par Barroo, Cédric 
;Lambeets, Sten ;De Decker, Yannick ;Devred, François ;Kruse, Norbert ;Visart de Bocarmé, Thierry
Référence International conference on advanced complex inorganic nanomaterials (2: 3013-07-13: Namur, Belgique)
Publication Non publié, 2013-07-16
;Lambeets, Sten ;De Decker, Yannick ;Devred, François ;Kruse, Norbert ;Visart de Bocarmé, Thierry Référence International conference on advanced complex inorganic nanomaterials (2: 3013-07-13: Namur, Belgique)
Publication Non publié, 2013-07-16
Communication à un colloque
| Résumé : | Nitric oxides (NOx) emissions from vehicles are a matter of concern for the public health. NOx abatement is highly desirable but the development of viable solutions still represents a major challenge for catalysts makers, especially in the growing market of lean-driven vehicles. NO is known to be oxidized to NO2 under lean-burn conditions in automotive engines, and subsequently converted into N2 in the presence of reducing species. In this work, we investigate the catalytic hydrogenation of NO over palladium (Pd) and platinum (Pt) and of NO2 over Pt and rhodium (Rh). The surfaces are conditioned as sharp tips the top of which is a good model of a single catalytic particle. Samples are studied by field emission techniques including field emission/ion microscopies (FEM/FIM) and one-dimensional atom probe (1DAP). Real-time FEM and FIM are powerful methods to monitor the dynamics of catalytic reactions, whereas 1DAP provides a direct local chemical analysis of the surface in its catalytically working state. The microscope is run as an open chemical reactor, through a constant supply of gaseous reactants and constant gas-phase pumping of the reaction chamber, ensuring that the system is kept far from its thermodynamic equilibrium. This may lead to nonlinear dynamic behaviors such as hysteresis, bistability and kinetic oscillations. 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 to the dissociative adsorption of NO. After a subsequent addition of hydrogen in the chamber, a reaction with the adlayer can be observed, leaving a dark field ion image. The phenomenon is reversible and exhibits a hysteresis behaviour. 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 differ 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 mechanism seems 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. NO2 hydrogenation is followed over Pt and Rh samples and shows kinetic oscillations of unprecedented regularity. Time series have been subjected Fourier transforms, temporal autocorrelations and dynamical attractors that demonstrate the existence and robustness of the kinetic oscillations. |
