Photon Factory Activity Report 2022 #40 (2023)

BL-9C, BL-12C, AR-NW10A/2022G527

Debye–Waller factor analysis of Co0 site on ZrO

2under UV–visible lightirradiation for CO photoconversion
Tomoki Oyumi, Rento Ishii, and Yasuo Izumi*
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku
Chiba 263-8522, Japan
1. Introduction
In contrast to irreversible fossil fuel consumption asfuels and resources, CO2 reduction into fuels and/orvaluable chemicals using a sustainable energy sourcecompletes a new carbon-neutral cycle [1,2].
Photocatalytic CO2 reduction has the advantage of being
direct and simple; however, the products have been
limited to CO, CH4, and CH3OH, in contrast to theelectrochemical production of formate, C2H6, C2H4,CH3CHO, C2H5OH, C3H8, C3H6, C3H7OH, acetate, andoxalate from CO2 and/or CO owing to the concentrated
electrons supplied from electricity. The photocatalyticallyformed C2 and C3 hydrocarbons (HCs) from CO2 haverecently been economically viable key chemicals (0.9–8$ kg−1) compared to CO and CH4 (0.06–0.18 $ kg−1) [3].
This study reports the EXAFS analysis for the switchoverof photocatalytic pathways from CO2 to C1–3 paraffinsversus from CO to selective C2H4 and C3H6, using a Co0–ZrO2 catalyst.

Figure 1. Correlation between the Debye–Waller factor (σ) andtemperature for (a) bulk sites (circle, ○) and (b) surface sites(vertical motion; square, □) in/on the Co metal generated using
the correlated Debye model with a FEFF8 code [9].

2. Experimental section

3. Results and Discussion

Cobalt K-edge XAFS spectra were measured in thetransmission mode at the Photon Factory, High Energy
Accelerator Research Organization (Tsukuba, Japan)
using a Si(1 1 1) monochromator, a Pt-coated mirror, anda piezo transducer [4,5]. The Co–ZrO2 samples weretreated in quartz U-tube and transferred to Pyrex cellfilled with reaction gases, which was equipped with aKapton film (Dupont, Wilmington, DE, USA; 50 μmthick) on one side for X-ray transmission and apolyethene terephthalate film (Toyobo, Japan, G2, 50 μmthick) on the other side for both UV–visible light and Xray transmission. The distance between the fiber light exitof the Y-shaped quartz light guide and the photocatalyst
was 20 mm.
The obtained Co K-edge XAFS data were analyzed
using the XDAP software package [6]. The pre-edge
background was approximated with a modified Victoreen
function, C2/E2 + C1/E + C0, where E is the photon energy
and C0, C1, and C2 are constants. Multiple-shell curve-fit
analyses were performed with the data obtained on theEXAFS using the empirical amplitude extracted from theEXAFS data for the Co metal foil (10 μm thick). Theinteratomic distance (R) and coordination number (N)values for the Co–Co and Co–O interatomic pairs were
set to 0.2502 nm and 12 [7] and 0.2131 nm and 6 [8],respectively.
The temperature dependence of the Debye–Waller
factor (σ) was calculated for the bulk site and surface siteof Co metal (Figure 1) using the correlated Debye modelwith the ab initio multiple-scattering calculation code,
FEFF8 [9], and bulk and surface Debye temperatures.

The σ value was calculated based on the correlated
Debye model for bulk and surface Co sites (verticalmotion versus surface) and the Debye temperature forbulk θD(Bulk) (445 K [10]) and surface θD(Surface,⊥) for thevertical motion of freedom (211 K [11]). We
approximated the mean Co nanoparticle temperature (T)as the arithmetic mean value based on θD(Surface,⊥) weightedusing 1/2·1/3 D for vertical translational motion at a freehemisphere surface and θD(Bulk) weighted using(1−D)+(1/2)D+(1/2·2/3)D for the bulk site, nonfree
hemisphere in contact with ZrO2, and horizontaltranslational motion at a free hemisphere surface.
Tnanoparticle = TSurface,⊥ × 1/6 D + TBulk × (1−1/6 D)(1)

The σ value for samples (σsample) was calculated usingeq. 2, taking the contribution of structural disorder(σdisorder) and the difference of the σsample value from the Co
metal foil (σXDAP) into account.
σsample2 = σCo metal, correlated Debye2 + Δ(σdisorder2) +Δ(σXDAP2)
At 296 K, σsample and σCo metal, correlated Debye values were0.007 (28) and 0.007 (05) nm, respectively, for thenanoparticle model above based on the correlated Debye
model (Figure 1) and eq. 1. The σXDAP value was given as0.001 (00) nm. Thus, σdisorder was evaluated as 0.001 (82)nm.
The time course of Fourier transform, σ value, and Co0site temperature under CO (2.3 kPa), H2 (2.3 kPa), Co(7.5 wt %)–ZrO2 reduced under H2 at 973 K, and UV–Photon Factory Activity Report 2022 #40 (2023)

visible light (Figure 2) behaved very similarly to thereport last year under CO2 (2.3 kPa), H2 (2.3 kPa), Co (7.5wt %)–ZrO2 reduced under H2 at 973 K, and UV–visible
light [12], irrelevant to reactants [3]. Thus, thetemperature change was concluded mostly owing to lightenergy.

Interpretation of X-Ray-Absorption Near-Edge Structure.
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[11] Hou, M.; El Azzaoui, M.; Pattyn, H.; Verheyden, J.; Koops,G.; Zhang, G. Growth and Lattice Dynamics of Co
Nanoparticles Embedded in Ag: A Combined MolecularDynamics Simulation and Mössbauer Study. Phys. Rev. B
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[12] Loumissi, T.; Oyumi, T.; Yu, J.; Ishii, R.; Izumi, Y. Photon
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2019G141.
http://pfwww.kek.jp/acr/2021pdf/u_reports/pf21b0145.pdf* yizumi@faculty.chiba-u.jp

Figure 2. Time course of Debye–Waller factor based on the Co
K-edge EXAFS analysis and the evaluated temperature of Co
nanoparticles in Co (7.5 wt %)–ZrO2 reduced under H2 at 973 K
using CO (2.3 kPa), H2 (2.3 kPa), and UV–visible lightirradiation based on correlated Debye model.

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