Photocatalytic activity of silver-modified titanium dioxide at solid–liquid and solid–gas interfaces

https://doi.org/10.1016/j.colsurfa.2007.11.030Get rights and content

Abstract

Silver-modified titania samples (Ag–TiO2) with varying silver content (0.1–1.0 wt%) were prepared. Silver-modification of titanium dioxide was examined by TEM, XRD, XPS and DR-UV–vis spectroscopy. Ag or AgOx particles on TiO2 surface could not be observed by XRD and TEM investigation, however the color of the Ag–TiO2 samples varied between light rose and purple-brown. XPS measurements revealed that silver exists mainly in oxide form. The photocatalytic activity of pure and Ag–TiO2 samples were compared both in solid–liquid and in solid–gas interfaces. In the liquid phase the 2,2′-thiodiethanol was used as test molecule. Ethanol photodegradation was examined in gas phase at dry initial condition. It was shown that the rate of photooxidation of organic compounds significantly enhanced by silver-modification of titania.

Introduction

Among numerous semiconductor materials TiO2 is the most widely used photocatalyst nowadays due to its optical and electronic properties, chemical stability, low cost and non-toxicity. Forasmuch as the TiO2 utilizes only a very small region of the solar spectrum due to its band-gap energy, the improvement of the response to the visible light (i.e. photosensitization) resulting in enhanced photocatalytic activity is one of the most important aspects of heterogeneous photocatalysis. Deposition of different metals (like Pt, Pd, Au, Ag, Fe, Nb and Cu) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] or oxides such as WO3 [17] onto titanium dioxide has been widely used as a technique to extend the light absorption to the visible region.

The usual methods for modification of TiO2 with noble metals or oxides are impregnation and photodeposition. The altering of the physical and chemical properties of titanium dioxide can be realised also by fixing it on different support materials (clay minerals, silica, etc.) [18], [19], [20]. Among the large number of publications in the literature Carp et al. give an excellent review on the synthesis, characterization and photoinduced reactivity of titania [21]. The effect of the doping agent in the degradation efficiency is not evident. Positive effect of additives in the decomposition of different organic compounds has been published [2], [3], [4], [5], [6], [7], [8], [9], but opposite results have been also reported [1], [7], [9]. This miscellaneous behaviour can be explained by the difference in the morphology, crystal structure, specific surface area and the surface density of the OH groups of the studied TiO2 catalysts, due to the various preparation methods applied in their synthesis [22].

The positive effect of metal deposits has been explained by the improved separation of electrons and holes on the surface of the photocatalyst. There was also observed, that some metals on TiO2 surface can have no influence or even a detrimental effect on the photocatalytic degradation of investigated organic pollutants [1], [7]. According to a possible explanation for this negative impact presented in the literature metal deposits may occupy the active sites on the TiO2 surface at higher loadings causing the photocatalyst to lose its activity.

The importance of pollutant's adsorption on the catalyst surface was stated for Pt, Pd and Ag doped titania/silica samples by Hu et al. [2]. They found that the silver-modified samples showed almost the same photocatalytic activity as the pure support material. In contrary silver loaded titania catalysts were found to be more effective in azo dye decomposition than the bare TiO2 sample [23]. Miscellaneous behaviour was observed in the case of Ag–TiO2 samples, silver enhanced the rates of sucrose oxidation due to the improved charge separation, however it posses similar activities to bare titania for salicylic acid and phenol decomposition [24]. Natural oestrogen's decomposition revealed no effect of silver addition, which also supports the theory on substrate and metal specificity in the organic compound's degradation [7].

In this paper we report a brief study on structural characterization of Ag-modified TiO2 samples. The photooxidation rate of 2,2′-thiodiethanol in liquid phase and ethanol in the vapour phase on the modified samples with various silver loading was investigated.

Section snippets

Sample preparation

The amounts of Ag loading were 0.1; 0.5 and 1.0% (w/w) with respect to the TiO2 amount. For each Ag-modified sample a 1 g amount of TiO2 was dispersed into 500 ml of AgNO3 (Molar, Hungary) solution in distilled water, with a respective concentration to ensure 0.1, 0.5 and 1% (w/w) Ag on the supports. In the photoreduction of Ag onto the TiO2 2-propanol was added as a sacrificial donor. The suspension was then irradiated with the UV-light using a 300 W of Xe-lamp (Hamamtsu L8251, Japan) for about 1 

Surface and optical properties

The specific surface area of the samples was determined by N2 sorption measurements. The results varied within the experimental error and were independent of the silver content of the samples, i.e. the value measured was 51 ± 2 m2 in all cases.

To characterize the optical properties of photocatalysts, the diffuse reflectance UV–vis spectra of pure and Ag-modified titania samples are compared in Fig. 3. In the case of Ag–TiO2 samples with 0.5 and 1% (w/w) loading, a new, broad absorption band

Conclusion

The structural and photocatalytic properties of pure TiO2 and Ag–TiO2 samples prepared by photodeposition were compared. The surface region of titania was shown to contain silver oxide, classified as amorphous by XRD. There was no detectable difference between the specific surface areas of the Ag–TiO2 samples with different silver contents; at the same time, however, Ag-modification of titania significantly increased the rate of ethanol photooxidation in gas phase and that of 2,2′-thiodiethanol

Acknowledgements

This work was supported by Deák Ferenc fellowship (2007/08) of Ministry of Education and Culture and by the Hungarian National Office of Research and Technology (NKTH) and the Agency for Research Fund Management and Research Exploitation (KPI) under contract no. RET-07/2005.

References (29)

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