Reversible temperature-induced shape transition of Pt nanoparticles supported on Al2O3.

Supported platinum catalysts are widely used in industry for hydrogenation reactions. The variations of the electronic and geometric properties of Pt nanoparticles due to temperature can greatly affect their reactivity. In this work, we use in situ X-ray absorption spectroscopy and environmental transmission electron microscopy to study the effect of H2 and temperature on the shape and electronic properties of 1.8 nm average diameter Pt nanoparticles supported on Al2O3. We utilize actively trained machine learning potentials with uniform acceptance force-bias Monte Carlo (fbMC) to estimate the structural distribution of Pt15/γ-Al2O3 (110) clusters at finite temperatures. Our predicted cluster geometries are consistent with experimental data showing the nanoparticles reversibly change shape from 3D hemispheres at low temperatures (35-100 °C) to 2-2.5D rafts at higher temperatures (200-400 °C). Furthermore, experiments and computations indicate that the contraction in Pt-Pt bond distances and higher electron density on Pt at higher temperatures are attributed primarily to the change in nanoparticle shape and associated increased interaction with Al2O3. Our results show the fluxional nature of supported Pt nanoparticles driven by temperature changes.