In the present study, the effect of humid air on the photocatalytic decomposition of ethylene was examined using TiO₂ immobilized on different catalytic supports (glass, silica, and activated carbon (AC)). Prior to use, the immobilized photocatalysts were either exposed to humid air for one day to adsorb H₂O molecules or dried by heating to desorb them. To increase the rate of photocatalytic decomposition, it is fundamental to increase the reactant concentration in the neighborhood of TiO₂. The AC support initially decreased the ethylene concentration by adsorption under both humid and dry conditions, but the AC-immobilized photocatalyst exhibited a very low activity for ethylene decomposition. This is probably due to deposition of iron released from AC on TiO₂, by which the photocatalytic reaction was strongly inhibited. Under the humid air condition, the silica support exhibited a low performance for ethylene adsorption and the silica-immobilized photocatalyst decomposed ethylene slowly, whereas the dried silica-immobilized photocatalyst improved the performance for ethylene adsorption, thereby increasing dramatically the photocatalytic activity for ethylene decomposition. The glass support hardly adsorbed ethylene under both dry and humid conditions, but the glass-immobilized photocatalyst constantly decomposed ethylene rapidly without being affected by humid air. These experimental results suggest that the photocatalytic decomposition of ethylene is strongly affected by humid air. To solve this problem, we attempted to co-deposit Pd and Pt on the glass-immobilized TiO₂ and also heat the immobilized photocatalysts, and consequently found that these treatments increase the rate of ethylene decomposition. The result for heating treatment is considered due to desorption of H₂O molecules on the immobilized photocatalyst by heating. In conclusion, the use of a glass support having a low performance for H₂O adsorption is appropriate for the photocatalytic decomposition of ethylene and it is also essential to remove H₂O molecules adsorbed on the immobilized photocatalyst.

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