Nanoconfined nitrogen hydrogenation on Ru(0001): Prediction of entropy related shifts in the reaction equilibria

Remarkable variations in NH_x (x = 1,2,3) equilibrium molecular fractions are revealed by statistical-mechanical computations for a system composed of only few reactant atoms that are confined to nanoscale Ru(0001) surface. Using one of two sets of reported DFT-based reaction energetics as input, NH formation on the flat surface is enhanced up to ∼ 50% compared to the thermodynamic limit (TL) of the macroscopic system. This nanoconfinement entropic effect on chemical equilibrium (NCECE), introduced and demonstrated by us before for some hypothetical reactions, originates from mixing entropy reduction in the small-system that increases the product molecular fraction in exothermic reactions, as is fully elucidated here for the case of NH. The other input set gives an opposite effect due to the reaction endothermicity on both flat and stepped surfaces. For concurrent ND and NH formation, including H - D exchange, the temperature and coverage dependent superposition of the three NCECE contributions have a significant effect on two of the three equilibrium constants of the coupled reactions, and increase/decrease the ND/NH fractions. The computed TL low fractions of NH₂ and NH₃ in the coupled three hydrogenation steps are further diminished by the NCECE, suggesting that from this aspect ammonia synthesis on smaller particles is expected to be less effective.