par Visart de Bocarmé, Thierry 
;Bar, Thomas ;Kruse, Norbert
Référence International Field Emission Symposium (47: 2001-07-29 to 2001-08-03: Berlin, Germany)
Publication Non publié, 2001-07-29
;Bar, Thomas ;Kruse, Norbert Référence International Field Emission Symposium (47: 2001-07-29 to 2001-08-03: Berlin, Germany)
Publication Non publié, 2001-07-29
Communication à un colloque
| Résumé : | The catalytic oxidation of hydrogen on Rh tips has been investigated using Field ion microscopy (FIM) and Pulsed Field Desorption Mass Spectrometry (PFDMS). The reaction was carried out in the 10-5 mbar range at T=400-550 K. Samples imaged by FIM at 9 V/nm were found to undergo significant morphological transformations. A reshaping towards polyhedral forms was observed under reaction conditions, similar to adsorption studies with pure oxygen. Variations in the FIM intensities were used to establish a phase diagram for the H2+O2/Rh system between 400 and 500 K. Using PFDMS, ~900 atomic sites close to the central (001) pole of the Rh tip were probed with varying pulse heights. During imaging in a wide range of H2+O2 gas mixtures, it was found that water ionization causes image formation. Decreasing the hydrogen partial pressure in the gas mixture led to the formation of a surface oxide growing anisotropically and finally covering the whole visible area (O-side of the diagram). Under these conditions, moderate intensities were found for RhO2+ , RhO+ and Rh2O+. On the other hand, by increasing the H2 pressure, the surface oxide was reacted off within a timescale of ~0.1 second leaving thereby a relatively dark surface (H-side of the phase diagram). Chemical probing in the latter case revealed lower amounts of water but nearly no oxygen-containing species were found in the mass spectra. |
