Table 2.

Catalytic oxidation air cleaning technology. Catalytic oxidation refers to a set of chemical treatment procedures designed to remove organic and inorganic materials in gas by catalysts. The common types for indoor air cleaning are photocatalytic oxidation (PCO), thermal catalytic oxidation (TCO) and ozone-catalytic oxidation.

Papers	Results by the authors	Research type/test procedure	Target pollutants/concentration	Airflow rate, Air velocity, or residence time	CADR (m3 h−1)/efficiency (%)	By-product tested or not and results
Photocatalytic oxidation
Ao and Lee (2003)	UV-TiO2/AC was more effective in BTEX removal and less affected by the increasing humidity than AC alone. AC acted as a local pollutant concentrator by adsorbing pollutants from the air stream.	Laboratory; single-pass test. Primary UV wavelength: 365 nm; UV intensity: 0.75 mW cm−2; temperature: 25 ± 1 °C.	NO: 200 ppbv; BTEX: 20 ppbv; humidity: 2100–22000 ppmv.	Airflow rate: 5–30 L min. Residence time: 0.6–3.7 min.	50% to over 90%. Varies with residence time and humidity levels. When the residence time is 1.2 min, the efficiencies of TiO2/AC are: BETX: over 60%, NO: over 70%.	Yes. NO2 is an intermediate from photodegradation of NO. If combining UV-TiO2 and activated carbon for BTEX and NO removal, there was no deactivation. But deactivation occurred if only using UV-TiO2.
Ao et al. (2003)	Humidity and residence time had significant influence on the conversion rate by UVPCO, especially for BTEX compounds. The presence of NO reduced the photodegradation of BTEX at moderate humidity levels.	Laboratory; Single-pass test. PCO reactor: plate type; Primary UV wavelength: 365 nm; UV intensity: 0.6 mW cm−2.	NO: 200 ppbv; BTEX: <100 ppbv; Humidity: 2100–22000 ppmv.	Residence time: 2.85–11.4 min.	24–86% for BTEX and over 90% for NO at low humidity level.	Yes. The reaction between NO and BTEX will generate NO2.
Ao et al. (2004)	The presence of SO2 inhibited the conversion of BTEX and NO compounds by UVPCO, but increased the generation of NO2 as a by-product from NO.	Laboratory; Single-pass test. PCO reactor: plate type. Germicidal UV lamp. Temperature: 25 ± 1 °C.	NO: 200 ppbv; SO2: 200 ppbv; BTEX: 20 ppbv; Humidity; 2100–22000 ppmv.	Residence time:1.24 min.	40% to >80% for TiO2. After irradiation by UV for 120 min, the efficiencies are: Benzene: 30–70%; Toluene: 50–80%. Ethyl benzene: 60–80%; o-xylene: 65–80% (Residence time: 1.2 min; Humidity: 2100 ppmv).	Yes. NO2 is the by-product from the degradation of NO.
Chen and Zhang (2008)	The interference effect among VOCs was small in 2-VOC and 3-VOC mixture tests. However, such effect became significant in 16-VOC mixture. There is a competitive adsorption among different VOCs.	Laboratory; Chamber test. PCO reactor: Honeycomb monoliths; Primary UV wavelength: 365 nm; UV intensity: 0.6 mW cm−2; Temperature: 23 ± 0.5 °C; Humidity: 50 ± 5%.	Single compound: 1 mg m−3 for each compound except formaldehyde and acetaldehyde. 16 VOCs mixture test: 2 mg m−3 for each compound.	Airflow rate: 1360 m3 h−1. Face velocity: 1.05 m s−1.	1.1–9.5% for all VOCs.	No.
Jeong et al. (2005)	The photodegradation of toluene and benzene by UV254+185 nmwas much higher than by UV254 nm or UV365 nm. Ozone generated by the UV-185 nm lamp enhanced this photodegradation effect.	Laboratory; Single-pass test. PCO reactor: Pyrex glass cylinder; Primary UV wavelength: 185–254–365 nm; Temperature: 24.9 ± 1 °C. Humidity: <1–90%.	Toluene, benzene: 0.6–20 ppmv.	Airflow rate: 1.0–4.0 L min−1. Residence time: 33.0–8.3 s.	Varies with airflow rate. At 1 L min−1, inlet concentration = 0.6 ppmv, RH = 40%, Toluene: 82.6–99.9%; Benzene: 67.1–94.2%.	Yes. CO2 and CO are the main degradation products, with some water-soluble organic intermediates.
Hodgson et al. (2007)	UVPCO can effectively reduce many VOCs, with conversion efficiency greatest for alcohols and glycol ethers and lowest for halogenated aliphatic hydrocarbons.	Laboratory; Single-pass test in dust. PCO reactor: Honeycomb monoliths; Primary UV wavelength: 254 nm; UV intensity: 6.0–6.5 mW cm−2. Temperature: 19.5–25 °C; Humidity: 42–65%.	Mixture contained 27 VOCs from office buildings; Mixture including 10 VOCs emitted by cleaning products.	Airflow rate: 165–580 m3 h−1 for 27 VOCs; 165–580 m3 h−1 for 10 VOCs. Face velocity: 0.51–1.79 m s−1.	Given for all VOCs in study. Toluene: 16%–45%	Yes. Formaldehyde, acetaldehyde, formic acid and acetic acid are the main by-products from the degradation of 27 VOCs. Formaldehyde, acetaldehyde and acetone are from 10 VOCs.
Mo et al. (2009)	The by-products of toluene by UVPCO were identified, such as benzaldehyde, methanol, acetaldehyde and acetone.	Laboratory; Single-pass test. PCO reactor: Plate type; Primary UV wavelength: 254 nm; UV intensity; 0.43–0.95 mW cm−2; Temperature: 24–26 °C; Humidity: 1.1–84%.	Toluene: 450–8000 ppbv.	Airflow rate: 0.55 L min−1. Residence time: 0.2 s.	–	Yes. Benzaldehyde, methanol, acetaldehyde etc. are the main gas-phase by-product.
Muggli et al. (1998)	A possible reaction mechanism of ethanol decomposed by UVPCO was proposed. The intermediates of ethanol such as acetaldehyde, acetic acid (acetate), formaldehyde, and formic acid (formate) were identified.	Laboratory; Single-pass test. PCO reactor: annular Pyrex reactor. Primary UV wavelength: 356 nm; UV intensity; 0.3 mW cm−2; Room temperature.	Ethanol.	–	–	Yes. Part of the ethanol reacts on the surface through the pathway: acetaldehyde → acetic acid → CO2 +formaldehyde → formic acid → CO2.
Obee and Brown (1995)	Competitive adsorption between water and trace (sub-ppmv) contaminants has a significant effect on the oxidation rate.	Laboratory; Single-pass test. PCO reactor: Plate type; Primary UV wavelength: 250–350 nm; UV intensity; <0.01–0.125 mW cm−2. Humidity: 0–20000 ppmv; Temperature: 12.8–60 °C.	Formaldehyde; toluene and 1,3-butadiene: 0–<20 ppmv.	Face velocity; 2.6–12 cm s−1.	Varies with initial pollutant concentrations, humidity levels, and temperature.	No.
Tsai et al. (2008)	The application of UV/TiO2/quartz or UV/TiO2/MCM-41 in the process to control toluene, ethyl benzene, xylene was viable and effective. UV/TiO2/quartz had a better reaction rate than that of UV/TiO2/MCM-41.	Laboratory; Cycling or decay. PCO reactor: batch packed-bed reactor; Primary UV wavelength: 365 nm; UV intensity; 1.67 mW cm−2; Humidity: 0–20000 ppmv; Temperature: 15–35 °C.	Toluene, ethyl benzene, xylene: 2–10 ppmv.	Residence time: 8.5–20 s.	Varies with residence time. Toluene: 71.4–98.4%; Ethyl benzene: 54.4–94.8%; Xylene: 56.4–95.1%.	No.
Tsoukleris et al. (2007)	P25 TiO2 nanoparticle paste is an effective photocatalyst for removal of VOCs.	Laboratory; Single-pass test. PCO reactor: packed-bed reactor; Primary UV wavelength: 350 nm; UV intensity; maximum 0.0715 mW cm−2; Humidity: 60%; Temperature: 25 ± 2 °C.	Toluene: 1170.4–1321.7 μg m−3; Benzene: 701–775.2 μg m−3; Xylene: 0–45.4 μg m−3.	–	Toluene: 86%; Benzene 100%; Xylene: 100% in 3 min.	No.
Wisthaler et al. (2007)	The concentration of most organic pollutants present in aircraft cabin was efficiently reduced by PCO and adsorption air cleaning. PCO had intermediate products of acetaldehyde and formaldehyde by degrading ethanol.	Laboratory; Chamber test. PCO reactor and adsorptive prefilter. Humidity: 21 ± 2%; Temperature: 23.2 ± 0.1 °C.	Ethanol, monoterpenes, acetaldehyde, acetone, formaldehyde, methanol and isoprene: less than 200 ppbv.	Face velocity: 50.3–64.3 cm s−1.	–	Yes. PCO produces un acceptably high levels of acetaldehyde and formaldehyde.
Yang et al. (2007)	Vacuum ultraviolet (VUV) improved the conversion of formaldehyde markedly on the basis of TiO2/UV. The hybrid process was more economical than the TiO2/UV process.	Laboratory; Single-pass test. PCO reactor: annular type; Primary UV wavelength: 254 nm; UV intensity; 0.25–2.8 mW cm−2; Humidity: 30–80%.	Formaldehyde: 150–500 ppbv.	Face velocity: 0.3–0.94 m s−1.	Varies with airflow rate, UV intensity etc. 20% (0.94 m s−1)–62% (0.3 m s−1).	Yes. Ozone was produced by VUV.
Yu et al. (2006a)	The rate constants of PCO for toluene, xylene and mesitylene ranged from 1.22 to 4.00 μmol m−1 s−1 and were proportional to kOH (VOC-OH· rate constants).	Laboratory; Single-pass test. PCO reactor: plate type; Primary UV wavelength: 254 nm; UV intensity; 0.25–2.8 mW cm−2; Humidity: dry-humid; Temperature: 25 ± 0.5 °C.	n-Hexane, Iso-butanol, Toluene, p-Xylene, m-Xylene, Mesitylene: 0.1–9.0 ppmv.	Airflow rate: 200–1200 mL min−1.	–	Yes. The photodegradation of VOC results in CO2 and residual intermediates at different rates.
Yu et al. (2006b)	Both toluene and formaldehyde can be removed using photocatalytic filters in a simulated HVAC system. The VOC removal efficiency increases with RH.	Laboratory; Single-pass test. A mechanical filter coated with P25 and 2 commercial photocatalytic filters; Primary UV wavelength: 254 nm; UV intensity: average 0.0489 mW cm−2; Total air changer rate: 0.5–1.5 h−1; Temperature: average 25.65; RH: 30–70%.	Toluene, formaldehyde: 2 ppmv.	Face velocity: 177–532 m h−1 (0.05–0.15 m s−1).	CADRs varies with face velocity. At a face velocity of 177 m h−1, Toluene: 0.0466–0.0840 m3 h−1; Formaldehyde: 0.0732–0.0947 m3 h−1.	Yes. The ozone concentration is less than 25 ppbv in the test chamber.
Zhang et al. (2003a)	Introduction of O3 to TiO2/UV systems can enhance the degradation of toluene.	Laboratory; Single-pass test. Primary UV wavelength: 254 nm and 365 nm; Humidity: 20–60%; Temperature: 20–22 °C.	Toluene: 1.0–20 ppmv.	Airflow rate: 1.0–5.0 L min−1. Residence time: 17.3–86.4 s.	Varied from less than 5% for ozone alone to > 80% with ozone and UV-TiO2.	No.
Zhang et al. (2003b)	A simple PCO reactor model was developed to analyze the VOC removal performance in PCO reactors with experimental validation.	Laboratory; Chamber test. PCO reactor: annular type. Primary UV wavelength: 254 nm; Humidity: 37–50%; Temperature: 24–26 °C.	Toluene: 7.66 ppmv; Formaldehyde 1.77–1.85 ppmv.	25 m3 h−1.	Toluene: 0.4%; Formaldehyde: 5%.	No.
Zhang et al. (2007)	An analysis of the UVPCO behavior for two compounds (toluene and benzene) was provided. The component impact factor between binary compounds was defined to describe the influence of one compound on the reaction coefficient of another compound.	Laboratory; Single-pass test. PCO reactor: plate type; Primary UV wavelength: 254 nm; UV intensity: 0.56 mW cm−2; Humidity: ∼40%; Temperature: 25–27 °C.	Toluene: 4.48 mg m−3; Benzene 1.82–4.08 mg m−3.	Airflow rate: 3 L min−1. Face velocity: 1 m s−1.	Varies with reaction conditions.	No.
Other catalytic oxidation
Ellis and Tometz (1972)	The room temperature catalytic efficiency in decomposing ozone of 35 materials was investigated. Activated carbon/charcoal removes most O3 under room temperature, while zeolites, glass wool, and several others remove less.	Laboratory; Single-pass test. Humidity: 15–20%; Temperature: 23 ± 2 °C.	O3: 45–1000 ppbv.	Airflow rate: 0.14–1.17 ft3 min−1 (3.96–33.1 L min−1).	0–100% depending on the materials. The efficiency of all materials tested degraded with time.	No.
Kwong et al. (2008a)	Catalytic oxidation rate of toluene was enhanced over 3 types of adsorbent: NaX, NaY and MCM-41 when ozone was injected (6 ppmv).	Laboratory; Single-pass test. Humidity: 0% (dry condition), 50%; Temperature: 25 °C.	O3: 6 ± 0.1 ppmv in regular tests and 24 ± 0.5 ppmv was also used in some cases; Toluene: 1.5 ± 0.03 ppmv.	Airflow rate: 0.21 m3 h−1; Residence time: 0.13 s. Face velocity: 1.54 m s−1.	For toluene under dry conditions without ozone, at a 200 mm bed length: 73% (MCM-41), 53% (NaY) and 45% (NaX). Dry condition with ozone, at a 200 mm bed length: 90% (MCM-41), 78% (NaY) and 71.6% (NaX); Over 98% inlet ozone was consumed.	Yes. Some aldehyde species were generated, such as formaldehyde, acetaldehyde and benzaldehyde.
Kwong et al. (2008b)	Use of zeolite and MCM-41 catalytic sorbents, during ozonation, can reduce by-product formation while removing toluene.	Laboratory test/single-pass. Humidity: 0%, 50%; Temperature: 23–25 °C.	Toluene: 0.3–4.5 ppmv; O3: 0–80 ppmv.	Airflow rate: 0.39–0.12 m3 h−1. Residence time: 0.07–0.23 s.	50% toluene via adsorption and another 20–40% was decomposed by ozonation.	Yes. Acetaldehyde, formaldehyde, benzaldehyde and formic acid are the main by-products.

By-products (e.g., formaldehyde, acetaldehyde, formic and acetic acid, etc.) were generated during the PCO decomposition of various pollutants. Combining TiO₂ with adsorption material (activated carbon etc.) may lower the generation of the by-products. The effect of multiple indoor pollutants on UVPCO performance needs further investigation and should not be neglected. Most of the researches on UVPCO are laboratory studies.