Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 62

Ochuma et al., 2007; Yang et al., 2007). Combination of ceramic membrane and TiO 2 is
extremely attractive to the water purification field because of mechanical properties,
chemical inertia, long working life and thermal stability. Moreover, the presence of
metals in support may also improve the activity of the photocatalyst by acting as co-
catalyst (Pirkanniemi and Sillanpaa, 2002).
One major advantage of fixed bed reactors is the possibility of continuous
operation without having to separate the ultrafine particles from the finished stream.
From catalysis point of view, two factors are essential to ensure the effectiveness of the
fixed bed reactors. The first one is good adherence on supporting materials, and the
second is that there must have no effect on the photoactivity of the catalyst during the
film coating process. In general, wash or sol-gel coating of TiO 2 thin film is the easiest
and cheapest method. However, the stability and durability of such films are common
drawbacks. Other advanced techniques, such as physical and chemical vapor deposition
(CVD), plasma-CVD, can offer durable supported films (Gogate and Pandit, 2004). The
activity of the catalyst obviously can be affected by multiple factors, such as band gap
modification through the coating, particle size and specific surface area of the nano-TiO 2
particles coated.
Although most of practical drawbacks of slurry reactors can be avoided when a
fixed bed reactor system is used, other problems also rise. As many researchers have
reported, the photocatalytic performance of the fixed bed reactor, measured in terms of
quantum efficiency, is 56 times lower than the slurry photocatalytic reactor with
similar design (Pozzo et al., 2000). Several factors contribute to this lower performance
of anchored catalyst: (a) low surface area to volume ratios, which limits the mass
transfer rate of species from the solution phase to the thin film surface; (b) significant
photon extinction, due to light absorption on and scattering by the support materials; and
(c) aggregation of TiO 2 particles (i.e., surface clumping) during film fixation which
reduces the available specific surface area of the nano-TiO 2 particles (Pozzo et al., 2000).
Many configurations of thin film photocatalytic reactors have been designed for
water purification in order to minimize the aforementioned influence, including rotating
disk photocatalytic reactors (Dionysiou et al., 2000; Hamill et al., 2001), corrugated
plate photocatalytic reactors (Zhang et al., 2004), plug flow tube photocatalytic reactors
(Ling et al., 2004; Biard et al., 2007a, b), spinning disk reactors (Yatmaz et al., 2001),
and Carberry type photocatalytic reactors (Figure 3.10) (Cernigoj et al., 2007).
One high efficiency configuration is the Carberry type photocatalytic reactor
(Figure 3.10), designed by the Stangar group (Cernigoj et al., 2007). Up to 12 glass
slides coated with TiO 2 are fastened on the central piece with holders at both ends.
Besides having the advantage of conventional thin film photocatalytic reactors, this
simple Carberry type reactor offers several unique advantages: (a) an increase in the