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

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