Assessment of mechanisms for enhanced performance of TiO2/YAG:Yb+3,Er+3 composite photocatalysts for organic degradation

Abstract

Wastewater remediation is a key component to a sustainable water management program and the use of photo-activated catalysis has emerged as a promising advanced oxidative process for the decomposition of organic contaminants to innocuous products. Although the process is feasible using UV photons, only ∼5% of solar insolation is comprised of UV photons and few advances have been made to improve upon commercial TiO2 (i.e., P25). Here, composite catalysts containing both an upconversion phase (Yb+3/Er+3/Y-Al garnet or YAG host) and a photocatalyst (TiO2) are assessed for enhancing the photocatalytic activity of TiO2 for organic degradation using simulated insolation. Samples of the upconverting phosphor (YAG host) were prepared utilizing the Pechini method with varying molar concentration of Yb+3 ions [0%, 10%, 15%, or 20%] and a constant concentration of Er+3 ions [2%]. The composite materials were then obtained via calcination of the various YAG:Yb+3,Er+3 samples with titania at molar concentrations of [10%, 15%, or 20%]. Structures and properties of the upconverting phosphors and composite materials were verified with characterization analyses including X-ray diffraction, diffuse reflectance spectroscopy, transmission electron microscopy, and photoluminescence spectroscopy. For rose bengal degradation in a batch slurry reaction using a simulated ‘daylight’ spectrum as the irradiation source, results indicated that a mixed phased, Y-Al oxide doped with 2mol% Er+3 and 15mol% Yb+3 with 10mol% doped YAG/balance TiO2 composite yielded a 42% higher rate constant and apparent quantum yield than TiO2 alone. Additional kinetic studies with portions of the solar spectrum (i.e., UV bulbs or UV or IR LEDs) were conducted to further probe the effect of photon energies and suggested that rate and apparent quantum yield enhancements were largely attributed to increased amounts of organic adsorption and close proximity between phases, which coincided with the presence of the monoclinic phase (not cubic) of the Y-Al oxide structure. Upconversion and decreased charge carrier recombination rates were deemed to provide minimal contribution to these enhancements.