Comparing Thermal and Chemical Removal of Nanoparticle Stabilizing Ligands: Effect on Catalytic Activity and Stability

The use of stabilizers is an essential part of colloidal catalyst preparation; however their impact on catalytic behavior is challenging to elucidate. This report evaluates three commonly used nanoparticle (NP) stabilizing ligands, oleylamine (OAm), dodecanethiol (DDT), and the polymer polyvinylpyrrolidone (PVP). Stabilizing ligands are removed using thermal and chemical pretreatments, and the surface chemistry of the NPs is assessed using X-ray photoelectron spectroscopy (XPS). The method of ligand removal significantly altered the catalytic behavior of colloidal NPs, within the Suzuki cross coupling reaction, which is sensitive to the oxidation state of Pd and the chemistry of the capping ligand. Chemical treatment is effective for weakly binding OAm but less effective in completely removing DDT and PVP ligands; however catalytic activity of the NPs could be improved by partial ligand removal. Thermal pretreatment decreased the activity of all the catalysts, even when the catalyst diameter and Pd surface chemistry were reasonably preserved. XPS analysis further reveals changes in the interfacial chemistry of the treated catalysts, such as the formation of oxidized sulfur species on DDT-capped Pd NPs and conformational changes in PVP capping ligands as a result of thermal annealing.