Stabilization of Pt Nanoparticles with Amine Ligands : an Alternative Approach to Supporting Nanoparticles for Catalytic Applications

The state-of-the-art approach to stabilize nanoparticles (NPs) for heterogeneous phase applications is to support them on inert inorganic material, which is limited to low loadings of the catalytic compund. However, applications such as thermoelectric gas sensing require a high density of catalytically active sites at a low total heat capacity. To offer an alternative to the supporting of NPs, in this work the stabilization of catalytic NPs as solids with organic ligands was investigated. Mono- and bifunctional ligands were applied in this study. The catalytic properties of such stabilized NPs was exemplary investigated by application in a thermoelectric hydrogen sensor. Pt NPs stabilized with mono-amines were almost completely covered with ligands, which almost completely suppresses the catalytic activity of the particles. In addition, the NPs aggregated over time, so that mono-amines do not facilitate sufficient stabilization of Pt NPs for catalytic applications. The molecular linkage of Pt NPs with bifunctional amine ligands (ligand-linking) results in three-dimensional porous networks with ligand-free surface sites. In addition to an enhanced activity, the stability of the NPs can be significantly improved by ligand-linking. One reason may be that the bifunctional ligand is anchored on the NPs by two head groups. However, the NPs are only stabilized as long as the ligands are intact. The criteria for ligand structures to enable a successful NP stabilization were identified. Para-phenylenediamine (PDA) combines the criteria of aromatic hydrocarbon backbone and the presence of two sp3 hybridized head groups. Consequently, by linking of Pt NPs with PDA a constant catalytic activity over more than 20 h on stream was achieved. Thus, organic ligands could be successfully applied to stabilize catalytic NPs. Furthermore, the ability of ligands to participate in a reaction was shown: Pt NPs functionalized with the mono-amine 1-naphthylamine (NaphA) show a electrochemically induced reversible redox reaction of the ligand and simultaneous electrocatalytic reactions at the Pt surface. Thus, a hybrid material is formed which exhibits a parallel electrochemical reaction of the metal surface and the ligand.