Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants
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
References (52)
- et al.
Catal. Today
(1993) - et al.
J. Catal.
(1985) - et al.
J. Catal.
(1987) - et al.
J. Catal.
(1979) - et al.
Stud. Surf. Sci. Catal., Elsevier Amsterdam
(1990) - et al.
J. Photochem. Photobiol. A: general
(1998) - et al.
J. Blanco Appl. Catal. B: Environmental
(1998) - et al.
Catal. Today
(1996) - et al.
Chemosphere
(1989) - et al.
Wat. Res.
(1990)
Chemosphere
J. Mol. Catal.
J. Catal.
J. Photochem. Photobiol. A: Chem.
J. Photochem. Photobiol. A: Chem.
J. Photochem. Photobiol. A: Chem.
Sol. Energy Mater. Sol. Cells
Solar Energy
Appl. Catal. B: Environmental
Chem. Rev.
Cited by (2398)
Metal halide perovskite-based photocatalysts for organic pollutants degradation: Advances, challenges, and future directions
2024, Colloids and Surfaces A: Physicochemical and Engineering AspectsTreatment of petroleum refinery wastewater by a combination of anodic oxidation with photocatalyst process: Recent advances, affecting factors and future perspectives
2024, Chemical Engineering Research and DesignSono-precipitation dispersion of CuO-doped ZnO nanostructures over SiO<inf>2</inf>-aerogel for photo-removal of methylene blue, congo red and methyl orange from wastewater
2024, Journal of Industrial and Engineering Chemistry