Hydrogen bond interactions at the TiO2 surface: Their contribution to the pH dependent photo-catalytic degradation of p-nitrophenol

We have obtained pKa values of p-nitrophenol–TiO2 by measuring the adsorption equilibrium constants of p-nitrophenol (PNP) on the TiO2 surface at different pH values. These values have been obtained from Langmuir isotherms and from a plot of 1/rate vs. 1/[PNP]o obtained during TiO2 catalyzed solar light photo-degradation of PNP. Two limit equilibrium constants are readily obtained depending on the solution pH: at pH 5 at which the TiO2 surface is mainly positively charged and at pH 8 when it is negatively charged. With these and other adsorption equilibrium constants and the PNP pKa value in solution, thermodynamic cycles are established in order to obtain the PNP pKa when it is adsorbed on positively charged, neutral and negatively charged TiO2 surfaces. From these pKa values useful information on the PNP–TiO2 interaction is readily obtained. For instance, the PNP nitro group interacts with the TiO2 surface via a hydrogen bond, arising from the complex of water molecules with the Ti4+ ions on its surface. The weaker the hydrogen bond donor, the stronger the oxygen nitro group basicity. Therefore, pKa changes on the phenolic hydroxyl group result from these interactions. Linear free energy correlations, maximum PNP adsorption capacity values (QL) and FTIR ATR, spectrum support this proposal. A kobs vs. pH degradation profile of p-nitrophenol is also provided.

With adsorption equilibrium constants and the pKa value of PNP in solution, thermodynamic cycles are established in order to obtain the pKa of PNP when it is adsorbed on positively charged, neutral and negatively charged TiO2 surfaces. From these pKa values useful information on the PNP–TiO2 interaction is readily obtained. The kobs vs. pH profile predicts an efficient photo-catalytic PNP degradation at pH > 8.Figure optionsDownload as PowerPoint slide