(24e) Gold Nanoparticle Catalysis: Colloidal Versus Supported Heterogeneous Catalysis and Methods for Colloidal Nanoparticle Recovery

Gold nanoparticles (AuNPs) have attracted enormous attention due to their unique catalytic activities. Potential advantages of colloidal AuNPs over supported AuNPs is the greater catalytic activity and selectivity. Colloidal catalysts also catalyze reactions under mild conditions and are very effective for chiral catalysis. Recovery of colloidal catalysts involve tedious methods like pH or temperature induced aggregation, solvent extraction, etc. The advantage of supported catalyst lies in its reusability and recovery by simple methods like centrifugation, filtration, sedimentation, etc. Supported catalysts are preferred over colloidal catalyst in industries because they can be used in a continuous reactor system. For colloidal systems, surface functionalization with ligands is a prerequisite to prevent aggregation. Surface passivation causes significant reduction in catalytic activity and selectivity. For supported catalysts on metal oxide supports, a certain portion of the active surface area of the catalyst is lost due to the immobilization process and simultaneously new edges at metal-metal oxide interfaces are created. In this work, we have attempted to characterize the catalytic activity and active sites of both colloidal and supported catalysts systems by employing a ligand adsorption based method in aqueous media. An in-depth study of the effect of nanoparticle stabilizing ligands such as thiolated polyethylene glycol (PEG) and 11-mercaptoundecanoic acid (MUA) on the nanoparticle catalytic activity has been carried out and we have quantified the loss of active surface sites due to the ligand stabilization. Decreasing PEG chain length and increasing surface coverage of PEG on AuNP reduces the catalytic activity. Moreover, the functionalization of AuNP with MUA completely passivates the surface and the AuNPs show no catalytic activity. Although, high MUA coverage on the AuNP are detrimental to the catalytic activity, it renders the AuNPs easily recoverable from the solution by pH induced aggregation and redispersion. To maintain both catalytic properties and reversible pH induced recovery, we have considered partial surface coverages of the AuNP with MUA. This work also reports the synthesis of thiolated poly (acrylic acid) (PAA) functionalized AuNPs and explored its application as a recoverable catalyst where the PAA provides pH responsive dispersibility in aqueous media. Thus, PAA-AuNPs are easily and completely recovered from the reaction mixture and reused in subsequent reactions. It was found that the AuNP-PAA catalyst was highly active and reusable with modest reductions in the reaction rate with up to four catalyst recycles. We have also supported ‘bare’ colloidal AuNPs (trisodium citrate stabilized) on metal oxides and quantified the loss of active surface area. We report the comparison of loss of surface area due to ligand stabilization and metal oxide supports and compare the effectiveness and recoverability of both system of catalysts. A direct consequence of being able to characterize active surface area of both colloidal and supported system simultaneously is the ability to measure the effect of the support on the catalytic activity of the AuNPs. We have quantified the metal oxide support effect on the catalytic activity of AuNPs for various metal oxide-AuNP catalysts.