Ag/white graphene foam for catalytic oxidation of methanol with high efficiency and stability

Article abstract of DOI:10.1039/c4ta07053d

Oxidation of methanol not only eliminates contamination due to typical volatile organic compounds (VOCs), but also provides clean energy. Here, we report Ag/white graphene foam to act as a catalyst for the oxidation reaction of methanol with both high conversion efficiency and high stability. The T₅₀ and T₉₅ of the Ag/white graphene foam catalyst are as low as 50 °C and 110°C, almost half that of Ag/γ-Al₂O₃ catalysts. Furthermore, when the temperature was fixed at 100°C, the 93% conversion efficiency of the Ag/white graphene foam was maintained with a fluctuation of less than 2% over 50 hours, while the conversion efficiency of Ag/γ-Al₂O₃ degraded sharply from 46% to 30%. Such outstanding catalytic performance of the Ag/white graphene foam is mainly attributed to the unique microstructure, especially the multi-level pores and few-layer atomically thin walls, which has been well confirmed by the SEM and TEM observations. This journal is

Full text of DOI:10.1039/c4ta07053d

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Ag/white graphene foam for catalytic oxidation of
methanol with high efficiency and stability†
Cite this: J. Mater. Chem. A, 2015, 3,
Huijie Zhao,‡a Jizhong Song,‡ab Xiufeng Song, Zhong Yan and Haibo Zeng
6679
b
b
ab
*
Oxidation of methanol not only eliminates contamination due to typical volatile organic compounds
(VOCs), but also provides clean energy. Here, we report Ag/white graphene foam to act as a catalyst for
the oxidation reaction of methanol with both high conversion efficiency and high stability. The T50 and
T95 of the Ag/white graphene foam catalyst are as low as 50 ꢀC and 110 ꢀC, almost half that of Ag/g-
Al2O3 catalysts. Furthermore, when the temperature was fixed at 100 ꢀC, the 93% conversion efficiency
of the Ag/white graphene foam was maintained with a fluctuation of less than 2% over 50 hours, while
the conversion efficiency of Ag/g-Al2O3 degraded sharply from 46% to 30%. Such outstanding catalytic
performance of the Ag/white graphene foam is mainly attributed to the unique microstructure, especially
the multi-level pores and few-layer atomically thin walls, which has been well confirmed by the SEM and
TEM observations.
Received 22nd December 2014
Accepted 4th February 2015
DOI: 10.1039/c4ta07053d
www.rsc.org/MaterialsA
it has poor thermal stability especially under conditions in
which the feed gas contains water. In addition, g-Al2O3
1 Introduction
undergoes a phase change to a-Al2O3 when the temperature is
increased to 800 C. The phase transformation results signi-
Oxidative conversion of methanol has attracted great interest
due to the demands of an environment-friendly and energy-
saving society. Methanol, an important organic chemical raw
material and clean fuel, is colorless, transparent, and easily
transported. Methanol as the typical representative of volatile
organic compounds (VOCs), can be used for direct methanol
fuel cells (DMFCs) and fuelled vehicles (methanol gasoline,
methanol diesel).1–6 However, it can cause serious environ-
mental damage due to its toxicity and volatility. Therefore,
reducing the pollution released by the VOCs (methanol)
becomes a topic of great importance and can be improved by
increasing the catalytic efficiency of VOC (methanol)
conversion.7–11
ꢀ
cantly in the decrease of surface area, the closure of pores and
the encapsulation of active catalytic sites. Other catalyst
supports such as SiO2 for example, have the disadvantages of
poor alkali resistance, low mechanical strength and easy sin-
tering reunion, and hence cannot meet the demands of a harsh
environment.9,11,13–17 Therefore, it is a matter of urgency to nd a
novel catalyst support to boost catalytic oxidation in harsh
environments. High specic surface area, high thermal
conductivity, hydrophobic properties, and good stability are the
desired factors to achieve this aim, but how to fabricate such a
catalyst support is still a great challenge.
Here, we report Ag/white graphene foam for the oxidation of
methanol with both a high conversion efficiency and high
stability. Notably, the catalyst supports are composed of three-
dimensional white graphene foams (the bulk material is
composed of a low-dimensional boron nitride nanosheet, in
order to facilitate the writing, called 3D BN) with multi-level
pores, atomically thin walls, and a specic surface area of 681
m2 gꢁ1. These structural features endow the white graphene
foams with superior advantages in the eld of catalyst supports,
and hence the supported Ag catalysts exhibit good catalytic
activity and stability. The T50 and T95 of Ag/white graphene foam
catalysts are as low as 50 ꢀC and 110 ꢀC, respectively, almost half
that of Ag/g-Al2O3 catalysts. Furthermore, when the tempera-
ture was xed at 100 ꢀC, the 93% conversion efficiency of the Ag/
white graphene foam was maintained with a uctuation of less
than 2% over 50 hours, while the conversion efficiency of
Ag/g-Al2O3 degraded sharply from 46% to 30%.
The conversion efficiency is mainly associated with the key
catalyst, co-catalyst, catalyst support and the third-group
compounds.12 As for a specic key catalyst, the use of supported
catalysts is an effective route for improving catalytic activity,
and the effect of a catalyst support on the catalytic process
should not be neglected. However, conventional catalyst
supports such as alumina, silica and carbon materials cannot
always fully meet the needs. For example, g-Al2O3 with a surface
area of 150–300 m2 gꢁ1 is widely used as a catalyst support, but
aState Key Laboratory of Mechanics and Control of Mechanical Structures & College of
Materials Science and Technology, Nanjing University of Aeronautics and Astronautics,
Nanjing 210016, China. E-mail: zeng.haibo@njust.edu.cn
bInstitute of Optoelectronics & Nanomaterials, College of Materials Science and
Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta07053d
‡ These authors contributed equally to this work.
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were calculated based on the production of CO2. The reaction
was also conducted under various ow rates to test the inu-
ence of mass-transfer resistance on the catalyst. No signicant
rate change was noted, meaning that the mass-transfer limita-
tion was negligible.
2 Experiments
2.1 Synthesis methods
In a typical procedure, ammonia borane (AB) was supplied as
the B and N source due to its stoichiometric ratio and appro-
priate decomposition performance, while thiourea was used as
the vesicant in the reaction process for its function in increasing
mutual cross-linking between the intermediate products,
improving the yield. Thiourea and ammonia borane (AB) with
different mass ratios (0 : 1–5 : 1) were loaded at the crucible,
then placed into the central region of a high-temperature
tubular furnace and heated to 1100–1400 ꢀC under N2. Aer 1–5 h
of growth, the furnace was cooled to room temperature and the
3D BN products were synthesized.
2.3 Characterization and application
A eld-emission scanning electron microscope (SEM, SU-4800,
Hitachi Corp.) and a transmission electron microscope with an
energy-dispersive X-ray analyzer (EDX) (TEM or HRTEM, Tecnai
F30 S-TWIN, FEI Corp.) were used to investigate the morphology
and crystalline structure of the as-prepared 3D BN and Ag/3D
BN, respectively. X-ray diffraction (XRD, D8 Advance, Bruker Cu
The as-grown 3D BN (2 mg mLꢁ1) products were dissolved in
poly(ethylene glycol) (PEG) by sonication and magnetic stirring.
Then the AgNO3 (2 mg mLꢁ1) was added to the solution under
stirring and exposure to an ultraviolet lamp (254 nm) for 30
min. Aer washing and drying, we can obtain Ag/3D BN
samples. The Ag/g-Al2O3 sample (Ag loaded on the g-Al2O3
support) and the Ag/C BN sample (Ag loaded on the commercial
BN support) were prepared using a similar method.
˚
Ka radiation (l ¼ 1.5406 A)) and X-ray photoelectron spectros-
copy (XPS, PHI QuanteraII, and Japan-US Nano Surface Analysis
Instruments Corp.) were used to test the composition. A contact
angle meter (OCA20, dataphysics) was used to measure the
hydrophobic property of 3D BN. Specic surface area tests (N2
adsorption–desorption isotherms, ASAP 2020M + C, American
Micromeritics Corp.) were used to detect the Brunauer–
Emmett–Teller (BET) surface area and non-local density func-
tional theory (NLDFT) pore-size distribution of the as-prepared
3D BN. The N2 physisorption isotherms were measured at 77 K.
Prior to the measurement, the samples were outgassed in
2.2 Catalytic performance test
ꢀ
All the catalytic oxidation experiments were performed in a
conventional xed bed continuous-ow reactor under atmo-
spheric pressure. It consists of a stainless steel tube (9 mm i.d.)
charged with the required amount of catalyst. An E-type ther-
mocouple was placed in the center of the catalyst bed to control
the furnace and record the reaction temperature. The volatile
organic compound (VOC) laden gas was generated by bubbling
air through the VOC saturators, and then carrying out further
dilution with another gas stream before reaching the reaction
bed. An O2/N2 stream through a methanol saturator was mixed
with another O2/N2 stream. In each test, the molar ratio between
O2 and N2 is controlled at 5%, 10% and 20%. The concentration
of methanol in the feed was regulated in the range of 1000–4000
ppmv by adjusting the saturator temperature. 300 mg of catalyst
was placed into the center of the reactor and the total ow rate
vacuum at 300 C for 10 h. The BET specic surface area was
calculated from the N2 adsorption data in relative pressure
ranging from 0.05 to 0.35. Due to the broad pore size distribu-
tion (PSD), ranging from micropores to mesopores, the NLDFT
method was used to calculate the pore widths and pore size
distributions (ASiQwin soware). An ultraviolet spectropho-
tometer (UV-VIS, UV-3600, Shimadzu Corp.) was used to verify
the appearance of silver nanoparticles on the 3D BN. A gas
chromatograph (GC, GC7890A, Agilent Corp.) was used to
assess the catalytic performance of the Ag/3D BN, Ag/g-Al2O3,
and pure 3D BN.
3 Results and discussion
was kept at 200 mL minꢁ1 with a corresponding gas hourly Firstly, the white graphene foams were synthesized through
space velocity (GHSV) of 40 000 hꢁ1
thermolysis of thiourea (CN2H4S) and ammonia borane (AB)
.
ꢀ
The reaction temperature varied from 20 to 250 C. Prior to (H6BN) mixtures. The former acted as a vesicant to release a
the activity studies, the catalyst was activated at 500 ꢀC in O2 for mass of gas during the formation of BN arising from the later
2 h, and then cooled to room temperature before introducing raw materials. Fig. 1 presents the fabrication process of the Ag/
methanol. All catalytic oxidations were performed in the steady- white graphene foam catalysts. Under ultraviolet (UV) light
state condition and started at a temperature that ensured zero irradiation, the color of pure AgNO3, 3D BN and their mixed
conversion of the methanol. The product stream was separated solutions were quite different, as show in Fig. 1a. The
by a capillary column and the VOC concentrations in the unchanged 3D BN poly(ethylene glycol) (PEG) solution shows
effluent were determined by GC. When the composition and the high stability of the 3D BN support under UV radiation.
concentration of the VOCs in the inlet streams were varied, only Compared with the slight yellow dyeing of the AgNO3 PEG
water and CO2 were detected in the methanol oxidation. No CO solution, the color of the AgNO3/3D BN mixed solution obvi-
was detected in any experiment. Carbon mass balances were ously changed from white to dark brown, indicating that the 3D
conducted for each set of experimental conditions and the BN support has a great inuence on the loading process. The
methanol conversion and selectivity to oxidation byproducts color change is mainly caused by the reducibility of the PEG
were calculated and found to be within a relative error of less solution and the UV irradiation, through the reduction of the Ag
than 5%. The conversion efficiencies of the oxidation reactions ions to atoms via electron transfer.18
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Fig. 2 (a) A photograph of the large amount of white graphene foam
produced. SEM image of 3D BN (b and c) and Ag/3D BN (d and e). It is
clearly seen that 3D BN consists of a network-like morphology (b) and
has a large lamellar structure with ultra-thin walls (c). The Ag NPs are
uniformly distributed on the 3D BN (d and e).
Fig. 1 (a) Photographs of pure AgNO3, immersed 3D BN and their
mixed solution before (left) and after (right) ultraviolet irradiation. (b)
Comparison of the UV-vis absorption spectra of the three solutions.
shown in Fig. 2c. Such a 3D structure with ultra-thin walls is
conducive to improving the loading of silver onto the unit mass.
In the process of loading the silver particles, the structure of the
3D BN support exhibits no obvious change even aer several
hours of ultrasound and magnetic stirring treatment, as shown
in Fig. 2d, indicating the structure stability of 3D BN. Fig. 2e
reveals the uniform distribution of Ag NPs on the 3D BN
surface.
The formation of Ag nanoparticles (Ag NPs) in the mixed
solution was further conrmed by the comparison of the
absorption spectra as shown in Fig. 1b. Aer irradiation, the
local surface plasmon resonance (LSPR) peak of the Ag NPs at
ꢂ450 nm was very strong from the mixed solution of 3D BN and
AgNO3 PEG, very faint from the AgNO3 PEG solution, and
immeasurable from the 3D BN PEG solution.18,19
The detailed microstructure of the Ag/white graphene foam
was further characterized by TEM, as shown in Fig. 3. The
nanosized pores on the BN walls can be clearly seen in Fig. 3a.
The white graphene foams were estimated to have a large
specic surface area (SSA) of 681 m2 gꢁ1 based on the Brunauer–
The white graphene foams with and without Ag NPs were
characterized in Fig. 2. Fig. 2a shows a photo of a white gra-
phene foam with a length more than 10 cm, demonstrating the
large amount prepared by the simple thermolysis method
which can be realized in industrial applications. During the
formation of the white graphene foam, the thiourea played an
important role.20,21 It can promote the mutual crosslinking
between the molecules of the transition state of AB, including
polymeric aminoborane (PAB), (H2BNH2)n, polyiminoborane
(PIB), (HBNH)n and borazine (B3N3H6), etc.22,23 More impor-
tantly, thiourea can release a large amount of gas such as, H2,
N2, NH3, etc., which initiated and gathered into bubbles and
induced volume expansion, signicantly improving the
production and porosity of the boron nitride products. There-
fore, we can get high quality 3D BN with a yield up to 62%
compared to the raw materials of AB, far higher than previously
reported data (40%).24 The 3D network-like morphology can be
clearly observed from the SEM image as shown in Fig. 2b. This
network is composed of an irregular skeleton structure and
typical akes of the ultra-thin walls with a large planar size, as
Emmett–Teller (BET) method. The pore volume is 0.58 cm3 gꢁ1
,
and the corresponding bipolar pore size distribution of 1.5 and
32 nm were observed (ESI, Fig. S1†).25 Fig. 3b conrms the three
layer thickness of the lamellar structure, and the layer spacing is
about 0.33 nm, which corresponds to the lattice spacing of the
h-BN structure (002) plane.26,27
Fig. 3c reveals that the Ag NPs are uniformly distributed on
the BN surface. Their average diameter is about 5 nm and they
have a size distribution from 2 to 10 nm (Fig. S2†). A typical
HRTEM of a Ag NP is shown in Fig. 3d. Ag (112) and Ag (111)
lattice fringes with spacing distances of 0.22 and 0.24 nm,
respectively, are highlighted by the yellow lines.19 Fig. 3e shows
the energy dispersive X-ray spectrum (EDX) mapping of B, N,
and Ag elements, further conrming the formation of Ag cata-
lysts on the ultrathin BN walls, which is consistent with the
following X-ray photoelectron spectroscopy (XPS) and X-ray
diffraction (XRD) measurements.
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BN, it can be said that the unique structural characteristics of
the 3D BN support make it show obvious advantages over a
commercial BN support. The Ag/3D BN exhibits outstanding
catalytic activity, which is even better than that of conventional
Ag/g-Al2O3 catalysts. Generally, the activity of a given catalyst
can be mainly characterized by two parameters, T50 and T95,
corresponding to the temperatures at which the conversion
efficiency reaches 50% and 95%, respectively. The activities for
the catalytic oxidation of several VOCs are summarized in Table 1,
showing that the Ag/3D BN can be used as an effective catalyst.
ꢀ
The light-off temperature (T50) of Ag/3D BN is as low as 50 C,
and the temperature of T95 is only 110 ꢀC. For comparison,
those two values for Ag/g-Al2O3 are 110 ꢀC and 180 ꢀC, far larger
than those of Ag/3D BN. Fig. 4b and c show the effects of O2 and
methanol concentrations on the catalytic performance of Ag/3D
BN. The conversion efficiency slightly uctuated with the
increase in both O2 concentration and methanol concentration,
which is typical behavior according to reaction dynamics.7,9,14,30
Besides the activity, stability is also crucial for catalysts.
Fig. 4d compares the catalytic stability of Ag/3D BN with Ag/g-
Fig. 3 TEM and HRTEM image of 3D BN (a and b, respectively) and Ag/
3D BN (c and d, respectively). 3D BN exhibits a porous microstructure
(a) and three layers of lamellar structure (b). TEM images (e) of the Ag
NPs on 3D BN and the corresponding EDX elemental mapping of B, N
and Ag.
ꢀ
Al2O3 when the temperature is xed at 100 C. The Ag/3D BN
catalysts exhibited a consistent conversion efficiency of around
93% with a uctuation of less than 2% over the 50 hours of
measurement. Besides the low conversion efficiency, the
degradation of the Ag/g-Al2O3 catalysts was so obvious that the
conversion rate reduced rstly from 46.5% to 42% in the initial
one hour and from 40% to 30% during the period from 10 to 50
hours. The initial degradation of the Ag/g-Al2O3 catalysts was
induced by the rapid accumulation of water vapor in the
micropores due to its high water absorption, and capillary
condensation. In sharp contrast, there was almost no such
initial degradation for the Ag/3D BN catalysts, which may be
attributed to the highly hydrophobic features of the white gra-
phene foams with a hydrophobic angle of 131ꢀ (Fig. S3†). The
second degradation of the Ag/g-Al2O3 catalysts aer a long time
of oxidation reaction is usually attributed to the structural
instability and weak thermal conductivity of g-Al2O3.11,13,31 The
absence of the long term degradation of the Ag/3D BN catalysts
is due to the intrinsic properties of BN, such as the high
structural stability, high thermal conductivity and hydrophobic
properties of BN materials. It has been reported that BN can
maintain stability in O2-containing air at a much higher
The catalytic performance of the Ag/white graphene foam
was characterized with the oxidation reaction of methanol as
shown in Fig. 4. Prior to the reaction activity measurements, the
catalysts were activated at 500 ꢀC in O2 for 2 h.28,29 Fig. 4a
presents the temperature-dependent conversion efficiencies of
methanol to CO2. Comparing the catalytic activity of pure 3D BN
with that of Ag/3D BN, we can claim that Ag is the primary active
site in such an oxidation reaction. Comparing the catalytic
performance of Ag/C BN (commercial BN) with that of Ag/3D
Table 1 The comparison of different catalysts for VOC oxidation
Catalyst
VOC
T50 (ꢀC)
T95 (ꢀC)
Reference
Ag/3D BN
Ag/g-Al2O3
Ag/C BN
6% Ag/g-Al2O3
0.1% Pd/g-Al2O3
0.1% Pt/g-Al2O3
Pt/h-BN
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Iso-hexane
Iso-hexane
Benzene
50
110
134
135
187
169
170
260
140
140
240
110
180
195
175
239
195
300
350
170
175
280
This paper
This paper
This paper
30
30
30
11
11
13
13
13
Fig. 4 (a) Temperature-dependent catalytic performance of Ag/3D
BN, Ag/g-Al2O3, Ag/C BN (commercial BN) and 3D BN, (10% O2, 2000
ppmv methanol). (b and c) Effect of the O2 (5%, 10%, 20% O2, 2000
ppmv methanol) and methanol (10% O2, 1000, 2000, 4000 ppmv
methanol) concentration on the methanol conversion by Ag/3D BN.
(d) Stability of Ag/3D BN, Ag/g-Al2O3 catalysts for methanol oxidation
at 100 ꢀC.
Pt/g-Al2O3
0.37 wt% Pt/b-BN
0.37 wt% Pt/a-BN
Pt/Al2O3
Benzene
Benzene
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temperature of about 950 ꢀC, and that the thermal conductivity interactions between boron nitride and silver, not that they are
can reach up to 2000 W mꢁ1 Kꢁ1 at room temperature.26,27
simply mixed together. Metal–support interactions have an
The catalytic mechanism of Ag based catalysts has been important inuence on the catalytic performance. Fig. 5c pres-
controversial, but it is generally believed to have a certain ents the high resolution Ag spectra of Ag/3D BN. Obviously, the
relationship with oxygen. The formation of subsurface oxygen oxygen activation shis the Ag 3d5/2 binding energy from 368 to
(Og) was considered necessary for the activation of Ag catalysts 367.4 eV, revealing that the Ag surface has been partially
for methanol oxidation. Surface-bound atomic oxygen (Oa) oxidized to Ag2O.29,35–37 Such speculation about partial surface
preferentially led to the formation of complete oxidation prod- oxidation was further conrmed by the X-ray diffraction (XRD)
ucts. The increase in the ratio of Og : Oa on the Ag surface would measurements (Fig. S5†).
result in an increase in direct methanol oxidation. Recent
The mechanism of 3D BN/Ag catalytic methanol oxidation is
studies have shown that the Ag–O interaction involving the described in Scheme 1. The high temperature oxygen treatment
formation of subsurface oxygen species exerts an important caused reconstruction of the Ag surface, forming subsurface
inuence on the surface structure and, eventually, the catalytic oxygen and surface-bound atomic oxygen. The existence of
properties of silver catalysts.28,29,32,33
subsurface oxygen facilitates the formation of active sites on
We used X-ray photoelectron spectroscopy (XPS) to detect the silver catalysts, which could enhance the catalytic capability.
chemical states of Ag/3D BN to gain further understanding of the Surface-bound atomic oxygen was inclined to form complete
catalytic mechanism. The survey spectra in Fig. 5a reveals the co- oxidation products. The 3D BN support provides a moderate
existence of elements B, C, N, O, and Ag. The carbon detected is surface area that can accommodate dispersed Ag metals, while
from surface contamination which is commonly observed by the still preserving the unique characteristics. Furthermore, the 3D
sensitive XPS method. The detailed B and N peaks are shown in BN support has the property of high thermal conductivity,
Fig. 5b, in which the main binding energies are located at 190.2 temperature stability, acid–base resistance, and hydrophobicity
eV and 397.9 eV, respectively. Notably, compared with the (thus preventing moisture condensation on its surface), which
symmetric N1s peak, the shape of the B1s peak shows obvious greatly improves the performance of the catalyst. First, meth-
asymmetry with a shoulder at 192.1 eV, indicating the appear- anol and O2 were adsorbed onto the surface of the silver then
ance of B–O bonds.34 Meanwhile, the loading of Ag and the high dissociated with improving reactivity, reducing the activation
temperature O2 treatment do not have much impact on the B1s energy and increasing the reaction rate. Second, methanol can
and N1s peaks. Only a slight movement of the peak position and directly react with oxygen at a low temperature to produce H2O
shape is observed (Fig. S4†), indicating the presence of and CO2. Finally, H2O and CO2 molecules desorbed from the
surface by diffusion. Then the adsorption–desorption cycle over
the silver surface was continued.
4 Conclusion
In summary, we apply white graphene foams to replace the
usual g-Al2O3 to act as a support of Ag catalysts for the high-
Fig. 5 (a) XPS spectra of the 3D BN and Ag/3D BN before and after O2
treatment. (b) The detailed B1s and N1s spectra of 3D BN. (c) The Scheme 1 Illustration of the reaction mechanism of Ag/3D BN cata-
detailed Ag 3d spectra of Ag/3D BN before and after O2 treatment.
lytic methanol oxidation.
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Acknowledgements
This work was supported by the National 973 project from
National Basic Research Program of China (2014CB931700), the
National Natural Science Foundation of China (61222403), the
Doctoral Program Foundation from the Ministry of Education
of China (20123218110030), the Fundamental Research Funds
for the Central Universities (30920130111017 and NE2012004)
and the Opened Fund of the State Key Laboratory on Integrated
Optoelectronics (IOSKL2012KF06), Natural Science Foundation
for Youths of Jiangsu Province of China (BK20140787), China
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Postdoctoral
Science
Foundation
funded
project
(2014M560425) and the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions.
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6684 | J. Mater. Chem. A, 2015, 3, 6679–6684
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