Elsevier

Microchemical Journal

Volume 112, January 2014, Pages 186-189
Microchemical Journal

Quantitative determination of carbon in titania photocatalysts by temperature programmed oxidation method

https://doi.org/10.1016/j.microc.2013.10.008Get rights and content

Highlights

  • Optimization of an effective approach for carbon determination in TiO2 photocatalysts

  • Use of the Temperature Programmed Oxidation as quantitative analytical technique

  • Construction of a calibration curve for the determination of the final C amount

  • Extension of TPO calibration curve for commercial C-doped TiO2

Abstract

This study was directed to the optimization of a reliable and innovative approach, for the qualitative and quantitative analysis of carbon-doped titania photocatalysts. In particular, we report an easy procedure, based on the Temperature Programmed Oxidation (TPO) method for the determination and prediction of the carbon amount on the final catalysts. The synthesized Ti(OH)4 photocatalysts were doped with different amount of succinic acid and an in-depth characterization was carried out by X-ray diffraction (XRD), physisorption of gas, elemental analysis, and temperature programmed oxidation (TPO). The final amounts of C, after calcinations, were determinate by TPO analysis through the setting up of a calibration curve.

Introduction

Titanium dioxide is known as the most important photocatalyst because of its non-toxicity, good chemical stability and high catalytic activity in various photo-oxidation reactions [1], [2]. Photocatalysis applications of wide reaching importance include water splitting for hydrogen generation [3], degradation of environmental pollutants in aqueous contamination and wastewater treatment [4], self-cleaning activity and air purification [5].

However, its large high band gap (approximately 3.2 eV) requires the use of UV light (λ < 387 nm) [6] and does not allow the much larger visible part of solar light to be utilized.

Since the solar energy includes only 5% ultraviolet radiation, environmental applications of pure TiO2 are limited. Consequently, during the last years, many efforts have been directed toward the development of a modified titania which would be photocatalytically active also with visible light.

One approach to synthesize modified-doped titania is the substitution of Ti with transition metal ions (such as V, Cr, Mn, Fe, and Ni) [7], [8], [9]. Unfortunately these doped materials are thermally unstable, and transition metal ions can act as electron-hole recombination sites, resulting in low efficiency [10]. An alternative reliable way to expand the wavelength range response of a semiconductor toward visible light is the modification of TiO2 with non-metal atoms, such as N [11], [12], [13], [14], S [15], [16] and C [17], [18], [19]. This kind of titania modification leads to a decrease of the bad gap energy in these materials [20]. C-doped materials are most promising since, according to the result of Janus et al. [18], photocatalytic ability of C-doped TiO2 could be improved by the decrease of the recombination rate in photogenerate electron-hole pair. This is allowed by the existence of electron scavenger carbon doped into TiO2 [21]. Furthermore, carbon stabilizes anatase structure and increases the adsorption of organic molecules on the photocatalyst surface. The enhancement of photocatalytic activity was attributed to either the band gap narrowing [18] or the formation of localized mid-gap state [21]. These modified materials can be synthesized using various approaches in which the doping step occurred after the preparation of pure TiO2 or during the synthesis. One of the most used carbon doping methods is mechanical mixing [22], [23] of TiO2 with various amounts of active carbon or other carbonaceous species or alcoholic compounds [21], [24]. Using this approach, it's possible to obtain doped materials with well known amount of carbon on the TiO2 samples but unfortunately this type of technique often leads to non-homogeneous samples and then, sometimes to irreproducibility in reaction collected data. Another modification approach considers some changes during the steps of titania synthesis (for examples using sol–gel synthesis with the addition of active carbon [25] or organic templates [26] as carbon source or using TiCl4 hydrolysis [27] with for examples tetrabutylammonium hydroxide [12]). The samples obtained by this technique are more homogeneous than the previous ones. However it's difficult to know in advance the effective amount of carbon in the final catalysts and only a post synthesis determination can be carried out.

The aim of this work is the synthesis of various reproducible C-TiO2 samples and the development of a fast and cheap test for their carbon amount determination. The attention was centered on the temperature programmed oxidation (TPO test), that is a handy and relatively cheap analytical technique. Actually, this method is used above all for qualitative analyses and its potentiality is not fully exploited.

Section snippets

Synthesis of C-doped TiO2

Titanium hydroxide was obtained by precipitation from titanyl sulfate (TiOSO4, Aldrich) aqueous solution by the addition of 9 M ammonia solution under vigorous stirring, as previously reported [28]. The Ti(OH)4 powder was suspended in distilled water (25 wt.% solids) [29] and stirred at 333 K. Different amounts of succinic acid (Aldrich), from 0 to 12 wt.% referred to the solid, were added and slowly stirred until the complete dissolution of the organic compound. The obtained suspension was

Results and discussion

In order to determine the optimal calcination treatment, TPO analyses were carried out. We propose to select a suitable treatment which allows to obtain well crystallized titania, with both high surface area and pure anatase polymorph phase and to maintain a suitable amount of carbon.

Fig. 1 shows the TPO analyses of three samples. In the TPO profile of the undoped sample (0.0%C-TiO2) no peaks were detected. This means that there is neither carbon deposits on the catalyst nor oxidable species

Conclusions

The goal of this paper is the possibility to synthesize C-doped photocatalysts with a desired amount of C after all preparation treatments introducing a well known mass of succinic acid. Moreover we have demonstrated the opportunity to calculate the amount of carbon present in doped titania photocatalysts by an easy, fast and cheap TPO analysis. The potential of this approach has been evaluated through the use of this technique by determination of the concentration of carbon in a commercial

Acknowledgments

The authors thank Sonia Ceoldo (Ca' Foscari University of Venice) for the carbon determination analysis and Prof. Giuseppe Cruciani (University of Ferrara) for the XRD data.

References (32)

Cited by (4)

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