Adsorption of NOx Molecules on the Hematite Surface: Isolated Versus Paired

Hematite is a promising catalyst to remove nitrogen oxide (NOx, x = 1 and 2) pollutants from the atmosphere. Improving its catalytic performance requires a clear understanding of the relevant surface processes, including NOx adsorption. Prior theoretical simulations were limited to isolated adsorbates at static conditions, with results not fully consistent with experiments. Here, we investigate the adsorption of NOx molecules on the hematite (0001) surface by using the density functional theory + U method with van der Waals corrections. We find that, at static conditions, NOx prefer to occupy two neighboring surface Fe and O sites simultaneously, due to a cooperative effect between two open-shell NOx molecules [J. Phys. Chem. B 2002, 106, 7405–7413]. In contrast, isolated adsorbates are favored at high temperatures (T) or low NOx partial pressures (PNOx) due to their higher configurational entropy. A surface phase diagram is constructed for the NOx adsorbates, delineating both their stability fields and equilibrium coverages. As the structure and abundance of NOx adsorbates depend strongly on T and PNOx, surface reactions at different T-PNOx conditions may follow quite different pathways.