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Hunting the Correlation Between FeC Surfaces and Their Activities on CO: The Descriptor of Bond Valence Yurong He, Peng Zhao, Yu Meng, Wenping Guo, Yong Yang, Yongwang Li, Chun-Fang Huo, and Xiaodong Wen J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 20, 2018

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The Journal of Physical Chemistry

Hunting the Correlation between Fe5C2 Surfaces and Their Activities on CO: The Descriptor of Bond Valence Yurong He,a,b,c Peng Zhao,a,b,c Yu Meng,a,b,c Wenping Guo, a,b,d 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. d) Inner Mongolia University for Nationalities, Tongliao, 028043, P. R. China.

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

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Abstract To hunt the correlation between the surfaces of Fe 5C2 and their corresponding activities, the CO adsorption and dissociation on a series of both low ((010), (001), (110), (111), (111 )) and high Miller index surfaces of Fe5C2((221), (411) and (510)) surfaces are systemically investigated. For the CO adsorption, configurations with bonding to surface Fe sites are much stronger than that of on C sites. For the CO dissociation, the direct C-O cleavage can take place on the (221), (510), (010), and (11-1) surfaces due to the low activation energy. More importantly, to correlate the surface character and the activity of CO dissociation we proposed a concept of sum bond valence. It is found that the adsorbed CO with more bond valence can dissociate easier, and a linear relationship between the activation energies and the CO bond valence can be established. It can be inferred that the activity of Fe5C2 surfaces for CO dissociation highly relies on the binding characteristics. The relatively stable (100) and (111) surface are not active for direct CO dissociation. In this work, the CO bond valence is suggested to be an important descriptor to correlate complicated surfaces and their activities. Further, such finding can guide the rational design of catalysts with desired activities.

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1. Introduction Based on iron, cobalt, and ruthenium catalysts, synthesis gas (CO+H2) can be converted into clean transportation fuels and valuable chemicals by Fischer-Tropsch synthesis (FTS), 1,

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which has been gaining increasing attention due to the foreseeable depletion of crude. FTS provides the world an alternative to oil resources as syngas can be produced from coal, natural gas, and biomass. It is known that iron based catalysts are effectively applied in industrial FTS for the advantages of high activity, low cost, flexibility in product distribution, and suitability for converting syngas with low H2/CO ratio derived from coal.3,

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In practical application, the

initial catalyst precursor (α-Fe2O3) is firstly reduced to magnetite (Fe3O4) and then converted to a mixture of iron oxide and carbide phases such as ε-Fe 2C, ε´-Fe2.2C, χ-Fe5C2, θ-Fe3C, and Fe7C3.5-11 Among them, the Hägg carbide (χ-Fe5C2) is found to be a main active phase and the amount of χ-Fe5C2 is related to the FTS activity.11-14 In experiments, due to the instability of Fe5C2 and the limitation of characterizations, it`s difficult to obtain the detailed surface characterization of the χ-Fe5C2 phase in spite of its importance.15 On the other hand, theoretical calculations have allowed researchers to get the fundamental knowledge of the surface catalytic properties at the atomic level.16 In the recent decade, theoretical researches on Hägg carbide phase have gained more and more attention.20-34 In our research team, Cao et al.17-19 carried out density functional theory (DFT) calculations to study the adsorption and coadsorption of CO and H2 on the Fe5C2(001), (110), and (100) surfaces. Based on these, the CHx formation, ketene hydrogenation, and chain growth mechanisms of FTS on the Fe5C2(001) surface were further investigated.20-22 Huo et al. 23 performed the calculation for CH4 formation pathways from surface carbon hydrogenation on Fe5C2(010) as well as Fe2C(011), Fe3C(001), and Fe4C(100) surfaces and found a linear relationship between the effective barrier and d-band center of the surface. In addition, the effective barrier of CH4 formation can be changed by the charge transfer induced by the potassium promoter.24 Cheng et al.25 calculated the CO dissociation, C1 hydrogenation, and C1+C1 coupling on B5 site on a Fe-terminated Fe5C2(100) surface and inferred higher activity and similar methane selectivity of Fe carbide compared with metallic Fe. While on 3



the C-terminated Fe5C2(100) facet, a Mars-van Krevelen-like Mechanism for CO hydrogenation and CHx(O) formation was found by Niemantsverdriet et al.26-28 On a high Miller index Fe5C2(510) surface, high C2+ selectivity is confirmed by the comparison between the C1−C1 coupling and the CH4 formation mechanisms.29 CO dissociation is the initial and important step in the FTS process, which has high relationship with the activity, selectivity and stability of the catalysts, hence some researches focusing on CO reaction on the Fe5C2 surfaces were reported.23,30-34 By means of the PBE functional, it was found that the direct dissociation has high activity on C-vacant sites on Fe5C2(010) surfaces.23 Based on the combination of RPBE-PAW and PW91-USPP results, Petersen et al.30 also found high CO dissociation activity on the carbon vacancy sites and the activation energy could be further lowered by the presence of steps. Given that several different Fe5C2 surfaces can be exposed in the FTS process, Sorescu31 performed GGA-PBE calculations for the CO adsorption and direct dissociation properties on seven low Miller index surfaces, (010)0.25, (111)0.00, (110)0.00, (111)0.00, (111)0.50, (110)0.50, and (100)0.00, of χ-Fe5C2 phase depending on the theoretical analysis of the surface stabilities by Steynberg et al.32 Furthermore, based on the assumption that the (510) surface has the highest proportion (34.9%) among the exposed crystal facets, Pham et al. 33 calculated direct and H-assisted CO activation pathways on the Fe5C2(510) surface using the GGA-PBE method, and concluded that the direct CO dissociation is the preferred activation pathway. Based on the equilibrium morphology of Fe5C2 under typical pretreatment conditions (600K, 1atm, H2/CO=2.5), Zhao et al.34 studied the CO adsorption and direct dissociation on the most exposed (111), (100), (111), (510), and (411) facets with the RPBE calculations. From the previous literatures, we can clearly see different surface structures exhibit different performance on CO dissociation, which are very vital to the catalytic activity, selectivity and stability. However, it is difficult to set up this relationship based on the reported data due to the varying terminations of the surfaces and diverse methods be adopted by different groups.22,23,30,31,33 To hunt the key factor for CO activation and get deep insight into the sensitivity of CO dissociation to surface structure, in this work, we have carried out the systemic analysis of CO adsorption and dissociation on a series of low and high index 4


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Fe5C2 surfaces. According to our previous research,35 fourteen Fe5C2 facets are exposed on the surface in equilibrium shape based on Wulff construction36 under the practical FTS pretreatment (600K, 1atm, syngas H2/CO = 2.5). Herein, nine low and high index Fe5C2 surfaces, (100), (010), (001), (110), (111), (111), (221), (411) and (510), which accounts for about 83% of the total surface area, were chosen to study the CO dissociation. The purpose of this work is to explore the relationship between the surface structures and the CO dissociation properties on Hägg carbide.

2. Computational details 2.1. Methods. All spin-polarized density functional theory calculations were carried out by using the Vienna Ab intio Simulation Package (VASP),37,38 with projector augmented wave (PAW)39,40 potentials and Perdew-Burke-Ernzerhof (PBE) functional41 of generalized gradient approximations (GGA).42 The cut-off energy used was 400 eV and the sampling of the Brillouin zone was performed using a Monkhorst-Pack scheme 43. Electron smearing was employed via a 2nd-order Methfessel-Paxton technique with the width of the smearing consistent to 0.2 eV.44 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 the minimum energy pathways, all transition states were located using the climbing image nudged elastic band (CI-NEB) method,45,46 and vibrational frequencies were analyzed to evaluate a transition state with single imaginary frequency along the reaction coordinate. To describe the thermodynamic and kinetic properties, adsorption energy, reaction energy, and energy barriers were calculated based on the DFT results. The adsorption energy is 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 Fe5C2 slab, and E(adsorbates) is the total energy of free adsorbates. In principle, the more negative is the Eads, the stronger is the adsorption. The reaction energy and energy barrier are calculated by ΔrE = E(FS) - E(IS) and Ea = E(TS) - E(IS), where E(IS), E(FS), and E(TS) are the energies of the corresponding initial state (IS), final state (FS), and transition state (TS), respectively. Furthermore, zero-point energy (ZPE) of the adsorbates were considered for all 5



the calculated energy data (EZPC = ∑hνi/2). 2.2. Models. The Hägg iron carbide has a monoclinic unit cell and the calculated 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 μB47). The nine χ-Fe5C2 surfaces were modeled by the (12), (22) or (11) super-cell of periodically repeated slabs with the thicknesses and vacuum region of about 5 Å and 15 Å, respectively. The size of the slabs and the vacuum space are tested and proved to be reasonable in the previous work.34 The detailed information of the slab models are presented in Supporting Information (Table S1).

3. Results and Discussion 3.1. CO adsorption. Since CO adsorption is considered as the initial state of dissociation, we first examined the adsorption behaviors of CO on -Fe5C2 facets. As shown in Figure 1, nine low and high Miller index facets, Fe5C2(100), (010), (001), (110), (111), (111), (221), (411) and (510), were systematically studied. All the possible CO adsorption sites were considered and named by the coordination number of the CO molecule with the surface atoms, including the Fe-top (T) sites, n-fold sites (nF, 2≤n≤6), and C-top sites (CT).

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Figure 1 Structures of the nine Fe5C2 surfaces and the optimized CO adsorption sites: Fe-top (T), C-top (CT), n-fold (nF, 2≤n≤6) sites; blue balls for Fe atoms, red balls for O atoms, and black balls for C atoms; the most stable sites in red circles and the most activated sites in blue circles

For CO adsorption on Fe5C2(100), (010), (001), (110), (111), (111), (221), (411), and (510) facets, 4, 9, 9, 8, 8, 9, 19, 16, and 11 stable sites were found and are shown in Figure 1. The most stable and most active adsorption configurations are shown in Supporting Information Figure S1, and the other less stable configurations are also given in Figure S2-S10. All the adsorption energies, key bond parameters and CO stretching frequencies of the CO adsorption configurations on the nine -Fe5C2 surfaces are listed in Table S2-S10. On the Fe5C2(100), (110), (111), (111) and (411) surfaces, all the most stable CO adsorption configurations are on the Fe-top sites with the similar C−O distance of 1.17~1.18 Å, and the C-O stretching frequencies vary from 1879 to 1970 cm−1 (see Table 1). Among 7



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them, (110), (111) and (411) are more preferred for CO adsorption with the adsorption energies of -1.91, -2.04 and -2.02 eV, respectively. In comparison, CO adsorption on (100) and (111) are markedly weak (-1.53 and -1.51 eV). On the (010), (001), (221), and (510) surfaces, the most stable CO adsorption configurations are on the hollow sites with three or more bonds connecting the surface. The longest C-O bonds on (100), (001), (111) and (111 ) surfaces (1.214, 1.254, 1.365 and 1.337 Å) are found in the CsCO configurations, and the corresponding adsorption energies are -0.62, -1.28, -1.37 and -1.18 eV, respectively, which is in agreement with the previous result,34 and is similar to the CO adsorption on Fe3C(100) facet.48

Table 1 Computed adsorption energies, bond lengths and stretching frequencies of the most stable and activated configurations of CO on nine χ-Fe5C2 surfaces Surface

Site

Eads/eV

d(C-O)/Å

d(Fe-C)/Å

d(C-C)/Å

d(Fe-O)/Å

/cm-1

T1

-1.53

1.170

1.764

/

/

1940

2F1

-0.62

1.214

1.970

1.416

/

1726

4F1

-1.97

1.212

1.911, 2.018, 2.272, 2.411

/

/

1659

5F1

-1.92

1.285

1.858, 1.967, 1.988

/

2.171, 2.233

1322

3F1

-1.86

1.209

2.068, 1.916, 2.083

/

/

1680

6F 1

-1.28

1.365

2.031, 2.155, 2.161

1.444

2.197.1.993

1053

T1

-1.91

1.169

1.784

/

/

1943

2F3

-1.03

1.210

1.907, 1.923

/

/

1699

T1

-1.51

1.166

1.770

/

/

1970

4F1

-1.37

1.254

1.974, 2.039

1.401

2.157

1544

T1

-2.04

1.176

1.765

/

/

1903

6F 1

-1.18

1.337

2.185, 2.039, 1.962

1.448

2.128, 2.078

1173

5F 2

-2.00

1.298

/

2.035

1215

5F 1

-1.88

1.318

1.943, 1.923, 2.049

/

2.072, 2.221

1167

T1

-2.02

1.179

1.779

/

/

1879

4F 1

-1.82

1.277

1.940, 2.041, 2.055

/

2.054

1319

(100)

(010)

(001)

(110)

(111)

(111)

(221)

(411)

2.097, 2.040, 2.207, 2.039

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(510)

3F1

-2.05

1.201

5F1

-1.68

1.267

1.941, 2.067, 2.052 1.898, 1.987, 2.206, 2.076

/

/

1729

/

2.253

1390

Among the nine surfaces, the more stronger CO adsorption occurs on the Fe 5C2(221), (010), (411), (510), (111) and (110) with the adsorption energies around -2 eV, while the highly activated CO are found on the (001), (221) and (111) on the basis of the computed C-O length (1.365, 1.318 and 1.337 Å, respectively). On the (100) and (111) surfaces, the CO adsorption is the relative weaker (with the largest adsorption energy of -1.51 and -1.53 eV, respectively) and less activated (with the longest C-O bond of 1.214 and 1.254 Å, respectively). In general, the CO adsorption on surface C (Cs) sites is much weaker than that on Fe sites, which is in agreement with the conclusion from Sorescu31 and Zhao et al.34 Consequently, all the most stable configurations of CO adsorption on nine χ-Fe 5C2 surfaces are on Fe sites (Figure 1). Since (221), (010) and (001) have lower surface C/Fe ratio, and can provide more surface Fe sites, the most stable CO adsorption sites on these surfaces are highly coordinated, similar to the CO adsorption on pure iron surfaces.49,50 While on the surfaces with higher C/Fe ratio and the Cs being evenly distributed, such as Fe5C2(100), (110), (111), (111) and (411), the most stable configurations for CO appear on the Fe-top sites. It is noted that in the most active CO adsorption configurations, both C and O atoms are interacting with the surface except for that on the Fe5C2(100) and (110) surfaces. Besides, it is also found a linear relationship between the C-O bond length and the coordination number of the adsorbed CO (R² = 0.88, Supporting Information Figure S12). The C-O bonds are longer when more bonds are formed between CO and the surface atoms in the CO adsorption configurations. 3.2 CO dissociation. On the basis of the adsorption, the direct CO dissociation from the most stable sites as well as the most activated sites on the nine surfaces was computed. All the structures of the initio (IS), transition (TS), and final (FS) states for CO dissociation as well as the corresponding energies are shown in the Supporting Information Figure S13-S15. The detailed data in the dissociation are listed in Table 2. At low converge, CO priors to 9



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occupy the highly stable sites. However, upon most occasions, it is hard for the CO on most stable sites to dissociate directly, especially for the CO adsorbed on Fe-top sites on (110), (411), (111), (100), and (111) surfaces. On the (001) and (510) surfaces, CO on the most stable 3F sites also have high dissociation barriers of 1.89 and 2.57 eV. However, the most stable adsorption site on (221) surface has a low dissociation barrier of 0.97 eV and high adsorption energy of -2.00 eV, which indicates the high activity for CO dissociation. As CO pressure increases, CO occupying the less stable sites becomes possible. On the most activated sites, the CO molecules have lower dissociation barriers than those on the most stable site (Table 2). The barriers reduce to 0.80, 0.79, 0.87, and 0.83 eV on (001), (221), (510), and (111) surfaces, respectively, which indicates the high activity for CO dissociation on Hägg carbide (shown in Figure 2). In addition, on the (010), (110), and (411) surfaces, the C-O cleavage from the most activated sites have moderate activation energy of 1.49, 1.49, and 1.37 eV, respectively, which also have possibility to occur at the practical FTS temperature (around 550 K ).

Table 2 CO dissociation on 9 Fe5C2 surfaces, CO adsorption site, adsorption energy (Eads), activation energy (Ea), reaction energy (ΔEr), bond length (dC-O) of transition state for CO dissociation, and sum bond valence of the initially adsorbed CO (SBV CO) Surface (001) (221) (510) (010) (110) (411) (111) (100) (111) Surface (001) (221) (510)

Site 3F1 5F2 3F1 4F1 T1 T1 T1 T1 T1 Site 6F1 5F1 5F1

Most stable configuration Eads / eV Ea / eV ΔEr / eV -1.86 1.89 0.86 -2.00 0.97 -1.26 -2.05 2.57 1.06 -1.97 1.54 -0.09 -1.90 / 1.49 -2.02 2.90 0.73 -2.04 2.81 0.71 -1.53 2.55 1.06 -1.51 2.95 0.95 Most activated configuration Eads / eV Ea / eV ΔEr / eV -1.28 0.80 -0.41 -1.88 0.79 -1.43 -1.68 0.87 -0.91 10


dC-O/ Å 1.99 1.83 1.79 2.00 / 1.88 2.02 1.88 1.94

SBVCO 7.056 8.864 6.878 7.167 4.797 5.154 5.121 4.972 4.884

dC-O/ Å 1.96 1.75 1.72

SBVCO 8.877 9.890 9.239

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(010) (110) (411) (111) (100) (111)

5F1 2F3 4F1 6F1 2F1 4F1

-1.92 -1.03 -1.82 -1.18 -0.62 -1.37

1.49 1.49 1.37 0.83 2.45 2.00

-0.14 1.02 -0.28 0.14 1.12 0.31

2.00 2.09 2.04 1.86 2.00 1.94

9.614 7.422 8.237 8.940 5.359 7.362

Figure 2 Energy data for CO dissociation, activation energy Ea is shown in the left coordinate, reaction energy Er is shown in the right coordinate, ms represents for the most stable site and ma represents for the most activated site

3.3 Descriptors for CO dissociation activity on Fe5C2. We performed systematical studies on adsorption and dissociation of CO on various Fe5C2 slabs as discussed above. Besides creating a complete data set of χ-Fe5C2 reactivity toward CO dissociation, it is needed to establish the relationship between the Fe5C2 surface structures and their corresponding activities. Such correlations can benefit to design more efficient catalysts with desired activities. On the basis of the computed activation energy and the reaction energy for direct CO dissociation on the most stable adsorption configurations, as well as on the most activated adsorption configurations, we checked the Brønsted−Evans−Polanyi (BEP) relation, as shown in Figure 3. Unfortunately, the correlation performs not well (R2 = 0.65) for CO 11



dissociation on these different Fe5C2 surfaces. This is attributed to the remarkable geometric differences of the IS, TS, and FS of CO dissociation form diverse adsorption sites, which is different with the N2 or CO dissociation from a range of transition metals with the same or similar surface structures.51,

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Figure 3 Correlation between direct CO dissociation barriers and dissociation energies

The low symmetry of Fe5C2 surface leads to various CO adsorption configurations with different local environments. Consequently, CO activation is site-dependent property which makes it difficult to find out a surface property to describe the CO activation. Here, the interaction between CO and the slabs were studied based on bond valence. The concept of bond valence has been widely applied in solid-state chemistry.53 It was developed from the concept of bond number by Pauling and can give a quantitative measurement of the relative strength of the various bonds. 54 According to the method suggested by the former researches,55 the bond valence is most commonly calculated by the empirical equation: facet Sij =exp

r0 -rij

(Equ. 1)

B

Where r0 is the bond valence parameter, rij is the bond length between the two atoms i and j, and B is a universal constant equal to 0.37 Å. The bond valences from a given atom i with valence Vi can be calculated by the sum of the Sij: Vi = ∑j Sij

(Equ. 2) 12


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In this work, the sum bond valence between the adsorbed CO and the slab atoms is defined as: SBVCO =VC +VO -SCO (Equ. 3) In Equ. 3, VC is the bond valences from all the interaction between C and the other atoms in the system. Similarly, VO is the bond valences from all the interaction between O and the other atoms. SCO is the bond valence of the C-O bond. In the calculation, the bond valence parameter (r0) for C-O is taken to be 1.144 Å, the bond length in the isolated CO molecule. For C-C, r0 is the bond length in isolated C2H6 (1.526 Å) molecule. For C-Fe and O-Fe, r0 are set to be 2.249 and 1.911 Å, which are the bond length in the χ-Fe5C2 and α-Fe2O3, respectively. In the calculation, all the r0 parameters are taken from the molecules or the crystal unit cells obtained by optimization with PBE. SBVCO is a site-dependent local property and can be calculated on each CO adsorption site. The values of SBVCO for the most stable and most activated CO are listed in Table 2. On a given Fe5C2 surface, there exist various CO configurations with different activation energies. Consequently, the local property is better than the general surface properties, for instance, d-band centre56,

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(Figure S16). According to Equ. 3, SBVCO is a parameter containing not

only the immediate coordination environment, that is, bond number and the corresponding bond length, of the C and O atoms, but also the activation degree of C-O bond, which makes SBVCO a promising factor to describe the CO dissociation. According to the above calculation, it is found that the energy barrier of CO dissociation has a linear relationship with the sum CO bond valence (SBVCO), as shown in Figure 4. The correlation coefficient between Ea and SBVCO is even higher than 0.9 (Figure 4a, R2 = 0.86), which confirm that the SBVCO is an available descriptor for CO dissociation activity on the complex surfaces like Fe5C2. It is known that the bond valence of C and O atoms are calculated directly from the observed bond lengths and can be summed around each atom to obtain a measurement of the atomic valences. The sum of the bond valences can be regard as the total number of electrons that is available for forming bonds with the surrounding atoms. In this way, the bond valences represent the relative strength of the interaction between CO and the surface.58 To analyze the relation between SBV and the CO activation, the orbital of CO adsorbed 13



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on the surfaces are studied by calculating the local density of state of the adsorbed CO molecules. As shown in Figure 5, three sites with low to high value of SBVCO are analyzed, that is, the most stable site on (100) with the SBVCO of 4.972, the most activated sites on (110) and (221) surfaces with the SBVCO of 7.422 and 9.890, respectively. According to the curves shown in the figure, CO on the most stable site on (100) surface has the localized 2π* orbital, which means the back donation from surface to the adsorbed CO molecule is relatively tiny. While the CO on the (110) and (221) surfaces, with the increase of the bond valence, the 2π* orbitals are remarkably delocalized, which indicates the increasing of back donation from surface to antibonding 2π* orbitals. Consequently, as the SBVCO increases, the C-O stretching frequency is lower and the C-O bond is more weakened (Figure 4b).59 That`s the reason why CO molecules with high SBVCO exhibit the relatively low dissociation barriers. Similar to the prediction of adsorption properties with coordination numbers,60,

61

the intrinsic physical and

chemical principles of SBV make it a promising descriptor for small molecule activation on complicated surfaces like Fe5C2.

Figure 4 The linear correlations between (a) sum CO bond valence and the activation energy of direct CO dissociation, (b) sum CO bond valence and the C-O vibration frequencies in adsorption state

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Figure 5 LDOS of the adsorbed CO molecules on Fe5C2(100), (110), and (221) surfaces, respectively, ms and ma represent for the most stable and most activated sites

In addition, we also find the linear relationship (R2 = 0.80) between CO dissociation barriers and the total bader charge of the adsorbed CO molecules (Figure S17). The adsorbed CO that has more net bader charge is more easily to dissociate. This is also an important aspect to explain the results that the electron donor promoter such as potassium can facilitate the CO dissociation on the substrates and copper (with negative charge) can increase CO dissociation barriers.62,63 So, the electron transfer within the promoters, the catalyst surface, and the CO molecule is expected to be a vital factor affecting the promotion effect on the activation of CO. In our future work, effects of more promoters, such as Mn, Co, Cu, on CO adsorption and dissociation on Fe5C2 will be studied. In practical FTS process, diverse CO configurations can possibly exist due to the facility of CO adsorption upon diverse sites on the Fe5C2 surfaces. Therefore, we perform a prediction for CO direct dissociation form all adsorption sites based on the above Ea-SBVCO correlation. As can be seen in the Figure 6, the activation energies of CO dissociation on several sites on Fe5C2(001), (221), and (510) surfaces are below 1.00 eV. The CO dissociation on these sites can easily take place. It is noted that the surface energy of (001) and (221) surfaces (2.545 and 2.302 J/m2, respectively, shown in Table S11) are higher than the other surfaces, which indicates the relatively unstable surfaces are favorable for the CO direct dissociation. In addition,

15



the CO direct dissociation can easily take place on two sites on (111) surface, which also indicates the high activity for C-O cleavage.

Figure 6 The linear relationship between direct CO dissociation barrier and SBVCO and the corresponding activation energy on the most stable and most activated sites as well as the dissociation barriers evaluated by the linear equation on other CO adsorption sites, dashed black and red horizontal lines represent for the activation energy of 1.50 and 1.00 eV, respectively

On the (010), (110), and (411) surfaces, as can be seen in the Figure 6, the lowest direct dissociation barriers are around 1.50 eV, indicating that the dissociation are less favorable than that on the (221), (001), (510), and (111) surfaces. It is noted that the barriers around 1.5 eV can also be overcome in practical FTS with the temperature about 550 K. The facial direct CO dissociation on various Fe5C2 surfaces demonstrates the high activity of iron-based FTS catalysts. While on the (111) and (100) surfaces, the CO dissociation can hardly occur because of the high energy barrier towards the desorption energy. It is interesting to note that, these two surfaces have lower surface energies (1.816 J/m2 for (100) and 2.014 J/m2 for (111) surface) and expose more area than the other surfaces according to the research by Zhao et al.35 Though the two surfaces are not active for CO direct dissociation, the CO molecules can possibly 16


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dissociate with the assistance of H atoms. So, H-assisted CO dissociation pathways on (100) and (111) surfaces through the intermediates, HCO, HCOH, and H2CO are calculated. The results are shown in Supporting Information S18 and S19. The effective barriers for H-assisted dissociation on the two surfaces are remarkably lower than the direct dissociation barriers, especially for the H2CO pathways, which makes the H-assisted dissociation possible for C-O bond cleavage. Above all, direct dissociation is crucial for CO cleavage on Fe5C2 and can be described with SBVCO. In addition, H-assisted CO dissociation is also significant for the reaction mechanism and the overall reactivity of the FTS. It is reported that HCO, COH, HCOH, and CH2O are possible intermediates on Fe, Co, Ru, W surfaces,64-69 and on Fe5C2 surfaces.33,70 For the comparison of potential CO activation pathways with the H participation on various Fe5C2 surfaces, more detailed study will be carried out in our next work.

4. Conclusions The present work is among the first that comparatively analyze the surface reactivity of diverse facets of the hägg carbide. The CO adsorption and dissociation on a series of both low and high Miller index Fe5C2(100), (010), (001), (110), (111), (111), (221), (411) and (510) surfaces are systemically investigated using spin-polarized density functional theory method. It is found that the CO adsorption configurations bonding to surface C sites are much weaker than on Fe sites. All the most stable configurations of CO adsorption on nine χ-Fe 5C2 surfaces are on Fe sites and the C-O bonds are longer when more bonds between CO and the surface atoms are found in the CO adsorption configurations. The concept of bond valence was adopted to study the relationship between surface properties and the CO dissociation activity. The linear relationship between the activation energy for direct C-O cleavage and CO bond valence was found. The adsorbed CO molecule with more sum bond valence will have more weakened C-O bond, consequently, the activation energies are lower. It is predicted that the bond valence is a promising descriptor for other small molecule activation on complex surfaces. According to the calculation, the activity of CO dissociation on different Fe 5C2 surfaces 17



is different from each other. The Fe5C2(100) and (111) surfaces with high stability are not active for direct CO activation. While the relatively unstable surfaces, (010) and (221), have high activity CO dissociation. The (111) and the high index (510) surfaces are also active for CO cleavage. On the Fe5C2(010), (110), and (4 11) surfaces, the direct CO dissociation have moderate activity. The remarkable differences of the CO adsorption and dissociation properties between the χ-Fe5C2 surfaces indicate the significance to control the morphology of the catalysts, and this work will be helpful to rationally design highly efficient FTS catalysts with desired activities.

Supporting Information. Key parameters for the slab models of the nine -Fe5C2 surfaces (Table S1); Computed adsorption energies, bond lengths and stretching frequencies of CO on the nine surfaces (Table S2-S10); The CO dissociation activation energy and the surface energy on 9 Fe5C2 surfaces (Tables S11); Most stable and activated configurations of CO adsorption on nine χ-Fe5C2 surfaces (Figure S1); Configurations of CO adsorption on the nine surfaces (Figures S2-S10); CO adsorption sites and the corresponding adsorption energy on the nine Fe5C2 surfaces (Figures S11); The linear relationship between CO coordination number with surface atoms and (a) the C-O vibration frequencies as well as (b) the C-O bond length in adsorption state (Figure S12); Detailed structures and energies of IS, TS, and FS for direct CO dissociation on the nine surface (Figures S13-S15); The correlation between the d-band centre of Fe5C2 surfaces and the activation energy of CO dissociation (Figure S16); The linear relationship between the net bader charge of adsorbed CO and the activation energy of CO dissociation (Figures S17); Energy profiles for the direct and various hydrogen-assisted CO dissociation pathways on Fe5C2(100) and (111) surfaces (Figures S18, S19).

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

Notes 18


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The authors declare no competing financial interest.

Acknowledgment. The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 21273261, No. 21473229, No. 91545121, No. 21403118), 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. 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) 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. ( 6) Davis, B. H.; Occelli, M. L. Fischer-Tropsch Synthesis, Catalysts, and Catalysis: Advances and Applications. CRC Press: Boca Raton, U. S. A., 2016. (7) 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. (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. 19



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The Descriptor of Bond Valence - ACS Publications - American

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