Deploying radical inter-transition from [rad]OH to supported NO3[rad] on Mono-dentate NO3--modified ZrO2 to sustain fragmentation of aqueous contaminants

Abstract

To advance aqueous pollutant degradation using OH, H2O2 (OH carrier) should be cleaved homolytically on a non-reducible metal oxide (ZrO2) rather than heterolytically on a reducible counterpart (MnO2), given the merits of H2O2 homolysis such as improved OH productivity, unnecessity to recover H2O2 activators (Lewis acidic metals; LA) via electron reduction, and minute LA leaching. This paper presents a methodology to exploit H2O2 homolysis with the rate-determining step of endothermic OH desorption, thereby proposing the coupling of H2O2 homolysis and exothermic radical inter-conversion of OH → NO3SUP (supported NO3) to create the overall OH → NO3SUP route. ZrO2 was modified with NO3− functionalities (NO3SUP precursors) to form ZrO2-N, where NO3-SUP species were located close to Zr4+ (LA) and Brönsted acidic -OH (BA) sites, whose acidic strengths must be elevated to facilitate OH desorption for reducing the energy barrier (EBARRIER) of the overall OH → NO3SUP route. NO3-SUP species were bound to the ZrO2 surface via mono-dentate configuration only, thereby avoiding LA loss (rate in a per-gram↑), escalating LA/BA strengths (EBARRIER↓), and imparting two free oxygens available to OH → NO3SUP (rate in a per-site↑). Moreover, NO3SUP species extract electrons from contaminants via electron transfer to recover NO3-SUP species used for recurring OH → NO3SUP, while sustaining pollutant fragmentation efficiency by circumventing surface poison accumulation. Hence, NO3SUP on ZrO2-N revealed higher efficiencies in fragmenting bisphenol A or recycling phenol degradation than OH evolved from ZrO2. In addition, ZrO2 outperformed MnO2 in exploiting NO3SUP species, thus showing greater recyclability in mineralizing textile wastewater, while leaching a negligible amount of Zr.