PtPd(111) Surface versus PtAu(111) Surface: Which One Is More

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PtPd(111) Surface versus PtAu(111) Surface: Which one is more Active for Methanol Oxidation? Guojian You, Jian Jiang, Ming Li, Lei Li, Dianyong Tang, Jin Zhang, Xiao Cheng Zeng, and Rongxing He ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02698 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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PtPd(111) Surface versus PtAu(111) Surface: Which one is more Active for Methanol Oxidation?

Guojian You,†,‡,# Jian Jiang,ǂ,# Ming Li,† Lei Li, ǂ Dianyong Tang,‡,* Jin Zhang,‡ Xiao Cheng Zeng,ǂ,* and Rongxing He†,* †

Key Laboratory of Luminescence and Real-Time Analytical Chemistry (Southwest University),

Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China ‡

Research Institute for New Materials Technology and Chongqing Key Laboratory of

Environmental Materials and Remediation Technologies, Chongqing University of Arts and Sciences, Chongqing 402160, China ǂ

Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska, 68588, USA

1 Environment ACS Paragon Plus

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ABSTRACT: We have investigated the heterogeneous catalytic mechanism of methanol oxidation on the PtAu(111) and PtPd(111) surfaces. Density-functional theory (DFT) calculations and micro-kinetics studies show that on the PtAu(111) surface, the non-CO pathway is more favored over the CO pathway for the methanol oxidation, whereas the CO pathway is more favored on the PtPd(111) surface. This result indicates that the PtAu(111) surface apparently has higher CO-poisoning tolerance than the PtPd(111) surface since PtAu can be more effective to avert CO formation. On the other hand, our complementary experiment indicates that PtPd(111) is actually more active for the methanol oxidation despite of its lower CO-poisoning tolerance than PtAu(111). To reconcile the apparent inconsistency between the computation and experiment, we have performed additional DFT calculations and found that the adsorbed CO on PtAu(111) cannot be fully removed during the methanol oxidation, thereby PtAu(111) can still be poisoned by the CO and give lower catalytic activity. In contrast, PtPd(111) entails more OH adsorption intermediates to facilitate both the oxidation and removal of adsorbed CO, thereby having higher number of active Pt sites for methanol oxidation and giving higher catalytic activity than PtAu(111). Our finding shows the importance of OHassisted CO removal from the PtPd(111) surface on the assessment of catalytic activity of PtPd catalysts, and offers insight into the catalytic mechanism for the methanol oxidation on the PtAu(111) and PtPd(111) surfaces. The comprehensive mechanistic study will benefit future design of more efficient and stable metal alloy catalysts for direct methanol fuel cell applications.

KEYWORDS: Catalytic mechanism; Methanol oxidation; CO elimination; Electrocatalysis; Density functional theory


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1. INTRODUCTION Direct methanol fuel cells (DMFCs) are promising power sources for various applications due to their relatively high conversion efficiencies, low operating temperatures, as well as environmental benignity.1-4 The methanol oxidation reaction (MOR) is the key electrode reaction in DMFCs. Metallic Pt is generally employed as the electro-catalyst for the MOR in DMFCs in light of its exceptional catalytic activity5. However, the high cost and limited supply of Pt hinder widespread commercialization of Pt-based DMFCs. Moreover, Pt catalysts tend to be easily poisoned by CO or CO-like intermediates formed during methanol oxidation.6-8 Hence, development of an effective strategy to lower the usage of Pt while enhancing the CO-poisoning tolerance of Pt-based catalysts has been an active area of research. In recent years, various strategies to enhance the catalytic activity of Pt-based catalysts for the MOR have been tested. One strategy involves alloying Pt with other metals to form metallic alloy catalysts. This strategy can enhance the catalytic activities based on bifunctional mechanism or electronic effect.9-11 Various PtM (M = Ru, Co, Ni, Cu, Au, and Pd) bimetallic catalysts with high electrocatalytic activities and high stabilities have been reported.12-18 Among bimetallic Pt-based catalysts, PtAu and PtPd have attracted special attention due to their much improved tolerances of CO poisoning and notably enhanced catalytic performance in MORs compared with pure Pt catalysts.16-17,

19-20

For example, Liu et al. suggested that in

methanol oxidation, the enhanced catalytic activity of PtAu with respect to pure Pt catalysts is due to much reduced amounts of CO and CO-like intermediates formed.21 Park et al. showed that the methanol oxidation on the PtAu catalysts can involve formation of formic acid intermediates, followed by their decomposition to CO2.22 The catalytic activity of bimetallic PtPd in fuel cells can be also greatly improved due in part to similar physical properties of Pd as Pt.23 Liu and co-


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workers reported that PtPd nanospheres show significant electrocatalytic activity in formic acid decomposition.24 Li et al. showed that PtPd catalysts can enhance oxygen reduction reactions.25 Chen and co-workers synthesized a series of PtPd catalysts with different compositions and investigated their relative electrocatalytic activities in MORs. They found that the catalyst with a Pt:Pd atomic ratio of 1:1 gives the highest catalytic activity for the methanol oxidation.26 Today, two prevailing mechanisms have been used to explain the enhanced catalytic activity for bimetallic catalysts. First, the weakened CO binding on Pt with the presence of Au or Pd, where Au or Pd serves as an electronic modifier to reduce CO poisoning effect on Pt. 18 This indicates that the added anti-CO-poisoning abilities of bimetallic catalysts are largely attributed to the enhanced ability of removing CO and CO-like species with the alloy catalyst.27-29 However, several studies have also indicated that the methanol oxidation can proceed via a nonCO pathway on the catalyst surface. In other words, the apparently enhanced anti-CO-poisoning ability can be attributed to the inhibition of CO formation rather than the weakened CO binding on Pt or the removal of adsorbed CO.21, 30-31 The activity and stability of bimetallic catalysts can be also attributed to ligand effects, e.g., the effects of bonding, charge transfer, and strain. But they also strongly depend on the ensemble effects, which are related to the synergistic behaviors of different constituents in specific arrangements.32 Many theoretical studies have been performed to determine the most energetically favorable pathway for methanol decomposition on PtAu catalysts.31, 33 However, detailed reaction mechanism for methanol decomposition via non-CO pathway or for the removal of CO adsorbed intermediate from the PtAu(111) or PtPd(111) surfaces remain incompletely understood. An improved understanding of the methanol oxidation mechanism will be helpful to the rational design of more efficient and stable catalysts for DMFCs.


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In this study, theoretical density functional theory (DFT) methods and kinetics studies were used to investigate different mechanisms of methanol oxidation on PtAu(111) and PtPd(111) surfaces, and relative electrocatalytic activities and CO-poisoning tolerance for PtAu and PtPd catalysts. To examine the theoretical results, PtAu and PtPd bimetallic catalysts, synthesized using an electrodeposition method, were used to evaluate the electrocatalytic performance of PtAu and PtPd catalysts toward the methanol oxidation. Through the joint experimental and theoretical study, we found fundamental differences between the two common bimetallic Ptbased catalysts for methanol oxidation: PtAu(111) decomposes methanol via a non-CO pathway but cannot fully avert CO poison effect, while PtAu(111) catalyzes methanol via the CO pathway while entails more OH adsorption intermediates to facilitate the removal of adsorbed CO. This new understanding provides mechanistic basis for rational design of alloy catalysts for DMFCs.

2. EXPERIMENTAL SECTION 2.1 Models and computational details All calculations were performed in the density-functional theory (DFT) framework using Dmol3 software.34 The generalized gradient approximation (GGA) for the description of exchange and correlation energies with the Perdeu-Burke-Ernzerhof functional was employed.35 Double-numerical plus p-function (DNP) basis sets with a real-space cutoff radius of 4.5 Å were adopted. Density functional semi-core pseudopotentials36 were selected to treat the core electrons of Pt, Pd, and Au. Other atoms were treated using an all-electron basis set. Brillouin zone integration was performed on a 4 × 4 × 1 grid, using the Monkhorst–Pack k points.37 It is well known that the surface orientations, such as (111), (110), or (100), exhibit different catalytic behaviors and have limited influence to the catalytic trend on these metal


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surfaces with the same crystal structures. However, the anisotropic growth of the crystal indicates that the Pt(111) plane is the most stable plane in the exposed basal planes of nanoparticles. As a result, the Pt(111) plane is generally chosen as the prototype surface in previouis experimental and theoretical studies of its catalytic activity.38 Here, the homogeneous 1:1 PtAu and PtPd alloys are used as the model for the DMFC catalyst. The Pt, Au and Pd have uniform face-centered cubic (fcc) lattice in the unit cell and their lattice constants are similar, which can form close-packing fcc structure. Based on this, the original Pt atoms in Pt crystal cell are replaced by the Au or Pd atoms to form 1:1 PtAu and PtPd alloys cell. The stable 1:1 PtAu and PtPd alloys are used to construct the PtAu(111) and PtPd(111) surfaces. The optimized structural parameters for PtAu and PtPd bulk materials are 3.999 (a = b = c) and 3.333 (a = b = c) Å, respectively. To discuss the effect of the chemical order on the reactivity of PtAu and PtPd alloys, the arrangement of surface and sub-surface atoms is tested. The test results indicate that the chemical order has little influence on the methanol oxidation on the PtAu(111) and PtPd(111) surfaces. Detailed results and discussions of the chemical order are presented in Supporting Information. The PtAu(111) and PtPd(111) surfaces were modeled using a periodic six-atomic-layer slab with a 15 Å vacuum region to separate the slabs, and a p(4 × 4) super cell with 16 atoms in each layer. The Pt atoms were arranged linearly in the surface layer; this is consistent with the experimental results reported by Cuesta et al., which showed that methanol oxidation to CO requires a surface ensemble with three contiguous Pt atoms.39 For the slab calculations, all atoms other than those in the bottom two layers were fully relaxed during the geometry optimization to simulate bulk constraints. The transition states (TSs) were searched using the linear and quadratic synchronous transit (LST/QST) method,40 and confirmed by relaxed potential energy


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surface scanning. The adsorption energy was defined as Eads = Eslab + Eadsorbate – Eadsoebate/slab, where Eslab and Eadsorbate are the computed energies of the bare slab and the free adsorbate, respectively, and Eadsoebate/slab is the total energy of the slab with the adsorbate. The competition of reactive configurations was taken into account through micro-kinetics analysis (details are given in Part I of Supporting Information). A realistic DMFC system is complex, therefore, methanol oxidation in the gas phase is considered as a basic reaction to investigate the mechanism in low-temperature methanol fuel cells.41-45 As a starting point for fundamental understanding of methanol oxidation on PtAu and PtPd alloy surfaces, calculations in the present study were therefore performed with consideration of the gas-phase condition. 2.2 Synthesis of PtAu and PtPd catalysts PtAu and PtPd alloy nanoparticles were synthesized using an electrodeposition method, and were supported on hydroxylated single-walled carbon nanotubes (SWCNTs-OH) as catalysts for the methanol electro-oxidation. Prior to the deposition of PtAu or PtPd alloy nanoparticles, the SWCNTs-OH were supported on a glassy carbon electrode (GCE) and scanned in 0.2 M H2SO4 aqueous solution saturated with N2 gas until a steady cyclic voltammograms was recorded. Electrodeposition of PtAu was performed at −1.0 V for 200 s in a solution of 1 mM HAuCl4 mixed with 1 mM H2PtCl6 in a 0.1 M NaClO4 electrolyte saturated with N2 gas. Pt and PtPd were electrodeposited on the SWCNTs at −0.8 V for 200 s from solutions of 1 mM H2PtCl6 without and with 1 mM PdCl2, respectively, in a 3 M KCl electrolyte saturated with N2 gas. The obtained catalysts are denoted as PtAu/CNTs and PtPd/CNTs. The electrocatalytic activities and stabilities of the PtAu and PtPd catalysts in methanol were studied using cyclic voltammetry (CV) in a solution containing 1 M methanol and 1 M NaOH for


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a certain number of cycles to examine decay of the peak currents during methanol oxidation. Linear sweep voltammetry (LSV) was performed in 1.0 M NaOH at a scanning rate of 2.0 mV s−1. All electrochemical measurements were performed at ambient temperature using a CHI660E electrochemical workstation. A GCE (Φ = 3 mm), a Pt wire, and an Ag/AgCl electrode were used as the working electrode, auxiliary electrode, and reference electrode, respectively. The electrode potentials were referenced to the Ag/AgCl electrode. The fabrication and characterization of the catalysts are described in Part II of Supporting Information. 3. RESULTS AND DISCUSSION This section contains three main subsections. In the first two subsections, the mechanisms of methanol decomposition on the PtAu(111) and PtPd(111) surfaces are analyzed. All possible reaction pathways, starting from C–H, O–H, and C–O bond scissions, respectively, are considered, together with the non-CO pathways. All possible reaction pathways are shown in Scheme 1.

Scheme 1. All possible methanol decomposition pathways on the catalyst surface.


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The primary reaction pathways predicted from the micro-kinetics analysis are in good agreement with the experimental results. In the last subsection, experimental measurements are used to further understand the mechanism of methanol oxidation and to verify the theoretical results. 3.1 Methanol decomposition on PtAu(111) surface The adsorption geometries and energies for the most stable adsorption intermediates were first investigated to achieve a better understanding of the mechanism of methanol oxidation on the PtAu(111) surface. The results are presented in Parts IV and V of Supporting Information. These geometric configurations are used to describe the elementary steps in the methanol decomposition via initial C–H and O–H bond scissions (see Figure S1). C–O bond scission was not considered because the computed activation barrier (2.27 eV) for C–O cleavage is much higher. The most stable adsorption configurations of intermediates were chosen as the initial states (IS), and the corresponding product species and atomic H at the most stable sites were chosen as the final states (FS). The related reaction energies and energy barriers for all the elementary steps are listed in Table S1 (see Part VI of Supporting Information). Initial C–H bond activation. This pathway involves initial C–H bond activation followed by sequential H abstraction to form adsorbed CO and H. Figure 1 shows that the C–H activation starts with rotation of the adsorbed methanol molecule. The methyl group moves toward the surface, and one of the C–H bonds is activated. The reaction is exothermic by 0.15 eV, and the corresponding activation barrier is 0.92 eV. CH2OH decomposition then occurs via both C–H and O–H bond scissions, generating CHOH and CH2O, respectively. The CHOH pathway involves simultaneous movement of atomic H and CHOH toward two adjacent Pt atoms, with an energy barrier of 0.62 eV. The alternative pathway, involving O–H bond scission of CH2OH,


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resulting in CH2O and atomic H, has to overcome an activation barrier of 1.00 eV (Figure S2). Scission of the C–H bond is therefore more favorable than O–H bond scission. There are also two pathways for CHO and COH formation from CHOH, i.e., via O–H and C–H cleavage, respectively. In the CHO pathway, a swag vibration of the adsorbed CHOH makes the H atom move away from the C atom, favoring O–H bond activation. This pathway is exothermic by 0.24 eV with an activation barrier of 1.01 eV. Alternatively, C–H bond scission of CHOH would give H and COH, which are located at a Pt-bridge site (Figure S3). The corresponding activation energy (1.13 eV) is 0.12 eV higher than that for O–H bond scission.

Figure 1. Potential energy surface (PES) of methanol decomposition on PtAu(111) to CO, initiated by C–H bond cleavage with optimized geometries of intermediates and transition states. Energies (in eV) are relative to the total energy of one gaseous CH3OH and clean slab.


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CHO dehydrogenation gives the final products, i.e., CO and atomic H. Scission of the C–H bond in CHO involves a three-center (H–Pt–C) TS, in which the breaking C–H bond is elongated to 1.312 from 1.114 Å. This reaction is highly exothermic, by 0.93 eV (see bottom panel in Figure 1), with an activation barrier of 0.21 eV. The most favorable pathway for methanol oxidation via initial C–H bond scission is therefore CH3OH → CH2OH → CHOH → CHO → CO, with rate-determining-step energy barrier of 1.01 eV. The corresponding activation energies for this reaction pathway on the Pt(111) surface are 0.67, 0.63, 0.43, and 0.23 eV, respectively. 46 These energy barriers are much lower than those on the PtAu(111) surface because of the availability of the linearly arranged Pt atoms. Initial O–H bond activation. The activation and reaction energies of methanol decomposition via initial O–H bond scission are shown in Figure 2. In initial O–H bond scission, the activated O–H bond is lengthened to 1.404 Å from 0.976 Å, and a Pt–H bond of length 1.562 Å is formed in the FS. The reaction is endothermic by 0.64 eV, and the corresponding activation energy is 1.03 eV, which is higher than that with the pure Pt(111) surface.46 However, methyl H abstraction from CH3O is easy, with an activation barrier as low as 0.05 eV, and the reaction is exothermic by 0.42 eV. It is clear that CH3O should decompose rapidly to CH2O on the PtAu(111) surface. In the process, the abstracted H is transferred to a Pt top site and the breaking C–H bond is elongated to 2.034 Å in the TS. After formation of the TS, the atomic H and CH2O move toward the adjacent Pt2-bridge site. Further decomposition of CH2O leads to co-adsorption of CHO and H, with an energy loss of 0.69 eV and an energy barrier of 0.46 eV. This process involves rotation of the C–O bond, resulting in Pt–O bond rupture, and the abstracted H atom binds to the surface Pt atom, leading to CHO adsorption on a Pt top site. Subsequent H abstraction from CHO forms the final product CO and atomic H. The process is exothermic by


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0.93 eV, with an activation barrier of 0.21 eV. The barrier is much lower than that with the PtAu(111) surface, which has three Pt hollow sites available.47 The corresponding activation energies in the O–H bond scission pathway on the Pt(111) surface are 0.81, 0.25, 0.10, and 0.23 eV. These barriers are much lower than those on the PtAu(111) surface; this is primarily attributed to different surface structures.

Figure 2. Potential energy surface (PES) of methanol decomposition on PtAu(111) to CO initiated by O–H bond cleavage, with optimized geometries of intermediates and transition states. Energies (in eV) are relative to the total energy of one gaseous CH3OH and clean slab.

Non-CO pathway. Many studies have shown that the intermediates produced during the methanol oxidation can react with adsorbed OH groups derived from the alkaline electrolyte


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solution or activation of H2O.8,

30-31, 48

The OH group plays a key role in the MOR. It can

transform adsorbed intermediates into HCOOH, and this promotes the reaction. Zhang et al.49 and Guo et al.50 showed that CHO oxidation by OH forms HCOOH, which then dehydrogenates to the final product, CO2. Figure 3 shows that CHO binds to a Pt top site through its C atom and the OH is adsorbed on a neighboring Au atom through its O atom. The addition reaction encounters an energy barrier of 0.01 eV. These results indicate that the CHO oxidation can occur spontaneously.

Figure 3. PES of CHO oxidation and its decomposition on PtAu(111), with optimized geometries of intermediates and transition states. Energies (in eV) are relative to the total energy of CHO and OH on the slab.

The newly formed HCOOHT (the subscript T denotes that the HCOOH species is in the trans form) isomerizes into unstable HCOOHC (the subscript C denotes that the HCOOH species is in the cis form), which is far from the surface, with two Pt–H bonds of length being 2.157 and 2.408 Å. The activation barrier is predicted to be 0.42 eV. Subsequent H abstraction


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from HCOOHC affords formate and co-adsorbed H through a three-centered (H–Pt–C) TS, with a barrier of 0.27 eV and a corresponding energy change of 0.31 eV. The decomposition of COOHT (the subscript T denotes that the COOH species is in the trans form) to CO2 is slightly endothermic, by 0.01 eV, with an activation barrier of 0.74 eV. CO elimination by OH. The above discussion shows that the activation energy for CHO decomposition on PtAu(111) is as small as 0.21 eV, and the adsorption of CO on the PtAu(111) surface is stronger than that on the pure Pt(111) surface. CO would therefore be adsorbed on the PtAu(111) surface, leading to less number of available active sites for the MOR, and further resulting in poor catalytic activity and durability. The MOR is generally thought to proceed via a bifunctional mechanism, in which adsorbed OH groups facilitate the oxidation and removal of CO adsorbed on the Pt active sites.51-53 More Pt active sites available would enhance the catalytic performance in methanol oxidation.54 Consequently, it is important to understand the mechanism of CO removal assisted by OH. Figure 4 shows that CO removal starts with an addition reaction of CO with OH to form COOHC (the subscript C denotes that the COOH species is in the cis form), with an energy barrier of 0.58 eV. The reaction is exothermic by 0.93 eV. The COOHC adsorbed on the PtAu(111) surface adopts an atop configuration via binding of a C atom to a Pt site. The COOHC then isomerizes into COOHT (the subscript T denotes that the COOH species is in the trans form) by rotation of the O–H bond, with an energy barrier of 0.50 eV. Finally, COOHT forms CO2 via breaking of the O–H bond, with a barrier of 0.74 eV; the reaction is endothermic by 0.01 eV.


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Figure 4. PES of CO oxidation to form intermediate COOH and its decomposition on PtAu(111), with optimized geometries of intermediates and transition states. Energies (in eV) are relative to the total energy of CO and OH on the slab.

In summary, the pathway involving initial C–H bond scission has three sizable barriers, and CHOH decomposition is the rate-determining step with an energy barrier of 1.01 eV. In contrast, the decomposition of methanol via O–H bond scission has only one major energy barrier to overcome, and the rate-determining step is the initial O–H bond scission with a barrier of 1.03 eV. These results indicate that both pathways are kinetically favorable because there is a little difference between the energies for initial cleavage of O–H and C–H bonds. The initial C– H bond cleavage is slightly favorable than O–H bond cleavage thermodynamically. However, along the initial O-H bond scission pathway, the overall reaction rate could be reduced by the following reactions. This hypothesis has been confirmed via our further micro-kinetics analysis, as can be seen in Figure S4. Therefore, the methanol oxidation tends to proceed through CH3OH


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→ CH3O → CH2O → CHO → HCOOH → HCOOHC → COOHT → CO2, where the activation energy barriers are 1.03 eV, 0.05 eV, 0.46 eV, 0.01 eV, 0.42 eV, 0.27 eV and 0.74 eV, respectively. In Figure S5, the maximum of the dependence of CO pathway is close to 5.74 percent when the system reaches the steady state. This result indicates that 94.3 percent of CO2 formation comes from the non-CO pathway (Figure S6 gives the CO2 formation coverage on PtAu(111) and PtPd(111) surfaces within 10 µs). Compared to this pathway, the pathway which starts from initial C-H bond scission, CH3OH → CH2OH → CHOH → CHO → HCOOH → COOH → CO2 with the activation energies being 0.92 eV, 0.62 eV, 1.01 eV, 0.42 eV, 0.27 eV and 0.74 eV, respectively, is less kinetically favorable due to higher activation energy for second and third steps. We can therefore conclude that methanol decomposition is mainly initiated by O–H bond scission. In the O–H bond scission pathway, C–H bond scission is preferred to O–H bond scission in both CH3O and CH2O, as reflected by the corresponding energy barriers. Therefore, the most favorable CO pathway for the overall reaction on the PtAu(111) surface is CH3OH → CH3O → CH2O → CHO → CO (1.03 eV, 0.05 eV , 0.46 eV, 0.21 eV). The above comparison shows that the barrier associated with the rate-determining step in the non-CO pathway is lower by 0.29 eV than that associated with the CO pathway. Methanol decomposition on the PtAu(111) surface therefore preferentially occurs via the non-CO pathway rather than the CO pathway. Clearly, the most favorable pathway is CH3OH → CH3O → CH2O → CHO → HCOOH → HCOOHC → COOHT → CO2. The enhanced electrocatalytic activity of the PtAu catalyst in methanol oxidation can therefore be attributed to the major reaction pathway changing from the CO pathway on the pure Pt surface to the non-CO pathway on the PtAu bimetallic surface.


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Additionally, the enhanced anti-CO-poisoning ability can be ascribed to inhibition of CO formation rather than facilitation of removal of CO adsorbed on the catalyst surface. 3.2. Methanol decomposition on PtPd(111) surface The physical properties of Pd are similar to those of Pt, including the face-centered cubic crystal structure and similar electronegativity. These inherent properties are favorable for binding methanol molecules. Methanol decomposition can therefore begin simultaneously with methanol molecules adsorbed on Pt and Pd sites. The computed adsorption geometries and energetics of the intermediates involved in methanol decomposition on the PtPd(111) surface are described in Part V of Supporting Information. All possible elementary steps via initial O–H and C–H bond scissions were simulated to explore the mechanism of methanol oxidation on PtPd(111) (see Figures S7 and S8). The reaction energies and energy barriers for the elementary steps are listed in Tables S2 and S3. Methanol decomposition at Pd sites. The decomposition of methanol starting with initial O–H bond scission at the Pd sites of PtPd(111) is shown in Figure 5. It is clear that the initial O– H bond scission is the most demanding step, with an activation energy of 1.06 eV, which is higher than that for the Pt(111) surface, i.e., 0.81 eV.46 However, CH3O dehydrogenation to formaldehyde is very easy, with an energy barrier of 0.06 eV; and the reaction is exothermic by 0.6 eV. These results suggest that CH3O can decompose rapidly to CH2O. Subsequent C–H bond scission of formaldehyde gives CHO and H, with an energy loss of 0.77 eV. This process involves a tilt of the C–H bond, favoring C–H bond activation. The breaking C–H bond is elongated to 1.311 from 1.103 Å. The reaction is exothermic by 0.77 eV. In the final step, a swag vibration of the adsorbed CHO makes the H atom move away from the C atom, and adsorption finally occurs at a Pd2Pt hole site. The formed CO forms a monodentate configuration, with the


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C atom bound to the surface. The reaction is exothermic by 0.79 eV, with an energy barrier of 0.26 eV, 0.47 eV lower than that on the PtAu(111) surface. These energy barriers are similar to the corresponding barriers on the pure Pt(111) surface.55

Figure 5. PES of methanol decomposition to CO initiated by O–H bond cleavage at Pd site of PtPd(111), with optimized geometries of intermediates and transition states. Energies (in eV) are relative to the total energy of one gaseous CH3OH and clean slab.

The pathway initiated by C–H bond scission is shown in Figure 6. The activation energy for the initial C–H bond scission is 0.75 eV. This process involves rotation of the C–O axis so that the methyl H is close to a surface Pt atom. This process is exothermic by 0.3 eV. For CH 2OH


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decomposition, this is followed by C–H and O–H bond scissions to give CHOH and CH2O, respectively. The corresponding activation energy for the CHOH-forming pathway is 0.62 eV, and the reaction is exothermic by 0.22 eV.

Figure 6. PES of methanol decomposition to CO initiated by C–H bond cleavage at Pd site of PtPd(111), with optimized geometries of intermediates and transition states. Energies (in eV) are relative to the total energy of one gaseous CH3OH and clean slab.

The energy barrier for the CH2O pathway (Figure S9) is 1.11 eV. This suggests that CH2OH is preferentially dehydrogenated to CHOH rather than CH2O. CHOH also has two dehydrogenation pathways, i.e., formation of CHO or COH. In the CHO-forming pathway, scission of the O–H bond in CHOH produces CHO and H, with an energy loss of 0.23 eV. The corresponding activation energy for this reaction is 0.83 eV. The activation energy of the COH pathway (Figure


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S10) is 0.88 eV, suggesting that O–H bond scission is more favorable than C–H scission. This situation is similar to that for CHOH decomposition on the Pd(111) surface. Finally, H abstraction from CHO forms CO and H, with an activation barrier of 0.26 eV; the reaction is exothermic by 0.79 eV. This situation is similar to that on PtAu(111), and C–H bond scission in CHO is more favorable. Consequently, CO is so strongly bound that it can poison the catalyst surface, resulting in poor catalytic activity and durability. Methanol decomposition at Pt sites. Significantly, the major reaction pathway in methanol decomposition at the Pt sites of PtPd(111) is CH3OH → CH2OH → CHOH → CHO → CO. The activation energies and reaction energies are presented in Figure 7. For the initial H abstraction from methanol, rotation of the adsorbed methanol makes the methyl group shift to the neighboring Pd atom and one of the C–H bonds can be activated. The activation barrier for this process is 0.95 eV, and the endothermic energy is 0.09 eV. Dehydrogenation of CH2OH can be followed by both C–H and O–H bond scissions, forming CHOH and CH2O, respectively. The CHOH pathway involves rotation of the C–O bond, and CHOH and atomic H simultaneously move to adjacent Pt sites, with an activation barrier of 0.54 eV. Alternatively, scission of the O– H bond in CH2OH results in formation of intermediates CH2O and H, with an energy barrier of 0.67 eV (Figure S11). This suggests that C–H bond scission is more favorable than O–H bond scission. The dehydrogenation of CHOH also has two reaction pathways, i.e., via formation of COH or CHO. In the CHO pathway, the Pd–C bond of length is broken with a distance of 2.956 Å in the TS, and CHO adopts a monodentate configuration. The activation barrier for this reaction is 0.03 eV, and the reaction is exothermic by 0.24 eV. Alternatively, scission of the O–H bond in CHOH gives COH and H (Figure S12), with an energy barrier of 0.72 eV. Scission of the O–H bond in CHOH is therefore more favorable than C–H bond scission, similar to the


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situation on PtAu(111). In the final step, CHO is dehydrogenated to CO via breakage of the C–H bond, with an activation barrier of 0.26 eV; the reaction is exothermic by 0.79 eV.

Figure 7. PES of methanol decomposition to CO, initiated by C–H bond cleavage at Pt site of PtPd(111), with optimized geometries of intermediates and transition states. Energies (in eV) are relative to the total energy of one gaseous CH3OH and clean slab.

A comparison of the two possible dehydrogenation pathways for methanol decomposition at Pd sites on the PtPd(111) surface. It can be seen that in the O–H scission pathway the initial H abstraction from methanol is the rate-determining step, with a barrier of 1.06 eV. In contrast, in the C–H bond scission pathway, CHOH dehydrogenation to CHO is the rate-determining step, with a barrier of 0.83 eV. Clearly, the barrier for the rate-determining step in the C–H bond scission pathway is lower by 0.23 eV than that in the O–H bond scission pathway. The C–H scission pathway is therefore more favorable kinetically than the initial O–H bond scission 21 Environment ACS Paragon Plus

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pathway, which is supported by the stronger adsorption of CH2OH than that of CH3O. A further comparison of methanol oxidations at Pt and Pd sites shows that that initial C–H bond scission at the Pd sites of PtPd(111) is more favorable kinetically (0.20 eV) and thermodynamically (0.39 eV) than that in methanol decomposition at the Pt sites of PtPd(111). In the overall reaction pathway, the first step in methanol dehydrogenation involves methanol adsorption on the Pd sites of PtPd(111), and C–H bond scission is more favorable than O–H bond scission. In the C–H bond scission pathway, O–H bond scission is preferred to C–H bond scission in the dehydrogenation of CH2OH, as reflected by the corresponding energy barriers. Dehydrogenation of CHOH to CHO is not feasible because of the high activation barrier of 0.83 eV. However, the reaction sites of methanol oxidation at the Pd and Pt atoms could cross each other. The probability of such a crossover was evaluated by investigating the migration of CHOH to Pt sites. Figure 8 shows that migration of CHOH from the Pd site to the Pt site has an energy of 0.30 eV, which is much lower than that of CHOH decomposition to form CHO at the Pd sites of PtPd(111). The reaction is endothermic by 0.12 eV. CHOH dehydrogenation to CHO via O–H bond scission is more favorable at the Pd sites of PtPd(111), with an energy barrier of 0.03 eV. Finally, the activation energy for CHO decomposition to CO is 0.26 eV. These activation energies are similar to those on pure Pt(111) surface.


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Figure 8. Energetics of CHOH migration from Pd site to Pt site on PtPd(111). Values in left, middle, and right column are adsorption energies, energy barriers, and net reaction energies, respectively. Bond lengths are in angstroms and energies are in eV.

Non-CO pathway. The non-CO pathway involves an addition reaction of CHO with OH to form HCOOH as an intermediate, which is then dehydrogenated to CO2, as shown in Scheme 1. The energetic data are listed in Table S2. The energy barrier for the addition reaction is 0.76 eV, and the reaction is exothermic by 0.28 eV. There are two pathways for further dehydrogenation of HCOOH to CO2, i.e., HCOOH → COOH → CO2 and HCOOH → HCOO → CO2. The corresponding energy barriers for the rate-determining steps in these two pathways are 1.41 and 2.25 eV, respectively, which are much higher than that in the CO pathway. The preferred pathway in methanol decomposition on PtPd(111) is therefore the CO pathway rather than the non-CO one, and CO formation contributes to poor catalytic activity and durability. CO elimination by OH. Scheme 1 shows that CO can be further oxidized by OH to CO2. The calculated energetic data for the pathway CO + OH → COOH → CO2 are shown in Figure 9. The first step (CO + OH → COOH) in the reaction involves co-adsorption of CO and OH intermediates on the PtPd(111) surface. The activation barrier for this reaction is 0.41 eV, and the reaction is exothermic by 0.33 eV. Subsequently, COOH is dehydrogenated to CO2 and atomic H through rupture of the O–H bond, with an energy barrier of 0.76 eV.


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Many experimental studies have shown that OH groups play a crucial role in methanol oxidation in alkaline electrolyte solution.8,

56-57

High surface coverage of OH promotes the

formation of hydrogen bonds between adsorbed OH groups, and this lowers the energy barrier for COOH dehydrogenation to form CO2. CO adsorbed on the PtPd(111) surface can therefore be removed by surface OH groups assisted.58

Figure 9. PES of CO oxidation to form intermediate COOH and its decomposition on PtPd(111), with optimized geometries of intermediates and transition states. Energies (in eV) are relative to the total energy of CO and OH on the slab.

In summary, the decomposition of methanol on PtPd(111) preferentially occurs via the CO pathway rather than the non-CO pathway, which is also confirmed by our micro-kinetic analysis. Figure S13 showing that CH3OH → CH2OH → CHOH → CHO → CO → COOH → CO2 pathway is more favorable energetically and kinetically, and the reaction proceeds


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simultaneously from methanol adsorbed on Pt and Pd atoms, although this should be a competitive pathway. The two reaction pathways can cross each other in the middle stage, i.e., CHOH formation, which promotes the reaction. The final product, i.e., CO, is adsorbed on the PtPd(111) surface, and can poison it. However, OH species facilitate oxidation and removal of the intermediate CO adsorbed on Pt active sites. Consequently, more Pt active sites become available for methanol oxidation. In addition, OH assistance decreases the energy barrier of COOH dehydrogenation to form CO2. The barrier for this step is governed by the interactions between the C atom of CO and the O atom of OH. The projected local densities of state (PDOS) for COOH formation from CO and OH are plotted. Figure S14 shows that the overlap areas between the d bands of Pt and those of the p bands of the C atom of CO gradually decrease, indicating that the interactions between them become increasingly weak and the energy barriers increase. Additionally, the overlap areas between the p bands of the O atom of OH and those of the p bands of the C atom of CO on the PtPd(111) surface are larger than those on the PtAu(111) surface. Clearly, the interaction between the O atom and the C atom on the PtPd(111) surface is stronger than that on the PtAu(111) surface, and a strong C–O bond can be formed. This lowers the energy barrier of the addition reaction between CO and OH to form intermediate COOH. The main advantage of PtPd(111) is that it enhances the removal of CO compared with CO removal from PtAu(111). 3.3. Experimental Study of Methanol oxidation on PtAu and PtPd catalysts PtAu and PtPd bimetallic catalysts were synthesized by electrodeposition to validate the theoretical results and further understanding the mechanism of methanol oxidation on both the PtAu(111) and PtPd(111) surfaces.


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Surface morphology of catalysts. The surface morphologies and crystalline structures of the obtained PtAu and PtPd catalysts were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD) (Figures S15 and S16). The SEM images show that PtAu and PtPd alloy nanoparticles were successfully deposited on the SWCNT surfaces. The diffraction peaks in the XRD patterns of the PtAu and PtPd catalysts are shifted to slightly higher 2θ angles compared with the corresponding peaks in the pure Pt catalyst, indicating the formation of PtAu and PtPd alloy catalysts. The compositions of the PtAu and PtPd alloy nanoparticles were evaluated using energy-dispersive X-ray spectroscopy. Figure S17 shows that the compositions (atomic %) of the PtAu and PtPd alloys were Pt1Au1.3 and Pt1.2Pd1, respectively. These results are consistent with the initial molar ratios and the corresponding atomic scales in the DFT calculations. Measuring electrochemical activity. The electrochemical activities of PtAu/CNTs and PtPd/CNTs were determined and compared, and the results are shown in Figure 10(a). It can be seen that the peak current for methanol oxidation on PtPd/CNTs is slightly higher than that on PtAu/CNTs, suggesting that the catalytic activity of PtPd/CNTs is slightly higher than that of PtAu/CNTs. Potential cycling over a long period shows that the stability of the PtAu/CNT catalyst is low, and the peak current sharply decreases with increasing CV cycling. In contrast, PtPd/CNTs show less loss of methanol oxidation peak current with CV cycling. These results indicate that the PtPd catalyst is more stable than the PtAu catalyst.


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Figure 10. (a) Variations in methanol oxidation peak current with cyclic voltammetry cycling on PtAu/SWCNTs (lower graph) and PtPd/SWCNTs (upper graph). (b) Linear sweep voltammetry curves recorded on PtAu/SWCNTs (red solid line, in lower graph) and PtPd/CNTs (blue solid line, in upper graph), together with corresponding curve for Pt/SWCNTs (black solid line).

Characterization of CO poisoning resistance. Generally, the ratio of the reverse current peak (Ib) to the forward oxidation current peak (If), i.e., Ib/If, can be used to represent the resistance to poisoning by CO.59 A smaller Ib/If ratio indicates improved tolerance of CO poisoning. It is clear that the Ib/If ratio of the PtAu/SWCNT catalyst is much smaller than that of the PtPd/SWCNT catalyst, suggesting that PtAu/SWCNTs exhibit better tolerance of CO poisoning. Alloy catalysts show the bifunctional mechanism in the MOR because the added metal can provide sites for adsorption of OH species, and these can facilitate the oxidation and removal of intermediate CO adsorbed on active Pt sites.11 We therefore speculate that the anti-COpoisoning ability of PtAu/SWCNTs is higher than that of PtPd/SWCNTs because of the 27 Environment ACS Paragon Plus

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different tendencies for OH formation on the Au and Pd surfaces. This inference was verified by performing LSV in N2-saturated 1.0 M NaOH at a scanning rate of 2.0 mV s−1. Figure 10(b) shows that the current for the PtPd/SWCNT catalyst is notably higher than that for the Pt catalyst. In contrast, the peak current for PtAu/SWCNTs is similar to that for the Pt catalyst. Clearly, the addition of Pd facilitates formation of Pd–OH on PtPd/SWCNTs more than the addition of Au facilitates Au–OH formation on PtAu/SWCNTs. Also, OH binds more strongly to the surface of the PtPd catalyst than to the PtAu catalyst. The addition of Pd metal to Pt to form a PtPd alloy catalyst is therefore an effective method for enhancing the catalytic activity and stability in methanol oxidation. A larger amount of OH species can also be adsorbed on the PtPd surface. The PtAu catalyst therefore has a superior anti-CO-poisoning ability and relatively low onset potential, but a lower stability and catalytic activity than the PtPd catalyst. The theoretical predictions suggest that the barrier for the rate-determining step in methanol oxidation on PtPd(111) is lower by 0.28 eV than that on the PtAu(111) surface. The catalytic activity of the PtPd catalyst is therefore higher than that of the PtAu catalyst. This provides another evidence of the high catalytic activity of the PtPd catalyst in methanol oxidation. The CO-poisoning tolerance of the PtAu catalyst is significantly better than that of the PtPd catalyst. As discussed above, this is because methanol oxidation on the PtAu catalyst proceeds via the non-CO pathway, which inhibits CO formation. However, CO adsorption on the surface during methanol oxidation cannot be completely prevented. There is still some adsorbed CO, which can poison the PtAu(111) surface, resulting in decreased catalyst stability. The PtPd catalyst is more stable than the PtAu catalyst, because OH coverage on the PtPd catalyst surface is higher than that on the PtAu catalyst surface. Consequently, hydrogen bonds can be formed between the adsorbed molecules, and this can facilitate CO removal from the PtPd catalyst


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surface, making more Pt active sites available for methanol adsorption and oxidation; this improves the catalytic activity and stability. 4. CONCLUSIONS In this joint computational/experimental study, the molecular mechanisms of methanol decomposition on PtAu(111) and PtPd(111) surfaces were investigated. Both theoretical and experimental results provide insights into the electrocatalytic activities and tolerances of COpoisoning of the PtAu and PtPd catalysts during the methanol oxidation. Major findings include: (1) In the initial dehydrogenation on PtAu(111), the methanol oxidation catalytic activity of PtAu(111) is clearly higher than that of a pure Pt catalyst, and the anti-CO-poisoning ability is higher than that of pure Pt(111) surface. The decomposition of methanol on the PtAu(111) surface proceeds referentially via a non-CO pathway, unlike that on pure Pt(111) surface. Hence, the enhanced catalytic activity and improved tolerance of CO-poisoning for the methanol oxidation on a PtAu(111) surface can be attributed to the change in the major reaction pathway, namely, from CO pathway on pure Pt to non-CO pathway on the PtAu catalyst. (2) The electrocatalytic activity of PtPd(111) is superior to that of PtAu(111) for the methanol oxidation for two reasons. First, the barrier for the rate-determining step on PtPd(111) is lower, by 0.28 eV, than that on PtAu(111). Second, the greater number of Pt active sites available improves the methanol adsorption and oxidation through OH-assisted CO removal with the PtPd catalysts. (3) The dehydrogenation of methanol on PtPd(111) preferentially proceeds via the CO pathway. Although the CO produced during the methanol oxidation can be easily adsorbed on the PtPd catalyst surface and poison the surface, the CO can be removed with the assistance of adsorbed OH species. In contrast, the CO adsorbed on the PtAu surface cannot be easily removed.


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Such a comparative study of the electrocatalytic activities and CO-poisoning tolerances of PtAu and PtPd catalysts not only reveals distinct mechanisms of methanol oxidation on PtAu(111) and PtPd(111) surfaces at the microscopic level, as well as explanations on why the PtPd catalyst is more stable than the PtAu catalyst, but also will benefit future development of more effective DMFC catalysts for wide fuel cell applications.

ASSOCIATED CONTENT Supporting Information Experimental details, structures, the detailed micro-kinetics analysis, effect of chemical order and energetics of species adsorbed on PtAu and PtPd. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

To whom correspondence may be addressed. Email: [email protected] (D. Tang), or

[email protected] (X. C. Z), or [email protected] (R. He) Notes The authors declare no competing financial interest. # These authors contributed equally to this work. ACKNOWLEDGMENTS


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This work was supported by the National Natural Science Foundation of China (Grant Nos. 21573030 and 21173169), the Chongqing Science & Technology Commission, China (Grant Nos. cstc2013jcyjA50028 and cstc2013jcyjA90015), the Scientific Research Foundation of Chongqing University of Arts and Sciences (R2013CJ03), and the program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011). The calculations were performed at the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Centre) and Holland Computing Center at University of Nebraska-Lincoln. REFERENCES (1) Ko, T. H.; Devarayan, K.; Seo, M. K.; Kim, H. Y.; Kim, B. S. Sci. Rep. 2016, 6, 20313. (2) Long, N. V.; Yang, Y.; Cao, M. T.; Minh, N. V.; Cao, Y.; Nogami, M. Nano. Energ. 2013, 2, 636-676. (3) Liu, X.; Xu, G.; Chen, Y.; Lu, T.; Tang, Y.; Xing, W. Sci. Rep. 2015, 5, 7619. (4) Wan, N. J. Power Sources. 2017, 354, 167-171. (5) Park, J. C.; Chang, H. C. J. Power Sources. 2017, 358, 76-84. (6) Lu, X.; Deng, Z.; Guo, C.; Wang, W.; Wei, S.; Ng, S. P.; Chen, X.; Ding, N.; Guo, W.; Wu, C. L. ACS Appl. Mater. Interfaces 2016, 12194-12204. (7) Xu, H.; Wang, A. L.; Tong, Y.; Li, G. R. ACS Catal. 2016, 6, 5198-5206. (8) Huang, W.; Wang, H.; Zhou, J.; Wang, J.; Duchesne, P. N.; David, M.; Zhang, P.; Han, N.; Zhao, F.; Zeng, M. Nature. Commun. 2015, 6, 10035. (9) Narayanamoorthy, B.; Datta, K. K. R.; Eswaramoorthy, M.; Balaji, S. ACS Catal. 2014, 4, 1-6. (10) Xia, B. Y.; Wu, H. B.; Wang, X.; Lou, X. W. J. Am. Chem. Soc. 2012, 134, 13934-13937. (11) Liu, Y.; Li, D.; Stamenkovic, V. R.; Soled, S.; Henao, J. D.; Sun, S. ACS Catal. 2011, 1, 1719-1723. (12) Zheng, J.; Cullen, D. A.; Forest, R. V.; Wittkopf, J.; Zhuang, Z.; Sheng, W.; Chen, J. G.; Yan, Y. ACS Catal. 2015, 5, 1468-1474. (13) Xia, B. Y.; Wu, H. B.; Li, N.; Yan, Y.; Lou, X. W.; Wang, X. Angew. Chem. 2015, 127, 3797-3801. (14) Xu, X.; Zhang, X.; Sun, H.; Yang, Y.; Dai, X.; Gao, J.; Li, X.; Zhang, P.; Wang, H. H.; Yu, N. F. Angew. Chem. 2014, 12, 12522-12527. (15) Du, X.; Luo, S.; Du, H.; Tang, M.; Huang, X.; Shen, P. K. J. Mater. Chem. A 2016, 4, 1579-1585. (16) Su, S.; Zhang, C.; Yuwen, L.; Liu, X.; Wang, L.; Fan, C.; Wang, L. Nanoscale 2015, 8, 602-608. (17) Liu, Y.; Chi, M.; Mazumder, V.; More, K. L.; Soled, S.; Henao, J. D.; Sun, S. Chem. Mater. 2011, 23, 4199-4203.


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ACS Catalysis

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PtPd(111) Surface versus PtAu(111) Surface: Which One Is More

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