Gas phase adsorption of volatile organic compounds onto titanium dioxide photocatalysts

Adsorption of pollutants onto photocatalyst surface plays a critical role in the efficacy of photocatalytic oxidation (PCO) technology for air purification applications. In this article, gas phase adsorption of toluene and methyl ethyl ketone (MEK) was studied using a small-scale single-pass continuous flow system. Equilibrium adsorption isotherms for toluene and MEK on six commercially available TiO2 photocatalysts (P25, PC500, PC105, UV100, PC-S7, and S5-300A) coated on nickel foam substrates were determined. The selected photocatalysts cover a wide range of important photocatalytic properties including surface area, porosity, acidity, and population of surface hydroxyl groups. The TiO2 coated filters were characterized by SEM technique to evaluate the quality of coating and homogeneity of titania powder on the support material. Furthermore, a thorough Fourier transform infrared spectroscopy (FTIR) was performed to investigate the nature of surface hydroxyl (OH) groups on various photocatalysts. Using the adsorption isotherm data, Langmuir model adsorption coefficients for toluene and MEK on various photocatalysts and humidity levels have been determined. Additionally, the impacts of challenge compound polarity and air relative humidity (RH) on the adsorption efficiency and Langmuir constants were assessed. The adsorption capacity for MEK (a highly polar compound) was much higher than that of toluene (a non-polar compound) on all photocatalysts, regardless of the humidity content. The results vividly showed that the relative humidity negatively affects the adsorption capacity for both MEK and toluene, however to different extents due to the differences in water solubility and polarity. FTIR characterization of MEK/toluene saturated titania samples revealed that isolated OH groups (both terminal (Ti-OH) and bridged (Ti-OH-Ti)) serve as highly active adsorption sites for volatile organic compound (VOC) molecules.