Dynamics of Reactive Species and Reactant-Induced Reconstruction of Pt Clusters in Pt/Al2O3 Catalysts

We report a complete experimental characterization of the surface Pt-hydride species on an industrial 5 wt % Pt/Al2O3 catalyst (average particle size of 1.4 +/- 0.4 nm) under different hydrogenation/dehydrogenation conditions. By combining inelastic neutron scattering, FT-IR spectroscopy, and synchronous DRIFT/XAS/MS, we identified n-fold coordinated Pt-hydrides and four different types of linear Pt-hydrides characterized by different adsorption strength, whose relative proportion depends on the experimental conditions. In particular, we observed that the n-fold coordinated hydrides convert into linear ones upon decreasing hydrogen coverage, and vice versa, and we traced this phenomenon to a morphological and electronic reconstruction of the Pt nanoparticles. Although only a fraction of the surface Pt-hydrides are directly involved in the hydrogenation of toluene, all the others play an indirect but fundamental role, maintaining the Pt nanoparticles electronically and morphologically stable during the reaction and hence avoiding the occurrence of deactivation processes. Our results, which are in good agreement with the theoretical predictions reported in the literature, offer a comprehensive picture of the dynamics of Pt nanoparticles in hydrogenation conditions. This always involves a change in the relative proportion of the Pt-hydride species, and only in some cases an electronic and morphological reconstruction of the Pt particles.