Photocatalytic Reduction of Nitrate in Water on Meso-Porous Hollandite Catalyst: A New Pathway on Removal of Nitrate in Water

Abstract

Photocatalysis of a hollandite compound K x Ga x Sn8−x O16 (x = ca. 1.8) was examined for the reduction of nitrate to N2 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 N2 from NO 3 − . Two factors, an increase in UV intensity and a lowering in pH of the solution, contributed to improvement in the selectivity for N2. The selectivity for N2 was improved to reach the perfect level by adjusting the factors. Although, in the previous report, the nitrate was mainly reduced to NH 4 + or NO 2 − , the present photocatalytic conditions converted it to N2. The observed photocatalytic reduction of NO 3 − to N2 with reducing agent CH3OH was conjugated to the partial oxidation of CH3OH to HCOOH. This high selective photocatalytic decomposition of NO 3 − to N2 may be a new pathway among the others reported so far, and it would be useful for environmental protection of water.