-
consuming, especially the template elimination which involves
in the use of corrosive chemicals and rigorous washing. Despite
much effort has been devoted to etching the templates, few
template residues still present in the product, which will pollute
the after synthesized catalysts, thus disturbing the expression
of intrinsic activity of catalysts. One of class of hard templates,
inorganic salts have drown many attentions in recent years
because they not only show a similar function as other hard
templates for synthesis of porous carbon materials but also
a. College of Chemistry and Materials Science, Sichuan Normal University, Chengdu
610068, P.R. China
*To whom correspondence should be addressed. Tel.: (+86) 28-84760802
Fax: (+86) 28 84767868
E-mail: zhangyun@sicnu.edu.cn(Y Zhang) fanguangyin@sicnu.edu.cn (G.Y Fan).
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display a high solubility in water.14-19 These advantages electrocatalytic hydrogen evolution, AB hydrolysis, and 4-
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guarantee the subsequent procedure of template elimination is nitrophenol reduction. Overall, this reDpOoIr:t1e0d.10s3a9l/tC9tGeCm0p39la8t6eD-
environment-friendly and simultaneously ensure the inorganic assisted approach for the synthesis of SMNCs is convenient,
salt could be removed rapidly and fully, thus, achieving economical, and environmental-friendly, which coincides with
satisfactory porous carbon materials to produce SMNCs with an many of the principles of green chemistry and sustainability,
excellently catalytic activity.20 It should be noted that most of such as a) the wide available starting materials, b) atom
the carbon-based SMNCs were mainly prepared through carbon economy from the multiple roles of the achieved catalysts in
support synthesis and subsequent load of metal NCs. Compared catalytic applications, c) the entire process not involving the
with the two-step strategy, the in-situ pyrolysis of metal-organic organic species (organic solvent, surfactants, and surface
molecule or polymer for one-step synthesis of carbon-based capping agents), and d) economic for the simplified synthetic
SMNCs without use of organic solvents, stabilizing and reducing process without using reducing agents for reducing the metal
agents is relatively convenient and low-cost. For instance, Lin et precursor and chemic agents for the template elimination, and
al. employed salt microcrystal as salt template for successful the recycled salt template of sodium sulfate.
synthesis of Rh NCs deposited on carbon nanosheets, which
showed high electrocatalytic activities for oxygen reduction and
Results and discussion
hydrogenation reactions.21 Based on the assistance of salt
template, the developed Ni3Fe/N-C as the electrocatalyst
showed boosted activities for both oxygen reduction and
evolution reactions.22 Despite these advances, the following
shortages of salt template method need to be overcome. First,
the roles of salt template in the synthetic process were not well
studied. Second, it is still difficult to control the dispersion and
Scheme 1. Synthetic illustration for synthesis of Ru/PC.
the size of metal NCs during high temperature pyrolysis. Third,
there still have room to optimize the synthesis technologies of
The preparation procedure for Ru/PC through one-step strategy
carbon-based SMNCs for multiple catalytic applications. Based
by a green and sustainable salt template-assisted pathway is
on above, developing efficient one-step synthesis strategy that
illustrated in Scheme 1. First, potassium oleate and ruthenium
is capable of largely improving the SSAs and constructing a well-
(Ⅲ) chloride were reacted to form the ruthenium oleate
complex, which could be applied as metal precursor and carbon
defined nanostructure is a desirable but challenging goal.
Here, we report a simple yet efficient salt template-assisted
approach through direct pyrolyzing the mixture of ruthenium
oleate precursor and sodium sulfate for one-step synthesis of
SMNCs, in which Ru NCs in a diameter of 1.50 nm are highly
distributed on the simultaneously produced porous carbon
(Ru/PC). It is discovered that sodium sulfate dominated the
generation of mesopores, enlarged the pore structure, and thus
promoted the formation of 3D architecture. The achieved
Ru/PC displayed excellent activities toward electrochemical
hydrogen evolution with an overpotential of 21 mV at 10 mA
cm-2 in 1.0 M KOH, ammonia-borane (AB) hydrolysis with a high
turnover frequency of 744 min-1, and a high activity for 4-
nitrophenol reduction with a reaction rate of 1.29 min-1 at room
temperature. The catalyst also shows high stability and
reusability for the reactions. Compared with prior reported
strategies for synthesis of the carbon-based SMNCs, this
developed method stands out for the following features. a) The
entire procedure avoids the use of a large amount of surfactant
and reducing agents, thus saving cost and reducing
environmental pollution. b) The salt template can be removed
by washing with the green solvent of water, thus overcomes the
shortcomings of soft template-based method and especially the
hard template-based method which generally requires strong
acid or alkali to remove the templates; Moreover, the sodium
sulfate can be easily reused through recrystallization, making
the synthesis cost-saving and sustainable. c) The highly efficient
Ru/PC as the catalyst with an excellent activity and a strong
durability can be applied in various catalytic reactions, i.e.,
source of SMNCs. Then, appropriate amount of ruthenium
oleate was introduced into the large quantity to form a uniform
mixture. When this mixture pyrolysis under Ar atmosphere, the
ligand of ruthenium oleate gradually carbonized to form the PC
under the assistance of salt template of sodium sulfate. At the
same time, Ru ions were reduced to Ru NCs by the just
generated PC and then loaded on this PC matrix. Ultrafine,
surface-clean and uniformly dispersed Ru NCs decorated on PC
matrix were easily obtained after removal of the sodium sulfate
by water rinsing. The Ru loading in Ru/PC was 3.49 wt%, as
determined by inductively coupled plasma-optical emission
spectrometry (ICP-OES), which is very close to that measured by
thermal gravimetric analysis (TGA) (3.20 wt%) (Fig. S1).
The morphology and microstructure of Ru/PC were firstly
characterized by TEM technique. It can be clearly seen in Fig. 1a,
b that NCs are uniformly anchored on the PC matrix. The
average size of NCs was determined to be 1.50 nm with a
narrow size distribution ranging from 0.50 to 2.50 nm (Fig. 1a
inset). High-resolution TEM (HRTEM) was used to verify the
crystal structure of detected NCs (Fig. 1b inset). It was found
that the NCs are highly crystalline with typical interplanar
spacing of 0.20 nm, corresponding well to the characteristic
feature of the (101) facet of the metallic Ru (JCPDS No. 06-
0663).23 This result clearly illustrate that these NCs are metal
nanocrystals. Scanning TEM-energy dispersive X-ray (STEM-EDX)
elemental mapping images demonstrate that Ru, C and O are
homogeneously distributed across matrix without visible S
signal (Fig. 1c-f). The selected area electron diffraction (SAED)
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pattern shows an obvious diffraction ring which can be indexed size distribution derived from the isotherm discovers the
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DOI: 10.1039/C9GC03986D
to the Ru (101) plane (Fig. 1g). To demonstrate the superiority sample with abundant mesopores in an average size of 7.38 nm.
of this developed synthetic strategy, two contrast samples were The single point total volume of pores at P/P0= 0.99 is 0.73 cm3
carefully designed, in which one was synthesized in parallel by g-1. The large SSAs and pore volume of Ru/PC should be
the same approach as that for Ru/PC except for no addition of attributed to the assistance of salt template of sodium sulfate,
sodium sulfate (Ru/C) and another was prepared by which are expected to facilitate not only the load of metal NCs
impregnation method using pre-synthesized PC as the carbon in a ultrafine size and good dispersion but also the mass
matrix (Ru/PC-IM). From the TEM images, Ru NPs in the transport for catalytic applications.24
diameters of 3.10 (Fig. 2a, b) and 5.78 nm (Fig. 2c, d) with a wide
size distribution were clearly observed in Ru/C and Ru/PC-IM,
respectively, suggesting that both of sodium sulfate and one-
step approach are very key for the formation of ultrafine and
uniformly dispersed Ru NCs in this work.
Fig. 2 TEM images for (a, b) Ru/C and (c, d) Ru/PC-IM. Insets in (a, c) are the size
distributions of Ru NPs.
Surface analysis of Ru/PC was performed by X-ray
photoelectron spectroscopy (XPS). XPS survey spectrum (Fig.
3c) evidence the coexistence of Ru, C and O elements in the
Fig. 1 (a, b) TEM images, (c-f) STEM-EDX mapping images, and (g) SAED pattern for
sample, in agreement well with the result of EDX experiment.
According to XPS analysis, the contents of Ru, C and O in Ru/PC
were measured to be 6.82, 85.53 and 7.65 wt%, respectively
(Table S1). It should be pointed out that Ru loading of Ru/PC
from ICP-OES (3.49 wt%) and TGA (3.20 wt%) analysis is much
lower than the outmost surface Ru content (6.82 wt%)
measured by XPS. This result helps us to conclude that the tiny
Ru NCs are anchored on the surface instead of embedded in the
PC.25 The high-resolution Ru spectrum was recorded to analyse
the chemical state of Ru active component. In this work the
Ru3p spectrum instead of Ru3d spectrum was used for further
study since the Ru3d overlaps with C1s in the XPS detection (Fig.
3d). The Ru3p core level consists of the spin-orbit components
of Ru3P3/2 and Ru3p1/2 with the binding energies of 462.5 and
484.8 eV, respectively. This observation suggests the formation
of metallic Ru in the sample.6 Compared with the standard
binding energy of Ru0, a shift of 0.3 eV was observed in Ru/PC,
indicating a strong interaction between metal and carbon
support.26, 27 The C1s and O1s spectra were further recorded to
obtain more information about the oxygen-containing species.
The C1s spectrum was fitted into three peaks with binding
energy of 284.7, 286.1, and 289.2 eV, which are accounted for
Ru/PC. Inset in (a) is the size distribution of NCs but that in (b) is the HRTEM image
of NCs.
The crystal structures of samples were further studied by X-
ray diffraction (XRD). The diffraction pattern for PC has two
broad peaks at 24 and 43°, which were characteristic of the
(002) and (101) planes of graphitic carbon, respectively (Fig. 3a).
Only one weak but broad diffraction peak at 44° attributing to
the Ru (101) plane (JCPDS No. 06-0663) was observed in both
Ru/PC and Ru/C samples, which should be ascribed to the low
loading and/or small-size of Ru NCs in the samples. For Ru/PC-
IM, distinct and sharp diffraction peaks of metallic Ru are
detected, indicating the existence of large Ru NPs in this sample.
These results are consistent of the TEM observations.
Brunauer-Emmett-Teller
(BET)
gas
sorptometry
measurement was performed to study the textural structure of
Ru/PC. Fig. 3b depicts the N2 adsorption/desorption isotherm
and the pore-size distribution (inset) of Ru/PC. The BET surface
area of Ru/PC determined from N2 isotherm at -197 ℃ was
found to be 575.16 m2 g-1. The isotherm was identified as type
Ⅱ, which was characteristic of mesoporous material. The pore-
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C-C, C-O and O=C-O bonds, respectively (Fig. 3e).28 The O1s demonstration, a control sample (Ru/PC-IM) was done through
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spectrum was deconvoluted into two components with binding impregnation method using PC as the mDaOtrIi:x1.0N.10o3t9e/Cth9GatC0th39e8R6Du
energy of 531.8 and 533.2 eV,29 which are ascribed to bridging NPs in Ru/PC-IM show a size of 5.78 nm, much larger than that
hydroxyls and physically absorbed water, respectively (Fig. of Ru/C (3.10 nm) and the NCs of Ru/PC (1.50 nm), which
3f).30-34 These results demonstrate the presence of massive strongly demonstrates the superiority of our developed salt
oxygen-containing groups on the surface of catalyst, which template-assisted strategy through one-step pyrolysis of the
enables a good dispersion of Ru NCs on PC matrix.
mixture of ruthenium oleate and sodium sulfate.
Hydrogen evolution from water splitting
Fig. 3 (a) XRD patterns of PC, Ru/PC, Ru/C and Ru/PC-IM. (b) N2 sorption isotherm
curve of Ru/PC (Inset is the pore size distribution). (c) XPS survey and high-
resolution XPS spectra of (d) Ru 3p, (e) C 1s and (f) O 1s for Ru/PC.
Above characterization results suggest that Ru NCs in an
ultrafine size and good dispersion with a clean surface loaded
on the PC matrix have been successfully prepared via our
developed one-step approach. It is well-documented that the
catalytic reaction over a metal catalyst is commonly size-
dependance.35, 36 One can thus expect that the small-sized Ru
NCs with narrow size distribution and clean surface in Ru/PC can
provide an abundance of accessible active sites, leading to a
distinct performance for catalytic applications.36
Note that the Ru NPs in control sample of Ru/C prepared
without the sodium sulphate template are relatively large with
size of 3.10 nm, indicating the vital role of sodium sulphate
template in maintaining the small size of Ru NCs during the
synthetic process. Except for as template, sodium sulfate has
been reported to be act as sulfur source for the synthesis of
molybdenum disulphide.37 However, the observations from EDX
and XPS suggest the undetectable S element in the Ru/PC
sample, suggesting that in this case sodium sulfate may not
serve as the S source for doping in Ru/PC. This was further
confirmed by a control experiment conducted via direct
calcination of conductive carbon/sodium sulfate, in which no
Fig. 4 (a) Polarization curves of PC, 20 wt% Pt/C, Ru/C, Ru/PC-IM, and Ru/PC in 1.0
M KOH. (b) Comparison of overpotential (10) with Ru/PC, Ru/C, Ru/PC-IM and
previous Ru based nanocatalysts. (c) TOF values, mass activity, and tafel slopes for
Ru/C, Ru/PC-IM, and Ru/PC. (d) Tafel plots and (e) capacitive current at middle
potential of CV curves as function of scan rates for Ru/C, Ru/PC-IM and Ru/PC. (f)
Polarization curves of Ru/PC before and after 1000 cycles (inset: V-t curve at 10
mA cm-2 for Ru/PC).
Hydrogen evolution through electrocatalyic water dissociation
has been regarded as green and sustainable route in
comparison with traditional methods.38-41 Pt has been
demonstrated as the sate-of-the-art electrocatalyst for HER as
it shows highly intrinsic activity. Considering the high-cost and
low reserve of Pt, however, non-Pt metals (Ru,42-46 Rh,8, 47-52 and
Ir7, 53-55) have been explored as alternatives for HER
electrocatalysis. Among the metals, Ru is cost-effective with
about ~4% price of Pt, which enables its widespread used as
electrocatalyst for HER.56, 57 Here, our developed Ru/PC was
firstly employed for HER in alkaline solution. For comparison,
control experiments using PC, Ru/C, Ru/PC-IM and commercial
any
S signal was detected (Fig. S2). Nevertheless, as
demonstrated by N2 sorption measurements, this operation
decreases the SSAs but enlarge the pore volume and pore size
(Fig. S3 and Table S2, S3). According to these results, the roles
of sodium sulfate for Ru/PC synthesis can be concluded as
follows: a) as a soluble template to produce nanostructured
hybrid of Ru/PC; b) acting as a pore-extending agent to enlarge
the pore volume and pore size. Additionally, it should be
pointed out that in this work the size and dispersion of Ru NCs
is still correlated to the synthetic methods. For
a
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Pt/C as electrocatalysts for HER were also performed. As shown
in Fig. 4a, Ru/PC displays the best electrolytic activity with low
overpotential of 21 mV at 10 mA cm-2, which is lower than that
of Ru/C (10=46 mV), Ru/PC-IM (10=86 mV) and Pt/C (10=23
mV). It is noteworthy that Ru/PC also displays a higher activity
than many recently reported Ru-based nanocatalysts, including
Ru@CN (10=32 mV)56, NiO/Ru@Ni (10=39 mV)58, Ru/CoO
(10=55 mV)59, Ru2P (10=50 mV)60, Pd@Ru (10=30 mV)61,
RuPx@NPC (10=74 mV)62 (Fig. 4b and Table S4). For exact
comparison, the catalytic activity of HER was normalized based
on the metal loading of each catalyst. Notably, Ru/PC still
presents the highest mass activity (884.8 A g-1) for HER at only
an overpotential of 30 mV, much higher than that of Ru/C (242.3
A g-1) and Ru/PC-IM (97.2 A g-1) (Fig. 4c). The TOF values for
three samples were also calculated according to the method
illustrated in Supporting Information. As shown in Fig. 4c, Ru/PC
exhibits the higest TOF of 0.20 s-1, while Ru/PC-IM shows the
lowest TOF of 0.019 s-1, indicating the superior intrinsic activity
of Ru/PC for HER.
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DOI: 10.1039/C9GC03986D
The superior electrocatalytic activity of Ru/PC for HER can be
further verified by studying the Tafel slope and capacitance of
double layer (Cdl), which is in line with ECSA. The Tafel slopes of
the samples at an overpotential of 30 mV are shown in Fig. 4c,
d. Ru/PC exhibits the lowest Tafel slope of 46.6 mV dec-1, while
Ru/PC-IM shows the highest Tafel slope of 72.8 mV dec-1,
indicating the superior kinetics of Ru/PC for HER in alkinline
solution. Generally, three typical reaction steps are presented
for alkaline HER. The first step refers to the Volmer reaction,
where a discharge route occurs with the generation of adsorbed
H*. Afterward, the H* must undergo two separated routes
including Heyrovsky and Tafel reactions to generate hydrogen56.
Consequently, the Volmer-Heyrovsky pathway should be
explored to explain the mechisam for HER using Ru/PC as the
electrocatalyst. Furthermore, it can be clearly seen that the Cdl
increases as the size of metal NPs decrease. Ru/PC with the
smallest Ru NCs (1.50 nm) gives the largest Cdl of 35.90 mF cm-2
while Ru/PC-IM with the largest Ru NPs (5.78 nm) has the
smallest Cdl of 9.60 mF cm-2 (Figs. 4e and S4). The largest Cdl of
Ru/PC means that abundant active sites are available for HER,
leading to the highest catalytic activity. The stability test of
Ru/PC for HER indicates a negligible variation of electrochemical
HER activity before and after 1000 cycles (Fig. 4f). Moreover,
the stability test by performing the V-t curve with continuous
electrolysis for 11 h shows that the overpotentail of Ru/PC only
has a slight decrease in alkaline solution (Fig. 4f inset). However,
significant acitive loss is oberved for Ru/C and Ru/PC-IM after
1000 cycles (Figs. S5 and S6). The results suggest that the Ru/PC
prepared by salt-template assisted synthesis has the highest
stability for HER in alkaline medium. The slight durability loss of
Ru/PC was ascribed to the increase of the size (3.37 nm) of Ru
NPs during stability testing, as demonstrated by the TEM image
of recovered Ru/PC (Fig. S7).
Fig. 5 (a) Plots of hydrogen generation versus time for AB hydrolysis with Ru/PC,
Ru/C and Ru/PC-IM (inset: TOF values). (b) Plots of hydrogen generation versus
time for AB hydrolysis with Ru/PC at varied temperatures (inset: Arrhenius plot).
(c) Plots of hydrogen generation versus time for reusability tests. (d) TOF values
for each cycle.
AB is regarded as a potential and eco-friendly hydrogen source
and has been widespread investigated for hydrogen production
by various kinds of strategies including hydrolysis, thermolysis,
and methanolysis.63 To study the catalytic performance of
Ru/PC, Ru/C and Ru/PC-IM, AB hydrolysis to release hydrogen
was chosen as the model reaction. Control experiments by using
PC matrix showed negligible hydrogen evolution, whereas large
amount of hydrogen was produced by Ru/PC, suggesting that
Ru was the catalytic active sites for this reaction (Figs. 5a and
S8). Note that the highest TOF value of 744 min-1 was observed
with AB and Ru concentrations of 200 and 0.6 mM, respectively,
better than that of Ru/C (532 min-1), Ru/PC-IM (217 min-1), and
many recently reported Ru based catalytic results (Fig. 5a and
Table S5). The kinetic studies on hydrogen generation from AB
catalyzed by Ru/PC were performed at varied Ru and AB
concentrations. Figs. S9 and S10 show the hydrogen generation
versus reaction time for AB hydrolysis over Ru/PC at varied AB
and Ru concentrations. The hydrogen generation rate (HGR)
versus Ru and AB concentrations both in logarithmic scales
were constructed, respectively. The slope of 0.46 of ln rate
versus ln [AB] indicates the less much correlation between HGR
and AB concentration for hydrolytic decompostion of AB with
Ru/PC, but it does not follow a zero-order kinetic in accordance
with previous results.64 The slope of 0.94 of ln rate versus ln [Ru]
leads to the conclusion that the hydrolytic decomposition of AB
on Ru/PC is regarded as first-order with Ru concentration.58
The time dependences of hydrogen evolution from AB
hydrolysis at varied temperatures were conducted (Fig. 5b),
from which the activation energy (Ea) was calculated by
Arrhenius plot. The Ea value of AB hydrolysis over Ru/PC was
39.11 kJ/mol (Fig. 5b inset), lower than many previous Ru
nanocatlysts (Table S5). The reusability of Ru/PC for AB
Catalytic hydrogen generation from AB
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hydrolysis was further explored. As illustrated in Fig. 5c, d, the further confirming the pseudo-first-order kinetic of 4-NP
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Ru/PC still can completely catalyze AB hydrolysis to generate reduction catalysed by Ru/PC. ConsequDenOtI:ly1,0.t1h03e9R/Cu9/GPCC03h9a8s6Da
sufficient hydrogen after six reusability tests, whereas the TOF very high reaction rate of 1.29 min-1 for 4-NP reaction (Fig. 6a
values decreased with increasing reusability tests, with 53.90% inset), which is higher than that of Ru/C (0.034 min-1) and
remained catalytic activity of initial run. However, the present Ru/PC-IM (0.198 min-1) under ambient conditions (Figs. 6b, c
Ru/PC still preserves comparable reusability for AB hydrolysis, and S13), suggesting the generality and superiority of Ru/PC for
compared with previous results of Ru nanocatalysts (Table S5). the reaction. The reusability testing results for 4-NP reduction
To clarify the activity loss, the recycled catalyst was toward 4-AP show that Ru/PC was reusable for seven successive
characterized by TEM and XRD. As shown in Fig. S11, no visible runs. Note that the conversion of 4-NP is still higher than 95%
characteristic peaks relative to Ru species were detected in the in the seventh recycling test compared with the initial one
XRD pattern of reused Ru/PC. The TEM image of spent catalyst (99%), suggesting a good reusability of Ru/PC for 4-NP reduction
indicates the slight increase of the size of Ru NPs from 1.50 to (Fig. 6d). The slight decrease in 4-NP conversion with increasing
2.14 nm, which must be accounted for the activity loss.
recycling runs should be ascribed to the increased size of Ru NPs
from 1.50 to 2.93 nm in the reused Ru/PC catalyst, which leads
to the partial loss of active surface area (Fig. S14).
Catalytic reduction of 4-NP
Removal of 4-nitrophenol (4-NP) from wastewater is of
significant importance in view of environment protection since
4-NP is a prevalent contaminant produced in industry and
The excellent catalytic performances of Ru/PC for the
hydrogen evolution, AB hydrolysis, and 4-nitrophenol reduction
can be contributed to the following featured advantages. a) The
salt-template assisted synthesis of PC with large SSAs and
porous structure boosts the in-situ formed ultrafine and
surface-clean Ru NCs homogenously dispersed on the surface of
simultaneously formed PC matrix, instead of embedded in PC
matrix. b) The in-situ transformation of ruthenium oleate
complex into Ru NCs and PC matrix via carbothermal approach
ensures the strong interaction between Ru NCs and PC matrix,
significantly improving the stability of Ru/PC. These features
make the Ru/PC with the enlarged atom utilization and boosted
stability, thus, showing excellent catalytic properties for the
investigated reactions.
66
agriculture.65, Therefore, the reduction of 4-NP toward 4-
aminophenol (4-AP) in the presence of NaBH4 was selected as a
model reaction to further confirm the generality of Ru/PC. The
conversion of 4-NP at different intervals was detected via
Conclusions
In summary, a salt template-assisted strategy via direct
pyrolyzing the mixture of ruthenium oleate precursor and
sodium sulfate was delicately developed for one-step synthesis
of Ru/PC, in which Ru NCs in a diameter of 1.50 nm are highly
distributed on the simultaneously generated porous carbon.
The resultant Ru/PC as the catalyst showed excellent activities
for electrocatalytic HER (10=21 mV), AB hydrolysis (TOF=744
min-1) and 4-NP reduction (reaction rate of 1.29 min-1) under
ambient conditions. The Ru/PC also displayed good durability
and reusability for the catalytic reactions. The excellent catalytic
properties of Ru/PC should be attributed to the one-step
approach and the salt template of sodium sulfate, which
ensures that the in-situ formed Ru NCs with an ultrafine size, a
clean surface and a good dispersion are solidly loaded on the
simultaneously produced PC matrix. This work may offer a new
opportunity to produce supported NCs in an ultrafine size and a
good dispersion for widespread catalytic applications.
Fig. 6 UV-vis absorption spectra of 4-NP versus time (inset: plots of ln (Ct/C0) versus
time) for (a) Ru/PC (b) Ru/C, and (c) Ru/PC-IM. (d) Reusability of Ru/PC for 4-NP
reduction.
ultraviolet-visible spectroscopy (UV-Vis). A shift of absorption
peak from 317 to 400 nm is observed when NaBH4 solution is
added because of the transformation of 4-NP into 4-
nitrophenolate ion (Fig. S12). When Ru/PC catalyst was added,
the intensity of characteristic peak at 400 nm quickly declined.
The reduction of 4-NP was completed within 5 min (Figs. 6a and
S13). The rate constant (k) was calculated according to the
pseudo-first-order kinetic equation (ln (Ct/C0)=-kt), in which C0
and Ct referred to the initial and residual concentration of
substrate at the time intervals, respectively. A good linear
correlation of ln (Ct/C0) versus time was observed (R2 = 0.992),
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21777109, 21905187).
6 | J. Name., 2012, 00, 1-3
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29. J.-X. Feng, H. Xu, S.-H. Ye, G. Ouyang, Y.-X. Tong and G.-R. Li,
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A table of contents entry
Porous carbon supported Ru nanoclusters developed by a salt template-assisted strategy show
excellent performances for multiple catalytic applications.