Review article
Volatile organic compounds in indoor environment and photocatalytic oxidation: State of the art

https://doi.org/10.1016/j.envint.2007.02.011Get rights and content

Abstract

Volatile organic compounds (VOCs) are the major pollutants in indoor air, which significantly impact indoor air quality and thus influencing human health. A long-term exposure to VOCs will be detrimental to human health causing sick building syndrome (SBS). Photocatalytic oxidation of VOCs is a cost-effective technology for VOCs removal compared with adsorption, biofiltration, or thermal catalysis. In this paper, we review the current exposure level of VOCs in various indoor environment and state of the art technology for photocatalytic oxidation of VOCs from indoor air. The concentrations and emission rates of commonly occurring VOCs in indoor air are presented. The effective catalyst systems, under UV and visible light, are discussed and the kinetics of photocatalytic oxidation is also presented.

Introduction

Indoor air quality (IAQ) has become an important community concern due to the increased amount of personal time spent in indoor environment. Nowadays, people generally spend more than 80% of their time in an indoor environment such as home, office, car and shopping centre. Some studies showed that the level of pollutants in indoor environment is actually higher than that in outdoor environment. Indoor air pollutants mainly include nitrogen oxides (NOx), carbon oxides (CO and CO2), volatile organic compounds (VOCs), and particulates. VOCs are well-known indoor pollutants. These pollutants are emitted from different sources such as combustion by-products, cooking, construction materials, office equipment, and consumer products. Many VOCs are known to be toxic and considered to be carcinogenic, mutagenic, or teratogenic (Alberici and Jardim, 1997). These VOCs have a close relation with the sick building syndrome (SBS), which is one of many terms used by occupants to describe symptoms of reduced comfort or health.

In general, three methods are suggested to improve the indoor air quality, namely source control, increase ventilation, and air cleaning. Source control is often ungovernable and unavoidable in the metropolis. Increased ventilation might even transport more pollutants from the outdoor environment. Thus, air cleaning remains to be the most feasible option to improve the indoor air quality. Traditional pollution control method such as adsorption by activated carbon merely transfers pollutants from gaseous phase to solid phase. Advanced oxidation processes (AOP) such as thermal oxidation destruction (Drysys, 1997, Everaert and Baeyens, 2004, Roark et al., 2004) and photocatalytic oxidation (PCO) (Zhao and Yang, 2003, Carp et al., 2004) are promising technologies for air purification because the pollutants can be oxidised to H2O and CO2. However, thermally catalytic oxidation requires high temperatures of 200–1200 °C for efficient operation and hence expensive. Furthermore, thermally catalytic oxidation is not economically feasible at low pollutant concentrations.

Photodegradation usually occurs at room temperature and pressure for air purification and may be more cost-effective than other conventional techniques such as activated carbon adsorption and chemical scrubbers, because the semiconductor catalysts are inexpensive and capable of mineralising most organic compounds effectively. However, this technique is still in the developmental stage for VOCs removal from indoor air. In the past two decades, a lot of investigations have been conducted in photocatalytic oxidation of gaseous VOCs and several types of photocatalyst systems have been developed (Hoffmann et al., 1995). However, most investigations concentrated on the VOC destruction at higher concentrations of the parts per million (ppm) levels. While concentrations in the ppm range are typical for chemical stream concentration, sub-ppm levels or parts per billion (ppb) concentrations are commonly associated with indoor (buildings, trains, vehicles, planes, etc.) VOCs. Extrapolation of the oxidation data collected at concentrations much higher than the intended application may not be valid (Obee and Brown, 1995). Several investigations have found that different inlet contaminant concentrations lead to different reaction rates (Zhao and Yang, 2003). In general, reaction rate is enhanced with the increasing of inlet concentration of pollutants. Thus it is necessary to investigate the catalytic performance of catalysts at low pollutant level. However, there are limited studies on the photodegradation of VOC pollutants at typical indoor levels. With the increasing concerns on the indoor air quality, more researches have focused on photocatalytic oxidation of VOCs at ppb level in recent years. In this review, we will present updated progress in this area. We will discuss the VOC exposure level at various indoor environments and summarise various catalyst systems for photocatalysis under UV and visible light. The important factors that influence the catalytic activity and the kinetics will be discussed. The future research directions will also be presented.

Section snippets

VOCs exposure in indoor environment

Indoor air was defined as the air in non-industrial areas of dwellings, offices, schools, and hospitals. Recently, air quality in cars and aircraft cabins has also raised much concern, because these vehicles are also typical indoor environments. According to the definition of the World Health Organisation (WHO), VOCs are referred as all organic compounds in the boiling point range of 50–260 °C, and excluding pesticides. Table 1 lists the classes of VOCs identified in indoor air, in which

Photocatalysts and photocatalytic oxidation

Fig. 1 presents the diagram of photocatalytic oxidation. For the photocatalytic oxidation, an important step of photoreaction is the formation of hole–electron pairs which need energy to overcome the band gap between the valence band (VB) and conduction band (CB). When the energy provided (photon) is larger than the band gap, the pairs of electron–holes are created in the semiconductor, and the charge will transfer between electron–hole pairs and adsorbed species (reactants) on the

Effect of water vapour and other gaseous compounds on photocatalytic oxidation

The influence of water vapour on the performance of titania oxide catalyst depends on the level of water. Widely differing effects of water vapour have been reported. In the absence of water vapour, the photocatalytic degradation of some chemical compounds (e.g., toluene, formaldehyde) is seriously retarded and the total mineralisation to CO2 does not occur. The surface of TiO2 catalysts is abundant in hydroxyl groups, which leads to adsorbing water via hydrogen bonding or aromatics via an

Reaction mechanism and kinetics

It is generally believed that the initial stages concerning photocatalysis processes are as below (Cao et al., 1999b).TiO2 +   h+ + eh+ + OH  radical dotOHTi4+ + e  Ti3+Ti3+ + O2ads  Ti4+ + O2adsradical dotOH + pollutant  oxidised pollutanth+ + e  thermal heat or luminescence

Hydroxyl radicals and super-oxide ions are highly reactive species that will oxidise VOCs adsorbed on the catalyst surface. Different intermediates will be produced depending on the VOCs and these intermediates will be further oxidised by O2radical dot or Oradical dot to produce CO2.

Summary and perspectives

Indoor air quality is currently a public intensive concern. VOCs are important air pollutants in various indoor environments such as houses, offices, cars and transportation cabins. These airborne pollutants can be emitted from diverse sources and a long-term exposure to these air toxics will cause health problem known as SBS. Measurements indicate that exposure level of VOCs in various indoor is similar and building materials and coverings are the major sources of VOCs.

Removal of VOCs from

Acknowledgement

We thank the Australian government, Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC-CARE), for supporting the project.

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