Photon Factory Activity Report 2019 #37 (2020)

BL-15A1/2017G516

Chemical State Analysis of Cerium Oxide under H2 atmosphereby Means of XAFS-XRD Simultaneous Measurement
Takashi Ukawa1, Kaho Nishide1, Yusaku Yamamoto1,Hirokazu Kitazawa1, Misaki Katayama2, and Yasuhiro Inada1, *1
Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University,
1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan2
SR Center, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan
1 Introduction
Cerium oxide is one of the important functionalmaterials for exhaust gas purification because of its
excellent property for the oxygen storage and release [1].
It is important for understanding this property to analyse
the chemical state conversion and crystal structure changeof the Ce species under reaction conditions. The XAFS
technique is a powerful tool for chemical state analysis,
and it provides the short-range information. On the other
hand, the long-range information is available using theXRD technique. Therefore, the XAFS-XRD simultaneous
measurement of CeO2 under the reaction condition leads toclarify the chemical conversion during the oxygenstorage/release process. The purpose of this research is toevaluate the chemical state and crystal structure change ofCeO2 during the temperature-programmed reduction(TPR) process.

CeO2. The formation of the non-stoichiometric CeO1.73
phase expands the plane spacing because of the partial
reduction of the Ce sites.
The present in-situ XAFS-XRD simultaneous
measurement revealed that the reduction process of CeO 2
was expressed as eq. 1.
CeO2 → CeO1.73 → CeO1.50(1)This study provides the formation conditions of the CeO1.73
phase with the O atom defects during the TPR process.
37.85724
37.65723
37.45722

3 Results and Discussion
The lower energy shift was observed for the Ce LIII-edge
during the TPR process, and the final edge energy becamealmost comparable to that of the Ce(III) species. In
addition, the diffraction line derived from Ce 2O3 wasappeared at 1020 °C. These results indicated that CeO2
reduced to Ce2O3. The absorption edge energy (E0) and theXRD angle (2) are plotted in Figure 1 as a function oftemperature. It was found that the reduction of CeO2
proceeded at two temperature range, 390-570 °C and 7801020 °C.
The intermediate state was found to generate in thetemperature range from 570 to 780 °C, and there was no
XRD lines of Ce2O3. It means that the intermediate state
disagrees with the mixture of CeO2 and Ce2O3, and it isthus assigned to the non-stoichiometric compound
represented by CeOx (1.50 < x < 2). The value of x was
estimated to be 1.73 according to the E0 shift. The spacingof (111) plane of CeO1.73 is ca. 0.03 Å larger than that of

37.2

E0 / eV

CeO2
111

5719

Intensity

5721

5720

Ce2O3011
1020 ºC
960 ºC
900 ºC840 ºC780 ºC750 ºC
720 ºC
660 ºC
570 ºC
510 ºC450 ºC390 ºC270 ºC
11 ºC

5718

37.02 / degree

2 Experiment
The in-situ XAFS-XRD simultaneous measurements ofstandard CeO2 were performed at BL-15A1 of Photon
Factory (KEK). The CeO2 powder diluted by BN washeated from room temperature to 1020 °C under H2atmosphere. The XAFS measurements were performed bythe transmission mode over the range from 5500 to 6150eV. After the XAFS measurement, the X-ray energy wasset to be 6150 eV for the XRD measurement. The intensityof diffraction X-rays was recorded by the PILATUS 100K.

36.8

36.6

36.4

36 37 38 39 40 41

2  degree

5717
0

200

400

600

800

36.2
1000

T / ºCFigure 1. The shifts of absorption edge energy (E0) anddiffraction line angle (2 angle of CeO2 111 reflection)
during the TPR process of CeO2. The XRD patterns atsome temperatures were inserted.
References[1] J. Kašper, P. Fornasiero, and M. Graziani, Catal. Today,
50, 285 (1999).
* yinada@fc.ritsumei.ac.jp