CO/Rh(111): Vibrational frequency shifts and lateral interactions in adsorbate layers

Abstract

High resolution electron energy loss spectroscopy ͑HREELS͒, low-energy electron diffraction ͑LEED͒, and thermal desorption spectroscopy ͑TDS͒ were used to study lateral interactions in the adsorbate layer of the CO/Rh͑111͒ system. The vibrational spectra show that CO adsorbs exclusively on top at low coverage. At about half a monolayer a second adsorption site, the threefold hollow site, becomes occupied as well. A steady shift to higher frequencies of the internal C-O vibrations is observed over the whole coverage range. The frequency of the metal CO ͑M-CO͒ vibration in the on-top mode hardly shifts at low coverage. However, upon the emergence of the second adsorption site the M-CO vibrations experience a shift to lower frequencies. The population of the second site is also accompanied by the development of a low temperature shoulder in the TD spectra, indicating an increasingly repulsive interaction in the adsorbed CO layer. Vibrational spectra of isotopic mixtures of 12 CO and 13 CO were used to assess the origin of the observed frequency shifts. They confirm that frequency shifts of the C-O stretching vibration at total CO coverage of 0.33 ML in the (ͱ3ϫͱ3)R30°structure arise purely from dipole-dipole coupling. Dilution of an isotopic species effectively suppresses frequency shifts arising from dipole-dipole coupling. Therefore, experiments with a small amount of 13 CO as a tracer to monitor the frequency shifts in the 12 CO adlayer were carried out over the entire coverage range of 12 CO. The results demonstrate that dipole-dipole coupling causes the frequency shifts at low coverage ͑Ͻ0.5 ML͒, whereas chemical effects set in at higher coverage ͑0.5-0.75 ML͒, connected with the population of the threefold sites. The results illustrate that HREELS in combination with isotopic dilution is a powerful tool in the assessment of lateral interactions between adsorbed molecules.

Atom–Molecule Interactions on Transition Metal Surfaces: A DFT Study of CO and Several Atoms on Rh(100), Pd(100) and Ir(100)

“…However, upon decreasing the coverage of CO on the rhodium surface, the adsorption energy remains the same, which indicates that there is no significant interaction between the CO molecules at either coverage. This finding is in agreement with the experimental results of Linke et al [39] and Kose et al [40] As a next step, coadsorbed atoms were added to these systems. We first discuss the effect of different coverages of these atoms and the CO molecule.…”

Simulating the Complexities of Heterogeneous Catalysis with Model Systems: Case studies of SiO2 Supported Pt-Group Metals

“…Spectral features associated with linearly bound CO (t CO ) and multiply bound (two, three-fold) CO are visible in the spectra. These vibrational features are consistent with linear and multiply bound vibrational features observed in previous investigations of CO adsorption on single crystal (IRAS, HREELS) [35,[46][47][48], model planar supported [49][50][51], and high surface area Rh catalyst surfaces. [52,53] As mentioned in the Introduction, it is now well-known that undercoordinated ''step-like'' sites on nanoparticle surfaces (surface atoms having \9 nearest neighbors on FCC(111) metals) can have much different catalytic properties than coordinated (''terrace-like'') sites on surfaces [54].…”

Methanol Adsorption on V2O3(0001)

“…9 the spectrum of the partially reduced oxide layer is shown, exhibiting shifted vanadyl vibrations. The shift is likely due to the reduced density of oscillators on the surface which reduces the vibrational dipole coupling, leading to a downward shift of the vibrational frequencies [25][26][27][28]. Upon methanol adsorption the intensity of the vanadyl vibrations decreases as evidenced by the features with positive intensity at 982-984 and 1,023-1,025 cm -1 in the spectra recorded after methanol adsorption.…”

“…There is probably also a 16 O contamination which could stem from the interaction of the oxide with the residual gas atmosphere. We assume that vibrational dipole coupling shifts intensity from the low energy band to the high energy band [25][26][27][28]. This can be a significant effect [27] so that a small concentration of 16 O can produce an intense vibrational absorption line as in Fig.…”