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Low-temperature catalytic oxidation of water containing 4-chlorophenol over Ni-oxide catalyst
Applied Catalysis A: General 248 (2003) 249–259
Low-temperature catalytic oxidation of water containing 4-chlorophenol over Ni-oxide catalyst M. Stoyanova, St.G. Christoskova∗ , M. Georgieva Department of Physical Chemistry, Paisii Hilendarski, University of Plovdiv, 24 Tzar Assen Street, 4000 Plovdiv, Bulgaria Received 5 November 2002; received in revised form 19 February 2003; accepted 19 February 2003
Abstract The catalytic oxidation of 4-chlorophenol (4-CP; chosen as an aromatic model pollutant) over Ni-oxide system in an aqueous medium has been studied. Experiments were carried out to investigate the effect of such parameters as pH, temperature, and catalyst dosage both on the reaction efficiency and on the reaction mechanism. The current concentrations of 4-CP as well as those of reaction intermediates—hydroquinone, benzoquinone, and 4-chlorocatechol—during the kinetic investigations were monitored by UV-Vis spectral analysis. Pseudo first-order kinetics with respect to the parent compound was observed. Experimental results demonstrated that 4-CP could be completely converted into the environmentally harmless CO2 , H2 O, and mineral acid under certain conditions. A probable kinetic model for the degradation pathway is proposed. © 2003 Elsevier Science B.V. All rights reserved. Keywords: 4-Chlorophenol; Catalytic oxidation; Ni-oxide catalyst; Kinetics study
1. Introduction Chlorinated organic compounds, especially chlorophenols, represent an important class of environmental water pollutants [1]. These toxic compounds are present in waste waters from petrochemical, coal tar, plastic, and pesticide chemical industries [2–4], which produce them as chemical intermediates or generate them during chlorination of effluents containing phenolic compounds. Chlorophenols show low biodegradability, and, therefore, are persistent pollutants, posing serious risk to the environment once mixed into natural water [5]. The increasing environmental concerns have focused research on the development of efficient treat∗ Corresponding author. Tel.: +359-32-261-534; fax: +359-32-235-049. E-mail address: [email protected] (St.G. Christoskova).
ment processes and technologies for the purification of waste waters, containing chlorophenols. An ideal waste treatment process must completely mineralize the toxic species present in the waste streams without leaving behind any hazardous residues and it should be also cost-effective. Chlorophenols can be oxidized by chemical [6–8], microbiological [9], uncatalyzed, and catalyzed photochemical processes [10,11]. Several catalysts such as CdS [12], Fe2 O3 , ZnO, and TiO2 [13,14] were applied in photocatalytic oxidation. Photocatalytic decomposition using TiO2 under UV light is cheap (i.e. does not require costly oxidants), non-toxic, and also TiO2 shows almost no catalytic deactivation for considerable time on-stream. In this study, a detailed investigation has been carried out on the aqueous phase catalytic oxidation of 4-chlorophenol (4-CP) on the Ni-oxide system and the possibility of its application to a catalytic reaction
0926-860X/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(03)00164-9
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2. Experimental
well as using the rate constant (k, min−1 ), determined in accordance with a kinetic equation of first-order reactions. The activation energy (Ea , kJ mol−1 ) was calculated from the rate constant versus temperature plot according to the Arrhenius equation.
2.1. Catalyst
2.3. Methods and apparatus
The Ni-oxide system was synthesized, according to the procedure described in detail by Leyva et al. [15]. It was characterized by means of IR, XPS, ESR, X-ray diffraction, and chemical analyses. The synergism of the results obtained gave us reason to conclude that the applied synthesis yields Ni-oxide with a high content of active oxygen (O∗ ∼ 8%), a high oxidation degree and an octahedral coordination of the metal ions. A weak surface Ni–O bond was established as well. In principle, due to all these attractive properties, a high catalytic activity of the sample in reactions of complete oxidation, carried out at low temperature, was expected.
The initial and current concentrations of 4-CP as well as those of its oxidation products, hydroquinone (HQ), benzoquinone (BQ), and 4-chlorocatechol (4-CCT), were monitored by UV-Vis spectroscopy. Absorbance at 278, 292, 244, and 253 nm was measured to determine the concentration of above compounds, respectively. The UV spectra and their first and second derivatives (D-Sp) were recorded on a Lambda-15 UV-Vis spectrophotometer. The derivative spectroscopy (D-Sp) was applied to identify more accurately the intermediate products of the 4-CP oxidation. Silver nitrate was used for qualitative detection of the liberated inorganic chloride as the final oxidation product of the organic-bound chlorine. It was also determined as well by a conductometric method. Conductivity measurements were carried out with Radelkis model conductometer. The liberation of CO2 during the experiments was qualitatively proved by continuously bubbling the gas outlet stream into a saturated barium hydroxide solution and determined by IR spectroscopy. The FT–IR-spectra were recorded on a Perkin-Elmer 1750 spectrophotometer in CaF2 cuvettes. The absorption band, characteristic of CO2 under the indicated conditions lies within the 2320–2340 cm−1 range, depending on the pH value of the medium. Leaching of Ni-oxide catalyst in the reaction mixture was assessed by measuring the concentration of dissolved metal components with flame atomic absorption (Spectro-Flama IOP-OES model spectrometer) using a monochromator system at λmax = 216.55 nm.
important for the ecology—4-CP neutralization in waste waters.
2.2. Oxidation reaction Kinetic experiments were carried out in a thermostatic reactor upon continuous stirring, thus providing an equal level of all parameters describing the state of the system (temperature, concentration, pH). The reactor has a tight cover with three ports for gas inlet and outlet, and sampling. In a typical run, a 100 cm3 of the aqueous 4-CP solution (200 mg dm−3 ) previously adjusted to the corresponding pH, was fed into the reactor. The solution was saturated with oxygen by bubbling air at atmospheric pressure for 30 min before adding the catalyst. Thereafter a given amount of fresh Ni-oxide catalyst (catalyst concentration 0.5–2.0 g dm−3 ; BET surface area of 110 m2 g−1 ; particle size in the range 0.6–1.0 mm) was suspended in the solution. The air was continuously bubbled during the runs, so that the steady-state concentration of oxygen dissolved in the solution was maintained constant. Representative samples were withdrawn periodically and the catalyst was immediately separated from the aqueous phase by centrifugation. Depletive oxidation experiments (without bubbling of air) were also performed in order to clarify the reaction mechanism. The activity of the catalytic system was evaluated both on the basis of 4-CP conversion degree (α, %), as
3. Results and discussion Experiments were carried out using model solutions with initial concentration of 4-CP of 200 mg dm−3 . The concentration of the model solutions was selected
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Fig. 1. UV spectrum profiles (a) and UV-D spectra of the reaction mixture in 2 (b) and in 180 (c) during the oxidation of 4-CP on the Ni-fresh. Experimental parameters: C0 = 200 mg dm−3 ; T = 308 K; mcat = 2 g dm−3 ; pH = 6.
to correspond to the real concentration of 4-CP in waste waters of chemical industry. The gradual decrease in 4-CP concentration was followed by recording the UV spectra as well as their first (D ) and second (D ) derivatives of aliquots of the reaction mixture in the range 200–350 nm. The recorded spectra are presented in Fig. 1. It is seen from Fig. 1 that 4-CP is oxidized under the studied experimental conditions. The following intermediates are detected in the course of catalytic oxidation: HQ (λmax = 292 nm), BQ (λmax = 244 nm), 4-CCT (λmax = 253 nm) (Fig. 1a). They are further completely oxidized (Fig. 1c). The final oxidation products of 4-CP are established to be CO2 and mineral acids. The results obtained reveal that catalytic oxidation of 4-CP with participation of Ni-oxide system runs
according to a first-order reaction with respect to 4-CP. The linear course of the lnC versus time plots (see Fig. 2) confirms this conclusion. Since oxidation is carried out under constant bubbling of air in the mixture, we postulate that the reaction is of zero order against oxygen. Experimental conditions provide stationary concentration of oxygen on the surface of the catalyst. This fact gives us grounds to assume that the reaction rate is independent of dissolved oxygen concentration. It should be noted that all experiments are carried out with the same concentration of dissolved oxygen. Kinetic curves expressing the change in 4-CP concentration in the course of both catalytic and depletive oxidation are shown in Fig. 3. It is obvious that the basic part of 4-CP is oxidized up to the 30th minute from the start of the process
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Fig. 2. Linear transformation ln C = f(t) and α = f(t) of the kinetic curves of 4-CP disappearance over Ni-oxide system. (䊏, 䊐) Ccat = 0.5 g dm−3 ; (䉱, ) Ccat = 1 g dm−3 ; (䊉, 䊊) Ccat = 2 g dm−3 .
when the latter is carried out with catalyst concentrations of 1 and 2 g dm−3 , respectively. Comparison of data in Fig. 3a and b indicates commensurable rates of catalytic and depletive oxidation of 4-CP. Based on these results it may be concluded that catalytic oxidation of 4-CP over a Ni-oxide system proceeds through a reduction/oxidation mechanism, i.e. the active oxygen of the catalyst (the amount of active oxygen depends on the catalyst mass) that takes part in the oxidation process while the dissolved oxygen reoxidizes the reduced catalyst surface. The effect of catalyst mass on the process efficiency, expressed through k (min−1 ) and ␣ (%) is illustrated in Table 1. The results reveal that the amount of the catalyst significantly affects the catalytic parameters of the oxidation process run both in presence and in absence of oxygen. As seen from Table 1, a four-fold increase in catalyst concentration leads to increase in the rate con-
Fig. 3. Kinetic curves: (a) catalytic oxidation; (b) depletive oxidation. Catalyst concentration: (䊏) 2 g dm−3 ; (䊉) 1 g dm−3 ; (䉱) 0.5 g dm−3 .
stant by a factor of 10, other parameters being constant (temperature and pH). With mcat = 2 g dm−3 , a complete conversion of 4-CP is practically achieved for 20 min, while with mcat = 0.5 g dm−3 the conversion degree at the 180th minute is only 50%. The established symbiotic dependence between efficiency and catalyst mass gives grounds to assume that the process runs on the surface of the catalyst, but not according to a mixed homogeneous–heterogeneous mechanism. Additional grounds for such a conclusion present the experimental result that the oxidation ceases as the catalyst is suddenly removed from the reaction mixture. In addition, no presence of HO2 − ion-radicals is detected in the liquid phase. The amount of the oxide system influences considerably the selectivity of the process as one can see
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Table 1 Effect of the catalyst concentration on the degree of 4-CP conversion and on the rate constant during catalytic and depletive oxidation Catalyst amount (g dm−3 )
0.5 1.0 2.0
Rate constant (k, min−1 )
Degree of 4-CP (α, %) α20
α180
Depletive oxidation
Catalytic oxidation
Depletive oxidation
Catalytic oxidation
33 55 100
37 66 100
46.6 100 100
50 100 100
Depletive oxidation
Catalytic oxidation
0.0205 0.0623 0.164
0.0231 0.0762 0.219
Reaction conditions: C0 = 200 mg dm−3 ; T = 308 K; pH = 6. Table 2 Effect of the catalyst concentration on the reaction selectivity Reaction time (min)
0 5 15 30 60 180
Qualitatively test for Cl−
Conductivity (G × 103 ) (S)
Ccat = 1 g dm−3
Ccat = 2 g dm−3
Ccat = 1 g dm−3
Ccat = 2 g dm−3
− + + + + +
− + + + + +
0.086 0.84 1.04 1.26 1.46 1.80
0.086 1.22 1.68 1.71 1.78 1.80
GH2 O = 18.5 × 10−6 S.
from the results demonstrated in Table 2 and Figs. 4 and 5. It is seen, that the increase in the mass of the Ni-oxide system leads both to enhanced amount of the intermediates (BQ and 4-CCT; Figs. 4 and 5) and to higher rate of their subsequent oxidation to final products—CO2 , H2 O, and mineral acids. An
additional support to this statement is the established increase in conductivity of the reaction mixture in the course of the process as well as the results of the quantitative test for Cl− (Table 3). The effect of pH on efficiency and selectivity of the oxidation process has been studied with a view to finding optimum conditions for complete transformation of 4-CP to harmless products. Experiments were
Fig. 4. Change in absorbance of the intermediate BQ during the catalytic oxidation of 4-CP.
Fig. 5. Change in absorbance of the intermediate 4-CP during the catalytic oxidation of 4-CP.
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Table 3 Rate constant (k, min−1 ) and degree of catalytic conversion (α, %) of 4-CP as a function of pH Time (min)
pH = 6.0
pH = 8.0
pH = 10.0
α (%) k (min−1 ) α (%) k (min−1 ) α (%) k (min−1 ) 5 10 15 20
76.3 88.5 97.7 100
0.253
54.0 73.0 86.0 92.5
0.138
3.5 6.3 8.1 8.4
0.007
Reaction conditions: T = 308 K; mcat = 2 g dm−3 .
carried out in the pH interval of 6.0 < pH < 10.0, having in mind the following considerations: • Waste waters of different manufactures containing 4-CP and being subjected to detoxification could have varying pH value. • Authors, who studied photocatalytic oxidation of 4-CP, have established that lower pH values (3.0–5.0) favor the catalytic process [15]. They assumed that under these conditions adsorption of organic molecule on the catalyst surface is facilitated thus promoting a stronger photocatalytic transition that essentially reduces the rate of the process. • Our preliminary investigations revealed that the Ni-oxide system has significant solubility at pH < 6.0 [16]. That is why the lower level of pH was chosen to be pH = 6.0. • It is a priori known that pH of the solution affects considerably the mechanism and efficiency of the process [3,15].
Fig. 6. The effect of pH on kinetics of 4-CP catalytic oxidation. Reaction conditions: C0 = 200 mg dm−3 ; T = 308 K; mcat = 2 g dm−3 .
The results of studies on the effect of pH are illustrated in Tables 3 and 4 and in Figs. 6 and 7. It is seen that the higher conversion degree of 4-CP is achieved at pH = 6.0 (α is ∼100% at the 20th minute). The increase of the alkalinity (with other constant parameters) causes a decrease in the conversion degree and the process is retarded. At pH = 10.0, only 8% of the substrate has been converted for 15 min and the oxidation process runs rather slowly (k = 0.007 min−1 ). The results obtained confirm available data about the effect of pH on the efficiency of catalytic oxidation of 4-CP with the participation of other oxide catalytic systems [16]. UV-D spectral analysis of aliquots of the reaction mixture reveals (Table 4; Fig. 7) that increasing the
Table 4 UV-absorbance maxima of 4-CP and of intermediates formed during the catalytic oxidation of 4-CP over Ni-oxide system Time (min)
5 10 15 20 30 120 180
pH = 6.0
pH = 8.0
(nm)
(nm)
220 (4-CP)
244 (BQ)
√ √ d √ d √ d – – – –
252 (4-CCH)
– √ √ √ √ √ √
– √ √ i √ i √ √ d √ d Traces
i i i d d Traces
278 (4-CP) √ √ d √ d √ d – – – –
Experimental conditions: C0 = 200 mg dm−3 ; T = 308 K; mcat
290 (HQ)
220 (4-CP)
√ √ d √ d √ d √ d – – – √ = 2 g dm−3 . , presence; – √ √ d √ d √ d Traces – –
244 (BQ)
252 (4-CCH)
278 (4-CP)
290 (HQ)
– √ √ √ √ √ √ √
– √ √ i √ i √ √ √ √
– √ √ d √ d √ d – – –
– √ √ d √ d √ d Traces – –
i i d d d d
d, decrease; i, increase.
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Fig. 7. UV-D spectra: (a) initial solution of 4-CP; (b, c, d, and e) reaction mixture at the 5th, 30th, 120th, and 180th minute, respectively, pH = 6.0; (b , c , d , and e ) reaction mixture at the 5th, 30th, 120th, and 180th minute, respectively, pH = 8.0.
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Fig. 7. (Continued ).
pH the amount of the intermediates as well as the rate of their subsequent complete oxidation are reduced. At pH = 6.0, only traces of BQ and 4-CCT have been detected at the 180th minute from the start of the process, while at pH = 10.0 the amount of intermediates is still significant. Similar effect of pH has been observed with liquid phase oxidation of phenol using the same catalytic system [17], as well as with the participation of a Co-oxide system prepared in the same way [18]. This give us grounds to suggest that catalytic oxidation of 4-CP over Ni-oxide fresh runs according to analogous mechanism as those suggested for the catalytic oxidation of phenol [18]. According to this mechanism, the process is initiated by a dissociation adsorption of the substrate on the surface of the of the catalyst via cleavage of a H atom from phenolic OH group and formation of surface complexes with varying degree of oxidation. The latter complexes are either decomposed (depending on the conditions), forming stable intermediates, or are further oxidized to products of complete oxidation. The cleaved H atom forms surface hydroxyl radicals (• OH) with surface active oxygen that take part in further oxidation steps. Reaction with participation of • OH is more likely to occur due to the fact that • OH are more reactive but less selective than oxygen in the attack of the chemical bond. The very low efficiency of the oxidation process at pH = 10.0 could be explained with the impossibility of H atom cleavage and formation of • OH, since
4-CP in alkaline medium is predominantly present as phenolate anion. The discussed results concerning the effect of the amount of Ni-oxide system support as well the suggested probable redox mechanism, namely, oxidation runs with the participation of the oxide active oxygen, while dissolved oxygen re-oxidizes the partially reduced catalyst. The larger is the amount of O∗ (oxide mass, respectively), the higher is the amount of formed • OH. As a result, the process runs with higher rate. The commensurable values of α and k in catalytic and depletive oxidation of 4-CP provide additional grounds as well (Table 1; Fig. 2). Additional experiments have been carried out with a view to elucidating the effect of pH on leaching the Ni-oxide system in the course of catalytic oxidation. Application of heterogeneous catalysts in purification of waste waters requires they should be stable towards metal ion exchange between the catalyst and the liquid phase under the operating conditions. It is important to study the solubility of heterogeneous catalyst for the following reasons: • continuous (gradual) leaching of the catalyst should lead to its progressive deactivation; • it is known that homogeneous catalysts, e.g. Ni2+ , are efficient in reactions of liquid phase oxidation. Hence, dissolved metal ions, included in the catalyst composition, could cause the occurrence of homogeneous catalytic reactions. In these cases one could assume the presence of homogeneous–
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Experiments were carried out as follows: aliquots of the reaction mixture were taken 60 min after the start of the process (operating parameters: temperature interval 298–318 K; catalyst concentration interval 0.5–2 g dm−3 ; pH interval 6.0–10.00). The catalyst was filtered and the filtrate analyzed for dissolved nickel ions content using atomic absorption spectrometry. It was established that the amount of dissolved nickel ions is negligible (less than 0.90 mg dm−3 ) [16], the latter being a strong evidence in support of heterogeneous catalysis. The effect of temperature was studied in the temperature interval of 298–318 K with model solutions of 4-CP with equal initial concentration (C0 ∼ 200 mg dm−3 ), pH (6.0), and mass of the catalyst (1 g dm−3 ). The study aims at examining if Ni-oxide system acts at low temperatures, which is very advan-
phase oxidation of 4-CP over different catalytic systems (5–20 kJ/mol) [19]. The results reveal that temperature (in the studied temperature interval) does not affect the value of the rate constant of liquid phase oxidation of 4-CP with participation of Ni-oxide system, which is an explanation of the low activation energy. This low value could be as well explained with the high mobility of active surface oxygen in the catalyst, respectively, low energy of the surface cation to active oxygen bond, provided by the high degree of oxidation of Ni ions in the oxide and their octahedral coordination [20]. The high mobility of the surface oxygen is the probable reason of the observed high selectivity of the process. The calculated values of activation energy are close to that, characteristic of heterogeneous catalytic reactions, running in inner diffusion area. Having in mind, however, that experiments are carried out with catalyst fraction of 0.6–1.2 mm, one can assume that the inner surface of the catalyst grain is used to a great degree and the reaction runs with high rate. This give us grounds to assume that diffusion of the reagents in the pores of catalyst is not the limiting stage of the oxidation process. The limiting stage is the interaction of the substrate with surface-active oxygen of the oxide. Based on all results obtained, we suggest the following scheme of catalytic oxidation of 4-CP with participation of Ni-oxide system:
tageous. The temperature interval was chosen with a view to further development of a new catalytic method and new technological scheme for purification of waste waters containing 4-CP. The results obtained are listed in Table 5. The calculated value for activation energy of the process is comparable with reported values for liquid
According to the scheme, we suppose the run of two parallel process of interaction of the substrate with surface • OH, yielding more strongly oxidized intermediates (HQ, quinone, and 4-CCT). The latter are further oxidized completely or partially to final products, CO2 , H2 O, and mineral acids, depending on experimental conditions.
Table 5 Effect of the reaction temperature on the rate constants T (K)
k × 103 (min−1 )
Ea (kJ mol−1 )
298 308 318
6.16 7.62 9.29
16.2
heterogeneous catalytic reaction rather than of pure heterogeneous process.
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Only traces of HQ have been detected in the course of catalytic oxidation, while the curve reflecting changes in concentration of BQ exhibits well-defined maximum (Fig. 4). This fact gives us grounds to assume that the rate of formation of HQ is lower than the rate of its subsequent transformation into BQ. CO2 (proved by means of chemical and IR spectral analyses) and mineral acids are the final products of catalytic oxidation. Some authors, who have studied catalytic and photocatalytic oxidation of 4-CP, explain yielding of CO2 as a result of the so called rake-mechanism reported in [21] which has been subsequently applied in photocatalytic oxidation processes [22,23]. Based on results of the present study as well as on literature data, the discussed mechanism can be schematically illustrated as follows:
weakly bound active surface oxygen (O∗s = 8%), provide low temperature complete conversion of 4-CP to CO2 , H2 O, and mineral acids. Data obtained about the effect of pH, temperature, and amount of oxide system on efficiency of the process allow to establish the optimum conditions for running the process as well as to throw light on the discussed mechanism of liquid phase catalytic oxidation and to develop a method for purification of waste waters containing 4-CP.
Acknowledgements Authors gratefully acknowledge the financial support from the University of Plovdiv Research Fund through project 01-X-03 and from the Bulgarian National Scientific Research Foundation. References
4. Conclusion The high specific surface of Ni-oxide system (110 m2 g−1 ), in combination with high content of
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Low-temperature catalytic oxidation of water containing 4-chlorophenol over Ni-oxide catalyst M. Stoyanova, St.G. Christoskova∗ , M. Georgieva Department of Physical Chemistry, Paisii Hilendarski, University of Plovdiv, 24 Tzar Assen Street, 4000 Plovdiv, Bulgaria Received 5 November 2002; received in revised form 19 February 2003; accepted 19 February 2003
Abstract The catalytic oxidation of 4-chlorophenol (4-CP; chosen as an aromatic model pollutant) over Ni-oxide system in an aqueous medium has been studied. Experiments were carried out to investigate the effect of such parameters as pH, temperature, and catalyst dosage both on the reaction efficiency and on the reaction mechanism. The current concentrations of 4-CP as well as those of reaction intermediates—hydroquinone, benzoquinone, and 4-chlorocatechol—during the kinetic investigations were monitored by UV-Vis spectral analysis. Pseudo first-order kinetics with respect to the parent compound was observed. Experimental results demonstrated that 4-CP could be completely converted into the environmentally harmless CO2 , H2 O, and mineral acid under certain conditions. A probable kinetic model for the degradation pathway is proposed. © 2003 Elsevier Science B.V. All rights reserved. Keywords: 4-Chlorophenol; Catalytic oxidation; Ni-oxide catalyst; Kinetics study
1. Introduction Chlorinated organic compounds, especially chlorophenols, represent an important class of environmental water pollutants [1]. These toxic compounds are present in waste waters from petrochemical, coal tar, plastic, and pesticide chemical industries [2–4], which produce them as chemical intermediates or generate them during chlorination of effluents containing phenolic compounds. Chlorophenols show low biodegradability, and, therefore, are persistent pollutants, posing serious risk to the environment once mixed into natural water [5]. The increasing environmental concerns have focused research on the development of efficient treat∗ Corresponding author. Tel.: +359-32-261-534; fax: +359-32-235-049. E-mail address: [email protected] (St.G. Christoskova).
ment processes and technologies for the purification of waste waters, containing chlorophenols. An ideal waste treatment process must completely mineralize the toxic species present in the waste streams without leaving behind any hazardous residues and it should be also cost-effective. Chlorophenols can be oxidized by chemical [6–8], microbiological [9], uncatalyzed, and catalyzed photochemical processes [10,11]. Several catalysts such as CdS [12], Fe2 O3 , ZnO, and TiO2 [13,14] were applied in photocatalytic oxidation. Photocatalytic decomposition using TiO2 under UV light is cheap (i.e. does not require costly oxidants), non-toxic, and also TiO2 shows almost no catalytic deactivation for considerable time on-stream. In this study, a detailed investigation has been carried out on the aqueous phase catalytic oxidation of 4-chlorophenol (4-CP) on the Ni-oxide system and the possibility of its application to a catalytic reaction
0926-860X/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(03)00164-9
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2. Experimental
well as using the rate constant (k, min−1 ), determined in accordance with a kinetic equation of first-order reactions. The activation energy (Ea , kJ mol−1 ) was calculated from the rate constant versus temperature plot according to the Arrhenius equation.
2.1. Catalyst
2.3. Methods and apparatus
The Ni-oxide system was synthesized, according to the procedure described in detail by Leyva et al. [15]. It was characterized by means of IR, XPS, ESR, X-ray diffraction, and chemical analyses. The synergism of the results obtained gave us reason to conclude that the applied synthesis yields Ni-oxide with a high content of active oxygen (O∗ ∼ 8%), a high oxidation degree and an octahedral coordination of the metal ions. A weak surface Ni–O bond was established as well. In principle, due to all these attractive properties, a high catalytic activity of the sample in reactions of complete oxidation, carried out at low temperature, was expected.
The initial and current concentrations of 4-CP as well as those of its oxidation products, hydroquinone (HQ), benzoquinone (BQ), and 4-chlorocatechol (4-CCT), were monitored by UV-Vis spectroscopy. Absorbance at 278, 292, 244, and 253 nm was measured to determine the concentration of above compounds, respectively. The UV spectra and their first and second derivatives (D-Sp) were recorded on a Lambda-15 UV-Vis spectrophotometer. The derivative spectroscopy (D-Sp) was applied to identify more accurately the intermediate products of the 4-CP oxidation. Silver nitrate was used for qualitative detection of the liberated inorganic chloride as the final oxidation product of the organic-bound chlorine. It was also determined as well by a conductometric method. Conductivity measurements were carried out with Radelkis model conductometer. The liberation of CO2 during the experiments was qualitatively proved by continuously bubbling the gas outlet stream into a saturated barium hydroxide solution and determined by IR spectroscopy. The FT–IR-spectra were recorded on a Perkin-Elmer 1750 spectrophotometer in CaF2 cuvettes. The absorption band, characteristic of CO2 under the indicated conditions lies within the 2320–2340 cm−1 range, depending on the pH value of the medium. Leaching of Ni-oxide catalyst in the reaction mixture was assessed by measuring the concentration of dissolved metal components with flame atomic absorption (Spectro-Flama IOP-OES model spectrometer) using a monochromator system at λmax = 216.55 nm.
important for the ecology—4-CP neutralization in waste waters.
2.2. Oxidation reaction Kinetic experiments were carried out in a thermostatic reactor upon continuous stirring, thus providing an equal level of all parameters describing the state of the system (temperature, concentration, pH). The reactor has a tight cover with three ports for gas inlet and outlet, and sampling. In a typical run, a 100 cm3 of the aqueous 4-CP solution (200 mg dm−3 ) previously adjusted to the corresponding pH, was fed into the reactor. The solution was saturated with oxygen by bubbling air at atmospheric pressure for 30 min before adding the catalyst. Thereafter a given amount of fresh Ni-oxide catalyst (catalyst concentration 0.5–2.0 g dm−3 ; BET surface area of 110 m2 g−1 ; particle size in the range 0.6–1.0 mm) was suspended in the solution. The air was continuously bubbled during the runs, so that the steady-state concentration of oxygen dissolved in the solution was maintained constant. Representative samples were withdrawn periodically and the catalyst was immediately separated from the aqueous phase by centrifugation. Depletive oxidation experiments (without bubbling of air) were also performed in order to clarify the reaction mechanism. The activity of the catalytic system was evaluated both on the basis of 4-CP conversion degree (α, %), as
3. Results and discussion Experiments were carried out using model solutions with initial concentration of 4-CP of 200 mg dm−3 . The concentration of the model solutions was selected
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Fig. 1. UV spectrum profiles (a) and UV-D spectra of the reaction mixture in 2 (b) and in 180 (c) during the oxidation of 4-CP on the Ni-fresh. Experimental parameters: C0 = 200 mg dm−3 ; T = 308 K; mcat = 2 g dm−3 ; pH = 6.
to correspond to the real concentration of 4-CP in waste waters of chemical industry. The gradual decrease in 4-CP concentration was followed by recording the UV spectra as well as their first (D ) and second (D ) derivatives of aliquots of the reaction mixture in the range 200–350 nm. The recorded spectra are presented in Fig. 1. It is seen from Fig. 1 that 4-CP is oxidized under the studied experimental conditions. The following intermediates are detected in the course of catalytic oxidation: HQ (λmax = 292 nm), BQ (λmax = 244 nm), 4-CCT (λmax = 253 nm) (Fig. 1a). They are further completely oxidized (Fig. 1c). The final oxidation products of 4-CP are established to be CO2 and mineral acids. The results obtained reveal that catalytic oxidation of 4-CP with participation of Ni-oxide system runs
according to a first-order reaction with respect to 4-CP. The linear course of the lnC versus time plots (see Fig. 2) confirms this conclusion. Since oxidation is carried out under constant bubbling of air in the mixture, we postulate that the reaction is of zero order against oxygen. Experimental conditions provide stationary concentration of oxygen on the surface of the catalyst. This fact gives us grounds to assume that the reaction rate is independent of dissolved oxygen concentration. It should be noted that all experiments are carried out with the same concentration of dissolved oxygen. Kinetic curves expressing the change in 4-CP concentration in the course of both catalytic and depletive oxidation are shown in Fig. 3. It is obvious that the basic part of 4-CP is oxidized up to the 30th minute from the start of the process
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Fig. 2. Linear transformation ln C = f(t) and α = f(t) of the kinetic curves of 4-CP disappearance over Ni-oxide system. (䊏, 䊐) Ccat = 0.5 g dm−3 ; (䉱, ) Ccat = 1 g dm−3 ; (䊉, 䊊) Ccat = 2 g dm−3 .
when the latter is carried out with catalyst concentrations of 1 and 2 g dm−3 , respectively. Comparison of data in Fig. 3a and b indicates commensurable rates of catalytic and depletive oxidation of 4-CP. Based on these results it may be concluded that catalytic oxidation of 4-CP over a Ni-oxide system proceeds through a reduction/oxidation mechanism, i.e. the active oxygen of the catalyst (the amount of active oxygen depends on the catalyst mass) that takes part in the oxidation process while the dissolved oxygen reoxidizes the reduced catalyst surface. The effect of catalyst mass on the process efficiency, expressed through k (min−1 ) and ␣ (%) is illustrated in Table 1. The results reveal that the amount of the catalyst significantly affects the catalytic parameters of the oxidation process run both in presence and in absence of oxygen. As seen from Table 1, a four-fold increase in catalyst concentration leads to increase in the rate con-
Fig. 3. Kinetic curves: (a) catalytic oxidation; (b) depletive oxidation. Catalyst concentration: (䊏) 2 g dm−3 ; (䊉) 1 g dm−3 ; (䉱) 0.5 g dm−3 .
stant by a factor of 10, other parameters being constant (temperature and pH). With mcat = 2 g dm−3 , a complete conversion of 4-CP is practically achieved for 20 min, while with mcat = 0.5 g dm−3 the conversion degree at the 180th minute is only 50%. The established symbiotic dependence between efficiency and catalyst mass gives grounds to assume that the process runs on the surface of the catalyst, but not according to a mixed homogeneous–heterogeneous mechanism. Additional grounds for such a conclusion present the experimental result that the oxidation ceases as the catalyst is suddenly removed from the reaction mixture. In addition, no presence of HO2 − ion-radicals is detected in the liquid phase. The amount of the oxide system influences considerably the selectivity of the process as one can see
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Table 1 Effect of the catalyst concentration on the degree of 4-CP conversion and on the rate constant during catalytic and depletive oxidation Catalyst amount (g dm−3 )
0.5 1.0 2.0
Rate constant (k, min−1 )
Degree of 4-CP (α, %) α20
α180
Depletive oxidation
Catalytic oxidation
Depletive oxidation
Catalytic oxidation
33 55 100
37 66 100
46.6 100 100
50 100 100
Depletive oxidation
Catalytic oxidation
0.0205 0.0623 0.164
0.0231 0.0762 0.219
Reaction conditions: C0 = 200 mg dm−3 ; T = 308 K; pH = 6. Table 2 Effect of the catalyst concentration on the reaction selectivity Reaction time (min)
0 5 15 30 60 180
Qualitatively test for Cl−
Conductivity (G × 103 ) (S)
Ccat = 1 g dm−3
Ccat = 2 g dm−3
Ccat = 1 g dm−3
Ccat = 2 g dm−3
− + + + + +
− + + + + +
0.086 0.84 1.04 1.26 1.46 1.80
0.086 1.22 1.68 1.71 1.78 1.80
GH2 O = 18.5 × 10−6 S.
from the results demonstrated in Table 2 and Figs. 4 and 5. It is seen, that the increase in the mass of the Ni-oxide system leads both to enhanced amount of the intermediates (BQ and 4-CCT; Figs. 4 and 5) and to higher rate of their subsequent oxidation to final products—CO2 , H2 O, and mineral acids. An
additional support to this statement is the established increase in conductivity of the reaction mixture in the course of the process as well as the results of the quantitative test for Cl− (Table 3). The effect of pH on efficiency and selectivity of the oxidation process has been studied with a view to finding optimum conditions for complete transformation of 4-CP to harmless products. Experiments were
Fig. 4. Change in absorbance of the intermediate BQ during the catalytic oxidation of 4-CP.
Fig. 5. Change in absorbance of the intermediate 4-CP during the catalytic oxidation of 4-CP.
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Table 3 Rate constant (k, min−1 ) and degree of catalytic conversion (α, %) of 4-CP as a function of pH Time (min)
pH = 6.0
pH = 8.0
pH = 10.0
α (%) k (min−1 ) α (%) k (min−1 ) α (%) k (min−1 ) 5 10 15 20
76.3 88.5 97.7 100
0.253
54.0 73.0 86.0 92.5
0.138
3.5 6.3 8.1 8.4
0.007
Reaction conditions: T = 308 K; mcat = 2 g dm−3 .
carried out in the pH interval of 6.0 < pH < 10.0, having in mind the following considerations: • Waste waters of different manufactures containing 4-CP and being subjected to detoxification could have varying pH value. • Authors, who studied photocatalytic oxidation of 4-CP, have established that lower pH values (3.0–5.0) favor the catalytic process [15]. They assumed that under these conditions adsorption of organic molecule on the catalyst surface is facilitated thus promoting a stronger photocatalytic transition that essentially reduces the rate of the process. • Our preliminary investigations revealed that the Ni-oxide system has significant solubility at pH < 6.0 [16]. That is why the lower level of pH was chosen to be pH = 6.0. • It is a priori known that pH of the solution affects considerably the mechanism and efficiency of the process [3,15].
Fig. 6. The effect of pH on kinetics of 4-CP catalytic oxidation. Reaction conditions: C0 = 200 mg dm−3 ; T = 308 K; mcat = 2 g dm−3 .
The results of studies on the effect of pH are illustrated in Tables 3 and 4 and in Figs. 6 and 7. It is seen that the higher conversion degree of 4-CP is achieved at pH = 6.0 (α is ∼100% at the 20th minute). The increase of the alkalinity (with other constant parameters) causes a decrease in the conversion degree and the process is retarded. At pH = 10.0, only 8% of the substrate has been converted for 15 min and the oxidation process runs rather slowly (k = 0.007 min−1 ). The results obtained confirm available data about the effect of pH on the efficiency of catalytic oxidation of 4-CP with the participation of other oxide catalytic systems [16]. UV-D spectral analysis of aliquots of the reaction mixture reveals (Table 4; Fig. 7) that increasing the
Table 4 UV-absorbance maxima of 4-CP and of intermediates formed during the catalytic oxidation of 4-CP over Ni-oxide system Time (min)
5 10 15 20 30 120 180
pH = 6.0
pH = 8.0
(nm)
(nm)
220 (4-CP)
244 (BQ)
√ √ d √ d √ d – – – –
252 (4-CCH)
– √ √ √ √ √ √
– √ √ i √ i √ √ d √ d Traces
i i i d d Traces
278 (4-CP) √ √ d √ d √ d – – – –
Experimental conditions: C0 = 200 mg dm−3 ; T = 308 K; mcat
290 (HQ)
220 (4-CP)
√ √ d √ d √ d √ d – – – √ = 2 g dm−3 . , presence; – √ √ d √ d √ d Traces – –
244 (BQ)
252 (4-CCH)
278 (4-CP)
290 (HQ)
– √ √ √ √ √ √ √
– √ √ i √ i √ √ √ √
– √ √ d √ d √ d – – –
– √ √ d √ d √ d Traces – –
i i d d d d
d, decrease; i, increase.
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Fig. 7. UV-D spectra: (a) initial solution of 4-CP; (b, c, d, and e) reaction mixture at the 5th, 30th, 120th, and 180th minute, respectively, pH = 6.0; (b , c , d , and e ) reaction mixture at the 5th, 30th, 120th, and 180th minute, respectively, pH = 8.0.
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Fig. 7. (Continued ).
pH the amount of the intermediates as well as the rate of their subsequent complete oxidation are reduced. At pH = 6.0, only traces of BQ and 4-CCT have been detected at the 180th minute from the start of the process, while at pH = 10.0 the amount of intermediates is still significant. Similar effect of pH has been observed with liquid phase oxidation of phenol using the same catalytic system [17], as well as with the participation of a Co-oxide system prepared in the same way [18]. This give us grounds to suggest that catalytic oxidation of 4-CP over Ni-oxide fresh runs according to analogous mechanism as those suggested for the catalytic oxidation of phenol [18]. According to this mechanism, the process is initiated by a dissociation adsorption of the substrate on the surface of the of the catalyst via cleavage of a H atom from phenolic OH group and formation of surface complexes with varying degree of oxidation. The latter complexes are either decomposed (depending on the conditions), forming stable intermediates, or are further oxidized to products of complete oxidation. The cleaved H atom forms surface hydroxyl radicals (• OH) with surface active oxygen that take part in further oxidation steps. Reaction with participation of • OH is more likely to occur due to the fact that • OH are more reactive but less selective than oxygen in the attack of the chemical bond. The very low efficiency of the oxidation process at pH = 10.0 could be explained with the impossibility of H atom cleavage and formation of • OH, since
4-CP in alkaline medium is predominantly present as phenolate anion. The discussed results concerning the effect of the amount of Ni-oxide system support as well the suggested probable redox mechanism, namely, oxidation runs with the participation of the oxide active oxygen, while dissolved oxygen re-oxidizes the partially reduced catalyst. The larger is the amount of O∗ (oxide mass, respectively), the higher is the amount of formed • OH. As a result, the process runs with higher rate. The commensurable values of α and k in catalytic and depletive oxidation of 4-CP provide additional grounds as well (Table 1; Fig. 2). Additional experiments have been carried out with a view to elucidating the effect of pH on leaching the Ni-oxide system in the course of catalytic oxidation. Application of heterogeneous catalysts in purification of waste waters requires they should be stable towards metal ion exchange between the catalyst and the liquid phase under the operating conditions. It is important to study the solubility of heterogeneous catalyst for the following reasons: • continuous (gradual) leaching of the catalyst should lead to its progressive deactivation; • it is known that homogeneous catalysts, e.g. Ni2+ , are efficient in reactions of liquid phase oxidation. Hence, dissolved metal ions, included in the catalyst composition, could cause the occurrence of homogeneous catalytic reactions. In these cases one could assume the presence of homogeneous–
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Experiments were carried out as follows: aliquots of the reaction mixture were taken 60 min after the start of the process (operating parameters: temperature interval 298–318 K; catalyst concentration interval 0.5–2 g dm−3 ; pH interval 6.0–10.00). The catalyst was filtered and the filtrate analyzed for dissolved nickel ions content using atomic absorption spectrometry. It was established that the amount of dissolved nickel ions is negligible (less than 0.90 mg dm−3 ) [16], the latter being a strong evidence in support of heterogeneous catalysis. The effect of temperature was studied in the temperature interval of 298–318 K with model solutions of 4-CP with equal initial concentration (C0 ∼ 200 mg dm−3 ), pH (6.0), and mass of the catalyst (1 g dm−3 ). The study aims at examining if Ni-oxide system acts at low temperatures, which is very advan-
phase oxidation of 4-CP over different catalytic systems (5–20 kJ/mol) [19]. The results reveal that temperature (in the studied temperature interval) does not affect the value of the rate constant of liquid phase oxidation of 4-CP with participation of Ni-oxide system, which is an explanation of the low activation energy. This low value could be as well explained with the high mobility of active surface oxygen in the catalyst, respectively, low energy of the surface cation to active oxygen bond, provided by the high degree of oxidation of Ni ions in the oxide and their octahedral coordination [20]. The high mobility of the surface oxygen is the probable reason of the observed high selectivity of the process. The calculated values of activation energy are close to that, characteristic of heterogeneous catalytic reactions, running in inner diffusion area. Having in mind, however, that experiments are carried out with catalyst fraction of 0.6–1.2 mm, one can assume that the inner surface of the catalyst grain is used to a great degree and the reaction runs with high rate. This give us grounds to assume that diffusion of the reagents in the pores of catalyst is not the limiting stage of the oxidation process. The limiting stage is the interaction of the substrate with surface-active oxygen of the oxide. Based on all results obtained, we suggest the following scheme of catalytic oxidation of 4-CP with participation of Ni-oxide system:
tageous. The temperature interval was chosen with a view to further development of a new catalytic method and new technological scheme for purification of waste waters containing 4-CP. The results obtained are listed in Table 5. The calculated value for activation energy of the process is comparable with reported values for liquid
According to the scheme, we suppose the run of two parallel process of interaction of the substrate with surface • OH, yielding more strongly oxidized intermediates (HQ, quinone, and 4-CCT). The latter are further oxidized completely or partially to final products, CO2 , H2 O, and mineral acids, depending on experimental conditions.
Table 5 Effect of the reaction temperature on the rate constants T (K)
k × 103 (min−1 )
Ea (kJ mol−1 )
298 308 318
6.16 7.62 9.29
16.2
heterogeneous catalytic reaction rather than of pure heterogeneous process.
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Only traces of HQ have been detected in the course of catalytic oxidation, while the curve reflecting changes in concentration of BQ exhibits well-defined maximum (Fig. 4). This fact gives us grounds to assume that the rate of formation of HQ is lower than the rate of its subsequent transformation into BQ. CO2 (proved by means of chemical and IR spectral analyses) and mineral acids are the final products of catalytic oxidation. Some authors, who have studied catalytic and photocatalytic oxidation of 4-CP, explain yielding of CO2 as a result of the so called rake-mechanism reported in [21] which has been subsequently applied in photocatalytic oxidation processes [22,23]. Based on results of the present study as well as on literature data, the discussed mechanism can be schematically illustrated as follows:
weakly bound active surface oxygen (O∗s = 8%), provide low temperature complete conversion of 4-CP to CO2 , H2 O, and mineral acids. Data obtained about the effect of pH, temperature, and amount of oxide system on efficiency of the process allow to establish the optimum conditions for running the process as well as to throw light on the discussed mechanism of liquid phase catalytic oxidation and to develop a method for purification of waste waters containing 4-CP.
Acknowledgements Authors gratefully acknowledge the financial support from the University of Plovdiv Research Fund through project 01-X-03 and from the Bulgarian National Scientific Research Foundation. References
4. Conclusion The high specific surface of Ni-oxide system (110 m2 g−1 ), in combination with high content of
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