Deploying radical inter-transition from radical dotOH to supported NO3radical dot on Mono-dentate NO3--modified ZrO2 to sustain fragmentation of aqueous contaminants

Abstract

To advance aqueous pollutant degradation using (OH)-O-center dot, H2O2 ((OH)-O-center dot 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)-O-center dot 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)-O-center dot desorption, thereby proposing the coupling of H2O2 homolysis and exothermic radical inter-conversion of (OH)-O-center dot -> NOSUP center dot (supported NO3 center dot) to create the overall (OH)-O-center dot -> NOSUP center dot route. ZrO2 was modified with NO3- functionalities (NOSUP center dot precursors) to form ZrO2-N, where NOSUP- species were located close to Zr4+ (LA) and Bronsted acidic -OH (BA) sites, whose acidic strengths must be elevated to facilitate (OH)-O-center dot desorption for reducing the energy barrier (EBARRIER) of the overall (OH)-O-center dot -> NOSUP center dot route. NOSUP- species were bound to the ZrO2 surface via mono-dentate configuration only, thereby avoiding LA loss (rate in a per-gram up arrow), escalating LA/BA strengths (E-BARRIER down arrow), and imparting two free oxygens available to (OH)-O-center dot -> NOSUP center dot (rate in a per-site up arrow). Moreover, NOSUP center dot species extract electrons from contaminants via electron transfer to recover NOSUP- species used for recurring (OH)-O-center dot -> NOSUP center dot, while sustaining pollutant fragmentation efficiency by circumventing surface poison accumulation. Hence, NOSUP center dot on ZrO2-N revealed higher efficiencies in fragmenting bisphenol A or recycling phenol degradation than (OH)-O-center dot evolved from ZrO2. In addition, ZrO2 outperformed MnO2 in exploiting NOSUP center dot species, thus showing greater recyclability in mineralizing textile wastewater, while leaching a negligible amount of Zr.