Impact of Photocatalytic Building Products on Indoor Air Quality under
Visible light Conditions
V. Bartolomei 1,2, S. Delaby 1, D. Boutry 2, M. Nicolas 1
1
Centre Scientifique et Technique du Bâtiment (CSTB), 24 rue Joseph Fourier, 38400, SaintMartin d’Hères, France
2
Commissariat à l’Energie Atomique et aux énergies renouvelables (CEA), 17 rue des Martyrs,
38054, Grenoble, France
Corresponding author: [email protected]
SUMMARY
Nowadays, more and more functionalized building products declaring indoor air purifying
properties are available on the market. However, most available research studies documented
photocatalytic performance of such products under UV light, even though available light
indoors is mainly visible light. This study focused on the characterization of photocatalytic
efficiency of building materials under visible light. The removal of three Volatile Organic
Compounds (VOCs) has been measured using six different building products claiming air
purifying properties selected from a panel. The experiments were performed in a stainless steel
chamber with an air exchange rate of 1 h-1. Formaldehyde, toluene and limonene were selected
as typical indoor pollutants and were generated at representative indoor concentrations (i.e.
between 15 and 50 µg m-3). The observed pollutants conversion fluctuates between 0 up to 35
% under visible light indoor conditions. In return, by-products formation has been observed,
traducing an incomplete photocatalysis process.
PRACTICAL IMPLICATIONS
This work shows that efficiency of photocatalytic products to remove pollution under visible
light is reduced in comparison with other studies considering UV light experiments.
KEY WORDS
photocatalysis efficiency, chemical reactivity, VOCs, safety, ageing process.
1 INTRODUCTION
The indoor air quality (IAQ) issue is more and more considered as it is one of human health
major issues over the world, as highlighted by World Health Organization (2014). VOCs are
frequently associated with indoor pollution (Shinohara et al., 2009). In order to improve IAQ,
many air remediation systems have been suggested such as photocatalysis. The photo-catalyst
the most frequently used is Titanium dioxide (TiO2), under nanoparticles form. It can be added
or embedded in building products, and is usually activated by UV light (λ<390nm) and also by
visible light, when doped (Mo et al., 2009). The theoretical basis of photocatalysis is to
transform air pollution into water vapor and carbon dioxide. However, the reaction chain can
sometimes be incomplete and form by-products which can be more harmful than their
precursors, like e.g. formaldehyde (Debono et al., 2013; Sleiman et al., 2009). Many building
products with air purifying self-declared properties are already available on the French market.
Their performances can be evaluated using existing standards, based on UV light. Indoor light
is mainly available in the visible spectra (400 < λ < 700 nm) (Bartolomei et al., 2014) and does
not active or only partially activates the catalyst (Salthammer and Fuhrmann, 2007). The
purpose of this study is to evaluate the effectiveness of building products available on the
market with announced pollutant reduction properties in respect with indoor conditions (i.e.
temperature, relative humidity and light available).
2 MATERIAL AND METHODS
Chamber set up:
The experimental set up is detailed on Figure 1.
Figure 1: Experimental set up. (F; mass flowmeter)
The chamber used is only made of stainless steel and has a total volume of 360 L. Two fans are
placed at the back of the chamber to obtain a more realistic air flow and homogeneous pollutant
concentration. The total inlet flow of 6 L min-1 is a mix of humidified air charged with three
selected pollutants: formaldehyde, limonene and toluene. Selected VOCs are generated by a
permeameter device (Calibrage Pul 200) at typical indoor concentrations i.e. 15 to 50 µg m-3.
The humidified air is obtained by a bubbler system adjusted with two flowmeters. A
hygrometer “hydrolog NT2” (Rotronic) with a “hygroclip SC04” probe is permanently
connected to the test chamber to monitor temperature and relative humidity. All experiments
were performed at 50±5% Relative humidity (RH) and 25±2 °C. The chamber is equipped with
two LEDs (MasterLED tube performance 21W Phillips) lightning system to ensure a typical
indoor condition of visible light without excessive heating of the setup. Still, higher
temperatures (23 to 27°C) were observed during the experiments due to heating by LEDs lamps.
The integrated spectral irradiance delivered by the lamps on the surface was 12.2 W m-2 (CAS
140 CT, Instrument System). Figure 2 show the light distribution delivered by the Leds. This
light intensity was also measured with a luxmeter between 4000 and 4800 lux.
Figure 2: Relative light distribution of Blue LED lamps placed in the test chamber
The experiments:
Different building products with self-declared air purifying properties have been selected from
the French market. Their photocatalytic activity has been determined according to a specific
experimental protocol based on the degradation of a colored compound. Six products presenting
the most relevant results have been selected: one paint (referred to as paint A), two wood
coatings (referred to as GM0.85 and GM2.2), two wall coatings (referred to as SM2 and SM5)
and ceramic tiles. They have been applied on stainless steel plates (except for ceramic tiles).
All prepared samples have been stored during 21 days at classic indoor conditions (Temperature
23°C and Relative humidity 50 %) in order to reach stabilize state.
The number related to samples above, as SM5, exhibit the nanoTiO2 content (w/w %)
NB: The SM0 is a wall coating with the same composition as SM2 and SM5, but without
nanoparticles. Additional experiments have been performed with this seventh building products
and are presented figure 3, 4 and 5 (without ageing process).
Prepared test specimens have been divided into three groups representing different simulated
ageing processes:
1. Reference (no ageing)
2. Ozone: Exposure to 100 ppb of ozone during 14 days
3. UV-A: Exposure to UV-A during 14 days
As it is well known that formaldehyde could be a by-product of photocatalysis, every chamber
experiment has been performed with two mixtures of pollutant. The first is composed of toluene
and limonene only (Figure 5). And the second one with toluene, limonene and formaldehyde
added (Figures 3, 4, 6 and 7). The concentrations were the same in both experiments for toluene
and limonene.
Sampling procedure:
Air samples of carbonyls compounds were collected on dinitrophenyl hydrazine (DNPH)
cartridges and others VOCs on Tenax-TA tubes. All tubes and cartridge were linked to Brooks
SLA mass flow controller and a pump (KNF lab). This air flow was measured with a calibrated
flowmeter (4140, TSI) certified before every sampling process. Air samples were taken at the
inlet of our experimental setup where selected VOCs generated concentrations can be controlled
and at the outlet of the test chamber.
Every sample is collected after a minimum of 6 hour of stabilization to reach steady state
between pollutant generation and the building products exposure in the test chamber.
Carbonyls (as formaldehyde or acetaldehyde) and main VOCs (as terpenes and toluene) were
analyzed according to ISO 16000-3 and ISO 16000-6 respectively.
The conversion rate is obtained by applying the following equation (1), where C0 and Cout are
inlet and outlet concentrations of pollutant, respectively.
Conversion rate (%)=
C0 -Cout
C0
× 100
(1)
Cout has been corrected from the blank chamber concentration performed before to start each
experiment.
Negative values shown on following figures have to be considered as a concentration more
elevated at the outlet of the chamber than at the inlet.
Blank experiments
Blank experiments have been performed under dark conditions with every material and the mix
of pollutant. Figure 3 shows conversion rate of pollutant mix for reference building products.
Formaldehyde is the most converted of the three pollutants in the case of wall coatings, even in
the case of SM0 (without nanoTiO2). That reveals an important adsorption effect of the wall
coating, probably due to the porosity of the material. However, toluene and limonene
conversion rate are very low for SM0 and SM2, and non-existent on SM5.
Figure 3. Pollutant removal (%) in dark conditions for all reference tested building products.
Paint A presents the most homogenous results, with an average conversion of 20%. The ceramic
tiles have no effect on the pollutants. Finally, the wood coatings behavior is closed to the wall
coating considering limonene and toluene, but with almost no effect on formaldehyde.
3-4 RESULTS AND DISCUSSION
Reference building products:
The result of the photocatalytic degradation of the three pollutants exposed to the six building
products (and the wall coating reference SM0) under visible light is shown in figure 4.
Figure 4. Pollutant removal (%) under visible light conditions for all reference tested building
products.
Figure 4 shows that the reduction of formaldehyde under visible light can reach 45% for the
two wall coatings containing nanoTiO2 (SM2 and SM5). Formaldehyde is in contrast less
converted (or more probably less adsorbed) under light conditions than in dark ones. The paint
do no removed anymore as well than in dark condition the limonene and the toluene. The
formaldehyde is no more converted instead of dark condition, and even small emission have
been measured. The ceramic tiles has still no effect on the any pollutants. And as for the paint,
the formaldehyde is more released than in dark condition (Figure 3). Wood coatings have a
distinct behavior, with a better conversion rate of almost 20 % for toluene and limonene,
although no effect on formaldehyde. This results could be associated to photocatalytic effect
for limonene and toluene due to those better conversion rates.
By-products formed during photocatalytic conversion are essentially formaldehyde (case of
paint and ceramic), acetaldehyde, 2-ethyl hexanol (for wall coatings mainly) and benzaldehyde.
Those species are essentially found with visible light conditions.
Figure 5 shows results obtained for the second mix of pollutant composed of toluene and
limonene only. Results obtain are in excellent agreement for the all the building products, and
the pollutants behavior is the same with and without formaldehyde, it could traduce no
competitive effect between formaldehyde and the two other pollutants. Small conversion rate
improvement can be noted for the SM2 and SM5.The ceramic has still no effect on the
pollutants. The wood coatings and paint results are slightly lower than on the Figure 4.
Figure 5. Pollutant removal (%) under visible light conditions for all reference tested building
products with the mix of toluene and limonene only.
Aged building products:
The pollutant conversion has been also tested for products aged by exposure to O3 to observe
the effects often neglected once the product set. Figure 6 reveals that ozone has little reduction
effect on the removal efficiency of the building products. The limonene and toluene are less
converted, in particular the limonene in the case of the wood coatings. No conclusive fact on
formaldehyde could be done, but it is less converted in the case of the wall coatings, with a very
important decrease of more than 20 % for the SM5.
Figure 6. Pollutant removal (%) under visible light conditions for all tested building products
aged with O3 (100ppb).
It can also be added that less by-products are released as there is a very reduce photocatalysis
conversion after this ageing process.
Figure 7 show results obtain for the second ageing process with UV A on the conversion of
pollutants. As a matter of fact, it reduces even more the conversion process than O3 ageing.
Conversions are extremely low, with almost no effect on pollutants. Mean values reach 10 %
of remediation, and 25% in best cases (SM2 for formaldehyde). Formaldehyde is release as byproduct for every building product, excepted for SM2 and SM5, like with the O3 ageing.
Figure 7. Pollutant removal (%) under visible light conditions for all tested building products
aged with UV A.
Conversion rates obtained for aged materials are lower than for reference ones. Those results
traduce a reduction of the photocatalytic effectiveness over the building product lifetime.
4 CONCLUSIONS
The air purifying properties of building products tested concerning the removal of three typical
indoor air VOCs fluctuate from 0 to 35 % under visible light conditions for reference materials.
Those low results are consistent with the fact that available indoor light may not be sufficient
to initiate a total phocatalyst excitement. Moreover, by-products as formaldehyde have been
found for paint or ceramic mainly, which emphasize the incomplete photocatalysis process.
More efforts are necessary to improve performances of those building products for a real and
significant reduction of pollution indoors.
ACKNOWLEDGMENT
The authors gratefully acknowledge CSTB and ANR for funding the IMP AIR, « Impact des
Matériaux Photocatalytiques sur la qualité de l’air intérieure” project within the French program
PRIMEQUAL.
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