Theoretically Predicted CO Adsorption and Activation on the Co-Doped hcp-Fe₇C₃ Catalyst

Abstract

The Hcp-Fe₇C₃ phase has attracted more attention due to the high catalytic activity in Fischer–Tropsch synthesis (FTS) reactions. In this work, the adsorption and activation of CO on a Co-doped hcp-Fe₇C₃ catalyst were investigated by density functional theory (DFT) in order to understand the effect of Co doping on the initial step of FTS reactions on iron-based catalysts. Different Co-doped hcp-Fe₇C₃ $\left(001\right)$ and $\left(1\overline{1}0\right)$ surfaces were constructed, and the CO adsorption configurations were studied. The calculated results show that the structure of the $\left(001\right)$ surface remains basically unchanged after doping with Co atoms, while the replacement of Fe or C atoms on $\left(1\overline{1}0\right)$ surfaces with Co atoms has a significant impact on the surface structure. The top sites on the doped Co atoms of hcp-Fe₇C₃ are disadvantages for the CO adsorption, whereas the T, 2F, or 3F sites around the doped Co atoms are beneficial for promoting the adsorption of CO. The CO direct dissociation pathways on the four types of Co-doped hcp-Fe₇C₃ $\left(001\right)$ surfaces are exothermic, while the H-assisted dissociation pathways of CO are endothermic. The H-assisted activation via HCO on the 3F1 site of the 2Co2-doped hcp-Fe₇C₃ $\left(001\right)$ surface shows the lowest energy barrier of 1.96 eV. For the Co-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surfaces, the H-assisted activation via HCO is the preferred activation pathway for CO on the Co-doped surfaces with the energy barrier of approximately 1.30 eV.

1. Introduction

Fischer–Tropsch synthesis (FTS) is a promising process for the conversion of syngas (from coal, biomass, and natural gas) into clean liquid fuels and chemicals [1,2,3,4,5]. Syngas, CO, and H₂ convert to mainly alkanes and olefins through a series of catalytic reactions as well as converting to oxygenated chemicals, such as alcohols, aldehydes, ketones, acids, etc. [6]. Two major categories, iron-based and cobalt-based catalysts, were widely used in industrial FTS processes [7,8,9]. The iron-based catalysts have been widely used in FTS processes on account of the superiorities of high activity, low cost, operational flexibility, and raw material adaptability [7,10,11]. Contrary to the lower catalytic activity toward long-chain hydrocarbons of iron-based catalysts, the much more expensive cobalt-based catalysts show high activity and selectivity [12,13,14].

Recently, iron carbides, such as χ-Fe₅C₂, Fe₇C₃, θ-Fe₃C, and ε-Fe₂C, were suggested as the active phases of iron-based catalysts during the FTS processes [15,16,17,18,19,20,21]. However, the density functional theory (DFT) study of carbides catalysts has demonstrated that the theoretically intrinsic selectivity toward long-chain hydrocarbons of iron carbides was higher than that of cobalt at relatively low reaction temperatures [22,23]. In practice, the observed lower selectivity toward long-chain hydrocarbons of iron carbides was caused by the high reaction temperature required for CO dissociation. Therefore, many efforts have been attempted to fabricate Co-doped iron-based catalysts in order to realize a high activity and selectivity at a relatively low temperature. Yang et al., fabricated the Fe₅C₂/Co heterostructured nanoparticles for low temperature FTS and studied the synergistic effects between Fe₅C₂ and Co parts in the catalysts [24]. In the Fe₅C₂/Co heterostructured nanocatalysts, Co was responsible for the CO dissociation, while Fe₅C₂ was responsible for the chain growth, which led to the enhanced catalytic performance in low temperature FTS reactions.

The FTS reaction is an extremely complicated system because of the hundreds of elementary reactions which are all interconnected with each other on the surface of catalyst. Moreover, both activity and selectivity of the catalyst are very important in the catalytic system. There have been many studies published in the literature aimed at investigating the adsorption and activation mechanism of CO on iron carbides. It is generally known that the CO direct dissociation pathway is the major CO-activated mechanism on (510) surfaces of χ-Fe₅C₂ catalysts, while the CO H-assisted dissociation pathway is preferred on the (100) and (001) surfaces of χ-Fe₅C₂ [18,25,26]. For Fe₃C catalysts, the reported results revealed that CO direct dissociation is favored on the (110), (101), $\left(1\overline{1}1\right),$ and (111) surfaces, and the CO H-assisted dissociation pathway is favored on the (100), (011), $\left(0\overline{1}1\right)$, and (001) surfaces, while both of the two dissociation pathways are possible on (010) surfaces for the similar overall barriers [27].

Recently, the hcp-Fe₇C₃ phase has attracted more attention due to the high catalytic activity in FTS reactions. Chang et al., prepared different iron carbide phases by the pretreatment of a binary Fe/SiO₂ model catalysts at different gas atmospheres and found that Fe₇C₃ has the highest intrinsic activity among the three candidate carbides (ε-Fe₂C, Fe₇C₃, and χ-Fe₅C₂) in typical medium-temperature FTS conditions [20]. Zhang et al., studied the CO activation mechanism on hcp-Fe₇C₃ (211) and found that the top site and four-fold site are the major adsorption sites for CO, and the surface carbon is able to be the CO activation site by the CO insertion dissociation pathway. The CO was activated through the H-assisted dissociation pathway on the top site, while it was activated through the direct dissociation pathway on 4-fold site [28].

To the best of our knowledge, theoretical studies of the adsorption and activation mechanism of CO on Co-doped iron carbides have rarely been reported. Herein, the adsorption and dissociation mechanism of CO on the Co-doped hcp-Fe₇C₃ catalyst were investigated by DFT. Firstly, the different Co-doped hcp-Fe₇C₃ $\left(001\right)$ and $\left(1\overline{1}0\right)$ surfaces were constructed. Then, the adsorption configurations of CO on different Co-doped hcp-Fe₇C₃ surfaces were calculated. Moreover, the dissociation pathway of CO on the most stable adsorption configurations of each Co-doped hcp-Fe₇C₃ surface are discussed.

2. Results and Discussion

2.1. Structures of Co-Doped hcp-Fe₇C₃ Surfaces

The selected $\left(001\right)$ and $\left(1\overline{1}0\right)$ surfaces of hcp-Fe₇C₃ are demonstrated in Figure 1, which are modeled by a (2 × 2) supercell of periodical repeated slabs with a thickness of three repeated units and a vacuum layer of 15 Å. The upper layers are allowed to relax, while the bottom layers are kept at their bulk positions. As shown in Figure 1, the top layer of $\left(001\right)$ surface consists only of Fe atoms, while the top layer of $\left(1\overline{1}0\right)$ surface consists of Fe and C atoms. As a result, the surface Fe and C atoms might be replaced by Co atoms to form the Co-doped hcp-Fe₇C₃ catalysts. Therefore, based on the considered symmetry of the hcp-Fe₇C₃ $\left(001\right)$ surface, different surface Fe atoms are replaced by Co atoms to form four types of Co-doped hcp-Fe₇C₃ $\left(001\right)$ surface, which are denoted as 1Co-doped $\left(001\right)$, 2Co1-doped $\left(001\right)$, 2Co2-doped $\left(001\right)$, and 3Co-doped $\left(001\right)$. On the $\left(001\right)$ surface, the center site of hexagon (labeled as T4) locates in the hollow in the surface. Therefore, the Co-doped $\left(001\right)$ surface with the Co at the T4 site is not discussed in our present work. For the more complicated composition of the hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface, either Fe or C could be replaced by Co atoms, thus four types of Co-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface denoted as 1Co1-doped $\left(1\overline{1}0\right)$, 1Co2-doped $\left(1\overline{1}0\right)$, 1Co3-doped $\left(1\overline{1}0\right),$ and 2Co-doped $\left(1\overline{1}0\right)$ are obtained. The structures of different Co-doped hcp-Fe₇C₃ $\left(001\right)$ and $\left(1\overline{1}0\right)$ surfaces are shown in Figure 2 and Figure 3, with xCoy-doped hcp-Fe₇C₃, where, in the names of Co-doped hcp-Fe₇C₃, x means the number of Co atoms, and y means the different types of Co-doped structures with the same number of Co atoms. It can be seen that the surface structure of $\left(001\right)$ surface remains basically unchanged after doping with Co atoms. On the contrary, the replacement of Fe or C atoms on $\left(1\overline{1}0\right)$ surfaces with Co atoms has a significant impact on the surface structure.

2.2. CO Adsorption on the Co-Doped hcp-Fe₇C₃ Surfaces

CO adsorption on FTS catalysts is critical for deep understanding of the catalytic reaction mechanism since the CO adsorption is the first step of the FTS reaction, which shows great influences on the C=O bond breaking and the subsequent chain reactions. The possible adsorption sites of CO on the hcp-Fe₇C₃ $\left(001\right)$ and $\left(1\overline{1}0\right)$ surfaces are labeled in Figure 1, in which the top and n-fold sites are labeled as T, 2F, 3F, and 4F, respectively. For the original clean hcp-Fe₇C₃ $\left(001\right)$ surface, the most stable adsorption site for CO is the 3F1 site with the adsorption energy of −2.47 eV [29]. The C-O bond length and the adsorption energies for CO on different Co-doped hcp-Fe₇C₃ $\left(001\right)$ surfaces are listed in

Table 1. The C-O bond length and the adsorption energies for CO on the Co-doped hcp-Fe₇C₃ $\left(001\right)$ surface.

Site	$\left(001\right)$		1Co-Doped $\left(001\right)$		2Co1-Doped $\left(001\right)$		2Co2-Doped $\left(001\right)$		3Co-Doped $\left(001\right)$
	d(C-O)/Å	Eads/eV	d(C-O)/Å	Eads/eV	d(C-O)/Å	Eads/eV	d(C-O)/Å	Eads/eV	d(C-O)/Å	Eads/eV
T1			1.174	−2.23	1.175	−2.22	1.174	−2.20	1.176	−2.17
T2			1.176	−2.15	1.176	−2.14	1.178	−1.62	-	-
T3			1.177	−2.12	1.177	−2.14	1.177	−1.98	1.186	−1.94
T4			1.181	−2.40	1.181	−2.03	1.175	−1.75	1.190	−2.33
T5			- *	-	1.180	−1.84	1.201	−2.38	-	-
2F1			1.201	−2.47	1.185	−2.01	1.201	−2.58	1.199	−2.27
2F2			1.179	−2.12	1.190	−1.83	1.202	−2.60	1.195	−2.30
2F3			1.181	−2.14	1.196	−2.54	1.186	−2.29	1.201	−2.38
3F1	1.202	−2.47	1.203	−2.48	1.203	−2.50	1.201	−2.61	1.200	−2.26
3F2			1.181	−2.40	1.201	−2.53	1.198	−2.34	1.204	−2.37
4F1			1.198	−2.04	1.203	−2.02	1.231	−2.08	1.233	−2.08
4F2			1.193	−2.11	1.191	−2.16	1.295	−2.17	-	-

*—denotes the equivalent symmetrical sites.

, and the most stable adsorption configurations of CO on different Co-doped hcp-Fe₇C₃ $\left(001\right)$ surfaces are demonstrated in Figure 4.

For the 1Co-doped $\left(001\right)$ surface, the most stable adsorption site for CO is the 3F1 site with the lowest adsorption energy of −2.48 eV, which is approximately the same as the clean hcp-Fe₇C₃ $\left(001\right)$ surface. The 3F1 site provides three bond connections to the adsorbed CO molecule, resulting in the longest C-O bond length of 1.203 Å. In contrast, the adsorption energy of CO on the top site of Co atom (T1 site) is −2.23 eV, and the C-O bond length is 1.174 Å, indicating the inferior adsorbability of surface Co atoms for CO. The results show that the replacement of the hcp-Fe₇C₃ $\left(001\right)$ surface with a single Co atom rarely changes the interactions between the $\left(001\right)$ surface and CO.

With the increasing Co content of the $\left(001\right)$ surface, there are great changes taking place in the adsorption behavior of CO on Co-doped $\left(001\right)$ surfaces. For the hcp-Fe₇C₃ $\left(001\right)$ surface replaced with two Co atoms, their adsorbabilities for CO are enhanced to various degrees. The most stable adsorption site for CO on 2Co1-doped $\left(001\right)$ is changed to the 2F3 site with the adsorption energy of −2.54 eV and the C-O bond length of 1.196 Å, but it is still energetically favorable for CO adsorption compared to the clean hcp-Fe₇C₃ $\left(001\right)$ surface. Although the 3F1 site is less stable than the 2F3 site, the corresponding adsorption energy −2.50 eV is still lower than that of the clean hcp-Fe₇C₃ $\left(001\right)$ surface. For the 2Co2-doped $\left(001\right)$ surface, the most stable adsorption site is the 3F1 site with the adsorption energy of −2.61 eV and the C-O bond length of 1.201 Å, which shows the strongest interaction between the four types of Co-doped hcp-Fe₇C₃ $\left(001\right)$ surfaces and CO. On the basis of the structure of the 1Co-doped $\left(001\right)$ surface, the 3Co-doped $\left(001\right)$ structure containing three symmetrical Co atoms was constructed. The most stable adsorption site for CO is the 2F3 site with the lowest adsorption energy of −2.38 eV, and the corresponding C-O bond length is 1.201 Å.

As a result, the top sites on the doped Co atoms of hcp-Fe₇C₃ $\left(001\right)$ are disadvantages for the CO adsorption, whereas the 2F site or 3F site around the doped Co atoms are beneficial for promoting the adsorption of CO.

The replacement of Fe or C atoms on $\left(1\overline{1}0\right)$ surface with Co atoms has a significant impact on the surface structure. For the original clean hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface, the most stable adsorption site for CO is the T1 site with the adsorption energy of −2.49 eV. The C-O bond length and the adsorption energies for CO on different Co-doped hcp-Fe₇C₃$\left(1\overline{1}0\right)$ surfaces are listed in

Table 2. The C-O bond length and the adsorption energies for CO on the Co-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface.

Site	$\left(1\overline{1}0\right)$		1Co1-Doped $\left(1\overline{1}0\right)$		1Co2-Doped $\left(1\overline{1}0\right)$		1Co3-Doped $\left(1\overline{1}0\right)$		2Co-Doped $\left(1\overline{1}0\right)$
	d(C-O)/Å	Eads/eV	d(C-O)/Å	Eads/eV	d(C-O)/Å	Eads/eV	d(C-O)/Å	Eads/eV	d(C-O)/Å	Eads/eV
T1	1.180	−2.49	1.178	−1.97	1.187	−3.26	1.181	−2.33	1.175	−2.01
T2			1.177	−1.95	1.179	−1.84	1.172	−1.91	1.175	−1.82
T3			1.178	−1.88	1.173	−1.81	1.176	−2.15	1.170	−1.62
T4			1.171	−1.90	1.177	−1.92	1.179	−1.88	1.186	−2.04
T5			- *	-	1.182	−2.18	-	-	1.181	−2.00
2F1			1.196	−2.02	1.194	−1.89	1.181	−1.88	1.194	−2.01
2F2			1.175	−1.81	-	-	1.177	−1.83	-	-
2F3			1.181	−1.89	1.187	−1.91	1.187	−1.91	1.190	−1.98
2F4			-	-	1.181	−2.11	1.173	−2.02	1.192	−1.91
2F5			-	-	1.176	−1.82	1.184	−1.93	1.196	−2.21
3F1			1.182	−1.89	-	-	1.196	−1.86	1.175	−1.87
3F2			1.184	−1.94	1.180	−1.87	1.179	−1.95	1.153	−2.01
3F3			-	-	1.179	−1.85	1.182	−1.88	1.177	−1.79
3F4			-	-	-	-	1.190	−1.93	-	-
4F1			1.188	−1.87	1.185	−1.99	1.186	−1.98	1.194	−1.87
4F2			-	-	1.190	−2.04	-	-	1.255	−1.74

*—denotes the sites that were not calculated.

, and the most stable adsorption configurations of CO on different Co-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surfaces are demonstrated in Figure 5.

For the 1Co1-doped $\left(1\overline{1}0\right)$ surface, the most stable adsorption site for CO is the 2F1 site with the lowest adsorption energy of −2.02 eV and the longest C-O bond length of 1.196 Å, which is higher than that of the clean hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface. However, for the 1Co2-doped $\left(1\overline{1}0\right)$ surface, the most stable adsorption site for CO is the T1 site with the lowest adsorption energy of −3.26 eV, which is much lower than that of the clean hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface, indicating the strong adsorbability of surface Fe atoms for CO. When the C atom on $\left(1\overline{1}0\right)$ surface is replaced with the Co atom, the surface structure of hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface is slightly changed. The most stable adsorption site for CO is still the T1 site with the lowest adsorption energy of −2.33 eV on the 1Co3-doped $\left(1\overline{1}0\right)$ surface.

With the increasing Co content of $\left(1\overline{1}0\right)$ surface, the most stable adsorption site for CO on 2Co-doped $\left(1\overline{1}0\right)$ is changed to the 2F5 site with the adsorption energy of −2.21 eV and the C-O bond length of 1.196 Å, which is energetically unfavorable for CO adsorption compared to the clean hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface. Similar to the Co-doped $\left(001\right)$ surfaces, the top sites on the doped Co atoms of hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ are disadvantages for the CO adsorption, but the T1 site on the 1Co2-doped $\left(1\overline{1}0\right)$ surface shows the strongest interaction between the four types of Co-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surfaces and CO.

2.3. CO Activation on the Co-Doped hcp-Fe₇C₃ Surfaces

CO activation has been proved to be the critical step of the FTS reaction pathway. After the chemical adsorption of CO on the catalyst surface, the CO activation pathways mainly include two categories, the CO direct dissociation and H-assisted dissociation. With the former, CO dissociates into reactive C and O species. For the latter, with the effect of reactive H species, CO dissociates into HCO or COH species. In the present work, the CO direct dissociation and H-assisted dissociation pathways on the most stable sites of Co-doped hcp-Fe₇C₃ $\left(001\right)$ and $\left(1\overline{1}0\right)$ surfaces are compared. The potential energy diagrams, including the elemental reaction barrier (E_a) and reaction energy (ΔE_r), for the activation of CO on the Co-doped hcp-Fe₇C₃ surfaces are demonstrated in Figure 6. The structures of the transition states for CO direct dissociation and H-assisted dissociation pathways on the most stable site of Co-doped hcp-Fe₇C₃ $\left(001\right)$ and $\left(1\overline{1}0\right)$ surfaces are shown in Figure 7 and Figure 8, respectively.

As shown in Figure 5a and Figure 6, though the CO dissociation and the adsorption site of dissociated pieces are different for the CO direct dissociation and H-assisted dissociation (to HCO or COH) pathways, the three pathways show similar reaction barriers (around 2.9 eV) on the 3F1 site of the 1Co-doped $\left(001\right)$ surface. For the 2F3 site on the 2Co1-doped hcp-Fe₇C₃ $\left(001\right)$ surface, the reaction barrier of CO direct dissociation is 2.39 eV, which is lower than that of HCO formation (2.79 eV) or COH formation (3.05 eV), indicating CO direct dissociation is the favorable reaction pathway. Moreover, CO hydrogenation to HCO and COH are both endothermic.

On the 2Co2-doped hcp-Fe₇C₃ $\left(001\right)$ surface, the 3F1 site is the most stable site for CO adsorption with the binding energy of −2.61 eV, thus the CO dissociation pathway on the 3F1 site is discussed. The reaction barrier of CO direct dissociation is 2.73 eV, while it is 1.96 eV and 2.09 eV for the HCO and COH pathways, respectively. Significantly, the H-assisted dissociation to HCO on the 3F1 site of the 2Co2-doped hcp-Fe₇C₃ $\left(001\right)$ surface shows the lowest reaction barrier among the four types of Co-doped hcp-Fe₇C₃ $\left(001\right)$ surfaces. As shown in Figure 6, for the H-assisted dissociation to HCO pathway, the nearby activated hydrogen atom moves to the 3F1 site, reacts with the adsorbed CO molecule, and finally generates HCO species.

For the 2F3 site on the 3Co-doped hcp-Fe₇C₃ $\left(001\right)$ surface, the reaction barriers of CO direct dissociation and H-assisted dissociation to HCO and COH are 3.26 eV, 3.23 eV, and 3.07 eV, respectively. The high reaction barriers indicate both the direct dissociation and H-assisted dissociation of CO is difficult on the 2F3 site of the 3Co-doped hcp-Fe₇C₃ $\left(001\right)$ surface. Moreover, it can be seen from Figure 5a that all the CO direct dissociation pathways on the four types of Co-doped hcp-Fe₇C₃ $\left(001\right)$ surfaces are exothermic, while the H-assisted dissociation pathways of CO are endothermic.

On the contrary, both the CO direct dissociation and H-assisted dissociation pathways are endothermic on the four types of Co-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surfaces, as shown in Figure 5b. The 2F1 site is the most stable site for CO adsorption with the binding energy of −2.02 eV on the 1Co1-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface, and thus the CO dissociation pathway on 2F1 site is discussed. The reaction barriers of CO direct dissociation and H-assisted dissociation to HCO and COH are 4.07 eV, 1.30 eV, and 3.18 eV, respectively. It can be seen that the direct dissociation of CO on the 2F1 site is difficult, but the H-assisted dissociation to HCO is energetically favorable. During the H-assisted dissociation of CO to the HCO pathway, as shown in Figure 7, the adsorbed activated hydrogen atom on the 2F4 site moves around the adsorbed CO molecule and finally generates HCO species on the 2F1 site. On the 1Co2-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface, the T1 site is the most stable site for CO adsorption with the binding energy of −3.26 eV, which is the lowest among the four types of Co-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surfaces. For the CO dissociation on T1 site, the H-assisted dissociation to HCO is energetically favorable with the lowest reaction barrier of 1.35 eV, while the reaction barrier of H-assisted dissociation to COH is 3.46 eV. Additionally, different from the 1Co1-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface, the reaction barrier of CO direct dissociation is reduced to 2.07 eV.

On the 1Co3-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface, the surface C atom is replaced with the Co atom, and the most stable adsorption site for CO is still the T1 site with the lowest adsorption energy of −2.33 eV. As shown in Figure 7, the reaction barrier of CO direct dissociation on this T1 site is 2.67 eV, while the H-assisted dissociation to COH has a relatively higher reaction barrier of 3.19 eV, and the H-assisted dissociation to HCO pathway just reduces to 1.38 eV. On the most stable adsorption site (2F5 site) for CO on the 2Co-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surface, the reaction barriers of CO direct dissociation and H-assisted dissociation to HCO and COH are 2.92 eV, 1.39 eV, and 3.25 eV, respectively. In summary, our calculations suggest that the H-assisted dissociation to HCO is the preferred activation pathway for CO on the Co-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surfaces.

3. Computational Details

All calculations in this work were conducted based on DFT and were carried out with the CASTEP package in Materials Studio of Accelrys Inc. The electron exchange and correlation energy were calculated by using the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional [30]. The energy cutoff for the plane wave basis set was set to be 400 eV. The total energy convergence was employed as 2 × 10⁻⁵ eV/atom, while the convergence accuracy of self-consistent field was employed as 2 × 10⁻⁶ eV/atom. The k-points sampling was set as 2 × 3 × 1 with the density of 0.25 Å⁻¹. A vacuum layer of 15 Å was set in the perpendicular direction to avoid the spurious interaction between the periodic repeating slabs for all calculations. The calculation parameters test, including supercell size, k-points, and vacuum layer, were carried out, and the calculation results indicated that the parameters employed in our work are sufficient to satisfy the computational accuracy and efficiency, as shown in Table S1.

In the present study, the crystal structure of bulk hcp-Fe₇C₃ with crystallographic parameters of a = b = 6.826 Å, c = 4.495 Å, and γ = 120° was used as the basal structure. The Co-doped hcp-Fe₇C₃ catalysts were modeled by partial replacement of Fe or C atoms on hcp-Fe₇C₃ $\left(001\right)$ and $\left(1\overline{1}0\right)$ surfaces with Co atoms. The adsorption behavior of CO on the Co-doped hcp-Fe₇C₃ catalysts was calculated. The adsorption energy (E_ads) was determined by E_ads = E_CO/slab − E_slab − E_CO, where E_CO/slab is the total energy of the crystal plane absorbed with CO, E_slab is the energy of the crystal slab, and E_CO is the energy of free CO molecule.

For the dissociation of CO molecules, the transition states were located using the LST/QST method in CASTEP [31]. The reaction energy (ΔE_r) was calculated by ΔE_r = E(FS) − E(IS), while the reaction barrier (E_a) was calculated by E_a = E(TS) − E(IS), where E(IS), E(TS), and E(FS) are the energies of the corresponding initial state (IS), transition state (TS), and final state (FS), respectively.

4. Conclusions

In this work, a systematically periodic DFT study was performed to understand the effect of Co doping on the adsorption and activation of CO on Co-doped hcp-Fe₇C₃ catalysts. It is found that the surface structure of the $\left(001\right)$ surface remains basically unchanged after doping with Co atoms, while the replacements of Fe or C atoms on $\left(1\overline{1}0\right)$ surfaces with Co atoms have a significant impact on the surface structure. The top sites on the doped Co atoms of hcp-Fe₇C₃ are disadvantages for the CO adsorption, whereas the T, 2F, or 3F site around the doped Co atoms are beneficial to promote the adsorption of CO. The CO direct dissociation pathways on the four types of Co-doped hcp-Fe₇C₃ $\left(001\right)$ surfaces are exothermic, while the H-assisted dissociation pathways of CO are endothermic. The H-assisted dissociation to HCO on the 3F1 site of 2Co2-doped hcp-Fe₇C₃ $\left(001\right)$ surface show the lowest reaction barrier of 1.96 eV. For the Co-doped hcp-Fe₇C₃ $\left(1\overline{1}0\right)$ surfaces, the H-assisted dissociation to HCO is the preferred activation pathway for CO on the Co-doped surfaces with the reaction barrier of approximately 1.30 eV.