Impact of organic-ligand shell on catalytic performance of colloidal Pd nanoparticles for alkyne gas-phase hydrogenation
Monodispersed Pd nanoparticles (NPs) have been prepared by colloidal technique and deposited on a structured support consisting of carbon nanofibers (CNF) grown on sintered metal fibres (SMF). The surface properties of Pd NPs have been fine-tuned by (i) changing the nature of stabilizing agent (electrostatic vs. steric), (ii) controlling Pd NPs size (2-10 nm) and (iii) grafting N-containing ligands onto the CNF/SMF surface. In the semi-hydrogenation of acetylene (T= 393 K; 13= 1 bar) catalytic response was insensitive to the nature of the reducing agent where equivalent activity/selectivity were obtained over Pd NPs with similar dispersion, prepared with the same stabilizer. A similar product distribution was recorded over Pd NPs with similar crystal size irrespective of the colloidal stabilizer (electrostatic vs. steric). In contrast, a stronger inhibiting effect on hydrogenation rate has been found with electrostatic stabilizer (sodium di-2-ethylhexylsulfosuccinate) as compared to the steric ones (polyvinylpyrrolidone or polyvinylalcohol) and assigned to geometric and electronic effects. Decrease (from 8 2 nm) in Pd NPs size results in a concomitant decrease in activity (antipathetic size-sensitivity), but higher selectivity to target ethylene product. Grafting of nitrogen-containing modifiers (polyvinylpyridine or polyethylenimine) on the CNF/SMF support results in a significant increase in olefin selectivity (up to 93%) where the catalyst shows remarkable stability during 120 h on-stream. This is explained by the electronic modifications promoted by interactions between the Pd NPs and the grafted ligands as confirmed by XPS analysis. In comparison, stabilizer-free Pd/CNF/SMF has low selectivity to ethylene (65%). In summary, controlled size Pd (core) nanoparticles with organic ligands ( shell) demonstrated increased selectivity and remarkable stability in catalytic gas-phase alkyne semi-hydrogenation opening new tools for rational catalyst design. (C) 2014 Elsevier B.V. All rights reserved.