Surface reactions of ammonia on ruthenium nanoparticles revealed by 15N and 13C solid-state NMR

Ruthenium nanoparticles (Ru NPs) stabilized by bis-diphenylphosphinobutane (dppb) and surface-saturated with hydrogen have been exposed to gaseous 15NH3 and studied using solid-state 15N CP MAS NMR. Three signals have been observed at 24.5, −12 and −42 ppm (reference external liquid ammonia) which are assigned to chemisorbed ammonia species RuNHx. Sample exposure to vacuum or aging leads to conversion of the 24.5 ppm species into the other ones, a process which is reversed by re-exposure to hydrogen gas. Exposure to a mixture of 15NH3 and 13CO leads to the formation of surface bound urea as demonstrated by 15N and 13C CP MAS NMR. To understand the surface reactions of ammonia and the 15N NMR results, quantum chemical calculations of the structures, energies and 15N chemical shifts of ammonia species on Ru6 and Ru55 model clusters have been performed. The calculations indicate that under the experimental conditions applied, the fractions of RuNH3 and RuNH2 species are similar, independent of the H2 pressure. No RuN and RuNH species are formed which are calculated to resonate at a lower field than the signals observed experimentally. However, the 15N chemical shifts of RuNH2 depend on the number of neighboring surface hydrogens and hence on the H2 pressure. Thus, the signal at 24.5 ppm is assigned to RuNH2 in a neighborhood rich in surface hydrogens. RuNH2 depleted in neighboring surface hydrogens and RuNH3 resonated both in a similar chemical shift range to which the signals at −12 and −42 belong. A change of the hydrogen pressure then leads to interconversion of hydrogen-rich and hydrogen-poor neighborhoods of RuNH2 but does not alter the fractions of RuNH3 and RuNH2 according to the calculated stability diagram. Nevertheless, dissociation of RuNH3 into RuNH2 and surface hydrogen is expected to take place during the initial ammonia adsorption process and at low H2 pressures and high temperatures. Finally, some preliminary quantum chemical calculations suggest stepwise binding of two NH2 groups to adsorbed CO leading to surface bound urea where the oxygen is coordinated to Ru.