Inhibition effect of SO₂on NO_xand VOCs during the photodegradation of synchronous indoor air pollutants at parts per billion (ppb) level by TiO₂

Sulfur-containing compounds are well-known catalyst poisons. To evaluate the feasibility of photocatalytic technology for indoor air purification, a typical atmospheric SO2concentration of 200 parts per billion (ppb) was selected. In order to further evaluate the impact of SO2on the photocatalytic activity of other typical indoor air pollutants, SO2was co-injected with 200ppb NO and 20ppb benzene, toluene, ethylbenzene, and o-xylene (BTEX) using TiO2(P-25) as photocatalyst coated on a glass fiber filter. A concurrent photodegradation of SO2with NO, SO2with BTEX, and SO2with NO and BTEX was also conducted. Results showed that no photodegradation of SO2was found. However, the blank glass fiber filter adsorbed more than 75% of the SO2. The conversion of NO decreased by 8% and the generation of NO2increased by 10% with the presence of SO2. A similar inhibition effect was found on the photodegradation of BTEX with the presence of SO2. The presence of SO2decreased the conversion of BTEX by more than 10%. Ion chromatography analysis on the TiO2glass fiber filter showed that sulfate ion was formed from the adsorption of SO2. The formation of sulfate ion inhibited the formation of nitrate ion, which increased the generation of NO2. It is suggested that the inhibition effect of SO2is due to the sulfate ion competing with the pollutant for adsorption sites on TiO2. The promotion effect of NO on BTEX was also reduced by the presence of SO2.