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 carbonate on conductive-diamond thin film electrodes in a sodium carbonate solution at room temperature. The effect of several parameters including: electrolyte concentration, current intensity, addition of Na2SiO3 as a stabilizing agent, and increments in temperature for sodium peroxocarbonate formation 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 reduction during water electrolysis. The optimal experimental conditions are: the current density of 34.3 mAcm-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 percarbonate (Na2CO3 – 1.5H2O2) [24]. Some authors also have reported good results in stabilizing peroxocarbonate Na2C2O6 in solution [20, 16]. During electrosynthesis of peroxocarbonate on BDD anodes, some factors can cause the decomposition of this oxidant and decrease the generation efficiency. One of these factors is the increase of temperature 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 2OH → 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 mAcm-2 to 37.5 mAcm-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. (8) (mol L ), Vs is the solution volume (L), and I repre- Figure 3 shows the influence of the concentration sents the applied current (A) at sampling time t (s). of Na2CO3 solution on the electrogeneration of SPOC. Experiments were carried out in 0.5, 1, and nFCVs η(%) 100. (7) -2 t 2M of Na2CO3 solutions, applying 31.2 mAcm in a  idt divided cell, with the addition of 4g L-1 of sodium 0 metasilicate. In all cases, SPOC concentration in- RESULTS creased rapidly for the first hour of electrolysis.

Effect of the presence of stabilizer However, after that, the velocity of accumulation decreased. The concentration as function of time Under open circuit conditions, no oxidant prod- became almost asymptotic in the 0.5M solution. In ucts were detected in any cell arrangement. The this case, the concentration of SPOC after one hour effect of the presence of sodium metasilicate was 9.1 mM, which was more than 70% of the total Na2SiO3 (SMSC) as a stabilizer of peroxocarbonate oxidant that was synthesized within 3 hours was evaluated by adding 4 gL-1 of stabilizer. (11.25 mM). Figure 2 shows the concentrations of SPOC obtained Changes in slopes using 1 and 2M solutions of -2 applying a current density of 31.2 mAcm under Na2CO3 are similar but not as pronounced as in the different electrolysis conditions in Na2CO3 1M at 0.5M solution. The accumulated concentration of pH 10.8: i) without stabilizer; ii) with 4g L-1 of stabi- SPOC in 1M solution reached 26.5 mM. This was lizer. A BDD was used as the working electrode, a more than the double quantity obtained in the 0.5M graphite bar as the auxiliary or counter electrode, solution of carbonate. However, when the concentra- and SCE as the reference electrode. The sweep rate tion of electrolyte was doubled again, to the concen- was 10 mV s-1 in reduction sense. tration of 2M, the SPOC produced was 30 mM at the Figure 2b shows that the presence of SMSC en- end of electrolysis. Differences in SPOC concentra- hanced the production of SPOC, reaching about tions obtained can be attributed primarily to the ef- 25 mM after 3 hour of electrolysis. This is 10 mM fect of temperature during electrolysis, because of more than is produced in the absence of a stabilizing the poor stability of SPOC. The final T with reagent. spect to the initial room temperature were: 5.8°C for During the first 1.5 hours of the synthesis, in both the 2M carbonate solution, 6.1°C for the 1M, and experiments, there was a linear increase in the con- 7°C for the 0.5M concentration. This is attributed to centration of SPOC. After that time, changes in con- the electrical resistance of the solution and mem- centrations were minimal. This can be attributed to brane and corresponds to the high cell voltage chemical decomposition and to the effect of temper- shown in the Table for each current. Heating on the ature [18], because the increment is about 6°C at the surface and around the anode may have caused ac- end of each experiment. The membrane also acted as celerated decomposition of the product. a resistive element in the electrochemical cell. This Evidently, since current efficiency is a function was confirmed by an increase in the temperature of of the concentration of, it shows a similar behavior. the solution mainly on the anodic side. It is worth Figure 3b presents the current efficiency for SPOC generation. The maximum efficiencies were 18.9, mentioning that Na2SiO4 did not protect the SPOC molecules from destruction by temperature. 25.7, and 29.1%, respectively, for each solution in This behavior is also reflected in the current effi- the early phase of the experiment. Later, the efficiency decreased dramatically, to the final values of ciency for SPOC production. In all cases the highest 6%, 15% and 17%, because of the difficulty of ac- efficiencies for both electrodes were obtained in the cumulation SPOC in a solution. first few minutes of polarization, with the current These results indicate that, although it is more ef- efficiency of 25% in the presence of a stabilizer and ficient to use a 2M solution, this does not improve of 22% without SMSC. After that, the efficiency the carbonate/oxidant relationship. In order to de- decreased over time as the rate of SPOC accumula- termine the relationship of the oxidant to carbonate, tion decreased until it reached the values of 15% and the content of the active oxygen was calculated by 28

(a) (b)

Fig. 2. Effect of addition of sodium metasilicate as stabilizer in a 1M Na2CO3 solution. (a) on SPOC concentration and (b) current -2 -1 efficiency at 31.2 mAcm .  in absence;  with addition of 4 mgL of sodium metasilicate.

(a) (b)

Fig. 3. Influence of concentration of Na2CO3 solution as supporting electrolyte and precursor of SPOC on (a) concentration and -2 (b) current efficiency.  0.5M solution;  1M solution;  2M solution current density applied 31.2 mAcm . titration with the permanganate method, using the levels reached during electrolysis; the raise of tem- equation: perature was registered in all experiments.

0.008 (9) Table. Influence of electrolyte concentration on the O%VBNA   100. W SPOC production in function of Na2CO3 concentration at BDD anodes in a divided electrochemical cell applying Where O% represents the weight percentage of the 31.2 mAcm-2. active oxygen, V is the required volume of titrant, B Na CO [ Na C O ] % O average E/V is the required volume of titrant needed for the 2 3 2 2 6 T/°C mM /% blank, N is the normality of titrant, A is the aliquot 0.5M 11.25 0.171 12 9.8 7 factor, and w corresponds to the grams of sample 1M 26.5 0.404 22 8.5 6.1 used (in solution). 2M 30 0.243 25 7.8 5.8 The Table summarizes the effect of electrolyte concentration on the active oxygen content (O%), SPOC reduction average current efficiency (), cell potential (E), and Evidently, SPOC concentration depends on the final increase in temperature (T) for producing -2 duration of electrolysis, and different quantities of SPOC on the BDD anode at 31.2 mA cm . As oxidant can be obtained by changing this duration. shown in the Table, the highest SPOC content In order to confirm the formation and electrochemi- (30 mM) and average current efficiency (12%) were cal reduction of SPOC, cyclic voltammograms were obtained in a carbonate 2M solution. However, the obtained after the electrolysis. The cyclic voltam- oxidant content (0%) was greater using only 1M mograms in Fig. 4 show the cathodic part after elec- with a value 0.404% obtained with an average effi- trolysis applying 37.5 mAcm-2. A for different times ciency of 22%. In a less concentrated carbonate so- (0, 30, 120, and 180 min) in a 1M Na2CO3 solution lution, the oxidant power was weaker and efficiency in the presence of sodium metasilicate. A 5 mL lower. It can be accounted for in the following way: sample was taken from the anodic compartment and after reaching a certain concentration of SPOC, the SPOC was quantified by titration with thiosulphate. charge transportation is limited to the charge trans- The concentrations obtained were 8, 22, and 26 mM. ference on the electrode surface, but not to the diffu- A cyclic voltammetry was made immediately in the sion on the electrolyte, as evidenced by high voltage reduction sense using the BDD as the work electrode 29

Fig. 4. Cyclic voltammograms of boron diad doped electrode Fig. 5. In situ generation and detection of SPOC on BDD elec- after polarization of electrode in a 1M Na2CO3 solution. trodes in 1M Na2CO3 solution. Lower line: polarization curve at Sodium peroxocarbonate concentrations quantified: (a) 0; (b) sweep rate 2.5 mVs-1 in anodic sense (W1). Upper line: simultane- 8; (c) 22; (d) 26 mM. Sweep rate 10 mV s-1. ous response on BDD at -1.45 V vs ECS for SPOC reduction.

(a) (b)

Fig. 6. Influence of current density on the anodic synthesis of SPOC in 1M Na2CO3 solution. (a) concentration and (b) current -2 -2 -2 efficiency.  31.2 mAcm ;  34.3 mAcm ;  37.5 mAcm . in a typical three electrode electrochemical cell. A sodium percarbonate is formed. Then it is carried by graphite bar was used as the counter electrode and diffusion to the second electrode where SPOC is SCE as the reference. The cyclic voltammograms in reduced, with a consequent increase in the reduction Fig. 4 show the cathodic part of the scans. Increases current in the electrode. SPOC reduction can be in current can be observed in potentials between 1 represented by equation (10). However, Osetrova and 1.5 V vs. SCE with respect to non-electrolyzed and co-workers [21] have suggested that several solution. The peak current increases with SPOC organic compounds, such as methane, ethylene and concentration quantified. This behavior has been formaldehyde, are also formed in a complex reduc- observed during synthesis of other peroxocom- tion mechanism. pounds obtained under similar conditions. These 2- - 2- C2O6 + 2e → 2CO3 . (10) peaks are attributed to the reduction of the electro- generated K2S2O8 [3]. When the anodic process is insulated by using a An experiment was carried out in a four-electrode cationic membrane, the SPOC that is generated can- electrochemical cell, using a 1 cm2 BDD as a not pass to the cathodic compartment, which avoids working electrode 1 (W1) and a 5 cm2 BDD as W2. its destruction by electro reduction in the cathode. In W1, a linear sweep at 2 mV s-1 in the oxidation These results confirm that SPOC is formed as the 2- direction was carried out, while in electrode W2 the result of CO3 oxidation by free radicals during potential was set at -1.45 V (SCE), enough to reduce water discharge according to equation 2. Then the SPOC formed during polarization on W1 SPOC is carried by diffusion to the second electrode, electrode. where SPOC is reduced, with a resultant increase in The lower graph shows the polarization curves in the reduction current in the electrode. the oxidation direction at 10 mV s-1 over DDB (W1). The upper graph shows the reduction current Effect of current on sodium response at the W2 electrode. It was observed that peroxocarbonate generation there was not a substantial response in W2 after the Figure 6 shows the effect of current density in the electrolysis of water in W1. Upon initiation of water range 31.2–37.5 mAcm-2 on anodic production of oxidation (> 2 V) in the diamond electrode W1, SPOC in a 1M Na2CO3 solution in a two compart- 30 ment electrochemical cell. It can be observed that Technology (CONACyT) for their financial support peroxocarbonate generation is strongly dependent on of this work. current density. 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