Recently, Heterogeneous photocatalysis through illumination of aqueous suspensions of TiO2 offers an advanced technology of wastewater treatments for its following advantages: chemical stability of semiconductor TiO2 in aqueous media and in a wide range of pH values, cheap chemicals in use, no additives required (only oxygen from the air), total mineralization achieved for various organic pollutants, efficiency of photocatalysis with halogenated compounds which are sometimes very toxic to bacteria in biological water treatment (Parra et al., 2002, 2004; Zhao et al., 2004; Zhu et al., 2004; Lee et al., 2003; Taborda et al., 2001; Lachheb et al., 2002; Vulliet et al., 2004; Florêncio et al., 2004; Sivalingam and Madras, 2004; Aguado et al., 2002). Heterogeneous photocatalysis consists of two important steps: the first step is the generation of active species (hole, electron and hydroxyl radical) by light excitation and the second step is the contact of active species with reactant by adsorption of reactant on the catalyst surface or diffusion of active species to the reactant.
There are many methods of producing TiO2 nanopowders, such as chemical precipitation (Dhage et al., 2004; Tryba et al., 2003), anodic spark deposition (Meyer et al., 2004), spray pyrolysis (Abou-Helal and Seeber, 2002), sol-gel (Keshmiri et al., 2004; Su et al., 2004; Jung et al., 2004) and hydrolysis (Gablenz et al., 1998; Vorontsov et al., 2001). However, the use of inorganic salt (chloride) precursor rather than organic alkoxide precursor not only can reduce the cost of synthesis, but also can avoid the use of organic solvent to decrease pollution.
The effect of post-heat-treatment temperature on the photoactivity becomes more significant factor, especially when titania is prepared by a liquid-phase reaction route (Chen et al., 2003). The increase of cristallinity is considered as the major reason why the photoactivity is improved by elevating the calcination temperature (Chan et al., 1999). However, very high calcination temperature results in aggregation and/or phase transformation and affects the microstructures as well as the properties of TiO2 nanoparticles. Anatase-phase TiO2 crystallites are generally found to be more active than rutile. Recently, it was found that anatase/rutile mixture (7/3) made the best photocatalyst for the oxidation of organic materials in the wastewater treatment (Su et al., 2004).
One important consideration in the TiO2-photocatalyzed reactions is the adsorption of the organic compounds on the surface of semiconductor particles. It has been reported that adsorption is a prerequisite in the photodegradation of organic compounds (Parra et al., 2004).
The aim of this research is to study the correlation between photocatalytic activity of TiO2 prepared from TiCl4 via hydrolysis synthesis at higher temperatures and its several properties, including crystal size and possible reaction site on the surface of TiO2. To examine the photocatalytic efficiency of the synthesized catalysts and compared with commercial TiO2 Degussa P-25, photodecomposition of the β-naphthol, a prototype molecule, was studied.
MATERIALS AND METHODS
Materials: All chemicals were used as received without farther purification. β-naphthol and titanium tetrachloride TiCl4 supplied by Fluka (> 99% purity). Acetonitrile and barium hydroxide, respectively, were purchased from Solvachim and Merck. Titanium dioxide Degussa P-25, a known mixture of 80% anatase and 20% rutile form with an average particle size of 30 nm, nonporous with a reactive surface area of 50±10 m2 g-1 and a density of 3.85 g cm-3.
Preparation and characterization of catalysts: The nanocrystalline titania catalysts reported in this study have been prepared at room temperature by aqueous hydrolysis of TiCl4 and precipitated by NaOH (10%). The resulting deposit (hydrated titanium hydroxide) was separated after 24 h by filtration and washed thoroughly with distilled water until the disappearance of Cl¯ ions from the liquid (tested as AgCl). Subsequently, the solid was dried at 100°C for 24 h in a vacuum system to remove water and then calcined in air for 3 h at temperatures of 400, 700 and 900°C, respectively.
Crystalline phase, particle size and morphology of titania nanocrystals were investigated by X-ray diffraction analysis (XRD) and Scanning Electron Microscopy (SEM), respectively.
XRD was carried out using CuKα (λ = 1.5418 Å) radiation in a Siemens D5000 diffractometer provided with a thin film attachment. SEM measurements were performed using a Jeol apparatus (model T-330A) operating at 10 kV on specimens upon which a thin layer of gold or carbon had been evaporated.
A infrared absorption spectrophotometer (IR) Shimadzu IR-460 was used to determine the specific functional groups.
Irradiation experiments: The experiments were carried out in water at room temperature in a static batch photoreactor of ca. 2 L, consisting of a Pyrex cylindrical flask, open to air, with several apertures to ensure the measurement of pH and of temperature as well as the introduction of oxygen. UV-light was provided by a high pressure mercury lamp (Philips HPK, 125 W) and the infrared radiations were filtered by a circulating water cell (thickness, 2.2 cm) equipped with a Pyrex cut-off filter.
The volume of the aqueous solution introduced into photoreactor was 1 L and the optimum mass of catalyst was 1 g. This quantity was selected as it gives the optimal specific degradation rate using commercial TiO2 Degussa P-25 as a reference (Assabbane et al., 2000). The initial concentration of β-naphthol was 5x10-4 mol L-1. Before each photocatalytic test, the suspension at natural pH (~6) was stirred magnetically for 60 min to reach the adsorption equilibrium in the dark prior illumination. Centrifugation or Millipore filters (0.45 μm diameter) were used to separate titania from the solution before analysis.
Analysis: The quantitative and qualitative analysis of the organic compounds in the samples was performed by high performance liquid chromatography HPLC (Jasco-type). The wavelength of detector was 280 nm. A reverse-phase column (length, 25 cm; internal diameter, 4.6 mm) ODS-2 Spherisorb (Chrompack) was used. The mobile phase was composed of acetonitrile (80%) and doubly distilled water (20%). The flow rate was 0.4 mL min-1.
Complete mineralization of organic samples to CO2 is very often obtained by using the photocatalytic method. The kinetics of the evolution of CO2 formed was followed by using the method of (Chemseddine and Boehm, 1990), which consists of flushing the CO2 produced by oxygen into a flask containing 500 mL of barium hydroxide (1.2x10-2 mol L-1) and to follow the conductivity of the solution with a conductimeter Orion model 150. CO2 precipitates as BaCO3, thus decreasing the ionic conductivity in water.
RESULTS AND DISCUSSION
Photocatalyst characterization: In Fig. 1 we show
the XRD patterns of TiO2 powders calcined at various temperatures
(400, 700 and 900°C). All samples are crystalline. The XRD pattern of the
powder formed at 400°C is characteristic of anatase structure. The XRD peaks
become sharper as the calcination temperature is increased. When the TiO2
was heated up to 700°C, the products became a mixture of anatase and rutile.
||XRD patterns of powders prepared at different calcination
temperatures (A: Anatase, R: Rutile)
||SEM surface morphology of TiO2 heat treated at
||IR spectrum of TiO2 powder prepared at 700°C
This suggests that there is a phase transformation from anatase to rutile (A→R)
which agrees with previous experiments (Su et al., 2004; Chen et al.,
2003). The primary formed TiO2 particles usually contain large portion
of defect sites, high temperature facilitates bond breaking as well as atoms
rearrangement and therefore the A→R occurs easily. At 900°C the transformation
to the rutile phase is complete.
||Kinetics of β-naphthol disappearance in the absence and
in the presence of various illuminated TiO2
||Linear transform ln (Co/C) = f(t) of the kinetic
curves from Fig. 4
Crystallinity of the titania powders calcined at various temperatures has been studied. The calculated lattice parameters for anatase are a = 3.780 Å and c = 9.657 Å at 400°C and for rutile are a = 4.596 Å and c = 2.912 Å at 900°C.
Kinetics of CO2 appearance during the photocatalytic
degradation of β-naphthol in the presence of various illuminated TiO2
|| Results of XRD measurements for TiO2 samples
|| Adsorption percentage of β-naphthol on the various TiO2
|| Apparent first-order rate constants (kapp) for
However, to quantify the crystallite size of TiO2, the d spacing at 3.52 Å (101) phase for anatase (2θ = 25.4°) and 3.25 Å (110) phase for rutile (2θ = 27.5°) were used. X-ray peak-broadening was analyzed by employing the Debye-Sherrers Eq. 1 (Guillard et al., 2004; Porter et al., 1999):
where L is the crystal size, k is a constant (= 0.9 assuming that the particles are spherical), λ is the radiation wavelength, δ is the line width at half maximum height of selected peak and θ is the Braggs angle of diffraction.
The crystallite sizes of each phase present in the samples at various temperatures
are listed in Table 1. It is apparent that thermal treatment
greatly affects the structure and the size of the resulting TiO2
crystal. The crystal sizes for both anatase and rutile increased with increasing
of calcination temperature. It has been observed that at higher calcination
temperatures, the crystallites formed are larger in size, which can be attributed
to the thermally promoted crystallite growth (Chan et al., 1999).
The SEM photograph of the titania sample heat treated at 700°C is shown in Fig. 2. TiO2 700°C presents small particles forming spongy like surface and aggregates of different sizes.
In Fig. 3 we show the IR transmission spectrum of TiO2 powder produced at 700°C. The absorption band at about 440 cm-1 is due to the stretching vibrations of Ti-O-Ti and Ti-O bonds (Chen et al., 2003).
Photocatalytic degradation of β-naphthol
Adsorption in the dark: A good determination of the photocatalytic activity of a sample can only be performed after having reached the adsorption equilibrium, since, otherwise, the initial rate of disappearance of a pollutant would simultaneously include both the initial rate of adsorption and the true, initial photocatalytic rate of reaction.
Under the same conditions as photodegradation, the amounts of β-naphthol (%) adsorbed on the various samples (TiO2) measured at the end of the previous adsorption period in the dark are reported in Table 2. It is noted that the quantity of β-naphthol adsorbed on TiO2, which is calcined with 700°C, is more important. Indeed, the capacity of adsorption of β-naphthol is approximately 1.5 times higher for TiO2 700°C than TiO2 Degussa P-25.
The differences of adsorption exhibited by the various samples depended on the calcinations temperature, although the influence of other physicochemical factors cannot be excluded.
Kinetics of β-naphthol disappearance: The decomposition of organic
molecules that occurs under irradiation of above synthesized TiO2
is of great importance. In the present research, the degradation of β-naphthol
was selected as a test reaction to verify the photocatalytic activity of different
TiO2 dispersion samples. Figure 4 plots the concentration
changes of β-naphthol as a function of illumination time of the TiO2
samples prepared by calcinating at different temperatures (Tc):
400, 700 and 900°C. It can be observed that the direct photolysis without
catalysts can be neglected with less than 4% of conversion within 3 h of UV-irradiation.
However, it is found that TiO2 photocatalysis efficiency increases
with the Tc up to 700°C reaching a maxima, i.e., TiO2
700°C is the most active to catalyze the photodegradation of β-naphthol.
This catalyst exhibits the highest photoactivity. The total pollutant destruction
is occurred within 90 min and 150 min of time irradiation for TiO2
700°C and TiO2 Degussa P-25, respectively.
Sample TiO2 400°C composed primarily of anatase phase. Nevertheless, the broad XRD feature for sample-400°C as shown in Fig. 1 indicated that anatase crystallisation of TiO2 treated at 400°C was incomplete, hence TiO2 400°C was expected to exhibit lower photocatalysis efficiency than that of the samples TiO2 700°C and TiO2 Degussa P-25.
The catalysts treated at 900°C showed poor activity for β-naphthol decomposition. This catalyst consists of rutile phase only that is known to have less activity that anatase phase. Poor photoactivity of rutile is connected with its fast recombination rate of generated electrons and holes (Tryba et al., 2003).
A better and more quantitative way of presenting the activities of a series of similar catalysts is the use of the rate constant k, which is independent of the concentration used.
The photocatalytic degradation of organic pollutants in water generally follows a Langmuir-Hinshelwood mechanism (Eq. 2) (Assabbane et al., 1997; Herrmann et al., 1997; Chen et al., 2004; Yang et al., 2004; Konstantinou et al., 2001):
where r is the oxidation rate of the reactant (mol L-1 min-1), k is the reaction rate constant (mol L-1 min-1), K is the equilibrium adsorption coefficient (L mol-1) and C is the concentration of the reactant at time t (mol L-1).
At low substrate concentrations, the term KC in the denominator can be neglected with respect to unity and the rate becomes the apparent first order Eq. 3.
The integral form, C = f(t) of the rate equation is Eq. 4:
where Co is the initial concentration of the reactant (mol L-1) and kapp is the apparent rate constant of the pseudo-first order (min-1). The linear transforms ln (Co/C) = kappt of the curves in Fig. 4 are given in Fig. 5. The slopes of the straight lines all of which pass through the origin yield the kapp which are given in Table 3.
The photoactivity of the TiO2 700°C particles determined from the apparent rate constant was much higher than that of commercial TiO2 Degussa P-25. The UV-light induced photodegradation rate of β-naphthol using TiO2 700°C particles was faster than that using TiO2 Degussa P-25, TiO2 400°C and TiO2 900°C as photocatalyst by 2, 5.75 and 11.5 times, respectively.
The photoactivity of the four photocatalysts can then be ranked in the following
Kinetics of CO2 evolution: The evolution of CO2 as a function of irradiation time in the photocatalytic degradation of β-naphthol is shown in Fig. 6. The same reactivity order was found for the total mineralization followed by CO2 evolution. Taking into account the fact that complete disappearance of β-naphthol in the irradiated reactor occurs after 90 min for TiO2 700°C, whereas the stoichiometric (Eq. 5) formation of CO2 is observed after 210 min under the same working conditions. This implies that total degradation requires a longer time for degrading all the possible reaction intermediate produced involved.
The microstructure characteristics of TiO2 powders produced by aqueous
hydrolysis of TiCl4 at higher temperatures were studied using XRD,
SEM and IR. Their photoactivities were also evaluated using β-naphthol
degradation and mineralization to CO2 as a test reaction. Anatase
phase powders are obtained at the calcination temperature of 400-700°C.
As the calcination temperature increased, the particle size increases. At 900°C,
the transition to the rutile phase is complete. TiO2 700°C exhibits
the highest photoactivity. The photodegradation rate of β-naphthol using
700°C particles was faster than that using TiO2 Degussa P-25
as photocatalyst by 2 times. However, it is clear from our measurements that
the calcination temperatures used in hydrolysis preparation of TiO2
samples significantly affect the photocatalysis efficiency of samples in the
photodecomposition of β-naphthol. Suitable adsorption capability and better
distribution of TiO2 nanoparticles was the main cause for the high
activity. The results of several tests show that the samples derived from TiCl4
exhibit the best results and the technique presented here seems an economical
and fast way for the preparation of a highly active photocatalyst.