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Dynamic Monte Carlo simulations are used to investigate the kinetics of the catalytic reduction of NO on Rh(111) surfaces. The reaction scheme where the production of [N.sub.2] is assumed to result mainly from a pathway involving the formation of a [(N-NO).sup.*] intermediate is supported by the study and defines a reliable model capable for predicting the changes in reaction rate as a function of temperature and NO:CO gas composition.

The thermal conversion of N2O on Rh(111) single-crystal surfaces is studied using a collimated effusive molecular beam technique coupled with mass spectroscoy detection. The decomposition of pure N2O was determined to occur at temperatures as low as 120 K, to follow first-order kinetics, and to lead to the stoichiometric production of N2(g) and atomic adsorbed oxygen.

The kinetics of the catalytic reduction of NO by CO on Rh(111) was simulated by using a Monte Carlo algorithm based on a lattice-gas model. The results reported here complement those reported before by incorporating new experimental results which reveal the formation of an N-NO intermediate as necessary for molecular nitrogen production as well as the formation of N-islands on the surface. The behavior of the steady-state phase diagram for the reaction is analyzed in terms of several parameters representing different reaction schemes. It is shown how the range of conditions under which the reaction can occur is influenced by the mechanism for the formation of the N-NO intermediate, by the growth of N surface islands, and by side steps such as NO surface diffusion and NO and CO desorption. [ABSTRACT FROM AUTHOR]

The nondissociative adsorption of gas phase molecules onto a spatially homogeneous square lattice was studied using a Monte Carlo computer simulation method which accounts for the existence of an extrinsic precursor state. The parameters affecting the adsorption kinetics were varied systematically in order to compare our simulations with a model for adsorption originally proposed by Kisliuk [J. Phys. Chem. Solids 3, 95 (1957)]. Our studies indicate that in the presence of precursor mobility, the sticking coefficients obtained from our simulations are consistently lower than those obtained using Kisliuk’s equation. Upon further investigation, we found two reasons for this discrepancy, namely, islanding of the adsorbates on the surface, and site revisiting in the precursor state. We found that some degree of islanding within the chemisorption layer occurs as a direct consequence of the nature of the precursor mediated adsorption process and that the extent of this clustering is controlled by the competition between mobility within the physisorbed layer and desorption; when mobility dominates, islanding formation is enhanced. Site revisiting within the precursor state was also found to affect the adsorption kinetics; we were able to derive here an analytical expression for the sticking coefficient that accounts for this factor and that correlates well with results from the corresponding simulation. Finally, we show that sticking coefficients are affected by surface geometry. [ABSTRACT FROM AUTHOR]

We have derived theoretical expressions for the incident beam angular dependence of the absorption cross section for ethylene chemisorbed on single crystal surfaces with linearly polarized light. The changes in the π and σ resonance intensities for C 1s near-edge x-ray absorption fine structure (NEXAFS) spectra can be related to the geometry of chemisorption relative to the surface. The derived equations have been used to analyze experimental data in order to determine the structure of chemisorbed ethylene on Ni(100). At low temperatures ethylene bonds with the molecular axis parallel to the surface but with the molecular plane tilted 50°. The geometry of vinyl fragments formed by annealing the crystal to 180 K was also determined. The C–C bond is bent 35° from the surface normal in this case, with the plane of the molecule tilted by 65°. [ABSTRACT FROM AUTHOR]

The nature of the species formed by CO chemisorption of Mo(100) has been investigated using angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and near-edge x-ray absorption spectroscopy. High-resolution electron energy loss spectroscopy (HREELS) indicates the formation of two distinct types of CO. At coverages greater than 50% of saturation, chemisorbed CO exhibits a CO stretching frequency of ∼2100 cm-1 corresponding to a CO molecule chemisorbed in an atop site. ARUPS indicates that at these coverages CO chemisorbs with its axis perpendicular to the surface in an analogous manner to that commonly observed on transition metal surfaces. At coverages less than 50% of saturation, CO exhibits an extraordinarily low stretching frequency of ∼1200 cm-1. Both NEXAFS and ARUPS measurements unequivocally indicate that at low coverages CO is tilted at ∼40° to the surface normal. Since CO is tilted with respect to the surface at low coverages, this effect cannot be ascribed to adatom–adatom interactions, and a careful measurement of the positions of the photoelectron peaks of the tilted molecule indicates that both the 1π and the 5σ orbitals participate in surface bonding. A bonding model is proposed that is in accord with these observations in which the CO molecule chemisorbs into a fourfold hollow site. [ABSTRACT FROM AUTHOR]