Adsorption and decomposition of pyridine, pyrrole, and ethylamine on molybdenum(110): Experiment and modeling

The adsorption and decomposition of pyridine, pyrrole, and ethylamine on Mo(110) were studied using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT). Low pyridine surface exposures (0.5 L) indicated one desorption peak at 375 K (Eads = 23.5 kcal mol-1), while at 1 L surface exposure two desorption peaks at temperatures of 370 and 530 with adsorption energy of 23.2 and 33.6 kcal mol-1, respectively, were observed. Using DFT and EELS at low surface exposures, it was determined the preferred adsorption mode is pyridine in a parallel configuration coordinated to three surface Mo-sites as mu3,eta6-Py-0°, while the second desorption feature results from a configuration with a bond to the surface through the nitrogen atom coordinated to one Mo-site as eta 1(N)-Py-90°. Pyrrole showed only one desorption feature for all surface exposures at an approximate temperature of 351 K with adsorption energy of 21.9 kcal mol-1, which suggests pyrrole adsorption on Mo(110) is not surface coverage dependent as the case for pyridine. Pyrrole is suggested to adsorb in a parallel mode with respect to the Mo(110) surface through its pi orbital similar to mu3,eta5-Pyr-0° with the possible coexistence of a second adsorption mode with the molecular plane slightly tilted as mu3,eta4(N,C2,C3,C4)-Pyr-5°, which is suggested to arise from the lateral interactions of adsorbed pyrrole on Mo(110). Both pyridine and pyrrole adsorb reversibly and irreversibly on Mo(110) indicating the surface is active for denitrogenation. This is indicated by the presence of surface carbon and nitrogen following TPD. Pyridine also showed desorbed hydrogen upon decomposition. Ethylamine adsorption showed irreversible reaction behavior, as hydrogen and acetonitrile were observed desorbing from the surface during TPD. Based on the DFT relative adsorption energies of ethylamine adsorption on Mo(110), ethylamine adsorbs molecularly on Mo(110) through the nitrogen atom with the ethyl group pointing away from the surface and CH2 interacting with the metal surface with an adsorption energy of 20.6 kcal mol-1. This research provides fundamental information regarding the energetics and mechanism of catalytic hydrodenitrogenation (HDN) on a model hydrotreating catalyst surface.