Activity and selectivity of photocatalysts in photodegradation of phenols

https://doi.org/10.1016/j.jhazmat.2011.11.078Get rights and content

Abstract

Photodegradation of phenol and 4-chlorophenol over six different TiO2 samples was tested in order to establish whether an interconnection between the activity and selectivity of photocatalysts exists. The obtained experimental data were analyzed using correlation analysis. Some correlations between the activity in phenol(s) photodegradation and selectivity toward formation of primary intermediate products were established. The type of correlations depends on the type of studied photoreactions. The discussion of the observed correlations between the activity and selectivity of photocatalysts is given in terms of the difference of surface concentrations of electrons and holes and corresponding surface active sites which might be dependent on the types of dominating surface faces. On the basis of the obtained results of correlation analysis it was assumed that a higher activity of photocatalysts could be achieved provided that both reduction and oxidation reaction pathways occur with equally high efficiency.

Introduction

Most attention in fundamental and applied studies in the area of heterogeneous photocatalysis is directed toward exploration of the factors, which determine three main characteristics of photocatalysts: photoactivity, spectral sensitivity and selectivity [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. The problem of the enhanced (photo)activity of photocatalysts has attracted major attention among researchers dealing with heterogeneous photocatalysis. This is explained by the fact that photocatalysis has found the most application for degradation of organic pollutants in water and air that requires the strong oxidation power sufficient to decompose most organic molecules to simple mineralized products. Obviously, such a problem does not require much attention to more delicate characteristics such as selectivity of photocatalysts. In fact even the problem of increased spectral sensitivity to visible light has been recalled as a tool to increase the overall photoactivity of photocatalysts by taking such a simplified approach as “the more light absorbed, the higher the activity would be”. Detailed research however, demonstrated that the reality is not so simple and flat and the consequences of the extension of the spectral range of photocatalyst activity to its extrinsic light absorption by various chemical or physical modifications are more complex [6], [8], [11], [12], [13], [14] and can lead to decay of photoactivity and chemical power of photocatalysts.

The least attention was ever attracted by such characteristics as selectivity of photocatalysts and its relation to other major parameters. Only a few studies have been focused on measurement of selectivity and exploration of the factors determining its magnitude, such as reagent concentration, pH of solution, light intensity and so on [6], [15], [16], [17], [18]. In our studies we have explored the connection between the origin of the extrinsic absorption responsible for the increase of the spectral sensitivity to visible light and selectivity of photocatalyst dependent on wavelength of the actinic light [18], [19], [20]. Recently the understanding of the practical significance of the photocatalyst selectivity has emerged. It is dictated by the fact that many hazardous organic contaminants are represented by complex molecules whose complete mineralization requires utilization of the energy of quite a large number of photons that makes the process inefficient energy-wise. Moreover, at some stages of the multistep photodegradation of organic molecules more hazardous and chemically stable intermediates can be formed. Thus, exploration of factors affecting photocatalyst selectivity becomes practically essential for learning how to drive the organic contaminant degradation toward formation of benign reaction products without complete mineralization. Of particular interest might be the search for the interconnection between the activity and selectivity of photocatalysts.

Section snippets

Activity and selectivity of photocatalysts

As demonstrated in our earlier studies [6], [17], [18], [19], [20] there are two possible causes for selectivity of a photocatalyst in the case when the rate of the surface photochemical reaction is dictated by the surface concentration of photocarriers (electrons for reduction or holes for oxidation reaction pathways, respectively). The first cause originates from the ratio between surface concentrations of electrons and holes at the surface of photoactive material. Consequently, alteration of

Experimental

Six TiO2 photocatalyst samples were tested in the present study, namely: Degussa P-25 (50 m2 g−1), Hombikat UV100 (300 m2 g−1), ST-41 (10 m2 g−1), ST-21 (50 m2 g−1) FTL-200 (10 m2 g−1 (nano-rods)) produced by Ishihara and (2.9 m2 g−1) produced by Kanto Chem. According to the XRD data Hombikat UV100, ST-41 and ST-21 samples possess the anatase crystal structure, FTL-200 and Kanto Chem. are the samples of rutile type modification, and P-25 consists of 78% anatase and 22% rutile. Phenol and 4-chlorophenol

Results and discussion

To compare the photoactivities and selectivities of different TiO2 photocatalysts and to perform the correlation analysis we measured the initial rates of phenols photodegradation and formation of corresponding intermediates. A necessary condition for such analysis is that the rate of corresponding photoprocess should scale linearly with the light intensity and be independent of reagent concentration. This can be achieved by alteration of reagent concentration due to effect of dependence of the

Conclusion

Application of the correlation analysis to experimental data obtained for phtodegradation of phenol and 4-chlorophenol over six tested TiO2 photocatalyst samples results in establishing the correlation between the activity and selectivity of photocatalysts. The type of this correlation strongly depends on the type of testing reaction. The negative correlation between the activity of photocatalysts toward photodegradation of phenol whose primary step occurs by oxidation pathway only, and

Acknowledgments

This work was supported by a Grant-in Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government, and by Core Research for Evolutional Science and Technology (CREST). A.V.E. also thanks the JSPS for a Short-Term Fellowship for Advanced Researchers.

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