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

Abstract

Hematite is a promising catalyst to remove nitrogen oxide (NO x , x = 1 and 2) pollutants from the atmosphere. Improving its catalytic performance requires a clear understanding of the relevant surface processes, including NO x 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 NO x 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, NO x prefer to occupy two neighboring surface Fe and O sites simultaneously, due to a cooperative effect between two open-shell NO x molecules [J. Phys. Chem. B 2002, 106, 7405−7413]. In contrast, isolated adsorbates are favored at high temperatures (T) or low NO x partial pressures (P NOd x ) due to their higher configurational entropy. A surface phase diagram is constructed for the NO x adsorbates, delineating both their stability fields and equilibrium coverages. As the structure and abundance of NO x adsorbates depend strongly on T and P NOd x , surface reactions at different T-P NOd x conditions may follow quite different pathways.