Distinguishing homogeneous advanced oxidation processes in bulk water from heterogeneous surface reactions in organic oxidation

Abstract

Clarifying the reaction pathways at the solid–water interface and in bulk water solution is of great significance for the design of heterogeneous catalysts for selective oxidation of organic pollutants. However, achieving this goal is daunting because of the intricate interfacial reactions at the catalyst surface. Herein, we unravel the origin of the organic oxidation reactions with metal oxide catalysts, revealing that the radical-based advanced oxidation processes (AOPs) prevail in bulk water but not on the solid catalyst surfaces. We show that such differing reaction pathways widely exist in various chemical oxidation (e.g., high-valent Mn 3+ and MnO X ) and Fenton and Fenton-like catalytic oxidation (e.g., Fe 2+ and FeOCl catalyzing H 2 O 2 , Co 2+ and Co 3 O 4 catalyzing persulfate) systems. Compared with the radical-based degradation and polymerization pathways of one-electron indirect AOP in homogeneous reactions, the heterogeneous catalysts provide unique surface properties to trigger surface-dependent coupling and polymerization pathways of a two-electron direct oxidative transfer process. These findings provide a fundamental understanding of catalytic organic oxidation processes at the solid–water interface, which could guide the design of heterogeneous nanocatalysts.

Hydrocarbon Degradation by Contact with Anoxic Water Microdroplets

“…To understand how hexadecane was cracked into C 2 hydrocarbons, the intermediate species in the emulsion were probed using MS without applied potential (Figure c, Figures S15–S17). To minimize the possible iron contamination that might lead to Fenton reactions at the water interface, , silica tubing instead of stainless-steel tubing was used to export the emulsion to the MS inlet. The cationic adducts of hexadecane, n -tetradecane, and n -dodecane with three H 2 O were observed at m / z 280, 252, and 224, respectively, indicating the occurrence of sequential C–C bond fragmentation which yields C 2 fragments.…”

Transformative Removal of Aqueous Micropollutants into Polymeric Products by Advanced Oxidation Processes

“…not only facilitate • OH production to aid hydroxylation/phenolation but also act as ligands in coordination reactions with phenolic compounds by catechol groups, leading to the formation of metal-coordinated polymers (metal–phenolic networks). , In addition, the concentrations of metal ions, oxidants, and organic compounds have a significant influence on coupling and polymerization products and kinetics. For example, increasing the [Fe 2+ ]/[H 2 O 2 ] ratio will decrease the polymerization rate in Fenton reactions with a low-concentration pollutant, and the products are benzenediols, benzenetriols (free radical adducts of phenol), and phenol dimers. , …”

“…Notably, the limited conductivity of metal oxides (semiconductor) restricts direct electron transfer over the substrate. Instead, the surface-activated peroxide directly attacks pollutants at the solid–solution interface or neighboring co-adsorbed organics. , The formation of organic radicals and ultimately polymer layers will cover the catalyst surface . In this process, the heterogeneous metal oxide surface plays three crucial roles: activation of peroxides, stabilization of radical monomer/ions, and accumulation of the polymer products, which are all thermodynamically spontaneous and kinetically favorable processes.…”

“…4,39 Recent research highlighted that the oxidation of organics predominantly occurred on the surface of metal oxide with high-valence species (Mn). 4 These heterogeneous catalysts trigger a direct oxidation pathway (DOTP) to enable surface-dependent coupling and polymerization. 40 Notably, the limited conductivity of metal oxides (semiconductor) restricts direct electron transfer over the substrate.…”

“…9,10 Recent AOP studies have revealed polymerization pathways, where organic contaminants will be transformed to organic radicals and undergo interlinking as monomers toward polymerization, resulting in polymeric products. 4,8,11 Additionally, the polymerization produces value-added byproducts and performs high selectivity toward target pollutants in real environmental scenarios. 12 The polymerization reaction in AOPs is influenced by multiple factors, including the radical and non-radical pathways, carbon/metal catalysts, as well as properties of organics (e.g., molecular structures, and functional groups).…”