Mechanisms of CO Activation, Surface Oxygen Removal, Surface

Subscriber access provided by Kaohsiung Medical University

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Mechanisms of CO Activation, Surface Oxygen Removal, Surface Carbon Hydrogenation and C-C Coupling on the Stepped Fe(710) Surface from Computation Teng Li, Xiao-Dong Wen, Yong-Wang Li, and Haijun Jiao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04265 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 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

The Journal of Physical Chemistry Mechanisms of CO Activation, Surface Oxygen Removal, Surface Carbon Hydrogenation and C‐C Coupling on the Stepped Fe(710) Surface from Computation Teng Li,a,b,c Xiaodong Wena,b Yong‐Wang Li,a,b Haijun Jiaoa,d* 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, PR China; d) Leibniz‐Institut für Katalyse e.V. an der Universität Rostock, Albert‐Einstein Strasse 29a, 18059 Rostock, Germany. E‐mail: [email protected] Abstract: To understand the initial steps of Fe‐based Fischer‐Tropsch synthesis, systematic periodic density functional theory computations have been performed on the single‐atom stepped Fe(710) surface, composed by p(3×3) Fe(100)‐like terrace and p(3×1) Fe(110)‐like step. It is found that CO direct dissociation into surface C and O is more favored kinetically and thermodyna‐ mically than the H‐assisted activation via HCO and COH formation. Accordingly, surface O removal by hydrogen via H2O formation is the only way. On the basis of surface CHx hydrogenation (x = 0, 1, 2, 3), surface CHx+CHx coupling and CO+CHx insertion result‐ ing in CHxCO formation followed by C‐O dissociation, surface C hydrogenation towards CH3 formation is more favored kinetically than the formation of CHx‐CHx and CHxCO as well as thermodynamically. Starting from CH3, the formation of CH4 and CH3CO has similar barriers and endothermic reaction energies, while CH3CO dissociation into CH3C+O has low barrier and is highly exother‐ mic. Therefore, turning the H2/CO ratio should change the selectivity toward C‐C formation and propagation.

~ 1 ~

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Page 2 of 22

Introduction Iron‐based catalysts applied in industrial Fischer‐Tropsch synthesis (FTS), which converts synthesis gas (CO+H2) generated from coal, natural gas and biomass into to value‐added fuels and chemicals, have attracted steady attention because of their high activity, low methane selectivity, low price and non‐toxicity.1,2 During the reaction process, iron‐based catalysts can have a mixed form of iron oxides, iron carbides and metallic iron.3,4 The variety of products and the complexity of catalysts make the reaction mechanisms too hardly understood in clear schemes. One simplified way is to use pure metal models to study the reaction mechanisms, i.e.; chain initiation (CO activation), chain propagation and finally chain termination. Apart from the extensive and plentiful experimental studies there are increasing computational studies for understanding the elusive reaction mechanisms. Both experimental and computational studies identified the metallic Fe(110),5 Co(0001)6 and Ru(0001)7 surfaces as the most stable single crystal terminations; and they have been used to model FTS mechanisms. By using high resolution electron energy loss spectroscopy (EELS), Erley et al.,8 studied FTS reaction on the Fe(110) surface at 300°C using a fixed H2/CO ratio (10/1) at 1 atmosphere total pressure and observed the formation of surface C, CH, CH2 and CCH2 as well as products from methane to higher saturated hydrocarbons and olefins. Similar results were found for a carbon pre‐covered iron surface in pure hydrogen atmosphere under the same conditions (1atm and 300°C). They proposed that methanation and FTS proceed through the hydrogenation of surface carbon and some or all these species are the reaction intermediates under syn‐ thesis gas conditions. Gonzalez et al.,9 prepared and cleaned the Fe(110) surface with argon ion sputtering, which was used to create surface defects. The sputtered Fe(110) exhibited higher activity for CO dissociation than the annealing Fe(110) and the β peak for CO desorption on the annealing surface may result from defect sites. Since it is very hard to prove the formation of sur‐ face CHx on the perfect Fe(110) surface experimentally and theoretically, the more opened surfaces, such as stepped and kinked surfaces, should be taken into consideration. Computationally CO dissociation on the Ru(0001) surface is found more difficult than CO hydrogenation to CHO10 and the sub‐ sequent CHxO hydrogenation to CHx+1O is more favored than CHxO dissociation into CHx+O kinetically and thermodynamically, and consequently, CO consecutive hydrogenation tends to form methanol rather than surface CHx (x = 0, 1, 2, 3, 4). The same results are found on the Co(0001) surface,11 where the methanol route is optimal, while CHxO dissociation into CHx as well as CHx coupling with CO are not favored kinetically and thermodynamically. Systematic comparative DFT computations on the clean Co(0001) surface show that CH3OH formation [CO+4H → CHO+3H → CH2O+2H → CH3O+H → CH3OH] is most favored along the minimum energy path,11 while the dissociation and the consecutive C‐C coupling [CHO → CH+O and CH+CO; CH2O → CH2+O and CH2+CO; CH3O → CH3+O and CH3+CO] are not competitive kinetically and thermodynamically. It is therefore concluded that the flat Co(0001) surface should not represent the active sites in Co‐based Fischer‐Tropsch synthesis; and the flat Co(0001) surface is unstable and undergoes strong reconstruction under reaction condition.12 Although a complete scheme from CO to CH4 or CH3OH was not reported on the Fe(110) surface, Ojeda et al.,13 proved that HCO hydrogenation to H2CO has lower barrier than HCOH formation and HCO dissociation, and H2CO should form H3CO rather than CH2+O via C‐O bond breaking, and methanol should be the final product. They also concluded that oxygen produced by CO dissociation can more likely form CO2, while oxygen formed by H‐assisted CO scission can produce H2O.13 On the clean Fe(100) surface,14 the path of CO direct dissociation and surface C hydrogenation is kinetically more favored than the alternative path of CO successive hydrogenation, and CH3OH formation has much high barrier and is strongly endothermic; neither kinetically nor thermodynamically favored. All these show that on these metallic surfaces C1 species should be the main intermediates and the formation of methane or methanol as preferred products does not agree with FTS in producing long chain hydrocarbons. Dahl et al.,15 used experiments and DFT computation to study N2 dissociation on the steps and terraces of the Ru(0001) sur‐ face and found that steps are more active than terraces due to electronic and geometric effects. The Ru(109) surface, composed ~ 2 ~


Page 3 of 22 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

The Journal of Physical Chemistry by a (001) terrace and a two‐atom step, was found more active for CO dissociation than the clean Ru(001) surface on the basis of the combined vibrational spectroscopy and thermal desorption measurement.16 These results are in consistence with the com‐ putations by Li et al.17, where the stepped Ru(0001) surfaces have lower CO dissociation barriers than the flat Ru(0001) surface (1.29 and 0.94 eV vs. 2.37 eV). On Ru(0001), Zhang et al.,18 concluded that methane should be the main product of CH hydroge‐ nation at high H2 fraction; while CH would like to couple with CH to produce hydrocarbon at high CO2/CO fraction. Weststrate et al.,19 showed that CO can readily dissociate on the defect sites of the Co(0001) surface, such as monoatomic step edges, at 330 K with an estimated barrier between 0.93 and 1.08 eV. CO dissociation has much lower barrier on the more open Co(10−12) and Co(11−20) surfaces than on the flat Co(0001) surface (1.34 and 1.39 vs. 2.46 eV) theoretically.20 Petersen found that CO prefers direct rather than H‐assisted dissociation on the step and kink sites of the Co(321) and Co(221) surfaces.21 On the Co(211) sur‐ face, H‐assisted CO activation has lower barrier than CO direct dissociation (1.27 vs. 1.47 eV).22 All these show that either direct or H‐assisted CO activation depends on the surface structures. Wang et al.,23 computed CO dissociation on low and high index Miller iron surfaces and found that the formation of surface carbide is easiest on Fe(310). Sorescu24 found that CO activation on the Fe(310) and Fe(710) surfaces have similar activation barriers (0.89 vs. 0.91 eV). Tian et al.,14 found that on the Fe(100) surface CO direct dissociation is more favored than H‐assisted CO activation (1.08 vs. 1.81 eV). In addition to CO activation, self‐coupling of CHx on Fe(100) have been discussed,25, 26, 27 and the coupling reactions have higher barriers than CHx hydrogenation,14 indicating that methane formation is more favored on Fe(100). On Fe(110), it is reported that surface defect created by removing iron atom artificially can reduce CO dissociation barrier.28 On clean Fe(111), CO diffuses from the shallow‐hollow site to bridge‐like site firstly and then dissociates into C and O. The effective barrier is 1.73 eV, which is reduced to 1.17 eV under hydrogen assistance.29 Li et al.,30 found that the energy penalty of CO dissociation is 1.73 eV, but the total energy barrier for H‐assisted CO dissociation to CH [CO+H → CHO → CH+O] is 1.69 eV on Fe(111), higher than the reported 1.17 eV,23 and the difference may be due to the too short C‐O distance in the transition state (1.51 Å30). Formation of C‐C bond on Fe(111) has also been reported by Li et al.,30 and HC formed by H‐assisted CO dissociation is more likely to couple with another CH with low activation barrier (0.54 eV). On the other hand, surface oxygen which has the ability to oxidize metal surfaces should not be ignored. Kizilkaya et al.,31 studied the removal paths of surface oxygen on flat and defect Co(0001) under ultra‐high vacuum. They found that H2O forma‐ tion is the main path and oxygen cannot be removed by CO event at higher temperature. The experiment results is in agreement with the computed results by Liu et al.32 They proved that CO2 formation is unfavorable both kinetically and thermodynamically. Since few studies reported CO dissociation, C‐C chain formation and oxygen removal systematically, here, the more open and stepped Fe(710) surface, composed by Fe(100) and Fe(110), was used to explore these reactions. In addition, surface carbon hydrogenation and carburization mechanism were simulated. Method and model All calculations were done on the basis of periodic slab model by using the plane wave based density functional theory (DFT) method implemented in the Vienna Ab initio Simulation Package (VASP).33,34,35 The projected augmented wave method (PAW)36 was used to describe the interaction of electron and ion. The electron exchange and correlation energies were calculated using the generalized gradient approximation method in the Perdew‐Burke‐Ernzerhof (GGA‐PBE) functional.37,38 Spin‐polarization was included to correctly account for the magnetic properties and this is found essential for energy calculations. The value of cut‐off energy for plane wave basis was set up to 400 eV. To make sure that the energy difference is less than 104 eV and the force per atom is less than 0.03 eV/Å, the second‐order Methfessel‐Paxton39 electron smearing was used (0.2 eV). The vacuum layer bet‐ ween the periodically repeated slabs was set as 15 Å. The adsorption energy (Eads) of adsorbate (X) is defined as Eads = EX/slab − Eslab – EX, where EX/slab is the total energy of the slab with adsorbate on it, Eslab is the total energy of the clean slab and EX is the total energy of the free adsorbate (X) in gas phase; ~ 3 ~



Page 4 of 22

and therefore, the more negative Eads, the higher (or stronger) the adsorption. For the adsorption energy of oxygen and hydrogen atoms, half of the total energy of O2 and H2 in gas phase was used as reference, respectively. For the adsorption of two species step by step, adsorption energy is found as Eads = E(X1+X2)/slab − EX1/slab − EX2. For reactions, the climbing‐image nudged elastic band (CI‐NEB) method40,41 was adopted to search the transition states and the frequency analysis was also processed to verify the transition state with only one imaginary frequency. The reaction barrier (Ea) is defined as Ea = ETS − EIS and the reaction energy (Er) is defined as Er = EFS − EIS, where EIS, EFS and ETS are the total energies of the initial, final and transition states, respectively. For direct comparison with the reported data, our reaction barriers and energies do not include the zero‐point energy (ZPE), unless otherwise noted. As given in Supporting Information (Table S1) ZPE corrections are rather small, i.e.; less than 0.12 eV for reac‐ tion barriers and less than 0.19 eV for reaction energies. For studying the adsorption and reaction appropriately on the Fe(710) surface, we used a large p(3x2) slab model which has totally 132 Fe atoms (Figure 1). The p(3x2) surface slab model can be divided into two p(3x3) Fe(100)‐type slab models at the terrace and one p(1x3) Fe(110)‐type slab model at the step. Similar as the clean Fe(100) surface, there are three four‐fold hollow sites (4F1, 4F2 and 4F3), four top sites (T1, T2, T3 and T4) and seven bridge sites (B1, B2, B3, B4, B5, B6 and B7) on the terrace. Similar to the Fe(110) surface, apart from the T1 and T4 as well as B1 and B7 sites shared with the Fe(100)‐type surface, there are also two bridge sites (B8 and B9) and two three‐fold hollow sites at the step (3F1 and 3F2), which is fcc‐ and hcp‐like, respec‐ tively. It is to note that the 4F1 and 3F1 sites share the same B1 edge; and the 4F3 and 3F2 site share the same B7 edge. It is to expect that both 4F1 and 4F3 sites should be more active than the 4F site on the clean Fe(100) surface on the basis of the report‐ ed surface energies (2.50 vs. 2.47 J/m2).24 In addition, Fe atoms has coordination number of eight at the B1 site and nine on the Fe(100) surface on one hand; and the interlayer distance (2.830 Å) at the B1 site is longer than that (2.809 Å) on the Fe(100) sur‐ face.

Figure 1. Top and side views of the Fe(710) surface with possible adsorption sites For surface reactions, 72 top Fe atoms together with surface species are allowed to relax, while 60 bottom Fe atoms are fixed at their equilibrium positions. For the diffusion of surface carbon atom into the subsurface, only 24 bottom Fe atoms are fixed at their equilibrium positions and all other atoms are allowed to relax. It is noted that our slab model is much larger than the pre‐ viously used one,24 where a p(2x1) slab model containing totally 38 Fe atoms was used and 22 top Fe atoms including adsorbed species were allowed to relax and 16 bottom Fe atoms were fixed at their equilibrium positions. Results and discussion (a) CO adsorption and dissociation: There are five sites for CO adsorption on Fe(710). The adsorption configurations and ener‐ gies as well as the C‐O distances are shown in Figure S1. In the most stable adsorption configuration (Figure 2), CO is tilted at the 4F1 site over four surface Fe atoms (1.956, 1.956, 2.188, 2.189 Å) and one subsurface Fe atom (2.128 Å), and the oxygen atom interacts with two surface Fe atoms (2.090, 2.095 Å). The C‐O distance is elongated from 1.144 Å in gas phase to 1.328 Å. The ad‐ sorption energy is –2.11 eV, close to that (–2.14 eV42) on the Fe(100) surface. On the Fe(110)‐like step site (Figure 2), CO prefers ~ 4 ~


Page 5 of 22 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

The Journal of Physical Chemistry the T1 site with Fe‐C and C‐O distances of 1.803 and 1.198 Å, respectively. The adsorption energy at the T1 site is –1.80 eV, close to –1.8823 and –1.99 eV43 on the Fe(110) surface.

Figure 2. CO adsorption configurations at different sites on (710) Prior to CO direct and H‐assisted dissociations, we computed the adsorption configurations of the corresponding surface inter‐ mediates (Figure S2‐S6). On clean Fe(710), surface H prefers the 3F1 site and the adsorption energy is –0.85 eV, close to that (– 0.76 eV44) at the 3F site on Fe(110), while larger than that (–0.42 eV14) at the 4F site on Fe(100). Surface O prefers the 3F1 site and the adsorption energy is –3.39 eV, close to that (–3.3614 and –3.4345 eV) at the 4F site on Fe(100) and that (–3.43 eV44) at the long‐bridge site on Fe(110). Surface C prefers the 4F3 site and the adsorption energy is –8.50 eV, close to that (–8.34 eV14) at the 4F site on Fe(100) and larger than that (–7.86 eV46) at the long‐bridge site on Fe(110). Surface CH prefers the 4F3 site and the ad‐ sorption energy (–7.16 eV) is close to that (–7.04 eV14) at the 4F site on Fe(100) and larger than that (–6.84 eV13) on Fe(110). The adsorption of surface HCO, COH and H2CO prefers the 4F1 site and the adsorption energy is –2.97, –4.37 and –2.09 eV, respec‐ tively, close to those (–2.83, –4.47 and –1.66 eV,14 respectively) at the 4F site on Fe(100), and lower than those (–3.02, –4.72 and –1.56 eV,13 respectively) on Fe(110). On the basis of these results we computed CO direct dissociation at the 4F1 site (reaction 1a, Figure S7). CO direct dissociation at the 4F1 site has barrier of 0.90 eV and is exothermic by 0.87 eV. In the transition state (TS1a, Figure 3), the breaking C‐O dis‐ tance is 1.894 Å and the O atom is at the B1 site with O‐Fe distances of 1.885 and 1.884 Å. In the dissociated state, the C atom is at the 4F1 site and the O atom is at the 4F3 site; and this is different from the stable co‐adsorption configuration of C+O (Figure S8), in which the C atom is at the 4F3 site and the O atom is at the 3F1; and the migration of C and O atoms is exothermic by 0.26 eV. Therefore, total CO dissociative adsorption at the 4F1 site is exothermic by 1.13 eV. Because of the highly activated C‐O at the 4F3 site (Figure S1), CO dissociation is computed. Considering the difference in adsorption energy, CO dissociation has apparent barrier of 0.94 eV and is exothermic by 1.34 eV (reaction 1b, Figure S6), close to those at the 4F1 site. The computed CO dis‐ sociation barrier is close to the previously reported values (0.89 eV24) on Fe(710) and lower than that (1.03 eV42) on the Fe(100) surface. On Fe(110), CO direct dissociation has barrier of 1.51 eV and is exothermic by 0.46 eV. On Fe(111),23 CO direct dissocia‐ tion has barrier of 1.17 eV and reaction energy of 0.06 eV. These CO dissociation barriers Fe(710) are close to those on the range of the (510), (021), (311), (312) and (011) surfaces of ‐Fe5C247 (1.17, 0.85, 0.98, 0.91 and 0.69 eV, respectively) as well as the most activated (001), (221), (510), (11–1) surfaces of ‐Fe5C248 (0.80, 0.79, 0.87 and 0.83 eV, respectively). ~ 5 ~



Page 6 of 22

Figure 3. Transition states for CO dissociation (TS1a), HCO (TS2a) and COH (TS3) formation, HCO dissociation (TS4) and H2CO formation (TS5) Apart from CO direct dissociation, we computed H‐assisted CO activation. Since H prefers the 3F1‐1 site on the CO pre‐covered surface (Figure S9), the H atom migrates to the 4F2 site to interact with CO at the 4F1 site to form HCO. For CO at the 4F1 site, H migration needs 0.23 eV and the distance between H and C is 3.642 Å. The subsequent HCO formation (reaction 2a, Figure S10) has barrier of 0.88 eV and is endothermic by 0.68 eV. In the transition state (TS2a, Figure 3), the H atom is located to the T3 site with the forming C‐H distance of 1.499 Å and the H‐Fe distance of 1.582 Å. In the final state, the formed HCO is at the 4F1 site with the C‐H distance of 1.117 Å and the C‐O distance of 1.393 Å, and the C atom is pulled up and bonds with two iron atoms at the B2 site with the C‐Fe distances of 2.001 and 1.991 Å. Considering the H diffusion energy, HCO formation has apparent barrier of 1.11 eV and is endothermic by 0.91 eV. Comparatively, we computed COH formation, an alternative way for CO activation. Since CO direct dissociation and hydrogena‐ tion to HCO prefer the 4F1 site, we only considered COH formation at the 4F1 site. For CO at the stable 4F1 site, H migrates from one 3F1 site to another 3F1 site (0.44 eV), which is much close to oxygen as the 3F1‐1 and 3F1‐2 with the C‐O distance of 2.405 Å (Figure S9). The formation of COH (reaction 3, Figure S10) has barrier of 1.60 eV and is endothermic by 0.90 eV. In the transition state (TS3, Figure 3), the forming O‐H distance is 1.344 Å, where the H atom is at the B1 site with the H‐Fe distances of 1.888 and 1.905 Å as well as the O‐Fe distances of 2.331 and 2.320 Å. In the final state, the O‐H distance is 0.981 Å and the C‐O distance is 1.421 Å. By considering the H migration energy, COH formation has apparent barrier of 2.04 eV, much higher than that of HCO formation and COH formation is endothermic by 1.34 eV, stronger than HCO formation. The formation barrier of HCO and COH at the 4F1 site is close to those (0.84 and 1.64 eV,14 respectively) on Fe(100), lower than those (1.65 and 2.06 eV,43 respectively) on Fe(110). On Fe(111), HCO formation has barrier of 0.99 eV.29 In addition to CHO formation, we computed HCO dissociation into CH+O (reaction 4, Figure S11) for checking the formation of the CH intermediate. It is found that HCO dissociation at the 4F1 site has barrier of 0.55 eV and is exothermic by 0.88 eV. In the transition state (TS4, Figure 3), the breaking C‐O distance is 1.817 Å and the O‐Fe distances are 1.880 and 1.881 Å. In the dis‐ sociated state, HC falls to the 4F1 site and O migrates to 3F1 site. However, this co‐adsorbed CH at 4F1 site and O at the 3F1 site do not represent the most stable co‐adsorption configuration, in which CH prefers the 4F3 site and O prefers another 3F1 site (Figure S12). After CH and O migration, HCO dissociation is exothermic by 1.48 eV. On the basis of the most stable co‐adsorbed CO and H, HCO dissociation into CH and O has apparent barrier of 1.46 eV and is exothermic by 0.57 eV. H‐assisted CO dissocia‐ tion forming co‐adsorbed CH+O has apparent barrier of 1.69 eV and is exothermic by 0.57 eV on Fe(111),30 while on Fe(100), the apparent barrier is 1.81 eV and the reaction is endothermic by 0.15 eV.14 For HCO hydrogenation to CH2O (reaction 5, Figure S11), the second H atom moves from the stable 3F1 site to the 4F2 site and the migration energy is 0.21 eV (Figure S13). Subsequently, CH2O formation has barrier of 0.64 eV and is endothermic by 0.12 eV. In the transition state (TS5, Figure 3), the H atom is at the T3 site and the forming C‐H distance is 1.612 Å and the H‐Fe distance is 1.592 Å. Compared with HCO dissociation into CH+O, HCO hydrogenation to H2CO is not favorable kinetically and thermodyna‐ mically. On the basis of the most stable co‐adsorbed CO+H, CH2O formation has apparent barrier of 1.76 eV and is endothermic by 1.24 eV. On the basis of the most stable co‐adsorbed CO+2H (4F1+3F1+3F1), the potential energy surface for direct and H‐assisted CO ~ 6 ~


Page 7 of 22

dissociations in Figure 4 shows obviously that CO direct dissociation at the 4F1 site has the apparent barrier of 0.90 eV (0.87 in‐ cluding ZPE) and is most exothermic (–1.13 eV or ‐1.14 eV including ZPE), while the apparent barrier (1.11 and 2.04 eV, respec‐ tively) of HCO and COH formation is much higher and the reaction is less exothermic and endothermic (–0.57 and 1.34 eV, respectively). These results show that CO direct dissociation is much more favored kinetically and thermodynamically. It is noted that the CO direct dissociation barrier on Fe(710) is lower than that on the Fe(100), Fe(110), Fe(111), Fe(210), Fe(211) and Fe(310) surfaces (1.03, 1.51, 1.17, 1.11, 1.06 and 0.98 eV,23 respectively). Since CO prefers direct dissociation, the subsequent reactions of C and O are discussed following. 3.6 3.5

CO(g)+H2(g) 3.52

2.04 (1.60)

2.0

1.76 (0.64) COH+H 4F1+3F1 1.34

1.5

1.46 (0.55)

1.11 (0.88)

1.0

Energy/eV

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

The Journal of Physical Chemistry

0.5

CO+2H (4F1+3F1)+3F1 0.44

0.90 (0.90)

HCO+H 4F1+3F1 0.91

HCO+H (4F1+4F2) 1.12

0.0

H2CO 4F1 1.24

HC+O+H 4F1+3F1+3F1 0.03

CO+2H 4F1+3F1 0.00

-0.5 C+O+2H 4F1+4F3+3F1 -0.87

-1.0

HC+O+H 4F3+3F1+3F1 -0.57

C+O+2H 4F3+3F3+3F1 -1.13

Figure 4. Potential energy surface of CO activation on Fe(710) on the basis of the most stable co‐adsorbed CO+2H (red line for CO direct dissociation; black line for H‐assisted CO hydrogenation to CHO and CH2O; and blue line for H‐assisted CO hydrogenation to CO (intrinsic barriers in parenthesis) (b) C diffusion and surface O remove by H and CO: Since CO prefers direct dissociation on Fe(710), we computed surface C diffusion into subsurface as well as surface O remove by hydrogen forming H2O and by CO forming CO2. Apart from the above discussed adsorption of CO, C, O, H, HCO and COH, we computed the adsorption of surface OH, H2O, CO2, COOH (Figure S14‐S17). Surface OH prefers the B1 site and the adsorption energy is −4.32 eV, closes to that (−4.28 eV) at the 3FH site on Fe(110)44 and that (‐4.14 eV) at the B site on Fe(100).45 Surface H2O prefers the T4 site with the Fe–O distance of 2.195 Å, and the adsorption energy is −0.46 eV, close to that (−0.41 eV45) at the T site on Fe(100) and that (−0.38 eV44) at the T site on Fe(110). For CO2, the most stable adsorption configuration is at the 4F1 site and the adsorption energy of –1.15 eV is close to that (–1.08 eV49) at the 4F site on Fe(100) and about double of that (–0.54 eV43) on Fe(110). Surface COOH prefers the 4F1 site and is parallel with the B4 site; the adsorption energy is –2.90 eV, close to that (–2.82 eV49) on Fe(100) and that (–2.88 eV43) on Fe(110). On the basis of the adsorption of surface C, we further computed the carburization possibility. For surface C diffusion into sub‐ surface (reaction 6, Figure 5), the C atom at the 4F3 site moves to the B7 site and the adjacent iron atoms are pushed away. It further moves into subsurface continuously and arrives at its transition state (TS6, Figure 5), where the Fe atom shared by B7 and B8 sites is pushed towards vacuum and the distances between the C atom and the four adjacent iron atoms are 1.829, 1.919, ~ 7 ~



Page 8 of 22

1.851 and 2.031 Å. The diffusion barrier is 1.44 eV (1.42 eV including ZPE), higher than that (1.18 eV50) on Fe(110) and close to that (1.47 eV51) on Fe(100). Finally, the C atom at the octahedral interstitial site is less stable than on surface by 0.69 eV (0.66 eV including ZPE), indicating that surface C diffusion into subsurface is not favored kinetically and thermodynamically, instead, the overflow of subsurface C atom is favored. However, carburization becomes possible and favorable on Fe(100) and Fe(110) at high carbon coverage,46 where the reaction energy becomes negative and the diffusion barrier is reduced to 0.56 eV at 0.5 ML on Fe(110) as well as to 1.20 eV at 1 ML on Fe(100). Therefore, one might expect such coverage effect on the (710) surface under high carbon coverage.

Figure 5. Diffusion of carbon atom from surface into subsurface (reaction 6) Since CO direct dissociation results in surface O as well as both H2O and CO2 are produced in FTS, it is necessary to remove sur‐ face O to suppress surface oxidation, which can reduce the catalytic activity and prevent surface carburization.28 Under synthesis gas condition, surface O can be removed by H and CO resulting in H2O and CO2, respectively. For H2O formation, there are two routes, i.e.; surface OH formation (O+H → OH) followed by H2O direct formation (OH+H → H2O) or OH disproportionation (OH+OH → H2O+O). For CO2 formation, there is a direct way (CO+O → CO2) and an indirect way (CO+OH → COOH → CO2+H). The corresponding optimized transition states are shown in Figure 6, and the potential energy surfaces are shown in Figures 7 and 8.

Figure 6. Transition states for O (TS7a) and OH (TS8a) hydrogenation, OH disproportionation (TS9a), CO2 (TS10a) and COOH (TS11a) formation Considering OH disproportionation after OH formation, we used a surface model with two co‐adsorbed O atoms and H added step by step for H2O formation. The co‐adsorption configuration of one O atom at the 3F1 site and another one at the 4F1 site is most stable (Figure S18), and the first H atom prefers the 3F1 site (Figure S19). For the co‐adsorbed O and H at both 3F1 sites (reaction 7a, Figure S20), OH formation has barrier of 1.59 eV and is endothermic by 0.45 eV, comparable to those (1.47 eV and 0.57 eV,44 respectively) on Fe(110) and those (1.32 and 0.47 eV,45 respectively) on Fe(100). In the transition state (TS7a, Figure 6), the forming O‐H distance is 1.363 Å and the H atom is located at the B9 site with the H‐Fe distances of 1.802 and 1.845 Å. Finally, the formed OH is at the B1 site. During the reaction, the O atom remains its 4F1 site. Assuming that the 3F1 site is refilled by the second H after OH formation immediately, four possible configurations for co‐adsorbed O+H+OH are considered, and the one of OH the B1 site, O on the 4F1 site and H on the 3F1 site is most stable (Figure S21). Next we computed H2O direct formation (OH+H → H2O). H2O formation on the 4F2 site (reaction 8a, Figure S22) has barrier of 1.17 eV and is endothermic by 0.70 eV, close to that (1.29 and 0.88 eV,45 respectively) on Fe(100) and lower than that (1.96 and ~ 8 ~


Page 9 of 22

1.28 eV,44 respectively) on Fe(110). In the transition state (TS8a, Figure 6), the forming O‐H distance is 1.323 Å; both OH and H interact with only Fe atom with the Fe‐O distance of 2.026 Å and the Fe‐H distance of 1.823 Å. In case of the migration of OH+H+O from the B1+3F1+4F1 sites to B5+4F2+3F1 sites (0.47 eV, Figure S21) before reaction as well as H2O from the T2 to T4 site (–0.11 eV, Figure S23) after reaction, the apparent barrier is 1.64 and the reaction is endothermic by 1.06 eV. H2O formation from OH disproportionation comes from two co‐adsorbed OH groups at the B5 (reaction 9a, Figure S24). The barrier is 0.63 eV and the reaction is endothermic by 0.53 eV, close to those (0.74 and 0.56 eV,45 respectively) on Fe(100) and those (0.78 and 0.38 eV,44 respectively) on Fe(110). In the transition state (TS9a, Figure 6), one OH diffuses to the T2 site as well as the breaking and forming O‐H distance is 1.169 and 1.274 Å, respectively. In the final state, the formed H2O is at the T2 site and the formed O is at the 4F2 site; and the distance between the H of H2O and the O atom is 1.908 Å, indicating hydrogen bond‐ ing. After disproportionation, the migration of the formed O atom to the 3F1 site and the formed H2O to the T1‐2 site releases 0.13 eV (Figure S23). By considering the migration energy (Figure S25), OH disproportionation has apparent barrier of 0.65 eV and is endothermic by 0.42 eV. On the basis of the co‐adsorbed two O atoms at the 3F1 and 4F1 sites as well as two H atoms at the 3F1 site (Figure 7), the en‐ ergy state of 2OS+H2(g) is above 1.30 eV. H2O formation from OH formation and disproportionation has apparent barrier of 1.74 eV and is strongly endothermic by 1.51 eV (1.88 and 1.79 eV including ZPE, respectively). It is noted that oxygen removal via the same pathway has lower apparent barrier and reaction energy on Fe(110) (1.64 and 1.24 eV,44 respectively) as well as on Fe (100) (1.31 and 1.00 eV,45 respectively). In turn, Figure 7 shows that the reverse reaction, H2O dissociation on both clean and O‐pre‐covered surfaces has much low barrier (0.58 and 0.10 eV, respectively) and is very exothermic (0.70 and 0.55 eV, respec‐ tively). By considering the energy state of two surface O atoms and a gaseous H2, however, H2O formation has apparent barrier of 0.44 eV and is endothermic by 0.21 eV. 11.0 10.5

2CO(g)+5H2(g)-2CH4(g) +slab 10.67

2.09 (1.17)

H2O(g)+O 3F1 1.98

1.74 (0.63)

OH2+O T2+4F2 1.64

2.0

1.59 (1.59)

Energy/eV

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


1.5

2OS+H2(g) 3F1+4F1 1.30

OH2+O T2+3F1 1.62 OH2+O T4+3F1 1.51

2OH B5 1.11

2OH B1+B5 1.09

1.0

OH+H+O B5+4F2+3F1 0.92

0.5

OH+O+H B1+4F1+3F1 0.45

0.0

2O+2H 3F1+4F1+3F1+3F1 0.00

Figure 7. Potential energy surface for H2O formation from O hydrogenation on Fe(710) (intrinsic barriers in parenthesis) Next to H2O formation, we computed the competitive CO2 formation, where we used a co‐adsorption model of CO+O+H. For the co‐adsorbed CO+O, CO at the 4F1 site and O at the 4F2 site is most stable (Figure S26); and the H atom used for OH forma‐ ~ 9 ~



Page 10 of 22

tion prefers the 3F1 site (Figure S27). Migration of CO from the 4F1 site to the 4F3 site and O from the 4F2 site to the 3F1 site costs 0.43 eV. The reaction of CO on the 4F3 site and O on the 3F1 site after migration (reaction 10a, Figure S28) has barrier of 1.35 eV and is endothermic by 0.82 eV, lower than those (1.87 and 0.99 eV,49 respectively) on Fe(100) as well as those (1.61 and 1.55 eV,43 respectively) on Fe(110). In the transition state (TS10a), the forming O‐C distance is 1.857 Å and CO has been lifted up from the 4F3 site with only two Fe‐C distances (1.989 and 1.935 Å). The formed CO2 occupies the B6 and 3F1 sites with O‐C‐O angle of 122.58°. CO2 migration to the 4F1 site releases 0.02 eV. Therefore, CO2 formation has apparent barrier of 1.78 eV and is endothermic by 1.23 eV (1.79 and 1.25 eV including ZPE, respectively). In addition, we computed COOH formation from the previously formed OH and the adsorbed CO. Coupling of OH at the B5 site and CO at the 4F1 site (reaction 11, Figure S29) has barrier 1.51 eV and is endothermic by 1.01 eV, lower than those (1.78 and 1.30 eV,49 respectively) on Fe(100) as well as those (2.11 and 1.27 eV,43 respectively) on Fe(110). In the transition state (TS11a, Figure 6), OH is on the T3 site and the forming C‐O distance is 1.795 Å. On the basis of the stable site of COOH and the energy difference (0.94 eV) between CO+OH (4F1+B5) and CO+O+H (4F1+4F2+3F1) (Figure S30), COOH formation has apparent barrier of 2.45 eV and is endothermic by 1.95 eV. Since these values are much larger than those of direct CO2 formation; we did not con‐ sider the transition state of COOH dissociation into CO2+H. The potential energy surface in Figure 8 shows that CO2 direct formation from surface O removal by CO has lower apparent barrier and reaction energy (1.78 and 1.23 eV, respectively) than COOH formation (2.45 and 1.95 eV, respectively). Similarly, CO2 direct formation is also preferred on Fe(100) and Fe(110).43, 49 By considering the energy state of one surface O atom as well one gaseous CO and half gaseous H2, however, CO2 direct formation has negative apparent barrier of 1.06 eV and is exothermic by 1.61 eV. Compared with H2O formation (Figure 7), the formation of CO2 and H2O has close apparent barrier (1.78 vs. 1.74 eV or 1.79 vs. 1.88 eV including ZPE). Since CO direct dissociation has much lower barrier than CO2 direct formation (0.90 vs. 1.78 eV), CO will dissociate directly and H2O formation remains the only pathway to remove surface O atom. On Fe(110), the formation of CO2 and H2O also has close apparent barrier (1.61 vs. 1.64 eV). In addition, CO2 formation is endothermic by 1.55 eV, while CO dissociation has barrier of 1.58 eV and is exothermic by 0.41 eV.43‐44 By considering the thermodynamic equilibrium and the much low barrier of CO2 dissociation, CO dissociation is preferred and surface O removal prefers H2O formation. On Fe(100), sur‐ face O removal also prefers H2O formation over CO2 direct formation on the basis of the apparent barriers (1.31 vs. 1.83 eV, res‐ pectively).45, 49

~ 10 ~


Page 11 of 22 7.5 7.4

2.5

2CO(g)+5/2H2(g)-CH4(g) +slab 7.45 O+CO(g)+1/2H2(g) 4F2 2.45 2.84 (1.51) CO2(g)+H 3F1 2.38

2.0 1.78 (1.35)

Energy/eV

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


CO2+H B6+3F1 1.25

1.5

1.0

COOH 4F1 1.95

CO+OH 4F1+B5 0.94

CO2+H 4F1+3F1 1.23

0.5 O+CO+H 3F1+4F3+3F1 0.43

0.0

O+CO+H 4F2+4F1+3F1 0.0

Figure 8. Potential energy surface for surface O removal by CO on Fe(710) (intrinsic barriers in parenthesis) (c) Surface C stepwise hydrogenation: Since CO prefers direct dissociation and surface O removal prefers H2O formation Fe(710), we computed surface C stepwise hydrogenation after surface O removal. Firstly, we computed the adsorption of the cor‐ responding surface intermediates, i.e.; CH2, CH3 and CH4 (Figure S31‐S33). The adsorption of CH2 prefers the 4F2 site and the ad‐ sorption energy is –4.54 eV, close to that (–4.35 eV14) at the 4F site on Fe(100) and larger than that (–4.28 eV13) on Fe(110). However, it is noted that CH2 on Fe(710) has five adsorption configurations in very close energy (Figure S31), the adsorption con‐ figuration at the 4F3 site (–4.49 eV) is used for the subsequent adsorption and reaction. The adsorption of CH3 prefers the B1 site and the adsorption energy is –2.30 eV, higher than that (–1.81 eV14) at the B site on Fe(100). In contrast, CH4 has negligible ad‐ sorption energy over the T4 site (–0.19 eV), which is higher than that (0.02 eV14) at the T site on Fe(100). For surface C stepwise hydrogenation to CH4, we used a model of surface C at the 4F3 site and H added step by step at the 3F1‐2 site. For surface C at the 4F3 site, the adsorption energy of the first H atom is –0.62 eV (Figure S34). The following CH formation has barrier of 0.82 eV and is endothermic by 0.44 eV (reaction 12, Figure S35), similar with those (0.67 and 0.24 eV,14 respectively) on Fe(100) as well as those (0.95 and 0.13 eV,30 respectively) on Fe(111). In the transition state (TS12, Figure 9), the forming C‐H distance is 1.561 Å; the H atom is above the B7 site with the Fe‐H distance of 1.682 Å. The formed CH is located on the 4F3 site and the C atom interacts with five surface Fe atoms (2.043, 2.049, 2.038, 2.043 and 2.092 Å). Since the 4F3 site is the stable site for CH adsorption (Figure S5), CH formation has apparent barrier of 0.91 eV and is endothermic by 0.53 eV by con‐ sidering the H migration energy from the 3F1‐2 site to 3F1‐1 site (0.09 eV).

Figure 9. Transition states for sequential CHx+H (x= 0, 1, 2, 3) reaction ~ 11 ~



The formation of CH2 from CH at the 4F3 site and H at the 3F1‐1 site has barrier of 0.80 eV and is endothermic by 0.55 eV (reaction 13, Figure S35), close to those (0.79 and 0.66 eV,14 respectively) on Fe(100) as well as those (0.75 and 0.28 eV,30 respectively) on Fe(111). In the transition state (TS13, Figure 9), the forming C‐H distance is 1.408 Å and the H atom is above the B7 site with the Fe‐H distance of 1.640 Å. The formed CH2 group is located on the 4F3 site and the C atom interacts with five Fe atoms (2.149, 2.114, 2.151, 2.118 and 2.142 Å). By considering the H migration energy from the 3F1‐2 site to the 3F1‐1 site (0.09 eV, Figure S36), CH2 formation has apparent barrier of 0.89 eV and is endothermic by 0.64 eV. For the CH2 pre‐adsorbed at the 4F3 site, the configuration with H at 3F1‐2 site is most stable (Figure S37). For CH3 formation, CH2 migrates from the 4F3 site to the 3F1 site, which is close to the 3F1‐1 site. CH3 formation has barrier of 0.78 eV and is slightly endothermic by 0.02 eV (reaction 14a, Figure S38), close to those (0.86 and 0.06 eV,14 respectively) on Fe(100). On Fe(111),30 CH3 formation has barrier of 1.53 and is slightly exothermic by 0.11 eV. In the transition state (TS14a, Figure 9), the forming C‐H distance is 1.599 Å and the H atom is at the 3F2 site and interacts with one Fe atom with the H‐Fe distance of 1.646 Å. By con‐ sidering the CH2 migration energy (0.12 eV), CH3 formation has apparent barrier of 0.90 eV and is endothermic by 0.14 eV. The co‐adsorption of CH3 at the B1 site and H at the 3F1 site is most stable (Figure S39) and the direct formation of CH4 has barrier of 1.21 eV and is endothermic by 0.26 eV (reaction 15a, Figure S40), larger than those (0.90 and 0.16 eV,14 respectively) on Fe(100). On Fe(111), CH4 formation has barrier of 1.15 and is endothermic by 0.87 eV.30 In the transition state (TS15a, Figure 9), CH3 and H share the same Fe atom at the T4 site with the forming C‐H distance of 1.552 Å, the Fe‐C distance of 2.102 Å and the Fe‐H distance of 1.582 Å. The formed CH4 is adsorbed on the T1‐1 site with one H atom oriented to surface. The potential energy surface in Figure 10 shows that C stepwise hydrogenation is endothermic and the transition state (TS15a) of CH4 formation represents the highest point and the rate‐determining step. It should be noted that the barriers for CH, CH2 and CH3 formation are nearly same (0.91, 0.89 and 0.90 eV, respectively). For CH4 formation, the apparent effective barrier is 2.52 eV and the reaction is endothermic by 1.57 eV; higher than those (2.06 and 1.31 eV,14 respectively) on Fe(100). On Fe(111),30 CH3 formation is the rate‐determining step, and the apparent barrier is 1.81 eV and the total reaction is endothermic by 0.91 eV. All these shows that CH4 formation on metallic Fe surfaces is not favored kinetically and thermodynamically. The higher energy level of TS15a than gas CH4 (2.52 vs. 1.76 eV) represents the easy desorption of methane once it forms on surface. By considering the energy state of one surface C atom and two gaseous H2 molecules, however, CH4 formation has apparent barrier of 0.04 eV and is exothermic by 0.91 eV. 6.0 5.5

CO(g)+3H2(g)-H2O(g) +slab 5.75

2.52 (1.21)

2.5

Enegry/eV

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

Page 12 of 22

2.0

CS+2H2(g) 4F3 2.48

2.07 (0.78)

1.42 (0.80)

1.5 0.91 (0.82)

1.0

0.5

0.0

(C+H)+3H (4F3+3F1-1) +3F1-2 0.09

C+4H 4F3+3F1-2 0.00

(CH2+H)+H (3F1+3F1-1) +3F1-2 1.29

(CH+H)+2H (4F3+3F1-1) +3F1-2 0.62

CH2+2H 4F3+3F1-2 1.17

CH3+H B1+3F1 1.31

CH4 T4 1.57

CH+3H 4F3+3F1-2 0.53

~ 12 ~

CH4(g) +slab 1.76


Page 13 of 22 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

The Journal of Physical Chemistry Figure 10. Potential energy surface of the sequential CHx+H (x= 0, 1, 2, 3) reaction (intrinsic barriers in parenthesis) (d) Self‐coupling of CHx (x= 0, 1, 2, 3): Apart CH4 formation from surface C stepwise hydrogenation, surface CHx species can

also couple to form C‐C bonds. Prior to CHx self‐coupling, we computed the adsorption configurations of the corresponding sur‐ face intermediates (Figure S41‐S44). On Fe(710), the carbon dimer CC prefers the combined 4F3 and 3F2 sites with adsorption energy of −9.11 eV, stronger than that (−8.78 eV26) at the 4F site on Fe(100). The adsorption of acetylene (HCCH) prefers the 4F1‐2 site with adsorption energy of −3.03 eV, stronger than that (−2.83 eV26) on Fe(100) in similar adsorption configuration, and that (−2.14 eV30) on Fe(111). The adsorption of ethylene (H2CCH2) molecule prefers the B1 site with one CH2 on the T4 site and the adsorption energy is −1.22 eV, stronger than that (‐0.83 eV26 ) at the 4F site on Fe(100) as well as that (−0.53 eV30) on Fe(111). For ethane (CH3CH3) adsorption, the B1 site is most stable with staggered structure and the adsorption energy is −0.22 eV, stronger than that (0.23 eV26) with C‐C perpendicular to the B site on Fe(100) and that (0.00 eV30) on Fe(111). To study the surface CHx+CHx coupling, we used a model with two surface CHx species and H added step by step. For the co‐ad‐ sorbed C+C, CH+CH and CH2+CH2 (Figure S45‐S47), the configuration of one surface species at the 4F3 site and another one at the 4F2 site is most stable; while the configuration of CH3+CH3 (Figure S48) with one CH3 at the B1 and one CH3 at the B2 site is most stable. Surface H prefers the 3F1 site next nearest to CHx at the 4F3 site (Figure S49). Starting from the stable C+C configuration (4F3+4F2), C+C coupling needs one C at the 3F1 and one C at the 4F3 site and the migration is endothermic by 1.14 eV. The subsequent C+C coupling has barrier of 1.29 and is slightly exothermic by 0.10 eV (reaction 16a, Figure S50). By considering the migration energy, C+C coupling has apparent barrier of 2.43 and is endothermic by 1.04 eV. On Fe(100), C+C coupling has apparent barrier of 2.31 eV and is endothermic by 1.43 eV after migration.26 Without migration,27 C+C coupling has barrier of 2.18 eV and is endothermic by 1.07 eV. In the transition state (TS16a, Figure 11), the C atom at the 3F1 site migrates to the B9 site and the C‐Fe distances are 1.799 and 1.797 Å as well as the forming C‐C distance is 1.906 Å. The formed C2 is located on the combined 4F3 and 3F2 sites (Figure S41).

Figure 11. Transition states for CHx+CHx (x= 0, 1, 2, 3) coupling Starting from the stable CH+CH co‐adsorbed configuration (4F3+4F2), CH+CH coupling needs one CH at the 4F1 site and one CH at the 3F1 site (reaction 17a, Figure S51); and the migration energy is 0.97 eV. The subsequent acetylene formation has barrier of 0.57 and is slightly endothermic by 0.16 eV, close to those (0.54 and ‐0.07 eV30, respectively) on Fe(111) and lower than those (1.44 and 0.76 eV27, respectively) on Fe(100). In the transition state (TS17a, Figure 11), the forming C–C distance is 1.909 Å, the CH, originally at the 3F1 site, migrates to the B1 site with two Fe‐C distances of 1.889 and 1.890 Å; and the Fe‐Fe dis‐ tance at the B1 site is extended from 2.641 Å in initial state to 3.157 Å. In the final state, HCCH is at the 4F1 site with one CH at the B1 site and this configuration is higher in energy than the stable one at the 4F1‐2 site by 0.44 eV (Figure S42). By considering the stable adsorption configurations of the initial and final states, CH+CH coupling has apparent barrier of 1.54 eV and is endo‐ thermic by 0.69 eV, while on Fe(100) surface,26 CH+CH coupling has barrier of 1.89 eV and is endothermic energy 0.64 eV. Starting from the stable CH2+CH2 co‐adsorbed configuration (4F3+4F2), CH2+CH2 coupling needs both CH2 at two nearby 3F1 ~ 13 ~



sites (Figure S47) and the migration energy is 0.22 eV. The following ethylene formation (reaction 18a, Figure S52) has barrier of 0.92 and is exothermic by 0.37 eV, close to those (1.06 and –0.57 eV,30 respectively) on Fe(111) and different from those (1.80 and 0.06 eV,25 respectively) on Fe(100). In the transition state (TS18a, Figure 11), one CH2 is above the 3F2 site and the C atom interacts with two Fe atoms from the B8 site with the Fe‐C distances of 2.046 and 2.155 Å, and another CH2 is on the B1 site and tilted to 3F1 site with the Fe‐C distances of 1.966 and 2.152 Å; and the forming C‐C distance is 1.907 Å. By considering the stable adsorption configurations of the initial and final states, CH2+CH2 coupling has apparent barrier of 1.14 eV and is exothermic by 0.15 eV, and they differ from those (1.94 and 0.53 eV,26 respectively) on Fe(100). Starting from the stable configuration of CH3+CH3 (B1+B2), ethane formation has barrier of 1.84 eV and is slightly exothermic by 0.16 eV (reaction 19, Figure S53), different from those (2.52 and –0.33 eV,25 respectively) on Fe(100) as well as those (0.97 and –0.62 eV,30 respectively) on Fe(111). In the transition state (TS19, Figure 11), both CH3 groups are at the nearby T4 sites and the forming C‐C distance is 1.927 Å. Finally, staggered ethane is formed on the B1 site. On the basis of the two‐co‐adsorbed C atoms and stepwise added H atoms, the potential energy surface for CHx+CHx coupling is shown in Figure 12. For the elementary steps, C+C coupling has the highest apparent barrier and is endothermic (2.43 and 1.04 eV, respectively), followed by CH3+CH3 coupling, which has apparent barrier of 1.84 and is exothermic by 0.16 eV. The most favored reaction is CH2+CH2 coupling, which has apparent barrier of 1.14 and is exothermic by 0.15 eV; and they are lower than that of CH+CH coupling (1.54 and 0.69 eV, respectively). On the basis of the energy level (3.76 eV) of 2C+3H2(g), the effective barrier of ethylene formation is 0.04 eV and the formation of gaseous ethylene is slightly exothermic by 0.03 eV; while ethane formation has effective barrier of 1.22 eV and the formation of gaseous ethane is exothermic by 0.56 eV. Due to its very strong adsorption energy, gaseous acetylene formation is endothermic by 0.97 eV, surface acetylene should be further hydrogenated rather than desorption into gaseous phase. Compared with surface C stepwise hydrogenation (CHx + H = CHx+1), CHx+CHx coupling has always higher apparent barrier, i.e.; 2.43 vs. 0.91 eV for C, 1.54 vs. 0.89 eV for CH, 1.14 vs. 0.90 eV for CH2 and 1.84 vs. 1.21 for CH3. Therefore, CHx+CHx coupling is less favored kinetically than CHx hydrogenation. 11.0 10.5 10.0

2CO(g)+5H2(g)-2H2O(g)+slab 10.14

HCCH(g)+4H 3F1 4.73

5

Energy/eV

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

Page 14 of 22

3.80 (0.92)

4

3

C+C+3H2(g) 4F3+4F2 3.76 2.43 (1.29)

2

1

0

4.98 (1.84)

2.55 (0.57)

(H2C+H2C)+2H (3F1+3F1)+3F1 2.88 H2C+H2C+2H 4F3+4F2+3F1 2.66

(HC+HC)+4H (4F1+3F1)+3F1 1.98

HCCH+4H 4F1-1+3F1 2.14 HCCH+4H

(C+C)+6H (4F3+3F1)+3F1 1.14

C2+6H 4F3++3F1 1.04

H2CCH2(g)+2H 3F1 3.73

H3C+H3C B1+B1 3.14 H2CCH2+2H B1+3F1 2.51

H3CCH3(g) +slab 3.20

H3CCH3 B1 2.98

4F1-2+3F1 1.70

HC+HC+4H 4F3+4F2+3F1 1.01

C+C+6H 4F3+4F2+3F1 0.00

Figure 12. Potential energy surface of CHx+CHx (x = 0, 1, 2, 3) coupling (intrinsic barriers in parenthesis) ~ 14 ~


Page 15 of 22 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

The Journal of Physical Chemistry (e) CO insertion mechanism: Since surface CHx prefers hydrogenation rather than CHx+CHx coupling, we computed CHx+CO

coupling for C‐C bond formation following the CO insertion mechanism. Another reason is that CO can be supplied continuously. Firstly, we computed the adsorption configurations of the corresponding coupling intermediates (Figure S54‐S57). Surface CCO prefers the 4F1+3F2 site and the adsorption energy is −6.19 eV. Surface CHCO prefers the 4F3+3F2 site with CO at the 4F3 site and CH at the 3F2 site; and the adsorption energy is −4.08 eV, higher than that (−3.45 eV30) on Fe(111). For surface ketene (CH2CO), the most stable configuration is at the 4F1 site with CH2 at the T3 site and the adsorption energy is −2.09 eV, higher than that (−1.45 eV30) on Fe(111). Surface CH3CO prefers the 4F1 site and the adsorption energy is ‐2.68 eV, higher than that (−1.89 eV30) on Fe(111). It is noted that the C‐O distance in CHCO, CH2CO and CH3CO is longer than that of CO at the 4F1 site (1.382, 1.385 and 1.392, respectively, vs. 1.328 Å), while the C‐O distance in CCO is shorter than that in CO at the 4F1 site (1.280 vs. 1.328 Å). Next, the dissociation of the C‐O bond in CHxCO was also computed. The corresponding transition states are shown in Figure 13 and the potential energy surface is given in Figure 14.

Figure 13 Transition states for CO insertion into CHx and CHxCO dissociation into CHxC+O (x= 0, 1, 2, 3) On the surface with pre‐covered C at the 4F3 site, CO prefers the 4F1 site with adsorption energy of –2.11 eV (Figure S58), similar with CO adsorption on the clean surface. In case of CCO formation, CO diffusion from the 4F1 site to the 3F1 site costs 0.51 eV. The following CCO formation has barrier of 0.86 eV and is endothermic by 0.42 eV (reaction 20a, Figure S59). In the tran‐ sition state (TS20a, Figure 13), the forming C‐C distance is 1.873 Å and CO migrates to the B8 site with the Fe‐C distances of 1.813 and 2.170 Å. The formed CCO is at the stable site. On the basis of the stable co‐adsorption configuration, CCO formation has barrier of 1.37 eV and is endothermic by 0.93 eV. Next to CCO formation, the subsequent C‐O breaking in CCO (reaction 21, Figure S59) has barrier of 0.93 eV and is exothermic by 1.09 eV. In the transition state (TS21, Figure13), the breaking C‐O dis‐ tance is 1.761 Å and O is moved to the 3F1 site. In the final state, C2 is kept in the 4F3+3F2 site and O is at the 4F1 site (Figure S60). From C+CO to C2+O, the apparent barrier is 1.86 eV and the reaction is exothermic by 0.16 eV. On the surface with pre‐covered CH at the 4F3 site, CO prefers the 4F1 site with adsorption energy of –2.13 eV (Figure S61). For HCCO formation, CO diffusion from the 4F1 site to the 3F1 site costs 0.51 eV (Figure S48). The following HCCO formation has barrier of 0.96 eV and is endothermic by 0.65 eV (reaction 22a, Figure S62), larger than those (0.74 and 0.27 eV,30 respectively) on Fe(111). In the transition state (TS22a, Figure 13), CH titled to surface results in the Fe‐Fe distance elongation in the B3 site from 2.811 to 2.955 Å; CO migrates to the T1 site with the Fe‐C distance of 1.796 Å and the forming C‐C distance is 1.983 Å. In the final state, HCCO locates on at the 4F3‐1 site with CH at the 4F3 site and the C‐O distance is 1.287 Å (Figure S55). Rotating HCCO to the most stable configuration releases energy of 0.30 eV. By considering CO migration from the 4F1 site to the 3F1 site (0.51 eV, Figure S61) and HCCO rotation, CHCO formation has apparent barrier of 1.47 eV and is endothermic by 0.86 eV. Next to ~ 15 ~



Page 16 of 22

CHCO formation, CHCO dissociation at the 4F3 site (reaction 23, Figure S62) has barrier of 0.70 eV and is exothermic by 1.30 eV. In the transition state (TS23, Figure13), the breaking C‐O distance is 1.906 Å, O is moved to the B3 site with the Fe‐O distances of 1.901 and 1.906 Å. In the final state, CHC is kept at the 3F2+4F3 site and O is at the 4F2 site, this configuration is 0.26 eV higher than that of O at the 3F1 site (Figure S63). The total reaction including CHCO formation and dissociation has apparent barrier of 1.56 eV and exothermic by 0.70 eV. On the surface with pre‐covered CH2 at the 4F3 site, CO prefers the 4F1 site with adsorption energy of –2.13 eV (Figure S64). For CH2CO formation, migration of CO and CH2 to the 4F3 and 3F1 sites costs 0.34 eV. The subsequent CH2CO formation has barrier of 0.73 eV and is endothermic by 0.31 eV (reaction 24a, Figure S65), while CH2CO formation on Fe(111) has barrier of 1.35 eV and is slightly endothermic by 0.09 eV.30 In the transition state (TS25a, Figure 13), the forming C‐C distance is 1.947 Å; CO molecule is pulled up and the C terminal interacts with two Fe atoms from the B7 site (1.894 and 1.990 Å). In the finial state, CH2 is at the T1 site with the Fe‐C distance of 2.148 Å and CO is at the 4F3 site; the C‐C distance is 1.485 Å. The migration of CH2CO to the 4F1 site releases 0.18 eV. Therefore, ketene formation has apparent barrier of 1.07 eV and is endothermic by 0.47 eV. Next to CH2CO formation, CH2CO dissociation at the 4F1 site (reaction 25, Figure S65) has barrier of 0.84 eV and is exother‐ mic by 0.64 eV. In the transition state (TS25, Figure13), the breaking C‐O distance is 1.900 Å; O migrates to the B1 site with the Fe‐O distances of 1.874 and 1.876 Å. In the final state, CH2C is at the 4F1 site, C interacts with three surface Fe atoms (1.892, 1.995 and 2.093 Å) and one subsurface Fe atom (2.197 Å), while C from CH2 interacts with Fe at the T3 site with the Fe‐C dis‐ tance of 2.132 Å. The migration of the formed O at the 3F1 site to another 3F1 site releases 0.61 eV (Figure S66). Considering the formation and dissociation of ketene, the total reaction has apparent barrier of 1.31 eV and is exothermic by 0.78 eV. The most stable co‐adsorption configuration of CH3+CO has CH3 at the B1 site and CO at the 4F1 site (Figure S67). For CH3CO formation (reaction 26a, Figure S68), CH3 migrates to the B2 site at first (0.18 eV) and the next CH3CO formation has barrier of 1.10 eV and is endothermic by 0.21 eV, close to those (1.05 and 0.37 eV30, respectively) on Fe(111). In the transition state (TS26a, Figure 13), the forming C‐C distance is 1.948 Å; CH3 migrates to T3 site and the Fe‐C distance is 2.160 Å. The formed CH3CO is at the stable site. Totally, CH3CO formation has apparent barrier of 1.28 eV and is endothermic by 0.39 eV. Next to CH3CO formation, CH3CO dissociation at the 4F1 site (reaction 27, Figure S68) has barrier of 0.71 eV and is exothermic by 0.56 eV. In the transition state (TS27, Figure13), the breaking C‐O distance is 1.981 Å; O is moved to the B1 site with the Fe‐O distances of 1.854 and 1.853 Å; CH3C is at the B2 site with the Fe‐C distance of 1.930 and 1.933 Å. In the final state, CH3C falls into the 4F1 site and O is at the 3F1 site. The stable co‐adsorption configuration of CH3C+O has CH3C at the 4F1 site and O at the next nearest 3F1 site, which is 0.66 eV lower in energy than the finial state (Figure S69). Considering the formation and dissociation of CH3CO, the total reaction has apparent barrier of 1.10 eV and is exothermic by 0.83 eV. On the basis of CO insertion into surface CHX (x = 0, 1, 2, 3) and subsequently dissociation, we plotted the potential energy surface (Figure 14), where the co‐adsorbed C+CO+H is chosen as reference and the energy level of C+CO(g)+1.5H2(g) is on the basis of the adsorption energy of CO at the 4F1 site (Figure S58) and H at the 3F1‐1 site (Figure S9). As shown in Figure 14, all CO insertion steps are endothermic. On Fe(111),30 the formation of CHCO, CH2CO and CH3CO is endothermic by 0.27, 0.09, 0.37 eV, respectively. On Co(0001)11 and Ru(0001),52 the formation of CHCO, CH2CO and CH3CO is endothermic by 0.32, 0.45, and 0.50 eV, respectively, as well as by 0.93, 0.66 and 0.54 eV, respectively. These results show that CHxCO formation is not favored thermodynamically. In addition, the formed ketene is difficult to desorb due to the high desorption energy (−2.09 eV). Next, the competitive dissociation of the C‐C and C‐O bonds can also be seen in Figure 14. The C‐C bond dissociation of CH2CO, CHCO and CCO has apparent barrier of 0.60, 0.61 and 0.44 eV, respectively, and is exothermic by 0.47, 0.86 and 0.93 eV, respec‐ tively; while the C‐O bond dissociation to CH2C, CHC and C2 has apparent barrier of 0.84, 0.70 and 0.93 eV, respectively, and is exothermic by 1.25, 1.56, 1.09 eV, respectively. For CHxCO (x = 0, 1, 2), the C‐C bond dissociation is more favored kinetically, while the C‐O bond dissociation is more favored thermodynamically. For CH3CO, the C‐O bond dissociation is more favored kine‐ ~ 16 ~


Page 17 of 22

tically (0.71 vs. 0.89 eV) and thermodynamically than the C‐C bond dissociation (−1.22 vs. −0.39 eV). Under equilibrium conditions, therefore, C‐O bond dissociation from CHxCO should be favored thermodynamically. 7,2 7,1 4,0 3,8

2CO(g)+5/2H2(g)-H2O(g) +slab 7.18

H2CCO(g)+H 3F1 3.81

CS+CO(g)+1.5H2(g) 4F1 4.16

2.97 (1.10)

2.79 (0.71)

2.56 (0.84)

2,5

2.32 (0.73) H2CCO+1H 4F3-2+3F1 1.90

2.04 (0.96)

Eenrgy/eV

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


2,0 1.86 (0.93)

1,0 (C+CO)+3H (4F3+3F1)+3F1 0.51

0,0

2.13 (0.70)

HCCO+2H 4F3-2+3F1 1.43

1.37 (0.86)

1,5

0,5

HCCO+2H 4F3-1+3F1 1.73

CCO+3H 4F3+3F1 0.93

(H2C+CO)+H (3F1+4F3)+3F1 1.59

(H3C+CO) (B2+4F3) 1.87 H2CCO+H 4F1+3F1 H3C+CO 1.72 B1+4F1 1.69

H2C+CO+H 4F3+4F1+3F1 1.25 CH3C+O 4F1+3F1 0.86

HC+CO+2H 4F3+4F1+3F1 0.57 C+CO+3H 4F3+4F1+3F1 0.00

H3CCO 4F1 2.08

CC+O+3H 4F3+4F1+3F1 −0.16

CHC+O+2H 4F3+3F1+3F1 −0.13

CH2C+O+H 4F1+3F1+3F1 0.47

Figure 14 Potential energy surface of CHX (x=0, 1, 2, 3) coupling with CO on Fe(710) surface (intrinsic barriers in parenthesis) On the basis of the reactions for surface carbon, we compared the apparent barriers and reaction energies of CHx hydrogena‐ tion and CHx+CHx coupling as well as CO insertion into CHx followed by C‐O dissociation step by step (Figure 15). Firstly, surface C hydrogenation to CH is much more favored kinetically (0.97, 1.37 and 2.43 eV, respectively) and thermodynamically (0.53, 0.93 and 1.04 eV, respectively) than C+CO insertion and C+C coupling; and the C‐O dissociation of CCO has high apparent barrier of 1.86 eV. Next, surface CH hydrogenation to CH2 is more favored kinetically (0.89, 1.47 and 1.54 eV, respectively) and thermo‐ dynamically (0.64, 0.69 and 0.86 eV, respectively) than CH+CH coupling and CH+CO insertion; and the C‐O dissociation of CHCO has high apparent barrier of 1.56 eV. The hydrogenation of CH2 to CH3 is more favored kinetically than CH2+CH2 coupling and CH2+CO insertion (0.90, 1.07 and 1.14 eV, respectively, or 0.93, 1.14 and 1.30 eV including ZPE, respectively), but the differences of the apparent barriers become smaller and surface CH2CH2 formation is most favored thermodynamically. In addition, the C‐O dissociation of CH2CO has high apparent barrier of 1.31 eV (1.41 eV including ZPE). Finally, the hydrogenation of CH3 to CH4 and the insertion of CO into CH3 have comparable barriers (1.21 vs. 1.28 eV, or 1.21 vs. 1.33 including ZPE) and reaction energies (0.26 vs. 0.39 eV, or 0.42 vs. 0.51 eV including ZPE). Most importantly, the C‐O dissociation of CH3CO has lower apparent barrier (1.28 eV) and is most exothermic (−0.83 eV). On the basis of the potential energy surface, surface C should be hydrogenated to CH3 at first and the formation of CH4 and the insertion CO into CH3 to CH3CO followed by C‐O dissociation into surface CH3C+O are competitive kinetically, and surface CH3C+O formation is more favored thermodynamically. This is the initial step of C‐C bond formation. This might provide the basis for turning the coverage of surface species or partial pressure of H2 and CO. ~ 17 ~


The Journal of Physical Chemistry 2.5

1.5

2.43

1.54

1.86

1.37 0.91

1.0

0.5

0.0

CC 1.04

CH2 0.64

0.0

CH 0.53 CC+O −0.16

-0.5

CH+H CH+CO CH+CH 0.00

2.0

1.14 1.0

-0.5

1.28 1.10 1.21

1.0

Energy/eV

CH3 0.14 CH2+H CH2+CO CH2+CH2 0.00

1.84

1.5

H2CCO 0.47

0.5

0.0

CHC+O −0.70

1.31

1.07 0.90

HCCO 0.86 HCCH 0.69

0.5

CCO 0.93

C+H C+CO C+C 0.00

0.89

Energy/eV

Energy/eV

1.0

1.5

1.56

1.47

2.0

Energy/eV

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

Page 18 of 22

H2CCH2 −0.15

0.5

H3CCO 0.39

0.0

CH4 0.26

-0.5

CH2C+O −0.78 -1.0

CH3+H CH3+CO CH3+CH3 0.00

-1.0

H3CCH3 −0.16 CH3C+O −0.83

Figure 15. Stepwise comparison of CHx hydrogenation, CHx+CHx coupling and CHx+CO insertion followed by C‐O dissociation on the stepped Fe(710) surface Conclusion Spin‐polarized periodic DFT has been used to model CO activation and dissociation at low coverage, surface carbon diffusion to the octahedral site in subsurface, oxygen removal via the formation of H2O and CO2, stepwise CHx hydrogenation to CHx+1 (x= 0, 1, 2, 3) and CHx+CHx coupling as well as CHx+CO insertion, followed by C‐O dissociation on the stepped Fe(710) surface, compos‐ ed by p(3×3) Fe(100)‐like terrace and p(3×1) Fe(110)‐like step. The goal of this study is the understanding of the initial stages of Fe‐based Fischer‐Tropsch synthesis. It is found that CO prefers the 4F1 site near the step and CO direct dissociation into surface C+O is much more favored kinetically and thermodynamically than H‐assisted HCO and COH formation (Figure 4), and this is similar with that on the Fe(100) surface; while CO direct dissociation is similar to H‐assisted dissociation to CH on the Fe(111) and Fe(110) surfaces. The apparent barrier for CO direct dissociation is similar to the reported value on Fe(710), where the H‐assisted CO dissociation and subsequent reactions for surface oxygen and carbon atoms were not reported. Diffusion of surface carbon at the 4F3 site to subsurface is un‐favored kinetically and thermodynamically, but expectable under high carbon coverage as reported on the Fe(100) and Fe(110) surfaces. Similar with the Fe(100), Fe(110) and Fe(110), surface oxygen hydrogenation O+H → OH at the step is more favored kinetically and thermodynamically than CO2 formation [O+CO → CO2] on Fe(710), H2O formation by OH disproportionation on terrace is the only pathway by considering the most favored CO direct dissociation. In contrast, the dissociation of H2O assisted by O [H2O+O → 2OH] and CO2 [CO2 → O+CO] is highly favored kinetically and thermodynamically than their formation on these four surfaces. Surface C successive hydrogenation towards CH4 formation [C+4H → CH+3H → CH2+2H → CH3+H → CH4] has been computed as well as CHx+CHx coupling and CHx+CO insertion for comparison. In the reactions of C, CH and CH2, hydrogenation has lower apparent barrier than coupling and CO insertion, and the energy differences become small. On Fe(100), hydrogenation of CH and ~ 18 ~


Page 19 of 22 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


CH2 is also easier than the coupling of CH+CH and CH2+CH2. However, 2CH → CHCH has lower barrier than CH hydrogenation and CO insertion on Fe(111) without considering the surface diffusion. No such simulations were done on Fe(110) so far. After the formation of CH3 by consecutive hydrogenation, CH3+CH3 coupling to ethane is inhibited by the highest apparent barrier (1.84 eV). The step site increases the CH4 formation barrier from 0.90 eV on Fe(100) to 1.21 eV on Fe(710), comparable to that (1.28 eV) for CO incorporating into CH3 at the terrace. In addition, strong exothermic of CH3CO dissociation makes the process from CH3+CO to CH3C+O favorable thermodynamically. The formed CH3C is regarded as the RC, which can participate in the next cycle by hydrogenation into RCH2 followed by RCH2CO formation and dissociation for chain growth in Fischer‐Tropsch synthesis. Since CH4 formation and CH3C formation as well as iron carburization have close apparent barriers (1.21, 1.28 and 1.44 eV), the selectivity of products is totally dependent on the concentrations of surface species. To obtain high selectivity for C‐C forma‐ tion and following chain growth, the related coverages of surface species can be modified by tuning the H2/CO ratio in the inlet gas mixture. Acknowledgment: This work was supported by the National Natural Science Foundation of China (no. 21273262&21273266), and the Chinese Academy of Science and Synfuels CHINA. Co., Ltd. We also acknowledge general financial support from the state of Mecklenburg‐Vorpommern. Supporting Information Effects of ZPE corrections on the reaction barriers and reaction energies; all possible configurations of the surface species; configurations of the reacted initial states, transition states and final states for favored and less favored pathways (PDF) The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxx Reference (1) Dry, M. E., Fischer‐Tropsch Synthesis over Iron Catalysts. Catal. Lett. 1990, 7, 241‐251. (2) de Smit, E.; Weckhuysen, B. M., The Renaissance of Iron‐based Fischer‐Tropsch Synthesis: on the Multifaceted Catalyst Deactivation Behaviour. Chem. Soc. Rev. 2008, 37, 2758‐2781. (3) Herbstein, F.; Smuts, J.; Man Niekerk, J., Quantitative Analysis of Fischer‐Tropsch Catalysts by X‐Ray Diffraction. Determination of α‐Iron, Magnetite, and Iron Carbides. Anal. Chem. 1960, 32, 20‐24. (4) de Smit, E.; Cinquini, F.; Beale, A. M.; Safonova, O. V.; van Beek, W.; Sautet, P.; Weckhuysen, B. M., Stability and Reactivity of ϵ−χ−θ Iron Carbide Catalyst Phases in Fischer−Tropsch Synthesis: Controlling μC. J. Am. Chem. Soc. 2010, 132, 14928‐14941. (5) Huo, C.‐F.; Wu, B.‐S.; Gao, P.; Yang, Y.; Li, Y.‐W.; Jiao, H., The Mechanism of Potassium Promoter: Enhancing the Stability of Active Surfaces. Angew. Chem. Int. Ed. 2011, 50, 7403‐7406. (6) Liu, J.‐X.; Su, H.‐Y.; Sun, D.‐P.; Zhang, B.‐Y.; Li, W.‐X., Crystallographic Dependence of CO Activation on Cobalt Catalysts: HCP versus FCC. J. Am. Chem. Soc. 2013, 135, 16284‐16287. (7) Honkala, K.; Hellman, A.; Remediakis, I. N.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Nørskov, J. K., Ammonia Synthesis from First‐Principles Calculations. Science 2005, 307, 555‐558. (8) Erley, W.; McBreen, P. H.; Ibach, H., Evidence for CHx Surface Species after the Hydrogenation of CO over an Fe(110) Single Crystal Surface. J. Catal. 1983, 84, 229‐234. (9) Gonzalez, L.; Miranda, R.; Ferrer, S., A Thermal Desorption Study of the Adsorption of CO on Fe (110); Enhancement of Dissociation by Surface Defects. Surf. Sci. 1982, 119, 61‐70. (10) Lu, X.; Wang, W.; Deng, Z.; Zhu, H.; Wei, S.; Ng, S.‐P.; Guo, W.; Wu, C.‐M. L., Methanol Oxidation on Ru (0001) for Direct Methanol Duel Cells: Analysis of the Competitive Reaction Mechanism. RSC Adv. 2016, 6, 1729‐1737. (11) Liu, S.; Li, Y.‐W.; Wang, J.; Jiao, H., Mechanisms of H‐and OH‐assisted CO Activation as well as C–C Coupling on the Flat Co (0001) Surface‐Revisited. Catal. Sci. Technol. 2016, 6, 8336‐8343. (12) Wilson, J.; de Groot, C., Atomic‐Scale Restructuring in High‐Pressure Catalysis. J. Phys. Chem. 1995, 99, 7860‐7866. ~ 19 ~



Page 20 of 22

(13) 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. (14) 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 2017, 530, 83‐92. (15) Dahl, S.; Logadottir, A.; Egeberg, R. C.; Larsen, J. H.; Chorkendorff, I.; Törnqvist, E.; Nørskov, J. K., Role of Steps in N2 Activation on Ru(0001). Phys. Rev. Lett. 1999, 83, 1814‐1817. (16) Zubkov, T.; Morgan, G. A.; Yates, J. T., Spectroscopic Detection of CO Dissociation on Defect Sites on Ru(109): Implications for Fischer‐Tropsch Catalytic Chemistry. Chem. Phys. Lett. 2002, 362, 181‐184. (17) Li, W.‐Z.; Liu, J.‐X.; Gu, J.; Zhou, W.; Yao, S.‐Y.; Si, R.; Guo, Y.; Su, H.‐Y.; Yan, C.‐H.; Li, W.‐X.; Zhang, Y.‐W.; Ma, D., Chemical Insights into the Design and Development of Face‐Centered Cubic Ruthenium Catalysts for Fischer‐Tropsch Synthesis. J. Am. Chem. Soc. 2017, 139, 2267‐2276. (18) Zhang, S.‐T.; Yan, H.; Wei, M.; Evans, D. G.; Duan, X., Hydrogenation Mechanism of Carbon Dioxide and Carbon Monoxide on Ru(0001) Surface: A Density Functional Theory Study. RSC Adv. 2014, 4, 30241‐30249. (19) Weststrate, C. J.; van Helden, P.; van de Loosdrecht, J.; Niemantsverdriet, J. W., Elementary Steps in Fischer‐Tropsch Synthesis: CO Bond Scission, CO Oxidation and Surface Carbiding on Co(0001). Surf. Sci. 2016, 648, 60‐66. (20) Liu, J.‐X.; Su, H.‐Y.; Li, W.‐X., Structure Sensitivity of CO Methanation on Co(0001), (10‐12) and (11‐20) Surfaces: Density Functional Theory Calculations. Catal. Today 2013, 215, 36‐42. (21) Petersen, M. A.; van den Berg, J.‐A.; Ciobîcă, 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. (22) van Helden, P.; van den Berg, J.‐A.; Ciobica, I. M., Hydrogen‐assisted CO dissociation on the Co(211) Stepped Surface. Catal. Sci. Technol. 2012, 2, 491‐494. (23) Wang, T.; Tian, X.‐X.; Li, Y.‐W.; Wang, J.; Beller, M.; Jiao, H., Coverage‐dependent CO Adsorption and Dissociation Mechanisms on Iron Surfaces from DFT Computations. ACS Catal. 2014, 4, 1991‐2005. (24) Sorescu, D. C., Plane‐wave DFT Investigations of the Adsorption, Diffusion, and Activation of CO on Kinked Fe (710) and Fe (310) Surfaces. J. Phys. Chem. C 2008, 112, 10472‐10489. (25) Nie, X. W.; Wang, H. Z.; Janik, M. J.; Chen, Y. G.; Guo, X. W.; Song, C. S., Mechanistic Insight into C‐C Coupling over Fe‐Cu Bimetallic Catalysts in CO2 Hydrogenation. J. Phys. Chem. C 2017, 121, 13164‐13174. (26) Govender, A.; Curulla‐Ferré, D.; Pérez‐Jigato, M.; Niemantsverdriet, J. H., First‐principles Elucidation of the Surface Chemistry of The C2Hx (x= 0‐6) Adsorbate Series on Fe (100). Molecules 2013, 18, 3806‐3824. (27) Lo, J. M.; Ziegler, T., Theoretical Studies of the Formation and Reactivity of C2 Hydrocarbon Species on the Fe (100) Surface. J. Phys. Chem. C 2007, 111, 13149‐13162. (28) Chakrabarty, A.; Bouhali, O.; Mousseau, N.; Becquart, C. S.; El‐Mellouhi, F., Influence of Surface Vacancy Defects on the Carburisation of Fe 110 Surface by Carbon Monoxide. J. Chem. Phys. 2016, 145, 044710. (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) 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. (31) Kizilkaya, A. C.; Niemantsverdriet, J. W.; Weststrate, C. J., Oxygen Adsorption and Water Formation on Co(0001). J. Phys. Chem. C 2016, 120, 4833‐4842. (32) Liu, S.; Li, Y.‐W.; Wang, J.; Jiao, H., Mechanisms of H2O and CO2 Formation from Surface Oxygen Reduction on Co(0001). J. Phys. Chem. C 2016, 120, 19265‐19270. (33) Kresse, G.; Hafner, J., First‐principles Study of the Adsorption of Atomic H on Ni (111), (100) and (110). Surf. Sci. 2000, 459, 287‐302. (34) Kresse, G.; Furthmüller, J., Efficiency of Ab‐initio Total Energy Calculations for Metals and Semiconductors using a Plane‐wave Basis Set. Comput. Mater. Sci. 1996, 6, 15‐50. (35) 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. ~ 20 ~


Page 21 of 22 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


(36) Blöchl, P. E., Projector Augmented‐wave Method. Phys. Rev. B 1994, 50, 17953‐17979. (37) Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865‐3868. (38) Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1997, 78, 1396‐1396. (39) Methfessel, M.; Paxton, A. T., High‐precision Sampling for Brillouin‐zone Integration in Metals. Phys. Rev. B 1989, 40, 3616‐3621. (40) Jónsson, H.; Mills, G.; Jacobsen, K. W. In Classical and Quantum Dynamics in Condensed Phase Simulations, World Scientific: 2011; pp 385‐404. (41) Henkelman, G.; Uberuaga, B. P.; Jónsson, H., A Climbing Image Nudged Elastic Band Method for finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901‐9904. (42) Wang, T.; Tian, X.; Li, Y.‐W.; Wang, J.; Beller, M.; Jiao, H., High Coverage CO Activation Mechanisms on Fe (100) from Computations. J. Phys. Chem. C 2014, 118, 1095‐1101. (43) Liu, S.; Li, Y.‐W.; Wang, J.; Jiao, H., Reactions of CO, H2O, CO2, and H2 on the Clean and Precovered Fe(110) Surfaces – A DFT Investigation. J. Phys. Chem. C 2015, 119, 28377‐28388. (44) Liu, S.; Tian, X.; Wang, T.; Wen, X.; Li, Y.‐W.; Wang, J.; Jiao, H., Coverage Dependent Water Dissociative Adsorption on Fe(110) from DFT Computation. Phys. Chem. Chem. Phys. 2015, 17, 8811‐8821. (45) Liu, S. L.; Tian, X. X.; Wang, T.; Wen, X. D.; Li, Y. W.; Wang, J. G.; Jiao, H. J., High Coverage Water Aggregation and Dissociation on Fe(100): A Computational Analysis. J. Phys. Chem. C 2014, 118, 26139‐26154. (46) Liu, X.‐W.; Huo, C.‐F.; Li, Y.‐W.; Wang, J.; Jiao, H., Energetics of Carbon deposition on Fe(100) and Fe(110) surfaces and subsurfaces. Surf. Sci. 2012, 606, 733‐739. (47) 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. (48) 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. (49) Liu, S.; Li, Y.‐W.; Wang, J.; Jiao, H., Reaction of CO, H2O, H2 and CO2 on the Clean as well as O, OH and H Precovered Fe(100) and Fe(111) Surfaces. Catal. Sci. Technol. 2017, 7, 427‐440. (50) Sahputra, I. H.; Chakrabarty, A.; Restrepo, O.; Bouhali, O.; Mousseau, N.; Becquart, C. S.; El‐Mellouhi, F., Carbon Adsorption on and Diffusion through the Fe(110) Surface and in Bulk: Developing A New Strategy for the Use of Empirical Potentials in Complex Material Set‐ups. Physica Status Solidi B 2017, 254, 1600408. (51) Jiang, D.; Carter, E. A., Carbon Atom Adsorption on and Diffusion into Fe (110) and Fe (100) from First Principles. Phys. Rev. B 2005, 71, 045402. (52) Chiu, C.‐c.; Genest, A.; Rösch, N., Decomposition of Ethanol Over Ru(0001): A DFT Study. Top. Catal. 2013, 56, 874‐884.

~ 21 ~



Page 22 of 22

TOC Graphic

~ 22 ~


Mechanisms of CO Activation, Surface Oxygen Removal, Surface

Recommend Documents