EVALUATION OF THE PHOTOCATALYTIC ACTIVITY OF SiO2@TiO2 HYBRID SPHERES IN THE DEGRADATION OF METHYLENE BLUE AND HYDROXYLATION OF BENZENE: KINETIC AND MECHANISTIC STUDY

Bruno C B Salgado,Antoninho Valentini

doi:10.1590/0104-6632.20190364s20190139

Bruno C B Salgado, Antoninho Valentini

Open Access

https://doi.org/10.1590/0104-6632.20190364s20190139

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Abstract

Abstract Silica spheres coated with titania (SiO2@TiO2) were synthesized using chitosan as template. The N2 adsorption/desorption isotherms of the spheres point to meso and macroporous characteristics and the elemental mapping by EDS shows uniform distribution of Ti on the surface of the silica spheres, leading to formation of an amorphous structure (XRD). The results from the model reaction of photocatalytic degradation of methylene blue (MB) show a good stability of the spheres regarding their reuse. The tests with various additives pointed to hydroxyl radical production as the main via of MB degradation. The photocatalytic activity of the spheres in the hydroxylation of benzene to form phenol, hydroquinone and benzoquinone was assessed. The kinetic data point to the formation of phenol as the limiting step; in addition, the phenol consumption occurs by parallel and consecutive reactions producing benzoquinone.

Highlights

Heterogeneous photocatalysis is applied in several reaction systems, especially for oxidation reactions of organic pollutants present in the air (Augugliaro et al, 1999; Adamczyk and Długoń, 2012) or wastewater (Ahmed et al, 2011; Vela et al, 2012; Affam and Chaudhuri, 2013)
Synthesis of SiO2 spheres coated with TiO2 The synthesis procedure consisted of preparing hybrid spheres composed of chitosan (CTS), tetraethylorthosilicate (TEOS, Si(OC2H5)4, 98%, Aldrich) and silica (SiO2, Aerosil®, Degussa Evonik) (Braga et al, 2009a) followed by calcination
For both samples, the isotherms showed profiles that suggest morphological characteristics represented by the mixture of types II and III (IUPAC) and a low hysteresis of type H3, which may be due to pores in the slot shape

Summary

Introduction

Heterogeneous photocatalysis is applied in several reaction systems, especially for oxidation reactions of organic pollutants present in the air (Augugliaro et al, 1999; Adamczyk and Długoń, 2012) or wastewater (Ahmed et al, 2011; Vela et al, 2012; Affam and Chaudhuri, 2013). Once formed, the electron/hole can either recombine or react with species adsorbed on the surface of material, which may be either an electron donor (e.g. hydroxide ions) or electron acceptor (e.g., molecular oxygen). This process results in the formation of highly reactive substances such as the hydroxyl (OH) and superoxide (O2-) radicals (Wilhelm and Stephan, 2007)

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