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Osmín Avilés-García, Jaime Espino-Valencia, Rubí Romero, José Luis Rico-Cerda, Reyna Natividad, "Oxidation of 4-Chlorophenol by Mesoporous Titania: Effect of Surface Morphological Characteristics", International Journal of Photoenergy, vol. 2014, Article ID 210751, 10 pages, 2014. https://doi.org/10.1155/2014/210751
Oxidation of 4-Chlorophenol by Mesoporous Titania: Effect of Surface Morphological Characteristics
Mesoporous nanocrystalline anatase was prepared via EISA employing CTAB as structure directing agent. The drying rate was used as a key synthesis parameter to increase the average pore diameter. The resultant mesoporous crystalline phases exhibited specific surface areas between 55 and 150 m2 g−1, average unimodal pore sizes of about 3.4 to 5.6 nm, and average crystallite size of around 7 to 13 nm. These mesophases were used as photocatalysts for the degradation of 4-chlorophenol (4CP) with UV light. Under the studied conditions, the mesoporous anatase degraded 100% 4CP. This was twice faster than Degussa P-25. 57% reduction of chemical oxygen demand (COD) value was achieved.
TiO2 is well known as the photocatalyst by excellence. It has been successfully applied to degrade and mineralize a vast amount of hazardous compounds in both air and water [1–5], under mild reaction conditions (low temperature and atmospheric pressure). Within photocatalysis area, it has been widely accepted that the catalyst feature determining its activity is the crystalline structure. Although this is true, other surface characteristics should not be completely left aside. This work aims to report the effect of surface morphological characteristics on mesoporous nanocrystalline anatase activity. The synthesis of the material was conducted by evaporation induced self-assembly approach (EISA). This method was elected since it is a powerful synthesis method to design technologically relevant and functional oxides in the fiber, particle, and film form at the nanoscale [6–9]. The method relies on using very dilute surfactant initial concentration from which a liquid crystalline mesophase is gradually developed upon solvent evaporation. The slow coassembly between the inorganic network and the liquid crystalline phase leads to the formation of long-range order of well-defined mesostructures. The preparation of mesoporous titania particles by EISA has been studied by independent research groups [10–13]. It is expected that a change in size, shape, and dimensions of the mesopore TiO2 modifies the accessibility, adsorption, and diffusion of guest molecules within the pore network, thereby achieving further degradation. The photocatalytic activity of the synthesized TiO2 was tested in the degradation of 4-chlorophenol (4CP).
Chlorinated aromatic compounds are a class of compounds widely used and constitute a particular group of priority pollutants. This is mainly due to their numerous origins (pesticide, paint, solvent, pharmaceutics, wood preserving chemicals, coke oven, and pulp industries) [14, 15] and toxic effects. They can be found in ground water, wastewater, and soil. In particular, chlorophenols (CPs) pose serious ecological problems as environmental pollutants due to their high toxicity, recalcitrance, bioaccumulation, strong odor emission and persistence in environment, and suspected carcinogen and mutagen effect on the living [16, 17]. The photocatalytic degradation of chlorinated phenols in TiO2 suspensions has been studied by many investigators [18–24]. The results show that phenolic compounds are degraded completely to CO2 and H2O through a mechanism involving hydroxylation of the aromatic ring. In particular, 4-chlorophenol (4CP) has been accepted as the standard pollutant for heterogeneous photocatalysis. The photocatalytic degradation of 4CP has been the topic of many investigations [25–29], and the kinetics of the photocatalytic degradation has been extensively studied [30–36]. Despite this vast literature, it still remains unclear whether there is an interaction or not between 4CP degradation and surface morphological characteristics of the employed photocatalyst. Therefore, it is relevant to conduct this study not only in the context of 4CP degradation but also in general within photocatalysis area.
2.1. Materials and Synthesis
Titanium (IV) ethoxide (C8H20O4Ti, 80% Aldrich) and titanium (IV) butoxide (C16H36O4Ti, 97% Aldrich) were used as precursors. Hexadecyltrimethylammonium bromide, denoted as CTAB (C19H42NBr, 99% Sigma), was used as the structure directing agent (SDA). Ethanol (99.6%, Sigma-Aldrich) was used as organic solvent. Nitric acid (65.2%, Sigma-Aldrich) was used as catalyst.
The synthesis was performed as follows. An alcoholic solution of the precursor was prepared. This solution was added to the SDA under vigorous stirring, and then nitric acid was added dropwise. The resultant solution was stirred at room temperature for 3 h and then was dried at room temperature. Samples with different drying rates were placed in a rotary evaporator at 100 rpm (Heidolph G3 model) using oil as a heating medium. The synthesized powders were then calcined at 350°C and 400°C. It is worth clarifying that samples calcined at 350°C were first calcined at 300°C for 1 hour and then at 350°C for 4 hours with controlled heating and cooling rate of 1°C min−1 to remove the SDA. The same heating rate was used for the samples calcined at 400°C. The molar ratio of the as prepared samples was 1 precursor : 3.55 HNO3 : 0.018 CTAB : 18.71 ethanol.
Mesoporous titania samples were analyzed by X-ray diffraction (XRD) in a Bruker D8 Advanced diffractometer with Cu Kα radiation and a LynxEYE detector. BET surface areas and N2 adsorption-desorption isotherms were obtained in an Autosorb-1 Quantachrome. Before measurements, samples were degassed at 250°C for 2 h. TEM images were taken with a JEOL-2100 200 kV LaB6 filament. The morphology and particle size of the mesophases were inspected with a SEM JEOL JSM-6510LV.
2.3. Heterogeneous Photocatalytic Oxidation of 4CP
Photocatalytic degradation studies of 4CP were performed in a batch photoreactor of cylindrical shape (see Figure 1). The photoreactor was provided with ports in the lower and upper section for the inlet and outlet of gases and for sampling. Mesoporous titania samples were placed in the glass reactor under continuous stirring (1000 rpm). The total reaction volume was 30 mL. Tests were performed using 0.8 g L−1 of mesoporous titania at an initial pH value of 2 and 4CP initial concentration [4CP]0 was 0.233 mmol L−1. The pH adjustment was made by using 0.003 M HCl solution. The temperature throughout the experiment was kept constant at 20°C. The UV lamp was placed at the center of the reactor as the source of UV radiation (254 nm at 0.786 watts cm−2). Oxygen flow of 50 mL min−1 was constantly fed at the bottom of the reactor and an oxygen trap was used to increase its residence time. Aliquots samples (0.5 mL) were withdrawn from the system every 30 minutes during 3 hours. Catalyst was removed before analysis.
At all experiments, the concentration of 4CP () was determined using UV/Vis spectroscopy in a Perkin-Elmer Model Lambda 25 UV/Vis spectrophotometer with a wavelength range of 200–360 nm, where the characteristic absorption peak for 4-chlorophenol is located at 280 nm. A calibration curve was constructed from 0 to 0.311 mmol L−1. A determination coefficient of and a slope of were obtained. The experiments were repeated three times to verify results reliability.
The chemical oxygen demand (COD) value was analyzed with a Hach UV/Vis Model DR-5000 spectrophotometer in order to determine the degree of oxidation of the 4-chlorophenol after the photocatalytic tests.
3. Results and Discussion
3.1. Catalysts Characterization
The textural properties of the synthesized mesoporous titania with different type of drying are summarized in Table 1. The mesoporous samples that were dried at room temperature exhibited specific surface areas of around 117 m2 g−1 and average pore size of approximately 3.4 nm. Furthermore, it was observed that the pore diameter remains constant and independent of the calcination temperature. However, the calcination conducted at 400°C decreases the specific surface area. Mesoporous samples dried in rotary evaporator exhibited specific surface areas of approximately 145 m2 g−1 and average pore size of around 4.9 nm. Furthermore, it was observed that the calcination temperature modifies both the specific surface area and average pore diameter. For this type of drying, the best textural properties were obtained using titanium butoxide as precursor. The reactivity of the precursor determines the rates of hydrolysis and condensation to generate the final inorganic oxide structure. Also, the time used in these processes must be sufficient to allow proper interaction between the SDA and the inorganic precursor and generate assembly and organization in regular structures that finally will lead to an ordered mesoporous structure. For this reason, the titanium butoxide as precursor provides a more controlled reactivity and easy handling, thus allowing control of the hydrolysis and condensation reactions, as well as the dimensions of the pores directly related to the size of the alkoxy groups [37, 38].
The textural properties of the mesoporous titania phases synthesized with titanium butoxide as precursor and different drying rate in a rotary evaporator are summarized in Table 2. Specific surface areas and average pore size of samples were around 150 m2 g−1 and 5.6 nm, respectively. When the drying rate became slower (at the same calcination temperature), the average pore diameter is observed to increase. Furthermore, the calcination temperature is found to be the only variable that modifies the specific surface area for these samples. Thus, the influence of the evaporation rate of volatile entities is a key parameter that determines the final mesostructure. The slow and gradual evaporation of the solvent promotes progressive increase in the concentration of the SDA by obtaining the critical micellar concentration (CMC), surfactant micelle formation, and their self-assembly with inorganic species at a specific time where the network is flexible enough leads to greater micellar arrangement .
The average crystallite size for all samples was estimated using the Scherrer equation and the FWHM of anatase (1 0 1) reflection. Crystal growth increases with a calcination temperature of 400°C for all samples. With increasing calcination temperature, the peak intensity of anatase increases (Figure 2), and the width of the (101) peak becomes narrower due to the growth of anatase crystallites. The pore diameter increase is caused by shrinkage of the mesoporous framework at higher temperatures .
The XRD patterns of all the mesophases exhibited only the characteristic reflections of anatase at approximately 2θ of 25°, 38°, 48°, 54°, 55°, and 63°. These correspond to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), and (2 0 4) planes, respectively, of tetragonal titania [13, 41] as shown in Figure 2.
Nitrogen adsorption-desorption isotherms and the Barrett-Joyner-Halenda (BJH) pore size distribution of synthesized samples are shown in Figure 3. All of these samples show a IV type isotherm with H2 hysteresis loop, which is representative of mesoporous materials .
The TEM images of the mesoporous anatase samples AB4 (dried at room temperature) and SR4 (slow drying rate in rotary evaporator) are shown in Figure 4. For these samples, the anatase crystals were determined to be approximately 13 and 10 nm in size, respectively. The spacing of 0.35 nm, measured for these two sets of fringes, coincides with 0.352 nm, that is, with the d-spacing of (1 0 1) type planes in the anatase form of titania, and this was confirmed by XRD data (Figure 2).
Figure 5 shows SEM images of the mesoporous anatase samples SR4 and AB4. The synthesized mesophase SR4 exhibited clusters of approximately 5–10 μm while the synthesized mesophase AB4 resulted in the formation of larger clusters of approximately 5–14 μm with irregular shapes.
The reference material Degussa P-25 contains anatase and rutile phases in a ratio of about 3 : 1. Anatase and rutile particles separately form their agglomerates and the average sizes of the anatase and rutile elementary particles are 85 and 25 nm, respectively . Furthermore, their specific surface area is 52 m2 g−1 and the average crystallite size is 30 nm .
3.2. 4CP Degradation
The mesoporous anatase phases were evaluated in the photodegradation of 4CP. The photoactivity of these samples was compared to that of the reference material Degussa P-25. Figure 6 shows the photocatalytic degradation profiles of 4CP over the mesoporous titania samples and Degussa P-25. All synthesized titania samples showed a higher percentage of degradation than titania Degussa P-25. This may be related to the smaller crystallite size and relatively ordered pore structure of the obtained mesophases. Also, the enhanced photocatalytic activity of the mesoporous titania samples can be partially attributed to the presence of pure anatase phase which is the primary photoactive phase .
The mesoporous samples dried at room temperature (Figure 6(a)) and the ones dried in a rotary evaporator (Figure 6(b)) degraded approximately 60–69% and 75–93% of 4CP, respectively, after 180 min. Degussa P-25 degraded only 57% of 4CP after the same time of exposure to UV irradiation. Although mesoporous titania samples dried at room temperature showed the same average pore diameter of 3.4 nm, in Figure 6(a) it is evident that there are differences in the percentages of 4CP degradation due to different specific surface areas. For mesoporous titania samples dried in a rotary evaporator, the highest percentage of degradation (approximately 93%) was obtained with the highest average pore diameter of 4.9 nm (sample RB4), despite not having the largest specific surface area.
The mesoporous titania samples synthesized with different drying rate in a rotary evaporator (Figure 6(c)) degraded approximately 77–100% of 4CP. In Figure 6(c) it is evident that the increase in the percentage of degradation of 4CP is related to the increase in average pore diameters of synthesized samples, since the sample SR4 (average pore diameter of 5.6 nm and specific surface area of 108 m2 g−1) achieves 100% of 4CP degradation in 180 minutes and the sample SR3 achieved only 95% of degradation at the same time, despite having higher specific surface area (147 m2 g−1) and smaller average pore diameter of 4.9 nm. The ordered pore architecture of the mesoporous samples as compared to Degussa P-25 may result in higher diffusion rates of the guest molecules, and therefore the photocatalytic reaction rate increases. The benefits of having an ordered mesopore structure for photocatalytic applications have been demonstrated by independent research groups [46–48].
Table 3 shows the initial rates of degradation of 4CP for all samples synthesized and Degussa P-25. No effect of the average pore diameter was observed on the initial rate of degradation for the samples dried at room temperature. This may be due to several factors such as crystal size, because, when this decreases, the surface density of active sites available for substrate adsorption increases, thus increasing the photocatalytic reaction rate [49, 50].
Figure 7 shows the removal of 4CP by photocatalysis, adsorption, and photolysis using mesoporous titania sample SR4 since 100% of 4CP degradation was achieved in 180 minutes. The effect of photolysis was studied by carrying out the experiment only in the presence of oxygen and UV light without mesoporous titania. The degradation of 4CP by direct photolysis is negligible, and the increase in concentration is due to an electronic effect that modifies the UV absorbance spectrum and appears as if it was an increase in concentration (Figure 8). This phenomenon has been described as a photoinduction period associated with reactions involving the formation of free radicals [51–53]. Furthermore, the removal of 4CP by adsorption is considered negligible (Figure 7).
The kinetics of photocatalytic reactions of organic compounds are usually adequately described by the Langmuir-Hinshelwood model [29, 51]. It relates the degradation rate and the concentration of organic compound C and is expressed as follows: where is the intrinsic rate constant and is the adsorption equilibrium constant. If adsorption is weak and concentration of organic compounds is low, the factor is negligible, and thus (1) can be simplified to the first-order kinetics with an apparent rate constant (), which gives the following, after integration in the interval :
Plotting ln() versus reaction time yields a straight line, where the slope is the apparent rate constant. The half-life of the degraded organic compound can then be easily calculated. Figure 9 shows the lineal plot of 4CP photodegradation, which adjusts well to a pseudo-first-order kinetic behavior. Apparent constant , 4CP half-life, and the linearization coefficient are summarized in Table 4. 4CP half-life is as short as 25.6 min, with mesoporous titania sample SR4, and nearly 80% of initial 0.233 mmol L−1 is degraded in 60 minutes.
Table 5 shows the COD values to identify the presence of organic matter in the aqueous solution after photocatalytic tests. 4CP photodegradation using sample SR4 reduced the initial value of COD by 57%, whereas Degussa P-25 reduced the initial value of COD by 23% after the same time of exposure to UV irradiation. These results support the removal of 4CP by adsorption and photolysis shown in Figure 7, without reduction in initial values of COD. Therefore, it is demonstrated that the organic compound is mineralized. In specialized literature [34, 36], hydroquinone (HQ), benzoquinone (BQ), and 4-chlorocatechol (4CC) have been reported as the major aromatic intermediates, identified by HPLC, LC-MS, and GC-MS techniques. Although interesting, such characterization is beyond the scope of this paper.
Mesoporous nanocrystalline anatase was found to provide a faster degradation rate than Degussa P-25 as consequence of different surface morphological characteristics. Among the studied variables, different drying rate in rotary evaporator was determined to be the one that affects the increase in average pore diameter, and this affects both the percentage of photodegradation and the chemical oxygen demand (COD) value. The diffusion towards the active sites and the accessibility of the active sites for adsorption due to the presence of large pores are key parameters for the photocatalytic degradation of 4CP. The photodegradation process was found to be controlled by the Langmuir-Hinshelwood model. The mesoporous anatase degraded 100% 4CP, while Degussa P-25 degraded 57%. The enhanced photocatalytic activity of the mesoporous titania samples when compared to Degussa P-25 was related to smaller crystallite size, presence of pure anatase phase, higher average pore diameter, and surface area. The reduction of 57% COD with mesoporous anatase compared with 23% Degussa P-25 shows that 4CP is mineralized.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors are grateful to PROMEP for financial support through Project 103.5/13/S257. Osmín Avilés-García thanks CONACYT for scholarship 290649 and CCIQS from UAEM for the granted support.
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