The Electronic Structure and Bonding of Acetylene on PdGa(110

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The Journal of Physical Chemistry

The Electronic Structure and Bonding of Acetylene on PdGa(110)

P. Bechtholda, M. Sandovala, E. A. González a, G. Brizuelaa, A. Bonivardib and P. V. Jasena1

a

Departamento de Física & IFISUR (UNS-CONICET) - Universidad Nacional del Sur Av. Alem 1253, (8000) Bahía Blanca, Argentina

b

Instituto de Desarrollo Tecnológico para la Industria Química (Universidad Nacional del Litoral and CONICET), Güemes 3450, (3000) Santa Fe, Argentina

Abstract We have studied the adsorption of acetylene on the PdGa(110) surface by Density Functional Theory (DFT) calculation. Our results predict the Hollow site is the most stable location for the adsorbate. In this site, both Pd and Ga atoms interact with acetylene. This molecule is bonded with the C-C bond almost parallel to the surface. A small tilt angle of 4.1° is computed. The C atoms present a rehybridization from sp

sp2. This

rehybridization is also present in Bridge site but it is not present in the Top configuration, where the C-C-H bond angle is about 160°. We computed the total Density of State (DOS) for the system and also the projection on Pd, Ga, C and H atoms. These plots show a shift to lower energies on C and H projected states, which is an indication of stabilization after adsorption. At the same time, a reduction in the density for Pd atom directly bonded to C atom. The Crystal Orbital Overlap Population (COOP) curves show an increase in the C-C overlap population (OP) after adsorption; while, the C2H2 rehybridizes to a near sp2 geometry. The acetylene withdraws charge from the surface indicating a donation-adsorption mechanism. The formation of Pd-C and Ga-C bonds and a decrease in OP for Pd-Pd and Pd-Ga bonds are detected. We also found, in the Hollow site, a reduction in the C-C bond stretching vibrational frequency. This is an indication of the

1Corresponding

author: [email protected] Tel: +54-291-4595101. Ext: 2843. Fax: 54 - 0291 - 4595142

1

ACS Paragon Plus Environment


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significant distortion in the adsorbed molecule. We have also found that the C-C bond breaking is unfavorable and that the semihydrogenation is 2.27 eV more stable than C2H2+H2 in the gas phase.

Keywords: PdGa, DFT, Intermetallic Compounds, Acetylene adsorption, Semihydrogenation.

1. Introduction Selective hydrogenation of acetylene is an important industrial process that removes traces of acetylene in the ethylene feed for polyethylene production. Acetylene poisons the polymerization catalyst; therefore, by using another catalyst, the acetylene content in the ethylene feed has to be reduced to the lowest ppm range1-6. The most efficient way to accomplish this purification is to convert the contaminant into the valuable reactant through catalytic semi-hydrogenation. In this case the “optimal” heterogeneous catalyst maintains the same properties during the reaction (stability), enhances the transformation to ethylene (activity), and hinders further reaction to unwanted products like ethane or heavier hydrocarbon species (selectivity)7. Moreover, typical hydrogenation catalysts are made of palladium dispersed on metal oxides. Palladium metal exhibits high activity at moderate temperature and hydrogen pressure, though it has a limited selectivity. The drawback of these catalysts is that they frequently deactivate under hydrogenation conditions by the formation of a carbonaceous deposit8. Several authors have performed theoretical calculations of acetylene adsorption on transition metals. Belleli et al. studied the acetylene adsorption on fcc Pd low index surfaces9. They found that the olefins adsorb tilted and also report an important back-donation in the adsorption process. Using DFT, Matczak studied the effect of doped Pd (100) with Sn or Pb on acetylene adsorption10. The author report that the geometry of the C2H2 molecule adsorbed in di-σ configurations is highly perturbed with respect to the structure of acetylene in the gas phase. By contrast, the geometry of acetylene adsorbed in π configurations on the doped surfaces shows a much smaller distortion.

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Vattuone et al. performed ab-initio calculations analyzing the interaction of acetylene and ethylene with Pd and Ni (100) surfaces11; they found that on Pd{100} acetylene adsorbs molecularly in a rehybridized state. In addition, the adsorption of acetylene on Pd, Pt, Ni and Rh (111) surfaces, using DFT calculations, was investigated by Medlin and Allendorf12. They compute an adsorption site in which C2H2 is oriented above a 3-fold hollow, with the acetylene tilted away from a metal-metal bond, as the most stable site on Pd, Pt, and Rh surfaces. Modification of palladium catalysts by adding promoters or alloys has shown an increased selectivity and a long-term stability in the hydrogenation of acetylene2. However, the catalytic performance of these modified Pd catalysts remains insufficient. Besides, further improvements in selectivity may decrease the costs of polyethylene production. Aside from an unsatisfactory selectivity, the long-term stability of palladium catalysts has to be improved. Catalyst deactivation by carbonaceous deposits requires frequent exchange or regeneration of the catalyst in the hydrogenation reactor. Moreover, fresh or regenerated catalysts show high activity and local overheating of the reactor and, consequently, lead to increased ethylene consumption and selectivity loss6. The limited selectivity of Pd catalysts in acetylene hydrogenation can be attributed to the presence of active-site ensembles on the catalyst surface2,6. Restricting the size of the active sites in a palladium-containing hydrogenation catalyst and, thereby, preventing the formation of ensembles of neighboring Pd atoms on the surface -so-called active-site isolation- may increase catalyst selectivity and long-term stability in acetylene hydrogenation. As well as the presence of neighboring Pd atoms, the formation of palladium hydrides under hydrogenation reaction conditions substantially influences selectivity. Reducing the amount of the hydrogen incorporated into the catalyst decreases hydrogen supply for the hydrogenation reaction and increases the selectivity of acetylene hydrogenation to ethylene. It should also be mentioned 3



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that the modification of Pd-based catalysts by the presence of a second component in acetylene hydrogenation impacts on, at least, two properties: the absorption of hydrogen and the formation of weakly adsorbed ethylene on the metal crystallite, or weakly π-bonded acetylene versus di-σ ethylene. Therefore, a depletion of (subsurface) hydrogen and/or a lower barrier of desorption of the carbonaceous intermediate -or acetylene adsorption -can modify the selectivity of palladium-based catalysts5. The concept of using intermetallic compounds with covalent bonding rather than alloys is a suitable way to aim for long-term stable catalysts with preselected electronic and local structural properties13. Recently, Armbrüster et al. reported that the presence of a Pd-Ga intermetallic compound is a highly active, selective, and stable catalyst for the semihydrogenation of acetylene in a large excess of ethylene14. The hydrogenation experiment detected no hydrogen uptake in PdGa, thus preventing a hydride formation that could lead to a reduction in catalytic activity15. Armbrüster et al. also described a strong covalent bond between Pd and Ga atoms providing long-term stability for the catalysts under reaction conditions16. The first synthetic route to single-phase nanoparticulate Pd2Ga and PdGa was developed by Armbrüster et al.17. Both systems show high selectivity and good long-time stability for the acetylene semihydrogenation. Moreover, catalytic properties can be transferred from a well-defined macroscopic model system to nanostructured materials prepared by co-precipitation of Pd, Ga, and Mg diluents18. Active and selective Pd2Ga intermetallic compounds supported on CNT were used in alkynes hydrogenation. Ordered structures form high barriers for subsurface chemistry and prevent large ensembles on the Pd surface19. Also, surface inspection of intermetallic PdGa ( 1 1 1 ) reveals a smooth surface with an (1 × 1) unit cell where no segregation occurs. Co-adsorption properties indicate a bulk-truncated intermetallic compound with Pd-Ga partial covalent bonding20. Prinz et al.7 recently studied the adsorption sites of the 3-fold-symmetric surfaces of the PdGa (111) and ( 1 1 1 ) surfaces, in a combined experimental and computational approach using CO as a 4


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test molecule. Krajčí et al.21 analyzed the activity and selectivity of these two surfaces for the semihydrogenation of acetylene to ethylene using ab-initio DFT based methods. Prinz et al.22 performed a similar study on the same PdGa surfaces, by combining scanning tunneling microscopy, temperature-programmed desorption, and ab-initio catalysis. Krajčí and Hafner23 also performed a detailed DFT study of the hydrogenation of acetylene on the PdGa (210) surface. Furthermore, these authors have studied the semihydrogenation of acetylene on the (010) surface of a related intermetallic compound, GaPd224. Bechthold et al. investigated the H adsorption on PdGa (110), (111), ( 1 1 1 ) and (100) surfaces by DFT methods8,25. In these works, the authors verified the site isolation concept, the fact that gallium acts just as a spacer in the catalyst and they also computed the adsorption sites for hydrogen in the different surfaces. For the semi-hydrogenation process the catalysts should be active and possess high selectivity. A conventional Pd catalyst has good activity but poor selectivity. For that reason we choose the PdGa intermetallic compound. In this intermetallic, the Ga atoms take the role of spacer with a strong Pd-Ga covalent bond. The P213 PdGa structure is close packed exposing preferentially low Miller index surfaces. According to this, and that the mean Pd-Pd distance in (110) plane is almost 5 Å, the Pd sites can be considered fully isolated. All this facts makes the PdGa(110) surface a good choice for the catalytic process under study. The aim of the present work is to analyze the interactions of acetylene on the PdGa (110) surface using DFT calculations, to determine the adsorption sites and to describe changes in the electronic structure and bonding of both the molecule and the substrate after adsorption.

2. Computational Method We performed first-principles calculations based on spin polarized DFT. The VASP code is used to solve Kohn-Sham equations with periodic boundary conditions and a plane wave basis set26-28. Electron-ion interactions were described by ultra-soft pseudopotentials29. We used semi-local 5



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exchange-correlation functional in the generalized gradient approximation (GGA) proposed by Perdew et al.30. The accuracy and shortcomings of several theoretical approximations, including GGA and the most common density functional in materials and surface science -PBE-, has been extensively compared in the case of carbon monoxide and benzene adsorption on transition metals where numerous experimental data are available31-35. In order to assess the accuracy of the He/Mg(100) interaction potential, Martinez-Casado et al. performed a comparison of experimental data, Hartree-Fock, local and hybrid-exchange density functional theory and post-Hartree-Fock ab initio calculations. They found that several functional and Hartree-Fock methods fail to predict one or another properties. Being the more suitable advice to use combinations of methods36. We used a kinetic energy cutoff of 700 eV for all calculations, which converges total energy to ~1 meV/atom and 0.001 Å for the primitive bulk cell. The Monkhorst-Pack scheme is used for k-point sampling37. We used a 7x7x1 k-point mesh for the Brillouin-Zone integration. The geometry optimization was terminated when the Hellman-Feyman force on each atom was less than 0.02 ev/Å and the energy difference was lower than 10-4 eV. Bader analysis as implemented by Tang et al. is used to calculate electronic charges on atoms before and after acetylene adsorption38,39. The adsorption energy is calculated using the following equation:

∆Eads = ETotal (C2 H 2 / PdGa(110)) − ETotal ( PdGa(110)) − ETotal (C2 H 2 )

(1)

Here the first term on the right-hand side is the total energy of the super cell plus one C2H2 molecule; the second term is the total energy of the intermetallic super-cell; and the third term is the C2H2 molecule total energy. The last one is calculated by placing C2H2 in a cubic box with 20 Å sides and carrying out a Γ-point calculation. We obtained for the acetylene molecule a C-C and C-H bond lengths of 1.207 Å and 1.071 Å respectively in fairly good agreement with experimental values40. In order to understand C2H2-PdGa interactions and bonding we used the concepts of Density of States (DOS) and Crystal Orbital Overlap Population (COOP) as described by Hoffmann41. The 6


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COOP curve is a plot of the OP weighted DOS vs. energy. Looking at the COOP, we analyzed the extent to which specific states contribute to a bond between atoms or orbitals. The SIESTA code was used to compute COOPs42, 43.

3. Surface Models The PdGa intermetallic compound presents a P213 structure with a lattice parameter of a0 = 4.909 Å44-46. We selected the (110) crystallographic plane because it is the cleavage plane, and as a low index plane it could be exposed in a catalytic reaction surface. Experimental studies of Verbeek et al. conclude that this plane shows no reconstruction47. We represented the (110) plane with a super-cell. In order to achieve the best compromise between computational time and accuracy of our model, we decided to use a seven-layer slab separated in the [110]-direction by vacuum regions. It should be pointed out that each “layer” is formed by three “sub-layers”, presenting atoms above and below (see Fig. 1). We also tested our calculations with 9 and 11 layers (and the corresponding sub layers) but the total energy values show not significative differences. The thickness of the vacuum region, corresponding to 15 Å, was enough in order to avoid the interaction of acetylene molecule on the surfaces. The thickness of the PdGa(110) slab should be such that it approximates to the electronic structure of 3D bulk PdGa in the innermost layer. The interlayer spacing in this PdGa(110) model is 1.745 Å. The (110) plane presents two possible terminated surfaces, Pd or Ga, but we analyzed only the former because it has better catalytic properties. Our recent calculations support the fact that Ga is a non-active site for H chemisorption25. To study C2H2 adsorption on the PdGa(110) surface at low coverage, the C2H2-surface distance was optimized considering relaxation for the first four layers of the metal slab until 1 meV convergence was obtained in the total energy, and maintaining the three remaining layers fixed (bulk like). Due to the Pd coordination in the bulk structure, almost any exposed plane presents

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isolated Pd sites, being the next Pd neighbor located at a mean distance of 4 Å or more. Figure 1 shows a schematic top and side view of the surface after C2H2 adsorption.

Figure 1: Top and side view of C2H2 adsorption sites and configurations on PdGa(110).

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4. Results and Discussion We analyzed the C2H2-adsorption over the PdGa(110) surface and we found three stable adsorption configurations (see Fig. 1). The most stable configuration results the Hollow site with an adsorption energy of -0.74 eV; however, we reported all of them because the energy difference between the sites is less than 0.14 eV (see Table 1). Medlin and Allendorf also found that the most stable location is a Hollow site for the acetylene adsorption on Pd, Pt, Ni and Rh (111) surfaces at ¼ ML12. Li et al.48 studying the ensemble and ligand effects on the acetylene adsorption on ordered PdxAg1-x /Pd(100) surface alloys investigated by periodic DFT study computed adsorption energies for acetylene on Pd(100) between 0.79 and 2.69 eV depending on the adsorption site at ¼ ML. Recently, Xie et al. reported -for the Pd (111) with a coverage of 1/4 ML- DFT results in the range 0.63 to 1.91 eV, being the most stable adsorption structure the parallel-bridge site49.

Table 1: Adsorption energies, vibrational frequencies and structural properties for PdGa surfaces after and before C2H2 adsorption. For comparison, values of acetylene on vacuum are also included. System Properties

C2H2/PdGa(110) Adsorption Site Hollow Bridge Top Pd -0.74 -0.63 -0.60

Eads (eV) ν (cm-1)

C-H sym C-H asym C-C Pd-C C-C-H angle (°) C-C-Surf. tilt angle (°) C-H (Å) C-C (Å) Pd-C (Å) Ga-C (Å)

3042.3 3004.8 1388.6 473.4 121.7 4.1 1.097 1.350 2.123 2.098

3097.1 3028.4 1504.4 461.7 128.5 9.7 1.093 1.312 2.084 -

3334.0 3269.6 1801.8 317.4 160.1 11.4 1.076 1.247 2.181 -

C2H2 (vacuum)

3446.2 3347.9 2011.7 180.0 1.071 1.207 -

In all considered cases, our results show that the acetylene molecule experiences a geometry distortion during the adsorption process (see Fig. 1). Resulting in a change in the C-C-H angle, an 9



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increase in the C-C bond distance and a small C-C-Surface tilt angle, as detailed in Table 1. The closer height between the molecule and the surface is 1.70 Å, 1.99 Å and 2.73 Å for Hollow, Bridge and Top site respectively. The changes in the C2H2 angles indicate a rehybridization of carbon atoms from sp towards sp2. The most important distortion computed for the molecule corresponds to the Hollow site, being the calculated C–C–H angles approximately 120°, which means that C atoms almost become sp2 hybridized. Kesmodel reported that acetylene is adsorbed non-dissociatively at room temperature on Pd(100), but in a strongly distorted and rehybridized state (∼ sp3) or di-σ-bonded acetylene50. We also found a small C-C-surface tilt angle of 4.1°. The computed height between C2H2 and surface is 1.70 Å. Matczak reported similar values for C–C–H bond angles, C-C-surface angle and height for acetylene adsorbed in a Hollow site (type I) of Pd(100) pure and doped with Sn or Pb10. In the case of Bridge site, the computed acetylene adsorption configuration and energy are in very good agreement with the results reported by Krajčí et al. on the PdGa (210) surface at equivalent site23. These authors reported an energy value of 0.61 eV, a H-C-C bond angle of 132° and a height of 2.11 Å, which are close to our computed values of 0.63 eV, 128.5° and 1.99 Å respectively. We computed a small C-C-surface tilt angle of 9.7°. Medlin and Allendorf also found a C-C-H angle of 128° in the case of C2-H2 adsorbed on fcc Pd and C-C-surface tilt angle of 14°12. Similar results on stepped fcc Pd(422) surface were reported by Belleli et al.19. Prinz et al. performed a combined experimental and theoretical study of the acetylene adsorption on the PdGa(111) surface and they reported that the Pd top site is the most favorable site7. The values of the adsorption energy and bond angle are -0.61 eV and 162° respectively. Those values are in good agreement with our computed values of -0.60 eV and 160° for the top site. Medlin and Allendorf found that the geometry of C2H2 is close to the gas phase on Top site, with an C-C-H angle of 150°12.

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Table 2 presents the computed Bader charges for the surfaces and C2H2 molecule. The C2H2 withdraw charges from the surfaces in all considered adsorption sites. The Pd atom in the hollow site that interacts directly with C atom decreases its charge from -0.884e- to -0.589e-. While Ga atom becomes less positive changing from 0.893e- to 0.656e-. The negative value of work function change, indicates that, in the presence of the acetylene molecules, the electrons are easily extracted from the surface, which also indicates that the adsorption is dominated by donation rather than backdonation effects48,51. Recent theoretical calculations of Xie et al, predict that carbon atoms from acetylene gain most electrons from the Pd(111) surface and H atoms after adsorption49.

TABLE 2: Bader charge for specific atoms on the PdGa(110) surface and isolated and adsorbed species+. Charge (e-)

Atom C2H2 (g)

PdGa(110)

Hollow

Bridge

Top

Pd1

-0.888

-0.828

-0.824

-0.871

Pd2

-0.884

-0.589

-0.891

-0.889

Pd3

-0.910

-0.920

-0.902

-0.588

Pd4

-0.863

-0.868

-0.848

-0.861

Ga1

0.893

0.656

0.818

0.901

Ga2

0.904

0.925

0.952

0.915

Ga3

0.919

0.914

0.944

0.919

C1

-0.002

-

-2.321

-2.412

-1.971

C2

-0.002

-

-2.335

-2.392

-1.966

H1

0.002

-

-0.016

-0.031

-0.001

H2

0.002

-

-0.021

-0.023

-0.001

+

Atoms labels are indicated in Figure 1. 11



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We also computed the stretching vibration frequencies for the adsorbed C2H2. In order to do this, we used a whole vibrational mode that contributes to C2H2 bonding. The results are shown in Table 1. We found C-C bond frequencies of 1388.6 cm-1, 1504.4 cm-1 and 1801.8 cm-1 for the Hollow, Bridge and Top sites respectively which are similar to the values reported by Matezcak for acetylene adsorption on Top Pd sites on Pd(100) pure and doped with Sn or Pb10. This author also reported frequencies of C-H asymmetric and symmetric mode that agree with our computed values of 3042.3 and 3004.8 cm-1, 3097.1 and 3028.4 cm-1 and 3269.6 and 3334.0 cm-1 for the asymmetric and symmetric mode for Hollow, Bridge and Top sites, respectively (see Table 1). The reduction on the C-C bond stretching vibrational frequency, in the Hollow site, is an indication of the significant distortion of the adsorbed molecule. Considering the electronic structure, the projected DOS for C2H2 becomes about 3 eV stabilized after adsorption in all analyzed sites. After the adsorption in the Hollow site, a sharp peak is present at -10 eV as a direct consequence of the interaction between C atom and Pd and Ga nearest neighbors (see Figs. 2 IX-XI and XIII). In the Bridge configuration, the PdGa presents a slightly reduced electron density at the Fermi level with respect to the clean surface (compare Figs. 2 I vs. XV). The total DOS curve shows two additional new sharp peaks at -7 and -9 eV (see Fig. 2 XV). These peaks arise mainly due to the interaction of Pd3 and Pd4 (as indicated in Fig. 1) with the carbon atoms in the acetylene molecule as can be seen in Figs. 2 XVI-XVII and XIX-XX. The Ga projected DOS shows no interaction with the acetylene molecule (see Fig. 2 XVIII). Finally, for the Top site, we found no interaction between Ga atoms and the C2H2 as expected for this configuration (see Fig. 2 XXV). The projected DOS curves reflect the interaction between Pd3 and the adsorbate by the presence of two sharp peaks at -8 and -10 eV (see in Figs. 2 XXII-XXIII and XXVIXXVIII). The total DOS, for the three configurations, presents a sharp peak around -15 eV, that corresponds mainly to carbon orbitals interacting with the surface. 12


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Figure 2: Total and LDOS for Pd, Ga and acetylene on the PdGa surface. (I-IV) Clean surface. (V-VII) Acetylene in vacuum. (VIII-XIV) C2H2 adsorbed in Hollow site. (XV-XXI) C2H2 adsorbed in Bridge site. (XXII-XXVIII) C2H2 adsorbed in Top site. Regarding the bonding, the OP values for the most relevant interactions are summarized in Table 3. Analyzing the changes in the OP, before and after adsorption, we can see which atoms are 13



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involved in the bonding of acetylene to the surface, and which changes are produced by the adsorption of the acetylene in the surface atoms. In the Hollow site, the C-C bond increases its length with respect to the gas phase (1.207 Å, which is similar to the values reported by Matczak10 and Krajčí et al.23) and its OP by 33.2% (see Table 3). At the same time a sp2 like hybridization appears. This fact could be useful to semihydrogenate the molecule without breaking the C-C bond and, consequently, making more difficult the appearance of carbon deposits. The increase in the OP value for C-C bond is also present in the Bridge and Top sites. It should be mentioned that the only site where a Ga-C interaction is detected is the Hollow site (see Fig. 2 XI). There is almost no change in the C-H bond in all considered sites (about 0.5% in OP). There is an e- transfer from Pd s and d orbitals to C 2p and H 1s orbital and some little backdonation to Pd p orbitals. In the case of the Hollow site, there is also a change in the Ga 4p orbitals, which increase from 0.38e- to 0.74e-. The Pd-C and Ga-C bonds are developed at expenses of Pd1-Pd2, Pd1-Ga1 and Pd2-Ga2 bonds that decrease by 57.6%, 76.7% and 32.0% respectively. The C-C COOP curves show that this interaction is always bonding in the considered energy range. In the Hollow site, there is a net Pd-C bond at -4 eV (see Fig. 3 III) and a decrease in the Pd-Pd and Pd-Ga bonding areas (compare dashed red line with solid black line before and after adsorption respectively in Figs. 3 I and II). The Ga-C bond also has shown an important bonding interaction (see Fig. 3 III dashed line). In the case of Bridge configuration, we computed a weakening for the Pd3-Pd4, Pd4-Ga3, and Pd3-Ga1 bonds, while for the Top configuration the most affected bonds are Pd3-Pd4 and Pd3-Ga1. Furthemore, this behavior is present in the COOP curves for these bonds, where the reduction in the number of bonding states is noticeable (see Figs. 3 VI-VII and XI-XII).

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TABLE 3: Electron orbital occupation, overlap population (OP), ∆OP% and distances for PdGa(110) before and after C2H2 adsorption (only the minimum adsorption site is summarized). Values of acetylene on vacuum are also included. Structure

Electronic occupation s p d

Bond type

OP

∆OP% Distances (Å)

1.01 1.00

1.00 0.00

0.00 0.00

C1-C2 C1-H1

0.968 0.873

1.207 1.071

0.74 0.79 1.73 1.76

0.36 0.32 0.38 0.33

9.79 9.81 0.00 0.00

Pd1-Pd2 Pd3-Pd4 Pd1-Ga1 Pd2-Ga2

0.111 0.111 0.193 0.178

3.010 3.010 2.544 2.516

0.62 0.48 1.76 1.60 1.06 1.02

0.48 0.38 0.32 0.74 3.26 0.00

9.74 9.72 0.00 0.00 0.00 0.00

Pd1-Pd2 Pd3-Pd4 Pd1-Ga1 Pd2-Ga2 C1-C2 C1-H1 Pd1-C1 Pd2-C2 Ga1-C1

0.047 0.107 0.045 0.121 1.289 0.869 0.178 0.569 0.432

C2-H2 (vacuum) C H PdGa(110) Pd1 Pd2 Ga1 Ga2 C2H2 Hollow site Pd1 Pd2 Ga1 Ga2 C H

-57.6 -3.6 -76.7 -32.0 +33.2 -0.5

2.903 3.043 2.735 2.601 1.350 1.097 2.307 2.123 2.098

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Figure 3: COOP curves for Pd-Pd, Pd-Ga, Pd-C, Ga-C (only for Hollow site), C-C and C-H for PdGa(110) surface with acetylene on Hollow (I-V), Bridge (VI-X) and Top sites (XI-XV) before (red dashed line) and after (solid black line) adsorption. It is important to study what’s happens after acetylene adsorption. Certainly, coking by breaking C-C bonds is one of the reactions that can take place over the surface and frequently produce deactivation. The other main reaction is partial hydrogenation and C-H bond formation. The relative rates of these two processes would be highly indicative of which way the catalysis would go. Starting at the most stable acetylene adsorption site (see Fig. 1 I, hollow site) we simulate the C-C bond breaking to a final state were two C-H species results more stabilized and bonded to Pd top sites. The C-C bond breaking is a non favorable process on PdGa(110). As can be seem in Fig. 4. The final state is less stable than the initial state with two activation barriers of about 6.5 eV 16


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and 2.2 eV. This results are in line with experimental results from Kesmodel50. The author found a non-dissociative adsorption of C2H2 at room temperature on fcc Pd(100) surface. In the case of fcc Ni(100) methylidyne (C-H) and ethynyl (C-C-H) species are founded at room temperature50.

Figure 4: C-C bond breaking total energy curve of a function of C-C distances. Vattuone et al. reported the energetics and kinetics of the interaction of acetylene and ethylene on Pd(100) and Ni(100) and compared it with others surfaces of Cu, Ni, Pd, Fe and Pt. They concluded that acetylene do not decompose on Pd(100) at room temperature as happens on Ni(100). In the case of PdGa(110) the Pd atoms are really isolated (Pd-Pd distances is approximate 5 Å ) with Ga atoms acting as spacer making almost impossible the C-C scission11. Medlin and Allendorf addressed theoretically the differences in acetylene binding on the (111) surface planes of Pd, Pt, Ni, and Rh. Among this four metals, the main differences are found on Ni(111), where acetylene is found to be strongly adsorbed12. The predicted stability in the C-C bond drives us to continue considering the partial hydrogenation reaction to C2H4. Krajčí and Hafner

23

performed a detailed DFT study on the

PdGa(210) surface. In the PdGa intermetallic compound this surface is isomorphic to PdGa(110), so we decided to follow the same reaction model as previously reported. 17



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Fig. 5 I-II shows a simplified scheme for the hydrogenation of C2H2 to CH2-CH- as determined by nudged elastic band method53. In agreement with previous results23 we found after H attack the C-H bond rotates around the C-C axis and the C atoms move towards the Pd atom (see Fig. 5 II). The activation energy for the first hydrogenation reaction is 0.70 eV and the final state more stable than the co-adsorbed state (C2H2+H2)(see Fig. 6). Then a second H approach to the CH2-CH- fragment with a migration barrier of 0.20 eV (see Fig 5 III). The last hydrogenation present an activation barrier of 0.75 eV and the final C2H4 adsorbed state (see Fig. 5 IV) is 2.73 eV more stable than the C2H2(g) +H2(g) with a desorption energy of only 0.46 eV (relative to C2H4(g)). The overall process is strongly exothermic (2.27 eV) (see Fig. 6). According to Krajčí and Hafner 23

desorption of C2H4 is favored over further hydrogenation, as is expected for a selective catalyst.

Figure 5: Atomistic scenario for the hydrogenation of acetylene to ethylene in the case of the C2H2 adsorbed in the hollow site: (I) initial state with co-adsorbed acetylene and atomic hydrogen, (II) hydrogenation a acetylene to vinyl, (III) co-adsorbed vinyl and atomic hydrogen, and (IV) hydrogenation of vinyl to ethylene. 18


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Figure 6: Potential energy profile for the reaction steps of the hydrogenation of acetylene to ethylene. The energies are given relative to the (C2H2+H2) in the gas phase.

5. Conclusion The electronic structure of acetylene on P213 PdGa intermetallic compound has been studied by DFT calculations. Only three stable adsorption sites were found on the PdGa(110) surface. The C2H2 adsorption energies are 0.74 eV, 0.63 eV and 0.60 eV for Hollow, Bridge and Top sites respectively. These adsorption energies are lower than those reported for fcc Pd. The DOS plots 19



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indicate orbital stabilization after C2H2 adsorption in all considered sites. In addition, a strong distortion of adsorbed acetylene molecule is present in the three configurations. In the Hollow site the DOS curves show a decrease in Pd electron density and a small interaction Ga-C. A strong rehybridization is detected while C-C-H bond angle decreases to 121° (from 180°). The acetylene molecule withdraws charge from the surface indicating that donation rather than backdonation effects dominates the adsorption. The reduction of the C-C bond stretching vibrational frequency is also an indication of the significant distortion of the adsorbed molecule. The orbital electron occupation and OP analysis have detected an increase in the C-C OP (about 33%) at expenses of Pd-Pd and Pd-Ga overlap. Pd-C and Ga-C bonds are also detected after C2H2 adsorption and almost no change in the bond length and OP is detected for C-H bond. The strong sp2 rehybridization and the increase in the OP value for C-C bond indicate that the semihydrogenation process could be more favorable on PdGa than on fcc Pd. The adsorption of acetylene on PdGa(110) surface is markedly different from other transition metal surfaces. This behavior is related with Pd isolates sites present in the intermetallic compounds. Our reaction energy calculations explain that C-C bond breaking on the surface is unfavorable while semi-hydrogenation is a strongly exothermic process. These results explain the low carbon deposits found experimentally in PdGa intermetallics compound and the good selectivity reported for this catalyst.

6. Acknowledgements We acknowledge useful suggestions from the reviewers. Our work was supported by ANPCyT through PICT 2012-2186, PICT 2012-1280 and PICT 2014-1351, PME 2006-311 and PME 2003-8, PIP-CONICET No.112-201301-00436, 2011 0278, and UNL PI N° 501201101 00311 research grants, as well as by SGCyT-UNS and CIC-BA. E.A.G., G.B., A.B. and P.V.J. are members of CONICET. P.B. is fellow researcher at this institution and M.S. is ANPCyT fellow researcher.

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Note The authors declare no competing financial interests.

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Captions for Figures Figure 1: Top and side view of C2H2 adsorption sites on PdGa(110). Figure 2: Total and LDOS for Pd, Ga and acetylene on the PdGa surface. (I-IV) Clean surface. (VVII) Acetylene in vacuum. (VIII-XIV) C2H2 adsorbed in Hollow site. (XV-XXI) C2H2 adsorbed in Bridge site. (XXII-XXVIII) C2H2 adsorbed in Top site. Figure 3: COOP curves for Pd-Pd, Pd-Ga, Pd-C, Ga-C (only for Hollow site), C-C and C-H for PdGa(110) surface with acetylene on Hollow (I-V), Bridge (VI-X) and Top sites (XI-XV) before (red dashed line) and after (solid black line) adsorption. Figure 4: C-C bond breaking total energy curve of a function of C-C distances. Figure 5: Atomistic scenario for the hydrogenation of acetylene to ethylene in the case of the C2H2 adsorbed in the hollow site: (I) initial state with co-adsorbed acetylene and atomic hydrogen, (II) hydrogenation a acetylene to vinyl, (III) co-adsorbed vinyl and atomic hydrogen, and (IV) hydrogenation of vinyl to ethylene. Figure 6: Potential energy profile for the reaction steps of the hydrogenation of acetylene to ethylene. The energies are given relative to the (C2H2+H2) in the gas phase.

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T O C

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The Electronic Structure and Bonding of Acetylene on PdGa(110

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