Mechanisms of Bimetallic Synergistic Fenton-like Catalysis over Metal Oxides and Regulatory Approaches

雙金屬氧化物的類芬頓協同催化機制及調控

Student thesis: Doctoral Thesis

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Author(s)

Detail(s)

Awarding Institution
Supervisors/Advisors
  • Wenwei Li (External person) (External Supervisor)
  • Wenjun ZHANG (Supervisor)
Award date31 May 2023

Abstract

Inadequate access to clean water continues to be a key global challenge to a sustainable society. Fenton-like processes, which allow oxidative destruction of various recalcitrant organics under mild conditions, offer a promising approach to obtain clean water from polluted waters. Among the various heterogeneous catalysts for water purification, bimetallic oxides transition metal oxides (TMOs) have attracted broad interest due to their low-cost, abundant availability, and flexibly-tunable surface properties. However, the limited decontamination activity and stability are key hurdles to their practical application. Addressing these challenges require a deep understanding and fine-tuning of bimetallic synergy. Therefore, this dissertation aims to develop fundamental insights into the synergized bimetallic Fenton-like catalysis and find new avenues for modulating the catalytic selectivity of such bimetallic oxide materials. To this end, several engineering strategies were employed in this work, including the regulation of bimetallic composition, geometry coordination, active site distance and crystallinity for optimizing catalytic activity and modulating catalytic selectivity. Assisted by a series of characterization and theoretical calculation, the origins of superior Fenton-like activity and selectivity of bimetallic TMOs and the intrinsic synergism are unveiled. The findings of this dissertation may lay a basis for rational design of advanced catalyst materials to favor the development of more sustainable water decontamination technologies. The main findings of this dissertation are as follows:

1) The critical role of Mn-O covalency in governing the intrinsic Fenton-like activity of spinel oxides: Bimetallic catalyst has been found to boost metal redox cycling for improved peroxymonosulfate (PMS) activation, but the underlying mechanisms are unclear. Here, Co-Mn spinel oxides with varied composition (Co3-xMnxO4) were constructed and the correlations between metallic coordination geometry and surface properties and their catalytic activity were deeply investigated. We discover a high correlation between the TM-O covalency of the catalyst and its intrinsic catalytic activity. Experimental and theoretical analysis reveal that the introduction of Co significantly raises the Mn valence and enlarges Mn-O covalency in octahedral configuration, thereby lowering the MnOh-PMS charge transfer energy. With appropriate Mn4+/Mn3+ ratio to balance PMS adsorption and activation, the Co1.1Mn1.9O4 exhibited remarkable catalytic activities for PMS activation and pollutant degradation. The improved insights on the origins of spinel oxides activity for PMS activation may inspire the development of more active and robust metal oxide catalysts.

2) The key driver of electron delocalization of spinel oxides in nonradical PMS activation: In view of the complex Fenton-like reactions catalyzed by TMOs which typically requires excess energy/ chemicals consumption, a selective oxidation of recalcitrant micropollutants based nonradical pathway is highly desirable. Here, we propose a facile structural engineering strategy to tune the catalyst selectivity of TMOs. A series of ZnFe2−xMnxO4 spinel oxides (x = 0−2.0) comprising exclusively Zn- occupied tetrahedra (Td) for structural modulation and Fe- and Mn-occupied octahedra as the major active unit were prepared. By systematically comparing their electronic structure and catalytic behaviors of these catalysts via both experimental analysis and theoretical calculation, we identified electron delocalization as the decisive factor defining the catalytic selectivity of Fe-Mn spinel oxides for PMS activation. Intriguingly, benefited from a strong superexchange electronic interaction between the edge-sharing Fe and Mn in the octahedra, the ZnFeMnO4 with high degree of electron delocalization enabled near 100% nonradical activation of PMS. Lastly, the high activity, stability and environmental robustness of the catalysts for selective decontamination were also demonstrated. This work offers new insights into bimetallic synergy in regulating catalytic selectivity of spinel oxides in PMS activation, which may guide the design of low-cost spinel oxides for more selective and efficient decontamination applications.

3) Insight into geometry-dependent bimetallic synergy for tuning the Fenton-like catalytic pathway: Bimetallic oxides may exist in different forms with varied degrees of metal-metal interactions, which would inevitably affect their synergy in Fenton-like catalysis. To distinguish such different synergisms, three Fe-Mn oxides with different bimetallic combination forms, including physical mixing, heterostructure, and lattice doping were fabricated, and their PMS activation activity and pathways were compared. The modified coordination environment, electronic structure of the active metal sites in these catalysts and their structure-catalytic behaviors relationship were deeply analyzed. It was found that the distance of bimetallic sites critically affect their surface properties, PMS adsorption configuration and binding strength to regulate the Fenton-like catalytic pathway. The Mn2O3@Fe2O3 heterostructure oxide, attributing to an appropriate Fe-Mn distance and the optimized geometry, exhibited the highest catalytic activity for BPA degradation via concerted active species (high valent Mn and PMS* complex). This work provides a new scientific basis for guiding the design of advanced oxidation catalysts and other heterogeneous metal catalysts in the reaction pathway regulation.

4) Crystallinity engineering for overcoming the activity-stability tradeoff of spinel oxide in Fenton-like catalysis: While crystalline TMOs are typically adopted for Fenton-like catalysis, here we propose the crystallinity as an extra dimension for structure engineering of such catalysts. CoMnOx spinel oxides with the amorphous, amorphous/crystalline (A/C) and crystalline skeletons were elaborated via a salt- assisted method with controlled annealing temperature. A transition of the Fenton-like pathway from radical to nonradical with increased crystallinity of CoMnOx nanosheets was identified and quantified through radical quenching experiments, electron paramagnetic resonance, in situ Raman, and electrochemical measurement. By analyzing atomic and electronic structures, abundant A/C boundaries, the highest Mn valence and moderate surface hydroxyl group were identified on A/C-CoMnOx surfaces, triggering concerted radical and nonradical oxidation process. These features render surface-confined BPA oxidation via improving effective utilization of highly-oxidative radicals and eliminating surface passivation of catalysts, thereby breaking the activity- selectivity tradeoff of heterogeneous Fenton-like catalysis.

    Research areas

  • Fenton-like reactions, Heterogenous catalysts, Peroxymonosulfate, Transition metal oxides, Bimetallic synergy, Electronic structure