TiO2 nanotubes and CNT–TiO2 hybrid materials for the photocatalytic oxidation of propene at low concentration
Introduction
In the last decade, the synthesis of nanostructured TiO2 materials has received a great deal of attention. Three approaches have been reported for the preparation of TiO2-nanotube materials: the hydrothermal synthesis [1], [2], [3], [4], [5], [6], [7], the anodic oxidation [8], [9], [10], [11] and the template synthesis [12], [13], [14], [15], [16], [17]. The advantage of the template-based synthesis route is the straightforward control over the morphology of the resulting TiO2 nanotubes. For instance, the initial dimensions of carbon nanotubes (CNTs) determine both, diameter and length of the TiO2 nanotubes, while the TiO2 nanotube wall thickness depends on the type and initial concentration of the Ti precursor with respect to the CNTs [15]. The use of CNTs as a template was further significant as they can also support the tubular morphology of TiO2 during the phase transformation from anatase to rutile, resulting in the first synthesis of pure rutile nanotubes [16]. A significant innovation was the addition of small amounts of benzyl alcohol as a linking agent, which enabled the use of pristine CNTs as templates without the need for covalent functionalisation [17].
The last few years have seen a boost in interest in TiO2 nanotubes, due to their improved performance as photocatalysts in heterogeneous photocatalysis [2], [7], [11], [18], [19], [20], [21], [22]. However, most of these studies have been devoted to photocatalysis in solution and only the work of Inagaki et al. deals with gaseous phase oxidation [22]. According to some authors, the advantages of TiO2 nanotubes are solely based on their high surface areas [2], [18], [19], [23]. Other authors showed that TiO2 nanotubes contain a high amount of hydroxyl groups in comparison with TiO2 in the form of powder [6], [24]. In general, nanotubes have a particular advantage in the way they enable three-dimensional mechanically coherent structures, which provide ready gas and radiation access to the material [14]. For example, Macak et al. [20] demonstrated that TiO2 nanotubes showed higher photocatalytic activity than P25, despite their smaller surface area. Eder et al. investigated anatase nanotubes, both pure and doped with iron ions, for the photocatalytic splitting of water and observed that – in contrast to the inactive P25 – the TiO2 nanotubes did produce a significant amount of hydrogen [21]. Both authors related this improvement to the structure of the TiO2 nanotubes, which favours short diffusion paths of the pollutant molecules and, in addition, also decreases the electron–hole pair recombination velocity.
Despite these promising studies, the photocatalytic performance of TiO2 nanotubes is far from completely understood. One important question involves the phase composition of TiO2, which has two main phases, anatase – typically formed upon synthesis – and rutile, the thermodynamically stable phase. Due to the synthesis conditions for rutile, which often require heat treatments at elevated temperatures (e.g. >600 °C), the crystal sizes in rutile are typically larger than in anatase, resulting in significantly smaller specific surface areas. The rutile nanotubes used in this work, however, have surface areas comparable with those of anatase nanotubes, and thus are ideal candidates for photocatalytic testing.
The test reaction in this work is the photocatalytic oxidation of volatile organic compounds (VOC) at low concentration, a very important catalytic process, with the aim to reduce their harmful effects for human health and environment. Among the different VOCs, this work concentrates on propene, which is one of the major sources of both outdoor (involved in vehicle emissions and in many industrial applications, such as in petrochemical plants and foundry operations) as well as of indoor (one of the principal components of tobacco smoke) air pollution. So far, very few studies have been reported on the photocatalytic oxidation (PCO) of propene in gas phase using TiO2 [25], [26], [27], [28] and to the best of our knowledge the performance of TiO2 nanotubes has not been explored at all for this application or, in general, for gaseous phase applications.
In this respect, the novelty of this work is a detailed investigation of various TiO2 nanotubes, consisting of either pure anatase or rutile, or of a mixture of anatase and rutile, for the photocatalytic oxidation (PCO) of propene. Furthermore, this work compares the best results obtained previously with hybrid TiO2 catalysts (containing activated carbon [26] or MCM-41 [27]) with CNT–TiO2 hybrids, a very promising new class of functional materials [29], and investigates the beneficial role of CNTs in photocatalysis.
Section snippets
Photocatalysts preparation
The TiO2 nanotubes used in this work were synthesised via a sol–gel process using sacrificial CNT as templates. Multi-walled carbon nanotubes (CNTs) were prepared by a modified chemical vapor deposition (CVD) process, using ferrocene as the catalyst precursor and toluene as the feedstock. These reactants were vaporised into a hydrogen/argon atmosphere at 760 °C. The average outer diameter of the nanotubes was 70 nm and the length was between 20 and 30 μm, but can be controlled by adjusting various
Nomenclature
X-ray diffraction (XRD) analysis of the as-prepared samples revealed that the TiO2 coating on CNTs in the as-prepared samples was amorphous and thus required heat treatments (shown in Table 1) to induce crystallisation and phase transformation [15]. From the pristine material, several samples were prepared. The first sample (CNT–TiO2) was simply obtained heating in argon at 350 °C, at which the amorphous phase crystallised into anatase without removing the CNTs, as confirmed by XRD in Fig. 1.
Conclusions
The present paper has studied the photocatalytic oxidation of propene at low concentration (100 ppm) using titanium dioxide nanotubes. The results of this study allow obtaining the following conclusions.
Nitrogen adsorption shows that all these TiO2 materials are macroporous, as the commercial TiO2 photocatalyst (Degussa-P25). The use of XRD and TEM techniques has shown that the different TiO2-nanotube samples have different crystalline structures, with different anatase and rutile percentages.
Acknowledgements
The authors thank MCYT (CTQ2005-01358/PPQ) and Generalitat Valenciana (ARVIV 2007/063 and Prometeo/2009/047) for financial support. M. Ouzzine thanks MAEC-AECID for a predoctoral fellowship.
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