Photocatalytic conversion of CO2 in water using fluorinated layered double hydroxides as photocatalysts

Article abstract of DOI:10.1016/j.apcata.2015.11.023

The photocatalytic conversion of CO₂, which can convert CO₂ into useful chemicals, such as CH₄, CH₃OH, HCOOH, HCHO, and CO, is attractive process for creating an artificial carbon cycling system. It was found that the synthetic Ni-Al layered double hydroxide (Ni-Al LDH) can reduce CO₂ into CO in an aqueous solution of NaCl under UV light irradiation, and chloride ions (Cl^-) in the reaction solution scavenge the photogenerated holes. In this study, we prepared fluorinated Mg-Al LDH and Ni-Al LDH for altering the chemical and/or physical properties of the LDH as a solid base, and investigated the effect of fluorination on the photocatalytic activity for the conversion of CO₂ in an aqueous solution. We found that the fluorination of LDH by incorporation of hexafluoroaluminate (AlF₆^3-) units in the hydroxide sheets clearly improved the amount of CO evolved as a reduction product of CO₂.

Full text of DOI:10.1016/j.apcata.2015.11.023

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Photocatalytic conversion of CO2 in water using fluorinated layered
double hydroxides as photocatalysts
Shoji Iguchia, Kentaro Teramuraa,b,c,∗, Saburo Hosokawaa,b, Tsunehiro Tanakaa,b,∗
a Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
b Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
c Precursory Research for Embryonic Science and
Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1- 8Honcho, Kawaguchi, Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The photocatalytic conversion of CO2, which can convert CO2 into useful chemicals, such as CH4, CH3OH,
HCOOH, HCHO, and CO, is attractive process for creating an artificial carbon cycling system. It was found
that the synthetic Ni–Al layered double hydroxide (Ni–Al LDH) can reduce CO2 into CO in an aqueous
solution of NaCl under UV light irradiation, and chloride ions (Cl−) in the reaction solution scavenge the
photogenerated holes. In this study, we prepared fluorinated Mg–Al LDH and Ni–Al LDH for altering the
chemical and/or physical properties of the LDH as a solid base, and investigated the effect of fluorination
on the photocatalytic activity for the conversion of CO2 in an aqueous solution. We found that the fluo-
rination of LDH by incorporation of hexafluoroaluminate (AlF63−) units in the hydroxide sheets clearly
improved the amount of CO evolved as a reduction product of CO2.
Received 31 August 2015
Received in revised form
10 November 2015
Accepted 13 November 2015
Available online xxx
Keywords:
CO2 reduction
Photocatalyst
© 2015 Elsevier B.V. All rights reserved.
Layered double hydroxide
Fluorination
1. Introduction
in water, for the selective conversion of CO2 is required for the
construction of a photocatalytic carbon cycling system.
The development of a chemical process that can fix CO2 using
natural energy sources is necessary for creating an artificial carbon
cycling system. The photocatalytic conversion of CO2 is considered
one of the most attractive processes for converting CO2 into useful
chemicals, such as CH4, CH3OH, HCOOH, HCHO, and CO, because,
in future, it might be able to use solar energy as the driving source.
Our research group has previously reported the photocatalytic con-
version of CO2 using H2 as a reductant over metal oxides such as
ZrO2 [1–4], MgO [5,6], ATaO3 (A = Li, Na, and K) [7], and Ga2O3 [8].
The results showed that the adsorbed CO2 species on the surface of
these photocatalysts play an important role in the photocatalytic
conversion of CO2. This indicates that solid base materials that have
the ability to provide suitable surface properties for CO2 adsorption
should be promising candidates for this photocatalytic reaction.
However, because their basic sites are easily poisoned by water,
these metal oxide photocatalysts do not show promising results
for the conversion of CO2 in aqueous solutions. The development
of a highly stable photocatalytic system, capable of functioning
Layered double hydroxides (LDHs) [9–11], which can function
as solid base materials in the presence of water [12], are presented
by the general formula [M2+1−z–M3+ (OH)2](An−
)
·mH2O. M2+
z
z/n
and M3+ are the divalent and trivalent cations, respectively. An− is
the interlayer anion of valence n and compensates for the positive
charge of the hydroxide sheets. The value z represents the molar
ratio of M3+/(M2+ + M3+) within the hydroxide sheets, and it has
been reported that the LDH structure can exist for z values in the
0.1–0.5 range [9]. The nature of M2+, M3+, z, and An− can be altered
over a wide range of values; therefore, by referring to published
reports, various isostructural materials with varied chemical prop-
erties can be successfully synthesized [13–28]. In recently years, the
application of various LDHs to photocatalytic reactions, including
the degradation of organic compounds [17,29–33] and water split-
ting [23,34,35], has been studied. For example, the photocatalytic
evolution of H2 from water using Ni–Ti LDH as the photocatalyst
was achieved in the presence of a sacrificial reagent under visible
light irradiation [35]. Previously, we applied a series of synthetic
LDHs to the photocatalytic conversion of CO2 in water and found
that Ni–Al LDH exhibited high activity for the CO evolution under
UV light irradiation [36,37]. The use of Ni–Al LDH enabled the selec-
tive formation of CO, because the amount of H2 evolved by the
reduction of H+ derived from water was much smaller than that of
∗
Corresponding authors at: Department of Molecular Engineering, Graduate
School of Engineering, Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto
615-8510, Japan.
E-mail address: teramura@moleng.kyoto-u.ac.jp (K. Teramura).
http://dx.doi.org/10.1016/j.apcata.2015.11.023
0926-860X/© 2015 Elsevier B.V. All rights reserved.
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other LDHs. Moreover, in the presence of Ni–Al LDH, the addition of
chloride ion (Cl−) to the reaction solution enhanced the conversion
of CO2 to CO, and a stoichiometric amount of hypochlorous acid
(HClO) was produced by the oxidation of Cl− [38].
diffuse reflection spectra of LDHs were collected on a UV–visible
Spectrometer (V-650, JASCO) equipped with an integrated sphere
accessory. Thermal analysis for the synthesized LDHs were carried
out using a thermobalance (Thermo Plus 2, Rigaku) under dried
air atmosphere at a flow rate of 80 mL min−1. The specific surface
areas of samples were estimated from the N2 adsorption isotherms
at 77 K using a surface area analyzer (BELSORP-miniII, BEL Japan,
Inc.). Prior to the measurements, LDHs were evacuated at 383 K
for 1 h using a pretreatment instrument (BELPREP-vacII, BEL Japan,
Inc.). X-ray photoelectron spectra (XPS) profiles of the synthesized
LDHs were collected by an X-ray photoelectron spectrometer (ESCA
3400, Shimadzu Corp.). The narrow range scans for Ni 2p and F 1s
spectra were performed using an Mg K␣ X-ray source, and the peak
positions were calibrated by the peak that can be attributed to C 1s.
The peak intensity of F 1s spectra was normalized with that of Ni
2p3/2 peak.
Many attempts have been made to modify the chemical and/or
physical properties of LDHs. A series of noble metal particles loaded
on the surface of Mg–Al LDH exhibited excellent catalytic activi-
ties for dehydrogenation [39], oxidation [40], and reforming [41]
processes. The intercalation of different kinds of functional com-
pounds, such as polyoxometalates (POM) [42–44], metal complexes
[45,46], and organic dyes [47,48], into the interlayer space of LDHs
was presented by many researchers. Moreover, some composite
materials with metal oxides, such as TiO2/Mg–Al LDH [49,50] and
CeO2/Mg–Al LDH [51], have also been reported to improve the
photocatalytic activity. In 2012, Lima et al. developed the fluori-
nated Mg–Al LDH using Na3AlF6 as a fluorine source, wherein some
(Al(OH)6)3− octahedral structures within the hydroxide sheets
3−
were partially substituted by AlF6 units [52]. The fluorination of
2.3. Photocatalytic conversion of CO2 in an aqueous solution
LDH via intercalation of fluoride anions (F−) in the interlayer space
has also been reported previously [53–55]. It is expected that F−
should ultimately be exchanged with other selective anions such
as CO32−, because F− is not stable as a charge compensating anion
of LDH. Therefore, the incorporation of fluorine, by substituting the
OH groups, as part of the brucite-like hydroxide layer is a new strat-
egy for altering the chemical and/or physical properties of the LDH
material as a solid base. In fact, Lima et al. have suggested that,
The photocatalytic activity of synthesized F(x) M2+–Al LDHs
(M2+ = Mg2+ or Ni2+) for the conversion of CO2 in water were
evaluated by using a quartz inner-irradiation type reaction vessel
connected to the closed circulation system. 0.5 g of photocatalyst
powder was dispersed in 350 mL of ultra-pure water, and the sus-
pension was degassed at room temperature. 7.6 mmol of CO2 gas,
which was purified by a vacuum distillation at liquid N2 tem-
perature, was introduced into the free gas space. The suspension
was irradiated under a 400 W high-pressure Hg lamp (HL400BH-9,
SEN LIGHTS Corp.) through a quartz filter equipped with a cooling
water system. The gas phase products were analyzed by a thermal
conductivity detector gas chromatography (TCD-GC) using a GC-
8A chromatograph (GC-8AIT, Shimadzu Corp.) fitted out a MS-5A
packed column. GC–MS analysis was performed using a quadruple-
type mass spectrometer (BEL Mass, BEL Japan, Inc.) in the case of
an isotopic experiment. The selectivity toward CO evolution among
reduction products was calculated by following equation.
because of its high electronegativity, the incorporation of fluorine
3−
as AlF6
units into the hydroxide sheets greatly influences the
basicity of the LDH [52]. We predict that the CO2 adsorption on the
surface of the LDH is enhanced by the fluorination. Thus, the selec-
tivity toward CO2 reduction should be improved. In this study, we
prepared fluorinated Mg–Al LDH and Ni–Al LDH with different fluo-
rine contents by following the previously reported procedures and
investigated the effect of fluorination on the photocatalytic activity
for the conversion of CO2 in an aqueous solution.
Selectivity toward CO evolution (%) = 100 × ACO/(ACO + AH2
)
2. Experimental
ACO: amount of CO evolved / ␮mol
AH2: amount of H2 evolved / ␮mol
2.1. Catalyst preparation
The amount of HClO produced in the reaction solutions
was determined by a DPD (N,N’-dimethyl-p-phenylenediamine)
method [56,57], which is one of the most popular techniques to
observe HClO in water. The DPD solution was prepared by dissolv-
ing 0.06 g of the DPD reagent, which is a mixture of DPD sulphate
and Na2SO4, in 3.0 mL of phosphate buffer solution (pH 6.5). Follow-
ing this, 1.0 mL of the prepared DPD solution was added to 5.0 mL
of the sample solution, and was shaken for 20 s. The transmittance
spectrum was measured immediately using a multiscan UV–vis
spectrometer (MCPD-7700, Otsuka Electronics Co., Ltd.). The con-
centration of HClO in the aqueous solution was estimated from the
absorbance at 515 nm.
Fluorinated layered double hydroxides (F(x) M2+–Al LDH,
M2+ = Mg2+ and Ni2+) were synthesized by a typical coprecipitation
method. Precursors of metal components, such as MgCl2·6H2O or
NiCl2·6H2O, AlCl3, and Na3AlF6, were dissolved in 1.0 L of ultra-
pure water. The value of x in F(x) M2+–Al LDH means a ratio of
Na3AlF6 in the total Al species (AlCl3 + Na3AlF6) when preparing
it. The aqueous solution of precursors was dropped into an aque-
ous solution of Na2CO3 at room temperature with vigorous stirring.
The pH of the suspension was strictly kept between 9.9 to 10.1 by
using a pH controller (NPH-660NDE, Nissin Rika) equipped with a
liquid feeding pump for an aqueous solution of NaOH. The result-
ing suspension was aged at 333 K for 12 h, and then collected by
filtration. The solid precipitate was washed with 1.0 L of ultra-pure
water and dried at 383 K in air atmosphere. The atomic ratio of M2+
to Al3+ (M2+/Al3+, M2+ = Mg2+ and Ni2+) was fixed at 3 in all sam-
ples. F(7.5) imp- Ni–Al LDH was prepared as a reference sample via
an impregnation method. F(0) Ni–Al LDH powder was dispersed in
an aqueous solution which contains a required amount of Na3AlF6,
and the suspension was dried up at 353 K under air atmosphere.
3. Results and discussion
Fig. 1 displays the XRD patterns of F(0) Mg–Al LDH and F(5)
Mg–Al LDH collected over (a) a wide and (b) a narrow range of
angles, respectively. Both diffraction patterns corresponded with
the typical structure of the LDH group; i.e., the layered structure
of the hydroxide sheets grew along the c-axis with respect to the
hexagonal crystalline units. As reported by Lima et al. [52], three
sharp reflection peaks observed at around 11.4◦, 22.9◦, and 34.6◦
in the XRD pattern of the fluorinated Mg–Al LDH were assigned
to the (0 0 3), (0 0 6), and (0 0 9) phases, respectively. These reflec-
tions are related to the thickness of the layer structure, especially
the d-spacing of the (0 0 3) phase (d(0 0 3)), which corresponds to
2.2. Catalyst characterization
The powder XRD patterns of a series of prepared LDHs were
measured by an X-ray diffractometer (MultiFlex, Rigaku) using Cu
K␣1 radiation (ꢀ = 0.154 nm) at a scan rate of 4.0◦ min−1. UV/vis
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shift the peak of the (0 0 3) phase reflection to a lower angle because
of the bulky structure of AlF63−; in contrast, the ionic radius of
CO32− is 178 pm [58]. The fluorination of LDH via the incorporation
3−
of the AlF6 unit into hydroxide sheets did not change the inter-
3−
layer distance, which indicated that AlF6 units were not present
in the interlayer space. On the other hand, the peak position of the
(0 0 3) phase diffraction peak was shifted slightly to a higher angle,
as shown in Fig. 1(b) . This change indicated that the decrease in
interlayer distance was caused by fluorination. Lima et al. proposed
that the enhancement of hydrogen-bonding between the hydrox-
ide sheets and interlayer water molecules decreased the interval
between hydroxide sheets, because fluorine is one of the most elec-
tronegative elements [52]. Other intense peaks located above 60◦
are attributed to the (1 1 0) and (1 1 3) phase reflections. The cation-
to-cation distance within the hydroxide sheet (parameter a) can be
calculated using the d-spacing of the (1 1 0) phase reflection (a = 2d
(1 1 0)). The peak position of the (1 1 0) phase diffraction was not
influenced by the fluorination of hydroxide sheets, indicating that
parameter a remained unchanged. Lima et al. presented the MAS
NMR results of 27Al and 19F and reported that the AlF6
units
3−
should be incorporated randomly in the hydroxide sheets [52].
Weight loss curves of F(0) Mg–Al LDH and F(5) Mg–Al LDH, obtained
using thermal gravimetric analysis are presented in Fig. 2(a). The
LDH group structure shows weight loss in two steps. The first
weight loss happens below 473 K and is attributed to the desorption
of water molecules from the interlayer. The second loss takes place
between 473 and 673 K and is attributed to two phenomena, viz.
the decomposition of interlayer charge-compensating anions and
the dehydration of hydroxide sheets accompanied by the collapse
of the layer structure. Both F(0) and F(5) Mg–Al LDHs exhibited
similar weight loss curves. No changes were observed with regards
to the weight loss temperature and the extent of weight loss. These
results indicated that the fluorination of Mg–Al LDH influenced nei-
ther the stability of hydroxide sheets nor the amount of carbonate
ions preserved in the interlayer space. Fig. 2(b) shows the UV/vis
diffuse reflectance spectra of F(0) and F(5) Mg–Al LDHs. Both the
shape of the spectrum and the absorption edge were not changed by
the fluorination of Mg–Al LDH. On the other hand, Na3AlF6 powder
exhibited no absorption peak in the 200–800 nm range. The fluo-
3−
rination of Mg–Al LDH via incorporation of AlF6
units into the
hydroxide sheets did not change the absorption spectrum. In the
present study, the fluorination of LDH is not considered “doping”,
but is considered a “solid solution”. This is because fluorine atoms
3−
are present as a component of the AlF6 units in the brucite-like
hydroxide sheets.
Fig. 3(a) displays the time courses of CO evolution in the pho-
tocatalytic conversion of CO2 in an aqueous NaCl solution using
F(5) Mg–Al LDH photocatalyst. F(5) Mg–Al LDH photocatalyst could
reduce CO2 under UV light irradiation, and CO and H2 were evolved
with high stability as the reduction product of CO2 and proton (H+),
respectively. Other reduction products, such as methane, methanol,
formaldehyde, and formic acid were not produced in this photocat-
alytic reaction system. We previously reported that the addition of
Cl− in the reaction solution improves the activity for the photocat-
alytic conversion of CO2 into CO over Ni–Al LDH because Cl− acts
as a hole scavenger [38] and hence advances the conversion. It is
proposed that Cl− in the reaction solution enhances CO formation
based on the same mechanism as in the case of Ni–Al LDH; i.e.,
Cl− captures the photogenerated holes and it should be oxidized
to Cl2. As shown in Fig. 3(b), the photocatalytic activity for CO for-
mation over F(0) Mg–Al LDH was also enhanced by the addition of
Cl−. Moreover, the fluorination of Mg–Al LDH obviously increases
the amount of CO evolved in the photocatalytic conversion of CO2
in an aqueous NaCl solution (see difference between (ii) and (iv)).
In this case, the amount of CO produced for F(5) Mg–Al LDH was
twice that for F(0) Mg–Al LDH. On the other hand, the amount of H2
Fig. 1. XRD patterns of (i) F(0) Mg–Al LDH (black) and (ii) F(5) Mg–Al LDH (red)
collected over (a) a wide and (b) a narrow range of angles. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
the interval of adjacent hydroxide sheets. It is well known that the
intercalation of anions, which have large ionic radii, results in an
increase in the layer distance in the LDH structure. For example, the
peak position of the (0 0 3) phase was shifted to a lower angle by the
incorporation of organic dye anions or metal oxy anions. Carriazo
et al. reported that Mg–Al LDH, which contains (H2W12O40)
6− as an
interlayer charge compensating anion, shows (0 0 3) phase diffrac-
tion peak at around 6.1◦ (2ꢁ, Cu K␣1) [43]. Taking this into account,
3−
the presence of AlF6 anion in the interlayer space of LDH should
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Fig. 3. (a) Time courses of H2 (blue) and CO (red) evolution in the photocatalytic
conversion of CO2 in an aqueous NaCl solution (0.1 M) using F(5) Mg–Al LDH pho-
tocatalyst. (b) Total amounts of H2 (blue) and CO (red) after 8 h of photoirradiation
in the photocatalytic conversion of CO2 in water. (i) F(0) Mg–Al LDH photocatalyst
with no additive, (ii) F(0) Mg–Al LDH photocatalyst with 0.1 M of NaCl additive,
(iii) F(5) Mg–Al LDH photocatalyst with no additive, and (iv) F(5) Mg–Al LDH pho-
tocatalyst with 0.1 M of NaCl additive. The selectivity toward CO among reduction
products is presented with black circle in Fig. 3(B). Photocatalyst weight: 500 mg,
water: 350 mL, CO2: 7.6 mmol, light source: 400 W high-pressure Hg lamp (quartz
jacket). (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
Fig. 2. (a) Weight loss curves in thermal gravimetric analysis and (b) UV/vis diffuse
reflectance spectra of (i) F(0) Mg–Al LDH (black) and (ii) F(5) Mg–Al LDH (red). (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
Ni–Al LDH as a photocatalyst particularly suppresses H2 formation.
Therefore, Ni–Al LDH is considered one of the most favorable pho-
tocatalysts for selective CO2 in an aqueous solution [37]. Fig. 4(a)
shows the XRD patterns of F(x) Ni–Al LDH (x = 0, 2.5, 5, 7.5, 10 and
20) and Na3AlF6, which was used as a precursor for the AlF63− unit.
The typical diffraction patterns corresponding to the LDH struc-
ture were observed over the entire range of x values without any
impurity phases such as Ni(OH)2, Al(OH)3, NaF, and Na3AlF6. This
indicated that the fluorinated Ni–Al LDHs were successfully syn-
thesized via the co-precipitation method. Fig. 4(b–d) show the XRD
patterns of F(x) Ni–Al LDH (x = 0, 2.5, 5, 7.5, 10 and 20) collected in
a narrow range to observe details of the (0 0 3), (0 0 6), and (1 1 0)
phase reflections, respectively. As discussed above, the intercala-
tion of AlF63− in the interlayer of the hydroxide sheets should cause
an increase in the interlayer distance (d(0 0 3)). However, the (0 0 3)
evolved was also affected by both Cl− addition and the fluorination
of Mg–Al LDH (by comparing (i) and (ii) in Fig. 3(b)). It is well known
that selective CO2 reduction is one of the most important indices
of photocatalytic activity for CO2 conversion in water, because the
reduction of protons (H+) takes place competitively in this reaction.
In this regard, F(5) Mg–Al LDH enabled the improvement in selec-
tivity toward CO formation in an aqueous NaCl solution. Hence, we
propose that the fluorination of Mg–Al LDH leads to an enhance-
ment in both the amount of CO evolved and the selectivity toward
CO formation.
As described in our previous work, the amount of CO evolved
for Ni–Al LDH is larger than that for Mg–Al LDH, and the use of
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Fig. 4. XRD patterns of F(x) Ni–Al LDH and Na3AlF6 collected over (a) a wide range and a narrow ranges of angles for (b) (0 0 3) phase, (c) (0 0 6) phase, and (d) (1 1 0) phase
reflections. x = (i) 0, (ii) 2.5, (iii) 5.0, (iv) 7.5, (v) 10, (vi) 20, and (vii) Na3AlF6.
Table 1
Peak positions of (0 0 6) and (1 1 0) phase reflections in the XRD patterns, the calculated value of d(003), the specific surface area (SBET), and the result of the photocatalytic
activity test for F(x) Ni–Al LDH (x = 0, 2.5, 5, 7.5, 10, and 20). Photocatalyst weight: 500 mg, water: 350 mL, CO2: 7.6 mmol, light source: 400 W high-pressure Hg lamp (quartz
jacket), photoirradiation time: 8 h.
ꢂ
Position / degree
(0 0 6)
d(0 0 3) / Å
SBET / m2g−1
Amountsofproducts/␮mol
Selectivity (%)
(1 1 0)
CO
H2
0
22.90
22.92
22.95
22.97
23.00
23.10
60.88
60.82
60.80
60.80
60.82
60.82
7.76
7.75
7.75
7.74
7.73
7.70
75.1
83.4
113
113
120
18.8
30.0
36.8
47.3
31.7
18.5
20.6
8.0
7.0
12.0
14.1
12.6
47
79
84
80
69
59
2.5
5.0
7.5
10
20
86.3
and (0 0 6) phase diffraction peaks of Ni–Al LDH, which are related
to the thickness of the layer structures, shifted to slightly higher
angles with an increase in the fluorination rate. This result indi-
cated that the marginal decrease in layer distance was caused by the
incorporation of fluorine species in the hydroxide sheets of Ni–Al
LDH, which was consistent with the results obtained by Lima et al.
[52]. Table 1 lists the position of the (0 0 6) and (1 1 0) phase reflec-
tions in the XRD patterns, the calculated value of d(0 0 3), the specific
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surface area (SBET), and the results of the photocatalytic activity
test for F(x) Ni–Al LDH (x = 0, 2.5, 5, 7.5, 10, and 20). The distance
between adjacent hydroxide sheets (d(0 0 3)), which was calculated
from the position of the (0 0 6) phase reflection in this study, grad-
3−
ually decreased with an increase in the amount of AlF6 units in
the hydroxide sheets. However, the position of the (1 1 0) phase
diffraction peak was not influenced by fluorination. The amount of
CO evolved after 8 h of photoirradiation was affected by the fluo-
rination of Ni–Al LDH, and F(7.5) Ni–Al LDH exhibited the highest
activity for CO formation. In particular, 47.3 ␮mol of CO was pro-
duced after 8 h of photoirradiation using F(7.5) Ni–Al LDH in the
photocatalytic conversion of CO2 in an aqueous NaCl solution. It was
found that the excess AlF63− incorporation (x = 10 and 20) caused a
decrease in the photocatalytic conversion of CO2 to CO. On the other
hand, F(2.5) Ni–Al LDH produced less H2 than F(0) Ni–Al LDH, indi-
cating that fluorination altered the selectivity toward CO formation
among the reduction products. In fact, after 8 h of photoirradia-
tion, the selectivity toward CO clearly changed with the extent of
fluorination (as shown in Table 1). By comparing with the results
previously reported from our group [37], the photocatalytic activ-
ity of Ni–Al LDH was clearly improved by the tuning of the reaction
condition (addition of NaCl) and the chemical properties of photo-
catalyst itself (fluorination). Furthermore, the actual amount of CO2
reduction product, 47.3 ␮mol of CO after 8 h of photoirradiation in
this study, is much larger value than others reports using LDHs
photocatalysts for the conversion of CO2 [59–62]. As presented in
Table 1, the specific surface area of F(x) Ni–Al LDH was also influ-
enced by the value of x. For x = 5, 7.5, and 10, the results showed
relatively larger surface areas. We have already reported that the
amount of CO evolved in the photocatalytic conversion of CO2 in
an aqueous NaCl solution using Ni–Al LDH strongly depends on the
surface area. This means that Ni–Al LDH with a large specific surface
area can achieve higher photocatalytic activity for the conversion
of CO2 to CO because the adsorption of CO2 molecules on the sur-
face of photocatalyst materials is an innegligible step to convert
CO2 [38]. Therefore, an increase in the specific surface area is one
of the reasons for the enhanced formation of CO upon fluorination
of Ni–Al LDH. Meanwhile, another change in the physical and/or
chemical properties because of fluorination would cause the dif-
ference in selectivity toward CO formation. Lima et al. suggested
that fluorine incorporation into LDH hydroxide sheets leads to the
formation of strong basic sites [52]. We presume that the change in
surface properties has a critical influence on the adsorption features
of CO2 molecules and the selectivity toward CO formation is there-
fore improved via suppression of H2 evolution by the fluorination of
Ni–Al LDH. In other words, the fluorination of the hydroxide sheets
would impart surface properties that facilitate the conversion of
CO2 to CO in aqueous solution. This is because the adsorbed CO2
species on the surface of LDH are considered responsible for this
reaction. More detailed information, such as changes in the amount
of adsorbed CO2, the heat of adsorption for CO2 upon fluorination
and the adsorbed CO2 species under the reaction conditions, is now
being obtained in further investigations.
Fig. 5. Total amounts of H2 (blue) and CO (red) after 8 h of photoirradiation in the
photocatalytic conversion of CO2 in an aqueous solution of NaCl. (A) F(7.5) Ni–Al
LDH, (B) F(0) Ni–Al LDH, (C) F(0) Ni–Al LDH (Na3AlF6 was dissolved into the reac-
tion solution as a second additive), (D) F(7.5) imp-Ni–Al LDH, (E) F(7.5) Ni–Al LDH
without CO2 (Ar was introduced instead of CO2), and (F) F(7.5) Ni–Al LDH with-
out irradiation. Photocatalyst weight: 500 mg, water: 350 mL, additive: NaCl 0.1 M,
CO2: 7.6 mmol (except for (E)), light source: 400 W high-pressure Hg lamp (quartz
jacket), photoirradiation time: 8 h. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
Fig. 6. F 1s XPS profiles of (i) F(0) Ni–Al LDH, (ii) F(7.5) Ni–Al LDH, (iii) F(7.5) imp-
Ni–Al LDH, and (iv) Na3AlF6. The intensities of (i), (ii), and (iii) were normalized with
that of Ni 2p3/2 peak.
Fig. 5 presents the results of various blank tests for the photocat-
alytic conversion of CO2 in an aqueous solution using F(7.5) Ni–Al
LDH as the photocatalyst. No photocatalytic activity was observed
without photoirradiation (see Fig. 5(F)). Table 1 shows that the
fluorination of Ni–Al LDH (x = 7.5) clearly improved CO formation,
while simultaneously suppressing the H2 evolution (see Figs. 5(A)
and (B)). The addition of Na3AlF6, which was used as a precursor to
AlF63− units, did not improve the activity for CO formation over F(0)
Ni–Al LDH (see Fig. (C)). The use of F(7.5) imp- Ni–Al LDH, which
improved the photocatalytic conversion of CO2 to CO. To confirm
3−
the existence of AlF6
units on the surface of F(7.5) Ni–Al LDH
and F(7.5) imp- Ni–Al LDH, F 1s X-ray photoelectron spectroscopy
was performed (as shown in Fig. 6) . The peak at around 684.0 eV
in the profile of F(7.5) Ni–Al LDH suggested that fluorine atoms
were present on the surface of this material. However, no peak was
observed in the profile of F(0) Ni–Al LDH. Furthermore, the normal-
ized intensity of F(7.5) imp- Ni–Al LDH was larger than that of F(7.5)
Ni–Al LDH, indicating that the concentration of AlF63− units on the
surface was higher in F(7.5) imp- Ni–Al LDH than it was in F(7.5)
Ni–Al LDH. The peak position of F(7.5) Ni–Al LDH shifted to a slightly
lower binding energy in comparison to that of F(7.5) imp- Ni–Al
3−
is prepared via an impregnation method and wherein all AlF6
units are present on the surface of F(0) Ni–Al LDH, did not result in
the enhancement of CO evolution. These results indicated that the
3−
incorporation of AlF6 units in the hydroxide sheets significantly
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7
indicated that the carbon source of CO evolved in this photocat-
alytic system is the CO2 gas that was introduced as a substrate.
In addition, Fig. 7(b) displays the amounts of products produced
after 20 h of photoirradiation for the photocatalytic conversion
of 13C-labeled CO2 using F(7.5) Ni–Al LDH. The total amount of
CO evolved (12CO + 13CO), which was evaluated by TCD-GC, was
93.3 ␮mol, whereas the amount of H2 evolved was 24.7 ␮mol.
These results indicated that the selectivity toward CO among the
reduction products was around 80% after 20 h of photoirradiation.
By determination of peak area in the profile of m/z = 29, it was esti-
mated that almost 94% of the CO evolved was 13C-labled CO. Since
it is difficult to detect small amount of unlabeled CO because of
the high base level of the m/z = 28 peak, which is attributed to the
contaminated air in the Q-Mass apparatus, the quantification of
13CO using the profile of m/z = 29 is necessary for confirming the
actual origin of CO. Additionally, a certain amount of HClO was
produced as an oxidation product in the reaction solution. As previ-
ously reported by our group, HClO is formed via disproportionation
of Cl2 in water, which is formed by the oxidation of Cl− [38]. We
previously confirmed the stoichiometric formation of HClO via two-
electron oxidation of Cl−, which can be detected by the DPD test
[56,57], with the evolution of reduction products such as CO and H2
over Ni–Al LDH. In the present study, the amount of HClO produced
in the reaction solution was less than the stoichiometric amount,
which indicated that some of the photogenerated holes have been
scavenged by another process. The photocatalytic activity clearly
decreased in the absence of NaCl additive. Therefore, we conclude
that the photocatalytic conversion of CO2 to CO over fluorinated
Ni–Al LDH (F(x) Ni–Al LDH) in an aqueous NaCl solution proceeds
using Cl− as a hole scavenger. The results also show that the fluori-
3−
nation of LDH by incorporation of AlF6 units in hydroxide sheets
affords surface properties suitable for the photocatalytic conver-
sion of CO2 in the presence of Cl−. Moreover, the structural change
was not observed in F(7.5) Ni–Al LDH which was used in the pho-
tocatalytic conversion of CO2 for 20 h as compared to that before
the reaction, indicating that the stability of the fluorinated Ni–Al
LDH photocatalyst is considered to be sufficient for the use in the
photocatalytic reactions.
Fig. 7. (a) GC–MS chromatogram for the photocatalytic conversion of 13C-labeled
CO2 in an aqueous solution of NaCl using F(7.5) Ni–Al LDH as a photocatalyst. (i) GC
chromatogram detected by TCD, ion current profile of (ii) m/z = 28, and (iii) m/z = 29.
(b) Amounts of products yielded after 20 h of photoirradiation quantified by using
the TCD-GC for “total CO” and “H2”, the Q-Mass for “13CO”, and the DPD test for
“HClO”.
4. Conclusion
We found that the fluorination of Mg–Al LDH and Ni–Al LDH
improved the photocatalytic conversion of CO2 to CO in an aqueous
NaCl solution under UV light irradiation. After 20 h of photoirradia-
tion over F(7.5) Ni–Al LDH, the amount of CO evolved via reduction
of CO2 and the selectivity toward CO among the reduction products
were 93.3 ␮mol and 80%, respectively. When compared to F(7.5)
imp- Ni–Al LDH, the incorporation of AlF63− units in the hydroxide
sheets causes significant improvement in the photocatalytic con-
version of CO2 to CO. The isotopic experiment revealed that almost
all of the evolved CO originated from the CO2 gas that was intro-
duced as a substrate. In addition, a certain amount of hypochlorous
acid (HClO) was produced as an oxidation product in the reaction
solution. We conclude that the fluorination of LDH by incorporation
LDH, while the peak of F(7.5) imp- Ni–Al LDH was found at almost
the same position as Na3AlF6. This result provides strong evidence
in support of the hypothesis stating that AlF63− units are randomly
incorporated in the hydroxide sheets, because the fluorine atoms
3−
of the AlF6 units that constitute the hydroxide sheets are nega-
tively charged. In other words, fluorine atoms that participate in the
structure of Ni2+–F–Al3+ within the hydroxide sheets have a higher
negative charge than those in bare AlF6 units (Al3+–F–Al3+). The
3−
F 1s XPS profile results reflect this change in the environment of
3−
fluorine atoms along with the incorporation of AlF6 units in the
Ni(OH)2 structure.
Fig. 5(E) shows that a certain amount of H2 and a small amount of
CO were produced when, instead of CO2, Ar was introduced. The CO
3−
of AlF6 units in the hydroxide sheets affords surface properties
suitable for the photocatalytic conversion of CO2 in the presence of
Cl−, which acts as a hole scavenger.
evolved in this experiment originated from the naturally adsorbed
2−
CO2 species and/or the CO3
preserved in the interlayer space.
Fig. 7(a) shows the results of GC–MS analysis for the photocatalytic
conversion of 13C-labeled CO2 (99 atom% of the isotopic purity) in
an aqueous NaCl solution using F(7.5) Ni–Al LDH as the photocat-
alyst. GC–MS analysis was conducted to reveal the carbon source
of the product. The peak at 600 s in GC chromatogram is assigned
to CO. The sharp peak in the profile of m/z = 29, which can be found
at the same retention time, is attributed to 13C-labeled CO. This
Acknowledgments
This study was partially supported by a Grant-in-Aid for
Scientific Research on Innovative Areas “All Nippon Artificial Pho-
tosynthesis Project for Living Earth” (No. 2406) of the Ministry
of Education, Culture, Sports, Science, and Technology (MEXT) of
Japan, the Precursory Research for Embryonic Science and Tech-
Please cite this article in press as: S. Iguchi, et al., Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.11.023

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8
S. Iguchi et al. / Applied Catalysis A: General xxx (2015) xxx–xxx
nology (PRESTO), supported by the Japan Science and Technology
Agency (JST), and the Program for Element Strategy Initiative for
Catalysts & Batteries (ESICB), commissioned by the MEXT of Japan.
Shoji Iguchi thanks the JSPS Research Fellowships for Young Scien-
tists.
[32] J. Sun, Y. Zhang, J. Cheng, H. Fan, J. Zhu, X. Wang, S. Ai, J. Mol. Catal. A: Chem.
382 (2014) 146–153.
[33] S.J. Xia, F.X. Liu, Z.M. Ni, J.L. Xue, P.P. Qian, J. Colloid Interface Sci. 405 (2013)
195–200.
[34] J. Hong, Y. Wang, J. Pan, Z. Zhong, R. Xu, Nanoscale 3 (2011) 4655–4661.
[35] Y. Zhao, P. Chen, B. Zhang, D.S. Su, S. Zhang, L. Tian, J. Lu, Z. Li, X. Cao, B. Wang,
M. Wei, D.G. Evans, X. Duan, Chemistry 18 (2012) 11949–11958.
[36] K. Teramura, S. Iguchi, Y. Mizuno, T. Shishido, T. Tanaka, Angew. Chem. Int. Ed.
Engl. 51 (2012) 8008–8011.
[37] S. Iguchi, K. Teramura, S. Hosokawa, T. Tanaka, Catal. Today 251 (2015)
140–144.
[38] S. Iguchi, K. Teramura, S. Hosokawa, T. Tanaka, Phys. Chem. Chem. Phys. 17
(2015) 17995–18003.
[39] T. Mitsudome, Y. Mikami, H. Funai, T. Mizugaki, K. Jitsukawa, K. Kaneda,
Angew. Chem. Int. Ed. 47 (2008) 138–141.
[40] T. Mitsudome, A. Noujima, T. Mizugaki, K. Jitsukawa, K. Kaneda, Adv. Synth.
Catal. 351 (2009) 1890–1896.
[41] C. Pendem, P. Gupta, N. Chaudhary, S. Singh, J. Kumar, T. Sasaki, A. Datta, R.
Bal, Green Chem. 14 (2012) 3107–3113.
[42] E.A. Gardner, S.K. Yun, T. Kwon, T.J. Pinnavaia, Appl. Clay Sci. 13 (1998)
479–494.
[43] D. Carriazo, S. Lima, C. Martín, M. Pillinger, A.A. Valente, V. Rives, J. Phys.
Chem. Solids 68 (2007) 1872–1880.
[44] S. Omwoma, W. Chen, R. Tsunashima, Y.-F. Song, Coord. Chem. Rev. 258–259
(2014) 58–71.
[45] A. Corma, V. Fornes, F. Rey, A. Cervilla, E. Llopis, A. Ribera, Catal. Air J. Catal.
152 (1995) 237–242.
[46] M. Elena Pérez-Bernal, R. Ruano-Casero, T. Pinnavaia, Catal. Lett. 11 (1991)
55–61.
[47] E.P. Giannelis, D.G. Nocera, T.J. Pinnavaia, Inorg. Chem. 26 (1987) 203–205.
[48] U. Costantino, N. Coletti, M. Nocchetti, G.G. Aloisi, F. Elisei, Langmuir 15
(1999) 4454–4460.
[49] S. Pausova, J. Krysa, J. Jirkovsky, G. Mailhot, V. Prevot, Environ. Sci. Pollut. Res.
Int. 19 (2012) 3709–3718.
References
[1] Y. Kohno, T. Tanaka, T. Funabiki, S. Yoshida, Chem. Commun. (1997) 841–842.
[2] Y. Kohno, T. tanaka, T. Funabiki, S. Yoshida, Chem. Lett. 26 (1997) 993–994.
[3] Y. Kohno, T. Tanaka, T. Funabiki, S. Yoshida, J. Chem. Soc. Faraday Trans. 94
(1998) 1875–1880.
[4] Y. Kohno, T. Tanaka, T. Funabiki, S. Yoshida, Phys. Chem. Chem. Phys. 2 (2000)
2635–2639.
[5] Y. Kohno, H. Ishikawa, T. Tanaka, T. Funabiki, S. Yoshida, Phys. Chem. Chem.
Phys. 3 (2001) 1108–1113.
[6] K. Teramura, T. Tanaka, H. Ishikawa, Y. Kohno, T. Funabiki, J. Phys. Chem. B
108 (2004) 346–354.
[7] K. Teramura, S. -i. Okuoka, H. Tsuneoka, T. Shishido, T. Tanaka, Appl. Catal. B:
Environ. 96 (2010) 565–568.
[8] H. Tsuneoka, K. Teramura, T. Shishido, T. Tanaka, J. Phys. Chem. C 114 (2010)
8892–8898.
[9] F. Cavani, F. Trifirò, A. Vaccari, Catal. Today 11 (1991) 173–301.
[10] B.F. Sels, D.E. De Vos, P.A. Jacobs, Catal. Rev. 43 (2001) 443–488.
[11] C. Delhoyo, Appl. Clay Sci. 36 (2007) 103–121.
[12] K. Ebitani, K. Motokura, K. Mori, T. Mizugaki, K. Kaneda, J. Org. Chem. 71
(2006) 5440–5447.
[13] G. Abellan, E. Coronado, C. Marti-Gastaldo, J. Waerenborgh, A. Ribera, Inorg.
Chem. 52 (2013) 10147–10157.
[14] F. Giovannelli, M. Zaghrioui, C. Autret-Lambert, F. Delorme, A. Seron, T.
Chartier, B. Pignon, Mater. Chem. Phys. 137 (2012) 55–60.
[15] M. Gong, Y. Li, H. Wang, Y. Liang, J.Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei, H.
Dai, J. Am. Chem. Soc. 135 (2013) 8452–8455.
[16] Y. Guo, Z. Zhu, Y. Qiu, J. Zhao, J. Hazard. Mater. 239–240 (2012) 279–288.
[17] L. Huang, S. Chu, J. Wang, F. Kong, L. Luo, Y. Wang, Z. Zou, Catal. Today 212
(2013) 81–88.
[18] T. Iwasaki, H. Yoshii, H. Nakamura, S. Watano, Appl. Clay Sci. 58 (2012)
120–124.
[19] B. Li, Y. Zhao, S. Zhang, W. Gao, M. Wei, ACS Appl. Mater. Interfaces 5 (2013)
10233–10239.
[20] H. Lin, Y. Zhang, G. Wang, J.-B. Li, Front. Mater. Sci. 6 (2012) 142–148.
[21] J. Prince, F. Tzompantzi, G. Mendoza-Damián, F. Hernández-Beltrán, J.S.
Valente, Appl. Catal. B: Environ. 163 (2015) 352–360.
[50] Z. Huang, P. Wu, Y. Lu, X. Wang, N. Zhu, Z. Dang, J. Hazard. Mater. 246–247
(2013) 70–78.
[51] J.S. Valente, F. Tzompantzi, J. Prince, Appl. Catal. B: Environ. 102 (2011)
276–285.
[52] E. Lima, M.d.J. Martínez-Ortiz, R.I.G. Reyes, M. Vera, Inorg. Chem. 51 (2012)
7774–7781.
[53] G. Wu, X. Wang, B. Chen, J. Li, N. Zhao, W. Wei, Y. Sun, Appl. Catal. A: Gen. 329
(2007) 106–111.
[54] G. Wu, X. Wang, W. Wei, Y. Sun, Appl. Cata. A: Gen. 377 (2010) 107–113.
[55] M.G. Álvarez, R.J. Chimentão, F. Figueras, F. Medina, Appl. Clay Sci. 58 (2012)
16–24.
[56] L. Moberg, B. Karlberg, Anal. Chim. Acta 407 (2000) 127–133.
[57] K. Carlsson, L. Moberg, B. Karlberg, Water Res. 33 (1999) 375–380.
[58] H.D.B. Jenkins, K.P. Thakur, J. Chem. Educ. 56 (1979) 576.
[59] N. Ahmed, Y. Shibata, T. Taniguchi, Y. Izumi, J. Catal. 279 (2011) 123–135.
[60] M. Morikawa, N. Ahmed, Y. Yoshida, Y. Izumi, Appl. Catal. B: Environ. 144
(2014) 561–569.
[61] M. Morikawa, Y. Ogura, N. Ahmed, S. Kawamura, G. Mikami, S. Okamoto, Y.
Izumi, Catal. Sci. Technol. 4 (2014) 1644–1651.
[62] K. Katsumata, K. Sakai, K. Ikeda, G. Carja, N. Matsushita, K. Okada, Mater. Lett.
107 (2013) 138–140.
[22] F.B. Saiah, B.L. Su, N. Bettahar, J. Hazard. Mater. 165 (2009) 206–217.
[23] C.G. Silva, Y. Bouizi, V. Fornés, H. García, J. Am. Chem. Soc. 131 (2009)
13833–13839.
[24] F. Song, X. Hu, J. Am. Chem. Soc. 136 (2014) 16481–16484.
[25] T. Takei, A. Miura, N. Kumada, K. Okada, J. Porous Mater. 20 (2012) 777–783.
[26] L. Tian, Y. Zhao, S. He, M. Wei, X. Duan, Chem. Eng. J. 184 (2012) 261–267.
[27] X. Xu, R. Lu, X. Zhao, S. Xu, X. Lei, F. Zhang, D.G. Evans, Appl. Catal. B: Environ.
102 (2011) 147–156.
[28] X. Zhang, Z. Wang, N. Qiao, S. Qu, Z. Hao, ACS Catal. 4 (2014) 1500–1510.
[29] L. Mohapatra, K.M. Parida, Sep. Purif. Technol. 91 (2012) 73–80.
[30] L. Mohapatra, K.M. Parida, Phys. Chem. Chem. Phys. 16 (2014) 16985–16996.
[31] M. Shao, J. Han, M. Wei, D.G. Evans, X. Duan, Chem. Eng. J. 168 (2011) 519–524.
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