Computational Investigation of FeâCu Bimetallic Catalysts for CO2

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Computational Investigation of Fe-Cu Bimetallic Catalysts for CO Hydrogenation 2

Xiaowa Nie, Haozhi Wang, Michael J. Janik, Xinwen Guo, and Chunshan Song J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03461 • Publication Date (Web): 14 Apr 2016 Downloaded from http://pubs.acs.org on April 18, 2016

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Computational Investigation of Fe-Cu Bimetallic Catalysts for CO2 Hydrogenation

Journal: Manuscript ID Manuscript Type: Date Submitted by the Author: Complete List of Authors:

The Journal of Physical Chemistry jp-2016-03461y Article 05-Apr-2016 Nie, Xiaowa; Dalian University of Technology, State Key Laboratory of Fine Chemicals, School of Chemical Engineering Wang, Haozhi; Dalian University of Technology Janik, Michael; Penn State, Chemical Engineering Guo, Xinwen; Dalian University of Technology, Chemical Engineering Song, Chunshan; Pennsylvania State University, Energy & Mineral Engineering, and

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Computational Investigation of Fe-Cu Bimetallic Catalysts for CO2 Hydrogenation Xiaowa Niea, Haozhi Wanga, Michael J. Janikb, Xinwen Guoa,*, and Chunshan Songa,b,c,* a

State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China b

EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802, USA

c

EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of Energy & Mineral Engineering, Pennsylvania State University, University Park, PA 16802, USA

Corresponding Author Dr. Chunshan Song Email: [email protected] Dr. Xinwen Guo Email: [email protected]

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Abstract: Density functional theory (DFT) calculations were carried out to investigate Fe-Cu bimetallic catalysts for the adsorption, activation, and initial hydrogenation of CO2. CO2 adsorption strength decreases monotonically as surface Cu coverage increases. For dissociation of CO2, the reaction energy and activation barrier scale linearly with surface Cu coverage. The reaction energy becomes less exothermic and the activation barrier increases with increasing surface Cu coverage from 0 to 1 ML. For initial hydrogenation of CO2, formation of a formate (HCOO*) intermediate is kinetically favored over carboxyl (COOH*) at all surface Cu coverages. A substantial decrease of the kinetic barrier for HCOO* formation is observed when surface Cu coverage increases to 4/9 ML. CO* is the preferred intermediate from CO2 dissociation at 2/9 ML surface Cu coverage or below, however, the favorable conversion path changes to CO2 hydrogenation to a HCOO* intermediate when surface Cu coverage increases to 4/9ML or higher. The composition and structure of the bimetallic catalysts determine the preferred intermediates and dominant reaction paths for CO2 conversion, and thus impact the catalytic activity and selectivity.

1. Introduction The potential environmental impacts of increasing atmospheric carbon dioxide (CO2) concentrations have caused great concern and controlling CO2 emission has become a major global challenge.1 Energy efficient conversion of CO2 to valuable industrial feedstock such as lower olefins, liquid hydrocarbons, and clean fuels is a promising way to mitigate the global CO2 emission which also reduces the dependence on petroleum.2-6


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CO2 hydrogenation provides an alternative path to hydrocarbon products currently produced from petroleum, as it utilizes abundant CO2 as chemical feedstock and hydrogen, a flexible energy carrier.7-9 Fe-based catalysts have shown good performance for CO2 hydrogenation process consisting of reverse water–gas shift and Fischer-Tropsch synthesis.10-11 To date, CO2 hydrogenation studies have mainly focused on conventional Fischer-Tropsch catalysts including Fe, Co, Ni, and Ru.12-16 Monometallic Ru and Ni catalysts produce mainly methane,12-14 whereas Co catalyst selectivity to higher hydrocarbons relative to methane increases with increasing CO2 : H2 ratio.15 Product distribution over Fe catalysts is less sensitive to CO2 pressure.16 We have concentrated our recent work on developing Fe-based bimetallic catalysts for selective CO2 conversion to higher hydrocarbons (C2+).17-19 Bimetallic combinations of Fe with small amounts of Co, Ni, Cu, and Pd transition metals, together with a K promoter, have led to significantly higher activities for olefin-rich hydrocarbon production18 than the previous state-of-the-art KFe/Al2O3 catalyst.20-21 These promising experimental results point to a strategy towards selective synthesis of higher hydrocarbons from hydrogenation of CO2 over novel bimetallic catalysts. However, the nature of the active sites in these bimetallic catalysts, adsorption behaviors, preferential intermediates, dominant reaction paths, and the mechanistic aspects that dictate CO2 hydrogenation activity and selectivity to C2+ products still remain unresolved. The first step towards catalyst optimization is identifying the structure of the active sites and establishing a mechanism to explain the observed catalytic activity and selectivity. Density functional theory (DFT) methods are uniquely positioned for exploring catalyst structures and reaction paths for such complex reaction sequences.22-31 The role of copper promoter in Fe-based FTS has been studied experimentally.32-34 The content of Cu was found to be important in determining FTS activities.35-36 Cu-modified FT


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catalysts, such as Cu-modified Fe catalysts, were considered as promising catalysts for higher alcohol synthesis (HAS).37-38 Our recent experimental work on selective CO2 conversion to higher hydrocarbons (C2+) shows that bimetallic combinations of Fe with small amounts of transition metals such as Cu, together with a K promoter, lead to higher activities for olefin-rich hydrocarbon production.18 There have been extensive theoretical studies on CO hydrogenation for higher alcohol synthesis (HAS) over Fe-based catalysts, with a focus on adding Cu to promote the product selectivity.39-44 Zhao et al.40 investigated CO dissociation on Cu-doped Fe(100) surfaces and found that Cu doping reduces CO dissociation activity. An H-assisted dissociation path is more favorable than the direct dissociation path. Their computational results partially validate the proposed “dual-site mechanism” for HAS over Fe-Cu bimetallic catalysts, in which CO mainly dissociates on the Fe-rich surfaces whereas the Cu-rich surfaces may be potential sources of physisorbed CO molecule for insertion reaction to form higher alcohols.40, 42, 44 Recently, Tian and co-workers used DFT calculations to explore the effect of Cu in CO adsorption and dissociation on the Fe(100) surface and found that increasing Cu content not only makes CO dissociation thermodynamically unfavorable but also increases CO dissociation barriers, and the Cu-substituted Fe(100) surfaces suppress CO adsorption and dissociation more strongly than the Cu-adsorbed Fe(100) surfaces.42 These theoretical studies indicate an important dependence of the catalytic performance on surface composition and structure, and adding Cu into Fe significantly modifies the structure and surface properties of the Fe catalysts, thus altering the activity and selectivity. To our knowledge, theoretical studies of CO2 hydrogenation over Febased bimetallic catalysts have not yet been reported but a mechanistic explanation has become important for our experimentally observed transition metal promotion of Fe-catalyzed higher


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hydrocarbons synthesis (HHS).17-19 In order to understand the promotion effect of Cu to Fe catalyst, we performed periodic DFT calculations to explore the composition, structure, and surface activity of Fe-Cu bimetallic catalysts for CO2 hydrogenation. We first identified Cu substitution tendencies and stable surface compositions. CO2 adsorption, activation, dissociation, and initial hydrogenation steps were systematically studied on several Cu-substituted Fe(100) surfaces, which serve as models for Fe-Cu bimetallic catalysts in HHS. The effect of Cu addition on CO2 adsorption and conversion were investigated, and the preferred intermediates together with favorable conversion paths were uncovered.

2. Computational Details 2.1 Electronic structure methods All calculations reported in this work were performed using the Vienna ab initio simulation program (VASP), a plane-wave DFT software package.45-46 The electron-ion interaction was simulated by the projected augmented wave (PAW) pseudopotentials.47 The exchange and correlation energies were computed using the Perdew, Burke, and Ernzerhof (PBE)48 functional within the generalized gradient approximation (GGA).49 The cutoff energy for the plane-wave basis was set to 400 eV. Spin-polarized calculations were performed considering the ferromagnetic nature of Fe. A k-space mesh of 5 × 5 × 1 within the Monkhorst−Pack scheme was used to sample the Brillouin zone of the surface unit cell. Geometries were relaxed using a damped molecular dynamics method until the forces on all atoms were less than 0.03 eV/Å. Transition states were searched using the climbing image nudged elastic band (CI-NEB)50 method. Vibrational frequencies were evaluated to confirm the minima and transition states.


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Bader charge analysis was performed using the code developed by Henkelman and coworkers.51-52 2.2 Surface models A 3×3 Fe(100) surface slab including 4 atomic layers was constructed to model the monometallic Fe catalyst, with a vacuum thickness of 12 Å to avoid interactions between the repeating slabs. Fe-Cu bimetallic catalyst models substituted top-layer Fe atoms with different numbers of Cu atoms, accordingly generating a series of Fe-Cu bimetallic catalysts with different surface Cu coverages. From 1 to 9 surface Fe atoms were successively substituted with Cu atoms, creating corresponding Fe-Cu bimetallic catalysts with Cu coverage (θ) of 1/9, 2/9, 1/3, 4/9, 5/9, 2/3, 7/9, 8/9, and 1 monolayer (ML), respectively. We started with 1 substituted Cu atom, and examined both top-layer and sub-layer substitution. The top-layer substitution was found to be energetically more favored, and this observation is consistent with the work by Zhao et al.40 and Tian et al.42 in their studies of CO adsorption and dissociation on Cu-substituted Fe(100) surfaces. Therefore, we only considered top-layer substitution in our subsequent work. For substitution with n = 2 to 9 Cu atoms, we initiated with the (n-1) energetically favored structures and then placed an additional Cu atom on the surface to replace another Fe atom in different positions. The most stable structure was chosen to represent the Fe-Cu catalyst model at a given Cu coverage. In the process of examining Cu substitution tendencies and stable surface composition of Fe-Cu catalysts, Zhao et al.40 and Tian et al.42 used similar approaches to identify the energetically most favorable substitution with Cu for Fe(100) surfaces at different surface Cu coverages (e.g. 1/4, 3/4, and 1 ML on a 2×2 supercell). To test the validity of the slabs containing 4 atomic layers, we also calculated the adsorption energy of both CO2* and H* on the slabs including 5 atomic layers of the Fe-Cu surfaces. The


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difference in adsorption energy of CO2* and H* is small (< 0.07 eV), as shown in Figure S1. Therefore, we chose 4 atomic layer slab in this work for consideration of computational cost while keeping the computation accurate. The 3×3 unit cell corresponds to a surface coverage of 1/9 ML for each single adsorbate, which approximates the low-coverage limit for the small adsorbates considered. In our calculations, the bottom two layers of the catalyst models are fixed at their original atomic positions in the bulk structure while all remaining layers and adsorbates are fully relaxed during structural optimization. The adsorption energy (Eads) of surface species is defined by Eads = Especies-slab – Eslab – Especies, where Especies-slab represents the total energy of the adsorbed species with catalyst surface, Eslab is the energy of the bare surface, and Especies is the energy of the species in gas phase. A more negative Eads dictates a stronger surface adsorption interaction.

3. Results and Discussion Our calculated equilibrium lattice constant for Fe is 2.832 Å, in good agreement with previous theoretical40, 53 and experimental values.54 We obtained a magnetic moment of 2.2 µB per Fe atom in the bulk structure, reproducing precisely the experimental result.54 Energetically favorable structures of the Fe-Cu bimetallic catalysts with different Cu coverages are shown in Figure 1, where θ = 0 ML denotes a monometallic Fe(100) surface and θ = 1 ML represents a Fe-Cu catalyst with the top layer occupied entirely by Cu atoms.


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Figure 1. Energetically favorable structures of various Fe-Cu bimetallic catalysts. θCu represents surface Cu coverage (ML) on the 3×3 Fe(100) surface. (θCu = 0 ML denotes a monometallic Fe(100) surface and θCu = 1 ML represents a Fe-Cu catalyst covered entirely with Cu atoms on the top layer. Orange = copper, purple = iron.)

3.1 H* and CO2* Adsorption Adsorption of H* on the Fe(100) and Fe-Cu surfaces was systematically examined, and the Eads was referenced to the energy of 1/2 H2 molecule in the gas phase and an empty Fe(100) surface. There are two unique sites for H* adsorption on Fe(100), which are the 4-fold hollow and 2-fold bridge sites. H* adsorption on the top site cannot be stabilized, as H* moves to an adjacent bridge site upon structural optimization. The adsorption energy of H* on the hollow and bridge site of Fe(100) is -0.44 and -0.40 eV, respectively. On the Fe-Cu surfaces, we examined various H* adsorption sites and found adsorption on the 4-fold hollow site is energetically more stable. The Fe-Cu surfaces are composed of various 4-fold sites of varying Cu content, as shown in Table S1(a-f). We calculated H* adsorption energies on all 4-fold hollow sites for each given Fe-Cu catalyst, and the results are given in Table S1. The adsorption energy of the most stable H* adsorption is almost constant from θCu = 0 to 7/9 ML (Eads = -0.42 ~ -0.45 eV). When the FeCu surface is dominated by 4-fold sites including majority Cu atoms (θCu = 8/9 ML), H*


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adsorption energy weakens to -0.36 eV. When the metal surface is covered with only Cu atoms (θCu = 1 ML), the H* adsorption energy further weakens to -0.25 eV. Figure 2 shows the adsorption energy of most stable H* on all surface models; collectively increasing the surface Cu coverage from 0 to 1 ML has a mild impact on H* adsorption strength. For CO2 adsorption, structures with the C atom bound on the adsorption sites are energetically favorable for all surface models. CO2 adsorption energies on various 4-fold sites of Fe(100) and Fe-Cu surfaces are included in Table S1. The adsorption energy calculated for most stable CO2* on all surface models is shown in Figure 2. When the surface Cu coverage is relatively low (θCu < 1/3 ML), CO2* adsorption energy changes slightly from -0.95 eV at θCu = 0 to -0.89 eV at θCu = 2/9 ML, and the energetically most favorable CO2* adsorption is on the 4fold site containing only Fe atoms. Further increasing Cu coverage to 1/3 ML results in a dramatic weakening of CO2* adsorption energy to -0.57 eV due to the disappearance of pure Fe 4-fold sites, and the most stable adsorption occurs on the 4-fold site including the least Cu atoms, i.e. 1 Cu inclusion. With increasing surface Cu content from 1/3 to 2/3 ML, the energetically favorable adsorption remains on the 4-fold site including 1 Cu atom, and thus CO2* adsorption energy remains almost constant, as shown in Figure 2. When the surface Cu coverage increases further to 7/9 ML, the 4-fold sites including 1 Cu disappear and the most stable adsorption occurs on the 4-fold site containing 2 Cu atoms, resulting in a slight weakening of CO2* adsorption energy to -0.46 eV. Further increasing the Cu coverage till the catalyst surface is dominated by 4-fold sites involving 3 or 4 Cu atoms, CO2* adsorption energy further weakens to almost zero, close to that (-0.02 eV) calculated on the monometallic Cu(100) surface. Just as reported in many theoretical studies,55-57 CO2 adsorption on copper was found to be very weak (i.e. physisorption), with the PBE functional unable to precisely describe this weak van der


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Waals interaction for CO2-Cu. We applied a PBE+D3 method to correct the dispersion interaction for CO2 adsorption on Cu(100), and obtained an adsorption energy of -0.28 eV, in good agreement with the experimental values between -0.26 ~ -0.31 eV.58 However, the CO2 adsorption on Cu is still much weaker than that on Fe-rich surfaces. Addition of Cu to the Febased catalyst significantly reduces the CO2 adsorption stability.

Figure 2. Adsorption energy of most stable H* and CO2* on Fe(100) and various Fe-Cu surfaces. θCu = 0 ML denotes a monometallic Fe(100) surface and θCu = 1 ML presents a Fe-Cu catalyst with entirely Cu atoms on the top layer. To explore the impact of surface Cu coverage on the catalytic performance for CO2 conversion, we selected five representative catalyst models (θCu = 0, 2/9, 4/9, 7/9 and 1ML) for subsequent study. These surface models demonstrate varied CO2 adsorption strength due to catalyst surface structure change upon Cu addition, which would potentially reveal the influence of Cu addition on CO2 conversion properties. Figure 3 illustrates the variation of CO2 adsorption energy on these representative surface models calculated with PBE and PBE+D3 methods. CO2 adsorption strength decreases dramatically (from -0.95 to -0.03 eV) with increasing surface Cu coverage θCu from 0 to 1 ML, and there is a nearly linear relationship between CO2 adsorption energy and surface Cu coverage. The dispersion correction does not change the trend of CO2


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adsorption energy versus surface Cu coverage, though it strengthens adsorption by ~0.2 eV. CO2 molecules preferentially adsorb on Fe-rich surfaces.

Figure 3. CO2 adsorption energy as a function of surface Cu coverage (θCu, ML) calculated with (a) PBE, and (b) PBE+D3 methods.

Table 1. The C-O bond length, O-C-O bond angle of CO2, Bader charge on the CO2 moiety in the adsorbed state, and adsorption energy on the representative catalyst surfaces. Surface Cu coverage (θCu, ML)

C-O bond length (Å)

O-C-O bond angle (̊)

Bader charge on CO2 (δ-)

CO2 adsorption energy (eV)

0

1.34

119.8

-1.68

-0.95

2/9

1.33

120.0

-1.65

-0.82

4/9 7/9 1

1.29 1.26 1.18

123.7 136.5 179.8

-1.16 -0.90 -0.04

-0.54 -0.46 -0.03

Table 1 lists the C-O bond length and O-C-O bond angle of CO2*, Bader charge on the CO2* moiety in the adsorbed state, as well as the adsorption energy on the representative catalyst surfaces. CO2 activation lessens with increasing surface Cu coverage, as reflected by the reduction of the C-O bond length and enlargement of the O-C-O bond angle. The degree of CO2 activation towards hydrogenation closely relates to metal-CO2 electron transfer properties. The Fe-rich surface transfers more electrons to the CO2 molecule, weakening the C-O bonds and activating CO2 towards subsequent reaction more significantly than the Cu-substituted bimetallic catalysts.


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Figure 4. Energy profiles of CO2* dissociation on the representative Fe-Cu surfaces. Optimized structures of the initial, transition and final states associated with this step are included. (Orange = copper, purple = iron, red = oxygen, gray = carbon.)

3.2 Dissociation of CO2* Dissociation of CO2* to an adsorbed CO* and O* was examined on the representative FeCu surfaces, and the energy profiles are given in Figure 4. Optimized structures of the initial, transition, and final states involved in this step are included in Figure 4. The most stable initial -12- Environment ACS Paragon Plus

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states of CO2* adsorption are used for transition state search, whereas, in the final states, the CO* and O* species are adsorbed at two adjacent sites that are geometrically favorable for dissociation to occur. The energy barriers for migration of the CO* and O* species from the final states to the most stable adsorption sites were calculated and these barriers are found to be very small (< 0.04 eV).

(c)

Figure 5. Linear relationships between CO2 dissociation (a) energy and surface Cu coverage, (b) activation barrier and surface Cu coverage, and (c) activation barrier and reaction energy on the representative Fe-Cu surfaces.

Figures 5(a) and (b) illustrate the impact of surface Cu coverage on CO2 dissociation energy and activation barrier. Increasing surface Cu coverage dramatically decreases the reaction energy from -1.19 eV at θCu = 0 ML to -0.17 eV at θCu = 1 ML, making the reaction less exothermic. The CO2 dissociation barrier increases from 0.80 to 1.21 eV when surface Cu coverage θCu increases from 0 to 1 ML. Linear relationships of CO2 dissociation energy and activation barrier versus surface Cu coverage are observed. Figure 5(c) plots the activation


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barrier against the reaction energy for dissociation of CO2; satisfying a linear correlation shown in equation (1) with each term in the unit of eV. Eact = 0.39 × Erxn + 1.26

(1)

The high R2 value of 0.96 for this correlation indicates a good linear relationship between the activation barrier and the reaction energy, consistent with the Brønsted-Evans-Polanyi (BEP)59-60 relationship between these two energy parameters.

Figure 6. CO* adsorption energy as a function of surface Cu coverage (θCu, ML). Adsorbed CO* is the product from CO2 dissociation, and the adsorption stability of this intermediate would impact the subsequent conversion and final product distribution. The adsorption properties of CO* on these Fe-Cu surfaces were examined, and the most stable adsorption configurations are shown in Figure S2. Figure 6 plots the adsorption energy of CO* as a function of surface Cu coverage; a good linear relationship (R2 = 0.99) is observed. On Fe(100), the CO* adsorption energy is calculated to be -2.14 eV, similar to that (-2.05 eV) reported by Zhao et al.40 and that (-2.10 eV) by Tian and workers42. Increasing surface Cu coverage from 0 to 1 ML results in a major reduction of CO* adsorption strength from -2.14 to 1.28 eV. When the top layer of the Fe-Cu catalyst is covered entirely with Cu atoms, the adsorption interaction between metal and CO* molecule becomes weaker. Compared with CO2* adsorption on these catalyst surfaces, the adsorption of CO* is found to be much stronger.


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3.3 CO2* Hydrogenation To construct an H*-CO2* co-adsorption initial state, we started with the most stable adsorption structure of CO2* for all surface models, and then placed an H* on a nearby 4-fold hollow site. Based on the stable co-adsorption configurations of H*-CO2*, we further investigated the plausible initial steps of CO2 hydrogenation to produce the possible formate (HCOO*) and carboxyl (COOH*) intermediates on the representative catalyst surfaces. Optimized configurations of the initial, transition, and final states involved in hydrogenation steps are shown in Figure 7, and key structural parameters are given in Table S2. Figure 8 plots the kinetic barriers for HCOO* and COOH* formation versus surface Cu coverage. For HCOO* formation as shown in Figure 7(a), we started with the most stable adsorption configuration of CO2* and placed an H* on the neighboring 4-fold-hollow site where the H* is apt to attack the C atom of CO2 to form a C-H bond. On Fe(100), the CO2 molecule adsorbs on the 4-fold hollow site with the molecular plane along the surface normal, the C atom in the hollow site and closest to the surface, and the two O atoms bound on two adjacent bridge sites. The surface H* is adsorbed on a nearby 4-fold hollow site with the C-H distance of 3.31 Å in this initial state. In the transition state, the bent CO2 leans towards the metal surface and the surface H* moves from the initial hollow site to a neighboring bridge site. The C-H bond is partially formed in this transition state, as reflected by the shortening of the C-H atomic distance from 3.31 Å in the initial state to 1.53 Å in the transition state. In the final state, the C-H bond length is 1.10 Å. The O-C-O bond angle is rather constant around 120° among the initial, transition, and final states. The formed HCOO* species binds to the surface with the two O atoms on two bridge sites of the metal, forming a ~48° angle relative to the metal plane. The H in HCOO* is far from


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the surface. The adsorption energy of HCOO* is calculated to be -3.95 eV. HCOO* formation has an energy barrier of 1.00 eV on Fe(100). On the Fe-Cu surface at θCu = 2/9 ML, the initial, transition, and final state configurations for HCOO* formation are similar to those formed on pure Fe(100) surface, as shown in Figure 7(a) and Table S2. The adsorption energy of HCOO* on this surface is -3.77 eV, and the activation barrier is calculated to be 0.97 eV. When the surface Cu coverage increases to 4/9 ML, the most stable adsorption configuration of CO2* shows an interesting feature as compared with those on the Fe-Cu surfaces at lower Cu coverage. As shown in Figure 7(a), the CO2 molecule adsorbs on the 4-fold site including 3Fe-1Cu with the C atom bound to a Fe atom and the two O atoms interacting with the other two Fe atoms within the 4-fold site. The strong interaction between CO2 and Fe drives CO2 away from surface Cu, forming a ~44° angle relative to the metal surface. The H* is adsorbed at an adjacent hollow site (2Fe-2Cu in the 4-fold site) orientating towards the C atom of CO2 to facilitate C-hydrogenation. In the transition state, the configuration of the CO2 moiety is almost retained. The C-H distance contracts to 1.76 from 3.42 Å in the initial state, and the H* moves from the initial hollow site to a neighboring top Fe site. For the final state, the formed HCOO* species perpendicularly adsorbs on the 4-fold site with the two O atoms bound with two surface Fe atoms and the C-H bond perpendicular to the metal surface. This particular adsorption geometry of the H*-CO2* initial state may facilitate the transition state formation and the energy barrier would be mainly contributed by H* motion from the initial adsorption position to that in the transition state. A significant reduction of the energy barrier is observed for HCOO* formation at 4/9 ML Cu coverage, which is only 0.39 eV. Bader charge calculation results shown in Table S3 indicate that there is a slight change of the charge (∆δ = 0.07) on CO2 moiety during


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the transition state formation at θCu = 4/9 ML, and the electron transfer between the metal and H*-CO2* is less significant as compared with that observed on other Fe-Cu surfaces. The particular binding geometries of H*-CO2* in the initial and transition states, and the resulting electron transfer properties are likely the main cause for the significant reduction of the kinetic barrier for CO2 hydrogenation to HCOO* at 4/9 ML Cu coverage. We obtained an adsorption energy of -3.73 eV for HCOO* on this Fe-Cu surface. Increasing the surface Cu coverage to 7/9 ML makes the Cu atoms dominating the surface, and the most stable CO2 adsorption occurs on the Fe-Fe bridge site with the O-C-O molecular plane almost perpendicular to the metal surface, as shown in Figure 7(a). The H* stays at the 4fold hollow site. The C-H distance is 2.32 Å in the initial state. In the transition state configuration, the CO2 molecule rotates to approach the H* with its C atom, and the H* moves up towards C to form a bond. The C-H atomic distance shortens to 1.42 Å. When the HCOO* final state is formed, the molecular plane becomes perpendicular to the metal surface with the CH bond stick up to be far away from the metal surface. The adsorption energy for HCOO* is calculated to be -3.73 eV, with an activation barrier of 0.51 eV. When the catalyst surface is covered entirely with Cu atoms (θCu = 1 ML), the adsorption of CO2 becomes very weak and the CO2 molecule retains a linear configuration, being ~3.5 Å from the surface plane. The H* adsorbs on a 4-fold hollow site and the C-H interatomic distance is 4.05 Å in the initial state. For the transition state, CO2 moves toward the metal surface and becomes bent to adsorb through the two O atoms, as shown in Figure 7(a). Meanwhile, the C-H atomic distance reduces to 1.56 Å. Large structural change is observed comparing the initial and transition states and there would be a large energy barrier for this hydrogenation. In the final state, the formed HCOO* species vertically adsorbs on the bridge site of the Cu-covered surface,


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and a -3.09 eV adsorption energy is obtained. The energy barrier is calculated to be 0.80 eV on this Fe-Cu surface. Increasing surface Cu coverage makes C-H bond formation more difficult due to electron interaction between the metals and the adsorbates.

(a)

(b)

Figure 7. Optimized structures of the initial, transition, and final states involved in CO2 hydrogenation to produce (a) HCOO* and (b) trans-COOH* intermediates on the representative Fe-Cu catalyst surfaces. (Orange = copper, purple = iron, red = oxygen, gray = carbon, white = hydrogen.)


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Figure 8. Activation barrieres for HCOO* and trans-COOH* formation from CO2 hydrogenation versus surface Cu convergae (θCu, ML). COOH* is another possible intermediate from hydrogenation of CO2. We started with the most stable adsorption configuration of CO2* and placed an H* atom on the neighboring 4-foldhollow site where the H* is apt to attack the O atom of CO2 to form an O-H bond. Based on the initial co-adsorption configuration of H*-CO2* state, a trans-COOH* species would be the preferred product over cis-COOH* because the H* comes from the metal surface and directly attacks one O atom of CO2 to form an O-H bond with the H* pointing to the surface. Figure 7(b) shows the initial, transition, and final states associated with trans-COOH* formation from CO2 hydrogenation on these representative catalyst surfaces; the key structural parameters are included in Table S2. The conversion paths for trans-COOH* to cis-COOH* on these catalyst surfaces were also examined and all states involved in this process are shown in Figure S3. For trans-COOH* formation occurred on Fe(100) and the Fe-Cu surface at θCu = 2/9 ML, the surface H* is adjacent to the adsorbed CO2* molecule and pointing to the O1 atom in the initial state. The H-O1 atomic distances are 2.55 and 2.56 Å on these two surfaces. In the transition state configurations, the H* moves toward the O1 atom of CO2* and the H-O1 distances become shortened to 1.29 Å for these two catalysts. The relative position and geometry of the


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CO2* moiety change slightly comparing the transition with the initial state generated in transCOOH* formation step, as shown in Figure 7(b) and Table S2. The energy required for this hydrogenation step would mainly derive from H* motion from the initial adsorbed positon to the position apt to form an O-H bond in the transition state. The formed trans-COOH* species is adsorbed on the 4-fold site with the C and O2 atoms bonded at two Fe-Fe bridge bonds. The energy barriers for trans-COOH* formation on these two surfaces are 1.16 and 1.15 eV, respectively. Bader charges given in Table S3 show similar charge transfer trend between the initial and transition states in trans-COOH* formation step on these two surfaces. Trans-COOH* formation on the Fe-Cu surface at θCu = 4/9 ML requires the H* adsorbed on a 4-fold hollow site to attack the O1 atom of CO2*, as shown in Figure 7(b). The H-O1 distance is 2.76 Å. In the transition state, the CO2* molecule moves to stay over the Fe-Fe bridge bond in the 4-fold site from a different adsorption position in the most stable initial state, and the H-O1 bond shortens to 1.42 Å. The adsorption positions of CO2* and H* show apparent variation comparing the transition with the initial state, thus the energy requirement would be high for this hydrogenation step. The final trans-COOH* adsorbs at the Fe-Fe bridge bond with the C and O2 atoms bound to the metal surface. It requires higher energy input for this hydrogenation step to occur and the kinetic barrier rises up significantly to 1.36 eV due to the particular initial adsorption configuration and largely changed geometry of the transition state formed on this FeCu surface. Bader charges calculated at 4/9 ML Cu coverage show noticeable variation on CO2* moiety (∆δ = 0.23) during transition state formation and more significant electron transfer between the metal surface and the H*-CO2* state is observed. When the surface Cu coverage increases to 7/9 ML, the surface H* on the initial hollow adsorption site attacks the O1 atom of CO2* that is adsorbed on the Fe-Fe bridge bond. The H-O1


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atomic distance contracts from 2.77 Å in the initial state to 1.40 Å in the transition state. The CO2* stays at the Fe-Fe bridge bond during the transition state formation. In the final state, a trans-COOH* species is formed on the Fe-Fe bridge bond with the C and O2 interact with the surface Fe and an O-H bond is formed (H-O1 distance of 0.98 Å). The kinetic barrier is calculated to be 1.27 eV. On the Fe-Cu surface covered entirely with Cu atoms, the CO2 molecule is physically adsorbed at the 4-fold hollow site and the H* stays at a neighboring hollow site, as shown in Figure 7(b). The H-O1 distance is 3.76 Å. In the transition state, CO2 moves towards the metal surface and is adsorbed on a Cu-Cu bridge bond through bonding with the C and O2 atoms. The surface H* approaches the O1 atom of CO2* and the H-O1 distance shortens to 1.50 Å. The O-CO bond angle shrinks from 179.8° in the initial state to 128.5° in the transition state. Large structural change of CO2* state from the initial to transition state would result in substantial energy barrier for this hydrogenation step. The activation barrier increases to a maxima of 1.50 eV for trans-COOH* formation at 1 ML Cu coverage. The charge on CO2* part has 0.72 δ variation comparing the transition with the initial state, resulting in more significant electron transfer from the metal surface to adsorbates. The reaction energy and activation barrier for conversion of trans-COOH* to cis-COOH* on these representative catalyst surfaces are given in Table S4. There is a small difference in the thermodynamic stability of these two isomers on these surfaces, and the energy barriers are moderate (< 0.5 eV) for this conversion. The adsorption energies calculated for trans-COOH* formation on the catalysts at surface Cu coverage of 0, 2/9, 4/9, 7/9, and 1 ML are -2.77, -2.61, -2.40, -2.39, and -1.91 eV, respectively, less energetically stable than formation of HCOO* on these surfaces. Compared


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with HCOO* production, formation of trans-COOH* is kinetically unfavorable due to large energy barriers as shown in Figure 8, and CO2 hydrogenation to a HCOO* intermediate is the preferred path on these catalyst surfaces. Once HCOO* is formed, it will be then hydrogenated to a HCOOH* intermediate and further conversion will proceed through these preferred intermediates to form the final products. However in the present work, we focus on examining the initial steps of CO2 hydrogenation on these representative catalyst surfaces which play important roles in determining the reaction pathways. The path going through a HCOO* intermediate likely proceeds without forming CO* due to the C-H bond already formed in the first hydrogenation step of CO2. The dissociation of CO2 produces CO* and the reaction energetics is apparently impacted by the composition and structure of the catalysts, as shown in Figure 4. Increasing surface Cu coverage not only makes CO2 dissociation thermodynamically unfavorable but also increases CO2 dissociation barriers. Figure 9 plots the energy barriers for CO*, HCOO*, and COOH* formation through CO2 dissociation and hydrogenation on these catalyst surfaces. COOH* formation is kinetically unfavorable on all the surfaces. On Fe(100) without Cu addition, the activation barrier for CO* formation from dissociation of CO2 is 0.20 eV lower than that for formation of HCOO* through CO2 hydrogenation, and thus CO* is the preferred intermediate on the monometallic Fe surface. Adding Cu into the catalyst to θCu of 2/9 ML, CO2 dissociation and hydrogenation become competitive in kinetics (Eact = 0.88 vs. 0.97 eV), therefore, both CO* and HCOO* are the main intermediates. Further increasing the surface Cu coverage enlarges the energy barrier differences for CO* and HCOO* formation, making HCOO* kinetically more preferred. Particularly at θCu of 4/9 ML, CO2 hydrogenation barrier for forming HCOO* is 0.67 eV lower than that for dissociation of CO2, and HCOO* formation


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proceeds ~106 faster than forming CO* on this catalyst under the experimental condition at 573 K18.

Figure 9. Activation barriers for CO*, HCOO*, and COOH* formation versus surface Cu convergae (θCu, ML) on the representative surfaces. Our calculation results demonstrate that the composition and structure of the catalysts determine the preferred intermediates and thus impact the reaction pathways and selectivity to final products. The major finding of the present work is consistent with that of Sykes and collaborators61-62 who reported on selective hydrogenation of alkenes and alkynes on Pd-doped Cu surface in that both the present and previous studies show that adding a second metal to a metal surface can not only vary the surface composition but also change the catalytic reaction pathway. Furthermore, this computational study provides useful references for experimental research and also a rational theoretical model for studying the synergic effects of different Fe-M (M = Co, Ni, Pd, etc.) bimetallic catalysts which is important for catalyst design and optimization. Rational design of the bimetallic catalysts could better realize their synergic effects for highly selective synthesis of desired hydrocarbon products. Tailoring the adsorption properties of hydrogen and carbon species through modulating the Fe–M formulation could modify the energy preferences of key intermediates. Meanwhile, controlling the kinetic favorability for C-O bond breaking, C-C bond forming versus C-H bond formation via -23- Environment ACS Paragon Plus

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optimization of the Fe–M formulation could provide a new strategic direction for tailoring desired hydrocarbon products. However, more thorough theoretical and experimental work still needs to validate these assumptions which we will shed light on in our future work.

4. Conclusions In this work, we perform density functional theory calculations to investigate the adsorption, activation, dissociation, and initial hydrogenation of CO2 on Fe-Cu bimetallic catalysts. Fe-rich surfaces activate CO2 more significantly than Cu-rich surfaces due to more electron transfer from the Fe-surface to CO2 molecule. The adsorption energy of CO2 scales linearly with respect to surface Cu coverage, and the adsorption strength decreases as surface Cu coverage increases. For dissociation of CO2 on the Fe-Cu surfaces, the reaction energy becomes less exothermic and the activation barrier increases as the surface Cu coverage increases from 0 to 1 ML. The activation barrier scales linearly with the reaction energy, consistent with the BEP relationship. CO2 hydrogenation to a formate (HCOO*) intermediate is found to be kinetically favored over formation of a carboxyl (COOH*) species at all surface Cu coverages. Comparing CO* formation from dissociation of CO2, CO* is the preferred intermediate at lower surface Cu coverage of 2/9 ML or below, however, the favorable conversion path turns to CO2 hydrogenation to a HCOO* intermediate when surface Cu coverage increases to 4/9 ML or higher. The calculation results in this work show a dramatic lowering of the kinetic barrier by ~0.4 eV in the favorable HCOO* formation path at θCu = 4/9 ML as compared with the favorable CO* formation path on the monometallic Fe(100) surface. Cu coverage of 4/9 ML corresponds to a Cu/(Cu+Fe) atomic ratio of 0.11 in our computational model and this Fe-Cu formulation becomes a useful reference for future experimental effort in design of the Fe-based HHS


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catalysts. Our calculation results indicate that adding small amount of Cu facilitates the kinetics for CO2 conversion to key intermediates, alters the dominant reaction paths, and thus influences the selectivity to final products.

ASSOCIATED CONTENT Supporting Information. Supporting Information associated with this article can be found, in the online version, at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 21503027 and No. 21503029), the Fundamental Research Funds for the Central Universities (No. DUT15RC(3)027 and No. DUT15ZD236) and the QianRen B Program of the Chinese Government.

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