Effect of the Residual Silver and Adsorbed Lead Anions towards the Electrocatalytic Methanol Oxidation on Nanoporous Gold in Alkaline Media

Abstract Nanoporous gold (NPG) obtained by potentiostatic dealloying of an AuAg master alloy was pulverized, filled into a cavity microelectrode and its surface electrochemistry was investigated in NaOH and KOH solutions. This method yielded highly resolved undistorted voltammograms of this polycrystalline material with clear evidences for surface segregation of Ag during storage in air. One cycle in 0.1 M H2SO4 can remove most of the residual Ag from the surface as evidenced by voltammetry after back transfer to alkaline solution and by X-ray photoelectron spectroscopy (XPS). After removal of residual Ag from the surface, the surface undergoes rephasing forming wider {111} and {110} terraces as evidenced by Pb underpotential deposition (UPD). By adjustment of the bulk concentration of PbII species, a selective partial coverage of the high internal surface area of NPG could be intentionally adjusted. Lead species remain attached to the NPG surface at potentials positive of the UPD regions either as plumbates or PbIV species as evidenced by XPS. Those tools enable to disentangle effects on the electrocatalysis of the methanol oxidation reaction (MOR) in different potential regions for 1 M methanol in different concentrations of NaOH and KOH. The MOR commences at lower potentials in the presence of residual Ag. At very high potential, the presence of Ag species promotes the oxidation to CO2/CO32-. Tafel analysis after selective blocking of specific facets shows that the {110} terraces promote especially the first step of MOR while the {111} terraces enhance the rate of further steps that are rate-limiting at higher potentials. While high coverages by Pb UPD layers or adsorbed plumbate anions inhibit electrooxidation, the behavior of NPG electrodes with low coverages of Pb species is reminiscent to NPG electrodes with residual Ag. This could be caused by promoting the methoxide and/or OH- adsorption at low overpotentials and catalyzing complete oxidation by a surface bound PbIV species at higher potentials. The catalytic currents increase with base concentration and are higher for KOH compared to NaOH. Comparable effects of base concentration on the parameters of the Tafel lines suggest that this effect is a result of slightly stronger deprotonation of methanol in bulk KOH solutions.