Ethylene adsorption on Pd(1 1 1) studied using infrared reflection–absorption spectroscopy

The adsorption of ethylene has been studied on clean and hydrogen-covered Pd(1 1 1) using reflection–absorption infrared spectroscopy and molecular beam methods. Using a correlation diagram, in which vibrational frequencies are plotted versus a σπ parameter proposed by Stuve and Madix, shows that ethylene is substantially rehybridized on Pd(1 1 1) having a σπ parameter intermediate between those of ethylene on Ni(1 1 1) and Ru(0 0 1). In contrast, when ethylene adsorbs on hydrogen-covered Pd(1 1 1), only π-bonded species are detected. An additional species appears exhibiting a characteristic frequency of 957 cm−1 when Pd(1 1 1) is cooled to ∼80 K and the system pressurized with ∼10−5 Torr of ethylene. This species also appears on a CO-saturated Pd(1 1 1) surface. Molecular beam measurements show that its coverage reaches ∼1.3 ML indicating that it is due to ethylene adsorbed in the second and subsequent layers.