DEFECT INDUCED SURFACE-CHEMISTRY - A COMPARISON OF THE ADSORPTION AND THERMAL-DECOMPOSITION OF C2H4 ON RH(111) AND RH(331)

The adsorption and thermal decomposition of C2H2 on Rh{111} is compared to the atomically stepped Rh{331} surface over a temperature range of 300 to 800 K. Using X-ray photoelectron spectroscopy (XPS) we find that the C 1s spectra as a function of C2H4 exposure exhibit a shift in binding energy (Eb) from 283.5 eV at 1 L C2H4 exposure on both surfaces to 283.8 eV on Rh{33 and to 284.1 eV on Rh{111} at saturation coverage (4 L). Careful analysis of the C 1s Eb value and full width at half maximum as a function of surface temperature after a 10 L exposure of C2H4 at 300 K reveals that a species consistent with a C2H adsorbate composition is formed between 400 and 450 K on Rh{111}. This species is also observed on Rh{331} although at the lower temperature of 375 K. Computer peak deconvolution of the C 1s spectra between 500 and 700 K suggests that a CHads or Cads surface fragment is formed and increases in concentration at the expense of the C2H species as the surface temperature increases. Above 750 K a graphite overlayer is formed on both surfaces. This overlayer, however, exhibits a low degree of carbon π-character bonding on Rh{331}. The adsorption and decomposition mechanisms suggest that the 300 K C2H4 adsorbate on Rh{331} is ethylidyne and that the stepped surface is more thermally reactive than the flat Rh{111} surface. © 1990.