Title：Dynamic In-Situ Reconstruction of Active Site Circulators for Photo-Fenton-Like Reactions
Journal: Nature Communications
IF: 14.7
Original link：https://doi.org/10.1038/s41467-025-58392-3
Reporter：Jiazhen Wang-25-master
Developing efficient and stable heterogeneous catalysts for the continuous activation of oxidants is crucial to mitigating the global water resource crisis. Guided by computational predictions, this research achieved this goal through the synthesis of a modified graphitic carbon nitride with enhanced catalytic activity and stability. Its intrinsic activity was further amplified by dynamic insitu reconstruction using the I−/I3− redox mediator system during photoreactions. Impressively, this reconstructed catalyst demonstrated the capability for at least 30 regeneration cycles while maintaining high purification efficacy. The mechanism underlying the in-situ reconstruction of active sites for periodate functionalization was elucidated through theoretical calculations, coupled with semi-in-situ X-ray photoelectron spectroscopy (XPS) and electrochemical analyses. The system’s capacity to detoxify recalcitrant pollutants was demonstrated through successful Escherichia coli cultivation and Zebrafish embryo experiments. The economic feasibility and environmental impacts are quantitatively assessed by the Electrical Energy per Order (EE/O) metric and Life Cycle Assessment (LCA), confirming the system’ sscalability and applicability in real-world scenarios. This dual-site constrained interlayer insertion, and controllable in-situ catalyst reconstruction achieve durable robustness of the photocatalyst, paving the way for the development of sustainable catalytic water purification technologies.
The development of an ideal Fenton-like system that utilizes visible light to activate oxidants for the degradation and mineralization of organic pollutants offers an efficient and sustainable method for wastewater purification. Despite its potential, several challenges impede its widespread adoption, including the incomplete oxidation of organic micropollutants, the reduction in activity due to the loss of active species, and practical difficulties in scaling the technology for industrial applications. A pivotal strategy to address these issues involves enhancing the sustained generation of potent oxidants throughout the activation process, necessitating the design of efficient and stable photocatalysts. In this regard, graphitic carbon nitride (g-C3N4, or CN) is a promising material, favored for its environmental compatibility, straightforward synthesis, and appropriate band structure conducive to photo-Fenton-like reactions Nevertheless, the intrinsic lowmobility of photo-generated carriers inbulk CN curtails its overall photocatalytic efficacy.
To mitigate these limitations, numerous strategies, including crystalline modification, molecular engineering, formation of hetero/homojunctions, defect introduction, and doping, have been system-atically investigated to amplify CN’s performance. Ion intercalation is particularly impactful, profoundly affecting CN’s physical attributes, surface states, and electronic structure, thereby optimizing catalytic active sites and facilitating photo-generated carrier mobility.The embedding of alkali metals, such as K+ or Na+,has been notably successful in enhancing light absorption and charge separation efficiency by introducing new energy levels. This tactic has garnered considerable interest for applications in H2 storage, H2O2 production, and CO2 reduction. The integration of non-metal elements like B, O, S, or I can also adjust CN’s band structure and intrinsic electronic properties, enhancing conductivity, narrowing the band gap, and boosting light capture capabilities. A strategy combining metal and non-metal modifications could significantly boost in-plane charge transfer in CN, thus enhancing its photocatalytic activity. However, while in-plane doping can bolster charge transfer within CN layers, it contributes minimally to charge transfer between layers, which is vital for fully realizing CN’s intrinsic activity. Achieving synergistic activation of oxidants by simultaneously introducing targeted elements into both the CN plane and interlayers through dual-site constrained interlayers is challenging. This complexity stems from the sensitivity of the doping process to reaction conditions, precursors, and the low thermal stability during CN synthesis, highlighting the need for innovative approaches in catalyst design.
The catalytic activity and stability of the chosen catalyst are critical for effectively deploying photo-Fenton-like systems. While many photocatalysts exhibit robust initial activity, their performance frequently declines after extended use due to diminished photo-generated carrier separation and photo-corrosion.A primary challenge arises when the rate of photo-generated charge transfer exceeds that of surface water oxidation, leading to an accumulation of photo-generated holes. This accumulation typically leads to surface charge carrier recombination and subsequent photo-corrosion of the catalyst. Therefore, there is an urgent requirement to develop superior modification strategies that enhance both the migration rates of photo-generated carriers and their effective utilization. Addressing these elements is crucial for concurrently advancing and maintaining the balance between catalytic activity and stability.
Furthermore, the long-term operation of photocatalysts can alter their composition and structure, a process known as dynamic catalyst reconstruction. This in-situ process is pivotal for improving the photo-stability of catalysts, as it often leads to the generation of new active species that significantly influence the binding strength between the catalyst and reactants, thereby affecting the rate of catalytic reactions. Historically, the dynamic reconstruction of catalysts in photo-Fenton-like systems has been poorly understood. Additionally, assessments of catalyst reconstruction phenomena have varied. Some studies have identified in-situ catalyst reconstruction as a primary cause of catalyst deactivatio, whereas others argue that catalysts undergoing reconstruction form the “real catalytic centers” essential for the target reaction, suggesting that prior to reconstruction, these catalysts function merely as “pre-catalysts“. Therefore, a profound understanding of catalyst reconstruction mechanisms and their effective modulation is crucial. Such knowledge will facilitate the development of catalysts that not only exhibit high activity but also maintain robust photo-stability, thereby advancing the field of photocatalytic water treatment.
Recent studies have highlighted the pivotal role of redox mediators, specifically I−/I3−, which serve as intermediate electron carriers or reservoirs in the development of advanced charge transfer energy storage systems. The dynamic cycling of these I−/I3− redox mediators has been demonstrated to improve reaction kinetics and continuously neutralize high oxidation state species, resulting in batteries characterized by high surface capacity and extended lifespans of up to 2600 h. Inspired by these findings, the integration of precise dual-site constrained interlayer insertion strategies with innovative in-situ reconstruction of I−/I3− redox mediators offers significant potential for enhancing both the activity and stability of modified CN in photo-activated oxidant systems. A notable challenge and an opportunity arises from the inherent photo-stability limitations of potassium iodide (KI), where abundant I− is susceptible to oxidation to I3−. Exploiting this behavior, I−/I3− redox mediators can be introduced in situ during photocatalytic reactions to facilitate catalyst reconstruction and achieve dual-site constrained insertion between CN layers. This method represents a strategic approach for designing innovative functional catalysts. Cycling I−/I3− redox mediators through catalyst reconstruction not only boosts the migration of photo-generated charge carriers but also efficiently consumes unreacted holes, thereby preventing surface recombination and reducing photo-corrosion. Additionally, this cycling temporarily stores photo-generated electrons, which are not immediately utilized in reactions and releases them in subsequent cycles to sustain reaction continuity.
Within this innovative framework,modifiedCN featuring dual-site constrained interlayer insertion functions as a “pre-catalyst” paired with the potent oxidant periodate (PI, IO4−), known for its high oxidation potential (+1.6 eV) and cost-effective storage and transport.This photo-Fenton-like system is designed to exploit the advantages of in-situ redox mediator introduction during photocatalytic reactions, thereby maximizing the catalytic activity and stability ofmodified CN. This innovative paradigm synchronously optimizes both activity and stability through structural refinements, heralding a transformative shift in catalyst design. Comprehending the structure-activity relationships amid dynamic structural adjustments and their influence on the catalytic activity and stability of these systems is paramount. This understanding has profound implications for enhancing robust Fenton-like reactions and offers a meaningful trajectory for future research and development.
In this work, we synthesize a modified CN with K+ and I− intercalation (CN-KI), using a recrystallized mixture of melamine and KI. This precursor is further adapted to generate I3− in situ (3I− -2e− → I3−) during a photo-activated periodate (PI) process, enabling the dynamic reconstruction of the photocatalyst with the I−/I3− redoxmediator (CN-KI-I3). Our research focuses on the degradation of the antibiotic sulfamethoxazole (SMX), conducting an extensive investigation into the photocatalytic activity of the CN-KI-PI system under various parameters. Additional studies explore the stability of the CN-KI-I3/PI system through detailed theoretical analyses and a series of semi-in-situ characterizations, aiming to elucidate the mechanisms behind the in-situ formation of the I−/I3− redoxmediator and the enhancements in stability they confer. Furthermore, the activation, role, and function-ality of PI within this strategically designed system are rigorously evaluated. The system’s sustainability and environmental impact are also thoroughly assessed, with toxicity levels tested from bacterial, plant, and animal perspectives through experiments involving Escherichia coli and Zebrafish embryo. Moreover, the application potential of CN-KI-I3/PI system is demonstrated in various scenarios using continuous flow reactors and pilot-scale setups, underscoring its scalability andpractical utility. A comprehensive Life Cycle Assessment (LCA) and Electrical Energy per Order (EE/O) analysis are conducted to evaluate the environmental and economic impacts of the CN-KI-I3/PI systemduring real wastewater treatment, re-assessing thepotential for industrial application of the designed system. This work sheds light on the dynamic reconstruction of redox mediators with in the catalyst and their operational mechanisms in catalytic reactions. It also aims to optimize the balance between activity and stability. Ultimately, the goal is to broaden the applicability of photocatalytic technologies for organic wastewater purification.
1. Theoretical predictions of constrained interpolation and catalyst reconfiguration
The electronic properties and formation energies of different intercalation structures (e.g., CN, CN-KI, CN-KI3, CN-KI-I3) are demonstrated by first-principles calculations. CN-KI-I3 exhibits the optimal charge distribution, the lowest formation energy, and the largest electron mobility potential, which indicates that the synergistic intercalation of K+ and I-/I3- can significantly enhance the photocatalytic ability of CN and provide a thermodynamic basis for its subsequent in-situ reconfiguration. Provide a thermodynamic basis for its subsequent in situ reconstruction.
2. Evaluation of the catalytic activity and cyclic stability of the reconstructed systems
The synergistic advantages of the CN-KI/PI/Vis system were clarified by comparing the removal efficiencies and rates of different systems (PI, CN-KI, CN-KI/PI/Vis, etc.), and the environmental adaptability of the CN-KI/PI/Vis system was verified under multi-factors, such as pH, co-existing ions, and diversity of organic matter. The environmental adaptability of the system was also verified under multiple factors such as pH, co-existing ions and organic matter diversity. Meanwhile, the high stability and reusability of the system were demonstrated through multiple rounds of cycling experiments (without oxidant/catalyst supplementation).
3. Catalyst structure characterization and evidence of in situ reconstruction
Comparison of the physicochemical and electrical properties of CN-KI and CN-KI-I3 by XRD, UV-Vis DRS, PL, and EIS is demonstrated, confirming the crystal structure change and enhanced light absorption/carrier mobility induced by the intercalation. In addition, semi-in situ XPS and photocurrent tests revealed the dynamic process of I- being oxidized to I3- in situ, confirming the gradual reconfiguration behavior of the catalyst during the reaction.
4. Identification of active species and construction of catalytic mechanism
The dominant ROS species (-O2- and ¹O2-) in the system were identified by free radical bursting experiments, ESR spectroscopy, UV-Vis and energy barrier calculations, and the -OH dominant pathway was excluded. It was also confirmed that PI was not a conventional oxidant in the system, but an electron acceptor to promote the oxygen reduction reaction. Combined with the differential charge density map and the adsorption energy of PI-photoelectric initiator, a “non-traditional photo-Fenton mechanism” was constructed, emphasizing the central role of O2 reduction in ROS generation.
5. Toxicity assessment of degradation products and indoor continuous flow application
The ecotoxicity (predicted + measured) of SMX degradation intermediates was evaluated, and the degradation products were shown to significantly reduce the toxicity by biological experiments such as E. coli, zebrafish embryo, and wheat rooting. The continuous flow microreactor with CN-KI immobilized was further demonstrated to achieve 24-hour stable operation in RhB-rhodamine and SMX purification, which verified the long-lasting stability and preliminary engineering potential of the material.
6. Outdoor light and in-industry testing, economic-environmental assessment
Stable performance and 24/7 operability of the catalyst under natural light conditions were demonstrated. High SMX and TOC removal efficiencies were demonstrated in a 1 m² continuous flow platform and a 20 L pilot reactor treating simulated/actual wastewater (pharmaceutical/coking), respectively, and EE/O and LCA analyses further indicated that the system is superior to the conventional systems, such as O3, H2O2, etc., in terms of energy consumption, carbon emission, and environmental impact, and has a promising future for practical applications.
In this study, we innovatively proposed and realized an in situ dynamic reconfiguration mechanism for photocatalysts based on I-/I3- redox cycling, and constructed a “precatalyst” CN-KI through a two-site constrained interlayer intercalation strategy of K⁺/I-, which was used to induce the transition from I- to I3- in the presence of light. Under the light condition, the reversible conversion of I- to I3- was induced to realize the self-reconstruction and continuous activation of the active sites; this mechanism not only significantly enhanced the catalytic activity and stability, but also verified its durability through multiple rounds of cycling, and combined with the theoretical calculations, biotoxicity assessment, continuous flow and pilot tests, it demonstrated the potential of the system for highly efficient and sustainable wastewater purification and its advantages of environmental protection, which provides a new paradigm for the photo-Fenton reaction system.