Photocatalytic Oxidation of Organic Compounds over Uranyl Modified Oxides under Visible Light
Methods of photocatalytic purification of water and air widely are developed over last decades. Titanium dioxide is well-known photocatalyst because it is the most active metal oxide semiconductor among heterogeneous photocatalysts. Unfortunately, titanium dioxide is active only under UV irradiation (λ<380 nm) which occupies only about 4% of the solar light spectrum. At the same time visible light (>400 nm) occupies about 43% of the Sun irradiation. This is the reason why many researchers are trying to develop visible-light-driven photocatalysts.
It is known that uranyl ions could be sensitized by visible light and after excitation the oxidizing potential of uranyl ions becomes as high as 2,6-2,7 V . At the same time the gas phase photocatalytic oxidation of organic species with uranyl-modified photocatalysts are weakly investigated. Spectral characteristics as well as uranyl quantity, nature of the support and electronic structure are not studied. These questions were the subject of our study.
Photocatalyst samples were synthesized by incipient wetness impregnation method with the use of TiO2 (anatase), CeO2, γ-Al2O3 and SiO2 as support. The UO2(NO3)2 content was varied from 0.2 to 10 wt.%. Photocatalytic oxidation of acetone, ethanol, diethylsulfide and other VOCs was investigated.
Although pure unmodified TiO2 was not active under visible light (λ>420 nm) it was found to be the best support for uranyl nitrate and the UO2(NO3)2/TiO2 sample demonstrated highest photooxidation rate up to 500 nm incident light wavelength .
Silica, alumina and titania samples with deposited uranyl nitrate were examined by the luminescence spectroscopy. It was found that UO2(NO3)2/TiO2 sample does not luminesce when excited with UV light (320 nm) and only slightly luminesce if excited with visible light (430 nm) unlike the uranyl modified silica and alumina which luminesce strongly in both cases.
Suggestion was made that uranium species becomes more labile if deposited on the surface of TiO2 and less labile if deposited on the SiO2 or Al2O3 surfaces and uranyl liability correlates with photocatalytic activity.
The subsequent experiments by the XPS in situ method demonstrated that indeed on the surface of titania samples U6+ is easily reduced to U4+ under UV illumination and its backward reoxidation under O2 exposure is quick. At the same time on the surface of silica and alumina samples U6+ is hard to be reduced to U5+ under UV illumination and backward reoxidation is negligible.
A possible explanation of the increase of uranyl photocatalytic activity when it is deposited onto the TiO2 surface is its photophysical interaction with the TiO2 energy states.