The Oxidation of Methanol on PtRu(111): A Periodic Density

Article pubs.acs.org/JPCC

The Oxidation of Methanol on PtRu(111): A Periodic Density Functional Theory Investigation Lianming Zhao,*,†,‡ Shengping Wang,†,‡ Qiuyue Ding,†,‡ Wenbin Xu,†,‡ Pengpeng Sang,†,‡ Yuhua Chi,†,‡ Xiaoqing Lu,†,‡ and Wenyue Guo*,†,‡ †

College of Science, China University of Petroleum, Qingdao, Shandong 266580, P. R. China Key Laboratory of New Energy Physics & Materials Science in Universities of Shandong, China University of Petroleum, Qingdao, Shandong 266580, P. R. China

‡

S Supporting Information *

ABSTRACT: Self-consistent periodic density functional theory (PW91-GGA) calculations are employed to study the oxidation of methanol on PtRu(111). Geometries and energies for all the intermediates involved are analyzed, and the oxidation network is mapped out to illustrate the reaction mechanism. On PtRu(111), the Ru atoms with less electronegativity are more favorable to binding the adsorbates than the Pt atoms. Alloying Pt with Ru weakens the bond of CO to Pt, but strengthens the bond of CO to Ru. All possible pathways through initial C−H, O−H, and C−O bond scissions are considered. The initial O−H bond scission is found to be the most favorable and bears an energy barrier comparable to that for methanol desorption. The further oxidation occurs preferentially via the non-CO path from species CHO. The most possible reaction pathway of methanol on PtRu(111) is CH3OH → CH3O → CH2O → CHO → CHOOH → COOH → CO2. Furthermore, the activation of H2O on PtRu(111) is more favorable than that on the pure Pt(111) surface. The enhancement of methanol oxidation catalytic activity of the PtRu alloy is due primarily to altering the major reaction pathways from the CO path on pure Pt to the non-CO path on the alloy surface as well as promoting adsorption of methanol and formation of active OH species from H2O.

1. INTRODUCTION The direct methanol fuel cell (DMFC) is considered as a promising power source in the 21st century, such as applications in mini mobile power and electric vehicles, because of its quick start-up in room temperature, abundant source, good reliability, high energy density, and environmental friendliness.1−3 Currently, Pt and Pt group metals are considered to be the best electrocatalyst used widely in both the anode and the cathode of DMFCs. In order to reduce the cost and improve the tolerance toward CO-poisoning of pure Pt catalysts, a common method is alloying Pt with other transition metals or noble metals. In recent years, many binary, ternary, quaternary, and even more metal alloy catalysts have been studied by experiments.4 Binary catalysts involve PtRu, 5−10 PtAu, 11−13 PtPd, 14−18 PtNi, 19−21 PtSn, 22,23 PtMo,24−26 PtCo,27−29 and so on; ternary catalysts include PtRuAu,30 PtRuFe,31 PtRuSn,32 and PtRuNi;33,34 and quaternary catalysts such as PtRuSnW35 have also been studied. Among the binary alloy catalysts, PtRu alloys have been shown to have the best performance because of the significant enhancement of electrocatalytic activity for methanol oxidation and the improvement of CO-poisoning tolerance.36 The PtRu/ C catalysts prepared by a microwave-assisted polyol process exhibit the highest electrocatalytic activity toward methanol oxidation, when the Pt:Ru atomic ratio is close to 1:1.37 The © XXXX American Chemical Society

CO oxidation potential on PtRu alloys reduces approximately by 0.25 V compared to that on pure Pt and 0.1 V compared to that on Ru.38 Methanol oxidation overpotential on PtRu/ SWCNT reduces to about 0.35 V.39 Both electrochemical activity and anti CO-poisoning ability of PtRu/C are higher than the existing commercial E-TEK Pt/C electrocatalysts at room temperature.40 According to experimental results, two reaction mechanisms (denoted as dual path mechanism) have been proposed for the catalytic oxidation of methanol; that is, the adsorbed CH3OHad converts directly into CO2 without intermediate CO involved, denoted as the non-CO pathway, and/or CH3OHad decomposes into COad, followed by the oxidization of COad with OHad to form CO2, as the CO pathway. Introduction of Ru is expected to aid the CO oxidation involved in the methanol oxidation via bifunctional and/or electronic effects. On one hand, transfer of a part of the d electrons from Ru to Pt in the PtRu alloys weakens the interaction between Pt and CO, mirrored by a red-shift of the infrared absorption frequency of COad.41 On the other hand, in the bifunctional mechanism, water is activated over Ru in the surface to form active hydroxyl Received: April 24, 2015 Revised: August 9, 2015

A

DOI: 10.1021/acs.jpcc.5b03951 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

to give the absolute binding energies. The under-reproducing of the total energies using the PW91 functional is expected to counteract with each other to some extent for the relevant intermediates, so that gives correctly the qualitative features of the PES. To take the relativity effect into account, the density functional semicore pseudopotential (DSPP) method was employed for the Pt and Ru atoms, whereas the C, H, and O atoms were dealt with an all-electron basis set. The valence electron functions were expanded into a set of numerical atomic orbitals by a double-numerical basis with polarization functions (DNP). A Fermi smearing of 0.01 hartree and a realspace cutoff of 4.7 Å were used to improve the computational performance. We used the homogeneous 1:1 PtRu alloy as the model for the DMFC catalyst. The bulk lattice constants are calculated to be 2.775 Å, which falls in between the theoretical values of 2.756 Å for PtRu2 and 2.78 Å for Pt2Ru reported by Koper and co-workers.45,58 Generally, the Pt(111) surface is the most stable crystal plane in Pt nanoparticles and thus is usually selected as the prototype in both experimental and theoretical investigations.36,37,43,53,54,57 For comparison, the PtRu(111) surface was considered here. Furthermore, the PtRu(111) surface was computed with the fcc structure, which is the experimentally observed structure.59 Gasteiger et al. have found that the annealed polycrystalline PtRu alloys have an fcc lattice in the bulk, while their surface compositions are exhibited by an fcc structure at the bulk composition of 35−100 at % Pt and an hcp structure at 9.5 at % Pt.59 The PtRu alloy with an hcp lattice (9.5 at % Pt) exhibits a larger segregation than the fcc alloys (>35 at % Pt), due to the lattice mismatch between the bulk and the surface face structure.59 A periodic four-layer slab with four atoms per layer was modeled representing a 2 × 2 unit cell and a vacuum of 15 Å in thickness was used to separate the surface from its periodic image in the direction along the surface normal. The reciprocal space was sampled with a 3 × 3 × 1 k-point grid generated automatically using the Monkhorst−Pack (MP) method.60 The accuracy and reliability of calculations using a 3 × 3 × 1 MP grid has been confirmed by Lin, Zhong, and co-workers.46,48 A single adsorbate was allowed to adsorb on one side of the unit cells, corresponding to the surface coverage of 1/4 ML. Indeed, under the initial conditions on the clean surfaces, methanol molecules would approach each other, and thus, a relatively high coverage can be reached even at medium pressures and low temperatures.57 In addition, we also calculated the adsorption of CH3OH (Ru-top site) and CH2OH (Ru2-bridge site) on a larger (4 × 4) PtRu(111) cell. The relevant adsorption energies (Eads = −0.75 eV (CH3OH) and −2.22 eV (CH2OH)) were found to be slightly larger than that (−0.70 and −2.14 eV) calculated on the (2 × 2) cell, suggesting that the effect of surface coverage is not significant for adsorption. In the slab calculations, the atoms in the two bottom layers were fixed, whereas those in the uppermost two layers were allowed to be fully relaxed at their bulk-truncated structure. The transition states were performed with the complete linear and quadratic synchronous transit (LST/QST) method in Dmol3.61 The rate constant k and pre-exponential A0 were estimated using conventional transition state theory62

species, which then oxidize CO bound to neighboring Pt sites.42 First-principles investigations are of great help for clarifying the reaction mechanisms. Using a density functional theoretical (DFT) analysis, Neurock and co-workers suggested that alloying Pt with Ru on Pt2Ru(111) helps to lower the water activation barrier and thus promotes CO oxidation.43 On the basis of a cluster model of PtnRu10−n, Ishikawa et al. investigated the initial C−H activation pathway of methanol oxidation and found that the electronic effect is relatively important compared to the bifunctional mechanism.44 However, periodic DFT study by Koper and co-workers indicated that mixing of Pt with Ru weakens the bonds of both CO and OH to the Pt sites, but the bonds of these species to the Ru sites are strengthened.45 Recent theoretical studies suggested that methanol reaction on the PtAu(111) and PdZn(111) surfaces is favored by dehydrogenation to CH2O via initial O−H activation, and then oxidation to CO2 through the major non-CO pathway, rather than the CO pathway on pure Pt surfaces.46−48 It is thus very tempting to clarify the methanol oxidation mechanism on PtRu catalysts. In this work, we report a periodic DFT investigation of the methanol oxidation on the PtRu(111) surface, considering the special nature of the cluster model such as the limited number of atoms as well as the presence of the size effects, surface effects, and boundary effects. All possible pathways involved in the methanol decomposition (initial C− H, O−H, and C−O activations) and oxidation (the CO and non-CO pathways) (Scheme 1) are considered, and electronic Scheme 1. Possible Reactions of CH3OH on PtRu(111)

structure analyses on some key steps are performed. The purpose of this paper is to elucidate the reason why the PtRu alloy possesses a high catalytic activity.

2. MODEL AND COMPUTATIONAL DETAILS All calculations were performed in the framework of DFT with the program package DMol3 in Materials Studio of Accelrys Inc.,49,50 using the generalized gradient approximation (GGA) in the form of exchange-correlation functional PW91,51,52 which has presented a good reliability in describing the potential energy surface (PES) of methanol reaction on Pt(111), Ru(0001), PtAu(111), and PdZn(111).46,47,53−56 The PW91 functional has the shortcoming that it does not contain dispersion contributions, but the contribution of van der Waals (vdW) dispersion terms is small. For instance, Garcι ́a-Muelas et al. reported that the inclusion of vdW interactions by applying Grimme’s DFT-D2 method could enhance the adsorption energies by 0.01−0.20 eV for the reactants and products involved in methanol decomposition on Cu, Ru, Pd, and Pt surfaces.57 Furthermore, our main goal in the present work is to describe the PES of methanol reaction. To this end, the ability to correctly reproduce the relative energies of relevant intermediates is of greater importance than

k= B

⎛ −E 0 ⎞ ⎛ −E ⎞ kBT QTS exp⎜ a ⎟ = A0 exp⎜ a ⎟ ⎝ RT ⎠ ℏ Q IS ⎝ RT ⎠

(1)


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The Journal of Physical Chemistry C in which kB is the Boltzmann constant, R is the gas constant, ℏ is the Planck’s constant, T is the reaction temperature, which was selected as 300 K, following the experimental conditions of PtRu-based catalysts,37−40 E0a and Ea are energy barriers with and without zero-point energy corrections, and QIS and QTS are the partition functions at the initial state and transition state, respectively. The calculated adsorption energy is presented as follows Eads = Eadsorbate/substrate − (Eadsorbate + Esubstrate)

(2)

where Eads is the adsorption energy of the adsorbate on metal surfaces, Eadsorbate/substrate is the total energy of the slab with an adsorbed molecule, and Eadsorbate and Esubstrate are the energies of the free adsorbate and the clean slab, respectively. Taking into account the complexity of real DMFC systems, methanol oxidation in the gas phase has been used as the prototype reaction to probe the mechanism of low-temperature methanol fuel cells.46,47,53−57 One of the major differences between the electrocatalytic reaction environment and the gas phase is the influence of solvation (namely water). Some studies have been devoted to elucidate the effects of water on the methanol reaction on single crystal surfaces.63−65 Although water present in the aqueous solution phase can influence the energetics of electrocatalytic processes, the aqueous-phase methanol dehydrogenation mechanism on Pt(111) was found to follow similar paths and the ordering of the C−H and O−H activation steps was also detected as in the gas phase.53,54,66,67 Therefore, we used the gas-phase model to fundamentally clarify methanol oxidation on the PtRu alloy surface.

Figure 1. Top view and side view of the most stable adsorption configurations for the intermediates involved in methanol reaction on PtRu(111). Blue, cyan, gray, red, and white denote Pt, Ru, C, O, and H atoms, respectively.

3. RESULTS In this section, we first give adsorption geometries and energies for the most stable adsorption configurations along the reaction pathway of methanol oxidation on PtRu(111). Then, we present the possible final state and clarify the reaction steps to gather a general understanding of the reaction process, including the data of thermodynamics and kinetics. Values of all energies are reported with the zero-point energy (ZPE) corrections unless otherwise stated. 3.1. Adsorption. The adsorption of methanol and various intermediates involved in the title reaction are examined, including CH3OH, CH3O, CH2OH, CH2O, CHOH, CHO, COH, CO, CH2OOH, CHOOH, COOH, CHOO, CO2, H2O, OH, and atomic H. The most stable configurations are shown in Figure 1, and the corresponding adsorption energies and structural parameters of the intermediates are listed in Table 1. 3.1.1. CH3OH. For the gas-phase methanol, the bond lengths are calculated to be 1.100 Å for C−H, 1.430 Å for C−O, and 0.968 Å for O−H, agreeing well with both the experimental values of 1.09, 1.43, and 0.95 Å68 and theoretical values of 1.086, 1.411, and 0.954 Å,69 respectively. It is generally considered that methanol adsorbs via donation of the lonepair electrons from oxygen to metallic surfaces,70−72 forming a weak adsorption state and thus an unstable adsorption precursor for further dissociation with the increasing of the system temperature. 73 Our calculation shows that the adsorption of methanol on PtRu(111) prefers a Ru-top site through its oxygen atom with the Ru−O distance of 2.314 Å (see Figure 1 and Table 1). The Ru−O−C angle is calculated to be 119.8°, facilitating the binding of methanol to the surface via the oxygen lone-pair orbital. The adsorption energy of methanol on the PtRu(111) surface (−0.70 eV) is comparable

to that on the pure Ru (0001) surface (−0.78 eV),56 but much higher than that on the pure Pt(111) surface (−0.33 eV),53 suggesting that methanol decomposition on the PtRu(111) surface is more possible than that on the pure Pt(111) surface. 3.1.2. CHxO (x = 0−3). On the PtRu(111) surface, methoxy (CH3O) prefers to adsorb at a Ru2-bridge site with the Ru−O bond lengths of 2.126 and 2.131 Å. The C−O axis is inclined 51.8° from the surface normal. This state gives an adsorption energy of −2.26 eV (see Table 1). Formaldehyde (CH2O) is apt to flat adsorb above a Ru2Pt-fcc site through the C and O atoms. The O atom bridges two Ru atoms (with dO−Ru of 2.165 and 2.167 Å), while the CH2 group binds to the neighboring Pt atom (with dC−Pt of 2.099 Å). The closed-shell configuration of formaldehyde predicts a relatively low adsorption energy (−1.13 eV). For comparison, the adsorption energies of CH3O and CH2O on Pt(111) are −1.54 and −0.50 eV, respectively.53 Similar to formaldehyde, the C−O axis of formyl (CHO) is also approximately parallel to the substrate surface. However, in this case, formyl prefers to adsorb above a Ru2-bridge site, forming simultaneously the C−Ru (1.956 Å) and O−Ru (2.167 Å) bonds. The alloying strengthens the adsorption of CHO with the adsorption energy of −2.70 eV, as compared to the values for pure Pt(111) (−2.36 eV) and Ru(0001) (−2.46 eV).53,56 Different from the hollow site adsorption on Pt(111)53 and Pd(111),69 carbon monoxide tends to upright adsorb via the C atom atop a Ru atom on PtRu(111); the C− Ru distance is 1.868 Å. The adsorption energy in this case (−2.10 eV) falls in between the values for Pt(111) (−1.82 eV) and Ru(0001) (−2.30 eV),53,56 similar to the situation of PtAu(111).48 In addition, Pt-top site adsorption of CO on PtRu(111) is found to give a much lower adsorption energy C


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Table 1. Adsorption Configurations, Adsorption Energies (in eV), and Structural Parameters (in Angstroms) for Intermediates Involved in CH3OH Oxidation on PtRu(111) species CH3OH* CH3O* CH2OH* CH2O* CHOH* CHO* COH* CO* CHOOH* CHOO* COOH* CO2* H2O* OH* H*

sitesa

configurationb

Eads

Ru-top Ru2-bridge Ru2-bridge Ru2Pt-fcc Ru2-bridge Ru2-bridge Ru2Pt-fcc Ru-top Ru2Pt-fcc Ru-top Ru2-bridge PtRu-bridge Ru-top Ru2-bridge Ru2Pt-fcc

η (O) η2(O) η1(C)-η1(O) η2(C)-η1(O) η2(C) η1(C)-η1(O) η3(C) η1(C) η1(O)-η1(C)-η1(O) η1(O) η1(C)-η1(O) η1(O)-η1(O) η1(O) η2(O) η3(H)

−0.70 −2.26 −2.14 −1.13 −3.58 −2.70 −4.75 −2.10 −0.54 −0.02 −0.88 −0.12 −0.70 −3.03 −2.86

1

dC−O 1.448 1.431 1.462 1.406 1.36 1.261 1.338 1.161 1.324, 1.318, 1.269, 1.172,

dO−Ru 2.314 2.126, 2.299 2.165, 2.114, 2.167 2.018,

1.499 1.206 1.336 1.176

dC−Ru/Pt

2.131 2.167 2.114 2.021

2.076, 2.201 2.022 2.167 3.266, 3.940c 2.326 2.138, 2.138

2.104d 2.099e 1.956d 2.005,d 1.995,d 2.180e 1.868d 2.144d 2.019d

Bridge, fcc, and top represent adsorption site. bηn(X) denotes that X atom interacts directly with n surface metal atoms. cThe distance of O−Pt bond. bThe distance of Ru−C bond. cThe distance of Pt−C bond. a

low adsorption energy, −0.12 eV, similar to the previous findings of CO2 on PtAu(111)48 and PdZn(111).46 3.1.5. H2O, OH, and H. Analogous with the theoretical result on PdZn(111),46 water is prone to flat adsorbing at a Ru-top site through the O atom (dO−Ru: 2.326 Å), with the adsorption energy of −0.70 eV. OH can adsorb stably at two different sites, i.e., the Ru-top site and the Ru2-bridge site. When it adsorbs at the Ru-top site, the O−Ru distance is calculated to be 1.949 Å and the H−O−Ru angle is 108.6°, in agreement with the DFT results for PtAu(111)48 and Pt(111).54 For the Ru2-bridge site adsorption (see Figure 1), both the O−Ru bonds are 2.138 Å. Similar bridge configurations have also been reported on Ru(0001) and Pd(111).56,74 Energetically, the Ru2-bridge adsorption (Eads = −3.03 eV) is more stable than the Ru-top adsorption (Eads = −2.76 eV), but is weaker than that on Ru(0001) (Eads = −3.29 eV).56 The H atom prefers a Ru2Pt-fcc site on PtRu(111), in accordance with the electron energy loss spectroscopy result.75,76 The corresponding H−Pt distance (1.834 Å) is slightly shorter than that of H−Ru (1.917 and 1.942 Å). The adsorption energy (−2.86 eV) of H on the alloy surface is lower than the theoretical value of −3.15 eV on Ru(0001),56 but is close to that of −2.58 eV on PdZn(111).46 3.2. Decomposition Reactions. For the methanol decomposition, the initial C−O bond scission is too hard to occur in general, while the O−H or C−H bond scission is favorable in different cases. For example, it proceeds preferably through the initial O−H bond scission on Ru(0001),57 Cu(111),57,77 and PtZn(111),46 but the C−H bond scission is favorable on Pt(111)54,57 and Pd(111).57,69 To confirm the reaction network, we describe all the possible elementary steps in the methanol decomposition via both the initial O−H and C−H bond scissions. The C−O pathway is not considered, because a much high energy barrier (2.00 eV) is calculated for its initial step. Generally, we choose the most stable sites of intermediates as the initial state (IS), and the corresponding product species in the most stable positions as the final state (FS). However, to facilitate the reaction on the complex PtRu alloy surface, we also use other substable forms at some point. For example, in some cases, the dissociated H atom is located at the substable Pt-top site as the final state of the dissociation step, considering the facile migration of atomic

(−1.47 eV), similar to the results reported by Ishikawa,44 Shubina,58 and co-workers. 3.1.3. CHxOH (x = 0−2). Hydroxymethyl (CH2OH) is formed by the C−H activation of methanol. Its most stable configuration is at a Ru2-bridge site through the C and O atoms, with dC−Ru of 2.104 Å and dO−Ru of 2.299 Å. The corresponding adsorption energy is calculated to be −2.14 eV. Although hydroxymethylene (CHOH) has a closed-shell electronic configuration, its CH end is still active. Thus, CHOH could bind stably at a Ru2-bridge site through its C atom (dC−Ru: 2.114 Å) with a very large adsorption energy of −3.58 eV. COH prefers both the Ru2Pt-fcc and the Ru2Pt-hcp sites through its carbon atom with the almost identical adsorption energies of −4.75 and −4.70 eV, respectively. Because the C atom has no H neighbor, the C−surface interaction is strong; thus, it is hard for COH to dehydrogenize or to be removed from the surface, similar to the situation on the Pt(111) surface.54 It is interesting to note that removal of one H atom from the CHx end of CHxOH results in the enhancement of the adsorption energy by 1.2−1.5 eV. In comparison, the adsorption energies are −1.98 eV (CH2OH), −3.24 eV (CHOH), and −4.45 eV (COH) on Pt(111),54 and −2.48 eV (CH2OH), −3.92 eV (CHOH), and −4.69 eV (COH) on Ru(0001).56 3.1.4. CHxOOHy (x, y = 0 and 1). Formic acid (CHOOH) adsorbs flat at a Ru2Pt-fcc site, with the C, O(hydroxyl), and O(carbonyl) atoms binding to the Pt, Ru, and Ru atoms, respectively. The distance is 2.144 Å for Pt−C, 2.201 Å for Ru− O(hydroxyl), and 2.076 Å for Ru−O (carbonyl) (see Figure 1). This configuration gives a very low adsorption energy (−0.54 eV). Formate (HCOO) has a monodentate “upright” adsorption configuration on PtRu(111) via an O atom atop a Ru site (dO−Ru: 2.022 Å), and the H atom points to the adjacent Pt atom. The adsorption energy is calculated to be −2.02 eV. Although COOH has two isomers, trans-COOH and cisCOOH,74 the trans-form favors further reaction and is considered here. It adsorbs stably at a Ru2-bridge site through its C and O(carbonyl) atoms (dC−Ru: 2.019 Å and dO−Ru: 2.167 Å), with the adsorption energy of −2.88 eV (see Table 1). Carbon dioxide (CO2) prefers a RuPt-bridge site thought two end O atoms (dO−Ru: 3.266 Å and dO−Pt: 3.940 Å), giving a very D


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respectively. In this process, the activated O−H distance is elongated from 0.985 Å in the IS to 1.894 Å in the TS, and 2.506 Å in the FS (Figure 2b). The reaction is calculated to be endothermic by 0.36 eV with an energy barrier of 0.63 eV. CH3O → CH2O + H. Subsequently, methyl H abstraction from methoxy could afford coadsorbed formaldehyde and hydrogen. In this process, the incline of the C−O bond brings the hydrogen atom close to the surface, favoring the C−H bond activation. The abstracted H transfers to a Pt-top site in the TS (dC−H = 2.386 Å), and then to a Ru2Pt-fcc site in the FS. Simultaneously, the C atom binds with the neighboring Pt, forming a Ru2Pt-fcc configuration of CH2O. The activation barrier for this reaction is 1.01 eV, and the reaction is exothermic by 0.18 eV. CH2O → CHO + H. Further decomposition of CH2O forms CHO and atomic H with an energy loss of 0.10 eV and an energy barrier of 0.38 eV. This process involves the rupture of one Ru−O bond, followed by the movement of the C atom toward the adjacent Ru atom, forming a Ru2Pt-fcc adsorption of CHO. Simultaneously, the corresponding C−H bond is activated, and the relevant H atom shifts to a Ru2Pt-hcp site. CHO → CO + H. A swag vibration in the adsorbed CHO makes O apart from the surface and the H atom binds to the surface atoms. In the TS, the Ru−O bond is broken with the distance of 2.818 Å and CHO forms a monodentate structure through the C atom. After the TS, the O atom moves to the top of C and the H atom departs from C and finally adsorbs stably at a Ru2Pt-fcc site in the FS. The energy barrier for this process is 0.37 eV, and the exothermic energy is 0.84 eV. Furthermore, to elucidate the influence of OH species on CHO decomposition, OH assisted C−H bond scission (CHO + OH → CO + H2O) was also calculated (see Figure S1 and Table S1). The corresponding energy barrier (0.63 eV) is calculated to be 0.26 eV higher than that of direct decomposition, suggesting that the OH group has a negative effect on CHO decomposition. 3.2.2. Initial C−H Bond Activation. This reaction pathway involves the initial C−H bond activation, followed by stepwise H-abstraction to form adsorbed CO and H. The calculated configurations and energies for the IS, TS, and FS are shown in Figure 3. CH3OH → CH2OH + H. The initial C−H bond activation on PtRu(111) starts with a rotation of the adsorbed methanol, such that the methyl group could move down to the neighboring Ru atom and one of the C−H bonds could be activated. In the TS, the activated C−H bond is elongated to 1.894 Å, while the C−O bond is shortened to 1.401 Å. After the TS, the atomic H shifts to a neighboring Ru2Pt-fcc site; the Ru−C distance is further shortened to 2.111 Å, forming a Ru2bridge configuration of CH2OH as the FS. This process is endothermic by 0.16 eV and experiences an energy barrier of 0.84 eV. This barrier is higher than that of the O−H scission, indicating on PtRu(111) that the O−H bond scission of CH3OH is more favorable. CH2OH → CHOH + H (CH2O + H). CH2OH could be followed by both C−H and O−H bond scissions, forming CHOH and CH2O, respectively. The CHOH channel involves simultaneous movements of CHOH and H toward adjacent Ru2-bridge sites. This process is exothermic by 0.15 eV, and the corresponding energy barrier is 0.45 eV. Alternatively, the O− H bond scission of CH2OH results in intermediate CH2O and H, both of which are located at Ru2Pt-fcc sites. The calculated energy barrier (0.61 eV) suggests that the O−H bond scission

H on the metal surface. The relevant thermodynamic and kinetic parameters are listed in Table 2. Table 2. Reaction Energies ΔE, Energy Barriers Ea (in eV), and Rate Constants k (in s−1) at 300 K for the Elementary Steps Involved in the Methanol Oxidation on PtRu(111) reactions

Ea

ΔE

CH3OH → CH3 + OH CH3OH →CH2OH + H CH3OH → CH3O + H CH3O → CH2O + H CH2O → CHO + H CHO → CO + H CH2OH → CHOH + H CH2OH → CH2O + H CHOH → COH + H CHOH → CHO + H COH → CO + H CHO + OH → HCOOH HCOOH → CHOO + H CHOO → CO2 + H HCOOH → COOH + H COOH → CO2 + H

2.00 0.84 0.63 1.01 0.38 0.37 0.45 0.61 0.58 0.74 2.39 0.03 1.03 0.19 0.58 0.97

−0.02 0.16 0.36 −0.18 −0.10 −0.84 −0.15 −0.20 −0.58 −0.42 −0.60 −0.26 0.36 −0.53 −0.37 0.22

k 9.65 3.11 8.94 1.05 3.78 1.35 1.22 2.66 5.11 1.79 1.15 2.66 8.63 1.64 1.61 2.38

× × × × × × × × × × × × × × × ×

10−17 10° 104 10−1 107 108 107 105 105 104 10−21 1011 10−1 1011 104 101

3.2.1. Initial O−H Bond Activation. This pathway involves the initial O−H bond activation, followed by sequential Habstraction to generate adsorbed CO and H. Calculated structures for the IS, TS, and FS are shown in Figure 2. CH3OH → CH3O + H. For the O−H bond scission, the IS is the Ru-top adsorbed methanol, while coadsorbed CH3O and H as the FS are located at the Ru2-bridge and adjacent Pt-top site,

Figure 2. Dehydrogenation of methanol via the initial C−O and O−H bond scissions on PtRu(111). Blue, cyan, gray, red, and white denote Pt, Ru, C, O, and H atoms, respectively. E


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H2O.43,78,79 In this section, we show the possible oxidation reactions in the methanol oxidation process. 3.3.1. Water Activation. Previous experimental studies have suggested that PtRu alloy can break the O−H bond of H2O more effectively than pure Pt, which is an important factor to enhance catalytic efficiency in PtRu DMFCs.43,78,79 In this study, H2O adsorbs stably at a Ru-top site in the IS. In the reaction process, an O−H bond is broken and the H is prone to migrate to the top site of adjacent Pt. As shown in Figure 4, the

Figure 4. Activation of water on PtRu(111). Blue, cyan, red, and white denote Pt, Ru, O, and H atoms, respectively.

O−H distance of H2O is elongated from 0.980 Å in the IS to 1.815 Å in the TS. Finally, the OH group adsorbs stably at a Ru2-bridge site and the H adsorbs at a Pt-top site. The reaction is endothermic by 0.30 eV with an energy barrier of 0.91 eV. We also calculated H2O activation on pure Pt(111). The corresponding energy barrier is 1.17 eV, and the reaction energy is 0.94 eV, suggesting that alloying with Ru favors H2O activation in both thermodynamics and kinetics. This is in agreement with that the addition of Ru into the Pt surface helps to lower the onset potential for the water activation.80 Alternatively, the OH group can also be obtained via the reaction between coadsorbed H2O and O, that is, H2O + O → 2OH.48 As shown in Figure 4, the H2O molecule adsorbs preferentially on a Ru-top site, and the O adsorbs at the nearest Ru2Pt-fcc site. The H2O activation occurs with a barrier of 0.41 eV, lower than that for the process on the PtAu(111) surface (0.47 eV).48 These facts suggest that it is easy to form the OH group on the PtRu(111) surface. 3.3.2. Oxidation Reactions. The oxidation step plays an important role in the CH3OH reaction on PtRu(111). It would transform carbonaceous intermediates into acids to promote the reaction. The calculated structures are shown in Figure 5. CHxO + OH → CHxOOH (x = 0−2). The reactions of intermediates CH2O, CHO, and CO with OH are considered to be the oxidation processes for PtRu DMFCs. Among them, CO reaction with OH is expected to be hard to occur, because the two possible outcomes COOH and CO2 + H are hindered by the energy barriers of 1.58 and 1.02 eV, respectively (see Figure S2 and Table S2). The energy barriers for the reactions of OH with CH2O and CHO are 0.53 and 0.03 eV, respectively, indicating that the CHO oxidation can occur spontaneously and is the most feasible channel. This process involves coadsorbed CHO at a PtRu-bridge site and OH at a Ru-top site as the IS. The distance between O(hydroxy) and C( formyl) atoms is shorten from 2.379 Å in the IS to 2.102 Å in the TS, and to 1.499 Å in the FS (Figure 5a). The exothermicity of this process is 0.26 eV. CHOOH → CHOO + H → CO2 + H. The O−H bond scission of CHOOH forms a monodentate CHOO on the

Figure 3. Dehydrogenation of methanol via the initial C−H bond scission on PtRu(111). Blue, cyan, gray, red, and white denote Pt, Ru, C, O, and H atoms, respectively.

of CH2OH is less favorable than the C−H bond scission, similar to the situation on Pt(111).54 CHOH → COH + H (CHO + H). CHOH also has two possible dehydrogenation channels (forming COH and CHO). For the COH channel, a swag vibration of the adsorbed CHOH makes the H atom depart from C to the adjacent Pt atom. In the TS, the activated C−H bond is obviously elongated to 1.388 Å, and the Pt−H distance is shortened to 1.388 Å. After the TS, atomic H moves toward the Ru2-bridge site and COH shifts to the Ru2Pt-fcc site, forming the FS (see Figure 3c). This reaction needs to overcome an energy barrier of 0.58 eV and is exothermic by 0.58 eV. Alternatively, the O−H bond scission in CHOH produces CHO at a Ru2-bridge site and atomic H at a Ru2Pt-hcp site. In the TS (see Figure 3c*), the O−H bond is elongated to 1.789 Å from 0.993 Å in the IS and the relevant H adsorbs at the top of adjacent Pt. This reaction accounts for an energy barrier of 0.74 eV and releases a heat of 0.42 eV. COH → CO + H. As discussed above, COH adsorbs much stably at a Ru2Pt-fcc site in the η3(C) configuration (Eads = −4.75 eV), suggesting that it is hard to react or dehydrogenate further. In the dehydrogenation process, the H atom in COH shifts to the top of the neighboring Pt; the corresponding CO moves toward the Ru-top site, forming an η1(C) configuration. The reaction is exothermic by 0.60 eV, but it is impeded by a very high energy barrier of 2.39 eV. 3.3. Water Activation and Oxidation Reactions. Many studies have reported that the intermediates of methanol can react with species OH, which is formed by the activation of F


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of 0.22 eV (exothermic). This process starts with COOH at the Ru2-bridge site (via the carbonyl O and C atoms) as the IS. In the TS, COOH remains at the Ru2-bridge site, while the O−H bond is elongated to 2.368 Å from 0.980 Å in the IS. Passing through the TS, CO2 arranges to a line structure located at the Ru2-bridge site, forming an η1(O)-η1(O) configuration, while the atomic H transfers to the adjacent Ru2Pt-fcc site.

4. DISCUSSION In this part, we first discuss the nature of adsorption on PtRu(111). Then, we analyze the factors influencing the reaction path. Potential energy surfaces are drawn and energy barrier contribution factors are listed to analyze the reaction mechanism. 4.1. Analysis of Adsorption Sites. As shown in Figure 1, CH3OH, CO, CHOO, and H2O groups prefer to adsorb at the Ru-top sites; CH3O, CH2OH, CHOH, CHO, COOH, and OH groups bind preferentially to the Ru2-bridge sites; and H, COH, CH2O, and CHOOH groups tend to adsorb at the Ru2Pthollow sites, suggesting that most of the intermediates are apt to adsorb around the Ru sites, rather than the Pt sites. This situation may be induced by the electronegativity of the metal atoms. It is well-known that the electronegativities of the Ru and Pt atoms are 2.20 and 2.28, respectively. Therefore, the Ru atoms in the PtRu alloy show some positive charges, while the Pt atoms are negatively charged. Mulliken analysis suggests that the Ru and Pt atoms on the PtRu (111) surface are populated by 0.058 and −0.143 |e|, respectively. Thus, compared to the Pt atom, Ru atom favors adsorbing the intermediates of radicals or having lone-pair electrons. The d-partial density of states (PDOS) of Pt(111), Ru(0001), and PtRu(111) is shown in Figure 6. Compared

Figure 5. Oxidation of CHO and its decomposition on PtRu(111). Blue, cyan, gray, red, and white denote Pt, Ru, C, O, and H atoms, respectively.

PtRu(111) surface. In the TS (Figure 5b), both the Ru− O(hydroxy) and Pt−C bonds are broken, and the O−H bond is elongated to 1.340 Å. Subsequently, CHOO undergoes an intrastructure reforming so as to form the η1(O) configuration at the Ru-top site and the H transfers to a Ru2Pt-hcp site simultaneously. This process has an energy barrier of 1.03 eV and a reaction energy of 0.36 eV. A swag vibration of the monodentate-adsorbed CHOO makes the H atom transfer from C to the adjacent Pt atom. In the TS, the breaking C−H bond is elongated to 1.551 Å. After the TS, CO2 moves to the PtRu-bridge site via two O atoms in a parallel manner, while the dissociated H is located at a Pt-top site. This step occurs with an energy barrier of 0.19 eV and releases heat by 0.53 eV. We also calculate the mode that formic acid generates the bidentate structure of CHOO (Figure S3), similar to the situation occurring on the Pd74 and PtAu48 surfaces. Although the formation of the bidentate CHOO has a low energy barrier of 0.21 eV, the subsequent CHOO dehydrogenation needs to overcome a high energy barrier of 1.20 eV, suggesting that this channel is less favorable than the monodentate CHOO channel. CHOOH → COOH + H → CO 2 + H. The other dehydrogenation pathway of formic acid is to form COOH via the C−H scission. In this process, the carboxyl hydrogen departs from C and then migrates to the top of adjacent Pt, and COOH arranges to the RuPt-bridge site simultaneously, forming a bidentate structure via the carbonyl O and C atoms (dO−Ru = 2.049 Å and dC−Pt = 2.220 Å) as the FS. In the TS, the Ru−OH bond is ruptured (dRu−OH = 3.337 Å) and the C−H bond is elongated (dC−H = 1.103 Å). The energy barrier for this process is 0.58 eV, and the reaction is exothermic by 0.37 eV. Once COOH formed, it could form CO2 through rupture of the O−H bond with a barrier of 0.97 eV and an energy change

Figure 6. d-partial density of states of Pt of the clean Pt(111) facet; PtRu, Pt, and Ru of the clean PtRu(111) facet; and Ru of the clean Pu(0001) facet. The Fermi energy is set as the energy zero. G


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The further dehydrogenation of CH3O proceeds via CH3O → CH2O → CHO → CO with the energy barriers of 1.01, 0.38, and 0.37 eV (Figure 7), respectively, indicating that the first C−H bond scission (CH3O → CH2O + H) is the ratedetermining step (k = 1.05 × 10−1 s−1 at 300 K). In order to further evaluate the effect of surface coverage, we also investigate CH3O decomposition into CO on a larger (4 × 4) PtRu(111) (see Figure S4 and Table S3). The energy barriers for CH3O → CH2O → CHO → CO on the (4 × 4) cell are calculated to be 0.90, 0.58, and 0.45 eV, respectively, showing the varieties of the energy barriers with the coverages. However, these changes do not change the CH3O decomposition pathway on PtRu(111) because, at these two coverages, the ordering of the relevant energy barriers is not changed. The CO oxidation by OH needs to overcome the high energy barriers of 1.58 and 1.02 eV to form COOH (k = 2.36 × 10−13 s−1) and CO2 + H (k = 1.07 × 10−2 s−1) (see Table S2), respectively. Alternatively, the CHO oxidation by OH could occur spontaneously to form CHOOH with a barrier of 0.02 eV, much lower than its OH-assisted decomposition, that is, CHO + OH → CO + H2O (Ea = 0.63 eV; see Table S1). The further dehydrogenation of CHOOH to CO2 has two pathways, i.e., CHOOH → COOH → CO2 (Ea = 0.97 eV) and CHOOH → CHOO → CO2 (Ea = 1.03 eV, Figure 8). The

to pure Pt(111), the d-bandwidth of PtRu(111) is broader and the number of the d-band peaks decreases, suggesting a stronger delocalizability of d electrons. Especially, the significant increase of the d-PDOSs for the PtRu(111) surface near the Fermi level favors the reaction activities. The d-band center is calculated to be −2.90 eV for Pt on pure Pt(111), −3.03 eV for Pt on PtRu(111), −2.18 eV for Ru on PtRu(111), and −1.95 eV for Ru on Ru(0001). After alloying Pt with Ru, the downward shifting of the d-band center of Pt accounts for the weaker adsorption (such as CO) at Pt sites than on pure Pt (111), while the higher d-band center of Ru predicts the stronger adsorption at Ru sites. 4.2. PESs and Reaction Mechanisms. 4.2.1. PESs. The detailed PESs for the methanol decomposition and oxidation on PtRu(111) are shown in Figures 7−9. The energy reference used in the figures corresponds to the total energy of one gaseous CH3OH molecule and the clean slab.

Figure 7. PES of dehydrogenation for methanol on PtRu(111). Energies (in eV) are relative to the total energy of one gaseous CH3OH molecule and the clean slab.

As shown in Figure 7, the PES of dehydrogenation for methanol is stated. The initial C−O bond scission pathway is too hard to occur due to a high energy barrier of 2.00 eV. Compared to the initial C−H bond scission path (Ea = 0.84 eV), the initial O−H bond scission to form CH3O has a lower barrier of 0.63 eV and, thus, is more favorable. Further calculations show that the rate constant of O−H bond activation (8.94 × 104 s−1 at 300 K, in Table 2) is much larger than that of the C−H bond activation (3.11 s−1). This situation is similar to the same reaction on PtAu(111) 48 and PdZn(111),46 but contrary to that on Pt(111), where the initial C−H bond scission of methanol is energetically preferable.54 A similar situation is also found on Ru(0001), where only methoxy species were observed in the temperature range from 180 to 340 K on the basis of experimental studies.81 By a DFT calculation, Garcι ́a-Muelas et al. suggested that the O−H bond breaking of methanol is more favored on Ru(0001) at high coverages.57 Furthermore, on pure Pt(111), the initial reaction energy barrier (0.67 eV) of CH3OH is much higher than the desorption energy (0.33 eV),54 suggesting that desorption rather than decomposition is more favorable. On PtRu(111), however, we found that the energy barrier of initial O−H bond scission (0.63 eV) is close to the desorption energy of CH3OH (0.70 eV), indicating that the PtRu surface is, to a large extent, more feasible for methanol reaction than the pure Pt surface.

Figure 8. PES of CHO oxidation and its decomposition. Energies (in eV) are relative to the total energy of one gaseous CH3OH molecule and the clean slab.

relevant rate constant of the rate-limiting step of the former (k = 2.38 × 101 s−1) is calculated to be about 28 times higher than that of the latter (k = 8.63 × 10−1 s−1), and about 222 times higher than that of CO oxidation (k = 1.07 × 10−2 s−1). In addition, for the CO and non-CO pathways, the transition states of the key steps, i.e., CO + OH → CO2 + H and CHOOH → COOH + H, are found to be located at −3.20 and −3.35 eV relative to the gas-phase CH3OH and clean slab, respectively, indicating that the energy barrier of the CO pathway is 0.15 eV higher than that of the non-CO pathway. All of these suggest that the reaction of CHO prefers to proceed via CHO → CHOOH → COOH → CO2 (non-CO pathway). It can be seen that the most favorable pathway of the whole reaction proceeds via CH3OH → CH3O → CH2O → CHO → CHOOH → COOH → CO2 (Figure 9) and the dehydrogenation of CH3O is the rate-determining step (Ea = 1.01 eV and k = 1.05 × 10−1 s−1), similar to the dehydrogenation of CH3OH H


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From the Table 3, we can see obviously that the substrate has a slight change in all reactions. For the three decomposition modes of CH3OH (O−H, C−H, and C−O bond scissions), the ISs are the same. The highest energy barrier for the C−O bond scission is due to a weak adsorption of the fragment CH3 in the TS (ETS A = 0.36 eV). For the O−H and C−H bond TS TS TS scissions, the differences of ΔEsub, ΔEdef AB + Eint , and EA + EB are calculated to be 0.08, −0.14, and 0.16 eV, respectively, suggesting that the interaction of intramethanol and adsorption energy of cracked fragments in the TS have nearly equal contributions to the lower energy barrier of the O−H bond scission. For the stepwise dehydrogenation of CH3O → CH2O → TS CHO → CO, the values of ETS A + EB are calculated to be 3.38, 3.57, and 3.75 eV, respectively. The slight fluctuation indicates that the influence of the adsorbed fragments on energy barriers is comparable. However, the dehydrogenation of CH3O experiences the largest structural deformation (ΔEdef AB = 1.62 eV) and accounts for the highest energy barrier. Compared to the decomposition of CHO to CO (CO path), the lower energy barrier of CHO oxidation (non-CO path) is due to the strong binding energy of CHO and OH in the TS TS (ETS A = 2.36 eV and EB = 2.63 eV). For the HCOOH dehydrogenation to CO2, the O−H bond scission is the ratedetermining step of both initial O−H and C−H bond scission pathways. As shown in Table 3, the high energy barrier of the reaction HCOOH → HCOO is mainly due to the strong interaction between HCOO and H in the TS (ETS int = 2.86 eV), while, for COOH → CO2+ H, it is because of the large adsorption energy of COOH in the IS. The analyses of the activation of H2O on Pt(111) and PtRu(111) are also performed. Compared to the pure Pt surface, the lower activation barrier on the PtRu surface is due to the stronger adsorption of H (ETS A : 2.66 eV (PtRu) vs 2.58 eV (Pt)) and especially OH (ETS : 2.78 vs 1.90 eV) in the TS. B 4.2.3. Electronic Structure Analysis. Figure 10 shows the Δd-PDOS of surface atoms, which is obtained by subtracting the d-PDOS of the clean surface from that of TS for the C−H, O−H, and C−O bond activations of CH3OH on PtRu(111).

Figure 9. Whole PES of methanol reaction on PtRu(111). The blue part is the dehydrogenation of CH3OH; the pink part is the oxidation of CHO to HCOOH; and the red purple part is the decomposition of HCOOH. Energies (in eV) are relative to the total energy of one gaseous CH3OH molecule and the clean slab.

on Ru(0001), where the dehydrogenation of methoxy to formaldehyde is the highest energy demand step.57 After the dehydrogenation of CH3O, the subsequent reactions are much faster and do not result in accumulation of intermediates. 4.2.2. Energy Barrier Analysis. The energy barrier analysis is presented to deeply understand the energy barriers in the reaction of methanol on PtRu(111). The energy barrier Ea is decomposed based on the following equation82−84 def IS TS Ea = ΔEsub + ΔEAB + EAB + E int − EATS − E BTS

(3)

where ΔEsub is the effect of the structural change of the IS substrate from IS to TS on the barrier (ΔEsub = ETS sub − Esub); def ΔEAB is the deformation energy, which refers to the influence of the structural deformation of AB on Ea; EIS AB is the adsorption energy of AB in the IS; and ETS int is the interaction between A TS and B in the TS. ETS A (EB ) is the binding energy of A(B) at the TS geometry without B(A). All the relevant data of methanol reactions on PtRu(111) are listed in Table 3.

Table 3. Energy Barrier and Contribution Factors (in eV) for the Methanol Oxidation on PtRu(111) reactions

ΔEsub

ΔEdef AB

EIS AB

ETS int

ETS A

ETS B

Ea

CH3OH → CH3 + OH CH3OH → CH2OH + H CH3OH → CH3O + H CH3O → CH2O + H CH2O → CHO + H CHO → CO + H CH2OH → CHOH + H CH2OH → CH2O + H CHOH → COH + H CHOH → CHO + H COH → CO + H CHO + OH → HCOOH HCOOH → CHOO + H CHOO → CO2 + H HCOOH → COOH + H COOH → CO2 + H H2O → OH + H H2O → OH + H (on Pt(111))

0.06 0.13 0.21 0.03 0.02 0.09 −0.03 0.14 −0.03 0.04 0.00 −0.01 0.00 0.04 0.02 0.11 0.15 0.16

3.42 1.15 3.91 1.62 −0.17 −0.15 0.23 2.48 0.52 1.78 −0.26 −0.32 0.07 0.52 −0.36 0.94 4.01 2.88

0.63 0.63 0.63 2.46 2.80 3.05 2.48 2.48 3.64 3.64 4.87 4.62 2.50 2.22 2.50 3.03 0.69 0.40

1.00 3.71 0.81 0.29 1.31 1.13 3.54 0.24 3.60 0.11 −0.07 1.00 2.86 0.46 1.64 0.14 1.50 2.24

0.36 2.18 2.24 0.67 2.42 1.92 3.37 2.08 4.45 2.14 1.86 2.63 1.94 0.45 2.27 0.47 2.78 1.90

2.72 2.59 2.69 2.72 1.15 1.83 2.40 2.66 2.69 2.70 0.30 2.63 2.46 2.60 0.95 2.79 2.66 2.58

2.00 0.84 0.63 1.01 0.38 0.37 0.46 0.61 0.58 0.74 2.39 0.03 1.03 0.19 0.58 0.97 0.91 1.20

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03951. Illustrations of configurations and tables of reaction energies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.Z.). *E-mail: [email protected] (W.G.). Notes

The authors declare no competing financial interest.

■

Figure 10. Variation of the d-projected density of states of the surface metal atoms due to their bonding with the TS complex in the initial C−H, O−H, and C−O bond scissions of methanol decomposition on PtRu(111).

ACKNOWLEDGMENTS This work was supported by the Program for NSFC (21003158), the Shandong Province Natural Science Foundation (ZR2015BQ009), the Taishan Scholar Foundation (ts20130929), the Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province (BS2012NJ015), and the Fundamental Research Funds for the Central Universities (12CX02014A and 15CX08010A).

The stabilization energy (Ed) of the d-states is done using the following formula84,85 EF

Ed =

∫ −∞

■

ε(ndTS − ndbare)dε (4)

REFERENCES

(1) Rabis, A.; Rodriguez, P.; Schmidt, T. J. Electrocatalysis for Polymer Electrolyte Fuel Cells: Recent Achievements and Future Challenges. ACS Catal. 2012, 2, 864−890. (2) Kamarudin, S. K.; Daud, W. R. W.; Ho, S. L.; Hasran, U. A. Overview on the Challenges and Developments of Micro-Direct Methanol Fuel Cells (DMFC). J. Power Sources 2007, 163, 743−754. (3) Kamarudin, S. K.; Achmad, F.; Daud, W. R. W. Overview on the Application of Direct Methanol Fuel Cell (DMFC) for Portable Electronic Devices. Int. J. Hydrogen Energy 2009, 34, 6902−6916. (4) Zhou, X. W.; Zhang, R. H.; Zhou, Z. Y.; Sun, S. G. Preparation of PtNi Hollow Nanospheres for the Electrocatalytic Oxidation of Methanol. J. Power Sources 2011, 196, 5844−5848. (5) Liu, H. X.; Tian, N.; Brandon, M. P.; Zhou, Z. Y.; Lin, J. L.; Hardacre, C.; Lin, W. F.; Sun, S. G. Tetrahexahedral Pt Nanocrystal Catalysts Decorated with Ru Adatoms and their Enhanced Activity in Methanol Electrooxidation. ACS Catal. 2012, 2, 708−715. (6) Nishanth, K.; Sridhar, P.; Pitchumani, S.; Shukla, A. Durable Transition-Metal-Carbide-Supported Pt−Ru Anodes for Direct Methanol Fuel Cells. Fuel Cells 2012, 12, 146−152. (7) Jin, X.; He, B.; Miao, J.; Zhang, Q.; Niu, L.; yuan, J. Stabilization and Dispersion of PtRu and Pt Nanoparticles on Multiwalled Carbon Nanotubes Using Phosphomolybdic Acid, and the Use of the Resulting Materials in a Direct Methanol Fuel Cell. Carbon 2012, 50, 3083−3091. (8) Harish, S.; Baranton, S.; Coutanceau, C.; Joseph, J. Microwave Assisted Polyol Method for the Preparation of Pt/C, Ru/C and PtRu/ C Nanoparticles and Its Application in Electrooxidation of Methanol. J. Power Sources 2012, 214, 33−39. (9) An, X. S.; Fan, Y. J.; Chen, D. J.; Wang, Q.; Zhou, Z. Y.; Sun, S. G. Enhanced Activity of Rare Earth Doped PtRu/C Catalysts for Methanol Electro-Oxidation. Electrochim. Acta 2011, 56, 8912−8918. (10) La-Torre-Riveros, L.; Guzman-Blas, R.; Méndez-Torres, A. n. E.; Prelas, M.; Tryk, D. A.; Cabrera, C. R. Diamond Nanoparticles as a Support for Pt and PtRu Catalysts for Direct Methanol Fuel Cells. ACS Appl. Mater. Interfaces 2012, 4, 1134−1147. (11) Choi, J. H.; Jeong, K.-J.; Dong, Y.; Han, J.; Lim, T. H.; Lee, J. S.; Sung, Y. E. Electro-Oxidation of Methanol and Formic Acid on PtRu and PtAu for Direct Liquid Fuel Cells. J. Power Sources 2006, 163, 71− 75. (12) Choi, J. H.; Park, K.-W.; Park, I. S.; Kim, K.; Lee, J.-S.; Sung, Y. E. A Ptau Nanoparticle Electrocatalyst for Methanol Electro-Oxidation

where nd is the normalized density of states of metal atoms (in electrons eV−1) with and without bonds to the adsorbates of TS, and ε is the energy level. The Ed for the C−O, C−H, and O−H bond scissions on PtRu(111) is found to be −0.50, −2.41, and −5.00 eV, respectively. This suggests that, among the three bond scissions, the O−H bond scission has the strongest interaction between the fragment and surface, resulting in the lowest activation energy of O−H bond scission.

5. CONCLUSIONS In the present study, methanol oxidation on PtRu(111) has been explored using the self-consistent periodic DFT method. We now have several useful conclusions. The Pt and Ru atoms on the PtRu(111) surface display some positive and negative charges, respectively, and thus, most of the intermediates are apt to adsorb around the Ru sites. Alloying Pt with Ru weakens the bond of CO to the Pt sites, but strengthens the bond of CO to the Ru sites. The initial dehydrogenation of CH3OH prefers the O−H bond scission rather than the C−H and C−O bond scissions. The reaction proceeds preferentially along the non-CO pathway from intermediate CHO. The most possible pathway thus occurs via CH3OH → CH3O → CH2O → CHO → CHOOH → COOH → CO2, in which dehydrogenation of CH3O is the rate-limiting step, and thus, the subsequent steps occur relatively fast without accumulation of intermediates. Furthermore, the H2O activation on the PtRu(111) surface is more thermodynamically and kinetically favorable than that on the pure Pt(111) surface. The enhancement of catalytic activity of the PtRu alloy toward methanol oxidation can be attributed mainly to altering the major oxidation pathways to the non-CO path from the CO path on pure Pt as well as promoting adsorption of methanol and formation of active OH species from H2O. Our study has revealed, to some extent, the mechanism of methanol oxidation on the PtRu(111) surface. This is necessary for people to understand profoundly the PtRu DMFCs and to improve the DMFCs performance. J


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The Oxidation of Methanol on PtRu(111): A Periodic Density

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