Applied Surface Science 447 (2018) 687–696

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Full Length Article

Optimized photoreduction of CO2 exclusively into methanol utilizing
liberated reaction space in layered double hydroxides comprising zinc,copper, and gallium
Lukas Anton Wein 1, Hongwei Zhang, Kazuki Urushidate, Masaya Miyano, Yasuo Izumi ⇑Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japana r t i c l e

i n f oArticle history:
Received 11 September 2017
Revised 26 February 2018
Accepted 5 April 2018
Available online 6 April 2018
Keywords:
Layered double hydroxide
CO2MethanolPhotoconversion
Interlayer
XANES

a b s t r a c t
Layered double hydroxides (LDHs), typically comprising Zn, Cu, and Ga, photoreduce CO2 into methanol
and CO, however, selective methanol synthesis using CO2 and heterogeneous photocatalyst is very rare. In
this study, the amount of interlayer water molecules is reduced to 31% of that for as-synthesized LDHs bypreheating the LDHs at 423 K in vacuum, and the performance for CO2 photoreduction using 0–0.28 MPa
of CO2 and 0–0.56 MPa of H2 was investigated. If the LDHs are preheated in vacuum and never in contact
with air prior to the photoreduction tests, methanol was produced exclusively in all experiments of thisstudy. LDHs comprising inlayer Cu sites were more active compared to [Zn3Ga(OH)8]2CO3mH2O, [Zn3Ga(OH)8]2Cu(OH)4mH2O, and [Zn3Ga(OH)8]2Pd(OH)4mH2O LDHs. A contour plot for methanol formation
rates was drawn for the most active [Zn1.5Cu1.5Ga(OH)8]2CO3mH2O and the volcano top positioned at
0.12 MPa of CO2 and 0.28 MPa of H2; 2.8 lmol-methanol h1 g1
cat and the selectivity was >97 mol%methanol. 13CH3OH formation in the presence of 13CO2 and [Zn1.5Cu1.5Ga(OH)8]2CO3mH2O confirmed
photocatalytic methanol synthesis. Under 0.12 MPa of CO2 and 0.28 MPa of H2, the intensity of the Cu
K preedge peak progressively decreased at the rate of 170 lmol-Cu h1 g1
cat upon the UV–visible lightirradiation for the [Zn1.5Cu1.5Ga(OH)8]2CO3mH2O LDH, demonstrating photogenerated electron
accumulation at the CuII/I sites for subsequent CO2 reduction.
Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction
Photocatalytic conversion of CO2 into fuels explores one of theroutes to carbon–neutral fuels [1,2], avoiding the net increase inatmospheric CO2 concentrations associated with fossil-derived
alternatives [2,3]. Layered double hydroxide (LDH), a kind of clay
families, comprising Zn and Cr was originally reported to generate
O2 from water at a rate of 1.1 mmol h1 g1
cat using a sacrificial oxidant (AgNO3) [4]. Then, LDHs comprising Zn, Cu, and Ga/Al werereported to form methanol and CO at 2.3 kPa of CO2 and 21.7 kPaof H2 as a new type of photocatalysts for the CO2 photoconversion[5]. The highest formation rate and selectivity to methanol were0.49 lmol h1 g1
cat and 88 mol%, respectively, using LDH with a formula [Zn1.5Cu1.5Ga(OH)8]2Cu(OH)4mH2O irradiated by Ultraviolet(UV)–visible light [6]. The Refs. [5] and [6] assume that H2 isobtained via photosplitting of water for the following reaction with
CO2, and the overall reaction is the combination of CO2 with water⇑ Corresponding author.
E-mail address: yizumi@faculty.chiba-u.jp (Y. Izumi).
On leave from Department of Chemistry, Ludwig-Maximilians-Universität
München, 81377 Munich, Germany.
1

https://doi.org/10.1016/j.apsusc.2018.04.046
0169-4332/Ó 2018 Elsevier B.V. All rights reserved.

to form methanol and oxygen (DGr = 689 kJ mol1) [5]. LDHs havebeen also reported to photoproduce methane [7,8], CO [9–16], andmethanol [15–18] starting from CO2 and water/hydrogen. Ultrathin
LDHs are defect-rich and expected to be better photocatalysts [12].
In a practical point of view, it is highly expected to boost themethanol formation rates from CO2 and H2 by the optimizationof LDH composition, pretreating conditions, and reaction pressure.
Methanol is advantageous to handle and transport. Recently, newsemiconductor-based photocatalysts reducing CO2 were developed, however, most of them formed methane [19] and/or CO[20] at the rates as high as 0.63 mmol h1 g1
cat using triethanol
amine [20]. In this study, owing to both the liberation of interlayer
reaction space in LDHs (31% removal of structural water) and highreactant pressure (0.40 MPa), the formation rate and selectivity tomethanol were significantly improved using [Zn1.5Cu1.5Ga(OH)8]2
CO3mH2O LDH irradiated by UV–visible light. Furthermore, theeffects of interlayer carbonates and inlayer Cu ions in LDHs are critically evaluated. These results are comparable to the best activities
among literature reporting selective methanol formation in theabsence of sacrificial reductant [2,3]. Furthermore, pressure
dependence of photocatalysis rates, blank tests of CO2 photoreduction, synthesis and the performance using LDH comprising

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L. Anton Wein et al. / Applied Surface Science 447 (2018) 687–696

interlayer [Pd(OH)4]2, isotope-labeled test using 13CO2, and the Zn
and Ga K-edge X-ray absorption near-edge structure (XANES)
under high pressure of CO2 and H2 irradiated by the UV–visible
light are reported.
2. Experimental section
2.1. Syntheses of LDH photocatalysts
Throughout the syntheses of LDHs in this study, no Ccontaining compounds including organic solvents were used
except for sodium carbonate. Deionized water (conductivity<0.055 lS cm1, total organic C <10 ppb) supplied by a RFU424TA
system (Advatec) was used and special care was taken not toinclude any contaminations. [Zn3Ga(OH)8]2CO3mH2O and [Zn3Ga(OH)8]2Cu(OH)4mH2O were synthesized via previously reported
procedures [5,6]. The color of LDHs are white and light turquoise
and the LDHs are denoted as Zn–Ga–CO3 and Zn–Ga–Cu(OH)4,
respectively, in the following (Table 1A).
[Zn1.5Cu1.5Ga(OH)8]2CO3mH2O, [Zn1.5Cu1.5Ga(OH)8]2Cu(OH)4mH2O, and [Zn3Ga(OH)8]2Pd(OH)4mH2O, denoted as Zn–Cu–Ga–
CO3, Zn–Cu–Ga–Cu(OH)4, and Zn–Ga–Pd(OH)4, respectively, weresynthesized by adding an aqueous solution (20 mL) of 0.75 M Zn
(NO3)26H2O and 0.25 M Ga(NO3)3nH2O (for Zn–Ga–Pd(OH)4) or
0.375 M Zn(NO3)26H2O, 0.375 M Cu(NO3)23H2O and 0.25 M Ga
(NO3)3nH2O (for the two LDHs comprising inlayer Cu) to an aqueous solution (100 mL) of 0.075 M Na2CO3 (for Zn–Cu–Ga–CO3),
0.025 M (NH4)2CuCl42H2O (for Zn–Cu–Ga–Cu(OH)4), or 0.025 M

Na2PdCl4 (for Zn–Ga–Pd(OH)4) dropwise under rapid stirring at
the rate of 900 rotations per minute. Thereby, the pH value wascontinuously adjusted to 7.6–7.9 except for 6.5 for Zn–Ga–Pd(OH)4 to prevent Pd(OH)2 falling out by the addition of aqueous
NaOH solution (1.0 M, 40–44 mL in total). Then, the pH value
was increased to 8 and the mixture was stirred for 2 h at 300 K.
Subsequently, the temperature was raised to 333–353 K and thesuspension was kept stirring for 22 h, yielding light turquoise, lightturquoise, and light brown precipitates, respectively (Table 1A).
Filtering through a polytetrafluoroethene-based membrane filter (Omnipore JVWP04700, Millipore; pore size 0.1 mm) followed
by washing with deionized water for five times (5  50 mL)
afforded the crude LDH product, which was wetly casted as thinfilm on a Pyrex glass plate and dried in a glove box under ambient
air for Zn–Ga–CO3 and Zn–Cu–Ga–CO3 and Ar for Zn–Ga–Cu(OH)4,
Zn–Cu–Ga–Cu(OH)4, and Zn–Ga–Pd(OH)4 LDHs for 5 d.
2.2. Characterization
X-ray diffraction (XRD) patterns were recorded on a Bruker D8
ADVANCE diffractometer for 5.0°  2H  60° with a scan step of
0.01° and a scan rate of 5 s per step. The measurements were performed under the conditions at 40 kV and 40 mA using Cu Ka
emission (k = 0.15419 nm) and a Ni filter. The crystalline size (t)was evaluated based on Scherrer equation:

t¼

0:9k
ðFull width at half maximumÞ cos hB

ð1Þ

Table 1
(A) Basic physicochemical properties and (B) the analysis of elemental metal composition of LDH photocatalysts synthesized in this study.
(A)
LDHs

Color

Interlayer distance (nm)a

Mean crystalline size (nm)a

Eg (eV) or n valueb
Extrapolationof abs. edge

ahm1/n plot versus hm

Eg

n

n

1
2

3
2

2

3

5.3
3.0
3.0
2.9
3.0

5.2
2.8
2.9
2.8
2.8

5.0
2.7
2.7
2.5
2.6

Eg
Zn–Ga–CO3
Zn–Cu–Ga–CO3
Zn–Ga–Cu(OH)4
Zn–Cu–Ga–Cu(OH)4
Zn–Ga–Pd(OH)4

a

White
Light turquoiseLight turquoiseLight turquoiseLight brown

59.5
45.9
25.7
24.0
21.3

5.5
3.9
4.4
3.3
3.8

(B)
Absorption edge

Zn K-edge

Change of mass attenuation coefficient D(l/q) at the edge (cm2 g1) [21](1)Zn–Ga–CO3
Zn–Cu–Ga–CO3
Zn–Ga–Cu(OH)4
Zn–Cu–Ga–Cu(OH)4
Zn–Ga–Pd(OH)4

218.6
240.1
Edge jump D(lt) observedc
4.02
–
2.01
2.30
3.20
0.587
1.34
2.05
3.85
–

Cu K-edge

0.95
0.95
0.99
1.03
1.00

5.7
4.3
5.0
4.0
4.7

Ga K–edge

Pd K-edge

190.4

49.0

0.765
0.783
0.633
0.537
0.702

–
–
–
–
0.287

(2)
Zn–Ga–CO3
Zn–Cu–Ga–CO3
Zn–Ga–Cu(OH)4
Zn–Cu–Ga–Cu(OH)4
Zn–Ga–Pd(OH)4

Atomic ratio in sample based on D(lt) listed in (1)a
1
–
0.21
0.50
0.54
0.21
1
0.17
0.21
0.5
0.72
0.22
1
–
0.20

–
–
–
–
0.20

(3)
Zn–Ga–CO3
Zn–Cu–Ga–CO3
Zn–Ga–Cu(OH)4
Zn–Cu–Ga–Cu(OH)4
Zn–Ga–Pd(OH)4

Atomic ratio of introduced metal salts for the LDH synthesis
1
–
0.33
0.50
0.50
0.33
1
0.17
0.33
0.50
0.67
0.33
1
–
0.33

–
––
–
–
0.17

Based on XRD pattern.
Based on UV–visible spectra.
c
The evaluation error was within 4%.

b

0.753
0.749
0.783
0.772
0.777

L. Anton Wein et al. / Applied Surface Science 447 (2018) 687–696

where k is wavelength for Cu Ka emission and HB is the Bragg angleof the peak.
UV–visible spectra were recorded on a JASCO V-650 spectrophotometer equipped with an integrated JASCO ISV-469 sphere
for diffuse reflectance detection within the range of 200 and 800
nm. The spectra were used to evaluate the band gap values (Eg)based on the extrapolation of the absorption edge to the x-axis
and then n value is evaluated as the slope of the plot of log(ahm)versus (hm  Eg) value derived from Davis–Mott equation (Taucplot), given by

ahm / ðhm  Eg Þnð2Þ

where a, h, and m are the absorption coefficient, the Planck constant,and the frequency of light for n value of 12, 32, 2, or 3 for allowed
direct, forbidden direct, allowed indirect, and forbidden indirectelectronic transitions, respectively [22]. Alternatively, the Eg value
was also evaluated by extrapolation of the (ahm)1/n plot versus hm
as the interpolation to x-axis based on Eq. (2).
2.3. Temperature programed desorption (TPD)
48.6 mg of Zn–Cu–Ga–CO3 were charged in a Pyrex glass reactor
connected to a closed circulating line connected to vacuum using
rotary and diffusion pumps (106 Pa). The temperature at the sample was elevated starting from 303 K to 773 K with a ramping rateof 4 K min1. The desorbed gas mixture was trapped using a loop
section cooled to 77 K and analyzed every 7.5 min using a gas chromatograph (GC) equipped with thermal conductivity detector(TCD) (model 8A, Shimadzu) equipped with a packed column ofpolyethylene glycol (PEG)-6000 supported on Flusin P (GL SciencesInc.). The system was evacuated for 30 s after sampling at every
7.5 min. Another TPD test was also performed prior to every photoconversion test following a similar procedure, but by elevating
the temperature from 303 K to 423 K in 30 min and keeping thetemperature at 423 K for 1 h.
2.4. Photocatalytic CO2 conversion tests
LDH catalyst powder was suspended in deionized water andagitated by ultrasound (430 W, 38 kHz, 303 K) for 3 min. The suspension was casted on a Pyrex glass plate (25 mm  25 mm  1mm) and dried overnight at 373 K. The mass of the films was 9 ±2 mg, the area that covers the plate was adjusted to 20 mm  20mm, and the film thickness was  10 lm according to crosssectional scanning electron microscopy (SEM) images [8]. Prior tophotocatalytic test, the film on Pyrex plate was heated to 423 K
with a ramping rate of 4 K min1 and kept at 423 K for 1 h at
106 Pa using rotary and diffusion pumps (Fig. 1). The film on Pyrex
plate was then introduced into a homemade high-pressure
stainless-steel reactor (double quartz windows, effective internal
volume of 98.4 mL) under Ar [8,23].
The reactor and the stainless metal lines were purged with thereaction gas of CO2 and H2 for 15 min [8,23]. The gas was previously mixed in cylinders with the molar ratio of 2:8, 3:7, 4:6, or
5:5. Pure CO2 (Purity >99.995%), H2 (Purity >99.99%), or He (Purity>99.9999%) was used for the blank tests. Subsequently, the pressure was elevated to the respective reaction pressure and the catalyst was irradiated through double quartz windows by UV–visible
light emitted from a 500-W xenon arc lamp (Ushio, model OPM2502) for 5 h at 300 K. The light intensity was 90.2 mW cm2 at thecenter of the photocatalyst. Thereby, the pressure increased by
1.7% within the first 10 min, however, gradual pressure decrease
later than that was slower. Therefore, first pressure increase is
exclusively due to the temperature change by light irradiation.
Assuming ideal gas behavior, temperature inside the reactor

689

should increase by 5 K within the first 10 min and negligibly changed later than that during photocatalytic tests of 5 h.
Thereafter, the reaction gas was analyzed using columns packed
with 13X-S molecular sieves (GL Sciences Inc.) for the separationand quantification of O2, N2, CH4, and CO and PEG-6000/Flusin P
for those of CO2, methanol, water, and formic acid both set in a
GC-TCD using helium (Purity >99.9999%) as a carrier gas. The
amounts of methanol and water were analyzed by concentrating
those gases in a trap cooled to 195 K with a mixture of diethyl
ether and dry ice to separate them from H2 and vast bulk of CO2[8,23,24].
2.5. Preliminary isotope-labeled photoreaction tests
Preliminary isotope-labeled photoreaction tests were performed using a GC–mass spectrometry (GCMS; model JMSQ1050GC, JEOL) equipped with quadrupole mass spectrometerto investigate the reaction mechanism into methanol. The sampling loop in a Pyrex glass system connected to model JMSQ1050GC using 1.5 m of deactivated fused silica tube (model
160-2845-10, Agilent; U = 250 lm) was vacuumed using rotaryand diffusion pumps (106 Pa). The fused SiO2 tube was maintained at 393 K during the analysis to avoid gas adsorption
inside the tube.
13
CO2 exchange (1.27 kPa; 13C 99%, chemical purity >99.5%)
with Zn–Cu–Ga–CO3 LDH (100 mg) was monitored at 300 K using
a quartz reactor [2,5] connected to a closed circulating Pyrex glass
system connected to the rotary and diffusion pumps (106 Pa). The
exchange with interlayer carbonates was monitored using a PEG6000/Flusin P column set in model JMS-Q1050GC.
2.6. Cu, Zn, Ga, and Pd K absorption edge for metal composition
analyses and XANES under high-pressure of CO2 and H2
Zn and Ga K-edge XANES spectra were measured in the Photon
Factory on beamline 12C at the High Energy Accelerator Research
Organization (KEK), Tsukuba. The storage ring energy was 2.5 eV
and the ring current was between 450.2 and 324.8 mA. A Si (1 11) double-crystal monochromator and a pair of bent cylindricalmirrors were inserted into the X-ray beam path. Cu, Zn, Ga, and
Pd K-edge XANES spectra were also measured in the Photon Factory Advanced Ring on beamline NW10A at KEK. The storage ringenergy was 6.5 eV and the ring currents were between 47.3 and36.2 mA. A Si (3 1 1) double-crystal monochromator, bent cylindrical mirror, and double flat mirror were inserted into the X-ray
beam path. The details for beam control and X-ray detection system were described previously [5,18,24–28].
Zn–Ga–CO3 and Zn–Cu-Ga–CO3 LDH samples were set in ahomemade stainless reactor equipped with a pressure gauge, diamond windows (thickness 0.50 mm) for X-ray and a quartz window (thickness 4.0 mm) for UV–visible light (Fig. 2). 0.12–0.18
MPa of CO2 and 0.28–0.42 MPa of H2 were introduced to the reactor and the LDH sample was irradiated by UV–visible light provided by a 500-W Xe arc lamp (Ushio, model SX-UID502XAM).
Obtained XANES data were analyzed using the XDAP software
package [29].
3. Results and discussion
3.1. XRD patterns
The XRD patterns for five LDHs synthesized are depicted inFig. 3. For carbonate-type LDHs (patterns a and b), peaks appearedat 2hBragg = 11.7, 23.5, 34.3, 36.9, 38.9, 43.7, 46.4, 52.6, 56.0, and59.4° those were assigned to diffractions by 0 0 3, 0 0 6, 0 0 9, 1

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L. Anton Wein et al. / Applied Surface Science 447 (2018) 687–696

Fig. 3. XRD patterns for LDHs. (a) Zn–Ga–CO3, (b) Zn–Cu–Ga–CO3, (c) Zn–Ga–Cu(OH)4, (d) Zn–Cu–Ga–Cu(OH)4, and (e) Zn–Ga–Pd(OH)4. Impurities: * Cu(OH)2, +CuO, # Zn(OH)2,  Ga2O3, and  PdO. y-Scale: (a) 1/2, (b) and (c) 2/3.

Fig. 1. A glass cell for heating the photocatalyst film (light turquoise) on Pyrex platein vacuum.

Fig. 2. Homemade in-situ reaction cell for X-ray absorption fine structure
measurements during high-pressure photocatalytic test.

0 4, 0 1 5, 1 0 7, 0 1 , 1 0 1 0, 0 1 1 1, and 1 1 0 planes, respectively.
The peak due to 0 1 2 diffraction overlaps with that due to 0 0 9
diffraction. The peaks for Cu/Pd hydroxide-type LDHs (pattern c–e), peaks shifted downward to 2hBragg = 11.3, 22.7, 34.0, 36.5,
38.3, 43.0, 45.5, 51.3, 54.2, and 59.1° especially for diffraction
planes perpendicular to the c-axis due to the difference of anions,those were assigned to diffractions by 0 0 3, 0 0 6, 0 0 9, 1 0 4, 0 1 5,
1 0 7, 0 1 8, 0 1 1 , and 1 1 0 planes, respectively. Based on the peak
position of 0 0 3 diffraction, the interlayer distance was evaluated
to 0.749–0.753 nm and 0.772–0.783 nm for carbonate-type LDHs
and Cu/Pd hydroxide-type LDHs, respectively (Table 1A). These
values fall almost within the range 0.751–0.753 nm and 0.772–
0.792 nm, respectively, reported for the two types of LDHs [6]. It
should be noted that formal Cu hydroxide-type [MII1xMIIIx(OH)2]2[Cu(OH)4]xmH2O dehydrates into [MII1xMIIIxOxy/2(OH)2xy/2]2[Cu(OH)4y]xmH2O + xyH2O (y = 1, 2, 3) during LDH synthesis in which
Cu ions are bound to the O atoms bonded to M ions in cationic layers
based on Fourier transform infrared and X-ray absorption fine structure analyses [18]. Namely, Cu hydroxide dehydrates with hydroxygroups of cationic layer and is bound to the cationic layer throughCu–O–M bonds. They are ‘layered double hydroxide salts’ [6,30]
and the minor difference of interlayer distance (D = 0.019–0.034nm) for carbonate-type LDHs and LDH salts is understandable toassume the LDH salt formation also for Zn–Ga–Pd(OH)4. Those values based on 0 0 6 or 0 0 9 peak width were in accord with thoseabove.
Under application of the Eq. (1), the crystallite sizes were estimated at 59 and 46 nm for the carbonate-type LDHs, 26 and 24
nm for the tetrahydroxycuprate-type LDHs, and 21 nm fortetrahydroxypalladate-type LDH (Table 1A). Relatively greater
crystallites were formed for the CO3-type LDHs due to the higher
thermodynamic stability compared to the Cu(OH)4-type and Pd(OH)4-type counterparts.
Besides, peaks derived from CuO impurity phase appeared at
32.5 and 35.5° and those from Cu(OH)2 impurity phase appeared
at 16.3, 40.0, 50.2, and 53.6° in the pattern for Zn–Cu–Ga–Cu(OH)4 (d) whereas the intensity of these peaks was quite weakfor Zn–Ga–Cu(OH)4 (c). Additionally, a peak appeared at 31.9°
due to PdO impurity phase in the pattern for Zn–Ga–Pd(OH)4(e) and two small peaks at 12.8–13.7 and 17.7° in the patternof Zn–Cu–Ga–CO3 (b) indicate minor Zn(OH)2 and Ga2O3 impurities, respectively.

L. Anton Wein et al. / Applied Surface Science 447 (2018) 687–696

691

3.2. Metal ion elemental composition based on X-ray absorption edgejump
The metal ion elemental composition for LDH samples (Table
1A) was analyzed based on X-ray absorption edge jump D(lt) at
Zn, Cu, Ga, and Pd K-edge. Based on the changes of mass attenuation coefficient D(l/q) at each absorption edge [21], the weight ofeach element melement was calculated:DðltÞ ¼ Dðl=qÞ  melement =A ¼ Dðl=qÞ  melement =0:502 p

ð3Þ

where l is linear absorption coefficient and t, A, and q are thickness,area, and the density, respectively, for disk samples of U = 1.0 cm.
Then, the atomic ratios of Zn, Cu, Ga, and Pd in these LDH samples
were evaluated (Table 1B2).
The values were basically consistent with the atomic ratios of
Zn, Cu, and Pd introduced during LDH syntheses as their salts(Table 1B3, see Section 2.1), however, the evaluated atomic ratios
for Ga K-edge (0.20–0.22) was inconsistent with introduced
amount of Ga(NO3)3nH2O (0.33). Because Ga ion is more soluble
compared to Zn and Cu at pH 6.5–7.9 during the LDH synthesisin this study [30], 1/3 of Ga ions would not be incorporated in
LDHs and the actual formula of LDHs (neglecting hydration ofinterlayer Pd for simplicity) would be [Zn3Ga0.64(OH)7.28]2(CO3)0.64mH2O (1 : 0 : 0.21), [Zn1.5Cu1.5Ga0.64(OH)7.28]2(CO3)0.64mH2O (0.5 :0.5 : 0.21), [Zn3Ga0.64(OH)7.28]2[Cu(OH)4]0.64mH2O (1 : 0.11 : 0.21),[Zn1.5Cu1.5Ga0.67(OH)7.34]2[Cu(OH)4]0.67mH2O (0.5 : 0.61 : 0.22),
and [Zn3Ga0.61(OH)7.22]2[Pd(OH)4]0.61mH2O (1 : 0.10 : 0.20) in
which atomic ratio of Zn : Cu (or Pd) : Ga are also noted in theparenthesis. Minor inconsistency in the ratio of Cu and Pd forcuprate/palladate type LDHs (Table 1B2) suggested not negligible
formation of Cu(OH)2, CuO, and PdO impurities with these LDHs
as proved by XRD (Fig. 3c–e).
This evaluation is consistent with the color of filtrates during
LDH syntheses. Any filtrates during the syntheses of LDHs in
Table 1A were not colored, demonstrating all the Cu and Pd ions(Cu nitrate aqueous solution: blue; ammonium Cu chloride aqueous solution: yellow; sodium Pd chloride aqueous solution: brown)
were incorporated in catalysts while a part of colorless Ga3+ ions
may be eluted during the washing step of LDH synthesis.
Furthermore, it was possible to distinguish inlayer Cu ion (present as octahedron in cationic sheet) and interlayer (dehydrated)
Cu ion [Cu(OM)y(OH)4y(OH2)2 where M is a divalent/trivalentcation in the cationic layer and the Cu is coordinated to two structural water molecules] [18]. All coordination environment for
inlayer Cu (Fig. 4A-b), Zn (Fig. 4B-b–d), and Ga (Fig. 4C-b–d), andaccordingly all the XANES pattern well resembled each other. In
contract, in Cu K-edge XANES for samples comprising interlayer(dehydrated) Cu sites (Fig. 4A-c and d), a shoulder peak appeared
at 8991 eV and the peak at 9016 eV became weaker (Fig. 4A,
arrows), strongly indicating the structural difference between
octahedral Cu site in LDH cationic layer and dehydrated Cu(OM)y(OH)4y(OH2)2 sites.
3.3. UV–visible spectra

Fig. 4. (A) Cu K-edge, (B) Zn K-edge, and (C) Ga K-edge XANES for (b) Zn–Cu–Ga–
CO3, (c) Zn–Ga–Cu(OH)4, and (d) Zn–Cu–Ga–Cu(OH)4.

To analyze the light absorption properties of the LDH catalysts,
UV–visible diffuse-reflectance spectra were recorded, shown in
Fig. 5. These spectra were used to evaluate the Eg value based on
Eq. (2). The comparison of the Eg values based on simple
absorption-edge extrapolation with the Eg values obtained fromthe (ahm)1/n plot versus hm indicates that the n value was between
1
and 32 (Table 1A), demonstrating that direct electronic transitions
2
occurred. Additionally, n values were also evaluated based on theplot of log(ahm) versus (hm  Eg) value based on simple absorption
edge extrapolation (Table 1A). The n values were in the range of

0.95–1.03 and hence in agreement with the discussion above.
Zn–Ga–CO3 sample exhibits direct electronic transitions from O
2p to Zn/Ga 4s and 4p levels; and Zn–Cu–Ga–CO3, Zn–Ga–Cu(OH)4, and Zn–Cu–Ga–Cu(OH)4 exhibit direct electronic transitions
from O 2p to Zn/Ga 4s and 4p and/or Cu 3d, 4s, and 4p levels. For
Zn-Ga-Pd(OH)4 sample, transitions from O 2p to Zn/Ga 4s and 4p
and/or Pd 4d, 5s, and 5p levels occur.
If interlayer carbonate is replaced by Cu(OH)2
4 , the UV absorption edge shifted to longer wavelength (59–90 nm) for both

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L. Anton Wein et al. / Applied Surface Science 447 (2018) 687–696

Fig. 5. UV–visible spectra for LDHs. (a) Zn–Ga–CO3, (b) Zn–Cu–Ga–CO3, (c) Zn–Ga–
Cu(OH)4, (d) Zn–Cu–Ga–Cu(OH)4, and (e) Zn–Ga–Pd(OH)4.

samples (Fig. 5, spectra a versus c, spectra b versus d). For Pd(OH)2
4 replacement (spectra a versus e), the absorption edge also
underwent a redshift for 101 nm. Additionally, inlayer Cu incorporation leads to an absorption edge shift of 63–95 nm towards
higher wavelengths in comparison with Zn–Ga–CO3 and Zn–Ga–
Cu(OH)4, respectively (spectra a versus b, spectra c versus d).
Furthermore, a weak d-d transition peak appeared for Zn–Cu–
Ga–CO3, Zn–Ga–Cu(OH)4, and Zn–Cu–Ga–Cu(OH)4 LDHs between
500 and 800 nm (Fig. 5b–d). For the Zn–Ga–Pd(OH)4 LDH, a d-dtransition with a high absorption characteristic was observed in
the region of 450–800 nm (Fig. 5e), mainly due to the presenceof minor impurity PdO, as evidenced by XRD (Fig. 3e).
In summary, Eg values decreased from Zn–Ga–CO3 (5.5 eV) to
Zn–Ga–Cu(OH)4 for 1.1 eV, to Zn–Cu–Ga–CO3 for 1.6 eV, to Zn–
Ga–Pd(OH)4 for 1.7 eV, and then to Zn–Cu–Ga–Cu(OH)4 for 2.2 eV(Table 1A) due to the presence of Cu 3d levels or Pd 4d levels.

3.4. TPD
Two kinds of TPD data are shown in Fig. 6. The 48.6 mg of Zn–
Cu–Ga–CO3 used for this study corresponds to 52.2 lmol catalyst if
the molar amount of structural water is a half of that of metal
cations, i.e. m = 4 [30]. The total amount of CO2 desorbed until
773 K was 85.8 lmol (Fig. 6A). The desorption centered at 690 K
should be due to the decomposition of metal carbonates to oxides
in agreement with literature [30,31]. The extra CO2 in the TPD

would be due to adsorbed CO2 on the LDH surface leading to ashoulder peak centered at 560 K. In contrast, no clear peak was
observed for water desorption, and total amount of H2O desorbed
until 773 K was 120 lmol. The desorption temperature range wassimilar to reported thermogravimetric analysis data for [Mg2Al0.9Ga0.1(OH)6]2CO3mH2O [32]. Major part of interlayer water desorbs
until 773 K, but metal hydroxides transformed from LDH also
remain at the temperature.
The desorption during the pretreatment of Zn–Cu–Ga–CO3 forphotocatalytic tests using CO2 and H2 was also monitored. The desorption rate for both water and CO2 increased until the 423 K werereached, and the desorption rate gradually decreased at 423 K for
1 h (Fig. 6B). The total amount of desorbed water during the pretreatment was 64.5 lmol corresponding to 31% of interlayer water
removed. The total amount of desorbed CO2 during the pretreatment was 7.47 lmol corresponding to 14% of carbonates in the
LDH, however, CO2 in Fig. 6B originates from both carbonates
and adsorbed CO2 on the LDH surface in comparison to peak pattern in Fig. 6A.
3.5. Photocatalytic CO2 conversion
Under reaction conditions of CO2 0.12 MPa and H2 0.28 MPa, theformation rates of C-containing products using LDH catalysts weremonitored. Compared to previously-reported low-pressure experiments at CO2 2.3 kPa and H2 21.7 kPa [5,6], the formation rates weresignificantly improved by factors of 1.8–12 times (Table 2a–d).
Interestingly, the performances under reactant pressure of 0.40
MPa were highly selective for methanol formation: more than 90–
1
97 mol% selectivity and rates of 0.78–2.8 lmol g1, mainly due
cat h
to the advanced pretreatment conditions at 423 K under vacuum(Fig. 1). When the LDHs were exposed to air prior to the photoreduction tests of CO2, the methanol formation rates were as low as
10.011–0.30 lmol g1[23]. One of the major reasons of thiscat henhancement is the liberation of interlayer space by removing
31% of interlayer water (Fig. 6B). The rate difference among LDH
catalysts (Table 2a–d) indicates that inlayer Cu(II) site accelerates
the formation rates whereas dehydrated tetrahydroxy cuprate site
attached to cationic sheet lowers them. The dehydrated tetrahydroxy palladate site was more effective as compared to cuprate
counterpart for the methanol formation (Table 2c, e), however, it
was less effective than inlayer Cu(II) site (Table 2b, e). Thus, inlayer
Cu comprising LDH shows the highest methanol formation rate of1
2.8 lmol g1. This value is superior to CO formation rate of 1.7
cat h
1 1
lmol gcat h using CO2, moisture, and CeII–CeIII LDH [13]. Assuming homogeneous energy distribution in the whole wavelength
range of the Xe arc light source and that the photons with theFig. 6. TPD of H2O and CO2 using Zn–Cu–Ga–CO3 between 300 and 773 K with the ramping rate of 4 K min1 (A) and between 300 and 423 K with the ramping rate of 4 K
min1 and keeping at 423 K for 1 h (B).

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L. Anton Wein et al. / Applied Surface Science 447 (2018) 687–696
Table 2
Photocatalytic test results using 0.12 MPa of CO2 and 0.28 MPa of H2 and LDH photocatalysts irradiated by UV–visible light for 5 h.
Entry

a
b
c
d
e
a
b

Photocatalyst

Zn–Ga–CO3
Zn–Cu–Ga–CO3
Zn–Ga–Cu(OH)4
Zn–Cu–Ga–Cu(OH)4
Zn–Ga–Pd(OH)4

1
Formation rate (lmol g1)
cat h

Selectivity to methanol (mol%)b

CH3OH

CO

R

Ratio compared to corresponding low pressure test

1.5
2.8
0.78
2.0
1.1

<0.08
<0.08
<0.08
<0.08
<0.08

1.5
2.8
0.78
2.0
1.1

12
11
1.8
3.6
–

a

>95
>97
>90
>96
>93

Ratio of total C-containing product formation rate in comparison to that under 2.3 kPa of CO2 and 21.7 kPa of H2 using 100 mg of photocatalyst [5,6].
1
Methane and formic acid were not detected above the detection limit of GC-TCD: 0.12 and 0.004 lmol g1, respectively.
cat hFig. 7. Contour plot of methanol formation rates depending on the partial pressuresof H2 and CO2 using Zn–Cu–Ga–CO3 LDH photocatalyst (10 mg). The values in the1plot are methanol formation rates in the unit of lmol g1.
cat h(CO2) higher than 0.19 MPa was even negative for the catalysis
compared to lower P(CO2) between 0 and 0.07 MPa. Especially,
the relatively high conversion rate in the test under H2 only is a
1
specific feature of LDH catalysis (1.9 lmol g1), implying thatcathinterlayer carbonate can take part in the photocatalytic reaction.
Following along the straight line of P(CO2) : P(H2) ratio of 3:7 in
1Fig. 7, the rates were almost invariant at 1.0 ± 0.2 lmol g1
with
cath
exceptions of a lower rate at the origin (control test in 0.40 MPa of
1
He; 0.73 lmol g1) and the volcano peak top region (2.8–1.9
cat h
1lmol g1) of P(H2) and P(CO2) ranges of 0.28–0.35 MPa andcath
0.12–0.15 MPa, respectively. The region of almost invariant rates
1of 1.0 ± 0.2 lmol g1
further extends to the straight lines of P
cath(CO2) of 0.12 MPa and lower P(CO2) than 0.12 MPa forming relatively flat halfway region of the volcano (light green region in Fig. 7).
As already noted above, even at 0.40 MPa of He as a blank test,
the observed methanol formation rate was not negligible: 0.73
1lmol g1. This result suggests that interlayer H2O is a reaction
cath
partner for methanol formation. Thus, it is necessary to further
investigate the reaction mechanism to explain these results using
isotope-labeled substrates.
3.6. Preliminary 13C-labeled tests

energy of band gap (3.9 eV, Table 1A) are responsible for photocatalytic methanol synthesis using Zn–Cu–Ga–CO3 (the formation rate
1
2.8 lmol-CH3OH g1, Table 2b), the quantum efficiency is calcat hculated to 44 ppm.
We tried to detect C-containing products other than methanol
throughout the tests in this study, however, CO, methane, and formic acid were not found above the detection limit of GC-TCD(Table 2). As the reason of exclusive formation of methanol in thisstudy, longer residence time of CO2-derived intermediate(s) in theinterlayer space of LDH is suggested during the multiple reduction
steps to methanol [33].
Although methanol was formed exclusively under the describedreaction conditions, the catalytic rate depended highly on the partial pressure ratio of CO2 : H2 and also on the total pressure (Fig. 7).
1
A maximum rate of 2.8 lmol g1
was observed for hydrogencathpressure P(H2) of 0.28 MPa and CO2 pressure P(CO2) of 0.12 MPa.
Shifting to higher P(CO2) of 0.19–0.28 MPa inhibited the methanol
formation significantly by a factor of 0.089–0.14 shown as a slope
in the contour plot (Fig. 7). Extra amount of CO2 should adsorb on/in the LDH and would block the once-liberated interlayer reactive
space and hamper the diffusion of reactant (H2) and/or product(methanol). In this context, the liberation of interlayer reactive
space is primarily due to the removal of 31% of interlayer water,
but the removal of 14% of carbonates (and adsorbed CO2) during
the pretreatment is also critical. Once-removed CO2 would berecovered to the LDH framework at the higher P(CO2) regime of
0.19–0.28 MPa, and electrostatically stabilize it to possess less surface charging and be unreactive [34].
At lower P(CO2) of 0–0.07 MPa, e.g. following on the straightline of P(H2) of 0.28 MPa, the methanol formation rates became
also lower by a factor of 0.39–0.68 (Fig. 7). It is evident that a P

By the photocatalytic test using Zn–Cu–Ga–CO3 film (10 mg),the best catalyst in this study (Table 2b), at a reactant pressure of
0.40 MPa (CO2 : H2 = 3:7) for 5 h, methanol formation was confirmed as the major peak of mass number (m/z) 31 (CH3O fragment, peak area 5.7  105; Fig. 8A, top) and 32 (CH3OH, area 2.0
 105; Fig. 8A, bottom).
In 1.27 kPa of 13CO2 using Zn–Cu–Ga–CO3 powder (100 mg) in
the darkness for 1 h, no methanol peak was detected at m/z of 31
and 32 (Fig. 8B, top and bottom). In contrast, irradiated underUV–visible light for 1 h, methanol peaks appeared at m/z of 31(area 3.9  103) and m/z of 32 (area 1.1  104) at the retention timeof 110 0500 –110 1200 with the peak area ratio of 1:2.9 (Fig. 8C, top versus bottom). As in Fig. 8A, mass number for methoxy fragment ismore pronounced for methanol detection in GCMS. It was observedthat methanol retention time became progressively later as theamount decreased: from 80 3900 (area 2.3  108, 0.501 kPa) to
90 1600 (area 1.5  107, 0.016 kPa), 90 3400 (area 5.7  105, Fig. 8A),
and then 110 1200 (area 2.9  103, Fig. 8C). This result demonstrates
that both gas-phase 13CO2 and interlayer carbonates photoreacted
into methanol, however, gas-phase 13CO2 preferably reacted toform 13CH3OH leading to greater peak of m/z = 32 due to 88% of
13
CH3O and 12% of 12CH3OH fragments at the initial 1 h irradiatedby UV–visible light. However, only 4.0% of carbonate exchanged
with 13CO2 irradiated by UV–visible light for 24 h (data not shown).
This fact suggested that only water near external LDH sites participated in the photocatalysis under low pressure of 13CO2 (1.27
kPa). The CO2 exchange reaction with Mg–Al–13CO3 LDHs via

CO2 ðgÞ þ H2 OðinterlayerÞ þ 13 CO2
3 ðinterlayerÞ
! HCO3 ðinterlayerÞ þ HO ðinterlayerÞ þ 13 CO2

ð4Þ

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L. Anton Wein et al. / Applied Surface Science 447 (2018) 687–696

Fig. 8. Photocatalytic test using Zn–Cu–Ga–CO3 LDH (10 mg), 0.12 MPa of CO2, and 0.28 MPa of H2 (A) and CO2 exchange test using Zn–Cu–Ga–CO3 LDH (100 mg) and 1.27
kPa of 13CO2 (B, C) monitored using GCMS. Under dark for 1 h (B) and following irradiation by UV–visible light for 1 h (C). 500-W Xe arc lamp was used for tests A and C.

Fig. 9. In situ normalized XANES spectra at (A) Zn K-edge and (B) Ga K-edge for Zn–Ga–CO3 LDH under CO2 (0.18 MPa) and H2 (0.42 MPa) and UV–visible light irradiation at
beamline (Fig. 2).

L. Anton Wein et al. / Applied Surface Science 447 (2018) 687–696

695

Fig. 10. (A) In situ normalized Cu K-edge XANES spectra for Zn–Cu–Ga–CO3 (25 mg) under CO2 (0.12 MPa) and H2 (0.28 MPa) and UV–visible light irradiation for 0 min, 62
min, and 120 min at beamline. (Inset) Expanded view of 1s-3d preedge peak region. (B) Time course of the intensity of 1s-3d preedge peak for 2 h.

in air was reported. Most of 13CO2
3 exchanged in 10 weeks, 7 h, and
1 h when the molar ratio of Mg: Al was 2, 3, and 4, respectively, inthe LDHs [35]. This fact suggests that the population of positive
charge in layer critically affects the exchangeability of carbonates.
3.7. XANES for LDHs under 0.60–0.40 MPa of CO2 and H2First, the Zn K-edge and Ga K-edge XANES spectra of Zn–Ga–
CO3 during CO2 photoreduction test at 0.18 MPa of CO2 and 0.42
MPa of H2 are shown in Fig. 9A and B, respectively. Under similar
conditions, CO2 is photoconverted into methanol (Table 2a). Corresponding to the performance, the whiteline peak intensity at
10379 eV near the Ga K-edge decreased by 2.1% when the sample
was irradiated by UV–visible light compared to that in darkness(Fig. 9B, Right). In contrast, the whiteline peak intensity at 9671
eV near the Zn K-edge decreased only by 1.2% when the UV–visible
light was irradiated as compared to that in darkness (Fig. 9A,
Right). This change of whiteline peak intensity under UV–visible
light irradiation was only observed at elevated total pressure of
0.60 MPa, but not at 0.10 MPa.
The decrease of whiteline intensity should be related to the partial occupancy of 4s and 4p states by the photogenerated electrons.
Preferable CO2 adsorption on Ga ions is known [32,36]. The
possibility of partially-reduced Ga ions adsorbed with intermediate species, e.g. carbonate and/or hydrogen carbonate, at elevatedpressure needs to be verified.
In-situ Cu K-edge XANES of the most active Zn–Cu–Ga–CO3(Table 2b) was monitored under reactant pressures of 0.12 MPa
CO2 and 0.28 MPa H2 irradiated by UV–visible light during measurement at beamline for 2 h (Fig. 2). The 1s-3d preedge peak
appeared at 8980.6 eV (Fig. 10A, inset). The peak intensity progressively decreased (Fig. 10B) due to the reduction of Cu(II) (3d9 configuration) to Cu(I) (3d10 configuration) by the diffusion ofphotogenerated electrons to the Cu sites. 1s-3d electronic transition is impossible to fully occupied 3d10 Cu(I) site. The peakdecreasing rate corresponds to 170 lmol-Cu h1 g1
cat for 25 mgdisk sample of U = 1.0 cm.
Under 2.1 kPa of CO2 and 21.7 kPa of H2, Cu site reduction rateof 580 lmol-Cu h1 g1
cat was reported under similar conditions for
170 mg-disk Zn–Cu–Ga–CO3 sample of U = 2.0 cm [18]. The reduction rate of Cu at 0.40 MPa was 10 times greater compared to its
methanol formation rate (Table 2b), taking the electron number(s) needed for each reaction (Cu(II)/(I) redox versus six electron
reduction to methanol) into account. Thus, the inlayer Cu sites
once accumulate photogenerated and diffused electrons. However,
the electrons transfer to CO2 (or CO2-derived species) more easily

under pressurized conditions (Fig. 10) in comparison to that at alower pressure (23.8 kPa) [18]. Thereby the net Cu reduction rateduring this experiment (Fig. 10B) seemed to be lower in comparison to that in Ref. [18].
4. Conclusions
LDHs comprising Zn, Cu, and/or Ga in the cationic layers andcarbonates, [Cu(OH)4]2, or [Pd(OH)4]2 as anions were synthesized and applied for the photoreduction of CO2 at total reactant
pressures of 0–0.80 MPa. By the pretreatment in vacuum at 423
K for 1 h, 31% of interlayer water desorbed to liberate the interlayer
reaction space for Zn–Cu–Ga–CO3 LDH. The highest methanol formation rate in this study was observed at 0.40 MPa of CO2 and H2
with the molar ratio 3:7 using Zn–Cu–Ga–CO3. The liberation
effects of interlayer space are apparent: methanol formation rate
1
1
of 2.8 lmol g1
versus 0.011–0.30 lmol g1
using ascat h
cat hsynthesized versus air exposed LDHs [23] tested under similar
reaction conditions. The pressure dependence of methanol photosynthesis rates on P(CO2) and P(H2) constituted a volcano-like
contour peaked at P(CO2) of 0.12 MPa and P(H2) of 0.28 MPa using
Zn–Cu–Ga–CO3. The reason of volcano-like pressure dependence iscomplicating because carbonates and interlayer water potentiallyreact to form methanol under the photocatalytic reaction
conditions. However, P(CO2) above 0.12 MPa led to excessive
CO2-derived species to block the diffusion of reactant and/or
product to/from the interlayer reaction space liberated by the pretreatment at 423 K in vacuum. Major 13CH3OH formation from13
CO2 was observed by GCMS analysis in addition to minor 12CH3OH formation from carbonate of the LDH catalysts. In-situ XANES
study indicated complex formation of CO2-derived species withoccupied Ga ion sites rather than Zn ion sites in Zn–Ga–CO3 LDH
and also the accumulation of photogenerated electrons at Cu ion
sites for subsequent CO2 reduction in Zn–Cu–Ga–CO3 LDH.
Acknowledgments
The authors are grateful for the financial supports from theGrant-in-Aid for Scientific Research C (17K05961, 26410204) fromJapan Society for the Promotion of Science and Leading Research
Promotion Program (2015–) from the Institute for Global
Prominent Research, Chiba University. X-ray absorption experiments were conducted under the approval of the Photon Factory
Proposal Review Committee (2016G577). The authors thank Akane
Kawashima for the technical support of TPD data acquisition.

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