Unveiling the effects of uva photocatalysis on soot particles deposited over lime mortars manufactured with titanium dioxide

Abstract

The adhesion of soot on building surfaces darkens their colour and leads to undesirable black crusts, which are one of the most serious problems on the conservation of cultural built heritage [1]. As a preventive strategy, self-cleaning systems based on the use of titanium dioxide coatings have been employed on building materials for degrading organic compounds deposited on building surfaces, improving their durability and performance [2]. In this study, the self-cleaning effect of TiO2-treated mortars coated with soot deposits has been appraised under laboratory conditions. The mortar samples were manufactured using lime putty and two different doses of TiO2 (2.5% and 5%), which were then coated with diesel engine soot and irradiated with ultraviolet A (UVA) illumination for 30 days [3]. The photocatalytic activity was evaluated by visual inspection, scanning electron microscopy (SEM) and colour spectrophotometry. Changes in the chemical composition of the soot particles were assessed by analytical pyrolysis (Py-GC/MS) and Solid state 13C NMR spectroscopy. SEM and colour spectrophotometry revealed that the lime-based mortars manufactured with TiO2 promoted a self-cleaning effect after UVA irradiation. Py-GC/MS at 500 ºC showed that the UVA irradiation caused the cracking of the polycyclic aromatic structures and of the n-alkyl compounds of soot particles deposited on the surface of the mortars. In addition, the pyrochromatograms revealed that TiO2 acted as catalytic agent of such transformations promoting the self-cleaning of the soot-stained mortars [3]. The 13C NMR spectra confirmed the results achieved by analytical pyrolysis, the cracking of the aromatic structures from the soot material and the amplification of such alteration in the presence of TiO2. We can conclude that even without a total photocatalytic oxidation of the soot, the irradiation with UVA in the presence of TiO2 is a promising proxy to clean lime mortars affected by soot deposition. References [1] G. Montana, L. Randazzo, I.A. Oddo, M. Valenza. The growth of ``black crusts¿¿ on calcareous building stones in Palermo (Sicily): a first appraisal of anthropogenic and natural sulphur sources. Environ. Geol. 56 (2008) 367-380. DOI: 10.1007/s00254-007-1175-y [2] E. Quagliarini, F. Bondioli, G. Battista Goffredo, C. Cordoni, P. Munafò, Self-cleaning and de-polluting stone surfaces: TiO2 nanoparticles for limestone. Constr. Build. Mater. 37 (2012) 51-57. [3] J.M. de la Rosa, A.Z. Miller, J.S. Pozo-Antonio, J.A. González-Pérez, N.T. Jiménez-Morillo, A. Dionisio. Assessing the effects of UVA photocatalysis on soot-coated mortars manufactured with titanium dioxide. Sci. Tot. Environ. (2017). In press.