Vibrational spectroscopic evidence for (NO)3 formation on Cu(111)

Abstract

The formation of (NO)3 on Cu(111) was recently reported based on scanning tunneling microscopy observations [A. Shiotari et al., J. Chem. Phys. 141, 134705 (2014)]. We herein report studies into this system using electron energy loss spectroscopy and verify the above findings through vibrational analysis. For the surface covered with mixed isotopes of N(16)O and N(18)O, we observed four peaks corresponding to N-O stretching vibrations, which were ascribed to the four isotopic combinations of the trimer. Dynamic coupling within the trimer was evaluated from model calculations of the coupled oscillators. Furthermore, we observed hindered rotation and translation modes in the dipole scattering regime, suggesting that the molecular axis is tilted from the surface normal. These results provide spectroscopic support for the formation of (NO)3 on Cu(111).

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

“…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.…”

“…The adsorption site of NO on Cu(111) was originally assigned to a 2-fold bridge site and later it was assigned to a 3-fold hollow site . Koshida et al revisited this system by EELS at 100 K and verified the formation of the NO trimer on Cu(111) that is observed by STM, and the mode at 190 meV is reassigned to the N–O stretching modes of NO trimer rather than the NO monomer at the fcc-hollow site . As will be discussed below, we found that the stretching modes are blue-shifted when NO absorbed in the trimer configuration, and those modes for the NO trimer agree well with the aforementioned experimental data.…”

“…The results indicate that the NO trimer is thermodynamically more stable than both the NO monomer and NO dimer, which is supposedly attributed to an enhancement of the electronic couplings between the 2π* orbitals upon the trimeric adsorption on Cu(111) . The formation of the NO trimer was further verified by vibrational spectroscopy using the electron energy loss spectroscopy (EELS) . The hindered rotation and hindered translation modes of the NO trimer are found to be dipole-active, implying that the NO molecules in the NO trimer are tilted from the upright configurations.…”

“…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. − …”