First-Principles Characterization of NOx Adsorption on MgO

Alkaline earth oxides are leading candidates as NO x ( x = 1, 2) sorbents; however, the understanding of adsorption and reaction of NO x on these surfaces is currently limited. In this paper, Mg(001) terrace cluster models and density functional theory (DFT) are used to characterize the adsorption properties of neutral and charged NO, NO 2, and NO 3 . Neutral NO and NO 2 molecules weakly adsorb on the MgO terrace, while neutral NO 3 adsorbs more strongly through oxidation and significant acid−base coordination with the surface. Chemisorbed nitrite can be formed either from adsorption of Lewis acidic NO + on Lewis base surface sites (O s 2- ) or from adsorption of Lewis basic NO 2 - on Lewis acid surface sites (Mg s 2+ ). Similarly, chemisorbed nitrate can form from NO 2 + adsorption on O s 2- sites or NO 3 - adsorption on Mg s 2+ sites. These charged adsorbates are not expected to exist in isolation on the MgO surface but rather are expected to form oppositely charged, cooperatively bound pairs with enhanced adsorption energy over the isolated neutrals. While results are demonstrated for adsorption on MgO, they have application to NO x adsorption on a wide range of oxide surfaces.