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Propagation of chemical waves on a single nanoparticle of catalyst

par Barroo, Cédric ;De Decker, Yannick ;Kruse, Norbert ;Visart de Bocarmé, Thierry
Référence Conference on Advanced Complex Inorganic Nanomaterials (3: 2015-07-14: Namur)
Publication Non publié, 2015-07-14

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Résumé :

In order to improve the understanding of catalytic processes, information regarding the structure of the catalyst, the catalytic behavior of the reaction, as well as the influence of structure on activity has to be gleaned.However, a detailed description of such effects down to the molecular level remains challenging due to the lack of experimental techniques. Field emission techniques, including field ion microscopy (FIM) and field emission microscopy (FEM), are especially suited for studying these effects: the nanosized metal tip used as a sample represents a good model for a single catalytic nanoparticle. Furthermore, the observation of the morphological changes and catalytic activity can be performed during the ongoing reaction, by real-time imaging and in direct space. Nanoscale lateral resolution is achieved and provides information about the local surface composition in a time-resolved manner. FEM is based on the emission of electrons from the sample, which can be locally affected by the presence of adsorbates. Local variations of the work function are detected in the form of a brightness pattern and the surface composition of the sample can be qualitatively investigated during the ongoing catalytic process, allowing for the determination of the elementary processes. In this work, we study the morphological and structural changes occurring over platinum samples after oxygen exposure. The effects of reconstruction on the local catalytic activity of the NO2 hydrogenation reaction are also investigated. For this purpose, the microscope is run as an open nanoreactor. Accordingly, the chemical system is kept far from its thermodynamic equilibrium, a necessary condition to observe nonlinear spatiotemporal behaviors. Aperiodic and highly periodic oscillations of various complexity have been reported for the NO2 reduction on platinum. By increasing the time resolution of the system, it is now possible to study the emergence of these oscillations and to observe the propagation of chemical waves at the nanoscale, on a single facet of a nanocrystal. The velocity of wave propagation is estimated to be in the μm/s range, which is in accordance with previous studies of catalytic reaction at the mesoscale. This proves the robustness of dissipative, nonlinear behaviors down to the nanoscale.As a conclusion, we show that nanoparticle dynamics must be accounted for in models describing the nonlinear features of catalytic reactions and, more generally, included in the description of catalytic properties of nanosized particles.