Selective Photocatalysis of Dimethylnaphthalene by Means of β-Cyclodextrin Molecular Recognition Sites

Abstract

Heterogeneous photocatalysis using titanium dioxide is a non-selective technique used for the degradation of target molecules such as dimethylnaphthalene. This work presents a novel approach for selective photocatalysis implementing molecular recognition sites (MRS) placed in the vicinity of titanium dioxide which attract target molecules and shuttle them to the surface of titanium dioxide, where they are more rapidly and selectively degraded. The photodegradation rate of dimethylnaphthalene was studied under two variables: surface area and kinetics of titanium dioxide, as well as the presence of MRS. While improvement under zero order kinetics was not noteworthy, it was discovered that presence of MRS in conjunction with first order kinetics significantly improves the degradation rate of dimethylnaphthalene. Introduction Photocatalysis is a technique used for the degradation of target molecules by means of creating OH radicals through ultraviolet radiation. Heterogeneous photocatalysis, then, incorporates the use of a catalyst characteristic of a different phase than that of the degraded material. Titanium dioxide is known to be an efficient photocatalyst that, by the same token, is inexpensive, non-toxic, chemically stable, and accessible. A key characteristic of titanium dioxide is its high surface area, which allows the rate of reaction to increase, as the efficiency of a photocatalyst is proportional to its surface area. The process of photodegradation on the surface of titanium dioxide, initiated by ultraviolet illumination that generates electron-hole pairs, forms OH radicals when photogenerated holes interact with adsorbed water. It is acceptable to assume that the rate of photodegradation on titanium dioxide is of the form: (1) ] S [ K 1 ] S [ kK Rate + = where Rate = rate of photodegradation, [S] = concentration of the target molecule (dimethylnaphthalene), K = equilibrium constant between the surface of titanium dioxide and dimethylnaphthalene, and k = reaction constant. When the concentration of the target molecule is very low, the rate can be approximated to be a first order rate: (2) Rate = kK[S] In this case, the coverage of the titanium dioxide surface is partial and is proportional to the concentration of the target molecule in the bulk. However, when the concentration of the target molecule is very high, the rate becomes zero order: (3) Rate = k Here, the coverage is thorough, and the bulk concentration has no influence on the rate of degradation; only the constant of the reaction has such an influence. In the process of using photocatalytic reactions to chemically degrade a contaminant, the problem arises that such reactions have low selectivity. Selectivity controls a reaction in such a way that one target molecule can be adsorbed preferentially by the system and become decomposed more rapidly. Research in the field of selective photocatalysis is relatively novel, but selectivity of TiO2 has previously been achieved in several ways: modifying surface conditions or pH, imprinting the target molecule into the TiO2 particle, adding zeolites, and more. This work presents a different approach to selective heterogeneous photocatalysis based on molecular recognition sites (MRS) located in the vicinity of titanium dioxide (see figure 1). The target molecule is attracted by the MRS and then shuttled towards the titanium dioxide by surface diffusion, where it is degraded. Au TiO2 MRS UV light

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Cite this paper

@inproceedings{Daich2003SelectivePO, title={Selective Photocatalysis of Dimethylnaphthalene by Means of β-Cyclodextrin Molecular Recognition Sites}, author={T . Daich and Nyra Goldstein and Mentor Y . Sagatelian}, year={2003} }