English

• The increased CO₂ level in the atmosphere has caused many damage in climate, such as greenhouse effect. Therefore, the topic on carbon capture and storage has been proposed as a solution. The captured CO₂ has also stimulated the research of CO₂ conversion [1-3]. In the meantime, CH₄ has been treated as an attractive source of hydrogen energy at present and can be obtained from coal-bed gas in large quantity [4]. The CO₂ conversion with the assistance of CH₄ can realize the synthesis of value-added chemicals such as HCOOH, CH₃COOH, CH₃OH, CO [5]. Among these products, the synthesis of CH₃COOH has been investigated extensively due to the 100% utilization of atoms in CO₂ and CH₄. Besides, CH₃COOH is a fuel with high energy density compared to other C1 products [6-9]. This strategy can alleviate human's dependence on conventional fossil fules and simultaneously reduce the detrimental impact of greenhouse gasses. The most widely studied reaction for simultaneous activation of CO₂ and CH₄ is the production of CO/H₂ syngas [10-15]. However, the study of the coupling of CH₄ and CO₂ to C2 products is still an emerging area. The major challenge of this reaction exists in the simultaneous activation of CH₄ and CO₂, especially the process of CH₄ dissociation into CH₃ and H. These have been extensively studied in our previous works [16-19]. Therefore, it is highly desired to develop novel catalyst with high efficiency. As reported in previous studies, the final products of CO₂~CH₄ co-activation mainly include CH₃COOH and HCOOH. The major intermediate of CH₃COOH production is CH₃COO, which derived from the combination between CH₃ and CO₂. For HCOOH production, there exists two intermediates: COOH and HCOO. In other words, the H atom can bond with CO₂ on both C and O atoms. In pioneering works, direct production of CH₃COOH via CO₂ and CH₄ has been carried out on several heterogeneous catalysts, such as Co-Cu metal oxides [7], zeolite based catalysts [20], (ZnO)₃-In₂O₃ [21], Pd/C [8], Pt/Al₂O₃ [9], Co-Pd/TiO₂ [22], NiO/NiF [23] and ZnO/r-GO [24]. While, the low conversion efficiency and poor selectivity still hinder the development of catalysts in this direction. The main issue that hinders the reactivity of catalysts exists in the effective adsorption and dissociation of inert CH₄. Wang et al. have obtained only −0.04 eV adsorption energy for CH₄ on Co(001) surface, which agrees with the following study of Zhang et al. (−0.06 eV) [25, 26]. Besides, The activation barrier for CH₄ dehydrogenation, *CH₄ → *CH₃ + *H, has reached to 0.95 eV in Zhang's study [26]. Deo et al. have also revealed the weak van der Waals interaction between CH₄ and Ni based alloys with only ~−0.02 eV adsorption energies [27]. Moreover, the reaction barrier of CH₄ dissociation has reached to 1.78 eV (0.92 eV) for Cu (Ni) [27].

  In order to improve the reactivity, many research has also focused on the reaction mechanism. The first step of coupling between CO₂ and CH₄ is CH₄ adsorption and further dehydrogenation to CH₃, which has been extensively studied on metals [21, 28-36]. Ge et al. have realized the C-H activation in CH₄ via introducing M-O pair at oxide-oxide interface [21]. Hu et al. has studied the transformation of CH₄ to C and H on Pd(100) surface from the aspect of valencies, which emerges clear relationship with the location of transition states [30]. Kim et al. have introduced OH groups onto Ir(110) surface and successfully achieved CH₄ activation at low temperature [37]. Jiao et al. has demonstrated that the rate controlling step of CH₄ dissociation is the adsorption process and the key intermediate is CHO on Ni(111) surface [32]. Baroni et al. has designed a complex of Rh@Cu(111) as model catalyst, which can hinder the second dehydrogenation step (CH₃ → CH₂ + H) with respect to the first (CH₄ → CH₃ + H) [33]. Boukelkoul et al. have doped Cu(100) surface with W, which proved to be effective for CH₄ dissociation [38]. Gong et al. has studied conversion between CH₄ and CH₃ on both flat and stepped Co(0001) surface and suggested that the reaction is not sensitive to structure [35]. However, to the best of our knowledge, the insertion of CO₂ into the metal-CH₃ site is also essential for CH₃COOH formation. Wang et al. has suggested that CO₂ insertion into Cu-CH₃ bond is the most favorable path in the formation of CH₃COOH via comprehensive comparison of different pathways [5]. Ge et al. has studied the C-C coupling reaction on a Zn-doped ceria catalyst. The results show that the Zn dopant may stabilize the methyl group and further contribute the formation of CH₃COOH from CH₄ and CO₂ [39]. Park et al. have indicated that the M⁺ cation in the M⁺⁻ZSM-5 catalyst can assist the activation of CO₂ and subsequent reaction with CH₃ [40]. However, the origin of reactivity still remains elusive. Besides, the mechanism of the interaction between adsorbed molecules and catalyst need to be revealed in more details in current studies.

  In our previous study, we have successfully predicted a two dimensional BC₃N₂ (2D BC₃B₂) substrate. This structure is like graphene with B, C and N atoms bonded together upon sp² hybridization. Compared with graphene, boron nitride and other graphene materials, each hexagonal primitive unit cell of the BC₃N₂ has one more electron than them. Such a multi-electron system may be beneficial to catalysis due to its advantages in electron transfer rate and effective mass, which is beneficial than other B/C substrates like B₂C [5, 41-44].

  Inspired by the pioneering works, we attempted to design a novel 2D BC₃N₂ based catalyst with single metals loaded on it (M@2D-BC₃N₂) and calculated properties via density functional theory. Among these candidates, Pt@2D-BC₃N₂ is the most promising catalyst to realize effective activation of CH₄ and CO₂ with ultralow barriers for CH₄ splitting (0.26 eV) and CO₂~CH₃ combination (0.86 eV). The selectivity for CH₃COOH is also very high, which mainly stems from the unique electronic properties of molecules and substrate: The delocalized π bonds in CO₂, caused by degenerated states of s, p_x, p_y and p_z between C and O, can interact dramatically with states of Pt(s), Pt(p_z), Pt(d_xz), Pt(d_yz) and Pt(z²) in Pt@2D-BC₃N₂. The kinetics model also proves that our system can promote CH₃COOH production via simply increasing the temperature or the coverage of CH₄ and CO₂. Our results provide a reasonable illustration in clarifying mechanism and propose promising candidates with high reactivity for further study.

  All the first-principles spin-polarized calculations were performed by using the Vienna ab initio simulation package (VASP) [45, 46]. The ion-electron interactions were described by the projector augmented wave method [47]. The generalized gradient approximation (GGA) is described in the Perdew Burke Ernzerhof form and the cutoff energy for plane-wave basis is set as 400 eV. The convergence criterion for the residual force and energy was set to 0.05 eV/Å and 10⁻⁵ eV, respectively, during the structure relaxation. And for all calculations, we considered the vander Waals force correction by using MBD approach in the Grimme scheme [48]. Supercells consisting of 3 × 3 × 1 2D BC₃N₂ unit cells were used and the Brillouin zones were sampled by a Monkhorst-Pack k-point mesh with a 2 × 2 × 1 k-point grid for structure relaxation, while denser k-points mesh of 7 × 7 × 1 were used for electronic property evaluations. The Bader charge analysis was employed for the charge transfer. A vacuum space of 15 Å was employed to avoid the interaction between two periodic units. The above methods have been widely used in pioneering studies especially in catalytic areas [49-81].

  Our predicted 2D BC₃N₂ exhibits a hexagonal network structure (Fig. 1 and Fig. S1 in Supporting information). In 2D BC₃N₂, each unit cell has one more electron, leading to a high electron-transfer rate (more details can be obtained in Ref [30]). This character may be beneficial to activate inert molecules such as CO₂ and CH₄. We thus choose 2D BC₃N₂ as the substrate for single metal atoms to construct a novel catalyst (denoted as M@2D-BC₃N₂) in production of CH₃COOH via CO₂ and CH₄.

  Figure 1

  

  Figure 1.  (a) The configuration of 2D BC₃N₂ with metal atom loaded on it. The atom in the red cycle denotes the doped metal atom. (b) Binding energies of metal atoms on 2D BC₃N₂.

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  We first discuss the adsorption sites for single metal atoms loaded on 2D BC₃N₂. To identify the most stable M@2D-BC₃N₂ configurations, metal atoms is placed on various sites above BC₃N₂ (Fig. S1). Each situation is fully relaxed and the most stable structures are confirmed based on systems with the largest binding energies (E_b) for metal atoms on 2D BC₃N₂ as summarized in Table S1 (Supporting information). The binding energies are generally larger than −2 eV except for Ag, indicating the M@2D-BC₃N₂ complex is stable. In other words, the binding strength between 2D-BC₃N₂ and metal atoms is big enough to prohibit the aggregation of single atoms into clusters.

  The optimum adsorption configurations of CO₂ are discussed comprehensively with different orientations. This is due to the linear character of CO₂ molecule, which may lead to various adsorption behavior on single-atom sites (Fig. S2 in Supporting information). After calculation of adsorption energies (E_ad), the favorable orientation is assumed as the CO₂ molecule adsorbed on metal-atom site with C-O bond pointing to the N atom in the hexagon unit. About the coordination number, CO₂ tends to binds M@2D-BC₃N₂ bidentately with both C and O atoms. Similar conclusions can be obtained in other M@2D-BC₃N₂ systems. The final structures after fully geometric optimization are displayed in Fig. S3 (Supporting information).

  As shown in Fig. 2, the adsorption energies for CO₂ are generally larger than −0.5 eV except for Ag and Au, which is remarkably larger than that of CH₄ (E_ad[CH₄] are generally lower than −0.3 eV). Therefore, the adsorption process of CH₄ is treated as the rate determined step for simultaneous activation of CO₂ and CH₄. Pt@2D-BC₃N₂ and Pd@2D-BC₃N₂ possess the largest adsorption energy of −0.40 eV and −0.45 eV. However, Pt possesses significant advantages in binding energy on 2D BC₃N₂ than Pd (E_b[Pt] = −3.91 eV vs. E_b[Pd] = −2.15 eV). We thus serve Pt@2D-BC₃N₂ system as the most promising candidate for CH₃COOH production.

  Figure 2

  

  Figure 2.  Adsorption energies for (a) CO₂ and (b) CH₄ on M@2D-BC₃N₂.

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  In order to determine the accurate reactivity of the reaction, we have performed many analysis such as reaction energy, transition states and frequency. The reaction begins with the adsorption of CH₄ on Pt@2D-BC₃N₂ and further dissociates to *CH₃ and *H. Additional combination between CO₂ and *CH₃ can form *CH₃CO₂ intermediate, which can eventually be hydrogenated into CH₃COOH as final product. For the co-adsorption state of *CH₃ and *H, the *H can spontaneously separate away from *CH₃ with a negative reaction energy of −0.12 eV. In contrast, CO₂ cannot adsorb together with CH₃ on Pt@2D-BC₃B₂ and combine directly with *CH₃ in gas phase to produce CH₃COO. We first consider the CH₄ → *CH₃ + *H step: The catalytic reactivity of CH₄ dehydrogenation is surprisingly high with an ultralow reaction barrier of 0.26 eV (Fig. 3), validating the ability of Pt@2D-BC₃N₂ in activating CH₄. The length of C-H bond in CH₄ is 1.10 Å at initial state and has enlarged into 2.15 Å after dissociation. The configuration of transition state is shown in the inset of Fig. 3 with a C-H bond length of 1.67 Å. The rate determined step (RDS) of CH₄ + CO₂ → CH₃COOH reaction is *CH₃ + *H + CO₂ → CH₃CO₂ + *H step with a reaction barrer of 0.86 eV, which can be easily overcome at only 70 ℃. The CH₃CO₂ product possesses a C-C bond length of 1.52 Å, which is 2.04 Å in transition states as shown in Fig. 3 and Table S2 (Supporting information). The summarized reaction processes for steps of CH₄ → *CH₃ + *H and *CH₃ + *H + CO₂ → CH₃CO₂ + *H are demonstrated in Fig. S4 (Supporting information) and the continuous process for CH₃COOH production is shown in Fig. S5. The only one imaginary frequency of −552.11 cm⁻¹ for CH₃~H (−419.23 cm⁻¹ for CH₃~CO₂) indicates the transition state has been searched correctly for these steps (Table S3 in Supporting information). The results calculated above has illustrated that Pt@2D-BC₃N₂ could be a dramatically efficient catalyst for activation of inert gasses into CH₃COOH.

  Figure 3

  

  Figure 3.  The reaction pathways of (a) CH₄ dissociation and (b) combination of CO₂ and CH₃. The configurations of transition states are displayed in the insets of (a) and (b). The configurations of (c) *CH₄, (d) *CH₃ + *H, (e) *CH₃ + H, (f) *CH₃ + CO₂, (g) *CH₃COO, and (h) CH₃COOH on Pt@2D-BC₃N₂.

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  In addition to CH₃COOH, HCOOH is also an important production that we cannot ignore. The major intermediates for HCOOH production include *COOH and *HCOO, which derived from the combination between *H and *CO₂. We thus calculate the reaction barriers of *CO₂ + *H → *COOH and *CO₂ + *H → *HCOO. The reaction barriers are 1.51 eV and 1.01 eV for *COOH and *HCOO pathways (Figs. S6 and S7 in Supporting information), which are all larger than CH₃COOH production (0.86 eV). This may indicate that HCOOH production may be hindered by *CO₂ hydrodyzation process, leading to the higher selectivity for CH₃COOH comapred to HCOOH. We therefore focuse our study on the CH₃COOH production on Pt@2D-BC₃N₂.

  Our designed Pt@2D-BC₃N₂ catayst possesses both notably high adsorption ability and low barrier compared to TM systems. Especially for CH₄ dissociation, this is a general rate controlling step on transition metals (TMs) in pioneering studies as shown in Table 1. The high barriers on TMs (0.92~1.78 eV) generally resulted from the weak binding strength (−0.02~−0.04 eV) as the situations on Co, Ni, and Cu. In comparison, Pt exhibits higher CH₄ adsorption energy of −0.25 eV, leading to a reduced activation barrier of 0.55 eV. Therefore, the effective capture ability of CH₄ can be treated as the origin for high reactivity of CH₄ splitting. The C-H bond has been enlonged to 1.67 Å, which is much longer than that on transition metals (1.44~1.55 Å). This may illustrate that CH₄ has been sufficiently activated on Pt@2D-BC₃N₂. Thus, the interaction of CH₄ with the substrate can weaken the strength of C-H bond and eventually decrease the dissociation barrier of CH₄.

  Table 1

  

  Table 1.  Various reactivity data of CH₄ dissociation including activation barriers (E_a), bond strength of C-H for transition states (d(C-H)), and adsorption energies of CH₄ (E_ad[CH₄]) for systems of Co, Ni, Cu, Pt, Ir, and our designed Pt@2D-BC₃N₂.

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  To clarify the ultrahigh reactivity of Pt@2D-BC₃N₂ in CH₃COOH production, we focus our research on the activation of CO₂ and CH₄ in this part mainly from the hybridizations between various states. For CH₄ adsorption, the charge density difference between CH₄ and Pt@2D-BC₃N₂ demonstrates the dramatic accumulation of electrons between Pt atom and C atom in CH₄, indicating a covalent bond nature (Fig. 4). The density of states (DOS) of Pt and C atoms can reveal the interaction as hybridization between C(p_z), Pt(s), Pt(P_z) and Pt(d_yz) states.

  Figure 4

  

  Figure 4.  (a) The charge density difference and (b) density of states (DOS) for hybridization between Pt and C atoms for CH₄ adsorbed on Pt@2D-BC₃N₂. The charge depletion and accumulation were depicted by cyan and yellow, respectively.

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  For CO₂ adsorption, the interaction of CO₂ molecules with active sites leads to changes in the geometrical properties of CO₂: The angle of O-C-O bond has changed to 117.22° after adsorption, which is in contrast with the value of 180° in free state for CO₂. This may indicate the initial steps of chemical adsorption. As mentioned above, CO₂ tends to bond with Pt atom with both C and O atoms, which possess strongly degenerate between states, including s, p_x, p_y, and p_z. This may indicate the existence of delocalized π bonds in CO₂ as shown in Figs. 5a and b. The degenerate s~p states of O and C will further interact with Pt states, such as s, p_z, d_yz, d_xz and d_z² (Figs. 5c and d). The overlapping of electronic density regions between Pt site and CO₂ may lead to charge transfer between substrate and adsorbed species (Fig. S9 in Supporting information). The bader charge analysis has shown that Pt@2D-BC₃N₂ has transferred −0.37 e charge to CO₂, leading to a colvalent bond interaction between CO₂ and Pt@2D-BC₃N₂. For the CO₂ adsorbed on Pt@2D-BC₃N₂, the valence state of C (O) atom is about +1.63 (−1) (Fig. S8 in Supporting information). The valence is quantified using bader charge with CO₂ negatively charged with −0.37 e, which is agreed with the situation of Cu. The electrons can move to the lowest unoccupied orbitals of adsorbed CO₂ and eventually stabilize the systems. The characters described above has comprehensively illustrated that CO₂ has been totally activated on Pt@2D-BC₃N₂. Above all, the high reactivity of Pt@2D-BC₃N₂ mainly stems from the high capture ability of CO₂ and CH₄ molecules. Moreover, Pt is a material with high cost, which may hinder the practical application. Thus, more research works should be done in our further studies to find out proper substitutes based on the theories proposed in this work.

  Figure 5

  

  Figure 5.  Density of states (DOS) of degenerate between s and p states in (a) O and (b) C. The DOS in (c) and (d) also exhibit the degenerate states of (c) O and (d) C with s, p and d states in Pt.

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  For quantitative analysis, we have calculated the rate constant of CH₄ dissociation and CO₂~CH₃ combination and listed in Table 2. As mentioned above, the barrier of RDS is 0.86 eV, indicating a temperature of 344 K is at least necessary to activate the reaction. We thus investigated the rate constant (k) of CO₂ + *CH₃ → *CH₃COO and its related step of CH₄ disscociation at a temperature range of 350~600 K.

  Table 2

  

  Table 2.  The rate constant and equilibrium constant of steps under various temperature.

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  The relation between the reaction rate (r) and corresponding k can be described as follows:

  (1)

  (2)

  (3)

  (4)

  (5)

  (6)

  (7)

  (8)

  where r_f (r_b) is the reaction rate of forward (backward) reaction; k is the rate constant; θ is the coverage of the adsorbate; A (B, C) is the frequency factor of the reactant (transition state, product); f_r (f_TS, f_p) is the frequency of the reactant (transition state, product); K is the equilibrium constant; E_b is the reaction barrier of RCS; N_A is the Avogadro constant. The reaction can reach the equilibrium state at the condition of r_f = r_b.

  From the formulas above, we can deduce the relationship between K and coverage of CH₄, CO₂ and CH₃COO:

  where K₁ (K₂) is the equilibrium constant of CH₄ dissociation (CO₂~CH₃ combination); K is defined as .

  From the data listed in Table 2, we can find that the K₁ (K₂) is (decreased) increased with increasing temperature due to the endothermic (exothermic) reaction character. However, K constantly exhibits increased tendency with increasing temperature. Based on the formula, the production of CH₃COO can be improved via increasing the temperature or the coverage of CH₄ and CO₂. This can be attributed to the neglegible change tendency between K₁ and T compared to K₂ situation as shown in Fig. 6. Therefore, K~T exhibits similar tendency as K₂~T.

  Figure 6

  

  Figure 6.  The correlations between temperature and K₁, K₂ and K.

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  In conclusion, we have performed density functional theory calculation to study M@2D-BC₃N₂ systems for CH₃COOH synthesis via CO₂ and CH₄. We find that Pt@2D-BC₃N₂ can dissociate CH₄ (aggregate CO₂ and CH₃) with an ultralow barrier of 0.26 eV (0.86 eV). The high reactivity and selectivity originate from the high capture ability for CO₂ and CH₄. This phenomenon can be interpreted from the hybridization between various states: The C(p_z) states for CH₄ can interact with states of Pt(s), Pt(p_z) and Pt(d_yz) for Pt@2D-BC₃N₂ complex. Besides, the degenerate between states of s, p_x, p_y and p_z in C and O in CO₂ has indicated the delocalized π bonds. This degenerate states can interact with states of Pt(s), Pt(p_z), Pt(d_xz), Pt(d_yz) and Pt(z²) for Pt@2D-BC₃N₂. From kinetics aspect, our system can promote CH₃COOH production via simply increasing the temperature or the coverage of CH₄ and CO₂. Our study has not only provided clues for catalyst design in CH₃COOH synthesis via CO₂ and CH₄, but also enriched the understanding of mechanism in the activation of CO₂ and CH₄.

  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.

  Acknowledgments

  This study was funded by the National Natural Science Foundation of China (No. 21603109), the Henan Joint Fund of the National Natural Science Foundation of China (No. U1404216), the Special Fund of Tianshui Normal University, China (No. CXJ2020-08), the Scientific Research Program Funded by Shaanxi Provincial Education Department (No. 20JK0676). We also appreciate the National Supercomputing Center in Zhengzhou to provide the computational sources

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.02.018.

  
  
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• 

  Figure 1  (a) The configuration of 2D BC₃N₂ with metal atom loaded on it. The atom in the red cycle denotes the doped metal atom. (b) Binding energies of metal atoms on 2D BC₃N₂.

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  Figure 2  Adsorption energies for (a) CO₂ and (b) CH₄ on M@2D-BC₃N₂.

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  Figure 3  The reaction pathways of (a) CH₄ dissociation and (b) combination of CO₂ and CH₃. The configurations of transition states are displayed in the insets of (a) and (b). The configurations of (c) *CH₄, (d) *CH₃ + *H, (e) *CH₃ + H, (f) *CH₃ + CO₂, (g) *CH₃COO, and (h) CH₃COOH on Pt@2D-BC₃N₂.

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  Figure 4  (a) The charge density difference and (b) density of states (DOS) for hybridization between Pt and C atoms for CH₄ adsorbed on Pt@2D-BC₃N₂. The charge depletion and accumulation were depicted by cyan and yellow, respectively.

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  Figure 5  Density of states (DOS) of degenerate between s and p states in (a) O and (b) C. The DOS in (c) and (d) also exhibit the degenerate states of (c) O and (d) C with s, p and d states in Pt.

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  Figure 6  The correlations between temperature and K₁, K₂ and K.

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  Table 1.  Various reactivity data of CH₄ dissociation including activation barriers (E_a), bond strength of C-H for transition states (d(C-H)), and adsorption energies of CH₄ (E_ad[CH₄]) for systems of Co, Ni, Cu, Pt, Ir, and our designed Pt@2D-BC₃N₂.

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  Table 2.  The rate constant and equilibrium constant of steps under various temperature.

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•