Mechanistic Study of Carbon Monoxide Methanation over Pure and

Article pubs.acs.org/JPCC

Mechanistic Study of Carbon Monoxide Methanation over Pure and Rhodium- or Ruthenium-Doped Nickel Catalysts José L. C. Fajín,*,† José R. B. Gomes,‡ and M. Natália D. S. Cordeiro*,† †

LAQV@REQUIMTE/Department of Chemistry and Biochemistry, Faculty of Sciences, University of Porto, 4169-007 Porto, Portugal ‡ CICECO/Department of Chemistry, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal S Supporting Information *

ABSTRACT: Carbon monoxide (CO) methanation has been studied through periodic density functional theory calculations on flat and corrugated nickel surfaces. The effect of doping the catalyst was taken into account by impregnating the nickel surfaces with Rh or Ru atoms. It was found that the methanation of CO as well as the synthesis of methanol from CO and hydrogen (H2) evolve through the formyl (HCO) intermediate on all the surfaces considered. The formation of this intermediate is the most energy-consuming step on all surface models with the exception of the Rh- and Ru-doped Ni(110) surfaces. In the methanation reaction, the CO dissociation is assisted by hydrogen atoms and it is the rate-determining step. Also, surfaces displaying low-coordinated atoms are more reactive than flat surfaces for the dissociative reaction steps. The reaction route proposed for the formation of methanol from CO and H2 presents activation energy barrier maxima similar to those of CO methanation on pure nickel and Rh- or Ru-doped flat nickel surfaces. However, the CO methanation reaction is more likely than the methanol formation on the doped stepped nickel surfaces, which is in agreement with experimental results available in the literature. Thus, the different behavior found for these two reactions on the corrugated doped surfaces can then be used in the optimization of Ni-based catalysts favoring the formation of methane over methanol. formation.15−17 Most of the alkanes produced tend to be straight-chained, i.e., apt to be used as synthetic fuel, though some branched alkanes are also formed. Apart from alkanes production, competing reactions yield also alkenes, alcohols, and other oxygenated hydrocarbons.18,19 Usually, only relatively small quantities of these nonalkane products are formed, although catalysts favoring some of these byproducts have been developed as well.7,16 Despite the fact that whenever aiming at producing synthetic fuels by the F-T process methane formation is a reaction to be avoided, the latter reaction is of great interest in other catalyzed processes such as the cleaning of hydrogen-rich fuel cell gas streams in which it is used for the elimination of CO, which poisons the platinum catalyst.20 Other reactions, such as the preferential oxidation (PROX) of CO, are also used for the same purpose but with the limitation of requiring the inclusion of an oxidative agent, usually O2.21,22 Both the CO-PROX and CO-methanation reactions can be catalyzed by catalysts based on the same transition-metal (TM) atoms.23 The conversion of CO2 to methane via a methanation reaction is of great interest in processes aiming at the cleaning of natural gas from this contaminant.24 In the case of the

1. INTRODUCTION The increase of crude oil prices during the last decades led to a search for alternative supplies to obtain synthetic substitutes for petrol derivatives. A potential method, for example, is to obtain hydrocarbons from conversion of the so-called “synthesis gas” or “syngas” (CO + H2) on solid catalysts, i.e., by the wellknown Fischer−Tropsch (F-T) synthesis process.1,2 In fact, the F-T process was used during World War II in the production of synthetic fuels because of the problems experienced by some countries in providing crude oil. The principal route of this process is that related to the production of alkanes, which can be described by the following chemical equation: nCO + (2n + 1)H 2 → CnH(2n + 2) + nH 2O

(n = 1, 2, 3, ...) (1)

Many different catalysts can be used for the catalysis of the above reactions, but the most common are based on Co, Fe, Rh, Ru, and Ni metals or alloys of these metals.3−13 The main aim of the process is to produce a synthetic petroleum derivative, typically from coal, natural gas, or biomass to be used as synthetic lubrication oil or as synthetic fuel. Furthermore, the F-T synthesis involves a variety of competing chemical reactions which lead to a series of desirable products and undesirable byproducts.7,8,14−16 Process conditions and catalyst composition are usually chosen to favor higher-order reactions (n > 1 in eq 1) and thus minimize methane © 2015 American Chemical Society

Received: February 24, 2015 Revised: June 24, 2015 Published: June 26, 2015 16537

DOI: 10.1021/acs.jpcc.5b01837 J. Phys. Chem. C 2015, 119, 16537−16551

Article

The Journal of Physical Chemistry C

with more reactive metals as those just referred to. For the CO methanation on nickel surfaces, it has been suggested that the CO dissociation is the rate-determining step,53 with the step sites being the most reactive, both in the presence or in the absence of hydrogen during the CO dissociation.54,55 It has been also proposed that the CO dissociation on Ni surfaces occurs through a COH intermediate, which implies a reaction in two steps, i.e., COH formation and COH dissociation.55 Despite that, it cannot be discarded that the CO hydrogenation evolves through a formyl (HCO) intermediate, as has been proposed for the CO methanation on surfaces of several metals.36,37 Presently, it is well-documented that the methanation of CO as well as the Fischer−Tropsch synthesis in which it is framed are affected by a series of reaction variables that might modify the activity and the selectivity of the reaction, namely, the catalyst structure;55,56 the metal loading, i.e., the concentration of the active metal phase on the support;57 the oxidation state of the active metal phase;58 the presence of additives on the catalyst;16 the presence of dopants on the catalyst;59 the variations in the reactor temperature, pressure and space velocity;60 the applied potential;61 and the support used.62 The work reported here is a detailed computational mechanistic study of the effect of doping the Ni catalyst with Rh or Ru in the reaction of CO methanation. The different steps of the CO methanation were studied on pure and doped flat Ni(111) and corrugated Ni(110) surfaces. The consideration of these Miller indices for the pure Ni and doped Ni surfaces was based on the fact that the (111) index of facecentered cubic (fcc) metals (e.g., Ni, the predominant element in our catalytic models) simulates the most stable plane found on large metal particles, with surface atoms possessing coordination number (CN) 9, while the (110) plane is associated with an open surface structure, with surface atoms possessing coordination number 7, usually conferring to this plane the highest reactivity of the three low-index planes of fcc metals. In addition, the examination of the reaction on both flat and corrugated surface models enables the comparison of the effects of low-coordinated atoms in the catalysis of such reaction (e.g., CN 9 or 7). Reaction routes corresponding to the direct CO dissociation on the catalyst or via the COH and HCO intermediates are considered. The route leading to the methanol formation is investigated as well to understand if the breakage of the C−O bond to obtain methane is energetically competitive with reactions keeping intact the C−O bond. This work is organized as follows: the computational methods are described in detail in section 2, and the calculated results are reported and discussed in section 3; finally, the most important conclusions are summarized in section 4.

elimination of CO from hydrogen streams through the methanation reaction CO + 3H 2 → CH4 + H 2O

(2)

available CO2 molecules can also be methanated, which will consume significant quantities of valuable hydrogen CO2 + 4H 2 → CH4 + 2H 2O

(3)

or can lead to formation of CO molecules through the reverse water−gas shift (RWGS) reaction CO2 + H 2 → CO + H 2O

(4)

with undesired consequences. Thus, catalysts with a high selectivity toward CO methanation in the presence of CO2 are being developed for the cleaning of hydrogen-rich gas streams.25−28 Regarding the CO and CO2 methanation mechanisms, it is generally accepted that they evolve through the same intermediate species, suggesting that CO is an intermediate in the CO2 methanation.29,30 In spite of that, it has been suggested that CO is not an intermediate for the CO2 methanation over catalysts such as ceria doped with transition metals,31 because they were able to methanate the latter but not the former species. On the contrary, for the CO2 methanation over γ-Al2O3 supported Ni catalysts, it has been suggested that CO can be an intermediate as long as a hydroxylated support is employed.32 Furthermore, the hydrogenation of CO2 seems to be easier than the hydrogenation of CO on cobalt and iron catalysts.33 The mechanism of the Fischer−Tropsch synthesis (in particular the reaction steps leading to methane formation) has been extensively studied during recent years. For example, Ojeda et al.6 proposed a series of reactions for the initial steps of the F-T synthesis on Fe- and Co-based catalysts, with the crucial step being the C−O bond breaking, which the authors advocated as being easier if its activation is assisted by hydrogen atoms on Co-based catalysts. Similar conclusions had been reached from the semiempirical unity bond index-quadratic exponential (UBI-QEP) method34 and from density functional theory (DFT) calculations35−37 for CO methanation on cobalt. The CO dissociation assisted by hydrogen atoms has also been proposed for the CO methanation on Ru-based catalysts from experiments.20,38 In 2001, Davis5 reported the results of a study about hydrocarbon chain growth on Fe- and Co-based catalysts and suggested that different mechanisms are followed over these, which was further corroborated by the experimental work of Schulz.19 The reaction steps involved in the F-T synthesis are habitually followed in the opposite direction during the steam reforming of alcohols or hydrocarbons,39 with the catalysts being applied on the catalysis of these reactions based on Ni,40 Co,41−43 and Rh.41 Another important aspect in the F-T synthesis mechanism is the poisoning effect that sulfur atoms deposited on the catalyst may have during the reaction.44,45 The CO methanation is usually carried out on Ni-based catalysts,9 but other metals can be even more reactive for this reaction, such as Ru, Rh, and Co.28,46 The latter metals are not usually employed in the elaboration of commercial catalysts because of their high prices; thus, their alloys with cheaper metals are being developed for the catalysis of the CO methanation, as for example Ni−Rh−Ru,26 Ni−Rh,24 Ni−Fe,47 Rh−Fe,48 Pt−Co,49 Co−Ru,50 Ca−Ni−Ru,51 and Ni−Ru−B.25 In fact, nickel surfaces have a considerable activity for the CO methanation,52 which is increased by the formation of alloys

2. THEORETICAL METHODS 2.1. Slab Models. The catalytic surfaces used in this work were modeled by the repetition of a slab with adequate symmetry in the three spatial directions. As usual, a vacuum region with a width of 10 Å was introduced in the z direction to separate the repeated slabs, this region being large enough to avoid interactions between neighboring replicas. The slabs used to model the Ni(111) and Ni(110) surfaces were obtained by cutting the bulk Ni metal, with a lattice parameter of 3.522 Å optimized in a previous work,63 along the corresponding Miller indices. In each case, the slabs correspond to 2 × 2 unit cells with respect to the minimal unit cell. With regard to the doped nickel surfaces, these were modeled by replacing a single Ni 16538


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associated with its single imaginary frequency were carefully analyzed in forward and backward directions. As is common practice, the reaction energies and activation energy barriers were computed as the energy differences between the final and initial states at their most stable configurations or between the transition and the initial states, respectively. The adsorption energies (Eads) of the several (co)adsorbed species include ZPV corrections and were calculated as Eads = Eslab−adsorbate(s) − Eslab − Eadsorbate(s), where Eslab−adsorbate(s) stands for the energy of the slab−adsorbate(s) supersystem, Eslab for the energy of the slab model, and Eadsorbate(s) for the energy of the adsorbate(s) in the gaseous phase. Consequently, negative adsorption energies indicate thermodynamically favored adsorptions. In addition, rate constants (k) for the different reaction steps involved in the CO methanation and in the methanol production from CO and H2 were estimated by applying the expression below derived from transition-state theory.76

atom in the top layer of the respective Ni-pure slabs by Rh or Ru atoms, respectively. In the case of the Rh@Ni(111) and Ru@Ni(111) surfaces, a Ni atom of the Ni(111) slab top layer was substituted by a Rh or Ru atom, while in the Rh@Ni(110) and Ru@Ni(110) surfaces, a Ni atom forming the Ni(110) combs was substituted by a Rh or Ru atom, respectively. Herein, we were interested in understanding the effects on catalysis by doping the topmost surface layer of Ni catalysts with Rh and Ru. The study of the effects caused by doping underneath the topmost layer is not the aim of the present study because metal segregation is not expected or should be fairly weak for Rh and Ru on Ni(111).64 On the basis of these assumptions, to obtain the Rh@Ni(111) and Ru@Ni(111) surfaces, a Ni atom of the Ni(111) slab top layer was substituted by a Rh or Ru atom, because all positions are equivalent, while in the Rh@Ni(110) and Ru@Ni(110) surfaces, a Ni atom in the crests of the Ni(110) surface (i.e., topmost atomic positions on this surface) was substituted by a Rh or Ru atom, respectively. For both Ru and Rh, substitution at positions in the valleys of the Ni(110) surface was found to be almost isoenergetic (cf. < 0.1 eV). 2.2. DFT Calculations. The VASP 5.2 computer code65−67 was used to carry out all the DFT calculations, considering the PW91 generalized gradient approach (GGA) exchange correlation potential68 and the projected augmented-wave (PAW) method as implemented in VASP69,70 to take into account the effect of core electrons in the valence electron density. The PW91 density functional was chosen here because this is a quite robust approach. It was ranked first (equal with PBE) in the “podium of density functionals for bulk metals”71 and was found (together with PBE) to provide quite good adsorption and activation energies for the water dissociation on the Cu(111) surface.72 Adsorption and activation energies calculated by different authors with the PW91, PBE, and RPBE functionals for adsorption and dissociation of O2 on metal surfaces have been found to differ by less than 0.1 eV.73 Last but not least, most of the data available in the literature for catalytic reactions on metal surfaces were computed with the PW91 or PBE approaches; hence, because the results obtained with these functionals are very much the same, the consideration of the PW91 functional in the present work allows comparison with such results. A 7 × 7 × 1 Monkhorst−Pack grid of special k-points74 was employed for the numerical integration in the reciprocal space along with a cutoff of 415 eV for the plane waves expansion. Spin-polarized DFT calculations were used in the determination of the adsorption energies and geometries of the most favorable adsorption configurations for the species implied in the reaction steps corresponding to the CH4 or CH3OH formation from syngas (CO + H2). The transition states (TS) for the different reactions steps were located by the Dimer approach.75 Very strict convergence criteria were employed (10−6 eV for the total energy change and 10−3 eV/Å for the forces acting on the ions) to avoid the algorithm converging to local minima. Harmonic frequencies were computed at the optimized geometries of the stationary points found (keeping the surface frozen), allowing their assignment as minima or transition states, and the determination of zero-point vibrational (ZPV) corrections as well as vibrational partition functions. To further verify that each located TS structure connects the putative reactants and products, the movements

⎛ k T ⎞⎛ q ⧧ ⎞ k = ⎜ B ⎟⎜⎜ ⎟⎟ exp( −Eact /kBT ) ⎝ h ⎠⎝ q ⎠

(5)

where kB and h are the Boltzmann and Planck constants, respectively; T is the absolute temperature, and Eact is the ZPV corrected activation energy; q⧧ and q are the vibrational partition functions for the TS and initial state, respectively. Two different temperatures were chosen for the calculation of the rate constants, namely, T = 503 K and T = 573 K, which correspond to the temperature interval limits for the CO methanation on Ni-based catalysts doped with Ru and Rh.26

3. RESULTS AND DISCUSSION The elementary steps considered for CO reaction toward methane and methanol formation on nickel surfaces are shown in Scheme 1. To begin, common possible initial steps to methanol and methane formation were analyzed (steps I−VII); then, we focused on exploring the reactions leading to the Scheme 1

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Figure 1. Adsorption positions at the (a) Ni(111), (b) Ni(110), (c) TM@Ni(111), and (d) TM@Ni(110) surfaces. TM stands for the two possible doping transition-metal atoms, i.e., for Ru and Rh atoms.

Table 1. Calculated ZPV Corrected Interaction Energies (in Electronvolts) for Isolated Species Deposited on Ni(111), Ni(110), Rh@Ni(111), Ru@Ni(111), Rh@Ni(110), and Ru@Ni(110) Surfaces species CO

COH

HCO

HCOH

CH2

surface

adsorption site

Eoads

Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110) Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110) Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110) Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110) Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110)

hollow fcc short bridge top Rh top Ru short bridge Rh−Ni short bridge Ru−Ni hollow fcc hollow-3 hollow fcc Rh hollow fcc Ru short bridge Rh−Ni short bridge Ru−Ni hollow fcc hollow-4 bridge Rh−Ni bridge Ru−Ni hollow-4 Rh hollow-4 Ru hollow fcc short bridge top Rh bridge Ru−Ni short bridge Rh−Ni short bridge Ru−Ni hollow fcc short bridge hollow hcp Rh hollow fcc Ru short bridge Rh−Ni short bridge Ru−Ni

−1.85 −1.83 −1.77 −1.95 −1.85 −1.89 −5.51 −5.24 −5.45 −5.63 −5.24 −5.36 −2.23 −2.61 −2.15 −2.18 −2.69 −2.75 −3.75 −3.85 −3.60 −3.71 −3.93 −4.05 −3.99 −3.97 −3.92 −4.11 −4.06 −4.28

species CH3

CH4

H2CO

H3CO

H3COH

surface

adsorption site

Eoads

Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110) Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110) Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110) Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110) Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110)

hollow fcc short bridge top Rh top Ru short bridge Rh−Ni short bridge Ru−Ni physisorbed physisorbed physisorbed physisorbed physisorbed physisorbed hollow hcp short bridge hollow hcp Rh bridge Ru−Ni short bridge Rh−Ni short bridge Ru−Ni hollow fcc short bridge hollow fcc Ni hollow fcc Ru short bridge Rh−Ni short bridge Ru−Ni physisorbed short bridge physisorbed top Ru top Rh short bridge Ru−Ni

−1.91 −2.11 −1.76 −1.82 −2.12 −2.19 −0.02 −0.01 0.00 −0.01 −0.07 −0.08 −0.62 −1.05 −0.54 −0.77 −1.05 −1.34 −2.37 −2.73 −2.52 −2.23 −3.23 −3.30 −0.05 −0.34 −0.05 −0.20 −0.34 −0.43

such reactions are discussed. These data are finally used to elucidate the most favorable mechanisms for the methane and methanol formation from syngas on the pure and doped nickel surfaces under study. 3.1. Structure and Stability of Reactants and Products. The most stable (co)adsorption configurations for the species involved in the methane and methanol formation from CO and H2 on the Ni(111), Ni(110), Rh@Ni(111), Ru@ Ni(111), Rh@Ni(110), and Ru@Ni(110) surfaces were

formation of methane or methanol via the formyl intermediate. Other possible steps were discarded because the formation of relevant intermediate species (e.g., C or COH77) was accompanied by high energy barriers (as will be shown below). In this section, we first present the (co)adsorption energetics and structure of the species implied in the methane and methanol formation (see Scheme 1) on the targeted surfaces. Then, the trends in the activation energy barriers, reaction energies, and rate constants for the different steps involved in 16540


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Table 2. Calculated ZPV Corrected Interaction Energies (in Electronvolts) for Pairs Deposited on Ni(111), Ni(110), Rh@ Ni(111), Ru@Ni(111), Rh@Ni(110), and Ru@Ni(110) Surfaces pair C+O

CO + H

C + OH

surface

HCO + H

hollow hcp/hollow fcc long bridge/long bridge hollow hcp Rh/hollow fcc Rh hollow hcp Ru/hollow fcc Ru hollow-3 Rh/hollow-3 Ni

−11.41 −12.62 −11.01 −11.65 −12.58

Ru@Ni(110)

long bridge Ru/long bridge Ru

−13.17

Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110)

hollow hcp/hollow fcc short bridge/hollow-3 hollow fcc Rh/hollow fcc Rh top Ru/hollow hcp Ru short bridge Rh−Ni / hollow-3 Rh

−4.60 −4.55 −4.22 −4.49 −4.53

Ru@Ni(110)

top Ru/hollow-3 Ru

−4.50



hollow fcc/hollow fcc hollow-4/short bridge hollow hcp Rh/hollow fcc Rh hollow hcp Ru/hollow fcc Ru long bridge Rh−Ni/long bridge Rh−Ni long bridge Ru−Ni/long bridge Ru−Ni hollow fcc/hollow hcp hollow-3/hollow-3 hollow fcc Rh/hollow fcc Ni hollow fcc Ru/hollow fcc Ru hollow-4 Rh/hollow-3 Ni

−11.10 −11.73 −10.71 −11.23 −12.05

Ru@Ni(110)

hollow-3 Ru/hollow-3 Ru

−12.33


hollow fcc/hollow fcc hollow-4/hollow-3 hollow fcc Rh/hollow fcc Rh short bridge Rh−Ni / hollow-3 Ni short bridge Rh−Ni/long bridge Ni−Ni short bridge Ru−Ni/long bridge Ru−Ni hollow hcp/bridge hollow-3/short bridge hollow fcc Rh/bridge Rh−Ni hollow hcp Ru/bridge Ru−Ni hollow-3 Rh/short bridge Ni−Ni hollow-3 Ru/short bridge Ru−Ni hollow fcc/hollow hcp hollow-3/hollow-3 hollow fcc Rh/hollow fcc Rh hollow hcp Ru/hollow fcc Ru short bridge Rh−Ni / short bridge Ni−Ni short bridge Ru−Ni / hollow-3 Ru

Ru@Ni(110) CH + OH

CH + H

Eocoads


Ru@Ni(110) CH + O

coadsorption sites

Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110) Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110)

−9.05 −11.11 −8.94 −9.40 −10.99

pair CH2 + H

CH3 + H

Eocoads


hollow fcc/hollow hcp short bridge/hollow-3 bridge Rh−Ni/hollow fcc Rh hollow fcc Rh/hollow fcc Rh short bridge Rh−Ni/short bridge Ni−Ni short bridge Ru−Ni / short bridge Ni−Ni hollow fcc/hollow hcp short bridge/short bridge top Rh/hollow fcc Ni top Ru/hollow fcc Ni short bridge Rh−Ni/short bridge Ni−Ni short bridge Ru−Ni/short bridge Ni−Ni hollow hcp/hollow fcc short bridge/hollow-3 hollow fcc Ni/hollow hcp Rh hollow fcc Rh/hollow hcp Ru short bridge Rh−Ni/short bridge Ni−Ni short bridge Ru−Ni/short bridge Ni−Ni hollow fcc/hollow hcp short bridge/short bridge hollow fcc Rh/hollow hcp Rh hollow fcc Ru/hollow hcp Ru short bridge Rh−Ni / short bridge Ni−Ni short bridge Ru−Ni / short bridge Ni−Ni hollow fcc/hollow hcp short bridge / short bridge hollow fcc Rh/hollow hcp Rh hollow fcc Ru/hollow hcp Ru short bridge Rh−Ni / short bridge Ni−Ni short bridge Ru−Ni / short bridge Ni−Ni hollow fcc/desorbed hollow-3/short bridge hollow fcc Rh/desorbed hollow fcc Ru/desorbed hollow-3 Ni/short bridge Rh−Ni hollow-3 Ni/short bridge Ru−Ni

−6.48 −6.52 −6.32 −6.56 −6.61

Ni(111) Ni(110) Rh@Ni(111) Ru@Ni(111) Rh@Ni(110) Ru@Ni(110)

H2CO + H


H3CO + H


CO + CO

−5.23 −8.79 −9.87 −8.64 −9.22 −9.79 −9.99 −8.87 −9.14 −8.65 −9.08 −9.80

coadsorption sites

Ru@Ni(110)

−11.33

−4.81 −5.05 −4.68 −4.78 −5.20

surface


C + CO2


−6.82 −4.31 −4.66 −4.23 −4.32 −4.66 −4.74 −3.04 −3.53 −2.63 −2.83 −3.56 −3.56 −4.90 −5.15 −4.98 −4.77 −5.75 −5.83 −3.47 −3.71 −3.31 −3.56 −3.71 −3.73 −6.84 −7.54 −6.75 −6.91 −7.58 −7.73

−8.54

determined by examining several possible surface adsorption positions (the notation for the adsorption positions is provided in Figure 1). The adsorption energies obtained for the isolated and for the coadsorbed species on the different surfaces considered are presented in Tables 1 and 2, respectively. Electronic energies without ZPV corrections are provided in Tables S1 and S2 in the Supporting Information, respectively. A representation of

the most stable adsorption configurations for each species on each surface can be seen in Figures S1−S6 of the Supporting Information. As can be seen in Table 1, the adsorption of isolated species on these surfaces is in most cases very exothermic, moderately exothermic in the case of formaldehyde, and weakly exothermic in the cases of methanol and methane. Methane interacts with all surfaces with adsorption energies between 0.00 and −0.08 16541


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Figure 2. Projected density of states (in arbitratry units) for clean Ni(111), Ni(110), Rh@Ni(111), Ru@Ni(111), Rh@Ni(110), and Ru@Ni(110) and for these surfaces covered with a methanol molecule. Solid green lines correspond to the d-band of Ni atoms, dotted black lines to the d-band of the dopant atom, and dashed red lines to the p-bands of the O atoms of methanol. The zero energy marked with a vertical blue line corresponds to the Fermi energy.

eV. The adsorption energies for methanol range between −0.05 eV, calculated for the pure Ni(111) and doped Rh@Ni(111) surfaces, and −0.43 eV on the doped Ru@Ni(110) surface. The results in Table 1 suggest reversible adsorption for both gases and show that low-coordinated atoms have a stronger influence on the adsorption energetics. In fact, a general beneficial influence of low-coordinated atoms on the adsorption energetics of all the species is seen. In addition, doping of the nickel surfaces with Ru atoms leads to more negative adsorption energies (i.e., adsorbate stabilization), whereas doping with Rh atoms leads to adsorbate destabilization in most cases. Still, even in the case of the Rh@Ni surfaces, adsorbates prefer to interact with the dopant atoms with the adsorption of methoxy species on Rh@Ni(111) being an exception. These findings corroborate the importance of the surface doping in the catalysis of these reactions, as previously verified experimentally.24,26 The coadsorption of the species implied on the methane and methanol production from syngas on these surfaces is characterized by very negative coadsorption energies (cf. Table 2), with the coadsorptions being in general more favorable on the surfaces having low-coordinated atoms. Similarly to what has been reported above for the isolated species, the doping of the nickel surfaces with Ru atoms leads to more favorable adsorptions while the doping with Rh atoms is accompanied by a slight destabilizing effect. Also, as can be seen in the representations of the most favorable configurations for the coadsorbed species depicted in Figures S1−S6 in the Supporting Information, the adsorption occurs preferably at positions near the doping atoms.

3.2. Local Electronic Structure and Implications in the Reactivity. We investigated the local electronic structure of the surfaces used in this work through a projected density of states analysis (cf. Figure 2). The d-band structures of the atoms in the topmost layer of the clean Ni(111) (or Ni(110)) surface is compared with those on the two TM@Ni(111) (or TM@Ni(110)) surfaces. As can be seen on the (111) surfaces, the doping of the surface leads to higher peaks under the Fermi level when compared to the DOS calculated for the pure Ni(111) surface. In the case of the (110) surfaces, the changes in the DOS by doping Ni(110) with Ru and Rh atoms are less evident than in the case of the (111) Miller index surfaces. The interaction of an adsorbate molecule (e.g., methanol) with the six catalyst model surfaces considered in this work is accompanied by important variations in the electronic structure of the p band of methanol when the interaction is strong (i.e., methanol chemisorption in the cases of Ru@Ni(111), Ni(110) and TM@(110)) and negligible differences when the interaction is weak (i.e., methanol physisorption on Ni(111) and Rh@Ni(111) surfaces). The center of the d-band on the doped Ni(111) and Ni(110) surfaces is slightly shifted to more negative energies when compared with the values calculated for the pure nickel surfaces (Ni(111), −1.14 eV; Rh@Ni(111), −1.49 eV; Ru@Ni(111), −1.59 eV; Ni(110), −1.42 eV; Rh@Ni(110), −1.57 eV; Ru@ Ni(110), −1.58 eV). The shifting to more negative energies is similar to that observed in a previous work63 where it was found that the d-band center of nickel surfaces with different Miller indices appears at more negative energies when more reactive 16542


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Table 3. Calculated Data for the Different Reaction Steps Involved in the Methanol and Methane Formation from CO and H2 on the Ni(111) Surface reaction

distancea

CO* + * → C* + O* CO* + H* → COH* + * COH* + * → C* + OH* CO* + H* → HCO* + * CO* + CO* → C* + CO2 + *

1.94 1.32 2.01 1.16 1.97

HCO* + H* → HCOH* + * HCOH* + * → HC*+ OH* HCO* + * → HC* + O* CH* + H* → CH2* + * CH2* + H* → CH3* + * CH3* + H* → CH4 + 2*

1.35 1.94 1.88 1.69 1.63 1.56

HCO* + H* → H2CO* + * H2CO* + H* → H3CO* + * H3CO* + H* → H3COH + *

1.57 1.54 1.54

Eadsb

Eoreactc

Common Steps 3.23 2.15 1.93 1.15 1.94 1.56 1.51 1.38 3.32 2.04 Methane Formation Steps 1.35 0.47 0.75 −0.06 1.08 0.21 0.64 0.37 0.49 −0.08 0.64 −0.15 Methanol Formation Steps 0.70 0.49 0.46 −0.27 1.07 0.59

υid

k (503 K)e

k (573 K)e

442 1540 271 260 342

2.84 4.16 3.06 5.06 7.15

× × × × ×

10−21 10−7 10−8 10−3 10−21

9.21 1.08 7.57 3.84 8.38

× × × × ×

10−17 10−4 10−6 10−1 10−17

1326 403 650 718 926 915

1.23 1.60 3.61 9.13 2.79 4.48

× × × × × ×

10° 105 101 106 108 106

6.87 1.46 7.94 6.43 1.27 2.84

× × × × × ×

101 106 102 107 109 107

806 883 768

1.49 × 106 7.73 × 108 1.74 × 103

1.20 × 107 3.26 × 109 4.73 × 104

a

Length of the bond cleaving or forming in angstroms. bZPV corrected activation energy barriers in electronvolts. cZPV corrected reaction energies in electronvolts. dImaginary wavenumbers in reciprocal centimeters for the TS structures. eReaction rate constants in reciprocal seconds computed at the temperatures indicated in parentheses.

Table 4. Calculated Data for the Different Reaction Steps Involved in the Methanol and Methane Formation from CO and H2 on the Rh@Ni(111) Surface reaction

distancea


1.84 1.28 2.01 1.16 2.03


1.40 2.03 1.81 1.58 1.63 1.56

HCO* + H* → H2CO* + * H2CO* + H* → H3CO* + * H3CO* + H* → H3COH + *

1.45 1.55 1.52

Eoactb

Eoreactc

Common Steps 3.30 2.47 1.42 0.83 2.18 1.63 1.25 1.09 3.23 1.97 Methane Formation Steps 0.73 0.56 0.72 0.02 1.36 0.51 0.49 0.22 0.40 −0.09 0.42 −0.23 Methanol Formation Steps 0.62 0.44 0.25 −0.35 0.75 0.18

υid

k (503 K)e

k (573 K)e

364 1542 285 239 292

8.22 2.03 8.28 1.76 3.80

× × × × ×

10−22 10−2 10−10 10° 10−20

9.17 1.21 4.14 6.16 3.45

× × × × ×

10−18 100 10−7 101 10−16

1298 381 374 886 971 926

1.66 1.92 5.98 1.76 1.00 3.58

× × × × × ×

106 105 10−2 108 109 109

1.49 1.58 2.84 7.88 3.36 1.36

× × × × × ×

107 106 100 108 109 1010

594 792 947

7.68 × 106 1.06 × 1011 1.71 × 106

4.76 × 107 2.44 × 1011 1.82 × 107

a


The activation energy barriers for the reaction steps showed in Scheme 1 follow a series of general trends. For example, by comparing the entries for the reaction steps on the Ni(111) surface with those on the Ni(110) surface (cf. data in Tables 3 and 6, respectively), one can see that the activation energy barriers of some reaction steps are strongly reduced when these are catalyzed by catalysts possessing low-coordinated atoms (i.e., atoms on the (110) surface plane). Importantly, the reduction of the activation energy barrier for the CO and COH dissociation steps is noticeable, in both cases by more than 1 eV. These findings are in agreement with data from the literature showing that these reactions display high sensitivity to the Ni catalyst structure53−55 as well as to the presence of hydrogen species on the catalyst surface. The reduction in the

(i.e., with lower activation energy barriers) for the water dissociation. 3.3. Energetics for the Reaction Steps. The activation energy barriers, reaction energies, rate constants at 503 and 573 K, along with the lengths of the bonds being broken or formed in each reaction step and the imaginary wavenumbers of the corresponding transition states are given in Tables 3−8. Electronic activation and reaction energies without ZPV corrections are provided in the Supporting Information (Table S3). In addition, views of the optimized configurations for the initial, transition, and final states of the most favorable path for each reaction step on the different catalyst models can be seen in Figures S1−S6 of the Supporting Information. 16543


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Table 5. Calculated Data for the Different Reaction Steps Involved in the Methanol and Methane Formation from CO and H2 on the Ru@Ni(111) Surface reaction

distancea


1.81 1.29 2.18 1.17 2.01


1.42 1.85 1.77 1.59 1.64 1.56

HCO* + H* → H2CO* + * H2CO* + H* → H3CO* + * H3CO* + H* → H3COH* + * H3COH* → H3COH + *

1.39 1.54 1.46 −

Eoactb

Eoreactc

Common Steps 3.06 2.02 1.61 0.93 1.98 1.33 1.53 1.32 3.39 2.04 Methane Formation Steps 0.67 0.54 0.47 −0.45 0.94 0.02 0.72 0.46 0.54 0.09 0.37 −0.15 Methanol Formation Steps 0.53 0.31 0.21 −0.33 0.70 0.30 0.20 0.20

υid

k (503 K)e

k (573 K)e

420 1618 208 227 224

1.87 1.61 1.14 1.59 2.71

× × × × ×

10−19 10−4 10−7 10−3 10−22

1.08 1.69 3.35 1.24 3.68

× × × × ×

10−15 10−2 10−5 10−1 10−18

1280 279 389 873 984 891

4.89 4.53 4.43 1.01 4.92 4.34

× × × × × ×

106 107 102 106 107 109

3.78 1.81 6.47 8.74 2.43 1.44

× × × × × ×

107 108 103 106 108 1010

309 941 1076 −

5.26 × 107 2.08 × 1011 7.79 × 106 −

2.53 × 108 4.32 × 1011 7.18 × 107 −

a


Table 6. Calculated Data for the Different Reaction Steps Involved in the Methanol and Methane Formation from CO and H2 on the Ni(110) Surface reaction

distancea

CO* + * → C* + O* CO* + H* → COH* + * COH* + * → C* + OH* CO* + H* → HCO* + * CO* + CO* → C* + CO2*

2.23 1.25 1.93 1.55 1.91

HCO* + H* → HCOH* + * HCOH* + * → HC* + OH* HCO* + * → HC* + O* CH* + H* → CH2* + * CH2* + H* → CH3* + * CH3* + H* → CH4 + 2*

1.42 1.88 1.98 1.68 1.74 1.53


1.57 1.49 1.36 −

Eoactb

Eoreactc

Common Steps 1.84 0.92 1.44 1.37 0.62 −0.75 0.95 0.65 2.87 2.28 Methane Formation Steps 0.70 0.67 0.33 −0.95 0.50 −0.43 0.90 0.65 0.27 −0.24 0.51 −0.01 Methanol Formation Steps 0.61 0.30 0.54 −0.11 0.86 0.68 0.34 0.34

υid

k (503 K)e

k (573 K)e

209 1296 234 663 340

2.32 1.18 2.51 8.36 6.25

× × × × ×

10−7 10−2 106 102 10−17

4.06 7.46 1.51 1.31 1.94

× × × × ×

10−5 10−1 107 104 10−13

1167 332 354 712 857 864

1.65 8.69 4.40 8.16 1.98 2.19

× × × × × ×

106 108 107 103 1010 106

1.36 2.36 1.92 1.12 4.61 7.70

× × × × × ×

107 109 108 105 1010 106

823 817 1263 −

2.06 × 107 1.36 × 107 5.68 × 104 −

1.27 × 108 6.55 × 107 7.49 × 105 −

a


coordinated atoms must be at the level of reactant stabilization; thus, for dissociative reactions, the reactants are less relatively stabilized by the interaction with low-coordinated atoms of the surface than reactants for associative reactions. Also, it is interesting to note that the reaction energies become much more negative with the presence of low-coordinated atoms on the catalytic surface when they are associated with reaction steps in which bonds are broken. Similar conclusions can be attained by considering the reaction steps showed in Scheme 1 over the surface pairs Rh@Ni(111)/Rh@Ni(110) and Ru@ Ni(111)/Ru@Ni(110). We thus believe that this result can be generalized for reactions on nickel surfaces.

activation energy barrier for other reaction steps is more modest, and even for some steps, the activation energy barrier on the Ni(110) surface is higher than on the Ni(111) surface, as for example in the case of the CH* + H* → CH2 step. In general, the picture arising from the data compiled in Tables 3 and 6 is that the activation energy barrier for reactions involving breaking of bonds is more affected by catalysts displaying lowcoordinated atoms than reactions in which bonds are being formed. Here, it must be mentioned that the obtained transition states are the same for direct and reverse directions of the reaction (associative/dissociative or dissociative/ associative reactions types). Therefore, the effect of low16544


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Table 7. Calculated Data for the Different Reaction Steps Involved in the Methanol and Methane Formation from CO and H2 on the Rh@Ni(110) Surface reaction

distancea


2.05 1.25 2.03 1.39 1.90


1.43 1.86 2.15 1.58 1.71 1.50


1.49 1.56 1.55 −

Eoactb

Eoreactc

Common Steps 2.16 0.98 1.41 1.28 0.69 −0.64 0.88 0.78 2.84 1.54 Methane Formation Steps 0.81 0.74 0.28 −0.80 1.01 −0.29 0.05 −0.35 0.38 −0.16 0.86 0.14 Methanol Formation Steps 0.62 0.44 0.64 −0.13 1.17 0.68 0.34 0.34

υid

k (503 K)e

k (573 K)e

137 1283 183 407 336

1.88 2.61 3.83 3.99 3.66

× × × × ×

10−10 10−2 105 103 10−17

8.02 1.49 2.87 5.06 1.06

× × × × ×

10−8 101 106 104 10−13

1156 346 292 932 743 925

4.89 2.65 4.29 3.86 1.74 3.53

× × × × × ×

104 109 102 1012 109 104

5.23 6.25 7.89 5.14 5.46 4.45

× × × × × ×

105 109 103 1012 109 105

670 748 272 −

4.44 × 106 1.51 × 106 1.65 × 101 −

2.64 × 107 9.60 × 106 4.70 × 102 −

a


Table 8. Calculated Data for the Different Reaction Steps Involved in the Methanol and Methane Formation from CO and H2 on the Ru@Ni(110) Surface reaction

distancea


1.92 1.26 2.20 1.31 1.93


1.45 1.82 2.09 1.50 1.62 1.53


1.48 1.52 1.35 −

Eoactb

Eoreactc

Common Steps 1.90 0.43 1.36 1.19 0.50 −0.82 0.93 0.74 2.82 1.41 Methane Formation Steps 0.78 0.65 0.27 −0.87 0.90 −0.50 0.35 −0.25 0.37 −0.02 0.59 0.20 Methanol Formation Steps 0.47 0.19 0.41 −0.21 1.03 0.67 0.43 0.43

υid

k (503 K)e

k (573 K)e

137 1289 243 333 314

1.40 1.07 2.87 1.15 8.72

× × × × ×

10−7 10−1 107 103 10−17

2.90 5.49 1.25 1.67 2.41

× × × × ×

10−5 10° 108 104 10−13

1101 367 324 774 849 817

1.14 2.93 5.02 3.86 1.60 7.92

× × × × × ×

105 109 103 109 109 106

1.14 6.69 6.87 1.16 4.93 4.22

× × × × × ×

106 109 104 1010 109 107

544 881 1249 −

1.36 × 108 2.52 × 108 1.59 × 102 −

5.26 × 108 8.09 × 108 2.96 × 103 −

a


barriers on pure nickel and on doped surfaces are quite large. Despite the fact that the variations of the activation energy barriers with the surface doping are generally modest, these variations can favor some reaction mechanisms on the doped surfaces with respect to those on the pure nickel surfaces, as will be shown in the next section. Regarding the rate constant values estimated for the several reaction steps included in Scheme 1, as can be seen in Tables 3−8, these are considerable higher for reaction steps having moderate or low activation energy barriers independently of the temperature.

Let us now analyze the surface doping effects in the catalysis of the different reaction steps considered in this study. By comparing the results obtained for the reactions on the pure surfaces with those on the doped surfaces (see Tables 3−8), we can conclude that the influence of the surface doping in the activation energy barriers is generally modest. In fact, activation barriers are reduced or increased by doping the nickel surfaces with Rh or Ru atoms without a clear tendency. This suggests that surface doping (de)stabilizes the transition and initial states similarly in most cases. The exceptions are the CH + H → CH2 and the HCO → CH + O reaction steps on Ni(110) based surfaces where differences in the calculated activation energy 16545


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Figure 3. Reaction schemes showing the different routes studied for the methane and methanol production from CO and H2 on Ni(111), Ni(110), Rh@Ni(111), Ru@Ni(111), Rh@Ni(110), and Ru@Ni(110) surfaces. Red lines represent the most favorable reaction paths. Values of the activation energy barriers for each reaction step are given in electronvolts. An extended version of this figure is given in Figure S7 in the Supporting Information.

(CO* + H* → COH* + * → C* + OH*; steps V and VI); and (iv) H-assisted C−O bond breaking through the formyl intermediate (CO* + H* → HCO* + * → HC* + O*; steps VII and X). The hydrogen adatoms arise from the dissociative adsorption of hydrogen on the model surfaces considered in this work. As is clearly seen from the energetic data included in Figures 3 and 4, the direct dissociation of the CO molecule is energetically very costly on the six model systems considered in this work. On the pure Ni catalysts, barriers are 1.84 eV in the case of the Ni(110) surface and 3.23 eV in the case of the

3.4. Reaction Mechanisms. On the basis of the reaction barriers described in the preceding section, we can now analyze the different reaction routes on the six catalyst models considered in this work. Figures 3 and 4 depict schematically these mechanisms; the most favorable paths are highlighted in red. The general mechanisms start with the breakage of the C−O bond via one of the following routes: (i) direct CO dissociation (CO* + * → C* + O*; step III); (ii) CO-assisted CO dissociation (CO* + CO* → C* + CO2; step IV); (iii) Hassisted C−O bond breaking through the carbide intermediate 16546


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Figure 4. continued

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Figure 4. Energy diagrams for the methane and methanol formation from CO and H2 on Ni(111), Ni(110), Rh@Ni(111), Ru@Ni(111), Rh@ Ni(110), and Ru@Ni(110) surfaces. Red lines represent the most favorable reaction paths. Values of the activation energy barrier and reaction energy for each reaction step are given in electronvolts.

Ni(111) surface. Barrier is 3.30 eV in the case of Rh@Ni(111). The two latter values are to some extent higher than that obtained by Anderson et al.55 for the reaction on Ni(111) surface (2.85 eV), while the result calculated for the Ni(110) surface is close to those obtained for the reaction on Ni(100), Ni(211), Ni(221), Ni(321), and Ni(431) surfaces by these same authors.55 Encouragingly, after incorporation of an energy correction based on the internal CO stretch vibration as proposed by Abild-Pedersen et al.,78 the activation energy barrier for the CO dissociation on Ni(111) is corrected to 2.88 eV, becoming in quantitative agreement with the barrier of 2.85 eV taken from ref 55 and calculated with another DFT approach and basis set. We also investigated the CO dissociation assisted by a coadsorbed CO molecule, the so-called Boudouard reaction, that is the responsible for carbon deposition on catalysts. On the six surface models considered in this work, the Boudoard reaction is associated with very large energy barriers, which are similar to or even larger than those calculated for the direct CO dissociation. Similarly, the reaction mechanism through the carbide intermediate also looks unfavorable because the formation of the intermediate requires also surpassing large energy barriers, though they are smaller than those required to break the C−O bond in the direct reaction route. The C−O bond break seems to be much more favorable through the formyl intermediate (cf. barriers below 1.53 eV),

which was considered to be the most important intermediate in the reactions leading to the formation of methanol and methane. The C−O bond breakage reactions are quite sensitive to the presence of H species on the catalyst surface and less sensitive to the presence of doping atoms. As can be also seen in the energetic data for (111) and (110) surfaces reported in Figures 3 and 4, the dissociation of the CO bond is more favorable on the (110) than on the (111) surface planes. The sensitivity of the reaction of CO dissociation to the structure of the catalyst and to the presence of hydrogen species was already found on pure Ni surfaces.53−55 Andersson et al.55 suggested that the CO dissociation is more favorable in the presence of hydrogen species through the COH intermediate on several Ni surface planes, namely, (100), (211), (311), (321), and (511), except in the case of the Ni(211) surface at low CO coverage. These results contrast with those reported here for the pure and Ru- or Rh-doped Ni(111) and Ni(110) surfaces, and also with those in the work by Inderwildi et al.36 in which the route through the HCO intermediate was found to be the more feasible for catalytic hydrocarbon reactions. Three different reaction pathways were analyzed in this work after the formation of the HCO intermediate. One of these paths concerns the direct cleavage of the C−O bond breakage in this intermediate species which will form CH and O species on the catalyst surface. The other two paths are related with the reaction of the formyl intermediate with hydrogen adatoms to 16548


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well with available experimental findings on nickel-based catalysts doped with Rh or Ru,24−26,51 where the presence of methanol was not detected or found in very small amounts.24 In fact, the addition of Ru25 or Rh26 to Ni-based catalysts was found to improve the conversion (and selectivity) of CO to methane.

produce H2CO or HCOH. The energetic data for these reactions show that the formation of formaldehyde is the most favorable pathway, except on the Ni(110) where the barrier for the HCO* + * → HC* + O* reaction is 0.11 eV smaller than that for the HCO* + H* → H2CO* + * reaction. From the formaldehyde species, methanol can be obtained through a series of hydrogen addition reactions, e.g., H2CO* + H* → H3CO* + * (+ H*) → H3COH* + 2* → H3COH + 3*. The formation of the methoxy intermediate has to surmount energy barriers comparable to those required for the formation of formaldehyde from the formyl species, but much larger barriers are associated with the H3CO* + H* → H3COH* + * reaction step. In fact, in the case of the Rh@Ni(110) and Ru@ Ni(110) surfaces, the formation of methanol from the methoxy radical is the most energy-consuming elementary step. The formation of methane can be achieved via the successive addition of H to the CH species from HCO dissociation, e.g., CH* + H* → CH2* + * (+ H*) → CH3* + 2* (+ H*) → CH4* + 3* → CH4 + 4*, or via the cleavage of the C−O bond in the HCOH intermediate to yield OH and CH surface species, the latter reacting with H adatoms to provide methane. From the energetic data reported in Figures 3 and 4, the former reaction path is more favorable on the pure nickel surfaces, while the latter reaction path becomes more favorable on the doped nickel surfaces. Note, however, that the differences between the energy barrier maxima via one or the other mechanism are in general quite small. As a summary of the results in this section, the formation of the HCO species from CO and H2 is the most difficult step in the reactions leading to methane or methanol on the Ni(111), Ni(110), Rh@Ni(111), and Ru@Ni(111) surfaces. In the case of the Rh@Ni(110) and Ru@Ni(110) surfaces, the formation of the HCO species is the bottleneck reaction in the mechanism of formation of methane, while the reduction of methoxy is the most energy-consuming step in the reaction leading to methanol. Also, in the former surfaces, the breakage of the C−O bond is more favorable than the reduction of methoxy, while in the latter surfaces these reactions have to surpass similar energy barriers. The consideration of surfaces with low-coordinated atoms reduces considerably the global activation energy barrier for the methane and methanol formation on pure and on Rh- or Ru-doped nickel surfaces. Thus, catalyst sites possessing low-coordinated atoms must be responsible for the catalyst activity in the methane and methanol formation from CO and H2. Doping of the flat Ni(111) surface with Rh atoms decreases the activation energy barriers while the doping of the corrugated Ni(110) surface favors the methane formation with respect to the methanol formation. A similar picture is obtained when the nickel surfaces are doped with Ru atoms. Interestingly, after formation of the HCO species, the comparison of the calculated highest activation barriers for the reactions toward methane or methanol on the six surfaces considered in this work shows that obtaining the former species is slightly more favorable (i.e., by more than 0.25 eV) on Ru@ Ni(110) and Rh@Ni(110), while in the remaining surfaces, barriers are similar for obtaining either methane or methanol (largest difference is 0.06 eV in the case of Ru@Ni(111) surface). Furthermore, these barriers are decreased on going from the Miller (111) to the (110) indices in the case of the pure nickel surfaces, while in the case of the doped surfaces, the barriers on the (111) surface planes are smaller than those on the (110) surface planes. The results presented above agree

4. CONCLUSIONS The reactions leading to the methane and methanol formation on pure and Rh- or Ru-doped nickel surfaces were studied by means of density functional theory calculations. Surface models mimicking flat and corrugated surfaces were considered. It was found that the formation of the formyl (HCO) intermediate is the rate-limiting step in the reaction toward methane. This step is also the most energy costly for the reaction toward methanol on the pure nickel surface and on Rh- or Ru-doped Ni(111) surfaces, while on the Rh- and Ru-doped Ni(110) surfaces the reduction of methoxy to methanol is, energetically speaking, the most expensive. Interestingly, the activation energy barrier corresponding to the reaction of HCO formation is decreased upon doping of the nickel surfaces with Rh atoms while it is kept almost unchanged by doping with Ru atoms. Doping of the nickel surface possessing low-coordinated atoms with Rh or Ru atoms leads to activation energy barriers for methane formation lower than those for methanol formation, which suggests that the production of methane over methanol can be favored on nickel surfaces by doping with Rh or Ru atoms if low-coordinated atoms are present on the catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

Extended Tables S1−S3 with most favorable adsorption positions, electronic adsorption, reaction or activation energies, and vibrational wavenumbers for different adsorbate−adsorbent systems; Figures S1−S6 with optimal initial, transition, and final configurations for the several elementary reactions studied in this work; and Figure S7 with an extended version of Figure 3 showing the schemes for the reaction routes toward methanol and methane formation from CO and H2. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b01837.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +351 220 402 659. *E-mail: [email protected]. Fax: +351 220 402 659. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to Fundaçaõ para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to FEDER for financial support to LAQV@REQUIMTE and to CICECO, projects UID/QUI/ 50006/2013 and PEst-C/CTM/LA0011/2013, respectively, and for Programa Investigador FCT. J.L.C.F. acknowledges FCT for Grant SFRH/BPD/64566/2009 cofinanced by the Programa Operacional Potencial Humano (POPH)/Fundo Social Europeu (FSE) and Quadro de Referência Estratégico Nacional 2009-2013 do Governo da República Portuguesa. 16549


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