Investigating Ligand-Nanoparticle Interactions and Catalytic Activity through Organic-Aqueous Tunable Solvents Recovery

Dispersed gold nanoparticles have greater catalytic activity than those deposited on a fixed support. Current methods to recover dispersed nanoparticles require either specialized synthetic methods or produce large quantities of solvent waste and energy. These methods are difficult to scale up and can affect the morphology of the nanoparticles resulting in decreased catalytic performance. We propose a recovery system that utilizes Organic-Aqueous Tunable Solvents (OATS): a homogeneous mixture of water and an organic solvent selected specifically for its ability to absorb CO₂. The mixture undergoes a phase separation under pressure from CO₂ to form a biphasic system consisting of water and a CO₂-expanded organic solvent. The ligand selected to disperse the gold nanoparticles was Poly(vinylpyrrolidone) (PVP) because it is water soluble and binds weakly allowing surface access to the reacting species. Gold nanoparticles of size 9.4 ± 1.4 nm were synthesized and subjected to four thermal treatments (no thermal treatment, 40, 50, and 60°C) as well as pressure treatment between 1-11.3 bar of absolute CO₂ (the pressure range at which phase separation in OATS occurred). Complete recovery of gold nanoparticles was achieved as determined by UV-vis extinction spectra of the localized surface plasmon resonance for each of the four treatments. The hydrogenation reaction of 4-nitrophenol was used to determine the catalytic activity of the nanoparticles. Catalytic activity decreased after the nanoparticles underwent thermal treatment or pressure recovery but remained relatively high. Reduction of catalytic activity can be linked to loss of active sites which is likely due to the passivation of the nanoparticle surface resulting from increased PVP carbonyls reaching the surface. Surface bound ligands prevented precipitation and growth in OATS, but they also slowed dynamic surface restructuring which decreased catalytic activity.