Unsaturated single-atom CoN3 sites for improved fenton-like reaction towards high-valent metal species
Graphical Abstract
Introduction
Peroxymonosulfate (PMS)-based advanced oxidation processes are central to water decontamination. Attempts to advance PMS activation efficiency have been exemplified by single atom catalysts (SACs) [1], [2]. This is not only due to the utmost atom utilization and excellent catalytic performance but also the simplified structure that provides an ideal platform to explore the structure-activity relationship at the atomic level [3]. Typically, single metal atoms loaded in a nitrogen-doped carbon matrix (M-N-C) are quite competitive candidates, which can be obtained by simple pyrolysis of commercially available metal precursors with nitrogen and carbon sources [4]. Among them, cobalt (Co)- and iron (Fe)-based M-N-C configurations have been recognized as leading catalysts for PMS activation [5], [6], with free radicals (•OH/SO4•-), singlet oxygen (1O2) or high-valent metal species being reactive species to oxidize pollutants. It is worth reminding that the formation of specific reactive oxygen species (ROS) to allow improved catalytic performance is still highly desired.
Note that regulating the coordination environment of single atoms helps to tune the intrinsic performance in Fenton-like reactions [7]. Accordingly, the geometrical structure and electronic state of single atoms are tunable in response to coordination environment changes, which reshapes the absorption activity of the substrate towards metal sites and thus tailors the catalytic performance [8]. More importantly, the altered coordination environment is expected to affect PMS activation pathways by tailoring electron flow between PMS and single atoms to afford oriented generation of specific ROS. For instance, the CoN4 configuration is identified as an efficient electron donor for PMS reduction to produce free radicals (•OH/SO4•-) [5]. However, when the Co atoms coordinate with N atoms to form two adjacent N-C fragment edges (denoted as CoN2+2), the lowered electropositivity of central Co atoms drives the electron flow from PMS to CoN2+2 and triggers the selective formation of singlet oxygen (1O2) [9]. Therefore, rational control of M-N-C configurations via coordination environment regulation to tailor electron flow between catalysts and PMS is promising to advance the intrinsic activity of PMS activation via radical or non-radical pathways.
Compared with free radicals and 1O2, high-valent metal species are prized for their appreciable oxidizing strength while leaving the catalyst support unaffected [6]. However, the generation of either free radicals or high-valent species requires PMS reduction by electron donation from SACs, making the specific strategy to selectively form high-valent metal species quite challenging. Fortunately, the formation of free radicals only requires one-electron transfer while the latter requires two or more and thus highlights the importance of intensive electron flow from catalysts to PMS [10], [11], [12], [13]. Establishing unsaturated metal sites offers the possibility to address this issue. Firstly, unsaturated metal sites with lower coordination numbers are presented with more accessible d-orbitals and decreased steric hindrance [14], which are expected to promote interaction with reactants or converted intermediates during the PMS activation process. Secondly, the site-specific contributions from the unsaturated configuration are further emphasized here, as their lower thermodynamic stability can be related to a more negative electron work-function, which again increases the electron density in the Co centers [15], [16]. Following these arguments, we propose that the development of unsaturated metal sites in the M-N-C configuration increases the availability of their electrons, which may trigger intensive electron flow to PMS to generate rather selectively the wanted high-valent metal species.
In this work, we successfully synthesized an unsaturated single-atom CoN3 catalyst as an example through rational regulation of pyrolysis conditions, which showed evidently boosted degradation rates of various organic pollutants compared to the traditional saturated CoN4 configuration. We found that unsaturated CoN3, different from the reported CoN4 or CoN2+2, could efficiently activate PMS to generate high-valent Co species (Co(IV)), which dominated the subsequent degradation of contaminants. Electrical tests and in-situ Raman measurements revealed that the formation of high-valent Co experienced a two-step oxidation process as Co(II)/Co(III) and subsequent Co(III)/Co(IV) transformation. The intrinsic driving force for Co(IV) conversion was primarily the intensive electron flow from the catalyst to PMS, which was a reflection of the enhanced electron-donating ability of unsaturated single Co atoms affording a higher spin state, and the one with lower spin states may only produce free radicals. Our findings unravel the formation mechanism of high-valent Co species and free radicals in PMS activation, upon which the rational design of tunable atomic active sites with both high reactivity and selectivity in Fenton-like reactions can be expected.
Section snippets
Materials synthesis and characterization
The CoN3 and CoN4 catalysts in this work were fabricated by one-pot pyrolysis process using chitosan and cobalt salt as precursors (Text S1). The morphology and coordination structure of single atoms were mainly characterized by aberration-corrected high-angle annular dark field-scan transmission electron microscopy (AC-HADDF-STEM), soft and hard X-ray absorption spectroscopy (XAS). Materials characterizations in more details are provided in Text S2.
Measurements and analysis on catalytic performance
Rhodamine B (RhB), Orange II, bisphenol A
Characterizations of CoN4 and CoN3 catalyst
The synthetic procedures of traditional CoN4 and unsaturated CoN3 catalysts are briefly depicted in Fig. 1a. Here, the earth-abundant polymer chitosan was selected as a natural chelating substrate due to its rich amino groups that showed strong affinity to transition metal cations. [18] By calcining the mixture of chitosan and cobalt salts through one-pot pyrolysis process, it is promising to synthesize single-atom Co materials. The thermogravimetric curve of the mixed precursors (Fig. S1a)
Conclusions
In summary, our experiments and theoretical analyses clearly demonstrated that unsaturated CoN3 dramatically enhanced the intrinsic activity of PMS activation. Compared to saturated CoN4, which catalyzed PMS to generate free radicals with weak electron flow, the intensive electron flow from the catalyst to PMS promoted the formation of high-valent Co species by establishing unsaturated metal sites (CoN3) at the atomic level. This work highlights the importance of electron-flow regulation in PMS
CRediT authorship contribution statement
N.N.H., Y.W. and Y.M. designed and supervised the project. J.S.S. conducted the project. X.C.L. helped with the catalytic experiment. M.A. contributed to the analysis of synthesized materials and mechanism study. J.S.S., N.N.H., M.A., Y.W. and Y.M. wrote and revised the manuscript, and all the authors discussed the results and commented on the manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We acknowledge the financial support from the National Natural Science Foundation of China (52200073, 52025101, U19A20108 and 51821006). Dr. Yang Wang thanks the Alexander von Humboldt Foundation for a postdoctoral fellowship. We thank the National Synchrotron Radiation Laboratory (NSRL, Hefei, China) for the help with the Soft XAS tests. The Supercomputing Center of USTC is acknowledged for the computational support.
Appendix A. Supplementary material
Materials synthesis and characterizations, details of catalytic performance
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