**1. Introduction**

The complete degradation of dye molecules is not possible by the application of precipitation, adsorption, oxidation, reduction and biological and electrochemical types of conventional methods. These methods may either end up with less efficiency or create a secondary sludge. Semiconductor photocatalysis is an advanced oxidation process (AOP) for the treatment of air and water streams and emerged as an important technology for the degradation of dye molecules. In summary, the mechanism of photocatalytic processes is initiated by the bandgap illumination and this leads to production of electron-hole pairs. After separation, electrons and holes migrate to the catalyst surface, induce redox reactions with adsorbed pollutants and eventually result in the degradation of the dye pollutants.

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ZnO is a wide band-gap (∼3.3 eV) semiconductor that has been extensively used because of its catalytic and photochemical properties along with its low cost [1]. There are many reports of ZnO having higher photocatalytic activities than other semiconductors in both air and aqueous media [2,3]. As a photocatalyst, the surface area plays an important role since reactions mainly occur between catalyst surfaces and pollutants. The nanoscale ZnO crystals have shown larger surface areas and higher photocatalyic performances than that of bulk materials [4]. There‐ fore,recent studies have focused on the synthesis of nanostructured ZnO with tunable size and shape [5,6]. However, ZnO nanoparticles are not stable in acidic and alkaline conditions and also show rapid deactivation in bulk due to increased tendency of aggregation [7]. Hence, the development of industrially viable, cost-effective, eco-friendly adsorbents with attractive multiple functions such as adsorption, and decomposition becomes important. Silicate adsorbents engineered with photocatalytic ingredients, for example, tailoring the aluminosili‐ cate layers and their surfaces with nanostructured semiconducting photocatalysts, can make them multifunctional composites and heterogeneous catalysts [8]. ZnO/clay system reveals the potential of this composite for various applications [9–12]. Among clay minerals, the usage of sepiolite as a supportmaterial is rarelyreported[13–18].Natural sepiolite is a verycheap,fibrous and hydrated magnesium silicate with a relatively high surface area. The presence of alkales‐ cent [MgO6] and acidic [SiO4] centers in the sepiolite structure enhances the adsorption of reactant molecules and their degradation possibility. Moreover, silicate layers appear as an attractive support for the assembly of small-sized metals and metal-oxide aggregates (clus‐ ters and nanoparticles) that have been mainly employed for catalytic purposes. The immobili‐ zation of nanoparticles on the inner and outer surfaces of inorganic supports results in the formation of nanocomposite materials. The synergy established among nanoparticles and support systems makes them attractive options for the degradation of pollutants. In the study of Xu et al., quantum-sized ZnO particles supported on sepiolite were prepared using a solgel method with the sepiolite of acid activation as carrier and zinc acetate dihydrate (Zn(CH3COO)2 2H2O) and lithium hydroxide monohydrate (LiOH H2O) as raw materials [13]. Theyfoundthatthenano-ZnOsupportedonsepiolite cannotonlysolve thedispersingproblem but also has a positive synergistic effect on the ZnO photocatalysis. Bautista et al. prepared TiO2-Sep supports of vanadium oxide in order to obtain a new TiO2 support with a high and thermostable surface area [14]. According to this study, vanadium oxides supported on TiO2 coated sepiolite and sepiolite characterization studies indicated that well-dispersed vanadi‐ um in both types of supports was achieved in the systems with vanadia loading below the theoretical monolayer. Above this vanadia loading, the formation of V2O5 nanoparticles with a mainly crystalline character took place as well as the formation of V-Mg mixed metal-oxide phases, especially in systems supported on sepiolite. The hydrophilic character and more open structure ofthe sepiolite was underlined in the study ofArques et al.[15].Accordingly, sepiolite appearedto be a convenient supportforpyryliumsalts to be employed as a heterogeneous solar photocatalyst. Also, promising results have been obtained testing the performance of the new material with ferulic acid as target pollutant, and important percentages of photo-oxidation were achieved. In another study, monolithic catalysts based on Rh/TiO2-sepiolite were developed and tested in the decomposition of N2O traces [16]. The system was found to be extremely sensitive to the amount of rhodium and is an attractive alternative for the elimina‐ tion of N2O traces from stationary sources due to the combination of high catalytic activity with a low pressure drop and optimum textural/mechanical properties. The structural and photoca‐ talytic properties of TiO2-supported sepiolite and sodium ion-treated sepiolite catalysts were also investigated for the degradation of β-naphthol molecule [17]. Sodium chloride treatment enhanced the attraction of sepiolite support through TiO2 nanoparticles. This study explored that sepiolite can be employed as a catalyst support for the photocatalytic degradation reactions in solution. Such supported catalysts can be readily separated from the suspension without filtration since they decant in minutes, while TiO2 (Degussa P-25) sample could not sediment over hours. This provided an important, practical advantage in the usage of TiO2-supported sepiolite catalysts.

In this chapter, a mixed structure of ZnO and sepiolite (ZnO-SEP) is examined for the degra‐ dation of an azo dye, methyl orange (MO). Such supported catalyst systems are prepared using a coprecipitation method with highly dispersed active ZnO nanoparticles and investigated in terms of the dark adsorption capacities and photoactivities. X-ray diffraction (XRD), surface area (BET) measurements, scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS) and UV-vis diffuse reflectance spectra (UV-vis DRS) techniques are used for the characterization of these catalysts. The decolorization of MO is followed under UV irradiation.
