Operando Insights into CO Oxidation on Cobalt OxideCatalysts by NAP-XPS FTIR and XRD

摘要

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 mechanismabove 100 °C, involving the Co³⁺/Co²⁺ redoxcouple and oxygen vacancy formation. Under steady state, the Co3O4 surface appeared oxidized and the amount ofreduced Co²⁺ species atear the surface remained low upto 200 °C. Only in pure CO, about 15% of surface reduction weredetected, suggesting that the active sites are a minority species.The operando spectroscopic studies also revealed additional reactionpathways: CO dissociation followed by carbon reoxidation and carbonateformation and its decomposition. However, due to their thermal stabilityin various atmospheres, the carbonates are rather spectators and alsoCO dissociation seems a minor route. This study thus highlights thebenefits of combining operando surface sensitive techniques to gaininsight into catalytically active surfaces.