Laurent Piccolo
email: laurent.piccolo@ircelyon.univ-lyon1.fr

Corentin Chatelier

Marie-Cécile De Weerd

Franck Morfin

Julian Ledieu

Vincent Fournée

Peter Gille

Emilie Gaudry
email: emilie.gaudry@univ-lorraine.fr

Catalytic properties of Al13TM4 complex intermetallics: influence of the transition metal and the surface orientation on butadiene hydrogenation

Keywords: Complex intermetallic compounds, Al 13 Fe 4, Al 13 Co 4, Al 13 Ru 4, Heterogeneous catalysis, Hydrogenation, Butadiene, Density functional theory, Single-crystal surfaces

come

Introduction

The interest for intermetallic catalysts, i.e. compounds of at least two elements located left and around the Zintl line -vertical line between group 13 and group 14 [START_REF] Meyer | Inorganic Chemistry Highlights[END_REF] -in the periodic table, with a defined crystal structure and stoichiometry [START_REF] Vignola | C 2 H 2 -Induced Surface Restructuring of Pd-Ag Catalysts: Insights from Theoretical Modeling[END_REF], has considerably increased in recent years [START_REF] Armbrüster | Intermetallic compounds in heterogeneous catalysis-a quickly developing field[END_REF][START_REF] Furukawa | Intermetallic Compounds: Promising Inorganic Materials for Well-Structured and Electronically Modified Reaction Environments for Efficient Catalysis[END_REF]. The potential of these compounds lies in their stability, which is ensured by strong metal-metal bonds with an ionocovalent character for several of them, and the possibility to tune the catalytic activity and selectivity based on electronic, geometric and ordering effects [START_REF] Furukawa | Intermetallic Compounds: Promising Inorganic Materials for Well-Structured and Electronically Modified Reaction Environments for Efficient Catalysis[END_REF]. Several examples can be found among noble metal-containing intermetallics such as Pd-Ga [START_REF] Kovnir | A new approach to well-defined, stable and site-isolated catalysts[END_REF][START_REF] Osswald | Palladium-gallium intermetallic compounds for the selective hydrogenation of acetylene: Part I: Preparation and structural investigation under reaction conditions[END_REF][START_REF] Osswald | Palladium-gallium intermetallic compounds for the selective hydrogenation of acetylene: Part II: Surface characterization and catalytic performance[END_REF][START_REF] Kovnir | Etching of the intermetallic compounds PdGa and Pd 3 Ga 7 : An effective way to increase catalytic activity[END_REF][START_REF] Armbrüster | Pd-Ga Intermetallic Compounds as Highly Selective Semihydrogenation Catalysts[END_REF][START_REF] Luo | Addressing electronic effects in the semi-hydrogenation of ethyne by InPd2 and intermetallic Ga-Pd compounds[END_REF], Pd-In [START_REF] Luo | Addressing electronic effects in the semi-hydrogenation of ethyne by InPd2 and intermetallic Ga-Pd compounds[END_REF], Pd-Zn [START_REF] Zhou | PdZn Intermetallic Nanostructure with Pd-Zn-Pd Ensembles for Highly Active and Chemoselective Semi-Hydrogenation of Acetylene[END_REF], and Pt-Sn [START_REF] Jugnet | Selective hydrogenation of 1,3-butadiene on Pt 3 Sn(111) alloys: comparison to Pt(111[END_REF], which are able to selectively catalyze alkyne (one triple C-C bond, as in acetylene) or alkadiene (two double C-C bonds, as in butadiene) hydrogenation. These reactions are of great industrial importance for, e.g., the processing of petrochemical products for polymer synthesis.

Pd-based catalysts are rare and costly but actually used in industrial processes. The addition of an inactive ancillary metal such as Ag, Au or Cu is known to improve the catalyst selectivity to partially hydrogenated products [START_REF] Vignola | C 2 H 2 -Induced Surface Restructuring of Pd-Ag Catalysts: Insights from Theoretical Modeling[END_REF][START_REF] Piccolo | Pd-Au single-crystal surfaces: Segregation properties and catalytic activity in the selective hydrogenation of 1,3-butadiene[END_REF][START_REF] Hugon | Selective Hydrogenation of 1,3-Butadiene in the Presence of an Excess of Alkenes over Supported Bimetallic Gold-Palladium Catalysts[END_REF][START_REF] Cooper | Design of surface sites for the selective hydrogenation of 1,3-butadiene on Pd nanoparticles: Cu bimetallic formation and sulfur poisoning[END_REF]. For the semi-hydrogenation of acetylene, low-cost alternatives to reference Pd-Ag in the form of Ni-Zn alloys were discovered through a theoretical approach based on the density functional theory [START_REF] Studt | Identification of Non-Precious Metal Alloy Catalysts for Selective Hydrogenation of Acetylene[END_REF]. However, under reaction conditions, such substitutional alloys may suffer from adsorption-induced surface segregation, accompanied with a decrease in selectivity [START_REF] Piccolo | Pd-Au single-crystal surfaces: Segregation properties and catalytic activity in the selective hydrogenation of 1,3-butadiene[END_REF][START_REF] Sachtler | Chemisorption Complexes on Alloy Surfaces[END_REF][START_REF] Borodziński | Selective Hydrogenation of Ethyne in Ethene-Rich Streams on Palladium Catalysts, Part 2: Steady-State Kinetics and Effects of Palladium Particle Size, Carbon Monoxide, and Promoters Catal[END_REF].

The recent identification of transition-metal (TM) aluminides such as Al 13 Fe 4 , Al 13 Co 4 , and Al 5 Co 2 as efficient catalysts for the semi-hydrogenation of acetylene or butadiene is a major breakthrough in the quest for noble metal-free catalysts [START_REF] Armbrüster | Al 13 Fe 4 as a low-cost alternative for palladium in heterogeneous hydrogenation[END_REF][START_REF] Armbrüster | Ordered cobalt-aluminum and iron-aluminum intermetallic compounds as hydrogenation catalysts[END_REF][START_REF] Piccolo | Al 13 Fe 4 selectively catalyzes the hydrogenation of butadiene at room temperature[END_REF][START_REF] Piccolo | The partial hydrogenation of butadiene over Al 13 Fe 4 : A surface-science study of reaction and deactivation mechanisms[END_REF][START_REF] Piccolo | Intermetallic Compounds as Potential Alternatives to Noble Metals in Heterogeneous Catalysis: The Partial Hydrogenation of Butadiene on γ-Al 4 Cu 9 (1 1 0)[END_REF][START_REF] Meier | Semihydrogenation of Acetylene on Al 5 Co 2 Surfaces[END_REF][START_REF] Kandaskalov | Adsorption Properties of the o-Al 13 Co 4 (100) Surface toward Molecules Involved in the Semihydrogenation of Acetylene[END_REF][START_REF] Kandaskalov | Catalytic Semihydrogenation of Acetylene on the (100) Surface of the o-Al 13 Co 4 Quasicrystalline Approximant: Density Functional Theory Study[END_REF]. Indeed, the chemical bonding network in these polar intermetallics, which are also quasicrystal approximants, may prevent any surface segregation process, ensuring the stability of the catalyst. The interest for polar intermetallic compounds, i.e. compounds with lower valence electron concentrations than those of the Zintl phases, but close to those of the Hume-Rothery phases [START_REF] Lin | Electron-Poor Polar Intermetallics: Complex Structures, Novel Clusters, and Intriguing Bonding with Pronounced Electron Delocalization Acc[END_REF], is not limited to catalysis. They have recently received extensive attention as attractive materials for a wide range of applications (permanent ferromagnets, magnetocalorics, shape-A c c e p t e d M a n u s c r i p t 3 memory alloys, superconductors, hydrogen storage materials, thermoelectrics). Several studies have compared the physical properties of Al 13 TM 4 compounds along the series (TM = Co, Fe, Ru, Rh), but no drastic differences were highlighted, in line with similar vibrational and electronic properties in the series [START_REF] Piccolo | Intermetallic Compounds as Potential Alternatives to Noble Metals in Heterogeneous Catalysis: The Partial Hydrogenation of Butadiene on γ-Al 4 Cu 9 (1 1 0)[END_REF]. These approximants to decagonal quasicrystalline phases present specific crystal structures, described as a stacking of atomic planes perpendicular to the pseudo 10-fold direction -[100] for orthorhombic o-Al 13 Co 4 , [010] for monoclinic m-Al 13 Fe 4 and m-Al 13 Ru 4 -each plane presenting pentagonal atomic arrangements [START_REF] Ledieu | Surfaces of Al-based complex metallic alloys: atomic structure, thin film growth and reactivity[END_REF]. This leads to an anisotropy in the magnetic properties, as well as in charge and heat transport properties [START_REF] Dolinšek | Anisotropic magnetic and transport properties of orthorhombic Al 13 Co 4[END_REF][START_REF] Popčević | Anisotropic physical properties of the Al 13 Fe 4 complex intermetallic and its ternary derivative Al 13 (Fe,Ni) 4[END_REF][START_REF] Smontara | Anisotropic transport properties of the Al 13 TM 4 and T-Al-Mn-Fe complex metallic alloys[END_REF].

The complex structure of such polar intermetallics, involving mixed iono-covalent bonding, may also be of interest for heterogeneous catalysis. Owing also to the site isolation concept [START_REF] Kovnir | A new approach to well-defined, stable and site-isolated catalysts[END_REF] already invoked to explain the high performance of Al 13 Fe 4 towards the semi-hydrogenation of acetylene [START_REF] Armbrüster | Al 13 Fe 4 as a low-cost alternative for palladium in heterogeneous hydrogenation[END_REF], the catalytic properties of Al 13 TM 4 compounds are thus expected to be quite different from those of their pure TM counterparts. The surfaces of these materials can expose Al pentagons centered by the TMs, which are believed to be the catalytically active centers [START_REF] Krajčí | Complex intermetallic compounds as selective hydrogenation catalysts -A case study for the (100) surface of Al 13 Co 4[END_REF]. Besides, Al 13 Fe 4 has been recently proposed as a template for substituting single Pt atoms at Fe sites [START_REF] Yamada | Probing Single Pt Atoms in Complex Intermetallic Al 13 Fe 4[END_REF].

Here, we focus on the comparison of the catalytic properties of Al 13 TM 4 compounds (with TM = Fe, Co, Ru) for the gas-phase hydrogenation of butadiene, which an important petrochemical reaction for the purification of C4 cuts and the production of butenes before polymerization or alkylation [START_REF] Schimpf | Selective Hydrogenation of Multiple Unsaturated Compounds Basic Principles in Applied Catalysis Springer Series in Chemical Physics[END_REF][START_REF] Bond | The hydrogenation of alkadienes. Part I. The hydrogenation of buta-1,3-diene catalysed by the Noble Group VIII metals[END_REF][START_REF] Boitiaux | Hydrogenation of highly unsaturated hydrocarbons over highly dispersed palladium catalyst: Part I: behaviour of small metal particles[END_REF][START_REF] Piccolo | Tuning the shape of nanoparticles to control their catalytic properties: selective hydrogenation of 1,3-butadiene on Pd/Al2O3[END_REF]. This reaction is industrially performed on Pd-based catalysts, but it is also catalyzed by non-noble metals such as Fe, Co, and Ni, although with a much lower activity [START_REF] Phillipson | The hydrogenation of alkadienes. Part III. The hydrogenation of buta-1,3-diene catalysed by iron, cobalt, nickel, and copper[END_REF][START_REF] Moyes | Electronic effects in butadiene hydrogenation catalysed by the transition metals[END_REF]. Our objective is to investigate the influence of the transition metal and the structure on the catalytic properties. Our rational approach combines experiments on oriented single-crystal surfaces, and theoretical calculations based on ideal infinite surfaces within periodic boundary conditions. The combination of experimental measurements with theoretical calculations is crucial here. Indeed, surface-science studies achieved under ultra-high vacuum (UHV) have already highlighted that the pseudo 10-fold surface structures of these compounds differ: a surface reconstruction is observed for Al 13 Ru 4 (010) [START_REF] Ledieu | Reconstruction of the Al 13 Ru 4 (010) Approximant Surface Leading to Anisotropic Molecular Adsorption[END_REF], not for Al 13 Co 4 (100) and Al 13 Fe 4 (010) [START_REF] Shin | Structure of the orthorhombic Al 13 Co 4 (100) surface using LEED, STM, and ab initio studies[END_REF][START_REF] Gaudry | Structure of the Al 13 Co 4 surface: Combination of surface x-ray diffraction and ab initio calculations[END_REF][START_REF] Ledieu | Structural Investigation of the (010) Surface of the Al 13 Fe 4 Catalyst[END_REF]. The surface structure and composition determined under UHV are also different between The paper is organized as follows. Materials, experimental and theoretical methods are described in Section 2. In section 3, the catalytic measurements are detailed, highlighting the superior performances of Al 13 Fe 4 (010) and Al 13 Co 4 (010) with respect to Al 13 Ru 4 (010) and Al 13 Co 4 (100). A rationalization of the catalytic properties of the three Al 13 TM 4 pseudo 10-fold surfaces is then proposed, based on electronic structure calculations including the shapes of the density of states and the adsorption energies. Our results are finally discussed in Section 4.

Methods

Materials

Centimeter-size Al 13 TM 4 (TM = Co, Fe, Ru) single crystals were grown from an Al-rich solution using the Czochralski method [START_REF] Gille | Single crystal growth of Al 13 Co 4 and Al 13 Fe 4 from Al-rich solutions by the Czochralski method[END_REF] and polished with diamond paste down to 0.25 µm grain size. For the investigation of the catalytic properties, samples with surfaces oriented perpendicular to the pseudo-10fold axis were cut from the ingot: o-Al 13 Co 4 (100), m-Al 13 Fe 4 (010) and m-Al 13 Ru 4 (010). An additional sample, o-Al 13 Co 4 (010), was used to investigate the influence of the surface orientation on the catalytic performances. In the following, when o or m prefixes are not mentioned, the crystal structures correspond to those of the above four samples.

Experimental methods

Surface preparation

For each sample, the surface was cleaned by repeated cycles of Ar + sputtering (2 kV) for 15 min and annealing for 20 min under UHV (base pressure ~5×10 -10 Torr). The annealing temperatures were approximately 550-650 °C and 800 °C for Al 13 Co 4 (100), 750 °C for Al 13 Fe 4 (010) [START_REF] Piccolo | The partial hydrogenation of butadiene over Al 13 Fe 4 : A surface-science study of reaction and deactivation mechanisms[END_REF], 550 °C and 800 °C for Al 13 Ru 4 (010), and 800 °C for Al 13 Co 4 (010). Figure 1 and Figure S1 (Supplementary Information) show the low-energy electron diffraction (LEED) patterns and Auger electron spectra (AES), respectively, of the clean annealed surfaces (for Al 13 Fe 4 (010) the reader may consult Ref. [START_REF] Piccolo | The partial hydrogenation of butadiene over Al 13 Fe 4 : A surface-science study of reaction and deactivation mechanisms[END_REF]). Figure S1 shows that no In the case of Al 13 Co 4 (100), the high-temperature (800 °C) annealing [START_REF] Shin | Structure of the orthorhombic Al 13 Co 4 (100) surface using LEED, STM, and ab initio studies[END_REF] led to a more disordered surface than the low-temperature one (650 °C), as shown by Figure 1. Conversely, the atomic order of Al 13 Ru 4 (010) increased with the annealing temperature (lower background intensity), although the atomic structure of the surface remained essentially unchanged (Figure 1), consistently with the work of Ledieu et al. [START_REF] Ledieu | Reconstruction of the Al 13 Ru 4 (010) Approximant Surface Leading to Anisotropic Molecular Adsorption[END_REF]. For both Al 13 Co 4 (100) and Al 13 Ru 4 (010), the two annealing temperatures led to similar near-surface compositions, as probed by AES. In the following, the results for Al 13 Co 4 (100) and Al 13 Ru 4 (010) relate to low-temperature and high-temperature annealing, respectively, unless explicitly mentioned. For Al 13 Fe 4 (010) and Al 13 Co 4 (010), only high-temperature annealing was performed and led to sharp LEED pattern (Figure 1). In the case of Al 13 Co 4 (010), which had not been investigated so far, the main diffraction spots appear connected to each other through continuous and parallel lines. The corresponding surface structure is presently undetermined. A c c e p t e d M a n u s c r i p t 7 hydrogen (N55) and argon (N56) was prepared in a separate chamber before its injection into the reactor. All the gases were purchased from Air Liquide. The standard initial conditions in the reactor cell were 0.5 mbar butadiene, 5 mbar H 2 , and 0.5 mbar Ar (argon was used for internal calibration). At the end of the reaction run, the products were evacuated to secondary vacuum using a turbomolecular pump, and the reaction/evacuation cycle could be performed additional times.

A c c e p t e d M a n u s c r i p t

During the reaction, the gases were continuously sampled through a leak valve and analyzed by a mass spectrometer (MS) evacuated by an oil diffusion pump capped with a LN 2 trap (base pressure 2×10 -10 Torr, analysis pressure 2×10 -8 Torr). The MS intensities for m/z = 2, 40, 54, 56, and 58 were recorded for hydrogen, argon, butadiene (C 4 H 6 ), butenes (C 4 H 8 ), and butane (C 4 H 10 ), respectively. In some cases, online gas chromatography (GC) was employed in addition to MS for determining the full distribution of 1,3-butadiene hydrogenation products, i.e. the three butene isomers (1-butene, trans-2-butene and cis-2-butene) and butane, using an automatic gas sampling device connected to an Agilent 6850 GC-FID [START_REF] Morfin | A versatile elevated-pressure reactor combined with an ultrahigh vacuum surface setup for efficient testing of model and powder catalysts under clean gas-phase conditions[END_REF].

With an Agilent HP-AL/KCl column (50 m × 0.53 × 15 µm) kept at 80 °C, a chromatogram was recorded every 10 min.

Theoretical methods

Bulk and surface models

The o-Al 13 Co 4 compound crystallizes in the Pmn2 1 space group [START_REF] Grin | Crystal structure of orthorhombic Co 4 Al 13[END_REF], while m-Al [START_REF] Piccolo | Pd-Au single-crystal surfaces: Segregation properties and catalytic activity in the selective hydrogenation of 1,3-butadiene[END_REF] The pseudo-10-fold surface models considered in this study are deduced from a cleavage keeping the cluster substructure intact at the surface (Fig. 2). Each protruding surface TM atom is surrounded by a pentagonal arrangement of Al atoms, leading to the isolation of TM atoms at the surface. Under UHV, such surface model has been experimentally observed in the case of m-Al 13 Fe 4 (010), but not for o-Al 13 Co 4 (100) and m-Al 13 Ru 4 (010) [START_REF] Ledieu | Reconstruction of the Al 13 Ru 4 (010) Approximant Surface Leading to Anisotropic Molecular Adsorption[END_REF][START_REF] Gaudry | Structure of the Al 13 Co 4 surface: Combination of surface x-ray diffraction and ab initio calculations[END_REF][START_REF] Scheid | Bonding network and stability of clusters: the case study of Al13TM4 pseudo-tenfold surfaces[END_REF]. However, some operating conditions may stabilize the previous surface model (Gaudry et al., in preparation), which has already been postulated to explain the catalytic activity of o-Al 13 Co 4 (100) towards the semi-hydrogenation of acetylene [START_REF] Krajčí | Complex intermetallic compounds as selective hydrogenation catalysts -A case study for the (100) surface of Al 13 Co 4[END_REF]. In addition, the consideration of the same surface model for all compounds should allow comparing the influence of the electronic factor (nature of the TM) on the adsorption properties.

Theoretical details

The theoretical ground state properties were deduced from calculations based on the density functional theory (DFT), using the plane wave Vienna ab initio simulation package (VASP) [START_REF] Kresse | Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set[END_REF][START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF][START_REF] Kresse | Ab initio molecular dynamics for liquid metals[END_REF][START_REF] Kresse | Ab initio molecular-dynamics simulation of the liquid-metalamorphous-semiconductor transition in germanium[END_REF]. The interaction between the valence electrons and the ionic core was described using the projector-augmented wave (PAW) method [START_REF] Kresse | From ultrasoft pseudopotentials to the projector augmented-wave method[END_REF][START_REF] Blöchl | Projector augmented-wave method[END_REF] within the generalized gradient approximation (GGA-PBE) [START_REF] Perdew | Generalized gradient approximation made simple[END_REF][START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF], considering the valences of the atoms to be 3s 2 3p 1 (Al), 4s 1 3d 7 (Fe), 4s 1 3d 8 (Co),and 4p 6 5s 1 4d 7 (Ru). The DFT-D3-BJ functional [START_REF] Grimme | Effect of the damping function in dispersion corrected density functional theory[END_REF] was also used, since it is expected that the consideration of the van der Waals A c c e p t e d M a n u s c r i p t 9 interactions influences the absolute values of the adsorption energies. The cut-off energy and the k-point grid were set to 450 eV and 5×9×1 (or equivalent), respectively, within the Brillouin zone. Spin polarization was not taken into account since it was shown to be unnecessary for such Al-rich complex intermetallic compounds [START_REF] Mihalkovič | First-principles calculations of cohesive energies in the Al-Co binary alloy system[END_REF]. The structures were plotted using the VESTA software [START_REF] Momma | VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data[END_REF].

Bulk structures were relaxed till the Hellmann-Feynman forces were as low as 0.02 eV/Å. The resulting theoretical bulk parameters are found in good agreement with the experimental ones. As shown by Table S1, the relative differences are smaller than 1% (resp. 2%) for Al 13 Co 4 and Al 13 Ru 4 (resp. Al 13 Fe 4 ).

Adsorption studies were performed using surfaces modeled with 7-layer-thick symmetric slabs separated by void (thickness ca. 12Å). The positions of all the atoms in the slab were optimized, while the volume and the shape of the simulation cell were set. The adsorption energy for atomic hydrogen [E ads (H)], and 1,3-butadiene or 1-butene [E ads (but)] was calculated as the sum of the energies of the relaxed surface and the gas-phase molecule minus the energy of the adsorbate-surface system:

E ads (H) = E(slab) + ½ E(H 2 ) -E(slab+H) E ads (but) = E(slab) + E(but) -E(slab+but)
where E(slab), E(H 2 ), E(but), E(slab+H) and E(slab+but) are the energies of the systems {slab}, {dihydrogen molecule}, {unsaturated hydrocarbon molecule}, {slab + atomic hydrogen} and {slab + unsaturated hydrocarbon molecule}, respectively.

With this definition, stable adsorption on the considered site corresponds to a positive value of E ads .

Densities of states (DOS) of the d electrons n d (ε) were calculated for Al 13 TM 4 compounds, as well as for the corresponding TMs (hcp Co, hcp Ru, bcc Fe). The d-band center is defined as:

= (1)
where ε represents the energy and E F the Fermi energy.

Projected DOS (pDOS) calculations were performed to investigate the molecule/surface interactions. The modifications of the molecular states following the adsorption process were evaluated from the comparison of the pDOS of the {molecule+surface} system, when the molecule is adsorbed at the surface and when the molecule is located far from the surface (distance larger than 10 Å). The clean annealed surfaces were exposed to a mixture of hydrogen (5 mbar) and butadiene (0.5 mbar)

under batch conditions at RT and 110 °C. In all cases except that of Al 13 Fe 4 , the reaction kinetics was slow at RT, i.e. only a small fraction of butadiene was converted after ca. 30 min, so that the tests at 110 °C were generally performed during the same reaction run. Figure 3 shows the results of such an experiment for Al 13 Co 4 (010) (see also Fig. S2). Figures S3-S4 and S5-S6 show catalysis data for Al 13 Co 4 (100) and Al 13 Ru 4 (010), respectively. For detailed catalytic data concerning Al 13 Fe 4 (010), the reader may consult Ref. [START_REF] Piccolo | The partial hydrogenation of butadiene over Al 13 Fe 4 : A surface-science study of reaction and deactivation mechanisms[END_REF].

Figure 3a shows that the selectivity of Al 13 Co 4 (010) to butenes is 100%, both at RT and 110 °C, butane being formed only after complete butadiene conversion. The butene selectivity is also 100% for Al 13 Ru 4 (010) at any temperature (Fig. S5). For Al 13 Co 4 (100), it is 100% at RT at low conversion (Fig. S4) and superior to 95% at 110 °C (Fig. S3). The butene selectivity of Al 13 Fe 4 (010) is lower, but remains superior to 90% at any temperature. This lower selectivity correlates with the fact that Al 13 Fe 4 (010) is the only system, among the four Al 13 TM 4 surfaces investigated, that exhibits a high activity for butene hydrogenation to butane (in the absence of butadiene). Butene hydrogenation over Al 13 Fe 4 (010) is even faster than butadiene hydrogenation in most conditions [START_REF] Piccolo | The partial hydrogenation of butadiene over Al 13 Fe 4 : A surface-science study of reaction and deactivation mechanisms[END_REF]. On the other surfaces, butene hydrogenation to butane is always slow (see Figure 3a for Al 13 Co 4 (010) and Fig. S5 for Al 13 Ru 4 (010), both at 110 °C).

In terms of initial butene distribution at RT (1-butene:trans-2-butene:cis-2-butene = 74:26:0%, Figure 3b), Al 13 Co 4 (010) behaves more like Pd(100) (73:24:3%) than Al 13 Fe 4 (010) (71:13:16%). For the latter system, an unusually low trans/cis 2-butene ratio, close to unity, was evidenced [START_REF] Piccolo | The partial hydrogenation of butadiene over Al 13 Fe 4 : A surface-science study of reaction and deactivation mechanisms[END_REF]. Conversely, for Al 13 Co 4 (010), cis-2-butene is not a primary product and its fraction remains lower than 10% throughout the reaction, as long as butadiene is present in the reactor, at both investigated temperatures. After butadiene conversion, the butenes distribution evolves towards a near-equilibrium value, 9:62:29%, through isomerization. As shown by Figure S4b, the butenes distribution for Al [START_REF] Piccolo | Pd-Au single-crystal surfaces: Segregation properties and catalytic activity in the selective hydrogenation of 1,3-butadiene[END_REF]

°C 24 °C

A l 1 3 C o4 ( 1 0 0 ) A l 1 3 C o4 ( 0 1 0 ) A l 1 3 R u4 ( 0 1 0 ) A l 1 3 F e4 ( 0 1 0 )

Figure 4. Comparison of initial butene-formation activities of Al 13 TM 4 surfaces at RT and 110 °C.

In the case of Al 13 Ru 4 (010), similar experiments were carried out at RT for the Ar ion-sputtered surface, the surface annealed at ca. 550 °C, and the one annealed at ca. 800 °C. As a result, the activity of the sputtered surface is lower than that of the 550 °C-annealed surface, which is itself much lower than the activity of the (more structurally ordered) 800 °C-annealed surface (Fig. S6). Similarly, a lower activity of the sputtered sample was previously measured for Al 13 Fe 4 (010) [START_REF] Piccolo | Al 13 Fe 4 selectively catalyzes the hydrogenation of butadiene at room temperature[END_REF], and is also observed here in the case of Al 13 Co 4 (010) (Fig. S2). Moreover, AES shows that the sputtered Al 13 TM 4 surfaces are richer in TM than the annealed surfaces (Fig. S1). These results indicate that the structural and chemical orders, and correlatively the active center isolation, are beneficial to the hydrogenation activity of the Al-TM systems.

A c c e p t e d M a n u s c r i p t °C (Fig. S5). Both Al 13 Co 4 (100) and Al 13 Co 4 (010) contain, after reaction, not only oxygen but also traces of carbon, while Al 13 Ru 4 (010) contains only a small amount of oxygen (Fig. S1). The higher resistance of Al 13 Ru 4 (010) is tentatively ascribed to the presence of the noble metal (Ru) instead of Co or Fe.

Calculations

For Again, these adsorption energies are lower than those calculated for Fe(110) (E PBE = 0.54 eV) and Ru(0001) (E ads PBE = 0.55 eV) [START_REF] Ciobica | Adsorption and Coadsorption of CO and H on Ruthenium Surfaces[END_REF][START_REF] Kristinsdóttir | A systematic DFT study of hydrogen diffusion on transition metal surfaces[END_REF]. These trends are also valid within the DFT-D3-BJ approach. The charge density deformation, defined as the charge density difference between the adsorbate-surface system and the sum of the charge densities for the relaxed slab and the adsorbate, plotted in Figure 5 for H/Al 13 Ru 4 (010), suggests a metal-H bond with an iono-covalent character. This is also consistent with the (Table 1). The trends are similar when considering the van der Waals interactions, with E ads (butadiene) increases from 1.59 eV (o-Al 13 Co 4 ) to 1.67 eV (m-Al 13 Ru 4 ) and 1.80 eV (m-Al 13 Fe 4 ). The adsorption energies of 1-butene on the same surfaces are significantly lower, and the difference between E ads (butadiene) and E ads (butene) is almost constant (0.39-0.46 eV), regardless of the used functional.

Within the d-band model of Nørskov and coworkers, the d-band position and the d-band width were shown to be key parameters for rationalizing the adsorption properties of transition-metal surfaces [START_REF] Kitchin | Role of Strain and Ligand Effects in the Modification of the Electronic and Chemical Properties of Bimetallic Surfaces[END_REF][START_REF] Mavrikakis | Effect of Strain on the Reactivity of Metal Surfaces[END_REF]. This model was further developed to include the influence of the shape of the electron DOS [START_REF] Vojvodic | Electronic Structure Effects in Transition Metal Surface Chemistry Top[END_REF].

To illustrate the role played by the TM atom isolation within the Al matrix, and in turn the electronic structure on the adsorption properties, we first compare the Al 13 Ru 4 (010) surface and the (low-energy) Ru(0001) surface, for which the adsorption energies of C 4 H 6 are calculated to be 1.16 eV (Table 1) and 1.71 eV, respectively. On Ru(0001), only one adsorption site and configuration have been considered for C 4 H 6 , as shown in Figure S7. The stronger adsorption of butadiene on Al 13 Fe 4 (010) than on Al 13 Co 4 (100) (Table 1), classically attributed to a decreased filling of the adsorbate-metal antibonding states [START_REF] Nørskov | Fundamental Concepts in Heterogeneous Catalysis[END_REF], is indeed in agreement with the lower-energy d-band center in the former case (E dbc (Al 13 Fe 4 ) = 1.41 eV, E dbc (Al 13 Co 4 ) = 1.97 eV).

A c c e p t e d M a n u s c r i p t 18 However, the previous scheme is not consistent with the results found for Al 13 Ru 4 (010) (E ads = 1.15 eV, E dbc = 2.48 eV), indicating that other parameters, such as the d-band width and shape, likely play a role.

Discussion

The comparison of butadiene-hydrogenation activities of Al 13 TM 4 surfaces (Fig. 4) reveals a variety of behaviors, from the poor activity of Al any of the systems may reconstruct under reaction conditions, choice has been made to compare the electronic structures of unreconstructed, cluster-terminated pseudo 10-fold surfaces through DFT calculations. This has allowed us to focus, for a given atomic structure, on the effect of the TM nature (Ru, Co, Fe) and isolation (for Ru only) in an Al matrix on its surface electronic structure and corresponding bonding to the relevant adsorbates. As a result, site isolation is seen to considerably narrow the Ru d-band (Fig. 6), which leads to an adsorption energy reduced by 1/3. From the comparison of the bulk electronic structures of TM and Al 13 TM 4 systems (Fig. 7), we have seen that TM isolation in the Al 13 TM 4 structure proceeds similarly for the three TMs, i.e. by a narrowing of the d-band.

Remarkably, the situation is here quite different from the one described in Ref. [START_REF] Greiner | Free-atom-like d states in single-atom alloy catalysts[END_REF], where the stronger adsorption on the Ag-Cu "single-atom alloy" dilute in Cu, with respect to pure Cu, is attributed to the narrower d-band of the alloy as compared to pure Cu.

Butadiene/butene adsorption energies (Table 1) follow the same order as hydrogenation activities:

Al 13 Fe 4 > Al 13 Ru 4 > Al 13 Co 4 . Hence, in this (low) adsorption energy range (1.1-1.3 eV for butadiene within PBE), the hydrocarbon adsorption energy can be considered as a semi-quantitative descriptor of the hydrogenation activity. On the pure metals, the much higher adsorption energies of the hydrocarbons may be detrimental to hydrogenation rates, as suggested by the lower activities of the TM-rich sputtered surfaces (SI). Moreover, the higher adsorption energy of butadiene with respect to butene is consistent A c c e p t e d M a n u s c r i p t 19 with a preference for butadiene adsorption and subsequent hydrogenation, i.e. to a high (near 100%) selectivity to butene [START_REF] Piccolo | Pd-Au single-crystal surfaces: Segregation properties and catalytic activity in the selective hydrogenation of 1,3-butadiene[END_REF][START_REF] Loffreda | Tuning catalytic reactivity on metal surfaces: Insights from DFT[END_REF]. In the sole case of Al 13 Fe 4 butene appears sufficiently adsorbed to be (fastly) hydrogenated [START_REF] Piccolo | The partial hydrogenation of butadiene over Al 13 Fe 4 : A surface-science study of reaction and deactivation mechanisms[END_REF], whereas butene hydrogenation on the other Al 13 TM 4 surfaces is very slow at any temperature (Section 3.1). The high butene-hydrogenation activity of Al 13 Fe 4 (010) is consistent with a selectivity to butene slightly lower than 100% in most butadiene-hydrogenation conditions.

In addition, the butenes distribution, and more specifically the trans/cis 2-butene ratio, is different between Al 13 Co 4 (close to 3 for both orientations, and even much higher at the beginning of the reaction)

and Al 13 Fe 4 one (close to 1). Moyes et al. studied a number of TMs for butadiene hydrogenation and suggested that the electronic state of surface metal atoms would be the dominant factor driving the products distribution [START_REF] Moyes | Electronic effects in butadiene hydrogenation catalysed by the transition metals[END_REF]. A high trans/cis 2-butene ratio was explained by the stabilization of π-allylic half-hydrogenated states which favored a selective pathway (as previously discussed for Al 13 Fe 4 (010) [START_REF] Piccolo | The partial hydrogenation of butadiene over Al 13 Fe 4 : A surface-science study of reaction and deactivation mechanisms[END_REF]) whereas πσ-adsorbed states favored anti/syn interconversion of the half-hydrogenated species and a more dispersed products distribution. Intermediate adsorbed species are not considered in the present work but a possible difference in the reaction pathways is consistent with the observed disparity in the apparent activation energy of the reaction over Al 13 TM 4 compounds, i.e. Al 13 Co 4 (010) is the least active surface at RT but the most active one at 110 °C (Fig. 4).

As far as hydrogen adsorption is concerned, the adsorption energies for the Al 13 TM 4 surfaces do not scale with the butadiene-hydrogenation activities (Table 1). However, the very low adsorption energy of H on Al 13 Co 4 (100) (E ads < 0.2 eV) may contribute to the low hydrogenation activity of this surface.

In the future, a more quantitative theoretical description of the catalytic properties will have to include the effects of adsorbate coverage, thermodynamic conditions (pressure, temperature) and kinetics (reaction pathways including activation barriers).

Conclusions

On

Al 13

 13 Co 4 (100) and Al 13 Fe 4 (010). Nevertheless, catalytic properties are measured under gas pressure (typically a few millibars of reactants), which may imply different surface structures as compared to the UHV case. The consideration of a common surface model for all explored Al 13 TM 4 pseudo 10-fold A c c e p t e d M a n u s c r i p t 4 surfaces ensures a sound comparison between the catalysts, and may help to conclude about the influence of geometric or electronic factors.

A c c e p t e d M a n u s c r i p t 5 contaminants

 5 such as C or O were detected by AES on any surface following the sputtering-annealing procedure.

  Figure 1 approxim 2.2.2. Ca

  Fe 4 and m-Al 13 Ru 4 are monoclinic (C2/m space group) [47-49]. Their bulk structure (102 atoms per conventional cell, Figure 2) consists in a stacking of two types of planes perpendicular to the pseudo-10-fold direction ([010] for m-Al 13 Fe 4 and m-Al 13 Ru 4 , [100] for o-Al 13 Co 4 ). Alternative descriptions involve polyhedral atomic arrangements, such as the ones proposed by Henley [50], based on geometric considerations.

8 Figure 2 .

 82 Figure 2. (a) Bulk structure of Al 13 Ru 4 as a stacking of Henley-type clusters (represented in blue). (b) Top view. Grey and green balls represent Al and Ru atoms, respectively. Shaded balls correspond to subsurface atoms. The crystal (a) and surface (b) directions are indicated by the unit vectors {a, b, c} and {a, c}, respectively.

Figure 3 .

 3 Figure 3. Hydrocarbon concentrations (a) and butene distribution (b) during butadiene hydrogenation over Al 13 Co 4 (010) at 24 °C, then 110 °C. The data points were obtained from GC analysis, whereas the lines in (a) were obtained from MS.

  fact that H adsorption energies do not scale with the Bader charges held by TM atoms of the TM-Al-TM molecular group in the bulk (-3.83e for Al 13 Fe 4 , -4.08e for Al 13 Co 4 , and -4.32e for Al 13 Ru 4 ).

Figure 5 .

 5 Figure 5. Charge deformation following atomic hydrogen adsorption, highlighting the TM-H interaction (isodensity 0.007 Å -3 , plotted for H on Al 13 Ru 4 (010); the influence of the neighboring Al atom is not visible for this isodensity value). Charge accumulation and depletion are plotted in blue and red, respectively. The H atom is located inside the big blue distribution.

A c c e p t e d M a n u s c r i p t 15 On

 15 the pseudo-10-fold surfaces, the adsorption energy of butadfiene adsorbed in a di-π configuration increases from 1.06 eV (o-Al 13 Co 4 ) to 1.16 eV (m-Al 13 Ru 4 ) and 1.30 eV (m-Al 13 Fe 4 ) within the PBE scheme

A c c e p t e d M a n u s c r i p t 16 Figure 6 .

 166 Figure 6. Projected density of states (pDOS): average C-p contribution of carbon atoms (butadiene, in green); Al-p (red) and Ru-d (orange) contributions of the closest Al and Ru atoms protruding at the surface and involved in the adsorption for Al 13 Ru 4 (010); Ru-d (orange) contribution of surface Ru atoms involved in the adsorption for Ru(0001). The Fermi energy was set to 0 eV.

Figure 7 .

 7 Figure 7. Contributions of s-, p-, and d-states to the density of states (DOS) for bulk Al 13 TM 4 and TM. The Fermi energy corresponds to 0 eV. To further look for a possible relationship between the d-band center position with respect to the Fermi energy (E dbc ) and the catalytic performances, the bulk electronic structures of Al 13 Ru 4 , Al 13 Fe 4 and Al 13 Co 4 are compared in Figure 7. The DOSs of Al 13 Fe 4 and Al 13 Co 4 are similar, with a free electron-like shape at low energy, and a strong maximum at higher energy caused by localized and weakly dispersive TM-d states. These DOSs considerably differ from those of the pure metals (bottom graphs in Fig. 7), which exhibit a much more spread d-band, attributed to TM-TM hybridization. The latter is almost absent for Al 13 TM 4 compounds, owing to the rather large nearest-neighbor TM-TM distance.

  the basis of catalytic tests and DFT calculations, this work has allowed us to evidence trends in the electronic structures, adsorption properties and catalytic performances of the Al 13 TM 4 intermetallic series. TM site isolation in the complex Al 13 TM 4 structure induces a narrowing of the d-band and a decrease of the butadiene adsorption energy, leading to an enhanced hydrogenation activity. While Al 13 Fe 4 (010) is the most active model catalyst at room temperature, Al 13 Co 4 (010) is revealed as the mostA c c e p t e d M a n u s c r i p t 20 active one at higher temperature (110 °C), while remaining 100% selective to butenes unlike Al 13 Fe 4 (010). In-depth investigations of Al 13 Co 4 (010) are needed to determine its structure and enable comparative DFT calculations. Moreover, a comprehensive understanding of the catalytic properties of these intermetallic surfaces requires in situ characterization (e.g. surface X-ray diffraction) to identify the actual working surface under gas-pressure conditions.

A c c e p t e d M a n u s c r i p t

  , if retained under reaction conditions, may play an important role in its good catalytic performance.Al 13 Ru 4 (010) exhibits an activity intermediate between those of Al 13 Fe 4 (010) and Al 13 Co 4 (010).

	1 10 100 1)0.1 Initial butene formation rate (µmol h -1 cm -2 )	M a n u s c r i p t
	Co 4 (100) throughout the reaction is similar to that for Al 13 Co 4 (010) (64:27:9% for the former vs. 68:24:8% for the latter, at t = 30 min and RT), although the initial 1-butene fraction is lower for Al 13 Co 4 (100) (64% vs 74%). In contrast, the surface orientation has a dramatic effect on the catalytic activity, Al 13 Co 4 (010) being much more active A c c e p t e d
	than Al 13 Co 4 (100) both at RT and 110 °C, as shown in Figure 4. Whereas in terms of activity at RT
	Al 13 Fe 4 (010) has no rival, Al 13 Co 4 (010) appears as the most active system at 110 °C while keeping a
	butene selectivity of 100% at any temperature. The faceting observed for Al 13 Co 4 (010) under UHV (Fig.
		12

13

  Our previous work on Al[START_REF] Piccolo | Pd-Au single-crystal surfaces: Segregation properties and catalytic activity in the selective hydrogenation of 1,3-butadiene[END_REF] Fe 4 (010) had shown that this surface is easily contaminated by oxygencontaining reactant impurities, most probably H 2 O[START_REF] Piccolo | Al 13 Fe 4 selectively catalyzes the hydrogenation of butadiene at room temperature[END_REF][START_REF] Piccolo | The partial hydrogenation of butadiene over Al 13 Fe 4 : A surface-science study of reaction and deactivation mechanisms[END_REF]. In the present work, when several subsequent reaction runs are performed on the same sample, the Co-containing surfaces show a loss of activity at any temperature (Figs.S2, S3), whereas Al 13 Ru 4 (010) deactivates at RT (Figs. S6) but not at 110

Table 1 .

 1 Al 13 Co 4 (100), a previous study suggested favorable sites for the adsorption of atomic hydrogen, acetylene (C 2 H 2 ) and ethylene (C 2 H 4 )[START_REF] Krajčí | Complex intermetallic compounds as selective hydrogenation catalysts -A case study for the (100) surface of Al 13 Co 4[END_REF]. Based on these results, only one type of site per adsorbate has been considered here for the pseudo 10-fold surface of Al 13 TM 4 compounds: a bridge Al-TM site for atomic hydrogen, di-π adsorption of 1,3-butadiene (C 4 H Adsorption configurations and energies (eV). The pictures were plotted using the Al 13 Ru 4 model.For atomic hydrogen adsorption, our result for Al[START_REF] Piccolo | Pd-Au single-crystal surfaces: Segregation properties and catalytic activity in the selective hydrogenation of 1,3-butadiene[END_REF] Co 4 (100) (E ads PBE = 0.18 eV) is in good agreement with the value reported in Ref. [32] (E ads PBE = 0.21 eV), and shows that atomic hydrogen is much less strongly bonded to Al 13 Co 4 (100) surface than to Co(0001) (E ads PBE = 0.48 eV [63]). On Al 13 Fe 4 (010) and Al 13 Ru 4 (010), considering the same type of Al-TM site, atomic hydrogen is bonded slightly stronger (E ads PBE = 0.39 eV).

			1,3-Butadiene	1-Butene	Hydrogen
	o-Al 13 Co 4 (100)	PBE	1.06	0.62	0.18
		D3	1.59	1.13	0.24
	m-Al 13 Fe 4 (010)	PBE	1.30	0.91	0.39

6 ) on neighboring Al and TM atoms, and π adsorption of 1-butene (C 4 H 8 ) on a TM atom. The corresponding geometries and adsorption energies (E ads ) are reported in Table 1.

Color code: Al=grey; Ru = green; C = red; H = black. Prefixes o and m relate to orthorhombic and monoclinic structures, respectively. PBE and D3 correspond to PBE and DFT-D3-BJ calculations, respectively.

  13 Co 4 (100) at any temperature to the high activities of Al 13 Fe 4 (010) at RT and Al 13 Co 4 (010) at 110 °C; Al 13 Ru 4 (010) shows intermediate activity, but superior stability. This implies that (i) for the pseudo 10-fold surfaces, Fe is the most active center; (ii) depending on the surface orientation, Co acts as a highly [(010)] or poorly [(100)] active center. If the Al 13 Co 4 (100) surface actually present in catalytic conditions has the same structure as under UHV, i.e. a bulk-terminated Al-rich termination without protruding Co atoms, it is indeed expected to be poorly active. However, since on the one hand the exact surface structure of Al 13 Co 4 (010) under UHV is unknown, and on the other hand

Acknowledgements

EG thanks Dr. David Loffreda for fruitful discussions. EG acknowledges financial support through the COMETE project (COnception in silico de Matériaux pour l'EnvironnemenT et l'Energie) funded by the "Région Lorraine". The work in Nancy was supported by the European Integrated Center for the Development of New Metallic Alloys and Compounds. High Performance Computing resources were provided by GENCI under the allocation 99642, as well as the EXPLOR center hosted by the Université de Lorraine (allocation 2017M4XXX0108).