Mechanism of Heterogeneous Fenton Reaction Kinetics Enhancement under Nanoscale Spatial Confinement

Abstract

Nanoscale catalysts that can enable Fenton-like chemistry and produce reactive radicals from hydrogen peroxide activation have been extensively studied in order to overcome the limitations of homogeneous Fenton processes. Despite several advantageous features, limitation in mass transfer of short-lived radical species is an inherent drawback of the heterogeneous system. Here, we present a mechanistic foundation for the way spatial confinement of Fenton chemistry at the nanoscale can significantly enhance the kinetics of radical-mediated oxidation reactionspollutant degradation in particular. We synthesized a series of Fe 3 O 4 -functionalized nanoreactors with precise pore dimensions, based on an anodized aluminum oxide template, to enable quantitative analysis of nanoconfinement effects. Combined with computational simulation of spatial distribution of radicals, we found that hydroxyl radical concentration was strongly dependent on the distance from the surface of Fenton catalysts. This distance dependency significantly influences the gross reaction kinetics and accounts for the observed nanoconfinement effects. We further found that a length scale below 25 nm is critical to avoid the limitation of short-lived species diffusion and achieve kinetics that are orders of magnitude faster than those obtained in a batch suspension of heterogeneous catalysts. These findings suggest a new strategy to develop an innovative heterogeneous catalytic system with the most effective use of hydroxyl radicals in oxidation treatment scenarios.

In-Situ H2O2 Cleaning for Fouling Control of Manganese-Doped Ceramic Membrane through Confined Catalytic Oxidation Inside Membrane

“…Thus, the oxidation efficiency of organic foulants by generated ROS would be significantly improved if the reaction space were restricted far below the diffusion length scale of ROS. This phenomenon was reported in previous studies as a spatial confinement effect [19,20].…”

“…In order to avoid the undesirable harm to activated sludge, H 2 O 2 solution could be pumped into the membrane through the inner cavity to control the catalytic oxidation of organic foulants occurring within membrane pores. Confined spaces in membrane pores could achieve catalytic performances that are orders of magnitude faster than those obtained in the bulk phase [19]. We found that the ozone decomposition rate inside the membrane pores was about 428 times faster than that in the bulk phase, which was confirmed as a confinement effect of nano-scale membrane pores [20].…”

“…It is well-known that the lifetimes of HO• (<1 µs) and 1 O 2 (~3 µs) in water are extremely short, resulting in elimination before they react with pollutants [52]. In heterogeneous catalytic reaction systems, the concentration of ROS is the highest on the catalyst surface and then rapidly decreases with increasing diffusion length [19]. Assuming the lifetimes (τ) of HO• and 1 O 2 are 1 µs and 3 µs, respectively, at a neutral pH [53], the diffusion lengths (λ L ) of HO• and 1 O 2 in the aqueous phase should be ~96 nm and ~159 nm, respectively, as calculated by λ L = 2 √ τ × D (where D is the diffusion coefficient; D HO• = 2.3 × 10 −9 m 2 s −1 and D1 O 2 = 2.1 × 10 −9 m 2 s −1 ) [54].…”

“…As shown in Figure 8, the confined reaction space within the Mn-doped membrane pores shortened the diffusion lengths of ROS and prevented their elimination, leading to enriched ROS within the membrane pores. Because of the substantially reduced diffusion length and concentrated reactants, ROS reacted with organic foulants within membrane pores with much higher efficiency than they did in the bulk phase [19,55]. In this way, organic foulants were efficiently removed, and membrane fouling was well controlled via in-situ H 2 O 2 cleaning confined in the Mn-doped membrane pores.…”

Manganese‐Based Micro/Nanomotors: Synthesis, Motion, and Applications

“…First, innovative structural design of Mn-micro/nanomotors coupling with green and active materials are conducive to realizing diverse functionalization in detection and removal of inorganic or organic pollutants. Second, various calculation models were used to simulate the motion trajectory [56] and degradation process [99] in recent advances. Further studies are expected in devoting to deepen insights into the operational and catalytic regimes of H 2 O 2 -fueled Mn-micro/nanomotors.…”