Enhancement effect of TiO2 immobilized on activated carbon filter for the photodegradation of pollutants at typical indoor air level

doi:10.1016/S0926-3373(03)00054-7

Applied Catalysis B: Environmental

Volume 44, Issue 3, 30 August 2003, Pages 191-205

https://doi.org/10.1016/S0926-3373(03)00054-7 Get rights and content

Abstract

The competition effect between water vapor and pollutants at ppb level for adsorption sites has been previously reported. It was found that at high humidity levels, the pollutant photodegradation rate reduced drastically. To improve the photodegradation of pollutants at ppb level and at high humidity levels, TiO₂ is immobilized on an activated carbon (AC) filter. 200 ppb of nitrogen oxide (NO) and 20 ppb of benzene, toluene, ethylbenzene and o-xylene (BTEX) were chosen as target pollutants. Sensitivity analyses were conducted for NO and BTEX under different residence times and levels of humidity. Results showed that the combination of TiO₂ and AC significantly increased NO and BTEX removals at short residence time and high humidity levels. TiO₂ immobilized on AC filter was less affected by the increasing humidity levels. NO₂, as an intermediate generated from the photodegradation of NO, was also successively suppressed from exiting the system. Deactivation from the photodegradation of NO was suppressed by immobilizing TiO₂ on an activated carbon filter. The use of activated carbon acted as a local pollutant concentrator by adsorbing pollutants from the air stream and thereby diffused to the TiO₂ for photodegradation.

Introduction

People generally spend more than 80% of their time in an indoor environment [1] and the quality of indoor air has a vital impact on human health [2]. Due to the energy crisis in the 1970s, buildings were designed to be more airtight to save energy. With less fresh air intake and pollutants generated from building materials, occupants complained about sickness associated with working in an indoor environment [3]. This phenomenon is known as sick building syndrome (SBS). In general, there are three mitigation measures to reduce SBS, namely source control, ventilation and air cleaning. The first two methods are usually impossible as the source is unreachable and the ventilation is often ungovernable by building occupants. Thus, air cleaning is a feasible and convenient method for the individual occupant to improve indoor air quality.

Photocatalysis is an emerging and promising technology for pollution remediation in the gaseous and aqueous phase [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Recently, studies [14], [15], [16], [17] showed that the combination of photocatalyst TiO₂ with adsorbents appeared to have higher photocatalytic efficiency. Takeda and coworkers [18], [19] showed that the combination of TiO₂ with mordenite has a higher photodegradation rate for gaseous pyridine and propionaldehyde by concentrating pollutant on adsorbent and diffused to TiO₂ for photodegradation. The above studies, however, were all conducted at ppm levels and the effects of humidity levels are not investigated.

In the present study, the objective is to investigate the photodegradation of indoor air pollutants at the ppb level using TiO₂ loaded on activated carbon (AC) filter. It is of special interest to use TiO₂ with adsorbent for indoor pollutant removal as the concentration is extremely low and the competition for adsorption sites between water vapor and pollutants is most critical. Nitrogen oxide (NO), benzene, toluene, ethylbenzene and o-xylene (BTEX) were chosen as target pollutants as they are most commonly found in many urbanized cities such as Hong Kong [20], [21], [22]. Previously, we reported [23] the water competition effect of NO and BTEX at ppb level. In this study, the effect of humidity levels and residence times are investigated by using TiO₂ immobilized on activated carbon filter. To the best of our knowledge, no detailed investigation has been reported on the photodegradation of NO and BTEX at ppb level under different levels of humidity, by TiO₂, loaded on an activated carbon filter.

Section snippets

Reagents and catalyst preparation

The experimental procedure was described previously, and the only change was the use of a smaller size reactor [23]. BTEX (spectra gases) with a mixing ratio of 1:1:1:1 and NO (BOC gases) were used as reactant gas and acquired from compressed gas cylinder at a concentration of $1 ppm ±2$ % and $50 ppm ±2$ % with nitrogen as balanced gas with traceable National Institute of Standards and Technology (NIST) standard. TiO₂ (Degussa P-25) was used as a photocatalyst. The catalyst was used as received without

Characterization of the catalyst and filter

The Brunauer–Emmett–Teller (BET) surface area of the activated carbon powder extracted from the activated carbon filter was determined by nitrogen adsorption–desorption isotherm measurements at 77 K on a Micromeritics ASAP 2000 nitrogen adsorption apparatus. The BET surface area of the TiO₂ powder and the activated carbon powder was 1115 and 46 m²/g, respectively. Scanning electron micrographs of the glass fiber filter, used as substrate, and the different views of fiber coated with TiO₂ were

Conclusion

The removal of NO and BTEX at typical indoor air ppb levels by the use of TiO₂ immobilized on an activated carbon filter has significantly improved the pollutant removal. The improved removal using TiO₂/AC increased with decreasing residence time and increasing humidity levels compared to TiO₂. At a humidity level of 22,000 ppmv, only less than 10% of NO and BTEX are removed using AC only, whereas 40% and less than 10% of NO and BTEX respectively, are removed by TiO₂ only. When TiO₂ is

Acknowledgements

This project is funded by the Hong Kong Polytechnic University (GW-047). The authors would like to thank Mr. W.F. Tam for technical support in the laboratory. The authors would also like to thank Mrs. Anson for her help in the preparation of this article.

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Enhancement effect of TiO2 immobilized on activated carbon filter for the photodegradation of pollutants at typical indoor air level

Abstract

Introduction

Section snippets

Reagents and catalyst preparation

Characterization of the catalyst and filter

Conclusion

Acknowledgements

Atmos. Environ.

Appl. Catal. B-Environ.

J. Catal.

Atmos. Environ.

Appl. Catal. B-Environ.

J. Photochem. Photobiol. A

J. Mot. Catal.

J. Catal.

J. Catal.

Atmos. Environ.

Atmos. Environ.

Atmos. Environ.

Appl. Catal. B-Environ.

Carbon

Gas. Sep. Purif.

J. Loss Prev. Process. Ind.

Atmos. Environ.

Enhancement effect of TiO₂ immobilized on activated carbon filter for the photodegradation of pollutants at typical indoor air level