Insight into Room-Temperature Catalytic Oxidation of Nitric oxide by Cr2O3: A DFT Study

Cr-based catalysts have drawn attention as promising room-temperature NO oxidation catalysts. However, the intrinsic active component and reaction mechanism at the atomic level remain unclear. Here, taking the Cr2O3, one of the most stable chromium oxides, as an object, we systematically investigated NO oxidation processes on Cr2O3(001) and -(012) surfaces by virtue of DFT+U calculations, aiming to uncover the activity-limiting factors and basic structure activity relationship of the Cr2O3 catalyst. It was revealed that NO oxidation could not proceed via a Mars van Krevelen mechanism involving the lattice oxygen on both surfaces. For the Cr2O3(001) surface exposing the isolated three-coordinated Cr-3c, the reactions are inclined to occur through the Eley-Rideal route, in which the NO couples directly with the molecular O-2* or atomic O* adsorbed at the Cr-3c site to form two key intermediate species (ONOO* and NO2*) following a barrierless process. Nevertheless, the overall activity is limited by the irreversible adsorption of NO2 species on the highly unsaturated Cr-3c. In contrast, on the (012) termination, which exposes the five-coordinated Cr-5c, the NO2* can be easily released, but the reactant O-2 cannot be efficiently adsorbed and also results in a limited overall activity at room temperature. To achieve a higher activity, a thermodynamically favored interface model of monochain CrO3 supported on Cr2O3(012) was proposed, which shows an improved 02 adsorption energy of -0.99 eV and thus an enhanced activity of Cr2O3(012), possibly accounting for the experimentally high activity of Cr-based catalysts usually involving the Cr3+/Cr6+ redox. This study demonstrated the catalytic ability of Cr2O3 for NO oxidation at room temperature, and the presented systematic picture may facilitate the further design of more active Cr-based catalysts.