An infrared spectroscopic and temperature-programmed desorption study of methyl iodide hydrogenation on Pd(1 1 1)

The adsorption and hydrogenation of methyl iodide is studied on clean and hydrogen-covered Pd(1 1 1) using reflection–adsorption infrared spectroscopy (RAIRS) and temperature-programmed desorption. Molecular methyl iodide desorbs from clean Pd(1 1 1) at 229 K and hydrogen desorbs at ∼360 K. Methyl groups hydrogenate in two states at 191 and 304 K for large methyl iodide exposures where RAIRS data suggest that the low-temperature state is due to reaction with tilted methyl species while the high-temperature state is formed by hydrogenation of perpendicular ones. A single state is found at low methyl iodide coverages at ∼219 K due to the hydrogenation of tilted methyl species where the kinetics is limited by the rate of α-hydrogen elimination. Saturating the surface with hydrogen completely suppresses the high-temperature methane desorption state and the methane desorbs at between 189 and 207 K depending on coverage.