the hydroxyl radicals predominantly exist (Turchi and Ollis, 1990). Indeed, there has

been a reported increase in destruction efficiency when the reactants are brought into

close contact with the surface of activated TiO2 particles. This increase could be due to

the greater concentration of the substrate around the catalyst surface due to adsorption

interactions, for it has been proven that the highest efficiency of photocatalytic

degradation of organic pollutants is observed at relatively high concentrations of reagents

(Emeline et al., 2000).


 H20


OH' .


Figure 2-4. Simplified diagram of a catalyst support.

 2.4 Catalyst Supports

 Adsorbents researched as possible catalyst supports include activated carbon,

silica gel, mordenite, alumina, zeolite, and aerogels (Takeda et al., 1995; Torimoto et al.,

1996; Yoneyama and Torimoto, 2000). The highest decomposition rate for propyzamide

(herbicide) was obtained with supports having medium adsorption strengths, such as

silica and mordenite (Torimoto et al., 1996). With supports of too low zeolitee) and too

high (activated carbon) adsorption strengths, no significant effect on the rate of

destruction was found.


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