The adsorption of CCl_4 on Ag(111): Carbene and C=C bond formation

摘要

The adsorption of CCl_4 on Ag(111) has been investigated from 100 K to 300 K using absolute sticking probability measurement, temperature programmed desorption, Auger electron spectroscopy, low energy electron diffraction, ultra-violet photoelectron spectros-copy and X-ray photoelectron spectroscopy. At 100 K, CCl_4 adsorbs molecularly with a sticking probability of 1.0, forming a (3 x 3) adsorption structure. At 300 K the following overall reaction occurs, 2CCl_(4(g)) → 4Cl_((chem)) + C_2Cl_(4(g)), with an initial sticking probability of S_0 = 0.25, indicating that activated dissociative adsorption is the initial step in the above reaction. The saturated surface at 300 K consists entirely of chemisorbed chlorine which undergoes an order/disorder transition to a sharp (3~(1/2) x 3~(1/2)) R30° structure on cooling below 180 K. Temperature programmed desorption of CCLi adsorbed at 100 K shows a multilayer peak at 140 K and a first layer peak at 203 K, where part of the molecularly adsorbed CCl_4 desorbs and the rest is converted to a surface carbene, :CCl_2, and chemisorbed chlorine. At 248 K the adsorbed carbenes react to form C_2Cl_4 which promptly desorbs, as it is formed above its normal desorption temperature of 210 K.