Metal Ion Assisted Channelling of Substrates in Gold Nanoparticle Catalyzed Reactions

Abstract

In the present thesis, we revisit one of the traditional and benchmark reactions in the area of nanoparticle catalysis, namely the gold nanoparticle (AuNP) catalyzed reduction of para-nitrophenol (PNP) to para-aminophenol (PAP) by sodium borohydride. It has been shown that the ability to govern and dictate the interactions between different reaction components can have prodigious effects on the reaction rates. In this direction, our group has recently shown that regulation of surface potential around a gold nanoparticle (AuNP) catalyst can render the same nanoparticle (NP) system as an efficient catalyst, or as a non-catalyst. It should be noted that most of the studies on AuNP catalyzed PNP reduction can be categorized into two directions:- a) finding the design principles to create best catalysts (like optimizing catalyst shape and crystallinity, ligand hydrophobicity, catalyst-reactant interactions, etc.), and b) finding the best reaction conditions for carrying out efficient catalysis (like the effect of light irradiation, dissolved oxygen, pH, etc.). To the best of our knowledge, all of the studies utilize sodium borohydride as the reducing agent, and relatively little is known about the effect of reducing agents on the PNP reduction reaction. With this in mind, the work presented here aims to find out the optimized reaction conditions (with respect to the reducing agent) for carrying out efficient catalysis. We demonstrate that the strengths of reducing agent (an intuitive parameter) is an incomplete descriptor governing the rate of PNP reduction reaction. Additionally, the metal cations constituting the borohydride salt, can differentially bridge with PNP molecules, thereby increasing the local concentration of reactant molecules around the AuNP catalyst. Thus, our studies reveal that both strength and bridging ability of the reducing agents plays a key role in the PNP reduction. Furthermore, similar trends were observed with AuNPs of different surface chemistries, demonstrating the generality of our findings. The idea of incorporating of bridging interaction between metal ions (from reducing agent) and reactants is powerful enough to convert a non-catalytic system to a catalytically active one. We believe that our study shows how simple variations in the reaction conditions can help in making significant impacts in the field of chemical transformations.