KINETICS OF SURFACE-REACTIONS ON THE ATOMIC-SCALE

The reaction of CO and NO with Rh crystals of almost hemispherical sha pe was studied at temperatures between 300 and 550 K. Strong morpholog ical changes of the crystals were observed with atomic resolution in t he field ion microscope (FIM). After field-free reaction with CO at 42 0 K the hemispherical Rh crystal appeared to be transformed into polyh edral forms. The reaction with NO did not produce as regular forms as with CO. In particular, high-index planes were largely absent after re action in CO while they were still present, though redistributed, afte r reaction with NO. Adsorbate-induced restructuring was found to be th ermally activated. Using pulsed field desorption mass spectrometry (PF DMS), the underlying reaction mechanisms and kinetics were studied in small selected areas of up to 150 atomic sites. The interaction with C O led to the formation of Rh-subcarbonyls, Rh(CO)(x) (x = 1-3). These species are most likely directly involved in the reconstruction proces s of the Rh sample. A reaction model is proposed according to which Rh -dicarbonyl is liberated into the terrace parts of the surface after R h-Rh bond breaking in kink site positions. This is a repeatable step w hich ends as soon as an open chain of atoms is used up. Accordingly, k ink sites are annihilated and facet structures with long chains of led ge atoms appear. The details of the reconstruction process with NO are not as clearly established as with CO. Multiple bonded NOad was not d etected by PFDMS. Instead, considerable decomposition into nitrogen an d oxygen atoms occurred which finding is in agreement with the catalyt ic activity known for Rh metal.