Density functional theory study ofNO−H2Ocoadsorption on Cu(111)

The coadsorption and complex formation of nitric oxide (NO) and water $({\mathrm{H}}₂\mathrm{O})$ on Cu(111) are studied theoretically using the nonlocal van der Waals density functional method. The energetics, adsorption geometries, and vibrational properties of several $n\mathrm{NO}\text{\ensuremath{-}}{m\mathrm{H}}₂\mathrm{O}$ complexes $(n=1--4$, $m=1--3)$ on Cu(111) are reported, and the relative stabilities of those complexes are compared with their respective NO and ${\mathrm{H}}₂\mathrm{O}$ clusters on Cu(111). We find that the mixed $n\mathrm{NO}\text{\ensuremath{-}}{m\mathrm{H}}₂\mathrm{O}$ complexes on Cu(111) are more stable than separated NO and ${\mathrm{H}}₂\mathrm{O}$ clusters due to the attractive $\mathrm{NO}\text{\ensuremath{-}}{\mathrm{H}}₂\mathrm{O}$ and NO-NO interactions on the surface. The attractive $\mathrm{NO}\text{\ensuremath{-}}{\mathrm{H}}₂\mathrm{O}$ interaction originates mainly from the hydrogen bonding between ${\mathrm{H}}₂\mathrm{O}$ and the negatively charged NO upon adsorption. Moreover, hydrogen bonding also induces an additional back donation process from Cu(111), further strengthening the $\mathrm{NO}\text{\ensuremath{-}}{\mathrm{H}}₂\mathrm{O}$ coadsorption. In addition to hydrogen bonding, the NO-NO interaction originating from $2{\ensuremath{\pi}}^*$ orbital hybridization further stabilizes the formation of the $4\mathrm{NO}\text{\ensuremath{-}}3{\mathrm{H}}₂\mathrm{O}$ complex, which is observed in the experiment. Even though the hydrogen bonding strength in $\mathrm{NO}\text{\ensuremath{-}}{\mathrm{H}}₂\mathrm{O}$ complexes is slightly weaker than the one in ${\mathrm{H}}₂\mathrm{O}$ clusters, due to the saturation of hydrogen bonding and the NO-NO interaction, NO and ${\mathrm{H}}₂\mathrm{O}$ tends to form a mixed $\mathrm{NO}\text{\ensuremath{-}}{\mathrm{H}}₂\mathrm{O}$ complex on Cu(111), in agreement with experiment. Our findings shed light on the role of intermolecular interactions between NO and ${\mathrm{H}}₂\mathrm{O}$ in the formation of the NO complex, which is important for understanding the reaction of NO in three-way catalysts.