Environ. Sci. Technol. 2007, 41, 6573-6578
Photocatalytic Surface Reactions on Indoor Wall Paint ,†,‡
T. SALTHAMMER* AND F. FUHRMANN Material Analysis and Indoor Chemistry, Fraunhofer Wilhelm-Klauditz Institute, Bienroder Weg 54 E, 38108 Braunschweig, Germany, and University of Applied Sciences, Salzdahlumer Strasse 46/48, 38102 Wolfenbuettel, Germany
†
TABLE 1. Summary of Conducted Experiments with T ) 23 °C, r.h. ) 50%, n ) 0.4 h-1 and Resultsa paint
COremoval
NO2removal
HCHOremoval
VOCremoval
secondary products
P1 P2 P3 R1
not tested not tested
++ ++ ++ +
not tested - (+)b -
-
++ ++ ++ +
a ++: strong effect, +: weak effect, -: no effect. b An effect could be observed under static conditions (no air exchange) at 1.5 mg/m-3 initial concentration.
The reduction of indoor air pollutants by air cleaning systems has received considerable interest, and a number of techniques are now available. So far, the method of photocatalysis was mainly applied by use of titanium dioxide (TiO2) in flow reactors under UV light of high intensity. Nowadays, indoor wall paints are equipped with modified TiO2 to work as a catalyst under indoor daylight or artificial light. In chamber experiments carried out under indoor related conditions it was shown that the method works for nitrogen dioxide with air exchange and for formaldehyde without air exchange at high concentrations. In further experiments with volatile organic compounds (VOCs), a small effect was found for terpenoids with high kOH rate constants. For other VOCs and carbon monoxide there was no degradation at all or the surface acted as a reversible sink. Secondary emissions from the reaction of paint constituents were observed on exposure to light. From the results it is concluded that recipes of photocatalytic wall paints need to be optimized for better efficiency under indoor conditions.
Moreover, wall paints are equipped with organic binders like acrylic blends, vinyl acetate, styrene, and unsaturated fatty acids, which are also sensitive to photoreactions (15). Therefore, undesired byproducts might be formed during irradiation. Finally, the lighting is different in the indoor environment. In a typical office room, the intensity of artificial light is about 0.1-0.2 mW cm-2 (VIS). Photocatalytic reactors use light intensities from 1 to 10 mW cm-2 UV-A (2, 4) or UV-B (8). We have studied the photocatalytic effect of wall paints in 1 m3 test chambers under indoor related conditions with and without air exchange using artificial daylight. Nitrogen dioxide (NO2), carbon monoxide (CO), formaldehyde, and different VOCs were used as target compounds. In popular scientific literature and advertisement, indoor photocatalysis is often recommended for air cleaning, and it is stated that pollutants are mineralized to carbon dioxide and water. However, a consequent scientific study of the applicability is still missing. Moreover, mineralization to carbon dioxide and water is only possible for molecules containing no other elements as carbon, hydrogen, and oxygen.
Experimental Section Introduction Photocatalysis has gained considerable attention for the removal of indoor air pollutants, and the photodegradation of miscellaneous substances in a photoractor is described in numerous publications (1-10). Titanium dioxide (TiO2) in the anatase modification is a semiconductor with an energy gap of about 3.2 eV (387 nm) between valence band and conduction band and is widely used as the catalytic substrate. The key step in photocatalysis is the formation of holeelectron pairs on irradiation with UV-light. In the presence of ambient air the photogenerated hole reacts with water via the oxidative pathway to give hydroxyl radicals (‚OH), while the electron reacts with molecular oxygen via the reductive pathway to produce the superoxide radical ion (‚O2-). To improve the photocatalytic efficiency and to make the method applicable for indoor use under the influence of indoor daylight or artificial light, the TiO2 surfaces can be doped with foreign atoms or ions (11-14). Nowadays, such modified TiO2-catalysts are used for the manufacture of building products like wall paints with the purpose of photocatalytic removal of air pollutants under indoor conditions. In this case the effective rate coefficient at the active surface is a function of the molecular gas kinetic properties, the diffusive motion, and the air exchange rate. * Corresponding author phone: +49-531-2155-350; fax: +49-5312155-905; e-mail:
[email protected]. † Fraunhofer Wilhelm-Klauditz Institute. ‡ University of Applied Sciences. 10.1021/es070057m CCC: $37.00 Published on Web 08/17/2007
2007 American Chemical Society
Three different products of commercially available waterbased paint (P1, P2, P3) and one nonphotocatalytic reference paint (R1) were investigated. Rutil titanium dioxide (TiO2) was used as the white pigment throughout. The percentage of anatase TiO2-catalyst and the chemical composition of binders and additives were unknown. All paints were applied on inert plates made of window glass. The application method was based on the specifications of the manufacturers (ca. 350 g m-2). The thickness of the dried layer (100-250 µm) was determined by optical microscopy. The experiments were carried out in 1 m3 glass chambers in accordance with DIN EN ISO 16000-9 (16). Sunlight simulating ULTRA VITALUX (Osram) lamps were placed outside the chamber to avoid temperature effects. The light intensity at the sample surface was 0.3-0.4 mW cm-2 (VIS), 0.1-0.2 mW cm-2 (UV-A), and
