Further Theoretical Evidence for Hydrogen-Assisted CO

Article pubs.acs.org/JPCC

Further Theoretical Evidence for Hydrogen-Assisted CO Dissociation on Ru(0001) Dominic R. Alfonso* National Energy Technology Laboratory, U.S. Department of Energy, Pittsburgh, Pennsylvania 15236, United States S Supporting Information *

ABSTRACT: Extensive calculations based on spin-polarized density functional theory were carried out to examine how CHx are formed from the dissociation of CO on Ru(0001) in the presence of hydrogen. Common pathways, such as the direct CO dissociation and H-assisted route leading to HCO or COH, including alternative routes that involve the formation of HCOH and CH2O, were examined. The reaction energy and barrier for each elementary step were calculated. The calculations show that the carbide mechanism is not the main reaction pathway for the conversion of CO on Ru(0001). Complementary microkinetic simulations utilizing results from first-principles quantum mechanical calculations indicate that a branch starting from the hydrogenation of CO to HCOH (via COH intermediate) and subsequent C−O bond cleavage is more plausible. In the case of the flat Ru(0001), which is of direct relevance to the present study, the conversion of CO on this surface via the carbide mechanism is not viable. Experimental work of Fuggle et al. demonstrated that the dissociation process is not facile and requires electron bombardment to promote C−O bond cleavage.12 As an alternative to the carbide mechanism, the works of Mitchell et al. and Ueta et al. point to the Hassisted route to account for the hydrogenated intermediates detected upon exposure of the CO layer on Ru(0001) to gasphase H at sample temperatures below 130 K13 and above room temperature.14 In this mechanism, C−O cleavage occurs by coupling of H with CO, followed by decomposition of the hydrogenated species. Further support for this comes from representative DFT calculations.15,16 A route that consists of hydrogenation of CO forming HCO, followed by C−O bond cleavage, was identified. This does not require the active sites of corrugated surfaces for CO activation. Hydrogenation and the accompanying tilted configuration on the surface lower the C− O dissociation barrier. Though this route is predicted to be more likely to happen in comparison to the carbide mechanism, it was noted that the key intermediate HCO is highly endothermic4 and the decomposition barrier is small (∼0.02 eV). Work by Morgan et al. also shows that, while HCO could form on the flat surface at low temperatures, such a species is highly unstable at 180−220 K.17 The aim of this study is to contribute to the understanding of the mechanism of C−O conversion on Ru(0001) in the

1. INTRODUCTION Liquid long-chain hydrocarbons produced from Fischer− Tropsch synthesis are considered as promising fuel substitutes that are cost-competitive with crude-oil-based petroleum products. The resultant colorless and odorless fuel is transportable, offering important emission benefits because of its low aromatic content and the virtual absence of sulfur and nitrogen impurities.1 On Ru-based catalysts, conversion of syngas (CO + H2) occurs with high selectivity toward long-chain alkanes and monoalkenes.2 This is attributed, in part, to the low barrier activation of CO that gives rise to precursor CHx intermediates necessary for initiation and growth of hydrocarbon chains.3 It was proposed that the cleavage of either the C−O bond of the molecule itself (carbide mechanism) or its partially hydrogenated intermediates (via H-assisted mechanism) is responsible for the formation of these important precursor species on Ru, with the morphology of the surface dictating which pathway dominates.4 On stepped and corrugated surfaces, for example, temperature desorption spectroscopy (TDS), high-resolution electron energy loss spectroscopy (HREELS), infrared reflection−absorption spectroscopy (IRAS), and low-energy electron diffraction (LEED) studies indicate that the carbide mechanism was preferred with direct dissociation occurring in the temperature range of 300−500 K.5−7 Subsequent density functional theory (DFT) studies based on model surfaces containing under-coordinated sites, such as steps, revealed that the molecule was tilted with both carbon and oxygen essentially interacting with metal atoms.8−11 The activation of the C−O bond resulting from the enhanced interaction with the surface was thought to be responsible for the low barrier dissociation of the molecule. © XXXX American Chemical Society

Received: April 17, 2013 Revised: September 4, 2013

A

dx.doi.org/10.1021/jp403828y | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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mesh.28 A Methfessel−Paxton smearing29 of σ = 0.2 eV was utilized to improve convergence, and the corrected energy for σ → 0 was employed. Eadsorbate was calculated by placing a molecule or an atom in a cubic box with dimensions of 20 Å sides and performing spin-polarized Γ-point calculations. On the basis of this definition, a negative Eads indicates a bound state. Adsorption energies and site preferences for the various species that participate in direct and H-assisted CO dissociation were described for 1/9 ML coverage. The whole reaction starting from the adsorption of gaseous CO and dissociation of gaseous H2 on Ru(0001) was examined:

presence of H. In many previous works,15−17 Ru(0001) was chosen as a model system as Ru-based catalysts display superior activity for the Fischer−Tropsch synthesis.2,4 We examined specific CO dissociation pathways that form CHx (x = 1, 2) monomers that were indicated in previous efforts as the species that initiate and grow hydrocarbon chains.18,19 For reference, the common direct CO dissociation pathway was examined. The H-assisted pathway via the formation of HCO was revisited due to the aforementioned studies15−17 that suggest that this species plays an important role in the conversion of surface CO to CHx monomers and also in the electrochemical hydrogenation of CO2 on Cu.20,21 These earlier contributions were complemented by considering an alternative branch, such as those that begin with hydrogenation of CO to HCOH (via COH intermediate), followed by C−O bond cleavage. Analysis of the DFT free energies along considered reaction pathways plus complementary microkinetic modeling that would facilitate investigations at realistic conditions indicated that this alternative route is more likely. The technical details of the calculations are outlined in section 2. In section 3, we describe and discuss the results and place them in a wider context. Our conclusion is drawn in section 4.

CO(g) ↔ CO

(1)

H 2(g) ↔ 2H

(2)

We examined five main routes leading to C−O bond scission on the Ru(0001) surface: (i) Direct CO dissociation or the carbide mechanism. In this route, the following were considered

2. COMPUTATIONAL APPROACH 2.1. Calculations of Activation Energies of Elementary Steps from DFT. Reaction and activation energies required to model H-assisted versus direct CO dissociation pathways on Ru(0001) surface were calculated using spin-polarized DFT as implemented in the Vienna Ab initio Simulation Package (VASP) code.22,23 This implementation includes total energy and atomic force calculations. We used the generalized gradient approximation (GGA) formulation of Perdew, Burke, and Enzerhoff (PBE).24 The electron−ion interaction was described by the projector-augmented wave (PAW) method.25 The Kohn−Sham one-electron valence eigenstates were expanded in terms of plane-wave basis sets with a cutoff energy of 400 eV. The Ru(0001) surface was represented by a four-layer slab with a (3 × 3) surface unit cell. Periodic boundary conditions were imposed in the two directions parallel to the surface. To ensure the decoupling of the consecutive slabs, a 12 Å thick vacuum region is employed. The lattice constant of the Ru(0001) slab was fixed to the value obtained from optimizing with DFT this constant for the bulk metal. The computed lattice constants for the bulk Ru are a = 2.73 Å and c = 4.31 Å. These are in very good agreement with the values aexp = 2.71 Å and cexp = 4.28 Å found in the literature.26 Adsorption was allowed on one side of the slab. Adsorption of species containing electronegative atom(s) gives rise to a surface dipole moment on the Ru substrate, and therefore, the electrostatic potential was adjusted accordingly. A dipole correction scheme was employed to neutralize the artificial electric field created by the asymmetric slab.27 This approach involves imposing an external electric field in the vacuum region in order to nullify the artificial field generated by the slab dipole moment. The binding energy was computed using the expression Eads = Eadsorbate+slab − (Eslab + Eadsorbate), where Eadsorbate+slab is the total energy of the relaxed adsorbate−surface system, while Eslab and Eadsorbate are the total energy of the relaxed bare surface and adsorbate in the gas phase (i.e., the adsorbate as a stable molecule or radical in the gas phase), respectively. The k-point sampling of the two-dimensional electronic Brillouin zone of the periodic supercells was performed using the Monkhorst−Pack 3 × 3 × 1 k-point

CO ↔ C + O

(3)

C + H ↔ CH

(4)

CH + H ↔ CH 2

(5)

where step 3 involves direct C−O dissociation, while steps 4 and 5 correspond to subsequent reactions with surface H, giving rise to hydrocarbon monomers. (ii) HCO mechanism. This consists of addition of H to the C atom in CO forming HCO, followed by decomposition to CH and O. That is CO + H ↔ HCO

(6)

HCO ↔ CH + O

(7)

(iii) COH mechanism. Alternatively, H may react with the O atom of CO to form COH, which would decompose subsequently to C and OH: CO + H ↔ COH

(8)

COH ↔ C + OH

(9)

(iv) HCOH mechanism. This route involves CO hydrogenation of HCO (formed in step 6) to HCOH, followed by C−O bond scission: HCO + H ↔ HCOH

(10)

HCOH ↔ CH + OH

(11)

A parallel branch that starts with hydrogenation of COH forming HCOH COH + H ↔ HCOH

(12)

followed by step 11 was also examined. (v) CH2O mechanism. Finally, a scenario that starts with the formation of HCO (step 6), H addition to the C atom of HCO, leading to CH2O

HCO + H ↔ CH 2O

(13)

and subsequent C−O bond cleavage CH 2O ↔ CH 2 + O

(14)

was considered as well. Our strategy in locating the transition states and calculation of energy barriers for each elementary step consisted of two steps. First, we identified the process local minima (i.e., initial B



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Table 1. Adsorption Properties of Adsorbates on Ru(0001) speciesk

Eads (eV) this work

Eads (eV) other calculations

CO C O H HCO COH HCOH CH2O CH CH2

−1.94 −7.64 −5.92 (−2.88i) −2.88 (−1.24j) −2.57 −4.75 −3.20 −1.07 −6.99 −4.45

−1.82 to −1.96a,b −7.13d −2.82e,i, −2.60f,i −2.97g

Eads (eV) expts

adsorption mode

−1.66c

top-bound thru C hcp hcp fcc brg-bound thru C, top-bound thru O hcp-bound thru C same as HCO same as HCO hcp hcp

−1.28h,j

a Ciobica et al.15 bGajdos et al.41 cPfnür et al.43 dCiobica et al.48 eCiobica et al.15 fStampfl et al.47 gGreeley et al.52 hJachimowski et al.51 iRelative to O2 molecule. jH2 dissociative adsorption energy. kTotal energies of the adsorbates in the gas phase are reported in Table S1 in the Supporting Information.

reactant and final reaction products), and then we determined the transition-state configuration along the path that the system is likely to follow to its transition from one of the identified local minima to another. The climbing image nudged elastic band (CI-NEB) method for finding saddle points and minimum energy paths between known reactants and products was employed.30 A discrete representation of the path was generated, with the points (movable images) along the path being relaxed using first derivative based information. In this work, five movable images were used and an initial chain of images was constructed between the initial reactants and final reaction products using linear interpolation between the two end points. The transition state of the optimized process coordinate was approximated by the image of highest energy. Calculations tests without spin polarization on selected elementary pathways yielded barriers that are different by only up to ∼30 meV compared to the case when this effect is taken into account. For the end points, a very small energetic difference of up to ∼1 meV was found. Thus, we expect calculations without spin-polarization would yield trends essentially similar to the ones predicted here. In the calculations, the coordinates of the three topmost metal layers and the adsorbate(s) were allowed to relax while the bottom layer was fixed to their calculated bulk positions. The reaction energy ΔErxn was calculated using the expression ΔErxn = ∑Eprod − ∑Ereact, where the first and second terms represent the sum of energies of products and reactants, respectively. On the basis of this convention, a negative ΔErxn corresponds to an exothermic reaction. A full reaction energy diagram that includes the carbide and the various H-assisted mechanisms considered here was constructed within the Gibbs free energy framework.31,32 This approach is described in more detail in the Supporting Information. 2.2. Microkinetic Analysis. The conversion of CO on Ru(0001) was modeled microkinetically using activation energies from DFT in order to identify the dominant pathway. The rate coefficient for each elementary step included in the model was calculated based on the transition-state theory33,34 as follows k=

kBT q+ ⎛ Ea ⎞ exp⎜ − ⎟ h q0 ⎝ kBT ⎠

function was taken into account (q+ = q+vib and q0 = q0vib), which was described as +(0) qvib

=

∏ i

(

) 1 − exp(− ) hν

exp −0.5 k Ti B

hνi kBT

(16)

The adsorption steps investigated in this work (steps 1 and 2) were found to be a nonactivated process, and the corresponding rate coefficient was calculated according to collision theory35,36

kads =

SAλ h

(17)

where S and A are the sticking coefficient and area of adsorption site, while λ = h/(2πmkBT)1/2, where m is the molecular mass of the gaseous molecule. Following our previous work,37 the corresponding reverse rate coefficients for adsorption steps kdes was obtained from the corresponding kads using the equilibrium relationship, kads/kdes = Keq = z0/z+, where z0 and z+ are the total partition function for the molecule in the adsorbed and transition states, respectively. Thus, kdes takes the form kdes =

+ + ⎛ E ⎞ SAkBT qrotqvib exp⎜ − des ⎟ · 2 0 R ads hλ (qvib) ⎝ kBT ⎠

(18)

The term Edes refers to the desorption barrier, while q0vib is the vibrational partition function for either adsorbed CO or H, where the superscript Rads = 1 and 2 for steps 1 and 2, + + respectively. qvib and qrot represent the CO(g) or H 2(g) vibrational and rotational partition functions. The latter is given by q+rot = 8π2IkBT/σh2, where I and σ are the moment of inertia and symmetry number, respectively. The given set of time-dependent rate equations was solved by the stiff differential equation solver DVODE.38

3. RESULTS AND DISCUSSION 3.1. Adsorption of Reactants and Relevant Intermediates. The adsorption of reactants and all possible intermediates associated with the carbide and H-assisted mechanisms was investigated. The energetics and selected structural properties are presented in Table 1 and Figure 1. Our analysis would focus primarily on the most favorable adsorption mode, but information on the other stable configurations would

(15)

where Ea is the activation energy. q+ and q0 correspond to the partition function of the transition and initial states, respectively. For surface reactions, only the vibrational partition C



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Figure 1. Top and side views of the most stable configurations of (a) CO, (b) HCO, (c) COH, (d) HCOH, (e) CH2O, (f) CH, and (g) CH2 on Ru(0001). Gray, red, white, and green spheres represent C, O, H, and Ru atoms.

be provided as well. In the following, we comment on each species separately. Carbon Monoxide. The calculations show that CO binds preferably on the top site, but we note that the hcp site is competing with an energy difference of 0.04 eV. In this

configuration, the molecule is upright and binds to the metal surface through its C atom with a calculated C−Ru distance of 1.89 Å (Figure 1a). The C−O bond length is 1.17 Å, slightly longer than the gas-phase molecular bond length of 1.15 Å. This typical increase in the internal bond length is attributed to D



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and 1.19 Å and HCO angle of 124° for the free radical agree very well with experimental values of 1.11 Å, 1.20 Å, and 127°, respectively.53 The C−H bond does not significantly change when the molecule is adsorbed, but the C−O bond is significantly elongated (0.13 Å) due to the formation of C− Ru and O−Ru bonds. The hydroxymethylidine (COH) species exhibits a clear preference for the hcp site (Figure 1c). C is bound directly to the hcp site with O above it and H bonded to O. The average C−Ru distance of 2.04 Å is about ∼0.1 Å shorter than in the case of CO on the same hcp site. The C−O distance is 1.35 Å, the O−H distance is 0.98 Å, and the COH angle is 109°. While the free COH is higher in energy than the isomer HCO by 1.83 eV, the bound COH has a larger adsorption energy than HCO. The calculated Eads for COH is −4.75 eV, while the value is −2.57 eV for adsorbed HCO. The HCO adsorption geometry we find here is similar to that calculated by Inderwildi et al. using DFT.16 To the best of our knowledge, the previous quantitative description of the energetics and structural properties of these species on Ru(0001) is not available. Hydroxymethylene and Formaldehyde. Hydroxymethylene (HCOH) and formaldehyde (CH2O)both of which are O bearing hydrocarbon adspecies we considered in this work prefer to adsorb on the surface in a bonding mode essentially similar to HCO (Figure 1d,e). That is, the adsorbate interacts with the substrate via both C (on off-bridge site) and O (on offtop site). For HCOH, the O−Ru and the average C−Ru distances are 2.24 and 2.07 Å, while for CH2O, the values are 2.04 and 2.34 Å. The C−H and O−H bonds point away from the surface, and the corresponding bond lengths in the adsorbed and free molecules differ by only few hundredths of an angstrom. This is not the case for the C−O bond in adsorbed HCOH and CH2O, which are found to be noticeably elongated by 0.15 and 0.13 Å, respectively, compared to the free molecule. Moreover, the C−O bond is longer (HCOH: 1.47 Å and CH2O: 1.34 Å) than those formed for adsorbed HCO (1.19 Å) due to the presence of an extra C−H and/or O−H bonds. Formation of adsorbed HCOH is more favored over formation of CH2O; the calculated adsorption energies are −3.20 and −1.07 eV, respectively, with respect to their counterpart gas-phase molecule. To the best of our knowledge, no computational studies on the Ru(0001) surface exist to which we can compare our results for these two adsorbates. CH and CH2. In agreement with previous DFT investigations,48,54 CH exhibits the strongest binding at the hcp site (Figure 1f). It binds to the surface vertically through C, with H directly above it. The calculated adsorption energy is −6.99 eV with respect to the gas-phase CH. The C−H distance does not significantly change upon adsorption. For the adsorbed case, the calculated value is 1.11 Å, which differs by about 0.03 Å compared to the free case. The distances between C and the three neighboring metal atoms are 2.02 Å. It should be noted that the predictive adsorption energies of C and CH are large because the reference species in the gas phase are markedly unfavorable from an energetic standpoint compared to their adsorbed counterparts. Large adsorption values for C and CH were also obtained in previous DFT work when gas-phase C and CH species were chosen for the reference state.48,54 The adsorption mode for CH2 (Figure 1g) is similar to that for CH (C sits near the hcp site), consistent with previous DFT studies.48,54 An adsorption energy of −4.45 eV with respect to the free CH2 radical was obtained. In this geometry, the molecular plane is perpendicular to the surface. One C−H

the chemical bonding of the molecule with the surface. DFT investigations at θCO = 1/9−1/3 ML were reported,39 and for 1/3 ML, adsorption on the top site was preferred with hcp adsorption disfavored by 0.12 eV, whereas, for 1/9 and 1/4 ML, both sites have the same stability. Other DFT studies for 1/4 ML coverage also exist, and it was found that the top position was the most favorable with hcp slightly disfavored by 0.05−0.09 eV.40−42 Our results for 1/9 ML coverage indicates that the top position is the most favorable, followed by hcp, with a slight preference for the top site. The preference for CO adsorption on the top site in the low coverage limit was reported in previous experiments.43 The experimental values for C−Ru and C−O bond lengths are 1.93 ± 0.04 and 1.10 ± 0.05 Å,44 respectively, which are in good agreement with our results. Analysis of the temperature-programmed desorption (TPD) spectra of CO desorption from Ru(0001) gives an estimated adsorption energy of 1.66 eV.43 The calculated DFT value of Eads = 1.94 eV is somewhat larger. However, it must be emphasized that the results of comparison of the predicted adsorption energies from those obtained by TPD experiments must be interpreted sometimes with some caution. As pointed out previously,45,46 interpretation of TPD data requires the knowledge of a frequency prefactor that is often unknown. Thus, determination of adsorption energies from TPD experiments is often based on some guessed value for the prefactor, which is typically 1013 s−1. Hydrogen, Carbon, and Oxygen. For both C and O, a clear preference for 3-fold hcp was found (see Figure S1a,b in the Supporting Information). An adsorption energy of −7.64 eV for C (relative to C atom) was predicted with this site more favorable over fcc by 0.52 eV. For O, an adsorption energy of −5.92 eV relative to the O atom (or −2.88 eV relative to O2) was found with the corresponding fcc disfavored by 0.35 eV. The three C−Ru and O−Ru bonds are 1.94 and 2.01 Å. The calculated hcp site preference for O is consistent with previous experimental work.47 The O−Ru bonds estimated from LEED measurements are 2.00−2.03 Å, which compare well with our value of 2.01 Å. DFT studies for C48 and O15,47 adsorption were previously undertaken. These calculations also identified the hcp hollow site as the most stable for adsorption for both atoms. The reported C adsorption energy at 1/4 ML coverage is −7.30 eV,48 whereas, for O, the values (relative to the O2 molecule) are −2.60 to −2.82 eV.15,47 Our values for O and C are larger, which could be due the fact that our calculations are done at lower coverage and also to some differences in the calculation details underlying these studies. For H, the fcc site is favored (Figure S1c, Supporting Information) with an adsorption energy of −2.88 eV relative to atomic H (or −1.24 eV with respect to H2), but we note that the hcp site is competing with an energy difference of 0.03 eV. The calculated H−Ru bonds are 1.91 Å. The preference for the 3-fold sites is in line with previous experimental studies.49,50 TPD spectra yield an estimated energy change of −1.28 eV for H2 desorption from Ru(0001),51 which compares well with our calculated value of −1.24 eV. The slight preference by H for the fcc site was also found in previous DFT investigations for 1/4 ML coverage.52 In that work, it is favored by about 0.04 eV over the next most stable hcp site. Formyl and Hydroxymethylidyne. Formyl (HCO) binds to the surface with C and O situated on off-bridge and off-top positions (Figure 1b). The C−Ru and O−Ru distances are 2.12 and 2.06 Å, whereas the values for C−H and C−O are 1.09 and 1.32 Å. The calculated C−H and C−O bond lengths of 1.14 E



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COH mechanisms were examined first, and the results are depicted in Figures 3a,b. The barrier for HCO formation is 1.14

group is nearly parallel to the surface and points toward an adjacent Ru, while the other one points toward the gas phase. The former has a higher C−H bond length of 1.22 Å, compared to 1.10 Å for the latter. 3.2. CHx Formation from Direct and H-Assisted CO Decomposition. We examined the formation of CH x precursor species via the carbide mechanism. Direct CO dissociation on Ru(0001) was studied starting from its most stable position, the top site. The calculations show that the admolecule moves to a nearby hcp site before dissociation takes place. This allows the CO to begin the dissociation process closer to the surface. In the transition state, O sits almost on a nearby fcc site with an elongated C−O bond. The overall energy barrier for this process is Eact = 2.13 eV using CI-NEB, and the reaction is slightly endothermic with a reaction energy of ΔErxn = 0.23 eV (Figure 2a). Our results are in line with

Figure 3. Same as in Figure 2, for (a) HCO mechanism and (b) COH mechanism.

eV and is endothermic by 1.04 eV. For COH, we predicted a barrier lower by 0.11 eV and the reaction is less endothermic by 0.34 eV. COH formation is, therefore, more favorable both thermodynamically and kinetically. However, the C−O dissociation barrier in HCO is significantly lower than that in COH. Our calculations predict Eact = 0.79 eV for dissociation of HCO to CH and O and Eact = 2.41 eV for the dissociation of COH to C and OH. The more weakened C−O bond in HCO is attributed not only to hydrogenation but also to the significant tilting of the O atom toward the surface. The C−O bond tilts from 0° relative to the surface normal to about 75°. Moreover, the decomposition of HCO is highly exothermic (ΔErxn = −1.08 eV) in comparison to COH (ΔErxn = 0.08 eV). In view of these, the formation of precursor species via the HCO intermediate is predicted to be more favorable than COH. This HCO mediated pathway was previously investigated using a different DFT setup.16 This study also came to the conclusion that, while the barrier for HCO formation is rather high (0.99 eV) and endothermic (0.97 eV), the barrier for the subsequent dissociation is relatively lower (0.76 eV), while the corresponding reaction energy is exothermic (−0.85 eV). For the HCOH mechanism that goes through HCO, the formation of HCOH is computed to be endothermic by 0.41 eV with an energy barrier of 1.04 eV (Figure 4a). As mentioned previously, a parallel path that goes through COH can also be envisioned. Here, the formation of HCOH is more endothermic (0.93 eV) with an essentially similar barrier of 1.05 eV (Figure 4b). For the CH2O mechanism, the HCO + H → CH2O step has an energy barrier of 0.51 eV and it is endothermic by 0.44 eV. A higher barrier of 1.17 eV was obtained for the subsequent CH2O → CH2 + O dissociation, and it is exothermic by −1.03 eV (see Figure S2 in the Supporting Information). Compared to CH2O formation, HCOH formation via HCO and COH is less favorable

Figure 2. (a) CO dissociation and (b) CH and (c) CH2 formation on Ru(0001). Reaction barriers Eact and reaction energy ΔErxn are in eV. Gray, red, white, and green spheres represent C, O, H, and Ru atoms.

previous periodic DFT calculations that CO migrates to a nearby hcp site before the actual dissociation starts.15 A slightly higher barrier of 2.24 eV on a 3 × 3 unit cell was predicted in that work using a different DFT setup. CH is formed by combination of adsorbed C and H, as shown in Figure 2b. CH formation is slightly exothermic by −0.19 eV. In the transition state, C essentially remains in its hollow, while H is located in a top position. The calculated barrier for this process is 0.77 eV. CH2 can be formed by hydrogenation of adsorbed CH. In the transition state, H is almost on top of a metal atom. The reaction in this case is endothermic by 0.46 eV with an energy barrier of 0.50 eV (Figure 2c). The weak exothermicity and marked endothermicity found here for C + H → CH and CH + H ↔ CH2, respectively, are in line with previous DFT studies that predicted a reaction energy of −0.13 and 0.54 eV using a 3 × 3 unit cell.48 Investigation of hydrogenation of C on a 4 × 2 unit cell55 reveals that, for the first hydrogenation process, a barrier of ∼0.7 eV is predicted while the reaction is slightly exothermic. For the hydrogenation of CH, it was found to be markedly endothermic with a barrier of ∼0.5 eV. We now discussed plausible pathways toward formation of CHx species with the assistance of hydrogen. The HCO and F



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is the cleavage of the C−O bond. For the HCO mechanism, both the hydrogenation of CO to form HCO and subsequent C−O dissociation would require a relatively smaller barrier. The overall barrier is found to be 1.98 eV, which is ∼0.2 eV lower than that in the carbide mechanism. These results show that, in comparison to the carbide mechanism, the HCO mechanism is a more favorable pathway. Previous analyses based on total energy profile yielded a similar conclusion.15,16 Inspection of Figure 5 indicates that the alternative COH mechanism has a computed overall barrier (2.60 eV) that is higher to compete with the carbide mechanism and, therefore, can be ruled out. The same observation can be reached for the CH2O and the HCOH mechanisms that go through the HCO intermediate. Interestingly, we find that the corresponding HCOH mechanism via COH possesses an overall barrier that is fairly comparable to the HCO mechanism. A key feature of this route is that it goes through the COH intermediate that has a lower free energy than HCO by 0.34 eV. Additionally, comparison of the reverse reaction barriersthat is, COH → CO + H (0.31 eV) vs HCO → CO + H (0.05 eV)indicates that it is relatively less likely for COH to decompose back to CO. This is a new finding of this work, as previous studies never considered the further hydrogenation of COH on Ru(0001). Examination of the corresponding total energy profile (Figure S3 in the Supporting Information) yielded findings essentially similar to the free energy profile. 3.3. Kinetic Modeling. The present work confirms the critical role of the H-assisted route in the generation of precursor species on Ru(0001). The DFT results suggest that CO hydrogenation leading to the formation of HCO, followed by C−O scission, is an energetically feasible pathway. CO decomposition via COH and HCOH intermediates may be another realistic route. To throw further light on this, complementary microkinetic studies were conducted. The following processes were included in the model: (i) CO/H2 adsorption process (CO(g) → CO; H2(g) → 2H) and CO conversion via (ii) carbide (CO → C + O; C + H → CH), (iii) HCO (CO + H → HCO; HCO → CH + O), (iv) HCOH via COH (CO + H → COH; COH + H → HCOH; HCOH → CH + OH) mechanisms. The formation of CH from the three routes was monitored in order to determine the main reaction pathway. CO and H2 adsorptions are found to be nonactivated processes, and the corresponding rate kads is calculated according to eq 17. The sticking coefficient S is assumed to be 1.0, and the area of adsorption site A is set to 1.07 Å2. The flat surface approximation plus the assumption that the area of a single adsorption site is the same regardless of the nature of the site were used in the evaluation of the quantity A.56,57 The rates of CO and H2 desorption are expressed according to eq 18. The desorption barrier, Edes, is considered from a detailed balance to be equal to the absolute value of the adsorption energy. Equations 15 and 16 were used to evaluate the rate for surface reaction. Experimentally, the binding energy of CO was found to be influenced by the CO coverageit decreases with increasing coverage.43,58 Additional DFT calculations were then carried out to evaluate the binding energy at different coverages. An exponential function was then fit to the data, yielding an equation describing the binding energy (in eV), Eads,CO, as a function of CO coverage θCO: Eads,CO(θCO) = −2.64 + 0.66 exp(θCO/1.43). Activation energies for reactions involving CO were adjusted commensurately with Eads,CO(θCO).59,60 That is, say for a given 0.1 eV change in

Figure 4. Same as in Figure 2, for (a) HCOH (via HCO) and (b) HCOH (via COH) mechanism.

kinetically. However, this is offset by the fact that the C−O bond cleavage in HCOH is more facile. That is, the calculated barrier to break this bond in HCOH is 0.21 eV compared to 1.17 eV in CH2O. To take our static DFT calculations to the next level, the independent steps discussed above are put together in a twodimensional Gibbs free energy surface above room temperature (500 K). The free energies are calculated with respect to gasphase CO and H2 at infinite separation from bare Ru(0001). The intermediate products are shown with their free energies corresponding to that of the surface species at infinite separation from each other. Using DFT-predicted vibrational frequencies, the transition-state free energy, GTS, was calculated TS TS according to the expression GTS = ETS + ETS ZPE + Uvib − TSvib, TS TS where E and EZPE refer to the electronic and zero point energy of the transition state, respectively. The vibrational components of the internal energy and entropy were evaluated using eqs S8 and S12 in the Supporting Information. The overall barrier for CHx generation via the carbide mechanism is 2.14 eV (Figure 5). The rate-limiting step for this specific path

Figure 5. Two-dimensional free energy surfaces for the direct and various hydrogen-assisted CO dissociation routes on Ru(0001). The vertical axis is in relative free energy (in unit eV). All species are adsorbed, and the dashed line indicates the free energy of gas-phase CO, gas-phase H2, and bare substrates. G



Article

the binding energy of CO, the activation energy for the reaction is adjusted by 0.1 eV in the appropriate direction. The studies were conducted using a feed syngas ratio of 1:2 (CO/H2) with a pressure of 0.01 mbar kept constant above the surface. A starting surface coverage of 0.2 ML CO and 0.4 ML H was assumed. The simulations were carried out for 106 s (about 278 h). Displayed in Figure 6 is the resulting conversion of CO as a function of temperature. At low temperatures, CO adsorbs on

Figure 7. COH and CH (from the different mechanisms) coverages as a function of time (T = 600 K).

We have not considered the effects of external applied potential or enzymes in this work. Although these effects would undoubtedly affect, for example, the COH intermediate in a profound way by further favoring its formation thermodynamically and/or kinetically, this topic is beyond the scope of the current work.

4. CONCLUSIONS In the present work, various mechanisms for CO conversion to CHx intermediates on Ru(0001) in the presence of hydrogen were investigated using first-principles DFT. In agreement with previous works, the hydrogen-assisted pathway by which CO is hydrogenated by surface H species, forming HCO, followed by subsequent decomposition to CH, is more favorable than the one that involves direct CO dissociation. The predicted free energy barriers for the former and latter pathways are 1.98 and 2.14 eV, respectively. Thus, direct CO dissociation is more difficult from the kinetic standpoint. A reaction network that begins with H reaction with CO to generate the isomer COH, further hydrogenation of COH to produce HCOH, and followed by C−O bond breaking was examined. This was predicted to go through a more energetically favorable COH intermediate with an overall free energy barrier comparable to the above-mentioned H-assisted route that involves the HCO intermediate. Comparatively, other mechanisms postulating CH2O and HCOH (from HCO) as reaction intermediates are not predicted to be favorable. Rate coefficients for the elementary pathways were computed using data obtained from first-principles DFT calculations. This enabled a microkinetic analysis to be carried out at realistic conditions. By these means, it was found that the carbide mechanism is not viable on the Ru(0001) surface. The model, instead, predicts that the alternative HCOH mechanism, through a more stable COH intermediate, is more likely to happen on Ru(0001). The present theoretical study quantified in detail the mechanism leading to the formation of precursor CH x intermediates necessary for the initiation and growth of hydrocarbon chains in a process of general importance. It provides further theoretical evidence that, on Ru(0001), it occurs through a H-assisted mechanism. Our model should serve as a natural starting point for the examination of the

Figure 6. CO, COH, and CH (formed from the different mechanisms) coverages as a function of temperature.

the surface. At 380 K, for example, the surface was more than half-covered by this molecule. We observe the reaction of CO and H leading to adsorbed COH. No HCO species was detected. CH species, through further hydrogenation of COH to HCOH, is present in a few percent of a monolayer. With increasing reaction temperature, there is a general decline in the coverage of CO. The enhanced thermal excitation makes the intermediate COH unstable, as evidenced by the rapid decrease of its surface coverage with increasing temperatures. CH formation from the HCOH via COH route, on the other hand, increases rapidly until the surface is covered with ∼0.3 ML CH at 500−520 K. Up to about 500 K, CH formation from the other mechanism is minimal. Above 520 K, CO conversion via the carbide and HCO mechanisms is initiated. However, the amount of CH formed is comparatively small, and hence, we conclude that these routes are not the most active conversion pathways. The simulation was taken to the next level by looking at the transient kinetics at 600 K and using a higher pressure of feed gas of 10 bar. The surface coverages of COH and CH produced from the various mechanisms as a function of time are shown in Figure 7. It can be seen that, initially, COH was formed and a small amount of CH, formed from the conversion of COH, is present. With time, COH at the surface decreases rapidly owing to its reaction with H and subsequent formation of CH. The production of CH from the carbide and HCO mechanisms is again minimal. Our microkinetic simulations clearly indicate that the main reaction pathway is the hydrogenation of CO, giving rise to COH and then HCOH, followed by the decomposition of this species to CH. H



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propagation cycle leading to the formation of long-chain alkanes and monoalkenes on this surface. Furthermore, our work to a certain degree represents further effort to utilize complementary microkinetic modeling to study a technologically important surface reaction system. A growing trend in such modeling is the use of guided parameters that build upon information obtained from first-principles calculations. With this multifaceted approach, atomic-level investigations of the conversion of CO on Ru(0001) at realistic conditions was achieved.

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ASSOCIATED CONTENT

S Supporting Information *

Details of Gibbs free energy calculations, Table S1, and Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (412) 386-4113. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Valuable advice by D. Blaylock on microkinetic simulations is strongly acknowledged. The author would also like to thank D. Sorescu, John Lo, and M. Salcicioli for fruitful discussions. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of author(s) expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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Further Theoretical Evidence for Hydrogen-Assisted CO

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