Critical Role of Molecular Adsorption on Electrocatalysis at Single Nanoparticles

We report a mechanistic study of the electrocatalytic response of single Pt nanoparticles (NPs) on a carbon ultramicroelectrode (UME) in a hydrazine (N₂H₄) solution. Using a NP collision approach, our study shows their catalytic response is characterized by a sharp, <50 μs-long current spike followed by a steady step-current signal. Our results suggest that the current spike is due to the quick oxidation of N₂H₄ molecules preadsorbed onto the NP surface, while the step current reflects the continuous catalytic oxidation of protonated hydrazine (N₂H₅⁺), which goes through a deprotonation and adsorption step on Pt. Since each N₂H₅⁺ molecule releases five H⁺ upon complete oxidation, a drastic decrease in local pH can be expected in the vicinity of the NP. This pH shift in turn limits the rate of adsorption and the steady-state oxidation current one can observe from each colliding particle. Our study reveals the key importance of molecular adsorption and the changing local chemical environment (e.g., pH) to the observed catalytic response of single NPs and highlights that steady-state currents in their measurement may be chemically or mass-transport limited.