Electrocatalysis in the electrochemical conversioncombustion of organic pollutants for w.pdf

Electrochimica Acto. Vol. 39, No. 1 l/12, 1857-1862, pp. 1994 Copyright 0 1994 Ekvier Scieaa Ltd.

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0013-4686(94)E0107-B

ELECTROCATALYSIS IN THE ELECTROCHEMICAL CONVERSION/COMBUSTION OF ORGANIC POLLUTANTS

FOR WASTE WATER TREATMENT

CHRISTOS COMNINELLIS

Institute of Chemical Engineering, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland

(Received 24 September 1993; accepted 28 October 1993)

Abstrsti-The electrochemical oxidation (or combustion) of organ& with simultaneous oxygen evolution has been investigated using different electrode material (Pt, Ti/lrO,, Ti/SnO,). A simplified mechanism for the electrochemical oxidation or combustion of organics is presented according to which selective oxidation occurs with oxide anodes (MO,) forming the so-called higher oxide MO,,, and combustion occurs with electrodes at the surface oflwhich dH radicals are accumulated. Detection of OH radicals formed by water discharge at different anodes using N,Ndimethyl-p-nitrosoaniline (RNO) as a spin trap and preparative electrolysis confirm the proposed mechanism.

Key words: electrocatalysis, hydroxyl radicals, spin trap, oxygen evolution, oxidation of organics.

1. INTRODUCTION

Besides inorganic pollutants, industrial waste water also contains organic pollutants which have to be treated before the water can be discharged. Bio- logical treatment is the most economic process and is usually used for treatment of readily degradable (Biocompatible) organic pollutants present in the waste water. The situation is completely different when the waste water contains toxic or/and refrac- tory (Non-Biocompatible) organic pollutants. In this case, another type of treatment must be used. The electrochemical method for the treatment of waste water containing organic pollutants has attracted a great deal of attention recently[ l-31.

There are two main applications for the electrochemical treatment.

(i) The electrochemical conversion method in which the Non-Biocompatible (NON-BIO) organics are transformed to Biocompatible (BIO) organics before the biological treatment:

Elcctrochcm. Biolop.

[NON-BIO] - [BIO] -CO, Ckmvcrsion trc.tm.

+ biomass.

The ideal electrode material which can be used in the electrochemical conversion method must have high electrochemical activity for aromatic ring opening (aromatic compounds are non- biocompatible) and low electrochemical activity for the further oxidation of the aliphatic carboxylic acids which are in general biocompatible.

(ii) The electrochemical combustion method, in which the organics are completely oxidized to C02. In this case, the electrode material must have high

electrocatalytic activity towards the electrochemical combustion of organics to CO2 and H,O.

The electrochemical oxidation (conversion or/and combustion) of all organic compounds is theoreti- cally possible before oxygen evolution (due to Hz0 discharge) but in practice, the oxidation reaction is very slow as a consequence of kinetic rather than thermodynamic limitations.

To increase the electrochemical rate of oxidation, electrocatalytic anodes have been proposed (Pt, Pd, . ..) but the main problem during oxidation of organics at a fixed anodic potential before oxygen evolution, is the decrease of the anode activity as the consequence of poison formation at the anode surface[4]. These poisoning species can be oxidized only at high anode potentials in the region of water discharge with simultaneous oxygen evolution, which allows regeneration of the anode surface during oxidation.

In previous works[5-81, the electrochemical oxidation of organic pollutants (phenol has been taken as a model pollutant) was carried out under conditions of simultaneous oxygen evolution using different electrode material. Analysis of reaction intermediates and measurements of current efficiency have shown that traditional anode material (Pt, Ti/IrO, , Ti/RuO,) favour electrochemical conversion (carboxylic acids are the final oxidation products) but with low current efficiency, contrary to the Ti/SnO, anode, which not only gives high current efficiency but favours also electrochemical combustion.

In the present investigation, the aim is to elucidate the mechanism of the electrochemical conversion/ combustion of organics, with simultaneous oxygen evolution, to select candidate oxide anodes which

1857

1858 CH. COMNINELLIS

favour electrochemical conversion or combustion of organics for waste water treatment.

2. MECHANISM OF THE ELECTROCHEMICAL

CONVERSION/COMBUSTION OF ORGANICS WITH SIMULTANEOUS

OXYGEN EVOLUTION

In Fig. 1, a generalized scheme of the electrochemical conversion/combustion of organics on oxide anode (MO,) is presented. In the first step [equation (l)], Hz0 (or OH-) in acid (or alkali) solution is discharged at the anode to produce adsorbed hydroxyl radicals according to the equation :

kl MO, + H,O -MO,(OH) + H+ + e. (1)

In a second step, equation (2), the adsorbed hydroxyl radicals may interact with the oxygen already present in the oxide anode with possible transition of oxygen from the adsorbed hydroxyl radical to the lattice of the oxide anode forming the so-called higher oxide MO,, i, equation (2):

k2 MO,(OH) -MO,+, + H+ + e. (2)

Thus, we can consider that at the anode surface two states of active oxygen can be present :

(i) physisorbed active oxygen (adsorbed hydroxyl radicals, OH); and

(ii) chemisorbed active oxygen (oxygen in the oxide lattice, MO,, i).

In the absence of any oxidizable organics, the physisorbed and chemisorbed active oxygen produce dioxygen according to the equations (3) and (4):

Lo MOJOH) - 50, + H+ + e + MO,, (3)

kd MO,+ t -MO,+ 302. (4)

Direct evidence for the last route of oxygen evolution at platinum oxide (PtO,) is provided by the

H++ c

MOx(OH) MO,+,

H++ c-

Fig. 1. Generalized schema of the electrochemical conversion[6]/combustion[!j] of organics with simultaneous oxygen evolution[3, 41: (1) H,O discharge; and (2) transition of 0 from OH to the lattice of the oxide anode.

work of Rosenthal[9] who used so as a tracer to show that portion of the evolved gas comes from oxygen already present in the oxide film. This view was put forward several times with other electrodes (RuO, , IrO, , NiCo,O,, NiOOH)[ lo].

Such a result may be explained in terms of the equations (MO = oxide anode):

2MsO + 2H, I60 + 2M180(16OH) + 2H+ + 2e,

2M1*0(16OH) + 2M1s1602 + 2H+ + 2e,

2M8/0, + Ms0 + Ml60 + 18i602.

In the presence of oxidizable organics we speculate (by similarity with the heterogeneous catalytic oxidation with 0, on oxide catalyst[ll]) that the physisorbed active oxygen (OH) should cause predominantly the complete combustion of organics, equation (5), and chemisorbed active oxygen (MO,+,) participate in the formation of selective oxidation products, equation (6):

kc

R + MO,(OH), - CO2 + ZH+ + Ze + MO,, (3

k. R + MO,+, -RO+MO,. (6)

2.1. Selective oxidation (conversion) of organics, equation (6)

For the selective oxidation of organics, the concentration of adsorbed hydroxyl radical on the anode surface must be almost zero. To satisfy this condition, the rate of transition of oxygen into the oxide lattice, equation (2), must be much more faster than the rate of hydroxyl radicals formation, equation (1):

rate of hydroxyl radical formation = k,[MO,], rate of transition of oxygen in the oxide lattice =

k2C 1,

k,C 1% k,CMO,l, (7)

where :

k, = electrochemical rate constant for H,O discharge;

k2 = electrochemical rate constant for transition of oxygen into oxide lattice;

[MO,] = concentration of active sites on the oxide anode; and

Cl = concentration of oxygen vacancies in the oxide lattice.

Thus, efficient anodes for selective oxidation (conversion) of organics must have low concentration of active sites on the anode surface and must have a high concentration of oxygen vacancies in the oxide lattice.

Oxides forming the so-called higher oxide (MO,,,) at potentials above the thermodynamic potential for O2 evolution can be considered as oxides having a high concentration of oxygen vacancies and can favour selective oxidation (conversion) of organics.

The current efficiency depends on the relative rate of the selective oxidation of organics, equation (6) to

Electrochemical conversion/combustion of organic pollutants 1859

rate of oxygen evolution (side reaction) by decomposition of the higher oxide, equation (4):

rate of selective oxidation of organics =

rate of O2 evolution by decomposition of MO,+ I = kAYI,

where :

2 = stoichiometry factor for selective oxidation; = rate constant for the selective oxidation of

organics, equation (6); k, = rate constant for 0, evolution, equation (4); [0] = steady state concentration of active oxygen

in the oxide lattice; and [R], = concentration of organics at a given time, t.

The Instantaneous Current Efficiency (ICE) can be given by the relation:

ICE = 4 k,CRI,

Z, k,CJU + kc, (8)

This relation shows that the ICE for the selective oxidation is independent of the anode potential (k, and kd are chemical rate constants) and depends on the reactivity of organics (kJ, on its concentration and on anode material (kd).

High ICE can be obtained with reactive organics and with anodes having low rate for the decomposition of their higher oxide.

2.2. Combustion of organics, equation (5)

For the combustion of organics, high concentration of hydroxyl radicals on the anode surface must be present. This is the case when the rate of hydroxyl radicals formation, equation (l), is much faster than the rate of oxygen transition into oxide lattice, equation (2).

RH + OH + R + H,O;

Reaction of the organic radical with dioxygen formed at the anode:

R+O,+ROO;

Further abstraction of a hydrogen atom with the formation of an organic hydroperoxide (ROOH) and another organic radical.

The following relation must be satisfied: ROO + RH + ROOH + R.

Since the organic hydroperoxides formed are relatively unstable, decomposition of such intermediates often leads to molecular breakdown and formation of subsequent intermediates with lower carbon numbers. These scission reactions continue rapidly until the formation of carbon dioxide and water.

k,CMO,I ,> k2[ 1. Thus efficient oxide anodes for combustion of organics must have a large number of active sites for the adsorption of hydroxyl radicals and must have a very low concentration of oxygen vacancies in the oxide lattice.

Oxides in which the oxidation state of the cation is the highest possible and/or which contains an excess oxygen in the oxide lattice (this can be achieved for example by doping the oxide with another metal oxide in which the oxidation state is higher than the base oxide) can be considered as oxides (or mixture of oxides) at the surface of which the hydroxyl radicals are accumulated and favour the combustion of organics.

The current efficiency for the combustion of organics depends on the relative rate of combustion of organics, equation (5), to the rate of oxygen evolution (side reaction) by discharge of the adsorbed hydroxyl radicals, equation (3):

rate of combustion of organics = Z, kJOH][R], , rate of 0, evolution by discharge of OH =

WOW, where :

ZC

k,

ko

= stoichiometry factor for complete combustion of organics;

= electrochemical rate constant for the combustion of organ& equation (5);

= electrochemical rate constant for 0, evolution, equation (3);

COH] = steady-state concentration of adsorbed hydroxyl radicals at the oxide anode; and

PI, = concentration of organics at a given time,

The Instantaneous Current Efficiency (ICE) can be given by the relation

ICE = z, kCR1,

ZckCRl, + k, (10)

This relation shows that the ICE for the combustion of organics depends on the nature of organ&, on its concentration, on the anode material and on the anode potential. High ICE for the combustion of organics can be obtained with anodes having low electrochemical activity for 0, evolution by discharge of hydroxyl radicals, equation (3).

We have to note that it is very probable that dioxygen participates also in the combustion of organics according to the following reaction schema:

Formation of organic radicals by a hydrogen abstraction mechanism :

3. EXPERIMENTAL DETAILS

3.1. Detection of hydroxyl radicals during electrolysis

The direct detection of hydroxyl radicals formed by water discharge at the oxide anode, equation (l), by electron spin resonance (esr) is possible only if the OH radicals are produced in relatively high concentration in the esr cavity by in situ electrolysis.

The indirect technique for the detection and iden- tification of low concentration of OH radicals involves trapping of the OH radical by an addition reaction (spin trap) to produce a more stable radical (spin adduct):

OH + spin trap -+ spin adduct.


A number of OH radical spin traps are available in the literature[12] but N,N-dimethyl-p-nitrosoaniline (RNO) has the following advantages[ 131:

(i) The reaction of RN0 with OH radicals has been reported to be very selective as neither singlet oxygen (0,) nor various Peroxo compounds destroyed the chromophoric group of RNO[ 131.

(ii) The high rate of the reaction with OH radicals (k = 1.2 x 101oM-ls-l).

(iii) The ease of application as one merely observes the bleaching of the sensitive adsorption band at 440nm(& = 3.44 x 10-4M-1cm-i).

Another advantage of RN0 for the detection of OH radicals formed by water electrolysis is that RN0 is electrochemically inactive at Pt, IrO, and SnO, anodes as has been shown by cyclic voltam- metry measurements; similar results have been reported in the literature at Pt and PbOz anodes[14]. In our experiment, RN0 has been used as spin trap and the bleaching of the yellow colour was measured during electrolysis:

RN0 + OH + R-NO

OH 3.1.1. Electrolytic cell and measuring method. A

two-compartment cell of 50ml capacity was used, the anode was in the form of plate (4cm*) and the cathode was a platinum spiral enclosed in a lOm1 porous porcelain pot; stirring was provided by a magnetic bar. The anodic compartment contains also a Fiber-optic spectrophotometer (Guide wave, Inc. Model 260) for the measure of the bleaching of RN0 at 440nm during constant current electrolysis with different electrodes. Owing to the large extinc- tion coefficient of RN0 (in neutral and alkaline solution) it was possible to measure accurately very small changes in concentration. The screening test of anodes has been carried out in phosphate buffer (pH = 7.1) containing 2 x 10m5 moldme RNO. All experiments were done at room temperature (25C).

In order to study the OH radical reaction with phenol (PhOH), studies were carried out at pH9 (Na,B,O, solution) in the presence of both, RN0 and PhOH in the electrolyte.

Under these conditions, RN0 and PhOH are simple competition reactions for OH radicals:

koH RN0 + OH - product,

kom PhOH + OH -product.

For competitive reactions it is easily shown that:

1 (11) where :

G(-RNO) = bleaching rate of RN0 in the presence of phenol ;

Go = bleaching rate of RN0 in the absence of phenol;

(PhOH) = phenol concentration in the electrolyte; and

(RNO) = RN0 concentraton in the electrolyte.

Thus a plot of l/(G(-RNO) vs. (PhOH)/(RNO) should yield a straight line with slope (l/G,) .(ko,/k,,), from which the relative rate constant for the reaction of OH radicals with phenol (/con) can be calculated.

3.2. Electrochemical oxidation of organics

3.2.1. Determination of the current efficiency. The Instantaneous Current Efficiency (ICE) for the electrochemical oxidation of organics with simultaneous oxygen evolution is given by the relation:

ICE = 10 - Uo,), IO

where:

= electrolysis current; and Ir,,), = partial current for 0, evolution at a given

time, t.

Two methods have been used for the determination of the partial current for 0, evolution; the Oxygen Flow Rate (OFR) method (in which the oxygen flow rate was measured during electrolysis) and the Chemical Oxygen Demand (COD) method (in which the Chemical Oxygen Demand was measured during electrolysis). From the ICE-t curve, the average current efficiency (defined as the Electro- chemical Oxidation Index, EOI) and the Electro- chemical Oxygen Demand (EOD) can be calculated. Details concerning these methods are given in a previous paperf61.

3.2.2. kectiode material, electrochemical cell and analvsis. The Ti/IrO, electrode was oreoared bv the thermal decomposition technique which consists of the following steps: dissolution in isopropanol of the coating components; varnish application on the pretreated titanium base by brush; drying at 80C; thermal decomposition; cooling and repeating the above operations until the desired amount of the coating is reached, finally post-heat treatment for 1 h. Many more details concerning electrode preparation and characterization are given elsewhere[l& 163.

The SnO, film electrodes doped with Sb were pre- pared on titanium base metal by Stucki by the stan- dard spray hydrolysis method; the best composition of the spray solution found by Stucki was log SnCI, x SH,O, 0.1 g SbCl, in lOOm1 of ethanol; details of

the preparation are given elsewhere[ 171. A two compartment cell of 150ml capacity was

used; the anode was made of Pt, Ti/IrO, or Ti/SnO, , and the cathode was a platinum spiral enclosed in a 10 ml porous porcelain pot; stirring was provided by a magnetic bar.

The disappearance of phenol and the appearance of its oxidation products were monitored by HPLC (Shimazu 8A). The progress of the electrochemical oxidation was monitored by measuring the Total Oxygen Carbon (TOC, XERTEX, Dohrman) and the Chemical Oxygen Demand (COD, Hach Dr 2000).

The oxygen formed in the anolyte during electrolysis was measured by a gas burette and analysed by gas chromatography.

Electrochemical conversion/cc rmbustion of organic pollutants

4. RESULTS AND DISCUSSION

4.1. Anode screening for the selective oxidation/combustion of organics

According to the proposed model (Fig. 1) high concentration of OH radicals on the anode surface favours complete combustion of organics.

For screening tests of anodes we have used RN0 as spin trap of OH radicals (see experimental part). Figure 2 shows the adsorption spectrum of aqueous RN0 solution (2 x 10-5moldm-3) at pH = 7.1 during galvanostatic electrolysis (20 mA cm - ?) with different anodes.

With Pt and Ti/IrO, anodes, there is only a slight decrease in optical density at 440nm during electrolysis contrary to the Ti/SnO, anode, for which there is a rapid decrease in the optical density.

The rate constant ken between phenol and OH radicals has been calculated from Fig. 3 using equation (11). The calculated value (3 x 10s M - s- ) is in good agreement with those reported in literature from y-radiolysis studies[18].

These results show that there is accumulation of OH radicals at the SnO, anode surface contrary to IrO, and Pt anodes for whichthe surface OH radical concentration is almost zero. Thus, according to the proposed model (Fig. l), the SnO, anode favours complete combustion contrary to the IrO, and Pt anodes which favour selective oxidation.

30

[PhOH)/[RNO]

Fig 3. Determination of the rate constant between phenol and OH radicals at pH 9 using equation (11). NB : The rate constant between RN0 + OH has been taken as

1.2 x lOM-s-t.

tion of phenol was studied at platinum anode (at the surface of which an oxide PtO, is formed during oxidation); the obtained results[6] have shown that the current elkiency for phenol oxidation is independent of current density (and anode potential) but increases with phenol concentration and with increasing pH. It has also been found[5] that at Pt anode benzene derivatives which contain electron withdrawing groups (-COOH, -NO,, -SO,H) are oxidized with low current efficiency, contrary to benzene derivatives of which the substituents are

4.2. Electrochemical oxidation of organics

To confirm the proposed mechanism for the electrochemical conversion/combustion of organics with simultaneous oxygen evolution (Fig. 1) and the screening test using RN0 (see Section 4.1) the oxidation of organ& has been investigated at platinum (selective oxidation) and SnO, (combustion) anodes.

4.2.1. Selective oxidation (conversion) of organics. In a previous paper[6], the electrochemical oxida-

I I 372 436

A, nm

Fig. 2. Adsorption spectra of aqueous RN0 solution (2 x 10-smoldm-3) obtained at 5min intervals during 2 h galavanostatic electrolysis with different anode material: i = 20rnAcn1-~; pH = 7.1;

T = 25C.


0 20 40 60 80 100

Ah dm-

Fig. 4. Evolution of: (1) phenol; (2) aromatic intermediates; (3) aliphatic acids; and (4) CO, during the electrochemical oxidation of phenol at Pt anode: i = 50mAcm-*, T = 70C. pH = 2 (const). NB: The progress of electrolysis is expressed in specific electrical charge (Ahdm-s) and all

concentrations are expressed in mg Cl-.

electron donating (-NH,, -OH) which increase the reactivity of the benzene derivates and are oxidized with high current effkiency[5]. These results are in concordance with those expected using relation 8 (the influence of pH can be explained by the fact that the phenate ion is much more reactive than phenol).

Analysis of the oxidation products during electrolysis of phenol at platinum anode[6] shows that oxidation occurs in two steps: in the first step aromatic intermediates (hydroquinone, catechol, benzoquinone) are formed and in the second step aromatic ring opening occurs with the formation of muconic acid which is further oxidized to C4 and C, aliphatic acids (maleic, fumaric and oxalic acid) these acids are stable toward further oxidation. Figure 4 shows the evolution of phenol, aromatic intermediates and aliphatic acids (expressed in mgC l- ) during the electrochemial oxidation of phenol at Pt anode; the amount of CO* (mg C l- ) also formed is given in the same figure.

4.2.2. Combustion of organics. The electrochemical oxidation of phenol has been studied using SnO, anodes[8]. Analysis of the oxidation products during electrolysis has shown that aliphatic acids (fumaric, maleic, oxalic) are the main intermediate products which are further oxidized to CO? (combustion).

0 20 40 60 80 100

Ah dmm3

Fig. 5. Evolution of: (1) phenol; (2) aromatic intermediates; (3) alinhatic acids; and (4) CO, during the electrochemical . , oxidaiion of phenol at thk Sn6* anode: for conditions see.

Fig. 4.

Figure 5 shows the evolution of phenol, aromatic intermediates and aliphatic acids during the electrochemical oxidation of phenol at the SnO, anode; the amount of CO, also formed is given.

5. CONCLUSION

A simplified mechanism for the electrochemical selective oxidation or combustion of organics with simultaneous oxygen evolution is presented. Accord- ing to this mechanism, selective oxidation of organics occurs with electrodes forming the so-called higher oxide MO,, 1 (chemisorbed active oxygen) and combustion occurs with electrodes at the surface of which OH radicals are accumulated (physisorbed active oxygen).

The detection of OH radicals formed by water discharge at different anodes using N,N-dimethyl-p- nitrosoaniline (RNO) as a spin trap has shown that at Pt and IrO, anodes the surface OH radical concentration is almost zero, contrary to the SnO, anode for which there is accumulation of OH radicals at its surface. Thus, according to the proposed mechanism, the SnO, anode favours complete combustion contrary to the IrO, and Pt anodes which favour selective oxidation. The model has also been confirmed by preparative electrolysis.

Acknowledgement-The author acknowledges the assist- ance of M. Vincent Schaller for the hydroxyl radical spin trap measurements.

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Electrocatalysis in the electrochemical conversioncombustion of organic pollutants for w.pdf