49.
Photocatalytic oxidation of methylpyridine isomers (2-methylpyridine, 3-methylpyridine, and 4-methylpyridine) was investigated
in a mixed solution of acetonitrile and water or acetonitrile using various kinds of TiO
2 powders as photocatalysts. The main products from methylpyridine isomers were pyridinecarboxaldehyde isomers (2-pyridinecarboxaldehyde,
3-pyridinecarboxaldehyde, and 4-pyridinecarboxaldehyde). Rutile large TiO
2 particles showed the highest level of activity for oxidation of 2-methylpyridine probably because band bending was necessary
for the oxidation of 2-methylpyridine. On the other hand, a fine particle having an anatase or rutile phase showed a higher
level of activity than large TiO
2 particles for oxidation of 3-methylpyridine. A rutile fine particle showed the highest level of activity for the reaction.
It was found that pure rutile or pure anatase particles were inactive for oxidation of 4-mathylpyridine. If the particles
are not extremely small, pure rutile and pure anatase powders show fairly high levels of activity, and those containing both
anatase and rutile phases show the highest level of activity. The activity of pure rutile particles was also enhanced by physically
mixing them with a small amount of small anatase particles, which were inactive for this reaction. These results can be explained
by the synergism between rutile and anatase particles. All of these reactions effectively proceeded even under anaerobic conditions.
Photocatalytic reduction of methylpyridine isomers concomitantly proceeded on TiO
2 particles under the conditions used. These results suggest that the activities of TiO
2 photocatalysts for oxidation of methylpyridine isomers are dominated by the oxidation potential of alkylpiridine and band
bending of TiO
2 particles.
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