Nitrogen on ruthenium single-crystal surfaces

Static and dynamic properties of nitrogen on Ru surfaces are reviewed. On R u(0001), N occupies the h.c.p. threefold-hollow site as demonstrated by sca nning tunneling microscopy and low-energy electron diffraction. For Ru(10 ( 1) over bar 0) and (11 (2) over bar 1) models for ordered structures are di scussed. There is general agreement that the dissociative adsorption of N-2 is the rate-determining step in the ammonia synthesis over Ru surfaces. Th e dissociation of N-2 is an activated, surprisingly slow process. Different groups have observed a sticking coefficient of 10(-12). By density functio nal theory the geometry and energies of the dissociation pathway have been identified. Very recently it has been shown that the activation barrier fou nd in experiments can only be modeled assuming the dissociation to take pla ce at monoatomic steps. Experiments and microkinetic modeling of N-2 dissoc iation and NH3 formation on Ru/Al2O3, Ru/MgO, and Cs-Ru/MgO model catalysts are discussed. Differences between the different catalysts and the single- crystal surfaces cannot be explained from the existing single-crystal resul ts. It seems that a detailed knowledge of the morphology of the 2 nm large Ru metal particles on the different catalyst supports is needed to further develop a detailed understanding of ammonia synthesis over Ru-based catalys ts.