Efficient visible-light-induced photocatalytic activity on gold-nanoparticle-supported layered titanate

Article abstract of DOI:10.1021/ja1083514

The visible-light-induced photocatalytic conversion of aqueous benzene to phenol on Au-nanoparticle-supported layered titanate was accelerated when the reaction was conducted in the presence of aqueous phenol.

Full text of DOI:10.1021/ja1083514

Published on Web 11/08/2010
Efficient Visible-Light-Induced Photocatalytic Activity on
Gold-Nanoparticle-Supported Layered Titanate
Yusuke Ide, Mizuki Matsuoka, and Makoto Ogawa*
Department of Earth Sciences and Graduate School of CreatiVe Science and Engineering, Waseda UniVersity,
1-6-1 Nishiwaseda, Shinjyuku-ku, Tokyo 169-8050, Japan
Received September 15, 2010; E-mail: makoto@waseda.jp
N2 adsorption capacity (Figure 1b, BET surface area increased from
Abstract: The visible-light-induced photocatalytic conversion of
0 to 53 m2 g-1) of the thiol-modified layered titanate after the
aqueous benzene to phenol on Au-nanoparticle-supported lay-
reactions revealed that Au-titanate was nanoporous where gold
ered titanate was accelerated when the reaction was conducted
nanoparticles were immobilized in the interlayer nanospace of the
in the presence of aqueous phenol.
titanate through thiol groups bounded on the titanate sheets to create
pores, which is schematically shown in the Figure 1d inset. The
FE-SEM image of Au-titanate showed that Au nanoparticles did
The visible light response of semiconductor photocatalysts such
not deposit on the particle surface (Figure S1 in the Supporting
as TiO2 is an important issue for environmental applications;
Information), supporting the intercalation of Au nanoparticles.
accordingly, efforts have been focused toward designing novel
Similar results have been reported for the synthesis of a chromia-
photocatalysts that function under visible light by such methods as
nanoparticle-pillared H0.67Ti1.83O4, where the basal spacing and BET
dye sensitization and heteroelement doping.1 Recently, it has been
reported that the immobilization of gold nanoparticles on TiO2
particles led to visible-light-induced photocatalysis for the oxidation
of aqueous organic substrates.2,3 Gold-nanoparticle-loaded titania
appears to be a potential material for further nanostructure design,4
because of the size and morphology-dependent light absorption of
gold nanoparticles from the visible to near-infrared region.5 In this
communication, we report the preparation of a new photocatalyst
composed of an ultrathin layer of titanium oxide and gold
nanoparticles. A layered titanate was chosen to be supported with
gold nanoparticles, since gold nanoparticles were deposited in the
interlayer space of a layered titanate to possibly give a “pillared”
clay-type material with molecular-sieving ability.6 Studies on the
catalytic conversion of organic substrates using zeolites7 and
mesoporous materials8,9 have demonstrated that the separation of
substrates from the target products on the catalysts plays an
important role in the efficient formation of the products.
surface area are ca. 1.7 nm and 100 m2 g-1, respectively.14 The gallery
height of Au-titanate (ca. 1.3 nm if the thickness of the titanate sheet
is 0.5 nm15) is comparable to that (ca. 1.3 nm) of the thiol-modified
titanate, which is explained by the conformational change of the
attached propyl group.11 The partial deintercalation of the attached
silyl group is another possible reason for the result.
The present hybrid photocatalyst was used for the direct oxidation
of benzene to phenol by visible light irradiation. Since phenol is
one of the most important chemicals in industry and few efficient
photocatalysts for direct benzene oxidation have been developed,9,10
benzene oxidation was chosen as the photocatalytic reaction to be
examined in the present study. During the study, we found that the
reaction efficiency was substantially modified when the reaction
was conducted in the presence of the product (phenol). In the field
of heterogeneous photoctalysis, the reaction efficiency is also one
of the most important problems requiring improvement. The present
very simple approach, just adding the product in the reactant, seems
to be versatile for various photocatalytic systems.
A gold-nanoparticle-pillared layered titanate (abbreviated as Au-
titanate) was prepared on the basis of the reported procedure
developed for the synthesis of gold nanoparticles in layered
solids.11,12 A layered titanate, K0.66Ti1.73Li0.27O3.93 13 was modified
,
with (3-mercaptopropyl)trimethoxysilane according to the reported
procedure.11a The particle surface of the pristine titanate was
preliminarily modified with another silane coupling reagent which
does not possess a thiol group, octadecyltrimethoxysilane, to avoid
the deposition of Au on the particle surface.11 The thiol-modified
layered titanate was mixed with HAuCl4 followed by reduction with
NaBH4. The increase in the basal spacing (Figure 1a) and enhanced
Figure 1. (a) Powder X-ray diffraction patterns, (b) N2 adsorption (open
symbol)/desorption (filled symbol) isotherms, and (c) ultraviolet-visible-near-
infrared spectra recorded in diffused reflectance mode of the alkanethiol-
modified layered titanate (bottom) before and (top) after gold deposition.
(d) TEM and SEM images of Au-titanate. Insets show the size distribution
of Au particles and schematic structure of Au-titanate.
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C O M M U N I C A T I O N S
As depicted in Figure 1c, Au-titanate exhibits absorption (g520
nm) due to the surface plasmon resonance of nanometer-scale gold
particles, in addition to the absorption edge at around 380 nm due
to the titanate framework. The absorption from visible to near-
infrared light is characteristic of the anisotropic-shaped gold
nanoparticles.5 Figure 1d shows the TEM image of Au-titanate
observed from a direction perpendicular to the basal plane. With
the FE-SEM result taken into account, almost all Au particles seen
in the TEM image are located in the interlayer space. From the
gallery height of Au-titanate and the particle size of Au (Figure 1d
inset), the morphology of the intercalated Au is considered to be a
disk with a thickness of less than 1 nm and diameter of ca. 3.5
nm.
irradiation (almost all the reacted benzene was mineralized to CO2
on the basis of the HPLC result). Unexpectedly, the catalytic
efficiency for phenol formation was trace though Au-titanate was
able to recognize benzene in the aqueous mixture with phenol,
which was shown on the left side against dashed lines in Figure
2b-d. For example, ca. 42% of benzene adsorbed while only ca.
3% of phenol adsorbed when Au-titanate was reacted with an
aqueous mixture of benzene and phenol (600 ppm for each
compound) under dark conditions (Figure 2b inset).
We then decided to conduct the photocatalytic reaction in the
presence of the product, “phenol”, to utilize the benzene/phenol
recognition ability of the presently designed photocatalyst. Surpris-
ingly, benzene was oxidized to phenol substantially when the
reaction was conducted in the presence of phenol (the right side
against dashed lines in Figure 2b-d). The turnover number of
phenol formation was much larger than 1 (see Supporting Informa-
tion for details). Higher reaction efficiency was attained when a
larger quantity of light was used (Figure S2 in the Supporting
Information). Moreover, the XRD pattern and absorption spectrum
of Au-titanate did not change upon reaction, indicating the material
remained unchanged after the reaction. These facts support the
phenol formation results from the photocatalysis of Au-titanate. As
shown apparently in Figure 3, the yield and selectivity of phenol
formation depended on the amount of the initially added phenol in
the water. When using 18 000 ppm of phenol, benzene was oxidized
to phenol in 62% yield and 96% selectivity (it should be noted
here that the ratio of the formed phenol to the initially added phenol
is much larger than the experimental error for two runs; for example,
3% > 0.7% and 5% > 0.7% for the initially added phenol of 18 000
and 6000 ppm, respectively). These values were superior or
comparable to those reported in other direct oxidations using
catalysts or photocatalysts (Table S1 in the Supporting Information):
for example, 34% yield and 81% selectivity were attained by a
UV-light-activated catalytic system using mesoporous titania as the
catalyst and water as the oxidant,9 or 34% yield and >98%
selectivity were obtained by a thermally activated catalytic system
using a titanium-containing molecular sieve as the catalyst and H2O2
as the oxidant.8 If compared with other systems for the direct
oxidation of benzene previously reported,17-24 the present system
exhibits two advantages: (i) mild reaction conditions (visible light
and room temperature), (ii) cheap source of oxidant (water).
Dissolved oxygen in water acted as an acceptor of the electron
injected to the titanate from Au nanoparticles (active center).3a,b
Figure 2. Visible-light-induced catalytic oxidation of benzene in water
with Au-titanate in the absence and presence of aqueous phenol. The
amounts of benzene and phenol in aqueous mixtures were denoted as (b)
and (O), respectively. The light irradiation started after 2 h. The amounts
of the initially added phenol are (a) 0, (b) 600, (c) 6000, and (d) 18 000
ppm. Inset shows the expanded version of the figures.
A similar reaction was conducted using other visible-light-
responsive photocatalysts which did not efficiently adsorb both
benzene and phenol from water, a layered titanate where gold
nanoparticles are deposited only at the particle surface (abbreviated
as Au(external)-titanate) and a ruthenium complex-sensitized titania
particulate (abbreviated as dye-P25).25 As shown in Table S1, only
a trace amount of phenol formed when the two materials were
reacted with an aqueous benzene and phenol mixture under the
same reaction conditions. There are several reasons for the low
photocatalytic abilities of Au(external)-titanate and dye-P25 com-
pared to that of Au-titanate, such as the variation in the oxidation
power.3b,26 We consider that the molecular recognition ability for
benzene of Au-titanate (the left side against dashed lines in Figure
2b-d) plays an important role in the unique photocatalytic ability.
The modification of the photocatalysis of semiconductor particles
for visible light response and substrate selective reaction has been
reported so far. For example, visible-light-induced water splitting
and the oxidation of organic substrates have been achieved by the
complexation of semiconductors with photosensitizing complex
ions27 and noble metal2,3 or metal sulfide nanoparticles28 and by
Figure 3. Variation in (O) yield and (0) selectivity of phenol formation
as a function of the amount of the initially added phenol. Yield and
selectivity were calculated as formed phenol (mol)/added benzene (mol)
and formed phenol (mol)/reacted benzene (mol), respectively.
The photocatalytic oxidation of benzene in water was investigated
using Au-titanate as the photocatalyst, which was activated by
visible light (g420 nm).16 After the adsorption of the substrate onto
the catalyst had reached an equilibrium, the irradiation was started.
Figure 2a demonstrates the variation in the amounts of the residual
benzene and generated phenol during adsorption (the left side
against dashed line) and photocatalytic reactions (the right side
against dashed line). As clearly shown in the Figure 2a inset, Au-
titanate effectively adsorbed benzene from water and subsequently
oxidized benzene to phenol to some extent upon visible light
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C O M M U N I C A T I O N S
doping with metal or nonmetal elements.4 Visible-light-induced
water splitting has also been achieved on polymeric semiconduc-
tors29 and systems involving two photoexcitation processes (so-
called Z-scheme-type systems).30 Substrate-selective photocatalysis
has been attained on molecular recognitive layer-coated titanium
dioxide particles,31 nanoporous titania-based compounds,9,32 and
layered titanates with controlled swelling ability.33 On the other
hand, to the best of our knowledge, this is the first report on the
modification of photocatalysis to a higher yield and product
selectivity by the addition of the product in the starting mixture. In
addition to the versatility, the idea is economically and environ-
mentally favorable; therefore, it is potentially applicable for
improving the efficiency and selectivity of existing photocatalytic
processes.
In summary, a layered titanate containing immobilized gold
nanoparticles in the interlayer space was found to catalyze the
oxidation of aqueous benzene to phenol by visible light irradiation.
In addition, we have unexpectedly found that the photocatalytic
reaction was substantially modified to a higher yield and selectivity
of phenol formation when the reaction was conducted in the
presence of aqueous phenol. Under visible light irradiation at room
temperature using water as the oxidant, the direct benzene oxidation
was accelerated to attain 62% yield and 96% selectivity for phenol
formation by simply adding phenol in the starting mixture.
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(12) Au-titanate was synthesized on the basis of the procedure developed for
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13
surface of a layered titanate, K0.66Ti1.73Li0.27O3.93
,
was modified with an
octadecyl group by the reaction of the titanate (1.0 g) with a toluene solution
(100 mL) of octadecyltrimethoxysilane (1.0 mL). After the concentration
of the mixture at 60 °C and 90 hPa, the product was washed with toluene.
The dodecylammonium-exchanged titanate (0.65 g), synthesized by the
reaction of the externally modified titanate with an aqueous dodecylamine
solution,13 was dispersed in a toluene (65 mL) solution of (3-mercapto-
propyl)trimethoxysilane (0.34 mL), and the mixture was concentrated at
60 °C and 90 hPa. The product was washed with a mixture of 0.1 mol L-1
HCl aqueous solution and ethanol (1:1 v/v). The titanate was selectively
modified with alkanethiol at the interlayer surface, and the alkyl group at
the particle surface thus obtained (165 mg) was allowed to react with an
ethanol solution (280 mL) of HAuCl4 ·3H2O (163 mg) followed by reduction
with NaBH4 (107 mg) in ethanol (26 mL). The solid product was separated
by centrifugation (4000 rpm, 20 min) and washed with ethanol.
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(16) Catalytic reaction was conducted at room temperature using 4.0 mg of the
catalyst and 40 mL of the aqueous solution (pH 6.2-5.8). The catalyst
was mixed with the aqueous solution, and the mixture was stirred in a
glass vessel. After the adsorption of the substrate onto the catalyst had
reached an equilibrium, the aqueous mixture was irradiated by visible light
from a 150 W Xe lamp (Ushio Inc.) with a cut-off filter (λ e 420 nm),
under stirring. The glass vessel was fixed 22 cm away from the light source.
Supernatants obtained by centrifugation of the sampled mixtures were
analyzed using HPLC to determine the amount of the substrate and product
in the water (an example is shown in Figure S3 in the Supporting
Information). The generated amount of phenol was calculated assuming
that all the increased phenol in the water after visible light irradiation was
ascribed to the formed phenol. A blank sample without the catalyst was
also prepared and reacted to estimate vaporization losses and the adsorption
of the reactant and products on the glass vessel.
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research (B) (19350103) from Japan Society for the
Promotion of Science. This work was also supported by the Global
COE Program of MEXT ‘Center for Practical Chemical Wisdom’.
Waseda University also supported us financially as a Special
Research Project (2009B-077 and 2009B-370).
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Supporting Information Available: Turnover number of the present
photocatalysis, examples of catalytic oxidation of benzene to phenol
(Table S1), FE-SEM image of Au-titanate (Figure S1), effect of quantity
of light on the photocatalysis of Au-titanate (Figure S2), and chro-
matographs of aqueous benzene/phenol mixture before and after the
reaction with Au-titanate (Figure S3). This material is available free
of charge via the Internet at http://pubs.acs.org.
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followed by reduction with NaBH4 (768 mg) in ethanol (185 mL). The
solid product was separated by centrifugation (4000 rpm, 20 min) and
washed with ethanol. Dye-P25 was synthesized as follows: a photosensitizer,
tris(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium, was adsorbed on P25
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