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L. Lukashuk, N. Yigit, R. Rameshan, E. Kolar, D. Teschner, M. Hävecker, A. Knop-Gericke, R. Schlögl, K. Föttinger, G. Rupprechter:
"Operando insights into CO oxidation on cobalt oxide catalysts by NAP-XPS, FTIR and XRD";
ACS Catalysis, 8 (2018), 1 - 8630.

Cobalt oxide Co₃O₄ has recently emerged as promising, noble metal-free catalyst for oxidation reactions but a better understanding of the active catalyst under working conditions is required for further development and potential commercialization. An operando approach has been applied, combining near ambient (atmospheric) pressure X-ray photoelectron spectroscopy (NAP-XPS), Fourier transform infrared spectroscopy (FTIR) or X-ray diffraction (XRD) with simultaneous catalytic tests of CO oxidation on Co₃O₄, enabling to monitor surface and bulk states under various reaction conditions (steady-state and dynamic conditions switching between CO and O₂). Based on the surface-specific chemical information a complex network of different reaction pathways unfolded: Mars-van-Krevelen (MvK), CO dissociation followed by carbon oxidation, and formation of carbonates. A possible Langmuir-Hinshelwood (LH) pathway cannot be excluded because of the good activity when no oxygen vacancies were detected. The combined NAP-XPS/FTIR results are in line with a MvK mechanism above 100 oC, involving the Co³⁺/Co²⁺ redox couple and oxygen vacancy formation. Under steady state, the Co₃O₄ surface appeared oxidized and the amount of reduced Co²⁺ species at/near the surface remained low up to 200 °C. Only in pure CO, about 15% of surface reduction were detected, suggesting that the active sites are a minority species. The operando spectroscopic studies also revealed additional reaction pathways: CO dissociation followed by carbon reoxidation and carbonate formation and its decomposition. However, due to their thermal stability in various atmospheres, the carbonates are rather spectators and also CO dissociation seems a minor route. This study thus highlights the benefits of combining operando surface sensitive techniques to gain insight into catalytically active surfaces.

CO oxidation, Co₃O₄, operando, NAP-XPS, FTIR, XRD, surface spectroscopy

http://dx.doi.org/10.1021/acscatal.8b01237