A water-promoted Mars-van-Krevelen reaction dominates low-temperature CO oxidation over Au-Fe2O3, but not over Au-TiO2

We provide experimental evidence that is inconsistent with often proposed Langmuir-Hinshelwood (LH) mechanistic hypotheses for water-promoted CO oxidation over Au-Fe2O3. Passing CO and H2O, but no O2, over Au-γ-Fe2O3 at 25 oC, we observe significant CO2 production, inconsistent with LH mechanistic hypotheses. Experiments with H218O further show that previous LH mechanistic proposals cannot account for water-promoted CO oxidation over Au-γ-Fe2O3. Guided by density functional theory, we instead postulate a water-promoted Mars-van-Krevelen (w-MvK) reaction. Our proposed w-MvK mechanism is consistent both with observed CO2 production in absence of O2, and with CO oxidation in presence of H218O and 16O2. In contrast, for Au-TiO2, our data is consistent with previous LH mechanistic hypotheses.