Photodegradation of chloroacetic acids over bare and silver-deposited TiO2: Identification of species attacking model compounds, a mechanistic approach

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Abstract

Photocatalytic degradation of chloroacetic acids (ClAAs) over various bare and silver-deposited Degussa P25 TiO2 particles was studied. Adsorption measurements carried out using TiO2 photocatalysts of different origin demonstrated significant dependence of the adsorption efficiency on the nature of semiconductor particles and on the number of chlorine atoms of the substrate. Irradiation of the reaction mixtures containing monochloroacetic acid (MCA), dichloroacetic acid (DCA) and trichloroacetic acid (TCA), respectively, over P25 titania were performed under anaerobic and aerobic conditions. The progress of photocatalysis was followed by measuring the substrate concentration, the total organic carbon content (TOC) and the concentration of the chloride ion in the liquid phase of reaction mixtures. Opposite trends in the photodecomposition rate of the substrates were obtained for aerobic vMCAvDCA>vTCA and for anaerobic experiments vTCA>vDCA>vMCA, respectively. The evolved CO2 was also measured under aerobic photodecomposition of DCA. Important role of hydroxyl radicals in the photomineralization of mono- and dichloroacetic acid was confirmed by using coumarin (COU) as a hydroxyl radical scavenger and oxalic acid as an efficient scavenger for holes. Silver deposition onto the TiO2 surface enhanced the efficiency of the semiconductor by a factor of 4 for the photooxidation of TCA and by a factor of 1.4 for DCA and MCA.

Introduction

Application of TiO2 photocatalysis has been widely reported as a promising alternative technology for removal of various organic and inorganic pollutants from contaminated water and air [1], [2], [3], [4], [5], [6], [7]. Chloroacetic acids are important medical intermediates and organic materials [8]. These compounds are widespread in the environment, and often occur in industrial waste as pollutants and in chlorinated drinking waters as major chlorination by-products as well [9], [10], [11], [12], [13]. ClAAs are carcinogenic and mutanogenic [10]. Moreover, they cannot be completely decomposed by biotechnologies [12], [13]. In this respect investigation of the applicability of the heterogeneous photocatalysis for degradation of ClAAs and the study of the mechanism of these photoreactions are considered as a research of great importance.

Decomposition of mono-, di- and trichloroacetic acid in aqueous titania suspensions was extensively studied by several research groups [13], [14], [15], [16], [17], [18], [19]. Different experimental evidences, hence various proposals have been reported concerning the reaction mechanism, particularly on the primary steps of the photocatalytic decomposition of ClAAs. It has been concluded that two types of reactions may be responsible for the TiO2-mediated photodegradation of chloroacetic acids: (1) direct reactions between the photogenerated charge carriers and the organic molecules [16], [20], [21], [22] and (2) reactions of hydroxyl radicals or other oxygen containing radicals with the organic molecules [15], [23], [24]. MCA and DCA are readily decomposed over UV-irradiated TiO2 catalyst in aqueous media to CO2 and HCl [14], [15]. On the other hand using bare TiO2 TCA is degraded with a very low efficiency [15]. TCA has no C–H bond, and such a molecule has been found to be hardly reactive in TiO2-based photocatalytic systems [15], [24].

Various approaches have been attempted to enhance the photocatalytic efficiency and visible light utilization of TiO2 [7]. Doping and partially coating the catalyst's surface with noble and transition metals (such as Cu, Fe, Ag, Cr, Pt, Pd, Rh, Ir, Os and Au) have been proved to be promising [17], [25], [26], [27], [28]. Photocatalytic deposition of nanosized silver clusters offered some advantages, such as a simple procedure and relatively low cost [26]. The silver clusters at the semiconductor surface can act as electron traps during the illumination [29], [30], [31]. Due to the electronic contact between the deposited metal and the semiconductor (SC) the electrons are removed from TiO2 into the vicinity of metal clusters resulting in the formation of Schottky-barriers at each Ag–TiO2 contact regions. This facilitates the charge separation and hence inhibits the recombination of photogenerated electron–hole pairs [26], [32], [33], [34]. Substantial amount of work have reported the effect of silver deposition on the photodegradation rate of organic substrates [26], [31], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]. In a recent study Tran et al. [44] claim to predict the effect of silver deposition on TiO2 by considering the molecular structure of the substrate, and they also suggest that an enhanced photocatalytic activity can be observed for those compounds that predominantly oxidized by holes instead of hydroxyl radicals [44], [47]. In this respect silver-deposited TiO2 can be applied not only to increase the photooxidation rate of the organics but also to provide data for revealing the mechanism of the studied photoreaction. In principle, some valuable additional information on the mechanism of photocatalytic degradation of ClAAs can be obtained using scavengers for hole (e.g. oxalic acid) and for hydroxyl radical (e.g. COU), respectively in competitive kinetic experiments.

The aim of the present contribution is to deepen our knowledge on the photodecomposition mechanism of ClAAs occurring over TiO2 nanoparticles exposed to photons of higher energy than the band gap of the semiconductor. Attention has been focused to reveal the primary electron transfer reaction and the role of the hydroxyl radical formed by one of these steps. We propose a strategy based on: (i) the comparison of the rate of photoreactions occurring under anaerobic and aerobic conditions; (ii) the analysis of the results of competitive kinetic measurements using COU as a component, which is weakly adsorbed on TiO2 surface and acts as hydroxyl radical scavenger producing highly fluorescent 7HC and (iii) the application of silver-deposited TiO2 for increasing the rate of photoreaction of the reactant that can be directly attacked by eCB or h+VB generated by photon absorption. The different affinity of MCA, DCA and TCA to the TiO2 surface is also exploited for confirming the proposed mechanism.

Section snippets

Materials

The titanium dioxide samples used in experiments were obtained from Degussa (P25: 70% anatase, 30% rutile; with a surface area of 50 m2 g−1), from Aldrich (mainly anatase, with a surface area of 9.6 m2 g−1) and from Fluka (mainly rutile, with a surface area of 9.7 m2 g−1).

Monochloroacetic acid, dichloroacetic acid and trichloroacetic acid of the purest grade were purchased from Fluka. Coumarin (COU) and 7-hydroxycoumarin (7HC) of the purest grade obtained from Sigma–Aldrich and Carlo Erba,

Adsorption studies

In principle, adsorption and desorption of reactants and intermediates have great influence on the efficiency of the overall degradation process in TiO2-mediated photocatalytic procedures. Organic compounds chemisorbed on TiO2 surface directly react with photogenerated holes, while reactants weakly or non-adsorbed in aqueous environment are generally attacked by radicals such as radical dotOH, or O2radical dot at the surface or in the bulk of aqueous phase. Thus adsorption experiments are considered as essential

Conclusions

Photocatalytic degradation of ClAAs over various bare (pure anatase, pure rutile and Degussa P25) and silver-deposited Degussa P25 TiO2 particles was studied in aerobic and anaerobic conditions. The experiments were extended to investigate the adsorption characteristics of the substrates using TiO2 photocatalysts of different nature. The progress of photocatalytic reaction was followed by measuring of the concentration of ClAAs, the total organic carbon content, the concentration of the

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