Photocatalytic reduction of carbon dioxide over Ag cocatalyst-loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) using water as a reducing reagent

Article abstract of DOI:10.1021/ja207586e

Ag cocatalyst-loaded ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) photocatalysts with 3.79-3.85 eV of band gaps and layered perovskite structures showed activities for CO₂ reduction to form CO and HCOOH by bubbling CO₂ gas into the aqueous suspension of the photocatalyst powder without any sacrificial reagents. Ag cocatalyst-loaded BaLa ₄Ti₄O₁₅ was the most active photocatalyst. A liquid-phase chemical reduction method was better than impregnation and in situ photodeposition methods for the loading of the Ag cocatalyst. The Ag cocatalyst prepared by the liquid-phase chemical reduction method was loaded as fine particles with the size smaller than 10 nm on the edge of the BaLa ₄Ti₄O₁₅ photocatalyst powder with a plate shape during the CO₂ reduction. CO was the main reduction product rather than H₂ even in an aqueous medium on the optimized Ag/BaLa ₄Ti₄O₁₅ photocatalyst. Evolution of O ₂ in a stoichiometric ratio (H₂+CO:O₂ = 2:1 in a molar ratio) indicated that water was consumed as a reducing reagent (an electron donor) for the CO₂ reduction. Thus, an uphill reaction of CO₂ reduction accompanied with water oxidation was achieved using the Ag/BaLa₄Ti₄O₁₅ photocatalyst.

Full text of DOI:10.1021/ja207586e

ARTICLE
pubs.acs.org/JACS
Photocatalytic Reduction of Carbon Dioxide over Ag
Cocatalyst-Loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) Using
Water as a Reducing Reagent
Kosuke Iizuka,† Tomoaki Wato,† Yugo Miseki,† Kenji Saito,†,‡ and Akihiko Kudo*,†,‡
†Department of Applied Chemistry, Faculty of Science, and ‡Division of Photocatalyst for Energy and Environment,
Research Institute of Science and Technology, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
S Supporting Information
b
ABSTRACT: Ag cocatalyst-loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) photocatalysts
with 3.79ꢀ3.85 eV of band gaps and layered perovskite structures showed activities for
CO2 reduction to form CO and HCOOH by bubbling CO2 gas into the aqueous
suspension of the photocatalyst powder without any sacrificial reagents. Ag cocatalyst-
loaded BaLa4Ti4O15 was the most active photocatalyst. A liquid-phase chemical
reduction method was better than impregnation and in situ photodeposition
methods for the loading of the Ag cocatalyst. The Ag cocatalyst prepared by the
liquid-phase chemical reduction method was loaded as fine particles with the size
smaller than 10 nm on the edge of the BaLa4Ti4O15 photocatalyst powder with a
plate shape during the CO2 reduction. CO was the main reduction product rather than H2 even in an aqueous medium on the
optimized Ag/BaLa4Ti4O15 photocatalyst. Evolution of O2 in a stoichiometric ratio (H2+CO:O2 = 2:1 in a molar ratio) indicated
that water was consumed as a reducing reagent (an electron donor) for the CO2 reduction. Thus, an uphill reaction of CO2
reduction accompanied with water oxidation was achieved using the Ag/BaLa4Ti4O15 photocatalyst.
’ INTRODUCTION
photocatalysts show the activities for CO2 reduction to form
HCOOH and CO with high quantum yields. Especially, the CdS
photocatalyst works under visible light irradiation. However,
electron donors such as triethylamine and 2-propanol are
indispensable for suppression of photocorrosion for these metal
sulfide photocatalysts. In contrast, when a UV-responsive ZrO2
photocatalyst with 5 eV of a band gap is employed, CO and H2
simultaneously evolve accompanied by O2 evolution in an
aqueous medium without electron donors.44 The selectivity for
the CO formation was about 10% when a Cu cocatalyst was
loaded on the photocatalyst, indicating the major reaction was
still H2 evolution by reduction of H2O. It is a challenging theme
to develop highly efficient photocatalysts for CO2 reduction
using water as an electron donor.
CO2 reduction to produce usable products is an important
topic from the viewpoint of not only an environmental issue but
also artificial photosynthesis. If one thinks artificial photosynthe-
sis accompanied by light energy conversion, water has to be used
as an electron donor and a hydrogen source for the CO2
reduction. A photocatalytic system is a candidate for the CO2
reduction of an artificial photosynthesis in an aqueous medium as
well as an electrochemical system combined with a solar cell. The
1
photocatalytic reduction of CO2 has been studied for a long
time as well as water splitting.2,3 When homogeneous photo-
catalysts such as an Re complex are used, CO2 is efficiently
reduced to form CO in the presence of electron donors such as
triethanol amine.4ꢀ7 However, water cannot be used as an
electron donor for the CO2 reduction accompanied by O2
evolution because of the lack of ability of water oxidation.
Heterogeneous photocatalysts have also been studied. It has
been reported that TiO2 and SrTiO3 photocatalysts give
HCOOH, HCHO, CH3OH, and CH4, depending on cocatalysts
loaded on the photocatalysts.8ꢀ31 Various hydrocarbons are
also detected using a mesoporous TiO2 photocatalyst.22 Mixed
metal oxides such as CaFe2O4,32,33 LaCoO3,34 BiVO4,35
InTaO4,36 ZnGa2O4,37 and Zn2GeO438,39 have also been repor-
ted as nontitanate photocatalysts. However, the amounts of
products are small, and O2 is not determined even in the absence
of a sacrificial reagent in many cases. Moreover, contaminations
adsorbed on photocatalysts sometimes give carbon-containing
products.31 On the other hand, colloidal ZnS40,41 and CdS42,43
We have reported various d0-type metal oxide photocatalysts
with wide band gaps for water splitting into H2 and O2.2
Electrons photogenerated in conduction bands of these photo-
catalysts possess high reducing potentials. Therefore, these
photocatalysts are expected to be active also for CO2 reduction
using water as a reducing reagent, if a suitable reaction site for
CO2 reduction is introduced as a cocatalyst on the surface of a
photocatalyst. ALa4Ti4O15 (A = Ca, Sr, and Ba) photocatalysts
with layered perovskite structure show high activities for water
splitting.45 These photocatalyst powders prepared by a polymerizable
complex method possess plate shape reflecting the crystal structure.
Received: August 11, 2011
Published: November 16, 2011
r
2011 American Chemical Society
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Table 1. CO2 Reduction over ALa4Ti4O15 (A = Ca, Sr, and Ba) Photocatalysts with Various Cocatalystsa
activity/μmol hꢀ1
photocatalyst
band gap/eV
cocatalyst (wt %)
loading method
H2
5.3
O2
2.4
CO
HCOOH
BaLa4Ti4O15
BaLa4Ti4O15
BaLa4Ti4O15
BaLa4Ti4O15
BaLa4Ti4O15
BaLa4Ti4O15
CaLa4Ti4O15
CaLa4Ti4O15
SrLa4Ti4O15
SrLa4Ti4O15
3.9
3.9
3.9
3.9
3.9
3.9
3.9
3.9
3.8
3.8
none
0
0
NiOxb (0.5)
Ru (0.5)
Cu (0.5)
Au (0.5)
Ag (1.0)
none
impregnation
58
84
29
0.02
0
0
photodeposition
photodeposition
photodeposition
photodeposition
41
45
51
0
96
0.6
0
110
10c
0
0
0.3c
7.0c
4.3c
0.07
2.3
0.06
1.8
1.3
0.6
2.1
0.5
1.8
0
Ag (1.0)
none
photodeposition
photodeposition
5.6
0.8
2.7
1.3
0
Ag (1.0)
0.5
a Catalyst 0.3 g, water 360 mL, CO2 flow system (15 mL minꢀ1), a 400 W high-pressure mercury lamp, an inner irradiation quartz cell. b Pretreatment:
Reduced at 673 K and subsequently oxidized at 473 K after impregnation (543 K for 1 h). c Initial activity.
The anisotropic property of BaLa4Ti4O15 is stronger than those
of SrLa4Ti4O15 and CaLa4Ti4O15 because of the ordered dis-
tribution of alkali earth metal and lanthanum cations in the
layered perovskite structure of BaLa4Ti4O15.46 The reduction
site is the edge of the plate, while the oxidation site is the basal
plane. These separated reaction sites are convenient to suppres-
sion of back reactions for photocatalytic reactions.
In the present study, we employed the ALa4Ti4O15 (A = Ca,
Sr, and Ba) photocatalysts loaded with various cocatalysts for
CO2 reduction using water as a reducing reagent. Factors
affecting the activity were discussed on the basis of the examina-
tion and characterization of cocatalysts.
reduction of water proceeded accompanied by sacrificial con-
sumption of HCOOH, in addition to decomposition of HCOOH
to form CO.
Particle size and condition of the Ag cocatalyst depend on the
loading method and should affect the photocatalytic activity. There-
fore, the loading method and amounts on ALa4Ti4O15 (A =
Ca, Sr, and Ba) photocatalysts were examined for the photo-
catalytic CO2 reduction (Table 2). These Ag-loaded ALa4Ti4O15
(A = Ca, Sr, and Ba) photocatalysts showed activities for CO2
reduction to form CO and HCOOH. The photocatalytic activity
of BaLa4Ti4O15 with the Ag cocatalyst loaded by an impregna-
tion method was improved by H2 reduction at 473 K for 2 h.
A liquid-phase chemical reduction was the best loading method
for the Ag cocatalyst. CO2 reduction to form CO and HCOOH
was superior to H2O reduction to form H2. This method was
more effective than a photodeposition method also for CaLa4-
Ti4O15 and SrLa4Ti4O15 photocatalysts, as shown in Table 1.
The reduction of CO2 predominantly proceeded at any
loading amounts of the Ag cocatalyst on BaLa4Ti4O15. The
optimum amount of the Ag cocatalyst was 2 wt %. Photocatalytic
CO2 reduction over the optimized Ag/BaLa4Ti4O15 is shown in
Figure 1. CO, H2, and O2 evolved with stoichiometric amounts.
The ratio of the number of reacted electrons to that of holes was
almost unity. Turnover numbers of electrons reacted for CO2
reduction to the number of moles of total Ag and surface Ag were
5.5 and 75 at 7 h, respectively. These results indicated that the
CO2 reduction photocatalytically proceeded. The ALa4Ti4O15
(A = Ca, Sr, and Ba) photocatalysts were calcined at 1373 K in
air during the preparation. No CꢀH stretching peak around
2900 cmꢀ1 was observed by IR measurement. When Ar was
flowed instead of CO2, water splitting proceeded to form H2 and
O2 without carbon-contained products as shown in Table 2.
These results have proven that the formed CO and HCOOH
were originated from not contaminations but CO2. This is also
supported by unity of the ratio of reacted eꢀ/h+. Thus, it was
found that Ag cocatalyst-loaded ALa4Ti4O15 (A = Ca, Sr, and Ba)
was the photocatalyst for CO2 reduction to form CO using water
as an electron donor.
’ RESULTS AND DISCUSSION
Photocatalytic Reduction of CO2. Table 1 shows effects of
cocatalyst, the loading method, and the loading amount on CO2
reduction in aqueous media over ALa4Ti4O15 (A = Ca, Sr, and
Ba) photocatalysts. Bare ALa4Ti4O15 (A = Ca, Sr, and Ba)
showed negligible activities for CO2 reduction. Loading pre-
treated NiO, Ru, and Au cocatalysts on BaLa4Ti4O15 enhanced
photocatalytic activity for water splitting, but not CO2 reduction,
because these cocatalysts are highly effective for H2 production
by water splitting.47,48 When a Cu cocatalyst was loaded, photo-
catalytic activities for water splitting and CO2 reduction in-
creased. Although the amounts of reacted electrons and holes
were large in the order of Au > Cu > Ru > NiOx > Ag, Ag was the
most active cocatalyst for CO2 reduction. Reduction of CO2 to
form CO and HCOOH competed with that of water to form H2.
The Ag cocatalyst was also effective for CO2 reduction over
CaLa4Ti4O15 and SrLa4Ti4O15 photocatalysts. It has been
reported that Ag is a highly active electrocatalyst for electro-
chemical reduction of CO2 to form CO selectively in an aqueous
KHCO3 solution as shown in eq 1.49,50
CO2 þ 2Hþ þ 2eꢀ f CO þ H2O
ð1Þ
This fact indicated that the Ag cocatalyst functioned as a CO2
reduction site to form CO. The amount of HCOOH formed was
small, because Ag was not a suitable cocatalyst for it. Moreover,
HCOOH should not accumulate in a liquid phase because it is
readily oxidized by photogenerated holes. This was confirmed experi-
mentally as shown in Figure S1. When a bare BaLa4Ti4O15 photo-
catalyst was irradiated with UV in an aqueous HCOOH solution,
Characterization of Ag-Loaded BaLa4Ti4O15 Photocata-
lyst. Diffuse reflectance spectra of BaLa4Ti4O15 with Ag cocata-
lysts loaded by various methods are shown in Figure 2 to see the
condition of metallic Ag loaded. The absorption with an onset
at 320 nm was due to the band gap transition of BaLa4Ti4O15.
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Table 2. Effect of Loading Method of Ag Cocatalyst on the Photocatalytic Activity for CO2 Reduction over ALa4Ti4O15 (A = Ca,
Sr, and Ba)a
activity/μmol hꢀ1
photocatalyst
loading amount/wt %
loading method
impregnationb
H2
8.2
O2
5.7
CO
HCOOH
BaLa4Ti4O15
BaLa4Ti4O15
BaLa4Ti4O15
BaLa4Ti4O15
BaLa4Ti4O15
BaLa4Ti4O15
BaLa4Ti4O15
BaLa4Ti4O15
SrLa4Ti4O15
CaLa4Ti4O15
1.0
1.0
0.5
1.0
2.0
3.0
5.0
1.0
1.0
1.0
5.2
8.9
11
0.2
0.3
0.03
0.4
0.7
0.1
0.02
0d
impregnationb+H2 red.c
liquid-phase reduction
liquid-phase reduction
liquid-phase reduction
liquid-phase reduction
liquid-phase reduction
liquid-phase reduction
liquid-phase reduction
liquid-phase reduction
5.6
4.5
5.6
8.7
6.8
12
19
10
16
14
22
9.7
19
4.8
20d
6.6
11d
5.8
6.6
12
0d
4.8
7.1
0.8
3.2
9.3
0.4
a Catalyst 0.3 g, water 360 mL, CO2 flow system (15 mL minꢀ1), a 400 W high-pressure mercury lamp, an inner irradiation quartz cell. b 723 K for 1 h.
c 473 K for 2 h. d Ar flow.
Figure 2. Diffuse reflection spectra over BaLa4Ti4O15 photocatalyst with
Figure 1. CO2 reduction over BaLa4Ti4O15 photocatalyst with Ag (2 wt %)
Ag (1 wt %) cocatalyst loaded by (a) photodeposition, (b) impregnation,
cocatalyst loaded by a liquid-phase reduction method. Catalyst 0.3 g, water
(c) impregnation+H2 reduction, (d) liquid-phase reduction, and (e) bare
360 mL, CO2 flow system (15 mL minꢀ1), a 400 W high-pressure mercury
BaLa4Ti4O15.
lamp, an inner irradiation quartz cell, H2 (O), O2 (b), CO (4).
loaded on the BaLa4Ti4O15 photocatalyst with the size smaller
than 10 nm. The Ag cocatalyst on the basal plane of BaLa4Ti4O15
A surface plasmon absorption around 400ꢀ500 nm and a wide
absorption band continuing to a near-infrared region due to large
particles of metallic Ag were observed in the case of photo-
deposition. The surface plasmon absorption was not observed for
the Ag cocatalyst loaded by an impregnation method, suggesting
the existence of silver oxide on the cocatalyst surface, whereas it
was observed after H2 reduction of the sample. A simple profile of
the surface plasmon absorption and small absorption due to the
large particle of metallic Ag were observed for the Ag cocatalyst
loaded by a liquid-phase chemical reduction, indicating Ag
cocatalysts were loaded as fine metallic particles with a size with
a relatively narrow distribution.
almost disappeared, and the number of the Ag particles on the
edge increased even after 1 h of the photocatalytic reaction. It has
been reported that reduction by photogenerated electrons and
oxidation by holes proceed mainly on the edge and basal plane
for water splitting on BaLa4Ti4O15, respectively.45 This aniso-
tropic property has been clarified by reductive photodeposition
of Au and oxidative photodeposition of PbO2 being similar to
TiO2.5,51 This result suggested that the Ag on the basal plane
dissolved by photogenerated holes and photodeposited again on
the edge at the initial stage of the photocatalytic reaction. The
size of rephotodeposited Ag on the edge was smaller than 10 nm
and more uniform than photodeposited Ag of Figure 3a. The
reason the size of the rephotodeposited Ag on the edge shown
in Figure 3g was smaller than the just photodeposited Ag shown
in Figure 3a is due to the rate of photodeposition. The concen-
tration of dissolved Ag+ ions in an aqueous solution for the
rephotodeposited sample was lower than that for the just
photodeposited sample. Absorption band due to surface plasmon
after the reaction still possessed a simple profile with a peak shape
as shown in Figure 4, being different from that in Figure 2a. This
result also indicated that the nature of the Ag cocatalyst loaded
Figure 3 shows SEM images of Ag cocatalyst-loaded BaLa4-
Ti4O15 photocatalysts. BaLa4Ti4O15 had a plate shape with
100 nm thickness and 1 μm width reflecting a layered perovskite
structure. Photodeposited Ag cocatalysts in situ were loaded on
the edge of the plate as nanoparticles with the size of 30ꢀ40 nm.
The particle size of photodeposited Ag in situ after 1 h of the
photocatalytic reaction was similar to that after 7 h. Ag cocatalysts
loaded by an impregnation method and subsequent H2 reduction
aggregated with the size of 50 nm. Ag cocatalysts prepared by a
liquid-phase chemical reduction were uniformly and dispersively
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Figure 4. (a) Diffuse reflection spectra of Ag (1 wt %)/BaLa4Ti4O15
before (ꢀ ꢀ ꢀ) and after (ꢀ) CO2 reduction reaction.
Figure 5. Mechanism of photocatalytic CO2 reduction over BaLa4-
Ti4O15 with Ag cocatalysts loaded by several methods.
Figure 3. Scanning electron microscope images of Ag (1 wt %)/
BaLa4Ti4O15. (a) Photodeposition, (b) impregnation+H2 reduction
(as prepared), (c) impregnation+H2 reduction (after photocatalytic
CO2 reduction for 1 h), (d) liquid-phase reduction (as prepared),
(e) magnified view of (d), (f) liquid-phase reduction (after photocata-
lytic CO2 reduction for 1 h), and (g) magnified view of (f).
catalytic activity for CO2 reduction. These results indicated that
the critical factor contributing to facilitate CO/HCOOH evolu-
tion was particle size of Ag loaded on the edge of a BaLa4Ti4O15
crystal. XPS measurements indicated that the metallic state of
Ag was maintained even after 20 h of photocatalytic reaction
(Figure S2).52 The stability of metallic state of Ag resulted from
the loading position, that is, edges of a BaLa4Ti4O15 crystal,
which acted as reduction sites.
Scheme of Photocatalytic Reduction of CO2 on Ag/BaLa4-
Ti4O15. The scheme of photocatalytic reduction of CO2 on
Ag/BaLa4Ti4O15 is illustrated in Figure 5. The edge of a BaLa4-
Ti4O15 photocatalyst is the reduction site, while the basal plane is
the oxidation site for water splitting. This property is due to
anisotropy of the crystal structure.45 The degree of the aniso-
tropy of BaLa4Ti4O15 is larger than those of SrLa4Ti4O15 and
CaLa4Ti4O15 due to distribution of the alkali earth metal cations
in layered perovskite structure. The most active BaLa4Ti4O15
photocatalyst with fine Ag cocatalysts loaded on the edge under
working state possesses the ideal structure for the photocatalytic
CO2 reduction. CO2 reduction proceeds on the Ag cocatalyst
loaded on the edge competing with H2O reduction, while O2
evolution mainly proceeds on the basal plane. The CO2 reduc-
tion to form CO predominated over the H2O reduction to give
by a liquid-phase chemical reduction, subsequent photodissolu-
tion, and rephotodeposition during the photocatalytic reaction
was different from that loaded by photodeposition in situ. The
difference in the photocatalytic activity between photodeposition
and liquid-phase reduction was due to the difference in the size
and homogeneity of loaded Ag cocatalysts even if the cocatalysts
were photodeposited on the edge under the working state.
Although the shape of the surface plasmon band after CO2
reduction was similar to that before the reaction, the intensity
after the reaction was larger than that before the reaction,
suggesting that the number of the metallic Ag fine particle
increased. The rephotodeposition was also seen in the case of
samples with impregnation+H2 reduction as shown in Figure 3c.
The size of Ag rephotodeposited on the edge was 10ꢀ20 nm.
Therefore, the order in the size of Ag cocatalyst under working
condition was liquid-phase reduction < impregnation and
H2 reduction < photodeposition, corresponding to the order in the
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H2 on the BaLa4Ti4O15 photocatalyst with Ag cocatalysts loaded
by liquid-phase reduction and impregnation+H2 reduction,
whereas it was reverse for photodeposition as shown in Tables 1
and 2. The separation of the reaction sites contributes to sup-
pression of back reactions such as oxidation of formed CO. The
reactions were carried out under CO2 flow. The bubbling of CO2
also assists to get the gaseous products out into a gas phase, resulting
in the suppression of back reactions. Moreover, the photocatalytic
reaction may proceed at the three-phase interface among the surface
of photocatalyst, liquid of water, and a gas phase. When CO2 gas was
flowed on an aqueous suspension of the photocatalyst without
bubbling, the photoctalytic activity was considerably decreased as
compared to the bubbling system (Figure S3), suggesting that the
photocatalytic reaction proceeded at the three-phase interface.
H2 gas flow at 473 K for 2 h if needed. In the liquid-phase chemical
reduction method,53ꢀ55 an aqueous AgNO3 solution (0.1 mol/L) was
added to 50 mL of an aqueous suspension containing 0.5 g of the
photocatalyst. After addition of an equimolar amount of NaPH2O2aq
(0.4 mol/L) with respect to Ag+ to the suspension, the mixture was
stirred at 333 K for 1 h. The obtained powder was washed with water and
dried at ambient temperature in air.
The photocatalysts obtained were identified by powder X-ray diffrac-
tion (Rigaku; Miniflex, Cu Kα). Diffuse reflection spectra were recorded
on a UVꢀvisꢀNIR spectrometer (JASCO; UbestV-570) and were
converted from reflection to absorbance by the KubelkaꢀMunk method.
Scanning electron microscopy (SEM) images were taken using a JEOL
JSM-6700F.
CO2 gas with a purity of 99.995% was used. This gas contained a
negligible amount of CO as compared to the amounts of produced CO
shown in Tables 1 and 2. 0.3 g of the photocatalyst powder was dispersed
in 360 mL of water. Photocatalytic reactions were carried out by
bubbling the CO2 gas with a flow rate at 15 mL minꢀ1 using an inner
irradiation cell made of quartz and a 400 W high-pressure mercury lamp
(a spectrum of incident light, see Figure S4) under ambient temperature
and pressure. Gaseous products of H2, O2, and CO were determined by
online gas chromatographs (Shimadzu, GC-8A) with a thermal con-
ductivity detector (MS-5A, Ar carrier) and a flame ionization detector
(MS-13X, N2 carrier) with a methanizer. An aqueous product of formic
acid was analyzed by an ion chromatograph (TOA-DKK, ICA-2000,
DS-plus).
’ CONCLUSIONS
BaLa4Ti4O15 with anisotropic structure showed higher photo-
catalytic activity for CO2 reduction than SrLa4Ti4O15 and CaLa4-
Ti4O15. The ratio of the number of reacted electrons to holes was
unity, indicating that water reacted as an electron donor. Ag was
the most active cocatalyst. The Ag cocatalyst functioned as a CO2
reduction site to form CO as seen in an Ag electrocatalyst for
electrochemical reduction of CO2. The photocatalytic activity
depended on the loading method of the Ag cocatalyst. The liquid-
phase chemical reduction was the best method to load fine Ag
particles, and the condition of the Ag cocatalyst changed at the
beginning stage of the photocatalytic reaction. CO mainly
evolved on the Ag cocatalyst loaded on the edge of plate
particle of the BaLa4Ti4O15 photocatalyst. High activity of Ag/
BaLa4Ti4O15 for the photocatalytic CO2 reduction using water
is due to separated reaction sites of reduction from oxidation,
and specific loading of the Ag cocatalyst on the edge. More-
over, a CO2 bubbling method contributed to suppressing of
back reactions and serving the three-phase interface for the
reaction.
’ ASSOCIATED CONTENT
S
Supporting Information. Decomposition of HCOOH
b
over a bare BaLa4Ti4O15 photocatalyst in an aqueous medium
(S1), Ag(M4N4,5N4,5) Auger spectra of BaLa4Ti4O15 photocat-
alyst loaded with Ag (2 wt %) by a liquid-phase chemical
reduction method before and after 20 h of photocatalytic
reaction (S2), photocatalytic CO2 reduction over Ag(2 wt %)/
BaLa4Ti4O15 with and without bubbling CO2 gas (S3), and a
spectrum of incident light from a light source of a 400 W high-
pressure mercury lamp + a quartz cell (S4). This material is
available free of charge via the Internet at http://pubs.acs.org.
’ EXPERIMENTAL SECTION
ALa4Ti4O15 (A = Ca, Sr, and Ba) photocatalysts were prepared by
a polymerizable complex method.45 Ethanol suspension containing
titanium-n-butoxide (Kanto chemical, 97.0%), citric acid (Kanto chemi-
cal, 99.5%), and propylene glycol (Kanto chemical, 99.0%) was stirred at
333 K. After complete dissolution of the citric acid, lanthanum nitrate
(Wako, 99.9%) and alkaline earth carbonates (CaCO3, Kanto chemical,
99.5%; SrCO3, Kanto chemical, 99.9%; BaCO3, Kanto chemical, 99.5%)
were added to this solution, and the mixture was heated at 393 K for 12 h
to afford gel assembly. The gel was thermally decomposed at 673 K in air,
followed by calcination at 1373 K for 10 h to obtain photocatalyst
powder.
Ru, Cu, and Au cocatalysts were loaded on photocatalysts by a
photodeposition in situ from aqueous solutions dissolving RuCl3,
CuSO4, and HAuCl4. A NiO cocatalyst was loaded by an impregnation
method in which water was evaporated from the suspension of the
photocatalyst in an aqueous Ni(NO3)2 solution on a water bath and the
resulting solid was calcined at 543 K. The photocatalyst powder loaded
with NiO was reduced by H2 gas at 673 K for 2 h and subsequently
oxidized by O2 gas at 473 K for 1 h after the impregnation as
pretreatment. An Ag cocatalyst was loaded on the photocatalyst by
photodeposition, impregnation, and liquid-phase chemical reduction
methods using AgNO3. Photodeposition was conducted in situ at the
beginning stage of a photocatalytic reaction. An aqueous AgNO3
solution and calcination at 723 K for 1 h were selected as the condition of
the impregnation method. Reduction treatment was performed under
’ AUTHOR INFORMATION
Corresponding Author
a-kudo@rs.kagu.tus.ac.jp
’ ACKNOWLEDGMENT
This work was supported by the ENEOS hydrogen foundation.
’ REFERENCES
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