H2‐Induced Reconstruction of Supported Pt Clusters: Metal–Support Interaction versus Surface Hydride

Platinum nanoparticles supported on γ‐alumina are widely used as highly dispersed heterogeneous catalysts, in particular in the presence of H2. In the present work, an atomic‐scale model for such catalysts is provided, taking into account operating conditions (temperature, hydrogen pressure), thanks to density functional theory calculations coupled to a thermodynamic model. In the absence of hydrogen, Pt13 clusters supported on γ‐Al2O3 preferentially lie in a biplanar (BP) morphology in strong interaction with the support’s surface. This structure has a strong affinity towards hydrogen. The increase of hydrogen coverage above 18 H atoms per cluster (H/Pt>1.4) induces a reconstruction from a BP to a cuboctahedral (CUB) morphology as shown by molecular dynamics. This reconstruction is driven by the ability of the CUB structure to adsorb a significant amount of hydrogen with moderate deformation cost. Electronic analyses reveal that a hydride phase is then obtained, with a partial loss of the metallic nature of the Pt13 edifice. Our model is supported by numerous experimental data (temperature‐programmed desorption, titration experiments, X‐ray absorption spectroscopy). Values higher than 1 for the H/Pt ratio, as measured in previous experimental analyses, are rationalized by the reconstruction process. Moreover, in reaction conditions such as catalytic reforming, the particle remains biplanar with moderate H/Pt ratio and retains its metallic character. The catalytic conditions therefore have a drastic influence on the nature of the catalyst surface.