Nm Of Tio <sub> 2 </ Sub> Photocatalytic Polymerization Lead To Ways To Study

Electron-hole pairs are generated when semiconductor nanoparticles are excited by irradiation and redox reactions will take place on the surface of the particles. Photocatalytic polymerization extends the function of semiconductor nanoparticles. The photocatalytic polymerization is ostensibly the reverse reaction process from photocatalytic degradation. This seeming conflict contains a profound theoretical problem that there are requirements for a consummate theoretical system to picture the two different process uniformly. The reaction of photocatalytic polymerization provides a new point of view to probe into the theoretical details of surface chemistry of semiconductor nanoparticles that are not yet completely resolved. So it is of significant scientific value.When investigating the reaction of photocatalytic polymerization of vinyl acetate (VAc) over TiO₂ nanoparticles, our research group discovered a new initiation mechanism that photocatalytic decarboxylation of acetic acid occurred over TiO₂ nanoparticles and the so-generated methyl radicals initiated the polymerization of VAc. Literature search indicates that decarboxylation reaction of acetic acid is called Photo-Kolbe reaction which is first presented by Kraeutler and Bard in 1977. They discovered that the acetate solution would produce ethane on TiO₂ electrode under UV illumination and figured out that methyl radicals generated from decarboxylation of acetic acid coupled with each other to produce ethane. In this study, we pay more attention to the problems that are of great research value which include: (1) whether other long-chain carboxylic acids could initiate the reaction of polymerization? (2) the decarboxylation of acetic acid only produces methyl radical and whether Photo-Kolbe reaction is universal for the long-chain.carboxylic acids which have more complex structures. (3) since the discovery of Photo-Kolbe reaction, the reaction process of decarboxylation has received much attention. It correlates with the surface chemical reaction of TiO₂ nanoparticles and the process is still not clear. This study tries to work from the view of polymerization in order to obtain some valuable information about the reaction process and we have got the following results:(1) We have achieved the polymerization of VAc by propionic acid, n-butyric acid, isobutyric acid and pivalic acid. ¹³C labeled n-butyric acid is employed and the results of ¹³C-NMR show that n-butyric acid decarboxylates to produce n-propyl radical which initiates the polymerization. According to these results, Photo-Kolbe reaction is extended to long-chain carboxylic acids and a universal reaction mechanism is established as follows:RCOO^- + TiO₂ (?) R^*+ CO₂ (1)R^*+Mâ†'RM^* (2)RM^*+(n-1)Mâ†'R(M)_n (3)(2) When comparing the results of the polymerization reactions initiated by n-butyric acid and sodium butyrate, we find out that carboxylate ion principally takes part in Photo-Kolbe reaction and we present the decarboxylation mechanism as reaction formula (1). Through the adjustment of the pH value of reaction system, the reaction rate and conversion of monomer change apparently along with the change of property of TiO₂ surface charge and interaction of carboxylic acids with TiO₂.(3) The interaction of acetic acid or n-butyric acid with the surface of TiO₂ is investigated by ATR-FTIR. Results show that both acetic acid and n-butyric acid adsorb onto TiO₂ surface through a bridging mode structure.(4) When irradiated by UV, CO₂ is produced, which confirms the decarboxylation reaction. When H₂O₂ is added into acetic acid solution, CO₂ is also produced under UV irradiation. Considering hydroxyl radical will be generated when H₂O₂ decomposes by UV, we presume that the photo generated hydroxyl radicals on TiO₂ are related to decarboxylation reaction.(5) The experiments show that benzoic acid could not initiate polymerization, but produces salicylic acid. And when coumarin, a scavenger of hydroxyl radicals, is added into the reaction system that contains carboxylic acid, the primary reaction is restrained. According to these results and phenomena, in this study, we consider that hydroxyl radical plays a relatively important role in the decarboxylation process of Photo-Kolbe reaction and may even be the direct reactive initiator of the decarboxylation reaction.