Photolysis of pyrene at the solid/air interface of unactivated
and activated silica gel proceeds slowly to give mainly
oxidized pyrene products. We have identified 1-hydroxypyrene,
1,6-pyrenedione, and 1,8-pyrenedione among the main
reaction products. The remaining minor products show
molecular weights and spectral properties consistent with
oxygenated pyrenes. Furthermore, small amounts of 1,1‘-bipyrene dimer are also formed at higher surface coverages
(2 × 10-5 mol/g). When photolysis is carried out at 5 ×
10-5 mol/g pyrene, photodegradation rate drops sharply and
pyrene loss becomes insignificant. No significant change
in the product distribution is observed when the photolysis
is carried out on unactivated or activated silica. Photodegradation rate is slightly faster on activated silica compared
to unactivated silica. Mechanistic studies indicate that
the precursor to photoproduct formation is pyrene cation
radical which is postulated to be formed by electron transfer
from pyrene excited state to oxygen (type I) or by
photoionization of pyrene. The cation radical reacts with
physisorbed water on silica to give the observed oxidation
products.
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