New carbon xerogel-TiO2 composites with high performance as visible-light photocatalysts for dye mineralization
Graphical abstract
Introduction
The increasing pollution level of water resources together with the appearance of new and more stable pollutant in solution (emerging pollutant) make necessary the development of new and more efficient protocols to deal with this new problem. Traditionally, environmental catalysis tries to respond adequately to this requirement, fitting or developing materials and processes for a progressively more industrialized society. A large battery of advanced oxidation processes (Fenton, ozonation, catalytic wet air oxidation (CWAO), etc) is being developed [1], [2], [3]. Between them, photocatalytic processes are very interesting, and sometimes are used in combination [4] with the previous ones (photoFenton, photoozonation, etc). Photocatalytic processes become especially interesting when are able to use the visible radiation, not only by the energy saving, but because in such a way can be easily used in developing countries.
It is well known that TiO2 is the most extensively used photocatalyst. It has been regarded as an efficient photocatalyst for degradation of organic pollutants from water due to its high stability, low cost and environmental friendliness [5], [6]. However, UV-light is necessary to create the hole-electron pairs needed for the photocatalytic reaction which limits their use in environmental applications. Only 3–5% of UV in solar spectrum can be absorbed by pure TiO2 due to its wide band gap (3.2 eV of anatase and 3.0 eV of rutile), which greatly restricts its photocatalytic applications in the visible-light range [7], [8].
Therefore, to improve the efficiency of TiO2 under solar (or visible) light is necessary to modify the material in order to facilitate the visible light absorption. Several approaches are made with this aim such us the introduction of doping agents or sensitizers to decrease the material band gap [9], [10]. The introduction of metal doping agents into TiO2 narrows the band gap by producing new hybrid states which confer significant visible light absorbance to TiO2. On the other hand, the improvement obtained using sensitizers is due to the direct absorption of visible light by the sensitizer and the release of electrons to TiO2 in a redox process.
However, different problems were also detected. Metal doping shows thermal instability of doped TiO2, electron trapping by the metal centres decreasing the photocatalytic activity and high processing costs [11], [12]. Alternatively, doping with non-metals such as N and S are used [13], [14] being now the main drawbacks (i) the difficulty to obtain N-doped TiO2 with high nitrogen concentration; (ii) the formation of defects which can act as recombination centres for carriers [15]; and (iii) the decrease of N concentration at the surface layer after irradiation [16].
The synergetic effect of the addition of carbon materials to TiO2 photocatalysts was initially presented using directly simple mixtures of both solids [17], [18]. Although activated carbons were initially used to enhance the TiO2 photocatalytic performance [19],nanocarbons including carbon nanotubes (CNT) [18], nanohorms, fullerenes and graphene [20], [21] have been combined with TiO2 by different approach for such objective. Carbon gels are a new type of nanocarbons [22] with high potential applications in catalysis due to their unique properties [23]. In spite that some carbon gels −TiO2 composites were previously described [24], until our knowledge they have not been used today as photocatalysts.
The incorporation of carbon materials to the photocatalyst improve the TiO2 photoactivity by different mechanism [25]: (i) carbon absorbs over a wide range of visible light producing band-gap tuning/photosensitization, (ii) minimisation of electron/hole recombination and (iii) promotion of the reactants adsorption. The higher porosity of carbon facilitates the titania dispersion and the adsorption of reactants, enhancing the active site number and the contact between reactants and catalysts [25]. Additionally, carbon is a good electron acceptor. Electron transfers to the carbon phase minimize the electron/hole recombination on the TiO2. Also the better dispersion of the semiconductor nanoparticles on the carbon phase reduce that recombination because mainly occurs at boundaries and defects [26]. Thus, if the particle size is reduced, the distance that the photogenerated electrons and holes need to travel through the surface reaction sites is reduced, thereby decreasing the recombination probability [27].
In this paper, new TiO2-carbon composites were prepared by a sol–gel process and the xerogels obtained were deeply characterized and tested in the degradation of organic pollutants under visible light. The photocatalytic activity of the composites was evaluated using Orange G (OG) as a target molecule, and the relationship of the photocatalytic activity with the physicochemical characteristics of composites was studied. The prepared new materials present a high performance for the complete mineralization of pollutants (OG) under visible light.
Section snippets
Synthesis of TiO2-carbon xerogel composites
TiO2-carbon xerogel composites were prepared by sol-gel synthesis using resorcinol-formaldehyde and titanium isopropoxide (IV) as carbon and titanium oxide precursor, respectively, in the presence of a non-ionic surfactant (Span 80). In a typical synthesis procedure, Span 80 (S) was dissolved in 900 mL of n-heptane and heated at 70 °C under reflux and stirring (450 rpm). Then a mixture containing resorcinol (R), formaldehyde (F) and water (W) was added dropwise into the above solution. Immediately
Results and discussion
The fraction of TiO2 present in the CTiX carbon composites was determined by TGA, after burning the carbon phase in air flow. The TiO2 percentages are slightly higher than the theoretical ones due to the fact that the weight lost during the carbonization is not exactly 50%, however the experimental data are much closed to the expected ones (Table 1). Morphology of sample was studied by scanning electron microscopy (Fig. 2). CTiX samples (Fig. 2a) show a typical structure of carbon xerogels [39]
Conclusions
Carbon xerogel-TiO2 composites (CTiX) were successfully synthesized by sol-gel techniques. Composites present a homogeneous and three-dimensional mesoporous structure, where both phases are also homogenously and intimately distributed. This procedure can be fitted to obtain TiO2 coating the carbon phase structured microspheres, obtaining microporous materials. Due to the interactions between both phases all composites present a high dispersion of anatase TiO2 nanoparticles on the carbon support
Acknowledgements
EBG acknowledges for a pre-doctoral fellowship to the MCINN project CTM2010-18889. This research is supported by the FEDER and Spanish projects CTQ2013-44789-R (MINECO) and P12-RNM-2892 (Junta de Andalucía).
References (58)
- et al.
Chem. Eng. Process.
(2016) - et al.
J. Environ. Manag.
(2016) - et al.
Appl. Catal. B: Environ.
(2016) - et al.
Sep. Purif. Technol.
(2016) - et al.
Appl. Catal. B: Environ.
(2012) - et al.
Appl. Catal. B: Environ.
(2007) - et al.
Thin Solid Films
(1999) - et al.
Chem. Phys.
(2007) - et al.
Appl. Catal. B: Environ.
(1998) - et al.
Appl. Catal. B: Environ.
(2005)
J. Hazard Mater
Appl. Surf. Sci.
Appl. Catal. B: Environ.
Carbon
Catal. Today
Appl. Catal. A: Gen.
Carbon
Chem. Eng. J.
Carbon
Microporous Mesoporous Mater.
Appl. Catal. B: Environ.
J. Colloid Interface Sci.
Carbon
Curr. Appl. Phys.
Stud. Surf. Sci. Catal.
J. Catal.
Appl. Catal. B: Environ.
Appl. Surf. Sci.
Appl. Catal. B: Environ.
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