Novel Pd/GdCrO3 composite for photo-catalytic reduction of nitrate to N2 with high selectivity and activity

Abstract

Although photocatalytic reduction of nitrate attracted much attention recently, undesired intermediate products (nitrite and ammonium) limited its application in practice. In this study, novel Pd/GdCrO3 composite was successfully synthesized to photo-catalytically reduce nitrate to N2 with high selectivity. Compared with TiO2 (P25), GdCrO3, Ag/GdCrO3 and Cu/GdCrO3, 1 wt% Pd/GdCrO3 achieved higher removal rate of nitrate (98.7%) and selectivity to N2 (100%). The kinetics constant of 1 wt% Pd/GdCrO3 was two times higher than that of P25. Even after six cycling runs, 1 wt% Pd/GdCrO3 still remained high photocatalytic reduction rate of nitrate (95.5%) and selectivity to N2 (98.4%). Such advantages were not only attributed to the negative conduction band value of GdCrO3 but also attributed to the co-catalyst effect of Pd. On one hand, nano particles Pd improved the utilization of charge carriers of GdCrO3. On the other hand, nitrite was easily reduced to NO* on nano particles Pd surface, followed by decomposition step to N* and O*, and then the reduction of nitrite in the vicinity of the N* could yield N2O* which would be preferentially transformed into N2. Accordingly, high concentration of nitrite accumulated initially could enhance the likelihood of N* encountering nitrite so as to increasing the N2 selectivity. In addition, electron paramagnetic resonance and hole scavenger experiments demonstrated that both photo-generated electrons and CO2[rad]− played important role in photocatalytic reduction process, suggesting a different mechanism from P25 where nitrate was mainly reduced by CO2[rad]−. Thus, the present work not only prepared a promising photocatalyst for reduction of nitrate in water/wastewater treatment process, but also provided a simple and efficient way to improve the photocatalytic activity and selectivity to N2of photocatalysts.