Photoenhanced degradation of veratraldehyde upon the heterogeneous ozone reactions
Abstract
Light-induced heterogeneous reactions between gas-phase ozone and veratraldehyde adsorbed on
silica particles were performed. At an ozone mixing ratio of 250 ppb, the loss of veratraldehyde
largely increased from 1.81 106 s1 in the dark to 2.54 105 s1 upon exposure to simulated
sunlight (l 4 300 nm). The observed rates of degradation exhibited linear dependence with the
ozone in the dark ozonolysis experiments which change in the non-linear Langmuir-Hinshelwood
dependence in the experiments with simultaneous ozone and light exposure of the coated particles.
When the coated silica particles were exposed only to simulated sunlight in absence of ozone the
loss of veratraldehyde was about three times higher i.e. 5.97 106 s1 in comparison to the
ozonolysis experiment under dark conditions at 250 ppb ozone mixing ratio, 1.81 106 s1.
These results clearly show that the most important loss of veratraldehyde occurs under
simultaneous ozone and light exposure of the coated silica particles. The main identified
product in the heterogeneous reactions between gaseous ozone and adsorbed veratraldehyde
under dark conditions and in presence of light was veratric acid.
Carbon yields of veratric acid were calculated and the obtained results indicated that at low
ozone mixing ratio (250 ppb) the carbon yield obtained under dark conditions is 70% whereas
the carbon yield obtained in the experiments with simultaneous ozone and light exposure of the
coated particles is 40%. In both cases the carbon yield of veratric acid exponentially decayed leading
to the plateau (B35% of carbon yield) at an ozone mixing ratio of 6 ppm. Two reaction products
i.e. 3-hydroxy-4-methoxybenzoic acid and 4-hydroxy-3-methoxybenzoic acid were identified
(confirmed with the standards) only in the experiments performed under simultaneous
ozonolysis and light irradiation of the particles.