- Email: [email protected]
Surface structure and reaction property of CuCl2-PdCl2 bimetallic catalyst in methanol oxycarbonylation: A DFT approach
Applied Surface Science 292 (2014) 117–127
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface structure and reaction property of CuCl2 -PdCl2 bimetallic catalyst in methanol oxycarbonylation: A DFT approach Qingsen Meng a , Shengping Wang a,∗ , Yongli Shen a , Bing Yan a , Yuanxin Wu b , Xinbin Ma a a Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China b School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430073, China
a r t i c l e
i n f o
Article history: Received 22 October 2013 Received in revised form 18 November 2013 Accepted 18 November 2013 Available online 25 November 2013 Keywords: Mechanism Dimethyl carbonate Methanol PdCl2 -CuCl2 Density functional theory
a b s t r a c t Surface structure of CuCl2 -PdCl2 bimetallic catalyst (Wacker-type catalyst) was built employing density functional theory (DFT) calculations, and the reaction mechanism of methanol oxycarbonylation over the CuCl2 -PdCl2 surfaces was also investigated. On the CuCl2 -PdCl2 surface, the active site for methanol oxidation was confirmed as Cu-Cl-Cu (Pd). Comparing with pure CuCl2 surface, the introduction of Pd atom causes the electron repopulation on the surface and lowers the energy barrier for methanol oxidation, but the number of the active site decreases with the increasing of Pd doping volume. Agreed with previous experimental results, the Pd site is most favorable for the CO insertion, indicated by the lowest activation barrier for the formation of COOCH3 on Pd atom. The lowest energy barrier for the formation of DMC appears when COOCH3 species adsorbed on Pd atom and methoxyl adsorbed on Cu atoms, which is 0.42 eV. Finally, the reconstruction of the unsaturated surface is a spontaneous and exothermic process. Comparing with other surfaces, the rate-limiting step, methanol oxidation, on CuCl2 -PdCl2 surface with Pd/Cu = 1:17 has the lowest energy barrier, which is agreed with the experimental observation that PdCl2 -CuCl2 catalyst with Pd/Cu = 1:20 has the favorable activity. The adsorbed methoxyl will further lower the activation barrier of methanol oxidation, which is agreed with experimental observation that the Wacker-type catalysts have an induction period in the methanol oxidative carbonylation system. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Dimethyl carbonate (DMC) is a potential fuel additive to replace methyl tert-butyl ether (MTBE), and a precursor for synthesis of carbonic acids and derivatives. It can also be used as a methylating agent to replace methyl halides and dimethyl sulfate, and as an intermediate for the synthesis of polycarbonates and isocyanates [1–4]. Dimethyl carbonate was originally made by the reaction of methanol with phosgene. However, because of phosgene’s extreme toxicity, it is now manufactured by the trans-esterification of propylene carbonate and methanol or the copper-catalyzed reaction of methanol, oxygen, and carbon monoxide. And one of the best ways to produce DMC is by oxidative carbonylation of methanol (2CH3 OH + CO + 1/2O2 → (CH3 O)2 CO + H2 O) [5]. Although a liquid slurry process employing a copper chloride catalyst has been commercialized [6], a gas-phase process is more desirable because the copper chloride is highly corrosive in the liquid phase [7]. Copper-based catalysts, including CuCl2 /AC [7,8], Cu(I)Y [9–12], CuCl2 -PdCl2 /AC [5,13], CuCl2 /composite supports [14,15] are active
∗ Corresponding author. Tel.: +86 2287401818. E-mail address: [email protected] (S. Wang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.096
for the vapor-phase oxidative carbonylation of methanol to DMC. The previous studies indicated that the Cu(I)Y catalysts prepared by the solid state ion-exchange method showed high DMC selectivity but low methanol conversion [10–12]. The activated carbon (AC)-supported CuCl2 catalyst showed high methanol conversion with DMC selectivity of 80–90% [8,16], and the catalytic activity can be further improved by the addition of palladium chloride [15,17]. Meanwhile, the favorable ratio of the Cu/Pd in AC-supported CuCl2 /PdCl2 catalyst is reported as 20:1 [15,17], but the explanations for this phenomenon are still absence. Earlier experimental work by Curnutt and Midland suggests a three-step mechanism to explain the high reactivity of the CuCl2 /AC catalysts [18]. Jiang et al. [19] have also proposed a reaction scheme by combining the XPS and XRD results to explain the high reactivity of catalysts and the synergic effect between palladium and copper in the CuCl2 -PdCl2 /AC catalyst. However, these proposed mechanisms are largely speculative, and thus a deep understanding on the reaction details and nature of active sites are desirable. This, in turn, makes fundamental investigations on kinetics and thermodynamics characters of the oxidative carbonylation of methanol over CuCl2 and CuCl2 -PdCl2 surfaces become necessary. Theoretical studies have received wide recognition for providing deep insights into reaction pathways and the key factors that
118
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
control reactivity and selectivity [20–25]. In recent years, the reaction mechanism of oxidative carbonylation of methanol has been investigated employing DFT calculation over Cu-Y [23], Cu- [26], ␥-Cu2 Cl(OH)3 (0 2 1) surface [27], and Cu2 O(1 1 1) surface [28]. But the molecular-level mechanistic understanding of oxidative carbonylation of methanol over CuCl2 -PdCl2 surface is still absent. Herein, we present a systematic DFT study regarding possible intermediates involved in the methanol oxidative carbonation as well as reaction mechanism over CuCl2 -PdCl2 surfaces. We focus on geometric and electronic parameters of the adsorbed species as well as on their kinetic and thermodynamic properties. We attempt to address three questions in this paper: (1) what is the rate-limiting step in the oxidation carbonation of methanol on catalyst surfaces; (2) what is the role of palladium in the CuCl2 -PdCl2 catalyst. (3) Why the low Pd/Cu ratio is favorable for the activity of the CuCl2 -PdCl2 catalyst. 2. Calculation methods All calculations were carried out by using the density functional theory and periodical slab model. The electronic structure and equilibrium geometries of the model systems were calculated by using the Dmol3 program package in Materials Studio [29–31], at the spin-unrestricted level. Exchange-correlation effects were described by the generalized gradient approximation (GGA) with Perdew and Wang (PW91) functional [32,33]. The DFT calculations coupled with a van der Waals-inclusive correction (DFT-D) [34] were carried out to improve the calculations of the energies associated with the interlayer interactions of CuCl2 structures, ˚ Extended numerical all-electron basis sets which is around 3.49 A. (double-zeta plus polarization, DNP) were used to describe the electronic structure of all atoms in the model systems except for copper and palladium, which is approximated by an effective core potential (ECP). A Monkhorst-Pack grid [35] of 3 × 3 × 1 was used to simple the Brillouin zone. Geometry optimizations were performed with a numerical displacement accuracy of the ˚ Possible transition states (TSs) were atom centers of 5 × 10−3 A. located by using the complete linear and quadratic synchronous transit (LST, QST) method connecting the reactants and products [36]. The nudged elastic band (NEB) method was used to calculate the MEP for all the reaction [37]. The MEP for each reaction was discretized by a total of ten images between the initial and final states. All TSs were confirmed by the vibrational frequency analysis. The convergence criteria were set to 2 × 10−5 Ha, 0.004 Ha/Å for energy, and force and field (SCF) density convergence threshold value of 1 × 10−5 Ha was specified. A Fermi smearing of 0.005 Ha was used to improve the calculation performance. Adsorption energies, Eads , were calculated as: Eads = Eadsorbate+surface − Efree adsorbate − Efree surface , while the reaction energies, Er , were calculated as: Er = Eproducts − Ereactants . In addition, the Mulliken population analysis and density of states (DOS) were performed using the ultrasoft pseudo-potentials generated from the “Ultrasoft” method implement within the CASTEP module [38]. The GGA with the PW91 functional was used and a plane-wave cutoff energy of 300 eV was applied for all calculations. The default convergence criteria of CASTEP were applied 0.05 eV/Å for the root-mean-square residual force on movable atoms. 3. Results and discussion 3.1. Surface models The crystal structure of the CuCl2 follows a base-centered monoclinic space group C2/m [39]. Cu atoms are positioned in an axially
distorted octahedral environment with four equatorial nearest ˚ calculated; 2.263(6) A, ˚ neighbors Cl atoms (band distance: 2.299 A, ˚ experiment [40]) and two axial Cl atoms (band distance: 2.977 A, ˚ experiment [40]). This octahedral environcalculated; 2.991(6) A, ment is a result of the John–Teller effect. Each Cu2+ Cl6 octahedron shares two Cl–Clequatorial edges with adjacent octahedron, and its apical Cl− ions are equatorial ligands for the adjacent octahedral [40]. This linkage results in corrugated octahedral sheets (Fig. 1(a)) of composition CuCl2 paralleled the (0 0 1) plane. Each sheet is electrostatically neutral, and linkage between adjacent sheets is based on Van der Waals forces [40]. In our work, the CuCl2 (0 0 1) surface is adopted to study the reaction properties. Fig. 1(b) shows a CuCl2 (0 0 1)-2 × 2 supercell including two corrugated octahedral sheets, and a vacuum of 15 A˚ was inserted in the direction perpendicular to the surface. Because of the same valence state of Cu2+ and Pd2+ and similar coordination between CuCl2 and PdCl2 , as well as the low Pd/Cu ratio reported experimentally [5,40–45], the Pd doped CuCl2 (0 0 1) surface by replacing one Cu atom with one Pd atom was built to represent the CuCl2 -PdCl2 surface in this work. The atomic concentration ratio of Pd to Cu is 1:17, which is close to that (1:20) most commonly used in the experiment [5,41–46]. The doped surface is shown in Fig. 1(c) (donated as Pd/CuCl2 (0 0 1)). In order to investigate the influence of the Pd/Cu ratio, the two Pd atoms doped surface (Fig. 1(d), donated as 2Pd/CuCl2 (0 0 1)) with the Pd/Cu ratio equal to 1:8 was also built and discussed. As the linkage between adjacent sheets of CuCl2 is by Van der Waals forces, the doped Pd atoms were all located at the first sheet of the surface to investigate the doping effect [47]. Additionally, only the latter sheet of CuCl2 was fixed during the calculation. Furthermore, in order to determine whether the palladium atom can incorporate into copper chloride readily by substituting Cu, we calculated the substitutional energy (Esub ) using the following equation [48]: Esub = EPd/CuCl2 + ECu − ECuCl2 − EPd where EPd/CuCl2 is the total energy of the Pd doped CuCl2 (0 0 1) surface, and ECuCl2 is the total energy of the pure CuCl2 (0 0 1) surface, ECu and EPd , are the energies per atom of bulk material Cu and Pd, respectively. Negative values indicate that the occurrence of the substitutional doping is easy [48]. Our calculations reveal that dopant Pd is easy to incorporate into the CuCl2 (0 0 1) surface by substituting Cu due to its lower substitutional energy (−0.93 eV for Pd/CuCl2 (0 0 1), −2.31 eV for 2Pd/CuCl2 (0 0 1)). 3.2. Reaction mechanism Based on the previous results [7,13,28,49,50], we have extended our discussion into four subsections regarding possible reaction steps involved in the oxidative carbonylation of methanol over CuCl2 (0 0 1), Pd/CuCl2 (0 0 1), and 2Pd/CuCl2 (0 0 1) surfaces. That is, (i) oxidation of methanol, (ii) CO insertion, (iii) DMC formation, and (iv) surface reconstruction. 3.2.1. Oxidation of methanol Experimentally, the presence of chlorine seems essential for designing a catalyst with high initial activity for methanol oxycarbonylation [4,16,42,51]. In this work, the top site of the Cl atoms in the first layer of the catalyst surfaces were treated as the adsorption sites for methanol, as reported in our previous work [27]. Upon optimization, the methanol was found positioned with the H atom toward the Cl atom in an a-top position with a hydrogen bond between them at all adsorption sites. The optimized adsorption configurations are shown in Fig. 2. The similar adsorption energies and the key structural characters between different adsorption
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
119
Fig. 1. Optimized structures of bulk CuCl2 (a), CuCl2 (0 0 1) surface (b), Pd/CuCl2 (0 0 1) surface (c), and 2Pd/CuCl2 (0 0 1) surface (d). The dark blue, light red, and green spheres represent Pd atom, Cu atom, and Cl atom, respectively. The color of each atom is consistence throughout this paper. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
configurations (Table 1) indicate that the methanol has the similar stability on different adsorption sites and surfaces. It also suggests that the doping of Pd has little influence on the adsorption of methanol. On the other hand, based on the mechanism proposed experimentally [7,13], the co-adsorbed methoxyl and HCl were created as the oxidative product of methanol. Fig. 3 shows the key structural of the adsorbed methoxyl and HCl on different sites. Upon optimization, the methoxyl species adsorbed on the surface through two or three O Cu(Pd) bonds with its C O axis normal to the substrate, and the adsorption energy is ranged from −2.48 to −2.29 eV. Notably, the corresponding O Cu bonds are slightly shorter and stronger than the O Pd bonds. It suggests that the adsorption of methoxyl bonded with the Pd atom is weaker, which should be beneficial for the following CO insertion. This deduction can be further confirmed through the comparison of the methoxyl adsorption energies. The formed HCl species weakly adsorbed on the surface with its adsorption energy around −0.22 to −0.13 eV, and the Mulliken analysis shows a weak interaction (bond population is around 0.02) between the Cl atom at the adsorption site and the H atom of HCl
in all calculated structures. Thus, we expect that the adsorption of HCl on the PdCl2 -CuCl2 surface is weak and removable [27], which could account for the phenomenon of the leaching of Cl species from experimental measurements [13,18,43,46]. The energy barriers for the reaction pathways that connect the adsorbed methanol and co-adsorbed methoxyl and HCl were calculated and summarized in Table 1. We found that the oxidation reaction of methanol at the 3 site on Pd/CuCl2 (0 0 1) or 2Pd/CuCl2 (0 0 1) surface has a lower barrier compared to the pristine CuCl2 (0 0 1) surface, suggesting that the doping of Pd could increase the activity of the CuCl2 (0 0 1) surface on methanol oxidation. However, activation energies of the oxidation taking place at the 2 site (Fig. 2), in which the Cl atom has strong interaction with doped Pd atom, are extremely high and can barely be overcome under realistic condition (413 K [5,8,13,16,52]). And this kind of Cl atom will cover the surface and lower the activity of the catalyst with the increasing of the Pd doping mount, which is also agreed with the experimental observation that the preferable Pd/Cu ratio for the catalyst with high activity is low (the ratio is 1:20 [5,41–46]). On the CuCl2 (0 0 1) surface, the Cu atom is six coordinated with Cl atom. Thus, the hybridization of the valence electron of Cu2+
Table 1 Calculated bond distance (Å), overlap population, adsorption energies (eV) of methanol and activation energies (eV), reaction energy (eV) of methanol oxidation on different surfaces. Configuration
*H Cl
*O H
Adsorption energy (eV)
Activation energy (eV)
Reaction energy (eV)
CuCl2 (0 0 1)-1 Pd/CuCl2 (0 0 1)-1 Pd/CuCl2 (0 0 1)-2 Pd/CuCl2 (0 0 1)-3 2Pd/CuCl2 (0 0 1)-1 2Pd/CuCl2 (0 0 1)-2 2Pd/CuCl2 (0 0 1)-3
˚ 2.520 Anewline 0.02 ˚ 2.572 Anewline 0.02 ˚ 2.624 Anewline 0.02 ˚ 2.506 Anewline 0.02 ˚ 2.521 Anewline 0.02 ˚ 2.531 Anewline 0.02 ˚ 2.625 Anewline 0.02
˚ 0.980 Anewline 0.55 ˚ 0.977 Anewline 0.54 ˚ 0.976 Anewline 0.54 ˚ 0.980 Anewline 0.54 ˚ 0.980 Anewline 0.54 ˚ 0.980 Anewline 0.55 ˚ 0.980 Anewline 0.55
−0.57 −0.57 −0.57 −0.85 −0.55 −0.55 −0.56
1.25 1.34 2.96 1.00 1.66 2.77 1.05
0.49 0.54 0.55 0.57 0.57 0.57 0.53
120
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
Fig. 2. Adsorption configurations of methanol on different sites; the gray, dark red, and white spheres represent the C atom, O atom, and H atom, respectively. The color of each atom is consistence throughout this paper. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Key structures and adsorption energies of methoxyl and HCl at different sites.
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
121
Table 2 Adsorption key parameters, reaction energies and activation energies of methanol on methoxyl pre-adsorbed surface.
CuCl2 (0 0 1) Pd/CuCl2 (0 0 1) 2Pd/CuCl2 (0 0 1)
H Cl
O H
Adsorption energy (eV)
Activation energy (eV)
Reaction energy (eV)
˚ 2.552 Anewline 0.02 ˚ 2.470 Anewline 0.02 ˚ 2.547 Anewline 0.02
˚ 0.979 Anewline 0.55 ˚ 0.980 Anewline 0.55 ˚ 0.979 Anewline 0.54
−0.31 −0.30 −0.24
0.66 0.80 0.88
0.57 0.72 0.58
is d2 sp3 , while the Cl− has a sp3 hybridization in its valence electrons. Nine electrons fill in the d orbitals of Cu2+ , which means, three extra d electrons of Cu2+ would join in the hybridization and fill in the anti-bonding orbitals of six Cu Cl bonds. Caused by the John–Teller effect, two of the Cu Cl bonds were extended [40], as shown in Fig. 1. However, the doped Pd2+ , which has eight electrons in its d orbitals, is only four coordinated with the Cl atoms based on our Mulliken population analysis. It indicates that the valence electron of Pd2+ is the dsp2 hybridization and no electron fills in the anti-bonding orbitals of Pd Cl bonds, implying the Pd Cl bond is stronger than the Cu Cl bond and the Cl atom in Pd Cl bond has smaller electron density than that in the Cu Cl bond. This result is also agreed with the Mulliken population analysis that the population of Cl at 2 site is −0.21 and the population is −0.27 on pure CuCl2 surface. On the other hand, the oxidation of methanol follows a nucleophilic attack mechanism; thus, the Cl atom with smaller electron density shows lower nucleophilicity, which has to overcome higher activation energy to oxidize methanol. Thus, the energy barrier for methanol oxidation increased seriously at 2 site. The partial density of states (PDOS) of local structure of Cu-Cl(2)Pd (2 indicates the Cl atom at 2 site, Fig. 2) were calculated and listed in Fig. 4. Comparing with the PDOS structure of the atom in the pure CuCl2 (0 0 1) surface, the doping of Pd introduced one new peak located around 1 eV in the PDOS of Cl and Pd atoms, which should be responsible for the anti-bonding orbitals of Pd Cl bond. However, in the same location, a small peak was observed in the PDOS structure of Cu atom that in the Cu-Cl(2)-Pd local structure, which indicates the hybridization between the Cl 2p, Pd 4d and slight Cu 3d orbitals. It also proves the electron transfers between the Cu atom and the doped Pd atom. Meanwhile, comparing with the Cu atom in the pure CuCl2 surface, the peak at the right side of the Fermi level was decreased in the PDOS of Cu atom in Cu-Cl(2)Pd local structure, which also indicates the electron transfer from Pd atom to the Cu atom; and this is agreed with the experimental expectation about the redox reaction between the Cu and Pd atoms [4,13,41,53]. Mulliken analysis also shows that the atomic population of the Cl at the 3 site (−0.41 in Pd/CuCl2 (0 0 1)-3, and −0.36 in 2Pd/CuCl2 (0 0 1)-3) are more negative than the Cl atom in the pure CuCl2 surface (atomic population is −0.27). The main reason could be the doping of Pd atom, which is four coordinated with the Cl atoms nearby, causes the disappearance of apical bond between the Pd atom and the Cl atom at the 3 site and leads to one lone electron pair left at the Cl atom. Thus, the Cl atom is less occupied and own more electron, which makes this Cl site is more nucleophilic and active for methanol oxidation. Thus, the energy barrier for methanol oxidation decreased at this Cl site. This electronic behavior can also be seen at the PDOS structure of the Cl atom at the 3 site that one new peak appeared at the left side of the Fermi level (Fig. 4), which is contributed by the new lone electron pair. Based on the reaction equation [5], two methanol molecules are needed to form one DMC; thus, the energy barrier for the oxidative reaction of methanol over methoxyl pre-adsorbed surfaces is also discussed. Fig. 5 displays calculated co-adsorption configurations of methanol and methoxyl on different surfaces. Additionally, considering the high activation barriers (e.g. 2.77–2.96 eV), only the adsorption configurations of methoxyl that have the lowest
formation barriers (methoxyl adsorbed at 3 site) were chosen for the following discussion. It is clear that the adsorption structure of methanol on the methoxyl pre-adsorbed surface did not changed obviously compared to the clean surfaces (Table 2). Interestingly, we also found that the pre-adsorbed methoxyl decreased the energy barriers for the oxidation. One reason could be that the adsorbed methoxyl, which has lower electronegativity and stronger electron giving ability than the Cl atom, leads to the electron transfer from it to the Cl atom nearby. In Fig. 6, the peaks located at the right side of the Fermi level in the PDOS structure of Cu and Cl atom in Cl-Cu-OCH3 local structure (on Pd/CuCl2 (0 0 1) surface) decreased and shifted to the lower energy region, which can prove the electron transformation. In the Cl(2)-Pd-OCH3 local structure (on 2Pd/CuCl2 (0 0 1) surface), the peaks located just at the right side of the Fermi level in the PDOS structure of Pd and Cl atom also shifted to the lower energy region. Meanwhile, in the PDOS structure of Pd atom in the Cl(2)-Pd-OCH3 local structure, the peak located next to the Fermi level (left side) increased comparing to that in the Cl-Pd-Cl local structure, which can also prove the electron transformation from the CH3 O species to Cl atom through Pd atom. Meanwhile, the electron transformation also leads the Cl atom in the Cl-Cu(Pd)-OCH3 local structure has larger electron density and stronger nucleophilicity. Thus, the activation barrier for methanol oxidation decreased at this site. Furthermore, the decreasing of the energy barriers also indicates that the catalyst may show higher activity for the methanol oxidation if the surfaces are covered with a mount of methoxyl during the reaction, which means the catalyst may have an induction period in the experiment.
3.2.2. CO insertion Experimentally, in the oxidative carbonylation and CO selective oxidation systems, it is proposed that the CO prefers to adsorb on palladium chloride to form Pd(CO)Cl2 intermediate first [13,54,55]. However, in our calculations, the Cu site and Pd site are all considered as the CO adsorption sites to confirm the effect of different CO adsorption sites. On the other hand, it is widely accepted that the CO adsorbed on the metal atom through the C atom [56–58]. Thus, only the configurations with CO adsorbed with C end are calculated (Fig. 7). Employing the Mulliken analysis, a C Cu(Pd) bond was found between CO and metal atom, suggesting the chemisorption of CO on the surface. Meanwhile, we notice that upon adsorption, the bond distance of CO is slightly extended compared to that in gaseous ˚ This is agreed with the Blyholder mechanism [56], state (1.142 A). in which the bond to the metal atom is described in terms of the charge donation from the CO 5 orbital to the metal atom and the back-donation from the metal to the CO 2* orbital, which rationalized the increase in CO bond length upon chemisorption [58]. Meanwhile, the PDOS structure of the C atom in CO at different adsorption sites (Fig. 8) also prove the electron transfer, in which all the peaks of adsorbed CO move to the deeper energy region compare with the CO atom at gaseous sphere. In turn, the Pd atom seems more active on the CO activation than the Cu atoms, indicated by the longer C O bond distance when CO adsorbed on Pd atom (Table 3).
122
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
Fig. 4. PDOS of the Cu, Pd, Cl atoms at pure and Pd doped CuCl2 (0 0 1) surface. The Fermi level is set as zero point.
Fig. 5. Co-adsorption configurations of methanol and methoxyl on different surfaces.
Based on the mechanism of oxidation carbonylation over the CuCl2 proposed by Curnutt and Harley [7] and over the Cu-Y catalyst that proposed by Richter et al. [49], the adsorbed CO may not reacted with two adsorbed methoxyl at once, but to form an intermediate (COOCH3 [7] or HOCOOCH3 [26,49]) first, then react with another methoxyl or methanol to form the DMC. Thus, the co-adsorption of COOCH3 and methoxyl has been calculated as the product of CO insertion in this work. Two main adsorption configurations of COOCH3 are calculated, one is COOCH3 adsorbed on one metal atom via the interaction between the carbonyl C atom and the metal atom (Fig. 9; Cu-2, 2Pd/Cu-2); the other is COOCH3 adsorbed on the surface through both oxyalkyl O atom and carbonyl C atom (Fig. 9; Cu-1, Pd/Cu-1, Pd/Cu-2, 2Pd/Cu-1).
Bond distance C Cu/Pd
Cu-CO Pd/Cu-CO Pd-CO Pd/Pd-CO
2.005 A˚ 1.996 A˚ 2.131 A˚ 2.382 A˚
3.2.3. DMC formation As the target product of methanol oxycarbonylation, DMC molecularly adsorbed on the surface with two oxyalkyl O atoms Table 4 Activation and reaction energies for the formation of COOCH3 species.
Table 3 Key structural parameters of CO adsorption at different sites. Configuration
From Table 4, it is clear that the insertion reaction occurs with the lowest energy barrier (0.25 eV) when methoxyl and CO coadsorbed on one Pd atom; and the formed COOCH3 adsorbed on the Pd atom through the carbonyl C atom. However, when the methoxyl and CO adsorb on different atoms or co-adsorbed on Cu atom, the formation of COOCH3 species is facile and the energy barriers range from 0.30 to 0.49 eV. When the CO adsorbed on the Pd atom, the energy release of the reactions are also larger compare to that on Cu atom.
Overlap population C Cu/Pd
Bond distance C O
0.19 0.15 0.14 0.13
1.146 A˚ 1.146 A˚ 1.150 A˚ 1.183 A˚
Cu-1 Cu-2 Pd/Cu-1 Pd/Cu-2 2Pd/Cu-1 2Pd/Cu-2
Reactant
Activation energy (eV)
Reaction energy (eV)
Cu-CO Cu-CO Pd-CO Pd/Cu-CO 2Pd/Cu-CO 2Pd/Cu-CO
0.30 0.48 0.33 0.49 0.30 0.25
−0.14 −0.54 −0.60 −0.19 −0.58 −1.08
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
Fig. 6. The PDOS of Cl, Cu atom and Pd atom in the Cl-Cu(Pd)-OCH3 on Pd-doped CuCl2 (0 0 1) surfaces. The Fermi level was set as the zero point.
Fig. 7. Adsorption of CO on different methoxyl pre-adsorbed surfaces.
123
124
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127 Table 5 Activation energies and Reaction energies for the formation of DMC.
CuCl2 CuCl2 Pd/CuCl2 Pd/CuCl2 2Pd/CuCl2 2Pd/CuCl2
Reactant
Activation energy (eV)
Reaction energy (eV)
Cu-1 Cu-2 Pd/Cu-1 Pd/Cu-2 2Pd/Cu-1 2Pd/Cu-2
0.81 1.13 1.22 0.42 1.23 1.17
−2.00 −1.61 −1.28 −1.93 −1.01 −1.13
most favorable active site for the DMC formation. On the other hand, the lowest activation energy appears when the COOCH3 adsorbed through one C Pd bond and two O Cu bonds, while methoxyl adsorbed on surface with two O Cu bonds near the COOCH3 (Fig. 9, Pd/Cu-2). It is suggesting that both Cu and Pd site are needed for facile formation of DMC.
Fig. 8. PDOS of C atom in CO at different adsorption sites, the Fermi level was set as zero point.
most close to the surface, and the adsorption energies ranged between −0.63 and −0.28 eV; while the strongest one appeared at Pd/CuCl2 (0 0 1) surface. The corresponding adsorption configurations are shown in Fig. 10, as well as the adsorption energies. Take the co-adsorption configuration of methoxyl and COOCH3 as the reactant, the reaction pathways of the DMC formation was calculated and the energy barriers are listed in Table 5. Upon calculation, we notice that when COOCH3 species and methoxyl coadsorbed on one Pd atom (Fig. 9, 2Pd/Cu-1, 2Pd/Cu-2), the barriers for the formation of DMC is not the lowest; the similar situation occurs when the reactants co-adsorbed on Cu atom (Fig. 9, Cu-1, Cu-2). It implies that only the Cu site or Pd site itself may be not the
3.2.4. Surface reconstruction In the stoichiometric proportion of the surface regeneration, four Cu/Pd sites would be oxidized by one oxygen molecule. Based on the discussion above, the methoxyl pre-adsorbed surface obviously increases the surface activity. It indicates that the methanol will further react with more top-slayer Cl atoms upon the first methanol is oxidized at the appropriate site. Thus, it is dependable for us to build the unsaturated surface by removing four Cl atoms from CuCl2 (0 0 1), Pd/CuCl2 (0 0 1), 2Pd/CuCl2 (0 0 1) surfaces. The reactant configuration for the surface reconstruction was built by introducing HCl and O2 molecules into the region that 4 A˚ high from the unsaturated catalyst surfaces. The configurations before and after geometric optimization are illustrated in Fig. 11. Upon geometry optimization, the H Cl bonds and O O bond were broken. The Cl atoms bond with the Cu or Pd atoms to restore the surface. And the H atoms react with O atom to form H2 O molecule,
Fig. 9. Co-adsorption structure of COOCH3 species and methoxyl.
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
125
Fig. 10. Adsorption of DMC over different surfaces.
which weakly adsorbed on the catalyst surfaces. Thus, the reduction of catalyst surface is a spontaneous and exothermic process. 3.3. Reaction network for oxidation carbonylation of methanol Fig. 12 summarizes the calculated activation energy for elementary steps of the oxidative carbonylation of methanol over CuCl2 (0 0 1) with different Pd loadings. Compares with the pure catalyst surface, the doping of Pd decreases the energy barrier for the oxidation of methanol, which indicates the introduction of Pd is favorable for the catalyst activity; and this is agreed with the experimental observation [5,46]. However, the Cl atom, which has strong interaction with the doped Pd (Cl at the 2 site, Fig. 2), has an extremely high activation barrier for methanol oxidation. Thus, the high Pd doping amount will decrease the activity of the surface by increasing the number of these sites. On the other hand, the catalyst surfaces have higher activity on C O bond activating in CO insertion upon Pd doping, indicated by the lowest activation
energies of CO insertion reaction; this is agreed with the experimental speculation that the Pd atom is the suitable active site for the CO insertion [4,13]. Meanwhile, the DMC formation is also much easier on Pd/CuCl2 (0 0 1) surface. Compare to other two surfaces, the rate-limiting step over Pd/CuCl2 (0 0 1) surface (Pd/Cu = 1:17) has the lowest activation barrier, which is agreed with the experimental results that PdCl2 -CuCl2 catalysts with Pd/Cu ratio was 1:20 has the best activity [42,44,46]. Based on the calculations, another two interesting phenomenon were observed: one is the rate limiting step over Pd/CuCl2 surface is the oxidation of methanol; the other is the adsorbed methoxyl will decrease the difficulty for the oxidation of methanol on the catalyst surface, even the Cl atom strongly bond with the Pd atom will also have an acceptable energy barrier (Table 2). It indicates that, when the Pd/CuCl2 surface is covered by an amount of methoxyl during the reaction, the catalyst will show higher activity as the number of active sites will increase upon reaction and the activation energy for methanol oxidation will also decrease. And this is well agreed
Fig. 11. Initial and final structures of the surface reconstruction.
126
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
Fig. 12. Reaction paths for oxidation carbonylation of methanol over CuCl2 (0 0 1), Pd/CuCl2 (0 0 1) and 2Pd/CuCl2 (0 0 1).
with the experimental results that the Wacker-type catalysts have an obvious induction period in the oxidative carbonylation system [13,41].
with the experimental results that PdCl2 -CuCl2 catalysts with Pd/Cu ratio was 1:20 has the best activity. Acknowledgements
4. Conclusions We have examined the reaction pathways for oxidative carbonylation of methanol over CuCl2 (0 0 1), Pd/CuCl2 (0 0 1) and 2Pd/CuCl2 (0 0 1) via a comprehensive periodic density functional theory calculations. Various adsorption models of the intermediates involved in the reaction were investigated from the electronic, energetic and geometrical viewpoints, and the mechanism has been clarified. The methanol was found positioned with the H atom toward the Cl atom in a close a-top position before the reaction. The formed HCl species weakly adsorbed on the surface, which can explain the phenomenon of the leaching of Cl species observed from experimental measurements. Upon calculation, we notice that the doping of Pd leads the surface electrons repopulation and decreases the activation barrier of methanol oxidation; but high mount of doped Pd will also decrease the number of the suitable active sites, which is agreed with the experimental observation that the Pd/Cu ratio of the PdCl2 -CuCl2 catalyst with high activity is low. However, the formed methoxyl will increase the activity of the surface, which is agreed with the existence of the induction period of Wacker-type catalyst in the methanol oxidative carbonylation system. The doping of Pd increases the activity of CuCl2 (0 0 1) on CO insertion, which is agreed with the experimental propose that the Pd atom is preferable for the CO insertion. The doping of Pd also decreases the energy barriers of DMC formation, but the reactants—COOCH3 species and methoxyl—have to adsorb on different metal atoms. Finally, the reconstruction of the Clunsaturated surfaces is a spontaneous and exothermic process, which needs the HCl and O2 at the same time. Furthermore, we also notice that the doping of Pd decreases the energy barriers of the rate-limiting step over CuCl2 (0 0 1); and the Pd/CuCl2 (0 0 1) surface (Pd/Cu = 1:17) has the lowest activation barrier, which is agreed
The financial supports from the National Natural Science Foundation of China (NSFC) (grant no. 20576093, 20876112, 20936003), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (grant no. 20090032110021), and the Program of Introducing Talents of Discipline to Universities (B06006) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Y. Ono, Pure Appl. Chem. 68 (1996) 367. Y. Ono, Appl. Catal. A: Gen. 155 (1997) 133. M.A. Pacheco, C.L. Marshall, Energy Fuels 11 (1997) 2. N. Keller, G. Rebmann, V. Keller, J. Mol. Catal. A: Chem. 317 (2010) 1. P. Yang, Y. Cao, J.-C. Hu, W.-L. Dai, K.-N. Fan, Appl. Catal. A: Gen. 241 (2003) 363. U. Romano, R. Tesel, M.M. Mauri, P. Rebora, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 396. G.L. Curnutt, D.L. Harley, Oxygen Complexes and Oxygen Activation by Transition Metals, Plenum Publishing Co., New York, 1988, pp. 215. K. Tomishige, T. Sakaihori, S.-i. Sakai, K. Fujimoto, Appl. Catal. A: Gen. 181 (1999) 95. S. Huang, Y. Wang, Z. Wang, B. Yan, S. Wang, J. Gong, X. Ma, Appl. Catal. A: Gen. 417/418 (2012) 236. S.T. King, Catal. Today 33 (1997) 173. L. Zhong, W. Ruiyu, Z. Huayan, X. Kechang, Fuel 89 (2010) 1339. J. Engeldinger, C. Domke, M. Richter, U. Bentrup, Appl. Catal. A: Gen. 382 (2010) 303. R. Jiang, Y. Wang, X. Zhao, S. Wang, C. Jin, C. Zhang, J. Mol. Catal. A: Chem. 185 (2002) 159. J. Haggin, Chem. Eng. News 65 (1987) 26. P.-Y. Yang, Y.-G. Zhou, ChemInform 35 (2004). M.S. Han, B.G. Lee, I. Suh, H.S. Kim, B.S. Ahn, S.I. Hong, J. Mol. Catal. A: Chem. 170 (2001) 225. W. Yanji, Z. Xinqiang, Y. Baoguo, Z. Bingchang, C. Jinsheng, Appl. Catal. A: Gen. 171 (1998) 255. G.L. Curnutt, M. Midland, Catalytic Vapor Phase Process for Producing Dihydrocarbyl Carbonates, 1991. R. Jiang, T. Monroe, R. McRogers, P.J. Larson, Haemophilia 8 (2002) 1. R. Zhang, L. Ling, Z. Li, B. Wang, Appl. Catal. A: Gen. 400 (2011) 142. Q. Meng, Y. Shen, J. Xu, J. Gong, Chin. J. Catal. 33 (2012) 407. C. Dupont, R. Lemeur, A. Daudin, P. Raybaud, J. Catal. 279 (2011) 276. X. Zheng, A.T. Bell, J. Phys. Chem. C 112 (2008) 5043.
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127 [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]
Q. Meng, Y. Shen, J. Xu, X. Ma, J. Gong, Surf. Sci. 606 (2012) 1608. R. Zhang, H. Liu, B. Wang, L. Ling, Appl. Catal. B: Environ. 126 (2012) 108. Y. Shen, Q. Meng, S. Huang, S. Wang, J. Gong, X. Ma, RSC Adv. 2 (2012) 7109. Q. Meng, Z. Wang, Y. Shen, B. Yan, S. Wang, X. Ma, RSC Adv. 2 (2012) 8752. R. Zhang, L. Song, B. Wang, Z. Li, J. Comput. Chem. 33 (2012) 1101. B. Delley, J. Chem. Phys. 92 (1990) 508. B. Delley, J. Phys. Chem. 100 (1996) 6107. B. Delley, J. Chem. Phys. 113 (2000) 7756. A.D. Becke, Phys. Rev. A 38 (1988) 3098. J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244. F. Ortmann, F. Bechstedt, W.G. Schmidt, Phys. Rev. B 73 (2006) 205101. H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. T.A. Halgren, W.N. Lipscomb, Chem. Phys. Lett. 49 (1977) 225. G. Henkelman, H. Jónsson, J. Chem. Phys. 113 (2000) 9978. M.D. Segall, P.J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M.C. Payne, J. Phys.: Condens. Matter 14 (2002) 2717. L.B. Bergasova, S.K. Filatov, Dokl. Akad. Nauk SSSR 270 (1983) 415. P.C. Burns, F.C. Hawthorne, Am. Miner. 78 (1993) 187. P. Yang, Y. Cao, W.-L. Dai, J.-F. Deng, K.-N. Fan, Appl. Catal. A: Gen. 243 (2003) 323. X. Ma, Z. Li, B. Wang, G. Xu, React. Kinet. Catal. Lett. 76 (2002) 179.
127
[43] R. Jiang, S. Wang, X. Zhao, C. Wang, C. Zhang, Appl. Catal. A: Gen. 238 (2003) 131. [44] Y. Cao, P. Yang, C.-Z. Yao, N. Yi, W.-L. Feng, W.-L. Dai, K.-N. Fan, Appl. Catal. A: Gen. 272 (2004) 15. [45] P. Yang, Y. Cao, X.H. Bao, W.L. Dai, K.N. Fan, Chin. J. Catal. 25 (2004) 995. [46] Z. Zhang, X. Ma, P. Zhang, Y. Li, S. Wang, J. Mol. Catal. A: Chem. 266 (2007) 202. [47] H. Fu, Z.-P. Liu, Z.-H. Li, W.-N. Wang, K.-N. Fan, J. Am. Chem. Soc. 128 (2006) 11114. [48] W. Chen, P. Yuan, S. Zhang, Q. Sun, E. Liang, Y. Jia, Physica B: Conden. Matter 407 (2012) 1038. [49] J. Engeldinger, M. Richter, U. Bentrup, Phys. Chem. Chem. Phys. 14 (2012) 2183. [50] R. Zhang, H. Liu, L. Ling, Z. Li, B. Wang, Appl. Surf. Sci. 257 (2011) 4232. [51] Y.B. Song, A.G. Luo, Y.H. Du, Y.W. Fang, Prog. Chem. 20 (2008) 221. [52] H. Itoh, Y. Watanabe, K. Mori, H. Umino, Green Chem. 5 (2003) 558. [53] Z. Zhang, X. Ma, J. Zhang, F. He, S. Wang, J. Mol. Catal. A: Chem. 227 (2005) 141. [54] K.I. Choi, M.A. Vannice, J. Catal. 127 (1991) 465. [55] B.M. Bhanage, S.I. Fujita, Y. Ikushima, M. Arai, Appl. Catal. A: Gen. 219 (2001) 259. [56] G. Blyholder, Vacuum 15 (1965) 42. [57] H. Aizawa, S. Tsuneyuki, Surf. Sci. 399 (1998) L364. [58] F. Abild-Pedersen, M.P. Andersson, Surf. Sci. 601 (2007) 1747.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface structure and reaction property of CuCl2 -PdCl2 bimetallic catalyst in methanol oxycarbonylation: A DFT approach Qingsen Meng a , Shengping Wang a,∗ , Yongli Shen a , Bing Yan a , Yuanxin Wu b , Xinbin Ma a a Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China b School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430073, China
a r t i c l e
i n f o
Article history: Received 22 October 2013 Received in revised form 18 November 2013 Accepted 18 November 2013 Available online 25 November 2013 Keywords: Mechanism Dimethyl carbonate Methanol PdCl2 -CuCl2 Density functional theory
a b s t r a c t Surface structure of CuCl2 -PdCl2 bimetallic catalyst (Wacker-type catalyst) was built employing density functional theory (DFT) calculations, and the reaction mechanism of methanol oxycarbonylation over the CuCl2 -PdCl2 surfaces was also investigated. On the CuCl2 -PdCl2 surface, the active site for methanol oxidation was confirmed as Cu-Cl-Cu (Pd). Comparing with pure CuCl2 surface, the introduction of Pd atom causes the electron repopulation on the surface and lowers the energy barrier for methanol oxidation, but the number of the active site decreases with the increasing of Pd doping volume. Agreed with previous experimental results, the Pd site is most favorable for the CO insertion, indicated by the lowest activation barrier for the formation of COOCH3 on Pd atom. The lowest energy barrier for the formation of DMC appears when COOCH3 species adsorbed on Pd atom and methoxyl adsorbed on Cu atoms, which is 0.42 eV. Finally, the reconstruction of the unsaturated surface is a spontaneous and exothermic process. Comparing with other surfaces, the rate-limiting step, methanol oxidation, on CuCl2 -PdCl2 surface with Pd/Cu = 1:17 has the lowest energy barrier, which is agreed with the experimental observation that PdCl2 -CuCl2 catalyst with Pd/Cu = 1:20 has the favorable activity. The adsorbed methoxyl will further lower the activation barrier of methanol oxidation, which is agreed with experimental observation that the Wacker-type catalysts have an induction period in the methanol oxidative carbonylation system. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Dimethyl carbonate (DMC) is a potential fuel additive to replace methyl tert-butyl ether (MTBE), and a precursor for synthesis of carbonic acids and derivatives. It can also be used as a methylating agent to replace methyl halides and dimethyl sulfate, and as an intermediate for the synthesis of polycarbonates and isocyanates [1–4]. Dimethyl carbonate was originally made by the reaction of methanol with phosgene. However, because of phosgene’s extreme toxicity, it is now manufactured by the trans-esterification of propylene carbonate and methanol or the copper-catalyzed reaction of methanol, oxygen, and carbon monoxide. And one of the best ways to produce DMC is by oxidative carbonylation of methanol (2CH3 OH + CO + 1/2O2 → (CH3 O)2 CO + H2 O) [5]. Although a liquid slurry process employing a copper chloride catalyst has been commercialized [6], a gas-phase process is more desirable because the copper chloride is highly corrosive in the liquid phase [7]. Copper-based catalysts, including CuCl2 /AC [7,8], Cu(I)Y [9–12], CuCl2 -PdCl2 /AC [5,13], CuCl2 /composite supports [14,15] are active
∗ Corresponding author. Tel.: +86 2287401818. E-mail address: [email protected] (S. Wang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.096
for the vapor-phase oxidative carbonylation of methanol to DMC. The previous studies indicated that the Cu(I)Y catalysts prepared by the solid state ion-exchange method showed high DMC selectivity but low methanol conversion [10–12]. The activated carbon (AC)-supported CuCl2 catalyst showed high methanol conversion with DMC selectivity of 80–90% [8,16], and the catalytic activity can be further improved by the addition of palladium chloride [15,17]. Meanwhile, the favorable ratio of the Cu/Pd in AC-supported CuCl2 /PdCl2 catalyst is reported as 20:1 [15,17], but the explanations for this phenomenon are still absence. Earlier experimental work by Curnutt and Midland suggests a three-step mechanism to explain the high reactivity of the CuCl2 /AC catalysts [18]. Jiang et al. [19] have also proposed a reaction scheme by combining the XPS and XRD results to explain the high reactivity of catalysts and the synergic effect between palladium and copper in the CuCl2 -PdCl2 /AC catalyst. However, these proposed mechanisms are largely speculative, and thus a deep understanding on the reaction details and nature of active sites are desirable. This, in turn, makes fundamental investigations on kinetics and thermodynamics characters of the oxidative carbonylation of methanol over CuCl2 and CuCl2 -PdCl2 surfaces become necessary. Theoretical studies have received wide recognition for providing deep insights into reaction pathways and the key factors that
118
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
control reactivity and selectivity [20–25]. In recent years, the reaction mechanism of oxidative carbonylation of methanol has been investigated employing DFT calculation over Cu-Y [23], Cu- [26], ␥-Cu2 Cl(OH)3 (0 2 1) surface [27], and Cu2 O(1 1 1) surface [28]. But the molecular-level mechanistic understanding of oxidative carbonylation of methanol over CuCl2 -PdCl2 surface is still absent. Herein, we present a systematic DFT study regarding possible intermediates involved in the methanol oxidative carbonation as well as reaction mechanism over CuCl2 -PdCl2 surfaces. We focus on geometric and electronic parameters of the adsorbed species as well as on their kinetic and thermodynamic properties. We attempt to address three questions in this paper: (1) what is the rate-limiting step in the oxidation carbonation of methanol on catalyst surfaces; (2) what is the role of palladium in the CuCl2 -PdCl2 catalyst. (3) Why the low Pd/Cu ratio is favorable for the activity of the CuCl2 -PdCl2 catalyst. 2. Calculation methods All calculations were carried out by using the density functional theory and periodical slab model. The electronic structure and equilibrium geometries of the model systems were calculated by using the Dmol3 program package in Materials Studio [29–31], at the spin-unrestricted level. Exchange-correlation effects were described by the generalized gradient approximation (GGA) with Perdew and Wang (PW91) functional [32,33]. The DFT calculations coupled with a van der Waals-inclusive correction (DFT-D) [34] were carried out to improve the calculations of the energies associated with the interlayer interactions of CuCl2 structures, ˚ Extended numerical all-electron basis sets which is around 3.49 A. (double-zeta plus polarization, DNP) were used to describe the electronic structure of all atoms in the model systems except for copper and palladium, which is approximated by an effective core potential (ECP). A Monkhorst-Pack grid [35] of 3 × 3 × 1 was used to simple the Brillouin zone. Geometry optimizations were performed with a numerical displacement accuracy of the ˚ Possible transition states (TSs) were atom centers of 5 × 10−3 A. located by using the complete linear and quadratic synchronous transit (LST, QST) method connecting the reactants and products [36]. The nudged elastic band (NEB) method was used to calculate the MEP for all the reaction [37]. The MEP for each reaction was discretized by a total of ten images between the initial and final states. All TSs were confirmed by the vibrational frequency analysis. The convergence criteria were set to 2 × 10−5 Ha, 0.004 Ha/Å for energy, and force and field (SCF) density convergence threshold value of 1 × 10−5 Ha was specified. A Fermi smearing of 0.005 Ha was used to improve the calculation performance. Adsorption energies, Eads , were calculated as: Eads = Eadsorbate+surface − Efree adsorbate − Efree surface , while the reaction energies, Er , were calculated as: Er = Eproducts − Ereactants . In addition, the Mulliken population analysis and density of states (DOS) were performed using the ultrasoft pseudo-potentials generated from the “Ultrasoft” method implement within the CASTEP module [38]. The GGA with the PW91 functional was used and a plane-wave cutoff energy of 300 eV was applied for all calculations. The default convergence criteria of CASTEP were applied 0.05 eV/Å for the root-mean-square residual force on movable atoms. 3. Results and discussion 3.1. Surface models The crystal structure of the CuCl2 follows a base-centered monoclinic space group C2/m [39]. Cu atoms are positioned in an axially
distorted octahedral environment with four equatorial nearest ˚ calculated; 2.263(6) A, ˚ neighbors Cl atoms (band distance: 2.299 A, ˚ experiment [40]) and two axial Cl atoms (band distance: 2.977 A, ˚ experiment [40]). This octahedral environcalculated; 2.991(6) A, ment is a result of the John–Teller effect. Each Cu2+ Cl6 octahedron shares two Cl–Clequatorial edges with adjacent octahedron, and its apical Cl− ions are equatorial ligands for the adjacent octahedral [40]. This linkage results in corrugated octahedral sheets (Fig. 1(a)) of composition CuCl2 paralleled the (0 0 1) plane. Each sheet is electrostatically neutral, and linkage between adjacent sheets is based on Van der Waals forces [40]. In our work, the CuCl2 (0 0 1) surface is adopted to study the reaction properties. Fig. 1(b) shows a CuCl2 (0 0 1)-2 × 2 supercell including two corrugated octahedral sheets, and a vacuum of 15 A˚ was inserted in the direction perpendicular to the surface. Because of the same valence state of Cu2+ and Pd2+ and similar coordination between CuCl2 and PdCl2 , as well as the low Pd/Cu ratio reported experimentally [5,40–45], the Pd doped CuCl2 (0 0 1) surface by replacing one Cu atom with one Pd atom was built to represent the CuCl2 -PdCl2 surface in this work. The atomic concentration ratio of Pd to Cu is 1:17, which is close to that (1:20) most commonly used in the experiment [5,41–46]. The doped surface is shown in Fig. 1(c) (donated as Pd/CuCl2 (0 0 1)). In order to investigate the influence of the Pd/Cu ratio, the two Pd atoms doped surface (Fig. 1(d), donated as 2Pd/CuCl2 (0 0 1)) with the Pd/Cu ratio equal to 1:8 was also built and discussed. As the linkage between adjacent sheets of CuCl2 is by Van der Waals forces, the doped Pd atoms were all located at the first sheet of the surface to investigate the doping effect [47]. Additionally, only the latter sheet of CuCl2 was fixed during the calculation. Furthermore, in order to determine whether the palladium atom can incorporate into copper chloride readily by substituting Cu, we calculated the substitutional energy (Esub ) using the following equation [48]: Esub = EPd/CuCl2 + ECu − ECuCl2 − EPd where EPd/CuCl2 is the total energy of the Pd doped CuCl2 (0 0 1) surface, and ECuCl2 is the total energy of the pure CuCl2 (0 0 1) surface, ECu and EPd , are the energies per atom of bulk material Cu and Pd, respectively. Negative values indicate that the occurrence of the substitutional doping is easy [48]. Our calculations reveal that dopant Pd is easy to incorporate into the CuCl2 (0 0 1) surface by substituting Cu due to its lower substitutional energy (−0.93 eV for Pd/CuCl2 (0 0 1), −2.31 eV for 2Pd/CuCl2 (0 0 1)). 3.2. Reaction mechanism Based on the previous results [7,13,28,49,50], we have extended our discussion into four subsections regarding possible reaction steps involved in the oxidative carbonylation of methanol over CuCl2 (0 0 1), Pd/CuCl2 (0 0 1), and 2Pd/CuCl2 (0 0 1) surfaces. That is, (i) oxidation of methanol, (ii) CO insertion, (iii) DMC formation, and (iv) surface reconstruction. 3.2.1. Oxidation of methanol Experimentally, the presence of chlorine seems essential for designing a catalyst with high initial activity for methanol oxycarbonylation [4,16,42,51]. In this work, the top site of the Cl atoms in the first layer of the catalyst surfaces were treated as the adsorption sites for methanol, as reported in our previous work [27]. Upon optimization, the methanol was found positioned with the H atom toward the Cl atom in an a-top position with a hydrogen bond between them at all adsorption sites. The optimized adsorption configurations are shown in Fig. 2. The similar adsorption energies and the key structural characters between different adsorption
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
119
Fig. 1. Optimized structures of bulk CuCl2 (a), CuCl2 (0 0 1) surface (b), Pd/CuCl2 (0 0 1) surface (c), and 2Pd/CuCl2 (0 0 1) surface (d). The dark blue, light red, and green spheres represent Pd atom, Cu atom, and Cl atom, respectively. The color of each atom is consistence throughout this paper. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
configurations (Table 1) indicate that the methanol has the similar stability on different adsorption sites and surfaces. It also suggests that the doping of Pd has little influence on the adsorption of methanol. On the other hand, based on the mechanism proposed experimentally [7,13], the co-adsorbed methoxyl and HCl were created as the oxidative product of methanol. Fig. 3 shows the key structural of the adsorbed methoxyl and HCl on different sites. Upon optimization, the methoxyl species adsorbed on the surface through two or three O Cu(Pd) bonds with its C O axis normal to the substrate, and the adsorption energy is ranged from −2.48 to −2.29 eV. Notably, the corresponding O Cu bonds are slightly shorter and stronger than the O Pd bonds. It suggests that the adsorption of methoxyl bonded with the Pd atom is weaker, which should be beneficial for the following CO insertion. This deduction can be further confirmed through the comparison of the methoxyl adsorption energies. The formed HCl species weakly adsorbed on the surface with its adsorption energy around −0.22 to −0.13 eV, and the Mulliken analysis shows a weak interaction (bond population is around 0.02) between the Cl atom at the adsorption site and the H atom of HCl
in all calculated structures. Thus, we expect that the adsorption of HCl on the PdCl2 -CuCl2 surface is weak and removable [27], which could account for the phenomenon of the leaching of Cl species from experimental measurements [13,18,43,46]. The energy barriers for the reaction pathways that connect the adsorbed methanol and co-adsorbed methoxyl and HCl were calculated and summarized in Table 1. We found that the oxidation reaction of methanol at the 3 site on Pd/CuCl2 (0 0 1) or 2Pd/CuCl2 (0 0 1) surface has a lower barrier compared to the pristine CuCl2 (0 0 1) surface, suggesting that the doping of Pd could increase the activity of the CuCl2 (0 0 1) surface on methanol oxidation. However, activation energies of the oxidation taking place at the 2 site (Fig. 2), in which the Cl atom has strong interaction with doped Pd atom, are extremely high and can barely be overcome under realistic condition (413 K [5,8,13,16,52]). And this kind of Cl atom will cover the surface and lower the activity of the catalyst with the increasing of the Pd doping mount, which is also agreed with the experimental observation that the preferable Pd/Cu ratio for the catalyst with high activity is low (the ratio is 1:20 [5,41–46]). On the CuCl2 (0 0 1) surface, the Cu atom is six coordinated with Cl atom. Thus, the hybridization of the valence electron of Cu2+
Table 1 Calculated bond distance (Å), overlap population, adsorption energies (eV) of methanol and activation energies (eV), reaction energy (eV) of methanol oxidation on different surfaces. Configuration
*H Cl
*O H
Adsorption energy (eV)
Activation energy (eV)
Reaction energy (eV)
CuCl2 (0 0 1)-1 Pd/CuCl2 (0 0 1)-1 Pd/CuCl2 (0 0 1)-2 Pd/CuCl2 (0 0 1)-3 2Pd/CuCl2 (0 0 1)-1 2Pd/CuCl2 (0 0 1)-2 2Pd/CuCl2 (0 0 1)-3
˚ 2.520 Anewline 0.02 ˚ 2.572 Anewline 0.02 ˚ 2.624 Anewline 0.02 ˚ 2.506 Anewline 0.02 ˚ 2.521 Anewline 0.02 ˚ 2.531 Anewline 0.02 ˚ 2.625 Anewline 0.02
˚ 0.980 Anewline 0.55 ˚ 0.977 Anewline 0.54 ˚ 0.976 Anewline 0.54 ˚ 0.980 Anewline 0.54 ˚ 0.980 Anewline 0.54 ˚ 0.980 Anewline 0.55 ˚ 0.980 Anewline 0.55
−0.57 −0.57 −0.57 −0.85 −0.55 −0.55 −0.56
1.25 1.34 2.96 1.00 1.66 2.77 1.05
0.49 0.54 0.55 0.57 0.57 0.57 0.53
120
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
Fig. 2. Adsorption configurations of methanol on different sites; the gray, dark red, and white spheres represent the C atom, O atom, and H atom, respectively. The color of each atom is consistence throughout this paper. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Key structures and adsorption energies of methoxyl and HCl at different sites.
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
121
Table 2 Adsorption key parameters, reaction energies and activation energies of methanol on methoxyl pre-adsorbed surface.
CuCl2 (0 0 1) Pd/CuCl2 (0 0 1) 2Pd/CuCl2 (0 0 1)
H Cl
O H
Adsorption energy (eV)
Activation energy (eV)
Reaction energy (eV)
˚ 2.552 Anewline 0.02 ˚ 2.470 Anewline 0.02 ˚ 2.547 Anewline 0.02
˚ 0.979 Anewline 0.55 ˚ 0.980 Anewline 0.55 ˚ 0.979 Anewline 0.54
−0.31 −0.30 −0.24
0.66 0.80 0.88
0.57 0.72 0.58
is d2 sp3 , while the Cl− has a sp3 hybridization in its valence electrons. Nine electrons fill in the d orbitals of Cu2+ , which means, three extra d electrons of Cu2+ would join in the hybridization and fill in the anti-bonding orbitals of six Cu Cl bonds. Caused by the John–Teller effect, two of the Cu Cl bonds were extended [40], as shown in Fig. 1. However, the doped Pd2+ , which has eight electrons in its d orbitals, is only four coordinated with the Cl atoms based on our Mulliken population analysis. It indicates that the valence electron of Pd2+ is the dsp2 hybridization and no electron fills in the anti-bonding orbitals of Pd Cl bonds, implying the Pd Cl bond is stronger than the Cu Cl bond and the Cl atom in Pd Cl bond has smaller electron density than that in the Cu Cl bond. This result is also agreed with the Mulliken population analysis that the population of Cl at 2 site is −0.21 and the population is −0.27 on pure CuCl2 surface. On the other hand, the oxidation of methanol follows a nucleophilic attack mechanism; thus, the Cl atom with smaller electron density shows lower nucleophilicity, which has to overcome higher activation energy to oxidize methanol. Thus, the energy barrier for methanol oxidation increased seriously at 2 site. The partial density of states (PDOS) of local structure of Cu-Cl(2)Pd (2 indicates the Cl atom at 2 site, Fig. 2) were calculated and listed in Fig. 4. Comparing with the PDOS structure of the atom in the pure CuCl2 (0 0 1) surface, the doping of Pd introduced one new peak located around 1 eV in the PDOS of Cl and Pd atoms, which should be responsible for the anti-bonding orbitals of Pd Cl bond. However, in the same location, a small peak was observed in the PDOS structure of Cu atom that in the Cu-Cl(2)-Pd local structure, which indicates the hybridization between the Cl 2p, Pd 4d and slight Cu 3d orbitals. It also proves the electron transfers between the Cu atom and the doped Pd atom. Meanwhile, comparing with the Cu atom in the pure CuCl2 surface, the peak at the right side of the Fermi level was decreased in the PDOS of Cu atom in Cu-Cl(2)Pd local structure, which also indicates the electron transfer from Pd atom to the Cu atom; and this is agreed with the experimental expectation about the redox reaction between the Cu and Pd atoms [4,13,41,53]. Mulliken analysis also shows that the atomic population of the Cl at the 3 site (−0.41 in Pd/CuCl2 (0 0 1)-3, and −0.36 in 2Pd/CuCl2 (0 0 1)-3) are more negative than the Cl atom in the pure CuCl2 surface (atomic population is −0.27). The main reason could be the doping of Pd atom, which is four coordinated with the Cl atoms nearby, causes the disappearance of apical bond between the Pd atom and the Cl atom at the 3 site and leads to one lone electron pair left at the Cl atom. Thus, the Cl atom is less occupied and own more electron, which makes this Cl site is more nucleophilic and active for methanol oxidation. Thus, the energy barrier for methanol oxidation decreased at this Cl site. This electronic behavior can also be seen at the PDOS structure of the Cl atom at the 3 site that one new peak appeared at the left side of the Fermi level (Fig. 4), which is contributed by the new lone electron pair. Based on the reaction equation [5], two methanol molecules are needed to form one DMC; thus, the energy barrier for the oxidative reaction of methanol over methoxyl pre-adsorbed surfaces is also discussed. Fig. 5 displays calculated co-adsorption configurations of methanol and methoxyl on different surfaces. Additionally, considering the high activation barriers (e.g. 2.77–2.96 eV), only the adsorption configurations of methoxyl that have the lowest
formation barriers (methoxyl adsorbed at 3 site) were chosen for the following discussion. It is clear that the adsorption structure of methanol on the methoxyl pre-adsorbed surface did not changed obviously compared to the clean surfaces (Table 2). Interestingly, we also found that the pre-adsorbed methoxyl decreased the energy barriers for the oxidation. One reason could be that the adsorbed methoxyl, which has lower electronegativity and stronger electron giving ability than the Cl atom, leads to the electron transfer from it to the Cl atom nearby. In Fig. 6, the peaks located at the right side of the Fermi level in the PDOS structure of Cu and Cl atom in Cl-Cu-OCH3 local structure (on Pd/CuCl2 (0 0 1) surface) decreased and shifted to the lower energy region, which can prove the electron transformation. In the Cl(2)-Pd-OCH3 local structure (on 2Pd/CuCl2 (0 0 1) surface), the peaks located just at the right side of the Fermi level in the PDOS structure of Pd and Cl atom also shifted to the lower energy region. Meanwhile, in the PDOS structure of Pd atom in the Cl(2)-Pd-OCH3 local structure, the peak located next to the Fermi level (left side) increased comparing to that in the Cl-Pd-Cl local structure, which can also prove the electron transformation from the CH3 O species to Cl atom through Pd atom. Meanwhile, the electron transformation also leads the Cl atom in the Cl-Cu(Pd)-OCH3 local structure has larger electron density and stronger nucleophilicity. Thus, the activation barrier for methanol oxidation decreased at this site. Furthermore, the decreasing of the energy barriers also indicates that the catalyst may show higher activity for the methanol oxidation if the surfaces are covered with a mount of methoxyl during the reaction, which means the catalyst may have an induction period in the experiment.
3.2.2. CO insertion Experimentally, in the oxidative carbonylation and CO selective oxidation systems, it is proposed that the CO prefers to adsorb on palladium chloride to form Pd(CO)Cl2 intermediate first [13,54,55]. However, in our calculations, the Cu site and Pd site are all considered as the CO adsorption sites to confirm the effect of different CO adsorption sites. On the other hand, it is widely accepted that the CO adsorbed on the metal atom through the C atom [56–58]. Thus, only the configurations with CO adsorbed with C end are calculated (Fig. 7). Employing the Mulliken analysis, a C Cu(Pd) bond was found between CO and metal atom, suggesting the chemisorption of CO on the surface. Meanwhile, we notice that upon adsorption, the bond distance of CO is slightly extended compared to that in gaseous ˚ This is agreed with the Blyholder mechanism [56], state (1.142 A). in which the bond to the metal atom is described in terms of the charge donation from the CO 5 orbital to the metal atom and the back-donation from the metal to the CO 2* orbital, which rationalized the increase in CO bond length upon chemisorption [58]. Meanwhile, the PDOS structure of the C atom in CO at different adsorption sites (Fig. 8) also prove the electron transfer, in which all the peaks of adsorbed CO move to the deeper energy region compare with the CO atom at gaseous sphere. In turn, the Pd atom seems more active on the CO activation than the Cu atoms, indicated by the longer C O bond distance when CO adsorbed on Pd atom (Table 3).
122
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
Fig. 4. PDOS of the Cu, Pd, Cl atoms at pure and Pd doped CuCl2 (0 0 1) surface. The Fermi level is set as zero point.
Fig. 5. Co-adsorption configurations of methanol and methoxyl on different surfaces.
Based on the mechanism of oxidation carbonylation over the CuCl2 proposed by Curnutt and Harley [7] and over the Cu-Y catalyst that proposed by Richter et al. [49], the adsorbed CO may not reacted with two adsorbed methoxyl at once, but to form an intermediate (COOCH3 [7] or HOCOOCH3 [26,49]) first, then react with another methoxyl or methanol to form the DMC. Thus, the co-adsorption of COOCH3 and methoxyl has been calculated as the product of CO insertion in this work. Two main adsorption configurations of COOCH3 are calculated, one is COOCH3 adsorbed on one metal atom via the interaction between the carbonyl C atom and the metal atom (Fig. 9; Cu-2, 2Pd/Cu-2); the other is COOCH3 adsorbed on the surface through both oxyalkyl O atom and carbonyl C atom (Fig. 9; Cu-1, Pd/Cu-1, Pd/Cu-2, 2Pd/Cu-1).
Bond distance C Cu/Pd
Cu-CO Pd/Cu-CO Pd-CO Pd/Pd-CO
2.005 A˚ 1.996 A˚ 2.131 A˚ 2.382 A˚
3.2.3. DMC formation As the target product of methanol oxycarbonylation, DMC molecularly adsorbed on the surface with two oxyalkyl O atoms Table 4 Activation and reaction energies for the formation of COOCH3 species.
Table 3 Key structural parameters of CO adsorption at different sites. Configuration
From Table 4, it is clear that the insertion reaction occurs with the lowest energy barrier (0.25 eV) when methoxyl and CO coadsorbed on one Pd atom; and the formed COOCH3 adsorbed on the Pd atom through the carbonyl C atom. However, when the methoxyl and CO adsorb on different atoms or co-adsorbed on Cu atom, the formation of COOCH3 species is facile and the energy barriers range from 0.30 to 0.49 eV. When the CO adsorbed on the Pd atom, the energy release of the reactions are also larger compare to that on Cu atom.
Overlap population C Cu/Pd
Bond distance C O
0.19 0.15 0.14 0.13
1.146 A˚ 1.146 A˚ 1.150 A˚ 1.183 A˚
Cu-1 Cu-2 Pd/Cu-1 Pd/Cu-2 2Pd/Cu-1 2Pd/Cu-2
Reactant
Activation energy (eV)
Reaction energy (eV)
Cu-CO Cu-CO Pd-CO Pd/Cu-CO 2Pd/Cu-CO 2Pd/Cu-CO
0.30 0.48 0.33 0.49 0.30 0.25
−0.14 −0.54 −0.60 −0.19 −0.58 −1.08
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
Fig. 6. The PDOS of Cl, Cu atom and Pd atom in the Cl-Cu(Pd)-OCH3 on Pd-doped CuCl2 (0 0 1) surfaces. The Fermi level was set as the zero point.
Fig. 7. Adsorption of CO on different methoxyl pre-adsorbed surfaces.
123
124
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127 Table 5 Activation energies and Reaction energies for the formation of DMC.
CuCl2 CuCl2 Pd/CuCl2 Pd/CuCl2 2Pd/CuCl2 2Pd/CuCl2
Reactant
Activation energy (eV)
Reaction energy (eV)
Cu-1 Cu-2 Pd/Cu-1 Pd/Cu-2 2Pd/Cu-1 2Pd/Cu-2
0.81 1.13 1.22 0.42 1.23 1.17
−2.00 −1.61 −1.28 −1.93 −1.01 −1.13
most favorable active site for the DMC formation. On the other hand, the lowest activation energy appears when the COOCH3 adsorbed through one C Pd bond and two O Cu bonds, while methoxyl adsorbed on surface with two O Cu bonds near the COOCH3 (Fig. 9, Pd/Cu-2). It is suggesting that both Cu and Pd site are needed for facile formation of DMC.
Fig. 8. PDOS of C atom in CO at different adsorption sites, the Fermi level was set as zero point.
most close to the surface, and the adsorption energies ranged between −0.63 and −0.28 eV; while the strongest one appeared at Pd/CuCl2 (0 0 1) surface. The corresponding adsorption configurations are shown in Fig. 10, as well as the adsorption energies. Take the co-adsorption configuration of methoxyl and COOCH3 as the reactant, the reaction pathways of the DMC formation was calculated and the energy barriers are listed in Table 5. Upon calculation, we notice that when COOCH3 species and methoxyl coadsorbed on one Pd atom (Fig. 9, 2Pd/Cu-1, 2Pd/Cu-2), the barriers for the formation of DMC is not the lowest; the similar situation occurs when the reactants co-adsorbed on Cu atom (Fig. 9, Cu-1, Cu-2). It implies that only the Cu site or Pd site itself may be not the
3.2.4. Surface reconstruction In the stoichiometric proportion of the surface regeneration, four Cu/Pd sites would be oxidized by one oxygen molecule. Based on the discussion above, the methoxyl pre-adsorbed surface obviously increases the surface activity. It indicates that the methanol will further react with more top-slayer Cl atoms upon the first methanol is oxidized at the appropriate site. Thus, it is dependable for us to build the unsaturated surface by removing four Cl atoms from CuCl2 (0 0 1), Pd/CuCl2 (0 0 1), 2Pd/CuCl2 (0 0 1) surfaces. The reactant configuration for the surface reconstruction was built by introducing HCl and O2 molecules into the region that 4 A˚ high from the unsaturated catalyst surfaces. The configurations before and after geometric optimization are illustrated in Fig. 11. Upon geometry optimization, the H Cl bonds and O O bond were broken. The Cl atoms bond with the Cu or Pd atoms to restore the surface. And the H atoms react with O atom to form H2 O molecule,
Fig. 9. Co-adsorption structure of COOCH3 species and methoxyl.
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
125
Fig. 10. Adsorption of DMC over different surfaces.
which weakly adsorbed on the catalyst surfaces. Thus, the reduction of catalyst surface is a spontaneous and exothermic process. 3.3. Reaction network for oxidation carbonylation of methanol Fig. 12 summarizes the calculated activation energy for elementary steps of the oxidative carbonylation of methanol over CuCl2 (0 0 1) with different Pd loadings. Compares with the pure catalyst surface, the doping of Pd decreases the energy barrier for the oxidation of methanol, which indicates the introduction of Pd is favorable for the catalyst activity; and this is agreed with the experimental observation [5,46]. However, the Cl atom, which has strong interaction with the doped Pd (Cl at the 2 site, Fig. 2), has an extremely high activation barrier for methanol oxidation. Thus, the high Pd doping amount will decrease the activity of the surface by increasing the number of these sites. On the other hand, the catalyst surfaces have higher activity on C O bond activating in CO insertion upon Pd doping, indicated by the lowest activation
energies of CO insertion reaction; this is agreed with the experimental speculation that the Pd atom is the suitable active site for the CO insertion [4,13]. Meanwhile, the DMC formation is also much easier on Pd/CuCl2 (0 0 1) surface. Compare to other two surfaces, the rate-limiting step over Pd/CuCl2 (0 0 1) surface (Pd/Cu = 1:17) has the lowest activation barrier, which is agreed with the experimental results that PdCl2 -CuCl2 catalysts with Pd/Cu ratio was 1:20 has the best activity [42,44,46]. Based on the calculations, another two interesting phenomenon were observed: one is the rate limiting step over Pd/CuCl2 surface is the oxidation of methanol; the other is the adsorbed methoxyl will decrease the difficulty for the oxidation of methanol on the catalyst surface, even the Cl atom strongly bond with the Pd atom will also have an acceptable energy barrier (Table 2). It indicates that, when the Pd/CuCl2 surface is covered by an amount of methoxyl during the reaction, the catalyst will show higher activity as the number of active sites will increase upon reaction and the activation energy for methanol oxidation will also decrease. And this is well agreed
Fig. 11. Initial and final structures of the surface reconstruction.
126
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127
Fig. 12. Reaction paths for oxidation carbonylation of methanol over CuCl2 (0 0 1), Pd/CuCl2 (0 0 1) and 2Pd/CuCl2 (0 0 1).
with the experimental results that the Wacker-type catalysts have an obvious induction period in the oxidative carbonylation system [13,41].
with the experimental results that PdCl2 -CuCl2 catalysts with Pd/Cu ratio was 1:20 has the best activity. Acknowledgements
4. Conclusions We have examined the reaction pathways for oxidative carbonylation of methanol over CuCl2 (0 0 1), Pd/CuCl2 (0 0 1) and 2Pd/CuCl2 (0 0 1) via a comprehensive periodic density functional theory calculations. Various adsorption models of the intermediates involved in the reaction were investigated from the electronic, energetic and geometrical viewpoints, and the mechanism has been clarified. The methanol was found positioned with the H atom toward the Cl atom in a close a-top position before the reaction. The formed HCl species weakly adsorbed on the surface, which can explain the phenomenon of the leaching of Cl species observed from experimental measurements. Upon calculation, we notice that the doping of Pd leads the surface electrons repopulation and decreases the activation barrier of methanol oxidation; but high mount of doped Pd will also decrease the number of the suitable active sites, which is agreed with the experimental observation that the Pd/Cu ratio of the PdCl2 -CuCl2 catalyst with high activity is low. However, the formed methoxyl will increase the activity of the surface, which is agreed with the existence of the induction period of Wacker-type catalyst in the methanol oxidative carbonylation system. The doping of Pd increases the activity of CuCl2 (0 0 1) on CO insertion, which is agreed with the experimental propose that the Pd atom is preferable for the CO insertion. The doping of Pd also decreases the energy barriers of DMC formation, but the reactants—COOCH3 species and methoxyl—have to adsorb on different metal atoms. Finally, the reconstruction of the Clunsaturated surfaces is a spontaneous and exothermic process, which needs the HCl and O2 at the same time. Furthermore, we also notice that the doping of Pd decreases the energy barriers of the rate-limiting step over CuCl2 (0 0 1); and the Pd/CuCl2 (0 0 1) surface (Pd/Cu = 1:17) has the lowest activation barrier, which is agreed
The financial supports from the National Natural Science Foundation of China (NSFC) (grant no. 20576093, 20876112, 20936003), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (grant no. 20090032110021), and the Program of Introducing Talents of Discipline to Universities (B06006) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Y. Ono, Pure Appl. Chem. 68 (1996) 367. Y. Ono, Appl. Catal. A: Gen. 155 (1997) 133. M.A. Pacheco, C.L. Marshall, Energy Fuels 11 (1997) 2. N. Keller, G. Rebmann, V. Keller, J. Mol. Catal. A: Chem. 317 (2010) 1. P. Yang, Y. Cao, J.-C. Hu, W.-L. Dai, K.-N. Fan, Appl. Catal. A: Gen. 241 (2003) 363. U. Romano, R. Tesel, M.M. Mauri, P. Rebora, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 396. G.L. Curnutt, D.L. Harley, Oxygen Complexes and Oxygen Activation by Transition Metals, Plenum Publishing Co., New York, 1988, pp. 215. K. Tomishige, T. Sakaihori, S.-i. Sakai, K. Fujimoto, Appl. Catal. A: Gen. 181 (1999) 95. S. Huang, Y. Wang, Z. Wang, B. Yan, S. Wang, J. Gong, X. Ma, Appl. Catal. A: Gen. 417/418 (2012) 236. S.T. King, Catal. Today 33 (1997) 173. L. Zhong, W. Ruiyu, Z. Huayan, X. Kechang, Fuel 89 (2010) 1339. J. Engeldinger, C. Domke, M. Richter, U. Bentrup, Appl. Catal. A: Gen. 382 (2010) 303. R. Jiang, Y. Wang, X. Zhao, S. Wang, C. Jin, C. Zhang, J. Mol. Catal. A: Chem. 185 (2002) 159. J. Haggin, Chem. Eng. News 65 (1987) 26. P.-Y. Yang, Y.-G. Zhou, ChemInform 35 (2004). M.S. Han, B.G. Lee, I. Suh, H.S. Kim, B.S. Ahn, S.I. Hong, J. Mol. Catal. A: Chem. 170 (2001) 225. W. Yanji, Z. Xinqiang, Y. Baoguo, Z. Bingchang, C. Jinsheng, Appl. Catal. A: Gen. 171 (1998) 255. G.L. Curnutt, M. Midland, Catalytic Vapor Phase Process for Producing Dihydrocarbyl Carbonates, 1991. R. Jiang, T. Monroe, R. McRogers, P.J. Larson, Haemophilia 8 (2002) 1. R. Zhang, L. Ling, Z. Li, B. Wang, Appl. Catal. A: Gen. 400 (2011) 142. Q. Meng, Y. Shen, J. Xu, J. Gong, Chin. J. Catal. 33 (2012) 407. C. Dupont, R. Lemeur, A. Daudin, P. Raybaud, J. Catal. 279 (2011) 276. X. Zheng, A.T. Bell, J. Phys. Chem. C 112 (2008) 5043.
Q. Meng et al. / Applied Surface Science 292 (2014) 117–127 [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]
Q. Meng, Y. Shen, J. Xu, X. Ma, J. Gong, Surf. Sci. 606 (2012) 1608. R. Zhang, H. Liu, B. Wang, L. Ling, Appl. Catal. B: Environ. 126 (2012) 108. Y. Shen, Q. Meng, S. Huang, S. Wang, J. Gong, X. Ma, RSC Adv. 2 (2012) 7109. Q. Meng, Z. Wang, Y. Shen, B. Yan, S. Wang, X. Ma, RSC Adv. 2 (2012) 8752. R. Zhang, L. Song, B. Wang, Z. Li, J. Comput. Chem. 33 (2012) 1101. B. Delley, J. Chem. Phys. 92 (1990) 508. B. Delley, J. Phys. Chem. 100 (1996) 6107. B. Delley, J. Chem. Phys. 113 (2000) 7756. A.D. Becke, Phys. Rev. A 38 (1988) 3098. J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244. F. Ortmann, F. Bechstedt, W.G. Schmidt, Phys. Rev. B 73 (2006) 205101. H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. T.A. Halgren, W.N. Lipscomb, Chem. Phys. Lett. 49 (1977) 225. G. Henkelman, H. Jónsson, J. Chem. Phys. 113 (2000) 9978. M.D. Segall, P.J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M.C. Payne, J. Phys.: Condens. Matter 14 (2002) 2717. L.B. Bergasova, S.K. Filatov, Dokl. Akad. Nauk SSSR 270 (1983) 415. P.C. Burns, F.C. Hawthorne, Am. Miner. 78 (1993) 187. P. Yang, Y. Cao, W.-L. Dai, J.-F. Deng, K.-N. Fan, Appl. Catal. A: Gen. 243 (2003) 323. X. Ma, Z. Li, B. Wang, G. Xu, React. Kinet. Catal. Lett. 76 (2002) 179.
127
[43] R. Jiang, S. Wang, X. Zhao, C. Wang, C. Zhang, Appl. Catal. A: Gen. 238 (2003) 131. [44] Y. Cao, P. Yang, C.-Z. Yao, N. Yi, W.-L. Feng, W.-L. Dai, K.-N. Fan, Appl. Catal. A: Gen. 272 (2004) 15. [45] P. Yang, Y. Cao, X.H. Bao, W.L. Dai, K.N. Fan, Chin. J. Catal. 25 (2004) 995. [46] Z. Zhang, X. Ma, P. Zhang, Y. Li, S. Wang, J. Mol. Catal. A: Chem. 266 (2007) 202. [47] H. Fu, Z.-P. Liu, Z.-H. Li, W.-N. Wang, K.-N. Fan, J. Am. Chem. Soc. 128 (2006) 11114. [48] W. Chen, P. Yuan, S. Zhang, Q. Sun, E. Liang, Y. Jia, Physica B: Conden. Matter 407 (2012) 1038. [49] J. Engeldinger, M. Richter, U. Bentrup, Phys. Chem. Chem. Phys. 14 (2012) 2183. [50] R. Zhang, H. Liu, L. Ling, Z. Li, B. Wang, Appl. Surf. Sci. 257 (2011) 4232. [51] Y.B. Song, A.G. Luo, Y.H. Du, Y.W. Fang, Prog. Chem. 20 (2008) 221. [52] H. Itoh, Y. Watanabe, K. Mori, H. Umino, Green Chem. 5 (2003) 558. [53] Z. Zhang, X. Ma, J. Zhang, F. He, S. Wang, J. Mol. Catal. A: Chem. 227 (2005) 141. [54] K.I. Choi, M.A. Vannice, J. Catal. 127 (1991) 465. [55] B.M. Bhanage, S.I. Fujita, Y. Ikushima, M. Arai, Appl. Catal. A: Gen. 219 (2001) 259. [56] G. Blyholder, Vacuum 15 (1965) 42. [57] H. Aizawa, S. Tsuneyuki, Surf. Sci. 399 (1998) L364. [58] F. Abild-Pedersen, M.P. Andersson, Surf. Sci. 601 (2007) 1747.