Semihydrogenation of Acetylene on Al5Co2 Surfaces - American

Article pubs.acs.org/JPCC

Semihydrogenation of Acetylene on Al5Co2 Surfaces M. Meier, J. Ledieu, V. Fournée, and É. Gaudry*

Institut Jean Lamour, UMR 7198 CNRS Université de Lorraine, F-54011 Nancy, France ABSTRACT: Density functional theory calculations are used to investigate the catalytic properties of the low-index surfaces of Al5Co2 toward the semihydrogenation of acetylene. Adsorption energies of species involved in the reaction (H, C2H2, C2H3, and C2H4) are calculated on the (001), (100), and (21̅0) surfaces. Hydrogen adsorption sites are found to be stabilized by subsurface cobalt atoms, showing an electron donor character. Scaling relations between the adsorption energies of C2Hx are established. Surface activity has been investigated on the (21̅0) surface. A possible reaction path for the semihydrogenation of acetylene is proposed using nudged elastic band calculations, starting from a 3-fold adsorption site made of three aluminum atoms. The corresponding activation energy of the rate-controlling step is calculated to be 60 kJ mol−1. The activation energy of the rate-controlling step toward total hydrogenation is evaluated to be 106 kJ mol−1, thus suggesting a selective surface for catalysis.

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−2 eV. They also present comparable local atomic arrangements. Indeed, pentagonal motifs are present in both structures, and these two intermetallic compounds are considered as loworder approximants to the decagonal phase. Catalytic performances are generally ascribed to electronic and geometric effects. The latter are related to the so-called site isolation concept, where the active site is made of small atomic ensembles containing an active transition metal.20 One could therefore expect similar activities for the two Al−Co approximants. The Al5Co2 compound is however slightly enriched in cobalt compared to Al13Co4, and its complexity is lower than that of Al13Co4, when evaluated by the number of atoms in the unit cell. This may affect the catalytic performances. One objective of this paper is then to discuss the catalytic properties of the Al5Co2 compound in comparison with those of the Al13Co4 catalyst. Recently, the three low-index surface structures [(001), (100), and (210̅ )] have been determined by a combination of surface science studies and ab initio calculations.16,21 For each low-index surface, reconstructions have been highlighted by low-energy electron diffraction (LEED); surface models with low surface energies have been identified theoretically; and a good agreement has been obtained between the experimental and calculated scanning tunneling microscopy (STM) images. These studies lead to the conclusion that the considered surfaces differ by their surface energies, their corrugation, their surface atomic densities, and their surface chemical composition, in particular the presence or absence of topmost Co atoms, etc. How the adsorption, activity, and selectivity

INTRODUCTION Acetylene (C2H2) is a byproduct formed during the process of ethylene (C2H4) production. It acts as a poison during the production of polyethylene, one of the most common plastic, from ethylene. The removal of traces of acetylene is then required in the production process. Demands for semihydrogenation catalysts in this reaction are high since the loss of ethylene by total hydrogenation to ethane has to be avoided, requiring excellent activity and selectivity.1 Mixtures of several metals, like palladium-based catalysts modified with silver, are traditionally used in the industrial process.2−5 However, the use of simple alloys as catalysts presents some drawbacks. In particular, they usually suffer from solid state transformations such as segregation processes6−9 leading to a lowering of the selectivity, up to a complete deactivation of the catalyst with time. In contrast to substitutional alloys with random site occupancy, intermetallic compounds exhibit a crystal structure different from those of the constituting metals. Their specific electronic structure generally results in a high stability under reaction conditions. Again, Pd-based compounds (PdGa, Pd2Ga, Pd3Ga7) are shown to be efficient catalysts for the considered reaction.10−12 More recently, the Al13TM4 (TM = Fe,Co) aluminum-based intermetallic compounds have been identified as efficient, low-cost, and environmentally benign semihydrogenation catalysts.13−15 Only a small number of complex intermetallic compounds have been investigated so far for their potential in catalysis. In this paper, we investigate the catalytic properties of the Al5Co2 intermetallic compound toward the semihydrogenation of acetylene. This compound presents several similarities with the Al13Co4 active and selective catalyst. Both compounds show close electronic structures,16−19 with a d-band located around © 2017 American Chemical Society

Received: November 3, 2016 Revised: February 14, 2017 Published: February 15, 2017 4958

DOI: 10.1021/acs.jpcc.6b11083 J. Phys. Chem. C 2017, 121, 4958−4969

Article

The Journal of Physical Chemistry C

Surface Models. The (001), (100), and (210̅ ) surface structures have been determined previously.16,21 The (001) surface shows a (√3 × √3)R30° surface reconstruction.16 It arises from a surface termination at incomplete puckered layers where 6 out of 9 aluminum atoms are missing per reconstructed unit cell. This model is the only one matching with all experimental and theoretical results

properties are affected by the different surface structures is essential for the development of Al−Co catalysts. We present here a detailed theoretical investigation of the adsorption, activity, and selectivity properties of the low-index surfaces of Al5Co2, using the intrinsic surface model derived from the combined experimental observations and ab initio calculations reported in refs 16 and 21. More than 40 possible adsorption sites, with different symmetries, are considered. The estimation of adsorption energies for a large number of possible adsorption sites allows us to define scaling relations among the adsorption energies of C2Hx. A possible reaction path for the semihydrogenation of acetylene is further proposed for the surface containing transition metal atoms ((210̅ ) surface). The corresponding activity and selectivity are discussed in view of previous works reported on other metallic catalytically active surfaces.

° and is termed P(√3×√3)R30 in the following. 6 Al miss The (100) surface structure shows a (2 × 1) reconstruction, forming atomic rows running along the c direction.21 The model which provides the best fit with the experimental and theoretical results contains only Al atoms in the topmost atomic layer (Figure 2) and is termed A+1 in the following.

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BULK AND SURFACE STRUCTURES Al5Co2 Bulk Structure. Bulk Al5Co2 crystallizes in the P63/ mmc (hP28) space group with the following parameters:22,23 a = b = 7.6717 Å and c = 7.6052 Å. Its structure can be described by a stacking of planespuckered and flat planes perpendicular to the [001] and [21̅0] directions (Figure 1(bottom)). Perpendicular to the [21̅0] direction, two kinds of

Figure 2. Top views (top) and side views (bottom) of the models considered for the three low-index surfaces of Al5Co2 (Al = blue, Co = red). Light colors indicate atoms lying in the subsurface plane. For the (001) and (21̅0) surfaces, the two topmost atomic planes are shown. In the case of the (100) surface, all atoms except the two topmost aluminum atoms per surface cell present light colors. Figure 1. (Top) Al5Co2 bulk structure (Al atoms in blue, Co atoms in red). The covalent-like Al−Co bondings are highlighted. (Bottom) Al5Co2 bulk structure as a stacking of atomic planes: {001} and {21̅0} planes.

The (210̅ ) surface also shows a (2 × 1) reconstruction with 12 Å wide rows running along the c direction, leading to an important surface corrugation (Figure 2). Among the different models tested, only two were found to match the experimental data. The two models PB and PB−4Co are variations of the same surface termination, differing only by the presence or absence of four Co atoms. The near-surface atomic densities and compositions of the three different surfaces, estimated from the first 4 Å below the surface selvedge, are around 25 at. % Co concentration and around 70 atoms nm−3 for the near-surface atomic density. Large structural and composition differences are observed when considering only the termination layers. The surface corrugation is much more pronounced for the (21̅0) surface. The (21̅0) surface is also the only one which presents protruding Co atoms at the surface. From Table 1, it appears

planes with different atomic surface densities alternate, a feature which is also found in the Al13Co4 intermetallic compound along the [100] direction.22,24,25 Perpendicular to the [001] direction, Al5Co2 presents two types of planes with the same atomic density but with different chemical compositions. Chemical bonding analysis reveals that two types of atomic ensembles, one three-dimensional (CoAl6) and the other planar (Co3Al3), can be identified on the basis of two-centered bonds that are formed around the shortest Al−Co distances (Figure 1(top)).16,26,27 The network of both covalent-like and metallic bonding in the bulk structure leads to original surface structures, described in the following. 4959


Article

The Journal of Physical Chemistry C Table 1. Surface Energies in J/m2 for the Most Stable Surface Models Considered in Refs 16 and 21 (Labels in Parentheses)a surface

(001)

(100)

(21̅0)

Al-rich limit Co-rich limit

1.34 ×√3)R30 °) (P(√3 6 Al miss 1.55 × 2) (P(2 10 Al miss)

1.38 (A+1) 1.58 (A−2)

1.27 (PB−4Co) 1.60 (PB)

quantities are then strictly only valid at T = 0 K and P = 0 atm. Our results in terms of catalytic performances (activity, selectivity) can be significantly modified by such effects. Slabs used for surface calculations are described in refs 16 and 21. In this paper, the void thickness is increased up to 19.5 Å. Simplified slabs have also been built. Tests have been run on the (21̅0) surface model, in order to estimate the precision of our setup. A few adsorption energies have been calculated on the original slabs, containing about 300 atoms and compared to the equivalent adsorption energies obtained on simplified slabs, containing approximately half of the initial atom number (∼150 atoms). A difference of around 12 meV (≃10−2 kJ/mol, corresponding to roughly 0.4%) in adsorption energy is observed for atomic hydrogen, as well as for C2H2 (about 10 meV). Thus, we evaluate the precision for calculated adsorption energies around 0.01 eV (1 kJ/mol). Transition states are determined using nudged elastic band (NEB) calculations, more precisely the climbing-image method.39,40 Calculation of Adsorption Energies. Adsorption energies ads Eads H and EC2Hx of H and C2Hx (x = 2−4) molecules are evaluated using

a

In particular, the values are given for the surface models considered in °, PB, PB−4Co, A+1). this study (P(√3×√3)R30 6 Al miss

that surface energies of all considered models are higher than the one of Al(111) (calculated value equals to 0.80 J/m228), although the topmost layer is made by Al atoms only. Then one expects the adsorption properties and activity of the Al5Co2 surfaces to be quite different from those of the Al(111).

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CALCULATION DETAILS Computational Details. Calculations based on the density functional theory (DFT) are performed with the Vienna simulation package (VASP).29−32 The interaction between the valence electrons and the ionic core is described using the projector-augmented wave (PAW) method,33,34 and the calculations are performed within the generalized gradient approximation (GGA-PBE).35,36 Atomic structures and charge transfer images are plotted using VESTA.37 Spin polarization is considered. A cutoff energy (Ecut = 450 eV) and a number of kpoints (grid set to 8 × 8 × 8) within the Brillouin zone are set such as to achieve an energy precision lower than 0.06 meV at−1 (6 × 10−3 kJ/mol) for bulk Al5Co2. k-point meshes are adjusted for surface calculations: 5 × 5 × 1 for the (001) surface, 1 × 4 × 8 for the (100) surface, and 1 × 2 × 8 for the (21̅0) surface. van der Waals interactions have not been considered due to the limitations in computational time. The determination of the adsorption energies and barriers at a finite temperature and as a function of H2 and C2H2 pressure is an ambitious undertaking.38 Temperature and pressure effects are not included here, and all evaluated physical

E Hads =

0 0 E H/surf − Esurf −

1 0 EH 2 2

0 ECads2Hx = EC2Hx /surf − Esurf − EC0 2Hx

(1)

where E0X/surf is the total energy of the system with molecule X on the surface, E0surf the clean surface total energy, and E0H2 and E0C2Hx the corresponding free molecule energies. Geometries for C2H2 and C2H4 are in agreement with the calculated ones in ref 15 and the experimental values.41 The structural complexity of the surfaces studied in this paper lead to a huge number of possible adsorption sites. Due to symmetries, the number of calculations required to get a complete picture of the surface adsorption properties is reduced. The smallest areas needed to describe the entire surfaces are shown in Figure 3, along with the symmetry axes.

Figure 3. (Left) Atomic H adsorption sites for (21̅0) PB, (001) P(√3×√3)R30 °, and (100) A+1 surface models (top views). Sites are numerated starting 6 Al miss with the most favorable site. Black lines indicate symmetry lines. (21̅0) PB shows also a rotational axis perpendicular to the surface (2-fold axis, indicated by a black cross). Symmetric positions are indicated for sites AH9, BH1, CH2 for illustration. (Right) C2Hx adsorption sites for the three surfaces. Only carbon atoms are drawn (top views). More stable sites are highlighted in green in the cases of the (21̅0) and (001) surfaces. The active site is highlighted in blue for the (21̅0) surface. 4960


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The Journal of Physical Chemistry C Table 2. Atomic Hydrogen Adsorption Properties of the (21̅0) Surfacea PB model site pos. top

bridge

hollow

PB−4Co model d(H−X) distances

site env.

label

Eads H

CoS−1 AlS−1 AlS Al−Co Al−Co Al−Co Al−Co Al−Al Al−Al Al−Al 2Al,Co 2Al,Co 2Al,Co 2Al,Co 2Al,Co 2Al,Co

AH6 AH16 AH3 AH12 AH5 AH8 AH9 AH14 AH7 AH11 AH2 AH1 AH4 AH15 AH10 AH13

−30 +40 −35 −11 −32 −25 −23 −7 −30 −14 −36 −38 −32 −4 −22 −9

d(H−X) distances

Al

CoS

CoS−1

2.0 1.6 1.6 2.0 1.9 1.9 1.9 1.8 1.7 1.7 2.1 2.1 1.9 2.0 1.9 1.8

4.2 6.2 3.8 1.5 1.6 1.6 1.6 3.2 3.7 3.6 1.6 1.6 1.6 1.6 1.6 3.2

1.5 3.3 3.7 4.4 4.0 4.1 4.2 3.3 3.4 3.4 3.3 3.4 4.1 3.3 4.0 3.8

label

Eads H

Al

CoS

CoS−1

A’H3

−36

1.6

3.7

3.8

A’H5

→

A’H4

A’H14

−12

1.8

3.2

3.4

A’H11 A’H2 A’H1 A’H4

−9 +19 +34 −29

1.6 1.9 1.9 1.8

3.8 -

3.8 3.1 3.2 3.7

Energies are given in kJ/mol and the closest d(H−X) distances in Å, where X refers to Al, CoS, or CoS−1 atoms. Sites are first listed according to their type (top, bridge, and hollow sites) and then ranked by the distance to the closest Co atom. Empty entries mean that these sites were not calculated. a

In order to precisely analyze the adsorption properties, charge density deformations are plotted. The latter are obtained by removing the charge density contributions of the surface alone and the molecule alone, from the charge density of the molecule adsorbed on the surface.

Table 3. Atomic Hydrogen Adsorption Properties of Al5Co2(001)a

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site

site

env.

label

Eads H

Al

CoS−1

CoS−1 AlS−1 Al−AlS−1 Al−AlS−1 Al−Al Al−Al Al−Al Al−Al Al (2Al−Co)S−1 3AlS−1 3Al

BH5 BH7 BH9 BH8 BH1 BH2 BH3 BH10 BH6 BH4 BH11 BH12

+16 +19 +44 +40 −40 −2 +4 +64 +16 +6 +70 +74

2.1 1.6 1.8 1.7 1.8 1.8 1.8 1.9 1.8 2.0 1.9 1.9

1.5 4.0 3.0 3.1 3.3 3.3 3.4 4.7 3.8 1.5 3.1 4.1

top

ADSORPTION SITES AND ENERGIES A high number of adsorption sites are considered for each lowindex surface. They are labeled Sn in Figure 3, where S denotes the surface (A for (210̅ ), B for (001), and C for (100)), and the index n ranks them as a function of their adsorption energies (S1 has a stronger adsorption energy than S2). In the case of atomic hydrogen adsorption, an H character is added to their label. Atomic Hydrogen Adsorption. Adsorption Energies. Adsorption energies are listed in Tables 2, 3, and 4. Adsorption sites are first sorted according to their type (top, bridge, edge, and hollow sites) and then by their distance to the closest Co atom. Adsorption energies range from −53 kJ/mol to +79 kJ/ mol. One can notice that atomic hydrogen adsorption is exothermic on many adsorption sites. This is in contrast with calculated values for H adsorption on pure aluminum surfaces which are endothermic, in the range of 3−4 kJ/mol and 10−48 kJ/mol on Al(111) and Al(100), respectively.15 The (21̅0) surface presents a large number of exothermic adsorption sites for atomic hydrogen (Table 2). More than 10 sites are identified with adsorption energies between −40 kJ/ mol and −20 kJ/mol (AHi, with i ∈ [1;10]). Most of them are close to a Co surface atom of the PB model (Co−AHi distance smaller than 1.6 Å). Exceptions are observed for sites AH3 and AH7. AH3 is located on top of an aluminum atom, the position being shifted toward the closest hollow site made of 3 Al atoms. AH7 is a bridge site between two aluminum atoms. Both AH3 and AH7 present large H−Co bondings (d(H−Co) > 3.4 Å) and quite strong binding with the surface (Eads H between −30 kJ/mol and −35 kJ/mol). Hydrogen atoms adsorbed close to the surface Co atom slightly below the mean position of the

d(H−X) distances

site pos.

bridge

edge hollow

a Energies are given in kJ/mol and relevant d(H−X) distances in Å, where X refers to Al or CoS−1 atoms. Sites are first listed according to their type (top, bridge, edge, and hollow sites) and then ranked by the distance to the closest Co atom.

surface plane (AH15 site) are not strongly bounded to the surface (Eads H = −4 kJ/mol). The absence of the protruding Co atoms leads to a drastic reduction of the number of exothermic adsorption sites at the (21̅0) surface in the PB−4Co model, i.e., getting closer to Al elemental surfaces. The adsorption energies of the two most stable sites (AH1 and AH2, both located in hollow sites made of 2 Al and 1 Co atoms) change from around −37 kJ/mol to +19 kJ/mol upon removal of the surface Co atoms. Only sites AH3 (on top Al) and AH4 (hollow site made of 2 Al and 1 Co atom) maintain a relatively high adsorption energy (−36 kJ/mol and −29 kJ/ mol, respectively) when protruding surface Co atoms are absent. The number of exothermic adsorption sites is drastically reduced on Al5Co2(001) and Al5Co2(100) surfaces. On Al5Co2(001), the only site which presents a large exothermic 4961


Article


The stability of atomic hydrogen atoms is different on the three considered low-index surfaces and involves different charge transfers. In the case of AH1, the closest CoS (H−CoS = 1.59 Å) and CoS−1 (H−CoS−1 = 3.40 Å) show a donor character. On the (001) surface, the BH1 site cannot be stabilized by charge transfer from surface Co atoms since the topmost atoms are only Al atoms. Here, the closest CoS−1 (H− CoS−1 = 3.33 Å) has a donor behavior, while the farther CoS−1 atom (H−CoS−1 = 4.42 Å) shows an acceptor character. A strong depletion of electrons is also highlighted between the two surface Al atoms close to the hydrogen adsorbate, suggesting that the stabilization of the hydrogen adsorbate does not only involve Co atoms. CH1, which is the overall most stable site of all three surfaces, shows a donor character not only of the closest subsurface CoS−1 neighbors (H−CoS−1 = 3.89 Å) but also of the CoS−3 atoms (small contribution, H− CoS−3 = 7.78 Å). The closest subsurface Co atoms of the strongest adsorption sites (AH1, BH1, and CH1) have a donor character, while they have an electron acceptor character for the unfavorable CH10 site. It suggests that hydrogen adsorption sites are partly stabilized by the electron donor character of neighboring cobalt atoms. Adsorption of C2Hx Molecules. Adsorption Sites and Energies. The stable adsorption sites for C2Hx molecules are shown in Figure 3 (right), and the corresponding adsorption energies are listed in Tables 5, 6, 7, and 8 for the three Al5Co2 surfaces. All favorable adsorption sites are located preferentially in the topmost surface planes, rather than above subsurface regions left apparent by the reconstruction. In the case of the (21̅0) surface, the most stable site for C2H2 corresponds to a distorted 4-fold site, in a bridge position between 2 Al atoms and 2 Co atoms (Tables 5 and 6). The C− C distance of the C2H2 molecule is 1.40 Å, which is in between the sp−sp and sp2−sp bond lengths (1.37 and 1.43 Å, respectively42). The corresponding adsorption energy is −261 kJ/mol. Here again, the topmost Co atoms have a great influence on the adsorption. If they are absent (as for the PB−4Co model), the most favorable adsorption site changes to A’3 (Eads C2H2 = −233 kJ/mol). The A1 adsorption site is the most stable adsorption site for vinyl (Eads C2H3 = −296 kJ/mol, d(C−C) = 1.43 Å). The situation is different for C2H4. In this latter case, the most favorable adsorption site is the 3-fold site A4 (Eads C2H4 = −87 kJ/mol, d(C−C) = 1.55 Å), made of three Al atoms, located quite far away from surface Co atoms (distance larger than 4 Å). It is worth noticing that, concerning the adsorption properties of C2H2, the A4 site is mainly unaffected by the absence of surface Co atoms (Eads C2H2 = −197 kJ/mol for both PB and PB−4Co surface models). In the case of the (001) surface (Table 7), the most stable site for C2H2 adsorption is a bridge site, made of two Al atoms (Eads C2H2 = −218 kJ/mol, d(C−C) = 1.40 Å). The B1 adsorption site is also the most stable adsorption site for vinyl (Eads C2H3 = −326 kJ/mol, d(C−C) = 1.54 Å). The molecule is slightly rotated so that the carbon atom of the CH2 group is forming one single bond with one surface Al atom (d(Al−C) ∼ 2.0 Å). While the C2H4 molecule is strongly adsorbed in B1, its most favorable adsorption site is B4 (Eads C2H4 = −97 kJ/mol, d(C−C) = 1.55 Å). In the case of the (100) surface (Table 8), the most stable site for C2H2 adsorption (C1) is located between two

Table 4. Atomic Hydrogen Adsorption Properties of Al5Co2(100)a d(H−X) distances

site

site

site

pos.

env.

label

Eads H

Al

Co

Al Al Cosubsurface Alsubsurface (Al−Co)subsurface (Al−Co)subsurface Al−Al (Al−Al)subsurface Al 3Alsubsurface

CH 1 CH 2 CH 6 CH 8 CH 3 CH 4 CH 7 CH10 CH 5 CH 9

−53 −25 −3 +36 −14 −11 +11 +79 −9 +60

1.6 1.6 2.1 1.6 2.0 1.9 1.9 1.8 1.7 1.8

3.9 3.8 1.6 3.5 1.5 1.5 1.7 2.9 3.3 3.1

top

bridge

edge hollow a

Energies are given in kJ/mol and the closest d(H−X) distances in Å, where X refers to Al or Co atoms. Sites are first listed according to their type (top, bridge, edge, and hollow sites) and then ranked by the distance to the closest Co atom.

adsorption energy is a bridge site (BH1, − 40 kJ/mol), lying in between two Al atoms forming the triangular-shaped motifs observed at the surface. This site is located quite far away from a Co atom: d(H−CoS−1) = 3.3 Å. Here, the close distance between the H adsorbate and Co atoms does not seem to be related to a favorable adsorption site. The site BH5 is located close to the subsurface CoS−1 atoms [d(Co−H) = 1.5 Å], but the corresponding adsorption energy is endothermic (+16 kJ/ mol). On Al5Co2(100), the most stable adsorption sites are located on top of protruding aluminum atoms (CH1 and CH2), again quite far away from any surface cobalt atom (distance larger than 3.8 Å). Altogether, the most stable adsorption site for atomic H on all three surfaces is located on top of a protruding Al atom (CH1, Eads = −53 kJ/mol), which has two covalent-like Al−Co bonds broken, compared to its environment in the bulk structure.16 Charge Density Deformation. The charge density deformation is plotted in Figure 4 for the most stable site of each surface (AH1, BH1, CH1) and the least stable one of all three surfaces (namely, CH10).

Figure 4. Charge density deformations for H adsorbed on AH1, BH1, CH1, and CH10 sites. Δρ < 0 in green (less electrons) and Δρ > 0 in dark blue (more electrons). Black arrows indicate the closest CoS−1 atoms. 4962


Article

The Journal of Physical Chemistry C Table 5. C2Hx (x = 2−4) Adsorption Properties of the (21̅0) Surface (PB Model)a site

d(C−X) distances

site

pos.

env.

label

Eads C2H2

T

Co 2Al, 2Co 2Al, 1Co 2Al, 1Co 3Al 2Al, 1Co 2Al, 1Co 2Al, 1Co (2Al, 1Co)S−1 2Al 2Al 2Al 2Al

A10 A1 A2 A3 A4 A7 A8 A9 A13 A11 A6 A12 A5

−148 (−138) −261 (−210) −242 −214 −197 (−158) −168 −160 −158 −75 (−60) −139 −180 −101 (−9) −191

H

B

Al

CoS

CoS−1

2.9 2.3 2.0 2.1 2.0 2.0 2.0 2.1 2.2 2.1 2.1 2.1 2.0

1.9 1.9 1.9 1.9 4.0 1.1 1.9 3.4 1.9 1.9 2.0 3.7 3.8

4.8 4.3 3.5 3.9 3.7 4.3 4.2 3.6 3.0 3.4 2.0 4.6 4.0

Eads C2 H 3

Eads C2H4

−296 (−286) −272 (−248) −255 (−270) −273

−73 −68 −54 −87 −32 +15

−279

−6 −5

a

Energies are given in kJ/mol. Values in parentheses are adsorption energies of C2H2 molecules (resp. C2H3 molecules) rotated by 90° (resp. 180°) compared to the previous value. The d(C−X) distances are given in Å, where X is Al, CoS, or CoS−1. T for top, H for hollow, B for bridge.

Table 6. Energy and Geometry of the C2H2 Molecule Adsorbed in Bridge Sites on the (21̅0) Surface (PB−4Co Model)a

Table 8. C2Hx (x = 2−4) Adsorption Properties of the (100) Surfacea pos.

env.

label

site

site

site

Eads C2 H 2

bridge

2Al 2Al 2Al 2Al Al Al 4Alsubsurf.

C1 C2 C3 C6 C4 C5 C7

−168 (−154) −157 −156 −71 −133 −88 −9

d(C−X) distances

site env.

label

Eads C2H2

2Al, 2Co 2Al, 1Co 2Al, 1Co 3Al

A’1 A’2 A’3 A’4

−168 −177 −233 −197

Al

CoS

CoS−1

2.0 2.0 2.0 2.0

4.3 3.6 4.2

4.1 3.5 3.8 3.7

edge

a

Energies are given in kJ/mol. The d(C−X) distances are given in Å, where X is Al, CoS, or CoS−1. The C2H2 molecule adsorbed in A’1 is rotated by 90° compared to the geometry shown in Figure 3(right).

hollow

site

site

pos.

env.

label

Eads C2 H 2

Eads C2 H 3

Eads C2 H 4

bridge

2Al 2Al 2Al 2Al 3Al 4Al 3AlS−1

B1 B2 B4 B5 B3 B6 B7

−218 −203 (−201) −170 (−38) −164 (−106) −182 (−182) −147 (−133) −41 (−6)

−326 (−326)

−82

−297 −262 (−258) −303 (−303)

−97 −9 −54

hollow

−256 −177 −163 −270

(−290) (−190) (−161) (−213)

Eads C2H4 −83 −83 +109 +23

a

Energies are given in kJ/mol. Values in parentheses are adsorption energies of C2H2 molecules (resp. C2H3 molecules) rotated by 90° (resp. 180°) compared to the previous value. The C2H3 adsorption energies of site C6 are given for a molecule shifted compared to the position of C2H2. Additional stable positions are calculated for C2H3 adsorbed in site C1: −299 kJ/mol and −225 kJ/mol. They correspond to position rotated by ±90° compared to the one presented in the table.

Table 7. C2Hx (x = 2−4) Adsorption Properties of the (001) Surfacea site

Eads C2 H 3

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HYDROGENATION REACTIONS In this section, we investigate the activity and selectivity of the Al5Co2 intermetallic compound toward the semihydrogenation of acetylene. Hydrogen Dissociation. Hydrogenation reactions require the presence of atomic hydrogen adsorbed on the surface. Hydrogen dissociation on aluminum terminations is unlikely. The simultaneous H2 dissociation and hydrogenation of C2H2 (using H2 from the gas phase) is also not expected. The activation energy is calculated to be 87 kJ/mol on Al5Co2(001), while the energy difference between the reactants and products is −79 kJ/mol. Among the three considered surfaces, the (21̅0) surface is the only one which contains surface Co atoms, on top of which the dissociation of the dihydrogen molecule occurs without high energy barriers. Indeed, calculations show that the corresponding activation energy is small (15 kJ/mol). Here, H2 is first adsorbed on a surface Co atom (Eads around −75 kJ/ mol), before it dissociates. C2H2 Hydrogenation. C2H2 → C2H4. Possible reaction paths have been tested starting from scenarios using the most stable adsorption sites for C2H2. In the previous section, we

a

Energies are given in kJ/mol. Values in parentheses are adsorption energies of C2H2 molecules (resp. C2H3 molecules) rotated by 90° (resp. 180°) compared to the previous value.

protruding surface Al atoms and precisely above a subsurface Co atom (Eads C2H2 = −168 kJ/mol). The calculated d(C−C) distance is 1.36 Å, close to the one of ethylene (d(C−C) = 1.34 Å), corresponding to sp2-hybridized carbon atoms with bond order equal to 2. Likewise, this site is also the most stable site for vinyl adsorption (Eads C2H3 = −290 kJ/mol). Here, the d(C−C) distance (1.58 Å) is close to the one in ethane. Rotating the molecule by 90° leads to a small stabilization of the molecule at the surface (Eads C2H3 = −299 kJ/mol). This C1 site is again the most stable site for C2H4 adsorption, along with C2 (Eads C2H4 = −83 kJ/mol, d(C−C) = 1.55 Å). 4963


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Figure 5. Energy profile of a possible reaction path based on the A4 site on model PB from the H2 dissociation to the formation of C2H6 with transition state energies. The green line (ED) corresponds to the desorption energy of C2H4. Also represented, the different occupied sites during the reaction.

Figure 6. Energy profile of a possible reaction path based on the A’3 site on model PB−4Co from the H2 dissociation to the formation of C2H5 with transition state energies. The green line (ED) corresponds to the desorption energy of C2H4. Also represented, the different occupied sites during the reaction.

The situation is similar starting from the C2H2 molecule adsorbed in sites A2 or A3: the reactants are again more stable than the products. The energy difference is larger than 40 kJ mol−1 and 65 kJ mol−1 for the A2 and A3 sites, respectively. Using site A4 as an adsorption site for C2H2, the energy difference ΔE between the reactants and the products is quite similar (ΔE ≃ 6 kJ mol−1). Here, the closest CoS atom is used to dissociate H2. In this configuration, the possible interactions between molecules adsorbed close to (i) the catalytic active site

have highlighted the strong binding of C 2H2 on the Al5Co2(21̅0) surface. Starting from the C2H2 molecule adsorbed in site A1, the reactants {C2H2 + 2H} are more stable than the products {C2H3 + H}. The corresponding adsorption energies are −326 kJ mol−1 and −282 kJ mol−1, respectively. These numbers were calculated with hydrogen atoms on sites AH5 and AH2, diffusing from a site where H2 dissociation may occur (topmost Co atom). This means that forming C2H3 is unfavorable. 4964


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The Journal of Physical Chemistry C Table 9. Activities and selectivities of Al5Co2(21̅0) and Al13Co4(100) Surfacesa H2 → 2H

C2Hx → C2Hx+1

C2H4 ↑

surface

model

Edis a

Ex=2 a

Ex=3 a

Ex=4 a

ECd 2H4

ΔEselect

Al5Co2(21̅0) Al5Co2(21̅0) GaPd2(010) Al13Co4(100) GaPd(210) AlPd(210)

PB PB−4Co G243

15 46 49 54

60 80 51 63 61 51

50 87 71 61 70 71

60 64 42 80 59 56

87 105 29 70 45 45

−59 −49 +39 −40 +14 +11

14,44 45 44

C H4

a dis Ea

and Eia are the activation energies for H2 dissociation and C2Hx hydrogenation, respectively. Ed 2 are given in kJ mol−1.

is the ethylene desorption energy. Energies

C2H4 is probably linked to the surface through a single bond (sp3 hybridization). On the other hand, the C−C bond length of adsorbed C2H2 is 1.40 Å, which is in between the sp−sp and sp2−sp bond lengths.42 It suggests that the C2H2 molecule is linked through three bonds to the surface, leading to an adsorption energy larger than the one calculated for C2H4. Figure 7 shows the adsorption energies of C2H3 and C2H4 as a function of that of C2H2 calculated for each adsorption site

(A4) and (ii) the topmost Co atoms where H2 dissociates are minimized. Indeed, the H2 dissociation has been tested with and without the presence of acetylene in A4, with almost no changes in the H2 adsorption energy (less than 2%). Starting from site A4, the complete reaction path is detailed in Figure 5. C2H4 → C2H6. The ethylene molecule obtained in the previous paragraph either desorbs or is transformed to C2H5. For the formation of C2H5, the best scenario considered here involves a rotation of the C2H4 molecule simultaneously with the incorporation of atomic hydrogen (Figure 5). It is worth looking at further hydrogenation of C2H5 to C2H6. A first possible scenario is the hydrogenation occurring at site A4. The resulting formation energy of C2H6 is equal to −14 kJ/mol, and the corresponding activation energy is high (183 kJ mol−1), suggesting that that the formation of C2H6 is unlikely. In the previous scenario, one should notice that the adsorption site A4 is not the most favorable one for C2H5. It is then worth analyzing a possible reaction path starting from site A3, which is more stable by 29.9 kJ/mol compared to site A4. Since the A3 site is not the most favorable one for C2H6, the hydrogenation of C2H5 is calculated including a migration of C2H6 from site A3 to site A4. Within this scenario, the energy difference between the activation energies for the formation of C2H4 and C2H6 is ECa 2H6 − ECa 2H4 = +56 kJ/mol. The resulting activation energy is 106 kJ/mol, i.e., substantially smaller than the one of the first scenario, but still quite high, then suggesting that the formation of C2H6 is unlikely. Influence of Surface Co Atoms. To evaluate the influence of the topmost surface Co atoms on the surface activity, the PB−4Co structural model is also considered. The assumption that hydrogen is adsorbed and freely available is made. It is related to the low hydrogen dissociation barrier and diffusion calculated on Al5Co2(21̅0) (see Discussion below). In this case, the most stable site for acetylene adsorption is site A’H3. The complete reaction path is detailed in Figure 6 and Table 9 (second row). The rate-controlling step is the formation of ethylene. The energy required to desorb ethylene is quite high with 105 kJ mol−1, but a neighboring H atom can decrease the desorption energy of ethylene by 29 kJ mol−1. The absence of the topmost Co atoms then leads to a decrease of the activity (the activation energies are larger in this scheme than on the PB model).

Figure 7. Adsorption energies of vinyl and ethylene as a function of that of acetylene.

considered in this study. An almost linear relationship between the adsorption energies of the C2Hx molecules is found ECads2H3 = 0.21ECads2H2 + ECref2H3

(2)

ECads2H4 = 0.60ECads2H2 + ECref2H4

(3)

where = −227 kJ/mol and = 66 kJ/mol. The few points which depart from this linear relation in Figure 7 correspond to sites presenting modifications of the adsorption geometry (shift from a 3-fold site to a bridge site, rotation of the molecule, etc.). Such scaling relations are quite frequent in catalysis. Initially developed for AHx-type of adsorbates with A = C, N, O, or S, adsorbed on transition metals,46 they were generalized to adsorption energies for C2Hx-type of adsorbates.47,48 On reactive transition metal surfaces, a simple bond order conservation model leads to coefficients equal to 0.75 and 0.5 48 ads ads for Eads This C2H3 and EC2H4 as a function of EC2H2, respectively. model has been checked to be in excellent agreement with the energetics obtained from DFT calculations, using transition metal surfaces (reactive surfaces) that differ mainly in their electronic structure. The change in adsorption energy across Eref C2H3

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DISCUSSION Correlation between Adsorption Energies on Al5Co2 Low-Index Surfaces. Overall, adsorption energies for ethylene are weaker than for acetylene, on a given site. This may be linked to the number of bonds between the molecule and the surface. Since the length of C−C bonding in adsorbed C2H4 is similar to the one of ethane (1.55 Å), each carbon atom of 4965

Eref C2H4


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with potentially higher barriers to dissociative adsorption of hydrogen.54−56 The H2 dissociation does not present a high barrier on Al5Co2(21̅0) (15 kJ/mol). This activation energy is similar to the one calculated on Al13Co4(100) (17 kJ/mol). The reason for such low H2 dissociation barriers on the two previous Al− Co compounds is related to the presence of protruding Co atoms at the surface. Indeed, while the barriers for H2 dissociation are quite high on Al(111) and Al(100) surfaces (between 86 and 124 kJ/mol),57−61 they are significantly lower on several transition-metal-doped Al or Mg surfaces.62−64 When Al and a transition metal are present at the surface, for instance on NiAl(110), the energy barrier for H2 dissociation is much lower over Ni sites compared to Al sites.65,66 On PdxRu1−x, the activity of Ru toward dissociative adsorption of molecular hydrogen is enhanced by the presence of nearestneighbor Pd atoms.67 The dissociation of H2 occurs on top of a protruding Co atom. Such a site is close to the reactive site (A4 site), where the C2H2 molecule is adsorbed. The migration of one atomic hydrogen atom is then required for the hydrogenation reaction (C2H2 → C2H4), but the corresponding barrier is quite low (20 kJ/mol). More generally, only low barriers (smaller than 20 kJ/ mol) have been calculated on Al5Co2(21̅0) for the hydrogen migration. It is consistent with the low activation energy calculated for hydrogen migration on pure and transition-metaldoped Al(100) (≃21 kJ/mol for Al(100) and below 14 kJ/mol for Sc-, Ti-, V-, and Nb-modified Al surfaces62). Adsorption Properties: Al5Co2 vs Al13Co4. Calculated H adsorption properties for both compounds slightly differ. The highest adsorption energies for atomic hydrogen are found to be −53 kJ/mol, −40 kJ/mol, and −38 kJ/mol on Al5Co2(100), (001), and (21̅0) surfaces, respectively, while it is only −22 kJ/ mol and −15 kJ/mol on two different models for the Al13Co4(100) surface.14,15 A large number of sites present large adsorption energies for C2H2 on both (21̅0) and (001) surfaces of Al5Co2 (sites A1, A2, A3, A’3, B1, and B2, with Eads < −200 kJ/mol), while the strongest adsorption energy for C2H2 on Al13Co4(100), using a surface model obtained by a simulated cleavage, is equal to −184 kJ/mol.14 There are however similarities between the most favorable C2H2 adsorption sites on Al5Co2(21̅0) and Al13Co4(100). They are usually made of two Al and one adjacent Co atoms (except site A1 made of 2 Al and 2 Co atoms). Another exception could be site A4 (hollow site made of 3 Al atoms), but in that case there is one Co atom located immediately below in the subsurface. The orientation of the C2H2 molecule adsorbed in this type of atomic environment is different in the case of Al5Co2(21̅0) and Al13Co4(100). No such large differences between the two Al−Co compounds are observed for the adsorption of C2H3 and C2H4. The strongest C2H3 adsorption energies are −296 kJ/ mol (A1), −326 kJ/mol (B1), and −299 kJ/mol (C1) on Al5Co2 surfaces, while it is −322 kJ/mol on Al13Co4(100).14 The strongest C2H4 adsorption energies are −87 kJ/mol (A4), −82 kJ/mol (B1), and −83 kJ/mol (C1, C2) on Al5Co2 surfaces, quite similar to the −70 kJ/mol calculated for Al13Co4(100).14 Activity: Al5Co2 vs Al13Co4. The different adsorption properties of Al5Co2(210̅ ) and Al13Co4(100) lead to different calculated activities. On Al5Co2(21̅0), the most stable C2H2 adsorption sites are found to be nonreactive. In addition, the strong adsorption of

transition metals is due to changes in the coupling to the delectrons (d-band model). The previous approach lacks structure sensititivy. Recently, ref 49 quantified the effects of adsorption-site geometry on scaling relations, on the basis of oxygen and oxygenates adsorbed on various surface termination of a few pure transition metals. Only electron-counting rules are found to control the slope of the scaling relations in that case. Usually, the linear relationships between adsorption energies of similar adsorbates are considered for most stable adsorption sites on a series of pure transition metal surfaces. In our case, the scaling relations are established for the low-index surfaces of a single intermetallic. They cannot be understood with a simple bond order conservation model. Other effects need to be taken into account, as already demonstrated for other studies on alloy surfaces. More subtle effects may occur at the surface. The consideration of site specificity is required to obtain reliable linear relations for hydrocarbons adsorbed on hexagonal transition metal surfaces from groups 3 to 5 and 7 to 11.50 A subtle environment-dependent behavior has been highlighted for adsorption of hydrocarbons on close-packed Ag/Ni(111): the binding of CH3 and CH2 is weakened, while the adsorption of CH and C is enhanced with the incorporation of Ag.51 Reference 52 shows that metal carbides do not in general follow the transition-metal scaling relations. Their specific surface activity can be partially rationalized with the valence configuration of the catalyst surface as well as the contributions of the metal-projected sp states to the adsorbate−surface bond. On Al5Co2 surfaces, a deep understanding of the origin of these scaling relationships would require the consideration of extended parameters, such as the local electronic and atomic structure around each adsorption site, which is beyond the scope of the present article. However, it suggests that complex intermetallic surfaces are promising to go over the well-known linear relationships based on a simple bond order conservation model and to optimize the catalyst performances. H2 Dissociation on Al5Co2(21̅0). The strongest adsorption site on Al5Co2(001) is a bridge site (BH1), while it is a hollow site on Al5Co2(21̅0) (AH1) and a top site on Al5Co2(100) (CH1). In all cases, the analysis of the charge deformation shows that both surface (if any) and subsurface Co atoms influence the stability of the investigated adsorption sites. They tend to stabilize the neighboring adsorption site when presenting an electronic donor character. The atomic hydrogen binding energies among pure metals and near surface alloys, defined as alloys wherein a solute is present near the surface of a host metal in a composition different from the bulk composition, have been rationalized in terms of the electronic structure of the clean surface. A reasonable linear correlation exists between the d-band center of the metallic substrates (pure metal surfaces and Ptterminated near surface alloy) and the atomic hydrogen binding energies.53 However, the slope between the two substrate types (pure metal or Pt-terminated near surface alloy) differs, suggesting that other factors are needed to fully explain the hydrogen adsorption on metallic surfaces. On Al5Co2 surfaces, the hydrogen adsorption energies are slightly different among the three considered low-index surfaces, while the corresponding surface d-band centers are reasonably similar (−1.70 eV, −1.76 eV, and −1.84 eV for the (21̅0), (100), and (001) surfaces, respectively). Hydrogen dissociation can be rate determining. It is the case on coinage metal surfaces (Cu, Ag, Au), which present lower hydrogen adsorption energies compared to transition metals, 4966



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CONCLUSION Adsorption energies of hydrogen, acetylene, vinyl, and ethylene have been calculated on Al5Co2(001), (100), and (21̅0) surfaces. As already observed for the Al13Co4(100) surface, Co subsurface atoms can play a stabilizing role on the adsorption sites. On the three low-index surfaces of Al5Co2, the adsorption energies of C2H2 roughly scale with the one of C2H3 and C2H4. While the C2H3 and C2H4 adsorption energies are found to be similar to the ones calculated on Al13Co4(100), atomic hydrogen and acetylene are more strongly bounded to Al5Co2 surfaces. This in turn affects the activity. In a second step, we focused on the activity of the Al5Co2(210̅ ) surface, as it is the only one (among the considered surfaces) which contains surface Co atoms favoring H2 dissociation. Here, the reactive site is a 3-fold site made with 3 Al atoms (A4 site). The rate-controlling step for the semihydrogenation (C2H2 → C2H4) is the formation of C2H3, with an activation energy calculated to be 60 kJ/mol. Further hydrogenation is limited since the barrier for the complete hydrogenation is quite high (C2H5 → C2H6, 106 kJ/mol). This suggests a selective catalyst. The (100) surface shows a less stable adsorption site for C2H2 (−168 kJ/mol), while maintaining a similar adsorption energy for C2H4 (−83 kJ/mol). One would expect a higher activity for this latter low-index surface. Here, the limiting factor may be hydrogen dissociation since there are no surface Co atoms. In all cases, experimental investigations of adsorption and activity properties of the low-index Al5Co2 surfaces would be interesting to assess the numerical approach.

the reactants at these sites, close to a surface Co atom, may block surface Co atoms for dihydrogen dissociations and then reduce the catalytic activity of the surface. In this work, the starting point for the considered reaction is site A4, an adsorption site made of three Al atoms which is less stable (by ≃60 kJ/mol) and which presents an C2H2 adsorption energy (−197 kJ/mol) similar to the one calculated on the reactive and selective Al13Co4(100) surface (−184 kJ/mol). While there are two A4 sites per reconstructed unit cell (≃200 Å2) on the Al5Co2(21̅0) surface (PB model), only one reaction site is available per surface unit cell (≃178 Å2) for Al13Co4(100). The rate-controlling step is similar on the two Al−Co compounds: it is the formation of vinyl (Table 9) with an activation energy equal to 62 and 63 kJ/mol on the Al5Co2(21̅0) and Al13Co4(100) surfaces, respectively. The presence of the topmost Co atoms on Al5Co2(21̅0) ensures this relatively small activation energy. Using the PB−4Co model, the ratecontrolling step becomes the formation of ethylene with a relatively high activation energy (Ei=3 a = 87 kJ/mol, see Table 9). As a conclusion, the efficiency of the Al5Co2 catalyst [(21̅0) surface] is not better than the Al13Co4 one [(100) surface]. These conclusions result from the differences calculated for the adsorption properties of the two Al−Co surfaces. According to Krajči ́ and Hafner,12 a decisive point for the selectivity of intermetallic catalysts toward ethylene is the change from the strong di-σ binding of acetylene and the vinyl moiety to the weak π binding of ethylene. Here, while the favorable adsorption sites for C2H2 on Al5Co2(21̅0) and Al13Co4(100) involve 2 Al and 1 Co atom, the orientation and the energy of the molecule on the two sites substantially differ, making the hydrogenation reaction almost impossible according to this scheme. Reaction Selectivity. Selectivity to ethene is commonly defined as the ratio between ethene and ethane production rates. Recently, M. Krajči ́ et al. proposed a criteria for the selectivity in the case of the semihydrogenation of acetylene catalyzed by complex intermetallic compounds (Pd−Ga, Pd− Al, Al−Co).12 It is based on the difference between the barrier for C2H4 hydrogenation to ethyl and the desorption of C2H4 (ΔEselect = ETS − ED, where ETS is the transition state energy for the ethyl formation and ED the reference energy of ethylene in the gas phase, ED = −219 kJ mol−1). It correctly reproduces the high experimental selectivity of Ga−Pd catalysts reported in the literature. Indeed, on GaPd2(010) and GaPd(210), the desorption of C2H4 is calculated to be more likely than further hydrogenation, leading to the selectivity of the corresponding semihydrogenation reaction (ΔEselect > 0 for GaPd2(010) and GaPd(210), see Table 9). Using the previous criteria leads to negative values for ΔEselect in the case of Al13Co4(100) and Al5Co2(21̅0). These results are not surprising since the selectivity cannot be accounted for only on the basis of the ethyl formation barrier minus the gas-phase ethene energy. It should also depend on the acetylene hydrogenation barrier, dihydrogen activation barrier, etc. According to ref 14, the selectivity of Al13Co4(100) mainly originates from hydrogen migration. Indeed, the quite high barrier of the rate-controlling step for the hydrogenation C2H4 → C2H5 (80 kJ/mol in ref 14) is related to atomic hydrogen migration close to the active site. This barrier is larger than the desorption energy of an isolated ethylene molecule (70 kJ/ mol). In this paper, we showed that the selectivity of Al5Co2(21̅0) mainly originates from the high activation energy calculated for the C2H5 hydrogenation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

É. Gaudry: 0000-0001-6546-8323 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the ANR CAPRICE 2011-INTB 1001-01 and the European C-MAC consortium. MM also acknowledges financial support from the “Région Lorraine” during his PhD. This work was granted access to the HPC resources of GENCI (grand équipement national de calcul intensif) under the allocation 96339 (IDRIS 99642).

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REFERENCES

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Semihydrogenation of Acetylene on Al5Co2 Surfaces - American

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