The populations of bridge and top site CO on Rh(100) vs coverage, temperature, and during reaction with O

Abstract

The intensities of the stretch modes of CO adsorbed in bridge and top sites on Rh(100) are presented as a function of coverage, temperature, and during reaction with O. Following adsorption at 90 K, the top site is predominantly occupied at low coverages, and at 0.5 atomic monolayers (ML) only the top site is occupied and a c(2×2) low energy electron diffraction (LEED) pattern is observed. With higher coverages the bridge site is increasingly occupied, and at saturation a p(4(2)1/2×(2)1/2)R45° LEED pattern is observed. The occupations of bridge and top sites at a fixed (0.5 ML) total coverage are observed to vary reversibly when the temperature is linearly ramped from 87 to 371 K and back down to 90 K; the difference in the binding enthalpy of bridge and top sites is determined to be 1.10±0.06 kcal mol−1. The enthalpy difference between bridge and top sites is also obtained from temperature programmed EELS (TP-EELS) which follows the site occupations as the temperature is increased past desorption. Surface order is found to have a significant effect on the normalized intensities of CO in conventional EEL spectra. The normalized intensities in angle integrated spectra, obtained by modulating the voltage difference applied to the two halves of the split analyzer input lenses, are insensitive to changes in the degree of surface order. The effect of low frequency frustrated rocking motions of diatomic molecules on the temperature dependence of the intensities observed in electron energy loss spectroscopy (EELS) is discussed. TP-EELS of coadsorbed CO and O allows the direct observation of the changes in the occupation of CO binding sites during reaction, and the results are correlated with LEED and temperature programmed reaction spectroscopy (TPRS) measurements of the reaction. Segregation of the reactants during the reaction is important.