The effects of preadsorbed CO on the chemistry of CH₃ and CH₃I on Pd{111}

The thermal decomposition of iodomethane on clean, I-precovered and CO-precovered Pd{111} is investigated using thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). On a clean surface, CH₃I decomposes to produce CH_3,ads and I_ads at temperatures between 175 and 200 K. No evidence of C₂-hydrocarbon formation is observed. The CH_3,ads produced by C-I bond activation hydrogenates to desorb as CH₄ at 200 K. The I_ads remains on the surface below 850 K and completely desorbs by 1100 K. The I_ads behaves as a site blocker which removes activation sites for C-I bond activation without changing the chemical activity of CH_3,ads. Compared with the results obtained from other laboratories, the thermal decomposition of CH₃I on Pd is a strong structure-sensitive reaction. It was found that the C-I bond breaks at a temperature as low as 95 K on Pd{100}, whereas the C-I bond cleavage can only be observed at temperatures above 175 K on Pd{111}. The presence of CO on Pd{111} inhibits the C-I bond breaking which simultaneously influences the thermal stability of CH_3,ads. Depending upon the initial coverage of CO, the hydrogenation of CH_3,ads to CH₄ occurs at temperatures which are up to 80 K higher than for the clean surface. The enhancement of the thermal stability of CH_3,ads may result from both site blocking and electronic effects induced by preadsorbed CO.