Elsevier

Catalysis Today

Volume 53, Issue 1, 15 October 1999, Pages 115-129
Catalysis Today

Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants

https://doi.org/10.1016/S0920-5861(99)00107-8Get rights and content

Abstract

Photocatalysis is based on the double aptitude of the photocatalyst (essentially titania) to simultaneously adsorb both reactants and to absorb efficient photons. The basic fundamental principles are described as well as the influence of the main parameters governing the kinetics (mass of catalyst, wavelength, initial concentration, temperature and radiant flux). Besides the selective mild oxidation of organics performed in gas or liquid organic phase, UV-irradiated titania becomes a total oxidation catalyst once in water because of the photogeneration of OHradical dot radicals by neutralization of OH surface groups by positive photo-holes. A large variety of organics could be totally degraded and mineralized into CO2 and harmless inorganic anions. Any attempt of improving titania’s photoactivity by noble metal deposition or ion-doping was detrimental. In parallel, heavy toxic metal ions (Hg2+, Ag+, noble metals) can be removed from water by photodeposition on titania. Several water -detoxification photocatalytic devices have already been commercialized. Solar platforms are working with large-scale pilot photoreactors, in which are degraded pollutants with quantum yields comparable to those determined in the laboratory with artificial light.

Section snippets

Principle of heterogeneous photocatalysis

Heterogeneous photocatalysis is a discipline which includes a large variety of reactions: mild or total oxidations, dehydrogenation, hydrogen transfer, O218–O216 and deuterium-alkane isotopic exchange, metal deposition, water detoxification, gaseous pollutant removal, etc. In line with the two latter points, it can be considered as one of the new ‘advanced oxidation technologies’ (AOT) for air and water purification treatment. Several books and reviews have been recently devoted to this problem

Catalysts and photoreactors

Various chalcogenides (oxides and sulfides) have been used: TiO2, ZnO, CeO2, CdS, ZnS, etc. As generally observed, the best photocatalytic performances with maximum quantum yields are always obtained with titania. In addition, anatase is the most active allotropic form among the various ones available, either natural (rutile and brookite) or artificial (TiO2–B, TiO2–H). Anatase is thermodynamically less stable than rutile, but its formation is kinetically favored at lower temperature (<600°C).

Mass of Catalyst

Either in static, or in slurry or in dynamic flow photoreactors, the initial rates of reaction were found to be directly proportional to the mass m of catalyst (Fig. 3(A)). This indicates a true heterogeneous catalytic regime. However, above a certain value of m, the reaction rate levels off and becomes independent of m. This limit depends on the geometry and on the working conditions of the photoreactor. It was found equal to 1.3 mg TiO2/cm2 of a fixed bed and to 2.5 mg TiO2/cm3 of suspension.

Photocatalytic mild oxidations versus total oxidations

The gas phase or the pure liquid organic phase oxidations using oxygen from the air as the oxidizing agent mainly concerned the mild oxidation of alkanes, alkenes, alcohols and aromatics into carbonyl-containing molecules [15], [16], [17], [18]. For instance, cyclohexane and decaline were oxidized into cyclohexanone and 2-decalone, respectively, with an identical selectivity of 86% [17]. Aromatic hydrocarbons [18] such as alkyltoluenes or o-xylenes were selectively oxidized on the methylgroup

Disappearance of the pollutant

Most of the pollutants which are in the non-exhaustive list, given in Table 1, disappear following an apparent first order kinetics (see Section 3.3). For aromatics, the dearomatization is rapid even in the case of deactivating substituents on the aromatic ring. This was observed for the following substituents: Cl [19], [20], NO2 [21], CONH2 [22], CO2H [19] and OCH3 [23]. If an aliphatic chain is bound to the aromatic ring, the breaking of the bond is easy as was observed in the photocatalytic

Inorganic anions

Various toxic anions can be oxidized into harmless or less toxic compounds by using TiO2 as a photocatalyst. For instance, nitrite is oxidized into nitrate [35], [36], sulfide, sulfite [37] and thiosulfate [38] are converted into sulfate, whereas cyanide is converted either into isocyanide [39] or nitrogen [40] or nitrate [41].

Noble metal recovery

Heavy metals are generally toxic and can be removed from industrial waste effluents [38], [42] as small crystallites deposited on the photocatalyst according to the redox

Polyphasic (solar) photoreactors

To perform the various types of photocatalytic reactions described above, different types of photoreactors have been built with the catalyst used under various shapes: fixed bed, magnetically or mechanically agitated slurries, catalyst particles anchored on the walls of the photoreactor or in membranes or on glass beads, or on glass-wool sleeves, small spherical pellets, etc. [1], [2], [3], [4]. The main purpose is to have an easy separation of the catalyst from the fluid medium, thence the

Conclusions

Water pollutant removal appears as the most promising potential application since many toxic water pollutants, either organic or inorganic, are totally mineralized or oxidized at their higher degree, respectively, into harmless final compounds. Besides some drawbacks (use of UV-photons and necessity for the treated waters to be transparent in this spectral region; slow complete mineralization in cases where heteroatoms are at a very low oxidation degree; photocatalytic engineering to be

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