Nanotechnology in manufacturing paints for eco-efficient buildings - Nanotechnology in Eco-efficient Construction - page 348

and photocatalytic glass surfaces (Ohko et al. , 2009; Huang et al. , 2010; Laufs
et al. , 2010).
In 1998, Negishi et al. (1998) proposed a mechanism by which NO is
photochemically oxidized into NO 2 , and NO 2 is either further oxidized to
HNO 3 on a generally slower step or photolytically reverts into the forma-
tion of NO and hydroxyl radicals:

( )

+

hTiOh ν ,

2 vb

⎯→ ⎯⎯⎯ ←⎯

OH i

NO NO ⎯→ ⎯⎯ HNO

⎯⎯⎯⎯

2 3

( )

−

hTiOe ν ,

2 cb

Such a mechanism was further confi rmed by two different kinds of experi-
ments. Thus, Dalton et al. (2002) evaluated the adsorption of nitric acid and
nitrate ions on the surface of the photocatalyst. Ohko et al. (2009), on the
other hand, confi rmed the formation of a mixture of NO and NO 2 in the
gaseous phase when the photocatalysis of either pure NO or NO 2 was
tested.

Photooxidation of volatile organic compounds

It is well known that volatile organic compounds (VOCs) are photocatalyti-
cally oxidized by TiO 2 when exposed to UV radiation. The potential for
cleaning indoor air has been demonstrated for pollutants such as formal-
dehyde, acetaldehyde, acetone, benzene or toluene in photochemical reac-
tors using UV radiation and anatase TiO 2 powdered fi lms and building
materials (Obee and Brown, 1995; Stevens et al. , 1998; Pichat et al. , 2000;
Strini et al. , 2005; Everaert and Baeyens, 2004; Maggos et al. , 2007). Further-
more, doping of TiO 2 photocatalyst with N has proved valuable for the
degradation of acetaldehyde, 2-propanol, acetone, toluene and ethylene
under visible irradiation (Asahi et al. , 2001; Ihara et al. , 2003; Miyauchi
et al. , 2004; Irokawa et al. , 2006). As we will show in Section 15.4.2, the use
of photocatalytic paints is more complex than the use of powdered fi lms
due to the potential formation of secondary emissions from photocatalytic
paint constituents.
One of the parameters that critically affects the performance of photo-
catalytic paints is the interaction between the paint components and the
photocatalyst. Thus, when incorporated into the paint matrix, the photo-
catalyst loses around 90% of its overall photocatalytical effi ciency (Aguia
et al. , 2011) which, in the case of the degradation of volatile organic com-
pounds, can be critical.
Salthammer and Fuhrmann (2007) have recently evaluated the effi ciency
of different photocatalytic indoor paints in terms of their degradation of
organic and inorganic air pollutants. In general, all the evaluated photocata-
lytic paints showed the same pattern. In this sense, even if NO 2 abatement
was satisfactory when irradiating the photocatalytic paints with a typical

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