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/TiO2 (C) catalyst prepared by the conventional chemical reduction method, the photoinduced synthesized Ag/TiO2 (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/TiO2 (P) catalysts. A great influence of hole scavengers on the catalytic activity was observed and can be described as follows: H2C2O4 > HCOONa > C2H3OOH > C2H5OH > C2H3OONa > Na2C2O4. The optimal average activity of approximately 19 mmol(N)center dot g(Ag)(-1)center dot min(-1) was achieved on the 1 wt % Ag/TiO2 (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)center dot g(metal)(-1)center dot min(-1)) in terms of metal usage. It is interesting that slight influences of other anions such as SO42-, CO32-, or HCO3- 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.