Unexpectedly High Levels of Organic Compounds Released by Indoor Photocatalytic Paints

Abstract

Photocatalytic paints based on titanium dioxide (TiO) nanoparticles represent a promising treatment technology for cleaning the air at our dwellings. A few studies have shown that instead of elimination of harmful indoor air pollutants the production of carbonyl compounds occurs from the photocatalytic paints. Herein, we report unexpectedly high concentrations of volatile organic compounds (VOCs) released upon irradiation of photocatalytic paints which are meant to clean the air at our dwellings. The concentrations of the VOCs were measured continuously and online by PTR-ToF-MS (Proton Transfer Reaction-Time of Flight-Mass Spectrometry) connected to a well-established flow tube photoreactor. The PTR-ToF-MS analysis revealed the presence of 52 ions in the mass range between 20 and 490 amu, among which 43 have been identified. In particular very high emission rates were estimated of two relevant indoor air pollutants, formaldehyde and acetaldehyde as 355 μg h and 257 μg h for 1 m, respectively. We suggest a detailed reaction mechanism responsible for the production of these harmful indoor air pollutants (formaldehyde and acetaldehyde, among the others). The hydroxyl radicals (OH) formed upon activation of TiO, react with the organic constituent (butyl acrylate and vinyl acetate) of the paint binder lead to generation of an important number of organic compounds. We demonstrate that the TiO quantity and the organic content of the binder is of paramount importance with respect to the formation of VOCs, which should be considered for future optimization of this air remediation technology based on TiO nanoparticles.

Application of a mineral binder to reduce VOC emissions from indoor photocatalytic paints

“…The flow tube reactor used in this work is similar to the one used in previous studies of VOC emissions from photocatalytic paints 17,23 . Here, the horizontal flow tube reactor experiments involved time-resolved detection of the selected VOCs emitted from the solid paint media using the High Sensitivity-Proton Transfer Reaction-Mass Spectrometry (HS-PTR-MS) (IONICON Analytik, Austria) in the presence of UV irradiation or under "dark" conditions, that is without UV light irradiation.…”

“…The schematic diagram of the apparatus has been published elsewhere 23,24 and is described in detail in Figure S1. Some experimental details that are particularly relevant to this work are given below.…”

“…To date, a majority of small scale laboratory and larger scale chamber 12 experimental studies have concentrated their research effort to study the photocatalytic consumption and/or degradation of NOx [13][14][15][16][17] (NOx = NO + NO 2 ) and VOC [18][19][20][21] pollutants on selected photocatalytic materials. Fewer studies have been reported in the literature that researched VOC emissions from the photocatalytic paint materials themselves 9,[21][22][23] . However, it has been proposed that the binder molecules present within the paint may react with reactive radical species, namely, OH radicals, present on nano-TiO 2 surfaces in the presence of light to emit VOCs, therefore, act as a point source of atmospheric pollutants 9,[21][22][23] .…”

“…Fewer studies have been reported in the literature that researched VOC emissions from the photocatalytic paint materials themselves 9,[21][22][23] . However, it has been proposed that the binder molecules present within the paint may react with reactive radical species, namely, OH radicals, present on nano-TiO 2 surfaces in the presence of light to emit VOCs, therefore, act as a point source of atmospheric pollutants 9,[21][22][23] . This is troublesome since practically all paint materials contain various amounts of organic binder matter or a solvent, that is, all paints include some sort and quantity of a binder because it is the binder itself that keeps the pigment in place after the paint dries.…”

“…This is troublesome since practically all paint materials contain various amounts of organic binder matter or a solvent, that is, all paints include some sort and quantity of a binder because it is the binder itself that keeps the pigment in place after the paint dries. In fact, an important increase in acetaldehyde and formaldehyde emissions from photocatalytic paints under UV irradiation have been observed 21,23 . Other compounds that include ethylacrolein, pentanal, 1hydroxy-butanone and hexanal have also been detected 21 .…”