DFT Study of Hydrogen-Assisted Dissociation of CO by HCO, COH

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A DFT Study of Hydrogen Assisted Dissociation of CO by HCO, COH and HCOH Formation on Fe (100) Sebastian Amaya-Roncancio, Daniel H. Linares, Helio Anderson Duarte, and Karim Sapag J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12014 • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 12, 2016

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The Journal of Physical Chemistry

A DFT Study of Hydrogen Assisted Dissociation of CO by HCO, COH and HCOH Formation on Fe(100) Sebastián Amaya-Roncancio, † Daniel H. Linares,†* Hélio A. Duarte ‡ and Karim Sapag † †

Departamento de Física, Instituto de Física Aplicada, Universidad Nacional de San Luis, CONICET, 5700. San Luis, Argentina. ‡

Department of Chemistry – ICEx, Universidade Federal de Minas Gerais; 31270-901 – Belo Horizonte–MG, Brazil. Email: [email protected] Phone number: +54 (266) 44520300. Int (2320) ABSTRACT DFT calculations using GGA-PBE exchange correlation functional were used to investigate the effect of the hydrogen in the assisted CO dissociation on the Fe(100) surface. The formation mechanisms of the primary products CH, OH, CH2 and H2O involved in the Fischer-Tropsch synthesis have been studied. Three different routes were investigated passing through the HCO, COH or HCOH intermediates. The energy barriers of the reactions were estimated using the nudged elastic band method (NEB). The energy profiles of assisted and double assisted dissociations of CO are presented. The formation energies of HCO, COH and HCOH intermediates are estimated to be endothermic with activation energies of 0.90eV, 1.07eV and 2.13 eV, respectively. The formation of CH2 is energetically more favorable with the global reaction energy estimated to be -1.10 eV. The other CH, OH and H2O intermediates have also endothermic formation energies with respect to the Fe(100)/(CO + H) system. The chemical bonding of the adsorbed intermediates and reactants were analyzed based on the population analysis, electron localization function and pseudo-differential charge density. A comparison with direct CO dissociation leads to the conclusion that hydrogen-assisted processes constitute viable routes for CO dissociation on Fe (100) and alkanes formation. KEYWORDS: Adsorption, assisted dissociation, carbon monoxide, density functional calculations, iron.

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1. INTRODUCTION

The adsorption of carbon monoxide and the coadsorption with hydrogen on the Fe(100) surface is particularly interesting because of its relevance to the Fischer-Tropsch synthesis (FTS).1 FTS has generally been visualized as a sequence of reactions where hydrogen and carbon monoxide are adsorbed dissociatively and subsequently rearranged to form hydrocarbons and water.2 A recent assumption is that both C and O are hydrogenated, yielding the intermediate CH2 (carbene) and H2O, i.e. the carbide mechanism. While H2O desorbs, adsorbed CH2 can undergo polymerization and hydrogenation reactions leading to alkane chains.3 In earlier work, Storch suggested a mechanism in which a hydrogen adatom is added directly to the non-dissociatively adsorbed CO molecule, leading to oxymethylidyne (HCO) species (often referred to as formyl).4 In spite of the interest in CO dissociation on iron catalysts for the chemical industry and the several experimental studies reported on the adsorption and dissociation of CO and H2 on iron surfaces, theoretical calculations are relatively limited.5-9 In relation to Fe-H, the interaction is due to the overlap of the 4s and the H 1s orbitals, with smaller contributions from 4p and 3d Fe orbitals.6,7 Fe-H bonding causes Fe-Fe bond weakening. H atoms on the surface are very mobile with barriers for the diffusion pathways estimated at 0.08 eV.8-10 The mechanism of CO interaction with the metal center has been intensely investigated.11-13 X-ray emission spectroscopy and ab initio calculations applied for investigating the strong metal-CO interaction showed that it is better viewed as a combination of two simultaneous interactions: the first is between the filled σ orbitals of CO and the metal, leading to Pauli repulsion.14-16 The rearrangement of the orbitals leads to a polarization of charge away from the Metal-CO bond, hence minimizing the repulsion. The second involves the 1π and 2π* orbitals of CO and the metal dπ orbitals leading to bonding, nonbonding and antibonding orbitals. Therefore, the metalCO interaction is better described as a mechanism of σ repulsion and π bonding interactions than the classical σ donation and π backdonation mechanism. In the same way, based on x-ray emission spectroscopy and DFT calculations, the CO adsorbed in the hollow sites of Fe(100) is better described in terms of the π-donation/π*-backdonation.17 To gain a better understanding of CO adsorption, Bromfield et al.5 used the PW91 functional and found that, at a coverage of 1/4 monolayer (ML), the tilted 4-fold hollow is the only true local minimum, and by far the preferred adsorption site (Eads = -2.56 eV). In line with the 4-fold configuration, at 1/2 ML of coverage, the on-top site is shown to be a local minimum. At 1 ML, the on-top, bridge, and hollow sites were local minima with the tilted 4-fold configuration as the preferred adsorption site. Therefore, Elahifard et al.18 and Sorescu19 have separately reported similar adsorption geometries and adsorption energies of CO in clean and preadsorbed surfaces. In spite of the evidence from all previous investigations, the role of H2 in CO dissociation and several mechanistic details remain unclear and speculative. For instance, the specific CO dissociation pathways assisted by hydrogen to form CH and OH and their kinetic consequences for chain growth remain largely unresolved.20 In contrast, CO and H2 adsorption on iron surfaces have been studied quite extensively. However, very little information is available for the hydrogen and carbon monoxide coadsorption systems.7 Recently, Amaya-Rocancio et al.21,22 have investigated the effect of the hydrogen in the adsorption and direct dissociation of the carbon monoxide and Carbon hydrogenation on Fe(100) surfaces using DFT/plane wave calculations. Their results indicate that the presence of coadsorbed hydrogen have a stabilization effect in the system, increasing the adsorption energy of CO and favoring its recombination. Nevertheless, the role of hydrogen in


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the assisted dissociation of CO has not been completely elucidated. In the present investigation, we have carried out comprehensive calculations within spin-polarized Density Functional Theory (DFT) to study the assisted dissociation of CO by the following routes: i) the formation of the adsorbed intermediate formyl (HCO) which consequently dissociates into CH + O and finally becomes adsorbed CH2; ii) the formation of hydroxyl-carbene (COH), which results in C + OH, the consequent formation of H2O and the subsequent desorption; iii) the double assisted dissociation of CO by oxymethylene (HCOH) formation, which generates CH + OH products. Our main goal is to understand the role of hydrogen in the assisted dissociation of CO and the initial stages of the formation of essential surface species as CH, CH2 and OH in FTS.

2. COMPUTATIONAL METHODS

All DFT calculations were done using the Quantum Espresso package.23 The electron-ion interaction was described by Ultra-soft pseudopotentials with scalar relativistic correction generated by Rappe-Rabe-Kaxiras-Joannopoulos method (RRKJUS).24,25 The generalized gradient approximation (GGA) for the exchange/correlation density functional Perdew–Burke– Ernzerhof (PBE) was used.26 The chosen energy cutoff was 40 Ry and the threshold for selfconsistency was 1×10−6 eV. Brillouin zone integration was approximated using the Monkhorst– Pack scheme with 4 × 4 × 1 k-point sampling.27 The energy cutoff and k-grid points used in this work are able to calculate the total energy with an error lower than 10-3 Ry in comparison with larger grids and energy cutoffs (see Figures S1 and S2). The calculated lattice parameter of the Fe bulk is 2.850 Å that might be compared to the experimental value of 2.8665 Å28, about 0.0165 Å larger than the calculated value. The total magnetization in the bulk was calculated to be 2.30 µB/Fe-atom which is consistent with the previous reported values of 2.31 µB/Fe-atom7 and 2.20 µB/Fe-atom29 (see Tables S1 and S2). The magnetization of the Fe slab model was self consistently optimized and is not larger than 2.64 µB/Fe-atom as shown in Table S3. One could argue that fixing the magnetization to the bulk value of 2.20 µB/Fe-atom would be a better approach. In fact, it has been shown that the difference in the hydrogen adsorption energy is less than 0.1 eV using such approach. The vacuum between the slabs was set in 10 Å thick along the [100] direction using a p(2x3) supercell with four-metal-layer. The first two layers were fixed at the bulk position and the other two layers were free to relax describing the (100) surface where the species are adsorbed. The adsorption energies of CO and hydrogen in different slab models are shown at Tables S1 and S2, indicating that the p(2x3) supercell with four-metal-layer is enough to describe the adsorption energy with 0.01 eV of accuracy. The hydrogen adsorption energy is converged with four-metal-layer with an error of 0.01 eV. The CO adsorption energy is converged for p(2x3) model with an error of only 0.01 eV in good agreement with CO adsorption energies on Fe(100) performed by Sorescu.19 The geometry relaxation was done using BFGS quasi-Newton algorithm until the forces on each atom were less than 10-5 eV/Å and the energy difference of consecutive steps was less than 10-5 eV. The study of the minimum energy paths was undertaken using the nudged elastic band method (NEB),27,30 and local minima were found through the conjugate gradient CG technique. All the molecular and density plots were made with the XCrySDen package.31


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3. RESULTS AND DISCUSSION

To investigate the crucial steps of the H-assisted dissociation of CO mechanism it is necessary to calculate the adsorption energies of the intermediates in their preferential adsorption sites on the Fe(100) surface, as previously reported by theoretical investigations.5-7,18-21 The adsorption energies (Eads) of the different species (Table 1) were calculated using the following equation: 𝐸𝑎𝑑𝑠 = 𝐸𝑠𝑦𝑠 − (𝐸𝑠𝑢𝑟𝑓 + 𝐸𝑚𝑜𝑙 )

(1),

Esys, Esurf and Emol are the total energy of molecule/slab, the total energy of the slab with preadsorbed hydrogen and the total energy of the molecule at vacuum, respectively.19-21 The pseudo charge density difference Δη was calculated using Equation 2, as proposed by Corso et al.32 𝛥𝜂 = 𝜂𝑠𝑦𝑠 − (𝜂𝑠𝑢𝑟𝑓 + 𝜂𝑚𝑜𝑙 )

(2),

Where η is the charge density and the subscripts sys, surf and mol refer to the molecule/slab, isolated slab and isolated molecule, respectively. Finally, the electron localization function (ELF) is used to describe the topology of electron density in the bindings of HCO, COH and HCOH molecules and their interactions with Fe surface. The ELF is expressed as: −1 𝐷(𝑟⃗) 2

𝐸𝐿𝐹(𝑟⃗) = [1 + (𝐷𝑜 (𝑟⃗)) ]

(3),

Here, 𝐷𝑜 (𝑟⃗) = 3/5(6𝜋 2 )2/3 𝜌5/3 ( 𝑟⃗ ) is the kinetic energy of the uniform electron gas, and 𝐷(𝑟⃗) is the function given by 𝐷(𝑟⃗) = 𝜏(𝑟⃗) − 1/4(𝛻𝜌(𝑟)2 /𝜌(𝑟)) where the quantities 𝜏(𝑟⃗) and 𝜌(𝑟)are the kinetic energy and the total electron density, respectively. Values of ELF equal to 1 are interpreted as a complete localization of electron density and values of 0.5 as uniform electron gas.33, 34

3.1 FORMATION OF INTERMEDIATES HCO, COH AND HCOH

The hydrogen-assisted dissociation of CO leading to the formation of formyl species (HCO), hydroxyl-carbene species (COH) and the intermediate oxymethylene (HCOH) were investigated. The initial state (IS) for the formation of the intermediates HCO and COH was adopted as an adsorbed CO in hollow place with two coadsorbed hydrogens in adjacent hollow sites. The adsorption energies and geometrical parameters are shown in Table 1 and Figure 1.a. The present initial state configuration has been adopted from previous studies regarding the direct dissociation of CO in the presence of hydrogen21,22, and it is in good agreement with the values reported for CO adsorption by Bromfield et al.5 who found an adsorption energy of -2.56 eV for CO in the hollow site. Similarly, Helden and Steen9 presented a model of coadsorption of CO and H in a p(2x2) supercell of Fe(100) and reported an adsorption energy of -2.14 eV for CO in coadsorption of 0.5 ML of hydrogen. Later, Helden and Steen10 studied the effect of carbon and CO preadsorbed in the hydrogen adsorption finding an adsorption energy of -2.49 eV for the CO in a p(2x2) supercell of Fe(100). On the same lines, Sorescu19 reported CO adsorption energies of -2.00 eV in clean Fe(100) surface and -2.69 eV to CO coadsorbed with K. Finally, Wang et


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al.35 studied the influence of several coverages of CO with values ranging from -2.14 eV in clean surface to -0.54 eV in 0.75 ML of CO coverage. For HCO, the transition state (TS) consists of a hydrogen atom in a bridge site approaching the carbon atom of the CO molecule as shown in Figure 1.b. The activation barrier (Eforward) of HCO formation was calculated to be 0.90 eV and the reverse activation energy (Eback) 0.16 eV (Table 2). The reaction energy is endothermic with a value of 0.74 eV. The final state (FS) of HCO formation was found by geometrical optimization and consists of an intermediate HCO adsorbed on two adjacent bridge sites, one bonding the oxygen, and the other bonding the carbon of the HCO molecule (Figure 1.c). The Eads calculated with Equation 1 for HCO with respect to the same molecule at vacuum was -6.49 eV. The distance calculated from carbon to the surface plane was 1.19 Å and the tilt angle with respect to the surface plane was estimated to be 7.2°, see Figure 1.c and Table 1. In comparison with adsorption energy of CO, the Eads of HCO displays higher stabilization on the surface. This reduces the possibility of desorption of the intermediate, and favors its dissociation or formation of long chains bonded to the iron. The values of adsorption energy and geometrical parameters of HCO are in agreement with previous studies carried out by Inderwildi et al.3 presenting activation energies for HCO formation on Co of about 1.31 eV. Similarly, Elahifard et al. 18 reported activation energies of HCO formation of 1.41 eV and Ojeda et al.20 estimated the activation energy for the HCO and COH on Fe(110) in 0.92 eV and 1.61 eV, respectively. The activation energy for the same reactions on Co(0001) was estimated in 1.43 eV and 1.30 eV, respectively. For the hydroxyl-carbene species (COH) formation, the TS is due to the hydrogen promoted towards the oxygen through a top site, whereas the CO molecule changes its tilt with respect to the Fe surface, as can be seen in Figure 1.d. The COH formation has an Eforward of 1.07 eV, higher than the activation barrier of HCO but 0.05 eV lower than the dissociation barrier of CO on a clean surface.6-11, 20-22 The COH formation reaction is strongly endothermic with an Eback of 0.32 eV (see Table 2 and Figure 2). The adsorption geometry of the COH intermediate adsorbed by carbon at hollow site is shown in the Figure 1.e. The distance between the carbon and the iron surface is 0.83 Å with an angle of 86.50° with respect to the surface plane. The C-O bound distance is predicted to be 1.42 Å, see Table 1 and Figure 1.d. The adsorption energy of COH in comparison with the same molecule at vacuum was calculated in -6.21 eV. The values presented show, as in the case of formyl, the high stability of the intermediates COH adsorbed on the surface (see Table 1). The pathway calculated for HCOH formation has its initial state with a COH adsorbed in a hollow site with a hydrogen coadsorbed in a neighbor hollow site as the final state of COH (Figure 1.e). The Transition state is reached when the hydrogen is promoted towards the HCO, passing through a bridge site (Figure3.a and 3.b). The hydrogen atom bonds to the carbon forming the HCOH. The final state is reached when the HCOH formed is rearranged and adsorbed on two bridge sites, one bonding the oxygen, and the adjacent bonding the carbon of the HCOH molecule, as shown in Figure 3.c. In this case, the adsorption energy with respect to the same molecule at vacuum was -4.04 eV. The distance of the C-O bond was calculated in 1.51 Å and the distance of C to the surface plane was calculated in 1.29 Å. Finally the tilt angle with respect to the horizontal plane was 10.9° (Table1). The HCOH formation from COH has an activation barrier of 1.38 eV and a dissociation barrier of 0.53 eV. By comparing these energies, it is possible to note the remarkable endothermicity of the HCOH formation, as shown in Table 2 and Figure 2. The pseudo-differential electronic density (Δη) was calculated using equation 2 with the aim of providing some insights about the nature of the bond along the reaction path. An accumulation of electronic density (η) around the bond generated by the interaction of Fe-CH and Fe-O, and a depletion of η around the HC-O bond are shown for HCO intermediate (Figure


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4.a). The accumulation of electron density from Fe through transfer of η to stabilize the interaction with HCO indicates that a covalent bond was formed. Additionally, the depletion of electron density in the HC-O bond favors the dissociation assisted by H through the HCO intermediate. Taking these details into consideration, the Mulliken population analysis shows that Fe transfers charge mostly to the oxygen of the HCO molecule and to its hydrogen. The presence of the hydrogen in this molecule increases the charge on the C and Fe centers (Table 3). The Δη calculated for COH (Figure 4.b) shows accumulation lobes of electron density around the carbon, oxygen and hydrogen. As in the previous cases, the Fe atoms near the carbon generate a covalent interaction where the electron density is accumulated in the vicinity of the Fe-C bond. As in the previous case, the bond carbon-hydroxyl shows a depletion of η, making the dissociation via COH formation possible. Similarly, the calculation of Mulliken population indicates that the hydrogen bonded to the oxygen takes charge from the CO and Fe, and that the covalent interaction of the Fe-C bond slightly favors the Fe surface (Table 3). The adsorption energy, adsorption geometry, and activation barriers of hydroxyl-carbene are in accordance with Elahifard et al.18 who compared the direct versus the assisted dissociation of CO in a p(2x2) supercell and found that for that system size, the assisted dissociation was exothermic. Along the same lines, Blyholder and Lawless36 reported activation energies of 1.39 eV and 2.14 eV for the formation of HCO and COH in a small cluster of 2 layers of Fe(100). Even when their reported values are based on small slab models and low coverage of hydrogen, the behavior of the adsorption energies and assisted dissociations agrees with the results of our present work. Finally, Δη calculated for HCOH shows a high depletion of η in the bonding of the HC-OH. At the same time, an accumulation of η on the Fe surface is observed (see Figure 4.c).The Mulliken atomic population shows a charge transfer from the carbon to the hydrogen bonded to it and to the hydrogen bonded to the oxygen as well. In contrast, the slight reception of charge from the surface shows the covalent character of the molecule-surface bonding (Table 3). Electron Localization Function (ELF) was also calculated for the HCO, COH and HCOH species adsorbed, enabling a better understanding of the bonding formation. Figure 5 shows the calculation along the transversal plane of each molecule. In the three cases, a homogeneous electron gas around each Fe atom is observed on the surface, this behavior is similar to the reactive carbon vacancies on the Mo2C catalyst.37 For the case of the molecules adsorbed, a high localization of electron density is observed for the hydrogen. In the same sense, the ELF reveals a localization of density along the C-Fe bound and as a lone pair of electrons around the oxygen (Figure 5. a-c). The shape observed is consistent with the pseudo-differential electron density (Figure 4. a-c). For HCO, ELF = 1 is present in CH bonding, which displays its covalent nature (Figure 5. a). The same behavior is observed for the OH of the COH molecule as shown in Figure 5.b. In HCOH, the hydrogen atoms have a significant localization of electron density around them (Figure 5.c). In contrast to the carbon there is no localization of charge between the oxygen and the Fe surface, indicating that it does not bond to the surface in spite of its short distance. This indicates that HCOH interacts with the Fe surface through the carbon atom.

3.2 DISSOCIATION OF HCO, COH AND HCOH INTERMEDIATES

The dissociation of HCO intermediate was calculated with the configuration shown in Figure 6.a taken as the initial state. The HCO dissociation has an activation barrier of 0.76 eV, which is 0.36 eV lower than the direct dissociation of CO, see Table 2. The reaction of dissociation of


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HCO presents an exothermic nature and its recombination barrier was estimated at 2.02 eV (see Table 2). The transition state of the dissociation of HCO consists of a CH and O passing through the bridge sites. Finally, both molecules are adsorbed in hollow places as shown in Figure 6.b and 6.c. The values calculated are consistent with those reported by Elahifard et al.18, where values of 0.64 eV in the dissociation and 2.36 eV in the recombination were reported for a (2x2) supercell in a clean surface of iron. In the same line, Zhao et al.38 reported a dissociation barrier of 0.62 eV and a recombination barrier of 1.60 eV on a (2x2) supercell of Fe(100). From the previous configuration (Figure 6.c), a second transition state can be calculated to obtain the CH2 formation. The CH reacts with the coadsorbed hydrogen that is promoted by the bridge site (Figure 7.a and 7.b). For the CH2 formation, the calculated activation barrier was 0.18 eV and the reverse barrier was 0.76 eV (Table 2 and Figure 2). The final state of CH 2 is adsorbed in hollow sites, as can be seen in Figure7.c. The same mechanism on (2x2) supercell model of Cu(110) surface was investigated by Zhang et al.39 They estimated the activation barriers for the forward and reverse reactions in 0.28 eV and 0.61 eV, respectively. Additionally, the transition and final state geometries of CH2 are in agreement with a previous work carried out on iron clean surfaces by Sorescu40, who reported an activation barrier of 0.64 eV for CH2 formation and a reverse barrier of 0.42 eV. For COH, the dissociation is carried out in two steps: first, the dissociation of COH producing C + OH. Here, the carbon remains standing on a hollow place, whereas the OH adsorbs in a bridge site (Figure 8.a and 8.b). The activation barrier for this step is 0.36 eV, lower than the CO and HCO dissociations. The recombination barrier was 0.64 eV (see Table 2 and Figure 2), in good agreement with Elahifard et al.18 who reported the values of 0.37 eV for dissociation and 0.57 eV for recombination. The second step is the H2O formation, in which the OH adsorbed in the bridge site bonds to the neighbour adsorbed hydrogen in the hollow site. Finally, the H2O formed is desorbed from the Fe surface (Figure 8.c). The activation barrier, in this case, was estimated to be 0.18 eV while the reverse barrier was 0.32 eV. It is exothermic and shows the H2O as a viable pathway for the rapid elimination of oxygen in FT synthesis, see Table 2 and Figure 2. The calculated geometries involved in the process of H2O formation are in good agreement with calculations performed by Zhang et al.39 They reported activation barriers of 0.43 eV and 0.75 eV for the forward and backward H2O formation on a (2x2) supercell model of Cu(110) surface, considering the same mechanism of COH dissociation. In the same line, our geometries are in agreement with those reported by Jung et al.41 and Freitas et al.42 The formation of H2O following the same mechanism on clean Fe(100) surface has a forward activation barrier of 1.15 eV and a reverse activation barrier of 0.35 eV.42 The HCOH dissociation through the breaking of the HC-OH bond was also investigated. The OH remains adsorbed in the bridge and the CH is moved to a hollow adsorption site (Figure 9). The HCOH dissociation was exothermic with an activation barrier of 0.16 eV and a backward barrier of 1.71 eV (Table 2 and Figure 2).The HCOH dissociation was predicted to be exothermic on clean Fe(110) surfaces by Ojeda et al.20 who calculated a dissociation barrier of 0.65 eV and a reverse barrier of 0.92 eV. Zhang et al.39 investigated the same reaction on Cu(100) surface estimating forward and backward activation barriers of 0.99 and 1.40 eV, respectively.


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4. FINAL REMARKS

The present work provides important insights about the mechanism of the FTS reactions on Fe(100). Three different plausible mechanisms were investigated indicating that the formation of COH and HCO is energetically similar with formation energies of 0.75 and 0.74 eV, respectively. The energy barriers were estimated to be 1.07 for COH and 0.90 eV for HCO. The HCOH formation from the COH intermediate is not favorable with an estimated energy barrier of 1.38 eV and global reaction energy of 1.60 eV. The channel based on the HCO dissociation and CH2 formation is the most energetically favorable, leading to the global reaction energy of – 1.10 eV. The COH dissociation leading to the H2O has global reaction energy of –0.03 eV, as shown in Figure 2. The barrier energy of 0.36 eV is smaller than the channel leading to the CH2 formation. Both channels are accessible under the experimental conditions (high pressure and temperature). The double H-assisted channel with the HCOH formation from COH intermediate leading to OH and CH species is unfavorable with global reaction energy of 0.05 eV and energy barrier of 1.38 eV. The present study indicates that the alkane chain growth involves the carbene formation on the metal surface, as observed experimentally43. Previous studies about the CO dissociation in the presence of hydrogen using the same level of theory estimated the activation energy of 1.16 eV that is larger than the barriers for the CH2 formation21. The ELF and the pseudo differential electronic density were calculated aiming to provide information about the nature of the chemical bonding of the intermediates on the surface. The chemical bonding analysis revealed that the adsorption of the intermediates occurs mostly through the carbon atoms, forming a Fe-C bond with large covalent character.

Associated Content Electronic Supporting Information available: CO and hydrogen adsorption energies calculated with different slab models, total energy calculations using different energy cutoffs and k-grid points and optimization of the lattice parameter of Fe bulk. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information


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Corresponding Author * D. H. Linares: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors would also like to thank members of GAECI for their services and help. This work was financially supported by UNSL, ANPCyT and CONICET (Argentina), and CNPq (Brazil). The ACQUA-INCT (Brazil) has also supported this work (http://www.acqua-inct.org).

5. REFERENCES

1 Rofer-DePoorter, C. K.A Comprehensive Mechanism for the Fischer-Tropsch Synthesis. Chem. Rev. 1981, 81 (5), 447–474. 2 Gong, X. Q.; Raval, R.; Hu, P. CO Dissociation and O Removal on Co(0001): a Density Functional Theory Study. Surf. Sci. 2004, 562, 247-256. 3 Inderwildi, O. R.; Jenkins, S. J.; King, D. A. Fischer-Tropsch Mechanism Revisited: Alternative Pathways for the Production of Higher Hydrocarbons from Synthesis Gas. J. Phys. Chem. C. 2008, 112, 1305-1307. 4 Schulz, H. Short History and Present Trends of Fischer–Tropsch Synthesis. Appl. Catal, A. 1999, 186, 3-12. 5 Bromfield, T.C.; Ferro, D.C.; Niemantsverdriet, J.W. A DFT Study of the Adsorption and Dissociation of CO on Fe (100): Influence of Surface Coverage on the Nature of Accessible Adsorption States. ChemPhysChem. 2005, 6, 254-260. 6 Juan, A.; Hoffmann, R. Hydrogen on the Fe(110) Surface and Near Bulk BCC Fe Vacancies A Comparative Bonding Study. Surf. Sci. 1999, 421, 1–16. 7 Jiang, D. E.; Carter, E. A. Adsorption and Diffusion Energetics of Hydrogen atoms on Fe(110) From First Principles. Surf. Sci. 2003, 547, 85–98. 8 D.C. Sorescu. First Principles Calculations of the Adsorption and Diffusion of Hydrogen on Fe(100) Surface and in the Bulk. Catal. Today. 2005, 105, 44-65.


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9 Helden, P. V.; Steen, E. V. Coadsorption of CO and H on Fe(100) J. Phys. Chem. C. 2008, 112, 16505–16513. 10 Helden, P. V.; Steen, E. V. A DFT Study of Hydrogen Dissociation on CO- and C-Precovered Fe(100) Surfaces. J. Phys. Chem. C. 2010, 114, 5932–5940. 11 Blyholder, G. Molecular Orbital View of Chemisorbed Carbon Monoxide. J. Phys. Chem. 1964, 68, 2772-2777. 12 Aizawa, H.; Tsuneyuki, S. First-principles study of CO Bonding to Pt(111): Validity of the Blyholder model. Surf. Sci. 1999, 399, L364-L370. 13 Sung, S. S.; Hoffmann, R. How Carbon Monoxide Bonds to Metal Surfaces. J. Am. Chem. Soc. 1985, 107, 578-583. 14 Föhlisch, A.; Nyberg, M.; Bennich, P.; Triguero, L.; Hasselström, J.; Karis, O.; Nilsson, A. The Bonding of CO to Metal Surfaces. J. Chem. Phys. 2000, 112, 1946-1957. 15 Föhlisch, A.; Nyberg, M.; Bennich, P.; Triguero, L.; Hasselström, J.; Karis, O.; Nilsson, A. How Carbon Monoxide Adsorbs in Different Sites. Phys. Rev. Lett. 2000, 85, 3309-3312. 16 Nyberg, M.; Föhlisch, A.; Triguero, L.; Bassan, A.; Nilsson, A.; Pettersson, L. G. M. Bonding In Metal–Carbonyls:A Comparison With Experiment And Calculations On Adsorbed CO. J. Mol. Struct.: THEOCHEM. 2006, 762, 123-132. 17 Gladh, J.; Öberg, H.; Li, J.; Ljungberg, M. P.; Matsuda, A.; Ogasawara, H.; Nilsson, A.; Pettersson, L. G. M.; Öström, H. X-ray Emission Spectroscopy and Density Functional Study of CO/Fe(100). J. Chem. Phys. 2012, 136, 034702-10. 18 Elahifard, M. R.; Jigato, M. P.; Niemantsverdriet, J. W. Direct versus Hydrogen-Assisted CO Dissociation on the Fe(100) Surface: a DFT Study. ChemPhysChem. 2012, 13, 89-91 19 Sorescu, D. C. Adsorption and Activation of CO Coadsorbed with K on Fe(100) Surface: A Plane-Wave DFT Study. Surf. Sci. 2011, 605, 401-414. 20 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. 21 Amaya-Roncancio, S.; Linares, D. H.; Sapag, K.; Rojas. M. I. Influence of Coadsorbed H in CO Dissociation and CHn Formation on Fe(100): A DFT study. Appl. Surf. Sci.2015, 346, 438– 442. 22 Amaya-Roncancio, S.; Linares, D. H.; Duarte, H. A.; Lener G.; Sapag, K. Effect of Hydrogenin Adsorption and Direct Dissociation of CO on Fe(100) Surface: A DFT Study. Am. J. Anal. Chem. 2015, 6, 38-46. 23 Giannozzi, P. et al. QUANTUM ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter. 2009, 21, 1-19. 24 Corso, A. D. www.quantum-espresso.org. http://www.quantum-espresso.org/wpcontent/uploads/upf_files/Fe.pbe-nd-rrkjus.UPF.March 1, 2014.


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25 H.pbe-nd-rrkjus.UPF, O.pbe-nd-rrkjus.UPF http://www.quantum.espresso.org. March 1, 2014.

and

C.pbe-nd-rrkjus.UPF.

26 Payne, M.C.; Teter, M.P.; Allan, C.; Arias, T.A.; Joannopoulos, J. D. Iterative Minimization Techniques for ab-Initio Total-Energy Calculations: Molecular Dynamics and Conjugate Gradients. Rev. Mod. Phys. 1992, 64, 1045-1097. 27 Henkelman, G.; Uberuaga, B.P.; Jonsson, H. A. Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys.2000, 113, 99019904. 28 Kohlhaas, R.; Dunner, P.; Schmitz, P. N. Z. Angew.Phys. 1967, 23, 245. See also www.webelements.com. 29 Tiago, M. L.; Zhou, Y.; Alemany, M. G.; Saad, Y.; Chelikowsky, J. R. Evolution of Magnetism in Iron from the Atom to the Bulk. Phys. Rev. Lett. 2006, 97, 147201-4. 30 Borthwick, D.; Fiorin, V.; Jenkins, S. J.; King, D. A. Facile Dissociation of CO on Fe {211}: Evidence from Microcalorimetry and First-Principles Theory. Surf. Sci. 2008, 602, 2325-2332. 31 Kokalj, A. Computer Graphics and Graphical User Interfaces as Tools in Simulations of Matter at the Atomic Scale.Comput. Mater. Sci. 2003, 28, 155-168. 32 Kokalj, A.; Dal Corso, A.; de Gironcoli, S.; Baroni, S. The Interaction of Ethylene with Perfect and Defective Ag(001) Surfaces. J. Phys. Chem. B. 2002, 106, 9839-9846. 33 Becke, A. D.; Edgecombe, K. E. A Simple Measure of Electron Localization in Atomic and Molecular-Systems. J.Chem. Phys. 1990, 92, 5397−5403. 34 Savin, A.; Jepsen, O.; Flad, J.; Andersen, O. K.; Preuss, H.; von Schnering, H. G. Electron Localization in Solid-State Structures of the Elements - the Diamond Structure. Angew.Chem. Int. Ed. 1992, 31, 187−188. 35 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. J. Phys. Chem. C.2014, 118, 1095-1101. 36 Blyholder, G.; Lawless, M. Hydrogen-assisted Dissociation of Carbon Monoxide on a Catalyst Surface. Langmuir. 1991, 7, 140-141. 37 de Oliveira, C.; Salahub, D. R.; de Abreu, H. A.; Duarte, H. A. Native Defects in α-Mo2C: Insights from First-Principles Calculations. J. Phys. Chem. C. 2014, 118, 25517−25524. 38 Zhao, Y.; Li, S.; Sun, Y. CO Dissociation Mechanism on Cu-Doped Fe(100) Surfaces. J. Phys. Chem. C. 2013, 117, 24920−24931. 39 Zhang, R.; Sun, X.; Wang, B. Insight into the Preference Mechanism of CHx (x = 1−3) and C−CChain Formation Involved in C2 Oxygenate Formation from Syngason the Cu(110) Surface. J. Phys. Chem. C. 2013, 117, 6594−6606.


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40 Sorescu, D. C. First-Principles Calculations Of The Adsorption and Hydrogenation Reactions Of CHx (X = 0, 4) Species On A Fe(100) Surface. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 155420-17. 41 Jung, S. C.; Kang, M. H. Adsorption Of A Water Molecule On Fe(100): Density-Functional Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 115460-7. 42 Freitas, R. R.; Rivelino, R.; de BritoMota, F.; de Castilho, C. M. Dissociative Adsorption and Aggregation of Water on the Fe(100) Surface: A DFT Study. J. Phys. Chem. C. 2012, 116, 20306-20314. 43 Burtron, H. Davis. Fischer–Tropsch Synthesis: Current Mechanism and Futuristic Needs. Fuel Process. Technol. 2001, 71, 157–166.


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Figure 1. HCO and COH formation on Fe(100): a) Initial state in HCO and COH formation; b) Transition state of HCO formation; c) Final state of HCO formation; d) Transition State of COH formation; e) Final state of COH formation. Fe in Yellow, C in Gray, O in Red, and H in Cyan.


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Figure 2. Energy profile of assisted and double assisted dissociation of CO. solid line COH and HCOH pathway; long dashed line HCO pathway; short dashed line H2O formation.

Figure 3. Formation of HCOH intermediate, a) promotion of hydrogen towards COH intermediate, b) HCOH formation, c) HCOH stabilization on Fe(100).


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Figure 4. Pseudo-differential electronic density (Δη) of: a) HCO adsorbed in Fe, b) COH adsorbed in Fe and c) HCOH adsorbed in Fe. Red lobules represent accumulation of electronic density; blue lobules represent depletion of electronic density; value of isosurface: 0.004eV/Å3.

Figure 5. Electron localization function of species on Fe(100): a) HCO, b) COH and c) HCOH.

Figure 6. HCO dissociation on Fe(100): a) HCO adsorbed as the initial state, b) HC-O transition state, c) CH coadsorbed with oxygen as final state.

Figure 7. CH2 formation on Fe(100): a) CH adsorbed in hollow place as initial state, , b) hydrogen promoting through bridge site as transition state, , c) CH2 formation adsorbed in Hollow place as final state.


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Figure 8. HCO dissociation on Fe(100): a) COH approaching to the hydrogen coadsorbed, b) OH dissociated adsorbed on a bridge site, c) OH bonding hydrogen to form H2O.

Figure 9. Dissociation of HCOH intermediate: a) HCOH stabilization on the Fe surface, b) dissociation of HCOH on the Fe surface, c) adsorption of CH in hollow and OH in bridge site.

Table 1. Adsorption energy (Eads), distance of CO to Fe surface, distance of CO bond and inclination angle of CO on the Fe surface of involved species (CO, HCO, COH and HCOH). Molecule

Eads (ev)

Distance OCsurface (Å)

C-O bond distance (Å)

CO-surface Angle (°)

CO(a)

-2.00

--

1.33

46.6

CO + 2H(b)

-2.26

0.64

1.30

45.8

HCO

-6.49

1.19

1.39

7.2

COH

-6.21

0.83

1.42

86.5

HCOH

-4.04

1.29

1.51

10.9

(a) Reported value of CO adsorption on a p(2×2) supercell of clean surface of Fe (100) by Soresco.19 (b) Reported value of CO adsorption on a p(2×3) supercell of Fe (100) with hydrogen preadsorbed by Amaya-Roncancio et al.21,22


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Table 2. Reaction barriers of direct and hydrogen assisted dissociation of CO on Fe (100). Eforw(eV)

Eback(eV)

ΔE

CO ↔ C + O

1.12

2.28

-1.16

CO + H ↔ HCO

0.90

0.16

0.74

HCO ↔ CH + O

0.76

2.02

-1.26

CH + H ↔ CH2

0.18

0.76

-0.58

CO + H ↔ COH

1.07

0.32

0.75

COH + H ↔ C+OH

0.36

0.64

-0.28

OH + H ↔ H2O

0.18

0.32

-0.50

1.38

0.53

0.85

0.16

1.71

-1.55

Reaction Direct*

H-assisted by H-CO bond

H-assisted by CO-H bond

COH+H ↔ HCOH Double H-assisted dissociation of CO

HCOH ↔ OH +CH

* Reported values of direct dissociation of CO on (2×2) supercell of Fe (100) by Elahifard et al.18


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Table 3. Electric charge of adsorbed atoms and catalyst obtained from Mulliken analysis System Atoms

HCO/Fe Charge (a.u.)

C

0.116

0.121

0.189

O

-0.315

0.048

-0.066

H (bond to C)

-0.201

--

-0.215

H (bond to O)

--

-0.128

-0.110

H (surface)

-0.039

-0.007

--

Surface

0.439

-0.034

-0.202

COH/Fe Charge (a.u.)

HCOH/Fe Charge (a.u.)


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Figure 1. HCO and COH formation on Fe(100): a) Initial state in HCO and COH formation; b) Transition state of HCO formation; c) Final state of HCO formation; d) Transition State of COH formation; e) Final state of COH formation. Fe in Yellow, C in Gray, O in Red, and H in Cyan. 530x216mm (72 x 72 DPI)



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Figure 2. Energy profile of assisted and double assisted dissociation of CO. solid line COH and HCOH pathway; long dashed line HCO pathway; short dashed line H2O formation. 347x279mm (72 x 72 DPI)


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Figure 3. Formation of HCOH intermediate, a) promotion of hydrogen towards COH intermediate, b) HCOH formation, c) HCOH stabilization on Fe(100). 446x77mm (72 x 72 DPI)



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Figure 4. Pseudo-differential electronic density (∆η) of: a) HCO adsorbed in Fe, b) COH adsorbed in Fe and c) HCOH adsorbed in Fe. Red lobules represent accumulation of electronic density; blue lobules represent depletion of electronic density; value of isosurface: 0.004eV/Å3. 327x75mm (72 x 72 DPI)


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Figure 5. Electron localization function of species on Fe(100): a) HCO, b) COH and c) HCOH. 443x105mm (72 x 72 DPI)



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Figure 6. HCO dissociation on Fe(100): a) HCO adsorbed as the initial state, b) HC-O transition state, c) CH coadsorbed with oxygen as final state. 445x62mm (72 x 72 DPI)


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Figure 7. CH2 formation on Fe(100): a) CH adsorbed in hollow place as initial state, , b) hydrogen promoting through bridge site as transition state, , c) CH2 formation adsorbed in Hollow place as final state. 496x105mm (72 x 72 DPI)



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Figure 8. HCO dissociation on Fe(100): a) COH approaching to the hydrogen coadsorbed, b) OH dissociated adsorbed on a bridge site, c) OH bonding hydrogen to form H2O. 484x80mm (72 x 72 DPI)


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Figure 9. Dissociation of HCOH intermediate: a) HCOH stabilization on the Fe surface, b) dissociation of HCOH on the Fe surface, c) adsorption of CH in hollow and OH in bridge site. 521x72mm (72 x 72 DPI)



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TOC 484x287mm (72 x 72 DPI)


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DFT Study of Hydrogen-Assisted Dissociation of CO by HCO, COH

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