Electrochemical and XPS Characterization of Glassy Carbon Electrode Surface Effects on the Preparation of a Monomeric Molybdate(VI)-Modified Electrode

Abstract

Evaluation of electrochemical surface pretreatment methods on the preparation of monomeric molybdate(VI)-modified glassy carbon electrode (GCE) in H2SO4 is studied. The results show that the cathodically treated GCE is unusually highly activated toward Mo(VI) adsorption, but the anodized electrode does not respond, unlike the previous reports for the other adsorbed systems. The analyses of the voltammograms of Mo(VI) on a cathodized electrode show three pairs of reversible peaks with a surface excess on the order of ≅1 × 10-10 mol cm-2 for each of the peaks. The interaction parameters for the three processes are 1.14 × 109, 1.38 × 109, and −5.50 × 109 cm2 mol-1, respectively. Strong interaction between the Mo(VI) ion and cathodically reduced GCE surface groups is indicated by the formation of Mo(V) (detected by x-ray photoelectron spectroscopy (XPS) and electrochemical polarization measurements) during the course of Mo(VI) adsorption at open circuit conditions. The Mo(VI) adsorption via the mixed potential mechanism has been formulated involving Mo(VI) reduction to Mo(V) coupled with surface alcohol group oxidation to hydroperoxide. XPS analyses show that the cathodized surface contains higher amounts of >C−O− surface groups. The quantities of >C−O− surface groups correlate with the observed differences in the response of the pretreated GCE surfaces toward Mo(VI) adsorption.