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.