Olefin hydrogenation on Pd model supported catalysts: New mechanistic insights

Hydrogenation of unsaturated hydrocarbon compounds, catalyzed by transition metals, is traditionally believed to be a structure-insensitive reaction. Recent progress in understanding the microscopic details of hydrogenation processes now challenges the universality of this common belief. In this perspective article, we present results on the hydrogenation of cis-2-butene: over Pd model nanoparticles supported on an Fe3O4/Pt(1 1 1) oxide thin film and over a Pd(1 1 1) single crystal. We provide direct experimental evidence that the hydrogenation activity of Pd is strongly dependent on the presence of hydrogen species absorbed in the subsurface region of the metal catalyst and is governed by the permeability of the surface for hydrogen. Since the formation of subsurface hydrogen species in Pd is a strongly structure-sensitive process, the low-coordinated surface sites, such as edges and corners on Pd nanoclusters, play a crucial role in replenishment of the subsurface hydrogen reservoir and maintaining the hydrogenation activity under steady-state conditions. We show that selective modification of these low-coordinated surface sites on Pd nanoclusters with carbon allows for faster subsurface hydrogen diffusion and thereby results in sustained hydrogenation activity, a result that cannot be achieved on C-free Pd nanoparticles, nor on C-free Pd(1 1 1), nor on C-containing Pd(1 1 1). Theoretical calculations support the proposed facilitation of subsurface hydrogen diffusion through C-modified low-coordinated surface sites on Pd nanoclusters and show the conceptual importance of atomic flexibility of nanoparticles in hydrogenation catalysis.