Highly selective and efficient photocatalytic reduction of nitrate in water by a tandem reaction system consisting of Pt/TiO2 and SnPd/Al2O3: A comparative study of the tandem reaction system with a typical semiconductor photocatalyst, SnPd/TiO2

Abstract

A tandem reaction system consisting of a photocatalyst (Pt/TiO2) and a nonphotocatalyst (SnPd/Al2O3) promoted the reduction of NO3− into gaseous products (mainly N2) under light irradiation (λ>300nm) in the presence of glucose as a hole scavenger. Photocatalytic H2 evolution (2H++2e−→H2) proceeded over Pt/TiO2, and conventional catalytic reduction of NO3− with H2 (NO3−+5/2H2→1/2N2+2H2O+OH−) occurred over SnPd/Al2O3. We optimized the loading amount of Pt on TiO2, the Sn/Pd ratio, the loading amount of SnPd on Al2O3, and the two catalyst dosages. The optimized tandem system gave a high reduction rate of NO3− and a high selectivity for gas (94%) from the photocatalytic reduction of NO3− in water. On the other hand, a typical semiconductor photocatalyst SnPd/TiO2 with an optimized Sn/Pd ratio and an optimized loading amount of SnPd bimetal on TiO2 reduced NO3− about two-thirds as fast as the tandem system and was less selective for gas (70%). The tandem system suppressed the wasted H2 formation, resulting in high light use efficiency for the NO3− reduction (95%), which is defined as the ratio of the number of electrons consumed for NO3− reduction to the total number of electrons consumed for both NO3− reduction and photocatalytic H2 evolution, though the tandem and SnPd/TiO2 systems consumed about the same total number of electrons. The tandem system has two advantages: (i) the Pt/TiO2 and SnPd/Al2O3 subsystems can be separately designed to give highly efficient photocatalytic and catalytic reactions, respectively; and (ii) the reaction rates of photocatalytic and catalytic reactions can be easily controlled by changing the catalyst dosage in the reactor. Those advantages brought about a high reduction rate for NO3−, high selectivity for gas, and high light use efficiency for NO3− reduction in the photocatalytic reduction of NO3− in water.