This work examined the spatial structure and bonding configuration of nitric oxide (NO) at a well-defined Rh(lll) electrode surface by in situ scanning tunneling microscope (STM) imaging under potential control. Immersing Rh(lll) electrodes into acidic KNO2 solutions (pH 2, 0.5 M HF) resulted in a long-range ordered (3 x 3) structure, possibly owing to irreversibly adsorbed NO molecules. Although this structure predominated between 1.0 and 0.3 V, cathodic polarization to 0.2 V or more negative caused local roughening. In the absence of HNO2, cathodic polarization of a Rh(lll) electrode to 0.05 V completely reduced the surface-bound NO molecules. Coulometric and in situ STM measurements revealed a saturated coverage of 0.48 and 0.44, respectively, for NO molecules in the ordered (3 x 3) structure. This work also proposed a tentative model of the (3 x 3) structure containing 4 NO molecules. One fourth of the NO molecules adsorbed at near-top sites, whereas the remaining resided at 2-fold bridging sites. Real-time in situ STM imaging provided a direct view of the reduction processes at potential negative of 0.2 V. Reactions preferentially occurred at atomically flat terraces, rather than at surface defects such as step edges, kinks, and vacancies. Moreover, the initial reaction fronts were spatially concentrated, rather than randomly distributed.
