Mixed-phase TiO2 photocatalysts: Crystalline phase isolation and reconstruction, characterization and photocatalytic activity in the oxidation of 4-chlorophenol from aqueous effluents
Graphical abstract
Introduction
Heterogeneous photocatalysis on semiconductor powders is recognized as a viable advanced oxidation process (AOP) for the degradation of priority organic pollutants from wastewaters. Photocatalysis is usually described as the process in which a chemical reaction is induced when a semiconductor, which acts as a photocatalyst, is irradiated with photons of energy higher than or equal to its band gap energy, to produce photo-electrons and photo-holes [1], [2]. During a photocatalytic reaction, reactants can be adsorbed and react with either photo-electrons (acceptor molecules like dissolved O2) or with the photo-holes (donor molecules like H2O, for example) to produce oxidizing species like superoxide radical anion or hydroxyl radical, respectively, which consequently will oxidize pollutant species [2].
Applications of photocatalysis in this field are determined by the characteristics (composition, flow, etc.) of the aqueous influents that are processed and the quality requirements for the treated effluent. Among many candidates for photocatalysts, TiO2 is almost the only material suitable for industrial use to date and also probably in the future [3].
TiO2 exhibits three distinct polymorphs (anatase, rutile and brookite) of which only anatase, with a band gap of 3.2 eV, is functional as photocatalyst. Although rutile exhibits an energy band gap of only 3.02 eV, its photocatalytic activity in oxidation processes remains very low. Mixed-phase titania photocatalysts, containing both anatase and rutile crystallites, have been reported to display enhanced photoactivity relative to single-phase titania. For example, Degussa (Evonik) P25, Aeroxide TiO2 P25, used as a de facto standard in photocatalysis by titania [4], is a mixed-phase titania photocatalyst. Moreover, this was the first paper discussing the photocatalytic activity of anatase and rutile particles separated from P25 as well as the synergetic effect of co-presence of anatase and rutile. The reason for this so-called synergistic effect is still not fully elucidated although some speculative explanations have been proposed. An improved charge carrier separation, possibly through the trapping of electrons in rutile and the consequent reduction in electron–hole recombination [5], [6], [7] is usually invoked to explain this behavior. Surface trapping of holes together with lattice trapping of electrons has also been reported [8], [9]. However, in order to check the synergistic effect of an anatase–rutile mixture such as Degussa P25, it is necessary to isolate anatase and rutile crystallites from the original material and to reconstruct it, by preparing physical mixtures of known phase compositions. Separation of rutile by selective dissolution of anatase from Degussa P25 in HF has been reported by Ohno et al. [10] Recently, Ohtani et al. [11] proposed a route to isolate anatase by selective dissolution of rutile from Degussa P25, by using a mixed solution of hydrogen peroxide and ammonia. These authors used these phases as standards for the calibration curves in the XRD, in order to calculate the precise crystalline composition of P25.
The first aim of this study is to validate the phase separation methods from Degussa P25 and to provide a thorough characterization of the isolated phases and of the reconstructed TiO2 powders, following the works of Ohtani et al. [4], [11]. Another objective of this research is to check the existence of the synergistic effect between anatase and rutile by studying the effect of the crystalline phase composition of different TiO2 physical mixtures. Anatase and rutile phases, isolated from Degussa P25 or from commercial sources, were mixed and tested in the photocatalytic oxidation of 4-chlorophenol.
Chlorophenols represent common priority organic pollutants in water discharged by several industries and have particularities that make them useful model pollutants. Their properties, such as: toxicity (even at low concentrations), formation of substituted compounds during disinfection and oxidation processes (such as those used currently for the treatment of natural surface water for drinking purposes), phytotoxicity and ability to bioaccumulate in organisms, have similarities with other persistent organic pollutants. 4-chlorophenol, in particular, is considered a representative model of priority pollutants in water [12], [13].
Finally, the possibility to replace the rather expensive Degussa P25 photocatalyst with other lower-price commercially available TiO2 powders (eventually with larger crystal sizes to facilitate material recovering [14], [15], was also studied. In this sense, the critical question to be answered is whether pure anatase should be used alone or the addition of rutile could enhance the photocatalytic activity of the material in the oxidation of priority organic pollutants from water.
Section snippets
Materials
Three TiO2 samples, from Sigma-Aldrich, were used in this study: anatase (denoted as An, product no. 232033), rutile (denoted as Ru, product no. 204757) and Degussa (Evronik) P25 (denoted as P25, product no.718467). The organic compound (4-chlorophenol, 4-CP) used for photocatalytic activity tests and the reagents used in phenol analysis (4-aminoantipyrine, potassium ferricyanide(III) and the ammonium chloride/ammonium hydroxide) were also supplied by Sigma-Aldrich.
Crystalline phase separation
Isolation of anatase powder
Results and discussion
The XRD patterns of the three commercial TiO2 powders are presented in Fig. 1A. The anatase phase was identified by the presence of intense diffraction peaks located at 25° and 48° while in the rutile phase the main peaks appeared at 27°, 36° and 55°. Although An and Ru are commercialized as pure anatase and rutile type materials, both are contaminated with small amounts of the other crystalline polymorph. A rough estimation, based on the intensities of the anatase (1 0 1) and rutile (1 1 0) XRD
Conclusions
In this study we consider the influence of the mixing ratio in anatase to rutile physical mixtures on the photocatalytic degradation of 4-chlorophenol from aqueous solution. For this purpose, we used TiO2 Degussa P25, “pure” anatase and “pure” rutile commercial powders as starting materials. Anatase and rutile were isolated as pure phases by selective dissolution in H2O2/NH4OH and in 10% HF, respectively. All TiO2 samples were thoroughly characterized by various techniques. The crystalline
Acknowledgments
This work was supported by the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, project number PN-II-PT-PCCA-2011-1491 and CNCSIS-UEFISCSU project number PN-II-IDEI code 368/2008. The authors acknowledge also the support of EURODOC Project “Doctoral Scholarships for research performance at European level” POSDRU/88/1.5/S/59410, (ID59410) financed by the European Social Found and Romanian Government.
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