Using radicalized NOₓ derivatives supported on metal oxides

NOX (X=1 or 2) emitted from stationery/mobile sources are conventionally deemed as notorious, anthropogenic precursors of ultrafine particulate matters (PM2.5) because NOX can undergo a series of SO2-assisted photochemical transformative stages to finally evolve PM2.5 functioning as an air pollutant. Recently, a research group in South Korea rectifies the general notion of NOX (vide supra) by proposing an interesting means to exploit NOX in creative fashion.

Aqueous recalcitrant compounds including phenolics and bisphenol A are typically eliminated from water matrices via sedimentation with the use of coagulants or via degradation into H2O and COY (Y=1 or 2) with the injection of OH shuttles such as H2O2, O3, etc. However, these methods require additional stages to recover coagulants or suffer from short lifespans and/or chemical instabilities innate to OH, H2O2, and O3, thus severely limiting the sustainability of H2O purification processes currently being commercialized. As a substitute of OH, NO3 can be particularly appealing due to its longer lifetime and/or greater oxidizing potential in comparison with OH, OOH, or O2•-, thereby being predicted to enhance the efficiency in degrading aqueous pollutants over the other radicals stated above. Nevertheless, NO3 production is not trivial and has a bunch of constraints such as the need of highly energized electrons in the presence of a radioactive element or highly acidic environments.

Advance in water purification: Using radicalized NOX derivatives supported on metal oxides
Fig: Schematic representation of (A)H2O2 scission cycle on surface Mnn+ species (n=2 or 3) and radical transfer from surface-unbound OH radical to NO2 radical or NO3 radical species supported on α-/β-/γ-MnO2 surfaces (NO2 radical SUP or NO3 radical SUP), leading to the production of supported NO2 radical (NO2 radical SUP in B) utilized for degrading aqueous pollutants. Illustration of porous architectures for α-MnO2(D).

The resulting NO3 species were demonstrated to escalate degradation efficiency of textile wastewater by five- or seven-fold compared to those provided by conventional radicals (OH/OOH/O2•-). Of significance, the catalyst (NO3-functionalized manganese oxide) discovered herein is ~30 % cheaper than a traditional commercial catalyst (iron salt) and is mass-producible. Of additional significance, the catalyst is reusable ten times or more. This is in contrast to a traditional catalyst that only guarantees one-time utilization in decomposing aqueous pollutants via homogeneous H2O2 scission (OH generation).The OH → NO3 technology  has been patented and sold to a domestic company (SAMSUNG BLUETECH). Given a plenty of merits imparted by the catalyst modified with NO3 functionalities, we basically expect to install the catalyst in a wastewater treatment unit so soon.

Jongsik Kim et al, Deciphering Evolution Pathway of Supported NO3• Enabled via Radical Transfer from •OH to Surface NO3– Functionality for Oxidative Degradation of Aqueous Contaminants, JACS Au (2021). DOI: 10.1021/jacsau.1c00124