Reduction of nitrate by heterogeneous photocatalysis over pure and radiolytically modified TiO2 samples in the presence of formic acid

Abstract

Heterogeneous photocatalytic reduction of nitrate (2 mM) in the presence of formic acid (FA, 10 mM) using pure and modified TiO2 samples under UV–vis irradiation was performed at pH 3, and the evolution of the intermediate and final products was analyzed. For this work, commercial samples (Evonik P25 and Cristal Global PC500 and PC10) were modified by γ-radiolysis with two noble metal nanoparticles known to behave differently in NO3− transformation, Ag and Pd. P25 was modified with Ag (0.5 and 2% w/w), while PC10 and PC500 were modified with Pd (1% w/w). The order of the photocatalytic activity of the materials for NO3− transformation was 2 Ag-P25 > PC500 > 0.5 Ag-P25 ≈ P25 ≫ 1 Pd-PC500 > PC10 > 1 Pd-PC10. Nitrite formation was observed in all cases but at low amounts, and its concentration was negligible (≤1 μM) after complete NO3− reduction. Ammonium was found as final product and remained in considerable amounts at the end of the irradiation. The nitrogen balance accounted for a large amount of non-identified nitrogen products formed during the photocatalytic reaction, probably N2 or NO; this amount was higher for the P25 and PC500 pure samples. The efficiency on the use of FA as donor was evaluated and PC500 was found to be the most efficient sample in this sense. A mechanism is proposed to clarify the still not well understood process of TiO2 heterogeneous photocatalytic transformation of nitrate, based on literature data and detected products.