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Electrochemical Production of Peroxocarbonate at Room Temperature Using Conductive Diamond Anodes

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Electrochemical Production of Peroxocarbonate at Room Temperature Using Conductive Diamond Anodes 25 Electrochemical Production of Peroxocarbonate at Room Temperature Using Conductive Diamond Anodes E. J. Ruiz-Ruiza, Y. Measb, R. Ortega-Borgesb, J. L. Jurado Baizabalb aUniversidad Autónoma de Nuevo León, UANL, Facultad de Ciencias Químicas, Av. Universidad S/N, Ciudad Universitaria San Nicolás de los Garza, Nuevo León, México b Centro de Investigación y Desarrollo Tecnológico en Electroquímica, Departamento de Electroquímica Parque Tecnológico Querétaro, Sanfandila, Pedro Escobedo, Querétaro C.P. 76703, México e-mail: [email protected] This work describes a method for synthesizing sodium peroxocarbonate by electoxidation of car- bonate on conductive-diamond thin film electrodes in a sodium carbonate solution at room tempera- ture. The effect of several parameters including: electrolyte concentration, current intensity, addition of Na2SiO3 as a stabilizing agent, and increments in temperature for sodium peroxocarbonate for- mation has been studied in an electrochemical H cell of two compartments separated by a cation- exchange membrane. The oxidant formation has been confirmed by cyclic voltammetry and in situ re- duction during water electrolysis. The optimal experimental conditions are: the current density of 34.3 mAcm-2, an electrolyte concentration of 1M, and the addition of 4 g/L of sodium metasilicate in the two compartments of the electrochemical cell. Keywords: peroxocarbonate, peroxosalts, diamond electrodes, electrosynthesis. УДК 546 INTRODUCTION compounds is very attractive, because they are strong oxidizing agents that can be used to degrade Electro-synthetic processes for producing stable organic pollutants [17], and in other syntheses appli- oxidant compounds of industrial or ecological cations. interest, particularly anodic oxidation of inorganic It has been reported [18] that there are two kinds ions on diverse electrode materials, have been inten- of “percarbonates”. One is chemical percarbonate sively studied in recent years [1–3]. (Na2CO3 – 1.5H2O2) and the other is electrolytic The electrochemical formation of those com- percarbonate. Na2CO3 – 1.5H2O2. The chemical per- pounds is mainly carried out in strong acidic media carbonate is usually obtained by simply mixing with high oxygen overpotential in order to minimize H2O2 and Na2CO3 in an alkaline solution. It is an the secondary reaction of oxygen evolution. Plati- effective oxygen-based bleaching agent used in tex- num (Pt) or metallic oxides, like PbO2, are common- tile, chemical, paper- and -wood industries as a ly used for the anode. Despite its lower overpotential source of active oxygen [19–21]. platinum is usually the material of choice because Among the oxidized forms of carbonate is PbO anodes corrode slowly and release Pb-ions in 2 Na2C2O6, with a redox potential of 1.8 V (NHE) solutions [4]. In this regard, over the last two [22], which has a higher redox potential than per- decades many researchers have studied electroche- chlorate (1.201 V) or periodate (1.653 V). In this mical oxidation as an alternative technology for the article the term SPOC will be used to refer to sodi- degradation of a wide variety of organic pollutants um peroxocarbonate (Na2C2O6) in order to differen- [4–6]. In addition, some studies have been focused tiate it from the sodium carbonate Na2CO3 – on the use of diamond electrodes in the synthesis of 1.5H2O2.The electrochemical production of SPOC oxidants that can be difficult to generate by more has been investigated by direct oxidation of car- usual synthesis methods, most of them in acidic bonate using Pt anodes [18, 23, 21] as well as BDD media [2, 7, 3, 8, 9]. Thanks to the high overpoten- anodes [16, 15]. In the respective literature, the tial for water electrolysis and the large electrochemi- mechanism of their formation on BDD anodes im- cal window, enough to produce hydroxyl radicals plies the generation of free radicals (OH): (OH) with high efficiency [4, 5, 10], boron-doped + − BDD + H2O → BDD(OH) + H + e . (1) diamond (BDD) anodes have been shown to be suitable for electrosynthesis of powerful oxidants. Free radicals (OH) on the electrode surface react 2- These include: ferrates (FeO4 ) [6, 7, 11] peroxodi- with bicarbonate or carbonate, oxidizing it into 4- phosphate (P2O8 ) [9], peroxyacetic acid peroxocarbonate, according to reaction (2) [15, 22]. 2- (CH3CO3H) [12], peroxodisulfaphate (S2O8 ) − 2− - 2HCO3 + 2BDD(OH) → C2O6 + 2H2O. (2) [3, 1, 8], perbromates (BrO4 ) [13] or chlorine 2- oxoanions [14], and even peroxocarbonate (C2O6 ) SPOC is obtained by electro-oxidation precipi- in an alkaline medium [15, 16]. Synthesis of those tates at 0–5°C, yielding a dry white powder with the _____________________________________________________________________________________ Ruiz-Ruiz E.J., Meas Y., Ortega-Borges R., Jurado Baizabal J.L., Электронная обработка материалов, 2014, 50(6), 25–31. 26 same XRD pattern as sodium percarbonate [15]. Figure 1 shows a schematic representation of the Unfortunately, peroxocarbonate in solution exhibits experimental setup. poor stability with a half-life estimated to be 1–2 min at 15ºC, pH 10, SPOC is hydrolyzed to peroxide [18]. In addition, heat or the presence of low levels of ions of heavy metals can accelerate its decomposition. Certain inorganic salts, e.g. sodium metasilicate (Na2SiO3), have been used to stabilize sodium per- carbonate (Na2CO3 – 1.5H2O2) [24]. Some authors also have reported good results in stabilizing pero- xocarbonate Na2C2O6 in solution [20, 16]. During electrosynthesis of peroxocarbonate on BDD anodes, some factors can cause the decomposi- tion of this oxidant and decrease the generation effi- ciency. One of these factors is the increase of tem- perature due to high over voltages on the electrode surface and membrane resistance. In addition to OH production and formation of SPOC, several parallel Fig. 1. Schematic representation of the H type electrochemical parasite reactions can occur, like the generation of cell used to produce peroxocarbonate. ozone O3 and hydrogen peroxide. The anodic side of the cell contained 80 mL of 2OH → H2O2 (3) carbonate solution Na2CO3 at different concentra- (H2O2) adsorbed → (H2O2) solution (4) tions, and separated from the catholyte by a cation- OH → O + H+ + e- (5) exchange membrane (Asahi) with a cross-sectional 2 O + O2 → O3 . (6) area of 6.2 cm . No stirring was applied to the anolyte except prior to sampling. The distance from H O generation is certainly not a parasite reaction, 2 2 the membrane to the anode was 5 cm and it was 7 since it can be used to form sodium percarbonate as cm to the cathode. Constant-current electrolyses shown in the formula Na CO – 1.5H O by simple 2 3 2 2 were conducted over a period for 3 h at current den- abduction of hydrogen peroxide to carbonate ions. sity levels from 31.2 mAcm-2 to 37.5 mAcm-2. The The primary objective of this work is to study the temperature was initially 25±2°C and it was recor- effect of current density and electrolyte concentra- ded during the electrolysis experiments. Every tion on the electrochemical synthesis of peroxocar- 30 min, a 5 mL sample was taken from each com- bonate or peroxodicarbonate at room temperature on partment and replaced by an equal volume of a fresh a BDD anode. solution. Total peroxocarbonate concentration in the EXPERIMENTAL solution was measured using a titration by the stand- Materials and reagents ard KMnO4 method [25]. Analytical-grade Na2CO3 and Na2SiO3 were ob- Cyclic Voltammetry tained from Aldrich and KMnO from J.T. Baker 4 Cyclic Voltammetry was carried out in a solution and used as received. All electrolytic solutions were immediately after the anodic synthesis of SPOC in a prepared with deionized water. BDD (from Fraunho- conventional three-electrode electrochemical cell. In fer IST) films were synthesized by the filament this cell, a BDD was used as the working electrode, chemical vapor deposition technique on conducting a graphite bar as the auxiliary or counter electrode, p-Si substrates. The thickness of the diamond film and saturated calomel electrode (SCE) as the was about 1 mm. The geometric area of the BDD reference electrode. The sweep rate was 10 mV/s-1 in anodes was 8 cm2. Graphite was used as a cathode the reduction sense. with a geometric area of 54 cm2. All electrochemical In situ detection of SPOC was done using an experiments were carried out with an Autolab H-type cell with 4 electrodes using and a poten- PGSTAT 30 potentiostat/galvanostat connected to a tiostat in the bi-potentiostat mode. In the cathodic PC for processing data. compartment a graphite bar is used as the counter Electro-synthesis of SPOC electrode. On the anodic side there was the reference electrode (SCE) and two work electrodes: BDD Anodic oxidation of carbonate was carried out in (8 cm2) designated as the working electrode one a two-compartment H-type cell in order to insulate (WE1) and another BDD (2 cm2) designated as the the anolyte and avoid SPOC reduction in cathode. working electrode two (WE2). The electrochemical 27 generation was done in WE1 for a sweep potential 9%, respectively. This result indicates that addition from 0 to 3.8 V (SCE) at 2.5 mV s-1. The WE2 was of sodium metasilicate increases the stability of maintained at – 1.45 V (SCE). The distance between SPOC and consequently the total current efficiency. the WE1 and WE2 was 1 cm. However, efficiency also is affected by the decom- Current efficiency was calculated using equation position of free radicals according to reactions (7). Where n is the number of electrons consumed to (3)–(6) and by the oxygen evolution directly on the generate one molecule of SPOC according to the anode (reaction 8) [18]: mechanism in EQ (n = 2), F is the Faraday constant + - 0 2H2O O2 + 4H + 4e E = 35V (SCE) at 25°C, (96,487 Cmol-1), C is the concentration of SPOC -1 pH 10.75.
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