Adsorption Properties of the o-Al13Co4 (100) Surface toward

Article pubs.acs.org/JPCC

Adsorption Properties of the o‑Al13Co4(100) Surface toward Molecules Involved in the Semihydrogenation of Acetylene D. Kandaskalov, V. Fournée, J. Ledieu, and É. Gaudry* Institut Jean Lamour, Université de Lorraine UMR CNRS 7198, Parc de Saurupt CS50840 54011 Nancy cedex, France ABSTRACT: This paper presents the adsorption properties of the o-Al13Co4(100) surface toward molecules involved in the semihydrogenation of acetylene. The energetically favored adsorption sites of H, C2H2, C2H3, and C2H4 are determined thanks to ab initio density functional calculations. The surface model used for this study has been determined previously [Phys. Rev. B 2011, 84, 085411], using an approach combining both experimental observations and density functional theory calculations. We show that although the surface termination layer of o-Al13Co4(100) is a dense Al-rich layer, its adsorption properties are quite different from pure elemental Al surfaces, especially for atomic hydrogen adsorption (exothermic on oAl13Co4(100) and endothermic on low index Al surfaces). The role of surface and subsurface cobalt atoms is investigated carefully. The electronic donor/acceptor character of subsurface cobalt atoms is shown to influence the adsorption properties. In particular, the subsurface cobalt atoms have a rather stabilizing effect on adsorption while the role of surface cobalt atoms located slightly below the mean position of the termination plane is moderately destabilizing. These atomic interactions ensure the isolation of a well-defined active site on the surface.

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INTRODUCTION Polyethylene, obtained from ethylene by a polymerization process, is one of the most common plastic, the annual global production being approximately 80 million tonnes.1 Ethylene involved in the polyethylene production is generally obtained by the cracking of light alkanes and contains typically about 1% of acetylene, which poisons the polymerization catalyst.2,3 To remove acetylene, ethylene has to be purified. One step in the purification process is the selective hydrogenation of acetylene to ethylene, which requires high selectivity to prevent the hydrogenation of ethylene to ethane.4 Palladium-based catalysts modified with silver are traditionally used in the industrial process.5 Pd−Au and Pd−Pb also show a high conversion rate and selectivity toward this reaction.6,7 However, the high price of this type of catalysts is obviously prohibitive for large scale production. More recently, an ab initio analysis over 70 non-noble metals and alloys predicted Ni−Zn to be an efficient catalyst.8,9 This theoretical finding was confirmed experimentally, yielding opportunities for new cheap catalysts. The use of simple alloys as catalysts still presents some drawbacks. In particular, they usually suffer from solid state transformations such as segregation processes,10,11 leading to a lowering of the selectivity, up to a complete deactivation of the catalyst with time. Complex intermetallics (CIMs) have recently been considered as alternative materials for the semihydrogenation of acetylene.12−15 These catalysts are generally more stable compared to simple alloys, due to the presence of covalent-like bondings in their structures. Their specific reactivity is related to their complex surfaces, presenting a large number of nonequivalent adsorption sites, with © 2014 American Chemical Society

generally very specific adsorption geometries. Among them, the Al-based complex intermetallic compounds Al13TM4 (TM = Fe, Co) attracted the attention of the scientific community as a promising low-cost catalyst for the semihydrogenation.14−16 In this paper, we focus on the o-Al13Co4 compound. This phase is structurally related to the monoclinic m-Al13Fe4, but the surfaces of these two compounds show some significant differences which might affect their chemical reactivity.17,18 In this paper, we focus on o-Al13Co4, which is the compound included in a recent patent.14 Its atomic structure (Pearson symbol oP102, space group Pmn21) is related to the decagonal Al−Ni−Co quasicrystal19−21 and presents a rather large crystal cell containing 102 atoms (a = 8.158 Å, b = 12.342 Å, and c = 14.452 Å).22,23 Its bulk structure can be described by a stacking of two types of planes along the [100] direction, called flat (F) and puckered (P) planes, with the sequence F0P0.25F0.5P0.75, where indices refer to positions along [100]. As most complex intermetallics, the structure of o-Al13Co4 can also be described in terms of clusters as structural building blocks. The first description was based mainly on geometrical considerations,24 using 23-atom pentagonal bipyramid shaped clusters. Most recent quantum chemical calculations identified o-Al13Co4 as a cage compound with strong covalent linear Co−Al−Co bonding (guest) surrounded by elongated cavities (host cage).23,25 From the descriptions of the o-Al13Co4 crystal structure presented previously, competition is expected between the Received: May 19, 2014 Revised: September 10, 2014 Published: September 10, 2014 23032

dx.doi.org/10.1021/jp504906r | J. Phys. Chem. C 2014, 118, 23032−23041

The Journal of Physical Chemistry C

Article

are the ones involved in the semihydrogenation of acetylene (H, C2H2, C2H3, C2H4). Two surface models are considered, which differ by the number of surface cobalt atoms, to account for experimental observations. The paper is organized as follows. Details about the computational framework are given in the section Methodology. Results are presented in the following section. The adsorption properties of the two considered models are summarized in the section Discussion and Conclusions and compared with the ones calculated for the surface model obtained by numerical cleavage. By a systematic investigation of the adsorption properties of the o-Al13Co4(100) surface (around 50 possible adsorptions sites with different geometries are considered), we show that atomic hydrogen adsorption is exothermically possible on the Al-rich terminations of this complex surface, which is not the case on pure low-index Al surfaces. In addition, contrary to the expected fact that Co atoms located just beneath the mean position of the surface layer stabilize the adsorption sites, we have found that their role is moderately destabilizing. At the same time, the role of subsurface Co atoms, located around 2 Å below the surface plane, is rather stabilizing. The isolation of active sites toward reactivity results then from such atomic interactions, and not from the presence of isolated transition metal atoms at the surface.

selection of bulk-truncated dense planes as surface planes and the preservation of the cluster structure up to the surface. While a simulated cleavage results in a split of the P layer into two complementary parts, preserving the cluster integrity,26 a detailed analysis coupling both experimental observations (scanning tunneling microscopy (STM) and dynamical low energy electron diffraction) and ab initio methods leads to the conclusion that the o-Al13Co4(100) surface structure consists of dense Al-rich layers with surface Co atom depletion.17,27 In this latter case, clusters are truncated at the surface. The surface structure deduced from the numerical cleavage26 is then quite different from the one deduced from surface energy calculations.17,27 Indeed, the numerical cleavage is done with a fixed number of atoms in the simulation supercell, while the approach chosen by refs 17 and 27 considers a large number of possible structural models for the surface, with different chemical compositions. It is worth noting that the surface structure also presents few heterogeneities: some Co atoms lying slightly underneath the mean position of the puckered surface plane can also be present, and some of the Al glue atoms, connecting bipentagonal motifs, are missing at the surface as evidenced by STM observations. While numerous studies have been devoted to the properties of simple surfaces toward the adsorption of molecules involved in the semihydrogenation of acetylene, very few investigations have been reported on complex metallic alloys surfaces. For simple metals, two types of adsorption geometries have been proposed for C2H2 based on vibrational spectroscopy data: coordination to four surface atoms28−30 and di-σ bridging coordination to either one or two adjacent surface atoms with extra π-interaction to the neighboring atoms.28,30,31 Similarly, two types of bondings have been identified for C2H4 on simple metallic surfaces: di-σ and π bondings. The degree of the double-bond character retained upon adsorption varies with the nature of the metallic surface, from C−C single-bonded species32−39 to adsorbates with a large double-bond character.40−47A previous theoretical study of the adsorption properties of the o-Al13Co4(100) surface found that C2H2 is bound in a di-σ configuration to two Al atoms in the energetically most favorable configuration, while C2H4 is adsorbed on top of Co atom (π-type bond).26 However, the starting point of these calculations was the surface model derived from a theoretical cleavage which appears to be inconsistent with the experimental observations. In particular, the surface model used by ref 26 contains protruding surface cobalt atoms, while the surface model deduced from an approach combining theoretical and experimental results17 points toward an almost Al pure termination layer. In this paper, we investigate the adsorption properties of the o-Al13Co4(100) surface using the surface model derived from the combined experimental observations and ab initio calculations reported in refs 17 and 27. Around 50 possible adsorption sites, with different symmetries, are considered. The different structure models raise some issues toward the adsorption properties. Are the preferred adsorption sites the one corresponding to the truncated clusters, as it is usually the case for quasicrystalline surfaces? Do the surface cobalt atoms, located slightly beneath the surface plane, play any role in the adsorption properties? In order to gain deeper insights into the physical and chemical parameters influencing these properties, we present here a detailed investigation of the adsorption on the o-Al13Co4(100) surface, using calculations based on density functional theory. Atoms and molecules included in this study

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METHODOLOGY Computational Details. Our calculations were performed using the Vienna ab initio simulation package (VASP).48 Selfconsistent Kohn−Sham equations were solved by means of the projected-augmented wave (PAW) method49 to describe the electron−ion interactions. The electron exchange and correlation is described by the generalized gradient approximation approach (GGA) with the PBE functional.50 Spin polarization was not taken into account since it has been shown to be not required for such an Al-rich CIM.17,20 The plane-waves energy cutoff was fixed to 400 eV for all systems. This is sufficient to converge total energies down to 1 meV for molecules and pure Al surfaces and up to 3 meV for the o-Al13Co4(100) surface models. Monkhorst−Pack meshes51 were used for the k-points sampling. We used a 1 × 8 × 8 mesh for Al(100), a 1 × 8 × 6 mesh for Al(111), and a 1 × 4 × 4 kmesh for the o-Al13Co4 surface models. The total energy does not vary by more than 1 meV within these criteria. In the following, the adsorption properties are evaluated through the determination of the adsorption energies of H2 and C2Hx (x = 2−4) (eqs 1 and 2): 0 0 E Hads = E H/surf − Esurf −

1 0 EH 2 2

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

(1) (2)

where E0X is the total energy of system X. The binding energy of atomic hydrogen is also evaluated by 0 0 E Hbind = E H/surf − Esurf − E H0

(3)

Free Molecules. The intrinsic properties of the free C2H2, C2H3, and C2H4 molecules have been calculated as a preliminary study. The results for C2H2 and C2H4 are summarized in Table 1. There is a good agreement between the calculated and experimental geometries. In addition, the C2H3 radical has been considered, leading to calculated C−C distances equal to 23033



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However, in this case, the previous A13 top site is replaced by an additional 5-fold site (P4) and the 5 Al−Co bridge sites B0(a−e), along with 5 Al−Al−Co 3-fold sites, do not exist anymore. The adsorption sites are represented in Figure 2. This paper includes also the calculation of the adsorption properties of pure aluminum surfaces for comparison. Two lowindex surfaces were considered: Al(100) and Al(111). The corresponding slabs were built with 12 atomic layers separated by a 16 Å void thickness, leading to interlayer relaxations smaller than 0.01%. A 2 × 4 surface unit cell (respectively 2 × 2) was used for Al(111) (respectively Al(100)). This leads to large supercells (96 and 108 atoms) to avoid adsorbate− adsorbate interactions from periodic pictures when studying adsorption properties.

Table 1. Geometries of the C2H2 and C2H4 Molecules; Comparison of the Calculated Values with the Experimental Ones (Ref 52) C2H2 C2H4

C−C C−H C−C C−H C−C−H H−C−H

calcd

expt

1.21 Å 1.07 Å 1.33 Å 1.09 Å 117.4° 121.8°

1.20 Å 1.06 Å 1.34 Å 1.09 Å 117.4° 121.3°

1.30 Å which indicated that the carbon atoms of C2H3 are closer to an sp2 hybridization rather than an sp one. Bulk Systems. The cell parameters and the cohesive energies of bulk aluminum and o-Al 13 Co 4 have been determined prior to the surface study. For aluminum, the cell parameter (4.04 Å) and the cohesive energy (3.48 eV) are in good agreement with the experimental data (4.05 Å, 3.34 eV).53 It is the same for o-Al13Co4: the calculated cell parameters are a = 8.20 Å, b = 12.35 Å, and c = 14.43 Å, while the experimental ones are a = 8.158 Å, b = 12.342 Å, and c = 14.452 Å.22,23 The formation enthalpy is calculated to be −0.39 eV/at., in good agreement with previous calculations.54 Surface Models. The o-Al13Co4(100) surface models were designed by means of a supercell approach, using symmetric slabs built by a stacking of atomic layers separated by a void thickness (18 Å). An 11-layer thick slab (≃ 275 atoms) is required to obtain well-converged interlayer distances d in the center of the slab (((d0 − d)/d0) ≤ 0.1%, where d0 is the corresponding interlayer distance in the bulk) and small forces F on atoms (F < 0.01 eV/Å). Due to the large number of possible adsorption sites, we use here a 7-layer thick slab (≃ 175 atoms), resulting in relaxations at the center of the slab (d0 − d)/d0 < 0.5% and forces on atoms smaller than 0.02 eV/Å. The calculated adsorption energies for C2H2 in three different adsorption sites (B22, S1b, B7; see the following paragraph for explanations of the site labels) are within 10 meV for the two slabs. Two different terminations were considered to account for the experimental observations. The surface model labeled B is a pure aluminum layer while the surface model A contains additional Co atoms lying slightly underneath the mean position of the surface plane (Figure 1). Surface models A and B differ only by two cobalt atoms in the top layer (Figure 1). By considering symmetry, the number of possible adsorption sites can be reduced to about 50 for each model. For model A, the considered adsorption sites include 13 atop sites (A1-A13), 22 bridge sites (B1−B22), 4 3-fold sites (T1-T4), 4 4-fold sites (S1−S4), and 3 5-fold sites (P1−P3). The same types of sites have been considered for model B.

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RESULTS

Atomic Hydrogen Adsorption. H on Al(111) and Al(100) Surfaces. We begin our study with the pure Al(100) and Al(111) surfaces as reference systems to the Al-rich oAl13Co4(100) model surfaces. This preliminary study is also a good opportunity to compare our results with other theoretical and experimental data in order to justify our theoretical approach. Our results are gathered in Tables 2 and 3. Atomic hydrogen adsorption on Al(111) leads to binding energies in the range [−1.86; −2.00] eV, in agreement with previous calculations, using either a self-consistent Kohn−Sham approach for an H-jellium system55 or DFT calculations with a thinner slab (6 atomic layers).56 The most stable adsorption site on Al(111) is identified to be a 3-fold fcc site, in agreement with ref 56. Concerning the Al(100) surface, the most stable adsorption sites are bridge sites, with a binding energy (−2.17 eV), in good agreement with the experimental value (−2.1 eV)58 and other ab initio studies.55,57 H on o-Al13Co4(100). Among all possible adsorption sites, it appears that only some of the bridge sites are stable. After relaxation, hydrogen atoms adsorbed on almost all atop, 3-, 4-, and 5-fold sites migrate to the nearest bridge sites. The corresponding adsorption energies and geometries are gathered in Table 4, where the bridge sites are classified according to their distance from the A13 site (see Figure 2 left), i.e. the site occupied by a surface cobalt atom (respectively vacancy) in model A (respectively model B): 1NN sites with dA13−H ≈ 1.85 Å, 2NN with dA13−H ≈ 3.20 Å, 3NN with dA13−H ≈ 3.90 Å, and 4NN with dA13−H > 4.85 Å. It means that all 1NN sites are located on the black circle represented in Figure 1 left, while 2NN sites lie on the red circle and 3NN sites on the purple circle of the same figure. 4NN sites are located farther from the central A13 site.

Figure 1. Terminating layers for surface models A and B, highlighting the two types of bipentagonal motifs, and subsurface layer (F) common to models A and B. The o-Al13Co4 bulk structure is represented on the right-hand side. 23034



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Figure 2. Adsorption site locations on surface model A. Bridge sites are represented in the middle (B1−B22), while 3-fold (T1−T4), 4-fold (S1− S4), and 5-fold (P1−P3) sites, along with atop sites (white numbers 1−13) are represented on the right-hand side. Bridge sites are classified according to their distance from the A13 site (1NN, 2NN, 3NN, 4NN). On the left-hand side, black circles represent locations where dA13−X ≈ 1.85 Å (1NN sites lie on black circles), red circles show locations where dA13−H ≈ 3.20 Å (2NN sites lie on red circles), and purple circles correspond to locations where dA13−H ≈ 3.90 Å (3NN sites lie on purple circles). b Table 2. Atomic Hydrogen Adsorption Eads H and Binding EH Energies (in eV) on Al(111), along with Al−H Bond Distances a (dAl−H in Å)

top

a

bridge

Eads H

EbH

dAl−H

0.32 0.5

−1.94 −1.9 −2.07

1.62 1.54

Eads H

EbH

0.5

→ 3-fold (fcc) −1.9 −2.10

3-fold (fcc) dAl−H

Eads H

EbH

1.80

0.26 0.7

−2.00 −1.7 −2.36

3-fold (hcp) dAl−H

Eads H

EbH

dAl−H

ref.

1.92 1.91

0.40 0.6

−1.86 −1.8 −2.16

1.94 2.01

this work 55 56

Atomic hydrogen initially positioned at bridge site relaxes to the 3-fold (fcc) site.

b Table 3. Atomic Hydrogen Adsorption Eads H and Binding EH Energies (in eV) on Al(100), along with Al−H Bond Distances (dAl−H in Å)

top Eads H

EbH

0.31 0.5 0.29

−1.96 −1.9

bridge dAl−H

Eads H

EbH

1.61 1.59

0.09 0.1 0.08

−2.17 −2.3

4-fold

For surface model A, the adsorption energies Eads H for bridge sites vary from −0.16 eV/atom up to +0.27 eV/atom, while they range from −0.14 to +0.14 eV/atom for surface model B. For model A, the five sites B0(a−e) are not stable: hydrogen atoms migrate to the nearest cobalt atop position (A13). The corresponding endothermic adsorption energy (Eads H = +0.04 eV) is of the same order of magnitude as the one calculated for the atop position on pure Co(100) (Eads H = +0.09 eV), 59 ads Co(0001) (Eads H = +0.08 eV), and Co(111) (EH = +0.01 eV). From Table 4, we notice that the Al−H distances for the bridge adsorption sites range from 1.73 to 1.92 Å, which correspond roughly to the Al−H distance found for atomic hydrogen in the most stable adsorption sites on Al(111) and Al(100): 1.80 Å for H in a bridge site on Al(100), 1.92 Å for H in a hollow fcc site on Al(111). However, although the oAl13Co4(100) surface models present Al-rich terminations, the corresponding adsorption energies calculated for the most stable adsorption sites (B2, B8, and B16 for model A; B2, B14, and B16 for model B) are exothermic and are quite different from the endothermic adsorption energies calculated for the most stable adsorption sites on pure aluminum models (+0.26 eV for the 3-fold fcc site on Al(111) and +0.09 eV for the bridge site on Al(100)).

dAl−H

Eads H

EbH

dAl−H

ref.

1.80 1.80

0.48 1.0 0.50

−1.77 −1.4

2.13 2.39

this work 55 57

From Table 4, we also notice that, for surface model A, all stable adsorption sites are located within a Co−H distance (dCoS−H) roughly equal to 3.77−4.05 Å (3NN). Yet, all possible 3NN adsorption sites are not necessarily stable. This means that the CoS−H distance is not the only parameter to take into account in order to interpret the o-Al13Co4 adsorption properties. The same observation is valid for the dCoS−1−H distance (dCoS−1−H roughly equal to 3.28−3.44 Å), for both models A and B. Deeper insight into the stability of the considered adsorption sites is provided by the following charge density study. Influence of Electronic Parameters on the Stability of Bridge Sites. The charge density differences induced by H adsorption is Δρ = ρ(H/surf) − ρ(surf) − ρ(H)

(4)

where ρ(H/surf), ρ(surf), and ρ(H) are the charge densities for the H/surface, the clean surface, and an isolated H atom, respectively. The change in charge density upon adsorption ΔP(x) as a function of x is calculated by integrating Δρ over planes of constant height along the surface normal (defined as the x-axis here): 23035



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Table 4. Adsorption Energies and Geometries for Atomic Hydrogen on o-Al13Co4(100)a model A

model B

type

site

Eads H

dAl−H

dCoS−H

dCoS−1−H

Eads H

dAl−H

dCoS−1−H

1NN 2NN

B0(a−e) → A13 B9 B13 B14 B5 B12 B8 B2 B16 B10 B20 B3 B15 B6 B11 B7 B18 B22 B19 B1 B4 B21 B17

+0.04 +0.03 +0.12 +0.27 → B3 → B10 −0.16 −0.15 −0.15 −0.12 −0.09 −0.02 +0.02 +0.10 +0.11 −0.03 +0.01 +0.02 +0.05 +0.15 +0.20 +0.19 +0.24

2.05 1.75/1.90 1.79/1.87 1.73/1.86 − − 1.78/1.88 1.78/1.83 1.77/1.84 1.79 1.79 1.80 1.77/1.82 1.80 1.75/1.86 1.77/1.81 1.77/1.83 1.79 1.77/1.92 1.81 1.79 1.79 1.79

1.55 3.24 3.23 3.15 − − 4.05 4.01 3.98 3.77 3.95 3.93 3.95 4.02 3.87 5.78 6.44 5.61 5.15 5.43 4.85 5.29 5.12

4.89 3.61 3.74 3.93 − − 3.28 3.31 3.33 3.42 3.35 3.36 3.44 3.42 3.34 3.89 4.13 3.92 3.30 3.32 3.96 3.37 4.25

− +0.04 −0.03 −0.13 −0.05 +0.02 −0.10 −0.13 −0.14 −0.05 −0.05 +0.02 +0.07 +0.08 +0.14 −0.04 +0.02 0.00 +0.07 +0.07 +0.01 +0.14 +0.08

− 1.75/1.85 1.73/1.87 1.76/1.86 1.77/1.83 1.78 1.82 1.75/1.86 1.80 1.79 1.77/1.81 1.71/1.92 1.77/1.84 1.78 1.78/1.89 1.79 1.79 1.78 1.78/1.89 1.78/1.86 1.79 1.80 1.79

− 3.81 3.84 3.88 3.81 3.81 3.28 3.32 3.34 3.39 3.35 3.39 3.45 3.81 3.30 3.90 3.90 3.95 3.29 3.32 3.95 3.35 4.10

3NN

4NN

a

Energies are given in eV, and distances in Å. Two values are given for the Al−H distancedAl−H if the bridge position is nonsymmetric. The dCoS−H and dCoS−1−H are the shortest distances where the Co atom is in the surface (S) and subsurface (S − 1) layer, respectively.

ΔP(x) =

∫ ∫ Δρ(y , z) dy dz

present a relatively small charge transfer, in the range of 0.04− 0.09 electrons, depending slightly on the integration range. This suggests a rather covalent metal-H bonding.60 This conclusion is also consistent with the plot of charge density differences, highlighting nonspherical charge density differences around atomic hydrogen (Figure 4). Figure 3 also shows that the variations of the charge density expand up to the subsurface layer. The subsurface cobalt atoms could then play a role in the adsorptions properties. This is demonstrated by the plot of the charge density differences for the most stable (B8) and less stable (B14) adsorption sites of surface model A (Figure 4). It appears that the electronic donor character of the subsurface Co atom, lying roughly underneath the hydrogen adsorbate, tends to stabilize the corresponding adsorption site (B8 site), while the electronic acceptor character of the subsurface Co atom tends to destabilize the corresponding adsorption site (B14 site). The role of the surface cobalt atom is also clear when comparing the stability of the B14 site on the two models A and B. While the subsurface cobalt atom presents an electronic donor character in model B which stabilizes this site, the same subsurface cobalt atom becomes rather electronic accepting in model A, leading to a destabilization of the corresponding B14 site. The same trend is observed for all investigated bridge sites. This charge density study also shows that, for 3NN bridge sites, the nearest surface cobalt atom plays only a minor role in the charge transfer toward the hydrogen adsorbate. Adsorption of Hydrocarbon Species: C2H2, C2H3, and C2H4. C2H2. We have tested several possible adsorption geometries for C2H2 on models A and B of the oAl13Co4(100) surface. Results are gathered in Table 5. It appears that the most favorable adsorption site are found among the 4-fold adsorption sites, for which adsorption

(5)

For all considered bridge adsorption sites, the ΔP(x) function shows that a small amount of charge depletion is observed for metallic surface atoms while charge accumulation is calculated for hydrogen adatoms (see Figure 3 where only the ΔP(x) for the most stable adsorption sites of surface models A are plotted). This charge transfer is evaluated more precisely by integrating ΔP(x) over a length of 1.8 Å centered on the hydrogen atom (pink region in Figure 3). All bridge sites

Figure 3. Change ΔP(x) in charge density upon adsorption, as a function of the distance x perpendicular to the surface, for three different adsorption sites: B14 for surface models A and B, and B08 for surface model A. The pink regions define the integration regions around H (two regions are used with lengths equal to 1.8 and 0.9 Å). The mean positions of the surface (first layer), subsurface (second layer), and subsubsurface (third layer) planes are represented by the thick blue lines. 23036



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suggests that the hybridization of the carbon atoms in the adsorbed acetylene is close to the sp3 hybridized state, with a C−C bond order equal to one. Two different orientations of the molecule on the surface are investigated, the C−C bond being either parallel or perpendicular to the long axis of the surface bipentagonal motif. From Table 5, the most favorable orientations are generally the ones with the C−C bond parallel to the long axis of the surface bipentagonal motif, with the exception of the S4∥ site of model A. Other adsorption geometries are less stable than the previous 4-fold one (Table 5). Adsorption energies at considered bridge sites range from −1.29 eV to −1.69 eV. The adsorption geometry of the adsorbed molecule (dC−C = 1.36 Å, H−C−C = 121°−122°) is close to that of isolated ethylene (dC−C = 1.34 Å, H−C−C = 120°). This suggests that the hybridization of the carbon atoms in the adsorbed acetylene is close to the sp2 hybridized state, with a C−C bond order equal to two. The adsorption energies for 3-coordinated sites are in the range [−1.82 eV; −2.24 eV], and the C−C distance (dC−C = 1.38 Å) is close to that of an isolated aromatic propenyl cation C3H3+ with bond order 1.33 (dC−C = 1.40 Å). The adsorption energy calculated for a few 5-fold adsorption sites range from −1.43 to −2.12 eV. Here, the adsorbed molecule reaches a quasi-4-fold site, with two types of Al−C distances (2.05 and 2.12 Å). As in the case of 4-fold sites, the hybridization of the carbon atoms is close to the sp3 hybridized state (dC−C = 1.48 Å, H−C−C = 113°). To highlight the influence of cobalt atoms, we first compare the adsorption properties of the o-Al13Co4(100) and Al(100) surfaces. The adsorption energy calculated for a 4-fold site on Al(100) is −1.87 eV. Then, most 4-fold adsorption sites present a more favorable adsorption energy on o-Al13Co4(100), although the geometries of the adsorbed molecule are similar in the two cases (dC−C = 1.50 Å, dC−H = 1.10 Å, H−C−C ≃ 115°, dAl−C ≃ 2.05 Å). Here, the presence of cobalt atoms tends to stabilize the favorable adsorption sites. To better understand the role of surface cobalt atoms on the adsorption properties, we compare calculated results for models A and B (Tables 6 and 5). For model B, the adsorption energies of Si∥ (i = 1−4) sites are all within −2.48 eV to −2.60 eV, while they range from −1.49 eV to −2.52 eV for model A. Here, there is a strong influence of the surface cobalt atoms: Eads C2H2 = −1.49 eV for the S4∥ site, located close to two surface cobalt ads atoms (dCoS−C = 3.36 Å), Eads C2H2 = −2.17 eV (S1∥), EC2H2 = −2.36 eV (S2∥) for the two 4-fold sites close to one surface cobalt atom (dCoS−C = 3.56 Å), and Eads C2H2 = −2.52 eV for the S3∥ site, quite far away from any surface cobalt atom (dCoS−C = 6.44 Å). The gradual removal of cobalt subsurface atoms possessing an electron acceptor character leads to a stabilization of the corresponding adsorption site, similar to what is observed for C2H2 on surface model B. C2H3. We have tested all possible 3-coordinated adsorption sites for C2H3 on o-Al13Co4(100). The considered sites (T1T4) are found to be stable sites, with similar adsorption energies, ranging from −2.72 eV to −3.11 eV (Table 7). The C2H3 molecule binds to the surface through three σ bondings to achieve a nearly sp3 hybridization for both carbon atoms (dC−H ≃ 1.10 Å, dC−C ≃ 1.53 Å, dC−Al ≃ 2.01 Å). The molecule is then oriented to form one σ bond with one Al atom (CH2 part of the molecule) and two σ bonds with two other Al atoms of the triangle (CH part of the molecule). The different orientations of the molecule on the surface lead to similar

Figure 4. Isosurfaces of charge density differences (0.0018 e−/Å3) for atomic hydrogen adsorbed in sites B14 (surface models A and B) and B08 (surface model A): Δρ > 0 in yellow and Δρ < 0 in blue.

Table 5. Adsorption Energies (eV) for C2H2 on Different Possible Adsorption Sites (o-Al13Co4(100))a site

label

model A

model B

top bridge

A13 B07 B18 B22 T2a T2b T2c T4a T4b T4c S1∥ S1⊥ S2∥ S2⊥ S3∥ S3⊥ S4∥ S4⊥ P3 S1−P2 S3−P2

−0.13 −1.58 −1.67 −1.29 −2.24 −2.19 −2.10 −2.15 −1.90 −2.03 −2.17 −2.12 −2.36 −2.29 −2.52 −1.97 −1.49 −2.22 −1.91 −2.12 −1.66

− −1.60 −1.69 −1.36 −2.11 −2.09 −1.98 −1.98 −1.82 −1.85 −2.56 −2.19 −2.60 −2.36 −2.50 −2.03 −2.48 −2.23 −1.75 −1.43 −1.68

3-fold

4-fold

5-fold

a Three different orientations of the molecule are considered for the 3fold sites (a, b, and c). Two different orientations of the molecule are considered for the 4-fold sites (parallel and perpendicular to the long axis of the bi-pentagonal motif).

energies range from −1.49 eV to −2.60 eV. The C−C distance increases upon adsorption (dC−C ≃ 1.50 Å), to reach a distance close to the C−C distance observed in isolated ethane (dC−C = 1.54 Å). The angles Al−C−C (106°−114° for most adsorption geometries) and H−C−C (113°−116°) are also comparable with the H−C−C angle in isolated ethane (109.5°). This 23037



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Table 6. Influence of the Surface Cobalt Atoms on the Calculated C2H2 Adsorption Energies (EC2H2, in eV)a

a

The number of close surface cobalt atoms (CoS) is indicated for each site. Isosurfaces of charge density differences (0.004 e/Å2) are plotted for each considered site (Δρ > 0 in yellow and Δρ < 0 in blue).

Table 7. Adsorption Energies (eV) for C2H3 on Triangular Shaped Sites of o-Al13Co4(100)a

Table 8. Adsorption Energies (eV) for C2H4 on Bridge Sites of o-Al13Co4(100) type

site

model A

model B

2NN

B9 B13 B14 B5 B12 B8 B2 B16 B10 B20 B3 B15 B6 B11 B7 B18 B22 B19 B1 B4 B21 B17

−0.64 −0.56 −0.55 −0.44 −0.26 −0.92 −0.87 −0.92 −0.66 −0.73 −0.53 −0.59 −0.52 −0.40 −0.67 −0.73 −0.28 −0.77 −0.54 −0.28 −0.33 −0.38

−0.28 −0.70 −0.74 −0.71 −0.04 −0.79 −0.88 −0.90 −0.29 −0.75 −0.57 −0.64 −0.46 −0.44 −0.69 −0.75 −0.40 −0.77 −0.67 −0.44 −0.48 −0.54

3NN

a Three different orientations of the molecule are considered for each site (a, b, c). The black line represents schematically the C−C axis of the adsorbed C2H3 molecule.

4NN

adsorption energies, within 0.25 eV (Table 7). Here, the presence/absence of surface cobalt atoms do not have a significant influence on the stability of the T1−T4 adsorption sites (comparison of surface models A and B; see Table 7). Indeed, for all considered sites, the surface cobalt atoms lie roughly at the same distance (about 3.3 Å) and a subsurface cobalt atom is located directly below the center of gravity of all these triangular shaped sites. C2H4. Several possible adsorption geometries for C2H4 on oAl13Co4(100) have been tested, and the most stable sites are found among the bridge sites. The C2H4 molecule binds to the surface through two σ bondings to achieve a nearly sp3 hybridization for both carbon atoms (dC−C = 1.53 Å, dC−H = 1.10 Å, H−C−C = 107°). Table 8 gathers calculated adsorption energies for bridge sites. From Table 8, it appears that, among bridge sites, the most stable one are the 3NN sites B2, B8, and B16. For model A, it corresponds to the same sites as the ones determined for atomic hydrogen. This similarity could be related to the fact that even if there are differences in the bonding to the surface between an atomic hydrogen and a C2H4 molecule (Al atoms form stronger C−Al bonds than H−Al bonds), the donor/ acceptor effect of Co atoms remains the same which leads to the same relative stability of the sites. The adsorption geometry

is almost the same for these 3 bridge sites (dAl−C = 2.01−2.04 Å, dCoS−C = 4.03−4.19 Å, dCoS−1−C = 3.98−4.04 Å). The presence of cobalt atoms leads to a stabilization of the considered adsorption sites: the adsorption energies calculated for C2H4 on Al(100) and Al(111) are quite low (−0.34 eV and −0.13 eV), while the ones calculated on o-Al13Co4(100) are roughly 0.6 eV higher (−0.92 eV). The same trend was already observed for C2H2, with a similar shift in the adsorption energies.

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DISCUSSION AND CONCLUSIONS We have studied the adsorption of atomic hydrogen and hydrocarbon species (C2H2, C2H3, C2H4) on two surface models of the o-Al13Co4 complex intermetallic alloy that differ only by the presence of cobalt atoms slightly below the mean position of the surface plane. For most stable sites, the considered hydrogen atom and hydrocarbon species are more strongly bounded on the Al-rich terminated o-Al13Co4(100) 23038



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toward the semihydrogenation of acetylene, with probably a lower conversion rate for the Al-rich o-Al13Co4(100) surface model, since an energy barrier needs to be overcome for the dissociation of the H2 molecule on the surface. This detailed adsorption study concern species involved with bridge, 3-fold, and 4-fold sites (T2−S3−T2 for surface model A) located just in between two truncated clusters. Due to the presence of surface cobalt atoms (CoS) in model A, the number of such sites is reduced compared to model B. In addition, since the most stable adsorption site for C2H2 is not exactly the same for both models (S3∥/S2∥), one could expect different reaction paths for the chemical reaction on both models. In all cases, these results may be a good starting point for the determination of the reaction mechanisms and to understand the origin of the specific reactivity of this surface toward the semihydrogenation of acetylene.

surface (A and B models) than on the pure low-index aluminum surfaces. This highlights the role of cobalt atoms on the adsorption properties, probably related to the shape and the orientation of the Co d-orbitals. The comparison of the results obtained for models A and B leads to the conclusion that Co atoms slightly underneath the surface (CoS) have a repulsive effect toward atomic hydrogen and C2Hx (x = 2−4) molecules adsorbed at the closest sites (distance between Co and the considered site smaller than 3 Å). In addition, we demonstrated here that not only surface Co atoms but also subsurface Co atoms influence the adsorption properties, as already observed for other systems such as nitrogen on Ni/ Pt(111) surface alloys61 or CO on Zn/Pd(111).62 This result would lead to a specific surface reactivity. In particular, the expected activation energy for H2 dissociation might be smaller on o-Al13Co4(100) than on Al(100), since the adsorption process is exothermic on o-Al13Co4(100). Concerning atomic hydrogen adsorption, the most stable sites are found among bridge sites. Both surface cobalt atoms (for model A) and subsurface cobalt atoms (for models A and B) influence the stability of the investigated adsorption sites. The electronic donor/acceptor character of cobalt atoms positioned in the subsurface layer (CoS−1), below the considered adsorption site, strongly influences the stability of the considered adsorption sites. When presenting an electronic donor character, its presence is favorable for adsorption. The corresponding adsorption energy is in line with trends observed on simpler metallic surfaces related to the linear dependence demonstrated between the d-band centers of various metals and the corresponding atomic hydrogen binding energies.63 We found that hydrogen adsorption is slightly exothermic (15 kJ/ mol, 0.15 eV/atom): even if H2 is not adsorbed at the surface, one can expect a dissociative adsorption of H2 at this Al-rich surface, contrary to what happens on pure aluminum surfaces. For the hydrocarbon species (C2H2, C2H3, C2H4), we found that carbon atoms of each molecule tend to reach the sp3 hybridization state where each carbon atom is surrounded tetragonally by x hydrogen atoms and 4 − x aluminum atoms. Thus, the most preferable sites for the C2H2 molecule are 4-fold sites, while they are 3-fold sites for the C2H3 radical and bridge sites for C2H4. Their relative stability decreases in the sequence: ads ads Eads C2H3 < EC2H2 < EC2H4. The high stability of C2H3 on oAl13Co4(100) is explained by the low stability of the free C2H3 radical. The adsorption energy of C2H2 molecules is higher than that calculated for C2H4 molecules: the first one presents twice as many σ-bonds to the surface than C2H4. Surface cobalt atoms present rather a destabilization effect for adsorbed hydrocarbon species. The present study highlights the influence of the surface model on adsorption properties. The presence of protruding surface Co atoms in the structural model built by numerical cleavage26 leads to atomic hydrogen more strongly bound to the surface (Eads H (bridge − sites) ∈ [−0.20; −0.23] eV/at. compared to Eads H (bridge − sites) ∈ [−0.10; −0.16] eV/at. in the present study). In contrast, the C2H2 and C2H4 molecules are linked more strongly in our Al-rich surface model (Eads C2H2(4f − sites) ∈ [−2.50; −2.60] eV/at. and Eads (bridge − sites) ≃ C2H4 −0.90 eV/at.) than on the highly corrugated surface model described in ref 26 (Eads C2H2(bridge − sites) ∈ [−1.85; −1.90] eV/at. and Eads (Co − top) = −0.73 eV/at.).26 Then, one C2H4 would expect a different reactivity for these two surface models

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Agence Nationale de la Recherche (Projet CAPRICE ANR-11-INTB-1001) is acknowledged for its financial support. This work was granted access to the HPC resources of the French institute IDRIS (Institut du Développement et des Ressources en Informatique Scientifique) under allocation 2013-99642 made by GENCI (Grand Equipement National de Calcul Intensif).

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Adsorption Properties of the o-Al13Co4 (100) Surface toward

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