Sub-Surface Boron-Doped Copper for Methane Activation and

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Subsurface Boron Doped Copper for Methane Activation and Coupling: First Principles Investigation of the Structure, Activity and Selectivity of the Catalyst Quang Thang Trinh, Arghya Banerjee, Yanhui Yang, and Samir H. Mushrif J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09236 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 27, 2016

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The Journal of Physical Chemistry

Subsurface Boron Doped Copper for Methane Activation and Coupling: First Principles Investigation of the Structure, Activity and Selectivity of the Catalyst Quang Thang Trinh,1 Arghya Banerjee,2 Yanhui Yang,1,2 Samir H. Mushrif 1,2,* 1

Cambridge Centre for Advanced Research and Education in Singapore (CARES), Nanyang

Technological University, 1 Create Way, Singapore 138602, Singapore. 2

School of Chemical and Biomedical Engineering, Nanyang Technological University, 62

Nanyang Drive, Singapore 637459, Singapore.

Corresponding Author * Email address: [email protected] (SHM)

ACS Paragon Plus Environment

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ABSTRACT. Copper (Cu) is a commercial catalyst for the synthesis of methanol from syngas, low-temperature water gas shift reaction, oleo-chemical processing, and for the fabrication of graphene by chemical vapor deposition. However, high barriers for C-H bond activation and the ease of formation of carbon/graphene on its surface limits its application in the utilization and conversion of methane to bulk chemicals. In the present paper, using first principles calculations, we predict that Cu catalyst doped with a monolayer of subsurface Boron (B-Cu) can efficiently activate the C-H bond of Methane and can selectively facilitate the C-C coupling reaction. Boron binds strongest at the subsurface octahedral site of Cu and the thermodynamic driving force for the diffusion of B from an on-surface to the sub-surface position in Cu is stronger than that for the experimentally synthesizable B-Ni (subsurface boron in Nickel) catalyst, providing a proof of concept for the experimental synthesis of this novel catalyst. Additionally, the first-principles computed free energy of the reaction to form B-Cu from boron precursor and Cu is also favorable. The presence of the monolayer subsurface B in Cu creates a corrugated step-like structure on the Cu surface and significantly brings down the methane C-H activation barrier from 174 kJ/mol on Cu(111) to only 75 kJ/mol on B-Cu. The subsequent dehydrogenation of the adsorbed CH3* to CH2* is also kinetically and thermodynamically feasible. Our calculations also suggest that, unlike most of the transition metals, complete decomposition of methane to carbon would not be favored on B-Cu. The dissociation of the surface CH2* moiety on B-Cu is limited due to the high activation barrier of 161 kJ/mol and lower relative stability of the resultant CH* species, under reaction conditions. The coupling of CH2* fragments however is kinetically and thermodynamically favorable, with an activation barrier of only 92 kJ/mol; suggesting that B-Cu catalyst would have higher selectivity towards C2 hydrocarbons. Furthermore, the formation of carbon from the adsorbed CH* moiety has a very high activation barrier of 197 kJ/mol and the


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completely dehydrogenated C* is relatively much less stable than CH*, under reaction conditions; predicting that coking might not be an issue on the B-Cu catalyst. Evaluation of C-H activation on Cu(110) surface, which has a similar step-like surface structure as B-Cu, and Bader charge and density of states analyses of B-Cu reveal that the geometrical/corrugation effect and the charge transfer from B to Cu synergistically promote the C-H activation on B-Cu, making it as active as other expensive transition metals like Rh, Ru, Ir and Pt.


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1. Introduction 1.1. Methane as a feedstock Methane is the smallest hydrocarbon with the highest hydrogen to carbon ratio and is the main component of many commercially exploitable resources such as natural gas, shale gas and gas hydrates.1 It has huge potential to become an important feedstock for the synthesis of fuel and value added chemicals.2 Especially, recent development in hydraulic fracturing technologies have significantly increased the supply of methane from shale gas, making methane conversion processes more attractive.1-3 However, despite its tremendous potential, methane still remains hugely underutilized.3-4 Majority of methane (> 90%) is used to supply heat and electricity via burning.2, 4 These processes produce lots of CO2, thus inducing global warming, climate changes and greenhouse gas effects.5-7 Current utilization of methane involves its conversion to syngas by catalytic partial oxidation or reforming processes, and subsequently to hydrocarbons via FischerTropsch synthesis.2, 4, 8 These processes require high temperature and pressure conditions and are therefore cost demanding.2,

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Direct conversion of methane to chemicals (via non-oxidative

coupling, partial oxidation, oxidative coupling) could be more economical and has been studied for decades; however it remains to be a commercial success and the lack of active and selective catalysts is one of the key reasons for it.2-4, 9

1.2. Catalysts for methane activation and utilization The C-H σ bonds in methane are very stable, with the bond dissociation energy of ~440 kJ/mol.9-10 Active transition metals such as Ru, Pt, Ni, Rh can activate methane; however, they


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also lead to the thermodynamically more favorable and undesired deep dehydrogenation of methane to carbon and this also deactivates the catalyst.2, 9-10 It is suggested that the ability of transition metals to activate the methane C-H bond reduces when moving from the left to the right of the periodic table due to poorer stabilization of the transition states by the metal surface.9 It could explain why open d-shell metals such as Rh, Pt and Ru are very good in activating C-H bonds, while filled d-orbital metals such as Cu are inactive. Stepped sites are also believed to be more active than the closed pack terrace sites in activating the C-H bond, since their unsaturatedcoordination helps to stabilize the transition state; but step sites on a catalyst surface are usually in minority.9, 11-13 Therefore, in the two most promising processes for the conversion of methane to chemicals, either via the coupling reaction forming C2 hydrocarbons or via partial oxidation to oxygenated products (methanol, formaldehyde), controlling C-H activations is believed to be the most crucial aspect in the design of an effective catalyst. To promote the synthesis of C2 hydrocarbons from methane, the strategy is to activate the first and second C-H bonds, while suppressing further dehydrogenation to preserve the precursors of the coupling reaction.2-3, 14 Bao et al. have recently synthesized a single-site iron silicide catalyst that was able to facilitate methane non-oxidative coupling and observed 48.4% ethylene selectivity for methane conversion of 48.4%.14 Neurock et al. used sulfur as a soft oxidant to facilitate the coupling of methane, and the high selectivity towards ethylene was observed because the coupling reaction between CH2 fragments is kinetically more favorable than other competitive reactions on the Pd16S7 catalyst.15 In the partial oxidation of methane, metals oxides are the most popular catalysts. Recently, surface chemisorbed oxygen and lattice oxygen of metal oxides were reported to be very effective in activating the C-H bond via a “four center mechanism” with a lower energy barrier, wherein surface lattice metal, lattice oxygen atoms and dissociated


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fragments formed the four center complex “CH3—M—H—O” in the transition state.10, 16 The involvement of surface lattice oxygen into the methane C-H bond activation is promising since it can be incorporated into the reactant later to form oxidative products.10, 17-19 However, products from methane activation are bound too strongly on the metal oxide surface, making them difficult to be subsequently utilized. There is also a possibility of over oxidation to CO/CO2 and that reduces the utilizable value of methane.10, 19-20 Hence, achieving a compromise between the selectivity and the activity in methane conversion on metal oxide catalysts is also a challenge.2, 10, 19

1.3. Boron doping in transition metals: As a stabilizer and as a promoter In order to enhance the activity, selectivity and stability, transition metals are modified by alloying with other metals21-24 or by doping with a promoter, such as S,15 K,25 B26-30. Recently, doping transition metals with metalloid elements, in particularly boron, to promote their catalytic properties has been receiving much attention.26, 30-31 The introduction of small amount of boron into the lattice of transition metals forming rich-surface boron architectures could improve the stability of the catalyst significantly without influencing its activity. Specific examples include Ni promoted with 1% B in the methane steam reforming27 and methane dry reforming32 and Co doped with 0.5% B in Fischer-Tropsch synthesis.28, 30 In these studies, the incorporation of small amounts boron only modified the near surface region of the catalyst, reduced the carbon deposition by ~80% and enhanced the stability of the catalyst against coking by a factor of six. It is also reported that the doping with B also improved both, the stability against coking and sintering, for small clusters of Pt on a metal oxide support.33 Besides the promotional effect of B


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in improving the stability of the catalyst, it also enhanced the activity for Ni catalyst in methane steam reforming and the presence of only 1% boron increased the initial conversion of Methane on Ni by 5%.27 The same prediction was also reported for toluene decomposition on Nickel doped with a monolayer sub-surface Boron,34 making boron the special promoter that can improve both, the catalytic activity and stability. The promotional effect of B in enhancing the activity of Pd and Ru catalysts was also revealed recently.29, 35-39 For example, Pd doped with small amounts of boron increased dramatically the selectivity in the partial hydrogenation of alkynes29,

35

and achieved the record high hydrogen formation rate in formic acid electro-

oxidation.37-38 The near surface-enriched boron of Ru catalyst also significantly improved its activity and reusability in the selective hydrogenation of benzene.39

1.4. Boron-doped Cu Active metals like Pt, Rh, Ru are expensive and they also lead to over-dehydrogenation of methane to carbon. Therefore, there is a strong incentive to find cheaper and more abundant catalysts. Although being a commercial catalyst in the synthesis of methanol from syngas,40 in hydrolysis of fatty esters or in the fabrication of graphene by chemical vapor deposition41 due to the cheap price and abundance, the high activation barrier of Copper (Cu) in dissociating the C-H bond of methane hinders its application in methane utilization.9-10 Since Cu has a full d-shell, it performs poorly in methane C-H activation.9 However, similar to that of Pd, Ni and Ru, the presence of Boron is suspected to alter characteristics of Cu-based catalysts. Indeed, Zhe et al. introduced small amounts of boron to Cu/SiO2 (Cu:B atomic ratio = 6.6) and greatly enhanced the stability and selectivity of the catalyst in the hydrogenation of dimethyl oxalate to ethylene


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glycol.42 Addition of B to SiO2 supported Cu catalyst also resulted in the enhanced catalytic activity and stability for glycerol hydrogenolysis to 1,2-propanediol, as reported by Shanhui et al.43 Similar behavior for Cu doped with Boron was also observed for other hydrogenation reactions.44-45 Moreover, the presence of boron was also reported to increase the corrosive resistance, mechanical and thermal stability of copper-based materials.46-48 It should be noted though that most of the aforementioned catalysis studies on boron-doped Cu had very little or no information about the structure of the catalyst and the active site. On the other hand, structural investigations by Shanhui et al.43 reported that the Cu:B molar ratio analyzed by XPS measurement was much lower than the value measured using ICP analysis, and that observation suggested that Boron might be located near the surface. Some earlier studies on the B-Cu alloy using the β-radiation-detected nuclear magnetic resonance technique (β-NMR) have revealed that boron impurity prefer occupying the octahedral interstitial site of Cu. It was also reported in the theoretical study by Lozovoi et al.,49 that the octahedral interstitial site for B in Cu is more stable than the substitutional lattice site. Hence, in order to fully evaluate the potential of the novel boron doped Cu as a catalyst, it is crucial to (i) elucidate the most stable microstructure of the catalyst, (ii) investigate the activity and selectivity of the catalyst and (iii) explain the role of boron in altering the activity and selectivity.

1.5. Scope and overview of the paper The present study employs density functional theory calculations to evaluate the thermodynamic feasibility of the presence of boron at different positions in the Cu lattice (onsurface and sub-surface) and to identify the most preferred binding position of boron, in section


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3.1. Additionally, we calculate the thermodynamic driving force for the formation of boron doped Cu catalyst (called B-Cu) and the optimized structure of B-Cu. In section 3.2, we study the activity of the most stable configuration of B-Cu catalyst for methane C-H bond dissociation and the subsequent dehydrogenation/C-C coupling reactions. Based on the comparison amongst different competitive pathways, from thermodynamics and kinetic points of view, we evaluate the potential of B-Cu in methane utilization. Finally in section 3.3, we correlate the activity of BCu catalyst with its geometric and electronic structure to explain the promotional effect of Boron.

2. Computational methods The ab-initio total-energy and molecular-dynamics program VASP (Vienna ab-initio simulation program) developed at the Fakultät für Physik of the Universität Wien50-51 was used to perform the density functional theory (DFT) calculations. All the calculations in this study were spin-polarized and utilized the Perdew-Burke-Ernzerhof (PBE) functional,52 a plane-wave basis set with a cut-off kinetic energy of 450 eV, and the projector-augmented wave (PAW) method53 as implemented in VASP. The Cu(111) surface was modeled as a 4-layer p(4×4) slab, with an optimized lattice constant of 3.64 Å, in good agreement with the experimental value (3.60 Å).54 To evaluate the relative driving force for Boron diffusion in different metals, from the on surface site to the subsurface site, 4-layer p(4×4) slabs of Ni(111), Co(111) and Pd(111) surfaces were used. Two stepped surfaces were used to study the geometrical effect of Boron doping: Cu(110) surface and the F4 step site. Cu(110) surface was modeled as a 8-layer p(3×4) slab and F4 step site was modeled as a 3-layer p(4×8) slab with the three missing rows on the top


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layer (Supporting Information, Figure S1). The two bottom layers were fixed to reduce the computational cost without influencing the accuracy of the simulations,13, 55 while the remaining top layers and the adsorbates were allowed to fully relax. The Brillouin zone was sampled with a 3 × 3 × 1 Monkhorst-Pack grid, and repeated slabs were separated by 20 Å to minimize periodic interactions. Geometries were fully relaxed until the energy changes by less than 0.1 kJ/mol. Transition states were located using the Nudged Elastic Band method, and subsequently fully optimized. Frequency calculations confirmed the nature of the transition states. Increasing the slab thickness to 5 layers, the slab size to a p(5×5) slab, the inter-slab spacing to 25 Å, or the grid to (5 × 5 × 1) only changed the calculated binding energies by less than 5 kJ/mol. Bader charge was also computed to evaluate the electronic effect of boron doping on Cu.30, 56 For an initial methane C-H activation, the reaction energy ∆EFS was computed on Cu(111), B-Cu and Cu(110) surfaces assuming the dissociated fragments (CH3 and H) were co-adsorbed on the same unit cell after the reaction to conveniently compare the activity between different active sites on B-Cu and analyze the transition state on different surfaces. For the subsequent C-H activations and C-C couplings on B-Cu, the reaction energy ∆Erxn was calculated considering the reactants (in C-C coupling reactions) or products (in C-H activation reactions) to be adsorbed at infinite separation on different slabs. To evaluate the thermodynamics of the reaction and the stability of different reaction intermediates during the reaction, the stability of an adsorbed reaction intermediate CxHy* was calculated relative to the reservoir of clean surface (*) and gas methane CH4 (gas) at different reaction temperatures of 1223 K, 1323 K, 1363 K and pressure of 1atm using the free energy ∆Grxn of the following reaction: xCH4 (gas) + (1+4x – y)* → CxHy* + (4x – y)H*

(1)


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where * denotes the clean surface, x is 1 or 2 and y can take values from 1 to 5, depending on the reaction stoichiometry. Zero point energies (ZPEs) were calculated for all species. Entropy and enthalpy correction for adsorbed species were calculated from statistical thermodynamics.21, 55, 57

For gas phase molecules, entropy and enthalpy correction were obtained from the standard

thermodynamics NIST-JANAF table and corrected at specified reaction temperatures.58 The choice of temperatures and pressure was based on the conditions where high conversion of methane and selectivity towards ethylene were achieved on a single site Fe catalyst14 and a PdS catalyst15 in the methane non-oxidative coupling reaction.

3. Results and Discussion. 3.1. Molecular structure of boron doped Cu catalyst. 3.1.1. Binding position of Boron (B) on Cu(111). We first examine the adsorption of a single B atom on different sites of Cu(111) surface. The binding energy of B to Cu surface Γ is calculated using the following equation, similar to previous studies.28, 35, 59-60 Γ = EB/Cu – ECu - EB

(2)

where EB/Cu, ECu and EB are total energies of B adsorbed on Cu(111) surface, of clean Cu(111) slab and of a single B atom, respectively. The results and the structures are presented in Table 1.


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From the results in Table 1, it can be seen that adsorption energies of B on the surface hcp, fcc and bridge sites are almost identical; while the adsorption at the top site is much weaker. However, the binding of B in the first subsurface layer at the octahedral position is 67 kJ/mol stronger than that on the most favorable surface site. This result is in excellent agreement with the isotope labeling study using β-active

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B probe nuclei, wherein the β-radiation-detected

nuclear magnetic resonance technique (β-NMR) has revealed that dilute B impurity atom indeed occupied the octahedral interstitial site in the Cu lattice.61-62 The adsorption of B at the subsurface tetrahedral and at deeper second subsurface octahedral (represent the bulk) is also 101 kJ/mol and 33 kJ/mol weaker than at the first sub-surface octahedral position right beneath the surface, respectively. Therefore among all the possible adsorption sites in the Cu(111) structure, Boron binds strongest at the sub-surface octahedral position and the same trend was also observed for Boron binding in other transition metals, such as Ni,34, 59-60 Co28, 30 and Pd.29, 35-36 3.1.2. Feasibility of the synthesis of subsurface B doped Cu catalyst. To introduce Boron into interstitial positions of transition metals in experiments, the source of Boron is boric acid, borane tetrahydrofuran or potassium borohydride. Impregnation method is used to first form the on-surface boron.27-30,

32, 42-43

After the impregnation, boron is likely to

diffuse into the more stable sub-surface position.60,

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To evaluate the feasibility of the

experimental synthesis of sub-surface boron doped Cu, we evaluate the relative driving force for the diffusion of Boron from the on-surface to the sub-surface position in different metals. We use the energy difference ΓD between the binding energy of Boron at the on-surface and in the subsurface octahedral site of Cu as a measure of the thermodynamic driving force for the diffusion (illustrated in Table 2) and link it to the corresponding values for other metals (Co, Ni, Pd) where sub-surface Boron was experimentally observed.27-30 The results are shown in Table 2. It is worth


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noting that although these values reflect only the potential energy contribution, the entropic contribution (T × ∆S) to the free energy difference between the on-surface and the sub-surface site is very small. The actual free energy difference is also provided in the Supporting Information, Table T1. For Co(111), the thermodynamic driving force for B diffusion from on surface to subsurface (B coverage of 1/16 monolayer (ML)) is the lowest, -24 kJ/mol. For Ni(111) structure, B binding at the octahedral site is 52 kJ/mol stronger than at the on-surface site. For Pd(111) structure, the driving force is -131 kJ/mol, agreeing well with the value of -127 kJ/mol reported by Hu et al. at the same coverage.35 For Cu(111), the computed B diffusion driving force is -67 kJ/mol, higher than that in Co and Ni and lower than that in Pd. Since the computed driving force for B diffusion for Cu is between the driving force for Ni (-52 kJ/mol) and for Pd (-131 kJ/mol) and the fact that Pd and Ni catalyst doped with subsurface B have been experimentally synthesized,27, 29

we can deduce that sub-surface boron in Cu could also be synthesizable via the similar

preparation method. Our conclusion is consistent with a study by Lozovoi et al. wherein the segregation of B at the coverage ranging from 1/2 ML to 3/2 ML from surface site into the interstitial subsurface position of Cu(310) surface was also reported.49 For Co structure, the smaller driving force might not be enough for B to diffuse from on-surface to sub-surface, and this results in the formation of the surface reconstructed p4g Co2B boride instead.26, 28, 30 To further evaluate the kinetics of diffusion, we have also calculated the barrier for Boron diffusion from on-surface to the sub-surface position in Cu(111) (Supporting Information, Figure S2). The observed computed diffusion barrier is only 29 kJ/mol, making the diffusion from onsurface to sub-surface not only thermodynamically favorable, but also kinetically feasible. B at the second subsurface layer is relatively less stable compared to the first subsurface layer (33


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kJ/mol less stable, Table 1), and the computed diffusion barrier from the first subsurface layer to the second subsurface layer is 138 kJ/mol (Supporting Information, Figure S2), much higher than the diffusion barrier from on-surface to the first subsurface layer. This value is in good agreement with the theoretically reported value of 127 kJ/mol by Lozovoi et al.49 for the diffusion of interstitial B in Cu using a 32 atom super cell model. This high barrier, along with lower relative stability would inhibit the deeper diffusion of boron from the first subsurface site into the bulk of Cu and this is consistent with the low solubility of B in the Cu alloy.64-65 Therefore, with only a small amout of boron doping, only the near surface region (top and second Cu layers) will be significantly modified and it is unlikely that large amounts of B could be present in the bulk Cu. It was also reported in the fabrication of thin Boron film on Cu foil via chemical vapor deposition (CVD) by Tai et al. that the deposited Boron atoms generated from B2O2 dimer source diffused from surface into the lattice Cu firstly and only after the Cu system had reached the supersaturation, the surface B atoms could then be nucleated on the Cu surface to form a thin film structure.63 3.1.3. A monolayer subsurface Boron in Cu(111). Since the stability of sub-surface boron is reported to increase at higher coverages in the case of Ni,60 we evaluated the trend in Cu too. The average binding energy Γavg of Boron in the octahedral subsurface site was computed at different coverages of B and is presented in Table 3, together with the corresponding structures. We also reported the differential binding energy Γdff, which is the energy difference for adding one more B atom to the existing structure. B atoms can adopt two binding modes at the subsurface octahedral positions, called the near configuration (B atoms are in the adjacent octahedral sites) and the far configuration (B atoms


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are at octahedral sites far away from each other), as shown in Table 3. In the far configuration, there is no significant interaction between the B atoms, separated by more than 4.5Å, and the differential binding energy Γdff is almost the same as the binding energy of the existing B atom. The binding energy calculation was also repeated using the optB88-vdW functional, which was reported to describe accurately the mid to long-range interactions.34,

66

The average binding

energies in the far configurations obtained using PBE and optB88-vdW functional differ only slightly (binding energies calculated using optB88-vdW and their comparison with PBE is reported in the Supporting Information, Table T2), again suggesting that the long range interactions between B atoms is negligible in these structures. As could be seen in Table 3, the average binding energies Γavg of B in the far configurations are not coverage dependent. However, on the contrary, the binding energy Γavg for the near configuration is higher than in the far configuration and increases at higher coverages. In the near configuration, coupling of Boron atoms at the adjacent octahedral sites induces the surface of Cu(111) to be reconstructed (inserted pictures in Table 3). The B-B agglomeration in sub-surface Cu enhances the binding energy of B, as was reported theoretically49 and was also observed experimentally.67-68 Therefore upon doping in Cu, Boron would preferentially bind at adjacent octahedral sites. It is worth noting that the average binding energy Γavg of B in the near configuration at a monolayer coverage is almost the same as the differential binding energy Γdff for adding one more B next to the existing one at lower coverages, indicating that the binding energy of boron reaches the maximum at a monolayer coverage. Moreover, the computed horizontal diffusion barrier of B atom between the octahedral sites in the first subsurface layer is only 12 kJ/mol (Supporting Information, Figure S2). Therefore, B atoms initially co-adsorbed far away from each other could easily diffuse within the sub-surface positions and couple to stabilize the structure. It has to


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be noted that deeper diffusion of B into the bulk, after the subsurface monolayer formation, is associated with a high barrier of 173 kJ/mol (Supporting Information, Figure S3) and reaction energy of +44 kJ/mol, making it both thermodynamically and kinetically unfavourable. This is in agreement with the experimentally observed Cu:B ratio of ~ 1:1 detected using X-ray Photoelectron Spectroscopy for the B doped Cu synthesized by Zhu et al.,43 suggesting the presence of a monolayer of subsurface B in Cu. To further evaluate the thermodynamic stability of boron in Cu, with respect to its source/precursor for catalyst preparation, we evaluated the free energy of the following reaction for different boron dopings and for different B-Cu phases (on-surface vs. subsurface). Simulation details and structures are provided in the Supporting Information. Diborane is taken as the source of boron since it is usually more stable than H3BO3 under the reaction conditions 28, 36, 59, 69 and free energies are reported in Table 4. Free energies were calculated at 1223K, 1323K and 1363K and under 1 atm pressure.

( ) + → + ( )

(3)

For boron to bind with Cu, the free energy for this reaction should be negative. It can be seen from Table 4 that at the coverage of 0.25 ML, the formation of on-surface boron is not feasible since free energies are positive at all temperatures. Sub-surface boron in Cu is thermodynamically feasible and higher temperatures are better. At a coverage of 0.5 ML all the considered structures are stable, but interestingly the unique p4g structure, which forms only at 0.5 ML coverage, is the most stable at this coverage. This structure involves the reconstruction of the Cu(111) surface forming the so-called “square-planar boron” unit (pictorial representation shown in Supporting information Figure S4). High stability of this unique structure at 0.5 ML


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was explained from the aromaticity generated for the interaction between B in the center of the square with 4 coordinated Cu atoms surrounding.26 When the coverage increases to 0.75 ML, the sub-surface B structure is more stable than the on-surface structure and its stability is also higher than that of the p4g structure. Upon further increase in the dosage of B to 1 ML, the stability of the on-surface structure of B significantly increased due to the B-B coupling on the Cu(111) surface but at all temperatures considered in our study, it is still less stable than the 1 ML subsurface B structure. And interestingly at higher temperatures, the stability gap widens, which suggests that under high temperature reaction conditions, sub-surface layer will be even more stable. Though we did not evaluate the relative stability for coverages greater than 1 ML, it can be inferred from trend in free energies in Table 4 that on-surface boron may become more stable at higher coverages. In summary, (i) the binding of Boron at the subsurface octahedral site is the strongest; (ii) the diffusion barrier from on surface to subsurface position is only 29 kJ/mol, (iii) the deeper diffusion from first subsurface layer into the bulk is inhibited due to a significantly high barrier, (iv) diffusion of boron within the sub-surface layer, to form a monolayer is associated with a small barrier of 12 kJ/mol, and (v) for coverages near 1ML boron doping, the sub-surface boron layer is thermodynamically the most stable structure, collectively support that the monolayer of subsurface Boron in the octahedral sites of Cu is stable and could be synthesized experimentally. 3.1.4. Corrugated structure of the sub-surface B monolayer doped Cu catalyst The optimized structure of Cu doped with a monolayer subsurface B is shown in Figure 1. It can be seen that the incorporation of a monolayer subsurface B (and the agglomeration of B atoms in the subsurface layer) induced corrugations within the top Cu surface, forming a step-


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like structure (Fig 1b). One row of Cu (Cu-up) is pushed 0.84 Å higher than the neighbouring row (Cu-down). This corrugation is more pronounced than that observed in monolayer B doped Ni (the Ni-up is pushed up only by 0.58 Å).34, 59 The similar reconstruction of the top surface in Cu(310) struture was also reported by Lozovoi et al. for the segregation of 0.5-1.5 ML coverage B from the surface to the subsurface interstitial site and was stated as a reason to stabilize the grain boundary of the Cu particle.49 A possible reason for corrugations in the top surface Cu layer could be the local lattice expansion of Cu arising from the decoration of interstitial Boron. Using NMR analysis with β-emitting nuclei

12

B, Minamisono et al. reported the expanision

magnitude of 11 ± 2% for the local lattice of Cu in the nearest octahedral surrounding of the interstitial 12B probe.70 Therefore the expansion of the surface region due to the incorporation of a small amount of B doped within the constraint of the inner layer might result into corrugations within the top surface Cu layer. It should be noted that more severe reconstructions, involving changes in stoichiometry, may occur under operating conditions,71-73 but those have not been evaluated here and are beyond the scope of this study. Similar to the coupling of subsurface B in Ni and Co,30, 59-60 adjacent subsurface B atoms also agglomerated in Cu to form the parallel line of connected lozenge B4 cluster (Fig.1a and the inserted picture). It is important to note that due to the surface reconstruction, there are three different types of Cu atoms on the surface: Cu4u, Cu2u and Cud. Superscripts u and d refer to the Cu atoms in the Cu-up row and Cu-down row, respectively and subscripts 4 and 2 indicate the number of B atoms in the subsurface layer coordinating with the surface Cu atom. Cu4u is the Cu on the upper row and located on top of four B atoms, and Cu2u is the Cu on the upper row and located on top of two B atoms (Fig.1a). Since the Cu4u and Cu2u sites are under-coordinated, it is expected that they will be more active for the methane C-H bond activation.34,

59

The next section will present the evaluation of the


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catalytic activity of a monolayer sub-surface boron doped Cu (named as B-Cu) and it will be shown that C-H activation barriers differ significantly on these different surface Cu sites.

3.2. Catalytic activity of B-Cu in C-H bond dissociation and C-C coupling reactions 3.2.1. Initial Methane C-H activation on different active sites on B-Cu. The computed barrier for methane C-H activation on pure Cu (111) surface is 174 kJ/mol and is in good agreement with reported values.10 This reaction is highly endothermic with the reaction energy ∆EFS of +91 kJ/mol. The high barrier of C-H activation on Cu(111) was explained on the basis of the strong repulsion between doubly occupied orbitals of adsorbates and high electron occupied d orbitals in the Nilsson-Pettersson model for transition metals which have the filled d-orbital structure like in Cu.9, 74 Next, we evaluate the activation of methane on B-Cu surface, which refers to the monolayer Boron doped Cu catalyst. It was inferred from our calculations that the Cud site (Cu site at down row) could not be the active site for methane dissociation since dissociated fragments CH3 and H were not able to bind to that site due to its highly coordinated nature and only Cu atoms on upper rows are active sites for the reaction. As mentioned earlier, there are two different sites of Cu on the upper row: Cu4u site and Cu2u site and they have different activities. Transition states for the first C-H activation and corresponding barriers on these two sites are presented in Figure 2. The C-H activation barrier on Cu4u site is 75 kJ/mol (TS1 in Fig.2a), whereas it is higher on Cu2u site, i.e., 106 kJ/mol (TS2 in Fig.2b). Although the activation barrier differs significantly on Cu4u and Cu2u sites, the reaction energies ∆EFS for both sites are almost identical with the values of -65


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and -67 kJ/mol, respectively. It is important to note that the activation barrier on the Cu4u site of B-Cu catalyst is drastically reduced by ~100 kJ/mol, as compared to the activation barrier of 174 kJ/mol on pure flat Cu(111) surface, demonstrating the promotional effect of Boron. The barrier of 75 kJ/mol for the first methane C-H bond activation on Cu4u site means that the cheap, abundant but inactive Cu, after doping with boron, becomes as active as expensive transition metals such as Rh and Pt in methane activation.9 Moreover, the first C-H dissociation on the surface of B-Cu is also exothermic, compared to that on Cu (111) (+91 kJ/mol), making it energetically more favorable. After the reaction, the dissociated fragments CH3 and H are preferentially bound on bridge sites on the Cu-up row (Supporting Information, Figure S5). In order to explain the difference in the activities of different surface Cu sites in the B-Cu catalyst, we plot the Density of states (DOS) projected on the Cu4u, Cu2u and Cud sites in Figure 3. We calculated the d-band center relative to the Fermi level (EF) for the specific Cu atom (Fig. 3) and use the d-band model proposed by Hammer and Nørskov23,

75

to evaluate the binding

strength of the adsorbate on different Cu sites. The d-band of Cu4u site is narrow and the d-band center (εd) value is -2.67 eV, while the d-band of Cu2u site is broader and the corresponding dband center value is -2.89 eV, shifted away from the Fermi level. According to the d-band model proposed by Hammer and Nørskov,23,

75

the closer the εd to the Fermi level, the stronger the

adsorption energy for the adsorbate. This suggests that the binding affinity of Cu4u site is stronger than the Cu2u site, resulting in stronger stabilization effect in the transition state and reduction in the activation barrier for methane C-H dissociation, as shown in Fig.2. The d-band center of Cud site, -3.11 eV, is shifted further away from the Fermi level, reflecting weakened binding affinity of the Cud site and could explain why it is not an active site.


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3.2.2. Subsequent C-H activations After the first C-H bond dissociation on B-Cu, the adsorbed CH3* fragment could be further dehydrogenated or can couple with other fragments to form longer chain hydrocarbons. The strategy for converting methane to value added chemicals is to efficiently activate the first C-H bond, while preventing complete dehydrogenation of dissociated hydrocarbon fragments and facilitate the C-C coupling reaction.2, 14-15 Recent examples for this strategy are (i) the single Fe sites embedded in a silica matrix synthesized by Bao et al.14 which gave very high yield of ethylene in the direct non-oxidative coupling of methane at high temperature conditions and (ii) the preferential C-C coupling over the C-H activation on metal sulfide (Pd16S7) catalyst by Neurock et al.,15 resulting in high ethylene selectivity. In both these studies, coke formation was negligible despite the experiment being conducted at a very high temperature. Keeping the aforementioned strategy in mind, we evaluated subsequent C-H activations and the C-C coupling reactions on Cu4u sites of the B-Cu catalyst. The transition states and activation barriers for sequential C-H dissociations are shown in Figure 4. The barrier for subsequent C-H activation (on a Cu4u site) in the CH3* fragment is 120 kJ/mol; 45 kJ/mol higher than the first methane C-H activation barrier (Fig. 4a). Similar to the trend observed for the first C-H activation, the barrier to activate the C-H bond in the CH3* fragment on the Cu2u site of B-Cu is higher, with the value of 138 kJ/mol (Supporting information, Figure S6). It is worth noting that C-H activation in the adsorbed CH3* would exhibit significantly lower entropic contribution (loss of entropy) than the initial CH activation of methane; however, the first activation is associated with collision (that can’t be captured in DFT calculations and would require ab initio molecular dynamics) and these collision effects would compensate the increase in the free energy of activation due to the loss of entropy.41


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The subsequent C-H activation in CH2* on the Cu4u site has a much higher barrier of 161 kJ/mol, as shown in Fig.4b (it is even higher on a Cu2u site), making it kinetically difficult. Due to the high activation barrier to further dehydrogenate the CH2* moiety, it could be possible for the CH2* fragments to couple to form C2 hydrocarbons, as was also observed for the single Fe site catalyst synthesized by Bao et al.14 and the PdS catalyst of Neurock et al.15 It is also important to point out that subsequent dehydrogenation of CH* forming surface carbon also has a very high barrier of 197 kJ/mol (Fig. 4c). Therefore, complete dehydrogenation of methane to carbon, which is an issue on active transition metals such as Ni, Rh, Pt during methane activation, would be kinetically inhibited on the B-Cu catalyst. In order to evaluate the thermodynamics of dehydrogenation of methane on B-Cu, we also reported the free energy for reaction intermediates in Table 5 at different temperatures, 1223 K, 1323 K and 1363 K (the conditions where methane was converted to give good C2H4 yield in reported experimental studies14-15). The surface adsorbed CH2* is more stable than CH3* on B-Cu at all the considered temperatures and the free energy for the conversion from CH3* to CH2* is ~ -5 kJ/mol, making it thermodynamically feasible even at high temperatures (Table 5). However, further dehydrogenation of CH2* to CH* on B-Cu is not as thermodynamically favorable because CH* is slightly less stable than CH2* (the free energy for CH2* conversion to CH* on B-Cu at 1323 K is +1.3 kJ/mol and is +1.6 kJ/mol at 1363 K) (Table 5). Since the conversion from CH2* to CH* is also kinetically (Fig. 4b) not favorable, it is likely that CH4 activation on B-Cu will favor generating surface reaction intermediate species CH3* and CH2*, keeping further decomposition limited. Finally, the surface carbon formation is also thermodynamically not favored (Table 5).


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3.2.3. C-C coupling reactions on B-Cu catalyst Next, we evaluate the (C-C) coupling between generated surface intermediates on an active Cu4u site during the reaction, and the transition states are shown in Figure 5. Note that we also tested the activity of Cu2u site for C-C coupling reactions and the activation barriers are much higher than those on Cu4u site (reported in Supporting Information, Figure S7). It should be noted that in order to facilitate C-C coupling, a state, where two CHx fragments are located on neighboring sites, needs to be generated. This can be achieved either via the activation of methane on a site next to the existing CHx fragment or via the diffusion of two CHx fragments which are generated far away from each other. Our calculations show that both the scenarios are feasible. The first C-H activation barrier increases by Cu(110) > Cu(111) surface) lower was the C-H activation barrier on the corresponding surface, as was depicted in Fig.7e. The corrugation effect in Cu surface promoted C-H activation, but the barrier was only slightly reduced by around ~30-35 kJ/mol. This suggests that surface geometry effects may not only be responsible to bring down the initial methane C-H activation barrier by as much as ~100 kJ/mol, as was observed on the B-Cu catalyst (Fig.7e).


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3.3.2. Effect of boron on the electronic structure of the catalyst Charge transfer between metal and dopant atoms was believed to influence to the binding affinity of the surface and its catalytic activity.21, 30, 39, 80 In copper-boron systems, charge transfer from B to Cu was also reported considering the slight difference in their electronegativities (the electronegativity for Cu and B are 1.90 and 2.04, respectively).43, 81 We calculated Bader charges on Cu and B atoms in the B-Cu catalyst to investigate the effect of B doping on Cu (cf Table 6). The charge on the surface Cu atom on bare Cu(111) surface is negligible, i.e., -0.023. Surface Cu atoms on the stepped Cu(110) surface also have a slight positive charge of +0.017. Additionally, when present on the surface, boron did not alter the charge on Cu significantly and remained almost neutral. However, charge transfer appeared more pronounced for sub-surface boron doped Cu. In the far configuration (cf Table 3 and Table 6) at lower dopings, the charge transfer from boron was distributed amongst the coordinated Cu atoms and hence did not result in significant net change in the charge on Cu. But at higher coverages, when two B atoms at the subsurface positions couple and induce the coordinated Cu atom to pop-out from the surface, the charge on the Cu atom is significantly more positive. When a monolayer of subsurface B is incorporated into Cu, the top surface Cu layer is corrugated and the charge on the Cu4u site is +0.171, as shown in Table 6. It is consistent with experimental findings where partial positive charge on Copper surface atoms, when doped with B, was detected using XPS and Auger electron spectroscopy (XAES) measurements and those sites were also suggested to be the active sites for hydrogenation.42-44 Moreover, the existence of positively charged Cu sites, induced by the incorporation of B, was also proposed to be the reason preventing the aggregation of Cu particles and improvement in the thermal stability.43-44 Hence, we can say that in addition to


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surface corrugations, the incorporation of a monolayer subsurface B into Cu lattice also induces positive charge on the active Cu4u site to enhance its activity for methane C-H bond cleavage. To further evaluate the effect of charge transfer from boron to Cu on the C-H bond activation ability, we studied the methane C-H activation on two model sites: Cu(111) with 1 B atom at the subsurface position, denoted as Cu-1B-sub (surface Cu charge is +0.042) and Cu(111) with 2 subsurface B atoms in the near configuration, denoted as Cu-2B-sub-near (surface Cu charge is +0.149), as illustrated in Figure 8. The activation barrier for C-H dissociation on Cu-1B-sub is 178 kJ/mol, almost similar to the value on Cu(111) surface (Fig. 8a). However, the barrier for CH activation on a popped-out Cu atom with the charge of +0.149 is only 109 kJ/mol (Fig.8b), significantly reduced from 174 kJ/mol on the Cu(111) surface. The reaction energy ∆EFS for C-H activation on Cu-2B-sub-near also became more favorable (+29 kJ/mol vs +91 kJ/mol on Cu(111) surface). These results support the argument that the positive charge on Cu will promote C-H activation, as was suggested in previous experimental studies.42-45,

82

In summary, we

suggest that with the incorporation of a monolayer subsurface B, the geometrical effect (corrugation of the top surface Cu layer generating under-coordinate sites) and the electronic effect (generation of positive charge active center Cu4u sites) synergistically promote C-H activation on the Cu4 site in the B-Cu catalyst.

4. Conclusions In this study, the structure, activity and selectivity of a novel sub-surface monolayer boron doped Copper (B-Cu) catalyst were investigated using density functional theory (DFT) calculations. Boron prefers to occupy the octahedral interstitial subsurface site in Cu. DFT


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calculations revealed that the thermodynamic driving force for Boron diffusion from an onsurface to the sub-surface position in Cu is larger than that in Ni, suggesting that Cu doped with sub-surface B could be fabricated in a fashion similar to that of sub-surface boron doped Ni. Additionally, deeper diffusion of Boron into the bulk of Cu is both, kinetically and thermodynamically unfavourable. The binding energy of B in the subsurface layer increases with B coverage, and reaches the maximum at a monolayer coverage. The formation of a subsurface monolayer of boron in Cu is also thermodynamically feasible (and more favourable than onsurface monolayer), when Diborane is used as boron source. The incorporation of a monolayer subsurface B induces corrugations into the top Cu surface layer, forming a step-like structure and generating under-coordinate sites. These sites can activate methane C-H bonds efficiently, with an activation barrier for the first C-H dissociation of only 75 kJ/mol, compared to the value of 174 kJ/mol on an inactive, pure Cu(111) surface. Generated surface adsorbed CH3* species subsequently undergo dehydrogenation to form the stable CH2* species, which is a precursor for the C-C coupling reaction. The C-C coupling reaction, resulting in the formation of surface adsorbed ethylene, is both thermodynamically and kinetically more favourable than the competitive dehydrogenation of CH2* to CH* and subsequently to C*, suggesting that methane could be converted to ethylene with high selectivity on the B-Cu catalyst. Hence, we predict that Cu doped with a monolayer subsurface boron can be a potential catalyst for the methane coupling reaction to form ethylene. The promotional effect of the sub-surface monolayer boron in enhancing the activity of Cu is due to the creation of corrugations on the Cu surface and the charge transfer from B to Cu, contributing synergistically. Due to the surface corrugations within the top Cu surface, the generation of graphene like structures on B-Cu would be limited, and hence coke formation would be significantly reduced.


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ASSOCIATED CONTENT. Supporting Information. F4 and B5 step sites on a model of a p(4×8) Cu(111) unit cell with three missing rows. Thermodynamics driving force and transition states for B diffusion. Comparison between the PBE and optB88-vdW functionals for the binding of subsurface B. Stability of different structures of Boron in Cu at different coverages. Optimized structure of adsorbed intermediates during the reactions. Transition states for C-H dissociation and C-C coupling on a Cu2u site. Activation of methane nearby existing adsorbed CHx fragment. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (SHM) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This research is supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. A.B acknowledges financial support from the Ministry of Education (MOE) Singapore under the Tier-II Grant (Grant ID: MOE2015-T2-1-082)


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Enhanced Stability with Boron Dopant. J. Catal. 2013, 297, 142-150. 45. Yin, A.; Qu, J.; Guo, X.; Dai, W.-L.; Fan, K. The Influence of B-Doping on the Catalytic Performance of Cu/HMS Catalyst for the Hydrogenation of Dimethyloxalate. Appl. Catal. A: Gen. 2011, 400, 39-47. 46. Suryanarayanan Iyer, R.; Frey, C. A.; Sastry, S. M. L.; Waller, B. E.; Buhro, W. E. Plastic Deformation of Nanocrystalline Cu and Cu–0.2 Wt.% B. Mater. Sci. Eng. A 1999, 264, 210-214. 47. Glikman, E. É.; Goryunov, Y. V.; Zherdev, A. M. Intergrain Impurity Adsorption and Cold Brittleness of Fcc Cu-Sb and Cu-Sb-B Solid Solutions. Sov. Phys. J. 1974, 17, 946-951. 48. Weber, L.; Tavangar, R. On the Influence of Active Element Content on the Thermal Conductivity and Thermal Expansion of Cu–X (X = Cr, B) Diamond Composites. Scripta Mater. 2007, 57, 988-991. 49. Lozovoi, A. Y.; Paxton, A. T. Boron in Copper: A Perfect Misfit in the Bulk and Cohesion Enhancer at a Grain Boundary. Phys. Rev. B 2008, 77, 165413. 50. Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558-561. 51. Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50. 52. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 53. Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. 54. Hao, P.; Fang, Y.; Sun, J.; Csonka, G. I.; Philipsen, P. H. T.; Perdew, J. P. Lattice Constants from Semilocal Density Functionals with Zero-Point Phonon Correction. Phys. Rev. B 2012, 85, 014111. 55. Trinh, Q. T.; Chethana, B. K.; Mushrif, S. H. Adsorption and Reactivity of Cellulosic Aldoses on Transition Metals. J. Phys. Chem. C 2015, 119, 17137-17145. 56. Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354-360. 57. Irikura, K. K. Appendix B. In Computational Thermochemistry, American Chemical Society: 1998; Vol. 677, pp 402-418. 58. Chase, M. W. NIST-JANAF Thermodynamical Tables, Fourth ed.; Journal of Physical and Chemical Reference Data, Monograph 9, 1998.


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59. Xu, J.; Saeys, M. First Principles Study of the Effect of Carbon and Boron on the Activity of a Ni Catalyst. J. Phys. Chem. C 2009, 113, 4099-4106. 60. Xu, J.; Saeys, M. Improving the Coking Resistance of Ni-Based Catalysts by Promotion with Subsurface Boron. J. Catal. 2006, 242, 217-226. 61. Jäger, E.; Ittermann, B.; Stöckmann, H. J.; Bürkmann, K.; Fischer, B.; Frank, H. P.; Sulzer, G.; Ackermann, H.; Heitjans, P. Lattice Location of 12B in Al and Cu Single Crystals Determined by Cross Relaxation. Phys. Lett. A 1987, 123, 39-42. 62. McNab, T. K.; McDonald, R. E. Lattice Location of 12B in Aluminum and Copper at 77 K. Phys. Rev. B 1976, 13, 34-38. 63. Tai, G.; Hu, T.; Zhou, Y.; Wang, X.; Kong, J.; Zeng, T.; You, Y.; Wang, Q. Synthesis of Atomically Thin Boron Films on Copper Foils. Angew. Chem. Int. Ed. 2015, 54, 15473-15477. 64. Wald, F.; Stormont, R. W. Investigations on the Constitution of Certain Binary Boron-Metal Systems. J. Less Common Metals 1965, 9, 423-433. 65. Jacob, K. T.; Priya, S.; Waseda, Y. Measurement of the Activity of Boron in Liquid Copper Using a Four-Phase Equilibrium Technique. Metall. Mater. Trans. A 2000, 31, 2674-2678. 66. Klimeš, J.; Bowler, D. R.; Michaelides, A. Van Der Waals Density Functionals Applied to Solids. Phys. Rev. B 2011, 83, 195131. 67. Dewing, E. W. Dimerization of Boron. Metall. Trans. A 1990, 21, 2609-2609. 68. Yukinobu, M.; Ogawa, O.; Goto, S., Activities of Boron in the Binary Fe-B, Co-B, and Cu-B Melts. Metall. Trans. B 1989, 20, 705-710. 69. Saeys, M.; Tan, K. F.; Chang, J.; Borgna, A. Improving the Stability of Cobalt Fischer−Tropsch Catalysts by Boron Promotion. Ind. Eng. Chem. Res. 2010, 49, 11098-11100. 70. Minamisono, T.; Nojiri, Y.; Matsuta, K. Local Lattice Expansion of Cu Arising from Dilute Interstitial Impurities, β-Emitting Nuclei 12B and 12N. Phys. Lett. A 1983, 94, 312-316. 71. Böller, B.; Ehrensperger, M.; Wintterlin, J. In Situ Scanning Tunneling Microscopy of the Dissociation of Co on Co(0001). ACS Catal. 2015, 5, 6802-6806. 72. Banerjee, A.; van Bavel, A. P.; Kuipers, H. P. C. E.; Saeys, M. Origin of the Formation of Nanoislands on Cobalt Catalysts During Fischer–Tropsch Synthesis. ACS Catal. 2015, 5, 47564760. 73. Banerjee, A.; Navarro, V.; Frenken, J. W. M.; van Bavel, A. P.; Kuipers, H. P. C. E.; Saeys, M. Shape and Size of Cobalt Nanoislands Formed Spontaneously on Cobalt Terraces During


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Fischer–Tropsch Synthesis. J. Phys. Chem. Lett. 2016, 7, 1996-2001. 74. Nilsson, A.; Pettersson, L. G. M. Chapter 2 - Adsorbate Electronic Structure and Bonding on Metal Surfaces. In Chemical Bonding at Surfaces and Interfaces, Elsevier: Amsterdam, 2008; pp 57-142. 75. Hammer, B.; Nørskov, J. K. Theoretical Surface Science and Catalysis—Calculations and Concepts. In Advances in Catalysis, Academic Press: 2000; Vol. Volume 45, pp 71-129. 76. Makino, T.; Okada, M.; Kokalj, A. Adsorption of C2H4 on Stepped Cu(410) Surface: A Combined TPD, FTIR, and DFT Study. J. Phys. Chem. C 2014, 118, 27436-27448. 77. Skibbe, O.; Vogel, D.; Binder, M.; Pucci, A.; Kravchuk, T.; Vattuone, L.; Venugopal, V.; Kokalj, A.; Rocca, M. Ethene Stabilization on Cu(111) by Surface Roughness. J. Chem. Phys. 2009, 131, 024701. 78. Kravchuk, T.; Vattuone, L.; Burkholder, L.; Tysoe, W. T.; Rocca, M. Ethylene Decomposition at Undercoordinated Sites on Cu(410). J. Am. Chem. Soc. 2008, 130, 1255212553. 79. Galea, N. M.; Knapp, D.; Ziegler, T. Density Functional Theory Studies of Methane Dissociation on Anode Catalysts in Solid-Oxide Fuel Cells: Suggestions for Coke Reduction. J. Catal. 2007, 247, 20-33. 80. Rodriguez, J. A.; Goodman, D. W. The Nature of the Metal-Metal Bond in Bimetallic Surfaces. Science 1992, 257, 897-903. 81. Liu, H.; Gao, J.; Zhao, J. From Boron Cluster to Two-Dimensional Boron Sheet on Cu(111) Surface: Growth Mechanism and Hole Formation. Sci. Rep. 2013, 3, 3238. 82. Itadani, A.; Sugiyama, H.; Tanaka, M.; Ohkubo, T.; Yumura, T.; Kobayashi, H.; Kuroda, Y. Potential for C−H Activation in CH4 Utilizing a CuMF-Type Zeolite as a Catalyst. J. Phys. Chem. C 2009, 113, 7213-7222.


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FIGURES.

Figure 1. (a) Top view and (b) side view of the corrugated surface of Cu after the incorporation of a monolayer sub-surface Boron (called B-Cu). Sub-surface Boron atoms form a connected lozenge-cluster line (yellow dash line). Distances between boron atoms and nickel atoms are shown in yellow color. Peach and salmon balls represent Copper (Cu) and boron (B) atoms, respectively. Pictures in stick and ball mode are inserted for better visualization of boron cluster (Fig.1a) and corrugated surface (Fig.1b).


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Figure 2. Methane C-H bond activation on (a) Cu4u site and (b) Cu2u site of the corrugated B-Cu surface (Cu sites are indicated). Inserted pictures show the side view of the transition states. Computed activation barrier Ea and reaction energy ∆EFS are included. Distances between boron atoms and nickel atoms are shown in yellow color. Big peach and salmon balls represent Copper (Cu) and boron (B) atoms, while small grey and white balls represent Carbon (C) and Hydrogen (H) atoms, respectively.


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Figure 3. Density of states (DOS) projected on d-band for different surface site of B-Cu. The dband center εd values (eV) relative to the Fermi level (EF) are also shown for specified atoms. Inserted picture are for illustration of Cu4u and Cu2u sites with top view and side view for one surface Cu-up row. The formula to calculate εd is also shown.


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Figure 4. C-H activations in (a) CH3*, (b) CH2* and (c) CH* fragments on a Cu4u site of the B-Cu catalyst. Activation barriers Ea and reaction energies ∆Erxn are included. Inserted pictures show the side view of the transition states. Color code is the same as Fig. 2.


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Figure 5. C-C coupling reactions between (a) two CH3* fragments forming ethane; (b) CH3* and CH2* fragments forming adsorbed Ethyl and (b) two CH2* fragments forming adsorbed ethylene on a Cu4u site of a B-Cu catalyst. Activation barriers and reaction energies are included. Inserted pictures show the side view of the transition states. Color code is the same as Fig. 2.


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Figure 6. Overall reaction network for methane conversion on B-Cu catalyst. Activation barriers (Ea) and reaction free energies at 1323K (∆G) are reported in kJ/mol. Bold blue arrows present the most favorable reaction pathway, while yellow boxes show the most feasible intermediates along the reaction.


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Figure 7. Transition states for methane C-H activation on (a) Cu(111) surface; (b) Cu(110) surface; (c) F4 step edge and (d) B5 step edge of a p(4×8) Cu(111) model with three missing rows (F4 and B5 step sites are highlighted). (e) Activation barriers and reaction energies for the first C-H dissociation in methane on different surfaces in kJ/mol (structures are shown as insets). Color code is the same as Fig. 2.


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Figure 8. Transition states of methane C-H activations on (a) Cu with 1B subsurface (Cu-1Bsub) and (b) Cu with 2B subsurface in near configuration (Cu-2B-sub-near). Charges on the active Cu center are indicated. Activation barriers Ea, reaction energies ∆EFS and C-H distances in the transition states are shown. Color code is the same as in Fig. 2.


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TABLES. Table 1. Binding energies (Γ) of B on different sites of Cu(111) surface, as calculated using equation (2). On surface

Γ (kJ/mol)

fcc

hcp

bridge

top

-411

-412

-419

-280

Structure

First subsurface layer

Γ (kJ/mol)

tetrahedral

octahedral

Second subsurface layer at the octahedral site

-385

-486

-453

Structure


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Table 2. Difference between the binding energies of Boron at the on-surface and at the subsurface positions for different transition metals, including Cu. Values reported in the literature for different coverages are also mentioned for reference.

ΓD = Γsubsurface octahedron - Γon-surface

ΓD (kJ/mol)

Co(111)

Ni(111)

Cu(111)

Pd(111)

-24

-52

-67

-131


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Table 3. Binding energy Γavg (computed using equation 2) and the differential binding energy Γdff of B in the sub-surface octahedral position of Cu(111) at different coverages. Energy values are reported in kJ/mol. 2/16 ML

3/16 ML

1/16 ML Γavg

-486

Γdff

far

near

far

near

-486

-514

-483

-521

-486

-542

-475

-535

Structure

4/16 ML 1 ML far

near

Γavg

-482

-529

Γdff

-481

-550

-551

Structure


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Table 4. Stabilities of different structures of B in Cu at different coverages and different temperatures. The values presented are free energies per B atom in the structure calculated using reaction (3). The most stable structures are highlighted in bold.

T (K)

0.25 ML

0.5 ML

0.75 ML

1 ML

on

sub

on

sub

p4g

on

sub

on

sub

1223

+53.6

-33.1

-15.2

-64.4

-74.3

-59.3

-76.6

-86.7

-86.9

1323

+41.9

-45.5

-27.2

-76.1

-86.2

-70.7

-88.2

-98.1

-99.1

1363

+37.3

-50.4

-32.0

-80.8

-90.9

-75.3

-92.7

-102.7

-103.9


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Table 5. Free energy of reaction intermediates relative to gas phase methane at different reaction temperatures, calculated using the free energy ∆Grxn of the reaction: xCH4 (gas) + (1+4x – y)* → CxHy* + (4x – y)H*

∆Grxn (kJ/mol)

Reaction intermediate

Temperature (K)

CH3*

CH2*

CH*

C*

C2H5*

C2H4*

1223

-19.9

-25.3

-24.5

-14.3

-6.9

-80.9

1323

-8.8

-13.5

-12.2

-2.2

+5.1

-57.3

1363

-4.6

-9.3

-7.7

+2.2

+9.6

-48.3


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Table 6. Computed Bader charges of the surface Cu and B atoms in Cu and B-Cu catalystsa Structure

Configuration

Charge on Cu

Cu(111) surface

-0.023

Cu(110) surface

+0.017

Charge on B

On-surface

+0.028

-0.066

Subsurface

+0.042

-0.261

Far configuration

+0.049

-0.272

Near configuration

+0.149b

-0.263c

+0.171/+0.021d

-0.241c

1/16 ML B in Cu(111)

2/16 ML subsurface B in Cu(111) B-Cu (1 ML subsurface B) a

For structures involving B, the average charge on Cu atoms coordinated with B is presented. Charge is computed for a Cu atom that is pushed higher on the surface. cAverage charge on all B atoms is presented. dCharge is computed for Cu4u/Cu2u site of the corrugated B-Cu surface.

b


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Table of Contents Graphic


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Sub-Surface Boron-Doped Copper for Methane Activation and

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