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Abstract
The simultaneous bimodal study of the photocatalytic oxalic acid degradation by aqueous TiO2 suspensions revealed that particular systems possess the capacity to protect a certain amount of oxalic acid from oxidation, thus hindering, to some extent, the photocatalytic reaction. While measurements of the oxalic acid concentration in the bulk liquid phase indicated full photocatalytic degradation; in situ pH-stat measurements allowed the quantification of the amount of oxalic acid remaining in the part of the nanoparticulate agglomerates where light could apparently not access. An explanation for this phenomenon takes into account the possibility of the formation of TiO2 agglomerates in which these molecules are hidden from the effect of the light, thus being protected from photocatalytic degradation. Studies of different TiO2 materials with different particle sizes allowed a deeper exploration of this phenomenon. In addition, because this property of encapsulating pollutant molecules by photocatalytic systems is found to be a reversible phenomenon, P25 appears to be more convenient and advantageous as compared to the use of large surface area photocatalysts.
Graphical Abstract
Fig.: Deaggregation of TiO2 particle agglomerates upon UV illumination.
The four TiO2 materials were characterized by Rietveld refined powder X-ray diffraction (XRD) for the identification of the constituting polymorph and the calculation of the mean crystalline domain which is usually used as the particle size. The measurements were carried out on a Bruker D4 Endeavour diffractometer (Cu Ka radiation, Ni filtered) at the Institute of Mineralogy of the Leibniz Universitaet Hannover.
Solid oxalic acid in both forms, anhydrous and dihydrated, reveals the disposition of the molecules interacting via the carboxylic groups on both sides directly, or mediated by two water molecules, forming chains along the C–C bond Jayatilaka and Grimwood (2001), Grimwood and Jayatilaka (2001).
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