Theoretical Insight into the Reaction Mechanism of Ethanol Steam

Article pubs.acs.org/JPCC

Theoretical Insight into the Reaction Mechanism of Ethanol Steam Reforming on Co(0001) Sen Lin,*,† Jing Huang,† Xiaomei Gao,† Xinxin Ye,† and Hua Guo‡ †

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, China ‡ Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States ABSTRACT: By using plane-wave density functional theory, the reaction mechanism of ethanol steam reforming (ESR) on the Co(0001) surface is investigated by systematically exploring the barriers and reaction energies of elementary steps. Our results suggest that ESR is initiated by decomposition of ethanol: CH3CH2OH* → CH3CH2O* → CH3CHO* → CH3CO* → CH3* + CO*. This is followed by the water−gas shift (CO* + OH* → COOH* → CO2* + H*) or direct oxidation (CO* + O* → CO2*) reaction to produce CO2. The reaction between CO* and OH*/O* is considered to be the key step in ESR. The proposed mechanism is consistent with most available experimental data and provides theoretical insight into the reaction pathways of the ESR process on cobalt catalysts.

I. INTRODUCTION The depletion of fossil fuels and environmental concerns have motivated the search for clean fuels for future transportation needs. Hydrogen is often touted as a clean fuel as it can power proton exchange membrane fuel cells (PEMFCs) with high efficiency and zero emission. However, its transport and storage remain problematic. As an alternative, low molecular weight alcohols such as methanol have been proposed to serve as onboard hydrogen carriers for PEMFCs, which can generate H2 via steam reforming.1 Ethanol steam reforming (ESR) has also received a great deal of attention because ethanol has low toxicity and a relatively high H2 content and is renewable, readily available, and compatible with the current infrastructure.2−4 Stoichiometrically, the endothermic ESR reaction

The CO2 selectivity is also quite high. Although the support was found to have a strong influence on catalysis (particularly on water dissociation), most of the experiments identified metallic Co as the active phase. Despite numerous experimental investigations,9,11−20 the mechanism of ESR on Co is still not well understood. Compared with the much better understood methanol steam reforming (MSR),21,22 ESR contains many more elementary steps and several submechanisms, namely, steam reforming, water−gas shift (WGS), and methane steam reforming.4 Theoretical studies of ESR on Co have recently been carried out by a few groups using plane-wave density functional theory (DFT).23−25 These studies have shown that the mechanism is significantly more complex than that for MSR, which has been elucidated recently on several metal surfaces.26−32 These DFT studies indicated that the barriers associated with bond cleavages in ethanol are often very high, particularly for the C−C bond scission. In addition, ESR may proceed through different pathways from MSR. However, the existing studies are not exhaustive, and many possible steps have not been studied. To gain further insights into the ESR reaction mechanism, we report here an extensive DFT study of the adsorption and reactions of pertinent species in the ESR reaction pathways on Co(0001). On the basis of the energetics of elementary steps, it is determined that the initial step of ESR on Co involves the O−H bond scission, followed by sequential dehydrogenation in

CH3CH 2OH + 3H 2O → 6H 2 + 2CO2 0 ΔH298 = 172.6 kJ/mol

is seemingly a simple reaction. However, ESR is very complex involving over a dozen potential products, including CO, CO2, CH4, and acetaldehyde (CH3CHO). Due to the need to cleave the C−C bond in the reactant, ESR is typically run at quite high temperatures (>400 °C). ESR catalysts have traditionally been based on noble metals such as Ru, Pd, Au, and Pt, which are expensive. Recent interests have focused on the search for catalysts based on earth-abundant metals such as Ni, Co, and Cu.4 It has been shown that supported Co catalysts are especially efficient as ESR catalysts due to their abilities to cleave the C−C bond.5−10 © 2015 American Chemical Society

Received: December 2, 2014 Revised: January 12, 2015 Published: January 13, 2015 2680

DOI: 10.1021/jp512000k J. Phys. Chem. C 2015, 119, 2680−2691

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The Journal of Physical Chemistry C

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

a

species

adsorption configuration

dCo−O (Å)

H2O OH O H CO CO2 CHO cis-COOH trans-COOH CH2CHO CH2CH2O CH4 CH3 CH2 CH C CH3CO CH3CHO CH3CH2 CH3CH2O CH3CHOOH CH3COO CH3COOH CH3CH2OH CH2O CH3COCH3 CH3O

top through O hcp through O hcp through O fcc through H hcp through C hcp through C and O hcp through C and O bridge through C and O bridge through C and O hcp through Cα and O and bridge through Cβ hcp through Cβ and another hcp through O weak adsorption fcc through C fcc through C hcp through C hcp through C hcp through Cα and O hcp through Cα and O hcp through C of methylene hcp through O hcp through O bridge through two Os top through O top through O hcp through C and O hcp through C and O hcp through O

2.21 2.01/2.01/2.00 1.86

2.07/2.10 2.02/2.11 2.04 2.02 2.03/2.05 1.99/2.00/2.16

2.07/2.12 2.03/2.05/2.09 2.00/2.01/2.02 2.00/2.01/2.02 1.97/1.98 2.05 2.24 1.99/2.01 2.05/2.05/2.05 1.99/1.99/2.02

dCo−C (Å)

1.96/1.96/2.00 1.97/2.18/2.23 1.86/2.05 1.89 1.88 2.04/2.10/2.26 2.17/2.19 3.87/3.93/3.94 2.12/2.14/2.15 1.94/1.95/1.97 1.86/1.86/1.86 1.78/1.78/1.79 1.83 2.00 2.14/2.15/2.17

1.99 2.04

adsorption energy (eV) −0.32 (−0.28) −3.57 (−3.46) −5.88 (−5.81) −2.80 (−2.62) −1.73 (−1.70) 0.14 (0.19) −2.31 (−2.22) −2.36 (−2.28) −2.32 (−2.26) −2.40 (−2.35) −0.46 (−0.43) −0.02 (−0.02) −2.08 (−2.01) −4.12 (−4.02) −6.39 (−6.23) −7.05 (−6.95) −2.07 (−2.02) −0.65 (−0.61) −1.65 (−1.60) −2.86 (−2.71) −2.71 (−2.58) −3.01 (−2.93) −0.50 (−0.51) −0.32 (−0.30) −0.84 (−0.78) −0.44 (−0.41) −2.78 (−2.63)

The numbers in parentheses are corrected by ZPE.

the α-position and C−C bond cleavage of acetyl (CH3CO*), leading eventually to CO*. Interestingly, OH* or O* plays a role only in the last reaction step, namely, the reaction between CO* and adsorbed OH* to produce carboxyl or the direct oxidation of CO* to generate CO2*. This publication is organized as follows. The plane-wave DFT method is outlined in section II. The results, including adsorption geometries and energies of pertinent species and the reaction barriers for important elementary steps, are presented in section III. This is followed by discussion in section IV, in which the putative mechanism is compared with previous theoretical results and experimental data. The final conclusions are given in section V.

consisting of five layers was used with the top two layers allowed to relax in all calculations. A vacuum space of 14 Å was used in the z direction. The adsorption energy was calculated as follows: Eads = E(adsorbate + surface) − E(free molecule) − E(free surface). The climbing image-nudged elastic band (CI-NEB) method40,41 was used to determine the reaction pathways with the energy (10−4 eV) and force (0.05 eV/Å) convergence criteria. Stationary points were confirmed by normal-mode analysis using a displacement of 0.02 Å and energy convergence criterion of 10−6 eV, and the vibrational frequencies were used to compute zero-point energy (ZPE) corrections.

II. THEORY All spin-polarized DFT calculations were carried out using the Vienna ab initio simulation package (VASP)33,34 with the gradient-corrected PW91 exchange-correction functional. The ionic cores were described with the projector augmented-wave (PAW) method,35 and for valence electrons a plane-wave basis set36 with a cutoff of 400 eV was employed. The Brillouin zone was sampled using 2 × 2 × 1 Monkhorst−Pack k-point grid,37 which was tested to be converged to ∼0.05 eV. The Methfessel−Paxton method38 was used with a smearing width of 0.10 eV to extrapolate the total energy to kBT = 0 eV. The optimized bulk lattice parameter a and the c/a ratio for Co were found to be 2.483 Å and 1.623, in good agreement with the experimental value (a = 2.497 Å and c/a = 1.633).39 The adsorption energies of various species were calculated on surface 3 × 3 unit cells, corresponding to 1/9 ML coverage. To obtain accurate results, a slab model for the Co(0001) surface

III. RESULTS A. Adsorption of Pertinent Species. The adsorption of pertinent species involved in the elementary steps of ESR on Co(0001) discussed below has been studied using the DFT method described above. The adsorption geometries and adsorption energies of these species are listed in Table 1. The adsorption configurations of several key species are displayed in Figure 1. H2O*. This species adsorbs on Co(0001) with its oxygen on the top of a surface cobalt atom, as shown in Figure 1. The distance between the O and surface Co atom is 2.21 Å, and the molecule is almost parallel to the surface. The H−O−H angle is found to be 105.75°. From the adsorption energy of −0.28 eV, it is clear that this species adsorbs physically on Co(0001), consistent with experimental observations.42 The adsorption energy is also similar to previous results (−0.34 eV without the ZPE correction25 and −0.22 eV32). 2681


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Figure 1. Adsorption geometries of pertinent species on Co(0001). Atom colors are Co (blue), O (red), C (black), and H (white).

OH*. The hydroxyl species produced by bond cleavage of H2O* adsorbs at an hcp hollow site of Co(0001) with O−Co bond distances of 2.00, 2.01, and 2.01 Å. There is experimental evidence that OH* is produced when water is adsorbed on Co(0001).42 The adsorption energy for this species is about −3.46 eV, which is in excellent agreement with previous reports.25,32,43 O*. The scission of the H−O bond in OH* produces adsorbed oxygen species. The atomic O* species binds strongly at an hcp hollow site of Co(0001) with the three computed Co−O bond distances to be 1.86 Å. A large adsorption energy of −5.81 eV (relative to O(g)) is obtained, consistent with previous theoretical values.32,43

H*. H* binds strongly with three Co atoms at an fcc hollow site, yielding an adsorption energy of −2.62 eV, which is in accordance with the previous calculation values (−2.81 and −2.63 eV).25,32 The distances between the three Co atoms and H atom are calculated to be 1.71, 1.72, and 1.73 Å. CO*. CO* prefers to adsorb at an hcp hollow site in a vertical configuration with an adsorption energy of −1.70 eV, with the three Co−C distances of 1.96, 1.96, and 2.00 Å. These computational results agree well with the previous observations.32,43−45 CO2*. Our calculation results show that CO2 adsorbs as a bent structure on the Co(0001) surface with a positive adsorption energy of 0.19 eV, which is consistent with the previous studies.32,46 This structure might be an unstable 2682


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calculated adsorption energy is −2.02 eV, which is similar to a previously calculated result (−2.11 eV).23 CH3CHO*. The aldehyde molecule adsorbs in a bidentate fashion on Co(0001), with both O and C interacting with the substrate, yielding an adsorption energy of −0.61 eV, which is much lower than that (−0.06 eV) on Rh(111).47 The distance of C−Co is 2.00 Å, and O−Co bond lengths are found to be 2.03, 2.05, and 2.09 Å, respectively. In addition, the bond distance of C−O was calculated to be 1.38 Å. CH3CH2*. The hcp hollow site of the Co(0001) surface is found as the most stable adsorption site for CH3CH2*, and the adsorption energy is computed to be −1.60 eV. The C−Co distances are found to be 2.14, 2.15, and 2.17 Å, respectively. CH3CH2O*. As shown in Figure 1, the hcp site is the most stable adsorption site for CH3CH2O* with the C−O bond perpendicular to the surface. The distances between the O atom and three surface Co atoms are 2.00, 2.01, and 2.02 Å, respectively. The adsorption energy is −2.71 eV, making it relatively stable on the Co(0001) surface. This species was experimentally found to adsorb on Co(0001) after the dissociation of ethanol between the temperatures of 160 and 330 K.48 CH3CHOOH*. From Figure 1, we can see that the C−O bond of this species is nearly perpendicular to the surface and the oxygen atom binds at an fcc hollow site with the O−Co distances equal to those found in CH3CH2O*. The adsorption energy is about −2.58 eV for this configuration. CH3COO*. This species adsorbs on Co(0001) with its two oxygen atoms at the bridge site with the methyl moiety pointing away from the surface, as shown in Figure 1. The distances between the adsorbing O atom and the two surface Co atoms are 1.97 and 1.98 Å, respectively. The O−C−O angle is found to be 124.19°. From the adsorption energy of −2.93 eV, it is clear that it adsorbs chemically on Co(0001). Unfortunately, this species has been explored before by neither experiment nor theory. CH3COOH*. Due to its closed-shell character, acetic acid adsorbs weakly on Co(0001) with a calculated adsorption energy of −0.51 eV, which is lower than that for formic acid on Cu(111). As shown in Figure 1, CH3COOH* adsorbs with its carbonyl oxygen on Co through its lone pair electrons. The distance between the carbonyl oxygen and the underlying Co atom is 2.05 Å. The methyl group points away from the surface, and the O−C−O angle is 123.65°, whereas the hydrogen atom at the oxygen points downward. The two calculated O−C bonds are found to be 1.25 and 1.33 Å, respectively. CH3CH2OH*. By donating the lone pair of oxygen, ethanol adsorbs at a top site with a C−O surface angle of 122.86° and an O−Co distance of 2.24 Å. Our calculated adsorption energy of −0.30 eV is consistent with previous DFT results of −0.36 eV without the ZPE correction.25 CH2O*. The formaldehyde species adsorbs in a bidentate fashion on Co(0001), with both the O and C atoms interacting with the substrate, yielding a binding energy of −0.78 eV, in agreement with the previous results (−0.77 and −0.86 eV).32,49 The O−Co and C−Co distances are found to be 1.99/2.01 and 1.99 Å, respectively. CH3COCH3*. As seen from Figure 1, the acetone molecule prefers to adsorb on an hcp site through both the carbonyl C and O atoms, resulting in an adsorption energy of −0.41 eV. The O atom binds with three Co atoms, and all of the Co−O distances are equal to 2.05 Å, whereas the Co−C bond length is 0.01 Å shorter.

reaction intermediate before its desorption from the surface, which has been confirmed by a recent DFT calculation that the conversion of the bent CO2* adsorbate into physisorbed linear CO2* requires a very small activation barrier of 0.10 eV.32 CHO*. Formyl formed by the C−C bond cleavage of CH3CHO* adsorbs at an hcp site through O anchoring at the bridge of two Co atoms and C connecting with two Co atoms. The C−O bond length is 1.33 Å, and the calculated binding energy was found to be −2.22 eV, in excellent agreement with the value (−2.20 eV) calculated by Cheng et al.23 COOH*. The carboxyl species has two possible conformations, namely, cis- and trans-COOH. Their adsorption patterns are, however similar, as shown in Figure 1, with the C−O moiety interacting at the bridge site in a bidentate fashion with two Co atoms. For cis-COOH*, one C−O bond is nearly parallel to the Co(0001) surface with an O−C−O angle of 116.77° and the C−Co and O−Co bonds are 1.89 and 2.04 Å, respectively. For trans-COOH*, the configuration is similar to those of cis-COOH except that the O−H bond is almost parallel to the surface. The cis isomer binds slightly more strongly (−2.28 eV) than the trans counterpart (−2.26 eV). Similar results (−2.11 eV for cis isomer and −2.07 eV for trans isomer, respectively) were found in a previous DFT study.32 CH2CHO*. As shown in Figure 1, the CH2CHO* species bestrides three hollow sites with its oxygen atom at the Co−Co bridge sites and the two carbon atoms binding with two other Co atoms. The two Co−O bond lengths are calculated to be 2.03 and 2.05 Å, respectively. The two Co−Cα bond distances are 2.26 and 2.10 Å, and the Co−Cβ bond distance is 2.04. This adsorption configuration leads to an adsorption energy of −2.35 eV. CH2CH 2O*. As a product from dehydrogenation of CH3CH2O*, CH2CH2O* interacts with the Co(0001) surface through its oxygen atom at an hcp hollow site and the carbon atom in the edge methylene at another hcp site. The Co−O bond lengths are calculated to be 1.99, 2.00, and 2.16 Å, respectively. This configuration affords a small adsorption energy of −0.43 eV. CH4*. This molecule is often observed in ESR at low temperatures but with a small yield. It is found to weakly adsorb over the Co(0001) surface with a small binding energy of −0.02 eV. The distance between the C atom and the catalyst plane is about 3.87 Å. CH3*. As shown in Figure 1, methyl preferentially adsorbs at the fcc site through its carbon atom. The distances between the C and surface Co atoms are 2.12, 2.14, and 2.15 Å, respectively. The adsorption energy was found to be −2.01 eV. CH2*. The removal of the first hydrogen from CH3* produces CH2*, which locates at an fcc site with an adsorption energy of −4.02 eV. The average distance of the C−Co bond is calculated to be 1.95 Å. CH*. This species adsorbs perpendicularly at an hcp site, yielding a binding energy of −6.23 eV. The C−Co bond lengths are equal to 1.86 Å. C*. As a final product of the dehydrogenation of methyl, the atomic carbon atom strongly interacts with Co atoms at an hcp site. The binding energy is about −6.95 eV, and the C−Co bond distances are 1.78, 1.78, and 1.79 Å, respectively. CH3CO*. CH3CO* adsorbs at an hcp site through O binding at the bridge of two Co atoms and C connecting with another Co atom. The C−O bond is found to be parallel to the surface with a bond length of 1.31 Å, only 0.11 Å longer than that in CO*. The methyl group is pointing away from the surface. The 2683


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The Journal of Physical Chemistry C CH3O*. In this work, CH3O* is produced by the reaction between CH3* and O*. It prefers to adsorb at the hcp site with a binding energy of −2.63 eV. The Co−O distances are calculated to be 1.99, 1.99, and 2.02 Å. B. Reactions. The barriers and reaction energies for several elementary reactions pertinent to ESR on Co(0001) are listed in Table 2, with the ZPE-corrected values in parentheses. The geometries of the transition states (TSs) for all reactions are displayed in Figure 2. It is important to note that many species coadsorb on the catalyst surface, and they might influence the reaction barrier. In this work, we will only focus on the chemical steps, with the caveat that the calculated barrier

heights and other properties may be altered by the presence of other species. In the following discussion, the ZPE-corrected barriers and reaction energies are used unless stated otherwise. R1: H2O* + * → OH* + H*. The reactant state in this reaction is represented by H2O* adsorbed with its oxygen atom on the surface Co atom. Its dissociation produces the adsorbed hydroxyl (OH*) species and hydrogen (H*) atom both located at hcp hollow sites. With a transition state featuring an elongated O−H bond (1.50 Å), this step has an energy barrier of 0.67 eV and an exothermicity of −0.59 eV, respectively, consistent with those (0.69 eV for the energy barrier and −0.67 eV for the exothermicity) reported by Luo and Asthagiri.32 The calculated barrier is consistent with the observations of lowtemperature water dissociative chemisorption on Co(0001).19,42 Note that this exothermic reaction step is thermodynamically favored. R2: OH* + * → O* + H*. The further dehydrogenation of the resulting OH* species from R1 leads to the adsorbed atomic oxygen (O*) and hydrogen (H*). This reaction requires a higher barrier (0.89 eV) than that for R1, as shown in Table 2. Remarkably, this is also an exothermic reaction with a reaction energy of −0.35 eV. Our results are in agreement with the previously reported DFT values (0.94 and −0.29 eV).32 In the transition state, the H−O bond has a length of 1.43 Å. After decomposition, the products are found to locate at two nearby hollow sites. R3: CH3CH2OH* + * → CH3CH2O* + H*. This reaction represents the initial O−H bond scission of the adsorbed ethanol, leading to the formation of ethoxyl (CH3CH2O*) and H*. The CH3CH2OH* species adsorbs weakly on Co(0001) at a top site, donating its oxygen lone pair to the Co atom. After reaction, CH3CH2O* is located at an hcp hollow site, whereas H* is located at its favorite hcp site. The exothermicity of this step is −0.74 eV, and its barrier of 0.57 eV is about 0.10 eV lower than that for water dissociation. This barrier height is similar to that for methanol on the same Co(0001) surface (0.60 eV).32 Experimentally, the O−H bond scission is found to occur between 160 and 330 K,48 consistent with a relatively low dissociation barrier. R4: CH3CH2O* + * → CH3CHO* + H*. In the initial state, the ethoxyl (CH3CH2O*) species is adsorbed at the fcc site. This step possesses an energy barrier of 0.64 eV with a reaction energy of 0.29 eV. Our calculated barrier (0.82 eV without ZPE correction) is somewhat lower than that (0.99 eV) reported by Ma et al.25 In the transition state, the breaking C−H bond length is found to be 1.59 Å, about 0.49 Å longer than that in the reactant. After reaction, the acetaldehyde (CH3CHO*) is located at an hcp hollow site with its methyl pointing away from the surface and a hydrogen atom at Cα goes to an fcc hollow site. In experiment, ethoxyl was found to stay bound to the Co(0001) surface up to 300 K,48 consistent with the significant forward barrier for R4 and backward barrier for R3 and a large adsorption energy. R5: CH3CH2O* + * → CH2CH2O* + H*. The dehydrogenation of ethoxyl (CH3CH2O*) at the hcp site produces CH2CH2O*, which weakly interacts with the Co surface through its Cβ and O atoms at two bridge sites. This reaction has a high barrier of 1.04 eV, and the dissociation energy is about 0.47 eV. The breaking C−H bond length at this transition state is 1.64 Å. After dissociation, the H atom adsorbs at a nearby hollow site. R6: CH3CH2O* + * → CH3* + CH2O*. From Table 2, it can be seen that the C−C bond breaking of ethoxyl (CH3CH2O*)

Table 2. Calculated Activation and Reaction Energies for Several Elementary Reactions on Co(0001) Studied in This Worka no.

elementary reaction

R1 R2 R3

H2O* + * → OH* + H* OH* + * → O* + H* CH3CH2OH* + * → CH3CH2O* + H* CH3CH2O* + * → CH3CHO* + H* CH3CH2O* + * → CH2CH2O* + H* CH3CH2O* + * → CH3* + CH2O* CH3CH2O* + * → CH3CH2* + O* CH3CHO + OH* → CH3CHOOH* + * CH3CHO* + * → CH3CO* + H* CH3CHO* + * → CH2CHO* + H* CH3CHO* + * → CH3* + CHO* CH3CO* + OH* → CH3COOH* + * CH3CO* + * → CH3* + CO* CH3COOH* + * → CH3COO* + H* CH3COO* + * → CH3* + CO2* CH3COOH* + * → CH3* + cis-COOH* CO* + OH* → cis-COOH* +* trans-COOH* + * → CO2* + H* CO* + O* → CO2* + * CH3CO* + CH3* → CH3COCH3* + * cis-COOH* → transCOOH* CH3* + H* → CH4* + * CH3* + * → CH2* + H* CH2* + * → CH* + H* CH* + * → C* + H* CH3* + O* → CH3O* + * CH2* + O* → CH2O* + * CH* + O* → CHO* + * C* + O* → CO* + *

R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29 a

activation energy E⧧ (eV)

reaction energy ΔE (eV)

0.87 (0.67) 1.06 (0.89) 0.77 (0.57)

−0.49 (−0.59) −0.25 (−0.35) −0.64 (−0.74)

0.82 (0.64)

0.42 (0.29)

1.23 (1.04)

0.63 (0.47)

2.62 (2.40)

0.91 (0.74)

1.30 (1.19)

0.07 (−0.05)

0.69 (0.68)

−0.08 (0.06)

0.15 (0.10)

−0.28 (−0.41)

0.49 (0.32)

0.09 (−0.04)

1.17 (1.06)

0.23 (0.09)

0.88 (0.89)

0.54 (0.62)

0.74 (0.68)

−0.58 (−0.69)

0.42 (0.22)

−0.46 (−0.55)

1.89 (1.77)

0.91 (0.75)

1.03 (0.94)

−0.22 (−0.33)

1.48 (1.48)

1.12 (1.19)

0.95 (0.72)

0.15 (0.01)

1.36 (1.31) 1.07 (1.10)

1.13 (1.13) −0.11 (0.02)

0.44 (0.40)

−0.05 (−0.04)

0.64 (0.61) 0.67(0.52) 0.24 (0.16) 1.03 (0.91) 1.43 (1.48) 1.60 (1.62) 1.28 (1.26) 1.45 (1.41)

−0.46 (−0.35) 0.15 (0.03) −0.30 (−0.36) 0.27 (0.21) 0.22 (0.37) 0.74 (0.84) 0.67 (0.69) −0.84 (−0.84)

Entries in parentheses are the ZPE-corrected values. 2684


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Figure 2. Side and top views of transition states (TSs) for several elementary reactions listed in Table 2.

the resulting CH3* species binds at an fcc hollow site, and the formaldehyde (CH2O*) is found to locate at an hcp site. R7: CH3CH2O* + * → CH3CH2* + O*. For the C−O bond scission, an energy barrier of about 1.19 eV needs be overcome. This reaction is nearly thermoneutral with ΔE = −0.05 eV. This process is much more facile that the C−C bond cleavage of CH3CH2O*. In the transition state, the Cβ−O distance is

is very difficult on Co(0001). The large value of the decomposition barrier (2.40 eV), similar to that (2.03 eV) calculated by Ma et al.,25 and the reaction energy of 0.74 eV suggest that this reaction is unlikely to occur. At the transition state, the distance between two carbon atoms is calculated to be 2.22 Å and the angle of O−C−C is 116.02°. In the final state, 2685


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R13: CH3CO* + * → CH3* + CO*. We found that direct cleavage of the C−C bond in CH3CO* species can produce carbon monoxide. The moderate energy barrier of 0.68 eV is found to be 0.21 eV lower than that for R12. The result agrees well with the value (0.90 eV without the ZPE correction) obtained in a previous calculation.23 Consequently, R13 might be more facile than R12 in cobalt-catalyzed ESR. The breaking C−C bond length is calculated to be 1.87 Å at the transition state. After reaction, CH3* and CO* are located at hcp and fcc hollow sites, respectively. R14: CH3COOH* + * → CH3COO* + H*. The dehydrogenation of acetic acid (CH3COOH*) to acetate CH3COO* has a low barrier (0.22 eV) and is exothermic (ΔE = −0.55 eV). At the transition state, the length of the breaking O−H bond was found to be 1.51 Å, and the hydrogen atom has already connected to two cobalt atoms with Co−H distances of 1.75 and 1.79 Å, respectively. After the O−H bond cleavage, the H* species moves to an fcc site and CH3COO* adsorbs in a bidentate fashion with both O atoms on top of Co atoms. R15: CH3COO* + * → CH3* + CO2*. Although the reaction of R14 easily occurs, the subsequent C−C bond cleavage is much more difficult, with an unfavorable barrier of 1.77 eV and a large endothermicity of 0.75 eV. As displayed in Figure 2, we can see that the distance between two carbon atoms is found to be about 2.00 Å. R16: CH3COOH* + * → CH3* + cis-COOH*. Another pathway for the decomposition of acetic acid (CH3COOH*) is the scission of the C−C bond. As shown in Table 2, an energy barrier of 0.94 eV is required for this step, which is also found to be exothermic with ΔE = −0.33 eV. However, it should be noted that the adsorption pattern of CH3COOH* is different from that for the most stable one, and the binding energy is about 0.56 eV lower. After this is corrected, the total energy barrier for this step becomes 1.50 eV, making it practically impossible. R17: CO* + OH* →cis-COOH* + *. To produce CO2, CO* must be oxidized. The available hydroxyl on the surface can react with the adsorbed CO* to form COOH*. The calculated barrier height (1.48 eV) and endothermicity (ΔE = 1.19 eV) are in good accord with those reported earlier on the same Co surface.32 At the transition state, the bond between the carbon and hydroxyl oxygen atoms is almost formed with a distance of 1.65 Å. After the transition state, cis-COOH* is found to locate at the bridge site. R18: trans-COOH* + * → CO2* + H*. The dehydrogenation of trans-COOH* leads to the formation of carbon dioxide. This reaction is almost thermoneutral (ΔE = 0.01 eV) but needs to overcome an energy barrier of 0.72 eV. These values are consistent with those obtained by Luo and Asthagiri.32 The resulting CO2* is adsorbed as a bent molecule on Co(0001) and needs extra energy (about 0.10 eV) to break loose from the surface. R19: CO* + O* → CO2* + *. Due to the available atomic oxygen on Co(0001), the CO* intermediate can also react directly with O* to form CO2*. The barrier of 1.31 eV is slightly lower than that for R17, and the reaction is rather endothermic (ΔE = 1.13 eV), which is consistent with that (1.26 eV for the barrier and 1.10 eV of endothermicity) calculated before by Luo and Asthagiri.32 R20: CH3CO* + CH3* → CH3COCH3* + *. The further reaction between CH3CO* and CH3* yields acetone, a species often observed in experiment, but in a small amount.32 As far as we know, no theoretical studies were carried out for this

elongated to 2.01 Å. The resulting CH3CH2* and atomic oxygen are adsorbed at two adjacent hcp hollow sites. Experimentally, the C−O bond cleavage is reported to happen between 300 and 340 K,48 in reasonable agreement with the calculated barrier. R8: CH3CHO + OH* → CH3CHOOH* + *. Next, we consider the OH* attack at the α-carbon of adsorbed acetaldehyde (CH3CHO*) on Co(0001). This reaction is analogous to the formation of CH2OOH* from formaldehyde (CH2O*) and hydroxyl (OH*) in MSR on Cu and PdZn, which was the key to the CO2 selectivity.26,30 The barrier for this step is 0.68 eV, much higher than those for the formation of CH2OOH* on PdZn (0.16 eV) and Cu (0.11 eV),28,30 but close to that of the same reaction (0.86 eV without the ZPE correction) on Co(0001).32 This process is nearly thermoneutral with ΔE equal to 0.06 eV. At the transition state, the distance of the incipient C−O bond is calculated to be 2.35 Å and the angle of O−C−O is about 95.92°. After reaction, the CH3CHOOH* species strongly interacts with Co atoms at an hcp hollow site with a large adsorption energy of −2.58 eV. The relatively high barrier can be attributed to the bidentate adsorption geometry of CH3CHO*, similar to reaction between bidentate CH2O* and OH* on Co(0001),32 which also has a higher barrier than the same reaction on Cu and PdZn, where CH2O* has a unidentate adsorption configuration. The bidentate adsorption makes it difficult for the OH* attack at the carbonyl carbon. R9: CH3CHO* + * → CH3CO* + H*. The direct Cβ−H bond cleavage on Co(0001) has a very low barrier (0.10 eV) and a moderate exothermicity (ΔE = −0.41 eV), consistent with the previous theoretical results (between 0.16 and 0.24 eV).23,50 The breaking Cβ −H bond has a length of 1.13 Å at the transition state. After the bond scission, the bidentate acetyl (CH3CO*) species prefers to adsorb at an hcp hollow site with H* at another hcp site nearby. R10: CH3CHO* + * → CH2CHO* + H*. In addition to the cleavage of the Cβ−H bond, CH3CHO* can also give up a hydrogen atom by breaking the Cα−H bond. In this case, the energy barrier (0.32 eV) is about 3 times that for R9, and the exothermicity is calculated to be −0.04 eV. However, a previous study gave an energy barrier of 1.10 eV for this step,50 but the reason for this large discrepancy is unclear. At the transition state, the C−H bond distance is 1.55 Å. After the dehydrogenation, the CH2CHO* species adsorbs at two adjacent hollow sites and the hydrogen atom moves to the hcp site. R11: CH3CHO* + * → CH3* + CHO*. We also explore another dissociation step of CH3CHO*, namely, the C−C bond scission. It is found that this process is not as facile as R8, R9, or R10 on Co(0001). The energy barrier and reaction energy are 1.06 and 0.09 eV, respectively. At the transition state, the α and β carbon atoms share the same adsorption site, with the C−C bond length of about 1.94 Å. At the final state, two smaller molecules (CH3* and CHO*) are generated on the surface. R12: CH3CO* + OH* → CH3COOH* + *. Similar to CH3CHO*, the addition of OH* to CH3CO* leads to the formation of acetic acid (CH3COOH*). In the initial state, OH* begins to attack the Cα atom with the initial distance of C−O to be 2.81 Å. The attack shortens it to 2.00 Å at the transition state. The barrier is calculated to be 0.89 eV. In the product state, CH3COOH* adsorbs through its Cα and carbonyl O interacting with two Co atoms sites in a bidentate configuration. 2686


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Figure 3. Network of reaction pathways of ESR on Co(0001). The barrier heights with ZPE corrections are in blue. The minimal energy pathway is in green.

IV. DISCUSSION On Co(0001), water dissociation is found to be thermodynamically favored with a moderate barrier of 0.67 eV, consistent with the reported values of previous theoretical studies.25,32 This is in contrast to other metal surfaces such as Cu and Ni, where the process has a higher barrier and is endothermic.51,52 The subsequent dissociation of OH* to atomic oxygen and hydrogen species is favored thermodynamically on Co(0001), and the reaction barrier (0.89 eV) for this step is not particularly high. Both OH* and O* are strongly bound to the surface and thus unlikely to desorb during the reaction, and their formation is thermodynamically favored. As a result, we will include both species in the subsequent discussion. As indicated by the previous DFT calculations of Wang et al.,24 the reaction barriers for the C−O bond cleavage (2.66 eV), C−C bond scission (1.02 eV), and dehydrogenation (H at Cβ) (1.34 eV) of ethanol on Co catalysts are much higher than that for O−H bond scission. Therefore, it is reasonable to assume that ethoxyl is the main product from the dissociation of adsorbed ethanol. Similar to the dissociation of water, the initial cleavage of the O−H bond in ethanol is also exothermic with a moderate energy barrier of 0.57 eV. This result agrees well with the previous experimental observations19,48 and theoretical data.25 As shown in Figure 3, there are four possible decomposition pathways for the resulting ethoxyl (CH3CH2O*) species, which is strongly bound to the surface. The cleavage of its C−C bond is unlikely as the energy barrier is exceedingly large (2.40 eV). For the scission of the Cβ−H and C−O bonds, the reaction barriers (1.04 and 1.19 eV) are found to be also quite high. The most likely pathway is dehydrogenation at the Cα position, which has a barrier of only 0.64 eV, leading to acetaldehyde (CH3CHO*). This conclusion is consistent with a previous theoretical calculation25 and experimental observations of acetaldehyde on Co-based catalysts at low temperatures.8,10,12,14,19 Unlike the rate-limiting methoxyl dehydrogenation in MSR on Cu,26,28 interestingly, our calculations indicate that the dehydrogenation of ethoxyl is not the rate-limiting step in ESR on Co(0001). The closed-shell acetaldehyde can proceed in several directions, as shown in Figure 3. The breaking of its C−C bond is not easy because of a high barrier larger than 1 eV. The possibility of OH* reacting with CH3CHO* to produce CH3CHOOH* is minor as it has a significant barrier of 0.68 eV, due apparently to the more stable bidentate adsorption pattern of acetaldehydes CH3CHO*. A similarly high barrier has been found for the OH* + CH2O* reaction on Co(0001).32 This is different from MSR on Cu and PdZn catalysts, in which the reaction between formaldehyde and

possible reaction during ESR. We found that the length of the incipient C−C bond at the transition state is 1.90 Å. The energy barrier for this step is 1.10 eV with an exothermicity of 0.02 eV. R21: cis-COOH* + * → trans-COOH*. It is difficult for cisCOOH* to dehydrogenate because the hydrogen atom is pointing away from the surface. Therefore, before dehydrogenation, this molecule must reorient itself. The energy barrier for this conformational change is about 0.40 eV, which is consistent with that (0.41 eV) found before by Luo and Asthagiri,32 and the reaction is nearly thermoneutral. In the final state, the hydrogen atom of trans-COOH* is found to point toward the Co(0001) surface, ready to break away. R22: CH3* + H* → CH4* + *. One possible reaction pathway for CH3* is hydrogenation. This step requires an energy barrier of 0.61 eV. At the transition state, the forming C−H bond length is calculated to be 1.48 Å. R23: CH3* + *→ CH2* + H*. The dehydrogenation of methyl produces CH2* and H* species. The energy barrier is 0.52 eV. In the initial state, CH3* is located at its most stable fcc site. At the transition state, the breaking C−H bond distance is about 1.71 Å. The exothermicity of this step is about 0.03 eV. R24: CH2* + * → CH* + H*. CH2* can give out a hydrogen atom very easily. The energy barrier for this step is only 0.16 eV. After dissociation, CH* and H* co-adsorb at fcc and hcp sites, respectively. R25: CH* + * → C* + H*. The energy barrier for this dehydrogenation process is computed to be about 0.91 eV. At the transition state, the breaking C−H bond length is about 1.67 Å. In the final state, C* and H* are found to adsorb at hcp and fcc sites, respectively. R26: CH3* + O* → CH3O* + *. CH3* can also react with the available surface O* to generate CH3O*. However, this step needs to overcome a high energy barrier of 1.48 eV, indicating that it is unlikely. R27: CH2* + O* → CH2O* + *. Similarly, CH2* might combine with O* species to produce CH2O*, and this step is found to have a barrier of 1.62 eV with a large endothermicity of 0.84 eV, thus also unlikely. After reaction, CH2O* locates at the hcp site. R28: CH* + O* → CHO* + *. The reaction between CH* and O* leads to CHO* with a barrier of 1.26 eV, yielding an endothermicity of 0.69 eV. At the transition state, the calculated C−O bond distance is about 1.79 Å and the angle of H−C−O is about 95.62°. R29: C* + O* → CO* + *. In the presence of O* species, CO* might be produced after C* reacting with O*. This step has a barrier (1.41 eV) comparable with that for R19. 2687


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Figure 4. Energetics of preferred pathways for the ESR on Co(0001) leading to CO2 and H2. Adsorbate−adsorbate interaction energies in the bimolecular reactions were included into the barrier heights with ZPE corrections.

0.61 eV, a species often observed in ESR, particularly at low temperatures.5,7,8,10,13,19 On the other hand, the sequential dehydrogenation steps of CH3* → CH2* → CH* → C* have barriers of 0.52, 0.16, and 0.91 eV, consistent with a previous DFT study by Liu et al.45 As a result, some dehydrogenation is likely, but the products can also be oxidized by OH* or O* on the Co surface. For example, the reaction between CH3* and OH* had been calculated by Ma et al.25 with a barrier of 0.95 eV. For CH2* + OH* → CH2OH* and CH* + OH* → CHOH*, the endothermicities are calculated in this work to be 0.67 and 0.76 eV, respectively. According to the previously reported barriers of the reverse processes,55 the activation barrier for two reactions should be larger than 1 eV. The calculated barriers for CH3* + O* → CH3O*, CH2* + O* → CH2O*, and CH* + O* → CHO* are shown to be 1.48, 1.62, and 1.26 eV, respectively, indicating the oxidation reactions of these species are more difficult than their dehydrogenation counterparts. The produced C*, which is presumably responsible for the observed coke deposition and might result in the deactivation of Co metal sites,7,8 can also be oxidized by the oxygen atoms (O*) to form CO2* with calculated activation barriers of 1.41 and 1.31 eV. In this case, both oxygen atoms in the produced CO2* are from water, consistent with the observation that 18CO2 was detected when H218O was fed in ESR.19 In another experiment by da Silva et al.,56 however, it was suggested that O and/or OH of support assists in cleaning coke from Co metal. From Figure 4, we can also see that after considering the adsorbate−adsorbate interaction, the barrier of C* + O* → CO* is increased to 1.71 eV, which might cause C* accumulation. This problem was also observed on Rh and Pt catalysts.4,57,58 Interestingly, the amount of carbon species formation is shown experimentally to be correlated with reaction conditions. For example, the increase in the reaction temperature will lead to a decrease in C* deposition.4 Another possible reaction pathway for CH3* is the combination with CH3CO* to generate acetone with a calculated barrier of 1.10 eV, which, however, cannot compete with that for the direct C−C bond scission of CH3CO*, thus consistent with the fact that acetone is often observed in small quantity.5,7,9,10,19 Our calculation also indicates that acetone can be generated from ethanol in the absence of H2O, consistent with the experimental observation that no 18O was seen in the acetone when H218O was fed.59

hydroxyl has a lower barrier than the dehydrogenation of formaldehyde, thus believed to be the key to selectivity.26,28,30 The C−O cleavage of CH3CHO* was found to have a comparable barrier of 0.72 eV.53 By contrast, the dehydrogenation of CH3CHO* to either CH3CO* or CH2CHO* is much more likely to occur due to their low barriers. Importantly, both processes can compete effectively with the desorption of acetaldehyde (Eads = −0.61 eV). The reaction from CH3CHO* to CH2CHO* is possible owing to its low barrier (0.32 eV). Previous theoretical studies have reported the barriers for the C−C (0.95 eV) and C−O bond (1.50 eV) cleavages.53,54 The dehydrogenation at both the Cα and Cβ positions has barriers of 0.76 and 0.58 eV, yielding CH2CO* and CHCHO*, respectively, followed by the C−C bond scission with barriers of 0.17 and 0.77 eV to produce CH2*/CO* and CH*/CHO*, respectively. We follow the lower barrier alternative (Ea = 0.10 eV), which produces acetyl (CH3CO*). Thanks to its large adsorption energy, CH3CO* is unlikely to desorb during the reaction. Both the C−O bond cleavage and the dehydrogenation of CH3CO* were found to have relatively high barriers (1.19 and 2.25 eV, respectively), in agreement with the previous results of Zhuo et al.50 We hereby consider two alternative pathways via the scission of the C−C bond of CH3CO* and the reaction between CH 3 CO* and OH* to form acetic acid (CH3COOH*). The former with a barrier of 0.68 eV is likely the more preferred pathway than the latter, which has a barrier of 0.89 eV. In the latter pathway, the O−H bond cleavage of the resulting acetic acid (CH3COOH*) to produce acetate (CH3COO*) is quite facile owing to a low barrier of 0.22 eV. This is consistent with a previous experiment that detected surface acetate using DRIFTS.19 However, the subsequent C− C cleavage of CH3COO* to yield CH3* and CO* needs to overcome a much higher barrier of 1.77 eV. On the other hand, the direct C−C bond cleavage of acetic acid requires a reaction barrier of 1.50 eV. Therefore, this alternative pathway is more likely a dead end. Indeed, Vohs and co-workers have observed the production of adsorbed methyl and CO in the decomposition of ethanol on metallic Co surfaces.20 For the resulting CH3* species from the decomposition of CH3CO*, it can readily react with the available surface hydrogen atom to form methane with a calculated barrier of 2688


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that on Cu, Ag, and Au.50 The proposed reaction pathway of ESR on Co(0001) shares many similarities with that on the Rh(211) surface50 but is somewhat different from that on Pt.61 On Rh(211),50 the proposed mechanism is CH3CH2OH → CH3CH2O → CH3CHO → CH3CO → CH3 + CO, followed by the WGS reaction to produce CO2. The reaction between CO* and OH* is also considered as the rate-determined step. On Pt surfaces, Sutton et al.61 suggested, on the basis of both theoretical and experimental data, a similar reaction network, namely, the decomposition of CH3CH2OH followed by WGS. However, the initial dehydrogenation of ethanol is found to be highly endothermic (ΔE = 0.53 eV) with a barrier of 0.81 eV. The reaction of CH3CO* decomposition to CH3* and CO* also has a high barrier of 1.40 eV, and the production of C1 products is controlled by the C−C scission of CHCO* from the dehydrogenation of CH3CO*. Interestingly, the extent of dehydrogenation is found to have a large effect on the C−C bond cleavage barriers, which tend to decrease with the level of hydrogenation. This trend has also been observed on Pt, an ESR catalyst, but not on Rh(211).57 From Table 2, it can be found that on Co(0001) the scission for a Cα−H bond tends to have a lower barrier than that for a Cβ−H bond.

The other product, CO*, is also a common product in ESR and has been observed in many experiments, particularly at low temperatures.5,7,8,10,13,19 The height of the dissociation barrier for CO* is 2.21 eV, in excellent agreement with the previously calculated results,44,45,50 suggesting that this reaction can be ruled out.48 Rather than desorbing from the Co(0001) surface, CO* is more likely to react with the available surface OH* to produce COOH* (Ea = 1.48 eV), which then dehydrogenates to form CO2*. This is essentially the WGS reaction. On the other hand, CO* can also combine with a surface atomic oxygen to yield CO2*, which has a barrier of 1.31 eV. In an experiment carried out by Batista et al.,13 when a H2/CO mixture was fed on an Al2O3-supported Co catalyst, methane dominated the products in the absence of water, whereas the CO2 was negligible at 400 °C and an inversion of the CH4/CO2 relative distribution is detected when water vapor is added to the system. A similar observation was also reported in another experiment by Haga et al.5 This strongly suggests that the WGS reaction is involved in ESR.17,59 A further piece of evidence was provided by Song et al., who showed that the oxygen in the ethanol oxidation products comes from both ethanol and water by analyzing the vibration peaks of CO2 from ESR.19 The resulting reaction network of ESR on Co(0001) is displayed in Figure 3, with the low-barrier pathways colored in green, and the preferred reaction pathway is shown in Figure 4. Our calculations thus suggest that the ESR catalyzed by Co is likely to proceed via CH3CH2OH* → CH3CH2O* → CH3CHO* → CH3CO* → CH3* + CO* → CH2* + CO* → CH* + CO* → C* + CO*, followed by the WGS/oxidation reaction to produce CO2 and H2. The calculated reaction energy (1.70 eV) for the overall ESR process is in good agreement with the experimentally estimated value (1.79 eV). This mechanism shares some features with that proposed recently by Song et al.19 Both are initiated by decomposition of ethanol to acetaldehyde, but our calculations do not support the proposed C−C bond cleavage of either acetic acid or acetate as suggested by these authors. On the other hand, the mechanism of Song et al. was specifically proposed for the Co/ ZnO2 and Co/CeO2 catalysts, in which the oxide support is assumed to participate in the catalysis. The proposed ESR mechanism here also shares similarities with the recently proposed MSR mechanism on the same Co(0001) surface by Luo and Asthagiri, who suggested, on the basis of DFT calculations, that the MSR pathway on Co(0001) differs from that on Cu or PdZn surfaces in that the formation of CO is the main product of methanol decomposition.32 As a result, WSG is needed to convert it to CO2. The step edges might also affect the ESR activity as reported by several theoretical studies.44,45,48,60 For example, the adsorption energy for the C* species is enhanced from −6.83 eV on the flat Co(0001) surface to −7.85 eV on Co (101̅2) and −7.22 eV on Co (112̅0). However, on C (101̅2) and Co(112̅0), the reaction barriers (1.92 and 1.71 eV) for C* + O* → CO* are close to that (1.77 eV) on the flat Co surface, whereas the reaction becomes more endothermic due to the stronger binding of the carbon atom species.45) Further investigations of the effects of defect sites toward ESR activity will be carried out in the future. It might be interesting to compare the proposed ESR mechanism on Co(0001) with those on other metal surfaces. The barrier for the initial dehydrogenation of ethanol on Co(0001) is similar to that on the 111 facet of transitional metal surfaces such as Rh, Ir, Ni, and Pd, but much lower than

V. CONCLUSIONS In this DFT study, we examined the reaction network of ESR on the Co(0001) surface. From our calculation results, it is suggested that the initial steps involve O−H bond scission in both ethanol and water, producing ethoxyl and hydroxyl species. Acetaldehyde (CH3CHO*) is an important intermediate, which can be produced from the dehydrogenation of ethoxyl (CH3CH2O*). However, further reaction of acetaldehyde (CH 3CHO*) with hydroxyl (OH*) to generate CH3CHOOH* is found to have a much higher barrier, due apparently to the bidentate adsorption of acetaldehyde, than that for the direct dehydrogenation of acetaldehyde to acetyl (CH3CO*). The DFT results indicate that the reaction of CH3CO* with OH* has a barrier comparable with that for the decomposition of CH3CO* to CO*. However, it is difficult to break the C−C bond in the acetate (CH 3 COOH*) intermediate due to a high barrier of 1.50 eV; thus, the CH3COOH* pathway assumes a minor role. The C* accumulation might cause the deactivation of catalyst because of the sizable energy barrier of 1.71 eV. In addition, the final reaction step between CO* and the OH* or O* species can be considered as key steps in the ESR process. Obviously, energetics alone is not sufficient to determine the mechanism and to calculate the rate constant. Kinetic simulations are required, but beyond the scope of this work. In addition, ESR on Co catalysts, particularly its selectivity, is known to be strongly affected by the support,5,6,8 which is not explored by the current work. The participation of the oxide support could conceivably lower the activation energy of the CO oxidation by providing a lattice oxygen,62,63 which can subsequently be replenished by OH* or O* formed by water dissociation.9,19 That may also result in the creation of Co2+ species, which have been implicated by several experiments.6,17,20 As a result, many more theoretical studies are needed to completely elucidate the mechanism of the ESR catalysis. 2689


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(18) Bayram, B.; Soykal, I. I.; von Deak, D.; Miller, J. T.; Ozkan, U. S. Ethanol Steam Reforming over Co-Based Catalysts: Investigation of Cobalt Coordination Environment under Reaction Conditions. J. Catal. 2011, 284, 77−89. (19) Song, H.; Bao, X.; Hadad, C.; Ozkan, U. Adsorption/ Desorption Behavior of Ethanol Steam Reforming Reactants and Intermediates over Supported Cobalt Catalysts. Catal. Lett. 2011, 141, 43−54. (20) Martono, E.; Vohs, J. M. Active Sites for the Reaction of Ethanol to Acetaldehyde on Co/YSZ(100) Model Steam Reforming Catalysts. ACS Catal. 2011, 1, 1414−1420. (21) Palo, D. R.; Dagle, R. A.; Holladay, J. D. Methanol Steam Reforming for Hydrogen Production. Chem. Rev. 2007, 107, 3992− 4021. (22) Sa, S.; Silva, H.; Brandao, L.; Sousa, J. M.; Mendes, A. Catalysts for Methanol Steam Reforming − a Review. Appl. Catal., B 2010, 99, 43. (23) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Lok, C. M. A First-Principles Study of Oxygenates on Co Surfaces in Fischer− Tropsch Synthesis. J. Phys. Chem. C 2008, 112, 9464−9473. (24) Wang, J.-H.; Lee, C. S.; Lin, M. C. Mechanism of Ethanol Reforming: Theoretical Foundations. J. Phys. Chem. C 2009, 113, 6681−6688. (25) Ma, Y.; Hernández, L.; Guadarrama-Pérez, C.; Balbuena, P. B. Ethanol Reforming on Co (0001) Surfaces: A Density Functional Theory Study. J. Phys. Chem. A 2012, 116, 1409−1416. (26) Gu, X.-K.; Li, W.-X. First-Principles Study on the Origin of the Different Selectivities for Methanol Steam Reforming on Cu(111) and Pd(111). J. Phys. Chem. C 2010, 114, 21539−21547. (27) Smith, G. K.; Lin, S.; Lai, W.; Datye, A.; Xie, D.; Guo, H. Initial Steps in Methanol Steam Reforming on PdZn and ZnO Surfaces: Density Functional Theory Studies. Surf. Sci. 2011, 605, 750−759. (28) Lin, S.; Johnson, R. S.; Smith, G. K.; Xie, D.; Guo, H. Pathways for Methanol Steam Reforming Involving Adsorbed Formaldehyde and Hydroxyl Intermediates on Cu(111): Density Functional Theory Studies. Phys. Chem. Chem. Phys. 2011, 13, 9622−9631. (29) Lin, S.; Xie, D.; Guo, H. The Methyl Formate Pathway in Methanol Steam Reforming on Copper: Density Functional Calculations. ACS Catal. 2011, 1, 1263−1271. (30) Lin, S.; Xie, D.; Guo, H. Pathways of Methanol Steam Reforming on PdZn and Comparison with Cu. J. Phys. Chem. C 2011, 115, 20583−20589. (31) Lin, S.; Xie, D.; Guo, H. First-Principles Study of the Methyl Formate Pathway of Methanol Steam Reforming on PdZn(111) with Comparison to Cu(111). J. Mol. Catal. A 2012, 356, 165−170. (32) Luo, W.; Asthagiri, A. Density Functional Theory Study of Methanol Steam Reforming on Co(0001) and Co(111) Surfaces. J. Phys. Chem. C 2014, 118, 15274−15285. (33) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for ab initio Total-Energy Calculations Using Plane Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (34) Kresse, G.; Furthmuller, J. Efficiency of Ab Initio Total Energy Calculations for Metals and Semiconductors Using Plane Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (35) Blöchl, P. E. Project Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (36) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (37) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (38) Methfessel, M.; Paxton, A. T. High-Precision Sampling for Brillouin Zone Integration in Metals. Phys. Rev. B 1989, 40, 3616− 3621. (39) Vincent, F.; Figlarz, M. Quelques Precisions Sur Les Parametres Cristallins Et Lintensite Des Raies Debye-Scherrer Du Cobalt Cubique Et Du Cobalt Hexagonal. C. R. Hebd. Ser. C 1967, 264, 1270. (40) Jónsson, H.; Mills, G.; Jacobsen, K. W. Nudged Elastic Band Method for Finding Minimum Energy Paths of Transitions. In

AUTHOR INFORMATION

Corresponding Author

*(S.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of the National Natural Science Foundation of China (21203026 to S.L.), the New Teachers’ Fund for Doctor Stations, Ministry of Education, China (20123514120001 to S.L.), and the U.S. National Science Foundation (CHE-0910828 to H.G.).

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Theoretical Insight into the Reaction Mechanism of Ethanol Steam

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