Carbon xerogels and ceria–carbon xerogel materials as catalysts in the ozonation of organic pollutants
Graphical abstract
Highlights
▸ Carbon xerogels (XC) and various ceria–XC materials are tested as ozonation catalysts. ▸ The pore size of XC plays a key role in the degradation of pollutants. ▸ Ceria–XC catalysts present higher performance than ceria and XC. ▸ The reaction mechanism involves surface reactions. ▸ A new and highly active ceria–XC catalyst was prepared by one-pot synthesis.
Introduction
Ozone is widely used in water treatment as disinfectant and oxidant. Transformation of organic compounds occurs via direct reaction with ozone or indirectly with hydroxyl radicals (HO), resulting from ozone decomposition in water [1]. Ozone reacts selectively with organic compounds attacking preferentially aromatic rings and unsaturated bonds, whereas HO is a less selective and more powerful oxidant. Despite several advantages of using ozone, there are a few disadvantages that limit its application in water treatment, such as the cost of generation and its highly selective oxidation power [2]. In order to overcome the latter problem, some ozone-based advanced oxidation processes have been developed; among them catalytic ozonation has received wide interest as a promising technology for removing refractory organic pollutants in water. Ozone combined with appropriate catalysts can improve the removal of taste, colour and organic contaminants.
Porous carbon materials prepared by polycondensation of hydroxylated aromatics (phenol, catechol, resorcinol, hydroquinone or phloroglucinol) and aldehydes (formaldehyde or furfural) in a solvent followed by drying and pyrolysis have been extensively studied [3]. The nature of the precursors, the gelation conditions and the drying method determine the texture of the carbon material obtained. For the preparation of carbon xerogels the most common precursors are resorcinol and formaldehyde, and the polymer is usually synthesized using water as solvent and Na2CO3 as catalyst. Carbon xerogels present excellent properties, such as high specific surface area, porosity and conductivity, controllable average pore size and they can be prepared in the desired form (monolith, thin film and powder) [4].
An extended mineralisation was observed when Mn, Co or Ce oxides supported on activated carbon were used in the ozonation of sulphanilic acid, aniline and a reactive dye [5]. The association of ozone and carbon xerogels with different surface chemical properties was reported in a previous study [6], and an improvement of the TOC removal from dye solutions was observed, especially with the more basic material. In a recent work, the effect of the composition and the influence of the surface chemistry and textural properties of carbon materials in the catalytic activity of ceria–carbon composites were evaluated. It was concluded that ceria–carbon xerogel composites presented better performance than carbon xerogel in the ozonation of oxalic acid [7].
The aim of this work is to study the ozonation of selected organic pollutants (oxalic acid and the textile dye CI Reactive Blue 5) in the presence of carbon xerogels with different properties, and ceria–carbon xerogel materials with different compositions and synthesized by different procedures (incipient wetness method, one-pot synthesis and precipitation method).
Section snippets
Materials and characterisation methods
Two organic molecules were selected for this study: oxalic acid (99%, Sigma–Aldrich) and a textile dye (CI Reactive Blue 5).
Different sets of catalysts were prepared. The first one consists of carbon xerogels prepared by the sol–gel process at different initial pH (identified as XC_pH = 5.45, XC_pH = 6.0 and XC_pH = 6.25). The second set of catalysts correspond to cerium oxide supported by the incipient wetness method on the carbon xerogel synthesized at pH 5.45, with 1, 5 and 10 wt% of CeO2 (samples
Characterisation of the catalysts
The properties of the prepared materials are presented in Table 1.
The main differences among the carbon xerogels (XC) prepared at different initial pH are in the average mesopore diameter. The carbon xerogel prepared at pH 5.45 has the largest mesopore sizes (dBJH = 24.4 nm). The catalysts of cerium oxide supported on carbon xerogel and the composite present similar textural properties. The sample 1%Ce/XC_in situ has a specific surface area and an average mesopore diameter smaller than XC pH 5.45.
Conclusions
Carbon xerogels with different textural properties and ceria–carbon xerogels materials with different compositions and synthesized by different routes were tested in the ozonation of oxalic acid and a reactive dye.
Pore sizes of carbon xerogels play a key role in the degradation of the selected pollutants: small pores are better for oxalic acid (small molecule) and the opposite is observed for the reactive dye (large molecule).
The catalyst prepared by one-pot synthesis and all materials of
Acknowledgments
This work was supported by project PEst-C/EQB/LA0020/2011 financed by FEDER through COMPETE. C.A.O. acknowledges the grant received from FCT (BD/45809/2008). The authors are indebted to Dr. Pedro Tavares (UTAD) for assistance with XRD analyses.
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