Theoretical study of p-block metal–nitrogen–carbon single-atom catalysts for heterogeneous Fenton-like reaction

Abstract

The heterogeneous Fenton-like reaction has been widely used in water purification and environmental remediation due to the highly reactive nature of hydroxyl radicals. Nevertheless, the intrinsic structure–activity relationship for heterogeneous Fenton-like reaction catalysts remains to be clarified. Metal/nitrogen/carbon (M/N/C) single-atom catalysts (SACs) provide an ideal opportunity to reveal the relationship between the structure and activity. In this work, the detailed catalytic mechanism and activity of H₂O₂ decomposition on p-block main-group metal/nitrogen/carbon (PM/N/C) catalysts were investigated systematically. A volcano relationship between the catalytic activity and the adsorption energies of reaction intermediates was found for H₂O₂ decomposition on PM/N/C SACs. PM-N₂C₂ and PM-C₄ exhibit higher H₂O₂ decomposition activity than PM-N₄, indicating that reducing the N/C ratio in the coordination environment can effectively adjust the catalytic activity. By altering the N/C coordination environment, it is possible to modify the p-band position of p-block main-group metal atoms in PM/N/C SACs, thereby enhancing the catalytic activity of H₂O₂ decomposition.

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Environmental significance

The heterogeneous Fenton process was considered as a promising strategy for the removal of organic pollutants from wastewater. Establishing the structure–activity relationship is crucial for the rational design of efficient heterogeneous Fenton catalysts. Metal/nitrogen/carbon (M/N/C) single-atom catalysts (SACs) provide an ideal opportunity to reveal the intrinsic structure–activity relationship. Herein, a volcano relationship was found between the catalytic activity of H₂O₂ decomposition and the adsorption strength of the reaction intermediates and the p-band center of p-block main-group metal/nitrogen/carbon (PM/N/C) catalysts. Furthermore, modifying the coordination environment can modulate the p-band position of PM atoms and enhance the catalytic activity of PM/N/C. This work can provide a valuable principle for the rational design of heterogeneous Fenton catalysts for efficient decontamination application.

Introduction

With the rapid development of urbanization and industrialization, environmental pollution caused by the continuous release of toxic agents into water environments has become an overwhelming worldwide issue, particularly in developing and underdeveloped countries.^1–3 The Fenton reaction, known as one of the most efficient advanced oxidation processes, is widely utilized for degrading persistent organic contaminants in wastewater treatment.^4,5 Unlike the traditional homogeneous Fenton process, which involves the reaction of hydrogen peroxide (H₂O₂) with dissolved ferrous ions, the heterogeneous Fenton process offers significant advantages such as functionality across a wide range of pH values, minimal metal leaching, controlled iron sludge precipitation, and convenient catalyst recycling.^6–8

The hydroxyl radicals (·OH) generated from the catalytic decomposition of H₂O₂ on heterogeneous catalysts can oxidize various organic pollutant molecules with high activity and non-selectivity.^9–11 Gao et al. found that sulfurized CoFe₂O₄ ensured effective H₂O₂ activation and sufficient generation of oxygen-containing radicals such as the hydroxyl radical and superoxide radical, which facilitated pollutant degradation.¹² Cao et al. found that N-doped hierarchically porous carbon with embedded FeO_x exhibited high activity and selectivity for H₂O₂ generation as well as effective H₂O₂ activation to ·OH.¹³ Up to now, most published research focused on H₂O₂ adsorption, activation and hydroxyl radical production.^14–16 Although significant progress has been made, a comprehensive understanding of the detailed catalytic cycle of heterogeneous Fenton-like reactions remains to be clarified.^17,18 Gaining a deep understanding of the physical and chemical mechanisms behind the decomposition reaction of H₂O₂ is important for the exploration of novel heterogeneous Fenton-like reaction catalysts.^19,20 It is crucial to understand the intrinsic relationship between the catalytic activity and the surface atomic structure of heterogeneous Fenton-like reaction catalysts.^21,22

Metal/nitrogen/carbon (M/N/C) single-atom catalysts (SACs) have attracted more and more attention as heterogeneous Fenton-like reaction catalysts because they hold great potential to provide unmatched high activity and selectivity for Fenton and Fenton-like reactions.^23–27 Nevertheless, these investigations primarily focused on transition-metals as active centers, while p-block main-group metals (PM) have been less explored.^28–32 Exploring diverse M/N/C configurations such as introducing extra metals or altering the coordination environment with different ligands is essential for modifying electronic and catalytic characteristics.³³

The catalytic activity of transition-metal SACs benefit from the localized character of d orbitals, while that of PM atoms originates from the hybridized p orbitals.^34–36 Although p-block main-group metal/nitrogen/carbon (PM/N/C) SACs are less reported for H₂O₂ decomposition, they show superior catalytic ability in electrocatalytic processes to achieve high activity and selectivity.^37,38 Unlike SACs based on transition metals, main-group metal SACs possess several unique characteristics. Firstly, C atoms can easily stabilize central main-group metals with covalent bonds rather than coordinating with N atoms, which contributes to the structural stability of single metal atoms (SAs). Secondly, main-group metals usually have the inherent poor ability of hydrogen adsorption, which can improve the reaction selectivity. Finally, experimental and theoretical research studies indicate that adjusting the electronic configuration of s/p orbitals in main-group metal SACs to a particular state can enhance the absorption of reactive entities. Recently, Jiang et al. found that Sb/N/C consisting of Sb–N₄ moieties anchored on N-doped carbon nanosheets could act as a CO₂ reduction reaction catalyst to produce formate efficiently.³⁹ Wu et al. demonstrated that Al and Ga can act as promising active centers in SACs toward the NO reduction reaction by the modulation of p-band filling of the PM atoms.⁴⁰ These pioneering research studies have confirmed that the p electrons in PM atoms can also be activated through engineering PM/N/C SACs, resulting in promising catalytic activity.^41,42 PM/N/C SACs, which featured atomically dispersed PM atoms with activating p electrons, could possess significant potential for heterogeneous catalytic reactions.^43–45 However, the potential of PM/N/C SACs as heterogeneous Fenton-like reaction catalysts still needs to be explored. The intrinsic structure–activity relationship remains to be clarified, which is necessary for the rational design of PM SACs.⁴⁶

In this work, the mechanism and catalytic activity of H₂O₂ decomposition on PM-N₄, PM-N₂C₂, and PM-C₄ moieties embedded in graphene were investigated based on density functional theory (DFT).⁴⁷ It was found that these PM/N/C SACs exhibit excellent catalytic activity toward H₂O₂ decomposition by changing the N/C coordination environment and modulating the p-band filling of the PM centers.⁴⁸ This work firmly demonstrates that PM atoms can serve as promising active centers in SACs toward H₂O₂ decomposition. This work not only elucidates the catalytic mechanism and reaction pathways of H₂O₂ decomposition, but also clarifies the intrinsic structure–activity relationship between PM/N/C SACs and their catalytic performance. Our work theoretically introduced PM atoms as active centers in SACs for H₂O₂ decomposition, which provides a potential new avenue for the rational design of heterogeneous Fenton-like catalysts, and sheds light on the further development of advanced oxidation process (AOP) catalysts for efficient decontamination application.

Results and discussion

Geometry structure and stability of PM/N/C SACs

Di-vacancy pyridine graphene, which can provide a range of coordination modes for active centers, has shown great stability and activity from the viewpoint of theoretical and experimental contexts.⁴⁹ In this work, pyridine graphene's di-vacancy defect site was selected to support the dispersed PM atoms.⁵⁰ In order to investigate the effect of the coordination number of N-atoms surrounding the metal atoms, three configurations (PM-N₄, PM-N₂C₂, and PM-C₄ shown in Fig. 1a) were considered to support the nine PM atoms (Al, Ga, In, Tl, Ge, Sn, Pb, Sb, Bi shown in Fig. 1b), resulting in 27 PM/N/C SACs.⁵¹ The full geometric optimization structures of the 27 PM/N/C SACs are illustrated in Fig. S1–S3 of the ESI.† Note that there are three allotropic structures for PM-N₂C₂ (Fig. S4†) and the most stable configuration with two para N atoms was considered. The corresponding energies of the three structures of PM-N₂C₂ are shown in Table S1 of the ESI.† The adsorption energies of the PM atoms in PM/N/C SACs and the cohesive energies of the nine PM atoms were calculated and summarized in Fig. 1c. The adsorption energy and cohesive energy decrease with the increase of the periodic number for PM atoms in the same group due to the decrease of electronegativity.⁵² In the three configurations, PM-N₂C₂ tends to show higher adsorption energy of the central metal atoms than those of PM-N₄ and PM-C₄, which suggests that PM-N₂C₂ is more stable. As depicted in Fig. 1c, the adsorption energies of PM in PM/N/C SACs (except TlC₄ and BiN₄) are higher than the cohesive energies of the corresponding bulk metals, indicating that the nine PM atoms can anchor stably on the majority of substrates except TlC₄ and BiN₄. In order to further investigate the thermodynamic stability, ab initio molecular dynamics (AIMD) simulation was conducted to evaluate the thermal stability of PM/N/C. As shown in Fig. S5,† the total energy oscillated within a fluctuation of less than 0.003 eV per atom and the geometric structure of InC₄ was perfectly retained without obvious distortion up to 10 ps at 300 K, further confirming its high stability and experimental feasibility. Therefore, the studied PM/N/C SAC moieties are favorable for experimental realization. Both the Mulliken and Hirshfeld charge analyses show that the supported PM atom donates electrons to the substrate in all the PM/N/C SACs, as listed in Table S2.† The resultant positively charged metal atoms become active sites for catalytic reactions.⁵³

Fig. 1 (a) Schematic illustrations of PM-N₄, PM-N₂C₂, and PM-C₄ structures. (b) PM atoms considered in this work. (c) The adsorption energies between the PM atoms and the adjacent N and C atoms in PM-N₄, PM-N₂C₂, and PM-C₄, and the cohesive energies (E_coh) of PM.

Reaction pathways of H₂O₂ decomposition

Given that recently published work about H₂O₂ dissociation always focused on the dissociation of H₂O₂ and the generation of hydroxyl radicals, understanding the complete reaction cycle for H₂O₂ decomposition was necessary. The decomposition of H₂O₂ on PM/N/C is a complex process in which a variety of close-shell species (H₂O₂, H₂O, O₂, H₂) and reaction intermediates (OH, OOH, H, O) can be present.⁵⁴ Various elementary reactions were involved (Table S3†) based on those close-shell species and reaction intermediates, which include adsorption, O–O bond scission, dehydrogenation, hydrogen transfer, and desorption.¹⁹ Here, we further compiled possible pathways for H₂O₂ decomposition on PM/N/C (as shown in Fig. 2), including the OH*-assisted pathway (path I), direct dehydrogenation pathway (path II), O*-assisted pathway (path III), and O*–O* recombination pathway (path IV). A complete pathway for direct dehydrogenation (path II) was not shown for simplicity, because the O–H bond in (* denotes the adsorbed state of the reactant molecule) is more difficult to be broken than the O–O bond (as shown in Table S4†). For the other three pathways, the first reaction step is the adsorption of H₂O₂ followed by the O–O bond scission to form two OH* or one O* and one H₂O*. Then the second H₂O₂ can adsorb and undergo continuous hydrogen-transfer steps with OH* in path I or O* in path III. Moreover, the second can directly decompose to form H₂O* and O*, and O* can recombine with the O* formed in the first H₂O₂ dissociation step to form (path IV). At last, the final products O₂ and H₂O desorbed from the surface of the catalyst and the catalyst was refreshed. These pathways show a detailed and complete catalytic cycle for H₂O₂ decomposition, which will assist in developing efficient Fenton-like systems.

Fig. 2 Reaction pathways of H₂O₂ decomposition.

Here, the reaction energies and barrier energies of all elementary reactions involved in the four pathways of H₂O₂ decomposition on the 27 PM SACs were calculated based on DFT calculations, which can help to establish the complete reaction energy diagram and then characterize the catalytic activity.⁵⁵ By comparing the barrier energies of the rate-determining step in four pathways, the favourable reaction pathway can be determined.⁵⁶

Reaction pathways and activity of H₂O₂ decomposition on the PM-N₄ SACs

Based on the four pathways of H₂O₂ decomposition mentioned above, the detailed catalytic mechanism was explored. InN₄ was first taken as an example (shown in Fig. 3). H₂O₂ adsorption is the first step in the catalytic cycle of H₂O₂ decomposition with an adsorption energy of −0.37 eV. The adsorbed can be directly dehydrogenated to form H* and OOH* by the O–H bond scission, or dissociated to form two OH* or one O* atom and one H₂O* by the O–O bond scission. As shown in Fig. 3c, direct dehydrogenation of needs to conquer a high barrier energy of 1.11 eV, indicating the low efficiency of this process. For comparison, the decomposition of H₂O₂ to form two OH* or O* and H₂O* via O–O bond scission only needs to overcome a small barrier energy of 0.08 eV and 0.11 eV, respectively, suggesting the high reaction rate of these two elementary steps. As the O–O bond scission is much more favourable than direct dehydrogenation due to the significantly lower energy barriers, one conclusion can be made, that is, the O–O bond breaking is the major pathway for the first adsorbed dissociation, excluding the possibility of direct dehydrogenation (path II).⁵⁷ Furthermore, the reaction energies of and are −2.88 eV and −2.15 eV, respectively, indicating that H₂O₂ can split into two OH* or one O* and one H₂O* easily on InN₄. ·OH can generate at the transition states of H₂O₂ dissociation to form two OH* or O* and H₂O* (Fig. 3c), indicating that ·OH can be easily formed on the InN₄ surfaces. And then, the second H₂O₂ can adsorb and undergo a hydrogen transfer step to the adsorbed OH* in path I and O* in path III with energy barriers of 0.21 and 0 eV, respectively. Meanwhile, the adsorbed second can directly decompose to form H₂O* and O* with an energy barrier of 1.59 eV, which is much higher than that of hydrogen transfer steps as shown in Fig. 3d, indicating that the hydrogen transfer step for the second adsorbed molecule is more energetically favorable than the direct dissociation step (path IV). Therefore, only path I and path III were discussed in the following part of this work.

Fig. 3 Reaction energies (a) and barrier energies (b) of elementary reactions along the four possible pathways for H₂O₂ decomposition on InN₄. The energy diagrams of the first (c) and second (d) adsorbed decomposition.

Due to the low efficiency of path II and path IV for H₂O₂ decomposition on InN₄, the reaction energy diagrams of the preferable pathways (path I and path III) were identified from the viewpoint of thermodynamics (shown in Fig. 4). In path I, the second adsorbed with an adsorption energy of −1.04 eV and then transfers an H atom to OH* to form H₂O*, characterized by an energy barrier of 0.21 eV and a reaction energy of 0.05 eV. The formed H₂O* was desorbed from the InN₄ surface with a desorption energy of 0.55 eV. Additionally, the OOH* adsorbed on InN₄ undergoes the second H-transfer step to the other OH* with a reaction energy and barrier energy of 0.70 and 0.71 eV, respectively. The final products H₂O* and are sequentially released from the InN₄ surface with a desorption energy of 0.44 eV and 0.60 eV, respectively. In path III, the first H₂O₂ dissociates to form O* and H₂O*. Subsequently, H₂O* desorbs from the InN₄ surface with a desorption energy of 0.66 eV, and then the second H₂O₂ adsorbs onto the InN₄ surface with an adsorption energy of −0.14 eV. The adsorbed second undergoes sequential H-transfer steps with O*. The barrier energies for the two sequential H-transfer steps were calculated as 0 and 0.71 eV, respectively, and the corresponding reaction energies were calculated to be −1.73 and 0.70 eV. At last, H₂O* and sequentially desorb from the InN₄ surface with desorption energies of 0.44 eV and 0.60 eV, respectively. The catalytic cycle of H₂O₂ decomposition was limited by the elemental step with the largest energy barrier in a reaction pathway, which is the rate-determining step for H₂O₂ decomposition. The reaction pathway with a smaller energy barrier of the rate-determining step holds higher potential toward H₂O₂ decomposition. Based on the thermodynamic analysis, the rate-determining step of H₂O₂ decomposition on InN₄ along path I and path III is identified to be with a barrier energy of 0.71 eV, indicating that path I and path III can occur simultaneously.

Fig. 4 The reaction energy diagram of H₂O₂ decomposition on InN₄via path I and path III.

As summarized in Table S4,† the reaction energies and barrier energies of the direct dehydrogenation of H₂O₂ (path II) were much larger than those of O–O bond scission (paths I and III) on all 27 PM SACs, indicating that the O–H bond was more difficult to break and path II was energetically unfavorable. Furthermore, when the active sites were covered by O* or OH*, the second H₂O₂ was hard to dissociate directly to form O* and H₂O* (path IV) due to the large barrier energy, suggesting that path IV was disadvantaged. Therefore, only paths I and III were discussed in the following part.

Fig. S6–S8 in the ESI† summarized the decomposition process of H₂O₂ on the remaining eight PM-N₄ (AlN₄, GaN₄, TlN₄, GeN₄, SnN₄, PbN₄, SbN₄, BiN₄) SACs. It was found that the rate-determining step of H₂O₂ dissociation on PM-N₄ is different. The desorption of is the rate-determining step for H₂O₂ decomposition on AlN₄ and GaN₄ with barrier energies of 1.80 and 1.37 eV, respectively. For InN₄, BiN₄, SbN₄, GeN₄, and SnN₄, the rate-determining step is the reaction of OOH* with OH* to form the final products H₂O* and with barrier energies of 0.71, 0.93, 0.82, 0.74, and 0.86 eV, respectively. For PbN₄, the reaction of the adsorbed O* atom with the second molecule to form OOH* and OH* is the rate-determining step with a barrier energy of 0.84 eV. For TlN₄, the O–O bond scission of the initial H₂O₂ molecule to form two OH* is the rate-determining step with a barrier energy of 0.91 eV.

From the discussion above, it was known that OH* and O* are two critical reaction intermediates in path I and path III, and the moderate adsorption strength of OH* and O* can ensure relatively high reaction rates for the whole process of H₂O₂ dissociation. Interestingly, as shown in Fig. 5a, the adsorption energy of OH* exhibits a linear relationship versus that of O* on PM-N₄ sites. Therefore, the adsorption energy of OH* can act as a valid descriptor to characterize the ability of PM-N₄ SACs for H₂O₂ decomposition. A straightforward volcano relationship (Fig. 5b) was observed between the barrier energies of the rate-determining step and the adsorption energies of OH*. For AlN₄ and GaN₄ in the left leg of the volcano curve, the release of the final product is the rate-determining step of H₂O₂ decomposition due to the strong adsorption strength of the reaction intermediates. In contrast, PM-N₄ in the right leg displayed improved activity for H₂O₂ decomposition due to the moderate adsorption strength of reaction intermediates. InN₄ exhibits the most excellent catalytic activity for H₂O₂ decomposition due to the favourable adsorption strength of reaction intermediates.

Fig. 5 (a) The linear relationship between the adsorption energies of OH* and O* on PM-N₄. (b) The volcano relationship of the barrier energies of the rate-determining step involved in the H₂O₂ decomposition process against the adsorption energies of OH* on PM-N₄. (c) The linear relationship between the adsorption energies of O* and OH* and the p-band centre (ε_p) of the PM atoms in PM-N₄. (d) The volcano relationship of the barrier energies of the rate-determining step involved in the H₂O₂ decomposition process against the p-band centre (ε_p) of the PM atoms in PM-N₄.

The adsorption strength of the reaction intermediates depends on the electronic interaction between the reaction intermediates and the metal centers. The electronic structure was investigated to further understand the reaction mechanism of the PM-N₄ SACs for the decomposition of H₂O₂.^58,59 By examining the partial density of states (PDOS) of the p-band for PM atoms in PM-N₄ (shown in Fig. 5c), a strong linear relationship was found between the adsorption energies of the reaction intermediates (O* and OH*) and the p-band centre (ε_p) of the PM atoms in PM-N₄. The higher the energy level of the p-band center, the weaker the adsorption strength of the reaction intermediate, indicating that the p-band center can also act as a descriptor to build the volcano curve (Fig. 5d). The energy level of the p-band near the Fermi level is responsible for the binding strength. With a higher p-band location (higher ε_p position), the resonance state after reactant adsorption is closer to the Fermi level, which results in the filling of anti-bonding states of the p-band and then the weaker adsorption strength (Fig. 6a).⁶⁰ According to orbital analysis, the bonding state of OH*/O* adsorption on the PM center depends on the coupling between the 2p states of oxygen and the p states of the PM atom. The PM–O interaction is mainly assigned to the head-to-head σ-bonds formed between p_z orbitals (Fig. 6b). As shown in Fig. 6c, the p_z-band of the central metal shifts to the left relative to the Fermi level as the central metals vary from Al to Tl. A downshift of the p_z states leads to a downward shift of the antibonding states, indicating the formation of weaker chemical bonds. Moreover, the crystal orbital overlap population (COOP) was adopted to gain deeper insight into the electronic interaction between PM-N₄ and O-containing adsorbates by revealing the corresponding bonding and antibonding states. As shown in Fig. 6d, the COOP value of O* adsorbed on AlN₄, InN₄, and TlN₄ near the Fermi level gradually decreases, indicating more occupied antibonding states near the Fermi level on TlN₄. This is consistent with the increase of the p-band center and the decrease of the adsorption strength of O*.

Fig. 6 (a) Schematic illustrations of the interactions between OH molecular orbitals and the p-band of the PM atom. (b) The p_z–p_z orbital hybridization between the central metal atom and O atom. (c) The PDOS of p_z in central metal atoms of AlN₄, GaN₄, InN₄, and TlN₄. (d) The COOP of O* adsorbed on AlN₄, InN₄, and TlN₄.

Reaction pathways and activity of H₂O₂ decomposition on PM-N₂C₂ and PM-C₄

Except for InN₄ and GeN₄, the barrier energies of rate-determining steps for the decomposition of H₂O₂ on PM-N₄ always exceed 0.8 eV, which is challenging to achieve at room temperature. Therefore, the effect of the N/C coordination environment on the catalytic activity of H₂O₂ decomposition was further investigated. Tables S4 and S5† show the reaction energies and barrier energies of the direct dehydrogenation of the first adsorbed and the direct decomposition of the second adsorbed on PM-N₂C₂ and PM-C₄. In path II and path IV, the efficiency is low because of the high barrier energies of the direct dehydrogenation of the first adsorbed and direct decomposition of the second adsorbed . The energy diagrams for H₂O₂ decomposition along path I and path III on PM-N₂C₂ and PM-C₄ are shown in Fig. S9–S14.† A nearly linear relationship was also found between the adsorption energies of OH* and the adsorption energies of O* (Fig. 7a). The volcano relationship between the adsorption energies of OH* and the barrier energies of the rate-determining step for H₂O₂ decomposition on PM-N₄, PM-N₂C₂, and PM-C₄ is shown in Fig. 7b. The two volcano curves in Fig. 5b and 7b show that PM/N/C SACs exhibit optimal catalytic activity for H₂O₂ decomposition when the adsorption energy for OH* is around −3.34 eV.

Fig. 7 (a) The linear relationship between the adsorption energies of OH* and O* on PM-N₄, PM-N₂C₂, and PM-C₄. (b) The volcano relationship of the barrier energies of the rate-determining step involved in the H₂O₂ decomposition against the adsorption energies of OH* on PM-N₄, PM-N₂C₂, and PM-C₄. (c) The linear relationship between the adsorption energies of OH* and the ICOOP. (d) The COOP of O* adsorbed on InN₄, InN₂C₂, and InC₄.

It can be observed from Fig. 7b and Table S6† that the barrier energies of the rate-determining step on AlN₂C₂, AlC₄, GaN₂C₂, and GaC₄ are significantly reduced compared to AlN₄ and GaN₄ due to the weak adsorption of the reaction intermediates. For Al, In, Ge, Sn, and Sb, the barrier energies of H₂O₂ decomposition gradually decrease with the reduction of N atoms in the coordination environment of PM/N/C SACs. In all the three configurations (PM-N₄, PM-N₂C₂, and PM-C₄), In presented the best catalytic efficiency for H₂O₂ decomposition compared with the other eight PM atoms. InC₄ presents the highest catalytic activity among the 27 PM SACs in this work, and the barrier energy of the rate-determining step for H₂O₂ decomposition on InC₄ is 0.59 eV, which can be overcome easily at room temperature.⁶¹ Furthermore, metal Ge also shows significant catalytic efficiency for H₂O₂ decomposition in PM/N/C. Generally, PM-N₂C₂ and PM-C₄ exhibit higher catalytic activity than PM-N₄ for H₂O₂ decomposition. The barrier energies of the rate-determining steps for the decomposition of H₂O₂ on PM-N₂C₂ and PM-C₄ were decreased to smaller than 0.80 eV, except for AlN₂C₂, AlC₄, and PbC₄, as shown in Table S6.† Based on the discussion above, we can conclude that changing the N/C coordination environments of the PM SACs can enhance the catalytic efficiency of H₂O₂ decomposition effectively.

The catalytic efficiency of H₂O₂ decomposition is governed by the adsorption strength of reaction intermediates, which in turn depends on the surface electronic states of catalysts. In order to gain further insight into the origin of the different catalytic activities of PM/N/C with different N/C coordination environments, the electronic structures of PM/N/C were considered. For a more quantitative comparison, the integrated COOP (ICOOP) was calculated between the PM atoms in PM/N/C and the absorbed OH*. As shown in Fig. 7c, a strong linear relation between the adsorption energies of OH* and the ICOOP value was found. Interestingly, the adsorption strength of intermediates (OH* and O*) on AlN₄, AlN₂C₂, and AlC₄ decreased with the decrease of the number of N atoms in the coordination environment, and the catalytic activity of H₂O₂ decomposition increased to near the apex of the volcano curve. The Mulliken charge value of the Al atom in AlN₄, AlN₂C₂, and AlC₄ gradually decreases (Table S2†), which implies that the electrostatic interaction between the reaction intermediates (OH* and O*) and Al atom is weakening. At the same time, the p-band center of the Al atom in AlN₄, AlN₂C₂, and AlC₄ shifts to the right (Fig. 8a), which results in the shifting of the 1π-adsorbate resonance state at the OH* molecule to the Fermi level (as shown in Fig. 9a), leading to the weaker adsorption strength. In contrast, the adsorption strength of intermediates (OH* and O*) on TlN₄, TlN₂C₂, and TlC₄ increased with the reduction of the number of N atoms in the coordination environment. The ICOOP of OH* adsorbed on TlN₄, TlN₂C₂, and TlC₄ gradually increased (Table S6†), indicating that the adsorption strength of OH* on TlN₄, TlN₂C₂, and TlC₄ gradually enhanced.⁶² Meanwhile, as the N atoms in the coordination environment decrease, the p-band center of the Tl atom in TlN₄, TlN₂C₂, and TlC₄ moves leftward (Fig. 8b), and the 1π resonance states after OH adsorption on Tl shift to the left, resulting in less occupied antibonding states near the Fermi level (Fig. 9b). Fig. 7b also reveals that InN₄, InN₂C₂, and InC₄ exhibit the best activity of H₂O₂ decomposition in PM-N₄, PM-N₂C₂, and PM-C₄, respectively, situated at the volcano curve's peak, exhibiting a steady rise in activity for H₂O₂ decomposition. However, the adsorption strength of intermediates (OH* and O*) on InN₄, InN₂C₂, and InC₄ doesn't change significantly, as demonstrated by the similar COOP value of O* adsorbed on InN₄, InN₂C₂, and InC₄ near the Fermi level (Fig. 7d). As shown in Fig. 9c, the hybridization between In-p states and O-p states becomes stronger as the number of N atom decreases.⁶³ The p_x and p_y orbitals of the In atom in InN₄, InN₂C₂ and InC₄ were further investigated based on the PDOS (shown in Fig. 10). It was found that the p_x and p_y orbitals of the In atom shift to higher energy position relative to the Fermi level as the N/C coordination environment varies from InN₄ to InN₂C₄ and InC₄, which results in the upward shift of the antibonding states and stronger adsorption strength of OH*.

Fig. 8 (a) The PDOS of the Al p-state in AlN₄, AlN₂C₂, and AlC₄. (b) The PDOS of the Tl p-state in TlN₄, TlN₂C₂, and TlC₄.

Fig. 9 The PDOS for OH vacuum states and OH* adsorbed on the AlN₄, AlN₂C₂, AlC₄ (a), TlN₄, TlN₂C₂, TlC₄ (b) and InN₄, InN₂C₂, InC₄ (c).

Fig. 10 p_x (a) and p_y (b) orbitals of In atoms for OH* adsorbed InN₄, InN₂C₂ and InC₄.

Conclusion

In this work, the catalytic cycles of H₂O₂ decomposition on PM/N/C SACs were systematically studied based on density functional theory calculation. Linear relationships were found between the adsorption energies of reaction intermediates and the p-band centre of the PM atoms in PM-N₄, and a straightforward volcano plot was found between the catalytic activity and the adsorption strength of the reaction intermediates for H₂O₂ decomposition on PM-N₄. Among the nine PM-N₄ sites, InN₄ and GeN₄ exhibit excellent catalytic activity toward H₂O₂ decomposition with low activation energies of the rate-determining step (less than 0.8 eV). The coordination environment also plays an important role in the activity of PM/N/C SACs. PM-N₂C₂ and PM-C₄ exhibit higher H₂O₂ decomposition activity than PM-N₄ by altering the N/C coordination environment and then modifying the p-band position of PM atoms in PM/N/C SACs. This means that reducing the N/C ratio in the coordination environment of PM/N/C SACs can adjust the catalytic activity of H₂O₂ decomposition efficiently. In other words, PM metals possess the capability to serve as viable active centers in PM/N/C SACs for the Fenton-like reaction to form ·OH for pollutant degradation, which is important not only in environmental remediation but also in the biomedical field.

Data availability

All data included in this study are available upon request by contact with the corresponding author.

Author contributions

Chen Zhou: investigation, writing – original draft. Haobin Tan, Shengbo Wang: formal analysis. Peng Zhang: conceptualization, supervision, writing – review & editing.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Guangdong Province (2024A1515012765), the National Natural Science Foundation of China (52350005, 51838005, and 21403092), and the basic research projects jointly funded by Guangzhou City and Guangzhou University (202201020194).

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Footnote

† Electronic supplementary information (ESI) available: Computational details; atomic structures; reaction energies; barrier energies; adsorption energies. See DOI: https://doi.org/10.1039/d4en00778f

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