Zn 1.5 PW 12 O 40 Mixed Oxide Nanotube: An Efficient and Stable Catalyst for Degradation of Phenol by Ultraviolet

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1 2011 International Conference on Biology, Environment and Chemistry IPCBEE vol.24 (2011) (2011)IACSIT Press, Singapoore Zn 1.5 PW 12 O 40 Mixed Oxide Nanotube: An Efficient and Stable Catalyst for Degradation of Phenol by Ultraviolet Huang Jiguo +, Wang Yu, Xia Tingting, Li Sen, Sun Baifeng Key Laboratory of Groundwater Resources and Environment, Ministry of Education, Jilin University, Changchun , PR China Abstract. As a continuation of our work to develop catalysts with high activity for catalytic oxidation process, degradation of wastewater containing phenol (0.3g/L) by ultraviolet oxidation over Zn 1.5 PW 12 O 40 nanotube catalyst was studied. It was found that Zn 1.5 PW 12 O 40 nanotube has good photocatalytic activity and performs well in the treatment of refractory pharmaceutical wastewater. The recycled catalyst after activated has little change in its catalytic characteristics. Keywords: Zn 1.5 PW 12 O 40 nanotube, catalytic oxidation, ultraviolet, phenol 1. Introduction Removal of pollutants such as dyes, phenols, and nitrobenzene in water is very important for protection of our environment. One of the promising options for the removal of such organic pollutants from wastewater is destruction of these contaminants by photocatalysis [1].The radicals can be easily produced using UV irradiation based on the homogenous photochemical degradation of oxidizing compounds like hydrogen peroxide and ozone. An alternative way to obtain free radicals is the photocatalytic mechanism occurring on the surface of semiconductors and this indeed substantially enhances the rate of generation of free radicals and hence the rates of degradation [2]. In the case of photocatalytic oxidation, the most common associated problem is the reduced efficiency of photocatalyst with continuous operation possibly due to the adsorption of contaminants on the surface and blocking of the UV activated sites, which makes them unavailable for the destruction. Thus, our aim should be to devise a technique for continuous cleaning of the catalyst surface during the photocatalytic operation [3]. Research published since circa 1980 has established the potential of polyoxometalates (POMs) as homogeneous oxidation catalysts [4-6]. POMs have been applied as acid and oxidation catalysts, including in several large-scale industrial processes. However, their applications are limited due to their inherent drawbacks, including their low specific surface area, low stability under catalytic conditions, and their high solubility in aqueous solution. Many strategies that immobilize POMs on various porous solid supports have been studied in an effort to improve their catalytic abilities [7]. Some POMs with Keggin or Dawson structures have the ability to participate in multiple, consecutive, and reversible multi electron reductions without decomposition to mixed valence species (called heteropolyblues) [8]. In addition to traditional inorganic and carbon-based porous materials, porous coordination polymers constructed from metal ions and doubly or multiply connecting ligands have recently appeared and become a new type of porous material [9, 10]. In this paper, an attempt is made to achieve the degradation of phenol in the presence of nanotube Zn 1.5 PW 12 O 40 catalyst under photocatalysis. The result shows that the self-made Zn 1.5 PW 12 O 40 nanotube has good photocatalytic activity. The catalyst can be separated easily by settling and shows great repeatability. 2. Experimental Details 62

2 2.1. Preparation of Zn 1.5 PW 12 O 40 nanotubes Zinc acetate (1.5g) was added into 5 ml ethanol. H 3 PW 12 O 40 23H 2 O (1g) was dissolved in 10 ml distilled water to form a clear solution. Six pieces of commercial filter paper was placed in a suction filtering unit. The zinc acetate solution and H 3 PW 12 O 40 solution were added dropwise into the filter funnel alternatively for about 20 times, while the solutions were slowly filtered through the filter paper. Then the filter paper was thoroughly washed by water and ethanol alternatively in order to remove the complexes which did not attach onto the surface of the filter paper. Then the filter paper was dried with air flow. The asprepared Zn 1.5 PW 12 O 40 /paper was calcined for 6 h at 460 C in air with heating rate 10 C/min. The resultant green powder (Zn 1.5 PW 12 O 40 nanotube) was obtained with yield of 0.6g Physical measurements Elemental analysis was carried out using a Leeman Plasma Spec (I) ICP-AES. TEM micrographs were recorded on a Hitachi H-600 transmission electron microscope. XRD patterns of the sample were collected on Japan Rigaku Dmax X-ray diffractometer with Cu Kα radiation (λ= nm). The IR spectrum of the nanotube was taken to verify the structure by Fr-IR. In order to further confirm the structure of Zn1.5 PW 12 O 40 nanotube, the 31 P MAS NMR spectrum of the sample was analyzed Catalytic procedure The oxidation reaction was determined by 0.3g/L of phenol degraded by ultraviolet at room temperature and atmospheric pressure in the presence of the catalyst. A procedure was carried out as follows: 0.5g of catalyst and simulated phenol waste water (500mL) were placed in the quartzose photocatalytic reactor (Fig.1). The mixture was uniform after magnetic stirring at room temperature. An ultraviolet source (8W, 365nm, 30000cd) was added in the photocatalytic device. The reaction happened under vigorous stirring. At given intervals, a sample of suspension was taken out by filtration. The clear filtrate solution of the catalytic process was tracked by GS/MS. The spectrophotometer was used in the experiment to monitor the degradation of phenol at 510 nm. 3. Results and discussion Fig.1: Photocatalytic reactor 3.1. Characterization of catalyst Zn 1.5 PW 12 O 40 nanotubes From the result of elemental analysis, the contents of the material are W, 75.9%; P, 1.0%; Zn, 3.2%. The ratio of W: P: Zn is 12:1:1.5, corresponding to the formula of Zn 1.5 PW 12 O 40 while the POM form the nanotube. The morphology and microstructure of the prepared Zn 1.5 PW 12 O 40 were investigated with transmission electron microscopy (TEM). The TEM image in Fig.2 shows that the sample displayed tube-like shapes with the average diameters from 20 to 25 nm, the length of the tube ranges from 300 nm, the width 21nm and the inner width 6 nm respectively. 63

3 Fig. 2: The TEM image of the Zn 1.5 PW 12 O 40 nanotubes To identify the structure of Zn 1.5 PW 12 O 40 nanotubes, XRD spectroscopy was performed and the result is given in Fig. 3. All the diffraction peaks can be readily indexed to tetragonal PW 12 O 40 phase [10]. (JCPDS no ). Fig. 3: The XRD pattern of the Zn 1.5 PW 12 O 40 nanotubes The IR spectrum of the Zn 1.5 PW 12 O 40 nanotube is shown in Fig. 4. It is in good agreement with that of its 3- parent PW 12 O 40 observed in macro-scale, with the four characteristic peaks reflecting the different vibrations of oxygen atoms of the Keggin-type structure PW 12 O 3-40 : 1083, 987, 895 and 810 cm -1 can be attributed to the asymmetry vibrations P-Oa (internal oxygen connecting P and W), W-Od (terminal oxygen bonding to W atom), W-Ob (edge-sharing oxygen connecting W) and W-Oc (corner- sharing oxygen connecting W 3 O 13 units. Fig. 4: The IR spectrum of the Zn 1.5 PW 12 O 40 nanotubes In order to further confirm the structure of Zn 1.5 PW 12 O 40 nanotube, the 31 P MAS (magic-angle spinning) NMR spectrum of the sample was analyzed. A characteristic peak at ppm shown in (Fig. 5) is the same as the starting H 3 PW 12 O 40 (-15.0 ppm), suggesting that during the synthetic process, the POMs PW 12 O 40 keeps its structure. Fig. 5: The 31 P MAS NMR spectrum of the Zn 1.5 PW 12 O 40 nanotubes 3.2. Effects of the catalyst The degradation of phenol is shown in Fig.6. It can be seen from comparison: the phenol removal rate under UV was 85%. After about 5min, the phenol concentration did not change significantly. However, in the Zn 1.5 PW 12 O 40 /UV system, the degradation of phenol has obvious increased. The efficiency could reach 99% within 8min. 64

4 Fig.6: The degradation of phenol The initial GC / MS detection diagram is shown in Fig.7. Carbon dioxide, formic acid, acetic acid, hydroquinone and diethyl phthalate are found. The result after a while is shown in Fig.8. The detected organic compounds are formic acid, acetic acid and hydroquinone. Fig.7: The initial GC / MS detection diagram Fig.8: The later GC / MS detection diagram Compared Fig.7 with Fig.8, obvious changes can be found. With the reaction time, diethyl phthalate (the peak of in Fig.7) is totally degraded. Hydroquinone (the peak of in Fig.7) decreased a lot. Formic acid (the peaks of in Fig.7 and in Fig.8) and acetic acid (the peaks of in Fig.7 and in Fig.8) both increased, especially acetic acid. Formic acid, acetic acid and hydroquinone detected in Fig.7 are in accordance with the intermediates of the free radical mechanism of the phenol degradation. Due to the decomposition in the column, some organics may not be detected, such as maleic acid, malonic acid and oxalic acid. The repeatability of the Zn 1.5 PW 12 O 40 nanotubes was also investigated. The treated wastewater was kept still for a while, and then the supernatant was discarded. The precipitate was rinsed with distilled water, dried at 100 and activated at 200 for 2.5h. The regeneration was completed after cooling in the dryer. The effect of the recycled catalyst was shown in Table 1. Table 1 The removal rate of phenol by reused catalyst Recyled times The removal rate of phenol/%

5 4. Conclusions The synthetic method of catalyst is a unique, convenient, low-cost and green chemical pathway, which is suitable for industrial production. When treating high concentration phenol (0.3 g/l), Zn 1.5 PW 12 O 40 nanotube exhibits an excellent catalytic activity under ultraviolet conditions, while phenol is completely mineralized into small inorganic species after a very short time (8 min). The removal rate increases from 85% to 99%. Moreover, Zn 1.5 PW 12 O 40 nanotube is insoluble in water and can be separated easily. The leaching test results indicate the stability and less leaching of leach, which can be reused for at least five times. 5. Acknowledgements This work was supported by the National Natural Science Foundation of China (No ) and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (No ). 6. References [1] Romero DD, Torres G, Arevalo JC, et al. Synthesis and characterization of TiO 2 doping with rare earths by sol-gel method: photocatalytic activity for phenol degradation. J Sol-Gel Sci. Techn.2010, 56(3): [2] Yawalkar AA, Bhatkhande DS, Pangarkar VG. Solar-assisted photochemical and photocatalytic degradation of phenol. J. Chem. Technol. Biotechnol.2001, 76(4): [3] Khokhawala IM, Gogate PR. Degradation of phenol using a combination of ultrasonic and UV irradiations at pilot scale operation. Ultrason. Sonochem.2010, 17(5): [4] Renneke RF, Hill CL. Homogeneous catalytic photochemical functionalization of alkanes by polyoxometalates. J Am Chem Soc.1986, 108(12): [5] Ellis S, Kozhevnikov IV. Homogeneous oxidation of methyl isobutyrate with oxygen catalysed by metal complexes: polyoxometalates versus metalloporphyrins and metallophthalocyanines. J Mol Catal A-Chem.2002, 187(2): [6] Liu Y.Y., Murata K, Inaba M. New peroxo-polyoxometalates pillared hydrotalcite catalyst for propylene epoxidation by molecular oxygen in methanol. Chem.Lett. 2006, 35(4): [7] Proust A, Thouvenot R, Gouzerh P. Functionalization of polyoxometalates: towards advanced applications in catalysis and materials science. Chem. Commun.2008, (16): [8] Li C.X., Zhang Y., Kevin P. O Halloran. Electrochemical behavior of an inorganic-organic hybrid based on isopolymolybdate anions and ethylenediamine. J Appl Electrochem. 2009, 39(7): [9] Huang J.G., Wang X.H., Li S. ZnO/MoO 3 mixed oxide nanotube: A highly efficient and stable catalyst for degradation of dye by air under room conditions. Appl. Surf. Sci. 2010, 257(1): [10] Chen C.C., Wang Q., Lei P.X., et al. Photodegradation of dye pollutants catalyzed by porous K3PW12O40 under visible irradiation. Environ.Sci.Technol.2006, 40(12):