Time-Resolved XAS Provides Direct Evidence for Oxygen Activation on Cationic Iron in a Bimetallic Pt-FeOx/Al2O3 Catalyst

Reducible oxides are effective aerobic oxidation catalysts being able to activate molecular oxygen. This ability is generally attributed to the high concentration of oxygen vacancies serving as oxygen activation sites. At the same time, the mechanism of oxygen activation remains unclear since surface oxygen activation sites cannot be easily detected using conventional methods. In this work, we unraveled the mechanism of oxygen activation over iron sites of Pt-FeOx/Al2O3 during carbon monoxide oxidation using a combination of in situ and operando methods. In situ/operando XAS at the Pt L-3 and Fe K-edges, in situ Fourier transform infrared (FTIR) spectroscopy, and carbon monoxide chemisorption showed that carbon monoxide activation takes place at metallic platinum sites and is not affected by the presence of cationic iron species. Operando time-resolved Fe K-edge X-ray absorption spectroscopy (XAS) demonstrated that the Fe2+/Fe3+ redox pair is directly involved in the mechanism of oxygen activation of Pt-FeOx/Al2O3. The detailed analysis of oxygen cutoff experiments demonstrated that after switching off oxygen, approximately one carbon dioxide molecule was formed for each Fe3+ ion reduced to produce Fe2+. At the same time, the steady-state carbon dioxide formation rate was about twice higher than the initial rate of Fe2+ formation after cutoff of oxygen from the catalytic feed. These experiments allude to a catalytic cycle involving electrophilic oxygen species adsorbed on iron centers as reaction intermediates. Similar mechanisms might be expected for other catalytic oxidation reactions over cationic iron of both chemical and biological importance.

Automated summary

This study unraveled the mechanism of oxygen activation over iron sites of Pt-FeOx/Al2O3 during carbon monoxide oxidation using in situ and operando methods. The experiments demonstrated that the Fe2+/Fe3+ redox pair is directly involved in the mechanism of oxygen activation, with oxygen species adsorbed on iron centers as reaction intermediates. The findings suggest a catalytic cycle involving cationic iron for oxidation reactions of both chemical and biological importance.