Photon Factory Activity Report 2016 #34 (2017)
BL-9A, BL-9C, BL-12C, AR-NW10A/2014G631

Behavior of Oxygen During Photocatalytic CO2 Conversion into Fuels Using Pd/TiO2
1

Hongwei Zhang1 and Yasuo Izumi1,*
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku,
Chiba 263-8522, Japan

1 Introduction
Photocatalytic conversion of CO2 into fuels is one of
the routes to producing carbon neutral fuels without a netincrease in atmospheric CO2 concentrations associatedwith fossil-derived alternatives [1]. Pd nanoparticlessupported on TiO2 (Pd/TiO2) were efficient forphotocatalytic conversion of CO2 [2], however, the photooxidation mechanism over the Pd/TiO2 photocatalyst isstill unclear. To clarify the mechanism of the photooxidation of water molecules, in situ Ti K-edge XAFS
study was performed.
2 Experimental Section
Ti K-edge XAFS spectra were measured in the KEK
Photon Factory on beamline 9A, 9C, and 12C. Pd K-edge
XAFS results measured on beamline NW10A werealready reported [3]. The storage ring energy was 2.5 eVand the ring current was 449.3–324.4 mA. A Si (1 1 1)
double-crystal monochromator and a pair of bent
conical/cylindrical mirrors and/or double flat mirrors
were inserted into the X-ray beam path. Spectra for thePd/TiO2 sample (10 mg) diluted by boron nitride weremeasured in the presence/absence of UV–visible
irradiation provided by a 500-W xenon arc lamp (Ushio)at the beamline. The Ti K-edge absorption energy wascalibrated to 4964.5 eV using the spectrum of the 5-μmthick Ti foil. XAFS data were analyzed using the XDAP
software package.
3 Results and Discussion
Under CO2 (100 kPa), moisture (2.2 kPa), and UV–
visible light, the change in coordination number N(Ti–O)
was monitored for Pd/TiO2 sample (Fig. 1A, 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
minutes after the light was off, the N(Ti–O) value
increased to the original value (6.0). N(Ti(–O–)Ti) valuesfollowed a similar trend: decrease from initial 11.3 to9.7–10.6 during the irradiation of UV–visible light andincrease to 10.9 at 30 minutes after the light was off (Fig.
1B). Taking the experimental and fit errors into account(Fig. 1A, B), the N(Ti–O) and N(Ti(–O–)Ti) values would
evaluate the concentration of oxygen vacancy (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. 1D–F).
In comparison to data for fresh sample under CO2 (100kPa), moisture (2.2 kPa), and dark (Fig. 1D: 0 min), thepeak pattern changed after the UV–visible light wasirradiated, especially in the region between 4968 and4971 eV (Fig. 1D: 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 [4,5] or 4
coordination Ti sites [5] in comparison to that at 4968.8
eV owing to 6 coordination Ti sites [4]. Please note that
there is minor energy calibration difference between
reference 4 and this study.
While the peak pattern negligibly changed during theUV–visible irradiation (Fig. 1E: 60, 100, 130, 150 min;Fig, 1F: 180 min), the pattern clearly changed after thelight was off (Fig. 1F: light off 0, 30 min). The last two
peak patterns were very similar to that of fresh sample
before light irradiation (Fig. 1D: 0 min), in consistent
with the reversible change of the N(Ti–O) and N(Ti(–O–)Ti) values based on EXAFS analyses (Fig. 1A, B).

Fig. 1: 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 hirradiated 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 k3weighted EXAFS χ function.

Photon Factory Activity Report 2016 #34 (2017)

We consider the oxidation reaction using moisture onTiO2 combined with Pd nanoparticles. Water is activatedby a hole to form the ·OH radical
H2O (g) + h+ → ·OH + H+ (surface).
The subsequent reaction of ·OH with a surface hydroxygroup/O atom is fast, e.g., to form O2, an OV site, and aproton(s) due to the nature of the radical reaction.
·OH + OH (surface) + 3h+ → O2(g) + OV + 2H+ (surface)·OH + O (surface) + 3h+ → O2(g) + OV + H+ (surface)In fact, the concentration of OV sites dramaticallyincreases on/in TiO2 as monitored by the decrease of theN(Ti–O) value from 6.0 for fresh sample to 4.9–5.7 andthe N(Ti(–O–)Ti) value from 11.3 for fresh sample to9.7–10.6 by the monitoring during the irradiation of UV–
visible light (3.5 h) and darkness (0.5 h) (Fig. 1). The
protons formed near the interface between the metal
nanoparticle and the TiO2 surface are transferred to themetal nanoparticle to form C-containing products or H2,
whether they encounter CO2-derived species andhot/excited electrons, or such electrons only [6].
Effectively separated redox sites are nicely conjugated totransport protons between them, similar to photosystems.
Acknowledgements
The authors are grateful for the financial supports fromthe Grant-in-Aid for Scientific Research C (26410204)
from Japan Society for the Promotion of Science andLeading Research Promotion Program (2015–) from theInstitute for Global Prominent Research, Chiba
University.
References[1] Y. Izumi, Coord. Chem. Rev. 257, 171–186 (2013).
[2] S. Kawamura, H. Zhang, M. Tamba, T. Kojima, M.
Miyano, Y. Yoshida, M. Yoshiba, and Y. Izumi, J.
Catal. 345, 39–52 (2017).
[3] H. Zhang, S. Kawamura, and Y. Izumi, Photon
Factory Act. Rep. 33, 2014G631 (2015).
[4] H. Yoshitake, T. Sugihara, and T. Tatsumi, Phys.
Chem. Chem. Phys. 5, 767–772 (2003).
[5] F. Farges, G. E. Brown, Jr., J. J. Rehr, Phys. Rev. B 56,1809–1819 (1997).
[6] H. Zhang, S. Kawamura, M. Tamba, T. Kojima, M.
Yoshiba, and Y. Izumi, J. Catal., in press (2017).
* yizumi@faculty.chiba-u.jp