NO_x on Al: The Unusual Adsorption Site Preference and the Attraction among Adsorbates

Abstract

The interactions between NO x , 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. NO x has two adsorption states (molecular and dissociated) on the Al surfaces, which are separated by a low energy barrier. The adsorption of NO x in either state does not favor Al nanoparticles (ANPs), opposite to its behavior on transition metals. In addition, NO x 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 (Al x N y ) of Al combustion in NO x 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 NO x 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.

Coverage-dependent adsorption and dissociation of H2O on Al surfaces

“…In contrast, typical adatoms (O* and N*) evidently prefer Al crystal surfaces over ANPs. 16,17 The discrepancy can be resolved by considering the two competing factors that determine the adsorption strength: (1) the intrinsic activity of the adsorption site. For systems of the same chemical composition, the coordination number (CN) of the atoms forming the adsorption site determines its activity.…”

“…After all, Al has exhibited unusual adsorption strength order for adatoms like O* and N* with respect to ANPs and Al crystal planes, as well as to the size of ANPs. 16,17…”

“…After all, Al has exhibited unusual adsorption strength order for adatoms like O* and N* with respect to ANPs and Al crystal planes, as well as to the size of ANPs. 16,17 That is, their adsorption is stronger on Al crystal planes rather than on ANPs and is weaker on smaller ANPs.…”

Morphology evolution of the aluminum surface in a fluorine-containing environment

“…O and N on Al surfaces). 23,24 Hence, we quantify the interaction between F adatoms over different surfaces. Here, the 1NN repulsion is obtained by subtracting the adsorption energy of a single F atom from the adsorption energy of two 1NN F atoms.…”

“…In our previous work, we investigated the coadsorption of N and O atoms on Al surfaces when discussing NOx on Al, which exhibit unusual inter-species attraction between neighboring N and O adatoms with a strength of 0.1-0.2 eV. 23,24 Although many studies have investigated the adsorption of F atoms on Al and Al 2 O 3 surfaces, more complex issues arise when F and O atoms co-adsorb on Al surfaces. The entanglement of F-F, O-O and F-O interactions strongly influences the interaction between the heterogeneous molecules (SO 2 F 2 and SOF 2 ) with Al.…”