Kinetics of surface reactions on the atomic scale

Abstract

AbstractThe reaction of CO and NO with Rh crystals of almost hemispherical shape was studied at temperatures between 300 and 550 K. Strong morphological changes of the crystals were observed with atomic resolution in the field ion microscope (FIM). After field‐free reaction with CO at 420 K the hemispherical Rh crystal appeared to be transformed into polyhedral forms. The reaction with NO did not produce as regular forms as with CO. In particular, high‐index planes were largely absent after reaction in CO while they were still present, though redistributed, after reaction with NO. Adsorbate‐induced restructuring was found to be thermally activated. Using pulsed field desorption mass spectrometry (PFDMS), the underlying reaction mechanisms and kinetics were studied in small selected areas of up to 150 atomic sites. The interaction with CO led to the formation of Rh‐subcarbonyls, Rh(CO)x (x=1‐3). These species are most likely directly involved in the reconstruction process of the Rh sample. A reaction model is proposed according to which Rh‐dicarbonyl is liberated into the terrace parts of the surface after Rh‐Rh bond breaking in kink site positions. This is a repeatable step which ends as soon as an open chain of atoms is used up. Accordingly, kink sites are annihilated and facet structures with long chains of ledge atoms appear. The details of the reconstruction process with NO are not as clearly established as with CO. Multiple bonded NOad was not detected by PFDMS. Instead, considerable decomposition into nitrogen and oxygen atoms occurred which finding is in agreement with the catalytic activity known for Rh metal.