Sulfated Fe₂O₃–TiO₂ synthesized from ilmenite ore: A visible light active photocatalyst

Abstract

Sulfated Fe₂O₃–TiO₂ (SFT) was synthesized by treatment of ilmenite ore with sulfuric acid. The presence of sulfated Fe₂O₃–TiO₂ and mixed phases of Fe₂O₃–TiO₂ was confirmed by DRIFT spectra and XRD. The dispersion of sulfate displayed thermal stability up to 500 °C. The adsorption–desorption of pyridine investigated by DRIFT spectra revealed the presence of both Brønsted and Lewis acid sites for the samples calcined up to 500 °C. The DRS/UV–vis spectra showed UV and visible light absorbance for samples calcined up to 900 °C. A band gap value of 2.73 eV is obtained for 500 °C calcined sample. The photocatalytic activity was evaluated by the oxidation of 4-chlorophenol (4-CP) in aqueous medium under UV–vis and visible light irradiation. SFT calcined at 500 °C demonstrated the highest photocatalytic activity. When compared with high surface area sulfated titania (275 m²/g), the photocatalytic activity was greater due to the presence of iron, despite the low surface area of the SFT samples (12–17 m²/g).

Introduction

An increasing awareness of the environmental impacts from pollution and stringent standards on emission regulations has prompted the development of catalytic routes for waste management. The development and practical application of systems that are clean and green have shown to be a formidable challenge for scientists and engineers. Photocatalytic technologies have shown practical application in antibacterial and deodorant filters for air purification owing to its property of promoting various chemical reactions such as the degradation of aqueous organic pollutants and sources of offensive odors using light. Titania-based materials have received considerable attention for their potential in environmental catalytic applications such as air purification, water disinfection, hazardous wastewater remediation, and deodorization [1], [2], [3], [4], [5], [6]. Recently, the application of titania with different architectures such as nanotubes has been examined [7], [8], [9]. Owing to its large band gap (E_g = 3.2 eV) titania, however, can only utilize photons in the UV region (<380 nm), which limits its practical application for sun light irradiation [7], [8], [10], [11], [12], [13]. One of the promising approaches to overcome this disadvantage is coupling titania with other narrow band gap semiconductors capable of promoting charge separation in the visible light spectrum [14], [15]. Many studies have reported sensitizer-loaded titania, such as CdS/TiO₂ [16], [17]. CdSe/TiO₂ [18], [19], Bi₂O₃/SrTiO₃ [20], Bi₂S₃/TiO₂ [17], [21], ZnMn₂O₄/TiO₂ [22], TiO₂/Ti₂O₃ [23] under visible light irradiation and have shown efficient visible light photoactivity. In most of these catalysts, the addition of sensitizers reduces the band gap of the material enabling the coupled material to absorb visible light. The conduction band (CB) of the loaded sensitizer has a more negative reduction potential than that of titania enabling visible light photoinduced electrons to be injected into the lower-energy CB of titania. However, the photogenerated holes of the sensitizer remain in the valence band (VB) resulting in an accumulation of holes on the sensitizer leading to photocorrosion of the catalyst. As a result, the stability of the composite photocatalyst becomes less [24]. Furthermore, most currently produced sensitizers are heavy metal chalcogenides (e.g., CdSe, PbS). These may constitute harm to ecological systems and humans as well due to their nanoscale and toxic metal release. A recent study by King-Heiden and coworkers [25] examined the toxicological effects of CdSe/ZnS nanoparticles on the growth of zebrafish embryos and showed Cd toxicity even at very low levels of CdSe nanoparticles.

To counter the potential negative environmental problems of using heavy metal sensitizers, iron as a dopant in titania-based systems has been investigated to enhance the photocatalytic efficiency under visible light irradiation [26], [27], [28], [29], [30]. Iron is one of the most abundant elements found in the Earth's crust. Similar to titania, iron and its oxides show promise as an eco-friendly catalyst in many applications. For example, FeTiO₃ has a band gap of 2.58–2.9 eV, [31], [32], [33], [34] and has been used as a chemical and as a photocatalyst. [32], [33] Ye et al. observed that under UV irradiation of TiO₂–Fe₃O₄ mixed oxide coatings exhibited higher photocatalytic efficiency than titania alone due to the formation of FeTiO₃ [33], [34], which may form a p–n junction with titania to induce spatial separation of the photogenerated electron/hole pairs. Iron-doped titania has shown to exhibit higher photocatalytic activity in visible light. A report by Choi and coworkers [35] shows that creating a shallow trap in the titania lattice due to its half-filled electronic configuration induces a red shift in the band gap and alters electron/hole pair recombination rates. Although the mechanism of narrowing the band gap and reducing the recombination rates of titania with the aid of iron remains in the realm of debate, [36], [37], [38], [39] in general, it is assumed that the photocatalytic behavior and efficiency are greatly influenced by the doping of iron oxides [40]. Hence, further investigations are essential to explore iron as a doping catalyst through appropriate synthesis processes. The objective of this study is to address some of the questions raised in previous related work and provide a photocatalyst which possesses high catalytic activity, non-toxic, inexpensive, and allows for the use of visible light directly to carry out photocatalytic reactions. Herein, we report the synthesis of sulfated Fe₂O₃–TiO₂ (SFT) using ilmenite ore and sulfuric acid as the starting material and its effect of photocatalytic activity is evaluated by the oxidation of 4-chlorophenol (4-CP) in water.