A novel TiO2 composite for photocatalytic wastewater treatment
Graphical abstract
A composite of crystalline Anatase and Ti3+ containing Rutile yields high photocatalytic activity in dye degradation.
Introduction
Heterogeneous photocatalysis is a promising technology for removal of organic pollutants from water and air. It has shown potential as an environmental friendly, low cost, and sustainable technology. TiO2 is by far the most frequently applied photocatalyst. The photocatalytic activity and stability of TiO2 are unprecedented in particular in wastewater treatment [1], in which organic compounds are totally converted into carbon dioxide and water [2], [3]. It has often been reported that the Anatase phase of titania is the most active phase, whereas the effects of the simultaneous presence of Rutile (or Brookite) are still unclear [4], [5], [6]. Another aspect of TiO2-based photocatalysis recently (re)addressed is the degree of reduction of the semiconductor [7]. Partially reduced titania crystals (TiO2−x, where typically 0 < x < 1) induce high photocatalytic rates in wastewater treatment [8], [9], air purification [10], hydrogen production by water splitting [11], [12], and selective oxidation in organic media [13]. However, the stability of Ti3+ centers, in particular if these are located on the surface, is generally low [13].
Several methods have been reported to create reduced titania crystals such as (i) plasma treatment [14], (ii) laser treatment [15], (iii) thermal treatment under vacuum, helium or hydrogen atmosphere [16], [17], (iv) bombardment with high energy particles (e.g., neutrons [18], or γ-rays [19]), and (v) thermal treatment in CO and/or NO [12]. While of interest for fundamental studies, various of these methods are inconvenient for synthesis of large quantities of reduced TiO2 required for practical applications.
Recently, Hashimoto et al. [20] reported a simple one-step synthesis procedure to prepare TiO2 samples consisting of a stable Rutile phase in contact with Ti3+ containing Rutile by thermal treatment of a mixture of commercially available Rutile and Ti2O3 at elevated temperature (up to 900 °C). However, the observed photocatalytic performance of the prepared material in the gas-phase reaction of 2-propanol to acetone was negligible without co-catalyst (CuO nanoparticles [20]).
Here, we report on an adaption of the synthesis method of Hashimoto et al. [20], by changing Rutile for high surface area Anatase in the synthesis procedure. Furthermore, we have analyzed the composites in detail by various microscopic and spectroscopic techniques. The photocatalytic activity of the thus prepared catalysts at variable calcination temperature was evaluated in the aqueous-phase decomposition of Methyl Orange (MO). The obtained photocatalytic rates will be discussed on the basis of the characterization data: crystalline Anatase in close contact with partially reduced Rutile provides for the highest rates. In addition, photodeposition of Pt will be demonstrated to further enhance the catalytic rates, as well as the stability of the composite.
Section snippets
Synthesis
A one-step synthesis procedure was applied to synthesize the composites. In a typical synthesis procedure, 1 g of commercial TiO2 powder of the Anatase phase (Hombikat UV 100, Sachtleben Chemie GmbH) was mixed and grinded with 1 g of Ti2O3 (Aldrich, 99.9%). The mixed fine powder was heated at different temperatures from 300 °C to 900 °C for 3 h in static air applying a heating rate of 5 °C/min. Samples are labeled M-x, where x is the heating temperature. Furthermore, two reference samples of Ti2O3
Thermal analysis of a physical mixture of Hombikat/Ti2O3
The thermal analysis of a mixture of Hombikat and Ti2O3 is shown in Fig. 1. Under nitrogen flow, there is only a small increase in sample weight at high temperature, while major heat effects are absent (not shown). This small increase is probably related to oxidation of Ti2O3 by small quantities of O2 present in the nitrogen environment (the oven was not absolutely gas tight). However, under oxygen flow, oxidation causes a significant weight increase from ∼400 °C upwards. The exothermic DSC
Characterization data
The composition and photocatalytic behavior of composites originating from thermally oxidized Ti2O3 [23] and a combination of thermally oxidized Ti2O3 and TiO2 (Rutile) have been described in the literature previously [20]. Generally, the characterization data of thermally treated Ti2O3 reported in our study are in agreement with those reported in [20,23]. TGA analysis, XRD patterns, Raman spectra, UV/Vis data, and XPS spectra all demonstrate the conversion of Ti2O3 into Rutile, containing more
Conclusions
Catalyst composites made by heating a mixture of Anatase/Ti2O3 showed very high photocatalytic activity in dye decolorization. Characterization results show that the composites exist of crystalline Anatase interacting with a well-defined Rutile phase containing bulk and surface oxygen vacancies, the quantity of which is depending on calcination temperature. The composite heated at 600 °C with an initial ratio of Anatase/Ti2O3 of 1:1 exhibits the highest photocatalytic efficiency. Photodeposition
Acknowledgments
W.H. Saputera thanks the University of Twente Scholarship (UTS) program and the Indonesian Ministry of National Education for his personal fellowship. The authors thank Mrs. F. Nami (Leiden University) for performing the EPR measurements. Mr. G. Kip, Mrs. L. Vrielink, and Mr. R. Veneman (Twente University) are acknowledged for recording XPS spectra, and conducting N2 physisorption, and TGA measurements, respectively.
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