ELECTROCATALYTIC OXIDATION PROCESSshodhganga.inflibnet.ac.in/bitstream/10603/33851/5/chapter5.pdf · 127 chemical for the synthesis of C-Ce0.8Gd0.2O2, C-Ce0.8Nd0.2O2 and C-Ce0.8Sm0.2O2

5

ELECTROCATALYTIC OXIDATION

PROCESS

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126

CHAPTER - 5

ELECTROCATALYTIC OXIDATION PROCESS

5.1 INTRODUCTION

Nanomaterials play an important role in many aspects of science and

technology, of which molecular recognition is becoming increasingly important for

environmental protection. Oxide materials exhibiting pyrochlore structure have

attracted scientific and industrial community due to their interesting properties and

potential applications. These compounds have high photocatalytic activity, piezoelectric

behavior, ferro-and ferrimagnetisms, order/disorder transformations, high thermal

expansion coefficient, range of electrical and ionic conductivities that include metallic,

semiconducting and superconductivity (Mims, 1995; Subramanian, 1996; Kennedy and

Vogt, 1996; Vassen et al., 2000; Eberman, 2002; Yonezawa et al., 2004). Nowadays, a

single process alone may not be adequate for the treatment of all the various organic

compounds. Hence, the researchers have attempted to combine two or more treatment

methods for the complete and successful removal. The combinations of electrochemical

oxidation and fenton reaction, ozonation, or photocatalysis have been extensively

studied as the pretreatment or mineralization process for the treatment of wastewater.

Semiconductor metal oxide nanostructures are highly attractive, so more attention has

been paid, because of their obvious optical and wastewater purification applications.

In this study, we have chosen IIIb-group metal complexes such as Gd3+, Nd3+ and

Sm3+ were prepared using citric acid combustion method and doped with cerium oxide.

Citric acid, cerium nitrate, gadolinium nitrate, Neodymium nitrate and samarium nitrates


127

chemical for the synthesis of C-Ce0.8Gd0.2O2, C-Ce0.8Nd0.2O2 and C-Ce0.8Sm0.2O2

nanostructures (Mangalaraja et al., 2009 and 2010). It is interesting that, we have

obtained sponge, platelets and flaky doped cerium oxide Gd3+, Nd3+ and Sm3+

nanostructures, respectively, from calcined at 700 °C for 2h and demonstrated

through X-ray spectroscopy, scanning electron microscopy, Raman spectrum and

FT-IR spectrum method.

5.2 CHARACTERIZATION OF CATALYST

5.2.1 XRD Patterns Studies

The X-ray diffraction patterns of the materials clearly identify the cubic (c),

tetragonal (t), and monoclinic (m) phases and their transformations with respect to the

calcining temperatures. The patterns also help to determine the crystallite size and the

relative amounts of the different phases. Figure 5.1 shows the XRD patterns of,

C-Ce0.8Gd0.2O2, C-Ce0.8Nd0.2O2 and C-Ce0.8Sm0.2O2 respectively, calcined at 700 °C.

In the case, single-phase cubic structure of doped cerium oxide Gd3+, Nd3+ and Sm3+

observed (JCPDS No.: 34-394), and structure was confirmed by position of the

different peaks at (111), (200), (220), (311), (222), (400), (331) and (420) lattice

planes. Since there were no peaks representing free Gd3+, Nd3+ and Sm3+ dopants, we

can confirm that the Gd3+, Nd3+ and Sm3+ dopant ions get substituted in the CeO2

lattice (Manglaraja et al., 2009).


128

2 0 3 0 4 0 5 0 6 0 7 0 8 0

(a )

(b )

(420

)(3

31)

(400

)

(222

)

(311

)

(220

)

(200

)

Inte

nsity

(a.u

)

2 θ (D e g re e )

(111

)

(c )

Figure 5.1 X-ray diffraction patterns of calcined C-Ce0.8Gd0.2O2, Ce0.8Nd0.2O2 and

C-Ce0.8Sm0.2O2 (at 700 ºC) powder

The peaks are significantly broader due to small crystallite size. The average

grain sizes were calculated from the XRD pattern according to the Scherrer equation

(3.6) (Cullity, 1978). The crystallite sizes were shown in Table 5.1 and found to be

18.11, 14.04 and 18.24nm, doped cerium oxide of Gd3+, Nd3+ and Sm3+ respectively.

Generally, high surface area to volume ratio enhances the catalytic activity

(Hoffmann et al., 1992). This can be explained in terms of an increase in number of

active sites per square meter (Xu et al., 1999), as well as yielding of radicals on the

catalyst surface.


129

Table 5.1 X-ray diffraction patterns for doped cerium oxide of Gd3+, Nd3+ and

Sm3+ nanoparicles

Sample 2θ h k l FWHM β Cos θ β Cos θ Kλ D (Å) AVG(nm)

Gd

3+do

ped

ceriu

m o

xide

28.6 111 0.5128 0.0089 0.969 0.0086 1.4481 167.05

18.11

33.1 200 0.6082 0.0106 0.9585 0.0101 1.4481 142.39

47.5 220 0.4863 0.0084 0.9153 0.0077 1.4481 186.49

56.3 311 0.5303 0.0092 0.8817 0.0081 1.4481 177.54

59.1 222 0.48 0.0083 0.8699 0.0072 1.4481 198.80

69.4 400 0.4889 0.0085 0.8221 0.0070 1.4481 206.53

76.6 331 0.5829 0.0101 0.7847 0.0079 1.4481 181.48

79 420 0.5705 0.0099 0.7716 0.0076 1.4481 188.57

Nd

3+do

ped

ceriu

m o

xide

28.5 111 0.6394 0.0111 0.9692 0.0108 1.4481 133.95

14.04

33 200 0.6413 0.0111 0.9588 0.0107 1.4481 135.00

47.3 220 0.6494 0.0113 0.916 0.0103 1.4481 139.55

56.1 311 0.7017 0.0122 0.8825 0.0108 1.4481 134.05

58.8 222 0.6983 0.0121 0.8712 0.0106 1.4481 136.45

69 400 0.5247 0.0091 0.8241 0.0075 1.4481 191.97

76.3 331 0.8218 0.0143 0.7863 0.0112 1.4481 128.46

78.6 420 0.8657 0.0151 0.7738 0.0116 1.4481 123.92

Sm 3+

dope

d ce

rium

oxi

de 28.5 111 0.4846 0.0084 0.9692 0.0081 1.4481 176.74

18.24

33 200 0.4379 0.0076 0.9588 0.0073 1.4481 197.71

47.4 220 0.4939 0.0086 0.9156 0.0078 1.4481 183.56

56.2 311 0.5227 0.0091 0.8821 0.0080 1.4481 180.04

58.9 222 0.4878 0.0085 0.8707 0.0074 1.4481 195.44

69.2 400 0.5029 0.0087 0.8231 0.0072 1.4481 200.54

76.4 331 0.6536 0.0114 0.7858 0.0089 1.4481 161.62

78.8 420 0.6552 0.0114 0.7727 0.0088 1.4481 163.96


130

5.2.2 Morphology and EDS Spectrum of Catalyst

Scanning electron microscopy (SEM) has been a primary tool for

characterizing the surface morphology and fundamental physical properties of the

catalyst. Figure 5.2 shows SEM images of doped cerium oxide of Gd3+, Nd3+ and Sm3+

shows morphologies like sponge, and flaky types. The microscopic observations

confirmed the powder also consisting of porous agglomerates. The chemical analysis

observed by the EDS attached with SEM for the calcined powders confirms that the

chemical composition is only C-Ce0.8Gd0.2O2, Ce0.8Nd0.2O2 and Ce0.8Sm0.2O2 respectively.

5.2.3 Raman Spectrum

It is known that the XRD technique for the bulk of the examined materials,

whereas the Raman technique, being based on a scattering effect, samples preferentially

the outermost layers of the examined materials. Raman studies were performed on

these nanomaterials at room temperature. It shows the presence of oxygen vacancies

in the materials and confirmed that the oxygen vacancy concentration. The Raman

spectrum of doped cerium oxide of Gd3+, Nd3+ and Sm3+ is characterized by peaks

resulting from scattering on optical phonon modes localized in the nanocrystal.

The position and the width of the peaks strongly depend on the size and structure of

the nanocrystal according to dispersion of localized modes. Figure 5.3 shows that

cerium oxide had shifted its position at 467, 461 and 464cm-1, and slight shifting

position at 559 and 611cm-1, this shifting due to doping of Gd3+, Nd3+ and Sm3+ is

observed in spectrum.


131

Figure 5.2 SEM with EDS images for the doped cerium oxide of (a) Gd3+ (b) Nd3+ and (c) Sm3+

(a) (b) (c)


132

The similar observation was reported by Taniguchi et al., (2008), Raman

spectroscopy confirms the information on the presence of nano - and micro clusters of

crystal silicon in various surroundings and it’s their size dispersion (Volodin, 1998).

No impurity peaks other than doped cerium oxide of Gd3+, Nd3+ and Sm3+ nanomaterial

were observed which shows the purity of the prepared samples.

100 200 300 400 500 600 700 800 900 1000

C-Ce0.8Sm0.2O2 - 700 0C

C-Ce0.8Nd0.2O2 - 700 0C

C-Ce0.8Gd0.2O2 - 700 0C

Rel

ativ

e In

tens

ity (a

.u)

Raman shift (cm-1)

(467

)(4

61)

(464

)

(559

)

(611

)

Figure 5.3 Raman spectra of doped cerium oxide of Gd3+, Nd3+ and Sm3+ after

thermal annealing at 700 °C


133

Raman first-order signals of bulk CeO2 with an intense peak at 464cm-1, two

weak peaks between 550 ~ 600cm-1 and second-order peaks at 261,

368, 670, 1184

and 1277cm-1. It is pointed out that major peak of 464 cm-1 shift to lower frequency if

the particle sizes are in nanoscale (Spanier et al., 2001). When the crystallite is larger

than 20 nm, there will be only one major shift at 463cm-1 in Raman spectroscopy,

however, with the crystal size decreasing below 10 nm, two more shifts at 270

and 315cm-1 appear and the intensities increase with the decreasing crystal sizes

(Wang et al., 2001).

5.2.4 FT-IR Spectrum

The results are summarized in Figure 5.4 and it shows the doped cerium oxide

of Gd3+, Nd3+ and Sm3+ respectively. The range between 3000 and 3750cm–1 with a

maximum at 3436cm–1, arises from the absorption of O–H groups. The second region

in the range between 1250 and 1750cm–1, originates from the absorption of COO–

(1620cm–1). It is difficult to assign the behavior of cerium doped Gd3+, Nd3+ and Sm3+

oxides by FTIR spectra as stated earlier by Kannan and Mohan (2001) in which the

appearance of peaks were observed from 1200 to 400cm-1. But the observed spectra

for Gd3+, Nd3+ and Sm3+ seems to be a noise pattern rather than the transmitted

intensity pattern. This is because C – phase (cubic) is stabilized over a very narrow

compositional range and also the structural distortion involving the displacement of

oxygen atom increases for the end member of the series.


134

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

Wavenumber (cm-1)

Sm3+

Nd3+

Gd3+

3440

.53

1596

.83

1348

.05

1025

.98

Figure 5.4 FTIR spectra of doped cerium oxide of Gd3+, Nd3+ and Sm3+ after

thermal annealing at 700 °C

5.3 ELECTROCATALYTIC OXIDATION MECHANISMS

Electro-oxidation of pollutants can occur directly on anodes by generating

physically adsorbed “active oxygen” (adsorbed hydroxyl radicals, •OH) or chemisorbed

“active oxygen” (oxygen in the oxidelattice, MOx+1) (Comninellis, 1994). This process is

usually called anodic oxidation or direct oxidation. The physically adsorbed “active

oxygen” causes the complete combustion of organic compounds (R), and the chemisorbed

“active oxygen” (MOx+1) participates in the formation of selective oxidation products:


135

R + MOx (•OH)z → CO2 + zH+ + z e + MO x (5.1)

R + MOx+1 → RO + MOx (5.2)

In general, •OH is more effective for pollutant oxidation than O in MOx+1.

Because oxygen evolution reaction (Dubpernell, 1978) can take place at the anode,

high over potentials for O2 evolution is required in order for reactions (5.1) and (5.2)

to proceed with high current efficiency. Otherwise, most of the current supplied will

be wasted to split water. The anodic oxidation does not need to add a large amount of

chemicals to wastewater or to feed O2 to cathodes, with no tendency of producing

secondary pollution and fewer accessories required. These advantages make anodic

oxidation more attractive than other electro oxidation processes. A free radical

reaction involves molecules having unpaired electrons. The radical can be a starting

compound or a product, but radicals are usually intermediates in reactions. Most of

the reactions discussed to this point have been heterolytic processes involving polar

intermediates and/or transition structures in which all electrons remained paired

throughout the course of the reaction. In radical reactions, homolytic bond cleavages

occur, with each fragment retaining one of the bonding electrons.

5.3.1 Electrocatalytic Oxidation

5.3.1.1 UV–Visible spectra study

A spectrophotometric scanning (200-700nm) of Reactive Orange 107, Reactive

Yellow 186 and Reactive Blue 198 have a maximum absorbance peak at 408, 425 and

625nm, respectively. Typical UV–Visible spectra for untreated and treated dye solutions


136

have been shown with changes in absorbance spectra in Figs 5.5 – 5.7. The dye

decolorization was apparent by a gradual decrease in absorbance at λ max. Absorbance

spectral pattern and decrease rate was not similar to each other and to initial one (0 min)

for each applied catalyst showing a change in affinity for dye for this nanocatalyst.

UV–Visible spectra for Reactive Orange 107 Finger 5.5 shows that the initial

spectra the wavelength of the maximum absorbance (λ max) was 408 nm in the

visible region and corresponds to the azo group. In addition, the aromatic structures

absorbance was observed between 200 and 300nm, in the UV region. Figure 5.5 a-c

shows that the UV-visible spectra of the RY 107 in aqueous solution using doped

cerium oxide of Gd3+, Nd3+ and Sm3+ nanoparticles, separately for 0 to 20min (time at

which maximum decolorization of dye was achieved) was scanned from 200 to

800nm. Maximum absorbance was observed at 408nm which is λmax of the dye

Reactive Orange 107. After 20min of electrocatalytic oxidation there was a significant

decrease in peak intensity almost equal to base line showing complete decolorization.

However, a small peak at 286 nm shows the formation of some phenolic derivative as

dye metabolites. Decrease in intensity of visible peak at λmax indicates disruption of

chromophoric group i.e. azo group. However, the peaks between 220 to 260nm at

20min of electrolysis were the absorption of * transitions due to amino group

bonded to naphthalene ring present in dye molecules (Wu et al., 2000; Xiong et al.,

2001). Appearance of a less intense peak at 286nm refers to the formation of some

phenyl derivative as metabolite but it can’t be amino substituted as they absorb at

lower side of UV-spectra. In the case of a base line absorbance occurs with no peak at


137

λmax which suggests breakage of azo nuclei leading to some aromatic intermediates

(phenyl or naphthyl) which finally could be cleaved leading to final products,

alcohols, or aliphatic hydrocarbons or even it could be mineralized completely

leading to CO2 and H2O during entire reaction.

Similarly in case of Reactive Yellow 186, maximum decolorization was

achieved after 20min of electrolysis for doped cerium oxide of Gd3+, Nd3+ and Sm3+

respectively. Figure 5.6 a-c shows the spectrophotometer analysis of 0 to 20min time

where maximum decolorization was attained λmax of RY 186 is 425nm. At start there

was a single major peak of absorbance at λmax showing no decolorization with

catalyst. After 20min, there was no other peak as well but decrease in absorbance

intensity at λmax was recorded which shows that color removal have been occurring

with no production of major metabolite. The results clearly showed that the

absorbance reduction rate at 425nm. The fact of this phenomenon was primarily due

to the cleavage of the chromophore structures in dye molecules and resulted in rapid

decolorization (Hao et al., 2000).

Where as in the case of Reactive Blue 198 Figure 5.7a-c displays the UV–vis

spectral changes of RB 198 in doped cerium oxides of Gd3+, Nd3+ and Sm3+ systems.

Before treatment, the UV–vis spectrum of RB 198 has two main absorption bands –

one in the UV region (297nm) and one in the visible region (625nm). After 20min

complete disappearance of absorbance peak is observed in all catalyst used.

Disappearance of the peak at 625nm demonstrates that dye structure has been broken

significantly especially conjugated molecular structure based on azo group has been


138

decomposed which was mainly responsible for the color of dye. For all of the systems

tested herein, the intensity of absorption at 625nm declines extremely rapidly;

however, the UV bands at 297nm disappeared more slowly than did the visible band.

The disappearance efficiency of the visible band is approximately double that of the

UV band. The presence of reactive group show that the C-Cl bond on the 1,3,5-

triazine ring is the weakest bond and the easy cleavage of this bond in the presence of

catalyst indicates that C-Cl bond on the ring is very susceptible to the hydroxyl

radical attack. Sakthivel (2001) observed a similar result for Reactive Black 5 in a

solar photocatalytic degradation system.

A marked difference in intensity of absorbance peak after complete

decolorization in visible region explains clearly that mechanism of decolorization for

the dyes. Even so, azo group is destroyed in electro oxidation (Figs. 4.7 a-c

respectively). Initial to the end of reaction (20min) there was a significant change in

peak intensity almost equal to base line showing decolorization of dye occurred

completely.


139

200 300 400 500 600 7000.0

0 .5

1 .0

1 .5

2 .0

2 .5

3 .0

3 .5

Abs

orba

nce

W avelength (nm )

0 m in 2 m in 5 m in 10 m in 15 m in 20 m in

200 300 400 500 600 7000 .0

0 .5

1 .0

1 .5

2 .0

2 .5

3 .0

3 .5

Abs

orba

nce

W aveleng th (nm )


200 300 400 500 600 7000 .0

0 .5

1 .0

1 .5

2 .0

2 .5

3 .0

3 .5

Abs

orba

nce

W aveleng th (nm )


Figure 5.5 UV -Vis spectra of Reactive Orange 107 during electrocatalytic oxidation at pH 9.4, NaCl concentration of 0.08 M and electrolysis of 20 minutes

A B

C


140

2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 00

1

2

3

4

5A

bsor

banc

e

W a v e le n g th (n m )

0 m in 2 m in 5 m in 1 0 m in 1 5 m in 2 0 m in

2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 00

1

2

3

4

5

Abs

orba

nce



2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 00

1

2

3

4

5

Abs

orba

nce



Figure 5.6 UV -Vis spectra of Reactive Yellow 186 during electrocatalytic oxidation at pH 3.9, NaCl concentration of 0.11 M and electrolysis of 20 minutes

C

BA


141

2 00 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 00

1

2

3

4

5

Abs

orba

nce

W av elen g th (n m )


200 300 400 500 600 700 8000

1

2

3

4

5

Abs

orba

nce

W avelength (nm )


2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 00

1

2

3

4

5

Abs

orba

nce

W a v e le n th (n m )


Figure 5.7 UV -Vis spectra of Reactive Blue 198 during electrocatalytic oxidation at pH7, NaCl concentration of 0.13 M and electrolysis of 20 minutes

C

B A


The catalyst generates the powerful radicals in this process thus, the catalyst

into an oxygen atom, eventually yielding more hydroxyl radicals can enhance the

decolorization of dyes. However, a small peak between 200 and 300nm shows the

formation of some carboxylic acids derivative as dye metabolites. Decrease in

intensity of visible peak at λmax indicates disruption of chromophoric group i.e. azo

group. In the case of visible regain, a base line absorbance occurs with no peak

between 200-700nm which suggests breakage of azo nuclei leading to some aromatic

intermediates which finally could be cleaved leading to final products, alcohols, or

carboxylic acids accumulated at the final stage of the oxidation or even it could be

mineralized completely leading to CO2 and H2O during entire reaction.

The reason behind this shifting is that conjugated π electron system of azo dye

refers to lower energy of absorption than respective aromatic amines. This sort of

shifting has been earlier reported in the case of methyl orange which was converted to

sulfanilic acid and N, N dimethyl aniline (Hsuch and Chen, 2007). If maximum

wavelength of dye (λ max) shifts from longer wavelength to shorter wavelengths with

the course of time of incubation (it means it will shift to UV region) which

corresponds to most of aromatic amines intermediates (Hsuch and Chen, 2007).

However, the peak was observed in UV region for the three dyes under study confirm

some metabolic by products formed during electrocatalytic oxidation process. This is

represents the residual TOC present in the effluent.


143

5.3.1.2 Mineralization studies

Mineralization of azo dyes by electrocatalytic oxidation processes has been

investigated. Mineralization of target pollutant is most important before discharging

the pollutant into the ecosystem. During the treatment process, the substrate undergoes

degradation and forms many intermediate products, which can sometimes be more

toxic than the parent compound. Therefore, the complete mineralization of the substrate

should be ensured before discharging the polluted water into the ecosystem. A rather

straight forward way of measuring oxidation process of an organic compound is the

determination of carbon content of the oxidation product mixture and this can be obtained

by monitoring the TOC content of the treated solutions. The comparative effects of

electro oxidation and electrocatalytic oxidation on the changes of TOC in aqueous

solutions of dye are presented in Table 5.2. Results of initial TOC values were about

48.26, 71.35 and 81.61 ppm, respectively for RO 107, RY 186 and RB 198 dyes.

The oxidation process of the three dyes using consent electrode surface area of 71.5cm2

and current density of 34.96 mAcm-2

An interesting observation was noted during the mineralization of Reactive

Orange 107 (Fig. 5.8) as with catalyst (doped cerium oxide of Gd3+, Nd3+ and Sm3+)

and without catalyst oxidation process. The TOC measurement could be performed as

well in experiments carried out with (500µM) were initial TOC concentration 49 ppm

in aqueous solutions by addition of Gd3+, Nd3+ and Sm3+ doped cerium oxide catalyst

under 20min mineralization were achieved of 32.8, 35.7 and 32.9 ppm respectively

(Table 5.2).


144

Table 5.2 Effect of electro oxidation and electrocatalytic oxidation on total organic carbon removal in aqueous Reactive

dyes solution

Name of dye

Before oxidation

After electro oxidation After electrocatalytic oxidation Electro

oxidation Electrocatalytic oxidation

TOC (ppm) ηTOC (%)

0 min 10min 20min 10min 20min

10min 20min 10min 20min

Gd3+ Nd3+ Sm3+ Gd3+ Nd3+ Sm3+. Gd3+ Nd3+ Sm3+ Gd3+ Nd3+ Sm3+

Reactive Orange

107 48.26 38.92 36.66 33.74 36.30 34.45 32.44 31.01 32.37 19.35 24.03 30.0 24.8 28.6 32.8 35.7 32.9

Reactive Yellow

186 71.35 68.45 48.46 60.39 61.46 59.58 33.74 48.56 45.98 4.06 32.08 15.4 13.9 16.5 55 31.9 53

Reactive Blue

198 81.61 69.12 61.8 63.61 67.06 61.81 47.71 49.29 44.28 12 24.27 22.1 17.8 24.3 41.5 40.0 45.7

η – denotes the reduction of TOC values (%).

.


145

During electro oxidation process TOC values were 37ppm at 20min. If compare

the both case TOC removal were obtained 24% and 36% for electro oxidation and

electrocatalytic oxidation, respectively at pH 9.4. Moreover, the color and TOC

removals indicates that the electrogenerated active species are quite selective for the

oxidation of the chromophore of the dye, and the oxidation of H2O or OH– to form

O2– with high activity and indirectly degrade the dye molecules in aqueous solution.

In the former the catalyst merely plays the role of an electron carrier (mediator)

whereas the transitory formation of catalyst-substrate adduct occurs in the latter

(Cheung et al., 2007).

Figure 5.8 Effect of electrocatalytic oxidation for the removal of TOC in RO 107

In addition, the electrons may then react with species adsorbed on the surface,

yielding radicals such as O2– as a result of the presence of hydroxyl groups, water, and

oxygen at the surface of the doped cerium oxide Sm3+ > Gd3+ >Nd3+ particles.

The results of TOC analysis indicate that organic compounds are not completely

oxidized to CO2 and water.


146

Similarly for RY 186, the decrease in TOC values were 32 and 55% after

20min for electro oxidation and electrocatalytic oxidation, respectively. The TOC

result shown in Figure 5.9 indicates the degree of mineralization of 32% (49 ppm)

achieved using electro oxidation process at acidic pH 4 after 20min. A significant

degree of mineralization was observed during electrocatalytic oxidation process.

The results of Reactive Yellow 186 degradation by catalytic doped cerium oxide of

Gd3+, Nd3+ and Sm3+ using different metal oxide catalysts have been summarized in

Table 5.2. A lesser degree of mineralization was obtained using doped cerium oxide

of Nd3+ 49 ppm (32% in 20min), and the significant mineralization was observed with

doped cerium oxide of Sm3+ 46 ppm (53% in 20min). Nevertheless, a degree of

mineralization of 55% (34 ppm) was achieved by the reaction over Gd3+doped cerium

oxide. It should be noted that 24% of such achievement was observed electrocatalytic

oxidation accomplished within 20 minutes. These results indicate the role of doped

cerium oxide of Gd3+, Nd3+ and Sm3+ in degradation of Reactive Yellow 186. In other

words, a doped cerium oxide is able to generate to hydroxyl radicals which enhance

the oxidizing activity of the system.


147

Figure 5.9 Effect of electrocatalytic oxidation for the removal of TOC in RY 186

In the case of Reactive Blue 198, is the electro oxidation processes the TOC

removal of were 12 and 25% at 10 and 20 min, respectively. The decolorization rate

substantially exceeds the TOC removal rate. Despite RB 198 was over 99%

decolorized, the TOC removal efficiency was 40 between 46% in all experiments.

This means that the degradation of Reactive Blue 198 and readily oxidizable

intermediates occur at the same time. The order of mineralization rates was Nd3+ >

Gd3+ > Sm3+, but the extents of TOC removal did not vary significantly among these

systems. In the mineralization of RB 198, the triazine ring was converted to cyanuric acid,

which was very stable as reported by Hu et al., (2003).


148

Figure 5.10 Effect of electrocatalytic oxidation for the removal of TOC in RB 198

The selective oxidation of organics on noble oxide catalytic anode was

attributed to the formation of “higher oxides” (Panizza and Cerisola, 2004) via

adsorption of hydroxyl radical and its interaction with the oxygen already present in the

oxide with the possible transition to higher oxide. Better removal of organics in the

presence of metal oxides could be attributed to the generation of •OH, O2•−, HOO•, HOCl

and H2O2 which are stronger oxidants compared to oxygen. Also, due to their higher

solubility, the build up concentration will become so high that the refractory organics could

be easily oxidized. From the present study it is evident that the degradation of organics in

presence of nanocatalyst was better compared to without nanocatalyst. The experimental

results reveal that the mineralization of all three reactive dyes was incomplete in electro

oxidation process, but in electrocatalytic oxidation process with addition of Gd3+, Nd3+ and

Sm3+ doped cerium oxides systems comparatively significance results were observed. This

study suggests that the rapid decolorization of the dye was followed by a much foster

mineralization of the subsequently formed intermediates.


149

5.3.1.3 FT-IR studies

To investigate the phenomena occurring during the electrocatalytic oxidation

process in RO 107, RY 186 and RB 198 analysis were carried out using FT–IR

spectroscopy. Compare the dye powder and electro oxidation process briefly

discussed in chapter IV section 4.2.11, Figures 4.18 – 4.20 respectively, shows the IR

spectra of before and after electro oxidation process at room temperature. In this

section electrocatalytic oxidation processes for all the three dyes used an experiment

conditions noted in Table 3.4.

The FTIR spectra of the dyes evidence absorption bands characteristic to the

functional groups (–NH2, –OH, –SO3Na, –COONa, aromatic nuclei). The main

absorption bands may be grouped as follows: the –NH2 group evidences intense

vibration bands in the 3300–3500cm–1 region, and two intense deformation bands

between 1590–1650 and 800–900cm–1, respectively; the phenolic group evidences

vibration bands of the O–H bond between 3200–3500 and 800–900cm–1 (which

overlaps with the ones of the –NH2 group), and intense vibration bands, characteristic

to the C–OH bond, between 1030–1085 and 1180–1260cm–1. The –SO3Na group

evidences an intense band between 1120–1230cm–1, while the bands characteristic to

the carboxylate group are present between 1700–1780cm–1 (the C=O bond) and

respectively 1050–1120cm–1 (the C–O bond). The C–H bonds show bands between

2800–3100cm–1, while the main bands characteristic to the aromatic nuclei appear

between 1400–1500 and 800–900cm–1 (Ayram and Mateescu, 1978).


150

FTIR spectrum of Reactive Orange 107 in anode treatment with Gd3+, Nd3+ and

Sm3+ doped cerium oxide, after electrolysis (Figure 5.11) the formation of peaks at

1648 cm-1 was observed, which can be assigned as the stretching of C=O in

carboxylic groups, aldehydes or ketones (Bauer et al., 2001; Zhang et al., 2005;

Li et al., 2006a). Furthermore, a new absorption peak of 977.7 and 619.1cm−1

indicates the aliphatic secondary amine, C–O–H stretching and •OH. This suggested

the cleavage of the vinylsulfonyl group (-SO2CH2CH2OSO3Na) after electrocatalytic

oxidation.

In similar way of Reactive Yellow 186, FTIR spectra suggest that treatment

with Gd3+ Nd3+ and Sm3+ doped cerium oxide, after electrolysis (Figure 5.12) the

formation of peaks at 1633.4, 981.1 and 620.9cm−1 indicates the aliphatic secondary

amine, C–O–H stretching and •OH. Raghu et al., (2009) reported that peak 620cm−1

suggests the OH, which may generally be present in the hydrogen peroxide.

The intensities of all the bands were attenuated, as expected because of the decrease,

result shown in Figure 4.19b and hence in moreover, there were no clear differences

between the relative intensities.


151

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0

% T

rans

mitt

ance

W a v e n u m b e r ( c m - 1 )

S m 3 +

N d 3 +

1128

.47

487.

92

977.

77

1390

.48

1648

.90

3455

.95

G d 3 +

Figure 5.11 FTIR spectrum of RO 107 after electrocatalytic oxidation (a) Gd3+

(b) Nd3+ and (c) Sm3+ doped cerium oxides

In the case of Reactive Blue 198 (Fig. 5.13), the wide absorption band at 3000-

3700cm-1 in the FTIR spectrum of all three samples, it in surface OH signal is

interfered with adsorbed water (Shu et al., 1997). Moreover, strong absorption bands

of 1367cm-1 were observed and assignable to C=N of triazine nucleus. Consisted of

peaks from sulfonate (1138.8cm-1; SO3

- and 900cm-1; R-SO

3

-Na

+) and chloride (600cm-1).

The results revealed that the ring structures of RB 198 were definitely degraded

because the peaks of the wave number between 670 and 870cm-1, which denote

aromatic rings, gradually disappeared with the increase of reaction time.


152

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0

% T

rans

mitt

ance

W a v e n u m b e r ( c m - 1 )

S m 3 +

N d 3 + 1160

.47

489.

83620.

90

981.

58

1384

.6316

33.4

1

3455

.81

G d 3 +

Figure 5.12 FTIR spectrum of the Reactive Yellow 186 after electrocatalytic

oxidation (a) Gd3+ (b) Nd3+ and (c) Sm3+ doped cerium oxides

Compare to optimum condition of electro oxidation and electrocatalytic

oxidation with Gd3+, Nd3+ and Sm3+ doped cerium oxides has highest dye removal

efficiency due to its produced powerful radicals. This variation in the FTIR spectra

can be explained by the complete degradation of organic compound and formation of

other intermediate organic compounds.


153

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0

% T

rans

mitt

ance

W a v e n u m b e r ( c m - 1 )

S m 3 +

G d 3 +

N d 3 +

475.

29

627.

76

964.

701138

.82

1367

.5216

39.5

2

3457

.88

Figure 5.13 FTIR spectrum of the Reactive Blue 198 after electrocatalytic

oxidation (a) Gd3+ (b) Nd3+ and (c) Sm3+ doped cerium oxides

5.3.1.4 GC MS analysis

The monitoring of UV-Vis spectra gave information about nature of color

removal with emphasis and shifts in maximum absorbance confirming presence of

some chemical change in extended conjugated double bond and chromospheres

groups of dyes. It did not provide any clue about products formed as a result of that

degradation. In order to identify dye metabolites formed after complete dye

decoloration GC/MS analysis were performed.

The intermediate compounds formed during the degradation of RO 107, RY

186 and RB 198 with cerium oxides doped lanthanide ions– Gd, Nd, and Sm were

dependent on both the role of the electrocatalytic oxidation and used identified by


154

GC/MS. Samples of electrocatalytic oxidation at 20min time intervals were collected

during the electrolysis of all the three dyes. It is necessary to discuss the role that

electrocatalyst and oxidation played in the combined process. In the above, it is found

that the removal of color and TOC is mainly attributed to the radicals’ is mainly due to

electrochemical generation of oxidant species (ie. hydroxyl radical, hydrogen peroxide,

hypochloric acid or hypochloric ions, depending on the pH), reaction pathway in

electrocatalytic oxidation. Owing to the unique actions of electrocatalytic oxidation,

several pathways for OH• radicals production are coexisted in the system, including the

electro generated surface OH• (Brillas, 1998a; Ezerskis and Jusys, 2001).

As shown in Table 5.3, the by produces were detected at 20min and at pH 9.4,

for both electro oxidation and electrocatalytic oxidation. Figure 5.14 also shows that,

after 20min of oxidation process, the major fragments generated in both degradation

process. Two different pathways such as S1 and S2 (Fig. 4.22) were proposed for the

degradation of RO 107. The degradation of the dye under catalytic conditions causes

significant mineralization which results in the formation of substituted N-(3-Amino-

phenyl)-acetamide, Benzene-1,3-diamine, Benzene-1,3-diamine, Benzenediazonium,

Benzene, Phenol and other lower molecular weight compounds. The chromophore in

the dye molecule should be split in the first step. The bonds C–N and C–N in the RO

107 molecule were probably cleaved by free radical attack, which led to the

decolouring of RO 107 in the bulk solution. Earlier studies suggested that the azo group

was decomposed due to the elimination of molecular nitrogen (Gahr et al., 1994).


155

N N NHCOCH3

NH2NaO3S

NaO3OSH2CH2CO2S

N2+

Benzenediazonium

OHPhenol

HN O

N-Phenyl-acetamide; compound with methane

NH2

NH2

Benzene-1,3-diamine

Benzene

C4H3 C2H2+

lower molecular weight compounds

C.I. Reactive Orange 107CAS No: 90597-79-8

N N N2NH NH4

+ + NO3-

SO3H SO42-

O2 -/ HOO / OH / HOCl / OCl- / Cl2

NH2

HN O

N-(3-Amino-phenyl)-acetamide

Figure 5. 14 The probable degradation pathway of C.I Reactive Orange 107

during electrocatalytic oxidation


156

Table 5.3 List of intermediate compounds generated during electrocatalytic oxidation

of RO 107

S. No Compound Molecular

weight Electro

oxidation

Electro catalytic oxidation

Gd3+ Nd3+ Sm3+

1 Sodium; 4-acetylamino- 2-amino-benzenesulfonate

253 √ - - -

2 2-Amino-benzenesulfonic acid 195 √ - - -

3 Sodium; benzenesulfonate 158 √ - - -

4 Benzenesulfonic acid 158 √ - - -

5 N-(3-Amino-phenyl)-acetamide 151 √ √ √ √

6 2-Amino-benzenesulfonic acid 195 √ - - -

7 N-Phenyl-acetamide; compound with methane

135 √ - √ -

8 Benzene-1,3-diamine 108 √ √ √ √

9 Benzenediazonium 105 √ √ - √

10 Phenol 94 √ - - √

11 Benzene 78 √ √ - √

12 Acetylene 26 - - √ √

However, the results indicated that the azo group might be converted to

ammonia and nitrate ion simultaneously. Takahashi et al., (1994) confirmed that only

trace amounts of nitrate and ammonium ions can be determined. Phenols are highly

reactive substrates in aromatic electrophilic substitution since the non bonded

electrons of the hydroxyl group stabilize the sigma complex that is formed due to an

electrophilic attack in the ortho or para position. A substitute electron donor as –OH


157

in the phenol molecule activates mainly the ortho and para positions and a substitute

electron subtracts or as the group –COOH disables these positions. Phenols oxidize

producing aromatic ketones or diketones named quinones once the quinone is formed, the

ring opening takes place to produce carboxylic acids. Tauber et al., (2005) reported that the

degradation of azo dyes using fungus and ultrasound will leads to the formation of the

reduction products such as Phenol, CO2, and carboxylic acids as Oxalic, Malonic, Formic,

Propionic and Acetic acid also the degradation of Phenol produces Catechol,

Hydroquinone and Benzoquinone.

During the electrocatalytic oxidation of Reactive Yellow 186 various organic

intermediates were produced. Using GC/MS techniques, 7 by-products were

identified as possible degradation products. Table 5.4 summarizes the GC–MS

compound name, the molecular weights and the characteristic fragmentation patterns

ions by-products. Based on the identification results, a tentative reaction pathway is

proposed, as schematically depicted in Fig. 5.15. In the first step of the degradation

mechanism of the dye is carried out a reduction of the molecule by the breaking of the

azo bond due to the transfer of electrons. As in the case of Reactive Yellow 186

mineralization pathway was similar to electro oxidation process. The mineralization

end reaction proceeds by 3-Methyl-pent-2-enedioic acid 1-amide 5-ethylamide, Sodium;

benzenesulfonate, 2-Chloro-[1,3,5]triazine and de-sulfonation of benzenesulfonic acid to

give benzene and cleavage of –NH– bonds of hydroxylated diphenyl amine compounds

resulting in the formation of benzene, aniline and phenol. Larson et al., (1991) that the


158

ring structure in substituted triazine cannot be broken down through photodegradation

by UV/Fe3 and also the similar result observed Feng et al., (2000).


of RY 186

S.No Compound Molecular weight

Electro oxidation


Gd3+ Nd3+ Sm3+

1

5-Diazenyl-1-ethyl-6-hydroxy-4-methyl-2-oxo-

1,2-dihydro-pyridine-3-carboxylic acid amide

224 √ - - -

2 Sodium; 4-amino-2-diazenyl-

benzenesulfonate 223 √ - - -

3

1-Ethyl-4-methyl-2,6-dioxo-

1,2,5,6-tetrahydro-pyridine-

3-carboxylic acid amide

196 √ - - -

4 Sodium; 4-amino-benzenesulfonate 195 √ - - -

5 Sodium; benzenesulfonate 180 √ √ - √

6 3-Methyl-pent-2-enedioic acid

1-amide 5-ethylamide 170 √ √ - √

7 6-Chloro-[1,3,5]triazine-2,4-diamine 145 √ √ - √

8 2-Chloro-[1,3,5]triazine 115 √ √ √ √

9 [1,3,5]Triazine-2,4-diamine 111 √ - - -

10 N-Chloromethylene-formamidine 90 √ √ √ -

11 Benzene 78 - √ √ √


159

S

HN

OO

OS

OO O-Na+

N N

N

Cl

NH

NS N

OH

NO

O-Na+

O

O

NH2O

H2N

N N

N

Cl

NH2

6-Chloro-[1,3,5]triazine-2,4-diamine

N N

N

Cl

H2N

N N

N NH2

2-Chloro-[1,3,5]triazine[1,3,5]Triazine-2,4-diamine

N

Cl

HN

N

Cl

N-Chloromethylene-formamidine

S OO-Na+

O

Sodium; benzenesulfonate

Benzene

O

HN

NH2O

3-Methyl-pent-2-enedioic acid 1-amide 5-ethylamide

+


C.I. Reactive Yellow 186(CAS 84000-63-5)

N N N2NH NH4

+ + NO3-

SO3H SO42-


Figure 5. 15 The probable degradation pathway of C.I Reactive Yellow 186

during electrocatalytic oxidation


160

As shown in Fig. 5.15, the substituted triazine departed from the dye molecule

may not be oxidized with the electro chemical treatment in this study. Phenol is

subsequently attacked by OH• radicals to form hydroquinone which can submit an

oxidative opening of the aromatic ring leading to low molecular weight aliphatic

carboxylic acids, such as formic and acetic acid. This result indicates that the –N N–

azo group was very likely transformed with 100% selectivity into gaseous N2 as

reported previously (Karkmaz et al., 2004) and only the –NH– amino group has been

transformed to ammonium which is subsequently oxidized to nitrate.

The degradation of the azo dye was not only a break of the azo bond, but also

degradation of the aromatic rings. Chen et al., (2001) have shown that the infrared

(IR) characteristic peak of the phenyl group disappeared after reaction with •OH,

proving that carboxylic intermediates were generated. Formic acid and acetic acid are

known to be the ultimate organic by-products of aromatic ring opening reactions

(Brillas et al., 1998b; Belhadj and Savall, 1999; Calindo et al., 2000). Our findings

have confirmed the formation of substituted benzene. This further leads to low

molecular weight formation in the system.

Comparing the intermediates identified in the process of electrochemical

oxidation of RB 198 with and without of catalyst (Gd3+ Nd3+ and Sm3+ doped cerium

oxide), it is obvious that the addition of nanocatalyst is helpful to the chromophoric

group stripping, and speed up for cleavage of the ring. As shown in Table 5.5,

cleavage of the C-N and C-N bonds seems to occur first on the dye molecule and led

to formation of compounds through pathways (Fig. 5.16). It can be seen that after


161

20min of electrolysis, compounds, such as 1,4-Dinitro-benzene, 6-Chloro-

[1,3,5]triazine-2,4-diamine, [1,4]Benzoquinone, Phenylamine and Benzene were formed.

The above organic compounds were further degraded to generate heterocyclic

compounds containing a six- and a five-atom ring, via a series of complicated

degradation reactions that cannot be identified in detail. These compounds later

underwent deep degradation to form benzene-type intermediates, the further oxidation

of which resulted in aromatic ring cleavage and generation of aliphatic acids, such as

hexanoic acid, enanthic acid and hexylacetic acid. Finally, these organic acids were

decomposed to CO2 to accomplish the entire mineralization process.

The mechanism of the electrocatalytic oxidation process using Gd3+, Nd3+ and

Sm3+ doped cerium oxides is briefly explained as follows: The electrogenerated electrons

can directly reduce the dye or react with electron acceptors such as O2 adsorbed on the

catalyst surface or dissolved in water to superoxide radical anion (O2• −). Besides, the

electrogenerated holes can oxidize the organic molecule to form R+ or react with OH− and

H2O oxidizing them into •OH. Other highly oxidant species such as peroxide radicals are

reported also to be responsible for the heterogeneous. The relevant reactions at the

semiconductor surface causing the degradation of dyes can be summarized as follows:

Dye + •OH, O2• − or HO2

• → SO42-, NO3

-, NH4+, CO2 and H2O (5.3)


162


of RB 198

S.No Compound Molecular weight

Electro oxidation


Gd3+ Nd3+ Sm3+

1 Sodium; 2-amino-6-hydroxy-benzenesulfonate 211 √ - - -

2 3,6-Diamino-2,5-dichloro-cyclohexa-1,4-diene-1,4-diol 207 √ - - -

3 3-Amino-2,5-dichloro-cyclohexa-1,4-diene-1,4-diol 199 √ - - -

4 2,5-Dichloro-[1,4]benzoquinone 177 √ √ √ -

5 2,5-Dichloro-cyclohexa- 2,5-dienylamine 164 √ √ √ -

6 1,4-Dinitro-benzene 168 √ √ - √

7 1,4-Dichloro-cyclohexa -1,4-diene 153 √ √ √ -

8 6-Chloro-[1,3,5]triazine- 2,4-diamine 145 √ √ √ √

9 2-Chloro-[1,3,5]triazine 115 √ √ - √

10 [1,4]Benzoquinone 108 - √ - -

11 Phenylamine 93 - √ √ -

12 Cyclopenta-2,4-dienone 80 - - - √

13 3-Chloro-2H-azirine 76 - - √ -

14 Benzene 78 - √ √ √


163

HN

SO3 Na

N N

NHN

NH

O

HN

NH

OHN

NH

N

NN

NH

Cl

Cl

Cl SO3 Na

SO3 Na

SO3 Na

Cl

N

N

N

Cl NH2

Cl

O

NH2

Cl

H2N

NH2

Cl

Cl1,4-Dichloro-cyclohexa

-1,4-diene

6-Chloro-[1,3,5]triazine-2,4-diamine

NH2Phenylamine

N

N

N

Cl2-Chloro-[1,3,5]triazine

N

Cl3-Chloro-2H-azirine

Benzene-1,4-diamine

O

Cl2,5-Dichloro-[1,4]

benzoquinone

O

O[1,4]Benzoquinone

OCyclopenta-2,4-dienone

ClH2N

2,5-Dichloro-cyclohexa-2,5-dienylamine

NO2

O2N1,4-Dinitro-benzene

Benzene



N N N2NH NH4

+ + NO3-

SO3H SO42-

C.I. Reactive Blue 198(CAS 124448-55-1)

Figure 5. 16 The probable degradation pathway of C.I Reactive Blue 198 during

electrocatalytic oxidation


164

These radicals (•OH, O2• − and HO2

•) are very strong oxidizing and they are

able to decompose the dyes to the oxidation products. Eventually, the parent

compounds and their intermediates will be oxidized into inorganic products

(SO42-, NO3

-, NH4+), CO2 and water (Bandara et al., 1999, Tanaka et al., 2000,

Galindo et al., 2000 and Daneshva et al., 2003). Even if this mechanism was involved

as primary step in degradation of dyes under study amines formed as a result were further

oxidized leading to low molecular weight non-toxic simpler organic compounds. In this

study it could be supposed that if azo oxidation acts as first part and aniline like

amino derivatives are formed on one side and naphthalene derivatives on other side

with further degradation particularly in C-N bond would be broken and domination

would result in form of oxidized phenyl derivatives or naphthyl derivatives.

As phenolic like catechols, resorcinols in way of degradation are easily

mineralized to aliphatic and aromatic carboxylic acid through ring opening. These

carboxylic acids can easily be changed to CO2 and H2O leading to complete mineralization

of recalcitrant organic dye molecules. Also, it is noteworthy that formation of organic

acids is consistent with the fact that the electrolyte pH was intended to shift toward

the slightly acidic region during the course of the reaction process.

Overall, the electrolysis decomposition of all the three reactive dyes in the

electrocatalytic system can be described by a series of consecutive degradation steps.

The TOC measurements have shown this study that the reaction follows an evolution

by way of a total mineralization. It is also to be mentioned that final degradation

products were mineral ions such as NH4+, NO3 - and SO4

2- according to the substituent


165

groups included in the initial molecule, as well as CO2 and H2O (Maletzky and Bauer,

1998; Tanaka et al.,2000; Joseph et al.,2000).

5.4 CONCLUSIONS

The results indicate that doped cerium oxides were acts as a co-catalyst for

electrocatalytic oxidation processes. The main electrochemical reaction for the

enhanced rate is that the rapid inhibition of the electro charge carriers by catalyst.

The beneficial effect of electron acceptors [Gd3+, Nd3+ and Sm3+] is to generate more

number of radicals, which in turn degrade the pollutants effectively through radical

chain branching mechanism. The results concludes that doped cerium oxide (Gd3+,

Nd3+ and Sm3+) can be efficiently used to degrade the dye, RO 107, RY 186 and RB

198 using electrochemical method.


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ELECTROCATALYTIC OXIDATION PROCESSshodhganga.inflibnet.ac.in/bitstream/10603/33851/5/chapter5.pdf · 127 chemical for the synthesis of C-Ce0.8Gd0.2O2, C-Ce0.8Nd0.2O2 and C-Ce0.8Sm0.2O2