Solvent-free selective photocatalytic oxidation of benzyl alcohol over modified TiO2

Article abstract of DOI:10.1039/c1gc15595d

Heterogeneous photocatalysis offers a promising route to realize green oxidation processes in organic synthesis. In this research, the solvent-free selective photocatalytic oxidation of benzyl alcohol to benzaldehyde in the presence molecular oxygen was s

Full text of DOI:10.1039/c1gc15595d

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PAPER
Solvent-free selective photocatalytic oxidation of benzyl alcohol over
modified TiO2
Wei Feng, Guangjun Wu, Landong Li* and Naijia Guan
Received 24th May 2011, Accepted 11th August 2011
DOI: 10.1039/c1gc15595d
Heterogeneous photocatalysis offers a promising route to realize green oxidation processes in
organic synthesis. In this research, the solvent-free selective photocatalytic oxidation of benzyl
alcohol to benzaldehyde in the presence molecular oxygen was studied by using TiO
TiO as photocatalysts. The surface modification of TiO by transition metal clusters dramatically
enhanced the photocatalytic oxidation activity. Ir/TiO prepared by photodeposition showed a
remarkably high activity for the photocatalytic oxidation of benzyl alcohol, and an average
2
and modified
2
2
2
-
1
-1
reaction rate of 14538 mmol h
loading and reaction conditions on the catalytic performance of Ir/TiO
Based on the catalytic and characterization results, the problem of product selectivity and the
reaction mechanism of the photocatalytic oxidation of benzyl alcohol over Ir/TiO are discussed.
g
cat could be obtained. The effect of preparation method, iridium
2
is investigated in detail.
2
Introduction
conditions, and oxygen is involved in the reaction instead of
toxic and corrosive oxidants. Therefore, heterogeneous photo-
catalysis offers an alternative green oxidation process. Although
the efficiency of heterogeneous photocatalysis is not as high
as conventional heterogeneous catalysis (non-photocatalysis),
heterogeneous photocatalysis still attracts great attention for its
chemical utilization of solar energy. Heterogeneous catalysis is
considered to be green chemistry with minimal environmental
The selective oxidation of alcohols to carbonyl compounds is
one of the most important organic transformations, which is of
great interest not only to fundamental organic synthesis but also
to the fine chemical industry. Conventionally, stoichiometric
oxidants, e.g. dichromate and permanganate, are employed in the
process of alcohol oxidation. These oxidants are usually toxic
and/or expensive, and large amounts of waste are produced
together with the desired products. To address this concern,
it has been proposed to use molecular oxygen as a clean and
1
,2
impact. TiO is a well known and applicable semiconductor
2
photocatalyst due to its chemical and physical durability, non-
15
toxicity and commercial availability. Up to now, TiO
2
has
3
cheap oxidant to realize a so-called green oxidation process.
been employed as a possible catalyst in various selective pho-
4–6
7–9
Numerous catalyst systems, e.g. Au and Pd catalysts, have
been developed for the selective oxidation of alcohols with
molecular oxygen as the oxidant.
Heterogeneous photocatalysis, which has been known since
the 1970s, has drawn great attention in past decades. In hetero-
geneous photocatalysis, holes in the valence band and electrons
in the conducting band are created when the semiconductor
is illuminated by photons of energy higher than its band gap.
Since the valence holes are powerful oxidants, they have been
extensively used in the degradation of organic pollutants.
Recently, the application of heterogeneous photocatalysis in
organic synthesis, e.g. most commonly in selective oxidation,
has been highlighted.
16–22
tocatalytic oxidation reactions under different conditions.
In the work of Mohamed et al., anatase TiO was employed
2
in the photocatalytic oxidation of 1-phenylethanol in dry
acetonitrile under UV irradiation, and the rate-determining step
was hypothesized to be the reaction between the superoxide and
16
the alcohol radical cation over the surface. Palmisano et al.
prepared rutile TiO at low temperature and employed it in
2
the photocatalytic oxidation of aromatic alcohols to aldehydes
in water suspensions under UV irradiation. The crystallinity
of the sample appeared to be a key factor controlling the
1
0,11
17
product selectivity, while the introduction of substituents into
the aromatic alcohols also changed the photocatalytic oxidation
12–14
Generally, heterogeneous photocat-
18
rate and product selectivity. More recently, Higashimoto et
alytic oxidation reactions are performed under mild reaction
al. reported that the photocatalytic oxidation of benzyl alcohol
and its derivatives into the corresponding aldehydes could be
achieved over TiO
UV and visible light.
was attributed to the characteristic surface complex formed by
2
in acetonitrile under the irradiation of both
19,20
The visible light response of TiO
2
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of
Education), College of Chemistry, Nankai University, Tianjin, 300071,
P. R. China. E-mail: lild@nankai.edu.cn; Fax: +86 22-23500341;
Tel: +86 22-23500341
the adsorption of the benzyl alcoholic compound on the TiO
2
19
surface.
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Despite the significant research on selective photocatalytic
oxidation reactions, several very important issues remain to be
solved. First of all, the rates of most photocatalytic oxidation
reactions are very low and far from sufficient for practical
applications. Moreover, the photogenerated holes formed in
photocatalytic oxidation processes have a high oxidative poten-
tial and may oxidize organic species indiscriminately, leading to
noted as Pd-Ir/TiO
also prepared by the co-instantaneous wet impregnation and
chemical reduction method. The final products were denoted as
2
-p. Bimetallic Pd-Ir/TiO samples were
2
Pd-Ir/TiO
2
-i and Pd-Ir/TiO -c, respectively.
2
Sample characterization
24
Transmission electron microscopy (TEM) images of samples
were acquired by a Tecnai G 2010 S-TWIN transmission
electron microscope at an accelerating voltage of 200 kV.
Diffuse reflectance ultraviolet-visible (UV-vis) spectra were
recorded in air against BaSO in the region 200–700 nm on a
poor product selectivity. For an ideal green oxidation process,
an environmentally benign catalyst with a high activity and
selectivity should be involved. Meanwhile, the oxidation reaction
should be performed under mild conditions and be free of
2
25,26
4
solvents or additives. However, with a few exceptions,
most
Varian Cary 300 UV-vis spectrophotometer.
selective photocatalytic oxidation reactions are performed in
water suspensions or even in organic solvents.
In the present study, a series of transition metal-modified
Photoluminescence (PL) spectra were recorded on a Spex
FL201 fluorescence spectrophotometer. Samples were dry-
pressed into self-supporting wafers and then illuminated by
a 325 nm He-Cd laser as the excitation source at ambient
temperature.
X-Ray photoelectron spectra (XPS) of samples were recorded
on a Kratos Axis Ultra DLD spectrometer with a pass energy
of 20 eV and a monochromated Al-Ka X-ray source. Accurate
binding energies (±0.1 eV) were determined with respect to the
position of the adventitious C 1 s peak at 284.8 eV.
TiO
free selective oxidation of benzyl alcohol. The newly-developed
Ir/TiO catalyst appears to be a promising choice to realize
selective photocatalytic oxidation processes. Based on the com-
prehensive physicochemical characterization of Ir/TiO , the
modification effects of iridium on TiO are discussed and the
2
materials are examined as photocatalysts for the solvent-
2
2
2
mechanism of the photocatalytic oxidation is proposed.
Experimental methods
Photocatalytic oxidation of benzyl alcohol
Preparation of M/TiO
2
catalyst
The catalytic oxidation of benzyl alcohol was performed in
a double-walled quartz cell cooled by water with a 250 W
high pressure Hg lamp (315–420 nm, main wavelength at
Commercial TiO (Degussa P25, 70% anatase, 30% rutile) was
2
used as a support and M/TiO (M Ag, Au, Pd, Pt and
Ir) catalysts were prepared by the so-called photodeposition
2
3
65 nm) as the light source. In each experiment, ca. 0.3 g
catalyst was dispersed in 0.3 mol benzyl alcohol in a quartz
cell under stirring. The suspension was then irradiated from
27,28
method.
In a typical synthesis of Ir/TiO
2
, a certain amount
and 10 mL of
of a 2 mM H
2
IrCl solution, 500 mg of TiO
6
2
-1
inside, with oxygen bubbled in at ca. 20 mL min . The organic
ethanol were added into a round-bottomed quartz flask under
vigorous stirring to form a slurry. The slurry was adjusted to
pH 11 ± 0.5 using either a 1 M HCl or a 1 M NaOH aqueous
solution and irradiated by a high pressure mercury light with
a main wavelength of 365 nm for 6 h under the protection
of nitrogen. After irradiation, the particles were filtered, dried
products were analyzed by GC (Shimadzu GC-2010 Plus)
and GC-MS (Shimadzu GCMS-QP2010 SE). Meanwhile, an
absorption apparatus containing a saturated Ba(OH)
was equipped downstream of the quartz cell to determine the
quantity of CO that might have formed during the reaction.
2
solution
2
under ambient conditions, reduced by 5 vol% H
2
/Ar at 573 K
Results and discussion
for 2 h and denoted as Ir/TiO -p. Ir/TiO samples with similar
2
2
Ir loadings were also prepared by the wet impregnation and
chemical reduction method. For wet impregnation, 50 mL of a
Photocatalytic oxidation of benzyl alcohol over M/TiO
2
2
mM H
2
IrCl
6
solution was added to 500 mg of TiO
2
, and then
The results of the selective photocatalytic oxidation of benzyl
alcohol over TiO and modified TiO are shown in Table 1.
No detectable oxidation reaction took place under irradiation
without catalyst, and therefore the homogeneous photochemical
oxidation contribution will be neglected in further discussions.
the mixture was evaporated in a rotary evaporator at a constant
temperature of 353 K. The as-obtained particles were carefully
washed with de-ionized water, dried under ambient conditions,
2
2
reduced by 5 vol% H
Ir/TiO -i. For chemical reduction, 500 mg of TiO
of a 2 mM H IrCl solution were added into a round-bottomed
quartz flask under stirring to form a slurry. Then, 10 mL of a
M KBH solution was dropwise added to the slurry under
2
/Ar at 573 K for 2 h and denoted as
2
2
and 50 mL
TiO
2
exhibits a very low activity in the photocatalytic oxidation
-
1
2
6
of benzyl alcohol, with a reaction rate of 325 mmol h
-
1
g
cat . The main oxidation products are carbonyl compounds
(benzaldehyde, benzoic acid and benzylbenzoate), while small
amounts of CO and other unknown products could be detected.
To enhance the photocatalytic activity of TiO , a series of
1
4
the protection of nitrogen. The particles were filtered, washed
with de-ionized water and dried under ambient conditions. After
2
2
reduction by 5 vol% H
denoted as Ir/TiO -c.
Bimetallic Pd-Ir/TiO
photodeposition process, and a mixture of H
solution was used for the photodeposition instead of a H
2
/Ar at 573 K for 2 h, the product was
transition metals were deposited on the surface. As expected,
certain noble metals show dramatic promotion effects on the
photocatalytic oxidation reaction. The work function of noble
2
2
was prepared by the co-instantaneous
2
IrCl
6
and PdCl
IrCl
2
metals are higher than that of TiO
electrons tend to migrate from TiO
2
, and the photogenerated
to metal clusters and be
2
6
2
solution. After reduction post-treatment, the sample was de-
trapped therein. In such a way, the deposition of metal clusters
3
266 | Green Chem., 2011, 13, 3265–3272
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a
Table 1 Selective photocatalytic oxidation of benzyl alcohol over modified TiO
2
Product selectivity (%)
g
Reaction rate /
b
c
d
e
f
-1
-1
Catalyst
TiO
Ag/TiO
Au/TiO
Metal loading (%) Metal size /nm Work function Conversion (%) Benzaldehyde Carbonyls Others mmol h g
cat
2
/
/
/
4.26
5.1
5.12
5.65
4.98
5.27
5.27
0.2
2.4
6.4
3.3
4.1
8.7
8.9
0
50
81
81
79
81
83
92
/
92
97
98
99
98
96
99
/
8
3
2
1
2
4
1
/
325
2
-p 3.5
-p 3.6
2.8
2.8
-p 3.2
2.8
5.7
3.6
3.3
3.1
2.9
2.9
3925
10475
5363
6688
14012
14538
0
2
Pt/TiO
2
-p
-p
Pd/TiO
Rh/TiO
2
2
Ir/TiO
Ir/TiO
2
-p
-p
3.5
3.5
h
2
a
b
c
Reaction conditions: catalyst 0.3 g, benzyl alcohol 0.3 mol, reaction T 333 K, reaction time 6 h. Analyzed by ICP. Average size observed by
d
e
f
2
TEM. Work function of modified metals, data from ref. 29. Including benzaldehyde, benzoic acid and benzylbenzoate. Including CO and other
unknown products. Rate of benzyl alcohol oxidation per unit of M/TiO catalyst per unit time. Without irradiation.
2
g
h
on the surface of TiO
erated electron–hole pairs and thus enhance the photocatalytic
activity of TiO . Meanwhile, the deposition of noble metal
clusters on the TiO surface may facilitate the adsorption and
activation of oxygen, which is good for the involvement of
molecular oxygen in photocatalytic oxidation processes. Among
2
can promote the separation of photogen-
chemical reduction and photodeposition, were adopted for the
preparation of Ir/TiO , and quite different results were ob-
tained. As seen in Table 2, with similar iridium loadings, Ir/TiO
prepared by photodeposition (Ir/TiO -p) exhibited the highest
photocatalytic activity, followed by Ir/TiO -i and then Ir/TiO
c. Photodeposition appears to be a more feasible method to
2
2
2
2
2
2
2
-
27,31
all the M/TiO
2
photocatalysts studied, Ir/TiO
2
, a rarely-studied
introduce modifiers, consistent with literature reports.
The TEM images of Ir/TiO prepared by different methods
are shown in Fig. 1, and it is seen that the iridium species tend
to form small clusters on the surface of the TiO . Typically,
homogeneous clusters of 2–4 nm, with an average size of 2.9 nm,
are observed to evenly disperse on the TiO support in Ir/TiO -p.
Similarly, clusters of 1.5–4.5 nm, with an average size of 3.2 nm,
are observed on the TiO support in Ir/TiO -i. Meanwhile for
Ir/TiO -c, larger non-uniform clusters of 2–6 nm are observed
example, exhibits the best catalytic performance with a high
2
-
1
-1
reaction rate of 14538 mmol h
than TiO
without irradiation, proving that all the products are derived
from the photocatalytic oxidation process. Ir/TiO appears to
g
cat (more than 40 times higher
2
) being obtainable. No oxidation reaction took place
2
2
2
2
be a very promising catalyst for the selective photocatalytic
oxidation of benzyl alcohol, and therefore a detailed study on
2
2
the modification effect of iridium on TiO
the following section.
2
will be presented in
2
on the support.
Fig. 2 shows the diffuse reflectance UV-vis spectra of TiO and
2
Ir/TiO
in the UV region, attributed to the band–band transition, is
observed for TiO P25. Besides the absorption in the UV
region, obvious absorption in the visible range is observed in
Ir/TiO -i and Ir/TiO -p, which should be related to oxygen
vacancy formation and the gap states introduced by iridium
2
prepared by the different methods. A strong absorption
Photocatalytic oxidation of benzyl alcohol over Ir/TiO
2
It is generally acknowledged that the efficiency of metal mod-
ification on TiO depends on the method to introduce metals
2
2
30
and the physicochemical properties of the final materials. In
the present study, the usual methods, e.g. wet impregnation,
2
2
a
Table 2 Selective photocatalytic oxidation of benzyl alcohol over Ir/TiO
2
Product selectivity (%)
Metal loading (%) Metal size /nm Reaction T/K Conversion (%) Benzaldehyde Carbonyls
f
Reaction rate /
mmol h g
cat
b
c
d
e
-1
-1
Catalyst
Others
Ir/TiO
Ir/TiO
Ir/TiO
Ir/TiO
Ir/TiO
Ir/TiO
Ir/TiO
Ir/TiO
Ir/TiO
Ir/TiO
Ir/TiO
2
2
2
2
2
2
2
2
2
2
2
-i
-c
3.8
3.6
3.5
3.5
3.5
3.5
3.5
3.5
0.7
1.9
6.6
3.2
4.3
2.9
2.9
2.9
2.9
2.9
2.9
2.2
2.5
3.7
333
333
333
333
333
293
313
353
333
333
333
4.5
1.8
8.9
6.1
11.5
3.0
4.2
10.5
5.5
6.8
3.6
89
93
92
93
90
65
80
78
96
93
92
98
99
99
99
99
100
99
97
99
99
98
2
1
1
1
1
0
1
3
1
1
2
7363
2925
14538
14952
14087
4851
6875
17203
9009
11138
5863
-p
-p
-p
-p
-p
-p
-p
-p
-p
g
h
a
b
c
General reaction conditions: catalyst 0.3 g, benzyl alcohol 0.3 mol, reaction time 6 h. Analyzed by ICP. Average size observed by TEM.
d
e
f
Including benzaldehyde, benzoic acid and benzylbenzoate. Including CO
2
and other unknown products. Rate of benzyl alcohol oxidation per unit
g
h
of catalyst per unit time. Catalyst 0.2 g, benzyl alcohol 0.3 mol, reaction time 6 h. Catalyst 0.4 g, benzyl alcohol 0.3 mol, reaction time 6 h.
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2
Fig. 1 TEM images and cluster size distributions of Ir/TiO prepared
by different methods.
Fig. 3 PL spectra of TiO
2
and Ir/TiO
2
prepared by different methods.
signals become much weaker with upon deposition of iridium on
the TiO surface, indicating that the iridium surface modification
2
can suppress both surface and bulk irradiative recombination.
This reasonably leads to a higher photocatalytic activity since the
photocatalytic reaction is evoked by photogenerated electrons
and holes. It is also observed that the different preparation
methods of Ir/TiO
the recombination of photogenerated electrons and holes. The
best suppression effect was obtained by Ir/TiO prepared by
photodeposition, followed by Ir/TiO -i and then Ir/TiO -c.
Based on the results of the UV-vis and PL spectra, we conclude
that the surface modification of TiO by iridium can change
the properties and surface states of the TiO semiconductor,
probably through iridium–TiO electronic interactions. The way
of introducing iridium to the surface of TiO , e.g. the preparation
2
lead to different suppression effects for
2
2
2
2
2
2
2
method, is a very important factor influencing modification
effects. Photodeposition appears to be a more feasible method,
which accordingly leads to better photocatalytic activities.
2 2
Fig. 2 Diffuse reflectance UV-vis spectra of TiO and Ir/TiO prepared
by different methods.
For Ir/TiO
iridium shows a distinct influence on the photocatalytic activ-
ity. The activity of Ir/TiO increases with increasing iridium
loading from 0.7 to 3.5%, reaching a maximal reaction rate of
2
prepared by photodeposition, the loading of
modification. It should be mentioned that the light absorption
of TiO at 315–420 nm (wavelength of the high-pressure Hg
2
2
-
1
-1
lamp employed in the photocatalytic reaction) is enhanced by the
iridium modification, and the order of the strength of absorption
14538 mmol h gcat , and then declines sharply with the iridium
loading further increased to 6.6%. The UV-vis spectra in Fig. 4
was observed to be Ir/TiO
2
-p > Ir/TiO
2
-i > Ir/TiO -c.
2
show that the maximal light absorption in the 315–420 nm region
Photoluminescence spectroscopy is a very useful approach for
disclosing the efficiency of charge carrier trapping, immigration
and transfer in semiconductors. Generally, photoluminescence
emission originates from the radiative recombination of photo-
generated electrons and holes. The room temperature emission
is obtained on Ir/TiO
spectra in Fig. 5 reveal that the photoluminescence intensity
of Ir/TiO -p, which represents the irradiative recombination of
photogenerated electrons and holes, decreases with increasing
iridium loading and then arrives at a minimum at an iridium
loading of 3.5%. The photoluminescence intensity increases
inversely with a further iridium loading increase to 6.6%. The
heavy iridium loading of 6.6% results in excess iridium clusters
2
-p at an iridium loading of 3.5%. The PL
2
PL spectra of TiO
methods are shown in Fig. 3. For TiO
2
and Ir/TiO
2
prepared by the different
P25, photoluminescence
2
signals at wavelengths of 400–500 nm are observed. Typically,
the signal at ca. 420 nm is attributed to the surface irradiative
recombination, while the signal at ca. 460 nm is attributed to
with larger sizes on the TiO surface, which may provide new sites
2
for the irradiative recombination of photogenerated electrons
and holes. The UV-vis and PL spectra clearly indicate that the
32,33
bulk irradiative recombination.
Both the photoluminescence
3
268 | Green Chem., 2011, 13, 3265–3272
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oxidation reactions performed in benzotrifluoride or acetonitrile
∑
solvent, where the generation of OH radicals and non-selective
19,23
autooxidation can be avoided.
of over 90% to benzaldehyde can be obtained for benzyl alcohol
photocatalytic oxidation over Ir/TiO -p after a time-on-stream
In this work, a high selectivity
2
of 6 h when the conversion is below 10%. Further extension
of the reaction time leads to an obvious change in the product
selectivity, as shown in Fig. 6. It is seen that the benzyl alcohol
conversion gradually increases to ca. 45% within a time-on-
stream of 40 h. The selectivity to benzaldehyde decreases with
increasing benzyl alcohol conversion, while the selectivity to
benzylbenzoate increases. Meanwhile, the selectivity to benzoic
acid and CO is kept at a low level of below 5% during the whole
2
reaction process. The experimental results from the photocat-
alytic oxidation of pure benzaldehyde indicate that benzaldehyde
can be easily oxidized to benzoic acid over Ir/TiO
2
-p with a
-
1
-1
reaction rate of ca. 30687 mmol h
gcat under similar reaction
conditions. However, we cannot observe the accumulation of
benzoic acid because the formed benzoic acid readily reacts
with benzyl alcohol to produce benzylbenzoate via a possible
photochemical process. Based on the time-dependent product
selectivity during the photocatalytic oxidation of benzyl alcohol,
it is easily concluded that the complete oxidation of benzyl
2
Fig. 4 Diffuse reflectance UV-vis spectra of Ir/TiO -p with different
iridium loadings.
alcohol to CO can be efficiently suppressed under solvent-free
2
∑
conditions when the formation of OH radicals can be avoided.
Meanwhile, the oxidation of formed benzaldehyde to benzoic
acid and subsequent esterification cannot be avoided, especially
at high levels of benzyl alcohol conversion. The formation of
acids and esters should be a common problem for the selective
photocatalytic oxidation of primary alcohols. If carbonyl com-
pounds (benzaldehyde, benzoic acid and benzylbenzoate) are
calculated as the desired products, a high selectivity of >95%
can be obtained, even at very high levels of benzyl alcohol
2
Fig. 5 PL spectra of Ir/TiO -p with different iridium loadings.
optimal iridium loading for surface modification on TiO
.5%, consistent with the photocatalytic activities.
2
is ca.
3
Product selectivity in photocatalytic oxidation reactions
In past decades, photocatalytic oxidation processes have been
extensively applied to the degradation of organic pollutants
10,11
to carbon dioxide.
To develop photocatalytic oxidation as
a green oxidation route for organic synthesis, a key point
is to achieve a high selectivity towards the desired product.
Meanwhile for the selective photocatalytic oxidation of alcohols
in water suspensions, a rather poor selectivity to carbonyl
compounds is obtained due to the generation of non-selective
Fig. 6 Benzyl alcohol conversion and product selectivity during
the selective photocatalytic oxidation of benzyl alcohol over Ir/TiO2
prepared by photodeposition up to a time-on-stream of 40 h.
∑
17,18
OH radicals.
Recently, it has been reported that high
-catalyzed photocatalytic
selectivity can be achieved in TiO
2
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conversion. It should be mentioned that for the photocatalytic
oxidation of secondary alcohols, a perfect selectivity to ketones
can be obtained since the further oxidation of ketones can be
easily avoided. For example in the experiment of a-phenylethyl
alcohol photocatalytic oxidation, a reaction rate of 17289
binding energy values at 60.3, 61.8, 63.2 and 64.6 eV are observed
for Ir/TiO before the reaction. Thebindingenergyvalues at60.3
2
and 63.2 eV are assigned to 4f7/2 and 4f5/2 of metallic iridium,
while those at 62.0 and 65.0 eV are assigned to 4f7/2 and 4f5/2 of
34
iridium oxides. It is clear seen that both metallic iridium and
iridium oxides exist in Ir/TiO prepared by photodeposition.
-
1
-1
mmol h
gcat and >99% selectivity to acetophenone can be
2
obtained over Ir/TiO
2
under reaction conditions similar to
After the photocatalytic reaction, the binding energy values
assignable to iridium oxides disappear, indicating the reduction
of iridium oxides to metallic iridium.
Further experiments reveal that iridium oxides cannot be
reduced by benzyl alcohol without irradiation, and therefore
iridium oxides should be reduced by photogenerated electrons
during the irradiation.
those employed for benzyl alcohol oxidation. From this point
of view, the selective photocatalytic oxidation of secondary
alcohols might be a more promising green oxidation reaction
for applications.
Mechanism for the photocatalytic oxidation of benzyl alcohol
In the Ti 2p region, binding energy values at 458.5 and
The changes in state of Ir/TiO
catalytic oxidation of benzyl alcohol were investigated by means
of XPS, and the results are shown in Fig. 7. In the Ir 4f region,
2
-p before and after the photo-
4+
4
64.2 eV, assignable to 2p3/2 and 2p1/2 of Ti in TiO , are
2
observed before the reaction. The appearance of binding energy
3+ 35
values at 456.2 and 461.8 eV, assignable to Ti , indicates
4
+
3+
the partial reduction of Ti to Ti during the photocatalytic
reaction. In the O 1 s region, binding energy values at 529.8,
5
31.9 and 533.3 eV are observed in Ir/TiO
2
I
-p before the reaction.
) is assigned to crystal
The binding energy value at 529.8 eV (O
lattice oxygen in O-Ti , while the binding energy values at 531.9
L
4
+
I
II
(
O
C
) and 533.3 eV (O ) are assigned to chemisorbed oxygen
C
36,37
in hydroxyl-like groups.
new binding energy value at 530.7 eV (O
oxygen in O-Ti , appears, which confirms the reduction of Ti
in Ir/TiO after the reaction. Meanwhile, the binding energy
value assignable to chemisorbed oxygen (O
After the photocatalytic reaction, a
II
L
), assignable to lattice
3
+
38
3+
2
II
C
) changes slightly
from 533.3 to 533.0 eV, probably related to the change in the
existence states of iridium.
A primary reaction scheme for the selective photocatalytic
oxidation of benzyl alcohol over Ir/TiO
in Fig. 8. Under ultraviolet irradiation, electron–hole pairs are
created on the semiconductor Ir/TiO . Since the work function
of iridium is higher than that of TiO , the photogenerated
electrons tend to migrate from TiO to the iridium clusters,
whereas the photogenerated holes are left in the TiO . The
2
is proposed, as shown
2
2
2
2
photogenerated electrons are trapped in the iridium clusters
and result in the reduction of iridium oxides to metallic iridium
species, as proved by the XPS results. During the photocatalytic
reaction, the benzyl alcohol molecules are adsorbed onto the
surface of the TiO
2
and react with the photogenerated holes
to produce carbon radicals, accompanied by the reduction
IV
III
of Ti in TiO to Ti through electron transfer. Molecular
2
Fig. 7 X-Ray photoelectron spectra of Ir/TiO
position before and after the photocatalytic oxidation of benzyl alcohol.
2
prepared by photode-
Fig. 8 Mechanism for the selective photocatalytic oxidation of benzyl
alcohol over Ir/TiO
2
.
3
270 | Green Chem., 2011, 13, 3265–3272
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oxygen prefers to be adsorbed onto the iridium clusters and
easily dissociates to chemisorbed oxygen atoms due to the
Ir/TiO
chemical reduction also showed higher activities than Ir/TiO
prepared by the same methods (Table 2 and Table 3). That is
to say, palladium is a universal effective modifier to Ir/TiO
no matter which preparation method is employed. UV-vis
and PL spectra results (not shown here) revealed that the
addition of palladium did not change the light absorption,
and recombination of photogenerated electrons and holes under
irradiation. Therefore, the modification effects of palladium on
2
prepared by co-instantaneous wet impregnation and
2
39
low dissociation barrier. The chemisorbed oxygen atoms can
diffuse to the adjacent TiO surface, where they react with
carbon radicals to produce benzaldehyde as well as oxidize Ti
2
2
,
III
IV
III
to Ti . Meanwhile, for the carbon radicals and the Ti located
far away from the iridium clusters, they can only be oxidized by
molecular oxygen. Because the oxidative reactivity of molecular
oxygen is much weaker than that of chemisorbed oxygen atoms,
the reaction between carbon radicals and chemisorbed oxygen
atoms make a major contribution to the total photocatalytic
oxidationactivity. Since molecular oxygen prefers to be adsorbed
onto the iridium clusters, the anaerobic photocatalytic oxidation
Ir/TiO are hypothesized to be related with the promotion of
2
oxygen activation and the enhancement of oxygen mobility
during the photocatalytic reaction. Further research on this
topic is in progress.
of benzyl alcohol on TiO may also proceed via a two-electron
2
23
transfer process, as suggested by Zhang et al. In the whole
III
photocatalytic process, part of the Ti species cannot be re-
Conclusions
IV
oxidized to Ti , and therefore can be detected by means of XPS.
The solvent-free selective photocatalytic oxidation of benzyl
alcohol in the presence of molecular oxygen can be achieved
Based on the characterization and catalytic results, the
modification effects of iridium on TiO
following two aspects. On the one hand, the deposition of
iridium clusters on the surface of TiO efficiently suppresses
the recombination of photogenerated holes and electrons in
the TiO semiconductor, which is certainly good for various
photocatalytic processes. On the other hand, the introduction
of iridium clusters onto TiO especially favors the activation of
2
are ascribed to the
using TiO
alysts under ultraviolet irradiation. The surface modification
of TiO by transition metal clusters can dramatically enhance
2
and transition metal-modified TiO as photocat-
2
2
2
its photocatalytic performance. The type of transition metal,
preparation method, metal loading and reaction conditions all
show obvious impacts on the photocatalytic oxidation of benzyl
alcohol. Typically, a high benzyl alcohol photocatalytic oxida-
tion activity as well as a high selectivity to carbonyl compounds
2
2
molecular oxygen and therefore promotes the participation of
molecular oxygen in selective photocatalytic oxidation reactions.
can be obtained on Ir/TiO
deposition of iridium clusters on the surface of TiO
the recombination of photogenerated holes and electrons in the
TiO semiconductor, which is good for photocatalytic reactions.
Moreover, the existence of iridium clusters on TiO especially
2
prepared by photodeposition. The
2
can suppress
Photocatalytic oxidation of benzyl alcohol over Pd-Ir/TiO
2
Ir/TiO prepared by photodeposition shows a promising activity
2
2
for the photocatalytic oxidation of benzyl alcohol, and we
have therefore tried to improve the photocatalytic activity of
2
favors the activation of molecular oxygen and greatly promotes
the participation of molecular oxygen in selective photocatalytic
oxidation reactions.
In the photocatalytic oxidation of benzyl alcohol, the reaction
between carbon radicals and chemisorbed oxygen atoms make a
major contribution to the total photocatalytic oxidation activity,
while the reaction between carbon radicals and molecular
oxygen, as well as the anaerobic photocatalytic oxidation of
Ir/TiO
screening, we found that only palladium appears to be an
effective second modifier of Ir/TiO , and Pd-Ir/TiO prepared
2
-p by introducing a second metal. After comprehensive
2
2
by co-instantaneous photodeposition showed a distinctively
improved activity for the photocatalytic reaction. It was found
that the Pd/Ir ratio greatly influenced the photocatalytic activity
of Pd-Ir/TiO
2
-p. As seen in Table 3, the optimal Pd : Ir ratio
-
1
was 1 : 1, where a very high reaction rate of 41713 mmol h
benzyl alcohol, on TiO may also proceed.
Under solvent-free conditions, the formation of OH radicals
is suppressed, and therefore the complete oxidation of benzyl
2
-
1
∑
g
cat
(ca. three-times higher than Ir/TiO
2
-p) with an 82%
selectivity to benzaldehyde could be obtained. Meanwhile, Pd-
a
Table 3 Selective photocatalytic oxidation of benzyl alcohol over Pd-Ir/TiO
2
Product Selectivity (%)
g
Reaction rate /
mmol h g
cat
b
c
d
e
f
-1
-1
Catalyst
Metal loading (%)
Pd : Ir
Metal size /nm Conversion (%) Benzaldehyde
Carbonyls
Others
Pd-Ir/TiO
Pd-Ir/TiO
Pd-Ir/TiO
Pd-Ir/TiO
Pd-Ir/TiO
Pd-Ir/TiO
Pd-Ir/TiO
2
2
2
2
2
2
2
-i
Pd: 1.9; Ir: 1.8
Pd: 1.8; Ir: 1.8
Pd: 1.8; Ir: 1.7
Pd: 3.2; Ir: 0.6
Pd: 2.6; Ir: 1.2
Pd: 1.2; Ir: 2.4
Pd: 0.6; Ir: 3.1
1 : 1
1 : 1
1 : 1
5 : 1
4 : 2
2 : 4
1 : 5
4.1
3.9
3.1
2.7
2.9
3.1
3.2
5.2
4.5
25.6
12.0
19.5
18.1
11.3
87
86
82
77
88
84
86
98
97
94
96
96
95
97
2
3
6
4
4
5
3
8403
7288
41713
19538
31789
29463
18425
-c
-p
-p
-p
-p
-p
a
b
c
d
Reaction conditions: catalyst 0.3 g, benzyl alcohol 0.3 mol, reaction T 333 K, reaction time 6 h. Analyzed by ICP. Designed ratio. Average
e
f
g
size observed by TEM. Including benzaldehyde, benzoic acid and benzylbenzoate. Including CO
alcohol oxidation per unit of catalyst per unit time.
2
and other unknown products. Rate of benzyl
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alcohol to CO
2
can be largely avoided. However, the further
15 M. D. Hernandez-Alonso, F. Fresno, S. Suareza and J. M. Coronado,
Energy Environ. Sci., 2009, 2, 1231.
oxidation of formed benzaldehyde to benzoic acid and subse-
quent esterification cannot be avoided, especially at high levels of
benzyl alcohol conversion. This is probably a common problem
for the selective photocatalytic oxidation of primary alcohols.
In contrast, perfect selectivity to ketones can be achieved in the
photocatalytic oxidation of secondary alcohols since the further
oxidation of ketones can be easily avoided.
1
6 O. S. Mohamed, A. M. Gaber and A. A. Abdel-Wahab, J. Photochem.
Photobiol., A, 2002, 148, 205.
17 S. Yurdakal, G. Palmisano, V. Loddo, V. Augugliaro and L.
Palmisano, J. Am. Chem. Soc., 2008, 130, 1568.
8 S. Yurdakal, G. Palmisano, V. Loddo, O. Alagoz, V. Augugliaro and
L. Palmisano, Green Chem., 2009, 11, 510.
9 S. Higashimoto, N. Kitao, N. Yoshida, T. Sakura, M. Azuma, H.
Ohue and Y. Sakata, J. Catal., 2009, 266, 279.
1
1
2
2
2
2
2
2
2
0 S. Higashimoto, N. Suetsugu, M. Azuma, H. Ohue and Y. Sakata, J.
Catal., 2010, 274, 76.
Acknowledgements
1 J. T. Carneiro, J. A. Moulijn and G. Mul, J. Catal., 2010, 273,
1
99.
This work was financially supported by the National Basic Re-
search Program of China (2009CB623502) and MOE (IRT0927).
Furthermore, L. L. would like to thank the support of the
State Key Laboratory of Catalytic Materials and Reaction
Engineering (RIPP, SINOPEC).
2 T. D. Bui, A. Kimura, S. Ikeda and M. Matsumura, J. Am. Chem.
Soc., 2010, 132, 8453.
3 M. Zhang, Q. Wang, C. Chen, L. Zang, W. Ma and J. Zhao, Angew.
Chem., Int. Ed., 2009, 48, 6081.
4 M. Zhang, C. Chen, W. Ma and J. Zhao, Angew. Chem., Int. Ed.,
2008, 47, 9730.
5 T. Ohuchi, T. Miyatake, Y. Hitomi and T. Tanaka, Catal. Today,
2
007, 120, 233.
6 T. Shishido, T. Miyatake, K. Teramura, Y. Hitomi, H. Yamashita and
References
T. Tanaka, J. Phys. Chem. C, 2009, 113, 18713.
1
R. A. Sheldon, I. W. C. E. Arends and A. Dijksman, Catal. Today,
000, 57, 157.
R. A. Sheldon, I. W. C. E. Arends, G. J. ten Brink and A. Dijksman,
Acc. Chem. Res., 2002, 35, 774.
T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037.
A. Abad, P. Concepcion, A. Corma and H. Garcia, Angew. Chem.,
Int. Ed., 2005, 44, 4066.
27 F. Zhang, R. Jin, J. Chen, C. Shao, W. Gao, L. Li and N. Guan, J.
Catal., 2005, 232, 424.
28 G. Wu, N. Guan and L. Li, Catal. Sci. Technol., 2011, 1, 601.
29 S. Halas and T. Durakiewicz, J. Phys.: Condens. Matter, 1998, 10,
10815.
30 M. I. Litter, Appl. Catal., B, 1999, 23, 89.
31 J. C. Crittenden, J. Liu, D. W. Hand and D. L. Perram, Water Res.,
1997, 31, 429.
32 A. Suisalu, J. Aarik, H. Mandar and I. Sildos, Thin Solid Films, 1998,
336, 295.
33 B. Liu, X. Zhao, Q. Zhao, X. He and J. Feng, J. Electron Spectrosc.
Relat. Phenom., 2005, 148, 158.
2
2
3
4
5
6
7
V. R. Choudhary, A. Dhar, P. Jana, R. Jha and B. S. Uphade, Green
Chem., 2005, 7, 768.
C. D. Pina, E. Falletta, L. Prati and M. Rossi, Chem. Soc. Rev., 2008,
3
7, 2077.
K. Mori, T. Hara, T. Mizugaki, K. Ebitani and K. Kaneda, J. Am.
Chem. Soc., 2004, 126, 10657.
34 C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and
G. E. Muilenberg, Handbook of X-ray photoelectron spectroscopy:
a reference book of standard data for use in X-ray photoelectron
spectroscopy, Perkin-Elmer, Eden Prairie, MN, USA, 1979.
35 L. Li, Q. Shen, J. Cheng and Z. Hao, Appl. Catal., B, 2010, 93, 259.
36 G. Pirug, C. Ritke and H. P Bonzel,, Surf. Sci., 1991, 241, 289.
37 H. Huang and D. Y. C. Leung, ACS Catal., 2011, 1, 348.
38 M. Yoon, M. Seo, C. Jeong, J. H. Jang and K. S. Jeon, Chem. Mater.,
2005, 17, 6069.
8
9
U. R. Pillai and E. Sahle-Demessie, Green Chem., 2004, 6, 161.
N. Jamwal, M. Gupta and S. Paul, Green Chem., 2008, 10, 999.
1
1
1
0 M. A. Fox and M. T. Dulay, Chem. Rev., 1993, 93, 341.
1 J.-M. Herrmann, Catal. Today, 1999, 53, 115.
2 D. Ravelli, D. Dondi, M. Fagnonia and A. Albini, Chem. Soc. Rev.,
2
009, 38, 1999.
1
1
3 G. Palmisano, E. Garcia-Lopez, G. Marci, V. Loddo, S. Yurdakal, V.
Augugliaro and L. Palmisano, Chem. Commun., 2010, 46, 7074.
4 R. E. Galian and J. Perez-Prieto, Energy Environ. Sci., 2010, 3, 1488.
39 W. L. Yim and T. Kl u¨ ner, J. Catal., 2008, 254, 349.
3
272 | Green Chem., 2011, 13, 3265–3272
This journal is © The Royal Society of Chemistry 2011