Photocatalytic oxidation over TiO2

[pirupiru]

Hi all,

This forum looks amazing ! I'm trying to get my head around photocatalytic oxidation but I don't have a strong background in chemistry. I was specifically looking into the decomposition of phenyl isothiocyanate and trinitrotoluene using photocatalytic oxidation over TiO2 (assuming complete mineralization) and I was trying to write down the reactions to arrive to the final product. Anyone can give me any help with even a minimum explanation for the reactions?

Thank you very much !

 

[TeethWhitener]

This is quite specific. Are you going off papers in the literature?

In general, TiO₂ as a photocatalyst operates via the following mechanism:
1) A photon excites an electron from the TiO₂ valence band into the conduction band, leaving behind a hole in the valence band.
2) The hole, which is a very strong oxidizing agent, pulls an electron from a substrate (either a solvent like water or directly from a reagent).
3) In the case of water, this results in the formation of hydroxyl radicals, themselves powerful oxidants, which can further react with other reagents in solution. In the case of direct reaction between the hole and the reagent, it's reaction specific.

I don't know about the specific TiO₂-photocatalyzed degradation of the two compounds in question, but generally speaking, toluenes (including TNT) are most easily oxidized at their methyl group. Uncontrolled, this would yield a substituted benzoic acid. Isothiocyanates can be oxidized to isocyanates given a strong enough oxidizer. I don't know if either of these are specifically what happens in the case of your question, but maybe it'll get you started.

 

[pirupiru]

Hi TeethWithener,

I'm just reading a lot and sometimes, to try understand better how everything works, I try to solve some exercise and example I find around literature. Your answer is really valuable and helpful, following your guidelines is this something that might look good?

 

[TeethWhitener]

The first equation doesn’t make much sense. The diradical product has the same number of electrons as the starting reagent, so the titanium dioxide hasn’t actually done anything. The other two reagents are a little less far-fetched. In this case, the photogenerated hole from the TiO₂ would react with the water (or more likely a hydroxide) to give the hydroxyl radical.