- Email: [email protected]
DFT study of selective hydrogenation of acetylene to ethylene on Pd doping Ag nanoclusters
Applied Surface Science 386 (2016) 125–137
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
DFT study of selective hydrogenation of acetylene to ethylene on Pd doping Ag nanoclusters D. Liu State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, 066004, China
a r t i c l e
i n f o
Article history: Received 13 April 2016 Received in revised form 31 May 2016 Accepted 2 June 2016 Available online 4 June 2016 Keywords: Density functional simulation Selective hydrogenation Acetylene Ethylene Mono-atomic catalysis
a b s t r a c t Recently, it has been reported that the reaction selectivity of catalytic hydrogenation of acetylene to ethylene can be significantly enhanced via the approach of Pd mono-atomic catalysis [Pei et al. ACS Catal. 5 (2015) 3717–3725]. To explain the catalytic mechanism of this binary alloy catalyst, C2 H2 hydrogenation reactions on Pd doping Ag nanoclusters are studied using density functional theory simulations. The simulation results indicate that H2 and C2 H2 can simultaneously bind with a single Pd doping atom no matter it is on vertex and edge sites of Ag clusters. The following H2 dissociation and C2 H2 hydrogenation are not difficult since the corresponding reaction barrier values are no more than 0.58 eV. The generated C2 H4 molecule can not be further hydrogenated since it locates on the top of Pd doping atom, which is the only adsorption site for H2 . On two Pd doping atoms at contiguous sites of Ag clusters, C2 H4 hydrogenation reactions can be carried out since there are enough sites for co-adsorption of H2 and C2 H4 . © 2016 Elsevier B.V. All rights reserved.
1. Introduction In petrochemical industries, ethylene is the important industrial material for polyethylene synthesis. Ethylene is usually produced by the decomposition of hydrocarbons. During the decomposition process, small amounts of acetylene are co-produced with ethylene, which is harmful for ethylene polymerization [1]. Catalytic hydrogenation (C2 H2 + H2 → C2 H4 ) is the preferred method for removing acetylene from an ethylene stream. Pd catalysts are commonly used for this purification process [2]. However, ethylene can be further hydrogenated to ethane (C2 H4 + H2 → C2 H6 ) on Pd catalysts [3]. After this purification process, the acetylene elimination may be accompanied by a large amount of ethylene loss, which is not cost-efficient for polyethylene synthesis. Adding a second metal element to Pd catalysts is an effective approach to increase the reaction selectivity of acetylene hydrogenation [4–6]. For example, Ag-Pd alloy nanoparticles (Ag/Pd atom ratio 1/1 or 1/3) growing on metal-oxides substrates have been prepared using chemical methods [7–9]. When C2 H2 hydrogenation reactions undergo on these Ag-Pd catalysts, the amount of C2 H4 can occupy 25%–60% of the total volume of products [7–9]. Recently, Pd doping Ag nanoclusters (diameters 1.5–3.5 nm) are prepared for C2 H2 hydrogenation [10]. In these Ag-Pd clusters, the content of palladium is very little (Ag/Pd atom ratio 40/1–200/1). Neverthe-
E-mail address: [email protected] http://dx.doi.org/10.1016/j.apsusc.2016.06.013 0169-4332/© 2016 Elsevier B.V. All rights reserved.
less, these binary metal nanoclusters show high C2 H2 conversion rate (> 90%) and reaction selectivity to generate C2 H4 (>80%) for the catalytic hydrogenation of acetylene [10]. Surface compositions of Ag-Pd alloy nanoparticles (Ag/Pd atom ratio 1/1 and 1/3) have been studied via Monte Carlo simulations [11]. Since the surface energy value of Ag is smaller than that of Pd [12], silver atoms are more easily to enrich at the surface of Ag-Pd nanoparticles. For AgPd3 nanoparticles (Ag/Pd atom ratio 1/3), silver occupies more than half of all surface atoms. When the number of Ag and Pd atoms is equal in Ag-Pd nanoparticles, surface silver concentration reaches 80%. At the surface of AgPd3 nanoparticles, most of Pd atoms (about 80%) segregate to form large ensembles. While at the surface of AgPd nanoparticles (Ag/Pd atom ratio 1/1), Pd atoms are mostly in the form of monomer and dimmer (about 70%) since the number of them is far less than that of Ag atoms. In addition, the distribution of Pd species at the surface of Pd doping Ag nanoclusters (Ag/Pd atom ratio 40/1–200/1) is probed by Fourier-Transform Infrared Spectra (FTIR) via CO chemical adsorption [10]. The FTIR results show that CO is mainly adsorbed on the top of Pd sites. The lack of CO adsorption on bridge sites proves that most Pd atoms are mono-dispersed at the surface of Pd doping Ag nanoclusters. On pure Pd and Pd-Ag alloy surfaces, C2 H2 is adsorbed on the two or three neighboring Pd atoms [13]. At the surface of Pd doping Ag nanoclusters, Pd monomer is the main species. The adsorption of C2 H2 on a single Pd atom may become weaker. On the other hand, the catalytic hydrogenation of acetylene needs the co-
126
D. Liu / Applied Surface Science 386 (2016) 125–137
adsorption of H2 and C2 H2 , which is not easy on a mono-atomic site. In our opinions, the limitation of reactants adsorption may bring quite a different reaction process and result. When C2 H2 is adsorbed on a single Pd atom of Pd doping Ag nanoclusters, its hydrogenated product is C2 H4 rather than C2 H6 , which induces significant increasing reaction selectivity. In this contribution, the catalytic hydrogenation of acetylene on Pd doping Ag nanoclusters is simulated via the density functional theory (DFT) method. The adsorption of reactants (H2 and C2 H2 ) and their combination process on a single Pd atom is investigated. The scientific question we want to clarify is that whether C2 H4 is the only product of H2 + C2 H2 reaction via this type of Pd mono-atomic catalysis. For comparison, C2 H2 hydrogenation on two Pd atoms at contiguous sites of Pd doping Ag nanoclusters is also studied. Based on these results, we can evaluate the possibility of C2 H6 production eventually after the co-adsorption and combination of H2 and C2 H2 on Pd diatomic sites.
may locate on separate sites and the situation is similar to that of a single Pd atom doping. In addition, two Pd atoms may also locate on contiguous sites as shown Fig. 1(d). The Pd-doping cluster is defined as Ag53 Pd2 . The adsorption energy value of reactants Ead is calculated based on the following equation, E ad = E cluster-re − (E cluster + E re )
(1)
where Ecluster and Ere separately denote the total energy of the cluster and reactants, Ecluster-re denotes the total energy after reactants are adsorbed on the cluster. In the calculation of TS search, the reaction barrier value Erb is defined as, E rb = E TS − E IS/MS
(2)
where ETS denotes the total energy of the reaction system at TS, EIS/MS denotes the total energy of the reaction system at the initial state (IS) or meta-stable intermediate state (MS) (depending on the reaction step).
2. Simulation details 3. Results and discussion In the first-principles DFT [14,15] simulations, the DMol3 module [16,17] was used for the geometric optimization and reaction process imitation. We employed the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof functional (PBE) [18] to describe exchange and correlation effects. During the geometric optimization process, the energy convergence, maximum force and maximum distance were 1.0 × 10−5 Ha, 2.0 × 10−3 Ha Å−1 and 5.0 × 10−3 Å. In the electronic setting, for reducing the computational cost, the DFT Semi-core Pseudo-potentials (DSPP) method [19] was used to replace core electrons by a single effective potential and introduce some degree of relativistic correction into the core. The self-consistent field (SCF) tolerance value was 1.0 × 10−6 Ha and the double numerical plus d-functions (DND) were chosen as the basis set [16]. For transition states (TS) searching, the calculation firstly performed a linear synchronous transit (LST) [20] maximum, which was followed by an energy minimization in directions conjugating to the reaction pathway. TS approximation obtained via LST/optimization was then used to perform a quadratic synchronous transit (QST) maximization to find more accurate transitional states. The convergence tolerance of the root mean square (RMS) force was 2.0 × 10−3 Ha Å−1 and the maximum number for QST step was set as 10. Vibration frequencies of the reaction system at TS were calculated for assessing whether the transition states are real ones. When the numbers of imaginary frequencies for reactants are more than one, TS optimization and TS confirmation was carried out for searching the real transitional states. In the calculation of TS optimization, the eigenvector following (EF) method [21] is employed, which seems like performing geometric optimization for the originally found TS. In the calculation of TS confirmation, a series of geometric optimization was performed along the reaction pathway to find the real TS. The diameters of Pd doping Ag nanoclusters are in the range of 1.5–3.5 nm [10]. At these dimensional sizes, metal nanoclusters keep quasicrystal icosahedral and decahedral structures [22]. For such small polyhedral structures, surface facets are limited and most surface atoms locate at vertex and edge sites. Considering the consumption time of calculation, a representative icosahedral structure including 55 atoms (the diameter about 1.5 nm) is chosen as the catalyst for C2 H2 hydrogenation. As shown in Fig. 1(a), Ag55 cluster has five-fold symmetric surface structures. Vertex and edge sites are the only two types of surface species. When a single Pd atom is doping on the surface of Ag55 cluster, it may locate at the vertex or edge site as shown in Fig. 1(b) and (c). The Pd-doping clusters are separately defined as Ag54 Pd1-vertex and Ag54 Pd1-edge . When two Pd atoms are doping on the surface of Ag55 cluster, they
During C2 H2 hydrogenation reactions (C2 H2 + H2 → C2 H4 ) on metal surface, C2 H2 and H2 molecules should be firstly co-adsorbed on contiguous sites. After the H2 molecule is dissociated as two H atoms, one of them combines with C2 H2 to form the intermediate product C2 H3 . In following, C2 H3 reacts with the other H atom to generate the final product C2 H4 . The reaction process of C2 H4 hydrogenation (C2 H4 + H2 → C2 H6 ) is similar to that of C2 H2 hydrogenation. For these saturated hydrogenation reactions, C2 H5 and C2 H6 molecules are separately the intermediate and final products. Firstly, the adsorption condition of reactants and intermediate products on Ag55 cluster is studied. All the possible adsorption sites are shown in Fig. 2 and the corresponding Ead values are given in Table 1. In Fig. 2(a)–(c), it is found that a hydrogen atom can be adsorbed on the hcp hollow, fcc hollow and bridge (connecting the vertex and edge) sites. However, H2 molecule can not be adsorbed on any site of Ag55 cluster. In Fig. 2(d) and (e), it is found that C2 H2 can be adsorbed on the top of vertex and edge sites. The corresponding Ead values are only −0.26 and −0.19 eV, which proves the weak bonding between acetylene and Ag surfaces. C2 H3 is an unsaturated intermediate product. It can be intensively adsorbed on the top of vertex, bridge (connecting the vertex and edge) and bridge (connecting two edges) sites as shown in Fig. 2(f)–(h). The adsorption of C2 H4 on Ag55 cluster is also very weak. In Fig. 2(i) and (j), it is found that C2 H4 can be adsorbed on the top of vertex and edge sites and the corresponding Ead values are only −0.31 and −0.21 eV. The intermediate product C2 H5 is moderately adsorbed on the top of vertex and edge, bridge (connecting the vertex and edge) and bridge (connecting two edges) sites as shown in Fig. 2(k)–(n). Since H2 can not be adsorbed on Ag55 cluster, the following catalytic hydrogenation of acetylene and ethylene is impossible. All the possible adsorption sites of reactants and intermediate products on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-vertex cluster are shown in Fig. 3 and the corresponding Ead values are given in Table 2. Generally, the doping Pd atom can give all the atoms and molecules stronger adsorption compared with the situation of pure Ag surfaces. In Fig. 3(a), it is found that H2 can be adsorbed on the top of vertex site, for which the Ead value is −0.22 eV. There are two sites for a hydrogen atom adsorption: the PdAg2 hcp hollow and Pd-Ag bridge as shown in Fig. 3(b) and (c). C2 H2 can be adsorbed on sites including the Pd atom in five types. The most stable adsorption site is the top of Pd atom [Fig. 3(d)], on which Ead reaches −0.78 eV. In Fig. 3(e), it is found that C2 H2 can be adsorbed on Pd-Ag bridge in type of tbt (topbridge-top). On the hollow site, there are two types for diatomic
D. Liu / Applied Surface Science 386 (2016) 125–137
127
Fig. 1. The geometric configurations of (a) Ag55 , (b) Ag54 Pd1-vertex , (c) Ag54 Pd1-edge and (d) Ag53 Pd2 clusters.
Table 1 Possible adsorption sites and the corresponding adsorption energy values of reactants and intermediate products on Ag55 cluster. Ag55 cluster Reactant
Site
Ead (eV)
Reactant
Site
Ead (eV)
H-I H-II H-III C2 H2 -I C2 H2 -II C2 H3 -I C2 H3 -II
hcp fcc bridge (vertex-edge) top (vertex) top (edge) top (vertex) bridge (vertex-edge)
−2.00 −2.18 −1.89 −0.26 −0.19 −1.30 −1.66
C2 H3 -III C2 H4 -I C2 H4 -II C2 H5 -I C2 H5 -II C2 H5 -III C2 H5 -IV
bridge (edge-edge) top (vertex) top (edge) top (vertex) top (edge) bridge (vertex-edge) bridge (edge-edge)
−1.54 −0.31 −0.21 −0.77 −0.88 −0.76 −0.74
adsorption: thob (top-hollow-bridge) and bhob (bridge-hollowbridge) [23]. Differentiating fcc and hcp hollows, the thob state can be sub-divided as tfb (top-fcc hollow-bridge) and thb (top-hcp hollow-bridge) ones. Similarly, the bhob state can be sub-divided as bfb (bridge-fcc hollow-bridge) and bhb (bridge-hcp hollow-bridge) ones. Considering the difference between Pd and Ag atoms, C2 H2 can be adsorbed on the PdAg2 hcp hollow in three types: thb (C on Pd top), bhb (C C bond parallel to Pd-Ag bridge) and bhb (C C bond parallel to Ag-Ag bridge) as shown Fig. 3(f)–(h). The unsaturated intermediate product C2 H3 is intensively adsorbed on the Pd-Ag bridge [Fig. 3(i)], for which Ead reaches −2.26 eV. In Fig. 3(j) and (k), it is found that C2 H4 and C2 H5 are adsorbed on the top of vertex site, for which Ead values are separately −0.75 and −1.47 eV.
The reaction process of C2 H2 hydrogenation on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-vertex cluster is shown in Fig. 4(a)–(h) and the corresponding potential energy change is given in Fig. 4(i). In usual, heterogeneous catalytic reactions need the co-adsorption of reactants on contiguous sites. At the surface of Ag54 Pd1-vertex cluster, C2 H2 adsorption on the Pd atom and its neighboring Ag atoms is much stronger than that on only Ag atoms. H2 can be only adsorbed on the top of Pd atom. Therefore, C2 H2 and H2 would share the Pd atom for adsorption. As mentioned above, there are five types for individual C2 H2 adsorption. When C2 H2 is co-adsorbed with H2 , it can only locate at the PdAg2 hcp hollow site (C2 H2 -IV type) as shown in Fig. 4(a). After the co-adsorption of reactants, H2 is easily dissociated as two H atoms with very small Erb value 0.05 eV. In Fig. 4(b) and (c), it is found that C2 H2 rotates during
128
D. Liu / Applied Surface Science 386 (2016) 125–137
Fig. 2. Possible adsorption sites of reactants and intermediate products (a)–(c) H, (d)–(e) C2 H2 , (f)–(h) C2 H3 , (i)–(j) C2 H4 and (k)–(n) C2 H5 on Ag55 cluster.
the H2 dissociation process. After the H2 dissociation, the C C bond of C2 H2 is parallel to the Ag-Ag bridge (C2 H2 -V type). In following, one of H atoms combines with C2 H2 to form C2 H3 as shown in Fig. 4(d) and (e). Erb value of this combination process is 0.58 eV. The remaining H atom can easily react with C2 H3 to form C2 H4 [Fig. 4(f) and (g)], for which the Erb value is only 0.21 eV. After the generated C2 H4 molecule has been desorbed [Fig. 4(h)], other C2 H2 and H2 molecules can be co-adsorbed on the Pd atom for catalytic hydrogenation reactions. In Fig. 4(i), it is found that the potential energy reduces −2.32 eV after C2 H2 hydrogenation on Ag54 Pd1-vertex cluster, which is just the reaction heat of C2 H2 + H2 → C2 H4 .
All the possible adsorption sites of reactants and intermediate products on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-edge cluster are shown in Fig. 5 and the corresponding Ead values are given in Table 2. In Fig. 5(a), it is found that H2 can be adsorbed on the top of Pd atom, for which the Ead value is −0.29 eV. There are three sites for a hydrogen atom adsorption: the PdAg2 hcp hollow, fcc hollow and Pd-Ag bridge (Ag at vertex site) as shown in Figs. 5(b)–(d). C2 H2 can be adsorbed on sites including the Pd atom in six types. The most stable adsorption site is the top of Pd atom [Fig. 5(e)], on which Ead reaches −0.73 eV. On the PdAg2 fcc hollow, there are two types for C2 H2 adsorption: tfb (C on Pd top) and bfb (C C bond parallel to Pd-Ag bridge) as shown in Fig. 5(f) and (g). In
D. Liu / Applied Surface Science 386 (2016) 125–137
129
Fig. 3. Possible adsorption sites of reactants and intermediate products (a) H2 , (b)–(c) H, (d)–(h) C2 H2 , (i) C2 H3 , (j) C2 H4 and (k) C2 H5 on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-vertex cluster.
Fig. 5(h)–(j), it is found that C2 H2 can be adsorbed on the PdAg2 hcp hollow in three bhb types. The difference between them is that C C bonds of C2 H2 molecules are separately parallel to three different types of bridges. In Fig. 5(k) and (l), it is found that C2 H3 can be adsorbed on two different types of Pd-Ag bridges (differentiating Ag atoms at vertex and edge sites). In addition, the only adsorption site for C2 H4 and C2 H5 is the top of Pd atom as shown in Fig. 5(m) and (n). The reaction process of C2 H2 hydrogenation on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-edge cluster is shown in Fig. 6(a)–(i) and the corresponding potential energy change is given in Fig. 6(j). As mentioned above, there are six types for individual C2 H2 adsorption. When C2 H2 is co-adsorbed with H2 , it can only locate at the PdAg2 hcp hollow site (C2 H2 -V type) as shown in Fig. 6(a). After the co-adsorption of reactants, H2 is dissociated as two H atoms with moderate Erb value 0.34 eV. In Fig. 6(c), it is found that the two H atoms are separately adsorbed on the Pd-Ag bridge and PdAg2 fcc hollow sites. In following, the H atom on the Pd-Ag bridge combines with C2 H2 to form C2 H3 as shown in Fig. 6(d) and (e). Erb value of this combination process is 0.43 eV. In Fig. 6(e), it is found that C2 H3 locates on the Ag-Ag bridge after the combination of H2 and C2 H2 . To react with the remaining H atom, C2 H3 moves to the Pd-Ag bridge as shown in Fig. 6(f). The remaining H atom can react with C2 H3 to form C2 H4 [Figs. 6(g) and (h)], for which the Erb
value is 0.39 eV. After the generated C2 H4 molecule has been desorbed [Fig. 6(i)], other C2 H2 and H2 molecules can be co-adsorbed on the Pd atom for catalytic hydrogenation reactions. Based on simulation results, it is concluded that the catalytic hydrogenation of acetylene to ethylene can be carried out on the doping Pd atom at both vertex and edge sites of Ag nanoclusters. The following catalytic hydrogenation of ethylene to ethane needs the co-adsorption of H2 and C2 H4 . As mentioned above, H2 can not be adsorbed on Ag sites. The adsorption of C2 H4 on the top of Ag vertex and edge sites is also very weak. Both of H2 and C2 H4 molecules are more likely to bind with the Pd atom. Considering the co-adsorption of H2 and C2 H2 , C2 H2 can be adsorbed on the PdAg2 hcp hollow, which makes space for H2 adsorption [Figs. 4(a) and 6(a)]. For C2 H4 binding with the Pd doping atom of Ag54 Pd1-vertex and Ag54 Pd1-edge clusters, it can only locate on the top of Pd vertex and edge sites [Figs. 3(j) and 5(m)]. In Fig. 7, it is found that the co-adsorption of H2 and C2 H4 on the Pd atom is impossible since the adsorption site of H2 has been occupied by C2 H4 . In the above analysis, the co-adsorption of H2 and C2 H4 is considered as the starting point, which follows a typical Langmuir-Hinshelwood mechanism. The reaction may be carried out in another path: H2 in air is directly dissociated on Ag sites and the generated H atoms react with C2 H4 on the Pd site, which follows an Eley-Rideal mechanism. Based on this consideration, the non-adsorbed H2 molecule
130
D. Liu / Applied Surface Science 386 (2016) 125–137
Table 2 Possible adsorption sites and the corresponding adsorption energy values of reactants and intermediate products on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-vertex and Ag54 Pd1-edge clusters. Ag54 Pd1-vertex cluster Reactant
Site
Ead (eV)
Reactant
Site
Ead (eV)
H2
Pd top
−0.22
C2 H2 -IV
−0.76
H-I
PdAg2 hcp
−2.43
C2 H2 -V
H-II C2 H2 -I C2 H2 -II C2 H2 -III
Pd-Ag bridge Pd top tbt (Pd-Ag bridge) thb (C on Pd top)
−2.41 −0.78 −0.55 −0.72
C2 H3 C2 H4 C2 H5
bhb (C C bond parallel to Pd-Ag bridge) bhb (C C bond parallel to Ag-Ag bridge) Pd-Ag bridge Pd top Pd top
−2.26 −0.75 −1.47
Reactant
Site
Ead (eV)
Reactant
Site
Ead (eV)
H2
Pd top
−0.29
C2 H2 -IV
−0.60
H-I
PdAg2 hcp
−2.44
C2 H2 -V
H-II
PdAg2 fcc
−2.57
C2 H2 -VI
H-III C2 H2 -I C2 H2 -II C2 H2 -III
Pd-Ag bridge (Ag at vertex site) Pd top PdAg2 tfb (C on Pd top) PdAg2 bfb (C C bondparallel to Pd-Ag bridge)
−2.36 −0.73 −0.66 −0.66
C2 H3 -I C2 H3 -II C2 H4 C2 H5
PdAg2 bhb (C C bond parallel to Ag-Ag bridge) PdAg2 bhb (C C bond parallel to Pd-Ag bridge, Ag vertex site) PdAg2 bhb (C C bond parallel to Pd-Ag bridge, Ag edge site) Pd-Ag bridge (Ag vertex site) Pd-Ag bridge (Ag edge site) Pd top Pd top
−0.64
Ag54 Pd1-edge cluster
dissociation on Ag sites of Ag54 Pd1 clusters are studied. The simulation results indicate that the Erb value for this dissociation process is as high as 1.59 eV, which means that H2 in air has no opportunity to directly decompose on Ag sites and further react with C2 H4 on the Pd site. Therefore, on single Pd atom doping Ag nanoclusters, C2 H4 can not be hydrogenated to form C2 H6 . All the possible adsorption sites of reactants and intermediate products on the two Pd atoms and their neighboring Ag atom of Ag53 Pd2 cluster are shown in Fig. 8 and the corresponding adsorption values are given in Table 3. Generally, the adsorption of reactants and intermediate products on Pd diatomic sites is stronger than that on Pd mono-atomic site. In Fig. 8(a), it is found that a hydrogen atom can be adsorbed on the Pd-Pd bridge, for which the Ead value is −2.78 eV. C2 H2 can be adsorbed on the Pd2 Ag hcp hollow in three bhb types as shown in Fig. 8(b)–(d). The difference between them is that C C bonds of C2 H2 molecules are separately parallel to three different types of bridges. C2 H3 can be adsorbed on the Pd-Pd bridge [Fig. 8(e)], for which the Ead value is −2.63 eV. In Fig. 8(f), it is found that C2 H4 locates on the two Pd atoms in tbt type. The reaction process of C2 H2 hydrogenation on the two Pd atoms and their neighboring Ag atoms of Ag53 Pd2 cluster is shown in Fig. 9(a)–(h) and the corresponding potential energy change is given in Fig. 9(i). For the co-adsorption of reactants, H2 binds with the vertex site and C2 H2 locates on the Pd2 Ag hcp hollow (C2 H2 -I type) as shown in Fig. 9(a). After the co-adsorption of reactants, H2 is dissociated as two H atoms with moderate Erb value 0.38 eV. In Fig. 9(b) and (c), it is found that C2 H2 rotates during the H2 dissociation process. The C C bond of C2 H2 is parallel to the Pd-Ag bridge (C2 H2 -III type) after the H2 dissociation. In Fig. 9(c), it is found that the two H atoms are separately adsorbed on the Pd-Ag bridge and PdAg2 hcp hollow sites. In following, the H atom on the Pd-Ag bridge combines with C2 H2 to form C2 H3 as shown in Fig. 9(d) and (e). Erb value of this combination process is 0.58 eV. In Fig. 9(e), it is found that the remaining H atom move to the neighboring PdAg2 hcp hollow after the formation of C2 H3 . The remaining H atom can react with C2 H3 to generate C2 H4 [Fig. 9(f) and (g)], for which the Erb value is 0.58 eV. After the C2 H4 molecule
−0.64
−0.66
−2.15 −2.07 −0.73 −1.39
has been desorbed [Fig. 9(h)], other C2 H2 and H2 molecules can be co-adsorbed on the two Pd atoms for catalytic hydrogenation reactions. The reaction process of C2 H4 hydrogenation on the two Pd atoms and their neighboring Ag atoms of Ag53 Pd2 cluster is shown in Fig. 10(a)–(h) and the corresponding potential energy change is given in Fig. 10(i). As mentioned above, H2 and C2 H4 can not be coadsorbed on the Pd atom of Ag54 Pd1-vertex and Ag54 Pd1-edge clusters. This problem is resolved on Ag53 Pd2 cluster since it can offer contiguous Pd sites for reactants adsorption. At the beginning of C2 H4 hydrogenation reactions, H2 and C2 H4 are separately adsorbed on the top of Pd edge and vertex sites [Fig. 10(a)], for which the Ead value reaches −1.06 eV. In Fig. 10(b) and (c), it is found that H2 can be easily dissociated as two H atoms with small Erb value 0.18 eV. After H2 dissociation, one of H atoms combines with C2 H4 to form C2 H5 as shown in Fig. 10(d) and (e). Erb value of this combination process is 0.75 eV. The generated C2 H5 molecule is far from the remaining H atom. For the following reaction, C2 H5 turns close to the remaining H atom as shown in Fig. 10(f). The remaining H atom can react with C2 H3 to generate C2 H4 [Fig. 10(g) and (h)], for which the Erb value is 0.62 eV. After the C2 H6 molecule has moven away, other C2 H4 and H2 molecules can be co-adsorbed on the two Pd atoms for catalytic hydrogenation reactions. In Fig. 10(i), it is found that the potential energy reduces −1.87 eV after C2 H4 hydrogenation on Ag53 Pd2 cluster, which is just the reaction heat of C2 H4 + H2 → C2 H6 . The cluster model used above has 55 atoms and a diameter about 1.5 nm. With the increasing cluster size (for example, 147 atoms and diameter 2 nm), the computational time cost would be huge. The geometric optimization of clusters including more than 150 atoms is almost impossible using the DMol3 module. Nevertheless, the reaction situation on Pd doping Ag clusters with diameters 2–3.5 nm can also be indirectly evaluated. On icosahedral Ag55 clusters, the coordinated numbers (CN) of atoms on surface edge and vertex sites are 8 and 6, separately. In the size range of 2–3.5 nm, metal clusters still keep quasicrystal icosahedral and decahedral structures [22]. On these larger clusters, CN of atoms on surface edge and vertex sites keep unchanged [24]. Therefore, the reactants
D. Liu / Applied Surface Science 386 (2016) 125–137
131
Fig. 4. (a)-(h) The reaction process and (i) potential energy change of catalytic hydrogenation of acetylene to ethylene on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-vertex cluster. Table 3 Possible adsorption sites and the corresponding adsorption energy values of reactants and intermediate products on two Pd atoms and their neighboring Ag atoms of Ag53 Pd2 cluster. Ag53 Pd2 cluster Reactant
Site
Ead (eV)
Reactant
Site
Ead (eV)
H
Pd-Pd bridge
−2.78
C2 H2 -III
−1.37
C2 H2 -I C2 H2 -II
Pd2 Ag bhb (C C bond parallel to Pd-Pd bridge) Pd2 Ag bhb (C C bondparallel to Pd-Ag bridge,Pd edge site)
−1.46 −1.24
C2 H3 C2 H4
Pd2 Ag bhb (C C bond parallel to Pd-Ag bridge, Pd vertex site) Pd-Pd bridge Pd2 tbt
adsorption and following reactions on the low-coordinated sites of larger Pd doping Ag clusters should be similar to the situation on Ag54 Pd1 and Ag53 Pd2 ones.
−2.63 −0.86
With the increasing cluster sizes, there are also small closepacked facets on the surface of Ag clusters [24]. The Pd doping atoms can also exist on these medium-coordinated sites (CN = 9).
132
D. Liu / Applied Surface Science 386 (2016) 125–137
Fig. 5. Possible adsorption sites of reactants and intermediate products (a) H2 , (b)–(d) H, (e)–(j) C2 H2 , (k)–(l) C2 H3 , (m) C2 H4 and (n) C2 H5 on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-edge cluster.
The atom arrangement of these facets on clusters is as same as that of bulk (111) surfaces. Therefore, Ag (111) slabs were built for representing the facets of Ag nanoparticles as shown in Figs. S1. The details for building these slabs are shown in supplementary data. In Figs. S1(a) and S1(b), one and two surface Ag atoms are separately replaced by Pd atoms [Ag-1Pd (111) and Ag-2Pd (111)], which denotes the single and contiguous Pd sites. In Table S1, it is generally found that the adsorption of reactants and intermediate products on Pd doping atoms at Ag (111)
is weaker than that on Pd doping atoms at Ag clusters. This conclusion is not reasonable since low-coordinated sites usually can give molecules and atoms stronger adsorption. On Ag-1Pd (111), H2 can be only adsorbed on the top of Pd atom. The adsorption of C2 H2 also needs the contribution of Pd atom. The co-adsorption of H2 and C2 H2 is impossible since the adsorption site of H2 has been occupied by C2 H2 as shown in Fig. S4. Even if C2 H2 is adsorbed on Ag-1Pd (111) in other adsorption types (C2 H2 -II, III and IV in Fig. S2), the co-adsorption with H2 is also impossible. Therefore, when
D. Liu / Applied Surface Science 386 (2016) 125–137
133
Fig. 6. (a)–(i) The reaction process and (j) potential energy change of catalytic hydrogenation of acetylene to ethylene on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-edge cluster.
134
D. Liu / Applied Surface Science 386 (2016) 125–137
Fig. 7. Illustration for proving the infeasibility for catalytic hydrogenation of ethylene on the Pd atom of Ag54 Pd1-vertex and Ag54 Pd1-edge clusters.
Fig. 8. Possible adsorption sites of reactants and intermediate products (a) H, (b)–(d) C2 H2 , (e) C2 H3 and (f) C2 H4 on the two Pd atoms and their neighboring Ag atom of Ag53 Pd2 cluster.
a single Pd atom is doping on the facets of Ag (111), the catalytic hydrogenation of acetylene can not be carried out. There is additional Pd site for H2 adsorption on Ag-2Pd (111). In Figs. S5 and S6, it is found that both C2 H2 and C2 H4 hydrogenation reactions can be carried out on Ag-2Pd (111) since the Erb values are in the range of 0.31–0.66 eV.
On the surface of larger Pd doping Ag clusters (diameters 2–3.5 nm), atoms at small close-packed facets still occupies minority. A doping Pd atom is more likely to locate on low-coordinated surface sites. The selective hydrogenation of acetylene is also feasible on larger Pd doping Ag clusters. It has been proved that both C2 H2 and C2 H4 hydrogenation reactions can be carried out on pure Pd surfaces, for which the Erb
D. Liu / Applied Surface Science 386 (2016) 125–137
135
Fig. 9. (a)–(h) The reaction process and (i) potential energy change of catalytic hydrogenation of acetylene to ethylene on two Pd atoms and their neighboring Ag atoms of Ag53 Pd2 cluster.
values are in the range of 0.65–0.80 eV [25]. Based on above simulation results, it is concluded that the saturated hydrogenation of acetylene to ethane can still be realized on only two Pd doping atoms at inert Ag surfaces. Therefore, Pd mono-atomic catalysis is the only method to increase the reaction selectivity for catalytic hydrogenation of acetylene to ethylene. 4. Conclusions C2 H2 and C2 H4 molecules are weakly adsorbed on Ag55 cluster. H2 can not bind with any site of this pure Ag nanocluster. The catalytic hydrogenation of acetylene and ethylene is impossible to carry out on Ag55 cluster. On Ag54 Pd1-vertex cluster, the co-adsorption of H2 and C2 H2 is possible when C2 H2 locates on the
PdAg2 hcp hollow in bhb adsorption type. In following, H2 is dissociated as two H atoms, which combine with C2 H2 separately to form C2 H4 . Erb values for these reactions are in the range of 0.05–0.58 eV. Similarly, H2 and C2 H2 can be co-adsorbed on Ag54 Pd1-edge cluster when C2 H2 locates on the PdAg2 hcp hollow in bhb adsorption type. Erb values for subsequent H2 dissociation and C2 H2 hydrogenation are in the range of 0.34–0.43 eV. H2 and C2 H4 can not be co-adsorbed on both of Ag54 Pd1-vertex and Ag54 Pd1-edge clusters since the only adsorption site of H2 is occupied by C2 H4 . On Ag53 Pd2 cluster, H2 and C2 H2 can be co-adsorbed when C2 H2 locates on the Pd2 Ag hcp hollow in bhb adsorption type. Erb values for H2 dissociation and C2 H2 hydrogenation are in the range of 0.38–0.58 eV. As for C2 H4 hydrogenation reactions, H2 and C2 H4 can be separately
136
D. Liu / Applied Surface Science 386 (2016) 125–137
Fig. 10. (a)–(h) The reaction process and (i) potential energy change of catalytic hydrogenation of ethylene to ethane on two Pd atoms and their neighboring Ag atoms of Ag53 Pd2 cluster.
D. Liu / Applied Surface Science 386 (2016) 125–137
adsorbed on the two Pd atoms. Erb values for H2 dissociation and C2 H4 hydrogenation are in the range of 0.18–0.75 eV. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.06. 013. References [1] A.N.R. Bos, E.S. Botsma, F. Foeth, H.W.J. Sleyster, K.R. Westerterp, A kinetic study of the hydrogenation of ethyne and ethene on a commercial Pd/Al2 O3 catalyst, Chem. Eng. Process.: Process Intensif. 32 (1993) 53–63. [2] W.T. McGown, C. Kemball, D.A. Whan, M.S. Scurrell, Hydrogenation of acetylene in excess ethylene on an alumina supported palladium catalyst in a static system, J. Chem. Soc. Faraday Trans. 73 (1977) 632–647. ´ G.C. Bond, Selective hydrogenation of ethyne in ethene-rich [3] A. Borodzinski, streams on palladium catalysts. Part 1. Effect of changes to the catalyst during reaction, Catal. Rev. 48 (2006) 91–144. [4] A. Sárkány, O. Geszti, G. Sáfrán, Preparation of Pdshell –Aucore /SiO2 catalyst and catalytic activity for acetylene hydrogenation, Appl. Catal. A: Gen. 350 (2008) 157–163. [5] J.H. Lee, S.K. Kim, I.Y. Ahn, W.J. Kim, S.H. Moon, Performance of Pd–Ag/Al2 O3 catalysts prepared by the selective deposition of Ag onto Pd in acetylene hydrogenation, Catal. Commun. 12 (2011) 1251–1254. [6] S.K. Kim, J.H. Lee, I.Y. Ahn, W.J. Kim, S.H. Moon, Performance of Cu-promoted Pd catalysts prepared by adding Cu using a surface redox method in acetylene hydrogenation, Appl. Catal. A: Gen. 401 (2011) 12–19. [7] Y. Han, D. Peng, Z. Xu, H. Wan, S. Zheng, D. Zhu, TiO2 supported Pd@Ag as highly selective catalysts for hydrogenation of acetylene in excess ethylene, Chem. Commun. 49 (2013) 8350–8352. [8] Y. Zhang, W. Diao, C.T. Williams, J.R. Monnier, Selective hydrogenation of acetylene in excess ethylene using Ag- and Au-Pd/SiO2 bimetallic catalysts prepared by electroless deposition, Appl. Catal. A: Gen. 469 (2014) 419–426. [9] C. Shi, R. Hoisington, B.W.L. Jang, Promotion effects of air and H2 nonthermal plasmas on TiO2 supported Pd and Pd-Ag catalysts for selective hydrogenation of acetylene, Ind. Eng. Chem. Res. 46 (2007) 4390–4395.
137
[10] G.X. Pei, X.Y. Liu, A. Wang, A.F. Lee, M.A. Isaacs, L. Li, X. Pan, X. Yang, X. Wang, Z. Tai, K. Wilson, T. Zhang, Ag alloyed Pd single-atom catalysts for efficient selective hydrogenation of acetylene to ethylene in excess ethylene, ACS Catal. 5 (2015) 3717–3725. [11] J. Tang, L. Deng, H. Deng, S. Xiao, X. Zhang, W. Hu, Surface segregation and chemical ordering patterns of Ag-Pd nanoalloys: energetic factors, nanoscale effects, and catalytic implication, J. Phys. Chem. C 118 (2014) 27850–27860. [12] Q. Jiang, H.M. Lu, M. Zhao, Modelling of surface energies of elemental crystals, J. Phys. Condens. Matter 16 (2004) 521–530. [13] D. Mei, M. Neurock, C.M. Smith, Hydrogenation of acetylene-ethylene mixtures over Pd and Pd-Ag alloys: first-principle-based kinetic Monte Carlo simulations, J. Catal. 268 (2009) 181–195. [14] P. Hohenberg, W. Kohn, Inhomogeneous electron gas, Phys. Rev. 136 (1964) B864–B871. [15] W. Kohn, L.J. Sham, Self-consistent equations including exchange and correlation effects, Phys. Rev. 140 (1965) A1133–A1138. [16] B. Delley, An all-electron numerical method for solving the local density functional for polyatomic molecules, J. Chem. Phys. 92 (1990) 508–517. [17] B. Delley, From molecules to solids with the DMol3 approach, J. Chem. Phys. 113 (2000) 7756–7764. [18] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [19] B. Delley, Hardness conserving semilocal pseudopotentials, Phys. Rev. B 66 (2002) 155125-1–155125-9. [20] T.A. Halgren, W.N. Lipscomb, The synchronous-transit method for determining reaction pathways and locating molecular transition states, Chem. Phys. Lett. 49 (1977) 225––232. [21] J. Baker, An algorithm for the location of transition states, J. Comput. Chem. 7 (1986) 385––395. [22] F. Baletto, R. Ferrando, Structural properties of nanoclusters: energetic, thermodynamic, and kinetic effects, Rev. Mod. Phys. 77 (2005) 371–417. [23] Y. Xu, M. Mavrikakis, The adsorption and dissociation of O2 on Pt-Co and Pt-Fe alloys, J. Am. Chem. Soc. 126 (2004) 4717–4725. [24] D. Liu, Y.F. Zhu, Q. Jiang, Site and structure dependent cohesive energy in several Ag clusters, J. Phys. Chem. C 113 (2009) 10907––10912. [25] D. Mei, P.A. Sheth, M. Neurock, C.M. Smith, First-principles-based kinetic Monte Carlo simulation of the selective hydrogenation of acetylene over Pd(111), J. Catal. 242 (2006) 1–15.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
DFT study of selective hydrogenation of acetylene to ethylene on Pd doping Ag nanoclusters D. Liu State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, 066004, China
a r t i c l e
i n f o
Article history: Received 13 April 2016 Received in revised form 31 May 2016 Accepted 2 June 2016 Available online 4 June 2016 Keywords: Density functional simulation Selective hydrogenation Acetylene Ethylene Mono-atomic catalysis
a b s t r a c t Recently, it has been reported that the reaction selectivity of catalytic hydrogenation of acetylene to ethylene can be significantly enhanced via the approach of Pd mono-atomic catalysis [Pei et al. ACS Catal. 5 (2015) 3717–3725]. To explain the catalytic mechanism of this binary alloy catalyst, C2 H2 hydrogenation reactions on Pd doping Ag nanoclusters are studied using density functional theory simulations. The simulation results indicate that H2 and C2 H2 can simultaneously bind with a single Pd doping atom no matter it is on vertex and edge sites of Ag clusters. The following H2 dissociation and C2 H2 hydrogenation are not difficult since the corresponding reaction barrier values are no more than 0.58 eV. The generated C2 H4 molecule can not be further hydrogenated since it locates on the top of Pd doping atom, which is the only adsorption site for H2 . On two Pd doping atoms at contiguous sites of Ag clusters, C2 H4 hydrogenation reactions can be carried out since there are enough sites for co-adsorption of H2 and C2 H4 . © 2016 Elsevier B.V. All rights reserved.
1. Introduction In petrochemical industries, ethylene is the important industrial material for polyethylene synthesis. Ethylene is usually produced by the decomposition of hydrocarbons. During the decomposition process, small amounts of acetylene are co-produced with ethylene, which is harmful for ethylene polymerization [1]. Catalytic hydrogenation (C2 H2 + H2 → C2 H4 ) is the preferred method for removing acetylene from an ethylene stream. Pd catalysts are commonly used for this purification process [2]. However, ethylene can be further hydrogenated to ethane (C2 H4 + H2 → C2 H6 ) on Pd catalysts [3]. After this purification process, the acetylene elimination may be accompanied by a large amount of ethylene loss, which is not cost-efficient for polyethylene synthesis. Adding a second metal element to Pd catalysts is an effective approach to increase the reaction selectivity of acetylene hydrogenation [4–6]. For example, Ag-Pd alloy nanoparticles (Ag/Pd atom ratio 1/1 or 1/3) growing on metal-oxides substrates have been prepared using chemical methods [7–9]. When C2 H2 hydrogenation reactions undergo on these Ag-Pd catalysts, the amount of C2 H4 can occupy 25%–60% of the total volume of products [7–9]. Recently, Pd doping Ag nanoclusters (diameters 1.5–3.5 nm) are prepared for C2 H2 hydrogenation [10]. In these Ag-Pd clusters, the content of palladium is very little (Ag/Pd atom ratio 40/1–200/1). Neverthe-
E-mail address: [email protected] http://dx.doi.org/10.1016/j.apsusc.2016.06.013 0169-4332/© 2016 Elsevier B.V. All rights reserved.
less, these binary metal nanoclusters show high C2 H2 conversion rate (> 90%) and reaction selectivity to generate C2 H4 (>80%) for the catalytic hydrogenation of acetylene [10]. Surface compositions of Ag-Pd alloy nanoparticles (Ag/Pd atom ratio 1/1 and 1/3) have been studied via Monte Carlo simulations [11]. Since the surface energy value of Ag is smaller than that of Pd [12], silver atoms are more easily to enrich at the surface of Ag-Pd nanoparticles. For AgPd3 nanoparticles (Ag/Pd atom ratio 1/3), silver occupies more than half of all surface atoms. When the number of Ag and Pd atoms is equal in Ag-Pd nanoparticles, surface silver concentration reaches 80%. At the surface of AgPd3 nanoparticles, most of Pd atoms (about 80%) segregate to form large ensembles. While at the surface of AgPd nanoparticles (Ag/Pd atom ratio 1/1), Pd atoms are mostly in the form of monomer and dimmer (about 70%) since the number of them is far less than that of Ag atoms. In addition, the distribution of Pd species at the surface of Pd doping Ag nanoclusters (Ag/Pd atom ratio 40/1–200/1) is probed by Fourier-Transform Infrared Spectra (FTIR) via CO chemical adsorption [10]. The FTIR results show that CO is mainly adsorbed on the top of Pd sites. The lack of CO adsorption on bridge sites proves that most Pd atoms are mono-dispersed at the surface of Pd doping Ag nanoclusters. On pure Pd and Pd-Ag alloy surfaces, C2 H2 is adsorbed on the two or three neighboring Pd atoms [13]. At the surface of Pd doping Ag nanoclusters, Pd monomer is the main species. The adsorption of C2 H2 on a single Pd atom may become weaker. On the other hand, the catalytic hydrogenation of acetylene needs the co-
126
D. Liu / Applied Surface Science 386 (2016) 125–137
adsorption of H2 and C2 H2 , which is not easy on a mono-atomic site. In our opinions, the limitation of reactants adsorption may bring quite a different reaction process and result. When C2 H2 is adsorbed on a single Pd atom of Pd doping Ag nanoclusters, its hydrogenated product is C2 H4 rather than C2 H6 , which induces significant increasing reaction selectivity. In this contribution, the catalytic hydrogenation of acetylene on Pd doping Ag nanoclusters is simulated via the density functional theory (DFT) method. The adsorption of reactants (H2 and C2 H2 ) and their combination process on a single Pd atom is investigated. The scientific question we want to clarify is that whether C2 H4 is the only product of H2 + C2 H2 reaction via this type of Pd mono-atomic catalysis. For comparison, C2 H2 hydrogenation on two Pd atoms at contiguous sites of Pd doping Ag nanoclusters is also studied. Based on these results, we can evaluate the possibility of C2 H6 production eventually after the co-adsorption and combination of H2 and C2 H2 on Pd diatomic sites.
may locate on separate sites and the situation is similar to that of a single Pd atom doping. In addition, two Pd atoms may also locate on contiguous sites as shown Fig. 1(d). The Pd-doping cluster is defined as Ag53 Pd2 . The adsorption energy value of reactants Ead is calculated based on the following equation, E ad = E cluster-re − (E cluster + E re )
(1)
where Ecluster and Ere separately denote the total energy of the cluster and reactants, Ecluster-re denotes the total energy after reactants are adsorbed on the cluster. In the calculation of TS search, the reaction barrier value Erb is defined as, E rb = E TS − E IS/MS
(2)
where ETS denotes the total energy of the reaction system at TS, EIS/MS denotes the total energy of the reaction system at the initial state (IS) or meta-stable intermediate state (MS) (depending on the reaction step).
2. Simulation details 3. Results and discussion In the first-principles DFT [14,15] simulations, the DMol3 module [16,17] was used for the geometric optimization and reaction process imitation. We employed the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof functional (PBE) [18] to describe exchange and correlation effects. During the geometric optimization process, the energy convergence, maximum force and maximum distance were 1.0 × 10−5 Ha, 2.0 × 10−3 Ha Å−1 and 5.0 × 10−3 Å. In the electronic setting, for reducing the computational cost, the DFT Semi-core Pseudo-potentials (DSPP) method [19] was used to replace core electrons by a single effective potential and introduce some degree of relativistic correction into the core. The self-consistent field (SCF) tolerance value was 1.0 × 10−6 Ha and the double numerical plus d-functions (DND) were chosen as the basis set [16]. For transition states (TS) searching, the calculation firstly performed a linear synchronous transit (LST) [20] maximum, which was followed by an energy minimization in directions conjugating to the reaction pathway. TS approximation obtained via LST/optimization was then used to perform a quadratic synchronous transit (QST) maximization to find more accurate transitional states. The convergence tolerance of the root mean square (RMS) force was 2.0 × 10−3 Ha Å−1 and the maximum number for QST step was set as 10. Vibration frequencies of the reaction system at TS were calculated for assessing whether the transition states are real ones. When the numbers of imaginary frequencies for reactants are more than one, TS optimization and TS confirmation was carried out for searching the real transitional states. In the calculation of TS optimization, the eigenvector following (EF) method [21] is employed, which seems like performing geometric optimization for the originally found TS. In the calculation of TS confirmation, a series of geometric optimization was performed along the reaction pathway to find the real TS. The diameters of Pd doping Ag nanoclusters are in the range of 1.5–3.5 nm [10]. At these dimensional sizes, metal nanoclusters keep quasicrystal icosahedral and decahedral structures [22]. For such small polyhedral structures, surface facets are limited and most surface atoms locate at vertex and edge sites. Considering the consumption time of calculation, a representative icosahedral structure including 55 atoms (the diameter about 1.5 nm) is chosen as the catalyst for C2 H2 hydrogenation. As shown in Fig. 1(a), Ag55 cluster has five-fold symmetric surface structures. Vertex and edge sites are the only two types of surface species. When a single Pd atom is doping on the surface of Ag55 cluster, it may locate at the vertex or edge site as shown in Fig. 1(b) and (c). The Pd-doping clusters are separately defined as Ag54 Pd1-vertex and Ag54 Pd1-edge . When two Pd atoms are doping on the surface of Ag55 cluster, they
During C2 H2 hydrogenation reactions (C2 H2 + H2 → C2 H4 ) on metal surface, C2 H2 and H2 molecules should be firstly co-adsorbed on contiguous sites. After the H2 molecule is dissociated as two H atoms, one of them combines with C2 H2 to form the intermediate product C2 H3 . In following, C2 H3 reacts with the other H atom to generate the final product C2 H4 . The reaction process of C2 H4 hydrogenation (C2 H4 + H2 → C2 H6 ) is similar to that of C2 H2 hydrogenation. For these saturated hydrogenation reactions, C2 H5 and C2 H6 molecules are separately the intermediate and final products. Firstly, the adsorption condition of reactants and intermediate products on Ag55 cluster is studied. All the possible adsorption sites are shown in Fig. 2 and the corresponding Ead values are given in Table 1. In Fig. 2(a)–(c), it is found that a hydrogen atom can be adsorbed on the hcp hollow, fcc hollow and bridge (connecting the vertex and edge) sites. However, H2 molecule can not be adsorbed on any site of Ag55 cluster. In Fig. 2(d) and (e), it is found that C2 H2 can be adsorbed on the top of vertex and edge sites. The corresponding Ead values are only −0.26 and −0.19 eV, which proves the weak bonding between acetylene and Ag surfaces. C2 H3 is an unsaturated intermediate product. It can be intensively adsorbed on the top of vertex, bridge (connecting the vertex and edge) and bridge (connecting two edges) sites as shown in Fig. 2(f)–(h). The adsorption of C2 H4 on Ag55 cluster is also very weak. In Fig. 2(i) and (j), it is found that C2 H4 can be adsorbed on the top of vertex and edge sites and the corresponding Ead values are only −0.31 and −0.21 eV. The intermediate product C2 H5 is moderately adsorbed on the top of vertex and edge, bridge (connecting the vertex and edge) and bridge (connecting two edges) sites as shown in Fig. 2(k)–(n). Since H2 can not be adsorbed on Ag55 cluster, the following catalytic hydrogenation of acetylene and ethylene is impossible. All the possible adsorption sites of reactants and intermediate products on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-vertex cluster are shown in Fig. 3 and the corresponding Ead values are given in Table 2. Generally, the doping Pd atom can give all the atoms and molecules stronger adsorption compared with the situation of pure Ag surfaces. In Fig. 3(a), it is found that H2 can be adsorbed on the top of vertex site, for which the Ead value is −0.22 eV. There are two sites for a hydrogen atom adsorption: the PdAg2 hcp hollow and Pd-Ag bridge as shown in Fig. 3(b) and (c). C2 H2 can be adsorbed on sites including the Pd atom in five types. The most stable adsorption site is the top of Pd atom [Fig. 3(d)], on which Ead reaches −0.78 eV. In Fig. 3(e), it is found that C2 H2 can be adsorbed on Pd-Ag bridge in type of tbt (topbridge-top). On the hollow site, there are two types for diatomic
D. Liu / Applied Surface Science 386 (2016) 125–137
127
Fig. 1. The geometric configurations of (a) Ag55 , (b) Ag54 Pd1-vertex , (c) Ag54 Pd1-edge and (d) Ag53 Pd2 clusters.
Table 1 Possible adsorption sites and the corresponding adsorption energy values of reactants and intermediate products on Ag55 cluster. Ag55 cluster Reactant
Site
Ead (eV)
Reactant
Site
Ead (eV)
H-I H-II H-III C2 H2 -I C2 H2 -II C2 H3 -I C2 H3 -II
hcp fcc bridge (vertex-edge) top (vertex) top (edge) top (vertex) bridge (vertex-edge)
−2.00 −2.18 −1.89 −0.26 −0.19 −1.30 −1.66
C2 H3 -III C2 H4 -I C2 H4 -II C2 H5 -I C2 H5 -II C2 H5 -III C2 H5 -IV
bridge (edge-edge) top (vertex) top (edge) top (vertex) top (edge) bridge (vertex-edge) bridge (edge-edge)
−1.54 −0.31 −0.21 −0.77 −0.88 −0.76 −0.74
adsorption: thob (top-hollow-bridge) and bhob (bridge-hollowbridge) [23]. Differentiating fcc and hcp hollows, the thob state can be sub-divided as tfb (top-fcc hollow-bridge) and thb (top-hcp hollow-bridge) ones. Similarly, the bhob state can be sub-divided as bfb (bridge-fcc hollow-bridge) and bhb (bridge-hcp hollow-bridge) ones. Considering the difference between Pd and Ag atoms, C2 H2 can be adsorbed on the PdAg2 hcp hollow in three types: thb (C on Pd top), bhb (C C bond parallel to Pd-Ag bridge) and bhb (C C bond parallel to Ag-Ag bridge) as shown Fig. 3(f)–(h). The unsaturated intermediate product C2 H3 is intensively adsorbed on the Pd-Ag bridge [Fig. 3(i)], for which Ead reaches −2.26 eV. In Fig. 3(j) and (k), it is found that C2 H4 and C2 H5 are adsorbed on the top of vertex site, for which Ead values are separately −0.75 and −1.47 eV.
The reaction process of C2 H2 hydrogenation on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-vertex cluster is shown in Fig. 4(a)–(h) and the corresponding potential energy change is given in Fig. 4(i). In usual, heterogeneous catalytic reactions need the co-adsorption of reactants on contiguous sites. At the surface of Ag54 Pd1-vertex cluster, C2 H2 adsorption on the Pd atom and its neighboring Ag atoms is much stronger than that on only Ag atoms. H2 can be only adsorbed on the top of Pd atom. Therefore, C2 H2 and H2 would share the Pd atom for adsorption. As mentioned above, there are five types for individual C2 H2 adsorption. When C2 H2 is co-adsorbed with H2 , it can only locate at the PdAg2 hcp hollow site (C2 H2 -IV type) as shown in Fig. 4(a). After the co-adsorption of reactants, H2 is easily dissociated as two H atoms with very small Erb value 0.05 eV. In Fig. 4(b) and (c), it is found that C2 H2 rotates during
128
D. Liu / Applied Surface Science 386 (2016) 125–137
Fig. 2. Possible adsorption sites of reactants and intermediate products (a)–(c) H, (d)–(e) C2 H2 , (f)–(h) C2 H3 , (i)–(j) C2 H4 and (k)–(n) C2 H5 on Ag55 cluster.
the H2 dissociation process. After the H2 dissociation, the C C bond of C2 H2 is parallel to the Ag-Ag bridge (C2 H2 -V type). In following, one of H atoms combines with C2 H2 to form C2 H3 as shown in Fig. 4(d) and (e). Erb value of this combination process is 0.58 eV. The remaining H atom can easily react with C2 H3 to form C2 H4 [Fig. 4(f) and (g)], for which the Erb value is only 0.21 eV. After the generated C2 H4 molecule has been desorbed [Fig. 4(h)], other C2 H2 and H2 molecules can be co-adsorbed on the Pd atom for catalytic hydrogenation reactions. In Fig. 4(i), it is found that the potential energy reduces −2.32 eV after C2 H2 hydrogenation on Ag54 Pd1-vertex cluster, which is just the reaction heat of C2 H2 + H2 → C2 H4 .
All the possible adsorption sites of reactants and intermediate products on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-edge cluster are shown in Fig. 5 and the corresponding Ead values are given in Table 2. In Fig. 5(a), it is found that H2 can be adsorbed on the top of Pd atom, for which the Ead value is −0.29 eV. There are three sites for a hydrogen atom adsorption: the PdAg2 hcp hollow, fcc hollow and Pd-Ag bridge (Ag at vertex site) as shown in Figs. 5(b)–(d). C2 H2 can be adsorbed on sites including the Pd atom in six types. The most stable adsorption site is the top of Pd atom [Fig. 5(e)], on which Ead reaches −0.73 eV. On the PdAg2 fcc hollow, there are two types for C2 H2 adsorption: tfb (C on Pd top) and bfb (C C bond parallel to Pd-Ag bridge) as shown in Fig. 5(f) and (g). In
D. Liu / Applied Surface Science 386 (2016) 125–137
129
Fig. 3. Possible adsorption sites of reactants and intermediate products (a) H2 , (b)–(c) H, (d)–(h) C2 H2 , (i) C2 H3 , (j) C2 H4 and (k) C2 H5 on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-vertex cluster.
Fig. 5(h)–(j), it is found that C2 H2 can be adsorbed on the PdAg2 hcp hollow in three bhb types. The difference between them is that C C bonds of C2 H2 molecules are separately parallel to three different types of bridges. In Fig. 5(k) and (l), it is found that C2 H3 can be adsorbed on two different types of Pd-Ag bridges (differentiating Ag atoms at vertex and edge sites). In addition, the only adsorption site for C2 H4 and C2 H5 is the top of Pd atom as shown in Fig. 5(m) and (n). The reaction process of C2 H2 hydrogenation on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-edge cluster is shown in Fig. 6(a)–(i) and the corresponding potential energy change is given in Fig. 6(j). As mentioned above, there are six types for individual C2 H2 adsorption. When C2 H2 is co-adsorbed with H2 , it can only locate at the PdAg2 hcp hollow site (C2 H2 -V type) as shown in Fig. 6(a). After the co-adsorption of reactants, H2 is dissociated as two H atoms with moderate Erb value 0.34 eV. In Fig. 6(c), it is found that the two H atoms are separately adsorbed on the Pd-Ag bridge and PdAg2 fcc hollow sites. In following, the H atom on the Pd-Ag bridge combines with C2 H2 to form C2 H3 as shown in Fig. 6(d) and (e). Erb value of this combination process is 0.43 eV. In Fig. 6(e), it is found that C2 H3 locates on the Ag-Ag bridge after the combination of H2 and C2 H2 . To react with the remaining H atom, C2 H3 moves to the Pd-Ag bridge as shown in Fig. 6(f). The remaining H atom can react with C2 H3 to form C2 H4 [Figs. 6(g) and (h)], for which the Erb
value is 0.39 eV. After the generated C2 H4 molecule has been desorbed [Fig. 6(i)], other C2 H2 and H2 molecules can be co-adsorbed on the Pd atom for catalytic hydrogenation reactions. Based on simulation results, it is concluded that the catalytic hydrogenation of acetylene to ethylene can be carried out on the doping Pd atom at both vertex and edge sites of Ag nanoclusters. The following catalytic hydrogenation of ethylene to ethane needs the co-adsorption of H2 and C2 H4 . As mentioned above, H2 can not be adsorbed on Ag sites. The adsorption of C2 H4 on the top of Ag vertex and edge sites is also very weak. Both of H2 and C2 H4 molecules are more likely to bind with the Pd atom. Considering the co-adsorption of H2 and C2 H2 , C2 H2 can be adsorbed on the PdAg2 hcp hollow, which makes space for H2 adsorption [Figs. 4(a) and 6(a)]. For C2 H4 binding with the Pd doping atom of Ag54 Pd1-vertex and Ag54 Pd1-edge clusters, it can only locate on the top of Pd vertex and edge sites [Figs. 3(j) and 5(m)]. In Fig. 7, it is found that the co-adsorption of H2 and C2 H4 on the Pd atom is impossible since the adsorption site of H2 has been occupied by C2 H4 . In the above analysis, the co-adsorption of H2 and C2 H4 is considered as the starting point, which follows a typical Langmuir-Hinshelwood mechanism. The reaction may be carried out in another path: H2 in air is directly dissociated on Ag sites and the generated H atoms react with C2 H4 on the Pd site, which follows an Eley-Rideal mechanism. Based on this consideration, the non-adsorbed H2 molecule
130
D. Liu / Applied Surface Science 386 (2016) 125–137
Table 2 Possible adsorption sites and the corresponding adsorption energy values of reactants and intermediate products on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-vertex and Ag54 Pd1-edge clusters. Ag54 Pd1-vertex cluster Reactant
Site
Ead (eV)
Reactant
Site
Ead (eV)
H2
Pd top
−0.22
C2 H2 -IV
−0.76
H-I
PdAg2 hcp
−2.43
C2 H2 -V
H-II C2 H2 -I C2 H2 -II C2 H2 -III
Pd-Ag bridge Pd top tbt (Pd-Ag bridge) thb (C on Pd top)
−2.41 −0.78 −0.55 −0.72
C2 H3 C2 H4 C2 H5
bhb (C C bond parallel to Pd-Ag bridge) bhb (C C bond parallel to Ag-Ag bridge) Pd-Ag bridge Pd top Pd top
−2.26 −0.75 −1.47
Reactant
Site
Ead (eV)
Reactant
Site
Ead (eV)
H2
Pd top
−0.29
C2 H2 -IV
−0.60
H-I
PdAg2 hcp
−2.44
C2 H2 -V
H-II
PdAg2 fcc
−2.57
C2 H2 -VI
H-III C2 H2 -I C2 H2 -II C2 H2 -III
Pd-Ag bridge (Ag at vertex site) Pd top PdAg2 tfb (C on Pd top) PdAg2 bfb (C C bondparallel to Pd-Ag bridge)
−2.36 −0.73 −0.66 −0.66
C2 H3 -I C2 H3 -II C2 H4 C2 H5
PdAg2 bhb (C C bond parallel to Ag-Ag bridge) PdAg2 bhb (C C bond parallel to Pd-Ag bridge, Ag vertex site) PdAg2 bhb (C C bond parallel to Pd-Ag bridge, Ag edge site) Pd-Ag bridge (Ag vertex site) Pd-Ag bridge (Ag edge site) Pd top Pd top
−0.64
Ag54 Pd1-edge cluster
dissociation on Ag sites of Ag54 Pd1 clusters are studied. The simulation results indicate that the Erb value for this dissociation process is as high as 1.59 eV, which means that H2 in air has no opportunity to directly decompose on Ag sites and further react with C2 H4 on the Pd site. Therefore, on single Pd atom doping Ag nanoclusters, C2 H4 can not be hydrogenated to form C2 H6 . All the possible adsorption sites of reactants and intermediate products on the two Pd atoms and their neighboring Ag atom of Ag53 Pd2 cluster are shown in Fig. 8 and the corresponding adsorption values are given in Table 3. Generally, the adsorption of reactants and intermediate products on Pd diatomic sites is stronger than that on Pd mono-atomic site. In Fig. 8(a), it is found that a hydrogen atom can be adsorbed on the Pd-Pd bridge, for which the Ead value is −2.78 eV. C2 H2 can be adsorbed on the Pd2 Ag hcp hollow in three bhb types as shown in Fig. 8(b)–(d). The difference between them is that C C bonds of C2 H2 molecules are separately parallel to three different types of bridges. C2 H3 can be adsorbed on the Pd-Pd bridge [Fig. 8(e)], for which the Ead value is −2.63 eV. In Fig. 8(f), it is found that C2 H4 locates on the two Pd atoms in tbt type. The reaction process of C2 H2 hydrogenation on the two Pd atoms and their neighboring Ag atoms of Ag53 Pd2 cluster is shown in Fig. 9(a)–(h) and the corresponding potential energy change is given in Fig. 9(i). For the co-adsorption of reactants, H2 binds with the vertex site and C2 H2 locates on the Pd2 Ag hcp hollow (C2 H2 -I type) as shown in Fig. 9(a). After the co-adsorption of reactants, H2 is dissociated as two H atoms with moderate Erb value 0.38 eV. In Fig. 9(b) and (c), it is found that C2 H2 rotates during the H2 dissociation process. The C C bond of C2 H2 is parallel to the Pd-Ag bridge (C2 H2 -III type) after the H2 dissociation. In Fig. 9(c), it is found that the two H atoms are separately adsorbed on the Pd-Ag bridge and PdAg2 hcp hollow sites. In following, the H atom on the Pd-Ag bridge combines with C2 H2 to form C2 H3 as shown in Fig. 9(d) and (e). Erb value of this combination process is 0.58 eV. In Fig. 9(e), it is found that the remaining H atom move to the neighboring PdAg2 hcp hollow after the formation of C2 H3 . The remaining H atom can react with C2 H3 to generate C2 H4 [Fig. 9(f) and (g)], for which the Erb value is 0.58 eV. After the C2 H4 molecule
−0.64
−0.66
−2.15 −2.07 −0.73 −1.39
has been desorbed [Fig. 9(h)], other C2 H2 and H2 molecules can be co-adsorbed on the two Pd atoms for catalytic hydrogenation reactions. The reaction process of C2 H4 hydrogenation on the two Pd atoms and their neighboring Ag atoms of Ag53 Pd2 cluster is shown in Fig. 10(a)–(h) and the corresponding potential energy change is given in Fig. 10(i). As mentioned above, H2 and C2 H4 can not be coadsorbed on the Pd atom of Ag54 Pd1-vertex and Ag54 Pd1-edge clusters. This problem is resolved on Ag53 Pd2 cluster since it can offer contiguous Pd sites for reactants adsorption. At the beginning of C2 H4 hydrogenation reactions, H2 and C2 H4 are separately adsorbed on the top of Pd edge and vertex sites [Fig. 10(a)], for which the Ead value reaches −1.06 eV. In Fig. 10(b) and (c), it is found that H2 can be easily dissociated as two H atoms with small Erb value 0.18 eV. After H2 dissociation, one of H atoms combines with C2 H4 to form C2 H5 as shown in Fig. 10(d) and (e). Erb value of this combination process is 0.75 eV. The generated C2 H5 molecule is far from the remaining H atom. For the following reaction, C2 H5 turns close to the remaining H atom as shown in Fig. 10(f). The remaining H atom can react with C2 H3 to generate C2 H4 [Fig. 10(g) and (h)], for which the Erb value is 0.62 eV. After the C2 H6 molecule has moven away, other C2 H4 and H2 molecules can be co-adsorbed on the two Pd atoms for catalytic hydrogenation reactions. In Fig. 10(i), it is found that the potential energy reduces −1.87 eV after C2 H4 hydrogenation on Ag53 Pd2 cluster, which is just the reaction heat of C2 H4 + H2 → C2 H6 . The cluster model used above has 55 atoms and a diameter about 1.5 nm. With the increasing cluster size (for example, 147 atoms and diameter 2 nm), the computational time cost would be huge. The geometric optimization of clusters including more than 150 atoms is almost impossible using the DMol3 module. Nevertheless, the reaction situation on Pd doping Ag clusters with diameters 2–3.5 nm can also be indirectly evaluated. On icosahedral Ag55 clusters, the coordinated numbers (CN) of atoms on surface edge and vertex sites are 8 and 6, separately. In the size range of 2–3.5 nm, metal clusters still keep quasicrystal icosahedral and decahedral structures [22]. On these larger clusters, CN of atoms on surface edge and vertex sites keep unchanged [24]. Therefore, the reactants
D. Liu / Applied Surface Science 386 (2016) 125–137
131
Fig. 4. (a)-(h) The reaction process and (i) potential energy change of catalytic hydrogenation of acetylene to ethylene on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-vertex cluster. Table 3 Possible adsorption sites and the corresponding adsorption energy values of reactants and intermediate products on two Pd atoms and their neighboring Ag atoms of Ag53 Pd2 cluster. Ag53 Pd2 cluster Reactant
Site
Ead (eV)
Reactant
Site
Ead (eV)
H
Pd-Pd bridge
−2.78
C2 H2 -III
−1.37
C2 H2 -I C2 H2 -II
Pd2 Ag bhb (C C bond parallel to Pd-Pd bridge) Pd2 Ag bhb (C C bondparallel to Pd-Ag bridge,Pd edge site)
−1.46 −1.24
C2 H3 C2 H4
Pd2 Ag bhb (C C bond parallel to Pd-Ag bridge, Pd vertex site) Pd-Pd bridge Pd2 tbt
adsorption and following reactions on the low-coordinated sites of larger Pd doping Ag clusters should be similar to the situation on Ag54 Pd1 and Ag53 Pd2 ones.
−2.63 −0.86
With the increasing cluster sizes, there are also small closepacked facets on the surface of Ag clusters [24]. The Pd doping atoms can also exist on these medium-coordinated sites (CN = 9).
132
D. Liu / Applied Surface Science 386 (2016) 125–137
Fig. 5. Possible adsorption sites of reactants and intermediate products (a) H2 , (b)–(d) H, (e)–(j) C2 H2 , (k)–(l) C2 H3 , (m) C2 H4 and (n) C2 H5 on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-edge cluster.
The atom arrangement of these facets on clusters is as same as that of bulk (111) surfaces. Therefore, Ag (111) slabs were built for representing the facets of Ag nanoparticles as shown in Figs. S1. The details for building these slabs are shown in supplementary data. In Figs. S1(a) and S1(b), one and two surface Ag atoms are separately replaced by Pd atoms [Ag-1Pd (111) and Ag-2Pd (111)], which denotes the single and contiguous Pd sites. In Table S1, it is generally found that the adsorption of reactants and intermediate products on Pd doping atoms at Ag (111)
is weaker than that on Pd doping atoms at Ag clusters. This conclusion is not reasonable since low-coordinated sites usually can give molecules and atoms stronger adsorption. On Ag-1Pd (111), H2 can be only adsorbed on the top of Pd atom. The adsorption of C2 H2 also needs the contribution of Pd atom. The co-adsorption of H2 and C2 H2 is impossible since the adsorption site of H2 has been occupied by C2 H2 as shown in Fig. S4. Even if C2 H2 is adsorbed on Ag-1Pd (111) in other adsorption types (C2 H2 -II, III and IV in Fig. S2), the co-adsorption with H2 is also impossible. Therefore, when
D. Liu / Applied Surface Science 386 (2016) 125–137
133
Fig. 6. (a)–(i) The reaction process and (j) potential energy change of catalytic hydrogenation of acetylene to ethylene on the Pd atom and its neighboring Ag atoms of Ag54 Pd1-edge cluster.
134
D. Liu / Applied Surface Science 386 (2016) 125–137
Fig. 7. Illustration for proving the infeasibility for catalytic hydrogenation of ethylene on the Pd atom of Ag54 Pd1-vertex and Ag54 Pd1-edge clusters.
Fig. 8. Possible adsorption sites of reactants and intermediate products (a) H, (b)–(d) C2 H2 , (e) C2 H3 and (f) C2 H4 on the two Pd atoms and their neighboring Ag atom of Ag53 Pd2 cluster.
a single Pd atom is doping on the facets of Ag (111), the catalytic hydrogenation of acetylene can not be carried out. There is additional Pd site for H2 adsorption on Ag-2Pd (111). In Figs. S5 and S6, it is found that both C2 H2 and C2 H4 hydrogenation reactions can be carried out on Ag-2Pd (111) since the Erb values are in the range of 0.31–0.66 eV.
On the surface of larger Pd doping Ag clusters (diameters 2–3.5 nm), atoms at small close-packed facets still occupies minority. A doping Pd atom is more likely to locate on low-coordinated surface sites. The selective hydrogenation of acetylene is also feasible on larger Pd doping Ag clusters. It has been proved that both C2 H2 and C2 H4 hydrogenation reactions can be carried out on pure Pd surfaces, for which the Erb
D. Liu / Applied Surface Science 386 (2016) 125–137
135
Fig. 9. (a)–(h) The reaction process and (i) potential energy change of catalytic hydrogenation of acetylene to ethylene on two Pd atoms and their neighboring Ag atoms of Ag53 Pd2 cluster.
values are in the range of 0.65–0.80 eV [25]. Based on above simulation results, it is concluded that the saturated hydrogenation of acetylene to ethane can still be realized on only two Pd doping atoms at inert Ag surfaces. Therefore, Pd mono-atomic catalysis is the only method to increase the reaction selectivity for catalytic hydrogenation of acetylene to ethylene. 4. Conclusions C2 H2 and C2 H4 molecules are weakly adsorbed on Ag55 cluster. H2 can not bind with any site of this pure Ag nanocluster. The catalytic hydrogenation of acetylene and ethylene is impossible to carry out on Ag55 cluster. On Ag54 Pd1-vertex cluster, the co-adsorption of H2 and C2 H2 is possible when C2 H2 locates on the
PdAg2 hcp hollow in bhb adsorption type. In following, H2 is dissociated as two H atoms, which combine with C2 H2 separately to form C2 H4 . Erb values for these reactions are in the range of 0.05–0.58 eV. Similarly, H2 and C2 H2 can be co-adsorbed on Ag54 Pd1-edge cluster when C2 H2 locates on the PdAg2 hcp hollow in bhb adsorption type. Erb values for subsequent H2 dissociation and C2 H2 hydrogenation are in the range of 0.34–0.43 eV. H2 and C2 H4 can not be co-adsorbed on both of Ag54 Pd1-vertex and Ag54 Pd1-edge clusters since the only adsorption site of H2 is occupied by C2 H4 . On Ag53 Pd2 cluster, H2 and C2 H2 can be co-adsorbed when C2 H2 locates on the Pd2 Ag hcp hollow in bhb adsorption type. Erb values for H2 dissociation and C2 H2 hydrogenation are in the range of 0.38–0.58 eV. As for C2 H4 hydrogenation reactions, H2 and C2 H4 can be separately
136
D. Liu / Applied Surface Science 386 (2016) 125–137
Fig. 10. (a)–(h) The reaction process and (i) potential energy change of catalytic hydrogenation of ethylene to ethane on two Pd atoms and their neighboring Ag atoms of Ag53 Pd2 cluster.
D. Liu / Applied Surface Science 386 (2016) 125–137
adsorbed on the two Pd atoms. Erb values for H2 dissociation and C2 H4 hydrogenation are in the range of 0.18–0.75 eV. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.06. 013. References [1] A.N.R. Bos, E.S. Botsma, F. Foeth, H.W.J. Sleyster, K.R. Westerterp, A kinetic study of the hydrogenation of ethyne and ethene on a commercial Pd/Al2 O3 catalyst, Chem. Eng. Process.: Process Intensif. 32 (1993) 53–63. [2] W.T. McGown, C. Kemball, D.A. Whan, M.S. Scurrell, Hydrogenation of acetylene in excess ethylene on an alumina supported palladium catalyst in a static system, J. Chem. Soc. Faraday Trans. 73 (1977) 632–647. ´ G.C. Bond, Selective hydrogenation of ethyne in ethene-rich [3] A. Borodzinski, streams on palladium catalysts. Part 1. Effect of changes to the catalyst during reaction, Catal. Rev. 48 (2006) 91–144. [4] A. Sárkány, O. Geszti, G. Sáfrán, Preparation of Pdshell –Aucore /SiO2 catalyst and catalytic activity for acetylene hydrogenation, Appl. Catal. A: Gen. 350 (2008) 157–163. [5] J.H. Lee, S.K. Kim, I.Y. Ahn, W.J. Kim, S.H. Moon, Performance of Pd–Ag/Al2 O3 catalysts prepared by the selective deposition of Ag onto Pd in acetylene hydrogenation, Catal. Commun. 12 (2011) 1251–1254. [6] S.K. Kim, J.H. Lee, I.Y. Ahn, W.J. Kim, S.H. Moon, Performance of Cu-promoted Pd catalysts prepared by adding Cu using a surface redox method in acetylene hydrogenation, Appl. Catal. A: Gen. 401 (2011) 12–19. [7] Y. Han, D. Peng, Z. Xu, H. Wan, S. Zheng, D. Zhu, TiO2 supported Pd@Ag as highly selective catalysts for hydrogenation of acetylene in excess ethylene, Chem. Commun. 49 (2013) 8350–8352. [8] Y. Zhang, W. Diao, C.T. Williams, J.R. Monnier, Selective hydrogenation of acetylene in excess ethylene using Ag- and Au-Pd/SiO2 bimetallic catalysts prepared by electroless deposition, Appl. Catal. A: Gen. 469 (2014) 419–426. [9] C. Shi, R. Hoisington, B.W.L. Jang, Promotion effects of air and H2 nonthermal plasmas on TiO2 supported Pd and Pd-Ag catalysts for selective hydrogenation of acetylene, Ind. Eng. Chem. Res. 46 (2007) 4390–4395.
137
[10] G.X. Pei, X.Y. Liu, A. Wang, A.F. Lee, M.A. Isaacs, L. Li, X. Pan, X. Yang, X. Wang, Z. Tai, K. Wilson, T. Zhang, Ag alloyed Pd single-atom catalysts for efficient selective hydrogenation of acetylene to ethylene in excess ethylene, ACS Catal. 5 (2015) 3717–3725. [11] J. Tang, L. Deng, H. Deng, S. Xiao, X. Zhang, W. Hu, Surface segregation and chemical ordering patterns of Ag-Pd nanoalloys: energetic factors, nanoscale effects, and catalytic implication, J. Phys. Chem. C 118 (2014) 27850–27860. [12] Q. Jiang, H.M. Lu, M. Zhao, Modelling of surface energies of elemental crystals, J. Phys. Condens. Matter 16 (2004) 521–530. [13] D. Mei, M. Neurock, C.M. Smith, Hydrogenation of acetylene-ethylene mixtures over Pd and Pd-Ag alloys: first-principle-based kinetic Monte Carlo simulations, J. Catal. 268 (2009) 181–195. [14] P. Hohenberg, W. Kohn, Inhomogeneous electron gas, Phys. Rev. 136 (1964) B864–B871. [15] W. Kohn, L.J. Sham, Self-consistent equations including exchange and correlation effects, Phys. Rev. 140 (1965) A1133–A1138. [16] B. Delley, An all-electron numerical method for solving the local density functional for polyatomic molecules, J. Chem. Phys. 92 (1990) 508–517. [17] B. Delley, From molecules to solids with the DMol3 approach, J. Chem. Phys. 113 (2000) 7756–7764. [18] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [19] B. Delley, Hardness conserving semilocal pseudopotentials, Phys. Rev. B 66 (2002) 155125-1–155125-9. [20] T.A. Halgren, W.N. Lipscomb, The synchronous-transit method for determining reaction pathways and locating molecular transition states, Chem. Phys. Lett. 49 (1977) 225––232. [21] J. Baker, An algorithm for the location of transition states, J. Comput. Chem. 7 (1986) 385––395. [22] F. Baletto, R. Ferrando, Structural properties of nanoclusters: energetic, thermodynamic, and kinetic effects, Rev. Mod. Phys. 77 (2005) 371–417. [23] Y. Xu, M. Mavrikakis, The adsorption and dissociation of O2 on Pt-Co and Pt-Fe alloys, J. Am. Chem. Soc. 126 (2004) 4717–4725. [24] D. Liu, Y.F. Zhu, Q. Jiang, Site and structure dependent cohesive energy in several Ag clusters, J. Phys. Chem. C 113 (2009) 10907––10912. [25] D. Mei, P.A. Sheth, M. Neurock, C.M. Smith, First-principles-based kinetic Monte Carlo simulation of the selective hydrogenation of acetylene over Pd(111), J. Catal. 242 (2006) 1–15.