- Email: [email protected]
Hydrogenation of ethene catalyzed by Ir atom deposited on γ-Al2O3(001) surface: From ab initio calculations
Physics Letters A 376 (2012) 1919–1923
Contents lists available at SciVerse ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
Hydrogenation of ethene catalyzed by Ir atom deposited on From ab initio calculations
γ -Al2 O3(001) surface:
Yongchang Chen b , Zhaolin Sun a,b , Lijuan Song a,∗ , Qiang Li a , Ming Xu a a b
Liaoning Key Laboratory of Petrochemical Engineering, Liaoning ShiHua University, Fushun, Liaoning 113001, China School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 19 March 2012 Accepted 19 April 2012 Available online 25 April 2012 Communicated by R. Wu Keywords: Ethene hydrogenation Iridium Ab initio Nudged elastic band
a b s t r a c t Ethene hydrogenation reaction, catalyzed by an iridium atom adsorbed on γ -Al2 O3 (001) surface, is studied via ab initio calculations based on density functional theory (DFT). The catalyzed reaction process and activation energy are compared with the counterparts of a reaction occurs in vacuum condition. It is found that the activation energy barrier is substantially lowered by the adsorbed Ir atom on the γ Al2 O3 (001). The catalyzed reaction is modeled in two steps: (1) Hydrogen molecular dissolution and then bonded with C2 H4 molecular. (2) Desorption of the C2 H6 molecular from the surface. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The developments of chemical industries are always closely related with the development of catalysts. Catalyzed chemical reactions at solid surfaces are studied intensively in the past decades. Many techniques from surface physics have been used to study the elementary processes involved in heterogeneous catalysis [1]. Recently, supported metal atoms or clusters become more and more important in heterogeneous catalysis, because of their intrinsic different physical properties from bulk metal or metal surfaces [2]. The catalytic efficiency of supported metal clusters as catalyst is related with not only the size, geometry and distribution of the metal cluster, the support itself also play important role in the catalysis reaction [3]. Among various oxides as catalyst supports, γ -Al2 O3 is studied extensively because of its application as both a catalyst and catalyst support [4,5]. γ -Al2 O3 possesses a defective spinel structure [6], while certain tetragonal distortion is observed when samples are prepared with different preparation conditions [7–10]. In the defective spinel structure, vacancies at octahedral Al sites are required in order to satisfy the stoichiometry of Al2 O3 [11–13]. As a catalyst or catalyst support, the surface properties of γ -Al2 O3 have also been studied by many researchers [14,15]. Recently, we have show that a dense Al–O(001) layer, containing both octahedral aluminum and oxygen atoms, are stable at various external
*
Corresponding author. Tel.: +86 413 6860658; fax: +86 413 6860658. E-mail addresses: [email protected] (Z. Sun), [email protected] (L. Song). 0375-9601/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physleta.2012.04.041
O2 pressures and temperature ranges [16]. With this stable surface model, we also calculated the binding energies and electronic structures of Ir atom deposited on the γ -Al2 O3 (001) surface [17], and the results show that top sites of O atoms are energetically most favorable sites for Ir adsorption. As a continuous work of our previous studies, in the current Letter the catalytic effect of γ -Al2 O3 supported iridium atoms are investigated by means of ab initio calculations within the DFT. Catalytic reactions are always very complicated and an accurate knowledge of the reaction mechanisms is usually difficult to be obtained experimentally. To the best of our knowledge, a general mechanism on the catalysis of the supported metal clusters is still not available in literature. Generally, most catalysts for hydrogenation reaction contain transition or noble metals [18,19]. The catalyzed hydrogenation reaction of C2 H4 to C2 H6 is simple but industrially very relevant. However, the catalytic mechanism of this simple reaction is still unclear from the atomic level. Recently, Chan and Radom [20] studied the zeolite-catalyzed hydrogenation of ethene from density functional theory, from which they proposed a three stage reaction mechanism. However, the details on atomic movement in the reaction process were not revealed. In the present work, the ethene hydrogenation reaction catalyzed by Ir atom adsorbed on the γ -Al2 O3 (001) is studied in details. 2. Computational details As a continuous work of our previous studies, the structural models of γ -Al2 O3 (001) and Ir adsorption on the surface are constructed according to our previous results. Please refer to Refs. [16,17] for details. An eleven layer slab model with thick Al–O
1920
Y. Chen et al. / Physics Letters A 376 (2012) 1919–1923
Fig. 2. Reaction energy barrier of the hydrogenation of ethene in vacuum condition. The initial, transition and final states are shown in this figure, and the grey (large) and white (small) spheres present C and H atoms, respectively.
Fig. 1. Schematic view of the relaxed structure of the bare γ -Al2 O3 (001) slab model. The red (middle sized), purples (small) and grey (largest) spheres present O atoms, Al atoms, and octahedral vacancies, respectively. AlT and AlO denotes tetrahedral and octahedral Al atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.)
layers exposed on both side of the slab has been constructed, as shown in Fig. 1. The stoichiometry of the slab is restricted to Al2 O3 , with 2 octahedral site vacancies in the middle of the slab. An Ir atom is adsorbed on the top of a surface O atom, which was shown to be energetically most favorable in Ref. [17]. All calculations are performed with DACAPO code [21,22], which is an ab initio simulation package based on the DFT and the plane-wave pseudopotential method [23]. The exchange and correlation energies were treated via the spin-polarized generalized gradient approximation (GGA) with the Perdew–Wang (PW91) exchange correlation functional [24,25]. The valence electron wave functions and the augmented electron density are expanded in the plane-wave basis sets with cutoff energies of 25 and 40 Ry, respectively. The Monkhorst–Pack [26] type 3 × 3 × 1 k-points mesh is used for sampling in the irreducible Brillouin zone. For relaxation of ethene and ethane molecule adsorbed at the Ir atom on the surface, atoms in the middle 3 layers were fixed while all the other atoms were fully relaxed until the final forces on all the relaxed atoms were smaller than 0.05 eV/Å. The ethene hydrogenation reaction on the iridium adsorbed γ -Al2 O3 (001) surfaces was simulated by using the NEB method [27,28]. 3. Results and discussions For comparison purpose, the hydrogenation reaction of ethene (C2 H4 ) in a vacuum was simulated, which is taken as reference for a non-catalytic reaction. The initial state was constructed as one ethene molecule and one separated hydrogen molecule (H2 ). The distance between them is far enough to ensure that the interaction between them is negligible. The final state is a single ethane (C2 H6 ) molecule. The total energy of the final state is about 1.8 eV lower than that of the initial state, indicating that the hydrogena-
tion reaction is thermodynamically favorable. As described in Section 2, the simulation was performed by using the NEB method. Fig. 2 shows the total energy changes of the gas phase system along the optimized reaction path. The reaction reaches its transition state when the H2 bond is broken, with one H atom bonds with one C atom while the other remains single. The evaluated energy barrier for hydrogenation of ethene in vacuum is 3.99 eV, which is quite large and the reaction is difficult to happen even under high temperature. A catalytic reaction is required to realize the reaction of hydrogenation of ethane. In the following context, we show how Ir adsorbed at γ -Al2 O3 (001) catalyzes this reaction. In order to simulate the catalytic behavior of the γ -Al2 O3 (001) supported Ir atoms with the NEB method, reasonable initial and final states should be built and optimized. The final state is simple (see Fig. 3c), which is defined as an ethane molecule in the vacuum layer of the slab. The Ir atom is located at most favorable site on the γ -Al2 O3 (001) surface. In our previous study, we have shown that the energetically most favorable sites are the top sites of the O atoms at the dense Al–O layer of the γ -Al2 O3 (001) surface [17]. The ethane molecule is far away from the surface, which makes sure that the interaction between them is negligible. For the initial state, an ethene molecule was first put on the top of the Ir atom. Different geometries of the molecule have been tried and the results show that the system is energetically most favorable when the ethene molecule plane is horizontal and parallel to the surface plane (see Fig. 3a). In the following context, we call it as “π -bonded” initial state just for simple description of the geometry. One H2 molecule was then put in the vacuum of the slab and located far enough from the surface to ensure that the interaction between the H2 molecule and the surface is small. Due to the small surface area of our model, we did not choose the initial H2 molecule geometry by adsorbing on the Al2 O3 surface. As the ethene molecule is initially “π -bonded” to the Ir atom and the energy of the system is most favorable, it can be understood that the product of the hydrogenation reaction could also have strong interaction with the Ir atom and the system energy is low. Therefore, we put an ethane molecule at the surface with the same way of putting the ethene molecule on the surface. Relaxation of the system with an ethane molecule on the top of the Ir atom shows that the C–C bond breaks and the resulted two methyl groups are bonded with the Ir atom, as shown in Fig. 3b. The energy of the relaxed system is even about 0.2 eV lower than the initial state, and therefore we choose it as the middle state of the Ir catalyzed process of the ethene hydrogenation reaction. This assumption is consistent with Argo et al.’s proposal on the
Y. Chen et al. / Physics Letters A 376 (2012) 1919–1923
1921
Fig. 3. Initial (a), middle (b), and final (c) states of the ethene hydrogenation reaction catalyzed by an Ir atom adsorbed on the γ -Al2 O3 (001) surface. The white (small), grey (middle sized), blue (large) spheres on the surface are H, C, and Ir atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.) Table 1 Charge and Magnetic Moments (MM) of the adsorbed Ir atom at different states.
Total charge Majority Minority MM (μ B ) a b c d e f g
a
Isob
Slabc
Inid
P1-4e
P1-6
P1-8
Midf
P2-4
P2-7
Fing
7.60 5.08 2.52 2.56
8.38 4.66 3.73 0.93
8.84 4.88 3.96 0.92
9.33 4.76 4.54 0.24
9.34 4.78 4.56 0.21
9.13 4.77 4.36 0.41
8.71 4.79 3.92 0.87
8.61 4.76 3.85 0.91
8.59 4.75 3.84 0.91
8.36 4.64 3.72 0.93
Obtained by integrating the charge density around the Ir atom within a sphere (radius: 1.5 Å). Isolated Ir atom in vacuum. Ir adsorbed on the γ -Al2 O3 (001) surface. Initial state, as shown in Fig. 3a. P1-4 refers to image No. 4 (transition state in reaction 1) in reaction path P1 (see Fig. 4). P1-6, P1-8, P2-4 and P2-7 are denoted in the same way. Middle state, as shown in Fig. 3b. Final state, as shown in Fig. 3c.
hydrogenation of propene catalyzed by MgO supported Ir4 clusters based on EXAEF and IR experiments [3]. The reason that the ethane molecule on the top of the Ir atom breaks down into two methyl groups is strongly related with the charge distributions around the Ir atom as discussed in our previous paper [17]. Extra charge around the Ir atom in the horizontal plane was observed and the energy levels of those electrons are very close to the Fermi level. To lower the energy of the system, the C–C bond breaks and two C–Ir bonds are formed, which lowers the energy level near the Fermi level of those Ir-5d electrons. As the two C atoms are also bonded with Ir, the energy levels of the C-2p do not change much after the C–C bond is broken. The charge analysis was given in Table 1, from which we can see that a small adjustment of the charge distribution around the Ir atom is occurred after the C–C bond is broken, resulting in a small change of the magnetic moments. To reach the final state defined above, association and desorption processes must occur, in which the methyl groups break from the Ir atom and reconstruct to an ethane molecule. Then the ethane molecule diffuses away from the surface. Based on the above discussion, the reaction was simulated in two steps: hydrogenation of the ethene (reaction from the initial state to the middle state shown in Fig. 3, described as P1) and desorption of the hydrogenation product (reaction from the middle state to the final state shown in Fig. 3, described as P2). The reaction path and the activation energy barrier of both steps were calculated with the NEB method. Fig. 4 shows the energy changes along the optimized reaction path for the hydrogenation of ethene (reaction P1) catalyzed by an adsorbed Ir atom on the γ -Al2 O3 (001) surface. As discussed above, the energy of the system at the middle state is about 0.2 eV lower than that of the initial state, implying that the reaction is thermodynamically favorable. The energy barrier of this reaction is ∼ 2 eV, which is about half of the energy barrier for the reaction takes place in vacuum (see Fig. 2). The optimized reaction path is very interesting, along which a local minimum (P1-IMG-6
Fig. 4. Reaction energy barrier of the hydrogenation of ethene (P1) catalyzed by an Ir atom adsorbed on the γ -Al2 O3 (001) surface. Three important images (IMG-4, IMG-6, and IMG-8) are also shown in this figure, and the color and size scheme is the same as in Fig. 3.
in Fig. 4) was found. With the H2 molecule moving down to the surface and approaching to the ethene molecule that was initially “π -bonded” to the Ir atom, the C–C bond length of the ethane molecule becomes longer, which increases the energy of the system and reaches a maximum at P1-IMG-4. The initial C–C distance is about 1.43 Å, and becomes 2.24 Å at the P1-IMG-4 where the C–C bond is already broken. The Ir–C covalent bond is then enhanced by the increase of the shared charge between the Ir and C atoms, resulting in a larger value of the total charge around the Ir atom as shown in Table 1. Interestingly, those enhanced charges are located in the minority spin channel, leading to an even small magnetic moment of the Ir atom. At this stage, the orientation of the H2 molecule is changed, but the H–H bond length remains unchanged.
1922
Y. Chen et al. / Physics Letters A 376 (2012) 1919–1923
length of ∼ 1.52 Å, only ∼ 0.1 Å larger than the bond length of a free ethane molecule. The Ir–C bond is weakened further with the C–C bond forming, which can also be seen from the total charge listed in Table 1. From the local minimum at the P2-IMG-7, the C atom bonded to the Ir atom moves gradually away along the reaction path, and finally the whole molecule is desorbed from the surface. 4. Summary and conclusions
Fig. 5. Optimized reaction path and energy barrier of desorption of ethane molecule (P2) from the Ir atom. Images numbers 4 and 7 (IMG-4 and IMG-7) used in the NEB calculations are also shown in the figure, and the color and size scheme is the same as in Fig. 3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.)
Further rotation of the H2 molecule, then, reduces the energy of the system, forming a local minimum along the reaction path (see P1-IMG-6), but the charge distribution around the Ir atom keeps constant (see Table 1). When the H2 molecule is almost parallel to the surface plane, the rotation stops and the H–H bond length starts to increase, which increases the energy of the system and reaches a maximum at P1-IMG-8 where the H–H bond is almost broken. The distance between the two H atoms in P1IMG-8 is ∼ 0.94 Å, about 0.2 Å longer than the bond length of free H2 molecular. With the H–H bond broken, the H atoms move to the nearby C atoms and two C–H bonds start to form, which lowers the total energy and the system becomes to a stable state, i.e., the middle state as mentioned above. At the middle state, the strength of the Ir–C bonds is slightly weakened by the decrease of the amount of shared charge between the Ir and C atoms (see Table 1). As expected, those reduced charge is from the minority spin channel, resulting in an increase of the magnetic moment of the Ir atom. Fig. 5 describes the energy changes along the reaction path of the association and desorption of the ethane molecule from the γ -Al2 O3 (001) surface, i.e., the reaction path of P2 from middle state (Fig. 3b) to the final state (Fig. 3c). The total energy of the final state is about 1.35 eV higher than that of the middle state, suggesting that the desorption process is an endothermic reaction. Here we mention that although P2 is endothermic, the whole reaction (C2 H4 and H2 to C2 H6 in vacuum) is exothermic, because the binding energy of C2 H4 on the surface is about −2.9 eV. To ensure the reaction happen, two conditions are necessary: (1) the resulting ethane molecules can diffuse away in time; and (2) the system is heated up. As shown in Fig. 5, the desorption process is rather complicated. Starting from the middle state (the starting image in this process), the two C atoms start to move away from the surface and get close to each other, resulting in an increase of the system total energy. The total energy reaches a maximum at P2-IMG-4 where the C–C distance is 2.39 Å, ∼ 0.34 Å smaller than that at the middle state. At the same time, the Ir atom also moves away from the surface and the Ir–O distance changes from 2.21 to 2.45 Å. Then, one C moves further up and the C–C distance becoming even smaller. The C–C bond starts to form, leading to a reduction of the energy of the system that reaches a local minimum at P2-IMG-7, in which only one C atom bonds with the Ir atom. However, the two C atoms are now strongly bonded with a bond
In summary, although with simple model, our results show that the ethene hydrogenation reaction can be catalyzed by even one isolated Ir atom adsorbed on the γ -Al2 O3 (001) surface. The adsorbed Ir atom triggers the ethene hydrogenation reaction in two major steps, i.e., ethene hydrogenation and desorption of the hydrogenation product. The ethene molecular is first binding to the Ir atom, and then the H2 molecule moves approach to the ethene. This process weakens the C–C and H–H interactions and gradually increases the energy of the system. Then, the system energy is lowered through forming C-H bonds. Therefore, the energy barrier of this process is mainly originated from the weakening or breaking down of C–C and H–H bonds. The obtained energy barrier is about 2 eV, which is still quite high in real application. The reason mainly lies in our simple model. In real cases, H2 molecule are first disassociated and adsorbed at the substrate or Ir clusters and thus the contribution from disassociation of H2 to the energy barrier in our simulation can be further lowered. Through the whole process, the strength of Ir–C bonds is changed time to time to adjust the charge equilibrium of the system. Then re-association of the two CH3 groups that are bonded to Ir atom starts to occur when the two C atoms move away from the Ir atom and close to each other. This process lowers the interaction between Ir and C atoms and thus increases the system energy. Then one C is escaped from the Ir atom, which strengthens the C–C interaction and thus decreases the system energy. Then the C2 H6 moves away from the surface by breaking down the remaining C–Ir bonding. This process needs overcome an energy barrier of about 1 eV. Acknowledgements The authors acknowledge the financial support from the Ministry of Science and Technology of the People’s Republic of China under the National Basic Research Program of China (973 Program) (2007CB216403). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
G. Ertl, Adv. Catal. 45 (2000) 1. B.C. Gates, J. Mol. Catal. A: Chem. 163 (2000) 55. A.M. Argo, J.F. Odzak, F.S. Lai, B.C. Gates, Nature 415 (2002) 623. J. van den Brand, P.C. Snijders, W.G. Sloof, H. Terryn, J.H.W. Wit, J. Phys. Chem. B 108 (2004) 6017. B.C. Gates, J.R. Katzer, G.A. Schuit, Chemistry of Catalytic Processes, McGraw– Hill, New York, 1979. R.S. Zhou, R.L. Snyder, Acta Crystallogr. B 47 (1991) 617. G. Paglia, C.E. Buckley, A.L. Rohl, B.A. Hunter, R.D. Hart, J.V. Hanna, L.T. Byrne, Phys. Rev. B 68 (2003) 144110. E.M. Proupin, G. Gutiérrez, Phys. Rev. B 72 (2005) 035116. M. Sun, A.E. Nelson, J. Adjaye, J. Phys. Chem. B 110 (2006) 2310. G. Paglia, C.E. Nuckley, A.L. Rohl, J. Phys. Chem. B 110 (2006) 20721. M.H. Lee, C.F. Cheng, V. Heine, J. Klinowski, Chem. Phys. Lett. 265 (1997) 673. C. Wolverton, K.C. Hass, Phys. Rev. B 63 (2001) 024102. G. Gutiérrez, A. Taga, B. Johansson, Phys. Rev. B 65 (2002) 012101. A. Vijay, G. Mills, H. Metiu, J. Chem. Phys. 117 (2002) 4509. H.P. Pinto, R.M. Nieminen, S.D. Elliott, Phys. Rev. B 70 (2004) 125402. C.Y. Ouyang, Z. Sljivancanin, A. Baldereschi, Phys. Rev. B 79 (2009) 235410. Y.C. Chen, C.Y. Ouyang, S.Q. Shi, Z.L. Sun, L. Song, Phys. Lett. A 373 (2009) 277. L.P. Nielsen, F. Besenbacher, E. Lægsgaard, I. Stensgaard, Phys. Rev. B 44 (1991) 13156.
Y. Chen et al. / Physics Letters A 376 (2012) 1919–1923
[19] D. Teschner, J. Borsodi, A. Wootsch, Z. Révay, M. Hävecker, A. Knop-Gericke, S.D. Jackson, R. Schlögl, Science 320 (2008) 86. [20] B. Chan, L. Radom, J. Am. Chem. Soc. 130 (2008) 9790. [21] B. Hammer, L.B. Hansen, J.K. Norskov, Phys. Rev. B 59 (1999) 7413. [22] https://wiki.fysik.dtu.dk/dacapo. [23] D. Vanderbilt, Phys. Rev. B 41 (1990) R7892.
1923
[24] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671. [25] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13224. [26] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. [27] G. Henkelman, H. Jonsson, J. Chem. Phys. 113 (2000) 9978. [28] G. Henkelman, B.P. Uberuaga, H. Jonsson, J. Chem. Phys. 113 (2000) 9901.
Contents lists available at SciVerse ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
Hydrogenation of ethene catalyzed by Ir atom deposited on From ab initio calculations
γ -Al2 O3(001) surface:
Yongchang Chen b , Zhaolin Sun a,b , Lijuan Song a,∗ , Qiang Li a , Ming Xu a a b
Liaoning Key Laboratory of Petrochemical Engineering, Liaoning ShiHua University, Fushun, Liaoning 113001, China School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 19 March 2012 Accepted 19 April 2012 Available online 25 April 2012 Communicated by R. Wu Keywords: Ethene hydrogenation Iridium Ab initio Nudged elastic band
a b s t r a c t Ethene hydrogenation reaction, catalyzed by an iridium atom adsorbed on γ -Al2 O3 (001) surface, is studied via ab initio calculations based on density functional theory (DFT). The catalyzed reaction process and activation energy are compared with the counterparts of a reaction occurs in vacuum condition. It is found that the activation energy barrier is substantially lowered by the adsorbed Ir atom on the γ Al2 O3 (001). The catalyzed reaction is modeled in two steps: (1) Hydrogen molecular dissolution and then bonded with C2 H4 molecular. (2) Desorption of the C2 H6 molecular from the surface. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The developments of chemical industries are always closely related with the development of catalysts. Catalyzed chemical reactions at solid surfaces are studied intensively in the past decades. Many techniques from surface physics have been used to study the elementary processes involved in heterogeneous catalysis [1]. Recently, supported metal atoms or clusters become more and more important in heterogeneous catalysis, because of their intrinsic different physical properties from bulk metal or metal surfaces [2]. The catalytic efficiency of supported metal clusters as catalyst is related with not only the size, geometry and distribution of the metal cluster, the support itself also play important role in the catalysis reaction [3]. Among various oxides as catalyst supports, γ -Al2 O3 is studied extensively because of its application as both a catalyst and catalyst support [4,5]. γ -Al2 O3 possesses a defective spinel structure [6], while certain tetragonal distortion is observed when samples are prepared with different preparation conditions [7–10]. In the defective spinel structure, vacancies at octahedral Al sites are required in order to satisfy the stoichiometry of Al2 O3 [11–13]. As a catalyst or catalyst support, the surface properties of γ -Al2 O3 have also been studied by many researchers [14,15]. Recently, we have show that a dense Al–O(001) layer, containing both octahedral aluminum and oxygen atoms, are stable at various external
*
Corresponding author. Tel.: +86 413 6860658; fax: +86 413 6860658. E-mail addresses: [email protected] (Z. Sun), [email protected] (L. Song). 0375-9601/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physleta.2012.04.041
O2 pressures and temperature ranges [16]. With this stable surface model, we also calculated the binding energies and electronic structures of Ir atom deposited on the γ -Al2 O3 (001) surface [17], and the results show that top sites of O atoms are energetically most favorable sites for Ir adsorption. As a continuous work of our previous studies, in the current Letter the catalytic effect of γ -Al2 O3 supported iridium atoms are investigated by means of ab initio calculations within the DFT. Catalytic reactions are always very complicated and an accurate knowledge of the reaction mechanisms is usually difficult to be obtained experimentally. To the best of our knowledge, a general mechanism on the catalysis of the supported metal clusters is still not available in literature. Generally, most catalysts for hydrogenation reaction contain transition or noble metals [18,19]. The catalyzed hydrogenation reaction of C2 H4 to C2 H6 is simple but industrially very relevant. However, the catalytic mechanism of this simple reaction is still unclear from the atomic level. Recently, Chan and Radom [20] studied the zeolite-catalyzed hydrogenation of ethene from density functional theory, from which they proposed a three stage reaction mechanism. However, the details on atomic movement in the reaction process were not revealed. In the present work, the ethene hydrogenation reaction catalyzed by Ir atom adsorbed on the γ -Al2 O3 (001) is studied in details. 2. Computational details As a continuous work of our previous studies, the structural models of γ -Al2 O3 (001) and Ir adsorption on the surface are constructed according to our previous results. Please refer to Refs. [16,17] for details. An eleven layer slab model with thick Al–O
1920
Y. Chen et al. / Physics Letters A 376 (2012) 1919–1923
Fig. 2. Reaction energy barrier of the hydrogenation of ethene in vacuum condition. The initial, transition and final states are shown in this figure, and the grey (large) and white (small) spheres present C and H atoms, respectively.
Fig. 1. Schematic view of the relaxed structure of the bare γ -Al2 O3 (001) slab model. The red (middle sized), purples (small) and grey (largest) spheres present O atoms, Al atoms, and octahedral vacancies, respectively. AlT and AlO denotes tetrahedral and octahedral Al atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.)
layers exposed on both side of the slab has been constructed, as shown in Fig. 1. The stoichiometry of the slab is restricted to Al2 O3 , with 2 octahedral site vacancies in the middle of the slab. An Ir atom is adsorbed on the top of a surface O atom, which was shown to be energetically most favorable in Ref. [17]. All calculations are performed with DACAPO code [21,22], which is an ab initio simulation package based on the DFT and the plane-wave pseudopotential method [23]. The exchange and correlation energies were treated via the spin-polarized generalized gradient approximation (GGA) with the Perdew–Wang (PW91) exchange correlation functional [24,25]. The valence electron wave functions and the augmented electron density are expanded in the plane-wave basis sets with cutoff energies of 25 and 40 Ry, respectively. The Monkhorst–Pack [26] type 3 × 3 × 1 k-points mesh is used for sampling in the irreducible Brillouin zone. For relaxation of ethene and ethane molecule adsorbed at the Ir atom on the surface, atoms in the middle 3 layers were fixed while all the other atoms were fully relaxed until the final forces on all the relaxed atoms were smaller than 0.05 eV/Å. The ethene hydrogenation reaction on the iridium adsorbed γ -Al2 O3 (001) surfaces was simulated by using the NEB method [27,28]. 3. Results and discussions For comparison purpose, the hydrogenation reaction of ethene (C2 H4 ) in a vacuum was simulated, which is taken as reference for a non-catalytic reaction. The initial state was constructed as one ethene molecule and one separated hydrogen molecule (H2 ). The distance between them is far enough to ensure that the interaction between them is negligible. The final state is a single ethane (C2 H6 ) molecule. The total energy of the final state is about 1.8 eV lower than that of the initial state, indicating that the hydrogena-
tion reaction is thermodynamically favorable. As described in Section 2, the simulation was performed by using the NEB method. Fig. 2 shows the total energy changes of the gas phase system along the optimized reaction path. The reaction reaches its transition state when the H2 bond is broken, with one H atom bonds with one C atom while the other remains single. The evaluated energy barrier for hydrogenation of ethene in vacuum is 3.99 eV, which is quite large and the reaction is difficult to happen even under high temperature. A catalytic reaction is required to realize the reaction of hydrogenation of ethane. In the following context, we show how Ir adsorbed at γ -Al2 O3 (001) catalyzes this reaction. In order to simulate the catalytic behavior of the γ -Al2 O3 (001) supported Ir atoms with the NEB method, reasonable initial and final states should be built and optimized. The final state is simple (see Fig. 3c), which is defined as an ethane molecule in the vacuum layer of the slab. The Ir atom is located at most favorable site on the γ -Al2 O3 (001) surface. In our previous study, we have shown that the energetically most favorable sites are the top sites of the O atoms at the dense Al–O layer of the γ -Al2 O3 (001) surface [17]. The ethane molecule is far away from the surface, which makes sure that the interaction between them is negligible. For the initial state, an ethene molecule was first put on the top of the Ir atom. Different geometries of the molecule have been tried and the results show that the system is energetically most favorable when the ethene molecule plane is horizontal and parallel to the surface plane (see Fig. 3a). In the following context, we call it as “π -bonded” initial state just for simple description of the geometry. One H2 molecule was then put in the vacuum of the slab and located far enough from the surface to ensure that the interaction between the H2 molecule and the surface is small. Due to the small surface area of our model, we did not choose the initial H2 molecule geometry by adsorbing on the Al2 O3 surface. As the ethene molecule is initially “π -bonded” to the Ir atom and the energy of the system is most favorable, it can be understood that the product of the hydrogenation reaction could also have strong interaction with the Ir atom and the system energy is low. Therefore, we put an ethane molecule at the surface with the same way of putting the ethene molecule on the surface. Relaxation of the system with an ethane molecule on the top of the Ir atom shows that the C–C bond breaks and the resulted two methyl groups are bonded with the Ir atom, as shown in Fig. 3b. The energy of the relaxed system is even about 0.2 eV lower than the initial state, and therefore we choose it as the middle state of the Ir catalyzed process of the ethene hydrogenation reaction. This assumption is consistent with Argo et al.’s proposal on the
Y. Chen et al. / Physics Letters A 376 (2012) 1919–1923
1921
Fig. 3. Initial (a), middle (b), and final (c) states of the ethene hydrogenation reaction catalyzed by an Ir atom adsorbed on the γ -Al2 O3 (001) surface. The white (small), grey (middle sized), blue (large) spheres on the surface are H, C, and Ir atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.) Table 1 Charge and Magnetic Moments (MM) of the adsorbed Ir atom at different states.
Total charge Majority Minority MM (μ B ) a b c d e f g
a
Isob
Slabc
Inid
P1-4e
P1-6
P1-8
Midf
P2-4
P2-7
Fing
7.60 5.08 2.52 2.56
8.38 4.66 3.73 0.93
8.84 4.88 3.96 0.92
9.33 4.76 4.54 0.24
9.34 4.78 4.56 0.21
9.13 4.77 4.36 0.41
8.71 4.79 3.92 0.87
8.61 4.76 3.85 0.91
8.59 4.75 3.84 0.91
8.36 4.64 3.72 0.93
Obtained by integrating the charge density around the Ir atom within a sphere (radius: 1.5 Å). Isolated Ir atom in vacuum. Ir adsorbed on the γ -Al2 O3 (001) surface. Initial state, as shown in Fig. 3a. P1-4 refers to image No. 4 (transition state in reaction 1) in reaction path P1 (see Fig. 4). P1-6, P1-8, P2-4 and P2-7 are denoted in the same way. Middle state, as shown in Fig. 3b. Final state, as shown in Fig. 3c.
hydrogenation of propene catalyzed by MgO supported Ir4 clusters based on EXAEF and IR experiments [3]. The reason that the ethane molecule on the top of the Ir atom breaks down into two methyl groups is strongly related with the charge distributions around the Ir atom as discussed in our previous paper [17]. Extra charge around the Ir atom in the horizontal plane was observed and the energy levels of those electrons are very close to the Fermi level. To lower the energy of the system, the C–C bond breaks and two C–Ir bonds are formed, which lowers the energy level near the Fermi level of those Ir-5d electrons. As the two C atoms are also bonded with Ir, the energy levels of the C-2p do not change much after the C–C bond is broken. The charge analysis was given in Table 1, from which we can see that a small adjustment of the charge distribution around the Ir atom is occurred after the C–C bond is broken, resulting in a small change of the magnetic moments. To reach the final state defined above, association and desorption processes must occur, in which the methyl groups break from the Ir atom and reconstruct to an ethane molecule. Then the ethane molecule diffuses away from the surface. Based on the above discussion, the reaction was simulated in two steps: hydrogenation of the ethene (reaction from the initial state to the middle state shown in Fig. 3, described as P1) and desorption of the hydrogenation product (reaction from the middle state to the final state shown in Fig. 3, described as P2). The reaction path and the activation energy barrier of both steps were calculated with the NEB method. Fig. 4 shows the energy changes along the optimized reaction path for the hydrogenation of ethene (reaction P1) catalyzed by an adsorbed Ir atom on the γ -Al2 O3 (001) surface. As discussed above, the energy of the system at the middle state is about 0.2 eV lower than that of the initial state, implying that the reaction is thermodynamically favorable. The energy barrier of this reaction is ∼ 2 eV, which is about half of the energy barrier for the reaction takes place in vacuum (see Fig. 2). The optimized reaction path is very interesting, along which a local minimum (P1-IMG-6
Fig. 4. Reaction energy barrier of the hydrogenation of ethene (P1) catalyzed by an Ir atom adsorbed on the γ -Al2 O3 (001) surface. Three important images (IMG-4, IMG-6, and IMG-8) are also shown in this figure, and the color and size scheme is the same as in Fig. 3.
in Fig. 4) was found. With the H2 molecule moving down to the surface and approaching to the ethene molecule that was initially “π -bonded” to the Ir atom, the C–C bond length of the ethane molecule becomes longer, which increases the energy of the system and reaches a maximum at P1-IMG-4. The initial C–C distance is about 1.43 Å, and becomes 2.24 Å at the P1-IMG-4 where the C–C bond is already broken. The Ir–C covalent bond is then enhanced by the increase of the shared charge between the Ir and C atoms, resulting in a larger value of the total charge around the Ir atom as shown in Table 1. Interestingly, those enhanced charges are located in the minority spin channel, leading to an even small magnetic moment of the Ir atom. At this stage, the orientation of the H2 molecule is changed, but the H–H bond length remains unchanged.
1922
Y. Chen et al. / Physics Letters A 376 (2012) 1919–1923
length of ∼ 1.52 Å, only ∼ 0.1 Å larger than the bond length of a free ethane molecule. The Ir–C bond is weakened further with the C–C bond forming, which can also be seen from the total charge listed in Table 1. From the local minimum at the P2-IMG-7, the C atom bonded to the Ir atom moves gradually away along the reaction path, and finally the whole molecule is desorbed from the surface. 4. Summary and conclusions
Fig. 5. Optimized reaction path and energy barrier of desorption of ethane molecule (P2) from the Ir atom. Images numbers 4 and 7 (IMG-4 and IMG-7) used in the NEB calculations are also shown in the figure, and the color and size scheme is the same as in Fig. 3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.)
Further rotation of the H2 molecule, then, reduces the energy of the system, forming a local minimum along the reaction path (see P1-IMG-6), but the charge distribution around the Ir atom keeps constant (see Table 1). When the H2 molecule is almost parallel to the surface plane, the rotation stops and the H–H bond length starts to increase, which increases the energy of the system and reaches a maximum at P1-IMG-8 where the H–H bond is almost broken. The distance between the two H atoms in P1IMG-8 is ∼ 0.94 Å, about 0.2 Å longer than the bond length of free H2 molecular. With the H–H bond broken, the H atoms move to the nearby C atoms and two C–H bonds start to form, which lowers the total energy and the system becomes to a stable state, i.e., the middle state as mentioned above. At the middle state, the strength of the Ir–C bonds is slightly weakened by the decrease of the amount of shared charge between the Ir and C atoms (see Table 1). As expected, those reduced charge is from the minority spin channel, resulting in an increase of the magnetic moment of the Ir atom. Fig. 5 describes the energy changes along the reaction path of the association and desorption of the ethane molecule from the γ -Al2 O3 (001) surface, i.e., the reaction path of P2 from middle state (Fig. 3b) to the final state (Fig. 3c). The total energy of the final state is about 1.35 eV higher than that of the middle state, suggesting that the desorption process is an endothermic reaction. Here we mention that although P2 is endothermic, the whole reaction (C2 H4 and H2 to C2 H6 in vacuum) is exothermic, because the binding energy of C2 H4 on the surface is about −2.9 eV. To ensure the reaction happen, two conditions are necessary: (1) the resulting ethane molecules can diffuse away in time; and (2) the system is heated up. As shown in Fig. 5, the desorption process is rather complicated. Starting from the middle state (the starting image in this process), the two C atoms start to move away from the surface and get close to each other, resulting in an increase of the system total energy. The total energy reaches a maximum at P2-IMG-4 where the C–C distance is 2.39 Å, ∼ 0.34 Å smaller than that at the middle state. At the same time, the Ir atom also moves away from the surface and the Ir–O distance changes from 2.21 to 2.45 Å. Then, one C moves further up and the C–C distance becoming even smaller. The C–C bond starts to form, leading to a reduction of the energy of the system that reaches a local minimum at P2-IMG-7, in which only one C atom bonds with the Ir atom. However, the two C atoms are now strongly bonded with a bond
In summary, although with simple model, our results show that the ethene hydrogenation reaction can be catalyzed by even one isolated Ir atom adsorbed on the γ -Al2 O3 (001) surface. The adsorbed Ir atom triggers the ethene hydrogenation reaction in two major steps, i.e., ethene hydrogenation and desorption of the hydrogenation product. The ethene molecular is first binding to the Ir atom, and then the H2 molecule moves approach to the ethene. This process weakens the C–C and H–H interactions and gradually increases the energy of the system. Then, the system energy is lowered through forming C-H bonds. Therefore, the energy barrier of this process is mainly originated from the weakening or breaking down of C–C and H–H bonds. The obtained energy barrier is about 2 eV, which is still quite high in real application. The reason mainly lies in our simple model. In real cases, H2 molecule are first disassociated and adsorbed at the substrate or Ir clusters and thus the contribution from disassociation of H2 to the energy barrier in our simulation can be further lowered. Through the whole process, the strength of Ir–C bonds is changed time to time to adjust the charge equilibrium of the system. Then re-association of the two CH3 groups that are bonded to Ir atom starts to occur when the two C atoms move away from the Ir atom and close to each other. This process lowers the interaction between Ir and C atoms and thus increases the system energy. Then one C is escaped from the Ir atom, which strengthens the C–C interaction and thus decreases the system energy. Then the C2 H6 moves away from the surface by breaking down the remaining C–Ir bonding. This process needs overcome an energy barrier of about 1 eV. Acknowledgements The authors acknowledge the financial support from the Ministry of Science and Technology of the People’s Republic of China under the National Basic Research Program of China (973 Program) (2007CB216403). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
G. Ertl, Adv. Catal. 45 (2000) 1. B.C. Gates, J. Mol. Catal. A: Chem. 163 (2000) 55. A.M. Argo, J.F. Odzak, F.S. Lai, B.C. Gates, Nature 415 (2002) 623. J. van den Brand, P.C. Snijders, W.G. Sloof, H. Terryn, J.H.W. Wit, J. Phys. Chem. B 108 (2004) 6017. B.C. Gates, J.R. Katzer, G.A. Schuit, Chemistry of Catalytic Processes, McGraw– Hill, New York, 1979. R.S. Zhou, R.L. Snyder, Acta Crystallogr. B 47 (1991) 617. G. Paglia, C.E. Buckley, A.L. Rohl, B.A. Hunter, R.D. Hart, J.V. Hanna, L.T. Byrne, Phys. Rev. B 68 (2003) 144110. E.M. Proupin, G. Gutiérrez, Phys. Rev. B 72 (2005) 035116. M. Sun, A.E. Nelson, J. Adjaye, J. Phys. Chem. B 110 (2006) 2310. G. Paglia, C.E. Nuckley, A.L. Rohl, J. Phys. Chem. B 110 (2006) 20721. M.H. Lee, C.F. Cheng, V. Heine, J. Klinowski, Chem. Phys. Lett. 265 (1997) 673. C. Wolverton, K.C. Hass, Phys. Rev. B 63 (2001) 024102. G. Gutiérrez, A. Taga, B. Johansson, Phys. Rev. B 65 (2002) 012101. A. Vijay, G. Mills, H. Metiu, J. Chem. Phys. 117 (2002) 4509. H.P. Pinto, R.M. Nieminen, S.D. Elliott, Phys. Rev. B 70 (2004) 125402. C.Y. Ouyang, Z. Sljivancanin, A. Baldereschi, Phys. Rev. B 79 (2009) 235410. Y.C. Chen, C.Y. Ouyang, S.Q. Shi, Z.L. Sun, L. Song, Phys. Lett. A 373 (2009) 277. L.P. Nielsen, F. Besenbacher, E. Lægsgaard, I. Stensgaard, Phys. Rev. B 44 (1991) 13156.
Y. Chen et al. / Physics Letters A 376 (2012) 1919–1923
[19] D. Teschner, J. Borsodi, A. Wootsch, Z. Révay, M. Hävecker, A. Knop-Gericke, S.D. Jackson, R. Schlögl, Science 320 (2008) 86. [20] B. Chan, L. Radom, J. Am. Chem. Soc. 130 (2008) 9790. [21] B. Hammer, L.B. Hansen, J.K. Norskov, Phys. Rev. B 59 (1999) 7413. [22] https://wiki.fysik.dtu.dk/dacapo. [23] D. Vanderbilt, Phys. Rev. B 41 (1990) R7892.
1923
[24] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671. [25] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13224. [26] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. [27] G. Henkelman, H. Jonsson, J. Chem. Phys. 113 (2000) 9978. [28] G. Henkelman, B.P. Uberuaga, H. Jonsson, J. Chem. Phys. 113 (2000) 9901.