par Visart de Bocarmé, Thierry ;Moors, Matthieu ;Kruse, Norbert
Référence European Conference on Applications of Surface and Interface Analysis (12: 2007-09-12: Bruxelles, Belgique)
Publication Non publié, 2007-09-12
Communication à un colloque
Résumé : This contribution reviews some of the recent progress made in local probing of catalytic surface reactions using video-Field Ion Microscopy (FIM) in combination with relaxation-type Pulsed Field Desorption Mass Spectrometry (PFDMS). For these two techniques, samples are conditionned as sharp tips the end of which mimicks one single catalytic crystallite in the absence of any ceramic support. FIM provides micrographs with atomic resolution and can be used to image surface processes during the ongoing reaction with nanoscale resolution. To provide local chemical analysis while these processes are occurring, short pulses of high voltage are applied to the sample and cause the surface species to desorb as ions, which are then identified by time of flight mass spectrometry (PFDMS). The catalytic oxidation of hydrogen on Rh tips was investigated using FIM and PFDMS. The reaction was carried out in the 10-3 Pa range at T=400-600 K. revealing tip morphologies during the ongoing water formation. The H2/O2 gas mixture was varied to demonstrate the occurrence of local structural changes as well as multistability in the surface reaction. A phase diagram was established for temperatures between 400 K and 500 K. At 550 K, self-sustained kinetic oscillations with cycles of ~40 s were observed in a continuous flow of O2-H2 (PO2 = 1.0x10-3 Pa and PH2 = 1,3x10-3 Pa). Using PFDMS, ~400 atomic sites close to the central (001) pole of the Rh tip were probed with varying pulse heights. This enabled us to correlate the different patterns visible by FIM with their respective surface composition. Under well chosen conditions, some regions of the visible surface area appear oxidized whereas others are in a metallic state. The species responsible for image formation are H2O+/H3O+ ions, indicating that the image brightness can be correlated to the local catalytic activity.Moreover, we present a comparative study on the NO-H2 reaction over Pd and Pt. Although occurring on both metals, the reaction mechanisms seem different. On palladium, NO dissociation takes place on the whole visible surface area leading to a “surface oxide” that can be reacted off by H2. On platinum, the catalytic reaction is restricted to specific zone lines of the Pt crystallites where NO dissociates to form Oad-species. This observation is compatible with the fact that Pd is more prone to oxidation than Pt. No oscillations are observed on Pd at this point in time but are easily produced on Pt.