Mechanism of catalytic decomposition of CH3I on the Cu(111) surface: A UBI-QEP approach

The method of unity bond index-quadratic exponential potential (UBI-QEP) an d the computer simulation of the temperature programmed desorption (TPD) pa tterns are employed to derive the kinetic and thermodynamic parameters asso ciated with the steps of the pathway we propose for the catalytic decomposi tion of methyl iodide on the Cu(111) surface. Assuming a highly reactive "h ot methyl" surface intermediate and on the basis of our calculations it is concluded that the desorption of a part of this species is responsible for the reported methyl radical TPD peak at 140 K, while a part of this surface species is trapped in the three-fold sites of the Cu(111) surface and deso rbs to form the reported TPD peak at 470 K. It is also concluded that the r ate-determining step of the formation of CH4, C2H4, C2H6, and C3H6, which a re the products of the interaction of methyl iodide with the Cu(111) surfac e and all desorb at 470 K in a TPD experiment, is the surface dissociation of the adsorbed methyl groups. The effect of the surface coverage on the ki netic parameters of the reactions, appearing as changes of the products' di stributions, is quantitatively accounted for.