DFT Study on the CO₂ Reduction to C₂ Chemicals Catalyzed by Fe and Co Clusters Supported on N-Doped Carbon

Abstract

The catalytic conversion of CO₂ to C₂ products through the CO₂ reduction reaction (CO₂RR) offers the possibility of preparing carbon-based fuels and valuable chemicals in a sustainable way. Herein, various Fe_n and Co₅ clusters are designed to screen out the good catalysts with reasonable stability, as well as high activity and selectivity for either C₂H₄ or CH₃CH₂OH generation through density functional theory (DFT) calculations. The binding energy and cohesive energy calculations show that both Fe₅ and Co₅ clusters can adsorb stably on the N-doped carbon (NC) with one metal atom anchored at the center of the defected hole via a classical MN₄ structure. The proposed reaction pathway demonstrates that the Fe₅-NC cluster has better activity than Co₅-NC, since the carbon–carbon coupling reaction is the potential determining step (PDS), and the free energy change is 0.22 eV lower in the Fe₅-NC cluster than that in Co₅-NC. However, Co₅-NC shows a better selectivity towards C₂H₄ since the hydrogenation of CH₂CHO to CH₃CHO becomes the PDS, and the free energy change is 1.08 eV, which is 0.07 eV higher than that in the C-C coupling step. The larger discrepancy of d band center and density of states (DOS) between the topmost Fe and sub-layer Fe may account for the lower free energy change in the C-C coupling reaction. Our theoretical insights propose an explicit indication for designing new catalysts based on the transition metal (TM) clusters supported on N-doped carbon for multi-hydrocarbon synthesis through systematically analyzing the stability of the metal clusters, the electronic structure of the critical intermediates and the energy profiles during the CO₂RR.

1. Introduction

The electrochemical CO₂ reduction reaction (CO₂RR), as a useful method to convert CO₂ into value-added chemical products, which not only helps to solve the energy and environmental problems caused by fossil fuel combustion but also achieves sustainable development [1,2,3,4]. The main products of CO₂RR are generally divided into C₁ products (e.g., CO, CH₄, CH₃OH, HCOOH, etc.) [5] and C₂ products (e.g., C₂H₄, C₂H₅OH, CH₃COOH, etc.) [6]. Cu and Cu-derived materials have been considered the most common electrocatalysts for the CO₂RR in the early stages [7,8]. Furthermore, Ag-based [9,10] and Au-based [11,12,13] catalysts can selectively reduce CO₂ to CO at low overpotentials. However, they suffer from low utilization of metal atoms and a low C₂₊ selectivity.

Recently, the single-atom catalysts (SACs) of metal loaded on carbon substrates (metal nitrogen-doped carbon-based catalysts) have become a rather hot frontier for the maximized atom utilization efficiency and defined active centers. Rossmeisl et al. [14,15] found that the transition metal nitrogen-doped carbon-based catalysts (M-N-C, M = Mn, Fe, Co, Ni or Cu) performed a high CO selectivity for CO₂RR. Furthermore, both Mn-N-C and Fe-N-C also possessed CO selectivity as well as trace amounts of CH₄, which was assigned to the stronger CO binding of the Fe and Mn porphyrine-like structures. Zu et al. [16] successfully synthesized atomically dispersed Sn sites on nitrogen-doped carbon, which performs excellent activity and stability for formate generation at a kilogram scale with a quick freeze-vacuum drying-calcination method. Many Ni-based, Fe-based and Co-based SACs have exhibited high electrocatalytic activity and Faradaic effectivity (FE) for the CO₂RR with CO as the primary product due to the moderate adsorption energies of *COOH and *CO intermediates, as well as the high activation barrier for the hydrogen evolution reaction (HER) [17,18].

Though the widespread study on the single-atom catalysts enhanced the utilization efficiency of metal atoms, most of the current studies are limited to the reduction of CO₂ to C₁ products. Compared to C₁ products, C₂₊ products have a higher economic and chemical utilization value [19,20,21]. Cu-based SACs, up to now, have still performed good electrochemical reduction of CO₂ to C₂₊ chemicals [22,23]. However, Karapinar [24] revealed that the atomically dispersed CuN_x sites could reversibly convert into Cu clusters during CO₂RR, which are suggested as the real multiple active sites for CH₃CH₂OH production. Considering the fact that a single metal atom can accommodate only a single CO, it is difficult to activate two CO₂ molecules simultaneously to trigger the C-C coupling reaction based on an isolated metal center. Thus C-C coupling proceeding on the single metal atoms is quite difficult. Therefore, the catalysts with multiple active sites need to be considered to achieve the conversion from CO₂ to C₂ products [25].

Transition metal (TM) clusters with precise atomic numbers can offer multiple active sites, tune the size-dependent catalytic activity [26], and allow them to find the highest reactivity for the activation and dissociation of strong chemical bonds from CO₂. Xu et al. [27] reported a facile underpotential deposition technique to fabricate Cu clusters on carbonaceous substrates via rationally introducing S dopants in graphite foam. The obtained free-standing electrode exhibited high activity and excellent long-term stability toward oxygen reduction reaction. Pei et al. [28] found the trimeric metal clusters anchored on N-doped porous graphitic sheets possess a good selectivity and superiority towards CO₂RR to multi-carbon products due to the multiple active sites.

Considering the loading of metal clusters with a precise number of atoms on the active substrate can not only avoid the problem of low stability of bare metal cluster catalysts at room temperature but also further enhance their stability and catalytic efficiency. Graphite-based materials are currently widely used as substrates for electrocatalysts. To access C₂ products more efficiently, herein, we employed density functional theory (DFT) calculations to explore the CO₂RR catalyzed by Fe_n (n = 1, 3–5) anchoring on N-doped carbon (Fe_n-NC) to C₂H₄ and C₂H₅OH in this work. Furthermore, the Co₅ cluster supported on N-doped carbon (Co₅-NC) was explored comparatively. We found that Fe₅ loaded on NC exhibit significant activity for promoting the reduction of CO₂ to C₂ products, while the Co₅ cluster has higher priority for the selective synthesis of C₂H₄. Our findings provide insights into the design of highly active catalysts for CO₂RR and create a platform for developing metal cluster-NC electrocatalysts.

2. Theoretical Method

First principle calculations were performed using DFT with spin polarization utilizing the Vienna Ab initio Simulation Package (VASP). The projected augmented wave (PAW) [29,30,31] was used, and the generalized gradient approximation (GGA) realized by the Perdew–Burke–Ernzerhof function (PBE) was adopted to incorporate the exchange-correction functional [32]. A 2 × 2 × 1 Monkhorst-Pack K-point was sampled in the Brillouin zone, and a cut-off energy of 500 eV was set for geometric optimization. The convergence criteria are of 10⁻⁵ eV in energy between two electronic iteration steps and 0.02 eV/Å in force for every atom [33]. Our calculations of catalytic performance are based on the computational hydrogen electrode (CHE) proposed by Nørskov et al. [34]:

$$H_{\left(aq\right)}^{+}+e^{-}\to \frac{1}{2}{H}_2\left(g\right)$$

(1)

The change of the free energy for the step $*A+H^{+}+e^{-}\to *AH$ can be equal to the reaction: $*A+\frac{1}{2}{H}_2\left(g\right)\to *AH$ at 0 V versus the reversible hydrogen electrode (RHE) at all pH values.

We employ five types of small iron clusters Fe_n (n = 1, 3–5) supported on nitrogen-doped carbon sheets as the calculation models. The defects of NC provide ideal anchor sites for the iron cluster. To estimate the stability of supported TM clusters, the binding energy (E_b) of TM_n cluster on NC is calculated by Equation (2)

$$E_b=E_{T{M}_n-NC}-E_{T{M}_n}-E_{NC}$$

(2)

Here $E_{T{M}_n-NC}$ is the total energy of the optimized TM_n cluster supported on NC. The terms $E_{T{M}_n}$ and $E_{NC}$ refer to the energies of isolated TM_n cluster and support. We calculated the cohesive energy (E_c) of each TM_n cluster to further evaluate the stability of TM_n-NC catalysts, with E_c defined as:

$$E_c=\left(E_{cluster}-n{E}_{TM}\right)/n$$

(3)

Here the $E_{cluster}$ and $E_{TM}$ represent the energy of the total energy of the TM_n cluster and the energy of single TM atom; n is the number of TM atoms in the cluster. The more negative cohesive energy (E_c) indicates a more stable structure.

The adsorption energy (E_ads) of every intermediate species is defined by Equation (4)

$$E_{ads}=E_{C_x{H}_y{O}_z-T{M}_n-NC}-E_{T{M}_n-NC}-E_{C_x{H}_y{O}_z}$$

(4)

where $E_{C_x{H}_y{O}_z-T{M}_n-NC}$ refers to the total energy of the adsorbed species on the supported TM_n cluster, and $E_{T{M}_n-NC}$ is the energy of supported TM_n cluster. $E_{C_x{H}_y{O}_z}$ refers to the energies of $C_x{H}_y{O}_z$ in gas phase, respectively. The more negative adsorption energy indicates a stronger binding between TM cluster and NC support. Gibbs free energy change (ΔG) [35,36] is defined as:

$$\Delta G=\Delta E+\Delta E_{ZPE}+\Delta \int C_{P}dT-T\Delta S+\Delta G_{pH}+\Delta G_U$$

(5)

where $\Delta E$, $\Delta E_{ZPE}$, $\Delta \int C_{P}dT$ and $\Delta S$ are the total energy difference, the zero-point energy difference, the difference in enthalpic correction and the entropy change between the products and reactants obtained from DFT calculations, respectively. The zero-point energies (ZPE) and total entropies of the gas phase were computed from the vibrational frequencies, and the vibrational frequencies of the adsorbed species were also computed to obtain the ZPE contribution to the free energy expression. Only vibrational modes of the adsorbates were computed explicitly, while the catalyst sheet was fixed (assuming that vibration contribution to the free energy from the substrate is negligible) [37,38]. T is the temperature (298.15 K). The influence of applied potential is: $\Delta G_U=-neU$, where U is the external potential versus RHE, e is the electron transfer, and n is the number of proton–electron pairs. $\Delta G_{pH}$ is the free energy correction due to the concentration of H⁺. $\Delta G_{pH}=-k_{B}T\times \ln \left[H^{+}\right]=k_{B}T\times \ln 10\times pH$, where k_B is the Boltzmann constant, and the value of pH was assumed to be zero for acidic conditions.

3. Results and Discussion

3.1. The STABILITY Analysis of Fe_n-NC

For the Fe_n clusters supported on the NC substrate (Fe_n-NC), n = 1, 3, 4 and 5 were chosen to be studied here since the Fe₂ cluster is unstable [39]. As shown in Figure 1, for the adsorption of a single Fe atom on the NC substrate (Figure 1a), the mono Fe atom coordinated with the four nitrogen atoms and Fe-NC maintains a perfect monolayer structure, which is in agreement with previous results [40]. For the adsorption of Fe_n clusters with n ranging from 3 to 5 (Figure 1b–d), one Fe atom is anchored at the same position with that in a single Fe atom. Two Fe atoms bound to the doped nitrogen atoms with distances of about 2.1~2.3 Å, respectively, while the other Fe atoms bound together through Fe-Fe metal bonds.

As listed in

Table 1. The Bader charge of Fe atom in various Fe_n-NC, binding energies (E_b) between the Fe_n cluster and NC, and the cohesive energy (E_c) of Fe atoms in various Fe_n clusters.

Catalyst	* Bader Charge (e)					Eb (eV)	Ec (eV)
	Fe 1site	Fe 2site	Fe 3site	Fe 4site	Fe 5site
Fe1-NC	−1.07	-	-	-	-	−9.01	0
Fe3-NC	−0.80	−0.32	−0.31	-	-	−9.73	−0.35
Fe4-NC	−0.81	−0.34	−0.33	−0.19	-	−9.79	−0.62
Fe5-NC	−0.81	−0.36	−0.35	−0.28	−0.01	−8.34	−1.01

* negative Bader charge means electron loss.

, all the binding energies of Fe_n clusters on NC supports are thermodynamically favorable (E_b < 0). With the increase in Fe atoms, the binding energy decreases except for the magic Fe₅ cluster, which means that the small Fe clusters may tend to aggregate from small clusters to bigger clusters on NC support. The reason for the decreased binding energy of the magic Fe₅ cluster lies in that the Fe^5site located at the top site, as shown in Figure 1d. Hence there is no interaction with the NC support. What is more, the cohesive energy of various Fe_n clusters was also explored according to Equation (3), as shown in Table 1. It can be found that with the increase in Fe atoms in the cluster, the cohesive energy becomes thermodynamically favorable.

3.2. Electrocatalytic CO₂RR

The charge difference between two active transition atoms plays a key role during CO₂RR, and the mixed oxidation state of the catalytic centers can boost the C-C coupling [41,42]. Thus the Bader charges of various Fe clusters adsorbed on NC were investigated. As listed in Table 1, one can find that the electrons can transfer from the clusters to the support, making the whole Fe cluster positively charged, and each Fe atom in the cluster is positively charged as well. Herein, the Fe_n clusters are favorable to the CO₂RR; thus the electron-accepting properties of the positively charged Fe and Co sites are favorable for stabilizing the CO₂RR intermediates [43]. The largest charge transfer can be determined for the single Fe atom configuration since the monatomic Fe interacts with four coordinated N atoms. Furthermore, significant discrepancies for each charged Fe atom in the Fe₃, Fe₄ and Fe₅ clusters can be determined, which is beneficial for the C-C coupling reaction.

The conversion of CO₂ to CO catalyzed by various Fe_n clusters was calculated, as shown in Figure 2; the PDS is the *CO to CO for all the Fe_n-NC with a maximum Gibbs free energy ($\Delta G_3=\Delta G_{CO}-\Delta G_{*CO}$). The ΔG₃ are 0.91, 1.14 and 1.58 eV for the Fe-NC, Fe₃-NC and Fe₄-NC, respectively. This value increases to 1.94 and 2.29 eV at two different sites of Fe₅-NC. Thus it can be deemed that the Fe-NC, Fe₃-NC and Fe₄-NC require a lower overpotential to drive the desorption of CO, indicating that these structures favor the conversion of CO₂ to CO. The $*CO\to CO\left(g\right)$ step with strong *CO binding leads to a positive ΔG of CO desorption. The relatively strong binding of *CO on Fe-N_x is fully supported by the experimentally confirmed exclusive ability of the Fe-N_x catalyst to produce the hydrocarbon CH₄ [44].

In simple terms, one could say that to produce subsequent reaction products from CO during the CO₂RR; the CO molecule must be bound strong and long enough to undergo subsequent dissociation and hydrogenation steps to arrive at CH₄ or other small organic molecules. Herein, our work focuses on the reduction of CO₂ to C₂ products, which requires improved selectivity and activity by inhibiting the unwanted, i.e., C₁ hydrocarbon reaction pathway, which favors both the stabilization of *CO on the catalyst surface and the formation of C-C bonds. The strongest interaction for CO on the two Fe₅-NC sites means that CO does not leave the iron cluster surface easily, which, in turn, favors the subsequent C₂ product conversion. Therefore, the following study focuses on the performance of electrocatalytic CO₂ reduction to C₂H₄ and CH₃CH₂OH over the Fe₅-NC cluster. In order to further extend this result to other systems, the Co₅ cluster supported on NC was chosen to be studied comparatively (Figure S1). As shown in

Table 2. The Bader charge of Co atoms in Co₅-NC, binding energy (E_b) between the Co₅ cluster and NC and the cohesive energy (E_c) of Co atoms in the Co₅ cluster.

Catalyst	Bader Charge (e)					Eb (eV)	Ec (eV)
	Co 1site	Co 2site	Co 3site	Co 4site	Co 5site
Co5-NC	−0.71	−0.34	−0.31	−0.24	0.06	−10.00	−1.39

, the E_b value between the Co₅ cluster and the NC is −10.00 eV, and the E_c value of the Co atom is −1.39 eV, which means the Co₅ cluster can adsorb stably on the NC.

To further study the mechanisms of CO₂RR catalyzed by Fe₅-NC and Co₅-NC, the optimized structures and the energy profiles along the reaction coordinate for CO₂RR to both C₂H₄ and CH₃CH₂OH on Fe₅-NC and Co₅-NC are calculated as shown in Figure 3 and Figure 4, respectively. It can be found that two strongly adsorbed CO molecules adsorbed on the two adjacent metal atoms through carbon atoms either on the Fe₅ or the Co₅ clusters before the C-C bond formation. The two CO molecules will couple with each other via the top Fe or Co atom in the following steps of CO₂RR. For the CO₂RR on Fe₅-NC, the Gibbs free energy for the hydrogenation of the *CO dimer is uphill with an energy value of 0.79 eV, which is the highest energy during the formation of both C₂H₄ and CH₃CH₂OH. Thus, it can be deemed that the C-C coupling reaction for the CO₂RR from CO₂ to C₂ chemicals is the PDS. Furthermore, it can be speculated that both C₂H₄ and CH₃CH₂OH products can be achieved with Fe₅-NC catalyst, and the amount of C₂H₄ should be much more than the CH₃CH₂OH. Because the Gibbs free energy for the hydrogenation of *CH₂CHO to C₂H₄ is energy thermodynamically favorable, while the hydrogenation of *CH₂CHO to *CH₃CHO is an uphill reaction with a free energy of 0.26 eV.

For the CO₂RR on Co₅-NC, the Gibbs free energy for the hydrogenation of the *CO dimer is uphill with an energy of 1.01 eV, which is 0.22 eV higher than that of the Fe₅-NC. However, the Gibbs free energy for the hydrogenation of *CH₂CHO to *CH₃CHO is (0.07 eV higher) comparable with that in the C-C coupling reaction, which means that the hydrogenation of *CH₂CHO to *CH₃CHO becomes the PDS. Thus, most of the final C₂ products should be C₂H₄. In general, the Fe₅-NC has good catalytic activity towards the C₂ chemicals with relatively lower free energy change (0.53 eV), while the selectivity is not as good as Co₅-NC. However, Co₅-NC possesses better selectivity while the activity is lower than Fe₅-NC.

The d-band center of the TM and its electronic occupancy can affect the bonding strength between the intermediate and the catalytic surface. As shown in Figure 5, the PDOS of Fe d orbit from the top- and sub-layer structures show much more differences than that of the Co d orbit on Co₅-NC. Furthermore, the d band center of the top Fe atom in the Fe₅ cluster is −3.48 eV, while it becomes −1.59 eV for the sub-layer atoms. However, the d band center of the top Co atom is −1.34 eV, and it only changes to −1.47 eV for the sub-layer atoms. A much bigger discrepancy of the d band center between the top Fe atom and sub-layer atoms than that in the Co₅ cluster may boost the C-C coupling reaction, which could be called the synergy effect between the top- and sub-layer metal atoms. Our findings are consistent with the synergy effect between Cu⁺ and Cu⁰, and the surface can significantly improve the kinetics and thermodynamics of both CO₂ activation and CO dimerization. Cu metal embedded in an oxidized matrix catalyst can promote CO₂ activation and CO dimerization for electrochemical reduction of CO₂ [41].

Our theoretical calculations found that the multiple active sites in both the Fe₅ and Co₅ cluster-based catalysts facilitate the stabilization of *CO on the catalyst surface and the formation of C-C bonds. Both geometrical effects and electronic effects are the key factors leading to the Fe₅ and Co₅ clusters exhibiting better activity and/or selectivity over the single metal component. Furthermore, the tunable synthesis of Fe and Co alloys supported on NC may promote both their activity and selectivity toward CO₂RR. Therefore, Fe₅, Co₅ and the related tunable alloy clusters show great potential applications in electrocatalytic CO₂RR, and our methods provide a concept for designing the improved CO₂RR electrocatalysts.

4. Conclusions

The stability of the Fe_n (n = 1, 3, 4, and 5) clusters was studied first, and it can be found that the Fe_n anchors stably on the nitrogen-doped carbon via a basic Fe-NC structure. With the increasing of Fe atoms in the cluster, both the binding energy and cohesive energy become thermodynamically favorable, which means a small cluster tends to aggregate to be a bigger one. While for the Fe₅ cluster, the binding energy decreases because there is no interaction between the topmost Fe atom with the NC support anymore. In addition, the CO desorption is the most difficult on the Fe₅ cluster, which is beneficial to the subsequent reaction products from CO. Hence, the Fe₅-NC cluster was chosen to be studied as our C₂ catalyst, and Co₅-NC was comparatively studied as well. The results show that Fe₅-NC has better activity towards CO₂RR, and the products should be the mixed C₂H₄ and CH₃CH₂OH, since the PDS is the C-C coupling reaction with a free energy change of only 0.79 eV. The free energy change is only 0.53 eV for the reduction of CH₂CHO to CH₃CHO, and the reduction of CH₂CHO to C₂H₄ is a spontaneous step without any free energy change. Considering the fact that C₂H₄ is a gas, Fe₅-NC should be a good catalyst for CO₂RR to liquid ethanol with a relatively lower yield since part of the C₂H₄ gas will also be produced. Furthermore, Co₅-NC possesses a relatively good selectivity, but bad activity since the reduction of CH₂CHO to CH₃CHO is the PDS, and the free energy change is 1.09 eV. The PDOS and d band center analysis demonstrates that the relative energy favorable C-C coupling reaction on the Fe₅ cluster could be attributed to the larger discrepancy of d electrons of the two CO-adsorbing Fe atoms. This paper predicts a good application prospect of TM clusters supported on nitrogen-doped graphene for CO₂RR, and the new insight into the relationship between selectivity and activity sheds light on a new route for understanding and designing highly efficient non-precious catalysts for CO₂RR.