Structure and photocatalytic performance of TiO₂/clay nanocomposites for the degradation of dimethachlor

Abstract

In the present study TiO₂/clay composites were synthesized by dispersion of TiO₂ on the surfaces of a natural montmorillonite and a synthetic hectorite in order to increase the sorption ability of TiO₂ and therefore its photocatalytic action. Six materials with different loading in TiO₂ (15, 30 and 55 wt%) were prepared and characterized by several analytical techniques including XRD, BET and SEM analysis. The synthetic procedure allows the development of delaminated layers for hectorite–TiO₂ samples, while in the case of montmorillonite–TiO₂ composites we have the formation of a more lamellar-like aggregation. It was found that, the greater the percentage of TiO₂, the greater the pore volume and the specific surface area of the montmorillonite–TiO₂ samples. On the contrary, in the case of hectorite–TiO₂ samples, as the content of TiO₂ increases, the surface area and pore volume decreases. The photocatalytic efficiency of the nanocomposite catalysts was evaluated using a chloroacetanilide herbicide (dimethachlor) in water as model compound. The primary degradation of dimethachlor followed pseudo-first-order kinetics according to the Langmuir–Hinshelwood model. All supported catalysts exhibit good photodegradation efficiency and their overall removal efficiency per mass of TiO₂ was better than that of bare TiO₂ produced by the sol–gel method. In conclusion, together with their good sedimentation ability the composite materials could be considered as a promising alternative for the removal of organic water contaminants.

Introduction

Photocatalysis mediated by the semiconducting TiO₂ is an established advanced oxidation process becoming more and more attractive for environmental applications [1], [2]. TiO₂ is used in many applications because of its outstanding properties, including deodorizing, defogging, anti-bacterial and anti-cancer action [3]. But one of the most significant features of TiO₂ is its strong decomposing power of organic pollutants in water and air. Advantages of utilizing titanium oxide in the decomposition of organic pollutants includes its low cost, ease of handling, radiation stability, and avoiding the need to use strong oxidizing agents [4].

The photocatalytic activity of TiO₂ is strongly influenced by its structure. For this purpose, the important role of structure has been studied extensively in the literature by preparing a series of compounds in which the structure of TiO₂ varied from layered and pillared titanates, microcrystalline TiO₂ pillared clays, mixed Ti/Si oxides, nanocrystals of TiO₂ dispersed in inorganic media, or on the surface of active, non-activated carbon [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. A major conclusion is that the activity of titanium-containing solids is affected by its crystal structures and chemical composition of the respective composite. In this way, it is important to stress that TiO₂ composites have been reported as highly effective photocatalysts for a range of catalytic performances [5], [10], [11], [12], [13]. Regarding TiO₂–clay composites a comparison between TiO₂ pillared clays and solid dispersions of anatase in swellable aluminosilicate minerals has shown that most TiO₂ particles are amorphous in pillared clays exhibiting a poor photocatalytic activity [3], [15] while composites from dispersion of TiO₂ particles in layered clays appeared to be more effective photocatalysts [10], [11], [12], [13]. Furthermore, the way to alter the pillar's size or the pore structure was a difficult procedure thus limiting the applications of the pillared materials. Recently, the synthesis of composite nanostructures of TiO₂ and swellable aluminosilicate minerals has been reported. These TiO₂-composites exhibited superior photocatalytic properties [10], [12], [13].

The present work sets out two objectives. At first, to prepare TiO₂ composites made up from the dispersion of the oxide to the surfaces of two different raw clays, namely (i) natural clay (montmorillonite) and (ii) synthetic hectorite (laponite RD). Six materials with different loading by weight of TiO₂ (15%, 30% and 55%) were prepared and characterized by several analytical techniques including XRD, N₂ adsorption–desorption isotherms, and SEM analysis. Secondly, to evaluate the photocatalytic efficiency of the nanocomposite catalysts using a chloroacetanilide herbicide (dimethachlor) as model compound. The molecular structure of the compound is depicted in Fig. 1.