Platinum/gold alloy nanoparticles-supported hydrotalcite catalyst for selective aerobic oxidation of polyols in base-free aqueous solution at room temperature

Article abstract of DOI:10.1021/cs400458k

As-prepared platinum/gold alloy nanoparticles-supported hydrotalcites (Pt_xAu_y-starch/HTs) by using a soluble starch as a green reducing and a stabilizing agent are found to be truly effective heterogeneous catalysts for the selective aerobic oxidation of glycerol (GLY) and 1,2-propanediol (PG) in base-free aqueous solution using molecular oxygen in atmospheric pressure at room temperature. The Pt_xAu _y-starch/HTs exhibited higher selectivities for oxidation of the primary hydroxyl group in GLY and PG toward glyceric acid (GA) and lactic acid (LA), respectively, with molecular oxygen in aqueous solution than those reactions over monometallic Pt-starch/HT or Au-starch/HT. Pt₆₀Au ₄₀-starch/HT was found to be the most active catalyst for selective aerobic oxidation of polyols. It showed 73% GLY conversion with 57% GA yield and 63% PG conversion with 47% LA yield and retained high selectivity in recycling experiments. XRD patterns of the Pt_xAu_y-starch NPs indicated the d-spacing of Pt atoms was changed by alloying with Au atoms. XPS and XANES analyses suggested that Pt atoms gained more electrons than Au atoms in Pt_xAu_y-starch/HTs as a result of the two types of electron transfers: (1) from the starch ligand to both Au and Pt atoms and (2) from Au to Pt atoms. We concluded that the high activity and selectivity of Pt_xAu_y-starch/HT can be explained in terms of alterations of geometric and electronic states of the catalytically active surface Pt sites by Au atoms and starch ligand.

Full text of DOI:10.1021/cs400458k

Research Article
pubs.acs.org/acscatalysis
Platinum/Gold Alloy Nanoparticles-Supported Hydrotalcite Catalyst
for Selective Aerobic Oxidation of Polyols in Base-Free Aqueous
Solution at Room Temperature
Duangta Tongsakul, Shun Nishimura, and Kohki Ebitani*
School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST) 1-1 Asahidai, Nomi 923-1292, Japan
S
* Supporting Information
ABSTRACT: As-prepared platinum/gold alloy nanoparticles-supported
hydrotalcites (PtxAuy−starch/HTs) by using a soluble starch as a green
reducing and a stabilizing agent are found to be truly effective
heterogeneous catalysts for the selective aerobic oxidation of glycerol
(GLY) and 1,2-propanediol (PG) in base-free aqueous solution using
molecular oxygen in atmospheric pressure at room temperature. The
PtxAuy−starch/HTs exhibited higher selectivities for oxidation of the
primary hydroxyl group in GLY and PG toward glyceric acid (GA) and
lactic acid (LA), respectively, with molecular oxygen in aqueous solution
than those reactions over monometallic Pt−starch/HT or Au−starch/
HT. Pt60Au40−starch/HT was found to be the most active catalyst for
selective aerobic oxidation of polyols. It showed 73% GLY conversion
with 57% GA yield and 63% PG conversion with 47% LA yield and
retained high selectivity in recycling experiments. XRD patterns of the PtxAuy−starch NPs indicated the d-spacing of Pt atoms
was changed by alloying with Au atoms. XPS and XANES analyses suggested that Pt atoms gained more electrons than Au atoms
in PtxAuy−starch/HTs as a result of the two types of electron transfers: (1) from the starch ligand to both Au and Pt atoms and
(2) from Au to Pt atoms. We concluded that the high activity and selectivity of PtxAuy−starch/HT can be explained in terms of
alterations of geometric and electronic states of the catalytically active surface Pt sites by Au atoms and starch ligand.
KEYWORDS: base-free polyol oxidation, heterogeneous catalyst, hydrotalcite, platinum/gold alloy, electronic effect, geometric effect,
starch ligand
reported during the past decade.14−19 Many publications have
detailed that the supported bimetallic NPs also possessed a
1. INTRODUCTION
Biomass is biological material derived from plants, and the
catalytic activity superior to the monometallic ones for
transformation of biomass-derived compounds into energy
oxidation of polyols in the presence of base (Supporting
sources and valuable chemicals will serve as an alternative
Information Tables S1 and S2).11,14,20−24 For example,
resource instead of fossil fuel.1−3 Biodiesel fuel (BDF) is the
one of biomass-derived energy produced by transesterification
Dimitratose et al. found that alloyed AuPd/TiO2 improved
activity and selectivity for PG oxidation to LA in the presence
of biomass (triglycerides) with alcohols, and a huge amount of
of NaOH under pressurized oxygen (10 atm) at 333 K.14 Brett
glycerol (GLY) is obtained as a byproduct during the
production of BDF.4−6 GLY-derived products can serve various
et al. observed that the addition of Pd to Au atoms significantly
enhanced the activity and retained high selectivity to methyl
substrates for the manufacture of high-end chemicals. For
lactate in the presence of sodium methoxide (NaOMe) at 373
instance, from the controlled partial oxidation of GLY, various
K under pressurized oxygen (3 atm).11
value-added chemicals, such as glyceric acid (GA), dihydrox-
yacetone, and tartronic acid (TA), can be provided.4,5,7
It is well-known that the catalysis for the oxidation reaction
of polyols (e.g., GLY) strongly depends on the basicity of the
Particularly, GA is a very important one, affording medicine
and skin care treatment.1,7−9 In addition, 1,2-propanediol (PG),
reaction medium because the external base can enhance a
the product of GLY hydrogenolysis,10 is a crucial starting
proton abstraction step from the hydroxyl group of GLY.
However, the salt of the products, such as glycerate, was formed
when a homogeneous base was used, and then the obtained
products required an additional neutralization or acidification.
material for the chemical synthesis of lactic acid (LA) by a
partial oxidation.5,6,11 LA is used in the industrial fields of food
and pharmaceutical products.12,13
General reaction pathways for the oxidation of GLY and PG
are shown in Scheme 1. Research on supported monometallic
nanoparticle (NP) catalysts, especially Pt, Au, and Pd catalysts,
for the oxidation of polyols in the presence of base have been
Received: June 17, 2013
Revised: August 13, 2013
© XXXX American Chemical Society
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Scheme 1. General Reaction Pathways for the Oxidation of GLY and PG
In advanced research, the oxidation reaction has been
performed with metal species supported on basic materials,
which permitted a base-free oxidation, producing the carboxylic
acid rather than the salt form. The modern oxidation process
prefers an atmospheric pressure of molecular oxygen (O2) as a
green oxidant because the classical oxidant, such as CrO3 and
KMnO4, had large harmful impacts for the environment.15,25,26
Recently, the catalytic oxidation of GLY over a bimetallic
catalyst in base-free aqueous solution under oxygen flow has
been reported. Hou and co-workers showed that PtCu/C and
PtSb/multiwall carbon nanotubes (MWCNTs) were more
active and selective toward GA than Pt/C and Pt/MWCNT,
respectively.27−29 Tomishige et al. also found that PdAg/C
showed higher activity and selectivity to dihydroxyacetone than
Pd/C for GLY oxidation with molecular oxygen (3 atm) under
neutral conditions at 373 K.30
Very recently, we succeeded in the synthesis of a novel
hydrotalcite-supported Pt−starch NPs (Pt−starch/HT) catalyst
using soluble starch as a green reducing and a stabilizing
agent.31 The Pt−starch/HT became an environmentally
friendly and a highly efficient catalyst which suppressed the
overoxidation of GLY to C1 products in comparison with the
bare Pt/HT catalyst32 for selective oxidation of GLY in base-
free aqueous solution with molecular oxygen under atmos-
pheric pressure. This achievement inspired us to further
challenge for the preparation of an efficient bimetallic NP
catalyst in selective oxidation of polyols.
Herein, we explore the efficient catalytic oxidation of polyols
(GLY and PG) in base-free aqueous solution under ambient
conditions over HT-supported PtxAuy−starch NPs (PtxAuy−
starch/HT) catalysts. The PtxAuy−starch/HTs were prepared
by a sol immobilization method with various Pt/Au molar
ratios (x/y) using soluble starch as a green reducing and
stabilizing agent.31,33 HT is well-known as a reusable basic
layered double hydroxide that is frequently used as a support
for Pd, Ru, and Au species.34,35 Following the results shown
below, we found that the PtxAuy−starch/HT catalysts exhibited
high selectivities for GA and LA formation in aerobic and
aqueous base-free oxidations of GLY and PG under ambient
conditions (i.e., room temperature and an atmospheric pressure
of molecular oxygen). The negative charge on Pt atoms
induced by the electron transfer from both neighbor Au atoms
and starch ligand were detected by XPS and XANES analyses.
We suggested that both geometric and electronic change of the
catalytically active Pt sites by adjacent Au atoms and starch
ligands contributed to improvement of the activity and
selectivity toward the target products.
2. EXPERIMENTAL SECTION
2.1. Chemicals. Hexachloroplatinic acid (H2PtCl6·6H2O,
99.9%), tetrachloroauric acid (HAuCl4·4H2O, 99.9%), 1,2-
propanediol (99%), and soluble starch were purchased from
Wako Pure Chemicals. Sodium hydroxide (NaOH, 97%) was
obtained from Kanto Chemicals. Glycerol (99.9%) was
provided by Nacalai Tesque. Hydrotalcite (HT, Mg/Al = 5)
was obtained from Tomita Pharmaceutical. 2,6-Di-tert-butyl-p-
cresol (BHT) was received from Tokyo Chemical Industry.
2.2. Preparation of Hydrotalcite Supported Platinum/
Gold Catalysts. First, the dispersions of PtxAuy−starch NPs at
various metal ratios were synthesized by a chemical reduction
method using soluble starch as a reducing and a stabilizing
agent. Different ratios of H2PtCl6·6H2O (x mmol) and
HAuCl4·4H2O (y mmol) were mixed (x + y = 0.05 mmol
(const.)) with 5 mL of aqueous starch solution (0.2 g starch
content) and stirred for 90 min, then the pH of the metal
solution was adjusted to neutral with 1 M NaOH. Thereafter,
an additional 5 mL of NaOH (0.05 M) was added before
heating to 373 K. After refluxing for 20 min, 0.5 g of HT was
added to the solution with vigorous stirring, then the mixture
was continuously refluxed for 1 h. The theoretical metal loading
on HT was 0.1 mmol·g−1. Finally, the solution was cooled to
room temperature, filtered, and washed with deionized water.
The solid catalyst was dried overnight at 373 K.
2.3. Catalytic Activity for Aerobic Oxidation of
Polyols. The catalytic activity of PtxAuy−starch/HT was
evaluated for oxidation of polyols (GLY and PG) under a
base-free aqueous solution at room temperature with an
atmospheric pressure of oxygen. All reactions were performed
in a 30 mL Schlenk tube attached to a reflux condenser. The
general reaction procedures were as follows: GLY or PG (0.5
mmol), H2O (2 mL), catalyst (20 mg), temperature (298 K),
stirring rate (500 rpm). After the reaction, the catalyst was
separated by filtration. The filtrate was analyzed by a high
performance liquid chromatography (HPLC) apparatus
equipped with an Aminex HPX-87H column (Bio-Rad
Laboratories) and a refractive index (RI) detector. The analysis
conditions were set as follows: eluent, aqueous solution of
H2SO4 (10 mM); flow rate, 0.5 mL·min−1; column temper-
ature, 323 K. Turnover frequency (TOF) was calculated by the
slope in the initial rate period (0−30 min) of the production of
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a
Table 1. Results of GLY Oxidation over PtxAuY−starch NPs/HT Catalysts
yield/%
conv. of
GLY/%
sel. of
GA/%
particle
size/nm
TOF
(h−1
c
entry
catalysts
GA
TA
HA
OA
total metal loading/mmol·g−1
c
)
1
2
3
4
5
6
7
Pt100−starch/HT
Pt80Au20−starch/HT
Pt60Au40−starch/HT
Pt40Au60−starch/HT
Pt20Au80−starch/HT
Au100−starch/HT
Au/HT
88
48
64
78
73
84
42
51
57
54
42
13
10
9
11
8
12
7
0.087
0.086
0.087
0.086
0.089
0.087
0.096
2.0
2.2
2.2
3.8
5.4
5.5
3.2
70.4
81.1
84.7
69.2
34.8
80
73
6
6
74
10
4
4
5
50
2
1
b
b
b
b
b
b
trace, 11
trace
−, 37
trace, 4
trace
0, 3
0
0, 3
0
0, 1
0
a
Reaction conditions: GLY, 0.5 mmol; H2O, 2 mL; catalyst, 20 mg; GLY/metal = 287; oxygen flow, 10 mL·min−1; room temperature, 298 K;
b
c
reaction time, 6 h. 333 K. Analysis by ICP−AES.
main product (GA or LA) per mol of total metal loading on the
PtxAuy−starch/HT catalyst in the reaction mixtures.36
(88%) was achieved by Pt100−starch/HT, although it exhibited
a low selectivity toward GA (48%) (Table 1, entry 1).
Because the sum of GA, TA, HA, and OA was ∼78% yield
(89% selectivity), another overoxidation product toward C1
product (ex. CO2) seemed to be expected in Pt100−starch/HT.
As the Au amounts increased from y = 20 to 80, the GLY
conversion gradually decreased with increasing selectivity for
GA and suppressed overoxidation to the formation of
byproduct (Table 1, entries 2−5), when the sum of product
selectivity is nearly 100%. Au100−starch/HT was inactive at
room temperature (RT), whereas it showed a small activity at
333 K (Table 1, entry 6), even though its exhibited particle size
of 5.5 nm was similar to that of Pt20Au80−starch/HT (5.4 nm).
The aerobic oxidation over bare Au/HT with 3.2 nm was also
inactive at RT (Table 1, entry 7). Thus, the aerobic oxidation
over a supported Au catalyst cannot proceed at RT. These
results suggest that the replacement of active Pt atoms with
inactive Au atoms likely has some roles for prohibition of
overoxidation, and it supports increasing the mass balance close
to 100%. The Pt60Au40−starch/HT showed the most effective
catalyst with a TOF of 84.7 h−1 at RT (Table 1, entry 3).
The time profile of the GLY oxidation over Pt60Au40−starch/
HT is plotted in Figure 1. The reaction was accomplished by
2.4. Characterization. The morphology of PtxAuy−starch/
HT was observed by transmission electron microscopy (TEM;
Hitachi H-7100) at 100 kV accelerating voltage. The PtxAuy−
starch/HT was dispersed in deionized water and dropped onto
a copper grid, then dried overnight in a desiccator. Powder X-
ray diffraction (XRD) patterns were obtained with a Rigaku
Smartlab X-ray diffractometer using Cu Kα radiation (λ = 0.154
nm) at 40 kV and 20 mA. Inductively coupled plasma atomic
emission spectroscopy (ICP−AES) was performed on a
Shimadzu ICPS-7000, version 2, to estimate the real
concentration of Pt and Au on the catalyst. Ultraviolet and
visible (UV−vis) spectra were measured using a Perkin-Elmer
Lambda35 spectrometer at room temperature with a light path
length of 1 cm. X-ray photoelectron spectroscopy (XPS) was
measured on a Shimadzu Kratos AXIS-ULTRA DLD
spectrometer using Al target at 15 kV and 10 mA. The binding
energies were calibrated with the C 1s peak (284.5 eV) as the
internal standard reference. X-ray absorption near-edge
structure (XANES) in the Pt L3-edge and Au L3-edge were
recorded at beamline BL01B1 of SPring-8 with the approval of
the Japan Synchrotron Radiation Research Institute (JASRI)
(Proposals Nos. 2011A1607 and 2012B1610). The PtxAuy−
starch/HT catalysts were grained and pressed to a pellet (ϕ ∼
10 mm) for XANES analysis. The obtained XANES spectra
were analyzed using the Athena software (version 0.8.056).
3. RESULTS AND DISCUSSION
3.1. Aerobic Oxidation of Glycerol. Results of activity
over PtxAuy−starch/HT catalysts for the aerobic GLY oxidation
under atmospheric conditions are summarized in Table 1,
together with total metal loading and mean particle size. All
catalysts except for Au100−starch/HT promoted the selective
oxidation toward GA, and minor amounts of tartronic acid
(TA), glycolic acid (HA), and oxalic acid (OA) were also
observed. The formation of dihydroxyacetone, oxidation at the
secondary hydroxyl group, was not detected in all cases. It was
consistent with previous reports that GA was obtained as the
major product, and it could be further oxidized to TA, HA, and
OA in the aerobic oxidation of GLY catalyzed by the Pt-based
heterogeneous catalyst.31,32,37−39 The highest GLY conversion
Figure 1. Time course of GLY oxidation catalyzed by Pt60Au40−
■
●
starch/HT: GLY conversion ( ), GA yield ( ), TA yield (Δ), HA
◊
▽
yield ( ), OA yield (☆), and GA selectivity ( ). Reaction conditions:
GLY, 0.5 mmol; H2O, 2 mL; catalyst, 20 mg; oxygen flow, 10 mL·
min−1; room temperature (298 K).
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a
Table 2. Results of PG Oxidation over PtxAuY−Starch/HT Catalysts
yield/%
particle
c
entry
catalysts
conv. of PG/% sel. of LA/%
LA
19
PA
AA
total metal loading/mmol·g−1
size/nm
TOF (h−1
)
1
2
3
4
5
6
Pt100−starch/HT
23
82
2
6
2
0.087
0.086
0.087
0.086
0.089
0.087
2.0
2.2
2.2
3.8
5.4
5.5
45.1
61.8
Pt80Au20−starch/HT
Pt60Au40−starch/HT
Pt40Au60−starch/HT
Pt20Au80−starch/HT
Au100−starch/HT
38
75
28
3
b
b
b
b
b
b
39, 63
38
74, 75
74
29, 47
28
6, 12
1, 3
1
62.3,77.2
43.6
7
4
0
27
75
20
1
21.6
trace
trace
0
a
Reaction conditions: PG, 0.5 mmol; H2O, 2 mL; catalyst, 20 mg, (PG/metal = 287; PG/metal = 179); oxygen flow, 10 mL·min−1; room
temperature, 298 K; reaction time, 6 h. Catalyst, 20 mg; PG/metal = 32 mg; reaction time, 16 h. Analysis by ICP−AES.
b
c
the oxidation of the primary hydroxyl group of GLY to GA,
together with the formation of side products (i.e., TA, HA and
OA) via the overoxidation. At the initial stage of the reaction
within 1 h, the highest selectivity to GA (87%) was achieved.
Nevertheless, the glyceraldehyde, which was expected to be an
intermediate, was not observed. The GLY conversion and yield
of GA are 68% and 52%, respectively, as the reaction time
reaches 5 h. When the reaction time was increased to 10 h,
nearly 80% conversion of GLY was obtained. There was no
significant increase in the GA yield after a reaction time of 5 h.
A small amount of the overoxidation of GA to TA and further
C−C bond cleavage to HA and OA scarcely occurred during
the reaction. Although the pH values were gradually decreased
with the progressive reaction that we reported on previously,31
the basicity of the HT support seems to be enough for further
reaction because the PtAu/HT catalyst was recyclable for 3
runs (vide infra). Previous reports on GLY oxidation at near
room temperature are summarized in Supporting Information
Table S1.21,22,40−42 Most reactions were carried out in the
presence of NaOH under pressurized oxygen (3−10 atm). The
oxidation of GLY under pressurized oxygen (3 atm) with GLY
conversions of 30% and 43% over AuPd (1:3)/MgO and AuPt
(1:3)/MgO has also been reported.37
Figure 2. Time course of PG oxidation catalyzed by Pt60Au40−starch/
■
●
HT: PG conversion ( ), LA yield ( ), PA yield (Δ), AA yield (☆),
◊
and LA selectivity ( ). Reaction conditions: PG, 0.5 mmol; H2O, 2
mL; catalyst amount, 32 mg; oxygen flow, 10 mL·min−1; room
temperature (298 K).
reports on PG oxidation are summarized in the Supporting
Information, Table S2.14−19 Most of them required basic
conditions and pressurized oxygen.
3.2. Aerobic Oxidation of 1,2-Propanediol. To confirm
the highly selective catalysis at the primary hydroxyl group of
C3 polyol under ambient conditions, PtxAuy−starch/HT
catalysts were further applied for the aerobic oxidation of PG.
The selective oxidation at the primary hydroxyl group of PG to
LA as a main product was observed over PtxAuy−starch/HT
except for Au100−starch/HT (Table 2). LA was also previously
obtained as the main product in the PG oxidation reaction.14,15
PG conversion of 23% was found over Pt100−starch/HT (Table
2, entry 1), and it was further enhanced as the Au content was
increased, y = 20−60 (Table 2, entries 2−4). Thereafter, the
conversion dropped to 27% and trace amounts at y = 80 and
100, respectively (Table 2, entries 5 and 6). The Pt60Au40−
starch/HT also showed the most effective catalysis for PG
oxidation with TOF of 62.3 h−1 at RT (Table 1, entry 3).
Figure 2 shows the time profile of PG oxidation over the
Pt60Au40−starch/HT. Lactaldehyde, which was expected to be
an intermediate, was not observed. After the reaction was
prolonged to 16 h, the high conversion of 63% was achieved
with selectivity to LA of 75%. In addition, the overoxidation of
LA to pyruvic acid (PA) followed by C−C bond cleavage to
acetic acid (AA) did not occur until after 4 h reaction. Previous
To examine the stability of the Pt60Au40−starch/HT catalyst,
the recycling experiments were carried out. After 6 and 16 h of
reaction for GLY and PG oxidation, the catalyst was
centrifuged, washed with deionized water, and dried under
vacuum, then it was employed for further oxidation reaction
under the same reaction conditions. Figure 3 indicates that the
catalyst slightly decreased in activity but retained high
selectivity during the recycling experiment in the oxidations
of GLY and PG. The morphology of the recycled catalysts from
glycerol oxidation was investigated by TEM. Although the
particle size was slightly increased after the reaction
(Supporting Information Figure S2), it scarcely influenced its
activity for glycerol oxidation.
To evaluate the reaction pathway of aerobic oxidation of
polyols over Pt60Au40−starch/HT, the reaction was operated in
the presence of a radical scavenger (BHT), as shown in Figure
4. BHT slightly influenced the initial oxidation rate of the
oxidation of the polyols; however, after prolonged reaction
time, the yields of GA and LA in the presence and absence of
BHT were the same. This result suggests that the aerobic
oxidation over Pt60Au40−starch/HT did not proceed through
the free-radical mechanism.
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Figure 3. Recycling of Pt60Au40−starch/HT catalyst. Reaction conditions: H2O, 2 mL; oxygen flow, 10 mL·min−1; room temperature (298 K). (A)
GLY, 0.5 mmol; catalyst, 20 mg; reaction time, 6 h. (B) PG, 0.5 mmol; catalyst, 32 mg; reaction time, 16 h.
■
□
Figure 4. Time course of aerobic oxidation of (A) GLY and (B) PG over Pt60Au40−starch/HT catalyst in the absence ( ) or presence ( ) of BHT.
Reaction conditions: H2O, 2 mL; oxygen flow, 10 mL·min−1; room temperature (298 K). (A) GLY, 0.5 mmol; catalyst, 20 mg. (B) PG, 0.5 mmol;
catalyst, 32 mg. BHT/polyol = 1/10 (11 mg).
3.3. Characterization of PtxAuY−starch/HT Catalysts.
First, the morphology of the PtxAuy−starch/HT catalysts was
analyzed by TEM. TEM images and mean particle size
distribution histograms for all catalysts are shown in Figure 5.
Bimetallic NPs were well-dispersed on the surface of HT and
showed a narrow size distribution. The particle size of PtxAuy−
starch/HTs was near constant with the mean sizes being 2.0,
2.2, and 2.2 nm for y = 0, 20, and 40, respectively. However,
increasing the amounts of Au resulted in increasing mean
particle sizes, to 3.8, 5.4, and 5.5 nm for y = 60, 80, and 100,
respectively (listed in Table 1). Supporting Information Figure
S1 shows the UV−vis absorption spectra of PtxAuy−starch NPs.
The absorption band (localized surface plasmon resonance) of
Au−starch NPs appears around 500−600 nm.43,44 This
characteristic peak of Au−starch NPs was not observed in all
PtxAuy−starch NPs spectra, indicating PtAu alloy formation
rather than phase-segregated bimetallic NPs.
Figure 6A exhibits the XRD patterns of PtxAuy−starch NPs.
All XRD patterns of PtxAuy−starch NPs describe the
characteristic of the face-centered cubic (fcc) structure of
metallic Pt or Au, the reflection corresponding to the planes
(111), (200), (220), (311), and (222).45 Furthermore, these
five diffraction peaks shifted to higher angle with a decrease in
the Au amount in each position. For example, the (111)
reflection of PtxAuy−starch NPs gradually shifted from 2θ =
38.3, 38.5, 38.8, 39.1, 39.4, and 39.7 with changes in y = 100 to
0. According to the previous literature,46 2θ = 38.3 and 39.7
correspond to the (111) reflection of metallic Au and Pt,
respectively. Lattice parameters of PtxAuy−starch NPs
estimated from the XRD diffraction pattern of (111)47 as a
function of the Au amount are plotted in Figure 6B. The lattice
parameters of metallic Pt and Au were 3.92 and 4.07,
respectively. Increasing the concentration of Au (y) in
PtxAuy−starch NPs leads to an increase in the lattice distance
from 3.92 to 3.96, 4.0, 4.03, 4.04, and 4.07 Å, and a linear
relationship between the lattice parameter and their composi-
tion was obtained. These results indicate that the nanostruc-
tures of PtxAuy−starch NPs are homogeneous PtAu alloys.45
XPS spectra of PtxAuy−starch NPs are shown in Figure 7.
Peaks at 85.6 and 82 eV in Figure 7A were assigned to Au 4f5/2
and Au 4f7/2, respectively. All spectra showed the large negative
shift to lower binding energy (BE), as compared with Au foil
(Au 4f5/2:88 eV and Au 4f7/2:84 eV). The same negative shifts
also appeared in all peaks around 72 and 68.7 eV attributed to
Pt 4f5/2 and Pt 4f7/2, respectively, as compared with Pt foil (Pt
4f5/2:74.4 eV and Pt 4f7/2:71.2 eV) in Figure 7B. These large
negative shifts in both Au 4f and Pt 4f in alloy PtxAuy−starch
NPs suggests that a negative charge is loaded on PtAu−starch
NPs,48 which is supposed by the interaction between PtAu−
starch NPs and starch ligand. It has been reported that the
electron donated from the stabilizer (for example, poly(vinyl
pyrrolidone) (PVP)) to a metal such as Pt, Au, and Ag NPs,
results in a negative shift of the BE in XPS spectra.43,49,50 In
addition, hydroxyl groups in soluble starch can facilitate the
complexation of metal ions to a molecular matrix.51,52
Therefore, it is supposed that the large negative shift in both
Pt 4f and Au 4f are due to electron donation from the starch
ligand to both Pt and Au atoms.53 Considering the peak of Au
4f the decrease in the Au content from y = 100 to 20 leads to a
more negative shift in Figure 7A, it was supposedly attributed
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diminished with a decreasing Au content (Figure 7B). On the
contrary, reductions in the negative shift in Pt 4f were found as
the Au content was reduced from y = 80 to 0. In other words,
the Pt 4f peaks gradually shifted to the lower side of the binding
energy. It is postulated that the excess electrons on Au atoms
transferred to the adjacent Pt atoms.
Figure 8 shows normalized Pt L3-edge and Au L3-edge
XANES spectra of PtxAuy−starch/HT. The white-line (WL)
feature in the L3-edge XANES was related to the electron
transfer phenomena induced by an X-ray absorption from the p
to d states in the element; thus, it indicated the unoccupied
densities of the d states.54,55 Au L3-edge XANES spectra have
no WL feature, since the 5d orbitals full in theory; however, the
s−p−d hybridization leads to electron transformation from 5d
to the s−p state, leading the small WL intensity in the Au L3-
edge XANES.55−57 Lower WL intensities than Au foil were
oberved for all Au L3-edges, implicating a Au gained electron
(Figure 8B). Figure 9 shows plots of WL intensities in Pt and
Au L3-edges XANES as a function of Au content. The heights
of the WL in the Pt L3-edges gradually became lower, whereas
that in Au L3-edges were higher with an increase of in the Au
content. The decreasing tendency of the Pt L3-edge WL
intensity is related to the increase in that of the Au L3-edge as
the Au content increases from y = 20 to 80. Previous works
have reported that the NPs’ size affected the WL intensity of
the XANES spectra. Small NPs reduced the number of metal−
metal bonds by the s−p−d hybridization change, resulting in a
decrease in the number of d-holes density, leading to a decrease
in the WL intensity.58 As the above result, an increase in the Au
content, the size of the PtxAuy−starch NPs increased while the
WL intensity of the Pt L3-edge decreased. Therefore, XPS
together with XANES confirmed that a negative charge was
present on both the Au and Pt atoms, in which the starch ligand
donates electrons, and the excess electrons on the Au atoms
can also transfer to the Pt atoms. There have been reports of
the electron transfer from Au (EN = 2.54) to Pt (EN = 2.28) in
the core−shell structure of NPs, leading to an increase in the
active oxygen species on the surface.59,60 In addition, it has also
been reported that a stabilizing/capping agent can play a direct
role in regulating the electronic structures.49,57,61,62 Soluble
starch is a linear polymer formed by the α-(1 → 4) linkages
between D-glucose units and adopts a left-handed helical
conformation in aqueous solution;52 therefore, it is supposedly
possible that the electrons can be transfer via complexation of
the starch ligand onto Pt and Au atoms, as illustrated in Scheme
2.
3.4. Oxidation Mechanism. As mentioned, the geometric
(d-spacing) and electronic changes of the catalytically active Pt
sites by adjacent Au atoms and the starch ligand can lead to
improvement of the activity and selectivity to target products.
The replacing of active atoms (Pt site) with inactive atoms (Au
site) changed the geometry of the Pt surface or modified the
strength of the surface adsorption then controlling oxygen
coverage.63−66 Pt60Au40−starch/HT showed the highest yield,
with a high selectivity for catalytic oxidation of GLY and PG
toward GA and LA, respectively, under an atmospheric pressure
of molecular oxygen because of both geometric and electronic
changes. The reaction mechanism of oxidation of polyols is
proposed in Scheme 3. First, O2 is adsorbed onto the surface of
negatively charged Pt atoms (1). It has been reported that
anionic metal can activate molecular oxygen by donating an
excess electron charge to the antibonding orbital, resulting in
the generation of anionic O2 such as superoxo or peroxo
Figure 5. TEM images of PtxAuy−starch/HT: (A1) Pt100−starch/HT,
(B1) Pt80Au20−starch/HT, (C1) Pt60Au40−starch/HT, (D1)
Pt40Au60−starch/HT, (E1) Pt20Au80−starch/HT, and (F1) Au100
−
starch/HT. (A2−F2) Their particle size distribution histograms.
to decreasing the particle size because the number of ligand-
capped surface sites will become larger with the reduced
particle size. Although the same phenomenon is also expected
to be encountered for Pt 4f, the enhancement of a negative shift
of Pt 4f was not observed; even their particle sizes were
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Research Article
Figure 6. (A) XRD patterns of (a) Au100−starch NPs, (b) Pt20Au80−starch NPs, (c) Pt40Au60−starch NPs, (d) Pt60Au40−starch NPs, (e) Pt80Au20−
starch NPs, and (f) Pt100−starch NPs. (B) Dependence of the lattice parameters for PtxAuy−starch NPs on the relative composition of the Au
amount.
Figure 7. XPS spectra of (A) Au4f and (B) Pt4f of PtxAuy−starch NPs: (a) Au100−starch NPs, 5.5 nm; (b) Pt20Au80−starch NPs, 5.4 nm; (c)
Pt40Au60−starch NPs, 3.8 nm; (d) Pt60Au40−starch NPs, 2.2 nm; (e) Pt80Au20−starch NPs, 2.2 nm; and (f) Pt100−starch NPs, 2.0 nm.
Figure 8. Normalized (A) Pt L3-edge and (B) Au L3-edge XANES spectra of PtxAuy−Starch/HT.
oxygen.67−70 Then the basic support abstracts a proton from
the polyol to form the alkoxide, which enhances the polyol’s
binding to the metal atom (2). The proton in the adsorbed
alkoxide carbon is transferred to the adsorbed oxygen; releasing
aldehyde (3) and H2O to generate back the peroxo form (4).
Aldehyde, thus formed, is attracted by the adjacent metal atom
with adsorbed oxygen (Au atoms) (5). This creates a partial
positive charge on the carbonyl carbon. The nucleophilic
oxygen atom of water attacks the electron-deficient carbonyl
carbon (6). The adsorbed oxygen abstracts the proton to yield
carboxylic acid (7) and is changed back to the peroxo form (8)
to complete the catalytic cycle.
4. CONCLUSIONS
We explored the efficient and recyclable heterogeneous
PtxAuy−starch/HT catalyst for the oxidation of polyols without
additional bases under an atmospheric pressure of molecular
oxygen. Bimetallic PtxAuy−starch/HT showed higher selectivity
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XRD revealed that PtxAuy−starch formed as an alloy, and their
d-spacing was changed from individual metals. XPS and
XANES showed the negative charge on both Au and Pt
atoms. It was supposed that the PtAu−starch NPs gained
charge from the starch ligand. Au L3-edge XANES spectra
revealed an increase in the white-line intensity as the amount of
Au was increased, indicating the excess electrons on the Au
atoms also transfer to the Pt atoms. The negatively charged Pt
atoms may enhance the oxygen absorption and generate
anionic O2 such as superoxo or peroxo oxygen to oxidize
polyols. We conclude that the geometric and electronic changes
of the catalytically active Pt sites by adjacent Au atoms and the
starch ligand lead to improvement of the activity and selectivity
to target products.
Figure 9. White line intensity of Pt L3-edge (at 11.565 keV) and Au
L3-edge (at 11.924 keV) XANES spectra of PtxAuy−starch/HT.
Dashed lines represent the white-line intensity of Pt and Au foils.
ASSOCIATED CONTENT
* Supporting Information
■
S
Scheme 2. The Possible Electron Transfer from Starch
Ligand to AlloyPtAu−Starch NPs and from Au Atoms to
Adjacent Pt Atoms
Catalyst performances for GLY and PG oxidation in the
previous reports, UV−vis spectra of PtxAuy−starch NPs, and
TEM images of recycled Pt60Au40−starch/HT catalysts. This
information is available free of charge via the Internet at http://
pubs.acs.org/.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Kohki Ebitani; ebitani@jaist.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
D.T. gratefully acknowledges support from a grant from the
dual Ph.D program between JAIST and Chulalongkorn
University. Parts of this research are supported by a Grant-in-
Aid for Young Scientists (B) (No. 25820392) and Scientific
Research(C) (No. 25420825) promoted by the Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
Japan. XAFS measurements of the PtxAuy−starch/HTs were
performed at the BL01B1 in the SPring-8 by approval of the
Japan Synchrotron Radiation Research Institute (JASRI)
(Proposals Nos. 2011A1607 and 2012B1610).
for the oxidation at the primary hydroxyl groups of GLY and
PG toward GA and LA, respectively, than those of each
monometallic ones. Pt60Au40−starch/HT is found to be the
most selective catalyst for oxidation of polyols. UV−vis and
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