Photocatalytic activity of anatase powders for oxidation of methylene blue in water and diluted NO gas

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Abstract

Two series of samples were prepared from titanium tetraisopropoxide (TTIP) through hydrolysis with different aging times, 10 min and 24 h, at room temperature and then annealed at different temperatures. Photocatalytic activities of these samples for methylene blue (MB) in water and for diluted NO gas were examined. For MB decomposition, rate constant k was determined from linear relation between logarithm of relative concentration of MB remained in water and UV irradiation time. For NO, its decomposition fraction after 10 h irradiation was determined. Difference in aging time gave different crystalline state of the precipitates, anatase phase with poor crystallinity after 10 min aging, but amorphous state after 24 h aging. These two precipitates showed different changes in crystallinity and in photoactivity for MB and NO with annealing temperature. Maximum rate constant k for MB decomposition was obtained at around 0.6° of full width at half maximum intensity (FWHM) of 101 diffraction line. For NO gas, a maximum decomposition fraction was obtained at FWHM of around 1.6°. Different crystallinity of anatase was shown to be required for the decomposition of MB in water and NO gas, high crystallinity of anatase phase for the former but poor crystallinity for the latter.

Introduction

Photocatalytic anatase-type TiO2 has attracted attention for the decomposition and elimination of various pollutants in air and water [1], [2]. It has been practically applied for purification of wasted water, decomposition of NOx in ambient air, anti-bacteria coating, etc. [3], [4]. A number of researches have still been carried out in order to get higher performance of photocatalytic activity through the control of structure and morphology of catalyst particles and the preparation of different composites of catalyst with other materials. Mounting of anatase particles on mesoporous silica and silica beads was reported with certain success [5], [6], [7]. Performance of anatase-mounted activated carbons was also studied in detail [8], [9], [10], [11]. Recently, we developed new photocatalyst, carbon-coated anatase, which could have certain advantages, better performance of photocatalysis than anatase itself, high adsorptivity, inhibition of interaction with organic binders, etc. [12], [13], [14], [15], [16]. Different methods for the preparation of anatase powders were proposed; hydrolysis under hydrothermal condition of titanium tetraethoxide above 250 °C gave the particles of 20–30 nm size [17], vapor hydrolysis of titanium tetraisopropoxide at 260 °C resulting in nano-sized particles [18], destabilization of aqueous titanium lactate below 100 °C giving thin films on various substrates [19], decomposition of titania-hydrate coated on hollow glass spheres [20], etc. The effects of sizes of primary particles (crystal size) and secondary particles (aggregates of primary particles) were also studied. For the decomposition of trichloroethylene gas, the maximum decomposition rate was observed on the primary particles with the size of about 8 nm and the rate decreased gradually with increasing particle size [21]. Among three anatase powders synthesized under hydrothermal condition with different crystallite sizes, 6, 11 and 21 nm, anatase with the intermediate size (i.e. 11 nm) was shown to have the highest photoactivity for the decomposition of CHCl3 gas [22]. For methylene blue in water, however, anatase particles with average size of 30 nm were reported to have the highest decomposition rate among the anatase powders with the particles size above 30 nm [23]. Degradation rate of C2HCl3 in its aqueous solution was reported to increase with increasing crystallite size up to about 15 nm [24]. In our previous papers on anatase powders synthesized under hydrothermal condition, crystallinity of anatase phase was pointed out to be one of important factors to govern their photocatalytic performance [25] and the maximum rate for methylene blue decomposition in water was obtained on the anatase powders with apparent crystallite size of about 36 nm [26]. These experimental results suggested that the crystallite size of anatase was one of important factors to govern its photoactivity, but crystallite size giving maximum decomposition rate might be different between pollutants in water and those in gaseous state.

In the present work, therefore, two series of anatase samples were prepared through hydrolysis of an alkoxide with different aging times and their photocatalytic activity was evaluated for the decomposition of methylene blue (MB) in water and NO diluted in air. Photoactivites for MB and NO were discussed in the relation to the crystallinity of anatase phase.

Section snippets

Experimental

Fine powders of TiO2 were prepared through hydrolysis of titanium tetraisopropoxide (TTIP) in its ethanol solution (5 mL TTIP in 10 mL ethanol). For hydrolysis of TTIP, its ethanol solution of 15 mL was gradually added in distilled water of 50 mL under stirring and the precipitates were aged for either 10 min or 24 h at room temperature in ethanol–water mixed solution under stirring. Hydrolysis and aging were performed in nitrogen atmosphere. The precipitates were separated from solutions by

Characteristics

In Fig. 1, XRD patterns of samples annealed at different temperatures are compared. Marked difference in structure between as-prepared sample after 10 min aging and that for 24 h aging was observed; the former shows the formation of anatase structure even though each diffraction lines are very broad, but the latter did not show any peak, amorphous phase. Even though diffraction lines of anatase phase improved with increasing annealing temperature for the series of 10 min-aged precipitates,

Discussion

On two series of samples prepared from the same alkoxide (titanium tetraisopropoxide, TTIP) through hydrolysis with different aging times, 10 min and 24 h, at room temperature and then annealed at different temperatures, photocatalytic activities for methylene blue (MB), which was dissolved in water, and for NO, which was diluted in ambient air, were examined. Activities were evaluated by rate constant k for MB decomposition reaction, which was determined from the linear relation between

Acknowledgments

The present work was partly supported by a grant of the Frontier Research Project “Materials for the 21st Century—Materials Development for Environment, Energy and Information” from Ministry of Education, Culture, Sports, Science and Technology.

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