Density Functional Theory Study of Methane Oxidation and Reforming

Article pubs.acs.org/IECR

Density Functional Theory Study of Methane Oxidation and Reforming on Pt(111) and Pt(211) Ying Chen† and Dionisios G. Vlachos*,† †

Catalysis Center for Energy Innovation (CCEI), Center for Catalytic Science and Technology (CCST), and Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716-3110, United States ABSTRACT: The activation barriers of elementary-like reactions pertaining to the oxidation and reforming of methane on Pt(111) and Pt(211) surfaces have been calculated using periodic density functional theory (DFT) calculations. We have investigated the adsorption of CHx(x=1−3)OH and CHx(x=1−3)O, all the O and OH-assisted dehydrogenation reactions of CHx(x=1−4), all the C−O bond coupling reactions forming C1 oxygenates, and their subsequent dehydrogenation. It has been found that (i) COH and CHO are the most stable C1 oxygenates on Pt(111) and Pt(211), repectively; (ii) In the presence of O on Pt(211), oxidative dehydrogenation of CH by O is more kinetically favorable than the pyrolytic CH dehydrogenation; (iii) CO can be generated by oxidation of C with a low reaction barrier on Pt(211); (iv) The reactions involving OH and the dehydrogenation of CHx(x=1−3)OH and CHx(x=1−3)O appear to be secondary reaction pathways on Pt. Based on the activation barriers, we conclude that the major reaction pathways on Pt(111) and Pt(211) are CH4 (g) → CH3ad → CH2ad → CHad → Cad, Cad + Oad → COad and CH4 (g) → CH3ad → CH2ad → CHad, CHad + Oad → Cad, CHad + Oad → COad, respectively. Low coordination sites, such as steps, exhibit lower barriers for pyrolytic dehydrogenation except for the reaction CH2ad + * → CHad + Had that is preferred on terraces. In addition, they are lower barrier sites in the oxidation of C and CH and, thus, are expected to play a key role in partial and total oxidation of methane. In methane steam reforming, OHad may play a role only in the last step of C oxidation and certainly in the water-gas shift reaction, and, thus, this process consists of nearly decoupled methane catalytic pyrolysis and water-gas shift reactions. exhibits high conversions and selectivities to syngas.4−7 Yet, a fundamental understanding of the reaction mechanism of methane oxidation over Pt remains elusive. Several surface reaction mechanisms for methane CPOX on Pt have been proposed.6,8−11 Schmidt and co-workers6 developed a model with a 19-step surface mechanism for the oxidation of methane and proposed CO and H2 are major products of methane CPOX via a pyrolysis mechanism under CH4-rich conditions at high temperatures followed by oxidation of C to CO (and possibly CO2). Using a hierarchical multiscale approach, Mhadeshwar and Vlachos11 developed a microkinetic model for methane CPOX on Pt, consisting of 104 elementarylike steps using the bond-order conservation (BOC) estimation method. They showed that CHx dehydrogenation mainly happens via the oxygen-assisted paths in the oxidation zone, followed by steam reforming reactions. Using a similar approach and massive kinetically relevant data-injection from the same lab, Maestri et al. developed a fairly comprehensive C1 mechanism for CPOX of methane on Rh12 and showed that CPOX may consist of up to three reaction zones.13 Recently, Iglesia and coworkers14,15 described CH4−O2 reactions on Pt nanoparticles (using O2, H2O, or CO2 as oxidants) using kinetic and isotopic data in combination with density functional theory (DFT) and showed multiple distinct kinetic regimes. In addition, they demonstrated different kinetically relevant steps in each regime.

1. INTRODUCTION Methane is an important feedstock for various industrially relevant chemicals such as methanol, ammonia, hydrogen, and potentially for liquid hydrocarbons via Fischer−Tropsch synthesis.1,2 Generally, the conversion of methane to synthesis gas (a mixture of CO and H2) is performed via steam reforming (SR), partial oxidation (POX) (catalytic or vapor phase), or dry reforming (DR), as shown in Figure 1. Catalytic POX (CPOX) of methane presents a promising alternative to commercial SR due to short residence times and the fact that it is an energy selfsufficient (autothermal) process eliminating the need for external heat supply that makes the process inherently slow.3 In addition, it has been demonstrated that CPOX of CH4 on noble metals

Received: Revised: Accepted: Published:

Figure 1. Schematic of methane chemistry. © 2012 American Chemical Society

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July 6, 2012 September 1, 2012 September 4, 2012 September 4, 2012 dx.doi.org/10.1021/ie301792g | Ind. Eng. Chem. Res. 2012, 51, 12244−12252

Industrial & Engineering Chemistry Research

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All reactions were simulated on the flat Pt(111) and the stepped Pt(211). In studying reactions on terraces, four layers of metals were modeled, and the vacuum region between slabs was around 15 Å. A p(2 × 2) unit cell and surface Monkhorst Pack meshes of 5 × 5 × 1 k-point sampling in the surface Brillouin zone were used for consistency to our previous work.19 The bottom two layers of metal atoms were fixed, and the top two layers and the adsorbates were relaxed. For steps, a repeated slab of twelve Pt(211) layers and a p(2 × 1) unit cell were employed. The surface Monkhorst Pack meshes of 3 × 4 × 1 k-point sampling in the surface Brillouin zone were used on the stepped surface. The top six layers and the adsorbates were relaxed. Convergence with respect to lateral unit cell dimensions has been checked on a 3 × 3 four-layer unit cell with the top 2 layers relaxed, and the surface Brillouin zone was sampled with 3 × 3 × 1 k-points. The reaction barrier of initial dehydrogenation of methane on 3 × 3 slabs (0.53 eV), which is the key step during methane activation, is only 0.1 eV lower than on 2 × 2 slabs (0.63 eV).19 This difference is insignificant for practical purposes. For example, fine-tuning of reaction parameters to match experimental data requires often larger changes in reaction barriers. In addition, coverage effects on reaction barriers are often much larger than cell size effects (e.g., the former can be of the order of 1 eV or more). The transition states (TSs) were identified using the new constrained Broyden minimization technique as described in detail elsewhere.30−33 The distance between the reactants is constrained at an estimated value, and the total energy of the system is minimized with respect to all the other degrees of freedom. The TSs are located via changing the fixed distance and are confirmed using two rules:30−33 (i) all forces on atoms vanish and (ii) the total energy is a maximum along the reaction coordinate but a minimum with respect to the rest of the degrees of freedom. Binding energies (BEs) are defined as

In three of these regimes, C−H bond activation is the only kinetically relevant step, but the active site varies as the O* coverage decreases. O2 dissociation on Pt clusters becomes the sole kinetically relevant step at a sufficiently low O2/CH4 ratio (low O* coverage). Recently, dehydrogenation of methane on Pt has been studied intensively via DFT.16−20 It was found that the activation barriers of dehydrogenation of CHx are less than 1 eV except for the final step (CH* → C* + H*) and the dehydrogenation barrier can be reduced at low-coordinated sites (like edges, corners). On the other hand, relatively few DFT studies have been devoted to the oxidation and reforming of methane on Pt. Au et al.21 have investigated partial oxidation of methane to syngas on a Pt10 cluster. It was claimed that H adsorption can be increased in the presence of oxygen on metal top sites, thereby promoting methane dissociation. The electro-oxidation of methane on a Pt10 cluster at the electrochemical interface has also been examined by Psofogiannakis et al.22 They concluded that dissociative adsorption of methane (CH4(g) → CH3* + H*) is the rate determining step in the electro-oxidation of methane. Even though the catalytic oxidation and reforming of methane on Pt has been studied both experimentally and theoretically, a comprehensive surface reaction network for methane oxidation and reforming on Pt via DFT is not readily available, especially comparing the effect of coordination number and going beyond BOC that does not treat accurately multidentate and oxygenated species. A fundamental understanding of the reaction mechanism is needed to improve catalysts and reactors for increased activity and selectivity. In this work, we examine all possible elementary steps in the CPOX and reforming of methane via DFT on both Pt(111) and Pt(211) surfaces. Elementary steps include oxidative dehydrogenation, C−O coupling reactions forming oxygenates, and dehydrogenation of these oxygenates. Finally, the major reaction pathways for methane oxidation on both surfaces are proposed and compared.

Ead = EA / Surface − (E A+Esurface)

2. METHODOLOGY All the DFT results present in this work were obtained using the SIESTA code.23 The Troullier-Martins norm-conserving scalar relativistic pseudopotentials24 as well as a double-ζ plus polarization (DZP) basis set were utilized. The DZP basis set has been tested quite extensively by several groups. The DZP basis offers quite well converged results comparable to those used in practice in most plane-wave calculations,25 striking a good balance between accuracy and computational cost for comparable accuracy. The localization radii of the basis functions were determined from an energy shift of 0.01 eV. A standard DFT supercell approach with the Perdew-Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA) functional26 was implemented with a mesh cutoff of 200 Ry. Calculations were considered converged when all forces on the atoms were lower than a tolerance of 0.05 eV/Å. A nonspin version of the code was utilized on the systems involving slabs with Pt atoms, as it has been previously determined that this is an adequate approximation.19 Spin polarization was included for calculations of gas-phase species. In our previous work,19 the basis set superposition error (BSSE) has been tested for CHx dehydrogenation reactions, which showed that the differences between the energies with and without BSSE correction were small (within 0.2 eV). The calculated equilibrium lattice constant of Pt bulk was 4.02 Å that is close to the experimental one (3.92 Å)27 and previous theoretical results.28,29

(1)

where EA/Surface, EA, and ESurface are the total energies of the catalyst with the adsorbate and the isolated adsorbate in vacuum and the clean surface, respectively.

3. ENERGETICS AND STRUCTURES In the methane oxidation mechanism, the following elementarylike (or groups of) reactions can occur: O2 (g) + 2* → O2ad → Oad + Oad

(2)

CHx(x = 1 − 4)ad + * → CHx(x = 0 − 3)ad + Had

(3)

CHx(x = 1 − 4)ad + Oad → CHx(x = 0 − 3)ad + OHad

(4)

CHx(x = 1 − 4)ad + OHad → CHx(x = 0 − 3)ad + H 2Oad

(5)

CHx(x = 0 − 3)ad + Oad → CHx(x = 0 − 3)Oad + *

(6)

CHx(x = 0 − 3)ad + OHad → CHx(x = 0 − 3)OHad + *

(7)

CHx(x = 1 − 4)Oad + * → CHx(x = 0 − 3)Oad + Had

(8)

COad + H 2Oad → CO2 (g) + H 2(g) + 2*

(9)

Reaction 2 (R2) describes the dissociative adsorption of oxygen, and R3 describes the dissociative adsorption of methane followed by pyrolytic dehydrogenation. R4 and R5 describe 12245

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least favorable and CH3O prefers a bridge site on Pt(111). Only adsorption of oxygenated C1 species on Pt(111) has been reported in previous work,35,36 whereas we calculated all the oxygenated C1 species on both Pt(111) and Pt(211) surfaces. Table 1 shows that all the CHxO and CHxOH species, except CH3O on Pt(111), are more stable than the reference state, and all the oxygenated C1 species on Pt(211) are more stable than on Pt(111). In addition, COH and CHO appear to be the most stable C1 oxygenates on Pt(111) and Pt(211), respectively. 3.2. Oxidative Dehydrogenation (ODH) of CHx by O and OH. In the presence of oxygen on the catalyst (catalytic combustion and upstream in CPOX), ODH of C1 species can occur. Thus, we calculated all the ODH reactions of hydrocarbon species on Pt(111) and Pt(211) surfaces by O and OH. All the reactions barriers and the distances between H and C at the TSs are listed in Table 2, and the TSs on Pt(111) and Pt(211) surfaces are shown in Figure 2 (a) and (b), respectively. From the configurations of the TSs, one can clearly see some general trends for O and OH: (i) At the TSs on Pt(111), most of the O atoms are at the bridge site and all the OH are at the top site; only at the TS of CHad + Oad → Cad + OHad, O is on the top site; (ii) At the TSs of ODH of CH4 and CH3 on Pt(211), the O atoms are at a ridge-atop site (not at the bridge site) and OH are on the top site; (iii) At the TSs of ODH of CH2 and CH on Pt(211), the O atoms are at a ridge-atop site and OH are at the top site of the terrace (not at the step edge); (iv) All the TS structures on both surfaces are similar to the ISs except for the ODH of CH3. The calculated activation barriers of ODH of CHx via O are significantly larger than those of the pyrolytic dehydrogenation of CHx on both surfaces. For example, the activation barrier of reaction 1 (CH4(g) + Oad → CH3ad + OHad) is over 1 eV, which is larger than that of methane dissociation on clean Pt surface19 (0.63 eV on Pt(111) and 0.21 eV on Pt(211)). O-assisted C−H bond breaking reactions have also been investigated by Chin et al.14,15 Indeed, they found that the methane dissociation barrier on an empty (*-*) site is much lower than on the *-O sites on a Pt201 cluster. Such large barriers are induced by the repulsive interactions between the adsorbed O and H-atom in CHx species which increase the TS energies. In addition, our obtained value (1.28 eV) of O-assisted methane dissociation on Pt(111) is consistent with their DFT calculation15 (1.26 eV). On the other hand, most of the OH-assisted C−H bond activation reactions on both surfaces exhibit much lower barriers than those of Oassisted dehydrogenation reactions. It is also interesting that the activation barriers of ODH of CH2 and CH via OH on Pt(211) are much higher than on Pt(111) because of larger repulsive interactions between O and H in the TSs on Pt(211) (O−H bond lengths in the TSs of reactions 7 and 8 on Pt(211) are shorter than on Pt(111), see Table 2). As a result of these findings, we do not expect that O* plays an active role in the breakdown of methane on Pt; rather, pyrolysis dominates. 3.3. O and OH Insertion Reactions. O and OH near adsorbed C1 species may lead to O and OH insertion reactions. In section 3.1, the stabilities and configurations of oxygenated C1 species have been summarized. Table 3 presents all the O and OH insertion reaction barriers and the distances between C and O at the TSs. The geometries of TSs on Pt(111) and Pt(211) surfaces are shown in Figure 3a (a)-(h) and Figure 3b (i)-(p), respectively. Figure 3 shows that on Pt(111) (i) O atoms and OH at the TSs are at the bridge site and on the top sites, respectively; (ii) only CH3 at the TSs is on the top site and other C1 species are at the bridge sites. On Pt(211), we examined several types of ISs at first and determined the most stable IS. Starting with the

oxidative dehydrogenation (ODH) that may become relevant when the coverage of oxygen is high. R6 and R7 describe addition of the oxidizer to the hydrocarbon fragment to form oxygenated species. R8 in turn describes dehydrogenation of C1 oxygenates. R9 depicts the overall water-gas shift (WGS) reaction and comprises several elementary reactions that have been studied in published work (not shown for simplicity). In our previous work,19,34 we have investigated the adsorption configurations and stabilities of C1 hydrocarbon fragments on Pt(111) and Pt(211) as well as dehydrogenation of CHx species (CHx(x=1−4)ad + * → CHx(x=0−3)ad + Had) and the WGS elementary reactions (COad + H2Oad → CO2(g) + H2(g) +2*) on both Pt surfaces. In this paper, we focus on adsorption structures of C1 oxygenates on Pt(111) and Pt(211) surfaces, oxidative dehydrogenation of C1 species via Oad and OHad (CHx(x=1−4)ad + Oad → CHx(x=0−3)ad + OHad and CHx(x=1−4)ad + OHad → CHx(x=0−3)ad + H2Oad), O and OH insertion reactions (CHx(x=0−3)ad + Oad → CHx(x=0−3)Oad + * and CHx(x=0−3)ad + OHad → CHx(x=0−3)OHad + *), and dehydrogenation of oxygenated C1 species (CHx(x=1−4)Oad + * → CHx(x=0−3) Oad + Had). These reactions are potentially important in reforming and oxidation of methane. 3.1. Adsorption of CHxO and CHxOH Species on Pt(111) and Pt(211). Most of CHxO and CHxOH species derived from methanol on Pt(111) have been examined by several groups.35,36 According to the most stable geometries provided, we calculated CHxO and CHxOH species on Pt(111) and Pt(211). A summary of energetics for the most stable CHxO and CHxOH species on Pt(111) and Pt(211) is provided in Table 1. In our work, the BEs Table 1. Energetics of CHxO and CHxOH Species on Pt(111) and Pt(211) (eV)b binding modea CHxO and CHxOH

Pt(111)

Pt(211)

Pt(111)

Pt(211)

CH3OHad CH3Oad + Had CH2OHad + Had CH2Oad + 2Had CHOHad + 2Had CHOad + 3Had COHad + 3Had

a1b1(O) a1b1(O) a1b1(C) a2b2(C, O) a1b2(C) a2b3(C, O) a1b3(C)

a1b1(O) a1b2(O) a1b1(C) a2b2(C, O) a1b2(C) a2b3(C, O) a1b3(C)

0 0.38 −0.50 −0.20 −0.82 −0.97 −1.39

0 −0.59 −0.75 −0.96 −1.56 −2.10 −1.88

a The nomenclature aibj designates that i atoms of the adsorbate are bonded to j-metal atoms on the surface. bReference state: CH3OHad and excess H adsorbs on a separate slab. The adsorption energies of CH3OH are both −0.34 eV on Pt(111) and Pt(211), respectively, and the adsorption energies of H are 0.62 and 0.78 eV on Pt(111) and Pt(211), respectively.

of CH3OH on Pt(111) and Pt(211) are −0.34 eV, which are in very good agreement with the values reported by Greeley and Mavrikakis35 and slightly lower than those of Desai et al.36 (−0.45 eV). Most of the optimized geometries of CHxO and CHxOH species are very close to previous results.35,36 For example, formaldehyde (CH2O) prefers a top-bridge-top (disigma) configuration on Pt(111), and the bonding distances of Pt−C and Pt−O are 2.11 Å and 2.06 Å, respectively. In previous work,35,36 the Pt−C and Pt−O bond lengths were 2.12−2.17 Å and 2.06−2.10 Å, respectively. There is a debate regarding the adsorption site of methoxy (CH3O) on Pt(111). The atop configuration is the most favorable in our work, which is the same as that reported by Greeley et al.;35 in contrast, Desai et al.36 reported that chemisorption of CH3O at atop sites is the 12246



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Table 2. Activation Barriers (Ea (eV)) and Distances (d (Å)) at the TSs of Oxidative Dehydrogenation (ODH) Reactions of C1 Species by O and OH and Reaction Energies (ΔH (eV)) on Pt(111) and Pt(211)a Pt(111)

a

Pt(211)

no.

reactions

Ea (eV)

d (Å)

ΔH (eV)

Ea (eV)

d (Å)

ΔH (eV)

1 2 3 4 5 6 7 8

CH4(g) + Oad → CH3ad + OHad CH3ad + Oad → CH2ad + OHad CH2ad + Oad → CHad + OHad CHad + Oad → Cad + OHad CH4(g) + OHad → CH3ad + H2Oad CH3ad + OHad → CH2ad + H2Oad CH2ad + OHad → CHad + H2Oad CHad + OHad → Cad + H2Oad

1.28 1.62 1.44 2.13 0.50 1.07 0.31 0.77

1.456 1.187 1.610 1.478 1.449 1.136 1.616 1.422

−0.13 0.22 −0.50 0.62 −0.86 −0.51 −1.22 −0.10

1.24 2.04 1.16 0.80 0.68 1.06 1.31 1.97

1.302 1.148 1.335 1.400 1.481 1.160 1.312 1.353

−0.88 −0.77 0.77 0.03 −0.25 −0.14 −0.13 0.65

The distances are those between H and O at the TSs. In the initial states (ISs), C1 species and O or OH are on separate slabs.

Figure 2. (a) Top view of the calculated TS structures for oxidative dehydrogenation of CHx(x=1−4) species via O and OH on Pt(111) (a)-(h): (a) CH4− O, (b) CH3−O, (c) CH2−O, (d) CH−O, (e) CH4−OH, (f) CH3−OH, (g) CH2−OH, (h) CH−OH. Pt atoms are represented with ● (blue), O atoms with ● (red), C atoms with ● (gray), and H atoms with ● (white). This notation is used throughout this paper. (b) Side view of the calculated TS structures for oxidative dehydrogenation of CHx(x=1−4) species via O and OH on Pt(211) (i)-(p): (i) CH4−O, (j) CH3−O, (k) CH2−O, (l) CH−O, (m) CH4−OH, (n) CH3−OH, (o) CH2−OH, (p) CH−OH.

Table 3. Activation Barriers and Distances at the TSs of the O and OH Insertion Reactions of C1 Species and Reaction Energies (ΔH (eV)) on Pt(111) and Pt(211)a Pt(111)

a

Pt(211)

no.

reactions

Ea (eV)

d (Å)

ΔH (eV)

Ea (eV)

d (Å)

ΔH (eV)

9 10 11 12 13 14 15 16

CH3ad + Oad → CH3Oad + * CH2ad + Oad → CH2Oad + * CHad + Oad → CHOad + * Cad + Oad → COad + * CH3ad + OHad → CH3OHad + * CH2ad + OHad → CH2OHad + * CHad + OHad → CHOHad + * Cad + OHad → COHad + *

2.04 2.18 1.69 1.92 2.02 1.05 1.17 1.08

1.994 2.002 1.912 2.027 2.012 1.879 1.981 2.015

0.34 −0.29 −0.42 −1.84 −0.18 −0.75 −0.42 −1.47

1.68 1.93 1.65 0.59 1.80 1.81 1.56 1.17

1.951 2.939 2.012 1.933 1.997 2.144 2.260 2.062

−0.11 −0.01 −0.70 −1.76 0.79 0.50 0.15 −0.50

The distances are those between C and O at the TSs. In the initial state, C1 species and O or OH are on separate slabs.

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Figure 3. (a) Top view of the calculated TS structures for O and OH insertion reaction on Pt(111) (a)-(h): (a) CH3−O, (b) CH2−O, (c) CH−O, (d) C−O, (e) CH3−OH, (f) CH2−OH, (g) CH−OH, (h) C−OH. (b) Side view of the calculated TS structures for O and OH insertion reaction on Pt(211) (i)-(p): (i) CH3−O, (j) CH2−O, (k) CH−O, (l) C−O, (m) CH3−OH, (n) CH2−OH, (o) CH−OH, (p) C−OH.

consistent with our results on Pt(111) (1.05, 1.17, and 1.08, respectively). Additionally, similar to previous work,19 we found a good linear relationship between FSs and TSs energies (relative to the IS gas-phase energies), with a high correlation coefficient (R2=0.97), as shown in Figure 4. The linear free energy relation

most stable ISs, the TSs have been identified. On Pt(211), there are three types of TS configuration: (1) both CH3 and OHx(x=0−1) are at the step edge (reactions (9) and (13)); (2) C1 fragments sit at the step edge and OHx(x=0−1) atoms are on the terrace (reactions (10), (11), (12), and (16)); (3) C1 species and OH are near the step edge and at the step edge, respectively (reactions (14) and (15)). We have found that all the activation barriers of oxidation of C1 species are above 1 eV except for the C oxidation on Pt(211) (0.59 eV) which is 1.33 eV smaller than on Pt(111). It is worth discussing the observation of a much lower barrier for the C oxidation on Pt(211). According to the literature,37 the association barrier (Eaass) for Cad + Oad → COad can be defined by the following equation: Eaass = (ECIS + EOIS) − (ECTS + EOTS) + EintTS, where ECIS and EOIS are the chemisorption energy of C and O at the IS, respectively; ECTS is the chemisorption energy of C at the TS geometry without O; EOTS is defined in a similar way; and EintTS is a quantitative measure of the interaction between C and O at the TS. Although the binding energies of C and O at the IS on Pt(211) are slightly higher than those on Pt(111), the interaction between C and O at the TS on Pt(211) is attractive (EintTS = −0.17 eV) and on Pt(111) repulsive (EintTS = 0.79 eV). In addition, the chemisorption of C and O at the TS on Pt(211) is 0.51 eV more stable than on Pt(111). Therefore, the activation barrier of C oxidation on Pt(211) is much smaller than on Pt(111). On both surfaces, O insertion is kinetically unfavorable. By comparison, OH addition is more favorable on Pt(111) but not so on edges. Three oxidation reactions of CHx species on Pt10 cluster have been examined by Psofogiannakis et al.22 They reported that the activation barriers of reactions (14), (15), and (16) are 1.03, 1.17, and 0.93 eV, respectively, which are

Figure 4. Correlation plot for the O and OH insertion reactions of C1 species on Pt(111) and Pt(211) surfaces calculated using DFT. Reaction direction is defined in the exothermic direction.

is ETS (eV) = 0.92 and EFS (eV) +1.36 (eV) for the O and OH insertion reactions of C1 species, and the mean average error (MAE) is 0.44 eV. Such correlations can be valuable in estimating reaction barriers on other catalysts or for larger hydrocarbons on Pt using the homologous series concept. Such transition state scaling (TSS) correlations (relating transition state to initial or final state energies) have become very popular in recent years compared to the traditional Brønsted-Evans-Polanyi (BEP) relations. 12248



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Table 4. Activation Barriers in This Work and in Previous Work (in Parentheses) and Distances at the TSs of Dehydrogenation Reactions of CHxO and CHxOH Species and O−H Bond Cleavage for CHxOH on Pt(111) and Pt(211) Surfacesa Pt(111)

a

Pt(211)

no.

reactions

Ea (eV)

d (Å)

ΔH (eV)

Ea (eV)

d (Å)

ΔH (eV)

17 18 19 20 21 22 23 24 25 26

CH3OHad + * → CH2OHad + Had CH2OHad + * → CHOHad +Had CHOHad + * → COHad + Had CH3OHad + * → CH3Oad + Had CH2OHad + * → CH2Oad + Had CHOHad + * → CHOad + Had COHad + * → COad + Had CH3Oad + * → CH2Oad + Had CH2Oad + * → CHOad + Had CHOad + * → COad + Had

0.77 (0.6738) 0.63 (0.77,22 0.6338) 0.62 (1.14,220.8038) 0.80 (0.8138) 0.99 0.54 (0.35,22 0.4338) 0.84 (1.18,22 0.9738) 0.24 0.14 0.36

1.471 1.604 1.366 1.513 2.030 1.597 1.337 1.709 1.424 1.195

−0.49 −0.33 −0.57 0.38 0.30 −0.14 −0.52 −0.57 −0.77 −0.95

0.48 0.51 0.96 0.49 0.99 0.62 0.92 0.54 0.25 1.11

1.448 1.689 1.357 1.771 1.700 1.548 1.311 1.686 1.404 1.315

−0.75 −0.81 −0.32 −0.59 −0.21 −0.54 −0.94 −0.37 −1.14 −0.72

The distances are those between C and H at the TSs. In the ISs, C1 oxygenates and adsorbed H are in the separate slabs.

Figure 5. (a) Top view of the calculated TS structures dehydrogenation of C1 oxygenates on Pt(111) (a)-(j): (a) H−CH2OH, (b) H−CHOH, (c) H− COH, (d) CH3O−H, (e) CH2O−H, (f) CHO-H, (g) CO-H, (h) H−CH2O,(i) H−CHO,(j) H−CO. (b) Side view of the calculated TS structures dehydrogenation of C1 oxygenates on Pt(211) (k)-(t): (k) H−CH2OH, (l) H−CHOH, (m) H−COH, (n) CH3O−H, (o) CH2O−H, (p) CHO-H, (q) CO-H, (r) H−CH2O, (s) H−CHO,(t) H−CO.

3.4. Dehydrogenation of C1 Oxygenates. During oxidation, it is likely to produce C1 oxygenates. Therefore, we considered all the relevant reactions of C1 oxygenates which consist of the dehydrogenation reactions of CHxO and CHxOH

species and O−H bond cleavage in CHxOH on Pt(111) and Pt(211) surfaces. All the reaction barriers and the distances between H and C and O and H at the TSs are listed in Table 4, 12249



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It is worth focusing on the initial dehydrogenation of methane which was identified as a rate-limiting step.22 Chin et al.14,15 calculated C−H bond activation barriers on *_*, *_Oad, and Oad_Oad site pairs as the Oad coverage increases. They showed that the activation barrier of methane dissociation on free sites (*_*) is much lower than on the other two types of sites. Consistently with their work, our activation barriers of dehydrogenation of methane on clean Pt(111) and Pt(211) surfaces are much smaller than those of the O-‘assisted’ dehydrogenation barriers. In this work, we also considered OH-‘assisted’ dehydrogenation of methane and found that OHassisted dehydrogenation barriers are similar to that of methane dehydrogenation on a clean Pt(111) surface but higher than those on a clean stepped surface Pt(211). However, the coverage of OH is likely low in methane oxidation. Thus, based on the activation barriers, pyrolytic methane dissociation is kinetically favorable on clean stepped surfaces, such as Pt(211). Figure 7 indicates that the major reaction pathways on Pt(111) and Pt(211) are CH4 (g) → CH3ad → CH2ad → CHad → Cad, Cad +Oad→ COad and CH4 (g) → CH3ad → CH2ad → CHad, CHad + Oad → Cad, Cad + Oad→ COad, respectively. Regarding steam reforming, given that the barriers for oxidative dehydrogenation of CHxad via OHad are usually comparable or higher than those of the pyrolysis steps, and the former are bimolecular reactions and probably limited from the OHad availability on the metal, we expect that OH chemistry becomes relevant only in the oxidation of C to COHad and then in the WGS chemistry (of COad via the COOHad intermediate) that is fairly fast.34 In other words, we expect methane catalytic pyrolysis chemistry followed by WGS chemistry of COad without significant coupling between the two process chemistries besides ‘cleaning’ of the surface sites from COad via the WGS reaction. This freeing up of sites in turn may have an effect on the overall reforming reaction rate, but this is expected to be small at high temperatures due to WGS being equilibrium limited, i.e., the overall effect on alumina support appears to be insignificant. Similar findings apply to the SR of oxygenates on Pt.39 Recently, Psofogiannakis et al.25 proposed a reaction pathway on Pt10 cluster: CH4 (g) → CH3ad → CH2ad → CHad → CHOHad → CHOad → COad. While it is slightly easier to convert CH to CHOH than to dehydrogenate CH to C on Pt(111), the surface OHad is likely a minority species. Therefore, we suggest dehydrogenation of CH to C rather than oxidation of CH as a major reaction pathway.

and the TSs on Pt(111) and Pt(211) surfaces are shown in Figure 5a (a)-(j) and Figure 5b (k)-(t), respectively. Table 4 shows that our DFT results are similar to those of previous studies.22,35,38 At the TSs, most of the breaking H atoms are at a ridge-atop or atop site. On Pt(111), most of the CHxO and CHxOH species at the TSs are close to the FSs, and only CH2O and CHO at the TS in dehydrogenation reactions are close to the IS. On Pt(211), all the reactions occur at the step edge except for reaction (22). Although all the reaction barriers for dehydrogenation reactions of CHxO and CHxOH species and O−H cleavage of CHxOH are less than 1 eV, we propose that the dehydrogenation reactions of C1 oxygenates are not major paths during methane oxidation reaction given the difficulty in forming these species. We have plotted FS energies against TS energies both relative to the IS gas-phase energy. A good linear free energy relationship with a high correlation coefficient (R2=0.96) is seen, as shown in Figure 6. The linear regression equation is ETS (eV) = 0.86 EFS (eV) + 0.66 (eV) and MAE is 0.23 eV.

Figure 6. Correlation plot for dehydrogenation reactions of CHxO and CHxOH species and O−H cleavage for CHxOH on Pt(111) and Pt(211) surfaces calculated using DFT values. Reaction direction is defined in the exothermic direction.

4. REACTION NETWORK The reaction networks of methane oxidation on both Pt(111) and Pt(211) surfaces have been summarized in Figure 7. Based on the activation barriers, the following observations can be made: (i) On Pt(111) and Pt(211), methane can dissociate with low barriers to methyl and then further dehydrogenate to C and the highest barrier is 1.29 eV in the CHad + * → Cad + Had; (ii) In the presence of O on Pt(111), the barriers of oxidative dehydrogenation and O insertion reactions are over 1 eV, and, thus, we do not believe that these steps are kinetically relevant, especially at low temperatures; (iii) In the presence of O on Pt(211), O-assisted dehydrogenation of CH (0.80 eV) is more kinetically favorable than dehydrogenation of CH (1.29 eV); (iv) C can be oxidized to form CO with low barrier (0.59 eV) on Pt(211); (v) Although the barriers of OH-assisted dehydrogenation reactions are low on both Pt(111) and Pt(211) surfaces, the coverage of OH is low in methane oxidation and reforming. Therefore, the reactions involving OH addition and dehydrogenation are most probably minor reaction pathways; (vi) The formation of CHx(x=1−3)OH and CHx(x=1−3)O is not favored during methane oxidation and reforming, and, thus, dehydrogenation of CHx(x=1−3)OH and CHx(x=1−3)O can be ignored.

5. CONCLUSIONS In this work, density functional theory (DFT) calculations have been ultilized to provide for the first time insights from first principles into the catalytic partial and complete oxidation and reforming of methane on Pt(111) and Pt(211) surfaces. We examined the structures and stabilities of C1 oxygenates, oxidative dehydrogenation of C1 species via Oad and OHad, O and OH insertion reactions, and dehydrogenation of oxygenated C1 species on both Pt(111) and Pt(211) surfaces. We found that most of the species are more stable on Pt(211) than on Pt(111) and COH and CHO are the most stable C1 oxygenates on Pt(111) and Pt(211), repectively. Key conclusions include the following: (i) On Pt(111) and Pt(211), methane can dissociate with low barriers to methyl and then further dehydrogenate to C; (ii) In the presence of O on Pt(211), O-assisted dehydrogenation of CH is more kinetically favorable than dehydrogenation of CH; (iii) C can be oxidized to CO with a low barrier on Pt(211), whereas this step is highly activated on terraces; (iv) 12250



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Figure 7. Reaction network of methane oxidation with activation barriers and reaction energies in parentheses (in eV) from DFT calculations on Pt(111) (a) and Pt(211). The major reaction pathways are highlighted in red. Values with superscript ‘a’ are from ref 19.

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The reactions involving OH addition and the dehydrogenation of CHx(x=1−3)OH and CHx(x=1−3)O appear to be minor reaction pathways on Pt surfaces in methane oxidation. The overall pathways of methane chemistry on Pt(111) and Pt(211) entail the following: CH4 (g) → CH3ad → CH2ad → CHad → Cad, Cad +Oad→ COad and CH4 (g) → CH3ad → CH2ad → CHad, CHad + Oad → Cad, Cad + Oad→ COad, respectively. With the exception of CH2ad + * → CHad + Had, which is preferred on closed packed surfaces, low coordination sites are expected to play a key role in methane oxidation in both pyrolysis steps (initial dehydrogenation reactions of CH4ad and CH3ad) and in late oxidative dehydrogenation of CHad to Cad and the oxidation of Cad to COad. These findings have important ramifications regarding the structure sensitivity of these reactions, and kinetic Monte Carlo simulations will be necessary to fully address this topic.34 Finally in steam reforming of methane, OHad may play a role only in the last step of Cad oxidation and certainly in the water-gas shift reaction. It appears that methane catalytic pyrolysis and watergas shift are nearly decoupled reactions. DFT-based microkinetic analysis and comparison to experimental data will be valuable future steps to provide quantitative insights into these complex processes.

AUTHOR INFORMATION

Corresponding Author

*Phone: +1 302-831-2830. Fax: 302-831-1048. E-mail: vlachos@ udel.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported initially by the NSF (CBET-0729701) and later on by the DOE (Grant Number DE-FG0205ER25702). The DFT calculations were performed using the TeraGrid resources provided by University of Illinois’ National Center for Supercomputing Applications (NCSA).

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REFERENCES

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