Unexpected Selective Photocatalytic Reduction of Nitrite to Nitrogen on Silver-Doped Titanium Dioxide

Photocatalytic reduction of nitrite ions on silver-doped titania catalysts prepared by photodeposition and chemical reduction methods was investigated. With respect to the Ag/TiO 2 (C) catalyst prepared by the conventional chemical reduction method, the photoinduced synthesized Ag/TiO 2 (P) samples showed a more homogeneous silver morphology and narrow size distribution. For the first time, selective conversion of nitrite ions to nitrogen instead of ammonia was achieved on the Ag/TiO 2 (P) catalysts. A great influence of hole scavengers on the catalytic activity was observed and can be described as follows: H 2 C 2 O 4 > HCOONa > C 2 H 3 OOH > C 2 H 5 OH > C 2 H 3 OONa > Na 2 C 2 O 4 . The optimal average activity of approximately 19 mmol N ·g Ag -1 ·min -1 was achieved on the 1 wt % Ag/TiO 2 (P) sample under the modification of 0.04 mol/L oxalic acid, which was greatly higher than that on most of the previously supported noble metal catalysts (1−3 mmol N ·g metal -1 ·min -1 ) in terms of metal usage. It is interesting that slight influences of other anions such as SO 4 2-, CO 3 2-, or HCO 3 - coexisting in drinking water were observed, which is different from previous works. Meanwhile, the effect of silver morphology as well as the photocatalytic mechanism of nitrite was discussed.