Effects Of Microwave Radiation On Preparation And Photocatalytic Process Of TiO₂

How to improve the low quantum efficiency of the photocatalysis by means of improving the structure and the properties of TiO₂ photocatalyst is currently a concerned issue in the photocatalysis domain. Microwave (MW) radiation, a high efficiency heating technique, has been proved to be very useful for the preparation of many chemical materials. Moreover, the MW technique has also attracted great attentions because it can improve the reaction rate and product selectivity by altering the interaction among reaction molecules. In view of these advantages, it was the first time for us to introduce the MW into the photocatalytic studies, and our results found that the preparation of TiO₂ and its photocatalytic process were remarkably affected by the MW radiation. In this paper, X-ray diffraction (XRD), N₂ adsorption analysis applying BET method, UV-Visible diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and surface photovoltage spectroscopy (SPS) were used to investigate the MW effects on the preparation process and the physicochemical properties of TiO₂. Additionally, morphological structure and properties of the TiO₂ prepared by using the conventional heating and microwave dielectric heating were also compared detailedly. Acetaldehyde was chosen as a model reactant to valuate the performance of the samples. Effect of MW (power 100¹70 mw, frequency 36 GHz) on the photocatalytic process under UV irradiation was also studied, and the time-resolved fluorescence probe technique was applied to monitor the yield of active hydroxyl radical in the MW-assisted photocatalytic system. The following main results are obtained: (i) The TiO₂ photocatalyst with small crystallite size, high surface area, narrow and uniform pore size distribution, and microporous frameworks could be prepared out by applying MW radiation in the preparation process. The microwave-induced fluorescence technique verified for the first time that microwave enhanced the formation rate of hydroxyl radical, leading to the increase in the photocatalytic efficiency. (ii) Compared with the conventional catalyst, TiO₂ prepared by MW dielectric heating owns a higher UV absorption ability, and its optical absorption edge and photovoltaic response are shifted toward the short wavelength region, resulting in increase of photooxidation efficiency. (iii) The photocatalytic conversion and the mineralization ratio of acetaldehyde are increased respectively by 30% and 40% over the TiO2 prepared by MW radiation when compared to the TiO2 prepared by the conventional heating. (iv) For the MW-assisted photocatalytic reaction over the MW-prepared TiO2, production of hydroxyl radicals is enhanced by 22.5%, and the photocatalytic conversion is accordingly increased by 20%. Similarlly, applying MW also increased the photocatalytic conversion and the mineralization ratio of acetaldehyde over TiO2/Al2O3 sample. (v) The product distribution of the acetaldehyde photooxidation is altered by applying MW to the reactor, indicating that the "non-thermal effect"of MW plays a primary role in the photocatalytic process and the acetaldehyde photooxidation proceeds according to the chain-type reaction induced by hydroxyl radical. The major innovations of this study are summarized as follows: (i) The experimental system with functions of photocatalysis and MW radiation was developed, and the problem of detecting reaction temperature in the MW-assisted photocatalysis was resolved. It is the first time to adopt the MW-induced fluorescence technique to verify yield enhancement of photoinduced hydroxyl radicals under the presence of MW filed, which agrees well with the increased photocatalytic efficiency. (ii) Remarkable differences in the structure and properties of TiO2 prepared by MW radiation and by the conventional heating were observed, and a mechanism of MW dielectic heating was suggested. According to the product distribution of microwave-assisted photooxidation of acetaldehyde, the mechanism for MW-assisted photocatalysis was proposed. (iii) Effect of MW radiation on photocatalytic process was attributed to its "non-thermal effect", and the mechanism was discussed. This paper provides a novel approach to improve the properties of TiO2 photocatalyst and to enhance the quantum efficiency of the photocatalytic process.