2D-Like Catalyst with a Micro-nanolinked Functional Surface for Water Purification.

In water purification, the performance of heterogeneous advanced oxidation processes significantly relies upon the utilization of the catalyst's specific surface area (SSA). However, the presence of the structural "dead volume" and pore-size-induced diffusion-reaction trade-off limitation restricts the functioning of the SSA. Here, we reported an effective approach to make the best SSA by changing the traditional 3D spherule catalyst into a 2D-like form and creating an in situ micro-nanolinked structure. Thus, a 2D-like catalyst was obtained which was characterized by a mini "paddy field" surface, and it exhibited a sharply decreased dead volume, a highly available SSA and oriented flexibility. Given its paddy-field-like mass-transfer routine, the organic capture capability was 7.5-fold higher than that of the catalyst with mesopores only. Moreover, such a catalyst exhibited a record-high O3-to-·OH transition rate of 2.86 × 10-8 compared with reported millimetric catalysts (metal base), which contributed to a 6.12-fold higher total organic removal per catalyst mass than traditional 3D catalysts. The facile scale preparation, performance stability, and significant material savings with the 2D-like catalyst were also beneficial for practical applications. Our findings provide a unique and general approach for designing potential catalysts with excellent performance in water purification.

Mn-doped carbon-Al2SiO5 fibers enable catalytic ozonation for wastewater treatment: Interface modulation and mass transfer enhancement.

Heterogeneous catalytic ozonation is an efficient approach to remove hazardous and refractory organic contaminants in wastewater. It is crucial to design an ozone catalyst with high catalytic activity, high mass transfer and facile separation properties. Herein, easily separable aluminosilicate (Al2SiO5) fibers were developed as carriers and after interface modulation, Mn-doped carbon-Al2SiO5 (Mn-CAS) fibrous catalysts were proposed for catalytic ozonation. The growth of carbon shells on Al2SiO5 fiber surface and the introduction of metal Mn provided abundant Lewis acid sites to catalyze ozone. The Mn-CAS fiber/O3 system exhibited superior reactivity to degrade oxalic acid with a rate constant of 0.034 min-1, which was about 19 times as high as Al2SiO5/O3. For coal gasification wastewater treatment, Mn-CAS fibers also demonstrated high catalytic activity and stability and the COD removal was over 56%. Computational fluid dynamic simulations proved the high mass transfer properties of fibrous catalysts. Hydroxyl radicals (•OH) were identified as the predominant active species for organic degradation. Particularly, the catalytic pathways of O3 to •OH on Mn-O4 sites were revealed by theoretical calculations. This work provides a novel fibrous catalyst with high reactivity and mass transfer as well as easy separation characteristics for catalytic ozonation and wastewater purification.

Single-Atom Fe-N4 Sites for Catalytic Ozonation to Selectively Induce a Nonradical Pathway toward Wastewater Purification.

Nonradical oxidation has been determined to be a promising pathway for the degradation of organic pollutants in heterogeneous catalytic ozonation (HCO). However, the bottlenecks are the rational design of catalysts to selectively induce nonradicals and the interpretation of detailed nonradical generation mechanisms. Herein, we propose a new HCO process based on single-atom iron catalysts, in which Fe-N4 sites anchored on the carbon skeleton exhibited outstanding catalytic ozonation activity and stability for the degradation of oxalic acid (OA) and p-hydroxybenzoic acid (pHBA) as well as the advanced treatment of a landfill leachate secondary effluent. Unlike traditional radical oxidation, nonradical pathways based on surface-adsorbed atomic oxygen (*Oad) and singlet oxygen (1O2) were identified. A substrate-dependent behavior was also observed. OA was adsorbed on the catalyst surface and mainly degraded by *Oad, while pHBA was mostly removed by O3 and 1O2 in the bulk solution. Density functional theory calculations and molecular dynamics simulations revealed that one terminal oxygen atom of ozone preferred bonding with the central iron atom of Fe-N4, subsequently inducing the cleavage of the O-O bond near the catalyst surface to produce *Oad and 1O2. These findings highlight the structural design of an ozone catalyst and an atomic-level understanding of the nonradical HCO process.

Bifunctional Fe for Induced Graphitization and Catalytic Ozonation Based on a Fe/N-Doped Carbon-Al2O3 Framework: Theoretical Calculations Guided Catalyst Design and Optimization.

Heterogeneous catalytic ozonation is regarded as a feasible technology in advanced wastewater treatment. Catalytic performance, mass transfer, and mechanical strength are the key elements for large-scale applications of catalysts. To optimize those elements, Fe was selected for its dual role in graphitization and catalytic ozonation. A Fe/N-doped micron-scale carbon-Al2O3 framework (CAF) was designed and applied to a fluidized catalytic process for the treatment of secondary effluent from coal gasification. The chemical oxygen demand removal rate constant and the hydroxyl radical generation efficiency (Rct) of the Fe/N-doped CAF were 190% and 429% higher than those of pure ozone, respectively. Theoretical calculations revealed that higher Fe valence promoted ozone decomposition, which implied increasing FeIII content for further catalyst optimization. The rate constant and Rct with a higher FeIII-proportion catalyst were increased by 13% and 16%, respectively, compared to those with the lower one. Molecular dynamics and density functional theory calculations were performed to analyze the reaction kinetics qualitatively and quantitatively. The energy barrier corresponding to FeIII configuration was 1.32 kcal mol-1, 27% lower than that for FeII configuration. These theoretical calculations guided the catalyst optimization and provided a novel solution for designing ozonation catalysts. The Fe/N-doped CAF demonstrated a great potential for practical applications.

A hybrid fluidized-bed reactor (HFBR) based on arrayed ceramic membranes (ACMs) coupled with powdered activated carbon (PAC) for efficient catalytic ozonation: A comprehensive study on a pilot scale.

Taking advantage of the high mass transfer in the bulk solution of fluidized-bed reactor (FBR), and the benefits of simultaneous particle separation and ozone catalysis on ceramic membranes, we proposed a hybrid fluidized-bed reactor (HFBR) based on arrayed ceramic membranes (ACMs) coupled with powdered activated carbon (PAC) for efficient catalytic ozonation. The optimum HFBR performance on a pilot scale was found at PAC addition of 3 g/L, ozone dosage of 25 mg/L, hydraulic retention time of 60 min and auxiliary aeration strength of 5 m3/h. During the 30-day treatment of coal-gasification secondary effluent (200 L/h), the HFBR system revealed not only a 117% increase in ozone utilization efficiency (ΔCOD/ΔO3) upon pure ozonation but also a highly purified effluent with better sterilization and low residual bromate (∼11 µg/L). Low-molecular-weight organic fragments and acids, as well as phthalate esters were identified as the main products in this process. By density functional theory (DFT) calculations, it was found the main functional groups (carbonyls, -C=O) on the PAC could be protected from direct ozonation in the presence of ozone-degradable organics (e.g. phenolic and aliphatic compounds) in the wastewater through an ozone-competing reaction, which prevented the rapid inactivation of the PAC in catalytic ozonation. A longer service life and cheaper materials for ceramic membranes would benefit low operation costs for the HFBR. Moreover, the addition of PAC could greatly reduce ozone demand by ∼60% in the HFBR, and therefore decrease energy consumption by ∼30%. Hence, the HFBR was proved to be a highly competitive technology for wide application in the near future.