Role of low-coordinated surface sites in olefin hydrogenation: a molecular beam study on Pd nanoparticles and Pd(111).

Hydrogenation of unsaturated hydrocarbon compounds catalyzed by transition metals is traditionally believed to be a structure-insensitive reaction. However, recent progress in understanding the microscopic details of this process challenges the universality of this common belief. Recently, several examples of catalytic systems were described in the literature where hydrogenation of the olefinic double bond was shown to crucially depend on the presence of hydrogen species absorbed in the subsurface region of a metal catalyst. Particularly for Pd nanoparticles, slow replenishment of these species required for hydrogenation was identified as a rate-determining process in a broad range of reaction conditions. In view of these results, the permeability of the metal surface for hydrogen can play a crucial role in its activity in hydrogenation catalysis. Since subsurface hydrogen diffusion is known to be a strongly structure-sensitive process on Pd surfaces, the degree of coordination of the surface Pd atoms can be very important in determining the formation rate of subsurface hydrogen species, and therefore may be decisive for the hydrogenation activity. In line with this suggestion, it was recently observed experimentally that modification of the low-coordinated surface sites of Pd nanoparticles, such as edges and corners, with carbon significantly affects the hydrogenation activity and results in a sustained hydrogenation rate that is not possible on the C-free surface. This observation was proposed to arise from faster subsurface diffusion of hydrogen through the edge sites modified with carbonaceous deposits. Recent theoretical calculations supported the hypothesis that carbon adsorbed in the vicinity of particle edges strongly reduces or nearly eliminates the activation barrier for subsurface hydrogen diffusion on Pd particles. This effect was ascribed mainly to C-induced expansion of the surface openings for penetration of H into the subsurface region. In contrast, no notable reduction of the activation barrier was found with carbon on the intrinsically rigid regular Pd(111) surface. The conceptual importance of atomic flexibility of sites near particles edges in subsurface hydrogen diffusion, demonstrated by the theoretical calculations, suggests that the low-coordinated surface sites can play a crucial role in the hydrogenation process. In order to obtain more insight into the role of low-coordinated surface sites in hydrogenation, we studied the reaction of cis-2-butene with deuterium on well-defined model Pd nanoparticles supported on a thin Fe3O4/Pt(111) film. Two complementary strategies were applied herein to address the role of low-coordinated surface sites experimentally: first, we compared the hydrogenation activity of Pd particles with an extended Pd(111) single crystal surface. Secondly, we selectively modified the low-coordinated surface sites by deposition of carbon, which has previously been determined to predominantly occupy edges and corners of Pd nanoclusters, and follow the reactivity changes upon such modification. Herein, we demonstrate that modification of the low-coordinated surface sites of Pd particles with carbon promotes sustained hydrogenation activity in a pronounced manner, while carbonaceous deposits adsorbed on the extended Pd(111) surface do not noticeably affect the reactivity. We ascribe this phenomenon to facilitation of hydrogen subsurface diffusion through carbon-modified low-coordinated sites of Pd nanoparticles, which are not available on Pd(111). Generally, alkene conversions promoted by transition-metal catalysts are accounted for by the Horiuti–Polanyi mechanism, which proceeds through a series of successive hydrogenation-dehydrogenation steps: