Synthesis and photocatalytic oxidation of different organic dyes by using Mn2O3/TiO2 solid solution and visible light

doi:10.1016/j.apsusc.2009.06.086

Applied Surface Science

Volume 255, Issue 22, 30 August 2009, Pages 9026-9031

https://doi.org/10.1016/j.apsusc.2009.06.086 Get rights and content

Abstract

Mn₂O₃/TiO₂ solid solution was prepared from two different oxides, manganese oxide (from KMnO₄ and ethanol) and TiO₂, these samples were characterized by BET, XRD, EDAX, SEM, FT-IR, ESR, XPS and UV–vis absorption spectroscopy. Photocatalytic activities of Mn₂O₃/TiO₂ powder was investigated by photooxidation of different dyes like Rhodamine B, thymol blue, methyl orange and Bromocresol green under visible light (300-W Xe lamp; λ > 420 nm). The results show that the alloy of TiO₂ with 1 mol% of Mn₂O₃ (MNT1) exhibit photocatalytic activity 3–5 times higher than that of P25 TiO₂ for oxidation of various dyes (RB, TB, MO and BG). The average particle size and crystallite size of MNT1 were found to be 100 nm and 12 nm measured from SEM and XRD, respectively. The EPR spectra of the Mn₂O₃/TiO₂ samples is a sharp five-line Mn(III) component centered on g_eff = 1.99.

Introduction

Photocatalytic degradation of various toxic hazardous organic compounds dissolved in water using TiO₂ and modified TiO₂ photocatalysts under visible light irradiation has been widely studied [1], [2], [3], [4], [5], [6], [7], [8], [9]. However, only the ultraviolet fraction of the solar irradiation (about 5%) is active in the photo excitation processes using pure TiO₂ solids. It is, therefore, necessary to develop a photocatalytic system which can be applied under visible or solar light irradiation [9], [10], [11], [12], [13], [14]. In recent years, the application of heterogeneous photocatalyst on the removal of contaminants in air and wastewater has acquired some interest due to the high photocatalytic activity and stability of titanium dioxide.

Thus, keeping the toxicity of the dye in view, various attempts have been made for the removal of dyes (RB, TB, MO, and BG) from water and wastewater. Doping titania with transition metal ions has been tested as a promising way of improving the photocatalytic activity of semiconductor oxides [15], [16], [17], [18]. The incorporation of metal ions into titania crystal lattice can significantly extend the absorption by the photocatalysts into visible range. The effect of doping is to change the equilibrium concentration of electrons or holes [19], [20], [21].

In our previous studies, the metal molybdate incorporated in titanium dioxide is used as a good photocatalysts for degradation of different organic dyes under UV light [22]. Moreover, studies on the phase formation characteristics of doped TiO₂ have shown stabilization of anatase phases for different experimental conditions. Arroyo et al. [23] reported that manganese (Mn²⁺) doped titania, indicates that the stabilization of the anatase phase for low doping levels, but stabilization of the rutile phase at higher concentration of the doping compounds due to segregation of the doping to the surface.

MnO₂ was among the oldest metal oxide catalyst and found to possess a potential activity in redox reactions. Unfortunately, it has been shown that manganese oxides are insoluble [24], [25]. However, the amount of manganese dissolved in solution can be increased considerably in acidic medium with the addition of organic compounds such as dyes [26]. But the further adsorption and oxidation of dye compound on the mineral particles interface reduce their surface areas and lead to inhibition of the process [27]. Recently, MnOx/TiO₂ heterogeneous mixed oxides have shown better photocatalytic activity than pure TiO₂ for oxidation of different organic compounds such as, indigo carmine dye [28], NO reduction [29], but it is inferior to Degussa P25.

In this paper, we have studied the preparation of Mn₂O₃ on titania by mechanochemical method and characterized by X-ray diffraction, EDAX, FT-IR, BET, SEM, ESR, XPS and UV–vis absorption spectra. Finally, the photooxidation of different dyes in the absence and presence of visible light is investigated for all the prepared samples.

Section snippets

Experimental

The Mn₂O₃/TiO₂ solid solution were prepared as follows: the TiO₂ and Mn₂O₃ (obtained from KMnO₄) powders with different mole ratio (manganese oxide in TiO₂ is 1 mol%, 3 mol% and 5 mol%) were grind and mixed thoroughly in ethanol in a mechanical grinding in a planetary ball mill. This mechanical milling was allowed for 4 h for complete mixing of the oxides forming a solid solution. The milling was performed in Fritsch Pulverisette No. 6 planetary ball mill, using a rotational speed of 250 rpm at a

XRD analysis

The XRD pattern of the different compositions of Mn₂O₃/TiO₂ solid solution heat treatment at 100 °C for 15 h in air atmosphere was shown in Fig. 1. It has been revealed that the phases prepared at different molar ratios of mixed oxides TiO₂ and Mn₂O₃ [Mn₂O₃ (1 mol%)/TiO₂ (MNT1), Mn₂O₃ (3 mol%)/TiO₂ (MNT2), Mn₂O₃ (5 mol%)/TiO₂ (MNT3), and TiO₂) have anatase phase up to 5 mol% of Mn₂O₃ (as per JCPDS No. 84-1285). The XRD pattern at TiO₂ sample shows distinct peaks of the anatase phases, without any

Conclusions

Mn₂O₃/TiO₂ solid solution synthesized by mechanochemical method showed the highest photocatalytic activity compared to P25 TiO₂ for oxidation of different organic dyes under visible light. This apparent activity can be due to the involvement of Mn³⁺ moieties as Mn₂O₃ in TiO₂ was responsible for improving electronic and thus optical properties of such nanometer sized particles. Initially the dye is sensitized by the catalyst in presence of visible light and the dye cation is formed. The dye

Acknowledgements

This work was supported by the Council of Scientific and Industrial Research, India. The authors are grateful for its financial support.

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Synthesis and photocatalytic oxidation of different organic dyes by using Mn2O3/TiO2 solid solution and visible light

Abstract

Introduction

Section snippets

Experimental

XRD analysis

Conclusions

Acknowledgements

Appl. Catal. B

J. Hazard. Mater. B

Stud. Surf. Sci. Catal.

Adv. Catal.

Appl. Catal. B: Environ.

Appl. Catal. B: Environ.

Catal. Today

Microporous Mesoporous Mater.

J. Mol. Catal. A: Chem.

Mater. Lett.