First-Principles Analysis of NOx Adsorption on Anhydrous γ-Al2O3 Surfaces

Abstract

The interaction of nitrogen oxides NOx (x = 1−3) with γ-Al2O3 has been investigated using first-principles density functional theory calculations. NO and NO2 weakly physisorb on the clean, dehydrated (100) and (110) surfaces of γ-Al2O3, whereas the adsorption of the NO3 radical is rather strong. Only the basic-like O-down adsorption configurations were found to be stable. The interaction between NOx and γ-Al2O3 can be described as a surface-mediated electron transfer process. For single NOx adsorption, greater electron transfer from the surface to the adsorbate (negatively charged) yields stronger interaction between NOx and the surface. The adsorption of four combinations of NOx + NOy (x = 1−3, y = 2, 3) pairs on the (100) and the (110) facets of γ-Al2O3 were investigated. Except for the NO2 + NO2 pair, a strong cooperative effect that substantially enhances the stability of NOx on both γ-Al2O3 surfaces was found. This cooperative effect consists of surface-mediated electron transfer processes resulting in a favorable electrostatic interaction between two adsorbed NOx species. The NO+δNO3−δ pair was found to be the thermodynamically most stable state among the coadsorbed NOx + NOy pairs on both γ-Al2O3 surfaces. The results are used to analyze the experimentally observed NOx evolution during temperature programmed desorption from NO2-saturated γ-Al2O3 substrates.