Photon Factory Activity Report 2018 #36 (2019)

BL-9C, NW10A/2017G534

XAFS Characterization of the Redox-active Ceria-based Mixed Oxides
Co-introduced with the First and Second Series Transition Metals
Satoshi MURATSUGU*1, Satoru IKEMOTO1, Xiubing HUANG1, Shoko NAGASE1,Gen-ichi YOKOTA1, Mizuki TADA1,2
1
2

Department of Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan.
Research Center for Materials Science (RCMS) & Integrated Research Consortium on Chemical
Science (IRCCS), Nagoya University, Furo-cho, Chikusa, Aichi 464-8602, Japan.

1 IntroductionIntroduction of different metal ions to cerium dioxide isone of the promising methods to reduce the redox
reaction temperature and increase oxygen storage/releasecapacity of cerium oxide. We prepared ceria-based mixed
oxides by co-introduction of first- and second- periodtransition metals to ceria and achieved high redox activity
at low temperatures. For example, the introduction ofboth Cr and Rh decreased the reduction temperature tobelow 373 K on ceria. The oxidation state changes of Ce,Cr, and Rh with H2 reduction process were investigatedby in situ Ce LIII-edge, Cr K-edge, and Rh K-edge
XANES to clarify which elements are responsible for thelow-temperature reduction.
2 Experimental
Ce LIII-edge and Cr K-edge XANES were measured ina transmission mode at the BL-9C station with a Si(111)
double-crystal monochromator. Ionization chambers
filled with pure N2/He (70/30 v/v) and pure N2 were used
to monitor incident and transmitted X-rays, respectively.
Rh K-edge XANES and EXAFS were measured in atransmission mode at the AR NW-10A station with aSi(311) double-crystal monochromator. Ionizationchambers filled with pure Ar and pure Kr were used tomonitor incident and transmitted X-rays, respectively.
In situ QXAFS measurements during H2 reduction wereperformed as follows. The sample was placed in an in situ
temperature-controlled gas flow cell. After flushing withN2 (100 sccm), the cell was heated to 303 K and kept at
this temperature for 5 min, at which QXAFS
measurement was started. After 5 min, the gas wasexchanged to H2 + N2 (50 + 50 sccm) and the temperature
was held for another 5 min. Then, the cell was heated to423 K at a rate of 2 K min-1. After keeping thetemperature at 423 K, the gas was changed to N2 (100sccm), and the cell was cooled to room temperature.
XANES spectra were analysed using ATHENA andARTHEMIS programs.
3 Results and Discussion
Figure 1 (A-C) show in situ Ce LIII-edge, Cr K-edge,and Rh K-edge XANES spectral changes of ceria-based
mixed oxide with Cr and Rh during H2 reduction. All
three species were involved in the reduction processes.
The Rh K-edge XANES spectra firstly changed at thelowest temperature of around 332 K. (Figure 1(A)).
Following the reduction of Rh, both the Cr K-edge andCe LIII-edge XANES spectra changed remarkably (Figure1(B, C)), which proved the oxidation state decrease inboth Cr and Ce ions. These results showed that the lowtemperature reduction of ceria-based mixed oxide with Crand Rh proceeded via the multi-reduction of the three
metal species, not by a single metal source in the oxide.
Figure 1(D) shows change in Rh K-edge EXAFS FT
before and after the reduction. Before the reduction, there
was only Rh-O bond at 0.204 ± 0.001 nm (coordinationnumber (CN) = 5.8 ± 1.3). After the reduction, Rh-Rh
bonds at 0.270 ± 0.002 nm (CN = 2.6 ± 0.7) wereobserved together with the Rh-O bond at 0.205 ± 0.002
nm. This result indicates that the formation of small Rh
nanoclusters were also important for the acceleration of
low-temperature redox process.

Figure 1. In situ (A) Rh K-edge XANES spectra (B) Cr
K-edge XANES spectra, and (C) Ce LIII-edge XANES
spectra of ceria-based mixed oxide with Cr and Rh during
H2 reduction. (D) k3-Weighted Rh K-edge EXAFS
Fourier transforms (k = 30–140 nm-1) of ceria-based
mixed oxide with Cr and Rh (a) before and (b) after thereduction.
* smuratsugu@chem.nagoya-u.ac.jp