Assessment of mechanisms for enhanced performance of Yb/Er/titania photocatalysts for organic degradation: Role of rare earth elements in the titania phase

The effect of doping titania with rare earth erbium and ytterbium ions on photocatalytic degradation of a real pollutant (phenol) and a model pollutant was studied under simulated solar radiation and using light of specific frequencies in the ultraviolet, green, red, and infrared regions via LEDs. The goal of this study was to quantify the effect of these dopants on photocatalytic degradation rates and critically assess the phenomena (upconversion, adsorption, recombination kinetics, etc) responsible for improved performance upon rare earth ion doping of titania. A hydrothermal process was modified to synthesize pure anatase and rare earth ion doped TiO2. The doped catalysts contained 2% Er and 0, 10, 15, or 20% Yb, on an elemental basis. These photocatalysts were characterized for structure, composition, surface area, particle size, bandgap, and band edge positions. The results indicated that the rare earth elements substituted in the titania phases, which were the crystalline anatase and amorphous titania phases. Aqueous-phase photocatalytic degradation rate constants for both organics under a variety of light irradiation conditions were quantified via slurry batch reactor studies in the liquid phase using pseudo first-order kinetics. On reactor volume and catalyst mass bases (same trends due to same catalyst loading), the 2% Er/TiO2 catalyst performed ∼3× better (compared to ∼10× on surface area basis) and the 2% Er 10%Yb/TiO2 catalyst performed ∼1.7× (∼3.5× better on surface area basis) than the pure titania sample for phenol degradation under simulated solar irradiation. Though there were minor differences because the model pollutant (Rose Bengal) adsorbed during dark equilibration and phenol did not, similar enhancements were observed for photodegradation of Rose Bengal. Photodegradation was further studied using high-intensity LED irradiation at different narrow wavelengths (405, 530, 660, and 940 nm) which matched the photon absorption energies related to upconversion of the doped samples. Phenol conversion was observed only for the UV (405 nm) source. Similar observations were made for Rose Bengal degradation. A conclusion was reached that upconversion did not contribution to the photocatalytic performance under simulated solar insolation. Combined with the lack of phenol adsorption during dark equilibration for any of the samples, photon energy of the excitation source relative to the band gap and differences in defect chemistry on key kinetic steps are the only plausible explanations for the enhanced performance of these rare earth doped samples compared to pure titania.