Journal of Catalysis 345 (2017) 39–52

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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat

Efficient volcano-type dependence of photocatalytic CO2 conversion intomethane using hydrogen at reaction pressures up to 0.80 MPa
Shogo Kawamura a, Hongwei Zhang a, Masayuki Tamba b, Takashi Kojima b, Masaya Miyano a,
Yusuke Yoshida a,1, Mao Yoshiba a, Yasuo Izumi a,⇑ab

Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japana r t i c l e

i n f oArticle history:
Received 30 August 2016
Revised 20 October 2016
Accepted 22 October 2016

Keywords:
CO2 conversion
Pressure dependence
Pd/TiO2
Electron trap
Oxygen vacancy
Interface site
Surface plasmon resonanceContour plota b s t r a c t
Photocatalytic conversion of CO2 into fuels could mitigate global warming and energy shortage simultaneously. In this study, the reaction pressure was optimized for CO2 reduction by H2. The major products
were methane, CO, and methanol, and the observed catalytic activity order was Cu or Pd on TiO2  Ag/
ZrO2  g-C3N4 > Ag/Zn3Ga-layered double hydroxide  BiOCl. Hot/excited electrons due to surface plasmon resonance could be transferred to CO2-derived species and the remaining positive charge could combine with excited electrons in the semiconductor. As the levels of hot/excited electrons became morenegative, the catalysts became more active, except for Ag/ZrO2 and Ag/Zn3Ga-LDH, probably due to lower
charge separation efficiency for intrinsic semiconductors or hydroxides. The reaction order was controlled by the partial pressure of H2, demonstrating preferable adsorption of H on Pd. The photoconversion of CO2 into methane was optimum at PH2 = 0.28 MPa and PCO2 = 0.12 MPa, but the rates gradually
dropped at higher partial pressures due to adsorption of CO2 being hindered by H.
Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction

followed by the production of nicotinamide adenine dinucleotide
phosphate (NADPH) by Photosystem I [9],Photocatalytic conversion of CO2 into fuels is one of the routes
to carbon-neutral fuels, avoiding the net increase in atmospheric
CO2 concentrations associated with fossil fuel-derived alternatives[1–3]. TiO2 has been investigated most intensively for this purpose
and is known to produce methane and/or CO. CuO supported on
TiO2 photoreduced CO2 into methanol [2,3] or methane [1–3].
However, when isotope-labeled 13CO2 was used, C-containing
impurities at the TiO2 surface were likely to be reduced to CO[3,4]. The product using Pd/TiO2 was carefully confirmed to be
13
CH4 [3,5].
We recently reported CO2 photoconversion into methanol using
H2 and layered double hydroxides (LDHs) [6–8]. Industrially, H2 isproduced via cracking and reforming fossil fuels. H2 can instead be
produced sustainably from water utilizing natural light, in a way
similar to Photosystem II,NADPþ þ Hþ þ 2e ! NADPH:

2H2 O ! O2 þ 4Hþ þ 4e ;ð1Þ⇑ Corresponding author.
E-mail address: yizumi@faculty.chiba-u.jp (Y. Izumi).
Current address: Innovation Research Center of Fuel Cells, University of ElectroCommunications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan.
1

http://dx.doi.org/10.1016/j.jcat.2016.10.024
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

ð2Þ

Our analogue of Photosystem I in heterogeneous catalysis is CO2
photoconversion using LDH and Pd/TiO2 to form methanol andmethane [6–8]:

CO2 þ 6Hþ þ 6e ! CH3 OH þ H2 O

ð3Þ

CO2 þ 8Hþ þ 8e ! CH4 þ 2H2 O

ð4Þ

2H2 O þ 2NADPþ ! O2 þ 2NADPH þ 2Hþ

ð1 þ 2Þ

4H2 O þ 2CO2 ! 3O2 þ 2CH3 OH

ð1 þ 3Þ

2H2 O þ CO2 ! 2O2 þ CH4

ð1 þ 4Þ

Although significant progress has been achieved by many researchers in the photoconversion of CO2 into fuels [1–3], most studies
have evaluated the photoconversion of dissolved CO2 in aqueous
solution, namely Eqs. (1) and (3) or (1) and (4). For example, thecritical role of oxygen vacancy (OV) sites in CO2/H2O was suggested[10–13], and enhanced photoconversion of CO2 was reported at ele-

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S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52

vated reaction pressure of CO2 and moisture [10,11] or liquid water[11,14–19]; however, most of the mechanisms are unknown. Thus,
the systematic dependence of CO2 photoconversion on reaction
pressure and an independent understanding of the reduction sites
and mechanism for CO2 conversion (Eqs. (3) and (4)) are still open
for discussion.
The present study screened semiconductor photocatalysts for
efficient photoreduction of CO2 into fuels using H2 as a reductant
at 0.12–0.80 MPa in order to clarify the role of Eq. (3) and/or (4).
The band gap (Eg) values of the semiconductors were varied
between 2.6 and 5.6 eV by employing ZrO2, g-C3N4, BiOCl, TiO2,and [Zn3Ga(OH)8]2CO3mH2O LDH while keeping the conduction
band (CB) minimum more negative than the reduction potential(E°) for CO2 to methane, CO, or methanol (0.32 to 0.11 V against
the standard hydrogen electrode (SHE)) [3,8]. The reaction pressure dependence for Reaction (3) and/or (4) was carefully
investigated.
Sastre et al. reported CO2 conversion using H2 and Ni-based catalysts irradiated by a solar simulator, and the rates were surprisingly as high as those using a photocatalyst: 55 mmol h1 g1cat[20]. The temperature and gas pressure reached 423 K and some
hundreds of kPa due to the exothermic nature of Eq. (4). On thebasis of blank tests below 453 K in the absence of light, photocatalytic activation of H2 to form Ni–H species followed by thermalhydrogenation of CO2 to formate on metallic Ni was suggested[2,20]. The possibility of a thermal assist for activation is also discussed in this study.
The reason that a volcano-type dependence of CO2 photoconversion on reaction pressure is obtained using the most active
Pd/TiO2 photocatalyst was investigated by monitoring the massbalance of reactions, the oxidation state of Pd, and the concentration of OV sites in TiO2 by X-ray absorption fine structure (XAFS).
This paper is the first of several in which the sites and mechanism
for reaction in CO2 + H2 are compared with those for reaction in
CO2 + H2O. As site separation between photo-oxidation (H2 or
moisture) and photoreduction (CO2), and the concentrations androles of OV sites on the catalysis are complicated issues, the photoconversion of CO2 using moisture is reported separately [21].

2. Experimental
2.1. Catalyst synthesis
An aqueous solution (1.0 mM) of Ag nitrate (>99.8%, Wako Pure
Chemical) was stirred with ZrO2 {JRC-ZRO-3, Catalysis Society ofJapan, specific surface area (SA) 94 m2 g1} for 24 h. An aqueous
solution of 40 mM of NaBH4 (>95%, Wako Pure Chemical) was thenadded in the molar ratio Ag:NaBH4 = 1:4 and the solution was stirred for 1 h. The solution was filtered using a polytetrafluoroethenebased membrane filter (Omnipore JVWP04700, Millipore; pore size
0.1 lm) and the collected yellow precipitate was washed with
deionized water (<0.055 lS cm1) supplied by a model RFU424TA(Advantec). The powder was dried under vacuum at 290 K for
24 h. The obtained yellow powder is denoted as Ag/ZrO2. The loading of Ag was 0.5 wt.% (Table 1a).
Ten grams of urea (>99%, Wako Pure Chemical) was heated at
823 K for 3 h and the resultant yellow powder was washed with
0.1 M of nitric acid (Wako Pure Chemical) and deionized water toobtain g-C3N4 [22].
BiOCl was synthesized via a solvothermal procedure describedin Ref. [23]. Briefly, 2.0 g of Bi nitrate pentahydrate (>99.5%, Wako
Pure Chemical) and 1.3 g of cetyltrimethylammonium chloride(>95%, Wako Pure Chemical) were dissolved in 80 mL of ethylene
glycol (EG, >99.5%, Wako Pure Chemical). The solution was stirred
for 1 h and 1.0 M of KOH (85%, Wako Pure Chemical) in EG wasadded. The reaction solution was maintained at 433 K for 12 h.
The solution was filtered and the collected white powder waswashed with deionized water and ethanol.
Sodium tetrachloropalladate (>98%, Sigma Aldrich) solution(1.0 mM) was stirred with TiO2 (P25, Degussa; anatase phase:rutile
phase = 8:2, specific SA 60 m2 g1) at 290 K for 24 h. Then 40 mM
of NaBH4 aqueous solution was added in the molar ratio Pd:
NaBH4 = 1:8 and the suspension was stirred at 290 K for 1 h. The
solution was filtered using a JVWP04700 filter and the collected
yellow precipitate was washed with deionized water before drying
under vacuum at 290 K for 24 h. The obtained gray powder was
denoted as Pd/TiO2. The loading of Pd was 0.5 wt.% (Table 1d).
Cu(NO3)23H2O (0.078 g; >99.9%, Wako Pure Chemical) and urea
(2.5 g) were dissolved in 50 mL of deionized water. A quantity of
1.0 g of TiO2 (P25) was added to this solution. Thereafter, the temperature of the suspension was increased to 353 K and kept constant for 4 h while stirring as a deposition–precipitation step. The
sample was then washed with deionized water (50 mL each) andfiltered five times using a JVWP04700 filter. After drying, the sample temperature was elevated at a rate of 1.25 K min1 and kept at
673 K in air for 2 h [24]. The loading of Cu was 3.2 wt.% (Table 1e).
The synthetic procedure for preparing LDH [Zn3Ga(OH)8]2CO3mH2O has been described previously [8]. A quantity of 1.0 mM
of Ag nitrate solution was stirred with [Zn3Ga(OH)8]2CO3mH2O
powder at 290 K for 24 h and then 40 mM NaBH4 aqueous solution
was added in the molar ratio Ag:NaBH4 = 1:4 and stirred for 1 h.
The solution was filtered using a JGWP04700 filter and the collected yellow precipitate was washed with deionized water. The
obtained yellow powder is denoted Ag/Zn3Ga-LDH. The loadingof Ag was 0.5 wt.% (Table 1f).
2.2. Characterization
X-ray diffraction (XRD) patterns were observed using a D8
ADVANCE diffractometer (Bruker) at the Center for Analytical
Instrumentation, Chiba University, at a Bragg angle (hB) of
2hB = 5.0–60° with a scan step of 0.02° and a scan rate of 3 s per
step. The measurements were performed at 40 kV and 40 mA using
Cu Ka emission (wavelength k = 0.15419 nm) [25,26] and a nickel
filter. Crystallite sizes (t) were estimated using the Scherrer
equation

t¼

0:9k
:
Peak width  coshB

ð5Þ

Ultraviolet (UV)–visible spectra were measured using a Model V650 (JASCO) spectrophotometer equipped with an integrating
sphere (ISV-469, JASCO) for diffuse reflectance measurements inthe wavelength range 200–800 nm. The Eg values were estimatedvia simple extrapolation of the absorption edge or by fitting to
the Davis–Mott equation,

ahm / ðhm  Eg Þn ;

ð6Þ

where a, h, and m are the absorption coefficient, Planck’s constant,and the frequency of light, respectively, and n is 1/2, 3/2, 2, or 3
for allowed direct, forbidden direct, allowed indirect, and forbidden
indirect electronic transitions, respectively [8,27,28]. Fits to Eq. (6)were performed assuming each n value, or using the n value evaluated based on fits to the log–log form of Eq. (6) using the Eg value
obtained by simple extrapolation of the absorption edge to thewavelength axis in the UV–visible spectra.
Transmission electron microscopy (TEM) images were observed
using a Model H-7650 transmission electron microscope (Hitachi)
at an accelerating voltage of 100 kV. A tungsten filament was usedin the electron gun and samples were mounted on conducting carbon with a Cu grid mesh (150 mesh per inch). The magnification

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S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52
Table 1
Basic physicochemical properties of the photocatalysts screened in this study.
Entry

Photocatalyst

Metal loading(wt.%)

Color

Peak due
to SPR (nm)Pd mean particle size
(nm)aAg/ZrO2

0.5

Yellow428b

g-C3N4–Lime yellow

2.9

c

BiOCl

–

White

3.6

d

Pd/TiO2

0.5

Gray(Too broad)e

Cu/TiO2

3.2

Light gray

440

3.2

f

Ag/Zn3GaLDH

0.5

Yellow

395

5.2

TEM

a
b

3.1(±0.9){3.9(±1.3)a}Eg (eV)

EXAFS

1.1(±0.1){2.3(±0.1)b}n obtained from
log–log formof Eq. (6)Extrapolated

Based on Eq. (6) (associated n)

5.1

5.2 (n = 1/2)
5.0 (n = 3/2)
3.0 (n = 1/2)
2.8 (n = 3/2)
3.8 (n = 1/2)
3.4 (n = 3/2)
3.4 (n = 1/2)
2.9 (n = 3/2)

1.25

3.4 (n = 1/2)
3.0 (n = 3/2)
5.6 (n = 1/2)
5.0 (n = 3/2)

0.98

3.1

0.99
1.06
1.05

1.32

After photocatalytic tests under CO2 (0.09 MPa) and H2 (0.21 MPa) for 5 h.
After photocatalytic tests under CO2 (2.3 kPa) and H2 (22 kPa) for 5 h.

was between 60,000 and 200,000 times. Cross-sectional scanningelectron microscopy (SEM) images were also observed using a
Model JSM-6510 scanning electron microscope (JEOL) at an accelerating voltage of 20 kV. A tungsten filament was used in the electron gun. The photocatalyst film on a Pyrex glass plate was cut andmounted on the Al sample holder by an adhesive. The incident
angle of electrons with reference to the normal of the sample surface was between 75° and 85°. The magnification was between 200
and 3000 times.
Palladium K-edge XAFS spectra were measured at 290 K intransmission mode in the Photon Factory Advanced Ring at theHigh Energy Accelerator Research Organization (KEK, Tsukuba)
on beamline NW10A [23,28,29]. The storage ring energy was
6.5 GeV and the ring current was 52.1–20.7 mA. A Si (3 1 1)
double-crystal monochromator and a Pt-coated cylindrical focusing mirror were inserted into the X-ray beam path. The X-ray
intensity was maintained at 67% of the maximum flux using a
piezo translator applied to the crystal to suppress higher harmonics. The slit opening was 1.0 mm (vertical)  2.0 mm (horizontal)
in front of the I0 ionization chamber. Samples (10 mg of film)
mounted on Pyrex glass plates for photoconversion tests in
0.12 MPa CO2 and 0.28 MPa H2 were measured at 290 K in the fluorescence mode using a Lytle detector and a 3 lm thick RuO2 filter
between the sample and the If ion chamber [30–32]. The Pd K-edge
absorption energy was calibrated to 24,348 eV using the spectrumof 12.5 lm thick Pd foil [25].
Titanium K-edge XAFS spectra were measured in transmission
mode in the Photon Factory at KEK on beamline 9A [33,34]. The
storage ring energy was 2.5 GeV and the ring current was 447.7–
342.7 mA. A Si (1 1 1) double-crystal monochromator and a pairof bent conical mirrors were inserted into the X-ray beam path.
Spectra for the Pd/TiO2 sample (10 mg) diluted by boron nitride
were measured under argon. The Ti K-edge absorption energy
was calibrated to 4964.5 eV using the spectrum of 5 lm thick Ti
foil [25].
XAFS data were analyzed using the XDAP software package[35]. The pre-edge background was approximated by a modified
Victoreen function C2/E2 + C1/E + C0. The background of the postedge oscillation was approximated by a smoothing spline function
and was calculated for the particular number of data points, using
Data
PointsXðlxi  BGi Þ2

i¼1

expð0:075ki Þ

2

6 smoothing factor;

where k is the angular wavenumber of the photoelectrons.

ð7Þ

Multiple-shell curve-fit analyses were performed [23,28,29,33]
for the Fourier-filtered k3-weighted extended XAFS (EXAFS) datain k- and R-space using empirical amplitudes extracted from the
EXAFS data for Pd foil, PdO powder, and rutile-type TiO2 powder.
The interatomic distance (R) and its associated coordination number (N) for the Pd–Pd, Pd–O, Ti–O, and Ti(–O–)Ti pairs were set to0.27509 nm with an N value of 12 [36], 0.2026 nm with an N valueof 4 [37], 0.1959 nm with an N value of 6, and 0.3058 nm with an N
value of 12 [33,38,39], respectively. The many-body reduction factor S20 was assumed to be equal for the sample and the reference.
The number of independent data points in the fit range was calculated based on the Nyquist theorem and the EXAFS-specificmodification by Stern [35],

m¼

2DkDR

p

þ 2:

ð8Þ

The number was 10.8–13.1 for Pd K-edge EXAFS data and 14.4–15.8
for Ti K-edge EXAFS data. Three-shell fits were also tried for Ti
EXAFS; however, the fit errors were significantly greater than thoseobtained in two-shell fits. Thus, two-shell fits (four variables in ashell) were chosen for both Pd and Ti K-edge EXAFS analyses.
2.3. High-pressure photoconversion tests of CO2
The synthesized/prepared catalysts (Table 1a–f) were immersed
in deionized water and agitated by ultrasound (430 W, 38 kHz) for
3 min. The suspension was mounted on a Pyrex glass plate(25  25  1 mm) and dried at 373 K for 12 h. The area of theobtained films was 20  20 mm. Films on plates or assynthesized fine powder samples in a Pyrex dish (Uinternal = 37mm) were introduced into a homemade high-pressure stainlesssteel reactor equipped with quartz double windows, a pressuregauge, and Swagelok valves (Fig. 1A, B) for photoconversion tests.
The effective internal volume of the reactor was 98.4 mL.
A mixed CO2/H2 gas (0.20–0.80 MPa in a ratio of 2:8, 3:7, 4:6, or
5:5) was introduced into the reactor. The photocatalyst was irradiated with UV–visible light provided by a 500-W Xe arc lamp
(Model OPM2-502, Ushio) through quartz windows in the reactor
chamber for 5 h (Fig. 1A, B). The distance between the light exit
and the photocatalyst surface was 82.7 mm. Light transmission
was checked using a photosensor and a counter (Model PCM-01,
Prede and Model KADEC-UP, North One, respectively). The lightintensity at the center of the photocatalyst was 90.2 mW cm2
(Fig. 2a). The quartz windows of the high-pressure reactor
absorbed/reflected 9.5% of the light (Fig. 2b), and 100 mg of Pd/

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S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52

After 5 h of irradiation, the reaction gas was analyzed using
packed columns of 13X-S molecular sieves and polyethylene glycol(PEG-6000) supported on Flusin P (GL Sciences) set in a gas chromatograph equipped with a thermal conductivity detector (Model
GC-8A, Shimadzu) [6–8,28,42]. Helium (>99.99995%) was used as acarrier gas for all analytes except H2, for which argon (>99.998%)
was used. The amounts of methanol and moisture were doublechecked by concentrating them using a trap made from a mixtureof diethyl ether and dry ice (192 K) to separate them from H2 andmost of the CO2.
The experimental errors of 4 Pa of methanol or moisture were
evaluated in this GC analysis procedure. The errors were 7.7%
and 8.4%, but increased to 41% and 26% by the adsorption of
methanol or moisture onto the surface of the Pyrex glass tube
and vacuum grease (Apiezon H). The gas adsorption could be minimized by completely heating the vacuum system using a dryer
just before the online GC analysis.
Blank tests were also performed by eliminating one of the threecontrol factors or adding one alternative control factor: (i) UV–visible light, (ii) reactants, (iii) catalyst, and (iv) temperature, i.e., in theabsence of any light at 298 or 353 K, in 0.12 MPa of CO2 only, in
0.28 MPa of H2 only, or 0.4 MPa of helium (the absence of any reactants) and in the absence of any catalyst.
Fig. 1. (A, B) High-pressure stainless reactor equipped with quartz windows and apressure gauge for CO2 photoconversion tests irradiated by UV–visible light from a
500-W Xe arc lamp (A: top view, B: side view). (C) Photocatalyst films(20  20 mm) of Ag/ZrO2, g-C3N4, BiOCl, Pd/TiO2, Cu/TiO2, and Ag/Zn3Ga-LDH (topto bottom).

TiO2 powder on a Pyrex dish and 10 mg of Pd/TiO2 film on Pyrex
glass plate absorbed/reflected/scattered 40% and 49% of the light,
respectively (Fig. 2c, d). Densely mounted films were advantageous
for light absorption efficiency compared with fine powders basedon the difference of their mean areal densities of 2.5 versus
9.3 mg cm2, respectively, and the exposed ratio of particles. Fine
powders should reflect/scatter more at exposed faces and less lightreach at the lower part of layers (Fig. 2c) compared with densely
mounted films (Fig. 2d).
As comparisons, photocatalytic tests were also performed using
an L42 filter (2.5 mm thick, Kenko) that filters the light less thank = 420 nm [40] or a U-330 filter (2.5 mm thick, Kenko) that filters
the light less than k = 250 nm and between k = 390 nm and 695 nm[41], set between the light exit and the high-pressure reactor(Fig. 1A, B).

3. Results
3.1. Characterization by XRD and UV–visible spectroscopy
The syntheses of g-C3N4, BiOCl, and Zn3Ga-LDH and the preparation of Ag/ZrO2, Pd/TiO2, Cu/TiO2, and Ag/Zn3Ga-LDH were verified by their XRD patterns (Fig. 3) and UV–visible spectra (Fig. 4).
For Ag/ZrO2 (Fig. 3a), the XRD peaks appeared at 2hB = 17.5°,
24.3°, 28.3°, 31.5°, 34.3°, 35.3°, 38.6°, 41.0°, 45.1°, 50.3°, 54.2°,
and 55.6° and were ascribed to 0 0 1, 0 1 1, 1 1 1, 1 1 1, 0 2 0, 0 0 2,
1 2 0, 1 1 2, 2 0 2, 0 2 2, 0 0 3, and 3 1 0 reflections of monoclinic
ZrO2 [43,44]. No peaks due to Ag metal or Ag2O nanoparticles wereobserved [28,29].
Broad peaks at 2hB = 13.3° and 27.5° and a weak peak at
2hB = 44° in pattern b appeared essentially at the same position
as those reported for g-C3N4 in the literature [22,45] and wereassigned to 1 0 0, 0 0 2, and 2 0 0 reflections, respectively. The peak
positions and relative intensities for peaks at 12.0°, 26.0°, 32.7°,
33.6°, 41.0°, 46.8°, 49.8°, 54.3°, and 58.8° in pattern c were in

Fig. 2. The light intensity at the photocatalyst position in the high-pressure stainless-steel reactor (Fig. 1) (a), in the presence of the quartz windows of the reactor (b), in thepresence of the quartz windows and 100 mg of Pd/TiO2 powder (c), and in the presence of the quartz windows and 10 mg of Pd/TiO2 film (d).

S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52

43

peak centered at 428 nm appeared due to the surface plasmon resonance (SPR) of the Ag nanoparticles (Fig. 4a). An SPR peak at
411 nm was reported for Ag particles with a mean size of 1.6 nm
on LDH [28].
The absorption edge shifted from the UV to the visible lightregion (435 nm) for g-C3N4 (Fig. 4b), corresponding to an Eg valueof 2.9 eV (Table 1b). The absorption edge in the UV region was
extrapolated to 344 nm, corresponding to an Eg value of 3.6 eV
for BiOCl, in agreement with the literature value (Table 1c andFig. 4c) [23].
The absorption edges were extrapolated to 394 and 384 nm,
corresponding to Eg values of 3.1 and 3.2 eV for Pd/TiO2 and Cu/
TiO2 (spectra d and e), respectively, in agreement with the literature for TiO2 [33]. Nearly flat background absorption extended overthe entire range of visible light due to various sizes of Pd nanoparticles, and a weak peak appeared at approximately 440 nm due to
SPR effects arising from the Cu.
3.2. TEM and cross-sectional SEM observations
Fig. 3. XRD patterns for Ag/ZrO2 (a), g-C3N4 (b), BiOCl (c), Pd/TiO2 (d), and Ag/
Zn3Ga-LDH (e).

TEM images were observed for the Pd/TiO2 photocatalyst. For
the fresh sample, polygonal particles of TiO2 were observed withsizes distributed between 30 and 50 nm (Fig. 5A). Smaller particles
that are clearly darker than the TiO2 were identified as Pd species
and their mean size was 3.1 ± 0.9 nm (Fig. 5A, histogram andTable 1d). The wavelength of the SPR peaks progressively
decreased as the metal particle size decreased due to the change
in the oscillation frequency of confined free electrons [48]. The size
distribution of the Pd species was between 1.0 and 6.0 nm, leading
to SPR peaks at various wavelengths with broad absorbance in the
wavelength range 400–800 nm (Fig. 4d).
TEM images were also observed for Pd/TiO2 samples after 5 h of
CO2 photoconversion testing in CO2 (0.09 MPa) and H2 (0.21 MPa).
The particle size distribution of Pd species slightly widened to 1–
9 nm and the mean size increased by 0.8 nm to 3.9 ± 1.3 nm(Fig. 5B, histogram and Table 1d).
SEM images revealed that the surface of fresh Pd/TiO2 photocatalyst films coated on Pyrex glass was quite flat and smooth(Fig. 5C1). Cross-sectional views were also observed (Fig. 5C2)
and the mean thickness of the photocatalyst films was 11 lm.
The mean particle size of the TiO2 was estimated to be 32 nm,
based on analysis of the 1 0 1 XRD peak of the anatase phase using
Eq. (5), which is in agreement with the size range 30–50 nm
observed by TEM (Fig. 5A).

Fig. 4. UV–visible spectra for fresh samples of Ag/ZrO2 (a), g-C3N4 (b), BiOCl (c), Pd/
TiO2 (d), Cu/TiO2 (e), and Ag/Zn3Ga-LDH (f).

3.3. Monitoring the active sites using Pd and Ti K-edge XAFS

accord with the literature reports for BiOCl and were assigned to
0 0 1, 1 0 1, 1 1 0, 1 0 2, 1 1 2, 2 0 0, 1 1 3, 2 0 2, and 2 1 2 reflections,
respectively, of the tetragonal layered structure [23].
In the pattern for Pd/TiO2 (Fig. 3d), peaks appeared at
2hB = 25.4°, 37.0°, 38.6°, 48.1°, 54.0°, and 55.1° and were assigned
to the 1 0 1, 1 0 3, 1 1 2, 2 0 0, 1 0 5, and 2 1 1 reflections, respectively,of the anatase phase, and the peaks that appeared at 2hB = 27.5°,
36.1°, 37.9°, 41.3°, 44.1°, and 56.7° were assigned to the 1 1 0,
1 0 1, 0 0 4, 1 1 1, 2 1 0, and 2 2 0 reflections, respectively, of the rutile
phase [46,47]. No peaks arising from Pd nanoparticles wereobserved above the detection limit.
As for pattern e for Ag/Zn3Ga-LDH, peaks appeared at
2hB = 11.7°, 23.5°, 33.5°, 34.2°, 36.9°, 38.9°, 43.6°, 46.4°, and 52.6°
and were assigned to the 0 0 3, 0 0 6, 0 0 9, 0 1 2, 1 0 4, 0 1 5, 0 1 7,
0 1 8, and 1 0 0 reflections of Zn3Ga-LDH, respectively, in accord
with the literature [7,8].
For Ag/ZrO2, the UV absorption edge was extrapolated to
242 nm, corresponding to an Eg value of 5.1 eV (Table 1a). A broadFirst, Pd K-edge XAFS spectra were measured for fresh Pd/TiO2(100 mg) and Pd/TiO2 after testing in CO2 (2.3 kPa) + H2 (22 kPa)
under UV–visible light irradiation for 5 h. The photocatalytic test
conditions were conducted at lower than atmospheric pressure[6–8,28]. The X-ray absorption near-edge structure (XANES) spectra are depicted in Fig. 6A–c and d, respectively. The energy values
at the tops and the bottoms of the post-edge oscillation for normalized oscillation lt coincided with those for Pd metal (Fig. 6A-b);
however, the pattern was quite different from that of PdO(Fig. 6A-a), demonstrating the dominant metallic Pd0 state for fresh
Pd/TiO2 samples and for those after CO2 photoconversion tests at
24 kPa. The amplitude of the post-edge oscillation was apparently
lower for Pd/TiO2 (Fig. 6A-c, d) than that for Pd foil (spectrum b),
indicating a Pd particle size of a few nanometers, as already shown
by TEM (Fig. 5A, B).
The best-fit results and fit errors of the Pd K-edge EXAFS for Pd/
TiO2 photocatalysts are summarized in Table 2. The data were wellfitted with two shells: Pd–O and Pd–Pd for fresh Pd/TiO2 (Fig. 6Bc3, c4, and Table 2-c). The coordination of the Pd–Pd interatomic

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S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52

Fig. 5. TEM images of Pd/TiO2 as prepared (A) and after CO2 photoreduction tests in CO2 (0.09 MPa) and H2 (0.21 MPa) (B), and cross-sectional SEM images of fresh Pd/TiO2(C1, C2).

pair at 0.276 nm (Fig. 6B-c2) was predominant (N = 5.9), which isconsistent with the assignment of the predominant Pd0 based on
the XANES data above (Fig. 6A–c). The minor coordination of Pd–
O (N = 1.9) arises from the interfacial bonding between metallic
Pd nanoparticles and the TiO2 surface [49–53]. The N value for
Pd–Pd (5.9) corresponds to a mean nanoparticle size of
1.1 ± 0.1 nm (Table 1d) [53]. The discrepancy compared with themean particle size based on TEM (3.1 ± 0.9 nm) is found because
particles smaller than 1 nm cannot be observed by TEM.
For the samples measured after undergoing photoconversion
under 24 kPa and light irradiation (see Photoconversion in CO2 andH2 at 0.20–0.80 MPa), the major Pd–Pd interatomic distance wasalso 0.276 nm (Table 2-d and Fig. 6B-d2, d4). The N value slightly
increased by 3.0 relative to the fresh sample (Table 2-c, d). Thisvalue corresponds to the mean particle size of 2.3 ± 0.1 nm, whichis 1.2 nm larger than for the fresh sample [53]. The particle growth
based on EXAFS was consistent with the corresponding value(0.8 nm) derived from TEM (Table 1d). The Pd–O interatomic distance increased by 0.01–0.21 nm and the N value decreased significantly from 1.9 to 0.57. Presumably, the reason for this is that the
number of OV sites increased at the interface between Pd and theTiO2 surface and/or the relative number of interface Pd sites in contact with the TiO2 surface decreased due to partial growth of the Pd
particles by 1 nm [49–53].
After 0.40 MPa of CO2 photoconversion testing using 100 mg of
Pd/TiO2, the peak energy and intensity in XANES were quite similar

to those of fresh samples (Fig. 6A–c, e). The N value of the Pd–Pdpair was 5.0 and did not significantly change from 5.9 for the fresh
sample based on the fit errors (Table 2-c, e and Fig. 6B-e3, e4),
whereas the N value for the Pd–O pair significantly decreased from
1.9 to 0.9 (Table 2-c, e). Thus, the increase in OV sites for a nearly
constant mean size of Pd particles during the photoconversion test
was suggested after the photoconversion test at 0.40 MPa. The reason that the mean Pd particle size increased only under lower pressure conditions but remained unchanged under higher pressure
conditions (Table 2-d, e) is unknown; however, the strong adsorption of H on Pd [54] may have prevented the coalescence of Pd
nanoparticles. The XANES spectrum (Fig. 6A–e) resembled that offresh Pd/TiO2 (c), supporting the interpretation based on EXAFS.
The XANES spectrum taken for 10 mg of Pd/TiO2 film (Fig. 1C)
after a photoconversion test at 0.40 MPa (Fig. 6A–e0 ) was quite similar to the spectrum in Fig. 6A–c for a fresh sample. In summary, Pd
sites in 10–100 mg of Pd/TiO2 photocatalyst remained Pd0 sites
before and after photoreduction tests in CO2 and H2 over a pressure
range from 24 kPa to 0.40 MPa.
To compliment Pd K-edge XAFS, Ti K-edge EXAFS was measured
for 10 mg of Pd/TiO2 photocatalyst after the photoconversion test
at 0.40 MPa. The N(Ti–O) and N(Ti(–O–)Ti) values were 6.1 and
11 (Table 3-e0 and Fig. 6C–e0 3, e0 4), similar to the values of 6 and
12 for the untreated rutile or anatase phase TiO2, respectively[33,36,37]. In contrast, the decrease in OV sites at the interface
between Pd and TiO2 surface sites under these conditions was sig-

S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52

45

Fig. 6. Pd K-edge XANES (A) and EXAFS (B) and Ti K-edge EXAFS (C) for PdO (a), Pd foil (thickness 12.5 lm; b), fresh Pd/TiO2 (c), and Pd/TiO2 after 5 h photocatalytic testing in
CO2 (2.3 kPa) and H2 (22 kPa) using 100 mg of Pd/TiO2 (d) and in CO2 (0.12 MPa) and H2 (0.28 MPa) using 100 mg (e) or 10 mg of Pd/TiO2 (e0 ). Spectra were measured in thetransmission mode except that the Pd K-edge of sample e0 was taken in the fluorescence mode. Panels in B and C: k3-weighted EXAFS oscillation (1), its associated Fouriertransform (2), and best-fit results in k-space (3) and R-space (4).

46

S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52

Table 2Best-fit results of Pd K-edge and EXAFS spectra for Pd/TiO2 photocatalysts.

a
b

Entry

Samples

Pd–O
R (nm)
N
D(r2) (105 nm2)

Pd–Pd
R (nm)
N
D(r2) (105 nm2)

Goodness of fit

c

Fresh

0.199 (±0.006)
1.9 (±0.4)
4.8 (±1.7)0.276 (±0.001)
5.9 (±0.5)
5.0 (±0.6)

1.2  103da

5-h photocatalytic test under CO2 (2.3 kPa) + H2 (22 kPa), 100 mg

0.21 (±0.04)
0.57 (±0.2)
7.9 (±1.7)0.276 (±0.002)
8.9 (±0.4)
3.3 (±0.3)

2.0  102eb

5-h photocatalytic test under CO2 (0.12 MPa) + H2 (0.28 MPa), 100 mg

0.2018 (±0.0004)
0.9 (±0.3)
6 (±2)0.2748 (±0.0003)
5.0 (±0.4)2.6 (±0.3)

1.1  103

After photocatalytic test listed in Table 4a.
After photocatalytic test listed in Table 4f.

Table 3Best-fit results of Ti K-edge EXAFS spectra for Pd/TiO2 photocatalysts.

a

Entry

Samples

Ti–O
R (nm)N
D(r2)(105 nm2)

Ti(–O–)Ti
R (nm)N
D(r2)(105 nm2)

Goodness of fit

e0 a

5-h photocatalytic test under CO2 (0.12 MPa) + H2 (0.28 MPa), 10 mg

0.1977 (±0.0002)
6.1 (±0.4)
1.6 (±0.4)0.313 (±0.002)
11 (±2)2 (±1)

9.9  104

After photocatalytic test listed in Table 4n.

nificant based on the N(Pd–O) decrease (Table 2-c, e) and should be
populated only at the interface with Pd nanoparticles after thephotoconversion test. The Ov sites were negligible in/on TiO2 apartfrom Pd nanoparticles.

rðH2 OÞ ¼ 2rðCH4 Þ þ rðCOÞ þ rðCH3 OHÞ

CO2 þ 4H2 ! CH4 þ 2H2 O

ð11Þ

3.4. Photoconversion in CO2 and H2 at 0.20–0.80 MPa

CO2 þ H2 ! CO þ H2 O

ð12Þ

CO2 photoreduction using H2 and 100 mg of Pd/TiO2 or Ag/Zn3Ga-LDH powder at a reaction pressure of 24 kPa [6–8] irradiatedwith UV–visible light produced methane, CO, and/or methanol,and the rates on a C basis were 0.14–1.0 lmol h1 g1cat(Table 4a, b). Table 4 lists normalized formation rates per photocatalyst weight for the practical comparisons to some of the literature,e.g., Refs. [10–12,14–17,55,56], but real formation rates in the unit
lmol h1 are also listed in Table S1 in the Supplementary Information for the comparisons with the other part of the literature
because photocatalyst weight had a critical influence on the photocatalytic activities in this section. If the reaction pressures were
elevated to 0.12 MPa of CO2 and 0.28 MPa of H2, the total rates
increased by a factor of 1.8–2.6 times (0.36–1.8 lmol h1 g1cat;
Table 4f, h). At a total reaction pressure of 0.40 MPa, the order of
the total photoformation rates using 100 mg of powder samples
on a C basis was as follows (Tables 4 and S1):CO2 þ 3H2 ! CH3 OH þ H2 O

ð13Þ

Cu=TiO2 ð2:8Þ > Pd=TiO2 ð1:8Þ >> Ag=ZrO2 ð0:64Þ
 g  C3 N4 ð0:53Þ > Ag=Zn3 Ga  LDHð0:36Þ
> BiOClð0:25Þ:ð9Þ

Ag/ZrO2, g-C3N4, and BiOCl mostly produced CO at formation rates
of 0.64, 0.53, and 0.15 lmol h1 g1cat, respectively (Table 4c–e),
whereas Pd/TiO2, Cu/TiO2, and Ag/Zn3Ga-LDH favored methane formation at rates of 1.7, 2.8, and 0.32 lmol h1 g1cat, respectively(Table 4f–h). A minor product was methanol, which was formed
at a rate of up to 0.003 lmol h1 g1
cat using these photocatalysts.
The theoretical formation rate of water, r(H2O), is given by theequationð10Þ

based on the stoichiometry of the equations

which account for the formation of methane, CO, and methanol.
For Ag/ZrO2, g-C3N4, Cu/TiO2, and Ag/Zn3Ga-LDH, the r(H2O)
values were calculated to be 0.64, 0.53, 5.6, and 0.68 lmolH2O h1 g1cat, which are in good agreement with the experimental
values of 0.64, 0.57, 6.3, and 0.65 lmol-H2O h1 g1cat, respectively(Table 4c, d, g, h) if compared with the maximum evaluation errorof H2O by GC (26%).
Conversely, the observed water formation rate using BiOCl(1.3 lmol h1 g1cat, Table 4e) was larger by a factor of 3.7 than ther(H2O) value based on Eq. (10) (0.35 lmol h1 g1cat), suggesting concurrent noncatalytic hydrogenation of reactive lattice O atoms at
the BiOCl surface [23,57]. Using Pd/TiO2, the observed water formation rate was 2.2 lmol h1 g1cat (Table 4f), which is a factor of
0.63 times lower than the r(H2O) value (3.5 lmol h1 g1cat) that
was a significant difference in comparison to maximum evaluation
error by GC (26%). Because Pd nanoparticles remained metallic andthe number of OV sites at the interface with the TiO2 surface
increased under the photocatalytic conditions (Table 2-e andFig. 6B–e), approximately 40% of O originating from CO2 should
be adsorbed onto the TiO2 surface away from Pd nanoparticles.
The quantity of missing O atoms based on the stoichiometry ofEqs. (11) and (12) was 0.65 lmol. The quantity of hydroxy groups
in the untreated Pd/TiO2 catalyst (7 species nm2) [58] amounted
to 69 lmol in 100 mg of Pd/TiO2 powder. These hydroxy groups
are plausible adsorption sites under the photocatalytic conditions
if 0.94% of them desorb as water, leaving behind OV sites during
the pretreatment of Pd/TiO2.

47

S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52

Table 4
Dependence of CO2 photoconversion on the amount of photocatalyst and reactant pressure, and control conversion results in the absence of UV–visible light, catalyst, or reactant.
Ent.

a
b
c
d
e
f
g
h
i
j
k
l
m
n
n0 b
n00 c
n0 00 dn00 00 e
n00 00 0
n00 00 00
n00 00 00 0 f
o
p
q
r
s
t
u
v
w
x
y

Photocatalyst

Reactants (MPa)Formation rates (lmol h1 g1
cat)

Type

Wt (mg)

CO2

H2

CO

CH3OH

CH4

H2O

O2

Pd/TiO2
Ag/Zn3Ga-LDH
Ag/ZrO2
g-C3N4BiOCl
Pd/TiO2
Cu/TiO2
Ag/Zn3Ga-LDH
Ag/ZrO2
g-C3N4BiOCl
Pd/TiO2100

0.0023

0.022
0.28

0.06
0.09
0.12

0.14
0.21
0.28

0
0.12
0
0.15
0.18
0.24
0.12
0.28
0.07
0.19
0.12

0.28
0
0
0.35
0.42
0.56
0.12
0.28
0.28
0.28
0.28

<0.0004
0.028
0.00210.0021<0.0004<0.0004
0.0030<0.0004<0.004<0.004<0.004<0.0040.18<0.0040.018
0.009<0.0040.016<0.0040.008<0.0040.17<0.004<0.004
0.013
0.020
0.14
0.08
0.056
0.027
0.10
<0.00004g

1.0<0.012<0.012<0.012
0.10
1.7
2.8
0.32
4.3
3.8
1.1
13
23
38
8.8
<0.12
3.0
1.3
<0.12
<0.12
<0.12
20
14
8.6
12
22
27
26
19
4.0
2.1
<0.0012g

5.0
–
0.64
0.57
1.3
2.2
6.3
0.65
6.5
4.1
37
38
43
68
11
7.2
5.1
9.8
11
19
8.2
28
12
9.6
25
58
44
39
33
56
5.3
<0.03g0

0.12<0.008
0.11
0.64
0.53
0.15
0.057
<0.008
0.044
<0.08
<0.08
1.9
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
<0.08
0.95
<0.0008g

Cu/TiO2
TiO2
Ag/Zn3Ga-LDH
No catalyst

10

0

P a
C

1.0
0.14
0.64
0.53
0.25
1.8
2.8
0.36
4.3
3.8
3.0
13
23
38
8.8
<0.21
3.1
1.3
<0.20
<0.20
<0.20
20
14
8.6
12
22
27
26
19
4.0
3.2
<0.0020g

a

Total molar amount of C-containing products.
Using U330 filter at the light exit (Fig. 1B).
Using L42 filter at the light exit (Fig. 1B).
d
In the absence of UV–visible light at 298 K.
e
In the absence of UV–visible light at 353 K.
f
In 0.40 MPa of helium.
g
In units of lmol h1.
bc

It seems contradictory that this effect of O uptake derived fromCO2 on OV sites on TiO2 was not confirmed using Cu/TiO2(Table 4g). One of the reasons for the difference is the different
final treatment temperature: 290 K under vacuum (Pd/TiO2) versus
673 K in air (Cu/TiO2) leading to fewer hydroxy groups for Cu/TiO2[58].
Next, at the same reaction pressure (0.40 MPa), the amount ofphotocatalyst was decreased to 10 mg. Using 10 mg of Pd/TiO2 film,
methane was exclusively formed at a rate of 38 lmol h1 g1cat,higher by a factor of 22 times than when 100 mg of Pd/TiO2 powder
was used (Table 4f, n). Samples of 10 mg of Ag/ZrO2, g-C3N4,
Cu/TiO2, and Ag/Zn3Ga-LDH films exclusively (Table 4i, j, v) or preferentially (Table 4x) formed methane at rates of 4.3, 3.8, 19, and
2.1 lmol h1 g1cat, factors of 6.7, 7.2, 6.8, and 8.9 times higher,
respectively, on a total C basis than when 100 mg of the corresponding catalyst was used (Table 4c, d, g, h). A quantity of 10 mg
of BiOCl film formed CO (64% selectivity) at a total formation rateof 3.0 lmol h1 g1cat (Table 4k), higher by a factor of 12 than when
100 mg of BiOCl was used (Table 4e).
The total photoformation rates on a C basis using 10 mg of photocatalyst at 0.40 MPa of CO2 and H2 (Table 4j, k, n, v, x andTable S1) were in the order

Pd=TiO2 ð38Þ > Cu=TiO2 ð19Þ  Ag=ZrO2 ð4:3Þ
 g  C3 N4 ð3:8Þ > Ag=Zn3 Ga  LDHð3:2Þ
 BiOClð3:0Þ:ð14Þ

Formation rates were boosted by a factor of 22–6.7 times when
10 mg of photocatalyst films was used; however, the activity order
did not change compared with Eq. (9) using 100 mg of catalyst,
except for a reversal of the order of the top two TiO2-based
catalysts.
One of the reasons for these rate increases was the improvement of light absorption efficiency for the photocatalyst film compared with that for powders: 49% versus 40% (Fig. 2c, d) due to thedifference in the exposed ratio of particles. As noted in Section 2.3,
the film of areal density of 2.5 mg cm2 absorbed more light compared to the powders of 9.3 mg cm2 due to the relatively greaterreflection/scattering by the powders. Furthermore, the activity
increase by a factor of 22–6.7 times using six kinds of photocatalysts suggested that the other control factor(s) also exist.
Mass transfer limitation was suggested for catalyst pellets
greater than 250 lm for the Fischer–Tropsch reaction [59]. A mean
0.7 lm-thick TiO2 catalyst film was not controlled by mass transfer
limitation for photocatalytic decomposition of formic acid [60]. No
mass transport effect was observed for 2–5 lm of Pt/Al2O3 catalyst
film for the CO preferential oxidation reaction [61]. Taking thelower reaction rates for photocatalytic CO2 conversion comparedwith these thermodynamically easier reactions [59–61], the masstransfer limitation seems negligible in the thin 10 mg films (thickness 11 lm, Fig. 5C2) and would not be significant in 100 mg of
powder (100 lm).
Instead, the anatase 1 0 1 and rutile 1 1 0 diffraction peaks,
especially the anatase 1 0 1 peak, were relatively weak in the

48

S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52

XRD pattern for 10 mg of Pd/TiO2 film in comparison to ones for
Pd/TiO2 powder (Figs. S1 and 3d and Table S2). The population ofthese planes in parallel with the sample holder of the XRD apparatus would be smaller and the other plane(s) favorable for CO2 photoreduction may be preferably exposed on Pd/TiO2 film (seeSupplementary Material).
Using 10 mg of Ag/ZrO2, g-C3N4, BiOCl, Pd/TiO2, Cu/TiO2, and Ag/
Zn3Ga-LDH, r(H2O), formation rates were predicted to be 8.6, 7.6,
4.1, 76, 38, and 5.3 lmol h1 g1
cat based on Eq. (10), and experimental values were 6.5, 4.1, 37, 68, 33, and 5.3 lmol h1 g1cat, respectively (Table 4i–k, n, v, x). Thus, CO2 conversion using Ag/ZrO2,
Pd/TiO2, Cu/TiO2, and Ag/Zn3Ga-LDH almost/exactly followed thestoichiometry of Eqs. (11-13) because the deviation between predicted and experimental values was 11–24% smaller than the maximum evaluation error for 4 Pa of water using GC (see Section 2).
This result is consistent with the exclusive Pd0 sites of Pd/TiO2(10 mg) before and after the photoreduction tests in CO2 and H2
at 0.40 MPa (Fig. 6A–e0 ).
Using Pd/TiO2, the amount of methane formed at 0.40 MPa in
5 h was 1.9 lmol (Table 4n), whereas the number of surface Pd
atoms in 10 mg of Pd/TiO2 catalyst was 0.19 lmol if we assume
the Pd dispersion is 0.40 for a mean nanoparticle size of 3.1 nm(Table 1d) [62], indicating a turnover of 10 and thus demonstrating
catalytic phenomena.
Using g-C3N4, the observed water formation rate was 0.54 times
lower than the calculated r(H2O) value; thus, approximately 50% of
O originating from CO2 would remain, or formed water readsorbedon the g-C3N4 surface. The water formation rate using BiOCl was afactor of 9.0 times higher than the r(H2O) value, suggesting concurrent noncatalytic hydrogenation of reactive lattice O atoms at the
BiOCl surface [23,57].
The possibility of reaction of CO2 with the product H2O was also
considered. At the end of the photocatalytic test under 0.12 MPa of
CO2 + 0.28 MPa of H2 for 5 h using the most active Pd/TiO2, theamount of formed water was 68 lmol h1 g1cat  5 h 
0.010 gcat = 3.4 lmol (Table 4n). In our preliminary photocatalytic
test under 0.12 MPa of CO2 + 2.3 kPa (94 lmol) of moisture using
the same catalyst (10 mg), the predominant methane formation rate
was 14 lmol h1 g1cat [21]. If the reaction for methane formation isassumed to be first-order in moisture, the rate under 0.12 MPa of
CO2 + 3.4 lmol of moisture would be 0.51 lmol h1 g1
cat at the final
stage of the photocatalytic test under CO2 + H2, which is negligible
compared with the observed methane formation rate 38 lmol h1g1cat (Table 4n). Thus, the contribution of the reaction with the
product H2O was negligible in the photocatalytic tests under highpressure CO2 + H2.
Wavelength dependence of irradiation [6,28,63–65] for the Pd/
TiO2 photocatalyst was also studied using filters between the lightsource and the high-pressure reactor (Fig. 1A, B). Using a filter ofU330 that cuts visible light and part of infrared light, the methane
formation rate was decreased to 23% of that under full irradiationfrom the Xe arc lamp (Table 4n0 ). Conversely, using a filter of L42
that totally cuts UV light, methane was not detected above thedetection limit of GC but methanol was formed very slowly00(0.009 lmol h1 g1cat; Table 4n ). Thus, the methane formation rate
under UV–visible light (Table 4n) was not the summation of that
under UV light and that under visible light, but a synergetic effectof UV and visible light was suggested.
Furthermore, the dependence of reaction rates on total pressure
was studied using the most active Pd/TiO2 film in Eq. (14) at pressures in the range 0.20–0.80 MPa (Table 4l–q). The product was
exclusively methane (Fig. 7). The reaction order of methane formation was 1.53, as determined by a log–log plot in the pressure
range 0–0.40 MPa. Methane formation rates reached a maximumof 38 lmol h1 g1
cat at 0.40 MPa and then suddenly dropped above

0.40 MPa and finally decreased to 8.6 lmol h1 g1
cat at 0.80 MPa(Table 4n, q and Fig. 7).
At total reactant pressures of 0.20, 0.30, 0.40, 0.50, 0.60, and0.80 MPa, r(H2O) values based on Eq. (10) were 26, 46, 76, 40, 28,
and 17 lmol h1 g1cat, and the observed formation rates were 38,
43, 68, 28, 12, and 9.6 lmol h1 g1cat, respectively (Table 4l–q).
Thus, the photocatalytic tests nicely followed the stoichiometry
at total reaction pressures of 0.30 and 0.40 MPa. Based on Pd Kedge and Ti K-edge XAFS results at 0.40 MPa of CO2 and H2 for
Pd/TiO2, Pd nanoparticles remained metallic and OV sites were only
plausible at the interface between Pd and the TiO2 surface, butwere not widely distributed over the TiO2 surface (Fig. 6A–e, e0 ,B-e, C-e0 and Tables 2-e and 3-e0 ). Hence, at 0.20, 0.50, 0.60, and0.80 MPa, approximately 32%, 30%, 57%, and 44% of O, respectively,
originating from CO2 would be adsorbed onto the TiO2 surface
away from Pd nanoparticles. The number of O atoms lacking basedon the stoichiometry of Eqs. (11) and (13) was 0.37–0.80 lmol
after the photoconversion tests at 0.50–0.80 MPa. The hydroxygroups of the untreated Pd/TiO2 catalyst (7 species nm2) [58] corresponded to 6.9 lmol in 10 mg of Pd/TiO2 film and are plausible
adsorption sites under the photocatalytic conditions if 5.4–12% ofhydroxy groups desorb as water to leave OV sites during the pretreatment of Pd/TiO2. The adsorption of O originating from CO2
on the TiO2 surface would be more pronounced at relatively lower
photocatalytic activity at 0.20, 0.50, 0.60, and 0.80 MPa.
Next, the pressure dependence was extended to create a twodimensional contour plot of total C formation rates, which were
essentially equivalent to the methane formation rates, as a function of H2 and CO2 partial pressures (Table 4l-s and Fig. 8). The
methane formation curve in Fig. 7 corresponds to a straight line
following P CO2 ¼ ð3=7ÞP H2 in Fig. 8. Photocatalytic tests were also
performed on three straight lines of P CO2 ¼ P H2 , P H2 = 0.28 MPa,and PCO2 = 0.12 MPa (Table 4r, s; Table 4n, n00 00 , s, t, u; Table 4n, n0000 0 ,r, respectively). The exclusive methane formation rates primarily
depended on P H2 ; the rates increased until P H2 reached 0.28 MPa
and then gradually dropped until PH2 increased to 0.56 MPa,
whereas they depended negligibly on P CO2 . However, within thezone P H2 = 0.20–0.35 MPa, the rates critically increased until a
PCO2 of 0.12 MPa was reached and then gradually decreased until
PCO2 increased to 0.28 MPa (Fig. 8). In this addition of data underboth CO2 and H2 (Table 4r–u), predicted and observed amountsof water were in agreement within the maximum evaluation error(26%).
Blank tests were performed by controlling four factors (see Section 2) to compare with the photoconversion tests, as listed inTable 4n using 10 mg of Pd/TiO2. In the absence of reactants or in
the absence of catalyst (Table 4n0000 000 , y), no carbon-containing
products were detected after 5 h of irradiation under UV–visible
light. Water formation at a rate of 8.2 lmol h1 g1
cat in the absenceof reactants was merely a desorption from Pd/TiO2 that was inadsorption/desorption equilibrium with adsorbed water molecules, as confirmed by FTIR [21].
In the presence of 0.28 MPa of H2 only, no C-containing products were detected above he detection limit (Table 4n0000 0 and
Fig. 8), demonstrating that CO2 was the only C source in this study.
Conversely, in the presence of 0.12 kPa of CO2 only, methanol was
00 0000
produced very slowly (0.008 lmol h1 g1and Fig. 8).
cat; Table 4n
As the rate was negligible compared with the methane formation
rate in the presence of CO2 + H2 (38 lmol h1 g1cat), hydrogen was
the predominant reductant for CO2 compared with adsorbed wateror surface hydroxy in this study.
In the absence of light at 298 K (the initial temperature 290 K in
the cell increased to 298 K after 10 min of UV–visible light irradiation; Table 4n00 0 ), thermal methane formation proceeded as in Eq.
(11), but it was only 7.9% of that under UV–visible light irradiation

S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52

49

Pd or Cu on TiO2 >> Ag=ZrO2  g  C3 N4> Ag=Zn3 Ga  LDH  BiOCl:

Fig. 7. The dependence of formation rates for CH4, CO, methanol, and water on total
reaction pressure using a Pd/TiO2 photocatalyst, where P CO2 =P H2 ¼ 3=7.

Fig. 8. Contour diagram for the total photoformation rates of C-containing products
from CO2 and H2 using a Pd/TiO2 photocatalyst as a function of the partial pressuresof H2 and CO2.

(Table 4n), demonstrating that most of the CO2 conversion from
CO2 and H2 in Table 4 proceeded photocatalytically.
To verify the thermal effects, blank tests in the absence of light
were also carried out at 353 K (Table 4n00 00 ). The methane formation
rate decreased by a factor of 0.43 times compared with that at 298 K.
Moreover, 73% of the water formed among 9.8 lmol h1 g1cat(Table 4n0000 ) was not stoichiometric according to Eq. (11), but was
merely simple desorption of adsorbed water from the TiO2 surface
at 353 K.
4. Discussion
4.1. Energetics of the photoconversion of CO2 and H2
The dependence of CO2 photoconversion using H2 on the typeand amount of photocatalyst and the reaction pressure was studied. For 10–100 mg of photocatalyst in 0.40 MPa of CO2 and H2,
the activity order Eqs. (9) and (14) for formation of all Ccontaining compounds (methane, CO, and methanol) can be summarized as

ð15ÞThis order was not correlated to the Eg values of the metal oxide orhydroxide semiconductors (3.1–3.2, 5.1, 2.9, 5.2, and 3.6 eV; Table 1)
and was almost in the opposite direction to the CB minimum potentials for the semiconductors (0.1, 1.0, 1.3, 1.3, and 0.6 V at
pH 0) [8,23,33,45] because more negative voltages are advantageous for CO2 reduction based on equilibrium control (Scheme 1B).
Thus, the (excited) energy level of metal nanoparticles (Pd, Cu, orAg) for the electron trap and the efficiency of electron injection into
CO2-derived species were the major factors that determined thereaction order of Eq. (15), as discussed below.
Electron traps that decrease the recombination of photogenerated charges were proposed for metal-doped TiO2 [11]. The work
functions (WFs) for Pd, Cu, and Ag are 5.22–5.6, 4.48–5.10, and
4.52–4.74 eV versus the vacuum level [66], corresponding topotentials of 0.78–1.16, 0.04–0.66, and 0.08–0.30 V versus SHE(pH 0), respectively, which are slightly positive compared with
the CB of TiO2 (0.1 V; Scheme 1B). The OV sites for BiOCl arereported to exist at 0 V at pH 2 [23,67]. Thus, the potentials forelectron trapping levels for Pd/TiO2, Cu/TiO2, Ag/ZrO2, g-C3N4, Ag/
Zn3Ga-LDH, and BiOCl are 0.78–1.16, 0.04–0.66, 0.08–0.30, 1.3,
0.08–0.30, and 0 V, respectively. Except for g-C3N4, the electron
trapping level was very close to or a little more positive than the
reduction potentials of methane (0.17 V at pH 0) or CO (0.30 or0.12 V at pH 0) [14,68]. In general, the electron trapping level
became progressively more positive as the CO2 photoformation
rates increased (Eq. (15)). Thus, the probability of direct electrontransfer from the trapping levels to CO2-derived species seems low.
Xie et al. reported a similar correlation between the potential ofelectron trapping sites and the photoformation rates of CO2 tomethane and CO using noble-metal-supported TiO2, which followed the order of the metal WFs: Pt > Pd > Au > Rh > Ag [11].
Thus, we propose that the electron transfer to CO2-derived species
was a combination of excited electron traps at Pd from TiO2 followed by transfer of hot/excited electrons produced by SPR to
CO2-derived species (Scheme 1B). Thus the proposed reaction
mechanism in the energetic diagram was in accord with the wavelength dependence of irradiation for Pd/TiO2 (Table 4n, n0 , n00 ). The
summation of rates under UV light and under visible light did notaccount for the rate under UV–visible light, implying a synergetic
effect, e.g., of UV for the band-gap excitation of TiO2 and of visible
light for the hot electrons by SPR. The SPR peak(s) were centered at
3.1–1.5, 2.8, 2.9, and 3.1 eV for Pd/TiO2, Cu/TiO2, Ag/ZrO2, and Ag/
Zn3Ga-LDH (Table 1a, d, e, f), respectively. Thus, the levels of hot/excited electrons due to SPR were estimated to be 2.3 to 0.3,
2.8 to 2.1, 2.8 to 2.6, 1.3, 3.0 to 2.8, and 0 V for Pd/
TiO2, Cu/TiO2, Ag/ZrO2, g-C3N4, Ag/Zn3Ga-LDH, and BiOCl, respectively, if we assume that the Fermi level for metal nanoparticles
does not shift.
In summary, as the levels of hot/excited electrons became morenegative, the catalysts became more active, except for Ag/ZrO2 andAg/Zn3Ga-LDH, probably due to lower charge separation efficiency
for intrinsic semiconductor (ZrO2) [69] or hydroxide (Zn3Ga-LDH)[70,71]. Essentially, the hot/excited electrons could be transferred
easily to CO2-derived species based on the energy difference, andthen the positive charge remaining in the metal nanoparticles
recombined with electrons that diffused from the semiconductors(Scheme 1B). However, hot/excited electron transfer by SPR was
not enough to complete the reactions (11)–(13), as confirmed byquite poor activity in the photocatalytic test irradiated by light ofk > 420 nm (Table 4n00 ). Thus, oxidation site of H2 to protons should
be on TiO2, close to the interface with Pd, but not on Pd(Scheme 1A).

50

S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52

Scheme 1. Proposed reaction mechanism of CO2 photoconversion using H2 (A) andthe corresponding energetic diagram (B; pH 0).

Ag/Zn3Ga-LDH and Cu phthalocyanine(CuPc)/Zn3Ga-LDH alsocollected electrons due to SPR of Ag [28] or HOMO–LUMO excitation of the CuPc and BG excitation of the LDH [6,28]. Zhang et al.
proposed a similar electron transfer to CO2 using TiO2 nanofibers
doped with Au/Pt [72]. The efficiency of trapping electrons in metal
nanoparticles that had diffused from the semiconductor was suggested to control the activity order in Eq. (15). Wang et al. proposed a similar CO2 photoconversion mechanism using small Pt
nanoparticles on approximately 2 lm of single-crystal TiO2 film
prepared by metal sputtering for 5–60 s [55]. Ultimately, small Pt
nanoparticles approach the WF level of 2.128 V for Pt1 and would
reduce CO2 directly. On the other hand, similar electron transfersfrom CB or OV sites to CO2 resulting in methane and/or CO formation were already reported for g-C3N4 [73] and BiOCl [53,74].
The total formation rates of all C-containing compounds using
100 mg of photocatalyst powder (0.28–0.025 lmol-C h1;
Table 4c–h) were clearly lower than those using 10 mg of photocatalyst film (0.38–0.030 lmol-C h1; Table 4i–k, n, v, w). Plausible
reasons for this are more effective absorption of UV–visible light incomparison to reflectance/diffraction (Fig. 2c, d) and possibly
preferable orientation of TiO2 crystallites for the 10 mg films compared with the 100 mg powders.
In summary, photoconversion rates of CO2 were controlled by
the energy of hot/excited electrons produced by SPR in metals,
the CB minimum of g-C3N4, and OV sites in BiOCl. Cu or Pd on
TiO2, especially as 10 mg films, was the most effective for photoconversion of CO2 and H2.
4.2. Reaction mechanism for photoconversion of CO2 and H2 using themost active Pd/TiO2 catalyst
The dependence of CO2 conversion rates for the most active Pd/
TiO2 photocatalyst on the partial pressures of CO2 and H2 wasspecific; the top of the volcano was searched for by following themethane formation rates along four straight lines following
PCO2 ¼ ð3=7ÞP H2 , P CO2 ¼ P H2 , PH2 = 0.28 MPa, and P CO2 = 0.12 MPa

and a steep volcano-like contour diagram could be drawn(Fig. 8). The rates reached a maximum at PCO2 = 0.12 MPa andPH2 = 0.28 MPa. In the zone P CO2 6 0.12 MPa and P H2 6 0.28 MPa,
the reaction order for methane formation was 1.53 and the dependence was primarily determined by PH2 , suggesting that the Pd surface was preferentially covered with H and the reaction ordershould be mostly dependent on PH2 . In the partial-pressure zone
at 0.30–0.40 MPa, the mass balance of Eq. (11) was preserved(Table 4m, n).
In the proposed mechanism (Scheme 1A), hydrogen dissociates
and is oxidized by holes at the interface between Pd and the TiO2
surface. The protons that form progressively combine with CO2derived species and electrons due to the SPR effects of Pd (Scheme 1B), finally forming methane and water. If the protons do not
encounter CO2-derived species, they can react with lattice oxygenat the interface between Pd and the TiO2 surface to form water andOV sites.
Conversely, in the zone P CO2 P 0.12 MPa and PH2 P 0.28 MPa,
exclusive methane formation rates progressively dropped (Fig. 8),
and deviations from the mass balance of Eq. (11) became more significant (Table 4o–q and Fig. 7). These trends were not due to thechange of Pd valence states. The Pd sites should be exclusively
metallic or even the OV sites at the interface with TiO2 surface
increased after the CO2 photoconversion tests, as confirmed by
Pd K-edge XAFS (Table 2A–d, e and Fig. 6A-d, e, e0 and B-d, e).
One of the reasons for the decrease in activity in this pressure zone
is that the Pd surface was predominantly covered with H, inhibiting the approach of CO2 (Scheme 1A). This difference is supportedby the difference in binding energies of CO2 (0.33 eV) and H(2.95 eV) on the Pd (1 1 1) surface [54]. Once the adsorption of
CO2 onto Pd sites became difficult, the conversion of CO2 tomethane and O2 decreased (Fig. 7), and intermediates and/or water
would remain on the TiO2 surface, e.g., on OV sites formed during
pretreatment.
In the proposed reaction mechanism, the reaction of CO2derived species, protons, and SPR electrons was plausible, but wesuspect that the thermally dissociated H species from H2 on thePd surface at 298 K would also combine with CO2-derived species.
In fact, 7.9% of the methane formed under UV–visible illumination(Table 4n) could be formed under 0.40 MPa of CO2 and H2 after 5 h
in the absence of light at 298 K (Table 4n00 0 ). Hence, dissociated H
from H2 on the Pd surface would be incorporated as a minor route
among the multiple reduction steps involved in converting CO2 tomethane. Overall, the CO2 conversion basically proceeded
photocatalytically.
Recently, CO2 activation at OV sites on TiO2 has often been suggested experimentally [10–13] and theoretically [75,76], in mostcases using water as a reductant via

CO2 þ OV ! CO þ OðsurfaceÞ

ð16Þ

and/or3þ

CO2 þ Ti

4þ

! Ti

 CO2 ;CO2 þ Hþ þ e ! CO þ OH :

ð17Þ
ð18Þ

The rate of CO2 photoconversion into methane using Pd/TiO2(38 lmol h1 g1cat; Table 4n) was higher than those into methaneand methanol using undoped TiO2 (4.0 lmol h1 g1cat; Table 4w) or
reported in the literature (0.15–17 lmol h1 g1cat) [11,12,56,72,77–
80] by 1–2 orders of magnitude, suggesting the importance of Pdsites for CO2 reduction. However, CO2 activation via Eq. (16) and/or (17) + (18) is, at most, only partial in this study (Scheme 1A).
The negligible activity using Pd/TiO2 irradiated by visible light00
only (0.009 lmol-CH3OH h1 g1cat Table 4n ), which was even lower
than that in the absence of light at 298 K (3.0 lmol-CH4 h1 g1cat

S. Kawamura et al. / Journal of Catalysis 345 (2017) 39–52

Table 4n000 ), needs attention. Under the irradiation of visible light
only, the formation of protons from H2 should be negligible andhot electrons by SPR effect diffused in Pd nanoparticles (Scheme 1A). The minor reaction path of CO2 with thermally formed H
on the Pd surface would be suppressed by the electrons because
the reaction of CO2 with negatively charged H should be difficultcompared with the coupling of CO2 with H+ or H.
Finally, the possibility of thermal CO2 conversion is considered.
This study employed a similar light source to Ahmed et al. [8] andYoshida et al. [65,81], in which the maximum temperature reached
313 K. The pressure decrease due to methane formation via Eq.
(11) at a rate of 38 lmol h1 g1
cat under 0.40 MPa of CO2 and H2(Table 4n) is as small as 93 Pa. As the critical pressures for CO2
and H2 are high (7.38 and 1.30 MPa, respectively) compared withthe reaction pressures (0.12 and 0.28 MPa, respectively), the reaction gas can be approximated as an ideal gas. The reaction pressure
before the irradiation of UV–visible light (0.400 MPa) gradually
increased to 0.412 MPa after approximately 10 min of irradiation
and remained constant for 5 h. After the UV–visible light was
turned off, the pressure decreased to 0.400 MPa in 30 min. Thus,
the initial temperature (290 K) reached thermal equilibrium at
298 K during the test. Under the same reaction conditions but
using a white TiO2 photocatalyst (Table 4w), the thermalequilibrium pressure was 0.406 MPa, corresponding to a reaction
temperature of 294 K. Furthermore, partial heating of the catalyst
also needs attention. The heat of reaction for Eq. (11)(165 kJ mol1) should elevate the Pd/TiO2 catalyst by 45 K (if themolar heat capacity at constant pressure is assumed to be
55.31 J mol1 K1 for anatase-type TiO2) at a maximum, althoughpart of the heat should diffuse to the Pyrex glass substrate beneaththe catalyst film (Fig. 1C). The blank test in the absence of UV–visible light using Pd/TiO2 in CO2 and H2 at 353 K, which is 18 K
higher than the theoretical maximum temperature for the Pd/
TiO2 catalyst, produced only 3.4% methane compared with theUV–visible light irradiated test (Table 4n, n00 00 ), reconfirming that
UV–visible light was indispensable for the conversion of CO2 using
H2 in this study.
In summary, no reactions proceeded in the absence of reactants(Table 4n0000 000 ) or catalyst (Table 4y). 7.9–3.4% of the methane wasproduced in the absence of UV–visible irradiation (Table 4n000 ,n00 00 ). Thermal processes accounted for a minor fraction of reaction(11), but their rate decreased to 0.43 times the rate at 298 K when
the reaction temperature was elevated to 353 K, due to the complex adsorption/reaction steps for reaction (11).

5. Conclusions
Photocatalytic CO2 conversion was investigated up to 0.80 MPa
using H2 and 10–100 mg of Pd/TiO2, Cu/TiO2, Ag/ZrO2, g-C3N4, Ag/
Zn3Ga-LDH, and BiOCl. The catalytic activities were in the order Pd
or Cu on TiO2  Ag/ZrO2  g-C3N4 > Ag/Zn3Ga-LDH  BiOCl. A twostep electron excitation mechanism was strongly suggested for
doped metal nanoparticles, in which hot/excited electrons arisingfrom SPR effects were estimated to exist at 2.8 to 0.3 V vs.
SHE, which is more negative than the potentials for the reductionof CO2 to methane or CO (0.12 to 0.30 V), and the remaining positive charge should combine with excited electrons that have diffused from the semiconductor. Therefore, as the levels of hot/excited electrons became more negative, the catalysts basicallybecame more active except, for intrinsic semiconductor (ZrO2)
and hydroxide (LDH) [82]. For the oxidation reaction, H2 was suggested to couple with photogenerated holes to form protons at theinterface between Pd and the TiO2 surface. The protons would
combine progressively with CO2-derived species and electrons at
the Pd to form methane and water, and would also react with lat-51

tice O to form OV sites at the interface between Pd and TiO2, as confirmed by Pd K-edge EXAFS.
Based on blank tests in the absence of UV–visible light at 298–
353 K, thermal CO2 conversion under H2 at 0.40 MPa was 7.9–3.4%of that under UV–visible light irradiation. Therefore, the CO2 conversion in this study is almost exclusively photocatalytic, but thermal H adsorption on Pd from H2 is also plausible. H was preferably
adsorbed on Pd, leading to the CO2 photoconversion efficiency
depending exclusively on P H2 . The methane formation rate reachedamaximum(38 lmol h1 g1
at
PCO2 = 0.12 MPa
andcat)
PH2 = 0.28 MPa, but the rate dropped at higher pressures due tohindrance of CO2 adsorption by H.
Acknowledgments
The authors are grateful for financial support from Grant-in-Aid
for Scientific Research C (26410204) from the Japan Society for the
Promotion of Science and the 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(2015G586 and 2014G631). The authors thank Sakino Oshiro forsample preparation and kinetic measurements. The authors thankEnago (www.enago.jp) for the English language review.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcat.2016.10.024.
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