Elsevier

Water Research

Volume 42, Issue 19, December 2008, Pages 4878-4884
Water Research

Photocatalytic degradation of Acid Red 88 using Au–TiO2 nanoparticles in aqueous solutions

https://doi.org/10.1016/j.watres.2008.09.027Get rights and content

Abstract

Metal loaded semiconductors in general possess greater photocatalytic activity than pure semiconductors. Hence, with an attempt to achieve higher photocatalytic activity, Au–TiO2 photocatalysts were prepared by deposition–precipitation method and used for the photocatalytic degradation of an azo dye (Acid Red 88; AR88). The materials were characterized by different analytical techniques. A possible mechanism for the photocatalytic degradation of AR88 by Au–TiO2 in the absence and presence of other oxidizing agents (peroxomonosulfate (PMS), peroxodisulfate (PDS) & hydrogen peroxide (H2O2)) has been proposed. The extent of mineralization of the target pollutant was also evaluated using Total Organic Carbon (TOC) analysis.

Introduction

Heterogeneous photocatalysis is an emerging technique for environmental remediation where semiconductor materials are used as photocatalysts. When organic pollutants are decomposed by heterogeneous catalytic reactions, the pollutant molecules adsorb on the surface of the catalysts, where chemical bonds are broken and formed on the surface and eventually small organic molecules are released as products. Most of the heterogeneous catalysts used today consist of small particles of a catalytically active material, typically with a diameter of 1–10 nm, anchored on a porous support. The use of nanoparticles results in a large contact area between the active material of the catalyst and the pollutants. This ensures that the catalytic sites are used effectively for the complete degradation of a wide variety of organic pollutants and microbial substances (Ollis and Al-Ekabi, 1993, Serpone and Pelizzetti, 1989, Schiavello, 1998).

One of the scientific and technological challenges associated with the use of nanoparticles as catalysts is the understanding of how the composition and atomic scale structure of nanoparticles produce the best catalytic activity. A second challenge is to synthesize these particles with a maximum control over the composition and structure. The modern methods involving nanotechnology clearly offer great potential for future developments in both characterization and synthesis of heterogeneous catalysts based on supported nanoparticles.

The assembly of metallic nanocrystals into functional (hierarchical) structures utilizing both quantum and classical properties, inherent to the individual “building blocks”, is an area of increasing interest (Gittinss et al., 2000, Valden et al., 1998, Kozlov et al., 1999). The control of size and the uniformity of nanocrystals rely upon either methodologically building larger clusters from smaller molecular precursors or arresting the growth of metallic nanocrystals at some intermediate stage between a molecular complex and a large colloidal particle. Previous studies (Schaaff and Blom, 2002, Lee et al., 2007, Moreau and Bond, 2007) have shown that the size and homogeneity of the supported metal particles are controlled by the metal ion concentration and the calcination duration and temperature. Haruta and co-workers (Haruta, 1997, Date et al., 2002, Akita et al., 2001, Tsubota et al., 1995) reported that the evolution of Au-nanocrystal size as a function of the above variables exhibited unusual catalytic activities. Their primary goal was to produce a catalyst where all of the gold (or other precious metals) loaded onto the metal oxide support was of the same size, which was found to have maximum catalytic activity.

Many studies of noble metal deposition on TiO2 have been focused on group VIII metals for UV-irradiated photocatalytic degradation of organic pollutants (Arabatzis et al., 2003, Badr and Mahmoud, 2007, Zhao et al., 1997, Sung-Suh et al., 2004, Anipsitakis and Dionysiou, 2004). Despite the great body of work available on the catalytic properties of Au–TiO2, papers devoted solely to a systematic description of the degradation/mineralization of organic dye molecules by Au–TiO2 photocatalysts using visible light irradiation have seldom been reported (Li and Li, 2001). The current study reports on the synthesis of Au doped TiO2 and characterize the catalysts using a number of analytical techniques. The efficiency of Au–TiO2 photocatalyst has been evaluated by following the kinetics and mechanism of the photocatalytic degradation of a textile dye (Acid Red 88; AR88) in the absence and presence of oxidants, such as PMS (peroxomonosulfate), PDS (peroxodisulfate) and H2O2 (hydrogen peroxide) under visible light irradiation.

Section snippets

Materials

Titanium dioxide (TiO2, Degussa P25, Germany) with a specific surface area of 57 m2 g−1 was used as a starting material to prepare Au–TiO2 photocatalyst. Potassium peroxomonosulphate, a triple salt with the composition 2KHSO5·KHSO4·K2SO4 from Janssen Chimica (Belgium) was used as received. Peroxodisulphate and hydrogen peroxide were analytical grade reagents purchased from E-Merck, India. Acid Red 88 (AR88) was a gift sample from Atul Ltd, India. Unless otherwise specified, all reagents used were

Characterization of the photocatalyst

During preparation, the white color of the original TiO2 sample was changed to purple color, which indicated the deposition of gold nanoparticles on the surface of TiO2 particles. Pure TiO2 particles exhibit strong absorption at λ < 400 nm (Duonghong et al., 1981), which corresponds to the 3.2 eV band gap energy of anatase. Pure colloidal gold exhibits an absorption band centered at ∼530 nm (Okitsu et al., 2005), which is normally attributed to the typical surface plasmon absorption of gold

Acknowledgements

The authors thank DST, New Delhi and DEST, Australia for the sanction of India–Australian strategic research fund (INT/AUS/P-1/07 dated 19 September 2007) for their collaborative research. The author (Mr. Sathish Kumar), thank AICTE, New Delhi, for the NDF fellowship.

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Work carried out under the Australia–India Collaborative Research Scheme funded by Department of Innovation, Industry, Science and Research, Australia and Department of Science and Technology, India.

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