Highly Active NaTaO3-Based Photocatalysts for CO2 Reduction to Form CO Using Water as the Electron Donor

Article abstract of DOI:10.1002/cssc.201601360

Doped NaTaO₃(NaTaO₃:A, where A=Mg, Ca, Sr, Ba, or La) has arisen as a highly active photocatalyst for CO₂reduction to simultaneously form CO, H₂, and O₂using water as the electron donor when used with an Ag cocatalyst, under UV irradiation, and with 1 atm (0.1 MPa) of CO₂. The ratio of the number of reacted electrons/holes was almost unity, indicating that water was consumed as the electron donor. A liquid-phase reduction method for loading of the Ag cocatalyst was superior to photodeposition and impregnation methods. The Ag cocatalyst-loaded NaTaO₃:Ba was the most active photocatalyst in water with no required additives. The addition of bases, such as hydrogencarbonate, was effective to enhance the CO formation for Mg-, Ca-, Sr-, Ba-, and La-doped NaTaO₃photocatalysts with an Ag cocatalyst. Ca- and Sr-doped NaTaO₃photocatalysts showed especially high activity along with the Ba-doped photocatalyst in the aqueous NaHCO₃solution. The selectivity for the CO formation [CO/(CO+H₂)] on Ca-, Sr-, and Ba-doped NaTaO₃photocatalysts with Ag cocatalyst reached around 90 %.

Full text of DOI:10.1002/cssc.201601360

Accepted Article
Title: Highly Active Alkaline Earth Metals and Lanthanum-Doped
NaTaO3 Photocatalysts for CO2 Reduction to Form CO Using
Water as an Electron Donor
Authors: Haruka Nakanishi, Kosuke Iizuka, Tomoaki Takayama,
Akihide Iwase, and Akihiko Kudo
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To be cited as: ChemSusChem 10.1002/cssc.201601360
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10.1002/cssc.201601360
ChemSusChem
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Highly Active Alkaline Earth Metals and Lanthanum-Doped
NaTaO3 Photocatalysts for CO2 Reduction to Form CO Using
Water as an Electron Donor
Haruka Nakanishi,[a] Kosuke Iizuka,[a] Tomoaki Takayama,[a] Akihide Iwase,[a, b] Akihiko Kudo*[a, b]
Abstract: Mg, Ca, Sr, Ba and La-doped NaTaO3 has arisen as
highly active photocatalysts for CO2 reduction to form CO using
water as an electron donor accompanied by O2 evolution under UV
irradiation and bubbling of 1 atmosphere of CO2 when an Ag
cocatalyst was loaded on those photocatalysts. CO, H2 and O2
simultaneously formed. The ratio of the number of reacted
electrons to that of holes was almost unity. This result indicated that
water was consumed as an electron donor. A liquid-phase reduction
method for loading of the Ag cocatalyst was superior to
photodeposition and impregnation methods. Ag cocatalyst-loaded
NaTaO3:Ba was the most active photocatalyst in water with no
additives. Addition of bases such as hydrogencarbonate was
effective to enhance the CO formation for Mg, Ca, Sr, Ba and La-
doped NaTaO3 photocatalysts with an Ag cocatalyst. Ca and Sr-
doped NaTaO3 photocatalysts especially showed high activity as
on water splitting and CO2 reduction. However, 12% of the
selectivity for the CO formation is not satisfying. We have also
reported a BaLa4Ti4O15 photocatalyst with 3.9 eV of a band gap
for water splitting and CO2 reduction.[8−10] The BaLa4Ti4O15
photocatalyst shows relatively high activity for water splitting
when a NiOx cocatalyst is loaded. Moreover, an Ag cocatalyst-
loaded BaLa4Ti4O15 photocatalyst is active for CO2 reduction to
form CO using water as an electron donor. Although the
selectivity for the CO formation is much higher than that over a
Cu cocatalyst-loaded ZrO2 photocatalyst, it still remains 69%. In
this case, the Ag cocatalyst plays an important role for the
photocatalytic CO2 reduction. Similarly, KCaSrTa5O15, ZnTa2O6,
SrO-modified Ta2O5, Zn-doped Ga2O3, ZnGa2O4, La2Ti2O7 and
CaTiO3 photocatalysts also show activity for CO2 reduction to
form CO by loading the Ag cocatalyst.[11−18] These metal oxide
well as the Ba-doped photocatalyst in the aqueous NaHCO3 solution. photocatalysts respond to only UV light because of their wide
The selectivity for the CO formation (CO/(CO+H2)) on Ca, Sr and
Ba-doped NaTaO3 photocatalysts with Ag cocatalyst reached around
90%.
band gaps. Recently, we have successfully demonstrated CO2
reduction to form CO accompanied by certain O2 generation due
to water oxidation under visible light irradiation using
a
powdered Z-scheme system consisting of CuGaS2 as a CO2-
reducing photocatalyst, CoOx/BiVO4 as an O2-evolving
photocatalyst and reduced graphene oxide (RGO) as a solid-
state electron mediator.[19] However, the number of the
photocatalysts for CO2 reduction is limited even under UV
irradiation at the present stage. Exploration of highly efficient
photocatalysts for CO2 reduction is still an important research
topic.
Mg, Ca, Sr, Ba and La-doped NaTaO3 (NaTaO3:A (A = Mg,
Ca, Sr, Ba and La)) photocatalysts with 4.1 eV of a band gap
show high activities for water splitting.[20−23] Their conduction
bands formed by Ta5d orbitals are thermodynamically preferable
for the CO2 reduction. Therefore, it is expected that the
NaTaO3:A (A = Mg, Ca, Sr, Ba and La) photocatalysts show high
activity for the CO2 reduction using water as an electron donor.
Recently, several groups have reported CO2 reduction on
NaTaO3 photocatalysts in an aqueous solution.[24−26] Small
amounts of hydrocarbons and alcohols are obtained. However,
O2 evolution is not observed despite the absence of sacrificial
electron donor, indicating that the ratio of the number of reacted
electrons to that of holes is not unity.
Introduction
Photocatalytic reduction of CO2 has attracted attention from a
viewpoint of artificial photosynthesis. A TiO2 photocatalyst has
mainly been studied so far.[1−5] It has been reported that the TiO2
photocatalyst produces formic acid, methanol and methane
without cocatalysts, and formic acid, formaldehyde, methane
and other reduction products with cocatalysts. Sayama and
coworkers have reported a ZrO2 photocatalyst with 5.0 eV of a
band gap for water splitting and CO2 reduction accompanied by
O2 evolution.[6−7] The ZrO2 photocatalyst possesses a high
conduction band level resulting in showing the activity for the
CO2 reduction without any cocatalysts. The selectivity for CO
formation is improved by loading a Cu cocatalyst.[7] They have
also found the positive effect of addition of Na2CO3 and NaHCO3
[a]
Ms. H. Nakanishi, Mr. K. Iizuka, Dr. T. Takayama, Prof. A. Iwase,
Prof. A. Kudo
Department of Applied Chemistry, Faculty of Science, Tokyo
University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-
8601, Japan
In the present study, effects of an Ag cocatalyst on
photocatalytic CO2 reduction over NaTaO3:A (A = Mg, Ca, Sr, Ba
and La) were examined. The effects of acid and base on the
photocatalytic CO2 reduction were also investigated to improve
the activity and to clarify the reactant species, CO32−, HCO3− or a
CO2 molecule.
E-mail: a-kudo@rs.kagu.tus.ac.jp
[b]
Prof. A. Iwase, Prof. A. Kudo
Photocatalysis International Research Center, Research Institute
for Science and Technology, Tokyo University of Science, 2641
Yamazaki, Noda-shi, Chiba, 278-8510, Japan
Supporting information for this article is given via a link at the end
of the document.
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10.1002/cssc.201601360
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Results and Discussion
Photocatalytic reduction of
CO2
Table 1 shows a loading effect
of cocatalysts on photocatalytic
CO2 reduction over NaTaO3:A (A
= Mg, Ca, Sr, Ba and La). We
Table 1. CO2 reduction over NaTaO3:A (A = Mg, Ca, Sr, Ba and La) photocatalysts with various cocatalysts.[a]
Activity / µmol h−1
Entry
Dopant
Cocatalyst
(wt%)
Loading method
CO selectivity
H2
152
49
O2
76
23
36
CO
0
%
0
1
None
None
−
have
reported
that
these
photocatalysts were active for
2
trace
0
−
0
Mg(5%)
Ca(5%)
Sr(5%)
Ba(5%)
La(2%)
None
None
−
water
splitting
without
water
cocatalysts.[20−23]
Only
3
74
None
−
splitting proceeded on the pristine
NaTaO3:A (A = Mg, Ca, Sr, Ba
and La) photocatalysts even
4
181
182
230
32
66
79
0
0
None
−
−
−
4
trace
trace
1.4
5
None
−
under CO2 bubbling of
1 atm
(Entries 1−6), indicating that they
were not active for CO2 reduction
without cocatalysts. In contrast,
all of Ag-loaded photocatalysts
109
16
6
None
−
7
Ag(1.0)
Ag(0.5)
Ag(1.0)
Ag(1.0)
Ag(0.5)
Ag(1.0)
Ag(1.0)
Ag(1.0)
Ag(1.0)
Ag(1.0)
NiO(0.2)
Ni(0.5)
Cu(0.5)
Ru(0.2)
Rh(0.5)
Pd(1.0)
Au(1.0)
Photodeposition
Photodeposition
Photodeposition
Photodeposition
Photodeposition
Photodeposition
Photodeposition
Impregnation[b]
Impregnation[b]+H2 red[c]
Liquid-phase reduction
Impregnation[b]
Photodeposition
Photodeposition
Photodeposition
Photodeposition
Photodeposition
Photodeposition
32
15
1.8
8
5
Mg(5%)
Ca(5%)
Sr(5%)
Ba(5%)
Ba(5%)
La(2%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
showed
activities
for
CO2
57
28
4.2
9
7
reduction to form CO (Entries
7−13). Formic acid and methane
were not detected. Ba was the
most effective dopant among
10
11
12
13
14
15
16
17
18
19
20
21
22
23
47
96
24
43
3.9
1.7
8
2
alkaline
lanthanum
earth
metals
under
and
the
65
30
7.4
1
10
1
experimental condition in Table 1.
A liquid-phase reduction method
for loading of the Ag cocatalyst
was superior to photodeposition
and impregnation methods for the
103
68
46
33
0.6
1.4
25
1
39
18
3
CO
formation
on
the
20
20
56
−
−
−
−
−
−
−
Ag/NaTaO3:Ba
photocatalyst
trace
(Entries 12, 14−16) as seen for
Ag/BaLa4Ti4O15.[9] The rate of CO
formation was similar to that of H2
formation when 1 wt% of the Ag
cocatalyst was loaded on the
NaTaO3:Ba photocatalyst by the
liquid-phase reduction method
(Entry 16). The activities for CO2
reduction using other cocatalysts
were negligible (Entries 17−23).
NiO, Ru and Au cocatalysts that
2778
234
218
788
31
1314
110
100
388
13
trace
trace
trace
trace
43
16
trace
trace
685
360
are good cocatalysts for water
just enhanced
water splitting even under a CO2
[a] Catalyst: 0.25−0.5 g, reactant solution: water (360−370 mL), CO2 flow rate: 30 mL min−1, light source: a 400 W
high-pressure mercury lamp, reactor: an inner irradiation quartz cell. [b] 723 K for 1 h, [c] 473 K for 2 h. CO
selectivity (%) = (Rate of CO formation)/(Sum of rates of H2 and CO formation)×100.
10, 27]
splitting[9,
atmosphere (Entries 17, 20 and
hydrogencarbonate on the photocatalytic CO2 reduction was
significant not only for the Ba-doped photocatalyst but also for
Mg, Ca, Sr and La-doped photocatalysts (Entries 24−28). High
activities were especially obtained when Ca, Sr and Ba dopants
were employed.
23). Cu was also an effective cocatalyst for water splitting under
the present experimental condition (Entry 19).
Effects of additions of acid and base on photocatalytic CO2
reduction
The effects of acid and base on the photocatalytic activity
were examined using Ag(3 wt%)/NaTaO3:Ba in detail. The
Effects of additions of acid and base into a reactant solution
on CO2 reduction over Ag/NaTaO3:A (A = Mg, Ca, Sr, Ba and
La) photocatalysts are shown in Table 2. The effect of
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and NaOH judging from the particle
Table 2. Effect of reactant solution on photocatalytic CO2 reduction over NaTaO3:A (A = Mg, Ca, Sr, Ba and
La) with Ag cocatalyst loaded by a liquid-phase reduction method.[a]
size
and
surface
plasmonic
absorption of the Ag cocatalyst.
These results indicated that the
change in the selectivity was not
mainly due to the change in the
states of the Ag cocatalyst. Next, we
Entry
Dopant
Ag cocatalyst
/ wt%
0.35
2.0
Additive
pH[b]
Activity / µmol h−1
H2 CO
CO selectivity
(mol L−1
)
O2
12
84
102
76
59
43
37
44
51
33
58
84
21
37
%
54
91
86
84
38
52
95
88
89
83
90
8
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Mg(5%)
Ca(5%)
Sr(5%)
Ba(5%)
La(2%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
Ba(5%)
NaHCO3 (0.1)
NaHCO3 (0.1)
NaHCO3 (0.1)
NaHCO3 (0.1)
NaHCO3 (0.1)
None
7.0
7.0
7.0
7.0
7.0
4.3
7.4
7.7
7.0
7.5
7.5
2.6
5.5
5.0
12
15
28
24
80
41
3
14
148
176
125
50
examined
the
reactants
for
photocatalytic CO2 reduction. In the
present experiment condition in
which CO formation was enhanced,
because CO2 gas is continuously
2.0
3.0
2.0[c]
3.0
bubbled,
carbonate,
hydrogencarbonate and solvated
CO2 molecules existed in addition of
Na+ or K+ in the aqueous solutions.
Therefore, we checked if carbonate
and hydrogencarbonate ions are the
44
3.0
Na2CO3 (0.05)
NaOH (0.1)
KHCO3 (0.1)
K2CO3 (0.05)
KOH (0.1)
63
3.0
10
10
9
74
reactants.
photocatalytic
Figure
3
shows
on
3.0
84
reactions
Ag/NaTaO3:Ba with not CO2 but Ar
bubbling in aqueous Na2CO3 and
NaHCO3 solutions. pH of an
aqueous Na2CO3 solution was about
11 indicating existence of carbonate
3.0
45
3.0
10
177
28
35
88
3.0
H2SO4
15
and hydrogencarbonate ions, and a
negligible amount of CO2 molecules
3.0
NaCl (0.1)
16
36
55
3.0
Na2SO4 (0.05)
42
under the equilibrium condition with
Ar bubbling.[28] Only water splitting
[a] Photocatalyst: 0.25−0.5 g, reactant solution: water (360 mL), CO2 flow late: 30 mL min−1, light source: a
400 W high-pressure mercury lamp, reactor: an inner irradiation quartz cell. [b] After CO2 bubbling, [c]
photodeposition. CO selectivity (%) = (Rate of CO formation)/(Sum of rates of H2 and CO formation)×100.
and negligible CO2 reduction were
observed under this condition. This
result indicated that carbonate and
selectivity for CO formation was 52% in water with no additives
as shown in Table 2, Entry 29. When basic compounds such as
NaHCO3 were added into the reactant solution, the selectivity for
CO formation drastically increased (Entries 27, 30−34). This is
due to suppressing H2 formation under the basic condition
and/or enhancement of CO2 supply as discussed later. Similar
results have been reported for a Zn-doped Ga2O3 photocatalyst
by comparing the performances in water and an aqueous
NaHCO3 solution.[15] The highest selectivity reached about 90%.
It indicated that the basic compound mainly affected the
selectivity rather than the numbers of reacted electrons and
holes. In contrast, when H2SO4 was added, H2 evolution was
enhanced and the CO formation was suppressed (Entry 35).
NaCl and Na2SO4 were not effective for improving the activity for
CO formation (Entries 36 and 37). We investigated to see
whether the change in the selectivity of CO formation by the
addition of basic compounds was due to the change in condition
of the Ag cocatalyst or the role of hydrogencarbonate and/or
carbonate. SEM and DRS were measured to clarify the condition
of the Ag cocatalyst as shown in Figures 1 and 2. The states of
the Ag cocatalysts after the reactions in water with no additives
were similar to those in additions of H2SO4, NaHCO3, Na2CO3
hydrogencarbonate ions were not directly reduced with
photogenerated electrons. On the other hand, carbonate,
hydrogencarbonate ions and molecular CO2 existed in an
aqueous NaHCO3 solution because the pH was about 9. CO2
was released from the aqueous NaHCO3 solution under Ar
bubbling and CO simultaneously formed. The rate of CO
formation decreased as the released rate of CO2 decreased.
These results indicated that the reactant was molecular CO2 as
concluded for electrochemical reduction of CO2.[29−31]
13CO2 was employed to confirm the carbon source of
produced CO using GC-MS with a molecular sieve-type column
(Figure S1). In this isotopic experiment, NaHCO3 was not added
to avoid an isotopic exchange that causes mixing of 13CO2 with
12CO2. Only 13CO was detected from 13CO2 whereas 12CO
formed from 12CO2. It was concluded from experiments using
aqueous Na2CO3 and NaHCO3 solutions and 13CO2 that CO2
molecules are the reactant of the photocatalytic reduction of CO2
to form CO.
The reaction scheme of photocatalytic CO2 reduction on Ag-
cocatalyst loaded photocatalysts in water and an aqueous
hydrogencarbonate solution is proposed in Figure 4.
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10
8
120
100
80
60
40
20
0
(a)
6
4
2
0
0
1
2
3
4
5
Time / h
400
300
200
100
0
300
250
200
150
100
50
(b)
0
0
1
2
3
4
5
Time / h
Figure 1. Scanning electron microscope images of (a) pristine
NaTaO3:Ba(5%), Ag(3 wt%)-loaded NaTaO3:Ba(5%) after photocatalytic CO2
reduction in (b) water and aqueous solutions containing (c) H2SO4, (d)
NaHCO3 and (e) Na2CO3.
Figure 3. Reduction of CO2 released from (a) 0.05 mol L−1 of an aqueous
Na2CO3 solution and (b) 0.1 mol L−1 of an aqueous NaHCO3 solution over
Ag(3 wt%)/NaTaO3:Ba(5%) photocatalyst. Photocatalyst: 0.5 g, reactant
solution: 350 mL, Ar flow rate: 30 mL min−1, light source: a 400 W high-
pressure mercury lamp, reactor: an inner irradiation quartz cell. H2 (○), O2 (●),
CO×10 (△) and CO2 (□). The Ag cocatalyst was loaded by a liquid-phase
reduction method.
CO2•− (ad) + H(ad) → CO + OH−
2H(ad) →H2
(3)
(4)
(5)
(6)
(7)
(8)
(9)
2−
2CO2•−(ad) → CO + CO3
CO2•− + H+ → COOH(ad)
COOH(ad) + e− → CO + OH−
CO2 + H2 → CO + H2O
HCO3− ↔ CO2 + OH−
(b)
(c)
On the surface of a photocatalyst;
(d)
(a)
2H2O + 4h+ → 4H+ + O2
(10)
250 300 350 400 450 500 550 600 650
Wavelength / nm
A CO2 molecule adsorbs on an Ag cocatalyst to form CO2•−(ad)
by capturing a photogenerated electron on the Ag cocatalyst (Eq.
1). The negative charge of CO2•−(ad) may be less than minus
Figure 2. Diffuse reflection spectra of (a) pristine NaTaO3:Ba(5%), Ag(3
wt%)/NaTaO3:Ba(5%) after photocatalytic CO2 reduction in (b) water, (c) an
aqueous H2SO4 solution and (d) an aqueous NaOH (NaHCO3) solution.
•−
one. The redox potential of the CO2/CO2 couple in water is
−1.9 V vs. NHE.[30] However, considering previous reports,[31]
CO formation by an electrochemical reduction of CO2 starts
around −0.7 V vs. NHE at pH 0 on an Ag electrode, and the
•−
On the surface of an Ag cocatalyst;
reason is considered to be the stabilization of CO2 by
CO2 + e− → CO2•−(ad)
(1)
(2)
adsorption on the Ag surface.[30] The conduction band level of a
NaTaO3:A (A = Mg, Ca, Sr, Ba and La) photocatalyst is about
H2O + e− → H(ad) + OH−
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Figure 4. Proposed mechanism of CO2 reduction to form CO over Ag-loaded NaTaO3:A (A = Mg, Ca, Sr, Ba and La) photocatalysts through Path
A; hydrogenation of CO2•−(ad) and Path B; reduction of COOH(ad).
−1.1
V
vs. NHE at pH0.[20−23] Therefore, photogenerated
1.0
(a)
electrons should possess the potential for the reduction of CO2
to form CO2•−(ad). H(ad) can be formed by the reduction of H2O
with a photogenerated electron on the Ag cocatalyst (Eq. 2). The
CO2•−(ad) species reacts with H(ad) to give CO and OH− (Eq. 3),
while a part of H(ad) is consumed to form H2 (Eq. 4), which
process is described as Path A in Figure 4. CO can also form by
disproportionation of CO2•−(ad) (Eq. 5). However, Eq. 3 is the
major route to form CO rather than Eq. 5 in an aqueous solution.
Another possible reaction path (Path B in Figure 4) is that H+
attacks the CO2•−(ad) species to form COOH(ad) (Eq. 6). The
COOH(ad) species successively captures an electron to release
CO and OH− (Eq. 7). It is not excluded that some photochemical
reactions of intermediates take part in the photocatalytic CO2
reduction. Although one might think that CO2 would react with H2
evolved by water splitting (Eq. 8), the reaction hardly proceeds
because dissociative adsorption of H2(g) to H(ad) hardly occurs
on the metallic Ag under ambient condition in an aqueous
solution. The reason why the addition of a base enhances CO
formation is that CO2 molecules are smoothly supplied to the Ag
surface of the reduction site of CO2 by releasing CO2 from
hydrogencarbonate ions due to equilibrium balance (Eq. 9). In
other words, hydrogencarbonate functioned as a buffer for
supplying CO2 molecules. On the other hand, O2 forms with
water oxidation by photogenerated holes on the photocatalyst
surface (Eq. 10).
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
6
7
Time / h
1.2
1.0
0.8
0.6
0.4
0.2
0
(b)
0
1
2
3
4
5
6
Time / h
2.0
1.5
1.0
0.5
0
(c)
Figure
Ag/NaTaO3:Ca,
5
shows CO2 reduction over the optimized
Ag/NaTaO3:Sr and Ag/NaTaO3:Ba
photocatalysts in an aqueous NaHCO3 solution. CO and H2 were
obtained as reduction products of CO2 and water, respectively,
accompanied by O2 evolution due to oxidation of water. The ratio
of reacted electrons to holes calculated by the equation (11) was
almost unity, indicating that water was an electron donor for the
CO2 reduction.
e−/h+ = (Sum of the number of electrons consumed for H2 and
CO formation)/(The number of holes consumed for O2 formation)
(11)
0
1
2
3
4
5
6
Time / h
Figure 5. CO2 reduction over (a) Ag(2 wt%)/NaTaO3:Ca(5%) photocatalyst, (b)
Ag(2 wt%)/NaTaO3:Sr(5%) and (c) Ag(3 wt%)/NaTaO3:Ba(5%) photocatalysts.
Photocatalyst: 0.5−1.0 g, reactant solution: 360 mL, CO2 flow rate: 30−100 mL
min−1, light source: a 400 W high-pressure mercury lamp, reactor: an inner
irradiation quartz cell; H2(○), O2(●), CO(△).
Turnover numbers of consumed electrons for CO formation to
the total numbers of Ag atoms in cocatalysts on Ag/NaTaO3:Ca,
Ag/NaTaO3:Sr and Ag/NaTaO3:Ba photocatalysts calculated by
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the equation (12) were 19, 22 and 13 at about 6 h of the reaction
time, respectively.
99.99%), MgO (Kanto Chemical; 99%), CaCO3 (Kanto Chemical;
99%), SrCO3 (Kanto Chemical; 99.9%), BaCO3 (Kanto
Chemical; 99.0%) and La2O3 (Kanto Chemical; 99.99%) were
mixed in a mortar at the ratio of Na:A:Ta = 1.05:0.02−0.05:1.
The excess amount of sodium (5 mol%) was added in the
starting mixture to compensate the volatilization. The mixtures
were calcined in air at 1173 K for 1 h and then 1423 K for 10 h
using a platinum crucible.
TONCO/Ag
formation)/(The total number of Ag atoms in a cocatalyst on a
photocatalyst) (12)
= (The number of electrons consumed for CO
The selectivities for the CO formation on Ag/NaTaO3:Ca,
Ag/NaTaO3:Sr and Ag/NaTaO3:Ba photocatalysts were 87%,
85% and 91% at about 6 h even in an aqueous medium,
respectively. Thus, CO2 reduction predominately proceeded in
an aqueous medium. Ag/NaTaO3:Sr and Ag/NaTaO3:Ba
photocatalysts (CO: 176 and 318 µmol h−1; Figure 5 (b) and (c))
with 4.1 eV of the band gap showed higher activity than a
Ag/ZnGa2O4 photocatalyst[16] (CO: 155 µmol h−1) with 4.7 eV of
the band gap under the similar experimental condition.
Assuming the level of the valence band maximum of NaTaO3:A
(A = Sr and Ba) formed by O2p is +3.0 V vs. NHE at pH 0, the
level of the conduction band minimum is −1.1 V vs. NHE at pH 0
because of 4.1 eV of the band gap.[21−23, 32] It has been reported
that the conduction band minimum of ZnGa2O4 is located at
more negative level than that of β-Ga2O3 (−1.1 V vs. NHE at pH
0).[33−35] Thus, Ag/NaTaO3:A (A = Sr and Ba) photocatalysts
would be thermodynamically inferior to the Ag/ZnGa2O4
photocatalyst for CO2 reduction. However, Ag/NaTaO3:A (A = Sr
and Ba) actually showed higher activity than Ag/ZnGa2O4. This
may be due to good affinity of an Ag cocatalyst with alkaline and
La-doped NaTaO3 photocatalysts that show high quantum yields
for water splitting.[21−23]
Loading methods of cocatalysts
Ni, Pd, Rh, Cu, Au and Ru cocatalysts were loaded on
photocatalysts by photodeposition in situ from aqueous solutions
dissolving Ni(NO3)2, PdCl2, Rh(NO3)3, CuSO4, HAuCl4 and RuCl3.
An NiO cocatalyst was loaded by an impregnation method.
Photocatalyst powder was dispersed in an aqueous Ni(NO3)2
solution. Water of the suspension was evaporated on a water
bath. The dried powder was calcined at 573 K for 1 h in air using
a
muffle furnace. An Ag cocatalyst was loaded by
photodeposition in situ, impregnation and liquid-phase reduction
methods using AgNO3. In the impregnation method, the dried
powder was calcined at 723 K for 1 h in air. The Ag-impregnated
photocatalyst was reduced by H2 gas at 473 K for 2 h to obtain a
metallic Ag cocatalyst. In the liquid-phase reduction method,
NaPH2O2 was employed as a reducing reagent. After addition of
an aqueous AgNO3 solution into an aqueous suspension
containing the photocatalyst, an aqueous solution dissolving an
equimolar amount of NaPH2O2 to Ag+ was added into the
suspension. The mixture was stirred at 333 K for 1 h. The
obtained Ag-loaded photocatalysts were washed with water and
were collected by filtration.
Conclusions
Characterization of photocatalysts
The photocatalysts obtained were identified by powder X-ray
diffraction (Rigaku; Miniflex, Cu Kα). Diffuse reflection spectra
were recorded on a UV-vis-NIR spectrometer (JASCO; UbestV-
570) and were converted from reflection to absorbance by the
Kubelka-Munk method. Photocatalysts and cocatalysts loaded
were observed using a scanning electron microscope (JEOL;
JSM-6700F).
Ag cocatalyst-loaded NaTaO3:A (A = Mg, Ca, Sr, Ba and La)
photocatalysts showed activity for CO2 reduction to form CO
using water as an electron donor. Among them, NaTaO3:A (A =
Ca, Sr and Ba) with an Ag cocatalyst loaded by a liquid-phase
reduction showed the highest activity. Experiments using
aqueous Na2CO3 and NaHCO3 solutions with Ar bubbling
revealed that the reactant of photocatalytic CO2 reduction was a
CO2 molecule. Hydrogencarbonate ions functioned as a buffer to
supply CO2 molecules smoothly to the Ag cocatalyst in addition
to CO2 gas bubbling as well as suppression of H2 formation. This
resulted in the improvement of the selectivity for the CO
formation up to about 90%. The synergetic effects of addition of
basic compounds and CO2 bubbling were found for
photocatalytic CO2 reduction using water as an electron donor.
This effect will be able to be applied to other photocatalysts with
high conduction band levels.
Photocatalytic reactions
Photocatalytic reactions were carried out using a gas-flow
system with an inner irradiation cell made of quartz equipped
with a 400 W high-pressure mercury lamp as a light source.
Photocatalyst powder (0.25–1.0 g) was dispersed in water (350–
370 mL). CO2 gas was bubbled into the suspension at about
30−100 mL min−1 of a flow rate. H2SO4 (Kanto Chemical; 96.0%),
NaCl (Kanto Chemical; 99.5%), Na2SO4 (SIGMA-ALDRICH;
98%), NaHCO3 (Wako Pure Chemical; 99.5%), KHCO3 (Wako
Pure Chemical; 99.5%), Na2CO3 (Kanto Chemical; 99.8%),
K2CO3 (Kanto Chemical; 99.5%), NaOH (Kanto Chemical;
95.0%) and KOH (Kanto Chemical; 86.0%) were added into the
reactant solution, if necessary. Gas products of H2, O2, CO and
CH4 were determined using gas chromatographs (Shimadzu;
GC-8A) with a thermal conductivity 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
Experimental Section
Preparation of photocatalysts
Mg, Ca, Sr, Ba and La-doped NaTaO3 powders were
prepared by a solid-state reaction.[20−23] The starting materials,
Na2CO3 (Kanto Chemical; 99.8%), Ta2O5 (Rare Metallic;
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Entry for the Table of Contents
FULL PAPER
Text for Table of Contents
Haruka Nakanishi, Kosuke Iizuka,
Tomoaki Takayama, Akihide Iwase,
Akihiko Kudo*
Loading of an Ag cocatalyst and
addition of bases realize highly active
and selective CO2 reduction to form
CO using water as an electron
Page No.1 – Page No.7
Highly Active Alkaline Earth Metals
and Lanthanum-Doped NaTaO3
Photocatalysts for CO2 Reduction to
Form CO Using Water as an Electron
Donor
source over NaTaO3:A (A = Mg, Ca,
Sr, Ba and La) photocatalysts.
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