CO Dissociation Mechanism on Pd-Doped Fe(100): Comparison with

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CO Dissociation Mechanism on Pd-Doped Fe(100): A Comparison with Cu/Fe(100) Wei Wang, Ye Wang, and Guichang Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00903 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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The Journal of Physical Chemistry

CO Dissociation Mechanism on Pd-Doped Fe(100): A Comparison with Cu/Fe(100) Wei Wang 1, Ye Wang 2, and Gui-Chang Wang*,1,3 (1Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and Collaborative Innovation Center of Chemical Science and Engineering(Tianjin),Nankai University, Tianjin 300071, P. R. China;

2

State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering

Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China; 3 State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P. R. China)

*Corresponding author: Gui-Chang Wang. Telephone: +86-22-23503824 (O)

E-mail: [email protected]

Fax: +86-22-23502458

1

ACS Paragon Plus Environment


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Abstract Spin-polarized density functional theory computations have been used to investigate the CO dissociation mechanisms and the different catalytic activities of the reaction on Fe(100) surfaces with different Pd coverages. CO can dissociate on Pd/Fe surfaces via three different mechanisms: direct and H-assisted mechanisms via HCO intermediate or COH intermediate. In our calculation, it was found that the activation barriers of direct CO and COH dissociation mechanisms on pure and Pd-doped Fe(100) surfaces were higher than that of HCO dissociation mechanism. Besides, energy barriers for the identical reaction pathway on Fe-rich Fe(100) surfaces were lower than those on Pd-rich Fe(100) surfaces, namely CO dissociation mainly occurs via the HCO intermediate pathway and the catalytic activity becomes lower with Pd coverage increasing toward CO dissociation in both direct CO and H-assisted CO dissociation mechanisms. As a result, CO dissociation mainly occurs on Fe-rich Pd/Fe surfaces leading to the formation of CHx and Pd-rich Pd/Fe surfaces can stabilize CO which may afford the high selectivity to oxygenate. The bimetallic catalysts will provide two different active sites that are synergetic for the formation of higher alcohols. Moreover, the difference between Pd-doped and Cu-doped Fe(100) systems was compared and analyzed based on the d-band model, and it was found that the d-band width of Cu/Fe(100) was more narrow compared to that of Pd/Fe(100), this was agreement with the calculation results that the energy barrier for C-O bond scission on Cu/Fe(100) was lower than that on Pd/Fe(100). We predicted that methane content decreases and methanol content increases with Pd coverage increases on Pd/Fe(100), and the selectivity of methanol on Pd/Fe(100) is higher than that on Cu/Fe(100). Importantly, a typical “ volcano curve” between ethanol synthesis and HCO dissociation barrier was gained, in which the selectivity for the ethanol synthesis is highest on Fe2Cu2/Fe(100) system among these studied bimetallic model catalysts due to its moderate catalytic activity for HCO dissociation.

2


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1. Introduction The concerns about shortage of petroleum products and the high crude oil price of the last few decades call for the promising fuel substitutes. Large efforts have been made to develop environmentally friendly routes.1 Fischer–Tropsch synthesis (FTS) converts synthesis gas (a mixture of CO and H2) into long-chain alkanes is a prospective and feasible route for the production of chemicals and liquid fuels from coal or natural gas.2-7 Higher alcohol synthesis (HAS) is an important FT-type process, and a fundamental understanding of the reaction is of utmost importance for developing active catalysts which have high selectivity of C2+ alcohols. The use of promoters is crucial for improving ethanol selectivity. Thus, bimetallic catalysts such as Rh–Fe,8 Cu-Fe,9 and Cu-Co10-12 show increased CO conversion rates and selectivity to C2-oxygenated chemicals. At present, Cu-modified FT catalysts were seen as prospective HAS catalyst.12-15 The active Fe, Co and Ru species is regarded as active catalysts which could dissociate CO to produce active hydrocarbon (CHx, x = 1−3) and long-chain alkanes,16-18 whereas the active Pd, Pt and Cu sites provide physisorption CO which was devoted to produce higher alcohols by inserting into the CxHy species and hydrogenation. Surface hydrocarbon (CHx, x =1−3) formation, growth of carbon chain, and CO insertion are also crucial pathways for higher alcohol formation, as suggested by Gupta et al.19 Previous experimental and theoretical studies proposed that the HAS reaction mechanism consists of a range of elementary pathways initiated by CO dissociation. CO adsorption and dissociation is of utmost importance for forming CHx species to grow hydrocarbon chains which improves the conversion of synthesis gas to ethanol. CO adsorption and dissociation on transition metals and alloys have been conducted by experimental studies.13,

20-25

CO adsorption and

dissociation on Ru, Fe, Co, Ni25-29 have been studied by theory and some investigations have also been carried out on different overlayer and bimetallic alloy surfaces such as Pt/Ru,28, 30 Cu/Fe31 and Al/Fe32. Theoretically, numerous researches on the mechanisms of CO decomposition on metals have been examined by systematic density functional theory DFT calculations (DFT). An alternative mechanism is H-assisted CO dissociation mechanism, Ojeda and co-workers proposed that the H-assisted CO dissociation was the main CO dissociation mechanism on Fe and Co catalysts kinetically.33 It has been suggested that the oxygenate synthesis process is initiated by H-assisted CO dissociation, in which CO is first hydrogenated to produce CHxO species,34 followed by C−O cleavage, resulting in the formation of CHx species. Rh-based catalysts have been promising 3



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catalysts in adsorbing and dissociating CO, the high cost and limited supply restricts the ability to be applied in industrial, so we choose Pd/Fe as catalyst. In our present work, we investigated the adsorption and dissociation of CO on Fe(100) surfaces with different Pd coverages. The most favorable CO dissociation pathway, the effects of Pd doping on the Fe catalysts, and the validity of the dual site mechanism were evaluated based in this study. Although FTS has been investigated on main bimetallic system such as Cu-Fe(Co), very little information on Pd/Fe system. In fact, Pd is an efficient catalyst for methanol synthesis from syngas as compared to that of metallic copper,35 and it can be expected that the Pd/Fe catalysts would show different catalytic behave with the pure and Cu-doped Fe catalysts, so it is worthy to study the CO dissociation, an initial step for FTS, on Pd/Fe(100) bimetallic model catalyst.

2. Calculation methods and models Methods: Self-consistent periodical DFT calculations were carried out to investigate the adsorption energy of each species, the reaction energy and the corresponding activation energy of each elementary step in the CO dissociation with the Vienna ab initio simulation package (VASP)36-37. The interaction between ionic cores and electrons was described by the projector-augmented wave (PAW)38-39 method. The generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE)40 functional was used to calculate the exchange-correlation energy. The kinetic cutoff energy was set at 400 eV to represent the electronic wave functions. Brillouin zone was sampled using a 3 × 3 × 1 Monkhorst−Pack k-point mesh.41 We considered spin polarization when we treat Fe due to the magnetic nature of Fe. The transition states (TSs) of the CO dissociation were located by three steps to determine the minimum-energy paths and calculate the energy barriers of each elementary react of CO dissociation: the general NEB42 method was employed to find an approximated TS, then the quasi-Newton algorithm was used to optimize the likely TS until the force acting on the atom is smaller than 0.03eVÅ-1, and last the frequency analysis was carried out to confirm the TS. The activation barrier (Ea) was defined as the total energy difference between the transition state (TS) and initial state (IS). The activation barrier (Ea) was calculated on the basis of the following expression:

Ea  E TS   E  IS  . The reaction energy was calculated as E  E  FS   E  IS  ,  E refers to the total energy gap between the final state (FS) and initial state (IS). The adsorption energy Eads was 4


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calculated using the formula of Eads  Eadsorbate / substrate   Eadsorbate  Esubstrate  . In this definition, Eadsorbate / substrate refers to the total energy of adsorbate–catalyst system, Eadsorbate and Esubstrate refer to

the energies of adsorbate species and free substrate, respectively. The more negative the Eads, the stronger the adsorption is. The d-band center model43-44 was applied to analyze the electronic structure of pure Fe(100), Pd-doped Fe(100). In the model, the whole d-band center was calculated by equation: 

 dc

  

 

E  d ( E ) dE



 d ( E ) dE

where ρd is the density of states (DOS) of the d-band of surface atoms at E (the ccupied d-band center calculation is shown in Supporting information). To calculate PDOS, the fine k-points of 7x7x1 were used. The detailed discussion of calculation parameters such as density functional, k-points, and cutoff energy can be found in the supporting information (Table S5, 6, 7 and 8). Models: A five-layer symmetric periodic slab was used to model the Fe(100) surface of which the uppermost three layers and the adsorbates were allowed to relax, while the bottom two layers of the slab were kept fixed at the bulk positions. The vacuum region of 15 Å between slabs was employed to simulate the surface to avoid interactions between the periodically repeating slabs using the p(22) surface unit cell. The Pd-doped Fe(100) bimetallic surface slab was modeled by partial or full replacement of surface layer of Fe atoms with Pd atoms in order to investigate the catalysis of different bimetallic catalyst surfaces for CO dissociation. Accordingly, a Fe−Pd bimetallic catalyst is generated with different surface Pd coverage as can be seen in Figure 1 and Figure S1. Replacing one, two, three or four Fe atoms on the surface of the slab with Pd atoms creating Pd/Fe surfaces with surface Pd coverage of 1/4, 2/4, 3/4 or 1 monolayer (ML), respectively. Our optimized lattice constant for Fe in the unit cell volume of Pd-doped Fe(100) is 2.866 Å, in good agreement with previous theoretical45 and experimental values.46

5



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Figure 1.The structure of pure Fe(100) and Pd-doped Fe(100) catalysts

3. Results 3.1 Stability of Pd-Doped Fe(100) models The stabilities of the surface structures were characterized by their surface energy, in the case of both the single-crystal Fe surface and bimetallic surfaces. The stabilities of the structures were compared on the basis of their surface energy ( E surf ), E surf is calculated according to Equation(1)47

Esurf 

bulk bulk E slab  N Pd EPd  N Fe EFe 2A

in which

E

surf

(1)

is the relaxed surface energy,

E

slab

is the total energy per unit cell of the slab,

bulk bulk EPd and EFe are the total energy of a bulk Pd atom and the total energy of a bulk Fe atom,

respectively. N Pd and

N

are the number of Pd and Fe atoms in the slab, respectively. A is the

Fe

surface area of one side of the slab. The lower the surface energy, the more stable the surface is. The surface energies for the Pd/Fe surfaces with the Pd atoms doping in the surface layer are always positive, 0.67, 0.60, 0.55, 0.51 and 0.46 J/m2 for Pd surface coverages of 0, 1/4, 2/4, 3/4, and 1 ML, respectively shown in Figure 1. Lower surface energy of the Pd-doped Fe(100) surface means that the Pd-doped Fe(100) surfaces are slightly more stable than the pure Fe(100) surface and the Pd is stable at the surface. It should be pointed that the present analysis based on the 0K DFT results, and the real reaction condition is usually at high temperature, so more detail analysis based on molecular dynamics (MD) simulation will be given on our future study. 3.2 Adsorption properties of possible species The adsorption configuration and energies of the various species (C,CO, COH, and HCO) on the different sites of Pd/Fe surfaces are shown in Figure2 and Figure S2 and the remaining O, H, CH, and OH species is shown in Figure S3, the key bonding parameters and the adsorption energies are listed in Table S1 and Table1.

Table 1. Adsorption Energies (Eads, eV) of the Various Adsorbates on Pd/Fe Surfaces

Species

Fe4/Fe(100) Site Eads

Fe3Pd1/Fe(100) Site Eads

Fe2Pd2/Fe(100) Site Eads 6


Fe1Pd3/Fe(100) Site Eads

Pd4/Fe(100) Site Eads

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C

4-hollow

-8.33

4-hollow

-7.91

4-hollow

-7.53

4-hollow

-7.19

4-hollow

-6.76

O

4-hollow

-6.71

4-hollow

-6.36

4-hollow

-5.97

4-hollow

-5.29

4-hollow

-4.09

H

4-hollow

-2.61

4-hollow

-2.63

4-hollow

-2.56

4-hollow

-2.56

4-hollow

-2.45

CH

4-hollow

-7.09

4-hollow

-6.78

4-hollow

-6.45

4-hollow

-6.09

4-hollow

-5.64

OH

4-hollow

-3.76

bridge

-3.69

bridge

-3.58

top

-3.40

4-hollow

-2.77

CO

4-hollow

top

-1.20

4-hollow

-0.97

HCO

4-hollow

t-b-tc

-1.96

bridge

-1.78

COH

4-hollow

4-hollow

-3.32

α

α

-1.94

α

4-hollow -1.62

4-hollow

-2.84

b-b-bb

-2.60

b-b-bb

-4.26

4-hollow

-3.93

4-hollow

α

α

-1.23 -2.29 -3.62 b

4-hollow

-3.45

Note: Both C and O atoms are bonded to the surface in this adsorption structure. b-b-b stands bridge-bridge-bridge. t-b-t stands c

top-bridge-top

We choose the most stable structures by calculating the adsorption energy of reactants. On Pd/Fe surfaces, C, O, H, and CH species take preferably place over the 4-fold hollow site with four metal atoms. The most favorable adsorption site of CO, OH, COH and HCO is slightly different for different Pd/Fe surfaces.

Figure 2. Optimized adsorption configuration of each species in the CO dissociation on Pd-doped Fe(100) surface 7



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The calculation results show that C, O, H, CH, and COH bind preferably to the 4-fold hollow for pure and Pd-doped Fe(100) surfaces. As for OH, on Fe4/Fe(100) and Pd-doped Fe(100) surfaces, the 4-fold hollow site of four metal surface atoms is the main adsorption site for OH species. But for Fe2Pd2/Fe(100), OH binds to the surface Fe atoms in the bridge mode. For CO, the most favorable adsorption site for the Fe4/Fe(100), Fe3Pd1/Fe(100) and Fe2Pd2/Fe(100) is the 4-fold hollow site with the C atom located at the center of the site and the O atom interacting with two metal atoms, as shown in Figure 2. For Fe4/Fe(100), CO is favored to interact with the surface through C and O. As for Fe1Pd3/Fe(100), CO absorbs on the top site through C. For Pd4/Fe(100), CO adsorbs at the 4-fold hollow site with C-O perpendicular to the surface. For HCO, on the Fe4/Fe(100) surface, HCO is favored at the 4-fold hollow site with the C atom located near the center of the site, and the O atom binds to the bridge sites, and the axis of C–O is almost parallel to the surface. For Fe3Pd1/Fe(100), Fe2Pd2/Fe(100) and Pd4/Fe(100), HCO is favored at the 4-fold hollow site with C adsorbed at the bridge site and O bonded to the bridge site. While on Fe1Pd3/Fe(100) surface, HCO prefers to adsorb at the bridge site, C in HCO species bestrides the top site of Fe atom while O atom points to the top site of a Pd atom on surface. Finally, from Table 1, the adsorption strength of all the above species except for H on the Pd/Fe surfaces becomes weaker when Pd coverage increases. For H, the adsorption energies differ very slightly for the Fe4/Fe(100), Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), and Fe1Pd3/Fe(100) surfaces (-2.61, -2.63, -2.56, and -2.56 eV, respectively), which are more negative than that on the Pd4/Fe(100) surface of −2.45 eV. The adsorption energy of CO on the Pd4/Fe(100) surface of −0.97 eV is much less negative than our calculated adsorption energy of CO on the Fe4/Fe(100) surface of −1.94 eV which is similar to the previous report (-2.05 eV)45. This shows the strong effect of surface Pd doping of the Fe(100) surface on CO adsorption. It is well-known that the binding strength of the adsorbates can be well measured by d-band center (See Table S3). The d-band center of Fe4/Fe(100) is −0.86 eV closer to the Fermi level energy than that for Fe3Pd1/Fe(100)(−1.03eV), Fe2Pd2/Fe(100)(-1.43 eV), Fe1Pd3/Fe(100)(-1.61 eV) and Pd4/Fe(100)(-1.82 eV) indicating that is more active than Pd-doped Fe(100) surfaces and interacts stronger with adsorbates. This is consistent with the fact that adsorption energies of adsorbates get lower with Pd coverage increasing. It is concluded that the surface structures can greatly alter adsorption of the adsorbates. Furthermore, the effect of 8


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parameters such as K-points, cutoff energy, and density functional on adsorption energies of the above species is discussed in Table S5, S6, S7, S8, and S9. 3.3 CO dissociation on Pd-dope Fe(100) surface 3.3.1 Direct CO dissociation The side view and the top view of structures in the initial states, transition states, and final states for direct CO dissociation on Pd/Fe surfaces are shown in Figure 3 and Figure S5. The energy barriers and reaction energies for CO dissociation are shown in Table 2. The potential energy profiles are shown in Figure 5. For the Fe4/Fe(100) surface, in the IS, CO is favored at the most stable site with the C-O of 1.32 Å. In the TS, C adsorbs at the 4-fold hollow site and O binds to the bridge site between two Fe atoms and the distance between C atom and the dropped oxygen atom is 1.93 Å. In the FS, the C still adsorbs at the 4-fold hollow site and dropped O tends to migrate to the neighboring 4-fold hollow site. The reaction energy of direct CO dissociation on the Fe4/Fe(100) surface is predicted to be -0.41 eV and the reaction barrier is 1.13 eV, in accordance with those reported in literatures of −0.44 and 1.11eV reported by Zhao et al.29 For Pd-doped Fe(100) surface, in the IS, on the Fe3Pd1/Fe(100) and Fe2Pd2/Fe(100) surfaces, CO species is favored at the 4-fold hollow site with C binding to the center site and O binding to the bridge site, and the C-O axis is not perpendicular to the surface. For Fe1Pd3/Fe(100), CO is initially located at the top site of a Fe atom. For Pd4/Fe(100), CO adsorbs at the 4-fold hollow site with C-O axis vertical to the surface. The distances of C-O are 1.30, 1.28, 1.17 and 1.20 Å for Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100), and Pd4/Fe(100). The predicted C−O lengths are shorter on the Pd-rich Pd/Fe surfaces than on the Fe-rich Pd/Fe surfaces, suggesting that CO is less activated on the Pd-rich surfaces. In the TS, for the Fe3Pd1/Fe(100), Fe2Pd2/Fe(100) and Pd4/Fe(100) surfaces, C adsorbs at the 4-fold hollow site and O binds with two surface metals. (d(C-O)=1.92, 1.92, and 2.23 Å). But for the Fe1Pd3/Fe(100) surface, C locates at the 4-fold hollow site and O locates at the top site of a Fe atom (d(C-O)=2.01). In the FS, C locates at the 4-fold hollow site and breaking O orientates to adsorb at the neighboring 4-fold hollow site for all Pd/Fe surfaces. The direct CO dissociation barriers are1.33, 1.42, 2.85 and 3.58 eV and endothermic by 0.17, 0.90, 0.68 and 2.94 eV for Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100), and Pd4/Fe(100), respectively. In general, the energy barriers for direct CO dissociation get higher significantly as the Pd 9



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surface coverage increases, from 1.13eV on the Fe4/Fe(100) surface to 1.33 eV on Fe3Pd1/Fe(100), 1.42 eV on Fe2Pd2/Fe(100), 2.85 eV on Fe1Pd3/Fe(100), 3.58 eV on Pd4/Fe(100). When the Pd surface coverage increases from 0 ML to 1ML, the reaction energy for direct CO dissociation becomes more positive and higher, from −0.41 eV on the Fe4/Fe(100) surface, to 0.17 eV on Fe3Pd1/Fe(100), 0.90 eV on Fe2Pd2/Fe(100), 0.68 eV on Fe1Pd3/Fe(100), 2.94 eV on Pd4/Fe(100). The C-O in CO species is almost vertical to the surface contributing to the higher barrier of these results to break the C-O bond. In the real reaction conditions, the catalyst may be covered with H species when the hydrogen pressure is relatively high, so we study direct CO dissociation on pure Fe(100) and Fe(100) surfaces with different Pd coverage in the presence of H species. The calculated results of direct CO dissociation barriers were 1.08, 1.35, 1.43, 3.02, and 3.50 eV on Fe4/Fe(100), Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100) and Pd4/Fe(100) at the H coverage of 1/4 ML, which were similar to the calculation results when no hydrogen is present. So the hydrogen pressure has a little effect on the direct CO dissociation on Fe(100), which is agreement well with the early theoretical results.29, 48 Table 2. Energy Barriers (Ea, eV) and Reaction Energies (ΔE, eV) for Direct and H-Assisted CO Activations on Pd/Fe Surfaces

Steps

Fe4/Fe(100) Ea ΔE

Fe3Pd1/Fe(100) Fe2Pd2/Fe(100) Fe1Pd3/Fe(100) Ea Ea Ea ΔE ΔE ΔE

Pd4/Fe(100) Ea ΔE

CO→C+O

1.13

-0.41

1.33

0.17

1.42

0.90

2.85

0.68

3.58

2.94

CO+H→HCO

0.77

0.56

0.75

0.41

0.65

0.17

1.09

0.98

1.11

0.53

HCO→CH+O

0.63

-0.92

0.81

-0.37

0.95

0.42

1.61

0.11

2.95

2.19

CO+H→COH

1.80

0.99

1.85

0.93

1.52

0.70

1.90

1.16

1.87

0.85

COH→C+OH

0.21

-1.46

0.38

-1.29

0.71

-0.31

0.72

-0.47

1.62

0.84

10


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Figure 3. Structures of the stationary states for direct CO dissociations on Pd/Fe surfaces

3.3.2 H-assisted CO dissociation Recent studies suggest the formyl species (HCO) to be crucial in synthesis from syngas.23, 33, 49-51

In the H-assisted CO dissociation mechanisms, CO is hydrogenated to HCO or COH, followed

by C-O breaking to form CH+O or C+OH. The side view and top view of structures in the initial states, transition states, and final states for H-assisted CO dissociation on Pd/Fe surfaces are shown in Figure 4 and Figure S6. The potential energy profiles are shown in Figure 5 with the energy barriers and reaction energies listed in Table 2.

11



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Figure 4.Structures of the stationary states for H-assisted CO dissociations of Pd/Fe surfaces (a) CO + H → HCO → CH + O and (b) CO+H → COH → C+OH. 12


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Figure 5. Potential energy profiles for direct (dash dot lines), HCO dissociation mechanism (solid lines) and COH dissociation mechanism (short dot lines) on Pd/Fe surfaces.

a) HCO dissociation mechanism CO+H→HCO For the Fe4/Fe(100) surface, in the IS, CO adsorbs at the 4-fold hollow site with C binding at the center site and O binding at the bridge site of two Fe atoms, and the C-O axis is not perpendicular to the surface, H adsorbs at the neighboring 4-fold hollow site. In the TS, CO adsorbs at the 4-fold hollow site with C binding at 4-fold hollow site, and O binds to the bridge site of two Fe atoms. H adsorbs at the bridge site of two Fe atoms. The C-H length is 1.51 Å which is much shorter than that of 2.55 Å in the initial state. The configuration of the final state is similar to that of the transition state with an even shorter C−H bond distance of 1.16 Å. The hydrogenation of CO also causes the C−O bond to elongate, from 1.30 Å in the initial state, to 1.32 Å in the transition state, 13



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1.38 Å in the final state. HCO formation overcomes activation energy of 0.77 eV and accepts an energy input of 0.56 eV. The calculated reaction energy and energy barrier for HCO formation agree well with the reported values of 0.59 and 0.78eV by Zhao et al.45 On the Pd-doped Fe (100) surface, the reaction is similar to the pure Fe(100) surface. In the IS, for the Fe3Pd1/Fe(100) and Fe2Pd2/Fe(100) surfaces, CO binds to the surface with O bonded to the bridge site of metal atoms. H adsorbs at the neighboring 4-fold hollow site. For the Fe1Pd3/Fe(100) surface, CO adsorbs at the top site of a Fe atom through C and H is located at the neighboring 4-fold hollow site, and the C-O axis is tilted away from the surface. For the Pd4/Fe(100) surface, CO connects to the surface with C-O axis almost perpendicular to the surface, H adsorbs at the neighboring 4-fold hollow site. In the TS, for the Fe3Pd1/Fe(100) and Fe2Pd2/Fe(100) surfaces, the transition state consists of a hydrogen atom in a bridge site approaching the carbon atom of the CO molecule, CO adsorbs at the 4-fold hollow site with O bonded with two surface Fe atoms. For Fe1Pd3/Fe(100), HCO binds to the surface with C locating at the top site of a Fe atom. For the Pd4/Fe(100) surface, CO binds to the surface with C-O axis tilted away from the surface, and the H atom moves to the bridge site of these two 4-fold hollow sites. For Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100), and Pd4/Fe(100) surfaces, the C–H distances are shortened to 1.47, 1.46, 1.14 and 1.43 Å from 2.78, 2.95, 2.70 and 2.98 Å. HCO formation are all endothermic on Pd/Fe surfaces. The reaction energies are 0.41, 0.17, 0.98 and 0.53 eV for Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100), and Pd4/Fe(100), respectively, and the calculated barriers of the C-H formation are 0.75, 0.65, 1.09 and 1.11 eV. The C−O bond stretches upon CO hydrogenation (1.28, 1.31, and 1.36 Å for the initial state, the transition state, and the final state on Fe3Pd1/Fe(100), 1.25, 1.31, and 1.35 Å for those on Fe2Pd2/Fe(100), 1.18, 1.23, and 1.25 Å for those on Fe1Pd3/Fe(100), and 1.19, 1.20, and 1.25 Å for those on Pd4/Fe(100)). Thus, CO hydrogenation into HCO leads to the increase of C-O distance making C−O weaker, whose cleavage can be expected to be easier than the direct CO dissociation. From Table 2, the activity barriers of the Fe4/Fe(100), Fe3Pd1/Fe(100) and Fe2Pd2/Fe(100) surfaces are 0.77, 0.75, and 0.65 eV, respectively, lower than that on Fe1Pd3/Fe(100) and the Pd4/Fe(100) surface of 1.09 and 1.11eV. HCO formation barrier increases generally with Pd coverage increasing, namely HCO is formed more easily on Fe-rich Fe(100) surface than Pd-rich Fe(100) surface. 14


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HCO → CH+O The cleavage of the C−O in HCO yields the CH and O species, effectively bringing about CO dissociation. For Fe4/Fe(100) surface, HCO is initially adsorbed at the 4-fold hollow site with C adsorbed at the center and O locating at the bridge site of two Fe atoms. In the TS, the CH remains located at the 4-fold hollow site and the O tends to migrate to the bridge site between two Fe atoms, the cleaving C–O is elongated to 1.85 Å from 1.38 Å in HCO. In the FS, CH is favored at the 4-fold hollow site, and the dissociated O adsorbs at the neighboring 4-fold hollow site, and the distance between CH and O is 2.83 Å. The barrier of C-O cleavage is computed to be 0.63 eV and its reaction energy is -0.92 eV which is approximate to the reported values of 0.62 and -0.97 eV by Zhao et al.45 Considering Pd-doped Fe(100) surfaces, in the IS, for the Fe3Pd1/Fe(100), Fe2Pd2/Fe(100) and Pd4/Fe(100) surfaces, HCO binds to the surface via C locating at the bridge metal site and O locating at the bridge site of two metal atoms. For the Fe1Pd3/Fe(100) surface, HCO binds to the surface with C deposited at the bridge site of two Pd atoms and O locating at the top site of a Fe atom. In the TS, for Fe3Pd1/Fe(100), Fe2Pd2/Fe(100) and Pd4/Fe(100) surfaces, the CH remains adsorbed at the 4-fold hollow site, the detaching O radical approaches to the bridge site of two metal atoms. But for Fe1Pd3/Fe(100) surface, the CH remains adsorbed at the 4-fold hollow site, the dissociated O connects to one surface Fe atom. The lengths of C–O are stretched to 1.85, 1.85, 1.95 and 2.28 Å, respectively which are much longer than those in the adsorbed HCO species 1.36, 1.35, 1.31 and 1.25 Å on the Fe3Pd1/Fe(100),Fe2Pd2/Fe(100), Fe1Pd3/Fe(100), and Pd4/Fe(100) surfaces. In the FS, CH is deposited at the 4-fold hollow site, and the dissociated O adsorbs at the neighboring 4-fold hollow site, and the C–Odistancesare2.70, 2.47, 3.50 and 2.87 Å for Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100) and Pd4/Fe(100), respectively. The process has reaction energies of -0.37, 0.42, 0.11, 2.19 eV and its calculated barriers of the C-O scission are 0.81, 0.95, 1.61, and 2.95 eV for Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100) and Pd4/Fe(100), respectively. In general, on the Pd-doped Fe(100) surfaces, the configuration of the transition states and final states for HCO dissociation into CH and O are similar to those on the Fe4/Fe(100) surface. The activity barriers of the Fe4/Fe(100), Fe3Pd1/Fe(100), and Fe2Pd2/Fe(100) surfaces for this step are very similar, 0.66, 0.81, and 0.95 eV, respectively, lower than that on Fe1Pd3/Fe(100) and Pd4/Fe(100) surfaces of 1.61 and 2.95 eV. HCO decomposition into CH and O needs to overcome higher energy barriers on the Pd-rich Pd/Fe surfaces than on the Fe-rich Pd/Fe surfaces, which indicates that Pd can 15



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play a role in stabilizing the HCO species on the Fe(100) surface. b) COH dissociation mechanism CO+H→COH CO can also be hydrogenated to COH, but the activation energy is higher than HCO formation. For the pure Fe(100) surface, starting from the co-adsorption structure of CO and H, CO binds to the surface with C deposited at the center site and O adsorbed at the bridge site of two Fe atoms, and H adsorbs at the adjacent 4-fold hollow site. In the TS, H is almost sitting at bridge site of two Fe atoms, the C–O bond is tilted away from the surface in order to coordinate H atom with O-H of 1.32 Å shorten from 2.56 Å with an activation barrier of 1.80eV, this reaction is endothermic by 0.99 eV. Considering Pd-doped Fe(100) surfaces, in the IS, for the Fe3Pd1/Fe(100) and Fe2Pd2/Fe(100) surfaces, CO is located at the 4-fold hollow site with C adsorbed nearly at the center and O binding to the bridge site of two Fe atoms, H adsorbs at neighboring 4-fold hollow site. For Fe1Pd3/Fe(100), CO locates onto the top site of a Fe atom, and H locates at the 4-fold hollow site. For Pd4/Fe(100), CO and H are deposited at the neighboring 4-fold hollow sites with C-O bond perpendicular to the surface. In the TS, for Fe3Pd1/Fe(100), Fe2Pd2/Fe(100) and Pd4/Fe(100), H is almost sitting at bridge site of two surface metal atoms and the C-O bond is tilted away from the surface to coordinate H atom. For Fe1Pd3/Fe(100), CO adsorbs at 4-fold hollow site with CO axis vertical to surface. The O–H distances are shortened to 1.36, 1.38, 1.32 and 1.35 Å from 2.57, 2.59, 3.16 and 3.54 Å for Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100) and Pd4/Fe(100) surfaces, respectively. The activation barriers of CO hydrogenation into COH are 1.80, 1.85, 1.52, 1.90 and 1.87 eV and reaction energies are 0.99, 0.93, 0.70, 1.16 and 0.85 eV, respectively. The activity barriers of COH formation on Pd/Fe surfaces are slightly higher than that on Fe-rich Pd/Fe surfaces. COH → C+OH For pure Fe(100) surface, COH is initially adsorbed at the 4-fold hollow site with C-O axis vertical to the surface. In the TS, the C atom still adsorbs at the 4-fold hollow site, and the detaching OH gets close to the top site of Fe atom, the cleaving C–O bond is stretched to 1.90 Å from 1.43 Å in COH. In the FS, C atom is deposited at the 4-fold hollow site, and the dissociated OH adsorbs at the neighboring 4-fold hollow site, and the C–O distance is 2.95 Å. The COH species on 4-fold hollow site could undergo the C-O cleavage yielding the C and OH species. Its reaction energy is -1.46 eV and the corresponding activation barrier is 0.21 eV. With regard to Pd-doped Fe(100) surfaces, COH is initially favored at the 4-fold hollow site 16


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with C-O axis perpendicular to the surface. In the TS, the C atom remains adsorbed at the 4-fold hollow site. For Fe3Pd1/Fe(100) and Fe1Pd3/Fe(100), the detaching OH migrates to the top site of surface metal atom, whereas for Fe2Pd2/Fe(100) and Pd4/Fe(100), detaching OH connects to two surface metal atoms. The lengths of the breaking C-O are stretched to 1.87, 1.91, 1.94 and 2.07 Å from 1.41, 1.40, 1.37 and 1.37 Å in COH for Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100) and Pd4/Fe(100), respectively. In the FS, for Fe3Pd1/Fe(100) and Fe1Pd3/Fe(100), C atom is located at the 4-fold hollow site, and the dissociated OH adsorbs at the bridge site of two metal atoms (d(C–O)=3.43 and 3.18 Å). For Fe2Pd2/Fe(100) and Pd4/Fe(100) surfaces, C atom is located at the 4-fold hollow site, and the dissociated OH adsorbs at the neighboring 4-fold hollow site (d(C–O)=2.62 and 3.17 Å). COH dissociation into C and OH occurs with the reaction energies of -1.46, -1.29, -0.31, -0.47 and 0.84 eV, the activation barriers are 0.21, 0.38, 0.71, 0.72 and 1.62 eV for Fe4/Fe(100), Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100) and Pd4/Fe(100), respectively. As Pd coverage increases, COH dissociation to C and OH gets more difficult. In conclusion, the energy barriers for both direct and H-assisted CO dissociations increase generally as Pd coverage increases. The formation of HCO has a lower barrier compared to the direct CO dissociation and COH formation, CO dissociation via HCO intermediate appears to be the most feasible. For HCO mediate mechanism, we predict the rate-determining step is HCO formation step on Fe4/Fe(100) surface, with Pd doping on Fe(100) surface, HCO dissociation is the rate-determining step. On the Fe4/Fe(100) surface, COH is hard to form due to its higher reaction barrier, and the HCO dissociation mechanism is preferred than direct CO dissociation although the difference between direct CO and HCO intermediate dissociation in their energy barriers is only about 0.3 eV. For the Pd4/Fe(100)surface, H-assisted CO dissociation including HCO and COH mediate is much preferred than direct CO dissociation. The situation for the Fe-rich Pd/Fe surface is similar to the Fe4/Fe(100) surface, whereas that for the Pd-rich Pd/Fe surface is similar to the Pd4/Fe(100) surface. Thus, for our model Pd/Fe catalysts, CO dissociation mainly occurs via the HCO dissociation mechanism although the direct CO dissociation may be exist on the pure or Fe-rich surfaces.

4. Discussions From our calculation results above, CO dissociate through three mechanisms, direct CO, HCO intermediate, and COH intermediate mechanisms. CO dissociation is favored to occur via HCO 17



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intermediate, and Pd doping can increase CO dissociation barrier. So we investigate the factors accounting to the lower barrier of HCO formation and Pd dopant effects on CO dissociation. 4.1 The Comparison of Different CO Dissociation Mechanisms CO can dissociate on Pd/Fe surfaces via three different mechanisms: direct, HCO dissociation and COH dissociation mechanisms which are shown in Figures 3, 4, and Table 2. From the results before, CO prefer to be hydrogenated to form HCO followed by dissociation into CH and O on pure Fe(100) together with Pd-doped Fe(100) surfaces. a) The comparison of HCO and COH intermediate dissociation mechanisms Compared HCO formation with COH formation, we find that at least three aspects accounting to the relative low barrier of HCO dissociation mechanism. We take Fe3Pd1/Fe(100) surface as an example to get insight of the favored mechanism. From the geometric point, in the IS, CO molecule is tilted away from the surface, in FS, CO in COH is vertical to the surface. The C-O bond in TS is very similar to that in IS in the tilted case. The stable vertical C-O bond in FS is achievable through a significant bending of the C-O bond with H orientating to CO. On the contrary, in HCO formation, the C-O bond in FS is very similar to that in IS in the tilted case. Meanwhile, thermodynamically, CO hydrogenation to COH is endothermic (0.93 eV) higher than to HCO (0.41 eV) and leads to a high barrier. CO is favored to hydrogenation to HCO rather than COH.29 To further understand the physical original why HCO intermediate mechanism is more favorable than that of COH one, the electronic analysis of TSs for CO+H→COH and CO+H→HCO was performed based on the projected COHP method (pCOHP) developed by Dronskowski group was used.52-53 In the pCOHP diagram, the positive and negative values correspond to the bonding and antibonding states, respectively. As seen from Figure 6, one can know that the Fermi level lines between bonding and antibonding region. Moreover, it was found that the population of bonding region for HCO system is much larger than that of COH system (the integrated pCOHP up to Fermi lever is 1.00 and 1.50 eV for COH and HCO respectively by reading from the intersect of Fermi lever and integral line), means a strongly interaction between HCO species and Fe3Pd1/Fe(100) surface at TS. Stronger interaction means that HCO is more stable than that of COH formation at TS, which leads to lower energy barrier. Therefore, HCO formation is more preferred than COH formation from geometrical, thermodynamical and electronic structure points. b) The comparison of HCO intermediate dissociation and direct CO dissociation mechanisms 18


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We compared HCO mediate dissociation with direct CO dissociation, and take Fe3Pd1/Fe(100) surface as an example, geometrically, the C−O bond in HCO has been weakened by the bonding H in HCO and the strong interaction between C-O bond and the surface metals. The length of C-O has been stretched to 1.36 Å from 1.28 Å in initial gas phase. As a result, the C-O cleavage can be expected to be easier than the direct CO dissociation. Thermodynamically, HCO dissociation is exothermic (-0.37 eV), whereas direct CO dissociation is endothermic (0.17 eV). Therefore, HCO dissociation is more preferred geometrically and thermodynamically than CO dissociation. In general, the H-assisted mechanism by which CO is hydrogenated to form HCO, followed by subsequent dissociation to CH, is more favorable than direct CO and COH dissociation mechanisms on Fe/Pd surfaces. This conclusion is true only at low CO coverage and low temperature. As pointed out by Andersson et al,54 under high CO coverage and relatively high temperature conditions, COH intermediate mechanism becomes most feasible. So, the pressure and material gap in heterogeneous catalysis should be considered, which is beyond the scope of the present theoretical study.

(a)

(b)

Figure 6.Projected Crystal orbital Hamilton population (pCOHP) between [C....O...H] and surface metals of Pd1Fe3/Fe(100) system at TSs: (a) CO+H→COH,(b) CO+H→HCO. 19



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4.2 Pd doping effect Pd doping was found to reduce the activity of the Fe(100) surface in catalyzing direct and H-assisted CO dissociations, so CO dissociations should occur primarily on Fe-rich surfaces. The further electronic structure analysis, thermodynamic analysis, and energetic factors analysis reaches the same conclusion. 4.2.1Pd effect--Electronic structure analysis For CO dissociation, since the CO and C/O are usually adsorbed on 4-hollow site, namely four surface metals make contribution to the reaction, so we calculate the d-band center of the four surface atoms as shown in Figure S11 and Table S3 in the supporting information. The d-band centers are -0.86, -1.03, -1.43, -1.61, and -1.82 eV for Fe4/Fe(100), Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100) and Pd4/Fe(100), respectively. On transition metal surfaces, the s band of most of the metals contributes similarly to the adsorbate-metal bonding due to the broadness and diffuse nature of the s band. And the difference in the chemisorption energy on different metals, primarily, is due to the variation in their d electron states.55 In general, the closer of d-band center to the Fermi level and the higher activity would be. Previous reports have shown that the d-band center of metallic Fe (-0.92 eV) is much closer to the Fermi level compared to those of Pd (-1.83 eV).56 From our precious calculation results, we find that the d-band centers of Fe-rich Pd/Fe surfaces are shifted closer to Fermi level energy compared to that of Pd-rich Pd/Fe surfaces, Pd doping lowers the catalytic activity for CO dissociation. 4.2.2Thermodynamical factors in CO dissociation on Fe(100) and Pd-dope Fe(100) surface

20


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Figure 7. Linear relation between (a) CO adsorption energy (Ead) and Pd surface coverage (XPd), (b) the ETS and the EFS for direct CO dissociation on Pd/Fe surfaces, (c) the calculated ETS and the EFS for HCO intermediate pathway on Pd/Fe surfaces, and (d) the ETS and the EFS for COH intermediate pathway on Pd/Fe surfaces, where EFS is the energy of the final state for the reaction, relative to the initial state gas-phase species and the clean slab. ETS is the energy of the transition state with the same reference. One data point deviates from the linear relationship in CO→C+O on Fe1Pd3/Fe(100) (labeled in the red frame).

The traditional Bronsted-Evans–Polanyi (BEP) correlation is used to examine the effect on reaction barriers from thermodynamical point. Previous study has shown a BEP relation for C-C, C-H and O-H cleavages on some metals surfaces.57-58 For the cleavage of C-O bonds over the different metals surfaces such as Pd/Fe surfaces, whether the BEP relationship retains or not is still unclear. Herein we have plotted the TS energy against the FS energy of C-O bond breaking reactions on pure and Pd-doped Fe(100) surface, as is shown in Figure 7. We have also plotted adsorption against XPd. 21



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As shown in Table 1, the adsorption energy of CO becomes less negative from −1.94 eV for the Fe4/Fe(100) surface to −0.97 eV for the Fe(100) surface with 1 ML of Pd surface coverage, indicating the significant influence of Pd surface coverage on CO adsorption energy. As shown in Figure 7a, XPd is Pd surface coverage, ranging from 0 for the Fe4/Fe(100) surface to 1 for the Pd4/Fe(100) surface. The R2 value is 0.918, suggesting that a good linear relationship between CO adsorption energy and Pd coverage, and CO adsorption energy becomes less negative at higher Pd surface coverage. This indicates the strong effect of surface Pd doping of the Fe(100) on CO adsorption. Weaker adsorption strength of CO on Pd-rich Pd/Fe surfaces means weaker interaction between CO and surface, which leads to higher barriers in CO dissociation. BEP correlation exists in the C-O bond cleavage in direct CO dissociation, HCO dissociation step, and COH dissociation pathway on Pd/Fe surfaces is shown in Figure 7b, 7c, and 7d. The high R2 value of 0.947, 0.924, and 0.983 indicate a reasonably good linear fit between the ETS and the EFS of direct CO dissociation, HCO dissociation, and COH dissociation, meaning the high endothermicity can bring about high barrier to some extent. The physical origin of the general BEP principle in Figure 7 can be understood as follows. BEP relationship is the case where the TS properties should be the same. The surface dissociation reaction generally owns a “late” transition state, where the configuration of transition state is similar to that of final state. One data point deviates from the linear relationship in CO→C+O (labeled in the Figure 7b) on Fe1Pd3/Fe(100) because the transition state is quite different from the final state. On one hand, in FS, the most optimal sites of C and O are next-nearest-neighbor sites along the diagonal line which is different to other Pd/Fe surfaces. On the other hand, by observing the bond distance of C-O (d(C-O)=2.01, 3.59 Å for TS, FS), the difference between CO lengths in TS and FS (Δd) of Fe1Pd3/Fe(100) surface (1.58 Å) is longer than that on other surfaces such as the Fe3Pd1/Fe(100) (d(C-O)=1.92, 2.73 Å for TS, FS, Δd=0.81 Å). Therefore the configuration of transition state is not similar to that of final state and the transition state properties of CO dissociation on Fe1Pd3/Fe(100) is not similar to that on other Pd/Fe surfaces, thus the general BEP rule cannot be applied to this model catalyst. Now it is necessary to analyze why the properties of Fe3Pd1/Fe(100) is different from other FePd models. For the bimetallic system, the previous theoretical study of Groß indicated that replacing smaller atoms by larger atoms such as Pd, the adsorption energies of CO on all top sites become less negative, the binding energies become smaller when the Pd concentration is increased, 22


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and the most favorable adsorption sites change to high coordinated Pd site with increasing Pd concentration30. On our model of Fe1Pd3/Fe(100) surface, CO adsorbed at the top site of a Fe atom, whereas on Fe2Pd2/Fe(100) surface, CO adsorbed at high coordinated 4-fold hollow site. Considering the low coordinated top site and compressive strain in the surface alloy monolayer on Fe1Pd3/Fe(100), CO binds less strongly with surface, and therefore a weaker interaction induce a higher activation energy barrier and lower reaction energy for CO dissociation on Fe1Pd3/Fe(100). So one data point deviates from the linear relationship in CO→C+O (labeled in the Figure 7b) on Fe1Pd3/Fe(100). From our results, direct CO dissociation is exothermic by -0.41 eV onFe4/Fe(100)surface and endothermic by 2.94 eV on Pd4/Fe(100) surface. Even though the endothermicity of CO hydrogenation to HCO was approximate for the different Pd/Fe surfaces, HCO dissociation to CH and O is exothermic by −0.92 eV on the pure Fe(100) surface and greatly endothermic by 2.19 eV on the Pd4/Fe(100) surface. Although CO hydrogenation to COH is endothermic approximately for different Pd/Fe surfaces, COH dissociation to C and OH is exothermic by -1.46 eV on the pure Fe(100) surface, and significantly endothermic by 0.84 eV on the Pd4/Fe(100) surface. From the above analysts, it indicates that C-O cleavage whether by direct or H-assisted on Pd-rich Pd/Fe surfaces is unfavorable from thermodynamic point. 4.2.3 Energetic factors controlling CO dissociation on Fe(100) and Pd-doped Fe(100) surfaces It is clear that the Fe-rich Pd/Fe surfaces have higher activity toward CO dissociation. Concerning Fe-rich Pd/Fe surfaces, the CO dissociation barrier is 1.1−1.5 eV via direct CO dissociation, 0.6−0.8 eV via the HCO formation and 1.5−1.9 eV via COH formation. On the Pd-rich Pd/Fe surfaces, CO dissociation barrier is 2.8−3.6 eV via direct CO dissociation, 1.0−1.1 eV via HCO formation and 1.8−1.9 eV via COH formation. From above results, it is clear that CO prefers to hydrogenation to HCO with lower barrier which is followed by dissociation to CH+O, and HCO formation on Fe-rich Pd/Fe surfaces is easier than that on Pd-rich Pd/Fe surfaces. CO dissociation mainly occurs via the HCO-intermediate pathway. As a result, we analyze the physical original of the energy barrier. The energy decomposition is a very useful tool to explore physical original.59 As the importance of the C-O cleavage in HCO in the whole CO dissociation process, in order to illustrate the possible factors affecting the HCO dissociation reaction activation energy, the chemical behavior of the step will be comprehensively understood from the part of adsorption energy.

23



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For dissociation reactions of generic form AB  A  B ,the sum of adsorption energy of A+B at the TS is decomposed as

E

TS

 E A  E B  E int   E  E int , TS

A B

where

TS

E E TS

TS

A

B

TS



TS

(2)

TS

is the adsorption energy of A (B) in the geometry of the TS, and the

interaction energy E TSint is defined by the above equation describing interaction between A and B in the TS geometry. The dissociation barrier can be represented as equations (3)

E

diss act

g

  E  E int  E AB   E AB TS

where

E

g AB

TS

g

IS

g

(3)

g

 E AB  E A  E B reflects the A−B bond energy in the gas phase,

E

IS AB

is the

adsorption energy of reactant. For the dissociation of HCO, the energy barrier is defined as

E

diss act

g

  E  E int  E HCO   E HCO TS

TS

IS

(4)

The first term is defined as the sum of adsorption energies of products (CH and O) in TS. The second term represents interaction between CH and O in the TS geometry, the third term is defined as desorption energy (-Eads) of the reactant (HCO). The last term is related to the gas phase dissociation of HCO and denoted as phase reaction items.

Table 3. Energy Decomposition of the Calculated Activation Barriers for the HCO Dissociation on Pd/Fe Surfaces. Fe4/Fe(100)

E

diss

E

IS

E

TS

act

HCO

int

E

TS

Note:

E

TS

E

diss act

Fe3Pd1/Fe(100)

Fe2Pd2/Fe(100)

Fe1Pd3/Fe(100)

0.66

0.81

0.95

1.61

2.95

-2.84

-2.60

-2.29

-1.96

-1.78

0.45

0.62

0.83

0.98

1.10

-12.18

-11.93

-11.69

-10.85

-9.35

is the activation energy for HCO dissociation,

E

is the sum of adsorption energies of CH and O at TS and

binding energies in the gas phase is −9.52 eV.

IS HCO

E

TS int

24


Pd4/Fe(100)

is the adsorption energy of the HCO, interaction between CH and O, and

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We investigate the main factors accounting to reaction barriers, the results are shown in Table 3. From Table 3 one can find the determining factor to the barrier is different for different Pd/Fe surfaces. For Fe4/Fe(100), Fe3Pd1/Fe(100), and Fe2Pd2/Fe(100) surfaces, we observe that the sum of adsorption energies of CH and O at the TS decreases by 0.25 eV (from Fe4/Fe(100) to Fe3Pd1/Fe(100)) and 0.24 eV (from Fe3Pd1/Fe(100) to Fe2Pd2/Fe(100)), but the contribution to the barrier is compensated by approximate increase in HCO adsorption energy 0.24 and 0.31 eV. The TS factor of the sensitivity for HCO dissociation barrier is the interaction energy at the TS ( Eint ), which

increases by 0.17 eV (from Fe4/Fe(100) to Fe3Pd1/Fe(100)) and 0.21 eV (from Fe3Pd1/Fe(100) to Fe2Pd2/Fe(100)). The results illustrate that the major contribution to reaction barrier is the interaction TS energy in TS for Fe4/Fe(100), Fe3Pd1/Fe(100), and Fe2Pd2/Fe(100) surfaces. The Eint

contains

direct Pauli repulsion which depends on the distance between two fragments, bonding competition caused by sharing the same substrate atoms between CH and O group, and electrostatic interaction between CH and O species at the TS configuration. We find that the distances between dissociated CH and O species are basically the same for Fe4/Fe(100), Fe3Pd1/Fe(100), and Fe2Pd2/Fe(100) surfaces, and CH and O bind with different metals, so the Pauli repulsion and bonding competition TS effect can be ignored. It means the Eint mainly depends on electrostatic interaction between CH and

O species at the TS, CH and O attract electrons from surrounding metal atoms, which are then charged and repel each other. However, as for Fe1Pd3/Fe(100) and Pd4/Fe(100) surfaces, since the sum of adsorption energy of TS HCO ( E ISHCO ) and the TS interaction energy ( Eint ) is almost equal, which have a little effect on the

distinction of the barriers, so the major contribution to energy barrier for Fe1Pd3/Fe(100) and Pd4/Fe(100) is the change of

E

TS

, namely from -10.85 to -9.35 eV as seen in Table 3.

In conclusion, the interaction between CH and O and the sum of adsorption energy of CH and O increase at TS lead to HCO dissociation barriers increase with Pd coverage increasing. 4.3 Comparisons with Cu-doping for the CO dissociation in Cu/Fe(100) system As we know that the Cu/Fe(100) system is an efficient catalyst for the methanol synthesis from syngas, and it can also improve higher alcohols conversion,60 so it is worthy to compare Pd/Fe(100) and Cu/Fe(100) directly. Although the reaction mechanism of CO dissociation on Cu/Fe(100) has 25



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been theoretically studied by Zhao et al.,45 the COH intermediate dissociation mechanism on Cu/Fe surfaces and CO dissociation mechanisms on Fe2Cu2/Fe(100) surface were not analyzed in the literature. The calculation model of Cu/Fe(100) is similar to that of Pd/Fe(100) in which the uppermost layer Fe atoms are replaced by Cu atom gradually. The surface energies of Cu-doped Fe(100) surface are 0.67, 0.66, 0.66, 0.65 and 0.63 J/m2 for Cu surface coverages of 0, 1/4, 2/4, 3/4, and 1 ML, respectively. Lower surface energy of the Cu-doped Fe(100) surface means that the Cu-doped Fe(100) surfaces are slightly more stable than the pure Fe(100) surface and the Cu is stable at the surface. The key bond distances, adsorption configurations and energies for the main species involved in CO dissociation on Cu/Fe(100) are shown in Table S2, Figure S4 and Table 4, and the related C-O dissociation energy barriers are given in Table 5. The optimal configurations of TSs are shown in Figures S7, S8, and S9. And the potential energy profiles are shown in Figure S10.

Table 4. Adsorption Energies (Eads, eV) of the Various Adsorbates on Cu/Fe Surfaces

Fe3Cu1/Fe(100) Site Eads



C

4-hollow

-7.98

4-hollow

-7.51

4-hollow

-7.10

4-hollow

-7.03

O

4-hollow

-6.54

4-hollow

-6.20

4-hollow

-5.84

4-hollow

-5.78

H

4-hollow

-1.64

4-hollow

-2.63

4-hollow

-2.48

4-hollow

-2.41

CH

4-hollow

-6.89

4-hollow

-6.59

4-hollow

-6.29

4-hollow

-6.16

OH

Bridge

-3.31

bridge

-4.00

4-hollow

-3.41

4-hollow

-3.38

CO

4-hollow

-1.62

4-hollow

-1.27

4-hollow

-1.02

4-hollow

-0.90

HCO

h-b-bb

-2.51

h-b-bb

-2.20

h-b-bb

-1.66

h-b-bb

-1.50

COH

4-hollow

-3.98

4-hollow

-3.67

4-hollow

-3.28

4-hollow

-3.08

Species

α

Cu4/Fe(100) Site Eads

Note: αstands both C and O atoms are bonded to the surface in this adsorption structure.bh-b-b stands hollow-bridge-bridge.

Table 5. Energy Barriers (Ea, eV) and Reaction Energies (ΔE, eV) for Direct and H-assisted CO Activation on Cu/Fe Surfaces

Steps CO→C+O

Fe3Cu1/Fe(100)

Fe2Cu2/Fe(100)

Fe1Cu3/Fe(100)

Cu4/Fe(100)

Ea

ΔE

Ea

ΔE

Ea

ΔE

Ea

ΔE

1.28

-0.04

1.34

0.34

1.86

0.66

2.22

0.93

26


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CO+H→HCO

0.77

0.55

0.96

0.47

1.01

0.65

1.06

0.79

HCO→CH+O

0.58

-0.74

0.65

-0.35

0.66

-0.33

1.13

-0.13

CO+H→COH

1.57

0.95

1.63

0.87

1.67

0.91

1.70

1.07

COH→C+OH

0.38

-1.38

0.66

-0.47

0.41

-0.95

0.73

-0.28

4.3.1CO dissociation on Cu/Fe(100) CO dissociation on Cu-doped Fe(100) surface via three different mechanisms: direct, HCO intermediate and COH intermediate dissociation mechanisms. HCO dissociation mechanism is much preferred among the three mechanisms. Starting from CO, COH formation is hard to occur with higher reaction barrier, the HCO dissociation mechanism is preferred than direct CO dissociation on the Fe4/Fe(100) surface, although the difference between energy barriers in direct CO and HCO intermediate dissociation is slight. For the Cu4/Fe(100)surface, H-assisted CO dissociation including HCO and COH intermediate is much preferred than direct CO dissociation. As expected, the situation for the Fe-rich Cu/Fe surface is similar to the Fe4/Fe(100) surface, whereas that for the Cu-rich Cu/Fe surface is similar to the Cu4/Fe(100) surface. Thus, for our model Cu/Fe catalysts, CO dissociation mainly occurs via the HCO dissociation mechanism, and on the pure or Fe-rich surfaces, the contribution of direct CO dissociation becomes greater than Cu-rich surfaces. As shown in Table 5, the energy barrier for CO dissociation whether by direct or H-assisted mechanism increases as the Cu surface coverage increases generally, which is similar to that of Pd/Fe(100) system. The activity barriers and reaction energies are similar to the previous results.45 4.3.2 d-band center of Cu-doped Fe(100) surface Similar to Pd-doped Fe(100), CO prefers to adsorb at the 4-fold hollow site of four surface metals which contributes to reaction of CO dissociation on Cu-doped Fe(100) surfaces, hence we calculate the d-band center of the four surface atoms as shown in Figure S12 and Table S4. Previous research results have shown that the d-band center of metallic Fe (-0.92 eV) is much closer to the Fermi level compared to that of Cu (-2.67 eV).56 Our precious calculation results of the d-band centers are -1.15, -1.43, -1.74, and -2.21 eV for Fe3Cu1/Fe(100), Fe2Cu2/Fe(100), Fe1Cu3/Fe(100) and Cu4/Fe(100) surfaces. The results indicate that the d-band centers of Fe-rich Cu/Fe surfaces are closer to Fermi level energy than that of Cu-rich Cu/Fe surfaces. The catalytic activity for CO 27



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dissociation becomes lower with Cu coverage increasing. 4.3.3 Active difference analysis between Pd/Fe and Cu/Fe surfaces Comparing Pd/Fe with Cu/Fe surfaces, it was found that the energy barrier of CO/HCO dissociation barriers on Pd/Fe surfaces are higher than those on Cu/Fe surfaces (see Table 2 and Table 5), and the possible physical original should be analyzed. In general, the catalytic activity of metallic Pd is higher than that of Cu due to the fact that the d-band center of metallic Pd is closer to the Fermi level compared to that of Cu, and this situation may be changed when they are deposited on the Fe(100) surface. As we know that the lattice constants of Pd is larger than that of Fe, whereas the lattice constants of Cu is smaller than Fe, and thus the lattice constants of Pd would be compressed and Cu lattice constants would be expend on the bimetallic Pd/Fe(100) and Cu/Fe(100) surfaces. Clearly, such kind of strain effect will make the Pd less active and Cu more active when compared to the corresponding metal surface.30 To further understand the different dopant effect of Cu and Pd on the Fe(100), the d-band center model was also used. However, it was found that the d-band center of Cu/Fe(100) is far away from the Fermi level than Pd/Fe(100) (see Table S3 and Table S4), which cannot correlate well the catalytic trends that the Cu/Fe(100) is more active in the activation of CO compared to that of Pd/Fe(100). The reason is clear because both the d-band center and d-band width contribute to the d-band properties, so here we calculated the d-band width which proposed by Vojvodic et al.61as shown in Table S3 and Table S4 in the supporting information to describes the transition metals activity. In the model, the d-band width was calculated by equation:

 dW   d W d / 2

(5)

mnc was defined as mnc  

E n  ( E)dE







 ( E)dE

,

(6)

Wd was the surface d-band width which was defined by equation: Wd  4 m2c ,

(7)

The d-band widths are calculated to 3.40, 3.31, 3.23, and 3.04 eV for Fe3Pd1/Fe(100), Fe2Pd2/Fe(100), Fe1Pd3/Fe(100), and Pd4/Fe(100) surfaces. For Cu/Fe surfaces, d-band widths are 3.25, 3.11, 2.96, and 2.83 eV for Fe3Cu1/Fe(100), Fe2Cu2/Fe(100), Fe1Cu3/Fe(100), and Cu4/Fe(100) surfaces. Wider d-band widths mean that lower activity. The d-band widths on Pd/Fe surfaces are 28


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wider than that on Cu/Fe surfaces, namely the reaction activities on Pd/Fe surfaces are lower, and hence the CO dissociation barriers on Pd/Fe surfaces are higher than that on Cu/Fe surfaces. 4.4 The bifunctional nature of bimetallic catalyst a) Methane, methanol, and ethanol synthesis on Pd/Fe(100) We investigate CO dissociation on pure and Pd-doped Fe(100) with different Pd coverages, and it was found that the HCO intermediate dissociation is the most favorable mechanism. HCO species is precursors for C2 oxygenates synthesis and is important intermediate involved in methanol synthesis from CO and H2. Dissociated HCO is then hydrogenated to produce surface hydrocarbon species (CHx), and CHx then further undergoes hydrogenation to produce methane (CH4). Methanol species is synthesized via HCO hydrogenation to form CH2O, CH3O and CH3OH at last. For the ethanol synthesis, it contains the steps of HCO insertion into CHx species to form a C-C bond followed by hydrogenation induce ethanol synthesis. Clearly, the ability of HCO dissociation is very important in determining the final product. When the decomposition rate of HCO is faster, HCO prefers to dissociate and hydrogenation to methane. The moderate decomposition rate of HCO induces ethanol formation and the lower decomposition rate of HCO make methanol main production. On pure Fe(100) surface, since the lower HCO dissociation barrier induce high selectivity of methane formation. With Pd coverage increases, HCO dissociation barrier increases, and thus the selectivity of CH3OH increases and the selectivity of methane decreases. For Fe2Pd2/Fe(100), it may be expected that the slight amount of ethanol can be synthesized, but the main production is methanol and methane. On Pd4/Fe(100) surface, as the HCO dissociation is hard to occur with high reaction barrier, and therefore the HCO species would go through the hydrogenation step to form CH3OH. In fact, the experimental results indicated the 1% ethanol formed on Fe2Pd2/Fe(100) and only methanol is produced on Pd4/Fe(100).62 b) Comparison of Methane, methanol, and ethanol synthesis on Pd/Fe(100) and Cu/Fe(100) Our DFT results suggest that CO dissociation via both direct and H-assisted mechanisms dominantly occur on Fe-rich Pd/Fe and Cu/Fe surfaces both kinetically and thermodynamically, and the HCO dissociation mechanism is the favorable one. Since the HCO dissociation barrier is an useful “descriptor” to measure the ability to form ethanol based on the our above analysis, so it is worthy to

29



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Figure 8.Relationship between the selectivity of ethanol and the calculated HCO dissociation energy barrier (Ea) on Fe4/Fe(100), Fe2Cu2/Fe(100), Fe2Pd2/Fe(100), Cu4/Fe(100), and Pd4/Fe(100) surfaces.

compare the catalytic difference between Pd/Fe and Cu/Fe systems directly. Interestingly, a typical “ volcano curve” between ethanol synthesis and HCO dissociation barrier was gained (see Fig.8), in which the selectivity for the ethanol synthesis is highest on Fe2Cu2/Fe(100) system among these studied bimetallic model catalysts due to its moderate catalytic activity for HCO dissociation. On pure Fe(100), Fe2Pd2/Fe(100) and Cu4/Fe(100) surfaces, slight amount of ethanol can be synthesized. On Pd4/Fe(100) surface, no ethanol can be produced.9, 62-63 In a word, with the addition of Pd and Cu, the HC−O bond cleavage step is inhibited both in kinetics and thermodynamics. Fe surfaces favor the formation of the CHx species, Pd and Cu surfaces are necessary to provide undissociated CO/HCO species, so the Pd/Fe and Cu/Fe catalysts will provide two different active sites that are synergetic for the formation of higher alcohols. It should be pointed that the present work focused on the CO dissociation reaction mechanism only, and the detailed reaction mechanism study of high alcohols synthesis from syngas over Pd(Cu)/Fe system will be given in our future work based on the DFT calculations and micorkintic simulation.

5. Conclusions In summary, we performed periodic DFT to investigate the mechanisms of CO dissociation on Pd-doped and Cu-doped Fe(100) surfaces. Comparing the mechanisms for these surfaces, some 30


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conclusions can be drawn by us: (1) On both Pd-doped and Cu-doped Fe(100) surfaces, CO dissociation via direct CO and H-assisted dissociation mechanisms including HCO intermediate or COH intermediate. The activation barriers of HCO formation are lower than COH formation and direct CO dissociation. HCO intermediate dissociation appears to be preferable. (2) The energy barriers of CO dissociation increase as Pd and Cu coverages increase. The effect of Pd has been analyzed as: Firstly, the d-band centers of Fe-rich Pd/Fe surfaces are closer to the Fermi level which means higher activity toward CO dissociation compared to those of the Pd-rich Pd/Fe surfaces; Secondly, reaction energy of C-O cleavage increases with Pd coverage increasing, which leads to increasing activity barrier; Thirdly, the interaction between CH and O and the sum of adsorption energy of CH and O increase at TS lead to HCO dissociation barriers increase with Pd coverage increasing. (3) Obviously, the energy barriers for C-O scission on Cu/Fe(100) are lower than that on Pd/Fe(100), the d-band model showed that the d-band width of Cu/Fe(100) is more narrow compared to that of Pd/Fe(100), which lead to lower energy barrier on Cu/Fe(100). (4) Fe exhibits a strong promotion effect on the activation of C-O breaking in both direct and H-assisted CO dissociation yielding long-chain alkanes. On the contrary, Cu and Pd show less promotion effect or even inhibit the activation of C-O cleavage, and can provide sites for CO and HCO physisorption which can undergo insertion reaction to promote chain growth toward C2 oxygenates. By observing HCO dissociation barriers on Pd/Fe and Cu/Fe surfaces, we conclude that with Pd coverage increases, methane content decreases and methanol content increases on Pd/Fe surfaces. Comparing Pd/Fe and Cu/Fe surfaces, the selectivity of methanol on Pd/Fe(100) is higher than that on Cu/Fe(100). The present result might help people to further understand the role of Pd(Cu) and their difference in the Fe catalyst in the F-T reaction, and design the efficient F-T catalysts to some extent.

Supporting Information The key bonding parameters, adsorption energies and configurations of key reaction intermediates, optimized initial, transition and final state configurations of key steps and energy profiles for reactions on Pd-doped Fe(100) surfaces and Cu-doped Fe(100) surfaces, d-band center and d-band width of Pd/Fe and Cu/Fe surfaces.

31



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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21421001, 21433008, 91545106) and the foundation of State Key Laboratory of Coal Conversion (Grant No. J17-18-908). References and Notes 1.

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CO Dissociation Mechanism on Pd-Doped Fe(100): Comparison with

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