Bulgarian Chemical Communications, Volume 42, Number 2

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Received: March 18, 2009

Additivity principle is employed to study heterogeneous redox catalytic process mechanisms by electrochemical means. The reaction mechanism of sulphide ions oxidation by air in alkaline medium is investigated. Assuming an electrochemically proceeding mechanism two partial reactions of sulphide oxidation and oxygen reduction are evaluated. Two highly active catalysts, suitable for industrial application, namely cobalt disulphophthalocyanine complex [CoPc(SO₃H)₂] and nickel hydroxide Ni(OH)₂ are studied. Hydrophobic gas diffusion carbon electrodes (GDE), are utilized.

The electrochemical mechanism of the process is proven by the good agreement between calculated by Faraday’s law values and converted quantity of sulphide ions, measured analytically. The sulphide ions oxidation reaction by oxygen proceeds essentially on spread micro-galvanic elements created in the Carbon-Teflon structure of the electrodes. The carbon in the GDE exhibits its own catalytic activity. The electrodes modified with CoPc(SO₃H)₂ and Ni(OH)₂ possess considerably higher catalytic activity (about 5-fold).

The studied systems are quite promising for industrial purification of waste waters containing H₂S.

Key words: Electrocatalytic oxidation; sulphide ions.

Introduction

Sulphide ion catalytic oxidation to elemental sulfur [1–3] is one of the most effective methods for eliminating this hazardous pollutant from waste waters. They are highly toxic and biological methods for cleaning become problematic [4]. Chemical pro-cesses for sulphide oxidation are generally homo-geneous and heterogeneous. Cobalt sulphophthalo-cyanine complexes, nickel, cobalt and iron salts [5–8] are used as catalysts in homogeneous media. Heterogeneous catalysts like Fe, Co and Pd salts as well as phthalocyanine and oxides of transition metals deposited on active carbon, silicagel or Al₂O₃ [9–12] have been studied. The mechanism of the sulphide oxidation process is complex and depend-ing on the working conditions produces either collo-idal sulphur or polysulphides.

Wagner and Traud [13] have proposed an addi-tivity principle that was employed and developed [14] to study heterogeneous redox catalytic process mechanisms by electrochemical means. Applying the additivity principle, we have aimed at gaining information on the reaction mechanism of sulphide ions oxidation in alkaline medium:

S^2– + 1/2O₂ + H₂O → S⁰ + 2OH^– (1)

The catalytic redox reaction (1) can be modelled by two partial reactions of oxidation and reduction [15, 16]. Assuming an electrochemically proceeding mechanism, the anodic and cathodic reactions may be presented as follows:

Anodic reaction

S^2– → 1/2S₂ + 2e^– (2)

Cathodic reaction

O₂+ H₂O + 2e^–→ HO₂^– + OH^– (3)

Total:

In this work two suitable for industrial appli-cation and highly active catalysts, namely cobalt disulphophthalocyanine complex [CoPc(SO₃H)₂] and nickel hydroxide Ni(OH)₂ have been studied.

Experimental

Hydrophobic gas diffusion carbon electrodes (GDE), developed in this laboratory (IEES, BAS) are utilized [17]. The electrodes are double layered and composed of gas diffusion and catalytic layers. The geometrical area of the GDE is 200 cm². The GDE’s catalytic layer surfaces are modified by two catalytically active substances. The GDE is used as a wall of a reactor’s cell as illustrated in Fig. 1.

The GDE is arranged in the reactor in such a way as to shape the reactor chamber (2) where the elec-trolyte is supplied. The GDE catalyst surface faces the electrolyte while its other side is in direct contact with the surrounding air. The sulphide ions containing electrolyte is supplied from the tank (7) to the reactor chamber (2) by means of the peri-staltic pump (6) via the electrolyte supply line (5). The waste solution is fed back to the tank (7) closing the cycle, where the electrolyte continually circulates with a defined flow rate. The waste electrolyte contains 10% Na₂S (15 g S^2–·l^–1) and 1% NaOH (0.25 N NaOH). In the course of the experi-ment a sample is taken from the tank (7) for analysis at 1 hour intervals. Mercury oxide (Hg/HgO) refer-ence electrode is used and all potentials in this paper are given in mV vs. E (Hg/HgO).

Fig. 1. Scheme of the reactor’s cell: 1 - GDE.; 2 - reactor chamber; 3 - waste solution line; 4 - reactor; 5 - solution supply line; 6 - peristaltic pump; 7 - solution tank;

The catalysts were deposited on the electrode surface. One of the substances, [CoPc(SO₃H)₂], was deposited by impregnation on the GDE catalytic layer from an ethylenediamine hydrochloride solu-tion. The surplus ethylenediamine was washed away and the electrode dried at 80ºC. Concentrated solu-tion of Ni(NO₃)₂ was used for impregnation of the electrode with nickel nitrate solution, followed by precipitation of the hydroxide on the surface through 10% NaOH solution treatment and finishing wash with distilled water, and drying at 100ºC.

The catalytic activity was measured by trans-porting a solution of 15 g S^2–·l^–1 and 0.25 N NaOH through the reactor cell. Under these conditions the sulphide ions were oxidized by air that has diffused through the GDE. The reaction rate (V) was evalu-ated by catalyst productivity expressed as equi-valents of converted sulphide ions at 20ºC per unit time onto a specific geometric surface, equal for non-modified and modified electrodes, at identical flow rates. The converted amounts of sulphide ions were determined by analytical method [18], which is based on precipitation of Na₂S in Cu(ClO₄) to CuS, which is titrated volumetrically with EDTA. The degree of conversion (Dc) of sulphide ions to ele-mental sulphur was used as criteria for the catalytic activity according to the Equation 5.

(5)

where D_c – degree of conversion; C_o – initial con-centration of S^2– ions; C – current concentration of S^2– ions (measured after some working time). In agreement with the Faraday’s law the reaction rate can be expressed as a current:

where: V (mol·sec^–1) is the reaction rate; n is the number of electrons exchanged by the two half-reactions; F is the Faraday constant and ∆NS^2– are the gram equivalents of S₂, oxidized over the time interval ∆t.

I. Mitov et al.: Heterogeneous catalytic oxidation of sulphide ions

The GDE electrochemical characterization was carried out by plotting partial polarization curves on GDE with or without deposited catalysts. The sulphide ions polarization curves were plotted by isolating the cell from oxygen and by utilizing the above mentioned electrolyte. The oxygen polariza-tion curves were plotted while admitting oxygen in the absence of sulphide ions. The electrolyte con-tains only 0.25 N NaOH. The E_mix values of the res-pective polarization curves for each electrode were compared with a measured value of the open circuit potential in the presence of both redox couples. The similarity between the values of mixed current I_mix (mA) and the reaction rate V (expressed as a current IF) as well as between mixed (E_mix) and open circuit (E_0c) potentials is an indication of the electroche-mical mechanism of the process.

Results and Discussion

The measurements of the sulphide oxidation reaction rate and the partial electrochemical curves commenced with the non modified GDE. In the first experiment the dependence of the reaction rate on the sulphide solution flow rate (v_r) has been studied. The results at 20ºC and various flow rates (between 14 and 53 cm³·min^–1) are depicted in Fig. 2. It is quite apparent that the degree of conversion in-creases with the solution flow rate. This is probably due to lower diffusion limitations and increase of circulation cycles. Flow rate of v_r = 24.15 cm³·min^–1 has been selected for next experiments.

With the chosen flow rate of solution, tests of the catalytic activity on non modified GDE at different temperatures have been performed. Fig. 3 shows the relationships between the degree of conversion and time on stream for three operating temperatures. The D_c increases with temperature but reaches a plateau, which is lower than 100% conversion of sulphide ions.

Fig. 2. Relationship between rate of sulphide oxidation by oxygen and flow rate of solution; T = 20ºC; electrolyte: 15 g S^2–·l^–1 + 0.25 N NaOH; non-modified GDE

Fig. 3. Relationships between degree of conversion and time on stream at different temperatures: T = 20, 40 and 60ºC; electrolyte: 15 g S^2–·l^–1 + 0.25 N NaOH + O₂ (Air);

I. Mitov et al.: Heterogeneous catalytic oxidation of sulphide ions

Fig. 4. Partial polarization curves: T = 20ºC; non-modi-fied GDE; v_r = 24.15 cm³·min^–1; SGDE = 200 cm²; cathodic curve (■) - 15 g S^2–·l^–1 + 0.25 N NaOH + Ar; anodic curve (●) - 0.25 N NaOH + O₂ (Air);

The catalytic activity of a modified GDE at different temperatures has been measured, too. Fig. 5 and Fig. 6 show the relationships between the degree of conversion and time on stream for three operating temperatures. The Dc increases with temperature. It should be noted that due to the high catalytic activity of the catalyzed GDE’s the complete sulphide ions conversion is achieved within a comparatively short period of time. The values calculated according to the Faraday’s law for the currents corresponding to the converted quantity of sulphide ions at 20ºС are respectively IF1 = 556 mA and IF2 = 908 mA. Here IF1 is the Faraday current of the modified with the [CoPc(SO₃H)] catalyst GDE, while IF2 is the Faraday current for the modified with the Ni(OH)₂ catalyst GDE.

Fig. 5. Relationships between degree of conversion and time on stream at different temperatures for CoPc(SO₃H)] catalyst on GDE: SGDE = 200 cm²; T = 20, 40 and 60ºC; electrolyte: 15 g S^2–·l^–1 + 0.25 N NaOH + O₂ (Air);

Fig. 6. Relationships between degree of conversion and time on stream at different temperatures for Ni(OH)₂ catalyst on GDE: SGDE = 200 cm²; T = 20, 40 and 60ºC; electrolyte: 15 g S^2–·l^–1 + 0.25 N NaOH + O₂ (Air);

Fig. 7 illustrates the partial polarization curves of anodic (') and cathodic (") reactions of sulphide ions oxidation and oxygen reduction, respectively, for GDE modified with [CoPc(SO₃H)₂] – 1’, 1” and Ni(OH)₂ – 2’, 2” catalysts. The nature of the curves (a steeper slope for the anodic curves) of shifting E_mix to cathodic values is a good reason to believe that the cathodic (oxygen) reaction is the rate-limiting step [15, 16].

The I_mix and E_mix values for the three catalysts studied on GDEs are given in Table 1. It is apparent that the modified electrodes exhibit considerably higher catalytic activity, about 5-fold, compared to the non-modified electrodes. The following order of catalytic activity: GDE < GDE/CoPc(SO₃H)₂ < GDE/Ni(OH)₂ corresponds to an increased rate of the rate-determining step of the oxygen cathodic reduction. Modification with a catalyst enhances the reversibility of both half-reactions and the polariza-tion curves exhibit steeper slopes and higher I_mix values, which is an indication for higher catalytic activity.

Fig. 7. Partial polarization curves: T = 20ºC; flow rate

S^2– + HO₂^– + H₂O → S⁰ + 3OH^– (6)

Conclusions

The sulphide ions oxidation reaction proceeds essentially via an electrochemical mechanism. The carbon in the GDE exhibits its own catalytic acti-vity. The electrodes modified with CoPc(SO₃H)₂ and Ni(OH)₂ possess considerably higher catalytic acti-vity (about 5-fold). For these electrodes, the carbon behaves as a conductor in the electrochemical mecha-nism implementation. The studied systems are quite promising for industrial purification of waste waters containing H₂S.

I. Mitov et al.: Heterogeneous catalytic oxidation of sulphide ions

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