Enhanced photoactivity for the phenol mineralization on ZnAlLa mixed oxides prepared from calcined LDHs

Catalysis Today 220–222 (2014) 56–60

Contents lists available at ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

Enhanced photoactivity for the phenol mineralization on ZnAlLa mixed oxides prepared from calcined LDHs F. Tzompantzi a,∗ , G. Mendoza-Damián a , J.L. Rico b , A. Mantilla c,∗ a

Universidad Autónoma Metropolitana-Iztapalapa, Departamento de Química, Av. San Rafael Atlixco No. 186, México 09340, D.F., Mexico Laboratorio de Catálisis, Facultad de Ingeniería Química, Universidad Michoacana de San Nicolás de Hidalgo, Edificio V1, C.U., 58060 Morelia, Mich., Mexico c CICATA, Instituto Politécnico Nacional, Av. Legaria No. 694, México 11500, D.F., Mexico b

a r t i c l e

i n f o

Article history: Received 25 April 2013 Received in revised form 13 July 2013 Accepted 15 July 2013 Available online 28 August 2013 Keywords: ZnAlLa mixed oxides ZnAlLa LDH Phenol mineralization Photodegradation of phenol

a b s t r a c t ZnAlLa layered double hydroxides (La/Zn molar ratio of 0.005, 0.01 and 0.03) were synthesized by the coprecipitation method and calcined at 500 ◦ C. XRD patterns showed lamellar type materials in fresh solids (dried) and a diminishing basal parameter denoting the incorporation of La in the ZnAl LDH structure. At low content of La, XRD does not show the formation of La2 O3 , which become detectable only in the sample with high La content. After calcination, the formation of zincite was detected in the LDH, indicating the destruction of the lamellar structure. The Eg band of the sample with low content of La, calculated from the UV–Vis spectra, showed a shift to low energy, while for the samples with high content of La the Eg shift is displaced to high absorption energy. Evaluation of the photoactivity in the degradation of phenol using a low intensity UV–Vis lamp (254 nm and 4400 ␮W/cm2 ), showed the maximum values in phenol mineralization 88–66% with the ZnAlLa samples, in comparison to that obtained with the ZnAl oxide (48%). These results show that ZnAlLa mixed oxides prepared from the thermal treatment of ZnAlLa LDH are promising materials for an effective phenol mineralization. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, the generation of alternatives to solve the problems caused by water, air and soil pollution, is becoming increasingly urgent. Phenol and its derivatives are categorized as one of the most hazardous contaminants of water, due to its toxic and carcinogenic nature arising from the high stability of the phenolic compounds that, besides, are very difficult to remove thus remaining into water for a long time. These compounds are frequently found in waste streams from chemical industries like petroleum refining industries (6–500 mg/L), coking operations (29–3900 mg/L), coal processing (9–6800 mg/L) and petrochemical processes (2.8–1220 mg/L) [1]. Several research papers are devoted to the treatment of water contaminated with phenol, among them, physical, biological, chemical and electrochemical methods, including those named Advanced Oxidation Process (AOP) [2–4]. For the AOP photocatalytic semiconductor materials are widely used to degrade organic contaminants in water [2,5,6]. The photocatalytic process is initiated with the irradiation of the semiconductor material using a light with similar/higher energy

∗ Corresponding authors. Tel.: +52 55 57296300x67752. E-mail addresses: [email protected] (F. Tzompantzi), [email protected], [email protected] (A. Mantilla). 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.07.014

to that of its own band-gap, resulting in the formation of the electron–hole pairs (e− /h+ ), which can recombine or migrate to the surface and then be captured by an acceptor/donor species thus starting the redox reaction. The most commonly solids used for APO processes are TiO2 and ZnO photocatalysts, where ZnO has the advantage of its lower cost and better performance in the photodegradation of organic compounds [3,4,7–9] but it has the drawback of its high electron–hole pairs recombination. Different methodologies have been tested in order to reduce the recombination rate of ZnO and TiO2 such as doping with rare earth metal oxides (mainly cerium and lanthanum) which have shown successful results [10–12]. On the other hand, layered double hydroxides (LDHs) or hydrotalcite-like materials belong to a class of anionic mineral clays, with structure derived from mineral brucite, Mg(OH)2 . When a fraction of Mg2+ ions is isomorphously substituted by a trivalent ion such as Al3+ , the positive charge generated on the metal hydroxide slab is compensated by the inclusion of anions generating the general formula: [M2+ 1−x M3+ x (OH)2 ]x+ (An− )x/n ·H2 O, where M2+ and M3+ are divalent and trivalent metal ions, respectively, and An− is an intercalated anion, being CO3 2− the most common. When LDH were subject to a controlled thermal decomposition, they transformed into the respective mixed oxides. Therefore, layered double hydroxides (LDH) can be used as a favorable and controllable method for the preparation of mixed oxides [13].

F. Tzompantzi et al. / Catalysis Today 220–222 (2014) 56–60

57

Some LDHs have been recently reported as a good alternative for the photodegradation of pollutant organic compounds like pesticides [14], methyl-orange [15], methylene-blue [16], methyl-violet [17], malachite-green [17,18] and phenolic compounds [19,20] in aqueous media as well as in the photocatalytic hydrogen production [21]. The present study aims to the synthesis of ZnAlLa mixed oxides, after calcination of ZnAlLa layered double hydroxides. The effect of the incorporation of lanthanum to ZnAl layered double hydroxides in the photocatalytic behavior of these materials during the degradation of phenol in the presence of UV light is used as photocatalytic test and the results are discussed. 2. Experimental 2.1. Preparation of catalysts ZnAlLa layered double hydroxides (LDHs) with atomic ratio (Zn/ZnAlLa) of 0.61 were synthesized by the co-precipitation method using Zn(NO3 )2 ·6H2 O, Al(NO3 )3 ·9H2 O (Fermont 99.9%) and LaCl3 ·6H2 O (Strem Chemicals 99.9%) as precursor salts; urea was used as precipitant agent. Procedure goes as follows: 2 L of solution was prepared dissolving, in deionized water, the amount of salts calculated to obtain the La/Zn molar ratio of 0.005, 0.01 and 0.03, respectively. This solution was stirred at 80 ◦ C for 12 h under reflux and then, urea was added in order to generate the corresponding hydroxides, maintaining the solution stirred for another 24 h. The precipitated was filtered under vacuum and washed several times with deionized water before drying at 100 ◦ C for 24 h. Prior to the catalytic test, the samples were heated up from room temperature to 500 ◦ C with a program rate of 1 ◦ C/min and calcined at that temperature for 5 h. Samples were named as ZnAlLa 05, ZnAlLa 1, ZnAlLa 3 for 0.005, 0.01 and 0.03 of La/Zn molar ratio preparation, respectively and as ZnAl 2 for the sample prepared without La. 2.2. Characterization XRD patterns of the dried (LDHs) and calcined materials were obtained with a Bruker D-8 Diffractometer by using Cu K radiation. BET specific surface areas were obtained from the nitrogen adsorption isotherms using a QUANTACHROME Autosorb-3B equipment following the BET equation whereas the pore size distribution was evaluated from the desorption isotherms using the BJH method. UV–Vis absorption spectra for the different calcined samples were obtained with a Cary-100 Varian spectrophotometer equipped with an integration sphere. The energy band gap (Eg) evaluation for the various samples was calculated from the value obtained by extrapolating the reflectance to the x-axis curve for y = 0. 2.3. Phenol photodegradation The activity of these materials was tested for the photocatalytic degradation of phenol. The evaluation was carried out in a slurry glass reactor, equipped with a cooling jacket to operate at constant temperature, according to the following procedure: 200 mL of water solution containing 40 ppm of phenol (0.42 mmol L−1 ) and 200 mg of catalyst where put in contact and maintained under constant stirring. The reaction mixture was irradiated with a Pen-Ray lamp (254 nm and 4400 ␮W/cm2 ), covered with a quartz bulb, as UV source. Air was continuously bubbled (1 mL/s) into the system during the experiment. Prior to the photocatalytic reaction, the UV source was turn off for an hour in order to assure the adsorption of phenol on the solid surface. The degradation of phenol was started after that time by turning on the UV lamp. The reaction was monitored by taking samples, using a syringe filter, and analyzing those aliquots with a UV–Vis spectrophotometer. The absorption band at 269 nm as a function of the irradiation time was used for phenol

Fig. 1. X-ray diffraction patterns of ZnAl and ZnAlLa layered double hydroxides (a) ZnAlLa 0.5, (b) ZnAlLa 1, (c) ZnAlLa 3 and (d) ZnAl 2.

detection. A calibration curve, which was previously obtained, was utilized to determine the concentration of phenol in each aliquot. 3. Results and discussion Diffraction patterns for all samples measured in the 2 range of 5–70, with a 2 step of 0.028◦ are presented in Fig. 1. The most intensive peaks can be observed at 12, 23, 34 and 39◦ , which represent the characteristic peaks of hydrotalcite-like materials [14]. It can be noted that, only in the sample with the highest content of La (ZnAlLa 3) the presence of La2 O3 can be observed (Fig. 1), denoting that some La was not incorporated in the LDH structure (Fig. 1). The parameter a corresponds to the cation–cation distance within the brucite-like layer of the LDH and c parameter is related to the thickness of the brucite-like layer and the interlayer distance. These parameters were calculated by the equations a = 2xd(1 1 0) and c = 3xd(0 0 3), respectively (Table 1), using the Bragg law: n = 2dsin ; n = 2d sin , where  is the wavelength corresponding to 1.5406 A˚ and  is the angle of diffraction. The interlayer distance, calculated assuming a thickness of 4.8 A˚ for the brucite lattice, was increased with the incorporation of lanthanum to the material. On the other hand, the value of the “a” parameter diminished in function of the La content in the LDH; this behavior has been reported as an evidence of the effective incorporation of La in the LDH framework [22–24]. On the other hand, XRD patterns of calcined ZnAl and ZnAlLa LDHs are shown in Fig. 2. Where the presence of zincite is noted; thus, the structure of the LDH has been destroyed after calcination. In Fig. 3 the nitrogen adsorption isotherms for the different calcined ZnAlLa LDH materials are presented. They correspond to Type IV isotherms with H2 hysteresis loop, usually observed in mesoporous materials. In Table 2, the specific surface areas are ranging from 98 to 130 m2 /g. Thus, the calcined samples showed high specific surface areas for any lanthanum content.

Table 1 Lattice parameter for the ZnAl and ZnAlLa layered double hydroxides. LDH

d(0 0 3) (Å)

Parameter c (Å)

Parameter a (Å)

Interlayer distance (Å)

ZnAl 2 ZnAlLa 05 ZnAlLa 1 ZnAlLa 3

7.688 7.389 7.446 7.427

23.066 22.169 22.337 22.281

3.0765 3.0578 3.0619 3.0578

2.888 2.589 2.646 2.627

58

F. Tzompantzi et al. / Catalysis Today 220–222 (2014) 56–60

Fig. 2. Adsorption isotherms of ZnAl and ZnAlLa LDHs.

Fig. 5. Photocatalytic behavior of the degradation of phenol in aqueous solution using ZnAl and ZnAlLa LDHs under UV light.

In Fig. 4, it is shown the absorbance (R) of the samples, as a function of the wavelength for the different LDHs. In comparison to the ZnAl 2 LDH (Table 2), it can be seen there that the energy of the band gap is slightly shifted to the blue region for the sample with high La content (ZnAlLa 3), whereas for the samples with low La content the Eg band was shifted to the low energy region. This red shift of the band gap for the samples with low La content can be attributed to the insertion of La in the ZnAl structure. In the opposite, when high La content is used, certain amount of La could have been segregated forming La2 O3 . Then, an important contribution of the segregated oxide could have modified the UV–Vis spectra of this sample, shifting the Eg to the blue region. 4. Photocatalytic behavior

Fig. 3. XRD spectra of ZnAl and ZnAlLa LDHs after calcination at 500 ◦ C. Table 2 Textural properties and band gap values of ZnAl and ZnAlLa LDHs. Catalyst

Specific surface area (m2 /g)

Pore size BJH (Å)

Pore volume (cm3 /g) 10−1

Band gap (eV)

ZnAl 2 ZnAlLa 05 ZnAlLa 1 ZnAlLa 3

124 122 98 130

34.0 33.8 33.9 34.2

3.12 2.4 2.6 4.4

3.587 3.481 3.521 3.660

Fig. 4. UV–Vis spectra of ZnAlLa and ZnAl LDHs.

The effect of the lanthanum addition to ZnAl LDH in the phenol photodegradation is shown in Fig. 5, where it is possible to note that activity was noticeably increased (97%) in the sample with low content of lanthanum (ZnAlLa 05); when the La content was increased (ZnAlLa 1 and ZnAlLa 3) the activity was 83 and 75%, respectively, reaching almost the same photo activity obtained with the sample without lanthanum (75%). However, the positive effect of La in the photoactivity was more evident in the results of phenol mineralization determined by TOC analysis (Fig. 6), where the presence of lanthanum in the samples increased their capacity of mineralization, reaching the maximum level (around 90%) for the LDH with

Fig. 6. Mineralization of phenol using the different ZnAl and ZnAlLa photocatalyst determined by TOC analysis.

F. Tzompantzi et al. / Catalysis Today 220–222 (2014) 56–60

59

Table 3 Photocatalytic behavior of calcined ZnAl and ZnAlLa LDH materials. Catalyst

t1/2 (min)

K (103 ) (min−1 )

Degradation (%)

Mineralization % (TOC)

K/specific surface area (×105 )

ZnAl 2 ZnAlLa 05 ZnAlLa 1 ZnAlLa 3

126.5 64.7 114.4 114.6

5.48 10.72 6.06 6.05

75.0 97.5 83.0 75.0

48.3 88.2 75.9 66.1

4.42 8.78 6.18 4.65

low La content (ZnAlLa 05). In samples with higher content of lanthanum, the mineralization was 75 and 65% for ZnAlLa 1 and ZnAlLa 3, respectively. Those values are higher than the ones obtained with ZnAl, which reached only 48% of mineralization. The phenol photodegradation follows a pseudo-first-order kinetic reaction and the rate constant was evaluated from Fig. 7. The values obtained for the apparent reaction rate constants (k) and the required time (t1/2 ) to decompose half of the phenol present in the irradiated solution with the different ZnAl and ZnAlLa materials, confirm that the photocatalytic degradation rate is much higher in the case of the ZnAlLa catalyst as compared to the value obtained with the ZnAl catalyst (Table 3). Some authors attribute [13] the increase in the photocatalitic activity shown by the materials based on La-doped ZnO at low content of La to the presence of a large number of oxygen vacancies, which cause strong absorption of OH− ions in the surface of ZnO. This site acts then as surface bound trap for the photogenerated holes, avoiding the electron–hole recombination as well as generating oxidizing species capable of breaking the different bonds present in the phenol molecule [13]. When the semiconductor material is excited by a source of UV light, the pair e− /h+ is generated. These holes (h+ ), present in the valence band, have an oxidative high enough potential to carry out the oxidation process of organic compounds by itself, or in the presence of hydroxyl radicals which can be formed by the decomposition of water or by the reaction of the hole with OH− radicals. The combination of hydroxyl radicals and the photo-generated holes present a very strong oxidative potential capable of degrading organic molecules in its surface [12]. ZnO + h → ZnO(e− + h+ )

(1)

h+ + Phenol → oxidationofPhenol

(2)

h+ + H2 O → H+ + • OH

(3)

h+ + OH− → • OH

(4)

Fig. 7. Pseudo-first-order kinetic for the photodegradation of phenol for ZnAl and ZnAlLa mixed oxides derived from LDHs.

On the other hand, superoxide anions can be generated by the reduction of molecular oxygen due to the e− of the conduction band, which may generate organic peroxides or hydrogen peroxides. Hydroxyl radicals, reported as the principal responsibles for the mineralization of organic compounds, can also be produced by the e− of the conduction band. La3+ + O2 − → La2+ + O2

(5)

e − + O2 → • O2 −

(6)

•O − 2

+ Ph → Ph-OO•

(7)

•O − 2

+ HO2 • + H+ → H2 O2 + O2

(8)

• OH

+ Phenol → degradationofPhenol

(9)

The effect of the addition of lanthanum to ZnAl LDH in its photocatalytic behavior is understandable since, under irradiation, La+3 ions work as electron scavengers, that may react with the superoxide species promoting the separation of the photo-induced holes–electrons (h+ /e− ), thus increasing the photo-oxidation efficiency, avoiding as well recombination [25]. According to Khatamian et al., at high content of La3+ , the space charge region becomes narrower because the surface barrier becomes higher causing a more efficient separation of the electron–hole pairs by a powerful electric field. Additionally, the depth of the space charge layer can be exceeded by the penetration of light into ZnO so promoting the recombination of photogenerated electron–hole pairs. Then, it is important that the content of La3+ ions be the proper one to equate the thickness of the charge layer to the depth of the light penetration for separating photoinduced electron–hole pairs. In the materials where the content of

Fig. 8. X ray diffraction patterns of ZnAlLa 05 mixed oxide derived from the calcination of LDHs and recovery after phenol photodegradation.

60

F. Tzompantzi et al. / Catalysis Today 220–222 (2014) 56–60

La+3 is higher (ZnAlLa 1 and ZnAlLa 3), La+3 could be segregated, losing photo-activity. In order to evaluate the stability of the mixed oxides and the presence of the memory effect, XRD of the samples recovered after the reaction was performed (Fig. 8). The spectra correspond to destroyed mixed oxide, thus in these samples the memory effect does not occur and the structure of the original LDH is not recuperated after put them in the phenolic solution and irradiated. 5. Conclusions In the present study, the synthesis of ZnAl and ZnAlLa mixed oxides by the thermal treatment of LDH materials is reported. The results obtained in the application of these materials for the photocatalytic degradation of phenol, allow us to propose that, ZnAlLa mixed oxides are promising materials for a very efficient mineralization of phenol, reaching 88% of total mineralization in comparison to the reference ZnAl mixed oxide, in which the phenol mineralization was only 48%. Acknowledgements The authors are grateful to CONACyT for the financial support (Projects 154994 and 169157). Angeles Mantilla acknowledges IPN for the financial support (Project SIP 20131470). Guadalupe Mendoza Damián thanks CONACyT for the scholarship granted (Scholarship number: 252081). References [1] G. Busca, S. Berardinelli, C. Resini, L. Arrighi, Technologies for the removal of phenol from fluid streams: a short review of recent developments, Journal of Hazardous Materials 160 (2008) 265–288. [2] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Advanced oxidation processes (AOP) for water purification and recovery, Catalysis Today 53 (1999) 511–559. [3] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO2 , Solar Energy Materials and Solar Cells 77 (2003) 65. [4] A.A. Khodja, T. Sehili, J.F. Pilichowski, P. Boule, Photocatalytic degradation of 2-phenylphenol on TiO2 and ZnO in aqueous suspensions, Journal of Photochemistry and Photobiology A: Chemistry 141 (2001) 231. [5] D. Prakasini, K. Parida, B. Ranjan, Journal of Molecular Catalysis A: Chemical 240 (2005) 1–6. [6] B. Naik, K.M. Parida, Industrial & Engineering Chemistry Research 49 (2010) 8339–8346. [7] A. Sharma, P. Rao, R.P. Mathur, S.C. Ameta, Photocatalytic reactions of xylidine ponceau on semiconducting zinc oxide powder, Journal of Photochemistry and Photobiology A: Chemistry 86 (1995) 197–200.

[8] K.M. Parida, S. Parija, Industrial & Engineering Chemistry Research 49 (2010) 8339–8346. [9] K.M. Parida, S.S. Dash, D.P. Das, Journal of Colloid and Interface Science 298 (2006) 787–793. [10] V.K. Pareek, A.A. Adesina, H.S. Nalwa (Eds.), Handbook of Photochemistry and Photobiology, vol. 1, American Scientific Publishers, Stevenson Ranch, CA, 2003, pp. 345–412. [11] Anandan, A. Vinu, T. Mori, N. Gokulakrishnan, P. Srinivasu, V. Murugesan, K. Ariga, Photocatalytic activity of La-doped ZnO for the degradation of monocrotophos in aqueous suspension, Journal of Molecular Catalysis A: Chemical 266 (2007) 149–157. [12] S. Anandan, A. Vinu, T. Mori, N. Gokulakrishnan, P. Srinivasu, V. Murugesan, K. Ariga, Photocatalytic degradation of 2,4,6-trichlorophenol using lanthanum doped ZnO in aqueous suspension, Catalysis Communication 8 (2007) 1377–1382. [13] M. Khatamian, A.A. Khandar, B. Divband, M. Haghighi, S. Ebrahimiasl, Heterogeneous photocatalytic degradation of 4-nitrophenol in aqueous suspension by La (La3+ , Nd3+ or Sm3+ ) doped ZnO nanoparticles, Journal of Molecular Catalysis A: Chemical 365 (2012) 120–127. [14] A. Mantilla, F. Tzompantzi, J.L. Fernández, J.A.I. Díaz Góngora, G. Mendoza, R. Gómez, Photodegradation of 2,4-dichlorophenoxyacetic acid using ZnAlFe layered double hydroxides as photocatalysts, Catalysis Today 148 (2009) 119–123. [15] E.M. Seftel, E. Popovici, M. Mertens, K. De Witte, G. Van Tendeloo, P. Cool, E.F. Vansant, Zn–Al layered double hydroxides: synthesis, characterization and photocatalytic application, Microporous and Mesoporous Materials 113 (2008) 296. [16] M. Parida, N. Baliarsingh, B. SairamPatra, J. Das, Copperphthalocyanine immobilized Zn/Al LDH as photocatalyst under solar radiation for decolorization of methylene blue, Journal of Molecular Catalysis A: Chemical 267 (2007) 202. [17] K.M. Parida, L. Mohapatra, Chemical Engineering Journal 179 (2012) 131–139. [18] K. Parida, L. Mohapatra, N. Baliarsingh, Journal of Physical Chemistry C 116 (2012) 22417–22424. [19] A. Mantilla, F. Tzompantzi, J.L. Fernández, J.A.I. Díaz Góngora, R. Gómez, Photodegradation of phenol and cresol in aqueous medium by using Zn/Al + Fe mixed oxides obtained from layered double hydroxides materials, Catalysis Today 150 (2010) 353–357. ˜ [20] F. Tzompantzi, A. Mantilla, F. Banuelos, J.L. Fernández, R. Gómez, Improved photocatalytic degradation of phenolic compounds with ZnAl mixed oxides obtained from LDH materials, Topics in Catalysis 254 (2011) 257–263. [21] K. Parida, L. Mohapatra, Dalton Transactions 41 (2012) 1173–1178. [22] S. Velu, K. Suzuki, T. Osaki, F. Ohashi, S. Tomura, Synthesis of new Sn incorporated layered double hydroxides and their evolution to mixed oxides, Materials Research Bulletin 34 (1999) 1707–1717. [23] D.Sh. Tong, H. Ch, Zh, M.Y. Li, W.H. Yu, J. Beltramini, Ch. X. Lin, Z.P. Xu, Structure and catalytic properties of Sn-containing layered double hydroxides synthesized in the presence of dodecylsulfate and dodecylamine, Applied Clay Science 48 (2010) 569–574. [24] N. Das, A. Samal, Synthesis, characterisation and rehydration behaviour of titanium(IV) containing hydrotalcite like compounds, Microporous and Mesoporous Materials 72 (2004) 219–225. [25] M. Khatamian, B. Divband, A. Jodaei, Degradation of 4-nitrophenol (4-NP) using ZnO nanoparticles supported on zeolites and modeling of experimental results by artificial neural networks, Materials Chemistry and Physics 134 (2012) 31–37.