Mechanistic Investigations on Thermal Hydrogenation of CO2 to

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Mechanistic Investigations on Thermal Hydrogenation of CO to Methanol by Nanostructured CeO(100): The Crystal-Plane Effect on Catalytic Reactivity 2

Wei Zhang, Xuelu Ma, Hai Xiao, Ming Lei, and Jun Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02120 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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The Journal of Physical Chemistry

Mechanistic Investigations on Thermal Hydrogenation of CO2 to Methanol by Nanostructured CeO2(100): The Crystal-Plane Effect on Catalytic Reactivity Wei Zhang,† # Xue-Lu Ma,†‡ Hai Xiao,§ Ming Lei*# and Jun Li*§, ¶ # State

Key Laboratory of Chemical Resource Engineering, Institute of Computational

Chemistry, College of Chemistry, Beijing University of Chemical Technology, Beijing, 100029, China. §

Department of Chemistry and Key Laboratory of Organic Optoelectronics &

Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China. ‡

School of Chemical & Environmental Engineering, China University of Mining &

Technology, Beijing, 100083, China. ¶

Department of Chemistry, Southern University of Science and Technology, Shenzhen

518055, China. † These authors contributed equally to this work. *E-mail: [email protected] (Ming Lei), E-mail: [email protected] (Jun Li).

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ABSTRACT: As key structural parameter, a crystal plane has a distinguished impact on catalytic performance. Different exposed crystal planes exhibit different reactivity. CeO2 nanostructured powders are usually exposed to three low-index surfaces, which are (111), (110) and (100) surfaces, respectively. Due to the unique structure and low stability of (100) surface, the investigation of the catalytic reaction mechanism on this surface is rarely involved. Here, DFT calculations suggest that CeO2(100) surface exhibits the strongest reactivity for H2 oxidation, attributed to the coordination unsaturation of surface oxygen atoms. For the hydrogenation of CO2 to methanol on defective CeO2(100) surface, CO2 prone to adsorb at oxygen vacancy in a nearly linear configuration, and formate pathway was verified as the dominant one. The bi-H2COO* can easily convert to bi-H2CO* with vacancy site filled, in which bi-H2CO* sever as the key intermediate in the methanol synthesis. This study aims at providing a better understanding of the catalytic reactivity of CeO2(100) surface and theoretical insights into the experimental design of thermal CO2-to-methanol conversion.

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1. INTRODUCTION Nowadays, the excessive CO2 emissions become one of the severe environmental issues. It is essential to seek efficient catalysts and methods to convert CO2 into chemical fuels or high value-added energy substances.1,2 However, due to the thermodynamically stability and chemically inertness of CO2, the efficient and selective CO2 conversion remains a great challenging task.3,4 So far, there are three main catalytic processes for CO2 reduction, i.e., electrocatalysis,5,6 photocatalysis7,8 and thermal catalysis.9-12 Electrocatalytic and photocatalytic CO2 reduction reactions currently suffer from some defects, including low energy efficiency, and by-products that are difficult to get around. In contrast, the thermocatalytic CO2 reduction reaction is relatively kinetically fast, and the corresponding target effective components are more or less controllable, thus having received more extensive attention.1,13 Among the different approaches of CO2 fixation, hydrogenation of CO2 to methanol (CO2(g) + 3H2(g) J CH3OH(g) + H2O(g)) has attracted extensive attention in heterogeneous catalysis. Methanol is of great value that can be used as both a starting feedstock for many chemical products and as the direct methanol fuel cells. There are three main reaction mechanisms proposed for the conversion of CO2 to methanol:2,14 (1) The formate (HCOO) route. In this route, C-H bond forms at first. The key intermediate, HCOO*, could be hydrogenated to H2COO* or HCOOH*, followed by further hydrogenation to H3CO* or H2COH*, and finally producing methanol. The corresponding catalysts are represented by the Cu-based catalysts, metal oxide catalysts and some binary or ternary complex catalysts, such as Cu/ZnO,2 Cu/ZrO2,15 Cu/SiO2,15 ZnO/ZrO211 , Pt/MoS216, and notably Cu-ZnO-ZrO217, which exhibited very high CO2 conversion (18.2%) and with a methanol selectivity up to 80.2%. Such excellent catalytic performance can be attributed to the ternary interactions between ZnO-ZrO2 3



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interface and Cu species. (2) The carboxylate (COOH) route. Here CO2 is first hydrogenated to COOH* with O-H bond formation, then COOH* is converted to COHOH*, and finally methanol is formed by COHOH*

COH*

CHOH*

H2COH

H3COH.18 (3) The reverse water-gas shift (RWGS) route, in which CO* is regarded as the key intermediate in this pathway to complete the subsequent hydrogenation reaction. Especially, Cu/CeO2 and Cu/TiO2/CeO2 are the distinguished catalysts following the RWGS route with the formate route inhibited.19 Obviously, electronic structures of catalysts have a major impact on reactivity and selectivity in the conversion of CO2 to methanol.19-22 Ceria (CeO2) as one of the important rare earth oxides, has attracted much interest in the heterogeneous catalysis.23-25 It has a splendid role in several redox processes, due to their high oxygen storage capacities, which is directly related to the oxygen vacancies.26,27 And the reduced CeO2 surface is beneficial to CO2-to-CH3OH conversion. Kumari et al. investigated the methanol synthesis from CO2 hydrogenation on stoichiometric and reduced CeO2(110) surface under electrocatalytic conditions,5 in which CO2 processed through the RWGS route to form methanol. Lo et al. used the thermodynamic and kinetic model to describe the hydrogenation of CO2 on the reduced ceria (110) surface.12 They suggested that the CO2 hydrogenation to methanol predominantly follows a formate (HCOO*) pathway, and the dissociation of H2COOH* into H2CO* is the rate-determining step (RDS). Different morphologies and crystal planes of nanostructured catalysts play an important role in their catalytic reactivity, chemical activity, and selectivity.28-34 For the three low-index crystal planes of CeO2, Zhang et al. has suggested that NiO/CeO232 and Fe2O3/CeO233 nanorods have higher catalytic activity for the selective catalytic reduction of NO with NH3 (NH3-SCR) than the pure CeO2 nanopolyhedra and 4


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Fe2O3/CeO2 nanopolyhedra (which mainly exposed {111} facets of CeO2), by means of extensive experimental and DFT studies. The predominately exposed {110} and {100} facets of CeO2 nanorods have been shown to have a crucial effect on the superior reactivity of catalysts. The majority of studies have been performed on the (110) and (111) crystal planes to unveil the surface reduction and catalytic hydrogenation of CO2,5,12,35-38 while few detailed studies of the catalytic CO2-to-CH3OH conversion mechanism on CeO2(100) surface are carried out. Due to the different sample preparation methods, the morphology control drives the CeO2 nanocube to expose the polar (100) surface,34,39 and the polar surface is stabilized via reconstruction that presents a half oxygen monolayer terminated configuration.39-41 Noteworthily, a theoretical study recently explored the complementary stabilization mechanism of the polar CeO2(100) surface and suggested that its intrinsic disordered configurations may have a significant effect on its catalytic behaviour.40 Significantly, experimental studies have already shown that the CeO2(100) facets are more active than the more stable and commonly used CeO2(111) facets for catalytic reactions.42,43 Li et al. utilized high-resolution transmission electron microscopy (HRTEM) revealed that the CeO2 nanorods predominantly exposed (100) and (110) planes exhibited unusually reactive rather than the stable (111) plane.42 Wang et al. demonstrated that due to the metal Ru promoted formation of oxygen vacancies on the (100) facet of CeO2, the Ru(3%)/CeO2 nanotubes possesses the highest concentration of oxygen vacancies, which facilitates the methanation of CO2.43,44 Albrecht et al. first reported experimental and theoretical studies that CO2 was stably adsorbed on CeO2(100) surface in the form of flat-lying, tridentate adsorption state.45 Gong et al. using DFT+U calculations analysed the atomic compositions, structure stability of CeO2(100) surfaces, and examined the reactivity of CO oxidation on this surface.46 5




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strong electron-correlation properties of CeO2 based on previous theoretical studies.23,55-57 Geometry optimizations were deemed to have converged when the forces dropped below 0.05 eV Å-1. For the electronic structure, the SCF energy convergence was set to a threshold of 10-4 eV. The bulk lattice parameter of ceria was optimized as 5.44 Å, in good agreement with experimental value of 5.41 Å.58 The CeO2(110)57 was modeled by p(2 × 3) five-atomic-layer supercells with the bottom two layers fixed.12 The oxygen-terminated CeO2(111)57 was modeled by p(3 × 3) nine-atomic-layer supercells with the bottom four layers fixed. (3 × 3 × 1) Monkhorst-Pack mesh k-points was used for (111) and (110) surfaces calculations. As the CeO2(100) has a nonzero dipole moment normal to the surface, to eliminate the surface dipole, a half of the O atoms from top layer are removed to the opposing face in a checkerboard style to fulfill the CeO2 formula.46,55,59,60 The (100) surface was modeled by an oxygen-terminated periodic nine-layer slab with the bottom four layers were fixed, and a (2 × 2 × 1) k-point grid generated with the Monkhorst-Pack scheme were found to give converged results. The vacuum gap was set as 15 Å to avoid the interaction between the periodic images. The adsorption energies of adsorbates were calculated following the definition Ead(M) = EM/S – EM – ES, where EM/S represents the total energy of a surface slab with the adsorbate, EM represents the energy of an adsorbate molecule, and ES represents the energy of a perfect or O-defective (111), (110) and (100) surfaces. The reaction energies of the reaction pathways were calculated as the total energy difference between the final and initial states: VE = Efin – Eini. According to this definition, the negative value indicates the exothermic process, whereas positive values are endothermic. The activation energy was calculated as the energy difference between the transition state and corresponding initial state: Ea = Etran – Eini. The oxygen vacancy formation energy 7



of (111), (110) and (100) surfaces were obtained from calculations as Eov x)

1/2E(O2)

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E(CeO2-

E(CeO2). The transition states are located by using climbing-

image nudged elastic band (CI-NEB) method,61,62 and the transition state was confirmed through harmonic vibrational frequency analysis to have only one imaginary frequency. 3. RESULTS AND DISCUSSION 3.1 H2 dissociation and oxygen vacancies formation on CeO2(111), (110) and (100) surfaces During the hydrogenation of CO2, the presence of surface oxygen vacancies facilitates its adsorption and activation, while oxygen vacancies are formed via the reduction with H2. The adsorption and dissociation forms of H2 on different surfaces of CeO2 are different, and the dissociation can be divided into the homolytic dissociation and the heterolytic dissociation pathway.59,63 Therefore, it is especially important to compare the oxygen vacancy formation energy of different surfaces and the dominant hydrogen dissociation path. The optimized prefect and oxygen defective (111), (110) and (100) surfaces are shown in Figure S1. The calculated surface energies and oxygenvacancy formation energies for the above three surfaces are given in Table S1. Compared to the more stable (111) and (110) surfaces,57,64 the (100) surface has a higher surface energy, which is 1.419 J/m2, close to the value reported by Skorodumova et al.60 The reaction mechanism for the formation of oxygen vacancy of H2 activation on CeO2(111), (110) and (100) surfaces was analysed based on the following elementary reaction steps:59,63,65 (1) hydrogen adsorption: H2 J H2* (the step is abbreviated as “R1”). The physical adsorption configuration of H2 on the CeO2(110), (100) and (111) surfaces is represented by 1H/1'H, leading to heterolytic dissociation pathway or 8


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homolytic dissociation pathway, respectively; (2) hydrogen heterolytic dissociation: generating a proton that binds with O site, and a hydride that binds with Ce site: H2* J HO* + HCe* (the step is abbreviated as “R2”), subsequently the hydride transfer from Ce site to the adjacent O site, forming two surface hydroxyl groups: HO* + HCe* J 2HO* (the step is abbreviated as “R3”); or two proton formed directly via homolytic dissociation on the surface after H2 adsorption: H2* J 2HO* (the step is abbreviated as “R2' ”). 2H and 3H/3'H represent the products of heterolytic and homolytic of H2, respectively. (3) the two HO* atoms further interact with each other to produce absorbed H2O. After H2O desorption, the adjacent Ce4+ is reduced to Ce3+ and one oxygen vacancy is formed at the same time: HO* + HO* J H2O* + OZ (the step is abbreviated as “R4”). Here an asterisk (*) represents the adsorbed state, Z represents the oxygen vacancy. 4H represents the product of reaction R4. The optimized structures and calculated energy profiles for the H2 heterolytic dissociation on the different CeO2 surfaces are shown in Figure S2 and Figure 1(a). H2 is physically adsorbed on the CeO2(100), (110) and (111) surfaces (1H) and dissociates through the R2 step with relatively low energy barriers, leading to a less stable intermediate 2H. In this step, CeO2(100) surface exhibits the lowest energy barrier (0.45 eV), whereas the CeO2(110) and (111) surfaces have higher activation barriers of 0.55 eV and 0.80 eV, respectively, which agrees with the previous DFT results.59,63 For the hydride transfer step (R3), the hydride at the Ce cation of 2H is not stable and tends to transfer to the O anion nearby, where the energy barrier of this step is the lowest on the CeO2(110) surface (0.22 eV), followed by the CeO2(100) surface (0.37 eV) and CeO2(111) surface (0.40 eV). It follows that R3 is most likely to occur on the (110) surface, but the hydride transfer process requires a slightly higher energy barrier on CeO2(111) surface due to the strong polarization of the H-H bond in the intermediate 9



2H.63 In general, the heterolytic dissociation of H2 is most likely to occur on the CeO2(100) surface, whereas the most difficult dissociation of H2 occur on the CeO2(111) surface. On the other hand, the homolytic dissociation of H2 (R2') begins with intermediate 1'H, leading two adjacent surface hydroxyl groups (3'H/3'H) on three different surfaces of CeO2, the energy profiles and optimized geometries for the intermediates and transition states are shown in Figure S3. The results indicate that the R2' process is most likely to occur on the CeO2(100) surface (with a barrier of 0.53 eV), compared to the CeO2(110) surface (with a barrier of 0.70 eV) and the (111) surface (with a barrier of 1.20 eV). However, the homolytic dissociation of H2 is still not favourable compared with the heterolytic pathway (R2).

(100) surface (A) (110) surface (B) (111) surface (C)

2

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

Heterolytic Path

(0.79) (0.005) (0.55) (-0.001) (0.36) (-0.09)

-2

(0.80) (1.18) (0.39) (0.61) (-0.20) (0.17)

(-2.19) (-2.79) (-3.25) (C) (B) (A)

1

1H

TS1H

2H

TS2H

3H

Figure 1. Energy profiles for the heterolytic H2 dissociation pathways on the three CeO2 surfaces For the oxygen vacancy formation (R4) on above three different surfaces, surface OH groups can further react with each other to produce absorbed H2O (Figure S4). On CeO2(110) surface, 3H requires a hydrogen transfer step to form 3'H. Although the energy difference between the two OH-containing intermediates is small, the conversion process from 3H to 3'H needs to climb an extremely high barrier (3.09 eV), 10


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via TS3H-3'H. Thus, for formation oxygen vacancy on CeO2(110) surface, H2 tends to generate 3'H after homolytic dissociation and then produce chemisorbed H2O and an oxygen vacancy via TS3H-4H, with a barrier of 0.90 eV (Figure S4). For oxygen vacancy formation on CeO2(100) and (111) surfaces, the activation barriers of TS3H4H(A) and TS3H-4H(C) are 1.39 eV and 3.36 eV, respectively (as shown in Figure S4). Therefore, CeO2(100) has a stronger ability to activate H2 than the CeO2(110) and (111) surfaces. However, (110) surface is more easily to generate oxygen vacancy,34 and the ability of generating OZ on above three surfaces is followed by the order of Eov, which is (111) > (100) > (110).66,67 In terms of the crystal-plane effect on the reactivity of H2 oxidation, the above computational results are consistent with the experimental results obtained by Désaunay et al., in which temperature-programmed reduction (TPR) was used to obtain the reactivity order of H2 on ceria, that is (100) > (110) > (111).68 Due to the different coordination environments of surface oxygen and Ce atoms on different surfaces, CeO2(111) is characterized with the threefold-coordinated O and sevenfold-coordinated Ce, which enhances the surface stability while also limits its reactivity. Similarly, CeO2(110) with threefold-coordinated O and sixfold-coordinated Ce behaves in the same way. However, CeO2(100) contains twofold-coordinated O and the sixfold-coordinated Ce, and the unsaturated surface O atoms give the (100) surface a strong ability to dissociate H2. It has been reported that reduced CeO2(100) surface can serve as an all-solid (frustrated Lewis-pair) FLP catalysts when a surface oxygen be removed.59,69 And it has an excellent ability to decompose hydrogen, and applied to partial hydrogenation. According to the above results, we select the oxygen defective CeO2(100) surface to investigate the reaction mechanism of CO2 hydrogenation to methanol in the following study.

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3.2 CO2 Hydrogenation to Methanol on Oxygen Defective CeO2(100) surface Electronic properties of perfect and oxygen defective CeO2(100) surfaces, 1 and 2, are characterized by using the electronic localization function (ELF) contour maps as shown in Figure 2. ELF maps clearly show that the ELF value is close to zero at the oxygen vacancy (OZ) site on the oxygen defective CeO2(100) surface, indicating a highly delocalized electron distribution at OZ site. Compared to the prefect CeO2(100) surface, there is a higher electron localization at the Ce1 and Ce2 sites adjacent to oxygen vacancy (OZ) on the oxygen defective CeO2(100) surface. The above ELF analysis results indicate that oxygen vacancy (OZ) is likely to serve as electron donors to donate partial electrons to the surface, making the CeO2(100) surface reduced. Pervious study suggested that the surface with an OZ could be regarded as a Lewis base, and the formation of the OZ site results in two unpaired electrons transferring to the Ce4+, leading to reduction to Ce3+.70

Figure 2. Electronic localization function contours of the perfect CeO2(100) surface (1) and oxygen defective CeO2(100) surface (2). Red and yellow spheres represent O and Ce, respectively. Large spheres indicate the first layer of O and Ce. As mentioned above, based on different activity sites of catalysts, the binding of CO2 on catalysts surface can be different, thus leading to different reaction mechanisms of CO2-to-methanol.2,12,14 The HCOO* and COOH* species are regarded as the key

12


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intermediates in the formate pathway and the carboxylate pathway, whereas in the reverse water-gas shift (RWGS) pathway, CO is the first intermediate for CO2 reduction.21,71 All the possible reaction pathways and intermediates for CO2 hydrogenation to methanol on oxygen-defected CeO2(100) surface are shown in the reaction network in Figure S5. And the key intermediates in the reaction network are shown in Figure 3. By starting from CO2 adsorbate (3) at oxygen vacancy, atomic H combines with carbon to form 5 (bi-HCOO*) or combines with oxygen to form 22 (COOH*). We firstly define the pathway through bi-HCOO* as the formate pathway, while the pathway through COOH* as carboxylate pathway. The formate pathway can be further divided into three channels leading to methanol based on the hydrogen binding site. H2 H1 C Oi Oii O 1 Ce Ce Ce2 17 (bi-H2COOH*) H3

path C

H1 C i

ii

O O Ce Ce1 Ce2 5 (bi-HCOO*) O

O

1 H2 H Oi C ii O O 1 Ce Ce2 Ce 14 (HCOOH*)

path B

Formate path Ce

H2 H1 C Oi Oii O 1 Ce Ce Ce2 7 (bi-H2COO*) path A

Ce1 Ce2 2 Carboxylate path H1 Oi Oii C O Ce Ce1 Ce2 22 (COOH*)

Oii 1 C H i O Ce O Ce1 Ce2 23 (CO*+OH*)

H2 H1 C ii i O O O O 1 Ce2 Ce Ce Ce 18 (mono-H2CO*+OH*) H3

2 H2 H1 H4 H H1 C C H3 ii Oi ii O O i O Ce Ce1 Ce2 Ce O Ce1 O Ce2 O Ce 8 (bi-H2CO*) 20 (H3CO*+OH*)

2

H3 H H1 Oi C O 1 Oii 2 Ce Ce Ce 10 (H2COH*)

H H C H i H O O 1 Oii 2 Ce Ce Ce 12 (H3COH*)

H1 H2 H2 Oii 1 C ii H C Oi O O 1 Oi 2 O Ce Ce Ce Ce Ce1 Ce2 25 (HCO*+OH*) 26 (HCOH*)

Figure 3. The reaction network of CO2 hydrogenation to methanol on oxygendefected CeO2(100) surface. CO2 adsorption is an essential first reaction step in the CO2-to-methanol conversion. 13



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On metal oxide surfaces, the monodentate and bidentate carbonates and CO2- species are regularly considered as shown in Figure S6. In this work, as shown in Figure 4, we studied three adsorption configurations of CO2 on oxygen defective CeO2(100) surfaces, named 1C, 2C and 3C, respectively. We labeled the two O atoms of the adsorbed CO2 as Oi and Oii. And the adsorption energies and structural parameters of absorbed CO2 are listed in Table 1. Oi

Oii C Oi

C Oii

2.84

2.95

2.97

3.85

Oi

C

Oii

2.63

Oi C Oii

Oi

Oi C

C Oii

1

2.6

1C (3)

2C

Oii

3C

Figure 4. Top and side views of optimized adsorption configurations of CO2 on oxygen defective CeO2(100) surfaces. Table 1. Adsorption energies, structural parameters of CO2 on the oxygen defective CeO2(100) surface. Configurations CO2 1C 2C 3C

Eads (eV)

C-Oi (Å)

C-Oii (Å)

-0.33 -0.13 0.75

1.18 1.17 1.18 1.25

1.18 1.19 1.18 1.26

Oi-C-Oii (deg) 180° 179.68° 177.20° 129.06°

For 1C, CO2 is adsorbed on the top of the oxygen vacancy vertically, with an adsorption energy of -0.33 eV. In 2C, CO2 is nearly parallel to the oxygen vacancy with the adsorption energy of -0.13 eV, which indicate a weaker interaction between adsorbed CO2 and the surface. CO2 was observed to be activated in a bent configuration (CO2-) in 3C with a positive Ead of 0.75 eV, indicating that the process of CO2 adsorption 14


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on the oxygen defective CeO2(100) surface to form 3C is endothermic and 3C is not stable. Therefore, in the following sections, we select 1C configuration for subsequent hydrogenation reactions, which is also the most beneficial structure for hydrogenation in the above configuration. As mentioned above, reduced CeO2(100) surface has an excellent ability to decompose hydrogen, with a little barrier of 0.08 eV.59 Therefore, we provide the hydrogen source by adding H atoms to the surface Ce or O atoms. In formate pathway, CO2 was shown to be adsorbed on Ce2 site of 4 and in a linear configuration with a single Ce2-Oii bond of 2.89 Å (Figure S7), which is more likely to participate in hydrogenation with the atomic H at Ce1.5,21 As shown in Figure 5, the H1 is further transferred to the C atom from 4 to 5 (bi-HCOO*) with a barrier of 0.22 eV. While in the first step of carboxylate pathway, the co-adsorption state of CO2* and HO* in 21 is more stable than 4 by 2.34 eV. However, the distance between the H1 and Oi atom of CO2 in 21 is farther away (4.33 Å). The proton transfers from 21 to 22 (COOH*) with an extremely high barrier of 2.99 eV as shown in Figure S8 and Figure S9. Therefore, CO2 is more likely to hydrogenate to form 5 (bi-HCOO*) rather than 22 (COOH*) in the initial step, and the preferred reaction pathway was verified to be the formate pathway. This result is consistent with previous theoretical and experimental studies indicating that bi-HCOO* is the dominant intermediate in the hydrogenation process of CO2 catalyzed by several metal oxides.11,12,21,44 After formation of 5 (bi-HCOO*), the adatom H2 either adsorbs at Ce2 site, leading to the path A, or adsorbs at O3 site, leading to the path B. In path A, the H2 binds at Ce2 site in 6, then transfers to the C atom to form 7 (bi-H2COO*) with a barrier of 0.53 eV as shown in Figure 5. The bi-H2COO* is formed on the surface oxygen vacancy with the Ce1-Oi bond (2.16 Å) and the Ce2-Oii bond (2.14 Å). Intermediate 8, bi-H2CO* 15



adsorption on prefect CeO2(100) surface, is another hydrogenation product of 5 (biHCOO*). Paralleled with Oii atom filling into the oxygen vacancy, the H2 migrates directly from Ce2 site in 6 to the C atom in 8 (bi-H2CO*) with a barrier of 0.68 eV. The adsorbate, bi-H2CO*, binds to the surface in a ^2-like configuration,72 in which the Oi of bi-H2CO* binds to the two surface Ce sites and the Oii atom drops at the oxygen vacancy. By contrast, the transformation form 7 (bi-H2COO*) to 8 is more easy-going with a tiny energy barrier of 0.04 eV, due to the presence of oxygen vacancy. The quick elementary step is a crucial driving force in the hydrogenation of CO2 to methanol on the CeO2(100) surface. Compared to the HCOO* hydrogenation on Cu(111) surfaces, the activation barrier from HCOO* to H2COO* was calculated to be over 1 eV.18,73 It is noteworthy that the bi-HCOO* hydrogenation is beneficial on the CeO2(100) surface. In the following process, 9 is then formed through the protonation of 8 (bi-H2CO*), and H3 is transferred from the O3 atom in 9 to the Oi atom in 10 with a barrier of 0.97 eV. For the final production 12 (methanol) formation step, H4 migrates from Ce2 site in 11 to 12 with a barrier of only 0.42 eV, which is close to the activation energy of methanol formation from H2COH* on the Cu(111) surface,18 but is much smaller than the barriers of that on the ZnO(0001) surface (1.66 eV)74 and In2O3(110) surface (2.52 eV).21 In the path B, beginning with the adatom H2 adsorbed at O3 site in 13, 14 (HCOOH*) is formed by the protonation of 4 (bi-HCOO*). And H2 is transferred from O3 atom in 13 to Oii atom in 14 (HCOOH*), which is strongly endothermic by 2.15 eV as shown in Figure S10 and Figure S11. Then hydrogenation of 14 (HCOOH*) produces 10 (H2COH*) from 15 with a barrier of 0.34 eV, and path B merges with path A at 10 to give the final production, methanol. Obviously, the RDS transformation from 13 to 14 (HCOOH*) in path B is not as favourable as the bi-HCOO* hydrogenation step from 6 16


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to 7 in path A. Besides, after forming 7 (bi-H2COO*), H3 absorbs at the O2 site in 16, leading to path C to form methanol. And 17 (bi-H2COOH*) is formed by the H3 migrating from O2 site in 16 with a barrier of 0.67 eV as shown in Figure S12 and Figure S13. Then C–Oii bond cleavage occurs from 17 to 18 barrierlessly, in which the monodentate H2CO* locate at Ce1 site and an OiH* filled in the oxygen vacancy. While the similar process occurred on CeO2(110) surface, in which H2COOH* dissociation is the RDS in CO2 hydrogenation to methanol with a barrier of 0.74 eV.12 Since 17 (biH2COOH*) is adsorbed just above the oxygen vacancy on defective CeO2(100) surface, and the Oi/Oii atoms are bonded with the Ce1/Ce2 atoms, respectively. Because of that, the C-Oii bond is easily broken and the Oi atoms are filled with oxygen vacancy to compensate for the charge loss at the OZ. However, on CeO2(110) surface, H2COOH* is more prone to adsorb with its C atom at the vacancy site,12 therefore different exposed crystal plane exhibits different reaction characteristics. Then the H4 is transformed from the Ce1 site in 19 to the C atom of 20 (H3CO*) with a barrier of 0.76 eV, and the 12 (methanol) is formed by the protonation of 20 with a barrier of 0.62 eV. Although the step of C-Oi bond cleavage in 17 (bi-H2COOH*) is accessible, the formation of 12 from 20 in path C is still not favourable compared with the methanol formation from 11 to 12 (with a barrier of 0.42 eV) in path A. According to the above proposed mechanisms of CO2 hydrogenation to methanol on CeO2(100) surface, the oxygen vacancy can serve as an active site during CO2 absorption, activation and transformation steps. Compared with path B and path C, path A in the formate pathway is more favourable to produce methanol, in which 5 (biHCOO*) is hydrogenated to form 7 (bi-H2COO*), and bi-H2COO* quickly drops at the oxygen vacancy with its Oii atom to form 8 (bi-H2CO*) on prefect CeO2(100) surface via path A. The process reflects a unique catalytic performance on CeO2(100) surface. 17



0

1 (0.00)

TS4-5 (0.12) 4 (-0.10)

TS6-8 TS6-7 (0.19) (0.03) 6 (-0.49)

TS11-12 11

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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TS7-8 (-1.09)

-1

(-0.55)

(-0.97)

7 (-1.13)

3 (-1.32)

8 (-1.53)

-2

TS9-10 (-2.22)

5 (-2.13)

12 (-1.96)

10 (-2.44)

-3 9 (-3.19)

Figure 5. Potential energy profile for CO2 hydrogenation to methanol on CeO2(100) surface via path A in formate pathway.

3.3 The characteristic of CO2-to-methanol conversion on CeO2(100) surface For the three low-index surfaces of CeO2, we have determined that the heterolytic dissociation of H2 is rather favorable than homolytic dissociation, and the CeO2(100) surface exhibits the best ability to activate H2. For the hydrogenation of CO2 to methanol on oxygen defected CeO2(100) surface, the formate pathway is more favorable than the carboxylate pathway, and methanol prefers to be obtained via path A of formate pathway. The bi-H2CO* protonation to H2COH* is the RDS with a barrier of 0.97 eV (from 9 to 10). In the terms of the hydrogenation of CO2 on CeO2(111) surface,36 the formation of COOH* (CO2 + H* J COOH*) is more favorable than formation of HCOO* (CO2 + H* J HCOO*). However, it is worth mentioning that CO2 is prone to dissociate into CO on oxygen defective CeO2(111) surface, which means that CO2 is likely to be hydrogenated through the reverse water gas shift (RWGS) path. For oxygen defective CeO2(110) surface,5,12,49 CO2 adsorbate prefers to exhibit a bent configuration with C atom of the CO2 bonded to the oxygen vacancy site. In addition, CO2 hydrogenation to 18


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methanol predominantly follows the formate pathway on oxygen defect CeO2(110) surface, and the RDS is the transformation from H2COOH* to H2CO* with the C-O bond cleavage.12 However, it is worth mentioning that CO2 hydrogenation to the COOH* (carboxylate) is accessible with the barrier of 0.30 eV and CO* is formed through the dissociation of COOH* with a barrier of 0.50 eV on the defective CeO2(110) surface.12 In contrast, on the CeO2(100) surface, the huge energy barrier of COOH* formation avoids the formation of CO, and the RDS of CO2-to-methanol conversion is the bi-H2CO* protonation to form H2COH*. It was suggested that CO2 has a stronger adsorption on CeO2(100) surface than on (111) surface. In addition, the presence of oxygen vacancy can improve the surface basicity and thus enhance the interaction between CO2 and surface.75 Because of the unique structure exhibited by CeO2(100) surface reconstruction and its rich surface vacancies, the adsorption and conversion characteristics of CO2 on CeO2(100) surface is determined. Interestingly, on defective CeO2(100) surface, due to the stronger coordination unsaturation of the surface oxygen atoms, CO2 and its hydrogenated adsorbates tend to interact with vacancy with their O-atom terminal. By contrast, on defective CeO2(110) surface, CO2 and subsequent intermediates are all apt to adsorb with their C atom terminal at the vacancy site. The above comparisons of three CeO2 surfaces indicate different exposed crystal plane could lead to different adsorbates and controlling the exposed crystal plane is a potential strategy to optimize the catalytic reactivity. These crystal-plane effects have been recognized in previous studies and the present work provide further insights on the origin of such effects.76,77

19



4 CONCLUSIONS In this study, density functional theory calculations are performed to study the dissociation of H2 and the creation of oxygen vacancies via H2 oxidation on the three low-index surfaces of CeO2. The calculated results have revealed that the heterolytic dissociation of H2 is rather favorable than the homolytic dissociation of H2 on CeO2(100), (110) and (111) surfaces. Significantly, the CeO2(100) surface exhibits the best performance for H2 activation. The mechanisms of methanol synthesis from CO2 hydrogenation are investigated based on the CeO2(100) surface with an oxygen vacancy. The carboxylate pathway seems not to be plausible due to the extremely high barrier of 2.99 eV for the COOH* formation. By contrast, CO2 is at first hydrogenated to biHCOO* via the formate pathway with the much lower energy barrier of 0.22 eV. The following fast elementary step from bi-H2COO* to bi-H2CO* is a crucial driving force in the hydrogenation of CO2 to methanol on the CeO2(100) surface. During the entire catalytic reaction, H2 is used as the hydrogen source to create defective surface, and the charge loss at oxygen vacancy promotes the unique catalytic CO2-to-methanol conversion on CeO2(100) surface. ASSOCIATED CONTENT Supporting Information. Additional comments and discussions of the findings in the paper, and supplemental data are shown in Figure S1 to S13 and Table S1.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Ming Lei), E-mail: [email protected] (Jun Li). 20


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Author Contributions † These authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (Nos. 21672018, 2161101308), and the Fundamental Research Funds for the Central Universities (12060093063) to M.L. Beijing Natural Science Foundation (2184105), and the Fundamental Research Funds for the Central Universities (2019QH01) to X.L.M., National Natural Science Foundation of China (Nos. 21590792, 91645203, and 21521091) to J.L. We also thank the National Supercomputing Center in Tianjin (TianHe-1) for providing part of the computational resources. REFERENCES (1) Li, W. H.; Wang, H. Z.; Jiang, X.; Zhu, J.; Liu, Z. M.; Guo, X. W.; Song, C. S. A Short Review of Recent Advances in CO2 Hydrogenation to Hydrocarbons over Heterogeneous Catalysts. RSC Adv. 2018, 8, 7651-7669. (2) Roy, S.; Cherevotan, A.; Peter, S. C. Thermochemical CO2 Hydrogenation to Single Carbon Products: Scientific and Technological Challenges. ACS Energy Lett. 2018, 3, 1938-1966. (3) Liu, X. M.; Lu, G.; Yan, Z. F.; Beltramini, J. Recent Advances in Catalysts for Methanol Synthesis via Hydrogenation of CO and CO2. Ind. Eng. Chem. Res. 2003, 42, 6518-6530. (4) Wang, W.; Wang, S. P.; Ma, X. B.; Gong, J. L. Recent Advances in Catalytic Hydrogenation of Carbon Dioxide. Chem. Soc. Rev. 2011, 40, 3703-3727. (5) Kumari, N.; Haider, M. A.; Agarwal, M.; Sinha, N.; Basu, S. Role of Reduced 21



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