Preparation of Au/CeO2 exhibiting strong surface plasmon resonance effective for selective or chemoselective oxidation of alcohols to aldehydes or ketones in aqueous suspensions under irradiation by green light

Article abstract of DOI:10.1021/ja305225s

Au/CeO₂ samples with various Au contents were prepared by the multistep (MS) photodeposition method. Their properties including Au particle size, particle dispersion, and photoabsorption were investigated and compared with properties of samples prepared by using the single-step (SS) photodeposition method. The MS- and SS-Au/CeO₂ samples were used for selective oxidation of benzyl alcohols to corresponding benzaldehydes in aqueous suspensions under irradiation by visible light from a green LED, and the correlations between reaction rates and physical properties of the MS- and SS-Au/CeO₂ samples were investigated. Difference in the two photodeposition methods was reflected in the average size and number of Au nanoparticles, for example, 92 nm and 1.3 × 10¹² (g-Au/CeO ₂)^-1 for MS photodeposition and 59 nm and 4.8 × 10¹² (g-Au/CeO₂)^-1 for SS photodeposition in the case of 1.0 wt % Au samples. Fixation of larger Au particles resulted in strong photoabsorption of the MS-Au/CeO₂ samples at around 550 nm due to the surface plasmon resonance, and the Kubelka-Munk function of the photoabsorption linearly increased with increase in Au content up to 2.0 wt %, in contrast to the photoabsorption of SS-Au/CeO₂ samples, which was weak and was saturated even at around 0.5 wt %. Due to the strong photoabsorption, the MS-Au/CeO₂ samples exhibited reaction rates approximately twice larger than those of SS-Au/CeO₂ samples with the same Au contents, and apparent quantum efficiency of MS-Au/CeO₂ reached 4.9% at 0.4 mW cm^-2. Linear correlations were observed between reaction rates (r) and surface area of Au nanoparticles (S) in both MS- and SS-Au/CeO₂ samples, though the two slopes of r versus S plots were different, suggesting that oxidation of benzyl alcohol occurred on the Au surface and that S was one of the important factors controlling the reaction rate. Photocatalytic oxidation of benzyl alcohol having an amino group revealed that the Au/CeO₂ photocatalyst exhibited high chemoselectivity toward the hydroxyl group of alcohol, i.e, the Au/CeO₂ photocatalyst almost quantitatively converted aminobenzyl alcohol to aminobenzaldehyde with 99% yield.

Full text of DOI:10.1021/ja305225s

Article
pubs.acs.org/JACS
Preparation of Au/CeO2 Exhibiting Strong Surface Plasmon
Resonance Effective for Selective or Chemoselective Oxidation of
Alcohols to Aldehydes or Ketones in Aqueous Suspensions under
Irradiation by Green Light
Atsuhiro Tanaka, Keiji Hashimoto, and Hiroshi Kominami*
Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, Kowakae, Higashiosaka, Osaka 577-8502,
Japan
ABSTRACT: Au/CeO2 samples with various Au contents
were prepared by the multistep (MS) photodeposition
method. Their properties including Au particle size, particle
dispersion, and photoabsorption were investigated and
compared with properties of samples prepared by using the
single-step (SS) photodeposition method. The MS- and SS-
Au/CeO2 samples were used for selective oxidation of benzyl
alcohols to corresponding benzaldehydes in aqueous suspen-
sions under irradiation by visible light from a green LED, and
the correlations between reaction rates and physical properties
of the MS- and SS-Au/CeO2 samples were investigated. Difference in the two photodeposition methods was reflected in the
average size and number of Au nanoparticles, for example, 92 nm and 1.3 × 1012 (g-Au/CeO2)−1 for MS photodeposition and 59
nm and 4.8 × 1012 (g-Au/CeO2)−1 for SS photodeposition in the case of 1.0 wt % Au samples. Fixation of larger Au particles
resulted in strong photoabsorption of the MS-Au/CeO2 samples at around 550 nm due to the surface plasmon resonance, and
the Kubelka−Munk function of the photoabsorption linearly increased with increase in Au content up to 2.0 wt %, in contrast to
the photoabsorption of SS-Au/CeO2 samples, which was weak and was saturated even at around 0.5 wt %. Due to the strong
photoabsorption, the MS-Au/CeO2 samples exhibited reaction rates approximately twice larger than those of SS-Au/CeO2
samples with the same Au contents, and apparent quantum efficiency of MS-Au/CeO2 reached 4.9% at 0.4 mW cm−2. Linear
correlations were observed between reaction rates (r) and surface area of Au nanoparticles (S) in both MS- and SS-Au/CeO2
samples, though the two slopes of r versus S plots were different, suggesting that oxidation of benzyl alcohol occurred on the Au
surface and that S was one of the important factors controlling the reaction rate. Photocatalytic oxidation of benzyl alcohol having
an amino group revealed that the Au/CeO2 photocatalyst exhibited high chemoselectivity toward the hydroxyl group of alcohol,
i.e, the Au/CeO2 photocatalyst almost quantitatively converted aminobenzyl alcohol to aminobenzaldehyde with 99% yield.
Oxidation by semiconductor photocatalysts such as titanium-
(IV) oxide and tungsten(VI) oxide has attracted much
attention and has been applied for purification of water and
air2 because holes and active oxygen species have strong
oxidation power. However, selective (or partial) oxidation of
organic compounds using semiconductor photocatalysts has
been difficult due to the strong oxidation power. Since
photocatalytic selective (or partial) oxidation occurs at room
temperature under atmospheric pressure using O2 molecules,
the oxidation is still attractive for organic synthesis. Recently,
several research groups have reported selective oxidation of
aromatic compounds to carbonyl compounds using a TiO2
photocatalyst.3 Palmisano and co-workers3c−g demonstrated
selective oxidation of benzyl alcohols to corresponding carbonyl
compounds by O2 in an aqueous solution by band gap
excitation of rutile-type TiO2 under UV irradiation. It is known
1. INTRODUCTION
Selective oxidation of aromatic alcohols to corresponding
carbonyl compounds such as aldehydes and ketones without
overoxidation to carboxylic acids and carbon dioxide remains a
challenging task. Carbonyl compounds are widely used as
intermediates for many fragrances, drugs, and vitamins.
Selective oxidations to these compounds are carried out using
stoichiometric oxidizing agents such as KMnO4, MnO2, CrO3,
and Br2. However, these oxidations also give reduced residues
of oxidizing agents. Since these reduced residues should be
removed from the reaction mixtures and adequately recovered
to prevent their diffusion in the environment, oxidations by
stoichiometric oxidizing agents are costly methods and should
be replaced with more environmentally friendly methods.
Heterogeneous and homogeneous catalytic oxidation of
alcohols to aldehydes or ketones with oxygen (O2) molecules
has been proposed as the most plausible method.1 Only water
(H2O) is formed as the byproduct in these reactions, if the
reactions ideally occur.
Received: May 30, 2012
Published: August 9, 2012
© 2012 American Chemical Society
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tetrachloroauric acid (HAuCl4) was injected into the sealed test tube
and then photoirradiated at λ >300 nm by a 400-W high-pressure
mercury arc (Eiko-sha, Osaka, Japan) under Ar with magnetic stirring
in a water bath continuously kept at 298 K. The Au source was
reduced by photogenerated electrons in the conduction band of CeO2,
and Au metal was deposited on CeO2 particles, resulting in the
formation of Au/CeO2. Analysis of the liquid phase after each
photodeposition revealed that the Au source had been almost
completely (>99.9%) deposited as Au on the CeO2 particles. The
resultant powder was washed repeatedly with distilled water and then
dried at 310 K overnight under air. This photodeposition of Au was
repeated several times to obtain an Au/CeO2 sample having a desired
Au content. This photodeposition method is called the MS
photodeposition method and, hereafter, a sample prepared by the
MS photodeposition method is designated as MS-Au(X)/CeO2, where
X means total Au content. The amount of Au loaded per a single
deposition in MS photodeposition was generally 0.5 wt %, and an MS-
Au(1.0)/CeO2 sample was prepared by performing the photo-
deposition two times. The MS photodeposition method can be used
flexibly, e.g., an MS-Au(1.0)/CeO2 sample can also be prepared by
first 0.50 wt % Au loading followed by 0.10 wt % loading five times
(1.0 = 0.50 + 0.10 × 5). General photodeposition method in which all
of the HAuCl4 solution containing the desired amount of Au was
injected, defined here as the SS photodeposition method, was used for
comparison with the MS photodeposition method, and a sample
prepared by the SS photodeposition method is designated as SS-
Au(X)/CeO2. It was confirmed that the Au source was also almost
completely (>99.9%) deposited as Au on the CeO2 particles when the
SS photodeposition method was used.
that UV light accounts for only about 5% of total solar energy,
whereas visible light accounts for about 50%. Therefore,
development of visible-light-responding photocatalysts is an
important topic for chemical syntheses using solar energy in
future. Many efforts have been devoted to synthesis of
photocatalysts that respond to visible light, and various types
of photocatalysts for mineralization of organic compounds have
been reported.4 However, to the best of our knowledge, there
are no reports on selective oxidation of organic compounds
using these visible-light-responding semiconductor photo-
catalysts. Different from these photocatalysts, selective
oxidation of benzyl alcohol to benzaldehyde under irradiation
by blue light from light-emitting diodes was reported by
Higashimoto et al.5 In their reaction system, benzyl alcohol was
adsorbed on the surface of TiO2 and electrons were transferred
from the surface complex to TiO2 under irradiation by visible
light, resulting in formation of benzaldehyde and reduction of
O2 molecules.
Unique optical properties of metallic nanoparticles have been
applied in many different fields including biochemistry, sensing
science, and catalysis. Nanoparticles of metals such as copper
(Cu), silver (Ag), and gold (Au) show strong photoabsorption
of visible light due to surface plasmon resonance (SPR).
However, there are limited reports on application of SPR-
induced photoabsorption to chemical reactions, i.e., oxidation
of organic substrates,6d,e,7a,b selective oxidation of aromatic
alcohol to a carbonyl compound,6c,7d hydrogen formation from
alcohols6f,7e and selective reduction of organic compounds.6g
We examined mineralization of organic acid in aqueous
suspensions of Au nanoparticles loaded on various metal
oxides (Au/MOx) under visible light irradiation and found that
Au loaded on cerium(IV) oxide (Au/CeO2) was the most
efficient photocatalyst among plasmonic Au/MOx photo-
catalysts.7a,b We also succeeded in shifting SPR to a longer
length in copper (Cu)-Au/CeO2 and mineralization of organic
acids in an aqueous suspension of Cu-Au/CeO2 even under
irradiation by visible light of λ = 700 nm.7c In our previous
communication,7d we briefly reported almost quantitative
oxidation of benzyl alcohols to benzaldehydes in aqueous
suspensions of Au/CeO2 under irradiation by green light from
an LED. By using the multistep (MS) photodeposition
method7c,e in which addition of metal sources and photo-
deposition of the metals on semiconductor particles were
repeated several times to obtain a desired metal loading, we
succeeded in preparation of Au/CeO2 exhibiting SPR
absorption stronger than that of Au/CeO2 prepared by using
a single-step (SS) photodeposition method. In this paper, we
report (1) characterization of Au/CeO2 samples prepared by
MS and SS photodeposition methods, (2) correlation between
properties and photocatalytic activities of Au/CeO2 in
benzaldehyde formation, and (3) chemoselective oxidation of
aminobenzyl alcohols to aminobenzaldehydes in aqueous
suspensions of MS-Au/CeO2 under irradiation by green light
from an LED.
2.2. Characterization. Diffuse reflectance spectra were obtained
with a UV−vis spectrometer (UV-2400, Shimadzu, Kyoto) equipped
with a diffuse reflectance measurement unit (ISR-2000, Shimadzu).
The morphology of Au/CeO2 particles was observed under a JEOL
JEM-3010 transmission electron microscope (TEM) operated at 300
kV in the Joint Research Center of Kinki University.
2.3. Oxidation of Aromatic Alcohols in an Aqueous
Suspension of Au/CeO2 under Irradiation of Visible Light.
The dried Au/CeO2 powder (50 mg) was suspended in distilled water
(5 cm3), bubbled with O2, and sealed with a rubber septum. Aromatic
alcohol was injected into the suspension and then irradiated with
visible light of a green light-emitting diode (HDMS8G, Hayashi Watch
Works, Tokyo, maximum energy at 530 nm, designated green LED
hereafter). The light intensity was fixed as 1.7 mW cm−2 unless
otherwise stated. Since an LED consumes less energy than a
fluorescent lamp, LEDs would be preferably used as light sources of
photochemical processes in industry because of their low energy
consumption. The amount of carbon dioxide (CO2) in the gas phase
was measured using a Shimadzu GC-8A gas chromatograph equipped
with a Porapak QS column. The amounts of aromatic alcohols and
aldehydes in the liquid phase were determined with a Shimadzu GC-
14B gas chromatograph equipped with a DB-1 capillary column (30 m,
0.25 mm). Toluene was used as an internal standard sample. The
reaction solution (1 cm3) was added to a diethyl ether/water mixture
(2:1 v/v, 3 cm3). After the mixture had been stirred for 10 min,
aromatic alcohols and aldehydes in the ether phase were analyzed. The
amounts of aromatic alcohols and aldehydes were determined from the
ratios of the peak areas of aromatic alcohols and aldehydes to the peak
area of toluene.
3. RESULTS AND DISCUSSION
3.1. Characterization of SS- and MS-Au(X)/CeO2
Samples. Figure 1a and b shows TEM photographs of SS-
and MS-Au(1.0)/CeO2 samples, respectively. In both cases,
fine Au nanoparticles were observed within relatively sharp
distributions. The average particle sizes of SS- and MS-
Au(1.0)/CeO2 samples were determined to be 59 and 92 nm
with standard deviations of 4.9 and 8.0 nm, respectively.
These results indicate that the size of Au nanoparticles
loaded on CeO2 is strongly affected by the type of
2. EXPERIMENTAL SECTION
2.1. Preparation of Au/CeO2. Commercial highly pure
(>99.99%) and fine spherical CeO2 particles having a cubic structure
and an average diameter of 0.2 μm were supplied by Kojundo
Chemical Laboratory. Loading of Au on CeO2 was performed by a
photodeposition method. Bare CeO2 powder (198 mg) was suspended
in water (10 cm3) in a test tube, and the test tube was sealed with a
rubber septum under argon (Ar). An aqueous solution of
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D
Au values of MS-Au(X)/CeO2 samples were always larger than
those of SS-Au(X)/CeO2 samples with the same X. The NAu
value of SS-Au(X)/CeO2 samples decreased with increase in X
and reached a minimum (4.8 × 1012 (g-Au/CeO2)−1) at X = 1.0
(Table 1). Further increase in X resulted in a slight increase in
NAu. These results suggest that balance of nucleation, growth
and aggregation of Au particles changed depending on
concentration of the Au source, which caused change of NAu
in the SS photodeposition method. It should be noted that the
NAu value of MS-Au(X)/CeO2 samples was constant (1.3 ×
1012 (g-Au/CeO2)−1), indicating that the Au source was
deposited only on Au particles previously formed in the range
from X = 1.0 to X = 2.0 in the MS photodeposition method and
that neither new particle formation nor particle aggregation
occurred.
In a previous study,7e it was found that the external surface
area of Au (not the Au content) was an important factor
controlling the photocatalytic activity of Au/CeO2 in SPR-
induced photocatalytic reaction.
Table 1 summarizes the external surface area (SAu) calculated
from DAu and NAu. The SAu value of Au nanoparticles in the SS-
and MS-Au(X)/CeO2 samples increased with increasing X. The
SAu values of MS-Au(X)/CeO2 samples were always smaller
than those of SS-Au(X)/CeO2 samples with the same X,
corresponding to the larger DAu values of MS-Au(X)/CeO2
samples.
3.2. Observation of Growth of Au Nanoparticles
Prepared by the MS Photodeposition Method. As stated
above, the NAu value of MS-Au(X)/CeO2 samples was constant
(1.3 × 1012 (g-Au/CeO2)−1) in the range of X = 1.0 to X = 2.0;
however, there was a large gap between MS-Au(0.5)/CeO2
(first 0.5 wt % Au deposition, identical to SS-Au(0.5)/CeO2)
and MS-Au(1.0)/CeO2 prepared by repeating 0.5 wt % Au
deposition (X = 1.0 = 0.5 × 2), i.e., the NAu value of the former
was 8.3 × 1012 (g-Au/CeO2)−1. This large decrease in NAu
suggests that the second added Au source decreased NAu. In
this experiment, the amount of Au added per photodeposition
was reduced to 0.1 wt %, and the effect of a small addition of
Au source on morphology and NAu of MS-Au(X)/CeO2
samples was examined. Figure 2a−d shows TEM photographs
of thus-prepared MS-Au(X)/CeO2 samples (X = 0.5 + 0.1 × n,
n = 0, 2, 3, and 5). The particle size increased with repetition of
photodeposition of 0.1 wt % Au, and contact between two
nanoparticles was observed in the sample of X = 0.8 (n = 3) as
shown in Figure 2c. This behavior is attributed to the decrease
in NAu in a series of MS-Au(X)/CeO2 samples. Changes in NAu
values of the MS-Au(X)/CeO2 samples are shown in Figure 2e.
The NAu values gradually decreased, indicating that contact
between two Au nanoparticles occurred at any n except n = 0
depending on the size of each Au particle at n = 0 and the
distance between two Au nanoparticles. Large decrease in the
NAu value was observed at around X = 0.7 and 0.8 (n = 2 and
3), and contact between Au particles occurred frequently at this
range.
Figure 1. TEM images of (a) SS-Au(1.0)/CeO2, (b) MS-Au(1.0)/
CeO2, and (c) size distributions of Au particles of SS- and MS-Au/
CeO2 samples.
photodeposition method and that there is a tendency for larger
Au nanoparticles to be formed by the MS photodeposition
method. Using SS and MS photodeposition methods, Au(X)/
CeO2 samples having various Au contents (X) were prepared.
Table 1 summarizes the average sizes of Au nanoparticles
determined by TEM (DAu) and number densities of Au
nanoparticles (NAu) calculated from DAu, the density of Au
metal (19.32 g cm−3) and the amount of Au loaded on CeO2
under the assumption that all of the Au nanoparticles were
spherical. The DAu values of Au nanoparticles in the SS- and
MS-Au(X)/CeO2 samples increased with increase in X, and the
Table 1. Various Properties of Au(X)/CeO2 Samples
Prepared by SS and MS Photodeposition Methods
b
c
d
Au loading, DAu
,
NAu, 1012 (g-Au/
SAu,
10−2 m2 (g-
a
method
SS
wt %
nm
CeO2)−1
Au/CeO2)−1
0.1
0.25
0.5
1.0
1.5
2.0
1.0
1.5
2.0
19
29
14
1.6
2.7
4.0
5.3
7.2
9.1
3.4
4.4
5.4
10
39
8.3
4.8
5.6
6.3
1.3
1.3
1.3
59
64
68
MS
92
105
115
a
SS: single-step photodeposition method. MS: multistep photo-
b
deposition method. DAu: average size of Au nanoparticles calculated
from the values of more than 50 particles observed by TEM. In the
case of non-sphere particles such as ellipse, the largest diameter (the
length of the major axis for ellipse) was regarded as the particle size.
c
NAu: number density of Au nanoparticles in g-Au/CeO2. This value
3.3. Photoabsorption Properties of SS- and MS-Au(X)/
CeO2 Samples. Figure 3 shows photoabsorption spectra of
MS- and SS-Au(1.0)/CeO2 samples. Strong photoabsorption
was observed in both spectra at around 550 nm, and the strong
photoabsorption was attributed to SPR of the supported Au
nanoparticles.6,7 It should be noted that MS-Au(1.0)/CeO2
samples exhibited photoabsorption much stronger than that of
SS-Au(1.0)/CeO2 samples, although the amounts of Au (X)
were the same. This result suggests that the intensity of
was calculated from DAu, density of metal (19.32 g cm−2), and the
amount of Au loaded on CeO2 on the assumption that all of the
particles were spheres. In the case of non-sphere particles such as
ellipse particles, the assumption causes overestimation of the particle
d
volume and underestimation of NAu. SAu: external surface area of
supported Au nanoparticles. This value was calculated from the
average size of Au nanoparticles and NAu. The values would be slightly
overestimated because of particle−particle contact and particle-
support contact.
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Figure 4. Kubelka−Munk function of photoabsorption at 550 nm of
■
●
SS-Au(X)/CeO2 ( ) and MS-Au(X)/CeO2 ( ) having various Au
contents (X).
is obvious that SS- and MS-Au(X)/CeO2 samples showed
different tendencies in the plots. The Kubelka−Munk function
at 550 nm of MS-Au(X)/CeO2 samples increased almost
linearly until X = 2.0. The linear correlation between the
Kubelka−Munk function and X in MS-Au(X)/CeO2 samples
indicates that Au nanoparticles fixed on CeO2 effectively
absorbed light corresponding to the amount (mass) of Au, as is
observed for some chemical species in solution that absorb light
corresponding to the concentration. On the other hand, the
Kubelka−Munk function at 550 nm of SS-Au(X)/CeO2
samples tended to be saturated (figure inserted inside). As
shown in Table 1, the DAu values of a series of SS-Au(X)/CeO2
samples were smaller than 70 nm. The smaller particle size
might be one of the reasons for the nonlinearity between SPR-
absorption and X. From these results, we reach the conclusion
that the MS photodeposition method is more effective for
preparation of Au/CeO2 exhibiting strong SPR.
Figure 2. TEM images of MS-Au(X)/CeO2 samples with various Au
contents (X): (a) 0.5 wt %, (b) 0.7 wt % (= 0.5 + 0.1 × 2), (c) 0.8 wt
% (= 0.5 + 0.1 × 3), and (d) 1.0 wt % (= 0.5 + 0.1 × 5). (e) Effect of X
on NAu.
3.4. Photocatalytic Oxidation of Benzyl Alcohol in
Aqueous Suspensions. Figure 5 shows the time courses of
photocatalytic oxidation of benzyl alcohol (33 μmol) in
aqueous suspensions of SS- and MS-Au(1.0)/CeO2 under
irradiation by green light from an LED. We confirmed that
benzyl alcohol was completely consumed after 20 h for SS-
Au(1.0)/CeO2 and after 15 h for MS-Au(1.0)/CeO2, and no
Figure 3. Absorption spectra of SS-Au(1.0)/CeO2 (broken line) and
MS-Au(1.0)/CeO2 (solid line), and light intensity of visible light
irradiated to reaction systems from a green LED (right axis).
photoabsorption due to SPR of Au was affected by the size of
Au nanoparticles (Table 1). Similar results have been obtained
in plasmonic Au/TiO2 photocatalysts7e and unsupported Au
nanoparticles.8 In the latter case, Shimizu et al.8 reported that
such observed features coincide with the prediction of the Mie
theory. As also shown in Figure 3, the photoabsorption
property of Au/CeO2 samples matched the wavelength of light
emitted from a green LED.
Figure 5. Time courses of benzaldehyde formation in photocatalytic
oxidation of benzyl alcohol (33 μmol) in aqueous suspensions of SS-
Figure 4 shows the Kubelka−Munk function at 550 nm of
SS- and MS-Au(X)/CeO2 samples having various values of X. It
■
●
Au(1.0)/CeO2 ( ) and MS-Au(1.0)/CeO2 ( ) under irradiation by
visible light from a green LED (1.7 mW cm−2) in the presence of O2.
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CO2 was detected in both cases during the photoirradiation. It
should be noted that benzaldehyde was formed with quite high
selectivity (>99%) at >99% conversion of benzyl alcohol,
indicating that benzyl alcohol was almost quantitatively
converted to benzaldehyde in the present photocatalytic
reaction system under irradiation by green light from an
LED. Since benzaldehyde increased linearly with photo-
irradiation time, the rates of benzaldehyde formation for SS-
and MS-Au(1.0)/CeO2 samples were determined to be 1.9 and
3.0 μmol h−1, respectively, i.e., MS-Au(1.0)/CeO2 exhibited a
rate of benzaldehyde formation ca. 1.6 times larger than that of
SS-(1.0)Au/CeO2. Intense SPR absorption of MS-Au/CeO2
due to the larger Au particles is one of the main reasons for the
larger activity for benzaldehyde production. Since MS-Au(1.0)/
CeO2 sample exhibited relatively strong photoabsorption even
at around 600−700 nm, photocatalytic reaction of benzyl
examined under irradiation by light from a green LED with
various light intensities (0.4−1.7 mW cm−2) as shown in Figure
7a. Figure 7b shows the effect of light intensity on the rate of
alcohol under irradiation with red light from a red LED (λmax
=
640 nm) was examined. After irradiation for 20 h, 5.2 μmol of
benzyl alcohol was consumed and 5.1 μmol of benzaldehyde
was formed, indicating that MS-Au(1.0)/CeO2 sample worked
for the oxidation of benzyl alcohol even under red light
irradiation.
Photocatalytic oxidation of benzyl alcohol in aqueous
suspensions of SS- and MS-Au(X)/CeO2 samples having
various values of X was examined using a green LED as the
light source, and their rates of benzaldehyde formation were
plotted against their SAu (Figure 6). Linear correlations between
Figure 7. (a) Light intensity of visible light irradiated to reaction
systems from a green LED and effect of light intensity on (b) the rate
of benzaldehyde formation and (c) apparent quantum efficiency
(AQE) in oxidation of benzyl alcohol to benzaldehyde in the presence
■
●
of SS-Au(1.0)/CeO2 ( ) and MS-Au(1.0)/CeO2 ( ).
benzaldehyde formation. The rates increased with increase in
light intensity in both cases, and the rates of MS-Au(1.0)/CeO2
samples were always larger than those of SS-Au(1.0)/CeO2.
Apparent quantum efficiency (AQE) under irradiation by green
light of an LED was determined to be 2.0% and 3.1%,
respectively, at 1.7 mW cm−2, i.e., MS-Au(1.0)/CeO2 exhibited
AQE ca. 1.8 times larger than that of SS-(1.0)Au/CeO2. The
effect of intensity of light on AQE is shown in Figure 7c,
indicating that Au/CeO2 samples worked effectively even under
irradiation by weak green light from the LED. With decrease in
the light intensity, AQE increased, and the value finally reached
4.9% and 2.7% for MS- and SS-Au(1.0)/CeO2 samples,
respectively, at 0.4 mW cm−2, suggesting that Au/CeO2 can
be used in selective oxidation of benzyl alcohol by visible light
from light sources such as an LED and the sun.
Photocatalytic oxidation of p-substituted benzyl alcohol in
aqueous suspensions of SS- and MS-Au(1.0)/CeO2 samples
was investigated. Benzyl alcohol and its derivatives substituted
by electron-donating groups (-NH2, -OCH3, and -CH3) and
electron-withdrawing groups (-Cl and -NO2) were converted to
the corresponding aldehydes with high rates of conversion and
selectivity (>99%) on Au/CeO2, while no other products were
observed. The rate of p-substituted benzaldehyde formation in
both Au/CeO2 samples decreased in the order -NH2 > -OCH3
> -CH3 > -H > -Cl > -NO2, and this order indicates that the
rate of p-substituted benzaldehyde formation is enhanced by
electron-donating substituents and retarded by electron-with-
drawing substituents. These rates showed a reasonably linear
Hammett correlation as shown in Figure 8, and Hammett σ
values of −1.77 (r2 = 0.99) and −2.83 (r2 = 0.99) were
obtained for SS- and MS-Au(1.0)/CeO2 samples, respectively.
Figure 6. Effect of external surface area of Au nanoparticles of SS-
■
●
Au(X)/CeO2 ( ) and MS-Au(X)/CeO2 ( ) on rate of benzaldehyde
formation from aqueous solutions of benzyl alcohol in the presence of
O2 under irradiation by visible light from a green LED (1.7 mW cm−2).
Values in the figure are contents of Au (X).
the rate and SAu were observed in both SS- and MS-Au(X)/
CeO2 samples. It should be noted that the slopes of the plots
were different, and MS-Au(X)/CeO2 samples showed a slope
larger than that of SS-Au(X)/CeO2 samples. These results
suggest that the external surface area of Au loaded on CeO2 and
the intensity of photoabsorption due to SPR of Au nano-
particles are important factors controlling the photocatalytic
activity of Au/CeO2 regardless of the type of photodeposition
method.
Oxidation of benzyl alcohol to benzaldehyde in aqueous
suspensions of SS- and MS-Au(1.0)/CeO2 samples was
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μmol) in aqueous suspensions of MS-Au(1.0)/CeO2 samples
under irradiation by light from a green LED.
Figure 9 shows time courses of photocatalytic oxidation of
ABAL. The amount of ABAL decreased linearly with
Figure 8. Hammett correlation study for photocatalytic oxidation of p-
substituted benzyl alcohols in aqueous suspensions of SS-Au(1.0)/
■
●
CeO2 ( ) and MS-Au(1.0)/CeO2 ( ) in the presence of O2.
On the basis of consideration of the above results, a possible
reaction mechanism for the photocatalytic oxidation of benzyl
alcohol to corresponding aldehydes over Au/CeO2 under
visible light irradiation is shown in Scheme 1. This scheme is
partially derived from the mechanism for electrochemical
oxidation of benzyl alcohols to aldehydes.9
Figure 9. Time courses of the amounts of aminobenzyl alcohol (a),
aminobenzaldehyde (b), nitrobenzyl alcohol (c), nitrobenzaldehyde
◆
(d), and material balance ( , right axis) in aqueous suspensions of
MS-Au(1.0)/CeO2 under irradiation by green light from an LED (1.7
Scheme 1. Assumed Reaction Mechanism for the
Photocatalytic Oxidation of Benzyl Alcohols over Au/CeO2
under Visible Light Irradiation
mW cm−2) in the presence of O2.
photoirradiation, while the amount of p-aminobenzaldehyde
(ABAD) as the oxidation product of ABAL increased. ABAL
was completely consumed after irradiation for 20 h, and no
CO2 was detected during the photoirradiation. Figure 9 clearly
shows that ABAD was formed with quite high selectivity
(>99%) at >99% conversion of ABAL with other oxidation
products, i.e., p-nitrobenzyl alcohol and p-nitrobenzaldehyde,
not being detected and that a high carbon balance (>99%) was
always preserved. These results indicate that ABAL was
chemoselectively and quantitatively converted to ABAD. The
quite high chemoselectivity can be explained by the fact that
ABAD formed was never converted to p-nitrobenzaldehyde
with prolongation of photoirradiation.
Under irradiation by visible light, incident photons are
absorbed by Au particles through their SPR excitation,
electrons may be injected from Au particles into the conduction
band of CeO2, and the resultant electron-deficient Au could
oxidize organic compounds to be recovered to the original
metallic state. The effective oxidation of electron-rich benzyl
alcohol may be ascribed to smooth electron transfer from
benzyl alcohol to electron-deficient Au. Since the present
reaction is performed in the presence of O2, electrons are
trapped by O2 and some active oxygen species would be
After photocatalytic oxidation of p-aminobenzyl alcohol, MS-
Au(1.0)/CeO2 was recovered from the reaction mixture and
was used for the same reaction. As shown in Figure 10, MS-
Au(1.0)/CeO2 worked as a reusable photocatalyst without loss
of its activity and chemoselectivity at least 5 times.
−
formed. The candidates are •O2 radical (O2 + e− → •O2)
and/or H2O2 (O2 + 2e− + 2H+ → H2O2). Multielectron
reduction of O2 is preferred to one-electron reduction of O2 in
photocatalytic oxidations in the presence of metal co-catalysts.
Very high selectivity of benzaldehyde in the present system
suggests that active oxygen species were removed from the
reaction system. Decomposition of H2O2 over Au nanoparticles
was reported by Naya et al.10
3.5. Intramolecular and Intermolecular Chemoselec-
tive Oxidation of Aminobenzyl Alcohol. Chemoselective
oxidation of an alcohol group of molecules that have an easily
oxidized amino group is a challenge. We noticed that a high
aldehyde yield (>99%) was obtained even in the presence of an
amino group that is easily oxidized. Therefore, we examined
photocatalytic oxidation of p-aminobenzyl alcohol (ABAL, 50
The chemoselectivity of Au/CeO2 for the hydroxyl group of
alcohol was further investigated in intermolecular competitive
reaction of benzyl alcohol and aniline (aminobenzene). In
photocatalytic oxidation of an equimolar mixture of benzyl
alcohol and aminobenzene in aqueous suspensions, we found
that MS-Au(1.0)/CeO2 gave only benzaldehyde as the product
from benzyl alcohol after irradiation for 20 h, as shown in
Figure 11. Oxidation of aminobenzene was observed after
benzyl alcohol had been completely consumed. These results
clearly demonstrated that the plasmonic Au/CeO2 photo-
catalyst system under visible light in the presence of O2 showed
complete chemoselective oxidation toward the hydroxyl group
of alcohol in the presence of an inter- and intramolecular amino
group.
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Table 2. Oxidation of Various Alcohols in Aqueous
Suspensions of MS- and SS-Au(1.0)/CeO2 Samples under
Irradiation by Green Light from an LED in the Presence of
a
O2
Figure 10. Chemoselective oxidation of p-aminobenzyl alcohol (50
μmol) to p-aminobenzaldehyde in an aqueous suspension of MS-
Au(1.0)/CeO2.
a
Photocatalytic reactions: Au/CeO2, 50 mg; water, 5 cm3; O2, 1 atm;
b
green LED, 1.7 mW cm−2. Determined by GC using the internal
standard method.
4. CONCLUSIONS
Using the MS and SS photodeposition methods, Au/CeO2
samples with various Au contents were prepared and their
properties including Au particle size, particle dispersion and
photoabsorption were investigated. Larger Au particles with
smaller number were loaded on CeO2 by the MS photo-
deposition method, whereas smaller Au particles with larger
number were loaded by the SS photodeposition method. The
MS-Au/CeO2 samples exhibited very strong photoabsorption
at around 550 nm due to surface plasmon resonance of Au
nanoparticles, and the strong photoabsorption was attributed to
the large Au particles. On the other hand, photoabsorption of
the SS-Au/CeO2 sample was weak, although Au content was
the same. Different behaviors were also observed in effects of
Au content on photoabsorption; the Kubelka−Munk function
of the photoabsorption of MS-Au/CeO2 samples linearly
increased with increase in Au content up to 2.0 wt % with
constant Au particle number, while the Kubelka−Munk
function of photoabsorption of the SS-Au/CeO2 sample was
saturated even at around 0.5 wt %. Both the MS- and SS-Au/
CeO2 samples exhibited high selectivity in oxidation of benzyl
alcohol to benzaldehyde in aqueous suspensions under visible
light irradiation from a green LED, i.e., benzaldehyde was
quantitatively formed in both cases. The formation rates by
MS-Au/CeO2 samples were ca. twice larger than those by SS-
Au/CeO2 samples, and apparent quantum efficiency of the MS-
Au(1.0)/CeO2 sample reached 4.9% at 0.4 mW cm−2. The
strong photoabsorption of MS-Au/CeO2 samples was attribut-
able to the large reaction rate. In both cases of MS- and SS-Au/
CeO2 samples, linear correlations were observed between the
reaction rates and SAu, although the slopes of the rate vs SAu
plots were different, indicating that the external surface area of
Au nanoparticles rather than the Au content is one of the
important factors controlling the reaction rate of Au/CeO2
under visible light irradiation. Au/CeO2 exhibited high
chemoselectivity in both selective oxidation of aminobenzyl
Figure 11. Intermolecular competitive oxidation of benzyl alcohol (a)
and aniline (aminobenzene) (b) to benzaldehyde (c) and nitro-
benzene (d), respectively, in aqueous suspensions of MS-Au(1.0)/
CeO2 under irradiation by green light from an LED (1.7 mW cm−2) in
the presence of O2.
3.6. Applicability of Au/CeO2 for Selective Oxidation
of Alcohols. To expand the applicability of Au/CeO2 for
selective oxidation, photocatalytic oxidation of various alcohols
other than benzyl alcohols in aqueous suspensions of SS- and
MS-Au(1.0)/CeO2 was examined under irradiation by green
light from an LED. As shown in Table 2, SS- and MS-Au(1.0)/
CeO2 photocatalysts were also effective in oxidation of
secondary aliphatic alcohol (2-pentanol), alicyclic alcohol
(cyclohexanol), secondary aromatic alcohol (1-phenylethanol),
and primary aromatic alcohol (2-phenylethanol) to correspond-
ing aldehydes or ketones with quite high selectivities larger than
99%, indicating that the plasmonic Au/CeO2 photocatalyst can
be used for selective oxidation of various alcohol compounds.
In all reaction systems, MS-Au(1.0)/CeO2 exhibited activities
higher than those of SS-Au(1.0)/CeO2 as well as oxidation of
benzyl alcohol.
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AUTHOR INFORMATION
Corresponding Author
hiro@apch.kindai.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partly supported by a Grant-in-Aid for Scientific
Research (No. 23560935) from the Ministry of Education,
Culture, Sports, Science, and Technology (MEXT) of Japan.
The authors (H.K. and A.T.) are grateful for financial support
from Iketani Science and Technology Foundation.
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