Heterogeneous catalysis with supported platinum colloids: A systematic study of the interplay between support and functional ligands

Abstract

Whereas colloidal metal nanoparticles have attracted considerable interest in homogeneous catalysis, the effect of organic ligands has been less systematically investigated in heterogeneous gas-phase catalysis. This paper aims at elucidating this aspect for nanoparticles capped with dodecylamine (DDA), which have been deposited on three different support materials with varying acid/base properties, namely γ-Al 2O 3, SiO 2 and MgO. For this purpose, a synthetic approach was applied that is based on the preparation of ligand- free Pt nanoparticles in ethylene glycol. By functionalizing these particles subsequently with ligands, it is possible to obtain ligand-free and ligand-capped particles with the same metal core (e.g. identical size and shape), thus allowing to investigate the influence of the ligands without changing any other parameter. After deposition on the different supports, the Pt nanoparticles were characterized by STEM, AAS and DRIFTS. The catalytic properties of these catalysts were investigated under two different reaction regimes: first, octadiene hydrogenation served as a test reaction to probe the influence of the ligands on larger molecules under reducing conditions at low temperatures ( T < 100 °C) where the ligand shell is intact. The results show that ligands can strongly modify metal–support interactions and exert a protecting effect with respect to support induced oxidation of Pt surface atoms that occurs during particle deposition. In particular, in the case of the Brønsted acidic SiO 2 support, where surface oxidation of Pt is most pronounced, the ligand-capped sample is significantly more active for octadiene hydrogenation than the ligand-free counterpart. Second, the samples were tested with respect to CO oxidation at high temperatures ( T ∼ 200 °C) where processes like decomposition/desorption and spillover of ligands on the support become important. Depending on the acid/base and adsorption properties of the different supports, the spillover of DDA turns out to be the main reason for diminishing the ligand coverage of the nanoparticles under these conditions. Whereas spillover is most pronounced on Lewis acidic γ-Al 2O 3, a specific interaction between the basic MgO and DDA, namely its catalytic transformation into a nitrile, leads to enhanced spillover when compared to the Brønsted acidic SiO 2. These observed ligand effects are not limited to catalysts synthesized with the ethylene glycol method but are also observed in the case of the particles prepared by a classical colloidal approach.