Operando XANES Reveals the Chemical State of Iron-Oxide Monolayers During Low-Temperature CO Oxidation.

We have used grazing incidence X-ray absorption near edge spectroscopy (XANES) to investigate the behavior of monolayer FeO films on Pt(111) under near ambient pressure CO oxidation conditions with a total gas pressure of 1 bar. Spectra indicate reversible changes during oxidation and reduction by O and CO at 150 °C, attributed to a transformation between FeO bilayer and FeO trilayer phases. The trilayer phase is also reduced upon heating in CO+O , consistent with a Mars-van-Krevelen type mechanism for CO oxidation.