The surface morphology and the reaction properties of chemisorbed N on a Pt(111) surface has been studied at the atomic level with low temperature scanning tunneling microscopy (LT-STM). At high coverages, two phases, p(2�2)-N and (v3�v3)R30�-N, are found to coexist at temperatures between 360 and 400 K. For both phases nitrogen occupies fcc-hollow sites. At temperatures of 400 K and slightly above, only the p(2�2)-N phase is present on the surface. At temperatures above 420 K, nitrogen starts to desorb. The p(2�2)-N phase shows a honey-comb structure in STM images with three nitrogen and three platinum atoms forming a six-member ring. At low coverages, incomplete (2�2) patches of N, but no (v3�v3)R30� structure, are observed on the surface. When N and O are coadsorbed on the surface, they form a mixed (2�2) overlayer of N+O at high coverages. N and O can be clearly distinguished from each other by their appearance in the STM images.After exposing the N-covered surface to hydrogen gas, NH molecules were found to occupy fcc-hollow sites, while H atoms occupy atop sites. Manipulated by the STM tip, dissociation of NH bond and lateral transition of H are observed, and in both cases the response of the motion against applied bias voltage were measured. The threshold voltages of NH dissociation and H lateral transition were found to be around 440 mV and 270 mV, correspond to the excitation energy of N-H stretching and the Pt-H stretching modes, respectively. Substitution of H by D results in an isotope shift of -30 mV and -23 mV of the threshold voltages for ND dissociation and D lateral transition, respectively. Based on the above findings, we conclude that both processes are vibrationally induced.

Formation and hydrogenation of atomic N on Pt(111)

247th National Meeting (2014)

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