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Author's personal copy Journal of Physics and Chemistry of Solids 72 (2011) 914–919
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TiO2/MgAl layered double hydroxides mechanical mixtures as efficient photocatalysts in phenol degradation Silvia P. Paredes a, Miguel A. Valenzuela a,n, Geolar Fetter b, Sergio O. Flores a a b
´lisis y Materiales, ESIQIE-Instituto Polite´cnico Nacional, Zacatenco, 07738, Mexico D.F., Mexico Lab. Cata ´noma de Puebla, 72570 Puebla, Pue., Mexico Facultad de Ciencias Quı´micas, Universidad Auto
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 November 2010 Received in revised form 7 March 2011 Accepted 25 March 2011 Available online 10 May 2011
MgAl layered double hydroxides (MgAl LDH) were synthesized by the sol–gel method using ultrasound irradiation in the crystallization step. The interlayer anions were nitrate and acetylacetonate-ethoxide. The solids were characterized by XRD, N2-physisorption and TEM. TiO2/MgAl LDH mixtures were prepared by mixing the MgAl LDH (as prepared) or the calcined sample with TiO2 (Aldrich, 99.9% anatase) in different weight ratios. Photocatalytic activities of the TiO2/MgAl LDH mixtures were evaluated through the degradation of phenol as model pollutant. TiO2/MgAl LDH mixture (1:1) was more photocatalytically active for the degradation of phenol than pure TiO2. The synergy effect was attributed to a higher production of OH radicals, which were formed from the structural hydroxides. Also, the hydrotalcite phase enhanced the phenol adsorption and transfer to the TiO2 sites where the phenol was photocatalytically degradated. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds B. Chemical synthesis C. X-ray diffraction C. Electron microscopy D. Crystal structure
1. Introduction The hydrotalcite-like compounds or layered double hydroxides (LDH) or simply called the hydrotalcite are the anionic equivalents of cationic clays. LDH structure may be easily visualized from brucite as hexagonal close packing of hydroxyls ions and magnesium ions occupying all the octahedral positions. If Mg2 þ ions are substituted by Al3 þ ions during the synthesis process, the LDH in brucite-type structure is obtained. The brucite-type layer (HO–Mg–O–Al–OH) is, then, positively charged and one positive charge is generated for each aluminum substituted by magnesium ion. To maintain the electrical neutrality, anions are required, and they are located in the interlamellar spaces [1–4]. These anions are normally hydrated. Fig. 1 shows the LDH structure. The LDH can be represented by the general formula II ½ðM II Þ1x ðM III Þx ðOHÞ2 x þ ðAm x=m ÞUnH2 O, where M is a divalent metal, MIII is a trivalent metal and A is a compensating anion (CO2,3 SO22 ,NO3 ,Cl ,y) with charge m and n is the number of water molecules. The most common LDH, known as hydrotalcite is constituted by magnesium and aluminum as MII and MIII metals and the positive charges are compensated by carbonate anions. The structure of these kinds of compounds, based on the stacking of positively charged layers with anions and water
n Correspondance to: IPN-ESIQIE, Edificio 8, tercer piso, Zacatenco, 07738 Mexico D.F. MEXICO Tel.: 5729 6000x55293; fax: 5255 55862728. E-mail address:
[email protected] (M.A. Valenzuela).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.03.017
confers relatively high mobility to the anions. It has been observed that the anions are easily exchanged in the following order [5]: 2 2 CH3 COO 4 NO 3 4ClO4 4Cl 4SO4 4CO3
The LDH can be obtained by direct or indirect synthesis [6]. Indirect synthesis can traditionally be conducted using three main techniques: (a) direct anion exchange, (b) anion exchange by elimination of the interlamellar precursor species susceptible to acid attack and (c) regeneration of the calcined LDH. The calcined LDH, which is composed by metal oxides, may be rehydrated again by contacting the solid with an anionic aqueous solution; the hydrotalcite structure is then recovered by incorporating the solution anions. Direct synthesis can be carried out by the coprecipitation or sol–gel methods. In a previous work [7] we studied the effect of microwave irradiation on the synthesis of sol–gel hydrotalcites intercalated with ethoxide and acetylacetonate species compared with a nitrated sample. The crystallite sizes of the microwaved LDH’s were smaller than those obtained by the conventional method and the resulting specific surface areas were bigger [8]. Photocatalytic oxidation processes are highly effective advanced technologies for the degradation of a wide variety of pollutants in wastewater [9]. The principle of photocatalysis is based on the generation of excited states of semiconducting materials by exposure to light. This allows exceptional redox reactions to take place. Nevertheless, rapid recombination of holes and electrons, large band gaps and mass transfer limitations
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was irradiated by ultrasound irradiation in an Elma D-78224 cleaner container operating at 2.4 kW and 25 kHz, the temperature was fixed at 80 1C. The final pH value of the mixture was adjusted to 11.5 with ammonium hydroxide. The resulting mixture was again irradiated for 10, 20 or 30 min under the same controlled pressure, temperature and stirring. The solid was separated by decantation and washed several times with ethanol. Ethanol was evaporated from the dispersion with a nitrogen flow. The solid was dried at 70 1C for 3 h. Samples with 10, 20 and 30 min of irradiation periods were referred to as UEA10, UEA20 and UEA30, respectively. All the samples were prepared under N2 atmosphere and the metallic molar ratio (Al3 þ /(Al3 þ þMg2 þ )) was maintained constant at 0.25. 2.2. Synthesis of MgAl-LDH nitrated
Fig. 1. Structure of a hydrotalcite-like compound.
are, among others, current issues to be improved for practical applications [10]. According to the extensive amount of publications related to titanium dioxide in several photocatalytic applications [9–11], it is clear that TiO2 will continue being used, due to the excellent photon absorption and excellent reactant adsorption compared with other semiconductors [12]. However, TiO2 has a very low photonic efficiency (i.e. less than 10% for most degradation reactions) than several aspects concerning to the synthesis and design of photocatalytic systems covering; structure and morphology, doping, metal coating, surface sensitization and composite semiconductors are being extensively studied [13]. In particular, the layer structure may improve efficiency of photocatalytic reactions by inducing electron transfer, preventing recombination of electrons and holes and avoiding agglomeration of nanoparticles [14,15]. LDH’s can be prepared with great flexibility both for metal ions and incorporated anions, thus a large variety of photocatalysts can be synthesized [6]. Works devoted to the use of LDH’s as photocatalysts have been focused to ZnAl-LDH, as-synthesized and calcined forms, obtaining homogeneous mixtures of oxides with small crystal size and hence high surface area [16], as well as polyoxometalates intercalated in layered double hydroxides [17] or pillared p-tungstate ions [18] or intercalated p-aminobenzoic acid in MgAl-LDH and ZnAl-LDH as sun screen formulation [19,20]. However, in our knowledge, there is no report about the synergist photocatalytic properties of mechanical mixtures of TiO2 with MgAl-LDH or with the calcined form. In this work, hydrotalcites were synthesized by the sol–gel method using ultrasound irradiation during the hydrothermal treatment step. These solids were mixed with titania (anatase) in different weight ratios and tested in the degradation of phenol in aqueous solution.
Nitrated hydrotalcite-like compound with an Al/(Mg þAl) molar ratio of 0.25 was synthesized as reported in a previous work [7]; a 1.86 N NaOH aqueous solution was added to a 2.5 M aqueous solution containing Mg(NO3)2 6H2O and Al(NO3)3 9H2O. The pH varied up to a final value of 13. The obtained gel was treated by ultrasound irradiation for 10, 20 and 30 min. The obtained samples were washed with deionized water and dried in an oven at 70 1C. These samples were designed as UNHT10, UNHT20 and UNHT30, depending on the ultrasound irradiation time. Sample UNHT10 was also calcined in air at 500 1C for 18 h (labeled as UNHT10C). 2.3. Characterization The X-ray diffraction patterns were obtained using a Siemens D-500 diffractometer coupled to a copper anode X-ray tube. A diffracted beam monochromator selected the Ka radiation. Compounds were identified by the conventional way using the JCPDS cards. The BET surface area, pore volume and mean pore diameter were determined by standard multipoint techniques using a Micromeritics ASAP 2010 instrument. A Jeol 1020 transmission electron microscope coupled to an EDX detector provided the sample micrographs and the corresponding elemental composition. 2.4. Photocatalytic activity test
2. Experimental
TiO2 (Aldrich, 99% anatase) was mixed by kneading with the LDH samples (UEA10, UNHT10 and UNHTC10) at weight ratios of: 1:1, 1:1.5 and 2:1 with each LDH. Photocatalytic phenol (Baker) degradation experiments were conducted using the following procedure. A solution of deionized water, phenol (50 ppm) and the catalyst under test were dispersed in an ultrasound bath and then poured into the reactor. Air was dispersed in the aqueous solution through six 1/4 inch rod spargers located in radial position to maintain a catalyst slurry. The solution was irradiated for 8 h by one fluorescent black light tube (UV Philips, 8 W, lmax ¼365 nm) in a batch annular photoreactor thermostated at 25 1C. A sample of 4 mL was extracted each hour from the photoreactor; the solids were separated by filtration of the sample (0.45 mm, Millipore). Phenol analysis was done using a UV–vis CINTRA 20-GBC apparatus, with lmax of phenol ¼270 nm.
2.1. Synthesis of MgAl-LDH with interlayered organic compounds (acetylacetonate-ethoxide)
3. Results and discussion
A series of samples of hydrotalcite-like compounds were synthesized following the procedure described elsewhere [7]. Magnesium ethoxide (Aldrich, 99%) was mixed with aluminum acetilacetonate (Aldrich, 99%), then, the mixture was added to ethanol and HCl aqueous solution as a catalyst. The obtained gel
3.1. Structure Figs. 2 and 3 correspond to the XRD patterns of UNHT and UEA series, respectively. These samples were synthesized by ultrasound irradiation for 10, 20 and 30 min. The X-ray diffraction
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003
009 015
Intensity (a.u.)
Intensity (a.u.)
006 110
018
UEA30
MgO
UEA20 UEA10 14
4
24
44
34 2θ
5 64
54
15
25
35
2θ
45
55
65
75
Fig. 4. X-ray diffraction pattern of MgAl LDH calcined sample (UNHT10).
Fig. 2. X-ray patterns of ultrasound prepared LDH with acetylacetonate-ethoxide as interlayered organic compounds at different irradiation times (UEA).
003
Intensity (a.u.)
006 009
5
10
15
20
25
30
015
35
40
110
018
45
50
55
60
65
70
2θ Fig. 3. X-ray patterns of ultrasound prepared LDH with nitrate as interlayered inorganic compounds at different irradiation times (UNHT).
peaks in all the cases were identified as LDH according to the JCPDS card 22-0700, then, pure hydrotalcite-like compounds were obtained. We observe in both series that by increasing the irradiation time, the crystallite size is also increased. Indeed, the smaller the particle size, a bigger surface area may result, which could favor the hydrotalcite adsorption capacity improving the photocatalytic process. Therefore, we selected the samples irradiated at 10 min (UEA10 and UNHT10) in order to have suitable conditions in the evaluation of the photocatalytic properties of the mixtures TiO2–MgAl LDH. If we compare the samples UEA10 and UNHT10, the interlamellar distances determined for UEA10 sample shows a small displacement to the left in the 00l diffraction peaks corresponding to an increase in the interlamellar distance (d) based on the 003 diffraction peak. The d value changes from 7.8 A˚ for the card value to 7.88, 7.81 and 7.86 A˚ for the samples UNHT10 and UEA10, respectively, which corresponds to an average of 3.05 A˚ of interlamellar space. This indicates that for the UNHT10 sample nitrate is the compensating anion because the average size reported for this anion was 3.99 A˚ [7]. For the LDH with interlayered organic compounds (acetylacetonate-ethoxide), the d value is smaller ˚ than any of the dimension of the acetylacetonate (4.81 3.64 A) ˚ ions, this kind of samples may and ethoxide (3.73 2.20 A) originate different species. Fig. 4 shows the X-ray diffraction pattern of the UNHT10 sample after calcination at 500 1C for 18 h. It can be observed that the characteristic peaks of magnesium oxide and a microcrystalline phase corresponds to a mixture of Al and Mg oxides.
Fig. 5. Transmission electron micrograph of the MgAl LDH, UEA10 sample.
The transmission electron microscopy image of the sample UEA10 as shown in Fig. 5 presents the local ordering of the brucite-type layers, in which the interlamellar distance (d) is clearly appreciated and represented by dark zones where the compensation anions (ethoxide and acetylacetonate) are located. The EDX measurements indicate that the corresponding metallic molar ratio (Al3 þ /(Al3 þ þMg2 þ )) was 0.25 for UEA10 and 0.25 for UNHT10 samples. Table 1 shows the textural properties of samples UEA10 and UNHT10. As can be seen, the lowest specific surface area (SSA) was obtained with UNTH10, though it is higher than that reported by other authors using autoclave method [6]. The calcined sample UNTH10C, showed a significant decrease of its SSA from 97 to 10 m2/g. It seems that the calcination temperature and time of treatment were not enough to complete the involved reactions that conventionally increase the surface area of LDH’s [21]. Note that the SSA of the UEA10 was 248 m2/g, this value is more than 100% higher than that of the UNHT10, and this result was attributed to the type of interlayered anion.
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Table 1 Textural properties of Mg/Al-hydrotalcites as-synthesized and calcined (C). Catalyst
Surface area (m2/g)
Pore volume (cm3/g)
Mean pore diameter (nm)
UEA10 UNTH10
248 97
0.245 0.082
3.7 3.3
10
0.036
2.6
UNTH10C
917
MgO (main compound, shown in Fig. 2), the reaction proceeds in the same way as that of the TiO2/LDH until 2 h of irradiation time, however, after this period the reaction is lowered significantly, but the final conversion was almost the same. An explanation to this behavior should be done in terms of the structure of LDH and the mixed oxides, the former is highly hydroxylated, and then these species are easily converted to OH radicals. On the contrary, MgO does not have structural OH species and must be taken from water and converted to OH radicals. UEA10-TiO2 mixture presented a lower conversion (80%) than UNHT10-TiO2; however, the degradation profile was the same, which confirms that the structural OH species surrounding TiO2 after irradiation are easily converted to OH radicals. The lower conversion for this last catalyst was attributed to the interlayer organic anion, which inhibits the adsorption of phenol on its surface. It is clear from these results that the addition of the same amount of LDH to TiO2 generates a kinetic synergy effect in phenol disappearance, with an increase of 9% and 21% conversion units using UEA10-TiO2 and UNTH10-TiO2, respectively, compared with pure TiO2 anatase. A synergist effect has also been found by mixing titania with activated carbon (AC) and was explained in terms of extended adsorption of phenol on AC followed by a transfer to titania, where the photocatalytic degradation occurs [12]. Nevertheless, samples UNTH10 and UNTH10C with small surface areas (97 and 10 m2/g) showed higher conversion when mixed with TiO2 (Fig. 7).
Fig. 6. Phenol degradation profiles of MgAl LDH (UNHT10 and UEA10 samples, as-prepared), photolysis and photocatalytic degradation using TiO2.
3.2. Photocatalytic degradation of phenol Phenol degradation profiles of UNHT10, UEA10 LDHs as prepared samples, the photolytic reaction and the photocatalytic reaction with TiO2 photocatalytic degradation are compared as shown in Fig. 6. This figure has the purpose of showing that only with the presence of TiO2 a significant degradation of phenol is observed (72%). The photolytic reaction and the UNHT10 sample achieved 15% of phenol degradation after 8 h of irradiation time although with a different degradation profile. This last sample defines the typical behavior of adsorption effect, whereas, the UEA10 sample practically did not show adsorption. Note that the photolytic reaction and the photocatalytic conversion with TiO2 showed an induction period of about 2 h, in which the reaction started. This behavior has been attributed to a long period required for the formation of OH radicals to carry out the photo-oxidation [12]. As shown in Table 1, the specific surface areas (SSA) of samples UNHT and UEA10 were 97 and 248 m2/g, respectively, in comparison with 50 m2/g belonging to TiO2, then, there is no correlation between the SSA with the adsorption effect on these solids. Even though, UEA10 had a high SSA compared with UNHT, the higher adsorption capacity is attributed to the type of interlayer anion. This means that the presence of organic anions inside the interlayer spaces inhibits the phenol adsorption. Usually, phenol degradation has a low reaction rates and it depends on the light intensity mainly [22], in this work we use only one black light lamp of 8 W, which could explain the long period to start the degradation. The phenol degradation profiles obtained with LDHs/TiO2 and mixed oxides/TiO2 catalysts are shown in Fig. 7. The weight ratio of TiO2 and LDH or the corresponding mixed oxides was 1/1. It is interesting to highlight that each mixture followed a different path. For instance, UNHT10-TiO2 sample presented a linear increase in phenol degradation of all the irradiation time, reaching a phenol conversion of 93%. If we compare the degradation path of the calcined sample UNHT10C/TiO2, it means that TiO2 is mixed with
Fig. 7. Phenol degradation profiles of TiO2/Mg–Al LDH (UNHT10, UNHT10C calcined and UEA10 samples) using a weight ratio MgAl LDH/TiO2, 1:1.
Fig. 8. Phenol degradation profiles of Mg–Al LDH (UNHT10 sample)/TiO2 using different weight ratios.
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In order to understand the effect of the weight ratio of LDHTiO2, three experiments by varying the amount of UNHT10 sample were carried out. As shown in Fig. 8, higher phenol degradation is obtained with the 1:1 weight ratio (93%) and by increasing the weight ratios from 1.5:1 or 2:1 a detrimental behavior is observed and the phenol conversion was 51% and 45%, respectively. The same experiments were performed with the UEA10-TiO2 and are shown in Fig. 9. A similar tendency can be observed than that observed with the UNTH10-TiO2 sample. Note that in all cases the reaction started after 2 or 3 h of irradiation time, which explains that the adsorption of phenol is not enhanced with the addition of high surface area UEA10 (220 m2/g) sample, see Table 1. These results revealed a linear photodegradation profile reaching a 81% conversion with the 1:1 weight ratio, and 51% and 32% using 1.5:1 and 2:1 weight ratios, respectively. The mineralization of phenol was checked in a qualitative way at the end of the reaction (8 h) by the formation of barium
Fig. 9. Phenol degradation profiles with Mg–Al LDH (UEA10 sample)/TiO2 using different weight ratios.
carbonate in a reservoir containing barium hydroxide. Although several reaction mechanisms have been reported for the photocatalytic degradation of phenol, it is accepted that the highly oxidative holes (h þ ) with E1¼ þ2.8 V, may directly react with the surface-sorbed phenol or indirectly be oxidized via formation of OH radicals according to the following reactions [23–25]: TiO2 þhu-e þh þ
þ
(1)
h þH2O- OHþH
þ
h þOH - OH þ
(2) (3)
In the presence of molecular oxygen, the negatively charged electrons are preferentially trapped by oxygen as O2 þe -O2
(4)
O2 þ2H þ2e -H2O2 þ
H2O2 þe -OH þ OH
(5) (6)
Therefore, materials rich in OH groups, as LDH, may favor the titania activity. Indeed, as shown in Figs. 6–8, for mixed 1:1 samples, the activities were higher than that of pure titania. Furthermore, a conversion of 99% is obtained for the sample UNHT10C-TiO2 (1:1), which is constituted by calcined hydrotalcite and titania. In this case, the surface OH groups should be more accessible than those of the hydrotalcite lamellar. On the other hand, the samples with organic interlayers (UEA10), may block more structural OH groups than the nitrated hydrotalcite (UNHT), which results in decrease of the photocatalytic activity, as shown in Fig. 9. When the amount of hydrotalcite:titania was increased to 1.5:1 and 2:1 the phenol conversion decreased to values lower than those for pure TiO2, Fig. 9. The OH radicals formation by electronic transfer mechanism could be affected by covering the TiO2 nuclei with hydrotalcite in excess. Thus, the distances from OH groups to Ti atoms could become bigger enough to block the electronic transfer. Also, a screen effect of the hydrotalcite, blocking the UV-light to TiO2 could not be discarded. A complete overview of the process was proposed and depicted in Fig. 10.
Fig. 10. A proposed scheme of the photocatalytic degradation of phenol in presence of a mixture of TiO2/Mg–Al layered double hydroxides.
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4. Conclusion MgAl-hydrotalcites prepared with ethoxide-acetylacetonate as interlayer anions presented higher surface area (220–248 m2/g) than that prepared with nitrate as interlayer anion (97 m2/g). Calcination at 500 1C of nitrate sample resulted in a significant decrease of its surface area to 10 m2/g, by appearance of the MgO phase. Nevertheless, it was these catalyst components, with low surface areas, which shows higher photocatalytic activities in the phenol degradation reaction. Even if the hydrotalcite promotes the phenol sorption capacity, a synergist effect leading to a higher production of radical species could explain the TiO2/LDH’s superior performance in comparison with each component.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Acknowledgments
[18] [19]
The authors gratefully acknowledge the ‘‘CONACYT’’ for financial support (Projects: 44118-Q and 79132) and Dr. Paz del Angel Vicente for the X-ray analysis.
[20]
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