Accelerated Reduction of 4-Nitrophenol: Bridging Interaction Outplays Reducing Power in the Model Nanoparticle-Catalyzed Reaction

General chemistry knowledge will intuitively predict that a stronger reducing agent will result in a faster reduction reaction. The present work, however, shows the counterintuitive role of reducing agents in a model reaction of gold nanoparticle (AuNP)-catalyzed reduction of 4-nitrophenol (4-NP) by sodium borohydride. The strength of reducing agents is varied, in situ, by adding appropriate metal ions in the reaction medium. The trend observed in the rate constants does not correlate fully with the strength of reducing agents. Surprisingly, Ca2+ outperforms Mg2+ in accelerating the AuNP-catalyzed reduction of 4-NP, despite Ca2+ forming a comparatively weaker reducing agent. A thorough investigation reveals that the reducing power is an incomplete descriptor, and additional ion-specific bridging interactions should be incorporated to explain the observed trend in the rate constants. The idea of combining increased reducing power with bridging interactions was then utilized to transform a traditionally noncatalytic AuNP to a catalytically active one.