The synthetic water was prepared by added potassium nitrate and formic acid (i.e. the hole scavenger) in deionized water. The initial concentrations of nitrate and formic acid were 7.14–59.3 mM, respectively, to provide the initial ratio of formate to nitrate of 8 to 1, as per the stoichiometric ratio described by Rengaraj and Li [18]. Water samples were periodically collected every 30 min from the beginning of the experiment until 360 min. The concentrations of nitrate, nitrite, ammonium and formate in the water samples were measured with the Dionex DX-120; AS12A column, Dionex IonPac CS10 column, Dionex IonPac; AS11-HC column ion chromatography, respectively.
Two additional sets of control experiments were also carried out. The first set was the experiments in which the uncoated stainless steel sheets were employed to verify whether the nitrate reduction could occur under only UV light without the photocatalysts. Meanwhile, the second set involved the experiments with UV light and both types of catalysts (i.e. TiO2 and Ag–TiO2) under the normal experimental conditions except that no nitrate was added to the synthetic water to determine the loss of formic acid from the system due to the photocatalytic process.
The nitrogen selectivity S(N2) was calculated from the balance of nitrogen by-products