Competitive Paths for Methanol Decomposition on Ruthenium: A DFT

Article pubs.acs.org/JPCC

Competitive Paths for Methanol Decomposition on Ruthenium: A DFT Study Ana S. Moura,†,‡ Jose L. C. Fajín,*,† Ana S. S. Pinto,† Marcos Mandado,§ and M. Natália D. S. Cordeiro*,† †

LAQV-REQUIMTE, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal ‡ Escola Superior de Estudos Industriais e de Gestão (ESEIG), Instituto Politécnico do Porto (IPP), 4480-876 Vila do Conde, Portugal § Department of Physical Chemistry, University of Vigo, Lagoas (Marcosende) s/n, 36310-Vigo (Pontevedra), Spain ABSTRACT: Methanol decomposition is one of the key reactions in direct methanol fuel cell (DMFC) state-of-the-art technology, research, and development. However, its mechanism still presents many uncertainties, which, if answered, would permit us to refine the manufacture of DMFCs. The mechanism of methanol decomposition on ruthenium surfaces was investigated using density functional theory and a periodic supercell approach. The possible pathways, involving either initial C−H, C−O or O−H scission, were defined from experimental evidence regarding the methanol decomposition on ruthenium and other metallic surfaces. The study yielded the O−H scission pathway as having both the most favorable energetics and kinetics. The computational data, which present a remarkable closeness with the experimental results, also indicate methanol adsorption, the starting point in all possible pathways, to be of weak nature, implying a considerable rate of methanol desorption from the ruthenium, compromising the reaction.

1. INTRODUCTION

Cathode: 3/2O2 + 6H+ + 6e− → 3H 2O

(2)

The past decade has seen a massive increase of interest in the economic market of portable electronic devices and engines, breaking the $1 billion mark in revenue from the sale of fuel cell systems for the first time in 2012.1 Predictions for the period of 2013−2019 indicate that such a value will become even more significant.2 Due to the increasing miniaturization of the devices, the need for an adequate power source is becoming paramount, and direct methanol fuel cells (DMFCs) are considered one of the most interesting possibilities due to their environmentally friendly residues or high energy density, for example.3,4 DMFC catalysts are mostly based on platinum since, within the fuel cell context, not only does it have the highest catalytic activities of all pure metals for methanol oxidation but also, as Holton and Stevenson state in an extensive review, it outperforms the other metals in the three key areas, namely: activity, selectivity, and stability.5 Several mechanisms have been proposed regarding methanol oxidation in DMFC, but it is accepted that hydration of the proton exchange membrane (PEM) leads to proton production in the anode and then begins its transportation through the membrane to the cathode. Therein, another catalyst is used to combine electrons, protons, and oxygen from the air, resulting in water vapor and CO2 production as methanol reacts in the anode.6−10

Overall: CH3OH + 3/2O2 → CO2 + 2H 2O

(3)

+

Anode: CH3OH + H 2O → CO2 + 6H + 6e

−

© XXXX American Chemical Society

The catalytic decomposition of methanol is an important part of this oxidative process, and as discussed amply and substantiated by experimental data, the methoxy radical (CH3O) is the first intermediate in the proposed mechanism.11−18 CH3OHgas → CH3OHads → CH3Oads → CH 2Oads → CHOads → COads

(4)

Thus, it appears that the dehydrogenation of the alcoholic group of the methanol is the first step of the main reaction pathway. In fact, recent experimental data have confirmed that, at room temperature and millibar pressure range, the rate of this process is sufficiently high since full dehydrogenation of methanol to CO on platinum surfaces is observed at ≈200 K.11 Therefore, understanding the energetics of this reaction represents understanding the fundamental step of the methanol oxidation mechanism, but other reaction routes rather than that given in eq 4 cannot be discarded as has been suggested by Greeley et al.19 from a microkinetics analysis of the methanol decomposition on Pt(111). That is, the authors suggested that the methanol Received: July 11, 2015 Revised: November 9, 2015

(1) A

DOI: 10.1021/acs.jpcc.5b06671 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C decomposition mechanism on Pt(111) at realistic reaction conditions starts with the C−H bond breaking on the catalyst.19 However, carbon monoxide, one of the products of the decomposition of methanol, poisons platinum catalyst, slowing down the kinetics of electro-oxidations and limiting the commercial advantages of DMFC.20 Due to this, a variety of catalysts have been studied toward diminishing the poison of platinum catalyst. Among others, ruthenium was found to be one of the most promising catalysts regarding this problem.21 In fact, platinum−ruthenium alloys are more tolerant electrocatalysts to carbon monoxide (CO) and therefore preferred as supporters for a feasible methanol oxidation reaction from an industrial point of view.22−26 Indeed, experimental data indicate that ruthenium itself is a viable catalyst for methanol decomposition, and more importantly, if one has the determination of the reaction mechanism as an objective, CH3O has been detected to be an intermediate species.27 In the past, several theoretical studies have been performed regarding the decomposition of methanol, analyzing the most energetically suited pathways in an attempt to unlock its reaction mechanism.19,28−33 Nevertheless, due to the nature of the Pt−Ru surface duality, the reaction mechanism is still elusive, and new approaches are ensued, as computational methods evolve and refine. On the other hand, methanol decomposition is also an important route in the steam reforming of methanol torward the goal of producing H2 from the methanol reaction with water. This process can proceed through the methanol decomposition followed by the CO oxidation toward CO2 and H2 production, following different routes depending on both the reaction conditions and catalysts considered.34,35 This article presents a systematic periodic self-consistent density functional theory (DFT) study of the various methanol decomposition pathways on Ru(0001), from the initial three bond scission possibilities, C−H, O−H, or C−O. Thermochemistry and reaction barriers of elementary reaction steps in these pathways are analyzed and detailed reaction mechanisms depicted, showing the interconversions of the various intermediates in each pathway.

Figure 1. Possible adsorption sites on Ru(0001), as used in the DFT calculations.

method due to Blöch41 and further implemented by Kresse and Joubert42 was employed, resourcing to the simultaneous use of a plane-wave basis set used to span the valence electronic states. A cutoff energy of 415 eV for the plane waves was applied since our previous studies have shown to be sufficient for energy and geometry convergence.43−45 The Monkhorst−Pack k-points grid used was 5 × 5 × 1, chosen after a systematic study of the geometry and energy convergence with the k-points grid for the methanol adsorption on Ru(0001). Transition states (TSs) along each reaction pathway involved in the methanol decomposition on Ru(0001) were located with the dimer approach,46 by applying very strict convergence criteria on energy and forces below 10−6 eV and 10−3 eV/Å, respectively. All stationary points found were characterized as minima or firstorder saddle points by harmonic vibrational analysis, which allowed as well the determination of zero-point energy (ZPE) corrections and thermal vibrational contributions. Reaction energies (Ereact) and activation energies (Eact) 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. Finally, rate constants (k) for the steps involved in the methanol decomposition on Ru(0001) have been estimated from transition state theory47 as follows ⎛ k T ⎞⎛ q ⧧ ⎞ k = ⎜ B ⎟⎜⎜ ⎟⎟e−Eact / kBT ⎝ h ⎠⎝ q ⎠

2. COMPUTATIONAL DETAILS 2.1. Slab Model. The three-dimensional (3D) periodic-slab approach was considered for modeling the infinite Ru(0001) surface and its interactions with the different reactants involved in the methanol decomposition. The positions of the Ru atoms in the slab used to represent the Ru(0001) surface have been optimized in a previous work;36 this slab consists of 16 ruthenium atoms distributed in four layers according with the (0001) Miller indices of the hexagonal crystal system. A vacuum region of 10 Å thick was introduced between repeated cells in the z direction to build a surface from the bulk metal for use in the 3D periodic calculations (notice that the (0001) face is parallel to the xy slab plane). The Ru(0001) surface is the most stable face of ruthenium, and it is characterized by a flat structure where there are four different adsorption positions as shown in Figure 1, that is top, hcp hollow, fcc hollow, and bridge. 2.2. DFT Approach. During the periodic spin-polarized DFT calculations, the ions in the uppermost two ruthenium layers and in the adsorbates were fully relaxed using the conjugate-gradient algorithm, while the bottom two layers were held frozen. These periodic DFT calculations were all carried out using the VASP 5.2.12 computer code37−39 and the GGA-PW91 functional proposed by Perdew et al.40 In order to describe the effect of core electrons on the valence shells, the projected augmented wave

(5)

where kB is the Boltzmann constant; T is the absolute temperature; h is the Planck constant; Eact is the calculated ZPE-corrected energetic barrier, whereas q⧧ and q are the vibrational partition functions for the TS and initial state, respectively, computed from the obtained harmonic vibrational frequencies. Moreover, typical temperatures for the methanol decomposition27 were considered, namely: T = 50, 150, 220, 340, 470, and 600 K.

3. RESULTS AND DISCUSSION The main objective of this work is the determination via computational methods of the decomposition mechanism of methanol on the Ru(0001) surface. The computational results and subsequent discussion are here divided in two main sections, i.e., the previous analysis of the possible pathways and the indication of the kinetically most favorable route, as concluded by the obtained computational data. 3.1. Possible Pathways for the Mechanism of Methanol Decomposition on Ru(0001). On the clean surface, the methanol decomposition proceeds by the overall equation CH3OH → CO* + 4H* B

(6) DOI: 10.1021/acs.jpcc.5b06671 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

site and geometry for the separated species (H2, CH3OH, CH3O, CH2O, CHO, CO, CH2OH, CHOH, COH); (2) an analogous study for the coadsorbed species (CH3O + H, CH2O + H, CHO + H, CO + H, CH2OH + H, CHOH + H, COH + H, H + H, CH3 + OH); (3) the calculation of the adsorption energies of the isolated species, Eads; (4) the calculation of the adsorption energies of the coadsorbed species, Ecoads; and (5) the determination of the most favorable pathway for each reaction step presented in Figure 2 and elucidation of the most favorable route for the methanol decomposition. The adsorption and coadsorption energies in steps (3) and (4) are calculated as usual according to the following equations

There are four possible reaction pathways considering the bond scission of the involved species, as illustrated in Figure 2 and described in the equations below.

Methanol decomposition starting by O−H bond scission − Blue Route (7)

Methanol decomposition starting by C−H bond scission − Red Route CH3OH → CH 2OH → CHOH → COH → CO

(8)

Methanol decomposition via hybrid route (I) − Black Route XI CH3OH → CH 2OH → CH 2O → CHO → CO

(9)

Methanol decomposition via hybrid route (II) − Green Route XII CH3OH → CH 2OH → CHOH → CHO → CO

(10)

Alternatively, one can envision also the following scenario as methanol adsorbs on the ruthenium surface CH3OH* → CH3* + OH*

(11)

Finally, as carbon monoxide is produced, there is another reaction of interest to be considered in the study of this process, i.e., the production of molecular hydrogen H* + H* → H 2

(13)

Ecoads = Eslab − (frag1 + frag2) − Eslab − Efrag1 − Efrag2

(14)

where Eslab refers to the electronic energy of the slab used to generate the ruthenium surface by its periodic repetition, Eadsorbate, to the molecular electronic energy on the gas phase of the adsorbed species; Eslab‑adsorbate refers to the electronic energy of the slab−adsorbate system; Efrag1 (or Efrag2) refers to the electronic energy for each fragment of the coadsorbed pairs in the gas phase; and Eslab‑(frag1+frag2) refers to the electronic energy of the slab-(frag1 + frag2) system. With this definition, negative values of Eads and/or Ecoads denote an exothermic or favorable adsorption and/or coadsorption, respectively. Unless stated specifically, all reported adsorption and coadsorption energies, as well as reaction and activation energies, are given with the ZPE corrections. Additional values for the (co)adsorption energies of all intermediates were also obtained employing a common gasphase reference state for this reaction, that is, comprising gasphase methanol plus the needed stoichiometric amount of gasphase molecular hydrogen (whenever necessary). Such energy values will allow a proper comparison of the relative stability of the species on the Ru(0001) surface. In the forthcoming sections, the results of the adsorption of the isolated and coadsorbed species and subsequent calculations of Eads and Ecoads and its implications in the methanol decomposition on ruthenium will be presented and discussed in detail. 3.2. Study of the Adsorbed Species Involved in Methanol Decomposition on Ru(0001). Table 1 presents the adsorption energies calculated for the isolated species involved in the methanol decomposition on the Ru(0001) surface. As can be seen, the most stable of the adsorbed species

Figure 2. Possible pathways for the mechanism of methanol decomposition on Ru(0001).

CH3OH → CH3O → CH 2O → CHO → CO

Eads = Eslab − adsorbate − Eslab − Eadsorbate

(12)

In view of that, this computational study will comprise the following steps: (1) a careful study of the most stable adsorption

Table 1. ZPE-Corrected Adsorption Energies (Eads, in eV) and Structural Parameters (d, in Å) for Isolated Species Adsorbed on the Ru(0001) Surface species

adsorption sitea

Eadsb

Eadsc

dd

CH3OH CH3O CH2O CHO CH2OH CHOH COH CO H2

top (O) hollow fcc (O) hollow hcp (C−O)

−0.28 −2.46 −0.88

−0.28 −0.29 +0.12

2.35 2.21; 2.21; 2.20 2.17 (O); 2.17 (O); 2.15 (C)

hollow fcc (C−O) bridge (C) hollow hcp (C) hollow fcc (C) desorbed

−1.80 −3.73 −5.67 −1.67 −0.02

+0.23 +0.44 −0.02 −0.17 −0.02

2.32 (O); 2.41 (C); 2.16 (C) 2.08; 2.10 2.05; 2.03; 2.09 2.15; 2.14; 2.21 3.96; 4.16

unstable

a The atom(s) through which the compound is adsorbed on the surface is indicated between parentheses. bAdsorption energy values determined following eq 13. cAdsorption energy values determined using a common gas-phase reference state, i.e., gas-phase methanol plus the needed stoichiometric amount of gas-phase molecular hydrogen. dDistances from the adsorbate to the surface nearest atom(s).

C


Article


Figure 3. Methanol decomposition reaction steps on the Ru(0001) surface. Values for the length of the bond breaking or forming for each reaction step (notation as in Figure 2) are given in Å.

determined using a common gas-phase reference state is CH3O on the hollow fcc, with an adsorption energy equal to −0.29 eV,

while the least stable of the adsorbed species is CHOH on the bridge site, with +0.44 eV. The majority of the adsorbed species, D


Article


Table 2. ZPE-Corrected Coadsorption Energies (Ecoads, in eV) and Structural Parameters (d, in Å) for Pairs of Species Coadsorbed on the Ru(0001) Surface pair

coadsorption sitea

Ecoadsb

Ecoadsc

CH3O + H CH2O + H CHO + H CO + H CO + 2H CH2OH + H CHOH + H COH + H H+H CH3 + OH

hollow fcc (O)/hollow hcp (H) hole hcp (C−O)/hollow fcc (H)

−5.06 −3.55

top (C)/hollow hcp (H) hollow fcc (C)/2 hollow fcc (H) hollow fcc (C−O)/top (H) bridge (C)/hollow fcc (H) hollow hcp (C)/hollow fcc (H) hollow fcc (H)/ hollow fcc (H) top (C)/ hollow fcc (O)

−4.70 −7.20 −4.32 −6.52 −8.43 −5.34 −5.16

−0.80 −0.45 unstable −0.81 −1.21 −0.33 −0.25 −0.68 −1.08 −0.87

dd 2.19; 2.20; 2.20/1.89; 1.89; 1.89 2.17 (O); 2.17 (O); 2.15 (C)/1.91; 1.85; 1.91 1.90/1.89; 1.89; 1.90 2.09; 2.34; 2.12/1.85; 1.87; 1.94 + 1.84; 1.88; 1.95 2.27 (O); 2.44 (C); 2.17 (C)/1.91; 1.87; 1.92 2.07; 2.09/1.87; 1.87; 1.98 2.03; 2.08; 2.04/1.90; 1.93; 1.90 1.88, 1.88, 1.93/1.88, 1.88, 1.93 2.14/2.20, 2.20, 2.20

a The atom(s) through which the compound is adsorbed on the surface is indicated between parentheses. bCoadsorption energy values determined following eq 14. cCoadsorption energy values determined using a common gas-phase reference state, i.e., gas-phase methanol plus the needed stoichiometric amount of gas-phase molecular hydrogen. dDistances from the adsorbate to the surface nearest atoms.

five out of eight, has preference for hollow-type sites. The exceptions are CH3 OH and CHOH, respectively, with preference for top and bridge sites and CHO, which is unstable. One should notice that these results agree with previous theoretical studies carried out with cluster surface models.29,48 As a final remark, since methanol interacts weakly with the surface (Eads = −0.28 eV), that might compromise the first step of the methanol decomposition due to possible desorption problems. Geometry clearly plays a role in the adsorption of the chemical species, as CH2O and CH2OH (planar structures) adsorb through interaction between their respective C−O bond and the surface, while CH3OH and CH3O (tetrahedral type) both adsorb through the oxygen atom. The remaining species, CHOH, CHO, and CO (angular and linear geometry), adsorb through the carbon atom. The distances between the adsorbate and the nearest interacting surface atoms are approximately the same in all situations, between 2.35 (top) and 2.03 Å (hollow hcp), and one verifies that indeed the highest energies correspond to the shortest interaction distances between the adsorbate and the nearest surface atoms. The analysis of the adsorption of chemical species, using a common gas-phase reference state, related to the methanol decomposition via O−H scission (Blue Route), i.e., the species CH3OH, CH3O, CH2O, CHO, and CO, yields the following stability order on the surface (adsorption site) for those chemical species: CH3O (hollow fcc) > CH3OH (top) > CO (hollow fcc) > CH2O (hollow hcp) > CHO (unstable). As regards adsorption energies, the difference between the ones calculated for CH3O and CH2O, that is the most stable and least stable chemical species in the Blue Route, is 0.41 eV. As regards the Red Route, the stability order (adsorption site) of the involved chemical species is as follows: CH3OH (top) > CO (hollow fcc) > COH (hollow hcp) > CH2OH (hollow fcc) > CHOH (bridge). In this case, the difference between the adsorption energies of CH3OH and CHOH, the most stable and least stable chemical species found in the Red Route, is 0.72 eV. There are two other possible reaction mechanism pathways that include chemical species from both the Blue Route and the Red Route. Let us first analyze the hybrid route named here as Black Route XI. The chemical species present in this route are CH3OH, CH2O, CHO, CH2OH, and CO, and the stability order (adsorption site) for these chemical species is as follows: CH3OH (top) > CO (hollow fcc) > CH2O (hollow fcc) > CH 2 OH (hollow fcc) > CHO (unstable). Concerning

adsorption energies, the difference between those of CH3OH and CH2OH, the determined most stable and least stable chemical species in the above-mentioned Black Route XI scenario, is 0.51 eV. Another possible pathway for methanol decomposition via a hybrid route is the one named here as Green Route XII, where bond scission may occur both via O−H and C−H. In this second hybrid route, the chemical species are CH3OH, COH, CHOH, CH2OH, and CO, and the stability order (preferential site) for such species is CH3OH (top) > CO (hollow fcc) > COH (hollow hcp) > CH2OH (hollow fcc) > CHOH (bridge). The difference between the calculated adsorption energies of CH3OH and CHOH, the most stable and least stable chemical species found in the Green Route XII, is 0.72 eV. Finally, the H2 adsorption on the Ru(0001) surface was also considered since the hydrogen recombination on the surface can lead to the H2 formation (see eq 12). As can be seen in Table 1, this species does not adsorb on the Ru(0001) surface; thus, if it is formed it will desorb immediately. 3.3. Study of the Coadsorbed Pairs Involved in Methanol Decomposition on Ru(0001). Regarding the reaction product coadsorptions (i.e., CH3O + H, CH2O + H, CHO + H, CO + H, CH2OH + H, CHOH + H, COH + H, CH3 + OH) on the Ru(0001) surface, several possible configurations were studied for each pair on the same unit cell considered in section 3.2, but only the most stable ones found for each pair are shown in the rightmost panels of Figure 3. The preferential adsorption sites, as well as energetic parameters, are depicted in Table 2. General adsorption preference resulted in positions pairing coadsorption in hollow-type sites which maximizes the interaction of the adsorbates with the surface by creation of a larger amount of adsorbate−surface bonds. Thus, one can expect the reaction to occur preferentially on or near hollow-type sites, though diffusion cannot be disregarded. The study also evidences a tendency for the larger fragments to interact with the surface through the carbon atom with the exception of the methoxide fragment which interacts through the O atom. The computational results present negative adsorption energies though not significant (ranging from −0.25 to −1.21 eV). The most stable of the coadsorbed pairs, according to calculations using gas-phase methanol and molecular hydrogen as the reference state, is CO + 2H, in which the CO fragment is on the hollow fcc interacting via its carbon atom along with both hydrogen atoms in hollows fcc, with a coadsorption energy of −1.21 eV, while the least stable of the coadsorbed species is E


Article


Table 3. ZPE-Corrected Energy Barriers (Eact, in eV), ZPE-Corrected Reaction Energies (Ereact, in eV), Imaginary Wavenumbers (υi, in cm−1), and the Rate Constants (in s−1) for the Different Reaction Steps Involved in the Methanol Decomposition on Ru(0001) step

Eact

υi

Ereact

CH3OH* + * → CH3O* + H*

1.47

0.49

−0.38

1070.9

CH3O* + * → CH2O* + H*

1.61

0.63

−0.02

766.8

CH2O* + * → [CHO]* + H* → CO* + 2H*

1.54

0.02

−1.10

687.6

CH3OH* + * → CH2OH* + H*

1.59

0.66

+0.13

927.7

CH2OH* + * → CHOH* + H*

1.45

0.70

−0.23

744.3

CHOH* + * → COH* + H* COH* + * → CO* + H*

1.46 1.32

0.00 1.07

−1.00 −0.70

506.6 1530.7

CH3OH* + * → CH3* + OH*

1.81

1.35

−0.45

529.6

CH2OH* + * → CH2O* + H*

1.40

0.43

−0.45

1249.5

CHOH* + * → [CHO]* + H* → CO* + 2H*

1.36

0.40

−1.43

1267.5

1.67

+1.68

-------

0.62

+1.12

CO* → CO↑ + * H* + H* → H2↑ + 2* a

distancea

----1.20

769.3

k (50 K, 150 K, 220 K, 340 K, 470 K, 600 K) 10−38/3.35 8

1.24 × × 10−5/7.38 × 100/8.91 × 104/1.15 × 107/ 1.93 × 10 2.29 × 10−52/8.65 × 10−10/5.14 × 10−3/7.34 × 102/3.30 × 105/ 1.12 × 107 8.23 × 109/6.92 × 1011/1.71 × 1012/3.82 × 1012/6.20 × 1012/ 8.49 × 1012 8.07 × 10−55/2.42 × 10−10/3.01 × 10−3/7.72 × 102/4.74 × 105/ 1.93 × 107 2.05 × 10−59/3.07 × 10−11/1.96 × 10−3/1.83 × 103/2.12 × 106/ 1.22 × 108 ----------5.60 × 10−97/3.1 × 10−24/1.46 × 10−12/1.12 × 10−3/3.92 × 101/ 1.54 × 104 8.46 × 10−125/2.23 × 10−34/4.25 × 10−20/2.43 × 10−9/6.60 × 10−1/ 7.66 × 10−1 2.16 × 10−32/6.85 × 10−3/3.89 × 102/1.73 × 106/1.34 × 108/ 1.69 × 109 3.15 × 10−29/1.54 × 10−1/5.31 × 103/1.68 × 107/1.10 × 109/ 1.24 × 1010 2.49 × 10−157/6.20 × 10−45/3.56 × 10−27/7.44 × 10−14/3.48 × 10−7/ 1.86 × 10−3 1.59 × 10−51/5.55 × 10−9/5.67 × 10−2/1.70 × 104/1.24 × 107/ 5.50 × 108

Length of the bond cleaving or forming in Å.

the pair of fragments and the surface correspond to the shortest interaction distances between the involved chemical species and the nearest metallic atom. Considering the case of adsorption of the pair of hydrogens on the ruthenium, in average the distance between the adsorbate and the nearest metallic atoms is very similar, independently of the calculated interaction energy. Let us first consider the coadsorbed chemical species in the methanol decomposition via O−H scission (Blue Route). The results presented in Table 2 showed that the stability order on the surface for the bigger fragment coadsorbed with hydrogen atoms on Ru(0001) is the following: CO + 2H > CO + H > CH3O + H > CH2O + H > CHO + H, the latter presenting a scenario of instability in the coadsorption. The adsorption energies vary from −1.21 eV, for coadsorbed CO + 2H, to −0.45 eV, in the case of CH2O + H. As can also be seen in Table 2, the coadsorbed species in the methanol decomposition via C−H scission (Red Route) present the following stability order for the interactions of a coadsorbed pair on Ru(0001) comprising a big fragment plus a hydrogen atom: CO + 2H > CO + H > CH3O + H > COH + H > CH2OH + H > CHOH + H. Here, the adsorption energies for the coadsorbed chemical species cover a spectrum from −1.21 eV, in the most stable interaction (the CO + 2H pair), to −0.25 eV, in the least stable case (in the case of the CHOH + H pair). As regards to the first hybrid route to be analyzed, the Black Route XI, the computational results for the stability order of the interactions between coadsorbed chemical species and the pure ruthenium surface present the following order: CO + 2H > CO + H > CH3O + H > CH2O + H > CH2OH + H > [CHO]. From these, only the first five revealed stability in the interaction with the ruthenium surface, CHO being unable to reach a platform of minimal adsorption energy. The least stable interaction, with a value of −0.33 eV, corresponds to the chemical pair CH2OH + H, and it adsorbs through the C−O bond in a hollow fcc site. The most stable interaction, with a value of −1.21 eV, corresponds to the final products CO + 2H, with carbon monoxide adsorbing through the carbon atom. Finally, analysis of the obtained

CHOH + H, in which the CHOH fragment is interacting via its carbon atom on the bridge site plus the hydrogen atom on the hollow fcc (Ecoads = −0.25 eV). For most of the coadsorbed species, the stablest interaction is achieved through coadsorption on hollow-type sites, either considering a bigger fragment or the hydrogen atom. The exceptions are CO + H, CH2OH + H, and CHOH + H, the first and the latter being exceptions for the bigger fragment (on the top and bridge sites, respectively, and interacting with the surface through the carbon atom) and the pair CH2OH + H for the hydrogen atom (adsorbing on a top site). Instability is met, in these computational results, only for the chemical pair CHO + H. Regarding geometric aspects and fragment−surface interactions, one can divide the attained results for the coadsorbed chemical pairs in the three cases mentioned in the previous section: (a) planar structures adsorbing through interaction between the surface and the bigger fragment C−O bond (pairs CH2O + H and CH2OH + H); (b) angular and linear structures adsorbing through interaction between the surface and the carbon atom in the bigger fragment (pairs CHOH + H, CHO + H, CO + H, and CO + 2H); and (c) tetrahedral-type structures adsorbing through interaction between the surface and the oxygen atom in the bigger fragment (CH3O + H). In what concerns the distances of the coadsorbed chemical pairs and the nearest surface atoms, one should consider the distance between the bigger fragment and the metallic surface nearest atom, as well as the distance between the coadsorbed hydrogen atom and the ruthenium surface nearest atom. In the first situation, the distances fluctuate between 2.03 Å (hollow hcp, interaction via the carbon atom) and 2.44 Å (hollow fcc, interaction through the C−O bond). In the second situation, the distances are approximately the same as well, between 1.85 and 1.94 Å, both distances being computed for the hydrogen adsorbed on ruthenium in hollow fcc sites. As in the case of the adsorption of solo chemical species, presented in Table 1 and analyzed in Section 3.2, in general the highest energies for the interaction of F


Article


Figure 4. Energy diagram for the methanol decomposition on the Ru(0001) surface. Values for activation and reaction energies (eV) are given with respect to the energy of the reactants into each step. * denotes species adsorbed on the surface, while the most favorable reaction path is highlighted in red.

ruthenium, thus indicating the Blue Route as the strongest hypothesis of the reaction mechanism.27,49 However, some of the chemical species only present in the Red Route, the Black Route XI, and the Green Route XII have been proposed as possible for the methanol decomposition on other metallic surfaces.19 To start with, by comparing the activation energy barrier with the adsorption energy of the corresponding reactants (determined using eq 13, which has the same scale as the reactant desorption barrier) for each reaction step of the four routes, one might establish the viability of the reaction step; that is, adsorption energies higher than the activation energy barrier prevent the desorption of the reactants. From this comparison (cf. Tables 1 and 3), there are some problematic conclusions that can be drawn about some of these steps. For example, in the Blue Route, the activation energy barrier for methanol evolution toward methoxy (0.49 eV) is superior to its corresponding adsorption energy on the ruthenium surface (−0.28 eV), thus implying that the methanol desorption will be significant and therefore the reaction rate will diminish. This aspect has also been studied experimentally, and it was observed that methanol desorption starts at about 180 K while showing a maximum at 215−220 K;27 however, after methanol dissociation toward methoxy, the formed species is already strongly adsorbed on the surface, preventing its desorption.27 Save for this first step, adsorption of the methanol on ruthenium, for the remaining steps of the Blue Route the reactants consistently have adsorption energies higher than the related activation energy barriers, in fact high enough to contain the reactant desorption. This means that issues regarding desorption or other possible obstacles to the effectiveness of the reaction are absent, and thereby a considerable reaction rate is expected. Another important theoretical finding from the data in Table 3 concerns the possible reasons for the experimental detection of CH3O as an intermediate in the methanol decomposition on ruthenium. The activation energy barrier for methoxy dissociation into form-

theoretical data regarding the second possible hybrid route, the Green Route XII, resulted in the following order for the most stable interactions between coadsorbed chemical pairs and the Ru(0001) surface: CO + 2H > CO + H > CH3O + H > CH2OH + H > CHOH + H. The similarity between the stablest sites and interaction fragments with the Black Route is clear, due to common chemical species for most of both pathways. However, the least stable interaction on the Green Route XII, of −0.25 eV, is lower than in the other hybrid route. 3.4. Reaction Kinetics and Favorable Pathways for the Methanol Decomposition on Ru(0001). As a final examination of the methanol decomposition on the Ru(0001) surface, this section will focus on the kinetics and favorable pathways for the reaction. The most stable adsorption (or coadsorption) configuration(s) for the reactant(s) taken as the initial states pertaining to all possible pathways were described in Subsection 3.3. The data scrutinized in that section will be crossreferenced with the one presented in Table 3, which displays the calculated data for the different reaction steps involved in the methanol decomposition on Ru(0001). The latter data include, for each reaction step, the lengths of the bonds being broken or formed, the activation energy barrier, the reaction energy, along with the imaginary wavenumber of the corresponding transition state and the reaction rate constants at different temperatures. A representation of the initial, transition, and final states for the most favorable pathway on Ru(0001) of each reaction step considered in this work can be seen in Figure 3, while a potential energy diagram of the overall mechanism is presented in Figure 4. The step reactions displayed in Table 3, appropriately conjugated, form the four possible pathways already defined in Subsection 3.1, i.e.: the Blue Route, the Red Route, the Black Route XI, and the Green Route XII. The reason to analyze these four pathways is rooted in experimental evidence, namely, the one that establishes the detectable presence of methoxy as an intermediate byproduct of the methanol decomposition on G


Article


notice that the rate constant for its formation at 220 K is 7.38 × 100 and for its evolution toward formaldehyde is 5.14 × 10−3, which suggests its accumulation on the surface, and hydrogen desorbs from the surface at 330−350 K since the rate constant for H2 formation at 340 K is 1.70 × 104. The cleavage distance of the C−H is 1.61 Å, and the imaginary wavenumber is 766.8 cm−1. However, this route also includes the fastest step of any possible pathway (an aggregation of two steps, in fact, of the successive dissociation of the last two hydrogen atoms), and considering the combination of the steps in it, the Blue Route not only is more energetically favorable and contains an experimentally detected intermediate species but also the most kinetically favorable pathway for the whole array of studied temperatures, including the above-mentioned temperatures of 220 and 340 K. Further, not only are the other pathways less favorable when it comes to the velocity of reaction but also in the Red Route one has the slowest step of the several possible steps in the four pathways, the dissociation of the hydrogen atom in the COH’s hydroxyl group, which presents values of k of 1.46 × 10−12 and 1.12 × 10−3 for the temperatures of 220 and 340 K, thereby making this route very slow if it occurs. For this step, the cleavage distance of the O−H is 1.32 Å, and the imaginary wavenumber is 1530.7 cm−1. In the case of the hybrid routes, the steps pertaining only to the Black Route XII and the Green Route XII, i.e., the dissociation of the hydrogen atom from the hydroxyl group in CH2OH and CHOH, respectively, and still for the two temperatures more close to the room temperature situation (220 and 340 K), the rate constants are within an interval of considerable values (from 102 to 107, the lower limit corresponding to 220 K and the upper limit to 340 K). In each situation, the cleavage distances of the O−H are as follows: 1.40 Å, with an imaginary wavenumber of 1249.5 cm−1, for the Black Route XI, and 1.36 Å, with an imaginary frequency of 1267.5 cm−1, for the Green Route XII. Nevertheless, the inclusion of these steps after the slowest step of the Red Route makes the hybrid routes with less kinetic advantages when compared with the Blue Route. The possibility of the methanol evolution toward CO through a primary C−O bond break on the surface (see eq 11) was investigated as well. However, as can be seen in Table 3, this step presents a high activation energy barrier which makes improbable the evolution through this reaction. This study also included the hydrogen formation step since the H2 desorption is also possible from the methanol decomposition on ruthenium; in fact, the activation energy barrier for H2 production on the Ru(0001) surface is quite low (0.62 eV) making this reaction feasible on this surface. This result is in good agreement with the experimental detection of hydrogen during the methanol decomposition on the Ru(0001) surface.27 Further, reaction energies computed show that all the steps of the methanol decomposition are exothermic reactions but the hydrogen and CO desorption which are endothermic processes.

aldehyde (CH3O → CH2O, step III) is sufficiently high to allow measurable accumulation of this chemical species during the process. Moreover, the activation energy barrier between the formed formaldehyde and the next species to be produced in the Blue Route, CHO, is extremely low, making the eventual experimental detection of the CHO species involved in the transition between formaldehyde and carbon monoxide (CH2O → CHO → CO, namely, step IV according to Figure 1) improbable, which can also account for the lack of its observation experimentally.27 Such is the reason for aggregating these two steps in a single one in Table 3: CH2O* + * → [CHO]* + H*→ CO* + 2H*. Further, this energetic-type scenario implies another aggregation of steps in the studied routes, though in the Red Route. Finally, to conclude the analysis of the steps in the Blue Route, one must point out that the transition from CH3O to CH2O is the energetically key step in this possible pathway for the methanol decomposition on Ru(0001). Actually the Blue Route seems to be the most likely pathway, due to both experimental evidence of the methoxy as an intermediate in the methanol decomposition and the computational results of this study, since it is the most energetically favorable route among the four possible pathways, and the activation energy barrier for its key step is lower than analogous ones in the other three routes. Nevertheless, let us follow up with the analysis of the obtained data for these other routes. In the Red Route, the first two steps (CH3OH* + * → CH2OH* + H* and CH2OH* + * → CHOH* + H*) present higher activation energy barriers than the step with the highest activation energy barrier in the Blue Route. However, the difference for the activation energy barrier in the first step of both routes is not very significant, and it is possible that the Red Route could be viable, if the remaining of the route did not persist with less favorable energetics. Even though the Green Route XI and the Black Route XII have part of the Blue Route steps, their initial two steps are common with the Red Route, and therefore they fail to have an energetics pathway as likely to be feasible as the one of the Blue Route. However, when comparing the Green Route XI and the Black Route XII with just the Red Route, both these can offer a more interesting energetic pathway than that of the Red Route itself. In fact, in certain circumstances, reproducible in laboratory eventually, the Black Route XII might function as a minority pathway but be viable even so. As stated, the first step of the Red Route (which is also the first step of Black Route XII) is not significantly different from the one of the Blue Route, from an energetic point of view. In fact, at high temperatures, the activation energy barrier for the first step of the Red Route/Black Route XII is not unlikely to permit the break of the C−H bond. The production of the CH2OH in the pathway of the Black Route XII would become almost as rapid as its decomposition, and this might explain its absence in experimental data even if the Black Route XII really occurs. The obtained computational data for the adsorption of the reactant chemical species of the first step (first break of a methanol C−H bond on the surface) in the Red Route also present an improbable scenario, reinforcing the Blue Route as the most likely pathway. The reaction rate constants of the different pathways are also very distinct. In the Blue Route, and considering the temperatures of 220 and 340 K, the step of dissociation of the second hydrogen is the slowest of this pathway with rate constants equal to 5.14 × 10−3 and 7.34 × 102, respectively. Moreover, the rate constant values at different temperatures for the Blue Route (see Table 3) go well along with the experimental findings.27 That is, methoxide is the principal species on the surface at 220 K

4. CONCLUSIONS In this article, we have examined by means of DFT calculations the several possible pathways for the methanol decomposition on the clean Ru(0001) surface, arising from three initial divergent bond scissions (either an initial C−H, C−O, or an O−H bond scission). In doing so, we obtained data pertaining to the adsorption and coadsorption of the several different reaction reactants, intermediates, and products, as well as regarding the thermochemistry and the reaction barriers of its elementary reaction steps. Several conclusions were reached, namely: H


Article

The Journal of Physical Chemistry C - Four possible pathways (the Blue Route, the Red Route, the Black Route XI, and the Green Route XII) were defined taking into account experimental observations of the methanol decomposition on ruthenium and other metallic surfaces. In particular, the Blue Route includes an experimentally detected intermediate byproduct of the methanol decomposition on ruthenium, the radical methoxy, which made it the most likely favorable pathway in the precomputational study phase; - Regarding the adsorption of all possible chemical species present in the four defined pathways, and according to the data obtained using a common gas-phase reference state, the stablest scenario belongs to CH3O, adsorbing in the hollow fcc site, while CHOH presents the least stable adsorption scenario, adsorbing in a bridge site. CHO is the only chemical species in the four pathways presenting instability in its adsorption. - Methanol adsorption, the prestep of the four pathways, is of weak nature, with computed adsorption energy of −0.28 eV, and such a situation might compromise effective methanol decomposition, as it implies enough desorption in reasonable rates. - The adsorbing interactions are clearly related with the chemical species geometries and three trends can be disclosed: (1) planar structures interact with ruthenium through the C−O bond; (2) tetrahedral type structures interact with the metallic surface through the oxygen atom; and (3) chemical species presenting angular and linear geometry adsorb on Ru(0001) through their carbon atoms. These trends replicate in the results for the study of the coadsorbed pairs on the ruthenium surface. - From the study of the reaction product coadsorptions on the Ru(0001) surface, we observed a general adsorption preference in positions pairing coadsorption in hollow-type sites and for larger fragments to interact through their carbon atoms. Such copairing maximizes the adsorption interaction of the chemical species with the surface by creating a larger amount of adsorbate−surface bonds. Though diffusion cannot be disregarded, these results make the occurrence of the reaction in such sites quite expectable. - For most of the coadsorbed species, employing a common gas-phase reference state in calculations, the stablest interaction is achieved through coadsorption in hollow-type sites. Either one is considering the big fragment or the hydrogen atom, as in the case of the most stable coadsorbed chemical species scenario, the coadsorbed pair CO + 2H, all adsorbing in hollow fcc sites, and the least stable of the coadsorbed species, CHOH + H, in the bridge site (big fragment, bridge/for the hydrogen atom, fcc). The array of studied coadsorbed big f ragment + hydrogen atom reached overall expressive negative adsorption energies. - For both solo chemical species adsorption and coadsorbed pair data, the highest energy for the energetic interaction of the solo species/paired fragment and the surface corresponds to the shortest distance between the adsorbate and the nearest metallic atom of the adsorbing surface. - The Blue Route appears to be the most likely pathway for the methanol decomposition on ruthenium. This is because: (a) it is the only pathway including an experimentally detected intermediate byproduct of the methanol decomposition on ruthenium, the radical methoxy; (b) though the activation energy barrier of methanol O−H bond scission in this route is superior to its corresponding adsorption energy on the ruthenium surface, indicating significant methanol desorption and slower reaction rate, the remaining steps of the Blue Route consistently have higher adsorption energies for the reactants in comparison with the forward activation energy barriers, making it the most

energetically favorable route among the four possible pathways; (c) the yielded activation energy barrier for methoxy dissociating into formaldehyde is high enough to permit experimental detection; (d) the data regarding the activation energy barrier between formaldehyde and the following chemical species to be formed in the Blue Route are sufficiently low enough to explain the lack of experimental detection of these species; (e) the Blue Route includes the fastest step of any possible pathway; (f) considering the overall reaction rate of the steps, the Blue Route is the most kinetically favorable pathway, namely, for temperatures of 220 and 340 K. - In the Blue Route, one should note the following: (a) the key step, when considering favorable energetics, is the transition from CH3O to CH2O; (b) the most stable adsorption of solo species corresponds to CH3O in a hollow fcc site; (c) the scenario for the highest coadsorption stability is the one of coabsorbed CO plus 2H species. - Nevertheless, in a certain temperature range, the Black Route XI might be active as a minority pathway. - Finally, for the possible hydrogen formation from the H atoms obtained in the methanol decomposition on Ru(0001), it was seen that this reaction is characterized by a quite low activation energy barrier, thus indicating that its formation is possible.

■

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Thanks are due to Fundaçaõ para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to FEDER for financial support to LAQV@ REQUIMTE, project UID/QUI/50006/2013. 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.

■

REFERENCES

(1) Fuel Cells Annual Report 2013, Market Development for Fuel Cells in the Stationary, Portable, and Transport Sectors, http://www. navigantresearch.com/research/fuel-cells-annual-report-2013 (Accessed on April 1st, 2015). (2) Research and markets − Market Research Reports − Micro Fuel Cell Market Opportunities, Strategies, and Forecasts; 2013 to 2019. http://www.researchmoz.us/stationary-fuel-cells-market-sharesstrategies-and-forecasts-worldwide-2013-to-2019-report.html (Accessed on April 1st, 2015). (3) Hamnett, A. Mechanism and Electrocatalysis in the Direct Methanol Fuel Cell. Catal. Today 1997, 38, 445−457. (4) Antolini, E. Formation of Carbon-supported PtM Alloys for Low Temperature Fuel Cells: A Review. Mater. Chem. Phys. 2003, 78, 563− 573. (5) Holton, O. T.; Stevenson, J. W. The Role of Platinum in Proton Exchange Membrane Fuel Cells. Platinum Met. Rev. 2013, 57, 259−271. (6) Wasmus, S.; Kuver, A. Methanol Oxidation and Direct Methanol Fuel Cells: a Selective Review. J. Electroanal. Chem. 1999, 461, 14−31. (7) Li, W. Z.; Liang, C. H.; Zhou, W. J.; Qiu, J. S.; Zhou, Z. H.; Sun, G. Q. Preparation and Characterization of Multiwalled Carbon Nanotubesupported Platinum for Cathode Catalysts of Direct Methanol Fuel Cells. J. Phys. Chem. B 2003, 107, 6292−6299.

I


Article


to Direct Methanol Fuel Cells. J. Am. Chem. Soc. 1999, 121, 10928− 10941. (29) Ishikawa, Y.; Liao, M.; Cabrera, C. Oxidation of Methanol on Platinum, Ruthenium and Mixed Pt−M metals (M = Ru, Sn): A Theoretical Study. Surf. Sci. 2000, 463, 66−80. (30) Desai, S. K.; Neurock, M.; Kourtakis, K. A Periodic Density Functional Theory Study of Dehydrogenation of Methanol over Pt(111). J. Phys. Chem. B 2002, 106, 2559−2568. (31) Greeley, J.; Nørskov, J. K.; Mavrikakis, M. Electronic Structure and Catalysis on Metal Surfaces. Annu. Rev. Phys. Chem. 2002, 53, 319− 348. (32) Delbecq, F.; Sautet, P. Low-Temperature Adsorption of Formaldehyde on a Platinum (111) Surface. A Theoretical Study. Langmuir 1993, 9, 197−207. (33) Delbecq, F.; Sautet, P. A Density Functional Study of Adsorption Structures of Unsaturated Aldehydes on Pt(111): A Key Factor for Hydrogenation Selectivity. J. Catal. 2002, 211, 398−406. (34) Wei, Z.; Sun, J.; Li, Y.; Datye, A. K.; Wang, Y. Bimetallic Catalysts for Hydrogen Generation. Chem. Soc. Rev. 2012, 41, 7994−8008. (35) Luo, W.; Asthagiri, A. Density Functional Theory Study of Methanol Steam Reforming on Co(0001) and Co(111) Surfaces. J. Phys. Chem. C 2014, 118, 15274−15285. (36) Fajín, J. L. C.; Cordeiro, M. N. D. S.; Illas, F.; Gomes, J. R. B. Descriptors Controlling the Catalytic Activity of Metallic Surfaces Toward Water Splitting. J. Catal. 2010, 276, 92−100. (37) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (38) Kresse, G.; Furthmüller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (39) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab-initio Total Energy Calculations Using a Plane-wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (40) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 6671−6687. (41) Blöchl, P. E. Projector Augmented-wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (42) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (43) Fajin, J. L. C.; Cordeiro, M. N. D. S.; Gomes, J. R. B. Adsorption of Atomic and Molecular Oxygen on the Au(321) Surface: DFT Study. J. Phys. Chem. C 2007, 111, 17311−17321. (44) Fajin, J. L. C.; Cordeiro, M. N. D. S.; Gomes, J. R. B. DFT study of the CO Oxidation on the Au(321) Surface. J. Phys. Chem. C 2008, 112, 17291−17302. (45) Fajín, J. L. C.; Cordeiro, M. N. D. S.; Gomes, J. R. B. DFT study of the Au(321) Surface Reconstruction by Consecutive Deposition of Oxygen Atoms. Surf. Sci. 2008, 602, 424−435. (46) Henkelman, G.; Jónsson, H. A Dimer Method for Finding Saddle Points on High Dimensional Potential Surfaces Using only First Derivatives. J. Chem. Phys. 1999, 111, 7010−7022. (47) Laidler, K. J. in: Chemical Kinetics, third ed.; Harper Collins: New York, 1987; p 193. (48) Cordeiro, M. N. D. S.; Pinto, A. S. S.; Gomes, J. A. N. F. A DFT Study of the Chemisorption of Methoxy on Clean and Low Oxygen Precovered Ru(0001) Surfaces. Surf. Sci. 2007, 601, 2473−2485. (49) Barros, R. B.; Garcia, A. R.; Ilharco, L. M. The Decomposition Pathways of Methanol on Clean Ru(0001), Studied by Reflection− Absorption Infrared Spectroscopy (RAIRS). J. Phys. Chem. B 2001, 105, 11186−11193.

(8) Liu, H. S.; Song, C. J.; Zhang, L.; Zhang, J. J.; Wang, H. J.; Wilkinson, D. P. A Review of Anode Catalysis in the Direct Methanol Fuel Cell. J. Power Sources 2006, 155, 95−110. (9) Ren, X. M.; Wilson, M. S.; Gottesfeld, S. High Performance Direct Methanol Polymer Electrolyte Fuel Cells. J. Electrochem. Soc. 1996, 143, 12−5. (10) Vidakovic, T.; Christov, M.; Sundmacher, K.; Nagabhushana, K. S.; Fei, W.; Kinge, S.; Bonnemann, H. PtRu Colloidal Catalysts: Characterization and Determination of Kinetics for Methanol Oxidation. Electrochim. Acta 2007, 52, 2277−2284. (11) Miller, A. V.; Kaichev, V. V.; Prosvirin, I. P.; Bukhtiyarov, V. I. Mechanistic Study of Methanol Decomposition and Oxidation on Pt(111). J. Phys. Chem. C 2013, 117, 8189−8197. (12) Liu, Z.; Sawada, T.; Takagi, N.; Watanabe, K.; Matsumoto, Y. Reaction Intermediates in the Oxidation of Methanol on a Pt(111)-(2 × 2)O Surface. J. Chem. Phys. 2003, 119, 4879−4886. (13) Akhter, S.; White, J. M. A. Static SIMS/TPD Study of the Kinetics of Methoxy Formation and Decomposition on O/Pt(111). Surf. Sci. 1986, 167, 101−126. (14) Wang, J.; Masel, R. I. Methanol Adsorption and Decomposition on (2 × 1)Pt(110): Enhanced Stability of the Methoxy Intermediate on a Stepped Surface. Surf. Sci. 1991, 243, 199−209. (15) Franaszczuk, K.; Herrero, E.; Zelenay, P.; Wieckowski, A.; Wang, J.; Masel, R. I. A Comparison of Electrochemical and Gas-Phase Decomposition of Methanol on Platinum Surfaces. J. Phys. Chem. 1992, 96, 8509−8516. (16) Diekhoner, L.; Butler, D. A.; Baurichter, A.; Luntz, A. C. Parallel Pathways in Methanol Decomposition on Pt(111). Surf. Sci. 1998, 409, 384−391. (17) Skoplyak, O.; Menning, C. A.; Berteau, M. A.; Chen, J. G. Experimental and Theoretical Study of Reactivity Trends for Methanol on Co/Pt(111) and Ni/Pt(111) Bimetallic Surfaces. J. Chem. Phys. 2007, 127, 114707-(1−12). (18) Davis, J. L.; Barteau, M. A. Decarbonylation and Decomposition Pathways of Alcohol’s on Pd(111). Surf. Sci. 1987, 187, 387−406. (19) Greeley, J.; Mavrikakis, M. Competitive Paths for Methanol Decomposition on Pt(111). J. Am. Chem. Soc. 2004, 126, 3910−3919. (20) Suh, D. J.; Kwak, C.; Kim, J.-H.; Kwon, S. M.; Park, T.-J. Removal of Carbon Monoxide from Hydrogen-rich Fuels by Selective Lowtemperature Oxidation over Base Metal Added Platinum Catalysts. J. Power Sources 2005, 142, 70−74. (21) Chen, T.-C.; Luo, T. − J. M.; Yang, Y.-W.; Wei, Y.-C.; Wang, K.W.; Lin, T.-L.; Wen, T.-C.; Lee, C. H. Core Dominated Surface Activity of Core−Shell Nanocatalysts on Methanol Electrooxidation. J. Phys. Chem. C 2012, 116, 16969−16978. (22) Salgado, J. R. C.; Paganin, V. A.; Gonzalez, E. R.; Montemor, M. F.; Tacchini, I.; Ansón, A.; Salvador, M. A. P.; Ferreira, P.; Figueiredo, F. M. L.; Ferreira, M. G. S. Characterization and Performance Evaluation of PtRu electrocatalysts Supported on Different Carbon Materials for Direct Methanol Fuel Cells. Int. J. Hydrogen Energy 2013, 38, 910−920. (23) Dillon, R.; Srinivasan, S.; Arico, A. S.; Antonucci, V. International Activities in DMFC R&D: Status of Technologies and Potential Applications. J. Power Sources 2004, 127, 112−26. (24) Jung, E. H.; Jung, U. H.; Yang, T. H.; Peak, D. H.; Jung, D. H.; Kim, S. H. Methanol Crossover through PtRu/nafion Composite Membrane for a Direct Methanol Fuel Cell. Int. J. Hydrogen Energy 2007, 32, 903−907. (25) Bunazawa, H.; Yamazaki, Y. Ultrasonic Synthesis and Evaluation of Non-platinum Catalysts for Alkaline Direct Methanol Fuel Cells. J. Power Sources 2009, 190, 210−215. (26) Wang, Z. B.; Wang, X. P.; Zuo, P. J.; Yang, B. Q.; Yin, G. P.; Feng, X. P. Investigation of the Performance Decay of Anodic PtRu Catalyst with Working Time of Direct Methanol Fuel Cells. J. Power Sources 2008, 181, 93−100. (27) Gazdzicki, P.; Jakob, P. Reactions of Methanol on Ru(0001). J. Phys. Chem. C 2010, 114, 2655−2663. (28) Kua, J.; Goddard, W. Oxidation of Methanol on 2nd and 3rd Row Group VIII Transition Metals (Pt, Ir, Os, Pd, Rh, and Ru): Application J


Competitive Paths for Methanol Decomposition on Ruthenium: A DFT

Recommend Documents