Photocatalytic Degradation of Nitro and Chlorophenols Using Doped and Undoped Titanium Dioxide Nanoparticles

Abstract

Pure and Ag-[subscript]TiO2[/subscript] nanoparticles were synthesized, with the metallic doping being done using the Liquid Impregnation (LI) method. The resulting nanoparticles were characterized by analytical methods such as scanning electron micrographs (SEMs), Energy Dispersive Spectroscopy (EDS), and X-ray diffraction (XRD). XRD analysis indicated that the crystallite size of [subscript]TiO2[/subscript] was 27 nm to 42 nm while the crystallite size of Ag-[subscript]TiO2[/subscript] was 11.27 nm to 42.52 nm. The photocatalytic activity of pure [subscript]TiO2[/subscript] and silver doped [subscript]TiO2[/subscript] was tested by photocatalytic degradation of p -nitrophenol as a model compound. Ag-[subscript]TiO2[/subscript] nanoparticles exhibited better results (98% degradation) as compared to pure [subscript]TiO2[/subscript] nanoparticles (83% degradation) in 1 hour for the degradation of p -nitrophenol. Ag-[subscript]TiO2[/subscript] was further used for the photocatalytic degradation of 2,4-dichlorphenol (99% degradation), 2,5-dichlorophenol (98% degradation), and 2,4,6-trichlorophenol (96% degradation) in 1 hour. The degree of mineralization was tested by TOC experiment indicating that 2,4-DCP was completely mineralized, while 2,5-DCP was mineralized upto 95 percent and 2,4,6-TCP upto 86 percent within a period of 2 hours.

(ProQuest: ... denotes non-US-ASCII text omitted.)

Recommended by William W. Yu

School of Civil and Environmental Engineering, National University of Sciences and Technology, H-12, Islamabad, Pakistan

Received 19 February 2010; Revised 13 April 2010; Accepted 1 June 2010

1. Introduction

Advanced oxidation processes (AOPs) are techniques used for the degradation of harmful organic pollutants resistant to conventional treatment methods. AOPs depend on in situ generation of highly reactive radical specie such as ^OH... that breakdown a number of organic compounds without being selective [1-3] using chemical or light energy. The process employing a semiconductor activated by UV or visible light to degrade aquatic or atmospheric pollutants is called photocatalysis which results in partial or complete mineralization of the organic compounds [4, 5].

Titanium dioxide (TiO₂ ), a metal oxide semiconductor has been found to be one of the most effective photocatalysts due to its high efficiency and stability. TiO₂ has a band gap of 3.2 eV that allows UV light to excite the valance electrons and inject these into the conduction band leaving holes in the valance band [6]. The high rate of electron-hole recombination in TiO₂ , however, limits the efficiency of the photocatalyst which can be arrested, to some extent, by doping with metallic elements such as silver [7]. On the other hand, the antibacterial action of silver, particularly in the colloidal form, is also well known [8].

Metal-doped titanium dioxide has been used for the degradation of many organic pollutants in general [2, 3] and for phenols in particular [9-11]. Chlorophenols have low taste and odour thresholds and exhibit high toxicity and carcinogenic character [9]. They are very toxic and poorly biodegradable compounds, that are not effectively degraded by direct biological methods [10]. Consequently, their elimination in wastewaters and drinking water is of great interest.

In the present work, TiO₂ nanoparticles were synthesized by calcination while metallic doping with silver was achieved using Liquid Impregnation [12]. These nanosized catalysts were characterized by the techniques such as X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS). Photocatalytic degradation using the nanosized particles was then studied with p -nitrophenol as a model compound followed by of 2,4-dichlorophenol, 2,5-dichlorophenol, and 2,4,6-trichlorophenol.

2. Experimental

2.1. Materials

TiO₂ (GPR, BDH Chemicals Ltd. Poole England), AgNO₃ , p -nitrophenol (purity 99%), 2,4-dichlorphenol, 2,5-dichlorophenol, and 2,4,6- trichlorophenol (GR, Merck, Germany).

2.2. Photocatalyst Preperation

In case of pure TiO₂ nanoparticles, TiO₂ was calcined at ^500° C for 3 hours, to abtain the nanosized crystal structure. Silver-doped TiO₂ nanoparticles were prepared by following the method of reference [12] described below.

1 g TiO₂ was added to 100 ml dionized water in a 500 ml Pyrex beaker. For silver doping 1% (molar ratio) of AgNO₃ was also added to the suspension. The resulting slurry was thoroughly mixed by vigorous stirring and allowed to settle, at room temperature, over night. The liquid so obtained was dried in an oven at ^100° C for 12 hours to get rid of any remaining moisture. The solid material resulting from this step was calcined, at ^500° C for 3 hours in a furnace. This resulted in fine particles of silver-doped TiO₂ , herein after referred to as Ag-TiO₂ .

2.3. Charaterization of TiO₂ and Ag-TiO₂ Nanoparticles

The phases of the synthesized particles were analyzed by JEOL JDX-II X-ray Diffractometer. Crystallite size of the prepared particles was determined from the broadening of the anatase main peak by the Scherrer equation. The external morphology of the particles was examined using an electron microscope JEOL JSM 6460. EDS analysis was carried out using the Oxford INCA X-sight 200 System.

2.4. The Photocatalysis Process

The schematic diagram of the experimental setup of photocatalysis experiment is shown in Figure 1. 0.004 g of high purity p -nitrophenol was dissolved in 200 ml deionised water in a 250 ml beaker and transferring it to a 1 L analytical flask. The beaker was thoroughly rinsed, twice, and the washings were added to the solution in the 1 L flask. The volume was made up, with deionised water, to 1 L to obtain a stock solution of 4 ppm p -nitrophenol.

Figure 1: Schematic Diagram of Experimental Setup of Photocatalysis.

[figure omitted; refer to PDF]

An aliquot of the solution was taken and its pH (monitored with the HACH Sension 1 pH meter) was adjusted to 7 with drop-wise addition of dilute NaOH (1N) solution. The resulting solution had a slight yellow tinge whose absorbance was measured at 400 nm in a 4 cm path length glass cell in a UV visible spectrophotometer (HACH DR 2400). Deionized water in a glass cell of the same dimensions served as a blank. The absorbance so obtained served as the reference value for determining the proportionate reduction in the concentration (_C° ) of the phenol, after exposing the solution to UV light under the conditions of (a) no TiO₂ , (b) with TiO₂ , and (c) with Ag-TiO₂ as described below.

To investigate the photocatalytic degradation, p-nitrophenol solution of 4 ppm was taken in a container. Ag-TiO₂ nanoparticles with a dose of 0.05 g/50 ml was added. It was placed under 20 W Blak-Ray UV lamp (with a wavelength peak at 365 nm and intensity of 1.4 mW/cm² ) and with continuous stirring. After every experiment the solution was centrifuged on (Sigma 204 instrument) at 4000 rpm for 10 minutes. The degradation of centrifuged solution was measured at 400 nm on UV visible spectrophotometer (HACH DR 2400) and the degree of mineralization was carried out using Analytik jena TOC multi N/C 3100 analyzer.

Similar experiments were performed with 2,4-dichlorphenol, 2,5-dichlorophenol, and 2,4,6-trichlorophenol. Stock solutions of these compounds were prepared by dissolving 0.004 gm of the above three compounds in dionized water and diluting to 1 liter. The cholorphenols were also exposed to UV light under conditions similar to those for p -ntirophenol. Here the baseline for the individual compounds was established by using the standard 4-aminpoantipyrene method [13].

3. Results

3.1. TiO₂ Nanoparticle Characterization

3.1.1. X-Ray Diffraction

The crystal phase of laboratory prepared nanoparticles was identified using JEOL JDX-II X-ray diffractometer. XRD patterns are shown in Figure 2. The data indicates that the major phase of all the prepared nanoparticles is anatase. The crystallite size of prepared nanoparticles determined from the broadening of the anatase main peak by Scherrer equation [12] is shown in Table 1. TiO₂ is in a range of 27 nm to 42 nm while the crystallite size of Ag-TiO₂ is 11.27 nm to 42.52 nm. Our results are in consonance with those reported earlier [7] where Ag-TiO₂ nanoparticles dimensions were reported to be between 15 nm and 37 nm, [figure omitted; refer to PDF] (see [12]).

Table 1: Crystallite properties of undoped and Ag-doped TiO₂ .

Sr. No.	Particles	Particles Size (nm)	Surface Area (m2 /g)	Pore Diameter (° A)
Maximum	Minimum
1	Undoped TiO2	42.52	26.72	74.30	131.06
2	Ag-TiO2	42.52	11.27	77.74	129.65

X-ray diffraction pattern (a) TiO₂ (b) Ag-TiO₂ .

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

3.1.2. EDS

The EDS-spectra of TiO₂ (Figure 3) and Ag-TiO₂ (Figure 4) was obtained using the Oxford INCA X-sight 200 equipment. In similarity to an earlier work using EDS [7], the presence of three distinct X-ray lines associated with O Kα , Ag Kα , and Ti Kα , is clearly evident. The results indicate that Ti, O, and Ag are the constitutive elements of the nanoparticles prepared by the Liquid Impregnation method and no extraneous elements are present.

Figure 3: EDS pattern of TiO₂ Nanoparticles prepared by Calcination.

[figure omitted; refer to PDF]

Figure 4: EDS pattern of Ag-TiO₂ Nanoparticles prepared by Liquid Impregnation Method.

[figure omitted; refer to PDF]

3.1.3. SEM

The SEM images of pure titania and Ag-doped titania obtained using (JEOL JSM 6460 Scanning Electron Microscope) are shown in Figures 5 and 6 whereby the porous and sponge-like network of irregularly shaped particles is amply demonstrated. Surface roughness and shaped complexity of the particles is high in case of Ag-TiO₂ which results in a high surface area. SEM images were taken from different sites of sample and then compared. The resulting image shows that silver is not uniformly deposited on the surface of titania nanoparticles, as found by other authors [12].

Figure 5: SEM micrograph of TiO₂ nanoparticles.

[figure omitted; refer to PDF]

Figure 6: SEM micrograph of Ag-TiO₂ nanoparticles.

[figure omitted; refer to PDF]

3.1.4. Surface Area and Pore Size Analysis

Figures 7 and 8 show the nitrogen adsorption isotherms and BJH pore size distribution curves of doped TiO₂ and undoped TiO₂ samples, respectively, obtained by Surface and Pore Size Analyzer NOVA WIN 2200e. The Brunauer-Emmett-Teller (BET) specific surface areas and pore volumes of doped TiO₂ and undoped TiO₂ are summarized in Table 1. Results reveal that doping results in positive impact on surface area and pore diameter of the TiO₂ nanoparticles.

Nitrogen adsorption curve for doped and undoped TiO₂ (a) Undoped TiO₂ (b) Ag-doped TiO₂ .

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

Pore size distribution curve for doped and undoped TiO₂ (a) Undoped TiO₂ (b) Ag-doped TiO₂ .

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

3.2. Optimization Studies

3.2.1. Photodegradation of p-Nitrophenol

For the optimization of the photocatalytic process, phototcatalytic degradation of p -nitrophenol with pure TiO₂ was studied by varying the parameters of light intensity, pH, and irradiation time. The concentration was determined by preparing the calibration curve of p -nirophenol on spectrophotometer.

3.2.2. Effect of UV Lamp Distance

The effect of UV light intensity on the degradation of p -nitrophenol was studied by varying the distance of UV lamp from the target. Figure 9 shows the relationship of UV light intensity in terms of lamp distance from the sample and percent photocatalytic degradation. As expected, the higher light intensity excited the TiO₂ particles to generate more electron hole pairs. The holes decompose the p -nitrophenol molecules adsorbed on the surface of TiO₂ particles and oxidize them to water resulting in their efficient degradation [14].

Figure 9: Effect of UV lamp distance on photodegradation of p - nitrophenol due to change in distance of UV lamp.

[figure omitted; refer to PDF]

3.2.3. Effect of pH

The degree of photocatlytic degradation of p -nitrophenol was found to be affected by a change in pH. Figure 10 shows that the photocatalytic degradation of p -nitrophenol was high between pH 2 and pH 4, while the degradation efficiency was lower in the alkaline environment above 7 pH. Titania surface will remain positively charged in acidic medium (pH<7 ) and negatively charged in alkaline medium (pH>7 ). Titanium dioxide is reported to have higher oxidizing activity at lower pH but excess H⁺ at very low pH can decrease reaction rate. This observation confirms the finding of other researchers [15].

Figure 10: Represent the effect of change in pH on photocatalytic degradation of p -Nitrophenol.

[figure omitted; refer to PDF]

3.2.4. Degradation versus Irradiation Time

One of the very important parameters that was studied for the photocatalytic degradation of p -nitrophenol was time of degradation. The degradation rate was observed to increase with increase in irradiation time as shown in Figure 11.

Figure 11: Photocatalytic degradation of p -Nitrophenol w.r.t irradiation time.

[figure omitted; refer to PDF]

In view of these results, subsequent experiments were caried out at a pH of 4 with UV lamp distance of 5 cm from the target surface of the solution in the china dish.

3.2.5. Comparative Degradation of p-Nitrophenol by Pure TiO₂ and Ag-TiO₂

Comparitive photocatalytic degradation of p -nitrophenol was also investigated in the presence of UV light. Ag-TiO₂ nanoparticles show better results (98% degradation) as compared to pure TiO₂ nano particles (83% degradation) in 1 hour as shown in Figure 12. This is due to the positive effect of silver on the photoactivity of TiO₂ at degradation of p -nitrophenol that may be explained by its ability to trap electrons, thus, reducing the recombination of light-generated electron-hole pairs at TiO₂ surface. These results encouraged the use of Ag-doped TiO₂ for the degradation of chlorophenols.

Figure 12: Comparitive photocatalytic degradation of p -nitrophenol with pure and Ag-doped Titania Nanoparticles.

[figure omitted; refer to PDF]

3.3. Photocatalytic Degradation of Cholophenols

When chlorination is done, phenols present in water react with chlorine to form chlorophenols. These chlorophenols are carcinogenic and impart an odour to the water making it unfit for secondary use. Three commonly known chlorophenols were selected which are 2,4-dichlorophenol, 2,5-dichlorophenol, and 2,4,6-trichlorophenol for the photocatalytic degradation using Ag-TiO₂ nanoparticles. The concentration was determined by preparing the calibration curves of chlorophenols using the 4-aminoantipyrine method [13].

As depicted in Figure 13, starting with the same initial concentration, the transformation rate of chlorophenols decreases as the number of chlorines on the phenolic ring increases. The addition of chlorine atom has previously been found to reduce the degradation rate of chlorophenols, [16]. In this study, it seems that the increase in the number of chlorine atoms on the phenolic ring makes the chlorophenols less responsive to UV degradation as the photo transformation rate of 2,4-DCP and 2,5-DCP was higher than that of 2,4,6-TCP, as shown in Table 2.

Table 2: Percent degradation of phenolic compounds in one hour.

Nanoparticles	Compounds	Percentage Degradation in 1hr
Pure TiO2	p -nitrophenol	83
Ag- TiO2	p -nitrophenol	98.3
Ag- TiO2	2,4-dichlorophenol	98.9
Ag- TiO2	2,5-dichlorophenol	98.55
Ag- TiO2	2,4,6-trichlorophenol	96.41

Figure 13: Photocatalytic degradation of phenolic compounds using Ag-TiO₂ nanoparticle on spectrophotometer.

[figure omitted; refer to PDF]

3.4. Total Organic Carbon (TOC) Analysis

In the experimental work, photocatalytic degradation of the above discussed phenolic compounds was primarily monitored using absorption spectroscopy. This, however, does not indicate the degree of mineralization of the organic compounds. Following an earlier study [17], the oxidative mineralization was therefore examined by measuring the Total Organic Carbon (TOC) in the solution. The TOC measurements were made using Analytik jena TOC multi N/C 3100 analyzer by direct injection of the aqueous solution after centrifugation.

As discussed above in photocatalytic degradation, the chlorophenols are converted into inorganic products like CO₂ , H₂ O and ^CL- [18]. Figure 14 gives the results of the TOC experiment indicating that 2,4-DCP was completely mineralized, while 2,5-DCP was mineralized up to 95 percent and 2,4,6-TCP upto 86 percent within the period of 120 minutes. The mineralization efficiency of the above said compounds was calculated by the following formula: [figure omitted; refer to PDF]

Figure 14: Percent minerlization of phenolic compound based on TOC analysis.

[figure omitted; refer to PDF]

3.5. The Reaction Kinetics

The photocatalytic degradation of organic pollutants generally follows the Langmuir-Hinshelwood mechanism [16], [figure omitted; refer to PDF] where k is the true rate constant which depends upon mass of catalyst, the flux of efficient photons, and the coverage in oxygen. K is the adsorption constant, t is the time, and C is the concentration of the organic pollutant. For the low initial concentrations of pollutants, the term KC , in the denominator, can be neglected and the photocatalytic oxidation rate approaches first order kinetics described by [figure omitted; refer to PDF] where ^{k[variant prime]} , the apparent rate constant, is also known as the pseudo-first order rate constant. The integral form of the rate equation is [figure omitted; refer to PDF] where _C0 is the initial concentration.

Values of ^{k[variant prime]} for experiments described in this study are given in Table 3^{k[variant prime]} values in Table 3, compare well with the vales reported in a previous study [7], which used pure and 1 to 2 mmol Ag-TiO₂ for degradation of p -nitrophenol as compared to 1% molar ratio in our case.

Table 3: Reaction Constants of photocatalytic process with TiO₂ and Ag-TiO₂ for phenolic compounds.

Nanoparticles	Compounds	K[variant prime] (min-1 ) × 10-2 a
Pure TiO2	p -nitrophenol	2.4
Ag-TiO2	p -nitrophenol	6.6
Ag-TiO2	2,4-dichlorophenol	7.7
Ag-TiO2	2,5-dichlorophenol	6.2
Ag-TiO2	2,4,6-trichlorophenol	3.6

^a Apparent first-order reaction Constants (^{k[variant prime]} ).

4. Conclusion

Pure TiO₂ (42.52-26.72 nm), and silver-doped TiO₂ (42.52-11.27 nm) nanoparticles were synthesized through calcination and Liquid Impregnation method. Ag-TiO₂ exhibited good potential for the photocatalytic degradation of phenolic compounds in acidic environment (pH 4). More than 95% photocatalytic degradation of phenolic compounds could be achieved in 1 hour using Ag-TiO₂ was achieved during 1 hour. Photocatalytic mineralization of 2,4-DCP (100%), 2,5-DCP (95%), and 2,4,6-TCP (86%) was achieved using Ag-TiO₂ , after a period of 2 hours.

Langmuir-Hinshelwood kinetic model provided a good fit to the photocatalytic degradation of phenolic compounds, used in this study.

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