Catalytic Semihydrogenation of Acetylene on the (100) Surface of the

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Catalytic Semi-Hydrogenation of Acetylene on the (100) Surface of the o-Al13Co4 Quasicrystalline Approximant: a DFT Study Dmytro Kandaskalov, Vincent Fournee, Julian Ledieu, and Émilie Gaudry J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06175 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 13, 2017

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The Journal of Physical Chemistry

Catalytic Semi-Hydrogenation of Acetylene on the (100) Surface of the o-Al13Co4 Quasicrystalline Approximant: a DFT Study ´ Gaudry∗,¶ D. Kandaskalov,†,‡ V. Fournée,¶ J. Ledieu,¶ and E. †Aix-Marseille Universitié, CNRS, IM2NP UMR 7334, Campus de Saint-Jérˆ ome, Avenue Escadrille Normandie Niemen, Case 142, F-13397 Marseille, France ‡Institut Jean Lamour, Université de Lorraine UMR CNRS 7198, Parc de Saurupt CS50840 54011 Nancy cedex, France ¶Institut Jean Lamour, Université de Lorraine UMR CNRS 7198, Parc de Saurupt CS50840 54011 Nancy cedex, France E-mail: [email protected]

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Abstract Density functional theory calculations are used to investigate the catalytic properties of the o-Al13 Co4 (100) surface towards the semi-hydrogenation of acetylene. The dissociation of the H2 molecule is the key process in this case, since the surface termination does not contain protruding Co atoms, according to a combination of surface science studies and ab initio calculations. However, compared to the pure Al(100) and Al(111) surfaces, H2 dissociation on o-Al13 Co4 (100) proceeds much easier, due to the presence of surface Co atoms slightly below the mean position of the termination plane. A possible mechanism for the hydrogenation is presented, highlighting the selectivity of the reaction on this Al-rich surface.

Introduction Alkene streams produced from cracking processes for use in the petrochemical industry contain small amounts of highly unsaturated hydrocarbons. Acetylene is one of these impurities. It poisons the polymerization catalysts even at low concentrations. It can have drastic consequences, for example in the production of polyethylene. The selective hydrogenation of acetylene to ethylene is used to increase the purity of alkene feedstocks without reducing their overall concentration. Therefore, materials that selectively catalyze the hydrogenation of acetylene to ethylene and prevent the hydrogenation of ethylene to ethane are of great interest. Pt- and Pd-based catalysts are traditionnaly used in the industrial process. 1 They are sometimes modified with an ancillary metal, inactive for hydrogenation (Ag, 2,3 Au, 4,5 etc), to improve the selectivity. 2 Under reaction conditions, such substitutional alloys suffer from surface chemical segregation, which leads to a modification of the surface composition and then a decreased selectivity. 6,7 The catalytic performances of the noble-metal free o-Al13 Co4 intermetallic compound for the acethylene semi-hydrogenation were recently recognized. 8 They were found similar to those of the m-Al13 Fe4 , a related complex intermetallic com2


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pound. 9 In contrast to substitutional alloys with random site occupancy, Al13 TM4 (TM = transition metal) intermetallics exhibit a well-ordered crystal structure differing from those of the constituting elements, that is supported by a network of covalent and ionic interactions which strongly modify the overall electronic structure. 10 Such a network of iono-covalent bondings causes a strong site preference, which can result in high stability under reaction conditions, avoiding surface segregation processes and maintaining isolated active sites. To investigate the surface reactivity of such complex surface first requires a realistic model. The o-Al13 Co4 phase is considered as an approximant of the decagonal Al-Ni-Co quasicrystal. 11–13 It crystallyzes in the P mn21 space group 14,15 (Pearson symbol oP102). Its orthorhombic cell is quite large (a=8.158 ˚ A, b=12.342 ˚ A and c=14.452 ˚ A) and contains 102 atoms (78 Al and 24 Co atoms). While bulk Al13 Co4 is traditionnaly described as a stacking of two types of atomic layers - a flat layer (F) and a denser puckered layer (P) - that alternates perpendicular to the [100] direction (Fig. 1, right), it can equally be described as an assembly of high-symmetry cluster building blocks (Fig. 1, left). The latter description was based mainly on geometrical considerations, 16 using 23-atom pentagonal bipyramid shaped clusters. Most recent quantum chemical calculations identified o-Al13 Co4 as an analogue to cage compounds with strong covalent linear Co-Al-Co bonding (guest) surrounded by elongated cavities (host cage). 10,15 Both experimental and theoretical studies of its (100) surface have been undertaken over the last few years. 17–21 The surface composition deduced from XPS (X-ray photoelectron microscopy) as a function of probing depth indicated no sign of chemical segregation. 18 Scanning tunneling microscopy (STM) images showed a A corresponding to a/2. terrace and step morphology with a unique step height of 4.2 ± 0.2 ˚ A large number of surface models have been constructuted and surface energy calculations identified 4 stable surface structures, depending on the value of the Al chemical potential. An agreement between the simulated and experimental STM images was obtained for only two of them, in which all Al atoms of the puckered planes are present but not all Co atoms. Protruding surface Co atoms were found to be absent, in agreement with a dynamical LEED

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(low energy electron diffraction) analysis, 18 while surface Co sites located slightly below the mean position of the topmost layer have been found to be partially occupied. Surface X-ray diffraction 21 measured occupancy of these Co sites equal to 0.6.

Figure 1: Al13 Co4 bulk structure, shown as a stacking of 23-atom pentagonal bi-pyramid shaped clusters and as a stacking of planes perpendicular to the [001] direction. Two types of Co atoms are present in the puckered planes, slightly above or below the mean position of the plane. Dark blue = Co, Light blue = Al. Such an Al-rich termination raises the question of the Al13 Co4 (100) surface reactivity towards hydrogenation reactions. Indeed, the dissociation of the H2 molecule on the catalyst surface is a key process for such reactions. It is well-known that hydrogen dissociation is unlikely on pure aluminium surfaces. 22–24 The dissociation probability has been measured to be less than 0.0001 at 280 K on Al(100), and atomic hydrogen has been shown to further recombine and desorbs from Al(100) around 350 K. 25 A sizeable activation barrier is indeed needed to dissociate H2 on Al(110) and Al(111) surfaces. 26–28 A previous theoretical study of the hydrogenation mechanism on Al13 Co4 (100) identified a possible reaction path, with barriers similar to the ones calculated for conventional Pd or Pd-Ag catalysts. 29 However, the starting point of these calculations was a surface model derived from a theoretical cleavage, presenting protruding surface Co atoms, which appears to be inconsistent with the experimental observations of the surface prepared by sputter-annealing under UHV. 21 To summarize, the Al13 Co4 compound has been identified as a promising catalyst towards the semi-hydrogenation of acetylene. To our knowledge, the only attempt to determine the reaction pathway on o-Al13 Co4 (100) was based on a surface model inconsistent with the ex4


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perimental observations of the surface prepared by sputter-annealing under UHV. Then, our first motivation for this paper is to investigate the possible mechanisms and related barriers for the semi-hydrogenation of acetylene on Al13 Co4 (100), based on (i) the surface model deduced from the combination of ab initio and surface science experiments, 18,21 and (ii) on a previous study of the adsorption properties of this surface towards molecules involved in the semi-hydrogenation of acetylene. The reaction path proposed here highlights the selectivity of the reaction, but presents a rate-limiting step with a relatively high activation energy (0.86 eV). The corresponding mechanism is quite different from the one proposed in Ref., 29 using a surface model obtained by a simulated cleavage. The comparison of both approaches allows us to discuss the role of protruding surface Co atoms on the C2 H2 hydrogenation.

Methodology Computational details All calculations are based on the Density Functional Theory and use the Vienna ab initio simulation package (VASP). Self-consistent Kohn Sham equations were solved by means of the projected-augmented wave (PAW) method 30 to describe the electron-ion interactions. The electron exchange and correlation is described by the generalized gradient approximation approach (GGA) with the PBE functional. 31 Spin polarization was not taken into account since it has been shown to be not required for such an Al-rich complex intermetallic compounds. 12,18 The plane-waves energy cut-off was fixed to 400 eV for low-index Al(100) Al(111) and Al13 Co4 (100) surfaces. Monkhorst-Pack meshes were used for the k-points sampling. 32 We used a 1×8×8 k-mesh for Al(100), a 1×8×6 k-mesh for Al(111), and a 1×4×4 k-mesh for Al13 Co4 (100). This is sufficient to converge total energies within 1 meV for molecules on pure Al surfaces and within 3 meV on Al13 Co4 (100). The Al(100) and Al(111) surfaces have been modelled with 12-layer thick symmetric slabs, each layer containing 8 atoms, separated by a void thickness larger than 16 ˚ A. 5



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Molecules are adsorbed on one side of the slab and we check that the parasite dipole moment does not impact significantly the calculated adsorption energy (the modifications are calculated to be less than 0.5 meV per molecule). To determine dissociation barriers and minimum energy paths (MEP), the nudged elastic band (NEB) method was used. 33,33 For the transition-state searches, convergence criteria from 10−4 to 10−5 eV for total energies and 0.06 eV/˚ A for forces acting on the atoms have been applied. The number of images in NEB calculations was chosen to be 6 for molecule migrations, at least 8 for atomic H migration and 10 for H2 dissociation with 10 complementary images for the interval around the transition state.

Structure of the Al13 Co4 (100) surface From the descriptions of the o-Al13 Co4 crystal structure presented in the introduction, competition is expected between the 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, 29 a detailed analysis coupling both experimental observations and ab initio methods leads to the conclusion that the o-Al13 Co4 (100) surface structure results from a plane selection and consists in dense Al-rich layers with surface Co atom depletion (Fig. 2a). 18,20,21 In this later case, clusters are truncated at the surface. The surface structure deduced from the numerical cleavage 29 is then quite different from the one deduced from surface energy calculations, 18,20,21 as shown in Fig. 2b. In the following, we focus on the surface model built by bulk truncation and selection of the puckered-type layer at the surface. The termination layer is Al-rich: it contains 22 Al and 2 Co atoms slightly below the mean position of the termination plane. To take into consideration the partial occupancy of the surface Co site (CoS ), we also examined the case where CoS atoms are absent. Such Al13 Co4 (100) surface structure induces a complex surface energy landscape, containing a large number of adsorption sites. In the following, we use the labels of Ref. 34 (Fig. 6


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(a)

(b)

Figure 2: Al13 Co4 (100) surface models. (a) Model used in this study, deduced from a combination of experimental and theoretical results. 18,21 Surface Co atoms (dark blue) are surrounded by five topmost Al atoms (light blue) and located below the mean position of the surface plane. (b) Model deduced from a theoretical cleavage. 29 The topmost plane contains protruding surface Co atoms surrounded by five Al atoms slightly below the mean position of the plane. Co (resp. Al) atoms in the subsurface atomic layer are represented in purple (resp. white). 3) to identify the Co top site (13), a few Al-Al bridge sites (B1-B19), a 3-fold hollow site (T2), two 4-fold hollow sites (S2-S3) and two 5-fold hollow sites (P1-P2).

Figure 3: Labels for the adsorption sites considered in this study.

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Results and discussion H2 dissociation and atomic H migration In the following, we first study the H2 dissociation on pure aluminum surfaces, step required to later compare these results with the Al-rich Al13 Co4 (100) surface. H2 dissociation on pure aluminum low-index surfaces Four and two different dissociation paths are considered for the Al(100) and Al(111) surfaces, respectively. The final states are described by atomic hydrogen adsorbed on bridge sites. The initial states are either hollow, bridge or top sites for Al(100). They are top sites for Al(111). The corresponding paths are represented in Figs. 4-5, while the corresponding activation energies are gathered in Tab. 1, along with the distance between the two hydrogen atoms of the transition state (r(H2 )) and the height of the transition state above the surface (Z). Our calculated activation energies for the hydrogen dissociation on pure Al(111) surface range over 1.03-1.09 eV, in good agreement with Refs. 37,39 (0.99-1.03 eV). The higher value found by Ref. 38 (1.28 eV) may be attributed to the thiner slab used for the calculations. On pure Al(100) surface, we found activation energies (0.98-1.37 eV) in reasonnable agreement with the activation barriers calculated in Refs. 35,36 (0.90-1.05 eV). Tab. 1 shows the influence a increases from 0.96 eV to 1.03 eV when the of the coverage on the activation energies: EH 2 √ √ coverage is divided by 2 (surface unit cells 2 × 2)R45o and (2 × 2)). The comparison of

the three aluminum low-index surfaces also shows that the activation energy increases with the surface density. H2 dissociation on Al13 Co4 (100) A previous study identified the most stable sites for atomic hydrogen adsorption on Al13 Co4 (100). 34 They are three bridge Al-Al sites close to subsurface CoS−1 atoms: B2 (Eads =-0.15 eV), B8 (Eads =-0.16 eV) and B16 (Eads =-0.15 eV). The calculated adsorption energies depend weakly 8


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a Table 1: Activation energies EH and transition state geometries for H2 molecule 2 dissociation on pure Al(100), Al(111) and Al(110) surfaces. Labels for transition states are those of Fig. 4. a EH 2 eV

unit cell

layer nb

√ √ ( 2x 2)R45o

TS1 TS2 TS3 TS4

1.03 0.96 1.05 1.16 0.98 1.27 1.37

4-6 4-6 6 12 12 12 12

TS5 TS6

1.03 1.28 0.99 1.09 1.03

Label

0.53 0.70 0.25

(2x2) √

√

(3 2x3 2)R45o

(2x2) (2x2) (2x2) (2x2) (2x2) (4x4) & (5x5)

(2x2) (2x2) (1x2) √

√

( 2x 2)R45o

(1x2)

6 12 12 5 4 5

Method

Transition state Initial (H2 )/ (r(H2 ), Z) Final (H) states Al(100) surface PW91 (1.03, 1.17) Hollow/Bridge PW91 (-, -) Hollow/Bridge PBE (1.1, 1.3) Top/Hollow PBE (1.37, 1.33) Bridge/Bridge PBE (1.16, 1.33) Top/Bridge PBE (0.84, 1.47) Top/Bridge PBE (0.92, 1.54) Hollow/Bridge Al(111) surface LDA (-, -) Top/Top PW91 (-, -) Bridge/Hollow PBE (-, -) Bridge/Hollow+Top PBE (1.37, 1.21) Bridge/Hollow PBE (1.37, 1.23) Bridge/Hollow+Top Al(110) surface GGA (1.23, 1.09) Long bridge/Top GGA (1.25, 1.12) Long bridge/Top LDA (1.37, 1.12) Long bridge/Top

Ref.

35 35 36 Our work Our work Our work Our work 37 38 39 Our work Our work 40,41 41 40

on the absence/presence of the surface CoS atom. The energies are are Eads =-0.13 eV for B2, Eads =-0.10 eV for B8 and Eads =-0.14 eV for B16 when CoS atoms are absent. In the following, we compare the H2 dissociation in the presence and absence of the acetylene molecule, adsorbed in the stable S3 site. The most probable sites for the dissociation are those leading to the most favorable atomic hydrogen adsorption sites (B2, B8, B16). In addition, since the sites around CoS atoms are slightly endothermic for atomic hydrogen (Eads = +0.04 eV), CoS atoms may stabilise transition states for the dissociation process. We also tested this site. Then, we figure out four possible dissociation schemes (Tab. 2). The C2 H2 molecule adsorbed in S3 has a weak influence on the activation and adsorption energies for all sites, except site T2, which is located quite close to S3. On the T2 and S2 sites of Al13 Co4 (100), the H2 dissociation occurs with a non negligible barrier (larger than 0.76 eV), similar to the ones calculated on pure Al surfaces. Barriers are slightly lower in 9



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Figure 4: Initial, Transition and final states (IS/TS/FS) for the studied dissociation on Al(100)

Figure 5: Initial, Transition and final states (IS/TS/FS) for the studied dissociation on Al(111) the case of the P1 or Co-top sites. In the latter case (Co-top site), the H2 dissociation takes place in two steps: molecular adsorption is followed by the dissociation of the adsorbed

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Table 2: H2 dissociation: adsorption and activation energies for a few sites on Al13 Co4 (100), calculated in the presence or absence of the C2 H2 molecule, adsorbed in site S3. Site P1 T2 S2 top Co

E ads , eV C 2 H2 -0.40 -0.37 -0.02 -0.28 -0.13 -0.20 +0.02 +0.02 -0.02 -0.02 -0.17 -0.17

E a , eV C2 H2 0.59 0.61 0.92 0.76 0.81 0.77 0.32 0.32 0.92 0.92 0.54 0.54

Initial

Final

B9 T2 S2 13 13 13

B2 + B8 B8 + B16 B15 + B16 13 B9 + B13 13 + B9

molecule. The first step has a moderate activation energy (0.32 eV). In the final state, the H-Co distance is equal to 1.61 ˚ A and the H-H bond of the adsorbed molecule (0.90 ˚ A) is A). The second step really dissociates the H2 larger than the one of a free H2 molecule (0.74 ˚ molecule, with a barrier equal to 0.54 eV: hydrogen atoms are located on top of the Co atom and on the bridge site B9, they are separated by a distance equal to 2.56 ˚ A. Dissociation at the P1 site (E a =0.59 eV) provides hydrogen atoms which are located closer to the C2 H2 molecule. Here, we can notice that the presence of surface Co atoms contributes to the lowering of the H2 dissociation barrier. Indeed, when absent, the barrier is larger (E a = 0.78 eV for the P1 site). Atomic H migration on Al13 Co4 (100) The source of atomic hydrogen for the hydrogenation reaction is provided by H2 dissociation. The dissociation on T2 sites directly gives two hydrogen atoms on the favorable B8 and B16 bridge sites, but the corresponding activation energy is high. Atomic hydrogen migration is then required to reach the C2 H2 molecule when the dissociation process occurs on the more favorable sites (top Co and P1). The migration of atomic hydrogen proceeds quite easily, with a maximum migration barrier of 0.27 eV to reach B8 and B16 sites (Tab. 3). The H2 dissociation on the P2 site leads to one hydrogen atom directly close to the C2 H2 molecule (B8), while the second hydrogen atom needs to migrate from B2 toward B8. This scheme

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seems appropriate, but other paths could also be a source of hydrogen atoms. The diffusion energies calculated on the Al13 Co4 (100) surface are low, and depending on the considered path, can be slightly larger or smaller than the ones calculated on pure Al(111) surfaces (0.12 eV, 39 0.16 eV 42 ). This is attributed to the complex surface energy landscape of Al13 Co4 (100), where exothermic adsorption is possible on several adsorption sites. Table 3: Atomic hydrogen migration in the presence of an adsorbed C2 H2 molecule: activation energy (E a , in eV) and energy difference between the initial (IS) and final (FS) states (EIS − EF S , in eV). Migration path B2 → B9 → B8 B8 → B16 B15 → B16 B9 → B8 B13 → B14 B14 → B9 B14 → B16

Ea 0.27 and 0.21 0.06 0.22 0.02 0.23 0.03 0.05

EIS − EF S -0.07 +0.03 -0.27 -0.22 +0.12 -0.17 +0.36

Influence of the Al13 Co4 (100) surface model and role of surface Co atoms A key step of any catalytic hydrogenation reaction is the H2 dissociation. In agreement with previous studies, the determination of the H2 dissociation barriers on pure Al surfaces (Ea =0.98-1.09 eV) exclude the possibility to dissociate H2 on these surfaces. The modified surfaces with d-element significantly facilitate the dissociation process. 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. 43,44 Dissociation barriers have been calculated to be considerably lower on several transition metal doped Al surfaces. 38,45 Replacing one surface Al atom by Co on Al(111) leads to a decrease of the H2 splitting energy (0.15 eV 39 ). The surface model considered here contains Co atoms in the topmost plane. However, they are not protruding but lie slightly below the mean position of the surface termination. 12


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The corresponding barrier for H2 dissociation is then much higher (0.54 eV) than the ones calculated with the previous transition metal doped Al surfaces. It is also much higher than the one computed using the Al13 Co4 (100) surface model based on a numerical cleavage, 29 which presents protruding CoSprotruding atoms (Ea =0.10 eV). It is of the same order of magnitude (0.15 eV) as the one calculated on a CoSprotruding atom of another Al-Co complex intermetallic compound (Al5 Co2 (2¯10)), 46 which also contains protruding surface Co atoms. Nevertheless, the CoSburied atoms play a non negligible role, since the H2 dissociation barrier calculated on a similar surface model, but where all surface Co atoms are absent (model B of Ref. 34 ) is found to be higher (0.78 eV).

C2 H2 Hydrogenation The adsorption energies of C2 Hn (n = 2, 3, 4) on the Al13 Co4 (100) model surface have been calculated in our previous paper. 34 The most stable sites for C2 Hn are (6 − n)-fold sites (carbon atoms are then sp3 hybridized). The acetylene molecule is adsorbed preferentially on the 4-fold S3 (Eads =-2.52 eV). The C2 H4 molecule is located at Al-Al bridge sites identified also for atomic hydrogen adsorption (B2, B8 and B16, Eads =-0.92 eV). In the following, a possible reaction path for the C2 H2 hydrogenation is described (Figs. 6-7). C2 H 2 → C2 H 3 We consider here a C2 H2 molecule localized on the S3 site. The C2 H3 molecule is formed by the reaction with an hydrogen atom localized in B8 or in B16, both leading to similar activation energies: 0.81 eV (B8) and 0.83 eV (B16). The overall reaction is slithtly endothermic: +0.10 eV (B16) and +0.07 eV (B8). Hydrogenation of acetylene to vinyl involves a simultaneous movement of the hydrogen atom and a rotation of the C-H group around the C-C axis with a slight displacement of carbon atoms away from aluminium atoms. The final C2 H3 molecule (C-C distance equal to 1.53 ˚ A) is roughly 3-coordinated as the 4th Al-C distance (2.31 ˚ A) is too long to be considered as a chemical bond. 13



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Figure 6: Possible reaction pathway for the C2 H2 hydrogenation on Al13 Co4 (100).

Figure 7: Summary of the reaction, migration and desorption steps involved in the reaction pathway (Fig. 6). Corresponding energies (Ea , Em , Edes ) are indicated. According to the reaction path described in the previous paragraph, the C2 H3 molecular group is formed on the S3 site, which is not the most stable site for vinyl adsorbed on Al13 Co4 (100). Indeed, the most favorable adsorption sites for C2 H3 are 3-fold sites. 34 However, the energy barrier related to a migration of C2 H3 from S3 to the closest 3-folded site (T2) is relatively high (1.09 eV). In the following, we present the vinyl hydrogenation in the S3 adsorption site.

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C2 H 3 → C2 H 4 The presence of the C2 H3 molecule breaks the local C2 symmetry of the topmost plane. The B8 and B16 sites, close to the CH2 part of the vinyl radical, are unreachable by atomic hydrogen atoms, due to steric effects. But the two B8 and B16 sites, close to the CH part of the vinyl radical, can be occupied by hydrogen atoms. The hydrogenation reaction, using H located on B8, presents a lower activation barrier (0.86 eV) compared to the one for H in B16 (1.04 eV). As for the C2 H2 → C2 H3 hydrogenation, it occurs with a simultaneous movement of the H atom and a rotation of the C-H group around the C-C axis with a slight lateral displacement. The C2 H4 molecule is located in a 4-fold site but it is bounded mostly to two ˚ ˚ long diagonally located Al atoms (dshort Al−C = 2.11 A, dAl−C = 2.56 A). Since the final C2 H4 molecule is not adsorbed on its one of its more favorable sites, the overall reaction is endothermic: +0.27 eV (B16) and +0.29 eV (B8). Further possible events include the migration and desorption of the C2 H4 molecule departing from S3. The C2 H4 desorption energy from S3 is calculated to be only 0.20 eV. The activation energies for the migration of C2 H4 to the closest B19 bridge site (Ea = 0.13 eV) and to the two most stable B8 (Ea = 0.11 eV) and B16 (Ea = 0.25 eV) bridge sites are also small. At this stage, the partial desorption of C2 H4 (S3) is expected as the energy difference between desorption and migration is less than 0.1 eV. C2 H 4 → C2 H 5 When located in the stable B8 bridge site, the C2 H4 desorption energy is 0.92 eV. This configuration sterically blocks the H2 dissociation in the surrounding P1, S2, T2 and S3 sites (red zones in Fig. 8). When considering further hydrogenation, the possible sources for hydrogen atoms are Co-top sites (13, with activation energies equal to 0.72 eV and 0.54 eV) or more distant P1 sites (Ea =0.59 eV). Thus, the energetically easiest way for the dissociation is the neighbor Co-top site (13), which gives atomic H atoms on B9 and B13 positions. We have also checked dissociation at P1 site located next to the adsorbed C2 H4 molecule (B8). 15



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This results in two hydrogen atoms on B8 and B2 sites. In the latter case, the nearest H atom (B8) needs to migrate up to the B18 site (B8 → B19 → B18 path) to reach the C2 H4 (B8) molecule.

Figure 8: Labels and activation energies for H2 dissociation on Al13 Co4 (100) with C2 H4 adsorbed in B8. The hydrogenation reaction C2 H4 (B8) + H(B18) → C2 H5 takes place with a high activation energy (1.15 eV and 1.20 eV, depending on the carbon atom considered in C2 H4 ). This implies that the complete hydrogenation of the C2 H2 molecule is unlikely and the desorption process is at least 0.23 eV more favorable than further hydrogenation of ethylene. Comparison of hydrogenation steps on two different o-Al13 Co4 (100) surface models The reaction path identified on the o-Al13 Co4 (100) surface model deduced from a simulated cleavage involves small barriers (≃ 0.6 eV), comparable or even lower than those calculated for conventional or Pd-Ag catalysts. 29,47–49 Acetylene is strongly bounded to Al atoms in a di-σ configuration, whereas ethylene is weakly π-bounded on to of the CoSprotruding atom. In this case, the reaction selectivity is due to hydrogen migration, which is found to occur with a quite high barrier (B2→D3, with the labels of Ref., 29 0.80 eV) in the presence of C2 H4 . The surface model deduced from a combination of surface science studies and ab initio

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calculations presents an Al-rich termination with only a fraction of slightly buried CoS atoms, which is a priori defavorable for H2 dissociation, a key step of hydrogenation reactions. The H2 disociation barrier calculated on o-Al13 Co4 (100) is however lower than the other elementary steps involved in the semi-hydrogenation of acetylene: C2 H2 → C2 H3 (0.81 eV), C2 H3 → C2 H4 (0.86 eV). Such high barriers are related to the strong adsorption energies of C2 H2 and C2 H4 , in a 4σ and 2σ geometry, respectively. The previous rather high activation energies suggest that the o-Al13 Co4 (100) does not present a high activity. It is however found to be selective, since further hydrogenation proceeds with a much higher activation energy (1.15 eV).

Conclusion We have determined a detailed atomistic scenario for the selective hydrogenation of acetylene on the o-Al13 Co4 (100) surface, which is an Al-rich bulk terminated surface without protruding surface Co atoms, but which contains isolated active region with elevated adsorption energies for the species involved in the reaction. The first important step is the H2 dissociation. Even if the topmost surface composition is Al-rich, the barrier for H2 dissociation is found to be smaller than 0.60 eV, i.e. much lower than the ones calculated on pure aluminium surfaces. The H atoms could easily migrate on the surface and approach the adsorbed acetylene molecule (barrier lower than 0.3 eV). The hydrogenation reaction presents three rate controlling steps with activation energies equal to 0.81/0.86/1.15 eV, which determines the formation of the C2 H3 /C2 H4 /C2 H5 species, respectively. The desorption of C2 H4 (0.23 eV on the S3 site, 0.92 eV on the most stable bridge stable site) is easier than the formation of the C2 H5 radical, which ensures the selectivity of this reaction. This study raises the question of the active surface orientation involved in the hydrogenation reaction. The Al13 Co4 (100) surface structure presents a relatively high atomic density, and a low surface energy (1.09 J/m2 , calculated for the Al chemical potential µAl = µbulk Al ).

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This is generally not favorable for a high catalytic activity. Other surface orientations might be more reactive, and should be investigated as well.

Acknowledgment This work was supported by the ANR-CAPRICE (ANR-11-INTB-1001) and the European C-MAC consortium. D. K. also acknowledge the Région Lorraine for its financial support. This work was granted access to the HPC resources of GENCI under the allocation 99642.

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Catalytic Semihydrogenation of Acetylene on the (100) Surface of the

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