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A theoretical study of CH4 dissociation on NiPd(111) surface
Surface Science 612 (2013) 63–68
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A theoretical study of CH4 dissociation on NiPd(111) surface Kai Li a, Zhongjun Zhou a, b, Ying Wang a,⁎, Zhijian Wu a,⁎ a b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, PR China
a r t i c l e
i n f o
Article history: Received 4 January 2013 Accepted 20 February 2013 Available online 27 February 2013 Keywords: NiPd(111) CH4 dissociation Reaction barrier
a b s t r a c t The CH4 dissociation on the NiPd(111) surface is studied by using the density functional theory (DFT). The possible adsorption sites are proposed and the favorite adsorption site(s) are determined. The potential energy curve for CH4 dissociation is presented. Compared with pure Ni(111) and Pd(111) surfaces, the dissociation of CH4 on NiPd(111) surface is more favored, especially on the Ni reaction center of NiPd(111) surface. The introduction of Pd improves the Ni catalytic ability for CH4 dissociation. A synergistic effect exists between Ni and Pd that results in an improved catalytic performance for CH4 disassociation over that of either parent metal. Bimetallic NiPd is predicted to be a good catalyst for CH4 dissociation, in good agreement with experiment. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The methane dissociation is the key to transform the resources of natural gas into useful products like hydrogen. However, the high stability of methane makes it hard to dissociate at room temperature. Thus, catalytic CH4 dissociation to produce hydrogen has attracted increasing attention in recent years [1–9]. Because of the good catalytic activity and low cost, Ni-based catalysts have become one of the most attractive catalysts in CH4 dissociation. However, carbon deposition is a severe problem for Ni-based catalysts and usually deactivates the catalysts [10–17]. Thus, improving the activity of Ni-based catalysts and suppressing coke formation are challenging technological problems. In order to improve the stability and activity of Ni-based catalysts, a bimetallic catalyst is often used, instead of pure Ni. Many Ni-based bimetallic catalysts have been proposed by experiments, such as Ni–Cu, Ni–Co, Ni–Pt, Ni–Au, Ni–Sn and Ni–Pd etc [18–41]. Among them, Ni–Au and Ni–Sn bimetallic catalysts show a suppressed coke formation, but at the cost of losing a catalytic activity [18,19]. Ni–Pt bimetallic catalyst with the lowest Pt/Ni ratio (Pt0.3Ni10) exhibited a higher catalytic activity and stability for CH4 dissociation than monometallic Ni [20,21]. In addition, Ni–Pd bimetallic catalyst has received much attention for CH4 dissociation because of the high activity and stability [22–27]. Takenaka et al. investigated the CH4 dissociation over M–Ni/SiO2 (M = Cu, Rh, Pd, Ir, Pt) and found that Pd–Ni catalysts had the highest yields of hydrogen with a mole ratio of Pd / (Ni + Pd) = 0.5 [22,23]. Steinhauer et al. investigated the catalytic activity of bimetallic Ni–Pd on different supports and found that they had a better performance than single metal Ni or Pd-catalysts for CH4 dissociation [24]. The superior catalytic performance of Pd promoted Ni catalysts came from the synergistic effect between Ni and Pd [25]. ⁎ Corresponding authors. Tel.: +86 43185262801. E-mail addresses: [email protected] (Y. Wang), [email protected] (Z. Wu). 0039-6028/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.susc.2013.02.012
On the theoretical side, there were also several studies available by using the density functional theory (DFT). This included the CH4 dissociation over a Ni-based bimetallic (111) surface such as Ni–Au [18,42,43], Ni–Co [44], Ni–Cu [45], Ni–Sn [19], or Ni–M surface (M = Cu, Ru, Rh, Pd, Ag, Pt, and Au) [46], etc. The results indicated that the introduction of Au and Sn to Ni made bimetallic surface less active for CH4 dissociation and weakened C adsorption [18,19,43]. The relative high energy barriers and the low C binding energy explained the experimentally observed coke resistance. To our knowledge, theoretical study of CH4 dissociation over Ni–Pd surface is still rare. Recently, the physical origin of the synergistic effect over Ni–M surface (M = Cu, Ru, Rh, Pd, Ag, Pt, and Au) with Ni/ M = 3 was conducted [46]. Although the electronic structure and adsorption properties had been investigated, the detailed reaction mechanism of CH4 dissociation over NiPd was not provided. In this work, we present a study on the possible reaction mechanism for CH4 dissociation on NiPd(111) surface by using the density functional theory (DFT). The Pd/Ni = 1 is selected based on the previous experiment, because at this ratio, the hydrogen yield is the highest [23]. The adsorption geometries, energies of CHx (x = 0–4) on NiPd(111) surface and activation energies for the four elementary steps involved in CH4 dissociation are investigated. For comparison, CH4 dissociation on elemental Pd(111) and Ni(111) surfaces is also given. 2. Computational details 2.1. Method The calculations were performed using Vienna ab-initio simulation package (VASP) [47–50]. The interactions between valence electrons and ion cores were treated by Blöchl's all-electron-like projector augmented wave (PAW) method [51,52]. The exchangecorrelation functional was the generalized gradient approximation
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with the Perdew–Burke–Ernzerhof, known as GGA-PBE [53]. The wave functions at each k-point were expanded with a plane wave basis set with a kinetic cutoff energy up to 400 eV and electronic occupancies were determined according to a Fermi scheme with an energy smearing of 0.1 eV. Brillouin zone integration was approximated by a sum over selected special k-points using the Monkhorst–Pack method [54] and they are set to 3 × 3 × 1. Geometries were optimized until the energy was converged to 1.0 × 10−6 eV/atom and the force converged to 0.01 eV/Å. Because of the existence of magnetic atom, spin polarization was considered in all calculations. The transition state (TS) structures and the reaction pathway were computed using the climbing image nudged elastic band (CI-NEB) method [55]. The minimum energy path (MEP) was optimized using a force-based conjugate-gradient method [50] until the maximum force is less than 0.01 eV/Å. In addition, TS was verified by only one imaginary frequency. 2.2. Model The NiPd alloy with Ni/Pd = 1 showed a remarkable performance in CH4 dissociation and reforming [22–24]. The formation of the Ni–Pd alloys was observed to include (111) and (200) surfaces by the XRD pattern [26]. Thus, we choose the ground state of Ni (face-centeredcubic lattice) [56,57] and replace half of the Ni atoms by Pd atoms to simulate the 1:1 Ni–Pd alloy. The calculations show that the lattice constants of alloy are a = 3.841 and c = 3.618 Å, different from the lattice constants of Ni bulk (3.518 Å) and Pd bulk (3.920 Å). Since experimentally, NiPd(111) and NiPd(200) surfaces are observed [26], in order to determine which surface is more stable, the surface energy (Esurf) is introduced which is defined as the energy of the slab related to its bulk reference [58]: Esurf
E −nEbulk ¼ slab 2A
where Eslab is the total energy of the surface slab, Ebulk is the total energy of the bulk NiPd, A is the surface area with a factor of 2 due to each slab containing two surfaces, and n is the number of NiPd formula units in the slab. The small Esurf means that the surface is more stable [58]. Thus, the calculated Esurf values of 0.045 eV for (111) and 0.103 eV for (200) indicate that NiPd(111) is more stable than (200). This is the reason why we study NiPd(111) surface in this work. The surface is obtained by cutting NiPd (fcc) bulk along (111) direction, the thickness of surface slab is chosen to be a three-layer slab which is proved to be a reasonable model and widely employed in previous literatures [43–45]. In our calculations, the atoms in the
bottom are fixed in their bulk positions, and those in the top and second layers are allowed to relax. A vacuum layer as large as 12 Å is used along the c direction normal to the surface to avoid periodic interactions. In order to reduce the interaction between adsorbates on the surface, a (2 × 2) supercell is used to model the coverage of a 1/4 monolayer. The chemisorption energy, Eads, is defined as follows ΔEads ¼ Eadsorbates=slab −ðEslab þ Eadsorbates Þ where Eadsorbates/slab is the total energy of adsorbates on (111) surface, Eslab is the total energy of the bare slab of the surface, and Eadsorbates is the total energy of free adsorbates. The first two terms are calculated with the same parameters. The third term is calculated by setting the isolate adsorbate in a box of 12 Å × 12 Å × 12 Å. The negative ΔEads indicates exothermic chemisorption, positive value suggests endothermic chemisorptions, while values closer to zero (slightly negative or slightly positive), are representative of weak physisorption. 3. Results and discussion 3.1. CHx adsorption on NiPd(111) There exist nine possible adsorption sites on the NiPd(111) surface (Fig. 1), they are, two different Top sites (TNi and TPd), Hexagonal Close Packed sites (HCPNi and HCPPd), and Face Center Cubic sites (FCCNi and FCCPd), as well as three different Bridge sites (BNi–Pd, BNi–Ni and BPd–Pd). Here, we have investigated all the possible adsorption configurations and energies of CHx (x = 0–4) and H on NiPd(111) surface (see Fig. S1 in the supporting information). 3.1.1. CH4 adsorption on NiPd(111) According to previous studies, CH4 adsorption is reported to be a physisorption process on pure metal surfaces with an extremely small adsorption energy [42–45]. Similarly, our results show that the introduction of Pd also does not increase CH4 adsorption energy significantly. The adsorption energy is calculated to be − 0.02 eV, close to the values on pure Ni(111) (− 0.02) and Pd(111) (− 0.03) surfaces, respectively. 3.1.2. CH3 and CH2 adsorption on NiPd(111) From the adsorption energies of CH3 and CH2 on NiPd(111) (Fig. S1, CH3(a) to CH3(h) and CH2(a) to CH2(g)), we find that CH3 and CH2 prefer to occupy the hollow site with two Ni atoms, that is, FCCPd site as shown in Fig. 2b and c. For CH3, all three H atoms point outward, without interaction between H and Ni/Pd. For the adsorbed CH2, it is seen
Fig. 1. Top view (a) and side view (b) of the possible adsorption sites on NiPd(111) surface.
K. Li et al. / Surface Science 612 (2013) 63–68
that the energy is lowered when one H atom is bonded to the Ni atom, which is favorable for the following dissociation of C–H. In the favorite CH3 and CH2 adsorption configurations (Fig. 2b and c), C\Ni bond lengths (2.115 Å in CH3, 1.859 and 1.980 Å in CH2) are shorter than the C\Pd bond lengths (2.330 Å in CH3, 2.072 Å in CH2). This might be an indication that the interaction between C and Ni is stronger than that between C and Pd. 3.1.3. CH and H adsorption on NiPd(111) CH prefers to locate at the hollow sites HCPPd and FCCPd, whose adsorption energies are −6.27 eV and −6.24 eV, respectively, slightly higher than −6.35 eV on the pure Ni(111) surface at the same level of theory (Fig. S1, CH(a) to CH(d)). This indicates that CH on NiPd alloy is less stable than on pure Ni surface. Similar situation is observed in the previous studies on NiCo(111) and NiCu(111) [44,45]. Similar to CH adsorption configurations, H also prefers to adsorb at hollow sites with four possible configurations (Fig. S1, H(a) to H(d)). The calculated adsorption energies (around − 2.7 eV) are much
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smaller than those for CH (around − 6.0 eV). In addition, the adsorption energies of the four H-adsorption configurations are very close and the diffusion barrier of H on NiPd(111) is only 0.15 eV, suggesting that the migration of H on the surface is relatively easy.
3.1.4. C adsorption on NiPd(111) Different from the configurations on Ni(111) and Pd(111) surfaces, there are only three stable geometries for C adsorption on NiPd(111) (Fig. S1, C(a) to C(c)). The initial structures FCCPd and HCPPd are all converged to the most stable fourfold site (denoted as FF in Fig. 2e). It is clear that when C is adsorbed on FF, the NiPd (111) surface deforms severely. In this adsorption configuration, compared to the isolated NiPd(111), the bond of Ni\Pd is shortened from 2.674 to 2.648 Å, while Ni\Ni and Pd\Pd bonds are elongated by 14.1% (from 2.723 to 3.106 Å) and 1.0% (from 2.723 to 2.749 Å), respectively. The surface distortion is consistent with the experiment observation by Takenaka et al. [22,23].
Fig. 2. The most stable adsorption geometries and selected parameters of CHx (x = 0–4) and H on NiPd(111) (only the top two layers of the NiPd slab are displayed in the top view, and the top layer in the side view). The gray balls denote C atoms and the white balls denote H atoms. The top H–Ni indicates one of the H atoms pointing to the Ni atom.
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3.2. CH4 dissociation on NiPd(111) In order to investigate the mechanism of CH4 dissociation on the NiPd (111) surface, we choose the most stable geometry as the initial state and co-adsorbed CHx − 1 + H species with the lowest Eads are set as the final configurations in the minimum energy path. Each step is divided into two possible pathways: one is with Ni as the catalytic reaction center and the other is with Pd as the catalytic reaction center. 3.2.1. CH4 → CH3 + H As mentioned above, CH4 on NiPd(111) is a physical adsorption process, and all configurations have almost the same adsorption energy. Therefore, we select the CH4 adsorption on the top of Ni or Pd configurations as the initial state. As FCCPd is the most stable site for CH3 adsorption, while H adsorption has nearly the same stability for the studied sites, the configurations of CH3 adsorbed at FCCPd and H located at FCCPd are chosen as the final state in the first step of CH4 dissociation on NiPd(111). Two transition states on two different reaction
centers are given in Fig. 3 (TS1 and TS1′, with the latter indicating Pd as a reaction center). In the two configurations, the CH3 fragment is adsorbed at the top site and the breaking H atom is located at the nearest hollow site. The breaking C\H bond increases by 45.7% (from 1.097 to 1.598 Å in TS1) or 48.3% (from 1.097 to1.627 Å in TS1′) compared to the C\H bond length in isolated CH4, and the forming Ni\H bond is elongated by 22.7% (from 1.723 to 2.114 Å in TS1) or 27.3% (from 1.723 to 2.194 Å in TS1′) with respect to the bond length of H adsorbed on FCCPd site. The elongation of the formed bond is smaller than that of the breaking bond, indicating that the two TSs are product-like. This means that the both reaction channels proceed via a “late” transition state. This character in the TSs is consistent with the calculated result that CH4 decomposition to CH3 and H is an endothermic process with the reaction energies of 0.30 and 0.48 eV for the Ni and Pd reaction centers, respectively, in agreement with Hammond's postulate [59]. It is worth to note that the reaction energy with the Ni reaction center is less endothermic by 0.18 eV compared to that of the Pd reaction center. In addition, the energy barrier height of the reaction with the Ni reaction center (0.84 eV,
Fig. 3. The transition state structures for CHx (x = 1–4) dissociation on NiPd(111)(only the top two layers of the NiPd slab are displayed in the top view, and the top layer in the side view). (a–d) for TS with the Ni reaction center and (e–g) for TS′ with the Pd reaction center. The C–H (rupture) indicates the breaking C\H bond. The vimag indicates the imaginary frequency.
K. Li et al. / Surface Science 612 (2013) 63–68
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Fig. 4) is lower than that of the Pd reaction center (1.01 eV). This suggests that taking Ni as the reaction center is favored both thermodynamically and kinetically.
on the Pd center also confirmed that the Ni reaction center is more preferred. Thus, Ni atoms have a higher catalytic activity than Pd for CHx (x = 1–4) dissociation.
3.2.2. CH3 → CH2 + H CH3 at the FCCPd site is the initial structure for CH3 dissociation, while both of the CH2 and H co-adsorbed at FCCPd site are the final states for the reaction with the Ni center. For the Pd reaction center, the configuration of CH2 at FCCPd site and H at HCPPd site is set as the final state. The two TS structures for CH3 dissociation on the NiPd(111) surface are shown in Fig. 3b and f. In Fig. 3b. The carboncontaining fragment CH2 is located at the FCCPd site and the detached H atom is positioned at the adjacent hollow site with dC–H = 1.620 Å. In Fig. 3f, the detached H is located on the top of the Pd atom with the C–H distance of 1.834 Å. It is clear to see that the breaking C\H bond length in the former case (Ni reaction center) is shorter than the latter (Pd reaction center) by around 0.2 Å, suggesting that CH3 dissociation is more difficult on the Pd reaction center than on the Ni reaction center. The calculated CH3 dehydrogenation with the Ni reaction center is endothermic only by 0.15 eV and the barrier height is 0.64 eV (Fig. 4), while those for the reaction with Pd are 0.19 and 0.90 eV, respectively.
3.3. Comparison with Ni(111) and Pd(111) surfaces
3.2.3. CH2 → CH + H CH2 adsorbed at the FCCPd site (Fig. S1, CH2(a) and Fig. 2c) with two Ni atoms is more stable than CH2 at the hollow site with two Pd atoms (Fig. S1, CH2(d)–CH2(e)). In addition, in the former configuration, one of the H atoms bonded to Ni with the dNi–H = 1.732 Å. Thus, the adsorbed H can be easily removed from CH2. Therefore, we only consider the CH2 dehydrogenation with the Ni reaction center. In TS3, as shown in Fig. 3c, the distance between C and the cleaved H atom is 1.502 Å, which increases by 36.9% (from 1.097 to 1.502 Å) compared to the C\H bond length in isolated CH4, while the Ni\H bond is elongated by 79.3% (from 1.723 to 3.090 Å). The extent of the elongation of the Ni\H bond is larger than that of the breaking bond, indicating that the TS is reactant-like. Thus, the reaction channel proceeds via an “early” transition state. This is consistent to the early character of TS with an exothermic process (− 0.27 eV) in Hammond's postulate [59]. It is noted that CH2 decomposition is the only exothermic process in the whole CH4 dissociation processes on NiPd(111) surface. Furthermore, the C–H dissociation barrier is the lowest with an energy of 0.28 eV compared to other dissociation processes (Fig. 4). 3.2.4. CH → C + H For the Ni reaction center, CH at FCCPd is selected as the initial structure, while for the Pd reaction center, CH at HCPPd is set as the initial state since the adsorption for CH at HCPPd is slightly more stable than FCCPd by 0.03 eV (Fig. S2, CH(a)–CH(b)). The configuration of C at FF and H at FCCPd sites is set as final state. The TS structures are shown in Fig. 3d and g. In Fig. 3d, the breaking C\H bond is 1.562 Å, and its orientation is almost parallel to the surface. From the calculated data, the deformation of NiPd(111) surface is obvious. This is because compared with an isolated NiPd(111) (planar) surface, the two Ni atoms bonded with C are enlarged by 11% (from 2.723 to 3.012 Å) on the xy plane, while the Pd and Ni bonded with C rise about 0.187 and 0.023 Å above the surface (along z axis). The energy barrier of CH dehydrogenation into C and H is 1.04 eV (Fig. 4) and the reaction is endothermic by 0.32 eV, which is the highest electronic barrier during the CH4 dissociation process. In Fig. 3g, the C atom locates at the middle between two Ni atoms with dC–Ni = 1.754 Å, and H moves toward the top of Pd with the breaking C\H bond of 2.198 Å. However, the deformation of the surface is not so large as that in the Ni reaction center. The activation energy for CH dissociation with the Pd center is 1.29 eV and the reaction is endothermic by 0.35 eV. The higher barrier and more endothermic
As discussed above, it is seen that Ni shows a higher catalytic activity than Pd on NiPd(111) surface, so the minimum energy path of the reaction should be CH4 dissociation on NiPd(111) with the Ni center. In order to show the performance of NiPd catalyst, we also investigated the mechanism of CH4 dissociation over pure Ni(111) and Pd(111) surfaces by the same level of theory (possible configurations are shown in Figs. S2–S4). The potential energy surface of CH4 dissociation on Ni(111), Pd(111) and NiPd(111) surfaces is shown in Fig. 4. It is seen that CHx (x = 1–4) dehydrogenation into CHx − 1 (x = 1–4) and H on NiPd(111) is more favored both thermodynamically and kinetically than that on Ni(111) and Pd(111) surfaces. A synergistic effect exists between Ni and Pd that results in the improved catalytic performance of NiPd for CH4 dissociation over that of either parent metal. This might be explained in the following. First, the introduction of Pd changes the geometrical structure of pure Ni. The lattice parameter increases from 3.518 Å in pure Ni to 3.841 Å for a and 3.618 Å for c in NiPd alloy. Second, it also makes the surface charge redistributed. Our calculations indicate that with the increase of x in CHx (x = 1, 2, 3), the average net charge of Ni is positive and changes from 0.27, 0.28 to 0.30 |e|, while that of Pd is negative from − 0.12, − 0.19 to − 0.22 |e|. The positive charge of Ni and negative charge of Pd are in agreement with the small electronegativity of Ni (1.91) and large electronegativity of Pd (2.20). This suggests that the Ni\Pd bond is polarized. Since the radicals of CHx (x = 1–3) have redundant electrons, they prefer to be adsorbed to the positive Ni. Thus, Ni becomes the reaction center. Hanmmer and Nørskov pointed out that the d-band center of similar transition metal correlates with the general catalytic activity and the reactivity of the transition metal system increases as the d-band center is shifted up [60,61]. Thus, to further explain why NiPd alloy has a higher catalytic activity and Ni atom is the reaction center for CHx dissociation, the d-band centers of the Ni(111), Pd(111) and NiPd(111) surfaces are examined. Our calculated d-band centers of pure Ni and Pd are − 1.47 eV and − 1.89 eV, respectively, while the average value for NiPd is − 1.71 eV. Obviously, the average d-band center of NiPd alloy surface is far away from a Fermi level compared with Ni(111). However, the d-band center of Ni on NiPd(111) surface is −1.14 eV, closer to a Fermi level than pure Ni, demonstrating that CHx adsorbed on the sites with two Ni atoms is more stable than on the other sites, and dissociations at the Ni reaction center are
Fig. 4. Potential energy curves for CH4 dissociation on Ni(111), Pd(111) and NiPd(111) surfaces, respectively.
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preferred. Thus, we conclude that Ni–Pd synergistic effect enhances Ni catalytic activity in alloy [46]. 4. Conclusions The mechanism of methane dissociation on NiPd(111) surface has been investigated by using the density functional theory. Our results show that CHx (x = 1–3) prefers to adsorb at the hollow sites with two Ni atoms (FCCPd or HCPPd) and CHx (x = 1–4) dissociation on the Ni reaction center is more favored than on the Pd reaction center. The largest electronic energy barrier of CH4 dissociation is CH dissociation to atomic carbon and hydrogen on NiPd(111) surface. By comparing with pure Ni(111) and Pd(111) surfaces, we observed that Ni–Pd alloy shows the highest catalytic activity. A synergistic effect exists between Ni and Pd that results in an improved catalytic performance for CH4 dissociation over that of either parent metal. We conclude that NiPd is a good catalyst to decompose CH4, consistent to the experimental observation. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (grant nos: 20921002, 21273219, 21203174). The authors are also thankful for the financial support from the Department of Science and Technology of Sichuan Province. The computational time is supported by the Performance Computing Center of Jilin University, China. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.susc.2013.02.012. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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A theoretical study of CH4 dissociation on NiPd(111) surface Kai Li a, Zhongjun Zhou a, b, Ying Wang a,⁎, Zhijian Wu a,⁎ a b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, PR China
a r t i c l e
i n f o
Article history: Received 4 January 2013 Accepted 20 February 2013 Available online 27 February 2013 Keywords: NiPd(111) CH4 dissociation Reaction barrier
a b s t r a c t The CH4 dissociation on the NiPd(111) surface is studied by using the density functional theory (DFT). The possible adsorption sites are proposed and the favorite adsorption site(s) are determined. The potential energy curve for CH4 dissociation is presented. Compared with pure Ni(111) and Pd(111) surfaces, the dissociation of CH4 on NiPd(111) surface is more favored, especially on the Ni reaction center of NiPd(111) surface. The introduction of Pd improves the Ni catalytic ability for CH4 dissociation. A synergistic effect exists between Ni and Pd that results in an improved catalytic performance for CH4 disassociation over that of either parent metal. Bimetallic NiPd is predicted to be a good catalyst for CH4 dissociation, in good agreement with experiment. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The methane dissociation is the key to transform the resources of natural gas into useful products like hydrogen. However, the high stability of methane makes it hard to dissociate at room temperature. Thus, catalytic CH4 dissociation to produce hydrogen has attracted increasing attention in recent years [1–9]. Because of the good catalytic activity and low cost, Ni-based catalysts have become one of the most attractive catalysts in CH4 dissociation. However, carbon deposition is a severe problem for Ni-based catalysts and usually deactivates the catalysts [10–17]. Thus, improving the activity of Ni-based catalysts and suppressing coke formation are challenging technological problems. In order to improve the stability and activity of Ni-based catalysts, a bimetallic catalyst is often used, instead of pure Ni. Many Ni-based bimetallic catalysts have been proposed by experiments, such as Ni–Cu, Ni–Co, Ni–Pt, Ni–Au, Ni–Sn and Ni–Pd etc [18–41]. Among them, Ni–Au and Ni–Sn bimetallic catalysts show a suppressed coke formation, but at the cost of losing a catalytic activity [18,19]. Ni–Pt bimetallic catalyst with the lowest Pt/Ni ratio (Pt0.3Ni10) exhibited a higher catalytic activity and stability for CH4 dissociation than monometallic Ni [20,21]. In addition, Ni–Pd bimetallic catalyst has received much attention for CH4 dissociation because of the high activity and stability [22–27]. Takenaka et al. investigated the CH4 dissociation over M–Ni/SiO2 (M = Cu, Rh, Pd, Ir, Pt) and found that Pd–Ni catalysts had the highest yields of hydrogen with a mole ratio of Pd / (Ni + Pd) = 0.5 [22,23]. Steinhauer et al. investigated the catalytic activity of bimetallic Ni–Pd on different supports and found that they had a better performance than single metal Ni or Pd-catalysts for CH4 dissociation [24]. The superior catalytic performance of Pd promoted Ni catalysts came from the synergistic effect between Ni and Pd [25]. ⁎ Corresponding authors. Tel.: +86 43185262801. E-mail addresses: [email protected] (Y. Wang), [email protected] (Z. Wu). 0039-6028/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.susc.2013.02.012
On the theoretical side, there were also several studies available by using the density functional theory (DFT). This included the CH4 dissociation over a Ni-based bimetallic (111) surface such as Ni–Au [18,42,43], Ni–Co [44], Ni–Cu [45], Ni–Sn [19], or Ni–M surface (M = Cu, Ru, Rh, Pd, Ag, Pt, and Au) [46], etc. The results indicated that the introduction of Au and Sn to Ni made bimetallic surface less active for CH4 dissociation and weakened C adsorption [18,19,43]. The relative high energy barriers and the low C binding energy explained the experimentally observed coke resistance. To our knowledge, theoretical study of CH4 dissociation over Ni–Pd surface is still rare. Recently, the physical origin of the synergistic effect over Ni–M surface (M = Cu, Ru, Rh, Pd, Ag, Pt, and Au) with Ni/ M = 3 was conducted [46]. Although the electronic structure and adsorption properties had been investigated, the detailed reaction mechanism of CH4 dissociation over NiPd was not provided. In this work, we present a study on the possible reaction mechanism for CH4 dissociation on NiPd(111) surface by using the density functional theory (DFT). The Pd/Ni = 1 is selected based on the previous experiment, because at this ratio, the hydrogen yield is the highest [23]. The adsorption geometries, energies of CHx (x = 0–4) on NiPd(111) surface and activation energies for the four elementary steps involved in CH4 dissociation are investigated. For comparison, CH4 dissociation on elemental Pd(111) and Ni(111) surfaces is also given. 2. Computational details 2.1. Method The calculations were performed using Vienna ab-initio simulation package (VASP) [47–50]. The interactions between valence electrons and ion cores were treated by Blöchl's all-electron-like projector augmented wave (PAW) method [51,52]. The exchangecorrelation functional was the generalized gradient approximation
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with the Perdew–Burke–Ernzerhof, known as GGA-PBE [53]. The wave functions at each k-point were expanded with a plane wave basis set with a kinetic cutoff energy up to 400 eV and electronic occupancies were determined according to a Fermi scheme with an energy smearing of 0.1 eV. Brillouin zone integration was approximated by a sum over selected special k-points using the Monkhorst–Pack method [54] and they are set to 3 × 3 × 1. Geometries were optimized until the energy was converged to 1.0 × 10−6 eV/atom and the force converged to 0.01 eV/Å. Because of the existence of magnetic atom, spin polarization was considered in all calculations. The transition state (TS) structures and the reaction pathway were computed using the climbing image nudged elastic band (CI-NEB) method [55]. The minimum energy path (MEP) was optimized using a force-based conjugate-gradient method [50] until the maximum force is less than 0.01 eV/Å. In addition, TS was verified by only one imaginary frequency. 2.2. Model The NiPd alloy with Ni/Pd = 1 showed a remarkable performance in CH4 dissociation and reforming [22–24]. The formation of the Ni–Pd alloys was observed to include (111) and (200) surfaces by the XRD pattern [26]. Thus, we choose the ground state of Ni (face-centeredcubic lattice) [56,57] and replace half of the Ni atoms by Pd atoms to simulate the 1:1 Ni–Pd alloy. The calculations show that the lattice constants of alloy are a = 3.841 and c = 3.618 Å, different from the lattice constants of Ni bulk (3.518 Å) and Pd bulk (3.920 Å). Since experimentally, NiPd(111) and NiPd(200) surfaces are observed [26], in order to determine which surface is more stable, the surface energy (Esurf) is introduced which is defined as the energy of the slab related to its bulk reference [58]: Esurf
E −nEbulk ¼ slab 2A
where Eslab is the total energy of the surface slab, Ebulk is the total energy of the bulk NiPd, A is the surface area with a factor of 2 due to each slab containing two surfaces, and n is the number of NiPd formula units in the slab. The small Esurf means that the surface is more stable [58]. Thus, the calculated Esurf values of 0.045 eV for (111) and 0.103 eV for (200) indicate that NiPd(111) is more stable than (200). This is the reason why we study NiPd(111) surface in this work. The surface is obtained by cutting NiPd (fcc) bulk along (111) direction, the thickness of surface slab is chosen to be a three-layer slab which is proved to be a reasonable model and widely employed in previous literatures [43–45]. In our calculations, the atoms in the
bottom are fixed in their bulk positions, and those in the top and second layers are allowed to relax. A vacuum layer as large as 12 Å is used along the c direction normal to the surface to avoid periodic interactions. In order to reduce the interaction between adsorbates on the surface, a (2 × 2) supercell is used to model the coverage of a 1/4 monolayer. The chemisorption energy, Eads, is defined as follows ΔEads ¼ Eadsorbates=slab −ðEslab þ Eadsorbates Þ where Eadsorbates/slab is the total energy of adsorbates on (111) surface, Eslab is the total energy of the bare slab of the surface, and Eadsorbates is the total energy of free adsorbates. The first two terms are calculated with the same parameters. The third term is calculated by setting the isolate adsorbate in a box of 12 Å × 12 Å × 12 Å. The negative ΔEads indicates exothermic chemisorption, positive value suggests endothermic chemisorptions, while values closer to zero (slightly negative or slightly positive), are representative of weak physisorption. 3. Results and discussion 3.1. CHx adsorption on NiPd(111) There exist nine possible adsorption sites on the NiPd(111) surface (Fig. 1), they are, two different Top sites (TNi and TPd), Hexagonal Close Packed sites (HCPNi and HCPPd), and Face Center Cubic sites (FCCNi and FCCPd), as well as three different Bridge sites (BNi–Pd, BNi–Ni and BPd–Pd). Here, we have investigated all the possible adsorption configurations and energies of CHx (x = 0–4) and H on NiPd(111) surface (see Fig. S1 in the supporting information). 3.1.1. CH4 adsorption on NiPd(111) According to previous studies, CH4 adsorption is reported to be a physisorption process on pure metal surfaces with an extremely small adsorption energy [42–45]. Similarly, our results show that the introduction of Pd also does not increase CH4 adsorption energy significantly. The adsorption energy is calculated to be − 0.02 eV, close to the values on pure Ni(111) (− 0.02) and Pd(111) (− 0.03) surfaces, respectively. 3.1.2. CH3 and CH2 adsorption on NiPd(111) From the adsorption energies of CH3 and CH2 on NiPd(111) (Fig. S1, CH3(a) to CH3(h) and CH2(a) to CH2(g)), we find that CH3 and CH2 prefer to occupy the hollow site with two Ni atoms, that is, FCCPd site as shown in Fig. 2b and c. For CH3, all three H atoms point outward, without interaction between H and Ni/Pd. For the adsorbed CH2, it is seen
Fig. 1. Top view (a) and side view (b) of the possible adsorption sites on NiPd(111) surface.
K. Li et al. / Surface Science 612 (2013) 63–68
that the energy is lowered when one H atom is bonded to the Ni atom, which is favorable for the following dissociation of C–H. In the favorite CH3 and CH2 adsorption configurations (Fig. 2b and c), C\Ni bond lengths (2.115 Å in CH3, 1.859 and 1.980 Å in CH2) are shorter than the C\Pd bond lengths (2.330 Å in CH3, 2.072 Å in CH2). This might be an indication that the interaction between C and Ni is stronger than that between C and Pd. 3.1.3. CH and H adsorption on NiPd(111) CH prefers to locate at the hollow sites HCPPd and FCCPd, whose adsorption energies are −6.27 eV and −6.24 eV, respectively, slightly higher than −6.35 eV on the pure Ni(111) surface at the same level of theory (Fig. S1, CH(a) to CH(d)). This indicates that CH on NiPd alloy is less stable than on pure Ni surface. Similar situation is observed in the previous studies on NiCo(111) and NiCu(111) [44,45]. Similar to CH adsorption configurations, H also prefers to adsorb at hollow sites with four possible configurations (Fig. S1, H(a) to H(d)). The calculated adsorption energies (around − 2.7 eV) are much
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smaller than those for CH (around − 6.0 eV). In addition, the adsorption energies of the four H-adsorption configurations are very close and the diffusion barrier of H on NiPd(111) is only 0.15 eV, suggesting that the migration of H on the surface is relatively easy.
3.1.4. C adsorption on NiPd(111) Different from the configurations on Ni(111) and Pd(111) surfaces, there are only three stable geometries for C adsorption on NiPd(111) (Fig. S1, C(a) to C(c)). The initial structures FCCPd and HCPPd are all converged to the most stable fourfold site (denoted as FF in Fig. 2e). It is clear that when C is adsorbed on FF, the NiPd (111) surface deforms severely. In this adsorption configuration, compared to the isolated NiPd(111), the bond of Ni\Pd is shortened from 2.674 to 2.648 Å, while Ni\Ni and Pd\Pd bonds are elongated by 14.1% (from 2.723 to 3.106 Å) and 1.0% (from 2.723 to 2.749 Å), respectively. The surface distortion is consistent with the experiment observation by Takenaka et al. [22,23].
Fig. 2. The most stable adsorption geometries and selected parameters of CHx (x = 0–4) and H on NiPd(111) (only the top two layers of the NiPd slab are displayed in the top view, and the top layer in the side view). The gray balls denote C atoms and the white balls denote H atoms. The top H–Ni indicates one of the H atoms pointing to the Ni atom.
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3.2. CH4 dissociation on NiPd(111) In order to investigate the mechanism of CH4 dissociation on the NiPd (111) surface, we choose the most stable geometry as the initial state and co-adsorbed CHx − 1 + H species with the lowest Eads are set as the final configurations in the minimum energy path. Each step is divided into two possible pathways: one is with Ni as the catalytic reaction center and the other is with Pd as the catalytic reaction center. 3.2.1. CH4 → CH3 + H As mentioned above, CH4 on NiPd(111) is a physical adsorption process, and all configurations have almost the same adsorption energy. Therefore, we select the CH4 adsorption on the top of Ni or Pd configurations as the initial state. As FCCPd is the most stable site for CH3 adsorption, while H adsorption has nearly the same stability for the studied sites, the configurations of CH3 adsorbed at FCCPd and H located at FCCPd are chosen as the final state in the first step of CH4 dissociation on NiPd(111). Two transition states on two different reaction
centers are given in Fig. 3 (TS1 and TS1′, with the latter indicating Pd as a reaction center). In the two configurations, the CH3 fragment is adsorbed at the top site and the breaking H atom is located at the nearest hollow site. The breaking C\H bond increases by 45.7% (from 1.097 to 1.598 Å in TS1) or 48.3% (from 1.097 to1.627 Å in TS1′) compared to the C\H bond length in isolated CH4, and the forming Ni\H bond is elongated by 22.7% (from 1.723 to 2.114 Å in TS1) or 27.3% (from 1.723 to 2.194 Å in TS1′) with respect to the bond length of H adsorbed on FCCPd site. The elongation of the formed bond is smaller than that of the breaking bond, indicating that the two TSs are product-like. This means that the both reaction channels proceed via a “late” transition state. This character in the TSs is consistent with the calculated result that CH4 decomposition to CH3 and H is an endothermic process with the reaction energies of 0.30 and 0.48 eV for the Ni and Pd reaction centers, respectively, in agreement with Hammond's postulate [59]. It is worth to note that the reaction energy with the Ni reaction center is less endothermic by 0.18 eV compared to that of the Pd reaction center. In addition, the energy barrier height of the reaction with the Ni reaction center (0.84 eV,
Fig. 3. The transition state structures for CHx (x = 1–4) dissociation on NiPd(111)(only the top two layers of the NiPd slab are displayed in the top view, and the top layer in the side view). (a–d) for TS with the Ni reaction center and (e–g) for TS′ with the Pd reaction center. The C–H (rupture) indicates the breaking C\H bond. The vimag indicates the imaginary frequency.
K. Li et al. / Surface Science 612 (2013) 63–68
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Fig. 4) is lower than that of the Pd reaction center (1.01 eV). This suggests that taking Ni as the reaction center is favored both thermodynamically and kinetically.
on the Pd center also confirmed that the Ni reaction center is more preferred. Thus, Ni atoms have a higher catalytic activity than Pd for CHx (x = 1–4) dissociation.
3.2.2. CH3 → CH2 + H CH3 at the FCCPd site is the initial structure for CH3 dissociation, while both of the CH2 and H co-adsorbed at FCCPd site are the final states for the reaction with the Ni center. For the Pd reaction center, the configuration of CH2 at FCCPd site and H at HCPPd site is set as the final state. The two TS structures for CH3 dissociation on the NiPd(111) surface are shown in Fig. 3b and f. In Fig. 3b. The carboncontaining fragment CH2 is located at the FCCPd site and the detached H atom is positioned at the adjacent hollow site with dC–H = 1.620 Å. In Fig. 3f, the detached H is located on the top of the Pd atom with the C–H distance of 1.834 Å. It is clear to see that the breaking C\H bond length in the former case (Ni reaction center) is shorter than the latter (Pd reaction center) by around 0.2 Å, suggesting that CH3 dissociation is more difficult on the Pd reaction center than on the Ni reaction center. The calculated CH3 dehydrogenation with the Ni reaction center is endothermic only by 0.15 eV and the barrier height is 0.64 eV (Fig. 4), while those for the reaction with Pd are 0.19 and 0.90 eV, respectively.
3.3. Comparison with Ni(111) and Pd(111) surfaces
3.2.3. CH2 → CH + H CH2 adsorbed at the FCCPd site (Fig. S1, CH2(a) and Fig. 2c) with two Ni atoms is more stable than CH2 at the hollow site with two Pd atoms (Fig. S1, CH2(d)–CH2(e)). In addition, in the former configuration, one of the H atoms bonded to Ni with the dNi–H = 1.732 Å. Thus, the adsorbed H can be easily removed from CH2. Therefore, we only consider the CH2 dehydrogenation with the Ni reaction center. In TS3, as shown in Fig. 3c, the distance between C and the cleaved H atom is 1.502 Å, which increases by 36.9% (from 1.097 to 1.502 Å) compared to the C\H bond length in isolated CH4, while the Ni\H bond is elongated by 79.3% (from 1.723 to 3.090 Å). The extent of the elongation of the Ni\H bond is larger than that of the breaking bond, indicating that the TS is reactant-like. Thus, the reaction channel proceeds via an “early” transition state. This is consistent to the early character of TS with an exothermic process (− 0.27 eV) in Hammond's postulate [59]. It is noted that CH2 decomposition is the only exothermic process in the whole CH4 dissociation processes on NiPd(111) surface. Furthermore, the C–H dissociation barrier is the lowest with an energy of 0.28 eV compared to other dissociation processes (Fig. 4). 3.2.4. CH → C + H For the Ni reaction center, CH at FCCPd is selected as the initial structure, while for the Pd reaction center, CH at HCPPd is set as the initial state since the adsorption for CH at HCPPd is slightly more stable than FCCPd by 0.03 eV (Fig. S2, CH(a)–CH(b)). The configuration of C at FF and H at FCCPd sites is set as final state. The TS structures are shown in Fig. 3d and g. In Fig. 3d, the breaking C\H bond is 1.562 Å, and its orientation is almost parallel to the surface. From the calculated data, the deformation of NiPd(111) surface is obvious. This is because compared with an isolated NiPd(111) (planar) surface, the two Ni atoms bonded with C are enlarged by 11% (from 2.723 to 3.012 Å) on the xy plane, while the Pd and Ni bonded with C rise about 0.187 and 0.023 Å above the surface (along z axis). The energy barrier of CH dehydrogenation into C and H is 1.04 eV (Fig. 4) and the reaction is endothermic by 0.32 eV, which is the highest electronic barrier during the CH4 dissociation process. In Fig. 3g, the C atom locates at the middle between two Ni atoms with dC–Ni = 1.754 Å, and H moves toward the top of Pd with the breaking C\H bond of 2.198 Å. However, the deformation of the surface is not so large as that in the Ni reaction center. The activation energy for CH dissociation with the Pd center is 1.29 eV and the reaction is endothermic by 0.35 eV. The higher barrier and more endothermic
As discussed above, it is seen that Ni shows a higher catalytic activity than Pd on NiPd(111) surface, so the minimum energy path of the reaction should be CH4 dissociation on NiPd(111) with the Ni center. In order to show the performance of NiPd catalyst, we also investigated the mechanism of CH4 dissociation over pure Ni(111) and Pd(111) surfaces by the same level of theory (possible configurations are shown in Figs. S2–S4). The potential energy surface of CH4 dissociation on Ni(111), Pd(111) and NiPd(111) surfaces is shown in Fig. 4. It is seen that CHx (x = 1–4) dehydrogenation into CHx − 1 (x = 1–4) and H on NiPd(111) is more favored both thermodynamically and kinetically than that on Ni(111) and Pd(111) surfaces. A synergistic effect exists between Ni and Pd that results in the improved catalytic performance of NiPd for CH4 dissociation over that of either parent metal. This might be explained in the following. First, the introduction of Pd changes the geometrical structure of pure Ni. The lattice parameter increases from 3.518 Å in pure Ni to 3.841 Å for a and 3.618 Å for c in NiPd alloy. Second, it also makes the surface charge redistributed. Our calculations indicate that with the increase of x in CHx (x = 1, 2, 3), the average net charge of Ni is positive and changes from 0.27, 0.28 to 0.30 |e|, while that of Pd is negative from − 0.12, − 0.19 to − 0.22 |e|. The positive charge of Ni and negative charge of Pd are in agreement with the small electronegativity of Ni (1.91) and large electronegativity of Pd (2.20). This suggests that the Ni\Pd bond is polarized. Since the radicals of CHx (x = 1–3) have redundant electrons, they prefer to be adsorbed to the positive Ni. Thus, Ni becomes the reaction center. Hanmmer and Nørskov pointed out that the d-band center of similar transition metal correlates with the general catalytic activity and the reactivity of the transition metal system increases as the d-band center is shifted up [60,61]. Thus, to further explain why NiPd alloy has a higher catalytic activity and Ni atom is the reaction center for CHx dissociation, the d-band centers of the Ni(111), Pd(111) and NiPd(111) surfaces are examined. Our calculated d-band centers of pure Ni and Pd are − 1.47 eV and − 1.89 eV, respectively, while the average value for NiPd is − 1.71 eV. Obviously, the average d-band center of NiPd alloy surface is far away from a Fermi level compared with Ni(111). However, the d-band center of Ni on NiPd(111) surface is −1.14 eV, closer to a Fermi level than pure Ni, demonstrating that CHx adsorbed on the sites with two Ni atoms is more stable than on the other sites, and dissociations at the Ni reaction center are
Fig. 4. Potential energy curves for CH4 dissociation on Ni(111), Pd(111) and NiPd(111) surfaces, respectively.
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preferred. Thus, we conclude that Ni–Pd synergistic effect enhances Ni catalytic activity in alloy [46]. 4. Conclusions The mechanism of methane dissociation on NiPd(111) surface has been investigated by using the density functional theory. Our results show that CHx (x = 1–3) prefers to adsorb at the hollow sites with two Ni atoms (FCCPd or HCPPd) and CHx (x = 1–4) dissociation on the Ni reaction center is more favored than on the Pd reaction center. The largest electronic energy barrier of CH4 dissociation is CH dissociation to atomic carbon and hydrogen on NiPd(111) surface. By comparing with pure Ni(111) and Pd(111) surfaces, we observed that Ni–Pd alloy shows the highest catalytic activity. A synergistic effect exists between Ni and Pd that results in an improved catalytic performance for CH4 dissociation over that of either parent metal. We conclude that NiPd is a good catalyst to decompose CH4, consistent to the experimental observation. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (grant nos: 20921002, 21273219, 21203174). The authors are also thankful for the financial support from the Department of Science and Technology of Sichuan Province. The computational time is supported by the Performance Computing Center of Jilin University, China. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.susc.2013.02.012. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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