In situ dynamic study of hydrogen oxidation on rhodium

The reaction of hydrogen/oxygen gas mixtures with rhodium single crystals w as studied using video-FIM (Field Ion Microscopy) at temperatures between 3 50 and 550 K and up to 2 x 10(-2) Pa total pressure. Imaging at 500 K in a hydrogen rich gas mixture (H-2: O-2 = 9) revealed considerable morphologica l changes of the {001}-oriented field emitter tip, i.e. the growth of low-i ndex at the expense of high-index planes and strong crystal coarsening. Dec reasing the hydrogen partial pressure led to chemical and structural change s of the Rh sample. Starting on the {IIO planes a surface oxide formed, whi ch spread anisotropically across the surface until it finally covered the w hole visible surface area. The transformation was reversible upon increasin g the hydrogen pressure back to its initial value. However, a hysteresis be havior was observed, i.e. a larger hydrogen partial pressure was found to b e necessary to re-establish the initial patterns of a reactive O-ad/H-ad la yer. By varying the temperature from 400 to 500 K a phase diagram was estab lished for the O-ad/H-ad system. Increasing the electric field proved to sh ift the phase diagram towards higher H-2 pressures. At 550 K self-sustained kinetic oscillations with a cycle time of similar to 40 s could be observe d. (C) 2001 Elsevier Science B.V. All rights reserved.