NMR Study of the Chemisorption and Surface Chemistry of Methylamine on Pd/SiO2

Abstract

Results of an investigation of the interaction of methylamine with a Pd/SiO2 catalyst utilizing nuclear magnetic resonance (NMR) spectroscopy are presented. Two sets of experiments were conducted. In one set, a sample of catalyst was initially exposed to 13CH315NH2 at 77 K and then subjected to a lengthy equilibration period at 190 K. A 13C spectrum at 77 K obtained on the sample at this point exhibited a single symmetric line at a frequency very close to the resonance frequency for methylamine in the absence of the catalyst. The line is attributed to methylamine adsorbed without dissociation. In 13C spectra obtained at 77 K on the sample after each of a series of subsequent annealing periods at successively higher temperatures in the range 253−298 K, the line exhibited a gradual broadening on the downfield side. Deconvolution of the spectra resolved the original symmetric line from a very broad downfield line attributed to partially dehydrogenated surface species designated by the formula (CN)Hx. In the second set of experiments, 13CH315NH2 was adsorbed on a sample of catalyst at 298 K. The sample was then annealed at successively higher temperatures in the range 325−450 K. Deconvolution of the 13C spectra obtained at 298 K after the various annealing periods revealed extensive formation of dimethylamine, reaching a maximum after the annealing period at 400 K. Ammonia and methane were first detected in the 15N and 13C spectra, respectively, after the annealing period at 375 K. During the annealing periods at 425 and 450 K, the dimethylamine decomposed almost completely, leaving only methane, ammonia, and residual surface species.