NOx on Al: The Unusual Adsorption Site Preference and the Attraction among Adsorbates
PQ Hai and C Wu and XD Ding, JOURNAL OF PHYSICAL CHEMISTRY C, 126, 11971-11980 (2022).
The interactions between NOx, N and O atoms, and aluminum (Al) surfaces including crystal planes and nanoparticles (ANPs) are systematically investigated by using density functional theory (DFT) calculations, canonical Monte Carlo (CMC) simulations, and reactive molecular dynamics (RMD) simulations. NOx has two adsorption states (molecular and dissociated) on the Al surfaces, which are separated by a low energy barrier. The adsorption of NOx in either state does not favor Al nanoparticles (ANPs), opposite to its behavior on transition metals. In addition, NOx does not show preference toward smaller ANPs or their vertices or edges; instead, it prefers threefold sites that resemble the fcc or hcp sites on a (111) surface. The big deformation energy of ANPs is found to be the reason. The N* adatoms are most stable at the tetrahedron interstitial sites between the surface and the first sublayer, where an aluminum nitride (AlN)-like structure is formed. Similar to O* adatoms on Al, the isotropic attraction between N* adatoms has a significant influence on their diffusion, both in-plane and in cross-layer. Besides, the attraction within a N-O pair as the first nearest neighbors (1NN) is roughly twice that of their respective attraction (about 0.1 eV/O-O or N-N). However, RMD simulations ignore the attraction among the N* adatoms as well as between the N* and O* adatoms, though O-O attractions are somehow incorporated. CMC simulations prove that the attraction still governs the configuration of the adatoms at 600 K with all adatoms congregating into a single polygonal island, and it is still influential even when reaching 1500 K. In addition, the fragmented products (AlxNy) of Al combustion in NOx according to DFT feature the tetra-coordinated Al centered inside the tetrahedron of four N atoms, different from the pyramidal unit structure with Al as the vertex by RMD simulations. Our results demonstrate that the unusual behaviors of NOx including the dissociated atoms on Al are caused by both the softness (low Young's modulus) of Al and the attractions among adsorbates, which have important implications for understanding the behaviors of gas molecules on main group metals in general.
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