A van der Waals Density Functional Investigation on the Improved Adsorption Properties of NO on the Rh_n/MgO (100) Interface

Using spin-polarized density functional theory calculations, we have studied the interaction and morphology of small Rhn clusters (n≤5) deposited on a MgO (100) surface, as well as the ability of Rhn/MgO systems to trap NO molecules. We show that Rhn clusters can tightly adhere to a MgO (100) surface because of hybridization of O 2p and Rh 4d orbitals. The most energetically preferred structure for each adsorbed Rhn cluster looks similar to the structure of the isolated cluster, except for an increment in the length of base edge(s) close to the substrate. We find that the Rhn/MgO system exhibits a much stronger ability to trap NO, especially for Rh2 and Rh4 clusters, than do the bare Rhn clusters. The hybridization of N 2p and Rh 4d orbitals is responsible for the strong adsorption of NO on Rhn/MgO. This can be explained by the increased Rh-Rh bond length that results in a decrease in the effective coordination number, and the presence of extra electrons obtained from the substrate, causing the N and O atoms to bond strongly to the Rh atoms, which in turn weakens the N-O molecular interaction. Additionally, binding to NO induces a reduction in the total magnetic moment of the Rhn/MgO system. Thus, putting Rhn on the MgO (100) surface can decrease competition between bonding and magnetism. However, whether it can be related to improve trapping of NO deserves further investigations.