Coadsorption of ethylene and potassium on Pt(111). 1. Formation of a π-bonded state of ethylene

High-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and ultraviolet photoelectron spectroscopy (UPS) have been combined to study the effect of preadsorbed potassium on the adsorption of ethylene on the platinum(111) single-crystal surface. Addition of potassium increases the relative amount of reversible ethylene adsorption at 100 K. Upon coadsorption with potassium, we also observe a new low-temperature ethylene desorption state at 150 K with E_d = 5-9 kcal mol^-1, which is about one-half the binding energy of ethylene adsorbed on clean Pt(111). The origin of this new desorption state is the formation of a weakly interacting ethylene species, which is only slightly distorted from the gas-phase hybridization, as determined primarily by vibrational spectroscopy. An explanation is given for these effects in which the increased charge density at the platinum surface due to coadsorbed potassium inhibits the strong σ-donation of adsorbed ethylene to platinum and greatly reduces the chemisorption bond strength.