par Visart de Bocarmé, Thierry
Référence Internaional Field Emission Symposium (52: 2010-07-05 -> 2010-07-08: Sydney, Australia)
Publication Non publié, 2010-07-05
Communication à un colloque
Résumé : Since the introduction of the catalytic converter in vehicles noble metals (Rh, Pt, Pd) have been used extensively to abate toxic exhaust gases. Today, the most difficult issues in pollution control by catalysis are related to the selective conversion of NOx species. These questions, amongst others, were intensively studied by Professor Jochen H. Block, whose seminal contributions in surface science studies by field emission techniques are inspiring a number of research groups. In this contribution, we undertake a comparative study on the NO-H2 reaction over Pd and Pt surfaces conditioned as sharp tips. Field Ion Microscopy (FIM) is employed at low temperature to image Pd and Pt tips with atomic resolution whereas at higher temperatures (300-600K), kinetic instabilities can be followed on the same surfaces in the presence of reactive gas mixtures (NO+H2). The local chemical composition can be monitored during the ongoing processes by means of a one-dimensional atom-probe (1DAP) dedicated for in-situ studies of model catalysts used in field ion microscopy studies.The interaction between Pd and pure NO at 450K shows the formation of an adsorbed layer that appears bright in field ion micrographs. Within seconds, this layer extends on the surface and covers the whole visible surface area. As the process indicates a modification in the image formation mechanism, we can conclude that the appearance of bright regions on the tip apex is correlated with the dissociation of adsorbed NO. When hydrogen gas is added to NO, the adlayer reacts off within a few tenths of a second leaving a dark field ion image. The phenomenon is reversible, easily repeatable and shows a hysteresis. A phase diagram has been established on this basis within the temperature range of 450 to 550 K. At present, no self-sustained kinetic oscillations have been observed on Pd. 1DAP experiments have established differences in the surface composition between the two stages of the reaction. Patterns are somewhat different when Pt is used as tip material. As in the Pd case, NO can dissociate and the resulting Oad layer can react with H2 in a non-linear manner. Strong anisotropy effects are observed during the catalytic reaction, i.e. bright wave fronts are seen to ignite and propagate along specific crystallographic directions where the surface density of kink sites is the highest. Although occurring on both Pd and Pt metals, the reaction mechanisms seem different. On Pd, NO dissociation takes place on the whole visible surface area leading to a “surface oxide” that can be reacted off by H2. On Pt, the catalytic reaction is restricted to specific zone lines of the crystallites where NO dissociates to form Oad-species. This observation is compatible with the fact that Pd is more prone to oxidation than Pt.