Multiscale analysis of the Pd/M(111)-H system (M = Pt, Au): coupling between electrochemistry and surface X-ray diffraction

(Dr Yvonne SOLDO-OLIVIER)

Palladium presents remarkable properties as a catalyst for hydrogen dissociation and is characterized by a high insertion/desorption kinetic. Compared to bulk Pd, the nanometric size of ultra-thin films is expected to induce deep modifications on the thermodynamic properties. This is the case for Pd nanoparticles, which present reduced hydrogen solubility.

In order to get a thorough comprehension of the mechanisms governing the hydrogen insertion into Pd ultra-thin films, we have considered the influence of the nanometric size and of the substrate on the electrochemical isotherms. In our approach, the global behavior of this electrochemical system (macroscopic characterization) is elucidated by measurements at the nanometer scales.

The original behavior of the different thermodynamic parameters, like the maximal hydrogen insertion rate, has been pointed out for Pd/Pt(111) and Pd/Au(111) films of different thicknesses. The effects of the substrate have been revealed, as well.

Thanks to the powerful approach coupling electrochemistry with in situ Surface X-ray Diffraction we have been able to give a deep insight into the strong relationship between the detailed description at the atomic level of the films structure and the isotherms behavior. In particular, we propose a model where the hydrogen insertion rate into the Pd deposit is deeply correlated with the substrate induced constraints.

Gold-based model catalysts studied by operando X-ray scattering at the gas/solid interface

(Dr Aude BAILLY)

The synthesis of model catalysts with controlled architecture is a promising way in order to understand the respective influence of parameters such as nanoparticles' (NPs) size, shape, chemical composition and crystallographic structure on the catalytic properties. Coupled to studies led during a chemical reaction (that is to say in operando conditions), this approach is intended to give a detailed description of the catalytic active sites. For instance, we have established the relationship existing between the size and the catalytic activity of gold NPs supported on TiO₂(110) for carbon monoxide oxidation. This study clearly showed the peculiar geometry of the active sites responsible for the catalytic activity maximum observed for NPs with a diameter of about 2 nm.

Recent results obtained on bimetallic Au-Pd NPs deposited on a nanostructured Al₂O₃ thin film grown on Ni₃Al(111) will also be presented. This type of substrate enables the growth of long range ordered NPs arrays with the possibility to precisely control the NPs' size (< 4 nm). The growth of such systems has been studied in situ in ultra-high vacuum by Grazing Incidence X-ray techniques (GIXRD and GISAXS). Some preliminary results obtained during butadiene hydrogenation will also be given.