Photocatalytic reduction of nitrate in water on meso-porous hollandite catalyst: A new pathway on removal of nitrate in water

Photocatalysis of a hollandite compound KxGaxSn8-xO16 (x = ca. 1.8) was examined for the reduction of nitrate to N-2 with a reducing agent of methanol in water under UV irradiation. Hollandites have a characteristic one-dimensional tunnel structure. The meso-porous hollandite was prepared by sol-gel method. This hollandite was used as the photocatalyst and its reaction process was quantitatively analyzed by using ion chromatograph and on-line mass spectrometry. The hollandite photocatalyst showed a significant activity for the formation of N-2 from NO3-. Two factors, an increase in UV intensity and a lowering in pH of the solution, contributed to improvement in the selectivity for N-2. The selectivity for N-2 was improved to reach the perfect level by adjusting the factors. Although, in the previous report, the nitrate was mainly reduced to NH4+ or NO2-, the present photocatalytic conditions converted it to N-2. The observed photocatalytic reduction of NO3- to N-2 with reducing agent CH3OH was conjugated to the partial oxidation of CH3OH to HCOOH. This high selective photocatalytic decomposition of NO3- to N-2 may be a new pathway among the others reported so far, and it would be useful for environmental protection of water.