Identification of Fenton-like active Cu sites by heteroatom modulation of electronic density

Developing heterogeneous catalysts with atomically dispersed active sites is vital to boost peroxymonosulfate (PMS) activation for Fenton-like activity, but how to controllably adjust the electronic configuration of metal centers to further improve the activation kinetics still remains a great challenge. Herein, we report a systematic investigation into heteroatom-doped engineering for tuning the electronic structure of Cu-N₄ sites by integrating electron-deficient boron (B) or electron-rich phosphorus (P) heteroatoms into carbon substrate for PMS activation. The electron-depleted Cu-N₄/C-B is found to exhibit the most active oxidation capacity among the prepared Cu-N₄ single-atom catalysts, which is at the top rankings of the Cu-based catalysts and is superior to most of the state-of-the-art heterogeneous Fenton-like catalysts. Conversely, the electron-enriched Cu-N₄/C-P induces a decrease in PMS activation. Both experimental results and theoretical simulations unravel that the long-range interaction with B atoms decreases the electronic density of Cu active sites and down-shifts the d-band center, and thereby optimizes the adsorption energy for PMS activation. This study provides an approach to finely control the electronic structure of Cu-N₄ sites at the atomic level and is expected to guide the design of smart Fenton-like catalysts.

The Fenton-like process presents one of the most powerful water treatment technologies to tackle persistent organic pollutants resulting from rapid economic development and unsustainable industrial and agricultural expansion (1 –4). The peroxymonosulfate (PMS)-based advanced oxidation process has attracted extensive attention due to its high efficiency at a wide pH range and ease of transport and storage (5 –7). However, the sluggish kinetics of PMS activation during oxidation processes results in prohibitive costs and substantial chemical inputs (8, 9). Therefore, developing efficient catalysts to accelerate the reaction kinetics of PMS is crucial toward efficient catalytic oxidation of recalcitrant organics. Although homogeneous first-row transition metals (Co²⁺, Fe²⁺, Cu²⁺, and Mn²⁺) generally exhibit remarkable capabilities for PMS activation, they also suffer problems such as poor recyclability and accumulation of sludge (10 –12). Comparatively, heterogeneous catalysts [e.g., transition metal oxides (13, 14), supported nanoparticles (NPs) (15, 16), and carbon-based materials (17, 18)] can be readily recovered and regenerated and are recognized as promising candidates for PMS activation. Nevertheless, the heterogeneity of NPs results in lower utilization efficiency of surface atoms (with 81.6% atoms buried and unavailable for 6-nm nickel NPs) and generally slower reaction kinetics than their homogeneous counterparts (19).Single-atom catalysts (SACs) featuring utmost atom-utilization efficiency and tunable electronic structure can break the limitations of heterogeneous catalysts in terms of the kinetics and catalytic activity (20, 21). Thus, SACs show a great potential to address the slow reaction kinetics of PMS for the Fenton-like process via maximizing the number of catalytic sites (22). For instance, a single-site Fe catalyst exhibited much faster reaction kinetics toward the degradation of phenol than the Fe NP catalyst, owing to the maximized atomic utilization (23). In addition, the synergetic effect between the atomic center and pyrrolic N site of supports endowed Co SACs with dual reaction sites and high activity for PMS-based oxidation (24). To further accelerate the reaction kinetics of PMS, various strategies have been developed to improve the intrinsic activity of single atomic sites. By controlling the configurations of single atomic sites, PMS was more favorable for adsorption and activation on the CoN_2 + 2 site than the CoN₄ site (25). Previous work shows that manipulating the electronic structure of single sites plays an essential role in mediating the intrinsic activity (26, 27). It is highly desirable to gain insights into tuning the electronic structure of single-atom sites to achieve superior PMS activation kinetics.Recent studies demonstrate that the electronic structure of isolated metal sites can be directly modulated by altering the coordinated atom species of the metal centers, favorable for expediting catalytic activity (28, 29). Notably, controlling the long-range interactions with suitable functionalities on the substrate of SACs can be a promising approach for tuning the electronic structure of metal centers (30). Indeed, the kinetic activity of single atomic sites was successfully tuned by the introduction of electron-withdrawing oxidized S groups or electron-donating thiophene-like S species into carbon supports of SACs (31). To this end, nonmetallic heteroatoms offer a substantial potential to serve as electron-withdrawing/donating functionalities on the carbon plane by chemical substitution (32, 33). Specifically, boron (B) with a vacant 2p_z orbital conjugating with the carbon π system extracts the electrons, while phosphorus (P) with a readily available lone electron pair and low electronegativity is expected to donate electron in graphene (34, 35). With this strategy, incorporating particular heteroatoms (B/P) into the substrate is a possible route to deplete/enrich the electronic density of metal centers, tuning the electronic structure of single sites to promote PMS activation kinetics.In this work, we designed a versatile strategy to systematically tune the electronic structure of Cu-N₄ sites by integrating specific heteroatoms (B/P) into N-doped carbon substrates of Cu SACs. Subsequently, the effect of the controlled electronic features of Cu centers on facilitating PMS reaction kinetics was explored. Here, the heteroatom modified Cu-N₄ catalysts were first prepared by using a hydrogen-bonding-assisted pyrolysis approach. Synchrotron X-ray adsorption spectroscopy and the projected density of states (PDOS) analysis verified the successful regulation of the electronic configuration of the Cu-N₄ SACs by different heteroatom functionalities. Furthermore, electron paramagnetic resonance and Raman spectra were employed to elucidate the PMS activation mechanism in the Cu-N₄/C-B/PMS system. This study opens an avenue to regulating the electronic structures of single active site of SACs to accelerate PMS activation kinetics for pollutant degradation.