Article pubs.acs.org/JPCC
Theoretical Study of Methanol Oxidation on the PtAu(111) Bimetallic Surface: CO Pathway vs Non-CO Pathway Wenhui Zhong, Yuxia Liu, and Dongju Zhang* Key Lab of Colloid and Interface Chemistry, Ministry of Education, Institute of Theoretical Chemistry, Shandong University, Jinan 250100, People's Republic of China ABSTRACT: By performing density functional theory calculations, we have studied the CO pathway and non-CO pathway of methanol oxidation on the PtAu(111) bimetallic surface. CO is shown to possess larger adsorption energy on the PtAu(111) surface than that on the pure Pt(111) surface, and the non-CO pathway on the bimetallic surface is found to be energetically more favorable than the CO pathway. These calculated results propose that the improved electrocatalytic activity of PtAu bimetallic catalysts for methanol oxidation should be attributed to the alternation in the major reaction pathway from the CO pathway on the pure Pt surface to the non-CO pathway on the PtAu bimetallic surface rather than the easier removal of CO on PtAu catalysts than on pure Pt catalysts.
1. INTRODUCTION Direct methanol fuel cells (DMFCs), as a viable power sources with potential applications in many systems, have attracted tremendous attention because of their low operating temperatures, ease of handling, and high-energy density.1−5 Pt and Pt group metals are known as the most excellent electrocatalysts for methanol oxidation,6−10 so they are extensively used as electrode materials of both the anode and cathode of DMFCs. However, it is well-known that pure Pt catalysts usually suffer from two major disadvantages, (i) high cost and limited resources,11 and (ii) poor performance durability resulted from the poisoning of CO and CO-like species produced during methanol oxidation at low temperatures.12−14 So, it is highly desired to find an effective approach that not only reduces the cost but also improves the tolerance toward CO-poisoning of pure Pt catalysts. Alloying Pt with another transition metal or noble metal that is generally less expensive than Pt could address these two challenging problems. In recent years, many efforts have been devoted to preparing various bimetallic Ptbased catalysts for use in DMFCs, including Pt−Mn,15 Pt− Fe,16,17 Pt−Co,18,19 Pt−Ni,20−22 Pt−Sn,23,24 Pt−Ru,25−29 Pt− Pd,30 Pt−Ag,31,32 Pt−Mo,33 and Pt−Au,34−36 which have been reported to have a positive effect on catalytic stabilities and activities with respect to methanol oxidation. Among Pt-based bimetallic alloy catalysts, Pt−Au nanostructures have attracted special attention due to their significantly improved tolerance toward CO-poisoning and enhanced electrocatalytic activity for methanol oxidation 37−40 as compared to pure Pt catalysts. In general, the enhanced CO antipoisoning ability of Pt−Au catalysts was attributed to the facile removal of CO and CO-like species adsorbed on the catalyst surface.34,38,41,42 However, several recent studies have proposed that the methanol oxidation could proceed through a non-CO-involved pathway,37,43,44 that is, the enhanced CO © 2012 American Chemical Society
antipoisoning ability of PtAu bimetallic catalysts should be attribute to their inhibition for CO formation rather than the facilitation for the removal of CO adsorbed on the catalyst surface. A very recent study reported by the group of Liu and Xing43 shows that the CO-poisoning problem during methanol electro-oxidation can even be eliminated on adjusted PtAu surface. Park et al.37 conjectured that the methanol oxidation on PtAu catalysts could involve formic acid intermediate, which dehydrogenates to form the final product CO2. To provide clue for rationally designing sustainable and costeffective catalysts for use in DMFCs, it is essential to clarify the methanol oxidation mechanism on PtAu catalysts. As far as we know, the non-CO pathway on Pt−Au bimetallic catalysts remains unelucidated theoretically at a molecular level, although several recent experimental studies have addressed this issue.37,43 Wu et al.44 have reported a computational study of the CO pathway of methanol decomposition on the PtAu(111) bimetallic surface. Based on calculated results, they proposed that proper arrangement of Au and Pt sites offers great opportunities for non-CO pathways for high H productivity in fuel cells. However, the relevant mechanism details of non-CO pathways keep unknown. In this work, we intend to make a comparison to the methanol oxidation on the PtAu(111) surface along the non-CO pathway with that along the CO pathway, from which we can elucidate the reason why Pt−Au bimetallic catalysts possess enhanced CO antipoisoning ability. Received: October 26, 2011 Revised: December 26, 2011 Published: January 5, 2012 2994
dx.doi.org/10.1021/jp210304z | J. Phys. Chem. C 2012, 116, 2994−3000
The Journal of Physical Chemistry C
Article
2. MODEL AND COMPUTATIONAL DETAILS It is well-known that formaldehyde (CH2O) is a generally accepted reactive intermediate during the methanol oxidation along both CO and non-CO pathways through the successive O−H and C−H bond cleavages (CH3OH → CH3O → CH2O) or C−H and O−H bond cleavages (CH3OH → CH2OH → CH2O).37,44−47 Thus, our present calculations start from CH2O along both the CO and non-CO pathways. As indicated in Scheme 1, the CO pathway involves two sequential
comparison, we here present the calculated results on the PtAu(111) surface. The calculations were based on the DFT-slab approach53,54 by using the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional,55,56 as implemented in the Cambridge sequential total energy package (CASTEP) code.57 The electronic wave functions were expanded in a plane wave basis set with a cutoff kinetic energy of 350 eV, and the ion cores were described by ultrasoft pseudopotentials.58 The vacuum region between slabs is 15 Å, which is sufficiently large to ensure that the interactions between repeated slabs in a direct normal to the surface are negligible. Monkhorst−Pack meshes with 2 × 2 × 1 k-grid sampling in the surface Brillouin zone were used. The criteria for energy and maximum force convergence used were 2.0 × 10−5 eV/atom and 0.05 eV/Å. The PtAu(111) surfaces were modeled using a periodic three-layer slab with a p(3 × 3) unit cell. Along the PtAu(111) surface normal, a slab geometry with two layers of Au was adopted. In the surface layer, each Pt has three first Pt neighbors, and the neighboring Pt atoms formed a chevron with 120° angle. In the slab calculations, the atoms in the top layer were allowed to be fully relaxed whereas those in the two bottom layers were fixed at their bulk-truncated structure. The transition states (TSs) were searched for with the linear and quadratic synchronous transit (LST/QST) complete search.59 Given the complexity of real-world DMFC systems, gas phase methanol oxidation studies have been used as prototype reactions to probe the mechanism of low-temperature methanol fuel cells. Therefore, as a starting point to fundamentally understanding methanol oxidation, our present calculations were performed using gas phase models.
Scheme 1. CO Pathway vs Non-CO Pathway of Methanol Oxidation
dehydrogenation processes (CH2O → CHO → CO) to produce the CO intermediate, which is further oxidized by a hydroxy group (OH) to the final product CO2,48−50 while the non-CO pathway relates to the formation of formic acid (HCOOH)37,44−47 through the reaction of CHO with OH and the final product CO2 is produced by the dehydrogenation reactions of HCOOH (CHO + OH → HCOOH → HCOO → CO2). Note that the OH group needed to complete methanol oxidation can be obtained via the reaction between coadsorbed H2O and Oad,51,52 that is, H2O + Oad → 2OH or from the alkaline electrolytes for the methanol electrooxidation under alkaline conditions.34,39−41 It is well-known that the Pt(111) surface is the most stable crystal planes in the exposed basal planes of nanoparticles, so it is generally chosen as the representative surface for both experimental and theoretical studies. In order to make a
3. RESULTS AND DISCUSSION 3.1. Structure and Adsorption Thermochemistry of Reaction Intermediates. Figure 1 shows the optimized adsorption configurations of seven species (CH2O, CHO, CO,
Figure 1. Optimized adsorption configurations of the species involved in the methanol oxidation on the PtAu(111) surface along the CO and nonCO pathways, together with the calculated adsorption energies (eV). Blue, yellow, gray, red, and white balls denote Pt, Au, C, O, and H atoms, respectively. Distances are in angstroms. 2995
dx.doi.org/10.1021/jp210304z | J. Phys. Chem. C 2012, 116, 2994−3000
The Journal of Physical Chemistry C
Article
Figure 2. Energetics of CH2O oxidation along the CO pathway (CH2O → CHO → CO) with the optimized geometries of intermediates and transition states involved in the reactions. Blue, yellow, gray, red, and white balls denote Pt, Au, C, O, and H atoms, respectively. Distances are in angstroms.
fashion, where the carbonyl oxygen binds to an atop Pt site with the OH group pointing down toward an adjacent atop Au site. The Au−H and Pt−O distances are 2.228 and 2.381 Å, respectively, and the calculated adsorption energy is 0.24 eV, which is even smaller than that of CH2O, implying that HCOOH molecules on PtAu(111) surface are also highly mobile. Formate (HCOO) binds in a bidentate configuration, where the molecule is adsorbed on a bridge site via Pt−O and Au−O bonds with a binding energy of 1.79 eV. Carbon dioxide (CO2) binds to a pair of Pt and Au atoms in a linear configuration through its two O atoms. The Pt−O and Au−O distances are 3.546 and 3.493 Å, respectively, and the binding energy is as small as 0.03 eV. The adsorption configurations of HCOOH,63−65 HCOO,66,67 and CO268 on the PtAu(111) surface are very similar to those on pure Pt(111). 3.2. CO Pathway. As shown in Figure 2, we present the energetics of the formaldehyde oxidation on the PtAu(111) surface along the CO pathway CH2O → CHO → CO. Once formaldehyde adsorbs on the PtAu(111) surface, the abstraction of hydrogen can occur via two successive dehydrogenation steps. The first C−H bond scission proceeds through a three center (H−Pt−C) transition state TS1, where the breaking C−H bond is elongated to 1.427 Å from 1.107 Å. This process is exothermic with a reaction energy of 0.41 eV but needs to overcome a barrier of 0.64 eV. As the dissociated H atom moves to an atop position on an adjacent Pt atom, the newly formed CHO binds to the surface through its C atom in an atop configuration. Then, the abstraction of the second H atom occurs through another three center (H−Pt−C) transition state TS2 with a barrier of 0.98 eV and a corresponding energy change of 1.03 eV (exothermic reaction step). As shown by TS2 in Figure 2, as the H atom is abstracted, the CO moiety tilts toward the surface to form a
OH, HCOOH, HCOO, CO2) involved along both the CO and non-CO pathways, together with the calculated adsorption energies according to Ead = −(Eadsorbate/substrate − Esubstrate − Eadsorbate). Formaldehyde (CH2O) prefers a top-bridge-top configuration through its O and C atoms binding to two adjacent Pt atoms with the Pt−O and Pt−C distances being 2.100 and 2.137 Å, respectively. The calculated adsorption energy is only 0.39 eV, indicating a relatively weak binding ability of CH2O on the bimetallic surface due to its close-shell configuration. Thus, CH2O molecules are expected to be mobile on the PtAu(111) surface. Formyl (CHO) preferentially binds to the Pt site through its C atom in an atop configuration with an adsorption energy of 2.30 eV, and carbon monoxide (CO) adsorbs on the bridge position of two adjacent Pt atoms through its C atom with an adsorption energy of 1.86 eV, which is in contrast with the fcc site adsorption of CO on Pt(111) surface with an adsorption energy of 1.77 eV. Note that the larger adsorption energy of CO on the PtAu(111) than that on pure Pt(111) surface does not support the argument that the enhanced CO antipoisoning ability of Pt−Au catalysts is due to the easier removal of CO adsorbed on the bimetallic Pt−Au catalyst surface than that on pure Pt catalyst surface.34,38,41,42 Alternatively, we believe that the alteration in the major reaction pathway may contribute to the enhanced CO antipoisoning ability of PtAu bimetallic catalysts. The adsorption configurations and adsorption energies of CH2O, CHO, and CO on PtA(111) are similar to those by Wu et al.44 The strengthened adsorption behavior of CO on the PtAu(111) is in good agreement with previous studies.60−62 Hydroxyl (OH) is found to bind through its O atom at an Au site with an atop configuration. The Au−O bond length is 2.119 Å, and the binding energy is 1.16 eV. Formic acid (HCOOH) adsorbs on the PtAu(111) surface in an upright 2996
dx.doi.org/10.1021/jp210304z | J. Phys. Chem. C 2012, 116, 2994−3000
The Journal of Physical Chemistry C
Article
Figure 3. Energetics of CH2O oxidation process along the non-CO pathway (CH2O → CHO → HCOOH → HCOOB → HCOOM → CO2) with the optimized geometries of intermediates and transition states involved in the reaction. Blue, yellow, gray, red, and white balls denote Pt, Au, C, O, and H atoms, respectively. Distances are in angstroms.
and 0.98 eV. This situation is very different from the corresponding dehydrogenation reactions of CH2O on a pure Pt(111) surface, where these two processes were found to have much lower barriers,
