Preparation and characterization of tungsten-loaded titanium dioxide photocatalyst for enhanced dye degradation

Abstract

Tungsten-loaded TiO₂ photocatalyst has been successfully prepared and characterized. TEM analysis showed that the photocatalysts were nanosize with the tungsten species forming layers of coverage on the surface of TiO₂, but not in clustered form. This was confirmed by XRD and FT-Raman analyses where tungsten species were well dispersed at lower loading (<6.5 mol%), but were in crystalline WO₃ at higher loadings (>12 mol%). In addition, loading with tungsten could stabilize the anatase phase from transforming into inactive rutile phase and did not shift the optical absorption to the visible region as shown by DRUV–vis analysis. PZC value of TiO₂ was found at 6.4, but the presence of tungsten at 6.5 mol% WO₃, decreased the PZC value to 3. Tungsten-loaded TiO₂ was superior to unmodified TiO₂ with 2-fold increase in degradation rate of methylene blue, and equally effective for the degradation of different class of dyes such as methyl violet and methyl orange at 1 mol% WO₃ loading.

Introduction

The presence of colour and its causative compound is undesirable for domestic or industrial uses as colour is visible and can be an indication of pollution [1]. WHO guidelines for drinking water quality, has set the maximum value for permissible colour at 15–20 units [2]. Several new technologies in wastewater decolourization have emerged with improved performance and more environmentally friendly. These include Advanced Oxidation Processes (AOPs) such as heterogeneous photocatalysis, Fenton and Photo-Fenton, and ozonation which have received considerable attention due to their compliance with Green Chemistry concept in promoting innovative technologies that reduce or eliminate the use or generation of hazardous substances in the design, manufacturing and use of chemical products [3]. Heterogeneous photocatalysis especially has several advantages, as it uses no reagent. The only chemical used, metal oxide photocatalyst such as titanium dioxide (TiO₂) is abundant and harmless. However, TiO₂ photocatalysis lacks efficiency due to the high rate of recombination of electrons and holes [4]. In addition, due to its large bandgap energy, E_g of 3.0−3.2 eV, it can only be activated by UV light, which accounts only 3–4% of sunlight spectrum [5]. Efforts have been made using chemical or physical methods, to enhance the photocatalytic activity of TiO₂ through modification with different group of metals such as alkaline metals [6], earth alkaline metals [7], transition metals [8], rare earth metals [9], and noble metals [10], but with varying degree of results.

TiO₂ has three natural phases—anatase, rutile, and brookite. Modification with certain metals such as Ni [11], Fe [11], [12], Th [12], Cu [12], V and Mo [13], Co [14], Sn [15], and Ag [16], may alter the phase transformation of TiO₂ from active anatase to inactive rutile by lowering the activation energy. The activation energy is further affected by metal dosage and method of preparation. On the other hand, metals such as Mg and Ba [17], Mn [18], Tb, Eu and Sm [19], La [20], and Sc and Nb [21] have been reported to inhibit phase transformation.

Degradation of pollutant can be influenced by both intrinsic and extrinsic factors. Intrinsic factors include the nature of the photocatalyst itself such as crystallinity, surface area, morphology, and optical absorption. Extrinsic factors are the process parameters that affect the photodegradation rate such as dye concentration, catalyst loading, pH, light wavelength and intensity, temperature, and oxygen pressure [22], [23], [24]. In addition, the presence of oxidants and dissolved metal ions and organic materials also affect the degradation rates [24]. From both application and economic point of view for operational effectiveness of a photocatalyst, the effects of extrinsic factors are important to be determined particularly the optimum decolourization conditions and limitations.

The objective of this study was to evaluate the effect of TiO₂ modification with tungsten trioxide, WO₃, using impregnation method, on the photocatalytic activity. Characterization of the photocatalyst was carried out using transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform Raman (FT-Raman), diffuse reflectance ultraviolet visible spectroscopy (DRUV-Vis) and point of zero charge (PZC) methods to understand the physico-chemical properties of the photocatalyst. The photocatalytic activity was tested against the decolourization of methylene blue (MB), methyl violet (MV), and methyl orange (MO) as model dyes in wastewater streams. The effects of process parameters such as initial dye concentration, catalyst loading, and initial pH were investigated.