Photon Factory Activity Report 2022 #40 (2023)

AR-NW10A, BL-9C/2019P018, 2021G136

XAFS characterization of Co-doped ZrO2 catalyst.

Abhijit Shrotri1,*, Kiyotaka Asakura1,2, Nazmul Dostagir1,3, Jin Ota1,2, Atsushi Fukuoka1
Institute for Catalysis, Hokkaido University, Kita 21 Nishi 10, Kita-ku, Sapporo, Hokkaido 0010021, Japan
2
Division of Quantum Science and Engineering, Graduate School of Engineering, Hokkaido
University, Kita 21-Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan
3
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13 Nishi 8,Kita-ku, Sapporo, Hokkaido 060-8628, Japan

11 Introduction
Recycling of CO2 is an attractive approach to avoid theuse of fossil fuels and produce value added chemical froma waste resource. Reduction of CO2 via reverse water gas
shift (RWGS) reaction produces CO, which is valuable forchemical synthesis. In RWGS reaction, achieving high CO
selectivity at high conversion and low temperature is achallenge because of the formation of methanol andmethane. Here we investigated the use of Co doped ZrO2
catalyst for CO2 hydrogenation.[1] XAFS analysis wasused to determine the nature of Co and Zr atoms in Co
doped ZrO2 catalyst at increasing Co loading.
2 Experiment
Co doped ZrO2 catalysts were prepared bycoprecipitation method with Co loading of 1-15, and 50
atoms%. XAFS measurements were carried out at BL9Cand NW10A beamlines of Photon Factory. For Co K-edge
XAFS analysis, the X-rays emitted from 2.5 GeV Storage
ring operating at 450mA were monochromatized by aSi(111) double crystal monochromator. X-ray beam wasfocused, and higher harmonics were rejected by a bent
cylindrical mirror. The incident and transmitted X-rays
were monitored by 17 cm and 34 cm long ionizationchambers filled with N2. The samples were diluted withboron nitride and pressed into pellets. Data analysis wascarried out using the REX package programs. Additional
analysis was done using Athena software using standard
parameters.
3 Results and Discussion
The presence of large amount of ZrO2 caused severe
absorption and limited the Co K-edge height to 0.1-0.2. In
the X-ray absorption near-edge spectrum (XANES), theedge position for stable doped catalysts CoZrOx (X = 5, 10,and 15) was at 7722 eV, which was assigned to divalent
Co2+ species. CoZrOx (50) was the exception among doped
catalysts and had a higher edge position of 7726 eV due tothe presence of Co3+ as found in Co3O4 (Figure 1a). In thepre-edge region, the height of the pre-edge peak around7710 eV was assigned to 1s to 3d transition, which wassmaller when centrosymmetry was present, for example, inCoO and Co(CH3COO)2 (Figure 1b). Larger peaks wereobserved for Co3O4/ZrO2 and CoZrOx (50), containing
Co3O4 having a tetrahedral structure without

centrosymmetry. The pre-edge peaks for doped catalysts
were larger than CoO and Co(CH3COO)2, indicating a lossof centrosymmetry in the Co2+ local structure owing to its
doping in the tetragonal ZrO2. The EXAFS region for theCoZrOx (10) catalyst is shown in Figure 1c. The noise inthe EXAFS region was high due to the large backgroundabsorbance of ZrO2. Nevertheless, the Fourier transform ofCoZrOx (10) differed from that of CoO and Co3O4. Curvefitting showed that the Co–O distance of 1.88 Å in CoZrOx(10) was shorter than that of CoO (Table 1). The small peak
appearing at 2–3 Å was well fitted with Co–Zr interaction
although with a short bond distance of 2.71 Å. Further
studies are necessary to confirm the presence of Co–Zr
interaction at this distance.

Figure 1: (a) XANES of doped and impregnatedcatalysts along with standard Co compounds. (b)Expanded spectra showing the pre-edge region for the Co
XANES. (c) Fourier transform of EXAFS region forCoZrOx (10) and reference Co oxides. (d) Fouriertransform of EXAFS region of Zr K-edge for CoZrOx (X)catalysts along with undoped ZrO2.
The phase transition in ZrO2 with Co loading was also
evident in the EXAFS analysis of Zr K-edge for doped

Photon Factory Activity Report 2022 #40 (2023)

catalysts (Figure 1d). Undoped ZrO2 and CoZrOx (1)exhibited a monoclinic structure with Zr–O and Zr–Zr
distances of 2.12 and 3.45 Å, respectively. Tetragonal ZrO2
was observed for CoZrOx (5), CoZrOx (10), and CoZrOx(15) catalysts as the Zr–O distance reduced to 2.08 Å andthe Zr–Zr distance increased to 3.65 Å. CoZrOx (50)showed features of monoclinic ZrO2 lacking long-rangeorder in EXAFS.
Table 1: Co-ordination number and radial distanceobtained for fitting of EXAFS data for CoZrOx (10) incomparison to CoO and Co3O4. S2 = 0.83 for Co-O which
was determined by using FEFF.
Sample

NCo-O
RCo-O (Å)σCo-O (Å)∆ECo-O (eV)NCo-Zr
RCo-Zr (Å)σCo-Zr (Å)∆ECo-Zr (eV)R (%)

CoZrOx (10)

2.3
1.88
0.63
2.0
0.502.71
0.03
4.68
6.38

Co3O4

6.0
1.94
0.054
26.8
1.3

CoO6
2.12
0.078
16.3
4.3

Through the above analysis we could conclude that theCo atoms were divalent in the doped catalysts and did notresemble the crystal structure of CoO or Co3O4. The
transformation of monoclinic to tetragonal ZrO2 happened
at Co loading between 1 – 5 atom%.
References[1] N. H. MD Dostagir, R. Rattanawan, M. Gao, J. Ota, J.
Hasegawa, K. Asakura, A. Fukouka, and A. Shrotri
ACS Catalysis 2021 11 (15), 9450-9461Research Achievements
1. Abhijit Shrotri, Outstanding Discussion Award, 129th
Catalysis Society Meeting, Catalysis society of Japan.
4 April 2022.
* ashrotri@cat.hokudai.ac.jp