Photocatalytic degradation of dye pollutant over Ti and Co doped SBA-15: Comparison of activities under visible light

Chemical Engineering Journal 176–177 (2011) 265–271

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Photocatalytic degradation of dye pollutant over Ti and Co doped SBA-15: Comparison of activities under visible light P.V. Suraja a , Z. Yaakob a,∗ , N.N. Binitha b , M.R. Resmi b , P.P. Silija a a b

Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Department of Chemistry, Sree Neelakanta Government Sanskrit College Pattambi, Palakkad 679306, Kerala, India

a r t i c l e

i n f o

Article history: Received 30 November 2010 Received in revised form 14 May 2011 Accepted 18 May 2011 Keywords: SBA-15 support Pollutant degradation Visible light activity

a b s t r a c t Thick-walled hexagonally ordered Cobalt and titanium loaded SBA-15 mesoporous photocatalysts were prepared by impregnating the metal precursor solution on the hydrothermally stable support. The catalysts were analyzed using various spectroscopic and diffraction techniques. XRD measurements revealed that Co exists as its spinel structure Co3 O4 and Ti is present in the anatase TiO2 phase. FTIR showed the absorption bands of Co3 O4 around 667 and 565 cm−1 . The visible light absorbance of the photocatalytic systems was studied by Diffuse Reflectance Ultraviolet–Visible spectroscopy (UV–vis DRS) measurements. Systems exhibited fairly good performance as photocatalysts for pollutant degradation under visible light. SBA-15 support was helpful in the easy separation of catalysts after the completion of the degradation. In the case of cobalt loaded SBA-15, activity is found to be maximum when the cobalt loading is 40% whereas 50% Ti loaded SBA-15 is found to be the most active among the prepared systems for the degradation of the dye pollutant methylene blue (MB). © 2011 Elsevier B.V. All rights reserved.

1. Introduction Photocatalysis is a widely used technique for the abatement of environmental pollutants and have attracted considerable attention in recent past. The superiority of photocatalytic technique in wastewater treatment can be attributed to its advantages over the traditional techniques, such as quick oxidation, no formation of polycyclic products, oxidation of pollutants in the ppb range, etc. The use of semiconductor heterogeneous photocatalysts for the degradation of organic pollutants is an area where a lot of research work is going on. Many semiconductor oxides like TiO2 , Fe2 O3 , ZnO, ZrO2 , Nb2 O5 , WO3 , Bi2 O3 , SnO2 , etc. have been employed as photocatalysts in the wastewater treatment [1–3]. TiO2 is the most investigated semiconductor photocatalyst in the past decade [4]. However, the major limitation of this technique is the poor visible light harvesting since the absorption wavelength of anatase does not conform to the visible light region of solar spectrum [5]. Many modification methods are available to shift the absorption of TiO2 to the visible region and now other transition metal ions are also effective in the field of pollution abatement [6]. Among transition metal

∗ Corresponding author. Tel.: +60 3 8921 6400; fax: +60 3 8921 6148. E-mail addresses: [email protected] (Z. Yaakob), [email protected] (N.N. Binitha). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.05.071

oxides, Cobalt oxides are reported to show good photoactivity for pollutant degradation [7–9]. One difficulty arising in the usage of nano TiO2 and CO3 O4 based catalytic systems is the separation of catalyst in the end. The filtration problems have been eliminated to a large extent by the development of supported photocatalysts in which metal oxides are immobilized on different adsorbent materials. In this context, molecular sieves have attracted greater attention due to their adsorption capacity that helps in pooling the pollutants to the vicinity of the metal oxide surface resulting in faster degradation [10–12]. Mesoporous materials are widely used as catalysts and hosts for nanomaterials synthesis because of their highly ordered and uniform mesoporous channels and large surface area (usually more than 800 m2 /g). Uniform ordered channels of mesoporous materials can control the particle size of TiO2 and Co3 O4 and can efficiently prevent particles from agglomeration [13]. Pure silica is having no active sites in their matrices. Active sites can be generated via chemical modification, i.e. by the introduction of heteroatoms into the silica matrix [14,15], and activity can be improved further. There are several reports in the literature about photocatalysis using transition metal incorporated mesoporous silicates such as Al, Ti, Cr, Fe, and Mn [16,17]. When 2,9-dichloroquinacridone sensitized Ti-SBA-15 (DCQ-Ti-SBA-15) was employed to decompose indigo carmine, high photocatalytic efficiency was shown, under UV light irradiation [18]. The photocatalytic performance of the

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titania–silica mixed oxides (mass ratio of Ti/Si = 0.5) with crystalline titania for the degradation of methyl orange dye under UV light has been reported by Li et al. [19]. Photocatalytic degradation of methylene blue (MB) had been reported over Ti and Co doped SBA-15 catalysts [20–24]. Most of the studies used UV light for irradiation. Xia et al. [25] had tested the activity of Co/SBA-15 under sunlight. There the samples are found to be amorphous in nature and no peaks of Co3 O4 spinel, the most photoactive form of cobalt oxide is observed. In the present study, efficient dispersion of Co3 O4 spinel and TiO2 is done on SBA-15 by impregnation method and the properties are analyzed using XRD, FTIR, FESEM, BET surface area – porevolume measurements and UV–vis DRS experiments. Activity for pollutant degradation is tested under visible light. Influence of the percentage metal loading on the properties and photodegradation activity is investigated. The SBA-15 support material is prepared using hydrothermal method. The visible light photoactivity is monitored by the degradation of a model aquatic pollutant methyleneblue (MB). 2. Experimental 2.1. Preparation of SBA-15 The support, SBA-15 material was prepared using the method already reported [26]. In a typical synthesis 4.4 g Pluronic P123 (Aldrich, Mavgerage = 5800 [EO20-PO70-EO20]), 120 g of 2 M HCl (R&M chemicals) and 9 g TEOS (Aldrich) were required. Typically 4.4 g of triblock copolymer was dispersed in 30 g-distilled water and stirred for 1.5 h. To the resultant solution, 120 g of 2 M HCl was added under stirring and the stirring was continued for 2 h. Finally, 9 g of TEOS was added drop wise and the mixture was maintained at 35 ◦ C for 24 h without stirring. The resulted heterogeneous mixture was subjected to hydrothermal treatment at 100 ◦ C for 48 h under static condition before recovering the solid material. The crystallized product was filtered, washed with distilled water and dried in air for 24 h and in oven at 70 ◦ C for overnight, and then calcined at 450 ◦ C for 8 h in air to remove the template completely. 2.2. Preparation of cobalt and titania loaded SBA-15 Cobalt and Titania loading was done on SBA-15 by simple impregnation method. The cobalt nitrate hexahydrate (Hamburg Chemical GmbH) was dissolved in ethanol. A predetermined amount of prepared SBA-15 was then added to this solution, followed by heating at 60 ◦ C for removal of the solvent. The amount of cobalt nitrate is varied to get different wt% of cobalt/g SBA in each impregnation. The concentrated sample was then dried in oven at 55 ◦ C for overnight. The dried sample was then calcined at 450 ◦ C for 3 h. The same procedure was repeated with Titanium isopropoxide (Aldrich) to get Titania loaded SBA-15 and the catalysts are designated as 40Co/SBA, 50Co/SBA, 40Ti/SBA, 50Ti/SBA where the numbers indicates the percentage metal loading. 2.3. Catalyst characterization XRD patterns of the samples were recorded for 2 between 10◦ and 80◦ on a Bruker AXS D8 Advance diffractometer employing a scanning rate of 0.02◦ /S with Cu K␣ radiation ( = 1.5418). Low angle XRD patterns of the samples were recorded for 2 between 0.5◦ and 10◦ on a Bruker D8 Advance diffractometer with Cu K␣ radiation ( = 1.5418). The formation of well ordered mesopores of SBA-15 is confirmed from TEM images (ZEISS LIBRA 200). The FTIR spectra were recorded in NICOLET 6700 FT-IR Thermoscientific spectrometer in the region 400–4000 cm−1 . BET surface area and pore volume were measured by TriStar 3000 V6.04 A,

50 Ti/SBA

40 Ti/SBA

Intensity

266

50 Co/SBA 40 Co/SBA SBA 10

20

30

40

50

60

70

80

2θ (degrees) Fig. 1. XRD patterns of the prepared systems.

using nitrogen physisorption at −195.800 ◦ C. The specific surface area was estimated by the BET method. The pore size distribution and pore volume was determined by the BJH method. The morphology studies were carried out using a Field Emission Scanning Electron Microscope SU6600 (HI-2102-0003). Diffuse Reflectance Ultraviolet–Visible spectroscopy (UV–vis DRS) of powder catalyst samples was carried out at room temperature using a Varian, Cary spectrophotometer in the range of 200–800 nm. 2.4. Photocatalytic degradation Photocatalytic activity of the prepared materials was evaluated for degradation of methylene blue as a model compound. The pollutant degrading capacities of the different systems were studied using a Rayonet type Photoreactor with visible light having 16 tubes of 8 W (Associate Technica, India). In the reactor, 5 glass tubes were concentrically arranged to get uniform illumination for all the systems. 50 mL of MB was placed in the glass tube, containing a definite amount of the catalyst and is irradiated with visible light under continuous stirring. In a typical experiment, the required amount of catalyst were suspended in 50 mL of the MB solution and kept for overnight in the dark to reach to the maximum adsorption equilibrium. The MB concentration was analyzed using a colorimeter (ESICO Microprocessor photo colorimeter model 1312) at a wavelength of 665 nm. For optimization studies, the catalyst weight, dye concentration, irradiation time, etc. are varied to compare the efficiency of catalysts. 3. Results and discussion 3.1. Structural characterization The photocatalytic activity of semiconductors depends on various parameters, such as crystallinity, impurities, surface area, and density of surface hydroxy groups. However, the most significant factor is their crystal forms. TiO2 is usually used as a photocatalyst in two crystal structures: rutile and anatase. Anatase generally has much higher activity than rutile. Fig. 1 represents X-ray diffraction patterns of the calcined SBA-15 and metal loaded samples. From the figure it is clear that titania is in most active anatase form. The strong peak at 2 = 25.41, belongs to anatase phase. In the case of Co/SBA samples the peak around 2 value 36.8◦ indicates the formation of spinel cobalt oxide, Co3 O4 . Both the Co/SBA catalysts showed peaks at 2 values of 31.48, 37.18, 59.38, and 65.28, which can be attributed to the presence of crystalline Co3 O4 (JCPDS 421467) [27]. Co3 O4 is reported to show greater photoactivity when compared to other cobalt oxides. The crystallite sizes calculated using Scherrer equation is tabulated in Table 1. TiO2 containing samples are found to show least crystallite size when compared to Co3 O4 crystallites and it is seen that the crystallite size increases

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267

Table 1 Surace area, porosity data and crystallite size (of the inserted metal oxide) of different catalytic systems. Photocatalyst

BET surface Area (m2 /g)

Pore volume (cm3 /g)

Pore diameter (A0 )

Crystallite size (nm)

SBA-15 40Ti/SBA-15 50Ti/SBA-15 40Co/SBA-15 50Co/SBA-15

683.9920 301.9147 320.0890 197.0784 127.6510

0.959917 0.192531 0.292841 0.419492 0.310881

60.869 34.328 32.284 65.288 77.840

– 4.31 6.89 11.12 12.97

Fig. 2. TEM images of SBA-15.

% Transmittance

SBA 50Ti/SBA 40Ti/SBA 50Co/SBA 40Co/SBA

3500

3000

2500

2000

1500

1000

vibration of Ti–O–Si bridges [29]. Also, in the low energy region of the spectrum the bands at 595 cm−1 is assigned to bending vibrations of Ti–O bonds. The absorption bands of Co3 O4 at around 667 and 565 cm−1 are clearly observed in the spectra of Co/SBA samples. Nitrogen adsorption–desorption isotherms (Fig. 4) were used to characterize the changes in the surface area as well as porosity of the SBA-15 after modifications. All the samples are featured by the type IV isotherms attributed to typical mesoporous structures. Materials of the present study exhibit pronounced hysteresis loops similar to those commonly observed for conventional mesoporous silicas. The prepared SBA-15 and Co doped SBA systems exhibits H1 hysteresis loops and indicates presence of open ended cylindrical pores lying in narrow range of radius. Whereas Ti doped systems exhibits H2 hysteresis loops and indicates presence of cage-like and inkbottle type large-pores. SBA-15 and Ti systems are showing low

40Ti/SBA

Volume adsorbed,cm2 /g at STP

with the percentage metal content. The low angle XRD patterns of the synthesized materials are typical for the SBA-15 (figure is not shown) [28]. Peak at 2 angle of 1.09◦ , corresponding to the (1 0 0) plane Bragg reflections confirms the hexagonal symmetry of the SBA-15 materials. The peak remains unaffected after metal loading which indicates that the incorporation of Ti and Co did not significantly change the hexagonal ordering of the SBA-15 framework. The formation of SBA-15 is again confirmed from TEM analysis of the prepared mesoporous silica. Images of SBA-15 in Fig. 2 show the highly ordered hexagonal arrangement of the channels along two directions, parallel and perpendicular to the c axis. Fig. 3 shows the FT-IR spectra of prepared samples. In the spectrum of raw SBA-15, the broad intense double bands appeared in the region 1000–1290 cm−1 can be attributed to asymmetric stretching vibrations of Si–O–Si bridges and the absorption bands observed at 467 and 812 cm−1 can be assigned to the asymmetric and symmetric Si–O stretching vibrations. The broad band from 3400 to 3700 cm−1 is assigned to stretching frequencies of hydrogen bound silanols. In the case of Ti containing samples, the absorption band appeared at about 947–961 cm−1 can be ascribed to the stretching

50Ti/SBA SBA 40Co/SBA 50Co/SBA

500

Wavelength (nm) Fig. 3. FTIR spectra of the metal doped and undoped SBA-15 samples.

Relative Pressure,(P/P0) Fig. 4. BET adsorption–desorption isotherms of metal modified SBA based systems.

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Fig. 5. FESEM photographs of the prepared photocatalysts.

pressure hysteresis, extending to the lowest attainable pressures which indicate the presence of micropores. Table 1 shows the BET surface area, pore volume and average pore diameter of prepared systems. The decrease in BET surface area and pore volume after metal doping may be due to plugging of the support pores by cobalt oxide and titania [30]. The plugging makes the pores inaccessible for nitrogen adsorption. This is consistent with the loss of specific surface area after metal doping due to the blocking of pore’s entrance by the clusters of metal oxides. From the surface area data, it is clear that pore blocking is more in the case of Co loaded systems when compared to Ti loaded ones; this may be one of the reasons for the lesser activity of Co/SBA samples compared to Ti/SBA samples [31]. The average pore diameter of Ti/SBA materials was found to be decreased after the impregnation process. These results can be explained by the formation of finely dispersed metal oxide species inside the channels of Ti/SBA15 supports. This may be lead to an alteration to the pores, which is evident from the isotherms that indicate inkbottle or cage-like pore structure of SBA upon Ti loading. But Co containing systems showed an increase in the average pore diameter eventhough the

surface area is found to be least for these systems, which may be due to leaching out of some Si from the matrix. The FESEM images of SBA-15, Co/SBA and Ti/SBA materials with different metal percentages are shown in Fig. 5. The particles of prepared SBA-15 are joined to form long fibrous macrostructures with a relatively particle size of several micrometers, the typical morphology of SBA-15 [32]. Almost no change in external morphology is observed for metal doped systems when compared to SBA-15. In the case of 50Ti/SBA visible particles are observed on the external surface by FESEM observation. These aggregated particles are suggested to be anatase titania evidenced by the result of wide-angle XRD pattern. Since in all SEM images, the fibrous structure is visible, addition of titanium or cobalt had retained the SBA-15 framework successfully. Fig. 6 shows the diffuse reflectance of UV–vis absorption spectra of prepared samples. The raw SBA-15 exhibited no significant absorption in the UV–vis ranges. With cobalt incorporation, the absorption in both visible and UV range increased compared with undoped SBA-15. The increased absorption over a broad range of 400–700 nm shown by Co/SBA samples compared to undoped SBA-

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Catalyst weight optimization

100 40CoSBA

0.8

269

90 80

50CoSBA

% Conversion

70

Absorbance

0.6

0.4

60 50 40 30

40Ti/SBA 50Ti/SBA 50Co/SBA

20 10

0.2

0

40Co/SBA

0.00

0.05

0.10

SBA

0.0 200

300

400

500

600

700

800

Absorbance

Wavelength (nm)

50Ti/SBA

40Ti/SBA

300

400

0.20

0.25

0.30

Fig. 7. Effect of photocatalyst dose on the photocatalytic degradation of MB (10 mg/L for Co/SBA, 25 mg/L for Ti/SBA) for 6 h irradiation except 50Ti/SBA (5 h irradiation).

of catalyst due to the increase in opacity of solution. 50Ti/SBA gave a degradation of 93% of MB with a catalyst weight of 3 g/L for 5 h run. In the case of Co/SBA samples, the optimizations were done by taking 10 mg/L MB solution. The activity is found to be less in general when compared to titania loaded samples. In the case of 40Co/SBA, 73.68% of MB is degraded with a catalyst weight of 2 g/L for 6 h run. With 50Co/SBA, the catalyst weight that gave maximum degradation of 70% is 5 g/L for 6 h run. The results are in agreement with UV DRS analysis where 40Co/SBA is showing higher visible light absorbance than 50Co/SBA. Thus the degradation is found to be more over 40Co/SBA. Surface area data also supports this observation. Among the different loaded systems, 50Ti/SBA is showing maximum surface area as well as photoactivity.

SBA

200

0.15

Catalyst weight

500

600

700

800

Wavelength (nm) Fig. 6. UV–vis absorbance spectra of different systems.

15 suggests higher activity under visible light [25]. Among the different Co loaded systems, 40Co/SBA exhibited higher visible light absorbance which may lead high photoactivity when compared to 50Co/SBA. The absorption edge of Ti/SBA shifts to visible region with increased percentage of Ti loading.

3.2.2. Effect of time Degradation rate was found to increase with increasing time. After 6 h of irradiation under visible light in TiSBA suspensions, more than 90% of dye was decomposed. The effect of time on degradability of MB over 40Ti/SBA and 40Co/SBA studied under optimum catalyst weight are shown in Fig. 8. The activity increases with time which is due to the reason that as time increases the

Time study

100

3.2. Photodegradation of methylene blue

3.2.1. Effect of catalyst weight The optimization of catalyst weight of Ti/SBA samples was done by taking 25 mg/L MB solution for 6 h run. The trend in photoactivity with increased amount of catalyst is shown in Fig. 7. With a catalyst weight of 1 g/L, Ti/SBA gave only 41.66%, but 94.59% activity was achieved with an increased catalyst weight of 3 g/L. However, there was a decrease in photoactivity with further increase in the amount

80

% Conversion

In order to obtain the real photodegradation rate due to photocatalysis, the decrease of dye concentration because of adsorption is deducted. The catalyst was mixed with the MB dye under dark conditions for overnight prior to photodegradation experiments to ensure that the absorbance was not influenced by removal of MB adsorbed on the catalyst when centrifugation was performed. The activity of different catalysts and the influence of reaction parameters were analyzed after deducting the decreased concentration due to adsorption.

60

40 50Ti/SBA 40Ti/SBA

20

40Co/SBA 50Co/SBA

0

0

50

100

150

200

250

300

350

400

Time (min) Fig. 8. Effect of time on the degradation of MB (10 mg/L for Co/SBA, 25 mg/L for Ti/SBA) with optimum catalyst weight (3 g/L for Ti/SBA, 2 g/L for 40Co/SBA, 5 g/L for 50Co/SBA).

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Table 2 Influence of initial pH of dye solution on the photocatalytic activity. Photo catalyst

Conversion (%) pH 3

Conversion (%) pH 8

Conversion (%) natural pH

40 Ti/SBA-15 50 Ti/SBA-15 40 Co/SBA-15 50 Co/SBA-15

8.33a 1.67b 11.86c 6.66d

25a 40b 70.59c 30d

94.59a 93b 73.68c 70d

a

Catalyst dose 3 g/L, time 6 h, MB conc 25 mg/L. Catalyst dose 3 g/L, time 5 h, MB conc 25 mg/L. Catalyst dose 2 g/L, time 6 h, MB conc 10 mg/L. Catalyst dose 5 g/L, time 6 h, MB conc 5 mg/L.

b c d

amount of incident radiation increases and this facilitates the degradation. 3.2.3. Effect of initial MB concentration The degradation results of Ti/SBA and Co/SBA for MB degradation over a range of initial MB concentrations were studied and the results are given in Fig. 9(a and b). MB degradation over Co/SBA decreased with increase in the initial concentration of MB. The possible reasons are (a) the absorption of light by the pollutant may be significant at higher concentrations and, (b) the screening effect dominated at higher concentrations there may be more screening effect and hence degradation efficiency decreased. But in the case of the most effective Ti containing SBA samples, dye degradation capacity remains more or less unaffected with increasing initial

(b) Molar concentration study-Ti/SBA

100 95

dye concentration. But after an initial concentration of 25 mg MB/L, there is a sudden decrease in the activity because at high concentration, the irreversible adsorption of the dye may occur on the catalyst surface leading to saturation during degradation [33].

3.2.4. Effect of pH For photodegradation experiments, the influence of two different pH conditions (3 and 8) was also studied in addition to the natural pH. MB shows slight adsorption at the natural pH of the solution and is more strongly adsorbed at alkaline pH. As expected, cationic dye, MB was not adsorbed at strongly acidic pH [33]. The photodegradation of the dye is found to be maximum under natural pH. Acidic pH has a detrimental effect on the photoactivity. Even at alkaline pH, the activity is not comparable with degradation under natural pH. Table 2 details the photoactivity of different systems with changing pH of the dye solutions. The data clearly indicates that best activity is observed for all photocatalysts under natural pH conditions.

% Conversion

90 85

4. Conclusion

80

The Ti/SBA and Co/SBA photocatalysts were synthesized by alcoholic impregnation method. A comparison of photocatalytic activity of MB solution was done by the prepared Ti and Co loaded SBA-15. Under visible light irradiation, the catalytic systems exhibited a good performance in the degradation of MB. Hydrothermally stable SBA-15 support provided a clean and easy recovery of the catalyst from the solution in the end. The characterization results showed that high loading of well-dispersed Co3 O4 and TiO2 can be achieved by impregnation method and the morphology of the support remained intact. The surface area of SBA-15 decreased upon loading which can be attributed to the plugging of the pores by the inserted metal oxides. This is evident from the decreased pore volume observed. TiO2 particles were finely dispersed inside the channels of SBA-15 supports, which may be responsible for the decreased average pore diameter of Ti doped systems. Adsorption studies were performed to get the actual degradation rates of prepared systems. Titania containing systems are found to be more active when compared to cobalt loaded systems. In the case of Co loaded systems the maximum activity is achieved at a weight percentage of 40, whereas further increase in Co concentration decreased the activity, which may be due to the agglomeration/big particles of cobalt oxides. But in the case of Ti containing systems even 50% metal loading is found to be effective over the support, and is found to show the highest degradation rate.

75 70 65

40Ti/SBA

60

50Ti/SBA

55

10

15

20

25

30

Concentration of MB (mg/L) (a) Molar concentration study-Co/SBA

80 70

% Conversion

60 50 40

40Co/SBA 50Co/SBA

30 20 10 0

5

10

15

20

25

30

Concentration of MB (mg/L) Fig. 9. Effect of degradation on initial concentration of MB under optimum catalyst weight and time.

Acknowledgement The authors acknowledge the UKM, the grant number UKMOUP-NBT-27-118/2009 for the financial support.

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