Photon Factory Activity Report 2017 #35 (2018)

12C, NW10A/2016G577

Monitoring of Electron Diffusion During High-Pressure CO2 Photoconversion inLayered Double Hydroxides
1Hongwei Zhang,1 Lukas Anton Wein,1 and Yasuo Izumi1,*Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku,
Chiba 263-8522, Japan

1 Introduction
Charges are spacially separated when semiconductors
are irradiated by ultraviolet (UV) or visible light whoselight energy is higher than the band gap ofsemiconductors. It is quite advantageous to reduce CO2into fuels using the separated electrons in semiconductors
in viewpoint of global warming and energy shortage [1].
We recently reported promoted CO2 photoconversion intomethanol using layered double hydroxides (LDHs)
preheated at 423 K. The optimum reaction pressure was0.40 MPa, i.e. 0.12 MPa of CO2 and 0.28 MPa of H2 [2].
In this report, electron diffusion created by UV–visible
light to the surface Cu active sites in/on LDHs for CO2
conversion was monitored by in situ X-ray absorption
near-edge structure (XANES).
2 Experimental Section
Zn, Cu, and Ga K-edge XAFS spectra were measured
in the KEK Photon Factory on beamline 12C and Photon
Factory Advanced Ring on beamline NW10A.

Fig. 1 Homemade high-pressure photocatalytic reactor forin situ XAFS measurements. (A) Side view and (B) topview.

[Zn3Ga(OH)8]2CO3·mH2O (abbreviated as Zn–Ga–CO3) and [Zn1.5Cu1.5Ga(OH)8]2CO3·mH2O (abbreviated asZn–Cu-Ga–CO3) LDHs were synthesized as described inRef. 2. The disk samples were set in a homemade
stainless reactor equipped with a pressure gauge, diamondwindows (thickness 0.50 mm) for X-ray and a quartz
window (thickness 4.0 mm) for UV–visible light (Fig. 1).
0.12–0.18 MPa of CO2 and 0.28–0.42 MPa of H2 wereintroduced to the reactor and the LDH sample wasirradiated by UV–visible light provided by a 500-W Xe
arc lamp (Ushio, model SX-UID502XAM). Obtained
XANES data were analyzed using the XDAP software
package.
3 Results and Discussion
In-situ Cu K-edge XANES of the most active Zn–Cu–Ga–CO3 [2] was monitored under reactant pressures of0.12 MPa CO2 and 0.28 MPa H2 irradiated by UV–visible
light during measurement at beamline for 2 h (Fig. 1).
The 1s-3d preedge peak appeared at 8980.6 eV (Fig. 2A,inset). The peak intensity progressively decreased (Fig.
2B) due to the reduction of Cu(II) (3d9 configuration) toCu(I) (3d10 configuration) by the diffusion ofphotogenerated electrons to the Cu sites. 1s-3d electronictransition is impossible to fully occupied 3d10 Cu site. Thepeak decreasing rate corresponds to 170 µmol-Cu h−1gcat−1 for 25 mg-disk sample of Φ = 1.0 cm.
Under 2.1 kPa of CO2 and 21.7 kPa of H2, Cu site
reduction rate of 580 µmol-Cu h−1 gcat−1 was reported
under similar conditions for 170 mg-disk Zn–Cu–Ga–
CO3 sample of Φ = 2.0 cm [3,4]. The reduction rate of Cu
at 0.40 MPa was 10 times greater compared to its
methanol formation rate (2.8 µmol-methanol h−1 gcat−1 [2]),taking the electron number(s) needed for each reaction(Cu(II)/(I) redox versus six electron reduction tomethanol) into account. Thus, the inlayer Cu sites once
accumulate photogenerated and diffused electrons.
However, the electrons transfer to CO2 (or CO2-derivedspecies) more easily under pressurized conditions (Fig. 2)
in comparison to that at a lower pressure (23.8 kPa) [3,4].
Thereby the net Cu reduction rate during this experiment(Figure 2B) seemed to be lower in comparison to that inreferences 3 and 4.

Photon Factory Activity Report 2017 #35 (2018)

Fig. 2 (A) In situ normalized Cu K-edge XANES spectra for Zn–Cu–Ga–CO3 (25 mg) under CO2 (0.12 MPa) and H2(0.28 MPa) and UV–visible light irradiation for 0 min, 62 min, and 120 min at beamline. (Inset) Expanded view of 1s-3d
preedge peak region. (B) Time course of the intensity of 1s-3d preedge peak for 2 h.

Acknowledgements
The authors are grateful for the financial supports fromthe Grant-in-Aid for Scientific Research C (17K05961,26410204) from Japan Society for the Promotion ofScience and Leading Research Promotion Program(2015–) from the Institute for Global Prominent Research,Chiba University. X-ray absorption experiments wereconducted under the approval of the Photon Factory
Proposal Review Committee.
References[1] Y. Izumi, Coord. Chem. Rev. 257, 171–186 (2013).
[2] L. A. Wein, H. Zhang, K. Urushidate, M. Miyano, andY. Izumi, Appl. Surf. Sci. 447, 687–696 (2018).
[3] M. Morikawa, N. Ahmed, Y. Yoshida, Y. Izumi, Appl.
Catal. B 144, 561–569 (2014).
[4] Y. Izumi, S. Kawamura, M. Morikawa, A. Naveed,
Photon Factory Act. Rep. 31(Highlights), 36–37(2014).
Research Achievements
Related work to this study monitoring electrons insemiconductuctorphotocatalysts
forCO2
photoconversion is reported as Highlights in Photon
Factory Activity Report 2017.
* yizumi@faculty.chiba-u.jp