22630
J. Phys. Chem. C 2010, 114, 22630–22635
Methane Formation on Corrugated Ru Surfaces Sharan Shetty,* A. P. J. Jansen, and Rutger A. van Santen Schuit Institute of Catalysis, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands ReceiVed: September 14, 2010; ReVised Manuscript ReceiVed: NoVember 12, 2010
In the present theoretical study, we investigated the formation of CHx (x ) 0-4) on corrugated Ru(101j0)B and Ru(112j1) surfaces. Our results demonstrate that on Ru(101j0)B and Ru(112j1) surfaces, C and CH intermediates are the most stable adsorbed CHx species respectively. This indicates that the distribution of the CHx adsorbed species will depend on the surface structure. The results obtained from the kinetic analysis show that the formation of CH4 on Ru(101j0)B surface is slower than that on the Ru(112j1) surface. Furthermore, the activation of CH4 is easier on the Ru(112j1) surface than on the Ru(101j0)B surface. 1. Introduction Natural gas, primarily consisting of methane, is considered to be one of the viable feed-stocks for the production of syngas, which is eventually used for the synthesis of clean fuel via the Fischer-Tropsch (F-T) process.1-3 The complex reactions involved in the F-T synthesis can be divided into three reaction steps, initiation, propagation, and termination. In the initial step, the necessary condition is to dissociate the adsorbed CO molecule at a low barrier, followed by the hydrogenation of the C and O adsorbed species to generate CHx intermediates and water, respectively. In the next step, the CHx intermediates couple to form long-chain hydrocarbons. Finally, the hydrocarbon chain is terminated and desorbs from the surface. The main byproducts in this process are methane and water, which are generated in the initial step. Hence, the CHx intermediates can be considered to be the building blocks for the formation of long-chain hydrocarbons via the C-C coupling reactions. If the hydrogenation of the C is faster than the C-C coupling step, then the selectivity toward methane will be higher than the formation of long-chain hydrocarbons. Considering this issue, Ru is favored over the Rh and Ni catalysts for the F-T synthesis.4-6 Although Ru has been proved to be the most active catalyst in the F-T synthesis, the use of Ru in the industrial application has been limited because of its high cost. If one equates Fe to 1.0, the relative cost of Fe:Co:Ru is 1.0:230:31 000 as reported by Dry.7 Several experimental and theoretical studies have been employed to investigate the step by step hydrogenation of the C adsorbed species or the reverse reaction, that is, dehydrogenation of methane on transition metal surfaces.8-13 Wu and Goodman used high-resolution electron energy loss spectroscopy (HREELS) in combination with temperature programmed desorption (TPD) experiments to analyze the decomposition of methane on Ru(0001) and Ru(112j0) surfaces.8 They reported that the methylidyne (CH), vinylidene (CCH2), and ethylidyne (CCH3) species are stabilized on the more open Ru(112j1) surface. However, on flat Ru(0001) surface, only methylidyne and vinylidene species are stable. Moreover, they found that CH is the most stable species in a wide range of temperature. In theoretical studies, Ciobica et al. demonstrated that the stability of CHx species on Ru(112j1) surface differs from that * Corresponding author. E-mail:
[email protected].
of Ru(0001).13,14 They showed that the CH and CH2 are stable species on Ru(0001) and Ru(112j1) surfaces, respectively. In recent experimental and theoretical work, Shimizu et al. demonstrated with the help of scanning tunneling microscopy (STM) that the CH are the most stable species on Ru(0001) surface.12 However, they suggested that the CH2 formation is a difficult step. Au et al. have studied the decomposition of methane on several transition metal clusters. Their results suggested that the Rh is the efficient catalyst for the decomposition of CH4.15 Hickman and Schmidt in a seminal experimental work proposed an alternative path of oxidative decomposition of CH4 on Rh and Pt.16 They proved that Rh is more active for CH4 conversion as compared to Pt. Bunnick and Kramer have shown from the periodic density functional calculations that methane on Rh(111) surface is more easily activated as compared to that on the Ru(0001) and Ni(111) surfaces.4 Recently, Chen and Liu did a detailed analysis with the help of microkinetic modeling and proposed that the hydrogenation of C on Rh has a lower barrier than that on Ru.5 Furthermore, on the basis of these results, they discussed the different selectivity of Rh and Ru. In another theoretical study, Cheng et al. performed a systematic analysis on the methane selectivity over a wide range of metal surfaces.17 They proposed that the CH4 selectivity decreases as Re > Ru > Fe > Co > Rh. Huo et al. demonstrated that the C hydrogenation is easier on the iron-carbide (FexCy) surfaces than the CO dissociation.18 Recently, in a combined experimental and theoretical work, Jones et al. showed that Ru and Rh are the most active catalysts for steam reforming.19 They proposed that the temperature and the reactivity of metals control the kinetics of CO formation and methane activation. Wie and Iglesia found from isotopic tracer and kinetic studies that the supported Pt clusters are more active for H2O and CO2 reforming of CH4.20 A direct route to the formation of hydrocarbons from methane was independently proposed by van Santen et al. and Amariglio et al.21,22 They proposed a nonoxidative two-step methane homologation at lower temperature. In the first step, the CH4 is decomposed to produce the carbonaceous species, and in the second step, these species are hydrogenated at low temperature to produce hydrocarbons. This route avoids the oxygen-assisted methane activation carried out by partial oxidation, steam, or CO2 reforming.
10.1021/jp108753a 2010 American Chemical Society Published on Web 12/06/2010
Methane Formation on Corrugated Ru Surfaces
J. Phys. Chem. C, Vol. 114, No. 51, 2010 22631
From recent studies, it is known that the CO activation is highly structure dependent.23-25 However, whether the hydrogenation or dehydrogenation of C is structure sensitive or insensitive is still a matter of debate.5,11,19,26-28 In the present theoretical work, we propose C hydrogenation pathways for the formation of CH4 on a corrugated Ru(101j0)B and Ru(112j1) surfaces where active sites for low barrier CO dissociation are available. We compare the reaction barriers obtained from the present results with the studies by Ciobica and van Santen on carbon hydrogenation pathways on flat Ru(0001) surface.13 From these analyses, we address the following issues regarding the formation of CHx building blocks for the chain propagation in the F-T synthesis on corrugated Ru surfaces; whether the CHx adsorption depends on the corrugation of the surfaces and how does the hydrogenation or dehydrogenation of the CHx species proceed are dependent on the corrugation of the Ru surfaces. 2. Model and Computational Details The calculations presented in this work have been performed by the Vienna ab initio simulation package (VASP) code.29 This is based on the periodic density functional theory (DFT), which uses the plane wave basis set for the valence electrons in conjunction with the projector augmented-wave (PAW) potentials for the core.30 The Kohn-Sham equations are solved selfconsistently with an iterative matrix diagonalization scheme. The kinetic energy cutoff for the plane wave basis set is set up to 400 eV. The exchange-correlation functional is expressed by the generalized gradient approximation (GGA) with Perdew-Becke-Ernzerhof (PBE) functionals.31 The k-point sampling was generated following the Monkhorst-Pack procedure with a 5 × 5 × 1 mesh. A coverage of 25% of the adsorbed species has been maintained on both Ru surfaces. The ionic relaxation has been carried by the conjugate gradient method. During the optimization, all the degrees of freedom of the system, that is, slab and adsorbed species, are relaxed. The reaction paths to determine the transition state (TS) have been computed using the nudged elastic band (NEB) method developed by Jo´nsson et al.32 The initial images between the optimized initial and the final state structures are obtained from the linear interpolation. These images are optimized simultaneously by the program. The TS is confirmed by the saddle point obtained from the additional frequency calculations. The rate constants are calculated at 300 K using the harmonic tranistion state theory as follows:33 ZPE/k T -Eact B
k ) ν·e
ν)
kBT QTS h QIS
(1)
(2)
In eq 1, k is the rate constant, ν is the pre-expeonential factor, EZPE act is the zero-point energy corrected activation energy for the forward and reverse reactions, kB is the Boltzmann constant, h is Plank’s constant, and T is the temperature. In eq 2, QTS and QIS are the vibrational partition functions for the transition and initial states. We have used Ru(101j0)B and Ru(112j1) surface models for the present calculations. These surfaces have been cleaved from the Ru(HCP) bulk. One can cleave two different surface layers from Ru(101j0) surface, that is, Ru(101j0)A and B. The surface of Ru(101j0)A is more compact as compared to that of the B cut. The A cut has 3-fold (3F) hollow, top, and bridge sites.
TABLE 1: Adsorption Energies (AE) of CHx (x ) 0-4) and H Species on Ru(101j0)B and Ru(112j1) Surfacesa Ru(101j0)B adsorbed species C H CH CH2 CH3 CH4
Ru(112j1)
sites
AE
sites
AE
4F B 3F B 4F B 3F B1 B2 B T B
-135 -76
4F B 3F 4F 4F B 3F B1 B2 B T
-91
-61 -122 -96 -84 -46 -75 -10 -3
Ru(0001) AE
-65 -67 -78
-21
-100 -65
-56 -11
-63 -9
-6 0
a For comparison, the sixth column shows the values on Ru(0001) surface from ref 13. The energies are in kJ/mol with respect to the CH4 in the gas phase. The adsorption sites are designated by four-fold hollow (4F), three-fold hollow (3F), top (T), and bridge (B). B1 and B2 are the staggered and eclipsed confirmations of the CH2 species in the bridge site, respectively. The AE of H atom is reported with respect to the H2 gas phase.
On the other hand, the B cut is more open and consists of 4-fold (4F) hollow sites, which are active for CO dissociation. For a detailed description of the Ru surface models used in the present study, we refer to our earlier work.24,34 3. Results and Discussion Initially, we will discuss the adsorption behavior of CHx (x ) 0-4) species on Ru(101j0)B and Ru(112j1) surfaces followed by the reaction pathways for the hydrogenation of C adsorbed species toward the formation of CH4 on both the Ru surfaces. 3.1. CHx (x ) 0-4) Adsorption. 3.1.1. C (Carbon). The adsorption energies of the CHx (x ) 0-4) species with respect to the CH4 in the gas phase at different sites on Ru(101j0)B and Ru(112j1) surfaces are presented in Table 1. The reference state for calculating the adsorption energy of the CHx (x ) 0-4) intermediates is the adsorption energy of the H atom (Table 1). The adsorption energies (AE) of the CHx (x ) 0-4) are calculated as shown in the following equation:
AE ) E[Ru-CHx] + E[(4 - x)Hads] E[Rusurf + CH4gas]
(3)
In the above equation, the first term is the energy of CHx adsorbed on the Ru surface, the second term is the H adsorption energy on the Ru surface, and the third and fourth terms correspond to the energy of the bare Ru surface and gas-phase CH4 molecule. On Ru(101j0)B surface, the adsorption energy of C in the 4F hollow site with respect to CH4 in the gas phase is -135 kJ/ mol (Table 1). The average Ru-C bond length in the 4F hollow site corresponds to 2.03 Å. The distance between the C atom and sublayer Ru atom is 2.43 Å, indicating a weak interaction with the sublayer Ru atom. In the bridge site, the average Ru-C distance is 2.13 Å. On flat Ru(0001) surface, the adsorption energy of C was reported to be -21 kJ/mol. This is about 114 kJ/mol less than that on the Ru(101j0)B surface. One should note that there are no 4F hollow sites present on the flat Ru(0001) surface. The second stable site for the adsorption of C on Ru(101j0)B surface is the bridge site with an adsorption energy of -76 kJ/mol (Table 1).
22632
J. Phys. Chem. C, Vol. 114, No. 51, 2010
In the case of Ru(112j1) surface (Table 1), the stable adsorption site for C is also a 4F hollow site with an adsorption energy of -91 kJ/mol. This is about 26 kJ/mol more stable than the adsorption in the 3F site. Ge and Neurock also reported the 4F hollow site to be the most stable for C adsorption on stepped Co(112j4) surface.35 An interesting point to note is that the C adsorption energy on the 4F hollow site of Ru(112j1) surface is 44 kJ/mol less than that of the Ru(101j0)B surface. This indicates the different reactivity of the 4F hollow sites for C adsorption, which is due to the differences in the surface structure of the Ru surfaces. 3.1.2. H (Hydrogen). The most stable site for the hydrogen adsorption on Ru(101j0)B and Ru(112j1) surfaces is the bridge site. The H atom adsorption on the other sites such as top, 4F, and 3F hollow is unstable and moves to the bridge site during the optimization. The adsorption energies of H at the bridge sites on Ru(101j0)B and Ru(112j1) surfaces correspond to -61 and -67 kJ/mol (Table 1), respectively, with respect to the H2 in the gas phase. One should note that the adsorption site of H can change in the presence of coadsorbed atoms or molecules.24 3.1.3. CH (Methylidene). The most stable site for CH adsorption on Ru(101j0)B surface is the 4F hollow site similar to C with an adsorption energy of -122 kJ/mol (Table 1). However, the CH species is 13 kJ/mol less stable than the C (Table 1). This is in contrast to that on the flat Ru(0001) surface, where it was shown that the CH species is more stable than the C.12,14 This is attributed to the 4F hollow sites available on the Ru(101j0) B surface for stable C adsorption, which are absent on Ru(0001) surface. The average Ru-C and C-H bonds correspond to 2.15 and 1.11 Å, respectively. One can see that, as the C forms bond with H, the Ru-C bond increases. This implies that the hydrogenation of the C adsorbed species can decrease the interaction of C with the surface metal atoms. The bridge site is 26 kJ/mol less stable than the 4F hollow site. If we compare the stability of CH adsorbed species with C adsorbed species at the bridge site, it is interesting to note that CH in the bridge is 20 kJ/mol more stable than the C species (Table 1). The top site is highly unstable for CH adsorption. The CH adsorption on the top site is 136 and 110 kJ/mol less stable than the 4F hollow and bridge sites, respectively. Moreover, the top site adsorption is 14 kJ/mol less stable with respect to the CH4 gas phase. On the Ru(112j1) surface, the CH adsorbed species is stable in the 3F hollow site with an adsorption energy of -100 kJ/ mol (Table 1). This is 22 kJ/mol more stable than the adsorption in the 4F hollow site. Moreover, the CH intermediate is 9 kJ/ mol more stable than the C adsorbed species. This is in contrast to that on the Ru(101j0)B surface where C was more stable than the CH intermediate. This is attributed to the differences in the surface structures of the two corrugated Ru surfaces. Ru(101j0)B surface is void of 3F hollow sites and hence cannot stabilize the CH species as in the case of Ru(112j1) surface, which consists of 3F hollow sites. 3.1.4. CH2 (Methylene). The CH2 adsorbed species has been suggested to be an important intermediate for the chain propagation in the F-T synthesis.36 In the 4F hollow site of Ru(101j0)B surface, we consider two configuration of CH2, that is, two H atoms pointing toward the bridge (staggered) and away from the bridge (eclipsed). CH2 adsorption in the staggered configuration is highly unstable and forms CH and H during the optimization. In the eclipsed configuration, the CH2 moves into a pseudo-3-fold position. On the bridge site, we again choose two different configurations for the CH2 adsorbed species, eclipsed and staggered. The H atoms that are aligned
Shetty et al. along the bridge, eclipsed configuration, are 38 kJ/mol less stable than the one in which H atoms are in the staggered confirmation (Table 1). In the former case, the CH2 bends, and one H atom interacts with the surface Ru atom. This creates a steric hindrance, which results in the decrease in the adsorption energy with respect to the later configuration where there is no such steric hindrance. In the stable bridge site, the Ru-C and C-H bond lengths are 2.05 and 1.10 Å, respectively. The top site is unstable, and the CH2 moves into the bridge site during the optimization. Similar to Ru(101j0)B surface, CH2 occupies a stable bridge site on Ru(112j1) surface. Even on Ru(112j1) surface, we choose two orientations for CH2 adsorbed species, where the two H atoms are oriented in eclipsed and staggered configurations with respect to the bridge site. The eclipsed configuration moves into a staggered configuration during the optimization. Hence, the stable configuration for CH2 adsorption on both the corrugated Ru surfaces is staggered, that is, the H atoms pointing away from the bridge site. The CH2 has an adsorption energy of -65 kJ/mol and is around 35 kJ/mol less stable than the CH intermediate (Table 1). The CH2 intermediate is unstable in the hollow site similar to that on Ru(101j0)B surface. 3.1.5. CH3 (Methyl). CH3 species is only stable on a bridge site. The adsorption energy corresponds to -75 kJ/mol with respect to CH4 in the gas phase (Table 1). The Ru-C bond lengths are 2.13 and 2.33 Å. This difference in the bond lengths is due to the H interaction with the Ru surface atoms. This trend was also observed on Ru(112j0) surface by Ciobica et al.14 The average C-H bond length is 1.11 Å. Because of the bulky nature of CH3 adsorbed species, one would expecta relatively high energy barrier for C-C coupling reactions in the F-T synthesis. On a stepped Ru surface, the barriers reported by Chen and Liu for CH3 coupling with C, CH, and CH2 are higher than 100 kJ/mol.5 This barrier increases with respect to the H atoms on C. In the top site, CH3 adsorbed species is unstable and moves to the bridge site during the optimization. The CH3 intermediate on Ru(112j1) surface is also adsorbed in the bridge site with one H atom interacting with the surface metal atom. The adsorption energy of the CH3 intermediate is -63 kJ/mol (Table 1). This energy is similar to the CH2 adsorption energy. The top site is 44 kJ/mol less stable than the bridge site. 3.1.6. CH4 (Methane). Methane is weakly adsorbed on the top site of both the corrugated Ru surfaces. The adsorption energy is only -10 and -9 kJ/mol on Ru(101j0)B and Ru(112j1) surfaces, respectively (Table 1). On both surfaces, the two H atoms of the CH4 molecule are pointing toward the surface, which are 1.11 Å from C, and the other two H atoms pointing away from the surface correspond to a C-H bond length of 1.09 Å. This indicates that there is a weak attractive interaction between the H atoms close to the surface with the surface Ru atoms. On the flat Ru(0001) surface, the adsorption energy of CH4 was reported to be 0 kJ/mol.13 Hence, we believe that the low coordinated surface atoms on the corrugated surface are responsible for an attractive interaction with CH4 as compared to that on the flat Ru surface. Moreover, our study reveals that the stability of the CHx species strongly depends on the surface structure. These different stabilities of the CHx species will decide the dominating adsorbed species as building blocks for the C-C coupling reactions in the F-T process. 3.2. CHx (x ) 0-4) Formation. In this section, we will discuss the step-by-step formation of CH4 on Ru(101j0)B and Ru(112j1) surfaces. 3.2.1. CH Formation. The geometries of the reaction path for the formation of CH4 on Ru(101j0)B surface are presented
Methane Formation on Corrugated Ru Surfaces TABLE 2: Geometries of the Reaction Path for the Formation of Methane on Ru(101j0)B Surfacea
J. Phys. Chem. C, Vol. 114, No. 51, 2010 22633 TABLE 3: Geometries of the Reaction Path for the Formation of Methane on Ru(112j1) Surfacea
a The forward (hydrogenation) and reverse (dehydrogenation) barriers are given in kJ/mol. Gray, yellow, and blue spheres correspond to the Ru, C, and H atoms, respectively. The values in the parentheses are for the flat Ru(0001) surface from ref 13. a
The forward (hydrogenation) and reverse (dehydrogenation) barriers are given in kJ/mol. Gray, yellow, and blue spheres correspond to the Ru, C, and H atoms, respectively. The values in the parentheses are for the flat Ru(0001) surface from ref 13. The second row indicates the diffusion of CH from four-fold hollow to three-fold hollow.
Figure 1. Reaction path for the hydrogenation of C (values in black) and dehydrogenation (values in red) of CH4 on Ru(101j0)B surface. The energies are with respect to the gas-phase CH4.
in Table 2, and the energy path is shown in Figure 1. The C and H atoms in the initial state (IS) are attached to the 4F hollow and bridge sites, respectively, 3.44 Å apart. The reaction path is described in Table 2a and Figure 1. The adsorption energy with respect to CH4 gas phase is -135 kJ/mol. This energy is the same as the adsorption energy of C when H is at infinite distance. This clearly indicates that there is no interaction between the C and H in a stable coadsorbed state. In the TS, the C and H atoms are 1.66 Å apart. In the final state (FS), the
H is attached to the C, and the CH adsorbed species is in the stable 4F hollow site. The barrier for the CH formation is 72 kJ/mol. Interestingly, this barrier is similar to that on flat Ru(001) surface, which has been shown to be 73 kJ/mol.13 The reverse reaction to dissociate the C-H bond is 59 kJ/mol. Surprisingly, this barrier on flat Ru(0001) surface is 108 kJ/mol, which is due to the strong adsorption of CH as compared to the C + H coadsorbed state. The geometries and the energy path for the CHx formation on the Ru(112j1) surface are presented in Table 3 and Figure 2, respectively. In the IS, C and H are coadsorbed in the 4F hollow and bridge sites, respectively (Table 3a), separated by 3.34 Å. In the TS, C and H atoms do not share the metal atoms, and the barrier corresponds to 85 kJ/mol. This barrier is 13 kJ/mol higher than that on the Ru(101j0)B surface. Because the CH intermediate is stable in the 3F hollow site, the reverse barrier for the dehydrogenation from this site is 94 kJ/mol. This is about 35 kJ/mol higher as compared to that on the Ru(101j0)B surface. Interestingly, the reverse barrier on Ru(112j1) surface is 14 kJ/ mol lower than that on the flat Ru(0001) surface. This implies that the hydrogenation and dehydrogenation of the carbon species depend on the structure of the surface. The diffusion energy of CH from the 4F hollow site to a more stable 3F hollow site (CH*) is 53 kJ/mol as represented in Figure 2. 3.2.2. CH2 Formation. CH2 formation on Ru(101j0)B surface is shown in Table 2b. In the IS, the CH and H are coadsorbed in 4F hollow and bridge sites, respectively. In the TS, CH and H are in a bent bridge configuration (Table 2b). The C-H distance is 2.17 Å. In the FS, the CH2 attains a stable bridge site. The barrier required for the hydrogenation of the CH is 96
22634
J. Phys. Chem. C, Vol. 114, No. 51, 2010
Shetty et al.
Figure 2. Reaction path for the hydrogenation of C (values in black) and dehydrogenation (values in red) of CH4 on Ru(112j1) surface. The energies are with respect to the gas-phase CH4. The diffusion of CH is indicated CH*.
kJ/mol. This reaction is 38 kJ/mol endothermic (Figure 2). This implies that, as compared to CH, formation of CH2 is difficult on Ru(101j0)B surface. On the flat Ru(0001) surface, the CH2 formation is 61 kJ/mol, while the dehydrogenation requires only 16 kJ/mol. On the Ru(112j1) surface, the CH intermediate occupies a more stable 3F hollow site and vacates the 4F hollow site unlike on the Ru(101j0)B surface. We further consider the CH in the 3F hollow site for the formation of CH2. In the TS (Table 3c), C and H atoms are at a distance of 1.6 Å. The CH intermediate moves from the 3F hollow site during the reaction. In the FS, the CH2 intermediate occupies the stable bridge site similar to that on the Ru(101j0) B surface. The barrier to form CH2 from the stable CH and H coadsorbed state is 83 kJ/mol. The reverse barrier is 48 kJ/mol. 3.2.3. CH3 Formation. The formation of CH3 on the Ru(101j0)B surface is described in Table 2c. In the TS, the C and H atoms are 1.67 Å apart (Table 2c). In the FS, the CH3 adsorbed species is in a bridge state. The barrier with respect to the stable coadsorbed state of CH2 and H is 109 kJ/mol
(Figure 1). This barrier is 55 kJ/mol higher than that on the Ru(0001) surface. On the flat Ru(0001) surface, the CH3 was shown to be stabilized on the top site. In contrast to this, we find that the CH3 moves from the top site to the bridge during the optimization. In the TS, on the Ru(112j1) surface, the H atom is attached to a single metal atom, and the C-H distance is 1.47 Å (Table 3d). In the FS, CH3 is attached to a bridge site. The barriers for hydrogenation and dehydrogenation with respect to stable configuration are 34 and 32 kJ/mol, respectively. One can clearly see that the formation of CH3 on Ru(112j1) surface is 76 kJ/ mol lower than that on the Ru(101j0)B surface. 3.2.4. CH4 Formation. This is the final step for the formation of CH4, and the reverse reaction is the initial step for the combustion process. In the TS on Ru(101j0)B surface (Table 2d), the coadsorbed CH3 and H are separated by 1.57 Å (Table 2d). In this configuration, the CH3 group is moving toward the top site and shares this site with the attacking H atom. In the FS, CH4 moves over the top site and has a weak interaction with the surface as discussed in an earlier section. The barrier for the hydrogenation of CH3 is 136 kJ/mol (Figure 1). In the case of Ru(112j1) surface, the C-H distance in the TS is 1.25 Å (Table 3e). In the FS, CH4 has the same configuration as that on the Ru(101j0)B surface. The barriers for hydrogenation and dehydrogenation on Ru(112j1) surface are 90 and 36 kJ/mol, respectively. If one considers the endothermic states in equilibrium with the most stable CHx state, then the overall barrier formation of CH4 will be the difference between the most stable CHx intermediate and the largest barrier. The overall barrier for the formation of CH4 on Ru(112j1) surface with respect to the most stable CHx intermediate, that is, CH, corresponds to 127 kJ/ mol. However, on Ru(101j0)B surface, with respect to the stable C, the adsorbed species is 195 kJ/mol. If one compares these overall barriers to the formation of CH4 on Ru(0001) surface, which is 141 kJ/mol, we can see that the formation of CH4 is difficult on the Ru(101j0)B surface as compared to that on the Ru(112j1) and Ru(0001) surfaces. The interesting point to note is that if one compares the CH4 decomposition on the Ru(112j1), Ru(101j0)B, and Ru(0001) surfaces, the initial step in the CH4 decomposition is easier on the Ru(112j1) surface by 34 and 49 kJ/mol with respect to that on Ru(101j0)B and Ru(0001) surfaces. This demonstrates that the CH4 decomposition is a difficult process on flat Ru(0001) surface as compared to that on the low coordinated corrugated Ru surfaces. In the hydrogenation process on all the Ru surfaces, the last step to form CH4 from the coadsorbed CH3 and H has the highest barrier as compared to the other steps. This step can be considered as the rate-limiting
TABLE 4: Rate Constants (k) and Pre-exponential (ν) Factors for the CHx Hydrogenation (Forward) and Dehydrogenation (Reverse) Reactions on Ru(101j0)B Surface at 300 K Reactions reaction path
forward k, s-1
reverse k, s-1
forward ν, s-1
reverse ν, s-1
C + H f CH CH + H f CH2 CH2 + H f CH3 CH3 + H f CH4
31.71 5.4 × 10-4 3.55 × 10-6 1.24 × 10-10
8.3 × 103 3.92 × 106 8.6 × 10-3 521
1.15 × 1013 1.5 × 1013 1.0 × 1013 8.9 × 1013
1.35 × 1013 1.06 × 1013 6.3 × 1012 1.6 × 1012
TABLE 5: Rate Constants (k) and Pre-exponential (ν) Factors for the CHx Hydrogenation (Forward) and Dehydrogenation (Reverse) Reactions on Ru(112j1) Surface at 300 K Reactions reaction path
forward k, s-1
reverse k, s-1
forward ν, s-1
reverse ν, s-1
C + H f CH CH + H f CH2 CH2 + H f CH3 CH3 + H f CH4
0.17 3.8 × 10-5 4.3 × 106 4.0 × 10-5
0.47 2.42 × 106 8.0 × 108 3.2 × 105
1.83 × 1013 8.3 × 1012 8.6 × 1012 8.1 × 1012
2.33 × 1013 7.05 × 1012 6.7 × 1012 1.3 × 1010
Methane Formation on Corrugated Ru Surfaces step in the hydrogenation process, which is in agreement with the earlier studies.17,37 3.3. Kinetic Analysis. The rate constants and pre-exponential factors calculated at 300 K from the transition state theory in a harmonic approximation (eqs 1 and 2) for the hydrogenation as well as dehydrogenation reactions on Ru(101j0)B and Ru(112j1) surfaces are presented in Tables 4 and 5, respectively. In the case of Ru(101j0)B surface, the rate of the reaction increases with respect to each hydrogenation step. Considering that the overall rate is controlled by the slowest step, we can see that on Ru(101j0)B surface the last step to form CH4 from coadsorbed CH3 and H has the smallest rate (Table 4). This clearly indicates that the last step is the slowest step for the formation of CH4 on Ru(101j0)B surface. Let us consider the reaction rates for the hydrogenation on Ru(112j1) surface (Table 5). We can see from Table 5 that there are two steps, which correspond to similar rate constants, that is, CH2 and CH4 formation. We can clearly see that the slowest step for the formation of CH4 on Ru(101j0)B surface (1.24 × 10-10) is 5 orders of magnitude slower than that on the Ru(112j1) surface (4.0 × 10-5). Moreover, the rate constants obtained for each step of hydrogenation and dehydrogenation of the CHx adsorbed species on Ru(101j0)B differ from that on the Ru(112j1) surface. We infer from the kinetic analysis that the CH4 formation on Ru(101j0)B surface is more difficult than that on the Ru(112j1) surface. Interestingly, the kinetic analysis also illustrates that the CH4 activation will be easier on Ru(112j1) surface than on the Ru(101j0)B surface. This clearly justifies the role of the corrugation of the Ru surfaces on the hydrogenation and dehydrogenation reaction pathways. 4. Conclusions In the present work, we have studied the hydrogenation of the C adsorbed species on corrugated Ru(101j0)B and Ru(112j1) surfaces. These results have been compared to the earlier work on the flat Ru(0001) surface. On the basis of these results, we make the following propositions. The stability of the CHx species on Ru surfaces is structure dependent. On Ru(101j0)B surface, C is the most stable adsorbed species in a 4F hollow site. On the other hand, on Ru(112j1) and Ru(0001) surfaces where 3F hollow sites are present, the CH adsorbed species is the most stable. The calculated rate constants at 300 K demonstrate that the hydrogenation and dehydrogenation are different on both corrugated surfaces. This clearly shows that the hydrogenation and dehydrogenation reactions depend on the surface structures. The present results as compared to the earlier results on Ru(0001) reveal that the stabilization of the CHx adsorbed species will be dependent on the surface structure. This is a crucial aspect for the C-C coupling reactions in the F-T synthesis. In the case of Ru(101j0)B surface, C adsorbed species will be the dominating species, while on Ru(112j1) and Ru(0001) surfaces, the CH adsorbed species will dominate.
J. Phys. Chem. C, Vol. 114, No. 51, 2010 22635 Acknowledgment. We would like to thank the National Computing Facilities (NCF) for providing computational resources (Grant: SH-067-10). References and Notes (1) Stynberg, A. P.; Dry, M. E. Fischer-Tropsch Technology. Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 2004; Vol. 152. (2) Schulz, H. Appl. Catal., A 1999, 186, 3. (3) Anderson, R. B. The Fischer-Tropsch Synthesis; Academic Press: Florida, 1984. (4) Bunnick, B. S.; Kramer, G. J. J. Catal. 2006, 242, 309. (5) Chen, J.; Liu, Z.-P. J. Am. Chem. Soc. 2008, 130, 7929. (6) Watwe, R. M.; Bengaard, H. S.; Rostrup-Nielsen, J. R.; Dumesic, J. A.; Nrskov, J. K. J. Catal. 2000, 189, 16. (7) Dry, M. E. Catal. Today 1990, 6, 183. (8) Wu, M.-C.; Goodman, D. W. J. Am. Chem. Soc. 1994, 116, 1364. (9) Wu, M.-C.; Goodman, D. W. J. Phys. Chem. 1994, 98, 5104. (10) Rostrup-Nielsen, J. R.; Sehested, J.; Norskov, J. K. AdV. Catal. 2002, 47, 65. (11) Egberg, R. C.; Ullmann, S.; Alstrup, I.; Mullins, C. B.; Chorkendorff, I. Surf. Sci. 2002, 497, 183. (12) Shimizu, T. K.; Mugarza, A.; Cerda, J. I.; Salmeron, M. J. Chem. Phys. 2008, 129, 244103. (13) Ciobica, I. M.; Frechard, F.; van Santen, R. A.; Kleyn, A. W.; Hafner, J. J. Phys. Chem. B 2000, 104, 3364. (14) (a) Ciobica, I. M.; van Santen, R. A. J. Phys. Chem. B 2002, 106, 6200. (b) Ciobica, I. M. Ph.D. Thesis, Technische Universiteit Eindhoven, 2002. (15) Au, C.-T.; Ng, C.-F.; Liao, M.-S. J. Catal. 1999, 185, 12. (16) Hickman, D. A.; Schmidt, L. D. Science 1993, 259, 343. (17) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Lok, C. M. J. Phys. Chem. C 2009, 113, 8858. (18) Huo, C.-F.; Li, Y.-W.; Wang, J.; Jiao, H. J. Am. Chem. Soc. 2009, 131, 14713. (19) Jones, G.; Jakobsen, J. G.; Shim, S. S.; Kleis, J.; Andersson, M. P.; Rossmeisl, J.; Abild-Pedersen, F.; Bligaard, T.; Helveg, S.; Hinnemann, B.; Rostrup-Nielsen, J. R.; Chorkendorff, I.; Sehested, J.; Norskov, J. K. J. Catal. 2008, 259, 147. (20) Wei, J.; Iglesia, E. J. Phys. Chem. B 2004, 108, 4094. (21) Koerts, T.; van Santen, R. A. J. Chem. Soc., Chem. Commun. 1991, 1281. (22) Belgud, M.; Pareja, P.; Amariglio, A.; Amariglio, H. Nature 1991, 352, 789. (23) van Santen, R. A. Acc. Chem. Res. 2009, 42, 57. (24) Shetty, S.; Jansen, A. P. J.; van Santen, R. A. J. Am. Chem. Soc. 2009, 131, 12874. (25) van Santen, R. A.; Neurock, M.; Shetty, S. G. Chem. ReV. 2010, 110, 2005. (26) Gates, B. C. Chem. ReV. 1995, 95, 511. (27) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley and Sons: New York, 1994. (28) Wei, J.; Iglesia, E. Angew. Chem., Int. Ed. 2004, 43, 3685. (29) (a) Kresse, G.; Hafner, J. Phys. ReV. B 1994, 49, 14251. (b) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15. (30) (a) Blochl, P. E. Phys. ReV. B 1994, 50, 17953. (b) Kresse, G.; Joubert, J. Phys. ReV. B 1999, 59, 1758. (31) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (b) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1998, 80, 891. (32) Henkelman, G.; Jo´nsson, H. J. Chem. Phys. 2000, 113, 9978. (33) Laidler, K. J. Theories of Chemical Reaction Rates; McGraw-Hill: New York, 1969. (34) Shetty, S.; van Santen, R. A. Phys. Chem. Chem. Phys. 2010, 12, 6330. (35) Ge, Q.; Neurock, M. J. Phys. Chem. B 2006, 110, 15368. (36) Turner, M. L.; Marsih, N.; Mann, B. E.; Quyoum, R.; Long, H. C.; Maitlis, P. M. J. Am. Chem. Soc. 2002, 124, 10456. (37) Geerlings, J. J.; Zonnevyelle, M. C.; de Groot, C. P. M. Surf. Sci. 1991, 241, 302.
JP108753A
