Operando Insights into CO Oxidation on Cobalt Oxide Catalysts by NAP-XPS, FTIR, and XRD

• Published in: ACS catalysis Vol. 8; no. 9; pp. 8630 - 8641
• Main Authors: Lukashuk, Liliana, Yigit, Nevzat, Rameshan, Raffael, Kolar, Elisabeth, Teschner, Detre, Hävecker, Michael, Knop-Gericke, Axel, Schlögl, Robert, Föttinger, Karin, Rupprechter, Günther
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 07-09-2018
• Subjects: CO oxidation; NAP-XPS; FTIR; surface spectroscopy; XRD; operando; Co3O4
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.8b01237

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 one to monitor surface and bulk states under various reaction conditions (steady-state and dynamic conditions switching between CO and O2). On the basis of 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 °C, 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.