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Reaction Mechanism of Ethanol on Model Cobalt Catalysts: DFT Calculations Meng-Ru Li,† Jun Chen,† and Gui-Chang Wang*,†,‡ †

Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, P. R. China ‡ State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P. R. China S Supporting Information *

ABSTRACT: In the present work, the density functional theory calculations analysis are performed to study the reaction mechanisms and catalytic activity of ethanol reactions over Co0, Co2+, and Co3+ sites. Adsorption situations and the reaction cycles for ethanol reactions on cobalt catalysts were clarified. The mechanisms include the dehydrogenation steps of ethanol and the C−C cleavage step. The present calculation results show that the mechanism of ethanol reaction on Co0 site is CH3CH2OH → CH3CH2O → CH3CHO → CH3CO → CH3+CO, and the final products are CO and H2. H2 is formed by the combination of adsorbed H species. On Co2+ site, the mechanism is CH3CH2OH → CH3CH2O → CH3CHO, and the main final product is CH3CHO species. On Co3+ site, the mechanism is CH3CH2OH → CH3CH2O → CH3CHO → CH2CHO → CH2CO → CHCO → CCO → COCO → CO → CO2, and the final products are CO2 and H2O. The rate-limiting step on Co0, Co2+, and Co3+ sites is the form of CH3CHO species. The possible reasons for the different catalytic activities may be the following two facts: First, Co3+ sites density in Co3O4 (110)-A is larger than that of Co2+ and tends to break the C−C bond to produce CO; second, Co3+ binds more oxygen atoms that the further oxidation of ethanol requires, which leads to the full oxidation of ethanol to CO2 on Co3+ sites. The present result may help people to design an ESR (ethanol stream reaction) catalyst by controlling its oxidation state, and the catalyst with modest oxidation state is benefit for the H2 formation. The proper catalyst should own the ability to break C−C to form CO but avoid the full oxidation of CO into CO2 which is needed to react with H2O in the water−gas shift reaction generating CO2 and H2. byproducts.27 The supports (CeO2, ZnO, MgO, Al2O3, zeolitesY, TiO2, SiO2, La2O2CO3, and CeO2−ZrO2) play a vital role in the catalytic activity, selectivity, and carbon deposition.28 Especially the Co/CeO229 catalysts doped with CaO have been proven to gain improved catalytic performance with the coexistence of CoO and metallic Co proportions, indicating that different oxidized and metallic cobalt could affect the ESR performance. It is obvious that different oxidized Co shows different reactivity toward ESR. Although the ESR mechanism has been theoretically studied on Co(0001),30,31 the catalytic activity for ESR on Co0, Co2+, and Co3+ sites is still unclear. As the mechanism of ethanol decomposition is similar to that of ESR involving the dehydrogenation forming CH3CHO species and the C−C cleavage,32 we will take ethanol decomposition as a probe reaction for ESR in our present theoretical work. Theoretically, numerous studies on the mechanisms of ethanol decomposition/oxidation on metals33,34 and metallic oxides35−37 have been examined by density

1. INTRODUCTION Both rising global energy demands and serious environmental problems call for the replacement of petroleum-based fuels with alternative fuels. Hydrogen, a promising environmentally friendly energy source, has attracted much attention.1 To date, ethanol steam reforming (ESR) stands out among numerous hydrogen production technologies as the steam reforming of methane,2−8 the steam reforming of methanol9−11 for high selectivity for H2.12 The renewable fermentation of the biomass synthetic route, the advantage in storage facilities, handling, and the transport safety, as well as the feasible H2 production applying in the fuel cell makes ESR more efficient in the application in the direct biorenewable resources and the fuel cell as the storage medium.13,14 The strongly endothermic ESR calls for a continuous heat supply at low temperature in production. The noble metal catalysts (Pt,15−17 Pd,18,19 Rh,20−22 Ir,23−25 and Ru26) have shown catalytic activity toward ESR reaction. However, the low H2 yield at low temperature, high methane selectivity, and high cost has limited the application of noble metal based catalysts. On the other hand, non-noble Co-based catalysts could obtain the low propensity to catalyze carbon deposition with low selectivity of © XXXX American Chemical Society

Received: April 21, 2016 Revised: June 1, 2016

A

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were allowed to relax to optimize the structures. The experimental Co3O4 lattice constant was applied (8.154 Å).47 The Coulomb (U) and exchange (J) interaction was taken into consideration due to the highly correlated electronic states of the cobalt oxide. The GGA+U48,49 method was used to recover the effect of 3d electron correlation on 3d transition metal oxide with the set U-J = 3.5 eV.38,39 The detailed calculation on how to choose of magnitude of effect (U-J) can be found in Table S1 in the Supporting Information. The d-band center model50−52 was applied to analyze the electronic structure of Co2+ and Co3+ sites. In the model, the location of the occupied d-band center was calculated by eq 1:

functional theory (DFT) calculations. It has been experimentally reported that ethanol undergoes decarbonylation forming CO and H2 on the clean Co foil, and with the exposure of O2, the Co0 site was oxidized into Co2+ and Co3+ sites. The acetaldehyde forms through sequential dehydrogenation steps on the Co2+ site, and on the more highly oxidized Co3+ site, ethanol would be converted fully into CO2 and H2O.32 To give a detailed exploration on the reactivity of ethanol decomposition on Co-based catalysts, a systematic DFT calculation of ethanol reaction on different Co sites like Co0, Co2+, and Co3+ was performed in this work. Herein the Co (0001) surface is utilized to simulate the reaction on Co0 site, and the Co3O4 (110)-A surface is used to simulate Co2+ and Co3+ sites since the Co3O4 (110)-A surface contains both Co2+ and Co3+ sites.38,39 In this paper, DFT study provides insight into the decarbonylation and decomposition reactions on Co0, Co2+, and Co3+ sites, which helps get a better understanding of the activity on different Co oxidation states on the Co-based catalysts.

E

εdc =

∫−∞f Eρd (E) dE E

∫−∞f ρd (E) dE

(1)

where ρd is the projected density of states (PDOS) of the dband of surface atoms at E and Ef is the Fermi level energy. To calculate PDOS, the fine k-points of 7 × 7 × 1 were used.

2. CALCULATION METHODS AND MODELS 2.1. Methods. The Vienna ab initio simulation package (VASP)40−42 was applied to investigate the ethanol reaction on the Co0, Co2+, and Co3+ sites by the self-consistent periodical DFT calculations with the projected augmented wave (PAW)43 pseudopotentials. All of the electronic structures were calculated using the Perdew−Burke−Ernzerhof (PBE)44 form of the generalized gradient approximation (GGA) expended in a plane wave basis with kinetic cutoff energy of 400 eV. The climbing image general nudged elastic band (CI-NEB)45 method was employed to locate the transition states (TSs). Spin polarization was included in the calculations. 2.2. Models. The Co (0001) surface containing the Co0 site was modeled by the p (3 × 3) unit cell of four layers of which the uppermost two layers were relaxed. The vacuum space of 15 Å was applied in the case of the spurious interactions normal to the surface. The 3 × 3 × 1 Monkhorst−Pack k-point mesh was used in the surface Brillouin zone.46 The adsorption energy (Eads), activation energy (Ea), and reaction energies (ΔE) were calculated by the following three formulas: Eads = EA/M − EA − EM, Ea = ETS − EIS, and ΔE = 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. The Co3O4 (110)-A surface shown in Figure 1 containing Co2+ and Co3+ sites was modeled by the p (2 × 1) unit cell of six layers that were separated by a 15 Å vacuum with the 1 × 3 × 1 Monkhorst−Pack k-point mesh. The uppermost four layers

3. RESULTS AND DISCUSSION 3.1. Adsorption of Pertinent Species. The adsorption configurations and energies for the main species involved in the ethanol reaction on the Co (0001) and Co3O4 (110)-A surfaces were investigated in detail using DFT method listed in Tables 1 and 2, respectively, and the corresponding configurations on Co0, Co2+, and Co3+ sites were displayed in Figure S2 and Figure S3, respectively, in the Supporting Information. 3.1.1. Ethanol. On the Co0 site, ethanol molecule tends to lie onto the top site of Co with the adsorption energy of −0.30 eV (d(C−Co) = 2.20 Å) similar to the previous results (−0.3631 and −0.3230 eV). On the Co2+ site, ethanol is adsorbed at the Co top site with the Co−O binding length of 2.15 Å. The hydroxyl in ethanol orientates to lattice oxygen O3f on the surface with the H−O3f of 1.85 Å contributing to a hydrogen bond between the H in hydroxyl and the O3f on the surface. The adsorption energy of ethanol on the Co2+ site is −1.08 eV. On the Co3+ site, ethanol interacts stronger with the Co reflected by a shorter Co−O length (2.09 Å), and the hydrogen bond turns weaker, shown in longer distance of H− O3f (2.05 Å). The adsorption energy is −1.06 eV on Co3+ and comparable to that on the Co2+ site. Compared to the adsorption of ethanol on the Co0 site, the hydrogen bond between the H in hydroxyl and O3f strengthens the affinity of ethanol on Co3+ and Co2+ sites. 3.1.2. Ethoxy. On the Co0 site, ethoxy binds to the surface at the hcp site (d(C−Co) = 2.00 Å). The O−C is almost perpendicular to the surface leaving the H(Ca) far away from the surface, which increases the barrier of the breaking of H− Ca. The hcp configuration yields higher stability with the affinity of −2.80 eV similar to the previous report (−2.86 eV30) due to more neighboring Co atoms to interact with. On the Co2+ site, the bond length of O−Co is 1.81 Å with an adsorption energy of −2.57 eV in the top-sited geometry. Ethoxy on the Co3+ site binds strongly with the bridge site of Co3+ with bond length of 1.94 Å. The adsorption energy is as high as −3.30 eV. 3.1.3. Acetaldehyde. On the Co0 site, the CH3CHO species is absorbed in a η2(O)-η1(Ca) mode. The O locates at the bridge site of Co with the length of O−Co being 2.03 Å. Meanwhile, Ca binds to the top site of Co (d(C−Co) = 2.00 Å). The adsorption energy is −0.33 eV in accordance with the

Figure 1. Structure of the Co3O4 (110)-A surface. O3f is the threecoordinate oxygen atom B

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Table 1. Adsorption Energies and Geometric Parameters for Various Pertinent Species in the Ethanol Reaction on Co (0001) species on Co

adsorption configuration

Eads(eV)

dCo−O (Å)

CH3CH2OH CH3CH2O CH3CHO CH3CO CH3COOH cis-COOH trans-COOH CO CO2 CH3 CH2 CH C

top through O hcp through O bridge through O and top through Ca top through O and top through Ca top through O and top through Ca top through O and top through C top through O and top through C top through C top through O fcc through C fcc through C hcp through C hcp through C

−0.30 −2.80 −0.33 −2.00 −0.08 −0.81 −0.74 −1.64 0.05 −1.95 −3.94 −6.32 −6.72

2.20 2.00 2.03/2.06 2.03 2.01 1.88 1.89

dCo−C (Å)

2.00 1.87 1.98 2.07 2.06 1.81

3.30 2.15 1.95/1.95/2.00 1.86/1.88/1.88 1.79/1.79/1.80

Table 2. Adsorption Energies and Geometric Parameters for Various Pertinent Species in the Ethanol Reaction on Co3O4 (110)-A species on Co3+ site

adsorption configuration

CH3CH2OH CH3CH2O CH3CHO CH2CHO CH3CO CH2CO CO CO2 CH3 CH2 CH C species on Co2+ site

top through O bridge through O top through O top through O and top through Cβ top through O and top through Ca top through O and top through Ca top through C top through O top through C bridge through C fcc through C fcc through C adsorption configuration

CH3CH2OH CH3CH2O CH3CHO CH2CHO CH3CO CH2CO CO CO2 CH3 CH2 C

Eads (eV) −1.06 −3.30 −0.69 −1.45 −1.83 −1.08 −0.81 −0.05 −1.45 −3.64 −4.04 −3.88 Eads(eV)

top through O top through O top through Oa top through O and top through Cβ top through O and bridge through Ca top through O top through C top through O top through C bridge through C bridge through C

−1.08 −2.57 −0.50 −1.28 −1.26 −2.18 −0.81 −0.17 −1.18 −1.48 −3.33

dCo−O (Å) 2.09 1.94/1.95 2.13 1.91 2.10 1.97

dCo−O (Å) 2.15 1.81 2.07 1.95 2.11 2.25

dCo−C (Å)

2.24 1.94 2.02 1.81 2.53 1.96 1.90/1.96 1.91/1.93/1.94 1.86/1.87/2.20 dCo−C (Å)

2.21 1.92 1.95 2.29 1.95 2.04/2.04 1.77/1.78

distance of 2.11 Å giving less adsorption energy of −1.26 eV. On the Co3+ site, the binding lengths of the Co−O and Co−Ca are 2.10 and 1.94 Å, respectively. The adsorption on the Co3+ site is stronger than that on the Co2+ site with a higher binding energy of −1.83 eV. 3.1.5. CH2CHO. On the Co2+ site, the CH2CHO species would locate in the η1(O)-η1(Cβ) mode. The lengths of O− Co2+ and Cβ-Co3+ are 1.95 and 2.21 Å separately. The binding affinity is −1.28 eV. On the Co3+ site, the CH2CHO species absorbed at the top site through O and Cβ (d(O−Co) = 1.91 Å, d(Cβ-Co) = 2.24 Å). The CH2CHO species is parallel to the surface plane leading to several binding sites of the surface reflected by the large adsorption energy of −1.45 eV higher than that on Co2+ site. 3.1.6. CH2CO. On the Co2+ site, O is the binding site to top Co (d(O−Co) = 2.25 Å) in the CH2CO species. The H in the CH2CO species interacts with the O3f through a hydrogen

previous study (−0.36 eV30). On the Co2+ site, the product acetaldehyde tends to adsorb in the top-O mode with the length of Co−O 2.07 Å. The adsorption energy is −0.50 eV. On Co3+ site the acetaldehyde binds to the surface in the top mode in which the length of Co−O is 2.13 Å and exothermic by 0.69 eV. The CH3CHO on Co3+ site is more stable than that on the Co2+ site, indicating that O interacts stronger with Co3+ than Co2+. Compared with the CH3CHO species on the Co0 site, the CH3CHO species on the Co2+ and Co3+ sites is a more stable precursor for further dehydrogenation. 3.1.4. CH3CO. On the Co0 site, the CH3CO species is produced in a η1(O)-η1(Ca) mode. The adsorption energy is −2.00 eV and is in agreement with the previous calculation result (−2.07 eV30), and the lengths of the O−Co and Ca−Co are 2.03 and 1.87 Å, respectively. On the Co2+ site, Ca in the CH3CO species bestrides the top site of Co2+ with the length of 1.92 Å while O points to the Co2+ atom on the surface with the C

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Figure 2. Calculated reaction networks and energy barriers (eV) on Co0, Co2+, and Co3+ sites. (The favorable paths are given in bold.)

site with the length of Co−C 2.15 Å. The stronger adsorption is reflected by the adsorption energy of −1.95 eV which is in good agreement with the previous study (−2.04 eV30). On the Co2+ and Co3+ sites, the top adsorption geometry yields adsorption energies of −1.18 and −1.45 eV with the C−Co lengths of 1.95 and 1.96 Å, respectively. 3.1.10. CH2. On the Co0 site, the CH2 species binds to the surface at the fcc site of Co. Compared with the CH3 species, the CH2 species is more stable with shorter lengths of 1.95, 1.95, and 2.00 Å with a rather high adsorption energy of −3.94 eV similar to the previous investigation (−4.12 eV31). On the Co2+ and Co3+ sites, the CH2 species is favored at the bridge site of Co (d(C−Co2+) = 2.04 Å, d(C−Co3+) = 1.96 and 1.90 Å). The adsorption energy on Co3+ is higher than that on the Co2+ site (−3.64 vs −1.48 eV). 3.1.11. CH. On the Co0 site, the hcp site of Co is the main adsorption site for CH species with the adsorption energy of −6.32 eV. A similar adsorption energy of −6.39 eV is found in a previous study.31 The binding lengths of C−Co are 1.86, 1.88, and 1.88 Å, respectively. The CH species on the Co3+ site is adsorbed occupying the fcc site of the Co with an adsorption energy of −4.04 eV (d(C−Co3+) = 1.91 and 1.93 Å and d(C− Co2+) = 1.94 Å).

bond with a distance H−O 3f of 1.99 Å. Hence, the configuration on the Co2+ site exhibits high stability with the adsorption energy of −2.18 eV. On the Co3+ site, the forming CH2CO species is adsorbed in the η1(O)-η1(Ca) mode with an adsorption energy of −1.08 eV. The lengths of O−Co and Ca− Co are 1.97 and 2.02 Å, respectively. 3.1.7. CO. On the Co0 site, the CO species is stably located at the hcp site through C with the adsorption energy of −1.64 eV. On Co3+ and Co2+ sites, CO holds almost the same adsorption energy of −0.81 eV. The lengths of Co−C on Co3+ and Co2+ sites are 1.81 and 1.95 Å. It is concluded that the CO on the Co0 site is more stable than that on the Co2+ and Co3+ sites and could hardly desorb from the surface. 3.1.8. CO2. On the Co0 site, the CO2 species takes preferable place over the top site of Co. The top-sited CO2 species vertical to surface could hardly be absorbed (d(O−Co)= 3.30 Å). The adsorption energy is 0.05 eV in accordance with the previous DFT result (−0.04 eV30). On the Co2+ site, CO2 would sit at the top site of Co2+ with the distance of 2.29 Å and adsorption energy of −0.17 eV. The forming CO2 at the Co3+ site could hardly bind to the Co3+ at the top site with the adsorption energy of −0.05 eV and the Co−O length of 2.53 Å. 3.1.9. CH3. On the Co0 site, the CH3 species is absorbed at the 3-fold fcc site of Co rather than the top site or the bridge D

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The Journal of Physical Chemistry C 3.1.12. C. On the Co0 site, C would site at the hcp site with the lengths of 1.79 Å. The significant stable C species would bind to the surface exothermically by 6.72 eV. On Co2+ site C could locate on the bridge site of Co2+ with the length Co−C of 1.77 Å. The adsorption energy is −3.33 eV. On Co3+ site, C species is adsorbed at the fcc site of Co. The adsorption energy is −3.88 eV giving the binding lengths of C−Co3+ 1.86 Å and the binding length of C−Co2+ 2.20 Å. From the above calculation results, we find that the oxygencontaining species adsorbed through O−Co binds strongly with the Co3+ site, followed by Co2+ and Co0 sites in general, so it is necessary to analyze the possible reasons behind such behavor. It is well-known that the d-band center is a good descriptor to measure the binding strength of the adsorbate. As the metallic properties are much different from the metal oxide due to the existence of the lattice oxygen, so only the Co2+ and Co3+ were taken into account. The d-band center is strongly dependent on the local adsorption configuration, and the best case is in a top adsorption mode. Since acetaldehyde species was adsorbed on Co3+ and Co2+ in the atop site through its oxygen atom, its adsorption properties were explored based on the d-band center model (see Figure S1). The d-band center of Co3+ is −4.27 eV closer to the Fermi level energy than that for Co2+(−4.94 eV), indicating that Co3+ is more active than Co2+ and interacts stronger with carbon species. This is consistent with the fact that acetaldehyde is adsorbed with a higher adsorption energy on the Co3+ site than the Co2+ site. 3.2. Ethanol Reaction Mechanisms on Co0, Co2+, and Co3+ Sites. 3.2.1. Ethanol Reaction Mechanisms on the Co0 Site. The ethanol on Co mainly gains CO and H2 as products rather than H2O and CO2.32 We have investigated the mechanisms on Co (0001) consisting of elementary steps listed in Table S2 in the Supporting Information. Figure 2 presents the ethanol reaction networks on Co0, Co2+, and Co3+ sites. The energy profile and the main configurations for the transition states are displayed in Figure S4 and Figure S5 in the Supporting Information. 3.2.1.1. Bond Scission in CH3CH2OH(A-1) Species. The CH3CH2OH species A-1 initially undergoes the O−H cleavage rather than Cα-H and Cβ-H cleavages,53 CH3CH2OH → CH3CH2O + H, overcoming a barrier of 0.79 eV (A-TS1). The value of the barrier is approximate to the previous study (0.77 eV31).The length of breaking O−H is 1.40 Å at TS. Then the resulting top-sited CH3CH2O species tends to migrate to the neighboring hcp site and the dropped H will locate at the fcc site at FS. The CH3CH2O species A-2 is formed exothermic by 0.62 eV. 3.2.1.2. Bond Scission in CH3CH2O(A-2) Species. The stable CH3CH2O species A-2 on the hcp site could undergo the Cα-H cleavage yielding the CH3CHO species A-3 with a high barrier of 1.20 eV (A-TS2) and endothermic by 0.71 eV. The initial CH3CH2O species located at the hcp site will approach to the less stable configuration in which O is adsorbed at the bridge site while the breaking H orientates to the top Co site (d(O− Co) = 1.96 and 1.98 Å and d(H−Co) = 2.14 Å), and the adsorption energy is decreased by 0.41 eV. The length of the breaking Cα-H is elongated from 1.14 Å in IS to 1.71 Å in TS, and the lower barrier of 0.78 eV corresponding to the less unstable IS is found in line with the previous calculation result (0.82 eV31). The detached H migrates to the neighboring hcp site at FS. The formed CH3CHO species may desorb from the surface since its relatively weak adsorption (the adsorption energy is 0.33 eV as seen in Table 1).

3.2.1.3. Bond Scission in CH3CHO(A-3) Species. CH3CHO could undergo the direct dissociation pathway or the H-assisted C−C cleavage: (i) CH3CHO → CH3CO + H,CH3CO → CH3 + CO; (ii) H(1) + CH3CHO→ CH4 + CO + H(2). For path (i), the Cα-H scission runs via transition state A-TS3 to form CH3CO A-4 species with the energy barrier of 0.15 eV and exothermic by 0.31 eV. The calculated barrier is found to be the same as the previous report.31 Then the resulting CH3CO gets dissociated into CH3 and CO A-5. The length of C−C is elongated to 1.86 Å at A-TS4. The barrier is computed to be 1.02 eV which is in good agreement with the reported result (1.05 eV54). For path (ii), as one H on surface attacks Cβ, the H(Cα) will get detached meanwhile the length of C−C is elongated to 2.71 Å at TS. The produced CH4 will desorb from surface and the resulting CO species is adsorbed through C yielding B-4. As displayed in Figure 2, the barrier of dehydrogenation in CH3CHO species in path (i) is much lower than the H-assisted C−C cleavage in path (ii). Hence, path (i) is favored over path (ii). 3.2.1.4. Bond Scission in CH3(A-5) Species. The resulting CH3 species A-5 could undergo either the C−H cleavage or the form of CH4 species. The activation energies of the C−H cleavage and the form of CH4 species are 0.77 and 0.99 eV respectively. The barrier of C−H cleavage is agreement with the DFT results (0.67 eV31). The CH3 and H species will locate at hcp sites stably, and H then could approach the fcc site neighboring CH3 species in the less stable configuration. The less stable configuration takes a barrier of 0.57 eV which consists with the reported results (0.64 eV31). However, the barrier of the form of CH4 is supposed to reach 0.99 eV corresponding to the most stable configuration. In conclusion, the CH3 species prefers the following dehydrogenation yielding the CH2 species A-6. The DFT result is in agreement with the small amount of CH4 detected in experiment and the reported DFT calculation results that CH3 species tends to remove H yielding CH2 species.31,32 The length of breaking C−H in CH3 species increases to 1.64 Å in TS. Then the detached H reaches the hcp site at FS. 3.2.1.5. Bond Scission in CH2(A-6) Species. The H in CH2 species A-6 tends to be detached from CH2 species due to its relatively long distance of 1.15 Å at IS, indicating the lower barrier of the C−H cleavage. In fact, the calculated barrier of the C−H scission is as low as 0.29 eV. The length of C−H in CH2 species is elongated to 1.65 Å in TS. The resulting CH species in A-7 locates at the hcp site. 3.2.1.6. Bond Scission in CH(A-7) Species. The C−H in CH species A-7 is almost vertical to the surface contributing to the higher barrier of 1.28 eV (A-TS7) to break the C−H bond. The barrier is somewhat higher than the previously reported result (1.03 eV31). This is caused by the different calculation models. As the Co catalyst model is modified into five layers from four layers, the barrier could reach 1.08 eV approximate to the reported value (1.03 eV31). For IS (CH species), the H orientates far away from the surface. The length of the breaking C−H is found to be 1.46 Å in TS. Then the H atom diffuses to the neighboring hcp site with the remaining C atom deposited at hcp site at FS. Moreover, the resulting C could bind to residue O on Co surface forming CO A-8 taking a barrier of 1.31 eV (A-TS8) as the high-temperature CO product in experiment.32 3.2.1.7. H2 Formation. The detached H could diffuse from one fcc site to another fcc site with a barrier of 0.16 eV and the diffusion is almost thermal-neutral with ΔE equal to 0.04 eV. E

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The Journal of Physical Chemistry C

Figure 3. BEP plots of Ea against ΔE for all the elementary steps in C−H and C−C cleavages on Co2+ and Co3+ sites.

CH3CH2O species C-2 shown in Figure 4 is adsorbed stably onto the top site of Co2+ site releasing energy of 0.18 eV. 3.2.2.2. Bond Scission in CH3CH2O(C-2) Species. The likely channels for the bond scissions in CH3CH2O species are Cα-H and Cβ-H cleavages. As O3f is sited by H species via the first dehydrogenation step, the mechanisms for the dehydrogenization of CH3CH2O (C-2) species involves: the leaving H could approach to O3f to form H(O3f); the leaving H could also bind to H(O3f) generating H2(O3f). That is, both O3f or H(O3f) can act as the oxidant. The resulting CH3CH2O species C-2 could undergo the Cα-H and Cβ-H cleavages via four reaction routes: (i) CH3CH2O + O3f → CH3CHO + H(O3f); (ii) CH3CH2O + H(O3f) → CH3CHO + H2(O3f); (iii) CH3CH2O + O3f → CH2CH2O + H(O3f); (iv) CH3CH2O + H(O3f) → CH2CH2O + H2(O3f). The O3f-assisted Cα-H cleavage (path (i) carries a lower barrier of 0.41 eV than that for HO3f-assisted Cα-H cleavage (2.19 eV), O3f-assisted (1.63 eV), and HO3fassisted Cβ-H cleavages (2.38 eV). Furthermore, the CH3CHO species is produced exothermic by 0.77 eV in path (i). The CαH scission produces CH3CHO species C-3 adsorbed in a η1(O)-η1(Cα) mode with the adsorption energy of 0.50 eV. The length of the breaking Cα-H bond is 1.24 Å and the distance of the detached H and O3f is shortened to 1.49 Å in TS (the distance of H(Cα)-O3f is 2.62 Å at IS). 3.2.2.3. Bond Scission in CH3CHO(C-3) Species. There are four possible routes following the remaining CH3CHO species C-3 on Co2+ site: (i) CH3CHO → CH3CO + H(O3f); (ii) CH3CHO + H(O3f) → CH3CO + H2(O3f); (iii) CH3CHO → CH2CHO + H(O3f) ; (iv) CH3CHO + H(O3f) → CH2CHO + H2(O3f). There is no barrier in the path (iii). Clearly, the Oassisted Cβ-H cleavage in path (iii) is the preferred mechanism. At IS the distance of H(Cβ)-O3f on surface is 2.83 Å, suggesting that relatively strong interaction lowers the barrier. Then the length of the breaking Cβ-H get elongated to 1.28 Å and the detached H approaches to O3f with a distance of 1.39 Å at TS. With the detached H binding to O3f, Cβ locates onto the neighboring Co2+ atom yielding CH2CHO and H(O3f) as C-4 exothermic by 0.44 eV. The desorption energy is approximate to 0 eV much lower than the adsorption energy, indicating that the H on surface could assist desorption of CH3CHO. 3.2.2.4. Bond Scission in CH2CHO(C-4) Species. The CH2CHO species C-4 can break its C−H bond via the following paths: (i) CH2CHO → CH2CO + H(O3f); (ii) CH2CHO + H(O3f) → CH2CO + H2(O3f); (iii) CH2CHO → CHCHO + H(O3f); (iv) CH2CHO + H(O3f) → CHCHO + H2(O3f). The CH2CHO species prefers the H(O3f)-assisted CβH cleavage in path (iv) possessing a lower barrier of 0.99 eV together with reaction energy of 0.56 eV. In IS the CH2CHO species is adsorbed in η1(O)-η1(Cβ) mode. At TS the distance

H2 forms with a barrier of 1.36 eV and the combination of H is endothermic by 0.69 eV. The length of the forming H−H is 0.88 Å at TS. Two mechanisms with similar barriers for the low-temperature CO formation on Co0 sites were proposed: (i) CH3CH2OH → CH3CH2O → CH3CHO → CH3CO → CH3+CO (Path A); (ii) CH3CH2OH → CH3CH2O → CH3CHO → CH4+CO (Path B). However, Path A is favored over Path B. This calculated reaction mechanism is in agreement with that proposed before.31 The rate-limiting step on Co0 site is found to be the form of CH3CHO resulting CO. 3.2.2. Ethanol Reaction Mechanisms on Co2+ Site. Exposing the clean Co foil to O2 gains the oxidized Co surfaces (Co2+ and Co3+ sites) in experiment.32 The reaction could start either on Co3+ or Co2+ sites due to similar adsorption energies of ethanol. The experimental results show that on the Co3+ site ethanol mainly undergoes the complete oxidation into CO2, CO, and H2O, and on the Co2+ site the major product is acetaldehyde.32 The DFT investigation on the different mechanisms on Co2+ and Co3+ sites was discussed below. For products are CH3CHO, CO2, and H2O on Co2+ and Co3+ sites, the dehydrogenation steps and C−C cleavage are the prime components in the pathway. In this paper, we will throw light on the mechanisms of dehydrogenation and C−C cleavage. Early theoretical calculations have shown that C−C cleavage occurs in the CHxCO species.31,55,56 As a result, we emphasized the C−C cleavage in CHxCO species in the present work. The possible reaction networks on Co2+ and Co3+ sites were illustrated in Figure 2. The barriers and the reaction energies for elementary reactions on Co2+ and Co3+ sites are listed in Table S3 in the Supporting Information. We first discussed the mechanisms on Co2+ site below. The energy profiles and the corresponding key configurations main path are shown in Figure S6. The structures for the side reactions were presented in Figure S7 in the Supporting Information. 3.2.2.1. Bond Scission in CH3CH2OH(C-1) Species. The initial possible bond scission involves the cleavages of O−H, Cα-H, and Cβ-H: (i) CH3CH2OH+O3f → CH3CHOH + H(O3f); (ii) CH3CH2OH + O3f → CH2CH2OH + H(O3f); (iii) CH3CH2OH + O3f → CH3CH2O + H(O3f). The Cα-H and CβH cleavages need to overcome barriers of 1.68 and 1.33 eV separately. The O−H cleavage is the most favorable path owing to the lowest barrier of 0.26 eV. The hydrogen bond lying in H(O) and O3f has elongated the breaking O−H from 0.98 Å in gas to 0.99 Å in ethanol adsorbed on Co2+ site, which lowers the barrier of the O−H cleavage. The length of the breaking O−H is calculated to be 1.50 Å at TS and the distance of the forming H−O3f is decreased to 1.07 Å. Then the forming F

DOI: 10.1021/acs.jpcc.6b04036 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C of H(Cβ) and HO3f on surface is shortened to 1.22 Å from the primary 2.64 Å and the length of breaking Cβ-H increases to 1.41 Å. Afterward, the forming H2(O3f) is apt to locate onto the top site of the neighboring Co with the resulting CHCHO species adsorbed in η1(O)-η1(Cβ) mode as FS C-5. The H2(O3f) needs additional energy of 0.53 eV to desorb from the surface yielding oxygen vacancy O3v in C-6. 3.2.2.5. Bond Scission in CHCHO(C-6) Species. The CHCHO C-6 is adsorbed through Cβ and O. The possible mechanisms for the CHCHO species C-7 were examined: (i) CHCHO → CHCO + H(O3f); (ii) CHCHO + H(O3f) → CHCO + H2(O3f); (iii) CHCHO → CCHO + H(O3f); (iv) CHCHO + H(O3f) → CCHO + H2(O3f); (v) CHCHO → CHCH + O. The barriers for the pathways above are 1.99, 2.95, 2.09, and 1.88 eV and more than 3 eV. Evidently, the CHCHO species disables to be further converted to CO2. 3.2.2.6. Formation of CO2. Another alternative path for the form of CO2 following CH2CHO species is CH3CHO → CH3CO → CH2CO → CHCO → CCO → COCO → CO → CO2 shown in Figure 3. The structures are listed in Figure S6 in the Supporting Information. However, the barrier of C−C cleavage in COCO has limited the form of CO2 for a high barrier of 0.98 eV. Therefore, CO2 could hardly form in low temperature on the Co2+ site and tends to generate at high temperature. In conclusion, the main product is the CH3CHO rather than CO2 due to the high barrier of the dehydrogenation in CH2CHO species. As a result, the CH3CHO is proposed to be formed mainly on Co2+ sites at low temperature: CH3CH2OH → CH3CH2O → CH3CHO. After desorption of CH3CHO, the remaining H binding to O3f could be removed from the surface by 2HO3f → H2O3f + O3f. The H on one O3f migrates to the neighboring HO3f forming H2(O3f) with a barrier of 0.63 eV and the reaction energy is −0.22 eV. The resulting water takes 0.84 eV to desorb from the surface yielding oxygen vacancy. Then O2 will be adsorbed into the vacancy to regenerate the catalyst as well as gain one atomic O filling another forming O3v in the subsequent reactions. 3.2.3. Ethanol Reaction Mechanisms on the Co3+ Site. The further oxidation of the Co2+ site produces the Co3+ site, on which ethanol could break C−C forming CO2. After the discussion of the elementary steps in Table S3, the favorable path can be selected and the energy profile is shown in Figure S8 as well as the corresponding structures. The geometry structures of transition states in a side reaction on the Co3+ site can be seen in Figure S9 in the Supporting Information. 3.2.3.1. Bond Scission in CH3CH2OH (D-1) Species. The initial possible bond scissions in ethanol D-1 involve the Cα-H, Cβ-H, and O−H cleavages: (i) CH3CH2OH + O3f → CH3CHOH + H(O3f); (ii) CH3CH2OH + O3f → CH2CH2OH + H(O3f); (iii) CH3CH2OH + O3f → CH3CH2O + H(O3f). The barrier of O−H cleavage is computed to be 0.18 eV lower than that for Cα-H (1.36 eV) and Cβ-H (1.41 eV) cleavages. Remarkably, the CH3CH2O species is formed exothermic by 0.86 eV. Therefore, the O−H cleavage is the most favorable path thermally and kinetically. The ethanol species adsorbs at the top site of Co, donating the oxygen long pair to Co. In TS, the length of the breaking O−H is 1.16 Å, 0.18 Å longer than that in IS. After reaction, the forming CH3CH2O (D-2) species is located at the bridge site of Co with the lengths of 1.93 and 2.03 Å. 3.2.3.2. Bond Scission in CH3CH2O (D-2) Species. The dehydrogenation processes in D-2 were examined: (i)

CH3CH2O + O3f → CH3CHO + H(O3f); (ii) CH3CH2O + H(O3f) → CH3CHO + H2(O3f); (iii) CH3CH2O + O3f → CH2CH2O + H(O3f); (iv) CH3CH2O + HO3f → CH2CH2O + H2(O3f). The barriers for the cleavages in mechanisms above are 1.13, 1.96, 1.73, and 2.17 eV and the related dissociation energies are 0.68, 1.41, 1.08, and 2.09 eV, respectively. Hence the O3f-assisted dehydrogenation in Cα-H cleavage (path (i)) is the most favorable path. For the case of path (i), the CH3CH2O species at top site of Co3+ produces CH3CHO species, which interacts with the surface through O with adsorption energy of −0.69 eV. The breaking Cα-H is 1.36 Å at TS. After dissociation, the dissociated H binds to O3f forming O3fH in D-3. The CH3CHO on Co3+ site could desorb from the surface taking 0.02 eV much lower than the adsorption energy. The H hindrance leads to the CH3CHO being easier to desorb. 3.2.3.3. Bond Scission in CH3CHO (D-3) Species. The CH3CHO species D-3 could proceed Cα-H and Cβ-H cleavages assisted by either O3f or HO3f. The Cα-H and Cβ-H scissions assisted by HO3f are excluded due to the rather high barrier (1.24 eV in Cα-H cleavage and 1.29 eV in Cβ-H cleavage). The O3f-assisted dehydrogenation steps in Cα-H has a higher barrier than that for Cβ-H (0.42 eV versus 0.26 eV). Therefore, the O3f-assisted Cβ-H scission in CH3CHO species should be considered as the dominant path and has a moderate exothermicity (−0.24 eV). The transition state is featured by an elongated length of Cβ-H 1.34 Å. This process yields the CH2CHO species closer to the surface adsorbed through O and Cβ in D-4. 3.2.3.4. Bond Scission in CH2CHO (D-4) Species. From Table S2 in the Supporting Information, it can be seen that the H abstraction in the CH2CHO species D-4 has four parallel paths: (i) CH2CHO + O3f → CH2CO + H(O3f); (ii) CH2CHO + H(O3f) → CH2CO+H2(O3f);(iii)CH2CHO+O3f → CHCHO +H(O3f);(iv)CH2CHO+H(O3f) → CHCHO+H2(O3f). The path (ii) and path (iii) are unlikely to occur for notable activation barriers of 1.80 and 2.81 eV. On the contrary, the path (i) and (iv) carry rather lower barriers of 0.77 and 0.50 eV producing CH2CO and CHCHO respectively. Both of the CH2CO and CHCHO related reactions were taken into consideration due to similar barriers. However, the subsequent scissions in CCHO species after path (iv) owns rather high barrier (1.45 eV) as shown in Figure 3, and thus CHCHO related reactions are ignored. As a result, CH2CHO species D-4 is supposed to decompose Cα-H into CH2CO species. The length of the breaking Cα-H is elongated to 1.44 Å, and the length of the forming H−O3f is shortened to 1.25 Å in TS. After the reaction, H will bind to O3f forming HO3f in D-5. 3.2.3.5. Bond Scission in CH2CO (D-5) Species. There are two possible channels for the CH2CO species D-5: (i)CH2CO + H(O3f) → CHCO + H2(O3f); (ii) CH2CO → CO + CH2. The large values of barrier and reaction energy in C−C cleavage suggest that the C−C cleavage is unlikely to occur (1.95 eV for barrier and 1.43 eV for reaction energy). For the Cβ-H cleavage, the barrier is 0.91 eV, and the reaction is endothermic by 0.47 eV. Therefore, the CH2CO species D-5 will give up H through breaking Cβ-H with the assistance of HO3f. In this case, the distance of the breaking Cβ-H is 1.34 Å while the distance of the forming H−O3f is 1.27 Å at TS. After the dehydrogenation, the detached H binds to the HO3f generating H2(O3f) in D-6. The H2(O3f) needs 0.46 eV to desorb from the surface leaving the vacancy and the CHCO species adsorbed in η1(O)-η2(Cβ) mode over the forming V3f as D-7. G

DOI: 10.1021/acs.jpcc.6b04036 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C 3.2.3.6. Bond Scission in CHCO (D-7) Species. The CHCO D-7 species can produce the CCO and CO species through the Cβ-H and the C−C cleavages: (i) CHCO + H(O3f) → CCO + H2(O3f); (ii) CHCO → C + CO. The Cβ-H cleavage is favored over the C−C cleavage due to the lower barrier of 0.13 eV in Cβ-H cleavage than that in the C−C cleavage (1.34 eV). Moreover, the Cβ-H cleavage is nearly neutral-thermal, and the C−C cleavage is endothermic by 0.95 eV. The DFT calculation results revealed that the CHCO species D-7 prefers to undergo the Cβ-H cleavage. The length of the breaking Cβ-H is 1.24 Å and the distance of the detached H and the H(O3f) decreases to 1.41 Å at TS. At FS, the CCO species and H2(O3f) species are generated on the surface identified as D-8. With the leaving of H2(O3f), one more oxygen vacancy in D-9 forms endothermic by 0.88 eV. 3.2.3.7. Bond Scission in CCO (D-9) Species. The C−C cleavage is the only path for CCO species D-9: CCO → C +CO. For the C−C scission, a barrier of 2.34 eV must be overcome. The direct C−C cleavage could hardly undergo. On the other hand, one neighboring vacancy is supposed to be occupied with O2 immediately exothermic by 4.16 eV. Meanwhile O2 is dissociated into lattice oxygen and atomic oxygen which attacks CCO species forming COCO species D10. For the forming COCO adsorbed in the η2(Cα)-η1(Cβ) type possesses an elongated C−C of 1.56 Å, the C−C cleavage will be rather facile with a barrier of 0.13 eV. The C−C will be elongated to 1.71 Å in TS. After C−C scission, the resulting two CO in D-11 migrate onto the top sites of Co3+ with distances of 1.78 and 1.82 Å: O2 + CCO + V3f → COCO + O3f, COCO → 2CO. Desorption of CO on the Co3+ site neighboring the forming oxygen vacancy V3f requires higher energy than that far from V3f (1.34 eV versus 0.40 eV). 3.2.3.8. Formation of CO2. CO2 is usually proposed to be produced in two ways: (i) the dissociation of acetate species and (ii) the CO oxidation by O2. The barrier of the acetate species dissociation is as high as more than 1.50 eV. On the other hand, CO2 forms easily through the oxidation of CO D11 by O2 due to the oxygen vacancy. This can be found in the previous study about the oxidation of CO.38 When O2 gets close to neighboring oxygen vacancy, CO will bind to the dissociated O forming CO2 D-12 exothermic by 5.74 eV: O2 + CO + V3f → CO2 + O3f. At the same time, the O3v site was filled by oxygen species. The forming CO2 desorbs from surface exothermic by 0.14 eV forming D-13. The remaining two hydrogens binding to lattice oxygen will undergo the H-shift process: 2HO3f → H2O3f + O3f. The barrier for the H-shift is 0.12 eV and the related reaction energy is −0.20 eV. The length of the breaking O−H is elongated to 1.36 Å while the length of forming O−H is shortened to 1.15 Å at TS. Then the H2O3f desorbs from the surface resulting new vacancy in D-15. Another O2 will be adsorbed into vacancy exothermic by 5.38 eV forming CO2 and O3f D-16. The forming CO2 takes 0.10 eV to desorb from surface forming D-17. The favorable mechanism for ethanol reaction on Co3+ site is calculated to be CH3CH2OH → CH3CH2O → CH3CHO → CH2CHO → CH2CO → CHCO → CCO → COCO → CO → CO2. The form of CH3CHO carries the relatively higher barrier, thus the rate-limiting step. The form of CO2 goes on by the oxidation of CO assisted by the O2 absorbed onto surface instead of the dissociation of the acetate species. 3.3. Thermodynamical Factors of the Selectivity Structure-Sensitivity on Co2+ and Co3+. 3.3.1. BEP Relationship. We have gained a large database of cleavages in

C−H and C−C. The traditional Bronsted-Evans−Polanyi (BEP) correlation is used to examine the thermodynamical effect on reaction barriers. In previous studies, a good linear relationship has been identified in barrier and reaction energy for C−H, C−C, and O−H dissociation on vast metal surfaces including Co.57,58 Whether the C−H and C−C cleavages on metal oxide states as Co2+ and Co3+ sites we emphases hold the BEP relation is not clear. Herein the relationship of the barrier and the reaction energy on Co2+ and Co3+ sites is plotted in Figure 3. A nearly linear BEP relationship is found in C−H and C−C cleavage on Co2+ and Co3+ sites, namely the high barrier is caused by the high endothermicity to some degree. As a result, it is worth analyzing the physical original of the reaction energy. As the importance of the formation of CH3CHO and the C−C cleavage in COCO in the whole reaction process, the chemical behavior of the two steps will be comprehensively understood from the aspect of adsorption energy. For the formation of CH3CHO, it could be described as CH3CH2O + H → CH3CHO + H + H, and here we assumed that one H atom produced from ethanol initial dissociation is coadsorbed on surface and acts as a spectator. The C−H cleavage could be considered as the combination of several steps: (1) reactant desorption: CH3CH2O → CH3CH2O(g); (2) the C−H gas phase dissociation: CH3CH2O(g) → CH3CHO(g) + H(g); and (3) product adsorptions: CH3CHO(g) → CH3CHO, and H(g) → H. Therefore, the reaction energy ΔE can be attributed to three kinds of energies: adsorption (or desorption), gas-phase reaction, and correction energies (Table S4, Supporting Information) expressed as eq 2: △E = [Eads(H) + Eads(CH3CHO) − Eads(CH3CH 2O)] + [Egas(H) + Egas(CH3CHO) − Egas(CH3CH 2O)] + [Ecorr (CH3CHO + H + H) − Ecorr (CH3CH 2O + H)]

(2)

The first three terms are identified as adsorption items and correspond to the adsorption energies of products (H and CH3CHO) and desorption energy (−Eads) of the reactant (CH3CH2O). The fourth to six terms are related to the gas phase dissociation of CH3CH2O and denoted as phase reaction items. The last two terms represent the correction energy from the adsorbate effect while H coadsorbed on surface as a spectator which can be expressed as correction items shown in eqs 3 and 4: Ecorr (CH3CH 2O + H) = Eads(CH3CH 2O + H) − Eads(CH3CH 2O) − Eads(H) (3)

Ecorr (CH3CHO + H + H) = Eads(CH3CHO + H + H) − Eads(CH3CHO) − 2Eads(H)

(4) 2+

Because the distinction of adsorption items on Co and Co3+ sites is relatively small(