Development and Performance Evaluation of Composite Modified Nano-TiO2 for Permeable Asphalt Mixtures: Focus on Exhaust Degradation and Dispersion Properties

Rapid urbanization has intensified challenges regarding urban waterlogging and vehicle exhaust pollution. While permeable asphalt mixtures mitigate waterlogging and nano-TiO2 offers photocatalytic exhaust degradation capabilities, the direct application of nano-TiO2 is hindered by agglomeration and low photocatalytic efficiency. This study developed a composite modified nano-TiO2 via metal ion doping and support treatment to enhance its performance in asphalt pavements. Specifically, nano-TiO2 was doped with Fe3+, Ag+, and La3+ via the sol–gel method, and supported on activated carbon (AC) or Al2O3. The exhaust degradation performance was evaluated using a custom-built system, while dispersion properties were assessed via fluorescence microscopy and UV-Vis spectrophotometry. Furthermore, X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy were conducted to investigate the microstructural mechanisms underlying the doping modification and support treatment. Photocatalytic permeable asphalt mixtures were prepared by partially replacing mineral powder with the composite modified nano-TiO2 to validate exhaust degradation and pavement performance. The results indicated that metal doping substituted Ti4+ in the lattice, inducing defects and reducing crystallite size to boost photocatalytic activity. The optimal doping concentrations are determined to be 1.0% for Fe3+, 1.5% for Ag+, and 1.0% for La3+. Among these, Fe3+-doped nano-TiO2 at 1.0% content exhibits superior exhaust degradation, achieving 46.7% efficiency for hydrocarbons (HC) and 33.5% for nitrogen oxides (NO). Regarding dispersion, while AC performs better at low support content, Al2O3 at 40% content provides superior dispersion properties by increasing active sites and surface hydroxyl groups. For photocatalytic permeable asphalt mixtures, replacing 40–50% of mineral filler with the composite modifier is recommended. The optimized mixture demonstrates superior exhaust degradation performance while maintaining the required high-temperature stability, low-temperature cracking resistance, water stability, and fatigue life. Specifically, compared to the control group, these indicators for the mixture with 50% of the mineral filler replaced by the composite modifier increases by 7.0%, 12.5%, 13.4%, and 22.9%, respectively. This study presents a viable technical solution for developing multifunctional asphalt mixtures with photocatalytic functionality as the core innovation and mechanical performance as the application baseline.