Density functional theory study of the energetics, electronic structure, and core-level shifts of NO adsorption on the Pt(111) surface

In this work, we report a first-principles investigation of the energetics, structures, electronic properties, and core-level shifts of NO adsorption on the Pt(111) surface. Our calculations are based on density functional theory within the framework of the ultrasoft pseudopotential plane-wave and the all-electron projected augmented-wave methods. We found that at 0.25, 0.50, and 0.75 monolayer, NO adsorbs preferentially in the fcc, $\text{fcc}+\text{top}$, and $\text{fcc}+\text{top}+\text{hcp}$ sites, respectively. The geometric parameters, adsorption energies, vibrational frequencies, and work-function changes are in good agreement with the experimental data. The interaction between NO and Pt(111) was found to follow a donation--back-donation process, in which the NO $\ensuremath{\sigma}$ states donate electrons to the substrate $\text{Pt}\text{ }d$ states, while the substrate $\text{Pt}\text{ }d$ states back donate to the NO $\ensuremath{\pi}$ states. Though there is an overall net charge transfer from the substrate to the NO adsorbate regardless of the adsorption sites and coverages, the spatial redistribution of the transferred electron is site dependent. The charge accumulation for NO in the top sites occurs closer to the surface than NO in the hollow sites, which results in the reduction of the Pt(111) surface work function for the top NO but an increase for the hollow NO. The core-level shifts of the topmost surface Pt atoms coordinated with top and hollow NO molecules at different coverages are in excellent agreement with experiments. In contrast, the N $1s$ core-level shifts between top and hollow NO $(\ensuremath{\sim}0.7\text{ }\text{eV})$ deviated significantly from the zero shift found in experiments. Our analysis indicates that the difference may come from the thermal vibration and rotation of adsorbed NO on the Pt(111) surface.