Hydrogenation of Ethylene and Dehydrogenation and Hydrogenolysis

J. Phys. Chem. C 2010, 114, 4973–4982

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Hydrogenation of Ethylene and Dehydrogenation and Hydrogenolysis of Ethane on Pt(111) and Pt(211): A Density Functional Theory Study Ying Chen and Dionisios G. Vlachos* Department of Chemical Engineering and Center for Catalytic Science and Technology (CCST), UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: September 23, 2009; ReVised Manuscript ReceiVed: February 2, 2010

Hydrogenation of ethylene and dehydrogenation and hydrogenolysis of ethane on Pt(111) and Pt(211) have been studied using density functional theory (DFT) calculations. Adsorption of CHx and C2Hx species on Pt(111) and Pt(211) has been investigated. All the dehydrogenation and hydrogenation elementary-like reactions of C2 species, all the C-C bond-cleavage reactions and isomerization reactions between C2 species have been calculated. The following have been found: (i) CH3C is the most stable C2 species on Pt(111) and Pt(211); (ii) on Pt(211), ethane dissociation to CH2CH2 and CH3CH is a rapid process at low surface temperatures; (iii) on Pt(111), CH3CH is expected to be rapidly consumed by dehydrogenation to CH3C, which is difficult to be further dehydrogenated to CH2C or hydrogenated to CH3CH; (iv) isomerization reactions are not energetically favored on Pt; (v) on Pt(111), the lowest barrier of C-C cleavage is 0.9 eV in CHC, whereas on Pt(211) the lowest barrier of C-C cleavage is 1.1 eV in CH3CH2. These results suggest that at high temperatures, C-C cleavage can happen most possibly via CHCH, CH3CH, and CH3CH2 intermediates. 1. Introduction Olefin hydrogenation over a small group of transition metals including platinum is widely used in the petroleum and chemical industries. Ethylene is particularly important because it is the basic unit for polyethylene and the selective hydrogenation of acetylene (in ethylene/acetylene mixtures) is a topic of interest.1 Ethane hydrogenolysis has been generally accepted as a probe reaction for studying the reactivities of various metal catalysts.2,3 So far, hydrogenolysis of ethane has been investigated extensively using experimental2–9 and theoretical10–13 techniques. On the basis of a kinetic scheme originally proposed by Cimino et al.,9 Sinfelt and co-workers3 suggested that the ratedetermining step may involve highly dehydrogenated C2Hx species. Dumesic and co-workers4–6,12,13 carried out comprehensive work on ethane hydrogenolysis over supported platinum catalysts using microcalorimetry, spectroscopy, and density functional theory (DFT). In their theoretical work,12,13 they estimated energetics for interactions of various C2Hx species with Pt and calculated the activation energies for C-C bond dissociation of various C2Hx species. Their DFT results13 showed that the primary reaction pathway for cleavage of the C-C bond takes place through activated complexes based on C2H5 and CH3CH species. Furthermore, they combined experimental and theoretical investigations and proposed the following reaction pathway for ethane hydrogenolysis on Pt:6 (1) hydrogen and ethane adsorb dissociatively; (2) further dehydrogenation of the adsorbed C2 species occurs accompanied by the C-C bond breaking leading to C1 species; (3) finally, hydrogenation of the C1 species takes place, followed by desorption of methane. Additionally, they suggested that adsorbed CH3C and CH2C, which are the most abundant hydrocarbon species on the surface, are not directly involved in the reaction pathway but block the active sites and affect the observed kinetic rates (spectators). Recently, King and co-workers investigated ethane dissociation on Pt(110)-(1 × 2).7,8,10,11 Using supersonic molecular beam * Corresponding author: [email protected]; tel. 302-831-2830.

experiments and DFT, they identified the stabilities of C2Hx species on Pt(110)-(1 × 2). The experimental study showed that the stable dissociation products of ethane on Pt(110)-(1 × 2) at all coverages are CH2C at 350-400 K and CCH at 440 K. They also calculated the stability of C2Hx species using DFT; thermodynamic analysis indicated that CH2CH2 and CH3C are favored at low temperatures, with CHC and CHCH becoming dominant at ∼400 and 600 K, respectively. Recent DFT calculations reported two distinct transition states for the initial dehydrogenation of C2H6 to C2H5 adsorbed on Pt(110)-(1 × 2) with activation energies of 0.38 and 0.42 eV.11 They used a novel application of supersonic molecular beam scattering to identify three low-energy pathways for ethane dissociative adsorption on Pt(110)-(1 × 2)8 with translational energies of 3-80 kJ/mol. The dissociation of ethane in the low Et regime occurs via a trapping mediated mechanism, and for Et greater than 40 kJ/mol, dissociation occurs via a direct activated process, consistent with the theoretically calculated barriers of around 0.4 eV.11 Finally, they examined all the dehydrogenation barriers of C2Hx species on Pt(110)-(1 × 2)14 and reached several important conclusions: (1) There are three distinct activation energies for ethane dehydrogenation on Pt(110): low barriers with values in the range of 0.29-0.42 eV for C2H6 conversion to CH2CH2 and CH3CH; medium barriers in the range of 0.72-1.10 eV for dehydrogenation of C2H4 to CHCH and CH2C; and high barriers >1.45 eV for further dissociation. (2) Using DFT, they reported the reaction barrier of 1,2-H shift between CH3CH and CH2CH2 to be 2.31 eV. (3) Based on their experiments,7 only the C-C bond scission in acetylidene was calculated and the barrier was reported to be 1.71 eV. While several computational studies have been performed, there is still not a comprehensive DFT study for all reactions on multiple surfaces. For example, Dumesic and co-workers12,13 only reported reaction barriers for cleavage of the C-C bond in CH3CH2, CH3CH, and CH2CH on Pt(111) and Pt(211) surfaces and did not calculate any isomerization reaction between C2 species, despite proposing these to be important.

10.1021/jp909163z  2010 American Chemical Society Published on Web 02/26/2010

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TABLE 1: Chemisorption Energies for CHx (x ) 0-3) Adsorbed on the Pt(111) and Pt(211) Surfaces (eV) our results Pt(111) CH3 CH2 CH C

2.16 4.12 7.31 7.05

previous results Pt(211) 2.29 4.61 7.46 7.19

Pt(111) 24,25

28,54

2.33, 2.05, 1.77 4.06,28 4.5224,25 6.84,28 6.71,55 7.2324,25 6.60,24,25 6.86,28 7.4055

On Pt(110), King and co-workers14 studied all the elementary steps for dehydrogenation of ethane, but they only computed C-C bond scission in CHC and 1,2-H shift between CH3CH and CH2CH2. In this paper, we perform a comprehensive DFT study of the reaction intermediates and reaction barriers of elementary like reaction steps in both hydrogenation of ethylene and its reverse (ethane dehydrogenation) and hydrogenolysis of ethane on Pt(111) and Pt(211). This is the first attempt to examine all elementary relevant steps for C-C cleavage in C2 species and isomerization reactions between C2 species on both surfaces. The paper is organized as follows. In section 2, we describe calculation details. Sections 3 and 4 present DFT results and discussion, respectively. Conclusions are summarized in the last section. 2. Calculation Details Inthiswork,theSIESTAcode15 wasusedwithTroullier-Martins norm-conserving scalar relativistic pseudopotentials.16 A double-ζ plus polarization (DZP) basis set was utilized. 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) functional17 was implemented with a mesh cutoff of 200 Ry. Spin polarization was included whenever necessary, e.g., for the gaseous species. Spin polarization on Pt has not been considered since it does not affect the results significantly. The BSSE correction was found to be small. The calculated equilibrium lattice constant was 4.02 Å, which is very similar to the experimental one (3.92 Å)18 and previous theoretical results.19,20 All reactions were simulated on the flat Pt(111) and the stepped Pt(211). In the study of reactions on terraces, four layers of metals were modeled and the vacuum region between slabs was around 10 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. The bottom two layers of metal atoms were fixed, and the top two layers and the adsorbates were relaxed. In a study of reactions at steps, a repeated slab of 12 Pt(211) layers and p(2 × 1) unit cell were used. 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 was checked on a 3 × 3 four-layer unit cell with the top two layers relaxed and the surface Brillouin zone was sampled with 3 × 3 × 1 k-points. The reaction barriers on four layers of 3 × 3 slabs are around 0.1 eV lower than on four layers of 2 × 2 slabs. For example, the dehydrogenation barrier of CH3CH2 to CH2CH2 is 0.69 eV on a 3 × 3 slab, which is 0.12 eV lower than that on a 2 × 2 slab. The transition states (TSs) are searched using a constrained optimization scheme.21–23 The distance between the reactants is constrained at an estimated value, and the total energy of the

Pt(110) 55

29

2.33 4.6129 6.7229 7.1929

Pt cluster 2.33,56 1.7757 4.5256 6.71,57 7.2356 7.4057

Figure 1. Side view of the calculated structures for the second type of acetylene adsorption on Pt(211). The small ball in gray represents the C atom, the small ball in white represents the H atom, and the big ball in blue represents Pt. This notation is used throughout this paper.

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:21–23 (i) all forces on atoms vanish; (ii) the total energy is a maximum along the reaction coordinate but a minimum with respect to the rest of the degrees of freedom. Chemisorption energies are defined as

Ead ) (Esurface + EA) - EA/surface

(1)

where EA, Esurface, and EA/surface are the total energies of the isolated adsorbate in vacuum, the clean surface, and the catalyst with the adsorbate, respectively. 3. Results 3.1. Adsorption of CHx Species and H on Pt(111) and Pt(211) Surfaces. C1 hydrocarbon fragments on Pt have been investigated by several groups.12,24–29 It is generally accepted that the CHx fragment on Pt(111) preferentially occupies a site that completes the carbon tetravalency such that CH3 adsorbs at an atop site, CH2 on the bridge site, and CH in a 3-fold hollow site. Indeed, our calculations confirm this rule. Carbon adsorbs on the fcc hollow site. On Pt(211), CH3 adsorbs preferentially on an edge atop site, CH2 and CH on edge bridge sites, and C on edge 3-fold hollow sites. It is worth mentioning that adsorption of CH on the edge bridge site is only 0.2 eV more stable than on the edge 3-fold hollow site. The calculated chemisorption energies for CH3, CH2, CH, and C are summarized in Table 1. It can be seen that our results are in agreement with previous results and the chemisorption energies of CHx (x ) 0-3) on Pt(211) are slightly higher than those on Pt(111). In this work, the adsorption site for H atom is on the fcc 3-fold site. The calculated adsorption energies for a hydrogen atom on Pt(111) and Pt(211) are 2.74 and 2.90 eV, respectively, which are similar to previous theoretical results.30 Overall, our results, consistent with previous studies, indicate that the

Hydrogenation of Ethylene

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TABLE 2: Energies (eV) of Adsorbed C2Hx Species on Pt(111) and Pt(211) Surfaces Relevant to CH3CH2ada Pt(111) C2Hx species

this work

Podkolzin et al.20,b

CH3CH2add CH2CH2ad + Had π-CH2CH2ad+ Had CH3CHad + Had CH2CHad +2Had CH3Cad +2Had CHCHad +3Had CH2Cad +3Had CHCad +4Had

0 -0.31 0.24 0.01 -0.29 -0.83 -0.27 -0.49 0.53

0 -0.20 0.15 -0.15 -0.78 -0.06 -0.20

Pt(211) this work

Pt(110) Anghel et al.10,c

0 -0.61 -0.27 -0.50 -0.67 -0.94 -0.53 -0.75 -0.26

0 -0.63 -0.40 -0.59 -0.72 -0.65 -0.62 -0.61

a

Reference state: CH3CH2ad. Excess H adsorbs on a separate slab. b DACAPO code with two fixed layers. c CASTEP code with six layers and top three layers relaxed. d The adsorption energies of CH3CH2ad on Pt(111) and Pt(211) are 2.03 and 2.21 eV, respectively.

TABLE 3: Activation Barriers and Distances at the TSs of Dehydrogenation Reactions of C2Hx Species and Reverse Barriers on Pt(111) and Pt(211)a Pt(111)

Pt(211)

reaction no.

reactions

reaction barrier (eV)

distance (Å)


distance (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

CH3CH3(g) + * f CH3CH2ad + Had CH3CH2ad + Had f CH3CH3(g) + * CH3CH2ad + * f CH3CHad + Had CH3CHad + Had f CH3CH2ad + * CH2CH2ad f π-CH2CH2ad π-CH2CH2ad f CH2CH2ad CH3CH2ad + * f CH2CH2ad + Had CH2CH2ad + Had f CH3CH2ad + * CH3CH2ad + * f π-CH2CH2ad + Had π-CH2CH2ad + Had f CH3CH2ad + * CH3CHad + * f CH2CHad + Had CH2CHad + Had f CH3CHad + * CH3CHad + * f CH3Cad + Had CH3Cad + Had f CH3CHad + * CH2CH2ad + * f CH2CHad + Had CH2CHad + Had f CH2CH2ad + * CH2CHad + * f CH2Cad + Had CH2Cad + Had f CH2CHad + * CH3Cad + * f CH2Cad + Had CH2Cad + Had f CH3Cad + * CH2CHad + * f CHCHad + Had CHCHad + Had f CH2CHad + * CH2Cad + * f CHCad + Had CHCad + Had f CH2Cad + * CHCHad + * f CHCad + Had CHCad + Had f CHCHad + *

0.54 1.02 0.88 0.87 0.64 0.09 0.81 1.12 1.21 0.63 0.71 1.02 0.28 1.13 0.84 0.82 0.70 0.90 1.33 0.99 1.03 1.01 2.22 1.10 2.12 1.22

1.512

0.08 0.89 0.27 0.77 0.36 0.01 0.44 1.06 0.60 0.25 0.92 1.10 0.86 1.30 0.57 0.63 0.66 0.74 1.30 1.11 1.13 0.98 1.67 1.20 1.19 0.93

1.559

a

1.592 1.513 1.868 1.579 1.450 1.601 1.478 1.717 1.474 1.577 1.500

1.625 1.532 1.455 1.580 1.500 1.540 1.549 1.774 1.540 1.607 1.497

Here asterisk (*) denotes an empty surface site. The distances are those between C and H at the TSs.

potential energy surface is fairly flat and adsorption on steps is only slightly stronger than that on terraces. 3.2. Adsorption of C2Hx Species on Pt(111) and Pt(211) Surfaces. Adsorption of C2Hx species on a clean Pt(111) surface has been investigated extensively.10,12,13,10,20,24,31 Watwe et al.12,13 have studied the stability and reactivity of C2 species on a Pt10 cluster, Pt(111), and Pt(211) via DFT. Most of our models are built based on theirs and most of the optimized structures are very similar to previous results. Like CHx (x ) 1-3) on Pt, the structures of C2Hx on Pt also achieve a saturated configuration: (i) C in CCHx (x ) 1-3) prefers fcc hollow sites; (ii) CH in CHCHx (x ) 1-3) prefers bridge site; (iii) CH2 in CH2CHx (x ) 1-3) adsorbs on the top site. Unlike CHC on Pt(110),10 we found that CHC is unstable on both Pt(111) and Pt(211) surfaces. In this paper, for brevity we only report details for adsorption of CH2CH2 and CHCH on Pt(111) and Pt(211) surfaces. The C-C bond length in the di-σ-bonded ethylene is 1.50 Å on both surfaces and is in good agreement with previous theoretical and

experimental values of 1.48-1.52 Å.10,12,13,20,24,31–35 Our calculations indicate that the adsorption energy of ethylene is 1.51 and 1.84 eV on Pt(111) and Pt(211), respectively. The binding energies of experimental36 and theoretical10,12,13,20,24,31–35 works range from 1.11 to 1.78 eV at low coverage. For CHCH on Pt(111), the calculated C-C bond length is 1.41 Å, which is similar to previous results.12,13,20 The binding energy of CHCH on Pt(111) in our work is 2.87 eV, which is larger than that of Dumesic and co-workers (2.17 eV)12,13,20 and Jacob and Goddard III (1.88 eV)31 but smaller than that of Kua and Goddard III (3.37 eV).24 Due to different DFT functionals and Pt structures, the difference between our and previous theoretical values is reasonable. Interestingly, on Pt(211), we found two types of adsorption sites for acetylene. The first one is very similar to that on Pt(111): CHCH adsorbs in the 3-fold hollow sites and forms two σ-bonds and one π-bond to three Pt atoms. The calculated C-C bond length is reported to be 1.42 Å, which is close to that on Pt(111); the binding energy is 2.83 eV. The

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Figure 2. Side views of the calculated TS structures for dehydrogenation of C2Hx species on Pt(111): (a) CH3CH2-H, (b) CH3CH-H, (c) CH2CH2-H, (d) CH2CH-H, (e) CH3C-H, (f) CH2CH-H, (g) CHCH-H, (h) CH2C-H, (i) CH2C-H, (j) CHC-H, (k) CCH-H, (l) π-CH2CH2-H.

second type of adsorption of CHCH is 0.49 eV more stable than the first one. The calculated C-C bond is 1.47 Å, indicating rehybridization from sp toward sp2 hybridization. The structure of this type of CHCH adsorption is shown in Figure 1. Although the second type of CHCH adsorption is more stable, we only consider the first type of CHCH adsorption for several reasons: (1) The first type of CHCH is the typical acetylene adsorption on Pt, similar to the one we found on Pt(111); (2) the formation of CHCH from dehydrogenation of CH2CH occurs on the upper step and diffusion is needed for formation of the second type of adsorbed CHCH, but this species does not easily diffuse; (3) the dehydrogenation of the second type of CHCH is more difficult due to its stable state. Table 2 lists energies of the most stable C2Hx species on Pt(111) and Pt(211) surfaces. The energy values in Table 2 are reported with respect to the reference state of the adsorbed CH3CH2 species. From Table 2, we can see that our results are 0.1-0.3 eV lower than those of Podkolzin et al.13 This difference is reasonable when considering the combined effect of slab thickness and of relaxing layers. Our results indicate that the stability of C2Hx species is similar to but slightly higher on Pt(211) than that on Pt(110). Our calculations show that ethylidyne (CH3C) is the most stable species on Pt(111) and Pt(211). 3.3. Surface Dehydrogenation Reactions. We calculated all the dehydrogenation reactions of C2Hx species on Pt(111) and Pt(211) surfaces. All the reaction barriers and the distances between H and C at the TSs are listed in Table 3 and the TSs on Pt(111) and Pt(211) surfaces are shown in Figure 2 and

Figure 3, respectively. On Pt, gaseous ethane dissociation to chemisorbed ethyl (CH3CH2) has been reported to be an easy process.11,14 Indeed, the reaction barriers of ethane dissociation are low: 0.54 and 0.08 eV on Pt(111) and Pt(211), respectively; the reverse barriers are much higher. Most activation barriers of dehydrogenation are around 0.2 eV lower than those reported by Podkolzin et al.13 In addition, most activation barriers of dehydrogenation on Pt(211) are very similar to those on Pt(110).11,14 For example, the barriers of reactions 3 and 13 are 0.33 and 0.90 eV on Pt(110), respectively,11,14 vs 0.27 and 0.86 eV on Pt(211), respectively. Table 3 indicates that most reaction barriers on Pt(211) are lower than those on Pt(111). From the configurations of the TSs in Figure 2, it can be seen that all of the geometries of TSs, except reaction 13, are very similar to those obtained by Podkolzin et al.13 Breaking H in our TS of reaction 13 is at a ridge-atop site (not at the bridge site). Interestingly, we find a linear relationship between the barriers (Ea) and enthalpy change (∆H) of dehydrogenation reactions on Pt(111) and Pt(211) with a high correlation coefficient (R2 ) 0.91), as shown in Figure 4. 3.4. Hydrogenation of CHx (x ) 0-3) Reactions and H-H Desorption. We calculated each elementary step on Pt(111) and Pt(211): C + H f CH, CH + Hf CH2, CH2 + H f CH3, and CH3 + Hf CH4. Figure 5 illustrates the geometries of the TSs of these elementary reactions on Pt. It can be seen that on both surfaces the reaction of H and CH3 is preferred on the top site, whereas the CH2 and CHx (x ) 0-1) are located on the bridge site and on the hollow site, respectively. The configurations of these TSs agree well with previous results and those on other



Figure 3. Top and side views (insets) of the calculated TS structures for dehydrogenation of C2Hx species on Pt(211): (a) CH3CH2-H, (b) CH3CH-H, (c) CH2CH2-H, (d) CH2CH-H, (e) CH3C-H, (f) CH2CH-H, (g) CHCH-H, (h) CH2C-H, (i) CH2C-H, (j) CHC-H, (k) CCH-H, (l) π-CH2CH2-H.

Figure 4. Barriers (Ea) of dehydrogenation of C1 and C2 species reactions against their corresponding enthalpy change (∆H) on Pt(111) and Pt(211) surfaces. In the FSs, C2 species and H are taken on separate slabs. Reaction is defined in the forward direction.

metals.28,37–40 All the reaction barriers and C-H distances at the TSs of hydrogenation of Cx (x ) 0-3) are listed in Table 4. Our results in Table 4 are consistent with previous results.28,29,37,41 For example, on Pt(111), Michaelides and Hu28,37 studied hydrogenation of CHx (x ) 0-3) and reported barriers

of 0.78, 0.72, 0.63, and 0.74 eV for hydrogenation of C, CH, CH2, and CH3, respectively. Our barriers of hydrogenation of C, CH, CH2, and CH3 are 0.82, 0.82, 0.76, and 0.92 eV, respectively. On Pt(211), our results are very close to those calculated on Pt(110).40 Petersen et al.40 reported that the barriers of CH2 and CH dehydrogenation are 0.56 and 1.20 eV, respectively. Our calculations on Pt(211) give activation energies of CH2 and CH dehydrogenation 0.55 and 1.29 eV, respectively. The dehydrogenation barriers follow the same linear free energy relationship as that of the C2 species (Figure 4). Panels i and j of Figure 5 show the geometry of H-H desorption on Pt(111) and Pt(211), and the reaction barriers are present in Table 4. In the TS of H-H desorption on Pt(111), the two H atoms are on the top side of one Pt atom while in the TS on Pt(211), one H atom is located at the bridge side and the other one is at the ridge-atop site. The barriers for the H-H desorption are high: 0.92 eV on Pt(111) and 1.66 eV on Pt(211). 3.5. C-C Bond Cleavage Reactions on Pt(111) and Pt(211) Surfaces. During hydrogenolysis of ethane, the C-C bond is broken to form C1 species. Therefore, calculating all the C-C bond cleavage reactions on Pt(111) and Pt(211) was necessary. Figure 6 illustrates the structures of the TSs. The activation barriers and the distances between the two C atoms in the TSs are listed in Table 5. One can clearly see some general trends in the configurations of the TSs: (i) In the dissociation of CCHx (x ) 1-3) species on Pt(111) and Pt(211), C is always sitting on the fcc hollow site. (ii) In the dissociation of CHCHx (x ) 1-3) species on Pt(111), CH is also located on the fcc

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Figure 5. Calculated TS structures for hydrogenation of Cx (x ) 0-3) and H-H desorption on Pt(111) (top views) and Pt(211) (side (insets) views). (a-d) Hydrogenation of CHx (x ) 0-3) on Pt(111) and (e-h) on Pt(211). (i and j) H-H desorption on Pt(111) and Pt(211), respectively.

TABLE 4: Activation Barriers of the Forward and Reverse Reactions and Distances at the TSs of Hydrogenation of Cx (x ) 0-3) and H-H desorption on Pt(111) and Pt(211)a Pt(111)

Pt(211)

reaction no.

reactions


distance (Å)


distance (Å)

27 28 29 30 31 32 33 34 35 36

Cad + Had f CHad + * CHad + * f Cad + Had CHad + H f CH2ad + * CH2ad + * f CHad + Had CH2ad + Had f CH3ad + * CH3ad + * f CH2ad + Had CH3ad + Had f CH4(g) + * CH4(g) + * f CH3ad + Had 2Had f H2(g) + 2* H2(g) + 2* f 2Had

0.82 1.29 0.82 0.17 0.76 0.83 0.92 0.63 0.91 0.58

1.673

0.96 1.29 1.00 0.55 0.64 0.18 0.78 0.21 1.66 0.11

1.636

a

1.550 1.630 1.483 0.996

1.490 1.646 1.436 0.860

The distances are those between C and H, and those between H and H in H-H desorption at the TSs.

TABLE 5: Activation Barriers and Distances at the TSs of C-C Cleavage Reactions in C2 Species and Reverse Barriers on Pt(111) and Pt(211)a Pt(111)

Pt(211)

reaction no.

reactions


distance (Å)


distance (Å)

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

CH3CH2ad + * f CH3ad + CH2ad CH3ad + CH2ad f CH3CH2ad + * CH3CH2ad + * f CH3ad + CHad CH3ad + CHad f CH3CHad + * CH2CH2ad + * f CH2ad + CH2ad CH2ad + CH2ad f CH2CH2ad + * CH3Cad + * f CH3ad + Cad CH3ad + Cad f CH3Cad + * CH2CHad + * f CH2ad + CHad CH2ad + CHad f CH2CHad + * CHCHad + * f CHad + CHad CHad + CHad f CHCHad + * CH2Cad + * f CH2ad + Cad CH2ad + Cad f CH2Cad + * CHCad + * f CHad + Cad CHad + Cad f CHCad + *

1.84 1.59 1.18 1.59 2.22 1.59 1.95 1.04 1.70 1.74 1.07 1.78 2.22 1.59 0.90 2.04

1.942

1.11 1.30 1.34 1.50 1.67 1.71 1.70 1.08 1.82 2.26 1.28 2.33 1.75 1.78 1.73 2.66

1.920

a

The distances are those between C and C at the TSs.

1.930 1.965 2.009 2.149 1.893 2.089 2.085

1.935 2.077 2.010 2.491 2.101 2.000 2.010



Figure 6. Side views of the calculated TS structures for C-C cleavage reactions in C2 species for Pt(111). (a-h): (a) CH3-CH2, (b) CH3-CH, (c) CH2-CH2, (d) CH2-CH, (e) CH3-C, (f) CH2-C, (g) CH-CH, (h) CH-C. Top and side (insets) views for Pt(211) (i-p): (i) CH3-CH2, (j) CH3-CH, (k) CH2-CH2, (l) CH2-CH, (m) CH3-C, (n) CH2-C, (o) CH-CH, (p) CH-C.

hollow site, while CH2 and CH3 are activated to the nearby top sites and CH is activated to the nearby hcp hollow site. On Pt(211), CH sits on the 3-fold hollow site in the dissociation of CHCH and CH3CH, but on the bridge site in the dissociation of CH2CH. (iii) For the dissociation of CH2CH2 and CH3CH2 on Pt(111) and Pt(211), CH2 sits on the bridge site and another CH2 group and CH3 are activated to the top site. CH is on 3-fold hollow site in the dissociation of CHCH and CH3CH. In general, the high-valence groups (C and CH) prefer to be on the 3-fold hollow site in the TS and CH3 prefers the top site in the TS. Watwe et al.12 have examined some of the C-C bond cleavage reactions on Pt clusters, Pt(111) and Pt(211). Their results are similar to ours. We take the dissociation of CH3CH on Pt(111) and CH3CH2 on Pt(211) as examples. In the TSs, our C-C distances are slightly shorter than theirs: 1.93 Å vs 1.99 Å for CH3CH and 1.92 Å vs 2.08 Å for CH3CH2. Our reaction barriers are slightly larger than theirs: 1.18 eV vs 1.09 eV and 1.11 vs 1.06 eV for C-C cleavage in CH3CH on Pt(111) and CH3CH2 on Pt(211), respectively. Interestingly, by means of the method suggested by Alcala et al.,42 we found a good linear relationship between final states (FSs) and TSs energies which are relative to IS gas-phase energies with a high correlation coefficient (R2 ) 0.99), as shown in Figure 7. The linear regression equation is ETS (eV) ) 0.99EFS (eV) + 1.88 and the standard error is 0.3 eV. 3.6. Isomerization of C2 Species. All isomerization reactions between C2 species have been studied. Table 6 gives the barriers of forward and backward isomerization reactions. All the structures of the TSs are shown in Figure 8. Table 6 shows that all the reaction barriers are very high and most of them are higher than 2 eV. Anghel et al.14 indicated that in the isomerization reactions, the H atom being transferred is pointing

Figure 7. Correlation plot for C-C bond cleavage reactions for C2 species on Pt(111) and Pt(211) surfaces calculated from DFT.

away from and does not interact with the surface; hence, the isomerization reactions are not surface mediated. Indeed, the reaction barriers of isomerization reactions are very similar on Pt(111) and Pt(211). For the 1,2-H shift between CH2CH2 and CH3CH on Pt(110), Anghel et al.14 have reported an activation energy of 2.31 eV, which is similar to our results on Pt(111) and Pt(211) (2.36 and 2.44 eV on Pt(111) and Pt(211), respectively). 4. Discussion Table 2 shows that CH3C is the most stable C2 species on Pt(111) and Pt(211). It is now generally accepted that the CH3C

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Figure 8. TS structures for isomerization between CH2CH2 and CH3CH, CH3C and CH2CH, and CHCH and CH2C on Pt(111) and Pt(211): Side views for Pt(111) (a-c) and top and side (insets) views for Pt(211) (d-f).

TABLE 6: Activation Barriers at the TSs of Isomerization between CH2CH2 and CH3CH, CH3C and CH2CH, and CHCH and CH2C, and of Their Reverse Reactions on Pt(111) and Pt(211) reaction barrier(eV) reaction no.

reactions

Pt(111)

Pt(211)

53 54 55 56 57 58

CH3CHad f CH2CH2ad CH2CH2ad f CH3CHad CH3Cad f CH2CHad CH2CHad f CH3Cad CH2Cad f CHCHad CHCHad f CH2Cad

2.04 2.36 2.50 1.96 2.38 2.60

2.32 2.44 2.32 2.03 2.14 2.36

surface intermediate is one of the most abundant species in ethane hydrogenolysis over close-packed fcc(111) and hcp(0001) metal surfaces.10,20,43–47 Although Anghel et al.10 suggested that CH3C and CH2CH have similar stability on Pt(110) (1 × 2), our calculations show that CH3C is about 0.5 and 0.3 eV more stable than CH2CH on Pt(111) and Pt(211), respectively. Using variable temperature scanning tunneling microscopy, Land et al.47 have imaged a long-range ethane superstructure on Pt(111) at 160 K, which upon annealing to 350 K gave CH3C as a stable surface species. It is worth emphasizing the effects of surface steps. First, the adsorption of CHx (x ) 0-3) and C2Hx species at steps is stronger than that on terraces. The reason has been explained elsewhere13,48 in terms of lower coordination number that can lead to a smaller local bandwidth and a higher d band center. Second, many dehydrogenation and C-C cleavage reaction barriers are lower at steps. Especially, the barriers of ethane dissociation, CH3CH2 dehydrogenation to form CH2CH2 and CH3CH and C-C cleavage in CH3CH2 decrease considerably. In addition, as mentioned in the last section, most reaction barriers on Pt(211) are similar to those on Pt(110), indicating that the effect of surface defects may be comparable for these reactions. A similar observation can be deduced from the DFT results of Tang et al.,49 who showed comparable energy barriers in water dimer dissociation on Cu(110) and (211). Although C2Hx species have been well-studied using DFT on Pt,10–14,20,24,31 there is no definitive agreement regarding the detailed pathway of ethane hydrogenolysis. Figure 9 illustrates the network of steps in hydrogenation of ethylene and hydrogenolysis of ethane. Elementary steps shown in Figure 8 include dehydrogenation, hydrogenation, C-C cleavage, and isomerization reactions. All the dehydrogenation and hydrogenation reaction barriers are summarized in Table 3. On the basis of the barriers, the following observations can be made: (i) On

Pt(211), ethane dissociation to ethyl and ethylidene is a rapid process at low surface temperatures. (ii) On Pt(111), ethylidene is expected to be rapidly consumed by dehydrogenation to CH3C which is difficult to be further dehydrogenated to CH2C and also hydrogenated to CH3CH. These large barriers can explain why CH3C is the most abundant species on Pt surface. Additionally, on Pt(111), the selectivity to CH3C is higher than that to CH2CH, whereas on Pt(211), the barriers of CH3CH dehydrogenation to CH3C and CH2CH are similar. (iii) The barriers of CH2CH formation from CH2CH2 and its reverse are nearly equal. (iv) It easier to form CH2C than CHCH by dehydrogenation of CH2CH. Indeed, there are several pieces of experimental evidence that show that the surface species of ethane dissociation is in fact CH2C and not CHCH.7,50 (v) The barrier of CHC formation is high. On the other hand, CHC can be hydrogenated more easily on both Pt(111) and Pt(211) surfaces; i.e., CHC is very unstable. Recently, Anghel et al.14 reported reaction pathways for ethane dehydrogenation on Pt(110) and obtained similar results. They indicated that ethane can be dissociated quickly to CH2CH2 and CH3CH around 230 K. However, the barrier of CH3CH dehydrogenation to CH3C on Pt(110) is much higher than that on Pt(111), so they did not predict facile formation of CH3C on Pt(110). Also, the barrier of CHC formation on Pt(110) is lower than those on Pt(110) and Pt(211) surfaces. Once C2Hx species form on Pt surfaces, they can dehydrogenate and be cracked through C-C bond scission. Table 5 shows that all the barriers for C-C cleavage reactions are above 1 eV except reaction 51. Anghel et al.14 have only characterized a pathway for C-C bond scission in CHC and found the barrier to be 1.71 eV. Consequently, they indicated that C-C scission is unlikely to play any significant role in ethane dissociation on Pt(110). In our calculations, the barrier of C-C bond cleavage in CHC on Pt(111) is only 0.9 eV, which is far lower


Figure 9. Elementary reactions (C-H bond formation, C-H bondbreaking, C-C cleavage, and H-1,2-shift reactions) in the decomposition of ethane on Pt.

than that on Pt(110). CHC, as mentioned before, is an unstable species and hard to form. Therefore, the possible pathways for C-C bond breaking are reaction 37 on Pt(211), reaction 39 on Pt(111), and reaction 47 on both surfaces. If we account for the stability of species, the most possible pathway for C-C bond breaking could be reaction 47. However, the selectivity to CHCH is lower than to CH2C. Therefore, C-C bond breaking is kinetically hindered at low surface temperatures. Indeed, Harris et al.7 suggested that C-C cleavage reaction can only happen over 540 K. They found that the products of ethane dissociation when exposed to deuterium on Pt(110) at 370 K gave no deuterated methane, which indicated that C-C bond cleavage does not occur at low temperatures. On the other hand, it is widely accepted that dehydrogenation of ethane to form various C2 species are quasi-equilibrated processes according to deuterium tracing experiments in which Zaera et al.51 showed that the deuterium exchange rates are 3 orders of magnitude faster than the rate of ethane hydrogenolysis over Pt(111) at temperatures between 475 and 625 K. On the basis of the assumption of quasi-equilibrated adsorption of ethane to form C2Hx species on Pt, C-C bond breaking may involve all the adsorbed C2Hx(x g 2) species. Using NMR, Klug et al.52 suggested that CH3C species, which forms upon adsorption of CHCH, may play a role in C-C bond scission over Pt. Dumesic and co-workers5,6 proposed that the primary reaction pathways for cleavage of the C-C bond on Pt take place through activated intermediates (CH3CH2 and CH3CH species). Our calculations indicated that C-C bond cleavage on Pt(111) and Pt(211) can take place via CHCH, CH3CH, and CH3CH2. The reaction scheme for ethane hydrogenolysis proposed by Dumesic and co-workers5,6 involves isomerization of C2Hx species. However, isomerization reactions appear to be energetically unfavored on Pt. Zaera and French53 proposed a threestep mechanism for conversion of CH2CH2 to CH3C: CH2CH2 first isomerizes to CH3CH and CH3CH dehydrogenates to CH3C. From our DFT results, it can be seen that CH2CH2 dehydrogenation to CH2CH is much faster than isomerization between CH2CH2 and CH3CH. Therefore, the most possible mechanism for conversion of CH2CH2 to CH3C involves the following steps: (i) CH2CH2 dehydrogenates to form CH2CH; (ii) CH2CH can be further dehydrogenated to produce CH2C; (iii) CH2C hydrogenates to give CH3C. Overall, we can rule isomerization reactions out in ethane hydrogenolysis on Pt. At low temperatures, CH3CH3 can easily dehydrogenate to form CH2CH2, whereas at high temperatures C2H4 can further dehydrogenate to form highly dehydrogenated C2Hx species and at the same time C-C bond cleavage can occur. 5. Conclusions In this paper, we has used massive density functional theory (DFT) calculations to provide for the first time insights into

J. Phys. Chem. C, Vol. 114, No. 11, 2010 4981 the “complete” pyrolytic C2 chemistry of hydrocarbons on Pt(111) and Pt(211) surfaces. Typical processes encompassing the paths explored herein include ethane hydrogenolysis, ethylene hydrogenation, and C2 hydrocarbon dehydrogenation. We determined all the structures and stabilities of C1 and C2 species. We found that most of the species on Pt(211) are more stable than those on Pt(111) and CH3C is the most stable C2 species on Pt(111) and Pt(211). In addition, we examined all the dehydrogenation and hydrogenation steps of C1 and C2 species, all the C-C bond cleavage reactions, and the isomerization reactions between C2 species. Key conclusions include the following: (i) On Pt(211), CH2CH2 and CH3CH can be formed very rapidly from ethane with low reaction barriers. (ii) On Pt(111), CH3CH can be dehydrogenated rapidly to form CH3C. (iii) The barriers of CHC formation are very high. (iv) Isomerization reactions do not appear to be energetically favored on Pt. (v) C-C bond breaking takes place most probably through CH3CH2, CH3CH, and CHCH. Microkinetic analysis using these DFT results and comparison to data will be valuable future steps to provide quantitative insights into these complex processes. Acknowledgment. The work was supported in part by the NSF (CBET-0729701). The DFT calculations were performed using the TeraGrid resources provided by University of Illinois’ National Center for Supercomputing Applications (NCSA). References and Notes (1) Miller, S. A. Ethylene and Its Industrial DeriVatiVes; Ernest Benn Limited: London, 1969. (2) Sinfelt, J. H.; Taylor, W. F.; Yates, D. J. C. J. Phys. Chem. 1965, 69, 95. (3) Sinfelt, J. H.; Yates, D. J. C. J. Catal. 1967, 8, 82. (4) Cortright, R. D.; Dumesic, J. A. Kinetics of heterogeneous catalytic reactions: Analysis of reaction schemes. In AdVances in Catalysis: Academic Press: San Diego, CA, 2002; Vol. 46; p 161. (5) Cortright, R. D.; Watwe, R. M.; Dumesic, J. A. J. Mol. Catal. A: Chem. 2000, 163, 91. (6) Cortright, R. D.; Watwe, R. M.; Spiewak, B. E.; Dumesic, J. A. Catal. Today 1999, 53, 395. (7) Harris, J. J. W.; Fiorin, V.; Campbell, C. T.; King, D. A. J. Phys. Chem. B 2005, 109, 4069. (8) Laffir, F. R.; Harris, J. J. W.; Fiorin, V.; King, D. A. Chem. Phys. Lett. 2007, 439, 342. (9) Cimino, A.; Boudart, M.; Taylor, H. J. Phys. Chem. 1954, 58, 796. (10) Anghel, A. T.; Jenkins, S. J.; Wales, D. J.; King, D. A. J. Phys. Chem. B 2006, 110, 4147. (11) Anghel, A. T.; Wales, D. J.; Jenkins, S. J.; King, D. A. Chem. Phys. Lett. 2005, 413, 289. (12) Watwe, R. M.; Spiewak, B. E.; Cortright, R. D.; Dumesic, J. A. J. Catal. 1998, 180, 184. (13) Watwe, R. M.; Cortright, R. D.; Norskov, J. K.; Dumesic, J. A. J. Phys. Chem. B 2000, 104, 2299. (14) Anghel, A. T.; Wales, D. J.; Jenkins, S. J.; King, D. A. J. Chem. Phys. 2007, 126, 044710. (15) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon, P.; Sanchez-Portal, D. J. Phys.: Condens. Matter 2002, 14, 2745. (16) Troullier, N.; Martins, J. L. Phys. ReV. B 1991, 43, 8861. (17) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (18) Wyckoff, R. W. G. Crystal structures, 2nd ed.; Interscience: New York, 1963. (19) Kokalj, A.; Causa, M. J. Phys.: Condens. Matter 1999, 11, 7463. (20) Podkolzin, S. G.; Alcala, R.; Dumesic, J. A. J. Mol. Catal. A: Chem. 2004, 218, 217. (21) Alavi, A.; Hu, P. J.; Deutsch, T.; Silvestrelli, P. L.; Hutter, J. Phys. ReV. Lett. 1998, 80, 3650. (22) Zhang, C. J.; Hu, P. J. Am. Chem. Soc. 2000, 122, 2134. (23) Zhang, C. J.; Hu, P.; Lee, M. H. Surf. Sci. 1999, 432, 305. (24) Kua, J.; Goddard, W. A. J. Phys. Chem. B 1998, 102, 9492. (25) Kua, J.; Goddard, W. A. J. Phys. Chem. B 1999, 103, 2318. (26) Papoian, G.; Norskov, J. K.; Hoffmann, R. J. Am. Chem. Soc. 2000, 122, 4129. (27) Minot, C.; Vanhove, M. A.; Somorjai, G. A. Surf. Sci. 1983, 127, 441.

4982


(28) Michaelides, A.; Hu, P. The Valency Effect on Reaction Pathways in Heterogeneous Catalysis: Insight from Density Functional Theory Calculations. In Theoretical Aspects of Heterogeneous Catalysis; Nascimento, M., Ed.; Kluwer Academic Publishers: Dordrecht and Boston, 2001. (29) Petersen, M. A.; Jenkins, S. J.; King, D. A. J. Phys. Chem. B 2004, 108, 5909. (30) Ford, D. C.; Xu, Y.; Mavrikakis, M. Surf. Sci. 2005, 587, 159. (31) Jacob, T.; Goddard, W. A. J. Phys. Chem. B 2005, 109, 297. (32) Ge, Q.; King, D. A. J. Chem. Phys. 1999, 110, 4699. (33) Watson, G. W.; Wells, R. P. K.; Willock, D. J.; Hutchings, G. J. J. Phys. Chem. B 2000, 104, 6439. (34) Koestner, R. J.; Stohr, J.; Gland, J. L.; Horsley, J. A. Chem. Phys. Lett. 1984, 105, 332. (35) Stohr, J.; Sette, F.; Johnson, A. L. Phys. ReV. Lett. 1984, 53, 1684. (36) Yeo, Y. Y.; Stuck, A.; Wartnaby, C. E.; King, D. A. Chem. Phys. Lett. 1996, 259, 28. (37) Michaelides, A.; Hu, P. J. Am. Chem. Soc. 2000, 122, 9866. (38) Gong, X. Q.; Raval, R.; Hu, P. J. Chem. Phys. 2005, 122. (39) Ciobica, I. M.; Frechard, F.; van Santen, R. A.; Kleyn, A. W.; Hafner, J. J. Phys. Chem. B 2000, 104, 3364. (40) Petersen, M. A.; Jenkins, S. J.; King, D. A. J. Phys. Chem. B 2004, 108, 5920. (41) Moussounda, P. S.; Haroun, M. F.; Rakotovelo, G.; Legare, P. Surf. Sci. 2007, 601, 3697. (42) Alcala, R.; Mavrikakis, M.; Dumesic, J. A. J. Catal. 2003, 218, 178.

Chen and Vlachos (43) Cremer, P. S.; Su, X. C.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 1996, 118, 2942. (44) Azad, S.; Kaltchev, M.; Stacchiola, D.; Wu, G.; Tysoe, W. T. J. Phys. Chem. B 2000, 104, 3107. (45) Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces; Wiley: New York, 1996. (46) Parlett, P. M.; Chesters, M. A. Surf. Sci. 1996, 358, 791. (47) Land, T. A.; Michely, T.; Behm, R. J.; Hemminger, J. C.; Comsa, G. J. Chem. Phys. 1992, 97, 6774. (48) Hammer, B.; Norskov, J. K. NATO ASI Ser., Ser. E 1996. (49) Tang, Q. L.; Chen, Z. X. J. Chem. Phys. 2007, 127. (50) Yagasaki, E.; Backman, A. L.; Masel, R. I. J. Phys. Chem. 1990, 94, 1066. (51) Zaera, F.; Somorjai, G. A. J. Phys. Chem. 1985, 89, 3211. (52) Klug, C. A.; Slichter, C. P.; Sinfelt, J. H. Isr. J. Chem. 1992, 32, 185. (53) Zaera, F.; French, C. R. J. Am. Chem. Soc. 1999, 121, 2236. (54) Michaelides, A.; Hu, P. J. Chem. Phys. 2001, 114, 2523. (55) Au, C. T.; Liao, M. S.; Ng, C. F. J. Phys. Chem. A 1998, 102, 3959. (56) Kua, J.; Faglioni, F.; Goddard, W. A. J. Am. Chem. Soc. 2000, 122, 2309. (57) Au, C. T.; Ng, C. F.; Liao, M. S. J. Catal. 1999, 185, 12.

JP909163Z

Hydrogenation of Ethylene and Dehydrogenation and Hydrogenolysis

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