Intermetallic Compound AlPd As a Selective Hydrogenation Catalyst

Article pubs.acs.org/JPCC

Intermetallic Compound AlPd As a Selective Hydrogenation Catalyst: A DFT Study M. Krajčí*,† and J. Hafner‡ †

Institute of Physics, Slovak Academy of Sciences, Bratislava SK-84511, Slovakia Faculty for Physics, Center for Computational Materials Science, Vienna University, Vienna A-1090, Austria

‡

ABSTRACT: Recently, it has been demonstrated that intermetallic compounds composed of Pd and Ga or Co and Al provide excellent selectivity for the hydrogenation of acetylene to ethylene. Motivated by experimental works on GaPd catalysts, we have performed a detailed ab initio study of acetylene hydrogenation by the pseudo 5-fold (120) surface of the isostructural and isoelectronic AlPd compound crystallizing in the B20-type structure. The structure of the surface can be described by a triangle−rectangle tiling, and we demonstrate that the most active sites for hydrogenation are triangular arrangements of two Al and one Pd atom. Acetylene is bound to bridge sites between two Al atoms. The most favorable adsorption site for ethylene is on top of the most strongly protruding Pd atom. Activation energies for all steps of the reaction have been calculated. We demonstrate that the activation energies for the rate-controlling steps are comparable to those on reference catalysts (Pd, Pd−Ag, and Al13Co4) and that a desorption energy for ethylene that is lower than the activation energy of ethylene to ethyl provides thus good selectivity. We show that the decisive factors for the activity and selectivity of the catalyst are the same on both intermetallic compounds AlPd and Al13Co4.

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INTRODUCTION The hydrogenation of acetylene to ethylene, C2H2+H2 → C2H4, is a chemical reaction of industrial importance. Ethylene produced by a steam cracking process contains a small fraction of acetylene. In the ethylene feedstock used for the production of polyethylene, any contamination by acetylene has to be removed to avoid poisoning of the polymerization catalyst.1 Usually Pd-based hydrogenation catalysts are used for the hydrogenation of acetylene to ethylene, and because further hydrogenation of ethylene to ethane is undesired, the selectivity of the catalyst plays a significant role. A typical hydrogenation catalyst contains metallic palladium dispersed on an inert oxidic support, e.g., Al2O3. Dispersed palladium exhibits high activity but unsatisfactory selectivity. The properties of Pd as catalyst for the hydrogenation of alkynes to alkenes have been studied experimentally and theoretically for many years. It has been demonstrated that pure Pd catalysts are selective if the reaction is performed under conditions where subsurface carbon is formed.2,3 However, the subsurface hydrogen promotes further hydrogenation to an alkane and hence reduces selectivity.4 It was also found that alloying Pd with other metals such as Ag, Au, Pb, or Ga can significantly improve selectivity. Osswald et al.5,6 compared the performance of various catalysts for acetylene hydrogenation under laboratory conditions. The relevant parameters are the rate of conversion (expressing the concentration of acetylene before and after the reaction) and the selectivity (measuring the concentration of the desired product, ethylene, after the reaction). For pure Pd © 2012 American Chemical Society

on an Al2O3 support, a conversion rate of 43% and a low selectivity of only 17% were reported. For a Pd20Ag80 alloy catalyst, the conversion rate increased to 83% and the selectivity to 49%. Alloying Pd with Ag can be considered as the current industrial solution for acetylene hydrogenation.7 Recently, a novel concept for the design of selective and stable catalysts for the hydrogenation of alkynes has been announced by Kovnir et al.:8 the isolation of the active sites on the surface of a complex intermetallic compound. On the surface of pure metals, every atom is a potential active site. As Pd atoms react strongly with alkynes and alkenes, the reactants are fully hydrogenated to alkanes and the catalysts have poor selectivity. The coordination of Pd atoms by less reactive atoms such as Ag in a Pd−Ag alloy catalyst reduces the adsorption energies of alkenes and favors their desorption over further hydrogenation to alkanes. Detailed ab initio density functional calculations, combined with kinetic Monte Carlo studies of Neurock et al.9−11 have demonstrated that electronic effects (changes in the local electronic density of states) are far less important than geometric effects (the number of Ag atoms occupying nearest neighbor sites around a Pd atom). In substitutional alloys, both components are randomly distributed, and the optimal local coordination is realized only around a fraction of the Pd atoms present on the alloy surface. In contrast, the surfaces of intermetallic compounds are ordered, Received: December 21, 2011 Revised: February 15, 2012 Published: February 21, 2012 6307

dx.doi.org/10.1021/jp212317u | J. Phys. Chem. C 2012, 116, 6307−6319

The Journal of Physical Chemistry C

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reliable structural model of the surface. Although the B20 structure is not too complex (8 atoms per elementary cell), little is known about the atomic structure of the surfaces. The B20 structure has a certain hidden relation to the structure of icosahedral Al−transition metal quasicrystals. We shall demonstrate that it is just the pseudo 5-fold (p5f) surface comparable to the 5-fold surface of quasicrystals that is responsible for the superior catalytic properties of Al(Ga)Pd compounds. Since DFT does not provide absolute values of adsorption and activation energies with chemical accuracy (i.e., within ±1− 2 kJ/mol), it is important to compare the results with those for a well-studied reference system. For acetylene hydrogenation, such a reference is the (111) surface of Pd. Ab initio DFT studies of acetylene hydrogenation over Pd(111) have been presented, e.g., by Sheth et al.9 For the C2H2+H → C2H3 reaction, activation energies of 66 and 50 kJ/mol are reported for surface coverages of 25% and 33%, respectively. At these high coverages, lateral interactions between the reactants cannot be neglected so that the comparison with a reaction at the surface of a complex intermetallic compound (where the distances between active sites are much larger) is not straightforward. We have calculated the energetics of acetylene hydrogenation over Pd(111) at a coverage of 4% and found that the activation energy for this reaction increases to 75 kJ/ mol. Calculations for lower coverages require large models, and therefore, they are computationally very demanding. Hydrogenation of acetylene to ethylene over Pd(111) proceeds through a Horiuti−Polanyi mechanism scheme via a vinyl intermediate. While the hydrogenation of acetylene to vinyl is almost thermo-neutral, the following hydrogenation of vinyl to ethylene is strongly exothermic and irreversible. Reactants, intermediates, and products are bound to the same Pd surface atoms. On the p5f surface of an AlPd compound, the hydrogenation reaction also follows the Horiuti−Polanyi scheme, but the active site is not the same at all steps of the reaction. Acetylene adsorbs preferentially in a bridge position between two Al atoms close to a Pd atom, and this is the active site for the hydrogenation to vinyl. During the second hydrogenation step from vinyl to ethylene, the molecular complex shifts from the bridge position to a position on top of a nearby Pd atom. Selectivity is than determined by the competition between further hydrogenation of Pd-bound ethylene to ethyl and desorption of ethylene. This scenario is very similar to that identified and reported for the Al13Co4 catalyst,16 where the active sites for the initial steps of the reaction are also Al atoms close to the transition metal (TM) atom and where selectivity is determined by the competition between desorption and reaction of transition metal bound ethylene.

and by appropriately choosing the composition and the crystal structure of the compound, the environment of the active sites can be optimized. The isolation of the active sites avoids undesired interactions and permits to control the activity, selectivity, and stability of the catalyst. Osswald et al.5,6 suggested to use intermetallic Ga−Pd compounds as a promising new class of highly active and selective catalysts for acetylene hydrogenation. They investigated the activity, selectivity, and long-term stability of GaPd and Ga7Pd3 under different reaction conditions. For a GaPd catalyst, both conversion and selectivity increased by 91% and 56%, respectively, in comparison with the reference catalysts Pd/Al2O3 and Pd20Ag80. The initially observed drawback of a low active surface area was overcome by milling and chemical etching.12 The superior properties of Ga−Pd catalysts are attributed to the isolation of the active Pd sites according to the active-site isolation concept. In the GaPd compound crystallizing in the B20 (FeSi-type) structure,13 each Pd atom is surrounded by 7 Ga atoms only. In the more complex structure of the Ga7Pd3 compound, Pd atoms are coordinated to 8 Ga atoms. However, so far the atomistic scenario of the catalytic reactions on GaPd surfaces is unknown. Enhanced covalency of Ga−Pd bonding prevents the formation of subsurface hydrids and reduces hydrogen supply for unselective hydrogenation, and this contributes to enhance selectivity.14 A GaPd compound exhibits also a significantly reduced electron density at the Fermi level and a shift of the Pd 4d band to higher binding energies compared to elemental Pd. In this work, ab initio DFT methods have been used to explore a complete atomistic scenario for the complex multistep hydrogenation process (coadsorption of hydrocarbons and molecular hydrogen, dissociation of hydrogen, and reaction of atomic hydrogen with hydrocarbon molecules and intermediates) on the surface of an AlPd compound. AlPd and GaPd are isostructural and isoelectronic; both crystallize in the B20 structure. The chemical properties of Al and Ga in the compounds with Pd are almost identical. The reason why we started our investigations with AlPd was the desire to compare the results with those available from our previous works on the complex intermetallic compound Al13Co4 as a selective hydrogenation catalyst.15,16 These studies have been motivated by preliminary reports on experiments demonstrating a superior performance and selectivity of this catalyst.17,18 The theoretical results demonstrated that the active sites for hydrogenation are Al atoms forming pentagons around isolated Co atoms. Because of the high stability of large pentagonal bipyramids forming the constituent building blocks of the complex orthorhombic structure of Al13Co4, the surface of the compound is strongly corrugated, and the active sites are exposed at the edges of stripes formed by Al5Co pentagons. It is hence possible that geometry effects are as important in promoting the catalytic activity of the Al atoms as their interaction with the Co atoms (to whom the role of active sites had originally been attributed). The comparison with a catalytically active Al-rich compound with a simpler crystal structure will thus provide further insight into the relative importance of structural, chemical, and electronic effects in promoting an improved catalytic performance. DFT methods allow to identify reaction centers, to determine adsorption and activation energies, and to search for optimal reaction paths, providing information not directly accessible to experiment. A prerequisite for any study is a

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COMPUTATIONAL METHODS AND MODELS Electronic structure calculations have been performed using the Vienna ab initio simulation package (VASP).19,20 VASP produces an iterative solution of the Kohn−Sham equations of density functional theory (DFT) within a plane wave basis. We used the semilocal exchange-correlation functional in the generalized gradient approximation (GGA) proposed by Perdew et al.21 The basis set contained plane waves with a kinetic energy up to Ecut‑off = 700 eV. The self-consistency iterations were stopped when total energies are converged to within 10−6 eV. The optimized geometries of the surface and of the adsorbate−substrate complexes were determined using 6308



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elementary cell. The B2 and B20 structures are the lowest approximants in this sequence. The electronic structure of FeSi and of isoelectronic compounds such as RuSi and AlRh is characterized by a narrow band gap in the Fe-d−Si-p bands around the Fermi energy. In AlPd with one electron more per formula unit, the gap reduces to a pseudogap located about 1 eV below the Fermi energy.26 The formation of a band gap is closely related to the topology of the lattice. In the undistorted B1 structure, the bands are multiply connected at the high symmetry points Γ and X. The distortion leading to the formation of the ideal B20 structure lifts the degeneracy and permits the opening of a gap. The further distortion from the ideal to the real B20 structure splits the shell of seven equidistant nearest Pd neighbors around each Al at 2.63 Å into 1 + 3 + 3 neighbors located at increasing distances of 2.58, 2.60, and 3.04 Å. The distortion correspond to the formation of −Al−Pd−Al−Pd− chains with alternating short (2.60 Å) and long (3.04 Å) distances. It leads to a stronger hybridization between Pd-d and Al-p states and contributes to a partially covalent character of the Al−Pd bonds resulting in the stabilization of the B20 over the B1 structure.26 Structure of the Pseudo 5-Fold Surface of B20. The determination of the stable surfaces of intermetallic compounds is a rather complex task. The still rather limited experience with the surface physics and chemistry of intermetallic compounds and quasicrystals has established a few simple rules:27−34 (i) the crystal cleaves preferentially along dense atomic planes separated by wide density gaps. (ii) Because the surface energy of Al is substantially lower than that of transition metals, for Al−TM compounds, Al-rich surfaces generally have lower surface energies. (iii) The sites on surfaces with a mixed Al− TM composition binding adsorbates most strongly are TM atoms or Al atoms close to TM atoms. In closely packed cubic crystals, the low-index surfaces (100), (110), and (111) fulfill these rules. For the cubic B20 structure, all atomic planes perpendicular to the [100] direction are identical, but they are relatively sparsely populated, and the distance between neighboring planes is only 1.49 Å. Perpendicular to the [110] direction, one finds many sparse atomic planes at small distances. Perpendicular to the [111] direction in one period of a√3, there are 9 atomic planes: three flat planes occupied by Pd only, three flat planes occupied by Al atoms only, and three slightly puckered Al−Pd planes. Because Al-rich surfaces have lower surface energies than terminations where many TM atoms are exposed at the surface, one can expect that the (111) surface will be Al-rich and that most Pd atoms will be buried ∼0.9 Å below the surface. Details of our study of the structural properties of the (111) surface and other surface terminations of the B20 compounds will be published elsewhere. In the B20 structure, one finds perpendicular to the (120) direction a stacking sequence of slightly puckered planes containing Pd and Al atoms in equal numbers, separated by a gap of 2.19 Å; see Figure 1. The atomic arrangement in these planes has pseudo 5-fold (p5f) symmetry, the angle between the pseudo 5-fold direction, and the [100] direction is β, sin β = τ/(τ + 2)1/2, β = 58.28°, where τ is the golden mean. This angle differs only slightly from the 60° between the [100] and [120] directions. Upon relaxation, the distance between the first Al− Pd layer, and the surface layer is reduced to 2.15 Å. Relaxation also reduces the puckering of the top layer by 9% but hardly

static relaxations using a quasi-Newton method and the Hellmann−Feynman forces acting on the atoms. Transition states were determined using the nudged elastic band method (NEB).22 In structural optimizations and transition state searches, convergence criteria of 10−4 eV for total energies and 0.1 eV/Å for forces acting on the atoms were applied. The surface of AlPd is represented by a slab cut from the bulk structure. The surface area x × y of the orthorhombic computational cell is 10.97 Å × 9.82 Å. It consists of the elementary surface cell doubled in the y direction. The thickness of the slab should be large enough to stabilize the surface. We have found that a thickness of four layers, with fixed bulk-like coordinates of atoms in the bottom layer is sufficient. Neighboring slabs are separated by a vacuum layer of 12 Å. The large lateral dimensions of the computational cell allowed to perform many exploratory calculations using the Γpoint, but all final results were calculated using a 2 × 2 × 1 kpoint mesh for Brillouin-zone integration. The accuracy of calculated adsorption and activation energies was verified for several configurations by comparison with calculations at a finer 3 × 3 × 1 k-point mesh. Crystal Structure of the B20 Compound. The B20 (FeSi-type) structure has space group P213 (No. 198). The Bravais lattice is simple cubic, but the overall point symmetry is tetrahedral. The space group consists of four 3-fold rotational axes oriented along the ⟨111⟩ directions and three 2-fold screw 21 axes consisting of a 180° rotation around a cubic axis, followed by a nonprimitive translation by (1/2,1/2,0)a and combinations thereof. The B20 structure has 8 atoms per elementary cell; the Pearson symbol is cP8. In FeSi, both Fe and Si are located at Wyckoff positions (4a) with coordinates (u,u,u), (u + 0.5,0.5 − u,−u), (− u,0.5 + u,0.5 −u), and (0.5 − u,−u,0.5 + u) where u(Fe) = 0.1358 and u(Si) = 0.844. The point symmetry of the Fe and Si sites is C3, i.e., a 3-fold rotation. In the AlPd compound, Al occupies Si sites, and Fe is replaced by Pd. The values of the experimental internal coordinates are23 u(Pd) = 0.143 and u(Al) = 0.847. The lattice parameter is a = 4.859 Å. The calculated values are a = 4.908 Å, and u(Pd) = 0.147 and u(Al) = 0.844. The FeSi structure may be considered as derived from the B1 (NaCl) structure by a distortion involving a displacement of Fe or Si atoms along a ⟨111⟩ direction to sites with u(Fe) = 0.25 and u(Si) = 1 − u(Fe) = 0.75 These distortions are rather large and reduce the space group symmetry from Fm3m̅ to P213. Another interesting view of the B20 structure is in terms of the ideal B20 structure proposed by Vočadlo et al.24 For u(Fe) = 1/ (4τ) = 0.1545 and u(Si) = 1− 1/(4τ) = 0.8455 (where τ = (1 + √ 5)/2 is the golden mean), each atom has exactly seven nearest neighbors of the opposite kind at a distance of a√3/ (2τ). Note that the positions of atoms in the AlPd compound are close to the ideal ones. The seven nearest neighbors occupy seven of the twenty vertices of a pentagonal dodecahedron centered at the atom. The 7-fold coordination and the arrangement of the coordinating atoms have led Dmitrienko25 to interpret the B20 structure as a low-order crystalline approximant to an icosahedral Al−TM quasicrystal (e.g., i−Al− Pd−Mn). The quasiperiodicity of icosahedral quasicrystal is closely related to the golden mean τ number. A systematic method to construct periodic approximants to the infinite quasicrystal consists of replacing this irrational number by a fraction of two subsequent Fibonacci numbers. One gets a sequence of periodic approximants with increasing size of the 6309



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Figure 1. Side view at the layered structure of AlPd projected onto the (001) plane. Positions of atoms are shown by circles: Al, light gray; Pd, dark gray. The blue dashed line shows the position of the cleavage plane determining the pseudo 5-fold (120) surface and the red dashed line the cleavage plane for the formation of a (100) surface perpendicular to the 2-fold screw axis. The dotted square marks one unit cell.

Figure 2. Top view of the pseudo 5-fold (120) surface of AlPd. A surface area of 3 unit cells is shown. Positions of atoms are shown by circles: Al, light gray; Pd, dark gray. The arrangement of atoms in the surface layer can be described by different planar tilings: a tiling consisting of hexagons and thick golden rhombi (full blue lines), or a triangle−rectangle tiling if all hexagons and rhombi are split as indicated by full red lines. The hexagonal tile can also be divided into one thick and two thin rhombi (as shown by the dashed lines), producing a Penrose tiling. If the hexagons are split into two irregular quadrangles, as shown by the dotted line, the tiling can be described as arising by a distortion of tiling describing the (110) surface of the B2 structure.

affects the lateral distances between the atoms in the surface layer. Figure 2 shows a top view at the p5f surface. The ordering of atoms in the surface layer can be described by a tiling consisting of golden thick rhombi and squashed hexagons (RH tiling) and the vertices being occupied alternatingly by Pd and Al atoms. The acute angles in the rhombi and the hexagons are 2π/5. We note that if each hexagon is decomposed into one golden thick rhombus and two golden thin rhombi (see the dashed lines in Figure 2), one gets a Penrose tiling. In the Penrose tiling, vertices linked by a short diagonal of a thin rhombus are left vacant. Alternatively, if each thick rhombus is split into two triangles and each hexagon is decomposed into two triangles and one rectangle (see full red lines in Figure 2), one gets a rectangle−triangle (RT) tiling with atoms at all vertices; see also Figure 3b. If the hexagons are split into two irregular quadrangles (see the dotted line in Figure 2), one creates a tiling that allows to see the relations with the (110) surface of the B2 (CsCl) structure. If the positions of the atoms in the surface layer are shifted a little such that all quadrangular tiles become rectangular, the structure becomes identical to that of the (110) B2 surface. The p5f surface of the B20 structure thus plays a role analogous to the intensively studied (110) surfaces of the B2-type Al−TM compounds.35,36 It is possible to assign a parity to each vertex in the RH tiling. The Al atoms are in say odd vertices, while Pd atoms occupy vertices with even parity. Moreover, there are two kinds of Al sites and two kinds of Pd sites. Pd sites of the first kind have 3 Al neighbors; Pd sites of the second kind have 4 Al neighbors (note the number of edges of the RH tiling with a common vertex). Each Pd site of the second kind is at the center of an incomplete pentagon of Al atoms: four Al atoms occupy the vertices of a regular pentagon, and the fifth is replaced by an Al−Pd pair occupying one edge of a rectangular tile. This configuration resembles to the

pentagonal configurations of atoms on the catalytically active (100) surface of Al13Co4.16 The p5f surface is slightly corrugated (see Figure 1). In Figure 3, the heights of all atoms in the relaxed surface relative to the average surface plane are given in angstroms. The height of the Pd atom in site P2 is at least by ∼0.3 Å greater than that of the surrounding Al atoms A1, A3, A1′, and A2′. However, the Pd atom at site P4 is more than ∼0.3 Å deeper than the surrounding Al atoms. A similar situation had been found on the puckered P plane of the (100) surface of the Al13Co4 compound.16 In the following, we shall use the p5f surface of B20-type AlPd to examine the catalytic properties of this compound.

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RESULTS The first step toward the exploration of an atomistic scenario for the multistep hydrogenation reaction must be the determination of the energetically most favorable adsorption sites of reactants, intermediates, and products, via the calculation of the binding energies at specific adsorption sites. Binding energies are defined by the energy difference between the adsorbate/substrate complex and the energies of the adsorbed species in the gas phase and of the clean substrate. Figure 3a shows a top view of the computational cell used in our simulations. The surface cell contains eight Al and eight Pd atoms. The relaxed surface is appreciably corrugated. The height of the Pd atoms relative to the average surface plane 6310



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Figure 3. (a) Atomic sites on the surface in the computational cell. The violet circles mark positions of Pd atoms; the light-blue circles are Al atoms. The atoms below the top surface layer are darker. On the p5f surface, there are four inequivalent sites for Al (labeled A1−A4) and four for Pd (labeled P1−P4). The surface is appreciably corrugated, with a difference of 0.68 Å in the heights of the most strongly protruding atoms and those below the average. The numbers inside the circles indicate the height of the atoms (in Å) with respect to the average height. (b) Valence electron density distribution in the surface layer, superposed by the rectangle−triangle tiling.

varies between +0.36 Å for the most strongly protruding P2 sites and −0.32 Å for the P4 buried rather deeply below. For the Al atoms, the height varies between +0.29 Å and −0.27 Å. Part b of the surface shows a contour plot of the valence electron density distribution in the average surface plane, superposed by the rectangle−triangle tiling. Note that the tiles are not flat, but skewed relative to the surface plane. Because of the corrugation of the surface, we have to distinguish between two types of rectangles (P1−P2−A1−A2 and P4′−P3′−A4− A3), and the triangles are also inequivalent. There are triangles with two Al and one Pd atoms (A1−A3−P2, A3−A1′−P2, A1′− A2′−P2, A4−A3−P4, A3−A1−P4, etc.) and triangles with one Al and two Pd atoms (A4−P4−P3, A2′−P1−P2, etc.), differing also in their inclinations relative to the surface plane. Molecular and Dissociative Adsorption of Hydrogen. Besides the active sites binding the hydrocarbon species, other active sites must be present, which promote the dissociative adsorption of hydrogen. Atomic hydrogen must also be sufficiently mobile to approach the reactants and intermediates. In this section, we determine the energetics of hydrogen dissociation and diffusion. On Pd(111) surface, hydrogen molecules are preferentially adsorbed on top of Pd atoms, 1.82 Å above the surface plane. On the p5f surface of AlPd, a H2 molecule is bonded very weakly; at most surface sites, it is not chemisorbed at all. The site promoting the strongest adsorption of the H2 molecule, with an adsorption energy of Eb = −7.6 kJ/mol, is on top of the most protruding P2 atom. On the other Pd atoms lying deeper in the surface, a H2 molecule does not bind at all. We have calculated hydrogen dissociation over the P2 and P4 sites. For each site, several dissociation paths are possible. Dissociated H atoms from the hydrogen molecule on top of the P2 site can find their final positions in, e.g., H2 + H4 or H4 + H3′ sites (for the definition of the adsorption sites, see Figure 4). The adsorption energies of H atoms coadsorbed at these sites are −21 and −26 kJ/mol, respectively. The adsorption energies of coadsorbed atoms are not a superposition of the adsorption energies of single H atoms presented in Table 1. The difference originates from repulsive interaction of the coadsorbates. For the P2 and P4 sites, we found activation energies for dissociation of Ea = 54 and 58 kJ/mol, respectively. The dissociated H atoms are located at the sites H3′ and H4 after dissociation over P2 and approximately at the sites H7 and H8 after dissociation over site P4. Hydrogen dissociation is an

Figure 4. Adsorption sites of H atoms Hi, i = 1−9, on the pseudo 5fold surface of AlPd. Two examples of diffusion paths are shown, one in x direction ([21̅0]) and one in y direction ([001]).

Table 1. Adsorption Energies Eb (in kJ/mol) and Height hb (in Å) above the Surface of Molecular and Atomic Hydrogen Adsorbed at Different Sites on the p5f Surface of AlPd (see Figure 4)a pseudo 5-fold surface of AlPd adsorbate

site

description

Eb

hb

H2 H

P2 H1 H2 H3 H4 H5 H6 H7 H8 H9

top hollow hollow hollow bridge bridge bridge hollow hollow hollow

−7.6 −2.3 −12.1 −5.0 −23.5 5.6 6.5 −0.5 −0.8 −21.8

2.24 0.90 1.13 1.18 1.24 1.42 1.03 1.13 0.90 1.06

a

Energies are calculated relative to the energy of the H2 molecule in the gas phase; heights are relative to the average surface plane.

exothermic process; for the P2 site, we found a heat of reaction of ΔEreact = −18 kJ/mol. The activation energy for the recombinative desorption of molecular hydrogen is thus 72 kJ/ mol. These values can be compared with the data for the dissociative adsorption of hydrogen on the Pd(111) surface37 where activation energies of 55 kJ/mol for H2 dissociation and 67 kJ/mol for the recombinative desorption were found. The 6311



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Figure 5. Energy profiles of the diffusion paths of adsorbed H atoms (a) in x direction and (b) in y direction; cf. Figure 4. Activations energies are given in kJ/mol.

Table 2. Adsorption Energies Eb (in kJ/mol, Relative to the Molecular Species in the Gas Phase), Height hb (in Å) of the Center of Gravity of the Molecule above the Average Surface Plane, Length dC−C (in Å) of the C−C Bond, Length dC−H (in Å) of the Shortest C−H Bond, Length dC−Pd (in Å) of the Shortest C−Pd Bond, and Average C−C−H Bond Angle ϕC−C−H (in deg) for Various C2Hx (x = 2−5) Species Adsorbed at Different Sites on the p5f Surface of AlPd (see Figure 6) pseudo 5-fold surface of AlPd adsorbate

site

description

Eb

hb

dC−C

dC−H

dC−Pd

ϕC−C−H

C2H2

P2 P1−P2 A1−A3−P2 A3−A1′−P2 A3−A4−P4 A1′−A2′−P1′−P2′ A3−A4−P3′−P4′ A1−A3−P2 A3−A1−P2 A1′−A2′−P1′−P2′ P1 P2 P3 P4 P2

top bridge T center T center T center R center R center T center T center R center top top top top top

−37 −47 −148 −131 −137 −150 −109 −294 −282 −255 −27 −45 −4 − −173

2.72 2.56 2.21 2.20 2.14 1.96 1.99 2.38 2.42 2.05 2.66 2.73 2.57 2.37 3.04

1.24 1.30 1.39 1.39 1.38 1.40 1.40 1.38 1.39 1.42 1.37 1.38 1.38 1.39 1.51

1.07 1.09 1.10 1.10 1.10 1.10 1.10 1.10 1.09 1.10 1.09 1.09 1.09 1.09 1.09

2.24 2.09 2.25 2.32 2.41 2.18 2.17 2.29 2.32 2.28 2.44 2.33 2.43 − 2.16

20.1 47.6 60.4 60.5 60.7 62.2 63.0 60.5 60.5 62.6 58.7 59.0 59.0 59.9 68.2

C2H3

C2H4 C2H4 C2H4 C2H4 C2H5

difference of ΔEreact = −12 kJ/mol corresponds to the adsorption energy of the coadsorbed atoms. All energies calculated for Pd(111) are thus comparable to those calculated for the p5f surface of the AlPd compound. On the p5f surface of AlPd, dissociated hydrogen atoms occupy preferably bridge or hollow sites formed by Pd and Al atoms. Figure 4 shows that the positions are all (meta)stable adsorption sites for H atoms. Adsorption energies are listed in Table 1. The binding energies were defined with respect to the gas phase, i.e., one-half of the binding energy of a H2 molecule has been subtracted. Small values around zero mean that the binding of atomic hydrogen to the surface is of comparable strength to the binding in the hydrogen molecule. Around the protruding P2 site, there are four stable H sites, while around the other Pd sites, only three H sites were found. If such sites are simultaneously occupied, the interaction between the coadsorbed H atoms is mostly repulsive. Diffusion of Hydrogen. Dissociated H atoms on the p5f surface can move via thermally activated diffusion jumps by locally stable adsorption sites. Figure 4 shows two possible

diffusion paths: one roughly along the x direction [210̅ ], connecting the sites H1, H2, H4, H5, H7, and H8, and one in the y direction [001] through the sites H2′, H4′, H3′, H2, H4, and H3. Figure 5 presents the energy profiles for both diffusion paths; activation energies for each step have been determined using the nudged elastic band method.22 The optimized diffusion paths show that H atoms tend to move around Pd atoms, following a region of lower electron density rather than to go over the top of the atoms. In contrast, H atoms jump over Al atoms, following almost a straight line between initial and final locations. It is evident that jumps over Al atoms require much higher activation energies than jumps of H around Pd atoms. The highest activation energies of 45 and 41 kJ/mol have been calculated for the jumps from H3 to H4 (across the Al atom in site A1) and from H5 to H7 (across the Al atom in site A3). All other activations energies are significantly lower. From the diffusion paths presented in Figure 5, it is obvious that a diffusion movement of H atoms along the [001] direction is much easier than that along the [21̅0] direction. The important 6312



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molecule, the T and R sites can be considered as reaction centers for the hydrogenation process. There is a significant difference in the character of bonding of acetylene on top of a Pd atom or in a bridge position between two atoms occupying the corner of a T or R tile. On top of Pd acetylene is bonded via the hybridization between the ppπstates of the molecule with d-orbitals of the transition metal, in the bridge position the bonding has di-σ character. In gas phase acetylene, the triple CC bond is formed by one sp hybrid orbital and two p orbitals. Upon adsorption, the intramolecular hybridization changes to sp2, the three sp2 hybrid promoting (i) the C−H bond (which is now tilted relative to the molecular axis), (ii) a C−C bond, and (iii) the bonding to the substrate. The second C−C bond is a ppπ bond; the elongation of the C−C distance reflects the change from a triple- to a doublebond. Vinyl (C2H3) preferably binds with the H−C end to one of the Al atoms from a T or R tile. In the nomenclature used in Table 2, the bonding Al atom is listed first, i.e., in the T(A1− A3−P2) reaction center, the C−H group of the C2H3 molecule binds to the A1 atom. Alternatively, the C−H group can also bind to the Al atom in the A3 site. The two configurations differ by 12 kJ/mol in the adsorption energy. The CH2 is shifted toward the Pd atom in site P2. In the gas phase, the molecule is planar; on the carbon atom of the C−H group, the orbitals form sp hybrids, and those of the C−H2 group form sp2 hybrids. Upon adsorption, the hybridization changes to sp2 in the C−H and to sp3 in the C−H2 group. All three C−H bonds are tilted away from the surface. On the C−H end, vinyl binds strongly to Al through one of the sp2 hybrids and the unpaired p-electron; on the C−H2 end, there is only a weak binding via one of the sp3 hybrids to the Pd atom. The C atom in the C− H2 group is by 0.86 Å higher above the surface than the other C atom, but the C−C distance is only slightly elongated compared to adsorbed acetylene from 1.38 to 1.42 Å. Ethylene (C2H4) is adsorbed rather weakly. It binds preferably on top of the protruding Pd at the P2 site. There is clear correlation between the height of a Pd atom above the surface plane and the adsorption energy; see Figure 7. On the P4 site buried in the surface, ethylene is not bound at all. Binding is promoted by the interaction of one of the sp3 hybrids on each C atom with the d states of Pd. For a molecule adsorbed at the P2 site, the center of gravity of the molecule is located 2.73 Å above the surface. The length of the C−C is 1.38 Å as in the gas phase, and the C−H bonds are tilted away from the surface. We shall see that the vertical position of the Pd atom above the surrounding Al atoms and consequently the large C-surface distance is an important factor determining both the activity and selectivity of the catalyst. Ethyl is more strongly bound, also on top of the protruding Pd atom at a height of about 3 Å. It is remarkable that the adsorption of hydrocarbons on the p5f surface of AlPd is very similar to that on the (100) surface of orthorhombic Al13Co4: binding of acetylene at Al−Al bridge sites close to TM atoms, shifting to adsorption on top of a TM atom. In the most stable sites, adsorption energies on Al13Co4 are higher by about 30 kJ/mol for acetylene, vinyl, and ethylene and by 140 kJ/mol for ethyl. The adsorption energy of Eb(P2) = −45 kJ/mol of the ethylene molecule on top of the protruding P2 atoms is significantly lower than the adsorption energy of −71 kJ/mol on top of protruding Co atoms in the case of Al13Co4 surface. On the buried P4 site, the ethylene does not bind at all. The

conclusion is that the activation energies for dissociation and diffusion of hydrogen are lower than that of the activation energies for the following hydrogenation steps, so that the catalytic activity is not controlled by the dissociation and mobility of hydrogen. Adsorption of Hydrocarbons. The adsorption energies of different hydrocarbon species (acetylene, vinyl, ethylene, and ethyl) at the most stable sites are listed in Table 2, together with the height of the center of gravity of the molecule above the average surface plane, the length of the C−C bond, length dC−H of the shortest C−H bond, length dC−Pd of the shortest C−Pd bond, and the average angle of the C−H bonds with respect to the C−C axis. All energies are calculated relative to the total energy of the same species in the gas phase and the clean AlPd surface. An acetylene molecule can bind, e.g., on top of Pd atoms, Eb(P2) = −37 kJ/mol or in a bridge position between two Pd atoms Eb(P1−P2) = −47 kJ/mol. However, on the p5f surface, the acetylene molecule is the most strongly adsorbed at special positions that are very close to bridge positions between two Al atoms, above the center of a triangular (T) tile, whose corners are occupied by two Al and one Pd atom, or of a rectangular (R) tile with two Al and two Pd atoms at its corners (see Figure 6). Because of the puckering of the surface layer, the T sites are not all equivalent; the

Figure 6. Preferred adsorption geometries of acetylene over triangular (T) and rectangular (R) tiles on the pseudo 5-fold surface of AlPd.

binding energies of the C2H2 molecule vary between −131 to −148 kJ/mol. Triangular tiles with two Pd and only one Al atoms are less favorable for acetylene adsorption. For instance, the position of the C2H2 molecule in the triangular tile A2′− P1−P2 is unstable. The molecule shifts to the bridge position P1−P2 already listed in Table 2. From the A1′−A2′−P2 triangle, the acetylene molecule is displaced into the A1′−A2′− P1′−P2′ rectangle. For two inequivalent R sites at the p5f surface, we have found the adsorption energies of −109 and −150 kJ/mol. The less favorable R sites are characterized by a larger difference of 0.7 Å in the heights of the atoms occupying the corners. All these adsorption energies are substantially larger than the binding energies at other sites. The strong adsorption at the T and R tiles also affects the molecular geometry of the adsorbates. While gas phase acetylene has a linear geometry, the C−H bonds of the adsorbate are tilted by about 60° relative to the C−C axis, away from the surface. The length of the C−C bond is about 1.39 Å, increased by 0.19 Å relative to the free molecule and almost equal to the length of a CC double bond in gas phase C2H4. The elongation of the C−C bond reflects the activation of the 6313



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coadsorbed at some distant P2 site Eb = −156 kJ/mol with respect to the energies of the molecules in the gas phase. The H2 molecule dissociates and one of the dissociated H atoms migrate via thermally activated diffusion to a position close to the acetylene molecule, e.g., to site H2. The second H atom remains at a larger distance, e.g., at site H8. The initial configuration of the reactants is presented in Figure 8a. The energy of coadsorbed acetylene and two H atoms is Eb = −160 kJ/mol. We note that coadsorbed species usually repel each

Figure 7. Correlation between the adsorption energy of ethylene Eb on top of a Pd atom and the vertical height hb of the P1−P4 sites relative to the surface plane.

weak binding of ethylene is an important factor in promoting selectivity of the catalyst. Acetylene to Ethylene Hydrogenation. Chemical reactions on the surface of a catalyst can proceed either via a Langmuir−Hinshelwood or an Eley−Rideal mechanism. In the former, the reactants are coadsorbed on the surface, and the reaction proceeds via thermally activated diffusion and reaction steps. The Eley−Rideal mechanism supposes that only acetylene is adsorbed and reacts directly with molecular hydrogen from the gas phase. This mechanism is disfavored by the strong Pauli repulsion between the H2 and C2H2 molecules, leading to a very high activation energy exceeding 200 kJ/mol. Hence, in the following, we will discuss a Langmuir−Hinshelwood reaction starting with coadsorbed acetylene and hydrogen. On the p5f surface, there are many adsorption sites for acetylene in T and R tiles (reaction centers). Atomic hydrogen can be placed into different adsorption sites in close vicinity. Therefore, there are also many possible initial configurations for a hydrogenation reaction. We have investigated several reaction paths and searched for the one with the lowest activation energies. Below, we describe in detail the most favorable reaction paths starting with acetylene adsorbed on a T or a R tile. Hydrogenation Reaction in the T Center. In Figure 6, the reaction center T(A1−A3−P2) is marked by a red triangle. This site was selected for a detailed analysis of the hydrogenation of acetylene to ethylene. The complete reaction path is a multistep process involving in addition to the chemical reactions C2H2+H → C2H3, and C2H3+H → C2H4, the diffusion of coadsorbed atomic hydrogen toward the hydrocarbon species. Figure 9 presents the energy profile of the rate controlling steps. For simplicity, the diffusion paths of the hydrogen atoms are not shown explicitly in Figure 9, they are indicated by the dashed lines. It is reasonable to assume that their energy profiles look like those presented in Figure 5 and that the activation energies for diffusion of H atoms do not exceed 50 kJ/mol. The energy of the initial configuration with the C2H2 molecule adsorbed in the T center and a H2 molecule

Figure 8. Atomistic scenario for the hydrogenation of acetylene to ethylene on the triangular reaction center. (a) Initial state with coadsorbed acetylene and atomic hydrogen, (b) hydrogenation of acetylene to vinyl, (c) coadsorbed vinyl and atomic hydrogen, and (d) hydrogenation of vinyl to ethylene. Small light green circles in panels b and d indicate the trajectory of the hydrogen from its initial state in site H2 to the final position in the CH2 group of vinyl. The position in the transition state is marked by a larger red circle.

other, but in some cases, their interaction energy can be also positive. Figure 8b shows the atomistic scenario for the hydrogenation of the acetylene to vinyl reaction as determined by the nudged elastic band method.22 All atoms of the reactants and all surface atoms are allowed to move. For simplicity, only the trajectory of the H atom attacking the acetylene molecule (with the position in the transition state marked in red) and the final position of vinyl are shown. Note that during the reaction, the C−H bond at the end of the acetylene molecule attacked by hydrogen rotates around the molecular axis and that the C atoms move from the Al to the Pd atom. The activation energy for the first hydrogenation step is Ea = 51 kJ/mol and the binding energy Eb = −186 kJ/mol. The step is exothermic by ΔEreact = −26 kJ/mol. The detailed energy profile for this first hydrogenation step is shown in detail in Figure 9b. The peak around the transition state corresponds to the formation of the C2H2−H bond, the following shoulder to the movement and relaxation of the activated C2H3 molecule to its equilibrium position. In the next step, the H atom in site H8 jumps over the P4 site and is coadsorbed with vinyl in the configuration shown in Figure 8c. The binding energy is Eb = −175 kJ/mol. The increase in energy is slightly larger than the difference in the adsorption energies of H in sites H8 and H6, i.e., the 6314



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Figure 9. (a) Potential energy profile for the rate-controlling steps of the hydrogenation of acetylene to vinyl and ethylene. Dashed lines correspond to diffusion of hydrogen atom adsorption sites. The activation energies are given in kJ/mol. Energies are given relative to the reactants in the gas phase. The green line is the energy of an isolated ethylene molecule. The reaction can continue by a desorption (green arrows) or by a further hydrogenation (blue arrow) of the ethylene molecule. (b) Detailed potential energy profile for the acetylene to vinyl reaction calculated by the NEB method.

interaction between the coadsorbed species is very slightly repulsive. During the reaction, the H atom moves first to a bridge position between sites P4 and A3, close to position H7. The C−H group of the vinyl molecule also shifts closer and binds with this hydrogen atom (this is the transition state); see Figure 8d. Then, in a concerted movement, the activated hydrocarbon complex shifts to its equilibrium position on the top of the P2 site. The activation energy for the C2H3+H → C2H4 reaction is Ea = 71 kJ/mol. The reaction is strongly exothermic. The binding energy in the final state is −265 kJ/ mol, and the heat of reaction is ΔEreact = −90 kJ/mol. The C2H4 adsorbed on top of the P2 site can either desorb with a desorption energy of 46 kJ/mol or the hydrogenation reaction can continue. Which scenario is more likely will be discussed below. We note that we have investigated also several other reaction paths. For instance, the vinyl molecule in Figure 8c can be attacked also by a hydrogen atom coadsorbed at the site H3′. The activation energy barrier of this alternative reaction path was found to be the same at 71 kJ/mol. Hydrogenation Reaction in the R Center. The adsorption energy of a C2H2 molecule in the R (A1−A2−P1−P2) center and a H2 molecule at some distant P2 site of the AlPd p5f surface is Eb = −158 kJ/mol with respect to the gas phase reactants. After dissociation, the atoms are coadsorbed with acetylene in the H2 and H8 sites; see Figure 10a. In the equilibrium position, an isolated acetylene adsorbed at the R(A1−A2−P1−P2) center forms approximately a bridge between the A1 and P1 sites. This unusual orientation is caused by the different heights of the A1 and A2 sites, while that of the A1 and P1 sites is almost the same. If a H atom is coadsorbed at site H2 (in the hollow of the P1−P2−A2 triangle), the C2H2 molecule is shifted toward the A1−A2 bridge position. The adsorption energy of the coadsorbed species is Eb = −131 kJ/ mol, i.e., by 31 kJ/mol higher than the sum of the adsorption energies of the isolated species in these sites and also higher by 27 kJ/mol than the initial configuration with molecular hydrogen. This means that in this configuration, there is a stronger repulsive interaction between coadsorbates caused by Pauli repulsion.

Figure 10. Atomistic scenario for the hydrogenation of acetylene to ethylene at the rectangular (R) reaction center. (a) Initial state with coadsorbed acetylene and atomic hydrogen, (b) hydrogenation of acetylene to vinyl, (c) coadsorbed vinyl and atomic hydrogen, and (d) hydrogenation of vinyl to ethylene. Small light green circles indicate the trajectory of the hydrogen atom along the reaction path; transition states are marked in red; see Figure 9a.

Figure 10b shows the atomistic scenario for the C2H2+H → C2H3 reaction. Coadsorbed acetylene and atomic hydrogen approach each other in a concerted movement, involving also a rotation of the C−H group around the molecular axis from a cis- to a trans-configuration. In the final configuration, vinyl is bound via the C−H group to the A1 atom. The C−H2 group points toward the P1−P2 bridge. The activation energy of the step is Ea = 57 kJ/mol, and the adsorption energy (relative to acetylene and molecular hydrogen in the gas phase) is Eb = −208 kJ/mol. This step is strongly exothermic by ΔEreact = −78 kJ/mol. To prepare the next hydrogenation step, the second H atom migrates by thermal diffusion to site H5, closer to the coadsorbed vinyl molecule in the configuration shown in 6315



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Figure 11. (a) Potential energy profile for the rate-controlling steps of the hydrogenation of (a) acetylene to vinyl and ethylene (red line), following the atomistic scenario shown in Figure 10. (b) Potential energy profile for the hydrogenation of ethylene to ethyl (blue line), according to the atomistic scenario shown in Figure 12. The green line in panel a is the energy of an isolated ethylene molecule. The green rectangle in panel b shows the range of desorption energies for ethylene in the presence of coadsorbed H atoms. The activation energies are given in kJ/mol. The energy scale is with respect to the reactants in the gas phase.

Figure 10c. The energy of the coadsorbed species is Eb = −192 kJ/mol. Hydrogenation occurs by a concerted movement of the reactants: vinyl moves to a position on top of site P2. Simultaneously, the C−H group rotates around the molecular axis to allow the hydrogen atoms to bind to the C atom. The activation energy for the C2H3+H → C2H4 reaction is Ea = 76 kJ/mol. The reaction is also strongly exothermic, the binding energy in the final state is −264 kJ/mol, and the heat of reaction is ΔEreact = −71 kJ/mol. Figure 10d shows the position of the C2H4 molecule after the reaction. From the trajectory of the attacking hydrogen atom and its position in the transition state, it is evident that in this case, a large shift of the vinyl molecule is required before the second hydrogenation can take place. The energy profile of the whole reaction path is shown in Figure 11a. For both reaction paths explored above, the rate-controlling step for the transformation of acetylene to ethylene is the second hydrogenation step from vinyl to ethylene, with activation energies of 71 and 76 kJ/mol for a reaction at the T and R center, respectively. All earlier diffusion and reaction steps have activation energies of 50 kJ/mol or lower. This has to be compared with the energy profile for the hydrogenation over the puckered surface of Al13Co4 where we have found activation energies of 61 ± 2 kJ/mol for all steps.16 It is remarkable that both reactions starting from different initial states at the R(A1−A2−P1−P2) and T(A1−A3−P2) centers finish in the same final configuration: C2H4 adsorbed on top of the most protruding P2 site. They differ only in a rotational orientation. The ethylene molecule can either desorb with a desorption energy of 45 kJ/mol or the hydrogenation reaction can continue. Which scenario is more likely is now analyzed. Ethylene Hydrogenation and Selectivity. The ethylene molecule is weakly bonded at the P2 site. Its equilibrium position is 2.33 Å above the Pd atom. A direct attack by atomic H coadsorbed at a nearby site is difficult because of the different heights of the coadsorbed species. The H atom essentially has to break the bond to the surface to approach the ethylene molecule high above the surface. This leads to a very

high activation energy comparable to that for an Eley−Rideal reaction. Nevertheless, the reaction is possible via a two-step mechanism. Similar to the reaction on the Al13Co4(100) surface, we have found that a reaction of ethylene with hydrogen is possible only if the H atom is first moved to a site just below the ethylene molecule. On Al13Co4, this first step requires a very high activation energy of 80 kJ/mol, and the Al13Co4 catalyst has a high selectivity because this energy is higher than the desorption energy of ethylene of 70 kJ/mol.16 The following reaction of H with C2H4 requires only an activation energy of 15 kJ/mol. On the p5f surface of AlPd, the reaction C2H4+H → C2H5 is also a two step process. The atomistic scenario is presented in Figure 12, the energy profile in Figure 11b. The activation energies for the first and second steps are 41 and 31 kJ/mol, respectively. In the initial step, ethylene is located on top of a

Figure 12. Atomistic scenario for the hydrogenation of ethylene to ethyl. (a) A diffusion jump of hydrogen atom toward ethylene and (b) reaction of the hydrogen atom with ethylene. Small light green circles indicate the trajectory of the hydrogen atoms along the reaction path; transition states are marked in red; see Figure 11b.

Pd atom at site P2 and coadsorbed with hydrogen in site H9. The coadsorption energy is Eb = −274 kJ/mol. The repulsion between the coadsorbates costs an energy of 12 kJ/mol. The first diffusion step bringing the H atom into position H2 (moving around a Pd atom in site P1; see Figure 12a) requires an activation energy of 41 kJ/mol. This step is endothermic by ΔEreact = 34 kJ/mol because site H2 is energetically less 6316



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presence and location of coadsorbed hydrogen. We also note that the strong exothermicity of the vinyl to ethylene reaction is important. It helps to enhance selectivity because the probability of a backwards reaction will be very low; but of course, despite our effort, we cannot claim that the reaction pathway we have found is the most favorable. This is also unnecessary: if the reaction is efficient and selective along the path we have found, this is sufficient to support our claim that AlPd is a good catalyst. Difficulties could arise only if there is a reaction path for ethylene to ethyl with a lower barrier, but this is unlikely because ethylene binds preferentially on top of P2, reducing strongly the number of possible reaction scenarios. In this configuration, the C−C bond of ethylene is very close to the length of a CC double bond in gas phase ethylene indicating the absence of any activation for further hydrogenation. Thus, our simulations parallel the experimental results5,6,8,12 for the isostructural and isoelectronic compound GaPd: both intermetallic compounds are efficient and selective catalysts for the hydrogenation of acetylene to ethylene. It is interesting to compare our results with the reaction path and its energetic profile constructed for other hydrogenation catalysts. For a Pd(111) surface, Mei et al.10,11 and Studt et al.3 reported activations energies of 66/74/72 kJ/mol for the hydrogenation of acetylene to vinyl, vinyl to ethylene, and ethylene to ethyl, compared to a desorption energy of 82 kJ/ mol for ethylene. The difference of 10 kJ/mol in favor of continued hydrogenation explains the poor selectivity of pure Pd. For a Pd−Ag alloy catalyst, the activation energies are lowered to 66/10/61 kJ/mol, but the activation energy for the formation of ethyl remains lower by 10 kJ/mol than the desorption energy of ethylene. It was argued that selectivity does not result from a low desorption barrier of ethylene but from the fact that the activation energy for dissociation of hydrogen from ethyl (and formation of ethylene) is lower than the activation energy for the hydrogenation of ethyl to ethane (and also much lower than the desorption energy of ethyl).9−11 For acetylene hydrogenation on Al13Co4, we have calculated activation energies of 63/61/80 kJ/mol for the successive hydrogenation steps, and a desorption energy of ethylene varying between 54 and 71 kJ/mol, depending on coadsorbates. Thus, the activation energy for the rate-controlling step of the hydrogenation of acetylene to ethylene is lower for the Al−Co compound, but the difference between the activation energy for the formation of ethyl and the desorption of ethylene is somewhat larger for the AlPd compound. Thus, the activity of the Al13Co4 catalyst is expected to be better; however, the selectivity of AlPd is expected to be somewhat better than that of Al13Co4. It is remarkable that only for the two intermetallic compounds selectivity results from a desorption energy for ethylene that is lower than the activation energy for complete hydrogenation. It is remarkable that the adsorption configurations of the reactants on the Al13Co4(100) surface and on the pseudo 5-fold surface of AlPd are quite similar; see Figure 13. In both cases the catalytically active sites are formed by a pentagonal arrangement of Al atoms around a slightly protruding transition metal atom. The most active reaction centers in both cases are triangular configurations of two Al atoms and one TM atom. The preferable adsorption geometry for acetylene is the Al−Al bridge position, forming a di-σ bond. Vinyl is adsorbed in a rather asymmetric configuration, with the strongest bond between the C−H group and an Al atom. An ethylene molecule is adsorbed via a π-bond on top of the TM

favorable than site H9 and because the repulsive interaction becomes stronger. The binding of the H atom with coadsorbed ethylene requires a rotational movement of the C−H2 group around the molecular axis such that a C−H3 group can be formed with the H atom approaching ethylene from below (see Figure 12a,b). Compared to Al13Co4, the activation energy is much lower for the first and higher for the second step. The first barrier is lower because diffusion of the H atom around the Pd atom at the P1 site requires less energy than the jump over an Al atom on the Al13Co4 surface. The second barrier is higher because a rotation of the C−H2 is required to make space for the additional H atom, while this is not the case on Al13Co4 where the configuration of ethylene remains unchanged. The selectivity of the catalyst is determined by the relationship between the activation energy for the hydrogenation of ethylene to ethyl and the desorption energy of ethylene. The hydrogenation is a two-step process, with atomic hydrogen coadsorbed very close to ethylene as an intermediate. The energy of the barrier for the second reaction step is 56 kJ/ mol higher than the initial state: this by 11 kJ/mol than the desorption energy of an isolated ethylene molecule. Hence, desorption is favored over hydrogenation, and therefore, the catalyst is selective. Selectivity is further enhanced by (i) a desorption energy for ethylene coadsorbed with hydrogen, which is considerably lower than the 45 kJ/mol calculated for isolated ethylene, and (ii) an activation energy for the backward reaction from the intermediate to the initial state, which is much lower than the barrier for the forward reaction (see Figure 11b). For the coadsorbed configuration shown in Figure 12b, the desorption energy is only 10 kJ/mol. In dependence on the position of coadsorbed hydrogen, the desorption energy varies in a 35 kJ/mol wide interval, between 10 and 45 kJ/mol as indicated in Figure 11b by a green rectangle.

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DISCUSSION AND CONCLUSIONS Ab initio DFT calculations have been used to construct an atomistic scenario for the selective hydrogenation of acetylene to ethylene catalyzed by the p5f surface of intermetallic AlPd crystallizing in the B20-type structure. The atomic order of the surface can be described by a tiling of golden rhombi and hexagons,or, for our purpose, more appropriately by a triangle− rectangle tiling. The surface is rather strongly corrugated. The height of atoms is differing by up to 0.68 Å. The dissociative adsorption of molecular hydrogen is an activated process with an activation energy of 54−58 kJ/mol. Atomic hydrogen is rather mobile on this surface; activation energies for diffusive motion between neighboring adsorption sites vary widely, between 2 and 45 kJ/mol as expected for such a corrugated surface. Two different reaction centers with acetylene adsorbed in Al−Al bridge positions in a triangular (T) or a rectangular (R) tile have been identified. For both reaction paths, the ratecontrolling step is the second hydrogenation from vinyl to ethylene with activation energies of 71 and 76 kJ/mol for the T and R center, respectively. In contrast to acetylene and vinyl, ethylene is bound to the surface not via an Al atom but rather on top of a Pd atom protruding from the surface. The hydrogenation of ethylene to ethyl is a two-step reaction, with an intermediate in the form of atomic hydrogen coadsorbed very close to but at a significantly lower height than ethylene. The height of the combined barrier for this reaction is with 56 kJ/mol, significantly higher than the desorption energy of ethylene, varying between 10 and 45 kJ/mol depending on the 6317



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theoretical investigation of catalytic properties of the isostructural and isoelectronic GaPd compound is under way.

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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 This work has been supported by the Austrian Ministry for Education, Science and Art through the Center for Computational Materials Science. M.K. also thanks the support from the Grant Agency for Science of Slovakia (No. 2/0111/11), from CEX FUN-MAT, and from the Slovak Research and Development Agency (Grant No. APVV-0647-10).

Figure 13. Comparison of the catalytically active surfaces of Al13Co4 (a) and AlPd (b). The violet circles represent the TM atoms. The red triangles mark the most active reaction centers. On both surfaces, the most preferable adsorption site for the acetylene molecule is a bridge position between two Al atoms. The vertical position of the neighboring TM atom is an important parameter controlling activity and selectivity of the catalyst.

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atom. During the hydrogenation reaction, the molecular complex shifts from the bridge position between the Al sites to the top position on top of the TM site where it is only weakly bound, favoring desorption. This is in contrast to the adsorption configurations on Pd or Pd−Ag surfaces where both acetylene and ethylene are bound via di-σ bonds to two Pd atoms, irrespective of surface composition. Another important factor with a significant influence on both activity and selectivity is the relative height of the Pd atom above the Al atoms in the reactive center. The Pd atom in the P2 site lies 0.36 Å above the average surface plane; see Figure 3. For comparison, the Co atom in the center of the Al-pentagon of Al13Co4 is 0.45 Å above surrounding Al atoms. On one hand, the binding energy of ethylene increases with the height of the Pd atom. This makes desorption more difficult, but a high binding energy of ethylene also contributes to reduce the activation energy for the second and rate controlling hydrogenation step. The location of adsorbed ethylene on top of a protruding Pd atom also increases the activation energy for the formation of ethyl. Similarly, as on Al13Co4, on the p5f surface of AlPd, the ethylene hydrogenation, C2H4+H → C2H5, is possible only if in a two-step reaction first, the H atom is brought to a position very close but significantly lower than the position of the ethylene molecule. In the second step, H is attached to the hydrocarbon molecule: this requires the loosening of the hydrogen-surface bond. The combined activation energy for this process is higher than for the desorption of ethylene. Hence, both the isolation of the TM atom (preventing the formation of a di-σ bond of ethylene with the surface) and its height in a corrugated surface are important factors influencing both activity and the selectivity of the catalytic reaction. We assume that the TM−Al2 triplet configurations that we identified as the catalytically active sites for acetylene hydrogenation can exist also at selected surfaces of other Al− TM intermetallics. However, our results suggest that the (111) or (100) surfaces of AlPd are presumably catalytically inactive. The top layer of the (111) surface is occupied by Al atoms; Pd atoms are buried 0.9 Å below the top plane and cannot contribute to the catalytic process. Our results demonstrate that a B20-type AlPd compound should be an active as well as selective hydrogenation catalyst. A 6318



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Intermetallic Compound AlPd As a Selective Hydrogenation Catalyst

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