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Selective C-C Coupling by Spatially Confined Dimeric Metal Centers

Direct conversion of carbon dioxide (CO2) to high-energy fuels and high-value chemicals is a fascinating sustainable strategy. For most of the current electrocatalysts for CO2 reduction, however, multi-carbon products are inhibited by large overpotentials and low selectivity. For practical applications, there remains a big gap of knowledge in proper manipulation of the C-C coupling process. Herein, we exploit dispersed 3d transition metal dimers as spatially confined dual reaction centers for selective reduction of CO2 to liquid fuels. Various nitrogenated holey carbon monolayers are shown to be promising templates to stabilize these metal dimers and dictate their electronic structures, allowing precise control of the catalytic activity and product selectivity. By comprehensive first-principles calculations, we screen the suitable transition metal dimers that universally have high activity for ethanol (C2H5OH). Furthermore, remarkable selectivity for C2H5OH against other C1 and C2 products is found for Fe2 dimer anchored on C2N monolayer. The correlation between the activity and d band center of the supported metal dimer as well as the role of electronic coupling between the metal dimer and the carbon substrates are thoroughly elucidated.


INTRODUCTION
Production of liquid fuels by catalytic convertion of CO2, the main greenhouse gas and meanwhile an abundant carbon feedstock, has been regarded as an appealing strategy to solve both energy and environmental crises, albeit facing great challenges (Birdja et al., 2019;Jia et al., 2019;Amal et al., 2017). Copper-based materials have been widely adopted as catalysts for electro-reduction of CO2 to multi-carbon (C2 or C2+) products . Although fairly good activity can be achieved by modification or morphology engineering of copper, such as sculpturing it into nanoparticles or nanocubes, doping or alloying, and making oxide-derived copper, the selectivity and efficiency of most copper-based electrocatalysts remain unsatisfactory for commercialization of the CO2 conversion technique to high-energy fuels and highvalue chemicals (Kim et al., 2017;Zhou et al., 2018;Gao et al., 2019;. Recenlty, transition metal atoms dispersed on nitrogen-doped porous carbon nanomaterials emerge as a promising category of electrocatalysts for CO2 reduction, which have maximum atomic efficiency, high electrical conductivity and good durability, and can be facilely synthesized in the laboratory Bayatsarmadi et al., 2017;Chen et al., 2019;Cheng et al., 2018). The transition metal atoms are usually anchored in the pores of the carbon matrix and coordinated with the nitrogen atoms, exhibiting unique electronic states and acting as isolated reaction centers for CO2 reduction. Remarkable activity and selectivity toward carbon monoxide (CO) has been observed for various dispersed transition metal atoms (Fe, Co, Ni, Mn, and Cu) on N-doped graphene, carbon nanosheets or nanospheres, with selectivity up to 97% and Faradaic efficiency above 80% (Jiang et al., 2018;Zhang et al., 2018;Yang et al., 2018;. First-principles calculations show that the activity highly depends on the type of metal atoms, which provide different binding strengthes with the reaction intermediates . The single metal sites also have an advantage of suppressing the competing hydrogen evolution reaction (HER), due to the unique adsorption configuration of H* species compared to those on the transition metal surfaces .
Furthermore, homonuclear and heteronuclear dimers of transition metal immobilized in carbon based nanostructures, such as Fe2 and Fe-Co on nitrogenated graphitic carbon materials, Fe-Ni on N-doped graphene, and Pt-Ru on g-C3N4, have been synthesized in the laboratory (Ye et al., 2019;Wang et al., 2017;Zhou et al., 2019). This opens up the windows for a broader range of chemical processes that require dual reaction centers either with enhanced activity or carrying different functionalities simultaneously. For instance, Ren et al. fabricated diatomic Fe-Ni sites embedded in nitrogenated carbon . By taking advantage of the strong binding capability of Fe with CO2 molecule and the weak adsorption of CO on Ni, they achieved impressively high selectivity of 99% for CO and Faradaic efficiency above 90% over a wide potential range from −0.5 to −0.9 V, reaching 98% at −0.7 V vs.
reversible hydrogen electrode (RHE). On the theoretical side, a Cu2 dimer supported on the C2N monolayer was predicted to have high selectivity for methane (CH4), while dimerization of two CO species leading to the formation of ethene (C2H4) is possible with an energy cost of 0.76 eV . Other C1 products have also been proposed for elementary and mixed metal dimers on various N-coordinated carbon nanostructures, including defective graphene and phthalocyanine (Li et al., 2015;He et al., 2018;Shen et al., 2017).
Intuitively, two adjacent metal atoms that are spatially confined in a hole of Ndoped carbon materials can provide unique active sites, which not only enable the simultaneous fixation of two CO2 molecules, but also sterically limit the reaction pathways that may be beneficial for C−C coupling toward C2 or C2+ products. Moreover, various combinations of metal dimers and carbon substrates give high degrees of freedom for modulating the catalytic performance. However, the atomistic mechanism and composition recipe of such heterogeneous catalysts remain largely unknown, which impede their rational design and experimental synthesis for practical uses.
Here we exploit 3d transition metal dimers immobilized on various nitrogenated holey carbon sheets for selective reduction of CO2 to C2 products. By systematic first-principles calculations, the detailed C−C coupling mechanism on the spatially confined dual metal centers has been elucidated for the first time. The suitable transition metal elements and carbon substrates that lead to high activity and selectivity for ethanol (C2H5OH) and C2H4 are screened, and the underlying electronic structure-activity relationship is unveiled. These theoretical explorations illuminate important clues for precisely engineering the dispersed metal catalysts on porous carbon nanomaterials for direct conversion of greenhouse gas to multi-carbon hydrocarbons and oxygenates.

RESULTS and DISCUSSION
In the laboratory, N-doped graphitic carbon materials with controllable doping contents (up to 16.7% of N content) and atomic geometries can be achieved via either direct synthesis or post treatment (Xue et al., 2012;Xu et al., 2018;Qu et al., 2010).
Here we focused on pyridine N dopants in graphene, which are the main doping species at high N contents and are usually associated with the vacancies or pores of the carbon basal plane (Sheng et al., 2011;Sarau et al., 2017). As displayed in Figure 1, we considered a series of N-doped holey graphene monolayers, comprising C vacancies of various sizes (denoted as Vn, n = 2, 3, 4, 6) with the edges coordinated with different numbers of N atoms (denoted as mN, m = 4, 5, 6). Specifically, 4N-V2, 5N-V3 and 6N-V4 systems can be viewed as four, five and six N atoms decorating the edges of divacancy, tri-vacancy and tetra-vacancy in graphene, respectively, all of which have been commonly observed in experiment (Lin et al., 2015;He et al., 2014;. Note that, the V6 pore in graphene is a favorable defect according to transmission electron microscopy experiment (Robertson et al., 2015), and our previous calculation showed that N-doped V6 (namely 6N-V6) has extraordinary thermodynamic stability (Luo et al., 2013). Besides the N-doped graphitic sheets, we also considered the synthetic carbon nitride monolayers, including g-C3N4 and C2N (Zhao et al., 2014;Mahmood et al., 2015). All these porous N-coordinated carbon sheets have formation energies (defined by Equation S1 in Supplemental Information) in the range of 0.16 ~ 0.21 eV/Å, while the N-free V6 is higher in energy by over 0.46 eV/Å than the others (Table 1). These nitrogenated 2D holey carbon materials are ideal templates to stabilize and disperse metal atoms or small clusters. Indeed, isolated Fe2, Fe-Ni, and Fe-Co dimers embedded in 6N-V4(a), as well as Fe2 and Pt-Ru dimers anchored on g-C3N4 have already been realized in experiment (Ye et al., 2019;Wang et al., 2017;Tian et al., 2018).
To evaluate the capability of various supported metal dimers for CO2 reduction toward C2 products, we first explored the atomic structures, electronic and adsorption properties of dimeric 3d transition metal clusters on the 6N-V6 monolayer (as will be shown later, this substrate gives metal dimers the highest activity for CO2 reduction).
As presented in Figure 1 and Figure S1, all metal dimers are embedded in the hole of the graphitic sheet, except that Sc2 with a larger atomic size induces a noticeable buckling of 0.94 Å in the out-of-plane direction. Four N−metal bonds are formed with bond length of 1.95 ~ 2.09 Å, and the metal−metal bond length ranges from 1.96 Å to 2.79 Å ( Table 2). The binding energy (defined by Equation S2 in Supplemental Information) between the metal dimer and the graphitic sheet is −4.29 ~ −10.28 eV, excluding the possibility of dissociation or aggregation of the metal dimer. The thermal stability of these carbon substrate anchored metal dimers was further assessed by ab initio molecular dynamics (AIMD) simulations, which manifest that they can sustain at least 800 K for 10 ps with small vertical displacement of metal atoms (< 0.2 Å) (see Figure S2 for details), suggesting superior thermal stability for practical uses.
A CO2 molecule can favorably chemisorb on these dispersed metal dimers except would provide stronger binding with CO2 (Hammer et al., 2000).
In addition, we examined the capability of various dispersed 3d transition metal dimers for activating two CO2 molecules simultaneously, which is a prerequisite for C−C coupling to yield C2 products. Several candidate systems including Sc2, Ti2, Cr2,  and 4N-V2, 5N-V3, and 6N-V4(a) increases with both N content and hole size. The bonding interaction between Fe2 and g-C3N4 or C2N is relatively weak, due to the electronic saturation of these two semiconducting carbon nitride monolayers (as manifested by their large band gaps). In sharp contrast, Fe2 is strongly anchored on the nitrogen-free V6 defect that has six unsaturated carbon atoms on the hole edge, thereby leading to the largest binding energy of −12.03 eV.
All the supported Fe2 dimers are able to chemisorb two CO2 molecules with total adsorption energies of −0.23 ~ −1.62 eV (compared to −0.11 ~ −1.58 eV for adsorption of single CO2 molecule), as revealed by Figure  The distinct binding capability of various supported Fe2 dimers with gas molecules can be related to the electronic coupling between Fe2 and the carbon substrate. As displayed in Figure 2c, the amount of charge transfer from Fe2 to the substrate varies from 0.71 e to 0.97 e. Generally speaking, less electron transfer leads to higher activity of the Fe2 dimer for CO2 and CO chemisorption, which is consistent with the trend of binding energies between Fe2 and the carbon templates discussed before (Table 1). It is the N content, the degree of electronic saturation of the hole edge, and the bond configuration of Fe2 in the hole, that jointly determine the coupling strength between the metal dimer and the carbon sheets. Therefore, the nitrogenated 2D holey carbon materials with diverse morphologies and controllable N contents can not only stabilize and disperse metal dimers, but also dictate the electronic structures and activity of the anchored metal dimers. This opens up a universal avenue for modulating the product selectivity for CO2 reduction, as will be discussed in the following contents.
Note that graphitic N species are inevitably present in the experimentally synthesized N-doped carbon materials (Lin et al., 2014). To clarify their effect on the activity of the dispersed Fe2 dimer, we investigated the CO2 adsorption on Fe2@6N-V6 containing various numbers of graphitic N atoms at different distances from the 6N-V6 hole ( Figure S4). For all the considered systems, the CO2 adsorption energies on the catalysts with and without substitutional N atoms on the graphene lattice differ by less than 0.16 eV, suggesting that existence of the graphitic N species has only minor impact on the catalytic properties of the Fe2 dimer supported on pyridine holes of 2D carbon substrates. Figure 4 shows the most efficient pathways for CO2 reduction toward possible C1 and C2 products on the Fe2 dimer immobilized on various nitrogenated carbon sheets, and the corresponding free energy diagrams of various model systems calculated by the computational hydrogen electrode (CHE) model (Peterson et al., 2010) are given by Figure 5, Figure S5 and Figure S6. We used point (.) to represent the co-adsorption of two carbon intermediates on the catalyst and strigula (−) to indicate the coupling between two carbon intermediates. The maximum Gibbs free energy of formation ∆G among all the reaction steps defines the rate-determine step (RDS) and is thus denoted as ∆GRDS. Overall speaking, formation of C2 products first requires the activation of dual CO2 molecules on the catalyst. By going through the carboxyl (COOH*) pathway, two CO* intermediates can be generated. Then, protonation of CO* leads to C1 products like methanol (CH3OH) and CH4. Alternatively, it paves a way to the coupling between two neighboring carbon intermediates, which is energetically favorable and kinetically easy, and finally yields C2 products (C2H5OH and C2H4).
Specifically, formation of two CO* species on most of the considered Fe2 dimers is uphill in the free energy profile, involving energy steps of 0.24 ~ 0.87 eV. Then, reduction of CO* gives rise to HCO* species, which is lower in energy by up to 1.13 eV than the other possible intermediates such as COH* ( Figure S7). The CO* → HCO* conversion is endothermic with ∆G = 0.49 ~ 1.01 eV. Further protonation of HCO* leads to HCOH*, and then produces a CH* species by release of a H2O molecule. The C−C coupling reaction is most likely to occur between a CH* (or CH2*) species and the neighboring CO*. Our nudged elastic band (NEB) calculations suggest that the CO−CH* coupling is exothermic and barrierless on all the considered Fe2 dimers, except for Fe2@C2N and Fe2@C3N4 that involve a small kinetic barrier of about 0.22 eV (Table S1). According to previous theoretical studies (Goodpaster et al., 2016;Jiang et al., 2018), Cu (211) and (100)  On the other hand, formation of C1 products is also possible on the dispersed Fe2 dimers. As discussed above, HCOH* can be reduced to CH*, followed by the CO−CH* coupling. Alternatively, HCOH* may be protonated to H2COH*. Then, reduction of H2COH* yields CH3OH, or produces CH2* with release of a H2O molecule followed by the generation of CH3* and CH4. For Fe2@4N-V2, Fe2@6N-V6 and Fe2@g-C3N4, the CO* → HCO* conversion is the rate-determine step for both C1 products. For  (Figure 5a, c). This would lead to high selectivity for C2 products on Fe2@C2N. Remarkable selectivity for C2H5OH is obtained for Fe2@C2N and Fe2@6N-V4(b) with ∆GRDS = 0.70 and 0.59 eV, respectively, notably lower than ∆GRDS values for the other products (above 0.94 and 0.85 eV, respectively). Hence, these supported Fe2 dimers have competitive activity but distinct selectivity with regard to the conventional Cu based catalysts. It is known that Cu crystals mainly produce CO under low electrode potentials, while CH4 and C2H4 are the main products at sufficiently high electrode potentials (about −1.0 V vs. RHE in experiment) (Dai et al., 2017;Mistry et al., 2016).
Previous calculations revealed that Cu(211) surface encounters ∆GRDS = 0.74 eV for CH4 and C2H4, while formation of CO is much more favorable with ∆GRDS = 0.41 eV due to the relatively weak adsorption of CO on the Cu surface (adsorption energy ∆E = −1.01 eV) (Peterson et al., 2010). Differently, release of CO is prohibited on the present Fe2 dimers that have strong adsorption energy of ∆E = −2.94 ~ −4.04 eV with CO molecule.
The unique geometry and favorable adsorption properties of the Fe2 dimers immobilized on carbon substrates bring about inimitable advantages for their catalytic behavior. First, CO as an inevitable and even dominant product of CO2 reduction on many metal catalysts, severely limits the formation of higher-energy-density products (Zhu et al., 2014;Sarfraz et al., 2016;Peng et al., 2018); but it would be largely suppressed on the anchored Fe2 dimers. Second, the adjacent dual metal centers and their strong binding with CO pave an efficient pathway for C−C coupling reaction; in contrast, C−C coupling only occurs on metal surfaces with homogenously distributed reaction sites when the coverage of CO is sufficiently high (Morales-Guio et al., 2018;Huang et al., 2017). Third, the difficult desorption of C2H4 from the Fe2 dimers may result in superior selectivity for C2H5OH, which is a clean liquid fuel with high heating value. For most of the Cu based catalysts, however, the yield of C2H5OH is quite low compared to C2H4 .
At last, we assess the activity of these supported Fe2 dimers for HER, which is a competing reaction against CO2 reduction and highly affects the efficiency of CO2 conversion (Zhu et al., 2016;Cui et al., 2017).  Fe2@C2N and Fe2@C3N4, the adsorption strength of H* species is notably weaker than that of CO2 molecule by 0.09 ~ 0.67 eV, implying that CO2 reduction would prevail over HER on these catalysts with either high activity or superior selectivity. For Fe2@4N-V2, Fe2@6N-V4(a) and , the H* adsorption strength is stronger than that of CO2, which may suppress the CO2 reduction. In short, our results confirm that the Fe2 dimers immobilized on various nitrogenated carbon materials universally possess outstanding activity for C2H5OH synthesis from CO2 reduction, and desired selectivity can be further achieved by selecting proper carbon substrates.

CONCLUSION
In summary, we exploited dispersed 3d transition metal dimers for CO2 reduction to selectively produce liquid fuels. Comprehensive first-principles calculations show that nitrogenated holey carbon materials not only serve as templates to stabilize small metal clusters, but also dictate their electronic structures. Specifically, controlling the metal-substrate coupling strength allows effective modulation of both activity and product selectivity. As a consequence, the spatially confined dual reaction centers within the carbon matrix exhibit the following advantageous catalytic behavior: (1) simultaneous fixation of two CO2 molecules, (2) prohibition of CO desorption,(3) exclusive pathway for C−C coupling, (4) high activity for C2H5OH production irrelevant to the type of substrate. A number of immobilized dimer systems with outstanding activity and selectivity have been obtained. In particular, a Fe2 dimer embedded in the C2N monolayer exhibits remarkable selectivity for C2H5OH against the other C1 and C2 products as well as HER. These theoretical findings provide vital guidance for atomically precise design of the dispersed metal clusters for converting the greenhouse gas to high-energy fuels and high-value chemicals.

Limitations of the Study
This study systematically exploited 3d transition metal dimers anchored on nitrogenated holey carbon monolayers for selective reduction of CO2 to liquid fuels, and screened suitable metal elements and carbon templates with high selectivity for ethanol. However, experimental realization of such superior subnano catalysts relies on the preparation of metal clusters with specific size supported on some given substrates, which may be challenging and requires the development of advanced synthesis methods.

METHODS
All methods can be found in the accompanying Transparent Methods supplemental file.