Using radicalized NOₓ derivatives supported on metal oxides

NO_X (X=1 or 2) emitted from stationery/mobile sources are conventionally deemed as notorious, anthropogenic precursors of ultrafine particulate matters (PM2.5) because NO_X 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 NO_X (vide supra) by proposing an interesting means to exploit NO_X in creative fashion.

The Korea Institute of Science and Technology (KIST) has announced that a KIST research group with principal investigators of Dr. Jongsik Kim and Dr. Heon Phil Ha has collaborated with a research team led by Prof. Keunhong Jeong in the the Korea Military Academy (KMA) to graft NO₃^– species on a metal oxide via chemical fusion between NO_X and O₂ under a low thermal energy (≤ 150 °C). The resulting supported NO₃^– species can then be radicalized to generate NO₃^• analogs that serve as degraders of refractory organic substances present in a wastewater.

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 CO_Y (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.

Dr. Kim and co-workers make it viable under a wastewater including H₂O₂ and NO₃^–-functionalized manganese oxide that surface manganese species (Mn²⁺/Mn³⁺) initially activate H₂O₂ for the formation of ^•OH, whereas ^•OH subsequently activates NO₃^– functionality for its transition into NO₃^• (denoted as ^•OH → NO₃^•), all of which are evidenced by density functional calculation (DFT) techniques alongside with a bunch of control experiments.

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).

Dr. Kim remarks that “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.”

The research was published in JACS Au.

Provided by
National Research Council of Science & Technology