First-principles methods for unraveling the structure–catalytic activity relationship and mechanism of δ-MnO₂-supported metal cluster catalysts in ambient temperature benzene to CO₂ degradation

Abstract

Benzene, a volatile cyclic aromatic hydrocarbon, is frequently discharged into the environment and poses serious threats to human health. However, the effective degradation of benzene at low temperatures remains a formidable challenge in the field of environmental remediation. In this study, a series of 49 catalyst samples, spanning from single atoms to trimers and tetramers, supported on δ-MnO₂ were meticulously constructed by employing the density functional theory (DFT) method. This approach facilitates an in-depth exploration of the structure–function relationship and enables the identification of prospective catalysts capable of degrading benzene into CO₂ and H₂O, thereby providing valuable insights for the development of efficient benzene degradation strategies at low temperatures. The obtained results unequivocally demonstrate that the separation between reaction products and reactants is substantially augmented with an increment in the number of active sites. Notably, the Ag₄ tetramer exhibits a remarkable enhancement in this regard. Further in-depth analyses reveal that the superior catalytic activity of Ag₄ relative to Pt₄ and Pd₄ tetramers can be ascribed to the closer energy alignment between the highest occupied molecular orbital (HOMO of Ag₄) and the lowest unoccupied molecular orbital (LUMO of benzene), which facilitates electron transfer and reaction initiation. To assess the practical catalytic effectiveness, a detailed analysis of the reaction pathways for benzene degradation was carried out. Remarkably, the energy barrier of the rate-determining step was determined to be merely 0.71 eV at room temperature, comparable to those achieved under photocatalytic and high-temperature conditions. This finding is of great significance as it represents the first successful degradation of benzene by precisely controlling the size of metal clusters. Overall, this study not only provides valuable theoretical insights for benzene abatement but also paves the way for remediating related pollutants, heralding a new approach in the pursuit of sustainable environmental protection strategies.

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1. Introduction

As one of the most prevalent volatile organic compounds (VOCs), benzene is renowned for its high toxicity, inflicting severe environmental deterioration and substantial harm to the human body upon short-term or long-term exposure.¹ Nevertheless, the efficient and economical decomposition of benzene remains a formidable challenge. Over the past several decades, a plethora of methodologies for benzene elimination, such as thermal catalysis, photocatalysis, and electrocatalysis, have been widely employed.^2,3 Among these, catalytic oxidation is regarded as one of the most efficacious approaches, whereby benzene can be completely converted into harmless CO₂ and H₂O at relatively low temperatures.^4,5 However, cleaving the benzene ring under mild conditions persists as an intractable obstacle in this process.

Supported noble metal nanoparticle catalysts exhibit effectiveness in benzene decomposition at low temperatures.⁶ Nonetheless, their extensive application is impeded by the high cost associated with them.⁷ Atomically dispersed catalysts (ADCs), which manipulate the configuration and composition of the central metal atom, possess advantages like high atom utilization and catalytic selectivity.^8,9 ADCs manifest diverse types based on the number of central metal atoms, encompassing single-atom catalysts (SACs), dual-atom catalysts (DACs), and clusters.^10–12 Among them, clusters, composed of several to tens of atoms, frequently display exceptionally high catalytic activity in comparison with nanoparticles and have achieved significant breakthroughs in VOC removal reactions.^13,14

However, the diminutive size of ADCs renders them prone to agglomeration and transformation into larger nanoparticles, thereby altering the electronic structure and considerably diminishing the catalytic activity.¹⁵ Consequently, maintaining the stability and efficiency of these ADCs represents a major challenge. It has been reported that techniques of loading single atoms onto carriers, such as metal oxides, graphene, and nanoparticles, can effectively forestall the aggregation of ADCs.^16,17 Zhang et al. reported that metal oxide carriers not only function as ligands for stabilizing catalytic metal centers but also directly partake in the activation of reactants and enhance the catalytic performance.¹⁸

As one kind of metal oxide, manganese dioxide (MnO₂) has been regarded as an efficient and environmentally friendly catalyst for the elimination of pollutants such as CO, NO, ozone, and volatile organic compounds (VOCs).^19–21 For example, ultra-thin birnessite-type manganese dioxide nanosheets (δ-MnO₂) have been demonstrated to possess high activity in converting ppm-level formaldehyde into harmless carbon dioxide at room temperature.²² However, on pristine manganese oxides, the decomposition of benzene is scarcely achieved below 200 °C, with a conversion rate of merely around 40% even at a high temperature of 400 °C.²³ In response to this, extensive investigations have been carried out based on layered δ-MnO₂ for benzene oxidation. Chen et al. enhanced the decomposition activity of MnO₂ towards benzene by modifying δ-MnO₂ with Ti atoms, leading to nearly complete removal of benzene at approximately 250 °C.²⁴ Liu et al. compared the effects of Mo⁶⁺ and Ce³⁺ as typical high-valence and low-valence modifiers in promoting the decomposition of benzene.²⁵ Zhang et al. achieved remarkable performance by doping single Pt atoms into MnO₂ through a hydrothermal method, which significantly boosted the catalytic activity for the degradation of toluene at low temperatures.²⁶ Specifically, 10 ppm of toluene was completely converted into CO₂ at 80 °C and fully oxidized to carbon dioxide at 220 °C. Nevertheless, although these catalysts have lowered the oxidation and degradation temperature of benzene, studies on the conversion of benzene to CO₂ at room temperature remain unreported.

In the present study, first-principles methods were utilized to predict the catalytic potential of benzene degradation at room temperature on a series of δ-MnO₂(001)-based atomically dispersed catalysts (ADCs). Twenty-three single-atom catalysts (SACs) and metal trimer catalysts with different work functions were investigated to explore the adsorption strengths of reactants (benzene) and products (H₂O and CO₂), respectively. By establishing the relationship between the catalytic activity and the amount of metal active sites, further ADCs of metal tetramers were studied to widen the gap between the adsorption energy of adsorbents and reactants. Among them, the Ag₄ cluster exhibited outstanding performance with an adsorption energy gap exceeding 3 eV. Moreover, the optimal pathway for benzene degradation at room temperature was proposed, featuring a limiting step free energy barrier of only 0.71 eV, which is comparable to those reported under photocatalytic and high-temperature conditions. This work presents a novel catalytic oxidation pathway for benzene degradation and offers theoretical insights for optimizing air quality by removing VOCs.

2. Calculation method

All computational procedures were carried out employing the Vienna Ab initio Simulation Package (VASP. 6.0), which is grounded in density functional theory (DFT).^27,28 To guarantee the precision of the calculations, plane-wave basis sets were implemented with a cutoff energy of 500 eV. The electron–ion core interactions were modeled by means of the Projected Augmented Wave (PAW) approach, and the General Gradient Approximation (GGA-PBE) developed by Perdew, Burke, and Ernzerhof was utilized to depict electron interactions.^29,30 Brillouin zone integration was executed on a 4 × 4 × 1 grid, with a convergence criterion set at 1 × 10⁻⁵ eV. In all theoretical computations, the spin polarization method was incorporated, and the DFT-D³ method was applied for van der Waals correction.^31–33 Additionally, the magnetic effects associated with δ-MnO₂ were also taken into account. The spin-polarized density functional theory with Hubbard U correction (DFT+U) was used to correct the standard DFT in describing the partially filled d-states for the Mn atoms, and the effective U value was set as 5.5 eV according to the literature.^34,35 Meanwhile the antiferromagnetic (AFM) configuration was adopted in the calculation as mentioned in previous studies.^36,37

The formation energy (E_form) of a single atom, trimer or tetramer on δ-MnO₂(001) was defined according to the following formula:³⁸

E_form = E_total − E_sub − nE_bulk/N

where E_total represents the total energy of the substrates and adsorbate, E_sub denotes the energy of the substrates, and E_bulk indicates the energy of the loaded metals. N signifies the number of atoms in the bulk, and n represents the number of atoms in the selected metal catalysts.

The adsorption energies (E_ads) were computed using the subsequent equation:³⁹

E_ads = E_total − E_sub − E_mol

where E_mol is the energy of the free molecule.

The free energy (ΔG) of each reduction step was obtained at zero bias potential using the expression:⁴⁰

ΔG = ΔE + ΔE_ZPE + TΔS

where ΔE is the reaction energy, ΔE_ZPE is the difference in zero-point energies, T is the temperature (298.15 K) and ΔS is the reaction entropy.

3. Discussion and results

3.1 The structural stability and electronic properties of δ-MnO₂(001) based SACs

The birnite type MnO₂ with the (001) plane was chosen as the substrate to support ADCs and enable the degradation of benzene to CO₂ and H₂O at room temperature. The catalytic potential of 23 single-atom catalysts (SACs) covering a diverse range of work functions (Sc, Ti, Zr, Ni, Mn, Fe, Cu, Ag, Nb, Co, Rh, Ru, Pd, Ir, Pt, Mo, Ta, Zn, Au, V, Cr, W, and Hf) on the δ-MnO₂(001) surface was initially investigated to elucidate the impact of active sites on catalytic activities. As depicted in Fig. S1,† there are two possible positions for single metal loading on the δ-MnO₂(001) surface, namely the “a site” and the “b site”. Through energy comparison, it was determined that all single metals favored the “a” deposition site and coordinated with three O atoms. To assess the resistance to sintering of SACs on δ-MnO₂(001), the formation energies (E_form) of the 23 types of SACs were primarily calculated. As shown in Fig. 1a and b, 15 SACs (Sc, Ti, Zr, Ni, Mn, Fe, Cu, Ag, Nb, Co, Rh, Ru, Pd, Ir, and Pt) exhibited negative E_form values, indicating high anti-sintering stabilities as illustrated in Fig. 1b. Additionally, the thermodynamic stabilities of these 15 SACs under the practical catalytic reaction temperature conditions (300 K) were simulated using the ab initio molecular dynamics (AIMD) method. It was observed that the SACs of Ag, Au, Mo, Co, Cu, Fe, Nb, Pd, and Rh maintained stable structures after 5 ps of molecular dynamics simulation (Fig. S2†). At this stage, 9 kinds of SACs possessing thermal, dynamic, and structural stability were identified, and subsequently, the structure and electronic properties of these catalysts, as well as their potential for catalyzing the degradation of benzene at room temperature, would be further explored.

Fig. 1 (a) Formation energy of single-atom catalysts (SACs) supported on δ-MnO₂(001). (b) Correlation model of SACs on δ-MnO₂(001). (c) Electronic localization functions of the representative SACs. (d) Bader charge between SACs and δ-MnO₂(001).

The chemical bonds and electronic interactions between SACs and δ-MnO₂(001) could be further probed through the electronic localization function (ELF) pattern. As presented in Fig. 1c, all single metals were surrounded by a greenish electronic cloud, while the O atom was surrounded by a reddish electronic cloud. This suggested that the metal atoms donated electrons to the adjacent O atoms and formed ionic bonds. Bader analysis also provided quantitative results for the interactions between SACs and the substrate as shown in Fig. 1d, where all SACs acted as electron donors and the O atom served as the electron acceptor. The direction of electron transfer was from the single metal atom to δ-MnO₂, with the number of transferred electrons exceeding 0.5 e. Notably, the interaction strength was correlated with the metal work function: the lower the metal work function, the greater the number of electrons transferred to δ-MnO₂, accompanied by stronger interactions.

It is well established that an ideal catalyst should exhibit a strong adsorption strength towards reactants for effective activation and a weak adsorption strength towards products to facilitate desorption. To investigate the catalytic performance of the single-atom catalysts (SACs) in the degradation of benzene at room temperature, the adsorption properties of the reactant (benzene) and the products (CO₂ and H₂O) were initially examined. As illustrated in Fig. 2a, the adsorbed H₂O binds to the SACs solely through its O atom, whereas multiple chemical bonds were formed between benzene, CO₂, and the SACs. This maximizes the interaction, which was manifested in the charge density difference (CDD) analysis presented in Fig. 2b. The blue electron cloud surrounding benzene indicated its role as an electron donor, contributing electrons to the δ-MnO₂(001)-based SACs. In contrast, CO₂ and H₂O capture electrons from the SACs, as evidenced by the yellow electron clouds around them. Furthermore, the Bader analysis in Fig. 2c corroborated the CDD results. The negative charge values of H₂O/CO₂ signify electron gain, while the positive values of benzene represent electron loss to the SACs. Notably, the aforementioned electron interactions were also correlated with the work function of the SACs. A smaller work function leads to a greater transfer of charge from the SACs to H₂O and CO₂. However, the interaction between benzene and the SACs follows an opposite trend. The SACs with larger work functions, such as Pd and Pt, were more prone to accepting electrons from benzene, where benzene functions as an electron donor on these SACs. The adsorption results of benzene, H₂O, and CO₂ on the SACs conformed to the above electron interaction principles (Fig. 2d). Specifically, a smaller work function of the SACs corresponds to a higher adsorption energy for H₂O and CO₂, whereas for benzene, the opposite relationship is observed. Although the difference in adsorption energy between benzene and CO₂ was significant, it is challenging to separate benzene and H₂O on these SACs.

Fig. 2 (a) Optimized structures of benzene, CO₂, and H₂O molecules adsorbed on δ-MnO₂-supported single-atom catalysts (SACs); (b) charge density difference of benzene, CO₂, and H₂O adsorbates; (c) adsorption energies of benzene, CO₂, and H₂O on δ-MnO₂-supported SACs; (d) Bader charge between the adsorbates and SACs.

3.2 Active site engineering of metal clusters on δ-MnO₂(001) for benzene degradation: from trimers to tetramers

To enhance the catalytic performance of ADCs, the addition of active sites was proposed to intensify the interaction between catalysts and benzene, which may widen the adsorption energy gap between reactants and products. Given the significant impact of the work function of single-atom catalysts (SACs) on the catalytic activity, 23 metal trimers (Sc₃, Ti₃, Zr₃, Ni₃, Mn₃, Fe₃, Cu₃, Ag₃, Nb₃, Co₃, Rh₃, Ru₃, Pd₃, Ir₃, Pt₃, Mo₃, Ta₃, Zn₃, Au₃, V₃, Cr₃, W₃, and Hf₃) within different work function ranges were considered. After optimization, some of the trimers underwent distortion or oxidation, and 17 types of trimers (Ag₃, Pd₃, Pt₃, Rh₃, Ru₃, Fe₃, Cu₃, W₃, Ir₃, Mo₃, Au₃, Cr₃, V₃, Nb₃, Ni₃, Hf₃, and Ta₃) exhibited structural stability (Fig. S3†). To further assess their stabilities, the formation energies were calculated, and 12 types of trimer ADCs (Ag₃, Pd₃, Pt₃, Rh₃, Ru₃, Cu₃, Fe₃, W₃, Ir₃, Au₃, Nb₃, and Ta₃) were screened out as shown in Fig. 3a. Moreover, the energy–time curves and structures obtained from molecular dynamics simulations at room temperature (300 K) for 5 ps as shown in Fig. S4† indicated that 7 trimers (Ag₃, Pd₃, Pt₃, Rh₃, Ru₃, Cu₃, and Fe₃) exhibited potential stability. Furthermore, the AIMD simulation at 300 K was extended to 50 ps for the selected δ-MnO₂-M₃, which showed that all of them had high thermodynamic stability. Compared with SACs, the increase in active metal atoms in the trimers led to an increase in the number of bonds between benzene, H₂O, CO₂, and the catalysts, resulting in a significant increase in their adsorption energies, especially for benzene (exceeding 2 eV) as shown in Fig. 3b. Notably, with the expansion of active sites, the adsorption energy difference among H₂O, CO₂, and benzene on Ag₃, Pd₃, and Pt₃ was substantially enlarged. Consequently, the products tended to desorb from the catalyst surface, thereby preventing the deactivation of the active sites.

Fig. 3 (a) Formation energy of metal trimers on δ-MnO₂(001); (b) adsorption energies of benzene, CO₂, and H₂O on the metal trimers; (c) Bader charge of the adsorbed benzene, CO₂, and H₂O on the metal trimers; (d) adsorption configurations of benzene, CO₂, and H₂O on the metal trimers; (e) charge transfer mechanism before and after benzene adsorption on Ag₃; (f) charge transfer mechanism before and after benzene adsorption on Fe₃.

The electronic interactions exhibited similar trends to the adsorption energies. In contrast to SACs, the charge transfer numbers between the trimers and the reactants/products gradually increased, especially for the Ag₃, Pd₃, and Pt₃ trimers. Interestingly, both H₂O and CO₂ obtained electrons from SACs and trimers, but for benzene, there was a difference. From the low work function trimers (such as Cu₃ and Fe₃), benzene acquired electrons, while benzene lost electrons to the high work function trimers (such as Pd₃ and Pt₃) as shown in Fig. 3c. Furthermore, when benzene was adsorbed on trimers, the interaction between the trimer and δ-MnO₂ showed two completely different patterns. For example, Pt₃ gained electrons from benzene while donating 0.56 e electrons to δ-MnO₂ (as shown in Fig. 3e). Conversely, Fe₃ donated electrons to benzene while causing a reduction of 0.74 e electrons on δ-MnO₂ (as shown in Fig. 3f).

Based on the above, the adsorption energy gap between benzene and the products was greatly increased with the increase in active sites, as benzene requires more active sites compared to CO₂ and H₂O. Through the trimer study, three potential single-cluster catalysts, namely Ag₃, Pd₃, and Pt₃, were identified. To explore whether further enhancing the active sites could optimize the performance, three types of tetramer catalysts based on trimers, namely Ag₄, Pd₄, and Pt₄, were constructed. These tetramers had two possible configurations, tetrahedron and parallelogram, as shown in Fig. S6.† Besides, the energy–time curve and structures obtained from molecular dynamics simulations for δ-MnO-M₄ at room temperature over a duration of 50 ps (Fig. S7†), as well as the phonon spectrum (Fig. S8†) were calculated. The results provided robust evidence to demonstrate the stability of the system.

Through energy comparison, all of them tended to form a parallelogram on the δ-MnO₂(001) surface, and their negative formation energies indicated high structural stability (Fig. 4b).

Fig. 4 (a) Adsorption configurations of benzene, CO₂, and H₂O on Ag₄, Pd₄, and Pt₄ tetramers; (b) formation energy of Ag₄, Pd₄, and Pt₄ tetramers on δ-MnO₂; (c) adsorption energies of benzene, CO₂, and H₂O on Ag₄, Pd₄, and Pt₄ tetramers; (d) lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) patterns of benzene and δ-MnO₂-Ag₄; (e) relative positions of the LUMO of benzene and the HOMO of Ag₄, Pd₄, and Pt₄ tetramers, respectively.

The adsorption properties of benzene, H₂O, and CO₂ on these tetramers are presented in Fig. 4a and c. Comparatively, the tetramers had larger adsorption energies for benzene and weaker adsorption strengths for CO₂ and H₂O than those on the trimers. These results were consistent with expectations, where the tetramers further enhanced the adsorption energy of benzene and reduced the adsorption of CO₂ and H₂O. Especially on the Ag₄ tetramer, the adsorption energy for benzene could reach up to 3.62 eV, while for CO₂ and H₂O it was less than 0.5 eV. In addition, the superiority of Ag₄ over Pt₄ and Pd₄ was investigated through the molecular frontier orbital model. As shown in Fig. 4d and e, the lowest unoccupied molecular orbital (LUMO) of benzene was highly compatible with the highest occupied molecular orbital (HOMO) of the Ag₄ cluster. Moreover, when compared with the Ag₄ and Pd₄ clusters, the HOMO of Ag₄ was more closely aligned with the LUMO of benzene. The above analysis demonstrated that the Ag₄ tetramer had outstanding advantages in the separation of benzene and its products and was a promising candidate catalyst for the degradation of benzene at room temperature.

3.3 Reaction pathway elucidation of benzene degradation on the Ag₄ tetramer: a comprehensive theoretical study

To dissect the theoretical catalytic characteristics of the Ag₄ tetramer during the degradation of benzene into CO₂ and H₂O, a comprehensive and methodical exploration into the particulars of reaction pathways was executed. The incipient oxidation of benzene could potentially transpire via three conceivable routes: activation instigated by O₂ molecules, direct dehydrogenation, or combination with the oxygen atom liberated from the decomposition of O₂ (Fig. 5a). Concerning the first pathway, the amalgamation between the O₂ molecule and the adsorbed benzene was highly arduous due to the positive adsorption energies. Instead, the O₂ molecule manifested a predilection for bonding with the Ag₄ tetramer, possessing adsorption and free energy values of −0.54 eV and −0.22 eV, respectively (Fig. S9†). In the context of the dehydrogenation pathway, an additional free energy of 1.09 eV was required to propel the reaction, which was disadvantageous for the overall reaction progression. Moreover, in the O-atom binding pathway, the adsorption site of the O atom was initially scrutinized. Here, the O atom also exhibited a propensity for bonding with the Ag atom, signifying an exothermic process and a relatively diminutive reaction free energy of −0.31 eV (Fig. S10 and S11†).

Fig. 5 (a) Reaction pathway of benzene oxidation and (b) reaction pathway of benzene degradation to a pentagonal ring on δ-MnO₂-supported Ag₄ tetramers.

Subsequently, our focus was centered on the pathways entailing benzene oxidation by O₂ molecules or O atoms. Along the former pathway, the adsorbed O₂ molecule abstracted an H atom from the adsorbed benzene, engendering a pair of adsorbates, C₆H₅ and OOH molecule, with a substantially elevated free energy value of 1.05 eV (Fig. S12†), which was inimical to a seamless reaction advancement. Up to this juncture, only the O atom binding pathway merited further scrutiny. The second step of benzene degradation via the O combination pathway was also partitioned into three alternatives, namely benzene oxygenation (addition of O₁) to form C₆H₆ + 2O or C₆H₆O + O, and dehydrogenation to form C₆H₅ + OH. The Gibbs free energies of these three paths were −0.25 eV, 0.32 eV, and −0.11 eV, respectively, where C₆H₅O + OH is the easiest intermediate product of this process (Fig. S13†). The ensuing step encompassed four potential routes: further oxygenation (addition of O₂) to form C₆H₅O + OH, C₆H₅ + OH + O, and C₆H₄ + 2OH, or dehydrogenation to form C₆H₄ + H₂O. The corresponding reaction free energies were −0.28 eV, 0.16 eV, 0.33 eV, and 0.76 eV, respectively (Fig. S14†), among which the C₆H₅O + OH path was the most preferable. In the subsequent step along the C₆H₅O + OH path, two courses persisted: the third O (O₃) binding to Pt and generating C₆H₅O + O + OH, or interconnecting with benzene to form C₆H₅O₂ + OH (Fig. S15†). The free energies were 0.12 eV and −0.68 eV, respectively, intimating that the latter reaction path was more favorable. It is noteworthy that when O₃ was appended to the benzene ring, three potential adsorption sites were contemplated: the ortho, meta, and para positions relative to the basal O₂ (Fig. S16†). Among them, the para adsorption site was the most stable, with the lowest structural energy.

In the further degradation process of the C₆H₅O₂ molecule, two possible paths subsisted: the fourth O (O₄) combining with C₆H₅O₂ and dehydrogenating to C₆H₄O₂ + H₂O, or O₄ being captured by Ag₄ and then converting C₆H₅O₂ to C₆H₃O₂. The reaction free energies were −0.43 eV and −0.26 eV, respectively, suggesting that the former path is more advantageous (Fig. 6a). After the first H₂O molecule was desorbed, the next step, whether dehydrogenation or oxidation, remained to be ascertained. As illustrated in Fig. 6b, in the dehydrogenation path, the OH groups on benzene were cleaved, accompanied by an endothermic reaction with a free energy of 0.58 eV. In contrast, in the oxidation path, the fifth O (O₅) continuously attracted the benzene ring, forming a five-membered carbon ring (C₅) and a CHO molecule. This was an exothermic process, exhibiting a favorable free energy of −0.29 eV. At this stage, the hexagonal structure of the benzene ring was completely disrupted, attaining the initial accomplishment of benzene degradation at room temperature. In fact, during the step of generating C₆H₅O₂, the conjugated π bond of the benzene ring is already perturbed, with the double C–C bond being elongated from a uniform length of 1.42 Å to 1.51 Å. Therefore, the steps of generation of C₆H₅O₂ and C₅H₂O₂ were both cardinal steps in the present benzene degradation work.

Fig. 6 (a) Reaction paths and Gibbs free energy of the fifth step in benzene degradation: combination of the fourth oxygen (O₄) with C₆H₅O₂ to generate C₆H₄O₂ + H₂O or capture of O₄ by Ag₄, leading to the conversion of C₆H₅O₂ to C₆H₃O₂; (b) reaction paths and Gibbs free energy of the sixth step in benzene degradation: dehydrogenation or oxidation (addition of O₅) process; (c) overall reaction pathways and Gibbs free energy of benzene degradation to CO₂ and H₂O on the Ag₄ tetramer catalyst.

Based on the above exploration, the reaction pathway for the cleavage of the benzene ring on the Ag₄ tetramer at room temperature was initially elucidated. Subsequently, a detailed study on the complete degradation process of benzene to CO₂ and H₂O was conducted. As depicted in Fig. 6c, the process involved benzene adsorption (forming *C₆H₆), oxidation and dehydrogenation (forming *C₆H₅O), disruption of the conjugated π bond (forming *C₆H₅O₂), decomposition of the hexagonal structure (forming *C₅H₂O₂ + CHO), further oxidation and dehydrogenation to *C₅HO₃ + CO on the Ag₄ tetramer, followed by the generation of the first CO₂. Subsequently, the C₅ moiety was disrupted (*C₅H₂O₂), with the second CO₂ being desorbed from the catalyst and a carbon chain structure (*C₄HO₂) being formed. As the continuous release of CO₂ proceeded, the carbon chain was transformed into *C₃HO₂ and then *C₂HO₂. During this process, the excess hydrogen was converted into water, which was released from the catalyst, and the adsorbed C₂HO₂ molecules underwent conversion to C₂O₃ and CO as intermediates, which were ultimately converted into CO₂. It was observed that six endothermic reactions occurred throughout the entire process of benzene degradation at room temperature. Among them, the generation of *C₄HO₃ from *C₄HO₂ was the rate-determining step, with a reaction free energy of 0.71 eV. This indicated that the catalytic activity of benzene degradation on δ-MnO₂-Ag₄ at room temperature was comparable to those under photocatalytic and high-temperature conditions from a theoretical perspective.^41,42 This study holds significant importance as it provides novel insights into the mechanism and kinetics of benzene degradation at room temperature, potentially facilitating the development of more efficient and environmentally friendly catalytic systems for pollutant elimination.

To thoroughly explore the fundamental reasons for the high catalytic activity of catalysts, we conducted in-depth electronic analyses on a series of δ-MnO₂-M₃ (Ag₃, Pd₃, Pt₃, Cu₃, Fe₃, Rh₃, and Ru₃) and δ-MnO₂-M₄ (Ag₄, Pd₄, and Pt₄) catalyst systems. Firstly, the projected densities of states of δ-MnO₂-Ag₄, δ-MnO₂-Pd₄, and δ-MnO₂-Pt₄ before and after benzene adsorption were calculated and are presented in Fig. S17.† As shown, there was a significant hybridization between the C 2p orbitals of benzene molecules and the d-electron orbitals of metal atoms, resulting in a strong resonance phenomenon. It indicated that throughout the entire catalytic reaction process, the d-electron orbitals of metals played a decisive role in the activation of benzene and are a key factor influencing catalytic activity. To further realize the determining reasons of the high catalytic activity, the work functions, d-band centers of δ-MnO₂-M₃ and δ-MnO₂-M₄, and the electronegativities of the relevant metals were summarized. The results in Fig. S18 and 19† showed that the work function and the d-band center values of δ-MnO₂-Ag₄ as well as the electronegativity of Ag were all in the middle position. Thus, δ-MnO₂-Ag₄ may possess a moderate adsorption energy for reactants (benzene) and products (H₂O and CO₂). This moderate adsorption energy ensures that the adsorption of products on the catalyst surface is neither too strong, thus avoiding the problem of difficult desorption, nor too weak, guaranteeing the effective activation of benzene molecules. It is precisely this optimal adsorption characteristic that endows δ-MnO₂-Ag₄ with relatively ideal catalytic activity, enabling it to exhibit excellent performance in the catalytic reaction. Through the above-mentioned supplementary calculations and analyses, we explored the reasons for the high catalytic activity of catalysts not only from a phenomenological level but also from the essential levels of electronic structure and chemical properties.

4. Conclusions

In this study, a comprehensive investigation was conducted on the catalytic properties of 49 types of ADCs (spanning from a single atom to a tetramer) supported on δ-MnO₂(001) with the objective of identifying promising candidates to facilitate benzene degradation at room temperature. The specific research conclusions are as follows:

1. Among the 23 SAC samples, 9 SACs were selected based on their high structural and thermodynamic stabilities. However, these SACs demonstrated relatively suboptimal separation capabilities between reactants and products, as the adsorption energies of benzene, CO₂, and H₂O were in close proximity.

2. To widen the adsorption energy gap between reactants and products, 23 metal trimers, which possess more active sites and enhanced metal utilization, were constructed. The results indicated that with the increment of active sites, the potential catalytic performance was improved, especially for the Ag₃, Pd₃, and Pt₃ trimers. Moreover, when the active sites were further augmented to form tetramers, the separation effect became even more pronounced, which could be ascribed to the stronger bonding interactions between benzene and the metal clusters.

3. The Ag₄ cluster emerged as the most promising candidate, exhibiting an adsorption energy gap exceeding 3 eV. This property not only enables the complete activation of reactants but also facilitates the smooth desorption of products. The superiority of the Ag₄ cluster over the Pt₄ and Pd₄ clusters can be attributed to the closer alignment of the highest occupied molecular orbital (HOMO) of Ag₄ with the lowest unoccupied molecular orbital (LUMO) of benzene.

4. The catalytic performance and reaction pathways of benzene degradation on the Ag₄ cluster were thoroughly explored at room temperature. An optimal pathway for the degradation of benzene to CO₂ and H₂O was proposed. The key intermediates in this process were identified as C₆H₅O₂ and C₅H₂O₂, with a rate-limiting step free energy barrier of merely 0.71 eV. These results are comparable to the catalytic activities reported under photocatalytic and high-temperature conditions.

In summary, this work has uncovered a novel catalytic oxidation pathway for benzene degradation and has successfully accomplished the conversion of benzene to CO₂ at room temperature, thereby providing valuable theoretical insights and laying a solid foundation for future research in this field.

Data availability

The data supporting this article have been included as part of the ESI.† And no primary research results, software or code have been included in this work.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National & Local Joint Engineering Research Center for New Noble Metal Catalysts R&D Technology; the Shaanxi Key Laboratory of Catalytic Materials and Technology and the Xian Key Laboratory of Catalytic Materials and Technology; and the National Key R&D and the Ministry of Program of China (2022YFA1503504). The authors gratefully acknowledge the support of the NSFC (Grant No. 22303068, 62204207), the Natural Science Basic Research Program of Shaanxi Province (2023-JC-QN-0123, 2024JC-YBQN-0079), the Natural Science Basic Research Program of Hebei Province (B2024201089), the Qinchuangyuan High-level Innovation and Entrepreneurship Talent Projects (QCYRCXM-2023-004), the Xi'an Youth Talent Lifting Program (959202413034) and the Northwest Institute for Nonferrous Metal Research Institute-Controlled Fund (YK2114).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr05227g

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