Mechanistic Insight Into C-C Coupling over Fe-Cu Bimetallic Catalysts

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Mechanistic Insight Into C-C Coupling over FeCu Bimetallic Catalysts in CO Hydrogenation 2

Xiaowa Nie, Haozhi Wang, Michael J. Janik, Yonggang Chen, Xinwen Guo, and Chunshan Song J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017

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The Journal of Physical Chemistry

Mechanistic Insight into C-C Coupling over Fe-Cu Bimetallic Catalysts in CO2 Hydrogenation Xiaowa Niea, Haozhi Wanga, Michael J. Janikb, Yonggang Chen d, 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, P. R. 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 d

Supercomputing Center , Dalian University of Technology, Dalian 116024, P. R. China

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 the mechanism of CO2 hydrogenation in production of C1 and C2 hydrocarbons over Cu-Fe bimetallic catalyst. CH* is found to be the most favorable monomeric species for production of CH4 and C2H4 via C-C coupling of two CH* species and subsequent hydrogenation. On the bimetallic Cu-Fe(100) surface at 4/9 ML Cu coverage, the energetically preferred path for CH* formation goes through CO2*HCOO*HCOOH*HCO*HCOH*CH*, in which both the HCOO*HCOOH* and HCO*HCOH* steps have substantial barriers. The bimetallic surface suppresses CH4 formation and is more selective to C2H4 due to the higher hydrogenation barrier of CH2* species relevant to those for C-C coupling and CH-CH* conversion to C2H4. On mono-metallic Fe(100) surface, CH* formation goes through a path of CO2*CO*HCO*HCOH*CH*, different from that identified on Cu-Fe(100). The hydrogenation of HCO* to HCOH* is the rate-limiting step that controls CO2 conversion to CH4 and C2H4. CH4 formation is kinetically more favored, with a 0.3 eV lower energy barrier, than C2H4 formation. The bimetallic combination of Cu and Fe enhances CO2 conversion by reducing the kinetic barriers, and alters the selectivity preference to more valuable C2H4 from CH4 on mono-metallic Fe surface. C2H6 can be produced from further hydrogenation of C2H4 with moderate barriers.


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1. Introduction The environmental influence of increasing concentration of CO2 in the atmosphere has caused global concern for climate change and controlling CO2 emission has become a major global challenge.1-2 One of the ways to reduce this problem is to convert CO2 with hydrogen produced from water using renewable energy to valuable industrial feedstocks such as lower olefins, liquid hydrocarbons, and clean fuels, as doing so reduces the dependence on petroleum.37

CO2 hydrogenation provides an alternative path to hydrocarbon products currently produced from petroleum, as it utilizes abundant CO2 as a chemical feedstock and hydrogen as the coreactant.8-11 Fe-based catalysts have shown good performance for CO2 hydrogenation processes including reverse water–gas shift (RWGS) followed by Fischer-Tropsch (F-T) synthesis.12-17 However, Fe catalysts show low selectivity to industrially important light olefins and higher hydrocarbons, which represents a general limitation in F-T synthesis.14 Many of the catalysts which are useful in F-T synthesis are also capable of catalyzing the hydrogenation of CO2 to hydrocarbons, such as Co, Ni, and Fe-based catalysts.17-26 Co catalysts are widely used in F-T synthesis due to high performance-cost evaluation. However, upon switching the feed from syngas to a CO2 and H2 mixture, Co performs as a methanation catalyst rather than as an F-T catalyst.21 For this reason, Fe-based catalysts are of more interest for CO2 hydrogenation. Potassium acts as a promoter for Fe-based catalysts in F-T synthesis.14,

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Our recent

experimental work has focused on developing novel Fe-based bimetallic catalysts for selective conversion of CO2 to C2+ olefins28 and hydrocarbons.29-32 Bimetallic combinations of Fe with small amounts of Co, Ni, Cu, and Pd transition metals, together with a potassium promoter, have led to significantly higher activities for olefin-rich hydrocarbons production30 than the previous state-of-the-art K-Fe/Al2O3 catalyst.33-34 These promising experimental results point to a strategy -3- Environment ACS Paragon Plus


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towards designing novel bimetallic catalysts for highly selective synthesis of C2+ olefins and liquid hydrocarbons. A particularly interesting feature of the product spectrum of CO2 hydrogenation over Febased bimetallic catalysts is the variation of the distribution of CH4 and C2+ hydrocarbons with changing the type and loading level of the second transition metals introduced to the Fe catalysts.28-30 If monometallic Fe, Co, Ni, Cu, or Pd catalysts are used, only Fe yields C2+ hydrocarbons whereas Ni, Co, and Pd catalysts yield mainly CH4 and Cu catalyst leads to merely CO.30 The combination of Fe with a small amount of each of these transition metals led to a notable enhancement of C2+ hydrocarbons from CO2 hydrogenation, while much lower Ni content was required for this enhancement than Co.30 This particular feature of promotion on CO2 hydrogenation to desired hydrocarbons with the bimetallic synergic effect has attracted our attention on a theoretical study since the mechanistic aspects of C1 and C2+ hydrocarbon synthesis in terms of C-O bond breaking, O-H bond forming, C-H bond forming, C-C coupling, and carbon chain growth pathway in CO2 hydrogenation over Fe-based bimetallic catalysts are still unresolved. Density functional theory (DFT) calculations are uniquely positioned for exploring catalyst structure and reaction mechanism for such complex reaction sequences.35-44 Cu-based catalysts are widely used in higher alcohol synthesis (HAS) from syngas and the key steps for C-C coupling and chain growth were determined as CO insertion into CHx species based on DFT calculations.45-49 Cu plays an important role in providing undissociated CO/HCO and reducing the barrier for CO/HCO insertion toward the oxygenate precursors formation.46, 48 In hydrocarbon synthesis from CO/CO2 hydrogenation, Cu catalysts are commonly used in electroreduction reactions; the dominant path for C-C coupling occurred through CO dimerization42, 50-54 or coupling of two CH2 species,37, 39, 42 depending on the Cu facets employed (e.g. 111 or 110) and the feed (CO or CO2), and also the preferred path was found to be sensitive -4- Environment ACS Paragon Plus

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to solvent under electroreduction environment. 37, 39, 54-55 Fe-based catalysts have shown good performance in F-T synthesis. A previous DFT work by Li and co-workers found that CH* is the most abundant CHx species on Fe(111), and it proceeds, kinetically more favored, via CH+CH coupling to form CH-CH which is then hydrogenated to hydrocarbons.56 A DFT study of C-C coupling on stepped metal surfaces demonstrated that the dominant chain growth pathway is C+CH3 coupling to C-CH3 on Fe(210).57 These studies indicate an important dependence of reaction pathways on the surface composition and structure. Adding Cu into Fe could modify the structure and surface properties of the Fe catalysts, which would impact the stability of the intermediates and transition states, and thus alter the dominant path. In our previous theoretical work,58 we constructed a series of Cu-Fe(100) bimetallic catalyst surfaces with top-layer Fe atoms substituted by different numbers of Cu atoms, thus generating a series of Cu-Fe bimetallic catalysts with surface 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. The surface of the bimetallic Cu-Fe catalysts plays an important role in CO2 hydrogenation since the synergic effect was observed in enhancing CO2 conversion and increasing selectivity to olefins upon adding a small amount of Cu to Fe, as evidenced from our experimental work.30 On these Cu-Fe bimetallic models, we found CO2 hydrogenation to a formate intermediate (HCOO*) is kinetically favored over formation of a carboxyl species (COOH*) at all surface Cu coverages examined from 0 to 1 ML. CO* is the preferred intermediate at the 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. Meanwhile, we observed a dramatic lowering of the kinetic barrier in the favorable HCOO* formation path at 4/9 ML Cu coverage within the surface. These calculation results suggest that specific amount of Cu addition facilitates the kinetics for CO2 conversion to key intermediates, alters the dominant reaction paths, and thus -5- Environment ACS Paragon Plus


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influences the selectivity to final products. Cu surface coverage of 4/9 ML corresponds to a Cu/(Cu+Fe) atomic ratio of 0.11 in our computational model, around which enhanced CO2 conversion and selectivity to olefins were achieved experimentally.30 In this work, we selected Fe(100) and the specific Cu-Fe(100) surface at 4/9 ML Cu coverage as the representative monometallic and bimetallic catalyst models to study the mechanism for C1 and C2 hydrocarbons synthesis during CO2 hydrogenation. DFT calculations were performed to examine the reaction intermediates and energetics for each elementary step proposed. According to the calculated reaction energy and activation barrier for individual elementary steps proposed, we elucidated the minimum energy path for CO2 conversion to CH4 and C2H4 on Fe(100) and the bimetallic Cu-Fe(100) surface.

2. Computational Methodology 2.1 Electronic structure methods All calculations were performed within the framework of density functional theory (DFT) by using the Vienna ab initio simulation program (VASP), a plane-wave DFT software package.59-60 The projected augmented wave (PAW)61 pseudopotentials were used to describe the interactions between ions and electrons. The exchange and correlation energies were computed using the Perdew, Burke, and Ernzerhof (PBE)62 functional within the generalized gradient approximation (GGA).63 A plane wave basis set with a cutoff energy of 400 eV was used. Spinpolarized 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. The atomic geometries were relaxed using the damped molecular dynamics algorithm as implemented in VASP code 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)64 -6- Environment ACS Paragon Plus

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method. The minimum energy path was examined using 4−6 images, including the initial and final state, during the transition state search. Vibrational frequencies were evaluated to confirm the minima and transition states. 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 10 Å to avoid interactions between the repeating slabs. The validity of using the slab containing 4 atomic layers was verified in our previous work.58 A 4/9 surface Cu content within Fe(100) constructed in our previous work58 was selected as the representative bimetallic Cu-Fe catalyst in this work to study the mechanisms of CO2 hydrogenation to lower hydrocarbons, because this surface model shows superior CO2 adsorption and initial hydrogenation when reacting with hydrogen.58 The bottom two layers were frozen at their equilibrium bulk positions, whereas the top two layers together with the adsorbates were allowed to relax.

3. Results and Discussion 3.1 Mechanism for formation of C1 species from CO2 hydrogenation In our previous DFT work,58 we have found that CO2 dissociation to CO* is preferred on Fe(100) and the kinetically favorable path changes to CO2 hydrogenation to a HCOO* intermediate when surface Cu coverage increases to 4/9 ML or higher. COOH* formation was found to be energetically unfavorable at all surface Cu coverages. In the present work, we further examined the reaction mechanism from HCOO* and CO* to important C1 species including C* and CHx* species on Fe(100) and the Cu-Fe(100) surface at 4/9 ML Cu coverage. Reaction networks examined to identify energetically favorable C1 species from CO2 hydrogenation on the two surfaces are shown in Figure 1(a) and (b), and the activation barrier for each elementary -7- Environment ACS Paragon Plus


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step is given. Figure 2 illustrates the optimized structures of the initial, transition, and final states associated with individual elementary step examined for C1 species formation in CO2 conversion on Fe(100) and the Cu-Fe(100) surface.

(a) (b) Figure 1. Reaction networks examined to identify energetically favorable C1 species from CO2 hydrogenation on (a) Fe(100) and (b) the Cu-Fe(100) surface at 4/9 ML Cu coverage. Activation barriers are given in eV. (The networks connected with red arrows represent the preferred path for CO2 conversion to CH*.)


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(a)



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(b) Figure 2. Optimized structures of the initial, transition, and final states associated with individual elementary step examined for C1 species formation in CO2 hydrogenation on (a) Fe(100) and (b) the Cu-Fe(100) surface at 4/9 ML Cu coverage (blue = iron, orange = copper, gray = carbon, red = oxygen, white = hydrogen).

On Fe(100), CO* is the more preferred intermediate than both HCOO* and COOH* formed from CO2 hydrogenation.58 CO* is then hydrogenated to HCO* with an energy barrier of 0.89 eV and formation of COH* from CO* hydrogenation is kinetically hindered due to a high energy barrier of 1.57 eV. Another path to produce HCO* through HCOO*HCOOH*HCO*+OH* is kinetically unfavorable as compared to that from CO* hydrogenation, as shown in Figure 1(a). Once HCO* is formed, it will be further hydrogenated to a HCOH* species, with a kinetic barrier of 1.32 eV. The C-O bond breaking of HCOH* is very facile on Fe(100), and the barrier is only 0.13 eV. The formed CH*+OH* species is quite stable on the catalyst surface and the reaction energy for this step is 2.21 eV exothermic. On Fe(100), formation of CH* species goes through a path of CO2*CO*HCO*HCOH*HC* and HCO* hydrogenation to HCOH* controls the rate. CO* hydrogenation to COH* has a significant barrier, thus we did not consider C* formation from C-O bond breaking of COH*. As for other CHx* species including CH2*, CH3*, and C* species, we further examined their formation from hydrogenation or decomposition of CH*. Optimized structures of the transition states associated with the hydrogenation reactions involving C* and CHx* species are illustrated in Figure 3(a). C-H bond formation reaction energies, activation barriers, and C-H distances (dC-H) at the transition states are presented in Table 1. The energy barrier for C* formation from CH* decomposition is calculated to be 0.45 eV, and formation of CH2* from hydrogenation of CH* has a barrier of 0.76 eV. Its further hydrogenation to CH3* needs to surmount an energy barrier of 0.93 eV. The barriers involving C* and CHx* species are moderate under reaction conditions.


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(a)

(b) Figure 3. Top and side view (inserted) of the transition state configurations associated with the hydrogenation reactions involving C* and CHx* species on (a) Fe(100) and (b) the Cu-Fe(100) surface at 4/9 ML Cu coverage (blue = iron, orange = copper, gray = carbon, white = hydrogen).

Table 1. Reaction energies, activation barriers, and C-H distances (dC-H) in the transition states for hydrogenation reactions involving adsorbed C and CHx species on Fe(100) and the CuFe(100) surface at 4/9 ML Cu coverage. Hydrogenation reactions C*+H* CH* CH*+H* CH2* CH2*+H* CH3* CH3*+H* CH4*

Fe(100) surface Erxn (eV) Eact (eV) dC-H (Å) 0.33 0.78 1.54 0.53 0.76 1.53 0.22 0.93 1.49 -0.20 0.82 1.53

Cu-Fe(100) surface Erxn (eV) Eact (eV) dC-H (Å) 0.13 0.77 1.56 0.44 0.73 1.55 -0.05 1.01 1.77 -0.39 0.64 1.56

On the Cu-Fe(100) surface at 4/9 ML Cu coverage, our previous DFT work demonstrated that HCOO* is the kinetically preferred intermediate from CO2 hydrogenation and the activation



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barrier for this step is only 0.39 eV, much lower than that for CO* and COOH* formation from CO2 dissociation and hydrogenation.58 Therefore, the first key intermediate in CO2 hydrogenation over Cu-Fe(100) is HCOO*, different from that identified over Fe(100) (on which CO formation is found to be more preferred). In this work, we then examined further conversion of the preferred HCOO* intermediate. HCOOH* formation from hydrogenation of HCOO* has an energy barrier of 1.27 eV and further conversion of HCOOH* to HCO* via C-O bond breaking has a small barrier, only 0.35 eV, as shown in Figure 1(b). HCO* formation from CO* hydrogenation was examined as well, and a barrier of 0.87 eV was found. In parallel, CO* hydrogenation to COH* was investigated, however, this step towards O-H bond formation has a significant barrier of 1.72 eV, and thus is blocked on the catalyst surface. In consequence, C* species will be unable to be produced through C-O bond breaking of COH*. Despite CO* hydrogenation to HCO* has a lower barrier (0.87 eV) compared to the larger barrier (1.27 eV) in the path from HCOO* to HCO*, the formation of CO* needs to overcome a large kinetic barrier (1.06 eV) than CO2 hydrogenation to HCOO* (0.39 eV). Therefore, HCO* formation goes through the path of CO2*HCOO*HCOOH*HCO*. Once HCO* is formed, it will be further hydrogenated to a HCOH* species with an energy barrier of 1.22 eV. Similar as observed on Fe(100), C-O bond breaking of HCOH* proceeds relatively fast on Cu-Fe(100), and a 0.32 eV of activation barrier was obtained for CH* formation, as shown in Figure 1(b). The preferred path associated with C1 species in CO2 conversion on the Cu-Fe(100) surface goes through CO2*HCOO*HCOOH*HCO*HCOH*HC*, and both the hydrogenation of HCOO* to HCOOH* and HCO* to HCOH* have substantial barriers. The CH* species can dissociate to C* or further hydrogenate to CH2* species, and the energy barriers are 0.64 and 0.73 eV, respectively, for these two elementary steps. Hydrogenation of CH2* to CH3* needs to overcome a barrier of 1.01 eV. Optimized structures of the transition states associated with the -12- Environment ACS Paragon Plus

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hydrogenation reactions involving C* and CHx* species are illustrated in Figure 3(b). C-H bond formation reaction energies, activation barriers, and C-H distances (dC-H) at the transition states are presented in Table 1. The relative stability of these C1 species including C* and CHx* was investigated on Fe(100) and the Cu-Fe(100) surface at 4/9 ML Cu coverage. The C*, CH*, and CH2* species are found to stably adsorb on the 4-fold hollow site whereas the bridge site is the energetically favorable adsorption site for CH3* on the two catalyst surfaces. Table 2 lists the relative stabilities of the CHx* (x = 1-3, Ex) species with respect to C*. Ex is defined as the total energy difference between the CHx* + (4-x)H* and C* + 4H* adsorbed on the metal surfaces. A positive Ex means the CHx* species is less stable than the adsorbed C*. The larger the Ex is, the less stable the C1 species. On Fe(100), adsorbed C* is the most stable C1 species and CH* is less stable than C*. CH2* and CH3* are unstable as compared to C* and CH* species. This trend of stability of C1 species does not change on Cu-Fe(100), as observed from Table 2. For the various Cu-Fe surfaces studied in our previous DFT work,58 we found that the 4-fold sites involving less Cu atoms are energetically favorable adsorption positions for CO2* and H*. On the Cu-Fe(100) surface at 4/9 ML coverage, the stable adsorption of these CHx* species occurs on the 4-fold site containing only one Cu atom, and the resulting surface adsorption does not alter the relative stability order of these CHx* species as compared to that on the Fe(100) surface. Based on the kinetic comparison for C-H bond formation reactions for C* and CHx* species, as well as their relative stability given in Table 2, it is presumed that these C1 species coexist under reactions conditions. Methane is one of the major products from CO2 hydrogenation on mono-metallic Fe and bimetallic Cu-Fe surfaces, and we further examined the reaction energetics for CH4 formation



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from hydrogenation of the CH3* species on the two surfaces. The activation barriers are found to be 0.82 and 0.64 eV, respectively, on Fe(100) and Cu-Fe(100), as observed from Table 1.

Table 2. Relative Stabilities of adsorbed CHx (x = 1-3, Ex) species with respect to adsorbed C. species. Relative stabilities Fe(100) Cu-Fe(100) (4/9 ML Cu) E1 0.34 0.15 E2 0.95 0.66 E3 1.04 0.75

3.2 C-C coupling and C-H bond formation mechanism for C2 species C1-C1 coupling reactions are key steps for lower hydrocarbons formation and carbon chain growth to produce higher hydrocarbons. We therefore systematically examined possible coupling of the C1 species involving C*, CH*, CH2*, and CH3* to form C2 intermediates, as well as subsequent C-H bond formation for C2 species to produce lower hydrocarbons. 3.2.1 C-C coupling and C-H bond formation on Fe(100) C-C coupling mechanism Optimized structures of the transition states produced from C-C coupling reactions involving C*, CH*, CH2*, and CH3* species on Fe(100) are illustrated in Figure 4. C*, CH*, and CH2* species are stabilized at the 4-fold hollow sites. For the initial states of C* + C*, C* + CH*, C* + CH2*, CH* + CH*, CH* + CH2*, and CH2* + CH2* steps, we placed the two reactants on adjacent 4-fold hollow sites. At the transition states for C* + C*, C* + CH*, C* + CH2* coupling steps, C* is always on the 4-fold hollow site with other reactants moving to an adjacent bridge site to form a C-C bond, as shown in Figure 4. This TS formation tendency is consistent with the relative stability of C1 species on Fe(100) as given in Table 2, showing that


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the C* species is more stable (less mobile) than CH* and CH2*. At the transition states of CH* + CH* and CH* + CH2* coupling steps, CH* species stays at the 4-fold hollow site while CH2* moves to the bridge site, in line with the relative stability order that CH* adsorption is much stronger than CH2* on Fe(100). When CH2* reacts with CH2*, they both move from the initial hollow sites towards the bridge site for C-C bond forming. CH3* species is found to be more stable at the bridge site, and when it reacts with C* or CH* species, the stabilized transition states show a migration of CH3* to an off-top site with the C* and CH* species remaining at the 4-fold hollow sites. CH3* coupling with CH2* produces a transition state where CH2* moves from the initial 4-fold hollow site to the bridge site and CH3* moves to an adjacent off-top site. For the transition state in coupling of two CH3* species, the reactants approach to each other from the initial adsorbed bridge sites by rotation of the molecules to form a C-C bond, as observed from Figure 4. C-C coupling reaction energies, activation barriers, and C-C distances (dC-C) at the transition states are given in Table 3. On Fe(100), the C-C coupling of two C* species is strongly endothermic with a reaction energy of 1.80 eV and the activation barrier is extremely high, around 2.0 eV. The C-C coupling of C* with CH* and CH2* species still requires overcoming large kinetic barriers of 1.92 and 1.68 eV, although the reaction energies are moderate (0.77 and 0.36 eV, respectively). When C* reacts with a CH3* species, the reaction energy becomes 0.24 eV endothermic and the activation barrier decreases to 1.10 eV. For CH* species, the activation barriers are 1.23, 1.51, and 1.30 eV, respectively for coupling with CH*, CH2*, and CH3* adsorbates, and the reaction energies are 0.91, 0.69, and 0.56 eV. For C-C coupling of two CH2* species, it requires to overcome a kinetic barrier of 1.80 eV. The activation barrier for CH2* coupling with CH3* is 1.33 eV and the reaction energy is almost zero. When CH3* reacts with another CH3* species, the formed CH3-CH3* product is weakly adsorbed on the metal surface. -15- Environment ACS Paragon Plus


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The energy barrier for this coupling step is extremely high, around 2.50 eV, therefore direct coupling of two CH3* species is unlikely on Fe(100). The C-C distances in the transition states for these C-C coupling reactions are with the range of 1.8 ~ 2.1 Å. Formation of CH-CH* and CCH3* intermediates have lowest barriers over Fe(100), suggesting that C2H4 formation most likely goes through a key CH-CH* intermediate via coupling of two CH* species. C2H6 might be produced through further hydrogenation of C2H4 on the metal surface or hydrogenation of C2 species such as C-CH3*.

Figure 4. Top and side view (inserted) of the transition states produced from C-C coupling reactions of C1 to form C2 species on Fe(100) (blue = iron, gray = carbon, white = hydrogen).

Table 3. C-C coupling reaction energies, activation barriers, and C-C distances (dC-C) in the transition states produced in C2 species formation reactions on Fe(100) and the Cu-Fe(100) surface at 4/9 ML Cu coverage. C-C coupling reactions C*+C* C-C*

Erxn (eV) 1.80

Fe(100) surface Eact (eV) dC-C (Å) 1.90 1.78

Cu-Fe(100) surface Erxn (eV) Eact (eV) dC-C (Å) 1.21 1.70 1.75


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C*+CH* C-CH* C*+CH2* C-CH2* C*+CH3* C-CH3* CH*+CH* CH-CH* CH*+CH2* CH-CH2* CH*+CH3* CH-CH3* CH2*+CH2* CH2-CH2* CH2*+CH3* CH2-CH3* CH3*+CH3* CH3-CH3*

0.77 0.36 0.24 0.91 0.69 0.56 0.06 0.05 -0.33

1.92 1.68 1.10 1.23 1.51 1.30 1.80 1.33 2.52

1.98 2.01 2.03 1.92 1.98 2.00 2.11 2.11 1.93

0.36 -0.05 -0.06 0.45 0.27 0.38 -0.20 -0.25 -0.72

1.27 1.27 1.04 0.76 1.17 1.09 1.38 1.08 2.58

1.93 2.11 2.04 1.96 1.99 2.05 2.00 2.13 2.11

(a)

(b) Figure 5. Top and side view (inserted) of the transition states produced from C-H bond formation reactions for C2 species on (a) Fe(100) and (b) the Cu-Fe(100) surface at 4/9 ML Cu coverage (blue = iron, orange = copper, gray = carbon, white = hydrogen).

C-H bond formation mechanism In the study of C-C coupling of C1 species on Fe(100) discussed above, we presumed that CH-CH* will be a key intermediate in formation of C2H4 and direct coupling of two CH2* species is kinetically unfavorable due to a large energy barrier of 1.80 eV. We, therefore, examined the reaction energetics for hydrogenation of CH-CH* to produce C2H4. Optimized configurations of the transition states produced from C-H bond formation reactions for C2 species are illustrated in Figure 5(a). The activation barrier for CH-CH* hydrogenation is 0.58



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eV and further hydrogenation of CH-CH2* to form CH2-CH2* requires to overcome a barrier of 0.59 eV, as listed in Table 4. Once the C1 species are formed, any C2H4 formation likely goes through C-C coupling of two CH* species, and then the produced CH-CH* intermediate further reacts with surface H* to form adsorbed CH2-CH2*. The desorption energy of CH2-CH2* to the gaseous C2H4 is 0.81 eV. CH3-CH3* formation from direct C-C coupling of two CH3* species is kinetically hindered due to a huge energy barrier of 2.52 eV. Once CH2-CH2* is produced on the metal surface, it may be further hydrogenated to C2H6. We calculated the activation barriers for hydrogenation of CH2-CH2* and CH2-CH3* to form CH3-CH3* on Fe(100), and these barriers are moderate, around 0.9 eV. The activation barriers for C-C coupling of CH3* with C*, CH*, and CH2* are 1.10, 1.30 and 1.33 eV, competitive with CH-CH* formation from coupling of two CH* specie with a barrier of 1.23 eV. Further hydrogenation of these C2 species such as C-CH3* and CH-CH3* likely have moderate barriers lower than C-C coupling barriers. These calculation results suggest that C2H6 would be produced from hydrogenation of C2 species such as C-CH3* or further hydrogenation of CH2-CH2* based on the calculation results on elementary reaction energetics, and of course the preference of these pathways also depends on the competitive adsorption and surface coverage of H* relative to those C1 species. Table 4. Reaction energies, activation barriers, and C-H distances (dC-H) in the transition states produced in C-H bond formation reactions for C2 species on Fe(100) and the Cu-Fe(100) surface at 4/9 ML Cu coverage. C-H bond forming reactions CH-CH*+H* CH-CH2* CH-CH2*+H* CH2-CH2* CH2-CH2*+H* CH2-CH3* CH2-CH3*+H* CH3-CH3*

Erxn (eV) 0.35 -0.03 0.00 -0.26

Fe(100) surface Eact (eV) dC-H (Å) 0.58 1.48 0.59 1.72 0.88 1.64 0.91 1.51

Cu-Fe(100) surface Erxn (eV) Eact (eV) dC-H (Å) 0.36 0.67 1.51 -0.05 0.76 1.77 -0.14 0.74 1.55 -0.49 0.58 1.62

3.2.2 C-C coupling and C-H bond formation on Cu-Fe(100) C-C coupling mechanism


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Optimized structures of the transition states produced from C-C coupling reactions of C1 species on the Cu-Fe(100) surface are illustrated in Figure 6. C*, CH*, and CH2* species are energetically more stable at the 4-fold hollow sites involving less Cu (i.e. 1 Cu atom inclusion) while CH3* prefers to adsorb on the bridge site of Fe-Fe. To examine the C-C coupling kinetics, we placed the reactants at two adjacent 4-fold sites including only 1 Cu atom, and found the transition state formation tendencies are quite similar as those observed on Fe(100). However, the C-C coupling kinetics would be different since the introduction of Cu modifies the surface electron distribution of the Fe catalyst, and the resulting change of catalyst composition and structure impacts the stability of the intermediates and transition states. The reaction energies, activation barriers, and C-C distances (dC-C) at the transition states for C–C coupling reactions on the Cu-Fe(100) surface are included in Table 3. With the exception of CH3-CH3 coupling, all C-C bond formation barriers are lowered due to the inclusion of Cu on the surface. Direct coupling of two C* species is strongly endothermic with large activation barrier of 1.70 eV. The energy barriers for C-C coupling of C* with hydrogenated C1 species CH*, CH2*, and CH3* are 1.27, 1.27 and 1.04 eV, respectively, quite lower than that for C-C coupling of two C* species. With regard to CH* species, direct coupling of two CH* has an activation barrier of 0.76 eV, and the reaction energy is 0.45 eV endothermic. When it reacts with CH2* and CH3* adsorbates, the energy barriers increases to 1.17 and 1.09 eV. C-C coupling of two CH2* species is 0.2 eV exothermic with a barrier of 1.38 eV. When it is coupled with a CH3* species, the barrier goes down to 1.08 eV. For direct coupling of two CH3* species, the CH3-CH3* product is weakly adsorbed on the metal surface. The energy barrier for this coupling step is extremely high, around 2.6 eV, and thus is blocked on the Cu-Fe(100) surface. The C-C distances in the transition states for these C-C coupling reactions are within the range of 1.7 ~ 2.1 Å. These calculation results on C-C coupling energetics show that CH2-CH2* formation from -19- Environment ACS Paragon Plus


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direct coupling of CH2* species is kinetically unfavorable and C2H4 production would proceed via a CH-CH* intermediate from C-C coupling of two CH* species, which has a moderate barrier of 0.76 eV. C2H6 formation through direct coupling of two CH3* species is kinetically prohibitive due to extremely high barrier and it might be produced via further hydrogenation of C2H4 on the metal surface or hydrogenation of C2 species such as C-CH3* since the coupling barriers of those C1 species are moderate.

Figure 6. Top and side view (inserted) of the transition states produced from C-C coupling of C1 to form C2 species on the Cu-Fe(100) surface at 4/9 ML Cu coverage (blue = iron, orange = copper, gray = carbon, white = hydrogen).

C-H bond formation mechanism Similar as discussed for Fe(100), we also examined C-H bond formation reactions for C2 species on the Cu-Fe(100) surface. Optimized structures of the transition states produced are shown in Figure 5(b), and reaction energetics are included in Table 4. The activation barrier for


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CH-CH* hydrogenation is 0.67 eV and further hydrogenation of CH-CH2* to form adsorbed CH2-CH2* requires to overcome an energy barrier of 0.76 eV. The desorption energy of CH2CH2* to the gaseous C2H4 is calculated to be 0.40 eV. The activation barrier for CH3-CH3* formation from direct C-C coupling of two CH3* species is extremely high (2.58 eV), therefore, this reaction is kinetically hindered. C2H6 formation may proceed through hydrogenation of C2H4. The activation barriers for hydrogenation of CH2-CH2* and CH2-CH3* to form CH3-CH3* on the Cu-Fe(100) surface are 0.74 and 0.58 eV, respectively. These results indicate that once C2H4 is produced on the catalyst surface, it can be further hydrogenated to C2H6 with moderate barriers.

3.3 Bimetallic effect on production of C1 and C2 hydrocarbons Based on the calculation results on reaction energetics for CO2 hydrogenation, we identified favorable pathways for production of CH4 and C2H4 on Fe(100) and the Cu-Fe(100) surface. Figure 7 illustrates the reaction networks for CH4 and C2H4 formation from CO2 on these two catalyst surfaces, with the kinetic barrier for each elementary step included. C2H6 can be produced through further hydrogenation of C2H4, and the barriers are also given in Figure 7. On mono-metallic Fe(100) surface, CH* is found to be the most favorable monomeric CHx* species for CH4 and C2H4 formation. The favorable path for producing CH* goes through CO2*CO*HCO*HCOH*CH*. Once the CH* species is formed, it can be further converted either to CH4 via a series of hydrogenation steps of the CHx species or coupled together to form C2H4 through subsequent hydrogenation. Comparing the calculated kinetic barriers for CH4 and C2H4 production through the CH* intermediate, CH4 formation is found to be kinetically favored over C2H4 since the energy barrier for C-C coupling of two CH* species is 0.3 eV larger than the highest hydrogenation barrier from CH* to CH4, as observed from Figure 7. The elementary kinetics drives a notable preference for CH4 selectivity on Fe(100), and this is -21- Environment ACS Paragon Plus


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consistent with the experimental observation conducted on monometallic Fe catalyst.30 On Fe(100), the rate limiting step for conversion of CO2 to CH4 is the hydrogenation of HCO* to HCOH* through forming an O-H bond, which needs to surmount a barrier of 1.32 eV. On the bimetallic Cu-Fe(100) surface, the dominant path for CH* formation goes through CO2*HCOO*HCOOH*HCO*HCOH*CH*, different from that identified on Fe(100). Once CH* is produced, it will be selective to C2H4 via C-C coupling of two CH* and subsequent hydrogenation rather than to CH4, as evidenced from the energy barriers given in Figure 7. Although CH* hydrogenation and CH*+CH* coupling have comparable barriers (0.73 vs. 0.76 eV), the subsequent hydrogenation of CH2* requires to overcome a larger energy barrier (1.01 eV) than those barriers (0.67 and 0.76 eV) for CH-CH* conversion to C2H4. Therefore, C2H4 formation proceeds faster than CH4 on Cu-Fe(100) based on elementary kinetics. However, once C2H4 is produced on the catalyst surface, it can be further hydrogenated to C2H6 with moderate barriers, which may account for why large amount of ethane over ethylene was observed experimentally without adding K promoter. The rate limiting step for CO2 conversion to C2H4 is the hydrogenation of HCOO* to HCOOH*, which needs to surmount an energy barrier of 1.27 eV. Compared to Fe(100), the bimetallic combination of Cu and Fe facilitates CO2 conversion by reducing the kinetic barrier in rate-limiting step, and alters the selectivity preference to more valuable C2H4, which is in contrast to the higher selectivity to CH4 on mono-metallic Fe surface. In literature, two possible paths have been proposed for CH3OH production from CO2 hydrogenation over Cu-based catalysts. One is the reaction path through a formate (-HCOO) intermediate without formation of CO.65-68 The other possible path involves a carboxyl (-COOH) intermediate through which CO2 is first converted to CO via reverse water gas shift (RWGS) reaction and then the CO is further hydrogenated to CH3OH eventually.69-71 These pathways for CH3OH synthesis do not go through CHx intermediates since one C-O bond retained. On -22- Environment ACS Paragon Plus

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Fe(100), we found that the CH* species is a key intermediate from CO2 hydrogenation whose formation goes through the path of CO2*CO*HCO*HCOH*CH*. Previous DFT studies also demonstrated that CH* is an important monomeric CHx species in CO hydrogenation for FT synthesis over Fe-based catalysts.72-75 On the Cu-Fe bimetallic surface studied in this work, CH* is also identified as the key monomeric species for production of CH4 and

C2H4,

however,

its

formation

goes

through

a

different

path

of

CO2*HCOO*HCOOH*HCO*HCOH*CH* comparing to that observed on Fe(100). For C-C coupling mechanism and carbon chain growth, CO insertion into CHx species was demonstrated as the key step in higher alcohol synthesis (HAS) from syngas over Cu-based catalysts.45-49 In hydrocarbon synthesis from electroreduction of CO/CO2 on Cu, the dominant path for C-C coupling occurs through either CO dimerization42, 50-54 or coupling of two CH2 species,37, 39, 42 depending on Cu facets employed and feed (CO2 or CO). DFT studies of C-C coupling on Fe catalysts showed that the preferred path for C-C coupling is sensitive to the Fe facets from which we observed that CH+CH coupling to CH-CH is favored on Fe(111)56 while C+CH3 coupling to C-CH3 is preferred on Fe(210).57 These studies indicate an important dependence of reaction pathways on the surface composition and structure. Adding Cu into Fe modifies the surface electron distribution of the Fe catalyst, and the resulting change of catalyst composition and structure impacts the stability of reactants, intermediates and transition states,25, 58, 76-77 and thus altering the dominant pathways. This computational study provides useful information for experimental efforts in design of novel Fe-Cu bimetallic catalysts for highly selective synthesis of lower hydrocarbons from CO2 hydrogenation. Based on the identification of rate-limiting and selectivity-determining steps for CO2 conversion to C1 and C2 hydrocarbons on Fe(100) and Cu-Fe(100) surfaces, the energy barrier for HCOO* hydrogenation to HCOOH*, HCO* hydrogenation to HCOH*, CH*+CH* -23- Environment ACS Paragon Plus


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coupling, or certain hydrogenation steps of CHx* species may be useful for developing reaction descriptors to screen other Fe-based bimetallic catalysts for CO2 hydrogenation. Rational design of the bimetallic catalysts could better realize their synergic effects for highly selective synthesis of desired hydrocarbons, nevertheless more comprehensive theoretical and experimental efforts are still needed to design novel bimetallic catalysts for CO2 hydrogenation.

Figure 7. Reaction pathways for production of CH4, C2H4, and C2H6 from CO2 on Fe(100) and the Cu-Fe(100) surface at 4/9 ML coverage. Kinetic barrier for each elementary step is given in eV.

4. Conclusions We performed density functional theory calculations to investigate the mechanism for CO2 hydrogenation to C1 and C2 hydrocarbons over Cu-Fe bimetallic catalysts. On mono-metallic Fe(100) surface, the CH* species is the most favorable monomeric CHx* species for production of

CH4

and

C2H4,

whose

formation

goes

through

a

path

of

CO2*CO*HCO*HCOH*CH*. Once the CH* species is formed, it can be converted either to CH4 via a series of hydrogenation steps or to C2H4 through C-C coupling of two CH* and subsequent hydrogenation. The Fe(100) surface is more selective to CH4 than C2H4 because the energy barrier for C-C coupling of two CH* species is 0.3 eV higher than the highest barrier for hydrogenation from CH* to CH4. The rate-limiting step for conversion of CO2 to lower


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hydrocarbons is the hydrogenation of HCO* to HCOH* through formation of an O-H bond. On the bimetallic Cu-Fe(100) surface, the preferred path for CH* formation goes through CO2*HCOO*HCOOH*HCO*HCOH*CH*, different with the one identified on Fe(100). Once CH* is produced, it will be more selective to C2H4 via C-C coupling of CH*, rather than its hydrogenation to CH4 due to the higher hydrogenation barrier of CH2 species relevant to the barriers for CH*+CH* coupling and subsequent conversion to C2H4. Both HCOO* hydrogenation to HCOOH* and HCO* hydrogenation to HCOH* have substantial barriers in the path for CH4 and C2H4 formation in CO2 conversion. The bimetallic combination of Cu and Fe facilitates CO2 conversion by reducing the kinetic barrier in rate-limiting step, and alters the selectivity preference to more valuable C2H4. C2H6 can be produced from further hydrogenation of C2H4 with moderate barriers.

ASSOCIATED CONTENT Supporting Information. Activation barriers without and with ZPE corrections for elementary steps examined for CH4 and C2H4 production in CO2 conversion on Fe(100) and Cu-Fe(100). The imaginary vibrational frequency corresponding to each transition state involved in CO2 conversion on Fe(100) and Cu-Fe(100). Reaction pathways with ZPE corrected barriers for CH* formation from CO2 hydrogenation on Fe(100) and Cu-Fe(100). Reaction pathways with ZPE corrected barriers for production of CH4 and C2H4 from CO2 on Fe(100) and the Cu-Fe(100) surface at 4/9 ML coverage. This material is available free of charge on the ACS Publication website at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author



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

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21503027), the National Key Research and Development Program of China (No. 2016YFB0600902), the Fundamental Research Funds for the Central Universities (No. DUT15RC(3)027), and the QianRen Program of the Chinese Government. We acknowledge the Supercomputing Center of Dalian University of Technology for providing the computational resources for this work.

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Mechanistic Insight Into C-C Coupling over Fe-Cu Bimetallic Catalysts

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