Photon Factory Activity Report 2022 #40 (2023)

BL-9C/2022G061

XAFS characterization for CoWO4@W18O49 catalyst
Rajaram Bal1,* and Takehiko Sasaki2
1
Light Stock Processing Division, CSIR- Indian Institute of Petroleum,
Dehradun-248005, India
2
Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa,
Chiba, 277-8561, Japan
1 Introduction
Among the most difficult and significant reactions in
the chemical industry is the selective oxidation ofcyclohexane in the presence of air to high value-addedintermediates such as cyclohexanone and cyclohexanol(the combination is named as ’KA oil,’ where ’K’ and ’A’represent ketone and alcohol, respectively). For theconversion of cyclohexane to KA oil a CoWO4@W18O49
nano-structured catalyst with abundant oxygen vacancy
sites was successfully synthesized via a co-precipitation
method. This catalyst selectively oxidized cyclohexane
to KA oil in solvent-free conditions, where under
optimum conditions, ~24 % cyclohexane conversion with
~93 % KA oil selectivity was achieved. The catalyst wascharacterized by EXAFS in addition to XRD, TEM, XPS
and Raman [1].
2 Experiment
CoWO4@W18O49 catalysts were prepared by a wet
chemical method.
Extended X-ray absorption finestructure (EXAFS) measurements at the Co-K edge werecarried out in transmission mode at room temperature at
the BL9C station of the Photon Factory at the Institute ofMaterials Structure Science, High Energy Accelerator
Research Organization in Japan (KEK-IMSS-PF). The
electron storage ring was operated at 2.5 GeV. A Si(111)channel-cut crystal was used as a monochromator.
Ionization chambers for incident X-rays (I0) andtransmitted X-rays (I) were filled with N2(100%) and25%Ar/N2 gas, respectively. EXAFS data were analyzed
and fitted using Athena and Artemis package based on
IFFEFFIT program Backscattering amplitude, and phaseshift were obtained by FEFF calculation.
3 Results and Discussion
Fig. 1 show the observed and fitted data for k3weighted Fourier transforms of Co K-edge for fresh
CoWO4@W18O49 catalyst. Since XRD confirmed theCoWO4 phase, the cif file for CoWO4 was used to obtainthe scattering paths from the Co atom as an X-ray
absorbing atom for Co K-edge EXAFS fitting procedure.
Fitting result was shown in Table 1. The first, second, third,fifthe and sixth paths are associated with the CoO bond,while the Co-Co and Co-W bonds were associated with thefourth and seventh paths, respectively. The results of theXRD are consistent with the existence of Co-W species inthe fresh catalyst.

Fig. 1 k3-weighted Fourier transforms of Co K-edge
for fresh CoWO4@W18O49 catalyst. (Amplitude (solidcurve) and imaginary part (dotted curve), thick curve(observed) and thin curve (fitted)).
Table 1: Summary of the EXAFS fitting results for thefresh CoWO4@W18O49 catalyst

PathR (10-1nm)

CN

Co-O1Co-O2
Co-O3Co-Co
Co-O4Co-O5Co-W

2.002±0.019
2.091±0.019
2.126±0.019
3.120±0.019
3.193±0.019
3.369±0.019
3.547±0.035

2.00±0.44
2.00±0.44
2.00±0.44
1.90±0.78
2.00±0.44
2.00±0.44
2.15±1.46

DW (10-5nm2)

5.9±2.5

Rf (%)

2.9

K range 3.0-11.0 (10 nm-1), R range 1.0-3.4 (10-1nm),∆E0 -2.5±2.9 (eV).
The sample used for the catalytic reaction was alsomeasured and analyzed in the same way (not shown). The
comparison of the FT-EXAFS spectra and the fitting
results confirmed that the catalyst phase is retained during
the catalytic reaction. These results are in consistent withother characterization methods [1].
References[1] S. Bhandari et al., Journal of Environmental ChemicalEngineering, 11, 109380 (2023).
* raja@iip.res.in