Characterization of adsorption trends of NO2, nitrite, and nitrate on MgO terraces

MgO and other alkaline earth oxides are currently studied as potential adsorbents for NOx (x=1, 2) molecules. This paper provides a first-principles understanding of the expected adsorption conformations of NO2, NO2− and NO3 on MgO(0 0 1) with the use of terrace cluster models and density functional theory (DFT). The ability to form chemisorbed nitrite (NO2−) and nitrate (NO3−) depends both on adsorbate electron affinity, an indicator of surface-oxidizing strength, and the local adsorption orientation on the surface, a predictor of electrostatic or acid/base coordination between the surface and adsorbate. NO2− more effectively binds to the surface to form adsorbed nitrite compared to NO2. NO3 is an effective surface-oxidizing agent, enabling the extraction of a monovalent charge from the surface to form adsorbed nitrate. In addition to effective electrostatic interactions, local adsorbate orientation also determines binding strength. Geometries that facilitate [Adsorbate Oxygen–Mg2+] and/or [N–Mg2+] coordination display strong binding and substantial charge transfer between the surface and adsorbate. In contrast, bidentate [Adsorbate Oxygen–Surface Oxygen] coordination results in weak binding and physisorption. This site-specific characterization of NOx adsorption on oxide surfaces will lead to a better understanding of acid–base chemistry between amphiphilic molecules and oxide surfaces as well as lead to a clearer path for practical NOx abatement strategies.