Mesoporous carbon-confined Au catalysts with superior activity for selective oxidation of glucose to gluconic acid

Article abstract of DOI:10.1039/c3gc36891b

A series of ordered mesoporous carbon (OMC)-supported Au catalysts were successfully prepared by nano-replication, followed by colloidal gold deposition method. Structural analysis showed that the mesopore sizes of the catalysts can be tuned controllably in the range of 3.2-7.6 nm by adjusting the dosage of boric acid used to prepare the carbon supports. TEM observations revealed that the Au nanoparticles were dispersed uniformly in the mesopore channels of the carbon supports. These Au/OMC catalysts were tested for the aerobic oxidation of glucose to produce gluconic acid at 40°C and pH 9. As demonstrated by the structural analysis and reaction results, the activities of these catalysts were closely related to their mesopore sizes. The catalyst with a mesopore size of 5.4 nm exhibited a superior catalytic activity with a TOF of 4.308 mol _glucose mol_Au^-1 s^-1 to the catalysts reported previously by other researchers. This high activity was mainly ascribed to its unique structure, consisting of 5.4 nm mesopore channels incorporated with 3.3 nm Au nanoparticles, which facilitates contact between glucose molecules and Au nanoparticles. Besides, the abundant active oxygen species existing on this catalyst surface also promote glucose oxidation.

Full text of DOI:10.1039/c3gc36891b

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Mesoporous carbon-confined Au catalysts with
superior activity for selective oxidation of glucose to
Cite this: Green Chem., 2013, 15, 1035
gluconic acid†
a
a
a
b
a
Chunyan Ma, Wenjuan Xue, Jinjun Li, Wei Xing* and Zhengping Hao*
A series of ordered mesoporous carbon (OMC)-supported Au catalysts were successfully prepared by
nano-replication, followed by colloidal gold deposition method. Structural analysis showed that the
mesopore sizes of the catalysts can be tuned controllably in the range of 3.2–7.6 nm by adjusting the
dosage of boric acid used to prepare the carbon supports. TEM observations revealed that the Au nano-
particles were dispersed uniformly in the mesopore channels of the carbon supports. These Au/OMC
catalysts were tested for the aerobic oxidation of glucose to produce gluconic acid at 40 °C and pH 9. As
demonstrated by the structural analysis and reaction results, the activities of these catalysts were closely
related to their mesopore sizes. The catalyst with a mesopore size of 5.4 nm exhibited a superior catalytic
activity with a TOF of 4.308 molglucose molAu s−1 to the catalysts reported previously by other research-
ers. This high activity was mainly ascribed to its unique structure, consisting of 5.4 nm mesopore channels
incorporated with 3.3 nm Au nanoparticles, which facilitates contact between glucose molecules and
Au nanoparticles. Besides, the abundant active oxygen species existing on this catalyst surface also
promote glucose oxidation.
−1
Received 27th November 2012,
Accepted 10th February 2013
DOI: 10.1039/c3gc36891b
www.rsc.org/greenchem
The performance of the supported Au catalyst seems to
1
Introduction
mainly depend on the size of the Au nanoparticles. Recently,
8
Over the last two decades, the catalytic transformation of Ishida prepared a series of Au catalysts with excellent catalytic
biomass-derived natural resources into valuable compounds performance for the oxidation of glucose by grinding a volatile
1
has emerged as a technique to minimise pollution. Gluconic dimethyl Au(III)acetylacetonate with various catalyst supports.
acid, resulting from selective oxidation of glucose, is applied It was found that the control of the Au particle size is critical
9
as an acidity regulator of food additives and is well suited for in glucose oxidation. Okatsu prepared a variety of Au catalysts
use in removing calcareous and rust deposits from metals or with various carbon supports by deposition reduction method.
other surfaces. In particular, the oxidation of glucose to gluco- Their results showed that the performance of the catalyst
nic acid over noble metal catalysts, including Pt, Pd and Au, increases with the decrease of the Au nanoparticle size.
2
–4
10
has been extensively studied. Compared with Pd and Pt cata- Ishida deposited Au nanoparticles directly onto cellulose by
lysts, supported Au catalysts have shown higher catalytic solid grinding with dimethyl Au(III)acetylacetonate followed by
activity with excellent selectivity to gluconic acid as H reduction. The results also demonstrated that the catalyst
2
5
–7
5
reported. Rossi and his co-workers attributed this high cata- with smaller Au nanoparticles possesses higher catalytic
lytic performance to the higher resistivity of Au catalyst to activity than that with larger Au nanoparticles. As reported by
1
1
overoxidation and self-poisoning compared to Pt or Pd.
Prüße, the supported Au/Al O catalysts had a long-term
2 3
stability for the catalytic oxidation of glucose, because the Au
nanoparticles did not sinter into larger particles. The previous
studies undoubtedly indicated that Au nanoparticles with
smaller size (<5 nm) are more catalytically active than those
with larger size (>10 nm) in the aerobic oxidation of glucose.
In addition to the particle size, the activity of the Au cata-
lysts seems to be affected by the nature and morphology of the
a
Department of Environmental Nanomaterials, Research Center for Eco-
Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, P. R. China.
E-mail: zpinghao@rcees.ac.cn; Tel: +86-10-62923564
b
School of Science, State Key Laboratory of Heavy Oil Processing, Key Laboratory of
Catalysis, China University of Petroleum, Qingdao 266555, P. R. China.
E-mail: xingwei@upc.edu.cn; Tel: +86-533-2788830
†
Electronic supplementary information (ESI) available: Small-angle XRD of the
8
12
support. Lately, Benkó prepared a series of Au catalysts sup-
ported on TiO , SiO and CeO by colloidal gold deposition.
OMC supports, nitrogen physisorption isotherms, influence of reaction time on
glucose oxidation, XPS Au 4f spectra, IR spectra and O
Au/OMC catalysts. See DOI: 10.1039/c3gc36891b
2
-TPD profiles of some
2
2
2
Those catalysts showed inferior catalytic performance to the
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carbon-supported reference catalyst. Al
2
O
3
-supported Au cata- sucrose and 0.07 g of H
2
SO
4
(98 wt%). The amount of sucrose
lysts were also found to be of lower activity than their active used is expected to just fill up the pores of SBA-15. The result-
4
,13,14
15
carbon-supported counterparts.
Au/TiO and Au/C catalysts by the gold sol method, and the 160 °C for another 6 h. Afterwards, the dried sucrose/silica
former showed lower activity for glucose oxidation than the composite was again impregnated with an aqueous solution
latter. To date, various active carbon-supported Au catalysts consisting of 0.4 g sucrose, 0.05 g H SO and 5.0 g H O. The
Mirescu
prepared ing sludge was dried at 100 °C for 6 h, and subsequently,
2
2
4
2
have been tested for the selective oxidation of glucose by many resulting mixture was dried again at 100 and 160 °C. The
researchers and active carbons are testified to be good sup- second sucrose infiltration step aims at completely filling up
ports for Au catalysts. However, Au nanoparticles with uniform the pores of SBA-15. The material thus obtained was heated to
size are rarely formed on active carbon due to their wide pore 900 °C in argon atmosphere. The sucrose was converted to
size distribution which provides a heterogeneously steric con- carbon by such a process using sulfuric acid as the catalyst.
1
6–18
finement for Au nucleation and deposition.
This diversity Finally, the silica framework was removed by dissolution in HF
in Au particle size would inevitably lower the activity and selec- (10 wt%) aqueous solution. The resulting porous carbon
tivity of the Au catalyst. Besides, the micropores of the active material is referred to as OMC-SBA-100.
carbons are also blocked easily by Au nanoparticles, which
would hinder the diffusion of reactants and products in the pared by a slightly modified Kim’s procedure. The procedure
pore texture of the active carbons. is quite similar to the nano-replication method used to
(2) OMC-MSU-x carbons. This series of carbons were pre-
2
4
Compared with traditional active carbons, ordered meso- prepare OMC-SBA-100, except for the use of MSU mesoporous
porous carbons (OMCs) have such merits as uniform/control- silica as hard template and a mixture of sucrose and boric acid
lable mesopore size and high surface area, and have been as the precursor solution. The boric acid content in the precur-
1
9–21
explored as a catalyst support in recent years.
It has been sor solutions was in the range of 2–50 wt% (boric acid/
demonstrated that metal nanoparticles can be dispersed and sucrose). Precursor solutions were prepared by adding various
confined uniformly in the ordered mesopores of OMCs and amounts of boric acid to a sucrose solution whereby the
the mesopore channels of OMCs are not easily blocked by sucrose concentration was kept constant. The resultant
1
9
metal nanoparticles, thus exhibiting high catalytic activity.
carbons were denoted as OMC-MSU-x, where x stands for the
As far as we know, OMC-supported Au catalysts have not yet weight ratio of boric acid to sucrose. A typical procedure for
been reported for the aerobic oxidation of glucose. Herein, the preparation of OMC-MSU-2 can be described as follows: a
OMCs with various pore diameters were prepared by a nano- precursor solution containing boric acid (0.025 g), sulfuric
replication method and used as supports for Au catalysts in acid (0.141 g), sucrose (1.25 g), and distilled water (4.0 ml) was
glucose oxidation for the first time. The effects of mesopore allowed to infiltrate the mesopores of 1 g MSU template. The
size and surface chemical nature of the OMCs on glucose oxi- composite material was then dried at 100 °C for 6 h and at
dation were investigated intensively in this work. It has been 160 °C for 6 h thereafter. The infiltration and drying processes
demonstrated that the activities of these catalysts were closely were repeated once more with an additional 3.57 g of precursor
related to their mesopore sizes. The catalyst with 5.4 nm meso- solution (66% of first infiltration), before the composite was
pores exhibited a superior catalytic activity, with a TOF of carbonized at 900 °C for 3 h under argon flow. Finally, the
−
1
−1
4
.308 molglucose molAu
s , compared to the catalysts OMC-MSU-2 material was obtained by removal of the in-
reported previously by other researchers. This high activity was organic framework using a 10 wt% HF solution.
mainly ascribed to its unique structure: 5.4 nm mesopore
Preparation of Au/Meso-C materials
channels incorporated with 3.3 nm Au nanoparticles, which
facilitates the contact between glucose molecules and Au nano- A suitable amount of PVP solution (2 wt%) was added to an
−
1
particles. Furthermore, the abundant active oxygen species aqueous HAuCl solution (0.39 mg Au ml ) under vigorous
4
existing on this catalyst surface also promote glucose oxi- stirring. Then, a freshly prepared solution of NaBH
dation. The study of catalysts presented here is critically added dropwise into the mixture (molar ratio NaBH
4
was slowly
/Au = 4).
4
important for exploring environmentally friendly catalysts with The as-prepared mesoporous carbon was added into the
superior activity, and thus high productivity of gluconic acid in mixture afterwards and stirred at room temperature for 2 h,
mild conditions can be achieved.
then filtered and washed until no Cl− was detected using
AgNO solution. After being dried at 70 °C for 5 h, the sample
3
was calcined at 200 °C for 3 h.
Material characterizations. Small-angle X-ray diffraction
2
Experimental
(
SAXRD) patterns were recorded on an X’pert PRO powder
diffraction system using Cu Kα radiation in the 2θ range of
1) OMC-SBA-100. The synthesis was performed using 0.7–6.0° with a scanning step size of 0.0016°. The textural
sucrose as the carbon source and 2-D hexagonal (P6mm) meso- properties of the samples were measured by N2 sorption at
porous silica SBA-15 as the hard template according to liquid nitrogen temperature, using a gas adsorption analyzer
Ryoo’s method. Briefly, 1 g of SBA-15 was mixed homo- NOVA 1200. The actual Au content in the samples was deter-
Preparation of ordered mesoporous carbons
(
2
2
2
3
geneously with 5 ml of aqueous solution containing 0.68 g of mined using an inductively coupled plasma optical emission
1
036 | Green Chem., 2013, 15, 1035–1041
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spectroscopy (ICP-OES) analyzer (OPTIMA 2000). Before
measurements, the solid sample was dissolved in aqua regia.
High resolution transmission microscopy (HRTEM) obser-
vations were conducted with a JEOL 2011 instrument at an
accelerating voltage of 200 kV. The specimens were prepared
by ultrasonic dispersion in ethanol, evaporating a drop of the
resultant suspension onto a carbon support grid. X-Ray photo-
electron spectroscopy (XPS) was conducted on a Kratos Axis
Ultra DLD spectrometer with a monochromated Al-Kα X-ray
source (hν = 1486.6 eV), hybrid (magnetic/electrostatic) optics
and a multi-channel plate and delay line detector (DLD). All
spectra were recorded using an aperture slot of 300 ×
7
00 microns. Survey spectra were recorded with a pass energy
of 160 eV and high-resolution spectra with a pass energy of 40
eV. Accurate binding energies (±0.1 eV) were determined with
respect to the position of the adventitious C1s peak at
Fig. 1 (A) Small angle X-ray diffraction of the catalysts and (B) pore size distri-
butions calculated from desorption branch for the catalysts.
2
84.8 eV. The Fourier transformed spectroscopy (FT-IR)
2
5
measurements were performed using a TENSOR 27 infrared peak corresponding to the 100 plane of P6mm symmetry. For
spectrometer with a DLaTGS detector. Infrared spectra in the Au/OMC catalysts, they still retain the mesoporous structure
−
1
range of 4000 to 400 cm were obtained by averaging 32 scans well after loading Au particles (as shown in Fig. 1A). The BET
−
1
with a resolution of 4 cm at room temperature. The catalyst surface area and pore structure parameters of these catalysts
powders were mixed with KBr at a mass ratio of 1 : 2000 and were determined by N adsorption and tabulated in Table 1.
2
then pressed into thin and transparent slices for analysis. An The as-synthesized Au/OMC catalyst has a mesoporous struc-
O
2 2
temperature programmed desorption (O -TPD) test was ture with a high surface area and large pore volume. Nitrogen
carried out using a Micromeritics Chemisorb 2720 apparatus. sorption isotherms (shown in Fig. S2†) indicate that the iso-
Prior to each TPD run, the catalyst was pretreated with a He therms of all the Au/OMC catalysts are typical type IV iso-
flow at 300 °C in a quartz reactor, and the reactor was then therms with large hysteresis loops.2
6,27
The capillary
allowed to cool to room temperature. The adsorption of O
2
was condensation partial pressure increases with the ratio of boric
measured by exposing the catalyst to a flow of O for 30 min acid to sucrose for Au/OMC-MSU samples, indicating the
2
1
−1
(
50 ml min− ). He gas was fed to the reactor at 50 ml min for enlargement in mesopore size. The mesopore size distribution
3
0 min to purge any residual oxygen. The catalyst was then (Fig. 1B) confirms the rise of mesopore size with the dosage
−
1
heated to 600 °C at a constant heating rate of 10 °C min
under He flow (50 ml min ). The desorption of oxygen was Au/OMC catalysts possess a monomodal mesopore distri-
monitored by a TCD detector.
bution and the mesopore diameter of Au/OMC catalysts can be
increase of boric acid. We can also see that the pores of all the
−1
Activity measurement of catalysts for glucose oxidation. tuned in the range of 3.2–7.6 nm by adjusting the ratio of
Glucose oxidation was carried out at atmospheric pressure by boric acid to sucrose.
bubbling molecular oxygen (100 ml min− ) through the
1
glucose (12.39 g) aqueous solution (125 ml). The catalyst
3.2 Au nanoparticle distributions
(0.25 g) was added into the glucose solution, and the aqueous
dispersion was stirred at 40 °C. The reaction was kept at pH 9 During catalyst preparation, the same impregnation procedure
by titrating with 2.5 M aqueous NaOH solution. Analysis of the was employed in order to obtain Au catalysts with similar load-
conversion of glucose was performed by HPLC on a Breeze ings and particle sizes, independent of the support used. The
1
525 instrument equipped with a water (210 nm) detector. A total gold loadings on the carbon samples were checked by
Luna Amino C18 column (4.6 × 150 mm, 5 μm) was used with ICP-OES analysis and the results were listed in Table 1. It was
3 4
ethanol and 1% H PO aqueous solution (9 : 1) as eluent (flow found that the gold loadings are proportional to the pore sizes
rate 1 ml min− ). The product of glucose oxidation was ident- of the catalysts, which may be due to the fact that large pore
1
ified as gluconic acid by comparison with the standard size facilitates the deposition of Au nanoparticles onto the
compound.
carbon pore surface. Actually, there is another clue to account
for this phenomenon. It has been reported that the surface
oxygen content of the OMC supports is proportional to the
boron doping because low-concentration boron doping favors
oxygen chemisorptions on the carbon surface. It is also
demonstrated by XPS analysis that the surface oxygen contents
3
Results and discussion
3
.1 The pore structure of Au/OMC catalysts
The as-prepared OMC supports exhibit an ordered 2D hexa- of OMC-SBA, OMC-MSU-10 and OMC-MSU-15 were 3.0, 4.9
gonal mesostructure, as can be seen from the XRD patterns and 5.7%, respectively. It is well known that the oxygenated
shown in Fig. S1.† The XRD patterns exhibit a well resolved groups are typically hydrophilic. Therefore, the prepared
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Table 1 Surface and physical properties and activity of Au/OMC catalysts
N
2
sorption analysis
Supported gold
Activity
BET (m g−1)
a
2
V
t
b
(cm g−1)
3
D
(nm)
Au content (wt%)
e
SAu (nm)
f
TOF (s−1)
Catalyst
S
p
c
Au/AC
Au/OMC-SBA
Au/OMC-MSU-2
Au/OMC-MSU-10
Au/OMC-MSU-15
Au/OMC-MSU-20
Au/OMC-MSU-25
1115
1024
749
597
910
630
1170
0.63
0.92
0.93
0.84
1.35
0.97
2.23
2.3
3.2
4.2
5.4
5.9
6.9
7.6
0.54
0.60
0.62
0.65
0.69
0.70
0.71
3.7
4.1
3.0
3.3
4.0
3.6
3.4
0.343
1.685
2.369
4.308
1.765
1.635
1.513
d
d
d
d
d
d
a
b
BET specific surface areas determined from the linear part of the BET equation (P/P
0
= 0.05–0.25). Total pore volumes obtained at P/P
0
= 0.99.
c
d
e
f
Average pore size. Pore size determined from the desorption branch using the BJH method. Obtained by ICP-OES analysis. Mean Au particle
size calculated from HRTEM images.
nanoparticles, imaged as black dots, are presented as pseudo-
spherical particles and incorporated uniformly into ordered
mesopores of the carbon supports for Au/OMC catalysts. The
mean Au particle sizes were also calculated from HRTEM
images. It was found that the Au particle sizes are independent
of the pore size of the carbon supports (Table 1). The Au par-
ticle sizes of all the Au/OMC catalysts were dispersed in a
narrow range of 3.0–4.1 nm. Considering that HRTEM instru-
mental and computational errors existed, such differences in
the Au particle sizes of the catalysts investigated can be neg-
lected. It was reported that small Au nanoparticles (e.g. less
than
oxidation.
5
nm) have
a
high catalytic activity in glucose
8
–10
In this regard, all the catalysts as-prepared in
this work are expected to be highly active. However, for the
Au/AC catalyst, it should be noted that the average pore size
of AC is 2.3 nm, which is obviously smaller than the mean size
of Au nanoparticles supported on AC. So, it is reasonable to
deduce that Au nanoparticles are dispersed on the outer
surface of AC support. By contrast, for Au/OMC-MSU catalysts,
the pore sizes of the carbon supports are larger than the sizes
of Au nanoparticles supported on them. As a result, Au nano-
particles can be dispersed and confined in the mesopore chan-
nels of the OMC supports.
In addition to Au loadings and dispersion, the valance of
Au nanoparticles was determined by Au4f XPS (Fig. S3†). It is
0
reported that the typical binding energies of Au4f7/2 for Au
2
8
2 3
and Au O are located at 83.8 and 86.3 eV, respectively. In
our Au4f XPS measurements, it was found that Au4f binding
energies in Au/OMC catalysts are 84.3 eV, located between 83.8
0
Fig. 2 HRTEM images of the catalysts. (a) Au/AC; (b) Au/OMC-SBA; (c) Au/OMC-
(
Au ) and 86.3 eV (Au
2
O
3
), suggesting the Au valence is
MSU-2; (d) Au/OMC-MSU-10; (e) Au/OMC-MSU-15 and (f) Au/OMC-MSU-25.
δ+
indexed to Au , namely Au with a fractional positive charge.
3.3 Analysis of surface functionality
carbon supports may show systematically increasing surface
hydrophilicity with the increase of the boron doping. Because Carbon surface chemistry plays an important role in both cata-
2
9
the HAuCl
4
aqueous solution as an impregnation precursor is lyst dispersion and catalytic activity.
The O1s spectra
hydrophilic, the carbon support with a more hydrophilic (Fig. 3A) of Au/OMCs can be resolved into four peaks
3
0,31
carbon surface has advantages in the infiltration of the Au pre- (Fig. 3A).
The peak at a binding energy of 531–531.9 eV
cursor, thus resulting in higher gold loading.
corresponds to the carbonyl oxygen of quinines; the peak at a
In order to clearly image the morphological dispersion of binding energy of 532.3–532.8 eV corresponds to CvO in
Au nanoparticles on carbon supports. HRTEM observations esters, anhydrides and phenolic groups; the peak at a binding
were conducted. It can be seen from Fig. 2 that Au energy of 533.1–533.8 eV corresponds to C–O in esters and
1
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peak below 300 °C is ascribed to the desorption of surface
−
−
active oxygen species, such as O and O , from the surface of
2
3
5
the catalysts, and the desorption peak above 350 °C is
attributable to the desorption of oxygen from the oxygen-con-
taining functional groups of carbon. It can be seen that a des-
orption peak is centered at 257 °C for Au/OMC-MSU-10, which
is obviously lower than the peaks of other catalysts. It is
obvious that the surface-active oxygen species on the Au/OMC-
MSU-10 desorb more easily than the other catalysts. Generally
speaking, the easier the surface active oxygen species desorb,
the higher the catalytic activity for oxidation reactions.
3.4 Catalytic activity
The oxidation of glucose was studied by bubbling oxygen
through a 10 wt% aqueous solution of glucose at a flow rate of
00 ml min− at 40 °C and pH 9 in the presence of meso-
1
1
porous carbon or Au catalysts under ambient pressure. All the
ordered mesoporous carbons without gold loading did not
exhibit catalytic activity under the same reaction conditions.
The activities of glucose oxidation over the supported Au cata-
lysts did not decrease with an extension of reaction time, indi-
-TPD cating the good stabilities of these catalysts (Fig. S6†). The
2
Fig. 3 (A) XPS O1s spectra, (B) IR spectra, (C) XPS C1s spectra and (D) O
profiles of the Au/OMC catalysts. (a) Au/OMC-SBA; (b) Au/OMC-MSU-10; selectivities to gluconic acid were very high even above 97%.
(c) Au/OMC-MSU-15.
Thus, the Au nanoparticles were thought to be the active sites
for the glucose oxidation. The highest catalytic activity was
observed for Au/OMC-MSU-10 with 5.4 nm pore diameter,
anhydrides, and the peak at a binding energy of 534.3–535.4 which showed a TOF of 4.308 molglucose molAu−1
s
−1
at 40 °C
eV corresponds to COOH. For Au/OMC-MSU-10, one main and pH 9. It is reported that the TOF over Au/Al O catalysts
2
3
peak at 532.6 eV (relative atomic percentage: 26.6%) and prepared by incipient wetness method is up to 1.97 mol
glucose
another main peak emerged at 533.5 eV (relative atomic per- molAu−
1
s
−1
at 40 °C and pH 9, and 1% Au/C and 0.5%
36
centage: 27.7%) are attributed to CvO and C–O of esters and Au/TiO catalysts showed a TOF of 0.023 and 0.978 mol
2
glucose
anhydrides, respectively. Compared to Au/OMC-MSU-10, we molAu−1
s
−1
at 40 °C and pH 11, respectively. The Au nano-
14
observe a decrease of the C–O of esters and anhydrides for particles deposited on nanoporous carbon with 2 nm meso-
Au/OMC-SBA and a decrease of the CvO of esters and an- pores and hollow cores only showed a TOF of 2.4 mol
glucose
hydrides for Au/OMC-MSU-15. The presence of each functional molAu−
1
s . Thus, the activity of Au/OMC-MSU-10 is higher
−1 8
group was confirmed by FT-IR measurements (shown in than those reported by other researchers, indicating the high
Fig. 3B and Fig. S4†). For example, a C–O stretching vibration suitability of OMCs as supports of Au nanoparticles, for
−
1
emerges in the 1000–1260 cm range, and a CvO stretching glucose oxidation to gluconic acid.
vibration in carboxylic acids or isolated carbonyl groups
The hypotheses for the active states of Au catalysts proposed
Fig. 3C shows the so far can be classified as follows: (1) specific size of the Au
evolution of the C1s spectra of Au/OMC catalysts. The C1s nanoparticles, (2) junction perimeter between gold and the
−
1
32,33
emerges in the 1550–1650 cm range.
2
37
8
peak centered at 284.5 eV is attributed to sp -hybridized supports and (3) the nature of support. Haruta et al.,
graphitic carbon. As for Au/OMC-MSU-10, the intensity of this reported that the metal–oxide-supported Au catalysts showed
3
4
peak is obviously larger than those of the other catalysts. It an increase in TOF with a decrease in Au particle size that
could be concluded that the sp -hybridized carbons sur- appears to be the most dominating parameter in glucose oxi-
2
3
8
rounded with two-oxygen atoms of esters or three-oxygen dation. However, Comotti et al., reported that the catalytic
atoms of anhydrides exist in Au/OMC-MSU-10. There are satu- activity of Au nanoparticles for glucose oxidation was inversely
rated bond oxygen atoms (CvO) present in the eaters and proportional to the Au nanoparticle size in the range of
anhydrides. Because there is a lone electron pair existing in 3–6 nm, but the catalytic activity per gold atom is independent
sp -hybridized carbon, the O species were formed by electron of the Au nanoparticle size. In our work, the TOF of Au/OMC
transfer from carbon to the unsaturated oxygen atoms of catalysts seems to be independent of the mean Au nano-
esters, and the O2− species were formed by electron transfer particle size (shown in Table 1). The TOF of Au/OMC-MSU-10
from carbon to the double unsaturated oxygen atoms of is higher than the others, and its mean Au size is smaller than
2
−
anhydrides.
the others, except Au/OMC-MSU-2. The TOF of Au/OMC-
The desorption of surface oxygen, such as O− and O − MSU-15 is higher than those of Au/AC and Au/OMC-MSU-25
2
2
species, was investigated by O -TPD (Fig. 3D and Fig. S5†). The catalysts, but the mean Au nanoparticle size of the former is
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between glucose molecules and Au nanoparticles. Besides,
abundant O− and O2− species were present on the Au/OMC-
MSU-10 surface and desorbed more easily for glucose oxi-
dation, which also contributed to the higher activity of this
catalyst.
4
Conclusions
In summary, we successfully synthesized a series of ordered
mesoporous carbons and deposited Au nanoparticles on the
carbon supports. Au nanoparticles with fractional positive
charge (Au ) were incorporated uniformly in the ordered
mesopore channels of the carbon supports. Au/OMC-MSU-10
Fig. 4 (A) The effect of mesopore size on the catalytic activity of the Au/OMC
δ+
catalysts; reaction conditions: 10 wt% glucose aqueous solution (125 ml), O
2
−
1
1
00 ml min , 40 °C, 6 h; the pH of the solution was kept at 9 by titrating with
2
.5 M NaOH. (B) proposed model for the active site of Au/OMC catalysts for the exhibited a very high catalytic activity with a TOF of 4.308
molglucose molAu−
1
s
−1
at 40 °C and pH 9. The high catalytic
activity of this catalyst can be attributed to two reasons: first,
.4 nm mesopore channels incorporated with 3.3 nm Au nano-
selective oxidation of glucose (light-gray ball: carbon atom; white ball: hydrogen
atom; red ball: oxygen atom; dark-gray ball: gold nanoparticles).
5
particles can facilitate the contact between glucose molecules
and Au nanoparticles. Second, the abundant active oxygen
species present on the catalyst surface promote the catalytic
activity for glucose oxidation.
also bigger. Therefore, the catalytic activity observed for
glucose oxidation over Au catalysts should be influenced more
significantly by the nature and structure of the carbon
support. As for all the catalysts investigated in this work, it is
found that their catalytic activity is closely related to the pore
size of the catalyst (Fig. 4A). With the rise in pore size of the
catalyst, the TOF value first increases to the upmost value of
Acknowledgements
−
1
−1
4
.308 molglucose molAu
s
, then decreases. Compared to the
This work is financially supported by the National Natural
Science Funds (21107124), the Dean’s Award Startup Funds of
the Chinese Academy of Sciences, the National Basic Research
Program of China (2010CB732300) and the National High
Technology Research and Development Program of China
Au/OMC catalysts, Au/AC shows a much lower TOF value of
−
1 −1
0
.343 molglucose molAu
s . This indicates that mesopore con-
finement could be an advantageous factor for the catalytic
activity of the catalysts. It is demonstrated that either too large
or too small a mesopore size deteriorates the catalytic activity
of the supported Au nanoparticles. As for the Au/OMC-MSU-10
catalyst, Au nanoparticles with a mean size of 3.3 nm can be
well-confined in the mesopore channels with 5.4 nm diameter.
In this case, the mesopores of the support were partially occu-
pied by Au nanoparticles but not blocked. It is worthwhile
noting that the molecular sizes of glucose and gluconic acid
are about 0.84 × 0.43 × 0.29 nm and 0.84 × 0.44 × 0.29 nm,
respectively, calculated by Gaussian 03. In such a mesoporous
carbon supported Au catalyst, therefore, the reactant molecules
can diffuse into the mesopores of this catalyst and have
sufficient contact with the supported Au nanoparticles, which
may be one of the dominating factors accounting for the fast
reaction rate on this catalyst.
(
2012AA063101).
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