Recombination rate of photogenerated charges versus surface area: Opposing effects of TiO2 sintering temperature on photocatalytic removal of phenol, anisole, and pyridine in water

The sintering temperature is an important parameter in many TiO2 synthesis processes. Increasing this temperature (1) reduces the number of defect sites and, accordingly, the rate of electron–hole recombination, which improves photocatalytic rates and (2) decreases particle surface area, which a priori may be expected to reduce photocatalytic rates. We have studied the photocatalytic removal in water of the title probe molecules over a series of Millennium Chemicals TiO2 prepared at increasing sintering temperatures in the order: PC500, PC105, PC50, and PC10. For phenol and anisole, reaction rates increase with increasing sintering temperature. For pyridine, the trend is opposite for PC500, PC105, and PC50, but PC10 outperforms all three. Therefore, the net effect of sintering temperature depends on the pollutant to be removed even for structurally closely related pollutants and is likely the result of an essentially surface-based reaction mechanism for pyridine versus a near-surface solution-phase mechanism for phenol and anisole. Hypotheses are presented regarding the higher activity of PC10 even for pyridine removal and the differences between phenol and anisole removal rates.