Removal of Indoor Air ppb Level Volatile Organic Compounds (VOCs) and NO x by Heterogeneous Photocatalysis

VOCs and NOx are the major pollutants in Hong Kong indoor environment. Traditional air purification method such as adsorption by activated carbon is not reliable and get saturated easily. Adsorption merely transfers pollutants from gaseous phase to solid phase and poses the disposal problem. Using photocatalysis, however, strong oxidant such as hydroxyl radical is generated and actually oxidized the pollutant to H2O and CO2. In this study, semi-conductor TiO 2 is used with 365 nm ultra violet light as activation energy. 35 ppb of benzene, toluene, ethylbenzene and o-xylene (BTEX) were used as target VOCs and 200 ppb NO was used. Sensitive analyses of the photodegradation of NOx and BTEX are conducted by investigating the effect of residence time and relative humidity. NO, NO 2, and NOx are simultaneously measured by a Chemiluminescence NO analyzer. BTEX were collected by Summa canister and analyzed by GC/MS. It was found that a minimum of 60% and a maximum of 85% of NOx are removed from the system under different variations. Removal efficiencies of NO x are higher at a longer resident time and at a lower relative humidity higher and light intensity. Relative humidity affected the photoefficiency most significantly. The presence of NO promoted the photodegradation of BTEX. The presence of BTEX, however, inhibited the photodegradation of NO. 1.0 INTRODUCTION People generally spend more than 80% of their time in indoor environment [1]. The quality of indoor air has a direct impact on human health in terms of lengthened exposure to pollutants by inhalation. NO and VOCs were chosen as target pollutants for this study. NO is one of the major pollutants in indoor environments. Among numerous compounds which belong to VOCs, benzene, toluene, ethylbenzene and o-xylene (BTEX) are chosen for this study. The reason is that BTEX is the major VOCs found in indoor environments in different countries [2,3]. In Hong Kong, BTEX are also the major VOCs found in an indoor environment, either in mechanically ventilated buildings or in residential homes. The concentration of BTEX usually found in this environment ranged from 10 ppb to 100 ppb [4,5]. There is a limited study on the photodegradation of pollutants at typical indoor levels. Photodegradation of toluene and benzene at several hundreds ppb levels was conducted with a conversion of around 80% and 95% respectively. However, the concentration used was several hundreds ppb, which is seldom found in normal and polluted indoor environment such as offices and residential homes. In this study, we investigated the photodegradation of BTEX and NO at parts per billion levels with reference to the typical indoor air pollutant concentrations in Hong Kong. As reported in previous study [6], the photodegradation rate increased with increasing concentrations of the pollutants in the range of several hundreds ppm level. It is vital to investigate if the photodegradation rate will reduce immensely at indoor ppb levels. The effects of residence time and levels of humidity on the photodegradation rate were also conducted. As NO and BTEX usually co-exist in indoor environments, the effects of concurrent photodegradation of NO and BTEX is also investigated to evaluate the feasibility of photocatalytic technology for indoor air purification. 2.0 EXPERIMENTAL BTEX (Spectra gases) and NO (BOC gases) were generated from standard gas. TiO2 (Degussa P-25) was used as a photocatalyst. The weight of TiO 2 imposed is determined by the weight difference before and after the coating procedure. In all experiments, the weight of TiO 2 imposed is 1.64 g ± 5%. A reactor with a volume of 57 liter (40.5H x 50.5L x 28W cm) with its surface coated by a Telfon film (BYTAC Type AF-21) was used for this study. Illumination was provided by a 6W UV lamp (ColeParmler) which emits a primary wavelength at 365 nm and its intensity was determined by a UV meter (Spectroline DRC-100X). Other experimental details are reported in elsewhere [7].