We have studied the mechanism of the partial dissociation of water on Ru(0001) by high resolution scanning tunneling microscopy (STM). The thermal evolution of water at submonolayer coverage has been tracked in the 110-145 K temperature range to identify the precursor structures for the partial dissociation. These were found to consist of hexagons arranged in thin stripes aligned along the close packed Ru [21¯1¯0] directions. The partially dissociated phase, on the other hand, contains a mixture of H2O and OH hexagons arranged into wider stripes and rotated by 30° with respect to the intact water stripes. The atomic structure of both types of stripes is determined with the aid of density functional theory and STM simulations, providing insights into the partial dissociation reaction path. The reaction is found to be exothermic by around 0.4 eV and initiating at the edges of the intact water stripes. Hydrogen atoms, from water dissociation or already present at the surface, are found to play an important role in the kinetics of the reactions.

Low-energy electron microscopy (LEEM) reveals a new mode of graphene growth on Ru(0001) in which Ru atoms are etched from a step edge and injected under a growing graphene sheet. Based on density functional calculations, we propose a model wherein injected Ru atoms form metastable islands under the graphene. Scanning tunneling microscopy (STM) reveals that dislocation networks exist near step edges, consistent with some of the injected atoms being incorporated into the topmost Ru layer, thereby increasing its density.

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The adsorption and dissociation of ammonia on Ru(0001) was studied by scanning tunneling microscopy (STM), density functional theory (DFT), and STM contrast simulations. Various NH x (with x = 0−2) species were formed by controlled STM tip manipulation. Each species shows a characteristic imaging contrast in STM measurements, changing from a protrusion for NH 3 and NH 2 , to a depression for NH and N. The adsorption sites of each species determined from the STM images and their contrast agree well with DFT calculations and STM image simulations. Ammonia was found to interact with hydrogen atoms present on the surface, leading to the formation of triangular-shaped NH 3 + 3H complexes. At submonolayer coverage, ammonia dimers were also identified and their formation and dissociation were observed upon tip manipulation.

Low-temperature scanning tunneling microscopy and density-functional theory ͑DFT͒ were used to study the adsorption of water on a Ru͑0001͒ surface covered with half monolayer of oxygen. The oxygen atoms occupy hcp sites in an ordered structure with ͑2 ϫ 1͒ periodicity. DFT predicts that water is weakly bound to the unmodified surface, 86 meV compared to the ϳ200 meV water-water H bond. Instead, we found that water adsorption causes a shift of half of the oxygen atoms from hcp sites to fcc sites, creating a honeycomb structure where water molecules bind strongly to the exposed Ru atoms. The energy cost of reconstructing the oxygen overlayer, around 230 meV per displaced oxygen atom, is more than compensated by the larger adsorption energy of water on the newly exposed Ru atoms. Water forms hydrogen bonds with the fcc O atoms in a ͑4 ϫ 2͒ superstructure due to alternating orientations of the molecules. Heating to 185 K results in the complete desorption of the water layer, leaving behind the oxygen-honeycomb structure, which is metastable relative to the original ͑2 ϫ 1͒. This stable structure is not recovered until after heating to temperatures close to 260 K.

We study the mechanism leading to the breaking of the N–H bonds in ammonia on Ru(0001) by means of scanning tunneling microscopy (STM). Our results support a model where injection of electrons or holes into antibonding (LUMO) and bonding (HOMO) orbitals of the molecule is far more effective than thermal excitations for molecular dissociation. We also found that a critical electric field between tip and surface is necessary to shape the tunneling barrier to obtain efficient rates of electrons from the tip or surface. First principle DFT calculations allow us to explain the observations and show that the applied electric field cannot by itself account for the observed dissociation. Since electron injection is the process governing photo- and electrochemical reactions, our STM study provides a detailed view of the reaction mechanism at the single molecular level in these nonthermal reactions.

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