Synthesis and characterization of titania photocatalysts: The influence of pretreatment on the activity
Introduction
Since the first preparation of the well-ordered mesoporous silica materials with high specific surface areas [1] there has been a great deal of interest in applying this preparative route for the synthesis of mesoporous transition metal oxide analogues [2], [3], [4], [5], [6] family of compounds is of particular interest because of their unique electrochemical, optical, semiconducting and redox properties. The surfactant templating synthesis provides the ordered periodic structure of the mesopores with a uniform pore size in addition to an unusually large surface area. These characteristics are expected to be favorable for most of the applications, catalytic supports, electronic materials, and optical devices. The sol–gel process has already been utilised for the synthesis of mesoporous Nb [2], Zr [4], [5], [6], Hf, Al, Sn, Ta and even mixed oxides [4], [5], [7]. However, the preparation of mesoporous titanium dioxide with high surface area, well-ordered structure, uniform pore size and thermal stability has still remained unsolved [3], [4], [5], [6], [7], [8], [9]. Experimental drawbacks are mainly associated with the calcination step, which is used for the removal of the template. During calcination the ordered structure of the mesoporous TiO2 collapses and/or the surface area decreases significantly.
The first mesoporous TiO2 has been synthesized by modified sol–gel method with phosphate-containing surfactants. The specific surface area of the product was found to be in the range of 200 m2/g after calcination, but a significant amount of remnant phosphorus was identified in it. Antonelli [3] managed to synthesize the first phosphorus-free pure mesoporous titania by using surfactants with amine head-groups. The template was removed by extraction, and the specific surface area was found to be 710 m2/g. This material was, however, thermally instable, because heat treatment in air at 300 °C resulted in a decrease in the specific surface area at a rate of ca. 50 m2/g/h. Uncontrolled hydrolysis of Ti-alkoxides or TiCl4 in solutions often leads to the rapid formation of inorganic networks, resulting in poorly structured materials. Stucky and co-workers [4], [5] applied for the first time P-123 and TiCl4 in nonaqueous solutions for the synthesis of hexagonal mesoporous TiO2. However, most of the reported syntheses used alkoxide/alcohol/water/template mixtures and acidic conditions [3], [8], [11], [12], [13], [14]. The complete dehydration of the product of hydrolysis (i.e., Ti(OH)4·xH2O) appears to be a crucial step in the formation of a durable mesostructure. The morphology, surface area and thermal stability of the mesoporous materials synthesized by the sol–gel method was found to depend on the type and concentration of the template [9], [11], [14], the temperature and ageing time [12], the pH [12] and the composition (in particular the water content) of the reaction mixture [15], [16].
Most frequently calcination or extraction was used for the removal of the structure directing template. It was observed, that the surfactant could never be completely removed by extraction. On the transmission electron microscopic (TEM) images of TiO2 synthesized this way, non-continuous or worm-like textures [10] are seen and regular structures are confined to relatively small areas of the entire sample. The transformation of amorphous TiO2 to one of its crystalline forms (anatase, rutile or brookite) may lead to the collapse and destruction of the mesostructure. Two methods are generally used for TiO2 crystallization: hydrothermal treatment and calcination. Both methods lead to increasing in the nanocrystal size with decreasing temperature and duration of heat treatment. The photocatalytic activity of TiO2 specimen and particle size distribution obtained by hydrothermal crystallization and calcination has been compared. It was observed, that samples prepared by hydrothermal treatment showed higher photocatalytic activity in the paraquat decomposition and the particles are very spherical and regular form opposite to the particles prepared by sol–gel method [17]. The degree of crystallinity and rate of the crystallization, as well as the specific surface area and the photocatalytic activity depends both on the water content of the reaction mixture and the applied organic solvent during the hydrothermal treatment [16]. The amorphous-to-anatase transition temperature also depends on the pH of the synthesis media [18] and the aging time and temperature [19].
The so-called sonochemically modified TiO2 synthesis was applied by Wang et al. [20] yielded wormhole-like structures and rutile phase besides anatase. The sonochemical preparation of mesoporous TiO2 in the presence of triblock copolymer and ethanol resulted brookite phase besides anatase and their ratio was found to depend on the ethanol–water ratio of the media [21].
Hwang et al. [8] attempted to synthesize mesoporous titanium dioxide from anatase nanoparticles. The nanoparticles were dispersed in a surfactant-containing solution and were aged. During calcination or after redissolution in ethanol, the mesoporous structure of TiO2 collapsed.
One of the major problems in the synthesis of the mesoporous TiO2 is associated with the removal of the template, which usually causes undesirable damages in the delicate mesostructure. Accordingly, various template elimination procedures, such as: (i) oxidation with ozone under mild conditions and (ii) solvent extraction, have been systematically investigated and compared with already established methods in order to identify the one that best preserves the textural properties of the material and ensures the complete removal of the block copolymer. For the syntheses, P-123 block copolymer surfactant (structure forming agent) and titanium isopropoxide as Ti source were used. The effect of the various treatments on: (i) the formation of mesoporous phases, such as anatase and/or rutile; (ii) the crystal size of these TiO2 derivatives; and (iii) the change in pore diameter occurring parallel to the phase transformation have also been studied. The phase transformation of the ‘as synthesized’ samples was performed both by hydrothermal treatment and by a two-step calcination process [10]. The catalytic performance of the differently prepared TiO2 samples was characterized by determining the apparent rate constant of the heterogeneous photocatalytic degradation of phenol.
The first aim of performing such experiments was to obtain evidences for synthesis and pretreatment conditions resulting in thermally stable titanate mesostructures. The second aim of the work was to find correlation, if there is any, between the photocatalytic activity of samples and their phase composition.
Section snippets
Syntheses
The preparative pathways used for obtaining the mesoporous TiO2 samples (hereafter MTiO2 stands for mesoporous TiO2) are shown on Scheme 1.
In a typical synthesis 1 g of (polyalkyleneoxide) block copolymer (P-123, BASF) HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H was dissolved in a mixture of ethanol and aqueous HCl (0.05 M). The mixture was stirred for 4 h at room temperature, in order to ensure the total dissolution and facilitate micelle formation. To the clear solution of the surfactant,
Thermal characteristics
The TG curves of the various samples are shown in Fig. 1A. The MTiO2-0 sample shows three separate weight-loss steps (see curve 1). The first, small (around 6%, w/w) step appearing at temperature <200 °C corresponds to the release of water (i.e., adsorbed water on the inner and outer surface. The second step seems to be a combination of several (two or more) weight loss processes. The block copolymer template decomposes in two steps and even some water formed in the dehydration of Ti(OH)4·x(H2O)
Conclusions
In this study, mesoporous titanate samples were prepared with a sol–gel procedure. The organic template was removed via different procedures. The samples were treated by hydrothermal and thermal treatments. As a result of both treatments, the total pore volume and specific surface area drastically decreased, while the anatase content and the nanocrystal grain size significantly increased. The observed mesostructure collapse is accompanied by the transformation of amorphous titania to anatase.
Acknowledgements
This work was supported by grants from the National Science Foundation of Hungary (OTKA T/04355 and T/15 043273), the National Development Plan (NFT, NKFP 3A/089/2004) and the Economic Competitiveness Operative Programme (GVOP, AKF 3.1.1.-2004-05-0259/3.0).
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