Improved efficiency of sol–gel synthesized mesoporous anatase nanopowders in photocatalytic degradation of metoprolol

https://doi.org/10.1016/j.materresbull.2012.11.098Get rights and content

Abstract

The structural and morphological properties of mesoporous anatase nanopowders, synthesized by sol–gel method, have been modified by varying the duration of calcination, in order to obtain more efficient photocatalyst than Degussa P25 in the degradation of relatively large pollutant molecules (>1 nm in size). According to X-ray diffraction analysis, the crystallite size was increased from 13 to 17.5 nm with the increase of calcination time from 1 to 7 h. The analysis of nitrogen sorption experimental data revealed that all samples were mesoporous, with a mean pore diameters in the range of ∼5–9 nm. The corrugated pore structure model was employed to evaluate pore structure tortuosity. Nanopowder properties have been related to the photocatalytic activity, tested in the degradation of metoprolol tartrate salt, selective β1-blocker used in a variety of cardiovascular diseases, with molecular size of 0.610 nm × 1.347 nm. The study has demonstrated that samples calcined for 4 and 5 h have displayed higher photocatalytic performance than Degussa P25, whereas the sample calcined for 3 h has shown comparable activity.

Highlights

► Mesoporous anatase nanopowders have been synthesized by sol–gel method. ► Structural and morphological properties of powders were varied by calcination time. ► Photocatalytic activity of samples was tested in degradation of MET under UV light. ► Improvement of photocatalytic efficiency was mainly influenced by pore structure.

Introduction

As an environmental-friendly material, titanium dioxide (TiO2) has been recognized as a most promising photocatalyst suitable for decomposition of various organic pollutants in the water and air. Most of all, the possibility to improve photocatalytic performance of nanosized porous TiO2 particles by controlling synthesis conditions and thus tailoring its properties, attracted extensive attention in recent decades. The photocatalysts based on nanostructured TiO2 are already used in a wide range of technological solutions to environmental problems, due to its band-gap energy suitable for redox reactions, mechanical and chemical stability, low cost and a variety of preparation methods [1], [2].

It is now recognized that photocatalytic performance of nanosized porous TiO2 particles is improved with higher surface area [3]. Namely, this material can be synthesized by diverse methods, with pore structure and crystallite size distribution tailored to enable larger surface area, offering more active adsorption sites and photocatalytic reaction centers [4]. However, for the photodegradation of larger organic molecules (with a molecular size of ∼1.5 nm), known as environmental pollutants on a global scale, mesoporous photocatalysts with large surface area have been designed. Several researches have confirmed higher photocatalytic activity of mesoporous TiO2 catalysts in comparison to the microporous materials with a restricted pore size [5], [6], [7]. Such mesoporous TiO2 nanostructures are suitable for facilitating the diffusion of the organic molecules to the active sites on material surface and enable its optimal exposition to the solar light [4], [7]. Mesoporous TiO2 was found to exhibit high photocatalytic activity in the degradations of various large organic molecules such as, for example, metoprolol [8], [9], methylene blue [3], [5], [6], 4-chlorophenol and methylene orange [6], β-cyclodextrin [10], etc., thus opening the field of designing photocatalysts with molecular size sensitivity.

The sol–gel synthesis of nanostructured materials was also intensively explored in the last decade [11], [12], [13], [14], [15], [16]. This technique provides synthesis of nanoparticles at ambient temperature under atmospheric pressure. Since this process occurs in a solution, many advantages over other preparation techniques are confirmed regarding the control of purity, homogeneity, doping, composition and stoichiometry, and most of all the simplicity of processing. Therefore the sol–gel synthesis is now widely used technique for designing nanostructured materials with targeted structural and morphological properties.

Metoprolol tartrate salt (1-[4-(2-methoxyethyl)phenoxy]-3-(propan-2-ylamino)propan-2-ol tartrate (2:1), CAS No. 56392-17-7, (C15H25NO3)2 C4H6O6, Mr = 684.81, MET) is commonly used as selective β-blocker in the treatment of cardiovascular diseases [17]. Continuous input of this pharmaceutical, and its persistence in the aqueous system, even at low concentrations, may result in an emerging environmental pollution. Although common concentrations can be defined as a trace, those pharmaceuticals are persistent against biological degradation and they retain their chemical structure long enough, so their presence in the environment is considered potentially dangerous. Metoprolol has been identified as potential endocrine disrupting compound [18]. Many authors [8], [19], [20] investigated the degradation of selected β-blockers in aqueous suspensions of TiO2 (Degussa P25) and proposed possible reaction pathways for the degradation of these compounds.

In this paper mesoporous TiO2 nanopowders have been synthesized by sol–gel method using titanium tetrachloride (TiCl4) as a precursor. The structural and morphological properties of prepared nanopowders have been modified by varying the duration of calcination in order to obtain photocatalyst more efficient than Degussa P25 in degradation of relatively large pollutant molecules (>1 nm in size). Several characterisation techniques have been employed to correlate structural and morphological properties of synthesized TiO2 nanopowders and their photocatalytic activity under ultraviolet (UV) irradiation. The effects of calcination time on the crystallite size and structure of the synthesized samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Raman scattering measurements.

The porous structure has been analyzed by Brunauer–Emmett–Teller (BET), Barret–Joyner–Halenda (BJH), and corrugated pore structure model (CPSM). The CPSM was employed to determine not only the pore size distribution, but also the pore structure tortuosity factor as a feature of primary importance in catalysis. Namely, the tortuosity factor provides the information on the connectivity among the pores, which is essential to describe transport dynamics in porous media, and consequently determines the time of the catalytic reaction [21], [22].

The properties of sol–gel synthesized anatase nanopowders have been analyzed and related to their photocatalytic activity in metoprolol degradation and compared to those of Degussa P25.

Section snippets

Synthesis

Anatase nanopowders have been synthesized by sol–gel process from titanium tetrachloride (TiCl4, Merck Chemicals) as the precursor. Namely, the Ti(OH)4 hydrogel was obtained by hydrolysis of TiCl4 at 0 °C, with controlled addition of 2.5 wt.% alkalic solution NaOH (MP Hemija) into the aqueous solution of TiCl4 (0.3 M) and careful control of the solution pH value (9.3) [9], [14]. After aging (fixed time of 5 h) [23], as-prepared hydrogel was filtered and rinsed out with distilled water to remove

X-ray powder diffraction

The X-ray powder diffraction (XRPD) patterns of anatase nanopowders are shown in Fig. 1. All samples have tetragonal anatase structure (ICSD 24276), clearly verified by 101 reflections at 2θ = 25.33°. Other TiO2 phases, like rutile and brookite, were not observed. Small amount of NaCl, about 4.2 wt.% is registered, as a consequence of sol–gel procedure applied. The peak at 2θ  32° indicates the presence of NaCl and is clearly visible only in the diffractogram of the sample calcined for 6 h (TA-6h),

Conclusion

Mesoporous anatase nanopowders have been synthesized by sol–gel method using TiCl4 as the precursor. The structural and morphological properties of these powders were intentionally varied by the duration of the calcination. The XRD analysis has shown that extending of calcination time caused slight growth of crystallites in synthesized samples (from 13 to 17.5 nm) with similar other structural properties. Raman scattering data confirmed the anatase as dominant TiO2 phase, with the presence of

Acknowledgements

This work was financially supported by the Serbian Ministry of Education, Science and Technological Development under the Projects No. III45018, ON171032, and ON172042, as well as SASA project F-134, and the Swiss National Science Foundation through Grant No. IZ73Z0-128169.

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