Evaluating the activities of immobilized TiO2 powder films for the photocatalytic degradation of organic contaminants in water

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Abstract

In a previous paper [J. Mater. Sci. 38 (2003) 823] we have described the preparation and characterization of conventional alkoxide sol–gel derived TiO2 films [J. Mater. Sci. 23 (1988) 2259] and of TiO2 powder enriched alkoxide sol–gel derived films. The powder films were prepared on flat stainless steel substrates and on glass beads. These films were characterized for various parameters like particle size, crystal phase, pore size, thickness and mechanical properties. In our current study, the photocatalytic activities of these sol–gel derived TiO2 films were studied utilizing a quartz batch reactor. The quartz batch reactor was characterized for parameters like mixing, recycle, aeration and UV radiation flux, and the TiO2 coated substrates were used as the photocatalyst. The activities of the catalyst films were evaluated by measuring the degradation rate of 4-chlorobenzoic acid used as a model organic pollutant. Immobilized TiO2 powder films on stainless steel containing a mixture of anatase and rutile phases were found to be more effective than films that were substantially composed of anatase phase particles. The activity of glass beads coated with TiO2 powder was compared to the activity of commercially available TiO2 catalyst beads. While the activity of the commercially available TiO2 catalyst beads was higher there was significant attrition of the TiO2 catalyst film. The films on the glass beads possessed better mechanical properties than the commercial catalyst beads and their activity can be significantly improved by optimizing the film synthesis process parameters.

Introduction

Photocatalytic processes to degrade organic pollutants in water by oxidation utilizing TiO2 as a catalyst either in the form of unsupported suspended TiO2 powder [3], [4] or in the form of TiO2 immobilized on substrates have been the subject of extensive research recently [5], [6]. Possible applications of these processes include production of ultrapure water for pharmaceutical and microelectronic industries and treatment of contaminated groundwaters in small to medium sized units [7], [8]. Photocatalytic reactors utilizing TiO2 catalyst irradiated with UV have been used to degrade numerous organic contaminants in water including herbicides [3], aniline [9], oil [10] and nitrophenols [11], [12]. Several different reactor models have been proposed and evaluated for the purpose of removing organic contaminants from water. Examples of these are the packed bed photoreactor [13], the annular photoreactor [14], the Taylor vortex reactor [15] and the falling film reactor [16].

Different models can be classified into two broad categories—slurry reactors utilizing suspended TiO2 powders and immobilized TiO2 catalyst reactors. Slurry reactor systems have relatively high surface area for mass transfer due to the high specific surface area of the suspended TiO2 powders. Surface areas of the powders used in slurry reactors in the range from 50 m2/g to higher than 300 m2/g as reported by various investigators [17], [18]. The major drawback of the slurry reactor systems is the requirement for a filtration unit for recovering the catalyst from the effluent stream. In practice, it is often difficult to remove the smallest catalyst particles from the effluent and this may be a cause of turbidity downstream. The filtration requirements also pose a considerable challenge to scale up bench scale reactors into the field.

For the above reasons, there is a preference to use immobilized TiO2 as the catalyst in photocatalytic reactor systems. However, such immobilized reactor systems suffer from some serious drawbacks that limit their use. Heterogeneous phase reactors using immobilized TiO2 may be limited by mass transfer [19], [20], [21] due to the relatively lower external mass transfer area available as compared to the slurry reactor system based on the area available for mass transfer per unit mass of the catalyst. Such reactors may have difficulty retaining a constant catalytic activity due to attrition of catalyst resulting from shear force induced at the surface of the catalyst by the flow of the fluid. This attrition may necessitate offstream reactor down time to replace the catalyst. Furthermore, as noted before, in the absence of a filtration unit to remove the separated catalyst, the catalyst particles may become a cause of turbidity in the effluent.

We have previously described in detail our attempt to produce an immobilized TiO2 catalyst film with excellent mechanical properties that can be coated on a variety of substrates including complex shapes [1]. In the current discussion we describe an extension of our earlier work in preparing and characterizing powder enriched TiO2 films, by studying their photocatalytic activities. We have evaluated the activities of powder enriched TiO2 films against conventional sol–gel derived TiO2 catalyst and commercially available TiO2 catalyst.

Section snippets

Chemicals

The following chemicals were used: 4-chlorobenzoic acid (Mol. wt. 156.57, 99%, Aldrich, Milwaukee, WI), potassium hydroxide (Mol. wt. 56.11, 86.4%, Fisher), acetonitrile (Mol. wt. 41.05, HPLC grade, Fisher), sulfuric acid (95.7%, Fisher), and nitric acid (68–71%, w/w, trace metal grade, Fisher). All the chemicals used in the potassium ferrioxalate actinometry method were reagent grade. Commercial extra pure titanium isopropoxide (TTIP) (Aldrich), isopropanol (i-prOH) (Fisher Scientific),

Pathway of photocatalytic oxidation via TiO2 catalyst

Anatase TiO2 has a band gap energy of 3.2 eV and rutile TiO2 has a band gap energy of 3.0 eV. When light below a certain wavelength is incident on these crystals, electron–hole pairs are generated.TiO2+hν→e+h+

The threshold wavelength is λ≤380 nm for anatase and λ≤413 nm for rutile. There are a variety of possible reactions with the ones of interest being [23],O2+e→[O2]The generation of the superoxide radical is followed by[O2]+H2OHO2+OHHO2+e→[HO2][OH2]+H2OH2O2+OHFinally the hydroxyl

Conclusions

We compared the activity of TiO2 films obtained on stainless steel substrates via a conventional alkoxide sol–gel method [2] and via a Degussa P-25 powder enriched (PE) variation of the conventional alkoxide method [1]. We find that the TiO2 film obtained from PE sol is significantly more active than the film obtained from the sol without modification. We attribute this increased activity to the increased fraction of rutile phase in the TiO2 film obtained from the PE alkoxide sol. The reasons

Acknowledgements

The authors would like to thank the Center of International Research for Water and the Environment (Centre International de Recherche Sur l’Eau et l’Environnement) of ONDEO Services for providing financial support to the present study.

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