SOLVENT-LIGAND-SURFACE INTERACTIONS OF NANOPARTICLES FOR APPLIED CATALYSIS

Shane Riley Reynolds

This work presents the research performed to understand solvent-ligand-nanoparticle interactions for applied catalysis. Dispersed gold nanoparticles can demonstrate greater catalytic activity than their supported counterparts but methods for nanoparticle recovery are difficult to scale up, require specialized synthesis techniques, or produce a vast amount of solvent or energy waste. Additionally, comparisons between nanoparticle syntheses are not possible due to the difficulty in measuring surface active sites. Traditional characterization techniques require dry powders, which causes the collapse of the ligand shell around the nanoparticle, possibly leading to growth. Previously, a separation technique called organic-aqueous tunable solvents (OATS) was developed that uses a single-phase mixture of a water and an organic to perform homogeneous reactions before separating the organic phase from the catalyst containing aqueous phase using pressurized CO2. Use of OATS has been limited to homogeneous catalysts and biocatalysts. This work presents the first use of gold nanoparticle separation and recovery using OATS. Gold nanoparticles were synthesized, and four different thermal treatments (none, 40, 50, 60 °C) performed. Complete recovery was achieved, but reduction of catalytic activity was observed after the nanoparticles underwent thermal treatments or were subjected to pressure separation in OATS. The decrease in catalytic activity can be attributed to previously unbound ligand passivating the surface of the nanoparticles. An in situ technique to quantify gold nanoparticle surface available for catalysis, using 2 mercaptobenzimidizole as a molecular probe is presented. Surface quantification allowed for turnover frequency to be calculated to give a comparable measure of catalytic performance. Hydrogenation of 4-nitrophenol was used to evaluate catalytic performance. This reaction has an induction time which was shown to be the result of ligand movement away from the nanoparticle surface induced by the presence of 4-nitrophenol, and gold leaching was eliminated as a possible location of the catalytic active site. Modeling of 2-mercaptobenzimidazole surface site quantification was carried out to understand the mechanisms involved in reactant surface adsorption. The Elovich kinetic model was determined to provide the best fit for the adsorption data. This model provided further evidence that removal of surface-bound ligand was responsible for the induction time during hydrogenation of 4-nitrophenol.

SOLVENT-LIGAND-SURFACE INTERACTIONS OF NANOPARTICLES FOR APPLIED CATALYSIS

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