Enhanced photocatalytic performance and moisture tolerance of nano-sized Me/TiO₂–zeolite Y (Me=Au, Pd) for gaseous toluene removal: activity and mechanistic investigation

This research developed a new type of photocatalyst for the gas phase photocatalytic oxidation of toluene under highly humid conditions to overcome the disadvantages of the traditional TiO₂ photocatalyst. By doping noble metal nanoparticles (Au, Pd) on the TiO₂ and adding the zeolite Y as an adsorbent, the adsorption-enhanced nanosized Me/TiO₂–zeolite Y (Me=Au, Pd) shown significant improvements in both photocatalytic activity and stability for toluene photocatalytic oxidation in a continuous annular reactor. The catalysts were analyzed using advanced techniques for understanding the structure and morphology of the materials such as X-ray diffraction (XRD), scanning electro-microscopy (SEM), transmission electro-microscopy (TEM) and N₂ adsorption at low temperature (77 K) for surface area analysis. The photocatalytic activity of the catalyst has been investigated under various reaction conditions such as different initial toluene concentrations in the range of 300–1300 ppmv, water concentration in the range 5.91–23.65 mg/L at two reaction temperatures (39 °C and 50 °C). The maximum photocatalytic efficiency of toluene removal was stable at 3.1 mmol g_TiO2⁻¹ h⁻¹ under highly humid condition. Among four kinetic models that have been tested, the Langmuir–Hinshelwood kinetic model in which toluene and water vapor adsorb on separated sites (\( -r=k\times \frac{K_A{C}_{Tol}}{1+{K}_A{C}_{Tol}}\times \frac{K_W{C}_W}{1+{K}_W{C}_W}\Big) \)was the most fitted to the experimental data and a reaction mechanism was discussed and proposed for the photocatalytic oxidation of toluene under humid condition using the nanosized Me/TiO₂–zeolite Y catalyst. The parameters for the rate equation at 39 °C were k = 2.847 × 10⁻³ mmol L⁻¹ g_{_TiO2}⁻¹ h⁻¹, K_A = 0.429 × 10⁻² ppmv⁻¹, and K_w = 0.194 × 10⁻³ ppmv⁻¹.