CeO2-x: A DFT

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The Mechanism of Steam-Ethanol Reforming on Co/CeO2-x: A DFT Study Meng-Ru Li, Yangyang Song, and Gui-Chang Wang ACS Catal., Just Accepted Manuscript • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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The Mechanism of Steam-Ethanol Reforming on Co13/CeO2-x: A DFT Study

Meng-Ru Li 1, Yang-Yang Song1 and Gui-Chang Wang 1,2,* (1 Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and the Tianjin key Lab and Molecule-based Material Chemistry, College of Chemistry, Nankai University, Tianjin 300071, P. R. China;

2

State Key Laboratory of Coal Conversion, Institute of Coal

Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P. R. China)

*Corresponding author: Gui-Chang Wang. Telephone: +86-22-23503824 (O)

E-mail: [email protected]

Fax: +86-22-23502458

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Abstract: In the present work, density functional theory calculations are performed to study mechanisms of ethanol steam reforming reactions on Co13/CeO2-x model. The related adsorption situations and reaction cycles were clarified. Ethanol will convert into CH3CO species through dehydrogenation steps on Cox+ site, followed by coupling with hydroxyl from water dissociation on CeO2-x , yielding acetic acid on interface. The acetic acid will spread to Co0 site to cleave C-C, and then convert to CO2. H2 forms on Cox+ site. The coke formation is mainly caused by CH accumulation on Co0 site, and could be released by CH oxidation on Cox+ site. The oxidation state of Co on surface affects the activity of ESR reactions. Higher oxidized Co site, featured with less ensemble size of Co, facilitates recombination reactions, e.g. H2, acetic acid formation and CH oxidation. On the contrary, more reduced Co site favors dissociation reactions, e.g. C-C scission. The Cox+ site is the most favorable site for the dehydrogenation of ethanol into CH3CO. CeO2-x will promote H2O dissociation via oxygen vacancy and lattice oxygen. On the hydroxylated CeO2-x surface, mobile O on CeO2-x has a higher tendency of oxidizing Co, while mobile OH is mainly responsible for releasing carbon deposition. In experiment, keeping high Co0/Co2+ ratio can gain high proportions of Co0 and Cox+ site, contributing to high ESR activity. Metal-oxide interaction should be strengthened to promote the spread of mobile OH. Enhancing metal-oxide interface formation is essential for CH3COOH formation. The redox property of CeO2 needs to be increased through doping with other elements, contributing to more oxygen vacancies. Adding O2 could help release carbon deposition. Keywords: Ethanol steam reform; Reaction mechanism; Coke formation; Co/CeO2-x; Density functional theory calculations.

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1. Introduction Both rising global energy demands and serious environmental problems call for the replacement of alternative fuels to the petroleum-based fuels. Hydrogen, the promising environmentally friendly energy source, has attached much attention. To date, ethanol steam reforming (ESR) stands out among numerous hydrogen production technologies, e.g. steam reforming of methane1-7, steam reforming of methanol8-10, owing to high selectivity for H211. The strongly endothermic ESR calls for the efficient catalyst at low temperature. The non-noble Co-based catalysts could obtain the low propensity to catalyze carbon deposition, and low selectivity of byproducts12. The supports (CeO2, ZnO, MgO, Al2O3, zeolites-Y, TiO2, SiO2, La2O2CO3, CeO2-ZrO2) affect catalytic activity, selectivity and carbon deposition significantly13. Acid supports, e.g. Al2O3, strongly facilitate carbonaceous deposition, caused by the dehydration of ethanol. On the contrary, the support oxides of basic nature lead to the condensation of ethanol to higher oxygenates14-15. The reducible metal oxides of only weakly basic, e.g. CeO2, turn to be the most suitable supports for ESR reaction16-23, owing to high oxygen storage capacity and oxygen mobility.24 Especially Pang et al. 25 observed that Co/CeO2 catalysts doped with CaO in ESR reaction possess high H2 yield (90%) under optimized conditions. The high performance of CeO2 can be rationalized from highly mobile lattice oxygen, assisting the removal of carbon and water dissociation26. Additionally, the oxidation state of cobalt is affected by the reversibly changing Ce4+/Ce3+, via storing/releasing oxygen27. Turczyniak et al.28 have reported the presence of Co(II) and metallic Co under higher pressure (4-20 mbar) of ethanol/water on Co/CeO2, gaining higher selectivity toward CO2 and H2. Notably, Del Río et al. 29 suggested that CoOx phase was required to obtain sufficient CO2 selectivity. The metallic Co site is proposed to be 3


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active for C-C scission, while the existence of Co2+ site promotes the selectivity toward CO2 30. Under ESR condition, the composition of cobalt phase stays in the dynamic equilibrium between metallic and oxidized Co phase18. Theoretically, extensive calculations have focused on ethanol decomposition/oxidation on metals31-32 and metallic oxides33-36. The ESR mechanisms on metallic Co site comprise consecutive dehydrogenation of ethanol, C-C scission, water dissociation, and water-gas shift sections37. Accounting for the activity of Co2+ and Co0 site toward ESR, our previous study attempted to explore the role of the metallic and oxidized Co sites, and proposed that controlling cobalt oxidation state is essential for gaining high selectivity of CO2 and H238. In experiment, supports dominantly determine Co oxidation state, changing reaction mechanisms 39-41. Mobile oxygen on reducible support is supposed to act as oxidant, affecting redox properties of Co.28 However, DFT calculation of ESR reactions on Co catalyst supported on reducible support, e.g. CeO2, remains lacking. Herein, we develop Co/CeO2 catalyst to explore ESR mechanism based on the work of Turczyniak et al.28.

Figure 1. Structures of Co13 (a), CeO2 (b), Co13/CeO2(111) model (side view in c and top view in e), and Co13/CeO2-x(111) with oxygen vacancy (side view in d and top view in f) Note: the atoms in yellow in (d) refer to the oxygen neighboring to the oxygen vacancy; the atoms in yellow in 4


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(e) refer to the lattice oxygen which will be occupied by oxygen vacancy in (f).

2. Calculation methods and models Methods The Vienna ab initio simulation package (VASP)42-44 was applied to investigate ethanol steam reforming reaction on Co catalyst supported on cerium oxide, by the self-consistent periodical DFT calculations with the projected augmented wave(PAW)45 pseudopotentials. All the electronic structures were optimized using the Perdew-Burke-Ernzerhof (PBE)46 form of the generalized gradient approximation (GGA) expended in a plane wave basis, with kinetic cut-off energy of 400 eV. The climbing image general nudged elastic band (CI-NEB)47 method was employed to locate the transition states (TSs). Spin polarization was included in the calculations. Models The CeO2(111) surface, as the most stable surface termination48-50, was represented by the slab model, featured with p(33) unit cell of three layers, which has been reported to be efficient for calculation51. The slab model was separated by a 20 Å vacuum with the 1 x 1 x 1 Monkhorst-Pack k-point mesh. The uppermost two layers were allowed to relax to optimize the structure. The density functional theory (DFT+U) method was used to treat the highly localized Ce 4f state. The GGA+U52-53 method was applied to CeO2 surface with the set U-J=5.0 eV19. The optimized Co13 cluster (See Figure 1a) was placed onto CeO2(111) surface (See Figure 1b), getting Co13/CeO2(111) model. Subsequently, Co13/CeO2(111) was simulated using extensive ab initio molecular dynamics (AIMD) simulation54 (See Figure 2a). The Co13 cluster and the uppermost two layers of CeO2 were relaxed in the canonical (NVT) ensemble, employing Nose-Hoover thermostats55 at 675 K for 2000 fs with 1 fs time step, to meet the experimental condition28. There is a significant change of Co13 configuration from its three-dimensional structure in vacuum (Figure.1a) to a nearly two-dimensional structure (Figure.2c). Afterwards, the resulted Co13/CeO2-x was optimized by DFT method to obtain 5


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the stable Co13/CeO2-x configuration. For oxygen vacancy on CeO2(111) could be formed through the reduction process: Co13/CeO2+xH2→Co13/CeO2-x+xH2O, the existence of oxygen vacancy should be taken into consideration. On Co13/CeO2, most lattice oxygen atoms on support will interact with Co13 cluster, remaining four lattice oxygen atoms available for oxygen vacancy formation. In order to get the thermodynamic stability of the Co13/CeO2(111) surface with different surface coverage of oxygen vacancy, the atomistic thermodynamic approach56 was used to calculate the reaction Gibbs free energy of the reduction process, with the number of oxygen vacancy x ranging from 1 to 4. The zero reference states of μ H 2O (T, p  ) and μ H 2O (T, p  ) are set to be the total energies of H2O and H2 ( μ H 2O (0, p  ) = E total and μ H 2 (0, p  ) = E Htotal ), the reaction Gibbs free energy can be expressed in H 2O 2 equation (1): ΔG = E(Co13 / CeO 2-x ) - E(Co13 / CeO 2 ) - x  μ(H2 O) - μ(H2 ) 

(1)

where E(Co13/CeO2-x) and E(Co13/CeO2) refer to the total energies of Co13/CeO2-x and Co13/CeO2 slabs separately. The chemical potentials of H2O and H2 under certain condition were calculated in equation (2):

μ(T, p) = μ(T, pΘ ) + E ZPE + RTln(p / pΘ ) = E ZPE + H(T, pΘ ) - T  S(T, pΘ ) + RTln(p / pΘ )

(2)

EZPE(H2O) and EZPE(H2) stand for the total energies of H2O and H2 with ZPE correction. The enthalpy and entropy at the temperature of T could be obtained from thermodynamic data table, and the temperature of 675 K was adopted to agree with the experimental condition28. The pH2O/ pH2 ratio varying from 0.01 to 1 was tested, aiming to get ΔG value with reaction going on (See Figure 2b). As shown in Figure 2b, oxygen vacancy will inhibit oxygen release under reduction condition, increasing reaction Gibbs free energy of oxygen release. Oxygen vacancy formation turns to be more 6


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difficult, with more oxygen releasing from support. Two oxygen vacancies were available with negative value of reaction Gibbs free energy, with pH2O/ pH2 ratio varying from 0.01 to 1. Subsequently, the Co13/CeO2-x(111) with two oxygen vacancies was optimized, keeping the Co13 cluster and the uppermost two layers of CeO2 surface relaxed. The charge distribution shows that the Co atoms, closer to support, are oxidized partially, with the charge between 0 and +2 (See Figures 2c and 2d), denoted as Cox+ site. The Cox+ site agrees with the equilibrium state between CoO and metallic Co during in situ ESR conditions reported by Lin et al57-58, and is originated from electron transfer with support. Hence, the Co13/CeO2-x(111) can be divided into four parts: Co0 site (top site of Co13 cluster), Cox+ site (bottom site of Co13 cluster), the interface between Co13 cluster and support, and support CeO2-x.We will discuss ESR mechanisms on different sites of Co13/CeO2-x. The adsorption energy (Eads), activation energy (Ea), and reaction energies (∆E) were calculated by the following three formulas: Eads=EA/M-EA-EM, Ea=ETS-EIS, and ∆E= EFS-EIS, respectively. Here, EA, EM, EA/M, ETS, EIS and EFS mean the calculated energies of the adsorbate, substrate, adsorption system, transition state, initial state (IS), and final state (FS), respectively.

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Figure 2. AIMD simulation trajectory for Co13/CeO2-x(111) (a), the calculated reaction Gibbs free energy with different number of oxygen vacancy under different pressure ratio of pH2O/pH2 (b), the charge of Co cluster in Co13/CeO2-x model (c) and the charge distribution of Co13/CeO2-x(d). Note: the charge in Figure 2c and 2d is calculated by Bader charge analysis59,60.

3. Results 3.1 Adsorption of Pertinent Species For Co13 cluster and support CeO2-x are main active sites for ESR reactions, we mainly discuss the adsorption of species on Co0, Cox+ site and support CeO2-x. It is observed that the adsorption energies of most species on Co0 site are higher, in comparison to Cox+ site (See Figure 3). This is caused by the high electron density of metallic Co site, and the observation agrees well with our previous calculation results38. Previous experimental study61 reported that CeO2 surface fails to cleave C-C bond, and is mainly responsible for H2O dissociation. Hence we mainly discuss the absorption of H2O, OH and H on support. For the adsorption of H2O, CeO2-x(111) is the most favorable site for H2O adsorption, releasing 1.07 eV, and H2O will get adsorbed over oxygen vacancy. Therefore, 8


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oxygen vacancy plays an essential role in promoting H2O adsorption in Co13/CeO2-x(111) model.

Figure 3. Optimized configurations for the main species involved in ESR reactions on Co13/CeO2-x(111) model (units: eV and Å). Note: the values in parentheses in Figure 3 are the adsorption energies of species.

3.2 ESR mechanisms on Co13/CeO2-x(111) model DFT calculations are applied on ESR reactions on different sites of Co13/CeO2-x(111) model, i.e. Co0 site, Cox+ site on Co13 cluster, CeO2-x(111) surface and Co13-CeO2-x(111) interface. The overall ESR reactions are divided into three sections: water dissociation, ESR reaction and water-gas shift 9


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reactions. For the metal-oxide interface is reported to be active for recombination reactions16,39, we emphasize on the essential recombination reactions on interface. The configurations of transition states for ESR reactions on Co0 site, Cox+ site and support are displayed in Figures S1, S2 and S3 in IS. In the last section, we will through light upon the surface distribution of CeO2-x. 3.2.1 Water dissociation mechanisms Figure 4 established the most favorable route for water dissociation on Co10 cluster and support: H2O→OH+H, 2OH→H2O+O. The rate-limiting step is primary water dissociation. Apparently, either OH or O is inclined to form on CeO2-x in Co13/CeO2-x(111) model, which agrees well with experimental report that support CeO2 could activate water dissociation significantly61. The enhancement of water dissociation on support is resulted from the basicity of lattice oxygen, and the presence of oxygen vacancy. Especially, oxygen vacancy can act as active site for water dissociation, in which the resulted OH and O species will fill the oxygen vacancy to regenerate the catalyst. In this way CeO2-x surface in Co13/CeO2-x(111) model promotes water adsorption and dissociation. 3.2.2 ESR reactions mechanisms As illustrated in Figure 4, the barriers of the dehydrogenation steps on Cox+ site in Co13/CeO2-x(111) model are lower than on Co0 site, in good agreement with the observation in experiment62, i.e. the oxygenated Co promoted the dehydrogenation of ethoxide. In this case, top-sited ethanol on Co13 cluster will undergo consecutive dehydrogenation steps, yielding CH3CO on Cox+ site in Co13/CeO2-x(111) model. The CH3CHO on Cox+ site could hardly desorb as side product, owing to its high desorption energy (0.96 eV). On the other hand, OH, resulted from water dissociation on support in Co13/CeO2-x (111) model, will spread to interface, overcoming a barrier of 0.38 eV (See Figure 5a). Subsequently, the CH3CO couples with OH on interface yielding CH3COOH, taking 0.58 10


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eV, which is lower than that for C-C scission in CH3CO (1.12 eV). The resulted CH3COOH on interface will spread to Co0 site, cleaving C-C, producing trans-COOH, requiring 0.52 eV. The spread of acetic acid is feasible, due to its similar adsorption energies on Co0 and Cox+ sites in Co13/CeO2-x(111) model. The trans-COOH can generate CO2 via dehydrogenation step, taking a barrier of 1.18 eV. The detached H will couple with each other on Cox+ site, owing to a lower barrier (1.22 eV), in comparison to that on Co0 site in Co13/CeO2-x (111) model.

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Figure 4. The reaction network for ESR reactions on Co13/CeO2-x(111) model (eV). Note: the values in black shown in Figure 4 are the activation barriers of the elementary steps, and the values in blue refer to the reaction energies of the elementary steps.

3.2.3 Water-Gas shift mechanisms As established in Figure 4, the barrier for cleaving C-C in CH3CO on Co0 site in Co13/CeO2-x(111) model is relatively high. Alternatively, the CH3CO on Co0 site could spread to Cox+ site, followed by coupling with OH producing CH3COOH on Cox+ site. Nevertheless, less CO can be generated from C-C scission on Co0 site, and could hardly get oxidized by OH or O. Therefore small amount of CO could also be obtained on Co0 site in Co13/CeO2-x(111) model, in good agreement with less CO observed in experiment28. 3.2.4 The surface distribution on CeO2-x Water could get adsorbed over oxygen vacancy on surface Ov-top or subsurface Ov-sub , followed by primary O-H scission, producing hydroxyl HOtop-lattice or HOsub-lattice, filling oxygen vacancy (See Figure 5b). Afterward, the resulted neighboring OH can undergo disproportion reactions, producing water on surface (H2Otop-lattice) or subsurface (H2Osub-lattice). However, the resulted H2O could hardly desorb from surface or subsurface, due to high desorption energy. The accumulation of H2O will hinder disproportion reactions producing H2O. In this way, the surface and subsurface get hydroxylated easily. The hydroxylated CeO2-x surface is also reported by Xu et al.63 that Ce(IV) facilitates OH formation, resulting high ESR efficiency. The hydroxyl HOtop-lattice or HOsub-lattice on CeO2-x can spread to Co site, taking part in acetic acid formation, as well as the release of carbon deposition. Subsequently, oxygen vacancy will be produced again through the consumption of hydroxyl. 12


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In conclusion, acetate pathway dominates CO2 formation in Co13/CeO2-x (111) model: CH3CH2OH→CH3CH2O→CH3CHO→CH3CO (Cox+ site), the resulted CH3CO will couple with OH from water dissociation on CeO2-x, yielding acetic acid: CH3CO→CH3COOH (interface). The resulted acetic acid will spread to Co0 site, cleaving C-C: CH3COOH→CH3+trans-COOH →CH3+H+CO2 (Co0 site). The acetate pathway was proposed to be prevailing with the existence of oxidized Co site, producing CO2 64. Strong synergetic effect was found in Co13/CeO2-x(111) model: the metallic Co site is responsible for C-C scission, and the Cox+ site favors the dehydrogenation of ethanol into CH3CO and H2 formation; The support is the most favorable site for water dissociation; Meanwhile the interface promotes acetic acid formation, which was reported to be essential for ESR reaction producing CO2 64. We find that the interaction between Co and CeO2-x is stronger, indicated by higher binding energy of -8.31 eV, compared with that on Co10/CoO(100) model (-5.49 eV38). The stronger metal-oxide interaction in Co13/CeO2-x(111) model leads to the lower barrier of acetic acid formation (0.66 eV), in comparison to that on Co10/CoO(100) (0.72 eV38).

Figure 5. The favorable pathway for ESR reaction on Co13/CeO2-x(111) (a) and the spread of oxygen vacancy on support of Co13/CeO2-x model (b) (unit: eV) 13


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Note: the different color regions in Figure 5(a) refer to the different reaction sites (Co0 site, Cox+ site, Co13-CeO2-x interface and CeO2-x surface) on Co13/CeO2-x(111) model; The values in back and blue in Figure 5 refer to the activation barriers and the reaction energies of the corresponding steps, respectively. The Osub-lattice, Ov-top, Osub-lattice H, H2Osub-lattice, Ov-sub in Figure 5(b) refer to lattice oxygen on subsurface of CeO2-x, oxygen vacancy on top surface of CeO2-x, hydroxyl formed by H binding to Osub-lattice, H2O formed on subsurface of CeO 2-x, and oxygen vacancy on subsurface of CeO2-x.

3.3 Coke formation mechanism on Co13/CeO2-x(111)model It is generally well known that ethylene, acetone and hydrocarbon are the precursors for coke formation65. For the hydrocarbon, the most favorable path for C2 species on metallic Co site is CH3CH2OH→CH3CH2O→CH3CHO→CH3CO, and the resulting CH3CO could hardly undergo the subsequent dehydrogenation step, producing CH2CO, with a high barrier of 1.46 eV. Instead, CH3CO species will bind to OH, yielding acetic acid, followed by cleaving C-C to form CHx species. Hence DFT calculations are applied on the form of ethylene, acetone and CHx species on Co13/CeO2-x(111) model. As shown in Figures 4 and 6, the barrier of Cβ-H scission in CH3CH2OH on Cox+ site is slightly lower than O-H scission. However, the O-H scission is exothermic, whereas the Cβ-H scission turns to be endothermic. As a result, CH3CH2OH on Cox+ site prefers to cleave O-H rather than Cβ-H, disfavoring CH2CH2 formation. Apparently, CH accumulation on Co0 site is the main factor for deactivating catalyst in Co13/CeO2-x(111) model (See Figure 6). Accordingly, Wang et al. 66 also proposed that CHx on metallic Co site, from C-C scission of acetic acid, is the precursor of carbon deposition. We can speculate that CH accumulation will deactivate metallic Co site, at the absence of the oxidized Co site. However, in this case, the oxidized Co will release CH accumulation via the route: CH+O→CHO→CO+H, requiring mobile O. On the hydroxylated CoO2-x surface, 14


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mobile O is covered with the detached H, producing mobile OH. The mobile OH will spread to Cox+ site, taking 0.69 eV, followed by dissociation reaction into O, aiming to oxidize CH. Therefore, mobile OH, from CeO2-x in Co13/CeO2-x(111) model, will release carbon deposition on Cox+ site via oxidation reaction67. In summary, based on the calculation results above, we compare the barriers for important reactions on different oxidized Co sites, support, and metal-oxide interface (See Table 1). It is observed that the activity of the dehydrogenation into CH3CO follows the trend: Cox+>Co0>Co2+ (0Cox+>Co0. The activity for C-C scission on different sites follows: Co0>Cox+>Co2+. More oxidized Co site will contribute to lower barriers of recombination reactions, e.g. acetic acid, H2 formation and CH oxidation. H2 formation and CH oxidation take place on either Co0 site or Cox+ site, following the activity sequence: Cox+>Co0. Acetic acid formation contributes to the activity trend: Co2+>Cox+>Co0. Nevertheless, the metal-oxide interface is the most active site for acetic acid formation in Co13/CeO2-x model. Accordingly, metal-oxide has been proved essential for ESR reaction in experiment68.

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Figure 6. The reaction energy profiles for coke formation on Co13/CeO2-x(111) model.

Table 1 The activation barriers for key ESR steps on various active sites (unit: eV) Reaction steps

Co0

H2O→H+OH

1.16

CH3CO→CH3+CO CH3CO+OH→ CH3COOH CH3COOH→CH3+trans-COOH 2H→H2 CH+O→CHO

Cox+

Co2+

interface

CeO2-x

0.91

0.79

-

0.27

1.00

1.12

1.70

-

-

0.85

0.66

0.08

0.58

-

0.52

1.06

1.38

-

-

1.40

1.22

-

-

-

1.31

0.59

-

1.15

-

Note: the Co0, Cox+, interface and CeO2-x refer to the reaction sites on Co13/CeO2-x(111) model, and the Co2+ site refer to CoO(100) surface with the data derived from our previous report38 and the related reaction information is 16


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shown in Figure S5 in IS.

4. Discussion 4.1 The ensemble size effect The ensemble size refers to the number of metal atoms in ensembles, reflected by the density of metal site. Obviously, metallic cobalt site provides larger ensemble size, compared with oxidized Co site. As reported in our previous study38, dissociation reactions, e.g. C-C scission, are incline to occur on Co0 site. By comparison, recombination reactions, e.g. CH3COOH, H2 formation and CH oxidation, will proceed on Co2+ site, with lower density of Co sites. Herein we will extend the analysis into the relationship between the selectivity towards dissociation/recombination reactions and oxidation state of Co. As established in Figure 7a, the higher the oxidation state of Co site is, the lower the barrier for dissociation reactions will be, and contrarily the higher the barriers for recombination reactions turn to be. Experimentally, previous report also proposed that more reduced Co site favors C-C scission, while more oxidized Co site promotes acetic acid formation and oxidation reactions30,62,69. Dissociation reactions, relating to bond breaking, favor the electron spread, from the distribution in the breaking bond, to the interaction between reactant and surface (See Figure 7b). Hence strengthening the interaction between reactant and surface, i.e. adsorption energy of reactant, promotes dissociation reactions. Less oxidized Co site, featured with larger ensemble size of Co, favors enhancing the adsorption of reactant, which will promote bond breaking in dissociation reactions. In the case of Co13/CeO2-x model, the metallic Co site, with higher adsorption energies of adsorbates, owns lower barriers for dissociation reactions (See Figure 7c). Recombination reactions, relevant to bond formation, need more electrons to spread to the 17


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distribution in the forming bond, from the interaction between reactant and surface. More oxidized Co site, providing less ensemble size of Co, facilitates the weakening of the interaction between reactant and surface, and thus favors recombination reactions. In this case, the interface is featured with less ensemble size, in comparison to Co site, and is the most active site for acetic acid formation. In conclusion, the higher the oxidation state of the Co site is, the less ensemble size the Co site will have, accompanied with the lower adsorption energy of reactant. Thus higher activity of recombination reactions will be gained, and contrarily dissociation reactions will have lower activity.

Figure 7. The relationship between the barriers for the dissociation reactions and recombination reactions and the charge of Co site (a), the spread of electron in recombination reactions and dissociation reactions (b), and the barriers for important reactions on different sites of Co13/CeO2-x(111) model (c). Note: the Co2+ site refer to CoO(100) surface with the data derived from our previous report38 and the related reaction information is shown in Figure S5 in IS. 18


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4.2 The dehydrogenation activity As illustrated in Figure 8a, compared with Co0 site, Cox+ site in Co13/CeO2-x(111) model has lower barriers of dehydrogenation steps, and the dehydrogenation reaction is redox reaction, associated with the oxidizing ability of surface. Obviously, the oxidized Cox+ site has higher ability to accept electron, compared with Co0 site (See Figure 3). Hence Cox+ site in Co13/CeO2-x(111) model is featured with lower barriers of dehydrogenation steps. We compared the dehydrogenation steps on Co0, Cox+ and Co2+ site, and found that the barriers for the consecutive dehydrogenation steps to produce acetaldehyde decrease, along with the oxidation of Co (Figure 8c). Previous study also reported that more oxidized cobalt is active to convert ethoxide groups to acetaldehyde39-41. However, the subsequent dehydrogenation in CH3CHO on Co2+ site carries much higher barrier than Co0 and Cox+ site, rationalized from much lower adsorption energy of CH3CO on Co2+ site (Figure 8d). The activity of dehydrogenation step relates to not only the oxidizing ability of surface but also the adsorption strength of intermediates. In this case, one can infer that the dehydrogenation of ethanol into CH3CO is more favored on partially oxidized Co site, in comparison with Co0 and Co2+ site 38-40.

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Figure 8. The barriers for dehydrogenation steps on Co0 and Cox+ site (a), , the barriers of dehydrogenation steps on different Co sites (b), and the adsorption energies of carbon species on different Co sites (c). Note: We collected the barriers of the dehydrogenation steps on different Co sites in Figure 8(a), the corresponding information is listed in Table S1 in IS; and the Co2+ site refer to CoO(100) surface with the data derived from our previous report38 and the related reaction information is shown in Figure S5 in IS.

4.3 The role of mobile O As established in Figure7b, despite of the less ensemble size of Co site on interface, the oxidation of CH by mobile O occurs on Cox+ site rather than the interface in Co13/CeO2-x(111) model. Now, we are in a position to discuss the insight of the activity for CH oxidation on interface and Cox+ site. We use the energetic method, developed by Hammer70, to decompose the barrier of CH oxidation step. For the recombination reaction A+B→AB, we divide the barrier into three terms in equation (3): A B A-B Ea = ΔEads + ΔEads + ΔEint

20


(3)

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A- B where ΔE A(B) ads stands for the adsorption energy change of A (B) from IS to TS, ΔE int refers to the

change of the interaction energy between A and B from IS to TS. As illustrated in Figure 9a, the O adsorption energy change of O from IS to TS on interface, i.e. ΔEads , is more positive, responsible for

the higher barrier on interface. Furthermore, we decompose the adsorption energy change of O from IS to TS into the deforming energy items and the interaction energy item between the deformed species shown in equation (4): deform catalyst  O ΔE Oads = ΔΔE catalyst + ΔΔE Odeform + ΔE int,ads

(4)

or m def or m where ΔΔE def stand for the deforming energy change of catalyst and O, upon the catalyst and ΔΔE O

adsorption of O from IS to TS. ΔE catalyst-O is the change of the interaction energy between the int,ads deformed catalyst and O, upon the adsorption of O from IS to TS. Therefore, the whole barrier for the CH oxidation could be divided into five items in equation (5) (See Figure 9b): deform deform catalyst  O CH-O Ea = ΔE CH + ΔE int,ads + ΔE int ads + ΔΔE catalyst + ΔΔE O

(5)

or m def or m -O , ΔE catalyst-O and ΔE CH where ΔECadsH , ΔΔE def catalyst , ΔΔE O int,ads int,ads are the adsorption energy change of CH from

IS to TS, the deforming energy change of catalyst upon O adsorption from IS to TS, the deforming energy change of O upon O adsorption from IS to TS, the change of the interaction energy between the deformed catalyst and O upon O adsorption from IS to TS, and the change of the interaction energy between CH and O from IS to TS. As shown in Figure 9b, for CH oxidation by mobile O on interface, the deforming energy change orm of catalyst upon O adsorption, i.e. ΔΔE def catalyst , is more positive, leading to the higher barrier for CH

oxidation on interface. The deforming energy of catalyst for CH oxidation on interface increases more significantly from IS to TS, in comparison to that on Cox+ site, indicative of stronger distortion of catalyst for CH oxidation on interface. The stronger distortion of catalyst for CH oxidation on 21


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interface is supposed to be related to more coordinated O atoms surrounding Co. As CH oxidation on interface proceed from IS to TS, mobile O on interface will bind to Co site, changing coordination number of Co, altering the geometry of Co catalyst strongly, reflected by higher ΔΔE

def o r m catalyst

.

Meanwhile, mobile O on interface will get involved into oxidizing Co site, indicated by the higher charge of Co sites for transition state of CH oxidation on interface (Figure 9d). It could be predicted that mobile O on interface in Co13/CeO2-x(111) model is more favorable to oxidize Co site instead of CH.

Figure 9. The decomposition of barrier for the oxidation of CH by O (a and b), the charge distribution of transition state for the oxidation of CH on Cox+ site (c) and interface (d) on Co13/CeO2-x(111) model. -O -O CH - O Note: ΔECH , ECH int int(TS) , Eint(IS) are the changing interaction energy between CH and O along the reaction from IS to TS,

H H the interaction energies between CH and O in TS and IS, respectively. ΔECadsH, ΔECads(TS) , ΔECads(IS) are the difference of

O O O CH adsorption energy in TS and IS, adsorption energies of CH in TS and IS separately; ΔEads , ΔEads(TS) , ΔEads(IS) refer

22


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to the difference of O adsorption energy in TS and IS, the adsorption energies of O in TS and IS; ΔEcatalyst-O , Ecatalyst-O int,ads int,ads(TS) , Ecatalyst-O int,ads(IS) are the interaction energy change between deformed catalyst and O from IS to TS, the interaction energies rm defo rm defo rm between deformed catalyst and O in TS and IS; ΔΔEdefo catalyst , ΔEcatalyst(TS), ΔEcatalyst(IS) are the deforming energy change of

rm rm defo rm catalyst from IS to TS, the deforming energies of catalyst in TS and IS; ΔΔEdefo , ΔEdefo are the deforming O O(TS) , ΔEO(IS)

energy change of O from IS to TS, the deforming energies of O in TS and IS. The charge in Figure 9c and 9d is calculated by Bader charge analysis59,60.

4.4 The ESR mechanism on Co catalyst with Co0, Cox+ and Co2+ The oxidation of Co site may lead to the complicated proportions of catalyst, comprising Co0, Cox+, Co2+ sites, metal-oxide interface, and support. Herein we extend the analysis of ESR mechanism on Co catalyst with the presence of Co0, Cox+, Co2+ site supported on CeO2-x (Figure 10). The Cox+ site is the favorable site for CH3CO formation, and CH3CO could hardly spread to Co2+ site, owing to much lower adsorption energy of CH3CO on Co2+ site. H2O dissociation prefers to take place on CeO2-x support, followed by the spread toward Co2+ site. The Co2+ site will assist H2O dissociation to some degree via oxidative dehydrogenation step. Hence OH will accumulate on Co2+ site, agrees well with OH-covered CoO surface reported by Luo et al.71. The OH on Co2+ site will couples with CH3CO on interface between Cox+ and Co2+, yielding acetic acid. Subsequently, acetic acid will spread to Co0 site, cleaving C-C. H2 will be formed on Cox+ site.

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Figure 10. The predicted favorable ESR mechanism on Co catalyst supported on CeO2-x with the proportions of Co0, Cox+ and Co2+ sites. Note: the different color regions in Figure 10 refer to the different reaction sites (Co0 site, Cox+ site, Co2+ site, the interface between Cox+ and Co2+ site and CeO2-x surface) on Co13/CeO2-x(111) model.

The calculation of ESR reaction mechanism can help modify experimental condition, including catalyst, reaction condition, gaining high selectivity and activity of ESR reaction. ESR catalyst should contain metallic Co site and oxidized Co site, favoring acetic acid pathway. Especially, enhancing partly oxidized and metallic Co proportions will contribute to higher activity of acetate pathway through high ratio of Co0/Co2+

. Secondly, reducible support, i.e. CeO2-x, is supposed

69, 72-74

to be the proper support for ESR. Thirdly, O2 is efficient for the release of carbon deposition. Besides, enhancing metal-oxide interaction helps promote the spread of O and OH.

4. Conclusions The mechanisms of ESR reactions on Co13/CeO2-x model have been explored by DFT in detail. The conclusions are summarized: (1) On Co13/CeO2-x model, main product CO2 is produced through acetate pathway: CH3CH2OH→ CH3CH2O→CH3CHO→CH3CO (Cox+ site). OH, produced from water dissociation on CeO2-x, spread to metal-oxide interface, coupling with CH3CO: CH3CO→CH3COOH (interface), 24


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CH3COOH spreads to Co0 site, cleaving C-C: CH3COOH→CH3+trans-COOH→CH3+CO2+H (Co0 site). H2 forms on Cox+ site. (2) The acetate pathway requires the involvement of Co0 site, Cox+ site and reducible support. The Co0 site is responsible for C-C scission, owing to large ensemble size of Co site. Cox+ site, with less ensemble size, has higher activity for recombination reactions, e.g. H2 formation, CH oxidation. Furthermore, Cox+ site owns higher oxidizing ability, favoring ethanol dehydrogenation into CH3CO. The CoO2-x support promotes water dissociation via oxygen vacancy and lattice oxygen. The interface between Co site and support, with less ensemble size, enhances acetic acid formation. (3) The coke formation is mainly caused by CH accumulation on Co0 site, and could be released by oxidation reaction on Cox+ site. On the hydroxylated support surface, mobile OH is mainly responsible for the release of carbon deposition. Adding O2 can depress carbon deposition efficiently. (4) The oxygen vacancy can form on surface or subsurface. However, the surface could get hydroxylated easily due to high desorption energy of H2O. On the hydroxylated surface, mobile OH is mainly responsible for the release of coke formation. The mobile O on support has a higher tendency of oxidizing Co site. (5) The oxidation state of Co determines the activity for ESR reaction. The Co oxidation state, relating to ensemble size of Co site, affects the activity towards dissociation/recombination reactions. The activity of acetic acid formation follows the trend: metal-oxide>Co2+>Cox+>Co0. H2 forms on Co site, and the activity is supposed to be Cox+>Co0. CH prefers to form on Co0 or Cox+ site, and could be released by oxidation reaction. The activity sequence for CH oxidation is supposed to be Cox+>metal-oxide interface>Co0. Contrarily, the activity trend for C-C scission is Co0>Cox+>Co2+. The oxidizing ability of surface, indicated by electron-accepting ability, determines the activity of 25


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dehydrogenation together with adsorption strength. The activity for the dehydrogenation into CH3CO is supposed to be Cox+>Co0>Co2+, while that for water dissociation is CeO2-x>Co2+>Cox+>Co0. (6) In experiment, measures should be taken to ensure the acetate pathway through: enhancing Co0/Co2+ ratio to ensure high proportions of Co0 and Cox+ site, using CeO2-x support with higher oxygen mobility (redox property), strengthening metal-support interaction, and promoting metal-oxide interface formation. Adding O2 can assist depress carbon deposition.

Supporting Information The calculated TS structures for ESR reactions on Co0, Cox+, metal-oxide interface and support in Co13/CeO2-x model; The calculated TS structures for coke formation on Co13/CeO2-x model; The calculated TS structures for ESR reactions on CoO(100) surface; The activation barriers of dehydrogenation steps on Co0 and Cox+ sites in Co13/CeO2-x(111) model.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21773123, 21421001, 91545106), the 111 project (B12015), the foundation of State Key Laboratory of Coal Conversion (Grant No. J17-18-908), and the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No.U1501501.

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CeO2-x: A DFT

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