Synergistic effect between transition metal single atom and SnS₂ toward deep CO₂ reduction

Summary

The electrochemical reduction of CO₂ is an efficient channel to facilitate energy conversion, but the rapid design and rational screening of high-performance catalysts remain a great challenge. In this work, we investigated the relationships between the configuration, energy, and electronic properties of SnS₂ loaded with transition metal single atom (TM@SnS₂) and analyzed the mechanism of CO₂ activation and reduction by using density functional theory. The “charge transfer bridge” promoted the adsorption of CO₂ on TM@SnS₂, thus enhancing the binding of HCOOH∗ to the catalyst for further hydrogenation and reduction to high-value CH₄. The research revealed that the binding free energy of COOH∗ on TM@SnS₂ formed a “volcano curve” with the limiting potential of CO₂ reduction to CH₄, and the TM@SnS₂ (TM = Cr, Ru, Os, and Pt) at the “volcano top” exhibited a high CH₄ activity.

Graphical abstract

Highlights

• •The relationships among the configuration, energy, and electronic properties of TM@SnS₂ for CO₂ reduction are explored by using density functional theory
• •The “charge transfer bridge” promotes the adsorption of CO₂ on TM@SnS₂, thus enhancing the binding of HCOOH∗ to the catalyst for further hydrogenation and reduction to CH₄
• •The binding free energy of COOH∗ on TM@SnS₂ forms a “volcano curve” with the limiting potential of CO₂ reduction to CH₄

Catalysis; Materials chemistry

Introduction

Excessive emissions of CO₂ can bring about various environmental problems such as the greenhouse effect,^1,2 and its transformation into industrial feedstocks such as HCOOH and CH₄ by chemical methods is significant for climate change as well as sustainable development of energy.^3,4,5 However, the C=O bond energy of CO₂ is as up to 803 kJ mol⁻¹, which is highly inert to the extent that it is hard to be activated and requires the involvement of catalysts.⁶ With mild reaction conditions and high conversion efficiency, electrochemical reduction is a cost-effective conversion approach.^7,8,9 And selectivity and limiting potential are two key dimensions of theoretical electrocatalysis research.⁹ Transition metal (TM) single-atom catalysts have the advantages of small size and high activity, and their rational design and screening can help facilitate efficient and highly selective targeting of product generation, which has vital investigative implications for the pioneering of CO₂ electroreduction (CO₂ER).^10,11,12,13

Earth-abundant and highly stable SnS₂ has been extensively studied in the CO₂ER. Unlike some TMX_n (X = S, Se, Te) and other ferromagnetic materials,¹⁴ the highly symmetric nonpolar surface of 1T-SnS₂ makes the activation of linear CO₂ extremely challenging, and the products are therefore mostly confined to 2e HCOOH, which cannot be deeply reduced, and inevitably have by-products such as CO and H₂ generated.^15,16,17,18 Combined with our calculations, CO₂ is weakly adsorbed on pure SnS₂ (−0.15 eV) and is indeed not effectively activated by hydrogenation. Currently, the catalysts for the reduction of CO₂ to hydrocarbon products are mainly Cu-based materials with a limiting potential of −0.74 V vs. RHE for the reduction to CH₄, which is poorly selective, and the presence of multi-carbon hydrocarbon products.^19,20 However, the loaded TM can supply an independent active site for CO₂ER on the one hand, which ensures the C1 product selectivity, and strengthen the coupling between the highly polarized catalyst and the intermediate, thereby ensuring the selectivity of the multi-electron products. We envision that firstly catalyst reduces CO₂ to HCOOH, which is difficult to desorb under the action of TM and further hydrogenated deeply to create CH(OH)₂∗, CH₂OOH∗, or HCO∗ + H₂O(g) until the formation of multi-electron products, which greatly promotes the electron efficiency.

In this study, the CO₂ER activity of TM loaded in SnS₂ (TM@SnS₂, TM detailed in Figure 1) was investigated using first-principle methods. We firstly studied the relationship between the configuration, energy, and electronic properties of the catalysts constructed by different TMs and reasonably predicted their activation and catalytic performance; then, we proposed the concept of “charge transfer bridge” and fully confirmed this conjecture, and proposed the intrinsic mechanism of different activation modes of CO₂; in addition, by constructing potential intermediates and comparing the free energy changes of each step in the whole reaction process, we determined the optimal pathway for different products, predicted the product composition based on the limiting potential, and revealed the intrinsic reaction mechanism. The results demonstrate that the loading of TM targeted to encourage CH₄ production by enhancing the coupling between TM@SnS₂ and HCOOH∗, and the “charge transfer bridge” constructed by the synergistic interaction between the active center TM and substrate SnS₂ broke the scalar relationship limiting the activity and selectivity. Moreover, the binding free energy of ∗COOH was found to follow a “volcano curve” with the limiting potential of CO₂ reduction to CH₄, which can be used as a reliable criterion to evaluate the CO₂ER performance of TM@SnS₂.

Figure 1.

Configuration, energy, and Bader charge of TM@SnS₂

(A) The optimized structures of TM@SnS₂ for fcc site loading.

(B) Binding energy of TM@SnS₂ and the Bader charge of TM after loading, in which the negative value represents the electron loss.

Results and discussion

Structural features and properties

The two-dimensional 1T-SnS₂ is characterized by Sn sandwiched between two S atomic layers. Non-polar 1T-SnS₂ surface makes it difficult for electrons to inject into the 2π_u orbital of the CO₂, hence CO₂ preserves natural linear shape with physisorption energy of only −0.15 eV. The strong sp hybridization that exists in carbon atom cannot be easily broken by physisorption.

The substrate SnS₂ influences the binding of the active center to the intermediate by modulating the electronic structure of TM. In order to enhance the CO₂ activation on the catalyst and modify the catalytic performance, 26 types of TM single atom shown in Figure 1A were modified on SnS₂ to obtain two types of catalysts. One is TM-doped SnS₂ with TM replacing a Sn atom (Figure S1A), the other loads TM on substrate SnS₂, named TM@SnS₂, while TM has three possible loading sites including fcc, hcp, and top sites (Figures S1B and S1C). We load the TM directly at the fcc site (Figure 1A, TM dimerization at SnS₂ requires migration from one cavity to another with high energy barriers, ensuring TM dispersion in Figures S1D and S1E).^21,22 However, the binding energy of the two types of catalysts (Figure S2, and Table S1) indicates that the synthesis of TM-doped SnS₂ is energetically unfavorable. Most importantly, the SnS₂ surface remains nonpolar after doping, resulting in a weaker trapping ability of TM-doped SnS₂ for CO₂ adsorption (the poor binding of the molecule to the material confines the product to 2e HCOOH, which is difficult to deeply reduce). Hence, we have focused in this work on the TM@SnS₂ for fcc site loading about the CO₂ER reaction in detail.

To have a more intuitive understanding on the structure of TM@SnS₂, the deviation of the horizontal location of TM from the fcc center in TM@SnS₂ and the longitudinal distance from the surface are presented in Figure S3. The deviation of TM when loaded at the SnS₂ fcc site is about 25% on average, with a slightly higher deviation of Hf. And the longitudinal distance from the surface is distributed around 1.0–1.3 Å, while the distance of Cd from the S surface is further away, reaching about 2 Å. The full arrangement of 4d orbitals weakens the orbital interactions between Cd and SnS₂, resulting in poor bonding ability between Cd and the substrate SnS₂. In addition to reflecting the stability of catalysts, the coupling strength between TM and substrate also significantly affects the effectiveness of catalysis (Figure 1B). The binding of TM and SnS₂ is quite strong when the d orbitals are less occupied, but with the increasing number of d electrons, the binding energy generally declines. Interestingly, when TM is Cr (3d), Mn (3d), Mo (4d), Re (5d), etc., the coupling between d orbitals and SnS₂ weakens, which is attributed to the semi-filled d⁵ orbitals, resulting in a decrease of binding strength, so that Cu, Ag, Zn, and Cd with d¹⁰ orbitals have the lowest binding energy to SnS₂. The p orbitals of S are dispersed near the Fermi energy level, resulting in interactions with TM d orbitals (Figure S4). Apart from that, the increasing number of d electrons is accompanied by the gradual filling of the d orbitals, and the lowering antibonding state energy level weakens the coupling between TM and SnS₂. Therefore, TM with lower d energy levels away from the Fermi level is more inclined to provide s orbitals participation in partial hybridization (Figures S4J and S4I), which adds the difficulty of electron transfer and intuitively manifests itself in poor binding and less charge transfer (Figure 1B, and Table S1). Moreover, the charge transferred from TM to SnS₂ has a scaling relationship with the workfunction of the TM@SnS₂ (Figure S5). With the intensified interaction between TM and SnS₂, TM^σ+ becomes more positive together with a decrease in the workfunction. TM reduces the minimum energy of electron spillover from catalyst, which is undoubtedly crucial for inert CO₂ capture and also implies an impact on the subsequent reaction mechanism and limiting potential (U_L).

CO₂ adsorption and activation

CO₂ capture plays an important role in carbon reduction catalytic reactions. As mentioned previously, CO₂ can only have a physisorption on pure SnS₂ without being effectively activated, and the first protonation requires energy of at least 1.47 eV, limiting further CO₂ reduction reaction (Figure S6). The adsorbed CO₂ always remain liner at a distance of 2.99 Å from the surface, with physisorption energy of only −0.15 eV. According to Norskov et al.,^23,24,25,26 the catalyst itself has a certain adsorption capacity for molecules, with a correlation between the strength of adsorption for reactants and products, suggesting that catalysts with poor CO₂ adsorption are also hard to bind with HCOOH∗ intermediates. In contrast to engaging in proton coupling and electron transfer (PCET), HCOOH∗ is easier to desorb directly, arguing for the previously reported SnS₂ as a typical formate catalyst. As a direct active site for adsorption and reduction of the catalyst, TM enhances the activation of CO₂ greatly. And crucially, the enhanced binding energy of TM@SnS₂ to HCOOH∗ makes it possible to synthesize 8e CH₄. Based on the correlation of CO₂, HCOOH, and CH₄ adsorption on TM@SnS₂ (Figure S7; Tables S2 and S3), the overall trend in adsorption energies remains consistent, with the pre-transition cycle of Hf-, Ta-, and Sc-loaded SnS₂ exhibiting relatively strong adsorption energy. However, the bonding between CO₂/HCOOH and TM@SnS₂ is mainly through TM−O, while CH₄ combines with TM@SnS₂ to form a weak TM−C bond. Therefore, the adsorption energy of CH₄ has a weak linear correlation with CO₂, and the relatively narrow CH₄ adsorption energy interval further leads to the dispersion of CH₄ adsorption energy.

Interestingly, the CO₂ configuration adsorbed on the TM@SnS₂ surface shows certain regularity (Figure S8A), in which most of the TMs and CO₂ bond as a C-TM-O bidentate ligand with a “V shape CO₂” (Figure S8B). With the increasing number of d electrons, the coupling between TM and C is gradually enhanced and the TM-O bond is gradually elongated (Figure S8A from α1 to α2). However, it does not apply to TM with d⁵ (Cr), d¹⁰ (Cu, Zn, Pd, Ag, Cd), which rely on monodentate TM-O and retain the linear configuration of CO₂ (α3 in Figure S8A).

It is obvious that d orbitals of TM have a significant effect on CO₂ activation. Molecular and surface interaction causes energy level splitting. The position of new antibonding state can be predicted by d band center of TM to judge the stability of the adsorbate.^27,28 Spin down electronic states tend to have higher energy levels,^29,30 which can be used to describe the strength of the interaction between the spin-polarized TM and the adsorbate, and it was also verified by our calculations (Figure S4). Apparently, there is no correlation between the spin down d band center of TM and the reactants CO₂ or possible products HCOOH and CH₄ (Figure S9). However, Figure S7 illustrates that as the interaction between CO₂ and TM@SnS₂ increases, so does the adsorption energy of HCOOH and CH₄, implying that the substrate SnS₂ has a great influence on CO₂ activation.

The difference in the binding morphology (α1, α2, and α3) of TM and CO₂ is due to different bonding mechanisms. In the case of bidentate adsorption (e.g., TM = Sc), the significant electron transfer between CO₂ and TM@SnS₂ (Figure 2A), accompanied by strong hybridization between the d orbitals of TM and 2p orbitals of C/O (Figures 2B, S10A, and S10B), exposes the intense interaction of CO₂ and TM@SnS₂. TM with empty d orbitals can accommodate electrons from the highest occupied orbital (1π_g) of CO₂ (Figure 2C), while TM feeds back electrons to the lowest unoccupied orbital (2π_u) of the CO₂ (Figure 2D), resulting in an eventual electron gain for CO₂. According to the Bader charge calculations (Table S3), the charge changes of TM are negligible before and after CO₂ adsorption. When loading on SnS₂, TM exhibits electron loss (Table S1). On application of the cathodic potential, it gains additional electrons through the TM-S bond as “charge transfer bridge” firstly, and the reduced TM then provides electrons for CO₂. Hence, we can observe the accumulation and loss of charge between the substrate and TM in Figure 2A. In the aforementioned processes, Sn achieves a transition of Sn⁴⁺ → Sn^x+ → Sn^σ+ (2 < x < σ < 4) (Figures 2E and 2F), as shown in the studies of Huang et al.^31,32 Transition metal with fewer d-occupied orbitals has a powerful interaction with the SnS₂ (Figure 1B) that prevents the formation of the reduced state, and the lack of electrons causes the activation of CO₂ to take on the α1 mode where TM is intensely coupled with O than C. Monodentate adsorption (α3) presents another bonding mechanism. In this case, weak interaction exists between TM and CO₂ (e.g., TM = Cr, Cu). Due to the relative inertness of the d orbital with a special electron arrangement (d⁵, d¹⁰), TM provides s orbital hybridization with the 2p orbital of O additionally (Figures S10C and S10D). The strong electronegativity prevents the generation of TM reduced state, and SnS₂ accepts few electrons from CO₂ using the TM-S bond as “charge transfer bridge” (Figures S10E–S10G) to realize Sn⁴⁺ → Sn^x+ → Sn^σ+, where 2 < σ < x < 4. At this moment, the highest occupied orbital (1π_g) of CO₂ transfers electrons to the empty s orbital (Figure S11A), and TM d orbital feeds back electrons to the lowest unoccupied orbital (2π_u) of CO₂ (Figure S11B). Note that, unlike the lower energy levels of d¹⁰, the Cr d orbitals are distributed near the Fermi energy level and it disperses to a certain extent in CO₂ adsorption. In conclusion, these two activation mechanisms reveal that CO₂ activation is synergistically facilitated by “charge transfer bridge” between TM and SnS₂. This is equivalent to the “electron sponge” proposed by Su et al.³³ We observed similar effect with Fe-N₄-C catalyst in CO₂ER,³⁴ while the multivalent state of Sn in the substrate SnS₂ facilitates the electron transfer even more than in graphite. Overall, we have explained the synergistic interaction between TM and substrate to promote the activation of CO₂ and further regulate the coupling strength of intermediates and TM@SnS₂ from a deep electron supply relationship.

Figure 2.

The electronic properties of CO₂ adsorbed on TM@SnS₂

(A) Charge density difference of CO₂ adsorbed on Sc@SnS₂ with the isovalue is set to 6 × 10⁻⁴e Å⁻³.

(B) Density of states of Sc@SnS₂.

(C and D) Mechanism of CO₂ binding to surface TM when the adsorption configuration is α1 or α2. Schematic diagram of net electron flow when (E) TM loading on SnS₂ and (F) CO₂ adsorbed on TM in α1 or α2.

The exotherm of CO₂ adsorption on the TM@SnS₂ increases with the period of TM, and CO₂ adsorption is enhanced by loading SnS₂ with pre-transition metals (Sc, Ti, Zr, Hf, Ta, W, etc.) for each period (Figure 3). CO₂ adsorption energy of most TM@SnS₂ is stronger than that of pure SnS₂, with the exception of Rh@SnS₂ and Cd@SnS₂ (−0.11 and −0.05 eV). In combination with the adsorption correlation aforementioned (Figure S7A), the poor binding of CO₂ to Rh@SnS₂ and Cd@SnS₂ adds to the difficulty of selectively generating CH₄ from CO₂ER. Furthermore, it is crucial to prevent the poisoning of H∗ at the monatomic active site. Re@SnS₂, Zn@SnS₂, and Cd@SnS₂ exhibit superiority of H∗ binding and hinder CO₂ adsorption. In summary, comparing the CO₂ adsorption energy on TM@SnS₂ with that on pure SnS₂ and the competition with H∗, we ignored the investigation of CO₂ reduction performance for Rh@SnS₂, Re@SnS₂, Zn@SnS₂, and Cd@SnS₂ and only considered the remaining 22 TM@SnS₂.

Figure 3.

Adsorption energy of CO₂∗ and H∗ on TM@SnS₂, where the blue dots are the difference in adsorption energy between them, i.e., E_ads (CO₂∗) − E_ads (H∗) and the red dashed line represents the CO₂ adsorption energy on pure SnS₂

Mechanisms and selectivity of reactions

Competition between COOH∗/HCOO∗ and H∗

The independent active site of TM@SnS₂ blocks the C−C coupling and ensures that CO₂ is reduced solely to C1 product.³⁵ We complement the potential reaction pathways for of CO₂ER proposed by Yuan et al. (Scheme S1).³⁶ CO₂ is progressively hydrogenated to HCOOH or CH₄ via PCET steps, with 35 possible radical reactions (Supporting Information R1-R35). The Gibbs free energy change for all reactions in the pathway was calculated (Supporting Information ES1-E35, Figure S12) to obtain the U_L for the CO₂ER. Brønsted-Evans-Polanyi relationship shows that lower Gibbs free energy change implies a smaller thermodynamic potential for the reaction.^37,38 As a side reaction of CO₂ reduction, not only the hydrogen reduction reaction (HER) reduces the Faraday efficiency, but the competing depletion of proton and electron pairs from electrolyte near the cathode also substantially slows the rate of CO₂ conversion. During the initial step of CO₂ hydrogenation, the proton may bond with O to form ∗COOH or transfer to C to generate HCOO∗ intermediates. ΔG (COOH∗/HCOO∗) and ΔG (H∗) reflect the competition between CO₂ER and HER (Figure 4A). All of HCOO∗ and most of COOH∗ intermediates exhibit energetic superiority over H∗, but the COOH∗ intermediates of TM@SnS₂ (TM = Ta, W, Os, Mn, Cu, Ag) require slightly higher energies than H∗. The independent active sites are occupied by COOH∗ and HCOO∗, inhibiting the occurrence of hydrogen precipitation side reactions and allowing TM@SnS₂ to exhibit superiority for CO₂ER. During the reaction, with similar surface adsorption bonds, there is a linear scalar relationship through the C-bound and O-bound intermediates, respectively.²³ In TM@SnS₂, the ∗COOH binding free energies bound to TM via C as a monodentate ligand and HCOO∗ bound by the bidentate ligand O-TM-O are also heavily correlated (Figure 4B), with HCOO∗ having a lower energy compared to COOH∗. The scaling relationship between HCOO∗ and HCOOH∗ diverging due to the weaken TM-O bond between the HCOOH∗ and TM (Figure S13). There is a poor correlation between the binding energies of the other subsequent intermediates. The scaling relationship imposes certain limitations on activity and selectivity in the involvement of transition metals in CO₂ER, which are broken by the “charge transfer bridge” between TM and SnS₂.

Figure 4.

The relationship with the free energies of reaction intermediates

(A) Binding free energy competition between COOH∗/HCOO∗ and H∗ intermediates.

(B) Comparison of the binding free energy of COOH∗ and HCOO∗.

Production of 2e HCOOH

CO₂ can form HCOOH∗ intermediates after two-step PCET, then HCOOH desorbed from surface with the relevant energy changes represented in Table S4. Figures 5A–5C show the free energy diagrams of HCOOH production after TM loading for 3d, 4d, and 5d periods, respectively, with ∗HCOO or COOH∗ intermediates being produced in the first step. On the one hand, the lower ∗HCOO energy increases the difficulty of converting ∗HCOO to ∗HCOOH, resulting in most TM@SnS₂ to yield formic acid through ∗COOH; on the other hand, the “charge transfer bridge” between TM and SnS₂ enlarges the coupling strength between TM and ∗HCOOH, making HCOOH desorption more difficult. The representative TMs are listed in Figure 5D. HCOOH∗ trapped on the surface through strong TM-O bonds is instead more readily hydrogenated and involved in reduction reactions than in desorption. The post-transition metals seem to exhibit high activity for HCOOH production, but the selectivity of CO₂ER depends on the difference in the limiting potential between the various products.

Figure 5.

The free energies of HCOOH generated on TM@SnS₂

Free energy diagrams of HCOOH production of TM@SnS₂ for (A) 3d, (B) 4d, and (C) 5d periods. The solid line is the energy of ∗COOH while the dashed line refers to ∗HCOO.

(D) Two unfavorable processes of HCOOH formation and the corresponding TMs for each step.

Production of 8e CH₄ and the “volcano curve”

Based on Scheme S1 and R1-R35 (Supporting Information), Gibbs free energy changes were calculated for 35 radical reactions (Figure S12). We can roughly see that in the hydrogenation of CO₂ to CH₄, the former part of the reduction (i.e., 1-4e) is more endothermic, while the latter part is mainly exothermic. In addition, R1 and R2 of 1e exhibits a tendency of increasing free energy change in each period; however, R34 and R35 of 8e show a gradual decline in energy change. The relatively insignificant variation in the middle part reaction and the limited linear scaling relationship favor the improved activity of the TM@SnS₂.

We screened the lowest energy pathway for CH₄ production from TM@SnS₂ (Figures S14 and S15) and obtained the U_L for CH₄ from CO₂ER. As aforementioned, while ∗HCOO is more energetically favorable than ∗COOH, it requires more energy to continue hydrogenation. Therefore most TMs (except Ni, Cu, Ta, Pt) get the lowest energy path for CH₄ generation via ∗COOH intermediate. We explored the relationship regarding the CO₂ adsorption energy and the ∗COOH binding energy of TM@SnS₂ relative to the U_L of the CH₄ production (Figure S16A) and found that the spontaneous adsorption of CO₂ simply acts as a knockon for reaction to take place and has almost no effect on activity of CH₄. And the corresponding CH₄ activity exhibits an advantage when the binding energy of ∗COOH remains between −0.4 and 0.2 eV. Furthermore, there is a correlation between ΔG (R35) and ∗COOH binding free energy, with a better activity when ΔG (R35) is in −0.6 to 0.2 eV (Figure S16B). The Sabatier principle suggests that the bond between the reaction intermediate and the catalyst should be neither too strong nor too weak in order to obtain the best catalytic performance.³⁹ There is indeed a “volcano curve” about ΔG (∗COOH) and the U_L (CH₄) in our system (Figure 6); when ΔG (∗COOH) < −0.4 eV, the binding between ∗COOH and TM@SnS₂ is too strong to proceed with hydrogenation, mainly in the pre-transition metal. Unexpectedly, Hf is farther away from SnS₂, making it overcoupled to COOH∗ and departing from the volcano curve. While ΔG (∗COOH) > 0.2 eV, ∗COOH formation becomes an endothermic process with a weak coupling, mostly distributed in the post-transition metal. Only if −0.4 eV < ΔG (∗COOH) < 0.2 eV, TM@SnS₂ has a limited potential for CO₂ER at the “volcano top” with significant CH₄ activity. In particular, Cr of 3d, Ru of 4d, as well as Os and Pt in 5d periods have excellent catalytic activity. Considering the tradeoff between the actual environment of electroreduction and computational pressure, we compared the free energy change of them before and after imposed implicit solvation model (Figure S15, and Table S5). With hydrogen bonding constraints, the energy of the H₂O molecule corrected by the implicit solvent model decreases by 0.31 eV, resulting in a slight energy decrease in the elementary reaction involving the removal of H₂O after one-step PCET. However, the overall free energy change of the elementary reaction is consistent, along with a subtle decrease in the limiting potential of the reaction.

Figure 6.

The “volcano curve” between ΔG (∗COOH) and the limiting potential U_L (CH₄)

Anticipated products of CO₂ER

In previous subsection, we have investigated the mechanism of CO₂ reduction to produce HCOOH and CH₄, exploring the optimal pathways about them. The ΔG_MAX about reaction reveals the catalytic activity of both products in the CO₂ER ultimately in Figure 7. The monoatomic loading enhances the coupling of ∗HCOOH to TM@SnS₂ and hinders HCOOH desorption. Overall, the U_L (HCOOH) is more negative relative to CH₄ production, with the former transition metal being more apparent in 4d and 5d, and the difference between the U_L reflecting the selectivity of the products. Sc and Y as pre-transition metals, Ni, Cu, Pd, Ag, Pt, as post-transition metals, and Mn are more superior for HCOOH synthesis and most TMs require less energy for CH₄ production. Of these TMs, Ti, Cr, Zr, Nb, Mo, Hf, Ta, and W have superior CH₄ selectivity with a larger difference in U_L. Finally, with its lower ΔG_MAX (0.32 eV) and excellent selectivity, Cr@SnS₂ is quite a promising catalyst for the formation of CH₄ in CO₂ER.

Figure 7.

Limiting potential for CO₂ER to HCOOH and CH₄ on TM@SnS₂, the blue dots are the difference in U_L between them, i.e., U_L (CH₄) − U_L (HCOOH), where the blue line represents U_L (CH₄) = U_L (HCOOH)

Conclusion

In conclusion, the TM@SnS₂ makes full use of the functional synergy between the TM and substrate, which strengthens the coupling between the intermediate and catalyst through “charge transfer bridge” effectively. Not only does it have a superior CO₂ adsorption capacity than pure SnS₂, but also TM@SnS₂ tends to reduce CO₂ to high-value CH₄ compared to conventional SnS₂, and which is limited to 2e HCOOH production. Our calculations indicate that with high selectivity and excellent CO₂ER performance, economical Cr@SnS₂ near the “volcano top” is an efficient catalyst for CH₄ production, and implicit solvation measurement did not significantly affect the limiting potential. This work provides valuable insights into the efficient design of single-atom electrocatalysts targeted to obtain desired products, and it is hoped that the aforementioned work will contribute to the further development of experimental and theoretical studies.

Limitations of the study

We have carried out a theoretical study of TM@SnS₂ for CO₂ reduction reaction. However, the reaction intermediates are susceptible to the influence of solvents in the microscopic state and the voltage under real reaction conditions, etc. The interactions between the catalyst, adsorbent, and electrolyte and the relationship between the reaction performance and the potential were not sufficiently considered in this work.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Software and algorithms
VASP code Kresse and Furthmüller40	VASP software GmbH	https://www.vasp.at
VESTA software package	JP-Minerals	https://jp-minerals.org/vesta/en/
VASPsol Mathew and Cramer47,48	Open source	https://www.opensourceagenda.com/projects/vaspsol

Resource availability

Lead contact

Further requests for resources should be directed to and will be fulfilled by the Lead Contact, Wei Lin (wlin@fzu.edu.cn).

Materials availability

This study did not yield new unique reagents.

Data and code availability

• •The published article includes all datasets generated or analyzed during this study.
• •This study did not generate new code.
• •Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

Experimental model and study participant details

Our study does not use experimental models typical in the life sciences.

Method details

Density functional theory (DFT) calculations were carried out by the VASP package.⁴⁰ The exchange and correlation interactions were depicted by the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE) functional.⁴¹ Electron-ion mutual effects were modelled by using Projector augmented wave (PAW) potentials.⁴² Bader analysis program^43,44 was employed to quantify the changes of electronic structure. The DFT-D3 correction was applied to assess the weak interactions.⁴⁵ Dipole correction in the z direction was opened to correct potential spurious terms brought by the asymmetry of the slabs.⁴⁶ The TM@SnS₂ was generated on a (3 × 3) SnS₂ supercell for probing the CO₂ reduction reaction mechanism. All of atoms were relaxed in any directions with the process of the structural optimization. A Monkhorst-Pack 3 × 3 × 1 k-point grid was adopted to sample the Brillouin zone during geometry optimizations, whereas a 7 × 7 × 1 k-point grid was utilized for electronic property computations. The convergence criteria for the structure relaxation were set to 0.02 eV·Å⁻¹ in force and 10⁻⁶ eV in energy, respectively. The kinetic cut-off energy of the plane wave basis was set to 450 eV. To avoid the interactions between the neighbouring slabs, the vacuum in the surface normal direction was set to 15 Å. Implicit solvation model^47,48 (using VASPsol) taking into account the aqueous electrolyte was used to simulate the reaction environment of Cr@SnS₂, Ru@SnS₂, Os@SnS₂ and Pt@SnS₂.

The binding energy was calculated by:

$$E_b=E\left({\text{TM}@\text{SnS}}_2\right)-E\left(\text{TM}\right)-E\left({\text{SnS}}_2\right)$$ (Equation 1)

Where E (TM@SnS₂), E (TM) and E (SnS₂) denote the total energy of fcc loaded TM@SnS₂, single TM and pure SnS₂.And the adsorption energy of molecular (CO₂, HCOOH and CH₄) was calculated by:

$$E_{ads}=E\left({\text{mol}-\text{TM}@\text{SnS}}_2\right)-E\left(\text{mol}\right)-E\left({\text{TM}@\text{SnS}}_2\right)$$ (Equation 2)

E (mol-TM@SnS₂), E (mol) refer to the energies of the system after molecular adsorption and the individual molecular, respectively. Deviation χ of the horizontal location of TM was calculated by:

$$\chi =\left(1-\frac{\sqrt{d_{TM-S}^2-H^2}}{d_{Sn-S}}\right)\times 100\%$$ (Equation 3)

Where d_TM-S and d_Sn-S represent the average distance of TM-S and Sn-S bond below TM, and H is the longitudinal distance from the surface.⁴⁹ The free energy of a proton/electron pair (H⁺ + e⁻) is equal to half of the gaseous hydrogen (1/2H₂) at an equilibrium potential, namely the free energy variations were calculated based on computational hydrogen electrode (CHE) model. The change of Gibbs free energy (ΔG) was computed as follows:

$$\Delta G=\Delta E+{\Delta E}_{\text{ZPE}}-T\Delta \mathrm{S}+\int C_{P}dT$$ (Equation 4)

Where ΔE, ΔE_ZPE, ΔS and ΔG_U signify the difference value of total energy, zero potential energy, entropy, and the free energy contribution related to enthalpic temperature correction, respectively, and T is the room temperature (298.15 K). Free energy change (ΔG) of the elementary reaction represents the free energy difference between the product and reactant and the formula of ΔG specific to each step is presented below (ES1-35) in supporting information. The limiting potential (U_L) of the reaction is obtained by U_L = −ΔG_MAX/e, where ΔG_MAX is the largest free energy change, corresponding to the potential determining step (PDS). The climbing image nudged elastic band (CI-NEB) method was adopted to locate the minimum-energy paths, and force convergence standard is 0.05 eV/Å.^50,51 The transition states (TSs) were verified using vibrational frequency calculations. The d band center (${\epsilon }_d$) is calculated by:

$${\epsilon }_d=\frac{\int n_d\left(\epsilon \right)\epsilon d\epsilon }{\int n_d\left(\epsilon \right)d\epsilon }$$ (Equation 5)

The $\epsilon$ and $n_d\left(\epsilon \right)$ are the electronic energy of states and the electronic density of states, respectively.

Quantification and statistical analysis

Calculate the variance of the adsorption energy linear relationship using Microsoft Excel.

Published: April 3, 2024