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Understanding All-Solid Frustrated-Lewis-Pair Sites on CeO2 from Theoretical Perspectives Zheng-Qing Huang, Li-Ping Liu, Suitao Qi, Sai Zhang, Yongquan Qu, and Chun-Ran Chang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02732 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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Understanding All-Solid Frustrated-Lewis-Pair Sites on CeO2 from Theoretical Perspectives Zheng-Qing Huang1, Li-Ping Liu1, Suitao Qi1, Sai Zhang1, Yongquan Qu2, Chun-Ran Chang1* 1

Institute of Industrial Catalysis, School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China 2 Center for Applied Chemical Research, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China ABSTRACT: The development of heterogeneous frustrated Lewis pair (FLP) catalysts from homogeneous FLP conception is of great promise in practical applications. While our recent discovery has shown that all-solid FLPs can be created on ceria via surface oxygen vacancy regulation (Nat. Commun. 2017, 8, 15266), a sound understanding of the intrinsic property and reactivity of the solid FLPs is still expected. Here we present a comprehensive theoretical study on the FLPs (Ce…O) constructed on CeO2(110) and (100) surfaces by using density functional theory calculations. We find that the creation of surface oxygen vacancy can enhance both the acidity of FLP-acid site and the basicity of FLP-base site. The enhanced acidity and basicity of Lewis sites together with the elongated distance of Lewis pairs (Ce…O) contribute to the high activity of solid FLPs. The dissociative activation of H2 on FLPs experiences a heterolytic pathway (H2 → Hδ+ + Hδ–) with a low activation energy of 0.07 eV on CeO2(110) and 0.08 eV on CeO2(100). Unlike the phenomenon on stoichiometric CeO2 surfaces that the dissociated hydride (Hδ–) adsorbed at Ce sites is prone to transfer to more stable O sites, the hydride on FLPs can be stabilized at Ce sites and thus benefits the hydrogenation of acetylene via an easier pathway. The rate-determining barriers of acetylene hydrogenation on FLP-CeO2(110) and FLP-CeO2(100) are calculated to be 0.58 eV and 0.88 eV, respectively. These results could help to understand the nature of solid FLPs and pave the way for rational design of heterogeneous FLP catalysts. KEYWORDS: Solid frustrated Lewis pairs, CeO2, Oxygen vacancy, H2 activation, Hydrogenation

1. INTRODUCTION Since the pioneering work by Stephan and co-workers in 2006 who reported the reversible, metalfree hydrogen activation catalyzed by sterically encumbered Lewis acid and Lewis base combinations, termed as “frustrated Lewis pairs (FLPs)”, FLPs have aroused considerable academic attention.1-6 In the past decade, the reactivity of FLPs has been evolved to a broad range of other small molecules including olefins, alkynes, heteroarenes, CO2, SO2, NH3, and so forth, showing a promising future of FLPs in catalysis, organic synthesis, and materials chemistry.7-14 However, as the concept of FLPs is mainly limited in homogeneous molecular-based chemistry and catalysis, further utilization of FLP catalysts in industrial applications is hindered by the problematic and costly catalyst recycling.15 Therefore, developing heterogeneous FLP catalysts using the knowledge

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of homogeneous molecular-based FLPs is of great promise. Recently, some studies have employed the concept of FLPs to describe heterogeneous hydrogenation of imine/nitrile by gold surface-imine/nitrile pairs16 and hydrogen dissociation by a silica-supported Lewis acid with a molecule Lewis base.17 These catalytic sites constructed by solid surface and adsorbed molecules can be regarded as solid-molecule FLPs. Then an experimental study reported that FLP sites are responsible for the activity of metal-free graphene materials in selective hydrogenation of acetylene and alkene.18 A subsequent theoretical study verified that the FLP sites are able to be designed on graphene by boron and nitrogen co-doping to activate hydrogen molecule.19 Johnson and Ye using DFT method designed Lewis pair-functionalized MOFs for CO2 hydrogenation to achieve the advantages of both homogeneous and heterogeneous catalysts, in which the molecular-based Lewis pairs were separated by covalently bonding with the specific sites of MOFs.20-22 These FLPs based on graphene and MOFs are fully heterogeneous catalysts and can be categorized as all-solid FLPs. However, the preparation of these solid FLPs is complex because two or more kinds of materials are needed and some methods are challenging in experiments. Therefore, seeking for all-solid FLP catalysts based on simple materials and methods is of great importance for practical applications. As metal oxides contain both metal cation as Lewis acid and oxygen anion as Lewis base, surface engineering on defects is likely to be a plausible method for designing solid FLPs. In this respect, Singh and co-workers reported the heterogeneous hydrogenation of CO2 on the surface of hydroxylated indium oxide with oxygen vacancies, ln2O3-x(OH)y, and proposed the reaction mechanism which was analogous to that in molecular FLP catalysis of CO2 and H2.23-26 Very recently, a concept of all-solid frustrated Lewis pair catalysts based on CeO2 was proposed by us,27 in which the FLPs sites were simply constructed by regulation of surface oxygen vacancies. As shown in Fig. 1, when the oxygen vacancies (VO) were created on CeO2(110) and CeO2(100), two adjacent surface Ce cations proximal to VO and one neighboring oxygen anion cooperatively construct the FLP sites. The distance between Ce cations and O anion is around 4 Å, much longer than classical Ce–O bonds (~2 Å) in stoichiometric CeO2, which satisfies the concept of “frustrated” Lewis pairs. Importantly, such a construction method of FLPs was realized by the preparation of porous nanorods of ceria which contains a high ratio of surface oxygen vacancies. The catalytic activity of as-designed FLPs was further verified by the efficient hydrogenation of alkenes and alkynes. This study provides a simple and effective method for designing FLP sites on metal oxides and is really important for moving forward the FLPs from homogeneous to heterogeneous catalysis.


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Figure 1. Schematic of solid FLPs on CeO2(110) and CeO2(100) constructed by surface oxygen vacancy regulation. Yellow and red balls represent Ce and O atoms, respectively. Atoms labeled by blue circles represent the Lewis acid (Ce) or Lewis base (O) of solid FLPs. The position of oxygen vacancy is labeled by VO in blue. Despite some interesting findings in that study, a few of theoretical issues regarding the intrinsic property and the reactivity of the solid FLPs still remain, which are also important for designing FLPs on other metal oxide surfaces: (1) the acidity and basicity of solid FLPs in comparison with acid-base adjuncts is not comprehensively studied. (2) the whole pathways of H2 dissociation are not complete and related mechanisms are still ambiguous. (3) the hydrogenation mechanisms on the solid FLPs are also not illustrated. Among these issues, the mechanisms of H2 dissociation at solid FLPs are the most important to understand the chemistry of solid FLPs and are the link to molecularbased FLPs. Currently, massive studies in both experiments and calculations have focused on CeO2catalyzed hydrogen activation and hydrogenation reactions.28-34 On metal oxide surfaces, two mechanisms were proposed for H2 dissociation: (1) homolytic (radicalary) dissociation that occurs at two O sites with the formation of two O−H groups; (2) heterolytic (polar) dissociation that occurs at Ce and O sites with formation of Ce−H and O−H species.30,32 Our previous study of H2 dissociation at solid FLPs on CeO2(110) reported the heterolytic dissociation process followed by a hydride transfer to the O site nearby to finally form a homolytic product.27 Though heterolytic cleavage of H−H bond was prone to occur at solid FLPs, the transfer of hydride from Ce to O was not clear in kinetics, which is relevant to the stability of hydride at Ce sites and vital for the subsequent hydrogenation reactions. Therefore, it is desirable to study in detail the hydrogen dissociation at FLP sites, especially the transfer ability of hydride from Ce site to O site. In addition, two reactivity models, i.e. electron transfer model and electric field model, have been proposed and debated to elucidate the facile heterolytic cleavage of H2 by molecular-based FLPs.35 The manner in which H2 dissociates on solid FLPs also deserves to be investigated. In the present work, we chose CeO2(110) and (100) as catalyst models, which are capable of being used to design FLPs by surface oxygen vacancy regulation according to our recent study,27 to unravel the nature of solid FLPs for H2 activation and acetylene hydrogenation. Our calculations show that the acidity and basicity of designed solid FLPs are both enhanced, which together with the elongated distance of Lewis pairs (Ce…O) contribute to the high activity for heterolytic H2 dissociation. The intermediate of heterolysis of H2, hydride, can be stabilized by the Ce cations of


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FLPs instead of transferring to surface O anions nearby, which favors the hydrogenation of acetylene via an easier pathway than those on stoichiometric CeO2 surfaces.

2. COMPUTATIONAL DETAILS All DFT calculations were performed using the Vienna Ab-initio Simulation Package (VASP).36-38 The exchange-correlation potential was treated by the Perdew-Burke-Ernzerh (PBE) generalized gradient approximation (GGA) with spin-polarized functional.39 The projected-augmented wave (PAW) pseudopotentials were utilized to describe the core electrons, and a plane-wave kinetic energy cutoff of 400 eV was adopted to treat the valence electrons.40 The DFT + U methodology was used to treat the on-site Coulomb and exchange interaction of the strongly localized Ce 4f electrons with an effective U = 4.5 eV.41-43 The van der Waals dispersion forces were considered using zero damping DFT-D3 method of Grimme to account for the weak interactions between adsorbates and surfaces.44 The Brillouin zone integration was sampled with 7 × 7 × 7 Monkhorst-Pack mesh k-points for bulk CeO2 calculations.45 The calculated lattice parameter for bulk CeO2 was 5.45 Å, in good agreement with the experimental value of 5.41 Å.46 The CeO2(110), a Tasker Type 1 surface, was modeled by a periodic five-layer slab with the bottom three layers fixed.47 The slab was separated by a 15 Å vacuum in the direction perpendicular to the surface and repeated in a p(2 × 2) surface (Fig. 2a). A 3 × 3 × 1 Monkhorst-Pack mesh k-points was used for CeO2(110) calculations. The reduced CeO2(110) surface (Fig. 2b) is constructed by removal of two adjacent atoms (OIII and OIV), which corresponds to a high defect density of 2.380 nm–2 (Table S1). The oxygen atoms neighboring to oxygen vacancies (OI, OII) on the reduced CeO2(110) tend to stay at the bulk position in plane, which is different from the one oxygen vacancy case that the neighboring oxygen will move to the bridge site of two Ce ions.27,48 The O-terminated CeO2(100), a Tasker Type 3 surface, was modeled by periodic seven-layer slab with a 15 Å vacuum and was repeated in p(2 × 2) or c(2 × 2) surface unit (Fig. 2c). The rationality of the seven-layer slab was verified in the Table S2 of Supporting Information. To eliminate the surface dipole, a half of the O atoms from top layer are removed to the opposing face, which has been applied in many previous computational studies.49-51 One surface atoms (OIII) was removed to construct the reduced CeO2(100) surface (Fig. 2d), leading to high defect density of 1.682 nm–2 for a p(2 × 2) supercell and 0.842 nm–2 for a c(2 × 2) supercell (Table S1). For calculations regarding H2 dissociation and hydrogenation reactions on CeO2(100), the c(2 × 2) surface unit with the bottom two layers fixed and a 2 × 2 × 1 Monkhorst-Pack mesh k-points were adopted. For calculations regarding probe molecule adsorption, the p(2 × 2) surface unit with the bottom four layers fixed and a 3 × 3 × 1 Monkhorst-Pack mesh k-points were used. All the structures were relaxed until forces on each ion were less than 0.02 eV/Å, and the convergence criterion for energy was 10−5 eV.


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Figure 2. Top view structures of optimized CeO2 surfaces. (a) Stoichiometric CeO2(110) and (b) reduced CeO2(110) in the p(2 × 2) supercell with two oxygen vacancies; (c) stoichiometric and (d) reduced CeO2(100) in the c(2 × 2) supercell with one vacancy. Yellow and cyan balls represent Ce4+ ions and Ce3+ ions, respectively. The balls in larger and smaller diameters are in the first and second atomic layer, respectively. The p(2 × 2) CeO2(100) surface unit is indicated in the blue frame and Lewis sites of CLPs and FLPs are also labeled in blue. The adsorption energy, Eads, was calculated using the following equation, Eads = Eadsorbate+surface – (Eadsorbate + Esurface), where Eadsorbate+surface is the total energy of surface covered with adsorbates, Eadsorbate is the energy of adsorbate, and Esurface is the energy of clean surface. Atomic charges were computed using the atom-in-molecule (AIM) scheme proposed by Bader.52 The reaction energy was calculated by energy difference between the product and the corresponding reactant, ΔE = Eproduct – Ereactant. The activation energy was determined as the energy difference between the transition state and corresponding initial state, Ea = Etransition state – Einitial state. To locate the transition state structure of reaction, the Nudged Elastic Band combined with minimum-mode following dimer method was used.53,54 All transition states were identified by vibration analysis. In order to confirm that the structures of reduced CeO2 surfaces we used were ground-state (i.e. minimum energy), we explore the influence of Ce3+ position using a measure in a two-step process as reported in the literature.48 As shown in Fig. S2 and S3, a series of reduced CeO2(110) and (100) structures with Ce3+ distributions at different sites were calculated, which was found that the most stable structures for CeO2(110) with two oxygen vacancies and CeO2(100) with one oxygen vacancy are indeed what we used in our study displayed in Fig. 2b and 2d, respectively. To simplify the following expressions we have some definitions, i.e., the Lewis CeI-OIV pairs on stoichiometric CeO2(110) (Fig. 2a) belonging to classical Lewis pairs (CLPs) are termed as CLPCeO2(110); the Lewis (CeI,CeII)-OVI pairs on reduced CeO2(110) (Fig. 2b) belonging to frustrated Lewis pairs are termed as FLP-CeO2(110); the classical CeIV-OIII Lewis pairs on stoichiometric CeO2(100) (Fig. 2c) are termed as CLP-CeO2(100); the frustrated (CeIII,CeIV)-OII Lewis pairs on reduced CeO2(100) (Fig. 2d) are termed as FLP-CeO2(100).


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3. RESULTS AND DISCUSSION 3.1 Acidity and Basicity of the Designed Solid FLPs In addition to the distance between frustrated Lewis acid and Lewis base, the acidity and basicity is another key characteristic of solid FLPs, which is also of great relevance to the reactivity of FLPs. To study the acidity and basicity of solid FLPs, those of classical Lewis pairs are also investigated for comparison. The adsorption energies of NH3 (Lewis base molecule) at Lewis acid site and BH3 (Lewis acid molecule) at Lewis base site are taken as the descriptor to quantify the Lewis acidity and basicity, respectively. The larger adsorption energy (more negative in values) means the stronger acidity or basicity. Similar approach has been utilized by previous study on γ-Al2O3 to determine the acidity of Al cations.55,56 Figure 3 displays the adsorption structures and adsorption energies of probe molecules on CLP and FLP sites. As shown in Fig. 3a, the adsorption energy of NH3 at the FLP-acid site of CeO2(110), namely the bridge site of CeI and CeII (denoted as (CeI,CeII) hereafter), is calculated as –1.06 eV, which is much higher that (–0.74 eV) on CLP-acid site of CeO2(110), indicating the acidity of the FLP-acid site is indeed improved. However, the increased adsorption energy of NH3 at (CeI,CeII) bridge sites might include a certain part of hydrogen bonding interaction between H atom in NH3 and surface O atom. Therefore, we re-evaluated the acidity of CLP-acid and FLP-acid sites by using PH3 as probe molecule since PH3 cannot form hydrogen bonding with surface O atom (Fig. S4f). The calculated results showed that at FLP-acid sites (CeI,CeII) the adsorption energy of PH3 (–0.61 eV) is also higher than that (–0.41 eV) at the CLP-acid site. As a result, the (CeI,CeII) site can have a stronger acidity than CLP-acid site. When it comes to the basicity of the FLP-base site on CeO2(110), the adsorption energy of BH3 (−2.93 eV) is higher than that (−2.60 eV) at CLP-acid site (Fig. 3c), indicating an enhanced basicity of the solid FLP-base site. For the CeO2(100) surfaces, the adsorption energies of NH3 at CLP-acid site and FLP-acid site are calculated to be –0.53 eV and –1.11 eV (Fig. 3b), respectively, suggesting the acidity of the FLPacid site is largely enhanced. Similar enhancement of the basicity of the FLP-base site on CeO2(100) is also found, which is evidenced by the higher adsorption energy of BH3 (–3.15 eV) at the FLPbase site than that (–2.67 eV) at CLP-base site (Fig. 3d). Overall, the calculated results of probe molecule adsorptions on CeO2(110) and (100) demonstrate that the surface oxygen vacancy regulation can enhance both the acidity of FLP-acid site and the basicity of FLP-base site. It is also interesting to note that although the removal of surface oxygen atoms reduces the oxidation state of Ce (from +4 to +3) as well as the acidity (Fig. S4), the constructed FLP-acid sites composed of two Ce3+ ions could bind NH3 more strongly than one Ce4+ ion and as a result present an enhanced acidity as a whole. In addition, we explored the dependence of acidity/basicity of Lewis sites on oxygen vacancy concentration. The calculated results in Fig. S5 and S6 indicate that both the Lewis


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acidity and basicity of FLPs are only slightly affected by further increasing the oxygen vacancy concentration as the variation of adsorption energies of probe molecules is less than 0.10 eV.

Figure 3. Optimized adsorption structures and adsorption energies of probe molecules on CeO2 surfaces. (a) NH3 adsorbed at Lewis acid sites of CeO2(110), (b) NH3 adsorbed at Lewis acid sites of CeO2(100), (c) BH3 adsorbed at Lewis base sites of CeO2(110), and (d) BH3 adsorbed at Lewis base sites of CeO2(100). The values in eV correspond to the adsorption energies of each adsorbate. The spin density isosurfaces indicate the position of the Ce ions in +3 oxidation state. 3.2 Hydrogen Dissociation on Solid FLPs Sites Activity of Solid FLPs for Hydrogen Dissociation on CeO2(110) and (100). In this section, three stepwise elementary steps of hydrogen dissociation were studied on both CeO2(110) and (100) surfaces: (1) hydrogen adsorption, H2(g) → H2*, where an asterisk (*) represents the adsorbed state; (2) hydrogen dissociation leading to one hydride binding with Ce cation and one proton binding with O anion, H2* → H*(O) + H*(Ce); (3) hydride transfer from Ce cation to O anion, H*(O) + H*(Ce) → 2H*(O). The calculated results are shown in Table 1 and Fig. 4. On CeO2(110), the adsorption of H2 is very weak at all the sites of stoichiometric CeO2(110), but becomes stronger at the FLP sites of reduced CeO2(110), indicating FLP favors the adsorption of H2. For the dissociation step, FLP sites also deliver superior activity than CLP sites from both thermodynamic and kinetic aspects. The dissociation of H2 on FLP sites is exothermic by 0.62 eV with an activation barrier of only 0.07 eV. In contrast, the dissociation of H2 on CLP sites is endothermic by 0.40 eV with a much higher activation barrier of 0.55 eV. The dissociated two H atoms on FLP sites anchored at Ce and O sites, H*(O)−H*(Ce), are distanced by 1.59 Å, indicative of a dihydrogen bond.57 On CeO2(100), the adsorption and dissociation of H2 on FLP sites are also


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more thermodynamically favorable than that on CLP sites, the dissociation energies of which are calculated to be −0.58 eV and −0.28 eV, respectively. Furthermore, the dissociation of H2 at FLP sites is nearly barrierless with an activation energy of merely 0.08 eV, much easier than that (0.38 eV) at CLP sites. For the third hydride transfer step, the large exothermic reaction heat (−3.01 eV) and low reaction barrier (0.25 eV) suggest that the hydride at the Ce site of CLP-CeO2(110) is not stable and prone to transfer to the surface O site nearby. However, at the Ce sites of FLP-CeO2(110) hydride transfer is not easy to occur as indicated by the high transfer barrier (1.58 eV). On CeO2(100) surfaces, the calculated results also indicate that the transfer of hydride from Ce to O is more difficult on FLPs than that on CLPs. Overall, the heterolytic cleavage of H2 could be enhanced by the created FLP sites with the decrease of the activation barrier of 0.48 eV on CeO2(110) and 0.30 eV on CeO2(100) compared to corresponding CLP sites. Moreover, the intermediate of H2 heterolysis, hydride, can be stabilized at Ce sites of solid FLPs, which is different with the trends at CLPs that hydride prefers to transfer to the more stable O sites. The stabilization of hydride on Ce site is shown to be advantageous for the subsequent hydrogenation reactions. Table 1. Reaction Energies ∆E (eV) and Activation Energies Ea (eV) for the Elementary Steps Involved in the Hydrogen Dissociation on the CeO2(110) and (100) Surfaces Reactions

CLP-CeO2(110) ∆E

Ea

FLP-CeO2(110) ∆E

Ea

−0.27

CLP-CeO2(100) ∆E

Ea

−0.23

FLP-CeO2(100) ∆E

Ea

H2(g) → H2*

−0.12

H2* → H*(O) + H*(Ce)

0.40

0.55

−0.62

0.07

−0.28

0.38

−0.58

0.08

H*(O) + H*(Ce) → 2H*(O)

−3.01

0.25

−0.93

1.58

−2.87

0.50

−2.41

0.72


−0.28

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Figure 4. Side and top view structures of the intermediates and transition states of hydrogen dissociation at (a) CLP sites on CeO2(110), (b) FLP sites on CeO2(110), (c) CLP sites on CeO2(100), and (d) FLP sites on CeO2(100). Original Chemistry of H2 Dissociation on Solid FLPs. To unravel the nature of FLP reactivity, we analyzed both the geometric and electronic structures of initial states (IS) and transition states (TS) of the hydrogen dissociation step, H2* → H*(O) + H*(Ce). As the hydrogen dissociation involves cleavage of H−H bond and formation of Ce−H and O−H bonds, the lengths of the three bonds mainly determine the potential energy profile of the reaction, where the H−H bond will get longer along the reaction to raise the potential energy, and the Ce−H and O−H bonds will get shorter to lower the potential energy. As shown in Table 2, from IS to TS the H−H bond length is increased by 0.37 Å at CLP-CeO2(110) sites, whereas only a 0.09 Å elongation of H−H bond occurs at FLPCeO2(110) site. The shrinkage of Ce−H and O−H bond lengths from IS to TS are 0.64 Å and 2.29 Å at CLP-CeO2(110) sites, compared with only 0.18 Å and 0.34 Å at FLP-CeO2(110) sites. Similar results can be also found at CLP and FLP sites on CeO2(100) in Table 2. In addition to the bond lengths, ∠CeHH and ∠HHO, and atomic charges in Table 2 also exhibit smaller variations from IS to TS at FLP sites than those at CLP sites on both CeO2(110) and CeO2(100) surfaces. Based on the analysis above, the transition states at FLP sites can be regarded as early transition states in


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hydrogen dissociation step, which is mainly attributed to the increased attacking abilities of Ce (strong electrophilicity) and O (strong nucleophilicity) atoms of FLP sites to H2 when the H−H bond is slightly elongated. For the Lewis acid sites (Ce) of FLP, the acidity of two Ce3+ is stronger than one Ce4+ of CLPs, which can contribute to the increased attraction of Ce to H2 molecules. For the Lewis base site (O) of FLPs, the increased basicity evidenced by probe molecule adsorption can enhance the nucleophilicity to H2 molecules. Overall both the enhanced electron transfer of Lewis base site and the increased polarization effect of Lewis acid site in FLPs cooperatively achieve the early transition state and lower activation energy of H−H bond breakage. Table 2. Structural Parameters and Atomic Charges (Q) of Initial States (IS) and Transition States (TS) in H2* → H*(O) + H*(Ce) at Lewis Pair Sites on CeO2(110) and (100) Surfaces Sites CLP-CeO2(110) FLP-CeO2(110) CLP-CeO2(100) FLP-CeO2(100)

States d(H−H) / Å d(Ce–H) / Å d(H–O) / Å

∠CeHH / °

∠HHO / °

Q/e HCe

HO

IS

0.75

2.97

3.45

84

115

−0.04

0.05

TS

1.12

2.33

1.16

79

159

−0.48

0.44

IS

0.80

3.00, 3.00

1.86

100, 104

178

−0.26

0.20

TS

0.89

2.82, 2.85

1.52

109, 111

176

−0.43

0.31

IS

0.76

2.91, 2.88

2.73

81, 84

123

0.00

0.00

TS

0.94

2.43

1.35

74

159

−0.37

0.34

IS

0.77

3.09, 3.07

2.12

91, 91

179

−0.12

0.09

TS

0.89

2.77, 2.85

1.53

98, 107

176

−0.37

0.28

Previous study of hydrogen activation using molecular-based FLPs reported that the acid-H-Hbase fragments in transition states present a special bent arrangement, namely the acid-H2 interaction is in a side-on mode and the base-H2 interaction is in an end-on mode.35 The special bent arrangement is attributed to seeking the optimum donor-acceptor overlaps, i.e., an side-on electron acceptor-H2 arrangement allowing an optimal overlap between the acceptor’s empty orbital and the σ bond of H2 (Fig. 5a, left), and an end-on electron donor-H2 arrangement allowing an optimal overlap between the donor’s lone-pair orbital and the σ* bond of H2 (Fig. 5b, left).35,58 Therefore, the O…H2 interaction in transition states will have a better O→σ*(H2) overlap to weaken the H–H bond when the ∠HHO is close to 180 ° (Fig. 5b, right). In Table 2, the ∠HHO at transition states are 159° at CLP sites on CeO2(110) and (100) surfaces, but increase to 176° at FLP sites on both CeO2 surfaces, indicating that the O→σ*(H2) overlap is enhanced at FLP sites compared with CLP sites. However, due to the polarization of σ/σ*, the optimum σ(H2)→Ce overlap does not occurs at the middle of H−H bond, therefore it is difficult to ascertain whether the σ(H2)→Ce overlap is enhanced at FLP sites. Herein, we use a simple model to explain how the optimum donor-acceptor overlaps (O→σ*(H2) in transition states) can be achieved by a suitable distance between Lewis acid and Lewis base. In Fig. 5c, two planar models of the CeHHO fragments with different distances between Ce and O are


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presented, where three distances, d(Ce−HCe), d(HCe−HO), and d(O−HO) are fixed. Two hydrogen atoms are only restricted on the corresponding solid parts of the circles. At short distances of Ce…O pairs (Fig. 5c, left), the ∠HHO cannot achieve 180 ° under the geometric restrictions, which is similar to cases of CeHHO fragments at CLP sites. At longer distances of Ce…O pairs (Fig. 5c, right), the ∠HHO can reach 180 °, which is analogous to the cases of CeHHO fragments at FLP sites. Detailed discussions on the relationship between geometric restrictions and ∠HHO are presented in the Supporting Information. Beside the enhanced acidity and basicity of the separated Lewis sites, the elongated distance between Lewis acid and base might also benefit the optimum donor-acceptor overlaps between H2 and FLP sites to achieve more efficient activation of hydrogen molecule.

Figure 5. (a) A side-on acceptor(A)-H2 interaction (left) and optimum orientations of σ(H2)→Ce overlap on solid FLPs (right). (b) A side-on donor(D)-H2 interaction (left) and optimum orientations of O→σ*(H2) overlap on FLPs (right). (c) Schematic of interactions between H2 and Lewis pairs at short (left) and long (right) distances of Ce…O pairs. The three distances follow an order in length, d(Ce−HCe)>d(O−HO)>>d(HCe–HO). To understand the origin of the enhanced interactions between FLP sites and H2, the electronic structures are analyzed by calculating the charge density difference, electron localized function (ELF), and projected density of states (PDOS) of the selected atoms (surface Ce and O atoms in CLPs or FLPs, and H atoms) of the transition state of hydrogen dissociation. In Fig. 6a, the electron density difference maps show that there are significant density increases around HCe and density decreases around HO, in agreement with atomic charges listed in Table 2. Though large density increases appear around HCe, there are no continuum and significant density increases in the region between HCe and Ce. In Fig. 6b, the ELF maps show that the electrons around HCe are highly localized and highly delocalized regions exist between HCe and Ce. Therefore, it can be inferred that the main interaction between H2 and Ce ions is Coulombic attraction. As for the regions between HO and O, significant density increases appear (Fig. 6a) and the ELF in these regions is around 0.5 (Fig. 6b), which both suggest that covalent bonding between HO and O in Lewis pairs. Overall,


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during the dissociation of H2 by CLPs and FLPs the electron transfer between H2 and Lewis pairs as well as the local electric field of the metal oxide surface contribute to the activity of Lewis pairs.

Figure 6. (a) Electron density difference (Δρ = ρ(TS) – ρ(surface#) – ρ(H2#)) maps and (b) electron localization function (ELF) maps of the TS of hydrogen dissociation at CLP and FLP sites on CeO2 surfaces. The atomic positions of surface# and H2# are identical to those in the TS. On CeO2(110) surface, the PDOS of H atoms are localized at −6.3 eV and −0.9 eV on CLP sites (Fig. 7a), and at −7.1 eV and −2.1 eV on FLP sites (Fig. 7b). Similarly, the PDOS of H atoms on CeO2(100) surface are localized at −6.5 eV and −1.4 eV on CLP sites (Fig. 7c), and −6.7 eV and −1.9 eV on FLP sites (Fig. 7d). Based on the ligand field theory of metal-ligand bonding59, the lowenergy state can be assigned to the H2…CLP/FLP bonding state, while the high-energy state can be assigned to the H2…CLP/FLP anti-bonding state. It is interesting to find that on CeO2(110) and (100) surfaces, both the low- and high-energy states at FLP sites shift to the lower energy side compared with those at CLP sites, which can elucidate that the transition states at FLP sites are lower in energy than that at CLP sites. Furthermore, we calculated the relative state occupations of H 1s orbitals in low and high energy levels (Table 3), respectively. At the FLP sites of both CeO2(110) and (100), the state occupations in high energy level of HCe 1s are decreased, but are increased in low energy level of both HO 1s and HCe 1s, indicating more electrons of H 1s are localized in low energy level at FLP sites compared with CLP sites. The orbital centers of H 1s, a descriptor reflecting the mean of electron energies of hydrogen atoms, are listed in Table 3. It can be found that the hydride HCe at FLP sites are much lower in energy than that at CLP sites, further demonstrating that the hydride prefers to be stabilized at FLP sites. Overall, the density of state analysis reveals that the interaction between H2 and FLP sites can result in lower energy-level of H2…FLP bonding state and contribute to the reduction of the potential energy of transition state.


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Figure 7. Projected density of states of the selected atoms of TS in hydrogen dissociation at (a) CLP sites on CeO2(110), (b) FLP sites on CeO2(110), (c) CLP sites on CeO2(100), and (d) FLP sites on CeO2(100). Table 3. Relative State Occupations of H 1s Orbitals in Low (H 1s-L) and High (H 1s-H) Energy Levels, and Orbital Center of H 1s (εs) of TS in Hydrogen Dissociationa Sites

Relative state occupation

εs (eV)

HO 1s-L

HO 1s-H

HCe 1s-L

HCe 1s-H

HO 1s

HCe 1s

CLP-CeO2(110)

0.34

0.04

0.10

0.44

–6.03

–2.08

FLP-CeO2(110)

0.43

0.03

0.38

0.21

–6.87

–5.50

CLP-CeO2(100)

0.38

0.04

0.28

0.30

–6.25

–4.16

FLP-CeO2(100)

0.42

0.03

0.38

0.20

–6.47

–5.18

a

Details on the related calculations are presented in Supporting Information.

3.3 Partial Hydrogenation of Acetylene by Solid FLPs. The high activity of solid FLPs for H2 dissociation has been thoroughly analyzed above. In this section, we select partial hydrogenation of acetylene as model reaction to explore the hydrogenation behavior of solid FLPs. The potential energy diagrams of both H2 dissociation and hydrogenation steps on CeO2(110) are shown in Fig. 8a. As mentioned above, the hydride originated from H2 heterolytic dissociation at solid FLP sites can be stabilized by Ce cations, whereas at CLP sites the hydride tends to transfer to the nearby O anion. Therefore, the hydrogenation of acetylene at CLP sites starts from the state D1-1 with two hydrogen atoms bound to surface O atoms (the black curve in Fig. 8a), while at FLP sites the reaction initiates from state C2-1 with two hydrogen atoms anchoring at Ce and O atoms (the red curve in Fig. 8a), respectively. At CLP sites, C2H2 first weakly adsorbs on surfaces (state E1-1) and then one carbon atom attacks the surface oxygen atom with another carbon atom grasping the adsorbed H atom to form C2H3* (state F1-1). The first hydrogenation step is an exothermic reaction with a large energy release of 1.51 eV. However, the


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second hydrogenation step (from G1-1 to H1-1) is difficult due to the large endothermic reaction energy of 1.63 eV, which is originated from the breakage of strong C−O bond in state G1-1. At FLP site, C2H2 also weakly adsorbs on surfaces (state E2-1). After overcoming a barrier of 0.28 eV C2H2* binds with the hydride adsorbed at Ce site to form C2H3* (state F2-1). Then the C2H3* changes to a more stable adsorption configuration (G2-1) with the unsaturated C atom approaching to the Ce cations and adsorbed H atom. The second hydrogenation step from G2-1 to H2-1 is nearly neutral and need to surmount an energy barrier of 0.58 eV, which is the rate-limiting step of hydrogenation of acetylene at FLP site. The hydrogenation at CLP sites is limited by high endothermic heat of 1.58 eV to break the C−O bond, which is supported by previous study.31 Overall, at FLP sites the hydrogenation step can go through a special pathway without formation of C−O bond and thus achieves a low activation barrier. Alternatively, we have also studied the concerted hydrogenation of C2H2 on reduced CeO2(110) where C2H2 is simultaneously hydrogenated by two adsorbed H atoms at FLP sites (Fig. S8). However, the activation barrier (0.89 eV) is higher than the ratelimiting barrier (0.58 eV) of sequential hydrogenation mechanism, thus the latter is more plausible in this case. Fig. 9 depicts the hydrogenation of acetylene on CeO2(100). At CLP sites, the first hydrogenation step is also readily to occur, but the second hydrogenation step is highly endothermic by 2.06 eV. While at FLP site, both the first and the second hydrogenation step are facile, the activation barriers of which are 0.51 eV and 0.88 eV, respectively. Overall, the hydrogenation of acetylene can be obviously accelerated on CeO2 surfaces by solid FLPs, which is mainly attributed to two reasons: (i) at FLP sites the H resource for hydrogenation could origin from hydride at Ce site which is more active than proton anchored at the surface oxygen sites, (ii) at FLP sites the intermediate can bind with surface Ce instead of O and thus the reactions avoid the cleavage of C– O bond, which is a strong endothermic step.


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Figure 8. Partial acetylene hydrogenation on CeO2(110). (a) Potential energy diagram of the reactions taken place on CLP sites (the black curve) and FLP sites (the red and blue curves). The zero energy reference corresponds for the sum of energy of H2(g), C2H2(g) and the corresponding clean CeO2(110) surfaces. (b-d) Selected geometric structures of the transition states and intermediates.

Figure 9. Partial acetylene hydrogenation on CeO2(100). (a) Potential energy diagram of the reactions taken place on CLP sites (the black curve) and FLP sites (the red and blue curves). The zero energy reference corresponds for the sum of energy of H2(g), C2H2(g) and the corresponding clean CeO2(100) surfaces. (b-d) Selected geometric structures of the transition states and intermediates.


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4. CONCLUSIONS The analyses of acidity and basicity of solid FLPs on CeO2(110) and (100) surfaces are firstly discussed, followed by the catalytic performance of solid FLP on hydrogen dissociation and hydrogenation reactions. Based on our DFT calculations, the following conclusions are drawn: (1) According to the calculated adsorption energy of the probe molecules (NH3 and BH3) on Lewis sites, the acidity and basicity of solid FLPs created via surface oxygen vacancy regulation have both been enhanced. (2) The heterolytic cleavage of H2 is accelerated at FLP sites with the decrease of the activation barrier of 0.48 eV on CeO2(110) and 0.30 eV on CeO2(100), which is attributed to the elongated distance and the enhanced acidity and basicity of Lewis pairs. (3) A special pathway of acetylene hydrogenation is proposed at FLP sites, which initiates from attacking the hydride adsorbed at Ce sites and avoid the formation of C−O bond. Therefore, the activation energies of the rate-determining step are relatively low at solid FLP sites with 0.58 eV on CeO2(110) and 0.88 eV on CeO2(100). Overall, these results provide a comprehensive understanding on the solid FLPs on ceria and will be of great help for developing novel efficient FLP catalysts on metal oxide as well as other materials.

ASSOCIATED CONTENT Supporting Information Effect of the excess charge on the stability of CeO2 surfaces, supplementary analysis of acidity and basicity of solid FLPs, supplementary discussion on the model presented in Fig. 5, calculation details on the results listed in Table 3 and structures of C2H2 hydrogenation on reduced CeO2(110) in the concerted mechanism

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledged the financial support from the National Natural Science Foundation of China (Grant 21603170, 91645203). This work is also supported by the Natural Science Basic Research Plan in Shaanxi Province of China (2016JQ2006), Young Talent fund of University Association for Science and Technology in Shaanxi, China (20170702), the Fundamental Research Funds for the Central Universities (Creative Team Plan No.cxtd2017004 in Xi’an Jiaotong University), and the Open Project of Shaanxi Key Laboratory of Catalysis. The calculations were performed by using HPC Platform at Xi’an Jiaotong University and National Supercomputing Center in Shenzhen.


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TOC Graphic


Understanding All-Solid Frustrated-Lewis-Pair Sites on CeO2 from

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