In situ dynamic study of hydrogen oxidation on rhodium

The reaction of hydrogen/oxygen gas mixtures with rhodium single crystals was studied using video-FIM (Field Ion Microscopy) at temperatures between 350 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 morphological changes of the {001}-oriented field emitter tip, i.e. the growth of low-index at the expense of high-index planes and strong crystal coarsening. Decreasing the hydrogen partial pressure led to chemical and structural changes of the Rh sample. Starting on the {IIO planes a surface oxide formed, which spread anisotropically across the surface until it finally covered the whole visible surface area. The transformation was reversible upon increasing the hydrogen pressure back to its initial value. However, a hysteresis behavior was observed, i.e. a larger hydrogen partial pressure was found to be necessary to re-establish the initial patterns of a reactive O-ad/H-ad layer. By varying the temperature from 400 to 500 K a phase diagram was established for the O-ad/H-ad system. Increasing the electric field proved to shift 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 observed. (C) 2001 Elsevier Science B.V. All rights reserved.