Hydrogen bond interactions at the TiO 2 surface: Their contribution to the pH dependent photo-catalytic degradation of p-nitrophenol

Abstract

We have obtained p K a values of p-nitrophenol–TiO 2 by measuring the adsorption equilibrium constants of p-nitrophenol (PNP) on the TiO 2 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 TiO 2 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 TiO 2 surface is mainly positively charged and at pH 8 when it is negatively charged. With these and other adsorption equilibrium constants and the PNP p K a value in solution, thermodynamic cycles are established in order to obtain the PNP p K a when it is adsorbed on positively charged, neutral and negatively charged TiO 2 surfaces. From these p K a values useful information on the PNP–TiO 2 interaction is readily obtained. For instance, the PNP nitro group interacts with the TiO 2 surface via a hydrogen bond, arising from the complex of water molecules with the Ti 4+ ions on its surface. The weaker the hydrogen bond donor, the stronger the oxygen nitro group basicity. Therefore, p K a changes on the phenolic hydroxyl group result from these interactions. Linear free energy correlations, maximum PNP adsorption capacity values ( Q L) and FTIR ATR, spectrum support this proposal. A k obs vs. pH degradation profile of p-nitrophenol is also provided.