Photon Factory Activity Report 2016 #34 (2017)%/9C, %/12C, $5NW10A/2016G577

Demonstration of In Situ High-Pressure Photocatalytic Cell for XAFS Measurements
1Masaya Miyano,1 Hongwei Zhang,1 and Yasuo Izumi1,*Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku,
Chiba 263-8522, Japan

1 Introduction
Charge transfer is a key step in photocatalysis andmore efficient charge transfer from surface to substrates
is expected if the pressure of reactants is elevated. We
recently demonstrated promoted photocatalytic CO2
conversion into fuels (methane, methanol, and CO) at
0.40–0.80 MPa using several semiconductor-based
photocatalysts including layered double hydroxides(LDHs) [1,2].
However, complex dependence of photocatalytic rates
on reactant pressure, i.e. P(CO2) and P(H2), was observed
when H2 was used as a reductant instead of H2O, resulting
in volcano-type dependence peaked at P(CO2) = 0.12 kPa
and P(H2) = 0.28 kPa [3]. Hence, the states of the active
sites in LDH photocatalyst were investigated by in situ
XAFS at high pressure in this study.
2 Experimental Section
Zn and Ga K-edge XAFS spectra were measured in theKEK Photon Factory on beamline 12C. The storage ringenergy was 2.5 eV. A Si (1 1 1) double-crystal
monochromator and a pair of bent cylindrical mirrors
were inserted into the X-ray beam path.

irradiated by UV–visible light provided by a 500-W
xenon arc lamp (Ushio). XAFS data were analyzed using
the XDAP software package.
3 Results and Discussion
The Zn K-edge and Ga K-edge XANES spectra during
CO2 photoconversion test at 0.60 MPa are shown in Fig.
2A and B, respectively. Under the condition, CO2 isphotoconverted into methane and methanol [1].
Corresponding to the performance, the whiteline peakintensity at 10379 eV near the Ga K-edge decreased by
2.1% when the UV–visible light was irradiated ascompared to that under dark. In contrast, the whiteline
peak intensity at 9671 eV near the Zn K-edge negligiblychanged or decreased only by 1.2% when the UV–visible
light was irradated as compared to that under dark. Thischange of whiteline peak intensity by the UV–visible
light irradiation was only observed at elevated pressure,and not at 0.1 MPa.

Fig. 1 Homemade high-pressure photocatalytic reactor
for in situ XAFS measurements. (A) Side view and (B)top view.
The [Zn3Ga(OH)8]2CO3·mH2O LDH sample was set ina homemade stainless reactor equipped with a pressuregauge, diamond windows (thickness 0.50 mm) for X-ray
and a quartz window (thickness 4.0 mm) for UV–visible
light (Fig. 1). 0.18 kPa of CO2 and 0.42 kPa of H2 wereintroduced to the reactor and the LDH sample wasFig. 2 In situ XANES spectra at (A) Zn K-edge and (B)Ga K-edge for [Zn3Ga(OH)8]2CO3·mH2O LDH under CO2(0.18 MPa) and H2 (0.42 MPa) and UV–visible lightirradiation at beamline (Fig. 1).
The decrease of whiteline intensity should be related tothe partial occupancy of 4s and 4p states by thePhoton Factory Activity Report 2016 #34 (2017)

photogenerated electrons. The possibility of partiallyreduced Ga ions adsorbed with intermediate species atelevated pressure needs to be verified.
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] M. Miyano, H. Zhang, M. Yoshiba, and Y. Izumi,Ener. Technol. 5, 892–900 (2017).
[2] H. Zhang, S. Kawamura, M. Tamba, T. Kojima, M.
Yoshiba, and Y. Izumi, J. Catal., in press (2017).
[3] S. Kawamura, H. Zhang, M. Tamba, T. Kojima, M.
Miyano, Y. Yoshida, M. Yoshiba, and Y. Izumi, J.
Catal. 345, 39–52 (2017).
* yizumi@faculty.chiba-u.jp