Seeing the subtle structural changes of Pt nanoparticles induced by hydrogen

Pt-based catalysts are used in a number of catalytic reactions due to their properties to dissociate hydrogen molecules and exchange hydrogen atoms with reactants thus promoting bond cleavage and hydrogen addition reactions. These processes occur at the of surface of Pt nanoparticles finely dispersed on metal oxides such as Al₂O₃, making analysis of effects of hydrogen on nanoparticle structure challenging. Using modulated excitation experiments while measuring X-ray total scattering patterns (in particular pair distribution function - PDF) of a Pt/Al₂O₃ catalyst both in gas phase and in cyclohexane solvent, we demonstrate that beside expanding in the presence of hydrogen small nanoparticles of the mean size of 1-3 nm also detach from the metal oxide support at high H₂ coverage. Both expansion ('breathing') and detachment processes are reversible and the structure of the nanoparticles reverts when H₂ is removed. The already subtle changes induced by H₂ are even smaller for larger nanoparticles and when the catalyst is exposed to liquid phase.

The data obtained at ESRF are supported by and confirm previous predictions by density functional theory.

Understanding the dynamic behaviour of metal nanoparticles is key to improve and design efficient catalysts for a number of chemical reactions.

This work is the result of a collaboration between us and the University of Torino (Italy), the ESRF (France), the ILL (France), IFP Energies nouvelles and Chimet SpA.

For more information, the full study can be accessed here.