Regulation of Nanoparticle-Reactant Interaction Outplays Ligand Poisoning in Metal Nanoparticle Catalyzed Reactions

Abstract

Surface ligands are ubiquitous in colloidal nanoscience. They provide the colloidal stability to nanoparticles (NPs) as well as dictate most of their physicochemical properties. However, in the area of catalysis, the ligands have a bad reputation of poisoning the catalyst, either by hindering the surface accessibility (due to steric effect) or by creating an insulating barrier for the movement of electron/holes. So, how to overcome this challenge of “ligand poisoning”? Traditional strategies include the deposition of NPs onto a support or use ‘ligand free’ NPs for catalysis. However, the available surface area and stability of NPs are compromised during the course of catalysis. Thus, nanoparticles and ligands are two inseparable entities and strategies have to be developed to accomplish catalysis by retaining as well as utilizing the ligands on the NP surface. In this direction, the present thesis focuses on introducing a new strategy based on NP-reactant interaction (emanated from surface ligands) to address the so called “ligand poisoning” effect. The first part of the thesis provides the much awaited and conclusive evidence for the existence of electrostatic effects in traditional gold nanoparticle (AuNP) catalysed reduction of nitro arenes to amines. The interactions between the substrates and AuNP surface was made favourable (attractive) or unfavourable (repulsive) by fine tuning the NP surface potential. An electrostatically assisted channelling of substrates due to the attraction between oppositely charged substrates and AuNP was responsible for the dominance of [+] AuNP catalysts over other NP systems. Thus by tuning the NP-reactant interaction we were not only able to achieve efficient catalysis at low NP concentration but also regulate the catalytic property between completely “on” and “off” states - rendering the same NP as catalyst and as non-catalyst. The photocatalytic activity of metal NPs predominantly depends on the rate of hot electron/hole transfer (within ~10-100 fs), which oftentimes is hindered by the insulating nature of the surface ligands. In a model photocatalytic one electron reduction of ferricyanide to ferrocyanide, we have successfully demonstrated that a favourable NP-reactant interaction can indeed improve the rate of hot electron transfer, and hence the photocatalytic activity. A detailed mechanistic analysis based on Marcus theory revealed that a higher local concentration of reactant molecules close to the NP surface is the limiting factor for the enhanced rate of photocatalysis. The last part of the thesis is focused on utilizing the “idea of interaction” for the photocatalytic multi-electron reduction of NAD+ to NADH. NADH is an expensive cofactor molecule and its regeneration from NAD+ is essential for the functioning of different oxidoreductase enzymes during various bio-catalytic reactions. The strong interaction between oppositely charged AuNPs and NAD+ helped in achieving an efficient photocatalytic reduction and regeneration of NADH. To the best of our knowledge, this is the first report on using plasmonic metal NPs as a sole photocatalyst for this important bio-reduction reaction. In summary, the present thesis introduces a new strategy of “Ligand Directed Catalysis” for addressing the long standing challenge of ligand poisoning in the area of NP catalysis.