Catalytic Wet Peroxide Oxidation of Chlorophenol ...

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Dec 13, 2013 - programmed reduction, and its activity in the catalytic wet peroxide oxidation of 4-chlorophenol (4-CP) and 2,4- dichlorophenol (2,4-DCP) was ...
doi 10.1515/ijcre-2013-0079

International Journal of Chemical Reactor Engineering 2013; 11(1): 1–9

Jia Zeng, Guilin Zhou*, Yongmei Ai, Ning Li, and Guizhi Zhang

Catalytic Wet Peroxide Oxidation of Chlorophenol Over a Ce0.86Cu0.14–xO2 Catalyst Abstract: A Ce0.86Cu0.14–xO2 catalyst was prepared by the citric acid complex method and characterized by X-ray diffraction, scanning electron microscopy, and temperatureprogrammed reduction, and its activity in the catalytic wet peroxide oxidation of 4-chlorophenol (4-CP) and 2,4dichlorophenol (2,4-DCP) was investigated. The results showed that the Cu2 þ ions dissolved into the CeO2 lattice to form a Ce0.86Cu0.14–xO2 solid solution with a coarse, interconnected, porous, and cotton-like morphology. The metal– oxygen bonds were weakened by solid-solution formation in the Ce0.86Cu0.14–xO2 catalyst. This weakening facilitated H2O2 activation and decomposition to form highly oxidative HO∙ species that can lead to significant chlorophenol mineralization. A total organic carbon removal rate greater than 80% was achieved after 2 h reaction at 50°C and at an initial 4-CP and 2,4-DCP concentration of 50 mg/L. The effects of H2O2 dosage, catalyst dosage, and initial chlorophenol concentration on catalytic efficiency were also determined. Keywords: catalytic wet peroxide oxidation (CWPO), 4chlorophenol, 2,4-dichlorophenol, CeCu oxide catalyst, solid solution

*Corresponding author: Guilin Zhou, Chongqing Key Labatory of Catalysis & Functional Orgnaic Molecules, Department of Chemistry and Chemical Engineering, Chongqing Technology and Business University, Chongqing 400067, China, E-mail: [email protected] Jia Zeng: E-mail: [email protected], Yongmei Ai: E-mail: [email protected], Ning Li: E-mail: [email protected], Guizhi Zhang: E-mail: [email protected], Chongqing Key Labatory of Catalysis & Functional Orgnaic Molecules, Department of Chemistry and Chemical Engineering, Chongqing Technology and Business University, Chongqing 400067, China

1 Introduction In recent years, concerns regarding environmental issues have steadily increased because of their effects on everyday life. Wastewater generated from industrial processes contain organic toxic compounds and have become a major social and economic focus as rigorous health-quality standards and environmental regulations are gradually being implemented. Priority pollutants, such as highly toxic and nondegradable organic compounds,

are of particular concern because of difficulties associated with their treatment by conventional wastewater treatment methods. Chlorophenols are representative compounds widely used in the pesticide, pharmaceutical, dye, and plastics industries and one of the major organic pollutants in wastewater. These compounds are of particular interest because of their high toxicity, low degradability, photochemical toxicity, and bioaccumulation even at low concentrations [1, 2]. As such, chlorophenols are listed by the United States Environmental Protection Agency as persistent organic pollutants in the Clean Water Act as well as in the European Decision 2055/2011/ EC (2001) [3]. In addition, the release of untreated chlorophenolic wastewater into natural recipients is prohibited because of the suspected mutagenic and carcinogenic properties of the compounds. Wastewater containing chlorophenols must be strictly controlled and properly handled prior to their discharge into the receiving water. A number of alternative technologies have been developed to treat wastewater containing highly toxic organic compounds. Advanced oxidation processes have been proposed as promising candidates [4, 5], particularly those based on the use of Fenton’s reagent (H2O2 þ Fe2 þ ). The Fenton process has drawn considerable research attention because of its simplicity of design and operation as well as its ability to process a diverse range of priority pollutants. In a Fenton reaction, H2O2 is activated by a catalyst to produce highly reactive hydroxyl radicals that can participate in nonselective oxidation of organic pollutants in aqueous solutions up to mineralization or to form small molecules. Fenton processes are normally performed under homogeneous conditions. Therefore, these processes have two main drawbacks, namely, loss of the active component and strictly limited pH. These disadvantages limit the practical application of the Fenton process. To overcome these problems, catalytic wet peroxide oxidation (CWPO), which uses a stable heterogeneous catalyst to replace the homogeneous catalyst of the Fenton process, was developed. Compared with the Fenton reaction, CWPO processes exhibit economic benefits, produce low secondary pollution, and exhibit strong adaptability to reaction conditions [6]. Both iron and copper oxides can be used as active components to react with H2O2 and produce highly

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J. Zeng et al.: CWPO of Chlorophenol Over a Ce0.86Cu0.14–xO2 Catalyst

oxidative HO∙ species in the CWPO process. As reported in the literature, the use of catalysts containing iron oxides is generally preferred by researchers. However, the use of copper-based catalysts in the CWPO process remains in the initial stages of experimentation [7]. Moreover, the majority of the CWPO studies reported thus far focus primarily on single active-component loading on the supporter materials. In this method, active species on the supported catalyst usually exhibit leaching. Thus, bimetal oxide catalysts are potentially useful as heterogeneous catalysts in CWPO, because their distinct crystalline structure allows active species to leach from the homogeneous catalyst system. CeO2 has attracted considerable attention in environmental catalysis because of its high oxygen storage capacity and Ce4 þ /Ce3 þ redox cycling property [8]. In particular, CuO–CeO2 system catalysts have been widely investigated for catalytic reactions [9], such as NO reduction, complete CO oxidation, preferential oxidation, and water–gas shifting, because of the presence of strong interactions between CuO and CeO2. However, studies on the application of this catalyst system in the CWPO process are limited [10]. Moreover, phenol is usually regarded as a target compound in CWPO processes, and only a small number of studies have focused on the use of chlorophenols as model pollutants. In addition, the use of 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP) as target compounds in the CWPO process has yet to be extensively studied. Therefore, an efficient catalyst with high catalytic activity in the CWPO process must be developed for potential practical applications. This work aims to synthesize a Ce0.86Cu0.14–xO2 catalyst via a simple and efficient citric acid complex method at 50°C for 2 h, for use in the CWPO process. The prepared Ce0.86Cu0.14–xO2 catalyst was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and H2 temperature-programmed reduction (H2-TPR). The catalytic performance of the prepared Ce0.86Cu0.14–xO2 catalyst was also investigated using the CWPO elimination of 4-CP and 2,4-DCP from wastewater. The effects of catalyst usage, H2O2 dosage, and concentrations of the target compounds on 4-CP and 2,4-DCP removal were further investigated.

2 Experimental

(CA) (nCe/nCu molar ratio ¼ 6.0; nCA/n(Ce þ Cu) molar ratio ¼ 1.8) were dissolved in a certain amount of deionized water. The mixture was then stirred at 80°C until the water completely evaporated. Afterward, the sample was dried at 100°C overnight and calcinated at 450°C in muffle furnace for 3 h. The resulting product is referred to as Ce0.86Cu0.14–xO2 catalyst.

2.2 Catalyst characterization 2.2.1 XRD characterization XRD patterns were recorded on a Rigaku D/Max–2500/PC diffractometer with a rotating anode using Ni filtered CuKα [as radiation source (λ ¼ 0.15418 nm)] radiation at 40 kV of a tube voltage and 200 mA of a tube current. The data of 2θ from 20° to 80° range were collected with the step size of 0.02° at the rate of 5°/min.

2.2.2 Structure characterization The structure characterization includes morphology and surface area characterization. The morphology and size of the crystallites were determined by scanning electron micrograph (SEM) images taken with a JSM-5900LV microscope. Specific surface area of the fresh and used catalysts were measured on Autosorb-6 at 77 K. Prior to the measurement, all samples were degassed at 473 K until a stable vacuum of about 5 mTorr was reached. The specific surface area was assessed using the Brunauer– Emmett–Teller (BET) method from adsorption data in a relative pressure range from 0.06 to 0.10.

2.2.3 H2-TPR studies H2-TPR was carried out as follows: 10 mg of the catalyst sample was placed in a U-type quartz tube, 5.0 vol.% H2/ Ar gas mixture was passed through the tube at a flow rate of 25 mL/min, and the reduction temperature was raised from room temperature to 360°C at a rate of 10°C/min. The consumption of hydrogen was measured by a thermal conductivity detector.

2.1 Catalyst preparation

2.3 Catalytic activity measurement

In a typical complex-method synthesis of CeCu oxide catalyst: Cu(NO3)2·3H2O, Ce(NO3)4·6H2O, and citric acid

The catalytic test was carried out in a 50-mL flask with a magnetic stirrer and a thermostatic water bath at

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J. Zeng et al.: CWPO of Chlorophenol Over a Ce0.86Cu0.14–xO2 Catalyst

where C0 and Ct are the initial and final TOC values of the 4-CP or 2,4-DCP aqueous solution, respectively. The dissolved Cu2 þ and Ce4 þ concentration in solution after reaction are detected by atomic absorption spectrometry (Hitachi Z-5000).

3 Results and discussion 3.1 XRD characterization The XRD patterns of the prepared Ce0.86Cu0.14–xO2 catalyst are shown in Figure 1. The prepared Ce0.86Cu0.14–xO2 catalyst shows strong XRD peaks at 2θ ¼ 28.6°, 33.1°, 47.5°, 56.3°, 59.1°, 69.4°, 76.7°, and 79.1°, which are in good agreement with the characteristic diffraction peaks of a cubic CeO2 fluorite structure (JCPDS 34–39). Meanwhile, the two weak diffractions at 2θ ¼ 35.5° and 38.7° are attributed to the bulk CuO phase. This result indicates that only a small amount of the CuO phase exists in the prepared Ce0.86Cu0.14–xO2 catalyst. The majority of the Cu2 þ ions dissolve into the CeO2 lattice to form a Ce0.86Cu0.14–xO2 solid solution at a Ce/Cu molar ratio of 6:1, leaving only a small amount of the CuO bulk phase in the catalyst. Figure 1 also shows that the prepared Ce0.86Cu0.14–xO2 solid solution exhibits a fluorite-like structure as a result of high-temperature calcination [11–13]. This result suggests that Cu2 þ , which features a small ionic radius (0.072 nm), is dissolved into the CeO2 lattice during high-temperature calcination and that the dissolution of Cu2 þ does not disrupt the CeO2 fluorite structure. The strong interactions between CuO and CeO2 are attributed to the formation of the Ce–Cu solid solution [10, 14]. In the Ce–Cu solid solution, strong interactions between

CeO2 CuO

Intensity / a.u.

atmospheric pressure. In a typical run, 1.0 g/L of prepared Ce0.86Cu0.14–xO2 catalyst (powder) was added to 30 mL of 4-CP or 2,4-DCP aqueous solution. When the reaction temperature was reached, a certain amount of 30% H2O2 was added into the reactor and the reaction started. For all runs, the total reaction duration was 120 min. Reaction samples were then collected and immediately analyzed after centrifugation. The 4-CP or 2,4-DCP concentration was determined by total organic carbon (TOC) measurement using a TOC-VCPN Shimadzu TOC analyzer. The catalytic activity in terms of the TOC removal efficiency was calculated as follows:   C0  Ct TOC removal ð X Þ ¼  100% ð1Þ C0

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Used catalyst

Fresh catalyst

20 25 30 35 40 45 50 55 60 65 70 75 80 2θ/o Figure 1 XRD patterns of fresh and used Ce0.86Cu0.14–xO2 catalyst

CuO and CeO2 can improve the charge transfer between Cu þ /Cu2 þ and Ce4 þ /Ce3 þ [8], which may provide highly active sites for the reactant molecules. On the other hand, strong interactions between CuO and CeO2 can weaken the Cu–O and Ce–O chemical bonds, which can easily dissociate to form reactive oxygen species. At the same time, Cu2 þ dissolved in the CeO2 lattice can lead to lattice distortion and form lattice defects [15]. These lattice defects can increase the oxygen vacancy content and enhance the migration and diffusion abilities of oxygen species in the prepared Ce–Cu solid-solution catalyst, thereby improving the ability of the catalyst to supply active oxygen species for the reactants. From Figure 1, the used and fresh catalysts have the same XRD patterns including their peak position, intensity, and area. The results indicate that the crystal structure of the used Ce0.86Cu0.14–xO2 catalyst did not obviously change in the process of the CWPO.

3.2 Structure analysis SEM images of the fresh and used Ce0.86Cu0.14–xO2 catalyst are shown in Figure 2. The surface of the prepared Ce0.86Cu0.14–xO2 catalyst exhibits a rough, loose, porous, and cotton-like morphology. The sizes of the particles present on the catalyst surface vary from 5 μm to 10 μm, and the small particles form a cotton-like structure with an abundance of interconnected pores. A large aggregated structure is also observed in the catalyst. The specific surface areas of the fresh and used Ce0.86Cu0.14–xO2 catalysts calculated by the BET equation are 63.4 and 62.8 m2/g, respectively. The morphology of the catalyst surface strongly depends on the selected preparation method. In this

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J. Zeng et al.: CWPO of Chlorophenol Over a Ce0.86Cu0.14–xO2 Catalyst

3.3 TPR studies

WD Mag Spot Det HV Pressure 8.8 mm 5,000 × 3.0 ETD 20.0 kV

20.0 µm

Used

WD Mag Spot Det HV Pressure 8.9 mm 5,000 × 3.0 ETD 20.0 kV

20.0 µm

Figure 2 SEM photograph of fresh and used Ce0.86Cu0.14–xO2 catalyst

work, the method used to prepare the Ce0.86Cu0.14–xO2 catalyst with a cotton-like structure is closely related to the citric acid complex method. During thermal treatment at 450°C, the precursor, which contains a large amount of citric acid and nitrates, can thermally decompose and gasify to produce an abundance of CO2, CO, H2O, and NOx gases. These generated gases act as pore-creating agents and are vital in the formation of loose pores as well as in the diversity of the cotton-like structure. This type of catalyst structure can provide suitable space for the adsorption and activation of reactant molecules. From Figure 2, the surface morphology of the used catalyst is similar to the fresh catalyst, and the specific surface area of used catalyst does not obviously change. The results indicated that the Ce0.86Cu0.14–xO2 catalyst has high structure stability.

H2-TPR has been extensively used to evaluate the reducibility of the prepared Ce0.86Cu0.14–xO2 catalyst. The H2-TPR profiles of the catalyst are shown in Figure 3. The Ce0.86Cu0.14–xO2 catalyst exhibits TPR reduction peaks with three different intensities: α, β, and γ at 150, 180, and 218°C, respectively. The TPR reduction peaks can be ascribed to the reduction of different reducible species, and the areas of the reduction peaks follow the order α < β < γ. The H2-TPR reduction temperature of the Ce0.86Cu0.14–xO2 catalyst is below 300°C. In addition, the H2 consumption curve of the catalyst clearly deviates from the baseline (as shown by the arrow) at ,70°C. The TPR curve results are attributed to the reduction of reducible species of the catalyst by H2 at the corresponding temperature. The reduction peak area is directly affected by the amount of reducible species. In general, the greater the H2 consumption in the TPR, the larger the amount of reducible species in the catalyst system. Moreover, the temperature location of the H2 consumption peaks strongly depends on the reducibility of the reducible species: a high reducibility corresponds to a low-temperature location of the H2 consumption peak. The reducibility of one species is closely associated with its dispersion. In general, a highly dispersed species is readily reduced by H2 at low temperatures. According to the literature [16], the reduction profile of pure CeO2 has two H2-TPR reduction peaks at approximately 450 and 800°C. These peaks correspond to the reduction of superficial and bulk CeO2, respectively. In addition, the reduction of pure CuO exhibits a single H2-TPR reduction peak at ,300°C [17, 18]. As shown in Figure 3, the reduction temperature of the prepared Ce0.86Cu0.14–xO2 catalyst γ

Intensity / a.u.

Fresh

β α

Fresh catalyst

Used catalyst 40

80

120

160

200

240

280

320

360

Temperature / oC Figure 3 H2-TPR profiles of fresh and used Ce0.86Cu0.14–xO2 catalyst

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J. Zeng et al.: CWPO of Chlorophenol Over a Ce0.86Cu0.14–xO2 Catalyst

5,500 mg/L. Figure 4 shows the relationships between the 4-CP and 2,4-DCP TOC removal and H2O2 amount at atmospheric pressure, a catalyst dosage of 1.0 g/L, 120-min reaction duration, 30 mL of 50 mg/L 4-CP and 2,4-DCP (initial concentration), and 50°C reaction temperature. Figure 4 clearly indicates that the H2O2 dosage significantly affects the catalytic wet oxidation of 4-CP and 2,4-DCP. When the H2O2 dosage is zero, the final TOC removal rates of 4-CP and 2,4-DCP are 1.4% and 8.9%, respectively. This result indicates that 4-CP and 2,4-DCP molecules can be adsorbed on the Ce0.86Cu0.14–xO2 catalyst from a corresponding aqueous solution and that the Ce0.86Cu0.14–xO2 catalyst has a higher adsorption capacity for 2,4-DCP than for 4-CP molecules. The presence of H2O2 can significantly enhance the TOC removal of 4-CP and 2,4-DCP. In addition, Ce0.86Cu0.14–xO2 exhibits higher catalytic performance for 4-CP molecules than for 2,4DCP molecules during CWPO at the same H2O2 and catalyst dosage. When the H2O2 dosage is 1,100 mg/L, the TOC removal rates in the 4-CP and 2,4-DCP solutions significantly increase to 46% and 37.7%, respectively. The TOC removal rates of 4-CP and 2,4-DCP in wastewater reach 86% and 46%, respectively, when the H2O2 dosage is increased to 2,200 mg/L. The 4-CP TOC removal curve is flat or shows a slightly decreasing trend when the H2O2 dosage in the 4-CP wastewater solution exceeds 2,200 mg/L. On the other hand, the TOC removal of 2,4-DCP in the wastewater solution continuously increases until the H2O2 dosage reaches 4,400 mg/L. The TOC removal of 4-CP and 2,4-DCP reach maximum values of 86% and 80.4%, respectively, at H2O2 dosages of 3,300 mg/L and

90 80 70 TOC removal / %

is clearly lower than that of the pure CuO and CeO2 species. This phenomenon is attributed to the change in dispersion as well as in the properties of the CuO and CeO2 species in the prepared Ce0.86Cu0.14–xO2 catalyst. The XRD results indicate that a larger number of Cu2 þ dissolves into the CeO2 lattice to form a Ce0.86Cu0.14–xO2 solid solution with a fluorite-like structure. The physicochemical properties of the overall components may change because of the presence of strong interactions between CuO and CeO2 in the Ce0.86Cu0.14–xO2 solid solution. Therefore, the reducibility of the CuO and CeO2 species can be enhanced, because the Cu – O and Ce – O chemical bonds are weakened as a result of strong CuO – CeO2 interactions. The SEM results show that the large, coarse, interconnected, porous, and cotton-like structures on the catalyst surface may facilitate the adsorption and reaction of H2, thereby promoting the reduction of reducible oxide species on the catalyst surface at low temperatures. The α, β, and γ peaks can be attributed to the reduction of different CuO species and the well-dispersed ceria on the surface of the catalyst. The α peak observed at 150°C can be attributed to the reduction of highly dispersed CuO species, which have strong interactions with CeO2. The β peak at 180°C can be assigned to the reduction of isolated CuO existing on the surface of the Ce0.86Cu0.14–xO2 catalyst. The γ peak at 218°C is registered to the reductions of Cu þ produced from the low-temperature reduction, and bulk CuO having strong interaction with CeO2 of the catalyst. The highly dispersed CeO2 on the surface can be reduced at relatively low temperatures because of the strong interaction between CuO and CeO2, as proposed in previous reports [19]. Moreover, the TPR curve clearly deviates from the baseline at 70°C, suggesting that the prepared Ce0.86Cu0.14–xO2 catalyst can be reduced by H2 at low temperatures. The formation of oxygen vacancies in the prepared Ce0.86Cu0.14–xO2 can improve the diffusion and migration ability of oxygen species in the catalyst system, thus promoting the reduction of the corresponding reducible species at low temperatures (