Daniel Curulla

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

High resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS) were used to study lateral interactions in the adsorbate layer of the CO/Rh(111) 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–CO) 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 12CO and 13CO were used to assess t...

Quantification of lateral repulsion between coadsorbed CO and N on Rh(100) using temperature-programmed desorption, low-energy electron diffraction, and Monte Carlo simulations

This has important implications for the lateral distribution of coadsorbed CO and N at different adsorbate coverages. Regarding the different lateral interactions and mobility of adsorbates, we propose a structural model which satisfactorily explains the observed effects of atomic N on the desorption of CO. Dynamic Monte Carlo simulations were used to verify the experimentally obtained value for the CO‐N interaction, by using the kinetic parameters and interaction energy derived from the temperature-programmed desorption experiments.

Theoretical study of CO2 activation on Pt(111) induced by coadsorbed K atoms

Abstract The adsorption of CO 2 on the clean and potassium-precovered Pt(111) surface has been studied by means of the cluster model approach within the hybrid B3LYP density functional theory-based method. On the clean surface, CO 2 is undistorted and weakly bound. The equilibrium position of this physisorbed species appears at a rather large distance from the surface. However, when coadsorbed K atoms are included in the model, a chemisorbed, bent CO 2 species on top of a surface Pt atom is found. The presence of the coadsorbed K is found to be necessary to promote CO 2 chemisorption and activation, the key step in activating the CO 2 molecule being a direct interaction with the coadsorbate. In addition, the calculated vibrational frequencies for this chemisorbed species are in agreement with available experimental data.

Reliability of the cluster model approach to the Stark tuning rate of adsorbates on metal surfaces: CO and OH− on Pt(111)

The Stark tuning rate, STR, of free and chemisorbed CO and OH− on Pt(111) is theoretically studied by means of a cluster model density functional theory, DFT, approach with the hybrid B3LYP potential for the exchange-correlation functional. The STR is obtained by explicit calculation of the corresponding vibrational frequencies and of their variation with respect to the intensity of a uniform external electric field. A point of special concern is the influence of the size of the model employed which has been tested by progressively increasing the number of metal atoms employed in the surface model. For both CO and OH− on Pt(111) results show that, except for extremely small cluster models, STR values do not appreciably vary with respect to cluster size. Therefore, the use of a cluster model does not introduce artefacts that may mislead the physical description of this property.

Ab initio cluster model study of electric field effects for terminal and bridge bonded CO on Pt(100)

Abstract The ab initio cluster model approach has been applied to investigate the electric field effects for CO on on-top and bridge sites of the Pt(100) surface modeled by Pt 41 and Pt 32 . For the available experimental data in the absence of external electric fields, a reasonable agreement between experimental and calculated results is found for CO chemisorbed on Pt(100). Electric field effects induce changes in the equilibrium distance and vibrational frequency of chemisorbed CO but also on the chemical bonding mechanism, in particular in the extent of the 2π* backdonation. The model calculations predict a linear relationship between the intensity of the electric field and the extent of the 2π* backdonation. The existence of this linear relationship means that a separation between electrostatic and chemical effects is not feasible.

Assignment of the vibrational features in the Rh(1 1 1)–(2×2)-3CO adsorption structure using density functional theory calculations

Calculated CO stretching frequencies for the Rh(1 1 1)–(2×2)-3CO structure are compared with recent high-resolution electron energy loss spectroscopy (HREELS) experiments. Two loss peaks in the HREELS spectrum at 2070 and 1861 cm−1 were attributed to ontop- and hollow-bonded CO molecules, respectively. Two weaker peaks, which had not been observed before due to the lack of resolution, appeared at 1785 and 1925 cm−1. DFT calculations reveal that the loss features at 1861 and 1785 cm−1 originate from the adsorption of CO molecules at hcp and fcc sites and correspond to the in-phase and out-of-phase normal modes resulting from dipole–dipole coupling.

A density functional study of the adsorption of CO on Rh(111)

Adsorption of CO on Rh{111} has been studied by means of density functional calculations within the cluster model approach. Optimized structures and C–O stretching frequencies appear to be in good agreement with experiment and they permit the confirmation of the structural model suggested by Beutler et al. (A. Beutler, E. Lundgren, R. Nyholm, J. N. Andersen, B. Setlik and D. Heskett, Surf. Sci., 1997, 371, 381, A. Beutler, E. Lundgren, R. Nyholm, J. N. Andersen, B. Setlik and D. Heskett, Surf. Sci., 1998, 396, 117) using HRCLS. Vibrational frequency shift dependence on surface coverage has been analyzed in terms of dipole–dipole coupling and chemical effects. Furthermore, we discuss a third mechanism that is likely to be responsible for the shift to higher frequencies at high coverage regimes.