Ethanol Reforming on Co(0001) Surfaces: A Density Functional

Article pubs.acs.org/JPCA

Ethanol Reforming on Co(0001) Surfaces: A Density Functional Theory Study Yuguang Ma, Liliana Hernández,† Carlos Guadarrama-Pérez, and Perla B. Balbuena* Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States ABSTRACT: A computational study using density functional theory is carried out to investigate the reaction mechanism of ethanol steam reforming on Co(0001) surfaces. The adsorption properties of the reactant, possible intermediates, and products are carefully examined. The reaction pathway and related transition states are also analyzed. According to our calculations, the reforming mechanism primarily consisting of dehydrogenation steps of ethanol, ethoxy, methanol, methoxy, and formic acid, is feasible on Co(0001) surfaces. It is also found that the reaction of formaldehyde yielding formic acid and hydrogen may not be an elementary reaction. The dehydrogenation of ethoxy possesses the highest barrier and is accordingly identified as the rate-determining step.

1. INTRODUCTION There is an increasing demand to replace fossil fuels with clean and renewable energy sources because of the concern of environmental pollution and dwindling oil supply. Hydrogen, as a clean and efficient energy carrier, has extensive applications in new energy technologies. For example, polymer electrolyte membrane fuel cells (PEMFCs) powered by hydrogen have been generally proposed as next-generation clean power sources for vehicle engines and stationary power units. However, hydrogen always exists in bound form and has to be produced from organic compounds or water. To date, various hydrogen production technologies have been developed. Among them, ethanol steam reforming is a promising approach, as ethanol has some advantages over other hydrogen sources, such as availability from renewable biomass, low toxicity, ease of transport, and high safety of handling. In recent years, studies of the ethanol steam reforming reaction have received increasing attention due to their importance in hydrogen energy technology. As the catalysts play a pivotal role in the efficiency of ethanol reforming, numerous experimental investigations have been made in this field.1−3 Metals such as Ni, Co, Cu, Rh, Ru, Pd, Ir, and Pt display good catalytic activity when supported on metal oxides such as Al2O3, CeO2, ZrO2, MgO, and so forth.4−8 Compared to noble metals, Co-based catalysts are much less expensive and exhibit very good catalytic performance as well: It has been reported that Co-based catalysts are able to catalyze ethanol steam reforming reactions at relatively low temperature (∼350− 400 °C).9−11 Moreover, Co catalysts also possess superior selectivity for the overall ethanol steam reforming reaction, ranking the best among 14 transition metals supported on Al2O3, according to the study by Haga et al.12 The entire ethanol steam reforming process is very complicated. Investigations on reforming mechanisms help in identifying intermediates, determining plausible pathways, and © 2012 American Chemical Society

designing novel catalysts. Various ethanol reforming reaction mechanisms have been proposed by experimental studies and kinetic models,10,13−20 approximately classified into two categories: dehydrogenation and dehydration pathways. The first pathway involves acetaldehyde intermediates converting to reforming products; while the second proceeds via ethylene first. On the other hand, first-principles computations have been used as a powerful tool in mechanism-elucidation studies. For example, reactions related to methanol and ethanol decomposition/synthesis have been widely investigated on Co21 Rh,22−25 Pt,26 Ni,27 Pd,28 Au,29 and other metal surfaces,30 using density functional theory (DFT) calculations. Theoretical/computational studies of ethanol steam reforming reactions on various metal surfaces have revealed some new insights into the possible reaction route and catalytic activities.31−35 For the steam reforming of ethanol, Co/Al2O3 shows the highest hydrogen selectivity among Co/Al2O3, Co/MgO, and Co/SiO2 catalysts.14 Some experimental evidence has indicated that the dehydrogenation of ethoxy is involved into the reforming mechanism over Co-based catalysts.36,37 Sahoo et al. proposed a series of reaction steps for the steam reforming of ethanol on Co/Al2O3 based on the product analysis and previous studies.38 Cheng et al. examined the possible pathways to produce formaldehyde and methanol via oxygenate intermediates on Co(0001) using DFT calculations.21 Notably, some reaction steps are shared with ethanol reforming according to the kinetic model. We examine the proposed ethanol reforming mechanism on Co(0001) surface using DFT. In particular, we aim at the identification of the surface adsorption properties and the energy barrier of each step in the reaction. Received: August 24, 2011 Revised: January 15, 2012 Published: January 17, 2012 1409

dx.doi.org/10.1021/jp208179e | J. Phys. Chem. A 2012, 116, 1409−1416

The Journal of Physical Chemistry A

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2. COMPUTATIONAL METHODS Spin-polarized periodic DFT calculations were performed using the Vienna ab initio simulation package (VASP),39−43 in which the Kohn−Sham equations are solved by self-consistent algorithms. The exchange-correlation functional was described within the generalized gradient approximation (GGA) proposed by Perdew, Burke and Ernzerhof (PBE).44 For improving the computational efficiency, the projector augmented wave (PAW) pseudopotentials45,46 were applied to the core electrons, while the valence electrons were described by plane wave basis sets with a cutoff energy of 400 eV. For surface adsorption calculations, the Brillouin zone was sampled with a 9 × 9 × 1 Γ-centered k-mesh in the reciprocal space, while a relatively small 4 × 4 × 1 Γ-centered k-mesh was chosen for transition state calculations. The Methfessel−Paxton method was employed to determine electron occupancies with a smearing width of 0.2 eV. The criterion of structure relaxation was set to convergence of the forces to 0.02 eV/Å. Crystalline Co has a hexagonal close-packed (hcp) structure. The DFT-optimized lattice parameters are in good agreement with the experimental ones.47 In this study, only the most straightforward (0001) surface was employed. A 4 × 4 unit cell was constructed to model the (0001) surface. Each cell consists of a three-layer slab and seven equivalent layers of vacuum space. The distance between two neighboring slabs is ∼14 Å, and the slab−slab interaction can be neglected according to our calculations. The structure of the adsorbates and the Co atoms in the top two layers were allowed to relax to their lowest energy configuration, while the atoms of the bottom layer were fixed to their bulk positions, keeping their optimized lattice constants. For the reactants, intermediates, and products, various adsorption models were examined to determine their favorable adsorption sites. The adsorption energy, Eads, is defined by the following equation: Eads = Eslab/ads − Eslab − Eads, in which Eslab/ads is the total energy of the slab with adsorbates, Eslab is the energy of the slab, and Eads is the energy of the adsorbates in gas phase. The nudged elastic band (NEB) method48,49 was used to identify transition states (TS) for each step of the reforming reaction and to compute the relevant energy barriers as well.

k4, −4

CH3CHO(1) + H(1) ←⎯⎯→ CH3(1) + HCHO(1) k5, −5

CH3(1) + OH(1) ←→ ⎯⎯ CH3OH(1) + S1

Co/Al2O3

k 6, −6

k 7, −7

k8, −8

k 9, −9

(10)

Their kinetic model suggested that the dehydrogenation of adsorbed ethoxy (eq 3) is the rate-determining step (RDS). In this work, the surface adsorption properties of the reaction intermediates on a Co(0001) surface are first studied, followed by the investigation of the reaction mechanism according to eqs 2−10. 3.1. Surface Adsorption. We study surface adsorption of the reaction species involved in the suggested mechanism, including ethanol (C2H5OH), ethoxy (CH3CH2O), acetaldehyde (CH3CHO), methyl (CH3), formaldehyde (HCHO), methanol (CH 3 OH), methoxy (CH 3 O), formic acid (HCOOH), water(H2O), hydroxyl (OH), hydrogen atom (H), and carbon dioxide (CO2). Possible adsorption sites are systematically examined for each species on a 4 × 4 Co(0001) surface unit cell. The calculated adsorption energies are used to determine the most stable adsorption mode, and the results are listed in Table 1. Table 1. Surface Adsorption Energy (Eads) of Reaction Species in the Ethanol Reforming Mechanisma

a

(1)

adsorbate

site

Eads (eV)

CH3CH2OH CH3CH2O CH3CHO CH3 HCHO CH3OH CH3O HCOOH H2O OH H CO2

O-top O-hcp O-top fcc O-brg-C-brg (p) O-top O-fcc carbonyl -O-top (v) O-top O-hcp fcc top

−0.36 −2.86 −0.36 −2.04 −0.87 −0.33 −2.84 −0.25 −0.34 −3.65 −2.81 −0.04

Only the most energetically favorable adsorption sites are included.

The oxygen atom is the main adsorption site for ethanol. We tested top, face-centered cubic (fcc) and hcp sites on Co(0001) with different orientations of the ethanol molecule. The geometry optimization unanimously leads to one binding mode: The oxygen atom is located on the top site of a Co atom and the Co−O distance is 2.17 Å, as illustrated in Figure 1. The calculation indicates a relatively weak adsorption energy (−0.36 eV). For ethoxy, the oxygen atom is the binding site on the surface. The intermediate can bind on several sites of the surface: The most favorable adsorption mode is hcp, with an adsorption energy of −2.86 eV. The binding strength on fcc sites is a little weaker, around 0.03 eV higher than on the hcp

(2)

k 2, −2

(3)

k3, −3

2S1 + H2O(g ) ←→ ⎯ OH(1) + H(1)

(9)

2S1 + HCOOH(1) ←⎯⎯→ CO2(1) + 2H(1)

k1, −1

S1 + CH3CH2O(1) ←⎯⎯→ CH3CHO(1) + H(1)

(8)

HCHO(1) + OH(1) ←⎯⎯→ HCOOH(1) + H(1)

According to the proposed Langmuir−Hinshelwood (L−H) surface reaction mechanism, the reforming process is primarily composed of the following steps:

2S1 + CH3CH2OH(g) ←→ ⎯ CH3CH2O(1) + H(1)

(7)

CH3O(1) + S1 ←⎯⎯→ HCHO(1) + H(1)

ΔH °

= 41.5 kcal/mol

(6)

CH3OH(1) ←⎯⎯→ CH3O(1) + H(1)

3. RESULTS AND DISCUSSION Sahoo et al. carried out a kinetic study of the ethanol reforming reaction over Co/Al2O3 catalysts.38 CH3CH2OH + 3H2O ←⎯⎯⎯⎯⎯⎯⎯⎯→ 6H2 + 2CO2

(5)

(4) 1410



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than that on fcc or hcp (−2.09 eV) sites. Similar to ethanol and ethoxy, the acetaldehyde species is also bonded primarily via the oxygen atom. Hollow site (fcc and hcp) adsorption is found not to be stable: the corresponding configurations are all converted into top and bridge adsorption mode after optimization. The top site configurations (Eads= −0.32−0.36 eV) exhibit higher stability than the bridge configurations (Eads= −0.26− 0.28 eV), in which the carbonyl O atom lies on the bridge site between two Co atoms. The optimized geometry shows that the methyl moiety may also interact to some extent with the surface in the top site adsorption considering the distance between C and Co (3.45 Å). The surface adsorption of methyl is straightforward. Three possible adsorption sites for the C atom, fcc, hcp, and top are identified. The fcc and hcp configurations possess comparable adsorption energies (−2.04 eV for fcc and −2.02 eV for hcp), whereas the top site adsorption is much weaker (Eads= −1.50 eV). The formaldehyde molecule has several different adsorption configurations on the Co(0001) surface. Starting from 17 initial geometries, we identified five adsorption modes: (1) bridge-O adsorption (Eads= −0.23 eV): The carbonyl O atom lies onto the bridge site, and the HCHO molecule is perpendicular to the surface. (2) top-O adsorption (Eads= −0.31 eV): The geometry is similar to the bridge site configuration, but the O atom is positioned onto the top site. (3) top-O and hollow-C adsorption (Eads= −0.74−0.76 eV): The HCHO molecule is roughly parallel to the surface, and the O and C atoms occupy top and hollow sites, respectively. (4) hollow-O and top-C adsorption (Eads= −0.79−0.80 eV): The molecule is parallel to the surface, but the O atom is onto a hollow site and the C atom lies onto a top site. (5) bridge-O and bridge-C adsorption (Eads= −0.87 eV): Both O and C are located onto a bridge site. Configuration 5 possesses the strongest adsorption possibly due to the multiple binding sites. According to the optimized structure shown in Figure 1, the O, C, and H atoms in formaldehyde are all interacting with the metal surface, leading to strong binding strength. The methoxy species has an adsorption mode analogous to that of methyl. The surface adsorption takes place on the top and hollow sites through the O atom in CH3O. The fcc site is the most energetically favorable one (Eads= −2.84 eV), followed by the hcp site (Eads= −2.82 eV) and then by the top site (Eads= −2.09 eV). Methanol is preferentially adsorbed on the top site of the Co(0001) surface with an adsorption energy of −0.33 eV. Other adsorption sitesfcc, hcp, and bridgeare found to be not favorable. The optimized O−Co distance (2.19 Å) is similar to that in ethanol (2.17 Å) and much longer than that in acetaldehyde (2.00 Å). The surface adsorption of formic acid is the most complicated among the reaction intermediates due to the existence of two major adsorption sites, the carbonyl, and the hydroxyl O atoms. Our calculation shows that the adsorption energy ranges from −0.03 eV to −0.25 eV, depending on the adsorption configurations. Basically, top side adsorption of the carbonyl O atom with a roughly perpendicular configuration can be observed in the energetically favorable adsorption modes. The hydroxyl O atom also closely interacts with the surface in the most stable configuration, with a Co−O distance of 3.03 Å. For water, top site adsorption was determined as the most favorable one. The long distance between O and Co atoms (2.23 Å) implies a weak interaction, which is verified by the calculated Eads (−0.34 eV). The preferred adsorption site of OH is hcp. The calculation indicates a vertical configuration and strong binding strength (Eads= −3.65 eV), as illustrated in

Figure 1. Optimized structures of adsorbed species involved into the ethanol reforming mechanism. The red, yellow, light blue, and bluish gray spheres represent oxygen, carbon, hydrogen, and cobalt atoms, respectively. The marked distances are in Å.

configuration. Ethoxy species adsorbed on the top sites is also observed. The adsorption energy, however, is much weaker 1411



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Figure 1. H is found to preferentially occupy hollow sites, in accordance with the adsorption on other (0001) surfaces.50 CO2 only shows very weak interaction (Eads= −0.04 eV) with the Co(0001) surface. Cheng et al. reported the most stable configurations of some oxygenate species;21 the geometry and adsorption energies found in this work are generally in good agreement with their results. 3.2. Ethanol Steam Reforming Mechanism. As indicated by the reaction mechanism by Sahoo et al., the first step of ethanol reforming is the dehydrogenation of ethanol to ethoxy. The optimized structure of the adsorbed ethanol on Co(111) is chosen as the initial geometry for the dehydrogenation. The O−H bond is dissociated when the H atom moves toward an fcc hollow and the O atom approaches an hcp site, yielding an adsorbed ethoxy species. In the transition state, the O−H distance is elongated to 2.75 Å, and the H atom is positioned between the bridge and fcc sites, as depicted in Figure 2.

Figure 2. Calculated reaction pathway and transition state structure for k1, −1

(1)

Figure 3. Calculated reaction pathways and transition state

(1)

k2, −2

2S1 + CH3CH2OH(g) ←⎯⎯→ CH3CH2O + H . The red arrows indicate the possible movement of corresponding species in the reaction step. The marked distances are in Å.

structures for (a) S1 + CH3CH2O(1) ←⎯⎯⎯→ CH3CHO(1) + H(1) , k4, −4

( b ) CH3CHO(1) + H(1) ←⎯⎯⎯→ CH3(1) + HCHO(1), a n d ( c ) S1+CH3CH2O(1) ↔ CH3(l)+HCHO(1). The red arrows indicate the moving direction of corresponding species in the reaction steps. The marked distances are in Å.

The products of this step, ethoxy and H, occupy their favorable adsorption sites, respectively. The step is exothermic by −0.72 eV. A low energy barrier (ΔE*) of 0.34 eV suggests that the step is kinetically favorable. Ethoxy transformation into acetaldehyde is proposed as the RDS in the reforming mechanism. We calculated the energy barrier and the structure of the transition state starting from the most stable configuration of the ethoxy species (hcp). During the reaction step, one of C−H bonds in the CH2 group is broken. The H atom migrates to an fcc site, and the oxygen atom transfers from the hcp site to a neighboring top site. The C−O bond is significantly shortened from 1.46 Å to 1.26 Å, indicating the formation of a CO double bond in the process. As shown in Figure 3a, the transition state is more like the dissociated species: The C−H bond is fully broken (dC−H = 2.61 Å), and the CO bond is shrunk to 1.24 Å. These results suggest an endothermic reaction of the ethoxy dehydrogenation, in agreement with the calculated reaction energy (ΔEreac) of 0.70 eV. The energy barrier of this step (0.99 eV) is much higher than the first step. The intermediates, CH3CHO and H, can rearrange on the surface to produce methyl and formaldehyde species (eq 5). We model the reaction based on the intermediate configuration generated in the last step. Compared to the dehydrogenation step of the adsorbed ethoxy, analogous reaction energy (0.72 eV) and barrier (0.92 eV) are obtained from the DFT calculation. The C−C decomposition, accompanying the capture of an adsorbed H atom, is observed in the transition state (Figure 3b). Notably, the HCHO species

is not positioned in its favorable adsorption site, mainly because of the existence of the nearby methyl species. Further adjustment of the adsorbed configuration of HCHO may decrease the reaction energy of the step. As the products of the ethoxy dehydrogenation step and the reactants of the rearrangement step are exactly the same, eqs 3 and 5 may be combined:

S1 + CH3CH2O(1) ↔ CH3(l) + HCHO(1)

(11)

This step is illustrated in Scheme 1a, in which the C−C bond of the ethoxy species is directly dissociated without the involvement of the dehydrogenation and hydrogenation processes. However, the reaction is heavily endothermic, with a reaction energy of 1.41 eV (Figure 3c). Moreover, our calculation indicates a very high decomposition barrier (ΔE*=2.03 eV) for the C−C bond cleavage. The transition state is composed of two developed species, CH3 and HCHO, adsorbed on the Co(0001) surface. The step is consequently neither thermodynamically nor kinetically favorable. Thus, considering the relatively low barriers given by eqs 3 and 5, the pathway described in eq 11 may not be feasible. It is well-known that decomposition of water is highly exothermic. Owing to the strong binding strength of H and OH species on metal surfaces, the reaction may proceed in mild conditions. This is verified by our calculations, which give a negative reaction energy of −0.63 eV and a moderate barrier of 1412



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Scheme 1. Alternative Reaction Pathway in Ethanol Reforming Reactiona

a (a) Direct decomposition of CH3CH2O to CH3 and HCHO; (b) rearrangement of CH3 and OH to CH3O and H.

0.69 eV associated with the process. The OH bond is slightly stretched to 1.06 Å in the transition state, and the OH and H are eventually relocated to their optimized adsorption site, as shown in Figure 4.

Figure 5. Calculated reaction pathways and transition state

Figure 4. Calculated reaction pathways and transition state structures k3, −3

(1)

k5, −5

structures for (a) CH3(1) + OH(1) ←⎯⎯⎯→ CH3OH(1) + S1, (b)

(1)

for 2S1 + H2O(g ) ←⎯⎯⎯→ OH + H . The red arrows represent the plausible movement direction of the H and OH species in the step. The marked distances are in Å.

k 6, −6

CH3OH(1) ←⎯⎯⎯→ CH3O(1) + H(1), and (c) CH3(1) + OH(1) ↔ CH3O(1) + H(1) The red arrows indicate the moving direction of corresponding species in the reaction steps. The marked distances are in Å.

According to eq 6, the methanol intermediate is formed by association of the CH3 and OH species. The reaction occurs much more difficultly on Co(0001) than in gas phase, because of the strong binding of CH3 and OH species to the surface (Eads= −2.04 eV for CH3 and Eads= −3.65 eV for OH), and weak binding of CH3OH. The calculation reveals a positive reaction energy of 0.88 eV. A product-like transition state is captured by the elastic-band method, and the calculated energy barrier is 0.95 eV (Figure 5a). CH3OH is further decomposed into CH3O and H in the next step (eq 7). The reaction, benefited from the strong surface adsorption of the CH3O and H species, is exothermic by −0.72 eV. The O−H bond dissociation occurs with the O atom moving away from the top site, to form an energetically favorable CH3O species onto the fcc hollow. The transition state indicates a modest energy barrier of 0.59 eV in the process. Analogous to eqs 3 and 5, eqs 6 and 7 can also be combined into one reaction step:

CH3(1) + OH(1) ↔ CH3O(1) + H(1)

CH3 and OH groups to the surface, preventing the species from moving out of their stable adsorption sites. According to the transition state structure in Figure 5c, the OH species transfers from the hcp site to the top site, and the CH3 species moves upward and is slightly tilted. The configuration makes the two species close to each other. Further dehydrogenation of methoxy to formaldehyde occurs in the following step. The transition state is searched starting from the most stable configuration of the adsorbed methoxy species. Surprisingly, the hydrogen atom does not migrate from the CH3 moiety to the hollow site as expected. Alternatively, the hydrogen atom transfers to the O atom (dOH = 0.99 Å) in the calculated transition state (Figure 6a). The C−O bond length (1.35 Å) is between a C−O single bond and a CO double bond. The reaction proceeds with a high barrier of 2.29 eV and a positive reaction energy of 1.01 eV. Since the pathway evolves with a high barrier, it is possible that (an)other pathway(s) for the reaction exist. We further tested an alternative reaction pathway involved in direct H adsorption on the metal surface, as shown in Figure 6b. The formaldehyde molecule adopts the bridge-O and bridge-C geometry. The reaction energy is less positive (0.65 eV) due to the more stable adsorption mode, and the energy barrier also reduces significantly to 0.80 eV. Both suggest that the pathway is

(12)

The CH3OH intermediate is dismissed in eq 12. As illustrated in Scheme 1b, the CH3 and OH species approach to each other, followed by the O−H dissociation and the C−O bond generation, eventually leading to the adsorbed CH3O and H species. The reaction is energetically feasible but kinetically unfavorable, owing to the presence of a huge barrier of 2.51 eV. The huge barrier can be attributed to the strong binding of the 1413



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Figure 8. Calculated reaction pathway and transition state structure for k 9, −9

2S1 + HCOOH(1) ←⎯⎯⎯→ CO2(1) + 2H(1) The red arrows indicate the moving direction of corresponding species in the reaction step. The marked distances are in Å.

reaction. The two H atoms are detached from the HCOOH molecule and adsorbed on the surface, and the remaining part is straightened to form a linear CO2 molecule. There is no obvious C−H or O−H bond dissociation observed in the transition state. The reaction proceeds with a low barrier of 0.22 eV and is exothermic by −1.25 eV. 3.3. Discussion in Relation to Experimental Results. Observations of intermediate species for ethanol steam reforming have been reported on the basis of DRIFTS (diffuse reflectance infrared fourier transform spectroscopy) analysis on cobalt based catalysts.51 DRIFTS studies the surface chemistry of high surface area powders showing functional group absorption bands of chemical species staying on a solid surface or in gaseous phase. Lin et al.51 reported bands attributed to gaseous ethanol, acetaldehyde, acetone, CO2, CO, and methane produced on a cobalt (hcp) catalyst without support. They claimed that, at the outset of the reforming reaction, the formation of carbon dioxide was observed along with that of acetone. In the proposed reaction pathway, ethanol dehydrogenates to acetaldehyde, which is converted into acetone and CO2 by aldol condensation. On the other hand, Song and Ozkan52 reported the presence of absorption bands for ethanol, ethoxy, acetate, and CO2 species on a Co/ZrO2 catalyst. These species were observed, in the same order, when the temperature was increased through the experiment. A reaction pathway was suggested where ethanol dehydrogenates to ethoxy species, which dehydrogenates, again, to produce acetaldehyde. Next, the latter one is oxidized to acetate species and finally decomposed to CO2 and methane. Similarly, Da Silva et al.53 reported the observation of absorption bands for ethoxy, acetyl, acetate, CO2, and methane species on a Co/ CeO2 catalyst. These species were detected, in the same order, at a series of increasing temperatures through the experiment. Accordingly, in their suggested reaction pathway, ethanol adsorbs as ethoxy species and may follow one of the two distinct routes: (i) decomposition and production of CO, CH4, and H2 or (ii) dehydrogenation to acetaldehyde and acetyl species, which can undergo oxidation to acetate species and decomposition to CO2 and methane. Our DFT calculations show that dehydrogenation steps of ethanol to ethoxy species (reaction 2) and ethoxy to acetaldehyde (reaction 3) are feasible, in agreement with DRIFTS observations reported in the literature over Co-based catalysts. Also, the DFT results show evidence that the decomposition step of ethoxy species to formaldehyde and methyl species (reaction 11) is not feasible. This suggests the need of consecutive dehydrogenation steps until the appearance of a chemical species where the C−C bond breaking may be

Figure 6. Calculated reaction pathways and transition state structures k 7, −7

for CH3O(1) + S1 ←⎯⎯⎯→ HCHO(1) + H(1) The red arrows indicate the moving direction of corresponding species in the reaction step. The marked distances are in Å.

energetically more favorable. In the transition state, the C−H bond stretch and H transfer to the surface were observed. The length of C−O (1.45 Å) indicates that a single-bond still remains in the TS. The formaldehyde molecule interacting with OH yields formic acid and hydrogen in the following step (eq 9). The process includes dissociation of the C−H bond and formation of the C−O bond. In the transition state, the C−H bond is broken, and the C−O bond is not formed, as illustrated in Figure 7. Our calculation reveals that a single-step reaction may

Figure 7. Calculated reaction pathway and transition state structure for k8, −8

HCHO(1) + OH(1) ←⎯⎯⎯→ HCOOH(1) + H(1) The red arrows indicate the moving direction of corresponding species in the reaction step. The marked distances are in Å.

not be feasible for the process due to the high energy barrier (2.23 eV). We therefore infer that the step is not an elementary reaction: The process may be composed of two steps: (1) dehydrogenation of HCHO and (2) association between CHO and OH. We are examining these steps, and the related results will be presented in a future report. Decomposition of the formic acid molecule yields the final product, CO2 (eq 10). As shown in Figure 8, the parallel configuration of HCOOH is favorable for the decomposition 1414



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feasible. Such chemical species may be acetyl, as suggested by Song and Ozkan52 and da Silva et al.53 Additionally, these DFT calculations proved the feasibility of CO2 and H2 formation from formic acid (reaction 10), which may suggest that is possible to get CO2 from the acetate species, as proposed by Song and Ozkan52 and da Silva et al.53 Similarly, the predicted feasibility of the reaction step for methanol formation from methyl and OH species (reaction 6) suggests that the acetyl oxidation step to acetate species, proposed by Song and Ozkan52 and da Silva et al.53 may be accomplished by the OH species (from water). Alternatively, the presence of the O species may be provided by an oxide support such as ceria.

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CONCLUSIONS The ethanol steam reforming mechanism on the Co(0001) surface was systematically examined using periodic DFT, following the reaction steps proposed by Sahoo et al. on the basis of experimental observations. The calculated adsorption configurations, energy barriers, and transition state structures were used to elucidate the reaction pathway. In general, the suggested mechanism is feasible. In particular, two intermediates, acetaldehyde and methanol, were found to be indispensable for the reaction mechanism. Direct decomposition of CH3CH2O to CH3 and HCHO results in a very high energy barrier and therefore is kinetically forbidden. The alternative pathway, dehydrogenation of CH3CH2O to CH 3CHO, followed by a rearrangement step, is kinetically favorable. Similarly, the rearrangement of CH3 and OH to CH3O and H via the intermediate CH3OH leads to much lower barriers. Furthermore, our study indicated that the proposed reaction of formaldehyde with OH yielding formic acid and hydrogen may not be an elementary step based on the calculated transition state and energy barrier. Instead, there could possibly be two or more steps. Except for this questionable step, the dehydrogenation of adsorbed ethoxy possesses the highest energy barrier of 0.99 eV, in agreement with the suggested mechanism.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Universidad Industrial de Santander, Bucaramanga, Colombia.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted with financial support by the Department of Energy, Basic Energy Sciences, grant DE-FG0205ER15729. Computational resources from Texas A&M University Supercomputer center, from Brazos computer cluster at TAMU, from the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DEAC03-76SF00098, and from the University of Texas at Austin TACC system are gratefully acknowledged.

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Ethanol Reforming on Co(0001) Surfaces: A Density Functional

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