CO Direct versus H-Assisted Dissociation on Hydrogen Coadsorbed Ï

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

CO Direct versus H-Assisted Dissociation on the Hydrogen Co-Adsorbed #-FeC Fischer-Tropsch Catalysts 5

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Yurong He, Peng Zhao, Junqing Yin, Wenping Guo, Yong Yang, Yong-Wang Li, Chun-Fang Huo, and Xiaodong Wen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06988 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 26, 2018

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The Journal of Physical Chemistry

CO Direct versus H-Assisted Dissociation on the Hydrogen Co-Adsorbed χ-Fe5C2 Fischer-Tropsch Catalysts Yurong He,a,b,c Peng Zhao,a,b,c Junqing Yin,a,b,c Wenping Guo, a,b Yong Yang, a,b Yong-Wang Li,a,b Chun-Fang Huo,a,b* and Xiao-Dong Wen,a,b*

a) State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China; b) National Energy Center for Coal to Liquids, Synfuels China Co., Ltd, Huairou District, Beijing, 101400, China; c) University of Chinese Academy of Sciences, No.19A Yuquan Road, Beijing, 100049, P. R. China.

Corresponding Authors: Xiao-Dong Wen ([email protected]) and Chun-Fang Huo ([email protected])

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Abstract The CO activation mechanism on the χ-Fe5C2 Fischer-Tropsch synthesis catalysts is studied by spin-polarized density functional theory calculations. A series of low ((100), (010), (001), (110), (111), (111ത )) and high Miller index ((221), (4ത 11) and (510)) surfaces are intensively investigated. Both the direct and H-assisted pathways through HCO, COH, HCOH and CH2O intermediates are systematically examined on the hydrogen co-adsorbed surfaces. Different activity and activation mechanisms are determined on the various surfaces as well as the varying sites. It is found that the high index Fe5C2(510) and the relatively unstable (010) and (221) surfaces are active for direct CO dissociation due to the existence of the highly activated sites. H-assisted dissociation pathways are preferred on the Fe5C2(010), (110), (4ത 11), (111ത) and (111) surfaces with the moderate stability. On the relatively stable (100) surface, CO dissociation can hardly take place. Such discrimination for the activity and the CO activation mechanisms on various Fe5C2 facets might guide the rational design of catalysts with desired activities.

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1. Introduction Iron-based Fischer-Tropsch synthesis (FTS) catalysts are gaining increasing attention due to their high activity and low cost in converting synthesis gas (CO+H2) into clean transportation fuels. 1 -4 In practical FTS, the catalysts inevitably undergo reduction and carburization processes from iron oxide (mainly Fe2O3) to carbide phases such as ε-Fe2C, ε´-Fe2.2C, χ-Fe5C2, θ-Fe3C, and Fe7C3.5-9 Among them, the Hägg carbide (χ-Fe5C2) has been proved to be responsible for the high activity of FTS.10-12 The CO conversion is normally regarded as the index of catalytic activity. However, it`s difficult to experimentally characterize the active site as well as active crystal plane of the iron-based catalysts due to the facility of phase transformation and the limitation of characterizations. 13 To fundamentally understand the initial process of FTS, it is very necessary and essential to study CO activation mechanisms on the surfaces of the catalysts. Theoretical researches on CO adsorption and dissociation mechanisms on Hägg carbide phase have attracted great attention.14 -27 Huo et al.14 studied the CO activation and surface carbon hydrogenation on Fe5C2(010), Fe2C(011), Fe3C(001), and Fe4C(100) surfaces using PBE functional and found that direct CO cleavage can take place on the vacancy sites produced by CH4 formation. The surface C-vacant sites were also proved to be active for CO dissociation on the Fe5C2(010), Fe5C2(110)0.00, and Fe5C2(110)0.80 surfaces by combining PAW-RPBE and USPP-PW91 calculations.15,16 Gracia et al. studied the CO hydrogenation using RPBE and found that with the surface carbon removed, the Fe5C2(100) facet shows high activity for both direct dissociation and CO hydrogenation. 17 However, it is difficult to make a fair comparison based on the diverse methods adopted by different groups. Many factors have been reported to have a significant effect on CO activation on the iron carbide catalysts, such as functional, surface carbon, terminations, and the adsorption sites. On the Fe-terminated Fe5C2(100) surface, by using Troullier-Martins norm-conserving scalar relativistic pseudopotentials, Cheng et al.18 proposed the facial direct CO dissociation and similar methane selectivity of Fe5C2 with metallic Fe. In contrast, on the C-terminated Fe5C2(100) surface, CO can hardly dissociate according to the study by Hans et al.19,20 On a high Miller index Fe5C2(510) surface, Pham et al.21 compared the direct and H-assisted CO 3



activation mechanisms initiating from the most stable site and suggested the direct CO dissociation as the preferred activation pathway. However, it was proved by Sorescu22 that diverse CO adsorption configurations can be located on Fe5C2 surface and different activities were found on the varying CO adsorption sites. On the base of the calculated equilibrium morphology of Fe5C2 under the practical FTS pretreatments (600K, 1atm, H2/CO=2.5),23 Zhao et al. investigated the direct CO dissociation on the Fe5C2(111), (100), (111ത), (510), and (4ത 11) facets. Considering the high barriers for CO direct dissociation initiating from the most stable sites, they suggested the H-assisted activation mechanism at low CO coverage.24 Most recently, increasing researchers are trying to find out the key factors determining the CO activation properties. Our group studied the direct dissociation on nine Fe5C2 surfaces and developed a site-dependent descriptor of sum bond valence of adsorbed CO (SBV) to predict the activation energy, which is potentially available in other complex systems due to its intrinsic physical and chemical principles.25 Chen et al. reported the relationship between CO direct cleavage barrier and the charge of involved surface Fe atoms in transition states.26 Broos et al. found the preference for the direct CO dissociation on the stepped surfaces while the H-assisted mechanism via a CHO intermediate on the Fe5C2 surfaces without a clear stepped character.27 It is seen from the previous literatures that there exist diverse CO adsorption sites on the Fe5C2 surface due to the low symmetry.22,25 Different surfaces and adsorption sites will show discrepant CO activation activities and mechanisms.24-26 Moreover, the particle morphology and the exposed surfaces are correlated to the FTS conditions.23,28 However, very few researches were reported on the systematic comparison of the CO activation mechanisms initiating from various practically exposed Fe5C2 surfaces and varying CO adsorption sites. To comprehensively study the surface reaction properties, apart from the most stable sites, CO activation mechanisms on other sites are also needed. It has been demonstrated by experimental techniques and computational researches that CO dissociation can occur on several Fe, Ru, Co, and W surfaces through HCO, COH, HCOH and CH2O intermediates.29-37 These intermediates also need to be taken into consideration for the H-assisted CO dissociation on the Fe5C2 catalysts. 4


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In the present work, to obtain the fundamental CO activation properties on various χ-Fe5C2 surfaces and varying adsorption sites, DFT calculations are carried out to study the CO activation mechanisms through both direct and H-assisted pathways. A series of both low and high index of (100), (010), (001), (110), (111), (111ത ), (221), (4ത11) and (510) Fe5C2 surfaces are systematically investigated on the base of the equilibrium morphology of Fe5C2 under practical FTS pretreatments (600K, 1atm, H2/CO=2.5).23,25,38 Not only the most stable CO adsorption sites but also the most activated sites have been taken to study the CO dissociation. Since hydrogen extensively exists in the FTS system and can easily dissociate on the iron-based catalysts,14,39 the CO dissociation in this work is studied on the hydrogen pre-adsorbed surfaces. Our goal is to distinguish the activity and mechanisms of CO activation on various χ-Fe5C2 surfaces.

2. Computational details 2.1. Methods. All the plane-wave-based density functional theory (DFT) calculations were performed with the Vienna Ab intio Simulation Package (VASP).40,41 The electron ion interaction was described with the projector augmented wave (PAW)42,43 potentials and the electron exchange and correlation energy was treated within the Perdew-Burke-Ernzerhof (PBE) functional44 of generalized gradient approximations (GGA).45 Spin-polarization was included for iron systems to correctly account for the magnetic properties. The cut-off energy of 400 eV and electron smearing via a second-order Methfessel-Paxton46 technique with the width of 0.2 eV were employed to ensure accurate energies with errors less than 1 meV per atom. The sampling of the Brillouin zone was performed using a Monkhorst-Pack scheme47. The convergence criteria for electronic self-consistent interactions and all forces were set to 10-4 eV and 0.03 eV/Å, respectively. To determine CO dissociation pathways with the minimum barrier, transition states were located using the climbing image nudged elastic band (CI-NEB) method,48,49 and stretching frequencies were analyzed to characterize a transition state with single imaginary frequency along the reaction coordinate. To describe the thermodynamic and kinetic properties of CO dissociation, adsorption energy, reaction energy, and activation energy were calculated. The adsorption energy is 5



defined as Eads = E(adsorbates/slab) - [E(slab) + E(adsorbates)], where E(adsorbates/slab) is the total energy of the slab with adsorbates, E(slab) is the total energy of the corresponding bare slab, and E(adsorbates) is the total energy of free adsorbates. The lower (more negative) Eads means the stronger adsorption. The reaction energy is defined as Er = E(FS) - E(IS). The activation energy is calculated by Ea = E(TS) - E(IS). E(IS), E(FS), and E(TS) are the energies of the corresponding initial state (IS), final state (FS), and transition state (TS), respectively. The effective barrier (Eeff) is the highest energy barrier to be overcome along the overall reaction pathway. 2.2. Models. The calculated Hägg iron carbide unit cell is a monoclinic phase. The crystallographic parameters (a = 11.545 Å, b = 4.496 Å, c = 4.982 Å, and β = 97.6°) and magnetic moment (1.73 µB) agree reasonably with the experimental data (a = 11.558 Å, b = 4.579 Å, c = 5.069 Å, β= 97.8°; and 1.72 - 1.75 µB50). For studying the CO activation mechanism on the χ-Fe5C2 catalyst, nine surfaces are considered: low ((100), (010), (001), (110), (111), (111ത)) and high Miller index ((221), (4ത11) and (510)) surfaces. The nine χ-Fe5C2 surfaces were modeled by the (1×2), (2×2) or (1×1) super-cell of periodically repeated slabs with the size about 10 Å ×10 Å. The vacuum region of 15 Å is set to avoid the strong interactions among slabs. The sizes of the slabs as well as the fixed and relax layers are the same with our previous work, which were tested and proved to be reasonable.24 The detailed information of the slab models are presented in Supporting Information (Table S1). The schematic top and side views of the nine Fe5C2 surface models are shown in Figure 1.

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Figure 1 Schematic top and side views of the nine Fe5C2 surface models, blue balls for Fe atoms and black balls for C atoms

3. Results and Discussion 3.1. CO adsorption. The CO adsorption on the catalyst surfaces is the first step of catalyzed FTS reaction. The adsorption behaviors of CO on the nine facets of χ-Fe5C2 were systematically investigated in our previous work.25 The energy and structure data for the most stable (ms) and most activated sites (ma) on the pure Fe5C2 surfaces is given in the Supporting Information (Table S2 and Figure S1). To compare the CO activation mechanisms on the uniform level, the CO dissociation through both direct and H-assisted pathways are studied on the hydrogen pre-adsorbed surfaces. Given the easy hydrogen dissociation and H diffusion on the iron-based catalysts,14,39 the steps of hydrogen dissociation and diffusion are neglected in this work. In practical FTS reactions, various CO structures are possible to exist due to the facility of CO 7



adsorption at varying adsorption sites on the Fe5C2 surfaces. On the base of the previous study, the co-adsorption of two H atoms and CO at the most stable and most activated sites are taken as the initial states. The corresponding co-adsorption configurations on the nine surfaces are presented in Figure 2 and the adsorption energies are listed in Table 1. In addition, the H-assisted mechanism through the HCO intermediate initiating from the one H and CO co-adsorption (H + CO) is also studied for comparison (Table S4 and Figure S8-10).

Figure 2 Most stable (ms) and activated (ma) CO adsorption configurations on 2H pre-adsorbed χ-Fe5C2 surfaces (blue: Fe atoms; black: C atoms of Fe5C2; purple: C atoms from CO; red: O atoms; yellow: H atoms)

3.2 CO dissociation. Starting from the co-adsorption configurations (2H + CO), the CO dissociation on the nine surfaces of the χ-Fe5C2 catalysts was studied. The reaction mechanisms are depicted in Figure 3. The direct pathway goes through the transition state (TS1) with the two adsorbed H atoms as spectators and ends up with the co-adsorption of C, O, and 2H on the surface. For the H-assisted activation, not only the first step of hydrogenation to HCO and COH but also the subsequent hydrogenation to H2CO and HCOH are taken into account. All the structures of the involved intermediates as well as the transition states are

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given in Supporting Information (Figure S2-S7). It is noted that the formation of COH always has high barrier when the O atom is pointing to the vacuum in the CO adsorption configuration.30-33 Consequently, the COH pathway is investigated on the condition that the O atom is bonding to the surface Fe atoms in the initial state.

Figure 3 Schematic mechanism of CO activation on the Fe5C2 surfaces, TSn is the corresponding transition state

On the (001) surface, the most stable CO configuration is located on the three-fold (3F) site. As is listed in Table 1, the direct dissociation from this site has a high barrier of 2.03 eV (Ea(1)), while the hydrogenation to the C atom need to overcome a 1.02 eV barrier (Ea(2)). The step of HCO dissociation has the barrier of 0.90 eV (Ea(3)), which is kinetically more favorable compared with the further hydrogenation to H2CO and HCOH (shown in Figure 4). Thus, the most facile pathway involved in H-assisted routes is CO + H → HCO → CH + O. The effective barrier for this path is 1.36 eV (Figure 7) and much lower than that of direct dissociation of CO, suggesting that the direct C-O cleavage do not contribute to CO activation on the most stable site. With respect to the most activated site, the CO molecule is six-fold (6F) coordinated with the surface atoms and the activation energy for the direct CO dissociation is as low as 0.83 eV. For the HCO formation, the H atom should go through a 0.92 eV barrier from the three-fold site to connect with the C atom. The barrier of deoxygenating of HCO is 1.27 eV, which is very close to that of the further hydrogenation to H2CO (Ea(4), 1.22 eV). The hydrogen addition to the O atom to form the COH and HCOH intermediates are also investigated and the calculated barriers are higher (Figure 4, Ea(8) vs. 9



Ea(2); Ea(5) vs. Ea(4)). So, the most facile hydrogen-assisted pathway is CO + H → HCO → CH + O. The overall barrier is 1.27 eV, which is higher than that of the direct route, indicating that the direct dissociation is the favorable path for the on the most activated site. In comparison, the CO dissociation on the (001) surface is facial and the direct dissociation from the most active site is easier. The highly activated 6F site makes the direct dissociation possible on this surface. In the case of the (221) surface, the most stable CO configuration is on a five-coordinated site (5F). As shown in Figure 4 and Table 1, the energy barrier of CO direct dissociation from the most stable site is 1.13 eV. This reaction is exothermic by 0.79 eV. The hydrogenation barrier for forming HCO is 1.19 eV. The further hydrogenation and dissociation from the HCO intermediate are also investigated. Among them, the reaction to form CH and O has the lowest barrier of 0.48 eV. The alternative hydrogenation to form COH gives very high activation barrier of 1.87 eV. So, the CO + H → HCO → CH + O is the main pathway for H-involved mechanism and the effective barrier for this path is 1.42 eV. One should note that the higher barrier required for the H-assisted pathway relative to that for the direct CO dissociation demonstrates that direct C-O cleavage is more favorable on the most stable site. We also examined the CO dissociation from the most activated site. As shown in Figure 2, the CO is located at the five-fold site with the C atom coordinating with three Fe atoms and the O atom coordinating with the two Fe atoms. The barrier for the direct dissociation is only 0.99 eV (Table 1). While the CO hydrogenation to form the HCO and COH intermediates have the energy barrier of 1.24 (Ea(2)) and 1.56 eV (Ea(8)), respectively, which are also higher than that of the direct dissociation. Overall, as shown in Figure 4, the CO dissociation via the direct pathways are energetically favorable on both the most stable and most activated sites, indicating the high activity for CO dissociation on the (221) surface mainly through the direct dissociation pathway.

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Figure 4 Energy profiles for the CO direct and H-assisted dissociation on Fe5C2 (001), (221), and (510) surfaces. The energies are given with respect to gas-phase CO + H2, ms: most stable site, ma: most activated site, the preferred activation pathway is shown with black line

On the (510) surface, with the co-adsorption of 2H, the most stable CO configuration is on the Fe-top site. The direct dissociation has a high barrier of 2.99 eV (Table 1, Ea(1)), suggesting the little possibility for the direct route. Alternatively, the H atom moves from the 3F site towards the C atom to form the HCO with a barrier of 1.18 eV. The second stage of HCO deoxygenating (HCO → HC + O) needs to overcome the barrier of 0.51 eV, which is more favorable than that of the further hydrogenation to form CH2O and HCOH (Figure 4). 11



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The H-assisted CO dissociation pathway via HCO is likely to occur with the effective barrier of 1.45 eV (shown in Figure 7). These results are in agreement with the work studying the CO activation mechanisms initiating from the most stable site on the (510) surface by Pham et al.21 However, when it comes to the most activated site, the adsorbed CO is on a four-fold site, which is favorable to the direct dissociation with a barrier of 1.18 eV. The reaction is exothermic by 0.35 eV. In comparison, the hydrogenation of CO molecule to HCO requires an activation barrier of 1.62 eV and is endothermic by 0.99 eV. The higher barrier for the endothermic reaction of HCO formation indicates that the hydrogenation route is not the preferred reaction path on the most activated site. Overall, it is inferred from the results that the most activated site has the higher activity for CO activation, and the direct dissociation is the dominant pathway.

Table 1 Computed adsorption energies Eads of the IS (adsorbed CO and 2H), and the activation energies Ea(n) corresponding to the TSn, all the energy data has the unit of eV Surface

Site

Eads(IS)

Ea(1)

Ea(2)

Ea(3)

Ea(4)

Ea(5)

Ea(6)

Ea(7)

Ea(8)

Ea(9)

ms

-3.34

2.03

1.02

0.90

1.05

2.01

0.93

0.76

/

/

ma

-3.17

0.83

0.92

1.27

1.22

2.19

/

/

1.77

/

ms

-3.77

1.13

1.19

0.48

0.64

/

/

/

1.87

1.09

ma

-3.73

0.99

1.24

0.33

0.88

/

/

/

1.56

/

ms

-3.90

2.99

1.18

0.51

0.83

/

0.72

/

/

/

ma

-3.78

1.18

1.62

0.54

0.56

1.57

/

/

/

/

ms

-3.63

1.89

0.96

0.56

0.64

1.83

0.65

0.07

/

/

ma

-3.39

1.53

0.72

0.56

0.64

1.83

0.65

0.07

1.86

/

ms

-3.06

3.51

1.13

1.69

1.07

1.65

1.71

0.77

/

/

ma

-2.52

1.61

0.98

0.33

0.86

2.08

0.50

0.19

/

/

ms

-3.92

3.26

1.03

1.08

0.66

1.85

0.96

0.32

/

/

ma

-3.31

2.11

1.03

1.32

0.87

1.94

1.32

0.36

1.47

0.49

ms

-3.48

3.64

1.01

0.99

0.19

1.52

1.12

0.23

/

/

ma

-2.56

0.96

0.51

0.81

0.64

1.60

/

0.46

2.19

0.91

(001)

(221)

(510)

(010)

(110)

(4ത 11)

(111ത )

12


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ms

-2.13

2.84

/

1.39

0.36

1.66

2.50

2.30

/

/

ma

-1.86

2.01

0.73

1.74

0.55

1.16

1.59

0.71

1.13

1.82

ms

-2.39

3.00

1.11

1.15

0.16

1.10

1.39

0.83

/

/

ma

-1.18

2.86

0.28

1.13

0.50

1.33

1.22

0.51

/

/

(111)

(100)

On the (010) surface, the most stable CO configuration is on the four-fold (4F) site with the C bonding with four Fe atoms and the O pointing to the vacuum. First, the direct dissociation is investigated by moving the O atom to the nearby three-fold site. The reaction has the relatively high energy barrier of 1.89 eV. Then the HCO formation by H migration to the C atom of the adsorbed CO is investigated. This hydrogenation step has a lower barrier of 0.96 eV relative to the direct pathway. The followed step of the C-O cleavage is computed to be exothermic by 0.58 eV with a low barrier of 0.56 eV. The possibilities of further hydrogenation from the HCO are also considered. As is shown in Figure 5, the formation of CH2O and HCOH intermediates require the barriers of 0.64 and 1.83 eV (Table 1, Ea(4) and Ea(5)), respectively, both of which higher than that of the HCO dissociation. The results demonstrate that the CO dissociation from the most stable site on the (010) surface prefers the CO + H → HCO → CH + O pathway with the effective barrier of 0.96 eV. Similarly, Broos et al.27 reported the preference of H-assieted H-assisted CO dissociation via HCO intermediate on this surface. In comparison, Petersen and van Rensburg obtained the very close energy barriers for direct and H-assisted CO activation on this (010) surface by using the RPBE method.16 We also studied the CO activation mechanisms on the most activated site. Comparing the energy profiles shown in Figure 5, we can find that the formation of HCO is energetically more facial than the direct CO dissociation. The subsequent C-O cleavage from HCO is preferred to the further hydrogenation and dissociation through CH2O and HCOH intermediates. As an alternative case for CO hydrogenation when the O atom is bonding to the surface Fe atoms, the formation of COH species by H migration to the O atom of the adsorbed CO (through TS8) is also examined. This reaction is endothermic by 1.10 eV and has a high barrier of 1.86 eV. Comparing the pathways of CO activation, we can conclude that on the (010) surface, where the direct CO dissociation has higher barrier, the H-assisted CO 13



activation through the HCO intermediate would be the favorable path on both the most stable and most activated sites.

Figure 5 Energy profiles for the CO direct and H-assisted dissociation on Fe5C2 (010), (110), and (4ത 11) surfaces. The energies are given with respect to gas-phase CO + H2, ms: most stable site, ma: most activated site, the preferred activation pathway is shown with black line

On the (110) surface, the most stable CO is adsorbed on the Fe-top site, on which the direct C-O cleavage can hardly take place. As listed in Table 1, the activation energy is as high as 3.51 eV. For the CO hydrogenation of H-assisted pathway, the HCO could form by 14


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overcoming a 1.13 eV barrier and deoxygenate by going over the barrier of 1.69 eV. The subsequent hydrogenation of the HCO species to CH2O and HCOH are also examined. Comparing the energy profiles in the Figure 5, we can find that the energetically favorable pathways for the CO activation are CO + H → HCO → CH + O and CO + 2H → HCO + H

→ HCOH → CH + OH, which have the similar effective barriers of 2.27 and 2.23 eV. The high barriers suggest that the most stable site is not active for CO activation. Likely, the CO activation form the most activated site (3F) is also investigated. For the direct dissociation path, the energy barrier is calculated to be 1.61 eV. With respect to the CO hydrogenation of H-assisted pathway, the HCO could form by C-H interaction. This step has a barrier of 0.98 eV (Ea(2)) and is endothermic by 0.58 eV. The following dissociative state is reached by the moving of O atom to the bridge site with a low barrier of 0.33 eV (Ea(3)). However, the subsequent hydrogenation of the HCO species to CH2O requires an activation barrier of 0.86 eV (Ea(4)), which is higher than that of the HCO dissociation. The alternative route of hydrogenation from HCO species leads to the formation of CHOH. This reaction has an high activation barrier of 2.08 eV (Ea(5)) and is excluded from the mechanisms. The dominated route for the CO activation is CO + H → HCO → CH + O. The corresponding effective barrier of 1.12 eV can be overcome in the practical FTS. So, it is inferred that the H-assisted CO dissociation can occur through the CHO intermediate initiating from the most activated sites, though the most stable site is inactive. On the (4ത11) surface, the most stable CO configuration is transferred to the bridge site with the co-adsorption and the direct dissociation starting from this site is thermodynamically equal with a high barrier of 3.26 eV. In the case of the H-assisted mechanism, as seen in the Figure 5, the kinetically preferred pathway is CO + H → HCO → CH + O. However, the effective barrier of this path is as high as 2.03 eV, indicating that the most stable site can hardly contribute to the CO activation. With respect to the most activated site, the C in the CO is bonding to four Fe and the O is bonding to one Fe atom in the surface. The direct dissociation proceeds by the moving of O atom to the bridge site with the barrier of 2.11 eV. While the CO hydrogenation steps to form HCO and COH require the lower activation 15



barriers (1.03 eV for Ea(2) and 1.47 eV for Ea(8)) than that of the direct C-O scission. The following steps of HCO and COH dissociation have the barriers of 1.32 and 0.49 eV, respectively. The further hydrogenation to CH2O only needs to overcome a barrier of 0.87 eV. But the dissociation of CH2O is difficult to occur. As shown in Figure 5, in the alternative hydrogenation pathways, the formation of HCOH is remarkably endothermic, which avoid the steps for HCOH formation from HCO or COH intermediates. We can find that the CO + H → HCO → CH + O and CO + H → COH → C + OH exhibit the similar energetic preferences with the effective barriers of 1.44 and 1.47 eV, respectively. The moderate energy barriers for CO activation through the H-involved pathways from the most activated site indicate the possibilities for these processes to take place on the (4ത11) surface. On the (111ത) surface, the most stable CO adsorption site also has the Fe-top structure. The direct C-O scission can hardly occur on such site (Table 1, Ea(1), 3.64 eV). As for the H-assisted mechanisms, the pathways of CO hydrogenation to HCO as well as CH2O and HCOH are all systemically investigated. As shown in Figure 6, compared with other routes, CO + H → HCO → CH + O is kinetically favored. Nevertheless, this path has the high effective barrier of 1.89 eV. Thus, neither of the direct and the H-assisted CO activation can take place initiating from the most stable site. In the case of the most activated site, the CO molecule is highly coordinated, with the C bond to four Fe and one surface C atom, the O bonding to two surface Fe atoms. Starting from this site, the direct C-O cleavage can occur with a low energy barrier of 0.96 eV and is slightly endothermic by 0.22 eV. The dissociation generates the surface C-C* species and is denoted as the C-assisted CO activation by Broos et al.27 Moreover, the CO hydrogenation provides a more favorable path. The HCO intermediate can easily form by the hydrogenation of the C atom in the CO with a tiny barrier of 0.51 eV. This step is exothermic by 0.50 eV, and the subsequent step of HCO dissociation only requires the activation energy of 0.81 eV. As shown in the Figure 6, the further hydrogenation of the CHO to form CH2O and HCOH are also studied. However, these routes together with the pathway through the COH species are not favored both thermodynamically and kinetically. The preferred pathway for the CO activation from the most activated site is through the HC-O 16


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cleavage with the effective barrier of only 0.81 eV. Above all, both the direct dissociation and the H-assisted dissociation have the significantly low overall barriers from the most active site, 0.96 and 0.81 eV, respectively, which indicates the possibility of high activity for CO dissociation on the (111ത) surface.

Figure 6 Energy profiles for the CO direct and H-assisted dissociation on Fe5C2 (111ത), (111), and (100) surfaces. The energies are given with respect to gas-phase CO + H2, ms: most stable site, ma: most activated site, the preferred activation pathway is shown with black line

17



On the (111) surface, the most stable site is also on the Fe-top site where the direct C-O scission can hardly occur (Table 1, Ea(1), 2.84 eV). As shown in Figure 6, the most favorable H-involved pathway is CO + H → HCO → CH + O. The high effective barrier of 2.20 eV avoids the CO activation via H-assisted mechanisms. So, the most stable site is not active for both direct and the H-assisted CO activation. When it comes to the most activated site with a 4F configuration, the direct CO dissociation cannot happen yet due to the high energy barrier of 2.01 eV. In contrast, the hydrogenation of the CO molecule to HCO requires a lower activation barrier of 0.73 eV. But the following dissociation step needs high activation energy of 1.74 eV. Further, the HCO is easily hydrogenated to CH2O through a barrier of 0.55 eV, while the cleavage of C-O bond in the CH2O intermediate is energetically difficult (Ea(6), 1.59 eV). The alternative hydrogenation to form HCOH and the subsequent dissociation require activation barriers of 1.16 and 0.71 eV, respectively. We also examined the possibility of H-assisted mechanism through the COH intermediate. This path is blocked by the high barrier of COH dissociation (Ea(9), 1.82 eV). As is shown in the Figure 6, the results demonstrate that the major CO activation pathway is CO + 2H → HCO +H → HCOH → CH + OH with the effective barrier of 1.16 eV. That is, the H-assisted CO dissociation is likely to occur via the HCOH intermediate on the (111) surface initiating from the most activated site. On the (100) surface, initiating from the most stable site (Fe-top), the dominant CO activation mechanism is the H-assisted pathway CO + 2H → HCO +H → CH2O → CH2 + O. However, this path has the high effective barrier of 1.86 eV. The results indicate that the Fe-top site on this surface is also not active for CO activation. The results on the ms site on the (100) surface are in line with the former researches by several groups.19,24 In the case of CO dissociation from the most activated site, the direct route is not favored both thermodynamically and kinetically, and thus should be ruled out (Figure 6). The H-assisted mechanism through the HCO intermediate is also investigated. The HCO forming by CO hydrogenation requires tiny activation energy of 0.28 eV (Table 1, Ea(2)). The subsequent dissociation to form surface CH and O species has the barrier of 1.13 eV (Ea(3)). This step is endothermic by 0.96 eV. The alternative routes of further hydrogenation from HCO lead to the formation of CH2O, with a barrier of 0.50 eV, rather than HCOH. The last step of C-O 18


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cleavage in the CH2O intermediate needs the activation energy of 1.22 eV. Comparing the energy profiles shown in Figure 6, we can find that the both pathways of CO + H → HCO → CH + O and CO + 2H → HCO +H → CH2O → CH2 + O are competitive. The effective barriers are 1.13 and 1.23 eV, respectively. However, it is noted that the adsorption of the CO on this site is very slight with the adsorption energy of only -0.62 eV (Table S2). So the desorption of the CO is very facial, which indicates the low possibility of CO activation from this site. Above all, CO molecules on the (100) surface can hardly dissociate. To comprehensively distinguish the CO activation mechanisms on the χ-Fe5C2 surfaces, the overall barriers of direct and the most favorable H assisted pathways initiating from the most stable (ms) and most activated (ma) sites are collected and schematically show in Figure 7. Different activity and activation mechanisms are found on the various surfaces as well as the adsorption sites, which demonstrated that the CO activation is the local site-dependent property on the Fe5C2 catalysts. The ms site is always less active than the most activated site. Moreover, the ms and ma sites could provide varying CO activation pathways on the same surface. On the (001) and (510) surface, initiating from the ms sites, the most favorable paths are CO + H → HCO → CH + O, with the overall barriers of 1.36 and 1.45 eV, respectively. However, the direct dissociation is preferred from the ma sites with lower overall barriers (0.83 and 1.18 eV, respectively). Though most of the ms sites have relatively low activities, it needs to be noted that the direct dissociation barrier on the (221) surface initiating from the ms site is only 1.13 eV due to the high CO activation level, which indicates the high activity for direct dissociation on the (221) surface. Likely, the H-assisted dissociation barrier on the (010) surface initiating from the ms site is as low as 0.96 eV, indicating the high activity for H-assisted dissociation on the (010) surface.

19



Figure 7 Computed overall barriers of CO dissociation through direct (Ea(D)) and H assisted (Eeff(H)) pathways on Fe5C2 surfaces initiating from the most stable (ms) and most activated (ma) sites

Generally, as can be seen in Figure 7, the Fe5C2(510) and the relatively unstable (001) and (221) surfaces (with surface free energy of 2.545 and 2.302 J/m2, respectively, shown in Table S3) are active for direct CO activation due to the low barriers from either ms or ma sites, which is in agreement with our previous study on the pure surface without co-adsorption of hydrogen atoms.25 On these surfaces, CO configurations with high coordination number are located. CO molecules on the highly coordinated sites are always more activated due the strong back donation from the surface atoms, which is similar with that on the surfaces with stepped character or C* vacancies.14-17,27 That is, it is the local property, special site, that makes the (510), (001), and (221) surfaces favorable for direct CO dissociation. The Fe5C2 (111ത) provides very low overall barriers for both the direct dissociation (0.96 eV) and the H-assisted dissociation (0.81 eV) on the ma site, which indicates the potential high activity for CO activation. Besides, the Fe5C2(010), (110), (4ത 11), (111), and (100) surfaces are predicted to be inert for the direct CO dissociation by using the sum bond valence (SBV).25 However, alternative H-assisted CO activation pathways are found to be possible on the Fe5C2(010), (110), (4ത 11), and (111) surfaces in spite of the high barriers for direct pathways. For the H-assisted mechanism, various pathways are possible. The pathway though 20


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COH needs the bonding between the O atom of CO and the surface atoms. For the pathways through HCO, CH2O, and CHOH intermediates, the stable configuration of HCO is necessary. Especially on the Fe5C2(010) surface, the HCO intermediates have high stability on both the most stable and most activated sites. So, it is also the local site-dependent property that determines the CO activation through the H-assisted pathways. Further, it is interesting to note that these surfaces always have moderate surface stabilities and have the moderate surface energies (Table S3) according to our previous calculation at the practical FTS conditions.23 On the Fe5C2(100) surface, it has low possibility for CO activation to take place due to the high overall barriers as well as the facile desorption of CO, which is consistent with the data reported by Hans and co-workers.19 From our previous research,23 it is proved that the (100) facet is the most stable (1.816 J/m2) among the fourteen exposed surfaces under the practical pretreatment conditions, noting that the stable surfaces would have the low activity for CO activation. As a result, it`s of significance to adjust the catalyst morphology to maintain the high catalytic activity.

4. Conclusions In this work, spin-polarized density functional theory calculations have been carried out to gain the fundamental knowledge of CO activation on the χ-Fe5C2 Fischer-Tropsch Catalysts. A series of low and high index (100), (010), (001), (110), (111), (111ത), (221), (4ത11), and (510) χ-Fe5C2 surfaces as well as varying adsorption sites are investigated to study the CO activation mechanisms. Both the direct and H-assisted pathways through HCO, COH, HCOH and CH2O intermediates are systematically examined on the hydrogen co-adsorbed surfaces. On the basis of the calculated reaction energy and activation barriers, different activity and activation mechanisms are found on the various surfaces. The high index Fe5C2(510) and the relatively unstable (001) and (221) surfaces are more active for direct CO activation due to the existence of the highly activated CO adsorption sites. H-assisted CO dissociation are found to be possible on the Fe5C2(010), (110), (111ത), (4ത 11), and (111) surfaces with moderate stability, although these surfaces are relatively inert for direct activation route. Among them, the Fe5C2(010) has the high activity for H-assisted dissociation. Specifically, the CO + H → 21



HCO → CH + O is the dominant pathway on the Fe5C2(010), (110), and (111ത) surfaces. On the (4ത11) surface, both CO + H → HCO → CH + O and CO + H → COH → C + OH are the possible routes. While on the (111) surface, the CO activation is likely to occur through the path of CO + 2H → HCO +H → HCOH → CH + OH. In addition, low possibility is found for CO activation on the relatively stable (100) surface. The remarkable activity and mechanism differences for CO dissociation on χ-Fe5C2 indicate the importance to control the morphology of the catalysts. The fundamental knowledge of CO activation from this work will be helpful to selectively adjust the exposed surfaces in rational designing of the FTS catalysts.

Corresponding Author Xiao-Dong Wen ([email protected]) and Chun-Fang Huo ([email protected])

Notes The authors declare no competing financial interest.

Acknowledgments. The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 21273261, No. 21473229, No. 91545121), and funding support from Synfuels China, Co. Ltd. We also acknowledge the innovation foundation of Institute of Coal Chemistry, Chinese Academy of Sciences, Hundred-Talent Program of Chinese Academy of Sciences, Shanxi Hundred-Talent Program and National Thousand Young Talents Program of China. The computational resources for the project were supplied by the Tianhe-2 in Lvliang, Shanxi.

Supporting Information. Key parameters for the nine χ-Fe5C2 surface slab models (Table S1); Computed dada on pure Fe5C2 surfaces (Table S2); Computed overall barriers for direct and H-assisted pathways (Table S3); H-assisted CO dissociation starting from 1H + CO

22


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co-adsorption (Table S3); Most stable and activated CO configurations on pure Fe5C2 surfaces (Figure S1); Structures for the key intermediates and the transition states involving in CO activation on the nine surfaces (Figure S2-S7); Structures and energies for H-assisted CO dissociation through the HCO pathway initiating from the 1H + CO co-adsorption (Figure S8-S10).

References (1) Fischer, F.; Tropsch, H. Über die Herstellung Synthetischer Ölgemische (Synthol) durch Aufbau aus Kohlenoxid und Wasserstoff. Brennstoff Chem. 1923, 4, 276-285. ( 2 ) Fischer, F.; Tropsch, H. Die Erdölsynthese bei gewöhnlichem Druck aus den Vergasungsprodukten der Kohlen. Brennstoff Chem. 1926, 7, 97-116. (3) Jin, Y.; Datye, A. K. Phase Transformations in Iron Fischer-Tropsch Catalysts during Temperature-Programmed Reduction. J. Catal. 2000, 196, 8-17. (4) Jothimurugesan K.; Goodwin, J. G.; Gangwal, J. S. K.; Spivey, J. J. Development of Fe Fischer-Tropsch Catalysts for Slurry Bubble Column Reactors. Catal. Today 2000, 58, 335-344. ( 5 ) Chang, Q.; Zhang, C.; Liu, C.; Wei, Y.; Cheruvathur, A. V.; Dugulan, A. I.; Niemantsverdriet, J. W.; Liu, X.; He, Y.; Qing, M.; et al. Relationship between Iron Carbide Phases (ε-Fe2C, Fe7C3, and χ-Fe5C2) and Catalytic Performances of Fe/SiO2 Fischer-Tropsch Catalyst. ACS catal. 2018, 8, 3304-3316. (6) Liu, X. W.; Cao, Z.; Zhao, S.; Gao, R.; Meng, Y.; Zhu, J.; Rogers, C.; Huo, C. F.; Yang, Y.; Li, Y.; et al. Iron Carbides in Fischer-Tropsch Synthesis: Theoretical and Experimental Understanding in Epsilon-Iron Carbide Phase Assignment. J. Phys. Chem. C, 2017, 121, 21390-21396. (7) Shroff, M. D.; Kalakkad, D. S.; Coulter, K. E.; Köhler, S. D.; Harrington, M. S.; Jackson, N. B.; Sault, A. G.; Katye, A. K. Activation of Precipitated Iron Fischer-Tropsch Synthesis Catalysts. J. Catal. 1995, 156, 185-207. (8) Ding, M. Y.; Yang, Y.; Wu, B. S.; Xu, J.; Zhang, C. H.; Xiang, H. W.; Li, Y. W. Study of Phase Transformation and Catalytic Performance on Precipitated Iron-based Catalyst for Fischer-Tropsch Synthesis. J. Mol. Catal. A 2009, 303, 65-71. (9) Niemantsverdriet, J. W.; Van der Kraan, A. M.; Van Dijk, W. L.; Van der Baan, H. S. 23



Behavior of Metallic Iron Catalysts during Fischer-Tropsch Synthesis Studied with Möessbauer Spectroscopy, X-ray diffraction, Carbon Content Determination, and Reaction Kinetic Measurements. J. Phys. Chem. 1980, 84, 3363-3370. (10) Herranz, T.; Rojas, S.; Pérez-Alonso, F. J.; Ojeda, M.; Terreros, P.; Fierro, J. L. G. Genesis of Iron Carbides and Their Role in the Synthesis of Hydrocarbons from Synthesis Gas. J. Catal. 2006, 243, 199-211. (11) Smit, E.; Beale, A. M.; Nikitenko, S.; Weckhuysen, B. M. Local and Long Range Order in Promoted Iron-based Fischer-Tropsch Catalysts: A Combined in Situ X-ray Absorption Spectroscopy/Wide Angle X-ray Scattering Study. J. Catal. 2009, 262, 244-256. (12) Yang, C.; Zhao, H.; Hou, Y.; Ma, D. Fe5C2 Nanoparticles: A Facile Bromide-Induced Synthesis and as an Active Phase for Fischer-Tropsch Synthesis. J. Am. Chem. Soc. 2012,

134, 15814-15821. (13) Liu, X.; Zhang, C.; Li, Y.; Niemantsverdriet, J. W.; Wagner, J. B.; Hansen, T. W. Environmental Transmission Electron Microscopy (ETEM) Studies of Single Iron Nanoparticle Carburization in Synthesis Gas. ACS Catal., 2017, 7, 4867-4875 (14) Huo, C. F.; Li, Y. W.; Wang, J.; Jiao, H. Insight into CH4 Formation in Iron-Catalyzed Fischer-Tropsch Synthesis. J. Am. Chem. Soc. 2009, 131, 14713-14721. (15) Petersen, M. A.; van den Berg, J. A.; van Rensburg, W. J. Role of Step Sites and Surface Vacancies in the Adsorption and Activation of CO on χ-Fe5C2 Surfaces. J. Phys. Chem. C 2010, 114, 7863-7879. (16) Petersen, M. A.; van Rensburg, W. J. CO Dissociation at Vacancy Sites on Hägg Iron Carbide: Direct Versus Hydrogen-Assisted Routes Investigated with DFT. Top. Catal. 2015, 58, 665-674. (17) Gracia, J. M.; Prinsloo, F. F.; Niemantsverdriet, J. W. Mars-van Krevelen-like Mechanism of CO Hydrogenation on an Iron Carbide Surface. Catal. Lett. 2009, 133, 257-261. (18) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Lok, C. M. Density Functional Theory Study of Iron and Cobalt Carbides for Fischer-Tropsch Synthesis. J. Phys. Chem. C 2010,

114, 1085-1093. (19) Ozbek, M. O.; Niemantsverdriet, J. W. Elementary Reactions of CO and H2 on C-terminated χ-Fe5C2(001) Surfaces. J. Catal. 2014, 317, 158-166. (20) Ozbek, M. O.; Niemantsverdriet, J. W. Methane, Formaldehyde and Methanol Formation 24


Page 24 of 28

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pathways from Carbon Monoxide and Hydrogen on the (001) Surface of the Iron Carbide χ-Fe5C2. J. Catal. 2015, 325, 9-18. (21) Pham, T. H.; Duan, X. Z.; Qian, G.; Zhou, X. G.; Chen, D. CO Activation Pathways of Fischer-Tropsch Synthesis on χ‑Fe5C2 (510): Direct versus Hydrogen-Assisted CO Dissociation. J. Phys. Chem. C 2014, 118, 10170-10176. (22) Sorescu, D. C. Plane-Wave Density Functional Theory Investigations of the Adsorption and Activation of CO on Fe5C2 Surfaces. J. Phys. Chem. C 2009, 113, 9256-9274. (23) Zhao, S.; Liu, X.; Huo, C.; Li, Y.; Wang, J.; Jiao, H. Surface Morphology of Hägg Iron Carbide (χ-Fe5C2) from Ab Initio Atomistic Thermodynamics. J. Catal. 2012, 294, 47-53. (24) Zhao, S.; Liu, X. W.; Huo, C. F.; Wen, X. D.; Guo, W.; Cao, D.; Yang, Y.; Li, Y.; Wang, J.; Jiao, H. Morphology Control of K2O Promoter on Hägg Carbide (χ-Fe5C2) under Fischer-Tropsch Synthesis Condition. Catal. Today 2016, 261, 93-100. (25) He, Y.; Zhao, P.; Meng, Y.; Guo, W.; Yang, Y.; Li, Y. W.; Huo, C. F.; Wen, X. D. Hunting the Correlation between Fe5C2 Surfaces and Their Activities on CO: The Descriptor of Bond Valence. J. Phys. Chem. C 2018, 122, 2806-2814. (26) Chen, B.; Wang, D.; Duan, X.; Liu, W.; Li, Y.; Qian, G.; Yuan, W.; Holmen, A.; Zhou, X.; Chen, D. Charge-Tuned CO Activation over a χ-Fe5C2 Fischer-Tropsch Catalyst. ACS

Catal. 2018, 8, 2709-2714. (27) Broos, R. J. P.; Zijlstra, B.; Filot, I. A. W.; Hensen, E. J. M. Quantum-Chemical DFT Study of Direct and H- and C‑Assisted CO Dissociation on the χ‑Fe5C2 Hägg Carbide. J.

Phys. Chem. C 2018, 122, 9929-9938. (28) Zhao, S.; Liu, X.; Huo, C.; Li, Y.; Wang, J.; Jiao, H. Determining Surface Structure and Stability of ε-Fe2C, χ-Fe5C2, θ-Fe3C and Fe4C Phases under Carburization Environment from Combined DFT and Atomistic Thermodynamic Studies. Catal. Struct. React. 2014,

1, 44-59. (29) Huo, C. F.; Ren, J.; Li, Y. W.; Wang, J.; Jiao, H. CO Dissociation on Clean and Hydrogen Precovered Fe(111) Surfaces. J. Catal. 2007, 249, 174-184. (30) Loveless, B. T.; Buda, C.; Neurock, M.; Iglesia, E. CO Chemisorption and Dissociation at High Coverages during CO Hydrogenation on Ru Catalysts. J. Am. Chem. Soc. 2013,

135, 6107-6121. 25



(31) Petersen, M. A.; Van Den Berg, J.; Ciobîc, I. M.; van Helden, P. Revisiting CO Activation on Co Catalysts: Impact of Step and Kink Sites from DFT. ACS Catal. 2017, 7, 1984-1992. (32) Li, H. J.; Chang, C. C.; Ho, J. J. Density Functional Calculations to Study the Mechanism of the Fischer-Tropsch Reaction on Fe(111) and W(111) Surfaces. J. Phys.

Chem. C 2011, 115, 11045-11055. (33) Shetty, S.; Jansen, A. P. J.; van Santen, R. A. Direct versus Hydrogen-Assisted CO Dissociation. J. Am. Chem. Soc. 2009, 131, 12874-12875. (34) Ojeda, M.; Nabar, R.; Nilekar, A. U.; Ishikawa, A.; Mavrikakis, M.; Iglesia, E. CO Activation Pathways and the Mechanism of Fischer-Tropsch Synthesis. J. Catal. 2010,

272, 287-297. (35) Inderwildi, O. R.; Jenkins, S. J.; King, D. A. Fischer-Tropsch Mechanism Revisited: Alternative Pathways for the Production of Higher Hydrocarbons from Synthesis Gas. J.

Phys. Chem. C 2008, 112, 1305-1307. (36) Huo, C. F.; Li, Y. W.; Wang, J.; Jiao, H. Formation of CHx Species from CO Dissociation on Double-Stepped Co(0001): Exploring Fischer-Tropsch Mechanism. J. Phys. Chem. C 2008, 112, 14108-14116. (37) Mitchell, W. J.; Wang, Y. Q.; Xie, J.; Weinberg, W. H. Hydrogenation of Carbon Monoxide at 100 K on the Ruthenium(001) Surface: Spectroscopic Identification of Formyl Intermediates. J. Am. Chem. Soc. 1993, 115, 4381-4382. (38) He, Y; Zhao, P; Guo, W.; Yang, Y.; Huo, C.; Li, Y; Wen, X. Hägg Carbide Surfaces Induced Pt Morphological Changes: A Theoretical Insight. Catal. Sci. Technol., 2016, 6, 6726-6738. (39) Tian, X.; Wang, T.; Yang, Y.; Li, Y. W.; Wang, J; Jiao, H. About Copper Promotion in CH4 Formation from CO Hydrogenation on Fe(100): A Density Functional Theory Study.

Appl. Catal. A: Gen. 2017, 530, 83-92. (40) Kresse, G.; Furthmüller, Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. J. Comput. Mater. Sci. 1996, 6, 15-50. (41) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. 26


Page 26 of 28

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42) Blochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. (43) Kresse, G. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method.

Phys. Rev. B 1999, 59, 1758-1775. (44) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. ( 45 ) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-gas Correlation Energy. Phys. Rev. B 1992, 45, 13244-13249. (46) Methfessel, M.; Paxton, A. T. High-Precision Sampling for Brillouin-Zone Integration in Metals. Phys. Rev. B 1989, 40, 3616-3621. (47) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. ( 48 ) Jónsson, H.; Mills, G.; Jacobsen, K. W. Nudged Elastic Band Method for Findingminimum Energy Paths of Transitions, in: B.J. Berne, G. Ciccotti, D.F. Coker (Eds.), Classical and Quantum Dynamics Condensed Phase Simulations, WorldScientific,

Hackensack, NJ 1998, 385-404. (49) Henkelman, G.; Jonsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978-9985. (50) Retief, J. J. Powder Diffraction Data and Rietveld Refinement of Hägg-carbide, χ-Fe5C2.

Powder Diffr. 1999, 14, 130-132.

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