pi-bonded ethene on Pt(111) surface studied by IRAS

Adsorption of ethene on clean and ethylidyne-covered Pt(111) surfaces was investigated by infrared reflection absorption spectroscopy (IRAS). Under gaseous ethene of similar to 10(-3) Pa, pi-bonded ethene was reversibly formed on a Pt(111) surface saturated with di-sigma-bonded ethene at 112 K. The pi-bonded ethene gave infrared absorption peaks at 954 and 3090 cm(-1) which were assigned to =CH2 wagging (out-of-plane) and CH2 asymmetric stretching modes, respectively. These two peaks indicated that the pi-bonded ethene was adsorbed on Pt(111) in such a way that the molecular plane is tilted with respect to the surface while the C=C bond is being kept parallel to the surface. The intensity of the C-H stretching band at 2908 cm(-1) of di-sigma-bonded ethene decreased with the increase of pi-bonded ethene. The heat of adsorption of pi-bonded ethene was estimated to be 40 +/- 10 kJ/mol. The presence of ethylidyne with saturation coverage was found to inhibit the pi-bonded ethene adsorption.