Two types of single-atom Si-embedded N-doped graphene sheets, denoted as SiNxC3−x and SiNxC4−x, were designed for CO2 activation and electroreduction. The first-principles calculations show that CO2 can be chemically adsorbed at the single-atom Si sites of SiN1C2, SiN2C1, SiN3C0, SiN3C1, and SiN4C0 monolayers with quite low-energy barriers and exothermicity to some extent. Unexpectedly, CO2 activation and capture at the atomically dispersed Si sites of SiNxC3−x and SiNxC4−x follow different electron mechanisms where the three-coordinated Si in SiNxC3−x behaves as an electron donor while the four-coordinated Si acts as an electron shuttle for the electron transfer from the SiNxC4−x framework to CO2. For SiNxC4−x, the low-energy Si-pz center is a prerequisite for the Si site to capture the electron from the support framework, which is beneficial for the electron transfer to CO2. The activity of SiNxC3−x depends on both the Si-pz band center and the electron population at the three-coordinated Si, resulting in the conventional linear correlation between the activity and the p-band center not being observed. Furthermore, the SiN3C0 sheet is predicted to be quite a promising electrode material for CO2 electrochemical reduction to HCOOH, CH3OH, and CH4 with quite low limiting potentials.
Catalytic conversion of CO2 is of great significance in reducing the emission of greenhouse gases and the optimal utilization of carbon resources. Over the past few decades, many transitional metals and their compounds have been demonstrated to be highly effective catalysts for CO2 reduction in value-added chemicals.1,2 However, metal catalysts often suffer from high cost, poor durability, and resource scarcity as well as causing environmental pollution, which may limit their large-scale applications.3–5 Nowadays, it is highly required to seek efficient alternatives to metal-based catalysts for CO2 transformation.
In recent years, nonmetallic catalysts have attracted considerable interest for their high earth abundance and environmental friendliness.6–8 Among metal-free catalysts, silicon (Si) exhibits potential activity for CO2 catalytic conversion, both experimentally9–11 and theoretically.12–15 At present, scientists have obtained an in-depth understanding of the activity of transition metals based on the well-known d-band center model proposed by Hammer and Norskov.16 Most recently, the p-band center model was also proposed for the p-block element.17–20 Intriguingly, since the p-band is much more delocalized than the d-band, they may have different bonding interactions with the adsorbate.21 Specifically, a deeper p-band center may correspond to either large19,22,23 or small13,15,24 adsorption energies. Accordingly, it is important to figure out the bonding mechanism for the p-block element toward molecular adsorption.
Previous calculations by Zhou et al. indicate that higher Si-p or Si-pz orbitals in energy lead to stronger binding to CO2 for Ag (111) supported silicene and silicon nanocages.13,15 Our previous calculations also revealed that CO2 can be easily captured and further reduced to HCOOH, CH3OH, and CH4 at the single Si site of the SiN4C4 monolayer.12 However, the working mechanism for CO2 activation by Si and the essence of its bonding environment effect on the catalytic performance are still not thoroughly understood. Herein, we further designed a series of Si-coordinated N-doped graphene monolayers and explored the mechanism of CO2 activation by different SiNx moieties. Since SiN3C0 and SiN4C0 show good CO2 capture ability, the CO2 electroreduction performances on both monolayer sheets were further investigated.
II. COMPUTATIONAL METHODS
Here, all density functional theory (DFT) calculations were performed by the Vienna ab initio simulation package (VASP).25 The Perdew–Burke–Ernzerhof generalized gradient approximation (GGA-PBE) functional with the plane-wave basis set was adopted to treat electron–electron interactions.26 The van der Waals (vdW) corrections were considered by using the DFT-D3 method proposed by Grimme et al.27 An energy cutoff of 500 eV was adopted, and the convergence criterion for energy and force was set to 10−5 eV and 0.01 eV/Å, respectively. The Brillouin zone was sampled by a 5 × 5 × 1 k-point grid. A vacuum space of 20 Å was set to avoid the effect between periodic images. The ab initio molecular dynamics (AIMD) simulations with the NVT ensemble were performed, and the VASPKIT code28 served as a postprocessing tool for the computational data of the VASP. The transition states of CO2 adsorption on the Si-embedded N-doped graphene sheets were identified by the climbing image nudged elastic band (CI-NEB) method.29
The binding energy (Eb) of single Si is calculated as
where , , and ESi represent the electronic energies of Si-embedded N-doped graphene, only N-doped graphene (without Si), and the Si atom referenced to the bulk Si, respectively.
The CO2 adsorption energy (Ea) on the SiNxCy sheet is defined as
where ESiNxCy@CO2,ESiNxCy, and ECO2 represent the electronic energies of the CO2 adsorbed SiNxCy sheet, the SiNxCy sheet, and the isolated CO2, respectively.
The Si-pz center may be used as a descriptor of CO2 adsorption strength, and it is estimated by
where D(E) is the density of state (DOS) of the Si-pz band at a given energy E.
Based on the computational hydrogen electrode (CHE) model proposed by Nørskov et al.,30 the Gibbs free energy change (G) of each elementary step is calculated by
where E denotes the electronic energy obtained by DFT calculations, ZPE and S represent the zero-point energy change and the entropy change, respectively. T is the room temperature (298.15 K), and pH is set to be zero. The limiting potential (UL) is defined as UL = −Gmax/e−. Thermodynamic correction data for the intermediates are shown in Table S1. In order to simulate an aqueous environment, the implicit solvation model implemented in VASPsol31 was adopted.
III. RESULTS AND DISCUSSION
A. Structure and stability
Based on the optimized pristine graphene sheet, whose lattice constants are a = 12.32 Å, b = 12.80 Å, and c = 20 Å and α = β = γ = 90°, two types of Si-embedded N-doped graphene nanosheets have been constructed. As Fig. 1(a) shows, the two newly designed structures are denoted as SiNxC3−x and SiNxC4−x, in which x is the number of N atoms bonded to Si, and correspondingly, 3 − x and 4 − x represent the number of C atoms bonded to Si in SiNxC3−x and SiNxC4−x, respectively. Their structural differences are shown in Fig. 1(a) and Table S2. In general, the Si atom in the SiNxC3−x monolayer protrudes from the modified graphene surface, while the SiNxC4−x sheet basically maintains a planar configuration for the Si-embedded N-doped graphene sheets. Similar spatial configurations have also been reported in metal-N4 porous carbon monolayers.32–35
The active-site atoms of single-atom catalysts may migrate during the catalytic reaction, which results in the agglomeration of individual atoms as the catalytic center.36,37 To evaluate the stability of single Si atoms anchored to the N-doped graphene, its binding energies in SiNxC3−x and SiNxC4−x sheets have been calculated and are shown in Fig. 1(b). Remarkably, the predicted binding energies for all structures with various Si-embedded moieties are negative, suggesting that they generally have high thermodynamic stability. Among them, SiN3C0 and SiN4C0 have relatively weak Si anchoring interactions with binding energies of −2.20 and −2.25 eV, respectively. Furthermore, the 9 ps AIMD simulations under 300 K have been further performed for SiN3C0 and SiN4C0, and selected snapshots of SiN3C0 and SiN4C0 at 9 ps and evolution of Si–N bond lengths are shown in Fig. S1. We note that both structural skeletons survive without any chemical bond breaking during the AIMD simulation, although the structures of SiN3C0 and SiN4C0 experience partial distortion and the Si–N bond lengths fluctuate near their equilibrium configurations. Accordingly, these Si-embedded N-doped graphene sheets are of high thermal stability.
B. Chemisorption of CO2
The interactions between CO2 and SiNxC3−x and SiNxC4−x have been studied by using first-principles calculations. For SiNxC3−x (x = 0–3), CO2 can be chemically adsorbed on SiNxC3−x monolayer sheets except for SiN0C3, indicating that the doping of N atoms may significantly improve the ability of SiNxC3−x to activate CO2. As Fig. 2 shows, for the CO2-chemisorbed state on SiN1C2–CO2, the Si–CO2 bond length and the bond angle of CO2 are predicted to be 2.01 Å and 141.3°, respectively. With the increase in N-doped atoms coordinated to Si, CO2 activation is further strengthened, and amazingly, Si is bonded to both C and O atoms in SiN2C1–CO2 and SiN3C0–CO2. In order to understand the intrinsic mechanism of Si–O bonding, we calculated the highest occupied crystal orbital (HOCO) and the next HOCO (HOCO − 1) at (0,0,0) points of SiN1C2 and SiN2C1. As shown in Fig. S2, HOCO is mainly distributed on the central Si atom for both SiN1C2 and SiN2C1, which accounts for the Si–C bonding through the electron donor–acceptor interaction. For SiN2C1, the Si atom has notable contribution to both HOCO and HOCO − 1, and such electronic features allow the Si center to behave not only as an electron donor for the Si–C bonding but also as an electron acceptor for the Si–O bonding, leading to the 2-CO2-chemisorbed configuration, as shown in Fig. 2.
For SiNxC4−x (x = 0–4), only SiN3C1 and SiN4C0 sheets can chemically bind CO2, and their chemisorption configurations are shown in Fig. 2. We note that predicted Si–C bond lengths and ∠OCO bond angles in SiN3C1–CO2 and SiN4C0–CO2 are 2.09 Å/1.97 Å and 141.2°/136.6°, respectively. Clearly, SiN4C0 shows stronger ability toward CO2 activation than SiN3C1. In general, the more N atoms coordinate with Si in both SiNxC3−x and SiNxC4−x sheets, the more remarkable the CO2 activation is.
The predicted transition-state (TS) configurations for formation of CO2-chemisorbed states on SiN1C2, SiN2C1, SiN3C0, SiN3C1, and SiN4C0 monolayer sheets are depicted in Fig. S3, and the corresponding energy barriers relative to their physical adsorption configurations are calculated to be 0.09, 0.12, 0.23, 0.22, and 0.05 eV, respectively, revealing the facile CO2 capture and chemisorption on these five single-atom Si-embedded N-doped graphene sheets. Herein, based on the unit cell of SiN4C0, a 2 × 2 supercell with four atomically dispersed Si sites has been constructed. Then, 16 CO2 molecules were randomly put into the supercell, and 5 ps AIMD simulations were further performed. Figure S4 shows the final configuration after 5 ps simulations and the evolution of the Si–CO2 distance over time, where one CO2 molecule is chemically adsorbed at the Si site at about 4 ps at room temperature, showing that CO2 is easily captured and activated by the SiN4C0 sheet.
C. Bonding mechanisms
It was known that the bending of CO2 may lower the LUMO energy level,38 which is beneficial for accommodation of excess electrons. Accordingly, the injection of electrons into the LUMO orbital of CO2 could activate and bend CO2. Figure S5 depicts the electron spin density of graphene, SiNxC3−x, and SiNxC4−x sheets. One can see that the introduction of Si and N atoms makes the electron spin density localize over different atoms. However, except that SiN1C2 has a net magnetic moment of 0.96 µB, the overall magnetic moment of other nine structures is zero. It is worth noting that the spin density is partially distributed on the Si atom for all sheets here, which is favorable for the bonding between Si and CO2.
The valence electron (VE) populations of the Si atom and the number of electrons occupied by the Si-pz orbital in SiNxC3−x and SiNxC4−x are compiled into Table I. One can see that as the number of N atoms increases, the valence electrons of the Si atom in both SiNxC3−x and SiNxC4−x increase first and then decrease. However, the electron population occupied by the Si-pz orbital always increases for SiNxC3−x, while for SiNxC4−x, it increases at first and then decreases. Such discrepancies may result from the strong conjugation interaction between Si-pz orbitals and their surrounding atoms for SiNxC4−x with a planar configuration. Here, charge transfer plays an important role in CO2 activation. In the process of CO2 chemisorption on SiN1C2, SiN1C2, SiN1C3, SiN3C1, and SiN4C0, the number of gain and loss electrons of Si and CO2 is presented in Table I. We note that for SiNxC3−x, the electrons acquired by CO2 are mainly from the Si atom, while for SiNxC4−x, the Si atom almost has no electron loss, and the electron transfer to CO2 basically comes from the N-doped graphene substrate. In brief, the Si atom donates electrons to CO2 for SiNxC3−x, while it acts as an electron shuttle to transfer electrons to CO2 from the SiNxC4−x framework. Overall, the net effect results in the electron injection into CO2 and in its activation and chemisorption.
|Structures .||VE of Si .||Si-pz electron .||Si-pz center .||CT of Si .||CT of CO2 .||Ea of CO2 .|
|Structures .||VE of Si .||Si-pz electron .||Si-pz center .||CT of Si .||CT of CO2 .||Ea of CO2 .|
Herein, the Si-pz centers [(pz)] as well as the adsorption energies of CO2 were also calculated and are shown in Table I. We note that the more the doping-N atoms, the lower the Si-pz center energy level. For SiNxC4−x, the low-energy Si-pz center corresponds to relatively strong chemisorption of CO2, while for SiNxC3−x, the chemisorption strength of CO2 depends on the balance between the Si-pz center energy level and the electron population in the Si-pz orbital and Si atom, as shown in Table I. Here, the Si center in SiNxC3−x acts as an electron donor in CO2 activation, and a high electron density at the Si site and a high-energy Si-pz orbital benefit the electron transfer to CO2, whereas the Si atom in SiNxC4−x serves as the electron shuttle to deliver the electron transfer from the SiNxC4−x substrate to CO2. The lower the Si-pz center energy level, the more favorable the electron capture is. In the transition state for CO2 chemisorption on SiN4C0, the single-atom Si has a valence electron population of 1.83, higher than that of the pristine SiN4C0 by 0.47 e−. Presumably, with the increase in electrons in the Si-pz orbital, its energy level increases, and the excess electrons may transfer to CO2. As shown in Fig. 2, SiNxC4−x sheets maintain an approximately planar configuration, and there should be strong conjugation interactions between Si and the SiNxC4−x framework, which are beneficial to the electron transfer.
In order to understand the intrinsic relationship between the Si-pz center and the CO2 adsorption energy, a detailed density of states analysis was further carried out. Figure 3(a) shows the projected density of states (PDOS) of Si-pz and N-pz of SiNxC3−x. As for SiN0C3, one can see that the Si-embedded N-doped graphene opens the bandgap of pristine graphene and the Si-pz orbital has a notable share of electronic state distributions in the valence and conduction bands near the Fermi level. As the number of N atoms increases, the conduction band around the Fermi level, contributed by the Si-pz orbital, is gradually occupied, as shown in the PDOS of SiN1C2. For SiN2C1, the conduction band with a big share of the Si-pz orbital is almost completely occupied near the Fermi level, and correspondingly, SiN2C1 possesses the strongest electron-donating ability and thus shows the strongest binding to CO2. As for SiN3C0, since the energy band contributed by the Si-pz orbital is already fully occupied and its energy level is further lowered, as shown in Fig. 3(a) and Table I, the extra electrons are populated into the conduction band contributed by the N-pz orbital, which accounts for the sharp change in the Si-pz center of SiN3C0 and relatively weaker binding ability toward CO2 than toward SiN2C1.
Figure 3(b) shows the PDOS of Si-pz and N-pz orbitals of SiNxC4−x. Apparently, there is a strong conjugate interaction between Si-pz and N-pz orbitals. Similar to SiNxC3−x, as the number of N atoms increases, the Si-pz unoccupied electronic states in SiNxC4−x are gradually occupied. It is worth noting that the Si-pz orbital of SiN4C0 still has electron state distributions in the conduction band near the Fermi level, indicating that more electrons can be accommodated. On the whole, the shift of the Si-pz center to the low-energy region results from its population of more electrons.
D. CO2 electroreduction
Since the SiN3C0 and SiN4C0 sheets show good CO2 activation ability, the electroreduction of CO2 (CO2ER) on SiN3C0 and SiN4C0 was further investigated by the first-principles calculations. Figure S6 depicts the proposed CO2ER pathways on SiN3C0 and SiN4C0 sheets. For the production of CO, the pathway is denoted as * + CO2 → *CO2 → *COOH → *CO → * + CO, which is the same for both SiN3C0 and SiN4C0. However, for the production of HCOOH, CH3OH, and CH4, there exist multiple pathways.
For both SiN3C0 and SiN4C0, the *OCHO intermediate may be involved in CO2ER. However, the predicted free energy change (G) of the *OCHO → *HCOOH step on SiN4C0 is 2.05 eV, and thus, the CO2ER pathway including *OCHO as a precursor is infeasible. Figure S7 depicts the optimal pathway for CO2ER on SiN4C0. As shown in Fig. S7, the hydrogenation of *COOH determines the speed of the entire CO2ER. Unfortunately, the limiting potentials (UL) are calculated to be −1.23 V for the production of CO and −1.15 V for generating HCOOH, CH3OH, and CH4, and such relatively high limiting potentials arise from the high stability of *COOH on SiN4C0. Accordingly, the SiN4C0 sheet may not be a kind of high-efficient electrode material, although it has an excellent ability toward CO2 activation.
Figure 4 shows the predicted optimal pathways for the production of CO, HCOOH, CH3OH, and CH4 on SiN3C0. For CO2ER on SiN3C0, the rate of the whole reaction is determined by the first three protonation steps, and the limiting potentials for the production of CO, HCOOH, and CH3OH/CH4 are calculated to be −0.95, −0.54, and −0.59 V, respectively; the corresponding rate-determining steps are *COOH → CO, *OCHO → *HCOOH, and *HCOOH → *CHO. Remarkably, the limiting potentials for products CO, HCOOH, CH3OH, and CH4 on SiN3C0 are comparable to those for CO2ER on Cu–C3N4, and such low limiting potentials were proved to have higher efficiency than Cu–NC and Cu(111) surfaces in selective electroreduction of CO2 to these C1 products.39 Intriguingly, the Si atom is bonded with both C and O of CO2 in the intermediates *HCOOH and *CH2O, as also observed in the configuration of CO2 adsorbed SiN3C0. Such bonding features can enhance the interaction between the Si atom and intermediate species and further produce more value-added chemicals, compared to the catalytic conversion by the metal-Nx porous carbon.35,40 In general, SiN3C0 has low limiting potentials for CO2 electroreduction and promises the selectivity for the formation of CO, HCOOH, and CH3OH/CH4 to some extent.
In the present work, newly designed Si-embedded N-doped graphenes are predicted to have high thermodynamic stability by the first-principles calculations. The increase in N atoms in coordination with the Si atom can enhance the electron-donating ability of SiNxC3−x and SiNxC4−x, and CO2 chemisorption can be achieved through the Si–CO2 bonding interaction. In chemisorption configurations of CO2 on SiN1C2, SiN2C1, SiN3C0, SiN3C1, and SiN4C0, the three-coordinated Si atoms in SiNxC3−x act as the electron donor, while the four-coordinated Si atoms in SiNxC4−x only behave as the electron shuttle to account for the electron transfer from the support framework to CO2. Accordingly, CO2 activation and chemisorption on SiNxC3−x and SiNxC4−x follow different electron mechanisms. For SiNxC3−x, the activity of a single Si site toward CO2 activation depends on its electron density and the Si-pz band center. The more the N-doping atoms, the larger the electron population at the Si site, while the deeper the Si-pz band center, and the CO2 chemisorption requires a balance among these factors. For SiNxC4−x, the low-energy Si-pz center is a prerequisite for the Si site to capture the electron from the support framework, which is beneficial to the electron transfer to CO2. The shift of Si-pz band centers originate from the change in the electron occupation of the Si-pz-based conduction band near the Fermi level. With the increase in N-doping atoms in coordination with Si, the electron population on the Si-pz-based conduction band increases, and the Si-pz band center gradually moves to the low-energy region. CO2 electroreduction to HCOOH and CH3OH/CH4 on SiN3C0 has low limiting potentials of −0.54 and −0.59 V, respectively, indicating that SiN3C0 as a novel electrocatalyst can be applied for CO2 capture and electroreduction.
See the supplementary material for the evolution of the Si–N distance and CO2 capture during AIMD simulations, the highest occupied crystal orbital, the transition-state configurations of CO2 chemisorption, the electron spin densities, and free energy profiles for CO2 electroreduction on SiN3C0.
This work was supported by the National Science Foundation of China (Grant Nos. 21873078, 21673185, and 21933009).
Conflict of Interest
The authors have no conflicts to disclose.
The data that support the findings of this study are available within the article and its supplementary material.