[Zurück]

K. Anic, L. Lukashuk, N. Yigit, Ch. Rameshan, K. Föttinger, G. Rupprechter:
"Operando studies of working cobalt oxide catalysts by XAS, NAP-XPS and infrared: applied and model catalysis";
Hauptvortrag: OperandoVI - 6th International Congress on Operando Spectroscopy, Estepona, Malaga; 15.04.2018 - 19.04.2018; in: "OperandoVI - 6th International Congress on Operando Spectroscopy", (2018), S. 21.

Contemporary studies of catalytic reaction mechanisms are typically carried out in the operando mode, i.e. performing spectroscopy, microscopy and/or diffraction on the functioning catalysts, while catalytic activity/selectivity are simultaneosly recorded. For many years our group has used synchrotron-based operando methods to evaluate catalytic processes [1,2]: near atmospheric pressure X-ray photoelectron spectroscopy (NAP-XPS), X-ray absorption spectroscopy (XAS, NEXAFS, EXAFS) and X-ray diffraction. This has been complemented by lab-based methods such as infrared (FTIR, PM-IRAS) and vibrational sum frequency generation spectroscopy (VSFG). Another asset of the group is to examine catalytic surface reactions on heterogeneous catalysts via a two-fold approach, employing both industrial-grade catalysts as well as surface science based planar model catalysts [3].
In this contribution we report operando studies of (preferential) CO oxidation on Co304 catalysts. XAS was utilized to examine the structure and oxidation state during reaction, and NAP-XPS at low photoelectron kinetic energies was applied to monitor surface composition changes. Despite the high surface sensitivity the concentration of oxygen vacancies was too low for detection under steady state reaction conditions so that Co304 appeared fully oxidized [5].
To obtain better insight, operando spectroscopy was performed upon switching between (pure) CO and 02, with NAP-XPS revealing significant changes in the Co3+/Co2+ fraction: in CO the Co3+ concentration decreased by up to 14% at 200°C. In the C 1s region, carbonates, molecular CO adsorbed on cobalt cations, and elementary carbon were observed. FTIR spectroscopy clearly indicated that CO formed surface carbonate species with Co304, whereas during reaction the amount of carbonates was lower. The combined results of NAP-XPS and operando FTIR spectroscopy indicate a redox Mars­ van-Krevelen mechanism. Moreover, the results point to additional reaction pathways towards C02: 1) carbonate formation and decomposition, and 2) CO dissociation followed by carbon reoxidation.

Cobalt oxide catalysts were also modelled in UHV by growing thin Co304 and CoO films on lr(100) single crystals. The interaction of 8 ML thick cobalt oxide films with CO has been examined using isotopes, 1802 for oxide growth and 13C160 for adsorption. C forms carbonates and desorbs as 13C160180, pointing to a Mars-van-Krevelen mechanism. Catalytic activity and composition changes in reactive C0+02 atmosphere at mbar pressure were simultaneously monitored by MS and NAP­ XPS, respectively, which allowed to identify the active state. The results of the thin film models will be contrasted to those obtained for technological Co304 and CoO powder catalysts.

References
1. G. Rupprechter, Advances in Catalysis, 51 (2007) 133-263; Textbook on Surface and Interface Science, K. Wandelt (Editor), Wiley-VCH , Weinheim, 2016 , 459-527 (ISBN: 978-3-527-41158-0) .
2. K. Föttinger, G. Rupprechter, Accounts of Chemical Research, 47 (2014) 3071-3079.
3. K. Anic, A. Wolfbeisser, H. Li, C. Rameshan, K. Föttinger, J. Bernardi, G. Rupprechter, Topics in Catalysis, 59 (2016) 1614 - 1627.
4. . L. Lukashuk, K. Föttinger, E. Kolar, C. Rameshan, D. Teschner, M. Hävecker, A. Knop-Gericke ,
N. Yigit, H. Li, E. McDermott, M. Stöger-Pollach, G. Rupprechter, Journal of Catalysis, 344 (2016) 1-15.

cobalt oxide; NAP-XPS; XAS; IR