The effect of interstitial boron on the mechanisms of acetylene hydrogenation catalyzed by Pd6: A DFT study

Computational and Theoretical Chemistry 1170 (2019) 112636

Contents lists available at ScienceDirect

Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc

The effect of interstitial boron on the mechanisms of acetylene hydrogenation catalyzed by Pd6: A DFT study

T

Jianfeng Wang1, Wenshu Hao1, Li-Juan Ma , Jianfeng Jia, Hai-Shun Wu ⁎

⁎

Key Laboratory of Magnetic Molecules & Magnetic Information Materials Ministry of Education, Shanxi Normal University, Linfen 041004, China The School of Chemical and Material Science, Shanxi Normal University, No. 1, Gongyuan Street, Linfen 041004, China

ARTICLE INFO

ABSTRACT

Keywords: Pd6B clusters Selectively Activity Density functional theory C2H2 selective hydrogenation

Detailed DFT calculations of C2H2 hydrogenation on Pd6, Pd6+ and Pd6B clusters were performed to explore the effect of interstitial B on the mechanisms of C2H2 hydrogenation catalyzed by Pd6. The results show that the Pd6B cluster has the lowest diffusion barriers of H atoms and dissociated barriers of H2; the interstitial B atom can simultaneously improve both the activity and selectivity of C2H2 hydrogenation to C2H4 on Pd6 cluster by altering the major product and the optimal pathway. Moreover, the higher charge is conducive to enhancing the adsorption of C2H2 and C2H4, rather than the selectivity of C2H4 formation. Our work provides some insight into the activity selectivity charge relationship of Pd-based catalysts in C2H2 selective hydrogenation.

1. Introduction Ethylene (C2H4) is widely used as an important monomer in the olefin industry [1]. However, approximately 0.1–1% of acetylene (C2H2) is produced during the process of C2H4 production, which will deactivate the downstream catalyst used for C2H4 polymerization [2,3]. Thus, C2H2 selective hydrogenation is an important industrial process not only to increase yield of C2H4 but also to purify C2H4. However, both C2H2 and C2H4 are unsaturated hydrocarbons that can be catalyzed easily to ethane (C2H6) [4,5]. Pd catalysts are widely and intensively used to hydrogenate C2H2 based on high catalytic activity [6,7]. Nevertheless, pure Pd catalysts exhibit a poor selectivity towards C2H4 formation. Meanwhile, oligomeric species can reduce the reaction rates [8]. Moreover, the subsurface C and H atoms decrease the selectivity of C2H4 on the Pd surface [9–21]. According numerous studies, three effective methods were adopted to improve the selectivity of Pd catalyst, i.e., incorporating additives, modifying the supports, and adding second metals, such as Cu [22], Al [23], Zn [24], Ag [25–28], Sn [29], and Au [30]. In the case of Pd-Zn/C and Pd-Ag/C systems, an increased distance between neighboring Pd atoms (2.82–2.89 Å) was observed [31]. PdZn [24] alloy has higher selectivity due to the weak adsorption of C2H4. Unfortunately, adding second metals is less effective in industrial applications given the high cost of these metals and the short lifetimes of catalysts.

Fortunately, Tsang [32,33] and found that Pd catalysts with interstitial boron (B) atoms exhibited ultra-selectivity and activities for a number of challenging catalytic reactions. Pd-B catalysts could effectively avoid the phenomena of isomerization and double-bond shifts. With the exception of alkyne hydrogenation reactions, Pd-B catalysts also exhibit ultra-catalytic performance in the reaction of formic acid decomposition to hydrogen [34], CO selective hydrogenation to methanol [35], and nitrobenzene selective hydrogenation [36]. However, few studies have been reported C2H2 hydrogenation to C2H4 catalyzed by Pd-B. Moreover, the effect of interstitial B on the mechanisms of C2H2 selective hydrogenation catalyzed by Pd remains unclear. In this study, to explore the mechanism of C2H2 selective hydrogenation on Pd-B catalyst, an ab initio density functional theory (DFT) method has been performed on Pd6B cluster. In addition, DFT calculations on Pd6 and Pd6+ were simultaneously performed to assess the effect of interstitial B. We choose the Pd6B cluster as a model based on the following considerations: (1) The surface of the cluster model also has the characteristics of the metal surface [37]. Moreover, Pdn clusters were used for the selective hydrogenation of CO to methanol and the production of H2 from formic acid, which considerably close to the real catalysts [38–40]. Recently, theoretical calculations indicated that Si modified Pd6 possessed high selectivity [41], which is consistent with previous experiments [42–44]. (2) The cluster model is very suitable for study

Corresponding authors at: Key Laboratory of Magnetic Molecules & Magnetic Information Materials Ministry of Education, Shanxi Normal University, Linfen 041004, China. E-mail addresses: [email protected] (L.-J. Ma), [email protected] (J. Jia), [email protected] (H.-S. Wu). 1 These authors contributed equally to this work and should be considered co-first authors. ⁎

https://doi.org/10.1016/j.comptc.2019.112636 Received 18 August 2019; Received in revised form 23 October 2019; Accepted 24 October 2019 Available online 13 November 2019 2210-271X/ © 2019 Elsevier B.V. All rights reserved.

Computational and Theoretical Chemistry 1170 (2019) 112636

J. Wang, et al.

due to its special electronic structures [45,46]. The size of Pdn clusters could directly affect the reaction pathway [47]. (3) Above all, Pd6B is the magic cluster and is the unit structure of Pd-B catalyst [48]. We expect that the results could be helpful to understand the contribution of interstitial B to the C2H2 hydrogenation reaction and to obtain some insight into the activity selectivity charge relationship of Pd-based catalysts in C2H2 selective hydrogenation.

Table 1 Bond length (Å), bond energy (kcal/mol) of H2, PdH and C2H2 systems, and electron affinity (eV) of the Pd6 cluster. This work

Exp.

H2

dH-H Eb dPd-H Eb dC-C dC-H Electron affinity

0.744 4.50 1.539 55.01 1.208 1.067 1.75

0.730 4.48 1.534 [59] 56 ± 6 [61] 1.210 [60] 1.070 [60] 1.65 ± 0.10 [62]

C2H2 Pd6

The hybrid B3LYP functional [49–52] was used to obtain fully optimized structures, normal-mode frequencies and Gibbs free energy at 298.15 K and 1 atm of reactants (R), products (P), intermediates (IMs) and transition states (TSs). All of the calculations were implemented with the Gaussian 09 program [53]. The all-electron basis set 6-31+ +G** [54] was adopted for C, B and H atoms and the LANL2DZ effective core pseudopotential basis set was adopted for the Pd atom. The geometrical structure was optimized without any symmetry restrictions. TSs were verified by the quadratic synchronous transit method [55] and by tracing the intrinsic reaction coordinate (IRC) [56,57]. In this study, we consider three possible pathways of C2H2 selective hydrogenation. Path I is C2H2 hydrogenation to form C2H4 via CHCH2 (H), followed by its desorption (red line in Fig. 1). Path II is C2H2 hydrogenation to form C2H4, and then C2H4 (2H) is hydrogenated into C2H6 via the intermediate C2H5 (H). Path III is C2H2 hydrogenation to form CHCH3, and then CHCH3 (2H) is hydrogenated into C2H5 (H) (blue line in Fig. 1). C2H5 (H) is easily hydrogenated to C2H6. Thus, given our aim to remove C2H2 impurities, suppressing Paths II and III and promoting Path I represents a better method to improve the selectivity of C2H2 hydrogenation to C2H4. The reference point of relative Gibbs free energies is defined as the total Gibbs free energy of Pd6/Pd6+/Pd6B cluster, C2H2 and two H2 molecules. The values of reaction energy (ΔG) and activation barrier (Ga) are as follows:

G = GP

GR

(1)

Ga = G TS

GR

(2)

H2, Pd–H in PdH, CeC and CeH in C2H2 with the B3LYP method are 0.744, 1.539, 1.208 and 1.067 Å, respectively. These values are very close to the experimental data [59,60] of 0.730, 1.534, 1.210 and 1.070 Å, respectively. Additionally, the calculated bond energies of H2 and PdH are 4.50 and 55.01 kcal/mol, which are consistent with the experimental values of 4.48 and 56 ± 6 kcal/mol [61]. The calculated electron affinity of neutral species is 1.75 eV, which is in perfect agreement with the experimental value of 1.65 ± 0.10 eV [62]. Moreover, in consideration of the effect of popular integration grids on DFT-computed free energy [63], B3LYP/6-31++G** electronic energy (E) and free energy (G) of Pd6, Pd6+ and Pd6B clusters were calculated using three popular integration grids: (75, 302), (99, 590) and (175, 974). There are negligible variations (< 0.007 kcal/ mol) in the relative electronic energies and free energies, indicating that the size of the grid has minimal effects on their Gibbs energies. The above results indicate that the calculation method employed is reliable and sufficiently accurate. 3. Results and discussion 3.1. Structures of Pd6, Pd6+ and Pd6B clusters All the optimized structures of the Pd6, Pd6+ and Pd6B clusters are displayed in Fig. 2. The Pd6 with Oh symmetry and Pd6B with D3d symmetry of Ref. [48] are adopted. The bond length of Pd-Pd in Pd6 is 2.702 Å, which is consistent with the result in Refs. [46,47,64]. When Pd6 loses an electron, the structure is distorted due to the asymmetric distribution of electrons. NBO [65] analysis demonstrates that the Pd atomic charges in Pd6+ are 0.152 and 0.173, respectively. There is a dihedral angle of 3.507 degrees on the symmetrical plane of Pd6+. Thus, the symmetry of Pd6+ is deviated from Oh to C2. The longest and shortest bond lengths of PdePd bonds in Pd6+ are 2.782 Å and 2.753 Å, respectively. Pd6B. Pd6, Pd6+ and Pd6B clusters exist in three adsorption sites: top (Pd atom), bridge (PdePd bond) and face (the fold of PdePdePd).

GP, GR, and GTS represent the total Gibbs free energies at 298.15 K of the adsorbed R, P and the TSs, respectively. As shown in Fig. 1, there are common steps of C2H2 + H → C2H3 and C2H5 + H → C2H6. Thus, starting from C2H3 (H) to C2H5 (H), the pathway with the lowest barrier is considered as the optimal pathway. The adsorption energies (Gads) of H2, C2H2 and C2H2 are evaluated as

Gtotal

Properties

Pd-H

2. Computational methods

Gads = Gadsorbate + Gcluster

Systems

(3)

where Gtotal is the Gibbs free energy of the total adsorbed system, Gadsorbate is the Gibbs free energy of H2/C2H2/C2H4, and Gcluster is the Gibbs free energy of the cluster. The desorption barrier of C2H4 in Path I is used as its adsorption energy because previous study [58] showed that the desorption barrier is approximately equal to the absolute value of adsorption energy. To test the reliability of our calculation, H2, PdH and C2H2 are calculated as shown in Table 1. The calculated bond lengths of HeH in

3.2. H2 adsorption and dissociation on Pd6, Pd6+ and Pd6B clusters Based on the calculation results, the H atom is likely to adsorb on the face sites of Pd6 and Pd6+ clusters, and on bridge sites of the Pd6B cluster. The diffusion barriers of H atoms shown in Fig. 3 are Pd6(3.91 kal/mol) > Pd6+(1.97 kal/mol) > Pd6B (1.04 kal/mol). These values are quite low compared with the reaction barriers. Pu [47] also Fig. 1. Three possible reaction pathways of C2H2 hydrogenation. The H in parentheses represents the H atoms adsorbed on the clusters.

2

Computational and Theoretical Chemistry 1170 (2019) 112636

J. Wang, et al.

hand, sufficient H atoms may diffuse rapidly on the clusters; namely, H exists dominantly in the atomic form on R, P, IMs and TSs. 3.3. Adsorption of C2H2 and C2H4 on Pd6, Pd6+, and Pd6B and the hydride complexes Pd6 (2H), Pd6+ (2H), and Pd6B (2H) The symmetries of HOMOs of C2H2/C2H4 shown in Fig. 5 match with the LUMOs of Pd6, Pd6+, and Pd6B clusters, allowing electrons to flow from orbitals of C2H2/C2H4 to the unoccupied orbitals of Pd. The LUMOs of C2H2/C2H4 also match with the d orbitals of Pd, resulting in the back-donation of Pd to C2H2/C2H4. The top, bridge and face are three possible adsorption sites of C2H2/C2H4 on Pd6, Pd6+ and Pd6B clusters. Fig. 5 demonstrates that C2H2 tend to bond on bridge site of Pd6 and Pd6+, but on the face site of Pd6B due to special HOMO and LUMO. Fig. 6 displays the stable adsorption configurations of C2H2, C2H4, and coadsorption configurations with C2H2/C2H4 and H2 molecules on Pd6, Pd6+ and Pd6B clusters. For C2H2 adsorption, the Gibbs free adsorption energies are Pd6+ (17.87 kcal/mol) > Pd6 (17.09 kcal/ mol) > Pd6B (16.89 kcal/mol). C2H2 tends to bond on bridge sites of Pd6 and Pd6+ but on face sites of Pd6B, which is consistent with that in Refs. [47,67], respectively. C2H4 is preferably adsorbed on the bridge sites of three clusters, and the corresponding free adsorption energies are Pd6+ (6.76 kcal/mol) > Pd6B (4.74 kcal/mol) > Pd6 (0.34 kcal/ mol). It is interesting to note that the adsorption energy of C2H2 and C2H4 increases as the cluster charge increases. The adsorption site of C2H2/C2H4 on metal hydride Pd6(2H), Pd6+(2H), and Pd6B(2H) is the same as that on Pd6, Pd6+ and Pd6B. The differences of adsorption energy between C2H2 and C2H4 species are 16.75, 11.11 and 12.15 kcal/mol for Pd6, Pd6+ and Pd6B clusters as well as 4.14, 13.12 and 11.85 kcal/mol for Pd6(2H), Pd6+(2H), and Pd6B(2H) clusters. Namely, Pd6B can effectively remove trace amounts of C2H2 in C2H4 feed.

Fig. 2. The optimized structures of Pd6, Pd6+ and Pd6B clusters (bond length unit: Å) and total charges of six Pd atoms based on natural bond orbital (NBO) analysis.

reported the migration barriers of H atoms on Pdn (n = 2–8) were in the range of 1.51–3.02 kcal/mol. Therefore, the H atoms may diffuse rapidly on clusters and the effect of highly mobile H atoms can be ignored in C2H2 hydrogenation. The results in Fig. 4 present that an H2 molecule is initially adsorbed at top Pd sites of Pd6, Pd6+ and Pd6B with Gibbs free adsorption energies of 5.33, 4.71 and 5.09 kcal/mol, respectively. This finding indicates that Pd6+ tends to have weak adsorption for H2 molecules, which was previously referred by Takenouchi [66]. Then, H2 is spontaneously dissociated into two adsorbed H atoms on Pd6, Pd6+ and Pd6B clusters with the corresponding barriers of 13.01, 12.13, 10.55 kcal/mol, respectively, and they are endothermic by 18.47, 13.60, 5.56 kcal/mol, respectively. These results show that the dissociation of H2 easily occurs, and sufficient H atoms can be provided for the C2H2 hydrogenation. The most stable Pd6 (2H), Pd6+(2H) and Pd6B(2H) clusters are presented on the right side of Fig. 4. The adsorption sites of H atoms in Pd6(2H), Pd6+(2H) and Pd6B(2H) clusters are similar to that in Pd6H, Pd6+H and Pd6BH clusters. The H atoms are well separated and the PdPd bond is elongated by less than 0.150 Å compared to the corresponding H2 adsorption configurations. According to the above analysis, the barriers of H migration and H2 dissociation follow the order of Pd6 > Pd6+ > Pd6B. On the other

3.4. The reaction pathway for C2H2 selective hydrogenation 3.4.1. C2H2 hydrogenation on Pd6 clusters As shown in Fig. 7, one H atom migrates to a C atom of C2H2 to form

Fig. 3. The adsorption and dissociation configurations of a single H atom on Pd6, Pd6+ and Pd6B clusters. 3

Computational and Theoretical Chemistry 1170 (2019) 112636

J. Wang, et al.

Fig. 4. The optimized adsorption (left) and dissociation (right) configurations of a single H2 molecule over Pd6, Pd6+ and Pd6B clusters, and potential energy diagram of H2 dissociation at 298.15 K. (Bond length unit: Å).

Fig. 5. The HOMOs and LUMOs of C2H2, C2H4, Pd6, Pd6+ and Pd6B.

the CeH bond. The Gibbs free energy of TS1 is −15.70 kcal/mol. Reaction from CHCH2 (H) to C2H4 occurs via TS2 with a barrier of 26.49 kcal/mol, and it is an exothermic reaction at 0.94 kcal/mol. Although C2H4 is formed in this reaction, H atoms are easily adsorbed to form C2H4 (2H). In Path I, C2H4 desorbs with the barrier of 0.34 kcal/

mol. In Path II, the formed C2H4 (2H) continues hydrogenation via TS3 to form C2H5 (H). It is an endothermic reaction at 6.14 kcal/mol and has a barrier of 20.33 kcal/mol. In Path III, CHCH2(H) goes through TS4 to form intermediate CHCH3, and CHCH3(2H) continues going through TS5 to form C2H5(H). The barriers of these two steps are 25.00 and 4

Computational and Theoretical Chemistry 1170 (2019) 112636

J. Wang, et al.

Fig. 6. The most stable adsorption configurations of C2H2 and C2H4 species involved in C2H2 hydrogenation on Pd6, Pd6+, Pd6B and corresponding metal hydride complexes. The Gibbs free adsorption energies at 298.15 K and 1 atm are also listed in parenthesis. The bond length unit is presented in Å, and the binding energy unit is presented as kcal/mol.

16.54 kcal/mol with reaction energies of −1.46 and 2.04 kcal/mol, respectively. The C2H6 is formed via TS6 with the activation barriers of 22.30 kcal/mol. Regarding the intermediate CHCH2 (H), the barriers of these three pathways are 26.49, 26.49 and 25.00 kcal/mol, respectively. Thus, competition exists between desorption and hydrogenation of C2H4. Moreover, C2H4 tends to desorb rather than be hydrogenated to C2H5 (H) (0.34 vs. 20.33 kcal/mol). The free energy of Pd6 + C2H4 (g) is lower than that of C2H5 (H) by 5.80 kcal/mol. Thus, the formation of C2H4 is favorable in terms of chemical thermodynamics only.

desorption and hydrogenation of C2H4. Although C2H4 tends to desorb rather than be hydrogenated to C2H5 (H) (6.76 vs. 13.39 kcal/mol). The free energy of Pd6+ + C2H4 (g) is higher than that of C2H5 (H) by 6.72 kcal/mol. 3.4.3. C2H2 hydrogenation on Pd6B cluster C2H4 tends to desorb rather than be hydrogenated to form C2H5 (H) (4.74 vs. 29.74 kcal/mol) as shown in Fig. 9. The highest barrier to form C2H4 (g) is much smaller than that to form C2H5(H) (14.17 vs. 31.68 kcal/mol and 29.74 kcal/mol). Moreover, C2H4 tends to desorb rather than be hydrogenated to C2H5 (H) (4.74 vs. 29.74 kcal/mol). The free energy of Pd6B + C2H4 (g) is lower than that of C2H5 (H) by 5.13 kcal/mol. Thus, Path I is the most favorable pathway in terms of chemical kinetics and thermodynamics.

3.4.2. C2H2 hydrogenation on Pd6+ cluster C2H4 prefers desorption rather than to be hydrogenated to C2H5 (H) (6.76 vs. 13.39 kcal/mol) as shown in Fig. 8. Meanwhile, almost no difference is noted between the highest barrier of the three paths (20.73 and 21.35 kcal/mol), which means that competition exists between

Fig. 7. Energy profile of C2H2 hydrogenation on Pd6 cluster at 298.15 K and 1 atm. 5

Computational and Theoretical Chemistry 1170 (2019) 112636

J. Wang, et al.

Fig. 8. Energy profile of C2H2 hydrogenation on Pd6+ cluster at 298.15 K and 1 atm.

at the B3LYP/6-31++G** level and Pd6 (21.76 kcal/mol) > Pd6+ (20.82 kcal/mol) > Pd6B (13.66 kcal/mol) by single-point CCSD(T) [68] calculations. The activity on the Pd6B cluster is smaller than that on Pd6, Pd6+, and Pd(1 1 1) and even AlPd alloy, indicating that the interstitial B atom can improve the activity of C2H2 hydrogenation to C2H4 on the Pd6 cluster. Fig. 10 also demonstrates that the highest energy barriers for C2H4 and C2H5 (H) are 26.49 vs. 25.00 kcal/mol on Pd6, 21.35 vs. 20.73 kcal/ mol on Pd6+ and 14.17 vs. 29.74 kcal/mol on Pd6B clusters, which roughly indicates that C2H2 hydrogenation is more effective with the interstitial B atom. This result can be obtained by single-point CCSD(T)

3.4.4. General discussion Next, the activity and selectivity of C2H2 hydrogenation on Pd6, Pd6+ and Pd6B are expounded, and a comparison with other catalysts containing Pd is shown in Table 2. To observe the activation energy of C2H4 formation on Pd6, Pd6+ and Pd6B more visually, Fig. 10 presents the simplified energy profile for the optimal pathway of C2H4 and C2H5(H). Though potential energy profiles indicate that the products of C2H4 and C2H5(H) are favorable thermodynamically on both Pd6, Pd6+and Pd6B clusters. As shown in Fig. 10 and Table 2, the activation energies for the formation of C2H4 are Pd6(26.49 kcal/mol) > Pd6+(21.35 kcal/mol) > Pd6B(14.17 kcal/mol)

Fig. 9. Energy profile of C2H2 hydrogenation on Pd6B cluster at 298.15 K and 1 atm. 6

Computational and Theoretical Chemistry 1170 (2019) 112636

J. Wang, et al.

Table 2 Comparison of the energy barriers in rate-determining step for C2H2 hydrogenation and selectivity on different catalysts (energies in kcal/mol). The data in parentheses represent the results obtained by single-point CCSD(T) calculations. Catalysts

Pd6 Pd6+ Pd6B Pd (1 1 1) AlPd

Energy barrier (kcal/mol) C2H2 + H2 → C2H4

C2H4 + H2 → C2H5(H)

Desorption of C2H4

Selectivitya

26.49 (21.76) 21.35 (20.82) 14.17 (13.66) 18.10 18.59

20.33 (18.68) 13.39 (10.02) 29.74 (25.18) 17.61 13.7

0.34 (2.18) 6.76 (6.42) 4.74 (8.10) 20.06 2.45–11.00

19.99 (16.50) 6.63 (3.60) 25.00 (17.08) 2.45 [69]b 2.70–11.25 [70]c

GSelectivity = G C2H4+H2→C2H5(H) - G Desorption of C2H4. a The selectivity of the catalyst is determined by the energy difference between the activation energy for the C2H4 hydrogenation to C2H5(H) and the desorption energy of C2H4. The higher barrier difference between C2H5(H) and C2H4(g) formation means the higher selectivity. The formula is noted as follows: b Experimental data. c Calculated using the Vienna ab initio simulation package (VASP) and the generalized gradient approximation (GGA).

calculations for the complex as noted in Fig. 10. The selectivity of the catalyst is determined by the relationship between the activation energy for the C2H4 hydrogenation to C2H5(H) and the desorption energy of C2H4. The higher barrier difference between C2H5(H) and C2H4(g) formation indicates higher selectivity. Table 2 shows that the hydrogenation barriers of C2H4 are 20.33, 13.39 and 29.74 kcal/mol, and it’s desorption barriers are 0.34, 6.76, and 4.74 kcal/mol on Pd6, Pd6+ and Pd6B clusters, respectively. Therefore, the selectivity is Pd6B (25.00 kcal/mol) > Pd6 (19.99 kcal/mol) > Pd6+ (6.63 kcal/mol) at the B3LYP/6-31++G** level and Pd6B (17.08 kcal/mol) > Pd6 (16.50 kcal/mol) > Pd6+ (3.60 kcal/mol) by

single-point CCSD(T) calculations. This finding indicates that interstitial B atom can improve the selectivity of C2H2 hydrogenation to C2H4 on Pd6 cluster. Selectivity is further enhanced because desorption energy for C2H4 coadsorbed with H2 is considerably lower than the isolated C2H4 as shown in Fig. 6. After comparison of the activity and selectivity for Pd6, Pd6+ and Pd6B, it can be concluded that the interstitial B atom can improve the activity and selectivity of C2H2 hydrogenation to C2H4 on the Pd6 cluster simultaneously. Of note, although the Pd atoms in both Pd6+ and Pd6B are positively charged, Pd6+ has less selectivity than Pd6. This finding indicates that the positive charged Pd atom is not the main

Fig. 10. The comparison of the simplified energy profile for the optimal pathway of C2H4 and C2H5(H) on Pd6, Pd6+ and Pd6B clusters (a) at the B3LYP/6-31++G** level; (b) single-point CCSD(T) calculations based on the optimized structures at the B3LYP/6-31++G** level. 7

Computational and Theoretical Chemistry 1170 (2019) 112636

J. Wang, et al.

factor for the ultra-selectivity and activities of the Pd-B catalyst.

[2] N.S. Schbib, M.A. García, C.E. Gígola, A.F. Errazu, Kinetics of front-end acetylene hydrogenation in ethylene production, Ind. Eng. Chem. Res. 35 (1996) 1496–1505. [3] B.M. Collins, Selective hydrogenation of highly unsaturated hydrocarbons in the presence of less unsaturated hydrocarbons, United States, US, 1978, 4126645. [4] S. Asplund, Coke formation and its effect on internal mass transfer and selectivity in Pd-Catalysed acetylene hydrogenation, J. Catal. 158 (1996) 267–278. [5] I.Y. Ahn, J.H. Lee, S.S. Kum, S.H. Moon, Formation of C4 species in the deactivation of a Pd/SiO2 catalyst during the selective hydrogenation of acetylene, Catal. Today 123 (2007) 151–157. [6] F. Huang, Y. Deng, Y. Chen, X. Cai, M. Peng, Z. Jia, P. Ren, D. Xiao, X. Wen, N. Wang, H. Liu, D. Ma, Atomically dispersed Pd on nanodiamond/graphene hybrid for selective hydrogenation of acetylene, J. Am. Chem. Soc. 140 (2018) 13142–13146. [7] H. Lindlar, Ein neuer Katalysator für selektive Hydrierungen, Ein neuer Katalysator für selektive Hydrierungen, Helv. Chim. Acta 35 (1952) 446–450. [8] L.D. Shao, B.S. Zhang, W. Zhang, D. Teschner, F. Girgsdies, R. Schlögl, D.S. Su, Improved selectivity by stabilizing and exposing active phases on supported Pd nanoparticles in acetylene-selective hydrogenation, Chem. Eur. J. 18 (2012) 14962–14966. [9] N.A. Khan, S. Shaikhutdinov, H.J. Freund, Acetylene and ethylene hydrogenation on alumina supported Pd-Ag model catalysts, Catal. Lett. 108 (2005) 159–164. [10] J. Sá, G.D. Arteaga, R.D. Daley, J. Bernardi, J.A. Anderson, Factors influencing hydride formation in a Pd/TiO2 catalyst, J. Phys. Chem. B 110 (2006) 17090–17095. [11] D. Teschner, J. Borsodi, A. Wootsch, Z. Revay, M. Kaveckre, A. Knop-Gericke, S.D. Jackson, R. Schlögl, The roles of subsurface carbon and hydrogen in palladiumcatalyzed alkyne hydrogenation, Science 320 (2008) 86–89. [12] H. Gabasch, K. Hayek, A. Knop-Gericke, R. Schlögl, Carbon incorporation in Pd (111) by adsorption and dehydrogenation of ethene, J. Phys. Chem. B 110 (2006) 4947–4952. [13] M. García-Mota, B. Bridier, J. Pérez-Ramírez, N. López, Interplay between carbon monoxide, hydrides, and carbides in selective alkyne hydrogenation on palladium, J. Catal. 273 (2010) 92–102. [14] F.M. McKenna, J.A. Anderson, Selectivity enhancement in acetylene hydrogenation over diphenyl sulphide-modified Pd/TiO2 catalysts, J. Catal. 281 (2011) 231–240. [15] A.J. McCue, F.M. McKenna, J.A. Anderson, Triphenylphosphine: a ligand for heterogeneous catalysis too? Selectivity enhancement in acetylene hydrogenation over modified Pd/TiO2 catalyst, Catal. Sci. Technol. 5 (2015) 2449–2459. [16] B. Bridier, N. López, J. Pérez-Ramírez, Molecular understanding of alkyne hydrogenation for the design of selective catalysts, Dalton Trans. 39 (2010) 8412–8419. [17] M. Armbruster, M. Behrens, F. Cinquini, K. Föttinger, Y. Grin, A. Haghofer, B. Klötzer, A. Knop-Gericke, H. Lorenz, A. Ota, S. Penner, J. Prinz, C. Rameshan, Z. Révay, D. Rosenthal, G. Rupprechter, D. Teschner, D. Torres, R. Wagner, R. Widmer, G. Wowsnick, How to control the selectivity of palladium-based catalysts in hydrogenation reactions: the role of subsurface chemistry, Chem. Cat. Chem. 4 (2012) 1048–1063. [18] S. Schauermann, N. Nilius, S. Shaikhutdinov, H.J. Freund, Nanoparticles for heterogeneous catalysis: new mechanistic insights, Acc. Chem. Res. 46 (2013) 1673–1681. [19] W. Ludwig, A. Savara, K.H. Dosert, S. Shauermann, Olefin hydrogenation on Pd model supported catalysts: New mechanistic insights, J. Catal. 284 (2011) 148–156. [20] W. Ludwig, A. Savara, R.J. Madix, S. Schauermann, H.J. Freund, Subsurface hydrogen diffusion into Pd nanoparticles: role of low-coordinated surface sites and facilitation by carbon, J. Phys. Chem. C 116 (2012) 3539–3544. [21] M. Wilde, K. Fukutani, W. Ludwig, B. Brandt, J.H. Fischer, S. Schauermann, H.J. Freund, Influence of carbon deposition on the hydrogen distribution in Pd nanoparticles and their reactivity in olefin hydrogenation, Angew. Chem. Int. Ed. 47 (2008) 9289–9293. [22] Z. Bo, Z. Riguang, H. Zaixing, W. Baojun, Effect of the size of Cu clusters on selectivity and activity of acetylene selective hydrogenation, Appl. Catal. A 546 (2017) 111–121. [23] M. Krajčí, J. Hafner, Intermetallic compound, intermetallic compound AlPd as a selective hydrogenation catalyst: a DFT study, J. Phys. Chem. C 116 (2012) 6307. [24] H. Zhou, X. Yang, L. Li, X. Liu, Y. Huang, X. Pan, A. Wang, J. Li, T. Zhang, PdZn intermetallic nanostructure with Pd-Zn-Pd ensembles for highly active and chemoselective semi-hydrogenation of acetylene, ACS Catal. 6 (2016) 1054. [25] D. Liu, DFT study of selective hydrogenation of acetylene to ethylene on Pd doping Ag nanoclusters, Appl. Surf. Sci. 386 (2016) 125–137. [26] G.X. Pei, X.Y. Liu, A. Wang, A.F. Lee, M.A. Isaacs, L. Li, X. Pan, X. Yang, X. Wang, Z. Tai, et al., Alloyed Pd single-atom catalysts for efficient selective hydrogenation of acetylene to ethylene in excess ethylene, ACS Catal. 5 (2015) 3717. [27] T. Mitsudome, T. Urayama, K. Yamazaki, Y. Maehara, J. Yamasaki, K. Gohara, Z. Maeno, T. Mizugaki, K. Jitsukawa, K. Kaneda, Design of core-Pd/shell-Ag nanocomposite catalyst for selective semihydrogenation of alkynes, ACS Catal. 6 (2016) 666. [28] M. Kuhn, M. Lucas, P. Claus, Long-time stability vs deactivation of Pd-Ag/Al2O3 egg-shell catalysts in selective hydrogenation of acetylene, Ind. Eng. Chem. Res. 54 (2015) 6683. [29] J. Zhao, S. Zha, R. Mu, Z. Zhao, J. Gong, Coverage effect on the activity of the acetylene semihydrogenation over Pd–Sn catalysts: a density functional theory study, J. Phys. Chem. C 122 (2018) 6005–6013. [30] C. Yang, G. Wang, A. Liang, Y. Yue, H. Peng, D. Cheng, Understanding the role of Au in the selective hydrogenation of acetylene on trimetallic PdAgAu catalytic surface, Catal. Commun. 124 (2019) 41–45. [31] D.V. Glyzdova, A.A. Vedyagin, A.M. Tsapina, V.V. Kaichev, A.L. Trigub, M.V. Trenikhin, D.A. Shlyapin, P.G. Tsyrulnikov, A.V. Lavrenov, A study on

4. Conclusions Using DFT calculations, the processes of C2H2 selective hydrogenation to C2H4 on Pd6, Pd6+ and Pd6B clusters have been studied, and the effect of interstitial B on the activation and selectivity are explored. H2 adsorption and dissociation on Pd6, Pd6+ and Pd6B clusters shows that the diffusion barriers of H atom are Pd6 (3.91 kcal/ mol) > Pd6+ (1.97 kcal/mol) > Pd6B (1.04 kcal/mol) and dissociation barriers of H2 are Pd6(13.01 kcal/mol) > Pd6+ (12.13 kcal/ mol) > Pd6B (10.55 kcal/mol). This finding indicates that sufficient H atoms will be provided for C2H2 selective hydrogenation, and the effect of highly mobile H atoms can be ignored. The adsorption behavior of C2H2 and C2H4 species shows that C2H2 tends to bond on bridge sites of Pd6 and Pd6+ but on face sites of Pd6B. The Gibbs free adsorption energies of C2H2 are Pd6+ (17.87 kcal/ mol) > Pd6 (17.09 kcal/mol) > Pd6B (16.89 kcal/mol). C2H4 is preferably adsorbed on the bridge sites of three clusters and the corresponding free adsorption energies are Pd6+ (6.76 kcal/mol) > Pd6B (4.74 kcal/mol) > Pd6 (0.34 kcal/mol). The differences of adsorption energy between C2H2 and C2H4 species are 16.75, 11.11 and 12.15 kcal/mol for Pd6, Pd6+ and Pd6B clusters, respectively. The highest energy barriers for C2H4 and C2H5 (H) are 26.49 vs. 25.00 kcal/ mol on Pd6, 21.35 vs. 20.73 kcal/mol on Pd6+ and 14.17 vs. 29.74 kcal/ mol on Pd6B clusters. Thus, competition exists between desorption and hydrogenation of C2H4 on these three clusters, while the formation of gas phase C2H4 on Pd6B cluster is the most favorable pathway in term of chemical kinetics. After comparison of the activation energy and selectivity for Pd6, Pd6+ and Pd6B, we note that the activation energies of C2H4 formation are Pd6 (26.49 kcal/mol) > Pd6+ (21.35 kcal/mol) > Pd6B (14.17 kcal/mol) and the selectivity is Pd6B (25.00 kcal/mol) > Pd6 (19.99 kcal/mol) > Pd6+ (6.63 kcal/mol). These results suggest that the interstitial B atom can improve the activity and selectivity of C2H2 hydrogenation to C2H4 on the Pd6 cluster simultaneously. Namely, Pd6B can effectively remove trace amounts of C2H2 in C2H4 feed. Moreover, we find that the higher charge is in favor of enhancing the adsorption energies of C2H2 and C2H4; however, it cannot increase the selectivity of C2H4 formation. After comparing the activation and selectivity of Pd6+ and Pd6B for the C2H2 selective hydrogenation, we deduce that the positively charged Pd atom is not the main factor for ultra-selectivity and activities of Pd-B catalyst. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work is financially supported by the National Natural Science Foundation of China (21805176), the 1331 Engineering and Education Reform Project (J2019098) of Shanxi Province of China, Scientific and Technologial Innovation Programs of Higher Education Institutions in Shanxi, the Doctor Fund (0505/02070359), Postgraduate Innovation Project (0109/01053005, 0109/01053020) and Education Reform Projiect (2018JGXM-13) of Shanxi Normol University. References [1] M.L. Derrien, Chapter 18 selective hydrogenation applied to the refining of petrochemical raw materials produced by steam cracking, Stud. Surf. Sci. Catal. 27 (1986) 613–666.

8

Computational and Theoretical Chemistry 1170 (2019) 112636

J. Wang, et al.

[32]

[33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52]

structural features of bimetallic Pd-M/C (M: Zn, Ga, Ag) catalysts for liquid-phase selective hydrogenation of acetylene, Appl. Catal. A 563 (2018) 18–27. C CWA, A.H. Mahadi, M.M.-J. Li, E.C. Corbos, C. Tang, G. Jone, W.C.H. Kuo, J. Cookson, C.M. Brown, P.T. Bishop, S.C.E. Tsang, Interstitial modification of palladium nanoparticles with boron atoms as a green catalyst for selective hydrogenation, Nat. Commun. 5 (2014) 5787. I.T. Ellis, E.H. Wolf, G. Jones, B. Lo, M.M. Li, A.P.E. York, S.C.E. Tsang, Lithium and boron as interstitial palladium dopants for catalytic partial hydrogenation of acetylene, Chem. Commun. 53 (2017) 601–604. J.S. Yoo, Z.-J. Zhao, J.K. Nørskov, F. Studt, Effect of boron modifications of palladium catalysts for the production of hydrogen from formic acid, ACS Catal. 5 (2015) 6579. P. Wu, B. Yang, Theoretical insights into the promotion effect of subsurface boron for the selective hydrogenation of CO to methanol over Pd, Phys. Chem. Chem. Phys. 18 (2016) 21720. X. Yu, M. Wang, H. Li, Study on the nitrobenzene hydrogenation over a Pd-B/SiO2 amorphous catalyst, Appl. Catal. A 202 (2000) 17.Li. V. Srivastava, R. Balasubramaniam, Theoretical modeling of metal–hydrogen interactions in Pd clusters, Mater. Sci. Eng. 304–306 (2001) 897–900. S. Abbet, A. Sanchez, U. Heiz, W.D. Schneider, A. Ferrari, G. Pacchioni, N. Rosch, Acetylene cyclotrimerization on supported size-selected Pdn clusters (1 ≤ n ≤ 30): one atom is enough, J. Am. Chem. Soc. 122 (2000) 3453–3457. G. Hogarth, S.E. Kabirb, E. Nordlanderc, Cluster chemistry in the Noughties: new developments and their relationship to nanoparticles, Dalton Trans. 39 (2010) 6153–6174. I. Efremenko, Method of determination of palladium concentration for C-Pd nanostructural films as a function of film thickness, roughness and topography, J. Mol. Catal. A: Chem. 173 (2001) 19–59. Z. Yu, Z. Mingyuan, K. Lihua, The DFT study of Si-doped Pd6Si clusters for selective acetylene hydrogenation reaction, J. Mol. Graph Model 83 (2018) 129–137. A.C. Reber, S.N. Khanna, F.S. Roberts, S.L. Anderson, Effect of O2 and CO exposure on the photoelectron spectroscopy of size-selected Pdn clusters supported on TiO2(110), J. Phys. Chem. C 120 (2016) 2126–2138. X. Liu, D. Tian, S. Ren, C. Meng, Structure sensitivity of NO adsorption-dissociation on Pdn (n = 8, 13, 19, 25) clusters, J. Phys. Chem. C 119 (2015) 12941–12948. B. Liu, J. Liu, T. Li, Z. Zhao, X.-Q. Gong, Y. Chen, A. Duan, G. Jiang, Y. Wei, Interfacial effects of CeO2-supported Pd nanorod in catalytic CO oxidation: a theoretical study, J. Phys. Chem. C 119 (2015) 12923–12934. L S-J, Z X, T W-Q, Theoretical investigations on decomposition of HCOOH catalyzed by Pd7 cluster, J. Phys. Chem. A 116 (2012) 11745–11752. D.W. Yuan, C. Liu, Z.R. Liu, Hydrothermal synthesis of CdTe quantum dots–TiO2–graphene hybrid, Phys. Lett. A 378 (2014) 408–415. L. Jun-Nan, P. Min, M. Chi-Cheng, T. Ye, H. Jing, G.E. David, The effect of palladium clusters (Pdn, n= 2–8) on mechanisms of acetylene hydrogenation: a DFT study, J. Mol. Catal. A: Chem. 359 (2012) 14–20. J. Wang, W. Hao, L.-J. Ma, J. Jia, H.-S. Wu, The structures, stabilities and electronic properties of PdnB (n = 1–10) clusters, Comput. Theor. Chem. 1164 (2019) 112554. A.D. Becke, Density-functional thermochemistry. III. the role of exact exchange, The J. Chem. Phys. 98 (1993) 5648–5652. A.D. Becke, Density-functional thermochemistry V Systematic optimization of exchange-correlation functionals, J. Chem. Phys. 107 (1997) 8554–8560. A.D. Becke, Density-functional exchange-energy approximation with correct asymptotic behavior, Phys. Rev. A 38 (1988) 3098–3100. C. Lee, W. Yang, R.G. Parr, Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density, Phys. Rev. B Condens. Matter. 37 (1988) 785–789.

[53] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian, 09, Gaussian Inc. Wallingford CT, 2010. [54] M.P. Andersson, P. Uvdal, New scale factors for harmonic vibrational frequencies using the B3LYP density functional method with the triple-zeta basis set 6–311+G(d, p), J. Phys. Chem. 109 (2005) 2937–2941. [55] C. Peng, P.Y. Ayala, H.B. Schlegel, M.J. Frisch, Using redundant internal coordinates to optimize equilibrium geometries and transition states, J. Comput. Chem. 17 (1996) 49–56. [56] H.B. C. Gonzalez, H.B. Schlegel, Reaction path following in mass-weighted internal coordinates, J. Phys. Chem. 94 (1990) 5523–5527. [57] C. Gonzalez, H.B. Schlegel, An improved algorithm for reaction path following, J. Chem. Phys. 90 (1989) 2154–2161. [58] C X.M., R. Burch, C. Hardacre, P. Hu, An understanding of chemoselective hydrogenation on crotonaldehyde over Pt (111) in the free energy landscape: The microkinetics study based on first-principles calculations, Catal. Today 165 (2011) 71–79. [59] L.B. Knight Jr., W. Weltner Jr, Hyperfine interaction and chemical bonding in the PdH molecule, J. Mol. Spectrosc. 40 (1971) 317. [60] L.E. Sutton (Ed.), Tables of Interatomic Distances and Configuration in Molecules and Ions, The Chemical Society, Burlington House, London, 1965. [61] M.A. Tolbert, J.L. Beauchamp, Homolytic and heterolytic bond dissociation energies of the second row group 8, 9, and 10 diatomic transition-metal hydrides: correlation with electronic structure, J. Phys. Chem. 90 (1986) 5015. [62] A.N. Bootsma, S. Wheeler, Popular integration grids can result in large errors in DFT-computed free energies, ChemRxiv. Preprint (2019), https://doi.org/10. 26434/chemrxiv.8864204.v4. [63] G. Ganteför, M. Gausa, K.H. Meiwes-Broer, H.O. Lutz, Photoelectron spectroscopy of silver and palladium cluster anions, J. Chem. Soc. Farad. Trans. 86 (1990) 2483–2488. [64] V. Bertin, E. Agacino, R. Lopez-Rendon, E. Poulain, The CO chemisorption on some active sites of Pd clusters: A DFT study, J. Mol. Struct. Theochem. 796 (2006) 243. [65] L.H. Sarah, M.S. Sonya, H.F. Amar, K.C. Gordon, The effect of reduction on Rhenium(I) complexes with binaphthyridine and biquinoline ligands: a spectroscopic and computational study, J. Phys. Chem. A 109 (2005) 3745–3753. [66] M. Takenouchi, S. Kudoh, K. Miyajima, Mafuné, Fumitaka, Adsorption and desorption of hydrogen by gas-phase palladium clusters revealed by in situ thermal desorption spectroscopy, J. Phys. Chem. A 119 (2015) 6766–6772. [67] Y. Bo, B. Robbie, H. Christopher, P. Hu, P. Hughes, Selective hydrogenation of acetylene over Pd-boron catalysts: a density functional theory study, J. Phys. Chem. C 118 (2018) 3664–3671. [68] J.A. Pople, M. Head−Gordon, K. Raghavachari, Quadratic configuration interaction. A general technique for determining electron correlation energies, J. Chem. Phys. 87 (10) (1987) 5968–5975. [69] H. Molero, B.F. Bartlett, W.T. Tysoe, The hydrogenation of acetylene catalyzed by palladium: hydrogen pressure dependenc, J. Catal. 181 (1999) 49–56. [70] M. Krajcí, J. Hafner, Intermetallic compound AlPd as a selective hydrogenation catalyst: a DFT study, J. Phys. Chem. C 116 (2012) 6307–6319.

9