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(001) 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. (C) 2003 Published by Elsevier B.V.