Rapid assessment of the photocatalytic activity in construction materials: Pros and cons of reductive inks and oxidative fl uorescence probes versus standardized NOx testing

The photocatalytic activity of a wide variety of photocatalytic construction materials with different intrinsic properties, mainly microstructure and TiO2 crystallographic phases, were obtained using alternative methods (Rz and NBT-reductive inks and TA-oxidative fluorescence probe) and the NOx removal standard ISO 221971:2007. The Rz-ink test was shown to be unsuitable for some of the materials due to spontaneous colour change in the dye upon contact with these (here denominated, reactive samples), in the absence of radiation. No such shortcomings were observed for either NBT-ink or TA-probe. Comparison of the three set of results showed a reasonably high correlation among the activities determined by the different methods, despite the wide variation in the characteristics of the photocatalytic materials tested, underlying reaction mechanisms and the physics and chemistry of the inter-phase contact (ink/probe/contaminant-substrate). The findings attest to the possible relationship between the rates of oxidative and reductive reactions in semiconductor photocatalysis. These alternative methods may be regarded as a new 'cheap and simple' approach to photocatalytic activity measurements.