Direct observations of surface chemical and structural changes resulting from the CO oxidation reaction on rhodium

Surface modifications resulting from the CO oxidation reaction on Rh have been investigated by imaging atom-probe mass spectroscopy and field ion microscopy. Rhodium field emitter samples were heated to 500--600 K in 1--15 Torr of the reactant gases and transferred under UHV to the analysis chamber. Imaging atom-probe analysis was used to detect changes in surface composition, and field-ion microscope analysis was used to detect changes in surface morphology. For reaction temperatures of 500 K and total pressures between 1 and 2 Torr, the surface composition was found to change from primarily CO and surface carbon to Rh2O3 as the O2/CO ratio was increased beyond 30/1. This ratio correlates with previous observations of the deactivation of the CO oxidation reaction providing direct evidence that the deactivation is oxide-induced. Field ion microscope images of Rh surfaces following the CO oxidation reaction in stoichiometric mixtures of CO and O2 at total pressures from 1.5--15 Torr indicate morphological changes which differ from changes due to thermal annealing alone. The nature of the changes are in qualitative agreement with previous studies of ammonia oxidation and decomposition on macroscopic Pt spheres