Electrocatalytic effects during redox reactions of arsenic at platinum nanoparticles in acid medium: Possibility of preconcentration, electroactive film formation, and detection of As(III) and As(V)

Abstract

Platinum nanoparticles, when deposited on an inert electrode substrate, exhibit electrocatalytic properties during both oxidation of As(III) and reduction of As(V) in acid medium (0.5 mol dm−3 H2SO4). Upon application of the appropriate potential, both As(III) and As(V) are preconcentrated on surfaces of Pt nanoparticles. Consequently, their presence can monitored under voltammetric conditions. The nature of arsenic stripping voltammetric peaks is strongly dependent on the preconcentration potential. While the reduction and deposition of As(III) is induced by adsorbed hydrogen on Pt formed at potentials lower than 0.25 V (vs. RHE), the adsorption and preconcentration of As(V) can be readily achieved at Pt oxides generated at Pt nanoparticles at potentials higher than 0.85 V (vs. RHE). From mechanistic point of view, voltammetric oxidation of bulk As(III) is catalyzed by Pt oxo species but electrocatalytic reduction of As(V) requires its prior adsorption. Electroreduction of the resulting As(V) adsorbates is catalyzed by both metallic platinum and by platinum on which hydrogen atoms are adsorbed; the resulting voltammetric peak current densities are dependent on concentration of As(V) in solution. A unique feature of the preconcentrated As(III) and As(V) oxo species is the ability to agglomerate and form stable redox-polymer-like polynuclear-electroactive-films on Pt nanoparticles. Their performance resembles the behavior of redox conducting polymers, and their redox transitions are fast and reversible with the electron self-exchange rate reaching 8·106 dm3 mol−1 s−1. Under such conditions, the stripping type voltammetric currents are enhanced. Electrochemical experiments are supported by the data from scanning and transmission electron microscopies and Raman spectroscopy.