Adsorption States of N/H Activated on Ru Nanoparticles Uncovered by Modulation-Excitation Infrared Spectroscopy and Density Functional Theory Calculations.

The adsorption states of N and H on MgO-supported Ru nanoparticles under conditions close to those of ammonia synthesis (AS; 1 atm, 250 °C) were uncovered by modulation-excitation infrared spectroscopy and density functional theory calculations using a nanoscale Ru particle model. The two most intense N adsorption peaks corresponded to the vertical chemisorption of N on the nanoparticle's top and bridge sites, while the remaining peaks were assigned to horizontally adsorbed N in view of the site heterogeneity of Ru nanoparticles. Long-term observations showed that vertically adsorbed N molecules gradually migrated from the top sites to the bridge sites. Compared to those adsorbed vertically, N molecules adsorbed horizontally exhibited a lower dipole moment, an increased N─N bond distance, and a decreased N─N bond order (i.e., were activated), which was ascribed to enhanced Ru-to-N charge transfer. H molecules were preferentially adsorbed horizontally on top sites and then rapidly dissociated to afford strongly surface-bound H atoms and thus block the active sites of Ru nanoparticles. Our results clarify the controversial adsorption/desorption behavior of N and H on AS catalysts and facilitate their further development.