Synergistic modification of TiO2 with Fe2O3, Fe3O4, and CaO to boost photocatalytic NOx oxidation, selectivity, and storage efficiency under UV light

Abstract

This study explores the oxidation and storage of NO_x gases (i.e., NO₂ and NO) on the surfaces of P25/Fe₂O₃/CaO and P25/Fe₃O₄/CaO photocatalysts. A photocatalytic flow reactor system is utilized, incorporating naturally occurring environmental parameters such as humidity, UV light intensity, gas pressure, temperature, and NO_x concentration (in ppm), thereby simulating realistic conditions for NO_x abatement and storage. The binary photocatalysts, P25/Fe₂O₃ and P25/Fe₃O₄, are synthesized via simple co-precipitation and hydrothermal methods, respectively, and subsequently combined with CaO through physical mixing to form the ternary oxide systems P25/Fe₂O₃/CaO and P25/Fe₃O₄/CaO, respectively. In this ternary structure, TiO₂ (P25) functions as the primary photocatalytically active component. The incorporation of Fe₂O₃ and Fe₃O₄ (narrow-bandgap semiconductors) into the TiO₂ matrix facilitates the efficient capture of photogenerated electrons, thereby suppressing electron–hole recombination and extending charge carrier lifetimes, which collectively enhance photocatalytic activity. The P25/Fe₂O₃ heterojunction also enhances NO_x to NO₃⁻ conversion and predominantly suppresses NO₂ release, thus outperforming the benchmark P25 catalyst. Additionally, the presence of iron oxides promotes oxygen reduction reactions and facilitates superoxide-mediated NO oxidation, thus inhibiting the reverse conversion of stored nitrate species to NO_x and thereby improving selectivity. The incorporated CaO serves as an NO_x storage medium, effectively trapping oxidized nitrogen species. Out of all synthesized catalysts, P25/Fe₂O₃/CaO proved to be an excellent photocatalyst compared with the benchmark P25 and other binary and ternary oxide catalysts with an outstanding NO conversion of 47%, an NO_x storage selectivity of 98% and a DeNO_x index value of 0.50.