Journal of Catalysis 352 (2017) 452–465

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

Is water more reactive than H2 in photocatalytic CO2 conversion into
fuels using semiconductor catalysts under high reaction pressures?
Hongwei Zhang a, Shogo Kawamura a, Masayuki Tamba b, Takashi Kojima b, 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 3 May 2017
Revised 10 June 2017
Accepted 12 June 2017

Keywords:
CO2
Methane
Oxygen vacancy
Hydroxy group
TiO2
Photocatalysis
Pressure dependence
Site separation
EXAFS
FTIR

a b s t r a c t
The reaction pressure in the photocatalytic conversion of CO2 into fuels is optimized between 0 and
0.80 MPa under CO2 and moisture. The higher reactivity of water than H2 was observed at higher pressure
and the reason was clarified using several 10-lm-thick semiconductor-based photocatalysts. The best
Pd/TiO2 photocatalyst produces methane with a reaction order of 0.39. The sum of independent total
formation rates of C-containing compounds under UV and visible light does not account for that underUV–visible light, demonstrating synergetic reaction mechanism on Pd for CO2 reduction by excited electrons via surface plasmon resonance and on TiO2 for water oxidation. Active metallic Pd and O vacancy
sites due to O2 formation from H2O are confirmed by in situ monitoring of EXAFS [N(Pd–Pd) = 5.9–6.2; N(Ti–O) = 5.2–3.5] and the decrease of the H-bound and bi/tri-coordinated OH peaks in FTIR. Effective
redox-site separation explains the higher reactivity of water than H2.
Ó 2017 Elsevier Inc. All rights reserved.

1. Introduction
Photocatalytic conversion of CO2 into fuels is one of the routes
to producing carbon neutral fuels without a net increase in atmospheric CO2 concentrations associated with fossil-derived alternatives [1–3]. TiO2 has been investigated most intensively for thispurpose and produces methane and/or CO [1–3]. The CO2 conversion reaction includes two steps: water oxidation, utilizing naturallight similar to Photosystem II (Eq. (1)) [4], followed by CO2 photoconversion using a photocatalyst to form methane (Eq. (2)), CO,methanol, formic acid, and formate ions [5–15] similar to Photosystem I, in which a strong reductant is formed capable of reducing
nicotinamide adenine dinucleotide phosphate (NADP+) to NADPH[4].

2H2 OðgÞ ! O2 ðgÞ þ 4Hþ ðsurfaceÞ þ 4eð1Þ

CO2 ðgÞ þ 8Hþ ðsurfaceÞ þ 8e ! CH4 ðgÞ þ 2H2 OðgÞð2Þ⇑ Corresponding author.
E-mail addresses: zhw9590@yahoo.co.jp (H. Zhang), dan556.1020@gmail.com(S. Kawamura), affa3553@chiba-u.jp (M. Tamba), tkojima@faculty.chiba-u.jp(T. Kojima), yubari.com@ae.auone-net.jp (M. Yoshiba), yizumi@faculty.chiba-u.jp(Y. Izumi).
http://dx.doi.org/10.1016/j.jcat.2017.06.016
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

Recently, studies of CO2 photoreduction, using CO2 and H2,
including a reduction step (Eq. (2)) were reported under a reaction
pressure up to 0.80 MPa [16,17]. The CO2 photoreduction rates followed the order

Pd=TiO2 > Cu=TiO2 >> Ag=ZrO2  graphitic  C3 N4> Ag=½Zn3 GaðOHÞ8 2 CO3  mH2 O  BiOCl

ð3Þ

as a function of the potential of hot/excited electrons and charge
separation efficiency for the semiconductors [16]. The best Pd/
TiO2 catalyst exclusively produced methane on Pd, starting from
CO2, coupled with (a) hot electrons consecutively excited by the
band-gap (BG) excitation of TiO2 and surface plasmon resonance(SPR) [18] on Pd and (b) protons formed from H2 at the interface
with the TiO2 surface.
Studies on the CO2 photoreduction into methanol were also
reported using H2 and various layered double hydroxides (LDHs)
photocatalysts [19–21]. Furthermore, the reduction step (Eq. (2))
and oxidation step (Eq. (1)) were combined using LDH and WO3,
respectively, in tandem cell separated by a proton-conducting film
[22].
For the CO2 photoconversion with water, on the oxidation reaction side (Eq. (1)), water/surface hydroxy activation by hole to forma hydroxy radial (OH) on TiO2 has been widely suggested [7].

453

H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465
þ

H2 OðgÞ þ h !  OH þ Hþ ðsurfaceÞ

ð4Þ

þ

ð4 Þ

ported Pd catalyst on TiO2 was prepared via liquid-phase reduction
starting from 1.0 mM of Na tetrachloropalladate (>98%, Sigma
Aldrich) and TiO2 (P25, Degussa; anatase:rutile phase = 4:1, specific surface area 60 m2 g1) stirred at 290 K for 24 h. Then, 40 mM of
NaBH4 aqueous solution was added in a molar ratio of Pd:
NaBH4 = 1:8 and the suspension was stirred at 290 K for 1 h, filtered, and the obtained gray precipitate was washed with deionized water before drying under vacuum at 290 K for 24 h. The
obtained catalyst was denoted as Pd/TiO2 and the Pd content was
0.5 wt% (Table 1).

0OHðsurfaceÞ þ h !  OH

The highly reactive OH radical is closely related to O2 formation
and O vacancy (OV) sites viaþOH þ OHðsurfaceÞ þ 3h ! O2 ðgÞ þ OV þ 2Hþ ðsurfaceÞ

OH þ OðsurfaceÞ þ 3h ! O2 ðgÞ þ OV þ Hþ ðsurfaceÞ

þ

ð5Þ
0

ð5 Þ

Furthermore, CO2 conversion into CO on OV sites on TiO2 is also
recently suggested experimentally [5–8] and theoretically [23,24]
via Eqs. (6) and/or (7) and (8).

CO2 ðgÞ þ OV ! COðgÞ þ OðsurfaceÞ
3þ

CO2 ðgÞ þ Ti
4þ

Ti

! Ti

4þ

2.2. Characterization
Cross-sectional scanning electron microscopy (SEM) images
were obtained using a model JSM-6510 (JEOL). A tungsten filament
was used in the electron gun, and the electron accelerating voltage
was 20 kV. The photocatalyst film formed on a Pyrex glass plate
was cut and mounted on the Al sample holder using an adhesive.
The incident angle of electrons with a reference to the normal lineof the sample surface was between 75° and 85°. The magnification
was between 200 and 3000 times. High-resolution transmission
electron microscopy (HR-TEM) images were observed using a
Model H-7650 transmission electron microscope (Hitachi) at anaccelerating voltage of 100 kV. A tungsten filament was used in
the electron gun and the samples were mounted on conducting
carbon with a Cu grid mesh (150 mesh per inch). The magnification
was between 60,000 and 200,000 times.
X-ray diffraction (XRD) patterns were measured using a model
Mini Flex (Rigaku) at a Bragg angle (hB) of 2hB = 20–60° with a scanstep of 0.02° and a scan rate of 0.3 s per step. The data scan was
performed at 30 kV and 15 mA using Cu Ka emission (wavelengthk = 0.15419 nm) [27] and a nickel filter. Crystallite sizes (t) were
estimated using the Scherrer equation

ð6Þ

 CO2

ð7Þ

 CO2 þ Hþ ðsurfaceÞ þ e ! COðgÞ þ OH ðsurfaceÞ

ð8Þ

The photoconversion of CO2 was accelerated at an elevatedreaction pressure of CO2 and moisture [5,6] or liquid water[6,9–12,25,26]. The formation rates of CO and methane on TiO2
were higher using moisture (12.3 kPa) rather than liquid water[6]. The pressure dependence has not been systematically reported
using CO2 and water/moisture in comparison to systematic studies
on CO2 photoreduction using CO2 and H2 at a reaction pressure of
0.10–0.80 MPa that showed volcano-type dependence [16]. Therefore, in this study, we systematically optimize CO2 photoconversion using moisture and Pd/TiO2 photocatalyst based on theinvestigations of catalyst optimization, pressure dependence, andreaction mechanism. As a result, moisture is more reactive than
H2 for CO2 photoconversion under the reaction conditions in thisstudy, contrary to the order of standard reduction potential forH2 [E°@298.15 K versus standard hydrogen electrode (SHE), pH 0](Eqs. (9) and (10)). The order predicts that H2 at a high potential
provides electrons more easily.

2Hþ ðsurfaceÞ þ 2e ! H2 ðgÞ; E ¼ 0 V

ð9Þ

O2 ðgÞ þ 4Hþ ðsurfaceÞ þ 4e ! 2H2 OðgÞ; E ¼ 1:229 V

ð10Þ

t¼

0:9k
:
Peak width  coshB

ð11Þ

Palladium K-edge XAFS spectra were measured at 290 K in thetransmission mode in the Photon Factory Advanced Ring at theHigh Energy Accelerator Research Organization (KEK, Tsukuba)
on beamline NW10A [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 focusing cylindrical 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 in front of the I0 ionization chamber had an opening
size of 1 mm (vertical)  2 mm (horizontal). Separately, during
the photoconversion tests, the samples (10 mg) on the Pyrex glass
plates were placed under 0.40 MPa CO2 and 2.3 kPa of moisture,and measurements took place at 290 K in the fluorescence modeusing a Lytle detector, placing 3-lm-thick RuO2 filter between
the sample and the If ion chamber [30]. The Pd K-edge absorptionenergy was calibrated to 24,348 eV using the spectrum of a12.5-lm-thick Pd foil [27].
Titanium K-edge XAFS spectra were measured in the KEK Photon Factory on beamline 9A, 9C, and 12C [31]. The storage ringenergy was 2.5 GeV, and the ring current was 449.3–324.4 mA. A

The reason for the higher reactivity of water in comparison toH2 under the conditions in this study is successfully explained
for the first time using X-ray absorption fine structure (XAFS)
and Fourier-transform infrared (FTIR) spectroscopy, and the massbalance in kinetic tests under various reaction conditions. To checkand confirm an advantage of the combination of CO2 and water on
spacially segregated redox surface sites as compared to that of CO2
and H2, Pd/TiO2 photocatalyst was tested that was already usedand proved to be the best for the studies under CO2 and H2 [16].
2. Experimental
2.1. Catalyst synthesis/preparation
The synthesis/preparation procedure of catalysts used in thisstudy (Table 1) was described in our preceding paper [16]. Sup-

Table 1
Basic physicochemical properties of Pd/TiO2 photocatalyst used for tests under CO2 and moisture in this study.
Pd loading (wt%)

0.5
*1

Color

Gray

Pd mean particle size (nm)TiO2 mean particle size (nm)TEM

HR-TEM

XRD

30–50 [16]25( ± 8) [16]3.1 [16]After photocatalytic tests under CO2 (3.4 kPa) + moisture (1.2 kPa) for 5 h.

EXAFS
*1

1.1( ± 0.1) {2.6( ± 0.3) }454

H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

Si (1 1 1) double-crystal monochromator and a pair of bent conical/cylinder mirrors and/or double flat mirrors were inserted into
the X-ray beam path. Spectra for the Pd/TiO2 sample (10 mg)
diluted by boron nitride were measured in the presence/absenceof ultraviolet (UV)–visible irradiation provided by a 500-W Xe
arc lamp (model OPM2-502, Ushio) at the beamline [32]. The Ti
K-edge absorption energy was calibrated to 4964.5 eV using thespectrum of the 5-lm-thick Ti foil [27].
XAFS data were analyzed using the XDAP software package[33]. 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 using Eq. (12) for the particular number of data
points i, where k was the angular wavenumber of photoelectrons, l
was the X-ray absorption coefficient, t was the thickness of sample,
and the typical values for smoothing factor were between 0.1 and3.
Data
PointsX

ðlt i  Backgroundi Þi¼1expð0:075ki Þ22

6 smoothing factor

ð12Þ

Multiple-shell curve fitting was performed [16,31] for the
Fourier-filtered k3-weighted extended XAFS (EXAFS) data in
k- and interatomic distance (R)-space using empirical amplitudes
extracted from the EXAFS data for Pd foil, PdO powder, andrutile-type TiO2 powder. The R and its associated coordination
number (N) for the Pd–Pd, Pd–O, Ti–O, and Ti(–O–)Ti pairs were
set to 0.275 09 nm with an N value of 12 [34], 0.2026 nm with
an N value of 4 [35], 0.1959 nm with an N value of 6, and
0.3058 nm with an N value of 12 [31,36,37], respectively. The
many-body reduction factor S20 was assumed to be equal for boththe sample and the reference.
The number of independent data points (m) in the fit range wascalculated based on the Nyquist theorem modified for EXAFS by
Stern based on the ranges of k and R [33].

m¼

2 Dk DR

p

þ2

ð13Þ

Based on the fitting and error analysis presented in our previous
paper [16], two shell fits (four variables in each shell) were chosen
for both Pd and Ti K-edge EXAFS analyses.
The adsorption of O2 was monitored for 100 mg Pd/TiO2 fine
powder that was previously irradiated using UV–visible light for
5 h in a quartz glass reactor connected to a Pyrex glass system
under vacuum connected to rotary and diffusion pumps (106Pa) [2]. Then, 2.3 kPa of O2 was introduced to the glass systemand the pressure change was monitored using a capacitancemanometer (models CCMT-100A and GM-2001, ULVAC) under
the irradiation of UV–visible light for 1 h and then in the absenceof light for 5 h. The temperature was 295 K and the total volumeof the glass system was 167.4 mL.
FTIR spectra were measured using a model FT/IR-4200 spectrophotometer equipped with a mercury-cadmium-tellurium-M
detector (JASCO) in the wavenumber region between 4000 and
650 cm1. 100 mg of the photocatalyst disk was set on a holder
in a quartz cell equipped with two NaCl windows on both sides
and connected to a glass system that could be evacuated using
rotary and diffusion pumps (106 Pa) [32].
Separately, the quartz cell was connected to the glass system,but 100 mg of the Pd/TiO2 powder was placed in quartz glass reactor connected to the glass system. The photocatalyst was irradiatedby UV–visible light from Xe arc lamp and the changes of reaction
gases [CO2 and moisture, 13CO2 (purity > 99.5%, 13C 99%, Cambridge
Isotope Laboratories) and moisture] was monitored by FTIR.

2.3. High-pressure photoconversion tests of CO2
The synthesized/prepared catalysts (Table 1) were immersed indeionized water and agitated by ultrasound (430 W, 38 kHz) for
3 min. The suspension was mounted on a Pyrex glass plate(25 mm  25 mm  1 mm) and dried at 373 K for 12 h. The areaof the obtained film was 20 mm  20 mm. The films on the plates
or the as-synthesized fine powder samples in Pyrex dishes(Uinternal = 37 mm) were introduced into a homemade
high-pressure stainless steel reactor equipped with quartz double
windows, a pressure gauge, and Swagelok valves [16,17]. The effective internal volume of the reactor was 98.4 mL.
CO2 gas (0.12–0.80 MPa) was introduced through a stainlesswater bubbler (Taiatsu Glass Kogyo, inner volume 120 mL; model
TVS-N2-120) at 293 K. The photocatalyst in the stainless steel reactor was irradiated with UV–visible light provided by a 500-W Xe
arc lamp (model OPM2-502, Ushio) through the quartz windows
for 5 h. The distance between the light exit and the photocatalyst
surface was 82.7 mm, and the light intensity at the center of thephotocatalyst was 81.6 mW cm2 [16]. The quartz window of thehigh-pressure reactor absorbed/reflected 9.5% of the light, and100 mg of Pd/TiO2 powder on Pyrex dish and 10 mg Pd/TiO2 film
on Pyrex glass plate absorbed/reflected/scattered 40% and 49% of
the light, respectively [16]. Densely mounted films were advantageous for light absorption efficiency compared with fine Pd/TiO2
powders based on the difference of their mean area densities of
2.5 mg cm2 and 9.3 mg cm2, respectively, and the exposed ratioof particles. Fine powders should reflect/scatter more light at
exposed faces, and less light reach the lower part of the layers compared to densely mounted films.
For comparison, photocatalytic tests were also performed using
L37, L38, L39, L42, Y48, and Y52 filters (2.5 mm-thick, Kenko) thatremoves light less than k = 370, 380, 390, 420, 480, and 520 nm,
respectively [38], or a U-330 filter (2.5 mm-thick, Kenko) thatremoves light less than k = 250 nm and between k = 390 nm and695 nm [39], which were set between the light exit and highpressure reactor.
After irradiation for five hours, the reaction gas was analyzed
using packed columns of 13X-S molecular sieves and polyethylene
glycol (PEG-6000) supported on a Flusin P (GL Sciences) set in a gas
chromatograph (GC) equipped with a thermal conductivity detector (model GC-8A, Shimadzu) [19,20]. Helium (>99.999 95%) wasused as a carrier gas for all analytes except H2, for which argon(>99.998%) was used instead. The amount of methanol and moisture was double-checked by concentrating them using a trap madeof a mixture of diethyl ether and dry ice (192 K) [17] in order toseparate them from H2 and most of CO2.
Blank tests were also performed by eliminating one of the four
control factors or adding an alternative control factor: (i) reactants,(ii) catalyst, (iii) UV–visible light, and (iv) temperature, i.e., in
0.40 MPa of CO2 only, in 2.3 kPa moisture only, or 0.4 MPa He or
Ar (in the absence of any reactants), in the absence of catalyst (orin the absence of Pd or OV sites), in the absence of any light at
299 K or 353 K, in the absence of UV light at 296 K, and in theabsence of (most of) visible light at 296 K.
3. Results
3.1. Cross-sectional SEM, HR-TEM, and XRD observation of films
The surface of the fresh Pd/TiO2 film (Fig. 1a1, a2) [16] and theone used for photoconversion test under CO2 (0.40 MPa) and moisture (2.3 kPa) for 5 h (Fig. 1b1, b2) both on Pyrex glass are flat andsmooth within the variance of 5 lm, based on cross-sectional SEM
images. In the close-ups (Fig. 1a2, b2), the mean film thickness is
14 lm ± 3 lm. In the comparison between fresh and used samples,H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

455

Fig. 1. Cross-sectional SEM images of Pd/TiO2, fresh (a1, a2) [16] and after CO2 photoreduction testing, under CO2 (0.40 MPa) and moisture (2.3 kPa) for 5 h (b1, b2). (a1, b1)
show wide views, and (a2, b2) show close-ups.

the homogeneous light gray contrast in the fresh sample (Fig. 1a1,a2) changes to more densely packed dark gray from the surface(b1) until the upper approximately 5 lm of the film (b2). Suchchange is observed in more than the half of the cross-sectionalviews for used samples, suggesting partial reduction of the upper
0
TiO2 layer to include more OV sites (Eqs. (5) and (5 )). To confirm
the crystalline phase, the XRD pattern is measured for these samples. The peak intensity and the width for anatase 1 0 1, 1 0 3, 1 1 2,
2 0 0, 1 0 5, and 2 1 1 and rutile 1 1 0, 1 0 1, and 0 0 4 diffraction
does not change significantly (by 4–10% and 13–19%, respectively,
except for very weak peaks) after the photoconversion test (Supplementary material, Fig. S1 and Table S1). The mean particle size
is evaluated at 25 ± 8 nm based on Eq. (11) for eight peaks [16](Table 1). The anatase 1 0 1 peak of low surface energy is relatively
weaker for the films compared to that for fine powder, suggestinghigher surface energy in the exposed crystalline face of the films[16]. The mean particle size was in accordance with the distribution obtained based on HR-TEM (30–50 nm, Table 1) if lower
detection limit for TEM (0.2 nm) is taken into account.
3.2. Monitoring the active sites using Pd and Ti K-edge XAFSThe Pd states are monitored by Pd K-edge X-ray absorption
near-edge structure (XANES). The post-edge pattern for fresh
Pd/TiO2 sample (Fig. 2c) is not the same as that for PdO or Pd foil(a, b) but mostly resembles that for the mixture of spectra for Pd
foil and PdO with the mixing ratio 3:2 (f; see also the comparisonin Fig. 2, right, top).
Then, 100 mg of Pd/TiO2 is placed under CO2 (3.4 kPa) and moisture (1.2 kPa) and UV–visible light for 5 h (Table 3A-a). The XANES
post-edge pattern (Fig. 2d) resembles more that of the mixture ofspectra for Pd foil and PdO with the mixing ratio 3:1 (Fig. 2g; seealso the comparison in Fig. 2, right, bottom). Conversely, thepost-edge pattern for 10 mg Pd/TiO2 under higher CO2 pressure(0.40 MPa) and moisture (2.3 kPa) and UV–visible light for 5 h(Table 3A-d) resembles more that of fresh Pd/TiO2 (Fig. 2c, e). Following, the sample of Fig. 2e is irradiated under UV–visible lightduring XANES measurement (Fig. 2e0 ). The intensity of the firstpost-edge peak at 24,370 eV becomes weaker, suggesting adecrease in the O-coordination around the Pd site.
As a complement to XANES, under CO2 (3.4 kPa) and moisture(1.2 kPa) and UV–visible light for 5 h (Table 3A-a), the N value
for the Pd–Pd interatomic pair based on EXAFS analysis is 9.2(Table 2A-d), corresponding to a mean particle size of 2.6(±0.3)
nm (Table 1) [40]. Accordingly, the Pd–Pd interatomic pair is more
pronounced in the Fourier transform (Fig. S2A-d), in consistency
with the XANES pattern with greater Pd0 contribution(Pd0: PdII = 3:1; Fig. 2d). Conversely, the N(Pd–Pd) value changes
negligibly from fresh Pd/TiO2 (5.9) to that after 0.40 MPa ofphotoconversion test for 5 h (6.2; Table 2A-c, e), demonstrating

Fig. 2. Pd K-edge XANES for PdO (a), Pd foil (b), Pd/TiO2 photocatalyst (100 mg; fresh) (c), under CO2 (3.4 kPa) and moisture (1.2 kPa) for 5 h irradiated by UV–visible light (d),under CO2 (0.40 MPa) and moisture (2.3 kPa) for 5 h irradiated by UV–visible light (e), Pd/TiO2 (10 mg) after the condition of spectrum e was irradiated by UV–visible lightduring XAFS measurement (e0 ), and the mixture of spectra a and b with the mixing ratio of 3:2 (f) and 3:1 (g). The spectra were obtained in the transmission mode, except for
Pd K-edge for samples e0 , which were taken in the fluorescence mode. (Right two panels) Good agreements between spectra c and f (Top) and between spectra d and g (Bottom).

456

Table 2Best-fit results of (A) Pd K-edge and (B) Ti K-edge EXAFS spectra for TiO2 and Pd/TiO2 photocatalysts.
Samples

Pd–O
R (nm)
N
D(r2) (105 nm2)

Pd–Pd
R (nm)
N
D(r2) (105 nm2)

Goodness of fit(A)
c [16]

Pd/TiO2, fresh, 100 mg

5-h photocatalytic test under CO2 (3.4 kPa) + moisture (1.2 kPa), 100 mg

e*2

5-h photocatalytic test under CO2 (0.40 MPa) + moisture (2.3 kPa), 100 mg

0.276 (±0.001)
5.9 (±0.5)
5.0 (±0.6)0.2753 (±0.0003)
9.2 (±0.3)
3.0 (±0.3)
0.2765 (±0.0001)
6.2 (±0.5)
3.6 (±0.4)

1.2  103d*10.199 (±0.006)
1.9 (±0.4)
4.8 (±1.7)0.216 (±0.009)
0.8 (±0.2)
0.2 (±0.2)0.201 (±0.001)
1.0 (±0.3)
2.6 (±1.3)

(B)
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

c-suppTiO2 (P25)

Pd/TiO2, fresh, 10 mg

e*3

5-h photocatalytic test under CO2 (0.40 MPa) + moisture (2.3 kPa), 10 mg

e0 *3,*4

5-h photocatalytic test under CO2 (0.40 MPa) + moisture (2.3 kPa), irradiated at beamline, 10 mg

0.312 (±0.02)
12 (±2)1 (±1)0.3128 (±0.0002)
9.28 (±0.05)
3.5 (±0.1)
0.31 (±0.01)
12 (±1)
2.4 (±0.7)
0.312 (±0.003)
10 (±3)2 (±2)

9.7  104c

0.197 (±0.001)
6 (±2)
3 (±1)
0.1989 (±0.0004)
5.4 (±0.1)
1.1 (±0.2)
0.1989 (±0.0002)
5.2 (±0.3)
0.2 (±0.4)0.2000 (±0.0002)
3.5 (±0.3)
1.7 (±0.5)*1,*2

After photocatalytic test listed in Table 3A-a*1 and A-b.*2
Similar conditions after photocatalytic test listed in Table 3A-d, but mixed with boron nitride.
*4
The sample was irradiated by UV–visible light for 1 h during XAFS measurements at beamline.

*3

3.9  102

1.0  103

3.2  104

5.8  1065.5  106

H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

Entry

Table 3
Dependence of CO2 photoconversion using moisture on the amount of photocatalysts and reactant pressure and control test results in the absence of UV and/or visible light, catalyst, or reactant (A) and the comparisons to CO2
photoconversion using H2 [16] at the same total pressure of reactants (CO2 and moisture versus CO2 and H2) (B).
Entry

Type

Wt (mg)

CO2

H2O

CO

CH3OH

CH4

H2

H2O

O2

Pd/TiO2100

0.0034
0.40
0.12
0.40

0.00120.002 300*7
0.002 3<0.0004
0.002
0.065
0.063
0.23
0.047
<0.004<0.004<0.0040.065<0.0040.52<0.004<0.004<0.00004*9

0.9
1.4
14
19
<0.12
7.4
<0.12
<0.12
<0.12
25
<0.12
30
<0.12
<0.12
<0.0012*9

6.9
17
55
181
<0.02
42
<0.02
<0.02
<0.02
37
106
291
6.7
4.2
<0.0002*9–00.40*700.80
0.40<0.008<0.008
<0.08
4.1
<0.08
<0.08
<0.08
<0.08
<0.08
5.4
<0.08
6.3
<0.08
<0.08
<0.0008*9

0.25
0.36
7.1
5.5
<0.08
1.2
<0.08
<0.08
<0.08
5.5
18
6.5
<0.08
<0.08
<0.0008*9

10

TiO2
No catalyst(B) Using Pd/TiO2 (10 mg)
Entry

0

Formation rates (lmol h

CO2

H2O

0.40
0.80

0.002 3

8.2
30
–

P*1
C

0.9
1.4
14
23
0.23
7.4
<0.20
<0.20
<0.20
31
<0.20
37
<0.20
<0.20
<0.0020*9(ii) CO2 Photoconversion under CO2 and H*102(i) CO2 Photoconversion under CO2 and moisture
Reactants (MPa)dgFormation rates (lmol h1 g1cat)

Reactants (MPa)

1

g1
cat)

Reactants (MPa)
P*1
C

23
37 (faster)

CO2

Formation rates (lmol h1 g1
cat)
P*1
H2
C

0.12
0.24

0.28
0.56

H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

(A)
a
b
c
d
d0 *2
d00 *3d00 0 *4d00 00 *5
d00 00 0 *6
e
f*6
g
h
h0 *8
i

Photocatalysts

38 (faster)
8.6*1

Total molar amount of products comprising C. *2,*3 Using L42 filter*2 and U330 filter*3 at the light exit. Temperature = 296 K. *4,*5 In the absence of UV–visible light at 299 K*4 and at 353 K. *5,*6 In 0.40 MPa of helium or argon.
Mixed gas of 0.10 MPa of CO2 and 2.3 kPa of H2O was flowed prior to the introduction of 0.40 MPa of CO2. *8 Catalyst was irradiated under UV–visible light for 4 h and vacuum prior to the test. *9 In the unit of lmol h1. *10 Results
using CO2 and H2 were reported in Ref. [16].

*7

457

458

H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

that the Pd valence state and the mean particle size change only
slightly under these conditions. In contrast, the N(Pd–O) value
decreases from 1.9 for fresh to 1.0 after the test at 0.40 MPa, suggesting an increase of the OV sites at the interface between thePd and TiO2 surfaces.
To complement the Pd K-edge results, Ti K-edge EXAFS is studied for Pd/TiO2 (10 mg) as prepared (Table 2B-c and Fig. S2B-c) andafter testing under 0.40 MPa CO2, moisture, and UV–visible lightirradiation for 5 h (Table 2B-e and Fig. S2B-e). The N(Ti–O) values
are 5.4 and 5.2, respectively, smaller compared to 6 for untreated
TiO2 (Table 2B-c-supp), or 6.1 for the sample under 0.40 MPa CO2
and H2, and UV–visible light irradiation for 5 h [16], suggesting
minor OV site formation during liquid-phase reduction to prepare
Pd/TiO2 catalyst. Conversely, the decrease (0.2) under CO2 andmoisture and UV–visible light is insignificant, taking the fit errors(±0.1 to 0.3) into account.
Pd/TiO2 after the test at 0.40 MPa for 5 h is irradiated by
UV–visible light during the Ti K-edge EXAFS measurement at
beamline (Table 2B-e0 and Fig. S2B-e0 ). The N(Ti–O) value decreases
to 3.5, suggesting a dramatic increase of the number of OV sites
during UV–visible light irradiation. The OV sites are reversibly filled
by O species after the light is turned off, because the N(Ti–O) value
increases to 5.2 in the absence of UV–visible light irradiation(Table 2B-c, e). During the irradiation of UV–visible light, the N(Ti–O) and D(r2) values may be difficult to determine exactly
due to the fast reaction with the OH radical to form OV sites onan ns or ls time scale [41]. Similar uncertainty of metal–O coordination is reported for BiOCl [N(Bi–O) = 4 ? 1.9] [28].

Under CO2 (100 kPa), moisture (2.2 kPa), and UV–visible light,
the change in N(Ti–O) values was monitored (Fig. 3A, C). In comparison to the initial value (6.0) under CO2, moisture, and dark,
the value significantly decreased to 4.9–5.7 (mean value 5.3) during light irradiation for 3.5 h. At 30 min after the light was off,
the N(Ti–O) value increased to the original value (6.0). N(Ti(–O–)Ti) values followed a similar trend: decrease from initial 11.3 to9.7–10.6 during the irradiation of UV–visible light and increase
to 10.9 at 30 min after the light was off (Fig. 3B). Taking the experimental and fit errors into account (Fig. 3A, B), the N(Ti–O) and N(Ti(–O–)Ti) values would evaluate the concentration of OV sites in/on
TiO2 semi-quantitatively.
Corresponding to the time course of Ti K-edge EXAFS, that of Ti
K pre-edge region was also plotted (Fig. 3D–F). In comparison todata for fresh sample under CO2 (100 kPa), moisture (2.2 kPa),
and dark (Fig. 3D: 0 min), the peak pattern changed after theUV–visible light was irradiated, especially in the region between
4968 and 4971 eV (Fig. 3D: 5, 30 min). This trend is in good agreement with the increase of pre-edge peak at 4967.6 eV owing to 5
coordination Ti sites [42–44] or 4 coordination Ti sites [43,44] incomparison to that at 4968.8 eV owing to 6 coordination Ti sites[42]. Please note that there is minor energy calibration difference
between reference 42 and this study.
While the peak pattern negligibly changed during the UV–visible irradiation (Fig. 3E: 60, 100, 130, 150 min; Fig, 3F: 180 min), thepattern clearly changed after the light was off (Fig. 3F: light off 0,
30 min). The last two peak patterns were very similar to that offresh sample before light irradiation (Fig. 3D: 0 min), in consistentFig. 3. Time course of Ti K-edge (A–C) EXAFS and (D–F) pre-edge region for Pd/TiO2 photocatalyst (10 mg) under CO2 (100 kPa) and moisture (2.2 kPa) for 3.5 h irradiated by
UV–visible light and subsequently for 0.5 h under dark. The changes of (A) N(Ti–O) values, (B) N(Ti(–O–)Ti) values, and (C) Fourier transform of k3-weighted EXAFS v function.

459

0.13
0.28
<0.0040.060
0.28<0.0040*50.002 30.40*50.40

Measured under UV–visible light for 1 h*1 and under dark for 5 h.*2
Using L42 filter*3 and U330 filter*4 at the light exit. Temperature = 296 K.
*5
Mixed gas of 0.10 MPa of CO2 and 2.3 kPa of H2O was flowed prior to the introduction of 0.40 MPa of CO2.
*1,*2

*3,*4

0.90
8.0
0.069
3.0
11
<0.06
2.6 (8.4 for 16 h)
6.6
0.018
1.8
6.0
0.17

7.0

24

0.75,*11.98 *2

e’s consumedfor H2 for
5 h (lmol)e’s consumed for
total C-containing
products for 5 h(lmol)
Surfacehydroxygroup(lmol)

Surfacehydroxy
consumed(lmol)
for 16 h

O2 uptake for
1 + 5 h (lmol)

TiO2

Based on Eq. (14), the theoretical O2 amounts formed using Pd/
TiO2, in the tests at 0.40 MPa is 6.6 lmol in comparison to thedetected O2 amounts of 0.28 lmol (Tables 3A-d and 4d). Thus, only
4.2% O2 is experimentally detected, suggesting that the formed O2
is readsorbed and/or intermediate O species remain on the photocatalyst surface after the tests. The ratios of consumed holes versus
consumed electrons is also listed in Table 4d, however, are notexactly the same as the ratios of experimentally detected O2 versus
theoretical one described above due to the contribution of side
reaction in Eq. (14). Using Pd/TiO2, O species are accumulated on
TiO2 after the photoconversion test, because some of the O sites
are lost on/in TiO2 during the photoconversion test but are restored
after the test, as confirmed by the Ti K-edge XAFS analysis(Table 2B-c, e, e0 and Fig. 3).

0.001 20.002 3

ð19Þ

0.003 4
0.40

2H2 OðgÞ () O2 ðgÞ þ 2H2 ðgÞ

100
10

ð18Þ

Pd/TiO2CO2 ðgÞ þ 2H2 OðgÞ ! CH3 OHðgÞ þ 1:5O2 ðgÞ

a
d
d0 *3
d00 *4
e*5
h

ð17Þ

H2O

CO2 ðgÞ þ H2 OðgÞ ! COðgÞ þ O2 ðgÞ þ H2 ðgÞ

CO2

ð16Þ

Wt (mg)CO2 ðgÞ þ 2H2 OðgÞ ! CH4 ðgÞ þ 2O2 ðgÞ

Type

The conversion of CO2 into C-containing products via Eqs. (16)–(18) and a competing side reaction (SR) of water splitting (Eq. (19))
are considered. The reverse reaction of Eq. (19) is also taken into
account to include the effect of a secondary reaction of formed
H2 with CO2 to produce C-containing products and water [16].

Theoretical O2 formedfor 5 h based onequation 14 (lmol)

ð15Þ

Reactants (MPa)rðH2 Þ ¼ rðCOÞ þ rðH2 ÞSR

Photocatalysts

ð14Þ

Entry

rðO2 Þ ¼ 2rðCH4 Þ þ rðCOÞ þ 1:5rðCH3 OHÞ þ 0:5rðH2 ÞSR

Table 4The balance of oxygen in the CO2 Photoconversion tests (5 h) and FTIR monitoring (16 h) under various reaction conditions.

First, photoconversion tests at 0.40 MPa CO2 and moisture are
performed by changing the amount of Pd/TiO2 photocatalyst
between 100 mg and 10 mg. This is the most active photocatalyst
among the catalysts in Eq. (3) under the reaction conditions
from CO2 and H2 in reference 16; under CO2 and moisture,
it is methane-selective among C-containing compounds(>99–82 mol%; Table 3A-b, d). The total formation rate of
C-containing products using 10 mg Pd/TiO2 (23 lmol h1 g1cat) ishigher by 16 times than that using 100 mg, probably due to thesmaller reflectance/scattering by 10 mg of film. Thus, light absorption of 10 mg of film is more effective, in comparison to 100 mg
powder. The peak due to low-energy anatase 1 0 1 plane is lessintense by 26% for film in comparison to that in powder while
high-energy anatase 1 0 3, 1 1 2, and ritile 1 0 1 planes are slightly
favored relatively [16]. The energetically unstable 1 1 2 plane ispreferred for a TiO2 film formed via aerosol chemical vapor deposition of titanium iso-propoxide on to the substrate at 773 K, andit enhances its electron mobility [13]. The difference of preferred
planes of film between this study (mean thickness 14 lm ± 3 lm,
Fig. 1) and reference 13 (mean thickness 1.5 lm) would be dueto the preparation procedure of film rather than the film thickness.
The total formation rates of C-containing compounds using
moisture (Table 3B-i) are compared to the corresponding ones atthe same total pressure using H2 (Table 3B-ii) for Pd/TiO2. Using
Pd/TiO2 at 0.40 MPa, the rate of 23 lmol-C h1 g1
cat usingmoisture (Table 3A-d) is lower by 0.61 times than that using H2(Table 3B-d-i, ii) [16].
Based on the results of the photoconversion tests, we examine
the mass balance under CO2 and moisture using the following
equations. r is the molar formation rate of the product.

O2 detected
for 5 h(lmol;
Table 3A)

3.3. Photoconversion in CO2 and moisture at 0.12–0.80 MPa

6.9
18<0.002
4.2
3.7
0.67

Ratio of consumed
h+’s/e’s based on
product amounts

with the reversible change of the N(Ti–O) and N(Ti(–O–)Ti) values
based on EXAFS analyses (Fig. 3A, B).

0.060
0.043
<0.0580.030
0.076<0.024

H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

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H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

Fig. 4. (A) The dependence of formation rates for CH4, CO, methanol, O2, and H2 on total reactant pressure and (B) on thickness of Pd/TiO2 photocatalyst film both under theirradiation of UV–visible light from 500-W Xe arc lamp. (C) The dependence of total formation rates for C-containing products on the wavelength of irradiated light plottedwith UV–visible absorption spectrum [16]. (A) Pd/TiO2 10 mg, CO2 0.12–0.80 MPa, and moisture 2.3 kPa. (B) Pd/TiO2 4.6–26 lm-thick (namely 2.9–16 mg), CO2 0.40 MPa, andmoisture: 2.3 kPa. (C) Pd/TiO2 10 mg, CO2 0.40 MPa, and moisture 2.3 kPa.

The pressure dependence of the photoformation rates on CO2
and moisture is investigated using Pd/TiO2 (see also Supplementary material, Section S3). Major methane formation is preserved(Fig. 4A). At 0.12–0.80 MPa using 10 mg catalyst, the reaction orderof methane, CO, and H2 is 0.39, 0.62, and 0.89, respectively(Table 3A-c, d, and g and Fig. 4A). H2 formation through Eq. (19)is accelerated by the increase of the CO2 pressure [P(CO2)], suggesting that it is correlated with the paths toward C-containing
products via Eqs. (16)–(19). Using Pd/TiO2 at 0.80 MPa, the total
C-containing product formation rate using moisture was
37 lmol h1 g1cat (Table 3A-g and Fig. 4A) that was higher by 4.3
times than that using H2 (Table 3B-g-i, ii).
The dependence of photoformation rates on Pd/TiO2 film thickness was also investigated (Fig. 4B). On the basis of rates in the unitof lmol h1 g1cat, formation rates for methane and CO gradually
increased as the mean thickness of Pd/TiO2 decreased from26 lm to 16 lm. In contrast, formation rates especially formethane dramatically boosted to 66 lmol-CH4 h1 g1
cat when themean thickness further decreased from 16 lm to 4.6 lm, suggesting that upper layer of Pd/TiO2 film was more effectively utilized
for the photocatalysis as suggested in the color change of top
5 lm by cross-sectional SEM for sample after photocatalytic test
(Fig. 1b2). H2 photoproduction increased linearly as the catalyst
thickness gradually decreased from 26 lm to 4.6 lm. The dependence for O2 formation rates on film thickness was not clear(Fig. 4B).
As a control, 0.10 MPa CO2 and 2.3 kPa moisture is input, andthen photocatalytic testing is performed only in 0.40 MPa CO2(Table 3A-e). In comparison to the corresponding standard test
under 0.40 MPa CO2 and 2.3 kPa moisture (Table 3A-d), theC-containing product selectivity of 82 mol% for methane, 18 mol%for CO, and 0.21 mol% for methanol (Table 3A-e) remains essentiallythe same at 82, 18, and 0.27 mol%, respectively (Table 3A-d). However, the total formation rates of C-containing products increase by
1.32 times to 31 lmol-C h1 g1
cat in the absence of moisture(Table 3A-e). This result suggests that water, which is stored asadsorbed or in the form of the hydroxy group during pretreatment,
photoconverts CO2 more efficiently. The theoretical O2 amountformed is 6.0 lmol based on Eqs. (14) and (15) and accounts for
only 4.6% of the experimental O2 evolution (5.5 lmol h1 g1
cat to0.28 lmol, Tables 3A-e and 4e), similar to the theoretical/experimental ratio of 4.2% for standard testing under 0.40 MPa CO2 andmoisture (Table 3A-d). 30 lmol h1 g1
cat water formation results
due to the non-catalytic desorption of adsorbed water.
The hydroxy group population on TiO2 is 7 nm2 based on thespecific surface area, the dependence of stretching vibration peakintensity of hydroxy (mOH) in FTIR on calcination temperature, desorbed water amount based on GC as a function of calcination

temperature, and/or the monitoring of the ratio of mOD/mOH in
thepresence
of
D2
gas
[45],
corresponding
to0.010  60  1018  7/(6.02  1023) = 7.0 lmol in the 10 mg
Pd/TiO2 photocatalyst used for the kinetic tests under CO2 only
(Tables 3A-e and 4e). The formation rates of the C-containing products and H2 (31 and 37 lmol h1 g1cat) correspond to 11 and3.7 lmol-e reduction, respectively (Tables 3A-e and 4e). The
oxidation reaction of the 7.0 lmol of surface hydroxy groups by0
photogenerated h+’s via Eqs. (4 ) and (5) (14 lmol-e oxidation)accounts for 98% of the reduction to the C-containing products
and H2 (total 15 lmol-e; Table 4e). surface O species and/or
adsorbed water are expected to participate in the oxidation via
0
Eqs. (5 ) and (4), respectively, as implied by the increase of theOV sites in/on TiO2 based on Ti K-edge EXAFS (Table 2B-e0 and
Fig. 3). The absence of moisture during irradiation testing would
alleviate the blocking of the OV sites for the activation of
CO2-derived species.
The other control tests are performed by controlling four factors(see the Experimental) in order to compare the results with those of
the test at 0.40 MPa using 10 mg Pd/TiO2 (Table 3A-d). In theabsence of reactants or in the absence of a catalyst(Table 3A-d00 000 , i), no C-containing products, H2, nor O2 are detected
after 5 h of UV–visible light irradiation. Water formation at the rateof 8.2 lmol h1 g1
cat in the absence of reactants results merely due
to desorption from the catalyst. Using TiO2, no C-containing products are detected, but H2 is formed at a 3.7% rate of that using Pd/
TiO2 (Table 3A-d, h), demonstrating that Pd is indispensable forconverting CO2 using moisture. OV sites are artificially created in
vacuum irradiated under UV–visible light (Fig. 5b). However, no
C-containing products are detected and H2 formation rate remains
low under the reaction conditions (Table 3A-h0 ), demonstratingthat the rate of Eq. (2) on Pd is by far higher than those of Eqs.
(6)–(8) on the OV sites [5–8,23,24]. The theoretical O2 amountformed is 0.17 lmol versus an experimental O2 evolution less thanthe detection limit of GC–TCD (<0.004 lmol), reconfirming that theO2 and/or O species are readsorbed/remain on TiO2.
A control test in the absence of CO2 produces no C-containing
products (Table 3A-f), demonstrating that the C source in the photoconversion tests is exclusively CO2 in this study. This result is
consistent with 13CH4 formation using 13CO2 and moisture monitored by FTIR (Fig. 7C-b). H2 is effectively formed at the rate of
106 lmol h1 g1
cat in the presence of moisture only. The rate
decreases by 0.59 times compared to that under 0.40 MPa CO2
and moisture (Table 3A-d, f), demonstrating that reduction
reaction paths to C-containing products and H2 (Eqs. (16)–(19))
are correlated.
Furthermore, we perform a control test by filtering out UV lightof k < 420 nm. In this case, no methane, CO, nor H2 is formed above

H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

461

detected above the detection limit, demonstrating that the CO2
conversion from CO2 and moisture shown in Table 3A proceeds
entirely photocatalytically.
3.4. O2 uptake measurements in the presence/absence of UV–visible
lightFig. 5. The color changes of TiO2 (P25) film as fresh (a), under vacuum and UV–
visible light for 4 h (b), under 0.40 MPa CO2 and moisture for 3 h (c), and after thetest and exposed to air (d).

Fig. 6. Monitoring of O2 uptake (2.3 kPa at 0 min) on Pd/TiO2 photocatalyst afterUV–visible light irradiation in vacuum for 5 h. Under UV–visible light for 1 hfollowed by 5 h under dark.

the detection limit (Table 3A-d0 ). A small amount of methanol is
detected, but the total formation rate of C-containing compounds
is only 1.0% of that irradiated under UV–visible light (Table 3Ad). The theoretical O2 amount formed is 0.018 lmol versus no O2
evolution above the detection limit (<0.004 lmol; Table 4d0 ). In
the other blank test conducted by removing UV light withk < 250 nm and most of the visible light (390 < k < 695 nm),methane is formed predominantly, but the rate is lower by 0.39
times in comparison to that under UV–visible light (Table 3A-d00 ,d). The theoretical O2 amount formed is 1.8 lmol against an experimental O2 evolution of 0.060 lmol (Table 4d00 ). Only 3.3% of the
stoichiometric O2 is detected, similar to 4.2% in the test under fulllight. The H2 evolution rate is lower by 0.23 times compared to thatin the test under full light. Thus, in both control tests, O2 adsorbs
again after irradiation, and the visible light contributes to producing C-containing products as well as H2 via SPR [16] on Pd.
In the absence of UV–visible light at 299 K (Table 3A-d000 , d), no
C-containing products, H2, nor O2 are detected. The photoreductionof CO2 using moisture and water photosplitting via Eqs. (16)–(19)are endothermal, in contrast that the photoreduction of CO2 intomethane using H2 that is exothermal [16]. In this context, to verify
the thermal effects, blank tests in the absence of UV–visible lightare also carried out at 353 K (Table 3A-d0000 ). No products are

The O2 formation in typical reaction conditions (0.40 MPa CO2
and moisture, 10 mg photocatalyst) was only 4.2% compared tothe stoichiometry of Eqs. (14) and (15) to form methane, CO,methanol, and H2 (Tables 3A-d and 4d). The Pd/TiO2 photocatalyst
was previously irradiated under UV–visible light in vacuum for 5 h.
O2 pressure decreased starting from 2.298 kPa to 2.287 kPa (Fig. 6,left), corresponding to 0.75 lmol of O2 uptake (Table 4a).
Next, the monitoring of O2 pressure was continued under dark
conditions (Fig. 6, right). The pressure quickly decreased in the first
30 min by more than 0.02 kPa and converged to 2.258 kPa after 5 h.
Therefore, 1.98 lmol of O2 adsorbed on Pd/TiO2 under dark
(Table 4a). The theoretical O2 amount formed using 100 mg Pd/
TiO2 in the photoconversion test at 4.6 kPa for 5 h was 2.6 lmol
based on Eqs. (14) and (15) and the rate in Table 3A-a, versus theexperimental formation of 0.13 lmol based on Eqs. (14) and (15)and Table 3A-a (Table 4a). The difference (2.5 lmol) can berationalized by the adsorption on TiO2, 0.75 + 1.98 = 2.73 lmol
(Fig. 6 and Table 4a).
The difference between the theoretical O2 amount formed(6.6 lmol) and the O2 amount detected (0.28 lmol) in the CO2
photoconversion test at 0.40 MPa using 10 mg Pd/TiO2 (Table 4d)
was greater (6.3 lmol) than that at 4.6 kPa using 100 mg of Pd/
TiO2 (Table 4a), mostly due to the difference in the reaction pressure and the thickness of the photocatalyst.
The oxygen balance was summarized in Table 4. StoichiometricO2 following Eqs. (16)–(19) was not detected for all the catalysts,
however, the unbalance would be consistently explained by the0participation of surface hydroxy groups via Eqs. (4 ) and (5) and/or readsorption of O2 after photocatalytic tests (Fig. 6).
Thus, the evidences of OV sites were the decrease of N(Ti–O) values based on Ti K-edge EXAFS (Table 2B and Fig. S2B), the decrease
and increase of both N(Ti–O) and N(Ti(–O–)Ti) values during theirradiation of UV–visible light and under dark, respectively(Fig. 3A, B), color change at surface 5 lm layer in cross-sectional
SEM (Fig. 1b2), color change of photocatalyst (Fig. 5b, c), and theO2 adsorption tests under UV–visible irradiation and under dark
(Fig. 6). These evidences strongly suggest the occurrence of0
Eq. (1) via Eqs. (4) and (5) and/or Eqs. (4) and (5 ), however, thepartial O2 readsorption even under light (Fig. 6) means that the
turnover of Eq. (1) was not completed and an O2-removing reagent
would be needed to proceed true catalysis following Eq. (1).
3.5. FTIR during the photoconversion in CO2 and moisture
The intermediate species during the efficient conversion of CO2
into major methane and H2 on Pd/TiO2 is monitored using FTIR. In
the spectrum for fresh Pd/TiO2 in vacuum at 293 K for 2 h, two sharp
peaks at 3673 and 3634 cm1, a very broad peak between 3500 and3000 cm1, a sharp peak at 1624 cm1, and weaker peaks at 1553,
1438, and 1358 cm1 are observed (Fig. 7A-a). These peaks can be
assigned to mOH of H-bound hydroxy, bi or tricoordinated hydroxy,and H-bound water [46], antisymmetric stretching vibration (mas)of hydrogencarbonate (HO–CO2Ti), mas of bidentate carbonate(O–CO2Ti), symmetric stretching vibration (ms) of hydrogencarbonate, and ms of bidentate carbonate [47], respectively.
When 3.4 kPa CO2 and 1.2 kPa moisture are introduced into thephotocatalyst (Fig. 7A-b), the peak intensity for both pairs ofhydrogen carbonate and bidentate carbonate increase, while a partof the two hydroxy peaks at 3673 and 3634 cm1 switches to a

462

H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

Fig. 7. (A) FTIR spectra for Pd/TiO2 photocatalyst in vacuum for 2 h (a), CO2 (3.4 kPa) and moisture (1.2 kPa) added (b), under irradiation of UV–visible light for 15 min (c) andfor 16 h (d), and light is turned off after 16 h of the reaction and evacuated for 20 min (e). (B) Difference spectra: A-b–A-a – the spectrum of CO2 (3.4 kPa) – the spectrum ofmoisture (1.2 kPa) (b), A-c–A-a – the spectrum of CO2 (3.4 kPa) – the spectrum of moisture (1.2 kPa) (c), A-d–A-a – the spectrum of CO2 (3.4 kPa) – the spectrum of moisture(1.2 kPa) (d), and A-e – A-a (e). (B0 ), (B00 ) Expanded views of B in the regions of stretching vibrations of O–H (B0 ) and O–C–O (B00 ). (C) FTIR monitoring of reactants and products
using Pd/TiO2, UV–visible light, CO2 (59 kPa), and moisture (2.3 kPa). Natural CO2 for (a) versus 13CO2 (13C 99%) for (b).

broad peak due to H-bound water (3550 cm1). These changes
are more evident if spectrum A-b is subtracted from spectrum
A-a, and further subtracted from the spectra of CO2 (3.4 kPa) andmoisture (1.2 kPa) (Fig. 7B-b and 7B0 -b; decrease of two OH peaks
versus increase of broad water peak).
Then, the Pd/TiO2 sample under CO2 and moisture is irradiatedwith UV–visible light for 15 min (Fig. 7A-c) and 16 h (Fig. 7A-d).
The extent of the change can be checked when the spectra underlight is subtracted from spectrum A-a and further subtracted fromthe spectra of CO2 (3.4 kPa) and moisture (1.2 kPa) (Fig. 7B-c, d).
The expanded views are shown for O–H region (Fig. 7B0 -c, d) andO–C–O region (B00 -c, d). The hydrogen carbonate and bidentate carbonate species are inert, and the peaks due to these species negligibly change starting from spectrum b to c then d (Fig. 7B00 ),whereas peaks due to H-bound and di/tri-coordinated hydroxygroups decrease in Fig. 7B-d compared to Fig. 7B-b and c via the0
reactions of Eqs. (4 ) and (5).

Finally, the UV–visible light is turned off, and the Pd/TiO2 photocatalyst is evacuated at 293 K for 20 min (Fig. 7A-e). The change
is more evident, if the difference between spectra B-e and B-d iscompared in order to extract the change of the surface species
compared to the original spectrum a (Fig. 7B-e). The expanded
views are also shown in Fig. 7B0 -e and B00 -e. The decrease of boththe H-bound and the bi/tri-coordinated hydroxy group during
the photoconversion test is canceled via the reaction

H2 OðgÞ þ OV þ OðsurfaceÞ ! 2OHðsurfaceÞ

ð20Þ

The amount of hydrogen carbonate and bidentate carbonate
species decreases to the level before the photoconversion test by
the evacuation of ambient CO2 (Fig. 7B00 -e). We assume that thesurface concentration of hydroxy is 7 nm2 for Pd/TiO2 evacuatedat 290 K (Fig. 7A-a) [45], and the peak area of total mOH in
Fig. 7A-a decreases by 0.66 times after the 16 h of the CO2 photoconversion test (Fig. 7A-d). Thus, 0.10  60  1018  7  0.34/

H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

463(6.02  1023) = 24 lmol of the hydroxy group on 100 mg of the Pd/
TiO2 disk are consumed during the FTIR measurements under
4.6 kPa CO2 and moisture and UV–visible light for 16 h
(Table 4a). Thus, 24 and 12 lmol O2 are formed via Eqs. (4) and0(5) and Eqs. (4 ) and (5), respectively. The O2 amount assuming0
the route of Eqs. (4 ) and (5) agrees more with the theoretical(stoichiometric) O2 formation (5.3 lmol h1 g1cat  0.100  16 =8.4 lmol; Table 4a) in the photoconversion test at 4.6 kPa CO2
and moisture based on the total formation of methane and H2(Table 3A-a) and Eqs. (14) and (15). The observed O2 formation(0.25 lmol h1 g1
cat corresponding to 0.40 lmol-O2) suggests readsorption of O2 on the TiO2 surface. In summary, the intensity of thehydrogen carbonate and bidentate carbonate peaks on the TiO2
surface partially increases in the presence of CO2 and moisture,but changes slightly due to UV–visible irradiation. The peak intensity due to the H-bound and the di/tri-coordinated hydroxy groupfor fresh Pd/TiO2 decreases by 0.66 times after the 16 h of CO2 pho0
toconversion, demonstrating that the reactions of Eqs. (4 ) and (5)for O2 formation proceed.
The reactants and products were also monitored using CO2(59 kPa), moisture (2.3 kPa), Pd/TiO2 photocatalyst, and UV–visible
light. When natural CO2 (12C 98.93%, 13C 1.07%) and moisture was
employed, a peak at 1306 cm1 appeared that was assigned tobending vibration mode (m4) of 12CH4 whereas a peak at
1297 cm1 appeared that was assigned to m4 of 13CH4 using 13CO2
and moisture (Fig. 7C). The isotope shift was in consistent with values in literature [48–50] taking the low concentrations of methane
due to kinetic tests using low pressure of 13CO2 (59 kPa) into
account. This result was in accordance with the blank test undermoisture (2.3 kPa) and UV–visible light in which no C-containing
products were detected (Table 3A-f).

4. Discussion
4.1. Electronic excitation of Pd/TiO2 photocatalyst
The performance of photocatalysts is compared at nearly
optimized conditions: using 10 mg photocatalyst film under
0.40 MPa CO2 and moisture (Table 3A-d). The photocatalytic
activity to form total C-containing compounds from CO2 and H2
was not correlated to the Eg values of the metal oxide (3.1–3.2 eV
for Pd/TiO2) or the conduction band (CB) minimum voltage forthe semiconductors (0.1 at pH 0, Scheme 1A) [16]. The work functions (WFs) for Pd are 5.22–5.6 eV versus the vacuum level [34],corresponding to potentials of 0.78–1.16 V versus SHE (pH 0),
which are slightly positive compared to the CB of TiO2 (0.1 V).
Namely, the potential for achieving electron trapping levels are alittle more positive than the reduction potentials of methane(0.17 V @pH 0) or CO (0.30 or 0.12 V @pH 0) (Scheme 1A)[9,51], and it is difficult to reduce CO2.
In UV–visible absorption spectra [16], the absorption extended
over the range between 400 and 800 nm of visible light for Pd/TiO2.
[Localized] SPR is a collective vibration of electrons near surface inthe resonant condition that natural frequency of the electrons
forced by the positive charge of nucleus coincides with the frequency of light. SPR results in elevated electric fields at the surfaceof the Pd nanoparticles (Scheme 1A). The elevated fields yield agreater number of electron–hole pairs [18,52]. It is also known thatthe resonance frequency of SPR increases as the metal nanoparticles size decrease [52]. In HR-TEM images for Pd/TiO2 photocatalyst, 1.0–6.5 nm of Pd nanoparticles were observed on TiO2
surface [16]. Hence, hot electrons due to SPR at Pd nanoparticles,
especially relatively smaller ones, would gain sufficient energy relative to the reaction potential of CO2/CH4 and transfer to CO2derived surface species (Scheme 1A).

Scheme 1. (A) Energetic diagram of CO2 photoconversion using moisture (pH 0), (B)proposed reaction mechanism of CO2 photoconversion using moisture, and (B0 ) thatof side reaction to form H2.

If the positive charge that remains at the Pd nanoparticles iscombined with excited electrons trapped from the CB of TiO2, theZ-scheme-like redox combination is completed. In this context,
the electron trapping at metal nanoparticles excited from thesemiconductors must be followed by the transfer of hot/excitedelectrons produced due to SPR on metal nanoparticles toCO2-derived species [16,21]. This proposed electron flow diagram(Scheme 1A) is well in accord with the wavelength dependenceof irradiation for Pd/TiO2 (Table 3A-d, d0 , d00 ). The sum of
C-containing product formation rates under visible light1(0.23 lmol h1 g1g1cat) and under UV light (7.4 lmol hcat) does
not account for the rate under UV–visible light (23 lmol h1 g1cat),
demonstrating a synergistic effect, e.g., by UV for the BG excitationof TiO2 and by visible light for the hot electron by SPR.
As the d-band positions at 2.4–4 eV below the Fermi energy (EF)level [18], the hot electrons in Pd/TiO2 catalyst would be mostly
due to intraband excitation within conduction band (visible lightregion) [18,52] of 1.0–6.5 nm of Pd nanoparticles rather than due
to interband transition between d-band and conduction band
(UV light region) [18,53].
Similar photoconversion rates in CO2 and moisture intomethane and CO are reported by the addition of metal to TiO2 inthe same order, Pt > Pd > Au > Rh > Ag, as that of WF [6]. Very small

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H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

metal nanoparticles are preferred, because the EF level rises, but
still remains below the CB minimum due to the quantum confinement for CO2 reduction [13]. The effects of quantum confinement
should be minimal for Pd particles with a mean size of 1.1–
3.1 nm (Table 1) in this study.

4.2. Cause of higher reactivity of moisture than H2 in CO2
photoconversion at 0.80 MPa
At 0.40 MPa, the total photoformation rates of C-containing
products in CO2 and moisture are lower by 0.61 times than thosein CO2 and H2 using Pd/TiO2 (Table 3B-d). Furthermore, using Pd/
TiO2 at 0.80 MPa, the total C-containing product formation rateusing moisture becomes higher by 4.3 times than that using H2(Table 3B-g). Thus, at the optimized reaction pressure, moistureis more reactive than H2 for pressurized CO2 photoconversion. In
our preliminary photocatalytic tests under 0.40 MPa of CO2 andmoisture using several semiconductor-based photocatalysts, i.e.
similar conditions to those in Table 3A-d, the total C-containing
product formation rates were higher than those in tests under corresponding reactant pressure of CO2 and H2 (Supplementary material, Section S4 and Table S2) [16]. The experimental O2 formation
was 40–3.2% of corresponding theoretical values based on Eqs. (14)and (15) (Table S3) in comparison to 4.2% for Pd/TiO2 (Table 3). In
contrast, quite fast photo-assisted methane formation wasreported under CO2, H2, visible light (k > 450 nm), and Ni/SiO2Al2O3. Thermal effect nearly at 423 K was suggested due to heat ofreaction for methane formation from CO2 and H2 [54]. In this context, the comparison of reactivity between moisture and H2
depends on reaction temperature. No thermal effects were foundusing Pd/TiO2 at 299–353 K in this study (Table 3A-d00 0 , d00 00 ). The
contribution of the secondary reaction of CO2 with formed H2 frommoisture is negligible (Supplementary material, Section S6).
Following, we consider the oxidation reaction using moisture or
H2 on semiconductors combined with Pd nanoparticles. Water isactivated by a hole to form the OH radical (Eq. (4)) [7]. The subsequent reaction of OH with a surface hydroxy group/O atom is fast0[41], e.g., to form O2, an OV site, and a proton(s) (Eqs. (5) or (5 ) andScheme 1B, left) due to the nature of the radical reaction. In fact,
the concentration of OV sites dramatically increases on/in TiO2 asmonitored by the decrease of the N(Ti–O) value from 5.4 for fresh
sample to as low as 3.5 under UV–visible light irradiation at beamline (Table 2B-c, e0 and Fig. S2B-c, e0 ). This change was supported by
the monitoring of N(Ti–O) and N(Ti(–O–)Ti) values during the irradiation of UV–visible light (3.5 h) and darkness (0.5 h) (Fig. 3). The
protons formed near the interface between the metal nanoparticle
and the TiO2 surface (Eq. (4)) are transferred to the metal nanoparticle to form C-containing products or H2, whether they encounter
CO2-derived species and hot/excited electrons, or such electrons
only (Scheme 1B, right). In clear contrast, OV sites are observed
only at the interface between Pd and the TiO2 surface under CO2
and H2 and UV–visible light [16], denying the radical oxidation
mechanism from H2. Effectively separated redox sites are nicely
conjugated to transport protons between them (Scheme 1B), similar to photosystems [4].
In contrast, both CO2 reduction and H2 oxidation may proceedon Pd using Pd/TiO2 [16]. Due to the competitive adsorption of
CO2 and H on Pd, the exclusive methane formation rates produce
a volcano-like maximum at P(CO2) of 0.12 MPa and at P(H2) of
0.28 MPa in the contour diagram. Thus, effective redox-site separation between TiO2 surface for H2O oxidation and Pd nanoparticles
for CO2 reduction is the reason of the observed higher reactivity ofmoisture than H2 using Pd/TiO2. Redox-site separation is also
deduced from the reaction order of methane and CO formationon P(CO2) (0.39 and 0.62, respectively; Fig. 4A), in clear contrast

to the volcano-like pressure dependence peaking at P(H2) =0.28 MPa and P(CO2) = 0.12 MPa using CO2 and H2 [16].
The best CO2 photoformation rate of C-containing compounds
in this study is 37 lmol h1 g1
cat using Pd/TiO2. The photo inprofile (action) spectrum for Pd/TiO2 photocatalyst peaked at
385 nm, but the action also increased in the UV light region basedon the tests using seven kinds of cut filters (Fig. 4C), suggesting theneeds of both UV light for BG excitation for TiO2 and UV/visible
light for SPR for Pd nanoparticles for the photoconversion(Scheme 1A). Please note that the peak centered at 385 nm was
drawn based on tests under the conditions of the presence of UV
light of k < 370 nm. Thus, if we simplify the quantum efficiency
as the ratio of total electrons needed to form C-containing products
from CO2 in the photo in-profile spectrum between 358 nm and412 nm (7.6  104 lmol-e s1) versus the photon number ofabsorbed/scattered/reflected UV–visible light between 358 nm
and 412 nm (2.9  1015 photons s1  0.49) based on the data that
51% of incident UV–visible light transmits the film (see Section 2.3),
quantum efficiency for Table 3A-g was 16%. This is the upper limitof true quantum efficiency because the proposed mechanism in
Scheme 1A requiring light for both BG excitation and SPR cannot
be plotted in simple two-dimensional action spectrum as inFig. 4C. In the evaluation above, the photon number of UV lightis not included.
The best rate in this study is lower compared to 1400–
2200 lmol-CH4 h1 g1
cat using Pt-TiO2 (mean thickness  1.5 lm)[13] or Au-Cu-TiO2 films (0.48 mg cm2) [5]. In the crosssectional SEM image, 5 lm of TiO2 layer including more OV sites
are detected at the upper part of the 15 lm ± 3 lm-thick Pd-TiO2
layer (Fig. 1b2). The upper 5 lm of the TiO2 layer absorbs more
photons to proceed reactions 4, 40 , 5, and/or 50 . In fact, the rate ofmajor methane formation dramatically boosted when the mean
thickness of Pd/TiO2 film decreased from 16 lm to 4.6 lm(Fig. 4B). Thus, even a thinner catalyst film is needed to improve
the photoconversion rate of CO2 in this study.
Liu et al. suggested that CO2 activation on an OV site forms CO
via Eqs. (6) and/or (7) + (8) [7]. OV sites should be distributed in
the upper 5 lm of the TiO2 layer (Fig. 1b2) during UV–visible light
exposure under 0.40 MPa CO2 and moisture (Fig. 1b2 and Table 2Ae, B-e0 ). In contrast, OV sites are detected only at the interface
between Pd nanoparticles and the TiO2 surface for Pd/TiO2 under
0.40 MPa CO2 and H2 [16]. Based on the test using artificially created OV sites on TiO2 (Table 3A-h0 and Fig. 5b), direct CO2 activation
on OV sites is negligible in comparison to that over Pd or the otherreduction sites in this study.
Stoichiometric O2 formation is not detected during CO2 photoconversion according to Eqs. (14) and (15), but only 4.2% O2 isexperimentally detected for Pd/TiO2 (Table 3A). The possibility of
H2O2 formation is investigated using Ti sulfate, but it is notdetected above the detection limit of 0.20 lmol (Supplementarymaterial, Section S5) in comparison to the theoretical (stoichiometric) O2 amount formed: 6.6 lmol using Pd/TiO2 at 0.40 MPa(Tables 3A-d and 4d). Thus, the possibility of H2O2 formation islow, but, instead, the formed O2 may be readsorbed by the photocatalysts after the photoconversion test but before the GC–TCD
analysis, as shown in Fig. 6.

5. Conclusions
Photoconversion of CO2 was optimized using moisture at
4.6 kPa–0.80 MPa. The result was successfully explained based on
the levels of hot electrons due to SPR or BG-excited electrons.
At 0.80 MPa, moisture was found to be more reactive than H2
for CO2 photoconversion. This finding was attributed to the effective site separation between CO2 reduction on Pd nanoparticles

H. Zhang et al. / Journal of Catalysis 352 (2017) 452–465

and water oxidation on the semiconductor surface, as confirmed
using in situ monitoring of Ti K-edge EXAFS and cross-sectional
SEM. The redox sites were conjugated in order to transport protons
effectively, in a way similar to photosystems. The mass balance of
O is explained by the readsorption of the formed O2 after the photoconversion test, because O2 adsorption on the OV sites wasquicker under dark rather than under UV–visible light. CO2 photoconversion rates increased up to 0.80 MPa CO2 and moisture in areaction order of 0.39–0.62 using Pd/TiO2, whereas the volcanolike maxima were at P(CO2) of 0.12 MPa and P(H2) of 0.28 MPa in
the contour plot.
Acknowledgements
The authors are grateful for the financial supports from theGrant-in-Aid for Scientific Research C (26410204) from Japan 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 conductedunder the approval of the Photon Factory Proposal Review Committee (2016G577, 2015G586, 2014G631). The authors thank
Masaya Miyano for the support of data analysis. In addition, theauthors would like to thank Enago (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.2017.06.016.
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