Photochemical removal of NO2 by using 172-nm Xe2 excimer lamp in N2 or air at atmospheric pressure

Photochemical removal of NO2 in N2 or air (5-20% O2) mixtures was studied by using 172-nm Xe2 excimer lamps to develop a new simple photochemical aftertreatment technique of NO2 in air at atmospheric pressure without using any catalysts. When a high power lamp (300 mW/cm2) was used, the conversion of NO2 (200-1000 ppm) to N2 and O2 in N2 was >93% after 1 min irradiation, whereas that to N2O5, HNO3, N2, and O2 in air (10% O2) was 100% after 5 s irradiation in a batch system. In a flow system, about 92% of NO2 (200 ppm) in N2 was converted to N2 and O2, whereas NO2 (200-400 ppm) in air (20% O2) could be completely converted to N2O5, HNO3, N2, and O2 at a flow rate of 1 l/min. It was found that NO could also be decomposed to N2 and O2 under 172-nm irradiation, though the removal rate is slower than that of NO2 by a factor of 3.8. A simple model analysis assuming a consecutive reaction NO2 → NO → N + O indicated that 86% of NO2 is decomposed directly into N + O2 and the rest is dissociated into NO + O under 172-nm irradiation. These results led us to conclude that the present technique is a new promising catalyst-free photochemical aftertreatment method of NO2 in N2 and air in a flow system.