Experimental and computational study on removal of nitric oxide using NH2-MIL-125: Revisiting removal mechanism

The NH2-MIL-125 (NM-125) or modified NM-125 materials are used for photocatalytic removal of nitric oxide (NO). Most researchers attributed few NO2 emission during NO photocatalysis to high selectivity towards the formation of nitrate ions due to the neglect of the principle of electroneutrality and pore structure of NM-125. This may cause misleading photocatalytic mechanism of NO removal. Herein, this work proposes a new mechanism behind NO removal using NM-125 based on experimental investigations and theoretical calculations. First, experimentally, the highest NO removal efficiency of 88.54 % with negligible NO2 emission occurs at the anhydrous condition. In addition, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements verify the presence and accumulation of NO2 as product during photocatalytic conversion of NO. Furthermore, density functional theory (DFT) calculations and grand canonical Monte Carlo (GCMC) computations demonstrate that there exist strong interactions between NO2 and -NH2 group within pores. Few NO2 is detected during experiments is probably due to its subsequent adsorption in the pores of NM-125. The new NO removal mechanism including photocatalytic conversion NO into NO2 and subsequent NO2 adsorption in the pores of NM-125 is thus proposed. This work provides a deeper insight into photocatalytic NO removal over NM125 and its variants.