Adsorption and Reactions of CH2I2 on Clean and Oxygen-Modified Ag(111): a RAIRS and TPD Study

The surface chemistry of CH2I2 on Ag(111) in the presence and absence of pre-adsorbed O, produced by NO2 adsorption at elevated temperature, has been examined using temperature-programmed desorption and reflection absorption infrared spectroscopy. There is good evidence for the formation of adsorbed methylene, CH2(a), that reacts with another CH2(a) to form and desorb ethylene, C2H4(g), in a reaction-limited process. Increasing the surface coverage of CH2I2 hinders both the dissociation and recombination processes indicated by the upward temperature shift in the formation of C2H4. Co-adsorbed O atoms strengthen the bonding of CH2I2 to the surface; the increased thermal stability is up to 60 K. The formation of C2H4 decreases with increasing amounts of pre-adsorbed O; the main reaction product is CH2O produced in a reaction-limited process. CH2O forms either on the chemisorbed or on the oxide phase with desorption peak temperatures of 225 and 270 K, respectively. The formation of gas-phase carbon dioxide suggests that a formate intermediate is involved in a secondary reaction pathway.