DossierThis paper is a part of the hereunder thematic dossier
published in OGST Journal, Vol. 70, No. 5, pp. 791-902
and available online here
Cet article fait partie du dossier thématique ci-dessous
publié dans la revue OGST, Vol. 70, n°5, pp. 791-902
et téléchargeable ici

DOSSIER Edited by/Sous la direction de : D. Uzio

IFP Energies nouvelles International Conference / Les Rencontres Scientifiques d’IFP Energies nouvelles

PHOTO4E – Photocatalysis for energy
PHOTO4E – Photocatalyse pour l’énergie
Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5, pp. 791-902
Copyright © 2015, IFP Energies nouvelles

791 > Editorial
M. Fontecave, A. Fécant and D. Uzio

799 > Solar Production of Fuels from Water and CO2: Perspectives and Opportunities fora Sustainable Use of Renewable Energy
Production solaire de carburants à partir de l'eau et de CO2 : perspectives etopportunités pour une utilisation durable de l’énergie renouvelable

R. Passalacqua, G. Centi and S. Perathoner

817 > Effect of Post-Synthesis Treatments on the Properties of ZnS Nanoparticles:

An Experimental and Computational Study
Effet des traitements après-synthèse sur les propriétés de nanoparticules de ZnS :une étude expérimentale et computationnelle

853 > Recyclable PhotoFuel Cell for Use of Acidic Water as a Medium

Cellule photocombustible recyclable pour l’utilisation d’eau acide en tant que
milieuY. Ogura, M. Yoshiba, and Y. Izumi

863 > Solar Hydrogen Reaching Maturity

L’hydrogène solaire arrive à maturité

J. Rongé, T. Bosserez, L. Huguenin, M. Dumortier, S. Haussener and J.A. Martens

877 > Design of Compact Photoelectrochemical Cells for Water Splitting

Conception de cellules photoélectrochimiques compactes pour la décomposition
de l’eau

T. Bosserez, J. Rongé, J. van Humbeeck, S. Haussener and J. Martens

E. Balantseva, B. Camino, A.M. Ferrari and G. Berlier

831 > Comparative Study on The Photocatalytic Hydrogen Production from Methanol
over Cu-, Pd-, Co- and Au-Loaded TiO2
Étude comparative de production d’hydrogène par photocatalyse à partir deméthanol et à l’aide de différentes phases actives (Cu, Pd, Co et Au)
supportées sur TiO2
P.P.C. Udani and M. Rønning

841 > Photocatalytic Conversion of Carbon Dioxide Using Zn–Cu–Ga Layered Double

Hydroxides Assembled with Cu Phthalocyanine: Cu in Contact with Gaseous Reactant
is Needed for Methanol Generation
Conversion photocatalytique du dioxyde de carbone par des hydroxydes doubles
lamellaires de Zn–Cu–Ga promus par la phtalocyanine de Cu : nécessité du contact
entre le Cu et le réactif gazeux pour la synthèse du méthanol

S. Kawamura, N. Ahmed, G. Carja and Y. Izumi

891 > Simultaneous Production of CH4 and H2 from Photocatalytic Reformingof Glucose Aqueous Solution on Sulfated Pd-TiO2 Catalysts
Production simultanée de CH4 et H2 par réformage photocatalytique d'une
solution aqueuse de glucose sur un catalyseur Pd-TiO2 sulfaté

V. Vaiano, G. Iervolino, G. Sarno, D. Sannino, L. Rizzo, J.J. Murcia Mesa,
M.C. Hidalgo and J.A. Navío

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5, pp. 841-852
Ó S. Kawamura et al., published by IFP Energies nouvelles, 2015
DOI: 10.2516/ogst/2015020
IFP Energies nouvelles International Conference
Rencontres Scientiﬁques d'IFP Energies nouvelles
PHOTO4E – Photocatalysis for energy
PHOTO4E – Photocatalyse pour l’énergie

Photocatalytic Conversion of Carbon Dioxide Using
Zn–Cu–Ga Layered Double Hydroxides Assembled
with Cu Phthalocyanine: Cu in Contact with Gaseous
Reactant is Needed for Methanol Generation
Shogo Kawamura1, Naveed Ahmed1, Gabriela Carja2 and Yasuo Izumi1*2

1
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522 - Japan
Department of Chemical Engineering, Faculty of Chemical Engineering and Environmental Protection, Technical University “Gh. Asachi” of Iasi,
Bd. Mangeron No. 71, Iasi 700554 - Romania
e-mail: x0s3033@students.chiba-u.jp - nedoanjum@yahoo.com - carja@uaic.ro - yizumi@faculty.chiba-u.jp* Corresponding author

Abstract — Photocatalytic conversion of CO2 into fuels is an attractive option in terms of both reducing
the increased concentration of atmospheric CO2 as well as generating renewable hydrocarbon fuels.
It is necessary to investigate good catalysts for CO2 conversion and to clarify the mechanism
irradiated by natural light. Layered Double Hydroxides (LDH) have been attracting attention forCO2 photoreduction with the expectation of sorption capacity for CO2 in the layered space andtunable semiconductor properties as a result of the choice of metal cations. This study ﬁrst clariﬁes
the effects of Cu doping to LDH comprising Zn and Al or Ga. Cu could be incorporated in the cationiclayers of LDH as divalent metal cations and/or interlayer anions as Cu(OH)42. The formation rates
of methanol and CO were optimized for [Zn1.5Cu1.5Ga(OH)8]+2Cu(OH)42mH2O at a total rateof 560 nmol h1 gcat1 irradiated by UV–visible light. Cu phthalocyanine tetrasulfonate hydrate(CuPcTs4) and silver were effective as promoters of LDH for CO2 photoreduction. Especially,
the total formation rate using CuPcTs-[Zn3Ga(OH)8]+2CO32mH2O irradiated by visible
light was 73% of that irradiated by UV–visible light. The promotion was based on HOMO–
LUMO excitation of CuPcTs4 by visible light. The LUMO was distributed on N atoms of pyrrole
rings bound to central Cu2+ ions. The photogenerated electrons diffused to the Cu site would
photoreduce CO2 progressively in a similar way to inlayer and interlayer Cu sites in the LDH in thisstudy.
Résumé — Conversion photocatalytique du dioxyde de carbone par des hydroxydes doubles
lamellaires de Zn–Cu–Ga promus par la phtalocyanine de Cu : nécessité du contact entre le Cu
et le réactif gazeux pour la synthèse du méthanol — La conversion photocatalytique de CO2 est
une option attractive pour limiter la concentration du CO2 atmosphérique quand elle a pour objectif
la production de produits hydrocarbonés utilisés comme carburants renouvelables. Néanmoins, desétudes sont encore nécessaires pour étudier les catalyseurs de conversion de CO2 et clariﬁer lesmécanismes réactionnels. Des Hydroxydes Doubles Lamellaires (HDL) sont des catalyseursintéressants pour la photoréduction de CO2 et l’on s’attend à obtenir une capacité d’adsorption duThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5842

CO2 dans l’espace interlamellaire et des propriétés semi-conductrices adaptables par le choix des cations
métalliques. La présente étude démontre tout d’abord les effets du dopage du Cu dans les HDL
comprenant du Zn et de l’Al ou du Ga. Le cuivre pourrait être incorporé dans les couches
cationiques des HDL sous forme de cations métalliques divalents et/ou d’anions inter-feuillets sous
forme de Cu(OH)42. Les taux de formation du méthanol et du CO ont été optimum pour le[Zn1.5Cu1.5Ga(OH)8]+2Cu(OH)42mH2O correspondant à une vitesse totale de 560 nmol h1 gcat1,
sous irradiation UV visible. Le tétrasulfonate-hydrate de phtalocyanine de Cu (CuPcTs4) et l’argent
se sont avérés être des promoteurs efﬁcaces de HDL pour la photoréduction du CO2. En particulier,le taux de formation total en utilisant du CuPcTs-[Zn3Ga(OH)8]+2CO32mH2O irradié par la lumière
visible représentait 73% de celui irradié par de la lumière UV visible. La promotion a été attribuée à
une excitation HOMO–LUMO du CuPcTs4 par la lumière visible. Le LUMO répartie sur l’atomed’azote des cycles des pyrroles lié à l’ion central Cu2+. Les électrons photogénérés diffusés sur lesite de Cu photoréduiraient le CO2 sur les sites cationiques (dans les couches) ou anioniques(interfeuillets) des HDL.

INTRODUCTION
Photocatalytic conversion of CO2 into fuels has emerged
as an attractive option, in terms of both reducing theincreased concentration of atmospheric CO2 as well as generating renewable hydrocarbon fuels that can be directly
supplied to our present energy infrastructure (Costentin
et al., 2013; Corma and García, 2013; Genevese et al.,2013; Habisreutinger et al., 2013; Indrakanti et al., 2009;
Izumi, 2013; Kubacka et al., 2012; Lewis and Nocera,
2006; Lv et al., 2012; Roy et al., 2010). The photocatalytic
conversion of CO2 involves two reaction steps:
2H2 OðgÞ ! 2H2 ðgÞ þ O2 ðgÞ;

Gr ¼ 689 kJ mol1

ð1Þ

CO2 ðgÞ þ 3H2 ðgÞ ! CH3 OHðgÞ þ H2 OðgÞ;Gr ¼ 2:9 kJ mol1

ð2Þ

It is important to investigate good catalysts for CO2 conversion into fuels using hydrogen as a reductant (Eq. 2),
which is potentially obtained from photocatalytic water
splitting (Eq. 1) (Izumi, 2013; Ahmed et al., 2011).
In this study, catalysts for converting CO2 intomethanol using hydrogen and UV–visible light were
investigated. Layered Double Hydroxides (LDH)[MII1xMIIIx(OH)2]x+X2x/2mH2O (MII = Zn, Cu; MIII = Al,
Ga; X = CO3, Cu(OH)4; m~1/2) were chosen with the expectation of:– sorption capacity for CO2 in the layered space,– tunable semiconductor properties as a result of the choiceof metal cations(Ahmed et al., 2013; Cavani et al., 1991; Fan et al., 2014; Liet al., 2014; Sideris et al., 2008; Zümreoglu-Karan and Ay, 2012).

Furthermore, to utilize visible light as the major part ofthe solar spectrum, dyes/nanoparticles were mixed with the
LDH. Especially, the effects of Cu phthalocyanine tetrasulfonate hydrate (CuPcTs4) combined with LDH werestudied.

1 EXPERIMENTAL SECTIONA LDH compound of [Zn3Ga(OH)8]+2CO32mH2O wassynthesized using a reported procedure from metal nitrates,
Na2CO3, and NaOH controlled at pH 8 (Ahmed et al.,2011). This compound is abbreviated as Zn3Ga|CO3.
Similarly, LDH comprising various compositions of[MII3MIII(OH)8]+2X2mH2O (X = CO3, Cu(OH)4) weresynthesized from nitrates of Zn, Cu, Al and Ga, and sodium
carbonate/ammonium tetrachlorocuprate dihydrate at pH 8
(Ahmed et al., 2012). The formula is abbreviated as
MII3MIII|X.
Na+4CuPcTs4 was purchased from Aldrich (purity>85%) and used without further puriﬁcation. For the ion
exchange method, we followed the procedure presented
in reference (Parida et al., 2007). Firstly, 0.50 g of
Zn3Ga|CO3 powder was immersed in an aqueous solutionof Na+4CuPcTs4 (0.60 mM, 25 mL) in a ﬂask and magnetically stirred at 900 rotations per minute (rpm) for 24 h.
The blue precipitates that were obtained were ﬁltered usinga polytetraﬂuoroethylene-based membrane ﬁlter (Omnipore
JGWP04700, Millipore) with a pore size of 0.2 lm andwashed well with deionized water (<0.06 lS cm1; total
250 mL). The precipitates that were obtained were dried
under vacuum at 290 K for 24 h. The blue sample wasdenoted as CuPcTs-Zn3Ga|CO3. The loading of Cu was
0.19 wt%.

S. Kawamura et al. / Photocatalytic Conversion of Carbon Dioxide Using Zn–Cu–Ga Layered Double Hydroxides Assembled 843
with Cu Phthalocyanine: Cu in Contact with Gaseous Reactant is Needed for Methanol GenerationPhotocatalytic Conversion of Carbon
Dioxide Using
Zn–Cu–Ga Layered Double Hydroxides Assembled
with Cu4
Phthalocyanine: Cu in Contact with Gaseous Reactant is Needed for Methanol
Anion exchange of Zn3Ga|NO3 with Na+4CuPcTs
was1/2, 3/2, 2 and 3 for allowed direct, forbidden direct, allowed
Generation

performed following the procedure in the literature (Abellán
et al., 2012). All of the procedure until the drying under vacuum was conducted under an argon atmosphere. 0.30 g of
Zn3Ga|NO3 powder was placed in a ﬂask and an aqueous
solution of Na+4CuPcTs4 (5.0 mM, 200 mL) was slowly
added and magnetically stirred at 900 rpm. Then, 30 mLof ethylene glycol and 30 mL of ethanol were added to theﬂask. The mixture was agitated by ultrasound (430 W,
38 kHz) for 10 min and then was magnetically stirred at
900 rpm for 3 days.
The precipitates that were obtained were ﬁltered using an
Omnipore JGWP04700 ﬁlter and washed well with ethanol(total 250 mL) and deionized water (total 250 mL). The precipitates that were obtained were dried at 290 K for 24 hunder vacuum. The sample was denoted as Zn3Ga|CuPcTs.
Ag nitrate was impregnated from aqueous solution with
Zn3Ga|CO3. The loading of Ag was 0.36 wt% (Kawamura
et al., 2015). The sample was denoted as Ag-Zn3Ga|CO3.
Separately, the assembly of Au nanoparticles with Zn3Ga|
CO3 was obtained using the structural reconstruction of
LDH in the aqueous solutions containing Au3+ (Carjaet al., 2013). Hence, 1.20 g Zn3Ga|CO3 powder was calcinedin an oven at 773 K for 8 h. Freshly calcined Zn3Ga|CO3
powder was directly added to 200 mL aqueous solution ofAu(III) acetate (>99.9%, Alfa Aesar; 0.10 g) stirring at a rateof 900 rpm. The pH of the solution was adjusted to 8.0 by theaddition of NaOH aqueous solution (0.10 M). The reaction
mixture was stirred at the rate of 900 rpm and 290 K for
20 min and at the rate of 150 rpm and 313 K for 5 h. Then,the precipitate was centrifuged at the rate of 10 000 rpm anddried in an oven at 353 K for 1 h. The color of the catalyst
obtained was light purple. The sample was denoted as
Au-Zn3Ga|CO3.
Optical spectroscopic measurements were performed
using a UV–visible spectrophotometer (model V-650,
JASCO) using D2 and halogen lamps for wavelengths
between 200 and 340 nm and 340 and 800 nm, respectively.
An integrating sphere (model ISV-469, JASCO) was used forthe Diffuse Reﬂectance (DR) measurements. The samples
were set in contact with the quartz window glass in gas-tight
DR cells. Measurements were performed at 290 K within the
wavelength range 200-800 nm using 70 mg of sample. DR
spectra were converted to absorption spectra on the basisof the Kubelka–Munk equation (Ahmed et al., 2011,2012). The band-gap (Eg) value was evaluated on the basisof either simple extrapolation of the absorption edge or theﬁt to the Davis–Mott equation:na  hm / hm  Eg

ð3Þ

in which a, h, and m are the absorption coefﬁcient, Planck’s
constant and the frequency of light, respectively, and n isindirect and forbidden indirect electronic transitions, respectively (Wooten, 1972).
The Brunauer-Emmett-Teller surface area (SBET) was calculated on the basis of eight-point measurements between 10 and
46 kPa in the N2 adsorption isotherm at 77 K. The X-Ray Diffraction (XRD) pattern was observed using a D8 ADVANCE
diffractometer (Bruker) at the Center for Analytical Instrumentation, Chiba University, at a Bragg angle of 2hB = 3-60° with ascan step of 0.01° and a scan rate of 5 s per step. The measurements were performed at 40 kV and 40 mA using Cu Ka
emission and a nickel ﬁlter.
Cu K-edge X-ray Absorption Fine Structure (XAFS)
spectra were measured at 290 K in transmission mode in
the Photon Factory Advanced Ring at the High Energy
Accelerator Research Organization (Tsukuba) on beamline
NW10A. The storage ring energy was 6.5 GeV and the ring
current was 46.6-36.1 mA. A Si (311) double-crystal monochromator and platinum-coated focusing cylindrical mirror
were inserted into the X-ray beam path. The X-ray intensity
was maintained at 65% of the maximum ﬂux using a piezotranslator set to the crystal. The slit opening size was
1 mm (vertical) 9 2 mm (horizontal) in front of the ionizationchamber. Part of the XAFS measurements were performed fora sample in a reactor equipped with polyethylene naphthalate
windows (Q51-16, Teijin) irradiated by a xenon arc lamp
(Morikawa et al., 2014a; Izumi et al., 2007). The Cu K-edge
absorption energy was calibrated to 8 980.3 eV for the
spectrum of Cu metal foil (Bearden, 1967).
The XAFS data were analyzed using an XDAP (X-ray
absorption ﬁne structure Data Analysis Program) package(Vaarkamp et al., 2006). The pre-edge background wasapproximated by a modiﬁed Victoreen function
C2/E2 + C1/E + C0. The background of the post-edge
oscillation was approximated by a smoothing spline function
and calculated by an equation for the number of data points,
where k is the wavenumber of photoelectrons:
Data
PointsX
i¼1ðlxi  BGi Þ2
 smoothing factor
exp ð0:075k 2i Þ

ð4ÞMultiple-shell curve-ﬁt analyses were performed for the
Fourier-ﬁltered k3-weighted Extended X-ray Absorption Fine
Structure (EXAFS) data in k- and R-space using empirical
amplitude extracted from the EXAFS data for Na+4CuPcTs4
(Izumi et al., 2005, 2009). The interatomic distance (R) and its
associated coordination number (N) for the Cu–N pair were
set to 0.1950 nm with a N value of 4 (Carrera et al., 2004).
The many-body reduction factor S02 was assumed to be equal
for both the sample and the reference.
As-synthesized and preheated samples of the LDH
CuPcTs-Zn3Ga|CO3, Ag-Zn3Ga|CO3 and Au-Zn3Ga|CO3

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5

2 RESULTS
2.1 Characterization
The interlayer interval and SBET of the LDH synthesized were between 0.751-0.792 nm and 35-70 m2 g1,
Hoya, web information of sharp cut ﬁlters: http://www.hoyaoptics.com/
color_ﬁlter/sharp_cut.htm, http://buyersguide.pennwell.com/Shared/User/
pr9f6c58fbba904eefa88c895cd0f5f336.pdf.

1

003

5 000

(a)
0

10

30
40
20
2-Theta (degree)

100
½11
110
100
½11
110

009

1 000

104

(b)

107
107
018
018

012

2 000

015

3 000

β-Ga2O3α-GaO(OH)
ZnO[ZnCO3]2·[Zn(OH)2]3

006

4 000

003 (LDH of CuPcTs)
006 (LDH of CuPcTs)

were tested for the photocatalytic conversion of CO2(Kawamura et al., 2015). The tests were conducted in aclosed circulating system (171 mL) equipped with aphotoreaction quartz cell that had a ﬂat bottom (23.8 cm2).
100 mg of the LDH catalyst was uniformly spread in thephotoreaction cell and was evacuated by rotary and diffusion
pumps (106 Pa) at 290 K for 2 h until the desorbed gas was
detected by an online Gas Chromatograph (GC). 2.3 kPa of
CO2 (0.177 mmol) and 21.7 kPa of H2 (1.67 mmol) wereintroduced to both intact and pretreated LDH photocatalysts
and were allowed to circulate for 30 min in contact with thecatalyst to attain sorption equilibrium before irradiation.
The photocatalyst was then irradiated with UV–visible lightfrom the 500-W xenon arc lamp (Ushio, model UI-502Q)
upward through the ﬂat bottom of the quartz reactor for
5 h. The distance between the bottom of the reactor and thelamp house exit window was set to 20 mm. The light intensity
was 42 mW cm2 at the center of the sample cell and 28 mWcm2 at the periphery of the bottom plate of the sample cell.
The temperature was within the range 305-313 K at the catalyst
position during the illumination for 5 h. As a comparison, thephotocatalyst was irradiated with visible light using one of
the following UV-cut ﬁlters between the light exit of
UI-502Q and the photocatalyst. Photocatalytic CO2 reduction
tests were performed systematically using L42, Y52, O58,
R62, R66 and W-R715 sharp cut ﬁlters (Hoya) between theirradiation light exit and photoreactor. The thickness was
2.5 mm except for W-R715 (2.0 mm). These ﬁlters pass thelight of wavelengths greater than 420, 520, 580, 620, 660
and 715 nm, respectively. The transmittance of nonﬁltered light
was more than 88% for L42, Y52, O58, R62 and R66, andalmost 100% for W-R715 (manufacturer’s web information(1)). Products and reactants were analyzed using packed
columns of a 13X-S molecular sieve and PolyEthylene Glycol(PEG-6000) supported on Flusin P (GL Sciences) set in the
online GC equipped with a thermal conductivity detector(Shimadzu, model GC-8A).
The molecular orbitals for CuPcTs4 were calculatedusing Gaussian 09 (Gaussian, Inc., Wallingford, Connecticut, USA) employing a polarized basis set of 6-31G(d) andin density functional theory mode to calculate the electron
correlation using the functional B3LYP (Rauf et al., 2012).

Intensity (counts per second)

844

50

60

Figure 1
XRD patterns for a) CuPcTs-Zn3Ga|CO3 and b) Zn3Ga|CuPcTs.

respectively, except for Zn3Ga|CuPcTs. The metal cations
existed in nearly identical octahedral MO6 coordination
environments in cationic sheets based on the similarity ofthe Zn K, Cu K and Ga K-edge X-ray Absorption Near-Edge
Structure (XANES) (Ahmed et al., 2011, 2012).
The layered structure of the LDH synthesized in this study
was conﬁrmed by XRD. The XRD pattern for CuPcTsZn3Ga|CO3 is depicted in Figure 1a. The diffraction peaks
at 2hB = 11.7°, 23.4°, 33.5°, 34.2°, 36.9°, 38.8°, 43.6°,
46.4°, 52.6°, 56.9° and 59.4° were assigned to (003),(006), (009), (012), (104), (015), (107), (018), (100), (12 11)[=(0111)] and (110) diffraction, respectively, for the regularlayered structure of the LDH. The interlayer interval value
was evaluated at 0.755 nm on the basis of the (003) diffraction angle and changed negligibly as compared with Zn3Ga|
CO3 (0.751 nm; Ahmed et al., 2011).
The pattern is compared with that for Zn3Ga|CuPcTs(Fig. 1b-line). Two peaks at 2hB = 4.04° and 7.91° appeared
assignable to (003) and (006) diffraction of the LDH with theinterlayer anions of CuPcTs4 (Abellán et al., 2012). Theinterlayer interval value was evaluated at 2.18 nm on thebasis of the (003) diffraction angle due to the intercalationof larger CuPcTs4 anions. The diffraction peaks at 2hB =11.6°, 23.3°, 33.5°, 34.2°, 36.3°, 38.7°, 43.2°, 46.2°,
52.3°, 56.6° and 59.3° also appeared, and suggested theby-product phase of Zn3Ga|CO3. The interlayer intervalvalue was evaluated at 0.760 nm on the basis of the (003)
diffraction angle for the by-product phase.
In the pattern for both samples, a very small amount of
impurity peaks was observed at 19.5-19.6° and 26.8° owing

S. Kawamura et al. / Photocatalytic Conversion of Carbon Dioxide Using Zn–Cu–Ga Layered Double Hydroxides Assembled 845
with Cu Phthalocyanine: Cu in Contact with Gaseous Reactant is Needed for Methanol GenerationPhotocatalytic Conversion of Carbon
Dioxide Using
Zn–Cu–Ga Layered Double Hydroxides
TABLE 1 Assembled with Cu Phthalocyanine: Cu in Contact with Gaseous Reactant is Needed for Methanol
Generation
678(b) CuPcTs4
The band-gap values and the type of electronic transition based on
UV–visible spectra for Zn-Cu-Ga LDH
Band gap (eV)e transition

Zn3Ga|CO3

5.6

DirectZn1.5Cu1.5Ga|CO3

3.5

Forbidden direct

Zn3Al|CO3

5.7

DirectZn1.5Cu1.5Al|CO3

4.1

DirectZn3Ga|Cu(OH)44.2 (impurity phase: 3.2)

DirectZn1.5Cu1.5Ga|Cu(OH)4

3.0

DirectAbsorbance (Kubelka–Munk)

Photocatalyst

Zn3GalCo3630220(a) Na4CuPcTs
337

3

612
Fresh
Used

340
2

660
1
Soret
band
0
200

Q-band
600

400

800

Wave length (nm)to b-Ga2O3 and a-GaO(OH), respectively (Fig. 1). In thepattern for Zn3Ga|CuPcTs, a small amount of impurity peaks
at 31.9°, 34.5° and 36.3° owing to ZnO and those at 14.2°
and 24.6° owing to [ZnCO3]2[Zn(OH)2]3 also appeared(b-line).
The Eg values were evaluated by the extrapolation of theabsorption edge in the UV–visible spectra. The values given
were within the range 5.6-3.0 eV for Zn3Ga|CO3,
Zn1.5Cu1.5Ga|CO3,
Zn3Al|CO3,
Zn1.5Cu1.5Al|CO3,
Zn3Ga|Cu(OH)4 and Zn1.5Cu1.5Ga|Cu(OH)4 (Tab. 1).
Besides the absorption edge for the LDH, an intense
absorption peak appeared at 678 nm for CuPcTs-Zn3Ga|
CO3 (Fig. 2b-line, solid line). The UV–visible spectrum
for CuPcTs-Zn3Ga|CO3 basically resembled that for aqueous
solution of Na+4CuPcTs4 (Fig. 2a-line); however, the peakintensity ratio in the range 600-700 nm was different:
678 nm (s), 630 nm (sh) and 612 nm (sh) for CuPcTsZn3Ga|CO3 versus 630 nm (s) and 660 nm (sh) for aqueous
solution of Na+4CuPcTs4. As the peaks at 678-660 nm or
630 nm would be assigned to HOMO (a1u)–LUMO (eg) electronic transition (Q-band, Fig. 3), the HOMO and/or LUMO
of CuPcTs4 were signiﬁcantly perturbed by the interaction
with the Zn3Ga|CO3 surface.
As compared with the Q-band, the peaks at 337-340 nm
were assigned to transition from a2u to LUMO (eg)(Soret band). The peak at 220 nm was only observed for
aqueous solution of Na+4CuPcTs4 (Fig. 2a-line). This peakwould be a transition from a slightly deeper level than a2u to
LUMO (eg) or from a2u to a slightly shallower level than
LUMO (eg) (Marom et al., 2008).
The UV–visible spectrum for CuPcTs-Zn3Ga|CO3 used
for photocatalytic tests under CO2 + H2 for 5 h was also measured (Fig. 2b-line, dotted line). The spectrum was essentially identical to that for the fresh sample (solid line),
indicating the stability of CuPcTs4 dispersed over Zn3Ga|
CO3 under the photocatalytic reaction conditions.

Figure 2
Diffuse-reﬂectance UV–visible absorption spectra for aqueous solution of a) Na+4CuPcTs4 (6.0 lmol L1) andb) CuPcTs-Zn3Ga|CO3. Solid line: fresh sample and dotted line:
sample used after the photocatalytic test for 5 h.

–

O3S

+
N

–

N

–
N

N
SO3–

N Cu N
–

O3S

N

N

O3S

O3S

N

N
+

SO3–

HOMO (a1u)

N
SO3–

N Cu N
–

N

N

N
+
–

SO3–

LUMO (eg)Figure 3
HOMO and LUMO for Na+4CuPcTs4 calculated using
Gaussian 09.

Ag-Zn3Ga|CO3 and Au-Zn3Ga|CO3 exhibited a major
absorption peak at 411 and 555 nm, respectively (notshown). These peaks are ascribed to Surface Plasmonic
Resonance (SPR) of Ag and Au metallic nanoparticles.

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5

846

The XANES spectra for Na+4CuPcTs4 and CuPcTsZn3Ga|CO3 are shown in Figure 4. The whole spectrum
pattern changed negligibly by the dispersion of CuPcTs4
over the Zn3Ga|CO3 surface as compared with

1.6
1.4
Normalized intensity

1.2
1.0
0.8
0.6
0.4 (b) CuPcTsZn3GalCO3
0.2(a) Na4CuPcTs
0
8 900
8 950

9 000
Energy (eV)

9 050

9 100

Figure 4

Na+4CuPcTs4 crystallines, suggesting that the framework
structure of CuPcTs4 was retained upon the dispersion overthe Zn3Ga|CO3 surface. In contrast, a sharp shoulder peak at
8 988 eV became relatively weaker upon the dispersion over
the LDH surface. This may be related to the perturbation of
LUMO by the interaction with the LDH surface based on
UV–visible spectrum change (Fig. 2).
The 1s-3d electronic transition peak appeared at 8 982 eV
for both Na+4CuPcTs4 and CuPcTs-Zn3Ga|CO3 (Fig. 5-I).
The transition is allowed for the CuII state of the 3d9 conﬁguration, whereas the peak disappeared for the CuI state of the
3d10 conﬁguration (Morikawa et al., 2014a). Utilizing thisdifference, the reduction of CuII sites in Na+4CuPcTs4
and CuPcTs-Zn3Ga|CO3 was monitored irradiated by UV–
visible light.
The Cu amount in Na+4CuPcTs4 charged was 118 lmol.
The decreasing rate of the 1s-3d peak irradiated by UV–
visible light for 64 min was 18.0% h1 or 21.2 lmol h1.
In contrast, the Cu amount in CuPcTs-Zn3Ga|CO3 charged
was 13.5 lmol. The decreasing rate of the 1s-3d peak
irradiated by UV–visible light for 180 min was 20.2% h1
or 2.72 lmol h1 (Fig. 5-II). The photoreduction to theCu(I) state was slightly faster for CuPcTs-Zn3Ga|CO3 thanfor Na+4CuPcTs4; however, exact comparison of the
diffusion rates of photogenerated electrons to the Cu(II)site (Morikawa et al., 2014a) was difﬁcult in this caseNormalized Cu K-edge XANES spectra for a) Na+4CuPcTs4
diluted by boron nitride and b) CuPcTs-Zn3Ga|CO3.

0.15

64 min56 min48 min40 min32 min24 min16 minUV-visible irrad.
after 8 min
Visible irradiation
after 64 min(a) Na4CuPcTs

Normarized intensity

0.10

0.05

0

180 min
153 min
136 min
119 min
102 min85 min
68 min51 min34 minUV-visible irrad.
after 17 min
before irradiation(b) CuPcTs-Zn3GalCO3
0.15
0.10
0.05
0
8 970

(I)

8 975
8 980
Energy (eV)(a) Na4CuPcTs
0.004

y = 0.0044227–1.6863×10-5x,
R = 0.92201

0.003(b) CuPcTs-Zn3GalCO3
0.012
0.008
y = 0.013935–4.8735×10-5x,
R = 0.81585

0.004

8 985

0
(II)

120 16040
80Time of photoirradiation (min)Figure 5I) Expanded view of the 1s-3d pre-edge peak region of normalized Cu K-edge XANES spectra of a) Na+4CuPcTs4 under vacuum andb) CuPcTs-Zn3Ga|CO3 under CO2 (2.3 kPa) + H2 (21.7 kPa) and UV–visible light irradiation. II) Time course of the 1s-3d peak intensity forthe two samples. The amount of Na+4CuPcTs4 and CuPcTs-Zn3Ga|CO3 charged in the photoreaction cell was 116 and 401 mg, respectively.

A)

0.1

5

0
-0.1

(b)

0
(a)

-5
(a) Na4CuPcTs
2 4 6 8 10 12
Wave number (10 nm-1)

-10
-15
B)

2 4 6 8 10 12 14
Wave number (10 nm-1)

Fourier transform (CF)6

k3χ (CF)

4
2
0
-2
-4
-6
D)

2

8 10 12
4
6
Wave number (10 nm-1)

Fourier transform

10

Zn3GalCO3
k3 χ

EXAFS χ oscillation

S. Kawamura et al. / Photocatalytic Conversion of Carbon Dioxide Using Zn–Cu–Ga Layered Double Hydroxides Assembled 847
with Cu Phthalocyanine: Cu in Contact with Gaseous Reactant is Needed for Methanol GenerationPhotocatalytic Conversion of Carbon
Dioxide Using
Zn–Cu–Ga Layered Double Hydroxides Assembled with15
Cu Phthalocyanine: Cu in Contact with
8 Gaseous Reactant is Needed for Methanol
0.2
Generation(b) CuPcTs6
4
2
0
-2
-4
-6
-8

0
1
2
3
4
C) Interatomic distance (10-1 nm)

6
4
2
0
-2
-4
-6

1
2
0
3
4
E) Interatomic distance (10-1 nm)

Figure 6A) Cu K-edge EXAFS v oscillation and B) k3-weighted EXAFS v oscillation for Na+4CuPcTs4 diluted by boron nitride (a) and
CuPcTs-Zn3Ga|CO3 (b), C) its associated Fourier transform, and D, E) best-ﬁt results in k-space D) and R-space E) for CuPcTs-Zn3Ga|CO3. Solidline: magnitude and dotted line: imaginary part C, E). Red (thick) line: experimental and blue (thin) line: ﬁt D, E).

due to the difference in the shape and light absorbance of thesamples: dense crystallines of Na+4CuPcTs4 versus ﬁne
powder of CuPcTs-Zn3Ga|CO3. At least, as the UV–visible
light absorbance was apparently higher for Na+4CuPcTs4,the possibility of faster electron accumulation for CuPcTsZn3Ga|CO3 than Na+4CuPcTs4 is high due to the injectionof excited electrons in Zn3Ga|CO3 irradiated by UV lightinto CuPcTs4 at the surface.
The EXAFS v oscillation changed negligibly when
Na+4CuPcTs4 was dispersed over the Zn3Ga|CO3 surface(Fig. 6A). This fact supported the retention of the CuPcTs4
framework over that of the Zn3Ga|CO3 suggested by
XANES. The curve-ﬁt results for the Cu–N interatomic pair
provided a distance of 0.198 nm (ﬁt error ±0.011 nm) with a
N value of 3.3 (ﬁt error ±1.3) (Fig. 6D, E).
2.2 Photocatalytic Tests under CO2 + H2
In the photocatalytic reaction tests under CO2 + H2 irradiatedby UV–visible light, Zn3Ga|CO3 produced CO and methanol(Tab. 2A, entry a). Using Zn1.5Cu1.5Ga|CO3, the methanol formation rate increased by a factor of 3.3 (Tab. 2A entry g).
Zn3Al|CO3 was more active than Zn3Ga|CO3, but the

major product was CO (entry h). When Zn1.5Cu1.5Al|CO3
was compared with Zn3Al|CO3, the methanol formation rate
was promoted by a factor of 3.3 (Tab. 2b entry i) similarly to
the Cu substitution into Zn3Ga|CO3. The methanol formation
rate using Zn3Ga|Cu(OH)4 was enhanced by a factor of 5.9
compared with Zn3Ga|CO3 (Tab.2A entries a, j). The methanol
selectivity was nearly the same as that obtained with
Zn1.5Cu1.5Ga|CO3 (71-68 mol%). Using Zn1.5Cu1.5Ga|
Cu(OH)4, the methanol formation rate and selectivity were
further improved to 0.49 lmol h1 gcat1 and 88 mol%(Tab. 2A entry k).
The results in Table 2 were independently reported in thisstudy and references (Ahmed et al., 2011, 2012; Kawamura
et al., 2015). However, the rates and selectivity arecomparable because common reaction apparatus connected
to common online GC were used in these studies. The reproducibility of rates was checked for Zn3Ga|CO3 in this study
and references (Ahmed et al., 2011, 2012; Kawamura et al.,2015), and the variation of formation rates of methanol andCO was always within 5%.
Furthermore, we reported control reaction tests in darkness, in the absence of a photocatalyst, and in the absenceof CO2. No products were found in these control tests except

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5

848

TABLE 2
Photocatalytic rates of CO2 reduction with H2 using LDH(a)Entry

Formation rate (nmol1gcat)

Photocatalyst
CH3OH

CO

R

Selectivity to
CH3OH (mol%)

ReferenceA) Irradiated by UV–visible lighta

Zn3Ga|CO3b

CuPcTs-Zn3Ga|CO3
(b)

51

80

130

39

Ahmed et al. (2011)

96

106

202

48

This work

28

41

69

41

This workb’

CuPcTs & Zn3Ga|CO3c

Zn3Ga|CuPcTs

<4

69

69

<5

This workd4

Na4CuPcTs

<4

38

38

<9.5

This worke

Ag–Zn3Ga|CO3

118

102

220

54

Kawamura et al. (2015)f

Au–Zn3Ga|CO3

(c)

30

201

231

13

Kawamura et al. (2015)g

Zn1.5Cu1.5Ga|CO3

170

79

250

68

Ahmed et al. (2011)h

Zn3Al|CO3

39

620

660

5.9

Ahmed et al. (2011)i

Zn1.5Cu1.5Al|CO3

130

370

500

26

Ahmed et al. (2011)j

Zn3Ga|Cu(OH)4

300

130

430

71

Ahmed et al. (2012)k

Zn1.5Cu1.5Ga|Cu(OH)4

490

70

560

88

Ahmed et al. (2012)

Ahmed et al. (2011)

B) Irradiated by visible light (k > 420 nm)
a

Zn3Ga|CO3

<4

<8

<12

<33

b

CuPcTs-Zn3Ga|CO3

75

74

148

51

This workc

Zn3Ga|CuPcTs

<4

33

33

<11

This workd

Ag–Zn3Ga|CO3

36

88

124

29

Kawamura et al. (2015)

e

(c)

<4

<8

<12

<33

Kawamura et al. (2015)

Au–Zn3Ga|CO3

aIn CO2 (2.3 kPa) + H2 (21.7 kPa). The catalyst amount was 100 mg.
Physical mixture with the weight ratio of 3:97.
c
Preheated at 373 K for 30 min.
b

for water from the interlayer space of the LDH. Whenthe reactant was switched from CO2 to CO2 + H2, methanol and CO began to evolve using Zn1.5Cu1.5Ga|CO3(Ahmed et al., 2011). When the Zn1.5Cu1.5Ga|CO3 andZn1.5Cu1.5Ga|Cu(OH)4 photocatalysts were recycled fourtimes (in total 20 h of reaction), the methanol and CO formation continued and the selectivity was kept at 68-57 mol%and 76-84 mol%, respectively (Ahmed et al., 2011, 2013).
The addition of CuPcTs4 to Zn3Ga|CO3 improved thetotal (methanol and CO) formation rates by a factor of
1.6 and the methanol selectivity increased to 48 mol%(Tab. 2A entry b; Fig. 7a-line). The formation of CO andmethanol continued for more than 5 h, demonstrating the stability of dispersed CuPcTs4 over the LDH irradiated by UV–visible light under CO2 + H2. In contrast, the performanceof Zn3Ga|CuPcTs was lower than that of Zn3Ga|CO3(Tab. 2A entry c and Fig. 7b-line). It should be noted that
unsupported Na+4CuPcTs4 generated CO at a rate of 48%
of that using Zn3Ga|CO3 (Tab. 2A entry d).
In a control kinetic test using the physical mixture of
3.0 wt% of Na+4CuPcTs4 and 97 wt% of Zn3Ga|CO3
LDH prepared by mixing using a mortar and pestle for 30
min, the formation rates of CO and methanol were 41 and28 lmol h1 gcat1, respectively (Tab. 2A entry b), suggesting the importance of close contact of CuPcTs4 with
the LDH surface for the CO2 photoconversion using
CuPcTs-Zn3Ga|CO3 (Fig. 7a-line).
The addition of Ag to Zn3Ga|CO3 similarly affected the
addition of CuPcTs4. Total formation rates increased by afactor of 1.7 compared with Zn3Ga|CO3 and methanol

200

H2O
CO

CO2 (μmol)

100

0.08

H2O

(c) Visible

CO
CH3OH

0

1

0
0.12

CO2
H2O

100

0.04

CO

(d) Visible

CO2

200

0

CO2

CH3OH

0

0.12

2

3 4 0 1 2
Time elapsed (h)

0.08
0.04

H2O
3

CO, CH3OH, H2O (μmol)

CO2

4

5

CO
0

CuPcTs-Zn3GalCO3

4

0.6
3

0.5
0.4

2

0.3

Kubelka–Munk(b) UV–visible(a) UV–visible

Product formation rate (nmol h-1 g-1 nm-1)

S. Kawamura et al. / Photocatalytic Conversion of Carbon Dioxide Using Zn–Cu–Ga Layered Double Hydroxides Assembled 849
with Cu Phthalocyanine: Cu in Contact with Gaseous Reactant is Needed for Methanol GenerationPhotocatalytic Conversion of Carbon
Dioxide Using
Zn–Cu–Ga Layered Double Hydroxides Assembled with Cu Phthalocyanine: Cu in Contact with Gaseous Reactant is Needed for Methanol
CuPcTs-Zn3GalCO3
Zn3GalCuPcTs
0.7
Generation
300

1
0.2
0.1
200

300

400 500 600
Wave length (nm)

700

0
800

Figure 8
Figure 7
Time course of photocatalytic tests under CO2 (2.3 kPa) andH2 (21.7 kPa) using UV–visible light and a) CuPcTs-Zn3Ga|
CO3 and b) Zn3Ga|CuPcTs; and using visible light(k > 420 nm) and c) CuPcTs-Zn3Ga|CO3 and d) Zn3Ga|CuPcTs.
CO2 (◆; diamond), H2O (N; triangle), CH3OH (■; square), andCO (; circle).

selectivity of 54 mol% (Tab. 2A entry e). The addition of Au
to Zn3Ga|CO3 promoted the total formation rates by a factorof 1.8, but the methanol selectivity became only 13 mol%(Tab. 2A entry f).
Next, photocatalytic reduction tests of CO2 by H2 wereconducted irradiated by visible light (k > 420 nm). Although
the Zn3Ga|CO3 LDH showed poor photoactivity irradiatedby visible light (Tab. 2B entry a), the total formation rateusing CuPcTs-Zn3Ga|CO3 irradiated by visible light was
73% of that irradiated by UV–visible light (Tab. 2B entry b).
The methanol selectivity was maintained at 48-51 mol%(Fig. 7 c-line). Again, under the condition of visible lightirradiation, the formation of CO and methanol continued
for more than 5 h, demonstrating the stability of dispersed
CuPcTs4 over the LDH under CO2 + H2.
The total formation rate using Ag-Zn3Ga|CO3 irradiatedby visible light was 56% of that irradiated by UV–visible
light (Tab. 2A entry c, Tab. 2B entry d). The selectivity tomethanol decreased from 54 mol% (irradiated by UV–
visible light) to 29 mol% (irradiated by visible light). Thus,
Ag was less effective than CuPcTs4 as a promoter under thecondition of visible light irradiation. In comparison with theaddition of CuPcTs4 and Ag, Au-Zn3Ga|CO3 did not exhibit photocatalytic activity above the detection limit of GC
irradiated under visible light (Tab. 2B, entry e).
The promotion effect of CuPcTs4 to Zn3Ga|CO3 was further investigated by plotting the in-proﬁle (action) spectrum

In-proﬁle spectrum for CuPcTs-Zn3Ga|CO3.

of total formation rates (methanol and CO) based on the photocatalytic tests irradiated by the light of wavelengths
progressively greater than 420, 520, 580, 620, 660 and
715 nm using sharp cut ﬁlters (Fig. 8). The in-proﬁle spectrum and UV–visible absorption spectrum coincided wellin the wavelength ranges 310 and 688 nm, demonstratingthat the visible light absorption by CuPcTs4 dispersed over
Zn3Ga|CO3 (Fig. 2b-line) led to electron excitation and thenthe reduction of CO2.
3 DISCUSSION
Zn3Ga|CO3 exhibited photocatalytic reduction of CO2 by H2
and UV–visible light at a total formation rate (methanol andCO) of 130 nmol h1 gcat1 (Tab. 2A, entry a). When theamount of photocatalyst varied between 25 and 100 mg,
the formation rate (the unit: mol h1) was essentially proportional to the amount (Ahmed et al., 2011; Yoshida et al.,2012).
Copper ions were doped as a part of MII cations in the cationic layer [MII1xMIIIx(OH)2]x+ of LDH and/or Cu(OH)42
anions between the cationic layers. Both inlayer andinterlayer Cu sites were effective for photocatalytic
reduction of CO2 (Ahmed et al., 2011, 2012). As a result,
the total formation rate using Zn1.5Cu1.5Ga|Cu(OH)4 was
560 nmol h1 gcat1 (Tab. 2A, entry k). Copper sites wereprimarily active for photocatalytic reduction of CO2 by H2.
The drawback of LDH comprising Cu was the limitationof the wavelength for excitation light. The estimated Eg values for the LDH used in this study were 5.6-3.0 eV (Tab. 1),
indicating that only UV light was effective for CO2 photoreduction using LDH comprising Cu. To overcome this drawback, CuPcTs4, Ag or Au was doped to Zn3Ga|CO3.

850

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5

Vaccum (eV)0

SHE (V)
-4

-1
-3

CB

-2

-2

e–

-3
-4
-5
-6

LUMO

-1

e–
BG
e–
e–
0
Excitation:
excitation:
2.2 eV
5.6 eV
1HOMO
CuPcTs2

CO2 + 6H+ + 6e – → CH3OH + H2O – 0.32 V– 0.11 V
CO2 + 2H+ + 2e – → CO + H2O

-7
-8

VB
Zn3Ga CO3 LDH

3Figure 9
The energy diagram and proposed electron ﬂows in
CuPcTs-Zn3Ga|CO3 during photocatalytic reduction of CO2.

CuPcTs4, Ag or Au in CuPcTs-Zn3Ga|CO3, Ag-Zn3Ga|
CO3 and Au-Zn3Ga|CO3 photocatalysts were effective
at boosting methanol and CO formation rates by a factorof 1.6-1.8 compared with Zn3Ga|CO3, both irradiatedby UV–visible light (Tab. 2A). The interlayer distance of
the LDH changed negligibly for these photocatalysts(Fig. 1), demonstrating CuPcTs4, Ag and Au were overthe exterior surface of the LDH rather than the interlayer
space (Kawamura et al., 2015). Under UV–visible light,
band-gap excitation of the LDH occurred by UV light(Tab. 1), while HOMO–LUMO excitation (2.2 eV, Fig. 9;
Giraudeau et al., 1980; Shang et al., 2011) of CuPcTs4
and SPR of Ag and Au nanoparticles occurred by visible
light.
The photocatalytic effects of UV and visible light wereclariﬁed by the photocatalytic tests irradiated by visible light
only (Tab. 2B), and also the in-proﬁle spectrum of CO2 photoreduction (Fig. 8). The formation rates of methanol and
CO irradiated by visible light were 73-56% of those irradiated by UV–visible light using CuPcTs-Zn3Ga|CO3 andAg-Zn3Ga|CO3, whereas Au-Zn3Ga|CO3 did not exhibit
photocatalytic activity above the detection limit of GC under
the irradiation of visible light. Thus, CuPcTs4 was the best
promoter for the LDH irradiated by visible light. The stability of CuPcTs4 during photocatalytic tests was demonstrated (Fig. 2, 7a-c-line). Because the Eg value for Zn3Ga|
CO3 was 5.6 eV (Tab. 1), the LDH did not participate in
the catalysis under visible light but just dispersed CuPcTs4
molecules on the external surface.
The energy diagram and proposed electron ﬂow during
CO2 photoreduction using CuPcTs-Zn3Ga|CO3 are depicted
in Figure 9. As the HOMO–LUMO gap positions between
Valence Bands (VB) and Conduction Bands (CB) of
Zn3Ga|CO3, band-gap excited electrons at CB and resultant

holes at VB transfer to LUMO and HOMO of CuPcTs4,
respectively, irradiated by UV light. In fact, photogenerated
electrons diffused to the central Cu(II) site of CuPcTs4 at
the rates of 2.7-21 lmol h1 for Na+4CuPcTs4 and
CuPcTs-Zn3Ga|CO (Fig. 5). Na+4CuPcTs4 was able tophotocatalyze CO2 to CO by H2 (Tab. 2A, entry d). When
CuPcTs-Zn3Ga|CO3 was irradiated by visible light only,
the band-gap excitation (5.6 eV) of Zn3Ga|CO3 did not takeplace and only HOMO–LUMO excitation (2.2 eV) of
CuPcTs4 was possible. The decrease in the formation rates
of methanol and CO under visible light to 73% of thoseunder UV–visible light (Tab. 2) can be explained by thedecrease in the electron and hole supply from Zn3Ga|CO3
to CuPcTs4.
The LUMO of CuPcTs4 does not exactly populate Cu
sites but N atoms neighboring Cu sites (Fig. 3). Thus, theelectrons excited to the CB of the LDH would transfer to
LUMO (and/or an unoccupied level near LUMO) of
CuPcTs4. The energy level diagram of the LDH and
CuPcTs4 supported this hypothesis (Fig. 9).
The HOMO dominantly distribute on C atoms of pyrrole
rings, while the LUMO dominantly distribute on both C andN atoms of pyrrole rings for CuPcTs4 (Rauf et al., 2012).
The N atoms of pyrrole are bonded to central Cu2+ ionsand thus able to transfer the photo-excited electrons to
Cu2+ (Fig. 3). CO2 would be progressively reduced by the
reduced Cu+ in a similar way to that by inlayer and interlayer
Cu sites (Morikawa et al., 2014a; Ahmed et al., 2011, 2012).
On the other hand, H2 would donate an electron to the position around HOMO (Fig. 3) to form H+. The proton combines the electron and CO2 at the Cu site and ﬁnallytransforms into CO and methanol.
The SPR effect of Ag nanoparticles was already discussed
in reference (Kawamura et al., 2015). In contrast, SPR of Au
was not effective for the photoreduction of CO2 due to thegreater work function of Au (5.31-5.47 eV) as comparedwith that of Ag (4.52-4.74 eV). Irradiated by UV–visible
light, the Au surface played the role of an electron trap from
Zn3Ga|CO3.
LDH compounds especially comprising Cu and Ga selectively photoproduced methanol from CO2 + H2 (68-88 mol%,Tab. 1). A reaction mechanism via hydrogen carbonate
bound to Cu was proposed based on FTIR and XAFS
(Ahmed et al., 2011; Morikawa et al., 2014a). These LDH
photocatalysts would be combined with a photooxidation
catalyst, e.g. WO3, to produce methanol from CO2 + H2O(Morikawa et al., 2014b).

CONCLUSIONSLDH comprising Zn and Al or Ga photoreduced CO2 to CO
and methanol by H2 and the irradiation of UV–visible light.

S. Kawamura et al. / Photocatalytic Conversion of Carbon Dioxide Using Zn–Cu–Ga Layered Double Hydroxides Assembled 851
with Cu Phthalocyanine: Cu in Contact with Gaseous Reactant is Needed for Methanol GenerationPhotocatalytic Conversion of Carbon
Dioxide Using
Zn–Cu–Ga Layered Double Hydroxides Assembled with Cu Phthalocyanine:
Contact M.,
with Okada
GaseousK.,
Reactant
Methanol
Carja Cu
G.,inBirsanu
García isH.Needed(2013)forComposite
When Cu ions were doped as cations in the cationic layer
Generation
plasmonic gold/layered double hydroxides and derived mixed

and/or as interlayer anions, the CO2 photoreduction rates
increased to as high as 560 nmol h1 gcat1 (Zn1.5Cu1.5Ga|
Cu(OH)4 photocatalyst). Due to the wide band-gap natureof these LDH, UV light was effective for CO2 photoreduction. The doping of CuPcTs4 and Ag to the Zn3Ga|CO3
LDH boosted CO2 photoreduction by a factor of 1.6-1.8
by H2 and the irradiation of UV–visible light. CuPcTs4
was especially effective doped to the LDH for CO2 photoreduction irradiated by visible light only. The LUMO of
CuPcTs4 was distributed on N atoms of pyrrole rings boundto central Cu2+ ions. The photo-excited electrons diffused tocentral Cu2+ would progressively reduce CO2 ﬁnally to CO
and methanol in a similar way to the inlayer and interlayer
Cu sites in the LDH in this study.

ACKNOWLEDGMENTS
The authors are grateful for the ﬁnancial support fromthe Grant-in-Aid for Scientiﬁc Research C (26410204,
22550117) from the Japan Society for the Promotion of
Science, Asahi Grass Foundation, Promotion Section ofTechnology Innovation, Daikin Industries, and from the
Romanian National Authority for Scientiﬁc Research (PN-IIIDEI-PCE-75/2013). X-ray absorption experiments wereconducted under the approval of the Photon Factory Proposal
Review Committee (2011G033, 2009G552, 2007G576) and
Grant of the Priority Program for Disaster-Affected Quantum
Beam Facilities (2011A1977, SPring-8 & KEK).

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Manuscript submitted in October 2014
Manuscript accepted in May 2015
Published online in September 2015

Cite this article as: S. Kawamura, N. Ahmed, G. Carja and Y. Izumi (2015). Photocatalytic Conversion of Carbon Dioxide Using
Zn–Cu–Ga Layered Double Hydroxides Assembled with Cu Phthalocyanine: Cu in Contact with Gaseous Reactant is Neededfor Methanol Generation, Oil Gas Sci. Technol 70, 5, 841-852.