Novel application of an in situ radiotracer method for the study of the formation of surface adlayers in the course of Cr(VI) reduction on a gold electrode

The electroreduction of Cr(VI) species to Cr(III) at noble metal electrodes in acidic solutions takes place via a ce (chemical-electron-transfer) mechanism where the heterogeneous chemical step which precedes the charge transfer involves anions (e.g. HSO4−/SO42−) leading to the formation of intermediate surface complexes. In this paper, we report some new findings obtained by in situ radiotracer and voltammetric studies of the simultaneous adsorption of anions (HSO4−/SO42− labeled with 35S, Cl− labeled with 36Cl) and Cr-containing species (labeled with 51Cr) on a gold electrode in the course of dichromate reduction in 1 mol dm−3 HClO4 supporting electrolyte. Moreover, special attention is paid to present a detection and calculation method elaborated for the quantitative evaluation of the surface excess of radiolabeled Cr species via measurement of the intensity of low energy X-rays (E=4.90 keV) emitted by 51Cr. From our experimental results it can be stated that: (i) at E<1.20 V the electroreduction of Cr(VI) particles presumably proceeds via a ce mechanism to yield a gold surface covered with an intermediate surface adlayer containing Cr(VI) species and added anions (HSO4−/SO42−, ClO4−); (ii) no data indicating the embedding of Cl− ions into the surface film could be detected; (iii) the maximum surface excess of Cr-containing species (Γmax=1.2×10−9 mol cm−2) as well as the molar ratio between the Cr species and HSO4−/SO42− ions (m≈6 at E=0.8 V) attests that the coverage of the gold surface with intermediate complexes does not exceed one monolayer.