ORIGINAL RESEARCH
published: 27 September 2018
doi: 10.3389/fchem.2018.00408

Why Is Water More Reactive Than
Hydrogen in Photocatalytic CO2
Conversion at Higher Pressures?
Elucidation by Means of X-Ray
Absorption Fine Structure and Gas
Chromatography–Mass
Spectrometry
Hongwei Zhang and Yasuo Izumi*
Department of Chemistry, Graduate School of Science, Chiba University, Chiba, Japan

Edited by:
Junguang Tao,
Hebei University of Technology, China
Reviewed by:
Ellen B. Stechel,
Arizona State University, United States
Federica Valentini,
Università di Roma Tor Vergata, Italy*Correspondence:
Yasuo Izumi
yizumi@faculty.chiba-u.jp
Specialty section:
This article was submitted toGreen and Sustainable Chemistry,a section of the journal
Frontiers in Chemistry
Received: 03 July 2018
Accepted: 20 August 2018
Published: 27 September 2018
Citation:Zhang H and Izumi Y (2018) Why Is
Water More Reactive Than Hydrogenin Photocatalytic CO2 Conversion atHigher Pressures? Elucidation by
Means of X-Ray Absorption Fine
Structure and Gas
Chromatography–Mass Spectrometry.
Front. Chem. 6:408.
doi: 10.3389/fchem.2018.00408

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Photocatalytic conversion of CO2 into mainly methane using Pd/TiO2 photocatalyst
proceeded faster at 0.80 MPa using water rather than hydrogen as a reductant.
The former reaction (CO2 + water) consists of two steps: first, water photosplitting
and second, the latter reaction (CO2 + hydrogen). It was paradoxical that total
steps proceeded faster than each step based on simple kinetics. To elucidate thereason, Pd and Ti K-edge X-ray absorption fine structure (XAFS) was monitored during
CO2 photoconversion using H2 or moisture and the exchange reaction of 13 CO2 at
Pd/TiO2 surface was also monitored. As a result, the coordination number, N(Ti–O)
and N[Ti(–O–)Ti] values, decreased from original values for TiO2 crystalline (6 and 12) to
4.9–5.7 and 9.7–10.6 under CO2 and moisture, respectively, in contrast to significantly
smaller decreases under CO2 and H2 and under Ar. The exchange of gas-phase 13 CO2with preadsorbed 12 CO2 reached the equilibrium in ∼20 h with a rate constant of 0.20
h−1 . The reason of the higher activity using water rather than H2 could be explained
owing to the oxygen vacancy (Ov ) sites as confirmed by XAFS. The reaction of TiO2
surface with water formed Ov sites responsible for water oxidation, specially separated
from Pd nanoparticle sites for CO2 reduction. In contrast, Pd nanoparticle sites werecompeted by CO2 and H species, and the photoconversion of CO2 was suppressed at
the elevated pressure of CO2 + H2 .
Keywords: CO2 , oxygen vacancy, X-ray absorption fine structure, gas chromatography–mass spectrometry,
13 CO
2

INTRODUCTION
Photocatalytic conversion of CO2 into fuels is one of the routes to producing C neutral fuels withouta net increase in atmospheric CO2 concentrations associated with fossil-derived alternatives (Izumi,2013). The reaction includes two steps: water oxidation to O2 followed by CO2 photoconversion toform methane, etc.

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2H2 O(g) → O2 (g) + 4H+ (surface) + 4e−+equipped with a thermal conductivity detector (GC-TCD; Model
GC-8A, Shimadzu).
The Pd K-edge XAFS spectra were measured at 290 K in thetransmission mode in the Photon Factory Advanced Ring at
the High Energy Accelerator Research Organization (KEK) on
beamline NW10A. An Si (3 1 1) double-crystal monochromatorand a Pt-coated focusing cylindrical mirror were inserted into the
X-ray beam path. A Piezo transducer was used to detune the Xray intensity to two-thirds of the maximum to suppress higher
harmonics (Izumi et al., 2001; Fujishima et al., 2015). The Pd
K-edge absorption energy was calibrated to 24348 eV using thespectrum of a Pd metal foil (Bearden, 1967).
The Ti K-edge XAFS spectra were measured in thetransmission mode in the Photon Factory at KEK on beamline
9A, 9C, and 12C. An Si(1 1 1) double-crystal monochromatorand a pair of bent conical/cylinder mirrors and/or double
flat mirrors were inserted into the X-ray beam path. A Piezo
transducer was used to detune to two-thirds of the maximum
X-ray intensity to suppress higher harmonics (Izumi et al.,
2001; Fujishima et al., 2015). Spectra for the Pd/TiO2 sample(10 mg) diluted by boron nitride were measured under CO2 (100kPa) and moisture (2.2 kPa), under CO2 (70 kPa) and H2 (30kPa), or Ar (100 kPa) in the presence/absence of UV–visible
light irradiation provided by a 500-W Xe arc lamp (the flux
intensity 81.6 mW cm−2 ) at the beamline (Izumi et al., 2007;
Morikawa et al., 2014). The Ti K-edge absorption energy wascalibrated to 4964.5 eV using the spectrum of a Ti metal foil
(Bearden, 1967). The obtained Pd and Ti K-edge XAFS data
were analyzed using XDAP software package using modified
Victoreen function, spline function, Fourier transform, andmultiple-shell curve fitting (Vaarkamp et al., 2006; Izumi et al.,2009).
Isotope-labeled exchange reaction tests were performed andmonitored with GCMS using a JMS-Q1050GC (JEOL) apparatus
equipped with a quadruple mass spectrometer. Helium (purity> 99.9999%) was used as the carrier gas. The sampling loop
comprised a Pyrex glass system vacuumed using rotary anddiffusion pumps (10−6 Pa) and connected to the JMS-Q1050GC
using 1.5 m of deactivated fused silica tube (No. 160-2845-10,
Agilent; internal diameter 250 µm) maintained at 393 K during
the analysis to avoid gas adsorption (Wein et al., 2018).
A photocatalyst sample (0.100 g) was placed in a quartz
photoreactor and evacuated at 295 K for 2 h while being
connected to a Pyrex glass circulating system (circulating section
206.1 mL) and rotary and diffusion pumps (10−6 Pa). Then,
13 CO (0.68 kPa; 13 C 99%, 17 O 0.1%, 18 O 0.7%, chemical purity2> 99.9%, Cambridge Isotope Laboratories, Inc.) photoexchangewith the photocatalyst surface was monitored at 295 K underirradiation by UV–visible light from a 500 W xenon arc lamp
(Model SX-UID502XAM, Ushio). The distance between the exitof the lamp window and the photocatalyst was 18 mm. The
exchange with adsorbed 12 CO2 at the surface was monitored
using a packed column of PEG-6000 supported on a Flusin P (GLSciences Inc.) set in the JSM-Q1050GC.
The exchange reaction rates were assumed to follow firstorder equilibrium kinetics and the rate constants kr and kr ′ arefor the exchange of gas-phase 13 CO2 with adsorbed 12 CO2 and(1)−CO2 (g) + 8H (surface) + 8e → CH4 (g) + 2H2 O(g)(2)
We systematically tested the photocatalytic reaction under CO2+ H2 (Table 1A), an essentially identical reaction to Equation (2),for photocatalytic conversion of CO2 into fuels, if it is combinedwith the water photo-oxidation step (Equation 1) (Kawamura
et al., 2017). Based on the formation rates of C-containing
products (CH4 , CO, and methanol) using Pd/TiO2 , water was
evidently more reactive (37 µmol-C h−1 g−1cat ) than H2 (8.6 µmolC h−1 g−1)
forCO
photoconversion
atthe higher reaction2
catpressures, e.g., 0.80 MPa (Table 1C, G). It was paradoxical that apart of the reaction steps, i.e., Equation (2), was slower than total
reaction steps, i.e., Equations (1, 2) based on simple kinetics. In
this study, the reason was investigated by X-ray absorption finestructure (XAFS) and gas chromatography–mass spectrometry(GCMS).

MATERIAL AND METHODS
A sample was prepared via liquid phase reduction (Kawamura
et al., 2017) from 1.0 mM aqueous solution of Na2 PdCl4(>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 addedin a molar ratio of Pd: NaBH4 = 1:8, and the suspension waswashed with deionized water (<0.055 µS cm−1 ) supplied by
an RFU424TA system (Advantec) before drying under vacuumat 290 K for 1 h and then filtered. The obtained precipitate waswashed with deionized water before drying under vacuum at
290 K for 24 h. The obtained sample was denoted as Pd/TiO2 andthe Pd content was 0.5 weight-% as metal.
For comparison, TiO2 was also synthesized in the laboratory.
Acid solution (1.2 mL of 5 weight-% H2 SO4 aqueous solution)and 1.37 mL of TiCl4 were added into 96 mL of deionized waterat 273 K under argon atmosphere and stirred at 900 rotations
per minute for 30 min. Then, 28 weight-% ammonia aqueous
solution was dropped into the solution at 368 K to adjust thepH 7.0. The solution was stirred at the temperature for 1 h andcooled to 290 K. The obtained suspension was filtered and washedby deionized water (total 500 mL) until no chlorine ions weredetected using silver nitrate. The thus-obtained powder was driedat 373 K overnight, calcined at 573 K for 2 h, and white TiO2
powder was obtained. The Pd was loaded on the homemade TiO2
in a similar manner to the liquid phase reduction for P25 as
described earlier.
The high-pressure photoconversion tests were performed inhomemade stainless steel reactor equipped with quartz windows(Kawamura et al., 2017; Zhang et al., 2017). A mixed CO2 /H2gas or CO2 gas/moisture was introduced into the reactor. A
film (10 mg of photocatalyst) in the reactor was irradiated by
a 500-W xenon arc lamp (Model OPM2-502, Ushio) for 5 h.
The reaction gas was analyzed using packed columns of 13X-S
molecular sieves and polyethylene glycol (PEG-6000) supported
on Flusin P (GL Sciences, Inc.) set in a gas chromatograph

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TABLE 1 | Results of photocatalytic tests of CO2 photoconversion for 5 h Using 10 mg of Pd (0.5 weight-%)/TiO2 Photocatalysts*a and TiO2 (P25) at 0.12–0.80 MPa.
Entry

Catalyst

Reactants(MPa)

A

Pd/TiO2Formation rates (µmol h−1 g−1
cat )
(kPa)

CO2

H2

0.12

0.28

TiO2

C

Pd/TiO2

0.24

D

0.12

E

0.40

F

TiO2

G

Pd/TiO2CH3 OH

CH4

Σ(C-products)*b

H2 O

a′
B

CO

References

0.56
2.3

0.80

< 0.08< 0.004

38

38< 0.08

0.034

5.2

5.3

Kawamura et al., 2017
This work< 0.08

0.027

4.0

4.0

Kawamura et al., 2017

< 0.08< 0.004

8.6

8.6

Kawamura et al., 2017

< 0.08

0.065

14

14

Zhang et al., 2017

4.1

0.063

19

23

Zhang et al., 2017

< 0.08< 0.004< 0.12< 0.20Zhang et al., 2017

6.3

0.52

30

37

Zhang et al., 2017*a Prepared using P25 TiO2 except for entry a′ (homemade TiO2 ).
*b Total formation rates for C-containing products.

gas-phase 12 CO2 with adsorbed 13 CO2 , respectively.

Photoconversion test was performed at 0.40 MPa of CO2 andH2 using Pd/TiO2 with the help of homemade TiO2 (Table 1a′ ).
Methane was the major product similar to the result using
Pd/TiO2 prepared from commercial P25 (Table 1A) (Kawamura
et al., 2017). The reason for the difference of the methane−1 g−1 ) is
formation rates (5.2 µmol h−1 g−1
cat
cat vs. 38 µmol h
not clear, but the crystalline phase (anatase vs. the mixture ofanatase and rutile phases) and specific surface area were different.
Although the impurity effects for photocatalysts using P25 weresuggested (Izumi, 2013; Cybula et al., 2015; Li et al., 2016; Dilla
et al., 2017; Grigioni et al., 2017) photoconversion of CO2 intomethane was confirmed using Pd/TiO2 prepared by not making
use of any organic reagents throughout the synthesis procedureof the photocatalyst (Table 1a′ ). For this problem, several black
tests were reported using Pd/TiO2 (P25), especially the one underUV–visible light and H2 excluding CO2 gas (Zhang et al., 2017).
No C-containing products were detected above the detection
limit of GC-TCD. Thus, a direct impurity effect is denied as
the reason for the rate difference (Table 1A, a′ ). The indirect
effect of gas-phase CO2 on impurity on/in catalyst, leading to theformation of C-containing products under light cannot be denied
as the reason for formation rate difference.
The other possibility of formation rate difference is criticalpressure dependence of TiO2 -based photocatalysts under CO2+ H2 (Kawamura et al., 2017). Due to the critical difference ofreactant pressure for the conversion rate maximum (volcano top)
for each catalyst, formation rate difference of by a factor of 7.2
times between Pd/TiO2 (P25) and Pd/TiO2 (homemade) may
result at a fixed reactant pressure (0.40 MPa).

dP13 CO2= −kr P13 CO2 + kr ′ P12 CO2
(3)
dt
n
o
kr
−(kr +kr ′ )t
P12 CO2 =P13 CO2 (initial) (4)′ 1−e
kr + kr
kr
P13 CO2 (initial)(5)
P12 CO2 (eqilibrium) =kr + kr ′on′(6)∴ P12 CO2 = P12 CO2 (eqilibrium) 1 − e−(kr +kr )t
Photocatalytic tests using 13 CO2 (2.3 kPa) and H2 (21.7 kPa) werealso performed in a similar apparatus and conditions to that of
13 CO exchange tests.
2

RESULTS AND DISCUSSION
Photoconversion Tests of CO2
First, reported kinetic results are summarized. Both the reactions
under CO2 + H2 and under CO2 + moisture critically depended
on the pressure of reactants using Pd/TiO2 (Kawamura et al.,2017; Zhang et al., 2017). The formation rates of C-containing
products under CO2 + H2 reached a local maximum at 0.40
MPa (0.12 MPa of CO2 , 0.28 MPa of H2 ) and drew a volcanolike
dependence (Table 1A, C). In Kawamura et al. (2017), thevolcanolike local maximum seems distributed relatively steep
based on 13 test data points. Methane was a dominant productunder the conditions. In contrast, the formation rates followed atypical Langmuir-type dependence, i.e., they gradually increased
as a function of reactants pressure under CO2 + moisture at
0.12–0.80 MPa (Table 1D, E, G) (Zhang et al., 2017). Minor CO
and methanol were also produced under the conditions.
Due to the balance of different pressure dependencies between
the two types of reactions, H2 was more reactive at lower than
0.40 MPa, whereas water was more reactive at higher than 0.40
MPa as a reductant for CO2 . In this context, the major questionof this study is the reason for the different pressure dependencies
of CO2 photoconversion rates using H2 vs. water.

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Active Site Monitoring by XAFSFor the morphological characterization of the photocatalysts,
transmission electron microscope images for the Pd/TiO2
catalyst, fresh and used one, after photocatalytic test in CO2+ H2 and cross-sectional scanning electron microscope (SEM)
images for fresh Pd/TiO2 were reported in Kawamura et al.
(2017). The morphology of the catalyst negligibly changed after
the photocatalytic test, but the mean Pd particle size slightly

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the original when the light was off (Figure 1C, Light off, 0 min).
In accordance with the peak intensity change, in comparison
with the initial N(Ti–O) value (6.0) under CO2 , moisture, anddark, the value significantly decreased to 4.9–5.7 during lightirradiation for 3.5 h. At 0.5 h after the light was off, the N(Ti–O)
value increased to the original value (6.0).
The peak intensity of Ti(–O–)Ti interatomic pair at 0.26 nm(phase shift uncorrected; Figure 1C) and the N[Ti(–O–)Ti]
values followed a similar trend: decrease from the initial
11.3 to 9.7–10.6 during the irradiation of UV–visible light(Figure 1B). Taking the experimental and fit errors into account(Figures 1A,B), the N(Ti–O) and N[Ti(–O–)Ti] values would
evaluate the concentration of Ov sites in/on TiO2 (Zhang et al.,2017).
Two control monitoring tests were also performed: (i) using
H2 instead of moisture as reductant for CO2 and (ii) using
only argon. First, under CO2 (70 kPa), H2 (30 kPa), and UV–
visible light, the changes in peak intensity of Ti–O and Ti(–
O–)Ti interatomic pair at 0.15 nm and 0.26 nm (phase shiftuncorrected), respectively (Figure 2E) and the resultant N(Ti–O) and N[Ti(–O–)Ti] values were monitored (Figures 2A,B).
The peak intensity of Ti–O interatomic pair decreased at 75 minof light irradiation, but no clear correlation between lightirradiation and the Ti–O peak intensity was found. The peakintensity of Ti(–O–)Ti interatomic pair did not change much.
In accordance with the peak intensity changes, in comparison
with the initial value (5.9) under CO2 , H2 , and dark, the N(Ti–O) value varied, but within a small range of 5.6–5.8 during
light irradiation for 125 min. At 50 min after the light wasincreased starting from 3.1 to 3.9 nm after the photocatalytic
test (Kawamura et al., 2017). Cross-sectional SEM images for
fresh Pd/TiO2 and used one after photocatalytic test in CO2 +moisture were also reported in Zhang et al. (2017). The colorof the top few micrometers of Pd/TiO2 catalyst film (total 11–
16 µm) changed after the photocatalytic test (Zhang et al., 2017).
In this study, monitoring of local active sites was performed inaddition to these morphological studies.
The X-ray absorption near-edge structure (XANES) at the Pd
K-edge suggested metallic Pd nanoparticles dispersed on TiO2 .
The extended X-ray absorption fine structure (EXAFS) also did
not show significant change during photoreactions under CO2+ H2 or CO2 + moisture based on the Pd–Pd shell, but thecoordination number N(Pd–O) decreased from 1.9 for fresh
sample to 0.6–0.9 after the photocatalytic tests under CO2 + H2
at 0.024–0.40 MPa (Kawamura et al., 2017) and to 0.8–1.0 underCO2 + moisture at 4.6 kPa−0.40 MPa (Zhang et al., 2017). The
decrease of N(Pd–O) values suggested the formation of oxygen
vacancy (Ov ) sites on/in TiO2 near Pd nanoparticles under thereaction conditions of CO2 photoconversion.
The Ti K-edge XAFS was also investigated under the reaction
conditions of CO2 photoconversion with moisture (Figure 1).
Under CO2 (100 kPa), moisture (2.2 kPa), and UV–visible
light, the change in N(Ti–O) values was monitored (Figure 1A).
The changes under light and then under dark were depicted
as the Fourier transform in Figure 1C. The intensity of peakfor Ti–O interatomic pair at 0.15 nm (phase shift uncorrected)
decreased when the light was on (Figure 1C, 5 min), remained
almost constant during the light irradiation, and increased to

FIGURE 1 | Time course of Ti K-edge EXAFS for Pd/TiO2 photocatalyst (10 mg) under CO2 (100 kPa) and moisture (2.2 kPa) for 3.5 h irradiated by UV–visible lightand 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 angular wavenumber k 3 -weighted
EXAFS χ function (Zhang et al., 2017).

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corresponds to 0.18 molecules of CO2 adsorbed per nm2 of Pd–
TiO2 that was an acceptable value after the evacuation at 295 K
for 2 h as pretreatment. The sum of rate constants kr and kr ′corresponds to total rate constant (Equation 6) to reach theequilibrium 13 CO2 and 12 CO2 (0.20 h−1 ).
As a comparison, if we assume that the dispersion of mean
3.1 nm Pd nanoparticles (0.50 weight-%) over TiO2 observedby transmission electron microscopy (Kawamura et al., 2017)is ∼50% (Kip et al., 1987) and the conversion to C-containing
products (methane, CO, and methanol) proceeded on the surfaceof the Pd atoms starting from CO2 , the formation rate of Ccontaining products (37 µmol h−1 g−1cat ) under 0.80 MPa of CO2
and moisture (Table 1G) corresponds to the turnover frequency(TOF)

off, the N(Ti–O) value increased to the original value (6.0).
In comparison with the monitoring under CO2 and moisture(starting from 6.0 to 4.9–5.7; Figure 1A), the decrease in N(Ti–O) value was effectively smaller under CO2 and H2 (1 = 0.1–
0.3) during photoirradiation. The N[Ti(–O–)Ti] value remained
between 12 and 11.7 under the irradiation of UV–visible light andnegligibly changed after the light was off: 12–11.2 (Figure 2B).
Under CO2 and moisture, the decrease was greater: from 11.3 to9.7–10.6 by the effect of light (Figure 1B).
Under Ar (100 kPa) and UV–visible light, the N(Ti–O) valuechanged between 5.7 and 6.0 during light irradiation for 100 min.
At 5 min after the light was off, the N(Ti–O) value remained at
5.9 (Figure 2C). The changes in N[Ti(–O–)Ti] values were also
minimal under Ar: 12–11 throughout the monitoring test underlight/dark (Figure 2D). These differences in N(Ti–O) and N[Ti(–O–)Ti] values were because more Ov sites were formed under
the photoreduction of CO2 using moisture rather than H2 orAr.

37×10−6 mol h−1 gcat

The 13 CO2 exchange reaction test was performed using Pd–
TiO2 (P25) photocatalyst. The amount of 13 CO2 gas gradually
decreased over 20 h by the exchange with 12 CO2 on Pd–TiO2
sample that was preadsorbed from air (Figure 3A). The changes
followed equilibrium kinetics as listed in Equations (3–6), andthe formation of 12 CO2 (n: mol) in gas phase was well fitted by
Equation (6′ ) (Figure 3A).
n12 CO2 (µmol) = 0.657 + 1.80 1 − e×10−2 gcat

= 1.6 h−1 . This comparison

is contradictory because the early adsorption equilibrium of
CO2 (0.20 h−1 based on Equation 6′ ) is slower than CO2
photoreduction to methane via much more difficult reaction
steps (1.6 h−1 ). However, the exchange rate (0.20 h−1 ) observed
under 13 CO2 (initial 0.68 kPa) in this study was by a couple oftimes higher compared with the formation rate of C-containing
products under initial 3.4 kPa of CO2 + 1.2 kPa of moisture (0.9µmol h−1 g−1cat ) (Zhang et al., 2017) corresponding to the TOF

Monitoring of 13 CO2 Exchange by GCMS

−0.20t−1

10−2 gcat ×0.005×0.5
106.42gPd mol−1

0.9×10−6 mol h−1 gcat

−1

×10−2 gcat

10−2 gcat ×0.005×0.5
106.42gPd mol−1= 0.04 h−1 . The CO2 exchange

was reported using isotope labeled Ti18 O2 irradiated by XeCl
excimer laser (308 nm) to form C18 O2 and 16 OC18 O (Civiš et al.,2011).
Furthermore, stability test of photocatalytic reaction using
13 CO (2.3 kPa) + H (21.7 kPa) and Pd/TiO was performed
2
2
2
for more than 50 h (Figure 3B). To avoid the possibility of

′

(6 )

Initial 0.657 µmol is the impurity (1.18 mol%) included in the
introduced 13 CO2 gas (0.68 kPa). The n12 CO2 (equiliburium) value(1.80 µmol) obtained by the fit of Equation (6′ ) to the data

FIGURE 2 | Time course of Ti K-edge (A–D) EXAFS for Pd/TiO2 photocatalyst (10 mg). (A,B) Under CO2 (70 kPa) and H2 (30 kPa) for 125 min irradiated by
UV–visible light and subsequently for 75 min under dark. (C,D) Under Ar (100 kPa) for 100 min irradiated by UV–visible light and subsequently for 20 min under dark.
The changes of (A,C) N(Ti–O) values and (B,D) N[Ti(–O–)Ti] values. (E) Fourier transform of angular wavenumber k 3 -weighted EXAFS χ function under CO2 (70 kPa)
and H2 (30 kPa).

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FIGURE 3 | Time courses of (A) 13 CO2 (0.68 kPa) photoexchange reaction with preadsorbed 12 CO2 on Pd/TiO2 (P25) and (B) photocatalytic reduction test in
13 CO (2.3 kPa) and H (21.7 kPa) on Pd/TiO (homemade).
222(Ji and Luo, 2016), anatase (1 0 1), and small TiO2 cluster (Leeand Kanai, 2012).

impurity effects, Pd/TiO2 (homemade) was used and a byproduct13 CO was monitored by GCMS rather than methane that may
be produced from impurity in TiO2 (Izumi, 2013; Cybula
et al., 2015; Li et al., 2016; Dilla et al., 2017; Grigioni et al.,2017). In accordance with the rate of 13 CO2 exchange reaction(Figure 3A), 13 CO started to be formed at ∼10 h of reaction
and constantly formed for more than 50 h (Figure 3B). As a
byproduct, the formation rate was low 0.0032 µmol h−1 g−1catcorresponding to the TOF 0.00014 h−1 . Nevertheless, the stability
test confirmed the availability of Pd/TiO2 photocatalyst for morethan 50 h of photoreaction test.
Thus, we should be careful in the isotope tracing tests as
12 CO could be included derived from preadsorbed 12 CO in2
2
the reactant 13 CO2 and 12 C-products could be included by thephotocatalytic conversion.
The Pd/TiO2 catalyst used in this study is advantageous
for CO2 photoconversion irradiated by visible light (Kawamura
et al., 2017; Zhang et al., 2017). The photocatalytic performancewas compared with TiO2 (P25) in 0.40 MPa of CO2 +H2 (Table 1A, B) or CO2 + moisture (Table 1E, F). The
formation rates of total C-containing products were 11% and<0.9% using TiO2 in comparison with corresponding values
using Pd/TiO2 , respectively. As TiO2 is mostly activated by
UV light based on the band gap value (3.2 and 3.0 eV for
anatase and rutile phases, respectively) (Izumi, 2013), theincreased photoactivity using Pd/TiO2 could be the effects oflocalized surface plasmon resonance of Pd nanoparticles owing
to visible light irradiation (Kawamura et al., 2017; Zhang et al.,2017).
Thus, the answer to the question as to why water was
more reactive than H2 in photocatalytic CO2 conversion at
higher pressures is that water forms hydroxy radical under theirradiation of UV–visible light (Equation 7) and then Ov sites areformed (Equation 8), but not by using H2 or Ar. The exchange
of CO2 (Figure 3A) and/or the elimination of C–O bond to form
CO are also suggested over Ov sites of Au–Cu–TiO2 (Neatu et al.,2014), MgO–Pt–TiO2 (Xie et al., 2014), TiO2 (Liu et al., 2012,2016; Li et al., 2017), and theoretical models of defective anatase

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H2 O + h+ → H+ + ·OH

(7)

+

+

·OH + O(surface) + 3h → O2 (g) + Ov + H (surface) (8)The reactions 7 and 8 proceeded as water oxidation over TiO2
spatially separated from CO2 reduction sites on Pd for Equation(2). Thus, the effective redox reaction of Equations (1, 2)
proceeded using water. Both the reactions 1 and 2 would proceedcompetitively on Pd under CO2 + H2 , and the efficiency followeda volcano-like dependence as a function of partial pressures ofreactants (Table 1A, B) (Kawamura et al., 2017).

CONCLUSIONS
The formation of Ov sites was monitored by means of Ti Kedge EXAFS using Pd/TiO2 . The coordination number around
Ti atoms clearly demonstrated the effective formation of Ov sites
under CO2 , moisture, and UV–visible light in contrast to themuch smaller population of Ov sites under CO2 , H2 , and UV–
visible light. This difference explained the effective red-ox site
separation for water oxidation via ·OH radicals and Ov sites andCO2 reduction over Pd nanoparticle sites under CO2 , moisture,
and UV–visible light. The exchange of 13 CO2 with preadsorbed
12 CO over Pd/TiO reached the equilibrium in ∼20 h. This2
2
result suggested the earlier step of CO2 photoconversion, but weneed to be aware that 12 C-products are formed via photocatalytic
reaction tests under 13 CO2 .

AUTHOR CONTRIBUTIONSThis study was planned by YI, most of the experiments andanalyses were performed by HZ, and this manuscript was written
together.

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September 2018 | Volume 6 | Article 408

Zhang and Izumi

CO2 Photoconversion and TiO2 Surface

ACKNOWLEDGMENTS

Research Promotion Program (2015–) from the Institute for
Global Prominent Research, Chiba University. X-ray absorption
experiments were conducted under the approval of the Photon
Factory Proposal Review Committee (2016G577, 2015G586,
2014G631).

The authors are grateful for the financial supports from theGrant-in-Aid for Scientific Research C (17K05961, 26410204)
from the Japan Society for the Promotion of Science and Leading

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Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2018 Zhang and Izumi. This is an open-access article distributed
under the terms of the Creative Commons Attribution License (CC BY). The use,
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author(s) and the copyright owner(s) are credited and that the original publicationin this journal is cited, in accordance with accepted academic practice. No use,
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September 2018 | Volume 6 | Article 408