Adsorption of NO2 and subsequent formation of nitrate species in the dark on TiO2 nanoparticles exhibiting different morphologies: An in-situ FTIR study

Abstract Nitrogen oxides (NOx) are among the major air pollutants, which are responsible for the photochemical smog and haze formation and can cause acid rain. The selective catalytic reduction (SCR) process is a promising method to control the NOx emission from various stationery and mobile resources. The chemisorption of NO2 is regarded as the crucial step in this heterogeneous catalytic process. Here, the adsorption and formation of nitrate species on TiO2 nanoparticles (NPs) with different morphologies, designated as TiO2 trigonal, TiO2 nanowires, TiO2 nanocubes was examined in the dark using an in-situ FTIR spectroscopy, under NO2 as the feeding gas. The N2O4 formation is the initial step as soon as TiO2 NPs are subjected to NO2 adsorption, that is simultaneously transformed into other nitrate species via intramolecular disproportionation reaction. However, TiO2 NPs with different morphologies displayed a distinguished trend towards the stability and formation of various nitrate species. In particular, formation of NO+ was noticed only in TiO2 nanowires and TiO2 nanocubes (10mL_HF/40 h). Also, NO2 was detected in TiO2 trigonal shaped NPs, TiO2 nanocubes (4mL_HF/24 h), and TiO2 nanocubes (10mL_HF/24 h). The formation of different NO3- species, including monodentate, bidentate, and bridged NO3- (m-, b-, br-NO3-) were also observed. This study provides valuable information to modify the TiO2-based supports for the selective catalytic reduction technology.