On-line Tracking Of The Interface Species And Reaction Mechanism Of The Electro-oxidation Of Sulfur(-?) Compounds On Pt Electrode

The electrocatalytic oxidation of sulfur?-??compounds?thiosulfate and thiourea?presents considerably complex behaviors.The oxidation products and reaction mechanism are dependent on the electrode potentials,electrode types,solution pH values and substrate concentrations.In this work,electrochemical techniques combined chromatography techniques?CV-HPLC,CV-CE?are applied to study on the electro-oxidation of thiosulfate and thiourea on polycrystalline platinum electrode.A micrometer sampling device was used to sample the species from the electrode surface on-line and later analyzed by HPLC.Through the combination of on-line sampling and off-line analysis,on the one hand,the problem of long time separation of chromatographic and electrophoretic analysis was overcome,on the other hand,species composition and concentrations on the electrode/electrolyte interface were measured in real time.The dependence relationship between distribution of oxidation products and electrode potential was revealed and the reaction mechanism of electrooxidation of sulfur?-??compounds was analyzed.The electro-oxidation of thiosulfate on platinum electrode is a very complicated reaction,because the oxidation products and electrochemical reactions are strongly dependent on the applied electrode potentials.The oxidation process involves the adsorption of thiosulfate on the electrode surface,electron transfer,mass transfer,surface reaction and homogeneous chemical reactions.A linear relationship between the current of the two oxidation peaks and the square root of the sweep rate is present,indicating that the oxidation of S₂O₃^2-thiosulfate is diffusion-controlled.The electrode reaction parameters n,a,n_a are calculated from the experimental results,wherein the transfer electron number n_a of the first oxidation peak is 1 eq.mol^-1,the electron transfer coefficient a is about 0.78,the transfer electron number n₁ of the reaction is4.2,the electron transfer coefficient a'is about 0.57,the transfer electron number n₂ of the second oxidation peak is 3.2.From E>0.35 V,liquid chromatography detected S₄O₆^2-,and its concentration reach a peak value at 0.8 V.The main reaction of this oxidation peak is the oxidation of thiosulfate to tetrathionate.Simultaneously,the homogeneous chemical reactions of tetrathionate to S₃O₆^2-and S₅O₆^2-are proceeding.SO₄^2-was detected from 0.5 V,and reach to the maximun concentration detected by capillary electrophoresis at 1.4 V.The generation of SO₄^2-was mainly originate from the oxidation of elemental S,and a little amount from the oxidation of SO₃^2-and S₄O₆^2-.According to the sulfur element conservation,the sulfur of all sulfur-containing products(S,S₃O₆^2-,S₄O₆^2-,S₅O₆^2-,SO₄^2-,SO₃^2-)are from S₂O₃^2-.According to the data obtained by on-line high performance liquid chromatography,the distribution of oxidation products at 0.8 V and 1.4 V was obtained.The electrochemical oxidation of thiourea on platinum electrode was analyzed by conbining electrochemical workstation with high performance liquid chromatography.At E<0.5 V,it is the thiourea adsorption process.At E>0.5 V,the oxidation intermediate formamidine disulfide was detected and its concentration is increased significantly to a platform between 0.9¹.2 V,where the premary reaction at this potential is the formation of formamidine disulfide.At E>1.1 V,the oxidation intermediate formamidine sulphinic acid begins to be detected and reaches a peak value at 1.4 V,which is corresponding to the second annodic peak at 1.4 V in the cyclic voltammetry curve,indicating the second annodic peak is corresponds to the oxidation of formamidine disulfide to formamidine sulphinic acid.At E>1.5 V,formamidine sulfonic acid was detected,and the decrease of the concentration of formamidine sulphinic acid is present at the same potential range,where formamidine sulphinic acid was oxidized to formamidine sulfonic acid.The study of the influence of sampling on the cyclic voltammetry curves of thiourea,formamidine disulfide(TU₂²⁺)indicates the current dencity of the annodic peaks is increased with the sampling rate due to the accelerated mass transfer of t substances in bulk solution that participate in electrochemical reactions.If the intermediate products on the surface of electrode continue to oxidize,sampling will leading to the reduce the concentrations of the intermediates on the surface of platinum electrode,thus decreased the current density of the annodic peak.The ekectrochemical reaction could be considered as diffusion controlled or electrochemical controlled through the analysis of the sampling process.CV-HPLC analysis of the electrooxidation products of thiosulfate and thiourea on the surface of polycrystalline Pt electrode indicates that two chemisorbed Pt-oxygenated anodic peaks present in the same applied potential(about 0.80 V and1.35 V?SCE?in the two systems,which are corresponding to the concentration peaks of the products/intermediates.Two chemisorbed platinum-oxygenated species(Pt-OH_ads and Pt-O_ads)that formed in the blank solution mediated the electrochemical oxidation reactions.Accordingly,thiosulfate and thiourea are oxidized indirectly mediated by chemisorbed Pt-oxygenated species rather than the direct charge transfer with polycrystalline platinum electrode.