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

Abstract

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 RuNH_x. 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.