Photodegradation of indoor air pollutants by photocatalyst TiO2

Abstract

Indoor air quality received great attention after the energy crisis in the 70's. Buildings were designed to be more airtight to save energy consumption. Consequently, employees complained about increasing sickness such as headache and irritating eyes. Later, studies showed that these sickness are related to the elevated indoor air pollutant concentrations, which is known as Sick Building Syndrome (SBS). Traditional gaseous pollutant removal method such as physical adsorption or air scrubbing is not suitable for indoor air purification since the quantity and the concentration of the pollutant in indoor air is too low. The aforementioned methods are costly and require frequent replacement. Photocatalysis offers a new alternative for gaseous pollutant removal. It actually oxidizes pollutant into harmless compound such as H2O and CO2. Furthermore, it works under room temperature and atmospheric pressure. However, the use of photocatalysis for indoor air purification is rare. This study aims to investigate the use of photocatalysis for indoor air purification. Common indoor air pollutants such as nitrogen monoxide (NO), nitrogen dioxide (NO2), carbon monoxide (CO), sulfur dioxide (SO2), volatile organic compounds (VOCs) and formaldehyde (HCHO) were selected as target compounds with reference to its typical indoor air pollutant concentration. Sensitive analyses were conducted with the aforementioned pollutants, either separately or concurrently, under different residence time, humidity levels, initial concentrations, irradiation time and ultra violet lamp intensities in a continuously flow reactor and photocatalyst TiO2. Inhibitory and promotion effect was observed between concurrent photodegradation of multiple pollutants. The photodegradation rate, in general, decreased with decreasing residence time, decreasing ultra violet intensity and increasing humidity levels. The adverse effect of humidity levels is the largest amount the sensitive analyses. To rectify the humidity levels problem, photocatalyst TiO2 was modified but the result was not significant. TiO2 was then immobilized on an activated carbon filter (TiO2/AC). Results showed that the pollutant removal efficiency, especially VOCs, was significantly increased even at high humidity levels. Apart from using glass fiber filter and activated carbon filter as coating substrate, TiO2 was also coated on stainless steel by liquid phase deposition (LPD). The TiO2 thin film was calcinated under different temperatures. Surface characterization methods such as Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), Scanning Electron microscope (SEM), Photoluminescence Spectra (PL), and X-ray Photoelectron Spectroscopy (XPS) were applied to characterize the photocatalyst calcinated under different temperatures. The as-prepared TiO2 thin films contained not only Ti and O elements, but also a small amount of F, N and Fe elements. No activity was observed when the TiO2 was calcinated at a temperature under 400 C since the TiO2 thin film was composed of amorphous TiO2. The amount of Fe3+, which was contributed by the diffusion from substrate, affected the photoactivity of the TiO2 thin film. An optimum content of Fe3+ was observed for the photodegradation of NO. The pollutant removal efficiency of the TiO2/AC filter was further evaluated by placing it inside a commercially available air cleaner. The air cleaner was tested inside an environmental chamber. Results also showed that the pollutant removal efficiency of the TiO2/AC filter is higher and faster than TiO2 only. Furthermore, the TiO2/AC filter with a set of ultra violet lamps was installed in the air duct inside an office to investigate the pollutant removal efficiency in a practical field. Results showed that pollutant removal, including bacteria was observed. The use of photocatalytic technology is suitable for indoor air purification. To increase its efficiency for practical application, it is necessary to combine photocatalyst with adsorbent. This method is successful and evaluated by the laboratory-scale reactor, practical application such as air cleaner and on-site testing (commercial office). Thus, the use of photocatalyst and adsorbent showed a promising direction for indoor air purification in which the pollutant concentration is low and high humidity levels are often encountered.