Photon Factory Activity Report 2019 #37 (2020)

BL-9C, AR-NW10A / 2019G141

Monitoring the role of gold combined with ZrO2 for CO2 photoconversion irradiatedby light
Hongwei Zhang, Takehisa Konishi, and Yasuo Izumi*
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan

1 Introduction
The conversion of CO2 into fuels utilizing sustainableenergy sources completes the carbon neutral cycle in an
ideal, sustainable society. Solar fuels, in particular, havebeen identified as promising alternatives to fossil fuels
and have thus been widely investigated.1 However, solar
fuel generation from CO2 is a highly endothermic,
unfavorable process, and clarifying a reaction pathway
from CO2 to fuels is essential for making this approach
viable and further improvement of catalysts.2 ZrO2 hasbeen shown to successfully promote the photoconversionof 13CO2 into 13CO, enabled by both UV light-induced
band-gap excitation and heat-induced promotion effects
converted from localized surface plasmon resonance(LSPR) via Au doping. The photocatalytic role of Au wasclarified using Au L3-edge EXAFS at KEK.

2 Experimental Section
Au L3-edge EXAFS spectra were measured at 290 K intransmission mode at the Photon Factory on the NW10A
beamline and 9C beamline. On both beamlines, a Piezo
transducer was used to detune the X-ray to two-thirds ofthe maximum intensity to suppress higher harmonics. The
Au L3-edge absorption energy was calibrated at 11 921.2
eV using the spectrum of Au metal. A disk of the Au (5.0wt %)−ZrO2 photocatalyst was set in a Pyrex glass reactor
equipped with a Kapton film for X-ray transmission and aPET film for both UV–visible light and X-raytransmission. The reactor was filled with 2.3 kPa of CO2and 21.7 kPa of H2. The sample was irradiated with UV–visible light from a 500-W Xe arc lamp through the PET
film at the beamline. X-rays transmitted the disk
perpendicularly while incident angle of UV–visible lightwas 45°. For the curve-fit analyses, the empirical
amplitude extracted from the EXAFS data for the Au2O3and Au metal was used.3 The Debye temperature [θD(Bulk),
165 K] was used for bulk Au while a “surface Debyetemperature” [θD(Surf,⊥)] of 83 K was used to account forthe motion of vertical freedom of translation of fcc Au(11 1) or the (1 1 0) surface.

3 Results and Discussion
Light-induced changes in the Fourier transformfunction of the EXAFS data were quantitatively evaluated.
For the Au (5.0 wt %)–ZrO2 photocatalyst, the N value
was 10.8 ± 0.5 before light irradiation (Figure 1A), 10.2–
11.1 during light irradiation, and 10.5–11.1 after the light
was turned off. Based on the mean particle size range of
4.3 nm obtained for Au in HR-TEM, the dispersion of Au
particle was calculated to be 0.31.3

The σ value was calculated to be 0.009 968 nm for Au
metal at 290 K using the correlated Debye model2 withthe ab initio multiple-scattering calculation code, FEFF8,
and the Debye temperature for Au (165 K). The initial σvalue of 0.010 54 nm for Au (5.0 wt %)–ZrO2 before lightirradiation quickly increased to 0.011 29 nm (10 min lightirradiation), remained nearly constant between 0.011 24
and 0.011 34 nm after 70 min of irradiation, andprogressively increased to 0.011 40 nm after 110 min oflight irradiation (Figure 1B).
Subsequently, this value quickly decreased to 0.010 72
nm after the light was turned off after 120 min ofirradiation. The value further decreased gradually to0.010 55 nm at 190 min, which was essentially the samevalue as that before light irradiation.
A change in the major peak intensity at 0.24 nm (phaseshift uncorrected) was not completely in accord with thechange in the σ value during irradiation of UV–visible
light for 120 min (Figure 1B, C). The peak intensity
progressively decreased during light irradiation for 110
min, whereas the σ value increased only during the first
10 min but essentially remained constant between 10–110
min of light irradiation. The complex correlation was
further analysed via log-ratio method analyses.3 The
result demonstrated that the combined effects of theN(Au–Au) and σ value changes resulted in a gradualdecrease in he peak intensity at 0.24 nm (phase shiftuncorrected), i.e. the initial sudden increase of the σ value
and subsequent gradual, slow decrease of the N value.
Furthermore, we evaluated the temperature at the Ausite based on the σ values. The temperature dependence ofσ value is derived from FEFF8 combined with thecorrelated Debye model for both bulk and surface Au
sites using θD(Bulk) and θD(Surf, ⊥ ) (Figure 1E-a and b,respectively). We assume preferable exposure ofthermodynamically stable fcc(1 1 1) or (1 1 0) face. Wealso approximated the mean Au nanoparticle temperatureas the arithmetic mean temperature based on θD(Surf, ⊥ )weighted by 1/2·1/3D [D: dispersion of nanoparticles(0.31), for an effective vertical degree of freedom at a freehemisphere surface] and that based on the θD(Bulk)
weighted by (1 – D) + 1/2D + 1/2·2/3D (bulk site, nonfree hemisphere in contact with ZrO2 and two lateraldegrees of freedom at a free hemisphere surface).
As a result, the initial temperature of 295 K before light
exposure rose to 316 K after 10 min of irradiation,
remained nearly constant between 314 and 318 K after 70
min of irradiation, and progressively increased to 321 K
after 110 min of light irradiation (Figure 1B). The
temperature quickly dropped to 301 K after the light wasturned off. The temperature gradually decreased further toPhoton Factory Activity Report 2019 #37 (2020)

its initial value before light irradiation (295 K) after 190
min.

In summary, the contribution of mean 4.3 nm of Au
nanoparticles was investigated using in-situ EXAFS.
Temperature rise from 295 K to 321 K under theirradiation of UV–visible light and drop to 295 K underdark were directly monitored based on Debye–Waller
factor change for Au–Au interatomic pair interference.
The lower Debye temperature of Au (165 K) compared toAg (225 K) resulted in greater heat capacity and thetemperature rise via LSPR was smaller, 26 K under CO2,
H2, and UV–visible light using Au (5.0 wt %)–ZrO2 incomparison to 106 K under similar conditions using Ag(5.0 wt %)–ZrO2.2 ZrO2 was effective in promoting both
CO and H2 formation under CO2, moisture, and UV–visible light conditions, but Au-doped ZrO2 directed theproducts toward H2 production (96%) via a reverse
reaction step. During this pathway, a proton is coupledwith an electron, and H2 formation occurs over the Au
surface.3

Acknowledgements
The authors are grateful for the financial supports fromthe Grant-in-Aid for Scientific Research (17K05961)
from Japan Society for the Promotion of Science, Grants
for Academic Research from the Japan Gas Association(2018), and Leading Research Promotion Program (2015–2019) from the Institute for Global Prominent Research,Chiba University.

References(1) Y. Izumi, Coord. Chem. Rev. 257, 171–186 (2013).
(2) H. Zhang, T. Itoi, T. Konishi, and Y. Izumi, J. Am.
Chem. Soc., 141, 6292–6301 (2019).
(3) H. Zhang, T. Itoi, K. Niki, T. Konishi, and Y. Izumi,
Catal. Today, DOI: 10.1016/j.cattod.2020.02.040.
* yizumi@faculty.chiba-u.jp

Figure 1. Time course changes of (A) N values, (B) σvalues, and (C, D) Fourier transform obtained from k3weighted Au L3-edge EXAFS χ-function for Au (5.0wt%)–ZrO2 under CO2 (2.3 kPa) and H2 (21.7 kPa) (C)irradiated by UV–visible light for 120 min followed by(D) dark conditions for 120 min. (E) The correlation
between the σ value and temperature for bulk sites (circle,◦) and surface sites (vertical motion; square, □) in/on theAu metal generated by the correlated Debye model using
an FEFF8 code.