DOI: 10.1002/ente.201600578

Selective Photoconversion of Carbon Dioxide into
Methanol Using Layered Double Hydroxides at 0.40 MPa
Masaya Miyano, Hongwei Zhang, Mao Yoshiba, and Yasuo Izumi*[a]CO2 photoconversion is a promising method to reduce atmospheric CO2 concentrations and mitigate energy problems
simultaneously. Among the various efficient and stable semiconductor photocatalysts used for this purpose, layered
double hydroxides (LDHs) have attracted attention as catalysts for CO2 photoconversion into CO and/or methanol.
In this study, various LDHs of the formula[MII3GaIII(OH)8]2A·m H2O (MII = ZnII, CuII ; A2@ = CO32@,[Cu(OH)4]2@) were synthesized and used for CO2 photoconversion at a reaction pressure of 0.40 MPa in the presence of
H2 to result in the exclusive production of methanol. Furthermore, the pretreatment of carbonate-type LDHs at 423 K

boosted the reaction rates by a factor of 7.5–20. Interestingly,[Zn3Ga(OH)8]2CO3·m H2O was the only LDH that produced
methane primarily by an eight-electron reduction (rather
than the production of methanol by a six-electron reduction)
at a total formation rate of 2.7 mmol h@1 gcat@1 after it was preheated at 423 K and protected by an Ar atmosphere. Conversely, the methanol photogeneration rates of tetrahydroxycuprate-type LDHs were suppressed to less than
0.1 mmol h@1 gcat@1 at 0.40 MPa. In summary, the contributionof the interlayer reaction space created by the partial removal of water molecules and/or carbonate ions of LDHs was
suggested.

Introduction
The photocatalytic conversion of CO2 into fuels is a method
to reduce atmospheric CO2 concentrations and mitigate
energy problems simultaneously.[1, 2] A further advantage ofthis process is the generation of methanol as a product for
further transportation and storage. The major products reported for CO2 photoconversion are methane, CO, the formate ion, and methanol. The selective production of methanol has few precedents, particularly for semiconductor-based
photocatalysts.[1, 2]
Layered double hydroxides (LDHs) of the formula[MII3MIII(OH)8]2A·m H2O (A2@ = CO32@, [Cu(OH)4]2@) produce methanol and CO photocatalytically from CO2 andH2,[3] and the combination of CuII and GaIII ions favors methanol formation.[3, 4] The selective formation of methanol waspreserved for a Zn–Cu LDH exposed to CO2 and UV/Vis
light (cathode), and a proton-conducting film was used toseparate it from a WO3 moisture photo-oxidation catalyst(anode).[5] The doping of LDHs by Ag or Au nanoparticles
and phthalocyanines can boost the visible-light-only photoconversion of CO2 into methanol and CO effectively.[6, 7]
In contrast, Ni-Al LDH; LDHs with MII = Ni, Mg, Zn andIII
M = Al, Ga, In; and Zn–Al LDH produced mainly CO,[8–12]
whereas Zn–Cu–Al and Ni–Cu–Al LDHs produced methane[13] and formic acid, formaldehyde, and methanol,[14] respectively, from CO2 in liquid water[8–11, 14] or moisture.[12, 13]
The diffusion of photogenerated electrons to CO2-derived
species in LDHs was monitored by using X-ray absorption
fine structure analysis (XAFS) and FTIR spectroscopy;[15]however, the origin of selective methanol production overthe Zn–Ga LDH from CO2 and H2 is still unclear.
Energy Technol. 2017, 5, 892 – 900

In this study, the photocatalytic conversion of CO2 overZn–Ga and Zn–Cu–Ga LDHs was investigated at 0.40 MPa
of CO2 and H2 (Figure 1). Following the water photo-oxidation step [Eq. (1)], H2 acted as a reductant for CO2 photoconversion [Eq. (2)].[3–7, 15] The metal ion profile of the cationic layers and the type of interlayer anions were varied,
and the mechanism of selective methanol formation is discussed below.
2 H2 O ! O2 þ4 Hþ þ4 e@

ð1Þ

CO2 þ6 Hþ þ6 e@ ! CH3 OHþH2 O

ð2ÞResults
Characterization of photocatalysts
The XRD patterns of Zn–Ga–CO3 and Zn–Cu–Ga–CO3
LDHs are depicted in Figures 2 a and b. The diffraction
peaks at 2 q = 11.7, 23.5, 34.2, 37.0, 38.8–38.9, 43.3–43.6, 46.4,[a] M. Miyano, H. Zhang, M. Yoshiba, Dr. Y. Izumi
Department of Chemistry
Graduate School of Science
Chiba University
Yayoi 1–33, Inage-ku, Chiba 263–8522 (Japan)
E-mail: yizumi@faculty.chiba-u.jp
The ORCID identification number(s) for the author(s) of this article canbe found under http://dx.doi.org/10.1002/ente.201600578.
This publication is part of a Special Issue on “CO2 Utilization”. To viewthe complete issue, visit: http://dx.doi.org/10.1002/ente.v5.6

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Figure 1. A) Zn–Ga–CO3 LDH film on a Pyrex glass plate; B) top and C) sideviews of a high-pressure stainless-steel reactor with quartz windows anda pressure gauge used for CO2 photoconversion tests under UV/Vis irradiation (500 W Xe arc lamp); D) diethyl ether/dry ice trap (192 K) used to concentrate the produced methanol and water.

Figure 2. XRD patterns of fresh samples: a) Zn–Ga–CO3, b) Zn–Cu–Ga–CO3,c) Zn–Ga–Cu(OH)4, and d) Zn–Cu–Ga–Cu(OH)4.

as 68 and 75 nm, respectively, based on their (003) peaks[Eq. (3), Experimental Section]. The XRD pattern of Zn–
Ga–CO3 LDH showed no peaks other than those of theLDH crystal structure (Figure 2 a); however, peaks of impurities at 2 q = 13.0, 18.4, 30.2, 35.6, and 57.38, ascribed toZn(OH)2, Ga2O3, Zn(OH)2, CuO, and Cu(OH)2 phases, respectively, were observed for Zn–Cu–Ga–CO3 LDH (Figure 2 b).
The XRD patterns measured for as-synthesized Zn–Ga–
Cu(OH)4 and Zn–Cu–Ga–Cu(OH)4 are shown in Figure 2 c, d. A shift to lower angles (by 0.3–1.68) was detected
for several peaks in the patterns of Zn–Ga–CO3 and Zn–Cu–
Ga–CO3 to result in peaks at 2 q = 11.2, 22.6, 33.9, 36.5, 38.3,
42.8, 45.4, 51.3, 54.4, and 59.18 for Zn–Ga–Cu(OH)4 and Zn–
Cu–Ga–Cu(OH)4. These peaks were assigned to the (003),(006), (009), (104), (015), (107), (018), (1010), (0111), and(110) lattice plane diffractions, respectively.[4] The interlayerinterval for Zn–Ga–Cu(OH)4 and Zn–Cu–Ga–Cu(OH)4 wasdetermined to be 0.785 nm from their (003) peaks (Table 1).
The mean thickness of the Zn–Ga–Cu(OH)4 and Zn–Cu–
Ga–Cu(OH)4 LDH crystallites was 60 and 59 nm, respectively [determined from Eq. (3) and the width of the (003)peak]. In addition to the characteristic LDH diffraction patterns, impurity peaks appeared at 2 q = 16.3, 32.5, and 39.8–
39.98, which originate from CuO and Cu(OH)2 impurity
phases in both Zn–Ga–Cu(OH)4 and Zn–Cu–Ga–Cu(OH)4LDHs (Figures 2 c and d).
The extrapolation of the absorption edges in the UV/Vis
absorption spectra (Figures 3 A a–d) yielded x intercepts ofl = 221, 370, 295, and 428 nm for Zn–Ga–CO3, Zn–Cu–Ga–
CO3, Zn–Ga–Cu(OH)4, and Zn–Cu–Ga–Cu(OH)4 LDHs,
which correspond to band gap (Eg) values of 5.5, 3.4, 4.2, and2.9 eV, respectively (Table 1). The Eg values of the above
samples, calculated from Equation (4) (Experimental Section) by applying different n values, were 5.8–5.4, 3.9–2.7,
5.0–4.0, and 3.6–2.7 eV, respectively (Table 1). In the case ofn = 1/2 and 3/2, the obtained values were similar to those obtained by simple absorption edge extrapolation (Table 1).
Alternatively, log–log plots of Equation 4 were constructedusing Eg values obtained by simple extrapolation (Table 1) togive n values between 0.85 and 1.02. As these values were
between 1/2 (allowed direct transition) and 3/2 (forbidden

52.6, 56.0, and 59.48 were assigned to the (003), (006), (009),(104), (015), (107), (018), (1010), (0111), and (110) latticeplane diffractions of the regular layered structure,
respectively.[3] The respective interlayer intervals
Table 1. Basic physicochemical properties of the LDH samples used in this study.
were estimated as 0.756 and 0.757 nm based on the
SampleColor
Eg [eV]n[b] Interlayer Crystallite(003) peak (Table 1). These intervals were consis[a]extrap.
based
on
Eq.
(4)
distance thickness
tent with the respective values of 0.378 X 2 andn = 1/2 n = 3/2[nm][c][nm][c]0.378 X 2 nm obtained from the diffraction angle ofZn–Ga–CO3
white
5.5
5.8
5.4
0.85 0.756
68
the (006) peak, whereas that of the (009) peak
Zn–Cu–Ga–CO3
gray
3.4
3.9
2.7
0.97 0.757
75yielded values of 0.262 X 3 and 0.262 X 3 nm, respecZn–Ga–Cu(OH)4
pale blue 4.2
5.0
4.0
1.02 0.785
60
tively. The observed small differences may be beZn–Cu–Ga–Cu(OH)4 light blue 2.9
3.6
2.7
0.95 0.785
59
Zn(OH)2
white
3.2
3.2
3.1
1.05
cause of the overlap of the (009) diffraction peak
Cu(OH)2
light blue 2.9
3.0
2.8
0.99
with that ascribed to the (012) lattice plane and/or
CuO
black
because of a minor incompleteness of the LDH
Ga2O3
white
4.6
4.7
4.5
1.15
crystallites (Figure 2).
[a] Extrap.: extrapolated value. [b] Obtained from log–log form of Equation (4).
The mean thicknesses of the Zn–Ga–CO3 and[c] Based on (0 0 3) peak.
Zn–Cu–Ga–CO3 LDH crystallites were determined
Energy Technol. 2017, 5, 892 – 900

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Figure 3. Diffuse reflectance UV/Vis absorption spectra of A) a) fresh Zn–Ga–
CO3, b) Zn–Cu–Ga–CO3, c) Zn–Ga–Cu(OH)4, and d) Zn–Cu–Ga–Cu(OH)4and B) e) Zn(OH)2, f) Cu(OH)2, g) CuO, and h) Ga2O3 reference samples.

direct transition),[3, 16] Zn–Ga–CO3 could exhibit a direct electronic transition from O 2p to Zn/Ga 4s or 4p levels, and Zn–
Cu–Ga–CO3, Zn–Ga–Cu(OH)4, and Zn–Cu–Ga–Cu(OH)4
could exhibit direct transitions from O 2p to Zn/Ga 4s or 4p
levels or Cu 3d, 4s, or 4p levels.
If the interlayer CO32@ ions of Zn–Ga–CO3 LDH were replaced with [Cu(OH)4]2@ (which results in Cu/Zn = 1:6 mol/mol), the UV absorption edge shifted to longer wavelengths
by 74 nm (Figure 3 A a, c). Accordingly, Eg decreased from
5.5 to 4.2 eV, mostly because of an electronic transition to
the Cu 3d level. Additionally, a d–d transition was observed
as a weak feature between l = 560 and 800 nm (Figure 3 A c).
If half of the Zn2++ sites in the cationic layers of Zn–Ga–
CO3 LDH were replaced with Cu2++ sites (Cu/Zn = 1:1 mol/mol), the UV absorption edge was further shifted to longer
wavelengths by 149 nm (Figures 3 A a and b), and Eg decreased from 5.5 to 3.4 eV, mostly because of electronic transitions to the Cu 3d level. Thus, the amount of Cu was a primary factor to determine the extent of the Eg shift (Figures 2 a–c). In other words, the density of states for Cu 3d in Zn–
Cu–Ga–CO3 was more diffuse than that in Zn–Ga–Cu(OH)4.
Finally, if we replaced both the interlayer and cationiclayer sites in Zn–Ga–CO3 LDH with Cu to produce Zn–Cu–
Ga–Cu(OH)4 (Cu/Zn = 4:3 mol/mol) the UV absorption edge
shifted to longer wavelengths by 207 nm, and Eg decreased
from 5.5 to 2.9 eV, mostly because of electronic transitions to
the Cu 3d level at inter- and in-layer sites. The observed decrease (2.6 eV) was greater than that obtained by replacing
interlayer sites with [Cu(OH)4]2@ (1.3 eV) or by replacing
half of the Zn2++ sites in cationic layers (2.1 eV). In accord
with the increasing Cu content, the intensity of the d–d tranEnergy Technol. 2017, 5, 892 – 900

sition between l = 560 and 800 nm increased in comparison
to that observed for Zn–Ga–Cu(OH)4 (Figures 3 A c and d).
Electronic transitions between the O 2p and metal energy
levels were observed for reference Zn(OH)2 and Ga2O3 samples in the UV region with no absorption detected in the visible region (Figures 3 B e and h). Consistently, these samples
were white (Table 1). In contrast, the reference CuO sample
was black and absorbed in the whole UV/Vis region (Figure 3 B g). Thus, the gray color (Table 1) and broad absorption peak between l = 330 and 800 nm (Figure 3 A b) observed for Zn–Cu–Ga–CO3 were because of the presence ofa CuO impurity, as evidenced by using XRD (Figure 2 b).
The Cu(OH)2 reference material was light blue (Table 1)
and its d–d transitions and the absorption edge caused by
O 2p to Cu 3d transitions lay in the visible and UV light regions, respectively. Cu(OH)2 exhibits a flat, sheetlike[17] structure that is somewhat similar to that of LDHs. The absorption edge of Cu(OH)2 was extrapolated to l = 433 nm (Figure 3 B f), which is shifted significantly to longer wavelengths
in comparison to those of Zn–Cu–Ga–CO3 and Zn–Ga–
Cu(OH)4 (Figures 3 A b and c and Table 1). Therefore, theweak shoulders at l = 310–330 and 300–400 nm observed for
Zn–Cu–Ga–CO3 and Zn–Ga–Cu(OH)4 (Figures 3 A b and c),respectively, can be ascribed to a Cu(OH)2 impurity. However, the concentrations of this impurity are very low, as estimated from the XRD peak intensity of Cu(OH)2 (Figures 2 b–d). Accordingly, the contribution of this impurity to
the d–d transition is marginal (Figures 3 A c and d).
Based on Equation (4), Eg values of 2.9–4.6 eV were determined for Zn(OH)2, Cu(OH)2, and Ga2O3, for which nvalues between 1/2 and 3/2 demonstrated direct O-to-metalelectronic transitions (Table 1).
The replacement of CO32@ with [Cu(OH)4]2@ was confirmed by the dramatic intensity decrease observed by using
FTIR spectroscopy for symmetric and antisymmetric stretches of CO32@.[15] The Cu/Zn molar ratios of Zn–Cu–Ga–CO3,
Zn–Ga–Cu(OH)4, and Zn–Cu–Ga–Cu(OH)4 (1:1, 1:6, and4:3, respectively) were confirmed by using X-ray absorption
spectroscopy. Interestingly, the octahedral coordination of
Cu2++ and O atoms was common in these LDHs, as observedby using X-ray absorption near-edge and extended X-ray absorption fine structure spectroscopy. Cu exhibited Cu(OH)6
motifs in the cationic layers of Zn–Cu–Ga–CO3 and Zn–Cu–
Ga–Cu(OH)4, whereas (m-O)nCu(OH)4@n(H2O)2 (n = 1, 2, or
3; m-O from the cationic layer; H2O from interlayer water)
units were present in the interlayer sites of Zn–Ga–Cu(OH)4and Zn–Cu–Ga–Cu(OH)4.[4, 15]
The SEM and TEM images of Zn–Ga–CO3 (Figure 4 A 1
and 6) show flakelike thin layers. The thickness distributionof a single layer (e.g., Figure 4 A 5) centered at 30–40 nm(Figure 4 A, histogram) and the mean value of (42 : 24) nm
were not consistent with the thickness of 68 nm obtained byusing XRD (Table 1) as electron-transparent layers werechosen for TEM measurements, which are thinner than thosecharacterized by using XRD. The observed hexagonal crystallites and outlines (perimeters) with thinner contrast are
depicted in Figure 4 A 2–4 and the outline (perimeter) width

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Photocatalytic tests using LDHs at 0.40 MPa

Figure 4. 1–5) TEM and 6) SEM images of fresh A) Zn–Ga–CO3 and B) Zn–
Ga–Cu(OH)4. Histograms-1 show the thickness and Histograms-2 the outline(perimeter) length of the flakelike layers.

was centered at 4–6 nm (Figure 4 A, histogram). The side
face of a single thin layer (thickness & 42 nm) was observed
at a nearly parallel angle.
TEM images of Zn–Ga–Cu(OH)4 (Figure 4 B) displayed
flakelike thin layers (Figure 4 B 1). The thickness distributionof a single layer (e.g., Figure 4 B 5) centered at 30–40 nm(Figure 4 B, histogram) with a mean of (43 : 22) nm wasfairly consistent with the value of 60 nm determined by using
XRD (Table 1). In analogy to the images of Zn–Ga–CO3,
outlines (perimeters) of crystallites with a thinner contrast
were observed, and the width distribution was centered at 2–
4 nm (Figure 4 B 2–4, histogram).

Energy Technol. 2017, 5, 892 – 900

CO2 photoconversion tests were performed at a total pressure of 0.40 MPa using four LDH catalysts and H2. The photocatalyst film formed on a Pyrex plate was dried at 373 K
for 12 h and transferred to the high-pressure reactor (Figure 1 B and C) under ambient conditions (Table 2 A). Methanol was formed as the only the C-containing product, andthe observed activity order was: Zn–Cu–Ga–Cu(OH)4 > Zn–
Ga–CO3 > Zn–Ga–Cu(OH)4 > Zn–Cu–Ga–CO3.
The selectivity trend at 0.40 MPa contrasted with the results of LDH low-pressure photoconversion tests performed
under 2.3 kPa of CO2+22 kPa of H2, in which both CO andmethanol were obtained as major products (Table 2 E 1–4).[3, 4]
Water was also formed (Table 2 A), however, its formation
rate was a factor of 780–5500 higher than that of methanol,
which demonstrates that it originated exclusively from desorbed interlayer water molecules and was not produced fromthe reactants. The rates of water formation among LDH photocatalysts varied by a factor of 5.6 (Table 2 A), which suggests that the LDH film preheated at 373 K (Figure 1 A) absorbed atmospheric moisture into the interlayer space before
it was introduced into the reactor (Figure 1 C).
Next, LDHs were pretreated at an elevated temperature(423 K) for 12 h to desorb approximately one third of the interlayer water.[3, 4] The LDH film on the Pyrex plate was preheated and transferred to the high-pressure reactor underambient conditions (Table 2 B). Again, methanol was observed as the only C-containing product; however, its formation rates were boosted by a factor of 7.5–20 compared to
the values obtained for catalysts preheated at 373 K(Table 2 A). Apparently, the photoconversion of CO2 was accelerated by removing some of the interlayer water molecules to liberate reaction space (Scheme 1 B).
Water desorption rates (Table 2 B) decreased to 56–57 %of the values obtained for catalysts preheated at 373 K(Table 2 A). Despite this decrease, the partial adsorption of
atmospheric moisture during sample transfer from the drying
oven held at 423 K to the high-pressure reactor could not be
excluded.
To address this issue, a preheated LDH film on a Pyrex
plate was transferred to the high-pressure reactor in an atmosphere of Ar to avoid contact with air and thus ensure thepartial removal of interlayer water molecules (Table 2 C).
Except for the Zn–Ga–CO3 photocatalyst, which favoredmethane production over that of methanol, all LDHs produced methanol exclusively. The total formation rate of Ccontaining compounds (SC) using Zn–Ga–CO3 LDH was
2.7 mmol h@1 gcat@1 (Table 2 C 1). The more effectively liberated
interlayer reaction space of the Zn–Ga–CO3 catalyst transferred under Ar favored a further two-electron reduction(Scheme 1 C). The catalytic activity based on total C-containing products was in the order: Zn–Ga–CO3 @ Zn–Cu–Ga–
CO3 @ Zn–Cu–Ga–Cu(OH)4 & Zn–Ga–Cu(OH)4.
The total formation rate of C-containing products for Zn–
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Table 2. Results of CO2 photoconversion and blank tests at elevated pressure using hydrogen and LDH photocatalysts (10 mg) and rates reported previously.

Entry
A
1
2
3
4
B
1
2
C
1
2
3
4
C’ [b]
1
D
5
6
7
8
E
1
2
3
4
1’
9
10
11

Formation rate [mmol h@1 gcat@1]
CH3OH
CH4
H2O
SC[a]

Photocatalyst

Reactant [MPa]CO2
H2Zn–Ga–CO3, 373 K, exposed
Zn–Cu–Ga–CO3, 373 K, exposed
Zn–Ga–Cu(OH)4, 373 K, exposed
Zn–Cu–Ga–Cu(OH)4, 373 K, exposed

0.12

0.28

< 0.08< 0.08< 0.08< 0.08

0.040
0.011
0.023
0.087

< 0.12< 0.12< 0.12< 0.12

66
61
18
100

0.040
0.011
0.023
0.087

Zn–Ga–CO3, 423 K, exposed
Zn–Cu–Ga–CO3, 423 K, exposed

0.12

0.28

< 0.08< 0.080.30
0.22< 0.12< 0.12

37
35

0.30
0.22

Zn–Ga–CO3, 423 K, not exposed
Zn–Cu–Ga–CO3, 423 K, not exposed
Zn–Ga–Cu(OH)4, 423 K, not exposed
Zn–Cu–Ga–Cu(OH)4, 423 K, not exposed

0.12

0.28

< 0.08< 0.08< 0.08< 0.08

0.049
0.19
0.028
0.034

2.6
< 0.12< 0.12< 0.12

13
59
24
41

2.7
0.19
0.028
0.034

Zn–Ga–CO3, 423 K, not exposed

0.12

0.28

< 0.080.020< 0.129.1

0.020

Zn(OH)2, 423 K, not exposed
Cu(OH)2, 423 K, not exposed
CuO, 423 K, not exposed
Ga2O3, 423 K, not exposed

0.12

0.28

< 0.087.3< 0.08< 0.08

0.048
0.45
0.14
0.059

< 0.12< 0.12< 0.12< 0.12

10
34
26
19

0.048
7.8
0.14
0.059

Zn–Ga–CO3, 290 K, evacuatedZn–Cu–Ga–CO3, 290 K, evacuatedZn–Ga–Cu(OH)4, 290 K, evacuatedZn–Cu–Ga–Cu(OH)4, 290 K, evacuatedAg/Zn–Ga–CO3 373 K, exposed
TiO2
g-C3N4BiOCl

0.002 3

0.022

0.080
0.079
0.13
0.070
0.95
< 0.08< 0.08
1.9

0.051
0.17
0.30
0.49
0.10
0.027
< 0.004< 0.004

5.3
56
4.1
37

0.13
0.25
0.43
0.56
3.2
4.0
3.8
3.0

0.12

0.28

CO

2.1
4.0
3.8
1.1

Ref.
this work

[3]
[4]
[20]

[a] Total formation rates of C-containing products. [b] In the absence of UV/Vis light at 303 K.

that obtained for the same catalyst transferred in air bya factor of 9.0 (Table 2 B), which indicates that sample transfer under Ar preserved the interlayer reaction space of thisLDH liberated by heating at 423 K.
However, the rate of exclusive methanol formation for
Zn–Cu–Ga–CO3 transferred in air was similar to that of thephotocatalyst transferred in Ar (Table 2 B, C). The exclusive
methanol formation rates for Zn–Ga–Cu(OH)4 and Zn–Cu–
Ga–Cu(OH)4 preheated at 373 K were almost unchanged if
these photocatalysts were preheated at 423 K and transferredunder Ar (Table 2 A, C). The reason behind these unimproved CO2 photoconversion rates is not known; however,two plausible reasons can be used to explain this observation: (1) the amount of water in the tests listed in Table 2 A
and B was not controlled well and was similar to that in thetests summarized in Table 2 C and/or (2) the interlayer spaceof these LDHs was not utilized effectively for catalysis as the
major active sites are located only on the outer surface(Scheme 1 B).
The results of a blank test using Zn–Ga–CO3 in the dark
at 303 K are shown in Table 2 C’. No CO and methane weredetected; however, methanol was formed at a rate of
0.020 mmol h@1 gcat@1. Thus, some methanol is formed thermally rather than photocatalytically if the catalyst activity is relatively low (Table 2 A 1–3, C 3–4). In contrast, the thermal
Energy Technol. 2017, 5, 892 – 900route is negligible for the entries in Table 2 A 4, B 1–2, andC 1–2, mainly because of the effect of the liberated LDH interlayer space.
The effects of Zn(OH)2, Cu(OH)2, CuO, and Ga2O3 impurities in Zn–Cu–Ga–CO3 (Figure 2 b) and Cu(OH)2 and CuO
in Zn–Ga–Cu(OH)4 and Zn–Cu–Ga–Cu(OH)4 (Figures 2 cand d) were evaluated. In photocatalytic tests under
0.40 MPa of CO2+H2, the total formation rates of C-containing compounds for Zn(OH)2 and Ga2O3 were low (0.048–
0.059 mmol h@1 gcat@1, Table 2 D 5 and 8), which indicates thatthe contribution of these impurities on the performance ofZn–Cu–Ga–CO3 (Table 2 C 2) was negligible.
In contrast, CuO photoconverted CO2 into methanol ata moderate rate (0.14 mmol h@1 gcat@1; Table 2 D 7), whichshows that a low concentration of this impurity may contribute to the lower rates observed for Zn–Ga–Cu(OH)4 andZn–Cu–Ga–Cu(OH)4 (0.028–0.034 mmol h@1 gcat@1; Table 2 C 3
and 4). As the photocatalytic performances of Zn–Cu–Ga–
CO3 and CuO were similar (Table 2 C 2 and D 7), the contribution of the CuO impurity to the rates observed for Zn–
Cu–Ga–CO3 is marginal because of its low concentration.
Surprisingly, the photoconversion of CO2 over Cu(OH)2
produced CO and methanol as the major and minor products, respectively, at a total formation rate of
7.8 mmol h@1 gcat@1 (Table 2 D 6). As no CO formation was ob-

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Scheme 1. Reaction space and products of CO2 photoconversion at A) low
pressure for LDHs pretreated at 373 K, B) high pressure for LDHs pretreatedat 373 K, and C) high pressure for LDHs pretreated at 423 K.

served for the LDHs studied, the effect of the Cu(OH)2 impurity can be excluded. Only the reference catalyst Cu(OH)2
was active because of the partial decomposition of bulk
Cu(OH)2 to CuO during pretreatment at 423 K, and the resulting mixture of Cu(OH)2 and CuO phases is advantageous
for CO2 photoconversion.
Discussion
Carbonate-type Zn–Ga and Zn–Cu–Ga LDHs and their tetrahydroxycuprate-type counterparts were synthesized andtheir high purity was confirmed by using XRD and UV/Vis
spectroscopy (Figures 2 and 3). Traces of impurities such as
Zn(OH)2, Ga2O3, CuO, and Cu(OH)2 were detected; however, their amounts were lower than those in materials prepared previously[3, 4] because of the slow crystallization duringdrying on a wide Pyrex plate for five days. Notably, no impurities were detected in the most active Zn–Ga–CO3 catalyst in this study, and these impurities were shown to be inert
at a reactant pressure of less than 0.10 MPa.[3, 4]
In this study, Zn(OH)2 and Ga2O3 impurities were inactiveat a reactant pressure of 0.40 MPa (Table 2 D 5, 8) and did
not contribute to the rates observed for Zn–Cu–Ga–CO3(Figure 2 b and Table 2 C 2). CuO exhibited a moderate activEnergy Technol. 2017, 5, 892 – 900

ity toward methanol production (Table 2 D 7), which possibly
affected the results of poorly performing Zn–Ga–Cu(OH)4and Zn–Cu–Ga–Cu(OH)4 but had a negligible influence on
Zn–Cu–Ga–CO3 if we take the concentration of this impurity
into account (Figures 2 b–d). Bulk Cu(OH)2 is probably activated by heating at 423 K to produce an interface between
Cu(OH)2 and CuO for the selective formation of CO(Table 2 D 6). Such effective CO2 photoconversion sites were
not formed in the LDHs studied herein, and no CO was produced (Table 2 A–C).
The salient point of CO2 photoconversion at 0.40 MPa(0.12 MPa of CO2 and 0.28 MPa of H2) is the exclusive selectivity toward methanol, whereas both CO and methanol
were produced over these LDHs at 24 kPa (2.3 kPa of CO2
and 22 kPa of H2 ; Scheme 1 A). Thus, above 0.40 MPa, thehigher concentrations of reactants and/or intermediates in
the reaction space of the LDHs facilitate the sequential sixelectron reduction of CO2 to methanol (Scheme 1 B) rather
than its two-electron reduction to CO.
For CO2 photoconversion at 0.40 MPa, the catalyst activity
depended critically on the pretreatment conditions of carbonate-type LDHs (Table 2 A 1, 2 A 2, 2 B 1, 2 B 2, 2 C 1, and2 C 2), and the methanol formation rates increased bya factor of 7.5–20 as the pretreatment temperature increased
from 373 to 423 K (Table 2 A and B). In the special case ofZn–Ga–CO3, the sample heated at 423 K and transferred tothe high-pressure reactor under Ar (Table 2 C) produced
methane (eight-electron reduction) rather than methanol(six-electron reduction). The total formation rate of C-containing products reached 2.7 mmol h@1 gcat@1, which corresponds to a 9.0-fold increase compared to that observed forthe same catalyst transferred in air (Table 2 B 1 and C 1). Thischange can be understood by assuming a subsequent twoelectron reduction path favored by the more efficient andopen interlayer reaction space in the tests listed in Table 2 C(Scheme 1 C). Exclusive methane formation from CO2 andmoisture has been reported for Zn-Ce and Zn-Ti LDHs irradiated by UV light (l = 185 nm).[18]In clear contrast, the rates of methanol formation for tetrahydroxycuprate-type LDHs were suppressed to less than
0.1 mmol h@1 gcat@1 at 0.40 MPa (Table 2 A 3, 2 A 4, 2 C 3, and2 C 4). The interlayer reaction space[19] created by the partial
removal of water molecules and/or carbonate ions of LDHs
is believed to be important for CO2 photoconversion.
For Zn–Ga–CO3, the rate of water formation decreased
progressively with the improvement of the pretreatment conditions (Table 2 A!B!C) and the CO2 photoconversion
rate increased concomitantly. In the case of Zn–Cu–Ga–CO3,
the water formation rate did not decrease simply because of
the improved pretreatment conditions; however, the extent
of this decrease correlated well with the CO2 photoconversion rate (Table 2 A 2, B 2, and C 2). In comparison to Zn–
Ga–CO3, Zn–Cu–Ga–CO3 was destabilized by the inclusionof 3d9 Cu2++ ions in the cationic layer because of the Jahn–
Teller effect,[3] and the desorption of interlayer water before/
during photocatalytic tests for 5 h was relatively difficult toreproduce quantitatively.

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The formation rates of C-containing products listed for
Zn–Ga–CO3 and Zn–Cu–Ga–CO3 in Table 2 B–C (0.40 MPa)
are comparable to those obtained at 24 kPa (Table 2 E 1–4),
except for the higher value of 2.7 mmol h@1 gcat@1 (Table 2 C 1).
The rates observed for Zn–Ga–Cu(OH)4 and Zn–Cu–Ga–
Cu(OH)4 (Table 2 A and C, 0.40 MPa) were one order ofmagnitude lower than those obtained at 24 kPa (Table 2 E 1–4). Thus, the reaction order was zero or even negative formost of the LDHs used in this study except for Zn–Ga–CO3
in contrast to the reported TiO2, g-C3N4, and BiOCl photocatalysts until 0.40 MPa,[20] presumably because of the strong
adsorption of reactants/intermediates in the interlayer reaction space of LDHs.
The photoconversion of CO2 at 0.40 MPa over Ag-doped
Zn–Ga–CO3 was reported to produce methane, CO, andminor amounts of methanol at a total formation rate of
3.2 mmol h@1 gcat@1 (Table 2 E 1’)[20] and a similar value was observed for Ag-free Zn–Ga–CO3 (2.7 mmol h@1 gcat@1;
Table 2 C 1) by pretreatment at 423 K and sample transfer
under Ar. The performance of Zn–Ga–CO3 at 0.40 MPa(Table 2 C 1) was equivalent to that of BiOCl (Table 2 E 11)
and close to that of TiO2 and g-C3N4 (Table 2 E 9 and 10).
Thus, the doping of LDHs with effective electron-trapping
and/or surface plasmon resonance sites and the optimizationof the pretreatment/transfer conditions (Table 2 C) is expected to further improve the catalytic performance of CO2 andH2 at 0.40 MPa.

Conclusions
The photoconversion of CO2 was studied for various layered
double hydroxides (LDHs). With H2 as a reductant, methanol production over the LDHs was achieved at less than
0.1 MPa and became exclusive at 0.40 MPa. The CO2 photoconversion rates observed for carbonate-type LDHs Zn–Ga–
CO3 and Zn–Cu–Ga–CO3 were improved by increasing thepretreatment temperature from 373 to 423 K (0.19–
2.7 mmol h@1 gcat@1), which suggests the liberation of the interlayer reaction space by the removal of approximately one
third of the interlayer water molecules. For Zn–Ga–CO3
heated at 423 K and transferred to the reactor under Ar,
methane was formed selectively rather than methanol, whichsuggests that the liberation of further interlayer sites enabled
the eight-electron reduction of CO2 to methane. In contrast,
tetrahydroxycuprate-type LDHs Zn–Ga–Cu(OH)4 and Zn–
Cu–Ga–Cu(OH)4 showed suppressed rates of 0.023–
0.087 mmol h@1 gcat@1, probably because only the outer surface[21] contributed to CO2 photoconversion for this type of
LDH.

Experimental Section
Synthesis of LDH photocatalysts
Two carbonate-type LDHs [Zn3Ga(OH)8]2CO3·m H2O and[Zn1.5Cu1.5Ga(OH)8]2CO3·m H2O were synthesized according toprocedures reported previously.[3] An aqueous solution (20 mL)
Energy Technol. 2017, 5, 892 – 900

of 0.75 m Zn(NO3)2·6 H2O (> 99.9 %, Wako Pure Chemical) and
0.25 m Ga(NO3)3·n H2O (n = 7–9, > 99.9 %, Wako Pure Chemical)
or 0.375 m Zn(NO3)2·6 H2O, 0.375 m Cu(NO3)2·3 H2O (> 99.9 %,Wako Pure Chemical), and 0.25 m Ga(NO3)3·n H2O was dropwise
added to a 0.075 m sodium carbonate (> 99.8 %, Wako Pure
Chemical) aqueous solution (100 mL) at a rate of 0.24 mL min@1
at 290 K under stirring at 900 rpm. The mixture was stirred at
290 K for 2 h, and the solution pH was adjusted to 8 by adding
1.0 m NaOH ( & 30 mL total; > 93 %, Wako Pure Chemical). Subsequently, the temperature was increased to 353 K, and the reaction solution was stirred for 22 h. The obtained precipitates were
filtered through a polytetrafluoroethene membrane filter (Omnipore JVWP04700) with a pore size of 0.1 mm and washed with
deionized water. In the drying step after the final washing step,a very thin intermediate product layer on a flat wide-area Pyrex
glass plate was produced in a glovebox over 5 days to improve
the purity of the crystalline LDHs. The produced LDHs were denoted as Zn–Ga–CO3 and Zn–Cu–Ga–CO3, respectively, inwhich m was generally equal to half of the total amount of metal
cations.
Two tetrahydroxycuprate-type LDHs [Zn3Ga(OH)8]2[Cu(OH)4]·m H2O and [Zn1.5Cu1.5Ga(OH)8]2[Cu(OH)4]·m H2O were synthesized according to procedures described previously.[4] An aqueous solution (20 mL) of 0.75 m Zn(NO3)2·6 H2O and 0.25 m Ga(NO3)3·n H2O
or
0.375 mZn(NO3)2·6 H2O,
0.375 mCu(NO3)2·3 H2O, and 0.25 m Ga(NO3)3·n H2O was dropwise
added to a 0.025 m aqueous solution (100 mL) of(NH4)2CuCl4·2 H2O (> 98 %, Wako Pure Chemical) at a rate of
0.32 mL min@1 at 290 K under stirring at 900 rpm. The mixture
was stirred at 290 K for 2 h, and the solution pH was adjusted to
8 by adding 1.0 m NaOH ( & 44 mL total). Subsequently, the temperature was increased to 353 K, and the reaction solution was
stirred for 22 h. The obtained precipitates were filtered througha JVWP04700 filter and washed with deionized water. In thedrying step after the final washing step, a very thin intermediate
product layer on a flat wide-area Pyrex glass plate was produced
in a glovebox over 5 days to improve the purity of crystalline
LDHs. The produced LDHs were denoted as Zn–Ga–Cu(OH)4and Zn–Cu–Ga–Cu(OH)4, respectively. Notably, the interlayer[Cu(OH)4]2@ reacted with the hydroxy groups of the cationiclayer in a dehydration reaction to result in Cu ion coordinationby oxo and hydroxy groups, similar to the coordination environment of Cu2++ in the cationic layer.[4, 15]
Zn(OH)2 was synthesized according to a procedure reported previously[22] from ZnO (1.0 g; > 99 %; Wako Pure Chemical) and
20 wt % aqueous NaOH (20 mL). Cu(OH)2 (> 90 %, Wako Pure
Chemical), CuO (> 99.9 %, Wako Pure Chemical), and Ga2O3 (>99.99 %, Wako Pure Chemical) were used as received. The lightblue color of the as-received Cu(OH)2 turned to dark brown at
423 K during the preparation of the photocatalyst film, which indicates its (partial) transformation into CuO.

Characterization
XRD patterns were recorded by using a diffractometer (model
D8 ADVANCE, Bruker) at the Center for Analytical Instrumentation, Chiba University, at 2 q = 5.0–608 with a scan step of 0.018
and a scan rate of 5 s step@1. Measurements were performed at
40 kV and 40 mA using CuKa emission (l = 0.15419 nm)[23] anda Ni filter. Crystallite sizes (t) were estimated using the Scherrer
equation [Eq. (3)]:T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

898

t¼

0:9l
ðFull width at half maximumÞ>cosqB

ð3Þ

UV/Vis spectra were recorded by using a model V-650 (JASCO)
spectrophotometer equipped with an integrating sphere (ISV469, JASCO) for diffuse reflectance measurements at 200–
800 nm. The Eg values were estimated by a simple extrapolationof the absorption edge or by fitting to the Davis–Mott equation[Eq. (4)]:ahn / ðhn @ Eg Þnð4Þ

in which a, h, and n are the absorption coefficient, PlanckQs constant, and the frequency of light, respectively, and n is 1/2, 3/2, 2,
or 3 for allowed direct, forbidden direct, allowed indirect, andforbidden indirect electronic transitions, respectively.[3, 16]
SEM imaging was performed by using a JSM-6510A instrument(JEOL) at the Center for Analytical Instrumentation, Chiba
University. A W filament was used in the electron gun, and theelectron accelerating voltage was 3.0 kV. Au particles were
coated by using a JFC-1100 instrument (JEOL). The employedmagnification was 10 000–20 000 X .
TEM imaging was performed by using a model H-7650 transmission electron microscope (Hitachi) at an accelerating voltage of
100 kV. The samples were mounted on a carbon-coated Cu gridmesh (150 mesh per inch) coated with a copolymer film of polyvinyl alcohol and formaldehyde (Formvar, Monsanto). A W filament was used in the electron gun. The employed magnification
was 6000–100 000 X .

Typically, the initial pressure of 0.400 MPa at 295 K increased
gradually to 0.407 MPa in approximately 10 min during the CO2
photoconversion tests. As the critical pressures of H2 and CO2
are relatively high (1.30 and 7.38 MPa, respectively), the reaction
gas in this study (0.28 MPa H2, 0.12 MPa CO2) can be approximated to be an ideal gas. Therefore, the temperature of the reaction gas reached 300 K. We confirmed that the external surfaceof the high-pressure reactor (Figure 1 C) reached the same temperature, which is in thermal equilibrium with the reaction gas.
The thin photocatalyst film (10 mg) on Pyrex glass and the reactor were in contact during photocatalytic tests. Even if we
assume that the catalyst film retained all the heat of methanol ormethane formation (15 and 130 nmol maximum), the maximum
temperature increase is calculated as 0.10 and 2.9 K, respectively,
based on Equations (5) and (6), with the catalyst heat capacity
approximated to that of Zn(OH)2 (72.4 J mol@1 K@1). Thus, wechose 303 K as a blank test temperature for LDH photocatalysts
under 0.40 MPa of CO2+H2 (Table 2 C’).

CO2 þ3 H2 ! CH3 OHþH2 O, DH r 2 ¼ @50 kJ mol@1

ð5Þ

CO2 þ4 H2 ! CH4 þ2 H2 O, DH r 2 ¼ @165 kJ mol@1

ð6Þ

Acknowledgements

CO2 photoconversion tests
LDH catalysts were immersed in deionized water(< 0.055 mS cm@1) produced by using an RFU424TA water purification system (Advantec) and agitated by ultrasound (430 W,
38 kHz) for 3 min. The suspension was cast on a Pyrex glass
plate (25 mm X 25 mm X 1 mm) and dried for 12 h at 373 or
423 K. The mass and area of the obtained films was 10 mg and
20 mm X 20 mm, and their thickness was & 12 mm (Figure 1 A).
These films were introduced into a homemade high-pressure
stainless-steel reactor with quartz double windows, a pressuregauge, and Swagelok valves (Figures 1 B, C) for photoconversion
tests. The effective internal volume of the reactor was 98.4 mL.
Subsequently, 0.40 MPa of a CO2/H2 mixture (3:7 v/v) was introduced into the reactor. The photocatalyst (Table 1) was irradiated by UV/Vis light emitted by a 500 W Xe arc lamp (model
OPM2-502, Ushio) through quartz windows in the reactor chamber for 5 h (Figure 1 C). The distance between the light exit point
and the photocatalyst surface was 82.7 mm. Light transmission
was checked by using a photosensor and a counter (model PCM01, Prede; and model KADEC-UP, North One, respectively).
The light intensity at the center of the photocatalyst was
90.2 mW cm@2. The quartz windows of the high-pressure reactor
absorbed/reflected/diffracted 9.5 % of incident light, whereas
10 mg of our standard photocatalyst film (Pd/TiO2) on the Pyrex
glass plate absorbed/reflected/diffracted 49 % of incident light.
After 5 h of irradiation, the gas mixture was analyzed by using
columns packed with 13X-S molecular sieves and polyethylene
glycol (PEG-6000) supported on Flusin P (GL Sciences) set ina GC equipped with a thermal conductivity detector (model GCEnergy Technol. 2017, 5, 892 – 900

8A, Shimadzu).[3–5, 15] He (> 99.999 95 %) was used as a carriergas. The amounts of produced methanol and moisture werechecked by concentrating them in a trap cooled by a mixture ofdiethyl ether and dry ice (192 K) to achieve separation from H2and most of the CO2 (Figure 1 D).

The authors acknowledge financial support from the Grantin-Aid for Scientific Research C (26410204, 22550117) provided by the Japan Society for the Promotion of Science and the
Leading Research Promotion Program (2015) of the Institutefor Global Prominent Research, Chiba University.

Keywords: copper · layered
photochemistry · reduction · zinc

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Manuscript received: September 15, 2016
Revised manuscript received: November 27, 2016
Accepted manuscript online: November 29, 2016
Version of record online: February 20, 2017

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