NMR study of the chemisorption and decomposition of methylamine on Rh/SiO2

Nuclear magnetic resonance (NMR) studies of the interaction of methylamine with the rhodium surface in a Rh/SiO2 catalyst at temperatures in the range of 253-298 K reveal two principal features in both the C-13 and N-15 spectra. One feature is a resonance occurring at a frequency very close to the resonance frequency for methylamine gas. This feature is attributed to methylamine molecules adsorbed without dissociation. Because the methylamine is not removed from the catalyst surface by evacuation to 10(-6) Torr, its interaction with the surface is reasonably strong. The silica support does not retain methylamine after such an evacuation, and therefore, the adsorbed methylamine is associated with the rhodium. The other feature in both the C-13 and N-15 spectra is a very broad resonance occurring downfield of the resonance for the nondissociatively adsorbed methylamine. It is attributed to partially dehydrogenated surface species designated by the generalized formula (CN)H-x. Incipient scission of carbon-nitrogen bonds is observed at room temperature. As the temperature is increased above room temperature, the extent of scission of the carbon-nitrogen bonds increases. Ammonia, methane, and higher-carbon-number hydrocarbons then appear in the gas phase. The nondissociatively adsorbed methylamine is continuously transformed into the species (CN)H-x, thereby releasing hydrogen on the surface. The hydrogen is consumed in the production of the ammonia and methane observed in the gas phase. Ammonia is formed much more readily than methane or the other hydrocarbons, with the result that the species remaining on the surface are rich in carbon relative to nitrogen.