Diverting the use of reduced TiO2 from a carrier of electron-hole pairs to a reservoir of Brönsted-Lewis acidic pairs for sustainable, non-optical H2O2 homolysis

Abstract

Defective TiO₂ bears intra-bandgap states (INTRA) and a valence band (VB) that host electrons and holes, respectively, upon photo-excitation, enabling ^•OH/O₂^•-/¹O₂ formation via semi-conducting pathways. However, the energetic window of INTRA/VB is often narrow or mismatches with that of semi-conducting pathways, restricting ^•OH/O₂^•-/¹O₂ generation. Here, TiO₂ was calcined at 300 °C (O300) and reduced at 300-600 °C (R300-R600) to tune quantities (N)/strengths (E) of Brönsted acidic (BA^--H⁺) and Lewis acidic sites with coordination numbers of 5 (LA_I) or ≤4 (LA_II). The catalysts revealed distinct N/E values with hierarchies of N_LAIILAI<BA--H+ or E_BA--H+LAI~E_LAII. Non-photonic H₂O₂ homolysis (H₂O₂→2^•OH) proceeded via BA^--H⁺/LA_II, yet, hinged on BA^--H⁺ with E_BA--H+ and N_BA--H+ dictating energy barrier (E_BARRIER) and lumped collision frequency (k_APP’), respectively, as corroborated by H₂O₂ consumption rate (-r_H2O2) law. -r_H2O2 values of O300/R300/R600 were challenging-to-assess, thus being assessed using their initial acetaminophen degradation rates in a per-BA^--H⁺ site (-r_{ACETAMINOPHEN, 0, CO2}) or in a per-gram (-r_{ACETAMINOPHEN, 0}). R600 showed the highest E_BA--H+ and smallest N_BA--H+, thus yielding the lowest E_BARRIER and lowest k_APP’, respectively, where the former overrode the latter to achieve the highest -r_{ACETAMINOPHEN, 0, CO2} for R600. R600 displayed the highest resistance to deposit poisonous oligomer/^•OOH on BA^--H⁺/LA_II or poisonous O₂ on LA_I due to its smallest N_BA--H+/highest E_BA--H+/highest E_LAII for oligomer/^•OOH and the smallest N_LAI/highest E_LAI for O₂, as also substantiated by density functional theory calculations. Consequently, -r_{ACETAMINOPHEN, 0} values of R600 were higher than those of O300/R300 throughout acetaminophen decomposition recycle runs, whereas R600 was moderately recovered upon regeneration. -r_{POLLUTANT, 0, CO2} values of R600 were 2-23-fold and 3-10-fold higher than those of O300/R300 and ZrO₂/UiO-66, respectively, in disintegration of diverse aqueous pollutants (analgesic/endocrine disruptor/pesticide/antibiotic).