ACETYLENE ADSORPTION STUDIES ON PD(111) AND STEPPED PD(111)

The adsorption of acetylene on Pd(111) and stepped Pd(111) surfaces as well as the influence and mechanism of a monatomic step have been inv estigated by theoretical calculations. On ideal Pd(111), the favourite acetylene adsorption site is the threefold hollow site. Our calculate d geometry configuration and vibrational frequencies of adsorbed acety lene are consistent with previous studies. Generally speaking, steps h ave active effects, zero effects or inactive effects in the adsorption process. For acetylene on stepped Pd(111), a monatomic step as the ac tive site makes the C-C bond of acetylene further weakened. Acetylene adsorption geometry in the second case (C-C bond axis tilts to the sur face normal and the molecular plane is vertical to the step direction) is similar to that of the molecule on the Pd(110) surface because of the (110) structure of the step. Furthermore, the vibrational frequenc ies of acetylene are also in agreement with the results of HREELS stud ies of acetylene on Pd(110).