Pt(111): A Density Functional Study

Applied Surface Science 369 (2016) 257–266

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Insight into the Reaction Mechanisms of Methanol on PtRu/Pt(111): A Density Functional Study Qiuyue Ding a,b , Wenbin Xu a,b , Pengpeng Sang a,b , Jing Xu a,b , Lianming Zhao a,b,∗ , Xiaoli He a,b , Wenyue Guo a,b,∗∗ a

College of Science, China University of Petroleum, Qingdao, Shandong 266580, PR China Key Laboratory of New Energy Physics & Materials Science in Universities of Shandong, China University of Petroleum, Qingdao, Shandong 266580, PR China b

a r t i c l e

i n f o

Article history: Received 8 July 2015 Received in revised form 21 October 2015 Accepted 10 November 2015 Available online 1 December 2015 Keywords: Reaction mechanism Methanol oxidation PtRu alloy Surface modification

a b s t r a c t Periodic DFT calculations have been performed to systematically investigate the mechanisms of methanol decomposition and oxidation on the PtRu/Pt(111) surface. Geometries and energies for the primary species involved are analyzed and the reaction network has been mapped out. The calculation shows that among three initial C H, O H, and C O bond scissions of methanol, the O H bond scission is found to be the most favorable and bears a lower energy barrier than the desorption of methanol. The decomposition of CH3 O occurs via the path CH3 O → CH2 O → CHO → CO with the limiting step of the first dehydrogenation. Although the oxidation of CO is hindered by a high barrier, the CHO oxidation to CHOOH could occur facilely. Further decomposition of formic acid to CO2 and/or CO could occur via four possible pathways, that is, initial C H, O H, and C O bond activations as well as simultaneous activation of C H and C O bonds, where the first pathway, HCOOH → COOH → CO2 , is the most favorable from a kinetic point of view. Compared to that on Pt(111), methanol on PtRu/Pt(111) prefers to decomposition rather than desorption and then oxidation via the favorable non-CO path with a lower rate-determining energy barrier of CH3 O → CH2 O for the whole reaction, which indicates that PtRu alloy can improved tolerance toward CO poisoning compared with pure Pt. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Global energy shortage and environmental pollution problems promote the demand for power sources with superior performance, which leads to the rapid growth of the portable electronics market [1,2]. Direct methanol fuel cells (DMFCs), as a viable power sources with potential applications in many systems, have attracted extensive attention due to their advantages such as high energy density and efficiency, low weight, fast recharge-time, and applications to portable systems. In the past several decades, platinum-based catalysts have obtained extensive attention as anode catalysts in the low temperature fuel cells [3–5], especially, DMFCs. Pt is known as the most excellent monometallic electrocatalyst for methanol oxidation [6,7], but active platinum catalyst sites are easily poisoned

∗ Corresponding author at: College of Science, China University of Petroleum, Qingdao, Shandong 266580, PR China. Tel.: +86 53286983372. ∗∗ Corresponding author at: College of Science, China University of Petroleum, Qingdao, Shandong 266580, PR China. E-mail addresses: [email protected] (L. Zhao), [email protected] (W. Guo). http://dx.doi.org/10.1016/j.apsusc.2015.11.104 0169-4332/© 2015 Elsevier B.V. All rights reserved.

by CO-like species [8–11]. An efficient way to improve catalytic activity is to modify Pt surfaces with a second transition metal or noble metal. Methanol oxidation has been extensively studied on various binary platinum alloy catalysts, such as PtMn [12], PtAu [13–15], PtFe [16,17], PtNi [18–20], PtSn [21,22], PtPd [23], PtCo [24,25], PtAg [26,27], PtMo [28], and PtRu [29–33]. Among those alloys, PtRu has shown the highest catalytic activity and been frequently used as an anode material in DMFCs due to its increased CO tolerance [34–36]. Although addition of Ru to Pt increases the “CO-tolerance” of platinum in DMFCs, the origin of such an enhancement still remains an incompletely resolved issue [37]. Within the ligand effect [38,39] approximation, the addition of Ru to Pt modifies the electronic structure of Pt, and thus weakens the adsorption of CO to Pt. The solid-state electrochemical nuclear magnetic resonance (EC NMR) study has found a role for Ru in weakening the Pt CO bond for CO on Pt, thereby increasing the CO oxidation rate [39]. On the other hand, the “bi-functional mechanism” [30,35] proposed that Ru provides an oxygenated surface species by dissociating water at the Ru sites at lower potentials than pure Pt sites, leading to accelerated conversion of CO to CO2 .

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Two principal models of bimetallic Pt–Ru electrodes, i.e., PtRu alloy and Ru-decorated Pt electrodes, are used to investigate the origin of the Ru enhancement [40]. In principle, the structures of two models are different considerably. However, to understand the reaction mechanisms of fuels on Pt–Ru electrodes in general, it is very essential to investigate the electrocatalytic activity of the Pt surfaces decorated with Ru adatoms [37]. Indeed, by using an EC NMR technique, Babu and coworkers found that Ru atoms in the PtRu alloy are enriched in the nanoparticle surface, yielding PtRu layers, and introduction of Ru to Pt nanoparticle catalyst by alloying and surface deposition has the same effect under fuel cell operation conditions [41,42]. Watanabe and Motoo have prepared polycrystalline Pt modified by Ru adatoms and created a new term: “electrocatalysis by adatoms”, which inspires many researchers to decorate Pt electrodes with foreign metal adatoms [43]. Wei et al. investigated the electrochemical oxidation of methanol on underpotentially deposited ruthenium-modified platinum electrode (upd-Ru/Pt), and found that the submonolayers of upd-Ru on a Pt electrode enhance the rate of methanol oxidation several times compared to that on a pure Pt electrode [44]. Chrzanowski and Wieckowski investigated electrochemical properties of Ru (up to ca. 0.4 monolayer) on the Pt(100), Pt(111), and Pt(110) single crystal surfaces [45]. It was suggested that the Ru/Pt(111) catalyst may be capable of an enhanced CO removal at the low potentials as compared to other surfaces [45]. Although a lot of papers claimed that a PtRu alloy (50:50) is the material with the highest catalytic activity for CO electrooxidation [46], the electrocatalytic activity of the Ru/Pt(111) catalyst toward CO oxidation was found to be higher than that of pure Ru and PtRu alloy (50:50) by Lin et al. [47]. Despite a lot of studies concerning the methanol reaction on Pt–Ru binary catalysts, the detailed potential energy surface (PES) of methanol oxidation on Pt–Ru bimetallic surface is still unclear. This seriously restricts the understanding of PtRu catalytic mechanism, especially clarifying the effect of addition of Ru to Pt. All these facts motivate us to perform periodic density functional theory (DFT) investigation on the methanol oxidation on the PtRu/Pt(111) surface. In this work, the adsorption and reaction of CH3 OH on PtRu/Pt(111) are examined. This includes an intact illustration of all possible adsorption of intermediates and reaction pathways, especially, a comparison between the non-CO and CO pathways. We expect this work could elucidate the CO-tolerant ability of PtRu/Pt(111) and offer guidance toward the identification of improved DMFC anode catalysts.

2. Computational details The periodic DFT calculations were carried out by using DMol3 package in Materials Studio of Accelrys Inc [48–50]. The exchangecorrelation energy was calculated with the generalized gradient approximation (GGA) using the form of the functional proposed by Perdew and Wang (PW91) [51,52], which has shown a good performance on description of the PES features of CH3 OH reaction on Ru(0001), Pt(111), PdZn(111), and PtAu(111) [53–56]. It is known that the contributions of dispersion term are not included in the PW91 calculations. However, the van der Waals dispersion contribution was found to be a small term for the reaction of CH3 OH on Ru(0001) and Pt(111) (0.01–0.20 eV) [57]. Furthermore, our main goal in this work is to describe the interesting properties of the PESs rather than to give accurate binding energies. The underestimation of the total energies is a systematic shortcoming of the PW91 functional, therefore, the relative energies of the PESs should be more correct. We chose the density functional semicore pseudopotential (DSPP) [58] to describe the ion cores of metal atoms and the

double-numerical basis with polarization functions (DNP) to represent the valence electron functions. To improve the performance of calculations, a Fermi smearing of 0.007 Ha and a real-space cutoff of 4.7 A˚ were employed. All computations were performed with spin-polarization. The tolerances of energy, gradient, and displace˚ ment convergence were 1 × 10−5 Ha, 2 × 10−3 Ha/Å, and 5 × 10−3 A, respectively. The PtRu/Pt(111) surface was modeled using a periodic fourlayer slab with a p(3 × 3) unit cell, while the Pt atoms at the top layer were adopted by Ru atoms with the ratio of 2:1 and three bottom layers remain unchanged. Periodic images of the slab were separated by a 15 A˚ vacuum gap. The (5 × 5 × 1) Monkhorst–Pack k-point grid sampling in the reciprocal space was employed in the calculations [59]. Only one side of the unit cells was adsorbed by an adsorbate. Correspondingly, the surface coverage was calculated to be 1/9 ML. In the full-geometry optimization, the atoms in the two topmost layers were relaxed, while those in the two bottom layers were fixed to the bulk distances. The adsorption energy Eads was calculated using the equation: Eads = Eadsorbate/sub − (Eadsorbate + Esub )

(1)

where, Eadsorbate is the total energy of the adsorbate, Esub is the total energy of the clean PtRu/Pt(111) slab, and Eadsorbate/sub is the total energy of the adsorbate on PtRu/Pt(111). By this definition, a negative Eads implies a stable adsorption. Transition state searches were performed at the same theoretical level with the complete Linear Synchronous Transit/Quadratic Synchronous Transit (LST/QST) method implemented in DMol3 [60]. We applied the transition state theory formalism to predict rate constant (k) for all the elementary steps involved [61]: k=

kB T QTS exp  QIS

 −E  a

RT



= A0 exp

−Ea0 RT



(2)

where, kB , , R, A0 , and T are the Boltzmann constant, the Planck’s constant, the gas constant, the pre-exponential, and the temperature of 300 K, respectively [62]; QIS and QTS are the partition functions at the initial state and transition state, respectively; Ea0 and Ea are energy barriers with and without zero point energy (ZPE) corrections, respectively. The calculated barrier Ea can be decomposed using the following equation [63–65]: def IS TS Ea = Esub + EAB + EAB + Eint − EATS − EBTS

where, Esub represents the influence of the change of the subfrominitial state to transition state on the barrier strate structure TS − E IS def means the influence of the structural Esub = Esub ; EAB sub IS , deformation of AB on the barrier (i.e. the deformation energy); EAB





TS , and E TS E TS are the adsorption energy of AB at the initial Eint B A state, the interaction between A and B in the transition state, and the binding energy of A(B) in the transition state geometry without B(A), respectively. The stabilization energy (Ed ) of the d-states was computed as follows [65,66]:



EF

Ed = −∞

ε(nTS − nbare )dε d d

where, nd is the normalized density of states of Pt or Ru (electron/eV) with and without bonds to the adsorbates in the transition state, and ε is the energy level. To clarify the mechanism of low-temperature DMFC, the reaction of methanol in the gas phase was generally used as a prototype reaction, because the actual DMFC systems are too complex [52–54,57]. Therefore, the gas phase model was used in our calculations to shed light on the reaction of CH3 OH on PtRu/Pt(111).

Q. Ding et al. / Applied Surface Science 369 (2016) 257–266

3. Results and discussion In this section, we first describe the adsorption geometries and energies of the important intermediates involved in CH3 OH oxidation on PtRu/Pt(111). Then, we discuss all possible elementary reaction steps in detail. Finally, we analyze the calculation result and give the most likely pathway. Values of all energies are reported with the zero point energy (ZPE) corrections unless otherwise stated. 3.1. Adsorption Here, we discuss the optimized structures and energies of various intermediates along the methanol oxidation reaction on PtRu/Pt(111) surface, including CH3 OH, CH3 O, CH2 OH, CH2 O, CHO, HCOOH, HCOO, COOH, CO, CO2 , H2 O, OH, and atomic H. The calculated adsorption energies and structural parameters of the intermediates are listed in Table 1, and the corresponding configurations are shown in Fig. 1. 3.1.1. CHx OH (x = 2–3) Methanol is stably adsorbed at the top site of Ru with the O Ru distance of 2.234 A˚ (Table 1), which is similar to the situation on Pd(111) [67], Pt(111) [55], and Ni(111) [76], but different with that on Ru (0001), where CH3 OH prefers to adsorb at the Ru2 bridge site [56]. The adsorption energy of methanol on PtRu/Pt(111) is calculated to be −0.82 eV, which is much larger than previously reported values on Pt(111) (0.33 eV) [55], Pd(111) (−0.31 to −0.39 eV) [67–70], and Ru (0001) (−0.78 eV) [56], suggesting it is more favorable to further dissociation on PtRu/Pt(111) than pure Pt(111). It is generally believed that methanol adsorbs via donation of the lone pair of oxygen to metallic surfaces [71–73]. Our result shows that the C O axis of adsorbed methanol is tilted relative to the normal of the substrate by about 33◦ , supporting this point. CH2 OH is an important intermediate in methanol decomposition and synthesis [74,75]. Different with the adsorption at top site on Pd(111) [67], Pt(111) [55] and Ni(111) [76] and fcc site on Ru (0001) [56], CH2 OH on PtRu/Pt(111) favors the ␩1 (C)–␩1 (O) configuration at PtRu bridge site (see Fig. 1). The chemisorption energy of the hydroxymethyl intermediate is calculated to be −2.39 eV, which is larger than the values of −1.98 eV on Pt(111) [55], and −2.31 eV on Pt(110) [77], but less than that of −2.48 eV on Ru (0001) [56].

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3.1.2. CHx O (x = 1–3) CH3 O from the O H bond scission of CH3 OH prefers to adsorb at the Ru2 bridge site through O atom with the O Ru bond lengths ˚ respectively. The corresponding adsorption of 2.104 A˚ and 2.105 A, energy of CH3 O is −2.43 eV. The bridge site favors to some extent the tilt of the C–O bond toward the PtRu/Pt(111) surface. Similar to the Ir(111) [78] surface, CH2 O adsorbs at the hcp site in the ␩1 (C)–␩2 (O) mode with the C atom binding at the Pt top site and the O atom binding at the Ru2 bridge site (see Fig. 1). The relative C O bond length is 1.401 A˚ and two Ru O bond lengths are ˚ respectively. The adsorption energy of CH2 O is 2.165 and 2.166 A, calculated to be −1.21 eV. CHO has been proposed as an intermediate in methanol dehydrogenation on PtRu surface by Palenzuela [79]. In our calculation, CHO is apt to adsorb at Ru2 bridge site with an adsorption energy of −2.81 eV, which is larger than the theoretical values of −2.36 eV on Pt(111), −2.46 eV on Ru (0001) [56], and −2.50 eV on Pd(111) [80]. Our calculated result shows that the C O bond tends to be parallel to the PtRu/Pt(111) sur˚ face with the C Ru and O Ru bond lengths of 1.960 A˚ and 2.145 A, respectively.

3.1.3. Hx COOHy (x, y = 0 and 1) HCOOH has two isomers, i.e., trans- and cis-HCOOH. Because the free trans-HCOOH in gas phase is more stable than the cis-species [80], only trans-HCOOH is considered here. Two initial adsorption modes of trans-HCOOH (see Fig. 1) are investigated, which are denoted as HCOOHa and HCOOHb , respectively. Both of them adsorb stably at the Ru top site through the carbonyl-O atom with the O Ru distance of 2.154 (HCOOHa ) and 2.207 (HCOOHb ) Å. For HCOOHa (HCOOHb ), the H atom of OH (CH) group is close to the substrate, which is in favor of the further O H (O H) bond activation to generate HCOO (COOH) species. The adsorption energy of HCOOHa (−0.88 eV) is 0.27 eV larger than that of HCOOHb . A similar situation is also found for adsorption of formic acid on Pd(111) [80]. HCOO is formed via the O H bond scission of HCOOHa . It prefers to adsorb at the Ru2 bridge site through two O atoms with the ˚ respectively. The lengths of two O Ru distance of 2.095 and 2.096 A, ˚ and fall in between two bonds C O bonds are the same (1.269 A), in HCOOHa . The adsorption energy is calculated to be −3.29 eV, which is larger than the theoretical values of −3.03 eV on Ru (0001) [56].

Table 1 Adsorption configurations, adsorption energies (in eV), and structural parameters (in Å) for intermediates involved in CH3 OH decomposing on PtRu/Pt(111) surface. Species CH3 OH* CH2 OH* CH3 O* CH2 O* CHO* HCOOH*a HCOOH*b HCOO* COOH* H2 O CO2 * CO* OH* H* a b c d e f g

Sitesa

Configurationb

Eads

Ru-top PtRu-bridge Ru2 -bridge Ru2 Pt-hcp Ru2 -bridge Ru-top Ru-top Ru2 -bridge Ru2 -bridge Ru-top PtRu-bridge Ru2 -bridge Ru2 -bridge Ru2 Pt-fcc

␩ (O) ␩1 (C)–␩1 (O) ␩2 (O) ␩1 (C)–␩2 (O) ␩1 (C)–␩1 (O) ␩1 (O) ␩1 (O) ␩1 (O)–␩1 (O) ␩1 (C)–␩1 (O) ␩1 (O) ␩1 (C)–␩1 (O) ␩2 (C) ␩2 (O) ␩3 (H)

−0.82 −2.39 −2.43 −1.21 −2.81 −0.88 −0.61 −3.30 −2.87 −0.80 −0.11 −1.88 −3.16 −2.86

1

Bridge, fcc, hcp, and top represent adsorption site. ␩n (X) denotes that X atom interacts directly with n surface metal atoms. The distance of O Pt bond. The distance of Ru C bond. The distance of Pt C bond. The distance of Ru H bond. The distance of Pt H bond.

dC–O 1.453 1.460 1.437 1.401 1.261 1.304, 1.242 1.328, 1.232 1.269, 1.269 1.334, 1.268 1.178, 1.169 1.183

dO–Ru 2.234 2.263 2.104, 2.105 2.165, 2.166 2.145 2.154 2.207 2.096, 2.095 2.136 2.269 2.750, 4.026c 2.121, 2.126

dC–Ru/Pt d

3.227 2.068e 3.154d 2.087e 1.960d 3.139d 3.095d 2.977d 2.016d

dH–Ru/Pt 2.565f 2.571f 2.674g 2.650g 2.696f 2.154g 2.841g 3.985f 2.863f 2.606f

3.735e 2.056d , 2.071d 2.598f 1.920f , 1.938f , 1.828g

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3.1.4. COx (x = 1 and 2) As shown in Fig. 1, CO2 interacts weakly with the PtRu/Pt(111) surface (Ea = −0.11 eV), agreeing with the experimental report that CO2 prefers to remain as a free species rather than adsorb on the metallic surface [81]. In the stable configuration, CO2 lies above the PtRu bridge site with C O bond lengths of 1.178 A˚ and ˚ respectively. CO prefers the Ru2 bridge adsorption site on 1.169 A, PtRu/Pt(111) with adsorption energy of −1.88 eV, which is less than that on Ru (0001) (−2.30 eV) [56]. It tends to coordinate to the surfaces through C atom, and the C O bond is vertical to the substrate ˚ respectively. with the C Ru bond lengths of 2.056 A˚ and 2.071 A, In addition, the adsorption energy of CO at the Pt top site on PtRu/Pt(111) is calculated to be −1.60 eV, which is less than that on Pt(111) (−1.81 eV) [82] and PtAu(111) (−1.86 eV) [83]. 3.1.5. Hx O (x = 1 and 2) H2 O prefers to adsorb at the Ru top site through the O atom and the H O H skeleton is almost parallel to the substrate (see Fig. 1). ˚ and corresponding The O Ru distance is calculated to be 2.269 A, adsorption energy is −0.80 eV, which is larger than the calculated values of −0.25 and −0.23 eV on PdZn(111) and PdZn(100) [84], respectively. Hydroxyl (OH) tends to adsorb at the Ru2 bridge site ˚ respectively. The with the O Ru distance of 2.121 A˚ and 2.126 A, stable configuration has an adsorption energy of −3.16 eV, which is close to the value of −3.14 eV on PdZn(111) [84], less than that on Ru (0001) (−3.29 eV) [56], but larger than that on Pt(111) (−2.56 eV) [82], Pt(221) (−2.45 eV) [82], and PtAu(111) (−1.16 eV) [83]. 3.1.6. Atomic H The most favorable site for H atoms is the Ru2 Pt fcc 3-fold hollow sites on PtRu/Pt(111), similar to Pd(111) and Ru (0001) [56,80]. The lengths of two H Ru bonds and one H Pt bond are ˚ and 1.828 A, ˚ respectively. The calculated adsorp˚ 1.938 A, 1.920 A, tion energy of −2.86 eV falls in between −2.71 eV on Pt(111) and −2.90/−3.15 eV on Ru(0001) [56,85]. From Fig. 1, we can find that the adsorbates prefer to adsorb at the Ru top sites for CH3 OH, HCOOH, and H2 O, the PtRu bridge sites for CH2 OH, the Ru2 bridge sites for CH3 O, CHO, HCOO, COOH, CO, and OH, the Ru2 Pt hcp sites for CH2 O, and Ru2 Pt fcc sites for atomic H, suggesting that most of the adsorbates are apt to adsorb around the Ru sites, rather than the Pt sites. Mulliken analysis suggests that the Ru and Pt atoms in the PtRu/Pt(111) surface are populated by about 0.104 and −0.080 |e|, respectively, indicating a part of electrons transfer from Ru to Pt in PtRu alloys. Therefore, compared to the negative Pt atom, the positive Ru atom is more favorable to adsorption of the species with radical and/or lone pair electrons. Furthermore, the d-band center is calculated to be −2.44 eV for Pt in pure Pt(111), −2.51 eV for Pt in PtRu/Pt(111), −1.86 eV for Ru in PtRu/Pt(111), and −1.95 eV for Ru in Ru(0001). This suggests a higher d-band center of Ru than Pt in PtRu/Pt(111) and a shifting down of d-band center of Pt in PtRu/Pt(111) compared to pure Pt(111), which accounts for a stronger adsorption at Ru site than Pt site on PtRu/Pt(111) and a weaker adsorption (such as CO) at Pt site on PtRu/Pt(111) than pure Pt (111). Fig. 1. The most stable adsorption configurations of intermediates involved in methanol reaction on PtRu/Pt(111) surface. Blue, cyan, gray, red, and white denoted Pt, Ru, C, O, and H atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

HCOOHb could decompose into COOH via the C H bond scission. The most stable configuration for COOH is the adsorption at the Ru2 bridge site with C Ru and O Ru bond lengths of 2.016 ˚ respectively. The distance of two C O bonds is 1.334 A˚ and 2.136 A, ˚ respectively. The stable configuration has adsorption and 1.268 A, energy of −2.86 eV, which is slightly less than the theoretical values of −2.95 eV on Ru (0001) [56].

3.2. Elementary reaction step Methanol decomposition could involve O H, C H, and/or C O bond scissions. However, our calculated results show that the initial C H and C O bond scissions on PtRu/Pt(111) needs to overcome a rather high energy barrier (1.35 eV and 2.23 eV, see Fig. 2 and Table 2), suggesting they are too hard to occur. Therefore, only the decomposition of CH3 OH via initial C H bond scission and its further oxidation are further discussed. For simplicity, we choose the most stable sites of involved intermediates as the initial state (IS) of reaction, and the corresponding product species in the most stable

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position are final states (FS). However, to facilitate the reaction on the complex alloy surface, we may use other minor stable forms sometimes. Configurations of IS, TS, and FS involved in the elementary steps are shown in Fig. 2 and Fig. 3, and the thermodynamic and kinetic parameters are listed in Table 2. 3.2.1. Decomposition of CH3 OH This process involves the initial O H bond activation followed by sequential H-abstraction to generate adsorbed CO and H. Calculated structures for the IS, TS, and FS are presented in Fig. 2. 3.2.1.1. CH3 OH → CH3 O + H. The O H path yields methoxy and atomic H, with the reaction energy of 0.35 eV. This step is hindered by 0.70 eV, which is apparently lower than that on Pt(111) (0.81 eV) [55] and Pd(111) (1.45 eV) [67], but higher than that on Ru (0001) (0.60 eV) [57] and Ni(111) (0.40 eV) [76]. Top bound CH3 OH is selected as the IS for this step. In the TS1-1, CH3 O remains at the initial site, and the departed H is close to the adjacent Pt top site. The C–O distance is shortened to 1.389 A˚ from 1.453 A˚ in the IS and the O H distance is elongated to 1.876 A˚ from 0.980 A˚ in the IS. In the FS, CH3 O and H bind at Ru2 bridge and Pt top site, respectively. 3.2.1.2. CH3 O → CH2 O + H. This step starts with CH3 O at Ru2 bridge site, ends with CH2 O at Ru2 Pt hcp hollow site and an atomic H at Pt3 fcc hollow site as the FS (see Fig. 2). The involved TS1-2 is structurally similar to the analogous TS found on Pd(111) [67], i.e., formaldehyde-like, in which the distances of H* to C and Pt ˚ respectively, indicating rupture of the C H are 1.806 and 1.619 A, bond and formation of the Pt H bond. Besides, the relevant C O ˚ which is shorter than that in CH3 O. This process distance is 1.298 A, is exothermic by 0.04 eV with an energy barrier of 1.10 eV. 3.2.1.3. CH2 O → CHO + H. CH2 O in the ␩1 -C–␩2 -O mode undergoes the C H bond scission to produce CHOhcp + Hhcp via transition state TS1-3 (see Fig. 2). In the TS1-3, one H atom tends to decompose from the C atom and the CHO group sits nearly at the hcp site; The C H bond are elongated to 1.121 A˚ from 1.097 A˚ in the IS. Following the TS1-3, both of H atom and CHO group move to the hcp site, giving rise to the FS. This step is hindered by a barrier of 0.52 eV with an exothermicity of 0.34 eV. 3.2.1.4. CHO → CO + H. Similar to the situation of formaldehyde, the C H path of formyl is activated with the help of the C H bond stretching vibration, producing H and CO, exothermic by 0.71 eV. In the TS1-4, the H and C atoms share one surface Ru atom with the ˚ respectively. After C H and Ru H distances of 1.216 and 1.994 A, the TS1-4, the CO entity remains in the ␩2 -C configuration at the Ru2 bridge site and tends to be perpendicular to the surface, and the atomic H moves to an adjacent Ru2 Pt fcc site, due to strong repulsion of CO. The barrier for this dehydrogenation process is calculated to be 0.49 eV.

Fig. 2. Elementary steps involved in the CH3 OH decomposition process on PtRu/Pt(111) surface. Blue, cyan, gray, red, and white denoted Pt, Ru, C, O, and H atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3.2.2. Oxidation reaction The oxidation reaction could transform the carbonaceous intermediates to acids to promot the reactions. In this section, the reactions of CH2O, CHO, and CO intermediates with OH are discussed as the oxidation processes for PtRu DMFCs. For CH2 O oxidation, the energy barrier is calculated to be 0.52 eV (see Fig. S1 and Table S1), which is equal to the CH2 O decomposition process. Based on the results above, we further calculated the decomposition process of CH2 OOH (see Fig. S1). CH2 OOH decomposition involves two possible pathways, that is, initial O H and C H bond activations. The energy barrier of initial O H bond activation is calculated to be 1.34 eV, suggesting it is too high to overcome (see Table S1). Besides, we did not locate a transition state that

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Table 2 Reaction energies E, energy barrier Ea , contribution factors (in eV), and rate constants k (in s−1 ) at 300 K for the reactions on PtRu/Pt(111). Reactions

Esub

def EAB

IS EAB

TS Eint

EATS

EBTS

Ea

E

k

CH3 OH → CH3 O + H CH3 OH → CH2 OH + H CH3 OH → CH3 + OH CH3 O → CH2 O + H CH2 O → CHO + H CHO → CO + H CHO + OH → HCOOH HCOOHa → HCOO + H HCOOHb → COOH + H HCOO → CO + OH COOH → CO + OH COOH → CO2 + H

−1.00 0.10 0.02 0.17 0.05 0.14 −0.04 0.06 0.07 0.65 0.17 0.23

3.95 1.44 3.41 1.39 −0.10 −0.08 −0.85 3.40 0.51 5.18 1.34 1.43

0.90 0.90 0.90 2.70 2.91 3.21 5.48 1.12 0.73 3.45 3.29 3.29

0.61 3.63 1.26 0.59 1.80 1.19 1.10 1.34 3.84 −0.72 1.04 0.09

1.23 1.94 0.51 0.88 2.68 1.94 2.76 2.42 2.20 6.03 1.95 1.32

2.52 2.78 2.85 2.86 1.47 2.04 2.79 2.82 2.25 1.77 2.00 2.76

0.70 1.35 2.23 1.10 0.52 0.49 0.14 0.68 0.69 0.77 1.88 0.96

0.35 0.29 −0.02 −0.04 −0.34 −0.71 −0.12 −0.37 −0.40 −0.18 −0.33 0.30

2.19 × 104 4.12 × 10−7 0 3.92 × 10−3 1.98 × 105 9.0 × 106 6.35 × 109 4.39 × 103 5.22 × 102 8.39 × 100 1.46 × 10−11 2.25 × 102

connects HCOOH and H in spit of our careful search. These indicate that CH2 O prefers to decomposition rather than oxidation. The energy barrier of CHO oxidation (CHO + OH → HCOOH) is 0.14 eV, which is much smaller than that of CHO decomposition (CHO → CO + H) (0.49 eV, see Table 2). In order to contrast CHO oxidation with its decomposition more fairly, we also calculated the OH-assisted decomposition process (CHO + OH → CO + H2 O, see Fig. S2), whose energy barrier is 0.70 eV (see Table S2). These results further testify that CHO prefers to be oxidized to HCOOH rather than decomposed to CO. Besides, compared to energy barriers of 0.52 eV for CH2 O oxidation (CH2 O + OH → CH2 OOH) and 1.68 (or 2.61) eV for CO oxidation (CO + OH → COOH (or CO2 + H)) (see Fig. S3 and Table S3), the CHO oxidation process can occur spontaneously and is the most favorable channel. Thus, the CHO oxidation and its further decomposition processes are discussed in details. It can be inferred that formic acid decomposition may involve four possible pathways, initial O H, C H, and C O bonds activations of HCOOH, as well as the activation of simultaneous C H and C O bonds of HCOOH. Further calculation suggests that the initial activations of C O bond and simultaneous C H and C O bonds are too hard to occur, due to the high the energy barriers (2.05 and 3.71 eV, see Fig. S4 and Table S4). 3.2.2.1. CHO + OH → HCOOH. This reaction process starts with CHO at PtRu bridge site and OH at Ru top site, ends with HCOOH at Ru2 Pt fcc hollow site (Fig. 3). In the TS4, CHO and OH remains at the initial site, whereas the distance between hydroxy O and formyl C ˚ When the reaction prodecreases from 2.666 A˚ in the IS to 2.058 A. ceeds, the C O distance continues to decrease, and is stabilized at 1.503 A˚ in the end. This step is slightly exothermic by 0.12 eV, and the energy barrier amounts to 0.14 eV, indicating that the oxidation path of CHO is much favorable. 3.2.2.2. HCOOH → HCOO + H. The decomposition of HCOOH could be initiated by the O H bond activation. It starts with HCOOHa at Ru top site, ends with HCOO at Ru2 bridge site and H atom at Pt top site. In the TS5-1, HCOO remains at Ru top site and the H atom tilts toward nearest Pt top site. The O H distance is elongated from ˚ The reaction energy for this step is 1.030 A˚ in the IS to 1.698 A. exothermic by 0.37 eV with the energy barrier of 0.68 eV. 3.2.2.3. HCOO → CO + OH. This step begins with the stable HCOOa at Ru2 bridge site with ␩1 (O)–␩1 (O) configuration and involves both C H and C O bond activations. In the TS5-2, one O atom moves to the adjacent Ru2 Pt fcc site, and the C H bond distance ˚ which is because that the decrease from 1.102 A˚ in the IS to 1.086 A, cleavage of C O bond leads to the stronger interaction of C H bond. Simultaneously, the other C O double bond in the TS5-2 occupies the Ru2 Pt hcp site and tends to be parallel toward the surface. Finally, the C H bond cleavages to form CO and OH at the Ru–Ru bridge as the FS. The activation barrier for this elementary reaction

is 0.77 eV, and the reaction is found to be slightly exothermic by 0.18 eV. 3.2.2.4. HCOOH → COOH + H. The other pathway for HCOOH decomposition is via the C H bond activation of HCOOHb . For initial C H bond scission, the IS (HCOOHb ) locates at a Ru-top site with ␩1 (O) configuration. The H atom of C H bond is striped from HCOOH, yielding COOH at Ru2 bridge site and H atom at Pt top site. In the TS6-1, the C H distance is elongated to 1.155 A˚ from 1.099 A˚ in the IS. The energy barrier is calculated to be 0.69 eV, and the reaction energy is slightly exothermic by 0.40 eV. 3.2.2.5. COOH → CO + OH. Alternatively, COOH could decompose into CO and OH via the C O bond scission. In the TS6-2, the single C OH bond is elongated to 1.865 A˚ from 1.334 A˚ in the IS. In the FS, both CO and OH are located at the Ru2 bridge sites. The energy barrier is calculated to be 1.88 eV with the reaction energy of 0.33 eV, suggesting it is too hard to occur. 3.2.2.6. COOH → CO2 + H. Decomposition of COOH could lead to CO2 and atomic H. For this reaction, COOH locates at Ru2 bridge site as the IS and forms CO2 at Ru2 bridge site and H at Ru2 Pt hcp site as the FS. In the TS6-3, the H atom tends to strip from COOH, and the length of O H bond is elongated to 2.565 A˚ from 0.978 A˚ in IS. The length of the single C OH bond decreases to 1.197 A˚ from 1.334 A˚ in IS. The energy barrier of this step is 0.96 eV and the corresponding reaction energy is 0.30 eV. 3.3. PESs and reaction mechanisms The PES of the methanol decomposition and oxidation on the PtRu/Pt(111) surface is shown in Figs. 4–6. In order to further understand the decomposition and oxidation processes of CH3 OH, the rate constant k at 300 K for all elementary reactions involved are calculated (see Table 2). From the PES we can see that the initial decomposition of CH3 OH prefers to take place via the O–H bond scission to form CH3 O and H atom with the energy barrier of 0.70 eV. Both C H and C O bond scissions are unfavorable due to the high energy barriers of 1.35 eV and 2.23 eV, respectively. The relevant rate constant of the initial O H bond scission (about 2.19 × 104 s−1 ) is much larger than initial the C H and C O bond scissions. This situation is similar to the methanol reaction on pure Ru(0001) at high coverage [57], but contrary to that on pure Pt(111), where the initial C H bond scission of methanol is energetically preferable [55]. To further understand the essence of the O H and C H and C O bond scissions of CH3 OH, the density of states projected onto the d-states (d-PDOS) of surface Pt and Ru atoms were calculated for the bare surface and that of the bond scission transition states (See Fig. S5). The stabilization energy (Ed ) is calculated to be −4.47, −0.37, and −0.01 eV for the O H, C H, and C O bond scissions of methanol on the PtRu/Pt(111) surface,

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263

Fig. 4. The PES of dehydrogenation for methanol on PtRu/Pt(111) (in eV).

Fig. 5. The PES of oxidation for methanol on PtRu/Pt(111) (in eV).

respectively. This indicates that the interaction between the fragments and surface in the O H bond scission is much stronger than that in the C H and C O bond scissions. In addition, the energy barrier of CH3 OH decomposition via the initial O H bond scission is less than its desorption energy of 0.82 eV, indicating that adsorbed CH3 OH favors decomposition to CH3 O rather than direct desorption. After CH3 O formation, successive dehydrogenation could occur via CH3 O → CH2 O → CHO → CO,

Fig. 3. Elementary steps involved in the oxidation reactions on PtRu/Pt(111). Blue, cyan, gray, red, and white denoted Pt, Ru, C, O, and H atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Fig. 6. The whole PES of methanol reaction on PtRu/Pt(111) (in eV).

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where the first dehydrogenation (CH3 O → CH2 O + H) is found to be the rate-determining step with the barrier of 1.10 eV and the rate constant of 3.92 × 10−3 s−1 at 300 K. For the oxidation reaction, the oxidation of CO by OH is found to be too hard to occur (Ea = 2.61 eV, see Table S3). Although the CH2 O oxidation to CH2 OOH undergoes a relative low energy barrier (Ea = 0.52 eV), it is hindered by further decomposition (CH2 OOH → CH2 OO + H, Ea = 1.34 eV, see Table S1). Interestingly, CHO and OH could undertake a much feasible reaction to form HCOOH due to a much low barrier of 0.14 eV and a very large rate constant of 6.35 × 109 s−1 , in contrast to the reaction process of methanol on Cu (111) [86]. For HCOOH decomposition, the energy barriers of initial C O bond activation and simultaneous C H and C O bonds activation of HCOOH are 2.05 and 3.71 eV (see Table S4), respectively, suggesting both of them are too hard to occur. Alternatively, initial O H and C H bond scissions experience relatively low energy barriers (0.68 and 0.69 eV, see Table 2). The further decomposition of HCOO involves two possible reaction modes, that is, the CO and non-CO pathways. However, we did not locate a transition state that connects HCOO and CO2 in spit of our careful searches. Alternatively, the energy barrier of HCOO decomposition into CO is only 0.77 eV and the corresponding rate constant is 8.39 s−1 . Analogously, COOH may also occur via the CO and non-CO pathways. The CO pathway to form CO + OH is unfavorable because of the high energy barrier of 1.88 eV and the small rate constant of 1.46 × 10−11 s−1 , while the non-CO pathway to form CO2 + H is favored by the low energy barrier of 0.96 eV and large rate constant of 2.25 × 102 s−1 . Interestingly, although the energy barrier of COOH → CO2 + H (0.96 eV) is slightly higher than that of HCOO → CO + OH (0.77 eV), the relevant rate constant of the former (2.25 × 102 s−1 ) is about 27 times larger than the latter (8.39 × 100 s−1 ). It is well known that k is not only related to the energy barrier Ea , but also related to the partition functions at the IS (QIS ) and TS (QTS ). The large value of QIS /QTS for COOH → CO2 + H results in a larger k in this process than that in HCOO → CO + OH. To compare with the reaction mechanism of methanol on pure Pt, we also calculated the pathways of methanol reaction on Pt(111) by using the same parameters with those on PtRu/Pt(111). The calculated configurations, energy barriers Ea , reaction energies E, and PES of the reactions involved are given as Supporting Information (Figs. S6–S8 and Table S5). The calculation indicates that methanol decomposition prefers to take place via the initial O H bond scission to form CH3 O and H with the energy barrier of 1.18 eV, which is the highest energy demand step for the CH3 OH decomposition process. The energy barrier for CHO oxidation (CHO + OH → HCOOH) is calculated to be the same as that for its decomposition (CHO → CO + H, Ea = 0.85 eV), suggesting CHO can further react via two competitive pathways. However, both HCOOH decomposition (Ea (HCOOH → COOH (HCOO) → CO2 ) = 1.33 (2.49) eV) and CO oxidation (Ea (CO + OH → COOH → CO2 ) = 1.31 eV) must overcome a high energy barrier, indicating it is hard for methanol to transform into CO2 on Pt(111). Furthermore, for the whole methanol reaction on Pt(111), the rate controlling step is the oxidation process rather than the dehydrogenation of CH3 OH on PtRu/Pt(111). All these indicate that the CO-like intermediates tend to accumulate rapidly on Pt(111), resulting in poisoning of Pt catalyst. For the methanol reaction on PtRu/Pt(111), the PESs and rate constant analysis show that the most favorable reaction pathway proceeds via CH3 OH → CH3 O → CH2 O → CHO + OH → HCOOH → COOH → CO2 with a rate-determining step of CH3 O → CH2 O (Ea = 1.1 eV). PtRu alloy can lower the energy barriers of the non-CO path and alter the rate-determining step of the whole reaction from the oxidation process on pure Pt to

the dehydrogenation of CH3 O, improving tolerance toward CO poisoning compared to pure Pt. 3.4. Energy barrier analysis To provide further insight into the energy barrier in the CH3 OH decomposition and oxidation process, the energy barrier analysis are presented in this section. The relevant data are listed in Table 2. From Table 2 we can see that except CH3 OH → CH3 O + H and HCOO → CO + OH, Esub values of other reactions are much small (−0.04 to 0.23 eV), indicating that the structural changes of the substrate originated from the change of adsorbates have slight effect on the barriers. In addition, the adsorption energy of H in the EBTS changes slightly in the whole reaction network owing to the slight alteration of adsorption energy for H at different sites, which indicates that EBTS has less effect on the change of Ea . The calculation results show that the energy barrier of initial O H bond activation of CH3 OH is much lower than that of initial C H and C O activations. It is mainly attributed to the reverse effect of Esub to the initial O H activation and the positive effect to the initial C H and C O activations. The absolute value of def decreases from 3.95 eV to 1.39 eV, and up to 0.10 eV along EAB with the dehydrogenation of CH3 OH → CH3 O → CH2 O → CHO. Correspondingly, the C H bond in the TS is gradually stretched to a less ˚ The dehydrogenation of CH3 O is extent: 0.896 A˚ > 0.704 A˚ > 0.024 A. found to be the rate controlling step for the whole CH3 OH reaction, which mainly originates from the weak binding of CH2 O group in the TS1-2. For CHO oxidation to HCOOH, it could occur spontaneously IS and E TS have the positive (Ea = 0.14 eV). We can find that only EAB int effects to the energy barrier, while other factors have the reverse effect for this reaction. For the further decomposition of HCOOH, the energy barrier of HCOO → CO + OH (0.77 eV) is much less than that of COOH → CO + OH (1.88 eV), which is mainly attributed to the stronger adsorption energy of CO group in the TS for the former (EATS = 6.03 (TS5-2) vs. 1.95 (TS6-2) eV). From Fig. 3 we can find that CO is adsorbed at the Ru2 Pt hcp site in the TS5-2, which leads to strong interaction between CO group and the substrate, whereas in the TS6-2, the co-adsorbed CO and OH fragments at the Ru top site result in weak interaction between CO and the substrate. By comparing the CO (COOH → CO + OH) and non-CO (COOH → CO2 + H) pathways of COOH decomposition, the favorable non-CO pathway is attributed to the weak  interaction between H and CO2 groups in  TS = 0.09 eV . the TS6-3 Eint 4. Conclusions First principle periodic DFT calculations have been used to study the thermochemistry and kinetics of methanol reaction on the PtRu/Pt(111) surface. Our results show that methanol on PtRu/Pt(111) favors decomposition rather than desorption. The decomposition of methanol proceeds preferably via CH3 OH → CH3 O → CH2 O → CHO → CO. The oxidation of CO and CH2 O by OH is forbidden by the high energy barrier, while the CHO oxidation to HCOOH could occur spontaneously. Further decomposition of formic acid to CO2 and/or CO involves four possible pathways, that is, initial C H, O H, and C O bond activations as well as simultaneous activation of C H and C O bonds. Among them, formation CO via path HCOOH → HCOO → CO is the most energetically favorable, but HCOOH → COOH → CO2 is the dominant pathway from a kinetic point of view. The calculated PESs suggest that the oxidation of methanol on PtRu/Pt(111) proceed preferentially via the non-CO pathway: CH3 OH → CH3 O → CH2 O → CHO → CHOOH → COOH → CO2 , with the rate-determining step of methoxyl dehydrogenation. It can be

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seen that modification of Pt by Ru could promote decomposition of methanol rather than desorption, lower the energy barriers of the non-CO path, and alter the rate-determining step from the oxidation process on pure Pt to the dehydrogenation of CH3 O, leading to an enhancement of catalytic activity and CO-tolerance of PtRu alloy. Acknowledgments This work was supported by the Program for NSFC (21003158), Shandong Provincial Natural Science Foundation (ZR2015BQ009), Taishan Scholar Foundation (ts20130929), 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). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2015.11. 104. References [1] M. Li, P. Liu, R.R. Adzic, Platinum monolayer electrocatalysts for anodic oxidation of alcohols, J. Phys. Chem. Lett. 3 (2012) 3480–3485. [2] C.V. Federico, M.T.M. Koper, A.S. Bandarenka, Tailoring the catalytic activity of electrodes with monolayer amounts of foreign metals, Chem. Soc. Rev. 42 (2013) 5210–5230. [3] A. Kongkanand, K. Vinodgopal, S. Kuwabata, P.V. Kamat, Highly dispersed Pt catalysts on single-walled carbon nanotubes and their role in methanol oxidation, J. Phys. Chem. B 110 (2006) 16185–16188. [4] M.D. Obradovic, A.V. Tripkovic, S.L. Gojkovic, The origin of high activity of Pt–Au surfaces in the formic acid oxidation, Electrochim. Acta 55 (2009) 204–209. [5] G. Wu, L. Li, J.H. Li, B.Q. Xu, Methanol electrooxidation on Pt particles dispersed into PANI/SWNT composite films, J. Power. Sources 155 (2006) 118–127. [6] R. Parsons, T. VanderNoot, The oxidation of small organic molecules: a survey of recent fuel cell related research, J. Electroanal. Chem. 257 (1988) 9–45. [7] T.D. Jarvi, E.M. Stuve, Fundamental Aspects of Vacuum and Electrocatalytic Reactions of Methanol and Formic Acid on Platinum Surfaces, Wiley-VCH, New York, 1998, pp. 75–153. [8] K. Kunimatsu, H. Kita, Infrared spectroscopic study of methanol and formic acid absorbates on a platinum electrode: Part II. Role of the linear CO(a) derived from methanol and formic acid in the electrocatalytic oxidation of CH3 OH and HCOOH, J. Electroanal. Chem. Interface 218 (1987) 155–172. [9] D.S. Corrigan, M.J. Weaver, Mechanisms of formic acid, methanol, and carbon monoxide electrooxidation at platinum as examined by single potential alteration infrared spectroscopy, J. Electroanal. Chem. Interface 241 (1988) 143–162. [10] S. Park, Y. Xie, M.J. Weaver, Electrocatalytic pathways on carbon-supported platinum nanoparticles: comparison of particle-size-dependent rates of methanol, formic acid, and formaldehyde electrooxidation, Langmuir 18 (2002) 5792–5798. [11] A. Miki, S. Ye, M. Osawa, Surface-enhanced IR absorption on platinum nanoparticles: an application to real-time monitoring of electrocatalytic reactions, Chem. Commun. 14 (2002) 1500–1501. [12] C.W. Xu, Y.Z. Su, L.L. Tan, Z.L. Liu, J.H. Zhang, S. Chen, S.P. Jiang, Electrodeposited PtCo and PtMn electrocatalysts for methanol and ethanol electrooxidation of direct alcohol fuel cells, Electrochim. Acta 54 (2009) 6322–6326. [13] J. Luo, P.N. Njoki, Y. Lin, D. Mott, L.Y. Wang, C.J. Zhong, Characterization of carbon-supported AuPt nanoparticles for electrocatalytic methanol oxidation reaction, Langmuir 22 (2006) 2892–2898. [14] J. Luo, M.M. Maye, N.N. Kariuki, L.Y. Wang, P.N. Njoki, Y. Lin, M. Schadt, H.R. Naslund, C.J. Zhong, Electrocatalytic oxidation of methanol: carbon-supported gold–platinum nanoparticle catalysts prepared by two-phase protocol, Catal. Today 99 (2005) 291–297. [15] C.X. Xu, R.Y. Wang, M.W. Chen, Y. Zhang, Y. Ding, Dealloying to nanoporous Au/Pt alloys and their structure sensitive electrocatalytic properties, Phys. Chem. Chem. Phys. 12 (2010) 239–246. [16] J.B. Xu, K.F. Hua, G.Z. Sun, C. Wang, X.Y. Lv, Y.J. Wang, Electrooxidation of methanol on carbon nanotubes supported Pt–Fe alloy electrode, Electrochem. Commun. 8 (2006) 982–986. [17] H. Uchida, H. Ozuka, M. Watanabe, Electrochemical quartz crystal microbalance analysis of CO-tolerance at Pt–Fe alloy electrodes, Electrochim. Acta 47 (2002) 3629–3636.

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