Water–NO Complex Formation and Chain Growth on Cu(111)

Abstract

We report the novel structure of a water–NO (nitric oxide) complex on Cu(111) by using scanning tunneling microscopy, electron energy loss spectroscopy, and noncontact atomic force microscopy. The fundamental motif of the complex is a triangular cluster, including four NO molecules and three water molecules. As coverage increases, the complex grows into a chain structure on the surface. The preferential formation of such unique complexes suggests that there is attractive interaction between water and NO that is strong enough to overcome NO–NO and water–water interactions. The origin of the interaction is argued in terms of electrostatics, where water donates a polar OH group to NO which is negatively charged via electron transfer from the surface.

Insight into Trimeric Formation of Nitric Oxide on Cu(111): A Density Functional Theory Study

“…The N–O stretching modes are blue-shifted by only ∼11 meV from NO monomeric to trimeric adsorptions, indicating that care must be taken in interpreting the experimental vibrational spectroscopic data. Our vibrational results further support the EELS and STM results that (NO) 3 is formed on Cu(111). − …”

“…The adsorption energies calculated by using vdW functionals increase in the sequence NO < (NO) 2 < (NO) 3 -I < (NO) 3 -II. The results using vdW functionals indicate that NO molecules tend to pair up to form dimers, and then trimers on Cu(111), consistent with the experimental observations. − Furthermore, we investigated a formation of NO tetramer ((NO) 4 ) on Cu(111) p (3 × 3), where one more NO molecule was added at the fcc-hollow site near (NO) 3 -II. The (NO) 4 has a rhombus geometry (Figure ) and the calculated E ads and E b are smaller than those for dimer and trimer, indicating that the NO tetramer is less stable than both the NO dimer and trimer.…”

“…We note that E ads of (NO) 3 -II in the p (3 × ) superstructure is slightly higher than the one in p (3 × 3) using the eight-layer slab (we confirmed this by using the 10-layer slab), implying that the p (3 × ) superstructure is thermodynamically more stable than p (3 × 3), which contradicts the experimental finding. − However, because of the different shapes of the surface unit cells, it is impossible to sample exactly equivalent k -point sets and fast Fourier transform meshes, and thus, the error cancellation is less likely. Therefore, considering the small energy difference (∼20 meV), we do not compare E ads values obtained using different surface unit cells.…”

“…On noble metal surfaces (Cu, Ag, and Au), the catalytic reduction of NO is predicted to proceed via NO dimer formation. − The open-shell structure of the 2π* orbitals is retained upon the monomeric adsorption on the noble metal surfaces, leading to a further intermolecular bonding between two adsorbed NO molecules to form a NO dimer. ,− The dimeric (NO) 2 formation was verified by (i) the detection of the N 2 O desorption because the NO dimer is an intermediate for N 2 O production; − by (ii) real-time observations of the NO dimer by scanning tunneling microscopy (STM); , and by (iii) first-principles calculations. − Generally speaking, on low-index Cu surfaces, molecularly adsorbed NO is assumed to be either monomer or dimer. ,− ,− Shiotari et al recently proposed that NO molecules adsorb in a trimer configuration on Cu(111) at 80 K. − The NO monomer initially adsorbs on Cu(111) at an fcc-hollow site in an upright configuration at 15 K . By STM manipulation, the NO dimer and NO trimer are produced and characterized.…”

“…Understanding the adsorption and reactivity of nitric oxide (NO) on metal surfaces is essential from fundamental as well as practical application points of view. − As an important step for catalytic NO reduction in automotive exhaust gas conversion, NO adsorption on metal surfaces has been extensively studied in the last two decades. − Unlike CO adsorption, − the NO adsorption on the metal surfaces exhibits a complicated behavior because of its open shell structure of 2π* orbitals. − On the metal surfaces, NO undergoes molecular or dissociative adsorption depending on coverage, surface temperature, and the nature of the metals. − At low temperature, molecular adsorption of NO − mainly takes place where NO adsorbs as a monomer in many different binding geometries, namely upright, − side-on, ,, tilted ones; − or as a dimer in a high-coverage regime owing to the intermolecular interaction. − Furthermore, NO can be dissociated into N and O adatoms at higher temperatures, but whether dissociation occurs strongly depends also on surface coverage. − The origin of NO adsorption on metal surfaces is the back-donation process where 2π* orbitals of NO hybridize with the d band of the metal surfaces. − …”