C-N/TiO2 photocatalysts: Effect of co-doping on the catalytic performance under visible light

https://doi.org/10.1016/j.apcatb.2014.05.015Get rights and content

Highlights

  • Optimization of a synthetic route of co-doped titania photocatalysts with both C and N.

  • Study of the influence of the amount of dopants on photoactivity.

  • Evaluation of photocatalytic activity in NO oxidation under visible light irradiation.

  • Correlation between the band gap and the catalytic activity under visible light irradiation.

Abstract

Aim of this work is the synthesis of titanium-based photocatalysts co-doped with both C and N, exhibiting improved photocatalytic activity under visible light irradiation. The preparation of the catalysts consists of a two step process: during the first one, the synthesis of the catalyst doped with nitrogen has been carried out, whereas in the second step carbon has been introduced in different amounts.

The photocatalytic activity has been evaluated in the gas phase reaction of NOx abatement under visible light irradiation. The characterization has been carried out by N2 physisorption, TPO (temperature programmed oxidation), DRS (diffuse reflectance spectroscopy) and XPS (X-ray Photoelectron Spectroscopy) in order to investigate both structural and physico-chemical features of the samples and their correlation with the catalytic activity. An effective promotion effect of the dopants has been observed; in particular, the synergic co-presence of both N and C produced a significant narrowing of the TiO2 band gap with the consequent extension of the photocatalytic activity towards the visible part of the light spectrum.

Introduction

In recent years, to face the problem related to the increasing emissions of pollutants, the research has been focused on the development of efficient and reliable technologies for the abatement of hazardous substances released in the atmosphere. Heterogeneous photocatalysis is an effective way to reduce pollution that has attracted particular interest and has been extensively studied because of the advantages associated with it, among which:

  • i.

    the degradation reaction already occurs at low concentrations of pollutants and in “environmental benign” conditions

  • ii.

    it is economical, as it allows to exploit sunlight;

  • iii.

    it requires no toxic additives.

These interesting features have turned this technique to be the most investigated for both air and water purification. The photocatalytic process involves the adsorption of the reactants on the catalyst surface which, when irradiated by photons with an appropriate wavelength, produces active chemical species with strong oxidizing power. Among the various systems, titanium dioxide is one of most studied photocatalytic materials, owing its success to the considerable advantage associated with its use: high chemical stability, low cost and low toxicity [1], [2], [3], [4].

However, its large band gap (approximately 3.2 eV) requires the use of UV light (λ < 387 nm), being the employment of the much larger visible part of solar light inhibited. Unfortunately the solar energy includes only the 3–5% [5], [6], [7], [8] of ultraviolet radiation, consequently environmental applications of pure TiO2 are limited. During the last years a lot of efforts have been directed toward the development of modified titania which would be photocatalytically active also under visible light irradiation (visible light covers >50% of the solar energy [9]). Among the various approaches proposed [10], the following are the main ones:

  • -

    the band gap reduction, obtained by using a photosensitizer which is excited by lower energy visible wavelengths and is able to transferring excited electrons or holes to the TiO2 matrix [11];

  • -

    the control of the recombination rate of photogenerated electron–hole pairs, which occurs at either boundaries and/or defects;

  • -

    decreasing the particles size, the distance that charges need to travel to reach the surface reaction sites is reduced, thus the recombination probability decreases [12];

  • -

    the promotion of oxidation reactions and absorption of reactants on the catalyst surface, providing both right amount and type of active sites, by using porous materials that increase the specific surface area [13], [14] or by using co-catalysts to introduce desirable active sites.

In particular, an increasing number of investigations have been focused on doped TiO2: through the catalyst modification with appropriate elements, it is possible to promote the catalytic efficiency of plain TiO2. A first type of doping concerns the introduction of transition metals, such as Fe, Cr, Ru, Mo, V and Rh [15], [16], [17], [18], [19], [20]: these are likely to favor an increase of the catalytic activity by acting as charge traps for electron and/or holes of electrons, inducing both lowering the recombination rate and increasing catalysts life time. However, the obtained advantages are attenuated by some negative aspects related to the use of substances that can render thermally unstable the catalyst [21] and that are often toxic. In addition, high concentrations of metals are negative for the photocatalytic efficiency, by acting as electron–hole recombination sites [22].

The doping with non-metals, such as N, C, F, S, P [9], [23], [24], [25], [26], [27], [28], [29], is an efficient alternative. Studies conducted on doping with N have brought to a considerable increase in the photocatalytic activity in the visible range [23], [30]. Several investigations have shown that N promotes the overlap of the 2p oxygen orbitals with the 2p nitrogen orbitals, thus lowering the band gap of TiO2 [31]. On the other hand, further studies propose the incorporation of N in the titania lattice (when added during the synthesis) as interstitial nitrogen by the creation of N–Ti–O and Ti–O–N bonds [32], [33], [34], [35], [36]. The generation of an intermediate energy state, between the conduction and the valence band, allows the promotion of electrons with light energy lower than in the undoped TiO2. If N is therefore proposed as a good promoter activity for TiO2, many studies have been devoted to the introduction of carbon [37], [38], [39]. The first studies regarding the introduction of C showed an appreciable increase of reaction rate in the electrolytic decomposition reaction of water into H2 and O2. As evidenced by Park et al. [40], carbon improves the photoactivity of titanium, but it is also likely to favor the stabilization of the anatase crystal phase and to promote the absorption of organic molecules on the catalyst surface.

In order to further increase the catalytic efficiency of TiO2 in the visible region, the researchers have addressed their efforts towards the investigation of the co-doping with C and N: it has been found to be particularly successful [21]. These co-dopants, non-toxic and inexpensive, enhance the separation of the photoexcited electrons and holes, improving the photocatalytic efficiency: therefore, C and N co-doping increases the activity of pure TiO2 under both visible and UV light irradiations [41]. Despite these numerous and interesting studies, many efforts are still needed to the optimization of an effective method which would allow the simultaneous introduction of dopants (C and N) during the synthesis step in order to maximize the subsequent catalytic activity.

The final goal of this work is then directed to both synthesis and characterization of stable TiO2-based co-doped photocatalysts with improved performance under visible light irradiation. The catalytic performance will be evaluated in the gas phase reaction of NOx abatement.

Section snippets

Synthesis

The following reagents were used as received: TiOSO4·xH2SO4·yH2O (Aldrich), NH3 (Riedel de Haen), succinic acid (HOOC-CH2CH2-COOH, Sigma–Aldrich).

The features and catalytic performances of the prepared catalysts were compared to those of the commercial Degussa P25 TiO2 used as a standard reference material.

The N-TiO2 samples were prepared by the precipitation method as previously reported [30]. Briefly, TiOSO4·xH2SO4·yH2O was dissolved in distilled water at room temperature under vigorous

Results and discussion

The principal goal of the work is the optimization of a reliable and effective approach for the synthesis of TiO2 photocatalysts with improved activity in the abatement of NOx under visible light irradiation. In a previous study [30] we have already verified the positive influence of N doping on the photocatalytic performance of TiO2 under visible light: in the present research in order to extend the investigation on these systems, the N-TiO2 catalysts have been reconsidered as starting point.

Conclusions

We have optimized a relatively easy and effective method for the preparation of active co-doped photocatalysts. The synergic presence of C and N greatly improves the photocatalytic activity (51%) that is higher than that of both a commercial standard (25%) and of a single-doped N-TiO2 systems (35%), acting on the decreasing of the apparent band gap energy which is one of the most effective limitations related to this type of catalysts.

This action makes possible the exploitation of such a

Acknowledgments

The authors thank Prof. Giuseppe Cruciani (University of Ferrara) for the XRD data and Tania Fantinel (Ca’ Foscari University of Venice) and Francesca Agostini for the excellent technical assistance.

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