No 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.

Palladium-based catalysts are known to promote the selective hydrogenation of acetylene to ethylene. Unfortunately, coupling reactions between the numerous surface intermediates generated in this process occur alongside. These side reactions are undesired, generating the so-called "green oil", i.e., C 4 + hydrocarbons that poison the active sites of the catalyst. The current work assesses the energetic and kinetic aspects of C 4 side products formation from the standpoint of computational chemistry. Our results demonstrate that the CC coupling of common surface species, in particular acetylene, vinylidene and vinyl, are competitive with selective hydrogenation. These CC couplings are particularly easy for intermediates where the C-Pd bond can largely remain intact during the coupling. Furthermore, the thus formed oligomers tend to be hydrogenated more easily, consuming hydrogen normally spent on acetylene hydrogenation. The analysis of site requirement suggests that isolated Pd 2 ensembles are sufficient for selective hydrogenation and would suppress oligomerization. However, upon aging, the PdAg alloy is likely to undergo reverse segregation and in this case, our computations suggest that the selectivity of the catalyst is lost, with enhanced CC couplings interfering even more strongly. Hence, small Pd ensembles are crucial to avoid oligomerisation side reactions of acetylene.

A systematic theoretical study of the adsorption of CO on the Pt{100}, Pt{110}, and Pt{111} surfaces is presented. The calculated equilibrium geometries and vibrational frequencies have been found to be rather independent of the cluster model chosen to represent the surface. However, calculated interaction energies are found to be very sensitive to the surface cluster model. The analysis of the chemisorption bond has been carried out by means of the constrained space orbital variation, CSOV, and of projection operator techniques. These analysis reveal that the bonding interactions are dominated by the π-back-donation although σ-donation plays a significant role. It is also clearly shown that all bonding mechanisms, other than Pauli repulsion, but specially π-back-donation, contribute to the observed red shift. However, the π-back-donation contribution to the red shift is very similar for CO on different sites. Hence, π-back-donation cannot be the mechanism responsible for the observed difference for the CO vibrational frequency on on-top and bridge sites. The CSOV decomposition reveals that the leading term contributing to this difference in vibrational frequency of chemisorbed CO is the initial Pauli repulsion or "wall effect"; this is a new, important and unexpected conclusion.

Temperature programmed desorption of CO coadsorbed with atomic N on Rh͑100͒, reveals both long-and short-range interactions between adsorbed CO and N. For CO desorption from Rh͑100͒ at low coverage we find an activation energy E a of 137Ϯ2 kJ/mol and a preexponential factor of 10 13.8Ϯ0.2 s Ϫ1 . Coadsorption with N partially blocks CO adsorption and destabilizes CO by lowering E a for CO desorption. Destabilization at low N coverage is explained by long-range electronic modification of the Rh͑100͒ surface. At high N and CO coverage, we find evidence for a short-range repulsive lateral interaction between CO ads and N ads in neighboring positions. We derive a pairwise repulsive interaction CO-N NN ϭ19 kJ/mol for CO coadsorbed to a c͑2ϫ2͒ arrangement of N atoms. 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.