ISO 22197-3: 2011 – Test method for air-purification performance of semiconducting photocatalytic materials. Part 3: Removal of toluene.
In the photocatalytic oxidation of toluene the major initial product appears to be benzaldehyde which is then subsequently oxidised to benzoic acid and eventually to carbon dioxide and water, provided the reaction intermediates do not adsorb so strongly to the surface of the titania as to render it photo-inactive. Typical test conditions are given below:
|Sample size||5 cm x 10 cm and typically 5 mm thick|
|Suitable sample type||Construction materials in flat sheet, board or plate shape; structured filter materials including honeycomb-form; woven and non-woven fabrics; plastic or paper materials if they contain ceramic microcrystals and composites.|
|Unsuitable sample type||Powder or granular photocatalytic materials|
|Sample pretreatment||16-24 hr UV irradiation at ≥ 1.5 mW/cm2|
|Test conditions||0.5 L min-1 of 1 ppmv Toluene in air adjusted to 50% RH; time: 3 hours; irradiance: 1 mW/cm2; temperature: 25 °C|
|Analytical method||Toluene: GC-FID|
|Information returned||Amount of toluene removed (mmol) and % toluene removed|
Toluene is a widely used chemical feedstock and industrial solvent. As a solvent, it is used in paints, paint thinners, silicone sealants, printing inks, glues, resins and disinfectants. It is also used as an octane booster in gasoline fuels. In industry it is also used in the manufacture of: benzoic acid, benzaldehyde, explosives, dyes and many other organic compounds. Toluene is toxic, and is one of the VOCs associated with sick building syndrome.
Many papers have been published on the removal of toluene via its photocatalytic mineralisation [1-7]. In the absence of water vapour the photoreaction quickly stops due to the inhibition of the hydroxyl regeneration process and the accumulation of reaction products, such as benzoic acid. In the presence of water vapour this deactivation process can be much slower, depending on how readily the reaction intermediates, such as benzoates, are adsorbed. For example Schiavello et al reported that whereas Merck TiO2 exhibited a stable photocatalytic activity, Degussa P25 continuously deactivated upon illumination . The following reactions summarise the photocatalytic oxidation of toluene :
A typical data set generated, i.e. toluene removed, is illustrated below. The hatched area B is proportional to the amount of toluene removed. The key points are the start of contact with the acetaldehyde feed (t=0), UV lights on (↓) and UV lights off (↑).
 V. Augugliaro, S. Coluccia, V. Lodo, L. Marchese, G. Martra, L. Parmisano and M. Schiavelllo, Appl. Catal., B, 20 (1999) 15.
 A.J. Maira, J.M. Coronado, V. Augugliaro, K.L. Yeung, J.C. Conesa and J. Soria, J. Catal., 202 (2001) 413.
 H. Einaga, S. Futamura and T. Ibusuki, Appl. Catal., B, 38 (2002) 215.
 M. C. Blount and J.L. Falconer, Appl. Catal., B, 39 (2002) 39.
 G. Marci, M. Addamo, V. Augugliaro, S. Coluccia, E. Garcia-Lopez, V. Loddo, G. Martra, L. Parmisano and M. Schiavello, J. Photochem. Photobiol. A, 160 (2003) 105.
 J.M. Coronado and J. Soria, Catal. Today, 123 (2007) 37.
 O. Debono, F. Thevenet, P. Gravejat, V. Hequet, C. Raillard, L. Lecoq and N. Locoge, Appl. Catal., B, 106 (2011) 600.
For the best evaluation of air purification performance of photocatalytic materials, it is recommended by ISO to combine one or more of the Air Purification test methods.
These methods are designed to obtain the air-purification performance of photocatalytic materials by exposing a test piece to model polluted air, in a photoreactor, under illumination by ultraviolet (UV) light. The concentration(s) of the analyte(s) of interest are monitored regularly and the concentration versus time data profile(s) are then processed so as to provide one or more measures of the efficiency of the test piece to remove photocatalytically the air-pollutant under test.