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

Abstract

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-FeO x /Al 2 O 3 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 Fe 2+ /Fe 3+ redox pair is directly involved in the mechanism of oxygen activation of Pt-FeO x /Al 2 O 3 . The detailed analysis of oxygen cutoff experiments demonstrated that after switching off oxygen, approximately one carbon dioxide molecule was formed for each Fe 3+ ion reduced to produce Fe 2+ . At the same time, the steady-state carbon dioxide formation rate was about twice higher than the initial rate of Fe 2+ 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.

A simple method to regulate surface hydroxy groups on Al2O3 for improving catalytic oxidation performance for HCHO on Pt/Al2O3

“…35,37,38 There was little difference in band position and relative intensity between Pt-Na/Al-5h and Pt-Na/Al-2h, demonstrating that the valence state of Pt species in both samples was similar. 39 This result also implied that the Pt species with different particle sizes could be reduced to species with similar valence states under the hydrogen reduction condition of 300 °C.…”

Sub-Nano Pt/β-FeOOH Quantum Dots for Photocatalytic Removal of Toluene: Catalyst Design, Preparation, and Benefits

“…The first Fe-O shell of FeOOH is at ~1.98 Å, attributing to a coordination number (N) of 5.8 (Sadykov et al, 2021), which indicates many oxygen vacancies in the matrix of crystal. As for Pt/FeOOH, the first Fe-O shell peak appears at ~1.94 Å, corresponding to a coordination number (N) of 6.0 (Table 3) (Sadykov et al, 2021), which is nearly the theoretical value of an octahedral structure. These results verified the good crystalline nature of Pt/FeOOH and the Pt-O-Fe interfaces.…”

“…Moreover, the shortened Fe-O bond and elongated Fe-Fe bond in Pt/FeOOH compared with those of FeOOH consolidate the Pt-O-Fe-rich interfaces. We tried to probe the local coordination chemistry of Pt atoms; unfortunately, the XAS (Liu et al, 2018;Ren et al, 2019;Sadykov et al, 2021).…”

Origin of the volcano-like ethanol oxidative dehydrogenation activity trend for VOx/CeO2 catalysts uncovered by operando time-resolved XAS

“…In our previous work, we demonstrated that operando quick XAS combined with a transient experimental strategy provides quantitative information on the redox activity of metal centers involved in the Mars-van Krevelen mechanism of carbon monoxide and ethanol oxidation on various catalysts. [28][29][30][31][32] In particular, for the bilayered VOx/TiOx/SiO2 catalyst, we have shown that V 5+ to V 4+ reduction upon ethanol oxidation is kinetically coupled with acetaldehyde formation, while the redox activity of titanium was shown to be not directly involved in the catalytic cycle. [32] In this work, we applied the same transient XAS methodology to uncover the redox processes involved in ethanol ODH over VOx/CeO2 catalysts.…”