CO-INDUCED MORPHOLOGICAL-CHANGES OF RH CRYSTALLITES - MECHANISMS, KINETICS, AND REAL-SPACE IMAGING ON THE ATOMIC-SCALE

The reaction of CO with Rh crystals of almost hemispherical shape (field emitter tips) was studied at various temperatures between 200 and 420 K and at pressures up to 10-1 Pa. Strong morphological changes of the crystals could be observed with atomic resolution by field ion microscopy (FIM). After field-free reaction with CO at 360 and 420 K the hemispherical Rh crystals appeared to be transformed into polyhedral shapes. High-index surface planes were absent on these crystals. i.e. kink sites were largely used up in faceting. More generally, the morphology of the reshaped crystals contained mainly planes with (001), (011), (111), and (113) symmetry. The CO-induced restructuring was found to be thermally activated: atomic displacements were first observed at a reaction temperature of 240 K but were absent at 200 K. Using (atom-probe) pulsed field desorption mass spectrometry (PFDMS), the underlying reaction mechanisms and kinetics were studied in small selected surface areas containing about 150 atomic sites. Besides CO the surface layer always contained Rh-subcarbonyls, Rh0(CO)x (x = 1-3). Time-resolved measurements (reaction times between 0.5 ms and 0.1 s) revealed temperature-dependent delay times for Rh0(CO)2 formation. This observation could be consistently interpreted on the basis of a reaction model leading to the liberation of mobile, adsorbed Rh0(CO)2 after RhRh bond breaking in kink site positions. More generally, the structural changes of Rh crystals observed in FIM were most likely caused by (thermally activated) Rh0(CO)2 formation and decomposition at kinks with intermediate diffusion across the surface. The results of the present study suggest Rh0(CO)2 to be the likely precursorspecies for Rh1(CO)2 formation as observed in studies with Rh/Al2O3 model catalysts. © 1993 Academic Press, Inc.