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Catalysis Today 185 (2012) 263–269

Contents lists available at ScienceDirect

Catalysis Todayjournal homepage: www.elsevier.com/locate/cattod

Photocatalytic conversion of carbon dioxide into methanol using optimized
layered double hydroxide catalysts
Naveed Ahmed a , Motoharu Morikawa b , Yasuo Izumi a,∗ab

Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
Department of Nanomaterial Science, Graduate School of Advanced Integration Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japana r t i c l e

i n f oArticle history:
Received 31 May 2011
Received in revised form 1 August 2011
Accepted 9 August 2011
Available online 1 September 2011
Keywords:
CO2
Photoreduction
MethanolLayered double hydroxide
Interlayer coppera b s t r a c t
The photocatalytic reduction of carbon dioxide into methanol was enabled between the Zn–Ga
or Zn–Cu–Ga hydroxide layers using hydrogen and was promoted by the partial desorption of
structural water stuffed between the cationic layers. The photoreduction rate obtained using[Zn1.5 Cu1.5 Ga(OH)8 ]+ 2 (CO3 )2− ·mH2 O was improved by replacing interlayer carbonate anions with
[Cu(OH)4 ]2− to 0.49 ␮molMethanol h−1 gcat −1 , and the methanol selectivity was 88 mol%. At the molar level,interlayer Cu species was 5.9 times more active than the octahedral Cu sites in the cationic layers. The
bandgap value was evaluated as 3.0 eV for the semiconductor [Zn1.5 Cu1.5 Ga(OH)8 ]+ 2 [Cu(OH)4 ]2− ·mH2 O.
Direct electronic transition from O 2p to metal 3d, 4s, or 4p was responsible for the photocatalysis excitedlargely by ultraviolet (UV), and to a lesser extent by visible light.
© 2011 Elsevier B.V. All rights reserved.

1. Introduction
There has been growing interest in the development of novel
artiﬁcial methods that capture and concentrate large quantities ofatmospheric carbon dioxide for subsequent conversion into fuels.
Use of these methods is considered to have the potential to help
alleviate major environmental problems relating to global warming and the scarcity of sustainable and secure energy sources [1,2].
Employing similarities that exist with the reaction of light during
photosynthesis in green plants, phytoplankton, and algae, many
researchers [3–5] have tried to attempt the photoreduction of CO2
coupled with water. However, due to thermodynamic limitations,the conversion of CO2 into fuels (such as methanol and formic acid)is extremely unfavorable [6]. In contrast, the conversion of CO2 andhydrogen into fuel shows promise for application in the near future
if H2 can be obtained from water using sunlight.
Recently, the photoreduction of CO2 and H2 into methanol
was reported for the ﬁrst time using semiconductor layered double hydroxide (LDH) photocatalysts that were formulated as [ZnII 1−x−y CuII y MIII x (OH)2 ]x+ 2/x (CO3 )2− ·mH2 O (M = Ga,
Al; 0 ≤ x ≤ 1/3; 0 ≤ y ≤ 1/2; m∼1/x) [6]. The methanol selectivity(26 mol%) obtained using Zn–Cu–Al LDH catalysts was improved to
68 mol% using Zn–Cu–Ga LDH catalysts. For practical applications,the catalytic rates of CO2 photoreduction are essential. Overall pho-

∗ Corresponding author. Tel.: +81 43 290 3696; fax: +81 43 290 2783.
E-mail address: yizumi@faculty.chiba-u.jp (Y. Izumi).
0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.08.010

tocatalytic rates (250 nmol h−1 gcat −1 ) obtained using Zn–Cu–Ga
catalysts [6] need to be accelerated by the optimization of theirelectronic state, concentrating the number of active sites, and thedesign of their interlayer reaction space.
In the interlayer space of LDH photocatalysts, CO2 was suggested for the reaction with the hydroxy group bound to the Cu sites
to form a hydrogen carbonate intermediate [6]. Under UV–visible
light, the Cu ions in the cationic layer facilitated charge separation
utilizing the reduction-oxidation (redox) of CuII  CuI . Hydrogencarbonate species were gradually reduced to formic acid, formaldehyde, and ﬁnally to methanol utilizing the trapped photogenerated
electrons as CuI ions [6]. Therefore, the interlayer space of theseLDH photocatalysts served as an active pocket for the reduction of
CO2 to methanol. An increase in the available reaction space would
lead to enhanced photocatalytic activity.
The shape and size controls of ordered one dimensional
nano/mesopores of metal oxides [7] and ordered two dimensional spaces between clay layers [8] were reported to lead to
improved catalysis. The LDH compounds are advantageous in thatthey increase the available reaction space between cationic layers by the desorption of structural water molecules and carbonate
anions while maintaining the regular stacked layer structure at
423–473 K [9,10]. As the quantity of CuII sites in the cationic layers increased, the photocatalytic formation rates of methanol wereenhanced [6]. If photoactive Cu sites can also be accommodated
as anion species of LDHs between the cationic layers, the photocatalytic rates per unit amount of catalyst would be further
optimized.

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N. Ahmed et al. / Catalysis Today 185 (2012) 263–269

In this study, the available interlayer reaction space wasincreased by the pretreatment of LDH catalysts. Also [Cu(OH)4 ]2−
anion species was introduced between the cationic layers of LDHs.
The effects of these modiﬁcations on the structure, physical properties, and photocatalytic performance were studied. In addition,
we demonstrated the feasibility of CO2 reduction coupled withwater.

2. Methods
2.1. Catalyst syntheses
and[Zn1.5 Cu1.5 Ga(OH)8 ]+ 2
[Zn3 Ga(OH)8 ]+ 2 (CO3 )2− ·mH2 O
2−(CO3 ) ·mH2 O LDH compounds were synthesized using areported procedure [6]. They are abbreviated as Zn3 Ga|CO3
and Zn1.5 Cu1.5 Ga|CO3 , respectively. Corresponding LDH samples
consisting of anionic copper species between the [Zn3 Ga(OH)8 ]+ or[Zn1.5 Cu1.5 Ga(OH)8 ]+ cationic layers were synthesized following asimilar procedure, in order to use (NH4 )2 CuCl4 ·2H2 O as the sourceof [CuCl4 ]2− anions.
For the synthesis of [Zn3 Ga(OH)8 ]+ LDH consisted of anionic Cu
species, 20 mL solution consisting of both 0.75 M Zn(NO3 )2 ·6H2 O
and 0.25 M Ga(NO3 )3 ·mH2 O was dropped at a rate of 0.6 mL min−1
into 100 mL of a 0.025–0.075 M (NH4 )2 CuCl4 ·2H2 O solution in aﬂask at 290 K under argon atmosphere while stirring at a rate of
900 rpm. The pH was adjusted to 8 by adding 1.0 M NaOH (∼40 mL
total), after which the mixture was continuously stirred at thesame rate at 290 K for 2 h. The pH was maintained at 8 by adding
1.0 M NaOH (∼1 mL total). Then, the temperature of the mixture
was raised to 353 K and stirred continuously for an additional
22 h, during which time the pH of the solution remained at 8.
The precipitates that were obtained were ﬁltered using a polytetraﬂuoroethene based membrane ﬁlter (Omnipore JGWP04700,
Millipore) with a pore size of 0.2 ␮m and washed well with
deionized water. The slurry of the precipitates was maintained
under an Ar atmosphere until the end of washing. The precipitates that were obtained were dried in ambient air at 290 K for 5
days.
During the catalyst synthesis, the [CuCl4 ]2− ions werehydrolyzed in the alkaline solution to form [Cu(OH)4 ]2− ions(see Section 3.1). The LDH compound obtained using 0.025 M(NH4 )2 CuCl4 ·2H2 O solution is [Zn3 Ga(OH)8 ]+ 2 [Cu(OH)4 ]2− ·mH2 O
andabbreviatedas
Zn3 Ga|Cu(OH)4 .
When
0.075 M
(NH4 )2 CuCl4 ·2H2 O solution was used, the molar quantity of [CuCl4 ]2− had increased by three times to form
Zn3 Ga|Cu(OH)4 . The compound that was obtained is denoted
as [Zn3 Ga(OH)8 ]+ 2 [Cu(OH)4 ]2− ·mH2 O-3 × ex and abbreviated as
Zn3 Ga|Cu(OH)4 -3 × ex.
Cu atoms were substituted at the ZnII sites of cationic layers
and were also intercalated as hydroxy anions following a similar procedure to obtain [Zn1.5 Cu1.5 Ga(OH)8 ]+ LDH consisting ofanionic Cu species. In order to set the molar ratio of ZnII , CuII ,and GaIII ions to 3:3:2, a mixed acid solution (20 mL) was prepared as 0.375 M Zn(NO3 )2 ·6H2 O, 0.375 M Cu(NO3 )2 ·3H2 O, and0.25 M Ga(NO3 )3 ·nH2 O. The 20 mL solution was dropped at a rateof 0.6 mL min−1 into 100 mL of a 0.025–0.075 M (NH4 )2 CuCl4 ·2H2 O
solution in a ﬂask at a temperature of 290 K and under an Ar atmosphere while stirring at a rate of 900 rpm. The subsequent steps
in the procedure were identical to those for Zn3 Ga|Cu(OH)4 . The
stoichiometric product starting from 0.025 M of (NH4 )2 CuCl4 ·2H2 O
is [Zn1.5 Cu1.5 Ga(OH)8 ]+ 2 [Cu(OH)4 ]2− ·mH2 O and abbreviated as
Zn1.5 Cu1.5 Ga|Cu(OH)4 . In the case where three times the quantityof (NH4 )2 CuCl4 ·2H2 O (0.075 M) was used, the compound that was
obtained is denoted as [Zn1.5 Cu1.5 Ga(OH)8 ]+ 2 [Cu(OH)4 ]2− ·mH2 O3 × ex and abbreviated as Zn1.5 Cu1.5 Ga|Cu(OH)4 -3 × ex.

2.2. Characterization
Nitrogen adsorption isotherm measurements were performed
at 77 K within the pressure range 1–90 kPa in a vacuum systemthat was connected to diffusion and rotary pumps (10−6 Pa) andequipped with a capacitance manometer (Models CCMT-1000A
and GM-2001, ULVAC). The Brunauer–Emmett–Teller (BET) surface
area (SBET ) was calculated on the basis of the eight-point measurements between 10 and 46 kPa (P/P0 = 0.10–0.45) in the adsorption
isotherm. As-synthesized samples were evacuated at 383 K for 2 h
or at 423 K for 1 h before the measurements were taken.
X-ray diffraction (XRD) data were obtained using a MiniFlex
diffractometer (Rigaku) at a Bragg angle of 2 B = 5–70◦ with a scanstep of 0.01◦ and a scan rate of 7 s per step for the sample powders.
The measurements were performed at 30 kV and 15 mA using Cu
K␣ emission and a nickel ﬁlter.
Optical spectroscopic measurements were performed using a
UV–visible spectrophotometer (JASCO, Model V-650). D2 and halogen lamps were used for wavelengths below and above 340 nm,
respectively, and an integrating sphere (JASCO, Model ISV-469)
was used for the diffuse reﬂectance measurements. Measurements
were performed at 290 K for wavelengths in the range 200–900 nm
using 100 mg of fresh samples. Diffuse reﬂectance spectra wereconverted to absorption spectra on the basis of the Kubelka–Munkfunction [11,12]. The bandgap (Eg ) value was evaluated on the basisof either simple extrapolation of the absorption edge or the ﬁt to
the Davis–Mott equation [12], given by˛ × hv ∝ (hv − Eg )nwhere ˛, h, and  are the absorption coefﬁcient, Planck’s constant,
and wavenumber, respectively, and n is 1/2, 3/2, 2, and 3 for allowed
direct, forbidden direct, allowed indirect, and forbidden indirecttransitions, respectively.
Cu K-edge X-ray absorption ﬁne structure (XAFS) spectra were
measured at 290 K in transmission mode in the Photon Factoryat the High Energy Accelerator Research Organization on thebeamlines of 9C or 7C. The storage-ring energy was 2.5 GeV, andthe top-up ring current was 450 mA. A Si(1 1 1) double-crystal
monochromator was inserted into the path of the X-ray beam. The
X-ray intensity was maintained at 65% of the maximum ﬂux using
a piezo translator to suppress higher harmonics. The size of theslit opening in front of the I0 ionization chamber was 1 mm (vertical) × 2 mm (horizontal). The I0 and Itransmit ionization chambers
were purged with N2 and Ar gases, respectively. The data accumulation time was 1 s for each data point. The Cu K absorption
edge energy value was calibrated to 8980.3 eV for the spectrum of
Cu metal [13,14]. The energy position of the monochromator was
reproduced with an error of ±0.1 eV.
The XAFS data were analyzed using an X-ray absorption spectroscopy data analysis programme (XDAP) package [15]. The
pre-edge background was approximated by a modiﬁed Victoreen
function C2 /E2 + C1 /E + C0 . The background of the post-edge oscillation was approximated by a smoothing spline function andcalculated by an equation for the number of data points, where
k is the wavenumber of photoelectrons.

(xi − BGi )2i=1

exp(−0.075ki )Data Points

2≤ smoothing factor

2.3. Photocatalytic conversion tests for CO2
As-synthesized and preheated samples of LDHs were tested forthe photocatalytic conversion of CO2 [6]. The tests were conductedin a closed circulating system (186 mL) equipped with a photoreaction quartz cell that had a ﬂat bottom (23.8 cm2 ) [16]. 100 mg

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N. Ahmed et al. / Catalysis Today 185 (2012) 263–269

of the LDH catalyst was uniformly spread in the photoreaction
cell and was evacuated by rotary and diffusion pumps (10−6 Pa)
at 290 K for 2 h until the desorbed gas was detected by an online
gas chromatograph (GC).
Reaction tests were also performed for samples of Zn3 Ga|CO3
and Zn1.5 Cu1.5 Ga|CO3 preheated at 423 K. The LDH sample was
evacuated at 290 K and the temperature was elevated to 423 K at
a rate of 4 K min−1 . The sample was kept at 423 K for 1 h, cooled,
and transferred to the photoreaction cell using an argon ﬁlled box
to prevent contact with air.
2.3 kPa of CO2 (0.177 mmol) and 21.7 kPa of H2 (1.67 mmol)
were introduced to both intact and pretreated LDH photocatalysts
and were allowed to circulate for 30 min in contact with the catalyst
to attain sorption equilibrium before illumination. The photocatalyst was then illuminated with UV–visible light from the 500-W
xenon arc lamp (Ushio, Model UI-502Q) from downward throughthe ﬂat bottom of the quartz reactor for 5 h. The distance between
the bottom of the reactor and the lamp house exit window was
set to 20 mm. The light intensity at a wavelength of 555 nm was
110 mW cm−2 at the center of the sample cell and 73 mW cm−2 atthe periphery of the bottom plate of the sample cell. The intensity
was measured at 555 nm, but the Xe arc lamp irradiated in a wide
spectrum between 200 and 1100 nm. The temperature was within
the range 305–313 K at the catalyst position during the illumination for 5 h [6]. The durability test for 20 h was also performed for
the Zn1.5 Cu1.5 Ga|Cu(OH)4 photocatalyst.
Products and reactants were analyzed using packed columnsof molecular sieve 13X-S and polyethylene glycol (PEG-6000) supported on Flusin P (GL Sciences) set in the online GC equipped witha thermal conductivity detector (Shimadzu, Model GC-8A).

3. Results
3.1. Nitrogen adsorption, XRD, UV–visible spectra, and Cu K-edge
EXAFS
The SBET values were measured by the N2 adsorption at 77 K
for LDH compounds treated at 383 K in a vacuum (Table 1).
The SBET value for Zn3 Ga|CO3 was 23% greater than thatfor Zn1.5 Cu1.5 Ga|CO3 . On increasing the preheating temperature to 423 K for these LDHs, SBET values increased by 18–19%(67–83 m2 g−1 ) due to the partial desorption of interlayer waterand carbonate ions.
When compared with the SBET value for Zn3 Ga|CO3 , the values
for Zn3 Ga|Cu(OH)4 and Zn3 Ga|Cu(OH)4 -3 × ex decreased by 50–53%
because of the substitution of carbonate ions with [Cu(OH)4 ]2−ions. On the other hand, when compared with the SBET value
for Zn1.5 Cu1.5 Ga|CO3 , the values for Zn1.5 Cu1.5 Ga|Cu(OH)4 andZn1.5 Cu1.5 Ga|Cu(OH)4 -3 × ex did not change signiﬁcantly (±8.8%).
The contribution of impurity phase(s) (see Section 3.1) may not be
negligible.
The XRD spectra measured for as-synthesized LDH samples
are shown in Fig. 1. The diffraction peaks exhibited typical patterns due to the diffraction of layer planes showing intenseand sharp peaks at 2 B = 10–35◦ , and also showing weak andbroad peaks at 2 B values greater than 35◦ [17]. There wasa downward shift in several peaks at 2 B = 11.7–11.8◦ , 23.5◦ ,34.3◦ , 36.9–37.0◦ , 38.9◦ , 43.6–43.7◦ , 46.4◦ , 52.6◦ , 56.0◦ , 59.4◦ ,60.8◦ , and 64.8◦ for Zn3 Ga|CO3 (spectrum a) and Zn1.5 Cu1.5 Ga|CO3(spectrum d) by 0.4–1.5◦ to 11.2–11.4◦ , 22.5–22.7◦ , 33.9–34.1◦ ,36.4–36.5◦ , 38.2–38.6◦ , 43.1–43.2◦ , 45.3–45.5◦ , 51.1–51.4◦ , 54.5◦ ,59.0–59.2◦ , 60.3–60.5◦ , and 64.0–64.1◦ for Zn3 Ga|Cu(OH)4 (spectrum b), Zn1.5 Cu1.5 Ga|Cu(OH)4 (e), and their 3 × ex analogues (cand f) that were assigned to (0 0 3), (0 0 6), (0 0 9), (1 0 4), (0 1 5),

265

Fig. 1. XRD spectra of as-synthesized samples of Zn3 Ga|CO3 (a), Zn3 Ga|Cu(OH)4(b), Zn3 Ga|Cu(OH)4 -3 × ex (c), Zn1.5 Cu1.5 Ga|CO3 (d), Zn1.5 Cu1.5 Ga|Cu(OH)4 (e), andZn1.5 Cu1.5 Ga|Cu(OH)4 -3 × ex (f). (*) and (+) marks indicate peaks derived from
Cu(OH)2 and CuO powders, respectively.

(1 0 7), (0 1 8), (1 0 1 0), (0 1 1 1), (1 1 0), (1 1 3), and (1 1 6) diffractions, respectively [18,19].
In addition to characteristic LDH diffraction patterns, peaks
derived from the CuO impurity phase appeared at 2 B = 32.4–32.5◦ ,35.5◦ , and 57.0–57.1◦ [20,21] in the spectra of Zn1.5 Cu1.5 Ga|Cu(OH)4and the 3 × ex analogue (Fig. 1e and f). Peaks derived fromthe Cu(OH)2 impurity phase appeared at 2 B = 16.2–16.3◦ , 39.4◦ ,50.1–50.2◦ , 53.5–53.6◦ , and 62.6–62.7◦ [22,23] for Zn3 Ga|Cu(OH)4 3 × ex, Zn1.5 Cu1.5 Ga|Cu(OH)4 , and Zn1.5 Cu1.5 Ga|Cu(OH)4 -3 × ex
(spectra c, e, and f). The low index diffraction peaks derived fromCuO or Cu(OH)2 were detectable for Zn3 Ga|Cu(OH)4 (b), which suggested a limited population of impurity phase(s). Diffraction peaks
due to CuO and Cu(OH)2 were not found for Zn1.5 Cu1.5 Ga|CO3 (d).
The interlayer interval was evaluated as 0.751 and 0.753 nm
on the basis of the (0 0 3) diffraction angle for Zn3 Ga|CO3 andZn1.5 Cu1.5 Ga|CO3 (Table 1A and B) [24]. The values were nearlyconsistent with the values of 0.756 and 0.758 nm, which were
evaluated based on the (0 0 6) diffraction angle. The interlayer interval values on the basis of the (0 0 9) diffraction were 0.784 and0.785 nm. The overlap of (0 1 2) peaks on (0 0 9) peaks may be thereason for the discrepancy (0.032–0.033 nm), when compared withthe corresponding values based on (0 0 3) diffraction.
On the basis of the (0 0 3) diffraction angle (Table 1A, C, andE), the interlayer interval increased from 0.751 nm for Zn3 Ga|CO3
to 0.792–0.784 nm for Zn3 Ga|Cu(OH)4 and Zn3 Ga|Cu(OH)4 -3 × ex.
This reﬂected the greater size of [Cu(OH)4 ]2− ions when compared to that of (CO3 )2− [25,26]. In a similar manner, by
substituting (CO3 )2− with [Cu(OH)4 ]2− (Table 1B, D, and F), theinterlayer interval increased from 0.753 nm for Zn1.5 Cu1.5 Ga|CO3
to 0.782–0.772 nm.
The in-plane (1 1 0) diffraction angle (0.155–0.156 nm, Table 1)
within a layer corresponds to R(Zn–O) of 0.219 nm if the complete
ZnO6 octahedra are assumed and remained unchanged for all of
the LDH samples. In this study, the value did not change for theZn-based LDHs [24].
The UV–visible absorption spectra for synthsized LDH compounds are depicted in Fig. 2. The Eg values were estimated byextrapolation of the absorption edge to the x-axis. For Zn3 Ga|CO3 ,the intersection with the x-axis was at 222 nm (spectrum a), whichcorresponded to an Eg value of 5.6 eV (Table 1A). The Eg values werealso estimated based on the ﬁt to the Davis–Mott equation, and they
were similar to 5.6 eV when n was 1/2 or 3/2 (Table 1A), suggesting
direct electronic transition from oxygen 2p to 4s or 4p levels of Zn
and Ga.
When the interlayer (CO3 )2− ions were replaced with
[Cu(OH)4 ]2− , the UV absorption edge shifted by 60–75 nm towards
the lower energy side (Fig. 2a and b). The absorption edge was
extrapolated to 297 nm, corresponding to the Eg value of 4.2 eV(Table 1C). In addition, a shoulder peak near the base that extended

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N. Ahmed et al. / Catalysis Today 185 (2012) 263–269

Table 1
Physicochemical characterization of [Zn3−x Cux Ga(OH)8 ]+ 2 [A]2− ·mH2 O (x = 0, 1.5; A = CO3 , Cu(OH)4 ) layered double hydroxides.
Eg (eV)
2

Entry

SampleA
B
C[Zn3 Ga(OH)8 ] 2 (CO3 ) ·mH2 O (Zn3 Ga|CO3 )[Zn1.5 Cu1.5 Ga(OH)8 ]+ 2 (CO3 )2− ·mH2 O (Zn1.5 Cu1.5 Ga|CO3 )[Zn3 Ga(OH)8 ]+ 2 [Cu(OH)4 ]2− ·mH2 O (Zn3 Ga|Cu(OH)4 )(impurity phase(s))[Zn1.5 Cu1.5 Ga(OH)8 ]+ 2 [Cu(OH)4 ]2− ·mH2 O(Zn1.5 Cu1.5 Ga|Cu(OH)4 )[Zn3 Ga(OH)8 ]+ 2 [Cu(OH)4 ]2− ·mH2 O-3×ex(Zn3 Ga|Cu(OH)4 -3×ex)(impurity phase(s))[Zn1.5 Cu1.5 Ga(OH)8 ]+ 2 [Cu(OH)4 ]2− ·mH2 O-3×ex(Zn1.5 Cu1.5 Ga|Cu(OH)4 -3×ex)

D
E

F
a,b

−1

SBET (m g

+

2−

)

Extrapolated

Fit to ˛×h ∝ (h – Eg )nInterlattice Distance(nm)n = 1232

2

3(0 0 3)(1 1 0)

5.9
4.2
4.8
4.6
3.2

5.4
3.2
3.6
3.0
2.8

5.2
3.0
3.4
2.8
2.7

5.0
2.6
2.7
2.5
2.6

0.751
0.753
0.792

0.155
0.155
0.156

62a

5.6
3.5
4.2
3.2
3.0

0.782

0.156

33a

3.6

4.2

3.1

2.8

2.6

0.784

0.156

52a

3.1
3.0

4.1
3.2

2.9
2.9

2.6
2.7

2.0
2.7

0.772

0.155

a

b

70 (83 )57a (67b )35a

Preheated at 383 K for 2 ha or 423 K for 1 hb under vacuum.

Fig. 3. Cu K-edge EXAFS spectra for Zn3 Ga|Cu(OH)4 -3 × ex sample. (A) k3 -weighted
EXAFS -function and (B) its associated Fourier transform. The solid and dotted lines
represent the magnitude and the imaginary part in (B).

Fig. 2. Diffuse reﬂectance UV–visible absorption spectra of as synthesized samplesof Zn3 Ga|CO3 (a), Zn3 Ga|Cu(OH)4 (b), Zn3 Ga|Cu(OH)4 -3 × ex (c), Zn1.5 Cu1.5 Ga|CO3(d), Zn1.5 Cu1.5 Ga|Cu(OH)4 (e), and Zn1.5 Cu1.5 Ga|Cu(OH)4 -3 × ex (f).

close to the visible light region (Fig. 2b) was extrapolated to 390 nm(Eg = 3.2 eV). The absorption edge further shifted by 40–50 nm
towards the lower energy side for Zn3 Ga|Cu(OH)4 -3 × ex comparedto that for Zn3 Ga|Cu(OH)4 (spectra b→c). The absorption edge was
extraporated to 348 nm (Eg = 3.6 eV; Table 1E). For spectrum c, theedge overlapped with the larger shoulder peak between 300 and
430 nm. The extrapolation of the larger shoulder resulted in an
estimated Eg value of 3.1 eV (397 nm) (Table 1E).
The ﬁts to the Davis–Mott equation resulted in similar Eg valuesof 4.2 and 3.6 eV, which were based on simple extrapolation (Fig. 2b
and c), for Zn3 Ga|Cu(OH)4 and Zn3 Ga|Cu(OH)4 -3 × ex, respectively,
when n was 1/2 or 3/2 (Table 1C and E), suggesting direct electronictransition from oxygen 2p to Cu 3d, 4s, or 4p levels and Zn/Ga 4s or
4p levels.
The shoulder corresponded to Eg values of 3.1–3.2 eV for
Zn3 Ga|Cu(OH)4 , and the 3 × ex analogue appeared to originate fromthe Cu(OH)2 and/or CuO impurity phase(s) in light of the XRD
spectra (Fig. 1b and c). The Eg values estimated by the ﬁt to theDavis–Mott equation were very similar to the values that werebased on simple extrapolation when n was 3/2 (Table 1C and E).
This demonstrates the forbidden direct electronic transition fromoxygen 2p to Cu 3d, 4s, or 4p levels of Cu(OH)2 and/or CuO phase(s).
On inclusion of Cu ions in the cationic layers of Zn3 Ga|CO3 ,the UV absorption edge shifted towards the lower energy sidefor Zn1.5 Cu1.5 Ga|CO3 (Fig. 2d) when compared to the corresponding LDH free from Cu (spectrum a). Simple extrapolation yieldedan Eg value of 3.5 eV (354 nm; Table 1B). Upon further replacing
interlayer (CO3 )2− ions with [Cu(OH)4 ]2− ions, the UV absorption
edge shifted by 65–75 nm towards the lower energy side (Fig. 2e).

Simple extrapolation of the absorption edge resulted in an intersection with the x-axis at 417 nm (Eg = 3.0 eV; Table 1D).
The absorption edge for the LDH sample for which three times
the amount of [CuCl4 ]2− was used for the synthesis (Fig. 2f)
was extrapolated to 407 nm (Eg = 3.0 eV, Table 1F). For the caseof n ranging from 1/2 or 3/2, the ﬁts of the spectrum to theDavis–Mott equation resulted in very similar estimates of the Eg
values based on simple extrapolation for Zn1.5 Cu1.5 Ga|Cu(OH)4 andZn1.5 Cu1.5 Ga|Cu(OH)4 -3 × ex (Table 1D and F).
In summary, the Eg value (5.6 eV) for Zn3 Ga|CO3 that is free fromcopper decreased to 4.2 and 3.5 eV on the inclusion of Cu between
and within layers, respectively, and decreased further to 3.0 eV on
the inclusion of Cu at both locations. The electronic transition wasconsidered to be directly from O 2p to metal (n–1)d, ns, or np.
In addition, the forbidden direct electronic transition (3.1–3.2 eV)from oxygen 2p to Cu 3d, 4s, or 4p levels of Cu(OH)2 and/or CuO
impurity phase(s) was observed.
The Cu K-edge EXAFS spectrum was tentatively measured for
the Zn3 Ga|Cu(OH)4 -3 × ex sample (Fig. 3). In the Fourier transform(panel B), two intense peaks appeared at 0.16 and 0.27 nm (phaseshift uncorrected). The peak that was derived from Cu–Cl bond(s)would appear at 0.195 nm [27] (phase shift uncorrected; true bonddistances 0.2270–0.2287 nm) [28,29], but no peak appeared at the
distance in Fig. 3B. Therefore, no Cu–Cl peaks were found in the
Fourier transform for this LDH photocatalyst (Fig. 3B), demonstrating the complete hydrolysis of [CuCl4 ]2− into [Cu(OH)4 ]2− .
3.2. Photocatalytic conversion of CO2
While Zn3 Ga|CO3 was CO selective (80 nmol h−1 gcat −1 ) in
2.3 kPa of CO2 and 21.7 kPa of H2 under the illumination of
UV–visible light (Fig. 4A and Table 2A), Zn1.5 Cu1.5 Ga|CO3 wasmethanol selective (170 nmol h−1 gcat −1 and 68 mol%; Fig. 4B and
Table 2B).

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N. Ahmed et al. / Catalysis Today 185 (2012) 263–269

267

Table 2Rates of photocatalytic conversion of CO2 with H2 into CH3 OH and CO over LDH photocatalysts.a
Entry

Photocatalyst

A
B
C
D
E
F
G
H

Zn3 Ga|CO3
Zn1.5 Cu1.5 Ga|CO3
Zn3 Ga|CO3 b
Zn1.5 Cu1.5 Ga|CO3 b
Zn3 Ga|Cu(OH)4
Zn1.5 Cu1.5 Ga|Cu(OH)4Zn3 Ga|Cu(OH)4 -3×ex
Zn1.5 Cu1.5 Ga|Cu(OH)4 -3×exab

Formation rate (nmol h−1 gcat −1 )CH3 OHCO

51(±4)
170(±14)50(±4)
310(±9)300(±9)490(±15)280(±8)430(±13)80(±6)
79(±6)
74(±6)
180(±2)130(±10)70(±6)
120(±9)48(±4)

130
250
120
500
430
560
390
480

Conversion(%, C-base)

Selectivity to
CH3 OH (mol%)

0.02
0.03
0.02
0.07
0.04
0.05
0.04
0.05

39(±4)
68(±4)
40(±4)
63(±1)
71(±2)
88(±2)
71(±3)
90(±1)

The catalyst amount was 100 mg. Values in the parentheses are experimental errors for evaluation.
Preheated at 423 K for 1 h under vacuum.

Fig. 5. Time course of photoreactions in CO2 (2.3 kPa) + H2 (21.7 kPa) using 100 mg
of Zn1.5 Cu1.5 Ga|Cu(OH)4 for 20 h. Other reaction conditions and legends are the sameas those for Fig. 4.

Fig. 4. Time course of photocatalytic reactions in CO2 (2.3 kPa) + H2 (21.7 kPa).
Hundred milligram of the LDH catalyst was charged: fresh Zn3 Ga|CO3 (A),Zn1.5 Cu1.5 Ga|CO3 (B), Zn3 Ga|CO3 preheated at 423 K (C), Zn1.5 Cu1.5 Ga|CO3 preheatedat 423 K (D), fresh Zn3 Ga|Cu(OH)4 (E), Zn1.5 Cu1.5 Ga|Cu(OH)4 (F), Zn3 Ga|Cu(OH)4 3 × ex (G), and Zn1.5 Cu1.5 Ga|Cu(OH)4 -3 × ex (H). The reactor was illuminated from a
500-W Xe arc lamp. CO2 (; diamond), H2 O (; triangle), CH3 OH (; square), andCO (䊉; circle).

When the Zn3 Ga|CO3 sample was preheated at 423 K in a vacuum, the change in the photocatalytic performance was negligible(Fig. 4C and A). In contrast, when Zn1.5 Cu1.5 Ga|CO3 was preheatedat 423 K (Fig. 4D), the methanol formation rate increased by a factor of 1.8 (310 nmol h−1 gcat −1 ; Table 2D and B). The availability ofinterlayer sites bound to Cu was considered to be critical for CO2
photoreduction.
The Zn3 Ga|Cu(OH)4 photocatalyst was then tested (Fig. 4E). The
methanol formation rate was enhanced by a factor of 5.9 comparedto that using Zn3 Ga|CO3 (Table 2E and A). The methanol selectivity was nearly the same as that obtained using Zn1.5 Cu1.5 Ga|CO3(71–68 mol%). Cu sites between interlayers and within cationiclayers may work similarly in photocatalysis. On the other hand,
the interlayer Cu sites boosted the methanol formation rates
by a factor of 5.9 as opposed to the promotion of Cu sites in[Zn1.5 Cu1.5 Ga(OH)8 ]+ layers, which boosted the methanol formation rates by a factor of 3.3 (Table 2E, B and A).

Using Zn1.5 Cu1.5 Ga|Cu(OH)4 (Fig. 4F), the methanol formation
rate and selectivity were further improved to 490 nmol h−1 gcat −1
and 88 mol% (Table 2F) owing to the combination of Cu sites withinlayers and Cu sites between layers.
In addition, photocatalysis using Zn3 Ga|Cu(OH)4 -3 × ex wascompared with an exact stoichiometric Zn3 Ga|Cu(OH)4 sample toevaluate the effects of Cu impurities. The decrease in the productformation rates was only 5.6–8.6% (Table 2E and G). The catalytic
effects of Cu impurities were greater for Zn1.5 Cu1.5 Ga|Cu(OH)4 3 × ex compared to its stoichiometric analogue (12–31%; Table 2H
and F). However, the effect was still relatively small.
In the kinetic tests (Fig. 4), the formation rates of water were
between 0.3 and 3.0 ␮mol h−1 gcat −1 . These rates exceeded thequantity of water that was catalytically formed as the products ofthe following equations:
CO2 + H2 → CO + H2 O

CO2 + 3H2 → CH3 OH + H2 OThis discrepancy is due to the desorption of interlayer watermolecules of LDHs [30,31].
Finally, the durability of the Zn1.5 Cu1.5 Ga|Cu(OH)4 photocatalyst (100 mg) was tested for 20 h in CO2 (2.3 kPa) + H2 (21.7 kPa)
illuminated with UV–visible light from the 500-W xenon arc lamp.
In the test, methanol formation continued and the selectivity was
76–84 mol% (Fig. 5). After every 5 h, the methanol formation rate
gradually decreased from 145 to 101, 61, and subsequently to
26 nmol h−1 gcat −1 , while the CO formation rate decreased from 28
to 23, 15 and ﬁnally 8.2 nmol h−1 gcat −1 . Throughout the test, theonly products that were identiﬁed were methanol, CO, and water.

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N. Ahmed et al. / Catalysis Today 185 (2012) 263–269

Table 3Rates of photocatalytic formation of CH3 OH and CO per speciﬁc surface area and the quantity of Cu in the LDH photocatalysts.a
Entry

Photocatalyst

A
B
C
D
E
F

Zn3 Ga|CO3
Zn1.5 Cu1.5 Ga|CO3
Zn3 Ga|CO3 b
Zn1.5 Cu1.5 Ga|CO3 b
Zn3 Ga|Cu(OH)4
Zn1.5 Cu1.5 Ga|Cu(OH)4abFormation rate per amount of Cu(nmol h−1 mmolCu −1 )Formation rate per speciﬁc
surface area (nmol h−1 m−2 )CH3 OHCO

CH3 OH

CO



0.73
3.0
0.60
4.7
8.6
7.9

1.1
1.4
0.89
2.7
3.6
1.1

1.9
4.4
1.5
7.4
12
9.1

–
51
–
97
300
120

–
24
–
56
130
17

–
77
–
150
430
140

The catalyst amount was 100 mg. Values in the parentheses are experimental errors for evaluation.
Preheated at 423 K for 1 h under vacuum.

4. Discussion
4.1. Improvement in photocatalysis by preheating of LDH
photocatalysts
The layered structure of the synthesized compounds of theformula for [Zn3−x Cux Ga(OH)8 ]+ 2 (CO3 )2− ·mH2 O was conﬁrmed by
XRD. The interlayer distance of these compounds ranged from 0.751
to 0.753 nm, and the SBET values ranged from 57 to 70 m2 g−1 . The Eg
value of Zn3 Ga|CO3 was 5.6 eV. Upon the addition of Cu sites in thelayers to form Zn1.5 Cu1.5 Ga|CO3 , the Eg value decreased to 3.5 eV.
Heating these samples under vacuum conditions at 423 K
resulted in the removal of one third of the water and interlayer
carbonate ions [6], while the SBET value increased by 18–19%. The
SBET increase for Zn1.5 Cu1.5 Ga|CO3 does not adequately explain theincrease of the methanol and CO formation rates by a factor of 1.8
and 2.3, respectively (Table 2D). The layered structure and the composition of the cationic layer did not change based on the XRD and
UV–visible absorption pattern (Figs. 1 and 2). The available interlayer space that was created at 423 K is expected to facilitate thediffusion of CO2 into the reaction space and the reaction with thesurface hydroxy groups that are bound to the Cu sites (the Graphicalabstract).
4.2. Role of in-layer and interlayer Cu in photocatalysis
Based on the tentative Cu K-edge EXAFS data, no chlorine
coordination was observed for the Zn3 Ga|Cu(OH)4 -3 × ex sample.
Therefore, during catalyst synthesis, the following reactions (atleast partially) took place.
[CuCl4 ]2− + 4NaOH → [Cu(OH)4 ]2− + 4NaCl(1)

2[Zn3 Ga(OH)8 ]+ + [Cu(OH)4 ]2− + mH2 O  [Zn3 Ga(OH)8 ]+2[Cu(OH)4 ]2− · mH2 O(2)2−[Zn3 Ga(OH)8 ]+· mH2 O  Zn6 Ga2 (OH)15 {( − O)
2 [Cu(OH)4 ]Cu(OH)3 } · mH2 O + H2 O

(3)

Zn6 Ga2 (OH)15 {( − O)Cu(OH)3 } · mH2 O  Zn6 Ga2 (OH)14{( − O)2 Cu(OH)2 } · mH2 O + H2 O

(4)Zn6 Ga2 (OH)14 {( − O)2 Cu(OH)2 } · mH2 O  Zn6 Ga2 (OH)13{( − O)3 Cu(OH)} · mH2 O + H2 O

(5)

By substituting (CO3 )2− anions for [Cu(OH)4 ]2− ions, the SBET
values decreased by 50–53% for [Zn3 Ga(OH)8 ]+ LDHs, and did notchange signiﬁcantly for [Zn1.5 Cu1.5 Ga(OH)8 ]+ LDHs (Table 1). The
formation rates per speciﬁc catalyst surface area and the formation
rates for speciﬁc quantities of Cu are summarized in Table 3.
The methanol formation rates per speciﬁc surface area weregreater for Cu containing LDHs, especially those which consisted of[Cu(OH)4 ]2− , whereas there was no signiﬁcant variation in the COformation rates per speciﬁc surface area. For the Zn1.5 Cu1.5 Ga|CO3
photocatalyst, the formation rates of methanol and CO were 51
and 24 nmol h−1 mmolCu −1 , respectively. These values increased to
97 and 56 nmol h−1 mmolCu −1 , respectively, when heated at 423 K
in a vacuum. For the Zn3 Ga|Cu(OH)4 photocatalyst, the formation
rates of methanol and CO were 300 and 130 nmol h−1 mmolCu −1 ,respectively. Therefore, the interlayer Cu sites were 5.3–5.9 times
more effective than Cu sites in cationic layers, if we assume that
the Cu atoms are primary active sites.
The methanol formation rate per unit amount of photocatalyst
was optimum when using Zn1.5 Cu1.5 Ga|Cu(OH)4 that consisted ofboth in-layer and interlayer Cu sites (Table 2F). The methanol formation rate per unit amount of Cu was 120 nmol h−1 mmolCu −1(Table 3F), which is in close agreement with the calculated valueof 110 nmol h−1 mmolCu −1 , which was contributed by the in-layer
Cu sites (3/4 × 51 nmol h−1 mmolCu −1 ) and the interlayer Cu sites(1/4 × 300 nmol h−1 mmolCu −1 ).
During the synthesis of LDH 3 × ex analogues, one third(2.5 mmol) of the (NH4 )2 CuCl4 ·2H2 O compound was used to
produce 2.5 mmol of LDHs (2.50–2.51 g). Two thirds (5.0 mmol)of the (NH4 )2 CuCl4 ·2H2 O would produce 0.40–0.48 g of CuO &Cu(OH)2 , depending on the population ratio. Therefore, the contents of Cu impurity phases are 14–16 wt% in Zn3 Ga|Cu(OH)4 -3 × ex
and Zn1.5 Cu1.5 Ga|Cu(OH)4 -3 × ex, which are in good agreementwith the photocatalytic rate decrease of 5.6–8.6% and 12–31%,
respectively (Table 2). The Cu impurity phases appear to beinactive.
The introduction of Cu in the interlayer space of LDHs accomplished two purposes. First, it resulted in an increase in theinterlayer distance from 0.751 nm for Zn3 Ga|CO3 to 0.792 nm for
Zn3 Ga|Cu(OH)4 , in effect expanding the interlayer reaction space.
A similar change was observed for [Zn1.5 Cu1.5 Ga(OH)8 ]+ layers
where the increase was from 0.753 nm for carbonate containing
LDH to 0.782 nm for [Cu(OH)4 ]2− containing LDH. Secondly, theintroduction of Cu resulted in the transformation to semiconductors as demonstrated by the decrease of the Eg values to 3.0–4.2 eV(Table 1). The UV–visible absorption edge shifted towards the lower
energy side and dramatically increased the excitation energy region
for the photocatalysis.
It is interesting to compare the catalytic performance between
interlayer Cu hydroxy anions and Cu octahedral sites surroundedby six oxygen atoms in the LDH cationic layers. Although the doped
quantity of Cu was one third for the former sites, the methanol formation rates increased by a factor of 5.9 due to the replacement ofAuthor's personal copy
N. Ahmed et al. / Catalysis Today 185 (2012) 263–269

carbonates with [Cu(OH)4 ]2− anions, as opposed to the increase bya factor of 3.3 due to the replacement of Zn sites with Cu (Table 2).
The steric availability (accessibility) of [Cu(OH)4 ]2− may be related
to the reactivity difference (the Graphical abstract), but the exact
speciation of doped Cu hydroxy groups (as in Eqs. (1)–(5)) is
required for detailed discussion.
In this study, hydrogen was used to achieve CO2 photoreduction,
but we also demonstrated the feasibility of using LDH photocatalysts and water to achieve CO2 photoreduction. 2.6 kPa of CO2
was applied to 7 mg of Zn1.5 Cu1.5 Ga|CO3 and subsequently converted to methanol at a rate of 5.1 ␮mol h−1 gcat −1 and coupled withhydrogen species activated from water using 20 mg of Pt/C catalyst
separated by a proton conducting polymer ﬁlm. Currently, heating at 413 K is required to make the polymer proton-conductive.
Also, the totally photocatalytic photoreduction of CO2 using water
is being researched.
5. Conclusions
The photoreduction of CO2 into methanol with hydrogen using[Zn1.5 Cu1.5 Ga(OH)8 ]+ 2 (CO3 )2− ·mH2 O was improved by a factor of
1.8 by preheating at 423 K in a vacuum. The available interlayer
space was created at 423 K and is expected to facilitate the diffusion of CO2 to the reaction space and the reaction with the surfacehydroxy groups that are bound to Cu sites.
By substituting the (CO3 )2− for [Cu(OH)4 ]2− , the methanol
formation rates using [Zn3 Ga(OH)8 ]+ 2 (CO3 )2− ·mH2 O and[Zn1.5 Cu1.5 Ga(OH)8 ]+ 2 (CO3 )2− ·mH2 O increased by a factor of
5.9 and 2.9, respectively. The hydroxy groups that were boundto Cu sites were important, and the effects of the interlayer[Cu(OH)4 ]2− were greater than those of the in-layer octahedral Cu sites because of its steric availability (accessibility) andsemiconductivity (Eg values of 3.0–4.2 eV).
Acknowledgements
The authors are thankful for ﬁnancial supports received fromthe Grant-in-Aid for Scientiﬁc Research C (2255 0117) from the
Ministry of Education, Culture, Sports, Science and Technology(MEXT) (2010–2011) and received from the Asahi Glass Foundation (2009–2010). The X-ray absorption experiments were269

performed with the approval of the Photon Factory Proposal Review
Committee (No. 2009G552). The authors thank Prof. Yoshitake forproviding the Xe arc lamp and Prof. Kaneko and Prof. Kanoh for the
XRD apparatus.
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