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.
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Notes
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.
The pKa constants at 25 °C and low ionic strength are 1.25 and 4.27 Smith and Martell (1979). Critical stability constants. New York, Plenum Press.
The Langmuir–Freundlich isotherm, i.e., \(y = \frac{{N \times K \times x^{(1 - C)} }}{{1 + K \times x^{(1 - C)} }}\), contains an experimental adjustable parameter C.
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Acknowledgments
We want to thank Prof. Thorsten Gesing for the XRD measurements and Dr. Bernhard Dringenberg for the BET measurements. We are especially grateful to Jenny Schneider from the Leibniz Universitaet Hannover and Luis Granone from the Universidad Nacional de Mar del Plata for very useful discussions. The authors gratefully acknowledge Ministerio de Ciencia, Tecnología e Innovación Productiva (MINCYT), Universidad Nacional de Mar del Plata (UNMDP) and Bildungsministerium für Bildung und Forschung (BMBF) for financial support: Project Numbers PICT 2683, EXA 701/14 and 033RC1012C-HyCats, respectively. Special thanks to MINCYT (Arg.) and BMBF (Ger.) for the financial support of the bilateral project AL1209. CBM is a member of the research staff of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).
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Ivanova, I., Mendive, C.B. & Bahnemann, D. The role of nanoparticulate agglomerates in TiO2 photocatalysis: degradation of oxalic acid. J Nanopart Res 18, 187 (2016). https://doi.org/10.1007/s11051-016-3495-x
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DOI: https://doi.org/10.1007/s11051-016-3495-x