In situ approach to investigations of CuFeAl-composite catalysts in catalytic oxidation of CO: X-ray adsorbstion spectroscopy
- Dr. Andrey SARAEV
- Dr. Andrey SARAEV (Boreskov Institute of Catalysis)
- Anna TSAPINA (Boreskov Institute of Catalysis)
- Dr. Alexander TRIGUB (NRC "Kurchatov Institute")
- Zakhar VINOKUROV (BIC SB RAS)
- Dr. Olga BULAVCHENKO (Boreskov Institute of Catalysis)
- Mr. Aleksandr FEDOROV (Boreskov Institute of Catalysis)
- Dr. Yan ZUBAVICHUS (National Research Centre Kurchatov Institute)
- Dr. Vasily KAICHEV (Boreskov Institute of Catalysis)
Keywords: In situ XAS, operando XRD, CuFeAl-composite catalysts, CO oxidation, catalytic combustion.
Catalytic oxidation of gasification products of solid fuels is allows utilizing low-grade fuels such as lignite, peat, and firewood as well as various industrial wastes. At the same time catalytic combustion produces a significantly lower amount of harmful emissions then “traditional combustion” of fuels. CuFeAl-composite catalysts demonstrate high activity and stability in catalytic oxidation of gasification products of solid fuels. Moreover, the catalysts are inexpensive and ecologically clean. In this contribution we present our first results of in situ investigations of the catalyst state in reaction conditions. Since carbon monoxide is the main product of gasification of solid fuels we performed investigation CuO, Fe2O3, and CuFeAl-composite catalysts in CO and CO+O2 mixture in a wide temperature range. We applied three methods: XANES, EXAFS, and additionally operando XRD. XANES is very useful for identification of different chemical states of copper and iron and allows us to study the chemistry of the catalysts under reaction conditions. X-ray diffraction techniques allow us to study the phase composition, but, unfortunately, the technique cannot identify nanoparticles and amorphous phases. This shortcoming can be eliminated by EXAFS which may clarify the structure of local environment of copper and iron atoms even when their concentration is extremely low. In situ XAS experiments were performed at the Structural Materials Science station at Kurchatov Center for Synchrotron Radiation. The spectrometer is equipped with high temperature chamber that allows collecting XAS spectra within temperature range from RT to 600°C in the gas mixture at atmospheric pressure . Operando XRD/MS experiments were carried out at the “High Precision Diffractometry II” station at “Siberian Synchrotron and Terahertz Radiation Center” and at lab Bruker D8 Advance diffractometer (Boreskov Institute of Catalysis). The both diffractometers are equipped with XRK 900 reaction chambers (Anton Paar GmbH) that allow observing the diffraction patterns within temperature range from RT to 900°C in the reactant mixture at atmospheric pressure . We found that fresh CuFeAl-composite catalysts consist of CuO, Fe2O3, and Al2O3. In a CO flow, the reduction of copper from Cu2+ to Cu1+ and Cu0 started at temperature about 200°C; at 600°C copper is mainly in the metallic state. At the same time the reduction of iron started at temperature about 400°C and at 600°C about 20% of iron is in the metallic state. Operando XRD study allows us to determine the phase transition of iron-containing phase during the reduction in a CO flow. The reduction process occurs in the next manner: Fe2O3 -> Fe3O4 -> FeO and Fe0. In CO:O2 = 2:1 mixture, the reduction of copper from Cu2+ to Cu1+ started at temperature about 400°C and at 600°C about 50% of copper is in the Cu1+ state, whereas iron is slightly reduced to Fe2+ state at 600°C. The following increase the partial pressure of O2 leads to shift initial reduction temperature to high temperature range. Thus, the use of complimentary methods (XANES, EXAFS, and XRD) allows us to determine the chemical state of copper and iron, phase composition in the catalyst during the oxidation of CO. The data presented can facilitate to clarify the mechanism of oxidation of CO.
Acknowledgement. This work was supported by the Russian Science Foundation, grant 17-73-20157.
 A.A. Chernyshov, A.A. Veligzhanin , Ya.V. Zubavichus, Nucl Instrum Methods Phys Res A 603 (2009) 95.  A.A. Saraev, Z.S. Vinokurov, V.V. Kaichev, A.N. Shmakov, V.I. Bukhtiyarov, Catal. Sci. Technol. 7 (2017) 1646.