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Stepped surfaces have widely been used as model catalysts as they provide active sites [1, 2] which trigger many catalytic processes.[3, 4] Molecular-dissociation processes on surfaces are expected to be sensitive to the surface structure and occur preferentially at defects, such as step edges and kinks. Various surface-science techniques that average over the entire surface have been employed for the investigation of catalytic properties.[3–6] Although such studies demonstrate the catalytic activity of a surface in general, they do not offer sitespecific information about the exact location of the reaction. Scanning tunneling microscopy (STM) provides a local probe, allowing site-specific information to be obtained about the structure and location of active sites on a catalytically active surface with atomic resolution.[7–10] STM studies have allowed the direct observation of the active sites of heterogeneous catalysts, as …