Insights into the stability of gold nanoparticles supported on metal oxides for the base-free oxidation of glucose to gluconic acid

Article abstract of DOI:10.1039/c3gc41362d

Gold (Au) catalysts have been rarely investigated for the oxidation of glucose in the absence of a base. These conditions are critical, however, to enable the sequential one-pot combination of cellulose hydrolysis and glucose oxidation. Here we evaluate the catalytic performance and stability of Au nanoparticles supported on metal oxides for the oxidation of glucose to gluconic acid under unadjusted pH and acidic conditions. The study provides insights into the deactivation of the catalysts caused by leaching and hydrothermal sintering of Au nanoparticles, as well as by adsorption of reactive species. We found that lowering the surface density of Au on metal oxides decreases the sintering rate of the Au nanoparticles and hence enhances the stability and activity of the catalysts.

Full text of DOI:10.1039/c3gc41362d

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Insights into the stability of gold nanoparticles
supported on metal oxides for the base-free oxidation
Cite this: DOI: 10.1039/c3gc41362d
of glucose to gluconic acid†
Yuran Wang, Stijn Van de Vyver, Krishna K. Sharma and Yuriy Román-Leshkov*
Gold (Au) catalysts have been rarely investigated for the oxidation of glucose in the absence of a base.
These conditions are critical, however, to enable the sequential one-pot combination of cellulose hydro-
lysis and glucose oxidation. Here we evaluate the catalytic performance and stability of Au nanoparticles
supported on metal oxides for the oxidation of glucose to gluconic acid under unadjusted pH and acidic
conditions. The study provides insights into the deactivation of the catalysts caused by leaching and
hydrothermal sintering of Au nanoparticles, as well as by adsorption of reactive species. We found that
lowering the surface density of Au on metal oxides decreases the sintering rate of the Au nanoparticles
and hence enhances the stability and activity of the catalysts.
Received 10th July 2013,
Accepted 12th August 2013
DOI: 10.1039/c3gc41362d
www.rsc.org/greenchem
1
2
conditions. It has been demonstrated that such conditions
can enable the one-pot combination of acid-catalyzed hydro-
1
. Introduction
2
2–24
Aldonic and aldaric acids have emerged as highly attractive lysis reactions with Au-catalyzed glucose oxidation.
This
chemical intermediates that can be used in a variety of kind of catalytic cascade reaction is primarily important for
1
–3
7,25–27
applications. One such compound, D-gluconic acid (100 000 the depolymerization and valorization of cellulose,
tons per year), is widely used in the food, pharmaceutical, although it might also be relevant to the catalytic processing
4
–6
paper and concrete industries.
are currently produced by the enzymatic oxidation of D-glucose
by Aspergillus niger and Gluconobacter suboxydans. One of the cerned the oxidation of glucose in the presence of a base.
impediments to the large-scale application of the fermentation Initially, Biella et al. studied the recycling of Au on activated
processes is that they necessitate the neutralisation of the acid carbon (Au/AC) at pH values from 7.0 to 9.5. They found a
Gluconic acid and its salts of other di-, oligo-, and polysaccharides.
Most studies on the stability of Au catalysts so far have con-
1
1
2
7
in order to avoid deactivation of the enzymes. This means that decrease in activity of more than 50% after four consecutive
there is a need to develop heterogeneous catalysts that can cata- runs, mainly due to leaching and sintering of the Au particles.
lyze the oxidation of D-glucose under base-free conditions.
In contrast, Au on metal oxide catalysts are typically more
The benefits of using gold (Au) catalysts for the aerobic oxi- stable under basic conditions. For example, Au/Al O and Au/
2
3
dation of carbohydrates have been well researched and docu- TiO catalysts could be successfully reused for the oxidation of
2
2
,8
6,10,28,29
mented in recent years. Key features include exceptionally glucose and disaccharides at pH 9.
Thielecke et al.
high catalytic activities and selectivities for the oxidation of even demonstrated the long-term stability of Au/Al O in a con-
2
3
9
,10
4,6,10–21
4,13
pentoses
tures (40–65 °C). Also, it has been shown that Au can catalyze about the stability of such catalysts under acidic conditions.
the oxidation of glucose without pH control, thus allowing the In this contribution, we report on the use of Au supported
production of gluconic acid, not its salt, under slightly acidic on nano- or microsized metal oxides. Au on nanosized ceria
Au/nCeO ), for example, has recently been shown to be a rela-
and hexoses
even at relatively low tempera- tinuous-flow system.
However, comparatively little is known
(
2
tively stable catalyst for the oxidative esterification of 5-hydro-
xymethyl-2-furfural into 2,5-dimethylfuroate under base-free
Department of Chemical Engineering, Massachusetts Institute of Technology,
7 Massachusetts Avenue, Cambridge, MA 02139, USA. E-mail: yroman@mit.edu;
Tel: +617-253-7090
Electronic supplementary information (ESI) available: Additional experimental
information, N adsorption/desorption isotherms of μCeO and nCeO , TGA
curve of the spent 0.02 wt% Au/μCeO catalyst, Au particle size distribution histo-
grams of the catalysts discussed in the Section 3.5 of the manuscript. See DOI:
0.1039/c3gc41362d
3
0
conditions.
The choice of support influences both the
7
dispersion and the electronic state of Au nanoparticles
†
31
(
Au NPs),
leading us to investigate a series of metal
2
2
2
oxides including ceria, titania and zirconia. The immobili-
zation of Au NPs on such supports can be achieved by the de-
2
32
1
position–precipitation method. Avoiding agglomeration and
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sintering of the Au NPs during thermal activation is a great operated at 120 kV or a JEOL TEM 2011 microscope operated
challenge since the catalytic activity markedly decreases for Au at 200 kV. Samples were deposited on the TEM grids after
3
3
particles larger than 10 nm. It is argued that Au species are ultrasonic dispersion in ethanol. The Au loading of the metal
comparatively more stable when formed by the deposition– oxide catalysts was determined by inductively coupled plasma
precipitation method than when they are prepared by e.g. anionic atomic emission spectrometry (ICP-AES) using an ICP analyzer
adsorption, leading to significantly smaller, and hence more (HORIBA JOBIN YVON). The Au loading of the Au/AC catalyst
3
2
active, Au NPs. Another factor that determines the size of Au was analyzed by X-ray fluorescence using a Bruker Tracer
1
1
particles is the metal–support interaction. It is understand- III-SD Handheld X-Ray Fluorescence spectrometer. Nitrogen
able that a higher Fermi level of the support material can lead adsorption–desorption isotherms were recorded using a Quanta-
to a stronger electronic interaction with the Au NPs and in chrome Autosorb iQ at liquid nitrogen temperature
3
1
turn a reduced tendency for agglomeration.
(−196 °C). Prior to the physisorption measurements, the
The aim of this study was to evaluate the catalytic perform- samples were degassed for 2 h under vacuum at 400 °C. X-ray
ance and stability of Au NPs supported on various metal photoelectron spectroscopy (XPS) was performed using a PHI
oxides for the base-free oxidation of glucose to gluconic acid. Versaprobe II equipped with a monochromatic aluminium
The key aspect that distinguishes our approach from most of anode X-ray source and a dual-beam charge neutralization
the previous studies is that the pH of the reaction solution was system with an electron neutralizer bias of 1.2 eV and an Ar
either unadjusted or lowered by the addition of H SO .
ion beam energy of 10 eV. The C1s peak was used as a refer-
ence and shifted to 284.8 eV for charge correction. Themo-
gravimetric analysis (TGA) was performed by heating the
2
4
catalyst under a flow of 10 mL min− N and 90 mL min air,
1
−1
2. Experimental section
2
using a Q500 thermal analysis system (TA Instruments). About
2
.1. Preparation of Au catalysts
Nanosized ceria and zirconia, hereafter referred to as nCeO
and nZrO , were synthesized according to a previously reported
−
1
3
6
0–50 mg of the sample was heated at 2 °C min up to
00 °C.
2
2
0
The dispersion of Au NPs was calculated as the ratio of the
3
method. Briefly, an aqueous solution of Ce(NO ) ·6H O or
3
3
2
number of external Au atoms (Nsurface Au) to the total number
of Au atoms (Ntotal Au). The values of Nsurface Au and Ntotal Au
were estimated based on the assumption that the Au NPs can
ZrO(NO
NH OH in deionized water (0.8 M, 1.1 L). After stirring for
0 min, the solution was aged at 100 °C for 24 h in a polyethy-
3 2 2
) ·xH O (0.8 M, 375 mL) was added to a solution of
4
3
4
3
be modelled as an fcc crystal lattice:
lene vessel. The mixture was cooled down, filtered and washed
with an excess amount of deionized water. The resulting particles
2
2
Nsurface Au ¼ 10m ꢀ 20m þ 12
ð1Þ
were dried under vacuum and calcined under an air flow of
3
2
ð10m ꢀ 15m þ 11m ꢀ 3Þ
1
1
00 mL min− at 400 °C for 4 h. The other support materials were
Ntotal Au ¼
ð2Þ
ð3Þ
3
purchased from Sigma Aldrich and used as received (see ESI†).
Au was deposited on the metal oxides by the deposition–
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
3
Dm ¼ 1:105 ꢁ Datom ꢁ Ntotal Au
3
0
4 2
precipitation method. A solution of HAuCl ·3H O (175 mg)
with m equal to the number of shells, D
determined mean diameter of the Au NPs, and Datom equal to
the atomic diameter of Au (0.288 nm).
m
equal to the TEM-
in deionized water (80 mL) was brought to pH 10 by adding a
NaOH solution (0.2 M). This solution was mixed with a sus-
pension containing the metal oxide (2 g) in deionized water
(
25 mL). After stirring for 18 h, the suspension was filtered
2
.3. Oxidation reactions and product analysis
and washed with an excess amount of deionized water until no
more chlorine could be detected by the AgCl test. The sup-
ported Au catalyst was dried overnight at 80 °C and then
The catalytic reactions were carried out with solutions of
glucose in deionized water (167 mmol L− ) at an initial pH of
4. In some experiments the initial pH was adjusted to 1.6 by
the addition of small amounts of H SO . The reaction mixture
1
−1
reduced for 4 h at 225 °C in 5% H in N (100 mL min ).
2
2
The Au/AC catalyst was prepared according to the procedure
2
4
1
2
reported by Biella et al. A solution of HAuCl
1
(
4
·3H
00 μg mL− ) was mixed with 2.5 g of a 2 wt% solution of poly
vinyl alcohol) (PVA, M ∼ 10 000). To this solution, 0.1 M
solution (20 mL) was added dropwise, leading to the
2
O (1 L,
2
was pressurized with O to 2.3 bar and stirred at 65 °C in a
1
pressure reaction vessel (Andrews Glass Co.). The catalysts
were recovered at the end of each run by centrifugation and
washed with deionized water for at least 5 times until the
supernatant reached the pH of deionized water. An alkaline
wash was done with a 0.1 M NaOH solution. In both cases, the
washed catalysts were dried at 80 °C prior to reuse. Regener-
W
NaBH
4
formation of metallic Au NPs. The PVA stabilized Au NPs were
then immobilized on activated carbon (Darco®, 100 mesh) by
adding 2 g of the support. The as-synthesized Au/AC catalyst
was filtered after 2 h and washed with deionized water.
ation of the Au/μCeO
2
catalyst was performed by calcination
for 4 h at the temperatures specified in Section 3.5 with a flow
2
.2. Characterization of the catalysts
−1
−1
rate of 10 mL min
2
N and 90 mL min air.
Transmission electron microscopy (TEM) measurements were
Liquid samples were syringe filtered (0.2 μm PTFE mem-
performed using either
a
JEOL TEM-200CX microscope brane) and analyzed using an Agilent 1260 HPLC. The
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carbohydrates were separated on a Bio-Rad Aminex HPX-87C CeO
column at 80 °C and detected with an evaporative light scatter- and to become reduced.
2
—the latter being known for its capability to store oxygen
3
7–40
There is, however, no correlation
ing detector, using deionized water as the mobile phase at a in Table 1 between the size of the Au NPs and the catalytic
−
1
flow rate of 0.6 mL min . In parallel, organic acids were sepa- activity.
rated on a Bio-Rad Aminex HPX-87H column at 30 °C and The formation of gluconic acid induced a significant pH
detected with a UV-Vis detector at 210 nm, using 8 mM H SO decrease of the reaction solution. For the experiments under
in deionized water as the mobile phase at a flow rate of 0.6 mL unadjusted pH, typical pH values measured at room tempera-
2
4
−
1
min . Dried aliquots of the samples were trimethylsilylated ture ranged from 4.0 to 2.5 before and after the reaction,
3
5,36
according to a previously reported protocol
by GC–MS. The conversion of glucose and the selectivity to under acidic conditions, parallel experiments were performed
gluconic acid were calculated on a molar basis. at an initial pH of 1.6 by the addition of small amounts of
SO (the last two columns in Table 1). Lowering the pH
caused a decrease of the reaction rate, which is in agreement
and analyzed respectively. To evaluate the performance of the catalysts
H
2
4
1
2
with the study by Biella et al. The inhibiting effect of the
acidic conditions is reflected in lower glucose conversions;
however, the impact on the catalytic activity is relatively small
for Au on nanosized metal oxides. The selectivity to gluconic
acid remained ≥88% regardless of the pH conditions used.
Entries 1–2 and 5–6 in Table 1 show the effect of Au loading
3
. Results and discussion
3
.1. Catalyst screening
A series of Au supported on metal oxides was prepared, charac-
terized and tested for the base-free oxidation of glucose to glu-
conic acid. Table 1 shows the Au loading, the mean particle
size and the dispersion of the Au NPs, as well as typical cataly-
tic results. Prefixes n and μ refer to nano- and microsized
metal oxide supports with particle sizes smaller than 30 nm
and larger than 100 nm, respectively. All of the Au NPs sup-
ported on metal oxides were highly active for glucose oxidation
under base-free conditions, with conversions of glucose
ranging between 24% and 91% after 2 h of reaction. For com-
parison, a benchmark Au/AC catalyst with 0.6 wt% Au loading
showed only 7% conversion.
on the catalytic activity of Au/nCeO2 and Au/μCeO . When
2
decreasing the Au loading of these catalysts, the conversion of
glucose remained approximately constant for Au/nCeO ;
2
however, for Au/μCeO under acidic conditions, the conversion
2
increased from 37% to 59% upon decreasing the loading from
1.1% to 0.6%. Hence, lowering the Au loading seems to
increase the activity of Au/μCeO under acidic conditions. This
2
result prompted us to study the stability of both catalysts
under the given conditions.
As for the selectivity to gluconic acid, the values in Table 1
agree well with previously reported data for Au-based catalysts
under acidic conditions. A GC–MS analysis after trimethyl- Recycling and reuse experiments provided further information
3
.2. Comparison of the stability of Au/μCeO and Au/nCeO
2
2
1
2
silylation of the product mixture showed the presence of glycolic on the stability of the Au catalysts. Fig. 1 compares the evolu-
acid, C
most important byproducts of this reaction.
Ishida et al. previously reported that the size of Au particles, washed with deionized water and dried after each run.
and hence their catalytic activity, depend strongly on the The experiments of Fig. 1(a) were carried out without pH
preparation method. The deposition–precipitation method adjustment of the reaction solution. It can be seen that by the
used in this study gave Au NPs with mean particle sizes fourth cycle, the conversion of glucose over Au/μCeO
ranging from 1.7 nm to 3.0 nm (fourth column in Table 1). decreased from 87% to 33%. Au/nCeO was less prone to de-
5
2 2
aldonic acids and the δ-lactone of gluconic acid as the tion of the activity of 0.6 wt% Au/μCeO and 0.5 wt% Au/nCeO
over consecutive reaction cycles. The catalysts were filtered,
11
2
2
The most promising results overall were obtained for TiO and activation, as in this case the conversion decreased from 87%
2
a
Table 1 Base-free catalytic oxidation of glucose with Au NPs supported on different metal oxides
Unadjusted pH
Conv. (%)
Initial pH of 1.6
Conv. (%)
b
c
d
Entry
Catalyst support
Au loading (wt%)
D (nm)
Dispersion
Select. (%)
Select. (%)
1
2
3
4
5
6
7
8
nCeO
nCeO
2
0.5
1.4
1.6
1.3
0.6
1.1
0.8
1.2
1.8
2.5
1.7
3.0
2.0
2.9
2.1
2.0
0.60
0.47
0.62
0.40
0.55
0.41
0.53
0.55
74
75
89
91
76
77
65
89
95
99
99
99
96
98
98
98
67
75
86
81
59
37
24
54
96
96
97
98
90
90
88
>99
2
nZrO
2
nTiO
μCeO
μCeO
2
2
2
μZrO
μTiO
2
2
a
Reaction conditions: glucose 167 mmol L−1, 12 mL, pO
b
2
= 2.3 bar, 65 °C, glucose/Au = 140, unadjusted pH, 2 h. Au loading determined by
d
c
ICP-AES. Mean Au particle diameter determined by TEM analysis and based on a count of at least 100 Au NPs. Dispersion of the Au NPs
calculated assuming a predominantly cuboctahedral structure of the Au NPs (see the Experimental section).
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3.3. Effect of the Au particle surface density on the catalyst
stability
Au leaching cannot fully account for the observed decrease in
activity, and hence we investigated the sensitivity of the Au
NPs to sintering in order to gain further insight into the de-
activation mechanisms. Fig. 2 shows TEM images of the 0.6 wt%
Au/μCeO and 0.5 wt% Au/nCeO catalysts prior to and after
2
2
reaction, while Fig. 3 displays the evolution of the Au particle
size distribution during several reaction cycles. The TEM-
based investigation reveals that the textural properties of the
metal oxide support, such as its size and surface area, are key
4
1
factors in determining the size and stability of the Au NPs.
The histograms in Fig. 3 indicate relatively narrow Au par-
ticle size distributions of the as-synthesized catalysts, irrespec-
tive of whether μCeO2 or nCeO2 is used as the support.
However, there is a vast difference in the evolution of the par-
ticle size distributions between the reused 0.6 wt% Au/μCeO
2
and 0.5 wt% Au/nCeO samples. The former is notably more
2
sensitive to sintering, which is consistent with the fact that
this catalyst showed higher deactivation rates (Fig. 1). Acidic
conditions accelerate Au sintering for both Au/nCeO and Au/
2
μCeO ; however, the shift towards larger Au NP sizes is more
2
pronounced for Au/μCeO
2 2
than for Au/nCeO . Indeed, the
mean Au particle size for Au/μCeO increased from 2.0 nm to
2
7
.1 nm after the last reaction cycle (Fig. 3c), whereas the mean
increased only from 1.3 nm to
.9 nm (Fig. 3d). We hypothesized that the relatively stable
2 2
Fig. 1 Conversion of glucose over Au/μCeO and Au/nCeO measured after
4
h of reaction (a) under unadjusted pH and (b) at an initial pH of 1.6. The cata- particle size for Au/nCeO
2
lysts were washed with an NaOH solution before the last reaction cycle. Reaction
1
conditions: glucose 167 mmol L− , 12 mL, pO
1
= 2.3 bar, 65 °C, glucose/Au =
2
nature of Au/nCeO is due to the low surface density of the Au
NPs. N
Teller (BET) surface area of nCeO2 (100 m
larger than the surface area of μCeO (4 m g ) (Fig. S1 in the
2
1
40.
2
physisorption showed that the Brunauer–Emmett–
2
−1
g
) is 25 times
2
−1
2
to 55%. Our initial strategy to recover the catalytic activity was
3
4
based on a study by Abad et al., who showed that a NaOH
wash can remove strongly adsorbed carboxylic acid species.
This method was applied to the catalysts recovered after
the fourth reaction cycle. In our case, however, the NaOH
wash did not result in significant changes of the catalytic
activity. Instead, we suspected that one of the major causes of
the deactivation of Au/μCeO was Au leaching. ICP-AES analy-
2
sis showed that this catalyst had lost about 55% of its original
Au loading during the oxidation reaction. Notably, the Au/
2
nCeO catalyst had lost less than 2% of its Au loading after five
cycles.
Consistent with the approach in Table 1, we also investi-
gated the effect of lowering the pH of the reaction solution.
Fig. 1(b) shows that the catalytic activity of 0.6 wt% Au/μCeO
2
decreased drastically under acidic conditions compared to that
under the unadjusted pH conditions. Specifically, the conver-
sion of glucose over 0.6 wt% Au/μCeO
2
decreased from 69%
(
run 1) to 12% (run 3) on adding H SO to the reaction solu-
2
4
tion. In contrast, only a slight decrease in catalytic activity was
observed for 0.5 wt% Au/nCeO . ICP-AES analysis showed no
significant Au leaching for Au/nCeO , while Au/μCeO lost 74%
2
2
2
of its Au loading after four reaction cycles. As such, the distinct
stability properties of the two catalysts required additional
studies.
Fig. 2 Representative TEM images of the 0.6 wt% Au/μCeO
2
(a, b) and 0.5 wt%
Au/nCeO
2
catalysts (c, d), before (left) and after use in glucose oxidation
(right).
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mediated Ostwald ripening via Au dissolution and redeposi-
tion. Ostwald ripening could explain the observed correlation
between the growth of the Au NPs and the extent of Au leach-
2 2
ing of Au/μCeO and Au/nCeO . The Au NPs are more suscep-
tible to sintering during reactions at a lower pH, which, again,
is in line with the general expectations given that the solubility
of Au is higher under more acidic conditions.
The size of the CeO support has important implications on
2
the stability of the Au NPs. Given its higher density of defect
2
sites (i.e., mainly oxygen vacancies), nCeO is known to have
3
0
more anchoring sites than μCeO2. One prevalent view is that
these anchoring sites can further stabilize the Au NPs against
3
2,40,44
particle growth.
In this study, a higher number of defect
sites on the surface of nCeO2 probably helps to stabilize a
higher number of Au NPs per unit surface area. In the case of
Au/μCeO , we anticipated that reducing the surface density of
2
Au NPs could improve their stability because of (i) a more pre-
ferred nucleation and growth of the Au NPs on the relatively
scarce defect sites of the μCeO support during the depo-
2
sition–precipitation process, and (ii) an increase in the inter-
particle spacing (vide supra). To validate our hypotheses, we
synthesized a Au/μCeO
.02 wt%. Since the dispersion for the 0.02 wt% Au/μCeO2
0.7) and that for the 0.5 wt% Au/μCeO (0.6) were virtually
2
catalyst with a Au loading as low as
0
(
2
identical, the former catalyst evidently had a significantly
lower surface density of Au NPs. The performance of these cata-
lysts and 0.4 wt% Au/nCeO was investigated for a longer reac-
2
tion time (72 h instead of 4–16 h) and a higher glucose/Au
ratio (9000 instead of 140). Fig. 4 shows the turnover number
(
TON, defined as moles of glucose converted per mole of
surface Au) as a function of reaction time. The results are in
good agreement with the observation made in Fig. 1 that
0.4 wt% Au/nCeO
2
is found to be more stable than 0.5 wt%
Au/μCeO . Although Au NPs supported on microsized metal
2
oxides were initially as active as Au NPs supported on nano-
sized supports, their activity decreased due to sintering. Con-
sistent with our previous findings, the increased maximum
2
Fig. 3 Au particle size distribution histograms of 0.6 wt% Au/μCeO (a, c) and
0
2
.5 wt% Au/nCeO (b, d). Reaction conditions: see Fig. 1.
ESI†). Since both catalysts have a similar Au loading, the
surface density of the Au NPs is about 25 times higher for Au/
μCeO than for Au/nCeO . This seems to suggest that the inter-
2
2
particle distance determines the coalescence rate of the Au
Fig. 4 Turnover number for glucose oxidation with 0.02 wt% Au/μCeO
.5 wt% Au/μCeO and 0.4 wt% Au/nCeO as a function of reaction time. Reac-
tion conditions: glucose 167 mmol L−1, 12 mL, pO
2
,
NPs and hence their tendency to sinter by the particle
0
2
2
4
1–43
migration mechanism.
An alternative mechanism that
2
= 2.3 bar, 65 °C, glucose/
sustains our experimental observations is based on a solution- Au = 9000.
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2
TON of 9090 for 0.02 wt% Au/μCeO shows that the lower
surface density of the Au NPs indeed leads to a significantly
higher activity and catalyst stability. A similar phenomenon
was reported by Abad et al. while studying the influence of Au
loading on the activity of Au/nCeO for the base-free oxidation
2
3
4
of cinnamyl alcohol in toluene. They observed that the turn-
over frequency (TOF, defined as moles of substrate converted
per mole of surface Au per hour) was more than three times
2 2
higher for 0.44 wt% Au/nCeO than for 1.80 wt% Au/nCeO ,
although both catalysts had almost the same particle size
distribution.
3
.4. Effect of the Au particle size on the turnover frequency
An important question concerning the impact of sintering is
4
5
how the TOF relates to the mean diameter of the Au NPs.
2
Fig. 6 XPS Au 4f spectra of (i) as-synthesized 0.02 wt% Au/μCeO , (ii) after
Studies by Ishida et al. have previously demonstrated the size 72 h of reaction with glucose and washing with deionized water, (iii) after
dependence of the TOF of Au supported on metal oxides for regeneration by calcination at 225 °C and (iv) after regeneration by calcination
1
1
at 325 °C. The open circles show raw experimental data. The C1s signal was
shifted to 284.8 eV for charge correction.
glucose oxidation at pH 9. They inferred that the reaction
might occur at the surface of the Au particles rather than at
the interface between the Au NPs and the metal oxide support.
Comparable trends were observed for the catalytic oxidation
reactions performed under our reaction conditions (Fig. 5),
which points to the more general nature of these findings. Au/
predominant oxidation state of the Au NPs on the as-syn-
thesized catalysts. The Au 4f signal shows two characteristic
peaks at 87.5 eV and 83.8 eV, corresponding to Au 4f5/2 and Au
nCeO , however, showed a significant decrease in TOF even for
2
4f7/2 transitions, respectively. After reaction with glucose, both
samples with almost the same mean diameter of Au NPs. This
suggests that the deactivation may be caused by factors other
than leaching or sintering of the Au NPs, such as the adsorp-
tion of reactive species on the Au surface.
peaks shifted to lower binding energies compared to the as-
synthesized sample. The negative shift (−0.5 eV) is tentatively
attributed to the electron transfer from intermediate species to
0
Au on the surface of the Au NPs. This electronic perturbation
can be anticipated from previous studies that showed a severe
deactivation of Au NPs by the strong adsorption of ketone
intermediates and condensation products of ketones. The
adsorption of species on the Au catalyst was also confirmed by
TGA. Spent catalyst samples were washed with deionized
water, dried at 80 °C and heated from room temperature to
3
.5. Inhibition of the catalytic activity by adsorption of
reactive species and regeneration by calcination
4
6
Compelling evidence for the adsorption of reactive species
came from TGA and XPS analyses of the Au/μCeO2 catalyst
before and after reaction, as well as after calcination at 225 °C
0
and 325 °C. The XPS spectra in Fig. 6 indicate that Au is the
6
00 °C under air while monitoring the weight losses (Fig. S2 in
the ESI†). The decomposition occurred at temperatures up to
25 °C, which is why we chose to calcine the spent catalysts at
3
this temperature. An additional calcination treatment was per-
formed at 225 °C to diminish the possible effect of sintering.
As shown in the XPS spectra (Fig. 6), the Au 4f5/2 and Au
4f7/2 peaks of the calcined samples gradually shifted back to
the band energies of the as-synthesized catalyst. According to
ICP-AES measurements, the recovered and regenerated cata-
lysts ((ii), (iii) and (iv) in Fig. 6) had almost the same Au
loading as the as-synthesized catalyst (i). Because of the
similar electronic state and loading of Au for the as-
synthesized catalyst and the one regenerated by calcination at
325 °C, it was expected that both catalysts would show a com-
parable catalytic activity. Our experiments, however, still
showed a significant decrease in catalytic performance. Specifi-
cally, the initial TOF of the as-synthesized catalyst was
Fig. 5 TOF per surface Au atom as a function of the mean diameter of the Au
NPs. The TOFs were measured after 20 min of reaction. Numbers 1–3 indicate
data points corresponding to the as-synthesized catalyst, the catalyst recovered
after the first cycle and the catalyst recovered after the last cycle in Fig. 1,
respectively. Reaction conditions: see Fig. 1.
−1
−1
3
100 molglucose molsurface Au
h , whereas the calcined
−
1
sample showed an initial TOF of 760 mol
h
molsurface Au
glucose
−
1
. A reaction promoted by the calcined catalyst gave a conver-
sion of 8% after 12 h, while the as-synthesized catalyst led to a
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conversion of 43% under the same reaction conditions. Note
that the catalyst calcined at 225 °C showed no significant cata-
lytic activity.
3 S. Van de Vyver and Y. Roman-Leshkov, Catal. Sci. Technol.,
2013, 3, 1465–1479.
4 N. Thielecke, M. Aytemir and U. Prüsse, Catal. Today, 2007,
121, 115–120.
Although part of the deactivation of Au/μCeO
2
can be
ascribed to the inhibition by reactive species, the severe loss of
activity after calcination at 325 °C is remarkable. Currently, we
hypothesize that the most likely deactivation pathway is
caused by agglomeration of the Au NPs, as the catalyst calcined
at 325 °C (5.7 ± 5.5 nm) showed significantly larger particle
sizes than the as-synthesized catalyst (1.7 ± 0.6 nm) (Fig. S3 in
the ESI†). Consequently, larger particles may feature drastically
lower amounts of the most active sites on the surface of the Au
NPs. A future challenge to be addressed is to tailor the calcina-
5 N. M. Xavier, A. l. P. Rauter and Y. Queneau, Top. Curr.
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6 C. Baatz and U. Prüße, J. Catal., 2007, 249, 34–40.
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1740–1763.
8 C. D. Pina, E. Falletta and M. Rossi, Chem. Soc. Rev., 2012,
41, 350–369.
9 B. T. Kusema, B. C. Campo, P. Mäki-Arvela, T. Salmi and
D. Y. Murzin, Appl. Catal., A, 2010, 386, 101–108.
tion conditions to remove the competitively adsorbed reactive 10 A. Mirescu and U. Pruesse, Appl. Catal., B, 2007, 70, 644–
species while preserving the original Au particle size
distribution.
652.
11 T. Ishida, N. Kinoshita, H. Okatsu, T. Akita, T. Takei and
M. Haruta, Angew. Chem., Int. Ed., 2008, 47, 9265–9268.
1
1
1
1
2 S. Biella, L. Prati and M. Rossi, J. Catal., 2002, 206, 242–
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3 N. Thielecke, K.-D. Vorlop and U. Prüße, Catal. Today,
4
. Conclusions
This work has shown that Au nanoparticles supported on
metal oxides are active and selective catalysts for the oxidation
of glucose to gluconic acid under base-free conditions. The
stability study offers some clues to how the irreversible de-
activation of the catalysts occurs through leaching and hydro-
thermal sintering of the Au nanoparticles. An easily applicable
approach for improving the catalyst’s stability against sintering
is lowering the Au loading on the metal oxides. Our results
indicate that the surface density of the Au nanoparticles
affects their tendency to agglomerate during the oxidation
reaction. Under the applied conditions, the highest stability
2007, 122, 266–269.
4 T. Ishida, K. Kuroda, N. Kinoshita, W. Minagawa and
M. Haruta, J. Colloid Interface Sci., 2008, 323, 105–111.
5 V. Matveeva, A. Bykov, V. Doluda, M. Sulman, N. Kumar,
S. Dzwigaj, E. Marceau, L. Kustov, O. Tkachenko and
E. Sulman, Top. Catal., 2009, 52, 387–393.
1
1
1
1
2
6 Y. Önal, S. Schimpf and P. Claus, J. Catal., 2004, 223, 122–
1
33.
7 C. Ma, W. Xue, J. Li, W. Xing and Z. Hao, Green Chem.,
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2
8 H. Yin, C. Zhou, C. Xu, P. Liu, X. Xu and Y. Ding, J. Phys.
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9 H. Okatsu, N. Kinoshita, T. Akita, T. Ishida and M. Haruta,
Appl. Catal., B, 2009, 369, 8–14.
2
was found for a 0.02 wt% Au/μCeO catalyst prepared by the
deposition–precipitation method. The reversible deactivation
of this catalyst is ascribed to the adsorption of reactive species,
which could be removed by calcination of the spent catalyst at
0 I. V. Delidovich, B. L. Moroz, O. P. Taran, N. V. Gromov,
P. A. Pyrjaev, I. P. Prosvirin, V. I. Bukhtiyarov and
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1 H. Zhang and N. Toshima, Catal. Sci. Technol., 2013, 3,
3
25 °C. Finally, our findings encourage further efforts to inves-
tigate the particle-size dependence of glucose oxidation under
base-free conditions.
2
2
2
2
2
2
268–278.
2 X. Tan, W. Deng, M. Liu, Q. Zhang and Y. Wang, Chem.
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Acknowledgements
3 D. An, A. Ye, W. Deng, Q. Zhang and Y. Wang, Chem.–Eur.
J., 2012, 18, 2938–2947.
Y. W. and K. K. S. thank DuPont for financial
support. S. V. d. V. thanks the Research Foundation – Flanders
4 J. Zhang, X. Liu, M. N. Hedhili, Y. Zhu and Y. Han, Chem-
CatChem, 2011, 3, 1294–1298.
(FWO), the Belgian American Educational Foundation (BAEF),
the “Plateforme pour l’Éducation et le Talent” and the Fulb-
right-Hays Commission for Educational Exchange between the
United States and Belgium.
5 S. Van de Vyver, J. Geboers, P. A. Jacobs and B. F. Sels,
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6 J. A. Geboers, S. Van de Vyver, R. Ooms, B. Op de Beeck,
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