Qualitative Differences in the Adsorption Chemistry of Acidic (CO2, SOx) and Amphiphilic (NOx) Species on the Alkaline Earth Oxides

Abstract

Density functional theory calculations and plane-wave, supercell models are used to examine trends in the adsorption chemistry of CO2, SOx (x = 1, 2), and NOx (x = 1, 2, 3) on the (001) surfaces of the alkaline earth oxides MgO through BaO. The Lewis acids CO2 and SOx adsorb at basic surface oxygen (Os2-) sites to form local carbonate (CO32-)-, sulfite (SO32-)-, and sulfate (SO42-)-like structures, with adsorption energies increasing uniformly down the series. In contrast, the most stable NOx-derived anions (nitrite, NO2-, and nitrate, NO3-) are not formed by Lewis acid−base reactions with acidic (Ms2+) or basic metal oxide surface sites alone. Rather, NOx adsorption involves both electron transfer and acid−base interactions with the oxide surface that combine to produce acid-like and base-like adsorption states; the increasing oxidizability and reducibility of the alkaline earth oxides contribute to the increasing NOx adsorption strength down the family. Combining NOx adsorbates in acidic donor and basic acceptor configurations produces structural modifications and systematically enhanced adsorption energies that arise from interadsorbate charge transfer. These “cooperative” pairs provide qualitatively correct representations of chemisorbed nitrite and nitrate, and comparisons with available experiment support their utility for quantitative description of adsorption energetics as well. The electron transfer and cooperative interactions that distinguish NOx from SOx and CO2 can potentially be exploited to tailor materials selective for NOx adsorptionan important goal for NOx emissions control.