Investigating the Potential Effects of Limestone and Bitumen Substrates on Photocatalytic NOx Degradation

NOx emissions commonly emitted by vehicles, pose environmental and health challenges worldwide. Photocatalytic asphalt pavements, used in urban settings, are in close contact with these emissions. In this study, the contribution and role of asphalt mix components—stone and bitumen—were analyzed in the degradation process. The effectiveness of TiO2 coatings on limestone-bitumen composites of varying ratios (100%, 75%, 50%, 25%, and 0%) was assessed using X-ray diffraction analysis (XRD), Confocal Laser Scanning Microscopy (CLSM), Fourier-transform infrared spectroscopy (FTIR), and the modified ISO 22197-1:2016 standard for NO removal. XRD verified the presence of calcite in limestone. CLSM revealed surface modifications and coating morphology, FTIR verified successful TiO2 PF2 coating deposition and NOx degradation quantified the NOx degradation (%), NO degradation (%) and NO2 formation (%) during photocatalytic activity. It was evident that samples with a higher ratio of stone-to-bitumen exhibited an elevated NOx degradation, reaching up to 29.11% for NOx, 43.79% for NO, and 13.96% for NO2 formation. Conversely, samples with a lower stone-to-bitumen ratio recorded values as low as 8.93% for NOx degradation (%), 10.30% for NO degradation (%), and 0.95% for NO2 formation (%). These outcomes firmly establish the inhibitory effect of the bitumen substrate on NOx and NO degradation but a positive effect on NO2 formation.