Importance of Interfacial Activation and Proton Transfers during Redox Processes of Highly Inert Systems: Electrocatalytic Detection of As(III) and As(V) in Acid Medium
Authors: Rutkowska Iwona Agnieszka , Postek Weronika , Cox James A. , Kulesza Pawel J. 
Published: February 2020

Noble metal nanoparticles (Pt, Rh, Ir Pd), when deposited on an inert electrode substrates, exhibit electrocatalytic properties during both oxidation of As(III) and reduction of As(V) in acid medium (e.g. 0.5 mol dm-3 H2SO4). For example, 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. Population of protons does affect the systems’ electrochemical characteristics. The nature of arsenic 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 adsorbed As(III) and As(V) oxo species is the ability to agglomerate and form stable polynuclear electroactive films on Pt nanoparticles. Their performance resembles behavior of redox conducting polymers, and their redox transitions are fast and reversible. Electrochemical experiments are supported by the data from scanning and transmission electron microscopies and Raman spectroscopy.
In conclusion, the present observations demonstrate the utility of Pt nanoparticles as the catalytic system for inducing of (otherwise highly inert) electrode processes of As(III) and As(V) in acid medium. While the electrocatalytic activity of platinum (particularly PtO) toward electrooxidation of As(III) was previously postulated, we clearly show here that arsenates(V) can be adsorbed, preconcentrated, and electrocatalytically reduced (without prior chemical treatment) and, thus, detected on surfaces of Pt nanostructures. On mechanistic grounds, As(V) must be first adsorbed on bare (metallic) Pt surface before the actual electron transfer can take place. In addition to the structural importance, involvement of protons is crucial to effective electrocatalysis. It is also noteworthy that the current signals can be correlated to concentration of As(V) in bulk solution. The present study also provides evidence of obtaining a stable redox-polymer-like As(III,V) oxo film capable of fast charge propagation (electron-self exchange rate on the level 8*106 dm3 mol-1 s-1) within the adsorbed layer existing on surfaces of platinum nanoparticles. The latter observation is of both fundamental and analytical significance. Although the experimental approaches are not optimized here, the detection of As(V) and As(III) down 5*10-6 and 2*10-6 mol dm-3, respectively, is feasible. The main drawback is that the baseline uncertainty limits its routine use in static solutions to concentrations above 2-5*10-6 mol dm-3. This level is not sufficient for practical purposes. For example, the US Environmental Protection Agency (EPA) defines the permissible level of arsenic in drinking water in the United States as 10 μg dm-3 (ca. 1.3*10-7 mol dm-3). Further research is being continued along this line.